摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2107-2111 Photoluminescence Spectra Resulting from Hydroxy Groups on Magnesium Oxide supported on Silica Hisao Yoshida, Tsunehiro Tanaka, Takuzo Funabiki and Satohiro Yoshida" Department of Molecular Engineering, Kyoto University, Kyoto 606-01,Japan Photoluminescent excitation and emission spectra resulting from hydroxy groups on magnesium oxide have been investigated using highly dispersed magnesium oxide species supported on silica. Since this sample has almost only one kind of Mg-0 species, reaction of the Mg-0 species with H,O produces hydroxy groups uniformly coordinated to Mg ions. Therefore, clear photoluminescent excitation spectra were obtained. Coordi- nation states of the hydroxy groups were elucidated from excitation spectra.The hydroxy group attached to the surface Mg ion is excited by 255 nm light. The other hydroxy groups which are coordinated to Mg ions within the silica matrix are excited by 265 nm light. Hydroxy groups on the sample exhibit almost the same broad emission spectra centred at 440 nm regardless of their coordination. The excited triplet states of photoactive sites on solid cata- lysts often play an important role in photocatalysis, and phosphorescence emitted from triplet states allows the photoactive sites to be studied. Consequently, photo-l~minescence'-~ and the photocatalytic activity6 of MgO have been studied extensively. We cannot disregard hydroxy groups on MgO when we examine the photoluminescence of MgO.These hydroxy groups are directly related to photoluminescent emission, and interfere with or mask the emission from Mg-0 ion pairs on the surface.lV2 Investigations into the properties of MgO bulk have been of particular interest'-7 and have revealed that hydroxy groups were removed by evacuation at high tem- peratures ; however, the assignment of luminescence by hydroxy groups remained unclear. Several coordination states of surface ions exist in the MgO bulk. The type of coordination affects the surface properties, for example, the basicity' or surface band-ga~.~ The properties of hydroxy groups on the surface may also be influenced by the coordination state. There have been some studies on the photoluminescence of dehydrated magnesium hydroxide7 and also of the hydrated surface of MgO.'V2 However, in these studies, coordination of the hydroxy groups, especially the surface ones, was not well defined, with the exception of the intrinsic hydroxy groups in Mg(OH), because of the variety of states involved.' In order to clarify the coordination, it is necessary to prepare a sample which contains uniform luminescent sites. Recently, we have found that highly dispersed magnesium oxide on silica exhibits a new type of photol~minescence~ and concluded that new Mg-0 bonds are the emission sites.The fine structure on the photoluminescence emission spec- trum reveals that the emission site, an Mg-0 bond, is dis-tributed uniformly. When hydroxy groups coordinated to these Mg ions are produced, or the 0 ions in Mg-0 bonds convert to hydroxy groups, the hydroxy group is expected to be highly uniform.In this paper, we describe the change in photoluminescence caused by adding water to this sample, and discuss the coordination states of the hydroxy groups produced. Experimental In this study, we used MgO supported on silica of 1 wt.% loading, MgO/SiO, (MS), because it gives a clear photolu- minescence spectrum due to the Mg-0 bond.g Silica (568 m2g-'), used as a support material, was prepared by the sol-gel method as described elsewhere." The sample was prepared as described in ref. 9. Prior to measurements, the sample was treated with 50 Torr 0, for 1 h at 1073 K, fol-lowed by 1 h evacuation at 1073 K. Deionized H,O was distilled, and purified by several freeze-pumpthaw cycles in a vacuum line before adsorption experiments.Photoluminescence spectra were recorded at 77 K with a Hitachi 850 fluorescence spectrometer using a UV filter (permitted wavelength >300 nm) to remove scattered light from the UV source. An in situ sample cell made of quartz (0.5 mm x 10 mm x 44 mm) was used. The amount of the sample in the cell was 200 mg, therefore the total amount of magnesium ions contained in the MS sample was 50 pmol. H,O was introduced to the sample cell at room temperature. Results The photoluminescent emission spectra of the sample vary with the wavelength of the excitation light. The characteristic spectra of the MS sample can be classified into two sets: A, emission spectra produced by excitation of the sample with light of 240 nm and excitation spectra monitored at 520 nm emission; B, emission spectra of the sample excited by 265 nm light and excitation spectra monitored at 430 nm emis- sion. Photoluminescence of SiO, Photoluminescence spectra of the silica support were re-corded as a blank test.The pretreatment was the same as that for the MS sample. Fig. 1 shows the photoluminescent emission spectra of SiO, excited by light at 240 nm (Fig. 1A) and 265 nm (Fig. 1B). In Fig. lA, a broad band centred at 480 nm was observed after evacuation at 1073 K. Upon adsorption of H,O, a component at ca. 440 nm was detected. Its intensity increased with further addition of H,O.(Note that the scales are different.) The emission at 440 nm was quenched by contact with 100 Torr He, indicating that the luminescent sites are on the silica surface. Therefore, it should be assigned to hydroxy groups produced on the silica surface. In Fig. lB, a band centred at 440 nm was already observed after evacuation at 1073 K. The peak position suggests that the band results from residual hydroxy groups on the silica. Fig. 2 shows photoluminescent excitation spectra moni- tored at 520 nm (Fig. 2A) and 430 nm (Fig. 2B) emission. After evacuation at 1073 K, a peak was observed at 250 nm. After adsorption of H,O, a peak at 240 nm appeared instead. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 A 6 "4,4Y I I I 1 I I I 1 0 400 500 600 700 0 400 500 600 700 wavelengt h/nm wavel en g t h/n m Photoluminescent emission spectra of SO, excited by A, 240 nm and B, 265 nm light; (a)after evacuation at 1073 K, (b)in the presence of 20 pmol H,O, (c) in the presence of 50 pmol H20, (d)exposed to excess H20, (e)followed by evacuation at room temperature.Intensities of recorded spectra are: A, (a) x 1.0, (b) x 1.0, (c) x 0.47, (d)x 0.40, (e) x 0.40;B, (a)1.4, (b)2.7, (c) 1,2, (d)1.2 and (e) 0.78. These results show that the luminescence, which was observed after the addition of H,O, is clearly attributed to the hydroxy groups on the SiO, surface. Therefore, we con- clude that the hydroxy groups on silica are excited by 240 nm light and that luminescence is emitted at 440 nm.A B nv) c.-C 4 v .-).c v) a3 c .-I I I L 0 300 400 200 300 400 wavelengt h/nm Fig. 2 Photoluminescent excitation spectra of SO, recorded by monitoring the emission at A, 520 nm and B, 430 nm; (a)-(e) see caption to Fig. 1. Intensities of recorded spectra are: A, (a) x 1.0, (b) x 1.3, (c) x0.80, (d)x 1.2, (e) x 1.1; B, (a) x 1.4, (b) x 1.0, (c) x0.60, (d)x 0.58 and (e) x 0.48. The luminescent site exhibiting a component of the emis- sion band at around 500 nm after evacuation at 1073 K, and the site exhibiting the excitation peak at 250 nm, are identi- cal. The luminescence was not quenched by the presence of 100 Torr He, indicating that the luminescent site exists in the silica matrix and not on the surface.This site presumably results from internal residual hydroxy groups or a radical species such as an oxygen dangling bond. Photoluminescence of tbe MS Sample: Set A In this section, the emission spectra of the MS sample excited by 240 nm light (Fig. 3) and the excitation spectra monitored at 520 nm emission (Fig. 4) are discussed. Fig. 3(a) shows the emission spectrum recorded after evacuation of the sample at 1073 K. It exhibits a band centred at 520 nm with fine structure due to the vibrational levels of the Mg-0 bond.g The excitation spectrum in Fig. 4(a)reveals a shoulder peak at 240 nm, suggesting that this photoactive Mg-0 band was excited by 240 nm light and emitted lights at 520 nm. Immediately after 20.3 pmol H,O was added at room tem- perature, the emission spectrum of the MS sample excited by 240 nm light changed; a broad band was observed with a maximum at 440 nm [Fig.3(b)] and its intensity was reduced to one third. Furthermore, the fine structure disappeared. These results suggest that H,O molecules interact with the Mg-0 bonds and new photoluminescent sites are produced giving rise to a broad peak. Subsequently, the sample was annealed at room temperature and left for 30 min. The emis- sion spectrum was recorded again at 77 K [Fig. 3(c)], and exhibits a broad band at 440 nm whose tail extends to the original band at ca. 520 nm. When a further 29.7 pmol H,O were added [Fig. 3(d)], the 440 nm band was not quenched but grew larger, whereas the intensity of the band at CQ.520 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I I I I 300 400 500 660 700 wavelengthlnm Fig. 3 Photoluminescent emission spectra of the MS sample excited by 240 nm light; (a)after evacuation at 1073 K, (b) immediately after addition of 20 pmol H20, (c)30 min later, (d) in the presence of 50 pmol H20, (e) exposed to excess H20 vapour followed by evacuation at room temperature. Intensities of recorded spectra are: (a) x 1.0,(b) x 2.9, (c) x 0.86, (d) x 1.3 and (e) x ca. 0.4. nm decreased. The sample was exposed to saturated H20 vapour at room temperature for 10 min and then evacuated. The spectrum in Fig. 3(e) changed to that comprising a single broad band centred at 440 nm, and the original band at 520 nm completely disappeared.These results indicate that the photoluminescent peak at 440 nm relates to hydroxy groups. The hydroxy groups would exist not only on the surface of magnesium oxide, but also on the surface of the silica support, and both would exhibit such photoluminescent emission spectra. It is quite probable that the Mg-0 species, which exhibits the emis- sion spectrum centred at 520 nm, reacts to give hydroxy groups. Fig. 4 shows the photoexcitation spectra. The spectrum after the pretreatment [Fig. 4(a)]exhibits a shoulder peak at 240 nm. On addition of a small amount of H,O (20 or 50 pmol: less than or comparable to the amount of Mg ions), the spectrum did not change as shown in Fig. 4(b) and (c). After an excess of H,O was added, followed by evacuation at room temperature, the shoulder shifted to 260 nm, as shown in Fig.4(d). Since silica does not exhibit such a peak at 260 nm, this excitation peak is assignable to hydroxy groups coordinated to Mg ions. On addition of a small amount of H20, the excitation spectra did not change although the emission spectra changed as mentioned above. On the other hand, when excess H20 was added, a new excitation peak appeared. From these results, we suppose that at least two kinds of OH groups are formed; one is formed initially by the adsorption of water molecules comparable to the number of Mg ions, and the other is subsequently formed by adsorption of an excess of water molecules. The spectral change clearly shows that they are distinguishable.Since magnesium ions in this sample exist as isolated monoatomic or raft-like crystallite species interacting at the 1 I 300 I 400 wavelengthlnm Fig. 4 Photoluminescent excitation spectra of the MS sample recorded by monitoring the emission at 520 nm: (a)after evacuation at 1073 K, (b) in the presence of 20 pmol H20, (c)in the presence of 50 pmol H,O, (d) exposed to excess H20 followed by evacuation at room temperature. Intensities of recorded spectra are (a) x 1.0, (b) x 1.5, (c) x 2.3 and (d) x ca. 0.4. silica surface," it is likely that at least two types of hydroxy groups coordinated to Mg ions are present on the surface; one is coordinated to only one magnesium ion and the other is bridged to Mg and Si.In this sample, it is probable that isolated Mg-0 bonds exist and that their oxygen is more reactive than the oxygen coordinated to both Mg and Si, so we suggest that the H,O molecule first reacts with this iso- lated Mg-0 bond to produce a new Mg-(OH) species. The result that the luminescence of the hydroxy groups replaces the luminescence of Mg-0 supports this assumption. It is likely that excess H20 molecules react with the oxygen bridging Mg and Si, or with MgO microcrystallites. Thus, we speculate that hydroxy groups produced by the addition of an excess of H,O are coordinated to Mg and Si, or that Mg(OH), microcrystallites are formed. The excitation wavelength of these hydroxy groups is 260 nm.Photoluminescence of the MS Sample: Set B In this section, the emission spectra of the MS sample excited by 265 nm light (Fig. 5) and the excitation spectra monitored at 430 nm emission (Fig. 6) are discussed. After pretreatment of the MS sample at 1073 K [Fig. 5(a)], the emission peak at 440 nm and the excitation peak at 265 nm [Fig. qa)] were observed. Such a peak in the excitation spectra is not reported for evacuated magnesium oxide at a 21 10 1 360 460 500 660 760 wavelength/nm Fig. 5 Photoluminescent emission spectra of the MS sample excited by 265 nm light: (a) after evacuation at 1073 K, (b) exposed to 50 pmol H,O vapour followed by evacuation at room temperature, (c) exposed to excess H,O followed by evacuation at room temperature.Intensities of recorded spectra are (a) x 1.0, (b) xO.85, and (c) x ca. 0.1 I I 3 300 400 wavelength/nm Fig. 6 Photoluminescent excitation spectra of the MS sample recorded by monitoring the emission at 430 nm: (a)after evacuation at 1073 K, (b)in the presence of 20 pmol H,O, (c)in the presence of 50 pmol H,O, (d)exposed to excess H,O followed by evacuation at room temperature. Intensities of recorded spectra are (a) x 1.0, (b) x 0.68, (c) x 1.0,and (6)x ca. 0.2. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 high temperature, and is not observed for silica (Fig. 2). The emission peak at 440 nm is assigned to hydroxy groups as described in a previous section in the case of silica. Therefore, this luminescence which is produced by excitation at 265 nm, and which emits at 440 nm is presumably due to hydroxy groups coordinated to Mg ions remaining even after evacu- ation at 1073 K for 1 h.Duley' reported that the excitations of the OH-in Mg(OH), are promoted by 267-276 nm light, and the emis- sion peaks are seen in the range 435-443 nm, and that OH- ions remain when Mg(OH), was evacuated for 1 h at lo00 K. These values are in good agreement with our observations. We found that luminescence was not quenched by contact with 100 Torr He, indicating that the luminescent species are not present on the surface. Therefore, the luminescence observed in the present work could be due either to hydroxy groups in the magnesium hydroxide crystallite which has been encapsulated in the SiO, matrix, or to hydroxy groups located on the inner interface of magnesium oxide species and silica.Hence, both hydroxide species attached to an Mg ion and the new Mg-0 species coexist in the MS sample evacuated for 1 h at 1073 K. Addition of a small amount of H20, less than, or compar- able to the Mg content, caused the excitation peak to shift to 255 nm as shown in Fig. 6(b) and (c').However, addition of an excess of H20 caused the peak to shift to 260 nm [Fig. 6(6)]. As described above, the Mg-0 species, which exhibit the fine structure in the photoluminescence excited at 240 nm, reacted with H20 molecules to produce a new Mg-(OH) species. Hence, we conclude that this new Mg-(OH) species is specifically excited by 255 nm light.On addition of an excess of H20 molecules, the peak in the excitation spectrum was replaced by a small broad maximum at 260 nm. It is likely that the excess of H,O molecules inter- feres with the localization of photoexcitation of the new Mg-(OH) species, and hence, the peak at 255 nm in the exci- tation spectrum vanished. This suggests that all Mg species may react with excess H20 molecules to form Mg(OH),-like species. Although the peak in the excitation spectrum changed, the emission spectra in Fig. 5 did not change on addition of H20. As the concentration of Mg ions in the MS sample is only 1 wt.%, one may conjecture that the emission bands centred at 440 nm are mainly due to the hydroxy groups coordinated to the silica surface.However, the presence of such obvious peaks in the excitation spectra indicates a certain amount of emission from hydroxy groups coordinated to Mg ions. Therefore, regardless of whether hydroxy groups are coordinated to Mg ions or Si ions, we can conclude that all hydroxy groups show the same photoemission spectra at 440 nm. Discussion After evacuation at 1073 K, the component of the emission band centred at 440 nm, excited by 240 nm light, resulting from hydroxy groups on the silica support was observed in a blank test, although it was scarcely observed on the MS sample, as shown in Fig. 3(a).The loading of MgO in the MS sample was 1 wt.% which corresponds to 0.25 mmol g- '. The population of residual hydroxy groups on silica calcined at 773 K was evaluated at 0.211 mmol g-'.? The photolumin- ~ t The population of surface hydroxy groups was estimated as follows.Silica was soaked in hexane. Hexane solution of n-butyl lithium was introduced into the mixture under an N, atmosphere. Evolved butane was estimated volumetrically. Si-OH + C,H,Li -,Si-OLi + C,HIof. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 escence results in this work suggest that luminescence-active silanols are not present on the surface of the MS sample. These silanols have presumably been lost by the reaction of OH with Mg(OCH,), in methanol during MS sample prep- aration. Si-OH + Mg(OCH,), --* Si-0-MgOCH, + CH,OH The photoluminescent emission spectra of other MS samples evacuated at 773 and 1073 K, which contain 20 and 1 wt.% of MgO, respectively, were reported in ref.9. The spectra of the sample evacuated at 773 K and excited by 240 nm light are the same as those of the hydroxy groups re- ported in the present paper. We mentioned in the previous paperg that the spectra result from three-coordinated Mg-0 ion pairs at the corners in MgO crystallites. Therefore, we concluded that crystallites of MgO are present on the silica surface in the sample evacuated at 773 K. Since the spectrum of the sample evacuated at 1073 K was assigned to the new Mg-0 bond, we reported that the MgO crystallites react with silica under evacuation at 1073 K, to result in the forma- tion of new Mg-0 bonds which interact with the silica.From the result described in the preceding sections, hydroxy groups excited by 240 nm light emit a broad band centred at 440 nm. Taking this into account, there are two possible considerations for the change of the photolumin- escent site due to evacuation at 1073 K. One is that the sample, after evacuation at 773 K, contained crystallites of MgO with surface hydroxy groups, and both dehydration and solid-solid reactions occurred during the evacuation at 1073 K to produce the new Mg-0 species. The other is that Mg ions were already dispersed on the silica surface and the new Mg-0 bonds, or the precursor, were formed after the evacuation at 773 K. In the latter case, the strong emission from the hydroxy groups on silica masked the emission from the Mg-0 bonds, and the dehydration at 1073 K promoted the removal of hydroxy groups to result in the spectrum exhibiting fine structure. Duley7 reported that hydroxide ions in a specific low- coordination site on Mg(OH), evacuated for 1 h at 1200 K exhibited 472 nm (2.63 eV) emission when it was excited by 270 nm light, and that this emission can be excited by the trapping of excitons created by absorption at a variety of 0:; and OH,, sites.The basis of his assignment was the change of the spectrum by subsequent evacuation of the sample for 6 h at 1200 K. The appearing emission excited by the 268 nm (4.63 eV) light with the emission spectrum centred at 386 nm (3.21 eV). He suggested that the excitation peak could be attributed to absorption by three-coordinated O2-ions3v4 on the MgO surface, and the emission band was assigned to the characteristic blue-violet emission of MgO.Almost all excitation spectra in his paper show a single peak at ca. 270 nm, regardless of the evacuation temperature. Duley argued that the emission site was excited by the trap- ping of excitons created at different absorption sites. However, the explanation is not clear because of discrep- ancies between the emission sites with the excitation sites. In the preceding sections, we have clearly assigned the peak at 255 nm in the excitation spectra and the band at 440 nm in the emission spectra to the surface hydroxy groups attached to an Mg ion. In our sample, hydroxy groups func- tion as both excitation sites and emission sites.Ordinarily, MgO crystallite has several kinds of hydroxy groups on the surface, and therefore the photoluminescence of hydroxy groups had not previously been assigned in detail because of the complexity and low resolution of photolumin-escent spectroscopy. We should make it clear that in the present paper, the sample MgO/SiO, has essentially only one kind of Mg-0 species. Conclusion Hydroxy groups coordinated to Mg or Si in MgO/SiO, are distinguishable from their pho toluminescen t excitation spectra. We conclude that for the new Mg-OH species, the hydroxy group attached to the surface Mg ion is excited by 255 nm light, and that hydroxy groups coordinated to Mg ions which are within the silica matrix are excited by 265 nm light.Surface hydroxy groups coordinated to Mg and Si, or that of Mg(OH), microcrystallites, produced by addition of an excess of H20 are excited by 260 nm light. The emission spectra relating to hydroxy groups show a broad band centred at 440 nm regardless of their coordination states. During sample preparation, the luminescence-active sila- nols are lost by reaction with Mg(OCH,),. The Mg-0 species are changed to hydroxy groups by addition of H,O molecules. We thank Professor Yamamoto at Department of Polymer Chemistry, Kyoto University for his help in the luminescence measurements. H.Y. acknowledges support by the Fellowship of the Japanese Society for the Promotion of Science (JSPS) for Japanese Junior Scientists.References 1 S. Coluccia, M. Deane and A. J. Tench, in Proc. 6th Znter- national Congress on Catalysis, ed. G. C. Bond, P. B. Wells and F. C. Tompkins, The Chemical Society, London, 1977, p. 171. 2 M. Anpo, Y. Yamada, Y. Kubokawa, S. Coluccia, A. Zecchina and M. Che, J. Chem. SOC., Faraday Trans. 1, 1988,84,751. 3 S. Coluccia, A. M. Deane and A. J. Tench, J. Chem. SOC., Faraday Trans. 1, 1978,742913. 4 S. Coluccia, A. J. Tench and R. L. Segall, J. Chem. SOC., Faraday Trans. I, 1979,75, 1769; S. Coluccia, A. Barton and A. J. Tench, J. Chem. SOC., Faraday Trans. I, 1981,77,2203; S. Coluccia and A. J. Tench, in Proc. 7th International Congress on Catalysis, ed. T. Seiyama and K. Tanabe, Kodansha, Tokyo, 1981, p. 1154. 5 A. J. Tench and G. T. Pott, Chem. Phys. Lett., 1974, 26, 590; V. A. Shvets, A. V. Kuznetsov, V. A. Fenin and V. B. Kazansky, J. Chem. SOC., Faraday Trans. 1,1985,81,2913. 6 M. Anpo, Y. Yamada and Y. Kubokawa, J. Chem. SOC., Chem. Commun., 1986, 714; M. Anpo, Y. Yamada, S. Coluccia, A. Zecchina and M. Che, J. Chem. SOC., Faraday Trans. I, 1989, 85, 609. 7 W. W. Duley, J. Chem. SOC., Faraday Trans. 1,1984,80,1173. 8 H. Kawakami and S. Yoshida, J. Chem. SOC.,Faraday Trans. 2, 1984,80,921. 9 T. Tanaka, H. Yoshida, K. Nakatsuka, T. Funabiki and S. Yoshida, J. Chem. SOC., Faraday Trans., 1992,88,2297. 10 S. Yoshida, T. Matsuzaki, T. Kashiwazaki, K. Mori and K. Tarama, Bull. Chem. SOC. Jpn., 1974,47, 1564. 11 H. Yoshida, T. Tanaka, K. Nakatsuka, T. Funabiki and S. Yoshida, in Acid-Base Catalysis 11, ed. H. Hattori, M. Misono and Y. Ono, Kodansha VCH, Tokyo, 1994, p. 473. Paper 3/05159E; Received 26th August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949002107

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2113-2118 21 13 Raman Band Shifts of y-Bi,MoO, and a-Bi,Mo,O,, exchanged with I8O Tracer at Active Sites for Reoxidation Takehiko Ono* and Nobuaki Ogata Department of Applied Chemistry, University of Osaka Prefecture, 1-1 Gakuen-cho, Sakai, Osaka 59I, Japan Catalysts in which small amounts of crystalline y-Bi,MoO, and a-Bi,Mo,O,, are present on supports have been prepared. The oxide oxygens of these samples were exchanged with l80tracer and the Raman spectrum shifts of the y and a phases were examined. With the y phase, all bands were shifted to lower frequencies, with increasing '*Ocontent, in the range 700-900 cm-'. With the a phase, the two bands at 860 and 845 cm-' were shifted preferentially to lower frequencies and other bands at ca.900 cm-' were not shifted so much initially. Applying the correlation between Raman band position and Mo-0 distance, the lattice oxygens, as oxidation sites, and the vacancies, as reoxidation sites, were discussed. With the a phase, the bands that were shifted correspond to the lattice oxygens of Mo tetrahedra with adjacent Bi ions, while the bands that were not shifted very much correspond to those situated next to Bi ion vacancies. In the catalytic oxidation of propene on the a phase, oxygen uptake also occurs at the terminal oxygen vacancies, corresponding to the bands at ca. 900 cm-'. y-Bi,MoO, and a-Bi,Mo,O,, are important phases of those of the unexchanged catalysts. ZrO, and SiO, were used Bi-Mo oxides which have high activities for partial oxidation as supports as there were no Raman bands in the range 750-and ammoxidation of alkenes.'-' These phases play impor- lo00 cm-'. Applying the correlation between Raman band tant roles in multi-component commercial catalysts. It is gen- position and Mo-0 distance, the active sites and reoxida- erally accepted that the reaction of the oxides with alkenes tion sites on the y and a phases in propene and butene oxida- and reoxidation by gaseous oxygen occur in different regions tion were discussed. of the catalyst surface.' We have studied the kinetics of, and lattice oxygen participation in, alkene oxidation over Mo-and Sb-containing oxide catalyst^.^.^ Experimental Grasselli et ~l.',~have studied the Raman band shifts of Catalystsy-Bi,MoO, exchanged with l8O tracer.They found that the shifts observed in butene oxidation are smaller than those in The a phase (Bi : Mo = 2 : 3) supported on ZrO, was pre- propene or methanol oxidation. Ueda et al.' found that the pared as follows. A nitric acid solution of Bi(N0,),.5H20 flow of lattice oxygen is different for propene and butene oxi- was first impregnated on ZrO, (50-55 m2 g-'), then dried dation using l80tracer over y phase catalyst. On the other and heated at 723 K. Mo oxide was supported next using the hand, Coulson et al." recently reported that there is no dif- ammonium heptamolybdate solution. After evaporation over ference between propene and butene oxidation over y phase a water bath, it was finally heated at 723 K for 10-15 h.The catalyst. Hoefs et al." found large spectral shifts in the y y phase (Bi : Mo = 2 : 1.04)supported on SiO, was prepared phase and little or no shifts in the a phase after propene oxi- as follows: the desired amounts of Bi(NO,), * 5H,O,ammon-dation using l80 tracer. Keulks and Krenzke', reported that ium heptamolybdate and SiO, were mixed in solution. After the lattice oxygens of crystalline y-Bi,MOO, diffuse rapidly evaporation over a water bath, the solution was heated at whereas those of a-Bi,Mo,O,, do not diffuse very much 723 K for 6 h. The y phase with Bi : Mo = 2 :1 had very low from the bulk to the surface during oxidation catalysis. Such activity and very poor selectivity according to Matsuura et In this case, the ratio was adjusted to 2 :1.04in order to an a phase, however, is suitable for l80studies since the "0 ~1.'~ tracer exchanged at the surface will not dilute into the bulk. achieve high activity and selectivity for partial oxidation.Cotton and Wing', reported that the IR intensities of Mo compounds correlate with Mo-0 distances, i.e. the higher the frequency, the shorter the length, and vice versa. Using Procedures this correlation, Matuura et a1." calculated the Raman and The reduction of catalysts with propene and butene was IR bands of some Bi-Mo oxides and made assignments carried out in a circulation system (ca. 290 cm3) at 673-723 K between calculated values and experimental ones. Hardcastle and at ca. 4 kPa. Then, reoxidation by 1802 (98%, MSD Co.and Wachs' '-' have more recently studied the direct Ltd.) was performed at the same temperature and at ca. 1 relationship between the metal-oxygen Raman stretching fre- kPa. The amount of l80substituted was determined from quencies and bond strength (bond length) using the diatomic the amount of product. Catalytic oxidation using alkene and approximation. They obtained good relationships between 1802 over these catalysts was also carried out. After the the V, Mo-0 distances and Raman peak positions for many desired reaction time, the products were analysed by gas V and Mo compounds, as far as the higher-frequency regions chromatography-mass spectrometry (GC-MS). The amount are concerned. of l80 substituted was calculated from the percentage of l80 In this work, catalysts where a small amount of crystalline in the products.y-Bi,MOO, and a-Bi,Mo,O, ,is present on supports were The catalysts (0.03-0.05g) exchanged with l80were mixed prepared. The y and a phases were exchanged with l80tracer with ZrO, in order to make a disc of ca. 10 mm diameter for by a reduction-oxidation method and by catalytic oxidation Raman spectral measurements. The Raman spectra were using 180, (98.7%). The laser Raman spectra of l8O-recorded on a JASCO NR-1000 laser Raman spectrometer. exchanged y and a phases were obtained and compared with An Ar-ion laser was tuned to the 514.5nm line for excitation. 21 14 The laser power was set at 150-200 mW. The data were stored on a computer and used for peak-shape analysis.Results and Discussion Catalysts and Their Raman Spectra Fig. 1(a) shows the Raman spectra of Bi-Mo (2 : 1) oxide on SiO, at 10 at.%. The bands at 855, 807 and 735 cm-' are attributed to crystalline y-Bi2MO06 .11*14 The band at 885 cm-' is not attributable to the y phase. As noted above, this catalyst contains an excess of Mo oxide, i.e. Bi : Mo = 2 : 1.04. According to Matuura et all4 this band comes from fi-Bi,Mo,O, produced on the y phase. Fig. l(b) shows the laser Raman spectra of Bi-Mo (2 : 3) oxide on ZrO, at 15 at.%. Sharp bands at 960,930,905, 860, 845 and 820 cm-' are observed, which are attributable to a-Bi,Mo,O,, ."714 The fractions of crystalline y-Bi2MO06 and a-Bi,Mo,O,, were determined by comparison of the X-ray diffraction or Raman peak intensities of the catalysts and mechanical mix- tures of each phase and support oxide. The results are shown in Table 1.With the Bi-Mo (2 :1, 10 at.%) oxide catalyst, the fraction of crystalline y phase is ca. 50%. The remainder seems to be dispersed on SO,. With the Bi-Mo (2 :3, 15 at.%) oxide catalyst, the a-phase fraction is 30-40%. Gener- ally, the Raman intensity is very strong for crystalline com- pounds and not for dispersed and non-crystalline c~mpounds.'~Thus, the bands in Fig. 1 mainly originate from crystalline y and a phase on supports. According to Hardcastle and Wachs," the band at 992 cm-' was observed u,.-v) a)-bb (b) c 0 .-C cn 0 0cv (3 co 1 1 I I J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 in addition to other a-phase bands. In this case, we have not found the band at 992 cm-' for the a phase. Other workers"-'4 also have not reported the band at 992 cm-' for the a phase. The band described above seems to be attribut- able to MOO,, which is formed owing to the excess of Mo. Raman Band Shifts of y-Bi,MoO, Catalyst exchanged with '*Oby a ReductiowOxidation Method The catalysts were reduced with but-1-ene and propene. After evacuation, they were reoxidized with 1602 or 1802. The yield of products, the amount of l8O replaced in pmol and the percentage exchange are described in the captions to the figures. The catalyst sample (0.03 g) has ca. 80 pmol oxygen atoms in the Bi-Mo oxides and half of them are present in crystalline y-Bi2MO06, as shown in Table 1.The Raman spectra in this case are due to crystalline y-Bi,MoO, as described above. Fig. 2 shows the Raman spectra in the case of but-1-ene. Fig. 2(a) is the reference spectrum due to a sample reoxidized by 1602, which is the same as that of the y phase before use in Fig. 1. With increasing percentage of exchange [Fig. 2(b), (c)], the band at 735 cm-' is shifted to ca. 715 cm-', whereas that at 807 cm-' becomes broader and that at 855 cm-' is slightly shifted to 850 cm-'. Therefore, the extent of band shift is greater for the 735 cm-' band than for others. The band at 885 cm-' is rapidly shifted and the original peak disappears gradually. The results in the case of propene are shown in Fig.3. Upon increasing the percentage of exchange [Fig. 3(b) and A 1. 1100 ioao goo 800 700 600 wavenumber/cm-' Fig. 2 Laser Raman spectra of y-Bi,MoO, exchanged with l8O after reduction by but-1-ene followed by reoxidation by l80,at 723 K. (a)No exchange, (b) 13 pmol(l6%) of '*O exchanged, (c) 22 pmol (27%) of l8O exchanged. 0.03 g of catalyst was used. The amount exchanged was calculated from the amounts of buta-1,3-diene (1,3- C,H,) and H,O produced. The selectivities to 43-C4H, were ca. 90%. Table 1 Properties of the catalysts used amount of crystalline material (Yo) catalysts Bi-Mo phase" particle size/A X-ray Raman -Bi-Mo (2/1)/SiO2' (10 at.%) Bi,MoO, 270 48-55 Bi-Mo (2/3)/ZrO, (15 at.%) Bi,Mo,O , 1600 44 32 From X-ray diffraction.* Surface area of ZrO, and SO, : ca. 50 m2 g-'. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1100 1000 900 800 700 600 wavenumber/cm-' Fig. 3 Laser Raman spectra of y-Bi,MoO, exchanged with "0 after reduction by propene followed by reoxidation by I8O2 at 723- 773 K. (a)No exchange, (b) 9.4 pmol(l2%) of "0exchanged, (c) 13.4 pmol (17%) of "0 exchanged. 0.03 g of catalyst. The amount exchanged was calculated from the amounts of CH,=CH-CHO (ca. 80%)and H,O produced. (c)], the band at 735 cm-' is shifted to 715 cm-', whereas that at 807 cm-' exhibits line broadening and that at 855 cm-' is shifted to 847 cm-'. These tendencies are similar to those observed in the but-1-ene results. The position of the band at 885 cm-' remains unaffected.Features of Raman Band Shifts over y-Bi,MoO, The band shifts observed for the but-1-ene and propene systems are compared at around 16-17% of exchange. Both bands [Fig. 2(b) and Fig. 3(c)] are shown in Fig. 4 with the same baseline. The band shifts at 735, 807 and 855 cm- ' for propene are larger by ca. 10 cm-' than those for but-1-ene. This tendency is similar to those reported by Glaeser et aL8 This seems to be caused by the greater consumption of lattice oxygen on the Mo octahedra of y-Bi,MoO, during propenal formation, i.e. oxygen insertion in the ally1 group. The band at 885 cm-', which seems to belong to the p phase, shifts rapidly in the case of but-1-ene. The a hydrogen of but-1-ene is generally abstracted by lattice oxygen ten times faster than that of propene.Therefore, the oxygen corresponding to the I >. c.-cn Q)c .-1 I I I I 1000 900 800 700 wavenumber/cm-' Fig. 4 Comparison between reactions of but-1-ene and propene over catalysts with similar percentages of "0exchange. (---) But-l-ene: result from Fig. 2(b);(-) propene: result from Fig. 3(c). 21 15 885 cm-' band seems to react and be exchanged more rapidly in the case of but-1-ene. Hoefs et al." have investigated IR and Raman band shifts after the catalytic oxidation of propene using "0, for the y phase. They have found that the Raman bands at 865, 812 and 728 cm-' were shifted to lower wavenumbers by 32, 42 and 28 cm-', respectively, after a sufficient time of exchange with l8O.In this study, the tendency for band shift of but-1-ene and propene is as follows: 855 -+850 < 807 + 800 < 735 4 715 cm-'. Band shape analysis was attempted for the 807 cm- ' band, which under- goes broadening, and the 735 cm- ' band, which is shifted to lower wavenumbers. The band at 807 cm-' was separated into two components at 807 and 770 cm-'. The 735 cm-' band was separated into components at 735 and 710-705 cm-'. The fraction of the shifted 710-705 cm-' band in the latter case was larger than that of the 770 cm- ' band in the former case. Thus, the band at 735 cm-' seems to be more easily shifted. The theoretical shifts at around 800-700 cm-' are calculated to be 40-34 cm-'. The shifts (25-30 cm-') observed or those obtained by shape analysis are somewhat smaller than the calculated ones in the y phase.These seem to be caused by simplifications such as the use of a diatomic Mo-0 model. Uda et ~1.~'reported that the y phase reduced by propene contained Mo4+ and Bio species and that the Mo4+ species were all reoxidized to Mo6+ by oxygen, while the Bio species were initially oxidized to Bim+ (rn = 1, 2) and then oxidized to Bi3+. This indicates that the reoxidation of bismuth vacancies takes place via transfer from molybdenum.' Thus, with the y phase, reoxidation may occur around the Mo oxygen vacancies, corresponding to the band at 735 cm-'. According to the diatomic approximation of Hardcastle et this lattice oxygen corresponds to an Mo-0 distance of 1.85 A, i.e., an oxygen layer.However, a more quantitative discussion will be required in the future for the y phase. Raman Band Shifts of a-Bi,Mo,O,, exchanged with "0 by a ReductioMxidation Method Fig. 5 shows the results of exchange with l8O after reduction by but-1-ene over the c1 phase. In 0.03 g of catalyst 8 and 17 pmol of oxygen were replaced with l80(this amount of cata- lyst contains ca. 80 pmol of Bi-Mo oxide oxygen when Bi : Mo = 2 : 3). Both bands at 860 and 845 cm-' decrease markedly. A new band at 790-800 cm-' appears. This feature is very different from that of y-Bi2MO06 as described above. With increasing fraction of "0 [Fig. 5(a)-(b)],the new band at ca. 800 cm-' increases and a new shoulder at around 900 cm -'appears.The decrease in the band intensities at 860 and 845 cm-' originates from their shifts to lower frequencies due to l80 exchange, since other peaks retain their positions before replacement. The shift due to the exchange Mo-160 to MO--'~O is calculated to be ca. 40 cm-' at around 800 cm-'. If the bands at 860 and 845 cm-' are shifted by ca. 40 cm-', new bands at 820 and 800 cm-' should appear. The new band at 800 cm- ' is consistent with the calculated one, although that at 820 cm-' overlaps with the original one. According to Newton and McDonald,,' a shift of ca. 40 cm-' was observed upon exchange with l80 for Mo-0 complex compounds. Hoefs et al." did not find such large shifts for the a-phase after exchange by catalytic oxidation of propene using except for the new band at around 800 cm-'.The shifts for the a phase are somewhat larger than those for the y phase. Fig. 6 shows the results when propene was used. After ca. 20% exchange with l80, the bands at 860 and 845 cm-' 2116 In0 b, > C...-v)c c .-1000 950 900 850 800 750 700 wavenum ber/cm -' Fig. 5 Laser Raman spectra of cl-Bi2M03012 exchanged with l80 by reoxidation with "0, after reduction by but-l-ene over Bi-Mo (2 : 3)/Zr02 (15 at.%) at 673 K. 0.03 g of catalyst was used. Amount of oxygen in the catalyst exchanged: (a)8 pmol (ca. lo%), (b) 17 pmol (ca. 20%). The selectivities to buta-lY3-diene were ca. 90%. decrease markedly, as for but-l-ene.The new band at ca. 800 cm-'grows and the band at 905 cm- 'exhibits line broaden- ing. The band shape around 850-750 cm-' is different from that in the case of but-l-ene (Fig. 5). This may be caused by "0 exchange producing broad bands due to non-crystalline Bi-Mo oxides dispersed on ZrO, . Lattice Oxygen Species estimated from Raman Peak Positions for a-Bi2Mo,012at Active Sites for Reoxidation Some workers have reported the structure of a-B~,Mo,O,,.~~,~~Fig. 7 shows the structure reported by 1000 950 900 850 800 750 700 wavenumber/cm -' Fig. 6 Laser Raman spectra of a-Bi2Mo,012 exchanged with "0 by reoxidation with "0, after reduction by propene. 0.03 g of cata- lyst was used. The amount replaced with "0was 2 15% at 703-723 K.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Fig. 7 Projection along the b axis of the structure of a-Bi2M03012 ~~reported by Cesari et ~1 al,.a2 and a3 denote the Mo tetrahedra. .~~Cesari et ~1 As shown in the figure, the a-phase consists of two kinds of twin tetrahedra, i.e. alal and a2 a,. The alal twin tetrahedra have a symmetry centre. The a1 and a2 have Bi ions adjacent to tetrahedra, while there is a Bi ion vacancy in the a, tetrahedron. The free molybdate ion, which has tetrahedral symmetry, has four vibrational modes, v,(A), v,(E), ~3(F2)and ~q(F2). v,(A) and ~3(F2)are connected with stretching modes. The three different and distorted MOO, (a1, a2, a,) have very complicated bands. Applying the Cotton-Wing relation,' Matsuura et al., l4 reported that the bands around 900 and 800 cm-' are attributable to stretching modes of each tetra- hedral species and that the six peaks of the a phase are assigned as follows: 960 (a,), 930 (a1),905 (a2),860 (al), 845 (a2)and 820 cm- (a,).Using the correlation between Raman bands and Mo-0 distances for the a phase by the diatomic approximation as proposed by Hardcastle and Wachs,' 5i16 the peak position, Mo-0 length and state of the tetrahedra were determined and are shown in Table 2; they correlate well with those reported by Matsuura et ~1.'~In this case, the bands at 860 and 845 cm-', i.e. the lattice oxygens at 1.72 A (a2)and 1.73 A (a1), are mainly reacted and exchanged, but the terminal oxygens corresponding to the bands at 959, 939 and 905 cm- 'do not participate in the oxidation very much initially.The activity for oxidative dehydrogenation is as follows: a2 = a1 > a,. The a, tetrahedra where there is a Bi ion vacancy are less active for reaction. Concerning the reoxidation step, the oxygens seems to be mainly inserted at the anion vacancy giving rise to the 860 and 845 cm-' bands i.e. oxygens at 1.72 and 1.73 A in the al and a2 tetrahedra. According to Elzen et aLY2,this a2 oxygen has a Bi-0 distance of 2.61 A, which is longer than for the eight oxygens around the Bi, cation. The oxygen giving rise to the 860 cm-' band in the a1 tetrahedron is bonded to Bi with a distance of 2.94 A, which is the longest of the eight Table 2 Assigned Raman bands, Mo-0 bond distances and tetra- hedral states of cl-Bi,Mo,012 Mo-0 bond tetrahedral Raman band"/cm- lengthb/A state' 960 1.68 930 1.69 905 1.72 860 1.72 845 1.73 820 1.78 a Band position of a-Bi2M03012 in this work.Values reported by Elzen et aL23 See Fig. 7.14,22 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 21 17 oxygens around the Bi, cation. Thus, the oxygens in the a phase are bonded to both Mo and Bi cations. Band Shifts of the a-Bi,Mo,O,, Catalyst in Catalytic Propene Oxidation using "0, Fig. 8 shows the results of catalytic oxidation over the Bi-Mo (2 : 3) 15 at.% catalyst. This catalyst (0.03 g) was exchanged with 20 pmol (25%) of l80after 15 min and 35 pmol (40%) after 50 min through CH,-CH-CHO and H20 formation.The amount exchanged was calculated from the percentage of l80 in CH2=CH-CHO. The percentage of l80in H,O was assumed to be the same as that in CH2=CH-CHO. The band at around 860 cm-' increases and the band at 845 cm-' decreases. A new band at ca. 800 cm-' appears. In order to elucidate the shift change in detail, the band changes are determined by shape analysis (Fig. 9). In Fig. 9, the band at 820 cm-' (aj)seems to show little shift in the initial stages, since the new band does not appear at around 780 cm-l. So, this band may be taken as a reference. Table 3 lists the band intensity (peak area) ratio. I905/I820 and I,,$,/I82, slowly decrease with reaction time. 1845/1820 decreases markedly with reaction time.On the other hand, I8,0/I820 increases. The results indicate that the band at 845 cm-' is more In 0 0 >. c.-In al c .-C 1000 950 900 850 800 750 700 waven u rnber/crn -' Fig. 8 Laser Raman spectra of a-Bi,Mo,O,, exchanged with "0 by propene oxidation using "0, over Bi-Mo (2/3)/Zr02 (15 at.%). 0.03 g of catalyst was used. Amount of oxygen in the catalyst exchanged: (a)20 pmol(25%) in 15 min; (b) 35 prnol(40%) in 50 min. The selectivities to propenal were ca. 90%. ~ 1000 900 800 700 wavenurn ber/cm -' Fig. 9 Band-shape analysis of the laser Raman spectra in Fig. 8. (a) Bands in Fig. 1 (no "0 exchange), (b) bands in Fig. 8(a), and (c) bands in Fig. 8(b). exchanged and that the band at 865 cm-' increases owing to overlap with some shifted band.The shifted band seems to be that at 905 cm-' because the other bands at 925 and 955 cm-' have too high wavenumbers. The calculated shift for ca.45 cm -'fits well here. The feature (Fig. 8) during catalytic oxidation is somewhat different from those (Fig. 5 and 6) where the exchange was performed separately by a reduction-oxidation method. The oxygen corresponding to the 905 cm-' band is attributed to the terminal one in the a2 state as shown in Table 2. In the case of a reduction-oxidation method, the band at 905 cm-' did not exhange very much. However, in the case of catalytic oxidation, i.e. in the presence of gaseous oxygen, the reoxida- tion takes place at terminal oxygen (905 cm-') vacancies in a2tetrahedra as well as at oxygen vacancies corresponding to the 845 cm-' band oxygen.Previously, one of us reported' that the rate of propene oxidation in the presence of gaseous oxygen is larger than in the absence of oxygen over the y and a phases. This suggests that the number of active sites is greater at the stationary state in the presence of oxygen. This features seems to be related to oxygen transfer in Mo tetra-hedra as described above. Table 3 Ratio of Raman band intensity of a-Bi,Mo,O,, substituted by l80during propene oxidation 0 1.o 1.o 1.o 1.o 1.o 15 1 .o 0.95 1.02 0.90 0.95 50 0.93 0.91 1.10 0.70 0.97 I denotes intensity in terms of Raman peak area. The values are divided by lezoand normalized.al, a2 and a3 denote the Mo tetrahedral species in a-Bi,Mo,O,, (see Fig. 7). 2118 The oxygens in the a,tetrahedra will react in the same way as those in a2,although this has not been confirmed owing to band overlap. The oxygens in a3 seem to be less active in oxidation reactions, similar to what is found with the reduction-oxidation method. The other lattice oxygens corre- sponding to the bands in the range 700-600 cm-' may be used for oxidation reactions; however, this is unclear at present. The authors thank Dr. H. Miyata for help with the computer band-shape analysis. References 1 T. D. Snyder and G. C. Hill Jr., Catal. Reu. Sci. Eng., 1989, 31, 43. 2 R. K. Grasselli and J. D. Burrington, Adu. Catal., 1981,30, 133.3 G. W. Keulks, L. D. Krenzke and T. M. Noterman, Adu. Catal., 1978,27, 183. 4 D. B. Dadyburjor, S. S. Jenur and E. Ruckenstein, Catal. Rev. Sci. Eng., 1979, 19,293. 5 T. Ono, T. Nakajo and T. Hironaka, J. Chem. SOC., Faraday Trans., 1990,86,4077. 6 T. Ono, M. Kiryu, M. Komiyama and R. L. Kuczkowski, J. Catal., 1991, 127, 698. 7 R. K. Grasselli, Appl. Catal., 1985, 15, 127. 8 L. C. Glaeser, J. F. Brazdil, M. A. Hazle, M. Mehicic and R. K. Graselli, J. Chem. SOC., Faraday Trans. I, 1985,81,2903. 9 W. Ueda, Y. Morooka and T. Ikawa, J. Chem. SOC., Faraday Trans. I, 1982, 78,495. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 10 D. R. Coulson, P.L. Mills, K. Kourtakis, P. W. J. G. Wijnen, J. J. Lerou and L. E. Manzer, in Proc. 10th International Congress on Catalysis, ed.L. Guczi, F. Solymosi and P. Teteni, Elsevier, Amsterdam, 1993, Part C, p. 2015. 11 E. V. Hoefs, J. R. Monnier and G. W. Keulks, J. Catal., 1979, 57, 331. 12 G. W. Keulks and L. D. Krenzke, in Proc. 6th International Congress on Catalysis, ed. G. C. Bond, P. B. Wells and F. C. Tompkins, The Chemical Society, London, 1977, vol. 2, p. 806. 13 F. A. Cotton and R. M. Wing, Inorg. Chem., 1965,4867. 14 I. Matsuura, R. Shut and K. Hirakawa, J. Catal., 1980763, 152. 15 F. D. Hardcastle and I. E. Wachs, J. Raman Spectrosc., 1990,21, 683. 16 F. D. Hardcastle and I. E. Wachs, J. Phys. Chem., 1991, 95, 10763. 17 F. D. Hardcastle and I. E. Wachs, Solid State Ionics, 1991, 45, 201. 18 F. D. Hardcastle and I. E. Wachs, J. Phys. Chem., 1991,95, 5031. 19 T. Ono, H. Miyata and Y. Kubokawa, J. Chem. SOC., Faraday Trans. I, 1987,83, 1761; F. R. Roozeboom, J. Medema and P. J. Gellings, 2. Phys. Chem. N.F., 1978,111,215. 20 T. Uda, T. T. Lin and G. W. Keulks, J. Catal., 1980,62,26. 21 W. E. Newton and J. W. McDonald, International Conference on the Chemistry of the Use of Molybdenum, Climax Molybdenum Company, 1970, pp. 25-30. 22 M. Cesari, G. Perego, A. Zazzetta, G. Manara and B. Notari, J. Inorg. Nucl. Chem., 1971,33,3595. 23 A. F. V. Elzen and G. D. Rieck, Acta. Crystallogr, Sect. B, 1973, 29,2433. Paper 4/oO14 1 I ;Received 10th January, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002113

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2119-2124 Hydrogenation Behaviour over Si0,-supported Lanthanide-Palladium Bimetallic Catalysts with Considerable Hydrogen Uptake Hayao Imamura," Koji Igawa, Yoshie Kasuga, Yoshihisa Sakata and Susumu Tsuchiya Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi University,2557 Tokiwadai, Ube 755 Japan Novel lanthanide-containing catalysts prepared by the use of dissolution of lanthanide metals in liquid ammonia have been studied. The hydrogenation of propene was carried out at 193-263 K over Si0,-supported lanthanide-palladium bimetallic catalysts (Ln-Pd/SiO,; Ln = Eu and Yb) obtained when the dissolved lanthanide in liquid ammonia reacted with 5 wt.% Pd/SiO, . Ln-Pd/SiO, showed remarkable synergetic effects between the lantha- nide and palladium metal involving considerable hydrogen uptake during the hydrogenation of propene.The rapid hydrogen uptake by Ln-Pd/SiO, occurred at the start of the reaction, followed by the hydrogenation of propene with a definite induction period. The catalyst contained reactive hydrogen species which were able to efficiently hydrogenate the adsorbed propene. The kinetic studies indicated that the hydrogenation predomi- nantly proceeds through a reaction path using hydrogen taken up by the catalyst. The presence of adsorbed propene on the catalyst surface was important to induce a promoting effect towards hydrogen uptake with subsequent hydrogenation of propene. Since lanthanide (Ln) elements have specific electron configu- Ind.Ltd.) was dried through a calcium oxide column and rations based on 4f orbitals, they are expected to catalyse through a sodium hydroxide column before use. Propene was various reactions that cannot be achieved with d-block tran- of research purity and further purified by triple distillation. sition metals. Recently, there has been increasing interest in the specific properties of lanthanides and related compounds as heterogeneous catalysts.' Procedures of Catalyst Preparation and Catalytic Reactions Eu and Yb readily dissolve in liquid ammonia to yield a The Si0,-supported Pd catalyst was prepared by impregnat- homogeneous solution containing solvated electrons.' When ing SiO, with aqueous solutions of PdCl, to 5 wt.% palla- transition-metal powder is added to this solution, the dis- dium. After drying and subsequent reduction at 623 K withsolved lanthanide metal in liquid ammonia is found to react flowing hydrogen as a standard pressure, palladium waswith the transition metal to form novel lanthanide metal detected in the form of small particles with a mean diameter overlayers and lanthanide-containing bimetallic compounds.of ca. 9 nm with the surface area of 55.8 m2 g- '. Reduced Pd By using the solvating ability of liquid ammonia for the powders were conventionally prepared by the incipientlanthanide metals, we have recently developed methods for wetness technique. the preparation of new catalysts containing lanthanides and The method of the lanthanide addition to the Pd catalyst have demonstrated that they exhibit specific catalytic proper- was similar to that previously described for the Ni catalyst8 ties. Our aim in the study of interactions of the lanthanide 5 wt.% Pd/SiO, that had been reduced was placed in awith the transition metal has been to reveal the correlation of Schlenk tube containing a solution of liquid ammonia (15-20 the electronic and geometric effects of bimetallic catalysts cm3) at 198 K. Eu or Yb was added to the Pd catalyst sus- with the catalytic pended in liquid ammonia with vigorous stirring.Upon dis- We now report catalytic behaviour of novel Si0,-solution of the lanthanide metal in liquid ammonia solvent, a supported Ln-Pd bimetallic catalysts (Eu-Pd/SiO, and blue homogeneous solution was immediately formed, which Yb-Pd/SiO,) obtained when the lanthanide metal dissolved was characteristic of solvated electrons., The blue colour in liquid ammonia reacts with 5 wt.% Pd/SiO, .Ln-Pd/SiO, gradually disappeared as a result of the reaction of the dis- shows synergetic effects between the lanthanide and palla- solved lanthanide metal with the Pd catalyst. On disap- dium metal involving considerable hydrogen uptake during pearance of the blue colour, the reaction tube was allowed to the hydrogenation of propene. The effects of the lanthanide warm to room temperature and the excess of ammonia was metal overlayer on hydrogen uptake and the related catalytic pumped off leaving Si0,-supported Eu-Pd and Yb-Pd behaviour over the surfaces of Si0,-supported palladium are bimetallic catalysts.Unsupported Ln-Pd catalysts were simi- investigated. Much attention is devoted to the phenomena larly prepared using the reduced Pd powders instead of associated with the action of hydrogen taken up in the cata- Pd/SiO,. The content of lanthanides in the bimetallic cata- lyst and their catalytic consequences in connection with the lyst was represented by the fraction of the at.%. All sample synergism of this bimetallic system. preparation steps were carried out in an atmosphere of dry argon without exposure to air, otherwise the catalysts became Experimental unreactive. Materials The catalytic reactions were performed on a recirculation reactor constructed of Pyrex glass and equipped with a Eu and Yb ingots (99.9%, Shin-Etsu Chemical Co.Ltd.) were mercury manometer. Prior to the reaction the catalyst was used in the form of turnings or granules. SiO, (380 m2 g-') subjected to evacuation treatment at 293-723 K for 2 h, set was the commercially available Degussa Aerosil 380. PdC1, at 193-263 K of the reaction temperature and then the (99.9% ; Rare Metallic Co. Ltd.) was commerically obtained hydrogenation was initiated by admitting H, and C,H,. The and used without further purification. Ammonia (Iwatani reacting gas in the system was periodically collected by a gas sampler and analysed by a Shimazu TCD gas chromato- graph with a column of active alumina. The gas composition during the reaction was determined by the mass balance between the quantities of propene and propane evaluated by gas chromatography (GC) and from the changes in pressure in the gas phase.Since the uptake of propene and propane by the catalyst was too small to be identified by manometric techniques, the hydrogen uptake was estimated from differ- ences between the quantities of the hydrogen obtained from the drop in pressure and the hydrogen used in the formation of propane. The hydrogenation rates per g of catalyst were found by measuring the rate of appearance of propane. The accuracy of the measurements on the recirculating reactor using GC and pressure changes was confirmed in the hydro- genation of propene which was carried out for reference over 5 wt.% Ni/SiO, (JRC-S3-5Ni; Reference Catalyst of the Catalysis Society of Ja~an).~ Analyses IR spectra were recorded on a JASCO FT-IR 7000 spectrom-eter equipped with a MCT detector.The samples for IR studies were prepared according to the method described above. To the solution of Eu or Yb in liquid ammonia was added, at 198 K, a disc of 5 wt.% Pd/SiO, that had been reduced at 623 K for 5 h. In a dry-argon glove bag the disc thus treated was carefully loaded into the IR cell of variable temperatures (193-673 K). IR spectra were obtained from the ratio of the background spectrum of catalysts to that of the species adsorbed on the catalysts. Extreme care was taken to prevent contamination by air. The X-ray diffraction patterns of the samples were obtained with a Shimazu X-ray diffractometer (VG-107R) using Cu-Ka radiation.Results and Discussion Hydrogenation of Propene over LR-Pd/SiO, :Influence of Lanthanide Addition 5 wt.% Pd/SiO, was highly active for the hydrogenation of propene at 193 K, whereas lanthanide metals exhibited very low or negligible activity under similar conditions.” Upon introduction of the lanthanide metal onto the Pd surface, the catalytic properties were markedly changed (Fig. 1). The hydrogenation was performed at 193 K over Yb-Pd/SiO, by the admission of a gaseous mixture of propene (35 Torr) and hydrogen (35 Torr). Yb-Pd/SiO, with lanthanide loading of 30 at.% [Yb(30”/.)-Pd(70%)/Si02] showed the rapid hydro- gen uptake over a period of ca. 10 min at the start of the reaction, followed by the hydrogenation of propene. No reac- tion of propene with hydrogen was observed during the initial hydrogen uptake; there was an obvious induction period for the hydrogenation. As shown in Fig.1, the catalyst contained reactive hydro- gen species able to hydrogenate propene efficiently as a means of extracting hydrogen. All of the hydrogen taken up by the catalyst was consumed, but the hydrogenation still continued (Fig. 1). From an analysis of the material balance between hydrogen and propene as reactants and the propane product, it seems that the hydrogenation of propene proceeds by consuming both the hydrogen taken up by the catalyst and the hydrogen in the gas phase. However, this does not necessarily imply that propane is produced through separate reaction paths, considering rapid hydrogen uptak-xtract cycles as shown later.Under the reaction conditions exam- ined, the hydrogen was not left in the catalyst when the reac- tion was completed. The general trend in the effects of 40. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 30 20 t 0 20 40 60 t/min Fig. 1 Hydrogenation of propene at 193 K over Yb(30”/,)-Pd(70’/,)/Si02. 0, Gaseous hydrogen ; 0, hydrogen uptake; a,propane. The catalyst (0.042 g) was evacuated at 633 K for 2 h before the reaction. P(C,H,) = 35 Torr, P(H2)= 35 Tom. lanthanide metals on the catalytic behaviour over the Pd metal surface was reproducible in runs using Ln-Pd/SiO, obtained from palladium samples prepared separately, although there were differences in degree.The Eu-Pd and Yb-Pd systems exhibited similar behaviour in the composi- tion ranges investigated. It has often been shown that Pd catalysts simultaneously absorb hydrogen during the reactions in which hydrogen par- ticipates.’ ’ However, X-ray diffraction spectra of Yb(30?Ao)-Pd(70%)/Si02 showed only the existence of metal- lic palladium in the cubic structure with non-distinguishable changes in the lattice parameters and showed no indication of PdH after the hydrogen uptake. The amounts (0.115 mmol) of hydrogen species taken up by Yb(30%)-Pd(7O%)/SiO2 (0.042 g) (shown in Fig. 1) exceeded those corresponding to stoichiometric palladium hydride (PdH) by about 12-fold.Even if hydrogen is fleetingly absorbed in the Pd metal, the absorbed hydrogen is so stable under these reaction conditions that it cannot be thermody- namically desorbed to react rapidly with propene as shown in Fig. 1.’’ Consequently, no absorption of hydrogen by palla- dium as an acceptor site is expected. The lanthanide com- ponent present on the catalyst can also absorb hydrogen to form more stable metal hydrides (YbH, and YbH3),I3 but the circumstances are similar for the lanthanide. Unsupported Yb-Pd catalysts prepared by the reaction of Pd metal powders with dissolved Yb in liquid ammonia were examined. Fig. 2 shows the progress of the hydrogenation with time; substantial bulk uptake of hydrogen occurred during the reaction and the formation of PdH was confirmed by XRD.The hydrogen taken up here was too stable to react with propene and remained intact in the catalyst, as opposed to the results observed for Ln-Pd/SiO, . Some questions still remain as to which sites the hydrogen is accepted in the Ln-Pd/SiO, catalyst system. The rates of hydrogen uptake (vH) as a function of Eu or Yb content in Ln-Pd/SiO, are shown in Fig. 3. The rate showed a tendency to decrease with increasing loading of the lanthanide metal on the Pd surface. For the separate constit- uent, Pd/SiO, or Ln/SiO,, the hydrogen uptake hardly occurred under the same reaction conditions. Thus the Ln-Pd bimetallic system is an interesting example of syn- J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 L$ 20 10 0 0 100 200 300 400 500 t/mi n Fig. 2 Hydrogenation of propene at 193 K over unsupported Yb(3%)-Pd(97%). 0,Gaseous hydrogen; 0,hydrogen uptake; 0, propane. The catalyst (0.098 g) was evacuated at 633 K for 2 h before the reaction. P(C,H,) = 34 Torr, P(H,) = 35 Torr. ergism involved in an enhancement of the ability to take up hydrogen. The existence of some synergetic effects between lanthanide and palladium metals rather than individual com- ponent elements constitutes active sites for the hydrogenation with considerable hydrogen uptake. However, the reasons for the hydrogen uptake and the composition dependence caused by this synergism have not yet been clarified. The initial rates of hydrogenation as a function of lanthanide content simi- larly decreased with increasing lanthanide loading (Fig. 4).This similarity between the dependence of hydrogen uptake and hydrogenation strongly suggests that a possible path for propene hydrogenation is associated with the hydrogen species taken up by the catalyst. For the hydrogenation of the olefin, the Ln-Pd system exhibited a composition dependence which was different from other lanthanide-containing bimetallic systems4-’ so far studied. This might be attribut- able to a reaction scheme in which such hydrogen species strongly participate. In addition, the thermal pre-treatment of Ln-Pd/SiO, under vacuum conditions affected the catalytic behaviour. For the catalyst pre-treated at a lower temperature (293 K), no hydrogen uptake was observed and the slow hydro- 1 1 I I 1 I 0 20 40 60 Eu, Yb (Yo) Fig.3 Rates of hydrogen uptake us. lanthanide content in Eu-Pd/SiO, (0)and Yb-Pd/SiO, (0).The catalysts were evac- uated at 633 K for 2 h before the reaction. P(C,H,) = 35 Torr, P(H,) = 165 Torr. 0.4-I 0, r I .-C -E g 0.2 E -2 0 0 20 40 60 content (Yo) Fig. 4 Rates of propene hydrogenation us. lanthanide content in Eu-Pd/SiO, (0)and Yb-Pd/SiO, (0). genation occurred without any induction period. The rates of hydrogen uptake and hydrogenation varied markedly with changes in evacuation temperature of Ln-Pd/SiO, and they increased with increasing temperatures (293-723 K). The hydrogenation activity of Yb(3O%)-Pd(7O%)/SiO, evacuated at 723 K was ca.five-fold greater than that of the catalysts treated at 293 K. It was also found that for Ln-Pd/SiO, the number of active sites, evaluated from CO chemisorption, increased with increasing evacuation temperature (293-723 K). As reported previously for Ln-Co,’ Ln-Ni,4i6 Ln-Cu’ and Ln-Ag5 bimetallic systems, we have demonstrated that such thermal treatments result in rearrangement of surface morphology or structure, leading to appearance of enhanced catalytic activity. For Nd/Cu(l 11)14 and Yb/Ni(100),15 it has been previously found that the overlayer-to-intermetallic transition occurs by heating at elevated temperatures. Effects of Co-existent Propene There was no indication of hydrogen uptake by the catalyst even when Ln-Pd/SiO, was brought into contact with only hydrogen at 193 K.A very important feature of the present catalyst system is that hydrogen uptake occurred only in the presence of propene; the effect of co-existent propene is strongly suggested. To determine how the hydrogen uptake is affected by presence of a second gas, we carried out two types of reaction differing in sequence of hydrogen and propene addition. As shown in Fig. 5, the first reaction method con- sisted of exposing Yb(30%)-Pd(70%)/Si02 to 35 Torr of hydrogen at 193 K for 30 min, followed by addition of propene (35 Torr) (referred to as H, +C,H,). The addition of propene resulted in rapid hydrogen uptake by the catalyst, but for the reaction between propene and hydrogen there was also a definite induction period.The uptake of hydrogen by the catalyst was observed during this induction period. However, its observation only in the presence of propene does not imply that a hydrogenated intermediate whose surface concentration builds up before substantial desorption of the propane product. This is because the carbon balance between propene and propane was always maintained during the reaction; hence the possibility of a decrease in pressure due to its disappearance from the gas phase can be ruled out. Thus it is to be expected that the adsorbed propene plays an important role in the rapid initiation of hydrogen uptake. When the pressure of propene varied from 6 to 67 Torr under a fixed pressure of hydrogen (169 Torr), the results obtained at higher pressures of propene showed higher rates of hydro- gen uptake (Fig.6), followed by faster hydrogenation with a definite induction period. The rate of hydrogen uptake was J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 20 I I t10 t 0 20 40 60 80 t/min Fig. 5 Effect of co-existent propene (introducing propene later; H, + C,H,). An arrow indicates the introduction of 35 Torr of propene. 0,Gaseous hydrogen; 0,hydrogen uptake; 0,propane. Yb(30%)-Pd(7O%)/SiO2 (0.044 g) was evacuated at 633 K for 2 h before the reaction. The hydrogenation was carried out at 193 K by introducing hydrogen (35 Tom) and subsequently propene (35 Torr). approximately proportional to the pressure of propene, reaching a value of 5.4 x lo-' mmol min-' g-' at 67 Torr.The rate of hydrogenation was equal to 2.0 x lo-' mmol min-' g-' at 6 Torr of propene, and increased in proportion to the propene pressure to reach a value of 1.1 mmol min-l g-at 67 Torr. Furthermore, at constant propene pressure, the reaction was performed at hydrogen pressures of 34-489 Torr. The rates of hydrogen uptake (Fig. 6) and hydro-genation were inversely proportional to the pressure of hydrogen. Upon addition of propene, hydrogen uptake immediately occurred; the adsorbed propene preferentially induces the hydrogen uptake irrespective of changes in its pressure. In marked contrast to this, there were still variable induction periods to initiate the hydrogenation.Thus it is reasonable to consider that there exists at least two separate sites on the surface of Ln-Pd/SiO, with which the adsorbed propene species are strongly associated: (i) hydrogen uptake and (ii) hydrogenation. c 1 0.1 11 I1111 I I I I11111 I I I I L 10 100 P/Torr Fig. 6 Dependence of hydrogen uptake on pressures of hydrogen(0)and propene (0).Yb(30%)-Pd(7O%)/SiO2 was evacuated at 633 K for 2 h before the reaction. The reaction was carried out at 193 K by introducing hydrogen and subsequently propene. $ 301 ;\In a second reaction, propene (35 Torr) was circulated over40 1 addition of C3H6 the catalyst at 193 K for 30 min and then hydrogen (35 Torr) was added into the circulating propene (C3H6+ H,; Fig.7). The induction period almost disappeared and immediately the hydrogenation of propene occurred. The production of propane usually agreed with the consumption of hydrogen in I quantities during the reaction; thus, little hydrogen uptake was observed in this case. In marked contrast to the case of the previous run (H,+C3H6), the rates of hydrogenation tended to increase proportionally with an increase in the pressure of hydrogen, leading to speculations of a different reaction pathway in this reaction condition, as described in the following section. It now seems certain that the predominant presence of propene on the catalyst surface is closely involved in occurrence of immediate hydrogenation as well as the ability to take up hydrogen.Upon addition of hydrogen and sub-sequent propene (H, -+ C,H,), rapid hydrogen uptake occurred but not in the reverse order (C3H, -+ H,); the sites for hydrogen uptake are formed in the presence of both hydrogen and propene, while the formation of sites for hydrogenation is possibre in the presence of propene alone. In the (H, 4C,H,) run, the active sites for hydrogenation are probably formed by the adsorption of propene on vacant sites of the catalyst surface mostly covered with hydrogen; therefore the induction period is required to a certain extent to form such catalytically active sites. IR spectra of adsorbed carbon monoxide on Yb-Pd/SiO, in the presence of co-adsorbed propene, which directly reflect variations in the nature of a catalyst surface, strongly sup-ported this view (Fig. 8).IR studies of adsorbed CO on Ln-Ni/SiO, have been reported.* Carbon monoxide was che-misorbed at 193 K with a pressure of 10 Torr. IR spectra of CO adsorbed on Pd/SiO, and Yb(20%)-Pd(8O%)/SiO2, which were evacuated at 673 K for 2 h as a pre-treatment, are shown in Fig. 8(a) and (b), respectively. Mainly two kinds of bands were observed at 2097 and 1993 cm-', which were assigned to the two adsorption states as linear and bridged adsorbed CO, respectively.16 IR spectrum of CO adsorbed on Yb-Pd/SiO,, which was exposed to propene (10 Torr) at 40 Jaddition of H2 30 L 5 *O 10 0 0 20 40 60 80 f/min Fig. 7 Effect of co-existent propene (introducing propene first; C,H,+H,).An arrow indicates the introduction of 35 Torr of hydrogen. 0,Gaseous hydrogen; 0,hydrogen uptake; 0,propane. Yb(30%)-Pd(7O%)/SiO2 (0.061 g) was evacuated at 633 K for 2 h before the reaction. The. hydrogenation was carried out by intro-ducing propene (35 Torr) and subsequently hydrogen (35Torr). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 L 2097 Q) CQ e8D 0.1 I 2071 II 2200 2000 1800 wavenumber/cm-’ Fig. 8 IR spectra of adsorbed CO on (a) Pd/SiO,, (b) Yb(20%)-Pd(8O%)/SiO2 and (c) propene-pre-adsorbed Yb(20%)-Pd(80%)/Si02 Fig. 9 Dependence of hydrogen uptake (0”) on pressures of hydro-..and propene 0,gen, Yb(3O%-Pd(7O%)/SiO, was evacuated at 193 K after the pretreatment, was shown in Fig. 8(c).It is noticed that the shapes of the spectrum in Fig. 8(c) were markedly changed compared to those in Fig. 8(a)and (b),e.g., the bands at 2097 and 1993 cm-’ decreased and the rela- tively low-frequency bands at ca. 2071 and 1925 cm-’ increased in intensity. It has been often argued from spectral studies that the presence of co-adsorbed hydrocarbons of dif- ferent types simply causes the frequencies of the CO peaks to shift to lower frequency.” In this case, it can be explained as follows: propene is an electron donor to the present bimetal- lic surface. Its pre-adsorption decreases the sites of CO adsorption attributable to the high-frequency bands and makes a greater charge density available for backdonation into the antibonding x* orbital of the adsorbed CO molecule, leading to the increase in intensity of the bands with such lower frequencies.Kinetic Behaviour over Ln-Pd/SiO, The reaction was carried out by introducing simultaneously a mixture of hydrogen and propene with varying partial pres- sures. Fig. 9 shows the initial rate (uH) of hydrogen uptake for Yb(30%)-Pd(7O%)/SiO2 as a function of C,H, and H, pres-sure. The initial rate of hydrogen uptake was proportional to the pressure of propene, rather than that of H, . The observed rate dependence on pressure may be described in the follow- ing form : kPPt)H = -(1) PH where k is the rate constant, and P, and P, the pressure of hydrogen and propene, respectively. Taking into consider- ation that the hydrogen uptake occurred only in the presence of propene and was in proportion to its pressure, it is reason- able to expect that the number involved in sites for the hydrogen uptake is proportional to coverage of adsorbed propene (O,), which is represented by assuming the Langmuir adsorption here.The number of the sites for hydrogen uptake is proportion- al to 0, KP PP8-‘-(l + KHPH + KpPp) where K, and K, are the adsorption coefficient of hydrogen and propene, respectively. The fraction (0,) of the surface 633 K for 2 h before the reaction. The reaction was carried out at 193 K by introducing a mixed gas of hydrogen and propene. covered with hydrogen is KH ‘H -(1 + KHPH + KpPp) The rate (uh) of hydrogen uptake seems to be proportional to the product of the concentrations of the sites (0,) and the surface hydrogen species (OH), which leads to where k’ is the rate constant.If the hydrogen adsorption is strong and the propene weak here, KH P, $ 1, K,Pp, the rate expression can reduce to k‘K, P, k”Ppuh=----(3)KHPH PH which is identical to U, [eqn. (l)] obtained experimentally, where k”( =k‘Kp/KH) is the constant. Fig. 10 shows the initial rate (up) of propene hydrogenation as a function of H, and C,H, pressure. As shown in Fig. 10, the rate of hydrogenation of propene over Yb-Pd/SiO, showed a negative dependence on the H, pressure and a 10 I I I I Ill11 I Ill 10 100 Pflorr Fig. 10 Dependence of propene hydrogenation (up) on pressures of hydrogen, 0and propene 0 I I \ 0,\o \ r \ I \ m \ 7 I C.- h E \ \ 0 c 0.1 4.0 4.5 5.0 103 KIT Fig.11 Arrhenius plots for the hydrogen uptake (0)and hydro- genation (0).Yb(30~o)-Pd(70'/,)/Si0, was evacuated at 633 K for 2 h before the reaction. The reaction was carried out by introducing a mixed gas of hydrogen (165 Torr) and propene (35 Torr). (---) See text. positive dependence on the C3H6 pressure, which was very similar to that observed for the hydrogen uptake (Fig. 9). The pressure dependence of vp was quite different from that for the lanthanide-free catalyst; it is generally accepted for con- ventional supported palladium catalysts that the hydro-genation is approximately first order with respect to hydrogen pressure and has little dependence upon alkene pressure.l8 Furthermore, the apparent activation energy for the hydrogen uptake and hydrogenation was almost equal between 193 and 263 K and was no more than ca. 3 kJ mol-' for some reason (Fig. 11). The close similarity in kinetic behaviour of hydrogen uptake and hydrogenation implies that the hydrogen taken up by the catalyst is strongly involved in a possible path for propene hydrogenation. In the (C3H6-+ H2)run which showed a different kinetic behaviour, an activation energy of 46 kJ mol- was determined for the hydrogenation of propene (see a dashed line in Fig. 11) and was significantly different from the above value. From Fig. 1 it seems as if some of the propane is produced by reacting propene with the hydrogen taken up by the cata- lyst and the rest arises from the reaction with gaseous hydro- gen.However, considering the similarity of the pressure dependence and the activation energy to those for the hydro- gen uptake, it can be concluded that the hydrogenation of propene over Ln-Pd/SiO, is by a rate-limited hydrogen uptake. Thus, the gaseous hydrogen is necessarily taken up to react with propene in this catalyst system and the reactivity of the hydrogen species taken up is high. Once the cata- lytically active sites for hydrogenation are formed, the hydro- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 gen uptake and subsequent hydrogenation process is rapid, and phenomena similar to spillover may be contained.The hydrogenation exclusively proceeds through a reaction path using the hydrogen taken up in the catalyst. We gratefully acknowledge the financial support of this work by the Asahi Glass Foundation. References 1 S. T. Oyama and G. L. Haller, Catalysis, Specialist Periodical Report, The Chemical Society, London, 1982, vol. 5, p. 333; W. E. Wallace, Chemtech, 1982,752; F. P. Netzer and E. Berter, in Handbook on the Physics and Chemistry of Rare Earths, ed. K. A. Gschneidner Jr. and L. Eyring, North-Holland, Amsterdam, 1983, vol. 5, ch. 3. J. C. Thompson, Electrons in Liquid Ammonia, Clarendon Press, Oxford, 1976. H. Imamura, T. Mihara, M. Yoshinobu, Y. Sakata and S. Tsu-chiya, J. Chem.SOC., Chem Commun., 1989, 1842. H. Imamura, K. Yoshimura, S. Hiranaka, Y. Sakata and S. Tsu-chiya, J. Chem. SOC.,Faraday Trans., 1991,87,2805. H. Imamura, M. Yoshinobu, T. Mihara, Y. Sakata and S. Tsu-chiya, J. Mol. Catal., 1991,69, 271. H. Imamura, S. Hiranaka, Y. Sakata and S. Tsuchiya, J. Chem. SOC., Faraday Trans., 1992,88,1577. H. Imamura, S. Hiranaka, M. Takamoto, Y. Sakata and S. Tsu-chiya, J. Mol. Catal., 1992, 77, 333. H. Imamura, H. Sugimoto, Y. Sakata and S. Tsuchiya, J. Catal., 1992,136,271. T. Uchijima, in Catalytic Science and Technology vol. 1, ed. S. Yoshida, N. Takezawa and T. Ono, Kodansha, Tokyo, 1991, p. 393. 10 H. Imamura, K. Kitajima and S. Tsuchiya, J. Chem. SOC., Faraday Trans. I, 1989,85, 1647; H. Imamura, A. Ohmura and S. Tsuchiya, J. Catal., 1985, %, 139. 11 W. Palczewska, Ado. Catal., 1975,25, 245. 12 F. A. Lewis, The Palladium Hydrogen System, Academic Press, London, 1967. 13 G. G. Libowitz and A. J. Maeland, in Handbook on the Physics and Chemistry of Rare Earths, ed. K. A. Gschneidner Jr. and L. Eyring, North-Holland, Amsterdam, vol. 3, 1979, p. 299. 14 R. M. Nix, R. W. Judd and R. M. Lambert, Surf. Sci., 1988,203, 307. 15 J. N. Andersen, J. Onsgaard, A. Nilsson, B. Eriksson, E. Zdansky and N. Martensson, Surf Sci., 1987, 189/190,399; J. N. Andersen, J. Onsgaard, A. Nilsson, B. Eriksson and N. Mar-tensson, Surf. Sci., 1988,202, 183. 16 Y. Soma-noto and W. M. H. Sachtler, J. Catal., 1974,32,315. 17 R. Quean and R. Poilblane, J. Catal., 1972, 27, 200; H. Ibach and G. A. Somorjai, Appl. Surf: Sci., 1979, 3, 293; M. Primet, J. M. Basset, M. V. Mathieur and M. Prettre, J. Catal., 1973, 29, 21 3. 18 G. C. Bond, Catalysis by Metals, Academic Press, London, 1962, p. 229. Paper 3/06544H ;Received 2nd November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949002119

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2125-2131 Surface Distribution and Heteroatom Removal Activity of Equilibrium Adsorption Prepared, Doubly Promoted (Zn,Co)Mo/Al,O, Catalysts H. Thomas, C. Caceres and M. Blanco Centro de lnvestigacion y Desarrollo en Procesos Cataliticos (CINDECA), CONICET-UNLP,Calle 47 No. 257, 1900-La Plata ,Argentina J. L. G. Fierro and A. Lopez Agudo* lnstituto de Catalisis y Petroleoquimica , CSIC, Campus de la UAM, Cantoblanco, 28049-Madrid, Spain Adsorption isotherms of Zn2+ and/or Co2+ ions on an 8 wt.% MoO,/alumina catalyst have been obtained. The Langmuir parameters derived from regression of the results indicate that preimpregnation of alumina with molybdate solutions notably decreased the adsorption of Zn2+ and Coz+ ions.In solutions containing both Znz+ and Coz+ ions, the saturation capacity of Zn2+ was practically unaffected, while that of Co2+ was significantly reduced owing to the higher intrinsic affinity of Zn2+ ions for alumina sites. Adsorption of Co2+ ions occurs on two type of sites, i.e. those on which Co2+ ions are loosely and strongly bound; the Co2+ on the latter type is not affected by the presence of Zn2+ ions. After calcination, the non-adsorbed Co, retained in the pores, was depos- ited heterogeneously as a surface cobalt oxide bound loosely to the A120,. However, the homologous Zn was more homogeneously deposited, owing to its higher reactivity with A120,. These differences result in an addi-tional promotional effect in the gas-oil hydrodesulfurization (HDS) reaction, which has been observed on the catalysts prepared by simultaneous incorporation of Zn and Co.This effect is explained in terms of changes in the distribution and dispersion of promoters. These changes appear to be detrimental in hydrodenitrogenation (HDN). The more commonly used catalysts in hydrotreating pro- cesses consist of molybdenum sulfide as the active phase, sup- ported on a y-alumina carrier. The incorporation of a second metal promoter, such as Ni, Fe, Zn or Mg, has often been practised to improve the HDS activity.'-4 Several studies of molybdenum catalysts have been reported in which the cata- lytic effect of Zn addition was found to be owing mainly to differences in preparation procedures involving catalyst composition and Zn loading.In a systematic study of a series of CoZn-Mo/A1203 catalysts in which both Co and Zn were simultaneously impregnated on an Mo/A1203 sample. Fierro et a1.' found a secondary promoting effect for gas-oil HDS when Co was partially replaced by Zn. This effect of Zn was attributed to a decrease in the formation of the catalytically inactive CoA1204 phase at the alumina inter- phase and, consequently, to an improvement in the amount of octahedrally coordinated surface Co, which was induced by the preferential coordination of Zn2 + to tetrahedral sites, rather than Co2+. This is consistent with the higher propen- sity of Zn than Co to form a surface spinel with alumina, as reported by Lo Jacono and Schiavello" and by Strohmeir and Hercules." On the other hand, Maezawa et a1." have shown that Zn did not significantly change the Mo dispersion on Zn-Mo/A1203 ,and that Mo could affect the Zn distribu- tion, depending on the order of impregnation of the Mo and Zn components.In order to clarify the origin of the observed secondary promotional effect of Zn on Co-Mo/Al,O, catalysts, we decided to examine in more detail the first step of the catalyst preparation. By studying the separate and simultaneous adsorptions of the different metal components on the alumina surface, it should be possible to identify the ion that is preferentially adsorbed and its maximum coverage. Washing of the samples, after ion adsorption, can give infor- mation on the strength of the adsorbate-support interaction and the possible presence of different types of adsorption sites.Moreover, characterization of the calcined forms of the precursors should reveal the chemical interactions taking place between the components in the calcination step and changes in the metal dispersion. In previous work,13 the adsorption isotherms of the simpler systems, Co/A1203, Zn/A120, and ZnCo/A1203 , were studied, and the resulting calcined samples were charac- terized. This study revealed that Zn2+ species are prefer- entially and more strongly adsorbed on an alumina surface than Co2 species, which leads to important differences in + the distributions of Zn and Co on the alumina surface upon calcination. Information on the adsorption and interaction of molybdates on different supports, but particularly on alumina, has been reported in many st~dies.'~-'~ Recently, it has been shown that coadsorption of Mo6+ species and Co2+ ions on the surface of y-alumina enhances the extent of adsorption of both Co2+ ions and Mo6+ species.23 Since the promoters are often impregnated in a second step, after molybdenum, a study of the adsorption of cobalt and zinc on a calcined Mo/A1203 sample is of great interest.Preliminary results of the equilibrium adsorption isotherms of Zn2+ and/or Co2 + ions on a molybdate-modified alumina support were recently presented.24 Here, a complete data analysis of the above-monitored adsorption isotherms and the results of an extensive characterization of a selection of representative calcined samples will be presented.Also, to corroborate the promoting effect of Zn on HDS activity' and to examine if such effects also occur in other hydrotreating reactions, addi- tional activity results for the simultaneous HDS and HDN of gas oil + pyridine on samples of Zn-, Co-and ZnCo-Mo/ A120, catalysts prepared by equilibrium adsorption (similar to the samples used for the adsorption isotherms) will be pre- sented and discussed. Experimental Sample Preparation The alumina support used was a commercial Girdler T-126 y-A1203(S,,, , 188 m2 g- ;pore volume, 0.4cm3 g- ') which was crushed and sieved to a particle size of 0.84-1.19 mm. An Mo/Al,03 sample was prepared by equilibrium adsorption of an ammonium heptamolybdate solution (20 g Mo 1-') on alumina.After impregnation, the solid was fil-tered, dried at room temperature for 24 h and then calcined in air at 823 K for 7 h. The Mo loading in the solid (8 wt.% MOO,) was determined by treating the sample with a strong acid solution and analysing the Mo in the solution by atomic absorption spectroscopy (AAS). The equilibrium adsorption experiments of Zn2+ and/or Co2+ ions were performed at 293 K as follows: 1 g of the above-prepared Mo/Al,O, sample was suspended in 4 ml of aqueous Zn solution and/or Co nitrate and stirred contin-uously for 24 h. The concentration of the solutions was varied from 1 to 50 g of active metal (Zn or Co) per 1. The Co :Zn molar ratio for all mixed solutions was 1:1.After equilibration, the solids were separated from the solution by centrifugation, and dried and calcined under the same condi-tions as the precursor Mo/Al,O, sample. The concentration of Zn and/or Co in the initial (Ci) and the equilibrium or final (C,) solutions was determined by measuring the metal content in such solutions by AAS. From Ci and C, the concentration of metal adsorbed (C,) on the Mo/Al,O, sample was calculated by mass balance and by assuming that the pore volume of Al,O, did not vary during adsorption. Details of the general procedure of adsorption and calculations of C, have been given else~here.'~*~~ To study the strength of adsorption of the ions on the support, wet samples (1 g) of the equilibrium adsorption experiments were washed repeatedly with 4 ml of deionized water for several days.The amounts of Zn and Co removed in each washing were determined by AAS. Three additional samples, Zn-, Co-and ZnCo-Mo/Al,O, , used for the catalytic activity measurements, were also pre-pared by equilibrium adsorption following the general pro-cedure described above. Specifically, the single promoted Zn-and Cc~Mo/A1,0, catalysts were prepared by suspending 7 g of MOO,@ wt.%)/Al,O, in 28 ml of an aqueous solution of zinc nitrate (22.1 g Zn 1-') or cobalt nitrate (20.2 g Co 1-'), respectively. The doubly promoted ZnCo-Mo/Al,O, catalyst was prepared in a similar manner using a solution containing 10.1 g Co 1-' and 11.1 g Zn 1-l.The separation of the solids and the calcination conditions were as mentioned above for the precursor Mo/Al,O, sample. The concentrations of adsorbed metal, both occluded in the pores and the total amount in the calcined catalysts, are summarized in Table 1. CharacterizationTechniques Diffuse reflectance spectra (DRS) were recorded on a Varian Super Scan 3 spectrophotometer using BaSO, as reference. X-Ray photoelectron spectra (XPS)were recorded on a Leybold Heraeus LHS 10 spectrometer equipped with an A1 anode. Each spectral region was signal-averaged for a number of scans to obtain good signal-to-noise ratios. The Table 1 Concentration of metal (wt.%) in the catalysts prepared by equilibrium adsorption catalyst C-Mo/AI 20, Zn-Mo/Al,O, ZnCc~Mo/Al,0, co Mo Zn Mo Zn Co Mo C, 0.45 -0.23 -0.25 0.13 -CP" 1.64 -2.35 -1.09 0.93 -C, 2.09 5.36 2.58 5.36 1.34 1.06 5.36 "Occluded in pores of the support.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 C ls,O ls,Al2p,Mo3d,Co2pand/orZn2pwererecordedforeach sample.Allbindingenergies(E,,)werereferencedtotheA12ppeakat 74.7. XPS intensity ratios were determined using the integrated areas of the Co 2p(and satellite),Zn 2p and A12p lines afterlinear background subtraction. Additional experiments involving chemical extraction af Co and Zn species were conducted by contacting lo0 mg of sample with 25 ml of a 3% (v/v) ammonia solution for 24 h. The metal content in the solution was determined by AAS. Activity Measurements Catalyst activity testing for the simultaneous HDS and HDN of gas oil and pyridine was carried out in a trickle-bed flow reactor at 30 kg ern-,, with a liquid hourly space velocity (uLHS)of 8.8 h-',and an H,(gas) :feed(1iquid) ratio of 408 :1, at 598, 623 and 648 K.A catalyst charge of 6 ml was diluted 1:1 with Sic of the same particle size. The catalysts were presulfided in situ at 623 K for 25 h with a 7% (v/v) CS,-gas oil mixture at 20 bar and a VLHs of 2. Sulfur and nitrogen contents were simultaneously determined with an Antek analyser system. From the percentages of sulfur and nitrogen removed, the activities of the catalysts for HDS and HDN, expressed as pseudo-second-order (kHDs) and pseudo-first-order (kHDN) rate constants, were calculated using the equa-tions : where xs and xN are the sulfur and nitrogen conversions, respectively.Results Adsorption Isotherms Fig. 1 shows the adsorption isotherms of Zn2+, Coz+ and (Zn2++Co2+) on MoO,/Al,O, at 293 K. They roughly obey the Langmuir isotherm. The solid lines of Fig. 1 are best fits of the experimental data to the Langmuir model. As in the adsorption on al~mina,'~the shapes of the adsorption Oa50 (c)t U n-0 c 0.25k+cb+ J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 isotherms for Zn2+ are slightly different from those for Co2+. The amount of metal adsorbed, Ca, increased and rapidly levelled off for Zn2+ adsorption, while C02 adsorption was + more gradual. The experimental results were fitted to the Langmuir iso- therm: (3) where K, is the adsorption equilibrium constant and C, the number of adsorption sites expressed in g metal per 100 g of support. From eqn.(3), the values of K, and C, for each isotherm were calculated; they are shown in Table 2. The K, values for Zn2+ are significantly higher than those for Co2+, indicating that Zn2+ ions have a higher affinity for alumina than Co2+ ions. The higher affinity of Zn2+ is related to the higher stability and preferential adsorption of the hydrolysed Zn2+ complex over the corresponding Co2+ 0ne.13 Also, K, for Zn2+ (for the coadsorption of Zn2+ and Co2+) is lower than that for the adsorption of Zn2+ alone, indicating that the affinity of Zn2+ for alumina is significantly decreased by the presence of Co2+ ions because both ions are competing for the same type of adsorption site.However, K, for Co2+ (during coadsorption) is higher than that for the adsorption of Co2+ alone, as was also found for adsorption on Al,O, without molybdena.13 In this paper we suggested that the majority of Co2+ ions may be adsorbed on the same sites as Zn", and that the remaining ions can be found on sites with stronger affinity, on which Zn2+ ions are not adsorbed.', Thus, the K, value of Co2+ for the coadsorption of Zn2+ and Co2+ may refer to the irreversible adsorption of Co2+ on the stronger sites. In line with this, Table 2 shows that the C, values of Zn2+ adsorption are practically equal for simulta- neous and monocationic adsorption, while the Cm for Co2+ is clearly smaller for the simultaneous adsorption case, since Zn2+ ions have a higher affinity for alumina than Co2+ ions.Moreover, the C, for Co2+ alone was clearly higher than that for Zn2+ alone, which is consistent with the presence of two distinct types of Co2 + adsorption site. Metal Leaching from Wet Samples The variations in Co and Zn content as a function of washing time for some representative samples of the adsorption iso- therms are shown in Fig. 2. The number in parentheses in the catalyst notation refers to the sequence order of the corre- sponding isotherm in Fig. 1. For both low- and high-Co- content Co-Mo/Al,O, samples the initial amounts of adsorbed Co, i.e. C, , decreased markedly with increasing washing time, and after about 75 h of washing the Co content remained practically constant. This means that most of the adsorbed Co2+ ions on Co-Mo/Al,O, samples are loosely bound species, which can be easily removed by repeated washing with water, and that only a small part of the Co species are strongly bound to the support, remaining adsorbed.However, Fig. 2A shows that for Zn,Co-Mo/Al,O, samples practically no extraction of Co Table 2 Equilibrium adsorption parameters system KJ(g Me)-' * C,/g(lOO g Mo/Al,O,)-' Co-Mo/Al 203 Zn-Mo/Al,O, 0.10 4.58 0.64 0.24 ZnCo-Mo/Al,O, 0.22 1.30 0.19 0.27 Me: metal. 2127 4 t - I I I 25 50 75 335 time of washing/h Fig. 2 Variation in A, Co and B, Zn content as a function of washing time for different wet samples of the catalysts in Fig.1. A, (a) ZnCo-Mo (10);(b) Co-Mo (10);(c) ZnCo-Mo (2), (d)Co-Mo (5). B, (a)Zn-Mo (9),(b) ZnCo-Mo (lo),(c)Zn-Mo (2), (d) Zn-O-Mo) (2). was observed. Therefore, in these samples only Co species strongly bound to the support were present. Notice also from Fig. 2A that the Co contents of the Co-Mo/Al,O, samples after about 14 days of repeated washing were approximately equal to the amounts of Co initially adsorbed in the counter- part ZnCo-Mo/Al,O, isotherm sample. A similar behaviour of the adsorbed Co species upon washing was also found for Co/Al,O, and ZnCo/Al,O, samples in the previous study.' The effect of washing on adsorbed Zn, if any, was very small for both Zn- and ZnCo-Mo/Al,O, samples, as Fig. 2B shows.The Zn content after repeated washings was equal to or slightly lower than its initial value. Hence, this behaviour indicates that all adsorbed Zn species are irreversibly bound, involving only one type of site. Diffuse Reflectance Spectra Diffuse reflectance spectra of the calcined Co-Mo/Al,O, and ZnCo-Mo/Al,O, samples show triplet bands at ca. 545, 580 and 630 nm, characteristic of tetrahedrally coordinated Co2 + (CoCT]) in the surface spinel CO/A~,O,.~~ The band at ca. 750 nm, due to octahedrally coordinated Co2+ (Co[O]) in CO~O,,~~was detected in samples with Co loadings above 0.15 wt.% for the ZnCo-Mo/Al,O, series, and at much higher contents, 3.5 wt.% Co, for the Co-Mo/Al,O, series.Fig. 3 shows that the ratio of Co[T] to Co[O], as determined by the Schuster-Kubelka-Munk function F(R,) ratio mea- sured at 580 and 750 nm, remained practically constant up to C, level of about 2 wt.% Co in the Co-Mo/Al,O, samples and then abruptly decreased to a very low value for samples with loadings >3.5 wt.% Co.This variation was reversed for the ZnCo-Mo/Al,O, samples, in which the ratio of Co[T] to CoCO] initially decreased very rapidly for contents up to about 0.3-0.5 wt.% Co and then became nearly constant or gradually decreased. Note that the ZnCo-Mo/Al,O, samples had a much lower Co[T] : Co[O] ratio, reflecting the lower C,/g Co (1 00 g AI,O,) -' Fig. 3 Variation of the Kubelka-Munk function ratio at 580 and 750 nm with Co content for calcined C+Mo/Al,O, (0)and CoZn-Mo/Al,O, (0)samples extent of adsorption of Co in the presence of Zn and, there- fore, the small amount of Co available for strong interaction with the alumina.Zinc Extraction Table 3 shows the effect of calcination temperature on Zn extraction by aqueous ammonia solution from four Zn-Mo/Al,O, and ZnCo-Mo/Al,O, representative samples, selected in the low and high regions of the corresponding adsorption isotherms. It is observed that for low-Zn-content samples the amounts of Zn removed were very low and did not vary significantly when the calcination temperature increased from 623 to 823 K, indicating that the fraction of Zn as ZnO (very soluble in ammonia) was very small.In such samples the percentage of Zn extracted was around 1%, which corresponds to the occluded Zn in pores being trans- formed upon calcination to extractable ZnO. However, for the high-Zn-content samples calcined at 623 K the percent- ages of Zn extracted were relatively high (15.7 and 5.5%), decreasing notably to ca. 1 and 0.3% upon calcination at 823 K. This decrease in Zn extraction with increasing calcination temperature means that a significant proportion of the Zn, present as ZnO in the dried samples, was transformed to a ZnAl,O, surface spinel which is only slightly solubilized by ammonia. X-Ray Photoelectron Spectroscopy The XPS data of the most representative calcined samples (used for the isotherms) are presented in Table 4.It is observed that the binding energies (Eb) of the A1 2p, Co 2p3,,, Zn 2p,,? and Mo 3d,,, lines did not vary to any significant extent, irrespective of the metal content and extent Table 3 Influence of calcination temperature on the extent of Zn extraction sample calcination temperature/K C," Ce" Zn-Mo (3) 623 2.7 0.004 823 2.7 0.03 Zn-Mo (12) 623 823 17.2 17.2 2.7 0.2 ZnCo-Mo (2) 623 1.4 0.012 823 1.4 0.016 ZnCo-Mo (12) 623 823 23.5 23.5 1.3 0.08 Zn concentration is given in mg per g support; t, total; e, extracted. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Binding energies (eV) of core electrons in Co-, Zn-and CoZn-Mo/Al,O, catalysts catalyst 0 1s Mo 3d,,, Co 2p3,, Zn 2P3,~ 8 Mo/Al 532.1 233.3 - - CoZn-Mo (3) CoZn-Mo (5) CoZn-Mo (6) CoZn-Mo (8) CoZn-Mo (9) CoZn-Mo (1 1) Co-Mo (4) Co Mo (5) Co-Mo (6) Co-Mo (8) Co-Mo (10) Zn--Mo (1) 531.9 531.7 531.9 531.8 532.0 532.0 531.7 532.0 531.9 531.9 532.8 53 1.9 233.0 233.1 233.3 233.2 233.0 233.0 233.0 232.8 232.7 232.8 232.9 232.9 780.4 780.8 780.8 780.6 780.3 780.4 780.9 780.8 780.9 780.9 780.6 1022.7 1022.7 1023.2 1022.8 1023.4 1022.7 1022.7 Zn-Mo (4) Zn-Mo (6) Zn-Mo (10) Zn-Mo (1 1) Zn-Mo (12) 531.9 532.0 531.8 531.9 53 1.9 232.9 233.0 232.9 233.0 233.0 1022.8 1022.7 1022.6 1022.7 1022.8 of adsorption, indicating, in principle, the presence of similar metal species in all calcined samples.The Ebs of co 2p3,, and Zn 2p,,, at ca. 780.6 and ca. 1022.6 eV, respectively, are very close to those of the CoAl,O, and ZnAl,O, surface spinels.' Quantitative XPS data of the Zn and Co dispersions are presented in Fig.4 as a function of the bulk metal content. The experimental intensity ratio, Izn/IA,, of the Zn-Mo/Al,O, samples increased linearly up to a Zn content of ca. 1.3 wt.%, where an inflection was observed, indicating a homogeneous deposition of Zn on the support up to the mentioned Zn content, and, beyond such a level, a change and decrease in the Zn dispersion. For the ZnCo-Mo/Al,O, samples, the change in the linearity of Iz,/IA, and, therefore, in Zn dispersion appeared at about 0.6 wt.% Zn. Another difference is that the ZnCeMo/Al,O, samples exhibited higher Izn/IA,ratios than the Zn-Mo/Al,O, counterparts, which were also greater than the theoretical intensity ratios for a monolayer distribution.This suggests that the oxidic Zn species of the ZnCO-Mo/Al,O, samples are more inhomoge- neously distributed, yielding some enrichment of Zn in the 4 6 8 10 12 10, (Me/Aub",k Fig. 4 Experimental XPS intensity ratios of (a) Z,,/ZA, (0,m)and (b) I,-,/IAl (0,0)as a function of the bulk metal ratio (chemical). Open symbols for Co- and Zn-Mo/Al,O, samples and full symbols for ZnCo-Mo/Al,O, samples. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I O' 014 OI8 112 1f6 210 C,Jg Me (1 00 g AI,O,) -' Fig. 5 Experimental XPS intensity ratio (ZMJZA,) as a function of the total metal (Co, Zn or Co + Zn) content in the calcined samples: (A) Zn-Mo/Al,O,, (0)CCFMO/AI,O, and (0)ZnCc+Mo/Al,O, outer part of the support grains.The variation of Ico/I,, with bulk Co content are shown in Fig. 4. The trends of these curves are, in general, similar to but the same as those for Zn, except that the deviation from linearity occurred at a much lower metal content, i.e. at about 0.7 wt.% Co for the Co-Mo/Al,O, samples and at about 0.3 wt.% Co for the ZnCo-Mo/Al,O, samples, indicating that Co became distrib- uted heterogeneously at much lower metal contents than Zn, as was also found for the samples without Mo.', Fig. 5 shows that the IM,,/IAlratios are practically the same for all samples, indicating that the dispersion of Mo was not significantly altered by the later adsorption of increasing amounts of Zn and/or Co.A similar finding was reported for catalysts prepared by using conventional impregnation methods. l2 This figure also shows that the parent Mo/A1,0, sample had an IMo/I,,ratio slightly higher than those of the Co-and Zn-containing samples. This is probably due to a dilution effect and/or a somewhat partial covering of the Mo with metal promoters. Catalytic Activity The activities of the three catalysts prepared by equilibrium adsorption for simultaneous HDS and HDN of gas oil and I f ;3 a 2 1 0 598 623 648 N' nt22 I -L 1 0 598 623 648 Fig. 6 Catalytic activity of Zn-, Co-and ZnCo-Mo/Al,O, cata-lysts for HDS and HDN of gas oil + pyridine at 598,623 and 648 K pyridine are shown in Fig.6. It is evident that the ZnCo doubly promoted catalyst exhibited higher HDS activity than the Zn-promoted catalyst, and the latter was more active than the Co-promoted catalyst; whereas for HDN, the Zn- promoted catalyst proved to be notably more active than the ZnCo doubly promoted catalyst and also much more active than the Co-promoted one. Discussion Influence of Preimpregnated Molybdena on Zn and/or Co Adsorption on the Surface of y-Alumina Comparison of the results in Table 2 with those previously obtained for adsorption on alumina', reveals that the afin- ities of both Zn2+ and Co2+ions for alumina sites decreased significantly (by at least a factor of two) after molybdena incorporation. Furthermore, all of the C, values of Table 2 (except for Co2+ in the coadsorption case, see below) for Mo/Al,O, are lower than those obtained on alumina alone.', In the present adsorption systems the Zn2+ and/or Co2+ ions are not coadsorbed with another negatively charged species, at variance with ref.23, because the molyb- date species were previously impregnated on the alumina and then calcined. This decrease in Co and Zn adsorption due to Mo incorporation cannot be explained if an electrostatic mechanism is assumed to describe the adsorption of M ions onto the alumina surface. As pointed out by Br~nelle,~~ the extent of cation adsorption is favoured when the pH of the solutions is higher than the isoelectric point (IEP) of the carrier, and this should be expected since the modification of alumina with 8% MOO, caused a decrease in the number of its basic hydroxy group^,'^-^' reflected by a decrease of cu.2 pH units in the IEP of alumina.28 This effect would cause the IEP of alumina to approach the pH of Zn2+ and Co2+ solu- tions, increasing the probability of having a negatively charged surface, and consequently it should favour the adsorption of cations, as also occurred upon doping the y-alumina with F-ions2, However, there is another phenome- non with the reverse effect. From electrophoretic migration rates, ca. 80% of the alumina surface was covered by molyb- date when the adsorption was allowed to take place,28 thus reducing the concentration of acidic hydroxy groups avail- able (for adsorption of metal ions) during calcination.The lower C, values in Table 2 relative to that for alumina alone', suggest that a fraction of molybdate is adsorbed on the same types of sites that are involved in adsorption of Zn2+ and most Co2+ ions. An alternative explanation is that molybdate prevents, probably by steric effects, the interaction between surface sites and the large aqueous metal ions, as has already been suggested for molybdate adsorption on phos- phated alumina.,' The exception in the C, values for Co2+ in the coadsorption of Co2+ and Zn2+, mentioned above, (these C, values were very similar for both ZnCo/Al,O, C0.16 g (100 g support)-'] and ZnCo-Mo/Al,O, C0.19 g (100 g support)-'] systems], is consistent with the previous assumption that a small fraction of Co2+ ions are probably adsorbed on differ- ent sites to those involved in Zn2+ adsorption. This conclu- sion is corroborated by the above results (Fig.2) of Co removed, from wet Co-Mo/A1203 samples, by washing with water. Such results confirm the presence of two types of alumina site (whose nature cannot be clarified from the present results) for the adsorption of Co2+ ions. In the case of coadsorption of Zn2+ and Co2+, the latter ions occupy only the sites on which they are strongly adsorbed. By con- trast, the Zn2+ ions are adsorbed very strongly on only one type of site. Dispersion and Distribution of Zn and/or Co in the Calcined Samples Upon calcination at 823 K of the Zn-, C* and ZnCo-Mo/Al,O, samples, as also occurred for samples that had been employed for adsorption on non-modified Al,O, ,13 the adsorbed and occluded Zn and/or Co were mostly incorporated into the defective lattice of Al,O, ,occupying, in general, tetrahedral and octahedral sites at low and high metal contents, respectively.Although the previous incorpor- ation of Mo into Al,O, considerably decreased the adsorp- tion of Zn and/or Co, compared with that on Al,O,, the segregation of bulk crystalline ZnO and Co,O, was not detected by X-ray diffraction in the present calcined samples. In agreement with this, the XPS E,s of the Zn 2p3,, and Co 2p,,, levels, which remained practically invariable for all samples, indicated that Zn and Co were present as ZnAl,O, and CoAl,O, surface spinels, respectively, rather than as ZnO and c030, structures.However, the observed changes in the linearity of the intensity ratios of Zn 2p3,, and Co 2p3,, to the A1 2p levels us. metal content (Fig. 4) clearly indicated that the surface distribution and dispersion of Zn and Co changed at certain metal loadings depending on the extent of adsorption. For the Zn-Mo/Al,O, samples, the change in Zn distribution occurred at about 1.3 wt.% Zn, which is slightly over the maximum Zn loading value of 0.24 wt.% obtained from the adsorption isotherm. This difference indicates that below 1.3 wt.% Zn most of the occluded Zn in the pores interacted with the alumina during the calcination step and formed similar ZnAl,04-like species to those resulting from adsorbed Zn, and that at higher Zn loadings not all of the occluded Zn can interact strongly with the alumina because the latter is partially covered by Mo and, therefore, some Zn remains as highly dispersed ZnO.In the ZnCo-Mo/Al,O, samples, this change in the homogeneous distribution of Zn appeared at lower Zn loadings (ca. 0.6 wt.%) because the percentage of occluded Zn was slightly higher than that for the Zn-Mo/Al,O, samples and also because of the presence of adsorbed and occluded Co, which can interact with alumina, although with lower reactivity than Zn. This phenomenon of the spreading of ZnO species, arising from occluded Zn, is supported by the Zn extraction results at different calcination temperatures for the different Zn-containing samples shown in Table 3.The change in the homogeneous distribution of Co, according to the XPS data (Fig. 4), occurred at Co contents of about 0.7 and 0.3 wt.% for the Co-Mo/Al,O, and ZnC*Mo/Al,O, samples, respectively. Such values were very close to the saturation coverages calculated from the adsorption isotherms, C, values are given in Table 2. This finding indicated that there was no significant spreading of the Co oxide species derived from occluded Co, owing to the relatively minor reactivity of the Co oxide species with alumina which may remain highly dispersed on the support, particularly for the Co-Mo/Al,O, samples with up to ca. 2 wt.% Co, on the basis of the DRS results.By comparing these results with the corresponding XPS and DRS results on Co/Al,O, samples,13 it is observed that the change in the slope of the XPS results and the Co[T] : Co[O] ratio appeared at 'lightly higher c0 loadings for the Co-Mo/A1203 than for the Co/A1203 Thisis consistent with the known fact that Mo improves the dis- persion of Co over A1,0, .12*30,31 Promotional Effect of Zn The activity results of Fig. 6(a) confirm the earlier findingg of an additional Promotion in HDS activity when the Zn and Co promoters were simultaneously incorporated into an J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Mo/Al,O, sample. The levels of activity of the present cata- lysts were, however, lower than those of the previous studyg in which the catalysts were prepared by conventional impreg- nation methods, but in both cases the relative promoting effect was similar.The increased HDS activity of the ZnCo-Mo/Al,O, cata-lyst, in comparison to the Zn- and Co-Mo/Al,O, catalysts, cannot be attributed to changes in the Mo dispersion since this remained essentially unchanged for all Co- and Zn- containing samples. The increase in HDS activity is, there- fore, associated with the observed changes in the distribution and dispersion of the promoters. According to Table 1, the doubly promoted ZnCo-Mo/Al,O, catalyst showed a lower proportion of adsorbed Co and a higher proportion of occluded Co than the singly promoted Co-Mo/Al,O, cata-lyst. This may lead, after calcination, to a higher proportion of Co[O] weakly bound to the alumina in the former cata- lyst, as the above characterization results indicate.Therefore, is seems that such Co[O] species, presumably in the form of Co oxide highly dispersed on the alumina surface, can form more easily a more abundant Co-promoted molybdenum active phase. The fact that the Zn-Mo/Al,O, catalyst was somewhat more active for gas-oil HDS than Co-Mo/Al,O, can, in prin- ciple, also be attributed to the lower C& ratio of the pro- moter in the former catalyst (Table l),which may lead to a higher proportion of promoter that is not strongly bound. In addition, note that the two catalysts differ in the total content of promoter, the wt.% of Zn was slightly higher than that of Co.The data of de Beer et aL3, also showed that at certain promoter : Mo ratios, the promoter action of Zn for thio- phene HDS was significantly higher than that of Co, Ni and Mn. As shown in Fig. qb), for HDN of pyridine, Zn-Mo/Al,O, was considerably more active than Co-Mo/Al,O, , which can be attributed to the comparatively higher hydrogenation activity of Zn respect to Co3, and also to the above- mentioned differences in promoter distribution. A similar large difference in HDN activity between Zn and Co was also found for molybdenum sepiolite-based catalyst^.,^ In this case, the higher surface acidity exhibited by the Zn-promoted catalyst relative to the Co-promoted one was suggested as another factor enhancing the HDN of the Zn-promoter cata- lyst.This could also occur with the present catalysts. From Fig. 6(b) it is also evident that in the ZnC*Mo/Al,O, catalyst no additional promotional effect for HDN activity appeared, in contrast to what is observed for HDS, since its HDN activity was below the sum of the corresponding activities of the Zn- and Co-promoted cata- lysts. This behaviour for HDN is consistent with the intrinsic activity of the singly promoted catalysts and the surface dis- tributions of both Zn and co. In the ZnCo-Mo/Al,O, cata-lyst most of the Zn is strongly bound to the support and, therefore, cannot contribute very much to the HDN reaction, whereas most of the Co is weakly bound to the support and is thus able to form active sites with Mo, but the latter are less active for HDN.Consequently, the HDN activity of this catalyst is relatively low. Financial support from the DGICYT (Spain), Project PB 87-0261, is gratefully acknowledged. The technical assistance of Mr. osiglio is also gratefully acknowledged. References 1 0.Weisser and S. Landa, Sulphide Catalysts, Their Properties and Applications, Pergamon, New York, 1973. 2 P. C. H. Mitchell, in Catalysis (ed. C. Kemball), The Chemical Society,London, 1977, vol. 1, p. 228. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2131 3 4 5 A. L. Hensley, US Pat., 3 849 296, 1974. H. Beuther, R. A. Flinn and J. B. McKinley, Ind. Eng. Chem., 1959,51, 1349. F. J. Lopez, H. Medina, N. P. Martinez and P. C. Mitchell, 21 22 N. Spanos, L.Vordonis, Ch. Kordulis and A. Lycourchiotis, J. Catal. 1990, 124, 301. P. G. Koutsoukos and A. S. Lycourghiotis, Colloids Su$, 1991, 55, 297. 6 React. Kinet. Catal. Lett., 1983, 1, 2, 153. F. Trifiro, P. L. Villa, P. Pasquon, A. Iannibello and V. Verti, 23 24 N. Spanos and A. Lycourghiotis, Langmuir, 1993,9,2250. H. Thomas, C. Caceres, M. Blanco, J. L. G. Fierro y A. Lopez 7 Chim. Ind. (Milan),1975,57, 173. G. Muralidhar, F. E. Massoth and J. Shabtai, J. Catal., 1984,85, 44. 25 Agudo, in Actas del XIZI Simposio lberoamericano de Catalisis, Segouia, Spain, 1992, vol. 1, p. 499. M. A. Aulman, G. J. Siri, M. N. Blanco, C. V. Caceres and H. J. 8 9 K. Jiratova and M. Kraus, Appl. Catal., 1986,27, 21. J. L. G. Fierro, A. Lopez Agudo, P. Grange and B.Delmon, in 26 Thomas, Appl. Catal., 1983, 7, 139. J. H. Ashley and P. C. H. Mitchell, J. Chem. SOC. A, 1969, 2730; 10 Proc. 8th Int. Congr. Catal., Berlin, 1984, Verlag Chemie, Weiheim, 1984, vol. 2, p. 363. M. Lo Jacono and M. Schiavello, in Preparation of Catalysts, ed. B. Delmon, P. A. Jacobs and G. Poncelet, Elsevier, Amsterdam, 27 28 1968,284. J. P. Brunelle, Pure Appl. Chem., 1978,50, 1211. M. N. Blanco, C. V. Caceres, L. Osiglio, H. J. Thomas and F. J. Gil Llambias, in Actas del XVZIZ Congreso Latinoamericano de 11 12 1976, p. 473. B. R. Strohmeier and D. M. Hercules, J. Catal., 1984,86,266. A. Maezawa, Y. Okamoto and T. Imanaka, J. Chem. SOC., 29 Quimica, Santiago, Chile, 1588, vol. 2, Pontificia Universidad Catolica de Chile, p. 255. J. A. R. van Veen, P.A. J. M. Hendriks, R. R. Andrea, E. J. G. M. 13 Faraday Trans. I, 1987,83,665. H. J. Thomas, M. N. Blanco, C. V. Caceres, N. Firpo, F. J. Gil Llambias, J. L. G. Fierro and A. Lopez Agudo, J. Chem. SOC., 30 31 Romers and A. E. Wilson, J. Phys. Chem., 1990,94,5282. F. E. Massoth, Ado. Catal., 1978, 27, 265. Y. Okamoto, T. Imanaka and S. Teranishi, J. Catal., 1980, 65, 14 15 Faraday Trans., 1990,86, 2765. L. Wang and W. K. Hall, J. Catal., 1980,66, 251; 1982,77,232. S. Kasztelan, J. Grimblot, J. P. Bonnelle, E. Payen, H. Toulhoat 32 488. V. H. J. de Beer, T. H. M. van Sint Fiet, J. F. Engelen, A. C. van Haandel, M. W. J. Woles, C. H. Amberg and G. C. A. Schuit, J. 16 and Y. Jacquin, Appl. Catal., 1983,7,91. C. V. Caceres, J. L. G. Fierro, A. Lopez Agudo, M. N. Blanco 33 Catal., 1972, 27, 357. N. P. Martinez and P. C. H. Mitchell, in Proc. Climax Third Int. 17 and H. J. Thomas, J. Catal., 1985,95, 501. J. A. R. van Veen, H. De Wit, C. A. Emeis and P. A. J. M. Con$ on Chemistry and Uses of Molybdenum, ed. H. F. Barry and P. C. H. Mitchell, Climax Molybdenum Company, Ann 18 19 Hendriks, J. Catal., 1987, 107, 579. N. P. Luthra and W. C. Cheng, J. Catal., 1987,107,154. J. A. R. van Veen, P. A. J. M. Hendriks, E. J. G. M. Romers and R. R. Andrea, J. Phys. Chem., 1990,94,5275. 34 Arbor, MI, 1979, p. 105. S. Mendioroz, J. M. Palacios, J. L. G. Fierro and A. Lopez Agudo, Bull. Soc. Chim. Belg., 1987, 96,891. 20 F. M. Mulcahy, M. J. Fay, A. Proctor, M. Houalla and D. M. Hercules, J. Catal., 1990, 124, 231. Paper 4/00451E; Received 25th January, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002125

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2133-2140 Selective Oxidative Coupling of Methane Catalysed over Hydroxyapatite Ion-exchanged with Lead Yasuyuki Matsumurat and John B. Moffat" Department of Chemistry and Guelph-Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G 1 Shigeru Sugiyama and Hiromu Hayashi Department of Chemical Science and Technology, University of Tokushima, Minamijosanjima , Tokushima 770,Japan Naoya Shigemoto and Kanako Saitoh Shikoku Research Institute lnc., Yashima-nishi, Takamatsu 761-01,Japan Oxidative coupling of methane to ethane and ethene can be effectively catalysed over hydroxyapatite ion- exchanged with lead at reaction temperatures as low at 700 "C,while hydroxyapatite itself catalyses methane oxidation mainly to carbon oxides.The rate of oxygen conversion over the former apatite is almost the same as that with the latter catalyst, suggesting that the oxidation sites on the two apatites are similar. The experimental results suggest that the lead ions on the surface of apatite play an important role in both the activation of methane and stabilization of methyl radicals on the surface. The catalytic conversion of natural gas to value-added products continues to occupy the attention of a significant number of researchers.' Since methane is the major com-ponent of natural gas, the oxidative coupling of methane to form ethane and ethene is, at least in principle, an attractive process for the generation of precursors for economically important chemical processes.In spite of a substantial amount of research during the last ten years,' both the con- version of methane and selectivities to C2+ compounds are still too low to justify implementation of the process on an industrial scale, at least under present economic conditions. Nevertheless the work has provided significant information both on the nature of the methane oxidation process as well as the surface and catalytic properties of the solids studied. Although a variety of solids of various compositions have been examined for the methane conversion process, those containing lead usually produce high selectivities to C, compounds'-27 and in particular, lead oxide, supported on basic materials such as alumina and magnesia, catalyses the reaction effectively.13-27 However, many of the catalysts are unstable because of the high reaction temperature. For example, PbO/MgO, possibly the most selective under the reaction conditions so far available, produced 71.9% selec-tivity to C2 compounds with 13.2%methane conversion from 13 kPa of methane and 1.4 kPa of oxygen at 750"C.2' The catalyst has been shown to lose lead" at the reaction tem- perature, presumably as a result of the relatively high volatil- ity of lead. However, the C2+ selectivity was as low as 38.5% over PbO/MgO at 700°C,'7 at which temperature the loss of lead should be considerably reduced. Although less volatile lead salts such as lead phosphate and lead sulfate appear to be appropriate for the reaction, the selectivities to C2 com-pounds on these materials was found to be relatively low, i.e.51% with 9% conversion of methane for Pb,(PO,), and 63% with 8% conversion for PbSO, from 66 kPa of methane and 8 kPa of oxygen at 740°C,6 while more recent work at a reaction temperature of 775°C produced a C, selectivity of 82% on lead@) phosphate, but with a considerably reduced conversion. Many of the properties of hydroxyapatite [Ca,,(PO,),(OH),], which is found naturally in hard tissues ~~ t Present address: Osaka National Research Institute, AIST, Mid-origaoka, Ikeda, Osaka 563, Japan. such as bone and teeth, have been known for at least 30 years.28 Although the lattice is believed to be stable up to lo00 "C, temperatures above 1500 "C convert the hydroxy- apatite to tri- and tetra-calcium phosphate.29 Hydroxyapatite has a hexagonal structure constructed from columns of Ca and 0 atoms which are parallel to the hexagonal axis.29 Three oxygen atoms of each PO, tetrahedron are shared by one column with the fourth oxygen atom attached to a neigh- bouring column.The hexagonal unit cell of hydroxyapatite contains ten cations located on two sets of non-equivalent sites, four on site 1 and six on site 2. The calcium ions on site 1 are aligned in columns while those on site 2 are in equi- lateral triangles centred on the screw axes. The site 1 cations are coordinated to six oxygen atoms belonging to different PO4 tetrahedra and also to three oxygen atoms at a larger distance.The site 2 cations are found in cavities in the walls of the channels formed between the cations and 0 atoms. The hydroxy groups are situated in these channels and prob- ably form an approximately triangular coplanar arrangement with the Ca ions. A number of substitutions are possible for the cations and anions contained within hydroxyapatite [Ca,o-,(HPo,),(Po4)6_,(OH),_,; 0 G Z < 1].30-37 These substitutions may alter the crystallinity, lattice parameters, morphology and the stability of the structure. For example, in the exchange of calcium by lead the size of the unit cell increases, as expected from the difference in ionic radii.36937 Although the hydroxyapatite structure contains two non-equivalent cation sites, there is strong evidence that, for com- positions of less than 50% lead atoms, the latter occupy mostly site 2.Hydroxyapatites can function as acidic and/or basic cata- lysts, depending on their comp~sition;~~-~~ for example, a stoichiometric material shows evidence of acidic and basic sites, while a non-stoichiometric composition is effective only in acid-catalysed proce~ses.~' Recently, it has been shown that hydroxyapatite ion-exchanged with lead stably catalyses the methane coupling with C2+ selectivity above 80% (86% at methane conversions of 10%)at a reaction temperature as low as 700 0C.44 The catalytic activity decreases gradually during the first 30 h but stabilizes subsequent to this time.In the present paper, it will be shown that the activity of lead-modified apatite is not only related to the content of lead, but is evidently also dependent on the parent apatite. Thus, it appears that the lead-modified apatite is a new and versatile catalyst for methane coupling and not merely an extension of the lead oxide system. Experimental Stoichiometric and non-stoichiometric hydroxyapatites were prepared from Ca(N0,),4H20 (BDH AnalaR) and (NH,),HPO,(BDH AnalaR) according to the method described in ref. 45. The resulting solid was heated in air at 500°C for 3 h. The Ca : P molar ratio of the prepared hydroxyapatite was determined by analysing the concentra- tions of Ca2+ and Po,,-ions in the remaining solution from the synthesis by ion chromatography (Dionex 4500i).The values for the samples (Ap,.,,, AP1.61, Ap1.52 and Ap1.51) are given in Table 1. The Ca : P molar ratios of the starting materials were 1.67 for Ap,.,, , 1.62 for Ap1.61, 1.40 for Ap1.52, and 1.50for Ap1.51. Formation of hydroxyapatite was confirmed by recording the X-ray diffraction (XRD) patterns of these sample^.^^,^' Lead cation was ion-exchanged into the hydroxyapatite by stirring the apatite sample (2.0 g) in 0.20 dm3 of aqueous solution of lead nitrate (BDH AnalaR) at room temperature. The conditions of doping are presented in Table 1. After washing with water, the sample was heated at 500°C for 1 h. The chemical composition of the sample was determined by analysing the concentrations of Pb2+ and Ca2+ ions in the filtrate by atomic absorption spectrometry (see Table 1).No Po4,-ions were detected in the filtrates by ion chromatog- raphy. Surface areas for the samples were determined by the Brunauer-Emmett-Teller (BET) method from nitrogen adsorption isotherms (see Table 1). The sample of b-tricalcium phosphate (TCP) was prepared from Ap1.51 by heating at 1OOO"C for 2 h. By recording its XRD pattern, it was confirmed that the apatite structure of Ap,.,, was almost perfectly transformed into that of P-tricalcium ph~sphate.~, Lead oxide supported on magnesium oxide (PbO/MgO ; content of lead oxide, 20 wt.%) or on b-tricalcium phosphate (PbO/TCP; 20 wt.%) was prepared by impregnation of lead nitrate on magnesium oxide (Fisher Scientific Company) or TCP, respectively.Both samples were heated in air at 500°C for 5 h after the impregnation. Methane conversion was performed in a conventional fixed-bed continuous flow reactor operated under atmo-spheric pressure. The reactor consisted of a quartz tube of 8 mm id and 35 mm in length sealed at each end to 4 mm id quartz tubes. The catalyst was sandwiched with quartz wool plugs whose contribution to the reaction was negligible. The reactants (CH,, 14-43 kPa; 0, , 2-7 kPa) were diluted with helium gas and the total flow rate was 0.9 dm3 h-I. Catalysts J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 (0.05-0.60 g) were preheated in the flow without methane (0,,6 kPa; total flow rate, 0.6 dm3 h-') usually at 700 "C for 1 h.The reactants and products were analysed with an on- stream gas chromatograph (HP 5880) equipped with a ther- mal conductivity detector. Two columns, one Porapak T (5.4 m) the other Molecular Sieve 5A (0.4 m) were employed in the analyses. The conversions and selectivities were calcu- lated on the basis of the amounts of reaction products formed as determined by the GC analysis. Powder XRD patterns were recorded with a Siemens D 500diffractometer using Ni filtered Cu-Ka radiation. Surface analyses by X-ray photon spectroscopy (XPS) were carried out using a Perkin Elmer Phi 5500 spectrometer. The samples were mounted on a sample holder in air and set into the spectrometer. After measurement argon-ion etching of the sample was carried out (4 kV, 1 min), and the spectra were measured again after etching. Charge correction of the XPS data was accomplished by assuming that the binding energy of the C 1s peak was at 284.6 eV.Results Methane Coupling over Stoichiometric Hydroxyapatite Ionexchanged with Lead It is known that stoichiometric calcium hydroxyapatite func- tions as a base catalyst for alcohol con~ersion~'-~~ while the non-stoichiometric form is an acid cataly~t.~~,~~ As a catalyst for methane oxidation, the stoichiometric calcium hydroxy- apatite (Ap1.65) produced mainly carbon oxides at 700 "C, while ethane, ethene, C, compounds, formaldehyde, water and hydrogen were also detected (Table 2.) The XRD pattern of Ap,.,, recorded after the reaction showed that the hydroxyapatite structure of Ap,.,, was preserved throughout the reaction. Ion-exchange of hydroxyapatite with lead produced a remarkable improvement in the catalytic activity of the stoi- chiometric hydroxyapatite at 700 "C.The methane conver- sion and selectivity to C2+ compounds increased with increase in the content of lead in the hydroxyapatite, while conversion was close to 100% (Fig. 1). In the case of 0.3 g of Pb1,Apl.,, , the methane conversion and selectivity to C,+ compounds were 15.6 and 66.7%, respectively. No formation of hydrogen was observed. No peaks other than those attrib- uted to hydroxyapatite were observed in the XRD pattern of Pb16Apl.65 recorded after 3 h on-stream. Methane Coupling over Non-stoichiometric Hydroxyapatite Ion-exchanged with Lead Non-stoichiometric calcium hydroxyapatites absorbed a larger number of lead ions in the ion-exchange process than Table 1 Composition and surface area of hydroxyapatites atomic ratio Pb conc.a time content of Pb BET surface area sample /mmol dmW3 P Ca :P Pb :P /wt.% /m2 g-' Ap1.6S Ap1.61 AP1.52 1.65 1.61 1.52 53.6 57.9 74.6 AP1.Sl 1.51 71.7 Pb5Ap1.65 Pbl 3Ap1.65 Pb16Ap 1.65 Pb,,Ap1.6 1 Pb2 -lAp1.52 Pb,3Ap1.S 1 Pb,6Ap1 .5 1 Pb26AP1.S1 3 25 50 25 75 50 50 75 1.61 1.56 1.52 1.32 1.32 1.32 1.05 1.32 0.04 0.11 0.14 0.27 0.26 0.21 0.56 0.25 5 13 16 27 27 23 46 26 53.0 47.6 49.0 38.6 51.1 43.3 37.5 51.4 a Initial concentration of lead ion in ion-exchanging solution.Duration of ion-exchange at room temperature.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Methane oxidation over hydroxyapatites at 700 "C after 3 h on-stream" conversion (Yo) selectivity (%) surface areab catalyst amount c3 /m2 g-' Ap1.65 0.05 7.1 63 40.0 54.3 0.0 1.2 4.6 0.0 31.4 Ap1.61 0.15 85 85 65.4 21.9 2.1 2.1 8.5 0.0 23.3 Ap1.51 0.15 6.7 60 62.3 14.1 5.3 4.6 11.3 2.4 16.9 TCP 0.30 4.4 41 66.0 23.0 3.7 1.3 6.0 0.0 4.1 Pbl 6Ap1.65 0.08 9.3 63 0.7 33.1 0.0 18.3 43.9 4.0 18.7 Pb27AP1.61 0.30 12.8 79 3.0 30.8 0.1 23.9 39.9 2.4 8.4 Pb23AP1.51 0.15 9.2 44 1.8 19.6 0.1 24.2 52.6 1.7 Pb23Ap1.51 0.30 13.0 67 1.3 20.4 0.1 30.2 45.0 3.1 10.9 Pb23Ap1.51 0.60 16.4 92 1.9 22.2 0.1 35.3 38.7 1.9 Pb46Ap1.51 0.30 14.2 87 0.4 32.1 0.1 24.5 40.9 2.0 7.6 PbOFCP 0.05 4.5 42 2.6 55.4 0.0 5.7 34.7 1.6 1.Y PbO/MgO 0.05 6.7 72 1.1 66.7 0.0 4.9 26.2 1.1 10.7' Reaction conditions: partial pressure of CH,, 29 kPa; 02,4 kPa; total flow rate, 0.9 dm3 h-'.* BET surface area after the reaction. The surface area was measured just after pretreatment at 700 "C for 1 h. was found with Ap,.,, , and the hydroxyapatites containing large quantities of lead can be easily prepared from non-stoichiometric forms (see Table 1). Non-stoichiometric apatites, Ap1.6, and Ap1.5 ,, produced significantly higher selectivity to carbon monoxide at a reac- tion temperature of 700°C than was observed with Ap,.,, (Table 2). Although the XRD pattern of Ap1.6, just after the reaction showed that the hydroxyapatite structure of Ap, ,6 1 was preserved throughout the reaction, in the pattern of Ap1.51 after 3 h on-stream, peaks which were not seen in the patterns recorded before the reaction appeared at 27.5, 30.8 and 34.1" in 28.The peaks are attributed to p-tricalcium phosphate [Ca,(P04),].46 Since the intensity of the XRD peaks attributed to hydroxyapatite in the pattern for Ap,,,, after the reaction decreased to ca. 2/3 of that for Ap,,,, as prepared, it is estimated that ca. 1/3 of the hydroxyapatite form in Ap,., , was transformed to p-tricalcium phosphate under the reaction conditions. Although Pb27Apl.61 contained a higher percentage of lead than Pb,,Ap,.,,, the former solid produced almost the same C2+selectivity as found in the latter (see Table 2).The formation of tricalcium phosphate and lead pyrophosphate, the latter with 28 peaks at 25.1 and 26.2" was evident from the XRD pattern of Pb,,Ap,.,, recorded after the reaction. The C2+selectivity produced with Pb,,Ap,,,, was higher than that found with Pb,,Ap,.,, or Pb,,Ap,,,, (see Table 2). The formation of considerable quantities of p-tricalcium phosphate and lead pyrophosphate was detected by record- ing the XRD patterns of Pb,,Ap,.,, after the reaction. The -100 60 95 h h515 -40 ,\" v .-: .->. 4-.->5 lo > -20 g-0 a O5 OYO 0 5 10 15 20 Pb content (wt.%) Fig. 1 Relationship between content of lead in stoichiometric hydroxyapatite and its catalytic activity. Reaction conditions : Pbl6Ap1.,,, 0.30 g; reaction temperature, 70°C;CH,, 29 kPa; 02, 4 kPa; time-on-stream, 3 h.(0)O,, (W) CO,, (0)CH,, (+) C,H6, (A)C2H4 9 (0)co. patterns for these samples after pretreatment at 700 "C were almost the same as those recorded after the reaction, showing that transformation to p-tricalcium phosphate and lead pyrophosphate is primarily the result of the higher tem-perature rather than the methane reaction species. The apatite containing a larger amount of lead, Pb4,Ap, ., ,,pro-duced a lower selectivity to C2+ compounds (67.4%) while the conversions of methane and oxygen were somewhat lower than those found with P~,,AP,.~,. In the case of Pb46Ap,.51, peaks attributed to lead oxide phosphate [5PbO Pb,(P04),] were observed at 28.2, 29.6 and 32.0" (28) in the XRD pattern recorded just after the preparati~n,~, while other peaks were attributed to hydro~yapatite.~~.~~ After the reaction peaks attributed to p-tricalcium phosphate and lead pyrophosphate were also observed.Methane Coupling over Supported Lead Oxide In order to compare the catalytic activity of hydroxyapatite ion-exchanged with lead with that of lead oxide catalysts which are known to be effective in methane coupling, the reaction with lead oxide supported on magnesium oxide or /I-TCP was also studied. At a reaction temperature of 700"C, 0.05 g of PbO/MgO produced a relatively low selectivity (32.2%) to C2+ products, while that found with PbO/TCP was somewhat higher (42.0%), suggesting that TCP is superior to MgO as a support for PbO in the methane coup- ling process (see Table 2).The catalytic activity of PbO/TCP was considerably larger than that of TCP (see Table 2, com-parable oxygen conversions were obtained with 0.05 g of PbO/TCP and 0.30 g of TCP). However, the selectivity to C,, compounds was significantly lower than that with hydroxyapatites ion-exchanged with lead. Comparison of Catalytic Activity between Unmodified and Modified Hydroxyapatite Since the ion-exchange of lead into hydroxyapatite signifi- cantly changes the catalytic activity of the apatite it is of interest to compare the catalytic activity of unmodified and modified apatites. In order to avoid oxygen limitations, 0.15 g of Ap1.51 was used as a representative unmodified catalyst.With this quantity of catalyst the oxygen conversion was 50-76% under the reaction conditions. Pb,,Ap,., , or Pb27Ap1,52(0.30 g of each) employed as modified catalysts, also produced oxygen conversions of 49-76%. The activities of both catalysts were almost the same as that of Pb2,Ap,.,, . The dependence of the conversion rates for methane and oxygen and the C,+ selectivity on the partial pressure of J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 n 80 s Y 0'-1.0v 60 2.-.-> 4-40 v) + 20 0" 0 0 2 4 6 8 pressure of O,/kPa Fig. 2 Dependence of conversion rate of methane and oxygen on the partial pressure of oxygen. Open symbols; Ap1.51, 0.15 g. Solid symbols; Pb,,Ap, ,5 ,,0.30 g.Reaction conditions: reaction tem- perature, 700 "C; partial pressure of CH,, 29 kPa; time-on-stream, 3 CH,, (I,h. (0,0) 0)0,;(A,A) C,+ selectivity. oxygen is shown in Fig. 2 for Ap1.51 and Pb26Apl.51. The conversion rate was calculated from the methane and oxygen conversions after 3 h on-stream and from the surface area of the catalyst subsequent to the reaction. Both the rate of oxygen and of methane conversion increased with increase in the partial pressure of oxygen with both the catalysts and the former rates were similar for the two catalysts. In contrast, the rate of conversion of methane and the C,+ selectivity were significantly larger with Pb,6Apl.5, than with Ap1.51 and with the former catalyst the selectivity of C2+ products decreased with increase in the OJCH, ratio in the feed stream.With Ap1.52 the rates of conversion of methane and of oxygen reached plateaus with an increase in the partial pressure of methane although the maximum conversion of oxygen obtained with these experiments was 66% (Fig. 3). However, with Pb,-,Ap,.,, , the rate of methane conversion continued to increase with increasing partial pressure of methane although the rate of oxygen conversion was similar to that found for Ap,.,, (the maximum oxygen conversion of Pb,-,Ap, .52 was 71%). Effectof the Structure Change on the Catalytic Activity of Non-stoichiometric Hydroxyapatite Ion-exchanged with Lead Although non-stoichiometric hydroxyapatites, AP1.61, Ap1,52 and Ap,., 1, showed no significant differences in catalytic activity, Pb,,Ap,.,, produced a lower selectivity to C2+ 0.6 100 NI E c 0.4-E" E--.a .-4-!2 > c..-v) -0.55 .-C 0 700 800 900 pretreatment temperature/"(= Fig.4 Effect of pretreatment temperature on the catalytic activity of Pb,,Ap,,,, at a reaction temperature of 700 "C. Reaction conditions: catalyst, 0.30 g; partial pressure of CH,, 29 kPa; 0,, 4 kPa; time- on-stream, 3 h. lapa,intensity of the peak at 34.1" for 8-tricalcium phosphate, I,, ,intensity of the peak at 26.2" for lead pyrophosphate. (A) ZTCP/Zapa; (V) Zp,,/Zapa; other symbols as in Fig. 1. compounds than Pb,,Ap,,,, (see Table 2). From the XRD patterns it can be shown that Pb,,Ap,.61 pretreated at 700 "C contains smaller quantities of /3-tricalcium phosphate and lead pyrophosphate than Pb,,Ap,., pretreated at 700°C.Hence, the difference in the catalytic activity may be related to the difference in the structure. In order to repro- duce the structural change in Pb,,Ap,.,,, samples were pretreated at temperatures higher than 700 "C. The C, + selectivity was observed to increase with an increase in pretreatment temperature while the methane and oxygen conversions decreased (Fig. 4). The C,, selectivity with the catalyst pretreated at 900°C was 81.5% with a methane con- version of 6.1%, while the surface area of the catalyst after the reaction was 2.2 m2 g-l. The XRD patterns after the reaction showed that the contents of p-tricalcium phosphate and lead pyrophosphate in the structure increased with increasing pretreatment temperature (see Fig.4). The surface area of the catalyst pretreated at 800 "Cafter the reaction was 9.6 m2 g- '. The effect of pretreatment temperature on the selectivities with the Pb,,Ap,.,, catalyst is particularly evident at the lower reaction temperature of 600°C (Fig. 5). Although Pb,,Apl.51 pretreated at 650°C produced almost the same conversions and selectivities as observed after pretreatment at 803h 60-> 4-.->.-4--8 40-% i 20-2 0.2.; a C C .-0 8v)6 C 600 700 800 900' -so pretreatment temperature/"(= 0 10 20 30 40 50 pressure of CHJkPa Fig. 5 Effect of pretreatment temperature on the catalytic activity of Pb,,Ap,,,, at a reaction temperature of 600°C.Reaction conditions: Fig. 3 Dependence of conversion rate of methane and oxygen on catalyst, 0.30 g; partial pressure of CH,, 29 kPa; 0,, 4 kPa; time- 0.15 g. Solid on-stream, 3 h. Zap., intensity of the peak at 25.5" for hydroxyapatite; the partial pressure of methane. Open symbols; AP~.~,, symbols; Pb2,Ap,,,2, 0.30 g. Reaction conditions: reaction tem- ZTcp, intensity of the peak at 34.1" for 8-tricalcium phosphate; I,,,, perature, 700 "C; partial pressure of 0, ,4 kPa; time-on-stream, 3 h. intensity of the peak at 26.2" for lead pyrophosphate. (V)Cz+ selec-Symbols as in Fig. 2. tivity; other symbols as in Fig. 1 and 4. J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 70 I 100 -80 A h v -60 5 C >.-.-cE? -40 'g> -P,0 8--20 =Ilo--" 0 I I I =I m+ 10 0 12 24 36 48 60 20 25 30 35 40 time on-stream/h 28fdegrees Fig. 7 Time course of the methane coupling over hydroxyapatite Fig. 6 Change in XRD patterns of Pb,,Ap,.,, with pretreatment doped with lead. Reaction conditions: PbZ6Ap,,,,, 0.30 g; reaction temperature:(a)600, (b)650, (c)700 and (d) 900 "C. The samples were temperature, 700°C; CH,, 29 kPa; 0, 2 kPa. Conversion: (0)taken from the reactions shown in Fig. 5. (0)Peaks attributed to (0)CH,. Selectivity: (A) C,, ,(m) CO,, (0)CO. O,, /3-tricalcium phosphate; (M)peaks attributed to lead pyrophosphate. 600"C,the selectivity to C,+ compounds and CO, increased Methane coupling catalysts containing lead have been abruptly after pretreatment at 700 "C, that to CO decreased found to deactivate due to the vaporization of lead from the and the conversions of methane and oxygen remained vir- solids.In order to estimate the stability of lead-modified tually unchanged. However, the C, + selectivity decreased apatite, the reaction was carried out for an extended period after treatment at 900 "C. The XRD patterns of these samples, of time (Fig. 7). At the initial stage of the reaction over lead- recorded after the reaction (Fig. 6) showed the formation of modified apatite, the conversion of methane increased slight- fl-tricalcium phosphate and lead pyrophosphate in the ly, but subsequently decreased after 2 h on-stream.However, apatite structure of the samples pretreated at 700°C or above after ca. 24 h on-stream the conversion became stable. The (Fig. 5). The values of the surface area for the samples after selectivity increased somewhat in the first 24 h and subse- reaction were 32.9 m2 g-' for Pbz3Ap1.,, pretreated at quently achieved a constant value. 600"C, 32.2 m2 g-' at 650"C, 18.7 m2 g-' at 700"C, and 2.8 m2 g- ' at 900 "C. In contrast, a similar C2+selectivity (66.7%)was observed with 0.30 g of Pb,,Ap,.,, pretreated at 900°C for 1 h as with Analyses by XPS the catalyst pretreated at 700 "C for 1 h [66.2% with 0.08 g of In order to characterize the surface of hydroxyapatites, XPS Pb,,Ap,.,, pretreated at 700°C and 66.7% with 0.30 g of analyses were carried out under the same pretreatment condi- Pb,,Ap,.,, pretreated at 700°C (see Fig.1 and Table 2)], tions as described previously. No significant differences in the while methane and oxygen conversions were 5.8 and 36%, binding energies of Ca 2p3,,, P 2p and 0 1s were observed respectively, for 0.15 g of Pb1,Apl,,, pretreated at 900°C. for Ap1.6, and Ap1.51 pretreated at 700°C for 1 h (Table 3) The conversions of methane are significantly smaller than although fl-tricalcium phosphate was partially formed in those produced with 0.08 and 0.15 g of the solids of the same Ap,.,,. No significant shoulders were found in the XPS peaks composition pretreated at 700 "C (methane conversions were (not shown). The surface composition was calculated from 9.3 and 15.6%, respectively).The XRD pattern on the cata- the intensities of the peaks. lyst just after the reaction showed that the structure of The binding energies of Ca 2p3/,, P 2p and 0 1s for the hydroxyapatite is preserved even after the pretreatment at apatites ion-exchanged with lead were almost the same as 900 "C. those for the unmodified apatites while the peak of Pb 4f,,, Table 3 Summary of XPS analyses' binding energyfeV atomic ratio' sample" 2P3/2 Pb 4f,,2 p 2P 0 1s Ca/P Pb/P o/p Ap1.65 346.9 133.1 530.7 4.1 (700 "C) (346.6) (133.1) (530.7) (3.9)AP1.51 346.7 1.33.0 530.5 4.1 (700 "C) (347.0) (1.33.3) (530.8) (4.0) Pbl 6Apl .6S 346.9 138.6 133.2 530.7 4.8 (700 "C) (347.0) (138.3, 136.2) (1 33.4) (531.0) (4.6)Pb23Ap1.S1 347.0 138.8 133.4 530.7 5.1 (600 "C) (347.2) (138.6, 136.4) (133.5) (53 1.0) (3.7)Pb,3Ap1 .S 1 347.0 138.8 133.2 530.8 5.8 (700 "C) (347.0) (138.1, 136.2) (1 33.2) (530.9) (4.6)Pb23Ap1.S1 346.7 138.6 133.1 530.4 5.3 (900 "C) (3 46.9) (138.1, 136.0) (133.2) (530.7) (3.6)Pb46Ap1.5 1 347.1 138.7 133.4 530.7 5.1 (700°C) (347.2) (1 38.4, 136.3) (1 33.4) (530.9) (4.3) a Values after argon-ion sputtering for 1 min." Pretreatment temperature. The surface composition was determined from the peak intensities of Ca 2p3i2, Pb 4f,,2, P 2p, and 0 1s using the sensitivity factors of 1.218, 4.786,0.355 and 0.711, respectively. was at 138.7 f0.1 eV before argon-ion sputtering. No signifi-cant shoulders were observed in the peaks. The peak observed on lead-modified hydroxyapatites at 138.7 eV can be attributed to Pb ions, while determination of the exact oxidation state is difficult because Pb 4f binding energies for the lead ion reported previously were not con~tant.~~?~’ After the sputtering, the latter peak divided into two peaks at 138.1-138.6 eV and 136.2-136.4 eV, which can be attributed to lead ions and metallic lead, re~pectively.~’ The surface atomic ratio of Pb/P (0.4) for Pbl6Ap1.6, was significantly higher than expected from chemical analysis (0.14, see Table 1).The surface ratio on Pb23Ap,,51 was similar to that on Pb,,Ap,.,, and increased after pretreat- ment at 900°C. The Pb : P ratio (0.5) of Pb4,Ap1~,, was appreciably lower than that found from chemical analysis (0.56).Discussion Effect of Lead Ions The addition of lead to hydroxyapatite by ion-exchange pro- duces a significant increase in the C2+ selectivity in the methane conversion (see Fig. 1 and Table 2). However, the rates of oxygen conversion observed with most of the hydroxyapatites ion-exchanged with lead are almost the same as that for unmodified apatites (see Fig. 2 and 3). In the case of Ap,.,, the rate of oxygen conversion is calculated as 0.69 mmol h-’ m-’ while the rate for Pb,,Apl~65 is 0.65 mmol h-’ rn-, from the data in Table 2. The rate for both Ap1.6, and Pb27Ap1.61 is 0.37 mmol h-’ m-’ although the oxygen conversion is relatively high (85 and 79%, respectively). On the other hand, only 0.05 g of PbO/TCP produces an oxygen conversion similar to that found with 0.30 g of TCP.Thus, lead oxide on the surface apparently enhances the interaction with oxygen although the C,, selectivity with the oxide is smaller than that produced with hydroxyapatite ion-exchanged with lead. The rate of oxygen conversion for Pb4,Apl.51 is substantially higher than with Ap1.51, for example, while the C,+ selectivity is lower than that pro- duced with Pb,,Ap,., ’.After modification of hydroxyapatite by ion-exchange with lead, the surface atomic ratio of 0 : P increased from ca. 4 to 5 (see Table 3). Since the ratio in hydroxyapatite is formally 4.3, oxygen atoms which are not included in the hydroxyapatite structure apparently exist on the surface. Although it is possible that these are associated with lead, possibly to form lead oxide, the binding energy of 0 1s for lead oxides is close to 531 eV,’’ and hence, such oxygen species cannot be differentiated from those producing the 0 1s XPS peak.However, no formation of lead oxide was detected from the XRD patterns for lead-modified apatite samples except Pb4,Ap1., ,,suggesting that the species con- taining the non-stoichiometric oxygen atoms and lead ions on the surface, if present, are not lead oxides. Since the rate of conversion of oxygen on ion-exchanged apatites is almost the same as that of unmodified apatite, it can be supposed that the sites responsible for the activation of oxygen are similar on these two catalysts. As shown in Fig. 2, the rate of conversion of methane increases with an increase in the pressure of oxygen in the feedstream on both modified and unmodified apatites.Hence, oxygen appears to play a particularly important role in the activation of methane during the coupling of methane over hydroxy-apatite. Since the methane conversion rate for hydroxyapatite ion-exchanged with lead increases with increase in the partial pressure of methane while that for unmodified apatite is almost constant regardless of the pressure of methane (see Fig. 3), it appears that the lead ions on hydroxyapatite J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 enhance the activation of methane. It is generally believed that methyl radicals are formed during the methane coupling process by the abstraction of a hydrogen atom from the methane molecule by active oxygen species.Since lead can bind through strong covalent bonding to carbon,,’ the lead ions on the hydroxyapatite surface may stabilize methyl rad- icals generated by the reaction between methane and active oxygen species on the surface. Sites of Lead Ions on Hydroxyapatite The Ca: P molar ratios of Pb23-27Apl~51-l~,lare 1.32 regardless of the Pb : P molar ratio and the Ca : P ratio in the original apatite (see Table 1). All of these samples were prepared by ion-exchange for 2-3 h, suggesting either the presence of calcium ion sites in the non-stoichiometric form which are comparatively stable against ion exchange or vacancies which become occupied by lead ions.After argon- ion sputtering the Ca : P ratios of Ap,.,, and Ap1.51 deter- mined by XPS increase from 1.4 to 1.6 and from 1.3 to 1.4, respectively (see Table 3), suggesting that even on stoichio- metric apatite the surface composition is non-stoichiometric. Although the Ca : P atomic ratio of Ap1.51 on the surface is 1.3, the atomic ratio of (Ca + Pb): P for the surface of Pb,,Ap,.,, pretreated at 600°C and 700°C and Pb4,Ap1.,, pretreated at 700°C is 1.7. The results of composition analysis (Table 1) show that the (Ca + Pb): P ratio of Pbl,Apl~6,(1.66) is slightly higher than the Ca: P ratio of Ap,.,,, suggesting that the majority of lead ions are exchanged with calcium ions in Pb,,Ap,~,, but only a small portion of the lead ions are bound in the apatite.The atomic ratio of (Ca + Pb) : P for Pb,,Ap,.,, determined by XPS is 1.7, a value which is significantly higher than the ratio of Ca : P (1.4)for Ap,.,, , but decreased to 1.5 after argon-ion sputtering (see Table 3). Thus, some portion of the lead ions apparently become fixed at the calcium-deficient sites on the surface of Ap,.,, during the ion-exchange process. Since the lead content on the surface of Pb,,Apl.,5 after the sputtering is reduced to 1/4 of the original value, it is evident that the sputtering process preferentially eliminates the surface lead species. The surface density of lead ions determined by XPS is also high on Pb,,Apl.5, and Pb4,Ap1.,,. As can be seen in Table 1, the number of lead ions exchanged into Pb,Apl~,l~,~5,was generally larger than that of the calcium ions exchanged out of the solid.For example, the atomic ratio of (Ca + Pb) : P in Pb46Ap,,51 is 1.61 and the ratio is apparently larger than that of the original hydroxyapatite (Ap1.51) by 0.10. The ratios for other samples are 1.53 for Pb,,Ap,~,,, 1.57 for Pb,,Ap,~,,, and 1.58 for Pb,,Ap,.,,. The surface atomic ratios of (Ca + Pb) :P for Pb,,Ap,.,, pretreated at 600 and 700°C and Pb,,Ap,.,, after argon-ion sputtering are no less than the ratio of Ca/P for Apl.51 after the sputtering (see Table 3). Thus after the ion-exchange process the lead ions are held in the solid structure primarily though the occupation of positions previously containing calcium ions and relatively small numbers of lead ions are held at the calcium-deficient sites both on the surface and in the bulk of non-stoichiometric hydroxyapatite. Since the for- mation of lead oxide phosphate was observed on Pb4,Ap,., ’, the large number of lead ions apparently occupying the calcium-deficient sites presumably result in the formation of lead oxide phosphate.Effect of Structure Change fl-Tricalcium phosphate is often produced from non-stoichiometric hydroxyapatite by dehydration at high tem- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 perat~re.~,~~~As can be seen in Table 2, 1.2 m2 of /3-tricalcium phosphate (0.30 g of TCP) produced a methane conversion of 4.4% and similar selectivities to those observed with the non-stoichiometric apatite, while 2.5 m2 of Ap,.,, (0.15 g) produced a conversion of 6.7%.Hence, the activity of the tricalcium phosphate phase on the surface appears to be appreciably higher than that of the non-stoichiometric apatite phase (Ca : P = 1.5). However, the result obtained with Ap1.61 (8.5% conversion with 3.5 m2) is similar to that of AplS5, while the activity of AP,.~, (7.1% conversion with 1.6 m2) is considerably higher than that of tricalcium phos- phate, although Ap1.65 and Ap1.61 did not contain /?-tricalcium phosphate. Thus, the major active species is concluded to be hydroxyapatite, although /3-tricalcium phos- phate, when present, contributes significantly to the reaction. Although Ap1.61 and Ap1.51 show no significant differences in catalytic activity, Pb27Apl.61 pretreated at 700 "cpro-duces lower selectivity to C, + compounds than Pb,3Apl,5, (see Table 2).In both catalysts a partial transformation of structure to /3-tricalcium phosphate and lead pyrophosphate occurs, but Pb,,Ap,., includes considerably larger quan- tities of these phosphates than Pb,,Ap,.,,. Since both the C2+ selectivity with Pb2,Ap1.,1 and the structure change into b-tricalcium phosphate and lead pyrophosphate increase with increasing pretreatment temperature the difference in selectivity appears to be due to the difference in the structure. The same tendency can be observed in the reaction at 600°C with Pb,3Ap,.,l as observed with Pb27Apl,61 at 700°C (see Fig. 5). The C,, selectivity abruptly increases at a pretreat- ment temperature of 700 "Cwhere the structure change takes place.However, the selectivity is reduced by the pretreatment at 900 "C while there are significant quantities of /3-tricalcium phosphate and lead pyrophosphate in the sample. Thus, these two species on the surface are not considered as active sites which produce high C,, selectivity. It would be supposed that formation of a significant quantity of fl-tricalcium phos- phate results in the low C2+ selectivity; however, the selec- tivity to carbon dioxide is larger than that observed with the catalyst pretreated at 700°C (see Fig. 5) while TCP produces high selectivity to carbon monoxide and low selectivity to carbon dioxide (see Table 2). It appears, therefore, that the low C,, selectivity is not directly caused by formation of /?-tricalcium phosphate on the surface.The formation of both /3-tricalcium phosphate and lead pyrophosphate implies that separation of lead and calcium in hydroxyapatite proceeds during the thermal treatment at a high temperature. This suggests that intermediate species may form during the transformation of a portion of the hydroxyapatite to calcium and lead phosphates. It can be supposed that such intermediate species may contain appre- ciably large concentrations of lead although the lead ions are still in the structure of hydroxyapatite. If lead ions in hydroxyapatite stabilize methyl radicals during the reaction, the high concentration of lead ions should enhance coupling of methyl radicals.Hence, it is speculated that the thermal treatment of non-stoichiometric apatite at a high temperature produces regions of high density of lead ions in the structure of hydroxyapatite and these effectively promote the coupling of methane. Although conversions of reactants usually depend on the surface area of the catalyst, the conversions in Fig. 4 and 5 do not relate to the surface area linearly. In Fig. 5 the conversion rather increases up to a pretreatment tem- perature of 700°C. Other work has shown that the surface activities of both stoichiometric and non-stoichiometric calcium hydroxyapatite, unmodified with lead, increase with increase in pretreatment temperature in the range of 600-900"C.54Thus, the change in the catalytic activity, although related to the density of lead ions on the surface, is evidently dependent on other factors including the crystallographic structure of the surface region.The financial support of the Natural Sciences and Engineer- ing Research Council of Canada is gratefully acknowledged. References 1 Y. Amenomiya, V. I. Birss, M. Goledzinowski, J. Galuszka and A. R. Sanger, Catal. Rev.-Sci. Eng., 1990,32, 163. 2 G. E. Keller and M. M. Bhasin, J. Catal., 1982, 73,9. 3 K. Otsuka, K. Jinno and A. Morikawa, Chem. Lett., 1985,499. 4 H. J. F. Doval, 0. A. Scelza and A. A. Castro, React. Kinet. Catal. Lett., 1987,34, 143. 5 J. A. Sofranko, J. J. Leonard and C. A. Jones, J. Catal., 1987, 103, 302. 6 J. A.S. P. Carreiro and M. Baerns, React. Kinet. Catal. Lett., 1987, 35,49. 7 J. M. Thomas, K. Xian and J. Stachurski, J. Chem. SOC., Chem. Commun., 1988, 162. 8 J. M. Thomas, W. Ueda, J. Williams and K. D. M. Harris, Faraday Discuss., Chem. SOC.,1989,87, 33. 9 K. C. C. Kharas and J. H. Lunsford, J. Am. Chem. Soc., 1989, 111,2336. 10 K. Aika, N. Fujimoto, M.Kobayashim and E. Iwamatsu, J. Catal., 1991,127, 1. 11 K. J. Smith, T. M. Painter and J. Galuszka, Catal. Lett., 1991, 11, 301. 12 T. Ohno and J. B. Moffat, Catal. Lett., 1992, 16, 181. 13 W. Hinsen, W. Bytyn and M. Baerns, in Proc. 8th Znt. Congr. Catal. Berlin, 1984, Dechema, Frankfurt-am Main, 1984, vol. 3, p. 581. 14 W. Bytyn and M. Baerns, Appl. Catal., 1986,28, 199. 15 K. Asami, S.Hashimoto, T. Shikada, K. Fujimoto and H. Tominaga, Chem. Lett., 1986,1233. 16 K. Asami, S. Hashimoto, T. Shikada, K. Fujimoto and H. Tominaga, Int. Eng. Chem. Res., 1987,245, 1485. 17 K. Fujimoto, S. Hashimoto, K. Asami and H. Tominaga, Chem. Lett., 1987, 2157. 18 J. A. Roos, A. G. Bakker, H. Bosch, J. G. van Ommen and J. R. H. Ross, Catal. Today, 1987, 1, 133. 19 J. P. Bartek, J. F. Brazdil and R. K. Grasselli, Catal. Today, 1988,3, 117. 20 K. Aika and T. Nishiyama, in Proc. 9th Int. Congr. Catal. Calgary 1988, Chem. Inst. of Canada, 1988, vol. 2, p. 907. 21 K. Fujimoto, S. Hashimoto, K. Asami, K. Omata and H. Tominaga, Appl. Catal., 1989,50,223. 22 S. K. Agarwal, R. A. Migone and G. Marcelin, J. Catal., 1990, 121, 110. 23 S.K. Agarwal, R. A. Migone and G. Marcelin, J. Catal., 1990, 123, 228. 24 K. J. Smith, T. M. Painter and J. Galuszka, in Preprints 21th Canadian Symp. Catal., Halifax, 1990, p. 18. 25 H. Meng, T. M. Painter and A. R. Sanger, in Preprints 11th Canadian Symp. Catal., Halifax, 1990, p. 34. 26 K. J. Smith, T. M. Painter and J. Galuszka, Catal. Lett., 1991, 11, 301. 27 M. Y. Sinev, D. G. Filkova, V. Y. Bychkov, A. A. Ukharskii and 0.V. Krylov, Kinet. Katal., 1991, 32, 157. 28 M. 1. Kay, R. A. Young and A. S. Posner, Nature (London), 1964, 204,1050. 29 D. E. C. Corbridge, The Structural Chemistry of Phosphorus, Elsevier, Amsterdam, 1974, p. 90. 30 D. McConnel, Apatites, Applied Mineralogy, Spnnger, New York, 1963, vol. 5. 31 J. Lang, Bull.SOC. Sci. Bretagne, 1981,53, 95. 32 G. H. Nancollas, in Biological Mineralization and Demineral- ization, ed. G. H. Nancollas, Dahlem Konferenzen, Springer, Berlin, 1982, p. 79. 33 R. Z. Le Geros and J. P. Le Geros, in Phosphate Minerals, ed. J. 0.Nizidgan and P. B. Moore, Springer, New York,1984, p. 351. 34 T. Suzuki, T. Hatsushika and Y. Hayakawa, J. Chem. SOC., Faraday Trans. I, 1981,77, 1059. 35 T. Suzuki, T. Hatsushika and M. Miyake, J. Chem. SOC., Faraday Trans. I, 1982,78,3605. 36 A. Bigi, A. Ripamonti, S. Bruckner, M. Gazzano, N. Roveri and S. A. Thomas, Acta Crystallogr., Sect. B, 1989,45247. 2140 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 37 38 39 40 41 42 43 A. Bigi, M. Gandolfi, M. Gazzano, A. Ripamonti, N. Roveri and S.A. Thomas, J. Chem. Soc., Dalton Trans., 1991,2883. J. A. S. Bett, L. G. Christner and W. K. Hall, J. Catal., 1969, 13, 332. C. L. Kibby and W. K. Hall, J. Catal., 1973,29, 144. C. L. Kibby and W. K. Hall, J. Catal., 1973,31, 65. H. Monma, J. Catal., 1982,75,200. Y. Imizu, M. Kadoya and H. Abe, Chem. Lett., 1982,415. Y. Izumi, S. Sato and K. Urabe, Chem. Lett., 1983, 1649. 49 50 51 52 C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder and G. E. Muilenberg, Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer Corp., Eden Prairie, MN, 1978. J. M. Thomas and M. J. Tricker, J. Chem. SOC., Faraday Trans. 2, 1975,71, 329. H. Shapiro and F. W. Frey, The Organic Compounds of Lead, Wiley, New York, 1968. J. A. S. Bett, K. G. Christner and W. K. Hall, J. Am. Chem. Soc., 1967,89,5335. 44 Y. Matsumura and J. B. Moffat, Catal. Lett., 1993,17, 197. 53 H. Monma, S. Ueno and T. Kanazawa, J. Chem. Tech. Biotech- 45 46 47 48 E. Hayek and H. Newesely, Znorg. Syn., 1963,7, 63. Index (Inorganic) to the Powder Diflraction File, ed. J. V. Smith, ASTM Publication No. PDIS-16i Pa, 1966. J. Huaxia and P. M. Marquis, J. Mater. Sci. Lett., 1991, 10, 132. C. L. Kibby, S. S. Lande and W. K. Hall, J. Am. Chem. Soc., 1972,94, 214. 54 nol., 1981,31, 15. Y. Matsumura, H. Kanai, S. Sugiyama, H. Hayashi and J. B. Moffat, to be published. Paper 4/00455H;Received 25th January, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002133

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2141-2146 Group Behaviour of SAP041 Molecular Sieves containing Various Metals (Mg, Zn, Mn or Cd, Ni, Cr) Jan Kornatowski" lnstitut fur Kristallographie und Mineralogie ,J-W.Goethe-Universitat, 60054 Frankfurt am Main, Germany and lnstytut Chemii, Uniwersytet M. Kopernika, 87-100 Torun, Poland Gerd Finger,? Karin Jancke and Jurgen Richter-Mendau Zentum fur Heterogene Kataluse, 12484 Berlin, Germany Dietrich Schultze Bundesanstalt fur Materialforschung und-prufung , 12484 Berlin, Germany Werner Joswig and Werner H. Baur lnstitut fur Kristallographie und Mineralogie, J-W. Goethe-Universitat, 60054 Frankfurt am Main, Germany Derivatives of SAPO-11 containing Mg, Zn, Mn, Cd, Ni and Cr as heteroatoms have been synthesized.These additions influence considerably both the properties and the morphology of the crystalline phases synthesized. The products noted as MeAPSO-11 were investigated with light microscopy, scanning electron microscopy (SEM), electron microprobe analysis (EPM), X-ray diffraction (XRD), temperature-programmed XRD, differential thermal analysis (DTA), thermogravimetry (TG), magic-angle spinning nuclear magnetic resonance (MAS NMR) and wet chemical analysis. The materials can be clearly divided into two groups: (1) Mg, Zn or Mn, or (2) Cd, Ni or Cr containing preparations. The results suggest that all metals are incorporated into the framework of SAPO-11 molecular sieve, though in rather different amounts. The content of heteroatoms decides the properties of the synthesized molecular sieves.Since 1982, when aluminophosphate molecular sieves were discovered,' several tens of structures and over two hundred compositions of this family have been reported, many of them by investigators from Union Carbide (now UOP).,v3 Numerous materials have been claimed on the basis of iso- morphous substitution of silicon and many other elements ('hetero-atoms') for phosphorus and/or aluminium in most structure types, as reported among others in ref. 2-4 and claimed in patents such as those given in ref. 5. The incorporation of Si and/or other heteroatoms into the framework of microporous aluminophosphates (AlPO,-TI) can result in the formation of acidic groups which can be catalytically active. It would be desirable if the properties could be controlled and fitted to a particular catalytic process by substitution of different heteroatoms in various amounts.In spite of the large number of materials ~ynthesized,~-~ conclusive evidence for incorporation of heteroatoms into the aluminophosphate molecular sieve frameworks is still not available in most cases. In particular, for AlPO,-ll and its derivatives (structure type code AEL6), the number of papers reporting isomorphous substitutions of metals is surprisingly low, e.g. ref. 7-13. For this reason we decided to synthesize the SAPO-11 molecular sieves containing Mg, Zn, Mn, Cd, Ni and Cr as heteroatoms. We noticed a considerable influ- ence of these heteroelements on the morphology of the resulting crystals.In testing for evidence for the incorpor- ation of these heteroatoms into the framework of SAPO-11, we discovered a surprising group behaviour of the synthe- sized molecular sieves. Experimental AEL-type molecular sieves were synthesized hydrothermally in PTFE-lined autoclaves in air-heated ovens at 463 K for t Present address: Markische Allee 84, 12681 Berlin, Germany. 36 h from gels of formal molar composition: (1 -42) A1203 . a Me"0 -P,05 + b Si02 -4.0DPA * 28OH,O * xH~SO, (1 -u) Al,03 * a Memo P20, * b SiO, 4.0DPA . 280H20 . xH2S04 with (I b molecular sieve AlPO,-ll - 0.1 SAPO-11 0.1 0.1 MeAPSO-11 Preparation of the gels was performed as for the AFI structure type materials,6 i.e.A1P04-5 l4 and SAPO-5," with the only difference that (a) di-n-propylamine (DPA) was used as templating agent instead of triethylamine, (b) for the MeAPSOs, Me" or Me'" sulfates were added before the DPA-H3P0,-H,0 solution was admixed. The sulfuric acid was used to adjust the pH value of the gels to 3.5 & 0.2. After a period of crystallization, the autoclaves were cooled and the resulting materials were decanted, filtered, washed, dried at 378 K, and sieved, if necessary. Calcination of the samples was performed in air for 12 h. The temperature was 873 K for the Mg, Zn and Mn preparations and 1023 K for the others, corresponding to the different thermal stability of the materials (cf. Results and Discussion). The crystals were examined by light microscopy, SEM, EPM, XRD, tempera- ture-programmed XRD (Guinier technique), DTA, TG, MAS NMR and wet chemical analysis.The Guinier XRD patterns were measured by an Enraf Nonius high-temperature X-ray camera within the tem-perature range 298-1273 K. The thermogravimetric investigations were performed by simultaneous TG-DTA measurements in flowing air or nitro- gen using the SETARAM thermobalance TAG 24. The 13C, 27Al, 29Si and 31P MAS NMR spectra were recorded on a Bruker MSL 400 spectrometer (details will be published in a separate paper16). Results and Discussion For all reaction batches, the experiments were successful in obtaining crystals of AEL-type molecular sieves. In most cases, the materials also contained some byproduct(s) which usually formed separate crystalline phases of different dimen- sions, habitus and specific density.Therefore, they could be completely or at least mostly separated by a simple decanta- tion treatment and/or by sieving. The X-ray patterns of the samples purified in this manner and used later for subsequent investigation are shown in Fig. 1. The appearance of the most frequently occurring bypro- duct is presented in Plate 1. It was identified as SAPO-31 by X-ray and 29Si MAS NMR methods." In spite of the fact that our procedure of synthesis was similar to that for growing large crystals of AFI-type material^,'^^' the present syntheses did not yield well formed monocrystals of AEL type. In every batch, the AEL phase was composed of growth aggregates, as shown in Plate 2.The only common feature in their appearance was a quasi-1 5 10 15 20 25 30 28/degrees Fig. 1 X-Ray powder patterns (Cu-Ka) of the investigated AEL samples: (a) AlPO,-11, (b) SAPO-11, (c) MgAPSO-11, (d) ZnAPSO-11, (e) CdAPSO-11, (f)MnAPSO-11, (9)NiAPSO-11, (h) CrAPSO-11 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Content of metals (Me) and silicon in the investigated AEL samples a/(mol.%) blatoms U.C. -a sample Si Me Si Me 1(Si + Me) SAPO- 1 1 4.1 - 1.6 - 1.6 MgAPSO-11 ZnAPSO-11 2.6 3.5 5.7 5.7 1.0 1.4 2.3 2.3 3.3 3.7 CdAPSO-11 3.2 1.1 1.3 0.4 2.7 MnAPSO-11 2.9 3.4 1.2 1.4 2.6 NiAPSO- 1 1 4.7 1.0 1.9 0.4 2.3 CrAPSO- 1 1 3.8 0.4 1.5 0.1 1.6 u.c., unit cell.hexagonal symmetry of the external habitus of the aggregates. The differences in morphology must obviously be related to the presence of various cations in the reaction gels. The dimensions of the growth aggregates were usually smaller than 80 pm. The morphology of the aggregates can be divided clearly into two groups: more sphere-like crystals for the Cd, Ni and Cr preparations and more spindle-like elon- gated crystals for the Mg, Zn and Mn preparations (Plate 2). Qualitative electron microprobe analyses (EPM) showed for all products that the heteroelements, except for Mg, added to the reaction batches were present in the MeAPSO- 11 crystals in easily observable amounts. Wet chemical analyses supported these findings (Table 1) showing a con- siderable content of the heteroelements in the samples investi- gated, also in the case of Mg.This indicates that the Mg atoms were most likely located in the bulk of the crystalline aggregates and not in the surface layer analysed by EPM. The X-ray patterns of the samples containing various het- eroatoms differ characteristically in their peak intensities. For the AlP0,-11 and SAPO-11 samples, the peak at about 28 = 21" shows the highest intensity. The same holds true for the Cd-, Ni- and Cr-containing samples which have a rela- tively low content of heteroatoms (Table 1). In contrast, for the samples with the highest heteroatom contents, namely ZnAPSO-11, MgAPSO-11 and MnAPSO-11, the intensities of the peaks at about 28 = 23" and 21" are similar.On the basis of the temperature-programmed XRD pat- terns (Plate 3), the samples may again be divided with respect to their thermal behaviour (Table 2) into the same two groups that are apparent from the morphology and the inten- sities of the lines in the XRD powder patterns: (a) AlPO-11 and SAPO-11, as well as the Cd-, Ni- and Cr-containing samples, remain thermally stable up to about 1300 K. In the case of the Zn- and Mg-containing materials, the spectra show the beginning of a transformation into the tridymite structure at about 973 K and for Mn at about 1073 K. (b) An irreversible change of lattice parameters shown by a shift of Table 2 Thermal behaviour of the investigated AEL samples sample TchangcelK TStabblK AlPO,-ll ca. 523 >1300 SAPO- 1 1 ca.523 >1300 CrAPSO- 1 1 ca. 523 >1300 NiAPSO-11 ca. 523 >130 CdAPSO-11 ca. 523 >1300 MgAPSO-11 ca. 673 973 ZnAPSO-11 ca. 673 973 MnAPSO-11 ca. 723 1073 a Temperature at which change of lattice parameters occurs. * Temperature up to which the sampje is thermally stable. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Plate 1 Scanning electron micrograph of the main byproduct occurring in as-synthesized AEL phases: SAPO-31 J. Kornatowski et al. (Facing p. 2142) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 (61 Plate 2 Morphology of crystalline growth aggregates of the (Me)AEL phases synthesized (SEM): (a)SAPO-11, (b)CdAPSO-11, (c) NiAPSO-11, (d) CrAPSO-11, (e)AlP0,-11, (f)MnAPSO-11, (9)MgAPSO-11, (h)ZnAPSO-11 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2Bldegrees Plate 3 Guinier-Lenne powder XRD pattern for the AEL materials investigated (*Pt-reflexes): (a)A1P04-11, SAPO-11 and MeAPSO-11 (Me: Ni, Cd, Cr); lattice parameters change at ca. 523 K, stable up to ca. 1300 K; (b) MnAPSO-11; lattice parameters change at ca. 723 K, transformation into tridymite at CQ.1073 K; (c) Mg- and Zn-APSO-11; lattice parameters change at ca. 673 K, transformation into tridymite at ca. 973 K J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2143 A I-..I *.I . I...I...l...I...I -40 -60 -80 -100 -120 '-140 6 Fig. 2 29Si MAS NMR spectra of the dehydrated calcined samples: (a) SAPO-11, (b) CrAPSO-11, (c) NiAPSO-11, (d)CdAPSO-11, (e) MgAPSO-11 and (f) ZnAPSO-11 the positions of XRD reflections is observed at about 523 K for the first group and at about 673 K for the second group of thermally less stable materials (Table 2).Similar effects were reported for AlPO,- 1 1, MgAPO-39 and MgSAPO-46.'8 The 29Si MAS NMR spectra indicated that the synthesized materials really contained Si within the framework, that is they were SAPO-11 (Fig. 2). This is shown by a narrow line at ca. -95 ppm, which can be assigned to isolated silicon atoms replacing phosphorus atom^.'^ This line dominates in the spectra measured for the group of SAPO-11 and Cr-, Ni- and Cd-containing samples. In the spectra of Mg- and Zn- containing samples, this line has an intensity similar to that of the second line at ca. -11 1 ppm, which is not related to isolated silicon atoms.16 Thus, the materials can be again divided into the same two groups based on their 29Si MAS NMR spectra.Thermal analyses of the synthesized materials were per- formed under streaming nitrogen (Fig. 3) and air (Fig. 4). Obviously, the calcination process proceeds in several stages (four under nitrogen, five under air), the DTA maxima of which are compiled in Table 3. Similar behaviour has been reported for A1P04-5 2o and metal-substituted A1P0,-5.21 After desorption of water (stage I), stages 11-IV reflect mainly an endothermal desorption and decomposition of the template molecules as observed under nitrogen. Under air, these processes are predominated by oxidation occurring in parallel and exothermal effects are observed.For AlPO,-11 itself, stage I1 is the only process of template removal which is in accordance with the TG measurements reported in ref. 18. Stage I11 remains an extremely weak effect for all studied preparations. The main process of the removal of the tem- plate proceeds under nitrogen in stages I1 and/or IV (Fig. 3). 373 573 773 973 1173 T/K B -24 t I 373 573 773 973 1173 TIK Fig. 3 Thermoanalytic curves of the AEL samples measured under flowing nitrogen atmosphere: A, DTA; B, TG. (a) AlP0,-11, (b) SAPO-11, (c) CrAPSO-11, (6) NiAPSO-11, (e) CdAPSO-11, (f) MgAPSO-11, (9)ZnAPSO-11, (h)MnAPSO-11.Table 3 Total mass loss and temperature of the DTA peaks for the AEL samples investigated (a) Nitrogen atmosphere (all four effects are endothermal) Tp, minK total mass sample I I1 111 IV loss (%) ~~ AlP0,-1 1 370 555 -10.8 SAPO-1 1 370 545 740 9.0 CrAPSO-1 1 370 -745 9.7 NiAPSO-1 1 370 560 745 7.0 CdAPSO-11 370 560 7 30 8.0 MgAPSO-1 1 370 -770 5.9 ZnAPSO-1 1 370 -755 5.7 MnAPSO-1 1 370 -? 760 7.3 (b) Air atmosphere (all effects, except for I, are exothermal) T,,maxK total mass sample I I1 I11 IV v loss (%) AlP0,-1 1 340 585 ---12.1 SAPO-1 1 340 560 685 735 775 10.1 CrAPSO-1 1 340 -685 735 795 11.8 NiAPSO-11 340 570 685 750 825 9.8 CdAPSO-11 340 545 -725 820 9.4 MgAPSO-11 340 --740 920 9.6 ZnAPSO-1 1 340 --740 870 10.3 MnAPSO-11 340 -685 740 860 8.8 A I 373 573 773 973 1173 1373 TIK B hs "f Y -3 -1 01 373 573 773 973 11'73 1373 TIK Fig.4 Thermoanalytic curves of the AEL samples measured under flowing air atmosphere. Labelling as for Fig. 3. Evidently, in stage IV, which is only slightly influenced by the heteroatoms present, a cracking and partial desorption/ oxidation of the template takes place. Certain amounts of these cracking products remain as coke, which can be burned out under air during stage V. This mechanism is supported by the experimental observation that samples calcined under air have a higher loss of mass than those calcined under nitrogen (Table 3). Note that stages IV and V are also reflec- ted in one or two corresponding steps in the TG curves [Fig.3(b) and qb)]depending on the atmosphere in which the cal- cination is performed. In contrast to stage IV under nitrogen, [Table 3(a) and Fig. 3A] the position, intensity and width of the exothermal effects in stage V under air [Table 3(b)and Fig. 4A] (burning out of the cracking products) are distinctly dependent on the heteroatoms present. In this process, SAPO-11 and CrAPSO- 11 are similar in their behaviour because a substitution of Crnl for A1 does not create any additional framework charge, i.e. apparently no new active centres. However, the effect in stage V is much stronger for CrSAPO-11 since stage I1 is not observed for this molecular sieve. This indicates that the low Cr"' content (Table 1) is suficient for a considerable change in the thermal stability of the template molecules.One might then expect the occurrence of interactions between the Cr and Si heteroatoms. The incorporation of Me" creates additional framework charges (additional active centres), which is followed by more remarkable changes of the exothermal peak V. Its broadening (Fig. 4) indicates the occurrence of differently bonded pro- ducts of template decomposition which cannot be burned out within a narrow and well defined range of temperatures. Their exothermal effects overlap forming a broader band. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Zn Mn Cd Ni Cr Si 760 I 1 I I > 4 .l '3I L 3 heteroatoms per U.C.Fig. 5 Dependence of the temperature of oxidative burning out of the template (stage V under air) on the content of heteroatoms in the Me, (a) (Me + Si).MeAPSO-11 samples. (0) Another clear change of the exothermal peak V is the shifting of its position with temperature. As is seen from Fig. 5, the temperature of oxidative burning out of the template increases linearly with the heteroatom content. This is true for both the Me heteroatoms and for the sum Me + Si. The parallel course of both lines (Fig. 5) indicates that Si plays no deciding role for the oxidation of the template. Consequently, the group of Mg-, Zn- and Mn-containing samples, which have the highest heteroatom content (Table l),shows: (a)the highest temperature of burning out of the template (Fig.5); (b)lack of stage I1 (Fig. 4); (c) the lowest thermal stability, as observed from temperature-programmed XRD. The total loss of mass (Table 3) decreases linearly with the number of heteroatoms per unit cell [Fig. 6(a)-(d)]. The most distinct effect is observed for the dependence of the mass loss on the sum Me + Si under a nitrogen atmosphere [Fig. 6(a)]. The same tendency, though weaker, holds for the Me hetero- atoms alone (without Si) [Fig. 6(b)]. Under an air atmo- sphere, such a tendency is only slightly pronounced for Me + Si [Fig. qc)] and practically no longer observed for Me heteroatoms alone [Fig. qd)].Such a dependence can be explained if one accepts that both Me and Si heteroatoms create charged centres which are able to bond to the template molecules: the higher the number of heteroatoms, the strong- er the bonding of the template and thus a clearly lower loss of mass under a nitrogen atmosphere (thermal decomposition of the template).Under an air atmosphere, the effect is weaker as the thermal processes are accompanied by oxida- tion reactions occurring in parallel. The above observations allow construction of the following interpretation of the thermogravimetric effects. Stage I1 at ca. 545-585 K reflects the removal of those template molecules that are simply occluded in the pores and able to diffuse freely during heat treatment. Such a situation occurs in 'pure' A1P04-11 and partially in the less substituted samples (owing to the appearance of stages IV/V and considerably weaker effects in stage 11). The other template molecules are bound to heterocentres.In order to leave the pore system, they have to be decomposed (stages I11 and IV) and/or oxidized (stage V). Therefore, the thermal effect of the decomposition under nitrogen is nearly temperature independent, as opposed to that of the oxidation reaction which shows a linear depen- dence on the content of bonding heterocentres. This linear dependence of the thermogravimetric effects on the number of heteroatoms can be accepted as a good indication of the incorporation of these heteroatoms into the framework. Otherwise, their influence would be random. It remains still an open question why the total mass loss differs between par- ticular samples under both nitrogen and air.A possible J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 l2 AlPOl210 jAlPO4-11 Cr i Si h ,\" 8-Y 8 6- :d 61 Ni Mn YgZn 24\ (a) , , , ~ 0 0 1 2 3 4 OJ 0 1 2 I (Si + Me) per U.C. I Me per U.C. Cr Zn. Mn Mg 0-l 0 1 2 (Si + Me) per U.C. Me per U.C. Fig. 6 Loss of mass at calcination as a function of the sum of heteroatoms (a), (c) or Me heteroatoms (b), (4under N, (a),(b) and air (c), (d) atmospheres explanation could be the bonding of the template molecules to the heterocentres as well. If such bonding is really able to hinder the diffusion of the template molecules, it should result in an immobilization of the template molecules during the synthesis process and, consequently, in a lower density of their packing.Conclusions All of the results presented show that the prepared materials differ with respect to both their morphology and properties and they can be clearly divided into two groups: (1) Mg, Zn, Mn or (2) Cd, Ni, Cr containing materials. The results show a strong influence of all heteroatoms even in the case of low contents of some of them. This indicates that the heteroatoms should not be only simply occluded within the pore system. None of the methods used offers by itself a definite proof for incorporation of the heteroatoms into the framework of SAPO-11. That holds true also for the elements which are commonly accepted as possible candidates for an incorpor- ation into microporous aluminophosphates, i.e.Mg, Zn and Mn. The group behaviour of the preparations investigated by us is likely to be due more to variations in the amount of particular heteroatoms present in the samples than to their physico-chemical nature. Thus, the nature of the metallic het- eroatoms and/or their compounds used for the syntheses seems to control the amount of incorporated heteroatoms. The content of heteroelements influences primarily the properties of the synthesized molecular sieves. The morphol- ogy, XRD and temperature-programmed XRD results, and especially the linear dependences of the thermal behaviour of the samples suggest that all of the metals studied can be incorporated into the framework of SAPO-11. The very low contents of some of the heteroatoms agree with the findings of Kevan and co-worker~~-~' and with our results for a group of MeAPO-3 1 materials2' The authors thank Dr.B. Zibrowius for MAS NMR mea-surements and stimulating discussions. The work was par- tially supported by the Bundesministerium fur Forschung und Technik, the Deutsche Forschungsgemeinschaft, and by the Polish Committee of Scientific Research (K.B.N.). References 1 S. T. Wilson, B. M. Lok and E. M. Flanigen, US.Pat., 4 3 10440, 1982. 2 E. M. Flanigen, B. M. Lok, R. L. Patton and S. T. Wilson, in New Developments in Zeolite Science and Technology, ed. Y. Murakami, A. Jijima and J. W. Ward, Stud. Surf. Sci. Catal. 28, Elsevier, Amsterdam, 1986, p.103. 3 E. M. Flanigen, R. L. Patton and S. T. Wilson, in Innooation in Zeolite Materials Science, ed. P. J. Grobet, W. J. Mortier, E. F. Vansant and G. Schulz-Ekloff, Stud. Surf. Sci. Catal. 37, Else-vier, Amsterdam, 1988, p. 13. 4 S. T. Wilson and E. M. Flanigen, in Zeolite Synthesis, ed. M. L. Occelli and H.E. Robson, ACS Symp. Ser. 398, Am. Chem. SOC., Washington, 1989, p. 329. 5 J. Kornatowski, M. Rozwadowski and G. Finger, Pol. Pat. Appl. 291 455, 1991; 291 459, 1991; 291 460, 1991; J. Kornatowski, M. Rozwadowski, G. Finger and W. H. Baur, Pol. Pat. Appl. 291456, 1991; 291458, 1991; B. M. Lok, C. A. Messina, R. L. Patton, C. F. Gajek, T. R. Cannan and E. M. Flanigen, U.S. Pat., 4440871, 1984; B. M. Lok, B. K. Marcus and E.M. Flani-gen, U.S. Pat., 4500651, 1985; C. A. Messina, B. M. Lok and E. M. Flanigen, U.S.Pat., 4 544 143,1985; S. T. Wilson and E. M. Flanigen, U.S. Pat., 4567029, 1986; L. J. Wright and N. B. Mile- stone, Eur. Pat. Appl., 141 662, 1985. 6 W. M. Meier and D. H. Olson, Zeolites, 1992, 12,20. 7 J. J. Pluth, J. W. Smith and J. W. Richardson Jr., J. Phys. Chem., 1988,92,2734. 8 B. Kraushaar-Czametzki, W. G. M. Hoogervorst, R. R. Andrea, C. A. Emels and W. H. J. Stork, J. Chem. SOC., Fatuday Trans., 1991,87, 891. 9 G. Brouet, X. Chen and L. Kevan, J. Phys. Chem.,1991, 95, 4928. 10 N. Azuma, Ch. W.Lee and L. Kevan, Prep. Am. Chem. SOC.,Div. Petr. Chem., 1993,38, 538. 2146 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 11 12 13 14 15 G.Brouet, X. Chen and L. Kevan, Proc. 9th ZZC, Montreal, 1992, ed. R. von Ballmoos, J. B. Higgins and M. M. J. Treacy, Butterworth-Heinemann, Boston, 1993, p. 489. M. P. J. Peeters, J. H. C. van Hooff, R. A. Sheldon, V. I. Zholo-benko, L. M. Kustov and V. B. Kazansky, in ref. 11, p. 651. S. M. Yang and S.Y. Liu, in ref. 11, p. 623. G. Finger, J. Richter-Mendau, M. Bulow and J. Kornatowski, Zeolites, 1991, 11,443. G. Finger, J. Kornatowski, J. Richter-Mendau, K. Jancke, M. Bulow and M. Rozwadowski, in Catalysis and Adsorption by Zeolites, ed. G. Ohlmann, H. Heifer and R. Fricke, Stud. Surf. 17 18 19 20 21 22 W. H. Baur, W. Joswig, D. Kassner, J. Kornatowski and G. Finger, Acta Crystallogr., 1994, in the press. A. Ojo and L. McCusker, Zeolites, 1991,11,460. C. S. Blackwell and R. L. Patton, J. Phys. Chem., 1988,92,3965. V. R. Choudhary and S. R. Sansare, J. Th.Anal., 1987, 32, 777. Ch. Minchev, V. Minkov, V. Penvchev, H. Weyda and H. Lechert, J. Them. Anal., 1991,37,171. G. Finger, J. Kornatowski, K. Jancke, R. Matschat, J. Richter-Mendau and W. H. Baur, to be published. 16 Sci. Catal. 65, Elsevier, Amsterdam, 1991, p. 501. B. Zibrowius, G. Finger, J. Kornatowski and E. Lomer, to be published. Paper 3/070G; Received 31st December, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949002141

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2147-2153 Mechanistic Study of sec-Butyl Alcohol Dehydration on Zeolite H-ZSM-5 and Amorphous Aluminosilicate M. A. Makarova,? C. Williams$ and K. 1. Zamaraev" Institute of Catalysis, Russian Academy of Sciences Siberian Branch ,Prospekt Akademika Lavrentieva , 5,Novosibirsk 630090,Russia J. M. Thomas* Davy Faraday Research Laboratory, The Royal Institution, 21 Albemarle Street, London, UK WIX 4BS The dehydration of sec-butyl alcohol has been studied by in situ FTlR and gas-chromatographic (GC) kinetic methods in the range 60-140°C on zeolite H-ZSM-5 and amorphous aluminosilicate (AAS) samples with a well characterized number and strength of Br~nsted acid sites. Under flow conditions (GC kinetic studies), the reac- tion yields butenes [but-1-ene, (Z)-and (€)-but-e-ene] and water, with an activation energy of 40 1 kcal mol-' determined from steady-state data.Under non-steady-state conditions, the so-called 'stop effect ' is observed : namely, an increase in the rate of butene evolution (as compared with that at steady state) when the flow of alcohol into the reactor is halted. The course of dehydration on H-ZSM-5 in a static IR cell was followed by the appearance and growth of a peak for adsorbed water (water deformation peak at 1640 cm-'). The rate constant determined from the kinetics of water formation in the FTlR experiments (1.1 x s-' at 70°C) is found to be 400 times as high as the rate constant calculated from GC steady-state kinetic data. All these anomalous pheno- mena observed under flow conditions (the low rate of reaction, the high activation energy and the 'stop effect') can be explained by the slowing down of dehydration under these conditions as a result of the reverse reaction, i.e.the hydration of the product butene with product water. When the zeolite pores are free from physically adsorbed reactants (in the FTlR experiments or during the 'stop effect'), the extent of the reverse reaction decreases and the rate of butene formation increases. On AAS, whiFh has acid sites of Fimilar strength, but which has a much more open surface (average pore diameter ca. 50 A compared with 5.5 A for ZSM-5), similar effects are observed, but they are much less pronounced. This probably arises from the lower reactant concen- tration in the AAS at steady state and hence, a lower concentration of water in the vicinity of the active sites.1. Introduction It is generally accepted that the presence of microporous channels is one of the most significant factors influencing reactions in zeolites.' Nevertheless, the mechanism of their influence is still under discussion.2 The present work is part of a series of investigations aimed at elucidating the influence of pore confinement on reactions in zeolites. As a model reac- tion we have chosen the dehydration of butyl alcohols in H-ZSM-5 and results for n-, iso- and tert-butyl alcohol have already been p~blished.~-~ This article is devoted to the fourth isomer, namely sec-butyl alcohol.For a better under- standing of the processes that the alcohol molecules undergo inside the zeolite channels, we have adopted the following approaches, refined by us in earlier studies: (1) comparison of the kinetic parameters of dehydration obtained from studying products remaining adsorbed in the zeolite pores (in situ FTIR studies) and products that are desorbed (GC studies); (2) comparison of the course of reaction under steady-state and non-steady-state kinetic regimes ; (3) comparison of the course of reaction taking place on 'enclosed' microporous ZSM-5 zeolite samples with that on a sample with an 'open' surface, such as amorphous aluminosilicate (with an average pore diameter of ca. 50 A). t Present address : Department of Chemistry, University of Man- Chester Institute of Science and Technology, P.O.Box 88, Manches-ter, UK M60 1QD.1C. Williams was also based at the Davy Faraday Research Laboratory, Royal Institution, London, when the work was carried out. Present address : Koninklijke/Shell-Laboratorium, Badhuisweg 3, 103 1 CM, Amsterdam, The Netherlands. 2. Experimental The samples used in the present study (four samples of ZSM-5 and one sample of amorphous aluminosilicate, AAS) were described in detail in ref. 4-6. Their main characteristics are summarized for convenience in Table 1. GC kinetic studies were carried out using a flow micro- reactor system with on-line GC analysis. Full details were given in ref. 5. Briefly, the reactor was a quartz U-shaped tube loaded with 0.005-0.05 g of a pelletized zeolite catalyst (0.3-0.5 mm fraction).The system allows one to feed into the reactor either pure helium or a helium-alcohol gas mixture (usually 2.0 mol% sec-butyl alcohol, residual water content <10 ppm). The gas flow rate was typically 30-40 cm3min-' at an overall pressure of 1 atm. In order to avoid adsorption of reactants on the tube walls, the sampling system was ther- mostatted at 70°C. Before reaction, the sample was treated Table 1 Characterization of aluminosilicate samples sample crystallite size/pm Si :Al N/1OZ0sites g-' ZSM-5 1 <1 42 : 1 2.3 2 0.5-4 20 : 1 3.3b 4 15-20 35 : 1 2.8 5 4-6' 35 : 1 2.8 AAS loood 1.1 Sample numbers are the same as in ref. 4-6, Scanning electron micrographs for the zeolite samples are given in ref.4 and 6. Detailed characterization of AAS is given in ref. 3(b). a Per g of dehydrated sample. Na :A1 = 0.35 : 1. Aggregates of 0.1 pm crystallites. Average pore diameter ca. 50 A. 2148 for 1 h in a flow of oxygen at 500°C and for 2 h in a helium flow at 450°C. The dehydration reaction was studied in the temperature range 80-140 "C. During reaction, conversion was kept to < 10%. The reaction was zero order with respect to the alcohol. The reaction rate (in terms of butene and water evolution) was defined as : W(C4H8) = F[C4H81/m W(H,O) = F[H,O]/m where F is the helium-alcohol flow rate (cm3 s-'), [C,H,] and [H,O] are the concentrations of products determined by GC (molecules cm-3) and m is the sample mass (g).The rate at which alcohol was incident on the sample or the rate of exit of unreacted alcohol was defined as before:' V = F[C,H,OH]/m where [C,H,OH] is the concentration of butanol determined by GC before and after the reactor. FTIR studies of the dehydration raction were carried out on a Bruker FTIR spectrometer (IFS-l13V), using a ther- mostatted in situ cell. The construction of the FTIR cell was described in more detail in ref. 5. The samples were pressed into self-supporting discs (mass typically 25 mg, p = 6-12 mg an-,). Before adsorption, the discs were calcined for 1 h in air and for 2 h in vacuum (lo-, Torr) at 450°C. The sample was then cooled to the desired temperature and the IR spec- trum of the dehydrated sample was recorded for reference purposes.After injection of the desired amount of alcohol into the cell ([alcohol] : [acid sites] = 0.5 : l), the kinetics of reaction were studied by following changes in the IR spectra with time. In these kinetic studies, spectra (of 10 scans each) could be collected every 25 s or so in the wavenumber inter- val 4000-1200 cm-' (resolution 4 cm-'). By plotting the dif- ference between these spectra and the spectrum of the purely dehydrated zeolite, the spectrum of the adsorbed species was obtained. For non-kinetic work a typical number of scans was 200. 3. Results 3.1 GC Kinetic Studies 3.1.1 Beginning of Reaction and Steady State Dehydration of sec-butyl alcohol on H-ZSM-5 was studied in the temperature interval 80-140°C. The only products of reaction are butene isomers and water; no ether formation was observed.Typical reaction kinetics at 126°C are shown in Fig. 1. At the beginning of reaction the trough in the V(t) curve shows alcohol adsorption. Simultaneously with saturat- ing the sample with alcohol, the rate of product formation increases, reaching a steady-state value when adsorption of alcohol is complete. Note that in the initial non-steady-state phase of reaction, the rates of formation of both products, butene and water, are almost identical. This significantly differs from the case of isobutyl alcohol dehydration described by us in ref. 5, where a much larger amount of water was evolved in comparison with butene before the sample became saturated with alcohol.At lower reaction temperatures, similar kinetics are observed to these at 126 "C. At higher reaction temperatures there is some decrease in the rate of reaction in the region following alcohol adsorption. This most likely arises from oli- gomerization of butene on the catalyst active acid sites (analogous to that observed for the isobutyl alcohol/H-ZSM- 5 and n-butyl alcohol/H-ZSM-5 systems described previously in ref. 3b and 4), resulting in gradual catalyst deactivation. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 i 31 'E mI I A I ,o 0 10 20 " 60 ti me/min Fig. 1 Kinetics of sec-butyl alcohol dehydration on H-ZSM-5. Flow microreactor, sample 2, m = 0.044 g, T = 126 "C, [ROH] = 1.0 mol%, V, = 3.1 x 10l8 molecules of sec-butyl alcohol gc;: s-'.(0) sec-butyl alcohol, (0)butene isomers, (@) H,O. Rate constants for butene formation on zeolite samples of various crystallite sizes are shown in the Arrhenius plot in Fig. 2. These constants were determined using as a reaction rate the value W,, (the reaction rate after saturation of the sample with alcohol) obtained in a series of experiments, k = W,,JN (where N is the number of active sites). At T < 130"C, W,,, is identical to the steady-state rate of reaction. At T > 130°C (the upper point of the plot), when there is no steady state, then Wsatis the rate of butene formation imme- diately on completion of alcohol adsorption, when catalyst deactivation is as yet still minimal (see ref.4 for more details of the W,,, definition). The fact that all the experimental points satisfactorily lie on one and the same line demon- strates the absence of any diffusion limitations for the dehy- dration of sec-butyl alcohol on H-ZSM-5 under these conditions. From this plot a reaction activation energy of 40 f2 kcal mol-' is determined. The kinetics of dehydration of sec-butyl alcohol on the AAS sample at 126°C are the same as those already seen for the ZSM-5 zeolite. The kinetics of butene formation at this temperature are shown in Fig. 3(4. It is very surprising that the rate of dehydration on the AAS is significantly higher than that on the ZSM-5 zeolite. Thus the rate per g is twice that on ZSM-5 [Fig.1 and Fig. 3(a)], while the rate per -3 I -5. n -7-c Iv)1-s = -9. -1 1--131 . . . . . . . . . 2.2 2.4 2.6 2.8 3.0 2 2 103 KIT Fig. 2 Arrhenius plot for the rate constants of butene formation on dehydration of sec-butyl alcohol on different samples of H-ZSM-5 : (0)sample 1, (A)sample 2, (0)sample 4, (0)sample 5 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 c I v) (a-2 2. 10 0 0s; U The Arrhenius plot for the rate constants of sec-butyl alcohol dehydration on AAS (in terms of butene evolution) obtained from experiments such as that described above, is shown in.Fig. 5. These data allow us to estimate two activa- tion energies: E,, ,,, = 18 f1 kcal mol- and E,, ,= 26 f1 kcal mol- '.Therefore, the two types of aluminosilicates, namely the ZSM-5 zeolite and the amorphous sample, behave quite dif- ferently in the dehydration of sec-butyl alcohol. First, for the zeolite samples, the decrease in the reaction rate as compared with Wsat becomes more significant as the reaction tem-perature is increased, and most likely indicates deactivation of the active sites as a result of butene oligomerization on them. For the AAS sample, the drop in the reaction rate as compared with W,,, becomes more prominent on decreasing the reaction temperature and so is apparently not connected with the oligomerization processes. Secondly, the rate con- stant for dehydration on AAS is higher than that on H-ZSM-5 (by a factor of six at 126°C and even more at lower temperatures).Thirdly, the activation energy in the case of H-ZSM-5 (40kcal mol-') is significantly higher than both activation energies determined for AAS by different methods $2$ 0 10 20 30 40 (18 and 26 kcal mol- I). ti me/min Fig. 3 Kinetics of sec-butyl alcohol dehydration on AAS at different temperatures, flow microreactor, rn = 0.0394 g, [ROH] = 2.0 mol%. T/"C: (a) 126, (b) 106, (c) 68. active site (using data from Table 1) is six times higher. This phenomenon was not observed either in the case of n-butyl alcohol dehydration3' where the rate constant (i.e. the rate of reaction per active site) for the H-ZSM-5 zeolite was slightly higher (by a factor of two to three) than that for the AAS, or in the case of isobutyl alcohol and tert-butyl alcohol dehy- dration, where the rate constants for zeolite and AAS were iden tical.4 At lower reaction temperatures the kinetics of butene for- mation on AAS differ from those observed on H-ZSM-5 [Fig.3(b), (c)]. After reaching W,,, (time ca. 3 min), one sees initially a decrease in the rate of butene formation and only after that, a steady-state region. Note that with decreasing reaction tem- perature, the difference between W,,, and W, (which is the steady-state rate) becomes increasingly pronounced. A more detailed picture of the kinetics of dehydration at an interme- diate temperature of 90°C is given in Fig. 4. It is seen that less water than butene is evolved before reaching the steady state.For instance, at the maximum rate of butene evolution (t = 3 min), water is not observed at all. A ..A b I 0 10 20 30 40 50 ti me/min Fig. 4 Kinetics of sec-butyl alcohol dehydration on AAS. Flow microreactor, m = 0.0535 g, T = 9O"C, [ROH] = 2.0 mol%. (0)sec-butyl alcohol, (0)butene isomers, ((3)H,O. 3.1.2 Non-steady-state Conditions :'Stop Efect ' Experiments under non-steady-state conditions were carried out in the following manner. After reaching steady state, the alcohol-helium reaction mixture was replaced by a flow of pure helium. This moment of switching flows was taken as time zero. The 'reply' of the zeolite catalyst to this change in reaction conditions was then followed. Results at 126°C are shown in Fig.6. Thus, after switching from alcohol-helium to pure helium, the rate of butene formation at first increases sharply and then subsequently decreases. Estimating the amount of butene desorbed from the sample during purging gives a value corresponding to less than half of the number of active sites in the catalyst. On returning to the alcohol- helium flow, the new rate of dehydration has dropped to ca. 50% of its original value. This decrease in the reaction rate can obviously be explained by butene oligomerization on the active sites during purging (when the zeolite channels are relatively empty and so the side reaction, oligomerization, is favoured). These results agree with those obtained for iso- butyl alcohol dehydration on H-ZSM-5, where, on switching \ 9-10--12 J4 2.2 2.4 2.6 2.8 3.0 3 2 103 U/T Fig.5 Arrhenius plot for the rate constants of butene formation on dehydration of sec-butyl alcohol on AAS: (0)WJN, (0)W,/N, usually for t = 1-3 min. Each pair of points at the same temperature refer to the same experiment. 2150 20 c Iv) 4J --m10 CJ, 15 h al al w 1s 10-1 al E I$5 'I0 r I Is-u-0 0 5 10 15 time/mi n Fig. 6 Dehydration of sec-butyl alcohol on H-ZSM-5 (sample 2) at 126°C and subsequent desorption of butene when alcohol flow is stopped. The horizontal line on the extreme right denotes the reac- tion rate after returning to the sec-butyl alcohol-helium mixture. back to a flow of reaction mixture, we observed a new reac- tion rate which was reduced by a factor of two as a result of deactivation of the zeolite active sites during the period of helium purge.4 At lower reaction temperatures, butene oligomerization on the active sites is not so prominent. This is shown by the results of a similar experiment to that described above, but at a lower temperature (96"C), Fig.7. In this case, the rate of butene formation increases gradually by one order of magni- tude after replacing the reaction mixture with pure helium, before eventually starting to decrease. On returning to the alcohol-helium stream, the original rate of reaction is fully restored. Moreover, estimation of the area of the butene desorption peak gives a value of one butene molecule desorbed per active site.Thus the picture observed at this temperature is not complicated by subsequent oligomeriza- tion of butene generated inside the zeolite pores. Since zeolite samples of different crystallite sizes behave in a similar manner, we conclude that there is no connection between the observed phenomena and diffusion limitations for butene molecules inside the zeolite pores under steady- state conditions. 201 I II reaction desorption 141 I p 10 0 58 6 4 2 0 0 10 20 30 " time/min Fig. 7 Dehydration of sec-butyl alcohol on different samples of H-ZSM-5 at 96°C and subsequent desorption of butene when alcohol flow is stopped. The horizontal line on the extreme right denotes the reaction rate after returning to the sec-butyl alcohol- helium mixture.(0) sample 4.Sample 1, (A)sample 2, (0) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 16 I 14 reaction - 12 & 10 d 98 I I 26 I I 4 I I 2 I I 0 0 10 20 30 40 50 a ti me/min Fig. 8 Dehydration of sec-butyl alcohol on AAS at 90°C and sub- sequent desorption of butene when alcohol flow is stopped. The hori- zontal line on the extreme right denotes the reaction rate after returning to the sec-butyl alcohol-helium mixture. For dehydration of sec-butyl alcohol on AAS, under non- steady-state conditions we also observed some increase in the rate of butene evolution (Fig. 8). The estimated value of the area of the butene desorption peak is close to the number of active sites in the AAS sample, 1.3 butene molecules desorbed per active acid site.The decay of the kinetics curve of butene evolution is exponential and so it is possible to calculate the rate constant for butene formation from these data. The start- ing point for the treatment of the function is denoted in Fig. 8 by *. This plot gives a rate constant of 1.7 x s-l, which is very close to that determined from W,,, at the beginning of this experiment (1.8 x s-' from Fig. 5) and is almost twice as much as the steady-state value (determined from W,) of 0.9 x s-' (Fig. 5). Therefore, for both H-ZSM-5 and AAS, replacing the reac- tion mixture with pure helium gives rise to the so-called 'stop effect': i.e.an increase in the rate of product (in our case butene) formation as compared with that at steady state. However, the effect is much more significant in the case of the zeolite samples. 3.1.3 Isomeric Distribution of Butenes For reaction on H-ZSM-5, the isomeric distribution of the reaction product, butene, is found to be independent of the zeolite crystallite size (Table 2) or contact time, and almost constant in the studied temperature range. The composition of butenes obtained on AAS also does not vary in the tem- perature range employed, but differs from that observed for the zeolite. For AAS the distribution is shifted more towards but-2-ene, while more but-l-ene is produced by the zeolite samples. In no case was 2-methylpropene (isobutene) observed; this is in good agreement with literature data on the absence of skeletal isomerization at such temperat~re.~ In addition, the results in Table 2 show that the mixture of Table 2 Butene isomeric distribution at 100 "C but-l-ene (E)-but-Zene (Z)-but-2-ene ZSM-5 sample (%) (%) ("/.I 1 27.8 45.5 26.7 2 26.4 43.9 29.7 4 27.3 44.9 27.8 AAS 11.0 45.5 43.5 equilibrium 7.9 63.4 28.7 mixt urea a From ref.7. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0) CC .-.-.e E 8 .-En c 13000 2000 wavenumber/cm -' Fig. 9 FTIR difference spectra for H-ZSM-5 sample 2, showing the course of reaction as a function of time after exposure to sec-butyl alcohol at 70 "C: (a)0.5,(b)5, (c) 17, (6)21, (e) 43 and (f)130 min butenes obtained on both H-ZSM-5 and AAS is far removed from the equilibrium one.3.2 FTIR Kinetic Studies Kinetic studies of reaction using in situ FTIR methods were carried out at 70 "C.Some typical spectra showing changes in the adsorbed species on increasing exposure time to alcohol, are given in Fig. 9. These are difference spectra, subtracting the spectrum of the purely dehydrated zeolite. Let us consider in more detail the first spectrum recorded 0.5 min after adsorption of alcohol. Stretching vibrations of the hydrocarbon skeleton (C-H vibrations) of alcohol mol- ecules lie in the range 2800-3000 cm-': the peaks at 2970 time/mi n Fig. 10 Kinetics of water formation in sec-butyl alcohol dehydra- tion on H-ZSM-5(sample 2) obtained from FTIR studies at 70°C (0)and on treating the data in first-order coordinates (a).Data are taken from Fig.9 (peak at 1640 cm-'). and 2885 cm-' arise from vibrations of CH, groups and the peak at ca. 2940 cm-' from those of CH, groups.8 Two peaks at 1460 and 1380 cm-' correspond to C-H deforma-tion vibrations: that at 1380 cm-' to CH, vibrations and that at 1460 cm-' to vibrations from both CH, and CH, groups.8 Two broad components at ca. 1500 and CQ. 2450 cm-' are associated with the mode of stretching vibrations of zeolite OH groups hydrogen-bonded to alcohol molecules, split because of Fermi re~onance.~,~ A weak peak at 1640 cm-' corresponds to deformation vibrations of water, which reflects the beginning of dehydration of the adsorbed alcohol molecules even at this early stage of reaction.As time increases, one can see a number of characteristic changes in the spectra, indicating the course of dehydration of the adsorbed alcohol molecules. These are: (1) an increase in the intensity of the peak at 1640 cm-', corresponding to an increase in the number of adsorbed water molecules; (2) the disappearance of the broad peak at ca. 1500 an-' and substitution of the broad peak of ca. 2450 cm-' by a rather narrower peak situated slightly to higher wavenumber, corre- sponding to decomposition of adsorbed alcohol molecules and formation of water molecules which are hydrogen-bonded to the zeolite OH groups; (3) an increase in the inten- sity of the peak at CQ.2940 cm-I and some decrease and broadening of the peak at 1380 cm-' corresponding to an increase in the number of CH, groups in the adsorbed reac- tion products at the expense of CH, groups (indicating product oligomerization as previously observed for tert-butyl alcohol6). For kinetic analysis of dehydration under these conditions, the water deformation peak at 1640 cm-' can be used (see ref. 5 and 6). At long reaction time (130 min) it is clear that alcohol decomposition is complete. First, the intensity of the peak at 1640 cm-' does not grow any further. Secondly, the intensity of this peak has reached a value which is four times greater than the intensity of the peak at 1460 cm-'. This is in agreement with the maximum ratio of 4 : 1 observed by us at the end of the reaction in previous studies involving dehydra- tion of other butanol isomer^.^^.^*^ Fig.10 shows the kinetics of water evolution derived from the change in the relative intensity of this peak. (1164(,)m is the intensity at the end of reaction (reaction time 130 min). On the same plot, the data are also given in first-order coordinates. From these data the rate constant for sec-butyl alcohol dehydration at 70 "C is determined to be 1.1 x lo-, s-'. 4. Discussion All the data concerning the rate constants for sec-butyl alcohol dehydration on H-ZSM-5 and AAS samples, esti- mated by the various methods described in Section 3, are summarised for convenience in the Arrhenius plot in Fig.11. 4.1 Reaction on AAS Let us consider the reason for the difference between initial and steady-state reaction rates on the AAS catalyst [Fig. 1 l(b), (c)]. Earlier, studying the dehydration of n-butyl alcohol and isobutyl alcohol, we considered in detail the mechanism of this reaction, shown in Fig. 12 [which has been analysed carefully in ref. 3(b) and 51. After water elimination, R I there are four ways for the active intermediate Al-0-Si to react : (i) desorption of butene (downwards); (ii) interaction with another butene molecule, i.e. oligomerization on active sites (upwards); (iii) interaction with another alcohol mol- ecule, i.e. ether formation (to the right); and (iv) interaction with a water molecule, i.e. back/reverse reaction (to the left).-11. -131 . . . . . -. . . I 2.2 2.4 2.6 2.8 3.0 3.2 103 KIT Fig. 11 Arrhenius plot for the rate constants of sec-butyl alcohol dehydration obtained by different methods on the different alumino- silicate samples. (a) Data are rate constants for H-ZSM-5 from Fig. 2, obtained by GC under steady-state conditions. (b), (c) Data taken from Fig. 5, where for dehydration as AAS, (b) corresponds to the initial rate of reaction (i.e. W,, ,immediately after adsorption) and (c) corresponds to steady-state rates (Wa).(d)Rate constant of dehydra- tion on AAS obtained from non-steady-state experiments (Fig. 8). (e) Rate constant for H-ZSM-5 estimated from FTIR data of Fig. 9 and 10. The last three pathways can, in principle, give rise to the observed slowing down of reaction with time.However, ether formation is unlikely to be of significance for sec-butyl alcohol dehydration. Di-sec-butyl ether is highly reactive and is easily decomposed on acid sites." Furthermore, no traces of ether are observed among the reaction products. As we noted in Section 3.1.1, the decrease in the reaction rate with time observed in the GC kinetic experiments is unlikely to be connected with catalyst deactivation resulting from butene oligomerization on the active sites. In fact, the decrease in rate most likely reflects the slowing down of the reaction by water. The following facts support this suggestion: (1) The decay of the kinetic curve of butene formation after satura- tion of the sample with alcohol is accompanied by retention of water (Fig.4). (2) After the falling phase in the kinetics of butene evolution, a clear steady-state rate is observed. If dehydration on AAS were complicated by oligomerization, such behaviour would not be expected. (3) The difference between Wsa,and W, increases with decreasing reaction tem- -__------.------.,-flu */-~BUR *-..I ..//// ,, ,, t .... ,.I i +Bu I (VI) RH R\?/R'0' I. ' +ROH OH --40 OR --oli I //// Fig. 12 Reaction scheme for butanol dehydration on alumino-silicates (see ref. 5 for more detail). R = C,H, ,Bu = C,H, . J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 perature, which indicates a greater degree of side reaction at lower temperatures.Oligomerization, on the other hand, would be expected to become more prominent as the tem- perature is increa~ed.~ (4) The observed 'stop effect' (Fig. 8) is not accompanied by deactivation of the active sites of AAS. The next question concerns the nature of this suppression of activity by water. Two explanations are possible. The first possibility is that product water molecules compete with alcohol molecules for the active sites and so bring about a decrease in the number of active sites that are interacting with alcohol molecules, and hence, a decrease in the reaction rate. However, the absence of such a reduction in reaction rate in the case of isobutyl alcohol and n-butyl alcohol dehy- dration at rather similar temperat~res~~~' makes this explana- tion unlikely, since one would expect that the adsorption properties of all three butanol isomers on the acid OH groups of AAS should be similar.The second possibility is connected with reverse reaction. Proceeding from the reac- tion scheme given in Fig. 12, it is seen that the active interme- diate *OR, generated on dehydration of a sec-butyl alcohol molecule, can in turn interact back again with product water molecules present in the sample. This suggestion is in agree- ment with all facts (1)-(4) above. It is clear that the more water molecules present on the sample (in the adsorbed layer), the higher is the rate of back reaction and, hence, the lower is the rate of butene evolution [point (l)].When all rates of the various processes: adsorption, desorption, forward and back transformations, become equal, the reac- tion comes to steady state [point (2)]. On decreasing the reaction temperature, the hydration4ehydration equilibrium shifts to the left towards alcohol, thus further slowing the rate of the forward dehydration reaction [point (3)]. Finally, during purging, the catalyst pores release adsorbed reactants (water in particular) and this reduces the possibility of back reaction and hence the rate of butene formation increases [point (4)]. Based on the above considerations, one can conclude that the points on Fig. 1 l(b) and the point at Fig. ll(d), lying on this line, are the true rate constants for sec-butyl alcohol dehydration on AAS; points lying on Fig.ll(c) are the effec- tive rate constants for dehydration in the presence of back reaction. Thus, E,,, = 18 kcal mol-' is the true activation energy of dehydration of sec-butyl alcohol on AAS (i.e. of the forward step of Section I1 in Fig. 12) while E, = 26 kcal mol-'is an effective value. 4.2 Reaction on Zeolite H-ZSM-5 Comparing the results obtained using two independent methods [the rate constants on Fig. 11 (a) obtained by GC studies and the rate constant plotted as Fig. ll(e) from FTIR studies], their striking difference becomes evident. At 70 "C, the rate constant from spectroscopic data is 400 times greater than the value determined from GC steady-state data. Again, as for the case of AAS, we believe that the low steady-state rate of reaction (GC studies) is connected with the back reaction. Based on the scheme in Fig.12, other explanations are hardly likely. Therefore, the decrease in activity cannot be explained by deactivation of the active sites as a result of butene oligomerization on them, because in that case catalytic activity would then depend on the zeolite crystallite size and the 'stop effect' would not be observed because deactivated sites could not produce more butene during purging. The decrease in the rate of butene formation also cannot be a result of a shift of reaction equilibrium to di-sec-butyl ether formation under steady-state conditions because, just as in the case of AAS, even traces of this ether are not detected among the reaction products (although this J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 can be a consequence of diffusion problems for such bulky molecules). Furthermore, the observed kinetics in the initial phase of reaction (period of alcohol adsorption) are com- pletely different to those observed previously for isobutyl alcohol and n-butyl alcoh01~~~~ where ethers are formed. As was mentioned in Section 3, in that case, water evolves ahead of butene, which reflects ether formation inside the zeolite pores. In the case of sec-butyl alcohol, this was not observed: the rates of formation of both products were equal. Analysis of the isomeric distribution of product butenes shows that sec-butyl alcohol is expected to be the only product of the back reaction, since it is known that in liquid- phase catalysis, both but-2-ene and but-l-ene give only sec- butyl alcohol on hydration.' In the FTIR experiments, back reaction does not occur and dehydration proceeds irreversibly.The reaction condi- tions in this case are different from those in the flow reactor and the reaction products are oligomers of butene adsorbed on the active sites (Section 3) which probably cannot be hydrated under these conditions. This means that from FTIR kinetic studies, we determined the true rate constant for sec- butyl alcohol dehydration, while the rate constants and the activation energy from GC kinetic studies are effective values. The idea of back reaction in the sec-butyl alcohol/H-ZSM- 5 system makes it possible to explain the whole complex of observed phenomena (Section 3): (1) the low rate constants under steady-state flow conditions; (2) the high value of E,; (3)the 'stop effect' and its magnitude.Comparison of the results for H-ZSM-5 and AAS shows that the true rate constants for both catalysts [Fig. ll(e) and the point from Fig. ll(b) at 70"C] are close, the difference being a factor of two or so, i.e. the same ratio that was observed by us for isobutyl and n-butyl alcohol dehydration earlier.3'.4 However, in the case of the zeolite system, the sup- pression of dehydration is stronger than for the AAS sample. This most likely arises from the specific adsorption properties of the zeolite pores.The higher the water concentration in the vicinity of the active intermediate, i.e. in the catalyst pores, the higher is the expected rate of back reaction. During the flow reaction at 126 "C, the zeolite micropores are completely filled with reactant molecules, while the macropores of the AAS, on the other hand, are virtually empty (see results for n-butyl alcohol dehydration3'). This, in its turn, is a result of the adsorption properties of the catalyst and hence of the type of pore structure. In the case of a flow reaction on H-ZSM-5 we do not see the initial drop in catalytic activity in contrast to AAS, but rather observe a low steady-state rate from the very begin- ning. This can be explained as follows. The time necessary for a system to reach a steady state is inversely proportional to the sum of the rate constants of the forward and back reac- tions.Therefore, having a higher rate of back reaction (as in the case of H-ZSM-5), the system reaches a steady state more quickly than in the case of AAS, and the initial non-steady- state region of the kinetics can occur during the period of sample saturation with alcohol. In conclusion, note that for the two aluminosilicate samples investigated, namely the AAS and the zeolite, the former gives rise to 'normal' heterogeneous solid-gas cataly- sis. In this case, the dehydration of sec-butyl alcohol under flow conditions proceeds almost irreversibly (or more exactly, with little reversibility). Just after the stage of water elimi- nation the active intermediate formed decomposes along pathway V (Fig.12) with high probability, releasing butene into the gas phase. Reaction on the zeolite catalyst in the temperature range studied is more like the reaction taking place in the liquid phase with a large contribution from the back reaction. In this case, the rate of reaction is lower than 21 53 expected because it is not possible to disperse or separate the reaction products and so they undergo back transformation. When the temperature is increased, the extent of back reac- tion on the zeolite is reduced, presumably owing to a release of adsorbed reactants from the pores. Indeed, extrapolation of Fig. ll(a) and (b)to higher temperatures suggests that at ca. 160°C the rate constant for H-ZSM-5 under flow condi- tions reaches a value twice that for the rate constant of AAS, which is the normal ratio typical for these catalysts (according to the results obtained for the other butyl alcohols and the ratio of the rate constants obtained by different methods for sec-butyl alcohol at 70 "C).5. Conclusions Comparison of sec-butyl alcohol dehydration in zeolite and amorphous aluminosilicate samples has shown the particular influence of pore confinement on reaction. Confinement of reactant molecules to the H-ZSM-5 micropores (in the tem- perature range 60-140 "C) results in an enhancement of the back reaction, namely hydration of the product butene with product water. At these low temperatures the catalyst pores are filled with reactants, in particular water, and this increases the probability of back reaction : all transformations proceed in a pseudo-liquid phase.Kinetically, the influence of pore confinement on this system manifests itself as: (i) anomalously low values of steady-state reaction rates under flow conditions ; (ii) an anomalously high value of the activation energy obtained from steady-state rate constants under flow conditions; (iii) an increase in the reaction rate after interrupting the alcohol- helium feed gas (the 'stop effect'). In the case of the amorphous aluminosilicate sample with larger pores (ca. 50 A diameter) and hence a more open surface, all of the kinetic effects mentioned above are also observed, but to a much lesser degree.The authors thank Dr. E. A. Paukshtis for assistance with the FTIR experiments, Dr. S. V. Dudarev for sample 1 and V. N. Romannikov for providing samples 3,4 and 5. We thank The Royal Society, USSR Academy of Sciences and the SERC for financial support. References 1 (a)N. Y. Chen and W. E. Garwood, Catal. Rev.-Sci. Eng., 1986, 28, 185; (b)M. E. Davis, Ind. Eng. Chem. Rex, 1991,30, 1675. 2 J. Weitkamp, in Catalysis and Adsorption by Zeolites, ed. G. Olhmann, J. C. Vedrine and P. A. Jacobs, Elsevier, Amsterdam, 1991, p. 21. 3 (a) M. A. Makarova, C. Williams, J. M. Thomas and K. I. Zamaraev, Catal. Lett., 1990, 4, 261; (b) M. A. Makarova, E. A. Paukshtis, C. Williams, J. M. Thomas and K. I. Zamaraev, J. Catal., in the press. 4 M. A. Makarova, C. Williams, V. N. Romannikov, K. I. Zamaraev and J. M. Thomas, J. Chem. SOC., Faraday Trans., 1990,86,581. 5 C. Williams, M. A. Makarova, L. V. Malysheva, E. A. Paukshtis, J. M. Thomas and K. I. Zamaraev, J. Chem. SOC., Faraday Trans., 1990,86,3473. 6 C. Williams, M. A. Makarova, L. V. Malysheva, E. A. Paukshtis, E. P. Talsi, J. M. Thomas and K. I. Zamaraev, J. Catal., 1991, 127, 377. 7 Yu. M. Zhorov, lsomerization of OIefins, Khimia, Moscow, 1977. 8 L. M. Sverdlov, M. A. Kovner and E. P. Krainer, Vibration Spectra of Polyatomic Molecules, Nauka, Moscow, 1970. 9 N. D. Sokolov, The Hydrogen Bond, Nauka, Moscow, 1981. 10 The Chemistry of the Ether Linkage, ed. S. Patai, Wiley- Interscience, New York, 1967. 11 I. Dostrovsky and F. S. Klein, J. Chem. SOC.,1955,440. 12 H. Knozinger, in The Chemistry of the Hydroxyl Group, ed. S. Patai, Wiley-Interscience, London, 1971, pp. 642-718. Paper 4/00095A; Received 7th January, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002147

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2155-2157 Electrode Kinetics for Chemists, Chemical Engineers and Materials Scientists. By E. Gileadi. VCH Wein- heim, 1993. Pp. xviii + 597. Price DM 189, f65 (hardcover). ISBN 3-527-89561-2. Gileadi writes with enthusiasm and in a very clear and read- able style which makes this book a pleasure to read. The book concentrates on the explanation of the phenomena rather than providing extensive derivations of the various models and equations and, as a result, the mathematical content is kept to a reasonable minimum. The main emphasis of the text is on the electrode surface and on reactions at the electrode surface. Thus there are extended discussions of the adsorption of species at electrode surfaces and the mecha- nisms and kinetics of inner sphere electrode reactions, such as the hydrogen evolution reaction.His treatment of these topics is sound. Where the text is very weak is in the dis- cussion of outer sphere electron transfer and of coupled homogeneous kinetics, a topic which is not mentioned at all. Also omitted are the whole collection of electroanalytical applications and techniques. The text is broadly divided between general considerations and fundamental concepts, techniques and applications. The techniques are focussed on those applicable to the measure- ment of the kinetics of electrode reactions, particularly those involving adsorbed intermediates. Thus fast transient mea-surements, coulostatic pulse techniques, sweep voltammetry and electrochemical impedance are all discussed.The applica- tions are directed towards the areas of batteries, fuel cells, electroplating and corrosion. These are discussed with authority and enthusiasm, and again the emphasis is on the role of adsorbed intermediates present on the electrode surface. In summary this is an attractive book and one which could profitably be read by those interested in the field of electro- catalysis and elect rochemis try postgraduate students. The treatment should be accessible to those with a background in materials science and in chemistry. The book has much less to offer as a first text in modern electrochemistry because of its narrow view of the subject, or for those interested in elec- troanalytical applications or in electrochemical reactions coup,led with homogeneous chemical processes.P. N.Bartlett Received 1st February, 1994 Advances in Photochemistry. Volume 18. Ed. by D. H. Volman, G. S.Hammond and D. C. Neckers. Wiley- Interscience, New York, 1993. Pp. ix + 406. Price f103.00.ISBN 0-471-59133-5. This latest volume in the Advances in Photochemistry series comprises four independent contributions on time-resolved FTIR emission spectroscopy, the effects of molecular entrap- ment on photochemical reactions, scale-up of photochemical reactions and the photochemistry of xanthene dyes. The con- tributions are of good to excellent quality with applications for both academic and industrial laboratories. Individuals will find only certain parts of interest, but overall the volume should appeal to a wide audience. The major section by far is the 167 page contribution from V.Ramamurthy, R. G. Weiss and G. S. Hammond entitled ‘A model for the influence of organised media on photo- chemical reactions ’. They collate a large amount of informa- tion on the effects of organised environments on trapped molecules, considering host materials ranging from individual organic molecules such as the cyclodextrins through micelles, LB films and liquid crystals to inorganic materials (silica, clays and zeolites). The effect of the host structure is con- sidered systematically in terms of the nature of reaction cavi- ties and the consequences of enclosure on reactions (mostly photoinduced) of trapped species.The authors then focus on Norrish I1 reactions of ketones, first discussing the processes in general then considering the effect upon them of inclusion of the ketone in various types of constraining host material. The latter half of this chapter will be of interest mostly to photochemists, while the first half is also a good general introduction to the nature and effects of constraining media on chemical reactivity. The other three sections are all of roughly half the length of the above. G. Hancock and D. E. Heard have written an interesting chapter on time-resolved emission studies using FTIR instru- mentation. This section should appeal to photochemists who are willing to construct their own instrumentation, or to hack commercial spectrometers.After a description of the basics of FTIR measurements Hancock and Heard consider and compare stop-scan and continuous scan modes of data col- lection, then review applications of time-resolved IR emission measurements. Photodissociation is considered first, followed by pulse laser-induced bimolecular reactions and finally kinetic studies of vibrational state changes during photo- chemical reactions. Industrial chemists may find useful the chapter on upscal- ing of photochemical reactions by A. M. Braun, L. Jakob, E. Oliveros and C. A. 0.do Nasciemento. This is a clear, well written guide to reactor design, scale-up of processes and modelling of reactor behaviour. Reactor designs, reaction media and safety requirements are introduced, followed by consideration of available light sources and the implications of the source properties on reactor design.Qualitative scale- up parameters are mentioned, followed by a discussion on mathematical modelling of reactor behaviour. TiO, photo- catalysed oxidative treatment of wastewaters is used as an example in the analysis of reactor performance. Finally, D. C. Neckers and 0.M. Valdes-Aguilera provide a chapter which is ‘straight’ photochemistry, considering the photochemical reactions of some of the many xanthene dyes. Fluorescein, Eosin, Rose Bengal and a new series of hydroxy- fluorones are discussed, with a brief history of each followed by a survey of applications. The use of Eosin as a photoinitia-tor in polymerisation of acrylates is discussed in some detail, and the properties and photoreactions of Rose Bengal receive considerable attention.The wide range of applications for the xanthenes (from polymer chemistry to insect control), mostly relying on photoinduced redox reactions, makes this an inter- esting and at times amusing read. The volume is a useful addition to the series and is well produced. D. Crowther Received 1st February, 1994 Scanning Tunnelling Microscopy and Spectroscopy : Theory, Techniques and Applications. Ed. by D. A. Bonnell. VCH, New York, 1993.Pp. xiv + 436. Price DM 196.00, f80.00. ISBN 3-527-27920-2. Given the spectacular advances in the development of the STM and related methods, there is no doubt a market for a book on this topic.The approach here has been to assemble a multi-author text, with chapters grouped under the head- ings of ‘Fundamentals.. .’, ‘Structure of sample and tip surfaces’ and ‘Related techniques and applications’. The strength of the book is the coverage of issues other than the high-profile application of topological imaging: the dangers of interpreting ‘images’ as topography are frequently flagged and well demonstrated; the detailed chapter on tun- nelling spectroscopy was particularly well done; inclusion of a chapter on surface thermodynamics and structure, includ- ing ‘traditional’ approaches to their study, gave perspective; there was detailed discussion of the importance of the elec-tronic, as well as morphological, structure of the tip.Sur- prisingly, only one author discussed the question of whether the small sampled areas are representative: perhaps the prac- titioners are better judges of this. As an interested outsider, I often question the nature of the relationship between the raw data and the exquisite pro- cessed images commonly presented. Chapter 7 includes figures making this comparison directly. This is typical of the commendable approach taken by the authors in confronting the difficult issues. As one interested in applications of the STM, I naturally found that section most interesting. The tabulation of electrochemically-based systems studied to date was very good: comprehensive but not obtrusive.The chapter on biological applications has an excellent exposition of the forces involved, with particular reference to the imaging of large, soft (biological) molecules. In presentational terms, a book on the STM has the benefit of a wealth of spectacular illustrative material. In terms of the text, there was an annoying number of unnecessary minor typographical errors. The dramatic rate of progress that creates the need for such a book also limits its life. With the exception of the last two chapters, the references run up to 1990. This must limit the value of the book for those working at the forefront of this area. Nevertheless, this is a useful book for those con- sidering entering the area and provides a very good snapshot of the field at the end of 1990.A. R. Hillman Received 18th February, 1994 ~ ~ Surfactant Science Series Volume 50. Fluorinated Surfactants. Synthesis, Properties and Applications. Ed. by Erik Kissa. Marcel Dekker, New York, 1993. Pp. vii + 469. Price $165.00. ISBN 0-8247-901 1-1. The author writes at the beginning of his preface: ‘fluorinated surfactants are truly the super surfactants’. Thus, the pub- lication of Vol. 50 of the Surfactant Science Series dedicated to the synthesis, properties and applications of fluorinated surfactants (FS) is fully justified by the widespread evolution of this subject over the last years. The work could be read and re-read with new information on each occasion because it is a coherent and stimulating handbook in this field of aca- demic and industrial research.For people in the field of colloid chemistry it is undoubtedly very important and is probably the most useful book to have appeared in the FS field to date. After a brief introductory chapter on the title compounds, grouped according on their different charge and structure and summarised in detailed tables, nine additional chapters follow with a comprehensive and exhaustive review of the properties of FS, of their analysis and use, including environ- mental and toxicological aspects. Each chapter is authori- tative and, usually, well organised with updated citations (over 2000 in total; many referenced papers and patents have been published in 1992). It contains concise explanations of the treated chemico-physical concepts and quantities so that J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the author’s intention to write a stand-alone book is widely satisfied. The second chapter, on chemical synthesis, is very detailed as most of the preparation procedures are reported, although yield data are only seldom given. A list of the commercial products together with their manufacturers could be of help to the reader, as well as a list of the abbreviations used for compounds quoted in the whole text. The physical and chemical properties of FS are treated in Chapter 3 with many figures and tables. Four further chap- ters follow on typical surface and aggregation properties. These are the core of the book, where the surface tension and the adsorption at the liquid/vapour, liquidfliquid, and liquid/ solid boundaries are dealt with in terms of the current theo- ries (Chapters 4 and 5).The behaviour of FS in solution and their self-assembled phases is presented in Chapters 6 and 7. The presentation of the theories tends to be rather long and there is some overlap among these chapters. The many tables (with the complement of figures) on surface tension data in both water and organic liquids, on solubilities, Krafft points and critical micelle concentrations, and on the effect of pressure, temperature and additives, are an indispensable source for each worker engaged in this field. In particular I wish to mention here the reference the author makes (in Chapter 5) to the preparation and characterisation of mono- and multi-layer films of FS, for their novelty and foreseeable applications in many fields.This is strictly related to the structure of micelles and mesophases discussed in Chapter 7 on the basis of proposed models for the micellization process in pure and mixed surfactants and of the results obtained with many experimental techniques, included scattering and magnetic resonance spectroscopies. It is a pity that the space dedicated to the literature on both mesophases and micro- emulsions and to the use of physical techniques for structural and dynamical characterisation is not so wide as it would deserve, and that the very recent applications of EPR spec- troscopy in micellar and lamellar FS systems did not find any place in this chapter.The applications and the analysis of FS are reviewed in Chapters 8 and 9 with particular emphasis on their applied and industrial aspects. The alphabetical list of applications starts with Adhesives and Antiflogging and ends with Wetting agents, through Coatings, Cosmetics, Emulsions, Fire-fightings, Metal finishing, Photography, Repellancy, etc. The enormous number of referenced patents in this chapter gives clear evidence of the industrial interest. The analytical methods for FS characterisation are con- sidered, in perhaps too much detail, in Chapter 9. This chapter does not seem to be in the proper location. Most of the quoted techniques are indeed presented here and they would perhaps have been more fruitful to have presented these in the previous chapters.The inclusion of tables with comparisons of data from different sources could be very sig- nificant. The final chapter deals with toxicology and environmental aspects of FS and this is the best conclusion for such an exhaustive handbook. The toxicological and physiological properties of FS are fully considered. The author proves to be an expert on this topic in particular. An enthusiastic approach is given to the use of fluorinated compounds and emulsions as oxygen carriers in blood substitutes and to their bio-compatibility, with a clear and convincing indication of the future perspective. Although minor points and a few typographical errors remained, I must recommend this book, which is clearly written throughout with an enormous number of diagrams and tables that are generally well chosen and clear.Dr. Kissa gives in his work a complete account of his personal interest J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 in this field without being biased towards his own particular interests; this is a positive sign as it occurs rarely in the liter- ature. G. Martini Received 4th March, 1994 Advances in Chemical Physics. Volume LXXXIII. Ed. by 1. Prigogine and S.A. Rice. Wiley, New York, 1993. PP. ix + 744. Price f136. ISBN 0-471-54018-8. This series has a well deserved reputation for the high level of expertise of its contributors, and the present volume main- tains that reputation.There are seven articles of varying lengths, each covering a fairly narrow area, but in consider- able detail. The topics chosen are spread across the field of chemical physics and do not appear to have any particular connection with each other. The article by Gina et a/. summarises their own recent theoretical work on geometric Berry phases. A model system is considered consisting of a ring of nuclei which can undergo Pseudo-rotation, represented by an elliptical distortion, the axes of which rotate. There is also a Jahn-Teller coupling of this distortion to the electronic quantum states of the mol- ecule. They then propose an experiment in which the mol- ecule is subjected to a pair of pulses separated by a time-delay.The initial pulse causes an excitation to an upper Jahn-Teller state, the phase of the electronic component of the wave function of this state then developing in time according to an expression which includes the so-called geo- metric contribution. Interference between this ‘developed’ excited wave function and the delayed pulse will then give information on the rate of geometric phase (Berry phase) development. Numerical evaluations of various possible out- comes to the experiment are discussed. Lee and Albrecht deal with the conservation of global energy in non-linear radiation/matter processes. In particular, they discuss the apparent breakdown of energy conservation in some so-called ‘passive’ processes (of which the most important spectroscopically are CARS and CSRS).No problem arises for ‘active’ (or direct) processes or for those ‘passive’ (or indirect) processes not involving resonance between field and material: conservation of energy is found, as has been previously realised. However, for resonant passive’ processes, previously thought to involve constancy of field energy but changing energy of the material, the present treatment exposes a coupling between radiation and matter by means of an in-phase component of the induced electrical polarisation. When this is taken into account the paradox is resolved and energy conservation survives. The arguments are carefully formulated, mainly in terms of Per-turbation theory. The chapter by Polimeno and Freed on ‘A many-body sto- chastic approach to rotational motions in liquids’ starts with a very informative introduction to the field, but particularly to Langevin and Fokker-Planck type treatments.The authors take as a model a solute molecule in a cage made UP of its nearest neighbour solvent molecules, which is in turn subjected to the influence of a fluctuating ‘field’ arising from the remaining molecules. The presence of the two kinds of neighbours leads to a biexponential decay of correlations. Results are presented for four different realisations of this approach and these are compared with the results of molecu- lar dynamics simulations (which can be regarded as exact evaluations for the molecular parameter values introduced). Agreement with molecular dynamics is claimed for various orientational correlation functions up to the maximum times for which molecular dynamics is feasible.The Fokker-Planck methods have the advantage of retaining their usefulness upto very much longer times. An interesting discussion of some of the new high-temperature superconductors by Burdett follows. This article concentrates on the crystal structures of Some of the families of cuprates which show superconductivity in Some of their members. In particular the way in which the ligand environ- ments of the CU ion varies with the composition of the solid, and how that, in turn, affects the energy levels of the d electrons and ultimately their localisation or delocalisation is discussed in detail. In this way a rationalisation of the switch from insUlatOr to metal is accounted for with a good deal of Success.The author does not delve into attempts to predict whether and when superconductivity might be found. The nearest he goes to this is some exploration of the connection between the CU-0 distance and electron localisation, cating that strong electron-phonon coupling would in Some cases be expected. COffeY et a/. give a detailed account of the relaxation of sinde-domain ferromagnetic particles, the latter being suffi-ciently small that the stable state of each is for a single domain to occupy the whole of the particle. The radius below which single domain magnetic particles are stable is of the order of 100 A. TWOtypes of magnetic relaxation are recog- nised: one which arises from motion of the whole particle (Debye relaxation), and the other which is caused by move- ment of the magnetic dipole with respect to the particle (Nee1 relaxation).The article largely ignores the case where the two processes are of comparable importance. The Debye- and Neel-dominated cases are, however, discussed in detail in terms of both Langevin and Fokker-Planck equations. The article by Hurtubise and Freed examines the nature and properties of effective Hamiltonian and other effective operators. In general, effective operators fail to conserve com- mutation relations between arbitrarily chosen pairs of oper- ators, although some definitions preserve the commutation rules between the Hamiltonian operator and an arbitrary operator and/or that between an arbitrary operator and any constant of motion.Other advantages and disadvantages of various choices of effective operator are also discussed. The volume is completed by a discussion of melting and liquid structure in two dimensions by Glaser and Clark. There is a good introduction, which dwells particularly on the theory of melting first proposed by Kosterlitz and Thou- less and subsequently developed by others. This is followed by much shorter discussions of several other theories, such as defect-mediated and grain boundary theories. The main thrust is contained in the following sections: those on com- puter simulation studies of melting, with a careful analysis of the onset of positional and bond-orientational disorder. In parallel with this is a section on studies on random Packing of hard discs. Insight obtained from the simulations is then used to develop a tiling model for melting, use being made of the dense random packing results. The last three sections draw heavily on work by the authors themselves. Readers may find the conclusions drawn to be heretical, but they are certainly interesting. The authors favour geometric (taking account of varying bond lengths), as distinct from toPologi- cal, defects as the main driving force for melting. In Particu- lar, the condensation of these geometric defects is regarded as the Source of the cooperative aspect Of melting. Some measure of topological constraint seems to be necessary in order to avoid the extremes of loss of the transition, on the one hand, or loss of all long-range interactions between the defects, on the other. Finally, the authors Put forward the provocative general conclusion that there is no real qualit- ative difference between two- and three-dimensional melting. Receioed 1st March, 1994N.G. Parsonage

ISSN:0956-5000    

DOI:10.1039/FT9949002155

出版商:RSC    

年代:1994

数据来源: RSC