J. Chem. SOC., FaradajJ Trans. 1, 1989, 85( 1 I), 3663-3673 Structure of Vanadium Oxides on ZrO, and the Oxidation of Butene Hisashi Miyata,* Mitsuru Kohno and Takehiko Ono* Department of Applied Chemistry, University of Osaka Prefecture, Sakai, Osaka 591, Japan Takashi Ohno and Fumikazu Hatayama School of Allied Medical Sciences, Kobe University, Suma, Kobe 654-01, Japan The oxidation of butene on V-Zr oxides prepared by the gas-phase and the impregnation method has been studied by F.t.i.r. spectroscopy as well as the microcatalytic method. V-Zr catalysts above 6.0 wt % vanadium loading show almost constant activity. V-Zr catalysts at low vanadium loadings show high selectivities to furan and buta- 1,3-diene, while high vanadium loading catalysts show high selectivities to acetaldehyde and acetic acid.The formation of a dihydrofuran-like intermediate before the formation of furan has been proposed. The oxyhydrative scission mechanism is proposed in the formation of acetaldehyde and acetic acid. This formation is attributed to the presence of Brnrnsted acidity. Vanadium oxide catalysts in combination with various promoters are widely used for selective oxidation of hydrocarbons. We have recently reported the characterization of various vanadium/metal oxides prepared by a gas-phase and have proposed that new species are formed by the reaction of vanadium oxytrichloride with surface OH groups on carrier oxides. Materials of this class are of growing interest in a wide range of practical applications. In previous papers,"*5 we have also reported the catalyst structures and catalytic activities of such oxide catalysts. Few studies for the selective oxidation of alkene on V-Zr catalyst have appeared in the literature.In the present work. by the selective immobilization of V,O, mono- and multi-layers on ZrO,, the influence of the support on the oxidation selectivity of alkene and intermediates in the oxidation has been investigated using Fourier- transform infrared spectroscopy as well as the microcatalytic method. Experimental The zirconium hydroxide was prepared from zirconium oxydichloride. The hydroxide was calcined at 383 K, followed by decomposition at 723 K. Two preparation methods were used, The first was a wet impregnation method with ammonium metavanadate (VZr-2.0-VZr-24, 2.0-24 wt % vanadium oxide as V,O,).The second was a gas-phase preparation method using vanadium oxytrichloride. In order to obtain samples with different vanadium layers, the circulation of vanadium oxytrichloride vapour was repeated several times. The vanadium contents ranged from 1.8 to 6.3 wt % vanadium oxide as V,05 (GVZr- 1.8-GVZr-6.3). The concentration of the surface OH groups on the zirconia, which controls the formation of the vanadium oxide surface phase, was fixed by appropriate choice of the pretreatment temperature of the zirconia. Details of the prepraration methods of catalysts were described previously.' The catalyst was heated to 723 K under evacuation and kept at that temperature under flow of oxygen (ca. 4 kPa) for 2 h. This treatment was repeated several times before each experiment.The physical parameters of those catalysts are listed in table 1 . 36633664 Structure and Butene Oxidation on V,O,/ZrO, Table 1. Physical properties of GVZr and VZr catalysts preparation catalyst methoda V,O, wt O h surface area/mz g-I VZr-2.0 VZr- 3.7 VZr-6.0 VZr-7.6 VZr-8.4 VZr- 15.6 VZr-24.0 GVZr- 1.8 GVZr-2.0 GVZr-6.3 2.0 3.7 6.0 7.6 8.4 15.6 24.0 1.8 2.0 6.3 67 68 60 44 53 49 39 62 61 53 a Gas = gas-phase preparation; wet = wet impregnation method. cycles of VOC1, circulation. Parentheses show number of Two types of apparatus were used. The first was a conventional closed-circulation system equipped with an i.r. cell in the circulation loop. The second was a pulse microreactor system directly connected with a gas chromatograph for analysis of the reaction products.Details of the apparatus and procedures were described in previous Fourier-transform infrared spectra were recorded on a Shimadzu FTIR-4000. After 100 accumulations had been stored the spectral data were transferred on an IEEE-488 bus line to a master computer (PC-9801 VX2, NEC). In order to obtain quantitative information on the spectral behaviour, some of the spectra were subtracted and deconvoluted by using a data acquisition system. The details of the data acquisition and analysis system were described previously. ' 7 ' 9 For a pulse microreactor study, the dried catalysts were tested in a fixed-bed reactor. The catalyst charge of 30 mg was preheated with flowing 0, for 1 h at 673 K, followed by flowing He for 1 h at reaction temperature (373-573 K).Following pretreatment, a gaseous mixture of Z-but-2-ene, oxygen, and helium (0.04,0.08, 1.7 mmol) was fed over the catalyst. Gaseous reaction products were measured by gas chromatography with a column of MS- 13X or GCPAC-54. Conversion and product selectivity were calculated as follows. Conversion (%) = 100 x (mole of butene reacted)/(mole of butene fed) Selectivity (YO) = 100 x (mole of product reduced to C,)/(mole of butene reacted). The characterization of the catalyst structures was carried out using XRD, F.t.i.r. and laser Raman techniques.'v Results and Discussion Microcatalyic Results Catalysts were tested for Z-but-2-ene oxidation in the temperature range 373-573 K. A fresh sample was used for each reaction temperature.As shown in fig. 1, the conversion of Z-but-2-ene is increased with increasing vanadium loadings and reached saturated value at vanadium loadings above ca. 6.0 wt %, suggesting that the active sites of catalysts are almost constant above 6.0 wt YO. Reaction products varied with the reaction temperatures and with vanadium loadings of catalysts. A trace of formic acid and maleic anhydride was formed on the catalysts at high vanadium loadings. Fig. 2 shows the selectivities of butene oxidation at various temperatures. The catalysts at lowH. Miyata et al. 3665 Fig. 1. Total v*o, (wt%) 473, (c) 573 K. Fig. 2. Products selectivity of 2-but-2-ene oxidation on V-Zr catalysts. (a) Furan, (b) buta-1,3- diene, (c) CH,CHO, ( d ) CH,COOH, CO+CO, (dotted lines); 0, 573 K; A, 473 K; a, 373 K.3666 Structure and Butene Oxidation on V,05/Zr0, 0.1 rl % 1'7 - 15 ' * 13 ' * 1 1 ' wavenumber/ lo2 cm- ' Fig.3. F.t.i.r. spectra of Z-but-2-ene adsorbed on GVZr-6.3. (a) Background, (b) after introduction of 2-but-2-ene (ca. 1 . 1 kPa for 30 min at 293 K) followed by 30 min evacuation at 293 K, (c) followed by 30 rnin at 353 K in oxygen, ( d ) 30 rnin at 413 K in oxygen, (e) 30 min at 473 K in oxygen. The spectra of (b)-(e) below 1800 cm-' shown after subtraction of background. vanadium loadings show high selectivity to furan and buta- 1,3-diene, while the catalysts at vanadium loadings above 6.0 wt YO show high selectivity to acetaldehyde and acetic acid. At high temperature the catalysts above 6.0 wt % show moderate selectivity to furan.Above 473 K total oxidation, as CO and CO, formation, occurred. The results showed that the nature of surface vanadia species on the catalysts at low vanadium loadings are different from those of high vanadium loading catalysts. It should be noted that pure zirconia showed little or no activity for the oxidation of Z-but-2-ene at these temperatures. F.t.i.r. Spectra of cis-But-2-ene on GVZr-6.3 and GVZr-2.0 As described above, V-Zr catalysts at vanadium loadings above 6.0 wt YO show almost the same activities as, but different selectivities from, low vanadium loading catalysts. Therefore, we focused on the catalysts with mono- and multi-layered vanadia (GVZr- 2.0 and GVZr-6.3) at vanadium loading below ca. 6 wt YO. Fig. 3(b) shows the spectrum of 2-but-2-ene adsorbed on GVZr-6.3 at room temperature.The spectrum exhibits the OH band around 3665 cm-l, the CH stretching band near 3000 cm-' and bands in the bending region. Sharp absorption bands at 1463 and 1385 cm-l are due to the bending modes of CH, group of adsorbed butene. The broad band at 1645 cm-' is assigned to v(C=C) of n-bonded b ~ t e n e , ~ suggesting that Z-but-2-ene is adsorbed as a n-complex on GVZr-6.3 at room temperature. Oxygen of 2.5 kPa was admitted to the GVZr-6.3 containing Z-but-2-ene, the temperature of the catalyst being raised in stages under circulation of oxygen [fig. 3(c)-(e)]. With increasing disc temperature the intensities of the bands due to the n-complex decreased ; simultaneously, new bands appeared at 1565, 1450, 141 5 and 1355 cm-l and were intensified.Surface acetate species have been found around 1550 and 1440 cm-l in the cases of various metal oxide^,^^^^-'^ which have been attributable to the asymmetric and symmetric stretching vibrations of acetate ions. Thus, the bands at 1565 and 1415 cm-' are assigned to va,(COO) and v,(COO) of surface acetate species, respectively. Considering that the asymmetric v(C00) may be overlapped, the bands at 1565 and 1355 cm-l are attributable to same modes of surface formate species.',H . Miyata et al. 3667 I , I . I . I 17 15 13 11 wavenumber/ 1 O2 cm- ' Fig. 4. F.t.i.r. spectra of Z-but-2-ene adsorbed on GVZr-1.8. (a) Background, (b) after introduction of Z-but-2-ene (ca. 1.1 kPa for 30 min at 293 K) followed by 30 min evacuation at 293 K, (c) followed by 30 min at 353 K in oxygen, ( d ) 30 min at 413 K in oxygen, (e) 30 rnin at 473 K in oxygen, (f) after introduction of small amount of dihydrofuran followed by oxidation at 353 K for 30 min, (g) followed by 30 rnin at 413 K in oxygen.The spectra of (b)-(g) below 1800 cm-' shown after subtraction of background. I k;; -' - 35 30 ' 1 I 1 1 17 15 13 11 I k;: - 35 30 ' wavenumber/ 1 O2 cm- ' Fig. 5. F.t.i.r. spectra of Z-but-2-ene adsorbed on ZrO,. (a) Background, (b) after introduction of Z-but-2-ene (ca. 1.1 kPa for 30 rnin at 293 K) followed by 30 rnin evacuation at 293 K, (c) followed by 30 rnin at 353 K in oxygen, ( d ) 30 rnin at 413 K in oxygen, (e) 30 rnin at 473 K in oxygen. The spectra of (6)-(e) below 1800 cm-' shown after subtraction of background.3668 Structure and Butene Oxidation on V,O,/ZrO, d / / 5 0 5 1( 00 i0 YO5 (wt%> Fig.6. Products selectivity of butan-2-one oxidation on GVZr-6.3 and on GVZr-2.0. (a) CH,CHO, (b) CH,COOH; 0, 573 K; A, 473 K; 0 , 373 K. 0.1 I I ,1 1 I 1 I , 1 , 35 30 17 15 13 11 wavenumber/102 cm-' Fig. 7. F.t.i.r. spectra of butan-2-01 adsorbed on GVZr-6.3. (a) Background, (b) after introduction of butan-2-01 (ca. 1.1 kPa for 30 min at 293 K) followed by 30 min evacuation at 293 K, (c) after 14 h of adsorption at 298 K, ( d ) followed by 30 min at 353 K in oxygen, (e) 30 min at 413 K in oxygen, (f) 30 min at 473 K in oxygen. The spectra of (b)-(f) below 1800 cm-' shown after subtraction of background. Similar experiments were carried out with GVZr-1.8 (fig.4). Only weak bands appeared at almost the same positions as those in the case of GVZr-6.3 at room temperature [fig. 4(b)], suggesting that the surface vanadia provides active sites for 2-but-2-ene adsorption. On oxidation with increasing temperature the spectral behaviour was observed. At 353 K new bands, which were not observed with GVZr-6.3,H. Miyata et al. 3669 1 "-R: 17 15 13 11 35 30 / wavenumber/lO? cm-' Fig. 8. F.t.i.r. spectra of butan-2-one adsorbed on GVZr-6.3. (a) Background, (b) after introduction of butan-2-one (ca. 1.1 kPa for 30 min at 293 K) followed by 30 min evacuation at 293 K, (c) after 15 h of adsorption at 298 K, (d) followed by 30 min at 353 K in oxygen, (e) 30 min at 413 K in oxygen. The spectra of (b)+e) below 1800 cm-' shown after subtraction of background. appeared at 1666, 1610, 1392, and 1356 cm-l [fig.4(c)]. As reported previ~usly,~ at low temperature the adsorbed 2-but-2-ene molecule on V,O,/TiO, incorporates 0.5 oxygen molecules. Thus, the formation of oxygen-containing intermediates is suggested in a similar reaction. Above 413 K, bands at 1561, 1446, 1383, and 1363 cm-' are intensified [fig. 4 ( 4 , (e)], suggesting that a similar reaction with GVZr-6.3 took place. In order to clarify new intermediate species formed above 353 K on GVZr-1.8, the spectra of oxygen-containing compounds adsorbed on catalysts and their spectral behaviour were studied. Fig. 4(f) shows the spectrum of 2,3-dihydrofuran adsorbed on GVZr-1.8, at 353 K. Bands were observed at 1662, 1615, 1453, 1390, and 1360 cm-l.Only this compound was found to give essentially the same spectra and spectral behaviour as the bands of Z-but-2-ene. Therefore the species formed at ca. 353 K are tentatively assigned to a dihydrofuran-like structure. Similar experiments were carried out with ZrO, alone. The resulting spectra are shown in fig. 5. Above 413 K the acetate and formate bands are observed at 1548 and 1447 cm-l, and 1570 and 1361 cm-l, respectively. These bands are much weaker than those with GVZr-6.3 and GVZr-1.8. In the oxyhydrative scission mechanism'? l4 of butene, the formation of enole-type intermediates is pr~posed.~ Thus, the adsorption of butan-2-01 and butan-2-one, and their oxidation over GVZr catalysts were investigated. Fig. 6 shows the results with the oxidation of butan-2-one over the GVZr-6.3 and GVZr- 1.8.The products were almost the same as those on butene oxidation. The GVZr-6.3 favours the formation of acetaldehyde and acetic acid. The spectrum of butan-2-01 adsorbed on GVZr-6.3 exhibits bands due to alkoxide species [fig. 7(b)]. On increasing the period of adsorption the 1660 cm-l band was intensified [fig. 7(c)], which can be assigned to the C=O stretching vibration of coordinately adsorbed ketone on Lewis-acid sites. After oxygen of 2.5 kPa was introduced onto the catalyst the temperature of the disc was raised in stages. At 353 K the 1550 cm-l band was intensified. In addition the OH band at 3660 cm-l was intensified, suggesting that the coordinated adsorbed ketone was3670 Structure and Butene Oxidation on V,O,/ZrO, I I ,# I .I . I . I 35 30 " 17 15 13 11 wavenumber/102 cm-' Fig. 9. F.t.i.r. spectra of butan-2-one adsorbed on GVZr-1.8. (a) Background, (b) after introduction of butan-2-one (ca. 1.1 kPa for 30 min at 293 K) followed by 30 min evacuation at 293 K, (c) after 15 h of adsorption at 298 K, ( d ) followed by 30 min at 353 K in oxygen, (e) 30 min at 413 K in oxygen. The spectra of (b)-(e) below 1800 cm-' shown after subtraction of background. Table 2. Characterization of the catalysts fraction of surface conc. amorphous of amorphous Raman catalyst v,o, (W oxide/pmol m-* i.r. band/cm-' band/cm-' VZr-2.0 VZr-3.7 VZr-6.0 VZr-7.6 VZr-8.4 VZr- 15.6 VZr-24 GVZr-2.0 GV Zr- 6.3 100 100 60 75 40 20 100 100 - 3 6 12 12 13 14 15 4 13 972 - - - 1005, 986 - 1008, 986 - 1018, 1002 - 1011, 983 1020 - 1025 - 978 1011, 995, 979 - - - - 1010-998 1023, 995 1032, 996 1032 - 1032 - 994 997 1020, 1000 1032 - - - dehydrogenated to enole-type species.The spectrum above 413 K resembles that of butene at 473 K, suggesting that a similar reaction occurred. Fig. 8 shows the spectra of butan-2-one on GVZr-6.3. On increasing the time of adsorption, the 1550 cm-l band, which can be attributed to the enol-type of adsorbed ketone, appeared. The spectra at higher temperatures are similar to those for butene and butan-2-01. In the case of GVZr-1.8, the enole-type bands appeared at rather low temperatures [fig. 9 (41. Structure Characterization of Catalysts The structure of vanadia species dispersed on zirconia was studied by XRD, F.t.i.r. and laser Raman techniques.XRD results showed that the VZr catalysts at vanadium loadings below 6.0 wt YO together with GVZr catalysts have no diffraction lines due toH. Miyata et al. 367 1 1 1 10 9 1 1 10 9 wavenumber/ 10' cm- ' Fig. 10. F.t.i.r. spectra of Z-but-2-ene adsorbed on (A) GVZr-6.3 and (B) GVZr-1.8. (a)-(e) As for fig. 3 and 4. crystalline V205, indicating that the most of the vanadia species are highly dispersed on zirconia. The amounts of crystalline V,O, and amorphous vanadia species on catalysts were estimated by using the method applied previously." The concentrations of amorphous surface vanadia species per surface area are listed in table 2. Although the calculation of amorphous species from XRD lines may include some errors, the concentrations of this species are almost constant for the catalysts at vanadium loadings above 6.0 wt %.Table 2 also summarizes the observed or calculated V=O bands of i.r.l and Raman spectra. The mono-layered sample (GVZr-2.0), which was prepared by a single cycle of VOCl, circulation, exhibits a single i.r. band at ca. 980 cm-', whereas the multi-layered catalyst (GVZr-6.3) shows complex bands, which were separated into three bands.l Generally, the i.r. band of V=O in crystalline V205 shows at 1020-1025 cm-' and the Raman band at 995 cm-l. On the other hand, the i.r. band at 980 cm-' and the Raman band at 1026 cm-l are attributable to those of polyvanadate. In the present study, in situ i.r. spectra of V=O region were observed. The spectra below 1100 cm-l corresponding to those in fig.3 and 4 are shown in fig. 10. The V=O band appeared at 1040cm-l after oxidation of GVZr-6.3 at 723 K [fig. IOA]. Introduction of butene on the catalyst caused the 5 cm-l shift to lower wavenumber and the slight reduction in intensity. The intensity and the position of this band were not changed appreciably on oxidation of butene above 473 K. A similar band appeared at 1030 cm-l for GVZr-1.8 [fig. lOB]. Thus, the V=O band shifts to a higher wavenumber in the reaction condition than that in the KBr disc.' Although further discussion will not be given in the present paper, those bands seem to be composed of two or more bands. As regards the character of V=O species of surface vanadate, the different reactivity would be expected with mono- or multi-layered vanadia species.121 F A R I3672 Structure and Butene Oxidation on V,O,/ZrO, butadiene furan p 3 w\ I w-!-cHsH3 r) O$"CHCH3 I? /-A C \ -v-0-v-0-v- + -v-0-v-0-4- + -0-0-0-0- * acetaldehyde acetic acid Fig. 11. Reaction scheme. Catalyst Structure and Catalytic Activities From the above considerations we propose that the oxidation of Z-but-2-ene proceeds as shown in fig. 11. Spectral behaviour together with high selectivity (77%) to acetaldehyde confirms that the cleavage of C-C bond takes place above 353 K. According to the proposal by Takita et aZ.,14 the butenes are converted to acetaldehyde and acetic acid in the presence of water on vanadium oxide catalysts by the oxyhydrative scission mechanism, where Brarnsted-acid sites play an important role in the hydration step.On the GVZr-2.0,Z-but-2-ene is mainly dehydrogenated to buta- 1,3-diene, and furan is formed at 373 K, although at high temperatures the formation of acetaldehyde is appreciable. This feature, together with the presence of the dihydrofuran-like species on this catalyst, suggests that the dehydrogenation proceeds via 71-ally1 species of butene, although the band due to 71-ally1 species (ca. 1600 ~ m - l ) ~ is not clear in fig. 3. Therefore furan formation on the GVZr catalyst at low vanadium loading occurs through the same pathway as that reported on the V-Ti As described above, the oxidation activities correlate well with the concentration of amorphous vanadia species. The difference of the selectivity between GVZr-2.0 (GVZr-1.8) and GVZr-6.3 seems to arise from the nature of surface vanadate, being accompanied by the concentration of amorphous vanadate or by the vanadium layers covered on zirconia.As described in the structural characterization by i.r. and Raman spectroscopies, the V=O species in a mono-layered vanadia is more weakened than that in a multi-layered vanadia by the interaction or bonding with ZrO,. Thus, the mono- layered vanadia favours the formation of furan. Low vanadium loading catalyst showed little or no concentration of Brarnsted acidity, while high vanadium loading catalyst showed high Brarnsted acidic concentration. l6 Considering high activity for acetaldehyde and acetic acid formation on GVZr-6.3, the C-C bond scission is closely related to Brarnsted acidity. Thus, a situation similar to that reported for the V,O,/TiO, system5 would be expected.References 1 H. Miyata, K. Fuji, T. Ono, Y. Kubokawa, T. Ohno and F. Hatayama, J. Chem. SOC., Faraday Trans. 2 H. Miyata, H. Nishiguchi and T. Ono, Chem. Express, 1988, 3, 243. I, 1987, 83, 675.H . Miyata et al. 3673 3 H. Miyata, K. Fujii and T. Ono, J. Chem. SOC., Faraday Trans. I , 1988, 84, 3121. 4 H. Miyata, T. Mukai, T. Ono, T. Ohno and F. Hatayama, J. Chem. SOC., Faraday Trans. I , 1988, 84, 5 T. Ono, T. Mukai, H. Miyata, T. Ohno and F. Hatayama, Appl. Catal., 1989, 49, 273. 6 T. Nakajima, T. Sonoda, H. Miyata and Y. Kubokawa, J. Chem. SOC., Faraday Trans. I , 1982,78,555. 7 H. 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