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. 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