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Ind. Eng. Chem. Res. 1997, 36, 328-334
High Selective Conversion of Methane to Carbon Monoxide and the Effects of Chlorine Additives in the Gas and Solid Phases on the Oxidation of Methane on Strontium Hydroxyapatites Shigeru Sugiyama,*,† Toshimitsu Minami,† Tomonori Higaki,† Hiromu Hayashi,† and John B. Moffat‡ Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, Minamijosanjima, Tokushima 770, Japan, and Department of Chemistry and the Guelph-Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Selectivities to carbon monoxide higher than 90% for conversions of methane greater than 10% were obtained from the partial oxidation of methane on stoichiometric strontium hydroxyapatite (SrHAp1.67) at 873 K during 6 h on stream. However, the activities decreased gradually with increasing the time-on-stream to 78 h due to the transformation of the apatite to Sr3(PO4)2. With small quantities of tetrachloromethane (TCM) added to the feedstream, the high selectivity to CO was retained while the conversion suffered a marked decrease with increasing the timeson-stream. In the presence of TCM the catalytic solid consists of a complex mixture of hydroxyapatite, chlorapatite, phosphate and chloride, each of which contributes dissimilarly to the catalytic process. Introduction Methane, a principal component of natural gas, is an abundant hydrocarbon resource which is employed as a relatively inexpensive and clean-burning fuel. However, the diminishing reserves of petroleum oil have focused attention on the possibility of making more efficient use of natural gas (Crabtree, 1995), particularly through the oxidative coupling of methane to C2H6 and C2H4 (Kalenik and Wolf, 1993; Lunsford, 1995) and the partial oxidation of methane to methanol (Wang and Otsuka, 1995) and formaldehyde (Banares et al., 1995). Methane is also employed in the production of synthesis gas (carbon monoxide and hydrogen), which can be converted to higher hydrocarbons, alcohols, and aldehydes by Fischer-Tropsch catalysis (Henrici-Olive and Olive, 1976; Iglesia et al., 1993). The re-forming of methane provides carbon monoxide, which is used in many carbonylation reactions of organic compounds such as the Reppe reaction, oxo synthesis, and the Monsanto process (Bahrman et al., 1980; Colgrehoun et al., 1991), as well as hydrogen itself. Carbon monoxide may be produced from methane by steam reforming (CH4 + H2O f CO + 3H2 (Van Hook, 1980)), on a nickel/alumina catalyst at 973-1073 K, a process which is highly endothermic and leads, in addition, to the formation of carbon dioxide by the shift reaction CO + H2O a CO2 + H2. Carbon monoxide may also be obtained from the partial oxidation of methane (CH4 + 0.502 f CO + 2H2), which is mildly exothermic (Kirk and Othmer, 1980) but operates at very high temperature (>1473 K). Recently, the partial oxidation of methane to CO and H2 has been achieved at temperatures less than 1080 K with various lanthanide ruthenium oxides (Ashcroft, 1990), silica-supported Rh (Kunimori et al., 1992; Nakamura et al., 1993), and Ru, Ni and Pt (Nakamura et al., 1993) as catalysts. * Author to whom correspondence is addressed. Tel: (+81886)567432. Fax: (+81-886)557025. e-mail: sugiyama@chem. tokushima-u.ac.jp. † The University of Tokushima. ‡ University of Waterloo. Tel: (519) 888-4567 ext. 2502. Fax: (519) 746-0435. e-mail:
[email protected]. S0888-5885(96)00210-2 CCC: $14.00
In previous papers (Sugiyama et al., 1996a-c), the introduction of a small quantity of tetrachloromethane (TCM) as a gas-phase additive into the feedstream of the oxidation of methane has been shown to enhance the selectivity to CO on stoichiometric and near-stoichiometric calcium hydroxyapatites, in apparent contrast to the previously published reports of the beneficial effects of TCM on the conversion of methane and the selectivity to C2 compounds on a wide variety of solid catalysts (Sugiyama et al., 1996e). The apatite structure [M10(XO4)6Z2], where M is a metal, X is P, As, Si, or S, for example, and Z is OH or one of the halogens, is evidently a solid of considerable flexibility in its elemental compositions (Kreidler and Hummel, 1970). One of the more common forms, calcium hydroxyapatite [stoichiometric form; Ca10(PO4)6(OH)2, Ca/P ) 1.67], has received considerable attention. Although various ions can replace calcium to a limited extent, strontium can be incorporated in the apatite structure over the entire composition range (Collin, 1959). Since the oxidative coupling of methanol appears to be related to the electronegativity of the cation of the catalyst (Sugiyama and Moffat, 1992; Sugiyama et al., 1993b), studies of the oxidation of methane on strontium hydroxyapatite are of interest and relevance. Earlier work has shown that nonstoichiometric strontium hydroxyapatites (Sr/P ) 1.63, 1.60, 1.56, and 1.52) catalyze nonselective oxidation of methane to CO2 and CO (Matsumura et al., 1994). In the present study, the oxidation of methane in the presence and absence of TCM has been studied on stoichiometric strontium hydroxyapatite (Sr/P ) 1.67), as well as those containing excess strontium (Sr/P ) 1.70 and 1.73) and excess phosphorus (Sr/P ) 1.61). The apatite-supported SrCl2 catalyst has also been examined to provide further information on the nature of the chlorinated species which are participating in the oxidation process. Experimental Section Catalysts. Strontium hydroxyapatites (SrHAp1.73, SrHAp1.70, SrHAp1.67, and SrHAp1.61, where the sub© 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 329 Table 1. Surface Areas and Bulk Densities of the Catalysts catalyst
surface area (m2/g)
bulk density (g/cm3)
SrHAp1.61 SrHAp1.67 SrHAp1.70 SrHAp1.73 5% SrCl2/SrHAp 10% SrCl2/SrHAp
72.4 60.3 67.9 65.5 38.5 42.8
0.42 0.45 0.40 0.43 0.45 0.43
scripts represent the Sr/P atomic ratio of each apatite determined by inductively coupled plasma (ICP)) were prepared from Sr(NO3)2 (Wako Pure Chemicals, Osaka, Japan) and (NH4)2HPO4 (Wako) (Matsumura, 1994) following the method described for the preparation of calcium hydroxyapatite (Hayek and Newesely, 1963). The resulting solids were calcined at 773 K for 3 h after drying in air at 373 K overnight. The X-ray diffraction patterns of these catalysts matched that of Sr10(PO4)6(OH)2, as provided in JCPDS 33-1348. Apatite-supported SrCl2 was prepared by impregnation of SrHAp1.67 with an aqueous solution of SrCl2‚ 6H2O (Kanto Chemical, Tokyo, Japan). The catalysts were dried in air at 373 K overnight and calcined at 773 K for 3 h. The loading of the catalyst was expressed as weight percent of anhydrous SrCl2 in each catalyst. Particles of 0.35-1.75 mm were employed as catalysts in the present study. The surface areas and bulk densities of the catalysts are summarized in Table 1. Apparatus and Procedures. The catalytic experiments were performed in a fixed-bed continuous-flow quartz reactor operated at atmospheric pressure. Details of the reactor design and catalyst packing procedure have been described elsewhere (Sugiyama et al., 1996a). Prior to reaction the catalyst was pretreated in situ in an oxygen flow (25 mL/min) at 873 K for 1 h. The reaction conditions were, unless otherwise stated, W ) 0.5 g, F ) 30 mL/min, T ) 873 K, P(CH4) ) 28.7 kPa, P(O2) ) 4.1 kPa, and P(TCM) ) 0 or 0.17 kPa; the balance to atmospheric pressure was provided by helium. Analysis and Characterization. The reactants and products were analyzed with an on-stream gas chromatograph (Shimadzu GC-8APT) equipped with a TC detector and integrator (Shimadzu C-R6A). The columns used in the present study and the methods employed in the calculation of conversions and selectivities have been described previously (Sugiyama et al., 1996a). The surface areas were measured with a conventional BET nitrogen adsorption apparatus (Shibata P-700). Powder X-ray diffraction (XRD) patterns were recorded with a CN-2011 (Rigaku) diffractometer, using monochromatized Cu KR radiation. Patterns were recorded over the range 2θ ) 5-60°. X-ray photoelectron spectroscopy (XPS) (Shimadzu ESCA-1000AX) used monochromatized Mg KR radiation. The binding energies were corrected using 285 eV for C 1s as an internal standard. Argon ion etching of the catalyst was carried out at 2 kV for 1 min with a sputtering rate estimated as ca. 2 nm/min for SiO2. The concentrations of Sr and P in each apatite were determined in an aqueous HNO3 solution by ICP spectrometry (Shimadzu, ICPS-5000). Results and Discussion Since SrHAp is stable at 873 K but easily converted to Sr3(PO4)2 at 973 K (Matsumura et al., 1994), all
Figure 1. Effects of the partial pressure of O2 on the oxidation of methane on SrHAp1.61 in the absence (A) and the presence (B) of TCM at 873 K: (a) 0.5 h on stream; (b) 6 h on stream. Catalysts were pretreated at 873 K in O2 (25 mL/min) for 1 h. Reaction conditions: W ) 0.5 g, F ) 30 mL/min, P(CH4) ) 28.7 kPa, and P(TCM) ) 0 or 0.17 kPa diluted with He.
Figure 2. Effects of the partial pressure of O2 in the feedstream on the oxidation of methane on SrHAp1.67 in the absence (A) and presence (B) of TCM at 873 K. Symbols and reaction conditions: same as those in Figure 1.
catalytic experiments have been performed at pretreatment and reaction temperatures below 873 K. Effects of the Partial Pressure of O2 on SrHAp. Figures 1-4 show the effects of the partial pressure of O2 on the oxidation of methane on SrHAp1.61, SrHAp1.67, SrHAp1.70, and SrHAp1.73, respectively, in the absence (A) and presence (B) of TCM at 873 K. In contrast to the results for calcium hydroxyapatites (CaHAp) (Sugiyama et al., 1996b), the conversion and the selectivities showed no systematic dependence on Sr/P, either with or without TCM. While, in the absence of TCM, with 1.61 and 1.70 SrHAp compositions, the conversion of methane shows little or no dependence on P(O2), with the 1.67 and 1.73 catalysts the conversion increases with P(O2). In the absence of TCM the selectivities to CO generally exceed 90% and frequently reach 95% at all P(O2) and for each of the four SrHAp compositions, in contrast to the significantly lower values found with
330 Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 Table 2. Surface Areas of SrHAp1.67a after Use in the Oxidation P(O2) (kPa) P(TCM) (kPa)
4.1
8.2
12.3
0.00 0.17
38.4 24.9
39.9 29.9
38.0 25.5
a Previously employed in obtaining results reported in Figure 2 but after 6 h on stream.
Figure 3. Effects of the partial pressure of O2 in the feedstream on the oxidation of methane on SrHAp1.70 in the absence (A) and presence (B) of TCM at 873 K. Symbols and reaction conditions: same as those in Figure 1.
Figure 5. XRD patterns of SrHAp1.67 previously employed in the oxidation of methane at 873 K. (A) Previously used in the reaction in the presence of TCM at P(O2) ) 4.1 kPa after 6 h on stream. (B) Previously used in the reaction in the presence of TCM at P(O2) ) 12.3 kPa after 6 h on stream. (C) Previously used in the reaction in the absence of TCM at P(O2) ) 12.3 kPa after 6 h on stream. (D) Previously used in the reaction in the absence of TCM at P(O2) ) 12.3 kPa after 78 h on stream. Figure 4. Effects of the partial pressure of O2 in the feedstream on the oxidation of methane on SrHAp1.73 in the absence (A) and presence (B) of TCM at 873 K. Symbols and reaction conditions: same as those in Figure 1.
CaHAp (Sugiyama et al., 1996b). Introduction of TCM into the methane/O2 feedstream led to decreased conversions for all compositions and all P(O2) but produced increases in the selectivities to CO to approximately 100% for all times-on-stream. Although H2 was found in the reactor effluent from each of the reported experiments either with or without TCM in the feedstream, H2 could not be quantitatively analyzed in the present study. Although both HCl and H2 can be generated under the conditions described here, the hydroxyapatite is capable of storing chlorine species in the form of chlorapatite through anion exchange with the hydroxyl group. In view of the absence of any systematic relationship between the catalytic properties in the methane oxidation process and the compositions of the catalysts, the remainder of the present paper will only be concerned with the stoichiometric (1.67) SrHAp. The drastic reduction in conversion observed with SrHAp1.67 on introduction of TCM into the feedstream cannot be related to changes in the surface area since
such decreases are relatively small (Table 2). The XRD patterns of SrHAp1.67 after use with TCM at P(O2) equal to 4.1 and 12.3 kPa (parts A and B of Figure 5, respectively) showed that the samples contained only strontium chlorapatite (Sr10(PO4)6Cl2). The surface regions of the same sample were identical with those of the corresponding fresh sample from XPS analyses except for the existence of a peak due to Cl 2p at approximately 199 eV, and the Cl/Sr ratio showed little or no dependence on P(O2) (Table 3). It should be noted that the measured Cl/Sr ratios fall in the range of 0.20.3 which are similar to but somewhat higher than would be expected for stoichiometric strontium chlorapatite. Since the XRD and XPS results provide evidence for the formation of chlorapatite throughout the catalyst, the inhibition of the reaction may, at least in part, be attributed to this transformation. Effects of Time-on-Stream on SrHAp1.67 in the Absence of TCM. Since the conversion of methane and selectivity to CO remained at relatively high values during 6 h on stream at P(O2) ) 12.3 kPa in the absence of TCM on SrHAp1.67, the effects of longer times-onstream have been examined. As shown in Figure 6, the conversion of methane increased in the initial 3 h on stream to 17%, followed by a gradual decrease to 8% at
Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 331 Table 3. XPS Analyses of the Near-Surface Region of Fresh and Useda SrHAp1.67 Srd )b
tc
3p3/2
3p1/2
Od 1s
Pd 2p
fresh
0 1 0 1 0 1 0 1
268.9 269.7 269.7 270.0 269.3 269.8 269.3 269.5
279.5 280.0 280.0 280.4 279.9 280.2 279.5 279.9
530.9 531.6 531.7 531.7 531.4 531.7 531.0 531.3
190.3 190.8 190.8 191.3 190.6 191.1 190.7 190.7
P(O2
4.1 8.2 12.3
Cld 2p
Sr/Pe
O/Sre
Cl/Sre
199.3 199.6 198.8 199.4 198.7 199.3
1.53 1.87 1.47 1.97 1.48 1.85 1.72 1.83
1.70 1.94 1.77 1.62 2.20 2.14 1.95 1.78
0.34 0.29 0.21 0.18 0.24 0.20
a Previously employed in obtaining results reported in Figure 2B but after 6 h on stream in the presence of TCM. b Partial pressure of O2 in the feedstream. c Etching time (min). d Binding energy (eV). e Atomic ratio.
Figure 6. Effects of the extended time-on-stream on the oxidation of methane on SrHAp1.67 in the absence of TCM at 873 K and P(O2) ) 12.3 kPa. Reaction conditions: same as those in Figure 1 except the partial pressure of O2. Table 4. Analyses of the Near-Surface Region on Useda SrHAp1.67 binding energy (eV)
atomic ratio
tosb
tc
Sr 3p3/2
Sr 3p1/2
O 1s
P 2p
Sr/P
O/Sr
SAd
6
0 1 0 1
269.0 269.5 269.0 269.7
279.2 279.5 279.0 279.3
531.2 531.6 531.1 531.5
190.7 190.9 190.5 190.8
1.57 1.94 1.74 1.91
2.35 2.29 2.39 2.23
38.0
78
18.8
Figure 7. Effects of the reaction temperature on the oxidation of methane on SrHAp1.67 in the absence (A) and presence (B) of TCM. Symbols and reaction conditions: same as those in Figure 1 except the reaction temperature and the partial pressure of O2 [P(O2) ) 4.1 kPa].
a Previously employed in obtaining results reported in Figure 5. b Time-on-stream (h). c Etching time (min). d Surface area (m2/g).
78 h on stream. The high selectivity to CO (90%) was retained during the initial 6 h on stream but decreased to 50% after 78 h on stream, concomitantly with a gradual increase in the selectivity to CO2. The XRD patterns of SrHAp1.67 after use in the reaction for 6 h on stream revealed a trace amount of Sr3(PO4)2, but after 78 h on stream the apatite was almost completely converted to Sr3(PO4)2 (parts C and D of Figure 5, respectively). Although the surface analyses by XPS showed that there were little differences in the binding energies of Sr 3p, O 1s, and P 2p and the atomic ratios of Sr/P and O/Sr between SrHAp1.67 used in the reaction after 6 and 78 h on stream, the surface area after 78 h on stream decreased to half the value measured after 6 h on stream (Table 4). On Sr3(PO4)2 (surface area 0.7 m2/g and apparent density 1.27 g/cm3) prepared by an independent method described elsewhere (Sugiyama et al., 1996d), under the same reaction conditions the conversion of methane and the selectivities to CO and CO2 at 6 h on stream were 0.6, 16, and 84%, respectively. These results show that both the gradual transformation of SrHAp1.67 to Sr3(PO4)2, as evidenced by the X-ray diffraction data shown in Figure 5, and the decrease of the surface area during the oxidation may contribute to the decrease of the conversion of methane and the selectivity to CO as described in Figure 6. Effects of the Reaction Temperature and Space Time (W/F) on SrHAp1.67 in the Presence and
Figure 8. Effects of the space time (W/F) on the oxidation of methane on SrHAp1.67 in the absence (A) and presence (B) of TCM at 873 K. Symbols and reaction conditions: same as those in Figure 1 except the partial pressure of O2 [P(O2) ) 4.1 kPa] and weight of the catalyst.
Absence of TCM. Studies of methane conversion at reaction temperatures lower than 873 K, at which the formation of Sr3(PO4)2 from SrHAp1.67 should be reduced, show, in the absence of TCM, a large decrease in conversion (Figure 7A). With TCM the conversion is relatively unchanged with reaction temperature although the conversion at 873 K is lower than that
332 Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 Table 5. Results of XPS Analyses of the Near-Surface Region on Fresh SrHAp1.67 Doped by SrCl2 and Pretreated with TCM Src catal.a
tb
3p3/2
3p1/2
Oc 1s
Pc 2p
Clc 2p
Sr/Pd
O/Srd
Cl/Srd
5 wt %
0 1 0 1 0 1 0 1 0 1
268.9 269.3 268.2 268.3 269.1 269.4 269.5 269.6 269.3 269.5
279.4 279.6 278.3 278.6 279.3 279.5 279.8 279.8 279.6 279.5
530.9 531.2 530.1 530.1 531.1 531.3 531.1 531.5 531.1 531.3
190.2 190.6 189.5 189.5 190.2 190.6 190.3 190.7 190.4 190.7
199.3 199.5 198.3 198.4 199.1 199.5 198.8 199.0 198.8 199.0
1.76 2.02 2.02 2.01 1.73 1.82 1.63 1.84 1.45 1.81
2.08 1.97 2.03 1.85 2.33 2.16 1.89 1.83 2.29 1.89
0.11 0.10 0.17 0.17 0.14 0.10 0.33 0.27 0.23 0.19
10 wt % 1h 3h 6h
a wt %: weight percent of SrCl in SrCl /SrHAp catalysts. h: pretreatment time with TCM (0.20 kPa) + O (total flow rate; 25 mL/ 2 2 2 min) at 873 K. b Etching time (min). c Binding energy (eV). d Atomic ratio.
Figure 10. Effects of the pretreatment time with TCM on the oxidation of methane on SrHAp1.67 in the absence of TCM at 873 K. Data were collected at 0.5 h on stream. Prior to the reaction, SrHAp1.67 was exposed to TCM (0.20 kPa) diluted with O2 (total flow rate; 25 mL/min) for the described time. Reaction conditions: same as those in Figure 1 except the partial pressure of O2 [P(O2) ) 4.1 kPa].
Figure 9. Effects of the loading of SrCl2 on the oxidation of methane on SrCl2/SrHAp in the absence (A) and presence (B) of TCM at 873 K. Symbols and reaction conditions: same as those in Figure 1 except the partial pressure of O2 [P(O2) ) 4.1 kPa].
observed where TCM was not present (Figure 7B). The selectivity to CO in the absence of TCM was greater than 90% regardless of the reaction temperature, but in the presence of TCM deep oxidation to CO2 occurred at both 773 and 823 K. Although selectivities to CO were high (>95%) at 873 K and relatively unaffected by W/F in the absence of TCM the conversion of methane increased by a factor of 3-4 as W/F was increased (Figure 8A). However, with TCM present and at all W/F the conversions after 6 h on stream were reduced by approximately 50%, although the COx composition was essentially all CO and remained as such for all W/F and times-on-stream investigated (Figure 8B). In the presence of TCM the formation of the chlorapatite during the reaction is evidently relatively fast. Effects of the Solid Phase Chlorine Species in SrHAp. Earlier work has shown that the formation of chlorides (Sugiyama et al., 1994) or oxychlorides (Sugiyama et al., 1993a) results in the improvement of the conversion of methane and the selectivity to C2 compounds, whereas with the chlorapatite formed from the apatite an improved selectivity to CO is observed. In the present section, the results of studies of SrCl2/
SrHAp and SrHAp pretreated with TCM, in the latter of which the chlorapatite is formed, are reported to provide comparisons of the participation of the chloride and chlorapatite in the oxidation of methane. Table 5 shows the surface composition determined by XPS on fresh catalysts doped by SrCl2 and pretreated with TCM. Although the binding energy of a given element is relatively uninfluenced by the addition of SrCl2 on pretreatment with TCM, the Cl/Sr ratios reflect the increase in the content of SrCl2 and the duration of exposure to TCM. It should be noted, however, that, although the Cl/Sr ratios, particularly at the higher pretreatment times, have increased, the values are similar to those shown in Table 3 and considerably different from those expected for stoichiometric SrCl2. The effect of the addition of SrCl2, with and without TCM in the feedstream, on the methane conversion process may be seen in Figure 9. In the absence of TCM (Figure 9A), on addition of SrCl2 the conversion of methane decreases while the selectivity to CO2 increases, with little effect of time-on-stream on either of these quantities. When TCM is present, the conversion increases when SrCl2 is added, and the selectivity to CO, which is relatively low at 0.5 h, increases after 6 h on stream. Further, significant selectivities to ethane and ethylene are observed after 6 h on stream with the 10% SrCl2 sample. Evidently, the nature of the participation of strontium hydroxyapatite and SrCl2 in the methane conversion process is significantly different.
Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 333
Pretreatment of SrHAp1.67 with TCM for various periods of time prior to use in the methane reaction without TCM shows that the selectivity to C2H4 is increased while that to CO is decreased (Figure 10) as the duration of pretreatment increases. Interestingly, these results, particularly the decrease in the selectivity to CO, appear to more closely reflect the formation of SrCl2 rather than the presence of the chlorapatite, although the correspondence is not exact. Although in the absence of TCM strontium hydroxyapatite is, after a period of time, completely converted to Sr3(PO4)2, in the presence of TCM the hydroxyapatite is converted, at least in large part, to the chlorapatite, as revealed by the XRD data. It is clear that, at some intermediate time, and in the presence of TCM, the original hydroxyapatite and the formed phosphate and chlorapatite will coexist together. However, the XPS data as well as that from the methane reaction suggest that SrCl2 is also present. This may result from the transformation of one of the three aforementioned components and/ or may be formed through the loss of surface oxygen as the methane oxidation reaction proceeds. If this rationalization is correct, it may be concluded that the presence of a chlorinated species as, for example, in the form of the chloride itself, is not necessarily beneficial in the formation of partial oxidation products, such as CO, but rather the catalytic solid must contain oxygen as part of its stoichiometry in order to optimize the production of the monoxide. Regardless of the aforementioned comments, gas-phase chlorinated species, presumably dissimilar to those in the solid phase, may be contributing to the methane conversion process although, in view of the present observations, to a lesser extent than those in the catalytic solids. Conclusions 1. High selectivities to CO (>90%) were obtained in the oxidation of methane on SrHAp1.67 at 873 K in the absence of TCM during the initial 6 h on stream. 2. Although the conversion of methane on each SrHAp composition decreased in the presence of TCM during 6 h on stream, the selectivity to CO under some conditions approached 100%. 3. The conversions and selectivities on SrHAp in the absence and presence of TCM showed little or no dependence on Sr/P. 4. A gradual decrease in the conversion of methane and the selectivity to CO during 78 h on stream was observed in the oxidation of methane on SrHAp1.67 in the absence of TCM apparently due to the transformation of SrHAp to Sr3(PO4)2. 5. Although the extent of the transformation of SrHAp to Sr3(PO4)2 would be expected to be reduced at lower reaction temperatures, a decrease in the reaction temperature from 873 to 823 K resulted in a substantial decrease in the catalytic activity. 6. SrHAp1.67, under the conditions employed in the present work and the presence of TCM, is rapidly converted to the corresponding chlorapatite. 7. During the methane conversion process in the presence of TCM the catalytic solid probably consists of a complex mixture of the hydroxyapatite, chlorapatite, phosphate, and chloride, each participating dissimilarly in the catalytic process. Acknowledgment This work was partly funded by a grant for Natural Gas Research from the Japan Petroleum Institute to S.S. and a grant from the Natural Sciences and Engi-
neering Research Council of Canada to J.B.M., to which our thanks are due. Literature Cited Ashcroft, A. T.; Cheetham, A. K.; Foord, J. S.; Green, M. L. H.; Grey, C. P.; Murrell, A. J.; Vernon, P. D. F. Selective Oxidation of Methane to Synthesis Gas Using Transition Metal Catalysts. Nature 1990, 344, 319. Bahrman, H.; Cornails, B.; Frohling, C. D.; Mullen, A. In New Syntheses with Carbon Monoxide; Falbe, J., Ed.; Springer: Berlin, 1980. Banares, M. A.; Hu, H.; Wachs, I. E. Genesis and Stability of Silicomolybdic Acid on Silica-supported Molybdenum Oxide Catalysts: in Situ Structural-Selectivity Study on Selective Oxidation Reactions. J. Catal. 1995, 155, 249. Colgrehoun, H. M.; Thompson, D. J.; Twigg, M. V. Carbonylation: Direct Synthesis of Carbonyl Compounds; Plenum: New York, 1991. Collin, R. L. Strontium-Calcium Hydroxyapatite Solid Solutions: Preparation and Lattice Constant Measurement. J. Am. Chem. Soc. 1959, 81, 5275. Crabtree, R. H. Aspects of Methane Chemistry. Chem. Rev. 1995, 95, 987. Hayek, E.; Newesely, H. Pentacalcium Monohydroxyorthophosphate (Hydroxyapatite). Inorg. Synth. 1963, 7, 63. Henrici-Olive, G.; Olive, S. The Fischer-Tropsch Synthesis: Molecular Weight Distribution of Primary Products and Reaction Mechanism. Angew. Chem., Int. Ed. Engl. 1976, 15, 136. Iglesia, E.; Reyes, S. C.; Madon, R. J.; Soled, S. L. Selectivity Control and Catalyst Design in the Fischer-Tropsch Synthesis: Sites, Pellets, and Reactors. Adv. Catal. 1993, 39, 221. Kalenik, Z.; Wolf, E. Oxidative Coupling of Methane. Specialist Periodical Reports; Catalysis Vol. 10; Royal Society of Chemistry: Cambridge, U.K., 1993. Kirk, R. E., Othmer, D. F., Eds. Encyclopedia of Chemical Technology, 3rd ed.; Wiley-Interscience: New York, 1980; Vol. 12, p 952. Kreidler, E. R.; Hummel, F. A. The Crystal Chemistry of Apatite: Structure Fields of Fluor- and Chlorapatite. Am. Mineral. 1970, 55, 170. Kunimori, K.; Umeda, S.; Nakamura, J.; Uchijima, T. Partial Oxidation of Methane to Synthesis Gas over Rhodium Vanadate, RhVO4: Redispersion of Rh Metal during the Reaction. Bull. Chem. Soc. Jpn. 1992, 65, 2562. Lunsford, J. H. The Catalytic Oxidative Coupling of Methane. Angew. Chem., Int. Ed. Engl. 1995, 34, 970. Matsumura, Y.; Sugiyama, S.; Hayashi, H.; Shigemoto, N.; Saitoh, K.; Moffat, J. B. Strontium Hydroxyapatites: Catalytic Properties in the Oxidative Dehydrogenation of Methane to Carbon Oxides and Hydrogen. J. Mol. Catal. 1994, 92, 81. Nakamura, J.; Umeda, S.; Kubushiro, K.; Kunimori, K.; Uchijima, T. Production of Synthesis Gas by Partial Oxidation of Methane over the Group VIII Metal Catalysts. Sekiyu Gakkaishi 1993, 36, 97. Sugiyama, S.; Moffat, J. B. Cation Effects in the Oxidative Dehydrogenation of Methane on Sulphates. Catal. Lett. 1992, 13, 143. Sugiyama, S.; Matsumura, Y.; Moffat, J. B. A Comparative Study of the Oxides of Lanthanum, Cerium, Praseodymium, and Samarium as Catalysts for the Oxidative Dehydrogenation of Methane in the Presence and Absence of Carbon Tetrachloride. J. Catal. 1993a, 139, 338. Sugiyama, S.; Satomi, K.; Hayashi, H.; Shigemoto, N.; Miyaura, K.; Moffat, J. B. Oxidative Coupling of Methane over Alkali Sulphates in the Presence and Absence of Tetrachloromethane. Appl. Catal., A 1993b, 103, 55. Sugiyama, S.; Satomi, K.; Kondo, N.; Shigemoto, N.; Hayashi, H.; Moffat, J. B. Role of Surface Chlorine Species in the Oxidation of Methane in the Presence of Tetrachloromethane on Magnesium Phosphate and Sulphate. J. Mol. Catal. 1994, 93, 53. Sugiyama, S.; Minami, T.; Hayashi, H.; Tanaka, M.; Shigemoto, N.; Moffat, J. B. Enhancement of the Selectivity to Carbon Monoxide with Feedstream Doping by Tetrachloromethane in the Oxidation of Methane on Stoichiometric Calcium Hydroxyapatite. J. Chem. Soc., Faraday Trans. 1996a, 92, 293. Sugiyama, S.; Minami, T.; Moriga, T.; Hayashi, H.; Koto, K.; Tanaka, M.; Moffat, J. B. Surface and Bulk Properties, Catalytic
334 Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 Activities and Selectivities in Methane Oxidation on Nearstoichiometric Calcium Hydroxyapatites. J. Mater. Chem. 1996b, 6, 459. Sugiyama, S.; Minami, T.; Hayashi, H.; Tanaka, M.; Shigemoto, N.; Moffat, J. B. Partial Oxidation of Methane to Carbon Oxides and Hydrogen on Hydroxyapatite: Enhanced Selectivity to Carbon Monoxide with Tetrachloromethane. Energy Fuels 1996c, 10, 828. Sugiyama, S.; Minami, T.; Hayashi, H.; Tanaka, M.; Moffat, J. B. Surface and Bulk Properties of Stoichiometric and Nonstoichiometric Strontium Hydroxyapatite and the Oxidation of Methane. J. Solid State Chem. 1996d, in press. Sugiyama, S.; Miyamoto, T.; Hayashi, H.; Moffat, J. B. Oxidative Coupling of Methane on MgO-MgSO4 Catalysts in the Presence and Absence of Carbon Tetrachloride. Bull. Chem. Soc. Jpn. 1996e, 69, 235.
Van Hook, J. P. Methane-Steam Reforming. Catal. Rev., Sci. Eng. 1980, 21, 1. Wang, Y.; Otsuka, K. Catalytic Oxidation of Methane to Methanol with H2-O2 Gas Mixture at Atmospheric Pressure. J. Catal. 1995, 155, 256.
Received for review April 15, 1996 Revised manuscript received October 28, 1996 Accepted November 8, 1996X IE960210D
X Abstract published in Advance ACS Abstracts, January 1, 1997.
