Furaduy Discuss. Chem. SOC., 1989, 87, 23-32 Mechanistic Relationships in the Activation of Methane and the Conversion of Methanol on Heteropoly Oxometallates Shamsuddin Ahmed, Slavik Kasztelant and John B. Moffat" Department of Chemistry and Guelph- Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3Gl Heteropoly oxometallates have been employed in the heterogeneous catalysis of the conversion of methanol to hydrocarbons and the partial oxidation of methane. In the former process methanol is first protonated at Bransted-acid sites, but at higher temperatures the heteropoly anions are partially methy- lated subsequent to scission of the C-0 bonds in protonated methanol. With silica-supported 12-molybdophosphoric acid the exchange of the pro- tons by cations effectively poisons the catalyst for the conversion of methane.Introduction of a chloro-additive to the feedstream in the latter process produces changes in the conversion and selectivity which are markedly different for the molybdenum- and tungsten-containing catalysts. With the former the conversion of methane is increased while the selectivity to partial oxidation products is decreased. With the latter the former comments are also applicable but selectivities to methyl chloride reach as high as 90 mol '30. The results from cation exchange and the addition of a chloro-additive are interpreted and a mechanism with common features is proposed for the methanol and methane conversion processes. Heterogeneous catalysts are many and varied, from monocationic oxides to multiele- mental complexes and from supported monometallic to the complex multicomponent solids such as those employed in ammonia synthesis.The functionality of a given catalyst is usually related in a complex way to its chemical composition, crystallographic structure and surface and bulk properties. Two catalysts with differing functionalities are invariably found to possess contrasting features of the aforementioned properties. Heteropoly oxometalates with anions of Keggin structure are interesting examples of isostructural catalysts, that is, those in which the elemental composition may be changed to produce an alteration of the catalytic properties while the structural features are retained. These heteropoly oxometallates are ionic solids with discrete anions and cations.' The latter may be simple one-atom inorganic ions or multi-atom organic species.The anions (fig. 1 ) are large cage-like structures with a central atom such as phosphorus which is surrounded by four oxygen atoms arranged tetrahedrally. Twelve octahedra with oxygen atoms at their vertices and a peripheral metal atom such as tungsten or molybdenum at their centres envelope the central tetrahedron and share oxygen atoms with each other and the central atom. The structures of the heteropoly anions are semiquantitatively similar for either molybdenum or tungsten as peripheral metal atoms, although the bond lengths are slightly different with, for example, the peripheral metal-terminal oxygen atom bond lengths being 1.66 and 1.70 8, for Mo and W, respectively.' There is considerable interest in the methanol-to-gasoline recess,* and in particular the mechanism by which the carbon-carbon bond is produced.'In addition, considerable t Permanent address: Direction de recherche CinCtique et catalyse, lnstitut Francais du PCtrole, 1 & 4, av.de Bois-PrCau BP 311, 92506 Rueil Malmaison Cedex, France. 2324 Mechanistic Relationships Fig. 1. Heteropoly anion of Keggin structure (KU, Keggin unit). Large circles, central atom and peripheral metal; small circles, oxygen atoms. attention has recently been directed to processes for the activation of methane and its conversion to a more suitable chemical feedsto~k.~-~ Earlier work in this laboratory has shown that methanol can be converted to hydrocarbons (>C,) on 12-tungstophosphoric acid (HPW), while methane is oxidized on 12-molybdophosphoric acid (HPMo); the anions of the two catalysts are structurally identical, but they contain tungsten and molybdenum, respectively, in the peripheral position.'-I4 Photoacoustic FTIR studies in this laboratory have shown that methanol is protonated at the Brflnsted-acid sites in HPW and at high temperatures the scission of the C-0 bond in CH30Hl results in the methylation of the oxygen atoms of the heteropoly anion.13914 The observation that methanol and methane can be converted on structurally similar but chemically different catalysts provides an interesting opportunity to explore the mechanistic relationships between such processes.In the present work the effects of additives to the solid and gas phase in the conversion of methane are reported and the rationalization of the observations is compared and contrasted with the mechanism for the methanol-to-hydrocarbons process on isostructural heteropoly oxometallates.Experimental The supported heteropoly oxometallates were prepared by impregnation of the silica (Grace-Davison grade 407, 740 m2 g-', 8-40 mesh) with aqueous solutions of the heteropoly acid, which was recrystallized before use. The solutions were evaporated to dryness at 80 "C and the solids calcined at 350 "C for 2 h. Particle sizes of 8-15 mesh were employed for the studies reported here. In the exchange studies, loadings of 16 wt O h HPMo on silica were employed, corresponding to 0.068 KU nm-* of support surface.In some cases 23.9 wt % HPMo was employed. The supported salts of the heteropoly acids were prepared by the impregnation of the calcined supported heteropoly acids with an excess of an aqueous solution of the cation in the form of the carbonate, acetate or nitrate, depending on the solubility. After evaporation to dryness at 80 "C the solids were calcined at 200 "C forS. Ahmed, S. Kasztelan and J. B. Mofat 25 cation (Cs)/KU ( x (Li)/KU (0) Fig. 2. Relative N 2 0 turnover rate of the CH4/N20 reaction after addition of caesium carbonate ( x ) or lithium carbonate (0) to silica-supported 12-molybdophosphoric acid (HPMo). X, 0 16 wt '/o HPMo; +, W 23.9 wt '/o HPMo. Reaction temperature 843 K, W = 0.5 g, F = 30 cm' min-', CH4 (67 mol %), N 2 0 (33 mol %).1 h. The reaction temperature was 570 "C, the catalyst weight was 0.5 g, the flow rate was 30 cm3 min-' and the feed composition was 67 mol '/o CH4, 33 mol YO NzO. Pretreat- ment was performed for 1 h in helium at the reaction temperature. A fixed-bed continuous-flow reactor was employed for studies of the catalysed process. The catalysts were preheated in helium for 1 h at the reaction temperature. An on-stream Hewlet-Packard 5890 gas chromatograph was used for all analyses. The chloro-additive was added to the feed stream by passing helium through a saturator containing the liquid held at 0 "C. The flow of helium was adjusted so that after dilution following introduction into the main flow of reactants ( CH4-N20) the desired concentra- tion of the additive in the feed was obtained.Experiments in the absence of CH, showed that the chloro-additive may also be oxidized, although methyl chloride is found in the effluent only when methane is present in the feed stream. To correct for the oxidation of the chloro-additive experiments were performed in duplicate, one in the absence, another in the presence of CH,, with care being taken to ensure that residence times were equivalent in each case. Since the experimental procedure followed provides results effectively obtained with a chloromethane-pretreated catalyst, the duration of the reaction performed in the absence of CH, was kept constant in each series of experiments. Results The exchange of the protons contained in the silica-supported heteropoly acids by various cations decreases the conversion and modifies the selectivity.Typical results are shown in fig. 2-4. It is evident that as the number of cations introduced into the catalysts increases, the turnover number, expressed in terms of the oxidant, decreases. In all cases the turnover number appears to approach that expected for the supported cation itself as the number of cations contained in the supported heteropoly oxornetallate is increased. Interestingly, while the selectivity to CO is larger than that to COz in the26 Mechanistic Relationships 0 2 4 6 8 cation (Cs)/KU Fig. 3. Selectivity of the CH,+ N 2 0 reaction after addition of caesium carbonate to silica-supported 12-molybdophosphoric acid (HPMo). Open symbols (16 wt YO HPMo). Closed symbols (23.9 wt YO HPMo).Open symbols not joined by line (Cs/Si02). Reaction conditions as in fig. 1. A, CO; 0, C02; 0, CH20. 50 A 0- 0 A lo O<O 0 4 -- - - I I 1 I I I I n I 0 2 4 6 8 cation (Li)/ KU Fig. 4. Selectivity of the CH4 + N 2 0 reaction after addition of lithium carbonate to silica-supported 12-molybdophosphoric acid (HPMo). Open symbols (16 wt '/O HPMo). Closed symbols (23.9 wt O/O HPMo). Open symbols not joined by line (Li/Si02 and SO2, respectively). Reaction conditions and symbols as in fig. 1.100 h $ e o - - E" 6 0 - x > 4 0 - 0 v c) .- .- CI - g 20- 0- S. Ahmed, S. Kasztelan and J. B. Moffat - :o co I h 3 - b 8 2 - ? v E: 0 .- E s 0 0- 1 I - A P 27 Fig. 5. Effect of addition of dichloromethane (DCM) or tetrachloromethane (TCM) to CH, feedstream over HPMo/Si02.(A, P refer to absence and presence, respectively, of chloro- additive). ( a ) Additive DCM, ( b ) - ( d ) additive TCM, ( a ) - ( c ) 20% loading of HPMo, ( d ) 10% loading. Reaction temperature 450 "C. F = 60 cm3 min-'. ( a ) w = 1.05 g, CH4/ N 2 0 = 1, DC M 0.30 mol YO ; ( b ) w = 1.0 g, C H,/ N20 = 4, TC M 0.12 rnol % ; ( c) w = 3 .O g, C H,/ N 2 0 = 4, TCM 0.17 mol YO; ( d ) w = l.Og, CH4/N20=4, TCM 0.17 rnol O/O. absence of the cations, the introduction of either lithium or caesium produces a reversal of the selectivities and again an approach to that expected with the cations themselves. In contrast, with barium and magnesium as the added cations the selectivities to CO and to COz are altered but the relative order is preserved.Both from the figures and other data (not shown) it is evident that poisoning of the catalysts resulting from the introduction of the particular cation approaches completeness as the number of cations inserted reaches 3-4 per heteropoly anion. This strongly suggests that the cations replace the exchangeable protons, and further that the protons play a vital role in the catalytic process. The addition of small quantities of a chlorine-containing additive to the feed stream in the conversion of methane also results in significant changes of the conversion and selectivities (fig. 5-7). With silica-supported 12-molybdophosphoric acid the addition of small quantities (<0.3 rnol "/o) of either dichloromethane (DCM) or tetra- chloromethane (TCM) increases the conversion of methane but decreases the selectivity to formaldehyde, while the selectivity to carbon monoxide increases.However, a substantial increase in the overall yield of H2C0 occurs. The production of carbon dioxide remains low while small quantities of methyl chloride are produced. As observed in the absence of any chloro-additive, the conversion in the presence of TCM is at a maximum for ca. 20 wt "/o loading of HPMo on the silica support and decreases with either decrease or increase of the loading from that at which the maximum is observed. In contrast the selectivity to formaldehyde is highest at low loadings. The effect of increasing residence time with the chloro-additive is semi-quantitatively similar to that observed where the additive is not present, with the conversion increasing and the selectivities to formaldehyde and carbon monoxide decreasing and increasing, respec- tively.With times-on-stream up to 8 h and a reaction temperature of 375 "C the selec- tivities to H2C0 and CO decrease and increase, respectively, within the first hour (but28 100 h 8 80- - 0 60- x c) .- .? 4 0 - * 0 0 - 20- 0 - h =" 7 0 8 v C 0 v) Q) > 0 .- 0 - Mechanistic Rela tionships - - I - A P A P I-I MC F MC Fig. 6. Effect of addition of dichloromethane (DCM) or tetrachloromethane (TCM) to CH, feedstream over HPW/Si02. (A, P refer to absence and presence, respectively, of chloro-additive). ( a ) Additive DCM, ( b ) - ( d ) additive TCM. ( a ) , ( b ) , ( c ) Reaction temperature 450 "C, w = 2.0 g ; ( a ) , ( b ) , ( d ) F=60cm'min-'; ( a ) , ( b ) , ( d ) CH,/NzO=4; ( d ) W=3.Og, T=525"C; ( c ) F = 11 cm3min-', CH,/N20= 1; ( a ) DCM=0.30mol YO; ( b ) TCM=0.21 rnol %; ( c ) TCM= 0.38 rnol YO; ( d ) TCM = 0.17 rnol %.to a relatively minor extent) and subsequently remain virtually constant. Concomitantly the conversion increases and, for times greater than 1 h, changes very little. The rates of product formation are also enhanced when a chloro-additive is present. The selectivity to and the rate of formation of H,CO are maximum for a CH4/N20 ratio between 4 : 1 and 8 : 1, while the conversion decreases continuously as this ratio is increased from 1 : 8 to 54: 1. Not surprisingly, the conversion of methane increases with increase in reaction temperature while the selectivities to H2C0 and CO decrease and increase, respectively.With 12-tungstophosphoric acid as catalyst in the conversion of methane the introduc- tion of either DCM or TCM to the feed stream produces changes which are distinctly different from those observed with 12-molybdophosphoric acid (fig. 6 and 7). For brevity comments will be restricted to observations obtained with TCM. With increasing amounts of TCM the conversion increases and appears to be approaching a constant value. Concomitantly the selectivity to H2C0 suffers a precipitous drop, while the production of CH&l increases and both attain virtually constant values with 0.1 mol '/O TCM. Note that selectivities to CH3Cl as high as 90 mol '/O are attainable with 0.55 mol YO TCM in the feedstream. A maximum in selectivity to methyl chloride is reached at a 5 wt '/O loading of HPW on the silica support with a decrease in this selectivity for further increases in the loading.However, the conversion of methane increases from the small value obtained on the support itself to a constant value at a loading of 20-30 wt %. Increases in the residence times produce increases in the conversion of methane and selectivity to CO, while that to CH3Cl decreases. With increase in the CH4/N20 ratio the selectivities to CO and CH3Cl decrease and increase, respectively, each passing through a point of inflection at a CH,/N20 ratio of ca. 1 : 1. Experiments in which the catalysts were pretreated as per usual with an He-N,O-CCl, mixture followed by exposure of the catalysts to a flow of CH4-N20, but with CCl, absent, provided evidence for the retention of chlorine on the catalyst (fig.8). WithS. Ahmed, S. Kasztelan and J. B. Moflat 29 0.4 h 2 0.3 0 8 v C .- 0.2 5 > s 0. I 0.1 0.2 0.3 0.4 0 . 5 TCM in feed (mol Yo) Fig. 7. Selectivity and conversion for various concentrations of TCM in the conversion of methane on HPW/Si02 ( w = 2 . O g , F=60cm3min-', T=45O0C, CH4/NzO=4). A, CO; 0, CO,; 0, H2CO; D, CH,CI; +, CH, conversion. both catalysts the conversions are initially relatively high but decrease to the values expected in the absence of the chloro-additive. The selectivities are initially similar to those expected when the chloro-additive is present in the feedstream but rapidly shift to those found when the chloro-additive is absent. It is interesting to note that the production of methyl chloride on HPW continues for ca.3 h. Although not shown in the figure, the rate of production of methyl chloride on HPW in the absence of CCl, from the feed is similar to that found in its presence. Discussion As noted in the introduction, earlier PAS FTIR studies from this laboratory have shown that with for example HPW, methanol interacts with the Brflnsted-acid sites to form protonated methanol (fig. 9), whose characteristic bands are identified in the infrared Since methanol is capable of diffusing into the bulk structure of HPW, all protons will, in principle, be capable of interaction with methan01.'~ Since the protons are coulombically bound to the terminal oxygen atoms of the heteropoly anion, the protonated methanol is presumably localized in this environment.While the protonated methanol exists at room temperatures and slightly above, a temperature of 150°C, for example, is sufficient to produce a C-0 bond scission to form water and methyl cations, which are evidently bound to the oxygen atoms of the heteropoly anions14 (fig. 10). Further heating produces oligomerization products which are believed to result through a carbene process.30 70 Mechanistic Relationships - - A Fig. 8. Selectivity to products and conversion of methane in the absence of chloro-additive in the feedstream after pretreatment of the catalyst with tetrachloromethane (TCM). Open symbols: 20 wt O/, HPMo/Si02. Pretreatment in helium/N20: 1 : 1 with 0.12% TCM for 1 h. Reaction temperature 450 "C, catalyst 1.05 g, flow 60 cm3 min-', CH,/N20 = 1 : 1.Filled symbols: 20 wt YO HPW/Si02. Pretreatment in helium/N20: 1 : 1 with 0.35 mol YO TCM for 2 h. Reaction temperature 450 "C, Catalyst 2.0 g, flow 1 1 cm3 min-', CH,/N20 = 1 : 1. A, A, CO; 0, 0, H2CO; V, CH,CI; 0, +, conversion. \ / \ / H' \ / \ 'H + Fig. 9. Protonated methanol. Exchange of the protons by larger cations in the silica-supported HPMo has the principal effect of reducing the activity of the catalyst in the methane-conversion process. Temperature-programmed desorption, exchange and reduction experiment^'^'^^ have shown that, at temperatures between 400 and 600 "C the protons in the heteropoly acids are desorbed as water apparently through a process which strips oxygen atoms from the heteropoly anion: KUOH + KUOH ---* KUO+ KUD + H20.In so doing a relatively small portion of the total oxygen atoms in the heteropoly anionS. Ahmed, S. Kasztelan and J. B. Moflat 8-0 31 0 Fig. 10. A fragment of the methylated heteropoly anion. are replaced by vacancies. The substitution of the larger cations for the protons effectively eliminates the vacancy production process. The oxidant N20 is believed to be capable of supplying oxygen to the vacant sites by a process of dissociative chemisorption and methyl cations or radicals will interact with these oxygen sites to produce oxygenated products. It is evident from the data that while the vacancy elimination process is undoubtedly the principal result of the cation exchange, other subsidiary processes are also operative.Since the results show a dependence on the nature as well as the number of cations, an electronic factor which may increase the electron density on the heteropoly anion is apparently also operative. This may favour the formation of active charged oxygen species (0") through the dissociation of N20: KUO + N20 -+ KUO" + N2 where O* may be 0- l7 or 02-,18 as suggested by various workers. Further, it is expected that the cations themselves may possess an activity in the conversion process. The effects of addition of carbon tetrachloride to the feedstream in the conversion of methane are strikingly different with the two silica-supported catalysts, HPMo and HPW. In the former case only a small quantity of methyl chloride is formed, while the conversion of methane is increased and the selectivities to CO and H2C0 are increased and decreased, respectively.In sharp contrast with HPW the conversion of methane and the selectivity to CH3C1 are both increased, while those to CO and H2C0 are decreased. It should be recalled that earlier work from this laboratory has shown that HPW is active and selective in the conversion of methanol to hydrocarbons (C > l), whereas with HPMo methanol is largely converted to deep oxidation products. Furthermore, the conversion of methane on HPW is a factor of 200 smaller than that on HPMo, although the selectivities to H2C0 are similar. Extended Huckel calculations have shown that the protons in HPW are more mobile and hence more acidic than those in HPMo, while the anionic oxygen atoms in HPW are more tightly bound than those in HPMo.19 The observations that hydrocarbons are formed from methanol on HPW while oxidation products are obtained on HPh!fo appears to be consistent with the results and interpretations of these calculations. It has been pointed out2' that the activation of the C-H bonds in methane becomes more difficult as the surface oxygen species become more stable.The lower conversion of methane on HPW, where the oxygen is more tightly bound, than observed with HPMo, where the oxygen atoms are more labile, is consistent with the aforementioned generalization. The effects of introduction of carbon tetrachloride provide some interesting insights into the mechanism for the conversion of methane. It is well known2' that alkanes react32 Mechanistic Relationships with carbon tetrachloride in the gas phase by a free-radical chain mechanism: R‘+CCl, ---* RCl+’CCl, RH + ‘CC1, + R’ + HCC1,.While the possibility of this purely gas-phase process cannot be entirely dismissed, other mechanisms appear to be more plausible in the present system. The observations that the products in the presence of the two catalysts HPMo and HPW are significantly different provides evidence for the direct involvement of the solid phase. The observation that the process takes place for a finite time in the absence of the chloroadditive subsequent to the pretreatment of the catalyst in the presence of tetrachloromethane provides further evidence for the direct participation of the catalyst. The near absence of chlorinated products other than CH3Cl (and HCl) rules out the dominance of a purely gas-phase process.Jt is of relevance to note that in the conversion of methanol on HPW at 350°C the dominant products are C2 and higher hydrocarbons, while at a temperature of 400°C methane is the major p r ~ d u c t . ~ As noted earlier, spectroscopic studies have shown that the oxygen atoms of the heteropoly anion are methylated in the conversion of methan01.’~”~ In the conversion of methane with HPMo the more labile oxygen atoms occupying the vacancies created by the loss of water can be removed to form oxygenated products, in contrast with HPW where the oxygen atoms are more tightly bound. In the presence of a chloro-additive, chlorine is evidently incorporated on the catalyst, although the nature of the binding is at this time unknown.Methyl species, possibly in radical form, may interact with the incorporated chlorine to produce methyl chloride. With HPW the chlorine is apparently less tightly bound and can be relatively easily removed from the surface by interacting methyl species. The financial support of the Canadian Natural Sciences and Engineering Research Council is gratefully acknowledged. References 1 M. T. Pope, Hereropoly and Zsopoly Oxomeralures (Springer-Verlag, Berlin, 1983). 2 C. D. Chang, Catal. Rev. Sci. Eng., 1985, 26, 323. 3 See e.g., G. J. Hutchings, L. J. van Rensburg, W. Pick1 and R. Hunter, J. Chem. SOC., Faraday Trans. 4 R. Pitchai and K. Klier, Card Rev. Sci. Eng., 1986, 28, 13. 5 N. R. Foster, Appl. Card., 1985, 19, 1. 6 J. S. Lee and S. T. Oyama, Card. Rev. Sci. Eng., 1988, 30, 249. 7 H. Hayashi and J. B. Moffat, J. Card., 1982,77,473; 1983,81,61; 1983,83, 192; in Catalytic Conversion of Synthesis Gas and Afcohols ro Chemicals, ed. R. G. Herman (Plenum Press, New York, 1984), p. 395. 8 S. Kasztelan and J. B. Moffat, J. Caral., 1987, 106, 512; 1988, 109, 206; 1988, 112, 54. 9 S. Kasztelan and J. B. Moffat, J. Chem. Soc., Chem. Commun., 1987, 1663. I , 1988, 84, 1311. 10 S. Kasztelan, E. Payen and J. B. Moffat, J. Card., 1988, 112, 320. 11 J. B. Moffat, in Methane Conversion, A Symposium on the Production of Fuels and Chemicals from 12 S. Ahmed and J. B. Moffat, Appl. Card., 1988, 40, 101. 13 J. G. Highfield and J. B. Moffat, J. C a r d , 1985, 95, 108. 14 J. G. Highfield and J. B. Moffat, J. Card., 1986, 98, 245. 15 B. K. Hodnett and J. B. Moffat, J. Card., 1984, 88, 253. 16 B. K. Hodnett and J. B. Moffat, J. Caral., 1985, 91, 93. 17 L. Mendelovici and J. H. Lunsford, J. Card., 1985, 94, 37. 18 R. EIAmrany, Y. Barbaux and J. P. Bonnelle, C a r d Today, 1987, 1, 147. 19 J. B. Moffat, J. Mol. Caral., 1984, 26, 385. 20 H. H. Kung, Ind. Eng. Chem. Prod. Res. Dev., 1986, 25, 171. 21 J. A. Dawari, S. Davis, P. S. Engel, B. C. Gilbert and D. Griller, J. Am. Chem. Soc., 1985, 107, 4721. Natural Gas (Elsevier, Amsterdam, 1988). Paper 8/04492K; Received 14rh December, 1988