Energy & Fuels 1994,8, 463-469
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Oxidative Coupling of Methane on Salts of Magnesium Doped by Alkali Carbonates Shigeru Sugiyamat and John B. Moffat’ Department of Chemistry and Guelph- Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada Received August 2, 1993. Revised Manuscript Received October 26, 1 9 9 P
The oxidative coupling of methane has been studied at 775 OC on MgO, MgSO4, and Mgs(POJ2 doped by carbonates of Li, Na, K, Rb, and Cs. The conversion and selectivity on the sulfate and phosphate in the presence and absence of tetrachloromethane (TCM) as a gas-phase additive were mainly dependent on the nature of the magnesium compounds but not on that of the solid-phase dopants, while those on the oxide were influenced by the nature of the alkali metals. The influence of TCM on the methane conversion process appears to result primarily from the interaction of TCM with the anions associated with the alkaline earth compounds.
Introduction In the oxidative coupling of methane, alkaline earth oxides doped by alkali metals (M) have been used and, particularly with Li/MgOld and Na/CaO1s2l4are reported to be active and selective for this reaction. In these systems, it has been suggested that since alkaline earth metal cations have radii similar to those of alkali metal cations, the former cations can be replaced by the latter to form active species as M+O- . l s 4 However, yields of C2 hydrocarbons less than those found with either of the aforementioned two catalysts were observed on Na/MgO and K/MgO, on which M+O-was not d e t e ~ t e dalthough ,~~~ these two catalysts have been reported to be active for the coupling reaction.21k7 The advantageous effect of the continuous addition of a small partial pressure of tetrachloromethane (TCM) into the methane conversion feedstream was first reported in 1988 in a publication from our laborator9 on the use of metal-oxygen cluster compounds as heterogeneous catalysts for this process. Since that time the effect of TCM has been examined in our laboratory with a variety of silica-supported and unsupported catalysts, including alkali metals, alkaline earths, sulfates, phosphates, and 1a11thanides.S~~The results of studies of the role of f Present addrese: Department of Chemical Science and Technology, University of Tokushima, Minamijosanjima, Tokushima 770,Japan. * To whom correspondence should be addressed. *Abstract published in Aduance ACS Abstracts, January 1, 1994. (1)Lunsford, J. H. Catal. Today 1990,6,235. (2)Amenomiya,Y.;Bhs,V. I,;Goledzinowski,M.;Galuszka, J.;Sanger, A. R. Catal. Reu.-Sci. Eng. 1990,32,163. (3)Lin, C-H.; Ito, T.; Wang, J-X.; Lunsford, J. H. J.Am. Chem. SOC. 1987, 109, 4808. (4)Lin, C-H.; Wang, J-X.; Lunsford, J. H. J. Catal. 1988, 111, 302. (5)Moriyama, T.;Takasaki, N.; Iwamatau, E. Chem. Lett. 1986,1165. (6) Kimble, J. B.; Kolta, J. H. Energy h o g . 1986,6,226. (7)Kimble. J. B.: Kolta, J. H. CHEMTECH 1987,501. (8) Ahmed;S.; Moffat, J. B. Catal. Lett. 1988,I , i41. (9)Ahmed, S.;Moffat, J. B. J. Phys. Chem. 1989,93,2542. (10)Ahmed, 5.;Moffat, J. B. Catal. Lett. 1989,2, 309. (11)Ahmed, S.;Moffat, J. B. Catal. Lett. 1989,118, 281. (12)Ahmed, S.,Moffat, J. B. J. Catal. 1990,121, 408. (13)Ahmed,S.;Moffat, J. B. Appl. Catal. 1990,58,83. (14)Ahmed, S.;Moffat, J. B. AppI. Catal. 1990,63,129. (15)Ahmed, 9.; Moffat, J. B. J. Catal. 1990,125,64. (16)Ahmed, S.;Moffat, J. B. Stud. Surf. Sci. Catal. 1991,61,57. (17)Ohno,T.; Moffat, J. B. Catal. Lett. 1991,9,23. (18)Sugiyama, 5.;Moffat, J. B. Catal. Lett. 1992, 13, 143.
chlorine in the partialoxidation of methane have also been reported by Burch and c o - w o r k e r ~ .OxyhalidesN ~~ and halides2’ have also been employed as catalysts in the methane conversion process. Most recently, Khan and Ruckenstein have reported the long-term effect of TCM addition on zirconia-supported catalystemHowever, there have been relatively few studies of the effect of the addition of TCM to the methane feedstream on catalysts composed of two active components such as Li/MgO. Thus, it is important to examine the effect of the addition of TCM on the coupling reaction over active supported catalysts such as Li/MgO in order to understand the nature and source of the effects resulting from the presence of the chlorine-containing species. In a previous paper,ll from this laboratory,= anion effects on the oxidative coupling of methane over three salts of lanthanum, the oxide, phosphate, and sulfate, both in the absence and presence of TCM in the feedstream have been shown to have a significant effect on both the conversion of methane and selectivity to CZ+hydrocarbons. These results suggest that the nature of the anion has considerable influence on the interactions between TCM and the catalyst. In our recent ~ a p e r s , ~we 8 *have ~ ~ also shown that the catalytic activity for the coupling reaction on alkaline earth18 and alkali metalu sulfates correlated with the electronegativity of each cation both in the presence and absence of TCM. Therefore, the study of catalysts containing combinations of an alkali metal and an alkaline earth and the effects of the addition of TCM should provide valuable data both on the effect of the (19)Ohno, T.; Moffat, J. B. Catal. Lett. 1992,16, 181. (20)Sugiyama, S.;Mataumura, Y.; Moffat, J. B. J. Catal. 1993,139, 338. (21)Ohno, T.; Moffat, J. B. Appl. Catal. 1993,93, 141. (22)Mataumura, Y.; Sugiyama, S.;Moffat, J.B. In Catalytic Selective Oxidation; Oyama, S. T., Hightower, J. W., Eds.; American Chemical Society, Washington, 1993; p 326. (23)Sugiyama, 5.;Moffat, J. B. Energy Fuels 1993,7, 279. (24)Sugiyama, S.;Satomi, K.; Hayashi, H.; Shigemoto, N.; Miyaura, K.; Moffat, J. B. Appl. Catal. A. Gen. 1993,103,55. (25)Burch, R.;Chalker, S.; Hibble, S. J. Appl. Catal. 1998,96,289. (28)Williams, J.; Jones, R. H.; Kent, J.; Thomas, J. M.; Catal. Lett. 1989. 3. 247. (27)Fujimoto, K.; Hashimoto, S.;h a m i , K.; Omata, K.;Tominaga, H. Appl. Catal. 1989,50,223. (28)Khan, A. Z.,Ruckenetein, E. J. Catal. 1993,139,304.
0SS7-0624/94/2508-0463$04.5~/0 0 1994 American Chemical Society
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464 Energy &Fuels, Vol. 8, No. 2, 1994
nature of the anion and the introduction of the chloromethane. In the present work, the results of comparative reaction studies, both in the absence and presence of TCM, on MgO, MgS04, and Mgs(P04)~doped by a series of carbonates of the alkali metals Li, Na, K, Rb, and Cs are described. Experimental Section AllreagentsMgO(99.99%,Aldrich),MgSO4(99.99%,Aldrich), MgS(PO4)2.8Hz0(Aldrich),LizCOa (99.15% ,Baker),NazCOs.Hz0 (ACSgrade, BDH),NalSO4 (ACSgrade, BDH),KzCOs (100.2%, Baker), RbzCOS (99.9+%,Aldrich), and CSZCOS (99.9%,Alfa) were high-purity materials and were used as received. The magnesium-containingcatalysts doped by alkali metals were prepared by adding an alkali carbonate or sulfate to an aqueous solution of the magnesium salt followed by evaporation of the water with vigorousstirring until a thick paste was formed. The paste waa dried at 110 OC for 2 h and waa then calcined at 775 OC for 3 h. The loading of the catalyst is expressed aa weight percent alkali metal against anhydrous magnesium salt. For convenience, each doped catalyst will be denoted as M/MgO, M/MgSO4, and M/Mgs(PO4)2, respectively. Undoped MgO, MgSO4, and Mgs(PO4)z were treated in the same manner, but without the addition of alkali carbonate or sulfate. The surface areas of MgO, MgSO4, and Mgs(P0h were 30.7,0.5, and 7.4 mZ/g, respectively. The catalytic experiment waa performed in a fixed-bedcontinuous flow reactor operated under atmospheric pressure. The reactor consisted of a 9 or 7 mm i.d. and 35 mm long quartz tube sealed at each end to 4 mm i.d. quartz tubes to produce a total length of 20 cm. The catalyst (W = 1.4 or 0.7 g) waa held in place in the enlarged portion of the reactor by two quartz wool plugs. In those experiments in which TCM waa added, the additive waa introduced to the main flow of C&, 0 2 , and diluent He by saturating a separate stream of He with TCM at 0 OC. In all experiments, the temperature of the catalyst was raised to 775 OC while maintaining a continuous flow of He and waa then conditioned at this temperature under a 25 or 12.5 mL/min flow of 0 2 for W = 1.4or 0.7g, respectively, for 1 h. The reactor waa then purged with He before introduction of the reactant (total flow rate; 30 mL/min for W = 1.4g or 16 mL/min for W = 0.7 9). The reactants and products were analyzed with an on-stream HP 5880 TCD-GC equipped with an integrator. Twocolumns,one Porapak T (18f t X 1/8in.) the other Molecular Sieve 5A (35cm X 1/8in.) were employed in the analyses. The conversionsand selectivities were calculated on the basis of the amount of reaction products formed aa determined by the GC analysis. Blank experiments conducted with CHI absent from the feed (02 + TCM + He) indicated that TCM undergoes oxidation producing CO and/or COZ. The data reported were corrected by running duplicate experiments with CIb absent under otherwise identical sets of process variables.
MgO
M93 [POL 2
M9S04
Figure 1. Conversionsand selectivities on Mg salts doped by Na2COs and Na&04. Conditions: C)4 215 Torr, 0 2 30.4 Torr, and TCM 1.3Torr when present. Reaction temperature 775 "C. Symbols: A, TCM absent; P, TCM present; Na(C), NazCOs; Na(S), Na2SO4. Columns: (Top) blank + hatched part, C& conversion;hatched part, CZ+yield in column of C K conversion + CSspecies; and Cs+yield. (Lower middle) filled part, CH*H blank part, CHs-CHs; and hatched part, CHpCH2 in column of CZ+selectivity. (Bottom) filled part, HCHO + CHsC1; blank part, CO2; and hatched part, CO in column of C1 selectivity.
n P
Results a n d Discussion Preliminary experiments using NazCO3 and NazSO4 aa a dopant were carried out, since it waa reported that the selectivity to CZ+hydrocarbon on CaO/MgA1~04doped by NazS04 waa higher ( 7 7 6 % ) than that (48-60% on the catalyst doped by Na~C03.l~As shown in Figure 1, selectivities to CZ+compounds on 1% Na/Mgs(PO,)z and 1% Na/MgSO4 were little affected by the anion in the dopant both in the presence and absence of TCM while the CZ+selectivity in the presence of TCM on 1%Na/ MgO doped by NaZSOr was lower than that in the absence of TCM. Since any advantage of the use of the sulfate as a dopant was not found with the present catalysts, carbonates were employed for the introduction of alkali metals. As is generally known, magnesia doped by alkali metals is subject to relatively rapid deactivation. In the present
"
Cs Figure 2. Conversions and selectivities on 1% M/MgO. Conditions, symbols, and columns as in Figure 1. MgO
Li
Na
K
Rb
work, conversions and selectivities a t 0.6 h on-stream were compared in order to compare the maximum activities on each catalyst. The principal products on each alkali-doped magnesium salt were CO, COZ,CZHI,and CZ&. C3 species, CZHZ.HCHO and CHaCl, the latter in the presence of
Methane Coupling on Mg Salts Doped by Alkali Salts
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Energy & Fuels, Vol. 8, No. 2,1994 466
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20r
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Figure 4. Conversions and selectivities on 1 % M/Mga(POd)z. Conditions, symbols, and columns as in Figure 1. TCM in the feed, were also detected in small amounts. Water was produced but is not reported here. Figures 2-4 compare conversions and selectivities in the absence and presence of TCM at 776 OC on 1% M/MgO, 1%M/MgSO4, and 1% M/Mg3(P04)2,respectively, with those on undoped catalysts. On 15% M/MgO, 0 2 conversion was more than 99% (not shown) while on MgSO4, M/MgSO4, MgdPOd2, Li/MgdPO4h and Na/MgdPO4)2 the conversion of 0 2 was considerably smaller. In the absence of TCM, methane conversion on MgO, MgSO4,
No Content
Figure 6. Effecte of loading on Li/MgSO, (I) and Na/MgSO4 (11). Conditions, symbols, and columns as in Figure 1.
and Mga(POd2 follows the order MgO > Mgs(P0412 > MgSOr while the selectivities to C2H4 and C2H8 are in the order MgSO4 > MgO > MgdPOdz. These orders are
Sugiyama and Moffat
466 Energy &Fuels, Vol. 8, No. 2, 1994 (1) No/MqjtPOL)Z
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TCM Concentration /Torr
Figure 9. Effects of concentrationof TCM on 0.5%Li/MgSO4 (I) and 5% Na/MgSO1 (11). Conditions, symbols, and columns as in Figure 1.
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similar to those for the corresponding anions for the lanthanum salts La203, La2(SO4)3, and LaP04, on which methane conversion was correlated with the negative charge on the oxygen atoms of the solid, estimated by use
of the semiempirical electronegativity equalization method.23t24In the presence of TCM, the enhancements of the conversions of methane and selectivities to ethylene followed the order MgSO4 > Mg3(P04)2> MgO and MgS04 > Mg3(P04)2 1 MgO, respectively. Therefore, the advantageous effects of the interaction of TCM appear to be optimized with those catalysts containing sulphate anions. As shown in Figure 2, the conversions of methane on 1%M/MgO and undoped MgO in the absence of TCM generally fall in the range between 10.5 and 13.1% with the exception of 1% Li/MgO and 1%K/MgO, for which values of 18.9 and 3.3 % , respectively, were obtained. As expected, relatively high values for the conversion of methane and selectivity to C1 compounds are found with 1%Li/MgO. It should be mentioned that the conversion on 1%K/MgO was quite different from that on other alkali-doped MgO catalysts. The conversion of methane and selectivity to ethylene and ethane on 1% Li, Na, and Cs/MgO were not improved, while those for WMgO and Rb/MgO were increased in the presence of TCM, particularly with the latter catalyst. Similar enhancements of the conversion and selectivity such as those observed with 1%Rb/MgO were observed on MgO doped by a small amount of Li or Na (Figure 5 and see below). Since TCM had little or no enhancement effect with Li/MgO, active oxygen species, 0-, on the catalyst14, appear to interact very little with TCM. On 1% M/MgS04, in the absence of TCM (Figure 3), the conversionsof methane and C2 selectivities where alkali metals have been introduced are similar to those found on MgSO4 itself, with the exception of the somewhat lower selectivity found with 1% K/MgS04. On each 1%
Energy & Fuels, Vol. 8, No. 2, 1994 467
Methane Coupling on Mg Salts Doped by Alkali Salts
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M/MgSOr, relatively high selectivity to HCHO was observed, although no HCHO was detected on MgSO4 where alkali metals were not present. The addition of TCM, produced 2-4-fold increases in the conversion of CH4 with MgS04 and M/MgS04 (Figure 3) as well as increases in the C2+ selectivities. As expected from our previous paper1*, a large amount of CH3C1was formed on undoped and doped MgSO4. On 1% M/Mg3(P04)2in the absence of TCM (Figure 41, the selectivities to ethylene and ethane on Na, K, and Rb/Mgs(POr)zwere factors of 2-3 higher than those found with undoped Mg3(P04)2, with methane conversion between 7.3 and 11.0%. However, the C2+ selectivities on the phosphates doped by Li2CO3 and Cs2CO3, which melt at alower temperature than the reaction temperature (775 "C), were improved by only 25-50% to approximately similar C2+ selectivities of 32.4 and 36.7 % ,respectively, in comparison with that on undoped Mg3(P04)2 (23.8%1. It is noteworthy that the influence of the melting point of each dopant (Li2CO3 720 "C; Na2C03 850 "C, K2C03 901 "C, RbzCOs 837 "C; and Cs2CO3 610 "C, respectively) is evident on the doped phosphates, but not on the oxides and sulfates. Although the increase in the conversion of methane resulting from the addition of TCM with the doped Mg3(P04)2 catalysts was substantially larger than that found for Mgs(P04)2 itself, the selectivities to C2+ hydrocarbons on the doped catalysts in the presence of TCM were generally similar to those on the undoped catalyst. Since the catalysts Li/MgO, Na/MgO, Li/MgSO4, Na/ MgSO4, Na/Mg3(P04)2,and K/Mg3(P04)2produced higher selectivities to C2+compounds and conversionsof methane and were relatively stable in the series of the doped catalysts these catalytic systems were selected for more
0
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conversion;0,C H d H 2 selectivity;W, C H d H s selectivity;A, CO selectivity; and A, CO2 selectivity. detailed examination. The effects of loading on the doped magnesium salts in the absence and in the presence of TCM are summarized in Figures 5-7. On Li/MgO (Figure 5 (part I)), the patterns of the improvement by TCM may be classified into two groups. Selectivities to ethylene and conversion of methane were increased to some extent on addition of TCM a t loadings as low as 0.5%. However, little or no changes were observed with TCM when the loading exceeded 1% . Similar trends were observed on Na/MgO (Figure 5 (part 11)). These resulta suggest that a discontinuous change in the nature of the MgO catalyst occurs as the quantity of alkali metal is increased. It appears that, with MgO containing relatively small quantities of either Li or Na, TCM interacts with MgO, while for larger quantities of the alkali metal, TCM is unable to penetrate to the oxide. On Li/MgSO4 (Figure 6 (part I)), the conversion of methane both in the presence and absence of TCM showed a maximum at a loading of 0.5%, but the ratios of the conversion and C2+ yield in the absence of TCM to those in the presence of TCM a t each loading showed similar values. The differing natures of MgO and MgSO4 thus appear to alter the effect of TCM. However, the C2Hs selectivity was increased with increased loading in the presence of TCM, although the selectivity to C2 hydrocarbons did not depend on the loading. In contrast with the observations for Li/MgSOr, selectivities both in the absence and presence of TCM on Na/MgS04 did not depend on the loading (Figure 6 (part 11)). With all loadings studied for Na/MgSOr, the addition of TCM produced favorable increases in the conversionof methane but the maximum in the latter, either with or without TCM, now appeared at a loading of 5 % .
Sugiyama and Moffat
468 Energy & Fuels, Vol. 8, No. 2, 1994 r
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Somewhat unexpectedly, for loadings from 0.5 to 9% with both Na/Mg3(P04)2 and K/MgdP04)2, little or no changes in the conversion and selectivities were observed either in the presence or absence of TCM. Although the improvement in selectivities to CZhydrocarbons in this loading range on addition of TCM was minor, the conversions were increased by factors of 3 or more. The effect of TCM on the reaction is evidently strongly influenced by Mg3(P04)2but relatively little by the alkali metal. As expected from the results of the effects of loading, the concentration of the added TCM had relatively little effect on the reaction behaviour over 1 % Li/MgO and 5% Na/MgO (Figure 8 (parts I and 11)). On 0.5% Li/MgSO4 and 5% Na/MgSO4, the trends in CZ selectivities with increasing concentration of TCM were similar, that is, the selectivity to ethylene increased with increasing TCM concentration (Figure 9 (parts I and 11)). The present improvement patterns of selectivities to CZspecies on both catalysts corresponded to those observed previously with La(S04)3.23From these observations it can be tentatively concluded that the influence of TCM on the methane conversionprocess is primarily the result of ita interaction with the anions, although it is evident that the role of the cation in the conversion process cannot be dismissed.These conclusions are also supported by the results with 1% Na/Mg3(P04)2and 1%K/Mg3(P04)~ which show that the alkali metal has a relatively minor effect on the influence of TCM. It should also be noted that increase in the partial pressure of TCM above 1.3 Torr produces relatively little enhancement in the conversion and selectivity. Finally, the stabilities of doped catalysts in the presence of TCM (2.6 Torr) were examined by using catalysts with approximately optimum loading. A significant deactiva-
0
1
2 3 4 Time on Stream /hr
5
6
Figure 13. Effects of time on-stream on 1 % Na/Mga(P04)2(I) and 1 % K/MgdPOr)z in the presence of TCM. Conditions and symbols as in Figure 11.
tion on 1%Li/MgO and 5% Na/MgO (Figure 11 (parts I and 11))was observed after approximately 3 h on-stream. On the contrary, 0.5% Li/MgS04, 5% Na/MgSO4, 1 % Na/Mg3(P04)2,and 1 % K/Mg3(P04)~ were comparatively stable up to 6 h on-stream (Figures 12 and 13). The dissimilar results observed for 5% Na/MgO, 5% Na/MgS04, and 1%Na/Mg3(P04)~on addition of TCM, although a common alkali metal was contained in each catalyst, strongly suggest that the enhancement of conversion and selectivity produced by the addition of TCM results from the interaction of TCM with the alkaline earth compound, as opposed to the alkali metal. As noted earlier in this report, there is also evidence that the beneficial interaction occurs between TCM and the anion of the alkaline earth compound. However, this is not to suggest that the alkaline earth and alkali metal play no role in the catalysis of the conversion process, but rather that some surface species such as an oxychloride may be formed between the oxygen atoms of the anion and the chlorine atoms of the TCM. The formation of such oxychlorides has previously been shown to have beneficial effects in the methane conversion process.20~22 Conclusions 1. The nature of the anion in the dopant has little or no effect on the methane conversion process in either the presence or absence of TCM. 2. In the absence of TCM, the conversion of methane follows the order MgO > Mgs(P04)~> MgSO4 while the C2+ selectivities decrease in the order MgSO4 > MgO > Mg3(P04)2. 3. The conversion of methane correlates with the negative charge on the oxygen atoms of the anions of magnesium.
Methane Coupling on Mg Salts Doped by Alkali Salts 4. In the presence of TCM the previously indicated orders are reversed: for methane conversion, MgSO4 > Mg3(P04)2> MgO; for CZ+selectivity, MgSOr > Mg3(P04)2 1MgO. The most advantageous effects are obtained with the sulphate. 5. In the presence of TCM, the conversion of methane is increased by as much as a factor of 4 and selectivities by a factor of 2, depending on the composition of the catalyst. 6. The advantageous effect of TCM, seen with many catalysts, is dependent on both the nature and concentration of the dopant. With MgO doped with 1% K or 1% Rb the effect is positive but nonexistent with Li, Na, or Cs. However, reduction in the concentration of Li introduces the enhancement effect with TCM. The larger
Energy & Fuels, Vol. 8, No. 2, 1994 469
quantities of the dopant may block the TCM from interacting with MgO. 7. Significant selectivities to formaldehyde are observed with 1%M/MgS04. 8. Doping with the alkali metals substantially increases the C2 selectivities obtained with Mg3(P04)2. 9. The effect of the introduction of TCM appears to be primarily the result of interaction with the anions of the alkaline earth compound.
Acknowledgment. The authors thank the Ministry of Education, Scienceand Culture, Japanese Government, for a Grant-in-Aid for Scientific Research No. 04750791 to S.S.and the financial support of the Natural Sciences and Engineering Research Council of Canada.
