Energy & Fuels 2001, 15, 671-674
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Methylation of Toluene with Methane over ZSM-5 Catalysts Moses O. Adebajo, Russell F. Howe,* and Mervyn A. Long School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia Received September 22, 2000. Revised Manuscript Received February 1, 2001
The methylation of toluene with methane in the presence of oxygen over zeolite catalysts is shown to be complicated by extensive disproportionation, hydrodealkylation, and hydrocracking reactions. However, it appears that these undesired reactions can be minimized, to enhance the methylation reaction, by using zeolites with reduced Bro¨nsted acidity.
1. Introduction The catalytic activation of methane by conversion to higher hydrocarbons including aromatic products is a process of considerable practical importance due to its potential for liquefaction of natural gas. A possible new route for methane conversion that has been investigated in recent times is the methylation of aromatic compounds with methane. For example, the direct methylation of naphthalene, toluene, and phenol with methane over various metal-substituted aluminophosphate molecular sieves in a high-pressure batch reactor has been demonstrated.1,2 More recent investigations3,4 have demonstrated, too, the incorporation of methane in the presence of Cu-β, H-β, and CuZSM-5 into a variety of organic substrates (e.g., benzene, toluene, phenol) which model petroleum, coal, and liquefaction residue structures. The methylation of benzene with methane over zeolite catalysts in a high-pressure flow reactor has been investigated, but in this case, however, no evidence was found for incorporation of methane into the observed reaction products.5 Our recent study of the methylation of benzene with methane at 400 °C over ZSM-5 catalysts in a highpressure batch reactor6 showed clearly that oxygen was required for the production of the methylated products. This implied a two-step mechanism involving the intermediate formation of methanol by (homogeneous) partial oxidation of methane followed by the methylation of benzene with methanol. Further support of this mechanism has been reported in a more recent publica* Corresponding author. Phone: 61-2-9385-4683. Fax: 61-2-93856141. E-mail:
[email protected]. (1) He, S. J. X.; Long, M. A.; Attalla, M. I.; Wilson, M. A. Energy Fuels 1992, 6, 498-502. (2) Long, M. A.; He, S. J. X.; Attalla, M. I.; Wilson, M. A.; Smith, D. R. In Stud. Surf. Sci. Catal.; Curry-Hyde, H. E., Howe, R. F., Eds.; Elsevier: Amsterdam, 1994; Vol. 81, pp 509-514. (3) Pang, L. S. K.; Wilson, M. A.; Quezada, R. A.; Prochazka, J. L.; Long, M. A.; He, S. J. X.; Gorbaty, M. L.; Maa, P. S. Fuel 1997, 76, 1091-1104. (4) He, S. J. X.; Long, M. A.; Wilson, M. A.; Gorbaty, M. L.; Maa, P. S. Energy Fuels 1995, 9, 616-619. (5) Kennedy, E. M.; Lonyi, F.; Ballinger, T. H.; Rosynek, M. P.; Lunsford, J. H. Energy Fuels 1994, 8, 846-850. (6) Adebajo, M. O.; Long, M. A.; Howe, R. F. Res. Chem. Intermed. 2000, 26, 185-191.
tion.7 We have therefore further investigated the toluene reaction with methane over ZSM-5 catalysts in a highpressure batch reactor containing residual air, to determine the contribution of direct methylation (via methanol) to the observed reaction products. 2. Experimental Section The catalysts used in this study were two commercial samples of HZSM-5 (PQ Corporation, SiO2/Al2O3 ) 35 and 140). The Na-exchanged catalyst which was also used was prepared by conventional ion exchange into the HZSM-5 sample having SiO2/Al2O3 ) 35 with aqueous sodium nitrate. The Co-, Mn-, and Cu-exchanged zeolites were then prepared from the NaZSM-5 produced by ion exchange with appropriate aqueous metal salts. All the reactions were carried out at 400 °C in a 71 mL Parr reactor fitted with a glass liner. In each experiment, 55 mg of catalyst and 570 µL of toluene were placed in the reactor which was then charged with 6.9 MPa chemically pure (> 99.0% pure) methane without removing the residual air present in the reactor. The details of the experimental procedures are similar to those reported recently.6 The details of the GC and GC/MS analysis are similar to those described in our recent paper.7 The measurement of the elemental compositions of the zeolite catalysts using the ICPAES technique and the characterization of zeolite acidities using FTIR spectra of adsorbed pyridine have also been described elsewhere.7,8
3. Results and Discussion 3.1. Characterization of Catalysts. Table 1 summarizes the results of the chemical analysis of the zeolite catalysts obtained from ICP-AES technique. It should be noted that CuZSM-5 is shown to have more than 100% copper exchange level and the lowest Na/Al ratio among the metal-exchanged catalysts. Similar observations have also been previously reported for CuZSM-59 and Cu-Mordenite,10 and this excessive ex(7) Adebajo, M. O.; Howe, R. F.; Long, M. A. Catal. Today 2000, 63, 471-478. (8) Adebajo, M. O. Ph.D. Thesis, University of New South Wales, Sydney, Australia, 1999. (9) Budi, P. Ph.D. Thesis, University of New South Wales, Sydney, Australia, 1997. (10) Kuroda, Y.; Kotani, A.; Maeda, H.; Moriwaki, H.; Morimoto, T.; Nagao, M. J. Chem. Soc., Faraday Trans. 1992, 88, 1583-1590.
10.1021/ef0002086 CCC: $20.00 © 2001 American Chemical Society Published on Web 04/07/2001
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Table 1. Characterization of ZSM-5 Zeolite Catalysts acid sites from FTIR of pyridine catalysts
Na/Al
HZSM-5 NaZSM-5 CoZSM-5 MnZSM-5 CuZSM-5
0.011 0.41 0.080 0.21 0.034
Ma/Al
Bro¨nsted sites (uc)-1
Lewis sites (uc)-1
0.28 0.28 0.71
4.2 2.5 1.5 1.4 0.6
1.0 2.3 2.2 2.4 1.5
a M ) transition metal exchanged into HZSM-5 (i.e., Mn, Co, and Cu).
Figure 1. FTIR spectra of pyridine adsorbed on fresh ZSM-5 catalysts at 100 °C in the region 1400-1700 cm-1.
change of copper has been attributed to the hydrolysis of copper ions leading to the formation of polynuclear complexes of the type [Cux(OH)y](2x-y)+ species during the ion exchange process.11,12 Figure 1 shows FTIR spectra in the region 1400-1700 cm-1 after pyridine adsorption on fresh ZSM-5 samples at 100 °C. Figure 1 shows well-resolved bands at about 1544, 1490, and 1453 cm-1. The bands at 1544 and 1453 cm-1 are assigned to pyridine adsorbed to Bro¨nsted (protonic) acid sites and to Lewis acid sites (coordinatively unsaturated Al3+), respectively, while the band at 1490 cm-1 is attributed to the adsorbed pyridine species on both Bro¨nsted and Lewis acid sites. The Bro¨nsted and Lewis acid concentrations were deter(11) Iwamoto, M.; Yahiro, H.; Mine, Y.; Kagawa, S. Chem. Lett. 1989, 213-216. (12) Kuroda, Y.; Kotani, A.; Uemura, A.; Yoshikawa, Y.; Morimoto, T. J. Chem. Soc., Chem. Commun. 1989, 1631-1632.
mined by measurement of the peak areas of the 1544 and 1453 cm-1 bands, respectively. Correction for wafer thickness was made by dividing the peak areas of these bands by the areas of the zeolite framework overtone bands at about 2000 cm-1. Corrections for absorptivity values13 were applied and it was assumed that the total acid concentration of the parent HZSM-5 was equal to the aluminum content of the zeolite. The calculated Bro¨nsted and Lewis acid concentrations of the zeolite samples per unit cell are presented in Table 1. Table 1 shows clearly that the order of increasing metal exchange level obtained by ICP-AES chemical analysis is exactly the order of decreasing Bro¨nsted acid concentration of the fresh catalysts. There is therefore a good agreement between the results of ICP-AES chemical analysis and the FTIR data. The reduction in the Bro¨nsted acid sites is reasonable as these sites are replaced by the metal ions. The metal ion exchange did not, however, totally remove Bro¨nsted acidity. The FTIR data presented in both Figure 1 and Table 1 also indicate that the decrease in Bro¨nsted acidity by increased metal ion exchange was accompanied by an increase in Lewis acidity and a decrease in total acid concentration. The increase in the number of Lewis sites is attributed to the electron acceptor function of transition metal cations as additional Lewis sites in a general sense.14 The shift in the coordinatively bound pyridine band from 1453 cm-1 to about 1443 and 1449 cm-1 (Figure 1) for NaZSM-5 and MnZSM-5, respectively, may be attributed to the weaker interaction of pyridine with the Na+ and Mn2+ cations. This is in agreement with an earlier report15 that, for a given coordination, the strength of interaction of cations which also function as Lewis acid sites increases with increase in the formal charge/radius ratio of the cation. 29Si and 27Al MAS NMR spectra showed that the zeolites used in this work contained very low levels of extraframework octahedral aluminum in addition to the normal tetrahedral framework aluminum. 3.2. Catalytic Reaction of Toluene with Methane. All the ZSM-5 catalysts used showed activity for the batch reaction of methane with toluene in the presence of residual air at 400 °C and 6.9 MPa methane. The major reaction products were found to be benzene and xylenes. Smaller amounts of ethylbenzene, trimethylbenzene, and other higher aromatics were also produced. Representative data are presented in Table 2. The toluene conversion decreased in the order HZSM-5 (SiO2:Al2O3 ) 35) > NaZSM-5 ∼ MnZSM-5 ∼ CoZSM-5 > CuZSM-5 ∼ HZSM-5 (SiO2:Al2O3 ) 140). This order agrees approximately with that described above for decreasing concentration of Bro¨nsted acid sites. Figure 2 shows a plot of toluene conversion against Bro¨nsted acid site concentration (the Bro¨nsted acid site concentration for the HZSM-5 (SiO2:Al2O3 ) 140) sample was assumed to be equal to the total aluminum concentration). For the conversion of toluene in the presence of methane:air mixtures, there is an approximately linear correlation between toluene conversion and Bro¨nsted acid site concentration. (13) Dakta, J. J. Chem. Soc., Faraday Trans. 1 1981, 77, 28772881. (14) Rhee, K. H.; Rao, U. S.; Stencel, J. M.; Melson, G. A.; Crawford, J. E. Zeolites 1983, 3, 337-343. (15) Pohle, W.; Fink, P. Z. Phys. Chem. N. F. 1978, 109, 205-220.
Methylation of Toluene with CH4 over ZSM-5 Catalysts Table 2. Conversion of Toluene and Selectivity to Products for the Reaction of toluene with (a) Methane + Air, and (b) under Nitrogen Methane + Aira selectivity (%) catalyst
conversion (%)
benzene
xylenes
C9+ aromatics
CuZSM-5 MnZSM-5 CoZSM-5 NaZSM-5 HZSM-5b HZSM-5
8.42 14.9 14.8 15.0 7.41 33.0
33.8 52.1 40.9 46.2 37.5 54.3
17.5 33.5 42.1 34.9 23.0 39.5
44.9 11.4 13.3 15.4 36.0 3.95
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both pathways. A further complication is the formation of aromatic coke through acid-catalyzed coupling reactions.16 The selectivity data in Table 2 show that disproportionation of toluene is a significant pathway over all catalysts when reaction is carried out in the presence of methane-air mixtures, in that benzene is a major product of reaction. It is noteworthy that over the catalyst showing the highest toluene conversion (HZSM5, SiO2:Al2O3 ) 35) the yield of C9 and higher aromatics is extremely low.
Nitrogenc selectivity (%) catalyst
conversion (%)
benzene
xylenes
C9+ aromatics
CuZSM-5 MnZSM-5 CoZSM-5 NaZSM-5 HZSM-5b HZSM-5
4.48 16.0 17.6 3.11 4.66 36.0
85.9 60.2 63.8 72.7 76.2 58.9
12.1 35.7 33.6 22.7 19.5 38.1
0.78 2.97 1.70 2.62 2.46 2.13
a 400 °C, 6.9 MPa methane, 4 h reaction time, no removal of residual air. b SiO2/Al2O3 ratio for this catalyst is 140. c 400 °C, 6.9 MPa nitrogen, 4 h reaction time, no removal of residual air. (Same reaction conditions as for methane, except that methane is replaced with nitrogen.)
Figure 2. Plots of toluene conversion against Bro¨nsted acid concentration for reactions carried out in methane:air and in nitrogen.
The conversion of toluene can in principle occur through two different reaction pathways: methylation by methane (via methanol, as in the case of benzene) and disproportionation, as shown below. Both pathways produce xylene products, but only disproportionation forms benzene. The further methylation of xylenes will form C9 and higher aromatics, but such molecules may also be formed by disproportionation of xylenes. It is thus difficult to quantify the relative contributions of
In an attempt to separate the methylation and disproportionation pathways, we carried out the reaction of toluene over the same catalysts in the presence of nitrogen alone. Under these conditions, disproportionation is the only possible pathway. Figure 2 plots the conversion of toluene under nitrogen versus Bro¨nsted acid site concentration, and the corresponding selectivity data are given in Table 2. Over HZSM-5 (SiO2:Al2O3 ) 35) the toluene conversion and product selectivities under nitrogen are closely similar to the corresponding values obtained in the presence of methane:air mixtures, indicating that disproproportionation is the only reaction pathway operating for this catalyst. For CoZSM-5 and MnZSM-5, the toluene conversions in nitrogen and in methane-air are very similar, suggesting likewise that disproportionation is the major pathway followed, although over these catalysts the yields of C9 and higher aromatics are slightly enhanced in the presence of methane-air. The largest differences are seen in the case of NaZSM-5, where the toluene conversion is substantially lower and the selectivity to benzene substantially higher in nitrogen than in methane-air. There are also significant differences between the conversions and selectivities in nitrogen and in methane-air for the CuZSM-5 and HZSM-5 (SiO2:Al2O3 ) 150) catalysts. We conclude from these data that in the reaction of toluene with methane in the presence of air the relative contributions of the methylation and disproportionation reactions depends on the concentrations and strengths of acid sites in the zeolite catalysts employed. The particular contribution of disproportionation is reduced in catalysts containing fewer acid sites and alkali metal cations. In catalysts such as NaZSM-5, CuZSM-5, and HZSM-5 (SiO2:Al2O3 ) 150) methylation appears to play a significant part. Since the metal-substituted aluminophosphate catalysts employed in the study of toluene (16) Bibby, D. M.; Howe, R. F.; McLellan, G. D. Appl. Catal. 1992, 93, 1.
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methylation reported earlier2 do not contain strong acid sites, it is likely that the xylenes observed in that work result from the methylation reaction rather than disproportionation. Disproportionation must, however, always be considered as a possible reaction pathway in reactions of all aromatic compounds with methane
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under these conditions in the presence of catalysts containing acid sites. Acknowledgment. The funding of this work by the Australian Research Council is gratefully acknowledged. EF0002086
