Energy & Fuels 1995,9, 616-619
616
Methylation of Benzene by Methane-13Cover Zeolitic Catalysts at 400 "C Simon J. X. He and Mervyn A. Long* School of Chemistry, University of New South Wales, Sydney NSW 2052, Australia
Michael A. Wilson* CSIRO Division
of Petroleum Resources, PO Box 136, North Ryde NSW 2113, Australia
Martin L. Gorbaty* and Peter S. Maa Exxon Research and Engineering Co., Annandale, New Jersey 08801-0998 Received January 26, 1 9 9 P
An isotopic study utilizing methane-13C shows that the product toluene and xylenes formed from its reaction at high pressure (5.5 MPa, 400 "C) with benzene, over Cuheta and H-beta zeolites, contain substantially methyl-13C substituents. It is therefore concluded that direct methylation of benzene with methane is possible under these conditions in a batch reactor. However, a second mechanism can also be recognized in which methylbenzene is formed by fragmentation and rearrangement of benzene without addition of methane gas. thalene contained principally one 13C atom and the dimethylnaphthalene contained two 13C atoms per The question as to whether methane gas may be molecule. NMR spectroscopy of the products confirmed catalytically activated for direct methylation of hydrothat the 13C atoms were in the methyl substituent. It carbons at modest reaction conditions (-400 "C, -6 was therefore concluded that these experimental condiMPa) is an important one t o answer in exploring ways tions, in which a high pressure of methane was used to of utilizing methane gas t o give higher value products. drive the reaction, were capable of bringing about some This question is the subject of some recent ~ t u d i e s . l - ~ methylation by methane of aromatic compounds, such An investigation of the conversion of benzene to as naphthalene and phenol. Thermodynamic calculamethylated aromatic products over H-Y and H-beta tions have shown3 that the fraction conversion of the zeolites a t 400 "C, 4.8 MPa in a flow reactor has been aromatic reactant to methylated products at equilibrium reported recently by Lunsford and co-w0rkers.l A wide is expected to be of the order of 18-20% under these range of methylated aromatic hydrocarbon products high-pressure conditions in which methane is in large were observed, with toluene constituting about 50% of excess relative to the aromatic reactant. In the case of the product. Over H-beta zeolite, methane when co-fed benzene4 this is estimated t o be 25%. with benzene enhanced the yield of toluene over that In view of the work reported for benzene using H-beta observed when nitrogen or hydrogen were co-fed with zeolite1we believe it necessary to prove or disprove that the benzene. However, an isotopic tracer experiment our related Cuheta and H-beta catalysts incorporate with 13CH4failed t o show the presence of detectable 13C methane. Hence we now report a 13C isotopic study of in the methylated products. It was concluded that the conversion of benzene in the presence of highbenzene served as the sole source of carbon in all of the pressure methane in a batch reactor over a variety of observed products under those reaction conditions. zeolitic catalysts carried out to explore further whether In contrast t o this, our earlier work2 had shown that there is evidence for direct methylation of benzene by methylation of naphthalene and phenol at 400 "C, 6.9 methane. A batch reactor rather than a flow system MPa in a batch reactor with Cu and other metal allows for the carrying out of several reactions without substituted ALP0 catalysts, where 13CH4 was used, the consumption of large amounts of highly expensive yielded methylated products which clearly demon13CH4. strated that the carbon of the methyl substituents added to the aromatic nucleus was dominantly 13C. Thus, for Experimental Section example, the methylnaphthalene formed from naph-
Introduction
Abstract published in Advance ACS Abstracts, May 15, 1995. (1)Kennedy, E. M.; Lonyi, F.; Ballinger T. H.; Rosynek, M. P.; Lunsford, J. H. Energy Fuels 1994, 8, 846. (2) He, S. J. X.; Long, M. A,; Attalla, M. I.; Wison, M. A. Energy Fuels 1994, 8, 286. (3) He, S. J. X.; Long, M. A,; Attalla, M. I.; Wilson, M. A. Energy Fuels 1992, 6, 498. (4)Long, M. A,; He, S. J. X.; Attalla, M. I.; Wilson, M. A,; Smith, D. R. Stud. Surf. Sci. Catal. 1994, 81, 509. @
0887-062495l2509-0616$09.00/0
Catalysts used in this study included two copper-exchanged zeolites, CdZSM-5 (SUA1 = 48) and Cuheta (SUA1 = 1001, and H-beta zeolite (SUA1 = 12.5). The copper-exchanged zeolites were prepared by a similar procedure to that described e l ~ e w h e r e .They ~ were prepared by oven drying under vacuum a t 100 "C overnight. Benzene, dichloromethane, and deuterated dichloromethane were all AR grade and obtained from Aldrich. They were used without further treatment.
0 1995 American Chemical Society
Methylation of Benzene by Methane-13C Table 1. Products from Reaction of Methane with Benzene over Beta Zeolites selectivityb wt benzene toluene xylenes TMBC tvue (me) convn(8) (8) (8) (%) 0 0.8 0.0 0.0 0.0 CdZSM-5 66 52.6 47.4 0.0 0.5 100.0 0.0 0.0 Cubeta 65 0.1 0.5 81.5 Hmeta 66 0.0 0.0 0.5 79.6 10.5 0.0 0 CdZSM-5 66 0.9 55.9 44.1 0.0 Cubeta 5.2 43.0 50.4 66 6.6 Hheta 67 12.3 58.8 33.3 1.9 3.9 43.8 53.7 Cuheta 66 2.5 2.9 82.0 15.9 2.2 Cu/beta 66 4 Hheta 7.5 70.4 13.1 0.0 66
gasa expt press. time no. (MPai (hi 1 6.9-N 4 2 6.9-N 4 3 6.9-N 4 4 6.9-N 4 5 6.9 4 6 6.9 4 7 6.9 4 8 6.9 4 9 5.5 4 4 10 5.5 11 5.5
Energy &Fuels, Vol. 9, No. 4, 1995 617 Table 2. Relative Peak Intensities in Mass Spectral Analysis of Products
catalyst
".
I
a The N in the gas pressure column indicates the gas was nitrogen in place of methane. Experiments 1-9 were conducted with methane-%, experiments 10 and 11with methane-13C. The balance of the products was unknown high molecular weight compounds. TMB = trimethylbenzenes.
The methylation reactions were conducted in a 71 mL Parr autoclave with a glass liner. A typical experimental procedure involved placing the catalyst and benzene in the glass liner inside the autoclave and charging the autoclave with methane or nitrogen to a pressure of 5.5-6.9 MPa. The system was heated a t a rate of 3 "Cfmin to 400 "C and held a t this temperature for 4 h. Some experiments were also performed with Cubeta for 8 h but conversions were only slightly higher and selectivities similar. After the reaction, the autoclave was cooled in a chloroform slush bath and the pressure released. Products were recovered by dissolution in dichloromethane for analysis. In the methane-13C experiment, deuterated dichloromethane was used as the solvent. The reaction conditions of each experiment are shown in Table 1. Details of the gas chromatographic analysis and GCIMS apparatus have been described e l ~ e w h e r e . Product ~ were identified on the basis of retention time, library matching, and known ionization pattern^.^ The method for calculation of benzene conversion and product selectivity was similar to that used p r e v i o u ~ l y .Only ~ the products visible by GC or GCIMS were used for calculations, and the presence of coking reactions cannot be discounted. lH and 13C solution NMR spectra of the reaction products were recorded on a Bruker AMX300 (AC300P) i n ~ t r u m e n t . ~ Chemical shifts are reported downfield relative to tetramethylsilane.
Results and Discussion Blank runs in the absence of catalyst under nitrogen (experiment 1) or methane (experiment 5) were conducted. The results (Table 1)indicate that the benzene was relatively unreactive. In a high-pressure nitrogen atmosphere, only some unknown high molecular weight products were observed with a total benzene conversion of 0.8%. Replacing nitrogen with methane in the absence of catalyst led t o a low benzene conversion of about 0.5%, with toluene as the dominant product together with the unknown high molecular weight compounds. Further blank runs were carried out with nitrogen in the presence of a catalyst (experiments 2, 3, and 4). For all the three selected catalysts, benzene appeared to be intact and no substantial benzene conversion was observed ((0.5%). However, in the presence of both methane and a catalyst, enhancement of the benzene conversion was observed, the extent depending on' the type of catalyst. (5) Davies, I. L.; Bartle, K. D.; Williams, P. T.; Andrews, G . E. Anal. Chem. 1988,60, 204.
product toluene mle = 90 mfe = 91 mfe = 92 mle = 93 mle = 94 p-xylene mle = 91 mle = 92 mle = 93 mle = 105 mle = 106 mle = 107 mle = 108 mle = 109 o-xylene mle = 91 mfe = 92 mle = 93 mle = 105 mle = 106 mle = 107 mle = 108 mle = 109
expt 9 Cufbeta "CH4
expt 10 Culbeta 13CH4
expt 11 H-beta 13CH4
0.0 193.0 100.0 7.0 0.0
10.5 43.8 208.0 100.0 7.6
92.0 256.0 316.0 100.0
458.3 33.3 0.0 14.6 100.0 8.3 0.0 0.0
116.7 330.3 72.7 27.3 51.5 78.8 100.0 12.1
340.0 464.0 76.6 0.0 136.0 157.0 100.0 10.6
314.0 22.9 0.0 48.6 100.0 11.4 2.9 0.0
108.6 311.4 82.9 0.0 40.0 65.7 100.0 11.4
392.0 436.0 92.0 60.0 110.0 142.0 100.0 16.0
0.0
The Cuheta-zeolite (experiment 7) showed a high catalytic activity for the methylation of benzene and gave an overall benzene conversion of 5.2% with selectivity directed exclusively toward methylated products, viz., toluene, xylenes, and trimethylbenzenes (TMB). Wbeta-zeolite (experiment 8) promoted the benzene conversion to an even higher level of 12.3%. The major products were again methylated benzenes but with lower selectivity t o xylene or TMB in comparison with that using Cuheta-zeolite. CdZSM-5 (experiment 6) appeared t o have a much lower activity in comparison with the other two catalysts. Nevertheless, the yield of the methylated benzene products was about double that of the blank runs. For the purpose of distinguishing between direct methylation of benzene and possible disproportionation type reactions of benzene itself, which may yield methylbenzenes, experiments using methane-13C were carried out in the presence of the two more active catalysts, Cuheta-zeolite (experiment 10) and Wbetazeolite (experiment 11). Since a maximum pressure of only 5.5 MPa of 13CH4was attainable, an additional run using this pressure of unlabeled methane (experiment 9) was conducted. At the lower methane pressure of 5.5 MPa, lower benzene conversions were observed than those for the corresponding catalysts at 6.9 MPa, but these remained an order of magnitude higher than those observed in blank runs without catalyst or methane. In the two 13CH4experiments the benzene conversions were 2.9% and 7.5% over Cuheta-zeolite and Hheta-zeolite, respectively. In both cases, toluene appeared to be the dominant product (> 70%). Repeat reactions were performed and variations of less than 10% in the conversions and selectivities were observed. GC/MS analysis (Table 2) of the reaction products of experiments 9-11 clearly indicated that the product toluene increased in molecular weight by one mass unit on substitution of 13CH4 for 12CH4 showing that the additional C atom originated from the methane itself.
618 Energy & Fuels, Vol. 9, No. 4, 1995
He et al.
L
PI
-? N
\
1
140
120
100
80
60
40
20 PPm
Figure 1. 13C NMR spectra: (a) toluene in CDC13, (b) products from experiment 10 in CD2C12, (c) products from experiment 11 in CDC13.
Furthermore, p-xylene and o-xylene each increased in molecular weight by two mass units, consistent with the incorporation of two 13C atoms from methane. In addition, inverse-gated 13C NMR spectroscopy demonstrated (Figure 1) that the methyl groups in toluene from the I3C experiments contained, substantially, 13C. Figure l a shows the spectrum of normal unlabeled toluene for comparison. The resonance at 21.4 ppm is due to the CH3 group, and the four resonances at 125.3, 128.2, 129.0, and 137.8 ppm are due to the aromatic carbon nuclei. Figure 1, b and c, shows the spectra of the reaction mixtures of experiments 10 and 11,respectively. In each reaction product mixture, benzene is predominant and less than 7% toluene is present. Each spectrum shows a strong signal at 128.8 ppm due to primarily benzene. The signal at 125.3 ppm is due to the aromatic carbons of toluene alone and is very low in intensity because the relative amount of toluene is small. It is just visible in Figure lb. However, the signals due t o the methyl groups of toluene (21.4 ppm) are very strong in each of the spectra in Figure 1, b and c, and thus the isotope abundance in the methyl group of the small concentration of toluene present is at least an order of magnitude above that of the aromatic nucleus. That is, the 13C
revealed in the mass spectrum of the toluene is incorporated in the methyl group of the molecule. The reaction products of experiments 10 and 11were further analyzed by 'H NMR and the methyl regions of these spectra are shown in Figure 2. The 13C/12Cratio in the methyl groups in each sample can be estimated by comparing the J coupled methyl proton resonances of H-13C (satellite peaks on left and right) and the nonsplit methyl resonance of H-12C (center signal). From the intensities of these peaks it is estimated that 82% of the methyl carbon originated from methane-'% gas when Cuheta-zeolite was used as the catalyst (experiment 10). In the reaction with the more acidic H/beta-zeolite as a catalyst (experiment 111,13Cmethyl groups constituted 53% of the total methyls, showing again that the majority, but not all, of the carbon of these methyls originates from the methane gas itself.
Conclusions The above results show that, under these experimental conditions, toluene formed from benzene derives the majority of its methyl carbon from the methane gas present. Additional experiments showed that toluene does not exchange its methyl substituents with methane
Methylation of Benzene by Methane-13C
Figure 2. 'H NMR spectra of reaction products: upper spectrum, experiment 10; lower spectrum, experiment 11.
gas over these catalysts under equivalent conditions. It is thus concluded that direct methylation of benzene is a reality. It is accompanied by other reactions which lead to methylbenzenes in which the source of carbon must be the benzene itself as has been demonstrated by Lunsford and co-w0rkers.l It is not surprising that this accompanying reaction appears t o be favored by a
Energy & Fuels, Vol. 9,No. 4, 1995 619
more acidic zeolite catalyst, since acidity in general promotes cracking and coking of hydrocarbons. The observation of'some darkening of the catalysts during reaction indicates some coking in all systems. As t o the role of metal ions such as Cu, this is not clear, but we note a recent papel.6 that Cu/ZSM-5 shows autooxidation behavior. The distinction between the results of this study and those of Lunsford et al.' may lie in the acidity of the catalysts used or, perhaps more likely, in the residence time difference between the two systems. Since SYAl ratios were not quoted in the previous study,l it is not possible t o compare the relative acidity of the H-beta zeolites between the two investigations, but the relative reactivities of the various catalysts used in the previous study appear to reflect catalyst acidity. A major distinction between the two studies lies in the fact that one is a batch reactor study and the other a flow system, and hence the effective residence times were markedly different. It is impractical to carry out extensive isotope studies in a flow system where large amounts of isotopic reactant will be consumed, and where, in the presence of phenomena such as catalyst coking, the time of product gas sampling after onset of reaction may be critical. It is concluded that there is substantial evidence that direct methylation of benzene by methane at high pressure may be observed, as it was for the methylation of other aromatics, with appropriate catalysts and reaction conditions. Under nonideal conditions it may be obscured by accompanying degradation reactions of benzene leading t o a mixture of methylated products which may not result from reactions of the methane itself.
Acknowledgment. We thank Dr J. Brophy for GC/ MS analyses. The Australian Research Council is acknowledged for financial assistance. EF9500200 (6)Larsen, S. C.; Aylor, A.; Bell, A. T.; Reimer J. A. J.Phys. Chem. 1994,98, 1153.
