Oxidative Benzene Methylation with Methane over MCM-41 and

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Oxidative Benzene Methylation with Methane over MCM-41 and Zeolite Catalysts: Effect of Framework Aluminum, SiO2/Al2O3 Ratio, and Zeolite Pore Structure Moses O. Adebajo* and Ray L. Frost Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia Received August 20, 2004. Revised Manuscript Received March 15, 2005

The effect of framework aluminum, SiO2/Al2O3 ratio, and zeolite pore structures on the oxidative methylation of benzene with methane in a high-pressure batch reactor have been studied using a range of MCM-41 samples and zeolites as catalysts. The presence of framework aluminum in MCM-41 has been shown to be important for providing the required Bro¨nsted acid sites for the reaction. Among the catalysts tested, HZSM-5, CoZSM-5, and H-beta were found to exhibit the best performance, while mordenite was found to be virtually inactive for the reaction. HY and NaY exhibited intermediate performance, but HY, due to its higher acidity, was more active than NaY. H-beta, being more acidic, showed slightly higher activity than the ZSM-5 catalysts due to extra contribution to methylated products formation by cracking of benzene without participation of methane. Differences in the catalytic activities and product selectivities of these catalysts have also been discussed in terms of the variations in the number of available Bro¨nsted acid sites, SiO2/Al2O3 ratios, and zeolite pore structures.

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. The methylation of aromatic compounds with methane is one possible new route for methane conversion that has been described recently in the literature. 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 reported.1,2 The incorporation of methane in the presence of Cu-beta, H-beta, and CuZSM-5 into various organic substrates (e.g., benzene, toluene, phenol), which model petroleum, coal, and liquefaction residue structures, has also been demonstrated.3,4 The methylation of benzene with methane over zeolite catalysts in both high-pressure flow reactor5 and batch reactor4 has also been published. However, the highpressure flow reactor study5 demonstrated, through the use of 13C-labeled methane, that the methyl groups in * Corresponding author. Tel.: 61-7-3864-2265. Fax: 61-7-3864-1804. 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.; 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, 10911104. (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.

the methylated aromatic products did not originate from the methane reactant. Our recent investigations6,7 have now shown clearly that the presence of oxygen is required for the production of the methylated products in the methylation of benzene with methane at 400 °C over ZSM-5 catalysts in a high-pressure batch reactor. It was also observed in these studies that when the batch reactor was flushed with oxygen in the absence of catalyst prior to adding methane, methanol was obtained as the major product (about 86% selectivity). On the other hand, methanol was not detected in the presence of the zeolite catalysts when oxygen was introduced into the reactor, indicating that all of the methanol, being more reactive than the initial methane reactant, was consumed in the reaction to form methylated products over the catalysts. Thus, the reaction was postulated to go via a two-step mechanism involving the intermediate formation of methanol by partial oxidation of methane followed by the methylation of benzene with methanol in the second step. That is, the two steps of the reaction are represented by eqs 1 and 2:

1 CH4 + O2 f CH3OH 2

(1)

CH3OH + C6H6 f C6H5CH3 + H2O

(2)

The overall reaction is thus given by eq 3: (6) Adebajo, M.; Long, M. A.; Howe, R. F. Res. Chem. Intermed. 2000, 26, 185-191. (7) Adebajo, M. O. Ph.D. Thesis, University of New South Wales, Sydney, Australia, 1999.

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1 C6H6 + CH4 + O2 f C6H5CH3 + H2O 2

Adebajo and Frost

(3)

Further support to this mechanism has also been reported in our later investigations.8,9 In one of these later investigations, the performance of the catalysts for the benzene methylation both with methanol (lowpressure flow reaction) and with methane in the presence of oxygen (high-pressure batch reaction) was found to show excellent correlation.8 In the other more recent investigation on the methylation of benzene with methane in a batch reactor,9 further evidence was provided for (i) the catalytic activity of the zeolite catalysts, (ii) the importance of the presence of methane, and (iii) the oxygen-limiting nature of the reaction. The results of the spectroscopic and powder X-ray diffraction of the various zeolite catalysts used for these benzene methylation reactions have also just been published.10 In this work, attempt has been made to extend the range of catalysts used for the oxidative methylation reaction to include mesoporous materials such as MCM41 of the M41S family of molecular sieves. These materials having pore sizes ranging from 1.5 to 10 nm were discovered some years ago by Kresge et al.11 and Beck et al.12 in the laboratories of Mobil Research and Development Corp. The M41S structures possess uniform, unidirectional pores synthesized via a liquid crystal templating procedure,12 whereby the pore size can be controlled by the choice of the organic template used in the crystallization mix. The main features of these materials are their large surface areas, their regular and narrow pore-size distribution, and moderate thermal stability that make them suitable for many catalytic applications. The wide pore-size range of these materials also makes them readily accessible to large molecules. Thus, they possess potential application for catalytic conversion of such large molecules. MCM-41 was therefore used in this study because of this potential catalytic application. MCM-41 possesses a uniform hexagonal array of unidirectional and hexagonally shaped pores embedded within a silica-based matrix. The role of aluminum in the oxidative methylation of benzene with methane was also investigated by using various samples of MCM-41 with and without aluminum in the framework. Such investigation was also conducted to confirm our earlier observation6,9 of the dependence of catalytic activity and product selectivity of zeolite catalysts for the oxidative benzene methylation reaction on the number of Bro¨nsted acid sites. Other zeolites, H and Na forms of zeolite Y and mordenite with different pore systems and acidities, were also used in the present work for the purpose of comparing their catalytic activities and product selectivities with those of ZSM-5 and H-beta used in our earlier investiga(8) Adebajo, M. O.; Howe, R. F.; Long, M. A. Catal. Today 2000, 63, 471-478. (9) Adebajo, M. O.; Long, M. A.; Frost, R. L. Catal. Commun. 2004, 5, 125-130. (10) Adebajo, M. O.; Long, M. A.; Frost, R. L. Spectrochim. Acta, Part A 2004, 60, 791-799. (11) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (12) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834-10843.

tions.6,9,13 Zeolite Y was chosen due to its low SiO2/Al2O3 ratio and relatively large 12-membered ring pore, while modernite was chosen to investigate the effect of its dual pore system consisting of both a 12-membered ring and an 8-membered ring on the catalytic performance. The effect of SiO2/Al2O3 ratio on the performance of HZSM-5 catalyst was also studied. The information that is provided by these investigations will be quite useful for further development of the catalysts for optimization of their activity and product selectivity in the oxidative benzene methylation reaction. 2. Experimental Section The MCM-41 samples used in the catalytic experiments were (1) pure silica MCM-41, (2) Cu-MCM-41 (Si/Cu ) 30), (3) Cu-exchanged Cu-MCM-41 (Si/Cu ) 30), (4) Cu-exchanged Al-MCM-41 (Si/Al ) 30), (5) Cu-Al-MCM-41 (Si/Al ) 20, Si/Cu ) 20), and (6) Al-MCM-41 (Si/Al ) 30). These materials were prepared using a procedure already described elsewhere.14 Commercial samples of HZSM-5 (PQ Corp., SiO2/ Al2O3 ) 35) and H-beta (PQ Corp., SiO2/Al2O3 ) 28) were also used. To prepare Co-exchanged ZSM-5, the HZSM-5 was first converted to NaZSM-5 by conventional ion exchange with aqueous sodium nitrate. The CoZSM-5 was then prepared from the NaZSM-5 produced by ion exchange with aqueous cobalt nitrate. Other zeolites used were commercial NaY (SiO2/Al2O3 ) 4.5; SiO2 ) 62.2, Al2O3 ) 23.6, Na2O ) 14.3 dry weight %) and mordenite (SiO2/Al2O3 ) 20.4; SiO2 ) 87.7, Al2O3 ) 7.3, Na2O ) 5.1 dry weight %). The HY used was prepared by converting the commercial NaY first to the ammonium form by ion exchange with aqueous ammonium nitrate solution followed by calcination in air at 500 °C for 6 h to remove ammonia. The ion-exchanged catalysts were then washed several times with deionized water and oven-dried overnight at 100 °C. A more detailed description of the preparation of these ion-exchanged zeolites has been reported elsewhere.7 All of the reactions were carried out at 400 °C and 6.9 MPa pressure (measured at room temperature) in a 71 mL Parr batch reactor fitted with a glass liner. In each experiment, 55 mg of catalyst and 465 µL of benzene were placed in the reactor, which was then charged with 6.9 MPa chemically pure (>99.0% pure CP grade) or ultrahigh purity (>99.7% pure UHP grade) methane. Other details of the experimental procedures for the catalytic experiments are similar to those published recently.6,13 The details of the GC and GC/MS analysis of the reaction products and the measurement of zeolite acidities using FTIR of adsorbed pyridine have been described elsewhere.7,8 The measurement of the elemental compositions of the zeolite catalysts using ICP-AES and XRF and characterization of the catalysts using 29Si and 27Al solid-state NMR have also been described in our recent paper.10

3. Results and Discussion 3.1. Performance of MCM-41 Catalysts. At the same batch reaction conditions of 400 °C and 6.9 MPa methane pressure in the presence 1 atm air, the same products that were observed previously on ZSM-5 catalysts6,9 were also obtained on all of the active MCM41 catalysts. Table 1 summarizes the results of the catalytic conversion of benzene by reaction with CP grade methane over all of the MCM-41 samples used. It should be recalled that we have previously demon(13) Adebajo, M. O.; Howe, R. F.; Long, M. A. Catal. Lett. 2001, 72, 221-224. (14) Budi, P. Ph.D. Thesis, University of New South Wales, Sydney, Australia, 1997.

Oxidative Benzene Methylation over MCM-41

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Table 1. Conversion of Benzene and Selectivity to Products for the Reaction of Benzene with Methane over MCM-41 Catalysts under Standard Conditionsa selectivity to products (%)

catalyst MCM-41 (pure silica) Al-MCM-41b Cu/Cu-MCM-41c Cu/Al-MCM-41d Cu-MCM-41e Cu-Al-MCM-41f a

30.

conversion (%) 0.11 2.53 0.17 2.05 0.14 1.34

toluene

ethylbenzene

xylenes

28.2 48.8 29.0 24.1 15.3

15.8

toluene/ ethylbenzene ratio

100 56.0 51.2 59.2 75.9 79.6

11.8 2.22

1.99 1.05 2.04 3.15 5.20

400 °C, 6.9 MPa CP grade methane, 4 h reaction time, no removal of residual air. Si/Al ) 30. Cu-exchanged Cu-MCM-41; Si/Cu ) Cu-exchanged Al-MCM-41, Si/Al ) 30. e Si/Cu ) 30. f Si/Al ) 20, Si/Cu )20. b

c

d

strated that the purity of methane used in the methylation reaction is critical in determining the conversion and product selectivities.13 It was observed in our previous results9,13 that when CP grade methane was used, the benzene conversion was about twice and the selectivity to toluene about one-half (with corresponding higher selectivity to ethylbenzene) the values obtained using UHP grade methane. This observation has been shown to be due to the presence of higher levels of ethylene and oxygen impurities present in the CP methane than in the UHP grade.13 Unfortunately, even when CP grade methane was used, the highest benzene conversions obtained using these materials were only about 1-2.5%, which are much lower than the values of 8-9% obtained in our earlier investigation using ZSM-5 catalysts under the same reaction conditions.9 Therefore, no further experiments were carried out using UHP methane. Nevertheless, it is significant to note that out of all of the MCM-41 samples used, only those containing aluminum, that is, Al-MCM-41 with Si/Al ) 30, Cu-exchanged Al-MCM-41 with Si/Al ) 30, and Cu-Al-MCM-41 with Si/Al ) 20 and Si/Cu ) 20, gave the highest conversions of 2.5%, 2.1%, and 1.3%, respectively. The siliceous (pure silica) MCM-41 and the other Cu-containing catalysts gave negligible conversions of e0.2%. Such low activities have also been previously observed for both silicalite15 and the coppercontaining MCM-4114 materials in the selective catalytic reduction (SCR) of nitrogen oxides (NOx) using propane and propylene, respectively, as the reducing agent. It should be noted that the MCM-41 samples used in this study were provided by Budi, another researcher in our research group, and they are the same samples used by this worker in his previous study of SCR of NOx with propane and propylene.14 Experimental data have been previously provided by Budi14 from the characterization of these samples using various techniques including XRD, ICP-AES, and FTIR of adsorbed pyridine. His characterization results indicated that copper was unable to substitute isomorphously into the silicon tetrahedral sites in the framework of the coppercontaining samples and that that there were no acid sites in these samples. Thus, Budi14 attributed the low activity of the copper-containing MCM-41 materials to the nonincorporation of copper into the framework tetrahedral sites and to the lack of acid sites in the materials. This worker also reported that there was no (15) Hamada, H.; Kintaichi, Y.; Sasaki, M.; Ito, T. Appl. Catal. 1990, 64, L1-L4.

proper incorporation of aluminum into these mesoporous MCM-41 molecular sieves and that only the aluminum-containing MCM-41 materials were observed to have both Bro¨nsted and Lewis acid sites and to possess medium acid strength. The much lower activity of the same MCM-41 samples used in this work than that obtained for ZSM-5 catalysts for the benzene methylation reaction can therefore also be attributed to the relatively lower number of acid sites resulting from lack of proper incorporation of copper and aluminum into the framework. However, the aluminumcontaining ones gave some activity because, as reported previously,14 FTIR spectra of adsorbed pyridine showed the presence of some acid sites in these Al-MCM-41 samples. Thus, useful information obtained from the use of these materials for the oxidative benzene methylation reaction is that the presence of aluminum in the framework of the catalysts is important for generating the Bro¨nsted acid sites that are necessary for the reaction to occur. This confirms our earlier observation6,8 that the function of the zeolite catalysts in the reaction is the provision of acid sites of moderate strength that catalyze the methylation of benzene with methane in the presence of oxygen. The inability of copper to substitute isomorphously into the silicon tetrahedral sites in the framework and the absence of acid sites are also reflected in the selectivity to products. Table 1 indicates that the experiments utilizing Al-MCM-41 and Cu-exchanged Al-MCM-41 samples exhibited approximately the same benzene conversion and product selectivity, giving about the same toluene/ethylbenzene ratio of 2. However, the only slightly active MCM-41 sample (Cu-Al-MCM-41) in which copper was attempted to be incorporated into the framework was observed to be more selective to toluene (∼80%) and less selective to ethylbenzene (∼15%) than the other more active aluminum-containing catalysts employed, giving a toluene/ethylbenzene ratio of about 5. It therefore appears that the lower is the number of acid sites, the smaller is the conversion and the higher is the selectivity to toluene, thus confirming our earlier observation.6,9 Although the results of the characterization of the MCM-41 samples used in this study were reported previously,14 the XRD patterns of the samples were again obtained in this work before and after the batch methylation reactions to determine the effect of the reaction on the crystallinity of the materials. The X-ray patterns of the MCM-41 catalysts remained unchanged

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Table 2. Benzene Conversion and Product Selectivity on Various Catalysts for the Reaction of Benzene with Methane under Standard Conditionsa selectivity to products (%)

catalyst

conversion (%)

H-beta HZSM-5b CoZSM-5 HY NaY Al-MCM-41c mordenite

5.53 4.17 4.36 3.87 2.21 1.26 0.16

HZSM-5 CoZSM-5 Al-MCM-41

9.32 8.14 2.53

toluene

ethylbenzene

Reactions Using UHP Methane 89.8 7.38 93.0 4.05 91.4 3.85 87.6 8.22 92.2 2.20

xylenes


2.80 2.90 4.75 4.18 5.65

12.2 23.0 23.7 10.7 41.9

100 Reactions Using CP Methane 65.7 17.4 48.1 31.5 56.0 28.2

15.5 18.5 15.8

3.78 1.53 1.99

a Reaction conditions as in Table 1. b 3 h reaction. c Benzene conversion over Al-MCM-41 estimated from the value obtained in Table 1 for reaction using CP methane on the basis of our earlier finding.13

Figure 1. XRD patterns for Al-MCM-41 before reaction (a) and after reaction (b).

after the reaction except that the relative intensities of the peaks decreased as observed for ZSM-5 and H-beta catalysts in our previous investigation.7,10 This is illustrated in Figure 1 for Al-MCM-41 with Si/Al ) 30.

This lowering of peak intensity has been attributed to X-ray shielding caused by absorption of carbonaceous material during reaction by the catalysts. A similar observation has been reported by Breck et al.16 on



Figure 2. XRD patterns for HZSM-5 before and after reaction.

Figure 3. XRD patterns for H-beta before and after reaction.

hydration of zeolites and by Long and co-workers1 for the methylation of naphthalene by methane over substituted aluminophosphate molecular sieves. The fact that the X-ray patterns of the MCM-41 samples remained unchanged after the methylation reaction, except for the intensity change, indicates that their crystalline structure remained intact after the reaction. 3.2. Characterization of Zeolite Catalysts. The results of characterization using ICP-AES, XRF, X-ray powder diffraction, FTIR of adsorbed pyridine, and solid state NMR of the various zeolite catalysts that are active for the oxidative benzene methylation with methane, including the samples used in this work, have been published previously.7,10,17 The characterization provided substantial information about the composition, structure, properties, and behavior of the zeolite catalysts before and after reaction. The XRD patterns of all zeolites catalysts remained unchanged after the reaction except for the decrease in relative intensities of the peaks, as observed for MCM-41 catalysts. This is illustrated in Figures 2 and 3 for HZSM-5 and H-beta, respectively. Thus, the crystalline structure of the zeolites remained intact after the reaction, in agreement with the earlier finding that the catalysts recovered (16) Breck, D. W.; Eversole, W. G.; Milton, R. M.; Reed, T. B.; Thomas, T. L. J. Am. Chem. Soc. 1956, 78, 5963-5972. (17) Adebajo, M. O.; Howe, R. F.; Long, M. A. Energy Fuels 2001, 15, 671-674.

Figure 4. FTIR spectra in the region 1400-1700 cm-1 of both fresh HZSM-5 and HZSM-5 used for the batch methane/ benzene reaction after pyridine addition.

from the reactor could be reused without loss of performance.10 The SiO2/Al2O3 ratios of ZSM-5 catalysts were measured using XRF to ascertain the accuracy of the manufacturer’s stated values. The measured values were found to be quite close to the values stated by the manufacturer. For example, an SiO2/Al2O3 ratio of 38.9 was obtained by elemental analysis for ZSM-5 with a manufacturer’s stated value of 35. Thus, the manufacturer’s stated values are simply used in this paper. The FTIR spectra of HZSM-5 before and after the methylation reaction in the region 1400-1700 cm-1 shown in Figure 4 illustrate the spectra of the zeolites after pyridine adsorption. Figure 4 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 sites (coordinatively unsaturated Al3+), respectively. The band at 1490 cm-1 is attributed to the adsorbed pyridine species on both Bro¨nsted and Lewis acid sites. The peak areas of the 1544 and 1453 cm-1 bands were used to determine the Bro¨nsted and Lewis acid concentrations, respectively. Our earlier paper10 had clearly shown that the order of increasing metal exchange level obtained by ICP-AES is exactly the order of decreasing Bro¨nsted acid concentration, thereby indicating a good agreement between the results of ICP-AES elemental analysis and the FTIR data. The reduction in the Bro¨nsted acid sites

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is reasonable as these sites are replaced by metal ions. However, FTIR data presented in our recent paper10 further showed that only a small amount of acid sites is actually necessary for a catalyst to be active because used catalysts containing highly reduced Bro¨nsted acidity were found to be reusable without loss of their activity. Nevertheless, our earlier work8 had also shown a linear correlation between the activity of the catalysts and the Bro¨nsted acid concentration, although the correlation could not be easily observed at temperatures higher than 250 °C due to increasing tendency for saturation of available acid sites at the higher temperatures. 29Si and 27Al magic angle spinning (MAS) NMR spectra presented in our earlier paper10 also showed all of the active zeolite catalysts to contain some extraframework octahedral aluminum in addition to the normal tetrahedral framework aluminum. However, the presence of this nonframework aluminum did not appear to have any significant effect on the crystallinity of the zeolites before and after the oxidative benzene methylation reaction. It was reported10 that there appeared also to be little or no dealumination of the zeolite catalysts during reaction because their catalytic performance was retained after use. 3.3. Performance of ZSM-5 and H-beta in Comparison with Other Zeolites. Table 2 shows the results of the oxidative benzene methylation experiments using commercial NaY with a low SiO2/Al2O3 ratio of 4.5, HY prepared from the commercial NaY by ion exchange, and mordenite with SiO2/Al2O3 ratio of 20.4. All of the reactions were conducted without removal of residual air at 400 °C and 6.9 MPa methane pressure measured at room temperature. Included in Table 2 are the conversion and product selectivity data obtained for HZSM-5, CoZSM-5, H-beta, and Al-MCM41 for comparison. Unfortunately, the data in Table 1 for MCM-41 samples used for reactions utilizing CP methane are the only data available for inclusion in Table 2. Therefore, the Al-MCM-41 that exhibited the highest conversion of 2.53% (Table 1) is the one included in this table. Thus, a benzene conversion of 1.26% was estimated for reaction involving the use of UHP methane for this Al-MCM-41 sample. The estimation is based on the assumption that the benzene conversion for reaction utilizing UHP methane is about one-half the conversion for the reaction conducted using the CP grade as reported previously.9,13 UHP grade methane was not used for the MCM-41 experiments due to the rather low conversion levels obtained even with the CP methane. Data are also provided in Table 2 for reactions utilizing CP grade methane over HZSM-5 and CoZSM-5 for a more realistic comparison with the data obtained for the Al-MCM-41 sample using the same type of methane. The data for the reactions using CP methane in Table 2 demonstrate the much higher conversion levels (about twice) than in the reactions using UHP methane. Al-MCM-41 is shown clearly in Table 2 to be much less active than all of the other catalysts except mordenite due to the rather low amount of acid sites present in the sample. HZSM-5, CoZSM-5, and H-beta catalysts are shown in Table 2 to exhibit the highest activity, although the H-beta is slightly more active than

Adebajo and Frost

H-ZSM-5 and CoZSM-5. The slightly higher activity of the H-beta may be attributed to our earlier finding that the mechanism of formation of methylated products over this catalyst involves a combination of both direct oxidative methylation via formation of methanol intermediate and cracking of benzene without participation of methane.7,13 The cracking of benzene on H-beta was possible because the catalyst was observed to be more acidic.9,10 The cracking of benzene pathway was found to be absent in the case of the less acidic ZSM-5 catalysts.7,13 Table 2 also shows that the toluene/ethylbenzene ratios for the reactions using CP methane are much lower that the values obtained for reactions using UHP methane. This observation can be attributed to the presence of trace amounts of ethylene in the CP methane that could easily react with the benzene to produce a relatively larger amount of ethylbenzene than in reactions utilizing UHP methane. Our earlier papers9,13 have demonstrated clearly that trace amounts of ethylene impurities in the methane used for the oxidative benzene methylation reaction over zeolite catalysts enhanced benzene conversions and selectivity to ethylbenzene. Thus, it was concluded that the purity of methane used in the reaction over zeolite catalysts is critical in determining conversions and product selectivities. It is not surprising that the NaY catalyst, being less acidic, was found to be much less active and more selective to toluene as compared to ethylbenzene (i.e., higher toluene/ethylbenzene ratio) than the ZSM-5 and H-beta catalysts. It should be noted too that the more acidic HY yielded higher benzene conversion and a greater proportion of ethylbenzene (i.e., lower toluene/ ethylbenzene) ratio than the parent NaY sample from which it was obtained by ion exchange. These observations therefore support the finding in section 3.1 that the higher is the number of acid sites, the higher is the conversion and the lower is the selectivity to toluene. In agreement with these observations, it has also been reported in our earlier investigation7 that an increase in the number of acid sites increases catalytic activity for the oxidative methylation reaction and increases the ability of the methanol intermediate to be converted to ethylene; the ethylene produced is then subsequently used to produce a greater proportion of ethylbenzene. It was, however, observed that the HY gave a lower toluene/ethylbenzene ratio of 10.7 comparable to the value of 12.2 obtained for H-beta. Again, this may be attributed to the larger 12-membered ring pore sizes of H-beta and HY zeolites than the 10-membered ring ZSM-5 catalysts. The larger pore sizes of both HY and H-beta favor the formation of more bulky intermediates such that toluene, the primary reaction product, is converted more easily to secondary reaction products leading to the lowering of the toluene/ethylbenzene ratio. Table 2 also indicates that mordenite was very inactive for the reaction and ethylbenzene and xylenes were not detected at the rather low conversion level obtained. In addition to the lack of acid sites in the sodium mordenite sample used in this work, the zeolite is also known to have two types of pores that are perpendicular to each other: 12-membered ring ellipti-



Table 3. Effect of SiO2/Al2O3 Ratio on Benzene Conversion and Product Selectivity for the Reaction of Benzene with Methane over HZSM-5 under Standard Conditionsa selectivity to products (%) reaction time and SiO2/Al2O3 ratio 2h 35 140 320 3h 35 4h 140 320 a

conversion (%)

toluene

ethylbenzene

xylenes


4.19 4.12 3.54

92.9 97.3 97.0

4.25 0.98 1.07

2.86 1.71 1.92

21.9 99.3 90.7

4.17

93.0

4.05

2.90

23.0

3.37 2.13

95.3 95.3

1.43 1.64

3.29 2.41

66.6 58.1

Reaction conditions as in Table 2, except the reaction was carried out at different reaction times.

cal pores (0.65 nm × 0.70 nm) and 8-membered ring pores (0.26 nm × 0.57 nm). It is therefore possible that a combination of lack of acid sites, diffusion limitations due to existence of the smaller 8-membered ring pores, and deactivation of catalytic sites by coke deposition resulting from cracking and secondary reactions at the larger elliptical pores might be responsible for the inactivity and low selectivity to higher aromatic products. It is worth mentioning that even H-mordenite has also been previously found to be completely inactive for the reaction of benzene with methane to produce toluene at 400 °C and 4.8 MPa in a flow reaction system.5 Our previous investigation on the methylation of benzene with methanol at 250-450 °C has also shown mordenite to be inactive for the reaction.8 3.4. Effect of SiO2/Al2O3 Ratio in HZSM-5 Catalysts. The SiO2/Al2O3 ratio is related to the number of Bro¨nsted acid sites, which has already been shown to influence the benzene conversion and product selectivity. The higher is the SiO2/Al2O3 ratio, the lower is the number of acid sites. Because HZSM-5 exhibited one of the highest benzene conversions, it was chosen to investigate more closely the effect of SiO2/Al2O3 ratio on the oxidative methylation reaction with UHP grade methane. The reactions were again carried out without removal of residual air at 400 °C and 6.9 MPa methane pressure. The results obtained are illustrated in Table 3. In the first instance, Table 3 shows that benzene conversion decreases with an increase in reaction time for the HZSM-5s with higher SiO2/Al2O3 ratios of 140 and 320, while there is no significant change in the conversion for the zeolite with a lower SiO2/Al2O3 ratio of 35. The leveling of, or decrease in, conversions with an increase in reaction time is usually caused by deactivation of catalysts, for example, by coke deposition. For instance, Smirniotis and Ruckenstein18 have previously observed that the activity of ZSM-5 and beta zeolites for the alkylation of benzene or toluene with methanol or C2H4 decreased with time on stream due to deactivation or blocking of active sites by coke formation. In general, coke formation has been found to occur during hydrocarbon transformations over highsilica acidic zeolites. A review on the formation, nature, and properties of coke in high-silica zeolites was previously published.19 It is clearly shown in this review that (18) Smirniotis, P. G.; Ruckenstein, E. Ind. Eng. Chem. Res. 1995, 34, 1517-1528. (19) Bibby, D. M.; Howe, R. F.; McLellan, G. D. Appl. Catal., A: General 1992, 93, 1-34.

deactivation of high-silica zeolites by coke deposition during methanol conversion usually causes a decrease in the number of acid sites and such a decrease in acid site concentration is more significant for zeolites containing more Al per unit cell (i.e., lower SiO2/Al2O3 ratio). We have previously observed a similar decrease in Bro¨nsted acid sites concentration in our earlier investigation of the oxidative methylation of benzene with methane over ZSM-5 and H-beta catalysts.7,10 However, it is unlikely that the lowering of benzene conversion with an increase in reaction time observed in this work for the higher silica HZSM-5s is due to catalyst poisoning by coke deposition, especially as this decrease is less significant for HZSM-5 with a lower SiO2/Al2O3 ratio of 35. Our earlier investigation has shown clearly that the ZSM-5 zeolites (SiO2/Al2O3 ratio ) 35) and H-beta (SiO2/Al2O3 ratio ) 28) that showed no significant change in benzene conversion and product selectivities on increasing the reaction time beyond 2 h could be reused without loss of performance.7,10 The leveling of the conversion was then attributed to the reaction being limited by the amount of oxygen in the reactor. Our subsequent work provided experimental data to support this oxygen-limiting nature of the reaction.9 The decrease in conversion with an increase in reaction time shown in Table 3 for the higher silica HZSM-5s can therefore also be attributed to this oxygenlimiting nature of the reaction. It is shown in Table 3 that there is a slight decrease in benzene conversion with increase in SiO2/Al2O3 ratio, especially after 4 h reaction. On the other hand, the selectivity to toluene is significantly enhanced with an increase in the SiO2/Al2O3 ratio to 140 and 320, resulting in much higher toluene/ethylbenzene ratios for the samples with lower number of acid sites. This observation is consistent with our earlier published report6,9 and the results obtained for the MCM-41 catalysts discussed in section 3.1 that indicate a decrease in catalytic activity and increase in selectivity to toluene with a decrease in the number of acid sites. This is reasonable because zeolites are usually used as acidic catalysts and have been observed to exhibit very high catalytic activity for major carbonium ion hydrocarbon transformations such as alkylation, cracking, and isomerization.20,21 Furthermore, it has also been reported22 that HZSM-5 has a high capability to generate light olefins (including (20) O’Connor, C. T.; Steen, E. V.; Dry, M. E. In Stud. Surf. Sci. Catal.; Chon, H., Woo, S. I., Park, S.-E., Eds.; Elsevier Science B.V.: Amsterdam, The Netherlands, 1996; Vol. 102, pp 323-362.

790


ethylene) from methanol, a reaction which is usually catalyzed by Bro¨nsted acid sites.23 The much higher toluene/ethylbenzene ratio exhibited by the samples with fewer acid sites may therefore be further attributed to reduced ability of methanol intermediate to be converted to light olefins, especially ethylene. The lower amount of ethylene produced would then have been subsequently used to produce a smaller proportion of ethylbenzene via ethylation of benzene. Table 3 also shows lower toluene/ethylbenzene ratios for the HZSM-5 with the higher SiO2/Al2O3 ratios at 4 h reaction time than the values obtained after 2 h reaction. This observation can also be attributed to increased ability, at the longer reaction time, for the methanol intermediate of the oxidative benzene methylation reaction to be converted to a secondary reaction product like ethylene. This would therefore have led to the ultimate small increase in the proportion of ethylbenzene produced from ethylation of benzene as compared to toluene. (21) Haag, W. O. In Stud. Surf. Sci. Catal.; Weitkamp, J., Karge, H. G., Pfeifer, H., Holderich, W., Eds.; Elsevier Science B.V.: Amsterdam, The Netherlands, 1994; Vol. 84, pp 1375-1394. (22) Hutchings, G. J.; Johnston, P.; Lee, D. F.; Warwick, A.; Williams, C. D.; Wilkinson, M. J. Catal. 1994, 147, 177-185. (23) Ono, Y. In Stud. Surf. Sci. Catal.; Imelik, B., Naccache, C., Taarit, Y. B., Vedrine, J. C., Coudurier, G., Praliaud, H., Eds.; Elsevier Science B.V.: Amsterdam, The Netherlands, 1980; Vol. 5, pp 19-27.

Adebajo and Frost

4. Conclusions In conclusion, it is evident from this study that even though the MCM-41 catalysts are not as active as the ZSM-5 and H-beta catalysts reported in our earlier investigation, useful information obtained from this study is that the presence of framework aluminum or copper in the MCM-41 catalysts is important for generating the Bro¨nsted acid sites that are necessary for the reaction to proceed. This provides further support to our earlier finding that the oxidative benzene methylation reaction is dependent on the availability of a moderate number of Bro¨nsted acid sites.6,8 Thus, the copper-containing samples with no aluminum were relatively inactive due to lack of acid sites and inability of copper to substitute isomorphously into the framework. It is also clear that the SiO2/Al2O3 ratio (and therefore the number of available Bro¨nsted acid sites) and the zeolite pore system (size and structure) play some important roles in determining the activities and product selectivities of the zeolite catalysts. Acknowledgment. The funding of this work by the Australian Research Council is gratefully acknowledged. We also express sincere thanks to Dr. P. Budi for providing the MCM-41 samples used in this study. EF049789F

Oxidative Benzene Methylation with Methane over MCM-41 and

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