Comment on “Efficient Conversion of Methane to Aromatics by Coupling Methylation Reaction”

Abstract

Liu et al. recently reported their results on coconversion of methane and methanol at 973 K over a typical methane dehydroaromatization (MDA) catalysts, Mo/HZSM-5. 1 In this work, the authors claimed that adding a small amount of methanol to a methane feed led to more than two times higher methane conversion, substantially higher xylene and toluene selectivities (i.e., combined ca. 80%, nearly an order of magnitude increase as compared to experiments without methanol), and improved catalyst stability to such an extent that no deactivation was observed during 60 h on stream. If reproducible, this result would be a significant achievement, because formation of coke in the MDA reaction has been considered inevitable hitherto. To support their experimental data, Liu et al. carried out a thermodynamic analysis, whose results were in good agreement with their experimental findings.

Reply to Comment on “Efficient Conversion of Methane to Aromatics by Coupling Methylation Reaction”

“…In summary, given the complexity of the reaction network, it is believed that the performance of methane–methanol cofeed reaction is highly dependent on the process conditions (such as CH 4 /CH 3 OH ratio, inlet temperature of CH 3 OH, conditions of inlet feed, and GHSV) and catalyst properties (such as preparation method, location of Mo species, and surface acid properties). For example, the catalyst employed in the work of Kosinov and Hensen deactivated much faster than ours, indicating that the catalyst properties are quite different from ours.…”

“…This result was also obtained by Iritani et al, as 13 CO isotopic labeling tracer studies showed that more than 80% of the carbon in a benzene molecule comes from 13 CO in the CO−CH 4 cofeeding reaction over silica-supported noble-metal catalysts under atmospheric pressures. 12 These results are in disagreement with the conclusion by Kosinov and Hensen et al 2 who claimed that the selectivity of aromatics was not affected by cofeed oxygenates.…”

“…We are very thankful to Dr. Kosinov and Dr. Hensen for their interest in our work on coupling of methane dehydroaromatization (MDA) and methanol methylation and their pertinent comments . In the commentary, they found that methanol readily decomposes to CO and H 2 over Mo/zeolites and concluded that the main aromatic products from MDA remained similar with and without methanol cofeed.…”

“…W e are very thankful to Dr. Kosinov and Dr. Hensen for their interest in our work on coupling of methane dehydroaromatization (MDA) and methanol methylation 1 and their pertinent comments. 2 In the commentary, they found that methanol readily decomposes to CO and H 2 over Mo/zeolites and concluded that the main aromatic products from MDA remained similar with and without methanol cofeed. Moreover, they cited several publications relevant to the influence of oxygenates on the MDA reaction, 3−10 and they concluded that the role of oxygenates in this reaction was merely to react with CH 4 and/or coke species to yield CO, which would improve catalyst stability due to the reduction of carbonaceous deposits, whereas the selectivity to aromatic products is independent of the presence of oxygenates in the feed.…”

“…These results suggest that methanol was more likely to react with benzene to produce methyl-substituted aromatics rather than directly convert to CO, CO 2 or hydrocarbons in this methane−methanol cofeed reaction. Furthermore, the thermodynamic analysis presented in the comment by Kosinov and Hensen et al 2 predicts that methanol can be completely converted over the whole temperature range of 300−1300 K. This result should have shocked anyone working in the field of catalytic conversion of methanol over zeolites as a bit extreme. After all, methanol conversion on acidic SAPO-34 zeolite (MTO) at substantially higher temperatures than 300 K is practiced on a multiple-milliontons scale today, and the industrial application of methanol conversion on HZSM-5 zeolite (MTG and MTA) is also proceeded at above 673 K. In addition, on the basis of the 13 Clabeled CH 3 OH solid-state 13 C CP/MAS NMR experiment in Figure 2c of our paper, 1 we observed either terminal methoxy species or strongly bonded 13 CH 3 OH residual on catalysts at a 13 C chemical shift of 51 ppm, 17 which proves that at least some of the methanol in our experiments was not decomposed even at 973 K in the MDA reaction.…”