Correspondence/Rebuttal pubs.acs.org/acscatalysis
Reply to Comment on “Efficient Conversion of Methane to Aromatics by Coupling Methylation Reaction” Yi Liu, Tianyun Wang, Defu Li, and Yi Zhang* State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
ACS Catal. 2016, 6 (8), 5366−5370. DOI: 10.1021/acscatal.6b01362 ACS Catal. 2017, 7, DOI: 10.1021/acscatal.7b00665
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catalyst at prolonged times-on-stream. 13CO isotopic labeling tracer studies showed that 13C from 13CO is easily incorporated into benzene.11 They speculated that carbon [C] derived from CO is effectively hydrogenated to a common active carbon species CHx similar to those from methane, which is converted to aromatics such as benzene and naphthalene. This result was also obtained by Iritani et al., as 13CO isotopic labeling tracer studies showed that more than 80% of the carbon in a benzene molecule comes from 13CO in the CO−CH4 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. Our group was not the first to report the impact of methanol on methane conversion by cofeeding methanol with methane over zeolite-based catalysts. There are several articles that described methanol-assisted methane conversion for higher hydrocarbon over the bifunctional zeolite catalysts (e.g., Zn/ Mo/HZSM-5 or H-GaAl-ZSM-5).13−16 In these works, it was found that the selectivity of alkylated aromatic product was enhanced in the presence of methanol. For example, Pant et al.14 clearly reported the coconversion of methane with methanol over the Zn−Mo/H-ZSM-5 catalyst at three different temperatures, viz., 823, 873, and 923 K. They found that the main products of the reaction were light hydrocarbons and aromatics such as benzene, toluene, ethylbenzene, and xylene. In some cases, the selectivity of toluene, ethylbenzene, and xylene was 3 times higher than that of benzene. This promotional effect of methanol on the selectivity of alkylated aromatic in the methane−methanol cofeeding reaction system was also mentioned by Choudhary et al. over H-GaAl-ZSM-5 or Mo−Zn/H-ZSM-5 at 773−823 K.16 These literatures demonstrated methanol participated in methylation reactions and was not fully decomposed. As shown in Table S1 of our paper,1 the dominant products for MDA with methanol cofeed are methyl-substituted aromatics, whereas the selectivity of C1− C5 (mainly consist of CO, CO2, and light olefins or paraffins) was only 0.9%. What is more, the selectivity of CO was as low as 0.4%, indicating that methanol was not decomposed to CO in this methane−methanol cofeed reaction. However, a comparison experiment with methanol feed only under the same conditions showed that methanol is mainly converted to CO, CO2, CH4, and light hydrocarbons (total selectivity of
e are very thankful to Dr. Kosinov and Dr. Hensen for their interest in our work on coupling of methane dehydroaromatization (MDA) and methanol methylation1 and their pertinent comments.2 In the commentary, they found that methanol readily decomposes to CO and H2 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 CH4 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. It should be noted that these references (refs 3−6 in the comment of Hensen et al.) almost all focus on the research about the influence of oxygenates such as O2, H2O, CO, and CO2 on the MDA reaction to remove the formed coke. Furthermore, ref 10 in the comment of Hensen et al. is about the MTO reaction. Only the references reported by Bhan et al. (refs 7−9 in the comment of Hensen et al.) were related to the effects of coprocessing HCOOH, CH3COOH, CH3CH2OH, and CH3CHO with CH4 on the net rate of benzene and total hydrocarbon production. More importantly, unlike the studies in our paper,1 the catalyst in the works by Bhan et al.8−10 was prepared via solid-state ion exchange starting with MoO3 powder and commercially available NH4-ZSM-5 zeolite with much lower Si/Al (11.7) and different Mo loading. In addition, in their works, methanol or other oxygenate was introduced with CH4 into the carburized catalyst at much lower CH4 space velocity (722 mL(CH4)/gcat h) and only when the benzene formation rate reached a steady-state, typically after 0.3−3.0 h pure MDA reaction depending on the catalyst loading. Only then can they create a stratified bed configuration consisting of upstream oxygenate reforming and downstream CH4 pyrolysis.8−10 As the pure MDA reaction would be rapid deactivation by quick coke formation and migration of Mo species, the properties of catalyst would be significantly changed from the initial state. Therefore, we proposed that the different catalyst preparation method, operational procedure, and reaction conditions lead to the different results with our work. Furthermore, note that Kosinov and Hensen et al. cited two reports by Ichikawa,5,6 but they failed to cite another important work also by Ichikawa published in a different article.11 Ichikawa et al.11 reported that the addition of a few percent of CO and CO2 to methane feed significantly increases benzene selectivity and improves the stability of the Mo/HZSM-5 © XXXX American Chemical Society
Received: March 28, 2017
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DOI: 10.1021/acscatal.7b01013 ACS Catal. 2017, 7, 4488−4490
ACS Catalysis
Correspondence/Rebuttal
Figure 1. (a) Equilibrium amounts (moles), 1 bar, starting with 1 mol CH4 and allowing only H2 and benzene as components. (b) Equilibrium amounts (moles), 1 bar, starting with 1 mol CH4 and allowing C(s), H2, and benzene as components. Calculated using Aspen Plus software.
allowing only benzene and hydrogen in the mixture. Solid carbon (coke), C(s), is not allowed.18 The equilibrium conversion of methane at 973 K is about 12%, which is well agreed with almost all reports about MDA.18,19 However, if we added C(s) as a possible product, a corresponding equilibrium plot for this reaction shows that virtually no benzene is formed at equilibrium (Figure 2b). Thermodynamically, methane goes to solid carbon [C(s)] and hydrogen, as expected. The equilibrium conversion of methane at 973 K is as high as 81%, which is much higher than the actual MDA reaction (10− 12%). Generally, for the actual MDA reaction with Mo/ZSM-5 catalyst, methane was converted to 50% benzene, 25−30% coke, 10−20% naphthalene, and 5−10% C2 with a conversion of 10−12% at 973 K.19,20 Hence, as reported in the Supporting Information of our paper,1 the direct conversion of methane to methyl-substituted aromatics by coupling methanol methylation reaction is simplified as follows:
>45.8%), as shown in Table S1. Additionally, the selectivity of CO was 13.3%. These results suggest that methanol was more likely to react with benzene to produce methyl-substituted aromatics rather than directly convert to CO, CO2 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 13Clabeled CH3OH solid-state 13C CP/MAS NMR experiment in Figure 2c of our paper,1 we observed either terminal methoxy species or strongly bonded 13CH3OH 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. Meanwhile, the 13C-labeled CH3OH experiment clearly proved the aromatic products contained 13C at methyl group of alkyl-substituted aromatics as shown in Figure 2a in our article, which strongly proves that the benzene was effectively methylated by methanol in MDA reaction. In the section of thermodynamic analysis on the methane− methanol cofeed reaction system, they claim that CO and CO2 must be added in the products because there are 3 independent reactions. Based on this view, for conventional MDA reaction, the number of independent reactions should be 6, because there are at least 8 species (CH4, H2, C6H6, C7H8, C10H8, C, C2H4, and C2H6) and a rank of 2. However, in most thermodynamic analysis cases for MDA, researchers only allow H2 and C6H6 in the possible product in order to simplify the calculation procedure.18 As reported in the Supporting Information of our paper,1 the thermodynamic analysis was performed similarly to what was reported in literatures. The direct conversion of methane to benzene is as follows: 6CH4(g) → C6H6(g) + 9H 2(g)
6CH4(g) → C6H6(g) + 9H 2(g)
△Gr o = +433 kJ mol−1
△Hr o = + 531 kJ mol−1
C6H6(g) + CH3OH(g) → C7H8(g) + H 2O(g) △Gr o = −74 kJmol−1
△Hr o = − 74 kJ mol−1
Only toluene is allowed as the product of methanol methylation reaction, just as Spivey and Hutchings18 only allowed the benzene as product, not allowing toluene and naphthalene for conventional MDA reaction. We speculate that the differences in reaction conditions or catalysts might have led to the apparently inconsistent results. Considering the rich chemistries involving methane and methanol conversions on Mo/ZSM-5, the competitions among the various types of reactions are strongly influenced by the catalyst properties or catalyst modification21 and can also be altered by the process conditions. Indeed, in our previous study, we conducted an experiment with decreased CH4/ CH3OH ratio to 15/1. The results showed that large amount of light hydrocarbons and benzene were formed as a result of the coconversion of methane and methanol, and the product distribution was wide and unselective. Furthermore, the CH4/ CH3OH ratio in our present work was also determined by the thermodynamic analyses of our coupled reaction (Figure 2). We found that when we decreased the CH4/CH3OH ratio, the equilibrium conversion of methane increased slightly; however,
△Gr o = +433 kJ mol−1
△Hr o = + 531 kJ mol−1
Figure 1a shows the result of a calculation of the equilibrium composition of a mixture of an initial 1 mol of CH4 and 4489
DOI: 10.1021/acscatal.7b01013 ACS Catal. 2017, 7, 4488−4490
ACS Catalysis
Correspondence/Rebuttal
(4) Kosinov, N.; Coumans, F. J. A. G.; Uslamin, E.; Kapteijn, F.; Hensen, E. J. M. Angew. Chem., Int. Ed. 2016, 55, 15086−15090. (5) Liu, S.; Ohnishi, R.; Ichikawa, M. J. Catal. 2003, 220, 57−65. (6) Shu, Y.; Ohnishi, R.; Ichikawa, M. J. Catal. 2002, 206, 134−142. (7) Lacheen, H. S.; Iglesia, E. J. Catal. 2005, 230, 173−185. (8) Bedard, J.; Hong, D.; Bhan, A. J. Catal. 2013, 306, 58−67. (9) Bedard, J.; Hong, D.; Bhan, A. Phys. Chem. Chem. Phys. 2013, 15, 12173−12179. (10) Bedard, J.; Hong, D.; Bhan, A. RSC Adv. 2014, 4, 49446−49448. (11) Ohnishi, R.; Liu, S.; Dong, Q.; Wang, L.; Ichikawa, M. J. Catal. 1999, 182, 92−103. (12) Naito, S.; Karaki, T.; Iritani, T. Chem. Lett. 1997, 26, 877−878. (13) Wang, W.; Buchholz, A.; Seiler, M.; Hunger, M. J. Am. Chem. Soc. 2003, 125, 15260−15267. (14) Majhi, S.; Dalai, A. K.; Pant, K. K. J. Mol. Catal. A: Chem. 2015, 398, 368−375. (15) Majhi, S.; Mohanty, P.; Dalai, A. K.; Pant, K. K. Energy Technol. 2013, 1, 157−165. (16) Choudhary, V. R.; Mondal, K. C.; Mulla, S. A. R. Angew. Chem., Int. Ed. 2005, 44, 4381−4385. (17) Yu, S. M.; Wu, J. F.; Liu, C.; Liu, W.; Bai, S.; Huang, J.; Wang, W. Angew. Chem., Int. Ed. 2015, 54, 7363−7366. (18) Spivey, J. J.; Hutchings, G. Chem. Soc. Rev. 2014, 43, 792−803. (19) Tang, P.; Zhu, Q.; Wu, Z.; Ma, D. Energy Environ. Sci. 2014, 7, 2580−2591. (20) Xu, Y. D.; Bao, X. H.; Lin, L. W. J. Catal. 2003, 216, 386−395. (21) Tian, P.; Wei, Y.; Ye, M.; Liu, Z. ACS Catal. 2015, 5, 1922− 1938.
Figure 2. Equilibrium conversion with different CH4/CH3OH ratio in methane−methanol cofeed reaction, 1 bar, starting with 1 mol CH4 and allowing H2, H2O, benzene, and toluene as components. Calculated using Aspen Plus software.
the equilibrium conversion of methanol involved in the methylation reaction dropped rapidly. Hence, we proposed that the preferred route for excess methanol conversion is the MTH (methanol to hydrocarbons) reaction, leading to the formation of gasoline (C5−C12) or light hydrocarbons (such as ethylene or ethane), as reported by other researchers.14,16 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 CH4/CH3OH ratio, inlet temperature of CH3OH, 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 Hensen2 deactivated much faster than ours, indicating that the catalyst properties are quite different from ours. Our original publication made some interesting observations, but clearly it also raised many unanswered questions. However, given the unprecedented opportunities with shale gas conversion, we felt that it was very important to publish the work to attract more participation from the scientific community to the direct conversion of methane to aromatics. In this regard, we welcome the comments, though critical, from Drs. Kosinov and Hensen.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for Y.Z.:
[email protected]. ORCID
Yi Zhang: 0000-0002-6730-4805 Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Liu, Y.; Li, D.; Wang, T.; Liu, Y.; Xu, T.; Zhang, Y. ACS Catal. 2016, 6, 5366−5370. (2) Kosinov, N.; Parastaev, A.; Wijpkema, A. S. G.; Vollmer, I.; Gascon, J.; Kapteijn, F.; Hensen, E. J. M. ACS Catal. 2017, DOI: 10.1021/acscatal.7b00665. (3) Yuan, S.; Li, J.; Hao, Z.; Feng, Z.; Xin, Q.; Ying, P.; Li, C. Catal. Lett. 1999, 63, 73−77. 4490
DOI: 10.1021/acscatal.7b01013 ACS Catal. 2017, 7, 4488−4490
