Mechanistic Investigation On The Activation And Conversion Of Methane By Solid-state NMR

Methane, the primary component of natural gas, is one of the most abundant and low-cost feed stocks available. Therefore,methane is considered as a potential alternative to petroleum energy. The utilization of methane in industry involves reforming reaction to produce syngas (CO and H2), and the subsequent conversion of syngas to methanol, followed by methanol transformation to other chemical products (indirect conversion). However, reforming of methane is a cost-intensive process and requires harsh reaction condition (T> 1000 K). Recent studies have found that the co-conversion of methane (direct conversion) can be achieved at low temperature,which makes the methane co-conversion having potential value of application. Meanwhile, the research also showed that the zinc-modified zeolites have high activity towards the co-conversion of methane.To understand the nature of the methane co-conversion and design the co-reactants, we need to study the reactivity of initial intermediates generated by methane activation. Hence, we isolate these three surface species after methane activation on zinc-modified H-ZSM-5 zeolite (denoted as Zn/H-ZSM-5 zeolite): zinc methyl species, zinc methoxy species, and surface formate species.Then, we use different probes (i.e., potential co-reactants in methane co-conversion) to study their reactivity. The result indicates that these three surface species possess distinct reactivity. For the first time, we found the reactivity of the zinc methyl species on Zn/H-ZSM-5 zeolite can be correlated with that of organozinc compounds in organometallic chemistry. In-depth understanding of the mechanisms of methane activation and conversion is not only of great significance in theoretical guidance, but also of importance for the rational design of efficient catalysts for practical application.Based on the research above, we investigate the activation and conversion of methane with DME, Br2, and carbon dioxide, respectively, on Zn/H-ZSM-5 zeolite, and we expect our understanding of the reaction mechanism could promote the research of methane activation and co-conversion. On Zn/H-ZSM-5 zeolite, we found for the first time that: (1) methane and DME could co-conversion into methyl substituted benzenes. (2) In the presence of Br2, methane could be activated to generate zinc methoxy species at room temperature. At 673 K, zinc methoxy species could transform into methyl bromide. (3) Methane and carbon dioxide could selectively co-conversion into acetic acid. Meanwhile, these reaction mechanisms were studied by solid-state NMR and GC/MS: (1) DME decomposed into aromatic compounds catalyzed by Bronsted H.Meanwhile, zinc methoxy species were generated on zinc-active sites by methane activation which catalyzed by Bronsted H. Zinc methoxy species reacted with aromatic compounds to generate one more methyl substituted aromatic compounds, and these methyl substituted aromatic compounds could yield alkanes by cracking-hydrogenation mechanism. (2) The Bronsted acid sites act to polarize the bromine molecules, and the bromine molecules transformed into Br?+ ions stabilized on the bridge oxygen of zeolites and HBr. Methane interact with the -Zn-O- species in the holes of zeolites and Br?+ ions to generate zinc methoxy species possible though the six-member ring transition state. Then, zinc methoxy species transformed into methyl bromide. (3)The zinc methyl species and Brdnsted proton were formed by methane activation on Zn/H-ZSM-5 zeolite. Then, the surface acetate species (-Zn-OOCCH3) were generated by the insertion of CO2 into the Zn-C bond of surface zinc methyl species. Finally, acetic acid was formed by surface acetate species abstracting the proton ion from Bronsted acid sites.Methane and carbon dioxide both are greenhouse gases and attractive chemical feedstocks.Hence, the utilization of these two C1-building blocks can protect the environment and meet the new energy demands at the same time, and then to get the win-win situation. Therefore, we investigate the catalytic effect of the direct conversion of methane and carbon dioxide on Zn/H-ZSM-5 zeolite. We expect that our exploration of the factors affecting the reaction,which could lay the foundation for the direct conversion of methane and carbon dioxide. The results indicated that the formation rate of acetic acid increases with increasing the reaction temperature,but drops down at higher temperature, and the best temperature is around 673 K. Within the scope of the glass tube reactor pressure ability, the formation rate of acetic acid gradually increases with increasing the pressure. The reaction gets balance after 5 min, and further extend the reaction time has no effect on the yield of acetic acid. The formation rate of acetic acid reached the maximum value of 240.7 ?mol·gcat-1·h-1 (CH4 conversion of 6.5%) at the condition of 0.6 cm3 volume of glass tube and 673 K. The selectivity of acetic acid keeps high (> 96.73%)at 673 K with different reaction pressure.