Theoretical Study Of The Mechanism Of The Conversion Of Methane And Carbon Dioxide To Acetic Acid Over Zn/ZSM-5 Catalysts

Methane(CH4) is a main component of abundant natural gas in the earth, and has been considered as a potentially clean energy source and cheap chemical raw material. The catalytic conversion of methane to higher valuable chemicals has attracted growing interest in the industrial perspective. Until now, the main way of the transforming of methane is indirect pathway, which transform methane to syngas(CO+H2), followed by further conversion to other chemicals. But this indirect conversion can only proceed in high temperature which means high energy consuming. So the direct conversion of methane and carbon dioxide(CO2) to acetic acid is considered a potential way to get rid of the disadvantage of indirect conversion. Besides, methane and carbon dioxide are main greenhouse gases which continuously draw worldwide attention about their influence on global warming. As a result, the direct co-conversion of methane and carbon dioxide is also meaningful in environment protection.However, due to the high chemical stability of methane and carbon dioxide molecules, this direct conversion is not favored in kinetics. Consequently, appropriate catalysts are needed to improve its practicability. Several catalysts such as Pd/C, Pt/Al2O3, Rh/Si O2, Co-Cu, etc, have been reported able to catalyze this conversion, but they are still away from practicability because the high operational temperature is still needed. Zeolites ion-exchanged by various metal are potential candidates to catalyze this reaction due to their ability of effective activation of the C-H bond of methane. Zn/ZSM-5 are one of which have got attentions of researchers in recent years, because it can active methane under mild condition, which means low energy consumption. But the mechanism of the conversion of carbon dioxide and methane over Zn/ZSM-5 is not clear yet, it requires exploration in the most important factors which influence the efficiency of catalysts.In present paper, a 128 T cluster model has been built to research the conversion of carbon dioxide and methane under different Zn/Al ratios and Br?nsted acidities. Result shows:1) The conversion start by methane activation is much favored compared with the co-activation of methane and carbon dioxide to zinc acetate intermediate complex.2) [ZnOZn]2+ loaded in ZSM-5 is much more active in the process of methane activation, compared with that of Zn2+.3)The co-existence of zinc species such as Zn2+ and [ZnOZn]2+ or bi-[ZnOZn]2+ result in lower energy barrier of the carbon dioxide insertion, because of the multiple interaction between the zinc species and carbon dioxide.4) The acidity of zeolite can reduce the energy barrier of the proton translocation remarkably.