Active sites and mechanism of the direct conversion of methane and carbon dioxide to acetic acid over the zinc-modified H-ZSM-5 zeolite

Abstract

Conversion of CH₄ and CO₂ to higher value chemicals will contribute to alleviating green-house gas emissions and the global fossil fuel shortage. Introducing metals into zeolites is used to catalytically convert CH₄ and CO₂ to acetic acid. Understanding the local structure of active sites and their functions in reaction mechanisms will help the design of highly-efficient catalysts and promote further application. In the present work, the effect of the local structure of active sites on the conversion of CH₄ and CO₂ over Zn modified H-ZSM-5 zeolites has been studied systematically using density functional theory. Herein, two different Zn-modified H-ZSM-5 structures are considered, one containing isolated Zn²⁺ cations and the other small ZnO clusters. It is found that the catalytic activity of the conversion of CH₄ and CO₂ is strongly dependent on the local structure of the active sites. Zn²⁺ cations located in the 5-membered rings of the sinusoidal and straight channels exhibit better catalytic activity than other isolated Zn²⁺ sites and small ZnO clusters due to the favorable adsorption properties of reactants and activation energies. The rate-determining step of the whole process on the isolated 5-membered Zn²⁺ in the sinusoidal channel is the insertion of CO₂ into the Zn–CH₃ bond, while that on the isolated 5-membered Zn²⁺ in the straight channel is the desorption of acetic acid.