Reaction Mechanism for Methane-to-Methanol in Cu-SSZ-13: First-Principles Study of the Z2[Cu2O] and Z2[Cu2OH] Motifs

Unni Engedahl,Adam A Arvidsson,Anders Hellman,Henrik Grönbeck

doi:10.3390/catal11010017

Unni Engedahl, Adam A Arvidsson + Show 2 more

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https://doi.org/10.3390/catal11010017

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Abstract

As transportation continues to increase world-wide, there is a need for more efficient utilization of fossil fuel. One possibility is direct conversion of the solution gas bi-product CH4 into an energy-rich, easily usable liquid fuel such as CH3OH. However, new catalytic materials to facilitate the methane-to-methanol reaction are needed. Using density functional calculations, the partial oxidation of methane is investigated over the small-pore copper-exchanged zeolite SSZ-13. The reaction pathway is identified and the energy landscape elucidated over the proposed motifs Z2[Cu2O] and Z2[Cu2OH]. It is shown that the Z2[Cu2O] motif has an exergonic reaction path, provided water is added as a solvent for the desorption step. However, a micro-kinetic model shows that neither Z2[Cu2O] nor Z2[Cu2OH] has any notable activity under the reaction conditions. These findings highlight the importance of the detailed structure of the active site and that the most stable motif is not necessarily the most active.

Highlights

The growing need for transportation makes a more efficient usage of combustion fuel necessary [1]
Z2 [Cu2 OH] in Cu-SSZ-13 are explored by first-principles calculations and micro-kinetic modelling
Methane dissociation occurs via a methyl radical state, which is responsible for the highest barrier of the reaction in both systems

Summary

Introduction

The growing need for transportation makes a more efficient usage of combustion fuel necessary [1]. Methane, which is extracted together with crude oil, is a not fully utilized bi-product. As methane is a gas at standard conditions with high sunlight absorption properties, methane is often flared into CO2 instead of being utilized [2]. Such a scenario can be avoided if it were possible to directly convert methane into methanol, which is a liquid at standard conditions. The management and distribution system is already in place to handle liquid fuels.

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