Source and major species of CHx (x = 1–3) in acetic acid synthesis from methane–syngas on Rh catalyst: a theoretical study

Abstract

Density functional calculations have been carried out to investigate the source and major species of CH_x (x = 1–3) involved in acetic acid synthesis from methane–syngas on the Rh(111) surface. All possible formation pathways of CH_x (x = 1–3) from methane and syngas have been systematically investigated. For CH_x formation from methane, our results show that CH is the most abundant species; for CH_x formation from syngas, all CH_x (x = 1–3) species form from CHO by CO hydrogenation, and the optimal formation routes of CH_x show that CH and CH₃ are the most abundant species rather than CH₂ and CH₃OH. On the other hand, CH formed by methane is more favourable than CH and CH₃ formed by syngas; meanwhile, CO insertion into CH_x species to form C₂ oxygenates as acetic acid precursors is more favourable than CO hydrogenation to CH and CH₃. As a result, in acetic acid synthesis from methane–syngas, CH_x (x = 1–3) species come from methane rather than syngas, and the corresponding primary species is CH. In addition, the CO in syngas is predominantly responsible for insertion reactions that produce CHCO, which is a C₂ oxygenate precursor leading to the formation of acetic acid. Furthermore, microkinetic modelling analysis shows that the major product of acetic acid synthesis from methane–syngas on the Rh(111) surface is CH₃COOH, and that the production of CH₃OH cannot compete with that of CH₃COOH.