On the redox mechanism of methanol carbonylation on a dispersed ReOx/SiO2 catalyst

Abstract

Acetic acid is industrially produced by methanol carbonylation using Ir- or Rh-based homogeneous catalysts and a corrosive HI promoter. Recently, a heterogeneous catalyst with atomically dispersed ReO₄ sites on an inert mesoporous SBA-15 support demonstrated high acetic acid yields and stability without the need for a promoter (J. Qi, J. Finzel, H. Robatjazi, M. Xu, A. S. Hoffman, S. R. Bare, X. Pan and P. Christopher, Selective methanol carbonylation to acetic acid on heterogeneous atomically dispersed ReO₄/SiO₂ catalysts, J. Am. Chem. Soc., 2020, 142(33), 14178–14189, https://doi.org/10.1021/jacs.0c05026). In this study, we investigate the reaction mechanisms of methanol carbonylation on monopodal –ORe(O)₃ sites using density functional theory calculations, natural bond orbital analysis, and the energetic span model. We find that the reduction of dispersed Re(VII) oxide by CO through an indirect mechanism is essential for catalyst activation. The C–C coupling of methyl and carbonyl ligands is favorable in both Re(V) and Re(III) complexes, with Re(III) being superior due to transition state stabilization by a metal-localized lone electron pair. The preceding C–O bond activation is favorable only on Re(V) and leads to a thermodynamic sink, posing challenges in interpreting the high carbonylation activity in terms of monopodal ReO_x site catalysis. We hypothesize that multi-nuclear sites or more exotic ligand environments drive the cooperative reaction mechanism of selective carbonylation.