CH₄ Carbonylation to Acetic Acid Using H₂O as an Oxidant on a Rh-Functionalized UiO-67 Combined with Oriented External Electric Fields: Selectivity and Mechanistic Insights from DFT Calculations

Acetic acid (CH₃COOH), as an industrially important petrochemical product, is predominantly produced via multistep energy-intensive processes. The development of a rhodium single-site heterogeneous catalyst has received considerable attention due to its potential to transform CH₄ into CH₃COOH in a single step. Herein, the reaction mechanism for the generation of CH₃COOH from CH₄, CO, and H₂O catalyzed by Rh-functionalized metal-organic framework (MOF) UiO-67 and the selectivity of products CH₃COOH, formic acid (HCOOH), methanol (CH₃OH), and acetaldehyde (CH₃CHO) under the oriented external electric fields (OEEFs) were systematically explored by density functional theory (DFT) calculations. The results reveal that the insertion of CO into Rh-CH₃ is the rate-determining step with a free energy barrier of 21.0 kcal/mol in CH₄ carbonylation to CH₃COOH. Upon applying an OEEF of <i>F</i>_<i>x</i> = +0.0050 au along the C-C bond, the rate-determining step shifts toward H₂O decomposition with the barrier of 19.6 kcal/mol, significantly improving the selectivity for CH₃COOH production, compared to the major competitive HCOOH route. The Brønsted-Evans-Polanyi (BEP) relationships between key transition states, field strength, and NPA charge transfer were established. This study may guide the rational design of atomically dispersed MOF catalysts for the selective coconversion of CH₄ and CO to CH₃COOH using H₂O as the oxidant under the OEEF.