(254b) On the Redox Mechanism of Methanol Carbonylation on the Dispersed ReOx/SiO2 Catalyst

Acetic acid (AA) is industrially produced via methanol carbonylation using the Cativa process, which employs a corrosive HI promoter and expensive homogeneous catalysts, [IrI₂(CO)₂]⁻ and [RuI₂(CO)₃].¹ High operational costs, energy-intensive catalyst recovery, and environmental concerns drive the search for sustainable heterogeneous alternatives. Recently, a SBA-15 – supported ReO₄ catalyst demonstrated promising methanol carbonylation performance, achieving over 93% AA selectivity and 60% methanol conversion in a 60-hour reaction.² However, identifying active catalytic sites and understanding atomistic mechanism remains challenging due to limitations in experimental observation of transient intermediates and the complexity of the catalytic environment.

In this study, we employ density functional theory (DFT), natural bonding orbital (NBO) analysis, and microkinetic modeling to investigate the methanol carbonylation mechanism on silica-supported monopodal ORe(=O)₃ sites, as proposed experimentally.³ By systematically evaluating potential pathways, we identify the crucial role of Re reduction in catalyst activation. We determined coordination environments conducive to critical elementary reactions, including C–O scission to form -CH₃, C–C coupling to generate -COCH₃, and C–O formation producing AA. However, the C–O scission and C–C coupling steps require distinct oxidation states and ligand environments, obscuring the interpretation of high catalytic activity through monopodal sites alone. Our findings suggest that the observed catalytic performance likely involves multinuclear sites or unconventional ligand environments enabling cooperative mechanisms. Interestingly, preliminary results for a proposed dual-site catalyst indicate enhanced Re(VII) reduction and facilitated C–O scission in high Re oxidation without impeding subsequent C–C coupling.

References

(1) Jones, J. H. Platinum Metals Review, 2000, 44, 94–105.

(2) Qi et al. J. Am. Chem. Soc. 2020, 142 (33), 14178–14189.

(3) Tran, Mironenko. React. Chem. Eng. 2025, 10 (3), 534–549.