Selective Methanol Carbonylation to Acetic Acid on Heterogeneous Atomically Dispersed ReO₄/SiO₂ Catalysts

Abstract

Methanol carbonylation to acetic acid (AA) is a large-scale commodity chemical production process that requires homogeneous liquid-phase organometallic catalysts with corrosive halide-based cocatalysts to achieve high selectivity and activity. Here, we demonstrate a heterogeneous catalyst based on atomically dispersed rhenium (ReO₄) active sites on an inert support (SiO₂) for the halide-free, gas phase carbonylation of methanol to AA. Atomically dispersed ReO₄ species and nanometer sized ReO_<i>x</i> clusters were deposited on a high surface area (700 m²/g) inert SiO₂ using triethanolamine as a dispersion promoter and characterized using aberration corrected scanning transmission electron microscopy (AC-STEM), UV-vis spectroscopy, and X-ray absorption spectroscopy (XAS). Reactivity measurements at atmospheric pressure with 30 mbar of methanol and CO (1:1 molar ratio) showed that bulk Re₂O₇ and ReO_<i>x</i> clusters on SiO₂ (formed at >10 wt %) were selective for dimethyl ether formation, while atomically dispersed ReO₄ on SiO₂ (formed at <10 wt %) exhibited stable (for 60 h) > 93% selectivity to AA with single pass conversion >60%. Kinetic analysis, <i>in situ</i> FTIR, and <i>in situ</i> XAS measurements suggest that the AA formation mechanism involves methanol activation on ReO₄, followed by CO insertion into the terminal methyl species. Further, the introduction of ∼0.2 wt % of atomically dispersed Rh to 10 wt % atomically dispersed ReO₄ on SiO₂ resulted in >96% selectivity toward AA production at volumetric reaction rates comparable to homogeneous processes. This work introduces a new class of promising heterogeneous catalysts based on atomically dispersed ReO₄ on inert supports for alcohol carbonylation.