Participation of linear methoxy species bonded to Ti4+ sites in the methanol carbonylation catalyzed by TiO2-supported rhodium: An infrared investigation

TiO2-supported rhodium samples synthesized from the Rh6(CO)16 complex were catalytically active and selective for the gas-phase methanol carbonylation at 140 °C and atmospheric pressure in the presence of methyl iodide as promoter. Infrared (IR) spectra recorded during catalysis allowed the identification of molecularly adsorbed methanol, together with linear, doubly, and triply-bridged surface methoxy species on Ti4+ sites of the support. IR bands characteristic of rhodium complexes that might be regarded as reaction intermediates were also observed, in addition to bands assigned to surface acetate species attributed to the formation of methyl acetate. Our results reveal that only linear methoxy species on Ti4+ sites react with flowing CO in the presence of CH3I to give methyl acetate, whereas bridged methoxy species and molecularly adsorbed methanol are only spectators in methanol carbonylation.