Ni-MoO_x bifunctional catalyst on SiO₂ for vapor halide-free methanol carbonylation: Insight into synergistic catalysis between Ni and MoO_x

A promising Ni-MoO_x bifunctional catalyst for halide-free methanol carbonylation is developed by H₂-reduction of NiO-MoO₃/SiO₂ obtained via facile impregnation method. Catalyst performance is strongly dependent on the calcination/reduction temperature. Over the preferable NiMo-350-600/SiO₂ catalyst obtained by calcining at 350 °C and subsequently reducing at 600 °C, a methanol conversion of 4.2 % and acetyls space-time yield of 1.37 mol kg_cat⁻¹ h⁻¹ are achieved with 22.1 % selectivity to acetyls at 290 °C and 3 MPa. Co-existence of Ni⁰ and MoO_x (especially MoO₂) markedly decreases formation of dimethyl ether (DME) and reversely increases acetyls formation, in nature, due to the catalyst acidity modulation and the partial electron transfer from Ni⁰ to MoO_x that tunes the CO adsorption strength on Ni⁰ sites. MoO₃ and NiMoO₄ are both favorable for formation of acetyls and DME whereas the latter mainly accounts for DME formation. MoNi₄ alone favors methyl formate formation while together with MoO₂ enhancing DME production.