Elucidating the nature and role of copper species in catalytic carbonylation of methanol to methyl acetate over copper/titania–silica mixed oxides

Abstract

In this study, a series of copper/titania–silica mixed oxide (Cu/TS) catalysts with different copper contents were prepared by a sol–gel method. The catalytic activity was evaluated by halide-free methanol carbonylation to methyl acetate (MA). The properties of the catalysts were mainly characterized by N₂ adsorption–desorption, X-ray diffraction, dissociative N₂O chemisorption, X-ray photoelectron spectroscopy, temperature-programmed desorption of ammonia and in situ Fourier transform infrared. The results show that the crystal size and aggregation degree of Cu increase with increasing Cu content. In addition, the amount of surface Cu⁺ firstly increases and then decreases, and the maximum is 1.560 mmol g⁻¹ for the 10.24 Cu/TS catalyst. Both the amounts of adsorbed CO and surface Lewis acid sites are found to be proportional to the amount of surface Cu⁺ species. The catalytic performance shows that the space time yield (STY) of MA is also strongly related to the amount of surface Cu⁺ species, and the 10.24 Cu/TS catalyst has a maximum of 1.770 mol h⁻¹ kg_cat⁻¹. It is found that the surface Cu⁺ species act not only as metal sites which adsorb CO but also as Lewis acid sites which promote the adsorption of methanol (methoxy). Furthermore, the activation of CO is the major factor for the MA synthesis.