Mechanism of microwave-induced carbothermic reduction and catalytic performance of Cu/activated carbon catalysts in the oxidative carbonylation of methanol

Abstract

Activated carbon–supported copper catalysts (Cu/AC) were prepared by impregnation of AC support with aqueous copper (II) nitrate solutions and subsequent microwave carbothermic reduction under different irradiation conditions. This was done to investigate the effect of the valence of copper on the vapor-phase oxidative carbonylation of methanol to dimethyl carbonate (DMC). Thermogravimetric analysis–mass spectrometry were carried out to explore the mechanism of carbothermic reduction in catalyst preparation. X-ray diffractometry, hydrogen temperature-programmed reduction, X-ray photoelectron spectroscopy, and scanning electron microscopy were employed to examine the bulk and surface properties of the Cu/AC catalysts. Microwave irradiation of the catalyst precursors rapidly induced a series of successive reactions, namely decomposition of Cu2(OH)3NO3 into CuO, reduction in CuO to Cu2O, and carbothermic reduction in Cu2O to Cu0. These reactions resulted in mixed-valence copper species, CuO, Cu2O, and Cu0, in the Cu/AC catalysts. CuO, Cu2O, and Cu0 were active in DMC synthesis, but the catalytic performance of Cu/AC was highly dependent on the Cu0 concentration. High irradiation temperatures resulted in higher Cu0 concentrations because of enhanced carbothermic reduction, and fast heating rates improved the degree of dispersion of Cu2O and Cu0. The catalyst irradiated at 540 °C at a heating rate of 48 °C min−1 showed the highest catalytic activity in the oxidative carbonylation of methanol.