Dimethyl carbonate (DMC) synthesis by oxidative carbonylation of methanol has been studied using vapor phase flow reaction system in the presence of CuCl 2 /NaOH/AC (AC, activated carbon) catalysts. A series of CuCl 2 /NaOH/AC catalysts have been prepared by the conventional impregnation method with varying the molar ratio of OH/Cu from 0.0 to 1.0. The catalytic activity of CuCl 2 /NaOH/AC catalysts has been found to increase continuously with the increase of OH/Cu molar ratio as compared with CuCl 2 /AC catalyst. The morphological analysis by X-ray diffraction, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) has shown that the improved performance of CuCl 2 /NaOH/AC catalysts was closely related with the existence of copper chloride hydroxides (Cu 2 (OH) 3 Cl). Two different crystal habits of Cu 2 (OH) 3 Cl were observed in the catalyst surface based on the OH/Cu molar ratio. One is β£-Cu 2 (OH) 3 Cl (atacamite) which is orthorhombic, while the other is β₯-Cu 2 (OH) 3 Cl (paratacamite) which is rhombohedral. The catalyst phase was revealed to change from β£-Cu 2 (OH) 3 Cl to β₯-Cu 2 (OH) 3 Cl with the increase of OH/Cu molar ratio. The catalyst phase was β£-Cu 2 (OH) 3 Cl dominant at the molar ratio of OH/Cu = 0.5, while at the molar ratio of OH/Cu = 1.0, β₯-Cu 2 (OH) 3 Cl dominant. The β₯-type catalyst was found to be more favorable to DMC synthesis than the β£-type catalyst.
The CO temperature-programmed desorption (TPD) experiments on the employed catalysts also strongly supported these observations.