Diethyl carbonate (DEC) was prepared via oxidative carbonylation of ethanol over a heterogeneous Wacker-type catalyst supported by activated carbon (AC). The active species of copper chloride hydroxide crystals has a great effect on the catalytic performance and chemical adsorption capacities. Cu(OH)Cl crystal is the most active catalyst for the diethyl carbonate synthesis. In situ XRD technique was used to analyze the effect of treatment temperature on the crystal structures of catalyst in the oxidative carbonylation of ethanol to diethyl carbonate. Cu(OH)Cl was easily formed at 523 K, which showed the best catalytic activities. gamma-Cu-2(OH)(3)Cl was formed at 573 K, which showed lower catalytic activities in the synthesis of DEC. CuO was formed at the treatment temperature of 673 K, which increased the yield of by-products-acetaldehyde and ethyl acetate. The catalytic performance of three catalysts Of CuCl2-PdCl2-KCl-NaOH/AC prepared at the different treatment temperatures above indicated the same trend which could be ascribed to the in situ effects on the catalysts under the reaction conditions. (c) 2006 Elsevier B.V. All rights reserved.