Surface hydrophobically modified PdCl2/Si-Cu-HMS-m materials were successfully synthesized via a simple silylation process using methyltrichlorosilane and phenyltrichlorosilane respectively. They were characterized by a series of techniques including FT-IR, powder XRD, nitrogen adsorption-desorption, and the contact angle measurement of the water droplet. It was demonstrated that the mesoporous structure of Cu-HMS was retained after modification and that hydrophobicity was enhanced. However, silylation agents had a significant influence on catalytic performance. The experimental results indicated that PdCl2/Si-Cu-HMS-CH3 showed a high catalytic activity for the gas phase oxidative carbonylation of ethanol to diethyl carbonate (DEC) and a small steric hindrance but a weak hydrophobicity in comparison with PdCl2/Si-Cu-HMS-Ben catalyst, demonstrating that catalytic performance was relative to both by-product water and structure of molecular sieves catalyst, but the latter was a main factor in the catalytic system. In addition, a probable mechanism has been proposed to explain this result that structure of molecular sieves was the main factor of influencing catalytic performance. (C) 2014 Elsevier B. V. All rights reserved.