Cu-exchanged beta and Y catalysts were investigated by oxidative carbonylation of ethanol in the gas-phase reaction. CLIP Catalyst has shown better catalytic selectivity for oxidative carbonylation of ethanol to diethyl carbonate (DEC), without the principal by-product 1,1-diethoxyethane (DEE) for CuY catalysts. In order to investigate the effect of zeolite structure oil the selectivity for products, computational analysis of molecular dimensions and diffusion parameters of DEC and DEE within Cu beta and CuY catalysts zeolite framework has been performed using molecular mechanics and quantum mechanics methods. The computational analysis results are in good agreement with the experimental results to some extent. DEC having a kinetic diameter of 3.663 angstrom and the lowest energy barrier was formed preferentially over both zeolites However, the DEE molecule was not detected among the products over Cup because Of Its greater kinetic diameter 6.059 angstrom and higher energy barrier. The special architecture of beta zeolite did not allow the diffusion of DEE molecules through its pores The formation of the higher sterically hindered DEE over CuY catalyst could be explained by involvement of the outer surface. (C) 2009 Elsevier B.V. All rights reserved.