Direct synthesis of dimethyl carbonate offers prospects for a "green chemistry" replacement of phosgene use for polymer production and other processes. The carbonylation of methanol to produce dimethyl carbonate over a Cu+X zeolite prepared by solid-state ion exchange has been investigated in a flow system at temperatures between 100 and 140 degreesC and a total pressure of 1 atm. Formation rates of dimethyl carbonate, methylal, and methyl formate are well described by a Langmuirian reaction mechanism, with quasi-equilibrated adsorption of methanol and oxidation of surface methanol to form methoxide, rate-limiting carbon monoxide insertion into the methoxide to form a carbomethoxide, and reaction of carbomethoxide with methoxide to form dimethyl carbonate. The by-products (methylal and methyl formate) are products of partial oxidation of the methoxide to formaldehyde. In situ FTIR experiments show the surface species present on the catalyst under reaction conditions and the effect of water on the system. (C) 2003 Published by Elsevier Science (USA).