A Combined Experimental and Theoretical Study on the Hydrolysis of Dimethyl Ether over H-ZSM-5

The catalytic hydrolysis of dimethyl ether (DME) over H-ZSM-5 was examined by experimental and theoretical studies. We experimentally observed that this reaction yields methanol as major product at low temperature (<300 degrees C) but produces other carbon-containing species at temperature above 300 degrees C. The insight into the reaction mechanisms of DME hydrolysis have been investigated by the M06/6-31G(d,p) method using the 128T cluster model of H-ZSM-S. Our calculations showed that DME hydrolysis catalyzed by H-ZSM-S occurs via two mechanisms, stepwise and concerted. For the stepwise mechanism the reaction starts with (i) the demethylation of DME to form a surface methoxide intermediate, followed by (ii) the hydrolysis between the methoxide intermediate and a water molecule to produce methanol as the product. The calculated activation barriers for the demethylation and hydrolysis steps are 168.6 and 81.8 kJ.mol(-1), respectively. For the concerted manner, the demethylation and hydrolysis take place simultaneously in a single step without forming the intermediate by using the activation barrier of 108.9 kJ.mol(-1). It was predicted that DME is preferentially hydrolyzed via a concerted mechanism and the rate-determining step is the DME demethylation step. The calculated apparent activation barrier for the DME hydrolysis is 75.5 kJ.mol(-1), which agrees well with our experimental observation that DME hydrolysis over H-ZSM-5 required energy of 76.5 kJ.mol(-1).