文摘
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 掳C) but produces other carbon-containing species at temperature above 300 掳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-5. Our calculations showed that DME hydrolysis catalyzed by H-ZSM-5 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路mol鈥?, 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路mol鈥?. 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路mol鈥?, which agrees well with our experimental observation that DME hydrolysis over H-ZSM-5 required energy of 76.5 kJ路mol鈥?.