Experimental and theoretical investigations on the hydrolysis of dimethyl ether to methanol over H-ZSM-5

摘要

The catalytic hydrolysis of dimethyl ether (DME) over H-ZSM-5 was studied by experimental and theoretical studies. We observed from the pack-bed reactor that this reaction yields methanol as product at low temperature (<300°C), but produces other carbon-containidg species at higher temperature (>300°C). However, these observations cannot give the details of the reaction mechanisms. To clarify that how the reaction proceeds, the insight into the reaction mechanisms of DME hydrolysis have been investigated by the ONIOM2(M06/6- 31G(d,p):UFF) method. Our calculations showed that DME hydrolysis catalyzed by H-ZSM-5 occur via two mechanisms; stepwise and concerted. For the stepwise mechanism the reaction starts from the demethylation of DME to form surface methoxide intermediate and then followed by the hydrolysis between methoxide intermediate and adsorbing water to produce methanol as the product. The activation barriers of this pathway are 194 and 112 kJ mol~(-1) for the demethylation and the hydrolysis steps, respectively. For the concerted mechanism, the demethylation and hydrolysis take place simultaneously in a single step by using 125 kJ mol~(-1) as the activation energy. Our results indicate that the rate-determining step of this reaction is the demethylation of DME in the stepwise mechanism of which the calculated apparent barrier is 76 kJ mall. This value agrees well with our experimental observation that the hydrolysis of DME to methanol over H-ZSM-5 required energy of 76 kJ mol~(-1).