Dehydration of Ethanol into Ethylene over H-MOR: A Quantum Chemical Investigation of Possible Reaction Mechanisms in the Presence of Water

The conversion of ethanol to ethylene, which is one of the most important feed stocks for the petrochemical industry, is of particular commercial interest. Two mechanisms, stepwise and concerted, of the ethanol dehydration to ethylene in the presence of water have been investigated by the ONIOM model (14T/120T) with B3LYP/6-31g(d,p):UFF method. In the stepwise mechanism, the coadsorbed water assists the protonation of ethanol by the acidic zeolite proton to form an ethoxonium ion (CH3CH2OH2). After that, the cation is dehydrated to form a surface ethoxide intermediate. The dehydration is found to be the rate-determining step with an activation barrier of 41.4 kcal/mol. The ethoxide is then deprotonated with a water molecule which results in the formation of the product of ethylene. The activation energy of the deprotonation is calculated to be 22.2 kcal/mol. The concerted mechanism differs in that the dehydration and the deprotonation occur simultaneously without the formation of the intermediate. In this mechanism, the activation barrier is higher at 54.7 kcal/mol. Therefore, we conclude that the stepwise mechanism should be the dominating one in hydrous ethanol dehydration.