Theoretical Study on Structures and Reaction Mechanisms of Ethylene Oxide Hydration over H-ZSM-5: Ethylene Glycol Formation

Abstract

Stepwise and concerted mechanisms of the ethylene oxide hydration reaction within a cluster model covering the nanocavity, where the straight and zigzag channels intersect, of H-ZSM-5 have been proposed and investigated by means of the ONIOM(B3LYP/6-31G(d,p):UFF) and embedded-ONIOM(B3LYP/6-31G(d,p):UFF) methods. For the stepwise mechanism, the hydration reaction of ethylene oxide starts from the ring opening of ethylene oxide by breaking of the C−O bond to form the alkoxide intermediate followed by hydration of the alkoxide intermediate to produce ethylene glycol as the product of the reaction. The calculated activation energies are computed to be 34.4 and 14.5 kcal/mol for the ring-opening and hydration steps, respectively. For the concerted mechanism, the ring opening and hydration take place simultaneously with a small activation barrier of 13.3 kcal/mol. This reaction is suggested as a more economical alternative to the present noncatalytic hydration of ethylene oxide via the concerted mechanism for p...