The influence of pore structure on reaction mechanism of propylene dimerization in zeolite: A theoretical viewpoint

Molecular sieves have been widely used in the petrochemical industry as environment-friendly catalysts. The pore structure is an important factor influencing the catalytic performance of the zeolite. In this work, a combined Our own N-layered Integrated molecular Orbital and Molecular mechanics method was used to study the mechanism of propylene dimerization in four zeolites (ZSM-5, BEA, MCM-22, and MOR) with different pore structures. Comparing the stepwise mechanism and the concerted mechanism, it is found that the two mechanisms compete with each other in the macroporous BEA, MCM-22, and MOR zeolites, and both mechanisms are possible. However, in ZSM-5 zeolite with medium pore size, the propylene dimerization reaction tends to proceed according to the stepwise mechanism. Furthermore, no matter which mechanism is adopted, the activation energy of propylene dimerization reaction in MCM-22 is the smallest in the four zeolites, indicating that its MWW-type structure (A framework type defined by the International Zeolite Association) may be the most favorable pore structure for the reaction and possesses the highest catalytic activity.