STRUCTURES AND REACTION MECHANISMS OF FRIEDEL-CRAFTS ACYLATION REACTIONS OVER ACID FAUJASITE ZEOLITE

摘要

Reactions of Friedel-Crafts acylation of toluene and acetic anhydride over nanoporous faujasite zeolite has been investigated by the well calibrated ONIOM (M06-2X/6-31G(d,p):UFF) and foll quantum cluster approaches. The 54T cluster is modeled to cover the intersection of the two supercages, which allows a large space for the reaction to take place. The reaction is proposed to proceed in two steps, the decomposition of acetic anhydride to generate an acylium ion intermediate and the acylation of toluene to p-methyl acetophenone product. The first step is found to be the rate determining step with the activation energy of 29.0 kcal/mol. The second step is more facile than the first step, and needs a much lower activation barrier of only 20.3 kcal/mol. This is due mainly to the effects of the zeolite framework on the stability of adsorption, intermediate and transition state complexes. The results indicate that H-FAU zeolite is a highly active catalyst for the the Friedel-Crafts Acylation reactions.