ALKYLATION OF BENZENE OR TOLUENE WITH MEOH OR C2H4 OVER ZSM-5 OR BETA-ZEOLITE - EFFECT OF THE ZEOLITE PORE OPENINGS AND OF THE HYDROCARBONS INVOLVED ON THE MECHANISM OF ALKYLATION

A comparative study of the alkylation of benzene or toluene with MeOH or C2H4 over a medium (ZSM-5) and a large pore (beta) zeolite of comparable acidities was carried out. It was observed that the reaction temperature in combination with the structure of the zeolite plays an important role in the reactions that take place. The maximum yield of either the primary or secondary alkylation products may occur at an intermediate temperature, which is lower over beta zeolite. Due to its pore structure, beta zeolite favors secondary alkylation reactions and also disproportionation reactions of the generated alkylaromatics to a higher extent than ZSM-5 does. MeOH generates both primary and secondary alkylation products, while C2H4 favors oligomerization reactions, primary alkylation reactions, and particularly, disproportionation reactions. Toluene is more reactive than benzene. The aromatic/alkylating agent molar ratio plays an important role in the relative importance of the reactions that take place. The size of the pores of the zeolite in combination with the sizes of the aromatic hydrocarbons and the alkylating agents employed determines whether the alkylation occurs via a Langmuir-Hinshelwood (LH) or Rideal-Eley (RE) mechanism. When a LH mechanism occurs, the alkylation rate passes through a maximum with respect to the concentration of the aromatic hydrocarbon employed; no such maximum occurs for the RE mechanism.