Mechanistic insight into low-temperature alkylation of thiophene with styrene over zeolites MCM-22 and β

Friedel-Crafts alkylation reactions are generally accepted to proceed by an electrophilic aromatic substitution (S_EAr) mechanism. Here, we observed that MCM-22 exhibited a high activity in the alkylation of thiophene with styrene at temperatures near ambient, while β was almost inactive. Three species were identified in the adsorption of thiophene on HMCM-22 and Hβ: the hydrogen-bonded thiophene, the protonated thiophene, and the thiophene oligomers. Styrene was chemically adsorbed on the two zeolites through its vinyl group. The infrared studies of the consecutive adsorption of thiophene and styrene on MCM-22 and β reveal that the major difference between the thiophene species on the two zeolites was the disappearance of the protonated thiophene on MCM-22. These results indicate that the reaction over MCM-22 occurs by the attack of a protonated thiophene molecule on the side chain of styrene that chemically adsorbed on the zeolite but not the conventional S_EAr mechanism.