Friedel–Crafts alkylation of aromatics with benzyl alcohols catalyzed by rare earth metal triflates supported on MCM-41 mesoporous silica

Abstract

Friedel–Crafts alkylation of aromatics with benzyl alcohol as alkylating agent has been investigated over rare earth metal triflates, Sc(OTf) 3, Hf(OTf) 4, La(OTf) 3, and Yb(OTf) 3 supported on MCM-41. The catalytic activity of triflates, particularly, Sc(OTf) 3 and Hf(OTf) 4 was enhanced by being loaded onto MCM-41 because of increased dispersion, and gave the benzylated product in high yield. They were reusable without the loss of the activity for their recycling. Water generated during the benzylation with benzyl alcohol (BnOH) did not affect on the catalytic activity of the Sc(OTf) 3 supported on MCM-41 (Sc(OTf) 3/MCM-41). The catalytic activity of Sc(OTf) 3/MCM-41 was not affected by the further addition of water equimolar to BnOH. The benzylation of aromatics (toluene, p-xylene, mesitylene, and naphthalene) with benzyl alcohol, and of benzene with benzyl alcohols (4-methylbenzyl alcohol and 4-chlorobenzyl alcohol) were studied over Sc(OTf) 3/MCM-41 at 353 K. The rate of the benzylation of benzene over Sc(OTf) 3/MCM-41 was accelerated by electron donating groups and retarded by electron withdrawing groups. The benzylation of aromatics proceeds with direct benzylation with benzyl alcohol, and the dehydration product, i.e., dibenzyl ether (DBE), which is a by-product formed during the benzylation of aromatics; however, it participates in the benzylation with benzene via BnOH with the decrease in BnOH.