Friedel-Crafts benzylation of aromatics with benzyl alcohols catalyzed by heteropoly acids supported on mesoporous silica

Heteropoly acids (HPA), such as tungstophosphoric acid (H3PW12O40 center dot XH2O) (HPW), molybdophosphoric acid (H3PMo12O40 center dot xH(2)O) (HPMo) and tungstosilicic acid (H4SiW12O40 center dot xH(2)O) (HSiW) were supported on mesoporous silica such as MCM-41, FSM-16 and SBA-15 by the impregnation method to enhance the catalytic activity of these solid acids by their dispersion on the support with high surface area. These supported solid catalysts were used in the benzylation of benzene and substituted aromatics with benzyl alcohol (BnOH). The immobilization enhanced the catalytic performances and HPW supported on MCM-41 exhibited the best activity for benzylation among the heteropoly acids, although dibenzyl ether (DBE) formation by the dehydration of BnOH also occurred. The mesoporous architecture of the silica enhances the activity of benzylation because of the high dispersion of HPW on the support with high surface area; however, no steric restriction by the pores of mesoporous silica was observed. The catalysts used in the present study retained their catalytic activity for five reaction cycles. The rate of benzylation of substituted benzenes and benzylating agents was influenced by the electronic nature of the substituent. Electron-donating groups enhanced the rate of reaction; however, electron-withdrawing groups retard the benzylation. These results show that the reactivity of benzene and benzyl alcohols was retarded by electron-withdrawing groups. The formation of polybenzylated products was influenced by the reactivity of diphenylmethane products. The benzylation accompanies the DBE formation by the dehydration of BnOH, particularly in the initial stages, because the benzylation of aromatics with BnOH is not as rapid as the dehydration of BnOH. However, direct benzylation of benzene occurs with DBE and DBE participates in the benzylation of benzene via BnOH after its hydrolysis. This is further supported by the effect of water on the benzylation of benzene by DBE, although there is a possibility of direct benzylation of DBE with benzene. (c) 2006 Society of Chemical Industry.