Photooxidation of benzene to phenol by ruthenium bipyridine complexes grafted on mesoporous silica FSM-16

Catalytic performances of bis- and tris(bipyridine) Ru complexes grafted on mesoporous FSM-16 were studied in the photooxidation of benzene to phenol using H2O2 as an oxidant, [Ru(bpy)(3)]Cl-2/FSM-16 showed a high activity under W-irradiation, and the turnover number (TON) of phenol was 430 based on Ru in 24 h, and the selectivity to phenol among the products was 98%. Non-grafted [Ru(bpy)(3)]Cl-2 complex gave a phenol TON of 170, thus demonstrating the promotion effect of grafting [Ru(bpy)(3)]Cl-2 on FSM-16. The hydroxylation of benzene to phenol by [Ru(bpy)(3)]Cl-2 on FSM-16 slightly occurred in the dark (TON = 34 in 24 h), but the irradiation remarkably increased the TON of phenol by a factor of 13. The absorption peak of [Ru(bpy)(3)]Cl-2 in the UV-VIS spectroscopy decreased under the reaction conditions; however, the recovered catalyst showed almost the same activity for phenol formation in the repeated runs. It is proposed that coodinatively unsaturated [Ru(bpy)(n)](2+) (n = 1,2) are generated by the UV-irradiation to [Ru(bpy)(3)]Cl-2 on FSM-16. These species activate H2O2 to give an OH radical that attacks benzene as in the Fenton-type mechanism. Grafting of the Ru complex on FSM-16 may enhance the reaction of a hydroxycyclohexadienyl radical with the isolated Ru center. (C) 2001 Elsevier Science B.V. All rights reserved.