Surface photochemistry: Benzophenone within nanochannels of H+ and Na+ ZSM-5 zeolites

This work reports a surface photochemistry study of benzophenone within nanochannels of MFI zeolites, in this case a series of H+ and Na+ ZSM-5 zeolites where the acidity of the host is varied systematically. Laser-induced room temperature luminescence of air equilibrated or argon purged solid powdered samples of benzophenone adsorbed onto the two sets of zeolites, which we will name HZSM-5 and NaZSM-5 zeolites, revealed the existence of three different benzophenone emissions. In the nanosecond time range a broad band centred at about 430 nm was detected and its origin was hydrogen bonded benzophenone. A second broad band peaking at about 470 nm and in the millisecond time range was also detected, which was the emission of the protonated benzophenone. Finally a vibrationally structured emission existed (448 nm), the normal room temperature phosphorescence emission of benzophenone. Triplet-triplet transient absorption of benzophenone was obtained for all zeolites under study, and clear evidence for the formation of hydroxybenzophenone radicals was obtained for the NaZSM-5 series (only Lewis acid sites). The more acidic the internal surface the more radical was detected. Other products resulting from a-cleavage of benzophenone were also identified. When the host was HZSM-5 (Bronsted and Lewis acid sites) benzophenone seemed to be less reactive, and no ketyl radicals from benzophenone were detected. All these findings clearly show that benzophenone is an excellent probe which is able to provide important information regarding the internal surfaces of MFI zeolites, namely the different photocatalytic activity of the different active sites of these zealites. The present study is paradigmatic in what regards the host influence on the photochemistry of the guest. (C) 2008 Elsevier Inc. All rights reserved.