Effect of SiO2 and zeolite surfaces on the excited triplet state of benzophenone, BT; a spectroscopic and kinetic study

Abstract

Laser flash photolysis has been used to study the triplet excited state of benzophenone B^T, on various surfaces, SiO₂, zeolites NaY, KY, NaX and KX, and in rigid media at room temperature, polyethylene and polymethylmethacrylate. The studies point to similarities of the spectroscopy and kinetics of B^T in fluid solution, in a solid matrix (polymers) and on a SiO₂ surface. However, stark differences are observed for B^T in zeolites where the absorption spectrum mimics that of the protonated ketone, and the reactivities of B^T with C₆H₁₂ and CH₃OH are an order of magnitude smaller than those in liquid C₆H₁₂ and CH₃OH. Inclusion of ammonia, which blocks acidic sites in the zeolite, produces a triplet spectrum which is similar to that in polar solution. The reactivity of the triplet with ammonia in a zeolite is also comparable to that observed for this reaction in polar solution. These data are discussed in terms of the interaction of benzophenone with acidic sites in the zeolites, and to restrictions placed on the reactants in the zeolite cages. The blocking of the zeolite acidic sites by ammonia produces spectral and kinetic data (reactivity with NH₃) of the triplet that are comparable to those observed in solution. This is one of the few cases where zeolites inhibit rather than promote reactions of a solute adsorbed in them.