Photooxidation of toluene and p-xylene in cation-exchanged zeolites X, Y, ZSM-5, and beta: The role of zeolite physicochemical properties in product yield and selectivity

The photooxidation of toluene and p-xylene with molecular oxygen and visibl e light has been investigated in several cation-exchanged zeolites. Ln gene ral, the yield of the photooxidation products, for fixed irradiation time a nd intensity, was found to correlate with the electric field intensity at t he cation sites within the zeolites. On the basis of measurements of CO vib rational frequencies, electric fields of approximately 3-7 V nm(-1) are ind icated for the cation-exchanged zeolites X, Y, ZSM-5, and Beta used in thes e studies. These large electric fields are thought to promote photooxidatio n by stabilizing an intermolecular charge transfer state (R+. O-2(-)) that is formed upon excitation with visible light. The measured electric field w as found to correlate with the product yield and was highest in divalent ca tion-exchanged zeolites with high Si/Al ratios, such as BaZSM-5 and BaBeta. For zeolites containing the same cation, the selectivity of toluene to for m benzaldehyde and p-xylene to form p-tolualdehyde was found to be higher i n zeolites X and Y (greater than or equal to 87%) compared to ZSM-5 and Bet a (<35%). Ln each case, the presence of residual Bronsted acid sites in the se zeolite materials was found to correlate with the loss of selectivity. T he electric field strength, the presence of Bronsted acid sites, and other physicochemical properties that affect yield and selectivity in these photo oxidation reactions are explored.