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 visible light has been investigated in several cation-exchanged zeolites. In general, the yield of the photooxidation products, for fixed irradiation time and intensity, was found to correlate with the electric field intensity at the cation sites within the zeolites. On the basis of measurements of CO vibrational frequencies, electric fields of approximately 3-7 V nm^-1 are indicated for the cation-exchanged zeolites X, Y, ZSM-5, and Beta used in these studies. These large electric fields are thought to promote photooxidation by stabilizing an intermolecular charge transfer state (R⁺·O₂^-) that is formed upon excitation with visible light. The measured electric field was found to correlate with the product yield and was highest in divalent cation-exchanged zeolites with high Si/Al ratios, such as BaZSM-5 and BaBeta. For zeolites containing the same cation, the selectivity of toluene to form benzaldehyde and p-xylene to form p-tolualdehyde was found to be higher in zeolites X and Y (≥87%) compared to ZSM-5 and Beta (<35%). In each case, the presence of residual Bronsted acid sites in these zeolite materials was found to correlate with the loss of selectivity. The electric field strength, the presence of Bronsted acid sites, and other physicochemical properties that affect yield and selectivity in these photooxidation reactions are explored.