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

Abstract

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 + ‚O 2 -) 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 (g87%) compared to ZSM-5 and Beta (<35%). In each case, the presence of residual Brønsted acid sites in these zeolite materials was found to correlate with the loss of selectivity. The electric field strength, the presence of Brønsted acid sites, and other physicochemical properties that affect yield and selectivity in these photooxidation reactions are explored.

Electric field convergence versus atomic basis sets in all‐siliceous zeolites

“…In another work, the ratio between the minimum EP value close to the bridge moiety of offretite and the sum of the AlO and SiO bond populations, thus with the proton position and acidity, has been assessed at the QM semiempirical and ab initio levels 2. Finally, let us mention that the importance of the EF has been emphasized to explain the product yield of photoinduced oxidation of toluene in X and Y type zeolites,3 whereas the dominant role of the EP has been established for cationic locations in NaA4 and NaNAT5 zeolites.…”

Recent Advances in Solid Waste Minimization using Nanotechnology

“…Porous crystalline solids with well-defined structures widely used for separations and catalysis. Recently nanometer-sized zeolites (10-100) has been developed to selectively oxidize hydrocarbons, such as toluene to benzaldehyde [2] in this approach oxidation reaction is provoked by visible light photons, which reduces energy consumption and secondly using visible region of light spectrum for low-energy reaction pathways that help eliminate wasteful secondary photoreactions and increase the yield of the desired product. In this study, selectivity for benzaldehyde using the nanostructures was 87%, compared to less than 35% for the same reaction with conventional zeolite material.…”

Peer Reviewed: Environmental Technologies at the Nanoscale

“…For example, aluminosilicate molecular sieves (zeolites) are porous crystalline solids with well-defined structures widely used for separations and catalysis. Nanometer-sized zeolites (10-100 nm) are being developed to selectively oxidize hydrocarbons, such as toluene to benzaldehyde (14). Using nanostructured zeolites makes this example more environmentally benign for two reasons.…”