No abstract

The interactions between the acetone carbonyl and the carbonyl and double bond of mesityl oxide and the Brönsted and Lewis acid sites of acid catalysts have been studied by Fourier Transform infrared (FTIR) spectroscopy. The acetone carbonyl interactions with bridging acidic OH and with bridging weakly acidic OH can be distinguished from one another semiquantitatively. This was substantiated by a quasi one-to-one relation between the number of bridging OH in a cluster Si−nAl containing only one Al and the number of strongly hydrogen-bonded acetone. The distinction between Brönsted and Lewis sites is not easy with acetone, while it is with mesityl oxide. The system acetone−mesityl oxide adsorbed an acid catalysts is rather unique, since the reagent and the reaction product of the condensation reaction are qualitatively and quantitatively detectable by FTIR spectroscopy. The quantitative determination of the coverage of the acid sites by the reagent and the reaction product was a prerequisite to the study of the reaction presented elsewhere.

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.

No abstract

No abstract