Photooxidation of 1-Alkenes in Zeolites: A Study of the Factors that Influence Product Selectivity and Formation

Abstract

Photochemical oxidation of hydrocarbons with molecular oxygen is potentially an environmentally benign method for the selective oxidation of hydrocarbons. In this study, in situ FT-IR spectroscopy and ex situ NMR spectroscopy were used to investigate the factors that influence product formation and selectivity in the room-temperature photooxidation of 1-alkenes in zeolites. Upon irradiation with broadband visible light, propylene, 1-butene, and 1-pentene loaded in BaY were photooxidized with molecular oxygen. As discussed in the literature, initial excitation of alkene and molecular complexes results in the selective formation of unsaturated aldehydes and ketones, proposed to occur through a hydroperoxide intermediate. In addition, epoxide and alcohol products are formed when the hydroperoxide intermediate reacts with an unreacted parent alkene molecule. Here it is shown that saturated aldehydes and ketones are formed as well through both a thermal ring-opening reaction of the epoxide in BaY and a second photochemical oxidation route involving a dioxetane intermediate. The yield of saturated aldehydes and ketones increased with decreasing wavelength, increasing temperature, and at a given temperature and wavelength, increasing chain length. Photooxidation of propylene in BaX, BaZSM-5, and BaBeta zeolites was also investigated. Photooxidation in BaX is very similar to that of BaY. In zeolites, BaZSM-5 and BaBeta, propylene polymerized upon adsorption. The polymer, polypropylene, also undergoes photooxidation with molecular oxygen to form an oxygenated polymer product. The results of this study show that product formation and selectivity in the photooxidation of 1-alkenes in zeolites depends on several factors. These factors include thermal reactions of the reactant and photoproduct molecules in the zeolite at ambient temperatures. Several reactions of 1-alkenes in cation-exchanged zeolites contribute to the loss of selectivity; they include: epoxide ring opening, double-bond migration, and alkene polymerization. Some of these reactions are proposed to occur at Brønsted acid sites that are present in various amounts in cation-exchanged zeolites.