Photooxidation of 1-alkenes in zeolites: A study of the factors that influence product selectivity and formation

Photochemical oxidation of hydrocarbons with molecular oxygen is potentiall y an environmentally benign method for the selective oxidation of hydrocarb ons. 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 discuss ed in the literature, initial excitation of alkene and molecular complexes results in the selective formation of unsaturated aldehydes and ketones, pr oposed 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 a ldehydes 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 ke tones 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. P hotooxidation in BaX is very similar to that of BaY. In zeolites, BaZSM-5 a nd BaBeta, propylene polymerized upon adsorption. The polymer, polypropylen e, also undergoes photooxidation with molecular oxygen to form an oxygenate d polymer product. The results of this study show that product formation an d selectivity in the photooxidation of l-alkenes in zeolites depends on sev eral factors. These factors include thermal reactions of the reactant and p hotoproduct molecules in the zeolite at ambient temperatures. Several react ions of l-alkenes in cation-exchanged zeolites contribute to the loss of se lectivity; they include: epoxide ring opening, double-bond migration, and a lkene polymerization. Some of these reactions are proposed to occur at Bron sted acid sites that are present in various amounts in cation-exchanged zeo lites.