Uranyl ion sensitised photooxidation of alkanes

Abstract

The focus of this work is on the development of strategies to use a transition-metal complex as a photocatalytic agent to activate/transform low molecular weight alkanes into oxygen-containing substances, mainly alcohols and ketones. In this work, cyclic, branched and straight chain materials have been chosen as representative of the three major alkane subcategories, and all of the experiments have been carried out in aqueous solutions at room temperatures and pressures. For these studies, uranyl ion was chosen to serve as the light antenna, or the "photo-catalyst". Results presented in this work show that at room temperature and room pressure along with visible light, the resulting excited uranyl ion *UO22+ is an effective species for oxygenation of all three alkane subcategories (cyclic, branched and straight chain hydrocarbons). Observed quantum yields of 0.022, 0.087 and 0.01 are found for the isobutane system, cyclopentane system and pentane system, respectively. Peroxydisulfate has been shown to be an effective amplification agent for all these processes. In the presence of 1 mM peroxydisulfate, quantum yields increase 4 to 50 times for correspondingly different systems. In the isobutane system, quantum yields higher than unity have been achieved even though only 6% of the excited uranyl ions are quenched by isobutane. Quantum yields increase with increased alkane and perchloric acid concentrations. In the absence of peroxydisulfate, uranyl ion concentration and light intensity have no significant influence on the quantum yield. However, in the presence of peroxydisulfate, increased light intensity leads to decreased quantum yield. Increased concentrations of peroxydisulfate favour higher quantum yields. In these uranyl-ion sensitised photooxidation processes, no net consumption of UO22+ can be detected in the presence of oxygen and/or of peroxydisulfate (pH $