Possible Trace of HMF Thermal Oxidation for Titanium-Based Catalysts: Succinic Acid, Furoic Acid, and Maleic Acid

Titanium dioxide is used extensively in ultraviolet-visible irradiation to induce oxidation. However, this method relies on daylight harvesting. Alternatively, our experiment utilized titanium dioxide directly under thermal excitation. Three different titanium oxide-based catalysts, i.e., commercial titanium dioxide Degussa P25, black titanium dioxide, and black titanium dioxide modified with copper oxide, were investigated at moderate temperatures (70 and 120 °C). The utilization of titanium-based catalysts with tert-butyl hydroperoxide (TBHP) in alkaline water is practical for oxidizing carbonyl derivatives of furan to one or two carbon-less carboxylic acids, such as succinic acid (selective to 38-87%), furoic acid (selective to 0-70%), and maleic acid (selective to 3-39%), under moderate conditions. These titanium-based catalysts differ in their physicochemical properties, resulting in variations in their catalytic activity and selectivity, and are active at different temperatures. Electron paramagnetic resonance (EPR) revealed no traces of oxygen vacancy sites on the titanium oxide. In contrast, black titanium dioxide consisted of an oxygen vacancy with one electron, whereas copper-modified black titanium dioxide consisted of a doubly ionized oxygen vacancy.