Simple TiO₂-based photocatalyst for HMF transformation under UV irradiation

5-hydroxymethylfurfural (HMF), a six-carbon heterocyclic organic compound containing both aldehyde and alcohol functional groups, is produced from the dehydration of hexose sugars under acidic conditions. The hydroxymethyl and carbonyl groups of HMF can be oxidized to form various important furanic chemicals, which serve as platform molecules for further transformations. During the last years, research in the field of photocatalysis has expanded from degradation of harmful compounds to synthesis as a green chemistry tool to replace dangerous and polluting routes. Different catalysts were already reported as active in the selective oxidation of HMF under UV or visible light irradiation. Doping of TiO₂ with rare earth metals is a very efficient way to improve its photocatalytic performance as a result of different factors. In our work, TiO₂ containing different amounts of Er and Yb was prepared using the sol–gel method and then calcined at 500 and 750^oC, respectively. The characterization of these samples showed that the phase distribution of these is different than that of the pristine TiO₂. While the pristine TiO₂ calcined at 500^oC contained about 5% anatase along with the predominant rutile phase, the doped samples contained 100% anatase. The increased calcination temperature led to a content of about 66% in rutile for TiO₂, compared to the doped samples, which contained a maximum of 28% rutile for 1%Yb-TiO₂. These catalysts were active under UV irradiation, more active than the pristine TiO₂, but most of them oxidized HMF to lactic acid (LA, major product) and glycolic acid (GA). The activity increased with the amount of dopant, and decreased with the increase of the calcination temperature. The only catalysts that led to 2,5-hydroxymethylfurancarboxylic acid (HMFCA) as the reaction product were 1%Er-TiO₂ calcined at 750⁰C and 1%Er-1%Yb-TiO₂ calcined at 500⁰C, the samples with the smallest amount of rutile. No base was added in our experiments.