Selective formation of glycolic acid by photocatalytic conversion of glycerol under sunlight with SnO2-based heterojunctions

Abstract

Glycerol is a byproduct of biodiesel synthesis and is one of the most readily available platform molecules originating from biomass. Heterogeneous photocatalysis is a strategic approach, and reactions run under ambient conditions convert glycerol into industrially relevant molecules such as glycolic acid. The p-n type heterojunctions are useful photocatalysts because they can decrease the bandgaps and facilitate electron promotion between the bands of materials, thereby optimizing the formation of reactive oxygen species (ROS) and increasing reaction efficiencies. In this study, photocatalysts were synthesized via coprecipitation with a 5 % w/w mixture of Cu2+ and Ni2+ precursors, which produced the p-n heterojunctions SnO2/CuO-5 % and SnO2/NiO-5 %, respectively. Was observed an increase in the surface areas of the materials, which were ∼19 and 62 m2/g for SnO2/CuO-5 % and SnO2/NiO-5 %, respectively. The addition of metals to the heterojunctions enabled the use of solar light, due to the decrease of the bandgaps of the materials to 1.9 and 2.0 eV for SnO2/CuO-5 % and SnO2/NiO-5 %, respectively; these were lower than of SnO2 (2.54 eV). The photocatalytic activities of the heterojunctions were tested via glycerol oxidation, and after 4 h, the SnO2/CuO-5 % heterojunction exhibited the highest reaction efficiency (19 %), followed by 13 % and 11 % for SnO2/NiO-5 % and SnO2, respectively. The selectivities indicated the reaction pathway, in which glyceraldehyde and tartaric acid were intermediates for glycolic acid production; glycolic acid is the precursor to polyglycolic acid (PGA), for which production levels are increasing. The results of this study point to new technological pathways enabling responsible use of the resources present in nature and reuse of their byproducts, as with the conversion of glycerol to economically attractive molecules and contribute to a sustainable economy.