A review of aerobic glycerol oxidation processes using heterogeneous catalysts: a sustainable pathway for the production of dihydroxyacetone

The world's biodiesel increasing production is leading to the accumulation of its main by-product, crude glycerol, with almost no economic value, which valorization is crucial to increase biodiesel production sustainability and competitiveness. Glycerol is a potential platform chemical, with several valorization routes identified. Among them, selective catalytic aerobic oxidation is an attractive and sustainable solution, as high added value products ensure the process robustness against raw material price fluctuations. When glycerol's secondary hydroxyl group is selectively oxidized, dihydroxyacetone (DHA) is obtained. DHA is a high added value compound, used in cosmetics as the active compound in sunless skin tanning lotions, and its current industrial production by bio-fermentation is not satisfactory; therefore a more efficient production process is needed to overcome the market deficit. The state-of-the-art of DHA production by glycerol aerobic catalytic oxidation in the liquid phase with water as solvent was reviewed and, although it is still in the lab-scale phase, some routes to reach a robust commercial application were already suggested. For DHA production, catalysts should be active under base free conditions, in order to achieve high DHA selectivity. Promoted Pt nanoparticles, as Pt-Bi and Pt-Sb supported in carbon and mesoporous materials, and Au nanoparticles, supported late transition metal oxides as Au/CuO and Au/ZnO, are among the most promising catalysts for high DHA yield processes. For a better understanding of the main variables associated with this process, the effect of catalyst support, particle size, preparation and activation methods, and catalyst deactivation problems were analyzed. In addition, the reaction conditions effect in catalyst performance, including the presence of crude glycerol impurities was considered. Finally, the main studies regarding DHA continuous flow production were reviewed, identifying the major obstacles to overcome, so that commercial DHA production processes through glycerol aerobic catalytic oxidation can finally be implemented.