Catalytic Conversion of Dihydroxyacetone to Lactic Acid with Brønsted Acids and Multivalent Metal Ions

At present, a vast majority of products such as plastics, cosmetics or pharmaceuticals are still petroleum-based. A shift towards renewable bio-based feedstock will be inevitable in the future. However, the use of biomass for chemical production implicates the redesign of existing processes and involves new challenges. Handling of aqueous reaction systems and the need for adapted separation techniques from aqueous solutions come to the fore. Retrieval of biomass which does not compete with the food production draws attention to the exploitation of residual materials. Glycerol (propane-1,2,3-triol) is a versatile molecule obtained as a by-product in the biodiesel manufacturing process. Due to increasing biodiesel production over the last decades, it may be regarded as a potential renewable feedstock chemical. Exploitation of glycerol for the production of lactic acid is a promising approach. Lactic acid (2-hydroxypropionic acid) is emerging as a building block in a new generation of biobased materials. With both a hydroxyl and a carboxylic acid group, lactic acid may undergo many reactions. It therefore represents a central feedstock for the chemical industry, e.g., in the production of propylene glycol, acrylic acid or different condensation products. Besides applications in the food and cosmetics industries, growth in demand is expected due to expanding polymer markets (biodegradable synthetics) and elevated demand in the chemical sector due to ecologically friendly solvents and oxygenated chemicals.1,2 Nevertheless, commercial success depends on the development of an effective production method for highly pure lactic acid from abundant non-food biomass. Oxidation of one of the three alcoholic functions of glycerol gives access to the synthesis of the trioses dihydroxyacetone (1,3-dihydroxypropan-2-one, DHA) and glyceraldehyde (2,3-dihydroxypropanal, GLAH), which may further be catalytically converted to lactic acid. Dihydroxyacetone from glycerol is accessible via different synthesis routes. Chemical synthesis involves liquid phase self-condensation of formaldehyde with thiazolium catalysts.3 Much research has been done in the area of noble metal catalysed oxidation of glycerol.4–6 Partial electrochemical oxidation of glycerol to dihydroxyacetone is carried out with different electrode types in neutral7, alkaline8–11 and acidic media10,11. The first microbiological dehydration of glycerol to dihydroxyacetone was reported in 1898 by Bertrand.12 Since then, several routes for microbiological synthesis of dihydroxyacetone have been developed and patented. Mainly vinegar bacteria type Acetobacter and Gluconobacter are used.13,14 Catalytic Conversion of Dihydroxyacetone to Lactic Acid with Brønsted Acids and Multivalent Metal Ions