Synthesis of lactic acid either through fermentation of carbohydrates or through chemical synthesis is well known. Glycerol based synthesis of lactic acid is a new approach. Glycerol is formed as a by-product of fat transesterification with alcohols. Increased production of biodiesel has led to a surplus of glycerol on the global market. Much effort in research and development has been focused on developing new and useful applications for glycerol.
Aerobic conversion or partial electrochemical oxidation of glycerol are efficient routes for converting glycerol to dihydroxyacetone. Whereas the isolation of dihydroxyacetone is difficult subsequent processing is advantageous.
Aim of this project was the catalytic transformation of dihydroxyacetone yielding lactic acid. The conversion reaction can either be carried out under caustic conditions or under acidic conditions. In both cases pyruvic aldehyde is formed as an intermediate. Subsequently pyruvic aldehyde is rearranged through hydrogen shift to lactic acid. This step can be seen as a kind of intramolecular Cannizzaro reaction. Whereas dihydroxyacetone is directly converted to pyruvic aldehyde under acidic conditions, the reaction runs over C6-adducts as primary intermediates under caustic conditions.
In this project homogeneous and heterogeneous catalysts were investigated for their capability of dihydroxyacetone conversion to lactic acid. The concentration of the catalyst, the reaction temperature as well as reaction time was varied. Heterogeneous catalysis under caustic conditions works well with caustic lime. The catalytic activity of amine based ion exchangers is poor. Acidic catalysts such as hydrochloric acid have proven best regarding selectivity and yield of lactic acid. Strong cation ion exchange resins can be used as well. The rearrangement reaction of pyruvic aldehyde to lactic acid is rate determing.