(591f) Conversion Of Dihydroxyacetone To Lactic Acid

Lux, S., Graz University of Technology
Stehring, P., Graz University of Technology
Mertlitz, V., Graz University of Technology
Hilber, T., BDI - BioDiesel International AG
Siebenhofer, M., Graz University of Technology
Marr, R., Graz University of Technology

The demand of sustainable energy sources increases due to rising prices for fossil fuel and the increasing climate awareness. Specific relevance comes up to biodiesel. In order to develop a profitable biodiesel production subsequent processing of the by-product glycerol is crucial in order to make production more economic and competitive on a long term scale. Since the recent past, much effort in research and development has been focused on developing new and useful applications for glycerol. Even though the consumption of glycerol is steadily increasing through substantial growth in larger applications demand cannot follow supply. Since biodiesel has to compete with regular diesel fuel from crude oil the by products of biodiesel production are expected to contribute to its feasibility. Among others lactic acid is a promising product for subsequent processing of glycerol. In the last few years lactic acid consumption for industrial applications has increased steadily. This is due to lactic acid-based biodegradable polymers which are highly promoted because of environmentally friendly properties. Whereas the direct conversion of glycerol is difficult good results have been achieved in the fermentation of glycerol to form dihydroxyacetone. The isolation of dihydroxyacetone from fermentation broth is difficult too but dihydroxyacetone has advantageous properties for subsequent processing. Aim of this project was the investigation of the alkaline catalyzed transformation reaction yielding lactic acid out of dihydroxyacetone. Latter substance is obtained from glycerol by biotransformation. The conversion of hydroxy ketones to acids, aldol condensation and aldol addition are well known reactions. Formation of lactic acid prefers alkaline operation conditions. These reactions can be used in order to convert dihydroxyacetone to lactate and consequently obtain lactic acid. Opposite to the complex conditions needed for DHA isolation the separation of lactic acid from the reaction mixture needs much less stringent operation conditions.

Under alkaline conditions ß-hydroxy carbonyl compounds such as glyceric aldehyde easily eliminate the ß-hydroxy group. In alkaline solutions glyceric aldehyde and dihydroxyacetone are in equilibrium and both substances form pyruvic aldehyde by elimination. Pyruvic aldehyde is an important intermediate in the formation of lactic acid as it is not stable under caustic conditions. By subsequent hydrogen shift it is transformed to lactate. This step can be observed as some kind of intramolecular Cannizzaro-reaction. The reaction pathway for forming lactic acid from dihydroxyacetone follows two first order parallel reactions. Whereas part of the dihydroxyacetone is converted directly to form lactate part of dihydroxyacetone dimerizes and the dihydroxyacetone dimers subsequently convert to lactate too. Aim of this work has been the investigation of the transformation reactions of dihydroxyacetone yielding in lactic acid formation.