Catalytical conversion of carbohydrates in subcritical water: A new chemical process for lactic acid production

Abstract

In the last years the production of lactic acid has increased due to growing polymer markets (biodegradable synthetics), elevated demand in the chemical sector (oxygenated chemicals, ecologically friendly solvents) and many applications in the food industry. At present, the annual lactic acid production is about 100,000 t, most of that from the fermentation of carbohydrates. The disadvantages of this process are small space-time-yields and the production of stoichiometric amounts of salt. In pure sub- and supercritical water (SCW) only small amounts of lactic acid are obtained from the degradation of carbohydrates. But by adding small quantities of metal ions such as Co(II), Ni(II), Cu(II) and Zn(II) to that reaction media, the lactic acid yield is increased up to 42% (g g −1) starting from sucrose and 86% (g g −1) starting from dihydroxyacetone at 300 °C and 25 MPa. Zn(II) gave the best results with regard to the lactic acid yield. The function of the catalyst in the complex reaction network of carbohydrate degradation is discussed. An alternative lactic acid production process is proposed.