WO3/SiO2 SOLID ACID CATALYST FOR SYNTHESIS OF GREEN SOLVENT: ETHYL LACTATE

Esterification of lactic acid to ethyl lactate is a very important transformation due to the applications of ethyl lactate as a green solvent. This esterification has been carried out using 10% WO3/SiO2 as a solid acid catalyst. Liquid phase esterification at 80°C, with 1:5 ratio of lactic acid:ethanol gave a maximum 61% yield of ethyl lactate. Commercial lactic acid (85%) contains about 55% monomers and about 43% oligomers. In the presence of ethanol, monomeric lactic acid and oligomers undergo esterification, and it is not very easy to quantify the lactic acid conversion as well as ethyl lactate yield; hence, the analysis of the reaction mixture is very crucial. The lactic acid conversion as determined by titration method showed 50% total lactic acid conversion, whereas analysis by HPLC showed 51% conversion of the lactic acid monomer. The results indicated depolymerization of the lactic acid oligomers present in a commercial 85% lactic acid solution used for esterification during the reaction due to the water present in the reaction system in the presence of a solid acid catalyst. During the reaction, the pH of the reaction mixture increased from ~ 2 to ~ 5, the pH at which tungsten oxide is reported to form complex with lactic acid. Detailed characterization of the fresh and used catalyst revealed a decrease in W concentration on the surface in the used catalyst, indicating the leaching of tungsten oxide as lactic acid complex, due to which the catalyst could not be recycled.