Low-cost racemic lactic acid oligomerization catalyzed by lipase in biphasic water/heptane system

Abstract

Lactic acid (LA) is an important building block for high added value polymers with technological and biomedical applications, such as 3d printing filaments, surgical sutures or food packaging. The manufacture of these materials implies the use of harmful organic solvents, metallic catalysts and high temperatures. The current trend in green chemistry demands a cleaner and more sustainable process. In this research, the esterification reaction of low-cost racemic LA was explored in a reusable biphasic water/heptane system at low temperature (30 ºC). The chosen catalyst was Candida antarctica lipase B (CALB), in the free and immobilized form. The functional groups of the recovered solids were identified by DRIFTS spectroscopy. The commercial immobilized CALB (Novozym435®) did not return any solid product under the explored conditions and significant leaching of the enzyme was observed by UV-Vis spectroscopy. On the other hand, the commercial CALB broth produced variable amounts of solid product in the water/heptane interphase with conversions (X) in the range of 2 ? 47 %, measured by HPLC. The highest product recovery was reached at 24 hours when conversion of LA achieved 37%. After that time the reaction went backwards. The LA polycondensation products were mostly water-soluble oligomers. The results suggest that CALB can start a new chain with either R or S-LA, but is stereoselective towards the R-LA for the chain growth. Additional experimental difficulties arose from the lipase broth?s excipients like sorbitol and glycerol which interfere in the esterification reaction. The outcomes presented herein provide a new starting point for LA polycondensation.