In-situ formation of hydrophobic deep eutectic solvents to separate lactic acid: Lactam as hydrogen bond acceptor and lactic acid as hydrogen bond donor

Lactic acid (LA) is a typical high added-value chemical and platform molecule derived from renewable biomass. To realize high efficiency and green separation of LA is of both scientific and practical significance. In this work, we proposed a novel mechanism of in-situ formation of hydrophobic deep eutectic solvents (HDES) to couple the separation of LA and synthesis of HDES. Due to the high solubility of LA as hydrogen bond donor in water, three hydrophobic lactams (N-Octyl-2-pyrrolidone (NOP), N-Dodecyl-2-pyrrolidone (NDP) and 2-Octyl-4-isothiazolin-3-one (OIT)) were optioned as hydrogen bond acceptors. The effect of lactam molecular structure on separation efficiency was discussed and compared in detailed. The compositions of HDES forming in-situ were found to be 1.50 for NOP and NDP to LA while 1.20 for OIT to LA using experiments of slope method and continuous variations method. The key evidence of formation of HDES was confirmed by FT-IR, frontier molecular orbital analysis and interaction region indicator analysis. The separation experimental results showed the in-situ formation of HDESs can transfer 99% of LA from fermentation broth to the HDES, and LA was separated completely from the impurities. This work provided a new perspective on lactic acid separation and its potential industrial applications.