Fundamentals And Processes For Separation And Purification Of Lactic Acid By Amide-Based Deep Eutectic Solvents

With the increasing concern about environmental issues,fully bio-based degradable materials have become one of the important alternatives to petrochemical-based plastics.Organic carboxylic acids,an important class of platform compounds produced from biomass fermentation,can be converted into bio-based degradable polymeric materials through several steps,including extraction,purification and polymerization.At present,the biomass fermentation technology is becoming more and more perfect and can meet the production demand,but the process of separating and purifying organic carboxylic acids from fermentation broth to prepare raw materials for polymer synthesis suffers from the problems that the selected solvents are not green enough,the selectivity of carboxylic acid solvents with similar properties is poor,and the separation mechanism of the system is not clear,which makes it difficult to obtain high purity carboxylic acid products.In this paper,the basic theory and process of the separation and purification of lactic acid（LA）from fermentation broth by hydrophobic DESs are studied to obtain the technical route and process parameters for the production of lactic acid with low production cost,high product purity,high yield and green environment protection,to reveal the influence mechanism of the structure and composition of hydrophobic DESs on the separation of lactic acid,and to discuss in depth the basic theory of the mechanism involved in the separation and purification process.A new system for the separation of carboxylic acids by forming hydrophobic DESs with amides was developed by using a simple liquid-liquid extraction method,and the distribution coefficients（K_D）for the interaction of carboxylic acids with amides of different structures under this system were systematically investigated.The K_D of carboxylic acids in hydrogen-bonding acceptor solvents containing amide structures is larger.The K_D of lactic acid in N,N-dibutylacetamide（DBA）is 2.17 and in N,N-dibutylbutyramide（DBB）is 0.90.The key factors affecting K_D were revealed by quantum chemistry calculation.The maximum electrostatic potential and polarity index on the surface of carboxylic acid were negatively correlated with K_D,and the hydrophobicity and molecular volume of carboxylic acid were positively correlated with K_D.The effects of different structural carboxylic acids on the stability of hydrophobic DESs were analyzed by thermodynamic data of amide and carboxylic acid interactions the binding constant K_ex and Gibbs free energyΔG.The essence of separation of the carboxylic acid via forming hydrophobic DESs was revealed by molecular dynamics simulations using lactic acid as an example,which was the competitive competition between amide acceptor molecules and water molecules for the carboxylic acid,and the dynamic reversibility of lactic acid between the organic and aqueous phases of the amide was used to purify it.Hydrophobic DESs were prepared using geraniol,menthol and thymol as hydrogen bond donors,amide DBA and DBB as hydrogen bond acceptors,and the separation selectivity for hydroxycarboxylic acid such as LA and malic acid（MA）and tartaric acid（TA）in fermentation broth was investigated by the interaction with carboxylic acid solutions using liquid-liquid extraction.The mechanism of geraniol,menthol and thymol improving the separation selectivity of hydroxycarboxylic acids with similar properties was revealed by analyzing the intermolecular interactions in the system using density functional theory DFT.Hydrogen bond donors and amides formed relatively stable hydrophobic DESs,bond lengths were 1.77～1.80（?）and bond energies were-11.48～-16.53 kcal/mol through single hydrogen bonds.The hydroxycarboxylic acids formed double hydrogen bonds with amides with bond lengths of 1.57～1.63（?）and bond energies of-16.40～-17.69 kcal/mol.The terpene donors competed with the carboxylic acids to improve the separation selectivity of hydroxycarboxylic acids with similar properties and mainly influenced the hydrophobic solventization effect of DESs.Using the Kremser method to simulate counter-current extraction,it was demonstrated that the addition of geraniol and menthol to hydrophobic DESs improved the separation selectivity of LA and MA and TA.In addition,the separation of LA and non-hydroxyl carboxylic acid in this system was achieved by using the external field to enhance the temperature effect,and the mechanism of the separation was explained microscopically by molecular dynamics simulations.In order to meet the needs of industrial production and achieve high yield and purity of LA,the tandem extraction theory was applied to the separation and purification of lactic acid from fermentation broth by hydrophobic DESs.The theoretical number of extraction stage was 13 and washing stage was 15 by setting the yield and purification multiplier,conducting material balance and mathematical equation derivation.The system reached steady state in 200 min,and the high-quality lactic acid was obtained with the yield and purity of more than 99%.The continuous bench experiments and the pilot test mutually verified the process parameters,purity and yield of cross-current extraction,counter-current extraction and fractional extraction.The experimental results proved the accuracy of theoretical simulation and bench test parameters.High quality lactic acid was obtained by fractionation extraction and characterized by NMR and HPLC.This study will expand the application of DESs in other carboxylic acid separation and help design new solvents to improve carboxylic acid separation,and it provides the direction for the design of green solvent for the selective separation of carboxylic acids in biological processes.