Study On Synthesis Of L-pyroglutamic Acid And Solid-Liquid Equilibrium Of Separation System

L-pyroglutamic acid and its derivatives are important organic compounds,which have widely used in various fields,such as medicine,food,daily chemicals,and agriculture.At present,most reports involve the synthesis of L-pyroglutamic acid,and there is a lack of research on its crystallization and phase equilibrium.This article mainly studies the equilibrium data and phase diagram separation of L-pyroglutamic acid in different solvents for improving its quality and yield in the production process.Firstly,L-pyroglutamic acid is prepared by dry pyrolysis with L-glutamic acid as raw material,and the process of the reaction is optimized through single-factor experiments.The optimal conditions for the preparation of L-pyroglutamic acid in the absence of a catalyst are determined:the temperature is 150℃,and the reaction time is 45 minutes.The type and dosage of the catalyst in this process are investigated,and the optimal conditions for preparing L-pyroglutamic acid are finally determined:the temperature is 150℃,the time is 45 minutes,and 1.0 mmol ZnSO4·7H2O is used as the catalyst;and the yield is 71.73%.In addition,the recrystallized product of L-pyroglutamic acid is characterized by Fourier transform infrared spectroscopy(IR)and hydrogen nuclear magnetic spectroscopy(1H NMR).Secondly,the solubility of L-Pyroglutamic acid in water,methanol,ethanol,n-propanol,isopropanol and four binary solvent systems(water+methanol,water+ethanol,water+n-propanol,water+isopropanol)is experimentally determined under atmospheric pressure by using the equilibrium method.The solubility of L-Pyroglutamic acid in all the tested pure and binary solvent systems monotonously increases with increasing of the temperature at the measurable range.The solubility order of L-Pyroglutamic acid in pure solvents is ranked as:water>methanol>ethanol>n-propanol ≈ isopropanol.Besides,ideal model,Xh model,Wilson model the Apelblat model and NRTL model are used to correlate the experimental solubility data of L-Pyroglutamic acid in five pure solvent systems and the solubility in binary solvent systems are also correlated by the Apelblat-Jouyban-Acree(A-JA)model and the Van’t Hoff=Jouyban-Acree(Van’t-JA)model.In pure solvents,the maximum relative average deviation(RAD)is 6.633%;in binary solvents,the RAD value of the NRTL model is less than 2.257%,and the other six models are all less than 2.0%.All of them give satisfactory correlations.Finally,the mixing and dissolution thermodynamic properties,including Gibbs energy,entropy and enthalpy of L-Pyroglutamic acid in all tested solvents are calculated by the Wilson equation and the solubility data.Thirdly,the solid-liquid phase equilibrium for the ternary system of L-Pyroglutamic acid+L-Glutamic acid+water is determined experimentally by the equilibrium method at temperatures of 298.15 K,308.15 K,318.15 K,and 328.15 K under normal atmosphere.According to the obtained solubility data,four ternary phase diagrams and the variable-temperature phase diagram are constructed.The crystallization regions of L-Pyroglutamic acid and L-Glutamic acid increase with the increase temperature.Moreover,at a fixed temperature,the crystallization region of L-Glutamic acid is a little larger than that of L-Pyroglutamic acid.The data of solid-liquid phase equilibrium are correlated and computed with the NRTL model and Wilson model.The RMSD value was no more 2.12×10’3 of the Wilson model and 40.3×10-3 for the NRTL model,the Wilson model provided correlation results better than those from the NRTL model for the ternary system at studied temperatures.Therefore,the Wilson equation is more consistent with the experimental data and more suitable for describing the ternary system of L-Pyroglutamic acid+L-Glutamic acid+water.Finally,based on the drawn ternary constant temperature/variable temperature phase diagram,two separation paths are proposed to separate the mixture of L-pyroglutamic acid and L-glutamic acid.The two separation paths are compared and analyzed,and the yields of the two substances during the cycle of the system are calculated.The results show that both separation routes could obtain pure L-pyroglutamic acid and pure L-glutamic acid,and the second separation route is simpler and more efficient.