High Performance Dehydration Membranes For Esterification-Pervaporation Coupling Process

Ethyl lactate, as a green solvent, has great development value and application prospect, its traditional production method suffer from a lot of drawbacks including the high temperature of dehydration, high consumption of energy and raw material and high cost. It is important to develop new technologies which dominated with high yield of ethyl lactate, low temparature, low consumption of energy and low cost. Recently, it was found that pervaporation-assisted esterification process has important research value because of its outstanding features including green technology, high effiency and low consumption of energy.The key technique of this coupling prosess is to develop a pervaporation membrane with excellent dehydration performance.In this study, chitosan was used as the bulk membrane material and the chitosan organic-inorganic hybrid membranes (including CS/SiO₂ and CS/TiO₂ hybrid membranes), the cross-linking modified chitosan membranes (the cross-linking q-CS/GA membrane) and the modified chitosan organic-inorganic hybrid membranes (q-CS/TiO₂ hybrid membrane) were developed. The prepared membranes were characterized by FTIR, SEM, XRD, DMA, TG/DTG, DSC, the measurement of contact angel and some other methods. The swollen and adsorption performance of these membranes, the dehydration performance of pervaporation and its function in the esterification of lactic acid and ethanol coupled with pervaporation were investigated.Furthermore, the effects of modification methods, such as organic-inorganic hybridization, quaternization and cross-linking, on the physicochemical properties, the configuration and structure and the swollen and adsorption performance of chitosan membrane were studied. Then, the dehydration performance of these modified chitosan membranes for pervaporation and esterification were investigated. It was found that, the hydrogen-bonding interaction between the chitosan moleculs was destroyed by introducing the inorganic nanparticles, the substituting group -(CH3)3Cl of -NH2 and the glutaric dialdehyde as the cross-linking agent. These factors resulted in the increasing hydrophilicity of membrane, the decrease of crystallinity and the increase of the membranes'pervaporating dehydration perfomance, consequently, the esterification of lactic acid of ethanol was enhanced. As a result, the yield of ethyl lactate in pervaporation-assisted esterifaction of lactic acid and ethanol could increased to 79.9, 78.2, 80.7 and 78.8wt% by using CS/SiO₂(02), CS/TiO₂(06) hybrid membrane, q-CS/GA(1.5) cross-linked membrane and q-CS/TiO₂(06) hybrid membrane, meanwhile, the yield of ethyl lactate by using chitosan membrane was 65.5wt%, which was only 60.8% without pervaporation.Using molecular dynamics simulation technique, the structure model of chitosan- SiO₂ hybrid membrane was constructed. The interaction between chitosan and SiO₂ particles was investigated. Next, the free volume properties of chitosan membrane and chitosan-SiO₂ membranes and the effect of introducing inorganic particles on the diffusion process of water and ethanol in chitosan hybrid membrane were studied. It suggested that the fractional free volume of hybrid membrane for ethanol molecule was decreased much more than that for the water molecule, accordingly, the diffusion selectivity of hybrid membrane increased more remarkably. Moreover, the smaller the SiO₂ particles, the more evidently the diffusion selectivity can increase.