Chemical Modifications of Polyimide Membranes for Pervaporation Dehydration

Author:Xu Sheng

Supervisor:wang yan


Degree Year:2019





Biofuels,especially bioalcohols,have gained increasing attention with the scarcity of fossil fuels and the aggravation of environmental pollution.Pervaporation,as a membranebased process,is extremely competitive for the separation of bioalcohols from the fermentation system as compared to traditional separation technologies,ascribed to its environmental benignity,low cost,and especially high efficiency for the separation of azeotropic mixtures.As well known,the membrane is one of the key factors determining the separation performance of pervaporation.In terms of the material selection of the membrane for pervaporation dehydration,polyimide has been proved as an excellent candidate,because of their superior physicochemical properties to most conventional hydrophilic polymers,such as high thermal stability and excellent mechanical properties.However,they still face some problems,including low flux and inevitable swelling.Herein,various chemical modifications,including thermal crosslinking,hydrazide crosslinking and thermal cyclodehydration,are carried out for the polyimide membranes,and the effects of different modification methods on the corresponding physicochemical properties,separation performance and long-term operation stability are investigated.Firstly,two novel carboxyl-containing polyimides are synthesized via chemical and thermal imidization methods,respectively,then a post thermal treatment is applied for the corresponding membranes.The results indicate that the thermal treatment exhibits similar effects on the physicochemical properties and separation performance of the two carboxyl group-containing polyimide membranes.The thermal treatment at a relative low temperature only leads to the thermal annealing,while it may cause the thermally-induced decarboxylation crosslinking at a higher temperature.Therfore,the separation performance of the polyimide membranes for ethanol dehydration can be improved by thermal crosslinking at a proper temperature.Benchmarking shows that the crosslinked polyimide membranes have superior performance to most other dense membranes.Secondly,a novel kind of crosslinker,1,3,5-benzenetricarboxylic acid trihydrazide(BTCH),is used to in-situ crosslink the polyimide membrane for isopropanol dehydration via pervaporation.The hydrazide group in BTCH can react with the imide group in polyimide to form a structure of network crosslinking.The effects of BTCH content and membrane-forming temperature on the physicochemical properties and separation performance of the polyimide membrane are studied.The optimized crosslinking results in the stable networking of polyimide chains,and therefore effectively enhances the separation factor of the corresponding polyimide membrane with a comparable flux.Thirdly,a thermal treatment is applied for the first time to modify the hydrazidecontaining polyimide membrane.The results show that the thermal treatment has two opposite effects,i.e.,thermal annealing and thermal cyclodehydration,which are both enhanced with the increase in the thermal treatment temperature.With a suitable thermal treatment temperature,an optimized separation performance of the thermallycyclodehydrated poly(imide-oxadiazole)membrane can be achieved for isopropanol dehydration,where the flux is enhanced by 2 folds and the corresponding separation factor is only slightly reduced.In addition,the optimized poly(imide-oxadiazole)membrane shows the excellent long-term operation stability,probably ascribed to the stable morphology and rigid chemical structure with effective thermal annealing and cyclodehydration.Finally,based on the above works,a combined modification is exploited for the polyimide membrane by terephthalic dihydrazide crosslinking and subsequent thermallyinduced cyclodehydration.The effects of the combined crosslinked and thermallycyclodehydrated modification on the physicochemical properties and separation performance of the polyimide membrane are investigated and characterized by various techniques.In the meantime,some other modifications(thermal annealing,p-xylenediamine crosslinking and terephthalic dihydrazide crosslinking)are also involved in this study for a comprehensive comparison.It can be found that,only the combined modification of crosslinking and thermal cyclodehydration can enable the polyimide membrane to overcome the trade-off effect between permeability and selectivity for pervaporation dehydration of alcohols(methanol,ethanol and isopropanol).After this combined modification,the polyimide membrane shows an improved overall separation performance by about 50% for ethanol dehydration,and a satisfactory separation performance for isopropanol dehydration with a prominent selectivity and an enhanced permeability.