Study On Regulation Of Interlayer Spacing And Separation Performance Of Pervaporation Membrane Based On Carboxylated Graphene

Graphene derivatives such as graphene oxide（GO）,as a class of typical two-dimensional nanomaterials,can be used to construct ultrathin separation layers of interlayer ordered nanochannels and endow it with excellent pervaporation separation performance,which has become one of the most promising nanobuilding blocks in the next generation of high-performance separation membranes.Among them,carboxylated graphene（CG）,as one of the derivatives of graphene oxide,has higher hydrophilicity and dispersibility than GO.However,for water-phase molecular separation,the hydrophilic carboxylated graphene interlayer channels can adsorb a large number of water molecules,resulting in increased interlayer spacing（swelling）and even membrane delamination.The prepared carboxylated graphene membranes usually have low separation coefficients and poor long-term stability.Therefore,improving the stability of the carboxylated graphene composite membrane is the key to ensure its application.In this paper,the carboxylated graphene composite membranes were prepared by chemical cross-linking.SEM,XPS,FTIR,XRD and water contact angle measurement were applied to characterize the physicochemical properties and microstructure of the as-prepared membranes,and the separation effect of the n-butanol/water mixture was investigated.The specific content mainly includes the following three aspects:（1）Using ethylene glycol,1,3-propanediol and 1,4-butanediol as cross-linking agents,covalently cross-linked carboxylated graphene/polyethersulfone（CG/PES）composite membranes were prepared on the dopamine-modified PES support layer by suction filtration.SEM characterization showed that the separation layer of the composite membrane was continuous and defect-free,and the thickness was between 60 nm and 64 nm.XPS characterization results demonstrated that the diols reacted successfully with the carboxyl groups on the CG nanosheets.XRD characterization showed that the introduction of the cross-linking agent could effectively control the interlayer spacing of the separation layer.With the increase of the diol molecular chain length,the interlayer spacing of the obtained composite membranes increased from 0.761 nm to 0.778 nm.The separation performance test showed that the CG/PES composite membrane had good separation performance for n-butanol/water mixture.When the feed liquid temperature was 50℃and the water mass fraction was 10%,the permeation fluxes of the composite membranes obtained by the three cross-linking agents reached to 0.79 kg·m^-2·h^-1,0.87 kg·m^-2·h^-1 and 0.96 kg·m^-2·h^-1,respectively.While the separation factor is an order of magnitude higher than that of the uncrosslinked composite membrane.Separation performance remained stable over 15 days of repeated testing.（2）Using ethanolamine,diethanolamine and triethanolamine as cross-linking agents,covalently cross-linked CG/PES composite membranes were prepared on the dopamine-modified PES support layer by vacuum filtration.XPS and FTIR characterizations demonstrated that reactions occurred between carboxylated graphene and alkanolamine molecules.XRD characterization showed that the interlayer spacing of the composite membranes increased from the original 0.731 nm to 0.895 nm,0.951 nm and 1.014 nm,respectively,after the introduction of alkanolamine molecules of different sizes into the CG nanosheets.The separation performance test shows that the composite membrane has good separation performance for n-butanol/water mixture.When the feed liquid temperature was 50℃and the water mass fraction was 15%,the permeation flux and separation factor of the ethanolamine-crosslinked composite membrane were 1.39 kg·m^-2·h^-1 and 1145,respectively,and the separation performance did not change significantly during the 15-day stability test.（3）Using p-phenylenediamine hydrochloride（PDHC）as a cross-linking agent,the composite membranes were prepared by layer-by-layer self-assembly（Lb L）deposition of CG nanosheets on a dopamine-modified PES support layer.XRD characterization shows that the interlayer spacing of the composite membrane prepared by the layer-by-layer self-assembly method is about 0.680 nm,which is significantly reduced compared with the composite membrane prepared by the vacuum filtration method.SEM characterization showed that the thickness of the separation layer was ultra-thin and relatively uniform.The separation performance test shows that the composite membrane deposited with 8 layers has the best separation performance for the n-butanol/water mixture.When the feed liquid temperature is50℃and the water mass fraction is 10%,the permeation flux and separation factor are 1.96kg·m^-2·h^-1 and 1146,the separation performance remained stable in 15 days of repeated tests.