Preparation And Separation Properities Of Metal-Organic Framework Membranes

With the development of modern society,the aggravating environmental problem,including rapidly elevating global carbon dioxide level and water pollution,has posed a serious threat on human beings.The use of fossil fuels takes major accounts for carbon dioxide emission.Developing effective carbon dioxide capture and storage?CCS?technology and using hydrogen as alternative energy sources were considered to be a feasible solution to this.The technology for pre-combustion is widely studied.In this case,the fossil fuels were first partially oxidized into syngas?H₂ and CO?,then transformed into CO₂ and H₂ through water-gas shift.The resulting CO₂ was sequestered while H₂ was further utilized,in which an effective H₂/CO₂ separation technology was needed.In addition,H₂/CO₂ separation technology is also necessary in hydrogen production since H₂ and CO₂ mixture is the final product in prevalent process via steam-methane reforming.Membrane-based separation technology is promising in gas separation applications owing to lower energy cost and smaller carbon footprint.The exploration of new membrane material received huge attention both academia and industry.Wastewater from industrial process contains contaminants including heavy metal ions,large amount of dyes and other organic pollutants,thus has to be properly treated before discharge in case of water pollution.Dyes are well known for their hazardous effects on the environment in general and human health in particular.Conventional physico-chemical methods such as adsorption are expensive and most importantly,dyes are of different properties,causing difficulties in selecting proper adsorbents.Membrane-based process including ultrafiltration and nanofiltration has shown great potential in wastewater treatment.Membranes with adequate pore structure can reject the passage of large dye molecules,therefore high purity water can be obtained.This process was not severely affected by chemical properties of dyes,and hence can be applied to remove a range of dyes from water.As an emerging material,metal-organic framework?MOF?membranes received huge attention.MOF membranes possess uniform pore structure and the size or properties can be readily adjusted by varying metal clusters or organic linkers,which gives them the opportunity to be precisely designed for specific separation applications.Earlier report by Lin et al.introduced the fabrication of a MOF-5 membrane by secondary growth.This membrane showed low gas selectivity and insufficient stability due to the structure of MOF-5.Herein,we take advantage of designability of MOFs and improve separation performance of MOF-5 membrane.We used2-methyl-1,4-dicarboxybenzene and 2-aminoterephthalic acid as substitution of1,4-dicarboxybenzene to synthesis methyl modified and amino modified MOF-5membrane on a Zinc oxide disc via‘twin Zinc source'method.Compared to original MOF-5 membrane,-CH₃ modified membrane showed an enhanced stability and–NH₂modified membrane showed an improvement in H₂/CO₂ separation factor,demonstrating the feasibility of using MOFs as platform to realize target modification.Regarding separation of H₂ and CO₂,the distinction in their kinetic diameters can be used to generate substantial difference in diffusion rate,and hence high selectivity can be achieved.Moreover,the diffusion rate of CO₂ can be further reduced by strong interaction between the membrane and CO₂ molecules,leading to a further increase in selectivity.Aperture size of the membrane plays a determining role in separation process,either too large or too small aperture results in low selectivity or low gas permeance.Based on previous experiment reports and simulation studies,we reckon the ideal aperture for H₂/CO₂ separation should be slightly larger than CO₂,in where high selectivity and gas permeance can be achieved simultaneously.We synthesized a mixed-ligand JUC-160 membrane as a‘proof of concept'.Aperture size of JUC-160was identified through gas adsorption measurements and was considered to be between CO₂ and Ar molecules.Gas adsorption measurements also revealed JUC-160possessed strong affinity to CO₂.In gas separation test,JUC-160 showed excellent separation performance as expected,with H₂ permeance of 9.75×10-7 mol m^-22 s^-1 Pa^-1and H₂/CO₂ separation factor of 26.3 under 200?.Besides,we studied the dye removal performance of JUC-160 membrane.JUC-160membrane possessed excellent stability,and the aperture size of JUC-160 membrane was between water molecules and dye molecules,thus can completely remove dyes from water theoretically.Three different dyes were used to evaluate removal performance of JUC-160 membrane,JUC-160 membrane exhibited strong size exclusion effect,the rejection rate of all three dyes was nearly 100%,along with high water flux.Based on these excellent performance of MOF membrane,we have confidence in this unique material being applied in separation process.