Design And Performance Of Gas Separation Membranes Based On Metal-Organic Framework Materials

H2 and CH4 are currently advocated as clean energy sources.Whether extracted from natural gas or prepared by chemical methods,they generally contain a lot of impurities.Due to the advantages of high separation efficiency,low energy consumption,no secondary pollution,and mild separation conditions,membrane separation technology shows great potential in these gas separation fields.However,traditional polymer membranes have a significant mutual restriction between permeability and selectivity(i.e.,the trade-off relationship),making it difficult to improve their gas separation performance to break the Robeson upper bound.Introducing microporous filler into the polymer matrix or preparing a pure microporous membrane are the potential ways to break through this limitation.In recent years,metal-organic frameworks(MOF)have shown great application potential in the gas separation field due to their unique advantages in high porosity,uniform pore size,and adjustable pore size.High performance gas separation membrane materials based on MOF have attracted more and more attention.In the preparation process of membrane materials,MOF can be made into mixed-matrix membranes(MMMs)in the form of fillers or pure MOF membranes.In the preparation process of MMMs,MOF particles ranging in size from nano-scale to micron-scale are often used.Nano-scale MOF particles have the advantages of easy preparation,larger contact area with the matrix and abundant open metal sites on the surface.They are often used to improve the gas permeability and selectivity of polymer membranes.However,the agglomeration problem of nano-scale MOF particles is serious at high loading content,which easily leads to the formation of non-selective aggregate voids and interface defects in MMMs,thus reducing the gas separation selectivity of MMMs.Although nano-scale MOF particles can be modified by using active groups or open metal sites,the common surface grafting modification methods tend to cause the plugging of MOF screening pores,which affects the performance of MOF screening.Micron-scale MOF particles have the advantages of low agglomeration and a high crystal structure,which can improve the gas permeability and selectivity of polymer membranes.However,compared with nano-scale MOF particles,the reduction of the contact area between the micron-scale MOF particles and the matrix makes MMMs more prone to interface defects in the process of membranes formation.Due to the high integrity crystal structure of micron-scale MOF particles,this issue is also difficult to be resolved by modification of active metal sites.The gas separation selectivity of pure MOF membrane strongly depends on its crystal structure and framework structure.Common MOF materials have the problems of insufficient framework rigidity and easy reversal of ligand.These problems easily lead to a large aperture size of MOF,thus affecting the accuracy of gas separation,especially for gas molecular separation with a similar kinetic diameter.Therefore,it is still very difficult to achieve accurate molecular separation.In view of the above scientific problems in the development of gas separation membrane technology,on the one hand,the interface design of the mixed matrix membrane with different sizes of MOF particles was carried out to enhance the interface interaction between the two phases,so as to achieve the highly selective separation of H2/CH4,H2/N2 and CO2/CH4 by MMMs under the high load of different sizes of MOF particles.On the other hand,organic ligands with "dual-arm" structure are introduced into the MOF frame structure,and the rigid and steric hindrance of such molecules are utilized to continuously and precisely regulate the aperture of MOF at sub-angstrom level,thus achieving efficient separation of H2/CH4 and CO2/CH4 in pure MOF membrane.At the same time,the relationship between the membrane and gas separation performance was systematically analyzed from the perspectives of material morphology,pore structure,adsorption performance and chemical structure.The main contents of the research are as follows:(1)Interface design of nanoscale MOF-based MMMs.In order to solve the problem of poor dispersion and easy agglomeration of nano-scale MOF particles,a polar molecule with two polar sides,isophthalic dihydrazide(IPD),was introduced between ZIF-8 and the polyimide(PI)matrix.The abundant amino groups in IPD can not only coordinate with the abundant uncoordinated Zn2+atoms on the outer surface of ZIF-8,but also form hydrogen bonds with the carbonyl groups on the PI matrix.Both the coordination effect and hydrogen bonds are beneficial to enhancing the interfacial interaction between MOF nanoparticles and PI matrix.IPD modified ZIF-8(IPD@ZIF-8)nanoparticles also showed excellent dispersion,with loading capacity up to 45 wt%in MMMs.In comparison with those without IPD modification,the corresponding ideal selectivities of H2/CH4 and H2/N2 gas pairs of PI/IPD@ZIF-8 MMMs are 15.1 and 13.0,which increase by 46.6%and 32.7%,respectively,exceeding Robeson upper bounds.The surface modification of IPD also enhances the CO2 plasticization resistance property of PI/IPD@ZIF-8 MMMs.This study provides a facile and easy-operated strategy for the surface modification of MOF nanoparticles,and opens up a new way for the preparation of MMMs with high filler loading and low defect.(2)Interface design of microscale MOF-based MMMs.In view of the problems of poor interface compatibility and low loading capacity of micron-sized MOF particles in MMMs,a type of micron-sized ZIF-11 filler(mZIF-11)containing benzimidazole groups is prepared,and a benzimidazole-based polyimide,named BIMPI,is synthesized and used as the polymer matrix.The benzimidazole group in polyimide will induce the formation of π-π interfacial interaction between polyimide and ZIF-11 particles.The successful construction of π-πinterfacial interactions in mZIF-11/BIMPI MMMs was confirmed by UV-vis and fluorescence characterization.Simulation results showed that the high binding energy between BIMPI and ZIF-11 greatly improves the interface force of MMMs.The obtained mZIF-11/BIMPI MMMs exhibited characteristics of high mechanical flexibility at 50 wt%loading content,and their tensile strength remained at 27.22 MPa.The selectivities of H2/CH4 and CO2/CH4 of this MMM is 261.3 and 77.2,respectively,and their comprehensive separation performance exceeds the Robeson upper bounds.This work provides an easy and feasible route for constructing micrometer-sized MOF-based MMMs with high mass loading and high separation performance.(3)Precise regulation of MOF aperture.The first two work explored the dispersion and interface defects of MOF particles at different sizes through interface design,and prepared a series of MMMs with high loading MOF.However,due to the flexible characteristics of MOF frameworks and the easy reversal of ligands,the separation selectivity of MOF in the process of CO2/CH4 mixed gas with very similar kinetic diameter is still to be improved.How to achieve more accurate aperture control of MOF is the key to giving full play to the advantages of MOF screening.Based on this,we proposed a "two-way placeholder" strategy for more accurate aperture regulation.The "dual-arm”ligand(2-chloromethylbenzimidazole(Clbim))was introduced into the framework of ZIF-8.The chloromethyl and benzimidazole benzene ring structures of Clbim ligands were extended to both sides respectively and could simultaneously affect the apertures of screening channels in multiple cages.In other words,the apertures of MOF can be continuously and precisely regulated at sub-angstrom scale by using the effect of bidirectional structure to achieve efficient separation of CO2/CH4.A novel zeolite imidazole hybrid MOF(Clbimx/ZIF-8)membrane was prepared by contra-diffusion method by mixing different proportions of Clbim with methylimidazole(mIm,the ligand of ZIF-8).The continuous variation trend of MOF apertures at a sub-angstrom scale is revealed by dynamics simulation.By analyzing the XRD and 1H NRM spectrum of Clbimx/ZIF-8 and the pore size of Clbimx/ZIF-8 calculated by the nitrogen adsorption isotherm of hybrid materials,it can be seen that Clbim not only successfully participated in the construction of the hybrid Clbimx/ZIF-8 framework,but also its pore size is accurately regulated.Clbim23/ZIF-8 membrane shows exceptional separation performance for CO2/CH4 and H2/CH4 with separation factor of 56 and 338,respectively.The comprehensive performance of CO2/CH4 and H2/CH4 of Clbim23/ZIF-8 largely exceed the Robeson upper bounds.This membrane has good stability for CO2/CH4 separation under different temperature,pressure and long-term operation.This work is the first to report the use of "dual-arm" ligand to achieve sub-angstrom level continuous precise control of MOF aperture,which provides a new idea for precise control of MOF aperture.