Preparation And Properties Of Cation Permselective Advanced Porous Framework Membranes

Membranes have great potential for separation due to their high separation efficiency,ease of operation,and environmental effectiveness.For a specific separation,ultra-thin separation membranes made of materials with uniform pores can meet the separation demand better,because the pore size plays a key role in separation.In recent years,advanced porous materials such as Metal-Organic Framework(MOF)and Porous Organic Cage(POC)are widely used in many fields due to their extremely high porosity,ordered and adjustable pore structure,and excellent post-modification.This thesis was devoted to study the preparation methods and the ions separation properties of advanced porous membranes(MOF membranes and POC membranes).By characterizing its morphology,microstructure,and ion separation performance,the effect of membrane structure and membrane surface chemistry property on ion separation performance was investigated.In Chapter 1,we introduced the background and the related application of the advanced porous materials.We first introduced MOF and the preparation methods of MOF membranes as well as their application progress in the field of ion separation.Then,we introduced POC and the preparation methods of POC membranes as well as application of POC membrane.Finally,the scope and content of this thesis are proposed.In Chapter 2,the preparation method of phase-pure UiO-66-NH2 membrane and the cation separation performance were studied.A simple and readily scalable contra-diffusion synthesis method has been developed for the controlled in situ smart growth of UiO-66-NH2 into leaf-like nanostructures with tunable density of the leaves and the surface layer thickness.By adjusting the concentration of metal ions and ligands,reaction duration,reaction temperature and other reaction parameters,the density and thickness of the UiO-66-NH2 layer can be adjusted.The membranes contain nanosized interstices among the MOF leaves,which enable maximum admission of ions within the membrane,and angstrom-sized inherent pores in every single UiO-66-NH2 crystal,which efficiently regulate the cation permselectivity.Consequently,the highest ever reported cation separations(Na+/Mg2+>200 and Li+/Mg2+>60)and excellent membrane stability during five sequential electrodialysis cycles are achieved.These characteristics position the fabricated MOF membranes as potential candidates for efficient extraction of pure lithium and sodium ions from salt lakes and seawater,respectively.In Chapter 3,the preparation method of phase-pure UiO-66-SO3H membrane and the separation performance were studied.Inspired by traditional membrane preparation methods,we think that the slow cation permeation is due to the absence of the permeation assisting groups(e.g.,acetate or sulfonate)in the MOF,which is a big issue and limits the widespread energy-efcient use of the MOF in the membrane technology applications.We propose a simple contra-diffusion synthesis method to fabricate the ultrathin(<600 nm)leaf-like UiO-66-SO3H membranes within a self-designed two-compartment reaction cell.By changing the feed ratio to control the content of sulfonic acid groups in the prepared membrane,a series of UiO-66-SO3H membranes with different contents of sulfonic acid groups were prepared.Such ultrathin and defect-free MOF-CPMs contain multidimensional sub-nanometer pores,which are highly suitable for selective transport and separation of ions(P(Na+/Mg2+)>140).Simultaneously,these MOF separation membranes have high monovalent ion permeation(three times of UiO-66 membrane)due to the introduction of the permeation assisting agents(-SO3H)in MOF nanostructures,which could accelerate the cation transport.In order to further explore the ion separation mechanism,the separation performance of the membrane for K+/Mg2+,Li+/Mg2+ and the transport characteristics of ions in single-component solutions were studied.In general,the presence of sulfonated sub-nano-sized ion transport channels in the prepared UiO-66-SO3H membrane can increase the cation permeation and achieve high ion selectivity.In Chapter 4,ultra-thin polyamide membrane with the functionalized UiO-66(Zr)-NH2 was used for cation separation.We hereby proposed a facile post-synthetic method to prepare UiO-66(Zr/Ti)-NH2 and further immobilized UiO-66(Zr/Ti)-NH2 in ultra-thin polyamide layer(?100 nm).Specifically,the Zr(?)in UiO-66(Zr)-NH2 is replaced with Ti(?),which neutralizes some of the positive charge and introduces negative charge in the porous framework of UiO-66(Zr)-NH2,represented hereafter as UiO-66(Zr/Ti)-NH2.The facile post-synthetic method could promote the fast transportation of ions through the pores of UiO-66(Zr/Ti)-NH2.Following the interfacial polymerization(IP),the acyl chloride groups of trimesoyl chloride(TMC)react with the UiO-66(Zr/Ti)-NH2 and diethylenetriamine(DETA)and produce a uniform polyamide layer containing embedded UiO-66(Zr/Ti)-NH2 nanoparticles.Briefly,the prepared ultra-thin(?100 nm)MOF surface layers containing specific ion transfer channels can simultaneously boost cation permeation and selectivity.The resulting thin-film nanocomposite(TFN)membranes offer high monovalent cation permeation and impressive selectivity for monovalent and divalent cations such as P(Na+/Mg2+)=13.44,(3.8× higher)and P(Li+/Mg2+)=11.38(5× higher)than the commercial state-of-art MCPM(Slemion CSO,Japan).Five continuous electrodialysis cycle test results show that the prepared TFN membrane has good stability and has good application potential in separation membrane technology.In Chapter 5,The POC membrane was studied for the cation separation.The previous chapters have used MOF membranes to achieve good cation separation performance.In this chapter,we use another porous material CC3 cages with a narrower pore size distribution(a kind of POC with excellent stability),and prepared a continuous and dense CC3 separation layer on a porous alumina substrate by contra-diffusion synthesis method.By optimizing the reaction parameters,a phase-pure CC3 membrane with excellent surface morphology was successfully prepared,and the characteristic diffraction peaks of CC3 could be obtained by XRD.By changing the growth time to control the surface morphology of the membrane and study its growth process,then further explore its cation separation performance.The bottom of the CC3 membrane(the part connected to AAO substrate)is dense,while the top of the membrane is relatively loose.The loose top structure will certainly promote the ion transfer due to the larger pore size of the surface.The results of electrodialysis show that the selectivity of the prepared CC3 membrane is at a higher level.In general,the CC3 membranes can effectively separate mono-/di-valent cations,which verify the potential of CC3 in the field of ion separation.In Chapter 6,the summary of this thesis is presented,suggestions and prospects for the future research work are also proposed.