Preparation Of MOF Membrane Through Self-Conversion Strategy Of Homologous Metal Compounds And Separation Application

Metal organic framework（MOF）has become ideal candidate for membrane-based separation due to the large surface area,tunable pore structure,structural diversity,variety of network structures,and relatively high chemical and thermal stability.Nevertheless,the improvement for separation performance of MOF membrane is still a great challenge,and the key factor is the preparation of high-performance membrane.In this study,a self-conversion strategy of homologous metal compounds was proposed and a series of MOF membrane were prepared based on this strategy,including highly oriented 2D MOF membranes,bimetallic MOF membranes and highly oriented 2D bimetallic MOF membranes.The prepared MOF membranes were applied to oil/water,gas and liquid separation,which expands application range of MOF membrane.The main contents and results are as follows:（1）Based on the principle of homologous metal oxide induced membrane formation,highly vertically oriented 2D Zn₂（b Im）₄ membrane was prepared by the self-conversion of ZnO nanorods in ligand synthesis solution with the stainless steel mesh as substrate.The conversion process of ZnO nanorods into Zn₂（b Im）₄ nanosheet membrane was researched and the growth mechanism was clarified,and it is found that ZnO nanorods provide a good guiding effect for the oriented growth of Zn₂（b Im）₄ nanosheets.The resulting nanosheets have width of 2μm,length of 2-4μm and thickness of 200 nm.In view of the hydrophobicity of Zn₂（b Im）₄ and the special microstructure of the 2D MOF membranes,the Zn₂（b Im）₄membrane was applied to the efficient separation of oil/water mixtures.Different membrane separators were fabricated by using stainless steel mesh with different pore sizes as substrate and applied in separating different kinds of oil/water mixtures.And the separation mechanism was explored.The separation efficiency of the membrane for dichloromethane/water mixture was more than 99.8%,and the separation flux was as high as 101400 L·m^-2·h^-1.The separation efficiency of the membrane was still higher than 99%after roasting at 300°C and20 cycle separation experiments,which showed excellent separation performance,good thermal stability and recyclability.This work expanded the application range of 2D MOF membranes.（2）In view of the difficulty of preparation of Co based MOF membrane and orientation regulation of 2D MOF membranes,highly oriented Co（OH）₂ nanosheets were introduced in the porous tubular substrate as precursor layer and then self-converted into highly oriented Co₂（b Im）₄ tubular membrane in organic ligand synthesis solution.The feasibility of self-conversion of Co（OH）₂ into 2D Co₂（b Im）₄ nanosheets and the effects of reaction parameters were investigated.It was found that Co（OH）₂ layer can efficiently induce the oriented growth of 2D Co₂（b Im）₄ membrane under the optimal synthesis conditions,because of the ultrathin thickness and high reaction activity of Co（OH）₂ itself.The achieved 2D Co₂（b Im）₄ membrane showed excellent gas separation factors with H₂/CO₂、H₂/N₂ and H₂/CH₄ binary mixtures separation factor of 19.0、12.7 and 21.5,respectively,which are much higher than the corresponding Knudsen diffusion coefficient,reflecting good separation performance.And this membrane exhibited good thermal stability and long-time operation stability,showing great potential for practical applications.（3）In view of the difficulty in preparing bimetallic MOF membranes,a vapor phase transformation（VPT）method was proposed to prepare highly oriented 2D bimetallic（Zn/Co）₂（b Im）₄ tubular membrane by self-conversion of hydroxy double salts（HDS）under ligand vapor treatment.The feasibility of the self-conversion of HDS into（Zn/Co）₂（b Im）₄ was explored,which laid a foundation for the regulation and synthesis of 2D bimetallic（Zn/Co）₂（b Im）₄ membrane.The formation mechanism of 2D bimetallic（Zn/Co）₂（b Im）₄membrane was explored through regulating reaction time,metal source and heating mode.Both HDS layer and VPT strategy have been identified to play decisive roles in the formation of highly oriented 2D bimetallic MOF membrane.The thickness of the membranes can be optionally adjusted by changing the growth height of the HDS layer.The prepared（Zn/Co）₂（b Im）₄ membrane with a thickness of 80 nm showed H₂ permeance of 1.68×10^-7mol·m^-2·s^-1·Pa^-1 and H₂/CO₂、H₂/N₂ and H₂/CH₄ ideal separation factors of 54.1,50.6 and67.9.The separation performance is better than the monometallic MOF membranes and bimetallic MOF membranes with disordered orientation,which reflects the synergistic effect of 2D MOF membrane and bimetallic MOF membrane.It shows the synergy and dual advantages of MOF nanosheet membrane and bimetallic MOF membrane.This synthesis strategy is a green and low-cost process,thus having potential for large scale preparation of MOF membranes.（4）In order to expand the application range of bimetallic MOF membranes,bimetallic Zn/Co-ZIF tubular membrane was prepared by the self-conversion of HDS in organic ligand synthesis solution.Firstly,the induced formation of bimetallic Zn/Co-ZIF powder was investigated and optimized,and the optimum conditions and influence rules of the self conversion process of HDS into Zn/Co-ZIF were obtained.Then,the HDS layer was introduced into the porous tubular substrate and the bimetallic Zn/Co-ZIF tubular membrane was formed by self conversion of HDS layer.The Zn/Co-ZIF membranes with different Zn/Co ratios were formed by adjusting the metal ratio of the HDS layer.Both adsorption experiments and simulation calculation showed the Zn/Co-ZIF membrane has higher adsorption selectivity for Me OH/MTBE than monometallic Zn-ZIF-8 and Co-ZIF-67.The obtained bimetallic Zn/Co-ZIF membrane showed an unexpected separation performance with flux of 1.96 kg·m^-2·h^-1 and separation factor of 6985 for 15 wt%Me OH/MTBE mixtures at 50℃and exhibited outstanding long-time operating stability.This method can also be used to synthesize other bimetallic MOF membrane.This study opens up a new avenue to fabricate high-performance bimetallic MOF membranes for efficient separations.