Studies On The Preparation And Separation Performance Of ZIF-95 Membrane

Membrane separation technology is a product of multidisciplinary intersection,which can improve the energy efficiency in molecular and ionic separation processes and also meet the need of modern industry for energy saving,high efficiency and environmental friendliness.In comparison with conventional distillation,adsorption and other separation methods,membrane separation technology shows obvious advantages,such as low cost,low energy consumption,no pollution,easy operation,etc.Metal organic frameworks（MOFs）are a new class of porous crystalline materials with a regular pore structure consisting of inorganic metal centers interconnected with organic ligands through covalent or ionic bonds,which have a wide range of promising applications in catalysis,separation,gas adsorption and storage.Among them,the pore size of ZIF-95 with POZ topology is about 0.37 nm and the hole is about 2.4 nm.The nitrogen-containing group in its backbone has a strong adsorption effect and can adsorb 82.6 times of its own volume of carbon dioxide,while it still has good thermal stability at temperatures over 500°C.Therefore,ZIF-95 is a new separation membrane material with very promising applications.Currently,there are many methods to prepare MOF membranes,such as in situ synthesis,secondary growth,interfacial synthesis,liquid phase epitaxy,vapor deposition and electrochemical deposition,which are usually performed in a solvent environment.However,most of the randomly oriented MOF membranes prepared in a solvent environment are polycrystalline membranes containing intergranular defects.At the same time,most of the solvents used in the solvent method are hazardous solvents,which can be harmful to the experimental personnel and the surrounding environment and do not follow the current development idea of green chemistry.Oriented membranes have perfect orientation with fewer intergranular defects and usually exhibit better gas separation properties.In order to prepare such oriented membranes,we often induce rapid crystal growth in a certain direction by pre-coating oriented seeds or with the aid of external forces.Overall,this thesis has put forward a new method for the preparation of oriented MOF membranes,i.e.,secondary growth of MOF membranes by solid-phase method.Mixed matrix membranes（MMMs）,prepared by incorporating inorganic porous materials into polymer matrices,combine the unique properties of selective adsorbents and polymers to greatly improve the gas separation performance of conventional polymer membranes,and are expected to break the bottleneck of conventional inorganic membrane preparation and application to achieve large-scale applications in industrial production.Two-dimensional metal-organic frameworks（2D MOFs）materials with high specific surface area and high porosity are being examined as suitable fillers to improve the separation performance of MMMs.In this paper,the following three studies are carried out on the zeolite imidazole framework of ZIF-95:（1）The morphology and size of ZIF-95 crystals are tuned by changing the molar ratio of reactants in solution,the amount of solvent,the reaction time and the reaction temperature.It is found that different reaction conditions or solution composition can prepare ZIF-95 sheets with different sizes and thicknesses.The c-axis oriented ZIF-95 sheets with the lateral size of about 20μm and the thickness of about 3μm can be successfully prepared by reacting at 150℃for 72 h.Meanwhile,the c-axis oriented ZIF-95 sheets with the lateral size of about 9μm and the thickness of about 500 nm can be obtained by reacting at 130℃for 168 h.（2）ZIF-95 sheets with a transverse dimension of about 9μm and a thickness of about 500 nm are synthesized by the solvothermal method and used as crystalline seeds,and a layer of highly oriented ZIF-95 crystalline seeds is uniformly covered on the surface of the porous alumina substrate by vacuum filtration.The separation factors of equimolar H₂/CO₂ and H₂/CH₄ mixtures are 81.8 and 78.6 at 25℃and 100k Pa,respectively,and the permeability of hydrogen is as high as 6120 barrer.（3）The c-axis oriented ZIF-95 sheets with a transverse dimension of about 20μm and a thickness of about 3μm are prepared by the solvothermal method.A certain mass of ZIF-95 sheets are incorporated into the polyimide（PI）matrix as fillers,and c-axis oriented ZIF-95@PI MMMs with different loading are prepared by the plate scraping method.At 100°C and 100 k Pa,the separation factors of H₂/CH₄,H₂/CO₂and CO₂/CH₄ mixture are 134.3,58.3 and 33.2,respectively,and the H₂ permeability is 967 barrer for the c-axis oriented ZIF-95@PI MMM with a 40 wt.%ZIF-95 loading.