| Compared with the conventional gas separation methods,such as distillation,extraction and adsorption,membrane separation technology with low energy consumption and simple operation has been widely applied and got rapid development.Metal organic framework materials（MOFs）are a new class of porous materials,fabricated with metal ions and organic connection agent,with good and adjustable pore structure,ultra-high specific surface area,excellent thermal and chemical stability.In recent years,MOFs,as new inorganic membrane materials,have been widely applied in gas separation,and the exploration of MOF membrane preparation has attracted the attention of researchers around the world.Conventional MOF membrane preparation usually relies on hydrothermal or solvothermal methods.However,the undesired crystal deposition from solution will cause random crystal arrangement and growth direction,and result in random orientated membranes.While the oriented MOF membrane with regular and uniform crystal arrangement can reduce the intercrystalline defects of the membrane and provide a single channel for gas permeability,thus improving the membrane permeability separation performance.Thus,preparation of oriented MOF membrane is an effective method to improve the morphology and the permeability separation performance of the MOF membrane.In recent years,the preparation of oriented MOF membrane has become a hot and difficult spot in membrane separation.In addition,when using solvothermal method to prepare MOF membrane,the use of harmful solvents will not only pollutes the environment,but also brings safety risks due to the self-generated pressure,which is not in line with the concept of green chemistry.Therefore,it is urgent to develop a new solvent-free synthesis route.Mixed matrix membranes are a kind of composite membrane which combine polymer membrane with inorganic nanoparticles.In actual production,the preparation of MOFs membrane has obstacles in terms of large-scale production,processability and economy,so it is not suitable for existing commercial technologies.While the production technology of polymer membranes has been relatively mature.So mixing porous MOFs particles into polymer to the produce mixed matrix membranes can solve the problem well.This paper takes ZIF-95 and NH2-MIL-125 as the main research objects and carries out the following research works:1.Solvent-free secondary growth as a new synthesis route was developed and used to prepare oriented ZIF-95 membrane.Through solvent-free secondary growth,c-oriented ZIF-95 membrane with thickness of 1.5μm was successfully prepared from20 mg synthetic powder after 72 h.And it was found that with the increase of the synthesis time from 12 h to 72 h,the gaps of ZIF-95 crystal gradually decreased until the continuous ZIF-95 membrane was formed.With the increase of powder quality from15 mg to 40 mg,the thickness of ZIF-95 membrane increases from 1μm to 4μm,while the orientation of ZIF-95 membrane decreases.In addition,the necessity of pre-coating seed layer and using Na HCO3 were also confirmed.Therefore,seed layer,sodium bicarbonate,sufficient time for growth and appropriate amount of synthetic powder are all the key to prepare oriented ZIF-95 membrane.2.The separation properties of ZIF-95 membranes prepared by different amounts of synthetic powders were tested and compared.With the increase of the amount of synthetic powder,the separation factor of mixed gas increased gradually,but the gas permeability decreased.The separation performance of ZIF-95 membrane prepared by20 mg synthetic powder was studied in detail.At 25℃and 1 bar,the permeability of single gas H2 is 2.51·10-7 mol·m-2·s-1·Pa-1,and the mixture separation factors of H2/CO2 and H2/CH4 are 184 and 140,respectively.Its permselectivity is much higher than that of MOF membranes reported in other literatures.On the one hand,the oriented membrane structure leads to superior separation selectivity,on the other hand,solvent-free condition also avoids point defects caused by solvent molecules.So the solvent-free secondary growth is an effective strategy to improve the morphology and the separation performance of membranes.In addition,ZIF-7 and ZIF-8 membranes were also synthesized by solvent-free secondary growth,which proved the generality of the synthesis strategy.3.There are barriers in the large-scale preparation of MOF membrane,so in this study,mixed matrix membranes（MMMs）were prepared to solve the problem.NH2-MIL-125 particles with submicron size were prepared as filler,and PI as matrix.Pure PI membrane and NH2-MIL-125@PI MMMs with different filler ratios were prepared.The MMMs membrane with 10%filler content had good compatibility and no obvious cavity was found.The H2/CO2 separation performance is not significantly weakened compared with pure PI membrane,but the permeability is increased by an order of magnitude.While the cavities in the MMMs with 20%and 30%filler provide non-selective channels for gas penetration,resulting in permeability increase but separation performance decrease.From further testing of the membrane with 10%filler,at 25℃and 1 bar,the permeability of single gas H2 is 1.3·10-7 mol·m-2·s-1·Pa-1,the ideal separation factors of H2/CO2 and H2/CH4 are 19.6 and 34.2,and the mixture separation factors are 10.8 and 24.4,respectively.It is expected to be applied in industrial hydrogen purification. |
