| Metal organic frameworks (MOFs), is a new type of porous material which has both organic and inorganic properties. Owing to its unique textural properties, like tunable pore structure, chemical functionality, large surface area and large pore volume, MOFs have been generating a great deal of interests for potential application in separation area. Membrane separation, a kind of separation technology which has lower energy consumption and easier operational procedure, has been widely used in industrial area. So, the development of MOFs membrane, which combine the promising application potential in separation area of MOFs and the advantages of membrane separation, has a great significance.NH2-MIL-125 is a titanium-terephthalate-based three dimensional porous MOF, which comprises two types of cages corresponding to octahedral and tetrahedral vacancies of a cc packing. It shows high CO2 adsorption capacity and excellent selectivity for CO2 over N2 due to the existence of amino group in its structure. Besides, the octahedral cages of NH2-MIL-125 can separate xylene isomers on the basis of differences in packing and interaction with the pore walls, as well as smaller tetrahedral cages, which are capable of separating xylene isomers by molecular sieving. Therefore, the research and development of NH2-MIL-125 membrane has great significance for the separation of CO2/N2 or xylene isomers, and to date there is no report on NH2-MIL-125 membrane. So, the purpose of this study was to develop NH2-MIL-125 membrane with separation performance.The study of NH2-MIL-125 crystal growth habits can provide theoretical guidance for the preparation of NH2-MIL-125 membrane, so first we conduct the research on the growth habits of NH2-MIL-125 crystal. NH2-MIL-125 crystals of different size (200nm-1μm) and different morphology (disk, square slice, decahedron, and octahedron) was fabricated from designed synthesis compositions and conditions, and on this basis NH2-MIL-125 crystal growth habits were analyzed. With the concentration of reaction solutions increasing, the size of NH2-MIL-125 crystal growing, and the morphology of NH2-MIL-125 crystal changing (disk to square slice to decahedron to octahedron), the possible reason for the NH2-MIL-125 crystal morphology changing is that the increase of reaction solutions concentration promote the growth of{002} crystal face. The ratio of reactants (Ti(C3H7O)4:NH2-H2BDC) has a distinct effect on the size of crystal, the most likely reason is that enough neutral ligands inhibit the growth of crystal and downsize the particle, moreover, the morphology of crystal changing (octahedron to decahedron) with the ratio of reactants decreasing. In the mixed solvent (DMF and MeOH), the crystal morphology changing (disk to square slice to decahedron) with the ratio of DMF and MeOH increasing, and when using other kinds of alcohols instead of MeOH, there is no NH2-MIL-125 crystal obtained because the hydrogen bond donor ability of MeOH is stronger than other kinds of alcohols. The morphology of NH2-MIL-125 crystal changing with prolonging the ultrasonic time of the reaction solution. The particle size increases with prolonging crystallization time, inferring that the NH2-MIL-125 crystal growth follows nucleation-growth mechanism. When the synthesis temperature below the critical value, there is no NH2-MIL-125 crystal obtained.Subsequently, crystal seeding method and substrate modification method were used for the research and development of NH2-MIL-125 membrane. In the process of preparing NH2-MIL-125 membrane by crystal seeding method, we discussed the effect of some key factors on the formation of NH2-MIL-125 membrane, like concentration of reaction solutions, ratio of reactants, ratio of solvents, crystallization time, crystallization temperature, types of substrate and so on. Besides, loading PEI, changing the preparation ways of precursor, decreasing the evaporation rate of cleaning solvent and repeated crystallization were conducted to improve the compactness and separation performance of NH2-MIL-125 membrane. Among these, repeated crystallization can obviously improve the compactness and separation performance, after repeating once, the permeation rate of N2 decreased from 8.27x10’7 mol m"2 s-1 Pa-1 to 4.25×10-7 mol m-2 s-1 Pa-1, decreased approximately 1-fold, and the separation factor of CO2/N2 increased from 1.78 to 6.56, increased approximately 2.5-fold. However, the separation performance of xylene isomers was relatively lower, the separation factor of pX/mX was 1.23, and the separation factor of pX/oX was 1.56, which may caused by the unstability of NH2-MIL-125 membrane.In the process of preparing NH2-MIL-125 membrane by substrate modification method, we modified substrate surface with three different types of TiO2 intermediate layer to increase the heterogeneous nucleation density, they are TiO2 nanoparticle layer, TiO2 membrane layer and TiO2 nanorods layer. When modifying support with TiO2 nanoparticle layer and TiO2 membrane layer, there is no continuous membrane obtained, the most likely reason for this result could be that anatase TiO2 can’t induce the growth of NH2-MIL-125 membrane. Besides, although modifying substrate with rutile TiO2 nanorods layer could obtain NH2-MIL-125 membrane, there are larger defects in this membrane. Moreover, TiO2 nanorods layer can be used as "metal source" to synthesize memebrane which has separation performance of CO2/N2. |
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