Study On Adsorption And Membrane Separation Mechanism Of Metal-organic Frameworks

Metal-organic fram ew orks (MOFs) having zeolite-like structure is recognized as a new kind of porous materials recently. Compared to traditional porous materials, MOFs dem onstrate some particular properties:(Ⅰ) H igher BTE surface area and larger porosity;(Ⅱ) Pore surface can be modified with different functional groups;(Ⅲ) Organic linkers inside frame work are substitutable;(Ⅳ) Frame work structure is flexible. Therefore, MOFs materials have attracted more attention in gas adsorption separation and mem brane separation technology. However, complexities of the properties limit understanding and application of the MOF material, further study of MOFs is highly requited. Computational simulation method is used to construct the new structure, study and predxt gas adsorption separation properties and optmize perform ancevia frame work and functional group modification of the MOFs. The method is of great in portance to conduct design and synthesis, speed up developm entand expand application of MOFs. M oreover, it is time and costsaving compared to com plicated experm ental works. In this work, a system atic molecular simulation and experm entalm ethod was carded out to study potential application of MOFsm aterials in four aspects: CO2adsorption, CH4/H2adsorption separation, VOCs adsorption and membrane separation. Them ain works are summarized as follows.For CO2adsorption of MOF materials, it is mainly focused on studying the functional group m odification. A m olecularm odeling method was used to modify a typicalhydrophobic MOF material: ZIF-8with am ine functional groups to form ZIF-8-NH2and ZIF-8-(NH2)2. GCMC sin ulation and DFT calculation were then used to study the adsorption properties of ZIF-8s before and after amine modification. The results indicated that addition of-NH2groups to ZIF-8helps in prove CO2adsorption capability and generate new CO2adsorption sites. DFT calculation result also showed that the addition of-NH2groups can enhance interaction between CO2molecules and ZIF-8framework. This work provides theoretical guidances for studying the functional group m odiBcation of MOFs and also provides a quick screening and designing method for the CO2adsorbing materials.For CH4/H2adsorption separation, the molecular sin ulation method was used. A molecular modeling method was originally used to construct new MIL-101s: MIL-101-R7_BDC by switching their organic linkers without changing the topologies of the MIL-101materials. Then a GCMC simulation was carded out to predict the CH4/H2adsorption separation properties of three m aterials. The effect of different organic linkers to adsorption separation properties of MIL-101s was also studied. The results indicated that MIL-101_NDC and MIL-101-R7_BDC with larger BET surface area can adsorb more CH4For H2,MIL-101that has larger pore volume and BET surface area exhibited stronger adsorption capability. The most favorite adsorption site of CH4in MIL-101s are inside ST cages.M oreover, the higher CH4/H2separation factor of MIL-101-R7_BDC indicated that the replacm entof ligands increasing BET surface area and reducing pore size and pore volume of MIL-101s, and then in proved the CH4/H2adsorption separation capability of MIL-101s. This work provided theoretical guidances for ligands modification of MOF materials. It also provides a quick screening method for the high efficient CH4/H2adsorption separation materialsFor VOCs adsorption in MOFs, the work first reported a forcefield for VOCs adsorption simulation in Cu-BTC base on Trappe forcefield. The establish of the this foiceBeld offers a possibility for the molecularsin ulation of VOCs adsoided in MOFs. The adsorption sites and adsorption mechanisni of VOCs adsorbed in Cu-BTC was first investigated with GCMC and MD simulation. The results indicated that the preferential adsorption sites of methanol and acetone inside Cu-BTC are near the BTC linkers and unsaturation metal site of the pocket cages. At lower pressure, the framework charge of Cu-BTC has larger in pact on methanol and acetone adsorption while it show s less effectathigherpressure. M D sin ulation w as also carried out to study the diffusion of VOCs inside Cu-BTC. The results showed that high tern perature is beneficial forthe diffusion of these VOCs inside Cu-BTC. This w ork provides a quick screening method for the MOFs thatapply to adsorlo VOCs.Formem brane separation of MOFs, the thin ZIF-8m em brane was synthetized by second growth method. The optimum synthesis conditions for ZIF-8mem brane are as follow: A0.05wt%seed solution was made by using60nm ZIF-8seeds and a uniform seed layer was prepared on the supportvia twice dip-coating. The second growth of ZIF-8seed layerwas carried outin a reaction solution with a Zn2+concentration of292mg/m lat120°C for8hr. A ZIF-8membrane of2.5μm thick was successfully prepared. The work was first characterized the diffusion properties and the flexibility property of ZIF-8mem brane with single (H2,He,N2, CH4, CO2and SF6) gases, It has great reference value for studying the gas transportproperties of MOFs. This work was also the first time to investigate the permeation and separation properties of binary (C3H6/C3H8) gases in ZIF-8m em brane. The experm ental results show ed that:1) Gas molecule with a larger kinetic diam eterexhiloits a smaller diffusivity in the ZIF-8membrane;2) N2(3.6A)andCH4C3-8A), with a larger diameter than ZIF-8m em brane pore (～3.4A), can permeate through the membrane due to its flexibility;3) C3H6demonstrates a higher pern eation flux than C3H8through the ZIF-8m em brane and make the mem brane suitable forC3H6/C3H8separation;4) The calculated diffusivity of C3H6and C3H8are126+0.05×10-8cm2/s and414±030×l0-10cm2/s.The activation energy of diffusion are12.7kJ/m oland38.8kJ/mol respectively.This work first reported using molecular dynam ic simulation to study the separation mechanism of C3H6/C3H8in ZIF-8membrane. The results showed that because of the "gate-opening" phenomenon of ZIF-8membrane, The C3H6gases diffuse more easier in ZIF-8UP) structure.The C3H6/C3H8selectivity in ZIF-8UP) structure is higher than that in ZIF-8(V/S) structure. The conclusions indicated that the "gate-opening" of ZIF-8mem brane caused by MOFs flexibility is the main reason for the separation of C3H6/C3H8. This work can provide theoretical guidances for the application of MOFs membrane in propylene/propane separation.