| The adsorption and separation of light hydrocarbons/organic dyes are of great importance from both industrial and environmental points of view.Metal-organic frameworks(MOFs)exihibit enormous potentials in the storage and separation of molecules,because of its large specific surface area,outstanding designability and structural adjustability.In order to further improve the storage and separation performance of MOFs,it's necessary to construct new functionalized MOFs.The main content of this paper is for the application of MOFs in the adsorption and separation area.A series of anionic MOFs were synthesized by introducing polar carboxyl groups,Lewis base active sites and increasing the length of ligands.The storage and separation performance of the series of MOFs were studied by combining experimental and computational methods.The main points are as follows:Athree-dimensionalporousCumetal–organicframework{[Cu2(L)·(H2O)2]·2H2O·3DMA·(CH3)2NH2}n(NEM-1)with Nb O topology based on a pentacarboxylate ligand(H5L)was synthesized and structurally characterized.NEM-1 with anionic skeleton and uncoordinated–COO-groups exhibits high adsorption capacities of CO2,C2H2,C2H4,C2H6,C3H6and C3H8 as well as large selectivities for CO2/CH4,C2H2/CH4,C3H6/CH4 and C3H8/CH4 at 295 K and 1 bar.Grand canonical Monte Carlo(GCMC)and density functional theory(DFT)simulations revealed the strong light hydrocarbons adsorption sites exist near the open CuII sites,the uncoordinated–COO-groups and the entrance windows across the framework pores.Moreover,NEM-1 show high size and charged state dependence on dye adsorption and separation,that is,NEM-1 can efficiently adsorb small cationic dyes in aqueous solution,such as methylene blue(MB+).To further increase the adsorption capacity and selectivity of the Cu metal–organic framework,we synthesized ligand H5N by introducing Lewis base sites into the H5L ligand,and then constructed{[Cu2(N)·(H2O)2]·2H2O·3DMA·(CH3)2NH2}n(NEM-4)with the same topology as NEM-1.Lewis basic nitrogen sites not only significantly enhance the interaction between framework and gas,but also increase the rotatory properties of the modified benzene ring to increase the pore volume of the NEM-4.Compared with NEM-1,NEM-4 with Lewis basic nitrogen sites,exhibits higher uptakes of C2H2,C2H4,C2H6,C3H6 and C3H8 at ambient conditions.According to the gas uptake data as well as the ideal adsorbed solution theory(IAST),NEM-4 is shown to have high selectivities for C2H2/CH4,C2H4/CH4,C2H6/CH4,C3H6/CH4,and C3H8/CH4.The GCMC and DFT calculations further show that Lewis basic nitrogen sites can further enhance the gas-framework interaction,so that NEM-4 exhibits better gas adsorption and separation performance than NEM-1.On the other hand,the anionic NEM-4 also shows efficiently selective adsorption of cationic dye in methanol solution.Increasing ligand length could enlarge pore volume of MOFs,which further improves the gas uptakes.Therefore,we construct a novel three-dimensional metal-organic framework[Cu2(L1)·(H2O)2]·2H2O·3DMF·(CH3)2NH2](NEM-5)through introducing a benzene ring into the H5L ligand(H5L1).Compared with NEM-1,NEM-5 has similar topology and larger pore size,but the gas adsorption capacities are significantly reduced.These results might be attributed to the large pore size and low framework density of NEM-5.Also,the partial collapse of the skeleton during the activation process leads to decreasing the gas adsorption capacities.In order to improve the C3/C2 hydrocarbon selectivity of MOFs,a novel anionic metal–organic framework{[Zn2(N)·(DMF)3·(CH3)2NH2]·(DMF)2}n(NEM-7-Zn)with constricted pores was synthesized by ligand H5N under solvothermal conditions.The metal center of NEM-7-Zn was converted from Zn to Cu through metal ion exchange,which resulted in a new anionic skeleton(NEM-7-Cu).Gas adsorption test results show that,the stability of NEM-7-Cu is improved,and it possesses high selectivities for C3H6/C2H4(8.61),C3H8/C2H2(7.1)and C3H8/C2H4(8.82)at 298 K.Moreover,the anionic NEM-7-Cu shows highly selective adsorption of cation organic dye molecules.Therefore,metal ion exchange is a simple and effective post-synthesis method that can be used to synthesize complexes with a target structure. |
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