Preparation And Separation Research Of Metal Organic Porous Materials Based Nanohybrid Membranes

Porous metal-organic materials（MOMs）,including metal-organic framework（MOFs）and metal-organic polyhedra（MOPs）have become one of popular hybrid membrane materials,due to their uniform pore structures in shape/size and rich functional sites.The challenge in the preparation and application of MOM-based hybrid membrane is to develop the functionality of MOM particles,fabricate hybrid membrane with homogeneous dispersion of MOF particles and construct channels with efficient mass transfer capability.In this work,a series of porous MOMs were employed as membrane materials to fabricate different hybrid membranes for the applications of pervaporation and nanofiltration.The microstructure,morphology,and composition of obtained membranes were characterized.The separation performance and stability of them were investigated.The incorporation of these functionalized MOMs with different structures is confirmed to be useful in improving chemical property and structure of the membranes,which achieved good separation performances.The research focused on the following four aspects.1.M-MOF-74/W3000（M=Co,Zn,Mg）hybrid membranes for pervaporation separation aromatic/aliphatic mixtureThe interaction between the d-orbital of the metal ion and theπ-electron clouds of the aromatic ring can promote the adsorption selectivity of the membranes towards aromatic hydrocarbons.Therefore,the coordination unsaturated metal incorporated in the MOFs can be used to fabricate hybrid membranes for improving their aromatic/aliphatic separation performance.In this study,the isostructural M-MOF-74with different metal ions were synthesized and used as fillers to fabricate M-MOF-74/W3000 hybrid membranes on the Al₂O₃ tubular substrate by the immersing-thermal crosslinking technology.These membranes were used to separate50 wt%toluene/n-heptane mixtures.Pervaporation results demonstrated that Co-MOF-74/W3000 hybrid membrane has the optimal separation performance among the three isostructural membrane,with the separation factor of 7.6 and the permeation flux of 90 g/m²h for the recovery of toluene from its 50 wt%mixture.In addition,the results of adsorption experiments showed that the toluene was much more strongly adsorbed by Co-MOF-74,which was attributed that Co²⁺with electron-unsaturated d orbital more easily produce conjugate effects with toluene than Zn²⁺and Mg²⁺.2.Hybrid membrane based on MOP-tBu molecule nanocages for aromatic/aliphatic hydrocarbon separation by pervaporationAlthough open metal unsaturated Co²⁺sites in the Co-MOF-74 can improve the membrane separation performance,Co-MOF-74 particles are large and easy to agglomerate,which resulted in the poor dispersion of fillers in the hybrid membrane.To improve the dispersion of fillers in the membrane,a soluble metal-organic polyhedra（MOP）,[Cu₂₄（5-tBu-1,3-BDC）₂₄（S）₂₄]（MOP-t Bu）as filler was fabricated and used in the pervaporation separation of aromatic/aliphatic hydrocarbons.It can be found from SEM images that no particle was found on the surface of the membrane,implying a good dispersion of MOP-t Bu molecules in the polymer.Pervaporation results showed that separation performance of W3000 membrane has been greatly improved after the incorporation of MOP-tBu molecules in the membrane.Both permeation flux and separation factor enhanced,up to 220.5 g/m²h and 19.0,respectively,for the pervaporation separation of 50 wt%toluene/n-heptane mixture.At the same conditions,the membrane can get a permeate flux of 392.3 g/m²h and a separation factor of 15.4 towards benzene,being comparable to some reported membranes.Furthermore,the separation factor and flux of the membrane are basically at a constant level in a 160 h period,implying a good stability of the membrane for recovering aromatic compounds.3.Functionalized MOP-X/W3000 hybrid membranes for aromatic hydrocarbons recovery by pervaporationIt was found that the open metal unsaturated sites,rich benzene rings in the MOP molecules and their porous structure indeed contribute to the good separation ability of the hybrid membrane.Besides these features,the functional groups on MOPs should have important influences on the separation performances of their hybrid membranes.To explore the effects of functional groups incorporated in the MOP-based hybrid membranes on the pervaporation separation of aromatic/aliphatic mixtures,a series of isostructural functionalized MOP-X（X=SO₃Na_n H_m（n+m=1）,OH,and tBu）molecules were used as the fillers and Boltorn W3000 as the polymer matrix,to fabricate hybrid membranes.The three MOPs with different functional groups are uniform in shape/size and soluble in solvents,which enable them to form isostructural hybrid membranes.Pervaporation results demonstrated that the MOP-SO₃Na_nH_m/W3000 membrane has the optimal separation performance with the separation factor of 8.03 and the permeation flux of 528 g/m²h for the recovery of toluene from its 50 wt%mixture,and those values are 8.4 and 540 g/m²h,respectively,for the recovery of benzene from 50 wt%benzene/cyclohexane mixture.It is proposed that the selectivity of these membranes is affected primarily by the polarity of functional groups in MOPs,which were further explained by the adsorption experiments and molecular simulations.4.The hierarchically structured MOF@LDH nanohybrid membrane for nanofiltrationBesides the membrane material,the membrane structure also has an important influence on the separation performance.In this work,an anionic 2D-MOF,namely BUT-203,was designed and synthesized.To obtain the nanohybrid multilayer membrane,the BUT-203 nanosheets were successfully obtained by exfoliating the bulk BUT-203 under the sonication.The nanohybrid membrane was constructed by assembling alternately BUT-203 and MgAl-LDH nanosheets onto the surface of PAN membrane under the electrostatic interaction.The assembly process was followed by SEM,UV-vis,CA,TGA and electrokinetic analyzer.The nanoindentation and thermogravimetric experiments showed that the BUT-203/MgAl-LDH membrane has good mechanical property and thermal stability.The BUT-203/MgAl-LDH hybrid membrane showed excellent nanofiltration performance in the removal of dyes from water.The retention of BUT-203/MgAl-LDH hybrid membrane towards congo red,methyl blue and eriochrome black T could reach 98%and the flux could be up 420L/m²hMPa.Moreover,BUT-203/MgAl-LDH hybrid membrane exhibited better separation performance towards negatively charged dyes than positively charged ones,which was due to the many negatively charged sulfonate groups on the surface of the hybrid membrane.Compared with PSS/MgAl-LDH and BUT-203/PDDA membranes,BUT-203/MgAl-LDH membrane exhibited excellent nanofiltration performances towards removal of dyes,which revealed that the desirable nanofiltration performance not only depends on the nature of the BUT-203 and MgAl-LDH nanosheets,but also relies on the synergistic effect between BUT-203 and MgAl-LDH nanosheets in the assembling process.