Preparation And Performance Investigation Of Metal-Organic Framework Membranes

Metal organic frameworks （MOFs） are emerging as an extensive class of novelnanoporous crystalline materials, the structure of which is composed of metal ions orclusters joined by a variety of organic linkers through strong coordination bonds. Theversatile organic ligands and inorganic components enable molecular engineering ofMOFs with topologically diverse and pleasing structures. A library of possiblestructures with different properties of large pore size range （3.0-48.3）, very highsurface areas, and adjustable internal surface properties make MOFs of interest in gasstorage, catalysis, magnetism, and other potential applications. Designing a targetstructure with specific properties and functionalities provides endless exploration inmaterials science.Unique properties of MOFs including uniform pore size, high surface areas andspecific adsorption affinities make MOFs attractive for assembling into membraneswith excellent performance. Besides the extraordinary degree of variability in MOFstructures themselves, tailored functionalization of MOFs offers new perspectiveswith expected properties and broadened applications. The target MOF materialNH₂-MIL-53（Al）, which is an analogue to MIL-53（Al） and share the same topologyof MIL-53, is composed of AlO₄（OH）₂clusters with2-amino terephthalate as linkersinstead of terephthalate. The framework of NH₂-MIL-53（Al） contains1-D diamondshaped channels with free standing amino groups, having a free diameter close to7.5. The NH₂-MIL-53（Al） membranes have been achieved via further solvothermaltreatment of assembled colloidal seeds. The sorption abilities of the NH₂-MIL-53（Al）material are studied by gas adsorption of H₂, CH4, N₂and CO₂. The obtainedmembrane is investigated for gas separations. Aiming at the potential application inhydrogen purification, a detailed investigation on both single and binary gaspermeations through the NH₂-MIL-53（Al） membrane was performed, including H₂,CH4, N₂and CO₂, and their binary mixtures. In addition, the influence of permeationtemperature on the separation of hydrogen from binary mixtures was systematicallystudied. Hydrogen separation powers of H₂over CO₂and H₂over N₂inNH₂-MIL-53（Al） membrane reach maximum values of6148and4442at313K and298K respectively. By comparing other MOF membranes in gas separation,as-synthesized NH₂-MIL-53（Al） membrane in the present study possesses a goodbalance between H₂permeance and selectivity, and exhibits a very high hydrogenseparation power. The NH₂-MIL-53（Al） membrane would be a good candidate in thehydrogen separation membranes.The host-guest chemistry of MOFs allows an implementation of desirableproperties by embedding guest molecules or clusters in the cavities. Therefore, the investigation of MOFs for host materials is attracting increasing attention although itis still in an early stage and thus remains a big challenge. The objective of the presentstudy is to assemble the photochromic molecules in the nanoporous films andinvestigate the photochromic properties for advanced photonics or optics applications.The big mesopores of JUC-120has been obtained via a microwave synthesisapproach. The structure of indium trimesate possesses the cubic zeolitic MTNtopology based on indium cluster and trimesic acid （H3BTC） ligand, which is a newanologue to MIL-100（Al, Fe or Cr） by XRD, FTIR, Raman,13C NMR, and N₂absorption-adsorption. Additionally, the titled spiropyran compound, BSP issuccessfully incorporated inside the supercages of JUC-120crystals by crystallizationassisted with microwave heating, named as BSP/JUC-120, and then formed films byspin-coating method. The crystallization inclusion process involves the directsynthesis of host materials from precursor and co-inclusion of dye molecules in themixture solution can occur simultaneously during hydrothermal/solvothermalcrystallization. Furthermore, the photochromatic behaviors of the BSP/JUC-120filmare studied by the UV-Vis and fluorescence spectroscopies. Upon photoirradiation ofthe BSP/JUC-120film, the stabilization of the open merocynaine isomer bleachs tothe closed spiropyran form, and the coloration is regained upon standing in the dark,exhibiting antidromic reversible photochromism. Moreover, the BSP/JUC-120filmshows high reversibility and thermal stability of photochromism. This highly efficientMOF film is expected to be promising in the applications of optical devices.In general, some specific physicochemical properties of MOFs are controlled andmodified by the judicious selection of organic ligands and metal centers. Theproton-conducting MOFs have recently been obtained, and tuned by introducingacidic and hydrophilic units. This includes carboxylate, phosphonate and sulfonategroups due to the presence of hydrophilic oxygen atoms to act as hydrogen-bondingacceptor. Furthermore, hydrophilic amine moiety is also utilized by either acceptingor donating proton to serve as proton carriers and hydrogen-bonding donor. A fewproton-conductive MOFs have successfully been synthesized by using theabove-mentioned synthetic strategies. However, most proton-conductive MOFs arelimited to being used under high-humidity conditions （¹00%relative humidity）, or athigh temperatures （>100°C）. Only a small number of studies on the protonconductivity of MOFs under ambient conditions （i.e. low humidity conditions and lowtemperatures） have been reported. We have successfully synthesized and characterizeda chiral two-dimensional MOF material containing protonated tertiary amines asintrinsic proton carriers and hydrogen-bonding chains as proton-conducting pathways.The structural feature indicates that JUC-121can be considered as a potentialcandidate for proton-conducting materials. Experimental results reveal that the JUC-121submicrorods show moderate proton conductivity at⁵3%RH and298K.Further, a series of composite membranes with different contents of MOF crystals areprepared by blending JUC-121submicrorods with PVP for the exploration of furtherapplication of MOFs in fuel cells under low humidity. Amazingly, high protonconductivity is observed in MOF1/PVP-50(2.8×10-5S cm^-1) sample at298K and⁵3%RH, obviously larger than those of pure submicrorods (6.9×10^-6S cm^-1) andpure PVP (1.4×10^-8S cm^-1) under the similar conditions. The conductionproperties/behaviors of JUC-121/PVP composite membranes for proton have beenstudied in details. We have found that the available proton carriers in the JUC-121structure provides a basis for the conductivity, and the large humidification effect ofPVP with adsorbed water molecules greatly contributes to the proton transport in thecomposite membrane. Moreover, proton transport mechanism has been brieflydiscussed. It has been found that the proton conduction in composite membranesmainly follows vehicle mechanism in couple with Grotthuss mechanism. We havepresented a new and universal approach to fabricate proton-conducting MOF-basedcomposite membranes by taking advantage of the humidification effect to enhanceproton conductivity of MOF materials under a low humidity. This strategy/conceptwould be considered as a significant stepping stone to prepare proton-conductivematerials with high proton conductivities.