Introducing Mesostructure Into MOF Materials By Self-assembly And Composite Assembly Strategy

Metal organic frameworks （MOF） have well developed to be an important class ofcrystalline materials. However, most MOF materials generally provide only onefunctional group or one porous structure. Self-assembly of surfactant micelles in thepresence of inorganic or organic framework precursors is considered as a commonand effective approach for the synthesis of mesostructured materials. Here, we focuson the effect of cationic surfactants on structure and morphology of mesostructuredMOF. Recently, MOF can be further developed and enhanced by serving as hostmatrices or templates to form new types of composite materials. It is a challenge todope complex component with bigger size and multidimensional morphology.Mesoporous MOF nanocrystals and size-tunable silica cross-linked micellar core-shell nanoparticles （SCMCSNs） are chosen to introduce into MOF to formcomposites. Major achievements have been shown as following:Four different charge density surfactants are chosen to investigate their effect onmeso-MOF formation. Phase changes in meso-MOF are observed if the chargedensity of surfactant is changed. The obtained meso-MOF have disordered, lamellar,p6mm and Pm-3n structures accordingly by increasing the charge density ofsurfactants (from C₁₆H₃₃N⁺（C₃H₇）₃, C₁₆H₃₃N⁺（C₂H₅）₃, C₁₆H₃₃N⁺（CH₃）₃to C18-3-1), and these materials have characteristic morphologies. We find that the chargematching is a very important factor in the formation of mesostructures. Orderedmesostructure of MOF can only be synthesized in the appropriate matching ofsurfactant and MOF. The morphology and particle size of product are sensitive to theconcentration of the surfactant. Comparing with the mesoporous silica synthesis, wefind that meso-MOF formation have a similar assembly process, which providesadvantages for further study. However, the relatively complicated formation of meso-MOF is due to their more complex characteristics. Therefore the synthesis system ofmeso-MOF has more variability than that of silica. The research of meso-MOF has abroad possibility in creating novel mesostructures, investigating unique properties anddeveloping new applications.The SCMCSNs are chosen to introduce into MOF to formmesoporous-microporous composite. SCMCSNs surface are modified by differentfunctional groups （carboxyl, or polyvinyl pyrrolidone PVP）, which decides silicananoparticles loading external or internal of MOF. External Eu（1,3,5-BTC）·6H₂Onanorods carry SCMCSNs modified with carboxyl. That make luminescent Eu（1,3,5-BTC）·6H₂O nanorods have mesopores. Due to the charge effect of the carboxylgroup of silica nanoparticles surface that Eu（1,3,5-BTC）·6H₂O nanorods have a gooddispersibility. SCMCSNs are chosen which are stable in the organic systems tomodify with PVP, and then added into the synthetic system of the ZIF-8to formSiO₂-PVP@ZIF-8. As is known that single micelle silica may change its pore sizethrough the use of different surfactant （a cationic surfactant, or a polymer surfactant）,or adding different swelling agent. And single micelle silica nanoparticles haveamphiphilic properties their internal hydrophobic and external hydrophilic. Whensilica nanoparticles hybridize with MOF to become composite by this method, silicananoparticles can also preload other functional molecules into the controlled pore toachieve multi-functional composite. A two-step methodology is used to design of MOF@MOF multifunctional crystals.Composite with two different topological MOF crystals using polyvinylpyrrolidone（PVP） as MOF particles surface modification was synthesized. MesoporousMIL-101（Fe） and luminescent Eu（1,3,5-BTC）·6H₂O are successfully encapsulatedinto a zeolitic imidazolate framework material（ZIF-8）. The structure and morphologyof the “core” crystal are remained during encapsulation process. Experimentsdemonstrated that the ratios of adding nanoparticle to2-methylimidazole are crucialfor good control morphology of the MOF nanocrystal@ZIF-8. The morphology ofcomposite will be controlled from n MIL-101@1ZIF-8to1MIL-101@n ZIF-8.The unique hierarchical porosity of MIL-101（Fe）@ZIF-8composite is demonstratedby fluorescein. This method cannot only carry out structure composite but alsoproperty composite. That explores new field of multi-functional MOF materials forcomplex application. Therefore MOF@MOF is a good strategy to achievemultifunction. The research of MOF@MOF has broad future in creating novelstructures, investigating unique properties and developing new application.