Synthesis And Characterization Of Metal-Organic Frameworks Based On Flexible Multicarboxylate Ligands

Metal-organic frameworks have emerged as a new type of porous materials withunique and outstanding properties, including high surface area, high porosity andeasily tunable pore size and function. These attributes make them great potentialapplications in the areas of gas storage, separation and catalysis. A large number ofMOFs with novel structures have been designed and synthesized according to thecrystal engineering approaches. However, the preparation of MOFs with predictablestructures is still a challenge. In recent years, the use of metal-organic polyhedra(MOPs) as supramolecular building blocks (SBB) for the construction of porousframeworks has been a powerful strategy, because MOPs themselves have cavitiesand possess high symmetry, leading to the possibility of a crystal engineering strategyfor MOFs fabrication with highly specific control over the topology. The use offlexible ligands to cross-link the MOPs can construct a few novel topology nets thatare inaccessible by using similar rigid ligands.In this thesis, the design and synthesis of MOFs as well as their applicaitons inthe fields of gas storage, separation and catalysis were reviewed. A series of novelthree-dimensional porous MOFs with MOPs as SBB have been synthesized basedupon the flexible tetracarboxylate ligands. The physicochemical properties andcatalytic behaviors of these MOFs have also been characterized. The main resultsobtained in this thesis are as follows:(1) Four flexible ligands,1,1-bis-[3,5-bis(caboxy) phenoxy]methane (H4L1),1,2-bis-[3,5-bis(caboxy) phenoxy]ethane (H₄L2),1,4-bis-[3,5-bis(caboxy)phenoxy]butane (H₄L3) and1,3-bis-[3,5-bis(caboxy) phenoxy] isopropanol (H4L4),have been prepared according to the literature. The molecular structures of the fourligands were characterized by means of Fourier transform infrared spectroscopy(FT-IR) and the proton nuclear magnetic resonance spectra (¹HNMR).(2) Five new porous MOFs, noted as complex1,2,3,4and5, respectively, havebeen prepared based upon the four flexible ligands under solvothermal reactionconditions. The crystal structures of the as-synthesized complexes were characterizedby means of single crystal X-ray diffraction. The physicochemical properties of theas-synthesized complexes were characterized by means of the techniques, such asFT-IR, powder X-ray diffraction (PXRD) and thermal gravimetric analysis (TGA).Among these MOFs, complex1is a pcu-type topological net based upon smallrhombihexahedron SBBs. Complex2and3are isostructural acs-type topological MOFs with trigonal prismatic MOP as SBBs. Complex4is a nbo-type topologicalMOFs. Up to now, the nbo-type topology net based upon flexible ligands has neverbeen reported.(3) The adsorption properties of complex1for N2, O₂, CH4and CO₂underatmospheric pressure and high pressure have been characterized. It was found thatcomplex1exhibited high performance for CH₄and CO₂adsorption. The amounts ofCH₄and CO₂adsorption at295K under6000mmHg were up to90and68cm3g-1,respectively. It is noteworthy that the CO₂sorption isotherm displays an obvioushysteresis. For most of the porous MOFs, hysteretic gas-uptake has been rarelyreported.(4) In order to prevent the formation of bridged μ-oxide dimers and oxidativeself-degradation for metalloporphyrin, two Por@MOFs type complexes, complex6and7, were prepared under solvothermal reaction conditions based upon the synthesiscondition of complex1. The physicochemical properties of complex6and7havebeen characterized by using the optical microscope photography, UV-Vis spectrum,ICP atomic emission spectrometry, elemental analysis, PXRD, N2adsorption-desorption, TGA and FT-IR.(5) The catalytic activities of complex6and7have been assessed for theoxidation of3,5-di-t-butyl-catechol (DTBC) to o-quinone. The results showed that thecomplex6exhibits a high catalytic activity, however, the catalytic activity of complex7is similar to complex1.