| Green chemistry is the inevitable trend for sustainable development of resourceand environment. To save resource and prevent pollution, green chemistry appliesinnovative chemical technology to change the traditional process of chemistry,promoting both the economical development and environmental protection. Mostprocesses in chemical industry are dependent on catalysts. Novel catalyst canenhance both the activity and the selectivity of chemistry reaction, leading to thereduced energy consumption and waste discharge. Therefore, catalysis is the coredomain of green chemistry. Most organic reactions are performed by using organicsolvents as reaction media. The volatilization and discharge of organic solventsconstitute the main sources of industrial pollution. Development of new organicreactions by using safe and clean water instead of organic solvents as reaction mediais one of the most important branches of green chemistry. To date, most studies arefocused on the homogeneous catalysis due to the dissolubility of organic compoundsin aqueous solution, which usually displays disadvantages in separation and reuse ofthe catalyst. Heterogeneous catalysts can solve the problem, but they commonlyexhibit poor activity and the selectivity in comparison with the correspondinghomogeneous catalysts.Metal-organic frameworks (MOFs) represent a new class of functional materialsformed by metal ions or clusters as nodes and polyfunctional organic ligands aslinkers. The controllable topology and geometry of framework and the tunable porefunctionality predict their versatile promising applications in catalysis, gas storage,chemical separations, sensors, drug delivery and light harvesting etc. MOFs arecharacterized by the defined crystal structure, the extended framework, metal ionsas nodes of the networks, and the ability to modulate the sizes of the micropores andthe topology by changing and modifying the organic ligand, which may make MOFsparticularly suitable for heterogeneous catalysts. The key to design and synthesizeMOFs-based catalysts is the incorporation of active sites into the frameworks. Twomethods are mainly employed.(1) Direct incorporation. The MOFs-based catalysts are directly synthesized by combining organic ligands with the active sites.(2) Postsynthesis modification. After the formation of MOFs, the active groups are modifiedinto the organic frameworks by covalent bond or coordinately grafted into theunsaturated metal sites.The paper involves design, preparation and characterization of novelMOFs-based materials with the aim to supplying powerful heterogeneous catalystsfor water-medium organic reactions. Through direct incorporation and postsynthesis modification, various active species are incorporated into the frameworksof MOFs with different crystal structure. By using different water-medium organicreactions as probes, their catalytic performances are carefully investigated andcompared with the heterogeneous catalysts prepared by traditional routes and thecommercial homogeneous catalysts. Meanwhile, MOFs-based multifunctionalcatalysts are synthesized and used in one-pot cascade reactions. Based on the crystalstructures of MOF, the influence of chemical microenvironment of active sitesincluding the micropore size, the chemical bond on the catalytic activity andselectivity are examined and discussed. In addition, the catalyst durability is alsodetermined and the reason of deactivation has been explored. This work includes thefollowing four parts:1. A novel Pd/Y heterobimetallic MOF (Pd/Y-MOF) catalyst is synthesized bycoordination of Pd2+and Y3+with2,2’-bipyridine-5,5’-dicarboxylate acid (bpydc)under microwave irradiation conditions. The crystal structure and other structuralcharacteristics are examined by XRD Rietveld refinement, FTIR, Raman, TG-DTA, XPSetc. It is found that the3D extended framework is constructed by linkingPd(bpydc)Cl2building blocks via Y3+coordinating to carboxylic groups. ThisPd/Y-MOF exhibits higher catalytic activity than Pd(bpydc)Cl2in water-mediumSuzuki-Miyaura coupling and Sonogashira coupling reactions owing to thecooperative effect between Pd2+and Y3+. The heterogeneity studies provide amechanistic evidence of the reaction proceeds on the surface of Pd2+ions in thecrystal framework. Thus, the Pd/Y-MOF exhibits remarkable size selectivity towardssubstrates. By using small-sized reactants in Suzuki-Miyaura coupling reaction, it displays comparable activities with the corresponding Pd(OAc)2homogeneouscatalyst. However, extremely poor activity is observed when bulk substrates1-iodonaphthalene and4-(tert-butyl) iodobenzene was used, which could beattributed to the inhibition of diffusion into the micropore channels taking intoaccount that the active sites are mainly incorporated in the pore channels ratherthan on the outer surface. Besides the high activity, the Pd/Y-MOF can be easilyrecycled and reused. The stabilizing coordination of Y3+with carboxylic group resultsin the high thermal stability of MOF framework. Meanwhile, the incorporation ofPd2+into the MOF framework could effectively prohibit Pd2+active species fromleaching.2. A facile approach has been developed to synthesize amine-functionalizedlanthanide MOF via the reaction between terbium and2-aminoterephthalic acidunder microwave conditions. The crystal structure is determined by XRD. Theas-prepared NH2-Tb-MOF is used as a basic catalyst in water-medium Knoevenagelcondensation and Henry reactions, which exhibits high activity and100%selectivityto the condensation product. Meanwhile, it can be easily recovered and reused.Moreover, we also find that the small-sized reactants favor while the bulk reactantssuppress the activity of NH2-Tb-MOF catalyst owing to the diffusion limit inmicroporous channels, which confirms that the amino groups are incorporated intomicropore channels rather than on the outer surface.3. One of the Uio series of Zr-based metal–organic framework (Zr-MOF) issynthesized by coordinated polymerization between ZrCl4and2,2’-bipyridine-5,5’-dicarboxylate acid under hydrothermal conditions. The Zr-MOF is used as supportsfor depositing Au nanoparticles. During the impregnation of Zr-MOF with HAuCl4ethanol solution, the Au/Zr-MOF is prepared by in situ reduction of Au3+into tiny Aunanoparticles (1.5nm). This catalyst exhibits high activity in various water-mediumA3-coupling reactions owing to the uniform dispersion of Au nanoparticles and themicroporous structure which facilitates the diffusion and adsorption of reactantmolecules. Moreover, the catalyst can be easily recycled and reused for at least5times without significant decrease in activity, which could be attributed to the excellent hydrothermal stability of the MOF structure. The strong interaction of Aunanoparticles with the MOF support inhibits the Au leaching.4. MIL series of materials formed by trivalent metal ions (Cr3+，Fe3+and Al3+）and1,4-benzenedicarboxylic acid are known MOFs with stable structure, giant poresand high surface area. Under microwave conditions, the NH2-MIL-101(Al) in purephase has been facilely synthesized via reaction between aluminum salt and organicligand containing amine functional groups. Then, after the amino group of theorganic ligand2-aminoterephthalate ligands is covalently modified withsalicylaldehyde to form the corresponding imine, NH2-MIL-101(Al)-Schiff, CuCl isfurther incorporated. With Cu-Schiff base complex lining on the pore walls, theNH2-MIL-101(Al)-Schiff-CuCl displayed high activity and selectivity in A3-couplingreactions of a wide range of reactants in water-medium, similar to the homogeneouscatalyst. More importantly, it could be easily recycled and used repetitively. |
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