Mesoporous Structure Of Organic Metal Catalyst Used In Aqueous Medium Clean Organic Synthesis

According to viewpoints of environmental protection and energy saving based on "Green Chemistry", the immobilized catalysts are easily separated and reused in comparison with the corresponding homogeneous catalysts, which could reduce cost and also diminish the environmental pollution from heavy metallic ions. Meanwhile, the use of water instead of organic solvents as reaction media could further decrease cost and the environmental pollutions from volatilization and discharge of organic solvents. Obviously, design of new and powerful heterogenous catalysts represents a keystone for achieving such goal. Herein, a number of heterogenized organometallic catalysts with mesoporous structure were prepared by co-condensation under hydrothermal conditions or evaporation-induced self-assembly (EISA), combined with other assisted technologies. These catalysts exhibited comparable activity and selectivity with their corresponding homogeneous catalysts in many water-medium clean organic reactions and could be used repetitively. Based on detailed characterizations, the correslation of catalytic performances to the structural properties were discussed. The detailed descriptions are listed as follows.1. Ethyl-bridged periodic mesoporous organosilicas (PMOs) functionalized with diphenylphosphino-ligands (PPh2) were synthesized by one-step evaporation- induced self-assembly (EISA), followed by coordinating with Pd(II) ions to prepare a novel heteogenized Pd(II) organometallic catalyst. The as-prepared Pd(II)-PPh2-PMO(Et) exhibited high activity in water-medium C–C coupling reactions and could be used repetitively. The high activity could be attributed to the high dispersion of Pd(II) active sites and ordered mesopore channels which effectively diminished the steric hindrance and thus, diffusion limit. Meanwhile, the ethyl-fragments and the PPh2-ligands in the silica walla could enhance surface hydrophobicity, which promoted the diffusion and adsorption of organic reactant molecules, leading to the high activities and selectivities. 2. Spherical organometallic catalysts with well-ordered two dimensional hexagonal mesoporous channels were synthesized by one-step aerosol-assisted self-assembly (AASA) between tetraethoxysilane (TEOS) and Pd(II), Rh(I) or Ru(II) organometallic silanes using cetyltrimethylammoniumbromide (CTAB) as template. Th catalysts exhibited excellent activity and durability in water-medium C-C formation reactions, such as Barbier and Sonogashira reactions etc. It seems reasonable to conclude that the excellent catalytic performance could be attributed to the high dispersion of Pd(II) active sites and the ordered mesopore channels which facilitated the diffusion and/or adsorption of organic reactant molecules.3. Spherical basic catalysts with centrosymmetric radial mesopores were synthesized by co-condensation method, which showed effective promoting effects on the catalytic performance owing to the surface pore structrure which could facilitate both the adsorption of reactant molecules and the desorption of product molecules to inhibit the formation of byproduct, leading to the enhanced activity and selectivity.4. By using PS spheres as the template, hollow spherical zeolitic imidazolate framework (ZIF-8) was synthesized in a simple way without traditional heator, microwave or ultrasound irradiation and even stabilizing agents. Such ZIF-8 catalyst displayed high activity in water-medium Knoevengael reaction owing to the high surface area and the unique ordered microporous structures.