Functional And Orderly Preparation Of Mesoporous Silica Materials In Green Organic Synthesis Applications

With the rapid growth of global industry and the rapid increase of world population, environmental pollution has become more and more serious. Green chemistry is widely attractive by government, academia and industry. Green chemistry deals with chemical processes aimming to decrease and even eliminate the pollution from fountainhead, including the designation of the non-toxic and safety catalysts with high efficiency, the use of clean raw materials, and the development of environmentally friendly reaction engineering process etc. Design of new catalysts for water-medium clean organic syntheses represents an important branch of green chemistry. Due to the limit of solubility of organic reactants in water, most organic reactions in water medium are conducted in the presence of homogeneous catalysts, which suffer from the difficult separation from the reaction system and could not be used repetitively. Heterogeneous catalyst could overcome these shortcomings, but their applications are still quite limited due to the poor activity and selectivity.The mesoporous materials display various superiorities including uniform and controllable mesoporous diameter, stable framework, suitable sized wall thickness, high surface area and easy modification of the pore surface. Preparation of immobilized catalysts onto these supports with ordered mesoporous structure may greatly enhance the dispersion degree of active sites. Meanwhile, the pore structure and size could be easily adjusted based on different templates, which can supply a promising way to diminish diffusion limits. Furthermore, the surface chemistry could be tailed by functionalization with various organic fragments. As a result, the catalysts immobilized onto these supports might exhibit matchable behaviors compared to the corresponding homogeneous catalysts and could be used repetitively, representing a new trend for designing heterogeneous catalysts for water-medium clean organic synthetic reactions.Previously, the immobilized catalysts onto supports with ordered mesoporous structure are mainly prepared by post-grafted method, which usually suffers from the pore blockage, leading to the decrease in catalytic efficiencies. Meanwhile, modification of the silica supports with hydrophobic groups could promote catalytic efficiencies owing to the facilitated adsorption and diffusion of organic reactants to active sites especially in water media. However, the introduction of large amount of organic groups also blocks the pore channels. In this thesis, ordered mesoporous heterogenized catalysts with active phases including organometallic complex, basic groups and the organic functionalities incorporated into silica walls were prepared by surfactant directed co-condensation. Their applications in the water-media organic reactions have been investigated. This thesis contains the following five sections:1. A facile approach to prepare Pd(II) organometal bridged periodic mesoporous organometalsilica was developed based on co-condensation of Pd {PPh2(CH2)2Si(OEt)3}3Cl₂ and (EtO)4Si, using surfactant P123 as template. The as-prepared Pd(II)-PPh2-SBA-15 catalysts possess ordered two-dimensional hexagonal mesoporous structure and large surface area. Compared to the catalysts prepared by the post-grafting method, the Pd(II)-PPh2-SBA-15 catalysts showed high catalytic efficiencies owing to the high dispersion of active sites, diminished pore blockage which facilitated the diffusion and adsorption of the organic reactants onto the catalyst. The Pd(II)-SBA-15 catalyst exhibited matchable catalytic efficiencies with the corresponding Pd(PPh3)2Cl homogeneous catalyst in water-medium Suzuki and Babier reactions. 2. A periodic mesoporous organogold(I)silica catalyst was synthesized by surfactant-directed co-condensation of Au{PPh2(CH2)2Si(OCH2CH₃)3}2Cl and (CH₃CH2O)3SiPhSi(OCH2CH₃)3. The as-prepared Au-PPh2-PMO(Ph) contained ordered mesopore channels with both phenyl (Ph) group and Au(I) organometal homogeneously embedded in silica walls. Such catalyst exhibited higher activity than free Au(PPh3)Cl in hydration reactions of various terminal alkynes. This could be mainly attributed to the high surface area, the ordered mesoporous channels and the enhanced surface hydrophobility, which facilitated the diffusion and adsorption of reactants. Meanwhile, the unique coordination model between Au(I) and PPh2 was also favorable for the alkyne hydration. The catalyst could be easily recycled and used repetitively owing to the high hydrothermal stability and the stabilized Au(I) active sites against leaching.3. A novel Au nanoparticle catalyst was prepared by in situ reduction of Au3+ with thiol in mercaptopropyl terminally bonded to the ethyl (Et)-bridged PMO support and the resulted Au nanoparticles were coordinated by thiol ligands. The as-prepared catalyst was denoted as Au-SH/SO₃H-PMO(Et), where SH refers to the HS-CH2-CH2-CH2 group and SO₃H refers to the SO₃H-CH2-CH2-CH2 group resulted from the oxidation of HS-CH2-CH2-CH2 by Au3+. The Au-HS/SO₃H-PMO(Et) displayed highly dispersed 1² nm Au naoparticles coordinated with HS-ligand and the ordered mesoporous structure with ethyl fragments embedded in silica walls. It acted as a bifunctional catalyst in hydration reactions of various alkynes in solvent free or water-medium organic reactions due to the presence of both the Au nanoparticles and the SO₃H acidic groups, which exhibited higher activity than homogeneous Au+ organometallic catalyst and could be recycled for more than 10 times. 4. Diamino-functionalized periodic mesoporous organosilicas, denoted as BD-PMO(Et), were synthesized directly by P123 surfactant directed co-condensation of 2-bis(triethoxysilyl)ethane (BTEE) and 1,4-bis-{3-(tirmethoxysilyl)-propyl} ethylenediamino (BTMSED) under acidic conditions. The content of N could be adjusted from1.5 to 3.9 mmol/g without destroying the ordered mesoporous structure. These samples were proven to be effective heterogeneous base catalysts in water-medium organic reactions, such as Knoevenagel and Henry condensation reactions as well as in the intermolecular cross-double-Michael addition reactions. They usually exhibited highly improved catalytic activities and could be used repetitively, which could reduce the cost and diminish the environmental pollution.5. BD-PMO(Et) was used as a support for immobizing Pd(II) organometallic catalyst by coordination reaction between N and Pd(II). The as-prepared Pd-en-PMO(Et) exhibited higher activity than the Pd(CH₃CN)₂Cl₂ catalyst in water-medium Sonaogashira reaction and could be reused. The high activity could be attributed to the high dispersion of Pd(II) active sites and ordered mesopore channels which effectively diminished diffusion limit and the steric hindrance. Furthermore, the ethyl-fragments incorparated into the support framework could enhance surface hydrophobicity, which facilated the adsorption and diffusion of organic reactant molecules in aqueous solution.