| Green chemistry deals with chemical processes with the aim to decrease and 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 solubility limit 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 reaction system and could not be used repetitively. Heterogeneous catalysts could overcome these shortcomings, but their applications are still quite limited due to the poor activity and selectivity. Preparation of supported metal catalysts 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 supply a promising way to diminish diffusion limits. Furthermore, the surface chemistry could be tailed by functionalization with various organic fragments. As a result, these supported catalysts might exhibit matchable behavirs with the corresponding homogeneous catalysts and could be used repetitively, representing a new trend for designing heterogeneous catalysts for water-medium clean organic synthetic reactions.In this thesis, a series of organic-inorganic hybrid silica materials with ordered mesoporous structure, such as Ph-SBA-15, NH2-Ph-SBA-15, NH2-PMOs-Et and H-PMOs-Ph etc., were synthesized by surfactant self-assembly. Based on these silica, supported Pd catalysts were prepared by impregnation and followed by reduction. Their catalytic performances were evaluated using iodobenzene Ullmann reaction in aqueous solution, which could be considered as an example in green chemistry since the water was used as reaction medium instead of traditionally organic solvents. Based on various characterizations and kinetic studies, the relationship between the catalytic behaviors and structural characters was investigated and discussed briefly and the promoting effects from both the unique pore channels and the surface functionalization were examined. The results are summarized as follows.1. Based on the co-condensation and surfactant self-assembly, the Ph-SBA-15 was synthesized and used as carrier for supporting Pd nanoparticles. The as-prepared Pd/Ph-SBA-15 was used in water-medium Ullmann coupling reaction with iodobenzene. It was found that the Ph-functionalization only slightly disturbed the ordered mesoporous structure and the Pd/Ph-SBA-15 exhibited considerably higher efficiency than Pd/SiO2, Pd/MCM-41, Pd/Ph-MCM-41 and Pd/SBA-15, obviously owing to the enhanced surface hydrophocility. Conducting the Ullmann reaction under CO2 atmosphere could further enhance both the activity and selectivity.2. Based on the co-condensation and surfactant self-assembly, the mesoporous silica functionalized with both Ph- and NH2-groups was synthesized used as carrier for supporting Pd nanoparticles. The Ph-modification could enhance the surface hydrophocility of the catalyst. While, the presence of NH2-groups could coordinate with Pd(II) ions, leading to the high and uniform dispersion of Pd active sites in the pore channels. As a result, the as-prepared Pd/NH2-Ph-SBA-15 catalyst exhibited high activity and selectivity in water-medium Ullmann reaction. The catalyst also displayed strong durability since the leaching of Pd active sites during reactions could be effectively inhibited.3. The Ph-functionalized PMO-type silica was prepared by using Ph-bridged silicane, followed by anchoring Si-H on the pore surface via grafting method. Then, the Pd/H-PMOs-Ph was prepared by in situ reducting Pd(II) ions. Such catalyst displayed highly ordered mesoporous structure with the Ph-gragments homogeneously embedded in the pore walls, which were were stable during heattreatment and chemical reactions in aqueous solution. Besides, the in situ reduction ensured the high dispersion of Pd active sites. As a result, the Pd/H-PMOs-Ph exhibited high catalytic efficiency and long durability during water-medium Ullmann coupling reactions. Based on our researching work, it was found that the catalytic efficiency of the Ullmann reaction in aqueous medium was strongly dependent on following factors: (1) the structure and surface chemistry of the mesoporous supports; (2) the dispersion of Pd naoparticles served as active sites; (3) the thermal stability, especially the hydrothermal stability of the mesoporous structure; (4) the quantity of reductant and base in the reaction system. Thus, more powerful catalysts could be designed for the water-medium clean organic reactions, which offered more opportunities for their industrial applications. |
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