Design, synthesis and catalytic applications of mesoporous silica molecular sieves

The discovery of the Mobil M41S family of mesoporous molecular sieves has stimulated great interest in surfactant-directed assembly of inorganic mesostructures. The mesopores in these molecular sieves provide new opportunities for catalytic conversion of large substrates in the liquid phase. Even though mesoporous molecular sieves exhibit much larger pore sizes than conventional zeolites, accessibility to the catalytically active sites in the framework of the mesopores is still very crucial for diffusion limited reactions. Therefore, it is practically useful and scientifically significant to find some ways to improve the accessibility to the active catalytic sites inside the mesopores.; In addition to framework mesopores, complementary textural mesopores are extremely important for enhancing the accessibility in catalysis. Our first objective was to tailor both the framework and the textural porosity of HMS silicas assembled through an electrically neutral SoI o mechanism. The approach involves a control of the solvent polarity, along with the use of organic auxiliary agent (i.e. mesitylene) to control particle nucleation rates and pore size. Increasing the polarity of the synthesis medium caused faster hydrolysis of silicon alkoxide in the presence of the alkyl amine surfactant, which led to meso-scaled fundamental particles and high textural porosity. Our second objective was to use organic assembly modifiers such as tartaric, oxalic, gluconic and citric acids to prepare a well-ordered MCM-41 with meoscaled particle size and textural pores or with flake-shaped particle morphology and pore channels running orthogonal to the flaky surface. The importance of the complementary textural pores and shorter channel lengths for improving the accessibility of the framework pores in catalysis was well demonstrated by liquid phase alkylation of 2,4-di-tert-butyl phenol in the presence of Al-substituted derivatives as the catalysts. By facilitating accessibility, significantly enhanced catalytic activity was observed for both Al-HMS and Al-MCM-41 with complementary textural mesopores and meso-scaled fundamental particles comparable to the pore channel lengths.; A new concept of altering the inorganic mesostructures by organic promoters through electrostatic S+I− assembly has been successfully illustrated. Mesoporous, silica molecular sieves with well ordered 3-dimensional hexagonal and cubic phases were hydrothermally assembled in the presence of organic modifiers using cetyltrimethyl ammonium bromide as the structure director and sodium silicate as the silicon source. In addition, a concept of incorporating structural order into mesoporous silica molecular sieves through the introduction of electrostatic interactions at the interfaces of electrically neutral surfactant micelles and neutral inorganic precursors has been successfully demonstrated. In stead of wormhole motifs, mesoporous silica molecular sieves with well ordered hybrid domains consisted of 3-dimensional hexagonal and spherical cubic (3-d-hex-cubic) phases and uni-dimensional hexagonal phase were assembled using alkyl polyethylene oxide surfactants and tetraethyl orthosilicate through a new counterion mediated (NoM +X−)Io pathway. The modifiers most likely bind to the surfactant head group to cause a significant change in the effective-surfactant-head-group-area, which alters the packing parameter g and, ultimately, the inorganic mesostructure.