The Synthesis Of Super-hydrophobic Layered Titanosilicates And Their Application In Olefins Epoxidation

Since the discovery of TS-1in1983, the titanosilicate materials (e.g. TS-1, TS-2, Ti-MOR, Ti-β) have attracted much attention because of their excellent catalytic activity in the selective oxidation of organic molecules using H2O2aqueous solution as the oxidant. One of the most important findings in this field is that the catalytic activity is not only related to the well-dispersed isolated tetrahedral Ti (Ⅳ) but also to the location of Ti in a hydrophobic/hydrophilic channel or cavity in the structure. It has been proven that the hydrophobicity can prevent poisoning of the active site by water as well as unproductive decomposition of H2O2and lead to a high catalytic performance. Two main strategies have been developed to synthesize hydrophobic TS materials via:1) grafting hydrophobic organic groups onto the surface;2) directly incorporating them into the frameworks. However, the restricted incorporated organic content and large portion of uncondensed silanol groups resulted in a limited enhancement of their hydrophobicity.In this thesis, a defect-less, layered organo-titanosilicate (denoted as LOTS) with covalently linked phenyl groups was synthesized through a simple and template-free synthetic route. Comparing with other TS catalysts, LOTS displays super-hydrophobicity, and exhibits superior catalytic activity in olefin epoxidation using H2O2as oxidants at room temperature.The layered organo-titanosilicate, LOTS, is synthesized using a simple, template-free, evaporation-induced self-assembly process in acidic ethanol solution, using organic trimethoxyl monosilane and tetrabutyl orthotitanate as precursors. The synthetic conditions were optimized by the variation of the organic monosilane, the aging temperature, the Ti/Si mole ratio, the acid concentration and other synthetic parameters. The layered structure is characterized by XRD, TEM, solid state NMR and XPS, which confirms that LOTS has a bilayer structure with a linear arrangement of two phenyl groups existing in its framework; and in the organo layer, a directional alignment of phenyl groups exists. The high content of phenyl groups (R:Si mole ratio is1:1) and high condensation of surface Si-OH (over90%), lead to a surper-hydrophobic surface of LOTS. Compared with other TS catalysts, LOTS exhibits excellent catalytic performance in the olefins epoxidation using30%H2O2as oxidant at room temperature.The low surface area of LOTS limits its catalytic activity due to less exposed Ti active sites. In order to increase the number of the surface active Ti sites, LOTS materials with a micro sphere morphology (denoted as ms-LOTS) were synthesized via the aerosol-assisted self-assembly (AASA) method. Ms-LOTS with a uniform sphere size of2-4micrometer was successfully prepared by the optimization of synthetic condition(the precursor concentration and the reaction temperature). In the olefins epoxidation. ms-LOTS show improved catalytic performance as compared to LOTS.In order to further enhance the catalytic activity and increase the surface active Ti sites, LOTS was coated on the surface of silica submicro spheres with the diameter about100nm, and leading to the formation of titanosilicate submicro spheres. A hydrophobic surface of silica micro-spheres is critical for a homogeneous coating. Thanks to the much smaller particle size and higher exposure of Ti active sites, this titanosilicate submicro spheres showed6-fold Ti active sites’utilization compared with normal LOTS, and about60-fold compared with TS-1, in the cyclohexene epoxidation.