Synthesis, Characterization And Catalytic Performance Of Silica-based Mesoporous Molecular Sieves In Selective Oxidation

Since Mobil researchers reported the discovery of a new family of silica-based mesoporous molecular sieves (M41S) materials in 1992, there has been a growing interest in the fields of materials, catalysis and separation. In recent years, great progress has been made in synthesis, modification, and novel application of mesoporous molecular sieves. Based on the investigation of the literatures, this dissertation was focused on synthesis, characterization and catalytic performance of heteroatom-containing silica-based molecular sieves. The main progress and research results have been obtained as follows:1. The synthesis and characterization of bismuth-containing SBA-15 mesoporous molecular sievesBismuth-containing SBA-15 mesoporous materials with different n_Si/n_Bi ratio was synthesized for the first time through a direct hydrothermal method, and characterized by various physico-chemical techniques. The content of Bi was quantified by inductively coupled plasma emission Spectrometry (ICP). XRD, N₂ adsorption/desorption and TEM analysis reveal that well-ordered hexagonal structure of SBA-15 is maintained after the incorporation of bismuth, and the synthesized materials have good crystallinity and high surface area. SEM analysis shows that Bi-SBA-15 has the morphology of rodlike particles. Diffuse-reflectance UV-visible (UV-vis) spectroscopy and Raman spectroscopy analysis show that bismuth atoms in Bi-SBA-15 exist in a highly dispersed state, and locate mainly at tetrahedrally coordinated sites. TG analysis shows that there exists difference in the removal of the organic template P₁₂₃ between SBA-15 and Bi-SBA-15. Moreover, XPS analysis shows that most of bismuth atoms enter the internal surface of SBA-15.2. Liquid-phase oxidation of cyclohexane catalyzed by Bi-SBA-15 In the catalytic test, the synthesized Bi-SBA-15 is found to be a very efficient catalyst for the oxidation of cyclohexane, in a solvent-free system, with oxygen as oxidant. Among the commonly used oxidants (H₂O₂, TBHP etc), molecular oxygen is the cheapest and cleanliest oxidant. Moreover, this catalytic system is economical and environmentally friendly, without solvents, co-catalyst or free-radical initiator. The influence of different parameters, such as reaction time, reaction temperature, oxygen pressure and the dosage of catalyst on the oxidation of cyclohexane is also investigated. In the condition of 1 MPa O₂ and 413 K for 4 h, the conversion of cyclohexane is 16.9%, and the total selectivity of cyclohexanol and cyclohexanone is 93%. Bi-SBA-15 is very stable in the system and can be reused. In addition, we confirm the radical initiated mechanism of this reaction.3. Liquid-phase oxidation of cyclooctane catalyzed by Bi-SBA-15In the catalytic test, the synthesized Bi-SBA-15 is found to be an efficient catalyst for the oxidation of cyclooctane, in a solvent-free system, with oxygen as oxidant. The influence of different parameters, such as reaction time, reaction temperature, oxygen pressure and the dosage of catalyst on the oxidation of cyclooctane is also investigated. In the condition of 1 MPa O₂ and 403 K for 6 h, the conversion of cyclooctane is 10.3%, and the total selectivity of cyclooctanol and cyclooctanone is 92%.4. Facile oxidation of aldehyde to esters using S·SnO₂/HMS-H₂O₂S·SnO₂/HMS catalyst was synthesized and characterized by XRD and N₂ adsorption/desorption. This catalyst is highly efficient for the oxidation of aldehyde to corresponding esters in alcohols under the mild reaction condition (323 K), with H₂O₂ as oxidant. This catalytic system can be used in the preparation of a broad range of esters. Moreover, S·SnO₂/HMS catalyst can be easily separated from the reaction system and reused several times.