Controllable Synthesis Of Ordered Mesoporous Alumina Materials And Theirs Performance Study

Ordered mesoporous alumina materials with regular pore structures,large specific surface area and pore volume,uniform and variable pore size,rich acid-base active sites,and adjustable surface properties,etc.have attracted extensive attention for their potential applications as catalysts and catalyst supports employed in catalytic cracking,adsorption and separation.However,for partially electropositive Al atoms,it is prone to occur nucleophilic reaction,leading to a fast and complicated hydrolysis-condensation behavior of aluminum precursors.As a result,the aluminum species existing in the synthetic solution cannot effectively interact and self-assemble with the template agent molecules.Therefore,it is still challenging to prepare ordered mesoporous alumina materials through a simple and repeatable synthesis method.In this thesis,a simple and easily reproducible synthesis method is sought to prepare mesoporous alumina materials with highly uniformed and ordered mesoporous structure,high surface area and pore volume,and significantly enhanced thermal stability.Based on the in-depth understanding on the hydrolysis-condensation behavior of aluminum precursors in synthesis process and their interaction with organic templating agents.Herein,a solvothermal pre-treatment induced self-assembly(STISA)method is firstly developed for the controllable synthesis of ordered mesoporous alumina materials.The effects of the solvothermal pre-treatment conditions and solvent evaporation conditions on the structure,texture and thermal stability of prepared materials were investigated in detail.According to the results obtained from multiple characterization techniques,the following conclusions have been drawn.(1)For the traditional evaporation induced self-assembly(EISA)method,the self-assembly between surfactant templates and aluminum species is triggered only during the solvent evaporation process by progressively increasing the surfactant concentration and hydrolysis and condensation degree of aluminum species.Therefore,many parameters that couldinfluencethesolventevaporationconditionsandtheprecursor hydrolysis-condensation reactions,such as temperature,relative humidity,catalysts,solvent evaporation rate,etc.,have impact on the structural and textural properties of final assembled alumina materials.Different from the traditional EISA method,in the STISA method,the introduction of the solvothermal pre-treatment process can promotes the complete hydrolysis of aluminum alkoxides by replacing the-OR with-OH groups and the formation of cluster-like aluminum hydroxide species with small sizes,which is advantageous for the enhanced interaction between aluminum species and the hydrophilic PEO headgroups of surfactant F127 by the hydrogen bond.As a result,the cooperative self-assembly between template F127 and inorganic aluminum species is largely promoted.More importantly,the STISA method can effectively avoid the effects of solvent evaporation conditions(such as relative humidity and solvent evaporation rate)on the synthesis of ordered mesoporous alumina materials,which is very important for the controllable synthesis of ordered mesoporous alumina materials.Compared to mesoporous alumina obtained by conventional EISA process,the resultant ordered mesoporous alumina materials prepared by the STISA approach exhibit higher specific surface area,pore volume and thermal stability.(2)The representative samples prepared with STISA method and EISA method were selected as the carriers to prepare a vanadium-based catalyst for the vapor phase oxidative dehydrogenation reaction of ethylbenzene to styrene with carbon dioxide as a mild oxidant.The catalytic results show that different from the vanadium-based catalyst with mesoporous alumina prepared by the traditional EISA method as support demonstrating a rapid inactivation,the vanadium-based catalyst with ordered mesoporous alumina obtained by STISA route as supported shows much more excellent catalytic activity,selectivity,and stability,which can be attributed to the excellent structural and textural properties of obtained ordered mesoporous alumina support.Its high specific surface area and pore volume can be favorable for a highly homogeneous dispersion of VO_x active species,and larger mesopore sizes can significantly improve the mass transfer and heat transfer of the ethylbenzene dehydrogenation reaction to inhibite the formation of coke deposition.(3)During the synthesis process of alumina mesoporous materials via proposed STISA method,the effects of solvent kinds and the introduction of organic pore-swelling agent(TMB)on the structural and textural properties of the obtained alumina materials were investigated in detail.The results indicate that the polarity and viscosity of solvent will seriously affect the structural and textural properties of the obtained materials.When ethanol and propanol are used as solvent,the small-angle XRD patterns of the obtained samples show obvious Bragg diffraction peaks,corresponding to ordered mesophase structures;while,when isopropanol,butanol,and isobutanol,etc,possessing a high-viscosity,are used as solvent,the resultant mesoporous alumina materials demonstrated a disordered mesostructure with a significantly increased specific surface area.In addition,in the synthesis system,the introduction of mesitylene(TMB)as a pore-swelling agent can successfully achieve the preparation of ordered mesoporous alumina materials with large-pore size.More importantly,the resultant alumina showed a superior thermal stability.The ordered mesostructure can be well retained even after high temperature thermal treatment at 900?.