Synthesis And Catalytic Performance Of Alumina With Novel Pore Structure

Alumina is widely applied as solid acid catalyst and support of catalysts in a varity ofpetroleum chemical processes. Mesoporous alumina offers high surface area, concentratepore size distribution, acidic surface functional group and excellent thermal stability, whichfacilitate the adsorption and promote catalytic properties. As a kind of non-silicon-basedmaterial, mesoporous alumina with various diameters and larger surface areas can becurrently prepared with surfactant templating strategies, which is an interaction betweenself-assembled aggregates of surfactant molecules and aluminum ploymers. Once an orderedphase is established, subsequent thermal processing is used to remove the organic andproduce a stable mseoporous alumina.This thesis is intended for the development of novel routes to hierarchically porousalumina, and avoiding the defetcts of previous approaches, such as mono-modal porestructure, mono-modal pore structure and so on. The as-synthesized alumina samples werecharacterized by X-ray diffraction (XRD), nuclear magnetic resonance, thermogravimetricand differential scanning calorimeter (TG-DSC), transmission electron microscopy (TEM),and N2adsorption-desorption analysis. The catalytic performance of catalysts were evaluatedin the hydrodesulfurization of4,6-Dimethyldibenzothiophene (4,6-DMDBT) in a fixed-bedreactor.A hierarchically mesoporous γ-Al2O3was successfully synthesized using sucrose andnonionic surfactant as templates in the hydrolysis process of aluminum isopropoxide. Thepore structure, crystal parameters and morphology of the alumina can be controlled bychanging the synthesis parameters, such as system pH value, templates content, aging time,etc. The as-synthesized alumina exhibits high surface area, large pore volume and a particularbimodal-pore structure with narrow pore size distributions. The smaller pore size isconcentrated around5nm and the larger one is around10nm. The bimodal pore structureendows this materials high thermal stability.A polyethylene glycol (PEG) controlled homogeneous precipitation route to bimodalmesoporous γ-Al2O3was developed. By adjusting the synthesis conditions, such as PEG content, solution pH, precipitant content, both of the mesopores sizes can be controlled. ThePEG molecules selectively adsorbed on certain faces of ammonium aluminium carbonatehydroxide (AACH) crystals through hydrogen bonding and induced the crystal growth alongthe specific direction to give1D nanorod morphology. The introduction of alumina withbimodal mesopore to the HDS (Hydrodesulfurization) catalysts can decrease themetal–support interaction, reduction temperature of molybdic oxide and diffusion resistance.High surface area of support contributes the higher dispersion of active phase and providesmore active sites to the catalysts. As a result, the CoMo-based catalysts prepared withbimodal mesoporous alumina have higher HDS activity for4,6-DMDBT compared to thecatalyst with mono-modal pores. Due to the steric hindrance effect of4,6-DMDBT on theDDS (Direct Desulfurization) pathway, the dominant HDS route in this study is HYD(Hydrogenation Desulfurization).Aluminum nitrate and urea were innovatively introduced to cross-linking polymerizationsystem of acrylic amide or styrene emulsion, in the synthesis of macro-mesoporous aluminafoam and monolith, respectively. The alumina foam and monolith exhibit three-dimensionalconnected macropores with a pore size ranging from submicrometers to several micrometers,and mesopore in the range of2–5nm in the macropore walls. This pore structure resultsrenders this novel material highly potential in the field of catalysis, adsorption and separation.