Preparation Of Macroporous Zirconia Monoliths Via Sol-gel Process Accompanied By Phase Separation

Zirconia is an important structural and functional material with high thermal and chemical stability, while porous materials possess many properties such as low density, high porosity, large surface area, and good adsorption. Thus, porous zirconia becomes a promising material to be used as catalyst carrier, and also in fields like separation, extraction, adsorption, and degradation. Sol-gel process accompanied by phase separation is a newly-developed technique to prepare porous ZrO^monoliths. However, reported research used expensive Zr-alkoxides as precursors, which made the process hard to control with rapid reaction rate.In the research of this thesis, ZrO2monoliths with controllable co-continuous macropores and skeletons were successfully fabricated via sol-gel process accompanied by phase separation in the presence of propylene oxide (PO) and poly(ethylene oxide)(PEO), where ZrOCl2-8H2O was used as a cheap ionic precursor. The preparation process, properties, pore structure, and hydrothermal treatment of the prepared ZrO2monoliths were deeply studied. The main results are listed as follows.(1) Preparation process optimization:the influence of PEO content, PO content, reaction temperature and solvent amount on the formation of pore structure was analysed and the mechanism of phase separation was discussed. The gelation of the system was mediated by PO as an acid scavenger, while PEO was added as a phase separation inducer. The driving force for phase separation is ascribed to the repulsive interaction between solvent mixtures and PEO adsorbed on ZrO2oligomers. The size of the macropores could be controlled by adjusting PEO content, while PO content, reaction temperature, solvent amount and proportion also had influence on the morphology of the monoliths. TEOS was induced in the solvent exchange process to prevent gel shrinkage during drying. Ideal co-continuous macroporous structure could be obtained with0.52mL PO,2.4mL H2O and EtOH each, and0.750-0.120g PEO under reaction temperature of80℃.(2) Heat-treatment and pore structure study:the phase transformation after heat-treatment and the pore structure were analysed. The crystallization of the prepared ZrOi monoliths during heat-treatment was inhibited for TEOS grafting on the gel skeleton. When heat-treated under600℃, ZrO2remained amorphous. Crystalline phase of tetragonal ZrO2only existed after heat-treatment at700-1000℃without spoiling the macroporous structure. The prepared ZrO2monoliths possessed a porosity of52.9%, narrow pore size distribution and surface area as large as172m2/g. But with the raise of heat-treatment temperature, the achieved surface area decreased rapidly.(3) Hydrothermal treatment:the influence of solution system, mineralizer content, hydrothermal duration and temperature on crystallization and pore structure were analysed. After solvothermal treatment with the ethanol solution of ammonia, tetragonal ZrO2monoliths with smooth skeletons and well-defined mesopores were achieved, and the surface area was enhanced to584m2/g. The effect of solvothermal treatment could be improved through increasing ammonia concentration, extending treatment duration or raising temperature.