Preparation Of Porous Complex Oxides Monoliths Via Sol-gel Process Accompanied By Phase Separation

Porous monoliths are a kind of monolithic shaped materials that contain a certain size and qualitity of pores with ordered structure. Due to its low bulk density, high surface area, excellent permeability and adsorption performance, porous monoliths are widely used as support materials in the fields of filtration, separation, adsorption and catalysis, and have been a hot research area in advanced materials. Many methods have been used to prepare porous monoliths. Sol-gel process accompanied by phase separation is a new wet chemical technique to prepare porous monoliths with controllable "co-continuous" structure in the micrometer scale. These monolithic materials show superior performances for applications as catalyst supports and liquid chromatography columns.This paper introduces the current situation of synthesis of porous monoliths. Based on the previous research of SiO2, Al2O3 and TiO2 system, preparation of porous complex oxides monoliths, mayenite (Ca12Al14O33), aluminium titanate (Al2TiO5) and aluminum phosphate (AIPO4) through sol-gel process accompanied by phase separation are studied. The effects of preparation parameters, such as the amount of poly(ethylene oxide) (PEO), propylene oxide (PO) and solvent, gelation and heat-treatment temperatures on the morphologies of samples are analyzed in detail. The mechanism of sol-gel process accompanied by phase separation of each system is investigated. The results are as follow:(1) Porous mayenite monolith is prepared by using aluminium chloride and calcium chloride as precursors, glycol (EG) as chelating agent, formamide (FA) as drying control chemical additive, PO as gelation agent, PEO (Mv=3×105) as phase separation inducer, water and ethanol as solvent. The addition of EG and FA suppresses the precipitation of calcium ions during sol-gel process and reduces the shrinkage of mayenite gels during evaporation drying, respectively. The mechanism of sol-gel process accompanied by phase separation of Ca-Al-0 system is relative to the ring-opening reaction of PO as well as the interaction between PEO and Al2O3 oligomers. Alkylene oxide segments in PEO form hydrogen-bonds with Al2O3 oligomers, which induces the polycondesation of Al2O3 oligomers and phase separation. PEO is preferentially distributed into the liquid phase. Under the condition of optimal starting compositions and gelation temperatures, monolithic mayenite xerogel with co-continuous structure is obtained. After 1000℃ heat-treatment, this amorphous gel transforms to crystalline phase Ca12Al14O32Cl2, while the macroporous structure is preserved. Porous mayenite monoliths possess narrow pore size distribution between 1-2 μm and high porosity of 72.9% after heat-treatment. This porous mayenite material will find applications in electrochemistry and catalyst support.(2) Porous aluminium titanate monolith is prepared by using aluminium chloride and tetrabutyl titanate as precursors, citric acid as chelating agent, FA as gelation agent, PEO (Mv=1×105) as phase separation inducer, water and ethanol as solvent. The phase separation of Al2O3-TiO2 system is a result of a compativite process between the phase separation of PEO-Al2O3 system and PEO-TiO2 system, and PEO is mainly distributed into gel phase after phase separation. Using the optimal starting compositions, monolithic aluminium titanate xerogel with well-defned macropores can be obtained. It has a pore size mainly distributed between 2-3 μm and a porosity of 63.3%. After heat-treated at 1300℃, the amorphous gel transforms to crystalline phase Al2TiO5, and the macroporous structure is optimized by the heat-treatment process. Compared with porous aluminium titanate ceramics prepared by a pore-forming method, the resultant monolith possesses co-contimuous structure with narrow pore size distribution, which is suitable for gas transfer. This porous Al2TiO5 material can be used as substrate material for diesel particulate filters.(3) Hierarchically porous aluminum phosphate monolith is prepared by using aluminium chloride and phosphate as precursors, PO as gelation agent, PEO (Mv=1×104) as phase separation inducer, water and methanol as solvent. The macroporous structure is optimized by adjusting the amount of PEO, PO and solvent. Ag nanoparticles (NPs) embedded aluminum phosphate monolith is synthesized by silver colloid immersion followed by heat-treatment. The silver colloid is prepared by using solvothermal method. Under the optimal process, silver NPs are distributed uniformly on the surface of skeletons. This Ag NPs-embedded porous aluminum phosphate material has potential application in precious metal catalysis.