Preparation of Thin Porous Silica Foam on Alumina Disk Substrate
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Catalysis Letters Vol. 106, Nos. 1–2, January 2006 (Ó 2006) DOI: 10.1007/s10562-005-9189-0
Preparation of thin porous silica foam on alumina disk substrate Ping-le Liua and Leon Leffertsb,* a College of Chemical Engineering, Xiangtan University, 411105, Xiangtan, P.R. China Catalytic Processes and Materials, Faculty of Science and Technology, IMPACT, University of Twente, AE Enschede 217 7500, The Netherlands
b
Received 10 August 2005; accepted 6 October 2005
This work reports how a novel macro-porous silica foam thin layer, applicable as a new catalyst support with high porosity, can be produced via spheres templating method. Three-dimensional close-packed crystals of polystyrene (PS) spheres were assembled on a porous alumina disk via vacuum filtration. The PS templates were filled with a silica colloidal solution, after annealing and adsorption of a surfactant, to generate interconnected macropores after calcination. KEY WORDS: polystyrene; template; filtration; macro-porous; silica; foam; alumina disk.
1. Introduction Fast reactions over solid catalysts in liquid phase can easily cause concentration gradients in reactors and catalysts because of relatively slow diffusion as well as low concentrations of reactants, e.g. dissolved gasses. Mass-transfer induced concentration gradients will influence both reaction rates as well as selectivity. Conventional strategies for gas–liquid–solid phase catalytic reactions comprise slurry reactors and trickle bed reactors. In trickle bed reactors, diffusion limitation occurs very frequently because catalyst particle are relatively large (typically 1 cm) because of hydrodynamic constraints. In slurry reactors, catalyst particles are usually much smaller (typically 20–100 lm) and mass transfer limitation is less likely, although fast kinetics will still induce significant concentration gradients, e.g. in hydrogenation of nitrate [1–4]. The disadvantage of slurry operation is the high cost of removal of the catalysts from the products. Therefore, development of structured catalysts is an active research area aiming to combine the advantages of slurry reactors and trickle bed reactors. Most work was done based on monoliths [5–7], but also corrugated plates, structured packing and glass fibers have been used, as reviewed by Centi et al. [8,9]. Our laboratory recently contributed to this field by preparing a new type of thin, highly porous carbon-nanofibres layers on monoliths and metal foams [10–12]. Porous materials are essential for applications including catalysis, sorption and separations. High surface area materials with limited control over pore size distribution without any control over the arrangement of pores are still the working horses in this area, e.g. activated carbon, alumina and silica. The development of synthetic zeolites has made available crystalline * To whom correspondence should be addressed. E-mail: l.leff[email protected]
materials with superior control over pores sized approximately up to 2 nm [13]. Later, mesoporous MCM-type of materials were discovered [14
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