Transport in multiscale porous extrudates

Abstract

When considering porous materials that are shaped under a specific form for a given application, a possible anisotropy of structure, that could be created by the synthesis procedure, have to be considered. This anisotropy may have consequences on the transport properties. To evidence such anisotropy, several methods are proposed here in the case of extrudates of alumina used as catalysts supports. The simplest one seems to be the release of probes, here a salt, in controlled geometry. Electrical measurements are also simple in principle but limited to aqueous solutions and to pore size ranges where the double layer can be screened. NMR is a very powerful tool, but the porous domain that is analyzed is often questionable. Finally, approaches such as band spreading analysis of chromatographic peak are also interesting in their ability to explore various pore domains by changing probe size, but a modeling is needed and the preparation of adapted columns is sometimes itself a difficult task. The tortuosity of the material, analyzed here in longitudinal and transverse directions, is used as an intercomparing parameter. Tortuosity measurements: Anisotropy of alumina extrudates. • Anisotropy due to the synthesis method is evidenced by simple experimental methods. • Tortuosities determined by diffusion and conductivity in all directions. • Transversal tortuosity of alumina extrudates higher than longitudinal one.