Pore Characteristics and Membrane Architecture

Porous materials have a very complex structure and morphology cmd many studies have been devoted to describing and characterising them [1-3,8]. Rouc-querol et al. [8] in their lUPAC report give useful advice for terminology, definitions and characterisation strategies. 

Parameters which influence transport properties are porosity, pore size distribution, pore shape, interconnectivity and orientation. Indirectly particle size distribution and shape are important in the way they affect the uniformity of the pore size distribution, the pore shape and the roughness of the internal surface area. 

A schematic picture of different t5q)es of pores is given in Fig. 9.1 and of main types of pore shapes in Fig. 9.2. In single crystal zeolites the pore characteristics are an intrinsic property of the crystalline lattice [3] but in zeolite membranes other pore types also occur. As can be seen from Fig. 9.1, isolated pores and dead ends do not contribute to the permeation under steady conditions. With adsorbing gases, dead end pores can contribute however in transient measurements [1,2,3]. Dead ends do also contribute to the porosity as measured by adsorption techniques but do not contribute to the effective porosity in permeation. Pore shapes are channel-like or slit-shaped. Pore constrictions are important for flow resistance, especially when capillary condensation and surface diffusion phenomena occur in systems with a relatively large internal surface area. 

A very important concept is the interconnectivity and the related tortuosity (t), as illustrated in pore d of Fig. 9.1. This parameter is used in almost all equations and will be discussed below in some detail. 

Systems used in practice have a spongy structure (porous glass or carbon) or have the structure common in ceramic membranes. The latter have an interconnected, tortuous and randomly oriented pore network with constrictions and dead ends (Fig. 9.1) and are formed by packing of particles. 

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