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Possible large applications of porous glass membranes as a representative example of inorganic porous materials attract attention on many researchers [1,2]. This paper describes the preparation, gas transport characterisation and possible modification of the porous glass membranes.
Porous glasses can be understood as the leaching products of phase-separated alkali borosilicate glasses. The pores that were formed during this process are in a wide range from 0.3 to 1000 nm in diameter, depending from glass composition and phase separation temperature. They are characterized by large specific surface areas, which are obtained by using standard nitrogen adsorption techniques. Porous glasses show in comparison with other porous inorganic solids high thermal stability, chemical resistance, high optical transparency, and good accessibility to the active sites inside the porous structure. They can be prepared in various geometrical forms i.e. as beads, rods, tubes or plates [2]. This material can be used as membranes for gas separation, as catalytic active membranes or as packing material for HPLC, all of which are influenced by or rely on diffusion.
In a wide range of different pore sizes starting from micro- to macro-porous materials, various mechanisms of gas diffusion can be considered: capillary condensation, Knudsen diffusion, surface diffusion, configurational diffusion, viscous flow and molecular diffusion [3]. The meso- and macro-porous membranes are relatively well studied, but in a case of micro-porous membranes the verification of transport mechanism is a very difficult task.
The availability of such membranes made it possible to employ a systematic approach of the permeation and adsorption properties. Four ultra thin, flat membranes in a range of pore sizes between 1.4 nm and 4.2 nm (1.4 nm; 2.3 nm; 3.1 nm; 4.2 nm) were selected for the investigation.
The dynamic permeability measurements for several gases (He, Ar, N2, Xe, CO2 and C3H8) in the temperature range from 293-453 K have been studied in a modified Wicke-Kallenbach cell. Additionally, adsorption measurements of these gases are examined by a well-known volumetric technique.
The observations of permeability for a membrane with a pore diameter of approximately 4 nm showed the dominance of Knudsen diffusion, what was already confirmed earlier [4]. With decreasing pore diameter, the diffusion is affected by the interaction energy between the gas molecules and the solid surface in the pore. The total flow for the membranes with 2.3 nm and 3.1 nm pore diameters is influenced by gas flow described with Knudsen diffusion and surface flow, which is decreasing with increasing temperature. As the pores become smaller (membrane with 1.4 nm pore diameter), it was found that the permeability coefficients of different gases are increasing with temperature. The permeation mechanism is changing from Knudsen regime to configurational diffusion. This fact is confirmed with strong correlation between molecular diameters rather than on molecular weight and permeability coefficients [5]. In addition, the resulting selectivity factors between different pairs of investigated gases are high while the Knudsen type of mechanism has an upper limit proportional to the square root of molecular masses. The micro-porous membranes might have a greater potential for gas separation compared to membranes with larger pores, but that does not mean that all problems concerning preparation and modification of porous glasses are solved.
An interesting task for the future work is to find the optimal material which could provide a high selectivity without loosing permeability. The modification of the surface with functional groups active for the interesting component is a possible solution.
[1] Kuraoka K., Chujo Y., Yazawa T., Journal of Membrane Science 182 (2001) 139.
[2] Enke D., Janowski F., Schwieger W., Microporous and Mesoporous Materials 60 (2003) 19-30.
[3] Hwang, S. T. and Kammermeyer K., Ind. Eng. Chem., Fundam., 7, 671 (1968).
[4] Tuchlenski, A., Uchytil, P., Seidel-Morgenstern, A., Journal of Membrane Science, 140 (1998) 165.
[5] Shelekin A. B., Dixon A. G., Ma Y. H., AIChE J., 41, 58 (1995).
