Membrane micro-porous silica

Figure 2.37 Permeability coefficients as a function of the gas kinetic diameter in micro-porous silica hollow fine fibers [58]. Reprinted from J. Membr. Sci. 75, A.B. Shelekhin, A.G. Dixon and Y.H. Ma, Adsorption, Permeation, and Diffusion of Gases in Microporous Membranes, 233, Copyright 1992, with permission from Elsevier...

R.J.R. Uhlhom, M.H.B.J. Huis in t Veld, K. Keizer and A.J. Burggraaf, High Permselectivities of Micro-porous Silica-Modified y-Alumina Membranes , J. Mater. Sci. Lett., 8 1135-38 (1989). 

State-of-the-art micro-porous membranes are based on silica, with sufficiently small pores, 2-10 A, to be selective towards hydrogen separation. One of the major problems with silica membranes under hydrothermal conditions is physical stability. Evaporation of silica-containing species is detrimental to long-term permselectivity and restricts the operation of these membranes to temperatures below 600 °C. Hydrogen permeances of >1 x 10 mol m s Pa with H2/CO2 permselectivity in the range 80-100 have, for instance, been measured with single deadend tubular micro-porous silica membranes for temperatures higher than 300 °C and with 4 bar pressure difference. These membranes were reported to be thermally stable for at least 2000 h at temperatures between 200 and 400 °C [50]. 

Tsai, C.-Y.,Tam, S.-Y., Lu, Y. and Brinker, C. J. (2000) Dual-layer asymmetric micro-porous silica membranes. Journal of Membrane Science, 169,255-268. 

Microporous and, particularly, ultramicropous membranes are more difficult to characterize. Different procedures based on the low-pressure part of the N2 adsorption isotherm have been proposed [36], but they often require knowledge of the shape of the pores and of gas-surface interaction parameters which are not always available. Small angle X-ray scattering (SAXS) is another technique which is well suited to micro-porous powders, but difficult to execute in the case of composite layers, as in microporous membranes. Xenon-129 NMR has recently been proposed [37] for the characterization of amorphous silica used in the preparation of microporous membranes, but the method requires further improvement. Methods based on permeability measurements appear to be limited by the lack of understanding of the mass transport mechanisms in (ultra)microporous systems. 

Zeolite membrane Ceramic membranes made up of a micro-porous support layer and a meso- or micro-porous active layer. Made from alumina, silica, titania, zirconia, or any other mixtures of these materials. 

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