Silica, micro-porous

The size separation of proteins has been routinely called gel filtration because of the historic use of cross-linked gels for this application. Specially modified Zorbax PSM columns, Zorbax GF-250 and Zorbax GE-450, are used for separating proteins by size. These columns are packed with porous silica micro-... 

A. Katz and M. E. Davis, Molecular imprinting of bulk, micro-porous silica, Nature, 2000, 403, 286. 

The first part of the book documents the history, structure, chemistry, formulation and characterizations of zeolites in Chapters 1-4. The past 60 years have seen a progression in molecular sieve materials from aluminosilicate zeolites to micro-porous silica polymorphs, microporous aluminophosphate-based polymorphs, metallosihcate and metallophosphate compositions, octahedral-tetrahedral frameworks, mesoporous molecular sieves and, most recently, hybrid metal organic frameworks (MOFs). 

Microporous particles are available in two sizes 20 to 40 jam diameter with longer pores and 5 to 10 fim with short pores (see Figure 3.14B). These are now more widely used than the porous layer beads because they offer greater resolution and faster separations with lower pressures. The micro-porous beads are prepared from alumina, silica, ion-exchanger resins, and chemically bonded phases (see next section). 

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). 

Lopez et al. [27] prepared Pd/SiC>2 catalysts under both acidic (pH = 3) and basic (pH = 9) conditions in the sol-gel step and reported that an acid medium promotes the formation of small metal crystallites. This finding is consistent with the formation of a micro-porous silica gel network at a low pH. By comparing samples prepared by the sol-gel method and impregnation, these authors found in the former a stronger metal-support interaction which they ascribed to the square planar palladium complex used as a precursor. Finally, their results showed that the method of preparation as well as the conditions used in each method impact on how these catalysts deactivate in the hydrogenation of phenylacetylene. 

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. 

Our experiments are carried out in the presence of various types of micro-porous solids. They consist principally of alumina (CA), silica (CS), and activated charcoal (EC). For irradiation purposes with fission fragments, uranium is incorporated as an oxide by impregnation or coprecipitation methods. Microporous solids with variable uranium content are made available in this way with, as supporting phase, alumina (CAU), silica (CSU), and activated charcoal (ECU) 8, 9), respectively. Quite apart from the chemical composition, the knowledge of the physical structure of these solids is particularly important. For this reason a series of systematic measurements have been carried out on these solids of granulometry between 15 and 30 mesh. 

For high-performance analytical applications, small polystyrene or micro-porous silica particles of 5- to 10-/xm diameter are used, with pore sizes of a few nanometers to several hundred nanometers. The controlled pore silica particles are coated with a hydrophilic phase to reduce solute adsorption. The polymeric particles can be used over a wider pH range (2 to 12) since silica is limited to pH 2 to 7. 

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]. 

A. Kiselev developed the adsorption-structural method of investigation [129], which made possible a rational classification of adsorbents [130-132]. Dubinin, Radushkevitch, Bering, Serpinsky, and others have developed on the basis of their experimental results a theory on the physical adsorption of gases and vapors in micro-porous adsorbents that they call the theory of volume filling of micropores. The theory is applicable to almost all the adsorption systems, including microporous silica gels and porous glasses [133,134]. 

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. 

Here, r = 470 nm and R = 500 nm. We thus obtain the value of /2 = 0.8013. The 0 value, for the flat silica surface without micro-pores, after modification, is experimentally about 114°. So the CA for our modified micro-porous sample can be estimated to be about 152.5° by Cassie s equation, equation (2), which is in good agreement with our result. 

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