Silica permeation, porous

Preparative-Scale Porous Silica Permeation Column. The preparative-scale columns were constructed with a 4 foot length of 1.5-inch i.d. borosilicate glass pipe fitted with Teflon endplates as shown in Figure... 

Merckogel Si Porous silica, controlled porosity, gel permeation... 

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...

Extensive research for substitutes for the expensive Pd membrane has been conducted. Porous membranes that consist of a highly porous metal or ceramic support with a thin top layer, tailored to have the desired selectivity, yield quite a high permeability but a relatively low selectivity. Some of the applications that have been tested on porous silica, vycor, alumina, and other membranes are listed in Saracco and Specchia [19] and Hsieh [20]. Most of the studies focused on selective permeation of products or reactants (mostly H2, in some cases O2) but the selectivity, which is determined by Knudsen diffusion, was very modest. While some improvement may be gained in ceramic membrane reactor when compared to conventional reactors it is often attributed to the dilution effect of the sweep gas [21]. 

Ishida, M., Tasaka, Y. and Aseada, M. 2005. A study on vapor permeation and pervaporation of acetic acid/water mixtures by porous silica membranes, 38(11) ... 

Domard, A. Rinaudo, M. Gel permeation chromatography of cationic polymer on cationic porous silica gels. Polym. Commun. 1984, 25, 55. 

Two classes of micron-sized stationary phases have been encountered in this section silica particles and cross-linked polymer resin beads. Both materials are porous, with pore sizes ranging from approximately 50 to 4000 A for silica particles and from 50 to 1,000,000 A for divinylbenzene cross-linked polystyrene resins. In size-exclusion chromatography, also called molecular-exclusion or gel-permeation chromatography, separation is based on the solute s ability to enter into the pores of the column packing. Smaller solutes spend proportionally more time within the pores and, consequently, take longer to elute from the column. 

Some bead materials possess porous structure and, therefore, have very high surface to volume ratio. The examples include silica-gel, controlled pore glass, and zeolite beads. These inorganic materials are made use of to design gas sensors. Indicators are usually adsorbed on the surface and the beads are then dispersed in a permeation-selective membrane (usually silicone rubbers). Such sensors possess high sensitivity to oxygen and a fast response in the gas phase but can be rather slow in the aqueous phase since the gas contained in the pores needs to be exchanged. Porous polymeric materials are rarer and have not been used so far in optical nanosensors. 

Size exclusion (SEC) (gel permeation GPC) Organic or aqueous Porous gels of silica, synthetic polymers or biopolymers with exclusion limits from 102 upto 108 Mainly synthetic and biopolymers of mol.wt >2000, but also smaller molecules. 

Kaupp and Sklarz [50] reported a clean-up method for the determination of polyaromatic hydrocarbons in plant samples including maize leaves. The two-step clean-up consisted of gel permeation chromatography on a porous styrene-divinylbenzene copolymer, followed by further clean-up on silica gel. 

In addition to the Pd-based membranes, microporous silica membranes for hydrogen permeation [8] can be produced by a special type of chemical vapor deposition [140] named chemical vapor infiltration (CVI) [141], A large amount of studies have been carried out on silica membranes made by CVI for hydrogen separation purposes [8,121], CVI [141] is another form of chemical vapor deposition (CVD) [140] (see Section 3.7.3). CVD involves deposition onto a surface, while CVI implies deposition within a porous material [141], Both methods use almost similar equipment [140] and precursors (see Figure 3.19) however, each one functions using different operation parameters, that is, flow rates, pressures, furnace temperatures, and other parameters. 

The author of this book has been permanently active during his career in the held of materials science, studying diffusion, adsorption, ion exchange, cationic conduction, catalysis and permeation in metals, zeolites, silica, and perovskites. From his experience, the author considers that during the last years, a new held in materials science, that he calls the physical chemistry of materials, which emphasizes the study of materials for chemical, sustainable energy, and pollution abatement applications, has been developed. With regard to this development, the aim of this book is to teach the methods of syntheses and characterization of adsorbents, ion exchangers, cationic conductors, catalysts, and permeable porous and dense materials and their properties and applications. 

Normally when one of the two performance indicators of a porous ceramic membrane for gas separation (i.e., separation factor and permeability) is high, the other is low. It is, therefore, necessary to m e a compromise that offers the most economic benefit Often it is desirable to slightly sacrifice the separation factor for a substantial increase in the permeation flux. This has been found to be feasible with a 5% doping of silica in an alumina membrane [GaBui et al., 1992]. 

Extensively studied is oxygen permeation through dense ceramic membranes (e.g. perovskhes). Temperatures > 600 °C are applied. Here, oxygen splits at the surface and is transited as 0 . Porous membranes include porous polymer films (cellulosics, polyamides) as well as amorphous inorganic materials (alumina, silica). 

A few publications have reported the permeation of capillary condensate in mesoporous materials. Caiman and Raal [14], measured permeability of CFiCb in Linde silica porous plugs at 240 and 251.5 K. Lee and Hwang [15], measured freon and water vapor permeabilities on vycor membranes. These permeabilities were found to exhibit maxima at relative pressures around 0.6-0.8, with values 20-50 times the Knudsen permeability. Ulhom et al. [16], reported a similar behavior for propylene at 263K in y-alumina membranes. Sperry et al. [17] demonstrated the ability of mesoporous y-alumina membranes in methanol separation at 473 K, provided the applied pressure... 

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