Silica supported membranes

Bowen et al. [94] made a B-MFI membrane on a monohthic support. The pervaporation fluxes and selectivities of several alcohol/water mixtures were comparable to similar tubular-based B-MFI membranes, demonstrating the scale-up, although, for pervaporation, the quality requirements are much more forgiving. Kuhn etal. tested a multicharuiel high-silica MFI membrane for ethanol/water separation. The membrane was supphed by NGK Insulators and, also, in this case, the multicharuiel membrane measures up to its tubular counterparts [95] (Figure 10.8). 

Porous supports like agarose, pol3mrethacrylate, or silica beads are generally used in current applications of affinity chromatography. However, in the past several years other types of supports have also become available commercially. Many of these newer materials have properties that give them superior performance in certain applications. Materials that fall in this category include nonporous supports, membranes, flow-through beads, continuous beds and expanded-bed particles. 

In summary, the main goal of the present work is the development of a hydrothermally stable microporous silica membrane with prescribed transport properties. Preferably, these steam stable membranes should have very high permselectivities. Because the permselectivity of a molecular sieving silica membrane will drop to the Knudsen value of the y-alumina supporting membrane when the silica membrane deteriorates under steam reforming conditions, a selectivity of the silica layer higher than the Knudsen selectivity is sufficient. In this way the measurement of the permselectivity is a powerful tool to assess the hydrothermal stability of a supported microporous membrane. 

The excellent separation properties of silica membranes prepared at temperatures as high as 825°C enables their use for high temperature applications, such as the dehydrogenation of H2S (chapter 8). Unfortunately no hydrothermal stability of the prepared layers could be tested because the mesoporous intermediate layer was not hydrothermally stable, but an indication of the hydrothermal stability of the unsupported material could be obtained from the specific surface area and XRD measurements. These measurements did not show any structural change in the material during SASRA treatment, which is a very hopeful result for the operation of real, supported, membranes at high temperatures and high pressures. 

Mesoporous membranes with high thermal stabilities to 1100°C have been reported by Chai et al. [65], These membranes were obtained by dip coating an alumina-silica support (a 20-step process) into a mixed sol consisting of an alumina sol to which about 11 wt% Ba or La was added in the form of salts. 

The comparison of the pore size measured on non-supported membranes by N2 absorption-desorption with that on supported silica membranes measured with gas permeation and separation with molecules of greatly different sizes indicates that the average pore diameter of supported silica layers is slightly smaller (0.40-0.45 nm). See Chapter 9 on gas transport. 

The experimental permeation results could be consistently described using Eqs. (9.43b) and (9.47) for Langmuir and Henry sorption respectively as shown by de Lange in a full analysis of sorption, permeation and separation results of five different gases [63]. This description requires knowledge of adsorption isotherms which could be measured only on unsupported membranes. To use these data for calculation of the permeation of supported membranes requires the assumption of equal pore characteristics in both cases. As discussed by de Lange et al. this is probably not correct in the case of silica layers. Based on sorption data a microporosity of about 30% and a pore size distribution with a peak at 0.5 nm is found. Analysis of permeation data point to a pore diameter of = 0.4 nm and a considerably smaller porosity. Table 9.7 summarises the sorption data. H2 and CH4 have relatively low (isosteric) adsorption heats (cf ) while CO2 and isobutane strongly adsorb. 

In asymmetric supported membranes the use of permeability data can give rise to much confusion and erroneous conclusions for several reasons. In most cases the layer thickness is not precisely known and usually it is not known whether this layer is homogeneous or has property gradients (e.g. a "skin" and a more porous part). In many cases the material of the layer penetrates the support to some extent and so it is not possible to separate properties of separation layer and support without giving account of the interface effect. Finally, even if all these complications can be avoided, a comparison based on separation layer properties expressed in terms of permeabilities can give a completely wrong impression of the practical possibilities (as done in e.g. Ref. [109]). This is illustrated by comparison of hydrogen permeabilities of ultra-thin silica layers (see Tables 9.14-9.16) with other materials such as zeolites and metals. The "intrinsic" material properties of these silica layers are not impressive ... 

The amount of colloidal silica present in an aqueous solution of high ratio alkali metal silicate can be determined for example by ultrafiltration. Ultrafiltration refers to the efficient selective retention of solutes by solvent flow through an anisotropic skinned membrane such as the Amicon Diaflo ultrafiltration membranes made by the Amicon Corporation of Lexington, Mass. In ultrafiltration solutes, colloids or particles of dimensions larger than the specified membrane cut-ofT are quantitatively retained in solution, while solutes smaller than the uniform minute skin pores pass unhindered with solvent through the supportive membrane substructure. 

Generally, the palladium-based membranes can be subdivided into supported and laminated ones. In the supported membranes, a thin dense layer of a palladium alloy is deposited onto a porous support such as porous Vycor glass (silica gel). Nevertheless, using this kind of support, the palladium layer is easily stripped off owing to the loss of an anchor effect [53]. 

Defect free DD3R supported membranes have been newly prepared by NGK insulators (Japan).The great advantage of this zeolite type with respect to the SAPO-34 and T zeolites, should be the chemical and thermal stability because of its all silica structure. Kapteijn and co-workers studied the permeation of various gases (carbon dioxide, nitrous oxide, methane, nitrogen, oxygen. 

Chen et al. (2005) synthesized high-reproducibility silicate-1 membranes on silica tubes, and all membranes showed high separation performance toward EtOH-water mixtures by PV. However, the membranes synthesized on alumina tubes showed much lower separation performance than the membranes on silica tubes this was caused by the decrease in hydrophobicity resulting from the dissociation of a-alumina tubes during hydrothermal synthesis. It was concluded that silica supports are more suitable for preparing high-performance and high-reproducibility silicalite-1 membranes. 

For the first time, siUca-filled poly(l-trimethylsilyl-l-propyne) (PTMSP) layers on top of UF membranes for the pervaporative separation of EtOH-water mixtures was reported by Claes et al. (2010). Reduction of the thickness of the separating PTMSP top layer and addition of hydrophobic silica particles resulted in a clear flux increase as compared with dense PTMSP membranes. The performances of the supported PTMSP-silica nanohybrid membranes were significantly better than the best conunercially available organophilic PV membranes. The developed composite PTMSP-silica nanohybrid membranes exhibited EtOH-water separation factors around 12 and fluxes up to 3.5 kg/m h, establishing a sevenfold to ninefold flux inCTcase as compared with dense PTMSP membranes. 

Hence, building on this approach, part of the protein of interest, an epitope, has been surface imprinted, instead of the whole, and the resulting binding sites were successfully used to capture the whole protein by recognition of the imprinted part (Fig. 4). In both cases, silica beads, membrane pores, or nanowire surfaces have been used as sacrificial supports and were easily removed by dissolution after polymerization occurred. 

Gu, Y., Oyama, S. T. (2009). Permeation properties and hydrothermal stability of silica-titania membranes supported on porous alumina substrates. Journal of Membrane Science, 345, 267-275. 

One of the features of inorganic membranes is their controlled pore structure. Anodic aluminum oxide membranes have uniform cylindrical pores, and were applied to an investigation of the analysis of transport mechanism [ 12]. Another route involves the application of a micelle template to membrane preparation [44]. Cubic mesoporous silica (MCM48) membranes were prepared on a stainless steel supports [45] to possible applications for filtration membranes and membrane reactions. 

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