Amorphous silica membranes

Other approaches to reducing the membrane pore size are being investigated. Many of them are based on the sol-gel process or chemical vapor deposition as discussed in Chapter 3. An example is the preparation of small-pore silica membranes. Amorphous silica membranes have been prepared from solutions of silicate-based polymers. More specifically, some strategies are employed aggregation of fractal polymeric clusters, variation of sol composition, the use of organic molecular templates and modification of pore surface chemistry [Wallace and Brinker, 1993]. 

Amorphous silica has also been mentioned as a starting metal oxide material for the preparation of particulate mesoporous membranes. These membranes were prepared from commercial sols, Ludox (DuPont) or Cecasol (Sobret), and coated on a macroporous a-alumina support [35]. In contrast to crystalline membrane materials such as alumina, titania or zirconia, the evolution of pore size with temperature of amorphous silica membranes was revealed to be more sensitive to drying conditions than to firing temperature (Table 7.1). When heat-treated for several hours at 800°C the silica top layer transformed from an amorphous state to cristobalite. 

Akamatsu K, Nakane M, Sugawara T, Hattori T, Nakao S (2008) Development of a membrane reactor for decomposing hydrogen sulphide into hydrogen using a high-performance amorphous silica membrane. J Membr Sci 325 16... 

Oda, K., Akamatsu, K., Sugawara, T., Kikuchi, R., Segawa, A., Nakao, S.-I. (2010). Dehydrogenation of methyl cyclohexane to produce high-purity hydrogen using membrane reactors with amorphous silica membranes. Industrial Engineering Chemistry Research, 49, 11287-11293. 

Most studies of microporous amorphous silica membranes assume, implicitly or supported by experiments, the occurrence of type 1 behavior. Studies of zeolite membranes, however, often consider separation of hydrocarbons with a much higher affinity and take the possibility of type 2 behavior into account. Type 2 behavior can also be expected with CO2 and H2O separation with amorphous silica membranes and may actually lead... 

Fully nondestractive techniques do not touch the membrane surface and may be used during operation. These requirements make it that such techniques are likely limited to in-line transport measurements and optical reflection measurements. The transport measurements are discussed under quasi nondestructive techniques. Spectroscopic ellipsometry provides the thickness and composition for layers with 0p < 50 nm and X < 5 p,m, mutually independent, for the optically smooth supports and membranes. The composition is in that case obtained from interpretation of the refractive index and may include information about 4>p and the amount of adsorbed molecules. The use of this method was demonstrated first in Benes et al. (2001) for the layer thickness and CO2 sorption in supported amorphous silica membranes, as shown in Figure 34.10, left. Spectroscopic ellipsometry is now used routinely in our laboratory for layer thickness of supported (-y-alumina) membranes. This analysis involves determination of the optical constants of uncoated macroporous AKP15 and AKP30 a-Al203 supports, described in Figure 34.11. As the -y-alumina membranes are optically transparent, the dispersion in refractive index n can be described as a Cauchy type material with the form... 

Akamatsu, K., Ohta, Y., Sugawara,T., Hattori,T. and Nakao, S. (2008) Prodnction of hydrogen by dehydrogenation of cyclohexane in high-pressnre (1-8 atm) membrane reactors using amorphous silica membranes with controlled pore sizes. Industrial Engineering Chemistry Research, 47(24), 9842-9847. 

The only ceramic membranes of which results are published, are tubular microporous silica membranes provided by ECN (Petten, The Netherlands).[10] The membrane consists of several support layers of a- and y-alumina, and the selective top layer at the outer wall of the tube is made of amorphous silica (Figure 4.10).[24] The pore size lies between 0.5 and 0.8 nm. The membranes were used in homogeneous catalysis in supercritical carbon dioxide (see paragraph 4.6.1). No details about solvent and temperature influences are given but it is expected that these are less important than in the case of polymeric membranes. 

Biomineralization. The processes controlling biomineralization are summarized in Fig. 6.1c. Organized biopolymers at the sites of mineralization are essential to these processes. In unicellular organisms these macromolecules act primarily as spatial boundaries through which ions are selectively transported to produce localized supersaturation within discrete cellular compartments. In many instances, particularity in organisms such as the diatoms that deposit shells of amorphous silica, the final shape of the mineral appears to be dictated by the ultrastrucure of the membrane-bound compartment. Thus, a diversity of mineral shapes can be biologi-... 

Amorphous silicas play an important role in many different fields, since siliceous materials are used as adsorbents, catalysts, nanomaterial supports, chromatographic stationary phases, in ultrafiltration membrane synthesis, and other large-surface, and porosity-related applications [16,150-156], The common factor linking the different forms of silica are the tetrahedral silicon-oxygen blocks if the tetrahedra are randomly packed, with a nonperiodic structure, various forms of amorphous silica result [16]. This random association of tetrahedra shapes the complexity of the nanoscale and mesoscale morphologies of amorphous silica pore systems. Any porous medium can be described as a three-dimensional arrangement of matter and empty space where matter and empty space are divided by an interface, which in the case of amorphous silica have a virtually unlimited complexity [158],... 

Amorphous microporous silica membranes as discussed here, consist of a macroporous a-alumina support (pore diameter -100 nm) with a mesoporous y-alumina intermediate layer (Kelvin radius of 2.5 nm) and a microporous silica top layer (pore diameter -4 A) [1,2],... 

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. 

In summary, both factors (AGexcXMA+/Na+ < 0 and ASadsXMA+ < 0) need to be fulfilled in order to disrupt the membrane sufficiently to cause lysis (Figure 6.2). This happens at the surface of quartz and other crystalline polymorphs of silica containing four-fold coordinated silicon. The AGexcXMA+/Na+ term distinguishes tetra-hedrally coordinated crystalline and amorphous silica polymorphs from other oxides. The ASads XMA+ contribution distinguishes tetrahedral silica polymorphs from amorphous silica.25... 

The determination of quartz dust in the air samples in industrial workplace is an established procedure. Although capable of collecting the particulates, organic polymer membranes can not be employed as an XRD substrate since the diffuse diffraction lines at or near the 10 angle of quaru makes polymeric membtanes not suitable for this application [Minneci and Paulson, 1988]. It is possible to quantify as low as 0.005 mg quartz under well controlled conditions (Bumsted, 1973]. Similarly, silver membranes can also be used as a collecting medium and XRD substrate for measuring crystalline and amorphous silica, lead sulfide, boron carbide and chrysotile asbestos [Leroux and Powers. 1970]. 

Silica membranes have also been studied by several investigators for use in gas separation and membrane reactors. They arc thermally very sublc up to about 500°C. Sintering and densification temperatures of silica membranes depend on the water/alkoxide ratio in the sol-gel process for making the membranes (Langlct et al., 1992]. Crystallization of amorphous silica particles in the membranes takes place at temperatures around 1,000°C [Larbot et al., 1989]. However, pore growth can gradually... 

With ceramic membranes (showing Knudsen diffusion) acting on e g. hydrogen in the presence of a hydrocarbon or of carbon dioxide, the theoretical separation ctor amounts to 4-6. For industrial processes, these values are too low [4]. Modification of ceramic membranes and supports by deposition of new materials improves the separation markedly. Wrth controlled modification of thin amorphous silica-layers on ceramics, membranes can be obtained showing sqraration values upto 150 for... 

In other respects, we can consider zeohte membranes as pertaining to the ceramic material category. Indeed zeolites are classified for the most part as microporous, crystalline silico-aluminate stmctures with different alumininum/silicon ratios. Thus, the chemical compositions are close to those of ceramic oxide membranes, in particular of microporous silica and alumina membranes. On the other hand, zeohtes are crystalline materials and they have a structural porosity very different from microporous amorphous silica [124]. Zeohte membranes are well adapted to the separation of gases, in particular H2 from hydrocarbons, but these membranes are not very selective for the separation of mixtures of noncondensable gases. 

Microporous membrane (pore diameter smaller than 2 nm) synthesis is still in its infancy. Microporous membrane layers of amorphous silica and sUica-ti-tania composites, zeolite, and carbon are reported on supports of (a or y) alumina (for silica and zeolite) or on stainless steel (for zeolite) or on carbon (for carbon or zeolite). Seeding up of the different processes used to obtain larger membrane surface areas have to be demonstrated. 

For amorphous silica layers the s)mthesis process is similar to that used for mesoporous membranes, except that now solutions of ultra small, polymeric silica particles, with fractal dimensions smaller than 1.5-2.0, are used as precursors. These are produced with a set of specific synthesis conditions (e.g. high acidity to control the relative rates of the hydrolysis and condensation reactions). 

Polycrystalline zeolite membranes consist of inter-grown zeolite crystals with no apparent cracks or pinholes (Fig. lA). These films are composed of only zeolite (i.e., there are no non-zeolite components such as amorphous silica or polymer). They are normally supported on a substrate although free-standing films have also been synthesized. Membranes can be prepared on different substrates such as silicon wafer, quartz, porous alumina, carbon, glass, stainless steel (SS), gold, etc. Polycrystalline films are primarily prepared by hydrothermal synthesis methods including in situ crystallization, seeded growth,and vapor transport, " and have potential use in all of the applications discussed in this entry. 

By comparison of the dif actograms, an increase of the ratio between the crystalline peak and the amorphous scattering, as well a shift of the crystalline peak to higher Bragg angles, is apparent in the Nafion-silica membrane with respect to Nafion 117. 

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