Microporous silica membranes support

Fig, 8.26. FE-SEM micrograph of a cross section of a microporous silica membrane supported by a y-alumina top layer with low roughness. From de Lange et al. [43,44]. 

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. 

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. 

This chapter is split in two parts. The first part will briefly treat the preparation of flat ceramic membrane supports by colloidal processing. In our laboratory, these supports are used to study stability and gas separation properties of microporous silica membranes because they are easy to prepare and demand less complex testing equipment. 

The above mentioned advantages make the supports very suitable for the preparation of flat microporous silica membranes for lab-scale tests. However, due to the almost perfect particle packing, the hydrogen permeance may be too low for application in process industry. For stability testing, on the other hand, the permeance of the membranes is of a far lower importance than the selectivity of the layer under investigation. More information about stability testing can be found in chapter 5 and 6 for the y-alumina and the silica layer respectively. 

Somewhat surprisingly, however, only a very limited amount of literature is available on hydrothermal stability of even the most commonly applied mesoporous membrane type, namely y-alumina membranes on OC-AI2O3 supports. These mesoporous y-alumina membranes are the common supports for the microporous silica membranes to be used in membrane steam reformers. In the investigations that finally led to the present study, delamination of the y-alumina membrane from the OC-AI2O3 supports in hot steam was found to be a major compli-... 

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],... 

For the application of microporous silica membranes in steam-containing environments it is of major importance that the silica membranes will be tested on hydrothermally stable supports. Silica membranes should be prepared on the basis of the results of the specific surface area measurements described in chapter 6. Unsupported silica membrane material of which the specific surface area does not change under SASRA conditions is most promising. An example is silica fired at 825°C (chapter 5). The need of doping the silica with foreign ions or atoms is currently uncertain. 

Recently Raman et al. [74] proposed a new approach to microporous silica membranes based on organic-inorganic polymers prepared by co-pol)oneriza-tion of tertraethoxysilane (TEOS) and methyltriethoxysilane (MTES). The hybrid polymers were deposited on commercial asymmetric alumina supports. Heat treatments were employed to density the inorganic matrix and pyrolyse the methyl ligands, creating a continuous network of micropores. 

As shown in Table 8.2, the minimum required roughness of the support to obtain crack-free membranes is smaller for Ti02 membranes than for y-alumina (mesoporous) membranes. To obtain microporous silica membranes even more stringent requirements prevail. 

Fig. 8.27. Three dimensional picture of a ultra thin microporous silica membrane on a y-alumina support (pore diameter 4 nm), obtained with atomic force microscopy. From de Lange et al. [46].

Brinker and coworkers [49] reported the synthesis of microporous silica membranes on commercial (membralox) y-alumina supports with pore diameters of 4.0 nm. Ageing of the silica sols was shown to be effective to form discrete membrane layers with an estimated thickness of 35 nm on top of the support and to inhibit pore penetration of the silica. Sols with gyration radii Rg < (radius of support pores) penetrate the support to a depth of about 3 im, which is the thickness of the y-alumina support layer. Minimization of the condensation rate during film formation was considered to decrease the width of the pore size distribution without changing the average pore radius, which was estimated to be 0.35 < Tp < 0.5 nm. The porosity of films deposited on dense supports was about 10% as calculated from refractive index measurements. 

C.L. Lin et al. [71] reported deposition of silica layers (plugs) with a thickness of about 1.5 pm within the pores of commercial, mesoporous y-alumina films (pore diameter 4 nm, thickness 1-3 pm) on a-alumina supports (US filter). The deposits were obtained by reaction of TEOS-oxygen (10-20%) mixtures in He as carrier gas applied in the OSG mode to the mesoporous layer. No further details (e.g., temperature or pressure) were given. Depending on these unknown conditions, dense as well as microporous silica membranes with pores down to estimated values of 0.4-0.6 nm were obtained. These membranes have interesting combinations of permselectivity and flux values for several gas combinations (see Chapter 9 on gas transport properties). 

Typical results for supported microporous silica membranes are given in Table 9.14 and are partly discussed in Section 9.4.3. 

Overview of typical fltix and separation data of supported microporous silica membranes made by different processes... 

The regeneration of supercritical carbon dioxide from a mixture containing caffeine by microporous MFI zeolite and mesoporous silica membranes supported on alumina was studied. The experimental data show that a caffeine rejection higher than 90% or 70% and a permeation flux of supercritical carbon dioxide more than 0.05 or 0.07 mol/m /s could be obtained at 10.5 MPa. 

Microporous silica membranes prepared by sol-gel processing and comprising a three-layer system consisting of a support prepared from a-alumina powder, y-alumina intermediate layer, and a molecnlar sieving silica top layer have been described by Benes et al. (2000). The surface polarity of sol-gel materials can be controlled by copolycondensation of MeSi(OR)3, with Si(OR)4. 

At the start of the project microporous sol-gel silica membranes were under investigation in various research groups. An extensive literature review is provided in the introduction of the thesis of De Vos [43], Common supports for sol-gel silica membranes are mesoporous y-alumina membranes [44-47], These mesoporous membranes are either home made [43-45] or obtained from commercial sources [46,47],... 

A common and well-known method to prepare silica membranes with molecular sieving properties is sol-gel coating [3-5], With this technique, microporous silica layers with a pore-size of about 0.5 nm are dip-coated on top of supported y-alumina membranes. The supports are porous a-alumina disks with pore diameters in the range from 100-200 nm. On top of these macroporous supports a 3 pm thick mesoporous y-alumina layer is coated, with a pore size of 3 nm. 

Selected signature libraries may be immobilized on a solid matrix such as activated silica resin, cellulose microporous modified membranes [66], Sepharose , magnetic beads based on MagaPhase technology. The affinity support obtained is used for IgM antibodies parting. 

The insertion of catalytically active guests, such as transition metal ions, is an example of the potentialities of zeolite membranes for applications in catalytic membrane reactors. The well-known catalytic properties of supported vanadium oxides for selective oxidations have recently prompted a number of studies on the possibility of inserting vanadium in the framework of crystalline microporous silica and aluminosilicate powders. " ... 

The support shown in Fig. 6.3 serves as a support for a microporous membrane, for example, a silica membrane. Figure 6.5 is a SEM photograph of such a membrane layer. The silica layer is orUy 200 nm thick and is supported by the system shown in Fig. 6.3. The substrate for the silica membrane is the very smooth mesoporous y-alumina layer 4. 

Silica is also employed to prepare microporous inorganic membranes suitable for gas separation. De Vos et al. [163] reported the preparation of silica membranes with a very low defect concentration. They employed a sol-gel synthesis starting from tetraethylorhosilicate. These membranes consist of a microporous layer on top of a supported mesoporous y-Al203 membrane. The support layer provides mechanical strength to the selective silica top layer. The prepared membranes have a thick... 

In other respects, we can consider zeolite membranes as pertaining to the ceramic material category. Indeed, zeolites are classified for the most part as microporous, crystalline silicoaluminate structures with different aluminum/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, zeolites are crystalline materials and they have a structural porosity very different from microporous amorphous silica [167]. Zeolite films can be grown as intergrown layers on porous metallic and ceramic membrane supports. These zeolite films constitute a special type of nanostructured interface capable of very specific interactions with individual molecules so that it can be used as membrane for the selective separation of molecular... 

With pervaporation membranes the water can be removed during the condensation reaction. In this case, a tubular microporous ceramic membrane supplied by ECN [124] was used. The separating layer of this membrane consists of a less than 0.5 mm film of microporous amorphous silica on the outside of a multilayer alumina support. The average pore size of this layer is 0.3-0.4 nm. After addition of the reactants, the reactor is heated to the desired temperature, the recyde of the mixture over the outside of the membrane tubes is started and a vacuum is apphed at the permeate side. In some cases a sweep gas can also be used. The pressure inside the reactor is a function of the partial vapor pressures and the reaction mixture is non-boiling. Although it can be anticipated that concentration polarization will play an important role in these systems, computational fluid dynamics calculations have shown that the membrane surface is effectively refreshed as a result of buoyancy effects [125]. 

The parameters and consequently the efficiency of PV strongly depends on the properties of the membrane material. Common membrane materials are various dense polymers and microporous inorganic membranes (zeolithes, silica,. ..) either with hydrophilic or organophilic character. Furthermore composite membranes offer the possibility to combine different materials for the dense active layer and the porous support layer. Besides membrane material fluid hydrodynamics influences the efficiency of separation. The pressure drop especially on the permeate side reduces the driving force of the most permeating components. 

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... 

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