Silica membrane properties

Gas transport properties through silica membranes have not been extensively studied. Especially the resistance of gas transport of small molecules like H2 through the thin SiC>2 layer are currently such that the resistance in the supporting layers should not be ignored or might even dominate the transport properties of the final membrane, see for example [50],... 

Benes and Verweij provide a thorough theoretical description of the multi-component mass transport in microporous systems [54], Lately, some systematic gas transport data has been obtained for different microporous membranes in our group [50], but more extensive measurements are necessary to get a good insight in the detailed transport properties of the different types of silica 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. 

The objective for the project, on which this thesis is based, is the extension of the applicability of H2 selective microporous silica membranes to higher temperatures and harsh environments by improvement of the material properties. Compared to the 1994 state-of-the-art, both H2 permeance as well as selectivity towards H2 had to be increased. In the present chapter, the conventional process is discussed first for the sake of comparison together with several catalyst issues. The membrane process is analysed after that and a comparison between both approaches is made. 

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 influence of firing temperature on the stability and transport properties of microporous silica membranes can largely be explained on the basis of the concentration of Si-OH groups... 

Until now, flat silica membranes were only tested at relatively low temperatures (up to 300°C) because of limitations in thermal stability of the polymer sealing rings [2], However, with the use of dense alumina rings together with carbon sealing it is possible now to measure membrane properties (e.g. permeance) at much higher temperatures (up to 600°C), which will be described below. 

Preparation, characterisation and properties of microporous silica membranes... 

Table 1 Permeance and permselectivity properties for different flat silica membranes. St. silica indicates...

As the permeance and permselectivity measurements show, it is possible to prepare high-quality doped silica membranes with excellent properties. Moreover it was possible to perform permeance and permselectivity measurements at temperatures up to 600°C on flat membranes. To the author s knowledge these are the first reliable measurements ever performed on flat membranes at such a high temperature. A more detailed discussion of the permeance and permselectivity results follows. It must however be noted that the relatively low hydrogen permeances obtained for the described membranes were at least partly due to the used AKP-30 supports, which had a bare-support hydrogen permeance of-8 10 mol/m sPa. 

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. 

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. 

Xomeritakis G, Naik S, Braunbarth CM, Cornelius CJ, Pardey R, and Brinker CJ Organic-templated silica membranes I. Gas and vapour transport properties. J. Membr. Sci. 2003 215 225-233. 

Tan et al. [4] proposed to use a thin silica membrane (prepared by them), which presents hydrophilic properties. They obtained very interesting results, i.e., arejection rate and a permeability of 0.98 and 4.4 x 10 mol s Pa , respectively (see Table 7.1). To underline the effects of adsorption, the same membrane was turned hydrophobic by chemical vapor deposition (CVD) treatment and tested. A very low rejection was obtained and no significant selectivity was given by this membrane. On the basis of the fact that caffeine has more affinity with hydrophilic surfaces, these results gave evidence that adsorption was the major mechanism. Therefore, adsorption phenomenon should be taken into account for membrane choice to control solute/membrane interactions. 

In a series of papers, Ma and his co-workers [1 ] systematically examined the interrelationship between adsorption, permeation and diffusion in microporous silica membranes. Both equilibrium and nonequilibrium properties of the microporous inorganic gas separation membranes were studied. Both high pressure and low pressure gravimetric units were used in their adsorption 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). 

The implication of the theoretical considerations given above is that the permeation can be increased in cases of low adsorption amd sticking coefficients by application of a mesoporous top layer with better sorption properties on top of the microporons membranes. Selective sorption should then also lead to an enhanced separation factor (see Eq. (9.71)). Indications for this effect are reported for dense membranes by Deng et al. [106] and for microporous silica membranes by Nair [107]. 

Heteropolyacid-modified polymer silica membranes for Direct Methanol Fuel Cells have been prepared and tested under high temperature operation conditions (145°C) in single cell configuration. A maximum power density of 0.4 W/cm in oxygen with 2 M methanol has been obtained with air at the cathode, this value decreased to 0.25 W/cm. The higher performance of the heteropolyacid-Nafion-silica membrane, with respect to Nafion-silica, is attributed to its better ion transport properties, since the measured cell resistance value is similar for both membranes. 

Heteropolyacid-modified silica-Nafion membranes showed suitable properties for operation at 145°C in direct methanol fuel cell. Since the cell resistance is similar to that of Nafion-silica membrane and the main improvement in polarization is observed at high current density, it is thought that the excellent oxygen solubility characteristics at the electrode/PWA interface are responsible for the significantly higher limiting current density. 

Nair BN, Okubo T, Nakao S. Structure and separation properties of silica membranes review. 

Lee D, Zhang L, Oyamaa ST, Niue S, Saraf RF (2004) Synthesis, characterization, and gas permeation properties of a hydrogen permeable silica membrane supported on porous alumina. J Membr Sci 231 117-126... 

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