Silica microporous membranes

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. 

Kurungot, S., Yamaguchi, T., Nakao, S.-L, Rh/y-AljOj catalytic layer integrated with sol-gel synthesized microporous silica membrane for combact membrane reactor applications, Catal. Lett. 2003, 86, 273-278. 

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. 

A novel type of membrane reactor, emerging presently, is the pervaporation reactor. Conventional pervaporation processes only involve separation and most pervaporation set-ups are used in combination with distillation to break azeotropes or to remove trace impurities from product streams, but using membranes also products can be removed selectively from the reaction zone. Next to the polymer membranes, microporous silica membranes are currently under investigation, because they are more resistant to chemicals like Methyl Tertair Butyl Ether (MTBE) [23-24], Another application is the use of pervaporation with microporous silica membranes to remove water from polycondensation reactions [25], A general representation of such a reaction is ... 

Clearly, in such a reaction the removal of the produced water will lead to an enhanced conversion. Commercially available polymer membranes cannot withstand the severe operation and cleaning conditions for this process (150-300°C) and microporous silica membranes again come into the picture. 

A third type of membrane is the sol-gel microporous silica membrane. This type of membrane is of major importance in this thesis. Below, a short overview will be provided of state-of-the-art silica membranes at the start of the project (1995). This has been the starting point from which the new membranes described in this thesis were developed. 

At the start of the project (1995) state-of-the-art microporous silica membranes as prepared by de Lange [45] and described above had a permselectivity of 43 of hydrogen towards methane and a hydrogen permeance of 1.6 10-6 mol/m2sPa. 

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. 

N.E. Benes, A. Nijmeijer and H. Verweij, Microporous Silica Membranes , to be published in Recent Advances in Gas Separations by Microporous Membranes , N. Kannellopoulos ed. 

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 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],... 

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... 

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

The results obtained for microporous silica membranes in the membrane steam-reforming project, described in this thesis, provide favourable perspectives to realise a Th-permselective membrane reactor for the dehydrogenation of H2S. Realisation of such a reactor, however, imposes significant scientific and technical challenges. 

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. 

When using these mesoporous layers for the preparation of microporous silica membranes, the following requirements should be fulfilled (of course depending on the application) ... 

Figure 25.17 illustrates some experimental results about a sol-gel-derived microporous silica membrane and related powder containing nanodispersed ZnO. An important result is that the microporosity is maintained after successive treatments of H2S chemisorption and regeneration under air. 

FIGURE 25.17 (a) SEM cross-section image of a microporous silica membrane with nanodispersed ZnO (designated as SZ4). (b) Evolution of the x-ray pattern for an SZ4 powder, (c) Evolution of the Nj adsorption-desorption isotherm for an SZ4 powder. A initial state B after contact with HjS Cl and C2 after thermal regeneration. (From Goswamee, R., Bose, E, Cot, D., El Mansouri, A., Lopez, M., Morato, F., and Ayral, A., / Sol-Gel Sci. Technol, 29, 97, 2004.)... 

Tsai CY, Tam SY, Lu YF, and Brinker CJ. Dual-layer asymmetric microporous silica membranes. J. Membr. Sci. 2000 169 255-268. 

Verkerk AW, Goetheer ELV, Van den Broeke LJP, and Keurentjes JTF. Permeation of carbon dioxide through a microporous silica membrane at subcritical and supercritical conditions. Langmuir 2002 18 6807-6812. 

Okamoto et al. [141] studied several water/organic systems that are listed in Table 10.6, and the performance of the zeolite A membrane was excellent for aU the separations. These results could be also compared with the ones obtained using microporous sflica membranes [153]. Sflica membranes, for a water/dioxane (10/90 wt%) mixture at 60°C, showed a separation factor of 125 and a water flux of 2.2 kg/m h. For dymethilformamide, (DMF), the results obtained for a mixture of water/DMF (13.2/86.8 wt%) were 30 and 0.225 kg/m h for the separation factor and water flux, respectively. In both separations, zeohte A outperforms the microporous silica membrane. 

Research on other types of materials for H2 separation has been motivated by relatively high cost of Pd and possible membrane degradation by acidic gases and carbon as summarized in Tsuru et al.76 These authors examined microporous silica membranes together with an Ni catalyst layer for SMR reaction. However, this type of membrane allows the permeation of hydrogen as well as other gases in reactants and products, which markedly reduces hydrogen selectivity and limits methane... 

Typical Langmuir isotherms are shown in Fig. 3.2(a) for several gases on a microporous silica membrane [1]. It is interesting to note that at low pressures, Eq. (3.4) reduces to a linear isotherm... 

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