Silica permselectivity

Fattakhova-Rohlfing, D. Wark, M. Rathousky, J. 2007. Ion-permselective pH-switchable mesoporous silica thin layers. Chem. Mater. 19 1640-1647. 

Uhlhom, R. J. R., M. H. B. J. Huis in t Veld, K. Keizer and A. J. Burggraaf. 1989, High permselectivities of microporous silica-modified x-alumina membranes. J. Mater. Science Lett. 8(10) 1135-39. 

Silica-alumina particles coated with a permselective silicalite membrane is almost completely selective in the formation of p-xylene in the disproportionation of toluene.402 Friedel-Crafts alkylations were performed in ionic liquids. The strong polarity and high electrostatic fields of these materials usually bring about enhanced activity.403 404 Easy recycling is an additional benefit. Good characteristics in the alkylation of benzene with dodecene were reported for catalysts immobilized on silica or MCM-41 405... 

Hydrogen-permselective silica membranes [8,10] can be, as well, synthesized by a particular application of solgel techniques [142,143], Additionally, hydrogen permselective asymmetric membranes composed of a dense ceramic of a proton-conducting perovskite over a porous support have been developed [40], However, some difficulties with respect to their stability is possible in certain reactive environments [121],... 

In theory CVI membranes are very promising. Especially membrane stability is expected to be very good, because the separative layer is located inside the support, where it is protected against mechanical damage and chemical attack. In addition, measured permselectivities of CVI-silica membranes are very high for H2 N2 values as high as 3000 have been measured [36,37],... 

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. 

On top of the newly developed supports a steam-stable intermediate layer was coated. The preparation of these layers is treated in detail in chapter 5. After this, the permselective silica layer was applied, which should be resistant against high temperature and steam-containing environments as well. The experimental procedure together with some transport and Rutherford BackScattering (RBS) studies are described in chapter 6. 

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

Table 2 Results of permeance and permselectivity measurements for tubular silica membranes.

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 very high H2/CO2 permselectivity for the 825°C fired standard silica membrane is remarkable. It is even more remarkable that the H2/CH4 selectivity is lower, which is contrary to the common observation that the H2/CO2 selectivity is lower than the H2/CH4 selectivity. 

As shown in chapter 6, silica membranes can nowadays be prepared at temperatures as high as 825°C while state-of-the-art steam-stable y-alumina membranes are prepared at 1000°C. This enables the use of such membranes in the high-temperature range needed for thermal dehydrogenation of H2S under conditions used in literature. The permselectivity of H2/CO2 of the silica membranes prepared at 825°C was >100 with a hydrogen permeance of... 

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. 

By performing the coating of the membrane layers under cleanroom conditions, silica membranes with a very high permselectivity were obtained (chapter 6). 

CVI silica membranes, which show reasonable permselectivity and permeance, were prepared at a relatively low synthesis temperature (chapter 7). 

Similarly, Sano et al. [1994] added colloidal silica to a stirred solution of tetrapropylammonium bromide and sodium hydroxide to synthesize a hydrogel on a stainless steel or alumina support with a mean pore diameter of 0.5 to 2 pm. The composite membrane is then dried and heat treated at 500 C for 20 hours to remove the organic amine occluded in the zeolite framework. The silicalite membranes thus obtained are claimed to be free of cracks and pores between grains, thus making the membranes suitable for more demanding applications such as separation of ethanol/water mixtures where the compound molecules are both small. The step of calcination is critical for synthesizing membranes with a high permselectivity. 

Thermal stability. Thermal stability of several common ceramic and metallic membrane materials has been briefly reviewed in Chapter 4. The materials include alumina, glass, silica, zirconia, titania and palladium. As the reactor temperature increases, phase transition of the membrane material may occur. Even if the temperature has not reached but is approaching the phase transition temperature, the membrane may still undergo some structural change which could result in corresponding permeability and permselectivity changes. These issues for the more common ceramic membranes will be further discussed here. 

Ulhom R.J.R., Huis In t Veld M.H.B.J., Keizer K. and Burggraaf A.J., High permselectivities of microporous silica-modified y-alumina membranes, 7. Mails, ScL Lett, 5 1135 (1989). 

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). 

H.Y. Ha, S.W. Nam, S.A. Hong and W.K. Lee, Chemical vapor deposition of hydrogen-permselective silica films on porous glass supports from tetraorthoethyl silicate. /. Membr. Sci., 85 (1993) 279-290. 

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