Silica-zirconia membranes

Yoshida K, Hirano Y, Fujii H, Tsuru T, Asaeda M. Hydrothermal stability and performance of siLica-zirconia membranes for hydrogen separation in hydrothermal conditions. J Chem Eng Jpn. 2001 34(4) 523-30. 

FIGURE 10.1.8 Molecular weight cut-off curves for silica-zirconia membranes [33],... 

Tsuru, T., Wada, S., Izumi, S., and Asaeda, M. (1998). Preparation of microporous silica-zirconia membranes for nanofiltration. J. Membr. Sci. 149 127-135. 

Asaeda, M., and Tsuru, T. (1996). Porous silica-zirconia membranes for separation of organic molecular mixtures by pervaporation, Proceedings of Fouth International Conference on Inorganic Membranes, pp. 68-78. 

Araki, S., Kiyohara,Y.,Imasaka,S., Tanaka, S. and Miyake, Y. (2011) Preparation and pervaporation properties of silica-zirconia membranes. Desalination, 266,46-50. 

M. Asaeda, P. Uchytil, T. Tsuru, T. Yoshioka, M. Ootani and N. Nakamura, Pervaporation of Metha-nol/MTBE Mixture by Porous Silica-Zirconia (10%) Membranes , pp 322-25 in Proc. ICIM5 June 22-28, Nagoya, Japan (1998). 

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. 

High quality microporous membranes are almost exclusively reported for silica or for binary silica-titania or silica-zirconia systems [42,46]. This is due to the very fast hydrolysis and condensation rates of the metal organic precursor of the metals relevant for membrane synthesis (Ti, Zr, Sn, Al). This usually results in too large particles in the precursor solution. Though many authors claim to have produced microporous materials by sol-gel methods (see e.g. Section 8.2.3), only a few have shown the synthesis of membranes of these materials and a still smaller number has characterised them with appropriate separation properties to be reasonably defect free. Therefore in the remainder of Section 8.2.1 a focus will be given to silica-based membranes. 

Silica and silica-titania/zirconia membranes with high quality combining high separation factors and high permeation values were first reported by Uhlhorn [12,58] and were further developed and analysed by de Lange et al. [43,46,47,60]. Further optimisation has been undertaken by Verwey and coworkers [59] in the same group. 

Inorganic membranes, usually appUed when high temperatures or chemically active mixtures are involved, are made of ceramics [171,172], zirconia-coated graphite [173],silica-zirconia [174],zeolites [168], or porous glass [175] among others [176]. Ceramic membranes are steam sterilizable and offer a higher mechanical stability [134], thus they may be preferably used in aseptic fermentations, since some hollow fibers are only chemically sterilizable and not very suitable for reuse. Composite materials, in which glass fiber filters are used as support for the polymerization of acrylamide monomers, were developed for the hydrolysis of penicillin G in an electrically immobilized enzyme reactor. By careful adjustment of the isoelectric point of amphoteric membranes, the product of interest (6-aminopenicillanic acid) was retained in an adequate chamber, adjacent to the reaction chamber, while the main contaminant (phenyl acetic acid), was collected in a third chamber [120]. 

There are several sPEEK membranes with very good relative selectivity and several sPEEK composites with silica, zirconia and their mixtures with heteropolyacids [370, 371, 377] which exhibit remarkable selectivities with high proton conductivities. Composites of sPAEK with epoxy resin [383] and crosslinked with PVA [386] also show excellent selectivities and high conductivities. Unfortunately, the performances of these promising membranes in DMFC have not been reported yet. 

FIGURE 10.1.9 Permeance of silica-zirconia composite membranes as a function of kinetic diameter at 500°C (S9-Z1, S7-Z3 and S5-Z5 have molar compositions of Si02-Zr02 of 9-1, 7-3, and 3-5, respectively) [56]. 

Aseada M., Sakou Y., Yang J., Shimazaki K. Stability and performance of porous silica—zirconia composite membranes for pervaporation of aqueous organic solutions. J. Membr. Sci. 2002 209 163-175... 

Addition of hydroscopic metal oxides such as silica, zirconia, or titania to a proton-conducting polymer is the most obvious way to improve water retention at elevated temperatures (Aparicio et al., 2003). Unfortunately, due to the negligible proton conductivity of these oxides, an increase in the overall resistance of the composite membrane is observed, especially at low temperatures. However, as the temperature is increased, the conductivity gain due to better hydration offsets the loss due to the excluded conducting volume, and the net fuel cell performance is improved, as compared to an unmodified membrane (Adjemian et al., 2002a,b). It should be stressed that there are limits to the water sorption capability of the oxides. While these membranes retain more water than traditional PEM materials such as Nafion at high temperature and low RH, water uptake is insufficient and ohmic losses are still unacceptably high for PEMFC applications. 

Generally, H2 is taken as a reference gas because of its higher permeability with respect to all the other gases. Nevertheless, in 1994, Ohya et al. (1994) prepared zirconia-silica composite membranes which allowed only H2O and HBr to permeate, but not H2. 

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