Titania alumina composite membranes

K.N.P. Kumar, K. Keizer and A.J. Burggraaf, Textural stability of titania-alumina composite membranes. /. Mater. Chem., 3 (1993) 917-922. 

Kumar [39] reports a considerably larger thermal stability for titania membranes in the rutile phase instead of the usual anatase form. The effect of the support on thermal stability has been reported by Kumar et al. [40,41]. Pure, non-supported titania (anatase) membranes lose their porosity completely when calcined at 600°C for 8 h, where as the supported titania membrane retained ca 30% porosity at 900°C (8 h). Unsupported titania-(50 wt%)alumina composite membranes retained a porosity of ca 40% at 700°C (8 h), supported ones retained porosity even at 900°C. 

Supported, multilayered (as5onmetric) - dense oxide or metal - porous ceramic membranes alumina, zirconia, titania, carbon - composite ceramic-metal, ceramic-ceramic layers on porous support tube, disk multilayers on porous support plate, disk, tube, monolith... 

Extensive stress measurements has been reported in mesoporous alumina [13,26], titania and alumina-titania composite membranes [13,27] supported by... 

On the other hand, Soria and Cominottim claimed that they had commercialized TiOi NF membranes having a MWCO of 1000 [26], Colloidal sols with a diameter of 15 nm, which showed an anatase phase and which were stable upto 600°C, were coated on porous alumina supports and fired at 500°C. The MWCO curves of titania (T), alumina (A), and titania-nafion composite (TN) membranes prepared by this research group are also shown in Figure 10.1.6 [27]. Table 10.1.4 shows MWCOs and water permeability along with recent reports [28, 29]. Table 10.1.4 indicates that titania membranes show a relatively large water permeability, in comparison with those of polymeric NF membranes [17]. 

CVD is an attractive technique for thin layer deposition. To perform CVD, a precursor gas (often diluted in carrier gases) is fed into a reaction chamber, where it reacts or decomposes to form a solid phase which is deposited onto the substrate. This technique was used by Gu et al. for the synthesis of ceramic composite membranes with excellent thermal stabiUty. Commerdal alumina supports of pore size 5 nm were coated with layers of titania and silica to prepare composite ceramic membranes. Titanium isopropoxide (TIP) and tetraethylorthosilicate (TEOS) were used as precursors of Ti and Si for CVD at 500-700°C, which provided top layers of 10-20 nm.The membrane prepared at 600°C using a molar ratio of TIP/TEOS = 0.10 exhibited the best performance permeation tests at 600°C showed a hydrogen permeance of 2.3xl0 mol m 2 s Pa and selectivities of H2/CH4 = 37 and HJCO2 = 57. 

Different supports are used, (see Section 10.6.4) with different geometry (discs or tubes), thickness, porosity, tortuosity, composition (alumina, stainless steel, silicon carbide, mullite, zirconia, titania, etc.), and symmetry or asymmetry in its stmcture. Tubular supports are preferable compared to flat supports because they are easier to scale-up (implemented as multichannel modules). However, in laboratory-scale synthesis, it is usually found that making good quality zeolite membranes on a tubular support is more difficult than on a porous plate. One obvious reason is the fact that the area is usually smaller in flat supports, which decreases the likelihood of defects. In Figure 10.1, two commercial tubular supports, one made of a-alumina (left side) and the other of stainless steel (right side) used in zeolite membrane synthesis, are shown. Both ends of the a-alumina support are glazed and both ends of the stainless steel support are welded with nonporous stainless steel to assure a correct sealing in the membrane module and prevent gas bypass. 

That bonds are formed between particles is inferred by the fact that the gel layers are able to bear considerable stresses. These bonds are sensitive to the presence of stresses and allow stress relaxation to occur. The relation between stress relaxation and cracking on one hand and particle shape on the other hand is not known. The relative ease of preparing y-alumina membranes might be due to the relative ease of rearrangement of the particles and easy stress relaxation in plate-shaped boehmite particles and the isomorphous transitions to plate-shaped y-alumina at about 300°C, the transition also being accompanied by a relatively small volume change [2-4]. With spherical particles (titania, zirconia) stress relaxation might be more difficult. The easier formation of defect poor composites of alumina and titania (with spherical particles) supports the beneficial effect of plate-shaped particles. 

Crooshnked organic—inorganic membranes cmitaining PVA and anion-exchange silica precursor with ammonium functionality [236] have been reported with conductivity up to 75 mS.cm. Composite of quaternized PVA with alumina exhibited conductivities between 35 and 48 mS.cm in the temperature range 30-70 °C [242], while in the same range of temperature a composite of quaternized PVA with titania... 

The integration of nanoMOFs onto surfaces has attracted major interest over the last few years, as it enables facile incorporation of MOF properties (e.g., porosity, magnetism, and luminescence) into functional metallic, metal oxide, and organic substrates as well as into porous alumina and titania supports. This method enables production of MOF thin films and membranes (or SURMOFs) with various compositions and controllable parameters (thickness and pore size, functionahties, and orientation). Direct deposition is a straightforward way to form polycrystalline MOF films it involves dip-coating... 

The selective layer of a membrane should be as thin as possible since the flux is inversely proportional to the membrane thickness. Thus, membranes for industrial applications are composite structures with a thin selective dense layer (0.5-5 micron) on a porous and mechanically stable support (Figure 20.2a) [20, 21]. This issue is particularly important for a soft and elastic material such as PDMS. The nature of the support can be organic (e.g., cellulose acetate, polysul-fone, polytetrafluoroethylene, polyvinylidine fluoride, etc.) or inorganic (e.g., alumina, titania). The support is often backed by a highly porous non-woven layer. In principle, the role of the support on the mass transport should be negligible, but it could be predominant in some cases [22]. 

---