Silica-titania/zirconia 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. 

Porous membranes can be made of polymers (polysulfones, polyacrylonitrile, polypropylene, silicones, perfluoropolymers, polyimides, polyamides, etc.), ceramics (alumina, silica, titania, zirconia, zeolites, etc.) or microporous carbons. Dense organic membranes are commonly used for molecular-scale separations involving gas and vapor mixtures, whereas the mean pore sizes of porous membranes is chosen considering the size of the species to be separated. Current membrane processes include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), gas and vapor separation (GS), and pervaporation (PV). Figure 1 indicates the types and sizes of species typically separated by these different separation processes. 

Zeolite membrane Ceramic membranes made up of a micro-porous support layer and a meso- or micro-porous active layer. Made from alumina, silica, titania, zirconia, or any other mixtures of these materials. 

As mentioned in Chapter 1, the catalyst in porous MRs may just be placed on the membrane (illustrated in Figure 1.12(a)). The reaction takes place in the catalyst phase and the membrane only serves either as a product extractor or as a reactant distributor but does not participate directly in chemical reactions. It is not always easy to obtain a true inert membrane since the porous membrane materials such as alumina, silica, titania, zirconia, zeolite or the components used to modify membrane permeation properties (e.g., pore-filling materials) can make a contribution to reactions. In order to reduce non-selective catalytic activity, the membrane used in selective oxidation reactions often has to be modified significantly by using controlled sintering to reduce surface area, or by doping with alkaline compounds to decrease surface acidity [19]. 

Ceramic membranes were first developed in the 1940s for uranium isotope enrichment processes. Important progress has been made since that time, mainly due to the improved knowledge of the physicochemical properties of the membrane precursors. Most CMR studies concern alumina membranes other oxides such as silica, titania, or zirconia are much less frequently mentioned. 

As to ceramic membranes [3,4] the focus has been so frir in particular on amorphous porous aluminas and silicas. Other inorganics studied include titania, zirconia, non-oxide ceramics (carbides), and microporous carbons. 

It should be stressed that this is a very simplified model. For example, the magnitude and size of the electrical charges on the pore wall and particle surface will be important in cases where pore and particle size are not too different. No data are available on initial membrane formation. A similar situation exists to explain the positive effect of some additions (e.g. PVA to boehmite solutions). The trends in experimental observations and the qualitative model discussed above holds also for the formation of other types of mesoporous membranes (titania, zirconia, silica) [6] (Chapter 7). 

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. 

Because the currently used y-alumina is not stable in all acid and basic environments used in industry [2], the development of mesoporous layers other than y-alumina deserves attention as well. Most common materials that can be used for the mesoporous layer are zirconia and ti-tania [3,4], but recently also the preparation of mesoporous hafnia is described [5], Hafnia seems to be a very interesting membrane material, because it can, unlike zirconia and titania, be fired up to 1850°C without a phase transformation of its monoclinic form. Hafnia also has a high chemical resistance toward acid and basic media. Another interesting material, currently under investigation by the group of Brinker is mesoporous silica [6,7], This material is especially interesting because a tailor made morphology and pore-size is possible. 

In the case of tape casting a membrane on a porous support, the membrane thickness and the quality of the support surface appear to be important issues. It has been found that alumina, but not titania or zirconia, can be tape cast into crack-free membranes on porous supports and any defect on the support surface (e.g., a protrusion or cavity) can yield a deposit layer that is either bare or cracked [Simon et al., 1991]. Alumina membranes have b tape cast on top of tape cast alumina supports. Borosilicate glass, but not silica, membranes have also been tape cast [Winnick, 19 ). 

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. 

Amorphous silica has also been mentioned as a starting metal oxide material for the preparation of particulate mesoporous membranes. These membranes were prepared from commercial sols, Ludox (DuPont) or Cecasol (Sobret), and coated on a macroporous a-alumina support [35]. In contrast to crystalline membrane materials such as alumina, titania or zirconia, the evolution of pore size with temperature of amorphous silica membranes was revealed to be more sensitive to drying conditions than to firing temperature (Table 7.1). When heat-treated for several hours at 800°C the silica top layer transformed from an amorphous state to cristobalite. 

Inorganic membranes are made of mainly ceramic and metallic materials. The ceramic ones are manufactured from a variety of materials including alumina, zirconia, titania and silica. The substrate (to give the thin membrane mechanical rigidity and strength) is either the same material as the membrane but with a larger pore structure, or a different material such as silicon carbide. 

Our research conducted at Cincinnati have been primarily focused on sol-gel synthesis of alumina, zirconia, titania and silica. These metal oxides not only are commonly used as adsorbent or catalyst support but also have recently emerged as excellent materials for ceramic membranes. The objective of this article is to report synthesis and properties of these sol-gel derived adsorbent materials with emphasis on development of a sol-gel granulation method and the properties of the sol-gel derived granular adsorbents. 

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