Silica membranes pore structure

Usually they are employed as porous pellets in a packed bed. Some exceptions are platinum for the oxidation of ammonia, which is in the form of several layers of fine-mesh wire gauze, and catalysts deposited on membranes. Pore surfaces can be several hundred mVg and pore diameters of the order of 100 A. The entire structure may be or catalytic material (silica or alumina, for instance, sometimes exert catalytic properties) or an active ingredient may be deposited on a porous refractory carrier as a thin film. In such cases the mass of expensive catalytic material, such as Pt or Pd, may be only a fraction of 1 percent. 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

Kusakabe K, Sakamoto S, Saie T, and Morooka S. Pore structure of silica membranes formed by a sol-gel technique using tetraethoxysilane and alkyltriethoxysilanes. Sep. Purif. Technol. 1999 16 139-146. 

General criteria for selection of materials for the processing of hydrogen separation membranes are discussed. Performance and stability standards required for applications in high temperature membrane reactors have been focused. The correlations between pore structure and stability issues of membranes made of amorphous materials, specifically silica membranes are discussed in detail. 

A representation of pore structure of silica membranes is shown in Fig. 16.4. As shown, the pores of silica membranes are probably formed, as in 3-cristobalite, by the 5, 6, 7, or 8 membered rings of Si-O [35]. The presence of solubility sites, shown in Fig. 16.4, with an opening of 0.3 nm is the most likely reason for the large ideal selectivity obtained for He (0.26 nm) and H2 (0.289 nm) molecules against molecules such as N2 (0.364 nm). Based on their studies, Oyama et al. state that molecules with sizes smaller than 0.3 nm permeate easily through silica membranes and larger molecules apparently permeate with difficulty. 

Fig. 16.4 Pore structure of silica membranes could be represented using this schematic of solubility site in /3-cristobalite. Reproduced with permission from [35]. Copyright 2004 Elsevier...

The study of the porous structure as discussed in the previous paragraphs would allow researchers to tailor more durable membranes for H2 separation. Making changes in nanostructure of the silica based porous material could be important in this respect. The partial substitution of Si or O in the pore structure is one possible direction (see Fig. 16.8). For example, it is reported that the partial substitution of O with N could increase the stiffness of the network [44]. This stiffness could change the activation enthalpy required by the gas molecules to diffuse through the network, but will increase the resistance of the network against any degradation. The... 

Nair BN, Keizer K, Okubo T, Nakao SI. Evolution of pore structure in microporous silica membranes sol-gel procedures and strategies. Adv Mater. 1998 10(3) 249-52. 

One of the features of inorganic membranes is their controlled pore structure. Anodic aluminum oxide membranes have uniform cylindrical pores, and were applied to an investigation of the analysis of transport mechanism [ 12]. Another route involves the application of a micelle template to membrane preparation [44]. Cubic mesoporous silica (MCM48) membranes were prepared on a stainless steel supports [45] to possible applications for filtration membranes and membrane reactions. 

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. 

Zhao D., Yang P., Chmelka B.F., Stucky G.D. Multiphase assembly of mesoporous-macroporous membranes. Chem. Mater. 1999 11 1174-1178 Zhao D., Yang P., Margolese D.I., Chmelka B.F., Stucky G.D. Synthesis ofcontinuous mesoporous silica thinfilms with three-dimensional accessible pore structures. Chem. Commun. 1998b 2499-2500... 

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