Surface area xerogel

Silica is the support of choice for catalysts used in processes operated at relatively low temperatures (below about 300 °C), such as hydrogenations, polymerizations or some oxidations. Its properties, such as pore size, particle size and surface area are easy to adjust to meet the specific requirements of particular applications. Compared with alumina, silica possesses lower thermal stability, and its propensity to form volatile hydroxides in steam at elevated temperatures also limits its applicability as a support. Most silica supports are made by one of two different preparation routes sol-gel precipitation to produce silica xerogels and flame hydrolysis to give so-called fumed silica. 

The second preparation route uses flame hydrolysis, a versatile way to produce all kinds of oxides with high specific surface areas. The advantages of fumed silica over xerogels are the better mechanical properties and higher purity of the former. 

The solid-state decomposition of OV[OSi(O Bu)3]3 occurs with a precipitous weight loss at ca. 200 °C (as observed by TGA) and a final ceramic yield that is 10% less than the expected ceramic yield [79]. This discrepancy results from volatihzation and loss of HOSi(O Bu)3. However, solution thermolyses of OV[OSi(O Bu)3]3 in n-octane produce xerogels with an approximate composition of V2O5 6Si02 (after drying) with a quantitative ceramic yield (i.e., with no loss of HOSi(0 Bu)3) that have a BET surface area of 320 m g ... 

Both xerogels and aerogels are characteristically high surface area materials (surface areas normally exceed 500 m2/g). Unlike wet gels, many uses exist for dried gels due to their high surface areas and small pore sizes (typically, < 20 nm diameters). Examples include catalyst supports (12.). ultrafiltration media (18), antireflective coatings (19-20), and ultra-low dielectric constant films. (Lenahan, P. M. and Brinker, C. J., unpublished results.)... 

Since both aerogels and xerogels have high surface areas and small pore diameters they are used as ultrafiltration media, antireflective coatings, and catalysts supports. Final densi-fication is carried out by viscous sintering. 

This xerogel has well developed porosity mainly in the mesopore range, which is partially regulated during synthesis, with specific surface area up to 300 m g" and total pore volume up to 1 cm g (Fig. 21.2). However, the hydrogel rather than xerogel has attracted most interest as an enterosorbent. 

When multicomponent alkoxide solutions, or a single alkoxide and a soluble inorganic salt, are mixed, a multicomponent alkoxide may result. In this way, such complex oxides such as the YBCO superconductor (cf. Section 6.1.2.4) can be formed. Sol-gel processing can also be used to coat fibers for composites and to form ceramics with very fine pore sizes called xerogels. A xerogel commonly contains 50-70% porosity, a pore size of 1-50 nm, and a specific surface area exceeding 100 m /g. 

Large-pore glasses, wide-pore Xerogels and compressed powders made from nonporous particles ( -100A in size and specific surface areas <300 m2/g). 

---