Support pyrogenic silica

The structure of these pyrogenic silicas has been discussed by Barby [5], particularly with reference to their specific surface area. It was concluded that the initially condensed particles are only about 1 rnn in diameter and that these are so closely packed (high coordination number) to secondary particles of 10 to 30 nm that only a small amount of nitrogen can penetrate the micropores between them. Thus the secondary particles are the ones that are commonly identified in electron micrographs and which determine the specific surface area. They are the primary particles in the voluminous aggregate structure and have a low coordination number of about 3 (see Fig. 8.3). Because of the low level of impurities this type of silica is often used as catalyst support in fundamental studies. 

We recently reported on the different reactivity of copper catalysts when supported on pure silica or on mixed oxides. Thus, the use of very pure, pyrogenic silica, allowed to obtain a catalyst showing excellent chemoselectivity in the hydrogenation of a,P-unsaturated ketones containing also an isolated olefmic bond [Ij. 

TABLE 31 LCB Levels in Polymers Made with Catalysts Supported on Pyrogenic Silicas... 

The Si02 supports used are pyrogenic silica from DEGUSSA, type Aerosil A 150, A 200 and A 300. (The number indicate the surface area in m /g) with low porosity and low number of Si-OH (1,8 to 2 by nm ). 

For commercial processes, formed supports are more useful. Compared with other supports, fumed oxide supports showed new catalytic effects [41]. Some intensively investigated applications for these supports are abstracted in the following. SiC>2 pellets have been successfully introduced in a new generation of precious metal supports in vinylacetate monomer production [42]. This resulted in better selcctivities and an up to 50% higher space-time yield compared with supports based on natural alumo-silicates. In alkene hydration fumed silica pellets serve as a support for phosphoric acid. In this case, an increased catalyst lifetime and a higher space-time yield were observed [43]. Pyrogenic TiC>2 powder can be used as a starting material for the manufacture of monolithic catalysts [44] for the selective reduction of NOv with ammonia. 

This result suggests that, for this family of structures at least, the value of 1.6 mL g 1 provides sufficient weakness to allow total breakdown and full access to all of the catalyst surface. This inference is also supported by a comparison of results obtained with the best commercial silica gels and with a pyrogenic or "fumed silica (Cabosil) formed by flame hydrolysis of SiCl4. The latter has no pore structure, and no such structural limitations. That the two exhibit similar activities indicates that the silica gel had disintegrated to the level at which nearly all of the surface contributed to the polymerization. Furthermore, once friability of the solid is obtained because the pore volume is sufficiently high, activity can still be influenced by the surface area. However, these are only general trends, and some small exceptions are evident in the data in the table as well. It is the structure itself, rather than any porosity measurement, that determines friability. 

The classical Ti-Si02 catalyst was initially prepared from TiCh and pyrogenic Si02 in 1969 [52]. Almost 30 years later, Maier et al. [53] synthesized calcinated xerogels via a sol-gel process with TEOS and various Ti-cyclopentadienyl complexes (entry 1, Table 5.3). In 1995, Baiker [54] demonstrated that sol-gel prepared titania-silica mixed aerogels showed better catalytic behavior in epoxidation of different bulky olefins than Ti zeolites [55] and silica supported titania described at that time [56]. The most common oxidant was cumene peroxide. The drying method, the titanium content and the calcination temperature were the most important parameters. Aerogels dried by semicontinuous extraction with supercritical CO2 at low temperature were found to be more efficient (entry 2, Table 5.3). In 2001 Baiker described the preparation of a series of titania-silica mixed... 

This paper will only be concerned with the surface chemistry of silica in vacuum and/or in the presence of an adsorbate. The IR spectrum of a typical self-supporting disc of a pyrogenic or fumed sifica after heating under vacuum for 1 h at 150 C is shown in Figure 25.1 A. Pyrogenic or fumed silicas [some trade names are AerosU and Cab-O-Sil] are made by the flame hydrolysis of SiCLt at 1000°C. These non-porous silicas have a low bulk density and adsorbed water can be removed by evacuation at 20°C. However, the spectral properties are identical when evacuation is carried out at 150 C evacuation at the latter temperature is preferred in order to remove any trace impurities which may be present. The spectrum is characterized by a sharp absorption band at 3747 cm with a broad tail to low wavenumber having a maximum near 3550 cm . The sharp peak is due to the OH... 

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