Silica mixing

An interesting recent example of successful application of the SSG process combined with ensuing supercritical drying is the design of titania-silica mixed oxides for the epoxidation of bulky olefins [16-18]. This example will be used to illustrate the opportunities the combined use of SSG and SCD provide for tailoring the chemical and structural properties of mixed oxides. 

Seo Y, Lee W, Yeh P. Improvements of oxide chemical-mechanical polishing performances and aging effect of alumina and silica mixed abrasive slurries. Microelectr Eng 2004 75 361-366. 

Figure 4 shows a hydrophilic and the corre nding hydrophobic precipitated silica mixed with dimethylpolysiloxane. The mixture with the hydrophilic silica gives an almost solid compound. In contrast, the well hydrophobized silica leads to a liquid silica-in-oil diqrersion. 

PREPARATION OF ZIRCONIA-SILICA MIXED OXIDE SUPPORTS... 

Kaneko et al. employed a coprecipitation method, which involved a base-catalyzed procedure that precipitated irregular particles of zirconia-silica mixed oxides. Specifically, a zirconium salt (zirconyl chloride octahy-drate) was added to an acidified solution of sodium me-tasUicate ennehydrate. The solution was made alkaline... 

Shalliker, R.A. Douglas, G.K. Rintoul, L. Comino, P.R. Kavanagh, P.E. The measurement of pore size distributions, surface areas, and pore volumes of zirconia and zirconia-silica mixed oxide stationary phases using size exclusion chromatography. J. Liq. Chromatogr. Relat. Technol. 1997, 20 (10), 1471-1488. 

They give as an example 75 kilos of finely divided silica mixed with 25 kilos of powdered wood clmrcoal, and heated in a stream of nitrogen for ten to twelve hours at I300°-i400 C., allowing the prixluet to cool in N gas. When the product is treated with saturated or superheated steam, ammonia is produced. 

R. Hutter, T. Mallat, A. Baiker, Titania-silica mixed oxides 11. Catalytic behavior in olefin epoxidation, J. Catal. 153 (1995) 177. 

Figure 2.15. Example of silica mixed-mode sorption on partially endcapped C-18.

The sol-gel titania-silica mixed oxide contained 20 wt% titania. The catalyst was synthesized imder acidic conditions [5]. The acidic hydrolysant was added to an isopropanohc solution of tetraisopropoxytitanitam(IV) modified by acetylacetone (molar ratio alkoxide acetylacetone =1 1) and tetramethoxysilicon(IV). The water alkoxide acid molar ratio was 5 1 0.09. The resulting gel was dried by semicontinuous extraction with supercritical COg at 40 °C and 240 bar, and stored in a closed vessel under Ar. After calcination in flowing air at 600 °C, the BET smface area was 648 m g and the specific pore volume 2.9 cm g". Details on the synthesis and characterization of the sol-gel titania-sihca by means of FTIR, Raman and UV-vis spectroscopies, Ng-physisorption, XRD, XPS, TEM and thermal analysis have been reported previously [5, 6, 14, 15]. 

The same spectral evolutions were observed for the silicas mixed with KBr, pretreated in the DR cell at high temperature, and then cooled down to room temperature. The successive variations of the vOH absorptions are shown in Figure 3 for silicas G and P. An apparent temperature of 500 °C is necessary to eliminate the most strongly H-bonded silanols (curve a). In contrast, the distribution of weakly perturbed silanols is desorbed between 540 and 580 °C (curve c), which corresponds to a rapidly increasing efficiency of the treatment above 540 °C. After a treatment at the apparent temperature of 580 °C, the i/OH profiles are similar to those of self-supporting silica disks pretreated at the real temperature of 645 °C. An intermediate distribution of desorbed silanols, with two broad peaks at about 3570 and 3675 cm-1, is. also shown in Figure 3 (curve b). 

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... 

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