Silica-titania

Type I isotherms are characterized by a plateau which is nearly or quite horizontal, and which may cut the p/p° = 1 axis sharply or may show a tail as saturation pressure is approached (Fig. 4.1). The incidence of hysteresis varies many Type I isotherms exhibit no hysteresis at all (Fig. 4.1), others display a definite loop, and in others there is hysteresis which may or may not persist to the lowest pressures ( low-pressure hysteresis ) (Fig. 4.2). Type 1 isotherms are quite common, and are no longer restricted, as seemed at one time to be the case, to charcoals. Many solids, if suitably prepared, will yield Type 1 isotherms the xerogcls of silica, titania, alumina... 

Both the Toth and Alcoa processes provide aluminum chloride for subsequent reduction to aluminum. Pilot-plant tests of these processes have shown difficulties exist in producing aluminum chloride of the purity needed. In the Toth process for the production of aluminum chloride, kaolin [1332-58-7] clay is used as the source of alumina (5). The clay is mixed with sulfur and carbon, and the mixture is ground together, pelletized, and calcined at 700°C. The calcined mixture is chlorinated at 800°C and gaseous aluminum chloride is evolved. The clay used contains considerable amounts of silica, titania, and iron oxides, which chlorinate and must be separated. Silicon tetrachloride and titanium tetrachloride are separated by distillation. Resublimation of aluminum chloride is requited to reduce contamination from iron chloride. 

Electron micrographs (scanning and transmission) showed that tungsten carbide is well dispersed on the surface of each support as nanosized particles (20 - 50 nm) as typified by the images in Figs. 3 (a b). However, BET surface area decreased in the order alumina > silica > titania > zirconia. With highest surface area obtained for each support being 240,133,18 and 9 m g respectively. 

Torma, V., Peterlik, H., Bauer, U., Rupp, W., Husing, N., Bernstorff, S., Steinhart, M., Goerigk, G. and Schubert, U. (2005) Mixed silica titania materials prepared from a singlesource sol-gel precursor A time-resolved SAXS study of the gelation, aging, supercritical drying, and calcination processes. Chemistry of Materials, 17, 3146-3153. 

Li, P., Ohtsuki, C., Kokubo, T., Nakanishi, K, Soga, N. and de Groot, K (1994) The role ofhydrated silica, titania and alumina in inducing apatite on implants. Journal of Biomedical Materials Research, 28, 7-15. 

C. Ingemar Odenbrand, S. Andersson, L. Andersson, J. Brandin, and G. Busca, Characterization of silica-titania mixed oxides, J. Catal. 125,541-553 (1990). 

Srinivasan, G, Pursch, M., Sander, L.C., and Muller, K., FTIR studies of C30 self-assembled monolayers on silica, titania and zirconia, Langmuir, 20, 1746, 2004. 

F-T Catalysts The patent literature is replete with recipes for the production of F-T catalysts, with most formulations being based on iron, cobalt, or ruthenium, typically with the addition of some pro-moter(s). Nickel is sometimes listed as a F-T catalyst, but nickel has too much hydrogenation activity and produces mainly methane. In practice, because of the cost of ruthenium, commercial plants use either cobalt-based or iron-based catalysts. Cobalt is usually deposited on a refractory oxide support, such as alumina, silica, titania, or zirconia. Iron is typically not supported and may be prepared by precipitation. 

This is apparent in Fig. 15, where the surface area of silica-titania cogels is plotted against calcining temperature. Pure silica catalyst exhibits very little drop in surface area, but when titania is added the surface area becomes unstable. The more titania added, the lower the temperature needed to cause sintering. 

This explains the melt index behavior of coprecipitated silica-titania catalysts which is shown in Fig. 16. With each catalyst, the MI rises with increasing calcining temperature until sintering begins, then it drops. The... 

Fig. 16. The relative melt index potential (RMIP) of a series of cogelled Cr/silica titania catalysts rises and then falls with calcining temperature, indicating first dehydroxylation then sintering. However, the more titania in the catalyst, the more easily it sinters and therefore the lower the temperature at which peak RMIP develops.

Silica-titania dehydrated at 870°C, then impregnated with 0.5% Cr from hexane, and calcined again in air at the temperature listed. 

Silica-titania was calcined at 870°C in the composition shown, impregnated with O.S % Cr as dicumenechromium in hexane, then calcined in air at 315-600°C. 

Fig. 18. A drop in surface area marks the onset of sintering in a series of Cr/silica-titania catalysts calcined in dry air or CO. Sintering is less severe in CO.

Fig. 19. The termination rate, plotted here as relative melt index potential (RMIP), reflects the extent of surface dehydroxylation in two series of Cr/silica-titania catalysts, calcined in (Y) air or ( ) CO and then air to reoxidize the chromium, both at the temperatures shown. The third series ( ) shows the additional benefit of low-temperature attachment. It was calcined in CO at the temperatures shown, then air at a lower temperature (760°C).

Fig. 20. After being reduced at 870°C, three series of Cr/silica-titania catalysts yield highest termination rates (RMIP) after reoxidation at 600°C. Catalysts reduced in CS2 display best results because CS2 is the most effective dehydroxylating agent. Carbon monoxide is second best. Trivalent samples calcined in N2 also show the benefit of low-temperature reoxidation, but without the effect of increased dehydroxylation.

Chromium oxide-based catalysts, which were originally developed for the manufacture of HDPE resins, have been modified for cthylcnc-< -olcfin copolymerization reactions. These catalysts use. a mixed silica-titania support containing from 2 to 20 wt % of Ti. 

Tanabe et al. (142, 143) find that silica-titania is highly acidic and has high catalytic activity for phenol amination with ammonia and for double-bond isomerization in butenes. Its acidity determined by n-butylamine titration varies with pretreating temperature and catalyst composition. The highest acidity per unit weight of catalyst was obtained when Ti02-Si02 (1 1 molar ratio) was heated at 500°C. 

The reaction of propylene on ZrC>2 exhibits the same characteristics as on other oxides. Propane-d2, for example, is selectively formed in the deuteration process, with no hydrogen exchange in propylene215. New features appear, however, when zirconia is dispersed on other oxides (alumina, silica, titania)215,216. A considerable rate increase is observed and exchange in propylene proceeds simultaneously with addition via the associative mechanism through the common intermediate n-propyl and s-propyl species. 

Figure 8.5 Plot of volume fraction ratio Vro/Vrf characterizing the swelling of an unfilled PDMS network relative to that of a filled PDMS network, against filler loading expressed as volume ratio of filler to PDMS is the volume fraction of filler).40 Types of filler were silica-titania mixed oxides ( ), silica (O), and titania (A).

The acid function of the catalyst is supplied by the support. Among the supports mentioned in the literature are silica-alumina, silica-zirconia, silica-magnesia, alumina-boria, silica-titania, acid-treated clays, acidic metal phosphates, alumina, and other such solid acids. The acidic properties of these amorphous catalysts can be further activated by the addition of small proportions of acidic halides such as HF, BF3, SiFit, and the like (3.). Zeolites such as the faujasites and mordenites are also important supports for hydrocracking catalysts (2). 

Castellani, A. M. Goncalves, J. E. Gushikem, Y. The use of carbon paste electrodes modified with cobalt tetrasulfonated phthalocyanine adsorbed in silica/titania for the reduction of oxygen. Journal of New Materials for Electrochemical Systems (2002) 5(3) 169-172. 

Table 8.1 Selected results for the selective oxidation of propene on titania- and silica-titania catalysts.

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