Alumina-Silica Mixed Oxides

Substantial uptake of Na and Cl is reported in [1186,3003] (at point of equal uptake, no independent estimation of PZC). 

16 Silica-Titania and Alumina-Silica-Titania Mixed Oxides 

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Not only has the Ti precursor been investigated, but also the structure of the molecular sieve has been heavily investigated. Thus we now have an array of silica and silica-alumina molecular sieve supported Ti catalysts. These include Ti on amorphous Si02,10,11 Ti on a variety of Si02 mixed oxides,12 Ti-0 (titanium-beta),13"17 Ti-MCM-48,18 Ti-MCM-41,19 Ti-HMS,18 titanium-... 

Silica-alumina is the most popular mixed oxide support, combining characteristic features of silica and alumina, including (i) high surface area, (ii) persistent OH population at high temperature and (iii) strong Lewis acidic sites. The predominant surface hydroxyl groups on silica-alumina are [=Si-OH], while [Als-OH] surface species have not been observed [79, 89, 90]. Note that the silica-alumina bulk is mainly composed of [=Si-0-Si=] along with [=Si-0-Als] moieties (Lewis... 

The 3M Company manufactures a continuous polycrystalline alumina—silica—boria fiber (Nextel) by a sol process (17). Aluminum acetate is dissolved in water and mixed with an aqueous dispersion of colloidal silica and dimethylformamide. This mixture is concentrated in a Rotavapor flask and centrifuged. The viscous mixture is then extruded through spinnerettes at 100 kPa (1 atm) the filaments are collected on a conveyor and heat-treated at 870°C to convert them to metallic oxides. Further heating at 1000°C produces the 10-pm diameter aluminum borosilicate fibers, which are suitable for... 

Acidic mixed oxides, including alumina and silica, as well as natural clays, and natural or synthetic aluminosilicates, are sufficiently (although mildly) hydrated to be effective as solid protic acids for the alkylation of aromatic hydrocarbons with olefins. The most studied of these catalysts are zeolites that are used in industrial... 

Aluminosilicates are the active components of amorphous silica—alumina catalysts and of crystalline, well-defined compounds, called zeolites. Amorphous silica—alumina catalysts and similar mixed oxide preparations have been developed for cracking (see Sect. 2.5) and quite early [36,37] their high acid strength, comparable with that of sulphuric acid, was connected with their catalytic activity. Methods for the determination of the distribution of the acid sites according to their strength have been found, e.g. by titration with f-butylamine in a non-aqueous medium using adsorbed Hammett indicators for the H0 scale [38],... 

The yield of the reaction (Table 1) clearly depends or the nature of the solid and on the experimental conditions (temperature, time). Thus, with silica and alumina, amorphous solids with a relatively low acidity, the acetophenone oxime molecule reacts with a very low yield, being the only reaction product the hydrolysis one, acetophenone. With the synthetic mixed oxide silica-alumina, that possesses simultaneously BrOnsted and Lewis acidic centres, the conversion is quantitative, being also the major product the hydrolysis one (3), when the reaction is carried out at 160°C. 

Much work has been done to characterize surface and catalytic properties of Si02—A1203 and other mixed-oxide systems. It is well known from these studies that Si02—A1203 possesses Lewis as well as Br nsted acid sites that are interconvertible. Different models for the site structures have been proposed. Tanabe 20) has recently reviewed these attempts to characterize the surfaces of silica-alumina mixed oxides as well as those of crystalline silica-aluminas. 

Until recently, when Peri 155) reported on a model of the silica-alumina surface, there were no detailed models for the surfaces of mixed oxides available. Beside the presence of Br nsted and Lewis acid sites, Peri 156) had proposed the existence of a sites on the Si02—A1203 surface, which he described as acid-base pair sites rather than simple Lewis acid sites. Various molecules, such as acetylene, butene, and HC1, are adsorbed very selectively on these a sites, whereas NH3 and H20 are also held by many other sites 157). To rationalize the formation of these sites, Peri 155) developed a semiquantitative surface model for certain silica-aluminas, which were prepared by reaction of A1C13 with the surface silanol groups of silica and subsequent hydrolysis and dehydration. The model is entirely based on a surface model of silica, which suggests an external surface resembling a (100) face of the cristobalite structure 158). It should be mentioned in this connection that Peri s surface model of silica may... 

As expected for silica-alumina as a mixed oxide (see also Section IV.B.5), the PyH+ and PyL species are observed simultaneously (160, 205,206,221-223). Two distinct types of Lewis acid sites could be detected (19b mode at 1456 and 1462 cm-1, respectively) on a specially prepared aluminum-on-silica catalyst (160). On water addition, the Lewis sites can be converted into Br nsted sites (160, 205, 221), The effect of Na+ ions on the acidity of silica-aluminas has been studied by Parry (205) and by Bourne et al. (160). It can be concluded from Parry s results that Na+ ions affect both types of acid sites, so that alkali poisoning does not seem to eliminate the Br nsted sites selectively. For quantitative determination of the surface density of Lewis and Br nsted acid sites by pyridine chemisorption, one requires the knowledge of at least the ratio of the extinction coefficients for characteristic infrared absorption bands of the PyH+ and PyL species. Attempts have been made to evaluate this ratio for the 19b mode, which occurs near 1450 cm-1 for the PyL species and near 1545 cm-1 for the PyH+ species (160,198,206,221,224,225). The most reliable value as calculated from the data given by Hughes and White (198) seems to be... 

Figure 9. Surface content of (a) A1 in fumed silica/alumina and (b) Ti in titania/silica and the maximum adsorption of (a) Pb(II) and (b) Ni(II) as a function of the total (a) alumina or (b) titania content in mixed oxides.

Figure 2. Optical spectra of (dimethylamino) azobenzene, DMAAB, adsorbed onto individual fumed alumina, silica A-300, and titania, and mixed oxides Si02/Ti02 (37 wt% Ti02), Al203/Si02 (30 wt% A1203) and Al203/Si02/Ti02 (22, 28, and 50 wt% res-pecttively).

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