Alumina-silica xerogels

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. 

Temperature Silica hydrogel Silica xerogel Silica-alumina Silica-alumina ... 

Granular xerogels such as alumina, molybdena-alumina, silica, and aluminosilicate were impregnated with a solution of niekel nitrate and then dried and heated for 2 hours at 500° in an air oven. 

In the application of WGS in non-Pd-based MRs, Bmnetti et al. (2007) performed the WGS reaction at low-medium temperature (493—563 K) and in the pressure range of 2.0—6.0 bar by using a composite membrane composed of a silica layer supported onto a porous stainless steel disk. The support was modified to make possible the reduction of the macropores by packing with silica xerogel (500 nm) and by coating with an intermediate layer of y-alumina via boehmite sol (y-AlOOH) by a soaking-rolling procedure. Therefore, this MR made it possible to reach a CO conversion of around 95.0% at 553 K, 4.0 bar, and stoichiometric feed ratio. 

A detailed study of the physical and chemical adsorption of water on three xerogels, ferric oxide, alumina and titania, as well as on silica (cf. p. 272) has been carried out by Morimoto and his co-workers. Each sample was outgassed at 600°C for 4 hours, the water isotherm determined at or near 20°C, and a repeat isotherm measured after an outgassing at 30 C. The procedure was repeated on the same sample after it had been evacuated at a... 

Aluminate cement, 2 415-416 Aluminate ions, silica sols and, 22 394s Aluminates, 2 273-279 analysis, 2 275-276 chemical reactions, 2 273-274 dispersants, 8 710t economic aspects, 2 275 health and safety factors, 2 276 manufacture, 2 274-275 physical properties of, 2 273-274 uses of, 2 276-277 Alumina trihydrate (ATH), 2 274 in synthetic fillers, 11 314-315 Alumina whisker reinforcement, 5 574t Alumina xerogels, X-ray diffraction of, 23 78... 

Silica aerogels, 1 753 applications, 1 760-767 physical properties of, 1 758, 761t versus xerogels, 1 758 Silica-alumina, 72.190-191 coke formation on, 5 268 poisons in representative reactions,... 

The vapor-phase esterification of ethanol has also been studied extensively (363,364), but it is not used commercially. The reaction can be catalyzed by silica gel (365,366), thoria on silica or alumina (367), zirconium dioxide (368), and by xerogels and aerogels (369). Above 300°C the dehydration of ethanol becomes appreciable. Ethyl acetate can also be produced from acetaldehyde by the Tischenko reaction (370—372) using an aluminum alkoxide catalyst and, with some difficulty, by the boron trifluoride-catalyzed direct esterification of ethylene with organic acids (373). 

Porous texture modifications of a series of silica and silica-alumina hydrogels and xerogels a thermoporometry study. 

Figure I Comparison of the texture of silica and silica-alumina (hydrogels and xerogels) from cumulative volume curves...

It can already be noted that the texture of silica gels (hydrogel and xerogel) is characterized by a very narrow pore size distribution while silica-alumina gels have a wider pore size distribution. 

Table 8 depicts the effects of the preparation temperature on the texture of hydrogels and xerogels of silica and silica-alumina. As the preparation temperature is increased the pore volume and the mean pore radius of hydrogels as well as xerogels, are increased. 

As thermoporometry is suitable for hydrogel as well as for xerogel materials, it has been possible to study the effects of the operating parameters of the preparation process on the texture of silica and silica-alumina and to follow the changes carried out by each step of the process. 

The silica hydrogel network constitutes the framework of the silica-alumina gel. From a textural point of wiev, a silica-alumina xerogel seems to be an "image" of the starting silica hydrogel. Depending on the alumina content the silica-alumina gels exhibit predominantly a silica-like texture or an alumina-like texture. 

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