Catalysts xerogels

The immobilization of the catalyst xerogel in a porous support was performed with a-Al203 foams with open structure provided by the Mendeleev University of Chemical Technology of Russia, Moscow. The aliunina foam was dipped in the sol-gel solution immediately after introduction of the aqueous ammonia. The impregnated a-alumina foams were removed fi"om the gel after aging and vacuum drying. The excess gel was used to prepare a reference sample. 

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 siUca gel (365,366), thoria on siUca 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). 

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. 

Both xerogels and aerogels are characteristically high surface area materials (surface areas normally exceed 500 m2/g). Unlike wet gels, many uses exist for dried gels due to their high surface areas and small pore sizes (typically, < 20 nm diameters). Examples include catalyst supports (12.). ultrafiltration media (18), antireflective coatings (19-20), and ultra-low dielectric constant films. (Lenahan, P. M. and Brinker, C. J., unpublished results.)... 

E. F. Murphy, L. Schmid, T. Burgi, M. Maciejewski, A. Baiker, D. Gunther and M. Schneider, Nondestructive Sol-Gel Immobilization of Metal (salen) Catalysts in Silica Aerogels and Xerogels, Chem. Mater., 2001, 13, 1296. 

Microporosity is a feature observed in many different materials (e g., activated carbons, aerogels, and xerogels). However, with regard to heterogeneous catalysis, zeolites are practically the only microporous catalysts used at present. The following chapter thus only addresses zeolites and their use in catalysis. 

Nickel silicate, as catalyst, 20 106-109 differential thermogram of xerogel, 20 107 infrared spectra of, 20 108 preparation by SHCP method, 20 106 properties and structure of, 20 107-109 X-ray diffraction pattern of, 20 109 Nickel sulfate hexahydrate, dehydration of, dislocations and, 19 389 Nickel sulfides... 

We attempted to improve the eatalytic performanee, including stability, of the silica-immobilized Co-POM catalysts by using hydrothermally stable supports, specifically, the mesostructured silicates SBA-15 and MCF, both modified with amino groups by grafting 3-aminopropyltrietoxysilane [97], The physico-chemical properties of three representative NH2-X (X = xerogel, SBA-15 and MSF) supported Co-POM catalysts are given in Table 1. The textural properties of the initial, POM-free supports are shown for comparison. 

Since both aerogels and xerogels have high surface areas and small pore diameters they are used as ultrafiltration media, antireflective coatings, and catalysts supports. Final densi-fication is carried out by viscous sintering. 

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