Alkoxides aerogels

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). 

Transition-metal mixed oxides active in combustion catalysis have been prepared by two main procedures i) classical coprecipitation / calcination procedures starting from metal nitrates and/ or alkoxides ii) preparation based on the supercritical drying of gels prepared from organic complexes (alkoxides, acetylacetonates or acetates), producing aerogels . Details on the second preparation can be found in Ref. 13. 

Ba,La-hexaaluminate Aerogel formation alkoxides hydrolysis in alcoholic medium and supercritical drying. Alkoxides 1300-1400 Ba-P-Al203 or MP extraphases always detected, Sa=10-15m2/g Catalytic combustion 18... 

Using alkoxides as precursors, carbon dioxide as tile drying agent, and a semicontinuous process the diying procedure became safer and faster. Introduction of organic aerogels. 

The applications of the SG methods for the preparation of catalysts, particularly in the form of aerogels (566), was recently reviewed (11a, 567). Attention may be drawn to other recent reviews (175) on the applications of the alkoxide derivatives of tin (175), as well as p block (39) and other main group metals (568) for the preparation of ceramic materials by the SG process. 

Making an aerogel involves base-catalyzed hydrolysis and polycondensation reactions of silicon alkoxides in an alcohol followed by 24 hours of aging in the alcohol solvent to produce an alcogel. Then the alcohol is removed from the pores of the alcogel... 

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