Hydrolysis and condensation of silicates: Effects on structure

Abstract

The hydrolysis and condensation reactions of monomeric alkoxysilanes and organylalkoxysilanes utilized in sol-gel processing are reviewed. Both reactions occur by acid or base-catalyzed bimolecular displacement reactions. The acid-catalyzed mechanisms are preceded by protonation of OH or OR substituents attached to Si, whereas under basic conditions hydroxyl or silanolate anions attack Si directly. Many of the observed structural trends are understood on the basis of the pH and [H20 ] dependence of the hydrolysis, condensation, and dissolution reactions.

Influence of Ni/SiO2 catalyst preparation methods on hydrogen production from the pyrolysis/reforming of refuse derived fuel

“…The initial composition used was TEOS: 0.1mol dm-3 HNO 3 aq: Ni(NO 3 ) 2 ·6H 2 O: urea = 18.7: 20.0: 2.34: 4.09 in wt.% to obtain the final concentration of 10wt.% NiO. Nickel nitrate and urea were added into an aqueous solution 0.1mol dm-3 of HNO 3 ; then TEOS was dropped into the solution that became homogeneous after constant stirring, as ethanol and silanol were formed and acted as homogenizing agents, promoting the hydrolysis of silanes [19]. The homogeneous solution was placed into an open container at 20 ºC until it became a wet gel; the wet gel was then kept in a sealed container at 50 ºC for one day.…”

A utilização de materiais obtidos pelo processo de sol-gel na construção de biossensores

“…Isto é devido a numerosos fatores, como a facilidade de fabricação, a flexibilidade do projeto de síntese e o fato das enzimas ocluídas na matriz de sol-gel manterem a sua respectiva atividade catalítica [27][28][29][30][31] . O processo de sol-gel pode ser facilmente reconhecido, pois trata de qualquer rota de síntese de materiais onde num determinado momento ocorre uma transição do sistema sol para um sistema gel.…”

“…Após secagem do gel, um xerogel é formado. As reações químicas que ocorrem durante a formação do sol, do gel e do xerogel influenciam fortemente a composição e as propriedades do produto final [28][29][30] . Uma compreensão bási-ca da cinética e das reações envolvidas no processo de sol-gel é de primordial importância na preparação de materiais com característi-cas controladas.…”

Studies of Wear and Tear and Hydrogen Bonding in Dendrimeric Fluorinated Polysilsequioxanes Coatings on an Aluminum Surface

“…Brinker 24 mentioned that a condensation reaction producing alcohol is favoured when water less than a stoichiometric amount is used, while water forming condensation is favoured when a large amount of water is used. However, the presence of a greater amount of water often promotes the depolymerizaion via reverse hydrolysis.…”