Fundamental studies by Stober [ ], Meyer and Heckerman [ ] , and Bering and Serpinskii [ ] indicate that silica surface consists of siloxane network in the bulk, while the hydroxyl groups are attached onto silicon atoms. However these groups are not equivalent in their adsorption or reaction behavior. Fig 1a represents a general arrangement on a silica surface. Belyakova et al. identifies that the number of hydroxyl (Silanol) group on different type of silica surfaces are same i.e. 4 -5 SiOH groups per nm2. Lange [ ] identifies that water associates with these silanol groups in two ways, by hydrogen bonding or by physically adsorbed. Dalton and Iler [ ] states that there is at least a monolyer of water immobilized on silanol groups due to hydrogen bonding, this “glassy layer” protects underlying silica network from foreign molecules. Klier and Zettlemoyer [ ] indicates that water sat “oxygen down” on silanol groups. De Boer and Vleeskens [ ] argued that around 120 oC in ambient air silica looses adsorbed water unless it is present in micropores which would otherwise tak...
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...ussion: Adsorption sites for salt as well as for alcohol are polar silanol groups (Fig 1c). As discussed earlier, alcohol is not adsorbed on all silanol groups (prefers isolated silanol group) and alcohol is more preferentially adsorbed than salt. So, anchorage of salt on the silica surface must be very distinct, reducing the possibility of irreducible mixed metal support formation. Also for substantial adsorption, silica gel has to be contacted with very concentrated salt-alcohol solution. In our opinion small amount of water cause ionization of alcohol which further induce ionization at silanol surface creating anionic sites suitable for esterification of ethoxy group. On the other hand, salt is mostly “associated” if the water is around 1-2% in the solution and the adsorbed salt will act as nucleation site for salt present mainly because of physical interaction.
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