| Abstract
| - Two samples of SBA-15 mesoporous silica with a different content of carboxylic groups (−COOH) wereprepared by a co-condensation route, using 4-(triethoxysilil)butyronitrile as the organosilane agent, then treatingthe samples with sulfuric acid, which removes the template and simultaneously hydrolyses the −CN to −COOHgroups, as shown by IR spectroscopy. Both incorporation of organosilane agents and subsequent acid treatmentdo not affect the ordered SBA-15 structure. The proton-donor ability of carboxylic groups, as well theiraccessibility to reactants, has been studied in the IR by dosing ammonia, which forms reversibly COO-groups and NH4+ ions. The related equilibrium constants have been determined by elaboration of IR data.Outgassing the samples at progressively increasing temperatures destabilizes to an increasing extent theammonium/carboxylate ion pair, because of the decrease in surface polarity brought about by dehydration: this decreases the related equilibrium constants. The amount of carboxylic groups undergoing reaction appearsinstead to be constant with dehydration, and to coincide with the whole population of COOH groups. Titrationwith alkali solutions yielded surface concentrations for the two samples of ca. 1.0 and 0.45 COOH/nm2.Proportionality between the surface concentrations and the intensities of the CO IR band is observed: thissuggests that COOH groups are noninteracting with each other and allows the computation of the relatedextinction molar coefficient.
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