Influence of surfactant alkyl length in functionalizing sol–gel derived microporous cobalt oxide silica?

RSC Advances Pub Date: 2014-08-15 DOI: 10.1039/C4RA05934D

Abstract

The size or alkyl chain length of cationic surfactants can be used to tailor both the pore morphology and the functionality or oxidation state of cobalt in silica materials. This work shows for the first time that these two mechanisms are interconnected. Cobalt oxide silica materials, with the same cobalt loading (Co?:?Si = 1?:?4), were prepared using an acid catalysed sol–gel method where the cobalt/surfactant ratio was systematically varied. The alkyl chain length of the cationic surfactant was also varied from 2 to 12 by using tetraethyl (C2-AB) ammonium bromide, and triethyl hexyl (C6-AB) and dodecyl trimethyl (C12-AB) ammonium bromides as the templating agents. Initial addition of C2-AB, C6-AB or C12-AB enhanced the oxidation of cobalt to cobalt (II,III) oxide in the xerogels. However, as more surfactant is added the enhancement effect is reversed and the cobalt (II,III) oxide content of the sample begins to decline. The point at which this transition occurs is a function of the alkyl chain length of the surfactant, with a longer chain indicating an earlier transition. Pore morphology was influenced in a similar fashion with the longer alkyl chain C12-AB surfactant undergoing an earlier transition (i.e. at lower concentrations) towards mesoporosity, than either of the smaller C2-AB and C6-AB surfactants. In both mechanisms it was the increased propensity of the C12-AB surfactant to aggregate (given its larger size and lower solubility) that was the controlling factor.

Graphical abstract: Influence of surfactant alkyl length in functionalizing sol–gel derived microporous cobalt oxide silica
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