Silica–alumina/sepiolite nanoarchitectures

Abstract

Novel silica–alumina/sepiolite nanoarchitectures have been prepared by an original approach based on sol–gel procedures. A commercial organo-sepiolite is used as a starting material whose organocations are provided by alkylchains that contribute to the controlled hydrolysis and polycondensation of silicon and aluminium alkoxides mixtures added to a non-aqueous suspension of that sepiolite. This procedure yields the coagulation of the alkoxides–organosepiolite suspension, resulting in the formation of a homogenous gel. After thermal treatment of the dried system, the organic matter is removed and the silica–alumina network consolidated. The characterization of the resulting materials reveals the formation of silica–alumina nanoparticles that remain anchored to the surface of sepiolite fibers through the silanol groups of the core silicate. These novel nanoarchitectures show specific surface areas in the range of 250–300 m² g⁻¹. The porosity of the resulting materials can be controlled depending on the nature of the oxide network generated. Thus, high silica contents generated microporous systems, whereas the incorporation of alumina enhances the mesoporous formation. The incorporation of a silica–alumina network on the surface sepiolite also permits the tuning of its surface acidity, typical values increasing from 0.1 to 1.4 mmol of acid sites per gram, according to the aluminium content. The acid–base properties of the new nanoarchitectures were evaluated as acid catalysts from the 2-propanol dehydration test. The catalytic behaviour is discussed in terms of the acidic properties of the involved solids.