Supported oxides catalysts for the dehydration of isobutanol into butenes: Relationships between acidic and catalytic properties
H4SiW12O40 heteropolyanions, WO3, TiO2 and SnO2 were supported over SiO2 at high loading by wet impregnation or grafting methods and evaluated in the dehydration of isobutanol to butenes. Their structural and textural properties were determined by different techniques such as XRD, TEM, IR, XPS and N2 liquid physisorption respectively. Most of the prepared compounds contained amorphous oxide clusters of few nanometers lying over SiO2 and were mesoporous. Their acidic properties (nature, density and strength) were investigated by pyridine and CO adsorption followed by FTIR. H4SiW12O40/SiO2 contained strong Brønsted acid sites while mostly moderate Lewis acid sites were present on TiO2/SiO2 and SnO2/SiO2. WO3/SiO2 had a mixed character with both moderate Brønsted and Lewis acid sites. The catalytic activity was related to the Brønsted acidity and the best selectivity to butenes close to 100% were obtained for the catalysts containing moderate and weak sites (WO3/SiO2 and TiO2/SiO2). Except for SnO2/SiO2 catalysts, which were unstable and unselective to dehydration products, a significant selectivity to linear butenes (ca 30%) was obtained with all the catalysts. It is proposed that this isomerisation activity is mainly related to the equilibrium between carbocations formed after E1 elimination of water. Its slight increase for H4SiW12O40/SiO2 (and to a lesser extent WO3/SiO2) was attributed to strong Brønsted acid sites able to catalyse the isomerization of isobutene to linear butenes.