Unravelling water effects on solid acidic catalysts: Case study of TiO 2 /SiO 2 for dehydration of isobutanol
Résumé
The understanding of water effects on solid acid catalysts is a key issue in developing efficient processes
to produce olefins by bio-alcohols dehydration. In this work, the effects of water on TiO2/SiO2 catalysts for
the gas phase conversion of isobutanol into linear olefins have been unraveled, using for the first time
in situ acidity measurements achieved with a flowing NH3 probe and water vapor containing gas mixtures
at adsorption equilibrium in the temperature range of the reaction. Such compounds, prepared
by grafting titania onto mesoporous silica, contain well-dispersed TiO2 amorphous clusters anchored
by TiAOASi linkages, leading to much higher catalytic activity than TiO2 and SiO2. They yielded only
dehydration products, among which 30% were linear butenes. Furthermore, their activity was significantly
improved by addition of water into the feed, whatever the contact time. Acidity measurements
indicated that TiO2/SiO2 mixed oxides were mostly of Lewis type after activation at 450 C. However,
in situ FTIR acidity measurements showed that addition of H2O to NH3/He gas mixture has no influence
on the number of Lewis sites, while weak Brønsted sites were formed on other sites. This formation, due
to a shift of equilibrium depending on both the temperature and the H2O partial pressure, corresponds to
the hydrolysis of TiAOASi bonds, generating OH acidic groups and enhancing catalytic activity under
water vapor. The novel type of acidity measurements used in this work appear powerful and can be
applied to other acidic heterogeneous catalysts.
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