investigation of water effects on heterogeneous acid catalysts: application to bio-alcohols dehydration into olefins

Z. Buniazet 1 Couble J. D. Bianchi 2 Cabiac A. Maury S. S. Loridant 1
1 ECI2D - IRCELYON-Energies, carburants, intermédiaires pour le développement durable
IRCELYON - Institut de recherches sur la catalyse et l'environnement de Lyon
2 ING - IRCELYON-Ingéniérie, du matériau au réacteur
IRCELYON - Institut de recherches sur la catalyse et l'environnement de Lyon
Abstract : Bio-alcohols dehydration is of prominent relevance because it represents a sustainable alternative route for olefins production. However, water present in bio-alcohols and generated by their dehydration often gives rise to structural, textural and acidity modifications of heterogeneous catalysts. In this work, a method based on in situ techniques has been developed to understand H2O effects on heterogeneous catalysts used for conversion of isobutanol into linear butenes. Our approach consists in correlating catalytic performances and in situ acidity measurements at the reaction temperature and under substantial water pressure. TiO2/SiO2 and H4SiW12O40/SiO2 oxides were prepared by different methods, the metal loading was maximized whilst amorphous and well dispersed supported oxides were maintained. FTIR spectra of absorbed pyridine revealed that TiO2/SiO2 possessed mostly moderate Lewis acid sites whereas strong Brønsted acid sites mainly compose H4SiW12O40/SiO2. The catalytic results revealed that Brønsted acid sites are more active in isobutanol dehydration and that high strength is required for isomerization. For TiO2/SiO2 catalysts, H2O addition to the feed significantly increased the conversion especially at high contact time (Fig 1.a) whereas the selectivity was unchanged. Similar positive effects of water on conversion were obtained for H4SiW12O40/SiO2 catalysts but to a lesser extent. Both catalysts deactivated with time on stream but Ti/SiO2 deactivated more slowly. This was shown to arise from coke formation and not from structural or textural modifications due to H2O exposition. In parallel, in situ FTIR measurements were performed with a specially design short path length IR cell reactor, allowing relevant data under water flow up to 3% to be obtained [1]. Acidity measurements were achieved by NH3 and H2O adsorption from 298 to 673 K. The number of Brønsted acid sites density increased four times for TiO2/SiO2 catalysts when H2O was injected in the system, while Lewis density was unchanged (Fig 1.b). Therefore, this density modification could not be attributed to Lewis acid sites conversion into Brønsted acid sites, as frequently mentioned in literature. These additional Brønsted sites were still present in the reaction temperature range (473-673 K) confirming they are active sites for isobutanol conversion and explaining the increase in the conversion with water. Furthermore, the heats of adsorption were determined from the IR bands evolutions using the AEIR method [2]. The values comparison clearly demonstrated that NH3 interactions with Lewis and Brønsted sites were not hindered by water at the reaction temperatures, water having much weaker heat of adsorption. Finally, as bridged OH groups were shown to be formed under water at the reaction temperatures on TiO2/SiO2 and to correspond to Brønsted sites, it was concluded that the activity improvement arises from such bridged OH sites. The water effect on isobutanol adsorption and butenes desorption is under investigation from in situ and operando FTIR experiments and the mechanism of Brønsted acid sites formation is also being considered. [1] T. Chafik, O. Dulaurent, J. L. Gass, D. Bianchi, J.Catal. 179 (1998) 503-514. [2] F. Giraud, J. Couble, C. Geantet, N. Guilhaume, E. Puzenat, S. Gros and D. Bianchi. J. Phys. Chem. C 119 (2015) 16089-16105.
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Submitted on : Tuesday, May 31, 2016 - 2:04:33 PM
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  • HAL Id : hal-01323940, version 1



Z. Buniazet, Couble J., D. Bianchi, Cabiac A., Maury S., et al.. investigation of water effects on heterogeneous acid catalysts: application to bio-alcohols dehydration into olefins. French Conference on Catalysis (FCCat), May 2016, Frejus France. ⟨hal-01323940⟩



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