in situ ftir approach for acidity measurements under water vapor flow: unravelling water effect on acid catalysts for bio-alcohol conversion
Résumé
Olefins production from bio alcohols represent an eco-efficient competitive route to petrochemistry. Water initially present in bio alcohols and generated by their dehydration gives rise to acid sites modifications on catalyst surface. However, the interaction mode and behavior of acid sites in presence of water raises contradictions in the literature because of the ex situ character and diversity of the characterization techniques used. Therefore, in situ characterization is required to well understand the effects of water and improve the performances of heterogeneous catalysts for bio-resources conversion.
Our approach goes further and consists in correlating catalytic performances in isobutanol conversion and in situ acidity measurements at the reaction temperature and under substantial water pressure. For that purpose, in situ FTIR was performed with a unique small internal volume IR cell allowing relevant data under water flow up to 3% [1]. Acidity measurements were obtained by unconventional NH3 adsorption in presence of water from room temperature to 673 K. Unprecedented results about the Lewis and Brønsted acid sites behavior in presence of water were obtained for TiO2/SiO2 and H4SiW12O40/SiO2 catalysts. For instance, we demonstrated that Brønsted acid sites density increased for TiO2/SiO2 when water was injected in the system, while Lewis density was unchanged (Fig.1a). Therefore, this density modification was not attributed to Lewis acid sites conversion into Brønsted acid sites. These additional Brønsted sites corresponding to OH groups with stretching vibration at 3660-3680 cm-1 (Fig1.b) were still present in the reaction temperature range (473-673K) suggesting they are active sites. Furthermore, the IR bands evolutions of absorbed H2O and NH3 allowed determining their heats of adsorption using the AEIR method [2]. In parallel, the effect of water on the catalytic activity has been investigated. The near future of this work will be focused on Operando experiments, consisting in coupling characterization and catalytic measurements under reaction mixture.