Origins of the deactivation process in the conversion of methylbutynol on zinc oxide monitored by operando DRIFTS
The catalytic behavior of zinc oxide samples was studied thanks to a model methylbutynol (MBOH) conversion reaction. The formation of acetone and acetylene is indicative of the basic properties of the zinc oxide surface. This basic catalyst, whose conversion level depends on the nature of the pre-treatment, was found to deactivate versus time on stream. Poisoning the basic sites by CO2 pre-adsorption only affects the active sites working at the beginning of the reaction, those still working at steady state being not impacted. This is consistent with the modification of the strength and population of the active sites during the catalytic reaction. Indeed, from operando DRIFTS experiments, at the beginning of the reaction, the strong acid base pairs generated upon inert treatment at 773 K promote the self aldol condensation of acetone, leading to oligomers. This reaction also produces water, which dissociates on the surface, filling up oxygen vacancies. This contributes to the lowering of the strength of the active sites, resulting in the quenching of polymerization of acetone at a certain stage, at the benefit of the increase of the amount of diacetone alcohol. Consistently, pre-adsorption of water was shown to lead to a decrease of the conversion level, resulting in a conversion profile similar to that obtained after 10 min of reaction in the absence of pre-adsorbed water. The poisoning mechanism proposed is based on the evolution of the IR bands versus time and is in line with the evolution of the deactivation profile versus time. It is concluded that both the control of the conversion level and the dependence of the reaction toward deactivation are associated to the formation of oxygen vacancies during the pre-treatment step but also on their stability in the conditions on the implemented catalytic reaction.
Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (190 K)Download as PowerPoint slideHighlights► The oxygen vacancy formation in ZnO controls the initial basic conversion of alcohols. ► The reaction deactivation profile is related to the stability of the oxygen vacancies during the reaction. ► The filling up of oxygen vacancies by water decreases the strength of the acid base pair active sites.
Journal: Catalysis Today - Volume 205, 30 April 2013, Pages 67–75