Role of oxygen vacancies in the basicity of ZnO: From the model methylbutynol conversion to the ethanol transformation application

Abstract

The parameters governing the basic reactivity of kadox and ex-carbonate zinc oxide samples towards alcohol conversion were investigated varying the atmosphere of pre-treatment for a model reaction, the methylbutynol (MBOH) conversion, and the ethanol conversion reaction, a transformation of potential higher industrial impact. The catalytic properties of ZnO can thus be tuned: enhanced activities were measured after a pre-treatment performed under inert gas as compared to oxidative conditions. If no significant variation of residual hydroxylation nor carbonation can account for this behaviour, a quantitative correlation between the variation of MBOH conversion induced by the different atmosphere of pre-treatment with that of the concentration in oxygen vacancies characterized by in situ EPR found, showing that oxygen vacancies play a key role towards basic reactivity. In fact, the electronic density of the oxygen of the active acid/base pair is directly influenced by the electron release or capture associated to the formation or filling up of oxygen vacancies, respectively. On the kadox samples, the presence of an additional weaker active site is responsible for the residual activity of kadox sample in the absence of oxygen vacancies. The existence and location on specific crystallographic faces of these sites are discussed in relation with the different morphologies of the two samples and with their different affinity with CO2. Even if the occurrence of oxygen vacancies still governs the conversion level for ethanol transformation, the reactivity inhibition after oxidative pre-treatment is less pronounced due to the higher reaction temperature that limits the efficiency of oxygen vacancies filling-up under oxidative atmospheres. The nature of active sites leading to the formation of acetaldehyde and ethylene is also discussed.