How Surface Hydroxyls Enhance MgO Reactivity in Basic Catalysis: The Case of Methylbutynol Conversion

This combined experimental and theoretical study aims at understanding why surface hydroxyl groups may enhance catalytic reactivity of MgO surfaces in basic catalysis, whereas hydroxyls are weakly deprotonating groups. We investigated that reactivity enhancement in the catalytic conversion of 2-methyl-but-3-yn-2-ol (MBOH). Reaction kinetics was experimentally determined on partially hydroxylated MgO: active sites were saturated with the reactant and catalyzed its conversion with an activation energy of 85 kJ center dot mol1. Reaction pathways were calculated over fully hydroxylated, partially hydroxylated, and dehydroxylated MgO surfaces by means of first-principles simulations. To highlight the effect of reactant coverage, we also calculated the reaction pathway on MgO precovered with MBOH molecules. The results show that the surface OH groups generated by dissociative adsorption of water induce a lowering in the activation energy barriers when they keep a bare Mg2+-O-2 pair available in the vicinity for MBOH to adsorb and react. Interestingly, OH groups do not directly interact with MBOH converting on the surface, but they modify the basic properties of the vicinal bare Mg2+-O-2 pair on which MBOH adsorbs and converts. A similar effect is predicted when MBOH converts on a bare Mg2+-O-2 pair in the vicinity of a second adsorbed MBOH molecule. The beneficial effect of coadsorbates on the reactivity of active sites is analyzed by means of a thermodynamic model and an electronic structure analysis.