Thermodynamic brønsted basicity of clean MgO surfaces determined by their deprotonation ability: Role of Mg 2+–O 2− pairs

Abstract

The deprotonation ability of clean (i.e., carbonate- and hydroxyl-free) MgO surfaces of various morphologies towards protic molecules, such as propyne and methanol, is evaluated by following the adsorption modes and amount of dissociated species by FTIR. Photoluminescence is used to identify the nature of the oxide ions involved in deprotonation. The same trend is observed for propyne and methanol: (i) the amount of dissociated species increases with the relative concentration of oxide ions of low coordination O LC 2−, (ii) Mg LC 2+ also are involved in deprotonation. Only ∼0.5% of O LC 2− are implied in propyne deprotonation. Quantitative adsorption and IR measurements suggest that the deprotonating Mg 2+O 2− pairs involve 3- and 4- coordinated ions: Mg 3CO 4C and Mg 4CO 3C. The pairs able to lead to deprotonation have to combine a strong Brønsted basic site O LC 2−, able to abstract a proton and a strong acidic Lewis site able to stabilize the anion generated by deprotonation. The deprotonation equilibrium position is related to the thermodynamic Brønsted basicity. The choice of protic molecules used to identify the catalytically active basic sites was justified a posteriori by the correlation between the relative amount of dissociated propyne and methanol species and the catalytic conversion of 2-methylbut-3-yn-2-ol (MBOH).