Role of Hydroxyl Groups in the Basic Reactivity of MgO: a Theoretical and Experimental Study

Abstract

Clean (i.e., carbonate- and hydroxyl-free) and hydroxylated MgO surfaces of various morphologies were used to study the correlation between thermodynamic Bronsted basicity (evaluated by the deprotonation of methanol and propyne followed by FTIR) and basic reactivity (evaluated by the conversion of 2-methyl-but-3-yn-2-ol (MBOH)). A correlation is evidenced for clean surfaces, but for hydroxylated surfaces, OH groups, though weak Bronsted bases, are much more active than oxide O2- ions. Some of the hydroxyl groups whose VOH infra-red stretching band is included in the narrow high-frequency OH band were shown to be responsible for such activity. In order to identify them, the irregularities of surfaces involving low coordination (LC) ions were modeled by DFT calculations (mono- and di-atomic steps for 4C ions, corners, kinks and divacancies for 3C ions). All of them dissociate water: except for kinks and divancancies where the OH formed can be considered as isolated, in all the other cases, OH groups are H-bonded. The i OH infra-red frequencies obtained by calculations are consistent with the experimental spectrum which exhibits two domains: the broad low-frequency () domain is assigned to H-bond donor OH groups (O3C H, O4C H and O5C H), whereas the high-frequency narrow band ( cm-1 ) corresponds to isolated OH (O2C H, O3C H and O4C H in kinks and divacancies) and H-bond acceptor OH groups (O1C H and O2C H). The latter are inferred to be implied in basic reactivity of hydroxylated surfaces toward MBOH.