Basic reactivity of CaO: investigating active sites under operating conditions

Abstract

A set of CaO samples was prepared from thermal decomposition of several precursors, leading to very different surface properties. During storage, CaO samples rehydrated quickly but reversibly. Before characterization, the samples were pre-treated at 1023 K under nitrogen flow to obtain CaO as the active phase. Although this pre-treatment led to almost the same specific surface areas for all samples, their basic reactivity levels toward 2-methylbut-3-yn-2-ol conversion were different from one preparation to another. In contrast with the properties of MgO pre-treated at the same temperature, the basic reactivity of CaO correlates neither with the concentration of surface defects (exposing ions in low coordination) determined by photoluminescence nor with the deprotonation ability toward methanol. In order to identify the active sites on CaO pre-treated under nitrogen in the temperature range 673 K–1023 K, OH groups were quantified with ¹H NMR: the higher the surface density of OH groups, the higher the basic reactivity. Even after pre-treatment at 1023 K, after which only a few hydroxyls remain, the basic reactivity is governed by the remaining hydroxylation of the surface. The higher reactivity of OH groups of CaO compared to those of Ca(OH)₂ and MgO is discussed.