Abstract
The stability and acid-base properties of MON2O mixed oxides (where M = Be, Mg, Ca; N = Li, Na, K) are studied by using ab initio methods. It is demonstrated that (i) the basicity of such designed systems evaluated by estimation of electronic proton affinity and gas-phase basicity (defined as the electronic and Gibbs free energies of deprotonation processes for [MON2O]H+) were found significant (in the ranges of 272–333 and 260–322 kcal/mol, respectively); (ii) in each series of MOLi2O/MONa2O/MOK2O, the basicity increases with an increase of the atomic number of alkali metal involved; (ii) the Lewis acidity of the corresponding [MON2O]H+ determined with respect to hydride anion (assessed as the electronic and Gibbs free energies of H− detachment processes for [MON2O]H2) decreases as the basicity of the corresponding oxide increases. The thermodynamic stability of all [MON2O]H2 systems is confirmed by estimating the Gibbs free energies for the fragmentation processes yielding either H2 or H2O.
Highlights
The alkaline earth metal oxides are classical base catalysts where oxide ions behave as bases whereas the metal cations serve as Lewis acids
The calculated proton affinity (PA) and gas-phase basicities (GPB) for alkaline earth metal oxides show a systematic growth with an increase of atomic number of M, see Table 1
The experimental PAs and GPBs are available only for MgO (PA = 236.14 kcal/mol; GPB = 229.30 kcal/mol) and CaO (PA = 284.56 kcal/mol; GPB = 277.80 kcal/mol)[27]. The comparison of those values with our calculated PAs and GPBs may indicate that the CCSD(T)/ aug-cc-pVTZ theoretical treatment somewhat overestimates the proton affinity and gas-phase basicity of MgO and CaO by 17.9–30.7 and 0.4–13.5 kcal/mol, respectively
Summary
The alkaline earth metal oxides are classical base catalysts where oxide ions behave as bases whereas the metal cations serve as Lewis acids They catalyse a variety of organic reactions, e.g. isomerization of olefins[1], aldol condensation[2,3,4,5,6], transesterification reactions [7,8,9,10,11], the Knoevenagel condensation[12, 13], the Michael addition[14,15,16], dehydrogenation reactions[17,18,19] and many other processes which require the cleavage of the C–H bond and the formation of carbanion intermediates[20, 21]. The high catalytic activity was attributed to the redox ability of active Co2+ site induced by alkali metal. This is consistent with our results reported for mixed nonstoichiometric MON oxides (where M = Be, Mg, Ca; N = Li, Na, K) [26]. We found that the introduction of alkali metal to alkaline earth metal oxide substantially affects the electron density distribution in the MO system (by reducing the partial charge on alkali earth metal atom) and rises the reductive ability (by ca. 2–3 eV with respect to the unmodified oxide)
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