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Mg-Ion Inversion in MgO@MgO-Al2O3 Oxides: The Origin of Basic Sites

Although MgO-Al2O3 is well known as having spinel structure and its inversion occurs by cation exchange of the trivalent (Al3+) and divalent (Mg2+) cations, the analytical study of degree of inversion has been still unknown. Herein we report simple methodology to identify the inversion by solid state nuclear magnetic resonance (NMR), and verify, for the first time, the correlation with CO2 capture capacity of MgO-rich MgO@MgO-Al2O3 spinel structures. Through 27Al-NMR, 25Mg-NMR, temperature-programmed CO2 desorption, and thermogravimetric analysis, we have found that higher inversion occurs at low Mg/Al ratio and the inversion decreases as Mg/Al ratio increases. Moreover, we can correlate the degree of inversion with CO2 sorption, which is associated with the medium-strength basic sites induced by formation of the unsaturated O2? species. These results will open new pathways of exploiting defects in complex oxides beyond spinels and their derivatives for desired applications. In this study we demonstrate MgO-Al2O3 for CO2 sorption, and thus can contribute to design new CO2 sorbents.
ChemSusChem 12, 2810-2818 (2019)
http://dx.doi.org/DOI : 10.1002/cssc.201900072