Effect of Tetrahedrally Coordinated Al on the Surface Acidity of Mg-Al Binary Mixed Oxides

Metal oxides (MOs) having Mg and Al with Mg/Al ratios of 1, 2, 3, and 4 were synthesized via calcination of the layered double hydroxides (LDH). The X-ray diffraction analysis revealed that all the MO consisted of periclase (MgO) crystallite with comparable crystallinity regardless of the metal ratio. According to the ²⁷Al magic-angle spinning nuclear magnetic resonance, the phase transformation from LDH to MO upon calcination facilitated the evolution of the Al³⁺ ions with unsaturated coordination at the surface of MO. The specific surface area values of MOs were not significantly different from each other, ranging between 100 and 200 m²/g, suggesting that the metal ratio did not strongly influence the porous structure of MO. The temperature-dependent desorption of ammonia demonstrated that the Lewis acidity of the Al-rich MOs was the largest with an Mg/Al ratio of 1, attributed to the efficient exposure of the surface-active site Al³⁺-O²⁻ pairs. The acidity of heterogenous Al-rich MOs significantly increased with the exposed tetrahedral Al site on the surface and dramatically diminished when the molar ratio (Mg/Al) was over two.