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

Abstract

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.

Highly Porous Layered Double Hydroxide and Mixed Metal Oxide by Sacrificial Bio-Template, Egg White Foam

“…This homogeneous distribution of the trivalent ion assures the even action of certain metal species, possibly the trivalent one, as an adsorption or catalytic site. Moreover, the pore structure evolution upon calcination enhances the specific surface area [15][16][17][18][19].…”