Revisiting the Structural Evolution of Hydrotalcite-Derived Mixed Metal Oxides upon Alkali Metal Doping and Its Impact on Base Catalysis

The structural evolution of classic coprecipitation-derived Mg-Al mixed oxide (MMO) upon doping of different alkali metals in a wide level (1-20 wt.%) was revisited. The pristine lateral MMO of aggregates into particulates, accompanied with the dramatic loss of the surface areas and the formation of aluminates. These phenomena become severer with the decrease of the atomic radius. The formation of NaAlO2 likely occurs via the gradual dealumination of the MMO, which is highly dependent on both the dopant content and the activation temperature. This leads to ultimately a new ensemble with MgO as the core decorated by NaAlO2 in the outer surfaces at high Na doping level and above 973 K. When evaluated in the base-catalyzed transesterification and acylation model reactions, the Na-doped MMO show enhanced performance and a plateau at increasing doping that might be explained by an interplay between the number and strength of strong basicity.