Tuning the acidic–basic properties by Zn-substitution in Mg–Al hydrotalcites as optimal catalysts for the aldol condensation reaction

Abstract

Mg–Zn–Al hydrotalcites and derived mixed oxides with different Mg2+-to-Zn2+ ratios were prepared by co-precipitation in super-saturated conditions, followed by thermal decomposition at 500 °C. The synthesized materials were evaluated as catalysts for the self-condensation of octanal in order to establish structure-to-functionality properties of the prepared materials. The presence of zinc affects the structural and textural properties of the as-synthesized hydrotalcites and derived mixed oxides, and provokes a remarkable modification on the acidic–basic properties of the materials as studied by CO2 and NH3-TPD. The presence of Zn2+ caused an increment in the concentration of surface acidic sites compared to the binary Mg–Al system. The samples characterized by a Zn/Mg ratio ≤1 showed the optimal ratio of acidic and basic sites and the best catalytic performance for the production of the α,β-unsaturated aldehyde. The reconstruction of the layered materials (starting from the mixed oxides) caused an increment in the concentration of surface OH− groups, further modifying the selectivity of the reaction.