Structure–property relationships in Mg–Al hydrotalcites and derived mixed oxides for biomass conversion

Mg–Al hydrotalcites are prototypical layered double hydroxides whose physicochemical properties arise from highly tunable local atomic environments and reversible structural transformations. Derived from brucite-like layers of edge-sharing MgO₆ and AlO₆ octahedra, these materials exhibit adjustable layer charge density, controlled cation distribution, and chemically responsive interlayer galleries composed of anions and hydrogen-bonded water networks. Variations in Mg/Al ratio, interlayer composition, synthesis strategy, and post-synthesis treatments strongly influence local coordination geometry, defect density, acid-base site distribution, and surface reactivity. Upon thermal activation, hydrotalcites undergo topotactic transformation into homogeneous Mg–Al mixed metal oxides, generating coordinatively unsaturated metal centers, lattice defects, and a balanced ensemble of Brønsted basic and Lewis acidic sites. Notably, these oxides retain a structural memory effect, enabling partial reconstruction of the layered framework under aqueous or reactive environments and imparting dynamic adaptability to the local surface structure. This reversible interplay between layered and oxide states governs metal anchoring, electronic metal-support interactions, and stabilization of highly dispersed mono- and bimetallic active phases. This review examines Mg–Al hydrotalcites and their calcined derivatives from a structural and surface chemistry perspective, correlating synthesis-induced structural features, local coordination environments, and catalytic functionality using insights from advanced diffraction, spectroscopic, microscopic, and operando techniques. By correlating coordination structure with catalytic behavior, this article provides a unified framework for the rational design of Mg–Al hydrotalcite-based materials as versatile catalyst supports and multifunctional catalytic systems.