Tuning the Mg–Al hydrotalcite structure through the Mg/Al ratio and K2CO3 impregnation for enhanced biodiesel catalysis

Abstract

A series of hydrotalcites were synthesized via co-precipitation, modified by incipient wetness impregnation and calcined to form layered double oxides (LDOs). Comprehensive characterization revealed that the catalyst with a Mg/Al ratio of 3 : 1 and 2.0 mol L⁻¹ K₂CO₃ solution exhibited enhanced thermal stability (up to 600 °C) with abundant medium-to-strong basic sites (1.87 mmol g⁻¹) and a hierarchical mesoporous structure (S_BET = 57.99 m² g⁻¹, pore size = 31.42 nm). XPS and EDS confirmed the effective incorporation and homogeneous dispersion of potassium species within the Mg–Al oxide framework and revealed strengthened interfacial interactions and modified local electronic environments, which collectively enhanced surface basicity (pK_a = 11.2), active-site accessibility, and reactant activation. Catalytic performance was evaluated in a three-component transesterification system (rapeseed oil/methanol/methyl acetate, 1 : 1 : 10 molar ratio) over 10 wt% K₂CO₃/MgAl(3)-Cl-Na₂CO₃-2.0-LDO at 60 °C, achieving a record biodiesel yield of 97.99% within 15 minutes and retaining 92% activity after seven cycles. Kinetic analysis showed that the reaction followed a pseudo-first-order model, with the optimal catalyst exhibiting a 2.3-fold higher apparent rate constant (k = 0.2197 min⁻¹) and an 18% lower activation energy (E_a = 12.1019 kJ mol⁻¹) compared to baseline samples, which established a clear structure–property–kinetic relationship for K-modified Mg–Al hydrotalcites, demonstrating that precise regulation of the Mg/Al molar ratio defines the intrinsic layered structure and pore architecture, while optimization of the K₂CO₃ impregnation concentration tailors surface basicity and active-site accessibility. This study provides a potential method for tailoring hydrotalcite-derived catalysts through composition engineering, offering insights for industrial applications. Although Mg/Al hydrotalcite catalysts have been widely applied in biodiesel production, this study uniquely combines Mg/Al ratio regulation with controlled K₂CO₃ impregnation, systematically tuning the surface basicity, pore architecture, and interfacial electronic environment. This allows a direct correlation between phase composition (K₃AlO₃/KAlCl₂O), medium-to-strong basic sites, and catalytic performance, providing insights into rapid glycerol-free biodiesel synthesis.