Tuning Mg-Al Hydrotalcite Structure through 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 characterizations revealed that the catalyst with a Mg/Al ratio of 3:1 and 2.0 mol/L K2CO3 solution exhibited enhanced thermal stability (up to 600 °C) with abundant medium-to-strong basic sites (1.87 mmol/g) and a hierarchical mesoporous structure (SBET=57.99 m2/g, 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 (pKa=11.2), active-site accessibility, and reactant activation. Catalytic performance was evaluated in a three-component transesterification (rapeseed oil/methanol/methyl acetate, 1:1:10 molar ratio) over 10 wt% K2CO3/MgAl(3)-Cl-Na2CO3-2.0-LDO at 60 ℃ achieved a record biodiesel yield of 97.99% within 15 minutes and retained 92% activity after seven cycles. Kinetic analysis showed 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 a 18% lower activation energy (Ea=12.1019 kJ/mol) 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. The study provides a perspective way 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 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 for rapid glycerol-free biodiesel synthesis.