Amine-Modulated Lanthanum-Based Metal-Organic Frameworks for Levulinic Acid Conversion to Alkyl Levulinate

Abstract

Lanthanide metal-organic frameworks (Ln-MOFs) have attracted considerable interest in heterogeneous catalysis due to their tunable channel sizes, stable network structures, and diverse active sites. Here, a series of lanthanum (La)-based MOFs were synthesized in the presence of various amine modulators of ethylene diamine (ED), 1,4-diazabicyclo[2.2.2]octane (DABCO), butylamine (BA), triethylamine (TEA), and heptylamine (HA) under identical conditions. These modulators play a crucial role in both the nucleation and growth processes of the La-MOFs. The impact of these modulators on crystallinity, porosity, acidity, thermal stability, bonding, morphology, and oxidation state of the metal was investigated. Textural analysis confirmed that the MOFs possess identical crystalline structures but different morphologies. The catalytic performance of the as-prepared La-MOF catalysts was evaluated for levulinic acid (LA) esterification. La-MOF-DABCO exhibited the highest yield, producing 92% ethyl levulinate (EL) in 2 h at 190 °C. Kinetic parameters indicated an activation energy of 20.7 kJ mol-1 and a reaction order of 0.5 with respect to LA. Thermodynamic analysis classified the reaction as endergonic, nonspontaneous, and more ordered. The study illuminated the influence of Brønsted-Lewis acid sites of La-MOF on catalytic esterification, highlighting a modulator-driven approach to MOF synthesis and its potential in heterogeneous catalysis. These findings offer insights for modeling and simulation studies in industrial applications.