UiO-66-NH₂-catalyzed biodiesel production: mechanistic insights from molecular dynamics simulation

To elucidate the performance mechanisms and limiting factors of UiO-66-NH₂ in catalyzing biodiesel production from waste oil at the molecular level, this study integrates experimental investigation with molecular dynamics (MD) simulations. Initially, UiO-66-NH₂ with high crystallinity and octahedral morphology was synthesized via an atmospheric solvothermal method. Single-factor experiments optimized the transesterification process parameters: alcohol-to-oil molar ratio of 15:1, catalyst loading of 3 wt%, reaction temperature of 50 °C, reaction time of 120 min, ultrasonic power of 90 W, and the addition of 50 wt% deep eutectic solvent (DES), achieving a biodiesel yield of 68.886%. The core analysis involved constructing a "waste oil-UiO-66-NH₂-ethanol" trilayer interface model for MD simulations, focusing explicitly on the diffusion behavior and adsorption mechanisms of reactants at the catalyst interface to elucidate their impact on yield. Analysis based on mean squared displacement (MSD) and radial distribution function (RDF) revealed that UiO-66-NH₂ exhibits mobility within the system and demonstrates dual adsorption towards both waste oil (represented by oleic acid) and ethanol. The catalytic advantage stems from the strong hydrogen-bonding interactions between the amino groups of UiO-66-NH₂ and ethanol molecules, which significantly enriches the local ethanol concentration at the interface, thereby promoting the reaction. However, the MD simulations critically identified the primary factor limiting yield enhancement: The inherently small pore size of UiO-66-NH₂ severely hinders the effective diffusion and mass transfer of large reactant molecules like oleic acid. This diffusion limitation induces significant steric hindrance near the interface, restricting sufficient contact between these molecules, the catalytically active sites, and the enriched ethanol. Consequently, by deciphering diffusion and adsorption behaviors, this study elucidates the concurrent molecular mechanisms in UiO-66-NH₂-catalyzed biodiesel synthesis: "interface reactant enrichment promotion" and "large-molecule diffusion limitation suppression." These findings provide crucial microscopic theoretical insights for understanding the experimental yield and guiding the future design and optimization of catalysts.

Molecular dynamics (MD) simulations were performed using Materials Studio 2020. The COMPASS III force field was applied, and a representative model of the oil component was constructed based on molecular configurations identified through GC-MS analysis. The UIO-66-NH₂ model was derived by modifying the UIO-66 framework according to its synthetic rationale. A three-layer interfacial system was subsequently assembled to simulate the relevant environment. The simulation results yielded dynamical parameters, including the mean square displacement (MSD) and radial distribution function (RDF) of the constituent molecules. These results elucidate the promoting and limiting factors influencing biodiesel yield by UIO-66-NH₂ at the molecular level.