Esterification of Free Fatty Acids Under Heterogeneous Catalysis Using Ultrasound

The efficient conversion of free fatty acids (FFAs) to fatty acid methyl esters via esterification is a crucial step in biodiesel production from low-cost high-FFA feedstocks, which supports global efforts toward renewable energy and reduced dependence on fossil fuels. However, this esterification process is hindered by slow reaction kinetics, high energy demand, and low catalyst efficiencies. This study investigates tungsten disulfide (WS₂) as a heterogeneous catalyst for the esterification of a mixture of oleic and linoleic acids with methanol under ultrasonic activation, aiming to improve catalytic performance, reaction efficiency, and enhance process sustainability. Four commercial WS₂ powders from various suppliers, varying in particle size (2 μm and 90 nm), were characterized using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Micron-sized WS₂ exhibited higher catalytic activity than nano-scaled WS₂ due to a higher density of edge defects and abundance of catalytically active edge sites. Variation in reaction parameters demonstrated that the ester yield increases from 7% to 53% as the catalyst loading rises from 2% to 32% and reaches a 95% yield at an FFAs-to-methanol molar ratio of 1:15 under ultrasonic activation at 75 °C for 1 h. Comparative experiments confirmed that ultrasound treatment increases the yield of esterification compared to thermal activation. The results suggest WS₂ as a heterogeneous catalyst appropriate for efficient sonochemical esterification in biodiesel production. These kinetic and catalytic data are valuable for future process design, scalability assessments, and techno-economic evaluations of sustainable biodiesel production.