Environmentally sustainable production of biodiesel from low-cost lipid feedstock using a zirconium-based metal-organic framework sulfonated solid catalyst

Heterogeneous acidic Zr-MOF (metal-organic framework) catalyst, UiO-66/SO₃H was synthesized for palm fatty acid distillate (PFAD)-methanol esterification. The characterizations for catalyst precursor and active catalyst were carried out using infrared spectroscopy, ammonia-temperature desorption analysis, thermogravimetric analyser, X-ray diffraction, surface textural analyser, and field emission scanning microscopy. The surface area of UiO-66 and UiO-66/SO₃H was 714.77 m²/g and 503.02 m²/g, respectively. Meanwhile, the acidity strength shown an increase in values, rising from 3.14 mmol/g to 7.98 mmol/g. Throughout the catalytic screening test under fixed parameters, UiO-66/SO₃H produced 72.3% of fatty acid methyl ester (FAME) while 45.9% catalyzed by UiO-66. Then, UiO-66/SO₃H was selected for response surface methodology-central composite design (RSM-CCD) optimization. Following 31 experiments, the optimized conditions were determined to be 75 ℃, 1.3 h, 4.2 wt% catalyst, and a methanol to PFAD molar ratio of 21:1, resulting in a yield of 98.6% FAME. Reusability tests demonstrated that the catalyst maintained its activity for seven cycles, averaging 72.4% yield but subsequently dropping to 53.8% after the eighth cycle. Environmental sustainability was evaluated using life-cycle assessment (LCA) across seven impact categories: global warming potential, stratospheric ozone depletion, acidification potential, terrestrial ecotoxicity, freshwater ecotoxicity, marine ecotoxicity, and fossil resource scarcity. LCA analysis revealed that the PFAD process had a substantial global warming impact, with the exception of microalgae-based biodiesel. The PFAD process has lower acidification potential than soybean or lignocellulosic biomass. Our advanced biodiesel production method, with minimal methanol and low electricity, is an environmentally friendly alternative.