Production of biodiesel via esterification of coffee waste-derived bio-oil using sulfonated catalysts

Catalytic esterification of acid-rich coffee waste-derived bio-oil was performed using sulfonated metal oxide catalysts (Al₂O₃, MgO, ZrO₂, and TiO₂) and ethanol to produce fatty acid alkyl esters. The potential of the sulfonated catalysts for esterification decreased in the following order: Ti-SO₄ > Zr-SO₄ > Al-SO₄ > Mg-SO₄. Particularly, Ti-SO₄ and Zr-SO₄ resulted in 91.2 % (peak area %) and 85.2 % esters, respectively. This is attributed to the contributions of well-dispersed Brønsted acid sites created by -SO₃H functional groups, additional Lewis acid sites formed by Ti and Zr oxides, and their appropriate pore size. Compared with HCl and H₃PO₄, the use of H₂SO₄ for TiO₂ treatment significantly enhanced ester formation. When using Ti-SO₄, increasing the catalyst-to-feedstock ratio (1/2 ∼ 1/10) significantly increased the esters' selectivity (38.7 %∼94.7 %). Ethanol utilization caused a superior selectivity for esters than methanol, while the increasing temperature favored ester production. This study proposes an eco-friendly and practical method for biodiesel generation.