Sulfonated mesoporous zinc aluminate catalyst for biodiesel production from high free fatty acid feedstock using microwave heating system

Methyl ester was derived from one-step esterification of palm fatty acid distillate (PFAD) in presence of mesoporous SO₃H–ZnAl₂O₄ solid acid catalyst using microwave irradiation heating system. The catalyst characteristics were studied by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), Fourier transform infrared spectroscopy (FT-IR), temperature programmed desorption (TPD) and thermogravimetric analysis (TGA). The mesoporous SO₃H–ZnAl₂O₄ catalyst possessed unique properties such as surface area of 376.26 m² g⁻¹, total pore volume of 0.16 cm³ g⁻¹, an average pore diameter of 3.55 nm and an acid density of 2.10 mmol g⁻¹. A sequence of experiments were carried out in order to assess the influence of reaction parameters as follows: catalyst concentration (0.5–3.0 wt%), methanol to oil molar ratio (3:1–15:1), reaction temperature (50–70 °C), and mixing intensity (200–500 rpm) for methyl ester production. The evaluation of the esterification was conducted by gas chromatographic (GC) analysis of the PFAD methyl ester at various reaction times. The highest FAME yield was achieved (94.59%) at 20 min under optimum esterification condition (catalyst concentration of 1.5 wt%, methanol to PFAD molar ratio of 9:1, reaction temperature of 60 °C, and mixing intensity of 300 rpm using microwave irradiation heating system. The recyclability experiments revealed that the synthesized catalyst was potential to stay stable for eight consecutive reaction runs with only 22.55% drop in ester synthesis. The quality of the produced ester was assessed by determination of some key fuel properties such as density, acid value, kinematic viscosity, water content, pour point, could point, and flash point. The PFAD methyl ester was establish to satisfy those of European (EN 14214) and American Standards for Testing Materials (ASTM) specifications.