Waste teak leaf litter ash as a mild solid acid-catalyzed esterification for free fatty acid reduction from WCO in biodiesel pretreatment production

Food waste is one of the most worrying aspects of the problems facing the global food system, and it is a major problem in the high-income countries and Thailand. Among the recyclable liquid food wastes is waste cooking oil (WCO). WCO can be recycled properly to lessen its negative environmental effects and to be utilized again in industrial processes like soap and biodiesel. Waste teak (Tectona grandis L.) leaf litter (WTLL) from the area of northern Thailand is considered a good and sustainable option for a mild solid acid catalyst because it is widely available. This study uses WTLL ash as a mild solid acid to help lower the free fatty acids (FFA) in WCO during the biodiesel production process. XRD, FTIR, LVSEM, EDX, and the Hammett indicator method were used to characterize the WTLL ashes. According to the XRD data, the main materials found in the ashes heated to 500 °C were SiO₂ (quartz) and Mg₀.₀₃Ca₀.₉₇CO₃, while at 700 °C, the ashes showed a less clear structure of K₂MgSi₅O₁₂ that had a small amount of K₂O. We saw the structures of SiO₂ (cristobalite), CaSiO₃ (wollastonite), and K₂MgSi₅O₁₂ when the temperature increased to 900 °C, and their functional groups and elements were confirmed using FTIR and EDX techniques. Hammett indicators verified only the WTLL ash at 500 °C that has a weak acid strength property with a total acidity of 0.7370 ± 0.0135 mmol g^–1 ash due to the ash containing high silica that can be suitable for the esterified FFA conversion of 79.50 ± 0.05% with the optimal yield of the esterified WCO that was 91.53 ± 1.49% under conditions of a 3:1 MeOH-to-WCO molar ratio, 2.5 wt.% catalytic loading, a stirred speed of 650 rpm, and the reaction temperature and time at 80 °C for 4 h; the maximum three catalytic reuse cycles were observed. Furthermore, GC–MS revealed the WCO biodiesel (WCOME) from the esterified WCO with high fatty acid methyl ester (FAME) conversion of 94.05%.