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The catalytic conversion of waste cooking oil (WCO) with high acid value (120.37 mg KOH/g) to biodiesel has been studied in low-cost SO3H-functionalized quaternary ammonium ionic liquid as catalyst. The ionic liquid (IL) catalyst was efficient in catalysing the simultaneous esterification and transesterification reactions of WCO and methanol. Moreover, it can be separated and reused for six cycles without any significant decrease in the biodiesel yield. Under the optimal reaction conditions (methanol/oil/IL molar ratio = 10 : 1 : 0.063, 120°C, 1 h), a maximum biodiesel yield of 95% was achieved. [ABSTRACT FROM AUTHOR]