Transesterification of used vegetable oils with a Cs-doped heteropolyacid catalyst in supercritical methanol

► Cs2.5PW12O40 (CsPW) exhibited the highest stability in supercritical methanol (SCM). ► Used vegetable oil was transesterified over CsPW in SCM to produce biodiesel. ► SCM can accelerate the low reaction rate of CsPW catalytic transesterification. ► CsPW mitigated the harsh operation conditions of the SCM process. ► The content of FAME reached 92% under the optimum reaction conditions.