Single-step conversion of waste cooking oil to biodiesel via a bifunctional calcium phosphotungstate heterogeneous catalyst

This study reports the development and evaluation of a bifunctional heterogeneous calcium phosphotungstate catalyst [Ca3(PW12O40)2], designed for effective single-step biodiesel production from waste cooking oil (WCO) subsequent to a pretreatment phase. The catalyst was synthesized by a solid-state process and comprehensively examined to verify its structural framework, surface textural characteristics, and distribution of active sites. XRD analysis revealed the formation of a well-defined scheelite-type crystalline phase, whereas FTIR spectra validated the preservation of the Keggin heteropoly anion configuration. BET and TPD analyses also revealed a mesoporous structure characterized by the presence of both potent acidic and basic functionalities; the catalyst enables simultaneous conversion of free fatty acids through esterification and triglycerides via transesterification. Under optimal conditions of 65 °C, a methanol-to-oil ratio of 11:1, 4 wt.% catalyst concentration, and a reaction period of 120 min, a maximum biodiesel yield of 97.1% was attained. Selected parameters of the produced FAMEs, such as viscosity, flash point, cold flow properties, density, acid value, and cetane number, were within EN 14214 and ASTM D6751 limitations. Kinetic evaluation produced an activation energy of 56.4 kJ mol−1, showing that the process adhered to a pseudo-second-order kinetic model. Furthermore, a proposed Eley-Rideal-type mechanism was consistent with the recorded kinetics. After 5 rounds of reutilization, the catalyst sustained over 90% of its activity, exhibiting commendable stability. In summary, Ca3(PW12O40)2 demonstrates significant potential as an effective and stable bifunctional catalyst for biodiesel production from low-graded feedstocks.