CaO-MgFe2O4@K2CO3 as a novel and retrievable nanocatalyst for two-step transesterification of used frying oils to biodiesel

In this study, biodiesel was generated from used frying oil (UFO) in the presence of CaO-MgFe2O4 @K2CO3 as a novel and effective nanocatalyst. Structural and morphological properties of CaO, CaO-MgFe2O4, and CaO-MgFe2O4 @K2CO3 nanocatalysts were evaluated by EDX/SEM, BET, FTIR, TEM, DLS, VSM, and XRD analyses. Also, life cycle assessment and reaction mechanism were investigated. The utmost biodiesel yield using CaO and CaO-MgFe2O4 @K2CO3 nanocatalysts was acheived 93.55% and 96.53%, respectively, at a alcohol/oil ratio of 15:1 for CaO and 18:1 for CaO-MgFe2O4 @K2CO3, catalyst percentage of 4%, temperature of 70 °C, and reaction time of 5 h, which indicates a significant increase in the biodiesel yield of CaO-MgFe2O4 @K2CO3 compared to CaO. Also, physical features of biodiesel such as density, viscosity, flashpoint, cetane number, pour point and cloudpoint were 882 kg/m3, 4.4 mm2/s, 168 ºC, 58.82, 3 ºC and − 2 ºC, respectively, which are within international standards. Moreover, thermodynamic and kinetic behaviors of transesterification were studied and the results showed that the biodiesel generation process using CaO-MgFe2O4 @K2CO3 is endothermic (ΔH°= 61.9 kJ/mol) and non-spontaneous (ΔG°>0). The activation energy of the transesterification reaction was 64.62 kJ/mol, showing the significant potential of the catalyst in the transesterification reaction. Besides, the reusability of CaO-MgFe2O4 @K2CO3 nanocatalyst showed that it can be used in 4 reuse cycles with high biodiesel yield (>90%). Life cycle assessment showed that CaO-MgFe2O4 @K2CO3 has less negative impacts on the environment than common catalysts such as KOH and NaOH. Also, collecting UFO and using it in biodiesel generation helps to reduce environmental pollution.