Potassium and zirconium promoted coffee husk ash catalysts for waste frying oil methanolysis: Kinetic and thermodynamic studies

The catalytic activity of the KNO3 and ZrO(NO3)2·H2O modified coffee husk ash alkali (KN/CHA) and bifunctional (ZN/CHA) catalysts was investigated for triglyceride-containing waste frying oil (WFO-TG) transesterification. The catalysts were characterized by the XRD, SEM, FTIR, BET, and pH and alkalinity measurements. Analyses of the catalysts' synthesis costs were also done. Both catalysts contained alkali active sites such as K2Ca(CO3)2, K2CO3, and K2O. In ZN/CHA, tetragonal zirconia (t-ZrO2) was also found. The catalyst loading (CL) (1–9 wt%), methanol-to-oil molar ratio (M/O) (9:1–21:1), reaction temperature (T) (45–75 oC), and reaction time (t) (20–120 min) effect on WFO-TG conversion (xME) to biodiesel (i.e. WFO methyl esters (WFOME)) was investigated. A refractive index technique correlated with 1HNMR measurements was employed to determine the xME. The conversions were 97.75 % using KN/CHA (at CL of 5 wt%, t of 70 min at T of 65 °C, and M/O of 15:1) and 93.65 % (at CL of 7 wt%, t of 80 min at T of 65 °C, and M/O of 15:1) using ZN/CHA. The WFO-TG transesterification followed first and second-order reaction kinetic models while employing KN/CHA and ZN/CHA catalysts, respectively. The activation energies for transesterification using KN/CHA and ZN/CHA were 28.52 and 35.70 kJ/mol, respectively. The respective positive changes in enthalpy (25.8 and 33 kJ/mol) and Gibbs free energy (91.65 and 87.5 kJ/mol) results indicated that the WFO-TG transesterification reactions were endothermic and unspontaneous. The recovered KN/CHA exhibited xME of 83, 66, and 53 % for three consecutive reuse cycles while for ZN/CHA reuse cycles were 50, 44, and 33 %. However, using recalcined spent ZN/CHA xME reached. 87 %. Compared to ZN/CHA, the KN/CHA catalyst was far more economical mainly due to the difference in cost of the chemicals employed for the coffee husk ash modification.