Preparation of chromium-doped ZIFs-8/ZnCo-LDH composite as a acid-base bifunctional catalyst for biodiesel production

This study reports the development of a new recyclable bifunctional composite catalyst composed of Cr-ZIFs-8/ZnCo-LDH for efficiently converting soybean oil into biodiesel. The synthesized catalyst is characterized by several techniques, including XRD, SEM-EDS, BET, FT-IR, and TGA. The catalytic performance of pure ZIFs-8 was affected by the introduction of Cr and ZnCo-LDH active constituents. The catalyst characterization revealed that the conversion behavior of the prepared catalyst is far better than pure ZIFs-8. The optimized yield of biodiesel is 96.32 % (molar ratio of Cr/Zn = 1:4, mass ratio of ZnCo-LDH/Cr-ZIFs-8 = 1:1, molar ratio of oil/methanol = 1:12, catalyst loading of 3.0 wt%, reaction temperature of 65 °C, and reaction time of 7 h). The reaction kinetics of the prepared catalyst for the conversion process showed that its activation energy was 9.1 kJ/mol, lower than that of a system with free fatty acid and water. Also, the values of ΔH◦, ΔS◦, and ΔG◦ were calculated to be 6.68 kJ/mol, −233.04 J/mol, and 81.95–85.45 kJ/mol, respectively. Meanwhile, the catalyst can be employed for four subsequent reaction cycles to evaluate catalytic stability. Further on, the physicochemical properties of the biodiesel produced showed that it has good compliance with international standards.