Mechanistic study of the catalytic transfer hydrogenation of biodiesel catalyzed by Raney-Ni under microwave heating

Abstract

The use of microwave heating (MH) constitutes a promising way to upgrade biodiesel via catalytic transfer hydrogenation (CTH) catalyzed by Raney-Ni. However, mechanistic reports on this reaction are rare and the mechanisms involved in CTH process remains unclear. Herein, we presented a thorough mechanistic study of CTH of Jatropha oil biodiesel catalyzed by Raney-Ni under MH, using isopropyl alcohol as hydrogen donor and water as solvent. The intensification effect under microwave radiation has been studied from three aspects of catalyst, water, and hydrogen donor. It was found that microwave made an adverse effect on the activity of Raney-Ni. The solvent of water could provide hydrogen to some extent for the CTH of biodiesel, and the influence of Raney-Ni from microwave radiation had a positive role for the water to provide hydrogen during CTH reaction. Under microwave irradiation, the activity of isopropyl alcohol would be enhanced, and the transfer of hydrogen atoms from hydrogen donors to hydrogen acceptors became easy. Based on the characterization of catalyst by SEM, XRD, FTIR and TGA, it was found that the reason for the catalyst deactivation was mainly the carbonaceous deposits of fatty acid methanol esters (FAMEs) on the surface of the catalyst. A schematic representation of mechanism of microwave-assisted CTH has been presented. Furthermore, combining theory of heterogeneous catalytic process with mechanism of six-membered cyclic transition state, the reaction course for the CTH of Jatropha oil biodiesel under microwave heating has been explained successfully. Lastly, the performance improvement of the upgraded Jatropha oil biodiesel was briefly analyzed in view of composition change of biodiesel. Overall, our results provide a systematic understanding of CTH of biodiesel catalyzed by Raney-Ni under microwave heating, and our findings help to develop a green technology for the upgrading of biodiesel in industry.