Molecular Mechanism Elucidation of the Nickel-Catalyzed Catalytic Transfer Hydrogenation of Furfural to Furfuryl Alcohol Nova Pratiwi Indriyani(a), Azi Ibnu Sulistia(a), Yessi Permana(a*), Muhamad Abdulkadir Martoprawiro(a), I Made Arcana(a)
a) Department of Chemistry, Faculty of Mathematics and Natural Science, Institut Teknologi Bandung
* yessipermana[at]gmail.com
Abstract
Current methods for hydrogenation of furfural are limited by their use of noble metals, such as Ru, Pd, Pt, Ir, and Au as catalysts at high pressures of hydrogen (up to 100 bar). In this work, the air-stable and inexpensive Ni (II) phosphine complex, NiCl2(PPh3)2, was used as a catalytic transfer hydrogenation (CTH) precursor in furfural (FAL) valorization in the presence of Zn as a reducing agent and ethanol as the hydrogen source, to produce furfuryl alcohol (FOL) under relatively mild conditions (7 h, 433 K). Experimental studies suggest the rule of Ni(0) phosphine complex as an active species in this catalytic reaction.
Quantum chemistry calculations were performed at density-functional theory level by employing the MN15L functional to elucidate the reaction mechanism in prior to an efficient catalyst design. The present work proposes two reaction schemes for the CTH of FAL, namely, via the formation of dihydride and monohydride of Ni(II). Based on the calculated free energy barriers, the hydride transfer is the rate determining step of the reaction scheme that involves the dihydride Ni(II) formation (Scheme 1). In contrast, the proton transfer is the rate determining step (RDS) for the reaction scheme via the monohydride Ni(II) route (Scheme 2). While the Scheme 1 cannot explain the effects of variation on the hydrogen source, Scheme 2 predicts a lower activation barrier for the proton transfer process. This work suggests that Scheme 2 is a more plausible mechanism for the CTH of FAL to FOL.
