Heterogeneous Electrosynthesis of Nitriles through Coupling of Primary Alcohols and Ammonia on Noble-Metal-Free Monometallic Catalyst

Abstract

Despite increasing interest devoted to the electrocatalytic refinery of renewable feedstocks for the sustainable production of value-added chemicals, the direct electrosynthesis of nitriles from biomass-derived alcohols is rarely studied. Nitriles are highly versatile intermediates for producing a wide range of chemicals, including biological materials, pharmaceuticals and polymers. The conventional chemical methods for nitrile synthesis, such as the Sandmeyer and the Rosenmund-von Braun reactions, are not benign as they utilize toxic starting materials, require severe reaction conditions and generate large amounts of chemical waste. More environmentally friendly chemical routes using alcohols and ammonia as the substrates, via oxidation or oxidant-free dehydrogenation coupled with imination, have been recently reported. However, they face certain issues, including the need for oxidants or high reaction temperatures, as well as poor selectivity due to possible over-oxidation and other side reactions. On the electrocatalysis front, the synthesis of hydrogen cyanide, an analogue of nitrile, has been demonstrated using methanol and ammonia as the substrates, albeit at an elevated temperature with costly solid electrolytes. Here, we report a benign one-pot synthesis of nitriles from primary alcohols and ammonia, in the presence of a simple nickel electrocatalyst under ambient temperature using aqueous electrolytes without the need for oxidants.In the initial screening stage, nine species of metal-based catalysts which were reported to be active in the thermocatalytic pathway of the same reaction, including Mn, Fe, Co, Ni, Cu, Zn, Ru, Pd and Pt, as well as carbon paper were studied using benzyl alcohol (BnOH) as a model compound (Figure 1a). Among them, Ni foam, bearing good resistances towards dissolution under oxidative potentials and poisoning by ammonia, has the capacity to produce benzonitrile as the main product under lower overpotentials. Several control experiments were conducted, revealing that the reaction pathway proceeds via two sequential steps: (1) The alcohol first undergoes dehydrogenation to produce an aldehyde intermediate, which equilibrates in the presence of ammonia to afford the corresponding imine. (2) The imine is subsequently dehydrogenated to form the nitrile product (Figure 1b). Based on the linear sweep voltammetry (LSV) results and in-situ Raman analyses (Figure 1c, d), we propose that the in-situ formed NiII/NiIII redox species serve as the active sites for the nitrile production through a hydrogen atom transfer (HAT) mechanism. The influences of applied potentials, alcohol/ammonia concentrations and pH on the reaction were also systematically investigated. A high benzonitrile faradaic efficiency (FE) of 63.0% and a formation rate of 90.8 mmol m-2 h-1 were achieved at applied potentials of 1.375 V and 1.425 V vs. RHE, respectively, under the optimum conditions: 20 mM BnOH, 1 M ammonia and pH 13 (Figure 1e). Subsequently, kinetic studies and mathematical modelling were performed, suggesting that the dehydrogenation from alcohol to aldehyde displays the smallest rate constant. Isotope labelling experiments (Figure 1f) further confirmed that the rate-determining step (RDS) likely involves the cleavage of α-carbon C-H bond in the hydroxyl group of the alcohol. Encouragingly, various aromatic, aliphatic and heterocyclic primary alcohols were transformed to the corresponding nitriles, exhibiting the broad feasibility of our strategy. This study offers a promising electrocatalytic system for the low-cost, green synthesis of high-value nitriles in the chemical industry. Figure 1