Direct Electrosynthesis of an Amino Acid from a Biomass Derivative

Abstract

The electrochemical synthesis of nitrogen-containing molecules from biomass-derived compounds under ambient conditions is demonstrated, relying only on green sources of feedstock, renewable energy, and water. In this study, we report a two-step method of electrochemically synthesizing 5-(aminomethyl)furan-2-carboxylic acid (AFCA) from 5-hydroxymethylfurfural (HMF) using hydroxylamine (NH₂OH) as the nitrogen source in an acidic electrolyte. In the first step, HMF was reductively aminated into (5-(aminomethyl)furan-2-yl)methanol (HMFA) using NH₂OH as the source of nitrogen. This was followed by a second step, involving the oxidation of HMFA to AFCA on a manganese oxide (MnO _x ) anode at the same pH. MnO _x was able to selectively oxidize the alcohol group on HMFA to produce AFCA with 35% Faradaic efficiency without affecting the amine group. As both of these reactions are completed in a pH 1 electrolyte, it eliminates the need to separate HMFA before proceeding with the second reaction. Among different metal electrodes (Ag, Au, Cu, Pb, Pt and Sn) tested for the electrochemical reductive amination reaction, Ag electrodes displayed the best performance to selectively aminate HMF to the intermediate species, HMFA, with up to 69% Faradaic efficiency at mild potentials of -0.50 V_RHE. Density functional theory calculations were carried out to explore a possible reaction pathway for the reductive amination on Ag(111), which suggests a thermodynamically feasible reaction even at 0 V_RHE. The cathodic experimental reaction parameters were optimized to reveal that an electrolyte pH of 1 is optimal for the electrochemical reductive amination reaction. Our work shapes the future possibility of an electrochemical synthesis to produce AFCA without the need for any product separation between steps by combining the Ag cathode reaction to the MnO _x anode reaction sharing the same electrolyte. Since both the cathode and anode reactions both involve four electrons transferred, combining both half reactions in a single electrochemical reactor can eliminate the need for energy-wasting auxiliary counter reactions such as hydrogen evolution or water oxidation.

Figure 1

Proposed membrane separated electrochemical reductive amination of HMF to produce HMFA on Ag cathodes and the subsequent oxidation of HMFA into AFCA on MnO_x anodes in acidic media. AFCA is able to be self-polymerized into the renewable polyamide poly(AFCA).

Figure 2

Linear sweep voltammograms (in the cathodic direction) of 10 mM HMF in the presence of 10 mM NH₂OH on (a) Ag, (b) Cu, (c) Pb, (d) Sn, (e) Au, and (f) Pt electrodes. Reaction conditions: 0.1 M HClO₄ electrolyte (pH 1); scan rate of 50 mV s^–1 at room temperature. Each LSV was the second measured scan.

Figure 3

Calculated free energy diagram of HMF reductive amination towards HMFA over the Ag(111) surface at 0 V vs. RHE. To focus on the reducible −CHO group in HMF, we denote HMF as R-CHO with R representing the unreacted “CH₂OH-furan-” group in the furanic compound. The thermodynamically favored pathway is colored in green, while the parallel but less favored states are in black. The side view for HMF* (R-CHO) and R-CHN* are shown as insets, with the other optimized structures summarized in Figure S1. Color codes: light gray, hydrogen (H); dark gray, carbon (C); red, oxygen (O); blue, nitrogen (N); gray, silver (Ag).

Figure 4

Electrochemical reductive amination of HMF into HMFA on Ag electrodes. (a) LSVs, (b) Faradaic efficiencies, partial current densities, and selectivity of HMF reductive amination at differing acidic pH values (0.5, 1.0, 1.5, and 2.0). (c) LSVs, (d) Faradaic efficiencies, partial current densities, and selectivity of HMF reductive amination at differing initial concentrations of HMF and NH₂OH (0, 5, 10, 20, and 40 mM for both). Reaction conditions: HClO₄ electrolyte solution adjusted for specific pH; scan rate for LSVs was 50 mV s^–1 in the cathodic direction, and all constant potential experiments were done at −0.50 V_RHE for 3 h. The HMF and NH₂OH concentrations for the reaction in (a) and (b) were 10 mM.

Figure 5

Electrochemical oxidation of HMFA using 200 nm MnO_x electrodes sputtered onto Freudenberg H17 carbon paper. (a) LSVs of MnO_x electrodes in pH 1 electrolyte before (“blank”, dashed line) and after (solid line) the addition of 10 mM HMFA. Inset depicts EDX image of Mn in a sputtered MnO_x on carbon electrode. (b) Faradaic efficiencies of detected liquid products after constant potential experiments at 1.60 V_RHE for 20 h. Error bars were produced after two separate experiments. Reaction conditions: 0.1 M HClO₄ electrolyte (pH 1); scan rate for LSVs was 50 mV s^–1. (c) ¹H NMR spectra of AFCA produced after electrolysis; yellow highlighted peaks represent HMFA. Full NMR spectra with integration values are shown in the Supporting Information.