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Electrocatalytic Hydrogenation of Pyridines and Other Nitrogen-Containing Aromatic Compounds
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  • Naoki Shida*
    Naoki Shida
    Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
    Institute of Advanced Sciences, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
    PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0003-0586-1216
  • Yugo Shimizu
    Yugo Shimizu
    Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
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  • Akizumi Yonezawa
    Akizumi Yonezawa
    Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
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  • Juri Harada
    Juri Harada
    Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
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  • Yuka Furutani
    Yuka Furutani
    Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
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  • Yusuke Muto
    Yusuke Muto
    Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
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  • Ryo Kurihara
    Ryo Kurihara
    Research Center for Solar Energy Chemistry, Graduate School of Engineering Science, Osaka University, 1−3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
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  • Junko N. Kondo
    Junko N. Kondo
    Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 225-8503, Japan
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    Orcidhttps://orcid.org/0000-0002-7940-1266
  • Eisuke Sato
    Eisuke Sato
    Division of Applied Chemistry, Graduate School of Environmental, Life, Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan
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    Orcidhttps://orcid.org/0000-0001-6784-138X
  • Koichi Mitsudo
    Koichi Mitsudo
    Division of Applied Chemistry, Graduate School of Environmental, Life, Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan
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  • Seiji Suga
    Seiji Suga
    Division of Applied Chemistry, Graduate School of Environmental, Life, Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan
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    Orcidhttps://orcid.org/0000-0003-0635-2077
  • Shoji Iguchi
    Shoji Iguchi
    Graduate School of Engineering, Kyoto University, Kyoto daigaku-katsura, Nishikyo-ku, Kyoto 615-8530, Japan
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    Orcidhttps://orcid.org/0000-0002-2522-1358
  • Kazuhide Kamiya
    Kazuhide Kamiya
    Research Center for Solar Energy Chemistry, Graduate School of Engineering Science, Osaka University, 1−3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
    Innovative Catalysis Science Division, Institute for Open and Transdisciplinary Research Initiatives (ICS-OTRI), Osaka University, Suita, Osaka 565-0871, Japan
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    Orcidhttps://orcid.org/0000-0002-6018-9277
  • Mahito Atobe*
    Mahito Atobe
    Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
    Institute of Advanced Sciences, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-3173-3608
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Journal of the American Chemical Society

Cite this: J. Am. Chem. Soc. 2024, 146, 44, 30212–30221
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https://doi.org/10.1021/jacs.4c09107
Published October 7, 2024

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Abstract

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The production of cyclic amines, which are vital to the pharmaceutical industry, relies on energy-intensive thermochemical hydrogenation. Herein, we demonstrate the electrocatalytic hydrogenation of nitrogen-containing aromatic compounds, specifically pyridine, at ambient temperature and pressure via a membrane electrode assembly with an anion-exchange membrane. We synthesized piperidine using a carbon-supported rhodium catalyst, achieving a current density of 25 mA cm–2 and a current efficiency of 99% under a circular flow until 5 F mol–1. Quantitative conversion of pyridine into piperidine with 98% yield was observed after passing 9 F mol–1, corresponding to 65% of current efficiency. The reduction of Rh oxides on the catalyst surface was crucial for catalysis. The Rh(0) surface interacts moderately with piperidine, decreasing the energy required for the rate-determining desorption step. The proposed process is applicable to other nitrogen-containing aromatic compounds and could be efficiently scaled up. This method presents clear advantages over traditional high-temperature and high-pressure thermochemical catalytic processes.

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Introduction

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According to a report by the International Energy Agency, the chemical industry is the largest consumer of energy, ranking third in terms of carbon emissions. (1) As energy conservation in chemical processes is critical for achieving a sustainable society, the shift from traditional thermal processes to electrochemical processes is urgently needed. The utilization of renewable energy sources in electrochemical processes can lead to sustainable chemical processes with reduced energy consumption and carbon emissions. Consequently, research has been conducted to electrify various organic chemical reactions. (2−5)
Cyclic amines are essential building blocks for fine chemicals. (6) For example, piperidine is one of the most common skeletons in drugs approved by the Food and Drug Administration (FDA; Figure 1a). (7,8) These compounds are also used in pesticides, functional polymeric material monomers, and other compounds that are integral to daily life. A typical synthetic method for piperidine involves the catalytic hydrogenation of the corresponding aromatic compound, pyridine, using H2 gas as a source of protons and electrons. (6) Although various small molecular catalysts have been developed, (9−12) heterogeneous catalysts have been actively developed owing to their applicability in industrial processes. (9,13−19) Pyridine can be easily reduced under mild conditions by activating the pyridinium cation using acid. (17,20) However, acid activation is not ideal because it increases waste production, production costs, and reactor corrosion. The hydrogenation of pyridine without acid activation generally requires elevated temperatures, pressurized H2 gas, or both of them, even with state-of-the-art heterogeneous catalysts (Figure 1b). (18,21−23) Thus, the hydrogenation of pyridine and its derivatives under mild conditions without the use of acidic additives remains challenging. In addition, these processes rely on the use of H2 gas as the reductant. Currently, most of the H2 gas in the market is gray hydrogen, i.e., hydrogen obtained through the steam reforming of methane, which is responsible for ∼3% of the global emission of CO2. (24,25)

Figure 1

Figure 1. Synthesis of piperidines via the hydrogenation of pyridines. (a) Examples of FDA-approved drugs containing the piperidine skeleton. (b) Thermal hydrogenation of pyridine to piperidine using a heterogeneous catalyst. (c) Electrochemical reduction of pyridines in a conventional electrolytic system via a radical anion intermediate. (d) Concept of this work.

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In this context, the electrochemical hydrogenation of pyridines has attracted significant attention (Figure 1c). (26−28) In 1896, Ahrens gave an early example of the electrochemical reduction of pyridine using a lead cathode in 10% sulfuric acid. (29) Some groups have recently reported the formation of piperidine through the electrochemical reduction of pyridinium cations. (30,31) However, no reliable methodology for the electrochemical hydrogenation of pyridine to piperidine with high energy efficiency, scalability, and sustainability has yet been reported. A plausible reason for this deficiency is that the electrochemical reduction of pyridine via outer-sphere single-electron transfer produces a highly reactive radical anion intermediate, leading to undesired side reactions such as dimerization and partial hydrogenation rather than the desired 6e–/6H+ hydrogenation (Figure 1c). (9) This fact suggests that the electrochemical hydrogenation of pyridine should rather rely on electrocatalytic hydrogenation, in which electrochemically generated adsorbed hydrogen species (Hads) react with organic molecules without generating ionic intermediates.
Herein, we report the electrocatalytic hydrogenation of nitrogen-containing aromatic compounds, particularly pyridines (Figure 1d). Using an anion-exchange membrane (AEM) electrolyzer equipped with carbon-supported Rh as the cathode catalyst, the hydrogenation of various pyridines was achieved at ambient temperature and pressure without any additives in the catholyte. This system is also applicable to quinoline, pyrazine, and pyrrole, demonstrating its versatility for various nitrogen-containing aromatic compounds. This paper proposes an innovative method for synthesizing valuable cyclic amines with high energy efficiency under mild conditions, thus contributing to the sustainable production of fine chemicals.

Results and Discussion

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Reaction in the AEM Electrolyzer

In this study, an AEM electrolyzer was used for the electrochemical hydrogenation of pyridine (1a) to piperidine (2a). AEM electrolyzers are widely used in H2O electrolysis (32) and CO2 reduction, (33) but are rarely applied to the electrosynthesis of fine chemicals. (34−36) Figure 2a–c show the setup of the AEM electrolyzer, a detailed description of the system components, and an enlarged image of the reaction site, i.e., the triple-phase boundary, respectively. Rh nanoparticles supported on Ketjen black (Rh/KB, loading amount: 0.5 mg cm–2) were used as the cathode catalyst and immobilized on the gas diffusion electrodes using an anion-exchange ionomer. A membrane electrode assembly (MEA) was constructed by sandwiching an AEM between the cathode and dimensionally stable electrode (DSE) anode to form a zero-gap configuration (Figures 2b and S1). A catholyte containing 1a and H2O was then introduced into the cathodic chamber. Hads is formed via the electrochemical reduction of H2O at the triple-phase boundary, concomitant with the generation of OH–. Hads react with the adsorbed 1a to produce 2a (Figure 2c). OH– migrates to the anodic chamber through the AEM and is oxidized to release O2 (Figures S2 and S3). This setup does not require an anolyte solution. In addition, nonaqueous organic solvents can also be used as catholytes, which are beneficial when the substrate is insoluble in aqueous media. In this case, the anolyte must contain H2O as a source of protons and electrons for Hads formation at the cathode (Figures S4 and S5).

Figure 2

Figure 2. Overview of the electrochemical hydrogenation of pyridine (1a) to piperidine (2a) in an AEM electrolyzer. (a) Schematic illustration and (b) detailed components of the AEM electrolyzer. (c) Enlarged image of the catalyst layer and triple-phase boundary. (d) Energy diagram of the cathodic and anodic reactions. (e) Plot of the current efficiency and cathodic potential for the electrochemical hydrogenation of 1a under various current densities. Electrolysis was conducted under single-flow operation at a flow rate of 12.4 mL h–1 in an AEM electrolyzer equipped with a Rh/KB. (f) Preparative electrocatalytic hydrogenation of 1a under circular-flow operation at a flow rate of 120 mL h–1. (g) Repeated preparative electrocatalytic hydrogenation of 1a under circular-flow operation at a flow rate of 120 mL h–1. (h) Comparison of various metal catalysts under circular-flow operation using MTBE as a cathodic solvent.

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Thermodynamics of Pyridine Hydrogenation

A thermodynamic comparison was made between the thermal and electrocatalytic hydrogenation of pyridine using H2O as the source of electrons and protons (Figure 2d). In a waste-free and sustainable thermal hydrogenation system, we postulate that H2 gas is obtained from electrolysis with renewable energy (green hydrogen). The voltage required to produce green hydrogen, which is independent of pH, is 1.23 V. The hydrogenation of pyridine with H2 is theoretically exothermic, although most instances of the heterogeneous hydrogenation of pyridine are performed under elevated temperatures and pressurized conditions. Thus, the energy input required for the thermal process using green hydrogen is 1.23 V.
On the other hand, the overall reaction formula for the hydrogenation of pyridine under basic conditions is shown in eqs 1–3.
anodicreaction:6OH−→3/2O2+3H2O+6e−E°=+0.40VvsSHE
(1)
cathodicreaction:pyridine+6H2O+6e−→piperidine+6OH−E°=−0.70VvsSHE
(2)
net⁢reaction:pyridine+3H2O→piperidine+3/2O2ΔE°=1.10V
(3)
Therefore, the electrocatalytic hydrogenation of pyridine proceeds with less energy than thermal hydrogenation using green hydrogen. Moreover, electrocatalytic hydrogenation at ambient temperature and pressure offers additional advantages over thermal processes in terms of energy consumption.

Electrocatalytic Hydrogenation of Pyridine

We performed the electrolysis using H2O as the catholyte prior to electrolysis. This pre-electrolysis treatment effectively improved the current efficiency and reproducibility of the system. Subsequently, a 100 mM aqueous solution of 1a was injected into the cathodic chamber using a syringe pump, and constant-current electrolysis was performed. When electrolysis was carried out at various currents with a fixed flow rate of 12.4 mL h–1, 2a was successfully obtained in a wide range of current densities (Figure 2e). The current efficiency profile peaked at 25 mA cm–2, reaching 75%. The decrease in current efficiency at higher currents was attributed to the concurrent hydrogen evolution reaction (HER), which is associated with a more negative cathode potential. Intriguingly, the low current efficiency was also observed at a lower current density, which may be due to the cathode potential being too positive, resulting in a smaller Gibbs free energy change for the electrochemical step (see First-principles Calculations section for related discussion).
Although many electrocatalytic studies have focused on the current efficiency with low conversion of the starting material, the product yield with complete conversion of the starting material should also be investigated, especially if the production of high-value chemicals such as pharmaceuticals is to be implemented. Therefore, preparative electrolytic synthesis was performed with a circular catholyte flow. Electrolysis was performed at 25 mA cm–2 under a circular catholyte flow with a rate of 120 mL h–1, and an electric charge of up to 9 F mol–1 was passed (Figure 2f). 1a was mostly consumed after passing 7.5 F mol–1, at which point only 1.2% of 1a was observed, and completely disappeared at 9 F mol–1. After passing 9 F mol–1, the desired product, 2a, was obtained in quantitative yield with a current efficiency of 66% (Figure 2f). Interestingly, the current efficiency was 99% at 5 F mol–1 and retained to 79% at 7.5 F mol–1. These values are even higher than those obtained in the single-flow experiment shown in Figure 2e, plausibly because of the difference in flow rate. These data suggest that this system enables the electrocatalytic hydrogenation of pyridine to piperidine with high energy efficiency even in a preparative-scale reaction. The data obtained herein was applied to the comparison with previously reported acid-free heterogeneous thermal catalytic systems (Table S1). The electrochemical system presented herein enabled the hydrogenation of pyridine into piperidine with a comparable production rate under much milder conditions. During the reaction, the formation of side products such as 1,2,3,6-tetrahydropyridine was not detected by gas chromatography analysis at any time point of electrolysis, and only 2a was observed as a product. Thus, it is suggested that the reaction proceeded directly from 1a to 2b without generating any side product.
To gain mechanistic insights, the electrolysis was performed using pyridine-d6 and D2O (Scheme S1, and Figures S24–S29). Gas chromatography–mass spectroscopy analysis of the reaction mixture indicated the formation of piperidine-d11, where the mass spectrum of synthesized piperidine-d11 corresponded to that of authentic piperidine-d11. This experiment demonstrates that water is a source of hydrogen atoms in the present electrocatalytic hydrogenation system.
Catalyst recyclability was examined under similar conditions (Figure 2g). After conducting electrolysis seven times under a circular flow, the starting material was completely consumed in all cases, and piperidine was obtained, with 2a yields of over 90% in all trials. Therefore, the high durability of the proposed system was successfully demonstrated.
Subsequently, catalyst screening was performed (Figure 2h). In addition to Rh/KB, platinum (Pt)-group metal (PGM) catalysts such as Ru/KB, Pt/KB, Pd/KB, and Ir/KB were used, and electrolysis was conducted for 36 min under a circular catholyte flow using methyl tert-butyl ether (MTBE) as the catholyte solvent. The results revealed that the use of PGM catalysts other than Rh resulted in a significant decrease in piperidine yield. This finding indicates that Rh/KB is a catalyst with uniquely high activity for the electrochemical hydrogenation of pyridine.

Electrochemical Measurements

Electrochemical measurements were performed using the AEM electrolyzer (Figure 3a). For comparison, the results obtained using a proton exchange membrane (PEM) electrolyzer are also presented (Figure 3b).

Figure 3

Figure 3. Electrochemical measurements. Comparison of the CVs recorded in the (a) AEM and (b) PEM electrolyzer at a scan rate of 10 mV s–1. The measurements were performed in the absence (black) and presence (red or blue) of 1a.

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Rh/KB was used as the cathode catalyst in both systems. When the AEM electrolyzer was used, the onset of a reduction current associated with hydrogen evolution was observed at approximately −0.8 V vs a standard hydrogen electrode (SHE) in the absence of 1a. Upon the addition of 1a, the current decreased slightly, and the passage of electricity was confirmed. By contrast, in the PEM electrolyzer, the cathodic current decreased significantly in the presence of pyridine, likely because pyridine is protonated to form pyridinium salts in the acidic reaction environment of the PEM electrolyzer, inhibiting ion conduction through the PEM. Because nitrogen-containing aromatic compounds are basic, their reduction in the PEM electrolyzer may be impractical. Therefore, the use of an AEM electrolyzer, which provides a basic reaction environment, is essential for the energy-efficient reduction of nitrogen-containing aromatic compounds without the use of additives.

In Situ X-ray Absorption Fine Structure Measurements

The electrolysis of 1a revealed that the current efficiency of 2a improved after pre-electrolysis with H2O. To gain insights into the reduction behavior of Rh/KB during electrolysis, we performed Rh K-edge in situ X-ray absorption fine structure (XAFS) measurements using a tailor-made electrolytic cell equipped with polyimide windows for the X-rays (Figure 4a). In situ XAFS measurement was performed during the 640 s of electrolysis using water as a catholyte (Figures S9 and S10). Linear combination fitting analysis of the Rh K-edge X-ray absorption near edge structure (XANES) spectrum of the Rh/KB catalyst before electrolysis clearly suggested that the Rh species were partially oxidized; the ratio of Rh(0) to Rh(III) was 62:38 (Figure 4b, red). When electrolysis was performed using H2O as the catholyte, the XANES spectrum of the Rh species changed, showing an isosbestic point, and Rh was nearly completely reduced to metallic Rh (Figure 3b). These data correspond to the results of X-ray photoelectron spectroscopy (XPS) measurements. Before electrolysis, the XPS spectrum of Rh/KB showed the presence of Rh(0) and Rh(III) (Figure S7a). By contrast, the postelectrolysis sample of Rh/KB showed the predominant presence of Rh(0) species (Figure S7b). This finding suggests that the surface of the oxide layer of Rh/KB is reduced to expose the catalytically active Rh(0) surface during pre-electrolysis treatment.

Figure 4

Figure 4. In situ XAFS measurements. (a) Schematic illustration of the AEM electrolyzer with a transmitting window for X-ray. (b) XANES spectra obtained during 640 s of electrolysis using H2O as the catholyte. The XANES spectrum of Rh foil is also shown as a reference.

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First-principles Calculations

After confirming that Rh(0) is the catalytically active species in the system, we conducted density functional theory calculations of pyridine hydrogenation on the Rh(0) surface to elucidate the relevant mechanism. For comparison, calculations were also performed for Pt, which showed lower catalytic activity than Rh.
First, calculations were performed for the adsorption process. We calculated the adsorption energy of flat and vertical configurations. The adsorption via the pyridine π-plane (flat configuration) was most stable, which corresponded with the previous report (Figures S11 and S12). (37) Although the adsorption energies of pyridine on Rh (2.20 eV) and Pt (2.27 eV) were similar, Pt was slightly more stable. Therefore, the energy calculations for the adsorption stage do not explain the difference in reactivity between Rh and Pt.
Next, we obtained a free-energy diagram for the pyridine hydrogenation process to elucidate the key steps. Li et al. reported comprehensive calculations on the hydrogenation of pyridine adsorbed by its π-plane on Pt surfaces, and proposed a reaction sequence starting from N-hydrogenation. (37) Based on this report, we calculated the energy of each intermediate in the pyridine hydrogenation process on the Rh(0) surface. As shown in Figure 5a, steps 8 and 9 (desorption of piperidine, nonelectron transfer steps) and steps 5 and 6 (electron transfer steps) exhibit high energy barriers of 1.18 and 0.69 eV, respectively, at 0 V.

Figure 5

Figure 5. Summary of the computational simulations for the electrochemical hydrogenation of pyridine. (a) Energy diagram for the adsorption, 6e–/6H+ reduction of 1a, and desorption process of 2a over the Rh catalyst. The order of hydrogenation position in 1a was defined according to the reported system. (37) (b) Energy diagram for the desorption of 2a from the Rh (red) and Pt (black) surface.

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Notably, under applied potential conditions, the product or intermediate states are stabilized based on the concept of a computational hydrogen electrode (CHE). The chemical potential of a proton–electron pair G(H+ + e–) is equivalent to one-half of the chemical potential of gaseous hydrogen [0.5G(H2)] at 0 V vs a reversible hydrogen electrode and the energy–eU, where U is the external potential (eq 4).
G(H++e−)=0.5G(H2)−eU
(4)
When we computationally applied potentials of −0.69 and −1.0 V vs CHE (Figure 5a), most of the elementary steps became exergonic. The theoretical onset potential, which is defined here as the potential at which all electron-transfer steps become exergonic, was −0.69 V for pyridine hydrogenation on the Rh(0) surface.
In contrast to the electron-transfer steps, the desorption of piperidine, a nonelectron-transfer step, remained endergonic (1.18 eV). The calculated desorption barrier decreased from 1.18 eV (25 °C) to 0.98 eV (50 °C) (Figures S14 and S15), indicating that the reaction could be facilitated at elevated temperatures. When the desorption steps of piperidine on Rh and Pt were compared, Rh showed a lower desorption barrier (1.18 eV) than Pt (1.43 eV) (Figure 5b). These results suggest that Rh exhibits extremely high catalytic activity compared with Pt because of the smooth progression of the product desorption process.

Expansion of the Synthetic Utility of the Proposed System

Subsequently, the substrate scope was examined. Electrosynthesis is generally evaluated on the basis of two criteria: current efficiency and yield. Current efficiency is important when emphasizing the energy efficiency of production and economic costs associated with electricity. However, yield becomes more significant when synthesizing high-value-added compounds, such as active pharmaceutical ingredients. This study focused on yields assuming the use of the proposed process in pharmaceutical production. Data on the current efficiency for small charge amounts are compiled in Figures S16–S23.
Table 1 summarizes the scope and limitations of the proposed system. A slight excess charge was required for pyridines that were simply substituted with a methyl or ethyl group, but the target compounds were obtained in very high yields (1b–f). The reaction proceeded smoothly with lutidine (1f), and the cis isomer was obtained diastereoselectively. Both nicotinamide (1g) and isonicotinamide (1h) successfully afforded the corresponding piperidine derivatives. Although nicotinic acid methyl ester (1i) gave the hydrogenated product in 7% yield, nicotinic acid (1j) afforded the corresponding piperidine derivative (2j) in 44% yield. Hydrogenation of pyridines with a 4-CF3 (1k) or 2-Ph (1l) group were performed using methyl tert-butyl ether (MTBE) as cathodic solvent due to the solubility issue in water. Reaction using 1k and 1l afforded desired product in relatively lower yield, although an excess charge afforded the desired compounds. The lower yields for 2j,k could be attributed both to the nature of the substrate or the effect of the solvent. The reaction progress using 1l was sluggish but proceeded sufficiently at an elevated temperature (50 °C). These findings correspond to the implications of the computational results.
Table 1. Substrate scope for the electrocatalytic hydrogenation of various nitrogen-containing aromatic compounds in an AEM electrolyzer equipped with a Rh/KB catalysta
a

Experimental conditions: catholyte, 5 mL solution of 100 mM (1a–l, 1q–r) or 50 mM (1m–p) substrate in water (1a–j, 1q), water/THF = 1/1 in vol. (1r), or MTBE (1k–p); anolyte, air (for aqueous systems) or 10 mM KOH (for nonaqueous systems); current density, 25 mA cm–2; anode, DSE anode; temperature, 25 °C.

b

Determined by GC.

c

Determined by 1H NMR.

d

Determined by HPLC.

e

50 mA cm–2.

f

50 °C. SM = starting material.

The reaction also proceeded smoothly with quinoline derivatives, and the desired tetrahydroisoquinolines were obtained in high yields (1m–p). The electrocatalytic hydrogenation of quinoline via a fluorine-modified cobalt catalyst in an alkaline medium was recently reported, (38) but no reports on its application in electrolysis without a supporting electrolyte in the catholyte, as presented in this study, have been published. Our system was also applicable to pyrazines (1q), providing piperazines (2q) in high yield. Pyrazine is a redox-active molecule that produces 1,4-dihydropyrazine through a 2e–/2H+ reduction process, (39,40) but no other examples of the electrochemical synthesis of piperazine, a 6e–/6H+ reduction product, have been reported. In addition, we performed the hydrogenation of pyrrole (1r). Compared with pyridine, pyrrole features an electron-rich aromatic ring; thus, its electrochemical reduction is generally more challenging. To our delight, 1r was quantitatively reduced to pyrrolidine (2r) in our system. Tetrahydroquinoline, piperazine, and pyrrolidine are the skeletons found in many approved drugs. (7,8) These examples demonstrate that an AEM electrolyzer equipped with Rh/KB could not only enable the hydrogenation of a wide range of nitrogen-containing aromatic compounds but also synthesize cyclic amines in high yields under ambient conditions. To the best of our knowledge, this study is the first to report the electrochemical hydrogenation of pyrazine to piperazine and pyrrole to pyrrolidine.
Electrocatalytic hydrogenations were also performed using 2-vinylpyridine, 2-ethynylpyridine, and indole, respectively, under the following reaction conditions (Figure S30). For 2-vinylpyridine and 2-ethynylpyridine, hydrogenation of both aromatic ring and double or triple bond occurred simultaneously, and 2-ethylpyridine was obtained as the sole product in both cases. It is noteworthy that the current efficiency of 2-ethynylpyridine was much lower compared to 2-vinylpyridine or 2-ethylpyridine, presumably due to the weaker adsorption of the starting material. Electrochemical hydrogenation of indole scarcely proceeded under the conditions shown below. These experiments give deeper insights into the mechanism and limitations of the system.
Then, large-scale electrolysis was performed to demonstrate the scalability of the proposed system (Figure 6a,b). Large-scale electrolysis was performed using 800 mL of an aqueous solution containing 6.3 g (80 mmol) of 1a. Electrolysis was carried out at 25 mA cm–2 under circular-flow operation at a flow rate of 120 mL h–1. After 318 h of electrolysis, 5.3 g (63 mmol, 78% yield) 2a was obtained. Importantly, the remaining 20% of 1a was also confirmed, suggesting the absence of significant side reactions, except the HER. During electrolysis, the cathode potential remained in the range of −0.78 to −1.1 V vs SHE, and the cell voltage increased from 2.7 to 4.5 V. These results demonstrate that our system is extremely robust and suitable for the large-scale synthesis of nitrogen-containing aromatic compounds (Figure 6c).

Figure 6

Figure 6. Large-scale electrolysis of 1a using an AEM electrolyzer. (a) Schematic illustration and (b) photograph of the experimental setup. (c) Plot of the yield and current efficiency of 2a as a function of the reaction time.

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Conclusions

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In this paper, we reported the electrocatalytic hydrogenation of pyridines and other nitrogen-containing aromatic compounds via electrolytic reduction using an AEM electrolyzer. The Rh/KB catalyst exhibited extremely high activity, allowing for the electrolytic hydrogenation of pyridines in quantitative yields with up to 99% current efficiency in the preparative reaction. In situ XAFS measurements confirmed that the oxide layer on the surface of the Rh nanoparticle catalyst was deoxygenated under electrolytic conditions to expose the Rh(0) surface, which acted as an active electrocatalyst. Furthermore, first-principles calculations revealed that the desorption of the product, piperidine, was the rate-determining step, and that Rh showed lower energy than Pt. Substrate investigations demonstrated that this system can be applied to a variety of pyridine and quinoline analogs. This study also presents the first successful complete electrochemical hydrogenation of pyrazine and pyrrole. In large-scale trials, we successfully obtained 5.3 g of the hydrogenated product from 6.3 g of the starting material, with a yield of 78%. This result demonstrates the scalability of the proposed system. The large-scale electrolysis showed a slight increase in the cell voltage during the 318 h of electrolysis, plausibly due to the increased resistance at AEM. The development of AEM is advancing rapidly with their use in water electrolysis and CO2 electrolysis. (32,41,42) Implementing advanced AEM, or more ideally, developing AEM specifically for organic electrosynthesis, could overcome the problem observed in this work.
Cyclic amines are an important group of compounds found in many pharmaceuticals, and their direct synthesis involves the reduction of nitrogen-containing aromatic compounds. Several methods for the thermocatalytic hydrogenation of nitrogen-containing compounds has been reported; however, such processes require high temperatures, high pressures, and additives such as acids. Very few reports on the electrochemical reduction of nitrogen-containing aromatic compounds have been published; specifically, no methodology for the electrochemical reduction of pyridine to piperidine has yet been introduced. This paper proposes the first practical electrochemical system that appropriately utilizes the basic reaction field of an AEM electrolyzer to reduce nitrogen-containing aromatic compounds, thus contributing to energy savings and waste reduction in the synthesis of pharmaceutical precursors. Further development of the current system, including an AEM electrolytic system using earth-abundant metals and application to real pharmaceutical precursors, is undergoing in our laboratory.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c09107.

  • General methods, description of the electrolyzer and experimental procedure, details of computation, chlormatgraphy data, and 1H NMR spectra (PDF)

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Author Information

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  • Corresponding Authors
    • Naoki Shida - Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan;  Institute of Advanced Sciences, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan;  PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan;  Orcidhttps://orcid.org/0000-0003-0586-1216;  Email: [email protected]
    • Mahito Atobe - Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan;  Institute of Advanced Sciences, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan;  Orcidhttps://orcid.org/0000-0002-3173-3608;  Email: [email protected]
  • Authors
    • Yugo Shimizu - Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
    • Akizumi Yonezawa - Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
    • Juri Harada - Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
    • Yuka Furutani - Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
    • Yusuke Muto - Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
    • Ryo Kurihara - Research Center for Solar Energy Chemistry, Graduate School of Engineering Science, Osaka University, 1−3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
    • Junko N. Kondo - Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 225-8503, Japan;  Orcidhttps://orcid.org/0000-0002-7940-1266
    • Eisuke Sato - Division of Applied Chemistry, Graduate School of Environmental, Life, Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan;  Orcidhttps://orcid.org/0000-0001-6784-138X
    • Koichi Mitsudo - Division of Applied Chemistry, Graduate School of Environmental, Life, Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan
    • Seiji Suga - Division of Applied Chemistry, Graduate School of Environmental, Life, Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan;  Orcidhttps://orcid.org/0000-0003-0635-2077
    • Shoji Iguchi - Graduate School of Engineering, Kyoto University, Kyoto daigaku-katsura, Nishikyo-ku, Kyoto 615-8530, Japan;  Orcidhttps://orcid.org/0000-0002-2522-1358
    • Kazuhide Kamiya - Research Center for Solar Energy Chemistry, Graduate School of Engineering Science, Osaka University, 1−3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan;  Innovative Catalysis Science Division, Institute for Open and Transdisciplinary Research Initiatives (ICS-OTRI), Osaka University, Suita, Osaka 565-0871, Japan;  Orcidhttps://orcid.org/0000-0002-6018-9277
  • Funding

    This work was financially supported by CREST (JST Grant No. JP65R1204400), PRESTO (JST Grant No. JPMJPR2373), and JSPS KAKENHI Grant Numbers 21H05215 (Digi-TOS), 23H04916 (Green Catalysis Science), 24H00394, Japan.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We thank Ms. Kaneda (Instrumental Analysis Center, YNU) for providing technical support for the TEM measurements. XRD, XPS, and NMR measurements were performed at the Instrumental Analysis Center (YNU). The XAFS measurements were performed at the BL14B2 beamline of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2023B1663). DFT calculations were achieved through the use of SQUID at the Cybermedia Center, Osaka University.

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    ACS Energy Letters (2022), 7 (4), 1322-1329CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    Conventional hydrogen sepns. from reformed hydrocarbons often deploy a water gas shift (WGS) reactor to convert CO to CO2, followed by adsorption processes to achieve pure hydrogen. The purified hydrogen is then fed to a compressor to deliver hydrogen at high pressures. Electrochem. hydrogen pumps (EHPs) featuring proton-selective polymer electrolyte membranes (PEMs) represent an alternative sepn. platform with fewer unit operations because they can simultaneously sep. and compress hydrogen continuously. In this work, a high-temp. PEM (HT-PEM) EHP purified hydrogen to 99.3%, with > 85% hydrogen recovery for feed mixts. contg. 25-40% CO. The ion-pair HT-PEM and phosphonic acid ionomer binder enabled the EHP to be operated in the temp. range from 160-220°. The ability to operate the EHP at an elevated temp. allowed the EHP to purify hydrogen from gas feeds with large CO contents at 1 A cm-2. Finally, the EHP with the said materials displayed a small performance loss of 12μV h-1 for purifying hydrogen from syngas for 100 h at 200°.
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    Designing anion exchange membranes for CO2 electrolysers
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    A review. New technologies are required to electrocatalytically convert carbon dioxide (CO2) into fuels and chems. at near-ambient temps. and pressures more effectively. One particular challenge is mediating the electrochem. CO2 redn. reaction (CO2RR) at low cell voltages while maintaining high conversion efficiencies. Anion exchange membranes (AEMs) in zero-gap reactors offer promise in this direction; however, there remain substantial obstacles to be overcome in tailoring the membranes and other cell components to the requirements of CO2RR systems. Here we recent advances, and remaining challenges, in AEM materials and devices for CO2RR. We discuss the principles underpinning AEM operation and the properties desired for CO2RR, in addn. to ing state-of-the-art AEMs in CO2 electrolyzers. We close with future design strategies to minimize product crossover, improve mech. and chem. stability, and overcome the energy losses assocd. with the use of AEMs for CO2RR systems.
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  • Abstract

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    Figure 1

    Figure 1. Synthesis of piperidines via the hydrogenation of pyridines. (a) Examples of FDA-approved drugs containing the piperidine skeleton. (b) Thermal hydrogenation of pyridine to piperidine using a heterogeneous catalyst. (c) Electrochemical reduction of pyridines in a conventional electrolytic system via a radical anion intermediate. (d) Concept of this work.

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    Figure 2

    Figure 2. Overview of the electrochemical hydrogenation of pyridine (1a) to piperidine (2a) in an AEM electrolyzer. (a) Schematic illustration and (b) detailed components of the AEM electrolyzer. (c) Enlarged image of the catalyst layer and triple-phase boundary. (d) Energy diagram of the cathodic and anodic reactions. (e) Plot of the current efficiency and cathodic potential for the electrochemical hydrogenation of 1a under various current densities. Electrolysis was conducted under single-flow operation at a flow rate of 12.4 mL h–1 in an AEM electrolyzer equipped with a Rh/KB. (f) Preparative electrocatalytic hydrogenation of 1a under circular-flow operation at a flow rate of 120 mL h–1. (g) Repeated preparative electrocatalytic hydrogenation of 1a under circular-flow operation at a flow rate of 120 mL h–1. (h) Comparison of various metal catalysts under circular-flow operation using MTBE as a cathodic solvent.

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    Figure 3

    Figure 3. Electrochemical measurements. Comparison of the CVs recorded in the (a) AEM and (b) PEM electrolyzer at a scan rate of 10 mV s–1. The measurements were performed in the absence (black) and presence (red or blue) of 1a.

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    Figure 4

    Figure 4. In situ XAFS measurements. (a) Schematic illustration of the AEM electrolyzer with a transmitting window for X-ray. (b) XANES spectra obtained during 640 s of electrolysis using H2O as the catholyte. The XANES spectrum of Rh foil is also shown as a reference.

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    Figure 5

    Figure 5. Summary of the computational simulations for the electrochemical hydrogenation of pyridine. (a) Energy diagram for the adsorption, 6e–/6H+ reduction of 1a, and desorption process of 2a over the Rh catalyst. The order of hydrogenation position in 1a was defined according to the reported system. (37) (b) Energy diagram for the desorption of 2a from the Rh (red) and Pt (black) surface.

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    Figure 6

    Figure 6. Large-scale electrolysis of 1a using an AEM electrolyzer. (a) Schematic illustration and (b) photograph of the experimental setup. (c) Plot of the yield and current efficiency of 2a as a function of the reaction time.

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      Vitaku, E.; Smith, D. T.; Njardarson, J. T. Analysis of the Structural Diversity, Substitution Patterns, and Frequency of Nitrogen Heterocycles among U.S. FDA Approved Pharmaceuticals. J. Med. Chem. 2014, 57 (24), 10257– 10274,  DOI: 10.1021/jm501100b
      7
      Analysis of the Structural Diversity, Substitution Patterns, and Frequency of Nitrogen Heterocycles among U.S. FDA Approved Pharmaceuticals
      Vitaku, Edon; Smith, David T.; Njardarson, Jon T.
      Journal of Medicinal Chemistry (2014), 57 (24), 10257-10274CODEN: JMCMAR; ISSN:0022-2623. (American Chemical Society)
      A review. Nitrogen heterocycles are among the most significant structural components of pharmaceuticals. Anal. of our database of U.S. FDA approved drugs reveals that 59% of unique small-mol. drugs contain a nitrogen heterocycle. In this review we report on the top 25 most commonly utilized nitrogen heterocycles found in pharmaceuticals. The main part of our anal. is divided into seven sections: (1) three- and four-membered heterocycles, (2) five-, (3) six-, and (4) seven- and eight-membered heterocycles, as well as (5) fused, (6) bridged bicyclic, and (7) macrocyclic nitrogen heterocycles. Each section reveals the top nitrogen heterocyclic structures and their relative impact for that ring type. For the most commonly used nitrogen heterocycles, we report detailed substitution patterns, highlight common architectural cores, and discuss unusual or rare structures.
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    8. 8
      Bhutani, P.; Joshi, G.; Raja, N.; Bachhav, N.; Rajanna, P. K.; Bhutani, H.; Paul, A. T.; Kumar, R. U.S. Fda Approved Drugs from 2015–June 2020: A Perspective. J. Med. Chem. 2021, 64 (5), 2339– 2381,  DOI: 10.1021/acs.jmedchem.0c01786
      8
      U.S. FDA Approved Drugs from 2015-June 2020: A Perspective
      Bhutani, Priyadeep; Joshi, Gaurav; Raja, Nivethitha; Bachhav, Namrata; Rajanna, Prabhakar K.; Bhutani, Hemant; Paul, Atish T.; Kumar, Raj
      Journal of Medicinal Chemistry (2021), 64 (5), 2339-2381CODEN: JMCMAR; ISSN:0022-2623. (American Chemical Society)
      A review. Perspective. In the present work, we report compilation and anal. of 245 drugs, including small and macromols. approved by the U.S. FDA from 2015 until June 2020. Nearly 29% of the drugs were approved for the treatment of various types of cancers. Other major therapeutic areas of focus were infectious diseases (14%); neurol. conditions (12%); and genetic, metabolic, and cardiovascular disorders (7-8% each). Itemization of the approved drugs according to the year of approval, sponsor, target, chem. class, major drug-metabolizing enzyme(s), route of administration/elimination, and drug-drug interaction liability (perpetrator or/and victim) is presented and discussed. An effort has been made to analyze the pharmacophores to identity the structural (e.g., arom., heterocycle, and aliph.), elemental (e.g., boron, sulfur, fluorine, phosphorus, and deuterium), and functional group (e.g., nitro drugs) diversity among the approved drugs. Further, descriptor-based chem. space anal. of FDA approved drugs and several strategies utilized for optimizing metab. leading to their discoveries have been emphasized. Finally, an anal. of drug-likeness for the approved drugs is presented.
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    9. 9
      Ratovelomanana-Vidal, V.; Phansavath, P.; Ayad, T.; Vitale, M. R. 8.21 Partial and Complete Reduction of Pyridine and Their Benzo Analogs. In Comprehensive Organic Synthesis II; Elsevier, 2014; pp 741– 793.
      There is no corresponding record for this reference.
    10. 10
      Wiesenfeldt, M. P.; Nairoukh, Z.; Dalton, T.; Glorius, F. Selective Arene Hydrogenation for Direct Access to Saturated Carbo- and Heterocycles. Angew. Chem., Int. Ed. 2019, 58 (31), 10460– 10476,  DOI: 10.1002/anie.201814471
      10
      Selective Arene Hydrogenation for Direct Access to Saturated Carbo- and Heterocycles
      Wiesenfeldt, Mario P.; Nairoukh, Zackaria; Dalton, Toryn; Glorius, Frank
      Angewandte Chemie, International Edition (2019), 58 (31), 10460-10476CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review on selective arene hydrogenation for direct synthesis of satd. carbo- and heterocycles is presented. Arene hydrogenation provides direct access to satd. carbo- and heterocycles and thus its strategic application may be used to shorten synthetic routes. This powerful transformation is widely applied in industry and is expected to facilitate major breakthroughs in the applied sciences. The ability to overcome aromaticity while controlling diastereo-, enantio-, and chemoselectivity is central to the use of hydrogenation in the prepn. of complex mols. In general, the hydrogenation of multisubstituted arenes yields predominantly the cis isomer. Enantiocontrol is imparted by chiral auxiliaries, Bronsted acids, or transition-metal catalysts. Recent studies have demonstrated that highly chemoselective transformations are possible. Such methods and the underlying strategies are reviewed herein, with an emphasis on synthetically useful examples that employ readily available catalysts.
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    11. 11
      Kim, A. N.; Stoltz, B. M. Recent Advances in Homogeneous Catalysts for the Asymmetric Hydrogenation of Heteroarenes. ACS Catal. 2020, 10 (23), 13834– 13851,  DOI: 10.1021/acscatal.0c03958
      11
      Recent Advances in Homogeneous Catalysts for the Asymmetric Hydrogenation of Heteroarenes
      Kim, Alexia N.; Stoltz, Brian M.
      ACS Catalysis (2020), 10 (23), 13834-13851CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      A review. The asym. hydrogenation of heteroarenes has recently emerged as an effective strategy for the direct access to enantioenriched, satd. heterocycles. Although several homogeneous catalyst systems have been extensively developed for the hydrogenation of heteroarenes with high levels of chemo- and stereoselectivity, the development of mild conditions that allow for efficient and stereoselective hydrogenation of a broad range of substrates remains a challenge. This Perspective highlights recent advances in homogeneous catalysis of heteroarene hydrogenation as inspiration for the further development of asym. hydrogenation catalysts, and addresses underdeveloped areas and limitations of the current technol.
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    12. 12
      Gunasekar, R.; Goodyear, R. L.; Silvestri, I. P.; Xiao, J. Recent Developments in Enantio- and Diastereoselective Hydrogenation of N-Heteroaromatic Compounds. Org. Biomol. Chem. 2022, 20 (9), 1794– 1827,  DOI: 10.1039/D1OB02331D
      There is no corresponding record for this reference.
    13. 13
      Cheng, C.; Xu, J.; Zhu, R.; Xing, L.; Wang, X.; Hu, Y. A Highly Efficient Pd–C Catalytic Hydrogenation of Pyridine Nucleus under Mild Conditions. Tetrahedron 2009, 65 (41), 8538– 8541,  DOI: 10.1016/j.tet.2009.08.011
      There is no corresponding record for this reference.
    14. 14
      Irfan, M.; Petricci, E.; Glasnov, T. N.; Taddei, M.; Kappe, C. O. Continuous Flow Hydrogenation of Functionalized Pyridines. Eur. J. Org. Chem. 2009, 2009 (9), 1327– 1334,  DOI: 10.1002/ejoc.200801131
      There is no corresponding record for this reference.
    15. 15
      Lückemeier, L.; Pierau, M.; Glorius, F. Asymmetric Arene Hydrogenation: Towards Sustainability and Application. Chem. Soc. Rev. 2023, 52 (15), 4996– 5012,  DOI: 10.1039/D3CS00329A
      There is no corresponding record for this reference.
    16. 16
      Lyons, T. W.; Leibler, I. N.-M.; He, C. Q.; Gadamsetty, S.; Estrada, G. J.; Doyle, A. G. Broad Survey of Selectivity in the Heterogeneous Hydrogenation of Heterocycles. J. Org. Chem. 2024, 89 (3), 1438– 1445,  DOI: 10.1021/acs.joc.3c02028
      There is no corresponding record for this reference.
    17. 17
      Tanaka, N.; Usuki, T. Can Heteroarenes/Arenes Be Hydrogenated over Catalytic Pd/C under Ambient Conditions?. Eur. J. Org. Chem. 2020, 2020 (34), 5514– 5522,  DOI: 10.1002/ejoc.202000695
      17
      Can Heteroarenes/Arenes Be Hydrogenated Over Catalytic Pd/C Under Ambient Conditions?
      Tanaka, Nao; Usuki, Toyonobu
      European Journal of Organic Chemistry (2020), 2020 (34), 5514-5522CODEN: EJOCFK; ISSN:1099-0690. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Hydrogenation of over a dozen arom. compds., including both heteroarenes and arenes, over palladium on carbon (Pd/C, 1-100 mol%) with H2-balloon pressure at room temp. is reported. Analyses using pyridine as a model substrate revealed that acetic acid was the best solvent, as using only 1 mol% Pd/C provided piperidine quant. Substrate scope anal. and d. functional theory calcns. indicated that reaction rates are highly dependent on frontier MO characteristics and the steric bulkiness of substituents. Moreover, the established method was used for the concise synthesis of the anti-Alzheimer drug donepezil (Aricept).
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    18. 18
      Murugesan, K.; Chandrashekhar, V. G.; Kreyenschulte, C.; Beller, M.; Jagadeesh, R. V. A General Catalyst Based on Cobalt Core–Shell Nanoparticles for the Hydrogenation of N-heteroarenes Including Pyridines. Angew. Chem., Int. Ed. 2020, 59 (40), 17408– 17412,  DOI: 10.1002/anie.202004674
      There is no corresponding record for this reference.
    19. 19
      Wei, Z.; Shao, F.; Wang, J. Recent Advances in Heterogeneous Catalytic Hydrogenation and Dehydrogenation of N-Heterocycles. Chin. J. Catalysis 2019, 40 (7), 980– 1002,  DOI: 10.1016/S1872-2067(19)63336-X
      There is no corresponding record for this reference.
    20. 20
      Hamilton, T. S.; Adams, R. Reduction of pyridine hydrochloride and pyridonium salts by means of hydrogen and platinum-oxide platinum black. XVIII1. J. Am. Chem. Soc. 1928, 50 (8), 2260– 2263,  DOI: 10.1021/ja01395a028
      There is no corresponding record for this reference.
    21. 21
      Chen, F.; Li, W.; Sahoo, B.; Kreyenschulte, C.; Agostini, G.; Lund, H.; Junge, K.; Beller, M. Hydrogenation of Pyridines Using a Nitrogen-modified Titania-supported Cobalt Catalyst. Angew. Chem. 2018, 130 (44), 14696– 14700,  DOI: 10.1002/ange.201803426
      There is no corresponding record for this reference.
    22. 22
      Qian, W.; Lin, L.; Qiao, Y.; Zhao, X.; Xu, Z.; Gong, H.; Li, D.; Chen, M.; Huang, R.; Hou, Z. Ru Subnanoparticles on N-Doped Carbon Layer Coated SBA-15 as Efficient Catalysts for Arene Hydrogenation. Appl. Catal., A 2019, 585, 117183  DOI: 10.1016/j.apcata.2019.117183
      22
      Ru subnanoparticles on N-doped carbon layer coated SBA-15 as efficient Catalysts for arene hydrogenation
      Qian, Wei; Lin, Lina; Qiao, Yunxiang; Zhao, Xiuge; Xu, Zichen; Gong, Honghui; Li, Difan; Chen, Manyu; Huang, Rong; Hou, Zhenshan
      Applied Catalysis, A: General (2019), 585 (), 117183CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
      The N-doped carbon layer coated SBA-15 support has been accomplished via a pyrolysis process. The ultra-low loading Ru nanoparticles (ca. 0.1 wt.%) was incorporated into the support by impregnation and the sequential redn. The images of HAADF-STEM revealed that the Ru particles with sub-1-nm size (0.2-0.7 nm) were uniformly dispersed on the support. The ultrafine Ru particles displayed the excellent activity for the hydrogenation of olefins, arenes, phenol derivs. and heteroarenes in aq. phase. The aliph. or alicyclic compds. were produced selectively without the hydrogenolysis of C-O and C-N bonds. The high turnover frequency (TOF) values can reach up to 10,000 h-1. Notably, the activity of these catalysts improved dramatically with decreasing the sizes of Ru particles. Meanwhile, the N-doped carbon layer coating endowed the high stability of the Ru catalysts and prevented the leaching of the Ru species owning to the strong interaction between doped-N atoms and the ultrafine Ru particles. Overall, this work provides a highly attractive strategy to construct the supported sub-1-nm Ru particles utilized for the aq. hydrogenation.
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    23. 23
      Martinez-Espinar, F.; Blondeau, P.; Nolis, P.; Chaudret, B.; Claver, C.; Castillón, S.; Godard, C. NHC-Stabilised Rh Nanoparticles: Surface Study and Application in the Catalytic Hydrogenation of Aromatic Substrates. J. Catal. 2017, 354, 113– 127,  DOI: 10.1016/j.jcat.2017.08.010
      23
      NHC-stabilised Rh nanoparticles: Surface study and application in the catalytic hydrogenation of aromatic substrates
      Martinez-Espinar, Francisco; Blondeau, Pascal; Nolis, Pau; Chaudret, Bruno; Claver, Carmen; Castillon, Sergio; Godard, Cyril
      Journal of Catalysis (2017), 354 (), 113-127CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      New Rh-nanoparticles (NPs) stabilized by N-Heterocyclic Carbenes (NHC) were synthesized by decompn. of [Rh(η3-C3H5)3] under H2 atmosphere and fully characterized. Surface studies by FT-IR and NMR spectroscopy employing isotopically labeled ligands were also performed. The Rh0.2 NPs are active catalysts in the redn. of various arom. substrates. In the redn. of phenol, high selectivities to cyclohexanone or cyclohexanol were obtained depending on the reaction conditions. However, this catalytic system exhibited much lower activity in the hydrogenation of substituted phenols. Pyridine was easily hydrogenated under mild conditions and interestingly, the hydrogenation of 4-Me and 4-trifluoromethylpyridine resulted slower than that of 2-methylpyridine. The hydrogenation of 1-(pyridin-2-yl)propan-2-one provided the β-enaminone in high yield as a consequence of the partial redn. of the pyridine ring followed by isomerization. Quinoline could be either partially hydrogenated to 1,2,3,4-tetrahydroquinoline or fully reduced to decahydroquinoline by adjusting the reaction conditions.
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    24. 24
      Wismann, S. T.; Engbæk, J. S.; Vendelbo, S. B.; Bendixen, F. B.; Eriksen, W. L.; Aasberg-Petersen, K.; Frandsen, C.; Chorkendorff, I.; Mortensen, P. M. Electrified Methane Reforming: A Compact Approach to Greener Industrial Hydrogen Production. Science 2019, 364 (6442), 756– 759,  DOI: 10.1126/science.aaw8775
      24
      Electrified methane reforming: A compact approach to greener industrial hydrogen production
      Wismann, Sebastian T.; Engbaek, Jakob S.; Vendelbo, Soren B.; Bendixen, Flemming B.; Eriksen, Winnie L.; Aasberg-Petersen, Kim; Frandsen, Cathrine; Chorkendorff, Ib; Mortensen, Peter M.
      Science (Washington, DC, United States) (2019), 364 (6442), 756-759CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Large-scale prodn. of hydrogen through steam reforming directly produces CO2 as a side product. In addn., the heating of reactors through fossil-fuel burning contributes further CO2 emissions. One problem is that the catalyst bed is heated unevenly, which renders much of the catalyst effectively inactive. Wismann et al. describe an elec. heating scheme for a metal tube reactor that improves the uniformity of heating and catalyst usage (see the Perspective by Van Geem et al.). Adoption of this alternative approach could affect CO2 emissions by up to approx. 1% of global emissions. Science, this issue p. 756; see also p.734.
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    25. 25
      Venugopalan, G.; Bhattacharya, D.; Andrews, E.; Briceno-Mena, L.; Romagnoli, J.; Flake, J.; Arges, C. G. Electrochemical Pumping for Challenging Hydrogen Separations. ACS Energy Lett. 2022, 7 (4), 1322– 1329,  DOI: 10.1021/acsenergylett.1c02853
      25
      Electrochemical Pumping for Challenging Hydrogen Separations
      Venugopalan, Gokul; Bhattacharya, Deepra; Andrews, Evan; Briceno-Mena, Luis; Romagnoli, Jose; Flake, John; Arges, Christopher G.
      ACS Energy Letters (2022), 7 (4), 1322-1329CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      Conventional hydrogen sepns. from reformed hydrocarbons often deploy a water gas shift (WGS) reactor to convert CO to CO2, followed by adsorption processes to achieve pure hydrogen. The purified hydrogen is then fed to a compressor to deliver hydrogen at high pressures. Electrochem. hydrogen pumps (EHPs) featuring proton-selective polymer electrolyte membranes (PEMs) represent an alternative sepn. platform with fewer unit operations because they can simultaneously sep. and compress hydrogen continuously. In this work, a high-temp. PEM (HT-PEM) EHP purified hydrogen to 99.3%, with > 85% hydrogen recovery for feed mixts. contg. 25-40% CO. The ion-pair HT-PEM and phosphonic acid ionomer binder enabled the EHP to be operated in the temp. range from 160-220°. The ability to operate the EHP at an elevated temp. allowed the EHP to purify hydrogen from gas feeds with large CO contents at 1 A cm-2. Finally, the EHP with the said materials displayed a small performance loss of 12μV h-1 for purifying hydrogen from syngas for 100 h at 200°.
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    26. 26
      Lund, H. Electrolysis of N-Heterocyclic Compounds. In Advances in Heterocyclic Chemistry; Elsevier, 1970; Vol. 12, pp 213– 316.
      There is no corresponding record for this reference.
    27. 27
      Lund, H.; Tabakovic, I. Electrolysis of N-Heterocyclic Compounds (Part II). In Advances in Heterocyclic Chemistry; Elsevier, 1984; pp 235– 341.
      There is no corresponding record for this reference.
    28. 28
      Cisak, A.; Elving, P. J. Electrochemistry in Pyridine-IV. Chemical and Electrochemical Reduction of Pyridine. Electrochim. Acta 1965, 10 (9), 935– 946,  DOI: 10.1016/0013-4686(65)80005-6
      There is no corresponding record for this reference.
    29. 29
      Keay, J. G. Partial and Complete Reduction of Pyridines and Their Benzo Analogs. In Comprehensive Organic Synthesis; Elsevier, 1991; pp 579– 602.
      There is no corresponding record for this reference.
    30. 30
      Kronawitter, C. X.; Chen, Z.; Zhao, P.; Yang, X.; Koel, B. E. Electrocatalytic Hydrogenation of Pyridinium Enabled by Surface Proton Transfer Reactions. Catal. Sci. Technol. 2017, 7 (4), 831– 837,  DOI: 10.1039/C6CY02487D
      30
      Electrocatalytic hydrogenation of pyridinium enabled by surface proton transfer reactions
      Kronawitter, C. X.; Chen, Z.; Zhao, P.; Yang, X.; Koel, B. E.
      Catalysis Science & Technology (2017), 7 (4), 831-837CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      Pyridinium is hydrogenated at Pt electrodes in electrochem. conditions consistent with those previously shown to yield selective redn. of carbon dioxide to methanol and formic acid. The hydrogenation proceeds through a heterogeneous reaction with chemisorbed hydrogen, which originates from 1-electron surface proton transfer reactions. Electrochem. methods are used to show that pyridinium adsorbs on the Pt surface, consistent with the proposed heterogeneous reaction mechanism. From this first observation of the electrochem. generation of a stable hydrogenated piperidinium-like near-surface species it logically follows that dihydropyridinium, the protonated form of the previously-proposed hydride-shuttling redn. catalyst, must transiently exist under these conditions near the Pt surface in the presence of carbon dioxide. Therefore partially hydrogenated heterocycles remain strong candidates for catalytically active species that enable selective carbon dioxide redn. More generally, the obsd. mild potentials required for electrocatalytic hydrogenation of stable orgs. implies that engineered transfer hydrogenations involving org. adsorbates can be a viable approach for achieving selective carbon dioxide redn. to fuels.
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    31. 31
      Olu, P.-Y.; Li, Q.; Krischer, K. The True Fate of Pyridinium in the Reportedly Pyridinium-catalyzed Carbon Dioxide Electroreduction on Platinum. Angew. Chem., Int. Ed. 2018, 57 (45), 14769– 14772,  DOI: 10.1002/anie.201808122
      There is no corresponding record for this reference.
    32. 32
      Du, N.; Roy, C.; Peach, R.; Turnbull, M.; Thiele, S.; Bock, C. Anion-Exchange Membrane Water Electrolyzers. Chem. Rev. 2022, 122 (13), 11830– 11895,  DOI: 10.1021/acs.chemrev.1c00854
      32
      Anion-Exchange Membrane Water Electrolyzers
      Du, Naiying; Roy, Claudie; Peach, Retha; Turnbull, Matthew; Thiele, Simon; Bock, Christina
      Chemical Reviews (Washington, DC, United States) (2022), 122 (13), 11830-11895CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. This Review provides an overview of the emerging concepts of catalysts, membranes, and membrane electrode assemblies (MEAs) for water electrolyzers with anion-exchange membranes (AEMs), also known as zero-gap alk. water electrolyzers. Much of the recent progress is due to improvements in materials chem., MEA designs, and optimized operation conditions. Research on anion-exchange polymers (AEPs) has focused on the cationic head/backbone/side-chain structures and key properties such as ionic cond. and alk. stability. Several approaches, such as crosslinking, microphase, and org./inorg. composites, have been proposed to improve the anion-exchange performance and the chem. and mech. stability of AEMs. Numerous AEMs now exceed values of 0.1 S/cm (at 60-80°C), although the stability specifically at temps. exceeding 60°C needs further enhancement. The oxygen evolution reaction (OER) is still a limiting factor. An anal. of thin-layer OER data suggests that NiFe-type catalysts have the highest activity. There is debate on the active-site mechanism of the NiFe catalysts, and their long-term stability needs to be understood. Addn. of Co to NiFe increases the cond. of these catalysts. The same anal. for the hydrogen evolution reaction (HER) shows carbon-supported Pt to be dominating, although PtNi alloys and clusters of Ni(OH)2 on Pt show competitive activities. Recent advances in forming and embedding well-dispersed Ru nanoparticles on functionalized high-surface-area carbon supports show promising HER activities. However, the stability of these catalysts under actual AEMWE operating conditions needs to be proven. The field is advancing rapidly but could benefit through the adaptation of new in situ techniques, standardized evaluation protocols for AEMWE conditions, and innovative catalyst-structure designs. Nevertheless, single AEM water electrolyzer cells have been operated for several thousand hours at temps. and current densities as high as 60°C and 1 A/cm2, resp.
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    33. 33
      Wiranarongkorn, K.; Eamsiri, K.; Chen, Y.-S.; Arpornwichanop, A. A Comprehensive Review of Electrochemical Reduction of CO2 to Methanol: Technical and Design Aspects. J. CO2 Util. 2023, 71 (1), 102477  DOI: 10.1016/j.jcou.2023.102477
      There is no corresponding record for this reference.
    34. 34
      Ido, Y.; Fukazawa, A.; Furutani, Y.; Sato, Y.; Shida, N.; Atobe, M. Triple-phase Boundary in Anion-exchange Membrane Reactor Enables Selective Electrosynthesis of Aldehyde from Primary Alcohol. ChemSusChem 2021, 14 (24), 5405– 5409,  DOI: 10.1002/cssc.202102076
      There is no corresponding record for this reference.
    35. 35
      Shida, N.; Atobe, M.; Ido, Y.; Shimizu, Y. Comparative Investigation of Electrocatalytic Oxidation of Cyclohexene by Proton-Exchange Membrane and Anion-Exchange Membrane Electrolyzers. Synthesis 2023, 55 (18), 2979– 2984,  DOI: 10.1055/a-2000-8231
      There is no corresponding record for this reference.
    36. 36
      Atobe, M.; Shida, N. Organic Electrosynthetic Processes Using Solid Polymer Electrolyte Reactor. Curr. Opin. Electrochem. 2024, 44, 101440  DOI: 10.1016/j.coelec.2024.101440
      There is no corresponding record for this reference.
    37. 37
      Liu, J.; Li, W.-Y.; Feng, J.; Gao, X. Molecular Insights into the Hydrodenitrogenation Mechanism of Pyridine over Pt/γ-Al2O3 Catalysts. Mol. Catal. 2020, 495, 111148  DOI: 10.1016/j.mcat.2020.111148
      37
      Molecular insights into the hydrodenitrogenation mechanism of pyridine over Pt/γ-Al2O3 catalysts
      Liu, Juan; Li, Wen-Ying; Feng, Jie; Gao, Xiang
      Molecular Catalysis (2020), 495 (), 111148CODEN: MCOADH ISSN:. (Elsevier B.V.)
      The hydrodenitrogenation of nitrogen-contg. heterocycles over noble metals has fundamental importance for energy and environmental science. To develop more efficient catalyst, mechanistic investigation has been conducted with a method combining in situ Fourier transformation IR expts. and d. functional theory calcns. in this work. The in situ expts. indicate flatly adsorbed pyridine mols. convert to pyridinium and α-pyridyl species at higher temp. on metallic Pt of Pt/γ-Al2O3 catalysts. Pyridine hydrogenation distinctly takes place at 150°C with the appearance of methylene stretching vibrations, and a stepwise mechanism is identified as the temp. further increases. The adsorption, hydrogenation and hydrogenolysis of pyridine on Pt are studied in detail by theor. calcns. In line with these findings, the geometry optimization confirms pyridine preferentially adsorbs on Pt(111) and Pt(211) both in a parallel configuration. Based on the Langmuir-Hinshelwood mechanism, the results show successive hydrogenation markedly lowers the energy barrier for subsequent hydrogenolysis. The C-N bond cleavage occurs via nucleophilic attack of pentahydropyridine, rather than piperidine, which dets. the reaction products, including piperidine, n-pentylamine and n-pentane. The comparative study reveals both hydrogenation and hydrogenolysis are kinetically and thermodynamically more competitive on Pt(211) than Pt(111). Esp. for hydrogenolysis, the coordinatively unsatd. Pt step atoms play an essential role in C-N bond cleavage. Thus, hydrogenolysis is more geometric-dependent than hydrogenation. This provides instructive information for the design of catalysts with adjustable product selectivity.
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    38. 38
      Guo, S.; Wu, Y.; Wang, C.; Gao, Y.; Li, M.; Zhang, B.; Liu, C. Electrocatalytic Hydrogenation of Quinolines with Water over a Fluorine-Modified Cobalt Catalyst. Nat. Commun. 2022, 13 (1), 5297  DOI: 10.1038/s41467-022-32933-6
      There is no corresponding record for this reference.
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      Klatt, L. N.; Rouseff, R. L. Electrochemical Reduction of Pyrazine in Aqueous Media. J. Am. Chem. Soc. 1972, 94 (21), 7295– 7304,  DOI: 10.1021/ja00776a009
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      Brolo, A. G.; Irish, D. E. SERS Study of the Electrochemical Reduction of Pyrazine on a Silver Electrode. J. Chem. Soc., Faraday Trans. 1997, 93 (3), 419– 423,  DOI: 10.1039/a605416a
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    41. 41
      Yang, Y.; Li, P.; Zheng, X.; Sun, W.; Dou, S. X.; Ma, T.; Pan, H. Anion-Exchange Membrane Water Electrolyzers and Fuel Cells. Chem. Soc. Rev. 2022, 51 (23), 9620– 9693,  DOI: 10.1039/D2CS00038E
      41
      Anion-exchange membrane water electrolyzers and fuel cells
      Yang, Yaxiong; Li, Peng; Zheng, Xiaobo; Sun, Wenping; Dou, Shi Xue; Ma, Tianyi; Pan, Hongge
      Chemical Society Reviews (2022), 51 (23), 9620-9693CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Anion-exchange membrane (AEM) water electrolyzers (AEMWEs) and fuel cells (AEMFCs) are technologies that, resp., achieve transformation and utilization of renewable resources in the form of green hydrogen (H2) energy. The significantly reduced cost of their key components (membranes, electrocatalysts, bipolar plates, etc.), quick reaction kinetics, and fewer corrosion problems endow AEM water electrolyzers and fuel cells with overwhelming superiority over their conventional counterparts (e.g., proton-exchange membrane water electrolyzer/fuel cells and alk. water electrolyzer/fuel cells). Limitations in our fundamental understanding of AEM devices, however, specifically in key components, working management, and operation monitoring, restrict the improvement of cell performance, and they further impede the deployment of AEM water electrolyzers and fuel cells. Therefore, a panoramic view to outline the fundamentals, technol. progress, and future perspectives on AEMWEs and AEMFCs is presented. The objective of this review is to (1) present a timely overview of the market development status of green hydrogen technol. that is closely assocd. with AEMWEs (hydrogen prodn.) and AEMFCs (hydrogen utilization); (2) provide an in-depth and comprehensive anal. of AEMWEs and AEMFCs from the viewpoint of all key components (e.g., membranes, ionomers, catalysts, gas diffusion layers, bipolar plates, and membrane electrode assembly (MEA)); (3) summarize the state-of-the-art technologies for working management of AEMWEs and AEMFCs, including electrolyte engineering (electrolyte selection and feeding), water management, gas and heat management, etc.; (4) outline the advances in monitoring the operations of AEMWEs and AEMFCs, which include microscopic and spectroscopic techniques and beyond; and (5) present key aspects that need to be further studied from the perspective of science and engineering to accelerate the deployment of AEMWEs and AEMFCs.
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    42. 42
      Salvatore, D. A.; Gabardo, C. M.; Reyes, A.; O’Brien, C. P.; Holdcroft, S.; Pintauro, P.; Bahar, B.; Hickner, M.; Bae, C.; Sinton, D.; Sargent, E. H.; Berlinguette, C. P. Designing Anion Exchange Membranes for CO2 Electrolysers. Nat. Energy 2021, 6 (4), 339– 348,  DOI: 10.1038/s41560-020-00761-x
      42
      Designing anion exchange membranes for CO2 electrolysers
      Salvatore, Danielle A.; Gabardo, Christine M.; Reyes, Angelica; O'Brien, Colin P.; Holdcroft, Steven; Pintauro, Peter; Bahar, Bamdad; Hickner, Michael; Bae, Chulsung; Sinton, David; Sargent, Edward H.; Berlinguette, Curtis P.
      Nature Energy (2021), 6 (4), 339-348CODEN: NEANFD; ISSN:2058-7546. (Nature Portfolio)
      A review. New technologies are required to electrocatalytically convert carbon dioxide (CO2) into fuels and chems. at near-ambient temps. and pressures more effectively. One particular challenge is mediating the electrochem. CO2 redn. reaction (CO2RR) at low cell voltages while maintaining high conversion efficiencies. Anion exchange membranes (AEMs) in zero-gap reactors offer promise in this direction; however, there remain substantial obstacles to be overcome in tailoring the membranes and other cell components to the requirements of CO2RR systems. Here we recent advances, and remaining challenges, in AEM materials and devices for CO2RR. We discuss the principles underpinning AEM operation and the properties desired for CO2RR, in addn. to ing state-of-the-art AEMs in CO2 electrolyzers. We close with future design strategies to minimize product crossover, improve mech. and chem. stability, and overcome the energy losses assocd. with the use of AEMs for CO2RR systems.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c09107.

    • General methods, description of the electrolyzer and experimental procedure, details of computation, chlormatgraphy data, and 1H NMR spectra (PDF)


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