An organic molecular mimic metal-free heterogeneous catalyst for electrocatalytic alkyne semihydrogenation

doi:10.21203/rs.3.rs-4302343/v1

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An organic molecular mimic metal-free heterogeneous catalyst for electrocatalytic alkyne semihydrogenation

https://doi.org/10.21203/rs.3.rs-4302343/v1

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The direct construction of metal-free catalysts on conductive substrates for electrocatalytic organic hydrogenation reactions is significant but still unexplored. Here, learning from the homogeneous molecular catalysts, an organic molecular mimic metal-free heterogeneous catalyst is designed and constructed in situ on a graphite flake electrode via a mild electrochemically oxidation-reduction relay strategy. The as-prepared–COOH- and –OH-functionalized metal-free catalyst exhibits an electrocatalytic alkyne semihydrogenation performance with a 72% Faradaic efficiency, 99% selectivity and 96% yield of the alkene product, which is comparable to that of noble metal catalysts. The removal of these oxygen-containing groups leads to the negligible activity. The experimental and calculation results reveal that the origin of the high activity can be assigned to the –COOH and –OH groups on graphite. A flow electrolytic cell delivers ten-gram grade hydrogenated products with 81% Faradaic efficiency. This metal-free catalyst is also suitable for gas-phase acetylene semihydrogenation and other electrocatalytic hydrogenation reactions.

Physical sciences/Chemistry/Electrochemistry/Electrocatalysis

Physical sciences/Chemistry/Green chemistry/Sustainability

The development of noble metal-free or metal-free catalysts to replace noble metal catalysts is significant for electrocatalytic organic hydrogenation reactions. Alkyne semihydrogenation (ASH) is a classical model reaction for creating hydrogenation methods and evaluating designed catalysts¹⁻⁶. Currently, impressive progress has been made in the development of metal-free catalysts, such as Cu- and Ni-based solid catalysts⁷⁻¹⁰ and Co or Ni-based molecular catalysts¹¹⁻¹⁴. Three days ago, Zhang et al. sprayed a well-designed deprotonated 2-thiolimdazole for acetylene hydrogenation¹⁵. The required addition of conducting binders may block the active sites, and the gradual removal of the catalyst from the substrate surface is still one main challenge. However, at present, the direct construction of new metal-free catalysts on conductive substrates for electrocatalytic organic hydrogenation reactions is lacking. Therefore, exploring metal-free ASH electrocatalysts on conductive substrates is highly desirable but highly challenging.

Organic molecular catalysts play an important role in organic synthesis¹⁶⁻²³. For instance, Brønsted acid catalysts were found to activate alkenes because their very high acidity enables easy alkene protonation¹⁸. Inspired by this, we speculate that Brønsted acids may be potential metal-free catalysts for catalytic alkyne hydrogenation because of the similar molecular structures of alkynes and alkenes, and alkynes are easier to protonate because they are more electron-rich. However, the poor stability and conductivity of organic molecular catalysts limits their applications in electrochemical reactions. In these regards, the in situ construction of Brønsted acid molecular mimic catalysts on stable and conductive electrodes is the key to solving the problems of electrode stability and conductivity to realize electrochemical ASH with metal-free catalysts.

In this article, we begin by screening a series of simple Brønsted acid molecular catalysts with the assistance of density functional theory (DFT) calculations and find that carboxyl (–COOH)- and hydroxyl (–OH)-containing molecular catalysts exhibit optimum activities for electrochemical ASH. Then, we construct –COOH and –OH groups in situ on stable and conductive graphite flake (GP) via a mild electrochemical oxidation‒reduction method. As a result, the electrochemically oxidized and reduced GP (EOR-GP) exhibits comparable noble metal performance (72% Faradaic efficiency (FE), 99% selectivity and 96% yield) for electrochemical ASH. Control experiments, spectral characterization and DFT calculations reveal that the origin of the high activity can be assigned to the –COOH and –OH groups. Moreover, EOR-GP is suitable for other electrochemical hydrogenation reactions, such as nitrobenzene hydrogenation and the dehalogenation of halides. This work opens the avenue for the construction of stable metal-free catalysts for electrocatalytic hydrogenation reactions by imitating organic molecular catalysts.

Organic small-molecule catalysts for electrochemical 2-methyl-3-butyn-2-ol (MBY) semihydrogenation

Oxygen-containing functional groups (OFGs), such as –OH and –COOH, can serve as Brønsted acid sites²⁴; thus, we began this study by testing the ASH activity of simple molecules with –OH and –COOH groups. For comparison, molecules with neutral OFGs, such as C–O–C and C = O, were also tested. 2-Methyl-3-butyn-2-ol (MBY), a key stock in vitamin A production, was used as the

model substrate. The ASH activity was first studied by cyclic voltammetry (CV) (Fig. 1a). No current density variation is observed for the glassy carbon before and after adding MBY. Interestingly, a pair of redox peaks arises after introducing the molecular catalyst, indicating that electron transfer occurs between MBY and the molecular catalyst. Then, an electrolysis experiment was conducted at −1.6 V vs. Ag/AgCl (all potentials are referenced to the saturated Ag/AgCl unless otherwise stated), and the products were identified and quantified by ¹H nuclear magnetic resonance (¹H NMR, Supplementary Fig. 1). The four molecular catalysts exhibit ASH activity, and the molecular catalysts with –OH and –COOH show better activities (Fig. 1b). No activity was found over the pure carbon electrode (Supplementary Fig. 1). Notably, the experimental results are consistent with the DFT calculations (Figs. 1c − d). However, the reaction rate is very slow due to the inadequate contact of homogeneous molecular catalysts with the cathode. These results inspired us to construct –OH and –COOH groups in situ on conductive and stable GP.

Synthesis and characterization of EO-GP and EOR-GP

We treated the GP with an electrooxidation method followed by an electroreduction treatment to enrich it with –OH and –COOH groups (Fig. 2a). GP was first oxidized in 0.5 M H₂SO₄ at a current density of − 200 mA cm^–2 for 0.5, 1.0, and 1.5 h to obtain electrooxidized GP (EO-GP-t, t = 0.5, 1.0, or 1.5) with different oxidation degrees. Then, a subsequent electroreduction treatment was conducted to remove reducible groups such as C–O–C and C = O groups to obtain EOR-GP-t. X-ray diffraction (XRD) patterns (Supplementary Fig. 2) of the EO-GP and EOR-GP samples exhibit diffraction peaks at 26.5°, 44.5°, 54.5°, and 77.6°, which are attributed to graphitic carbon (PDF no. 01-089-8487). Compared with those of the initial GP, the peak intensities of EO-GP and EOR-GP decrease, suggesting that the crystallinity of the GP decreases after electrochemical treatment²⁵. Scanning electron microscopy (SEM) images (Supplementary Fig. 3) show that the GP edges become curled and thinner after electrooxidation and electroreduction. Raman spectra (Supplementary Fig. 4) show that the I_D/I_G ratio increases with increasing oxidation time, indicating an increase in carbon defects²⁶. The defects remain unchanged after electroreduction.

Spectral characterization was used to characterize the OFGs on the GP before and after electrochemical treatment. X-ray photoelectron spectroscopy (XPS) survey scans (Fig. 2b and Supplementary Fig. 5a) show that a new O 1s peak emerges after electrooxidation and then becomes more obvious with increasing electrooxidation time. After electroreduction, the peak become weaker due to the removal of the reducible OFGs (Fig. 2b and Supplementary Fig. 5b). In the O 1s spectra (Fig. 2c), the initial GP shows a weak peak of –OH (533.3 eV)²⁷ from aromatic phenols (Ar–OH) or aliphatic alcohols (C–OH), possibly due to slight oxidation during storage and cleaning. After electrooxidation, the –OH peak increases greatly, and new oxygen species of C = O (531.3 eV, ketone and carboxylic acids, etc.)^24,26,27,29, O–C = O (532.2 eV, carboxylic acids, etc.)^24,26,28,29, and C–O–C (534.4 eV, furan, etc.)^24,26,28,29 appear. As the oxidation time increases, the three new peaks increase, while the –OH peak decreases, and O–C = O and C–O–C are the dominant groups in EO-GP-1.5 (Supplementary Fig. 6a). After electroreduction, the C–O–C peak disappears, the C = O peak becomes much weaker, while the –OH peak increases, and the O–C = O peak remains unchanged (Supplementary Fig. 6b). These results show that C–O–C and C = O are reduced to –OH, but O–C = O cannot be reduced during the electroreduction treatment. Thus, –OH and O–C = O are the dominant groups in EO-GP-1.5. Similar results are obtained for the C 1s spectra (Supplementary Fig. 7).

Additionally, soft X-ray absorption near edge structure (XANES) was used to further confirm the OFGs. In the O K-edge XANES spectra (Fig. 2d), a weak peak at 534.5 eV for the initial GP is attributed to the aromatic phenol (σ* Ar–OH)^24,28 After electrooxidation, a broad peak attributed to C = O (542.1 eV)^{21, 25}, O–C = O (539.7 eV)^24,26,28,29, and C–O–C/C–OH (537.4 eV)^24,26,28,29 appear simultaneously. Another new peak at 533.5 eV is attributed to the π* C = O in quinone. After electroreduction^24,26,28, the π* C = O and σ* C–O–C peaks decreased significantly, while the O–C = O and C–OH peaks are preserved. In the C K-edge XANES spectra (Fig. 2e), four new peaks at 286.9, 287.7, 286.5, and 288.5 eV arise in the spectrum of EO-GP-1.5, which are attributed to π* C–O–C, π* C = O, C–OH and σ* O–C = O, respectively^24,28,29. After electroreduction, only σ*, O–C = O and C–OH are retained in EOR-GP-1.5. These results are consistent with the XPS data. A similar variation is observed in attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy (Supplementary Fig. 8). These results demonstrate the successful construction of –COOH- and –OH-enriched EOR-GP.

EOR-GP for electrochemical 2-methyl-3-butyn-2-ol (MBY) semihydrogenation

The electrochemical ASH performance of GP and EOR-GP was tested. CV curves (Fig. 3a and Supplementary Fig. 9) show that the hydrogen evolution reaction (HER) performance of EOR-GP is better than that of GP, and EOR-GP-1.5 shows the best HER performance. After the addition of MBY, GP exhibits a higher onset potential and a lower current density compared with that of no MBY, possibly due to the larger electrolyte resistance caused by MBY. In contrast, EOR-GP shows a much lower onset potential and a noticeable increase in current density after the introduction of MBY, indicating the rapid conversion of MBY on EOR-GP. Constant potential electrolysis experiments show that the electrochemical ASH performance increases with increasing oxidation degree, and EOR-GP-1.5 exhibits the best performance of 72% FE, 96% yield, 99% selectivity and 0.48 mmol cm^− 2 h^− 1 yield rate of MBE at the optimum potential of − 1.5 V (Figs. 3b−d and Supplementary Fig. 10). Notably, the EOR-GP-1.5 performance is comparable to that of Pd-based catalysts³⁰. In comparison, GP shows negligible performance for electrochemical ASH (Fig. 3d). The ASH performance and catalyst composition can be maintained for 40 h, verifying the good durability of EOR-GP-1.5 (Fig. 3e and Supplementary Fig. 11).

Control experiments were performed to gain insight into the origin of the activity of EOR-GP. To eliminate the activity difference caused by the active area, the normalized yield rates of the electrochemical surface areas (ECSAs) were calculated (Supplementary Fig. 12). EOR-GP-1.5 shows much better activity, ruling out the contribution of a high ECSA to the performance enhancement (Supplementary Fig. 13). Furthermore, to observe whether the defects contributed to the performance enhancement, EOR-GP-1.5 was further treated in a mixed hydrogen/argon atmosphere (denoted as EOR-GP-1.5-H₂) to remove the OFGs but retain the defects³¹. XPS and Raman data (Supplementary Fig. 14) reveal that this hydrogen reduction process removes most of the OFGs while the defects are still retained. As a result, EOR-GP-1.5-H₂ shows very negligible electrochemical ASH activity (Fig. 3f), excluding the contribution of defects. Additionally, we treated GP via chemical oxidation through concentrated nitric acid at 100 ℃ for 25 h, and the obtained COR-GP was enriched with –COOH and –OH and exhibited activity comparable to that of EOR-GP-1.5 (Fig. 3f and Supplementary Figs. 15−17). These results suggest that the activity of EOR-GP-1.5 originates from the OFGs on the carbon surface.

Mechanistic studies

Further study was conducted to elucidate the reaction mechanism. Generally, two mechanisms are involved in electrochemical reduction: electrocatalytic hydrogenation (ECH) and direct electroreduction³². ECH involves strong interactions with the electrode surface and reactants, requiring H_ads produced via H₂O splitting as a hydrogen source. In direct electroreduction, protonation occurs in solution and does not require direct chemical interactions with the electrode surface. A previous study on metal electrodes for electrochemical ASH proposes an ECH mechanism². However, carbon-based electrodes are reported to favour the direct electroreduction mechanism in the electrochemical reduction reaction^{32, 33}. This finding inspires us to explore the mechanism of ASH on EOR-GP-1.5. First, a kinetic isotopic effect (KIE) for the HER is observed for EOR-GP-1.5 (Fig. 4a). The CVs in Fig. 4b show that MBY reduction also experiences a KIE with a similar magnitude to that of the HER, indicating that the ASH reaction may share a common step with the HER and involve reactions with H_ads (ref.³²). As expected, a decreased production rate of MBE is observed in the D₂O electrolyte at − 1.5 V (Fig. 4c). Moreover, a long-chain thiol, which can block surface sites but allow electron tunneling^{32, 34}, are employed to modify the EOR-GP-1.5 electrode. Almost no MBE is detected after thiol modification, demonstrating that the specific adsorption of MBY or H is

indispensable for MBY reduction (Fig. 4d). Similar results are obtained by electrochemical in situ Raman spectroscopy (Fig. 4e). In the spectrum of EOR-GP-1.5, peaks from the stretching vibrations of C ≡ C (2107 cm^− 2) and C − H (2943 and 2992 cm^− 2)³⁵ in MBY increase as the applied potential increases. Accordingly, the peak attributed to the stretching vibration of C = C (1448 cm^− 2)³⁵ arises at − 1.3 V and increases with increasing potential, which is consistent with the experimental results. In comparison, no Raman signals of MBY or MBE are observed for the glassy carbon electrode without OFGs. These results indicate that the adsorption of MBY is the precondition for realizing MBY reduction and that the OFGs probably play a key role in promoting MBY adsorption, as also suggested by DFT calculations (Supplementary Fig. 18). Moreover, the reaction intermediates of the enol and active H species are confirmed by high resolution mass spectrometry (HRMS, Fig. 4f). Based on the above analysis, a possible ECH mechanism is proposed (Fig. 4g). First, MBY adsorbed on the EOR-GP-1.5 surface and activated by receiving an electron from the cathode. Then, the activated substrate processes the addition of two active H from H₂O splitting to produce MBE, which can desorb quickly to avoid overhydrognation.

Computational analysis

DFT calculations were conducted to further understand the activities of the OFGs towards ASH (see the supporting information for calculation details, Supplementary Tables 1–3 and Supplementary Fig. 19). The adsorption energy of C₅H₉O* (G_ad (M-C₅H₉O*)) is used as the descriptor of MBY reduction activity based on the scaling relationships between the adsorption energies of the adsorption intermediates involved in the reaction (Supplementary Fig. 20−21). The limiting potential U_L is defined as the lowest potential³⁶ at which the free energy of all reaction steps decreases and is considered a metric of activity. Figure 5 and Supplementary Fig. 22 show the calculated U_L as a function of G_ad (M-C₅H₉O*) based on the OFGs grafted onto graphene (graphene-OFGs, blue dots) or the OFGs containing molecules (molecular-OFGs, red dots). Among the different graphene-OFGs, the –COOH and –OH groups on the basal plane and at the edge of the graphene are located near the summit of the volcano. Thus, we reasonably speculate that –COOH and –OH on EOR-GP-1.5 are both active sites for electrochemical ASH. For molecular-OFGs, we found that –COOH is located closest to the summit of the volcano, with a theoretical overpotential of 0.53 V vs. RHE. –OH and C = O are the second closest to the volcanic summit, with overpotentials of 0.92 and 1.00 V vs. RHE, respectively. C–O–C is slightly farther from the summit of the volcano, with an overpotential of 1.14 V vs. RHE. Pure C exhibited the least activity on the right side of the volcano plot due to the weak bond energy of M-C₅H₉O*, with an overpotential of 1.54 V vs. RHE. The calculation results are not exactly the same for graphene-OFGs or molecular-OFGs, probably due to the difference in the adsorption scaling relationship between the two catalysts (Supplementary Fig. 21). For example, although graphene-OH and molecular-OH have similar binding energies for MBY (C₅H₈O*), graphene-OH has a much stronger binding energy for the further hydrogenated product of MBY (M-C₅H₉O*), resulting in better molecular-OH activity (Fig. 5). These DFT results confirm that the activity probably originates from the –COOH and –OH groups on the GP.

Utility and universality study

The utility and universality of the catalyst were studied. We successfully implemented a flow electrolytic cell for the electrosynthesis of gram-scale MBE from MBY electroreduction with 81% FE and 10.4 g yield (Fig. 6a and Supplementary Fig. 23). Moreover, EOR-GP-1.5 is also applicable in gas-phase acetylene electrocatalytic semihydrogenation, where a 92% FE is achieved at a potential of − 1.7 V (Fig. 6b). Additionally, a series of alkynes with electron-donating and electron-withdrawing groups on the phenyl ring and aliphatic terminal alkynes are converted to alkenes with high selectivity and yield (Fig. 6c and Supplementary Fig. 24). Furthermore, EOR-GP-1.5 is suitable for other electrocatalytic hydrogenation reactions, such as the hydrogenation of nitrobenzene to aniline and the dehalogenation of halides (Fig. 6d and Supplementary Fig. 25), showing the promising potential of this metal-free catalyst in other electrocatalytic hydrogenation reaction systems.

In summary, we experimentally and theoretically demonstrate the electrocatalytic activity of oxygen-containing organic molecular catalysts for 2-methyl-3-buten-2-ol hydrogenation. To improve the conductivity and stability of organic molecular catalysts, an organic molecular mimic metal-free catalyst was designed on a stable and conductive graphite flake via a mild electrochemical oxidation‒reduction treatment strategy. The as-prepared –COOH- and –OH groups functionalized metal-free catalysts show an outstanding electrochemical ASH performance of 72% FE, 96% yield, 99% selectivity and 0.48 mmol cm^–2 h^–1 yield rate of MBE, which is comparable to that of noble metals. An electrocatalytic hydrogenation mechanism is proposed by a combination of kinetic isotope experiments, thiol-electrode modification, electrochemical in situ Raman, and DFT calculations. Based on spectral characterization and DFT calculations, we can assign the origin of the high activity to the –COOH and –OH groups on GP. Additionally, our catalyst can also be applied in gas-phase acetylene semihydrogenation and other electrocatalytic hydrogenation reactions. This work not only paves the way for constructing organic molecular mimic heterogeneous catalysts on conductive carbon but also inspires the design of efficient metal-free catalysts for electrocatalytic hydrogenation reactions.

Synthesis of EOR-GP

The EOR-GP was synthesized through electrochemical chronopotentiometry oxidation and reduction of high-purity GPs (Beijing Jinglong Special Carbon Technology, 99.99%) in 0.5 M H₂SO₄ and 1.0 M KOH, respectively. For the fabrication of EOR-GP, a piece of high-purity GP (1 cm × 1 cm) was ultrasonically cleaned in diluted hydrochloric acid (Aladdin, analytically pure), acetone (Sigma‒Aldrich, analytically pure), ethanol (Aladdin, chromatographically pure), or ultrapure water (Aladdin, chromatographically pure) for 40 min. After vacuum drying at 60 ℃ for 12 h, the cleaned GP was used as the working electrode. A carbon rod and saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively, in a 0.5 M H₂SO₄ electrolyte (prepared with ultrapure water). EO-GP could be formed by continuous electrolysis for 0.5, 1.0 or 1.5 h at a current density of 200 mA cm⁻² (CorrTest, Wuhan). For the synthesis of EOR-GP, the as-prepared EO-GP was used as the working electrode, and carbon rods and Ag/AgCl electrodes were used as the counter electrode and reference electrode, respectively, in a 1.0 M KOH electrolyte (CorrTest, Wuhan). EOR-GP could be formed by continuous electrolysis for 24 h at a current density of − 100 mA cm⁻².

Electrochemical measurements

Electrochemical tests were carried out using a CS350MA (CorrTest, Wuhan) electrochemical workstation in a two-chamber electrochemical cell consisting of a working electrode, a graphite rod plate counter electrode, and a Ag/AgCl reference electrode. All the potentials in this work were referenced to Ag/AgCl without iR correction unless otherwise stated. The electrolyte contained 8 mL of 1 M KOH (prepared with ultrapure water) and 1 mmol of MBY. Then, chronopotentiometry was carried out under magnetic stirring (800 rpm). Every measurement was performed on a freshly prepared electrode. The double layer capacitance (C_dl) was calculated from the slope of the linear fit plot of the current density vs. scan rate. The cyclic voltammograms (CVs) were recorded in the nonfaradaic region, where charging of the double layer is responsible for the current density. All electrolytic devices and electrolytes are completely isolated from metal sources.

Detection and quantification of MBE

The produced MBE was analysed by ¹H NMR with solvent (H₂O) suppression. Then, 0.1 mL of D₂O (containing 0.0625 mmol maleic acid) as a deuterated solvent was mixed with 0.5 mL of electrolyte. Maleic acid was used as the internal standard (ISD). The concentration of the product was calculated based on the ISD quantitative method (maleic acid, ~ 5.78 ppm; MBE, 1.07 ppm).

In situ Raman spectroscopy measurements

In situ electrochemical Raman spectroscopy was performed using potential-dependent in situ methods on a Renishaw inVia reflex Raman microscope using an excitation of a 633 nm laser under controlled potentials by an electrochemical workstation. The electrolytic cell was constructed in-house by Teflon with a piece of round quartz glass as the cover. The working electrode was set to maintain the plane of the sample perpendicular to the incident laser. Pt wire was used as the counter electrode, and Hg/HgO with an internal reference electrolyte of 1.0 M KOH was used as the reference electrode.

Acknowledgements

We acknowledge the National Natural Science Foundation of China (22271213 to B.Z.) and the China Postdoctoral Science Foundation (BX20230256 to Y.W.) for financial support.

Author Contributions

B.Z. conceived the idea and directed the project. Y.W. and B.Z. designed the experiments. Z.S. and Y.W. carried out the experiments and analysed the data. R.Y. performed the DFT calculations. X.L. assisted with some experiments. Y.W. wrote the paper. B.Z. revised the paper. All authors discussed the results and commented on the paper.

Competing interests

The authors declare no competing interests.

Data and materials availability

All the data needed to evaluate the conclusions in the paper are presented in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.

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An organic molecular mimic metal-free heterogeneous catalyst for electrocatalytic alkyne semihydrogenation

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Abstract

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Introduction

Results

Organic small-molecule catalysts for electrochemical 2-methyl-3-butyn-2-ol (MBY) semihydrogenation

Synthesis and characterization of EO-GP and EOR-GP

EOR-GP for electrochemical 2-methyl-3-butyn-2-ol (MBY) semihydrogenation

Mechanistic studies

Computational analysis

Utility and universality study

Discussion

Methods

Synthesis of EOR-GP

Electrochemical measurements

Detection and quantification of MBE

In situ Raman spectroscopy measurements

Declarations

References

Additional Declarations

Supplementary Files

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