Direct activation of C−H bonds via selective oxidation of hydrocarbons is of great interest for organic hydrocarbons1,2. As a typical and important transformation path of C−H bonds, selective dehydrogenation of C−H bonds has been widely used for the production of high value-added compounds such as alcohols, and ketones and ethers3-5. However, the chemical stability of C−H bonds without activating neighborhood effects makes C−H activation quite challenging, and either extreme and rather toxic oxidants as chromium or selenium compounds or noble-metal-catalysts (based on rhodium or palladium) at high temperatures have to be applied to obtain acceptable conversions6-8. Moreover, the as-formed side product water from the cleavage of C−H bonds via oxydehydrogenation is free of value. As a result, novel and sustainable strategies are highly desirable to further decrease the economic and environmental footprints of C−H activation processes.
Electrochemical transformation is recognized as an environmentally friendly method for the production of various functional molecules driven by electricity under mild conditions9-11. Pioneering works of electrochemical synthesis using homogeneous catalysts have demonstrated the advantages of this technique for C−H activation11, which includes selective oxidation12, amination13, epoxidation14 and dehydrogenative coupling reactions15. Most of these reactions have high atom economy and excellent compatibility with flow reactors for continuous synthesis16,17. The current strategies to boost the transformation of specific substrates mainly rely on the involvement of functional additives11 (e.g., organic ligands, bases and mediators), a high work potential and/or sacrificial transition metal electrodes18, which all will severely limit real-industry applications. Principally, the preparation of cost-effective and active electrode materials is at least as important as the development of a new methodology for selective C−H bonds activation19-26, and only non-targeted, commercial first generation electrodes (such as carbon rod, platinum and reticulated vitreous carbon) are applied as the current collectors in electrochemical organic synthesis at the moment. The significant progress reported using well-designed reaction-specified electrodes in improving the catalytic activity for water splitting, nitrogen reduction reactions and even carbon dioxide reduction reactions27-30 further manifests the huge gap between the design of novel electrode materials and the requirements of sustainable electrochemical organic synthesis.
Herein, we present the proof-of-concept application of electron-deficient W2C nanocrystal-based electrodes for the highly efficient electrochemical activation of C–H bonds, highlighting the key importance of the modified physicochemical properties of electrode materials in boosting additive-free C–H activation reactions. A nanoheterojunction composed of W2C nanocrystals and nitrogen-doped carbons has been rationally designed to control the number of electrons flowing from W2C nanocrystals to nitrogen-doped carbons by increasing the doping concentration in the carbon supports to enhance the interfacial Schottky effect. The as-formed electron-deficient W2C nanocrystal-based electrode acts as a functional anode to simultaneously facilitate the alkoxylation of ethylbenzene with methanol on the anode and the balancing hydrogen evolution reaction on the cathode. Both the experimental and theoretical results indicate the key role of the electron deficiency of the W2C nanocrystals in capturing ethylbenzene on the anode to substantially increase the reaction rates of alkoxylation and hydrogen evolution reaction processes simultaneously and ensure the long-term stability of the anode without scarifying the current collector.
The W2C/NC catalysts were prepared via a modified nanoconfinement method (Supplementary Fig. 1) from a mixture of dicyandiamide and ammonium tungstate, followed by N2-protected thermal pyrolysis at high temperatures. The nitrogen contents (x at.%) of the W2C/NxC samples could be tuned from 3.0 via 2.3 to 1.4 at.% (Supplementary Fig. 2 and Table 1) by elevating the condensation temperatures from 1000 to 1200 °C (for experimental details please see the experimental section). The morphology (Supplementary Fig. 3), surface area (Supplementary Fig. 4) and W content (Supplementary Table 1) of W2C/NxC samples are well maintained, as reflected by their scanning electron microscopy (SEM) images. Transmission electron microscopy (TEM) observations (Fig. 1a-c and Supplementary Fig. 5-7) further reveal the presence of few-layer-graphene-supported W2C nanocrystals with a mean size of 2.5 nm (Fig. 1a and Supplementary Fig. 8) and a typical lattice fringe of 0.24 nm (Fig. 1b), which corresponds to the (002) plane of α-W2C31,32. The formation of W2C is doubly confirmed by its X-ray diffraction (XRD) pattern (Supplementary Fig. 9), matching well with that of typical α-W2C (JCPDS# 35-776)31. Detailed elemental mapping images (Fig. 1c) exhibit nanometer-sized W-rich areas with a homogeneous distribution of N atoms along with the whole carbon support, indicating an integrated structure of W2C nanocrystals on the nitrogen-doped carbons.
The highly coupled structure of W2C/NC dyads makes it possible to form a rectifying interface for modulation of the electron density of W2C nanocrystals. The density functional theory (DFT) calculation results (Supplementary Fig. 10,11) predict electron transfer from W2C to nitrogen-doped carbons, resulting in more pronounced electron-deficient regions in W2C nanocrystals suggested by the charge density difference (CDD) stereograms (Fig. 1d) of the same W2C model supported on pristine carbons (W2C/C). The mean number of electrons transferred from the W2C nanocrystal to the nitrogen-doped carbon support (Fig. 1e) increases from 0.338 to 0.397 as more nitrogen atoms (from 1.4 to 3.0 at.%) are doped into the carbon support models (Supplementary Fig. 12), which were constructed based on the X-ray photoelectron spectroscopy (XPS) analysis results33. As depicted in Fig. 1f, the nanoheterojunction of W2C and NC has a rectifying contact, with electrons flowing from the W2C side with a lower Fermi level (EF) to the NC side, generating electron-deficient W2C due to the interfacial Schottky barrier34,35. Indeed, the electron donation from the W2C nanocrystals to the nitrogen-rich carbon supports is experimentally confirmed by the gradual shift in W 4f XPS peaks to higher energy (Fig. 1g) from 34.2 via 34.4 to 34.5 eV for W2C/N1.4C, W2C/N2.3C and W2C/N3.0C, respectively, resulting in gradually increased work functions (Fig. 1h and Supplementary Fig. 13) from 5.4 via 5.6 to 5.7 eV. A similar trend for the electron density of W2C nanocrystals in W2C/NxC samples is also demonstrated by the most positive W M4,5 peak (Supplementary Fig. 14) of the W2C/N3.0C materials among all samples36. All of the above results indicate the formation of electron-deficient W2C nanocrystals and the successful further enhancement of electron deficiencies by increasing the nitrogen contents in the carbon supports.
Inspired by the success in modifying the electron density of W2C nanocrystals, we further evaluated the possible catalytic activity of W2C/NxC catalysts for electrochemical alkoxylation of ethylbenzene with methanol under mild conditions as a model reaction. Considering that the reported methods for alkoxylation of C–H bonds usually require highly active additives/oxidants and/or a high reaction temperature, we initially tested the possibility of additive-free alkoxylation of ethylbenzene with methanol using only a simple electrolyte containing lithium perchlorate and W2C/NxC-based electrodes under ambient conditions (Fig. 2 and Supplementary Fig. 15). No product was detected without applying a working potential for various electrodes in our electrochemical system (Supplementary Fig. 16), illustrating that the methoxylation reaction cannot proceed spontaneously. Surprisingly, a complete conversion of ethylbenzene can be achieved on the W2C/N3.0C electrode with high selectivity to the target product (1-methoxyethyl)benzene (Fig. 2a,b and Supplementary Fig. 17) and a total carbon balance of approximately 95%, confirming the possibility of highly efficient alkoxylation of C–H bonds on a well-designed heterogeneous electrode without scarifying additives. The fact that control electrodes with the same amount of bare NC sample, W2C catalyst or a mechanical mixture of the two components (Fig. 2e) give much lower conversions of ethylbenzene than the W2C/N3.0C electrode under fixed conditions further indicates a synergistic effect between W2C and N3.0C components in facilitating the transformation of ethylbenzene.
Unlike the oxidative alkoxylation reaction of C–H bonds by using various oxidants for dehydrogenation to generate water37, our heterogeneous electrochemical system could achieve the full use of as-formed protons from the activation of C–H bonds and methanol for subsequent hydrogen evolution reactions, generating hydrogen gas bubbles on the cathode (Supplementary Fig. 18). Moreover, the calculated Faradaic efficiencies (Fig. 2d and Supplementary Fig. 19) are similar for the conversion of ethylbenzene to (1-methoxyethyl)benzene on the W2C/N3.0C anode (FE: 42-46%) and hydrogen production on the Ti cathode (FE: 42-55%), implying a cascade transformation of protons generated from the anode into hydrogen gas on the cathode. Even with an excess amount of methanol in the reactor, only a trace amount of formaldehyde (0.006 mmol) formed during the conversion of 0.5 mmol of ethylbenzene (Supplementary Fig. 20), well explaining the comparable Faradaic efficiencies for the reactions on anode and cathode without the obvious contribution of methanol dehydrogenation to the total FE for hydrogen evolution reactions. Remarkably, the electron-deficient W2C in the W2C/N3.0C-based electrode substantially promotes the hydrogen evolution rate on the Ti cathode to 880 μmol (Fig. 2c), which is above 10 times that on the same Ti cathode (85 μmol) when using bare carbon cloth as the anode. The constant current density of the W2C/N3.0C anode under fixed conditions with different cathodes (Fig. 2f), including Pt mesh, Ti mesh and carbon rod, further demonstrates that the activation and deprotonation of ethylbenzene on the W2C/N3.0C electrode is the rate dominating step for the whole reaction. Indeed, the alkoxylation reaction could be selectively quenched by butylated hydroxytoluene (BHT) (Supplementary Fig. 21), indicating a radical-based pathway on the W2C/N3.0C anode, as indicated in Fig. 2a18.
The role of the electron-deficient W2C nanocrystals and the interfacial effect of the heterojunction catalysts on the electrochemical alkoxylation of C–H bonds were simulated via theoretical calculations and then validated by experimental evidence (Fig. 3). The optimized geometry (Fig. 3a,c) of ethylbenzene presents preferred adsorption of benzylic C–H bonds on the W2C surface dependent of the electron-deficiency of W2C, indicating the feature role of W2C as an active component. This role was further validated by more negative onset potentials (<1.4 V versus SCE) for the electrochemical alkoxylation reaction on W2C/NxC anodes than that (>1.6 V versus SCE) of the bare carbon cloth electrode (Supplementary Fig. 22). However, the polarization of adsorbed C–H bonds is enhanced by the electron-deficient surface of the W2C-0.08e- model, as reflected by the more pronounced electron density difference (Hirshfeld charge) of the preadsorbed C–H bonds (Fig. 3c and Supplementary Fig. 23) and a much lower calculated adsorption energy for ethylbenzene (Fig. 3e). Such strong adsorption of ethylbenzene molecules over the electron-deficient W2C surface was then experimentally validated by the temperature-programmed desorption (TPD) analysis results (Fig. 3f), exhibiting gradually elevated adsorption capacities over those of more electron-deficient W2C/NxC samples with similar surface areas. It should be noted that the bare carbon support (NC sample in Fig. 3f) provides a low adsorption capacity, only 21% of the best-in-class W2C/N3.0C sample (Supplementary Fig. 24). More importantly, the electron deficiency-induced adsorption behavior of ethylbenzene on the final W2C/NxC-based anodes under a fixed bias in the electrochemical reactor was well expressed with the same trend in adsorption capacities (Fig. 3g) as that revealed by TPD results, making successive C–H dissociation process more favorable.
Indeed, the stronger interaction between preadsorbed ethylbenzene molecules and electron-deficient W2C significantly reduces the Gibbs free energy of each step of the whole alkoxylation reaction pathway (Fig. 3e). The dissociation of C–H bonds of ethylbenzene on the electron-deficient W2C catalyst (W2C–0.08e- model) is the rate-limiting step with a free energy change of only 0.34 eV, and the subsequent coupling of *C8H9 and *CH3O• (*C8H9• + *CH3O•) and desorption of as-formed (1-methoxyethyl)benzene (*C9H12O) proceed automatically. With similar configurations, the last three steps for the catalytic conversion of preadsorbed ethylbenzene molecules on the pristine W2C catalyst (W2C model) are thermodynamically uphill with a larger free energy change of 0.4 eV for the (*C8H9• + *CH3O•) step, again indicating the key role of electron density in facilitating the whole reaction and desorption processes on the W2C surface. This electron-deficiency-dependent promotion effect on the activity of W2C was then unambiguously confirmed by the gradually increased catalytic activities (Fig. 3h) and FE values (Fig. 3i) for producing (1-methoxyethyl)benzene on more electron-deficient W2C/NxC-based anodes under fixed work potential.
The W2C/N3.0C anode also shows excellent electrochemical stability for long-term use. The composition (Supplementary Fig. 25) and morphology (Supplementary Fig. 26) of the used W2C/N3.0C materials were maintained well. Most importantly, the W2C/N3.0C anode can be recycled at least four times without an obvious decrease in FE (41-46%) (Fig. 4b and Supplementary Fig. 27). It should be noted that inert metal anodes for alkoxylation of ethylbenzene, including stable metals (exemplified by Ti mesh) and active metals (exemplified by Ni plate), decompose rapidly within 5 h (Fig. 4a and Supplementary Fig. 28), illustrating the key importance of the high activity of the W2C/N3.0C anode to keep itself from corroding. As a durable anode, the electron-deficient W2C electrode exhibits satisfying activity for electrochemical alkoxylation of various aromatic C–H bonds using a series of aliphatic alcohols (Supplementary Table 2) with good to high conversions and high selectivity in 18 h, suggesting an excellent tolerance of our electrode material to various functional groups. As the best-in-class anode in this work, the W2C/N3.0C electrode provides a high turnover frequency (TOF) value of 18.8 h-1, which is comparable to or even higher than the reported values, mostly of homogeneous catalysts, for similar alkoxylation reactions (Fig. 4c and Supplementary Table 3-5)38,39.
In summary, we have demonstrated the key role of electron-deficient W2C nanocrystals as electrode materials in boosting the activity and durability for electrochemical activation of C–H bonds via a heterogeneous pathway. We successfully tuned the electron density of W2C nanocrystals by constructing Schottky heterojunctions with nitrogen-doped carbons to achieve preferred adsorption of benzylic C–H bonds of ethylbenzene on the W2C surface and facilitate subsequent C–H activation, which is the rate-limiting step. Unlike conventional oxidative alkoxylation to generate water, the as-formed protons on the W2C anode could be simultaneously converted to hydrogen gas in our additive-free electrochemical reactor under mild conditions. This two-birds-with-one-stone strategy illustrates the significant potential of powerful designer electrode materials to substantially increase catalytic efficiency, atomic economy and electricity utilization for organic electrosynthesis and hydrogen energy production in one electrocatalytic system. In addition to the hydrogen evolution reaction, the reduction process might be compatible with other important reactions (e.g. carbon dioxide reduction reaction or N2/NOx reduction reactions) to create novel or more complex cascade reaction pathways for the production of high value-added compounds from abundant hydrocarbons and even waste gases. This work may also boost the development of zero-additive and zero-emission electrosynthesis systems through the design of novel electrode materials.