Electrocatalytic hydrogenation (ECH) provides an attractive method towards the use of renewable electricity in the chemical sector, ranging from the conversion of biomass to the production of fine chemicals. Protons from water are here reduced using electrons and then immediately transferred to the unsaturated substrate, therefore avoiding the need for H₂ storage, transport and handling. These processes can be operated at mild temperature and pressure.^1-2 However, in aqueous media this technique is limited by the low solubility of most organic substrates and the competing hydrogen evolution reaction producing H₂. Organic electrolytes can be used to improve the substrate solubility, but the lower electric conductivity of such electrolytes leads to lower energy efficiency. Furthermore, the separation of the products from the electrolyte can be challenging.^3-5

In the present work, we developed a Pickering emulsion-based ECH system to address the above issues. Pickering emulsions consist of two immiscible phases, with one phase being dispersed into the other as micrometer-sized droplets. These droplets are stabilized by solid particles at their interface.⁶ Everyday examples for Pickering emulsions include homogenized milk, mayonnaises and many skincare products. Such systems have also been applied to catalysis, e.g by the use of solid catalysts as stabilizers for emulsions. This approach enables facile catalyst recycling and products separation without requiring additional extraction steps. The vast interfacial area between the two phases also improves the mass transport as compared to typical biphasic systems.⁷

In our work, organic substrates were dissolved in oil droplets (containing cyclohexane as organic solvent) surrounded by an aqueous electrolyte phase (Figure 1).

Figure 1. Illustration of our emulsion ECH system. The Ag working electrode is located within an oil-in-water Pickering emulsion. The emulsion is stabilized by carbon nanotubes (CNTs) which are molecularly modified to introduce positive charges, increasing the adsorption of the CNTs on the electrode. Due to the high conductivity of CNTs, the network of emulsion droplets connected with the working electrode vastly increases the active electrode surface. The CNTs are decorated with catalytically active Pd nanoparticles. At the oil-water interface, protons from the aqueous electrolyte enable the ECH of water-insoluble alkenes on the inside of the emulsion droplets. 

Such a configuration simultaneously benefits from the high ionic conductivity of the aqueous phase and good substrate solubility in the emulsion droplets. By employing conductive carbon nanotubes (CNTs) as stabilizer for the emulsion, electricity is transported from a silver cathode to the oil/water interface. The CNTs were decorated with Pd nanoparticles that enable the reduction of protons from the aqueous phase. Inside the oil droplets, these hydride species adsorbed on Pd nanoparticles can hydrogenate the substrates. Under the applied reaction conditions, the surface of the silver electrode is negatively charged. To ensure that there is sufficient contact between the electrode and the surrounding emulsions droplets, we modified the surface of the CNTs molecularly to generate a positively charged electrocatalyst, noted as Pd/CNTs(⁺) (Figure 2).

Figure 2. Synthesis and characterization of Pd/CNTs(+). (a) Pristine CNTs were treated with acids, forming negatively charged CNTs(⁻), which are then functionalized in multiple steps to positively charged CNTs(⁺); (b) transmission electron microscopy image of Pd/CNTs(⁺); (c) light microscope image of the Pd/CNTs(⁺)-stabilized Pickering emulsion. 

Using styrene as a model substrate and applying a potential of -0.65 V vs a reversible hydrogen electrode (V_RHE), the system showed a high current density (proxy of the reaction rate) and hydrogenated styrene into ethylbenzene in excellent yield (95.1%) and Faradaic efficiency (FE, 95.0%). These results mean that 95.0% of electrons entering the system were effectively involved in the formation of the desired product ethylbenzene, with the remaining 5.0% participating to the generation of hydrogen gas as a side reaction. Compared to state-of-the-art Pd membrane reactors, improvements of the FE by 30-50% and of the Pd-normalized current density by one order of magnitude were observed with the emulsion ECH system.⁸ The positively charged surface of CNTs was found critical to connect the emulsion droplets and the Ag current collector, and thus to reach high performance. By performing multiple control experiments, we could exclude reaction mechanisms based on electrochemically promoted thermal hydrogenation and evidence that the hydrogenation process is indeed electrocatalytic in nature. Hydride formation or transport at the interface are proposed to be rate-determining in the system, which would point to an ECH mechanism that involves the transfer of adsorbed hydrides to the substrates.

Figure 3. Examples for substrates used in our emulsion ECH system. The reactions were conducted at −0.65 V_RHE and stopped as soon as the amount of the passed charge was sufficient to convert all substrate (2 mmol).

Similar high current densities and FEs could be reached for a variety of alkenes (see Figure 3 for some examples). Moreover, with styrene as a substrate it was also possible to work under neat conditions, without any additional organic solvent. As an additional benefit, the emulsion system enables facile product isolation, as organic and aqueous phase separate after filtering off Pd/CNTs(⁺). High purity products were readily collected after evaporating the cyclohexane (e.g. 86% isolated yield for the product of the ECH of 4-chlorostyrene). However, for long-chain aliphatic alkenes such as 1-decene, the product yield was lower due to double-bond migration and restricted accessibility of the resulting internal olefins for our ECH system.

Due to the simplicity, versatility, and efficiency of the emulsion system, we believe to potentially open a way to the practical application of ECH to a wide range of water-insoluble organic substrates, such as biomass or biocrude.

If you would like to know more details, please take a look at our article published in Nature Catalysis: https://doi.org/10.1038/s41929-022-00882-4

We hope you enjoy it!

References

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