Ternary Alloys Enable Efficient Production of Methoxylated Chemicals via Selective Electrocatalytic Hydrogenation of Lignin Monomers

Efficient electrocatalytic hydrogenation of guaiacol via construction of electron-rich platinum active centers through alloying

The electrocatalytic conversion of lignin model compounds into high-value chemicals represents a sustainable and environmentally friendly approach. In this study, a PtRuCo alloy catalyst was synthesized through a simple, cost-effective electrodeposition method. This catalyst demonstrated exceptional performance in the mild and efficient electrocatalytic hydrogenation (ECH) of guaiacol, achieving an impressive 84.6 % yield and showing significant potential for hydrogenating other related monomers. The PtRuCo exhibited remarkable stability and superior electrochemical performance under acidic conditions. The electron distribution within the alloy was thoroughly investigated using X-ray photoelectron spectroscopy (XPS) and theoretical calculations. These analyses provided valuable insights into its outstanding ECH properties. Moreover, in-situ Fourier Transform Infrared Spectroscopy (FTIR) was employed to monitor the evolution of reaction intermediates, establishing a robust experimental foundation for elucidating the reaction pathway. This study offers theoretical guidance for designing tailored catalysts for specific target reactions through detailed electronic structure analysis.

Modulating product selectivity in lignin electroreduction with a robust metallic glass catalyst

Converting the lignin into value-added chemicals and fuels represents a promising way to upgrade lignin. Here, we present an effective electrocatalytic approach that simultaneously modulates the depolymerization and hydrogenation pathways of lignin model compounds within a single reaction system. By fine-tuning the pH of the electrolyte, we achieve a remarkable shift in product selectivity, from acetophenone (with selectivity >99%) to 1-phenylethanol (with selectivity >99%), while effectively preventing over-hydrogenation. The robust metallic glass (MG) catalyst, endowed with an amorphous structure, demonstrates high stability, activity, and full recyclability across over 100 consecutive cycles in ionic liquid electrolytes. The relatively strong affinity of the MG catalyst for the substrate during the initial reaction stage, in conjunction with its weaker binding to the phenolic product, as the reaction progresses, creates a delicate balance that optimizes substrate adsorption and product desorption, which is pivotal in driving the cascade hydrogenation process of acetophenone. This work opens versatile pathways for lignin upgrading through integrated tandem reactions and expands the scope of catalyst design with amorphous structures.

Development status of electrocatalytic hydrogenation of biomass small molecules and prospects for industrial production

Biomass is the only renewable organic carbon resource in nature, and utilization of biomass is important for carbon neutrality. Currently, depolymerizing biomass macromolecules into small organic monomers via thermocatalytic pyrolysis is a well-established technique. Further valorization of these biomass small molecules to value-added products has attracted increasing attention, especially via electrochemistry coupling green electricity. Electrocatalytic hydrogenation (ECH) directly uses hydrogen from water and operates under mild conditions (e.g., ambient temperature and pressure), which plays an important role for upgrading biomass small molecules and avoids substantial CO2 emission. In this review, we will provide a summary of recent achievements in ECH of biomass small molecules, with a review focus on the research about pushing ECH toward industrial-scale productivities. We will also discuss the existing problems and challenges in this field and propose an outlook for the future developments.

Understanding the Dynamics Governing Electrocatalytic Hydrodeoxygenation of Lignin Bio-Oil to Hydrocarbons

Electrocatalytic hydrodeoxygenation (EHDO) is a promising approach for upgrading biomass-derived bio-oils to sustainable fuels without the use of high-pressure hydrogen gas and elevated temperatures. However, direct EHDO for realistic hydrophobic lignin-based oil production remains challenging. Herein, we discuss the molecular dynamics that govern the EHDO of lignin bio-oil over Pt/C in an acidic electrolyte added with 2-propanol or a surfactant. Excellent conversion (98.1%) and a high yield (79.0%) of hydrogenated products, including 40.5% propyl-cyclohexane, are achieved under ambient temperature and pressure. Experimental results and various investigations on molecular dynamics suggest that EHDO occurs at the water-solvent-catalyst three-phase boundary. Proton transfer significantly influences the current density of EHDO. Factors such as cluster size and vector of lignin-based oil to electrode govern the selectivity and Faradaic efficiency of EHDO. This work advances the understanding of dynamics for EHDO and suggests governing factors to improve it.

Modulating the Coverage of Adsorbed Hydrogen via Hydrogen Spillover Enables Selective Electrocatalytic Hydrogenation of Phenol to Cyclohexanone

Selective electrocatalytic hydrogenation (ECH) of phenol is a sustainable route to produce cyclohexanone, an industrially important feedstock for polymer synthesis. However, attaining high selectivity and faradaic efficiency (FE) for cyclohexanone remain challenging, owning to over-hydrogenation of phenol to cyclohexanol and competition of hydrogen evolution reaction (HER). Herein, by employing hydrogen spillover effect, we modulate adsorbed hydrogen species (Hads) coverage on Pt surface via migration to TiO2 in an anatase TiO2-supported Pt catalyst. In ECH of phenol, a high selectivity (94 %) and good FE (63 %) for cyclohexanone are obtained, showing more advantageous performance compared with previous reports. Cyclic voltammetry (CV) tests and electrochemical Raman spectroscopy reveal that Hads migrated from Pt to TiO2. We propose that TiO2-induced hydrogen spillover contributes to low Hads coverage over Pt, which effectively hinders over-hydrogenation of cyclohexanone and HER. We establish a scaling relationship between the intensity of hydrogen spillover and cyclohexanone selectivity by varying the types of anatase TiO2, and show the universality of the strategy over other reducible metal oxides as the support (rutile TiO2, CeO2 and WO3). This work showcases an effective strategy for tuning hydrogenation selectivity in electro-catalysis, by taking advantage of thermo-catalytically well-documented hydrogen spillover effect.

Recent Progress in Electrocatalytic Conversion of Lignin: From Monomers, Dimers, to Raw Lignin

Lignin, as the second largest renewable biomass resource in nature, has increasingly received significant interest for its potential to be transformed into valuable chemicals, potentially contributing to carbon neutrality. Among different approaches, renewable electricity-driven biomass conversion holds great promise to substitute a petroleum resource-driven one, owing to its characteristics of environmental friendliness, high energy efficiency, and tunable reactivity. The challenges lie on the polymeric structure and complex functional groups in lignin, requiring the development of efficient electrocatalysts for lignin valorization with enhanced activity and selectivity toward targeted chemicals. In this Review, we focus on the advancement of electrocatalytic valorization of lignin, from monomers, to dimers and to raw lignin, toward various value-added chemicals, with emphasis on catalyst design, reaction innovation, and mechanistic study. The general strategies for catalyst design are also summarized, offering insights into enhancing the activity and selectivity. Finally, challenges and perspectives for the electrocatalytic conversion of lignin are proposed.

Electrokinetic Analyses Uncover the Rate-Determining Step of Biomass-Derived Monosaccharide Electroreduction on Copper

Electrochemical biomass conversion holds promise to upcycle carbon sources and produce valuable products while reducing greenhouse gas emissions. To this end, deep insight into the interfacial mechanism is essential for the rational design of an efficient electrocatalytic route, which is still an area of active research and development. Herein, we report the reduction of dihydroxyacetone (DHA)-the simplest monosaccharide derived from glycerol feedstock-to acetol, the vital chemical intermediate in industries, with faradaic efficiency of 85±5 % on a polycrystalline Cu electrode. DHA reduction follows preceding dehydration by coordination with the carbonyl and hydroxyl groups and the subsequent hydrogenation. The electrokinetic profile indicates that the rate-determining step (RDS) includes a proton-coupled electron transfer (PCET) to the dehydrated intermediate, revealed by coverage-dependent Tafel slope and isotopic labeling experiments. An approximate zero-order dependence of H+ suggests that water acts as the proton donor for the interfacial PCET process. Leveraging these insights, we formulate microkinetic models to illustrate its origin that Eley-Rideal (E-R) dominates over Langmuir-Hinshelwood (L-H) in governing Cu-mediated DHA reduction, offering rational guidance that increasing the concentration of the adsorbed reactant alone would be sufficient to promote the activity in designing practical catalysts.

A Ru/RuO2 heterostructure boosting electrochemistry-assisted selective benzoic acid hydrogenation

Electrocatalytic hydrogenation of benzoic acid (BA) to cyclohexanecarboxylic acid (CCA) at ambient temperature and pressure has been recognized as a promising alternative to thermal hydrogenation since water is required as the hydrogen source. So far, only a few Pt-based electrocatalysts have been developed in acidic electrolyte. To overcome the limitations of reactant solubility and catalyst corrosion, herein, carbon fiber-supported Ru electrocatalysts with abundant Ru/RuO2 heterojunctions were fabricated via cyclic electrodeposition between -0.8 and 1.1 V vs. Ag/AgCl. In an alkaline environment, a Ru/RuO2 catalyst achieves an excellent ECH reactivity in terms of high BA conversion (100%) and selectivity towards CCA (100%) within 180 min at a current density of 200/3 mA cm-2, showing exceptional reusability and long-term stability. 1-Cyclohexenecarboxylic acid (CEA) was identified as the reaction intermediate, whose the selectivity is governed by the applied potential. Kinetic studies demonstrate that ECH of BA over Ru/RuO2 follows a Langmuir-Hinshelwood (L-H) mechanism. In situ Raman spectroscopy and theoretical calculations reveal that the Ru/RuO2 interface enhances the adsorption strength of CEA, thereby facilitating the production of fully hydrogenated CCA. This work provides a deep understanding of the ECH pathway of BA in alkaline media, and gives a new methodology to fabricate heterostructure electrocatalysts.

Synergistic Roles of the CoO/Co Heterostructure and Pt Single Atoms for High-Efficiency Electrocatalytic Hydrogenation of Lignin-Derived Bio-Oils

Electrochemical hydrogeneration (ECH) of biomass-derived platform molecules, which avoids the disadvantages in utilizing fossil fuel and gaseous hydrogen, is a promising route toward value-added chemicals production. Herein, we reported a CoO/Co heterostructure-supported Pt single atoms electrocatalyst (Pt1-CoO/Co) that exhibited an outstanding performance with a high conversion (>99%), a high Faradaic efficiency (87.6%), and robust stability (24 recyclability) at -20 mA/cm2 for electrochemical phenol hydrogenation to high-valued KA oil (a mixture of cyclohexanol and cyclohexanone). Experimental results and the density functional theory calculations demonstrated that Pt1-CoO/Co presented strong adsorption of phenol and hydrogen on the catalyst surface simultaneously, which was conducive to the transfer of the adsorbed hydrogen generated on the single atom Pt sites to activated phenol, and then, ECH of phenol with high performance was achieved instead of the direct hydrogen evolution reaction. This work described that the multicomponent synergistic single atom catalysts could effectively accelerate the ECH of phenol, which could help the achievement of large-scale biomass upgrading.

Electrocatalytic valorization of lignocellulose-derived aromatics at industrial-scale current densities

Electrocatalytic hydrogenation of lignocellulosic bio-oil to value-added chemicals offers an attractive avenue to use the increasing intermittent renewable electricity and biomass-derived feedstocks. However, to date the partial current densities to target products of these reactions are lower than those needed for industrial-scale productivity, which limits its prospects. Here we report a flow-cell system equipped with a Rh diffusion electrode to hydrogenate lignocellulose-derived aromatic monomers, such as furans and lignin monomers, to value-added chemicals. We achieve high faradaic efficiencies up to 64% at industrial-scale current densities of 300-500 mA cm-2, representing high productivities to target products. A screening of electrocatalysts indicates that only by highly-electrolyte-permeable Rh diffusion electrodes are we able to unite current density with faradaic efficiency. We apply in-situ infrared reflection-absorption spectroscopy to investigate the electrode-potential-dependent reaction pathways and intermediates, confirming a wide potential window for efficient electrocatalytic hydrogenation of lignocellulose-derived aromatics to target products.

Supramolecular Polyaniline-Metal Ion as Chiral Nanozymes for Enantioselective Catalysis

Understanding origin of asymmetric information encoded on chiral nanozymes is important in mediating enantioselective catalysis. Herein, the supramolecular chiral nanozymes constructed from P/M-polyaniline (P/M-PANI) nanotwists and metal ions (M2+ , M = Cu, Ni, Co, and Zn) are designed through thioglycolic acid (TA) without chiral molecules to show the regulated catalytic efficiency and enantioselectivity. With combination of chiral environment from supramolecular scaffolds and catalytic center from metal ions, the P-PANI-TA-M2+ as nanozymes show preference to 3,4-dihydroxy-S-phenylalanine (S-DOPA) oxidation while the M-PANI-TA-M2+ show better selectivity to R-DOPA oxidation. Among them, though the Cu2+ doped supramolecular nanotwists show the highest catalytic efficiency, the Co2+ doped ones with moderate catalytic efficiency can exhibit the best enantioselectivity with select factor as high as 2.07. The molecular dynamic (MD) simulation clarifies the mechanism of enantioselective catalysis caused by the differential kinetics with S/R-DOPA enantiomers adsorbed on chiral PANI surface and free in solution. This work systematically studies the synergistic effect between the chiral supramolecular nanostructures assembled by achiral species and metal ions as peroxidase-like catalytic centers to regulate the enantioselectivity, providing deep understanding of the origin of asymmetric catalysis and serving as strong foundation to guide the design of nanozymes with high enantioselectivity.

Developing electrochemical hydrogenation towards industrial application

Electrochemical hydrogenation reactions gained significant attention as a sustainable and efficient alternative to conventional thermocatalytic hydrogenations. This tutorial review provides a comprehensive overview of the basic principles, the practical application, and recent advances of electrochemical hydrogenation reactions, with a particular emphasis on the translation of these reactions from lab-scale to industrial applications. Giving an overview on the vast amount of conceivable organic substrates and tested catalysts, we highlight the challenges associated with upscaling electrochemical hydrogenations, such as mass transfer limitations and reactor design. Strategies and techniques for addressing these challenges are discussed, including the development of novel catalysts and the implementation of scalable and innovative cell concepts. We furthermore present an outlook on current challenges, future prospects, and research directions for achieving widespread industrial implementation of electrochemical hydrogenation reactions. This work aims to provide beginners as well as experienced electrochemists with a starting point into the potential future transformation of electrochemical hydrogenations from a laboratory curiosity to a viable technology for sustainable chemical synthesis on an industrial scale.

Selective electrocatalytic hydrogenation of lignocellulose-derived 5-hydroxymethylfurfural with superior productivities

Recently, valorization of biomass to value-added chemicals has drawn increasing attention due to carbon neutrality and sustainability. 5-Hydroxymethylfurfural is an important lignocellulose-derived biomass molecule. Herein, we have demonstrated the efficient electrocatalytic hydrogenation of 5-hydroxymethylfurfural to value-added 2,5-bis(hydroxymethyl)furan. An optimized electrolyzer with a highly electrolyte-permeable Pd cathode well balanced the selectivity, faradaic efficiency, and productivity. We have achieved high selectivity (97%) and faradaic efficiency (72%) at 50 mA cm-2 current density, and a record high productivity of 0.923 mmol cm-2·h-1 at 100 mA cm-2 current density, ∼2 times advanced compared with the best productivity in prior reports. We applied in situ infrared reflection-absorption spectroscopy to investigate the electrode-potential-dependent reaction pathways and mechanism, confirming that the highly selective hydrogenation of HMF is due to the tilted adsorption geometry through carbonyl group bonding to the surface of electrode. This work offers an opportunity for the sustainable electrocatalytic valorization of renewable lignocellulose-derived biomass with superior productivities approaching industrial level.

Simultaneous Oxidative Cleavage of Lignin and Reduction of Furfural via Efficient Electrocatalysis by P-Doped CoMoO4

Electrochemical oxidative lignin cleavage and coupled 2-furaldehyde reduction provide a promising approach for producing high-value added products. However, developing efficient bifunctional electrocatalysts with noble-metal-like activity still remains a challenge. Here, an efficient electrochemical strategy is reported for the selective oxidative cleavage of C_α -C_β bonds in lignin into aromatic monomers by tailoring the electronic structure through P-doped CoMoO₄ spinels (99% conversion, highest monomer selectivity of 56%). Additionally, the conversion and selectivity of 2-furaldehyde reduction to 2-methyl furan reach 87% and 73%, respectively. In situ Fourier transform infrared and density functional theory analysis reveal that an upward shift of the E_d upon P-doping leads to an increase in the antibonding level, which facilitates the C_α -C_β adsorption of the lignin model compounds, thereby enhancing the bifunctional electrocatalytic activity of the active site. This work explores the potential of a spinel as a bifunctional electrocatalyst for the oxidative cracking of lignin and the reductive conversion of small organic molecules to high-value added chemicals via P-anion modulation.

Selective Electrochemical Hydrogenation of Phenol with Earth-abundant Ni-MoO2 Heterostructured Catalysts: Effect of Oxygen Vacancy on Product Selectivity

Herein, we report highly efficient carbon supported Ni-MoO₂ heterostructured catalysts for the electrochemical hydrogenation (ECH) of phenol in 0.10 M aqueous sulfuric acid (pH 0.7) at 60 °C. Highest yields for cyclohexanol and cyclohexanone of 95 % and 86 % with faradaic efficiencies of ∼50 % are obtained with catalysts bearing high and low densities of oxygen vacancy (O_v ) sites, respectively. In situ diffuse reflectance infrared spectroscopy and density functional theory calculations reveal that the enhanced phenol adsorption strength is responsible for the superior catalytic efficiency. Furthermore, 1-cyclohexene-1-ol is an important intermediate. Its hydrogenation route and hence the final product are affected by the O_v density. This work opens a promising avenue to the rational design of advanced electrocatalysts for the upgrading of phenolic compounds.

Recent Progress in Electrochemical Upgrading of Bio-Oil Model Compounds and Bio-Oils to Renewable Fuels and Platform Chemicals

Sustainable production of renewable carbon-based fuels and chemicals remains a necessary but immense challenge in the fight against climate change. Bio-oil derived from lignocellulosic biomass requires energy-intense upgrading to produce usable fuels or chemicals. Traditional upgrading methods such as hydrodeoxygenation (HDO) require high temperatures (200−400 °C) and 200 bar of external hydrogen. Electrochemical hydrogenation (ECH), on the other hand, operates at low temperatures (<80 °C), ambient pressure, and does not require an external hydrogen source. These environmental and economically favorable conditions make ECH a promising alternative to conventional thermochemical upgrading processes. ECH combines renewable electricity with biomass conversion and harnesses intermediately generated electricity to produce drop-in biofuels. This review aims to summarize recent studies on bio-oil upgrading using ECH focusing on the development of novel catalytic materials and factors impacting ECH efficiency and products. Here, electrode design, reaction temperature, applied overpotential, and electrolytes are analyzed for their impacts on overall ECH performance. We find that through careful reaction optimization and electrode design, ECH reactions can be tailored to be efficient and selective for the production of renewable fuels and chemicals. Preliminary economic and environmental assessments have shown that ECH can be viable alternative to convention upgrading technologies with the potential to reduce CO2 emissions by 3 times compared to thermochemical upgrading. While the field of electrochemical upgrading of bio-oil has additional challenges before commercialization, this review finds ECH a promising avenue to produce renewable carbon-based drop-in biofuels. Finally, based on the analyses presented in this review, directions for future research areas and optimization are suggested.

In situ reconfiguration of plasma-engineered copper electrodes towards efficient electrocatalytic hydrogenation

Defect engineering of Cu via O2-plasma is introduced to accomplish efficient electrocatalytic hydrogenation, in which the in situ reduction of CuOx to defective Cu promotes the kinetics.