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.

Electrocatalytic Hydrogenation of Biomass-Derived Organics: A Review

Sustainable energy generation calls for a shift away from centralized, high-temperature, energy-intensive processes to decentralized, low-temperature conversions that can be powered by electricity produced from renewable sources. Electrocatalytic conversion of biomass-derived feedstocks would allow carbon recycling of distributed, energy-poor resources in the absence of sinks and sources of high-grade heat. Selective, efficient electrocatalysts that operate at low temperatures are needed for electrocatalytic hydrogenation (ECH) to upgrade the feedstocks. For effective generation of energy-dense chemicals and fuels, two design criteria must be met: (i) a high H:C ratio via ECH to allow for high-quality fuels and blends and (ii) a lower O:C ratio in the target molecules via electrochemical decarboxylation/deoxygenation to improve the stability of fuels and chemicals. The goal of this review is to determine whether the following questions have been sufficiently answered in the open literature, and if not, what additional information is required:(1)What organic functionalities are accessible for electrocatalytic hydrogenation under a set of reaction conditions? How do substitutions and functionalities impact the activity and selectivity of ECH?(2)What material properties cause an electrocatalyst to be active for ECH? Can general trends in ECH be formulated based on the type of electrocatalyst?(3)What are the impacts of reaction conditions (electrolyte concentration, pH, operating potential) and reactor types?

Study on the Electrochemical Hydrogenation of Soybean Oil under H2 Conditions

The solubility of H₂ in electrolytes, H₂ reaction consumption and the conductivity of electrolytes under different pressures in an electrochemical hydrogenation reactor were studied. It was found that with an increase in H₂ pressure, H₂ was electrolyzed at the anode, accompanied by the generation of H⁺. The solubility of H₂ in the electrolytes and the conductivity of the electrolytes also increased. At first, the reaction consumption increased, followed by a tendency to be stable at 3 MPa. Therefore, the electrochemical hydrogenation of soybean oil was carried out at a H₂ pressure of 3 MPa. When the current was 120 mA, the temperature was 50°C, the agitation speed was 300 rpm, and the time was 7.5 h, the IV of hydrogenated soybean oil was 99.6 g I₂/100 g oil, and the TFA content of the oil was 4.3%.

Enhanced Electroreductive Removal of Bromate by a Supported Pd-In Bimetallic Catalyst: Kinetics and Mechanism Investigation

In this work, the electroreductive removal of bromate by a Pd₁-In₄/Al₂O₃ catalyst in a three-dimensional electrochemical reactor was investigated. A total of 96.4% of bromate could be efficiently reduced and completely converted into bromide within 30 min under optimized conditions. On the basis of the characterization results and kinetics analysis, a synergistic effect of Pd and In was observed, and Pd₁-In₄/Al₂O₃ had the highest reaction rate constant of 0.1275 min^-1 (vs 0.0413, 0.0328, and 0.0253 min^-1 for In/Al₂O₃, Pd/Al₂O₃, and Al₂O₃). The results of electron spin resonance and scavenger experiments confirmed that both direct electron transfer and indirect reduction by atomic H* were involved in the bromate removal process, while the direct reduction played a more important role. Moreover, the introduction of In could increase the zeta potential of Pd₁-In₄/Al₂O₃, facilitating bromate adsorption and its subsequent reduction on the catalyst. Finally, a reaction mechanism for bromate reduction by Pd₁-In₄/Al₂O₃ was proposed based on all the experimental results.

Ion-exchange membranes in chemical synthesis – a review

The applicability of ion-exchange membranes (IEMs) in chemical synthesis was discussed based on the existing literature. At first, a brief description of properties and structures of commercially available ion-exchange membranes was provided. Then, the IEM-based synthesis methods reported in the literature were summarized, and areas of their application were discussed. The methods in question, namely: membrane electrolysis, electro-electrodialysis, electrodialysis metathesis, ion-substitution electrodialysis and electrodialysis with bipolar membrane, were found to be applicable for a number of organic and inorganic syntheses and acid/base production or recovery processes, which can be conducted in aqueous and non-aqueous solvents. The number and the quality of the scientific reports found indicate a great potential for IEMs in chemical synthesis.

Graphene-modified Pd/C cathode and Pd/GAC particles for enhanced electrocatalytic removal of bromate in a continuous three-dimensional electrochemical reactor

Bromate (BrO3(-)) is a carcinogenic and genotoxic contaminant commonly generated during ozonation of bromide-containing water. In this work, the reductive removal of BrO3(-) in a continuous three-dimensional electrochemical reactor with palladium-reduced graphene oxide modified carbon paper (Pd-rGO/C) cathode and Pd-rGO modified granular activated carbon (Pd-rGO/GAC) particles was investigated. The results indicated that the rGO sheets significantly promoted the electrochemical reduction of BrO3(-). With the enhanced electron transfer by rGO sheets, the electroreduction of H2O to atomic H* on the polarized Pd particles could be significantly accelerated, leading to a faster reaction rate of BrO3(-) with atomic H*. The synergistic effect of the Pd-rGO/C cathode and Pd-rGO/GAC particles were also exhibited. The atomic H* involved in various electroreduction processes was detected by electron spin resonance spectroscopy and its role for BrO3(-) reduction was determined. The performance of the reactor was evaluated in terms of the removal of BrO3(-) and the yield of Br(-) as a function of the GO concentration, Pd loading amount, current density, hydraulic residence time (HRT), and initial BrO3(-) concentration. Under the current density of 0.9 mA/cm(2), BrO3(-) with the initial concentration of 20 μg/L was reduced to be less than 6.6 μg/L at the HRT of 20 min. The BrO3(-) reduction was inhibited in the presence of dissolved organic matter. Although the precipitates generated from Ca(2+) and Mg(2+) in the tap water would cover the Pd catalysts, a long-lasting electrocatalytic activity could be maintained for the 30 d treatment. SEM and XPS analysis demonstrated that the precipitates were predominantly deposited onto the Pd-rGO/C cathode rather than the Pd-rGO/GAC particles.

Effects of hydrophobicity of diffusion layer on the electroreduction of biomass derivatives in polymer electrolyte membrane reactors

For the first time, the hydrophobicity design of a diffusion layer based on the volatility of hydrogenation reactants in aqueous solutions is reported. The hydrophobicity of the diffusion layer greatly influences the hydrogenation performance of two model biomass derivatives, namely, butanone and maleic acid, in polymer electrolyte membrane reactors operated at atmospheric pressure. Hydrophobic carbon paper repels aqueous solutions, but highly volatile butanone can permeate in vapor form and achieve a high hydrogenation rate, whereas, for nonvolatile maleic acid, great mass transfer resistance prevents hydrogenation. With a hydrophilic stainless-steel welded mesh diffusion layer, aqueous solutions of both butanone and maleic acid permeate in liquid form. Hydrogenation of maleic acid reaches a similar level as that of butanone. The maximum reaction rate is 340 nmol cm(-2)  s(-1) for both hydrogenation systems and the current efficiency reaches 70 %. These results are better than those reported in the literature.

Electrocatalytic reduction of acetone in a proton-exchange-membrane reactor: a model reaction for the electrocatalytic reduction of biomass

Acetone was electrocatalytically reduced to isopropanol in a proton-exchange-membrane (PEM) reactor on an unsupported platinum cathode. Protons needed for the reduction were produced on the unsupported Pt-Ru anode from either hydrogen gas or electrolysis of water. The current efficiency (the ratio of current contributing to the desired chemical reaction to the overall current) and reaction rate for acetone conversion increased with increasing temperature or applied voltage for the electrocatalytic acetone/water system. The reaction rate and current efficiency went through a maximum with respect to acetone concentration. The reaction rate for acetone conversion increased with increasing temperature for the electrocatalytic acetone/hydrogen system. Increasing the applied voltage for the electrocatalytic acetone/hydrogen system decreased the current efficiency due to production of hydrogen gas. Results from this study demonstrate the commercial feasibility of using PEM reactors to electrocatalytically reduce biomass-derived oxygenates into renewable fuels and chemicals.

Selective Electrocatalytic Hydrogenation of Linolenic Acid onPd/Al2O3andPd-Co/Al2O3Catalysts

Electrochemical hydrogenation of linolenic acid as a model for polyunsaturated acids was studied on Pd and Pd/Al2O3catalysts in acidic and alkaline media. The results are presented in terms of number of double bonds in the polyunsaturated fatty acid and interpreted in terms of the adsorption capacity of the catalysts in these media. The highest hydrogenation yield was obtained with Pd/Al2O3at pH 13, in good correlation with the adsorption power of linolenic acid and its first hydrogenation product, linoleic acid, measured in this solution. A preliminary electrochemical hydrogenation study was conducted on Pd/Al2O3catalyst containing Co, in the optimum electrolysis conditions, showing a cooperative effect of the noble metals regarding thecis/transselectivity with preferential formation ofcis-oriented monounsaturated compound. All the products were characterized by gas chromatography after derivatization of the samples; fifteencis-transisomers of monounsaturated fatty acid which could be identified are presented here.

Hydrogenation for Low Trans and High Conjugated Fatty Acids

Hydrogenated vegetable oils contain of trans fatty acids. Because of the increased health concern about trans fatty acids, new hydrogenations have been studied to seek ways for substantial reduction of the trans fatty acids in the hydrogenated vegetable oils. This paper reviews new hydrogenation processes such as electrocatalytic hydrogenation, precious catalyst hydrogenation, and supercritical fluid state hydrogen, which have shown promising results for the reduction of trans fatty acids below the level of 8%. This paper also reviews the recently introduced hydeogenation technique for high accumulation of conjugated linoleic acids, beneficiary functional components. The hydrogenated vegetable oils with high quantity conjugated linoleic acid might provide the possibility for the utilization of hydrogenated oils as health-prompting food ingredients.