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

Electrocatalytic hydrogenation of cyanoarenes, nitroarenes, quinolines, and pyridines under mild conditions with a proton-exchange membrane reactor

An electrocatalytic hydrogenation of cyanoarenes, nitroarenes, quinolines, and pyridines using a proton-exchange membrane (PEM) reactor was developed. Cyanoarenes were then reduced to the corresponding benzylamines at room temperature in the presence of ethyl phosphate. The reduction of nitroarenes proceeded at room temperature, and a variety of anilines were obtained. The quinoline reduction was efficiently promoted by adding a catalytic amount of p-toluenesulfonic acid (PTSA) or pyridinium p-toluenesulfonate (PPTS). Pyridine was also reduced to piperidine in the presence of PTSA.

Electrochemical hydrogenation and oxidation of organic species involving water

Fossil fuel-driven thermochemical hydrogenation and oxidation using high-pressure H2 and O2 are still popular but energy-intensive CO2-emitting processes. At present, developing renewable energy-powered electrochemical technologies, especially those using clean, safe and easy-to-handle reducing agents and oxidants for organic hydrogenation and oxidation reactions, is urgently needed. Water is an ideal carrier of hydrogen and oxygen. Electrochemistry provides a powerful route to drive water splitting under ambient conditions. Thus, electrochemical hydrogenation and oxidation transformations involving water as the hydrogen source and oxidant, respectively, have been developed to be mild and efficient tools to synthesize organic hydrogenated and oxidized products. In this Review, we highlight the advances in water-participating electrochemical hydrogenation and oxidation reactions of representative organic molecules. Typical electrode materials, performance metrics and key characterization techniques are firstly introduced. General electrocatalyst design principles and controlling the microenvironment for promoting hydrogenation and oxygenation reactions involving water are summarized. Furthermore, paired hydrogenation and oxidation reactions are briefly introduced before finally discussing the challenges and future opportunities of this research field.

Removal of organic pollutants in shale gas fracturing flowback and produced water: A review

Shale gas has been developed as an alternative to conventional energy worldwide, resulting in a large amount of shale gas fracturing flowback and produced water (FPW). Previous studies focus on total dissolved solids reduction using membrane desalination. However, there is a lack of efficient and stable techniques to remove organic pollutants, resulting in severe membrane fouling in downstream processes. This review focuses on the concentration and chemical composition of organic matter in shale gas FPW in China, as well as the hazards of organic pollutants. Organic removal techniques, including advanced oxidation processes, coagulation, sorption, microbial degradation, and membrane treatment are systematically reviewed. In particular, the influences of high salt on each technique are highlighted. Finally, different treatment techniques are evaluated in terms of energy consumption, cost, and organic removal efficiency. It is concluded that integrated coagulation-sorption-Fenton-membrane filtration represents a promising treatment process for FPW. This review provides valuable information for the feasible design, practical operation, and optimization of FPW treatment.

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.

Electrochemical hydrogenation of enones using a proton-exchange membrane reactor: selectivity and utility

Electrochemical hydrogenation of enones using a proton-exchange membrane reactor is described. The reduction of enones proceeded smoothly under mild conditions to afford ketones or alcohols. The reaction occurred chemoselectively with the use of different cathode catalysts (Pd/C or Ir/C).

Electrocatalysis with Molecular Transition-Metal Complexes for Reductive Organic Synthesis

Electrocatalysis enables the formation or cleavage of chemical bonds by a genuine use of electrons or holes from an electrical energy input. As such, electrocatalysis offers resource-economical alternative pathways that bypass sacrificial, waste-generating reagents often required in classical thermal redox reactions. In this Perspective, we showcase the exploitation of molecular electrocatalysts for electrosynthesis, in particular for reductive conversion of organic substrates. Selected case studies illustrate that efficient molecular electrocatalysts not only are appropriate redox shuttles but also embrace the features of organometallic catalysis to facilitate and control chemical steps. From these examples, guidelines are proposed for the design of molecular electrocatalysts suited to the reduction of organic substrates. We finally expose opportunities brought by catalyzed electrosynthesis to functionalize organic backbones, namely using sustainable building blocks.

Upgrading of Bio-Oil Model Compounds and Bio-Crude into Biofuel by Electrocatalysis: A Review

Limited availability of fossil energy and serious environmental pollution have caused the emergence of bio-oil, which can serve as an alternative and promising green energy source. However, bio-oil generated from the rapid pyrolysis of biomass cannot be utilized immediately owing to its corrosivity, instability, and low heating value. Herein, the electrocatalytic hydrogenation (ECH) process towards bio-oil upgrading is reviewed. Specifically, the ECH integrates the advantages of mild operating conditions, no petrochemically derived hydrogen and good controllability. The influence of different factors on the conversion of bio-oil components and product selectivity in the ECH process are presented comprehensively. In addition, various reaction mechanisms are discussed in the designed ECH systems. Finally, some challenges need to be further overcome for real bio-oil reduction in the ECH process: exploration of efficient multifunctional electrocatalysts for specific bio-oil components and determination of the dominant steps in the complicated reaction path network.

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?

Greener Routes to Biomass Waste Valorization: Lignin Transformation Through Electrocatalysis for Renewable Chemicals and Fuels Production

Lignin valorization is essential for biorefineries to produce fuels and chemicals for a sustainable future. Today's biorefineries pursue profitable value propositions for cellulose and hemicellulose; however, lignin is typically used mainly for its thermal energy value. To enhance the profit potential for biorefineries, lignin valorization would be a necessary practice. Lignin valorization is greatly advantaged when biomass carbon is retained in the fuel and chemical products and when energy quality is enhanced by electrochemical upgrading. Though lignin upgrading and valorization are very desirable in principle, many barriers involved in lignin pretreatment, extraction, and depolymerization must be overcome to unlock its full potential. This Review addresses the electrochemical transformation of various lignins with the aim of gaining a better understanding of many of the barriers that currently exist in such technologies. These studies give insight into electrochemical lignin depolymerization and upgrading to value-added commodities with the end goal of achieving a global low-carbon circular economy.

Membrane-based separation technologies: from polymeric materials to novel process: an outlook from China

During the past two decades, research on membrane and membrane-based separation process has developed rapidly in water treatment, gas separation, biomedicine, biotechnology, chemical manufacturing and separation process integration. In China, remarkable progresses on membrane preparation, process development and industrial application have been made with the burgeoning of the domestic economy. This review highlights the recent development of advanced membranes in China, such as smart membranes for molecular-recognizable separation, ion exchange membrane for chemical productions, antifouling membrane for liquid separation, high-performance gas separation membranes and the high-efficiency hybrid membrane separation process design, etc. Additionally, the applications of advanced membranes, relevant devices and process design strategy in chemical engineering related fields are discussed in detail. Finally, perspectives on the future research directions, key challenges and issues in membrane separation are concluded.

Electrochemical Conversion of Biomass-Based Oxygenated Compounds

Dwindling fossil fuel resources and substantial release of CO2 from their processing have increased the appeal to use biomass as a sustainable platform for synthesis of chemicals and fuels. Steps toward this will require selective upgrading of biomass to suitable intermediates. Traditionally, biomass upgrading has involved thermochemical processes that require excessive amounts of petrochemical-derived H2 and suffer from poor product selectivity. Electrochemical routes have emerged as promising alternatives because of ( a) the replacement of petrochemical-derived H2 by protons generated in situ, ( b) mild operating temperatures and pressures, and ( c) the use of electrode potential to tune reaction rates and product selectivity. In this review, we highlight the advances in the electrocatalytic hydrogenation and oxidation of biomass-derived platform molecules. The effects of important reaction parameters on electrochemical efficiency and catalytic activity/selectivity are thoroughly discussed. We conclude by summarizing current challenges and discussing future research directions.

Electrochemical Hydrogenation of Acetone to Produce Isopropanol Using a Polymer Electrolyte Membrane Reactor

Electrochemical hydrogenation (ECH) of acetone is a relatively new method to produce isopropanol. It provides an alternative way of upgrading bio-fuels with less energy consumption and chemical waste as compared to conventional methods. In this paper, Polymer Electrolyte Membrane Fuel Cell (PEMFC) hardware was used as an electrochemical reactor to hydrogenate acetone to produce isopropanol and diisopropyl ether as a byproduct. High current efficiency (59.7%) and selectivity (>90%) were achieved, while ECH was carried out in mild conditions (65 °C and atmospheric pressure). Various operating parameters were evaluated to determine their effects on the yield of acetone and the overall efficiency of ECH. The results show that an increase in humidity increased the yield of propanol and the efficiency of ECH. The operating temperature and power supply, however, have less effect. The degradation of membranes due to contamination of PEMFC and the mitigation methods were also investigated.

Electrocatalytic valorisation of biomass derived chemicals

Recent progress in electro-valorization of biomass-derived intermediates is reviewed, while a perspective on future R&D in this field is provided.

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.

Influence of electrode assembly on catalytic activation and deactivation of a Pt film immobilized H+ conducting solid electrolyte in electrocatalytic reduction reactions

Symmetric assembly deactivated the electrocatalytic nitrate reduction process due to oxidation of copper atoms on the anode surface. In case of assymetric assembly, H₂ evolution eroded copper oxides causing catalytic activation.

Liquid fuel cells

The advantages of liquid fuel cells (LFCs) over conventional hydrogen-oxygen fuel cells include a higher theoretical energy density and efficiency, a more convenient handling of the streams, and enhanced safety. This review focuses on the use of different types of organic fuels as an anode material for LFCs. An overview of the current state of the art and recent trends in the development of LFC and the challenges of their practical implementation are presented.

Selective glycerol oxidation by electrocatalytic dehydrogenation

This study demonstrates that an electrochemical dehydrogenation process can be used to oxidize glycerol to glyceraldehyde and glyceric acid even without using stoichiometric chemical oxidants. A glyceric acid selectivity of 87.0 % at 91.8 % glycerol conversion was obtained in an electrocatalytic batch reactor. A continuous-flow electrocatalytic reactor had over an 80 % high glyceric acid selectivity at 10 % glycerol conversion, as well as greater reaction rates than either an electrocatalytic or a conventional catalytic batch reactor.

Electricity storage in biofuels: selective electrocatalytic reduction of levulinic acid to valeric acid or γ-valerolactone

Herein, we report an effective approach to electricity storage in biofuels by selective electrocatalytic reduction of levulinic acid (LA) to high-energy-density valeric acid (VA) or γ-valerolactone (gVL) on a non-precious Pb electrode in a single-polymer electrolyte membrane electrocatalytic (flow) cell reactor with a very high yield of VA (>90 %), a high Faradaic efficiency (>86 %), promising electricity storage efficiency (70.8 %), and a low electricity consumption (1.5 kWhL(VA)(-1) ). The applied potential and electrolyte pH can be used to accurately control the reduction products: lower overpotentials favor the production of gVL, whereas higher overpotentials facilitate the formation of VA. A selectivity of 95 % to VA in acidic electrolyte (pH 0) and 100 % selectivity to gVL in neutral electrolyte (pH 7.5) are obtained. The effect of the molecular structure on the electrocatalytic reduction of ketone and aldehyde groups of biomass compounds was investigated. Whereas LA can be fully electroreduced to VA though a four-electron transfer, the C-O groups are only electroreduced to -OH by a two-electron-transfer process when glyoxylic acid and pyruvic acid serve as feedstocks.