The production of future transportation fuels and chemicals requires the deployment of new catalytic processes that transform biomass into valuable products under competitive conditions. Furfural has been identified as one of the most promising chemical platforms directly derived from biomass. With an annual production close to 300 kTon, furfural is currently a commodity chemical, and the technology for its production is largely established. The aim of this review is to discuss the most relevant chemical routes for converting furfural to chemicals, biofuels, and additives. This review focuses not only on industrially produced chemicals derived from furfural, but also on other not yet commercialised products that have a high potential for commercialisation as commodities. Other chemicals that are currently produced from oil but can also be derived from furfural are also reviewed. The chemical and engineering aspects such as the reaction conditions and mechanisms, as well as the main achievements and the challenges still to come in the pursuit of advancing the furfural-based industry, are highlighted.

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Furfural: a renewable and versatile platform molecule for the synthesis of chemicals and fuels

Slow-pyrolysis of biomass for the production of biochar, a stable carbon-rich solid by-product, has gained considerable interest due to its proven role and application in the multidisciplinary areas of science and engineering. An alternative to slow-pyrolysis is a relatively new process called hydrothermal carbonization (HTC) of biomass, where the biomass is treated with hot compressed water instead of drying, has shown promising results. The HTC process offers several advantages over conventional dry-thermal pre-treatments like slow-pyrolysis in terms of improvements in the process performances and economic efficiency, especially its ability to process wet feedstock without pre-drying requirement. Char produced from both the processes exhibits significantly different physiochemical properties that affect their potential applications, which includes but is not limited to carbon sequestration, soil amelioration, bioenergy production, and wastewater pollution remediation. This paper provides an updated review on the fundamentals and reaction mechanisms of the slow-pyrolysis and HTC processes, identifies research gaps, and summarizes the physicochemical characteristics of chars for different applications in the industry. The literature reviewed in this study suggests that hydrochar (HTC char) is a valuable resource and is superior to biochar in certain ways. For example, it contains a reduced alkali and alkaline earth and heavy metal content, and an increased higher heating value compared to the biochar produced at the same operating process temperature. However, its effective utilization would require further experimental research and investigations in terms of feeding of biomass against pressure; effects and relationships among feedstocks compositions, hydrochar characteristics and process conditions; advancement in the production technique(s) for improvement in the physicochemical behavior of hydrochar; and development of a diverse range of processing options to produce hydrochar with characteristics required for various industry applications.

A comparative review of biochar and hydrochar in terms of production, physico-chemical properties and applications

The direct synthetic organic use of electricity is currently experiencing a renaissance. More synthetically oriented laboratories working in this area are exploiting both novel and more traditional concepts, paving the way to broader applications of this niche technology. As only electrons serve as reagents, the generation of reagent waste is efficiently avoided. Moreover, stoichiometric reagents can be regenerated and allow a transformation to be conducted in an electrocatalytic fashion. However, the application of electroorganic transformations is more than minimizing the waste footprint, it rather gives rise to inherently safe processes, reduces the number of steps of many syntheses, allows for milder reaction conditions, provides alternative means to access desired structural entities, and creates intellectual property (IP) space. When the electricity originates from renewable resources, this surplus might be directly employed as a terminal oxidizing or reducing agent, providing an ultra-sustainable and therefore highly attractive technique. This Review surveys recent developments in electrochemical synthesis that will influence the future of this area.

Electrifying Organic Synthesis

Hydrothermal conversion of biomass waste to activated carbon with high porosity: a review.

Hydrothermal carbonization (HTC) is a thermochemical conversion technique which is attractive due to its ability to transform wet biomass into energy and chemicals without predrying. The solid product, known as hydrochar, has received attention because of its ability to prepare precursors of activated carbon in wastewater pollution remediation, soil remediation applications, solid fuels, and other carbonaceous materials. Besides the generally lignocellulose biomass used as sustainable feedstock, HTC has been applied to a wide range of derived waste, including sewage sludge, algae, and municipal solid waste to solve practical problems and generate desirable carbonaceous products. This review presented the critical hydrothermal parameters of HTC, including temperature, residence time, heating rate, reactant concentration, and aqueous quality. The chemical reaction mechanisms involved in the formation of hydrochar derived from single components and representative feedstock, lignocellulose, and sludge termed as N-free and N-rich biomass, were elucidated and summarized to better understand the hydrochar formation process. Specifically, hydrochar physicochemical characteristics such as surface chemistry and structure were investigated. Current knowledge gaps, and new perspectives with corresponding recommendations were provided to further exploit the great potential of the HTC technique and more practical applications for hydrochar in the future.

A review of the hydrothermal carbonization of biomass waste for hydrochar formation: process conditions, fundamentals, and physicochemical properties.

Subcritical water, that is, water above the boiling and below critical point, is a unique and sustainable reaction medium. Based on its solvent properties, in combination with the often considerable intrinsic water content of natural biomass, it is often considered as a potential solvent for biomass processing. Current knowledge on biomass transformation in subcritical water is, however, still rather scattered without providing a consistent picture. Concentrating on fundamental physical and chemical aspects, this review summarizes the current state of knowledge of hydrothermal biomass conversion in subcritical water. After briefly introducing subcritical water as a reaction medium, its advantages for biomass processing compared to other thermal processes are highlighted. Subsequently, the physical-chemical properties of subcritical water are discussed in the light of their impact on the occurring chemical reactions. The influence of major operational parameters, including temperature, pressure, and reactant concentration on hydrothermal biomass transformation processes are illustrated for selected carbohydrates. Major emphasis is put on the nature of the carbohydrate monomers, since the conversion of the respective polymers is analogous with the additional prior step of hydrolytic depolymerization.

Subcritical Water as Reaction Environment: Fundamentals of Hydrothermal Biomass Transformation

The electrochemical conversion of furfural to 2-methylfuran (MF) and of 5-hydroxymethylfurfural (5-HMF) to 2,5-dimethylfuran (DMF) by means of a room-temperature electrocatalytic hydrogenation is presented. This is the first time that the electrochemical conversion of 5-HMF to 2,5-dimethylfuran is demonstrated, and it is the first time that the electrochemical conversion of furfural to methylfuran is achieved at high selectivity. The electrochemical hydrogenation may provide a path for the storage of electric energy into liquid organic fuels.

Electrochemistry for biofuel generation: production of furans by electrocatalytic hydrogenation of furfurals

By means of a two-step electrochemical conversion of levulinic acid to octaneviavaleric acid we propose the use of electrochemistry for the production of renewable chemicals and biofuels. The reactions can be performed in water and at room temperature and thus fulfil the major criteria of green chemistry.

Electrochemistry for biofuel generation: Electrochemical conversion of levulinic acid to octane

The oxidative and reductive electrochemical conversion of levulinic acid to its primary products valeric acid, γ-valerolactone, 2,7-octanedione, 4-hydroxy-2-butanone and 3-buten-2-one is studied in detail. The reactions were performed in aqueous solutions and at ambient temperature, following the principles of green chemistry. The obtained primary reaction products were studied with respect to the oxidative and reductive electrochemical formation of secondary products, such as n-octane, 1-butanol and 1,3-butanediol. It is shown that the choice of electrolyte composition, educt concentration and the nature of the electrode material has a strong influence on the selectivity of product formation. For instance it is demonstrated that in alkaline solutions γ-valerolactone can be gained from levulinic acid at iron electrodes with similar Coulombic efficiency (∼20%) but higher selectivity (S = 70%) than on lead (S = 50%). Furthermore, for the first time the electrochemical two-step reaction of levulinic acid to 1-butanol via 4-hydroxy-2-butanone is reported. For some of the reaction pathways the main product is water insoluble, which allows a direct separation of the product and the potential electrolyte reuse in a semi-continuous process. Especially the use of the electrocatalytic hydrogenation may provide a path for the storage of electricity into liquid organic fuels as shown by a basic energetic assessment of all electrochemical conversions.

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Electrochemistry for the generation of renewable chemicals: electrochemical conversion of levulinic acid

Electroorganic synthesis can be exploited for the production of biofuels from fatty acids and triglycerides. With Coulomb efficiencies (CE) of up to 50 %, the electrochemical decarboxylation of fatty acids in methanolic and ethanolic solutions leads to the formation of diesel-like olefin/ether mixtures. Triglycerides can be directly converted in aqueous solutions by using sonoelectrochemistry, with olefins as the main products (with a CE of more than 20 %). The latter reaction, however, is terminated at around 50 % substrate conversion by the produced side-product glycerol. An energy analysis shows that the electrochemical olefin synthesis can be an energetically competitive, sustainable, and--in comparison with established processes--economically feasible alternative for the exploitation of fats and oils for biofuel production.

Peter Nilges

Papers

Subcritical Water as Reaction Environment: Fundamentals of Hydrothermal Biomass Transformation

Electrochemistry for biofuel generation: production of furans by electrocatalytic hydrogenation of furfurals

Electrochemistry for biofuel generation: Electrochemical conversion of levulinic acid to octane

Electrochemistry for the generation of renewable chemicals: electrochemical conversion of levulinic acid

Electrochemistry for biofuel generation: transformation of fatty acids and triglycerides to diesel-like olefin/ether mixtures and olefins.