ReviewLignin valorization toward value-added chemicals and fuels via electrocatalysis: A perspective
Graphical abstract
This review provides a brief summary and perspective on lignin valorization toward value-added chemicals and fuels via electrocatalysis.
Introduction
The heavy global dependence on fossil feedstocks for chemicals and fuels has imposed tremendous negative impacts on the global environment and economy, which has promoted societal interest in clean and sustainable resources, such as wind, solar, and biomass energy. As a typical biomass energy source, lignocellulosic biomass has drawn considerable attention in recent years because it is a major component of trees, grasses, and straws. More importantly, it contains 15 wt%–35 wt% of lignin that is the largest natural large-scale aromatic sources with more energy dense [1, 2, 3]. These features enable lignocellulosic biomass, especially lignin, to be a promising alternative to traditional fossil feedstocks for the direct production of abundant chemicals and fuels via a renewable approach [4, 5]. Normally, three procedures are needed to convert lignocellulosic biomass to value-added chemicals and fuels: lignocellulose fractionation, lignin depolymerization, and valorization (Fig. 1). Lignocellulosic biomass is first transformed into lignin using various well-developed fractionation processes [6]. Because lignin is a complex cross-linked macromolecule consisting of rich aromatic rings connected via ether (C–O–C) and carbon-carbon (C–C) bonds, the obtained lignin is commonly depolymerized into simple aromatic lignin monomers that fit current industrial processes. Finally, in most cases, the obtained lignin monomers must be further upgraded to value-added chemicals and fuels [7].
Approximately 20 years ago, lignin was produced from pulp and paper industries and was mostly used for fuels [8]. Gradually, as the energy crisis was proposed, many other value-added applications of lignin were found, including in cement, adhesives, agriculture, and medicine [9, 10, 11, 12, 13, 14]. Lignin is a complex cross-linked macromolecule composed of functionalized aromatic units [15, 16] (Fig. 2). These units are classified into three different types: p-coumaryl alcohol (H), coniferyl alcohol (G), and sinapyl alcohol (S) (Fig. 2) [17, 18]. They are randomly connected by C–O–C and C–C bonds [19]. Typically, lignin is distinguished by the type of plant (e.g., softwood, hardwood, and grass lignin). It should be noted that softwood lignin mostly contains coniferyl alcohol, hardwood lignin contains sinapyl alcohol and coniferyl alcohol, and grass lignin contains all three alcohol types [20, 21] (Table 1).
As shown in Fig. 2, linkage is also an important component in lignin and acts as a “bridge” to link the lignin units. The type and ratio of linkages largely depend on the plant source. Normally, C–O–C and C–C bonds are the most fundamental linkages in the lignin structure. The C–O–C bond includes β–O–4, 4–O–5, and α–O–4 types and the C–C bond presents β–5, β–1, 5–5, and β–β types [18, 22]. β–O–4 is rich in lignin (>50%) and is the most common linkage [2, 23]. From Fig. 2, it can be clearly seen that lignin also contains various functional groups (e.g., hydroxy, methoxy, carboxyl) [20, 24]. These linkages and functional groups make lignin chemically reactive, which enables lignin-based biomass to be a feedstock for high-value-added chemicals and fuels. Linkage is a significant aspect for evaluating the properties of model chemicals, and it helps predict potential products that can be used in various synthetic methods. For example, β–O–4 is the major linkage type in softwood and hardwood, and this makes its cleavage especially significant in lignin depolymerization [25, 26, 27, 28, 29, 30, 31]. The diversity of functional groups gives lignin versatile processing possibilities. For example, the hydroxy group is beneficial for further modification of lignin, and the carbonyl group gives rise to alcohol production during the hydrogenation process [25, 32].
In the past decades, the lignocellulose fractionation process has been well developed, whereas lignin depolymerization and valorization processes need to be further improved. Currently, thermocatalytic methods have been widely studied and employed for depolymerizing lignin and upgrading lignin monomers [33]. However, this type of method is costly for large-scale production owing to their energy-intensive and harsh conditions (e.g., high temperatures of 240–650 °C and high hydrogen pressures of 250–315 bar) [20, 34, 35]. To utilize lignin with a reduced carbon footprint, an increasing number of researchers have turned their attention to electrocatalysis for lignin upgrading in a water-based electrolyte. Compared to the traditional thermocatalytic route, electrocatalysis normally occurs under mild conditions such as at (or near) room temperature and pressure, and it can be powered by renewable electricity (e.g., solar and wind), lowering the carbon footprint of the total process. Moreover, it has the potential for controllable reaction activity and product selectivity by simply varying the experimental parameters (e.g., voltage and current density), illustrating that electrocatalytic technology has the feature of excellent controllability, which is promising for large-scale industrialization. Furthermore, either electrochemical reduction or oxidation reactions can produce in situ free radicals by water splitting, which then serve as reductants or oxidants during the reaction process, which means this method can avoid the utilization of additional hydrogen or oxygen (hydrogen peroxide) at a high temperature and/or pressure, resulting in a more economical, safe, efficient, and exercisable process. Additionally, a tandem electrocatalytic system can be realized via cathodic (anodic) lignin depolymerization and lignin monomer upgradation. However, the current electrocatalytic performance is not satisfactory for large scalable applications because of the following reasons:
- (1)
Electrocatalytic upgradation of lignin (both polymer and monomer) is a multi-electron-involved procedure.
- (2)
The reaction energy barrier is high for activating lignin-based compounds (e.g., hydrogenating the delocalization in aromatic rings, cleavage of C–C bonds).
- (3)
The competing electrocatalytic reaction of water splitting (hydrogen evolution reaction (HER) or oxygen evolution reaction (OER)) is more thermodynamically favored than the electrocatalysis of lignin.
To solve these challenges, it is necessary to provide a summary and outlook for the recent achievements in electrocatalytic lignin valorization. This will aid in the understanding of possible reaction mechanisms and design a high-performance electrocatalytic system for lignin valorization.
Section snippets
Typical electrocatalytic reaction
In the past decades, several electrocatalytic approaches have been developed for the valorization of lignin in biofuels and bulk chemicals. These approaches include electro-hydrogenation, hydrodeoxygenation, hydrogenolysis, oxidation, and carbonylation. For example, electro-hydrogenation at the cathode is a promising method for upgrading lignin-derived aromatics to products with increased hydrogen content and stability, producing important chemicals and fuels, such as cyclohexanol, cyclohexane,
Characterization methods for lignin
Because lignin is a complex biomass polymer without any organized structures or repeated units, it can only be characterized by analyzing the functional group, linkage, relative abundance of specific lignin monomers, or condensation degree of the polymer. Both destructive and nondestructive methods have been used for lignin characterization. The nondestructive method is limited in characterizing lignin because it is not possible to identify and qualify all of the structural features of lignin,
Electrocatalytic depolymerization of lignin to monomers
Lignin valorization via an electrocatalytic approach normally includes the electrocatalytic depolymerization of lignin into lignin monomers and electrocatalytic valorization of lignin monomers to value-added chemicals and fuels [43, 53, 60, 61]. In Section 4, we detail the electrocatalytic depolymerization of lignin (the first stage). Next, we illustrate the electrocatalytic valorization of lignin monomers (the second stage) in Section 5. For the target products, it should be pointed out that
Electrocatalytic upgrading of lignin monomers
In this section, we detail the electrocatalytic valorization of lignin monomers. Similar to electrocatalytic lignin depolymerization, in water-based electrolytes electrocatalytic upgrading of lignin monomers can be performed at both the cathode and anode mainly via direct electron-transfer pathways. The reactions at the cathode or the anode are mainly dominated by electro-hydrogenation or electro-oxidation for upgrading the lignin monomer, which increases the hydrogen or oxygen content in the
Summary and outlook
From this perspective, we provide a comprehensive overview of electrocatalytic lignin depolymerization and lignin monomer upgradation for value-added chemicals and fuels, which provides insight into the utilization of lignin in nature. Electrocatalytic lignin valorization powered by green electricity benefits from mild operating conditions (ambient temperature and pressure) and offers routes to tune product selectivity. However, current electrocatalytic lignin valorization suffers from low FE
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Role of methoxy and C<inf>α</inf>-based substituents in electrochemical oxidation mechanisms and bond cleavage selectivity of β-O-4 lignin model compounds
2024, Green Energy and EnvironmentCitation Excerpt :Currently, common methods of depolymerization used in industry and academia include chemical oxidation [11–17], hydrogenolysis [18–26], photocatalysis [27–32] as well as electrochemical oxidation [33–37]. Among them, as a sustainable conversion approach, lignin depolymerization by electrochemical oxidation (ECO) becomes more promising, because the oxidation process following a Green Chemistry concept, which is relative to electron transfer alone, only requires milder reaction conditions and largely avoids using oxidative and acidic reagents [36,38–41]. Comparing with C–C bonds such as β-1 and β-5, β-O-4 bond has a lower bond energy and it is more suitable for conversion at a lower oxidation potential by electrochemistry [36].
Available online 20 August 2021
This work was supported by the Shenzhen Science and Technology Program (JCYJ20190808150615285) and Sichuan Science and Technology Program (2020YJ0162).
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These authors contributed equally to this work.