2G waste lignin to fuel and high value-added chemicals: Approaches, challenges and future outlook for sustainable development

Highlights

• 2G Lignin as a bio-based biorefineries platform for highly advanced products preparation.

• Thermocatalytic and biotechnological approaches were emphasized for lignin value addition.

• Thermo catalytic approach is an effective process for lignin valorization in bulk scale.

• In biological valorization, adipic acid production is promising towards scale-up.

Abstract

Lignin is produced as a byproduct in cellulosic biorefinery as well in pulp and paper industries and has the potential for the synthesis of a variety of phenolics chemicals, biodegradable polymers, and high value-added chemicals surrogate to conventional petro-based fuels. Therefore, in this critical review, we emphasize the possible scenario for lignin isolation, transformation into value addition chemicals/materials for the economic viability of current biorefineries. Additionally, this review covers the chemical structure of lignocellulosic biomass/lignin, worldwide availability of lignin and describe various thermochemical (homogeneous/heterogeneous base/acid-catalyzed depolymerization, oxidative, hydrogenolysis etc.) and biotechnological developments for the production of bio-based low molecular weight phenolics, i.e. polyhydroxyalkanoates, vanillin, adipic acid, lipids etc. Besides, some functional chemicals applications, lignin-formaldehyde ion exchange resin, electrochemical and production of few targeted chemicals are also elaborated. Finally, we examine the challenges, opportunities and prospects way forward related to lignin valorization.

Introduction

Increased energy demand coupled with economic growth resulted in massive fossil fuel consumption causes huge environmental concerns, which motivates the scientific communities for the development of a bio-based economy in recent years. Lignocellulosic biomass (LCB) is the most abundantly available organic matter on the earth, which has been recognized as a renewable, cheap potential feedstock for the production of a variety of fuels and commodity chemicals (Yoo et al., 2020; Satari and Jaiswal, 2021). LCB mainly comprises of three major cell wall components, i.e. cellulose (30–45%), hemicelluloses (15–25%), and lignin (15–25%) in the majority (Li and Takkellapati, 2018; Bhatia et al., 2020). These cell wall components may serve as basic precursors for the transformation in liquid/solid fuel, fibers, aromatics (Chinnappan et al., 2016). In commercial lignocellulosic biorefinery, polymeric sugars, i.e. cellulose/hemicelluloses could be extracted following many chemicals, biochemical treatment and could be enzymatically hydrolyzed into monomeric fermentable sugars, which on downstream processing is used for the production of bioethanol and being used as a transport fuel (Zhao et al., 2020; Gohain et al., 2021). The leftover lignin-rich solid fermented residue is a heteropolymer of propenyl phenol have a molecular weight of several thousand. In order to valorize lignin, two approaches can be adopted 1) top-down 2) bottom-up (Bajwa et al., 2019). In the top-down approach, high molecular weight polymer needs to be converted into small molecules, which can either be separated and purified in various molecules like, vanillin or all these components as a mixture can be dissolved in the toluene or xylene and hydro processes to produce fuels. Whereas, in the second approach different layers of functional groups can be grafted into surface-active materials. As per the product requirement, functional groups either having positive charge or negative charge can be created. For example, for a positive charge, the lignin molecules can be grafted with amino, azido groups etc., whereas for negative charge it can be grafted with carboxylate groups or sulpho groups (Georgouvelas et al., 2020). This will result in a series of surface-active compounds having application as a binder in cement, ion exchange materials, ink, biomedicine etc (Chen et al., 2021; Ramdas et al., 2021). The top-down approach includes depolymerization and further processing into final products. However, in bioethanol process in biorefinery, >400 MT/day of residual lignin-rich solid could be left as a reaction by-product, leaving behind a mixture of sugars (unreacted) and lignin as waste material for burning, with almost no economic value (Svensson et al., 2020). Pulping industries produce a huge amount of lignin as waste and used to preheat boilers. Out of this lignin; lignosulphonate contributes (∼88%), Kraft lignin’s (∼9%), organosolv (∼2%) of total production (Bajwa et al., 2019). Presently, with increased fuel demand, the cellulosic ethanol market size is expected to grow by USD 47.8 billion during 2020–2025 (Valdes et al., 2016). Currently, several commercial plants have been set like of these plants are close or running at much lower capacity due to high operation cost and poor economics. For instance, the USA’s largest ethanol (51-million-gallon) plant in Minnesota is struggling to sustain its existence due to market uncertainty and high cost of production (Araújo, 2016). Similarly, Corn Plus ethanol plant started at Winnebago, the USA was shut down due to low commodities prices.

Lignin retains a huge polydispersity, poly functionally, intrinsic polyaromatic units, which limits its industrial applicability for downstream processing (Xu et al., 2020). Although, lignin from the pilot plant contains nearly 50–60% moisture, considerably very low calorific value and releases toxic gases in the environment due to burning and causes ash deposition in the inner lining of the reactor, which reduced overall efficiently of the process (Zygarlicke et al., 2000). However, the aromatic structure of lignin makes it a promising platform for biobased high value-added chemicals, i.e. vanillin, phenolics, aldehydes etc. alternative to petroleum-based with high yields and selectivity, resulting in efficient GHG reduction and conversion to target products alternative to fossils fuels (Gomes and Rodrigues, 2020). A comparative LCA study of catechol synthesis from lignin isolated from candlenut shells showed an overall 2, 7 and 59% GHG reduction, ecotoxic effects reduction and petroleum depletion respectively when compared with fossil-based catechol (Montazeri and Eckelman, 2016). In another report, biobased lignin-derived adipic acid leads to 4.87 kgCO₂eq/kg, with a 62–78% greenhouse gas reduction when compared to petroleum-based adipic acid (Corona et al., 2018). Hence, advancement approaches in lignin valorization for value-added chemical, material production may lead to the establishment of greener, economic and environmentally friendly development under intgegretated biorefinery instead of burning in boilers with reduced carbon footprints.

Lignin from 2G biorefinery and pulp industry can be chemically, biochemically or in bulk can be used to produce essential green value-added phenolics, i.e. vanillin, polyhydroxyalkanoates, BTX, phenolic aldehydes, composites, high-performance lignin-derived carbon fibers, adhesives, graphene’s, battery electrodes for energy storage, resin, fillers, admixtures, pigments, additives in cement industry, adsorbents, flocculants, dye preparations etc. alternative to petroleum-based chemicals (Ten and Vermerris, 2015). Low molecular weight lignin’s (C₆–C₁₂) have opened the various opportunity for drop-in fuels for economic shit from fossils industry. Polyhydroxyalkanoates are biodegradable, biocompatible properties can be produced from lignin monomers under biochemical approaches, which may substitute the synthetic plastics. Besides, that, commercially, lignin could be used to prepare bio-composites with natural reinforcing fibers (e.g. PHA, PLA, Bio-PET, bio-BE) (Rico-García et al., 2020). Industrial lignin could also be used for the preparation of industrial hydrogels adhesive due to abundant phenolic units in lignin and could replace the petroleum-based phenol (Rico-García et al., 2020). By virtue of polyfunctionality and antioxidant, anti-microbial, anti-diabetic properties, lignin could also be preferred for therapeutic applications, i.e. controlled drug deliver action and nano-tube preparation for DNA delivery (Bhat et al., 2020). Chemical modification of lignin could lead to producing novel, food packaging materials, tailored polymer system (lignin mulch films) for agricultural purposes for control release of fertilizers or pesticides.

Emerging integrated lignocellulosic biorefineries may produce a variety of value-added chemical commodities, polymers, fuels etc. from renewable feedstocks and have tremendous potential to replace fossil-based economy. However, the production of low-cost ethanol is highly challenging compared to molasses and corn ethanol. Thus, in order to offset the higher cost and make the process economically sustainable; residual lignin valorization is highly essential. Therefore, emerging biofuel/bioenergy technologies are performing various R&D project to develop value-added products from biorefinery/pulp lignin to make the entire bioconversion process cost-effective. Hence, in this vast review, we have focused and represent a holistic approach for the overall economic development for future biorefinery with the integrated development of lignin valorization for value-added chemicals. This is highly desirable and considered as a greener technology approach for economic viability improvement with sustainable development. Hence, in this review, we attempt to systematically arrange the recent developments for lignin valorization using distinctive approaches, i.e. 1) Chemical approaches 2) Bio-chemical approaches 3) Functional compounds from lignin etc. to produce fuel and chemicals from biorefinery lignin. Furthermore, the review will also emphasize the brief introduction of some of the emerging chemicals/materials synthesized from lignin, i.e. vanillin, PHA (Polyhydroxyalkanoates), ion exchange resin, lignin formaldehyde resins and electrochemical applications. Additionally, it also provides the future perspectives of 2G industry and lignin-based commodities chemicals with similar or better chemical and physical properties.

Lignocellulosic biomass (LCB) is a renewable and globally abundant raw material which is being considered as potent feedstock for transformation into energy, fuels and chemicals. LCB includes vegetative materials, i.e. grasses, hardwood softwood, sea plants, algae etc. (Kapoor et al., 2015; Raj et al., 2015, 2018). Chemically, LCB biopolymer mainly consists of cellulose (25–45%), hemicellulose (15–25%) and lignin (15–25%), besides some ash and extractives (Calcio Gaudino et al., 2019; Sjoede et al., 2020). The composition of these cell wall components varies depending upon the biomass type, nutrient condition, species, genes, soil fertility, harvesting method, geographical regions (Raj et al., 2015).

Cellulose is a homopolymer of glucose units, in which, glucose monomeric units are inter/Intra linked together linearly via. strong hydrogen bonds interaction, β-1,4-glycosidic bond and weak van der Waals forces, which impart a very rigid, semi-crystalline highly ordered cellulose structure imparting a high degree of polymerization (>10,000 units/chains) (Glińska et al., 2021; Liu et al., 2021). However, hemicellulose is a hetero amorphous polymer, a 2nd most available organic material comprising 15–25% of whole biomass. It is mainly composed of pentoses, i.e. xyloglucans, xylans, mannans and glucomannans, and β-(1→3,1→4)-glucans etc. (Banerjee et al., 2019; Hu et al., 2020). Additionally, the side chain consists of several α-l-arabinofuranosyl, d-galactopyranosyl, β-d-glucuronopyranosyl, or 4-O-methyl-β-d-glucuronopyranosyl subunit (Sun et al., 2021). Cellulose fibrils are hydrogen-bonded with branched hemicelluloses chains (amorphous polymers containing both pentoses, i.e. xylose, arabinose) and hexoses, i.e. galactose, glucose, mannose) and uronic acid (e.g. glucuronic acid) (Mugwagwa and Chimphango, 2020).

Lignin is amorphous, high molecular weight, branched, irregular three-dimensional heteropolymer present in the lamella of the plant cell wall (Pandey and Kim, 2011; Lu et al., 2017; Ma et al., 2018b). It constitutes about 15–40% of the total weight of biomass and provides cell support, plant integrity, maintain transport phenomenon and provide natural recalcitrance towards any biotic and abiotic deconstruction (Moura et al., 2010; Liu et al., 2018c; Tarasov et al., 2018). In the plant cell structure, these three biopolymers are interlinked with various ionic, covalent, hydrogen, and aromatic bonding, which make a highly recalcitrant structure due to strong lignin carbohydrate complex. This makes, lignin insoluble in most of the traditional solvents, i.e. water, acetone, chloroform etc. Lignin can be extracted from biomass either using alkali, organosolv, sulphite treatments or is generated as a major byproduct during bioethanol synthesis.

2G biorefinery involves the production of cellulosic ethanol and chemicals from lignocellulosic waste biomass, i.e. rice straw, wood chips, stalks, leaves, grasses etc. For fuels and chemical production from lignocellulosic biomass, 2G process involves three steps, i.e. 1) Pretreatment 2) Saccharification; 3) Fermentation followed by distillation of cellulosic ethanol (Semwal et al., 2019) (Fig. 1). In 2G ethanol biorefinery, the primary step involves the extraction/fractionation of holocellulose, which on downstream processing of saccharification using cellulases followed by fermentation of monomeric sugars into bioethanol. After distillation of cellulosic ethanol, the leftover solid residue enriched in lignin was achieved by centrifugation of fermentation broth/or evaporation of water is hereby described as fermentation residues or technical lignin. This fermentable residue/or technical lignin is found to have 30–40% of the lignin with 34–45% of ash and 10–25% unconverted sugars. Hence, these aromatic lignin-rich solid acts as a potent, cheap source for the production of various commodity chemicals, i.e. aromatics, phenolics, bio-oils etc. Besides that, in another approach, this lignin-rich solid residue could be used as such for binders, thermoplastics materials and epoxy resins; ion exchange resins preparation (Gioia et al., 2020; Ortiz-Serna et al., 2020). Hence, with increased green bio-refinery concept and economics, this solid residue un-fold the new research opportunities for the development of new strategies for lignin extraction and valorization.

In plants, lignin biosynthesis follows the phenylpropanoid pathway, involves oxidative condensation of three monolignols units catalyzed by a specific enzyme (laccases) having Cu²⁺ ion at the reactive center. Lignin is highly complex heteroaromatic structures and mainly consist of 3D amorphous biopolymers of methoxylated phenylpropane subunits (Fig. 2), i.e. coniferyl, guaiacyl units, p-hydroxyphenyl units. Lignin chemical structure primarily consists of three monomeric units, i.e. sinapyl (3, 5-dimethoxy-4-hydroxycinnamyl), coniferyl (3-methoxy-4-hydroxycinnamyl); and p-coumaryl (4-hydroxycinnamyl) alcohols, which are linked via. C–C and ether bonds (Zhang and Wang, 2020). These monomers are also known as monolignols as guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H) units, respectively (Fig. 3a). In-plant, lignin is associated with cellulose and hemicelluloses via. different bond and act as glue to hold the cell wall matrix to provide structural strength and rigidity (Li et al., 2019a). However, the lignin structure varies depending upon feedstock type, plant type, genus, species, nutrient condition, age and within structural parts of the plant. For example, softwood contains about 30% lignin by weight and comparatively, hardwood contains about 20%–25% of lignin and grass shares only 10–15% of the total plant weight (Guo et al., 2019). Lignin structure may also vary depending upon the isolation process, i.e. dilute acid, alkali, steam explosion, organosolv, pulping processes etc. For instance (Zhang et al., 2019b), reported that Kraft processes were used in pulping in the paper manufacturing industry, which used a mixture of Na₂S and NaOH and solubilized lignin at 150–170 °C into lower molecular wt. components with increased β-O-4 units in depolymerized liquid (Belkheiri et al., 2014, 2018; Yuan et al., 2016; Shi et al., 2017; Ma et al., 2018a). Chemical structure determination of depolymerized unit showed that about 32% β-O-4, 40% secoisolariciresinol, 24% β–β′ & 4% β-5′ linkages are main units were present at the end-groups.

Sinapyl alcohol consist of two methoxy groups attached with a phenyl group, however, only one methoxy group it attached to phenyl unit in coniferyl alcohol and p-coumaryl alcohol has none. Chemically, the three-dimensional structure of lignin in cellulosic biomass is highly recalcitrant due to extreme inter and intra-molecular linkage (ether and C–C linkage) within lignin. Ether linkages in lignin polymer are predominant, which comprise about 2/3 of total linkages present. These linkages in monolignols structures are further categorized, depending upon the linkage at the C-atoms inside chains, which are named as α, β, and γ (Laskar et al., 2013; Dutta et al., 2017; Du et al., 2020). In majority, aromatic carbons in above phenolic units are numbers from 1 to 6. In major, there are six kinds of linkages present in all lignin structures, i.e. β-O-4 (β-aryl ether), β−β (resinol), and β-5 (phenylcoumaran), α-O-4 (α-aryl ether), 4-O-5 (diaryl ether), 5−5, α-O-γ (aliphatic ether) and β-1 (spirodienone) as shown in Fig. 3a and b.

The global market of lignin is expending with increase bioeconomy (Table 1). Globally, approximately 100 million tones/year lignin is produced in 2015, which valued about USD 732.7 million. Furthermore, it is further expected to increase with a growth rate (CAGR) of 2.2% to $913.1 million by 2025 with a growing bio-economy (Rodrigues et al., 2018). Most of the commercial enterprises are producing lignosulfonate, which account ∼85% of global lignin market (1 M ton/year). Overall production lignin production from pulp and paper industry contributes to 1.1 M tones of lignosulfonates (potential available 3 M ton), and 160,000 ton/year (Miller et al., 2016). For instance, MeadWestvaco in Charleston USA) commercially produce lignosulfonates in USA (30,000 ton/year). In addition to that, 2G biorefinery lignin and organosolv lignin segment are expected to grow over 5% CAGR from 2016 to 2025 (Bajwa et al., 2019). Kraft lignin production is performed at large scale from black liquor obtained in the pulp and paper industry and purified lignin has a huge market globally. For instance, BioChoice™ (Domtar) and BioPiva™ (UPM), Stora Enso, Domtar, UPM, Metsä Fibre, Suzano and Fibria, are some commercial pulp and paper manufacturing partners, which are producing Kraft lignin as a bioproduct scope. Domsjö Fabriker is the world’s 2nd largest producer of powdered lignin from sustainable Swedish forestry (Hägglund). LignoBoost initially placed a demo plant in Bäckhammar, Sweden in 2007., which isolate lignin from pulp mill and have a capacity of 8000 ton/year. After this LignoBoost™ from Valmet extracts lignin from Kraft black liquor from pulp industry using Nordic softwood, pine and spruce. They have installed first two commercial plants at Domtar-Plymouth plant in North America in 2013 with a production capacity of 25,000 MT/year and another is Sunila LignoBoost™ plant installed in 2015 at Stora Enso’s Sunila mill in Finland with an annual production capacity of 50,000 MT/year (Enso) Emerging Stora Enso, Finland recently invested Eur 10 million to set up a pilot facility for the conversion of lignin into value-added products (Granström, 2015; Coons, 2017). Borregaard LignoTech, Norway and Rayonier Advanced Materials, USA is another global supplier lignin-based product, i.e., lignosulphonates, dispersants, additives for concretes, batteries, textile dyes, pesticides application etc. and have a production capacity of 230,000 ton Cellulose, 120,000 ton lignin and 20,000-ton bioethanol (Rødsrud et al., 2012). Both Rayonier Advanced Materials, Florida and Borregaard, LignoTech have owned about 45 and 55% of shares respectively (Bruijnincx et al., 2016), which have a production capacity of 100,000 metric ton.

Pulping industry widely uses woody and LCB as a feedstock for the production of pulping materials. Where woody and LCBs are treated with a strong base to fractionate lignin and hemicelluloses from crystalline cellulose for paper manufacturing. In literature, various types of the pulping process, i.e. Kraft-lignin, soda and organosolv lignin processes are used. In the sulfite process, the LCB material is subjected to an acid catalytic digestion for the dissolution of lignin at a temperature ranging from 130 to 160 °C with the addition of SO₃²⁻ or HSO³⁻ (Gratzl and Chen, 1999; Ek et al., 2009). The sulfonate moiety inserted into the lignin molecule and make them soluble in the liquid fraction, thus eliminating lignin from the complex LCB moiety (Strassberger et al., 2015). This sulfite rich lignin called as lignosulfonates (Sundin, 2000), which is produced around 8 to 10 million ton per year. This sulphonate based lignin could be used as dispersants in the cement industry and have huge application in dye preparations (Aro and Fatehi, 2017). Additionally, these lignosulphonates varying molecular weight may be used for preparation of composites for enhancement of hydrophilicity etc. (Northey, 2002; Bahrpaima and Fatehi, 2019; Hemmilä et al., 2019).

In Kraft-lignin alkaline process, LCB is reacted with sodium sulfide and sodium hydroxide at temperature up to 170 °C, which provides the efficient separation of lignin in the form of dissolved black liquor from the cellulosic fiber (Chakar and Ragauskas, 2004; Tomani, 2010). The Kraft-lignin process dissolves low molecular weight lignin compound and it can be recovered by acidifying the black liquor solution around pH 2–3 in the presentence of H₂SO₄ or CO₂ (Asina et al., 2017). The Kraft-lignin process is advantageous over the lignosulfonate process, in which the chemicals involved in the Kraft-lignin process can be recycled and further used in the Kraft-lignin process. Similarly (Chio et al., 2019), reported that the combination of sodium hydroxide and hydrosulphide (NaHS) might drastically enhance the cleavage the β-bond with lignin solubilization. Kraft lignin can be further precipitated by acidifying (<pH 5.0). Though, both methods widely used in the pulp industries, it generates a surplus amount of Sulfur during the process steps. An alternative to the sulfur-free lignin removal is the soda and organosolv process, in which the lignin was recovered as black liquor. In the soda pulping process, the concentrated NaOH (13–16%) solution was mixed with anthraquinone for the separation of lignin from LCB at 140–170 °C (de la Torre et al., 2013). In organosolv process, the LCB was heated at 130–220 °C under an organic solvent, i.e. CH₃CH₂OH or/and CH₃OH, it dissolute the lignin and hemicelluloses degradation byproducts in the form of liquor and the residual solid fraction rich in cellulosic fraction. After the organosolv solvents, the organic compounds were recovered using distillation and recycled.

Despite, the challenges used in the various existing methods for the removal of lignin from the LCB matrix, further purification of the lignin and application of the underutilized lignin for various applications could be explored for developing a cost-effective and sustainable biorefinery ((El Mansouri and Salvadó, 2006; Wang et al., 2019b). Hence, lignin from pulping industry could also serve as a feedstock for lignin valorization for production of low value sulfonates and high-value products (Upton and Kasko, 2015). estimated that about 2700 ton of Kraft lignin was produced and could be used for production various aromatics. However, the high cost of alkali and neutralization of alkaline media limits its application for large scale process development.

Biorefinery concept assumes a multi-output system, and the majority of the 2G plant is focused on cellulosic ethanol production. In cellulosic ethanol biorefinery process, the lignin-rich semi-solid slurry is obtained at the end of fermentation and distillation of ethanol (Kumar et al., 2009; Alvira et al., 2010; Agbor et al., 2011; Brodeur et al., 2011; Auxenfans et al., 2012; An et al., 2015; Chandra et al., 2015; Aditiya et al., 2016; Jönsson and Martín, 2016; Robak and Balcerek, 2018). At present, this lignin-rich solid is centrifuge/concentrated (30–40% moisture) to obtain lignin-rich solid, which is burnt to produce approximately 10–13 MW of power surplus. A recent report said that for ethanol production ∼43% of the lignin residue, comprising 33% lignin by mass, could be extracted at a minimum selling price ranging from $43 to $70 per ton (Obydenkova et al., 2019). These lignin residues vary in structural composition, molecular weights, polydispersity etc. In order to utilize the left-over solid material, the main challenge is to recover and purify the lignin from solid residue using an environmentally friendly extraction system. Therefore, various approaches for fractionation of pure lignin from lignin residue can be performed using various non-toxic sequential solvents, i.e. formic acid, ethanol, ethyl acetate, methanol, propanol, butanol, propylene glycol, DMSO, dichloromethane etc. are shown in the literature (Liao et al., 2020; Xu et al., 2020). Furthermore, the combination of co-solvents, methanol-water, acetone-water-methanol, iso-propanol-tetrahydrofuran and certain alkaline media, i.e. 1-10% NaOH, KOH could also be used for lignin solubilization at a temperature ranging from 35 to 120 °C (Ropponen et al., 2011; Duval et al., 2016; Passoni et al., 2016; Jiang et al., 2017; Domínguez-Robles et al., 2018; Wang et al., 2018e). The solubilized lignin can be reprecipitated by removing organic solvents or changing the pH of the media or by antisolvent addition. This resulted to a lignin-rich solid fraction containing an increasing molar mass, a decreasing polydispersity and free from impurities like ash and residual carbohydrates and proteins and could pave a way to unlock the potential of the residual lignin in a biorefinery scheme and generate extra revenue to improve the economics of the 2G based cellulosic ethanol plant. The schematic block diagram shown in Fig. 4 showed the various steps involved in lignin valorization.