Catalytic depolymerization of lignin by metal and metal oxide: a review

ABSTRACT

Lignin, with its rich reserves of phenolic compounds, holds great promise as a candidate for renewable energy and valuable chemical production. However, the complex molecular structure and low reactivity of lignin have impeded progress in this research direction. Consequently, the depolymerization of lignin into high-value small-molecule chemicals has become a new area of focus. Metal and metal oxides have emerged as a promising catalyst to overcome this obstacle due to their high selectivity in depolymerizing lignin and the mild reaction conditions required. This paper reviews the properties, and products of various metal and metal oxides used for lignin depolymerization under microwave, pyrolysis, hydrogenolysis, and oxidation conditions. The research prospects and challenges of metal oxide degradation of lignin are summarized to pave the way for future applications and development of lignin depolymerization.

KEYWORDS:

• Lignin
• catalytic depolymerization
• metal
• metal oxide
• mechanism

1. Introduction

As fossil fuels continue to decrease and the ecological environment deteriorates, biomass is gaining global attention as an important renewable energy source [1]. The biomass industry has the potential to efficiently produce energy and organic materials by converting agricultural wastes and organic pollutants into valuable resources. This process not only renders these materials harmless but also makes them useful. Lignocellulosic biomass is composed of three primary components: cellulose, hemicellulose, and lignin. Among these, lignin has emerged as the most promising biomass source for obtaining aromatic compounds due to its rich aromatic components (i.e. high carbon content), high biodegradability, and thermal stability [2,3]. However, the high-value utilization of lignin has not been achieved due to the complexity of its structure [3]. For a considerable period of time, the paper industry and agriculture have generated a significant amount of lignin, which has been used as a fuel for direct combustion. However, this practice has resulted in a waste of resources [4]. Effectively improving the utilization efficiency of plant lignin has gradually become an important research topic to achieve resource conservation.

Lignin is a promising raw material for the synthesis of value-added bioproducts through biorefinery processes due to its functional chemical groups, high antioxidant capacity, and biodegradability [1]. The structure of lignin is highly branched and amorphous. It is formed by the polymerization of three monolignol radicals through different types of C-C and C-O bonds, such as β-β, β-1, 5–5′, β-O-4, α-O-4, and 4-O-5 [5–7]. Zhou et al. reported several functional groups in the lignin molecule, including methoxy, aliphatic hydroxyl, phenolic, benzyl, noncyclic phenyl ether, and carbonyl chemical groups [1]. These groups make lignin a valuable candidate for producing various compounds, such as hydrocarbons (e.g. benzene and homologues of benzene), simple phenols (e.g. catechols, eugenol, vanillin, and quinones), polymeric macromolecules (e.g. carbon fibers and thermosets), nutritional products, pharmaceuticals, and cosmetics [8].

The efficient utilization of lignin depends on the selective cleavage of specific C-O and C-C bonds within its structure [1]. Catalysts are essential in these processes. Commonly used acid or base catalysts are primarily used to break ether bonds [9]. The industrial application of acid or alkali catalytic depolymerization of lignin faces challenges such as corrosion and the complexity of catalyst recovery [10]. Furthermore, the use of acid or alkali catalysts typically requires harsh reaction conditions, including high temperature and pressure, leading to a significant rise in depolymerization expenses [9]. The cost increase can be attributed to the energy required to maintain these conditions. Metals and metal oxides can reduce the energy barrier in the conversion process and maximize the yield and selectivity of lignin depolymerization [11]. Metals and metal oxides possess unique chemical properties as catalysts. The incorporation of certain metal oxides not only lowers the activation energy of the reaction, but also enhances the selectivity toward specific products [12]. This paper discusses the main methods of depolymerization of lignin using metal and metal oxides, focusing on microwave, pyrolysis, hydrogenolysis, and oxidation depolymerization. It also explores the extraction methods of lignin and the reaction mechanism involved in the depolymerization process. The goal is to evaluate the advantages and limitations of using metal and metal oxide for lignin depolymerization.

2. Lignin type

Lignin is a three-dimensional reticulated macromolecule composed of a phenylpropane structure. It contributes to the strength of plant cell walls by filling the space between cellulose and hemicellulose and binding the lignocellulosic matrix together [1,13].

Figure 1 depicts the three precursors and basic units of lignin. The primary precursors of lignin polymers are p-coumaryl alcohol, pineal alcohol, and sinapyl alcohol, which give rise to the three fundamental units of hydroxyphenyl (H), guaiacyl (G), and syringyl (S), respectively [1,5]. The aromatic moieties are linked by various types of C-C and C-O bonds, such as β-β, β-1, 5–5′, β-O-4, α-O-4, and 4-O-5 [5]. As shown in Figure 2, Chio et al. illustrated the molecular structure of lignin, including its typical chemical bonds. The C-O bond is represented by a solid square, while the C-C bond is represented by a dashed square [9].

Figure 1. Three precursors and basic units in lignin [1,5].

Figure 2. Molecular structure of lignin and these typical chemical bonds. The bonds marked by solid squares are C-O bonds and the bonds marked by dashed squares are C-C bonds [9].

The structure and properties of lignin vary depending on extraction methods, plant species, environmental factors, and even plant genotypes [14]. This variation is mainly related to the content of coumaryl alcohol, pineal alcohol, and sinapyl alcohol [15]. The recovered lignin exhibited variations in average molecular weight, polydispersity, solubility, and abundance of functional groups, depending on the extraction method employed [16]. This section focuses on the classification of lignin based on extraction methods.

2.1. Milled wood lignin

Milled wood lignin is a type of lignin that is obtained by grinding raw biomass materials, such as wood, at normal temperature and pressure, using a neutral solvent as a medium, and then extracting them with a suitable solvent [17]. Bjorkman was the first to propose a treatment for milled wood lignin by grinding wood powder to the micron level using a vibratory ball mill. This was followed by extraction of the wood powder with aqueous dioxane, using a 9:1 or 8:2 ratio of dioxane to water. After the solvent was extracted and removed by spinning, the resulting residue (crude milled wood lignin) was dissolved in 90% acetic acid. The mixture was then injected into water to purify the milled wood lignin [18].

The structure of milled wood lignin is similar to that of natural lignin. However, this method typically yields less than 30%. Additionally, mechanical processing unavoidably results in depolymerization and chemical modification of lignin [19].

2.2. Klason lignin

Klason lignin, is a type of lignin sulfate and is currently the most commonly used form of lignin [20]. Klason lignin was prepared by treating wood with 72% concentrated acid at room temperature (10 ml of concentrated acid per 1 g of wood). Initially, the polysaccharide component of the wood dissolved, forming a viscous solution that eventually became a thin liquid. After 2 hours, twice the volume of water was added, and the mixture was allowed to stand for a short time before being diluted to 3% [20].

However, even under mild conditions, lignin is highly sensitive to inorganic acids and susceptible to condensation reactions. These reactions enable the positive carbon ions on the side chains of the molecule to combine with other structural units. Therefore, the yield of vanillin obtained from the oxidation of Klason lignin with nitrobenzene was only 1.5%. Based on this result, it is evident that Klason lignin is not a suitable material for studying the structure of lignin itself. Compared to other types of lignin, Klason lignin is expected to contain more carbon-carbon bonds [20,21]. Additionally, Klason lignin is highly susceptible to degradation under strong acidic conditions, leading to condensation or polymerization that can compromise its original active groups. As a result, its use is limited [22].

2.3. Dioxane lignin

Lignin can be extracted with dioxane at room temperature or high temperature using a mixture of dioxane and dilute acid in a ratio of 9:1. This extraction process does not react with lignin. To extract lignin from wood powder that has been degreased using benzene-ethanol extraction, a 9:1 mixture of dioxane-water (9:1) is added along with 0.2 equivalents of hydrochloric acid. The mixture is then heated to reflux at 90–95°C. The lignin extract was concentrated through spin evaporation and subsequently injected into water to facilitate precipitation. This process was carried out with minimal condensation reaction. The chemical structure of lignin macromolecules extracted by this method changed little due to the stability of dioxane and lignin. This stability ensures that dioxane lignin can better represent the overall characteristics of lignin [23].

2.4. Kraft lignin and sulfite lignin

Kraft lignin is a type of lignin that is produced through the Kraft pulping method. This process involves the addition of sulfide or polysulfide, such as Na₂S, to extract the lignin, which is then mixed with NaOH and cooked at a temperature range of 150–180°C for a duration of 2 hours. During the cooking process, the components of lignocellulose undergo separation, and the aryl ether bond of lignin is broken. This triggers a series of depolymerization reactions, resulting in the formation of small lignin fragments. The addition of Na₂S lowered the pH of the system and caused lignin molecules to self-aggregate, reducing their water solubility. This led to the gradual precipitation of Kraft lignin [24].

Sulfite lignin is derived from the treatment of biomass feedstock with sulfite or bisulfite solutions. The resulting sulfur content ranges from 4–8%, and the available cations include sodium, ammonium, manganese, or calcium ions. These cations contribute to the excellent water solubility of sulfite lignin. Sulfite lignin have a larger relative molecular weight compared to Kraft lignin, with a typical heavy average molecular weight ranging from 1000 to 140,000 g/mol [25,26].

2.5. Alkaline lignin

Alkaline lignin is produced through the hydrolysis of lignocellulose in an alkali solution, typically using sodium hydroxide, under relatively mild conditions. The mechanism for preparing alkaline lignin is similar to that of Kraft lignin, which also has low molecular weight and hydrophobic properties. Alkaline lignin does not contain sulfur, and the processing time for its preparation is longer [27,28]. However, this method consumes a significant amount of alkali and alters the structure of lignin [19].

2.6. Organosolv lignin

Organosolv lignin is a widely used and promising form of lignin extraction. The process typically involves the use of organic solvents or water-miscible organic solvents, such as ethanol, acetone, methanol, or organic acids (e.g. formic and acetic acid) [29]. The use of organic solvent methods for lignin extraction results in the retention of most of the β-O-4 structure. To enhance extraction efficiency, it is common practice to add inorganic acids or bases to facilitate the decomposition of lignin [30].

Most of the extracted organosolv lignin is insoluble in water at pH 2–7, but it can be dissolved in alkaline solutions and many polar solvents. Organosolv lignin has a low relative molecular mass, and it hardly alters the original structure of lignin, which facilitates the preparation of low molecular weight phenolic and aromatic hydrocarbon chemicals. The organosolvent method has a significant drawback, which is the challenge of utilizing and retrieving substantial quantities of organic solvents. This issue makes the entire process expensive. Additionally, the organosolv lignin obtained is insoluble in water, which restricts its application [14].

2.7. Pyrolytic lignin

Pyrolytic lignin is a product of biomass pyrolysis. This process involves heating biomass to approximately 500°C in the absence of air, with a gas phase fraction residence time of less than 2 seconds. The result is a significant amount of pyrolysis oil, which constitutes approximately 75% of the weight of the pyrolysis feedstock. The remaining by-products are gas and solid carbonaceous residue. The pyrolysis oil obtained was added to ice water to produce pyrolytic lignin, which appears dark brown after washing and drying. The primary distinction between pyrolytic lignin and native lignin is its lower molecular weight, resulting from a greater degree of lignin depolymerization during pyrolysis. The heavy average molecular weight (M_w) of pyrolytic lignin ranges from 600–1300 g/mol, while the number average molecular weight (M_n) ranges from 300–600 g/mol. However, a significant drawback of the pyrolytic lignin preparation process is the substantial waste of cellulose and hemicellulose [31].

2.8. Steam blast lignin

Steam bursting is a method that utilizes pressure changes to rapidly separate the individual components of lignocellulose [32]. This is accomplished by exposing the biomass feedstock to high temperature and high-pressure water vapor, followed by instantaneous decompression to create a decompression explosion [33]. The explosion acts on the tissue cell layers of lignocellulose, breaking the bonds of its three cross-linked components and causing their separation [34,35].

The steam-blasting of lignin results in a low molecular weight product that is non-polluting and aligns with the principles of green chemistry. This process not only consumes low energy but also yields a high amount of lignin [36]. However, steam blasting lignin has certain disadvantages. Part of the lignin matrix cannot be completely destroyed when the xylan fraction is hydrolyzed. Additionally, the partial destruction of the lignin β-O-4 structure can reduce the yield of subsequent products [37].

2.9. Enzymatic lignin

Enzymatic lignin refers to a type of lignin that retains its intact structure following the process of enzymatic hydrolysis. Enzymatic lignin involves the biological treatment of lignocellulose. This process requires the use of microorganisms or enzymes to break down the wood, hydrolyze the cellulose and hemicellulose, and subsequently precipitate the lignin. The enzymes typically used for hydrolysis in biological processes are cellulase and xylanase, which are used to break down cellulose and hemicellulose, respectively. The pretreatment stage is a crucial step in the production of biofuels and other value-added products from lignocellulosic biomass. During this process, the bonds connecting lignocellulose complexes are disrupted, leading to the separation of components and extraction of lignin [19,38].

The treatment of enzymatic lignin does not require excessive energy consumption. However, it typically involves a lengthy processing time, ranging from hours to days, and results in lignin that contains impurities [19,38].

2.10. Cosolvent enhanced lignocellulosic fractionation lignin

“Cosolvent enhanced lignocellulosic fractionation (CELF) is an emerging pretreatment method that can decompose lignocellulosic biomass into various components [39]. CELF utilizes dilute acid as a catalyst and tetrahydrofuran as a solvent to fractionate biomass components. This method improves the purity of lignin [39–41]. Compared to ethanol organosolv lignin and kraft lignin, CELF lignin has a higher carboxylic acid OH content, a lower molecular weight and a higher purity [39,41]. CELF lignin can be used as an ideal candidate for bio-value materials [39]. However, the method also faces the challenge of solvent recovery. Furthermore, the method described is currently limited to laboratory-scale experiments. Scaling up the production of lignin on an industrial level remains a significant hurdle that needs to be overcome [39].

In the work of lignin value, the extraction and purification effect of lignin plays a very important role in its value. Molecular weight and purity are important factors affecting the depolymerization of lignin [1,9]. The extraction of lignin using the aforementioned methods presents both advantages and disadvantages, making it challenging to determine the optimal approach as a general method. However, among these methods, the CELF lignin extraction technique appears to be the most effective for depolymerizing lignin, as it maintains higher purity and yields lower molecular weight. Nevertheless, the issue of solvent recovery cost remains a concern. Further improvements and optimizations are necessary to fully meet the requirements for lignin valorization.

3. Depolymerization of lignin

The metal and metal oxides demonstrated excellent performance in catalyzing the depolymerization of lignin under various conditions, including microwave, pyrolysis, hydrogenolysis, and oxidation. Metal and metal oxides exhibit various mechanisms for depolymerizing lignin under different conditions, but they can selectively break specific chemical bonds in lignin. The β-O-4 bond is a significant chemical bond in the lignin structural unit, accounting for almost 50% or more of the total chemical bonds in lignin [9]. The cleavage of the β-O-4 bond is considered to be an important step in lignin depolymerization [42].

3.1. Microwave

The microwave method is a safer form of thermal catalysis for applying high-energy electromagnetic radiation to lignin molecules [9]. The polar molecules in lignin undergo dipole rotation and ionic conduction in response to radiation, resulting in constant friction between the particles and generating significant amounts of heat [43]. Compared to traditional heating methods, microwave heating offers several advantages. Microwave heating is more economical than traditional heating in terms of both heating time and energy consumption. This method minimizes heat wastage and reduces environmental pollution [44,45]. Catalysts with varying metal compositions demonstrate distinct catalytic activities in the process of microwave-assisted depolymerization. Research has indicated that specific metal sites can facilitate the cleavage of the C-O bond in lignin [46]. The presence of an appropriate metal or metal catalyst can render the depolymerization conditions of lignin more gentle and decrease the microwave power required [9,12].

Nickel is a widely used catalyst that can specifically cleave C-O bonds and carbon-hydroxyl structures on side chains, allowing them to attach to alkanes. The linkage structure in lignin can be more effectively cleaved under the stimulation of microwave heating [47]. Zuliani et al. synthesized pure magnetic nickel metal using a single-mode microwave reactor. The synthesized metal exhibited high catalytic activity for the hydrogenolysis of the lignin-type compound benzyl phenyl ether. The metallic nickel showed significant selectivity for C-O bonds and achieved the best performance with a yield of 71% at 250°C and 5 min of microwave irradiation [48]. And loading Ni on a support is a viable method, as evidenced by studies demonstrating that the catalyst prepared by depositing Ni nanoparticles onto aluminosilicate exhibited excellent catalytic performance. Aluminosilicate possesses excellent hydrothermal stability and a large specific surface area, which facilitates the deposition of Ni particles. The catalyst containing 10% Ni exhibited the highest depolymerization rate, resulting in a bio-oil yield of 30%. The primary products were phenolic monomers and dimeric phenolic compounds [49].

Furthermore, mixed metal catalysts based on nickel have also demonstrated exceptional performance. Zhou et al. investigated the effect of different ratios of CuNiAl-based catalysts on lignin cleavage in methanol. They found that the catalysts were highly selective for α-O-4, β-O-4, and C_α-C_β bonds in the lignin unit. The main products were p-hydroxyacetophenone, guaiacol, p-hydroxyacetyl vanillin, and eugenol [46]. Liu et al. conducted a study on the depolymerization process of lignin using a Cu Ni Al catalyst with a hollow structure zeolite as a carrier. The researchers discovered that the highest bio-oil yield, reaching 69.8%, was achieved when the Cu/Ni/Al ratio was 1.5:4.5:2. In addition, the content of lignin-derived dimers/trimers in bio-oil is about 65.2–67.5% [50]. Mixed metal catalysts appear to be a more cost-effective and safer option when conducting microwave-assisted depolymerization of lignin with lower microwave heating.

Other metal catalysts, such as Cu-Fe bimetallic catalysts, show excellent performance in the microwave-assisted cracking of lignin. The dissociation of H₂O₂, under the combined action of Cu and Fe, generated free radicals that cleaved the α-O-4 and β-O-4 bonds in lignin. The Cu-Fe bimetallic catalyst exhibited the highest selectivity for the products eugenol and acetosyringone at relatively low microwave power, with yields of 54.64% and 23.65%, respectively [51]. Table 1 displays the reaction parameters of the catalysts. It appears that the depolymerization effect of Ni catalysts is superior to that of other metal catalysts. Additionally, the reaction conditions for mixed metal catalysts are milder compared to those for Ni catalysts.

Table 1. Reaction parameters of each catalyst in microwave-assisted depolymerization.

Catalyst	Temperature(°C)	Time(min)	Degradation Product	Conversion (%)	Ref.
Ni	200	-	4-ethyl-guaiacol, 4-propyl-guaiacol	91	[47]
Ni nanoparticles	250	5	Phenol, benzene, toluene	71	[48]
Ni10%AlSBA	140	30	Simple phenols	30	[49]
CuNiAl	160	80	Guaiacol, p-hydroxyacetophenone, p-hydroxyacetovanillon, syringaldehyde	60.1	[46]
CuNiAl-HSZ	140	80	Bio-oil	67.9	[50]
Cu-Fe/Al2O3	-	30	Syringol, acetosyrigone	54.6, 23.65	[51]

Currently, microwave-assisted lignin depolymerization faces technical challenges related to cost and process. While microwave heating technology has numerous advantages over traditional thermochemical methods, it often produces local heating effects due to the heterogeneous structure of lignin molecules [52]. Several studies have shown that metal catalysts can effectively depolymerize lignin at low microwave power. However, more research is needed to develop a catalyst system that is cost-effective and can optimize the lignin conversion process [1].

3.2. Pyrolysis

Pyrolysis is one of the most extensively studied and commercially used techniques for depolymerizing lignin [1]. Pyrolysis is a high-temperature thermal treatment of lignin in the absence or presence of a small amount of oxygen [52]. In this case, the final products typically contain significant amounts of aromatic monomers [53]. The first stage of pyrolysis typically occurs within the temperature range of 200–400°C. It is generally accepted that 400°C is the critical point for lignin pyrolysis [1]. At this stage, most of the ether bonds are cleaved, with non-phenolic ether bonds being cleaved more readily than phenolic ether bonds. Figure 3 illustrates the cleavage mechanism of the β-O-4 bond pyrolysis [9]. When the temperature exceeds 400°C, the pyrolysis progresses into the second stage, where the free radical reaction becomes dominant [1]. During this stage, most of the linkages are decomposed, and the lignin may even undergo gasification [9]. The use of metal catalysts for lignin pyrolysis offers several advantages, including high safety, efficiency, and product selectivity [9].

Figure 3. Cleavage mechanism of β-O-4 bond pyrolysis [9].

Catalysts containing only one metal exhibit good selectivity. For instance, the H-zeolite Socony Mobil-5 (HZSM-5) type molecular sieve is an excellent catalyst for lignin pyrolysis, displaying high selectivity for aromatics [54]. Jia et al. further modified this catalyst by incorporating metal oxides (La, Mg Ce, or Zn oxides). The addition of metal elements adjusted the acidic strength and strong acidic sites of the catalyst, which effectively reduced the formation of coke while increasing the yield of monocyclic aromatics. Experimental results showed that the La₂O₃-modified HZSM-5 catalyst exhibited good activity for the isomerization of m-xylenes and o-xylenes. The conversion of m-xylene was 19.8%, and the selectivity of p-xylene was 46.3% [55]. Ryu et al. prepared MgO/C catalysts using carbon as a carrier, which exhibited a good acid-base balance and specific surface area, and demonstrated excellent performance in the catalytic cracking of lignin. The addition of MgO promoted the ketonization of acid and the aldol condensation reaction of ketones, thereby enhancing the selectivity of aromatic hydrocarbons. The experimental findings revealed a 33% increase in the yield of aromatic hydrocarbons [56]. Vincent et al. reported significantly higher yields of mono-aromatics, di-aromatics, and olefins after single-tank pretreatment and fractionation of biomass lignin using a Pd/C catalyst at 250°C [57]. In addition, the calcium catalyst also exhibits unique chemical properties. Chen et al.showed that CaO was conducive to free radical reaction, and the decomposition rate of lignin reached 65%, showing selectivity to low-carbon monohydroxyphenols and CH₄ [4].

The bimetallic catalysts would show a significant mutual promotion effect in depolymerizing lignin. Zhang et al. demonstrated that the use of a single Ni catalyst or a single Sn catalyst resulted in petroleum ether soluble yields of only 10% and 8%, respectively. However, when Ni-Sn bimetallic catalysts were employed, an improvement in the depolymerization of lignin was observed, resulting in a significant increase in the petroleum ether soluble yield to 60% [58]. Pu et al. utilized silica-based polymetallic oxide spheres for the degradation of lignin. The study revealed that the Ni component of the catalyst broke the β-O-4 bond, while Mn effectively broke the C-C bond in the original lignin. The combined effect of the two metals efficiently prevented the methoxy group from departing the aromatic hydrocarbon structure, thereby reducing lignin condensation and enhancing depolymerization efficiency [59]. Insyani et al. reported that the TiO₂-based NiFeO_x core-shell (ternary heterostructure) structure can generate abundant oxygen vacancies under high-temperature pyrolysis. This can lead to highly selective direct deethoxylation-hydrogenation reactions, resulting in cyclic alcohols [60]. Binary metal catalysts, such as CoO/MoO₃ catalysts, have been found to produce only small amounts of aromatic hydrocarbons directly from the aromatic structure of lignin via a direct deoxygenation mechanism [61]. Na/ZrO₂ catalysts resulted in the highest combined yields of monomer and alkylphenols. The optimal catalyst to lignin ratio was found to be 3:1, and the reaction temperature of 500°C was deemed the most effective. The study also demonstrated an enhanced recovery rate of 17.5 wt% of total phenols, which included 6 wt% of alkylphenols. These findings suggested that Na/ZrO₂ catalysts, when used in the appropriate ratio and temperature, can significantly improve the yield and recovery of phenols from lignin [62]. Atanda et al. found that MgB₂O₄ (where B=Al or Fe) facilitated the deoxygenation of aromatic rings more effectively by providing additional active sites when deposited on the surface of zeolites [63]. The Co/CeO₂ catalyst exhibited the highest efficiency for acetophenone, 1-(2-hydroxy-5-methylphenyl), and acetylbutanedione, displaying remarkable selectivity. Additionally, the Co/TiO₂ catalysts demonstrated good selectivity for H-type phenols [5].

Polymetallic catalysts exhibit superior catalytic performance compared to binary and monometallic catalysts. Yeardley et al. synthesized mixed metal catalysts using Na₂CO₃, CeO₂, and ZrO₂, which resulted in reduced coke formation during pyrolysis when compared to Na₂ZrO₃ catalysts. However, the mixed metal catalysts retained the ability to produce phenol-rich bio-oil similar to that of the Na₂ZrO₃ catalysts [64]. According to Zheng et al., the WO₃-TiO₂-Al₂O₃ composite catalyst can selectively break the C-O bond in lignin, resulting in the formation of aromatic hydrocarbons through direct deethoxylation and dehydration [65]. Metal-rich red mud containing Fe₂O₃, Al₂O₃, and TiO₂ was an excellent catalyst for lignin pyrolysis due to its high specific surface area, layered pores, and acidity [66].

Although metal catalysts have the characteristics of multivalence and catalytic reduction, increase the yield of bio-oil, the selectivity of the target product during pyrolysis are still not ideal. One way to achieve the future industrialization of lignin pyrolysis is by constructing a series system that combines catalysts with different properties. However, there is still a long way to go to achieve industrial production of pyrolysis lignin [67].

3.3. Hydrogenolysis

Reductive depolymerization is an efficient method for depolymerizing lignin [68]. Under reducing conditions, the cleavage of the ether bond becomes relatively easy, leading to further deoxygenation of the phenolic units. This process results in the formation of benzene derivatives in the presence of a sufficient catalyst. Figure 4 illustrates the lignin depolymerization reaction that occurs under reducing conditions [69]. Hydrogenolysis is a chemical reaction that takes place between hydrogen or hydrogen donor reactants and lignin molecules. In the presence of a catalyst, this reaction breaks C-O bonds, particularly β-O-4 bonds, through hydrogenation, resulting in the decomposition of lignin into two hydrogenation products. This process is used to produce aromatic platform chemicals from lignin. Catalytic hydrogenolysis is one of the effective methods for depolymerizing lignin, which has many advantages, including high conversion and product selectivity, as well as a significant reduction in char content [70,71]. Typically, metal catalysts play an important role in catalytic hydrogenolysis process [72].

Figure 4. Lignin depolymerization reaction occurring under reducing conditions [69].

Noble metal catalysts, including Pd, Pt, Ru, and Rh, exhibit exceptional catalytic performance among single metal catalysts [72]. Figure 5 depicts the reaction pathway for the hydrogenolysis of catechol glyceride catalyzed by Ru catalyst [73]. The Ru catalyst exhibited a strong selectivity for the C-O bond. Upon activation, the lignin model compound underwent simultaneous hydrogenolysis of the C_α-O and C_β-O bonds, resulting in the production of caffeyl alcohol. Caffeyl alcohol was subsequently hydrogenated to produce propenylcatechol and allylcatechol. The hydrogenation of the C_α-O and C_β-O bonds occurred much faster than other reactions due to the action of catalyst. According to the results, the predominant compound in the final product was propenylcatechol, which was obtained with a yield of 51% and a selectivity of 77% [73]. Gómez et al. supported the Ru catalyst on one-dimensional multi-walled carbon nanotubes, exposing the metal clusters directly to the reaction medium. As a result, 80% of the β-O-4 bonds in lignin disappeared, compared to only 20% in the uncatalyzed group [74]. Héroguel et al. utilized TiO₂ as a carrier for the Ru catalyst. The oxygen-philic properties of TiO₂ increased the accessibility of the catalyst to the reactants, resulting in a 32% yield of cyclohexane and cyclohexanol [75]. Furthermore, a monomer yield of 15.1% was achieved using a Pt nanoparticle catalyst on an alkaline support [76]. Figure 6 illustrates the reaction mechanism of hydrogenolysis of organosolv lignin using a Pt catalyst. The synergistic effect of the Pt and alkaline support facilitated the gradual breakdown of the C_α-O, C_β-O, C_β-C_γ, C_α-C_β, and C₁-C_α bonds within the lignin structure, ultimately resulting in the cracking of lignin into monomeric products [76].

Figure 5. Reaction pathway of Ru catalyst-catalyzed hydrogenolysis of catecholignans [73].

Figure 6. Reaction mechanism of Pt catalyst hydrogenolysis of organosolv lignin [76].

Although precious metals exhibit good catalytic performance, their usage in large quantities is limited due to cost constraints. However, non-precious metal catalysts have also demonstrated good catalytic performance in the study [72]. Ni catalysts have been extensively investigated for the depolymerization of lignin. Jiang et al. prepared a single-atom nickel catalyst (xNi/CeO₂-S) and found that the highest yield of lignin oil (84.3 wt%) and monomer yield (32.6 wt%) were achieved when x = 1.0. The nickel catalyst demonstrated high selectivity for C-O and C-C bonds during the hydrogenolysis process, effectively enhancing the hydrodeoxygenation reaction of methoxy-containing aromatic compounds into alkylphenols [77]. Bie et al. synthesized the Ni/TiN catalyst, which was able to convert p-benzyloxy benzene to toluene (64%) and phenol (60%) with high selectivity. The reaction was carried out at 250°C and 1 MPa H₂ for 3 hours [78].

The bimetallic catalyst exhibits exceptional catalytic performance for the lignin hydrogenolysis reaction. The excellent performance is attributed to the synergistic effect of the two metals [72]. According to reports, the Ni element exhibited a robust synergistic effect with Ru, which enhances hydrogenation catalytic activity. This effect was attributed to the improved electron transfer between Ru and Ni, leading to enhanced hydrogen adsorption. This, in turn, was a critical factor in improving hydrogenolysis performance [79,80]. Shu et al. utilized a RuNi/C bimetallic catalyst to catalyze the hydrogenolysis of lignin, resulting in a monomer yield of 20.4 wt% at 280°C. This yield was significantly higher than the 11.8 wt% obtained using a ruthenium carbon catalyst under the same conditions, thus confirming the effectiveness of the bimetallic catalyst [81]. Zhu et al. prepared a Ni-Co/C catalyst to selectively break the β-O-4 bond while minimizing the destruction of the benzene ring. The highest yield of monophenols achieved was 55.2%, with a selectivity of guaiacol as high as 70.3% [82]. The catalyst consisting of Ni₃Fe₁ nanoparticles supported on zirconium phosphate achieved a lignin conversion rate of 77.0% and a monomer yield of 19.6%. The interaction between Ni and Fe facilitated the degradation of the lignin structure and hindered the recondensation of phenolic oligomers [83].

Bifunctional catalysts (including multifunctional catalysts) exhibit a significantly improved depolymerization effect, similar to that of bimetallic catalysts [72]. Gurrala et al. developed a 2Pd-5Mo/ABC catalyst supported on activated biochar (ABC), which exhibited a selectivity of 57.3% for phenolic compounds and a maximum monomeric phenol yield of 22 wt%. The introduction of Mo in the catalyst inhibited the hydrogenation of aliphatic C_α=C_β bonds in lignin, resulting in high selectivity for t-isoeugenol [84]. Yan prepared metal-modified Mo₂C catalysts supported on activated carbon (AC), among which the Ni₇Fe₃-Mo₂C/AC catalyst exhibited the highest catalytic efficiency. In the actual test, the yield of phenolic monomer was 35.42%, and the liquid yield was 85.11% [85].

Copper-doped porous metal oxide (CuPMO) is one of the metal oxide catalysts that exhibits exceptional catalytic performance in the hydrogenolysis of lignin, among others. CuPMO is primarily composed of MgO and Al₂O₃, with some of the Mg²⁺ ions being replaced by Cu²⁺ [86]. CuPMO has the ability to catalyze the hydrogenolysis or hydrogenation of lignin. Initially, the phenyl ether bond in lignin is hydrogenated, followed by gradual hydrogenation of the aromatic ring [87]. Barta et al. used supercritical methanol as the reaction medium and CuPMO as the catalyst to convert organic solvent lignin into propylcyclohexanol derivatives at a temperature range of 300–320°C. The yield obtained was 86% [87]. Bernt et al. demonstrated that the use of CuPMO in lignin degradation can enhance the selectivity of aromatic products. It was achieved by reducing the reaction temperature and limiting the contact time [88]. Chui et al. reported that the Sm(III)-modified CuPMO catalyst demonstrated exceptional catalytic activity in the hydrogenolysis of aryl ether bonds, particularly for α-O-4, β-O-4, and 4-O-5 bonds. The inclusion of Sm(III) hindered the hydrogenation of phenol and enhanced the production of aromatic monomers [86]. Additionally, Fe oxide catalysts exhibit exceptional catalytic performance. Zeng et al. prepared a Fe-Pd catalyst that was reacted at 1 MPa H₂ and 320°C for 120 min, resulting in a lignin conversion rate of 98.17% and an aromatic monomer yield of 27.92%. The presence of Fe facilitated the dispersion of Pd and reduced the particle size of Pd from 6.1 nm to 2.5 nm. Fe was present in an oxidized state, while Pd was present in a zero-valent state. The electrons of Pd tended to transfer to Fe, forming a structure that enhanced H₂ activation and increased the number of active H atoms.

The catalytic hydrogenolysis of lignin can convert all oxygen-containing functional groups into hydroxyl groups and C=C bonds, resulting in the highest final product selectivity. Furthermore, the reaction process does not produce free radicals or easily condensed intermediates, such as aldehydes, which significantly inhibits coke production. Therefore, reductive hydrogenolysis of lignin is suitable for the industrial preparation and production of aromatic compounds [89]. However, the reduction conditions can cause a reduction in the value of the product by reducing the side chain functional groups of lignin. This reduction may also hinder the further functionalization of lignin [68]. In the future, new methods may be employed to prepare catalysts that exhibit strong anti-deactivation properties, high selectivity, cost-effectiveness, and recyclability [89].

3.4. Catalytic oxidation

Oxidative depolymerization is one of the most common methods used to utilize lignin [9]. In general, oxidative lignin depolymerization aims to produce polyfunctional aromatic compounds by using metal oxides, molecular oxygen, or hydrogen peroxide as oxidants [90]. During this process, functional groups and chemical bonds in lignin will be selectively oxidized, including side group links, phenolic hydroxyl groups, aromatic rings, and other structures [91]. The cleavage of the β-O-4 bond is a crucial step in the depolymerization of lignin. Figure 7 illustrates the reaction mechanism of Pt catalyst in cracking lignin [92]. The process involved two primary pathways for lignin oxidation. The first and most significant pathway involved the degradation of aromatic units through ring-opening reactions, resulting in the production of aliphatic small molecules. The second pathway involved the breaking of links between lignin units, leading to the generation of low-molecular-weight aromatic compounds, such as vanillin, which undergo condensation or ring-opening reactions. The Pt catalyst accelerated the condensation reaction, thereby enhancing the selectivity of aromatic compounds. The findings indicated that the yield of aromatic aldehydes increased from 3.4 wt% to 7.7 wt%. The results suggested that the Pt catalyst was effective in promoting the cracking of lignin and improving the yield of valuable aromatic compounds [92].

Figure 7. The reaction mechanism of Pt catalyst catalytic cracking lignin [92].

Among single metal catalysts, the Ni catalyst is the most widely used. Du et al. loaded Ni onto carbon nanofibers to prepare a highly stable Ni catalyst that can be reused for three cycles without losing catalytic activity. The results showed a conversion rate of lignin as high as 91%, with 7% phenol and 87% light lignin fragments obtained [93]. Ni-based catalysts in the form of compounds have also demonstrated excellent performance. For instance, the Ni₂P catalyst prepared by Panpian et al. exhibited a selectivity of 80% for phenol. This catalyst promoted the cleavage of C-C and C-O bonds, resulting in the production of phenol [94].

Single metal oxides are known to have good catalytic performance as catalysts. Ma et al. investigated the effect of metal oxides on the oxidation of lignin by peracetic acid and found that Nb₂O₅ had the most significant effect on the process. The yield of monomeric phenolic compounds was approximately 47% [95]. Luo et al. prepared an N-doped Co₃O₄ catalyst capable of oxidative cleavage of β-O-4 bond at room temperature. The yields of phenol and methyl benzoate were 96% and 73%, respectively, while the yield of the main byproduct, benzoic acid, was 26% [96]. Kumar et al. aimed to enhance the catalytic performance by doping Co on the surface of CeO₂. The presence of Co²⁺ and Co³⁺ on the surface facilitated the conversion of lignin cloves into carbonyl groups. The pure CeO₂ catalyst yielded a bio-oil of 49 wt%, while the Co-doped catalyst exhibited a selectivity of 78% towards acetosyringone [97]. Dong et al. investigated the correlation between metal oxide structure and catalytic performance using Eu₂O₃. They discovered that Eu₂O₃ with a nanorod structure demonstrated the highest catalytic activity due to its exposure of more (222) active crystal planes. As a result, the conversion rate of lignin increased from 54.6% to 91.2% [98].

Monometallic chlorides exhibit good performance in catalytic oxidation. Figure 8 illustrates the working principle of this FeCl₃/NaNO₃/O₂ catalytic system. During the depolymerization process, Fe³⁺ in the catalyst oxidizes lignin into aromatic compounds, and Fe²⁺ produced by oxidation is reoxidized to Fe³⁺ by NO₃⁻. The main products of the catalytic system were vanillic acid and vanillin, with a yield of 5 wt%, which was higher than the yield reported in previous literature [90]. Ren et al. investigated the CuCl₂ catalyst, which is capable of producing a significant amount of highly active aromatic compounds through selective cleavage of C-C and C-O bonds. The results indicated that the overall selectivity of the catalyst toward aromatic monomers exceeded 96%, while the conversion rate reached 88%. These results demonstrated the effectiveness of the catalyst in producing desirable aromatic compounds [99].

Figure 8. Catalytic mechanism of FeCl₃/NaNO₃/O₂ catalytic system [90].

Bimetallic catalysts demonstrate improved catalytic performance due to the synergistic effect between metals. Research has indicated that metals such as Fe and Ni exhibit good activity in selectively breaking the C-O bond in lignin, and are commonly used as catalysts for the depolymerization of lignin [100]. In particular, the nickel exhibits a high selectivity for α-O-4 and β-O-4 bonds. Xiu et al. prepared a Ni-Fe bimetallic catalyst that yielded 67.14 wt% of bio-oil at 240°C, with a selectivity of 98.5% for G-type phenols. Additionally, the catalyst demonstrated high stability and depolymerization activity over seven consecutive cycles [100]. Chen et al. demonstrated that the Co-Mo bimetallic catalyst exhibited remarkable selectivity towards phenolic compounds. The inclusion of Mo induced a shift in the metal oxide phase and an elevation in oxygen vacancies (V_O). Additionally, the interaction between the two metals decreased the acidity of the catalyst and slowed down the repolymerization of the intermediate product. The experimental findings indicated that the phenolic monomer yield was 37.58 wt%, with the highest yield of 4-methylguaiacol at 8.11 wt% [101]. Chen et al. found that the size of Mn-Mo catalyst particles, the concentration of oxygen vacancies (V_O), and the concentration of surface acidic sites were affected by different Mn/Mo ratios. The highest yield of phenol, at 85.61%, was obtained when the Mn/Mo ratio was 3 [102]. Mottweiler et al. investigated the catalytic properties of Cu-V bimetallic catalysts using molecular oxygen as an oxidant. The study demonstrated that the catalysts exhibited high catalytic activity and selectivity, with veratric acid as the main product [103].

The process of oxidative depolymerization requires milder conditions compared to reductive depolymerization, with a reaction temperature of approximately 100°C [68]. The oxidation of hydroxyl groups in the side chain can facilitate the depolymerization of lignin. However, the addition of oxidants also leads to an increase in the oxygen content of the resulting product and a reduction in the calorific value of bio-oil [89]. Some reaction conditions are still too harsh to be applied in industry [104].

4. Conclusions and perspectives

This paper provides a summary of various metal and metal oxide catalytic depolymerization methods commonly used for lignin. Each method has its unique mechanism, advantages, and limitations. It is challenging to recommend the best lignin depolymerization scheme as they employ different lignin models, depolymerization conditions, and product distribution. Therefore, selecting the appropriate method for a specific purpose and conditions is crucial.

The utilization of metal and metal oxides for depolymerizing lignin is a highly promising approach. Metal and metal oxides offer a significant advantage in terms of high selectivity, enabling the production of specific aromatic compounds through the use of different catalysts. Furthermore, the catalytic conditions associated with metal and metal oxides are typically milder compared to other catalysts used in the same depolymerization method and mechanism. These factors make metal and metal oxides an attractive option for lignin utilization. The use of metal catalysts can typically facilitate the depolymerization of lignin at a lower temperature, thus enhancing safety during the process. Metal and metal oxides are typically processed and synthesized into smaller particles, resulting in a larger specific surface area and higher efficiency for catalytic depolymerization. However, despite the ability of metal and metal oxides to selectively convert lignin into specific aromatic compounds, they possess several limitations. Noble metal catalysts, which exhibit superior depolymerization effects, are often more expensive. Additionally, the use of metal and metal oxides as catalysts presents economic challenges due to the costs associated with processing and synthesis. It is worth mentioning that the low conversion rate of metal and metal oxides, in comparison to other catalysts, is a significant limiting factor for their application. Other acidic or basic catalysts have the ability to almost completely convert lignin into other valuable chemicals during the depolymerization process.

Additionally, the depolymerization process is also influenced by the quality of lignin. However, the molecular structure of lignin remains uncertain due to various factors, including plant growth environment and extraction methods. These factors impact the chemical properties of lignin, leading to inconsistent product quality. There are still many deficiencies in lignin prepared by various extraction methods. Among them, CELF lignin is more in line with the needs of depolymerization in terms of chemical structure and molecular weight, but this method is currently difficult to be put into use on a large scale. The separation and purification of lignin still need to be improved and optimized.

There is no doubt that the use of metal and metal oxides for catalytic depolymerization of lignin is a promising method for producing valuable chemicals. While current research is primarily focused on laboratory-scale experiments, the high selectivity and milder reaction conditions of metals and metal oxides suggest good prospects for practical application. The transition from single metal catalysts to bimetallic catalysts and subsequently to multifunctional metal catalysts is facilitating the process of chemical production through lignin depolymerization, in a stepwise manner. The development of depolymerized lignin presents both opportunities and challenges, which form the foundation for a sustainable future.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research was supported by the Jiangsu Agricultural Science and Technology Innovation Fund [Grant No. CX(21) 3072]; Jiangsu Provincial Postgraduate Practice Innovation Plan [Grant No. SJCX23_2074]; Sponsored by Key Laboratory For Crop and Animal Integrated Farming, Ministry of Agriculture and Rural Affairs [Grant No. 202301]; Exploring and Overturning Innovation Program of Jiangsu Academy of Agricultural Sciences [Grant No. ZX(21) 1221].