Structurally, lignin is a three-dimensional amorphous phenolic polymer that consists of monomers such as phenylpropane unit, C3C6 including p-coumaryl, sinapyl, and coniferyl alcohol. It contains β-O-4 (40% to 60%), biphenyl (3.5% to 25%), α-O-4 (3% to 5%), and β-5 (4% to 10%) linkages. The different structural and chemical properties of lignin lead to the production of a wide variety of aromatic chemicals. Therefore, lignin was observed as the major aromatic source of the bio-based economy. Dimethyl sulfide, vanillin, and dimethyl sulfoxides are the chemicals, manufactured from lignin on a large scale. Several researchers have summarized the applications of lignin as a renewable resource, such as emulsifier, bio-dispersant, polyurethane foams, wood panel products, resins, automotive brakes, and precursors for the synthesis of thermoplastic materials in the industry. In addition, the production of aromatics from depolymerization of lignin is considered as the most promising process for the sustainable utilization of lignin. Aromatics can be derived from monomeric C6 fragments from depolymerized lignin. The maximum theoretical obtainable yield of benzene, toluene, and xylene (BTX) from lignin is approximately 36 to 42%, as lignin contains 60% to 65% carbon in C6 aromatic rings. The main challenge in producing aromatics is to selectively deoxygenate and dealkylate the C6 aromatic structure (typically with hydrogen) without hydrogenating C6 aromatic rings. The difficulty in the valorization of lignin originates from its complex polymeric structure, which differs from one lignin to another depending on the botanical origin and the pretreatment used for its separation from carbohydrates (cellulose and hemicellulose).
The catalytic pathways, including base-catalyzed depolymerization, pyrolysis, and Lewis acid-catalyzed solvolysis, have been investigated and studied for the conversion of lignin to valuable compounds. Low product yields and severe treatment conditions, as well as complex product mixtures, have been major drawbacks for lignin conversion. However, lignin’s aromatic nature and its versatile functional groups suggest that it can be a valuable source of chemicals, particularly monomeric phenolics. Hydrothermal liquefaction of lignin in the presence of water as the solvent leads to the production of bulk aromatic compounds. Bio-oil obtained after lignin decomposition contains monomeric phenols and oligomeric polyphenols. The monomeric phenols are valuable chemicals; however, the oligomeric polyphenols that existed in bio-oils were volatile and viscous, which makes them more difficult for conversion into useful products. As a result, the conversion of lignin to monomers instead of oligomers is highly desirable.
The thermochemical conversions such as catalytic fast pyrolysis and microwave pyrolysis were commonly used processes in the presence of effective catalysts to enhance reactions, including cracking, decarbonylation, deoxygenation, and decarboxylation. Recently, several studies have been reported for lignin depolymerization to obtain monomeric phenols. The monomers of phenols such as alkylated phenol and guaiacol have found applications as intermediates for the production of polymers, antioxidants, resins, medicines, and pesticides. The preparation of phenolic resins such as phenol-formaldehyde using phenolic-rich pyrolysis oils is well known.
Phenolic compounds are obtained from lignocellulosic biomass after treatment with alkali. A large number of different methods have been discussed, but the processes reported are complex, low yielding, cost-ineffective, and energy inefficient. The most challenging aspect of the production of chemicals from lignin-derived monomeric phenols using catalytic hydrotreatment is the synthesis of catalysts that can perform deoxygenation without saturating the aromatic rings in the phenol deoxygenation processes. This will help to decrease the hydrogen consumption. For this process, mainly conventional metal sulfide, metal oxide, transition metal phosphide, metal carbides, and bi-metallic catalysts were used. Bi-metallic catalysts are found to be more suitable than monometallic catalysts for deoxygenation of phenols.
Ni-based catalysts were used for lignin hydrogenation/hydrogenolysis in 1940. Ni catalysts supported on carbon and magnesium oxide were found to be active for C-O bond cleavage of model compounds as well as selectively hydrogenating the aryl ether C-O bonds of β-O-4 without disturbing the arenes. The alcohol solvents were used as a hydrogen donor for the hydrogenolysis of lignin. The platinum group metals (palladium, Pd, platinum, Pt, ruthenium, Ru, rhodium, Rh, and iridium, Ir) bear higher intrinsic activity than Ni catalysts and hence were widely reported for hydrogenolysis of lignin. Zn in Pd-based catalysts was found to be far more effective than the Pd/C catalyst and Zn-based catalysts were effective for the cleavage of β-O-4 bonds in lignin model compounds.
Oxidative depolymerization of lignin leads to the production of polyfunctional aromatic compounds. These compounds include aromatic aldehydes and carboxylic acids, such as 4-hydroxybenzaldehyde, vanillin, muconic acid, and syringaldehyde, which are good alternatives to crude oil-based chemicals. The depolymerization of lignin in 1-ethyl-3-methyl-imidazolium trifluoromethanesulfonate with nitrate catalysts yielded pure 2,6-dimethoxy-1,4-benzoquinone. The catalytic systems for lignin oxidation involve organometallic catalysts, metal-free organometallic catalysts, acid or base catalysts, metal salt catalysts, photocatalytic, and electrocatalytic oxidation. Methyltrioxorhenium (MTO) in combination with H2O2 catalyzed lignin oxidation reactions is the most promising.
This catalytic system leads to extensive oxidation on the aliphatic side-chain and aromatic-ring cleavages.
Lignin-Derived Polymers
After the depolymerization and production of aromatic compounds from lignin, the consequent processes do not require much advancement. The mature technologies already exist for the transformations of aromatic compounds into commodity monomers and polymers. The commodity polymers that can be derived from lignin are polyethylene terephthalate (PET), Kevlar, polystyrene, polyanilines, and unsaturated polyesters. The alternatives to fossil-based aromatic polymers could be accomplished by the full valorization of lignin. The synthesis of bio-based PET can be realized by the preparation of ethylene glycol (EG) and p-terephthalic acid from renewable biomass. Bio-based p-xylene can be used as the raw material for p-terephthalic acid to produce a 100% plant-based PET. Sulfur-free lignin derivatives have been widely used as a raw material for wood panel products, polyurethane foams, automotive brakes, biodispersants, and epoxy resins for printed board circuits. Cornstalk-derived bio-oils were used to synthesize phenol-formaldehyde resins.
An integrated biorefinery approach will optimize the utilization of renewable biomass for the production of bioenergy, biofuels, and bio-derived chemicals for the short- and long-term sustainability. For an integrated biorefinery, the concept of usage of platform intermediates as precursors to different products by chemo-catalytic routes will be of highest importance. This will offer the refinery the necessary adaptability to product demand. This review summarizes the production of platform chemicals from lignocellulosic biomass components. The three main components of lignocellulosic biomass, cellulose, hemicellulose, and lignin are valuable precursors for numerous chemicals having valuable applications. The target chemicals include furan derivatives, such as 5-hydroxymethylfurfural (5-HMF), 2,5-dimethylfuran (2,5-DMF), sugar alcohols and organic acids, such as levulinic acid, lactic acid, succinic acid, and aromatic chemicals. These chemicals can be further converted to a range of derivatives that have potential applications in the polymer and solvent industries. The chemo-catalytic routes were found to be most promising ones for the conversion of biomass feedstocks to these high-value chemicals.
Production from Sugars
Cellulose and hemicellulose are the polymers of C6 and C5 sugar units that are linked by ether bonds. Cellulose consists of D-glucose units connected by β-1-4 linkages and extensive hydrogen bonding which makes the hydrolysis process difficult. Acid and enzymatic hydrolysis are commonly used to liberate the monosaccharide glucose units. Hemicellulose contains C5-sugars, such