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 as xylose, galactose, mannose, and arabinose. The dehydration of C5 sugars can yield furfural, which is a platform chemical that has applications ranging from solvents to resin and fuel additives. The large-scale synthesis of organic chemicals and chemicals based and on furan from sugars is an important alternative to crude oil-based energy resources.
Hydroxymethylfurfural
Hydroxymethylfurfural (5-HMF), also 5-(hydroxymethyl)furfural (5-HMF) is the most important platform chemical from renewable feedstock for the next-generation plastic and biofuel production. The derivatives such as levulinic acid, 2,5-bis(hydroxymethyl)furan (2,5-BHF), 2,5-dimethylfuran (2,5-DMF), and 2,5-diformylfuran (2,5-DFF) were synthesized from 5-HMF. Other derivatives are 1,6-hexanediol, 5-hydroxymethyl-2-furan carboxylic acid (HMFCA), 2,5-furfuryldiamine, 2,5-furfuryldiisocyanate, and 5-hydroxymethyl furfuryliden ester. These derivatives have found applications as precursors for the synthesis of materials such as polyesters, polyamides, and polyurethane. The synthesized polymeric materials exhibit good properties. Polyurethane demonstrates high resistance to thermal treatments; photoreactive polyesters have been used for ink formulations, and Kevlar-like polyamides exhibit liquid crystal behavior.
The formation of furan-based derivatives from the hydroxymethylfurfural by catalytic oxidation and hydrogenation processes is reported in Figure 8. The chemicals obtained include 2,5-diformylfuran (2,5-DFF), 2,5-dimethylfuran (2,5-DMF), 2,5-furan dicarboxylic acid (2,5-FDCA), and 2,5-bis(hydroxymethyl)furan (2,5-BHF). 2,5-DFF is produced by the selective and partial oxidation of 5-HMF. It is used in the synthesis of fungicides, drugs, and polymeric materials. Several promising catalytic routes for 2,5-DFF production are reported in the literature. The complete transformation of 5-HMF with a 90% yield of 2,5-DFF was achieved with a vanadium oxide titanium oxide (V2O5/TiO2) catalyst in the presence of air and toluene or methyl isobutyl ketone (MIBK) as the solvent.
Furfural
Furfural is also considered a key chemical produced in lignocellulosic biomass refineries. Hemicellulose, which contains a large amount of C5 sugars xylose and arabinose, can serve as a raw material for the production of furfural. This industrial chemical is mainly obtained from xylose by dehydration. Furfural has been used as a foundry sand linker in the refining of lubricating oil. The use of furfural as an intermediate for the production of chemicals such as furan, furfuryl alcohol, and tetrahydrofuran (THF) has been reported. Reviews have been published on the chemistry of furfural.
Commercially, furfural is produced by the acid-catalyzed transformation of pentosan sugars; C5 polysaccharides are first hydrolyzed by H2SO4 to monosaccharides (mainly xylose), which are subsequently dehydrated to furfural. Furfural is then recovered from the liquid phase by steam stripping to avoid further degradation and purified by double distillation. Several reports on the conversion of raw biomass into C5 sugars and furfural using mineral acid and solid acid catalysts were published. The use of these catalysts makes the reaction system more corrosive, which increases the capital costs of the processes. The use of ionic liquids for furfural manufacture has been widely discussed. An ionic liquid plays a role as an acidic catalyst for pentose dehydration
