substrate, mainly alcohol, forming an alkene or other unsaturated product depending on the substrate. The reaction is commonly catalyzed by Lewis or Brønsted acids, as the hydroxyl group is a poor leaving group. Dehydration in the presence of a Brønsted acid catalyst occurs by first protonating the hydroxyl group, as the protonated hydroxyl group (R-H2O+) is a better leaving group than the hydroxyl group. As a result, the catalyst is eliminated as water. Simultaneously, a carbon-carbon double bond (C=C) is formed in the carbon skeleton of the substrate, as per Zaitsev’s rule.
In Lewis acid-catalyzed reactions, however, the reaction proceeds through the bonding of Lewis acid to the lone electron pair of hydrogen-oxygen. The electrophile nature of the Lewis acid lowers the electron density in the alcohol carbon-oxygen bond, which results in the cleavage of the alcohol carbon-oxygen (C-O) bond and the formation of alkene and Lewis acid hydroxide specie. The Lewis acid hydroxide reacts with the released β-proton, forming water and the original catalyst species.
Due to the abundances of hydroxyl groups in a wide variety of natural resources, dehydration reactions are the most common and important ways to valorize biomass. As a result, different dehydration products can be obtained from biomass, and are used as high-value chemicals.
Hydrogenation
Hydrogenation is a reaction in which hydrogen atoms are added to an unsaturated compound to reduce the double and triple bonds. Molecular hydrogen (H2, gaseous) and other compounds (transfer hydrogenation) can be used as a hydrogen source in the reaction. However, the addition of hydrogen does not take place without a catalyst; therefore, the reaction is catalyzed by homogeneous and heterogeneous catalysts to increase the feasibility of reactions at the laboratory and industrial scales within short time durations. Most commonly, heterogeneous systems with solid metal hydrogenation catalysts and molecular hydrogen are used as catalysts for biomass conversion reactions.
Hydrogenation with a heterogeneous solid metal catalyst and hydrogen follows the Horiuti-Polanyi mechanism. First, the hydrogen molecule is chemisorbed on the surface of the catalyst, followed by the scission of a hydrogen-hydrogen (H-H) bond producing two adsorbed hydrogen atoms. Next, the unsaturated reactant is adsorbed on the catalyst. The opening of a double bond through chemisorption follows this. The hydrogen atoms are transferred to the chemisorbed reactant on the surface of the catalyst in a stepwise manner of which the first hydrogen transfer is reversible. The second hydrogen transfer forms the reduced reaction product, and then it is desorbed from the surface of the catalyst, thus completing the reaction cycle.
Hydrogenation is the most fundamental reaction in chemistry. Nature produces many different unsaturated products including carbon-carbon double (C=C) bonds, in carbonyl groups, in the structural aldoses and ketoses of cellulose and hemicellulose. The hydrogenation of these biomass-derived monosaccharides in lignocellulosic biomass produces sugar alcohols. For example, hydrogenation of glucose and xylose, the main components in lignocellulosic biomass, produces sorbitol and xylitol, respectively. In addition, the dehydration products can be further upgraded through hydrogenation. These hydrogenation products can be used as solvents, monomers, and biofuels. The synthesis and uses of hydrogenation of biomass-derived substrates will be covered in more detail.
Hydrodeoxygenation
Hydrodeoxygenation (HDO) is a hydrogenolytic reaction in which the removal of the oxygen atom from the reactant occurs in the presence of hydrogen (H2). The removal of oxygen-containing functionalities can occur through direct hydrogenolysis (C-O bond cleaved with hydrogen), dehydration (C-O bond cleaved through the removal of water), decarbonylation (removal of carbon monoxide, and decarboxylation (removal of carbon dioxide. The most common hydrodeoxygenation pathways depend on the oxygen moieties. Hydrodeoxygenation also needs selective catalysts to facilitate the formation of the desired reaction products. Catalysts typically contain noble metals as the hydrogenation catalyst, as well as Brønsted or Lewis acidic sites for cleavage of the carbon-oxygen bonds. The hydrodeoxygenation mechanisms of different oxygen functionalities depend on the reaction conditions and catalysts used.
In the case of biomass or biomass-derived substrates, hydrodeoxygenation reactions are used to reduce high oxygen content. Typically, these hydrodeoxygenation reactions require a high temperature and high pressure, possibly resulting in the formation of product mixtures through cleavage of carbon-carbon bonds and carbon skeleton rearrangements. In this context, new catalytic systems need to be developed to remove the oxygen-containing functionalities.
Production from Lignin
Lignin is the key constituent of the lignocellulosic biomass and responsible for the structural and mechanical integrity of plants. Lignin is a polymer with wide variability in structure. Its components depend on the biomass source and are most often combined with cellulose and hemicelluloses. It is considered the least susceptible to chemical and biotransformation techniques. Therefore, lignin often becomes a low-value waste product of biomass processing technologies, such as in the conventional paper and pulp industry and in the modern bioethanol-fuel-production industry. Therefore, lignin valorization in relation to energy, chemical, and biotechnological application is creating considerable interest to researchers.
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
