The development of processes and technologies to convert lignocellulosic biomass to fuels and value-added chemicals remains a significant challenge. In this context, the major difficulty in producing a high yield of target chemicals and fuels is the complex chemical composition of lignocellulosic biomass feedstocks. Structurally, cellulose contains anhydrous glucose units and hemicellulose consists of different C5 sugar monomers. On the other hand, lignin is a complex, three-dimensional, and cross-linked biopolymer having phenylpropane units with relatively hydrophobic and aromatic properties. Due to these differences in their chemical composition and structure, cellulose, hemicellulose, and lignin have different chemical reactivities. In addition to the complex nature of bio-resources, the inert chemical structure and compositional ratio of carbon, hydrogen, and oxygen in molecules in biomass present difficulties in the chemo-catalytic conversion of biomass to fuels and chemicals.
A variety of methods can be employed to obtain different product portfolios of bulk chemicals, fuels, and materials. Biotechnology-based conversion processes can be used to ferment the biomass carbohydrate content into sugars that can then be further processed. For instance, the fermentation path to lactic acid shows promise as a route to bio-degradable plastics and has been demonstrated commercially. An alternative is to employ thermochemical conversion processes which use pyrolysis or gasification of biomass to produce a hydrogen-rich synthesis gas. This synthesis gas can then be used in a wide range of chemical processes.
While the concept of exploiting the wide range of chemicals from plants may appear novel, the published literature shows that large numbers of metabolites have already been identified and characterized from a wide variety of plant species. For example, over 37,000 different potential and unexploited materials can be identified. These have a wide range of chemical, physical, and biological properties and include phenolics, nitrogen containing compounds, and terpenes (terpenoids). The variety of molecular compounds is vast. For example, in the terpene group, there are six sub-groupings of molecules with a large number of applications including use in anti-cancer drugs.
Extraction procedures can have a major impact on the availability of these chemicals, and, to ensure optimal exploitation, some of the well-established extraction procedures may need to be revised. For example, in winter rapeseed, the harvested seed is crushed and rapeseed oil extracted mechanically. The residual meal is then treated with hexane to extract the remaining oil, before being used as feed, primarily for ruminants. Rapeseed oil components have numerous applications including use in bio-diesel, and specialty chemicals.
However, innovative oil-extraction procedures could allow greater exploitation of protein-based metabolites in the rapeseed, which can comprise 25% or more of the rapeseed mass. Research from studies, such as the EC-funded Enhance project, has demonstrated that this separation would allow products to be produced for numerous applications (see diagram) with base cellulose material and some other metabolites remaining in the residual meal.
See also: Biofuels, Petrochemicals.
Biochemicals - Production
Basic knowledge of the mechanisms of common reactions such as dehydration, hydrogenation, and hydrodeoxygenation involved in biomass upgradation processes is discussed in the following section.
Dehydration
Dehydration is a reaction in which a water molecule is removed from the 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