Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 146
Biochemicals - Production
ОглавлениеBasic 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 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 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 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 in aqueous media, eventually in the presence of organic solvents. These can also act as additives for improving the furfural yields in the reaction media comprised of xylose or xylan, organic solvent, and acidic catalysts. Ionic liquids can also serve as a reaction medium for furfural manufacturing from pentoses, higher saccharides made up of pentoses, or pentosans.
The important chemical obtained from furfural is furfuryl alcohol (FA), and approximately 65% of the overall furfural produced is consumed for the production of FA. FA is currently manufactured industrially by hydrogenation of furfural in the gas or liquid phase over Cu-Cr catalysts. However, chromium in these catalysts causes serious environmental problems because of its high toxicity. Therefore, current studies are focused on exploring more environmentally acceptable catalysts that could selectively hydrogenate the carbonyl group while preserving the C=C bonds. The hydrogenation of furfural over Raney Ni modified by impregnation with heteropolyacid (HPA) salts, such as Cu3/2PMo12O40, that produced a 96.5% yield of furfuryl alcohol was reported. Recently, novel catalyst synthesis methods such as atomic layer deposition (ALD) and encapsulation in metal organic frameworks have been reported.
2-Methylfuran (2-MF) is another industrial chemical that can be synthesized from furfural. 2-MF is also a biofuel component. Another one, tetrahydrofurfuryl alcohol (THFA), is typically produced from furfural via furfuryl alcohol as an intermediate. The hydrogenolysis of tetrahydrofurfuryl alcohol to 1, 5-pentanediol (1, 5-Ped), a promising biofuel component, was disclosed using Rh-MoOx/SiO2 and Rh-ReOx/SiO2 catalysts with 85% and 86% yields, respectively. Furfuryl alcohol and THFA are widely used as green solvents for the synthesis of resins. These can also be used as raw materials for the synthesis of fuels and fuel additives. Cyclopentanone (CPO) is another C5 chemical that can be synthesized from furfural. CPO can be widely used in the production of fuels and polymeric materials. Mainly, Cu-based catalysts are used for the transformation of furfural to cyclopentanone.
The decarboxylation of furfural leads to the production of furan. The hydrogenation of furan produced tetrahydrofuran (THF). Furan and THF are also important industrial chemicals. Furfural can be decarboxylated in both gas- and liquid-phase reactions. Supported noble metal catalysts (Pd, Pt, Rh) and mixed metal oxides, such as Zn-Fe, Zn-Cr, Zn-Cr, and Mn were investigated. The decarboxylation has been found to be most efficient with Pd-based catalysts at a high pressure of hydrogen and a high and reaction temperature. These rigorous reaction conditions result in catalyst deactivation. Additionally, the noble metals used are expensive and limited in abundance. Therefore, alternate active and selective catalysts need to be explored.
The oxidation of furfural can also lead to the production of C4 chemicals such as maleic anhydride (MAN), maleic acid (MA), and succinic acid (SA). The use of vanadium oxide-based catalysts has been studied for gas-phase oxidation of furfural to maleic anhydride with oxygen. The use of oxidants such as oxygen and hydrogen peroxide (H2O2) was also discussed for the oxidation of furfural. The combination of copper nitrates with phosphomolybdic acids selectively converts furfural to maleic acid with a 49.2% yield or maleic anhydride with a 54% yield in a liquid medium using oxygen as an oxidant.
Sugar Alcohols
Lignocellulosic-based sugar alcohols, such as sorbitol, mannitol, xylitol, and erythritol, are potential fuels and chemicals widely used for polymer, food, and pharmaceutical applications. These are extensively used as moisturizers, sweeteners, softeners, texturizers, and food for diabetic patients. Currently, sorbitol and mannitol can be synthesized through hydrogenation of fructose and glucose. Xylitol and erythritol can be prepared by the conversion of xylose and glucose, respectively. Many catalytic systems and methods have been reported for the conversion of cellulose into sorbitol and mannitol via hydrolysis followed by hydrogenation. The use of noble metal-based catalysts Pt/SBA-15 and Ru-PTA/MIL-100(Cr) for the conversion of glucose and cellulose into sorbitol, respectively, were reported. However, cheaper non-metal catalysts (supported on TiO2, Al2O3, SiO2, MgO, ZnO, and ZrO2) have been found to be effective in converting cellulose into sorbitol and mannitol.
Industrially, xylitol is synthesized by the hydrogenation of pure xylose, while xylose can be obtained through acidic hydrolysis of hemicellulose biomass (corncobs and hardwoods). The first report on the synthesis of xylitol through the hydrogenation of xylose over a Raney Ni catalyst was carried out in a three-phase slurry reactor. The acid-transition metal or bi-functional catalysts were used for the hydrolysis and hydrogenation of cellulose to sugar alcohols in the presence of hydrogen pressure.
Erythritol is a C4-sugar alcohol, mainly found in food ingredients. It occurs as a metabolite or storage compound in fruits, such as grapes, pears, seaweed, and fungi. Pentose sugars (arabinose and xylose) are the precursors for producing C4-sugar alcohols. The most efficient route for the synthesis of erythritol from pentose sugars is selective cleavage of a carbon-carbon bond. The production of erythritol and threitol is mostly carried out at a high temperature range of 200 to 240°C (390 to 465°F) with a pressure of hydrogen pressure of in alkaline conditions. Very few findings relate to the selective bond cleavage to produce erythritol.
Succinic Acid
Succinic acid is one of the 12 high-value bio-based chemicals investigated by Werpy and Peterson as a compound that has the potential to improve the profitability and productivity of biorefineries. Conventionally, succinic acid is produced from maleic acid using Pd/C heterogeneous metal catalysts. Other methods reported for succinic acid production are oxidation of 1,4-butanediol with nitric acid; the carbonylation of ethylene glycol, acetylene, and dioxane; hydrogenation of fumaric acid in the presence of Ru catalyst; and the condensation of acetonitrile to produce butanedinitrile, which can be subsequently hydrolyzed to succinic acid.
Lactic Acid
Lactic acid (2-hydroxypropanoic acid) is an important chemical. It is an alternative for producing alkyl lactates, propylene glycol, propylene oxide, acrylic acid, and poly (lactic acid). Lactic acid has applications in food, pharmaceuticals, and cosmetics. In particular, the biopolymers from lactic acid have created a strong interest. Conventionally, lactic acid is produced via fermentation from carbohydrates.
Lactic acid can be made from different reagents such as lignocellulosic materials, cellulose, carbohydrates, sugars, trioses, glycolaldehyde, and glycerol. The production of lactic acid involves complex reactions of several types of transformations such as aldol condensation, retro-aldol condensation, dehydration, and 1,2-hydride shifts.
Several homogeneous and heterogeneous catalysts have been reported for the production of lactic acid from biomass. The catalysts such as alkali metal ions, tin chloride, tin dioxide (SnO2), acidic resins, zeolites, metal-modified zeolites, mesoporous materials, tungstated alumina, mixed-oxides, and carbon-silica hybrid materials have been reported in the literature. The use of Rh/C, Ru/C, Ir/C, Ir/CaCO3, and Pt/C catalysts for the transformation of glycerol to lactic acid was discussed. Typically, the highest yield of lactic acid has been achieved in the presence of an inert gas and alkaline medium (CaCO3). An alkaline platinum-calcium carbonate (Pt/CaCO3) catalyst has been shown to be an efficient catalyst for glycerol transformation to lactic acid, with 54% selectivity for lactic acid at 45% conversion in the presence of borate esters at 200°C (390°F). The Rh/Al2O3 catalyst also gives high selectivity for lactic acid, i.e., 69% in the presence of borate derivatives.
Chemicals such as pyruvic acid, acrylic acid, 2,3-pentanedione, polylactic acid (PLA), lactic acid esters, and 1,2-propanediol (1,2-PDO) can be synthesized from lactic acid. The polylactic acid can be synthesized by two ways: direct polycondensation of LA and ring-opening polymerization of the lactide monomer (cyclic). The direct poly-condensation of lactic acid is a difficult process because of the strong equilibrium between polylactic acid, water, and lactide that limits the synthesis of high molecular weight products. The most commonly used process is using the lactide intermediate. The lactide intermediate was polymerized via a homogeneously catalyzed ring-opening polymerization (absence of water), which produces two lactoyl units in the growing chain. This process suffers from racemization. Consequently, when commercial L-lactic acid was used, a 5% to 12% yield of undesired product (meso-lactide) was produced. The properties of polylactic acid depend mainly on the stereo-composition of the feed used. Therefore, stereo-pure L,L-lactide is used to obtain stereo-pure poly-L-lactic acid. As a result, improvements in catalysts, process parameters, and configuration have been reported.
Catalytic dehydration of lactic acid leads to acrylic acid, while propanoic and pyruvic acids were obtained by lactic acid reduction and dehydrogenation, respectively. 2,3-pentanedione can be produced by condensation and acetaldehyde by either decarbonylation or decarboxylation. The lactic acid conversion to acetaldehyde was investigated on silica-supported heteropolyacid with an 83% yield.
Hydrogenation of lactic acid to 1,2-propanediol was performed in both the liquid as well as the vapor phase using Ru/C and Cu/SiO2 catalysts, respectively. The direct hydrogenolysis of lactic acid seems an attractive option, but deactivation of catalyst makes the process undesirable. The catalyst deactivation occurs as a result of the polymerization of lactic acid and formation of the side-products, propionic acid. Therefore, to avoid this problem, carboxylic acids are usually converted into more readily reducible esters.
In summary, the catalytic conversions of sugars to commodity chemicals are widely discussed, but the industrial applications are limited. Therefore, further research for the improvements of the catalytic conversion and selectivity are still required for achieving the goal of integrated biorefineries. The areas that need attention are the search for novel reaction media to use efficient catalysts for the biomass conversion processes and the extraction/purification steps to isolate the chemicals with high yield and purity.
See also: Biochemicals, Carbohydrates, Coniferyl Alcohol, p-Coumaryl Alcohol, Lignin, Sinapyl Alcohol, Sugars and Starch.
