Encyclopedia of Renewable Energy - James G. Speight

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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.

       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.

       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

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