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
