Furfural from pentoses

Absorption spectra of the phenol-sulfuric acid solutions tested for total sugars show that 5-hydroxymethylfurfural from hexoses is more common in the uppermost Silurian and Devonian samples than in the earlier deposits. Furfurals from pentose sugars evidently form the bulk of the residual carbohydrates in these samples, however. There is no definite evidence as to the marine or terrestrial origin of the hexose products in the samples. 

Furfural (from pentoses) Hydroxymethyl-5 furfural (from hexoses)... 

The formation of furfural from pentoses, of 5-methylfurfuraI from methylpentoses, and of 5-hydroxymethylfurfural from hexoses under acidic conditions has long been known.1 There are only a few more recent investigations to be mentioned. 

The researches have dealt with each phase of the pentosan determination. Many modifications have been introduced in the distillation procedure, in attempts to obtain ma.ximal yields of furfural. For e.xample, some workers prefer to distil with 23 % hydrobromic acid, rather than with the conventional 12% hydrochloric acid. Others distil in the presence of added sodium chloride, to avoid changes in acid concentration. Steam distillation has been used by a number of workers, who claim theoretical yields of furfural from pentoses, but Launer and Wilson found no advantage either in salts or in steam in the analysis of pulps and papers. Interfering substances are of two types materials other than pentosans which form furfural in the pentosan analysis, and substances which yield products which may be determined as furfural. TJronic acids and polyuronides yield furfural, although not quantitatively, and, in the case of materials containing appreciable quantities of these substances, it is usual to make a correction. The value of the correction to be applied has been determined experimentally by several workers, with somewhat differing results. 

The subsequent formation of furfural from pentose involves the liberation of three molecules of water per molecule of pentose. Any such major transformation of a molecule does not take place concertedly but in steps. A plausible mechanism is illustrated in Figure 2. The initial pentose is shown in its prevalent ring form representing an intramolecular hemiac-etal. The open-chain aldehyde form in equilibrium with the ring form can be disregarded as it amounts to less than one percent of the total pentose present. The transformation steps shown consist of two 1,2-eliminations and one 1,4-elimination of water. The 1,2-eliminations must... 

Formula 4.59 shows examples of products obtained on the decomposition of 3-deoxy-osones. The best known compounds are 5-hydroxymethylfurfural from hexoses (HMF, II in Formula 4.59) and furfural from pentoses (I in Formula 4.59). Taking the furanoid structures of 3-deoxyosone as a basis (Formula 4.55), 3,4-dideoxyosone is obtained after ring opening, enolization, and water elimination (Formula 4.60). Water elimination from the hemiacetal form of 3,4-dideoxyosone directly yields HMF. Taking into account the water elimination required to form 3-deoxyosone (cf. Formula 4.55), 5-hydroxymethylfurfural is formed from hexose by the stoichiometric elimination of 3 mols of water. 

Furfural comes from pentose sugars in cereal straws and brans. Furfural is the precursor of furfuryl mercaptan and its disulfide, difurfuryl disulfide, which are both important chemicals for coffee, meat and roasted flavours. They are prepared by the reaction of furfural and hydrogen sulfide (Scheme 13.15). 

Furfural occurs in many essential oils and is produced in small quantities in many organic reactions,1 particularly those involving the decomposition of various carbohydrate materials. Pentoses when subjected to the action of hydrochloric acid are decomposed to give practically quantitative yields of this aldehyde.2 It is well known that carbohydrate materials such as corn cobs, wood, bran, etc., when heated with steam under pressure or distilled with dilute hydrochloric or sulfuric acids, yield appreciable quantities of furfural.3 Particularly good yields, however, are obtained from ordinary corn cobs, and this material therefore appears best for the production of large amounts of furfural in the laboratory. Improvements in the production of furfural from carbohydrate material have appeared recently in the patent literature 4 but those do not appear as satisfactory as the ones described.5... 

The fermentation inhibitors include furan aldehydes, aliphatic acids, and phenolic compounds. The furan aldehydes, furfural, and hydroxymethyl furfural (HMF), are formed from pentoses and hexoses, respectively (4,5). Several studies indicate that furfural inhibits Saccharomyces cerevisiae, at least when present in high concentrations (6-10). HMF has a similar effect (11,12). 

Furfural identified in beef diffusate appears to be a prominent meat flavor intermediate. It is a dehydration product of pentoses similar to formation of hydroxy methyl furfural from hex-oses. These compounds are formed by dehydration of 1,2-enediols derived from deamination of Amadori compounds (51). 

A few years later it was confirmed that the degradation of carbohydrates with acids gave rise also to furfural (II). This had been obtained fifty years earlier by Dobereiner by the action of sulfuric acid and manganese dioxide on sugar but its significance was not realized at the time. Furfural however was obtained only from pentoses, under normal acid conditions. 

Among the other sugars detected in the soil, only the 6-deoxyhexoses have been quantitatively determined. In Delaware soils, rhamnose and fucose, determined by quantitative paper-chromatography, amounted to 20% of the sugars. Under the conditions of furfural formation from pentoses, the 6-deoxyhexoses yield 5-methyl-2-furaldehyde this has been determined by the differential solubilities of the phloroglucides in alcohol. The proportion of 6-deoxyhexoses in some cases exceeded that of pentoses. 

The first process to make furfural from sulfite liquor was offered by VOEST-ALPINE of Austria in 1988. In this process, shown schematically in Figure 32, the sulfite liquor is first thickened to a dry solids content of 30 %. After heating the concentrate to 180 °C, and after holding it at this temperature in a tube reactor for a period of time sufficient to convert some pentose to furfural, the reaction mixture is passed into a distillation column where the furfural is stripped by steam. The treatment of providing residence time at 180 C in a tube reactor to convert more pentose to furfural, followed by removal of the furfural in a stripping column, is repeated two times. In this fashion, the furfural is removed stepwise soon after its formation, to reduce losses by furfural reacting with itself, with intermediates of the pentose-to-furfural conversion, and with other constituents of the liquor. 

Figure 38. The Formation of 5-Methyl Furfural from 5-Methyl Pentose.

Furfural is easily produced from pentoses rich biomass by cyclodehydration. Various acidic reaction conditions have been applied [110, 111]. The precise order of the different dehydration steps is not known for certain. A plausible mechanism is depicted in Scheme 20 [112]. The worldwide production is currently about 300,000 tons per year. Thus, furfural is a low cost, polyfunctional substrate for the production of numerous bulk and fine chemicals. 

Many efforts have been made to base polymers on furfural made from pentoses.159 The polymers may be useful, but tend to have lower thermal stability than the usual synthetic polymers. Polyesters based on furfural were mentioned earlier. The acid-catalyzed polymerization of furfuryl alcohol is used in foundry cores.160 Furfural has been condensed with cardanol (m-pentadecadienylphenol) from cashew nut shell oil in the presence of other phenols to produce polymeric resins.161 Cardanol and hydrogenated cardanol have been polymerized with horseradish peroxidase to soluble polymers in up to 85% yield.162 Plasticizers that are effective in polyvinyl chloride, such as (12.31), have been made from furfural.163... 

Furaldehyde (furfural from Latin for bran) is obtained industrially from plant residues which are rich in pentoses, e.g. bran, by treatment with dilute sulfuric acid followed by steam distillation ... 

The reaction products of the Maillard reaction, such as l-amino-l-deoxy-2-ketose (Amadori product) or 2-amino-2-deoxyaldose (Heyns product), do not contribute to flavor directly but they are important precursors of flavor compounds [48]. These thermally unstable compounds undergo dehydration and deamination reactions to give numerous rearrangement and degradation products. The thermal degradation of such intermediates is responsible for the formation of volatile compounds that impart the characteristic burnt odor and flavor to various food products. For example, at temperatures above 100 C, enolization products (such as l-amino-2,3-enediol and 3-deoxyosone) yield, upon further dehydration, furfural from a pentose and 5-hydroxy methylfurfural and 5-meth-ylfurfural from a hexose [2]. 

In catalytic biomass cmiversion, the dehydration of polyol moieties is a key reaction, forming either an olefin bond, an ether, or a carbonyl group (after tautomerization). This type of reactimi usually requires the aid of acid catalysis. Both Br0nsted and Lewis acids are known to catalyze dehydration. The most famous dehydration in the context of biomass conversion is the formation of HMF from six-carbon sugars. As can be seen in Fig. 12, HMF from fructose (or glucose) preserves the F.C value of 1.17, while the FI index falls from 2 to 1. Similarly, pentoses lead to furfural in aqueous media under acid catalysis. The catalytic formation of HMF and furfural from hexoses and pentoses, respectively, and HMF production directly from cellulose, have been reported frequently over... 

FA is a bio-based material, produced by hydrogenation of furfural on an industrial scale. Furfural has been prepared in commercial quantities for many decades from pentose-rich agricultural residues, including rice hulls, bagasse, oat huUs, and corn cobs. Furfural can also be derived from wood and wood products, which represent a second natural storehouse for furfural [62],... 

However, it was originally prepared (over a century ago) from pentoses (five-carbon carbohydrates see Chapter 11), which, on sulfuric acid treatment, produce furfural (furan 2-carboxaldehyde) (Scheme 8.75), and which, in turn, can be easily oxidized to the corresponding carboxylic acid (2-furoic acid). The acid (2-furoic add) is then readily decarboxylated (i.e., loss of CO2) to furan (oxa-2,4-cyclopentadiene) and the resulting product is catalytically reduced with hydrogen (H2) (Scheme 8.102) over a Ni catalyst to yield the desired five-membered heterocyclic compound (oxalane, oxacyclopentane,THF). 

Scheme 8.102. The formation of furfural (furan 2-carboxaldehyde) from pentoses (typical examples, D-ribose and its proton tautomer D-ribulose, are shown) on treatment with sulfuric acid (Scheme 8.75).The conversion to furan 2-carboxylic acid,furan (oxa-2,4-cyclopentadiene), and tetrahydrofuran (THF, oxacyclopentane, oxolane) follows.

The steps in the formation of 5-hydroxymethyl furfural (HMF) from 1,2-enediol is shown in Formula 4.37. HMF is also used as an indicator for the heating of carbohydrate containing food, e. g., honey. The (retro-Michael addition) water elimination at C-3 and subsequently at C-4 leads to a 1,2-diulose (3,4-dideoxyosone), which after cyclization to a hemiacetal, a dihy-drofuran, releases another molecule of water, producing HMF. In the same way, e. g., furfural can be made from pentoses and 5-methylfurfural from the 6-methylpentose rhamnose. 2-Hydroxyacetylfuran, which is preferentially formed from fructose, can be obtained starting from the corresponding 2,3-enediol by water elimination at C-4, followed by C-5 (Formula 4.38). 

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