5-hydroxymethyl-2-furfural

Sugar is destroyed by pH extremes, and inadequate pH control can cause significant sucrose losses in sugar mills. Sucrose is one of the most acid-labile disaccharides known (27), and its hydrolysis to invert is readily catalyzed by heat and low pH prolonged exposure converts the monosaccharides to hydroxymethyl furfural, which has appHcations for synthesis of glycols, ethers, polymers, and pharmaceuticals (16,30). The molecular mechanism that occurs during acid hydrolysis operates, albeit slowly, as high as pH 8.5 (18). 

In some cases, the hydrolysis reaction liberates the sugars from the biomass and converts them directly into derivatives such as furfural, hydroxymethyl furfural and/or levulinic acid. These derivatives can be further converted into various chemical intermediates. We will not discuss these further conversions as they are extensively reported in the literature, e.g., for furfural [15, 44], hydroxymethyl furfural [15, 44, 50] and levulinic acid [15, 44-47]. 

A probable pathway for the degradation of hemicelluloses via free-radical intermediates has been proposed by Fengel and Wegener (1989) and is shown in Figure 5.1. Hemicellulose polymers are depolymerized to form oligosaccharides and monosaccharides, which are dehydrated to form furfural (pentoses) and hydroxymethyl furfural (hexoses). 

Dextrose is widely used as an isotonic media in parenteral formulations. Sterilisation using autoclaving has been reported to induce the formation of fructose via an isomerisation reaction, with the resultant formation of 5-hydroxymethyl-furfural [90]. 

Scheme 6 Fructose can be transformed into 5-hydroxymethyl furfural (HMF) via acid-catalyzed dehydration. Solid acid catalysts applied to facilitate the reaction are zeolites, ion-exchange resins and solid inorganic phosphates. With sporadic success, notably with inorganic phosphates, other carbohydrate sources such as inulin can also be transformed into HMF.

Hydroxymethyl furfural. 2,5-diformylfuran (DFF) is a furan derivative that has many uses, including use as a polymer building block. By utilizing a platinum catalyst supported on carbon, and running the reaction in water at high temperatures, DFF is produced as the major product in neutral solution. If low temperatures and high pH are employed, 2,5-furandicarboxylic acid results. 

The major soluble components of acid hydrolysates are sugars, such as xylose, glucose, and cellobiose furfurals, such as furfuraldehyde and hydroxymethyl furfural and organic acids, such as levulinic acid, formic acid, and acetic acid (13). When natural sources of cellulose are acid-hydrolized, numerous products can result, largely because of the hemicellulose materials. These make it difficult to produce a relatively pure sugar product and limit the utility of the acid hydrolysis process. 

The KIE in the dehydration of fructose to 5-(hydroxymethyl) furfural has been determined624 and interpreted as arising in the rate-determining addition of H+ to... 

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). 

Early researchers (24.251 identified 5-hydroxymethyl furfural from sucrose solutions heated above 150°C. Later reports (26.271 listed a series of aliphatic and aromatic carbonyl compounds identical to those associated with the thermal degradation of monosaccharides. 

Since the compound hydroxymethyl furfural is a major product of ketohexose dehydration, its presence can be used as an indication that caramel has been added as a coloring agent in a nonheated food system (66). In contrast, ketopentose dehydration produces furfural. Another dehydration product, bis-5,5 -formylfurfurly ether, can also be used to confirm the addition of caramel color (66). 

Furfuryl alcohol 2-Hydroxyacetyl furan 5-Hydroxymethyl furfural Isomaltol... 

Table 7.2 Conversion and selectivity to 5-hydroxymethyl furfural (HMF) for dehydration of fructose and precursors over H-Mordenite (Si/AI — II) at 165 °C in water/methyl isobutyl ketone (1 5 by volume)...

Finally, the formation of derivatives of pyridine from 5-hydroxymethylfurfural and related compounds on treatment with ammonia 0 may be considered in this section on the furan ring. 5-Hydroxymethyl-furfural itself gives 2-hydroxymethyl-5-hydroxypyridine (XLIV) with ammonia, which is presumably formed by opening of the ring in the intermediate aldehyde-ammonia compound followed by closure to give the 6-membered ring. 

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