Hydroxymethylfurfural and Levulinic Acid

It has often been proclaimed that 5-hydroxymethylfurfural (HMF, Fig. 8.35) could be an ideal cross-over compound between carbohydrates and petrochemistry [184], as it is a bifunctional heteroaromatic compound that is accessible from fructose in one step. It was expected that HMF could be developed into a valuable synthetic building block and that its derivatives, such as furan-2,5-dicarb-oxylic acid (FDA), would be able to compete with fossil-derived monomers for use in thermostable polyesters and polyamides. 

Fructose, or a fructose-containing polysaccharide, can be transformed into HMF by aqueous sulfuric acid at 150 °C via a number of isomerization and dehydration steps [185]. Presumably, such a procedure was at the basis of the pi-lot-scale production of HMF by Siidzucker (Germany) [186]. Major problems are the competing rehydration of HMF into levulinic acid (4-oxopentanoic acid, LA, see Fig. 8.35) and the formation of polymeric side-products, which necessitate chromatographic purification. Yields have stayed modest (approx. 60%), in consequence. 

Various strategies, such as the use of solid acids, reaction in biphasic media or in anhydrous DM SO, have been attempted to improve the yield and selectivity [187]. Recently, HMF has been prepared in 1,2-dimethoxyethane, combined with water removal [188], in ionic liquid media [189] and in supercritical acetone [190] but the selectivity for HMF could not be improved beyond approx. 80%, which is undesirably low. 

In conclusion, the selective conversion of biomass into HMF is still a formidable obstacle after 50 years of study. Due to the modest selectivity, the DSP is too elaborate for a commodity chemical and the target price of 20001-1 [190] is still elusive. 

The interest in FDA arises from its possible application as a renewable-derived replacement for terephthalic acid in the manufacture of polyesters. A multitude of oxidation techniques has been applied to the conversion of HMF into FDA but, on account of the green aspect, platinum-catalyzed aerobic oxidation (see Fig. 8.35), which is fast and quantitative [191], is to be preferred over all other options. The deactivation of the platinum catalyst by oxygen, which is a major obstacle in large-scale applications, has been remedied by using a mixed catalyst, such as platinum-lead [192]. Integration of the latter reaction with fructose dehydration would seem attractive in view of the very limited stability of HMF, but has not yet resulted in an improved overall yield [193]. 

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Hydroxymethylfurfural is not volatile by steam. It is prepared from hexoses in the presence of an acid catalyst by short heat treatment to avoid further degradation to levulinic acid. After recovery by solvent extraction hydroxymethylfurfural is purified by distillation. Levulinic acid can be prepared in good yield from hexose-based polysaccharides by heating with acids. In this reaction formic acid is liberated and levulinic acid is easily lactonized to form a- and /3-angelica lactones (Fig. 2-31). 

Levulinic acid and formic acid are end products of the acidic and thermal decomposition of lignocellulosic material, their multistep formation from the hexoses contained therein proceeding through hydroxymethylfurfural (HMF) as the key intermediate, while the hemicellulosic part, mostly xylans, produces furfural.A commercially viable fractionation technology for the specific... 

Blanksma5 continued the investigation of the formation of hydroxy-methylfurfural and confirmed Kiermayer s results that it was formed from hexoses by elimination of three molecules of water on acidic degradation and showed that ketoses reacted more readily than aldoses, a fact which was observed by Kiermayer when sucrose was treated with aqueous oxalic acid. Kiermayer had observed also that levulinic acid was obtained when hydroxymethylfurfural was treated with aqueous oxalic acid under pressure and this received further confirmation by Van Ekenstein and Blanksma.6 It was these authors who first pointed out that the complete degradation of hexoses to levulinic acid took place through the intermediate formation of hydroxymethylfurfural. 

The most important aspect of the chemistry of the furan ring in 5-hydroxymethylfurfural is its scission under the influence of acidic reagents. In the very earliest work on the acidic degradation of hexoses, levulinic and formic acids were obtained which were shown subsequently to have arisen from the decomposition of 5-hydroxymethylfurfural. This degradation was studied by Teunissen53 87 who measured its rate and showed it to be a unimolecular reaction. He proposed the scheme represented by XLIV-XLIX for the conversion of 5-hydroxymethyl-... 

More advantageous is the use of solids acids and especially bifunctional catalysts, exhibiting acid sites for hydrolysis in combination with a metal (Rh, Pt, Ru, and Ni) for hydrogenation, which can be used, in appropriate solvents, for the direct conversion of cellulose into value-added chemicals (e.g. sorbitol, mannitol, ethylene glycol, hydroxymethylfurfural, 2,5-dimethyltetrahydrofuran, levulinic acid, valerolactone, and isosorbide) [33 36]. 

An opposite reaction to the acid-catalyzed hydrolysis is the above-mentioned reversion. Acids can also catalyze the formation of anhydro sugars (see Section 2.3.4). Reversion tends to result in formation of (1—>6)-glycosidic bonds. The degradation of pentoses and uronic acids into furfural and of hexoses into hydroxymethylfurfural, levulinic, and formic acids are also important acid-catalyzed reactions, which, however, require concentrated acid and higher temperatures (Fig. 2-31). 

Fig. 2 -31. Reactions of sugars in the presence of concentrated mineral acids, (a) Pentoses (R = H) yield furfural and hexoses (R = CH2OH) hydroxymethylfurfural. (b) On further heating hydroxymethylfurfural is fragmented under liberation of formic acid. The rest of the molecule is rearranged to levulinic acid, which is lactonized to form a- and /3-angelica lactones.

Phenolysis of Carbohydrates. The carbohydrates are also modified by reaction with phenol under comparable conditions as has been demonstrated by Mathur (11) Among the products identified were levulinic acid, furfural, and hydroxymethylfurfural. All are capable of forming carbon-carbon bonds with phenol. 

In acid solution, pentoses and hexoses form furan compounds. Pentoses give high yields of 2-furaldehyde (furfural). Hexoses give 5-(hydroxymethyl)-2-furaldehyde (hydroxymethylfurfural), which can further react to give levulinic acid and polymeric materials. 

Fig. 8.35 Transformation of D-fructose into 5-hydroxymethyl-furfural and furan-2,5-dicarboxylic acid. Compounds Fru, D-fructose HMF, 5-hydroxymethylfurfural FDA, furan-2,5-dicarboxylic acid, LA, levulinic acid.

The hexoses can also be converted directly to levulinic acid and formic acid on acid treatment without isolation of 2-hydroxymethylfurfural. Note that levulinic acid can be synthesized from both pentoses and hexoses, but is usually manufactured from cane sugar or starch by boiling with hydrochloric acid. A process for the manufacture of levulinic acid has also been developed for paper-mill sludge feedstocks (Fitzpatrick and Jarnefeld, 1996). 

Preparation. Tollens discovered that levulinic acid can be obtained by the action of dilute mineral acids on carbohydrates starting materials include sucrose, starch, glucose, furfural," and 4-hydroxymethylfurfural." In the Organic Syntheses procedure a solution of 500 g. of sucrose in 1 1. of water is treated with 250 ml. of... 

The Biofine process produces levulinic acid, furfural and formic acid from lignocellulose in a two-step process using dilute sulfuric acid as a catalyst [45], The first step depolymerises the polysaccharides and converts the resulting 6-carbon sugars into hydroxymethylfurfural by a dehydration reaction at 210-220 °C and 25 bar. The second step converts hydroxymethylfurfural to the products at 190-200 °C and 14 bar (Scheme 1). 

Methylfurfural 5-Hydroxymethylfurfural Phenols (including guaiacol, syringol and alkyl derivatives) Levulinic acid and lactones Unidentified... 

Yields of hydroxymethylfurfural as high as 54 % and of levulinic acid as high as 69 to 79 % have been reported by the use of sucrose as the initial... 

Levulinic acid can be manufactured by the acid treatment of starch or the C6-carbohydrates in lignocellulosic biomass via the hydration of hydroxymethylfurfural (HMF), an intermediate in this reaction. A side product of this reaction is formic acid, which is produced in equimolar amounts. It is also possible to produce levulinic acid from the five carbon carbohydrates in hemicellulose (eg, xylose, arabinose) by the addition of a reduction step (via furfuryl alcohol) subsequent to the acid treatment. Levulinic acid has been endorsed as a significant biorefinery building block due to its high yield from six carbon carbohydrates (Bozell and Petersen, 2010). Levulinic acid contains two reactive functional groups that permit a great number of synthetic transformations. 

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