Dehydration of carbohydrate

The reaction of fructose in alcohols in the presence of Amberlyst 15 at 373 K for 20h gives the laevulic ester and hydroxymethylfurfuryl ether mixtures.  

The dehydration of sorbitol and oligosaccharide can be effected by cation-exchange 45) 

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Concentrated sulphuric acid Concentrated acid Spray Dehydration of carbohydrates... 

Hydroxymethylfurfural (HMF) can be readily obtained from acid-catalysed dehydration of carbohydrates. On the other hand, FFA can be further reacted to produce 2,5-furandiacetic acid and 5-carboxyfuran-2-acetic acid which could form polymers, much like tereftahc acid. Therefore the carbonylation of MF can be regarded as the first step of the green manufacture of polymers on the basis of renewable (carbohydrate) raw materials. 

Concentrated aqueous salt solutions were used for dehydration of carbohydrates catalyzed by RuCh + Ag2S04 ( RUSO4 ) [47]. Such solvents may also help in constracting aqueous-organic biphasic media with good phase separation properties. Selective dehydroxylation of polyols and sugars was achieved in aqueous solutions with the use of anionic rathenium carbonyls, as well [48]. 

Konig and co-workers also reported that Amberlyst 15 can promote the dehydration of carbohydrates to HMF using safe concentrated low melting mixtures consisting of choline chloride (ChCl) and about 50 wt% of carbohydrates. From fructose, glucose, sucrose, and inulin, HMF was produced with 40, 9, 27, and 54%, respectively within 1 h of reaction at a temperature around 100°C. Montmorillonite has also been used as a solid acid catalyst affording HMF with 49, 7, 35, and 7% yield from fructose, glucose, sucrose, and inulin, respectively [97]. 

The formation of furan derivatives in acid-catalyzed dehydrations of carbohydrate substrates is a well known reaction, first reported by Dobereiner186 in 1832. Among the plethora of compounds formed, 2-furaldehyde is the main product obtained from all of the pentoses, whereas 5-(hydroxymethyl)-2-furaldehyde is the major product... 

Possibilities include (a) the hydrolysis of the lignin-carbohydrate bond by small amounts of water present in the wood despite all precautions to make the system anhydrous, (b) the dehydration of carbohydrates under these conditions to produce water and hence hydrolytic conditions, (c) transacetalization of the extracting acetal with carbohydrates to liberate alcohol and create conditions of alcoholysis, and (d) transacetalization of the extracting acetal with free or bound keto groups in the wood to liberate acetone and create conditions of acetonolysis. 

As discussed in detail in Chapter 1 [1], the chemical-catalytic approach to biomass valorization is poised to come to the fore of biorefinery operations due to its advantages over microbial and thermochemical processing of lignocellulosic feedstocks. Below, we consider three mainstream platform chemicals, collectively referred to as furanics, that are derived from the acid-catalyzed dehydration of carbohydrates. The first, 5-(hydroxymethyl)furfural, or HMF 1, is an icon of the biorefinery movement. With derivatives that branch out over multiple product manifolds, HMF is a recognized commercial opportunity for whoever can manage to produce it economically, and approaches towards the realization of this aim will be discussed. 

Brasholz M, von Kanel K, Homung CH, Saubem S, Tsanaktsidis J (2011) Highly efficient dehydration of carbohydrates to 5-(chloromethyl)furfural (CMF), 5-(hydroxymethyl)furfural (HMF) and levulmic acid by biphasic continuous flow processing. Green Chem 13 1114-1117... 

The dehydration of carbohydrates to furfural derivatives has been investigated by Tsanaktsidis et al. [26] (Scheme 8.4). The carbohydrates such as sucrose, o-fructose. 

Scheme 8.4 Dehydration of carbohydrates to 5-(chloromethyl)furfural under biphasic conditions.

The water naturally present in green coffee and water formed by the dehydration of carbohydrates during roasting escapes as steam, along with other volatile products of pyrolytic reactions. 

Figure 3.8. Mechanisms in the dehydration of carbohydrates by strong acid.

The dehydration of carbohydrates also yields furfural, furanone, and other similar compounds. Mild hydrogenation will reduce the alkenes to alkanes and the aldehyde and ketone groups to alcohols. The overall positive effect of this reaction is that the heat of combustion of the products is higher than that of the starting material. The ahphatic and... 

Weingarten R, Tompsett GA, Coimer Jr WC, Huber GW. Design of solid acid catalysts for aqueous-phase dehydration of carbohydrates the role of Lewis and Brpnsted acid sites. J Catal 2011 279 174-82. 

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