Dehydration of Polyols

One classical polyol dehydration reaction is the transformation of glycerol to acrolein in the presence of fused, powdered KHSO4 [94]. Another important example is the dehydration of substituted 1,2,5-triols (8) to dihydropyran derivatives 

Sorbitan esters can be produced by transesterification in the presence of basic catalysts [104], This process does not, however, give satisfactory results-dehydra-tion of sorbitol is incomplete and, therefore, sorbitol esters can be the major products [105], Nowadays the acid-catalyzed direct esterification is the major route practiced in industry. Under these conditions both intramolecular dehydration and esterification occur to satisfactory extents [106]. When fatty acids in large excess are reacted for longer times sorbitan triesters are formed these can be used as water-in-oil emulsifiers. 

A wide range of solid catalysts can be used in the dehydration of monohydric and polyhydric alcohols. The materials active in these processes are mostly acidic in nature. Despite competing reactions various valuable products (alkenes, ethers, carbonyl compounds, dienes and cyclic ethers) can be synthesized selectively over carefully selected catalysts under appropriate reaction conditions. 

Meerwein in Methoden der Organischen Chemie (Houben-Weyl), Vol. 6/3, Thieme, Stuttgart, 1965, pp. 11-19. 

Hooz in The Chemistry of Functional Groups. The Chemistry of the Ether Linkage (Ed. S. Patai), Wiley, London, 1967, pp. 457-460. 

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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... 

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