Thermal behavior of carbohydrates

Thermal analysis of a hardwood reflected the pyrolysis of its main components including xylan and cellulose. Thermal properties of these components were investigated with model compounds consisting of α‐D‐xylose, substituted phenyl β‐D‐xylopyranosides, β‐Dglucopyranosides and α‐D‐arabino‐hexopyranosides and 1,6‐anhydro‐β‐D‐glucopyranose.

At the lower temperatures these molecules displayed anomerization, loss of water and phase change, which were studied with a variety of physical methods. Calorimeteric and wide line NMR measurements showed that 1,6‐anhydro‐β‐D‐glucopyranose and related anhydro sugars undergo plastic crystalline transition, involving reorientation of the molecules about their centers of gravity and self‐diffusion before melting.

At more elevated temperatures the above compounds showed cleavage of the glycosidic group, polymerization of the sugar moiety, decomposition and evaporation of the pyrolysis products. This involved some heterolytic reactions, which could be catalyzed by acid or alkaline materials.

Kinetics of the pyrolysis process, including the rates and energies of activation were determined by thermal analysis and ESR spectroscopy. These data indicated that cleavage of the glycosidic bond is directly influenced by variation of its electron density and constitutes the rate determining step in pyrolysis of the carbohydrate compounds.