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Chemical Products from Lignocellulosics*

Published online by Cambridge University Press:  29 November 2013

Wolfgang G. Glasser
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Lignocellulosics represent that mass of organic matter produced by land-growing plants in the form of trees, shrubs, and agricultural crops. Lignocellulosics are renewable, and they sustain living conditions on our planet by recycling carbon dioxide to oxygen. Lignocellulosics serve the planet as a carbon sink. Chlorophyll is the essential catalyst and sunlight the necessary energy source that drive this carbon dioxide reduction to an organic mass that varies little from the carbon-to-oxygen ratio of carbon monoxide.

There is, however, no distinct “lignocellulose” molecule. Instead, nature has found it necessary to formulate a multiphase material consisting of cellulose, hemicelluloses, and lignin. The composition of this multiphase material can be likened to a fiber-reinforced organic glass where cellulose serves as lightweight fiber, lignin serves as a continuous (glassy) matrix, and hemicelluloses serve as coupling agents. The overall composition suggests that there is 35–45% cellulose, 25–35% hemicelluloses, 20–25% lignin, and various minor constituents.

In chemical terms, cellulose is a linear homopolysaccharide with a high degree of crystallinity. It consists of 1,4-β-linked D-glucopyranose units connected in syndiotactic fashion (on alternating sides of the main chain). Cellulose molecules in nature occur with molecular weights exceeding 2 million. Hemicelluloses, by contrast, are branched heterosaccharides of lesser molecular weight. They rarely exceed 20,000 daltons. Hemicelluloses are either rich in glucomannan chains (softwoods), or they consist primarily of branched xylans (hardwoods and annual crops). The chemical composition of hemicelluloses is extraordinarily similar to cellulose (i.e., polyanhydro-pyranoside), but their morphological structure is vastly different.

Type
Reprocessing Paper and Wood-Based Materials
Information
MRS Bulletin , Volume 19 , Issue 2 , February 1994 , pp. 46 - 48
Copyright
Copyright © Materials Research Society 1994

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Footnotes

*

This article is based, in part, on a paper given at the International Workshop on Environmentally Compatible Materials and Recycling Technology, Tsukuba, Japan, November 15–17, 1993.

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