Chemical Products from Lignocellulosics

Abstract

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.