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CARBOHYDRATES AS RENEWABLE RAW MATERIALS: A MAJOR CHALLENGE OF GREEN CHEMISTRY

Frieder W. Lichtenthaler

Institute of Organic Chemistry, Technische Universität Darmstadt, Darmstadt, Germany

A raw material as feedstock should be renewable rather than depleting wherever technically and economically practicable.1

INTRODUCTION

Coal, oil, and natural gas, the fossil resources built up over eons, are not only our main energy suppliers but they are also raw materials for a large variety of man-made products ranging from gasoline and diesel oil to bulk, intermediate, and fine chemicals. However, as our fossil raw materials are irrevocably decreasing and as the pressure on our environment is building up, the progressive changeover of chemical industry to renewable feedstocks for their raw materials emerges as an inevitable necessity,2-4 that is, it will have to increasingly proceed to the raw materials basis that prevailed before natural gas and oil outpaced all other sources.

The present overreliance of the chemical industry on fossil raw materials has its foreseeable limits, as these materials are depleting and are irreplaceable. The basic question today is not "When will affordable fossil fuels be exhausted?"—fossil

Methods and Reagents for Green Chemistry: An Introduction, Edited by Pietro Tundo, Alvise Perosa, and Fulvio Zecchini

Copyright © 2007 John Wiley & Sons, Inc.

oil will be around for a long time, even if it has to be isolated eventually from oliferous rocks or shale—but "When will the end of cheap oil be?" or, stated more appropriately, "When will fossil raw materials have become so expensive that biofeedstocks are an economically competitive alternative?" Experts realistically prognosticate this for 2040.5

The transition to a more biobased production system is hampered by a variety of obstacles: Fossil raw materials are not only more economic at present, but the process technology for their conversion into organic chemicals is exceedingly well developed and basically different from that required for transforming bio-based raw materials into products with industrial application profiles. This situation originates from the inherently different chemical structures of the two types of raw materials. Compared to coal, oil, and natural gas, terrestrial biomass is considerably more complex, constituting a multifaceted array of low and high-molecular-weight products: sugars, hydroxy and amino acids, lipids, and biopolymers such as cellulose, hemicelluloses, chitin, starch, lignin, and proteins. By far the most important class of organic compounds in terms of volume produced are carbohydrates, as they represent roughly 75% of the annually renewable biomass of about 180 billion tons (Figure 2.1). Of these, only a minor fraction (ca. 4%) is used by man, the rest decays and recycles along natural pathways.

Figure 2.1 Renewable biomass (180 billion tons/year).

Thus, carbohydrates, a single class of natural products—aside from their traditional uses for food, lumber, paper, and heat—are the major biofeedstocks from which to develop the industrially and economically viable organic chemicals that are to replace those derived from petrochemical sources.

The bulk of the annually renewable carbohydrate biomass is polysaccharides, yet their nonfood utilization is confined to the textile, paper, and coating industries, either as such or in the form of simple esters and ethers. Organic commodity chemicals, however, are usually of low molecular weight, so they are more expediently obtained from low-molecular-weight carbohydrates than from poly-saccharides. Accordingly, the constituent repeating units of these polysacchar-ides—glucose (cellulose, starch), fructose (inulin), xylose (hemicelluloses), or disaccharide versions thereof, most notably sucrose—are the actual carbohydrate raw materials for organic chemicals with tailor-made industrial applications: they are inexpensive, ton-scale accessible, and provide a resulting chemistry that is better worked out and more variable than that of their polymers.

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