The lignin Problem M O R , than 100 years ago, the major noncellulosic material comprising around 25% of the woody tissue in plants was given the name “lignin.” A great many investigations of lignin have now been completed and much has been learned about where lignin is situated in plant tissue, how it can be extracted, and what its properties are. Chemists have established that lignin is a polymer made up of structural units which are at least largely phenolic propane types and are somewhat different in softwoods and hardwoods. However, the complete chemical structure of lignin has still not been elucidated and many important theoretical and practical problems remain to be solved. Thus the 1956 Lignin Symposium of the AMERICAN CHEMICAL SOCIETY was held at Atlantic City as part of the program of the Cellulose Division. Its objective was to present and discuss results of research obtained since the 1951 Lignin Symposium in New York. Twenty-five research papers were contributed to the 1956 symposium. Research results concerning a wide range of lignin topics were reported : biochemical mechanisms of lignin formation (Freudenberg, Nord, Schubert) ; extraction of lignins with neutral solvents (Bjorkman), with enzymes (Pew), by kraft pulping (Gierer, Alfredsson), and by alkaline hydrogenations (Schuerch) ; isolation of water-soluble lignins (Lyness, Schenker) and of hardwood lignins (Browning, Harris, Hogan, Pearl, Beyer) ; characterizations of lignins by color reactions (Kitaura, Nakamura), by methoxyl group stability (von Wacek), by ultraviolet absorption spectra (Aulin-Erdtman, Goldschmid, Maranville), by thermal plas-
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ticity (Kratzl), by molecular weights (Doughty, Lawson, Gordon, Mason, Felicetta, McCarthy), by solution viscosity (Luner), and by reactions of oxidation (Kratzl, Pearl, Smith, Purves), of alcoholysis (Sarkanen, Schuerch), and in the presence of mineral acids (Adler, Pepper) ; the nature of the structural units of lignins (Adler) ; and the remaining research questions (Erdtman). This topical diversity manifests the great scope of the lignin problem. Symposium papers thought to be of interest to the general readers of INDUSTRIAL AND ENGINEERING CHEMISTRY are published here; the other papers appear elsewhere. The results reported demonstrate that substantial progress is being made toward elucidation of the genesis, extraction, physical properties, and chemical structure and reactions of lignin. They also seem to foreshadow the development of important and profitable industrial silvaproducts and silvachemicals from this interesting phenolic polymer, which can be had in such huge amounts from almost universally available and perpetually renewable natural sources. These products and chemicals can come directly from woods, or from the spent liquors of wood cellulose pulp production-for example, some 1,000,000,000 pounds of lignin sulfonates and 250,000,000 pounds of sugars are now available each year in sulfite spent liquors in the Pacific Northwest alone. The economic utilization of these materials is a continuing challenge to chemists and chemical engineers and their business colleagues throughout the world. JOSEPHL. MCCARTHY Chairman
ERICH ADLER Chalmers University of Technology, Gb’teborg, Sweden
Structural Elements of lignin A detailed knowledge of lignin structures will furnish a solid base for closer understanding of processes involved in pulp manufacture, and open new possibilities in commercial utilization of lignin
E L U C I D A T I O N of the structure of lignin is of twofold importance. Primarily, it may be looked upon as a purely scientific problem, lignin being a natural product synthesized in tremendous amounts in plant tissues. Secondarily, it is of considerable practical importance in giving a closer understanding of the processes involved in pulp manufacture, and opening new possibilities in the commercial utilization of lignin. Although it is still impossible to present a correct structural picture of the lignin macromolecule or macrornolecuIes,
recent views of the structural elements occurring in lignin are summarized here, For the sake of simplicity, the discussion is confined to the lignin of coniferous wood, or “guaiacyl lignin” according to the nomenclature suggested by Nord (25). Because the lignin isolated by BjBrkman (7475) from finely divided wood, in its structural features, is reasonably close to protolignin, examination of the analytical composition of lignin may be based on Bjorkman lignin. Elemental analysis and methoxyl analysis of a Bjorkman spruce lignin prep-
aration which had been extracted with dioxane-water (25 to 1) from wood meal disintegrated in toluene suspension under nitrogen by means of a vibrational ball mill, gave the following formula for a phenylpropane or CQunit :-CQH8.8302. 81(OCHs)n.s,. ’ The ’ox$gen belongs to a variety of functions and groupings. Of the 2.37 atoms of oxygen present per CQ (except in the methoxyl group), 1.0 must be the phenolic oxygen of the guaiacyl nucleus; only 0.29 oxygen atom, however, occurs in the form of free phenolic hydroxyl
VOL. 49, NO. 9
SEPTEMBER 1957
1377
