VINCENT A. SULLIVAN, JR., and GEORGE D. PALMER University of Alabama, University, Alabama
L ~ w r s h a s ~ o i n t eout d (1) that approximately 35,000,000 tons of lignin in the form of agricultural and sawmill waste are discarded each year in this country alone. In addition, approximately 2,500,000 tons of by-product lignin from the pulp and paper mills are either burned or run into streams and rivers for disposal. It is surprising that in most elementary textbooks little mention is made of lignin which constitutes a considerable part of most plants. In fact the lignin content of most full-grown trees generally lies between 20 and 30 per cent. There are apparently two good reasons why a description of lignin is not ordinarily given. The chemistry of lignin is extremely obscure and no practical use for it on a large scale baa been found. The early chemists and botanists believed that wood was a definite chemical substance, and it was not until the year 1838 that its heterogeneous character was discovered (2). The manner in which lignin is' joined to the cellulose is still not clear, although experimedal results indicate the existence of a "lignocellulose bond" ( 3 , 4 , 6, 6, 7) which is always destroyed by the present methods of isolating lignin. It is believed now that lignin is the bmding material for the cellulose, or, in other words, it is the material responsible for the "woody" portion of the tree. The word "lignin" is sometimes confusing, and when used, it should be described, since the nature of this substance is determined by many factors, and a:single;formula or definition cannot be given. More properly, a lignin should be identified as that lignin obtained from a certain tree or plantby a particular method. Generally speaking, the original character of a lignin depends upon its source and upon its mode of isolation from the lignocellulose material. The separation of "protolignin," i. e., lignin as it exists in the plant, has been a problem of prime importance to the investigators in this field, since any other form of lignin will yield only indirect evidence as to its nature in the plant. It might be mentioned, incidentally, that Brauns (8) has prepared a "native lignin" which is reported to resemble the protolignin closely. ISOLATION
The methods of isolating lignin may be divided into two general classes. The separation of lignin is effected by dissolving it while leaving the carbohydrate components as an insoluble residue, or by hydrolyzing the carbohydrate material to soluble sugars and leaving the lignin as an insoluble residue. The methods of the first class form the basis for most commercial delignification processes in the manufacture 90:z
of wood pulp for the pulp and paper industry. From the viewpoint of industry, the two important delignification processes of this class are the sulfate process and the sulfite process. Sulfate Process: This process is responsible for the largest part of the paper pulp manufactured a t present, and is especially popular because it can be employed to process nearly any type of wood. However, since the process was particularly developed to remove the oils and resins from coniferous woods, these woods are ordinarily used. The original process consisted of digesting the wood with a caustic soda solution whereby the lignin and hemicelluloses are rendered soluble, and an insoluble and impure cellulose pulp remains. Basically, this is the "soda process' which is still used extensively today, but it is limited to deciduous woods. The sulfate process diiers from the soda process only in the addition of sodium sulfide and sulfur to the cooking liquor. The sodium sulfide compensates for the loss of alkali during the digestion, while the sulfur bleaches the pulp to a l i i t e d extent and also increases the rate of solution of the lignin. Owing to the heterogeneous reactions involved and the fact that both the lignin and the carbohydrates decompose in the7presenceof alkalies a t high temperatJres, the chemical reactions which occur are very complex and little known. Since the sulfate process competes with other p u l p ing methods, the recovery of the spent liquor is an important factor. The black liquor coptains over 95 per cent of the total alkali which was initially charged into the digestor. Most of this alkali is present as sodium carbonate and various sodium organic compounds. Usually the liquor, containing about 20 per cent solids, is evaporated to a concentration between 45 and 70 per cent solids. The hot concentrated liquor is charged into a furnace where the water is evaporated and the organic salts are decomposed. The resulting liquor is then discharged into a smelting furnace where the carbonaceous matter (including the lignin) is burned away and the inorganic substances are melted. From the* residue the sodium salts are recovered for use in the cooking liquor. The Mead Corporation has developed a process (9) whereby a highly reactive alkali liguin is removed from the "soda ~rocess"black liouor bv carbon dioxide. The purified &in is sold com&ercihy under the name of Meadol. The crude black liquor is first evaporated to a qolotion containing 10 to 20 per cent solids, of which 60 per cent is organic matter. Flue gas, containing 17 to 18 per cent of carbon dioxide, is passed through the
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liquor a t 20' to 30°C. and a under a pressure of 5 pounds per square inch. Smce the absorption of the carbon dioxide is not complete in the first tank of liquor, the gas is passed into a second tank where complete stripping of the remaining carbon dioxide from the flue gas occurs. Meadol begins to separate as a fine precipitate a t a pH of 9.0, but treatment N t h carbon dioxide is continued to a pH of 8.5 to 8.6. Smce filtration is extremely difficult a t this point, the precipitated liquor is coagulated by heating it to 90°C. and subsequently cooling it to 40%. The alkali lignin is then filtered on an acidproof rotary vacuum filter, washed with water followed by a dilute sulfuric acid wash which reduces the ash content. It is 6nally dried to a moisture content of 2 to 5 per cent and pulverized. Sulfite Process: This process consists of digestingwood chips a t 130" to 150°C. in an aqueous solution containing calcium and magnesium bisulfites together with an excess of sulfur dioxide. There are two methods of preparing the cooking liquor in wide use, buteachultimately serves the same purpose. The active ingredients of the liquor are the calcium, hydrogen and acid sulfite ions and sulfur dioxide. During the digestion the lignin combines with the sulfur dioxide or acid sulfite and is rendered soluble. The less resistant hemicelluloses are hydrolyzed to water-soluble products, while a portion of the cellulose is decomposed. The mechanism by which the lignin is converted into soluble lignoeulfonates is still unknown, although many theories exist. The fact that lignin is present in solution as the calcium salt of a sulfonic acid has been definitely established (10). Certain analytical evidence indicates that the lignosulfonic acids are actually mixtures (11). The waste liquor resulting from this process was, in the past, run into near-by streams or rivers, but recent legislation on the prevention of stream pollution has practically stopped this procedure. When evaporated to a high concentration, the liquo'r is often used as a fuel, a road binder, and a binder for briquetting material. Other proposed uses include utilization as a fertilizer, cattle food, and tanning material. The MarathonHoward Process (12) utilizes the sulfite waste liquor by treating it with lime. The liquor is separated into (a) calcium sulfite product which is used in preparing fresh cooking acid, (b) a calcium lignosulfonate which is used as a fuel, as a source of vanillin, and as a raw material in the manufacture of lignin plastics, and (c) a material which is suitable for discharge into streams without causing serious pollution. The methods of the second class, typified by the wood saccharification process, are used extensively in Europe and in Japan for the preparation of alcohol by the fermentation of the soluble sugars. Recently a plant was completed in Springfield,Oregon, to hydrolyze wood and ferment the resulting sugars. Obviously the successful operation of this plant will depend largely upon the outlet of the lignin, since there are more economical methods for preparing alcohol in this country. I t has been stated (13) that for such a plant as this to operate
economically the by-product lignin must be sold for more than approximately one cent per pound. It can readily be seen, then, that if the lignin could be sold for only two or three cents per pound, this process would become commercially feasible. Wood Saccharificatwn: This process consists essentially of treating wood chips or sawdust shavings with dilute sulfuric acid a t temperatures between 160' and 19YC under pressures varying from 8 to 10 atmospheres. The cellulosic material is hydrolyzed to soluble sugars, while the lignin remains as a solid residue. This lignin is very pure, has little ash, and briquettes easily. At present it is being used only as a fuel to supply heat for plant operations. The laboratory methods of preparation may be divided into the same two classes as were the commercial delignification processes. However, a preliminary treatment of the lignocellulose material is necessary before attempting to prepare lignin by any of the standard chemical methods. This includes the removal of extractives, such as resins, volatile oils, fats, etc., with organic solvents, preferably with a 1: 2 alcohol-benzene solution. A dilute sodium hydroxide solution may be used for the extraction, although there is danger of losing some of the lignin which is soluble in this reagent. It must be remembered that lignins isolated by any twomethods are not exactly alike owing to the different chemical reactions which occur during the separation. The reagents which are commonly used in the laboratory to remove lignin, as a soluble product, from the lignocellulose are aqueous and alcoholic alkali solutions, alcohols, phenols, and organic acids. Those which are used to separate the lignin as, aninsoluble residue are concentrated sulfuric acid, fuminghydrochloric acid, anhydrous hydrofluoric acid, and cuprammonium solution. CHEMICAL STRUCTURE
I t was mentioned previously th&.the constitution of ligniu varies with its source and mode of isolation. However, certain constituent groups have been found present in almost every .lignin; and it may be rightfully assumed that these groups are definitely a part of the lignin molecule. Freudenberg and his associates (14) have concluded that one or more methoxyl groups are attached to an aromatic nucleus. They discovered that the rate of removal of this group from lignin and vanillin by hydriodic acid was approximately the same. On the other hand, from an aliphatic compound, such as 3-methylglucose, 96 per cent of the total methoxyl was removed in 20 minutes. With lignin and vanillin a much longer time was required. Rather conclusive evidence for the presence of hydroxyl groups is given by the alkylation and acetylation of lignin (15, 16). Complete methylation of the hydroxyl groups can be effected with dimethyl sulfate and alkali. Since diazomethane methylates only a part of the total bydroxyl content, it may be concluded that some of these groups are phenolic or enolic.
JOURNAL OF CHEMICAL EDUCATION The presence of the carbonyl group in lignin prepara- dently inhibits the crystallization of lead sulfate on the tions has not been definitely established although there lead plate during discharge, thus increasing the disis reason to believe that it does ekst. Powell and Whib charge capacity. Sulfite waste liquor is often used in taker (17) reported that their lignin reduced Fehling's its natnral form as an adhesive or it may be compounded solution and concluded that an aldehydic group was to serve the same purpose. The partially concentrated present. Brauns (8) treated native lignin with meth- liquor is useful as a dust binder on unpaved roads. anol-hydrochloric acid and found that the lignin com- Other minor uses of sulfite waste liquor depend upon its Vied with two methoxyl groups which were removed colloidal nature in the preparation of aqueous emulsions when the product was treated with 72 per cent sulfyric for insecticides, asphalt for road treatment, and deteracid. He considers these results as evidence for the pres- gents for the textile industry. ence of a carbonyl group. This same investigator has It is less likely that lignin will he a liability rather also prepared a hydrazone containing 13.3 per cent than an asset, now that the versatile hands of a few methoxyl from native lignin which further supports chemists and wood technologists have revealed some of its secrets and potential power as an industrial raw mathe theory that lignin contains a carhonyl group. The aromatic nature of lignin has been established terial. The tremendous annual waste of lignin is one of beyond much doubt owing chiefly to the fact that when the greatest challenges to our scientists and industriallignin is decomposed by means of dry distillation, oxida- ists. There should be much more applied and fundation, hydrogenation, etc., many different types of aro- mental research on such a cheap and abundant material matic compounds are produced. In addition, there are -a prodigy of the plant kingdom. formed many simple aliphatic compounds, and it is b e lieved that these must be a part of the end groups and LITERATURE CITED oxygen residues of the aromatic nuclei. (1) LEWIS,H. F.,C h a . & Eng. News, 23, 1074 (1945). Freudenberg (18) has postulated that if the oxygen (2) PATEN.A,. Comet. rend.. 7. 1052. 1125 (1838): 8. 61. 169 and oxygen residues in lignin were removed through ~-.-~ -~,,, - ~,~ hydrogenation, phenylpropane would he obtained, and (3) CROSS,C. F., E. J. BEVANAND C. BEADLE,"CeUulose. that this is the basic unit in the lignin molecule. He asAn Outline of the Chemistry of the Structural Elements of Plants, with Reference to Their Natural History and sumes that these phenylpropane units are joined through Industrial Uses." Longmans, Green and Co., London, ether linkages between a phenolic hydroxyl and an al1903, p. 89. cohol group in the side chain. Hibbert obtained prod(4) HAGGLUND, E., AND C. B. BJORKMAN, Biochem. Z., 147, 74 ucts of the phenylpropane type and has thus given (1924). more basis for the views of Freudenberg. The most (5) PHILLIPS, M., J. Am. Chem. Soc., 50,1986 (1928). AND R. L. MITCHELL, (6) HARRIS,E. E., E. C. SHERRARD, interesting substances produced by Hibbert to subibid., 56, 889 (1934). stantiate Fruedenberg's theory are: vanillin; P-hyNORMAN, A. G.: AND J. G. SHR~KHANDE, Biochem. J . , 29, droxyl-a-(4-hydroxyl-3-methoxylpheny1)-cqqmmne; (7) 2259 (1935). a-hyrdoxyl-a-(4hydroxyl-3-methoxylphenyl)-~-propa- (8) B R A U NF. ~ ,E., J. Bm. Chem. Soe., 61, 2120 (1939); Paper Trade J., 111, No. 14,33 (1940). none; a-(4-hydroxyl-3-methoxyl-pheny1)-a, P-proThe Chemwgic Digest, 2, No. 11, 97 (1943). panedione; and a-(4-hydroxyl-3-methoxylpheny1)-P- (9) BROOKBANK, (10) LINDGEY. J. B.. AND B. TOLLENS. Ann.. 267. 341 118921. propanone (19). ~
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, G. C., Chem. an; $el. ~ ; g . , 4 6 ,618 (1939). (12) HOWARD, P. H., "Unit Processes in Organic Synthesis," (13) GROGGINS, 3rd ed., McGraw-Hill Book Co., Inc., New York and London, 1947, p. 695. I
