SULFITE COOKING PROCESS Effect of Pretreatment of Spruce Wood on the Reaction
T
HE chemical reactions F. E. BRAUNS A N D D. S. BROWN droxyl groups with a group not actually taking place beeasily split off, the reaction beThe Institute of Paper Chemistry, Appleton, Wis. tween the so-called lignin of tween sulfurous acid and this wood and sulfurous acid Tn the covered hydroxyl group can no sulfite cooking process are still unknown. Many theories have longer take place. It seems useless to study the reaction bebeen proposed, but all of them lack unquestionable experitween sulfurous acid and lignin, using isolated lignin, as there mental proof. When Bucherer (4) found in 1904 that certain is no doubt that all lignin preparations, regardless of what phenols can be converted into sulfonic acids by bisulfite, he, method may have been used for their isolation, are more or as well as other investigators, used this fact in attempting an less changed from the native lignin. Hilpert (24) even denies explanation of the chemical reactions taking place in the the presence of lignin in vegetable plants and Qonsiders that sulfite cook. Bucherer assumed that a phenolsulfurous acid the isolated lignin is a secondary reaction product. ester is the intermediate product in the reaction of phenols Since it is well known that lignin in wood is readily changed with bisulfite; Fuchs ( I O ) claimed the primary formation by treatment with dilute acids or alkalies, it is evident that of an addition compound of the bisulfite with a carbonyl only a reaction which occurs without any marked change in group resulting from the tautomerization of the phenol. the lignin can be applied. Such a reaction is methylation The latter idea had already been proposed in 1893 by Klason with diazomethane. The concept that diazomethane methyl(i7),who considered lignin sulfonic acids as bisulfite addition ates only acidic or enolic hydroxyl groups holds only in the productr; of a tautomerized phenol. Freudenberg (9) asabsence of water; in the presence of water, carbohydrates sumed that lignin is built u p of guaiacol-glycerol units and such as starch can easily be partially methylated by diazobelieved that ligninsulfonic acid is formed by addition of methane. The effect of diazomethane methylation of spruce sulfurous acid to a double bond; since lignin does not have a wood on the sulfite cooking process was therefore studied. sufficiently large number of such double bonds, he proposed the idea that they are formed during the reaction by splitting Methylation of Spruce with Diazomethane off of water. If this were the case, however, with the enThe methylation of wood by diazomethane is not new. It trance of each sulfonic group, ligninsulfonic acid should have was first carried out by Ungar (ZO),who obtained a methylone less hydroxyl group. Hagglund and Carlsson ( l a ) , on ated spruce wood with a methoxyl content of 10.9 per cent. the other hand, found that in the formation of ligninsulfonic He did not isolate the lignin, and therefore its methoxyl conacid a new hydroxyl group is formed for each sulfonic group tent is not known. Fifteen years later Fuchs and Horn (11) entering the lignin, which led them to conclude that the prepared a methylated wood with 15.8 per cent methoxyl, and sulfurous acid opens an oxygen bridge. After taking these the lignin isolated from it with 42 per cent hydrochloric acid results under consideration and finding that this new hyhad 19.7 per cent methoxyl. droxyl group is phenolic in character, Freudenberg (8) postuWhen spruce wood meal (100-mesh) , previously extracted lated that the sulfonation may occur in two steps, which may with benzene-alcohol and water, is treated with diazomethane or may not be mutually dependent. I n the first stage a ring in an aqueous-free suspension in cyclohexanol, methylation system is opened with formation of a phenolic and of a takes place slowly and to a very small degree. Even this secondary hydroxyl group; in the second stage the latter is slight reaction may be due to the presence of traces of moissubstituted by a sulfonic group, either directly or after ture. When water is present, however, it can be methylated previous splitting off of water. The possibility that in the to a methoxyl content of 17.6 per cent, as contrasted with 4.7 sulfite cooking process the bisulfite reacts with an alcoholic per cent for the original wood. The progress of methylation hydroxyl group according to the equation can be seen from Table I. =C*OH HSOIH +=C.SO,H H20
+
+
was proposed by Holmberg (15) as a result of his investigation of the action of thioglycolic acid on lignin. I n addition to these theories, which hold that a hydroxyl group takes part in the sulfonation of lignin, other theories have been advanced. Klason ( l 7 ) , for example, made the suggestion that an ethylene oxide ring is opened with addition of the sulfurous acid so as to form a sulfonic acid and a hydroxyl group. Freudenberg (8) supports the theory of the opening of an oxygen ring by model experiments with socalled Erdtman’s acid, which is obtained on oxidation of dehydrodiisoeugenol. If the theories mentioned are correct in stating that one or the other of the hydroxyl groups in native lignin participates in the sulfonation, it is evident that by covering these hy-
TABLEI. METHYLATION OF SPRUCEWOOD WITH DIMOMETHANE Methylation Number 0 2 3 5 8 9‘
Days of Methylation 0 9 26 68 145 193
Per Cent Methoxyl Content of Wood 4.67 12.64 15.02 16.81 17.23 17.60
Per Cent Methoxyl Content of Lignin 15.02 18.02 19.30 19,85 20.23 20.22
The lignin isolated from the methylated wood has a methoxy1 content of 20.42 per cent (calculated on the ash-free basis). This is an increase of 5.4 per cent over that of the lignin from the original wood; and this value is the same as that found 779
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for lignin isolated according to Freudenberg's method and methylated with diazomethane (3). The diazomethane-methylated spruce wood no longer gives the characteristic color reaction with phloroglucinol and hydrochloric acid. As a result of this observation, other lignins and lignin derivatives, such as Willstiitter, Freudenberg, and Klason lignins, and methanol-, glycol-, cyclohexanol-, and benzyl alcohol;-lignins ( 8 ) were tested with phloroglucinol and hydrochloric acid before and after methylation with diazomethane. It was found that all except the lignin isolated according to Klason with 72 per cent sulfuric acid give the purple coloration before methylation but that after methylation the color fails to appear. This indicates that the hydroxyl group capable of methylation is responsible for the color reaction. It also indicates that this hydroxyl group is still present in the isolated lignins and lignin derivatives, with the exception of the Klason lignin; this shows that by the action of sulfuric acid a deeper change in the lignin has taken place. This conclusion is supported by the fact that Klason lignin is insoluble in phenol in the presence of hydrochloric acid in contrast t o Willstatter and Freudenberg lignins ( 5 ) .
Methyl8tion of Mildly Hydrolyzed Spruce Wood I n the preparation of lignin according to Freudenberg ( 6 ) , it is necessary to apply a mild hydrolysis previous to the extraction, and the question arises as to what effect such hydrolysis may have. Is the lignin combined with a carbohydrate, making necessary the cleavage of this linkage, or does the hydrolysis remove some carbohydrate and so loosen up the structure of the wood as to make possible the attack on cellulose by cuprammonium hydroxide? I n the former case it might be possible that a phenolic or enolic hydroxyl group is set free which is capable of methylation with diazomethane. Spruce wood was therefore hydrolyzed according to Freudenberg by boiling it with 1 per cent sulfuric acid for 2 hours, or with 7 per cent oxalic acid for 4 hours. A quantitative experiment showed that, in both cases, 18 per cent of the wood went into solution with a corresponding increase in the lignin content of hydrolyzed wood from 28.8 to 35.1 per cent and an increase of the methoxyl content from 4.85 to 5.6 per cent. The methoxyl content of the lignin isolated from the hydrolyzed wood was 15.5 per cent, as compared with 15.1per cent for the lignin of the original wood. A methylation of the hydrolyzed spruce wood with diazomethane was carried out in a moist etherous pension. It was found that the methylation of the lignin of the hydrolyzed wood meal occurs more quickly than that of the original wood meal, as can be seen from Table 11. OF MILDLY HYDROLYZED WOODWITH TABLE11. METHYLATION DIAZOMETHANE
Methylation ' Days of Number Methylation
Per Cent Methoxyl Content of Wood
Per Cent Lignin Content
Per Cent Methoxyl Content of Lignin
The results of two additional methylation experiments of hydrolyzed wood meal are given in Table 111. Tables I1 and I11 show that the methoxyl content of methylated hydrolyzed wood was 3.0 per cent lower than that of the methylated original wood; this may be due to the removal of hemicellulose by the hydrolysis, which may also be the cause of the quicker rate of methylation of the lignin. The methoxyl content of the lignin isolated from the hydrolyzed diazo-
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methane-methylated wood meal is 20.7 per cent, or about the same as that of the diazomethane-methylated original wood. This would appear to demonstrate that no hydroxyl group capable of methylation with diazomethane is liberated. I n one experiment (Table 11) the methoxyl content of the lignin was 21.6 per cent (on ash-free basis), but this value could not be duplicated. Experiments on the factors which influence the methylation of wood are in progress. TABLE111. METHYLATIOW OF MILDLY HYDROLYZED WOOD MEALWITH DIAZOMETHANE Methylation Number 4 5 10
Per Cent Methoxyl Content of Wood 12.22
Per Cent Methoxyl Content of Lignin 20,36
13.4
20,77
...
20.7
Methylation Kumber 3 5
7 10 14 15
Per Cent Methoxyl Content of Wood
...
10.0
10.9 , . .
13.1 13.5
Per Cent Methoxyl Content of Lignin 19.0 19.93 20.2
20.3 20.5 20.75
Effect of Mild Hydrolysis on Hydrolyzed Spruce Wood Methylated with Diazomethane I n order to determine the effect of hydrolysis on the methoxyl content of diazomethane-methylated hydrolyzed wood meal, a sample was again hydrolyzed by boiling it with a 7 per cent solution of oxalic acid for 4 hours. The lignin content after this second hydrolysis was 35.5 per cent, indicating that only a minute amount of carbohydrates was removed. The methoxyl content of the wood was 11.6 per cent, a decrease of 1.3 per cent as compared with that of the methylated hydrolyzed wood ; the methoxyl content of lignin decreased from 21.6 to 19.9 per cent. This shows that the methoxyl groups in the lignin from diazomethane-methylated spruce wood are not completely stable towards hydrolysis with 7 per cent oxalic acid.
Methylation of Mildly Hydrolyzed Spruce Wood (Premethylated with Diazomethane) with Dimethyl Sulfate and Sodium Hydroxide Since there seems to be no difference in the methoxyl content of spruce wood methylated with diazomethane before and after hydrolysis, the diazomethane-methylated hydrolyzed wood meal was further methylated with dimethyl sulfate and sodium hydroxide a t 20' to 30' C. The methylation mixture was kept slightly alkaline. On dilution of the filtrate from the first methylation, a white flocculent precipitate separated out in about 10 per cent yield. When this precipitate was purified by dialysis and by precipitation from its chloroform solution with petroleum ether, it was obtained as a snow-white powder with a methoxyl content of 38.25 per cent. This product seems to be methylated pentosans, for which Heuser and co-workers (15) found 38.55 per cent after two methylations; 38.75 per cent is the calculated value for such a product. After a second methylation, the methoxyl content of the wood meal was 35.5 per cent which did not increase after two more methylations. The lignin determinations with this fully methylated wood according to Willstiitter and the TAPPI standard method did not give concordant results. Studies of this disagreement are in progress.
Effect of Chlorine and Pyridine-Alcohol on Fully Methylated Wood In an earlier investigation ( 1 ) it was shown that after methylation with diazomethane, unbleached pulp is more difficult to bleach, owing to a partial methylation of the lignin left in the pulp. An investigation was therefore made of the behavior of completely methylated spruce wood towards bleaching reagents; by means of the method of Ritter and Kurth
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INDUSTRIAL AND ENGINEERING CHEMISTRY
(I@, an attempt was made a t the same time to isolate the holocellulose, which should be a methylated holocellulose. The experiment failed. After six alternating chlorinations and extractions with pyridine-alcohol, the wood meal still gave a dark colored solution with the solvents. The product was then treated with bleaching liquor and washed, when it had the appearance of a normal untreated wood meal. The methoxyl content was 30.2 per cent, and its lignin content, determined according to WillstBtter's method, was 32.0 per cent. This indicates that it is not possible to isolate a methylated holocellulose, since the lignin cannot be removed without a simultaneous decomposition of the carbohydrates. A similar observation has been made by Suida and Titsch (19) with acetylated wood. In this case also it was impossible to separate the lignin from the cellulose according to the Cross and Bevan method. Effect of Diazomethane Methylation of W o o d o n the Sulfite Cook In order to study the effect of diazomethane methylation of spruce wood on the reaction in the sulfite cooking process, unhydrolyzed diazomethane-methylated spruce wood meal with a methoxyl content of 17.6 per cent, or 20.4 per cent for the lignin, was digested under exactly the same conditions as the unmethylated original wood; a cooking liquor was used which contained 4.5 per cent total and 0.86 per cent combined sulfur dioxide a t 125" C. for 4 hours. Such mild conditions were applied in order to obtain a more uniform lignin sulfonic acid. It was shown by Freudenberg (7) and later by King, Brauns, and Hibbert (16) that under such conditions a ligninsulfonic acid is obtained completely precipitable with quinoline1 or naphthylamine hydrochloride; this behavior indicates that only a-ligninsulfonic acid is present. The original wood gave a pulp with 11 per cent lignin; the methylated wood was apparently unchanged although slightly darker in color. The waste liquor was absolutely colorless. After removal of the sulfites and sulfates by precipitation from the waste liquor with neutral lead acetate, basic lead acetate gave less than 0.1 per cent of a lead salt with a methoxyl content of only 3.3 per cent; the lead salt of ligninsulfonic acid prepared from original spruce wood under the same conditions has 7 to 8 per cent methoxyl. The pulp which was obtained in a yield of about 80 per cent had a lignin content of 35.6 per cent, the same as that of the hydrolyzed wood; this indicates that a removal of carbohydrates by hydrolysis with the sulfite liquor had taken place. The methoxyl content of the lignin isolated from it by means of 72 per cent sulfuric acid was 19.3 per cent. Both pulps were then recooked with one hundred times their weights of cooking liquor for 8 hours. This time the original wood gave a normal pulp with about 3 per cent lignin. The waste liquor of the methylated wood pulp was again absolutely colorless. The pulp had the appearance of an unmethylated, mildly hydrolyzed wood meal. Its lignin content, isolated according to Willstatter, was 33.65 per cent, with a methoxyl content of 19.0 per cent. The pulp had a methoxyl content of 12.0 per cent and a sulfur content of 0.51 per cent. The lignin, isolated by means of 42 per cent hydrochloric acid, had a sulfur content of a t least 0.8 per cent, which is undoubtedly too low, since the lignin, when treated with nitric acid, caught fire and burned with a bright flame. A continuation of these experiments is now in progress.
Conclusions The cooking experiments with diazomethane-methylated wood confirm the assumption that the hydroxyl group, which is methylated by diazomethane, plays the decisive role in the The statement contradicting this by Hibbert [ J . A m . C h e n . Soc., 58, 340 (1936)l has already been disputed by Freudenberg [Be?.. 70, 508 (1937)] and Hhgglund [Suensk P a p p e r s t i d n . , 39 347 (1936)l.
781
sulfite cooking process. It is not surprising that more drastic reactions, such as a treatment of the wood with acids or alkalies, change the lignin by condensation or polymerization in such a way that the reaction with sulfurous acid is hindered or a t least reduced; but it is surprising that such a mild reaction as methylation with diazomethane brings about a change in the lignin molecule which prevents the dissolution of the lignin. This change cannot be very marked, since the methylation occurs in neutral solution a t low temperature. It is evident that, by covering a certain hydroxyl group by a methoxyl group, the dissolution of the lignin is prevented. This does not mean that a reaction of the lignin with sulfurous acid does not take place. The results of the analysis show clearly that the lignin, after methylation with diazomethane, is still capable of binding sulfurous acid, but the amount is less than the amount which is taken up by untreated wood. The sulfur content of the methylated wood after treatment with cooking liquor is 0.51 per cent, about half as high as the sulfur content of the untreated wood meal sulfonated under mild conditions. The results of the sodium bisulfite cook of the diazomethane-methylated wood seem to indicate that the dissolution of the lignin occurs in two stages. I n the first stage, addition of sulfurous acid to the lignin complex forms the so-called solid ligninsulfonic acid; and in the second stage, a rearrangement of the sulfonic group in the lignin complex with the participation of a hydroxyl group takes place, forming soluble ligninsulfonic acid. I n the diazomethane-methylated spruce wood only the fist stage occurs because, by the covering of a specific hydroxyl group in the native lignin, the rearrangement and therefore the dissolution of the lignin is hindered. It is also possible, although not very probable, that the covering of the hydroxyl group may prevent a hydrolysis of a lignin-carbohydrate linkage. That a t least a part of the lignin is combined with carbohydrate in the wood was shown in a recent experiment in which a compound was isolated which is soluble in certain solvents, which does not reduce Fehling solution, and which, on hydrolysis with 72 per cent sulfuric acid, gives a ligninlike precipitate with 10.5 per cent methoxyl content, whereas the solution contains sugars strongly reducing to Fehling solution. The investigation of this lignin-carbohydrate compound is in progress.
Literature Cited J., 103, No. 5, 36 (1936). ( 2 ) Brauns, F. E., unpublished experiments. (3) Brauns, F. E., and Hibbert, H., Can. J. Research, 13B, 78 (1935). (4) Bucherer, H., 2: angew. Chem., 17, 1068 (1904), 19, 1649 (1906), 20, 877 (1907); Ber., 53, 1457 (1920). (5) Buckland, I. K., Brauns. F. E., and Hibbert, H., Can. J. Research, 13B, 61 (1935). (6) Freudenberg, K., Ann., 518, 62 (1935). (7) Ibid., 518, 70 (1935). (8) Freudenberg, K., Ber., 70, 500 (1937). (9) Freudenberg, K., Cellulosechem., 12, 263 (1931). (10) Fuchs, W., Ber., 52, 2281 (1919). (11) Fuchs, W., and Horn, O., Ibid., 62, 1691 (1929). (12) Hagglund, E., and Carlsson, G. E., Biochem. Z . , 257, 467 (1933). (13) Heuser, E., and co-workers, Ber., 55, 2084 (1922). (14) Hilpert, R. S., Ibid., 68, 380 (1935). (15) Holmberg, B., Papir-J., 23, 81, 95 (1935). (16) King, E. G., Brauns, F. E., and Hibbert, H., Can. J. Research, 13B, 88 (1935). (17) Klason, P., T e k . T i d . A f d . K e m i Metallurgi, 23, 49 (1893). (18) Ritter, G. J., and Kurth, E. F., IND. EXG. CHEM.,25, 1250 (1933). (19) Suida, H.. and Titsch, H., Monatsh., 53/54, 687 (1929). (20) Ungar, dissertation, Zurich, 1914. (1) Brauns, F. E., Paper Trade
RECEIVED April 8, 1938. Presented before the Division of Cellulose Chemistry at the 94th Meeting of the American Chemical Society, Rochester, N. Y . , September 6 to 10, 1937. This paper represents a portion of a thesis submitted in partial fulfillment of the requirements of The Institute of Paper Chemistry for the degree of master of science from Lawrence College ,Bppleton W'is.
