1430
ELWIN E. HARRIS AND BILL G. LANG
HYDROLYSIS OF WOOD CELLULOSE hV1) DECOMPOSITION OF SUGAR I N DILUTE PHOSPHORIC ACID' ELWIN E. HARRIS
AKD
Forest Products Labnmlory,2 Forest Service,
BILL G. LANG
u.8.Department of Agriculture
Received M a y 8 , 1947 INTRODUCTION
The rate of hydrolysis of cellulose and the rate of the decomposition of the hydrolytic products are of great importance in a study of the saccharification of wood. The ideal conditions would be those in.which cellulose would be hydrolyzed at a satisfactory rate with minimum decomposition of the hydrolytic products. Economic considerations limit the conditions to the use of dilute acid for the hydrolysis. In earlier investigations, Liiers (8) showed that the rate of hydrolysis of a hydrocellulose in dilute sulfuric acid was approximately equal to the rate of the decomposition of the hydrolytic products and that both reactions were unimolecular reactions. Neuman (9) had previously carried on similar hydrolytic work, but he contended that t.he rate of decomposit,ion of the hydrolytic product,s a t temperatures above 175°C. was so great that it was useless to study hydrolysis above that temperature. More recent work at the Forest Products Laboratory (12, 14) on the hydrolysis of wood in dilute sulfuric acid indicated that the rate of hydrolysis of wood cellulose increased more rapidly than the rate of decomposition of the hydrolytic products when either the t,emperature or the acid concentration was increased. The complexity of the problem of hydrolysis when dealing with wood is increased because wood contains a mixture of carbohydrate materials called hemicellulose, in addition to the stable alpha cellulose. The hemicellulose hydrolyzes very rapidly, its rate of hydrolysis being dependent upon easily hydrolyzable linkages and upon molecules that are relatively smaller in' size. When hydrolysis had proceeded sufficient,ly to hydrolyze the hemicellulose in the Laboratory study, the cellulosic material remaining had lost most of its fibrous structure. Under conditions drastic enough to cause the hydrolysis of the resistant cellulose t,o occur in a few hours' time, the hydrolysis of the hemicellulose and the loss of fibrous properties occurred in periods of time too short for the usual methods of measurement. Several investigators, in working with cellulosic fibers, have assumed that the difference in rat,e of reactivity bekeen the highly reactive and the more resistant carbohydrate material is due to a greater reactivity of the amorphous portion of the molecule. Hess and Trogus (7), Xickerson (lo), and Conrad and Scroggie (2) employed hydrolysis and oxidation. Goldfinger, Mark, and Siggia (6) oxidized cellulose wit,h periodic acid, and Xzssf, Haas, and Purves (1) subjected 1 Presented before the Division of C ~ l l u l o sChrniistry ~ at the 111th Meeting of the American Cheniical Societ.y, w h i r h was held xt Atlantic City, Kew Jersey, April 11-15. 19-27, 2 Maint.ainet1 at hfxdisrm 5 . \\'isconsin, i n coiiprrstion w i t h the University of Wisconsin.
HYDROLYSIS OF CELLULOSE BY PHOSPHORIC ACID
1431
cellulose t o thallation in efforts to throw light upon the relationship between amorphous and crystalline cellulose. They did not consider the possibility of the presence of hemicellulose or of modified cellulose that would hydrolyze a t a more rapid rate. Pacsu and coworkers (11) explained the differences in the reactivity of cellulose as being due to hemiacetal linkages and hydrogen bonding in the reactive portion, and to hydrolysis of true 1,4-linkages in the resistant portion of cellulose fibers. The latter factor appears to account for the reactions that occur when wood is hydrolyzed. A difference in the energies of activation of the hydrolysis of cellulose in concentrated acids and in dilute acids may be accounted for by assuming a difference in the type of reactive groups. In concentrated acid the energy of activation has been shown to be between 27,800 and 29,800 cal. (3, 4, 16), while the value in dilute sulfuric acid has been found to be between 42,000 and 44,000 cal. (12, 14). Concentrated acid with its higher reactivity toward hydrogen bonds and also acetal groupings puts cellulose into solution, and therefore the energy due to these reactions changes before the measurements are made. With dilute acid both solvation and hydrolysis occur and give higher values. The reaction occurring in the presence of dilute acid may'be expressed as follolvs: Wood
A)-+ Insoluble wood cellulose
(3) Insoluble hydrocellulose -+ lignin
+
+ simple sugars, etc. + lignin
Soluble polysaccharides /
+ lignin
/
+
Glucose lignin Reactions 1, 2, and 4 are rapid in dilute solution and incapable of measurement under the conditions of the experiment. Reaction 3 is slow and is measured by the rate of hydrolysis. The energy of activation, however, includes both 3 and 4. With concentrated acid the reaction may be expressed as follow: Wood
--%Soluble hydrocellulose ester with strong acid (lignin
+ simple sugars, etc.)
+
+
Soluble polysaccharides lignin --+ (3) Glucose lignin Reaction 1 occurs very rapidly and before the measurements are made. Reactions 2 and 3 take place slowly a t low temperatures. Stamm and Cohen (17)
1432
ELWIN E. HARRIS AND BILL 0. LANQ
found that both of these reactions occur very slowly a t room temperature in 85 per 'cent phosphoric acid. Wolfrom and coworkers (18) found that hydrolysis with strong hydrochloric acid in the presence of ethyl mercaptan did not produce the glucose mercaptan derivative until the reaction had proceeded for long periods of time. Similar results were found by Gladding and Purves (5) in their work with hydroxylamine and hydrogen chloride on cellulose. The rate of decomposition of the glucose controls the simple sugars that will be available after periods of hydrolysis. Decomposition is also catalyzed by the presence of acid. The rate of decomposition vi11 be dependent upon the hydrogen-ion concentration, but will not necessarily be the same with all types of acid because the nature of the acid will also influence the type of reaction involved. The tendency to form complex derivatives with the acid indreases with the complexity of the acid. The decomposition of the sugars may be expressed as follows: Simple sugar
I
t -
acid (1)
Complex of acid with simple sugar
Soluble decomposition products l(4)
Insoluble decomposition products
It may be assumed that the loss in reducing power is represented by reaction 2, which is the one measured by the conditions of the experiment, although in more concentrated solutions the reaction may also proceed by reactions 1 and 3,which may result in a lower rate of decomposition in higher concentrations of complex acids. MATERIAL USED FOR EXPERIMENTAL WORK
The present work describes the hydrolysis of wood cellulose and the decomposition of simple sugars in the presence of phosphoric acid in concentrations from 0.2 to 3.2 per.cent a t temperatures from 18OOC. to 195°C. A measurement of the pH of the phosphoric acid solutions with a pH meter gave the following values a t 20°C.: 3.2 per cent solution, pH 1.4; 1.6 per cent, pH 1.57; 0.8 per cent, pH 1.73; 0.4 per cent, pH 1.91; and 0.2 per cent, pH 2.10. The sulfuric acid solutions used in previous work when tested in a similar manner gave for 1.6 per cent solution, pH 0.5; for 0.8 per cent, pH 0.8; and for 0.4 per cent, pH 1.12. On the basis of hydrogen-ion concentration, the 3.2 per cent phosphoric acid should have less reactivity than the 0.4 per cent sulfuric acid. .The wood used for the hydrolysis tests was air-dried Douglas fir that had been
HYDROLYSIS O F CELLULOSE BY PHOSPHORIC ACID
1433
ground until it remained on an 80-mesh screen and passed a 40-mejh screen. The samples were kept sealed, except when sampling, and were used in air-dried form to eliminate the possibility of decomposition during drying. The cotton used for the tests was a sample of purified linters that had been obtained commercially. AYALYSIS
In order t o eliminate from results errors that might arise from the use of different analytical procedures, all measurements were designed so that the same method of analysis would be used for all results. In tests of the decomposition of sugar, pure glucose was used for the experiments, and its decomposition was measured by determining the loss in reducing power as determined by the Schaffer and Somogyi (14, 15) method for sugar analyses. The rate of hydrolysis was determined by converting to sugar the cellulosic material in the residue remaining after the hydrolysis (13) and measuring the sugar by the Schaffer and Somogyi method. In this manner it was possible t o avoid corrections for decomposition products. EXPERIMENTAL
Samples for experiments on the hydrolysis of wood and on the decomposition of sugars were sealed in glass tubes and heated by direct steam in a rotating digester. About 1; min. was required to reach the desired temperature, and about the same length of time was allowed for it to drop back so that no appreciable amount of hydrolysis or decomposition would occur. Pressure was maintained a t =kl lb. per square inch with the pressure under manual control. The digester was pre!ieated for a t least 1 hr. before the test so as to minimize errors due to failure t o heat the equipment in a short time. All samples were run in duplicate or triplicate. Samples usually checked within 0.5 per cent.
Decompositzon OJ sugars Since glucose is the principal sugar in the Douglas-fir hydrolyzate, it was chosen for determination of the rate of decomposition. Ten-milliliter samples of 5 g. of glucose in 100 ml. of solutions containing 0.2, 0.4, 0.8, 1.6, and 3.2 per cent phosphoric acid were heated at three temperatures for various periods of time. The results of these experiments are shown in figure 1. The straight lines indicate that at the temperatures and acid concentrations used the reactions were of the first order. Table 1 gives the calculated first-order reaction constant and the half-life of glucose, both based on the loss of reducing power. When the logarithms of the reaction rates were plotted as a function of pH, the series of straight lines shown in figure 2 was obtained. The reaction rate increased by a constant multiple when the pH was decreased by a constant multiple. The values for the slope M of the lines in figure 2 for a constant temperature were obtained by the following equation :
1434
ELWIN E. HARRIS A S D BILL G . LANG
FIG.1. Decomposition of glucose in dilute phosphoric acid at various tempertltures TBBLE 1 The decomposition o j glucose in 0.8,0.4, 0.8, 1.6, and 9 2 per cent phosphqric acid at
carious temperalures k
-
‘C
.
PKOSPKORlC ACID CONCEXTRATION
IIST-ORDER REACTION COB STANT BASED ON LOSS OF R E D U C E S POWER
Am-LIFE BASED OK MSS 01 REDUCRTQ POWER
per cen1
minulcs-1
minutes
179.1
0.2 0.4 0.8 1.6 3.2
0. MI29 0,0036 0.0052 0.00775 0.0120
238 192 133 89 0 57 5
188.9
0.2 0.4 0.8 1.6 3.2
0,0064
0.0110 0.0169 0.0254
108 90.5 62 5 41 0 27 2
0.2 0.4 0.8 1.6 3.2
0.0092 0.0118 0.0171 0.0247 0.0380
74.5 58.5 40.5 28.0 19.0
194.5
0.0076
1435
HYDROLYSIS O F CELLCLOSE 13Y PHOSPHORIC ACID
3.P
PHOSPKlplC ACID CONCENTRATION (PERCEN T ) 1.6 0.8 0.4
?a o m
2 P
I a 0100 $ a m $0 oodo
E.
g a m
go.ms-2 i:
&oiwJ
6 gam10
$ a m EO.oooS
PH
I:] G . 2. Relation of first-order wartion coiistnnt k t o 1111 i n the decoiiil:osition of glucose 111 the presence of phosphoric acid :ti wrioiis teinperatures. TEMPERATUREP C )
-.'
E
P
0.loo 0.080
0.c80 O
M
t
0.030 P
;
0020
B 2 Y 2
0.010
o m 0.006
B8 o E
m
O B 3
O m
O.oOPx,
O.a)2x) O.WZ15 RECIPRWAL OF ABSOLUTE TEMPERATURE f $ 1
O.W.?3
F l G . 3 . Relation of first-order reaction constant k t o the temperature in the dec.omposit ion of ' glucose with phcsphoric acid of various strengths.
where ill had an average value bf 1.007 over the temperature range 180-195°C. This corresponds to, a 20 per cent increase in the rate of decoqpposition for 8 decrease of 0.1 in pH.
1436
ELWIN E. HARRIS AND BILL G. LANG
When the logarithms of the reaction constants in table 1 were plotted against the reciprocal of the absolute temperature, a series of straight lines was obtained, as shown in figure 3. The energy of activation was obtained by multiplying the slope of the lines in figure 3 by -4.56. The average of the values for the energy of activation for the various acid concentrations was approximately 32,000 cal. At temperatures of 180-195°C. the rate of decomposition increased 114 per cent for an increase of 10°C.
E8 t
8
P
&
TlU5 lMlhWrESl
FIG.4 Hydrolysis of Douglas fir in dilute phosphoric acid a t various temperatures
Hydrolysis of Douglas-jir wood Douglas-fir sawdust, 40 t o 80 mesh, was sealed in tubes with solutions of 0.2, 0.4, 0.8, 1.6, or 3.2 per cent phosphoric acid, using a liquid-solid ratio of 10 to 1 a t 179.1°, 188.9", and 195.7"C. for various lengths of time. Figure 4 shows a plot of the logarithms of the residual potential sugar as a function of time. Table 2 gives the calculated values for the first-order reaction constants k and for the half-life of the hydrolysis of the resistant cellulose in Douglas-fir mood, both based upon loss of reducing power. When the logarithms of the first-order reaction constants were plotted as a function of the pH of the solutions used for hydrolysis, the series of lines shown in figure 5 was obtained. By using the equation
1437
HYDROLYSIS OF CELLULOSE BY PHOSPHORIC ACID
TABLE 2 T h e hydrolysis of DouglaS f i r i n 0.9, 0 . 4 , 0.8, 1.6, and 3.9 per cent phosphoric acid at carious temperatures k IZMPERAlVRE
PEOSPEOPIC ACID CONCENIPATION
IPST-ORDER REACTION COh PTANT BASED ON LOSS OF PEDUCING POWER
ALP-LIFE BASED ON LOSS OF REDUCING POWEP
"C.
pE? C W l l
minu1rr-1
minuter
179.1
0.2 0.4 0.8 1.6 3.2
0.00047 O.MM97 0.00161 0.00291 0.00474
1470 712 428 237 146
188.9
0.2 0.4 0.8 1.6 3.2
0.00160 0.00256 0.00395 0.00670 0.0117
432 268 175 103 59
195.7
0.2 0.4 0.8 1.6 3.2
0,0036 0,0062 0.0088 0.0142 0.0250
198 115 81 50 28
PW-IL'
I.
20.003
ACID CCNCENTRATION (PERCENT)
\
E 0.002
FIG.5. Relation of first-order reaction constant k to pH in the hydrolysis of Douglas 6 r with phosphoric acid at various temperatures.
1438
E L W S E. H.'.RRIS
AND BILL G. L.iSG
TC-IQATUR€
I*Cl
FIG.6. Relation of first-order reaction constant k to tlic temperature in of Douglas fir \Yith pliusphorir acid of varicus slrrnytbs.
?hi.
hytlrt>lysis
s
TIME (MINUTES)
FIG 7 . A comparison betneen the hydrolysis of cotton cellulose in dilute phosphoric acid and in sulfuric acid at IQO'C.
HTDROLTSIS O F CELLULOSE €31- PHOSPHORIC .ICID
1439
!og x.2 - log x.1 = 11.1 pHi - pHz the slope of the lines was obtained. The average values for the elope for the t,hree lines is about 1.43. This corresponds to an increase of 40 per cent'in the rate of hydrolysis for each drop of 0.1 in the pH of the solution. Plotting the logarithm of the first-order reaction constants, - k , against the reciprocal of the absolute temperature, 1/T, gave the curves shown in figure 0. From the slope of these curves an average value of 40,303 cal. \vas obtained for the energy of activation of the reaction for the various concentrations of dilute phosphoric acid. For each 10°C. rise in temperature in t,he range of 180-195"C., the rate of reaction was increased 158 per cent. The rate of hydrolysis of a purified cotton cellulose was determined at 190°C. with 0.2,0.4,0.8, 16, and 3.2 per cent solutions of phosphoric acid. The values for the residual sugars at various intervals of time are shown in figure 7. The value for resistant cellulose extrapolates to 94 per cent. The half-life of the resistant cellulose a t 190°C. for 3.2 per cent acid was 110 min.; for l.G per cent acid, 182 min.; for 0.8 per cent acid, 223 min.; and for 0.4 per cent acid, 288 min. DISCUSSION
The decomposition of glucose in dilute phosphoric acid has approximately the same energy t f activation, 32,000 cal., as that found previously for its decomposition in dilute sulfuric acid, 32,700 cal., for a similar temperature range, a fact that indicates n similar relationship of the acidity of the solution to the rate of decomposition. The energy of activation for the hydrolysis of Douglas-fir Tvood cellulose is slightly lower, 40,300 cal., in dilute phosphoric acid than in sulfuric acid, in which it was found to be 43,900 cal. This difference would indicate that the factors controlling the rate are not the same in the two cases. The value for phosphoric acid is intermediate between the value for dilute sulfuric acid and dhat for concentrated acid (27,800 to 29,800 cal.) found by Freudenberg and others. It. is generally assumed that a reaction occurs between the concentrated acid and the carbohydrate, and it is possible that there was some tendency for similar reactions here. A comparison of the half-life of the Douglas-fir cellulose with the decomposition of glucose in dilute sulfuric acid and in dilute phosphoric acid as a function of the pH of the solutions, shown in figure 8, thus reveals that the disappearance of the cellulose and the decomposition of the glucose are controlled by the pH of the solution. It also reveals that an increase in acidity or decrease in pH makes t,he relationship of hydrolysis to decomposition more favorable. Concentrations of phosphoric acid from 0.2 to 3.2 per cent did not provide conditions where the rate of hydrolysis equaled the rate of decomposition. Dilute sulfuric acid in concentrations from 0.4 to 1.6 per cent did produce conditions where the rate of hydrolysis mas equal to the rate of decomposition a t temperatures from 18OOC. to 190OC.
1440
ELWIN E. HARRIS AND BILL G. LANG
FIG, 8. Relationship of the half-life of cellulose and glucose to the pH of solutions of sulfuric and phosphoric acids a t various temperatures. CONCLUSIOKS
It may be concluded from the results of this study that within the temperature range of 180-195"C., phosphoric acid in concentrations of 0.2 to 3.2 per cent is not as suitable for hydrolysis as sulfuric acid. The rate of hydrolysis of cellulose and of the decomposition of glucose is a function of the pH and not of the acid concentration. Yields of sugars in wood hydrolysis should be higher with the use of higher concentrations of acids. Equal yields may be obtained a t lower temperatures with higher concentrations of acid. Provided means can be obtained to remove the sugars as snon as theyare produced in wood saccharification, higher yields of sugar should result from the use of higher temperatures for hydrolysis. REFEREBCES (1) AZSAF, A. G., HAAS,R . H., AND PVRVEB, C. B: J. Am. Chem. Soc. 66,59, 66 (1914). (2) CONRAD, AND SCROGGIE, A . G.: Ind. Eng. Chem. 37, 592 (1945). (3) EISENHUT,O., AND SCHWARTZ, E . : Die Chemie 66, 380 (1942). (4) FREUDENBERG, K., AND BLohiavIaT, G . : Ber. 68B, 2070 (1935). (5) GLADDING, E. K., AXD PURVES,C. B.: Paper Trade J. 116, 261 (1943).
c. c.,
SOLUBILITY OF THORIUM KITRATE TETRAHY DRATE
1441
GOLDFINGER, G., M A R K , H . , AND S I G G I A , S.: Ind. Eng. Chem. 36, 1083 (1943). H E S S , K., A N D TROGUS, C.: Z.physik. Chem. BlS, 157 (1931); Kolloid-2. 68, 168 (1934). LUERS, H . : Z. angew. Chem. 43, 455 (1930); 46, 399 (1932). NEUMAN, J . : Dissertation, Polytechnic Institute, Dresden, 1910. KICKERSON, R . F . : Ind. Eng. Chem., Anal. Ed. 13, 423 (1941); Ind. Eng. Chem. 33, 1022 (1941); 34, 85, 1480 (1942). (11) PACSU, E . , AND COWORKERS: Textile Research J. 16, 243, 318, 490, 5M (1946). (12) SAEYAB, J. F . : Ind. Eng. Chem. 37, 43 (1945). (13) SAEMAN, J. F., BUBL,J. L . , . ~ N DH A R R I S , E. E . : Ind. Eng. Chem., Anal. Ed. 17, 35
(6) (7) (8) (9) (10)
(1945). (14) SAEMAN, J. F . , H A R R I S , E. E . , A N D KLIBE,A. A , : Ind. Eng. Chem., Anal. Ed. 17, 95 (1945). (15) SCHAFFER, P. A,, ABD SOMOGYI, 9.: J. B i d Chem. 100, 695 (1923). (16) SCHULTZ, G . V., A X D LOHMAN, H . J . : J. prakt. Chem. 167, 238 (1941). (17) STAYM, A. J., AND COHEN,W. E.: J. Phys. Chem. 42, 921 (1938). (18) WOLFROM, M. L., AND COWORKERS: J. Ani. Chem. Soc. 69, 282 (1937); 60, 1026, 3009 (1938); 61, 1072 (1939).
T H E SOLUBILITY OF THORIUM XITRATE TETRAHYDRATE I N ORGANIC SOLVESTS AT 25OC.l CHARLES C . TEMPLETON
AJD
NORRIS F. HALL
Depaitment of Chemistry, University of Wisconsin, Madison, Wisconsin Received July 14, 1947
Preliminary t o research into the liquid-liquid extraction of thorium salts from aqueous solutions, an investigation has been made of the solubility of thorium nitrate tetrahydrate in a wide range of organic solvents. No such comprehensive study has been previously reported. Misciattelli (4) made a complete study of the system thorium nitrateAiethy1 ether-water. Wells ( 5 ) measured the ether solubilities of the nitrates of thorium, zirconium, and seven of the rare earths t o determine if they interfered with a method of Hillebrand (1) for dissolving uranyl nitrate in ether t o separate from it the last traces of the rare earths. Imrie (2) extracted thorium nitrate from aqueous solution with ether, and Misciattelli (3) used ether to extract uranyl nitrate from an aqueous nitric acid solution also containing thorium. The solubility of the tetrahydrate, rather than that of the anhydrous salt, was investigated because it was desired t o use the data t o make qualitative predictions concerning extraction from aqueous solution. The anhydrous salt was prepared in Misciattelli's work, but it was necessary t o use nitrogen pentoxide. 1 This paper is based upon the thesis submitted by C. C. Templeton to the Graduate Faculty of the University of Wisconsin in partial fulfillment of the requirements for the degree of Master of Science, April, 1947. Presented before the Meeting-in-Miniature of the Wisconsin Section of the American Chemical Society, April 26, 1947.
