344
ELIVIN E. H A R R I S ;1ND ALBERT A . K L I N E
HYDROLYSIS O F WOOD CELLULOSE W T H HYDROCHLORIC ACID AND SULFUR DIOXIDE AND T H E DECOMPOSITION OF ITS HYDROLYTIC PRODUCTS E L W I S E. HARRIS Forest Proditcis
AND
ALBERT A . KLIFE
Forest Set vice, U . S . Dcpaihncnt of AgriczLltlt/e
Received February $0, 1948
In previous reports (3, 4) it was shown that stable cellulose is hydrolyzed by the action of dilute sulfuric and phosphoric acids a t elevated temperatures and also that the rate of hydrolysis follows a first-order reaction. Information on the rate of hydrolysis of cellulose as compared with the rate of decomposition of the products of hydrolysis was made the basis for determining the conditions for the hydrolysis of wood in a pilot plant (2) and commercial plants for the saccharification of wood (G). It was shown that phosphoric acid was not so satisfactory for hydrolysis as sulfuric acid, both because much larger amounts were required and also because the rate of decomposition of the hydrolytic products was greater than the rate of hydrolysis of the cellulose. Recent reports on the hydrolysis of mood by aqueous sulfur dioxide (1) indicated that fundamental information would be useful in the evaluation of sulfur dioside as a hydrolytic agent. Other investigators (7, 8) have proposed the use of hydrochloric acid, because it gives more hydrogen ions than do other hydrolyzing acids. This work describes work done at the U. S. Forest Products Laboratory on the hydrolysis of wood cellulose with hydrochloric acid and sulfur dioxide, using procedures previously employed (3, 4). This work was limited t o the study of the hydrolysis of stable cellulose and the decomposition of its hydrolytic products. The hemicellulose, which represents 20-25 per cent of the n-ood, hydrolyzes readily in periods of time too short to be measured when conditions are employed that will hydrolyze the stable cellulose. EXPERIMENTAL
The stalile cellulose in Douglas fir was hydrolyzed in dilute aqueous hydrochloric acid a t concentrations of 0.2, 0.4, 0.8, 1.G, and 3.2 per cent, using a liquid-solid ratio of 10 to 1, by heating a t lGO", l i O " , 180", and 190°C. for various lengths of time. The extent of the hydrolysis was determined by analysis of the carbohydrate content of the residue (5). A series of curves for each temperature was drawn in which the log of the cellulose or potential sugar left in the residue was plotted as a function of time. Figure 1 illustrates the curve for hydrolysis a t 170°C. Similar curves were drawn for the results of the hydrolysis a t lGO", 180", and 190°C. The slope of the curves multiplied by -2.303 gave the first-order reaction constant for loss in cellulose or potential reducing sugar. The half-life of the stable cellulose was calculated from the point a t which half 1 Pi escnted before the Division of Cellulose Chemistry a t the 113thiLIeeting of the Amrri c m Chemical Society, which \vas held i n Chicago, Illinois, April 19-23, 1918. 2 nlniiitaiiied at Madison, Wiscoiisiii, in cooperation with the University of Wisconsin.
345
HYDROLYSIS O F JYOOD C E L L U L O S E
0
F I G .1 . Hydrolysis of Douglns-fir wood \\-it11 dilute hydrochloric acid a t 170°C. TABLE 1 H!ydrol!ysi.s uf Doccylns-$1, cellitlose in 0.2, 0.4,0.8, atid 3.2 p / e m pcr,al //,res 1'EhIPER.AI'URE
,
.ACIDC(IXCEKTRAT1ON
per cenl
"C.
160.. . . . ..........
I T O . . . . . . . . . . . . . . . . i.
I '
i i
,~
nides
c.e,it liydrochloi,ic acid at
FIRST-ORDER RE.ACTION C O N S T I N T , fi SiiI
0.055 0.11 0.22 0.44 0.88
0.002034 0.00486 0.01075 0.02GI 0.0672
0.2
0.055 0.11 0.22 0.44 0.55
0,00568 0.0148 0.0361 0.0852 0.230
0.055 0.11 0.22 0.44
0.0100 0.0455 0.111 0.284
0.4
0.8 1.6 3.2
0.4
0.8
0.055 0.11 0.22
~
i
~
.-'
0.2 0.4 0.8 1 .6 3.2
0.2
190
,
i '
O.OG27 0.149 0.357
iiarious
H A L F - L I F E OF CBLLULOSE
niiu.
314.0 142.0 64.3 26.5 10.1
j i I
121 . 0 46.5 10.2 8.1 2.9 36.2 15.1 6.2
2.43
~
11.0 4.65 1.95
~
of the potential reducing material \vas removed. Table 1 gives the first-order reaction constant [I; ( m i n . ~ ' ) ]and the half-life of the cellulose, in minutes, for
346
ELWIN E. H A R R I S AND A L B E R T 9. KLINE
FIG.2. Decoinposition of glucose in dilute hydrochloric acid a t 170°C.
TABLE 2 D e c o m p o s i t i o n of glucose in d i l u t e hydrocliloric acid TEMPERATURE
“C.
ACID CONCENTRATION
FIRST-ORDER REACTION COXSTANT, fi
HALF-LIFE OF CELLU. LOSE
per ceiil
moles
160.. . . . . . . . . . . . . . . .
0.2 0.4 0.8 1.6 3.2
0.055 0.11 0.22 0.44 0.88
0.00322 0.00655 0.0141 0.0298 0.0861
214.0 105.0 49.0 23.2 8.0
170.. . . .
0.2 0.4 0.8 1.6 3.2
0.055 0.11 0.22 0.44 0.88
0.00767 0.01535 0.0317 0.072 0.147
90.0 45.0 21.8 9.5 4.7
180 . . . . . . . . . . . . . . . . .
0.2 0.4 0.8 1.6 3.2
0.055 0.11 0.22 0.44 0.88
0.0207 0.0391 0.0841 0.179 0.482
33.3 17.5 8.0 3.86 1.43
0.2 0.4 0.8 1.6 3.2
0.055 0.11 0.22
0.0488 0.107 0.218 0.406 0.715
14.1 6.45 3.16 1.7 0.97
190. ................
0.44 0.88
miu-1
min.
,
347
HYDROLYSIS O F W O O D CELLULOSE
the various concentrations of acid a t the different temperatures. It may be noted that an increase of' 10 degrees in temperature had a much greater effect on the rate of hydrolysis than doubling the acid concentration. The principal sugar resulting from the hydrolysis of Douglas-fir wood was glucose. Pure glucose was used to determine the rate of decomposition of the products of hydrolysis. Samples of glucose solution containing hydrochloric acid in concentrations of 0.2, 0.4, 0.8, 1.6, and 3.2 per cent were sealed in glass tubes and heated at temperatures from 160" t o 190°C. for various lengths of time, after which the tubes were opened and the residual sugar determined. The log of the unchanged sugar was plotted as a function of time, and curves for the resitlual sugar were drawn similar to figure 2, which s h o w the decomposiTABLE 3 _
_
_
H y r l i d y s i s of cellitlose w i l h stilfui, dioxide
~ I
I
1 HALF-LIFE of ~-
TEMPERATCRE ~
DECOJIPOSITION O F R E D U C I S G S1IC.IR I N SULFCR DIOXIDE
H \ D R O L P S I S O F C E L L C L O S E IYITK S U L F U R DIOXIDE
First-order
Sulfur dioxide con-
i
CELLULOSE
Concentra tion
of sulfur
centration
H A L F - L I F E OF
CLGCOSE
First-order reaction :onstant, k miir .-I
1GO.. .
170... . . .
, ~
1
1 so
,
miii.
0,00256
270
3.00
0.117 1 0.234 0.4GS
0.00235 0.00263 0.0035s
0.00230 0.00338 0.00498
300 223 139
0.75 1.50 3.00
O.lli 0.234 0.4GS
0.00502 0.00629 0.00843
0.00543 0,00777 0.0106
127.5 89.3 G5
0 75 1.50 3.00
0.11: 0,234
0.01275 0.0212 0.0255
51.2 32.6 27.2
O.4GS
1
~
i 1
0.0125 0.0159 0.0213
I
5i
,
42 32
1
I ,
0.75 1.50 3.00
t,ion at 1'70°C. The decomposition follows a first-order reaction. The reaction coiist,a,nt [I; was determined from the slope of the line, and the half-life of the sugar as the point when half of the sugar had disappeared. Table 2 gi\.es the calculated half-life and first-order reaction constant [k (min.-')] for the decomposition of glucose. Hydi~o1ys.i~ of stable cellulose and the decomposition of the hydrolytic product's in aqueous sulfur dioside were determined in small nickel-alloy bombs. Samples of wood and of glucose \\.ere mixed with aqueous sulfur dioside in eoncentrations of 0.75, 1.50, and 3.0 per cent and heated a t temperatures from 150" to 180°C. for various lengths of time. Curves for the residual carbohydrate were plotted from 11 hich the first-order reaction constant and half-life of the resistant' cellulose and the sugni. \\-eredetermined. These values are shown in table 3.
348
E L W I N E. HARRIS B N D .4LBERT A . K L I Z E
DISCUSSION O F RESULTS
Calculated values for the first-order reaction constants [A: (min.-I)] a t the various temperatures and acid concentrations for. hydrochloric acid and sulfur dioside from tables 1 and 3 and those for sulfuric acid (2) and phosphoric acid ( I ) presented previously were assembled in table 4 for purposes of comparison. In all cases the constant increased as the concentration of the acid increased TABLE -1. First-order reaction constanl f o r the hydrolysis o j ccllztlose w i t h aqueoiis solutions o j s i t l j u r i c , phosphoric, a n d hydrochloric acids a n d s u l f u r dioxide TEMPERATURE
ACID C O W E N TRATION
'
FIRST-ORDERREACTION CONSTANT,
Sulfuric acid
"C.
160. . . . . . . , , . . . , . .
170. . . . . , , . . . . . . . .
180..
190.
195.
Phosphoric acid w&,-1
0.04 0.08 0.16 0.32 0.88
0.04 0.08 0.16 0.32 0.85
0.00355 0.00886 0.0222
0.04 0.08 0.16 0.32 0.88
0.00905 0.0258 0.0664
0.00007 0.00161 0.00291 0.00474
0.04 0.08 0.16 0.32
0.0299 0.0725 0.183
0.00256 0.00305
0.04 0.08 0.16 0.32
O.OOG7
Sulfur dioxide
k Hydrochloric acid
min.-l
miii.-l
0.0019 0.0025 0.0031 0.0045
0.0013 0.0033 0,0078 0.0183 0.0672
0.0042 0.0055 0.007 0.01
0.0043 0.0105 0.025 0.058 0.230
0.011 0.0145 0.0185 0.027
0.0137 0.033 0.078 0.18
0.045 0.105 0.25
0.0117 0.0062 0,0088 0,0014 0.025
and also as the temperature increased. An increase of 10 degrees in temperature caused a gveater increase in the constant than doubling the acid concentration. The four acids exhibited wide differences in their first-order reaction constant. When expressed on a mole basis, sulfuric acid gave reaction constants of the same order of magnitude as hydrochloric acid. This indicates that, although the second hydrogen ion in sulfuric acid is highly dissociated in these acid concentrations, it does not enter into the reaction of bringing about the hydrolysis
HYDROLYSIS O F WOOD CELLCLOSE
349
of cellulose and also that the acids react as molecules rather than as ions. Sulfur dioside when expressed on a mole basis gave values ranging from about one-half to one-twentieth those given by sulfuric and hydrochloric acids. For esample, at 180°C. an 0.88 molar (5.6 per cent) solution of sulfur dioside would give about the same rat'e of hydrolysis as an 0.08 molar (0.8 per cent') solution of sulfuric acid. The values for the first-order reaction constant for sulfur dioxide do not increase so much with increase in concentration as do those for sulfuric acid. This may indicate eit,her that sulfur dioside is not soluble or that it is associtited in solution and that only a few of the molecules are available to catalyze the hydrolysis of cellulose. Phosphoric acid gives values that range from 1/100 to 1/200 of those given by sulfuric and hydrochloric Reid :it the same temperatures. On the basis of this low rtite of hydrolysis phosphoric acid was considered unsatisfactory for hydrolysis, I n work on hydrolysis sulfur dioxide, which is R volatile acid, is recoverable. The only loss is that oxidized to sulfuric acid or combined with the wood constituents. The requirement for high concentrations presents only the problem of pumping more gas. In order to be suitable for commercial use, the agent promoting hydrolysis must give a high rate of hydrolysis (k,) as compared with the rate of sugar decomposition (X.2). The calculated values for kllX.2 for the various acids a t various acid concentrations and temperatures were assembled in table 5 for comparison. A value of 1 indicates that the isate of hydrolysis of the stable cellulose is equal t'o the rate of decomposition of the glucose produced by the hydrolysis. Values over 1 indicate conditions favorable for high yields. Values of kl/l;2 for sulfuric, phosphoric, and hydrocbloric acids increased as either the acid concentration or the temperature w:ts increased. When sulfur dioxide was used as the hydrolj7tic agent, the relationship between 1 ~ 1and 1 : ~appeared to be less favorable with an increase in acid concentration and showed little change wit'h the increase in t,emperature. This Jvould indicate that the use of high concentrations of sulfur dioxide in order to promote a mpid hydrolysis may not' produce so high a yield of sugar as is produced by the a,mount of sulfuric or hydrochloric acid required tjo give the same rate of hydrolysis. Phosphoric acid failed to give values favorable to t,he production of high yields of sugar. This condition, along with the low rates of hydrolysis shown in t,able4, indicates that phosphoric acid is not so suitable as any of the other three hydrolytic substances. Pilot-plant tests were condud.ed a t the Forest Products Laboratory on the hydrolysis of Douglas-fir -wood chips with dilute aqueous solutions of sulfuric acid and sulfur dioxide. A solution containing between 0.5 and 0.G per cent sulfuric acid WLLS allowed to trickle through a 400-lb. charge of wood (dry hnsis) a t the rate of 20 lb. of dilute acid per minute at an initial temperature of 150°C. for 30 min. Thereafter, the temperature ]vas raised 5°C. in each 10-min. period until a temperture of 185°C. was reached; then the temperature was held a t 185°C. untJil the end of the hydrolysis. By this process about 90 per cent of the carbohydrate was
350
ELM'IN E. H A R R I S AXD ALBERT A . K L I N E
hydrolyzed in a total t,ime of 3 hr. If the sugars were removed continuously as produced, the recovery of reducing sugar including the hydrolyzed hemicellulose was about 80 per cent of the carbohydrate hydrolyzed. When aqueous sulfur dioxide was used under the same temperature conditions, the use of 2.0 per cent sulfur dioxide for 7 hr. was required to hydrolyze 90 per cent of the carbohydrate. When the sulfur dioxide solution was allowed to pass through the chips continuously and removed continuously a t the rate of 10 111. of sulfur dioxide solution per TABLE 5 R u l i o os Lhc d e of hydrolysis of cellulose ( k l ) to the rate of glncose decomposition ( k , ) f o r vai,ioiis acids ACID CONCENTRATION
TEMPERATURE
OC .
160.,. ,
170.. . .
150. . . .
190.. . .
195.. . .
moles
. . . . . .., .
. . , . . ... .
,
.
,
., . . . .. .
I
.
.
.
.
.
.
.
. . ......
0.04 0.08 0.16 0.32 0.88 0.04 0.08 0.16 0.32 0.88
0.04 0.08 0.16 0.32
0.04 0.08 0.16 0.32
'I 1
ki/Ri
Sulfuric acid
Phosphoric acid
Sulfur dioxide
Hydrochloric acid
1
1 I
i
1 ~
I
I
!,
0.62 0.84 1.00
1
i
j
1
0.81 1.07 1.31
1.11 1.36 1.68
0.269 0.31 0.375 0.395
0.337 0.36 0.40 0.46
1.o 0.74 0.71
0.60 0.70 0.79 0.84 1.03
0.95 0.90 0.81 0.i8
0.84 0.95 0.96 1.16 1.5 1.o
0.98 0.90 0.82
1.11 1.23 1.39 1.60 1.32 1.44 1.60 1.70
0 525 0.52 0.57 0.67
0.04
I
0.32
minute, the recovery of sugars was 65 to 75 per cent of the carbohydrate hydrolyzed. SUMMARY
The rates of hydrolysis of Douglas-fir cellulose in dilute hydrochloric acid and in aqueous sulfur dioxide were determined. These rates mere compared with those in sulfuric and phosphoric acids, which had been previously determined.
SOLUBILITY O F S I L V E R B R O M I D E I N N I T R I C ACID
35 1
On a molar basis, hydrochloric acid hydrolyzes cellulose a t about the same rate as sulfuric acid, while sulfur dioxide gives about one-seventh the rate of sulfuric acid. The net yield of reducing sugar was not so high with sulfur d i o d e as with hydrochloric or sulfuric acid. REFERENCES (1) .hr-WuoRINEN, 0.: Suoinen Keniistilehti 15A, 31 (1942). (2) HARRIS,E. E . , A N D BEGLINGER, E.: Ind. Eng. Cheni. 38, SO0 (1946). E. E., AND LANG,BILLG . : J. Phys. Colloid Cheni. 51, 1430 (1047). (3) HARRIS, (4) SAE,MAN, J. F.: Ind. Eng. Cheni. 37, 43 (1915). (5) S.~E,JIAK, J. F., BUBL,JANET, A N D HARRIS,E. E.: Ind. Eng. Cliem., .Stid. Ed. 17, 35 (I 945). (6) Staff Report: Pulp S: Paper Ind. 20, 3G (Septetnber, 1916). (i'j \ ~ ~ O L F R O AAI. I , L., AND GEORGES, L. w.: J. Ani. Chem. Soc. 59, 282-6 (1937). (Sj WOLFROM, h4. L., A K D SNOWDEN, J. C . : J. Am. Cliem. SOC.60,3009-13 (1938).
DETERRIINATION OF T H E SOLUBILITY OF SILI'ER. BROMIDE I N NITRIC ACID SOLUTION GSING RADIOBROJ'IINE ROSWELL RUIiA1
. ~ N DJ.
E. WILLARD
Depai tinent of Chemistry, U n i z e r s i l y of Wisconsin, M a d i s o n , W i s c o n s i n Received June 23, 1948
The most direct and sensitive method for the determination of the solubility of silver bromide appears to be the determination of dissolved silver or bromine with the aid of radiosilver or radiobromine. Radiobromine is no\\. readily available with a specific activity sufficient to give of the order of 1000 counts per minute per milliliter of saturated silver bromide solution. We have used this material t o obtain semiquantitative information, which was needed in our laboratory, on the effect of acidity and of temperature between 0" and 25°C. on the solubility of silver bromide. 11'Isny previous investigators have made determinations of the solubility in neirtd solutions by other methods. Their results :ire given in table 1. E X P E R I M E N T A L PROCEDURE*
The general procedure employed in this work was to precipitate a small amount (about 0.025 mg.) of silver bromide in a 15-ml. centrifuge cone by mixing equivalent amounts of dilute solutions of potassium bromide containing Brs2 (34 hr.) and silver nitrate. The nearly invisible precipitate was then centrifuged onto the walls of the tubes, and rinsed several times with nitric acid solution of Present address: Westinghouse Research Laboratories, East Pittsburgh, Pennsylvania. -4more complete discussion of this work is included as part of a Master's degree thesis filed with the Li1)rai-y of the University of Wisconsin by Roswell J. Ruka in Februaw, 104s. 2
