I
C. J. MALM, L. J. TANGHE, and J. T. SCHMITT Cellulose Technology Division, Eastman Kodak Co., Rochester, N. Y.
Catalysts for Acetylation of Cellulose Why is sulfuric acid the preferred catalyst?
b b (I
It concentrates on cotton linters at the site of acetylation It reacts with cellulose, and cellulose acetate sulfate triester has greater solubility in the acetylating bath than cellulose triacetate
SULFURIC
ACID is the most commonly used catalyst for the acetylation of cellulose, and its role in the reaction has been studied in considerable detail (3, 4 ) . Other catalysts are of potential interest for the acetylation of cellulose and many have been proposed in the literature. A successful catalyst must be effective in the acetic acid system. Some of the traditionally strong acids in water are weak in acetic acid. I n the solution process for making cellulose acetate the catalyst must do more than promote acetylation of hydroxyl groups; it must lead to a completely soluble product having the desired viscosity. T h e present work is focused on the effectiveness of various catalysts for the acetylation reaction itself.
Experimental I n addition to acetylation of cellulose fiber, simpler acetylations were studied, including reactions of acetic anhydride with water and with methanol. T h e program was then extended to the hydroxyls of a dissolved cellulose ester and finally to cellulose itself. T h e experiments with cellulose were carried out with cotton linters and with cellulose regenerated from cellulose acetate which had been made by a solution process. All these reactions are strongly exothermic, and the reaction rates were determined from the rates of temperature rise under adiabatic conditions. As the total temperature rise is a measure of the amount of reaction (7), so also is the rate of temperature rise a measure of the rate of reaction. T h e rate of reaction was evaluated from the time-temperature plot. I t is valid for the concentrations of reactants and catalyst present a t the temperature of measurement. Reaction rates were measured using perchloric, sulfuric, methanedisulfonic, sulfoacetic, and methanesulfonic acids for the reaction of acetic anhydride with each of the substances mentioned above. Several concentrations of catalyst were used for each of the above catalystreactant combinations. Experimental details are given on pages 364-5.
Results Reaction b e t w e e n Acetic A n h y d r i d e a n d Water. T h e temperature rise was proportional to the amount of water and was 24.5' C. per mole under the conditions used in the rate measurements. T h e total temperature rise for the reaction of various amounts of water with acetic anhydride (0.001M perchloric acid catalyst) is: Moles H*O/L. 0.05
Temp. Rise, 1.3
O
dc/dt = the rate of disappearance of water, in moles per liter per minute,
and c
= the molar concentration of water
at any time, t. Retaining this expression in differential form, the pseudo first order rate constant is simply
C.
4.7
0.19 0.33 0.47 0.61
tion of water with a large excess of anhydride :
8.25 11.5 14.85
Figure 1 (left) shows the time-temperature curves for this reaction catalyzed by 0.0025M methanesulfonic acid. T h e initial concentrations of water a t 25" C. were 0.60 and 0.32M which were reduced to 0.40 and 0.12M, respectively, upon reaching 30' C. Figure 1 (right) gives the time-temperature curves for the same reactions catalyzed by 0.0001M perchloric acid. I n this case the relative positions for the curves starting with the two different amounts of water are reversed. T h e tangents to the curves at 30" C. gave the rate of temperature rise; dividing by 24.5 gave the moles per liter of water reacting per minute. T h e expression for the rate of a reaction obeying pseudo first order kinetics is dc/dt = kc. As applied to the reac-
where the numerator is the moles per liter of water reacting per minute at 30" C., obtained from the rate of temperature rise. T h e denominator is the moles per liter of water remaining after the temperature has reached 30" C. Pseudo first order rate constants for this reaction are calculated in Table I. I n order of decreasing activity in this reaction the catalysts were: perchloric equal to sulfuric acid, then methanedisulfonic, sulfoacetic, and methanesulfonic acid, All catalysts showed increased activity, as measured by pseudo &st order rate constant, when the concentration of water was decreased. For methanesulfonic acid the effect was small, but for the perchloric and sulfuric acids the rate constant was six times greater a t 0.12M than a t 0.40M water. T h e factor column of Table I1 gives the increase in rate constant. Sulfuric and perchloric acids showed this effect to
Minutes
Minutes
Figure 1 . In the reaction between AczO and HzO catalyzed b y 0.0025M CHBSO~H, an increase in HzO concentration accelerates the reaction (left); when catalyzed by 0.0001M HC104, an increase in HzO concentration retards the reaction (right) VOL. 53, NO. 5
M A Y 1961
363
the same degree. The increase in activity was progressively less for methanedisulfonic, sulfoacetic, and methanesulfonic acid than for perchloric acid, but was not affected by reducing the concentration of any given catalyst. Effect of Water o n Acidity Function, H,. Ludwig and Adams ( 2 ) have shown that decreasing the amount of water in acetic acid strongly increases the acidity (gives a lower numerical value for Ho) of 0.001M perchloric acid. These measurements have been confirmed and extended to methanesulfonic acid :
0.72
0.40
0.22
0.12
0.50" 0.50b -0.14 -0.05
Reference ( 2 ) .
0.00 -0.11
Present work.
iYhen the water content was decreased from 0.40 to 0.12M, H , decreased 0.6 unit for 0.001M perchloric acid but only 0.11 unit for 0.05M methanesulfonic acid. A decrease of one unit in H, should increase the rate constant 10-fold. Hence the increase in rate constant for perchloric acid should be 4-fold and for methanesulfonic acid, 1.?+fold. T h e observed increases in rate constants were 6-fold and 1.25-fold, respectively. T h e small discrepancies are probably due to the fact that H , was measured in the binary system acetic acid-water and the rate constants were determined in the unstable ternary system acetic acidacetic anhydride-water. Since the increase in acidity with decreased water content is much greater for perchloric acid than for methanesulfonic acid, the reversal of the timetemperature curves of Figure 1 can be explained.
Effect of Concentration of Catalyst o n Reaction between Acetic Anhydride and Water. Several concentrations of each catalyst were used. covering the range for which this method of measuring reaction rates is suited. Figure 2 shows plots of log pseudo first-order rate constant us. log concentration of catalyst. Straight lines were obtained having a slope of nearly unity. T h e concentration of water at the point \\,here the reaction rate was measured was 0.40M (Figure 2 4 and 0.12M (Figure 2B). Acetylation of Methanol. Methanol was acetylated using several concentrations of each catalyst. Figure 2C shows straight lines obtained by plotting log concentration of catalyst us. log pseudo first-order rate constant a t 30' C., and ar: 0.51M methanol. Except for sulfuric acid, less catalyst was required with methanol than with water to obtain a given rate constant.
was pressed out as before and the process was repeated. Before use the cellulose was given a similar treatment with acetic acid-acetic anhydride ( 2 to 1, aged); the final wet cake was about 30 grams. Regenerated cellulose was prepared from high quality commercial cellulose acetate. The cellulose acetate was dissolved in acetone, precipitated, and washed in distilled water, and hydrolyzed with 14% ammonium hydroxide for two days at room temperature. The regenerated cellulose was washed in distilled water and centrifuged to a wet cake containing 207, solids, as determined by drying a sample of 110 O C . overnight. The water was displaced from 500 grams of wet cake by seven changes of 1500 ml. each of acetic acid, as described for cotton linters. The wet cake was tumbled overnight at room temperature to obtain a uniform mixture. The acetic acid content by titration was 83.3%. Before use, 90 grams of the cellulose-acetic acid mixture (15.0 grams cellulose) was given a final dewatering with 500 ml. of acetic acid-acetic anhydride (2 to 1, aged). Reaction between Acetic Anhydride a n d Water. These reactions were carried out in a 1500 ml. bottle containing one liter of freshly mixed acetic acid-acetic anhydride (2 to 1 by volume). The temperature was adjusted to 26' C., and water (five or 10 ml.) was added. This caused the temperature to drop a few tenths of a degree. The bottle was placed in an insulated 4-liter metal beaker. A thermometer graduated in 0.1 C. and an electrically driven glass stirrer were set into position. A final temperature adjustment to 25.0 =t 0.1' C. was made by immersing a test tube filled with warm or cold water, as required. The catalyst was added as a stock solution in acetic acid, stirring vigorously during the addition and slowly thereafter. Time-temperature readings were taken up to about 32' C. In some cases, 10 ml. 0.1M perchloric acid in acetic acid solution was added and the maximum temperature noted from which the amount of watcr present at 30' C. could be vezified. A maximum temperature of 39.8 C. indicated the presence of 0.40 mole per liter of water at 30 C. Acetylation of Methanol. A quart Dewar flask was used containing 700 ml. acetic acid-acetic anhydride (2 to 1, aged) and the desired amount of catalyst.
This solution was adjusted to approximately 25 C. before placing in the flask, and then to 25.0 i: 0.1' C., as described above. The methanol was added with vigorous stirring, and time-temperature data were recorded as above. Disappearance of Free Sulfuric Acid. To 500 ml. acetic acid-acetic anhydride (2 to 1, aged) was added 20 mi. methanol and 50 ml. 0.1M sulfuric acid in acetic acid solution. After 30 seconds the temperature had risen IOo C. The catalyst was then neutralized by adding quickly 120 ml. of 0.1M potassium acetate in acetic acid solution. To 100 ml. of the resulting solution were added 100 ml. of water and 10 ml. of 5% aqueous barium chloride. No precipitate formed, even on standing overnight at room temperature. In a control experiment, substituting water for methanol, an immediate copious precipitate of barium sulfate formed. Acetylation of Dissolved Cellulose Ester. In a quart screw-cap bottle were placed 25.2 grams of the ester (dried two hr. 110' C.) and 455 ml. acetic acidacetic anhydride (2 to 1 aged). The mixture was stirred with a Lightnin mixer for one minute at full speed, then capped and placed on a shaker for about an hour until the ester was completely dissolved. The cap was replaced with a sheet of polyethylene which was held in position with a rubber band. A thermometer was inserted through the sheet, and the temperature was adjusted to 28 I=! 0.5" C. A quart Dewar flask was rinsed with acetic acid-acetic anhydride and allowed to drain a few minutes. The ester solution was poured into the flask and allowed to drain into it for one minute. This delivered 25.0 grams of cellulose acetate and 450 ml. solvent to the flask. The glass stirrer was wiped with tissue just before using. After stirring at full speed for 15 seconds, the speed was reduced, and thc steady reading on the thermometer was recorded after one or two minutes. The catalyst solution was prepared just before use by diluting the required amount of stock solution to 50 ml. with acetic acid-acetic anhydride (2 to 1). A starting temperature of 27.5' C. was obtained by warming or cooling the catalyst, as needed, before addition to the ester solution. If the temperature of the ester solution was above 27.5" C., the catalyst was adjusted
EXPERIMENTAL DETAILS Catalysts. Perchloric and sulfuric acids were reagent grade, 72 and 95 wt. %, respectively. Sulfoacetic acid was prepared by heating sulfuric acid in acetic acid-acetic anhydride (1 to 1) for 6 hrs. at 70 O C. The straw-colored solution was 0.7M sulfoacetic acid, free from sulfate. Methanedisulfonic acid was obtained from Tennessee Eastman Co. as a 52% aqueous solution. Methanesulfonic acid was an Eastman Kodak product. Stock solutions of 0.01 or 0 . l M catalyst in acetic acid were prepared. In some instances methanesulfonic acid was used undiluted. Solvents and Reactants. Acetic acid and anhydride were comniercial products of high quality from Tennessee Eastman Co. and were used without further purification. The acetic acid had a melting point of 16.2O C. The acetic anhydride was 98% pure. A mixture of two parts acetic acid and one part acetic anhydride was allowed to stand two days or more at room temperature, after which it was shown to be anhydrous by failure of perchloric acid to increase the temperature. Distilled water was used. Methanol was distilled over magnesium turnings ; it contained less than 0.02T0 water by the Karl Fischer method. The starting material for the acetylation of cellulose hydroxyls in solution was obtained by further hydrolysis of a commercial cellulose acetate butyrate of low viscosity (Type EAB-272-3, supplied by Tennessee Eastman Co.). Ten pounds of this material was dissolved in 50 lb. of acetic acid and 10 lb. of water, and the solution warmed to 45 O C. A mixture of 7 lb. of acetic acid, 2 lb. of water. and 250 ml. of concentrated hydrochloric acid was stirred in After three days at 45' C. the ester was precipitated and washed in distilled water. The product contained 1.03 acetyl, 0.88 butyryl, and 1.09 hydroxyl groups per glucose unit, and had an intrinsic viscosity of 1.01 in methylene chloride-methanol (90 to 10, by weight). Cotton linters were activated in 10-gram quantities as needed. The cellulose was soaked in distilled water overnight and pressed as dry as possible on a Buchner funnel with a rubber sheet. The wet cellulose was placed in a quart bottle with 500 ml. of acetic acid and shaken occasionally for 15 minutes. The liquid
364
INDUSTRIAL AND ENGINEERING CHEMISTRY
O
O
ACETYLATION O F CELLULOSE Table I.
Reaction between Acetic Anhydride and Water Calculation of rate constants a t 30' C. from experimental data Pseudo First Rate of Moles/L., Order HrO Rate Molar Temp. Concn. Rise, Reacting/ Constant/ Acid Catalyst Molarity of HzO O C./Min. Min. Min. Perchloric 0.0001 0.4O5 0.34 0.014 0.036
Sulfuric MethanedisulfonicC
.
Sulfoacetic Methane sulf onic a
0.0001 0.00025 0.00025 0.00025 0.00025 0.0005 0.0005 0.0025 0.0025
O.12* 0.40 0.12 0.40 0.12 0.40 0.12 0.40 0.12
0.65 1.07 1.91 0.96 1.47 0.74 0.70 1.05 0.39
0.026 0.044 0.078 0.039 0.060 0.030 0.028 0.043 0.016
0.22 0.109 0.64 0.098 0.47 0.076 0.23 0.107 0.133
Factor (See Text) 6 6
5 3 1.25
Starting with 0.60M HzO at 25' C. Starting with 0.32M HzO at 25' C. Dibasic in acetic acid; all other catalysts are monobasic.
to 9 " C. below 27.5' C. for every degree the ester solution was above 27.5 O C. For example, when the ester solution was 27.7" C., the catalyst owas adjusted. to 25.7' C. to give 27.5 C. on mixing. Similarly, if the temperature of the ester solution was below 27.5" C., the catzlyst was warmed sufficiently to give 27.5 C. on mixing. The solution was stirred at full speed during the five seconds required for the addition of the catalyst, and for 20 seconds afterwards. Slow stirring was maintained throughout the reaction, except for five seconds of high speed stirring after each of the first three temperature readings. After taking several temperature readings above 30" C., 10 ml. of 0.1M perchloric acid in acetic acid-acetic anhydride (2 to 1) was added to catalyze quickly the remainder of the reaction. If the total rise was less than 4.8" C. or greater than 5.2" C., the run was repeated. Uncatalyzed acetylation was negligible during the time required to prepare the solution in acetic acid-acetic anhydride (2 to 1); after three days at room temperature only 10% of the hydroxyls had acetylated. Acetylation of Linters. Ten grams of activated linters with about 20 grams of adhering acetic acid-acetic anhydride (2 to 1) was placed in a freshly rinsed Dewar flask and 430 ml. of acetic a$d-acetic anhydride ( 2 to 1, aged) at 25 C. was added. The thermometer and stirrer were fitted in position and the mixture stirred at full speed for three seconds each minute until a steady reading on the thermometer was obtained. The catalyst solution was prepared and its temperature adjusted as described above, but to give a calculated starting temperature of 25.0" C. The stirring routine was similar to that of the dissolved cellulose ester, except for five seconds of high speed stirring after each temperature reading throughout the experiment. This gave adequate mixing, and avoided generation of heat by excessive stirring. Continuous stirring at moderate speed gave a temperature rise of about 0.1 C. per minute in the absence of catalyst, and a hump in the timetemperature curve during the interval of two to five minutes after the addition of catalyst. Intermittent high speed stirring
gave a temperature rise of only 0.01 O C. per minute in the absence of catalyst, and a smooth time-temperature curve after the addition of catalyst. Acetylation of Regenerated Cellulose. Regenerated cellulose (15 grams plus about 7 5 grams of adhering acetic acid-acetic anhydride, 2 to 1) was transferred to a freshly rinsed Dewar flask, and 600 ml. acetic acid-acetic anhydride (2 to 1, aged) at 25' C. was added. The catalyst was added in 75 ml. of acetic acid-acetic anhydride (2 to 1). The acetylations were carried out following the details and precautions given for cotton linters. Table I1 summarizes the acetylation conditions. Sorption of Catalysts by Cellulose. Catalyst solutions of the desired concentrations were prepared in acetic acid or in acetic acid-acetic anhydride (2 to 1). Aliquots of 25 to 200 ml., depending on the concentration, were titrated with 0.1M potassium acetate in acetic acid solution to determine their exact concentrations. T o 500 ml. of each catalyst solution was added 10 grams of activated linters. The catalyst concentration was corrected for the acetic acid or acetic acid-acetic anhydride ( 2 to 1) adhering to the cellulose. After shaking intermittently for 30 minutes with the catalyst solution in acetic acid, or for two minutes with the catalyst solution in acetic acid-acetic anhydride (2 to l), the supernatant liquid was recovered by
Table II.
Substance Acetylated Water Methanol Dissolved Ester Activated Linters Regenerated Cellulose
Acetylation Conditions
Temp., O C., at Rate MeasureStart ment Finish 25.0 25.0 25.0 25.0 25.0 27.5 25.0 25.0
30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0
32.9 39.7 33.9 42.8 51.7 32.5 35.0 34.5
25.0
30.0
34.5
Sulfuric acid, however, appeared relatively much weaker with methanol than with water. With water sulfuric acid had the same catalytic activity as perchloric acid, but with methanol it has only the same activity as sulfoacetic acid. After the addition of sulfuric acid to acetic anhydride and methanol, no sulfate ion could be detected. This appears to be due to the conversion of sulfuric acid to methyl hydrogen sulfate under the conditions of the experiment. Hence methyl hydrogen sulfate, rather than sulfuric acid, is equal in catalytic activity to sulfoacetic acid. Table I11 shows that methanol decreased the catalytic activity in the same manner as water, although the effect did not appear to be as great. As with water, this depressing effect was less pronounced in going from strong to weak
filtration by gravity through a plug of borosilicate glass wool. The cellulose treated with sulfuric acid in acetic acidacetic anhydride ( 2 to 1) became too mushy to filter in this way and was filtered with suction on a Buchner funnel through dry filter paper. I n all cases except sulfuric acid, titration of an aliquot of the sorption filtrate gave the percentage of the original catalyst remaining in solution; the amount sorbed by the cellulose was calculated. I n case of sulfuric acid after titrating a 100-ml. aliquot of the sorption filtrate from the experiments with sulfuric acid in acetic acid-acetic anhydride ( 2 to l), the solution was diluted with one liter of distilled water. About 5 ml. of 0.5N barium chloride solution was added and the mixture allowed to stand overnight at room temperature. The clear liquid was decanted, one liter of water was added, and the precipitate was digested on the steam bath overnight to improve its filterability. The barium sulfate was collected on a sintered crucible (Selas 2001). Effect of Water o n Acidity Function, Stock solutions in 99.9% acetic acid were prepared, containing 0.25M methanesulfonic acid, and 0.0004M recrystallized N,N-diethyl-2,4-dinitroaniline, m.p., 80 C. Into each of six 25-ml. volumetric flasks was pipetted 5.0 ml. of each of the above stock solutions. Calculated amounts of 90% acetic acid were added, so that after filling to the mark with 99.9% acetic acid, the amounts of water would be 0.1, 0.2, 0.5, 1.O, 2.0, and 4.0 %, respectively. For controls, solutions were prepared containing 0.1 and 4.OyG water, and with the indicator converted completely to its acidic and basic forms at both percentages of water. I n each of two 25-ml. flasks was placed 5.0 ml. of indicator solution. T o one of these flasks was added a few drops of 7 2 y 0 perchloric acid, and to the other a few drops of 0.1M sodium acetate in acetic acid solution. These solutions were diluted to the mark with 99.9% acetic acid for the desired concentration of indicator. They contained 0.1 70 water. Two similar solutions, but containing 470 water, were prepared. Absorbances were read at 372 mfi on a Beckman DU spectrophotometer, and the acidity function, Ho, was calculated based on a jbK, of 0.30 for the indicator. VOL. 53, NO. 5
MAY 1961
365
05c
Table 111. Acetylation of Methanol Effect of concentration of methanol on rate constant Pseudo First Order Rate Constant/Min. at 30° C. Molarity 0.86M MeOH" 0.51M MeOHb 0.1511.1 MeOHC
0 25-
"it
005,
Acid Catalyst 0.00007 0.063 0.13 0.47 Perchloric 0.0005 0.15 0.23 0.45 Sulfuricd 0.15 0.17 0.22 Methanesulfonic 0.003 * Starting with 0.71M a t 25O C. Starting with 0.35M a Starting with 1.06iM a t 25' C. Converted to methyl hydrogen sulfate under conditions of the experiment. a t 25" C . Table IV.
Acid Catalyst Perchloric Methanesulfonic
0.00015 0.00015
25 50
27.5 25
30.0 30.0
32.5 35
0.60 0.75
1.20
0.05 0.05
25 50
27.5 25
30.0 30.0
32.5 35
0.42 0.56
0.84
&So4
Sulfoacetic Acid CHaSOsH a
0.014 0.066 0.0054 0.46 2.9
0.000012 0.000033 0 * 00021 0.0004 0.0056
1100 2000 25 1200 500
Reaction starting a t 25O C. and giving a total rise of 10' C. Reaction starting a t 27.5' C. and giving a total rise of 5' C.
366
05
0 II
INDUSTRIAL AND ENGINEERING CHEMISTRY
Table VI.
0001
0000l
.-
001
Molar concentration
01
of catolyst
Figure 2. The catalytic activity of HzS04 equals that of HCIOd for the reaction of AczO with HzO, A and B, but is inferior for the reaction with MeOH, C
than perchloric acid, but it was about twice as active as sulfoacetic acid. Under the conditions of the experiment the sulfuric acid combined quantitatively with the cellulose (4). Effect of Dissolved Cellulose Ester on Activity of Catalyst. Since water and methanol decreased the acidity of catalysts, it was thought that the hydroxyls of a dissolved cellulose ester might have a similar effect. Because the hydroxyl groups in cellulose have different reactivities, the design of the experiment had to be modified so that a t the temperature of measurement there would be different molar concentrations of total hydroxyl, but the same array of primary and secondary hydroxyls. This was accomplished by increasing the amount of ester from 25 to 50 grams and starting the acetylation a t 25.0" C., while retaining the same amount of catalyst. T h e rates of reaction were measured a t 30" C. since this was the midpoint of the acetylation in each case. If the dissolved cellulose ester had no effect on the activity of the catalyst, the rate of temperature rise shouid be twice as great in the solution containing twice the molar concentration of hydroxyl groups. Table IV shows, how-
Table V. Amounts of Catalyst Required for Comparable Rates of Esterification of Cellulose in Fibrous and Dissolved States Molar Concn. of Catalyst Ratio of Required for Rate of Temp. Amounts of Rise at 30' C. of Catalyst 0.05' C./min. h'eeded 0.1' C. min. in dissolved Fibrous: in fibrous Catalyst statea stateb Dissolved
CHZ(SOBH)Z
+.
Effect of Cellulose Hydroxyls on Activity of Catalyst Temp., O C . Rate of Temp. Rate Rise, " C./Min. Grams measureTheoMolarity Ester Start ment Finish Observed retical
catalysts. Sulfuric acid showed the effect much less than perchloric acid, which is in accord with its conversion to the weaker methyl hydrogen sulfate. Acetylation of Dissolved Cellulose Ester. A cellulose ester of low viscosity and high hydroxyl content was used i o obtain sufficient temperature rise from acetylation and to avoid temperature rise from heat of stirring. These acetylations were started a t 27.5' C. and gave a temperature rise of 5.0 O C. T h e cellulose ester contained hydroxyl groups in the 2-, 3-, and 6-positions, all of differing reactivities. T h e relative amounts of these hydroxyl groups changed during the course of the acetylation, leaving larger proportions of the less reactive ones as the reaction proceeded. This is in contrast to water and methanol where all hydroxyl groups are alike, Pseudo first-order rate constants were therefore not calculated. T h e rate of temperature rise in degrees C. per minute a t 30" C., the midpoint of the esterification, was taken as a measure of the rate of reaction, and this value was plotted us. concentration of catalyst in Figure 3A. Straight lines were obtained as before. Again sulfuric acid was far less active
HCIOi
0251
L----L-00001
0001
001
-
10
01
Molar concentration of cotaiyst
Figure 3. HzS04 i s the best catalyst for the acetylation of activated linters, B, but is surpassed by HC104 and CHz(S03H)zfor cellulose regenerated from acetate, C, and for hydroxyls (1.09 per glucose unit) of a cellulose ester in solution, A See Figure 2 for symbols
Sorption of Catalysts by Cellulose
% ' Catalyst Sorbed Ac0H:AciO Regenerated Activated celluActivated lintersC loseD lintersc AcOH"
Catalyst Perchloric Acid Sulfuric Acid Methanedisulfonic Acid Sulfoacetic Acid Methanesulfonic Acid
8 10 4 1 1
20 20 40
5 10
(2:l)b Regenerated cellulosed
1
7
60e
208
1 0.5 1
10 6 11
0.05mole catalyst/lO g. cellulose/ After 2 min. a After 30 min. 500 ml. liquid. 0.005 mole catalyst/lO g. cellulose/500 ml. liquid. e
See Table V I I I ,
A C E T Y L A T I O N O F CELLULOSE
Table VII. Distribution of Sulfuric Acid Catalyst during Acetylation
Cellulose Linters Regenerated None (control) a
In Solution, % ' CataCellulyst lose Sorbed," Sulfuric acetate % acidb sulfatec 60 20
.. .
28 18 99
12 62
...
By titration of sorption filtrate:
ference from total available. tation of Bas04 from filtrate. of filtrate minus sulfuric acid.
difBy precipiTotal acidity
ever, that the rate of temperature rise was not doubled by doubling the concentration of cellulose hydroxyls. I n a separate experiment it was shown that 25 grams of cellulose triester (isolated from previous acetylation experiments) had no effect on the rate of acetylation of 25 grams of cellulose ester, starting a t 27.5' C. with 0.05M methanesulfonic acid. Additional cellulosic hydroxyl in solution must have decreased the activity of the catalyst. Acetylation of Linters. T h e linters for this work was activated by soaking in water to provide a highly active, reproducible starting material. T h e water was then completely displaced so that it would not contribute to the heat of the reaction. T h e acetylations were started at 25' C. and the rate of temperature rise was measured a t 30 ' C. The rates of reaction a t several concentrations of each catalyst are given in Figure 3B. The amount of catalyst had to be increased tremendously-in some cases over a thousandfold (Table V)to obtain rates of reaction comparable to that of the dissolved cellulose ester. I n these acetylations sulfuric acid became the most active of the five catalysts due to its high degree of sorption by, and chemical combination with, the cellulose (Table VI). T h e other catalysts were not appreciably sorbed by the cellulose and were therefore relatively much less active than sulfuric acid. A reaction solution free from graininess was readily obtained only with sulfuric acid catalyst. This is because cellulose acetate sulfate triester has much better solubility than cellulose triacetate in the acetylating bath. Acetylation of Regenerated Cellulose. Smaller amounts of catalysts were required for regenerated cellulose, Figure 3C, than for activated linters, Figure 3B. When the degree of polymerization of native cellulose was reduced to that of the regenerated cellulose by acid hydrolysis in aqueous suspension, no change in its rate of acetylation was found.
Table VIII. Comparison of Catalysts for Acetylation Point of Comparison Relative Amounts of Various Acids for Equal pseudo first Rates of Reaction (Compared to Perchloric Acid) Rate of temp. order MethMethrise rate PeranediSulfoanesulC./min. constant chloric Sulfuric sulfonic acetic fonic
Substance Acetylated la 0.10 0.98 Water, 0.40M 0.10 1 0.29 Water, 0.12M 1 0.10 Methanol, 0.51M 1.25 Dissolved cellu1 0.25 lose ester 1 0.10 Activated linters Regenerated 0.25 1 cellulose a Comparisons valid only on horizontal rows.
... ...
...
Hence the greater reactivity of the regenerated cellulose is not d u e to its lower degree of polymerization. T h e greater sorption of catalyst by the regenerated cellulose (Table VI) would tend to promote more efficient use of the catalyst. T h e relative position of sulfuric acid is again noteworthy, since with regenerated cellulose it was no longer the most active catalyst as it was for activated linters. Rather, it assumed the same relative position as it had in the acetylation of dissolved cellulose ester. The apparent sorption of sulfuric acid from acetic acid-acetic anhydride by regenerated cellulose was much less than by activated linters (Table V I ) . However, the determination of catalyst sorption by titration of the sorption filtrate did not differentiate between free sulfuric acid and dissolved cellulose acetate sulfate. A separate determination of free sulfuric acid showed that the sorption filtrate from regenerated cellulose contained mainly dissolved cellulose acetate sulfate, whereas the sorption filtrate from activated linters contained mainly sulfuric acid (Table V I I ) . The regenerated cellulose had reacted even more rapidly than activated linters with sulfuric acid, but the cellulose acetate sulfate from regenerated cellulose was more easily soluble than that from activated linters. After the cellulose acetate sulfate had gone into solution, the combined sulfate was evenly distributed throughout the reaction mixture and not concentrated on the unreacted cellulose. Relative Position of Sulfuric Acid among t h e Catalysts in Various Acetylations. Sulfuric acid was the only catalyst which did not retain the same relative position among the catalysts as various substances were acetylated (Table V I I I ) . I n the reaction between acetic anhydride and water, sulfuric acid was as strong as perchloric acid. This was the only case where sulfuric acid remained as such under the conditions of the reaction.
1.0 1.0 3.2
1.1 1.3 1.2
17 0.36
2.8 4.5
36 32
470 200
2.9
35
330
9
4.3 4.8 3.2
9 33 27
When methanol was substituted for water, three times as much sulfuric acid was required to give the same reaction rate as perchloric acid-because of its conversion to methyl hydrogen sulfate. I n the acetylation of the hydroxyls of a cellulose ester in solution there was a much wider spread among the various catalysts; seventeen times as much sulfuric acid was required to give the same rate as perchloric acid. Sulfuric acid combined with the cellulose during the acetylation. Sulfuric acid was the best catalyst for the acetylation of activated linters, requiring only one third as much as perchloric acid. This was due to the retention of the sulfuric acid on the fibers a t the site of the acetylation (Table VI). I n the acetylation of regenerated cellulose, sulfuric acid resumed the relative position it had during the acetylation of methanol and of the hydroxyls of a cellulose ester in solution. I n those cases it was present throughout the solution in the form of methyl hydrogen sulfate or cellulose acetate hydrogen sulfate. This was also the case for regenerated cellulose, since the cellulose acetate hydrogen sulfate first formed a t the surface of the regenerated cellulose passed rapidly into solution. The anomalies in the behavior of sulfuric acid are thus readily explained and give a clearer insight into the behavior of it and of other catalysts in the acetylation of cellulose. literature Cited (1) Greathouse, L. H., Janssen, H. J., Haydell, C. H., AnaZ. Chem. 28, 357 (1956).
( 2 ) Ludwig, F. J., Adams, K. H., J. Am. Chem. Sod. 76, 3853 (1954). ( 3 ) Malm, C. J., Tanghe, L. J., Ibid., 47, 955 (1955). (4) Malm, C. J., Tanghe, L. J., Laird, B. C., IND.END.CHEM.38, 77 (1946). RECEIVED
for review October 27, 1960 ACCEPTED February 7, 1961
VOL. 53, NO. 5
M A Y 1961
367
