ESTERIFICATION I N PRESENCE OF SILICA GEL' BY C. H. MILLIGAN, J. T. CHAPPELL AND E. EMMET REID
It is well known that esterification in the vapor phase is catalyzed by certain oxides. Sabatier and Mailhe2 studied the effect of titania and thoria on a mixture of equivalent amounts acetic acid and ethyl alcohol and, in the most favorable cases, obtained 67% esterification which was very close to Menschutkin's value3 of 66.7. It has been assumed by Sabatier and Mailhe4 that the limit in gas phase reached in the presence of these catalytic oxides would be the same as that in the liquid phase. They found about 70% esterification at 300' and attributed the slightly higher value to the higher temperature since Berthelot had found 65 .6y0at 100' and 67.370 a t 200'. This view seems to have been coramonly accepted, particularly as no measurements are on record of the equilibrium in the gas phase. Berthelot and PBan de Saint-Gilles5made several attempts t o measure the equilibrium in the gaseous phase but did not succeed on account of the extreme slowness of the reaction. I n parallel experiments in which equivalent amounts of alcohol and acetic acid were heated IO hours a t 200' there was 65.270 esterification in the case in which the volume of the tube was 2.6 cc. per gram of mixture and only 10.0% where this volume was 1351 cc. I n another experiment in which the volume was 1562 cc. the esterification was only 49.0% after 458 hours at 200'. Starting from the other end, they heated ethyl acetate hydrolysis in 0.5 hour with 2 equivalents of water at 200' and found 11.57~ where the volume was 2.3 cc per gram but with the volume 476 cc. the decomposition of the ester was "insensible" in 1 4 2 hours. These experiments demonstrate the extreme slowness of the reaction in the gaseous phase and, by contrast, serve well to show the enormous accelerating influence of the catalytic oxides. They ran two series of experiments6 in which the volume per gram of mixture was varied and with equivalent amounts of ethyl alcohol and acetic acid at 200' obtained the following results: Time hours Vol t o I g. yo Esterification 22 66.4 5.4 37 * o 72.3 77 77 53 * o 76 .o IO
2.6
65.2
22
5.3
22
8.3
22
21.2
66.4 66.8 7I.7
22
24.4
72-9
Contribution from the Chemical Laboratory of Johns Ho kins University. * Sabatier and Mailhe: Compt. rend. 152, 358, 494,669 a n 1044 (1911). 'Ann. 195, 334 (1879)~ Ann. chim. phys. (5)20, 289; 23, 64 (1880). 4 "Catalysis in Organic Chemistry,' p. 277 (1922). 8Ann. chim. phys. [3]66,54 (1862). 6 Ann. chim. phys. [3]66,59 (1862); 68,239 (1863). 1
B
ESTERIFICATION IN PRESENCE O F SILICA GEL
873
They conclude (p. 243) that increase of volume favors the combination of acids and alcohols and enfeebles the inverse action of water on the esters. It appears then, that if the limit of esterification could be measured in the gaseous phase, it would be found to be high, apparently very much higher than the 66.5% found for the liquid phase. With silica gel as catalyst we have obtained esterification up to 8g.6%, the percentage depending on the time of contact, but have no reason to Lhink that equilibrium has been reached. In this system in the liquid phase, esterification goes on 4 times as rapidly as saponification, in the vapor phase this ratio is apparently much higher. The 8 9 . 6 % esterification which we have reached corresponds to a ratio of 74 and the true ratio is probably higher yet. Recently Edgar1 has measured the limit for ethyl alcohol and acetic acid by a distillation method and his preliminary results indicate a limit well above go%. Perhaps the excellent yields of esters obtained by Bodroux2 and the remarkable results obtained in plant processes for ethyl acetate from very dilute solutions of acetic acid as described by Whitaker3 may be connected with the more favorable location of the equilibrium point in the vapor phase. We find silica gel to be more active in esterification than either thoria or titania is said to be. The Present Investigation The experiments in Tables I and I1 were carried out by C. H. Milligan in 1921and reported a t the Rochester meeting of the Society; those in the later
tables are by J. T. Chappell, using the same furnace and same methods but with different materials and standard solutions and working a year later. Apparatus and Methods. The catalyst tube was pyrex glass 18 mm. inside diameter. The catalyst, IOO g. of silica gel,4 occupied 450 mm. of this tube of which the portion in the furnace was 570 mm., the vacant space being nearly all a t the forward end to allow the gases to come to temperature. This end of the tube projected IOO mm. out of the furnace portion being surrounded with asbestos and heated by an extra coil wire, thus serving as a volatilizing chamber for the liquid mixture. The electric tube furnace was automatically controlled and kept the desired temperature within &IO for a week a t a time. The mixture of exactly equivalent amounts of pure acetic acid and absolute alcohol was admitted through a needle valve dropping device which could be regulated to admit the liquid at so many drops per minute. By weighing a number of the drops the rate could be determined. All rates are stated in number of cubic centimeters of mixed vapors at the temperature of the furnace. By turning the regulating screw this rate could be set from 4 drops to As the free 160 per minute or from 40 cc. to 1600cc. of vapors per minute, space in the catalyst chamber was 82 cc. the time each molecule was in this space can be calculated. The amount of esterification was determined by Professor Edgar has kindly let us see some of his results in advance of publication. See J. Am. Chem. SOC.,46,64 (1924). * Compt. rend. 156, 1079 (1913);157, 1428 (1913). 3Chem. Met. Eng., 28, 108 (1923). For which we are indebted to Professor Patrick.
C. H. MILLIGAN, J. T. CHAPPELL, E. EMMET R E X
874
titrating the free acid in a weighed sample of the condensate. To check the figures thus obtained, the ester was determined in a number of samples by saponification. The values obtained in the two ways checked very closely. I n Table I are given a number of analyses of successive samples of the condensate when the furnace was set a t different temperatures and the rate of the vapors varied.
TABLE I No.
Temp.
Rate cc. per min.
Time in Furnace sec.
Percent Esterification
89.6 82
I 2
3 4
85 85 87
5 6
68.7 43
7 8
77.5
9
72.5
IO
74.0 76 .o 76 .o 76 .o 76.4 69 .o 71 .o 64.0 64.6 69.5 70 .o 57.5 60.7
I1 I2
I3
I4 I5
16 17
18 I9
I3 1,
20 21 22
8 21
It appears that the percentage of ester formed at a given temperature is closely related to the rate a t which the mixture is passed over the catalyst. To account for the high percentages of esterification obtained our first supposition was that water was being retained by the gel, the equilibrium being shifted according to the mass law by diminishing the concentration of that constituent. For 66y0 esterification the partial pressure of the water vapor would be 2 53 mm. and it was thought that the gel might absorb a considerable amount of water a t this concentration. Quantitative experiments were made to determine the amount of water taken up by the gel, a t several temperatures, from air containing this amount of water vapor. A weighed sample of silica gel was heated in a U-tube in an oil bath to the desired temperature and air which had bubbled through water a t 7 2 ' was drawn over it. This was continued to constant weight.
ESTERIFICATION IN PRESENCE O F SILICA GEL
875
The amount of water retained by silica gel a t 253 mm. partial pressure is: 9.27%; a t 200' 7.25%; a t 250' 5.38%. At 150' The sample of gel used was the commercial gel and had become saturated with moisture under usual atmospheric conditions and contained 17.95% of water. When heated up to the temperature a t which it was used, a part of this water was given off. When it was used a t a higher temperature and then a t a lower, some water would, of course, be taken up and might account for the high esterification figures, a t least till it had acquired the proper amount of water. I n order to test this a run was made in which the mixture of acid and alcohol was passed continuously for 108hours a t 150' in which time 420 g. of the mixture passed over the gel and about 50 g. of water was set free, or 5 times the total water content of the lot of gel a t 150'. During this run the rate was varied from time t o time and samples taken at the different rates, but not till the rate had remained constant long enough for conditions to become constant. The results are given in the table below:
TABLE I1 Esterification a t No. I 2
3 4 5 6 7 8 9
Rate 9c. per min.
82 82 82
82
Time in furnace sec.
60 60 60 60 123 60 44 60 60 60
150'
Amount that had passed through, g.
Percent of esterification
76.5 76.6 74,2 75.0
85 .o 82 74 IO0 65.5 82 74 82 72 IO 82 79 *6 I1 89.6 40 12 82 70 I3 83 50 82 I4 75.2 I 20 60 I5 Most of the samples were small but No. 1 2 and 14 were each from IOO g. of the condensate. It appears from this long run that when the rate is the same, the percentage of esterification is nearly the same. At very slow rates the percent of esterification is very high, even approaching 90%. On account of the length of the run and the large amount of water involved the excess of esterification cannot be accounted for by supposing retention of water by the gel. The high values are all the more remarkable when the water content of the gel is considered. If we think of the alcohol and acid as being in solution in the water in the gel we must have a rather dilute solution, even taking into account the displacement of a part of the water by alcohol and 40
C. H. MILLIGAN, J. T. CHAPPELL, E. EMMET REID
876
acid. Under constant conditions the amount of water in the gel is fixed, yet it is the amount of the water and not the individual molecules of water that is fixed. They are free to go and come and to react as other water molecules.
TABLE I11 Esterification at various temperatures. NO.
Rate, cc.
I
4
84 56 89 I03
2
3
5
58
6 7 8 9
89 62 roo 119
IO
103
I1
136
% Esterification
Time, sec.
57
44.5
86 54 47 82 54
73.9 57.8 51.3
78.5 71.3
78 48
76.4 54.9 70.6 68.6 63 .o
40 46 35
TABLE IV Esterification at I75' Rate, cc.
Time, sec.
yo Esterification
I
56
2
79 89 93
86 61 54
61.o
51
52.7
No.
3 4
73 * 8
57
5
103
47
51.3
6 7
107
45 43
47.9 48.4
I12
TABLE V Esterification a t NO.
Rate, cc.
200'
Time, sec.
I
4.9
975
2
20
2 44
3 4
35 38 44 49 54 59 88 I I8 138 I97
I39
5
6
7 8 9 IO I1 12
12.5
108 99 87
81 54 41 35 24
yoEsterification
.
84.4 81.3 80.4 80.6 79.1
77.3 77.4 77.8 71.3 64.8 53 * 7 39.5
877
ESTERIFICATION IN PRESENCE O F SILICA GEL
At 200' there was a slight diminution of activity of the gel with time as is shown by two runs a t two constant rates: TABLE VI Esterification and Time of use. Time, days 0 7 9 I4 I5 80.7 79.4 78.3 Rate, 38 cc. 83.9 __ Rate, 44 cc. 81.o 79.7 76.6 In a given run when conditions are kept constant the percentage of esterification is nearly, but not quite constant as is shown by the following run. TABLE VI1 Variation of Esterification Temp. 175'
Rate 107 cc.
Time, min. yo Esterification
52.8
289 297 362 369 377 449 456 464 540 547 5 2 $ 4 52 .I 52 . I 51 .g 51.5 51.2 51.5 51.2 53.3
552 51.2
In the figure the percentage of esterification is plotted against the time the vapors remained in the furnace. The numerals on the curves refer to the tables -o--..zp' from which the figures are taken. From this it is apparent that high 20 40 80 120 160 zoo nrzf /A/ C,4T/1LYSr TUDE -5€CdAJN temperature makes for rapid esterificaFIG.I tion; that the percentage of esterification Percentage Esterification Against increases with the time up t o a certain Time in Catalvst Tube point where it becomes nearly constant as if a limit were being approached but this apparent limit is lower for higher temperatures, From observations of Brown and Reid1 only 16% of ethyl alcohol is decomposed by silica gel a t 360°, when passes at a comparable rate, from which it is probable that it would break up very slightly at ~oo', or even a t 250'. It is possible that the ethyl acetate may be less stable in the presence of silica gel and may decompose with the re-formation of acetic acid. In a g hour run, the products of collected weighed 473. and the reactants put in, 52 g., the loss being too great t o be accounted for by evaporation. In both series of experiments commercial silica gel was used but the first sample, Tables I and 11, appears to have been considerably more active than the second. After 2 7 days of use the gel was quite black. One run was made at 200' in which 2 equivalents of alcohol were used to I of acid. The results are given in Table VIII. TABLE VI11 Esterification a t 200°, 2 equiv.. alc. Time in see. 62 49 39 7 0 Esterification 89.g 82.5 75.5 do for I:I 77.6 74.7 71.5 ,I
Paper t o appear in a subsequent number of this Journal.
878
C. H. MILLIGAN, J. T. CHAPPELL, E. EMMET REID
The figures in the last line are taken from the curve for 200' in Figure The excess of alcohol speeds up the esterification considerably.
I.
Comparison of Catalysts Taking the results of Sabatier and Mailhel we may make a rough comparison of silica gel with their catalysts. They ran 0 . 2 mol of the mixture per hour which would mean 236 cc. of vapor per minute at 150'. Their results and ours are brought together in table below.
TABLE IX Temp.
Comparison of Catalysts. % Esterification
Rate
I SO0
236
I 70'
248
230'
277
2 50'
290
Thoria
cc.
Titania
I1
20
26
-
45
-
Silica Gel
60
-
Summary Silica gel has been found to be an excellent catalyst for esterification, better than thoria or titania. The percentage of esterification attained depends on There rate of passage of the vapors, being about 90% for slow rates a t 150'. is no indication that the limit has been reached but this limit must be far higher in the vapor phase than in the liquid. 1 Sabatier and Mailhe as quoted by Sabatier: "Catalysis in Organic Chemistry," 278 ( 1 9 2 2 ) .
Baltimore, M d .
