SYMPOSIUM ON CHEMICAL UTILIZATION OF PETROLEUM HYDROCARBONS' Direct Synthesis of Esters from Olefins
and Organic Acids
T. W. EVANS, K. R. EDLUND, AND M. D. TAYLOR Shell Development Company, Emeryville, Calif.
T
HE fact that esters can be produced from olefins and organic acids has been known for some time. Bouchardat and Lafont in 1886 treated camphene with acetic acid to secure isoborneol acetate (3); Bertram and Wahlbaum in 1894 used a sulfuric-acetic acid mixture with camphene to secure the same product (2). BBhal and Desgrez in 1892 secured heptyl and octyl acetates by heating acetic acid with heptylene and octylene in a sealed tube (1); Kondakov prepared tert-amyl acetate from trimethylethylene and acetic acid, using zinc chloride as a catalyst (11). I n recent years this general reaction has attracted considerable industrial attention, largely because of the increasing demand for esters as lacquer solvents and the realization that the olefins available from cracking processes can serve as source materials for these esters. Thus from a butene-butane fraction, butyl acetate can be prepared by reaction with acetic acid, isopropyl acetate from a propene-propane fraction, etc. The large number of patents along these lines, with more issuing continually, shows that much time has been spent on this subject by industry during the past several years. The present work comprises an investigation of this subject with particular reference to the development of a commercially feasible synthesis from the available starting materials. This has necessitateddeterminations of the equilibria involved, rates of reaction, and means of recovering the products formed. The experimental results have been restricted mainly to the use of acetic acid, since this is the cheapest and most used acid available. I n general, a catalyst is desirable for the direct addition of olefins and organic acids, and a large number of substances have been used. Of the numerous materials suggested, however, sulfuric acid is probably the most practical since i t is cheap, easily handled, and generally satisfactory from the standpoint of rate of reaction and quality of product formed. The olefins employed in the literature have ranged from ethylene through octylenes and higher. The present discussion is limited largely to the four- and five-carbon olefins, since they are readily available and yield esters of desirable boiling points.
From the butylenes two acetates are available-namely, terl-butyl acetate from isobutylene and sec-butyl acetate from 1- and 2-butenes. The amylenes similarly can provide one tert-amyl and three sec-amyl acetates, the sec-amyl acetates resembling one another so closely that they can be treated as a single compound for all practical purposes. Commercially, the secondary esters have found numerous uses, but the tertiary esters are virtually unused. Study of some of the patents in this field would lead to the conclusion that complete conversion of both olefin and acid to ester is relatively simple. Actually, the reaction reaches an equilibrium which is incomplete. Also, i t has been mentioned that, depending on the proportions of reactants taken, there may be one or two liquid phases present, and that it is desirable to have but a single liquid phase (6). The writers have found, on the contrary, that the presence of two liquid phases is desirable. The reaction time specified in various patents ranges from 2 to 72 hours (4, 9, 10). The present work describes conditions under which this figure can be reduced to 15 minutes or less. Also, new and simpler methods are described for recovering the ester and the unreacted carboxylic acid from the reaction mixture, the acid being recovered in a form suitable for immediate re-use.
Nature of Starting Materials The cheapest and most readily available lower olefins a t present are those produced in the manufacture of gasoline by cracking processes. The lighter materials from the cracking furnaces may be condensed and fractionated to give liquid products consisting essentially of isomers containing the same number of carbon atoms. Thus propene-propane, butenebutane, and higher fractions can be prepared. In the case of the three-carbon fraction only one olefin, propylene, exists. Consequently, it yields only isopropyl esters. I n the fourcarbon fraction three olefins are found, and these can give rise to sec- and tert-butyl esters. Similarly, the five-carbon cuts give rise to sec- and tert-amyl esters. The present work is concerned mainly with sec-butyl and sec-amyl acetates. The butene-butane fraction obtained by rectification of cracking gas varies in composition with the nature of the
1 Presanted before the Division of Petroleum Chemistry a t the 94th Meeting of the American Chemical Society, Rochester, N. Y . . September 6 t o 10,
1937
55
INDUSTRIAL AND ENGINEERING CHEMISTRY
56
cracking stock and process employed, and also with the degree of rectification achieved. Often, a material containing about 50 per cent butanes, 20 per cent isobutylene, and 30 per cent 1- and 2-butenes is secured. The pentene-pentane fraction also varies in composition with circumstances, but an average composition of 50 per cent pentanes, 20 per cent terl-pentenes, and 30 per cent sec-pentenes is common.
completely analogous to the method previously described for the production of ethers by the catalytic addition of alcohols to olefins (6). With this reaction scheme i t is clear that the lower phase becomes in effect a part of the apparatus, since it is recirculated continuously and does not leave the system. Consequently, most interest attaches to the composition to be ex-
TABLEI. PERCENTAGE ANALYSISOF UPPERPHASEPRODUCED IN Amyl Acetate Temp.,
" C . AmOAc
HOAc
HzSOi
SO:
Polymer
C&z
The reactivities of the tertiary and secondary olefins are far different, the tertiary being the more reactive. Therefore it is impractical to esterify both types of olefin a t one operation, since they demand different conditions. Furthermore, the tertiary esters have not proved as desirable commercially as the secondary esters. Consequently, although the tertiary esters may be prepared by direct addition of an acid to the double bond, the tertiary olefins are not esterified but are removed by conversion to other derivatives prior to esterification of the secondary olefins. This removal may be accomplished by selective polymerization, absorption in acid, or other means. Hence, in general, the starting material for the preparation of the secondary esters is essentially a mixture of paraffins and secondary olefins, the olefin content running around 35 per cent.
Preparation of sec-Butyl and -Amyl Acetates The addition of acetic acid t o an olefin is best carried out in the presence of a catalyst. A large number of catalysts are known, and, as mentioned previously, sulfuric acid is one of the best, possessing the advantages of cheapness, high activity, and ease of handling. I t has been claimed ( 4 ) that sulfuric acid is too strong a condensing agent, promoting extensive polymerization. This statement is true unless the proper conditions are employed, when there is little or no trouble in this respect. Consequently, the present paper is restricted t o the use of sulfuric acid as a catalyst. The fundamentals of the process may be understood best by considering a typical experiment. A mixture is made of 106 grams of acetic acid, 109 grams of sulfuric acid (95 per cent), and 419 grams of a pentene-pentane mixture containing 65 per cent pentanes, 2 per cent tert-pentenes, and 33 per cent sec-pentenes. This mixture is stirred under pressure in an autoclave a t 60" C. for 2 hours, then cooled and drained. Two liquid phases are obtained in this may. The upper phase amounts t o 422 grams and on analysis gives. Pentene Pentane Amyl acetate Polymers Acetic acid
9.9%
60.6 18.3 3 6 3 0
Sulfuric acid Sulfur dioxide Diamyl sulfate -I-amyl hydrogen sulfate undetermined losses
+
+
THE
PREPARATION OF AMYLAXD BUTYL ACETATES Butyl Acetate
r
CdGo
0.9%
0.03 3.7
These figures show that very little sulfuric acid is present in the upper layer, and that the amyl acetate concentration is six times the acetic acid concentration. The sulfuric acid and the greater part of the unreacted acetic acid are confined to the lower layer, which amounts to 212 grams. These facts suggest that only the upper phase be worked to recover its amyl acetate, and that the lower phase, after the addition of the small amount of sulfuric acid necessary to compensate for that removed in the upper layer, be re-used as a catalyst with fresh olefin and acetic acid ('7). This is
VOL. 30, NO. 1
Total
Temp., C. BuOAc
HOAc
HI SO^
502
Polymer
ClHO
C4Hlo
pected in the upper layer. This composition is conditioned by two equilibria, a chemical equilibrium expressed by the reaction CsHio
+ HOAC a CjHii0A~
and a physical equilibrium involving the distribution of the materials between the two phases. This latter equilibrium is reached rapidly with moderate agitation of the phases, whereas the chemical equilibrium is established comparatively slowly. Table I gives the compositions of upper layers obtained from various batch runs similar to those described above. These results are fairly close to equilibrium as judged by the fact that longer reaction time produced little change in composition, but the equilibrium value was not verified by approaching from the other side. The data in Table I illustrate the effects on the ester concentration of varying the acetic acid and olefin concentrations and temperature. Increasing either of these two concentrations tends to increase the ester. Practically, however, it is generally desirable to operate with a free acidity somewhere in the range of 0.3 t o 1.5 normal. The conversion increases only slowly with increasing acid Concentration above 1.5 normal, while the reaction rate decreases. I t the lower acid concentrations the olefin conversion is becoming too low to be attractive. By operating near the middle of this range the process is consequently more flexible than a t either extreme. The olefin concentration is not ordinarily subject to much variation, being fixed by the starting material available. An exception t o this would occur if pure olefin were available as the starting material; commercially it is usually not available. The effect of higher temperatures is t o give a less favorable equilibrium and a greater tendency for polymerization; these points must be balanced against the increase in rate of reaction with temperature. d further point of interest is the water content of the sulfuric acid used as cata1y.t or of the lower catalytic layer. It has been the writers' experience that the sulfuric acid can range from 100 per cent acid to 80 per cent or less and still be used satisfactorily. The weaker acid has the advantage that less free sulfuric acid and diamyl sulfate occur in the upper layer when it is used, whereas the stronger acid gires a greater reaction velocity. Hence in practice it becomes a question of whether the increased reaction velocity is worth the additional trouble in working up the upper layer. Data are given in Table I1 to illustrate the successful batch operation of this process, employing re-use of the catalytic lower layer. For these runs the initial lower layer was made up with 80 per cent sulfuric acid, whereas 100 per cent sulfuric acid was added in each run to replace the small amount removed in the upper layer. Each run was carried out for 1.5 hours a t 80" C. and cooled, the phase. were sepa-
INDUSTRIA4LAND ENGINEERING CHEMISTRY
JANUARY, 1938
rated, and the lower layer was reacted with fresh pentene-pentane and acetic acid. The data in Table I1 are the analyses of the upper layer from each run for free acid (almost entirely acetic acid) and amyl acetate. The initial pentenepentane for these runs contained 79 per cent pentanes and 21 per cent sec-pentenes. The forty runs show little variation in acid and ester content. This demonstrates the ease with which the reaction is controlled and the long life of the catalytic layer. I n this particular series i t was desired to use an acetic acid content of 0.6 to 0.7 normal, which was accomplished by properly adjusting the ingoing ratio of acid to hydrocarbon. A lower acidity could have been obtained by reducing the acetic acid input, or a higher acidity by increasing this input. Any change in acidity in the upper layer, however, is reflected in a change in the lower layer; consequently, once a steady state has been reached, a change in this state will change the lower layer, and provision should be made for it. OBTAINED BY RE-USE TABLE11. ANALYSESOF UPPERLAYERS OF LOWER LAYERAS CATALYST Run No. 1 2 3 4 5
6 7 8 9 10 11 12 13
14 15 16 17 18 19 20
Free Acid Normdiiy
0.584 0.696 0.676 0.705 0.690 0.715 0.690 0,642 0.700 0,715 0.655 0.616 0.600 0.636 0,610 0.645 0.616 0.605 0.586 0.589
Amyl Acetate
Run No.
Gram/cc.
0.116 0.108 0.116 0.118 0.114 0.111 0.117 0.111 0.110 0.108 0.107 0.105 0.104 0.105 0.104 0.107 0.108 0.106 0.106 0.105
21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
Free Acid
Amyl Acetate
Normalily
Gram/cc.
0.680 0.663 0.655 0.629 0.637 0.610 0.620 0.637 0.618 0.613 0.665 0.670 0.674 0.674 0.680 0.694 0.706 0.685 0.617 0.618
0.106 0.104 0.104 0.102 0.104
0.103 0.105 0.098 0.096 0.094 0.099 0.100 0.101 0.100 0.106 0.104 0.105 0.102 0.106 0.105
For the runs in Table I1 the ratio of lower to upper layer by volume a t the end of each run was about 1 to 4. Xow since this lower layer is in effect the catalyst for the process and does not leave the system, it is clear that this ratio can be varied a t will. High values of the ratio correspond to a mixer which is nearly full of lower layer and hence contains little product, whereas low values correspond t o nearly all product layer. Which choice is best in practice can be determined only by experiment, since the aim is to produce the maximum amount of ester per unit time. To investigate this point, a series of comparable batch runs was made in which only the volume ratio of the phases was changed, and the time required to reach a substantially constant ester content in the hydrocarbon phase was determined. This time requirement decreased as the proportion of lower layer increased. Thus, for one set of conditions 60 minutes were required at a ratio of lower t o upper layer of 1to 7.5,30 minutes a t a ratio of 1 to 2, and 15 minutes a t a ratio of 1 to 1. Hence, the volumes of upper layer which could be produced per hour per unit volume of reactor (neglecting time for charging and discharging) for these three ratios are 0.89, 1.33, and 2.0. Consequently, in continuous operation where time of charging and discharging does not enter, greatest throughput is achieved by maintaining a fairly large amount of lower layer in the system (6). The numerical values given obviously vary with the conditions of the experiment, such as temperature and acidity, but the same general trend has been found under a variety of circumstances. A suitable continuous reactor consists of a coil or other reaction vessel through which the phases are forced in turbulent flow, followed by a separator from which the upper layer is removed as product and the lower layer is returned
57
to the reactor. I n actual operation of such a reactor, this recycle of the lower layer has been found advantageous in line Kith the predictions from batch experiments. I n the foregoing discussion it has been assumed generally that tertiary olefins were substantially absent from the hydrocarbon feed. If this is not the case under the conditions used, these tertiary olefins are largely polymerized. (The tertiary esters are secured by using milder conditions-i. e., lower temperatures or a lower ratio of sulfuric to acetic acid.) Since these polymers are difficult to separate from the secondary esters and hence give an impure product, their presence is undesirable. For this reason the tertiary olefins are preferably removed in advance of the esterification step. It has been shown that nearly complete conversion of the olefin to ester in one step is impractical, since this calls for too high an acetic acid concentration. To secure good conversions of the o l e h , i t is consequently desirable to re-use the once-reacted material. Thus the 21 per cent pentene-pentane used in the experiments of Table I1 was the hydrocarbon recovered from esterification of an initial 33 per cent pentene material. By using a second reactor in this fashion, it is readily possible to esterify 65 per cent or more of the pentenes, depending on the starting material and the conditions under which the process is run.
Recovery of Ester and Unreacted Acid So far only the preparation of the reacted upper layer has been considered. The h a 1 step of the process is the isolation from this layer of the ester in salable form and the unreacted acid in condition for re-use. Direct distillation of the material is generally unsatisfactory because the small amount of sulfuric acid present concentrates along with the ester as the hydrocarbon is removed and promotes decomposition. It has been proposed to overcome this difficulty by reacting the sulfuric acid with solid calcium acetate before distillation; this is difficult to accomplish because it involves reacting a solid and a liquid. Washing with water will remove the sulfuric acid, but it also simultaneously removes acetic acid which is undesirable. To avoid these steps a different procedure has been devised (8). It involves contacting the upper layer with an aqueous acetic acid solution of such concentration that it is substantially a t equilibrium with the upper layer as it leaves the esterification reactor. In addition, this aqueous solution contains a metal acetate in solution. By this means the free sulfuric acid is extracted readily into the aqueous phase while practically no exchange of acetic acid takes place. There the sulfuric acid reacts with the metal acetate to give acetic acid plus metal sulfate, and the latter builds up until the solution is saturated. At this point the sulfate starts precipitating and is filtered. The final result is to replace the free sulfuric acid by an equivalent amount of acetic acid, which, by the nature of the process, is reextracted into the hydrocarbon layer. The metal acetate used may be a material such as calcium acetate or sodium acetate; the latter has the advantage that sodium sulfate filters more readily than calcium sulfate, but the calcium acetate is the cheaper. This procedure takes care of the free sulfuric acid easily and completely, and also any alkyl hydrogen sulfate that may be present. The dialkyl sulfates are not removed by this treatment. They are present in very small concentrations normally, and the presence of a little barium or calcium acetate during the subsequent distillation renders them innocuous. In this way the upper layers from the amyl acetate process can be freed of sulfuric acid and then distilled to give recovered pentene-pentane, acetic acid, and amyl acetate. With an ester like sec-butyl acetate, however, this distillation step is not satisfactory because of the difficulty in separating secbutyl acetate and acetic acid by fractionation.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Consequently, a different procedure is required in butyl acetate manufacture. The acetic acid can be water-washed from the upper layer and the ester then recovered by distillation. This is undesirable, however, for it is then necessary to concentrate the acetic acid before re-use. To avoid this difficulty, advantage is taken of the fact that in the system butyl acetate-water-acetic acid no ternary azeotrope exists, and only one binary azeotrope-that between the ester and water. Accordingly the upper layer is first freed of sulfuric acid as described above, the butene-butane is flashed off, and the remaining ester-acetic acid is fed to a distilling column. Sufficient water is present in the column so that the ester separates a t the top as the water-ester azeotrope; a t the bottom there is anhydrous acetic acid. This water-ester azeotrope splits into two phases on condensation, and only the upper ester phase is withdrawn as top product; the lower aqueous phase is totally refluxed. In this way the top product is butyl acetate saturated with water, and the bottom product is anhydrous acetic acid. The ester hydrolyzed by this process is negligible, and in any event the hydrolytic product, butyl alcohol, is acceptable and in fact present in some lacquer
VOL. 30, NO. 1
solvents. This same recovery technic is not limited to secbutyl acetate but may be applied equally well to many other esters. Finally, to produce finished product the crude ester secured by either of the means discussed is neutralized to remove the last traces of free acid and distilled. I n this way, ester of 97 to 100 per cent purity is secured, the impurities being hydrocarbons and alcohols.
Literature Cited (1) BQhalandDesgree, Compt. rend., 114,676 (1892). (2) Bertram and Wahlbaum, J. prakt. Chem., 49,l (1894). (3) Bouchardat and Lafont. Compt. rend., 102, 171 (1886). (4) Brooke, U.S. Patent 1,894,662(1933). (6) Davis and Harford, Ibid., 1,790,521(1931). (6) Edlund and Evans, Ibid., 1,968,601(1934); IND.ENO.&EM., 1186 (1936). (71 Edlund and Evans, U. S. Patent 2,006,734(1935). (8) Ibid., 2,042,218(1936). (9) Frolich and Young, Ibid., 1,877,291(1932). (IO) Isham, Ibid.. 1,929,870(1933). (11) Kondakov, J. prakt. Chem., 48,479 (1893).
28,
RECEIVED August 9,1937.
Polymerization of Propylene by Dilute Phosphoric Acid
T
HE published results of Ipatieff and his associates (7-11)
throw much light on the polymerization of olefins by strong phosphoric acid. They show that in the presence of 100 per cent acid and under proper conditions, propylene yields polymers which are largely olefinic, and which contain dimer, trimer, and other polymers of propylene. Ipatieff's isolation and identification of monoalkyl ester as a source of polymer (8) led him to the belief that, under conditions which he studied, the initial reaction involved takes place as follows: After combination of dissolved o l e h with acid to form monoalkyl ester, two molecules of ester react to yield a molecule of dimer and two of regenerated acid. On the other hand, Berthelot (1)was the first, according to Kondakov (IS), to propose an alternative mechanism, by which reaction of an ester molecule takes place not with another molecule of ester, but with one of olefin, to produce dimer:
L. A. MONROE AND E. R. GILLILAND Massachusetts Institute of Technology, Cambridge, Mass.
from 260' to 350' C., and at pressures from 170 to 410 atmospheres. The reaction was carried out in a cylindrical coper-lined steel reactor 50 cm. long, having a volume of 675 ml. fn each run the acid catalyst was charged first, the usual charge being 250 ml. of the liquid, measured cold. After adjustment of temperature, propylene was admitted to raise the vessel's contents rapidly to the desired pressure, and more propylene was added intermittently through the experiment to maintain constant pressure. The reactor was held at high temperature by an electric heating coil wound externally, and its temperature was controlled within about 5" C. of the desired value by manual regulation of the flow of cold water through a small copper cooling coil inside the reactor. Temperatures were read by means of a copper-constantan thermocouple, silver-soldered into the reactor so as to extend t o its center and standardized against the vapor pressure of water by means of an accurately calibrated Bourdon gage. The reactor was agitated by an external rocking
Whitmore (19) proposed a third mechanism involving the direct catalytic action of hydrogen ion rather than of molecular acid. During his work on the propylene-isopropyl alcohol equilibrium, Majewski (16) observed the formation of polymers from propylene in the presence of very dilute acids, and showed that these polymers were similar in nature to others previously described (6).
Experimental Procedure Propylene was polymerized in the presence of dilute phosphoric acid at concentrations from 10 to 50 per cent by weight, at temperatures
FIQURE1. DIAGRAM OF APPARATUS
