INDUSTRIAL AND ENGINEERING CHEMISTRY
58
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 This water-ester azeotrope splits into two phases on condensation, and only the upper ester phase is withdrawn as top product; the lower aqueous phase is totally refluxed. I n 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., 28, 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). 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 a n alternative mechanism, by which reaction of a n 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 t h a t 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
JANUARY, 1938
device, ordinarily at 40 cycles per minute, the reactor swinging lengthwise twice per cycle through about a 20" arc of a circle of o n e - f o o t radius. A diagram of the apparat u s w i t h major items roughly to scale is given in Figure 1. The phosphoric acid used as catalyst was of c . P. grade. Propylene was p r e p a r e d by dehydration of c o m m e r c i a 1 constant-boiling isopropyl alcohol over alumina at about 450" C. As fed t o the reactor this material was about 99 per cent unsaturated and had an average molecular weight of 42.7. At the completion of each polymerization e x p e r i m e n t t h e h o t reactor was tilted outlet end down, and its c o n t e n t s were released rapidly through an ice-cooled coil to receivers. During this process the aqueous catalyst c o n t a i n i n g little dissolved gas appeared first in the receiver, giving evidence that in t h e r e a c t o r i t c o n s t i t u t e d the heaviest' phase present. After removal of this material, an apparently uniform mixture of propylene, polymer, and some aqueous layer appeared until the reactor pressure had dropped to atmospheric. Aqueous layer and liquid polymer were collected in a graduated glass receiver in an ice bath; uncondensed gas passed from this receiver to a gas holder. After separating and neighing the acid and liquid p o l y m e r layers, one sample of the former was titrated with standard caustic and another analyzed for isopropyl alcohol by the method of Ponndorf (18). A 30-ml. sample of the cold polyrier was fractionated in a 3-fOOt (91.4-cm.) laboratory column consisting of 6-rnm. glass tubing containing a spiral of 22-gage piano wire, and the distillate was analyzed for isopropyl alcohol. Gases initially dissolved in this polymer sample were collected in an aspirator bottle at the beginning of the distillation; during this period a slight vacuum was maintained in the apparatus to prevent outward 1e a k a g e . These gases were later analyzed in an Orsat apparatus for unsaturates and for air w h i c h h a d e n t e r e d the column d u r i n g c o l l e c t i o n of the gases. Their average m o 1 e c u 1a r weight was taken by weighing in a glass bulb, to permit calculation as dimer
IXDUSTRIAL AND ENGINEERING CHEMISTRY
propylene, as well as a certain amount of heavier material. I n addition, an intermediate fraction appears between dimer and trimer, which careful redistillation (TableI) shows to consist largely of compounds boiling between the two latter. Ingeneral, as illustrated by Figure 2, comparatively s m a 11 amounts of such intermediate-boiling compounds occur in polymers made at 275' C. and lower; they become somewhat more noticeable a t 305" C.; and at higher temperatures (5) they compose an important part of the polymers. For simplicity, d i s t i l l a t i o n analyses are interpreted below in terms of so-called dimer, trimer, and tetramer fractions only. The dimer fraction is taken as t h a t portion of the polymers boiling up t o 88" C.; the trimer, as that boiling from 88" to 150" C.; and the tetramer, or residue, as all that boiling above 150" C. The tetramer fraction so defined contains material less volatile than tetramer; but t h a t the average molecular weight of this fraction closelyapproximates t h a t of its main constituent, tetramer, is shown by Table I. Figure 3 is a plot of polymer composition against the corresponding extent of polymerization of the feed, and is made u p of points for runs at 260-305" C., 170-410 atmospheres, and 10-30 per cent H3P04. Study of Figure 3 brings out that within this favorable operating range the only operating variable which appreciably affects the composition of the polymers is the extent t o which the feed is polymerized in making them. At constant percentage reaction of the feed under these conditions, temperature, p r e s s u r e , and acid catalyst concentration have no effect on product composition detectable by the analytical methods used. At low-percentage pol~ymerizationof the feed, the polymers a-ill be seen t o consist of nearly pure dimer fraction; at 50 per cent polymerization two-thirds of the total is dimer fraction; and even when the feed is almost completely polymerized, this fraction amounts to 3 6 3 0 per rent of the total polymer. T h e significance of these facts is twofold: (1) They show t h a t one can control t h e volatility of polymers from normal olefins through a wide range by varying the percentage polymerization of t h e feed-i. e., in continuous operation by varying t h e recycle ratio. (2) They contrast strongly with the results (7, 8) of using 100 per cent phosphoric acid, which &ould seem t o indicate t h a t relatively little dimer formation from propylene occurs with this catalyst, trimer being the predominant product. These low dimer yields with 100 per cent acid cannot have resulted from use of excessively long times of reaction, for in t h e cases cited the data show that only 50 t o 80 per cent of the gaseous olefinic reactant initially in t h e feed was polymerized. T h e conclusion seems unavoidable that,
Propylene is polymerized catalytically by dilute phosphoric acid at temperatures above 250" C. and at pressures of 150 atmospheres or higher. The polymerization is stepwise. The first product is dimer; this in turn reacts with more propylene to form trimer, and the latter compound then combines with more propylene to form tetramer. Somewhat above 300" C., and also at concentrations of HBPOl over about 30 per cent, the character of the polymer starts to change, an excess of heavier compounds forming at the expense of the yield of dimer. Below these limits, however, the composition of the products depends solely on the extent of the polymerization of the feed and varies from practically pure dimer initially to about 35 per cent dimer at nearly complete reaction. In this range the rate of dimer f o r m a t i o n from propylene is closely proportional to the square of the reactant concentration in the gas phase above the acid catalyst, as well as to the volume and concentration cf the latter.
analysis was carried out' on the uncondensed gas from the reactor. Combined polymers from numerous experiments were washed with water to remove alcohol, dried over calcium chloride, and distilled in a laboratory fractionating column equivalent to about ten theoretical plates, but with rather high liquid hold-up. The cut between dimer and trimer was redistilled and broken up into narrower fractions in the small column previously described. Of the several fractions so ohtained, densities, refractive indices, and bromine numbers by the method of McIlhiney (23, 14, 15) were measured. In certain cases blending values, lead susceptibilities, ultimate analyses, and molecular weights by the freezing point method in benzene were determined. Blank runs showed that a negligible quantity of t,hermal polymer is formed in the absence of acid, and at temperatures, propylene pressures, and reaction times in the ranges here discussed.
Character of Polymers As shown in Figure 2 b y laboratory distillation curves of crude polymer samples, dilute-acid polymers made under the conditions here described contain large proportions of sixand nine-carbon compounds-i. e., the dimer and trimer of
59
INDUSTRIAL AND ENGINEERING CHEMISTRY
60
VOL. 30, NO. 1
bromine addition numbers showing them to be roughly one-fourth saturated, are all almost idenMcIlhiney tical with those of the olefinic hydrocarbons of Bromine corresponding boiling point and are near those of Nos." Cent Den- Refractive sp. Mol. Per the corresponding paraffins, but are quite far reSubBoiling by sity, Index Refrac- Refrac- Addistitu- Mol. moved from those of the naphthenes and aromatFraction Range, C. Vol. d:: n%' tion tion tion tion Weight ics of the same boiling point. The indication Dimer 52-72 4 0 . 5 0.682 1 . 3 9 5 7 0.3526 29.6C 155d 8 86 Cut 1 69-78 3 . 0 0.698 1.4000 0.347 ,., 132 10 . . is that the saturated materials present are largely 2 78-85 2 . 0 0.705 1.4010 0.344 .., 128 13 , . paraffinic, but that they may contain a certain 3 85-90 1 . 6 0.709 1.4056 0.347 . ., 130 15 , . 45 90-109 34 .. 09 0 11 .. 44 11 17 20 00 .. 33 44 1 118 17 proportion of members of other series. Such a 109-119 0 .. 7 72 33 5 2 ., .. ,. 110 19 .. 6 119-128 2 . 6 0.739 1 . 4 2 0 0 0.342 96 25 . conclusion is in general agreement with analyti7 128-130 3 . 8 0.739 1.4225 0,344 .. .. ., 94 29 .. cal results of Komarevsky (7) on the saturated 8 130-134+ 5 . 6 0.741 1.4250 0.345 91 29 43.'3j 92g 29 125 portion of a propylene polymer made-with 100 per Trimer 132-138 15.5 0.747 1.4272 0.344e Residue 13813.6 0.790 1.4463 0.339 ... 44 38 167 Loss ..... 4.0 cent phosphoric acid a t 204' C. and 50 atmosa I n centigrams of bromine introduced i n t o t h e molecule per gram of sample. pheres. b Theoretical for hexylene = 0.349. Theoretical for hexylene = 29.3. Table I11 (6) lists blending values of polymers d Theoretical for hexylene = 191. in an aviation gasoline base fuel of 75 octane Theoretical for nonylene = 0.342. I Theoretical for nonylene = 43.1. number, as well as the results of addition of lead. I Theoretical for nonylene = 127. Blending values are roughly equal to those of other olefinic hvdrocarbons. but. as would be expected, the lead susceptibility of such blends ';s poor. as the concentration of a phosphoric acid polymerization catalyst is increased, the nature of its catalytic action changes in such a way as to cause marked decrease in the amount of TABLE 11. CLTIYATE AN.4LYSES O F POLYMER FRACTIONS dimer formed and corresponding marked increase in the Dimer Intermediate Trimer amount of heavier polymers produced. Within their preciElement Fraction Fraction Fraction Residue sion the present data give evidence of no such change up to a 83.94 83.03 85.22 C 84.08 13.79 13.81 14.26 H 14,50 Concentration of 30 per cent; but analyses of polymers from a 0.03 0.01 0.01 0.01 s (2 2.24 3.15 0.51 1.41 0 (by difference) few experiments with 40 and 50 per cent acid (runs 109, 121, Atomic ratio H / C 2.05 1.98 1.99 1.96 128) 'show dimer contents 10 to 20 per cent lower, a n d Rates of Reaction trimer contents correspondingly The curves of Figure 5 show typical relations between higher, than would amount of the several polymer fractions formed (as defined be read from t h e above) and time a t constant pressure, temperature, and acid curves of Figure 3 concentration. I n each run plotted, the 675-ml. reactor was a t the same values of t h e percentage DATAON PROPYLENE POLYMERS (6) TABLE 111. BLEXDING polymerization of Calcd. Octane No. t h e f e e d . It apClear C . F. R. Blending a i t h 3 M1. pears, t h e ref o r e , Blend Octane No. Value Lead that for p r a c t i c a l Base fuel 75.0 88.5 1 0 7 L. B.a dimer 76.1 ss:o 87.8 purposes the point 25 L. B. dimer 77.2 83.8 87.5 75.5 80.0 87.4 where this change 1 0 g . H . B.bdimer 84.8 25 H, B. dimer 76.1 79.4 in catalytic action 1 0 % trimer 88.0 25!Z trimer 7e:5 si.'o 88.0 begins lies a t an acid a Lowboil& 58 4-67.0° C. concentration beb High-boilin;, 6?.0-73.0° C. tween 30 and 40 per cent. A similar contrast IO0 charged with 250 ml. of liquid acid catawith the data shown 80 lyst (approximately in Figure 3 results VOLUME PER CENT DISTILLED 25 per cent H~POI), f r o m a n a l y s i s of LT w measured. a t room FIGURE2. TYPICAL DISTILLATION propylene polymers 5 60 CURVES OF CRUDE POLYMERS t e m p e r a t u r e . At (5)made with dilute P o p e r a t i n g condiacid but a t 325'or I 40 tions the reactor was 350" C. I n this case, however, the decrease in dimer content thus about half full of the polymers appears to be counterbalanced by an increase of liquid acid phase in their content, not of trimer but of heavier compounds. I40 each time. Figure 3 Tables I and I1 give physical and chemical data on the sev= 6 gives a rough idea eral individual fractions of a large sample of propylene poly20 of the effect of acid mers all made with acid concentrations not over 30 per cent, concentration, presand a t temperatures of 305" C. or lower. The presence of 0 2o 40 60 80 100 sure, and temperasaturated hydrocarbons, shown by the bromine numbers ture o n t h e r a t e , WEIGHT PER CENT OF FEED POLYMERIZED tabulated here, is analogous to that reported by Ipatieff (8) in w h e n t h e reactor strong phosphoric acid polymer. It is noteworthy, however FIGURE 3. VARIATIONOF POLYMER was described. charged as just (Figure 4),that the densities, molecular weights, and molecuc~MP~SITI~ WITH N EXTENT O F POLYMERIZATION OF FEED lar refractions of close-cut dimer and trimer fractions, with TABLEI.
PHYSICAL AND CHEMIC.4L PROPERTIES O F POLYMER FRACTIONS
7
O
,
, ,
C
8
JANUARY, 1938
INDUSTRIAL A S D ENGINEERING CHEMISTRY
61
POLYMERZATIONOF PROPYLENE
IN 6 7 5 M L REACTOR
w
BOILING FOINT,'C
FIGURE 4. SPECIFICGRAVITY us. ROILIXG POINT FOR HYDROCARBONS
YJ
The low initial slopes of the J'TETRAMCRtrimer and tetramer fraction us. 0 I 2 0 time curves in Figure 5 and elseT I M E . HOURS where show clearly that, in comFIGURE 5. EFFECT OF TIMEox parison with the rate of dimer POLYMERIZATIOS formation, the rate of formation of these higher polymers directly from propylene-is extremely low. It follows that the predominant reaction involved in producing trimer is the combination of dimer with propylene.
1
This point xas checked by an experiment in which a sample of dimer (boiling range, 58.4' to 73.0" C.) was agitated with 350 ml. (measured cold) of 25 per cent phosphoric acid in the usual reactor, at 305" C. and 272 atmospheres for 0.75 hour. The product of this run, in addition to showing 2 or 3 per cent distilling below the initial boiling point of the dimer used as feed stock, included only about 20 per cent of material boiling above the end point of the feed. This heavy portion contained both trimer and tetramer, as well as material of intermediate and higher boiling points. Comparison with data on runs made with propylene feed shows that, in the latter, polymerization of dimer alone without intervention of propylene must have been relatively inconsequential; at most 15 or 20 per cent of the trimer present was produced in this way. A similar run with close-cut trimer as feed, made under the same conditions as that with .dimer feed but for 3 hours, resulted in a product containing around 7 per cent boiling lower than the initial of the feed, and only about 16 r)er cent above its end point. The conclusion here is that direct ormation of tetramer from either dimer or trimer alone is very slow, and that in ordinary runs the tetramer which constitutes most of the polymer boiling higher than trimer must have resulted from reaction of trimer with propylene. I n runs with propylene as feed and a catalyst of low acid concentration, the data show the isopropyl alcohol formed to
I
0
1.0
2.0
3.0
TIME. HOURS
FIGURE6. EFFECTO F OPERATING L'ARIABLES POLYMERIZATIOX
ON
reach a n apparent equilibrium with propylene in a third or less time than that required for the polymerization of a n equivalent amount of propylene. Since formation of either alcohol or polymer requires diffusion of propylene from gas to liquid phase, it follows that such diffusion is considerably faster than the observed polymerization rate. Coupled with the fact that alcohol formation may even be controlled by chemical reaction and not by diffusion, this forces one to conclude that, for polymerization under the present conditions, the diffusional process involved is relatively rapid compared to the chemical reaction. This conclusion is confirmed by the fact that 100 per cent variation in the speed of agitation of the reactor shows no measurable effect on the rate of the reaction within it. It follows from this rapidity of diffusion that, essentially, equilibrium is maintained between the dissolved propylene in the liquid phase and that in the gas. Study of the present data from the standpoint of specific reaction rate requires operations such as differentiation, which magnify experimental errors acceptable in measurement of
TABLE IV. SUMMARIZED DATAAND RESULTS R u n No Pressure, a t m . C Temp Time,'hours H a p o i present, grams Cold acid out, ml.
v.,
ml.
ec.2 va/ v g
CsHs found, gram moles Dimer fraction, gram moles Trimer fraction, gram moles Tetramer fraction, gram moles D (sum of 3 preceding Items) I~o-CJHIOH found, gram moles k X 10-4 R u n No. Pressure, a t m . Temp., C. Time, hours H3P04 present, grains Cold acid out, ml. V., ml. BCaVo/ Vg CaHs found, gram moles Dimer fraction, gram moles Trimer fraction. gram moles Tetramer fraction, gram moles D (sum of 3 preceding items) I s o - C ~ H I O Hfound, gram moles k X 10-4
123
116
110
106
109
121
128
E7
E5
115
124
272 275 0.50 34.6 272 346 0.58 1,74 0.23 0.00 0.00 0.23 1.04 4.16
122 272 275 1.0 31.7 280 356 1.01 1.68 0.31 0 02 0.00 0.34 0.99 3.16
105
204 305 0.50 69.6 261 361 1.12 0.71 0.33 0.13 0.02 0.47 0.16 16.4
108 272 275 1 0 26 8 277 352 0 83 1 55 0 36 0 01 0 00 0 37 1 04 4 75
104
177 275 1.0 26.0 267 347 1.04 1.20 0.19 0.01 0.00 0.20 0.60 5.23
272 275 0.25 71.5 277 352 0.56 1.55 0.26 0.02 0.00 0.28 0.59 5.62
272 275 0.50 73.0 279 354 1.14 1.51 0.43 0.05 0.00 0.47 0.71 4.34
272 275 0.75 72.0 271 344 .1.65 1.37 0.48 0.11 0.00 0.59 0.69 4.42
272 275 0.25 133 272 346 1 02 1 72 0 33 0 09 0 00 0 42 0 44 3 64
272 275 0.50 120 267 340 1.82 1.47 0.47 0.17 0 02 0 66 0 47 3.86
272 275 0.50 166 255 324 2.37 1.49 0.42 0 25 0 05 0 72 0 23 3.18
272 275 2.0 62 252 320 3.53 1.23 0.67 0.25 0.01 0.93 0.15 3.48
272 275 3.25 63 254 322 5.87 0.56 0.75 0.41 0.05 1.21 0.19 7.27
272 305 0.50 68.5 255 324
0.74 0.57 0.25 0.03 0.84 0.17 25.3
408 260 1.0 23.4 291 352 0.69 1.96 0.38 0.00 0.00 0.38 1.88 3.59
120
111
125
117
113
118
114
112
126
127
119
E3
E2
E4
E6
408 275 1.0 28.2 290 362 0.91 1.80 0.42 0.04 0.00 0.46 1,35 3.82
408 275 0.50 65.4 265 33 1 0.96 1.71 0.64 0.09 0 00 0 73 0 87 6.19
408 275 1.0 31.6 185 232 0.72 2.21 0.70 0.13 0.01 0.84
408 305 0.25 26 269 355 0 21 1 73 0 31 0 05 0 01 0 37 0 76 15 8
408 305 0 50 25 7 270 356 0 45 1 39 0 53 0 14 0 02 0 68 0 36 18 5
408 305 1.0 26 271 358 0.83 1 10 0 65 0 22 0 04 0 91 0 21 15 7
408 305 0.25 72.5 273 360 0.58 1.29 0 61 0.23 0.04 0.87 0.34 16.7
408 305 0 50 71 0 250 330 1.04 0 82 0 75 0 39 0.08 1.22 0 16 24 0
408 305 0.50 68 7 265 350 1.07 1 00 0.68 0.32 0 04 1.03 0.27 15.2
408 305 0.50 70.0 263 348 1.08 1.02 0.62 0.32 0.06 1.00 0.27 12.7
408 305 0.75 70.5 263 34s 1.64 0.54 0.77 0.46 0.10 1.32 0.08 26.1
408 305 1.00 63 0 250 330 1.84 0 41 0.71 0.52 0.09 1 32 0 09 34 5
408 305 1.25 56 207 273 1.76 0.52 0.83 0.57 0.11 1.51 0.09 32.4
408 305 1.42 64 248 328 2.64 0.41 0.76 0 59 0.11 1.46 0.11 24.9
408 305 1.47 66 254 335 2.87 0.39 0.75 0.60 0.13 1.48 0.04 23.5
1,oo
7.55
0.98
INDUSTRIAL AKD ENGINEERING CHEMISTRY
62
absolute quantities. Further, the data cover an undesirably narrow range of operating variables. Kevertheless, as a whole they exhibit regularities which are felt to justify application of the usual concepts of reaction rate to their interpretation. I n this connection only the polymerization of propylene to its dimer will be considered, for the smaller propor-
VOL. 30, NO. 1
power of the propylene concentration in the same layer. Since this last quantity is not obtainable from the data at hand, it must be assumed proportional to the propylene concentration, P/V,, in the gas phase, which has already been shown to approach equilibrium with the liquid phase, as far as propylene concentration is concerned. The rate equation for formation of dimer from propylene is thus:
The exponents m and n will be taken as integers, on the assumption that any slight departure found from the best integral value of either results from experimental error or from failure of the materials involved to behave as perfect solutes. The value of exponent m, which expresses the effect of acid concentration on rate of polymerization, is determined most simply by plotting the molal concentrations of reactants and products, ( P / V , ) , ( D / V g ) ,etc., found in runs a t constant temperature and total pressure, against corresponding values of the quantity Corn( V J V,) e, trying various possible values of m until one is found which correlates the data satisfactorily. Figure 7 illustrates typical results of this procedure. This type of figure shows that the data of Table I V are well correlated by the assumption of m equal to one. I n other words, the rate of polymerization is closely proportional to the first power of the concentration of the dilute phosphoric acid catalyst in which it occurs.
'0
IO
20
30
40
50
60
FIGURE 7. DIMERFRACTION FORMED PER UNIT VOLUME US. eC,mv,/v, tions of higher hydrocarbons in the reaction products described prevent the obtaining of the minimum accuracy in analysis required for effective study of their rates of formation. Likewise, polymerization of higher hydrocarbons is complicated in the present case by reactions leading to saturation of polymers. I n the following discussion of reaction rates, the number of moles of dimer formed in a given experiment is taken as equal to the total number of moles of dimer, trimer, and higher polymer fractions found in its products; reliance is placed on the earlier conclusion that these compounds are formed largely as the result of reaction of dimer with propylene. The acid concentration is taken in each case as that measured by titration of the catalyst after its withdrawal from the reactor, which agrees within the experimental error with that of the acid fed. Volumes occupied by liquid catalysts in the reactor under polymerizing conditions are estimated from their volumes measured cold after completion of the reaction, together with measured vapor pressures and with thermal expansion coefficients estimated from recent steam table data. The propylene content of the gaseous phase a t the end of each run is taken as equal to the total quantity of this gas found to be present; rough measurements showed its physical solubility in dilute acid cataIysts under polymerizing conditions to be so low as to introduce negligible error. Focusing attention on a unit volume of gas space in order to derive a n expression for the rate of dimer formation, it is apparent that the number of moles of dimer formed, d(D/V,), in this unit volume will be proportional to the time, &, and also to the volume of catalyst, V,/V,, available for conversion of the propylene contained in this space. Likewise, the amount of dimer formed will be proportional to some power of the acid concentration, C,, in the catalyst layer and to some
FIGURE 8. DIMERFRACTION FORMED PER UNIT VOLCME vs. ec.v,Iv,
Incorporating this conclusion for the sake of simplicity, Equation 1 becomes:
The data can be applied to determining the constants k and n. For two sets of runs, each a t the same temperature and pressure, and each with approximately the same value of V,, the quantity D / V , is plotted against BC.V,/V, in Figure 8. Slopes of the curves through these points give values of the left-hand side of Equation 2. When these slopes are plotted on logarithmic paper against P/V,, Figure 9 results; the slopes of the lines obtained should equal n. Due consideration being given the difficulty of obtaining precision with this method, it is apparent that the lines of Figure 9 (with slopes of 2) represent the data better than would lines of any other equal integral slopes. I n other words, the rate of polymerization is best taken as proportional to the square of the concentration of propylene in the gas phase above the catalyst. The lines of Figure 9 lead to values of 4.2 X l o 4 and 19 X lo4 for k , the specific reaction rate of dimer formation (in
INDUSTRIAL AND ENGINEERIIL'G CHEMISTRY
JANUARY, 1938
m1.2 / gram moles X hours X gram moles HBPOI) a t 272 atmospheres and 275' C., and a t 408 atmospheres and 305" C., respectively. Approximate calculations, using the data of other runs than those just considered, seem to show that a t a given temperature the value of the rate constant is not affected within the precision of its determination by change of the total pressure under which polymerization is conducted. It appears desirable to accept the values of k just stated as the best available for any pressure in the range covered, a t their respective temperatures.
20
used dilute acid or in the polymers made with it. It appears from measurements of the vapor pressure of phosphoric acid solutions that the activity of the water in them is nearly equal to the product of its mole fraction by the activity of pure water, and hence that the activity of Hap04 in the solutions used here should be nearly proportional to its mole fraction or in dilute solution to its concentration as measured. It is therefore a consequence of assuming the second of Berthelot's reactions to be a slow one controlling polymerization, that the rate of polymerization should be approximately proportional to the square of the gas-phase propylene concentration times the first power of the HaPo4 concentration. From the present viewpoint the agreement of this deduction with experimental results shows the acceptability of Berthelot's mechanism in explanation of olefin polymerization by dilute phosphoric acid. S o other of the three proposed mechanisms mentioned above leads directly to consistency with observed results as does that of Berthelot. This fact is by no means to be taken as proof that Berthelot's is necessarily the correct mechanism, but the others mentioned cannot be accepted as satisfactory in the present case. This statement, apparently inconsistent with Ipatieff's evidence for the occurrence of polymerization by the mechanism he describes, may not improbably be reconciled with Ipatieff's views by the following assumption: In general, polymerization reactions following these two, and possibly other, mechanisms occur side by side; but under certain circumstances (as in dilute acid) one of these mechanisms is predominant over the others. This assumption can best be tested by further study of rate data.
10
Acknowledgment Thanks are due G. H. Cummings for the use of data from
Discussion of Mechanism Acceptance of the above result-that the rate of polymerization is proportional to the square of gas-phase propylene concentration and to the first power of acid concentrationyields a criterion by which to test the applicability of mechanisms proposed to explain the means by which acids polymerize olefins. Consider, for example, the mechanism of Berthelot ( I ) , which suggests that polymerization occurs through initial formation of monoalkyl ester, and that this compound then reacts with a second molecule of olefin t o yield dimer. I n the present case the theories of Bronsted (3) and Bjerrum ( 2 ) on reaction in imperfect solutions lead us to expect the rate of the second of these reactions to be DroDortional to the product of the activity of isopropyl phosphaie by that of djssolved propylene, other things being equal. If we assume ~~
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. I
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his thesis on blending values of polymer fractions and for his experimental confirmation of certain other results reported here.
08 06
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Nomenclature = = =
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810
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molal concentration of HsPOI in liquid phase dimer fraction formed up to time 8 , gram moles second-order reaction rate constant for dimer formation, ml.2/gram moles X hours X gram moles of HaPo, constant exponent on acid concentration in rate equation exponent on propylene concentration in rate equation propylene present, gram moles volume of liquid acid layer in reactor, ml. volume of gas phase in reactor, ml. time, hours
1000 P/V#
FIGI-RE9. CORRECTED RATE FORMATIOX O F DIMERF R . 4 C T I O s
Literature Cited OF US.
PROPYLENE CONCENTR.4TIOrV
this second reaction of Berthelot to be so slow relative to the first that it controls the over-all rate of polymerization almost completely, it follows that the reactants and products in the first reaction will approach thermodynamic equilibrium with one another-i. e., that a t any time the activity of isopropyl phosphate will be nearly proportional to the product of the activities of propylene in the gas phase and of phosphoric acid in the liquid phase. Combining this result with the relation just given, it follows that the rate of polymerization should vary with the square of the gas-phase propylene activity multiplied by the activity of dissolved phosphoric acid. It is probable that propylene obeys the Lewis and Randall fugacity rule, a t least approximately, in the gas phase above dilute acid. Phosphoric acid is but little dissociated into ions under the present conditions (17). KO appreciable amount of phosphoric esters is found (compare citation 4 ) in
Berthelot, "Die Chemische Synthese," p. 84, Leiprig, F. A. Brockhaus, 1877. Bjerrum, 2. phusik. Chem., 108, 82 (1924). Bronsted, Ibid., 102, 169 (1922). Cavalier, .4nn. chim. phus., [7] 18, 449 (1899). Cummings, 9 . M. thesis, Mass. Inst. Tech., 1936. Gayer, IXD.ENG.CHEM.,25, 1122 (1933). Ipatieff, "Catalytic Reactions a t High Pressures and Temperatures," pp. 549-52, 570-650, New York, Macmillan Co., 1936. ENQ.CHEM.,27, 1067 (1935). Ipatieff, IKD. Ipatieff and Corson, I b i d . , 27, 1069 (1935). Ipatieff, Corson, and Egloff, I b i d . , 27, 1077 (1935). Ipatieff and Egloff, Refiner, .Vatatural Gasoline Mfr.? 14, 249 (1935). Kondakov, J. prakt. Chem., [2] 54, 442 (1896). 16, 275 (1894). McIlhiney, J . A m . Chem. SOC., I b i d . , 21, 1084 (1899). Ibid., 24, 1109 (1902). Majewski, Sc.D. thesis, Mass. Inst. Tech., 1934. Noyes, J . Am. Chem. Soc., 30, 335 (1908). Ponndorf, 2. anal. Chem., 80, 401 (1930). ENG.CHEM.,26, 94 (1934). Whitmore, IND. RECEIVED July 2 2 , 1937.
