I
D, E. WINKLER and G. W. HEARNE
Shell Development Co., Emeryville, Calif.
Liquid Phase Oxidation of Isobutane High yields of tert-butyl hydroperoxide, an equimolar mixture of tertbutyl hydroperoxide and tert-butyl alcohol, or tert-butyl alcohol can be obtained, depending on the conversion and the concentration of metal ions capable of decomposing hydroperoxides
LUMINOUS
literature exists on the air oxidation of hydrocarbons, but very little has been reported on the liquid phase oxidation of normally gaseous hydrocarbons in the absence of a solvent. High pressure requirements with attendant need for barricade facilities because of explosion hazards have undoubtedly kept many from this field. These laboratories have earlier described a unique method (2) for vapor phase oxidation of isobutane where tert-butyl hydroperoxide and di-tertbutyl peroxide were produced in high yields at relatively low temperatures by the use of hydrogen bromide as a chain initiator. Special equipment is, however, required to minimize corrosion caused by this initiator. Loder ( 3 ) oxidized isobutane in acetic acid solvent a t 155' C. in the presence of cobalt salts and obtained acetone and methyl acetate as the chief products. tertButyl alcohol was not mentioned as a product. Morgan and Robertson (4) refer to the catalytic oxidation of normally gaseous hydrocarbons such as propane, n-butane, isobutane, pentane, etc., in the presence of a solvent, but only give examples of n-butane oxidation. Powers (5) states that butane cannot be oxidized in the liquid phase without a solvent because the temperatures involved are above the critical temperature. A similar statement is indicated for isobutane, which is reported to give acetone and methanol as the chief products with smaller quantities of tertbutyl alcohol when oxidized in a solvent. The use of a metal catalyst was implied. This article describes work on the liquid phase air oxidation of isobutane in the absence of any solvent other than oxidation products, and for the most part, rigorous precautions were taken to keep metal ions a t a minimum. I t is possible to prepare either the tertbutyl hydroperoxide as the chief product or a mixture with tert-butyl alcohol capable of conversion to di-tert-butyl peroxide. The yields are high and the system is not corrosive.
Experimental The oxidation reactor was constructed of 304 stainless steel and had a pressure rating of 1000 p.s.i.a. The oxidizing chamber was a jacketed tube, 6 . 2 cm. I. D. and 88 cm. high. Near the top of this, an opening led to a water-cooled condenser. T h e condensate dropped down to a Jerguson sight glass from which water, if any, was drained to the outside and the hydrocarbon overflowed back into the reactor. A normal charge consisted of 800 grams of isobutane (pure grade from Phillips Chemical Co.) and 5 grams of peroxide initiator, di-tert-butyl peroxide or tert-butyl hydroperoxide. The reactor was charged a t room temperature and then heated by adding steam to the jacket. Temperature control was achieved by a steam regulator. Compressed air was metered through a calibrated rotameter and entered the reactor near the bottom through a compressed disk filter which provided good dispersion. Spent gases were vented from the condenser through a nitrogenloaded Grove regulator. After passing through cold traps, cooled with dry ice and acetone, a part of the gas was analyzed continuously for oxygen by a Hays oxygen analyzer. The volume of the exit gases was determined with a wet test meter. Air was added to the reactor a t such a rate that the exit gases contained 4 to 7% oxygen by volume. The normal operating temperature, measured by a thermocouple inside the reactor, was 125' C. At this temperature isobutane has a vapor pressure of 460 p.s.i.a. Normal operating pressure was 140 p.s.i. higher to minimize losses of isobutane in the exit gases. At the end of a batch run, the product was cooled to room temperature, then drained into the cold traps, and transferred to a still. Isobutane was removed under reflux to a kettle temperature of 30' C. The heat source was removed, and, after the column was allowed to drain, the last isobutane was taken off by
evacuating to 150 mm. for a few minutes. I n continuous runs, samples were slowly drawn from the hot reactor directly into cold traps. The isobutane was distilled slowly without a reflux to room temperature; only 1% was lost this way. Then dissolved isobutane was removed by evacuating to 150 mm. The product was analyzed for tertbutyl hydroperoxide by refluxing an isopropyl alcohol-acetic acid solution with excess potassium iodide for five minutes and then titrating the liberated iodine with sodium thiosulfate. A sample of the product was analyzed for total carbon by a micro-combustion technique and the result served as the basis for calculating conversions and yields. An aliquot of the product was reduced with sodium sulfite and then steam distilled and analyzed by gas-liquid chromatography (GLC) for acetone, methanol, tert-butyl alcohol, and isobutyl alcohol, The tert-butyl hydroperoxide was obtained by direct analysis as explained above; the tert-butyl alcohol before reduction was equal to the difference between the total tert-butyl alcohol after reduction and the tert-butyl hydroperoxide. Total acids were titrated, and the amount of formic acid was obtained by oxidation with mercuric chloride. Carbon monoxide and dioxide were determined on exit gas by absorption. Reactor Condition The reactor used for studying the oxidation of isobutane had been used for other oxidations in which cobalt naphthenate had been used as a catalyst. I t had been rinsed thoroughly with water and acetone and appeared clean by visual inspection. I n the first runs (Table I) substantial quantities of hydroperoxide were produced ; however, the goal of an equal number of moles of tert-butyl hydroperoxide and tert-butyl alcohol was not reached. Such a product can be converted directly into di-tert-butyl peroxide by the addition of sulfuric acid (6). VOL. 53, NO. 8
AUGUST 1961
655
Table I. Varia bles Influence Production of tert-Butyl Hydroperoxide Sodium Pyrophosphate; 8s. Water Rinse so-
so-
dium pyro. phosphate Water Water
Nitric Acid Treatment of Reactor Before After
dium pyrephosphate
Peroxide Initiator Di-tert-Butyl tert-Butyl peroxide hydroperoxide
27b
246
23b
576
54b
52b
47b
48b
49b
50b
66"
67b
64"
50b
61'
125
125
125
125
125
125
125
125
125
125
110
110
125
125
125
125
125
Hours
3
4
6
4
7.5
10
5
5
5
5
11
9
3.5
5
4
6
17.5
Conversion tert-Butylhydroperoxide, equivalents/100 g.
12
19 0.78
30
10
23
35
16
19
19
16
22
5.6
15
16
3.3
7.6
27
0.87
0.58
1.53
1.36
1.39
1.40
1.24
1.28
1.36
1.17
1.60
0.93
1.36
1.72
1.65
1.52
Yield of tert-butyl hydroperoxide, To
39
31
23
67
60
60
61
54
54
60
56
72
45
60
83
75
66
Yield of tert-butyl alcohol,
54
61
70
28
36
35
34
40
40
35
35
18
47
35
14
21
30
0.7
0.5
0.3
2.4
1.7
1.7
1.8
1.4
1.4
1.7
1.6
4.0
1.0
1.7
5.9
3.6
2.2
29
25
27
32
13
12
17
32
170
56
67
Run Temp.,
C.
65d
% Moleratiooftert-butyl hydroperoxide to tertbutyl alcohol
Mole ratio of tert-butyl 19 18 19 47 41 alcohol to acetone i n reduced product Charge, 800 g. isobutane plus initiators as specified by footnotes. a
25 g. Di-tert-butyl peroxide.
5 g. Di-tert-butyl peroxide.
A more thorough cleaning of the reactor-Le., heating for 2 hours a t 100' C. with 30% nitric acid followed by rinsing with water until neutral-resulted in marked improvements in the yield of hydroperoxide. The nitric acid treatment did not have to be repeated between runs to ensure high yields of hydroperoxide as long as metal ion oxidation catalysts were not added to the system. Before each run, the reactor was normally rinsed with a 2y0 solution of sodium pyrophosphate which served as a neutralizer and metal ion scavenger. I n addition to showing the effect of nitric acid treatment of the reactor on the yield of hydroperoxide, the data in Table I also show the effect of converTable II.
4 g. tert-butyl hydroperoxide.
sion on yields of hydroperoxide and alcohol. Table I gives mole ratio of tertbutyl alcohol to acetone in the reduced product. Before acid treatment of the reactor about one out of every 20 isobutane molecules attacked underwent p-scission to produce acetone. After the treatment, there was less than half as much 0-scission at a similar conversion. Sodium Pyrophosphate Rinse When isobutane is oxidized, a slight acidity develops (the isobutane-free product is about 0.11V in acid when oxidizing without a metallic catalyst). This could slightly corrode the stainless steel which would produce metal ions
Semicontinuous run at 125" C. with Gradually Increasing Conversion Charge: 800
Sample No. Hours, total Wt. of sample, g. Isobutane free product,
1
g.
isobutane, 5 g. di-tert-butyl peroxide
2
5
3
0
7
8
9
10
4
6
8
13
16
21
27
35
137
154
163
158
154
164
163
172
97
22
32
36.5
83
117
76.5
16.4
19.6
20.8
22.4
26.6
42
44.5
72.5
28.9
44.2
45
g.
Conversion,
Yo
Product, yo wt. of sample tert-Butyl hydroperoxide, equiv./100 g.
11.4 16.1 1.56
1.57
1.57
1.55
32.5
1.48
1.37
50.9 1.34
59.5
72.2
68.0
78.9
1.27
1.14
Yield of tert-butyl hydroperoxide, yo
64
57
52
46
Yield of tert-butyl alcohol, %
32
38
42
48
Mole ratio of ferf-butyl hydroperoxide to tertbutyl alcohol Mole ratio of tert-butyl alcohol to acetone in reduced product
656
2.0
57
INDUSTRIAL AND ENGINEERINGCHEMISTRY
1.5
32
1.2
23
0.96
27
8 g. tert-butyl hydroperoxide.
capable of catalyzing the decomposition of tert-butyl hydroperoxide. The relative effectiveness of rinsing the reactor before each run with a 270 solution of sodium pyrophosphate or with water is shown in the center of Table I. O n the basis of the tert-butyl-hydroperoxide concentration in the isobutane-free product, the phosphate rinse has a n advantage. During these experiments, the solutions were merely drained from the reactor so a small amount of the sodium pyrophosphate was held up in the reactor and could exert its influence during the oxidation. Other metal ion scavengers such as sodium stannate and Perma Kleer 50 (the tetra-sodium salt of ethylenedinitrilo tetraacetic acid) were tried but did not offer any advantages. Comparison of Initiators Di-tert-butyl peroxide or tert-butyl hydroperoxide was added as initiators in all runs to get the oxidation started and avoid long induction periods. The effects of large and small amounts of di-tert-butyl peroxide and of di-tertbutyl peroxide us. tert-butyl hydroperoxide are shown in Table I. Di-tert-butyl peroxide is a more effective initiator judged by the degree of conversion in a given time; however, it appears either to destroy tert-butyl hydroperoxide or to decrease the amount formed. At 110" C. or 125' C., 25 grams of di-tert-butyl peroxide in the initial charge produced a lower yield of tert-butyl hydroperoxide than did 5 grams. At 125' C. 5 grams of di-tertbutyl peroxide was poorer than a n equal amount of tert-butyl hydroperoxide as an initiator for producing high yields of teit-butyl hydroperoxide. An ex-
LIQUID PHASE OXIDATION Table 111. Charge: 600 Grams Isobutane. Sample No.
Semicontinuous Run a t 135" C. and High Conversion
357 grams product from previous runs containing 1.45 equivalents of tert-butyl hydroperoxide/lOO g.
5
6
7
2.5
3.5
3 3.0
4
Hours between samples
3.0
2.0
2.0
2.0
2.0
3.0
3.0
2.5
2.5
2.5
2.0
2.0
2.0
2.0
Hours, total
2.5
6
9
12
14
16
18
20
23
26
28.5
31
33.5
35.5
37.5
39.5
41.5
Weight of sample, g.
179
157
159
159
150
149
149
145
160
146
159
153
150
144
145
144
846
Isobutane free product, g.
128
104
109
104
92
87
85
80
99
85
101
95
95
91
91
88
503
Conversion,
1
%
2
58
52
8
9
11
10
52
12
13
46
14
15
16
55
17
55
Product, % weight of sample
71.5
66.2
68.5
65.3
61.3
58.4
57.1
55.1
61.9
58.2
63.9
62.1
63.3
63.2
62.7
61.1
59.5
tert-Butyl hydroperoxide, equivalents/100 g.
1.23
1.18
1.18
1.17
1.17
1.18
1.16
1.18
1.19
1.20
1.19
1.17
1.16
1.17
1.16
1.14
1.14
Yield of tert-butyl hydroperoxide, %
48
48
48
48
48
47
Yield of tert-butyl alcohol,
46
47
46
47
47
47
Mole ratio of tert-butyl hydroperoxide to tertbutyl alcohol
1.o
1.0
1.0
1.0
1.o
1.0
Mole ratio of tert-butyl alcohol to acetone in reduced product
12
13
12
13
14
12
%
planation as to why tert-butyl hydroperoxide yields are decreased when ditert-butyl peroxide is used as an initiator is given in the discussion on mechanism. Although the type of initiator used has a marked effect on the product distribution in batch runs, it has little significance in continuous operation, for any initiator added to start the reactor would soon be consumed or drained from the reactor. In continuous operation it is not necessary to add initiator continuously, for tertbutyl hydroperoxide is always being formed and it will serve as initiator. Di-tert-butyl peroxide is not produced during the oxidation. Semicontinuous Run Gradually Increasing Conversion. A run was made at 125' C. to determine how the product distribution changes with increasing conversion and to see how far one could carry the conversion before the mole ratio of tert-butyl hydroperoxide to tert-butyl alcohol dropped below one. The semicontinuous run was started like a batch run, the charge being 5 grams of di-terl-butyl peroxide and 800 grams of isobutane. At indicated intervals, samples were withdrawn into a cold trap and analyzed as previously described. A precut of about 20 grams was taken before each sample to clear the lines. After sampling, isobutane was added to the reactor to bring the level up to the starting position. The results of this run are presented in Table I1 and show the gradual decrease in yield of tert-butyl hydroperoxide and increase in yield of tert-butyl alcohol as the conversion was increased. The mole ratio of tert-butyl hydroperoxide to
tert-butyl alcohol became one a t about 70% conversion. The rate of oxygen consumption remained essentially constant between the fourth and 35th hour or between 11% and 60y0 isobutane conversion. The increase in hydroperoxide concentration with the accompanying increase in free radical initiators counteracted loss in oxidation rate which might be expected as the result of hydrocarbon depletion. The average rate of production throughout the run was 22 grams of product per hour. Most of this represents a production rate where the initiator is tert-butyl hydroperoxide; the ditert-butyl peroxide added at the start was not detectable by GLC after the fifth sample. R u n at 135' C. The results of a semicontinuous run at 135 O C. are given in Table 111. This run was made to
Table IV.
Semicontinuous Run a t
determine the production rate at the higher temperature and to determine the conversian a t this temperature which would give a product containing equal moles of tert-butyl hydroperoxide and tert-butyl alcohol. Because 135' C. is 1' above the critical temperature of isobutane, it was necessary to use a solvent, and for this, a reaction product from a previous run was chosen and charged to the reactor at the beginning of the run along with the isobutane. Samples were withdrawn a t time intervals indicated and the reactor recharged with isobutane to bring the level back to the starting point. The conversions given in the table are those which existed in the reactor at the time the sample was taken. This per cent conversion level would drop about 10 points when make-up isobutane was added ; the average conversion was
135' C. with Cobalt Catalyst
Charge: 750 g. isobutane, 100 g. product from another run contaming 0.26 equiv. of tert-butyl hydroperoxide/lOO g., 5 g. cobalt naphthenate s o h . = 350 p.p.m. cobalt on total charge Sample No. 1 2 3 4 0 6 7 8 Total hours 10.5 5" 6.5 8.5 12.5 14.5 16.25 17.75 Sample weight, g. 148 157 149 155 1150 153 148 808 Isobutane free product, g. 65 82 88 96 100 94 484 89 49.8 Conversion, % 44.7 58.1 52.1 57.4 43.9 52.2 59.0 Product, yo wt. of sample 64.0 65.3 63.5 59.9 tert-Butyl hydroperoxide, 0.53 0.47 0.40 0.35 0.29 0.25 0.26 0.26 equivJ100 g. Yield of fert-butyl hydroper18 13 10 9 oxide, % Yield of tert-butyl alcohol, % 72 76 77 77 Mole ratio of tert-butyl hydro0.25 0.17 0.13 0.12 peroxide t o tert-butyl alcohol 12 Mole ratio of tert-butyl alcohol 10 8 7 to acetone in reduced product a
First four hours, a t 125' C.
VOL. 53, NO. 8
AUGUST 1961
657
about five points lower than given in the table, or about Soy0,. The data in Table I11 indicate that one can operate continuously in a single stage reactor a t 135" C. a t an isobutane conversion level of 50% and obtain a product in which the mole ratio of tertbutyl hydroperoxide to tert-butyl alcohol is one. The average production rate a t 135" C. was 40.5 grams per hour which is nearly twice the rate a t 125' C. Cobalt Catalyst. The importance of kceping metal ions a t a low level if one wished to obtain high yields of tcrtbutyl hydroperoxide has already been stressed, and the results of minute traces of such contaminants were shown in Table I. Table I V presents results on the oxidation of isobutane at 135' C. in the presence of added cobalt naphthenate. The reactor for this run was charged with 730 grams of isobutane, 100 grams of product from a previous run containing 0.26 equivalents of tert-butyl hydroperoxide, and 5 grams of cobalt naphthenate (equal to 350 p.p.m. of cobalt on total charge). The reactor was run for 4 hours at 125' C. to build u p oxidation products to act as a solvent for the isobutane before raising the temperature to 135" C. At about SOY0 conversion the yield of tert-butyl hydroperoxide appears to have leveled off a t 9 to loyo,while the yield of tert-butyl alcohol was 77%. The ratio of tert-butyl hydroperoxide to trrt-butyl alcohol was only 0.12 compared to 1.0 in the absence of added cobalt at a similar conversion and the same temperature (Table 111). Yields of terl-butyl hydroperoxide plus tertbutyl alcohol were 86 to 87% with catalyst compared to 94 to 977, without catalyst. The ratio of rut-butyl alcohol to acetone in the reduced product dropped to seven, indicating a higher per cent of scission products. The catalyst increased the production rate to 56 grams per hour of isobutane-free product.
Preparation of Di-fert-butyl Peroxide To demonstrate that di-tert-butyl peroxide can be produced from isobutane oxidation products, 288 grams of product containing 1.61 moles of tert-butyl hydroperoxide and 1.75 moles of tertbutyl alcohol was charged to a 1-liter kettle with a stirrer, thermometer, and dropping funnel. T o this was added 246 grams of 70% sulfuric acid in 10 minutes a t 13 to 15' C. The reactants were heated to 40 to 45" C. and kept at that temperature for 30 minutes. The product was diluted with 300 ml. of Ivater, separated, and the organic phase washed with bicarbonate solution and then water. Recovered di-tert-butyl peroxide measured 232 grams which represents a 98.5% yield on the charged tert-butyl hydroperoxide and a 91.370 yield on isobutane. The infrared spec-
655
trum of this sample was identical to the spectrum of commercial di-tert-butyl peroxide. An analysis for active oxygen indicated the sample to be 98.6y0 pure.
Mechanism of lsobutane Oxidation Oxidation reactions require a free radical source for initiation. For most of the runs described in this work a small amount of di-tert-butyl peroxide was charged with the isohutane to serve as a source of radicals. The following equations in which R is tert-butyl (C4H,) are believed to represent some of the reactions. RO
ROOR 2RO. +RH+ROH
+ + RH
H.
ROO
0 2
+ R.
+ROO. ROOH
+ R.
+
(1) (2) (3) (4)
Di-tert-butyl peroxide is a more effective initiator than tut-butyl hydroperoxide judged by the rate of oxidation, but its use results in lower yields of A possible tert-butyl hydroperoxide. explanation for the latter observation lies in Equations 5 and 6. 2R00.
RO
.--f
+ ROOH
+
2RO. ROH
+
0
2
+ ROO.
(5) (6)
The addition of an excess of di-tertbutyl peroxide to the system effectively increases the total radical concentration. As this happens Equation 5 leading to R O . becomes more significant for it increases as the square of ROO. concentration. RO . then reacts according to Equation 2 or 6 , the latter leading to the destruction of some tert-butyl hydroperoxide. To produce high yields of tert-butyl hydroperoxide it is therefore necessary to maintain the radical concentration as low as possible, consistent with a reasonable reaction rate. In continuous runs the small amount of di-tert-butyl peroxide added a t the start soon disappears, and so the oxidation product, tert-butyl hydroperoxide, which decomposes a t a slower rate than di-tevtbutyl peroxide, becomes the chief source of initiating radicals. I t probably gives rise to free radicals by two mechanisms as suggested by Bateman ( 7 ) . At low concentrations the decomposition is believed to be unimolecular and a t higher concentrations a bimolecular decomposition probably predominates.
+
ROOH -+ K O . OH. 2ROOH -+ R O . f ROO. H20
+
(7) (8)
I n addition to hydrogen abstraction reactions (Equations 2 and 6), RO. may decompose to acetone and a methyl radical. RO.
-+
CHSCOCH3
+ CH,.
(9)
The ratio of hydrogen abstraction by R O . plus R O O . to decomposition of RO . was found to be greater than 50 for
INDUSTRIAL AND ENGINEERING CHEMISTRY
some low conversion runs at 125O C., about 13 a t 135' C. in the absence of metal ions, and about six a t the same temperature in the presence of cobalt catalyst (see mole ratio of tert-butyl alcohol to acetone in reduced products in the tables). Products derived from CH8. are methanol, formic acid, CO, and COS. The latter two frequently could not be detected in the exit gas from 125" C. runs by absorption methods. Most of the acid which is present is believed to be formic. This is difficult to prove since acidity is very low, varying from 0.01 equivalent per 100 grams in noncatalytic runs at 123' C. to 0.045 equivalent per 100 grams in cobalt-catalyzed runs at 135" C. These values are on the isobutane-free product a t conversions greater than 50%. Formate determinations varied from 50 to 80% of total acidity, but at these low values may not be too reliable. The identification of about 1% yield of isobutyl alcohol by infrared spectrum of a sample separated by GLC shows that some attack is taking place at the primary carbon-hydrogen bonds. When isobutane is oxidized in the presence of cobalt, low yields of tertbutyl hydroperoxide are obtained. This is due to the catalyzed decomposition of the hydroperoxide by cobalt according to the following reactions (7). Co+*
+ ROOH
+ ROOH
-f
-+
Co'3 $- R O .
-+
+ OH-
CO* ROO.
+ H+
(10) (11)
The net result is given in Equation 8. By decomposing the ROOH catalytically, the concentration of RO . is kept at a much higher level than when thermal decomposition is controlling. This results in a faster rate of oxidation, more tert-butyl alcohol, and more R O . decomposition products.
Acknowledgment The authors are indebted to F. F. Rust and W. E. Vaughan for discussions on the oxidation mechanism.
Literature Cited (1) Bateman, L., Quart. Rev. 8, 147 (1954). (2) Bell, E. R., Dickey, F. H., Raley, J. H., Rust, F. F., Vaughan, W. E., I N D . ENG. &EM. 41,2597 (1949). (3) Loder, D. .T., U. S. Patent 2,265,948 (1941).
(4) Morgan, C. S., Robertson, N. C., I&., 2,659,746 (1953); 2,704,294 (1955). (5) Powers, E. T.,"Chemicals by Direct Oxidation of Hydrocarbons." Fourth World Petrol. Congr., Rome, Italy, April 1955. (6) Vaughan, W. E., Rust, F. F., U. S. Patent 2,403,771 (1946). RECEIVED for review December 8,1960 ACCEPTEDMarch 28,1961
