OXIDATION OF TOLUENE AND OTHER ALKYLATED AROMATIC HYDROCARBONS TO BENZOIC ACIDS AND PHENOLS WARREN W. KAEDING, ROBERT 0 . L I N D B L O M , R. G. T E M P L E , AND H . I . M A H O N The Dow Chemical Go., Walnut Creek, Calif.
A new process utilizes air for the conversion of toluene to phenol by a two-step catalytic oxidation process. Benzoic acid is the intermediate.
The hydroxyl group becomes attached to the aromatic ring a t a position Meta-substituted phenols, therefore, are obtained from both
adjacent to thle departing carboxyl group. ortho- and para-substituted benzoic acids.
is the most important source of aromatic raw are utilized by the chemical industry. Advanced technology has made available increasing numbers and larger quantities of pure derivatives of this type a t lower cost. Toluene remains the cheapest and potentially the most abundant primary source of the aromatic ring. .4ir is an exceedingly desirable substance to use as a chemical reagent. No other material is so readily and conveniently available. has such a uniform and reliable composition, and (except for sea water) is so abundant. Disposal of the unrracted or undesired fractions is usually simple. The search for new ways to control and increase the specificity of its chemical reactivity will continue to challenge the imaginations of marly chemists and engineers. This phenol process combines air and toluene in a two-step oxidation process, where benzoic acid is the intermediate. ETROLEUM
Pmaterials which
Oxidation of Toluene tot Phenol
T h e conversion of toluene to phenol can be visualized as a complete oxidation of the methyl group to form carbon dioxide and water (Equation 1). CHJ
OH
Stable intermediates which represent successive stages of oxidation are benzyl alcohol, benzaldehyde, and benzoic acid. The latter is the most stable compound. Although certain reagents and conditions of reaction permit the formation of the other intermediates. benzoic acid is the usual end product for a considerable variety of oxidizing agents such as persulfate ( 7 1 ) . chromic acid (7411, alkaline permanganate (79), dilute nitric acid (73), chlorine followed by hydrolysis (4, 70), and oxygen (5, 76, 20). Oxygen in air is not only the most attractive oxidizing agent from a cost standpoint, but also appears to present the simplest system for product isolation when toluene can be used as the solvent. A catalytic. liquid-phase method utilizing this system has been reported (27). T h e yield of benzoic acid was low and a study to optimize the conditions of reaction was required. The conversion of benzoic acid to phenol and carbon dioxide itas a more formidable problem. The desired product was not only very sensitive to further oxidation but the chemistry of this transformation was unknown.
It has been reported that small amounts of phenol and related compounds, such as salicylic acid and phenyl benzoate, were produced when cupric benzoate was pyrolyzed (3. 7 7 , 77) or when benzoic acid was heated with a variety of copper salts ( 7 , 9 ) . These experiments were verified. Phenol was produced, although the yields were low and decomposition to form tar was predominant. Subsequent studies designed to elucidate the chemistry of this reaction eventually proved that attractive yields of phenol could be obtained in dilute aqueous solutions utilizing cupric ion as the specific oxidizing agent (7). T h e desired reaction also occurred in various organic solvents with the cupric salt of benzoic acid. Much subsequent work has led to the proposal that cupric benzoate is the key reactant for this transformation (2: 8, 78). Molten benzoic acid was the most attractive solvent because its high boiling point permitted the reaction to proceed a t atmospheric pressure, it had excellent solvent properties for the catalyst and promoters, and it provided a large reservoir of starting material. Experimental
Benzoic Acid Production. The apparatus utilized for the production of benzoic acid is shown schematically in Figure 1. Since elevated pressures were required, metal construction was used throughout the system. T h e 8-cm. tubular reactor had an operating liquid level of 110 cm. and a volume of 5 litt=rs OPERATION PROCEDURE.The reactor was charged with 5 liters of toluene, in which the soluble cobalt catalyst was dissolved, by means of the toluene feed reservoir, F. A smaller feed reservoir, Q, was available for the addition of other liquids to the reactor. After the desired temperature had been reached, air was introduced into the bottom of the reactor. R, through a d i p tube, A . T h e reactor temperature was measured by a thermocouple inserted in a well, W . The reactor was heated electrically by means of Nichrome resistance tape. T h e pressure was allowed to build u p to the desired value and maintained at a constant reading by adjusting the exit valve leading from the phase separator vapor space, S. Two pressure-control switch mechanisms (not shown on the drawing) were attached to the system. A warning bell sounded if the pressure varied 1 2 p.s.i.g. from the desired value. Vapors leaving the reactor were separated from the air not used in the oxidation with a condenser, K Two phases separated, a lower aqueous phase which contained some formic acid, and an upper toluene layer. The toluene was returned to the reactor by means of a seal leg. T h e water was periodically drained. Make-up toluene was added by means of reservoir, F. to maintain the proper level, indicated by the sight glass, G. A dry ice trap and charcoal absorber. T . was used to remove the last traces of toluene from the noncondensable gas stream. VOL. 4
NO. 1
JANUARY
1965
97
with a n equal volume of ethanol. T h e pFI was adjusted to 2.6 with HCI. When the oxime was formed, an equivalent amount of HC1 was liberated. This was titrated to the p H of the original reagent (2.6). .4 blank sample was also run. Separate analyses, where benzoic acid and benzyl alcohol were added in amounts expected in actual samples, were run. No interference was detected. Toluene was determined by distilling a 150, to 200-gram sample through a 1 5-tray, Sls-inch Oldershaw column. A sharp break in the distillation curve appeared when the toluene was removed. Neutral material, determined by difference, was primarily benzyl alcohol and benzyl benzoate. T h e composition of the filtrate recycled (not including toluene added to rinse the benzoic acid or for additional make-up) was benzoic acid, 11 to 167,; benzaldehyde. 2 to 37,; benzyl alcohol, benzyl benzoate (and other unidentified hydrocarbons), 4 to 9%. The remainder was toluene. PHENOLPRODUCTION. T h e apparatus and procedure for the oxidation of benzoic acid to phenol and carbon dioxide have been described (8). T h e only modification for the experiments reported here was in the reactor design. Agitation was achieved entirely by the flow of air through the molten benzoic acid solution. T h e tubular reactor, with a diameter of 2l!2 inches and a n operating volume of 1500 ml., was constructed of glass and electrically heated with Nichrome resistance tape. 'The phenol and water used for hydrolysis were collected together as a two-phase liquid system. Results and Discussion
Figure 1.
Apparatus
T h e concentration of oxygen in the effluent gas stream and the water production rate were sensitive indicators of the rate of reaction. T h e induction period before reaction began could usually be measured in minutes. T h e concentration of benzoic acid increased a t a steady rate u p to a level of 50 to 55% by weight. However, a lower value of 25 to 30% was usually maintained by draining 600 to 700 ml. of reactor liquid each hour into a large flask. T h e liquid level in the reactor was re-established by the immediate addition of toluene (and/or recycle filtrate). T h e reactor liquid accumulated over a n 8-hour period was allowed to stand for an additional 4 hours (to reach room temperature) and the benzoic acid which had precipitated was then removed by filtration .and washed with a portion of the make-up toluene. T h e filtrate was recycled to the reactor. A small amount of fresh cobalt catalyst (13 mg. per hour in run 17) was added each hour in the recycle toluene filtrate. T h e solid benzoic acid on the Buchner filter contained from 15 to 20% (by weight) of toluene, which was removed by heating in a 100' C . oven or by distillation. T h e benzoic acid was a finely divided white solid with a purity of better than 99%. T h e equipment was operated on a 24-hour-per-day basis. METHODS O F ANALYSIS. A weighed sample of benzoic acid (ca. 10 grams) was placed in a 600-ml. beaker and dissolved in 100 ml. of methylene chloride. After 300 ml. of water were added, the two-phase mixture, rapidly stirred, was titrated to p H 7.8 with standard base. A weighed sample of benzaldehyde (10 grams) was dissolved in a 100-ml. aliquot of standard hydroxylamine hydrochloride solution. This reagent was prepared by dissolving 35 grams of hydroxylamine hydrochloride in 160 ml. of water and diluted 98
l&EC
PROCESS DESIGN AND DEVELOPMENT
Oxidation of Toluene to Benzoic Acid. Early experiments indicated that toluene could be oxidized to benzoic acid a t $20" to 175' C., with a n air rate of 0.1 to 1.0 liter per minute per kilogram of toluene, with a soluble cobalt catalyst a t a concentration of 0.1 to 0.37, by weight. With the establishment of practical conditions of reaction, operating techniques and equipment were quickly perfected to the point where process conditions could be simulated. T h e product was separated and purified by crystallization, unconverted intermediates were recycled, catalyst activity was maintained indefinitely, a n d yield and conversion rates were calculated. A number of typical runs of this type are summarized in Table I. Reasonably good material balances were obtained despite large inventories outside the equipment. Runs 16 and 17 (Table I) were actually a continuation of run 15. T h e reactor was shut down over the week end and restarted with the filtered reactor liquid the following week. This demonstrated that catalyst activity could be maintained indefinitely and that a considerable fraction of the intermediates recycled eventually was converted to benzoic acid. A yield approaching 90% was indicated. This work confirmed for us that benzoic acid of high quality could be prepared from toluene by oxidation with air in a continuous manner, with acceptable yields and rates. T h e conditions and characteristics of the reaction were defined and materials of construction were tested. Economic evaluations could be calculated and a preliminary plant design developed. Oxidation of Benzoic Acid to Phenol. A similar approach was adopted for the oxidation of benzoic acid to phenol. This was a more difficult system to study because of the high melting point of the benzoic acid solvent (120' C.) and its tendency to sublime with large volumes of air bubbling through the molten liquid. Cupric benzoate was recognized as the key reagent which thermally decomposed to produce phenol precursors (Equation 2). In benzoic acid solution, this decomposition began a t approximately 190' C. The half life a t the operating temperature of 230' to 240' C. is measured in minutes.
c
d
2 h r-
m
0 rm
a
*
G.
3 c
oc d
3
m
m 3
d
m
hl d
d N
c'!
G.
m
c
a
m
m
N
0
0
m
m hi m
t
m
c
m d
u
r-
a
m
c
m
hl
r-
ir, m
m.
m
m
0
m
r-
m
0
m * m
3 m
m m m
N
f
F
N
m 0
9 2
m
m
rr,
m
m
c
m
r-
0
m m
m c
0
m N 0
W N N
d
N 0
N r-
N d
0
9
0
0
N
3 m
N
0
0
fm
m m
hl
a
4
e
c e
*
In
a
3
COOH
COOH
m
d
m
COOH 0
m m
m
hl
The presence of benzoylsalicylic acid was first indicated by observing the rate of carbon dioxide evolution. It began slowly, reached a maximum value. and then declined in a manner typical of consecutive reactions. The presence of this intermediate was verified by identification with a high resolution infrared spectrophotometer. In the presence of steam. the benzoylsalicylic acid hydrolyzed to give benzoic and salicylic acids (Equation 3). The latter decarboxylated very rapidly in the reaction medium to give phenol and carbon dioxide (Equation 4). T h e half life for this reaction is measured in seconds. When air. was bubbled through the benzoic acid solution containing the reduced copper salts, cupric benzoate was rapidly regenerated (Equation 5). Copper metal \vas also oxidized under these conditions of reaction.
m 0
.c h
si
i=
COOH
?H
When water was introduced simultaneously with the air, the principal reaction product was free phenol. This net reaction is shown by Equation 6 as the sum of all of the previous reactions. The fortunate difference in boiling points of the product and starting material and the relatively high temperature of reaction permitted the immediate and convenient removal of phenol from the reaction zone by distillation. An alternative mechanism of reaction would involve decarboxylation of the benzoylsalicylic acid prior to hydrolysis. I n this event, phenylbenzoate would be an intermediate. This ester would be the principal product if the reaction \vere carried out in the absence of water. The results of a number of runs are summarized in Table 11. A working set of conditions and rates of reaction was established and yields were calculated. An ample supply of phenol was also made available for product evaluation. A complex mixture of nonvolatile products, appropriately labeled "tars," slowly accumulated in the reactor. A continuous reaction would ultimately require some processing to remove tars. This was accomplished subsequently by distillation of the volatile components or by selective solvent extraction. T h e rate of reaction was affected by the concentration of cupric benzoate. When the concentration was doubled (Table 11), the phenol production rate increased by approximately 30%. The concentration of oxygen in the reactor inlet gas VOL. 4
NO. 1
JANUARY
1965
99
Table II.
Run
No. 3
Reactor Inlet Gas Reactor Rate, Temp., I./ Oxygen, a p b. min. yo 230 2.7 21
Time Run, Hr. 33
Water Rate,
MI./ Min. 1.55
~
Catalyst, Gram Moles
0.40
Mg 1.17
Gun
Oxidation of
Benzoic Acid, G r a m Moles Phenol AV. AV. addn. rate, Initial Makerate, Totalb mole/ charge up Total molelhr. moles hr.
Carbon Dioxide Ai.. .Mole rate, ratio Total mole/ COzl moles hr. PhOH
12.3
25.9
38.2
0.79
22.6
0.69
25.6
0.78
1.13
6
39
230
2.7
21
1.82
0.80
1.17
12.3
37.2
49.5
0.96
35.4
0.91
36.1
0.93
1.02
7
23
230
2.7
21
1.71
0.80
2.34
12.3
22.6
34.9
0.98
20.9
0.91
22.2
0.97
1.06
8
40.5
230
2.7
21
1.80
0.40
1.17
12.3
33.2
45.5
0.82
31.0
0.77
34.3
0.85
1.10
9
16.5
230
2.7
21
1.60
0.40i
1.2
12.3
12.3
24.6
0.75
12.2
0.74
12.2
0.74
1.00
10
21
240
2.7
21
1.70
0.80
1.2:',12,3
24.6
36.9
1.17
22.4
1.07
23.0
1.10
1.02
11
18
230
2.7
42b
1.61
0.80
1.2
12.3
24.6
36.9
1.37
22.6
1.25
25.2
1.40
1.11
230
2.7
70k
1.00
0.96
1.2
13.1
13.3
26.4
1.97
l5,l
2.22
14.9
2.21
0.99
230
2.7
21
1.41
0.40
1.2
11.11
28.3
39.4
0.63
26.6
0.59
28.7
0.64
1.08
12 14
6.8 45
b Includes phenol present as phenyl benzoate. c Recovered f r o m dry ice trap. a Gram moles. d Includes benzoic acid presmt in phenol distilled and in samples withdrawn for analysis. 6 Includes samples of reactor liquid w i t h d ~ a w nf o r analysis, approx. 75-150 g. f Assume GO? = ' / z 02, all of neutral fraction wasphenyl benzoate, andmol. w t . of tar = 100. 0 Molar yield = l O O X (Phenol) (phenyl benzoate)/(total B z O H a d d e d ) - (BzOHrecovered) -
+
stream had a more dramatic effect, not completely reflected in the average production rate shown in Table 11. T h e column size and perhaps the amount of water added did not permit removal of the phenol a t a n optimum rate. Phenyl benzoate rapidly accumulated in the reactor a t the expense of benzoic acid needed for the reoxidation. With run 12 (Table 11) the rate of reaction was increased to the point where the heat of reaction maintained the reactor liquid a t the operating temperature a n d provided the column reflux. No external heat source was required. I t was even necessary to reduce the oxygen conce7tration somewhat to prevent the reactor temperature from increasing above the desired value. T h e presence of soluble iron salts of benzoic acid had a definite retarding effect on the phenol production rate (run 14, Table 11). A slight decrease was also observed with run 9, where the iron was added as finely divided metal. T h e rate of solution was very slow. Most of the iron did not dissolve. Derivatives of T o l u e n e a n d Benzoic Acid. Cresols were obtained as products when the toluic acids were oxidized in a similar manner. However, rn-cresol was the exclusive phenolic product isolated from both o- and p-toluic acids. A mixture of 0- and p-cresol was obtained from rn-toluic acid. This steric result can be explained by postulating an attack by a n oxygen atom a t a position adjacent to the original carboxyl group during the decomposition of the cupric salt (R = CGHSCO) (Equation 7). This accounts for the formation of the benzoylsalicylic acid intermediate found in the reaction medium.
procedure by starting with the appropriate substituted toluene a n d benzoic acid derivative. There are certain practical limitations as the type and degree of substitution become more complex. For example. the iilitial oxidation of pseudocumene to the corresponding monocarboxylic acid would produce a mixture of three isomeric dimethvlbenzoic acids. Conversion of the acid mixture would give four xylenol isomers. Certain specific compounds, however, such as mesitylene or durene would give only one monocarboxylic acid and phenol (2>4dimethylphenol and 2,3,j-trimethylphenol, respectively). When toluene is substituted with alkyl groups containing two or more carbon atoms, an indiscriminate attack occurs to give a mixture of products, with the notable exception of the tert-butyl group. The latter is very resistant to oxidation under the conditions of reaction described for this process (6). p-tert-Butyltoluene may be converted to p-tert-butylbenzoic acid a n d the latter subsequently oxidized to m-tertbutylphenol by a similar procedure and with identical equipment used to produce phenol from toluene. When a methyl group is located a t a position ortho to the carboxyl group, partial oxidation may occur to give a n ester eventually by reaction with the adjacent carboxyl group. For example, phthalide is a significant side reaction product in the production of rn-cresol from 0-toluic acid (Equation 8 ) .
0
I
R
R
This mechanism has been verified conclusively for benzoic acid itself by Schoo and coworkers utilizing C14-labeled benzoic acid (75). A series of alkyl-substituted benzoic acids and phenols, therefore, may be prepared by the same general 100
l&EC PROCESS DESIGN A N D DEVELOPMENT
Since phenols are sensitive to further oxidation, it is imperative to remove them immediately from the reaction zone. This is most easily accomplished with phenol because of its relatively low boiling point in comparison \\ ith the temperature of reaction. Alkyl-substituted benzoic acids are oxidized to their corresponding phenols in the same tcmperature range (220' to 250' C . ) required for the parent compound. The additional substituents increase the boiling point of the product
Benzoic Acid to Phenol .\laterial Balance1 PhenolQ Aromatic Grams rings,/ Yield, Attack, Shutin out znjout yo doionsh 76 5167 3 ~8.2 89 67 4 4935 36.3 6707 4 9.. 5. ~ 94 77 0 6646 49.9 4722 34.9 93 65 0 4607 3410 45.5 6109 91 75 0 ~5999 44.1 24.6 3282 92 54 2 ~3203 24.2 36.7 4946 95 64 2 4887 36.8 4981 36.9 91 61 2 4982 37.0 3568 2 6_. 5 85 67 1 _ 3386 26.0 5525 39 4 93 12 0 -~ 5403 40.2 ~~
Benzoic
~
~-
Final Reactor Liquid -~ Benzoic Phenol, .Yeutral, acid, CL 5 c; 77.6 5.7 7.5 ~~~
~~
Tar,
12.6
Total. g.e 1602
0 36
11.7
1-88
13.8
0 36
12.3
1676
2.9
0 38
11.3
1643
9.1
73.4
5 5
9.3
1.3
1.3
0 02
11.4
1536
3.6
82.0
2.7
6.5
1.7
1.91
0 37
13.2
1629
5.4
77 6
4.6
8.3
2.7
1.4
0 27
12.1
1830
Benzene,
Acid,
.Molec
‘Vfolesd
0 19
c7 /c
5.6
1.8
2.0
~
66.8
5.7
10
2.18
7
0.98
~~
~~
80.6
4 1
2.90
6 2
3.21
~
~~~
~~~
11.7
64.2
6.5
13.9
2.5
1.2
~
0 38
8.8
1622
10.9
1 706
10.7
42.8
11.8
30.5
2.8
1.4
~~
13.5
69.6
4.2
8 6
2.0
l.lm
‘*
24 g . of powdered iron added (phenyl benzoate); attack = 7OOX ( B z O H consumed)!(BzOH ~ u , t ) p / ? e d ) . Shut doion ourmight. .Vat a continuous run. initially. 2 Plus l.6yoiron. k Inlet air (27cI,)enriched w i t h p u r e o.xypen 10 obtain concentration shoion. 1 0.48 mole of ferric benroat? addrd initially. m Plus 7.0cIc iron.
and it is more difficult to remove it from the reaction z o ~ i rby distillation. This promotes tar formation and adversely afl-ects the yield. If dicarboxylic acids are oxidized by this method, an iritermediate hydroxyhenzoir: acid is formed: Jvhich tends to d r carboxylate to give phenol rather than produce a dihydroxybenzene. Sitrobenzoic acids produce the corresponding nitrophenols. T h e high melting and boiling points of the compounds, however. presented serious operating difficulties. T h e hydroxyl group became attached a t a position adjacent to the original carboxyl group. This rule has applied without variation to every benzoic acid derivative tested. Commercial Production. At the time of writing. three commercial phenol plants utilizing this process are under construction or in production. T h e first to be started i s located a t Ladner, British Columbia, Canada, and is being operated by Dow Chemical of Canada, Ltd. T h e second is located a t Kalama, Wash.. and is being operated by the \.Vestern Division of T h e Dow Chemical Co. T h e third plant is located a t Rosenburg. T h e Netherlands, and is operated by Dutch State Mines. Ac knowledgrnent
A number of people have given valuable advice and asTistance \$ith the early process research work and with studies draling with the oxidation of benzoic acid to phenol Grate-
ful acknolvledgment is given to W. Hirschkind, T. R. Norton R . D. Barnard, R. H. Meyer, and TY. E. Brown. literature Cited (1) Bamdas, E. M., Shemyakin, M. M., Z h . Obshch. K h i m . 18, 324 (1948). (2) Barnard, I
