OXIDATION OF AROMATIC HYDROCARBONS WITH HBr/H20 PROMOTERS A R I E H
LIN A N D
E P H R A I M K E H A T
Chemical Engineering Department, Technion-Israel Institute of Technology, Haifa, Israel The addition of water vapor to the oxidation of toluene, ethylbenzene, cumene, and benzene, promoted by hydrogen bromide, results in a different spectrum of products than for oxidation promoted by dry hydrogen bromide. The experimental work was conducted in a continuous reactbt at atmospheric pressure in the vapor phase, temperatures of 100' to 450°C., residence times of 0.2 to 2 3 0 seconds, promotion by HBr and HCI, and a wide range of feed concentrations. Selectivities of 30% of toluene to frans-stilbene with HBr, 15% of toluene to benzaldehyde with HCI, and 50% of ethylbenzene to acetophenone with HBr were obtained at 104' to 130°C., residence times of 0.5 to 2 minutes, and high ratios of promoter to hydrocarbon. Selectivifies of up to 48.2% of benzene to phenol with a yield per pass of 8.21% and a low selectivity of biphenyl were obtained for a high ratio of water to benzene in the feed, a t 285' C. The loss of HBr to halogenated products was negligible.
THEmajor
product of commercial reactions of toluene is benzene. Other minor commercial products are toluene diisocyanate and phenol, through benzoic acid (Stobaugh, 1966b). New products of reactions of toluene are desired commercially. Commercial methods for the production of phenol from benzene involve the preparation and separation of intermediates, such as cumene or chlorobenzene. A singlestage process of oxidation of benzene to phenol is highly desirable. However, because of the relatively small difference in cost between benzene and phenol, higher conversions than achievable a t present are required. Stobaugh (1966a) reports one commercial U. S. plant, with a production capacity of 20,000,000 pounds of phenol, using direct oxidation by an undisclosed process. This is a very small fraction of the 1966 synthetic phenol production in the United States of 1285 million pounds. For the oxidation of toluene to benzoic acid in the liquid phase, Kaeding et al. (1965) reported for 140"C., 30 p.s.i.g., a soluble cobalt catalyst, and long residence times, production rates of about 0.5% per hour with a total yield of benzoic acid of close to 9070, and production of small amounts of benzaldehyde and condensation and degradation products. Parks and Yula (1941) reported for the oxidation of toluene in the vapor phase, at 365" to 507"C., vanadium oxide catalysts and residence times of about 1 second, yields up to 52% of maleic and benzoic acids, and smaller yields of benzaldehyde. At high air-to-toluene ratios, almost half the toluene was burnt to COYand HzO, while in the absence of catalyst, a t 45OoC., yields were very low. Minkoff and Tipper (1962) reviewed the vapor phase oxidation of benzene. Benzene reacts with oxygen a t temperatures above 280°C., with conversions of up to 50%. In general, the yield of phenol is increased by lowering the temperature, increasing the pressure, raising the feedto-oxygen ratio, and by the addition of water and cyclohexane Denton et al. (1950) found that many additives in436
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
crease the yield of phenol and lower the optimum reaction temperatures. The yield per pass of phenol was in most runs below 4%, and the conversion of benzene was low. The use of liquid H F as an additive under high pressures gave high yields per pass, but a t very low conversions of benzene (Simmons and McArthur, 1947). Barnett et al. (1949) used hydrogen bromide as a promoter, temperatures of 195-9" C., and low residence times to obtain conversion of 4sCr of toluene, yields of 28% of benzoic acid, 345% of benzylbromide, a small amount of bromophenols, 17.7LT~ of unidentified products, and a loss of about 15% to degradation products. They also found that benzene is very resistant to oxidation promoted by hydrogen bromide, and that at temperatures as high as 250°C., no oxidation products and only small quantities of bromination products were formed. The present work attempts to broaden the last study by the addition of water vapor to the reacting mixture, the use of HC1, and temperatures up to 450°C. The addition of water vapor prevents the loss of hydrogen bromide to bromination products, enables the recovery and recirculation of the promoter, and promotes condensation reactions. Experimental
Figure 1 is a schematic drawing of the experimental equipment. The reactor was an inclined quartz tube, heated by resistance wires and heavily insulated. The main reactor type used for most experiments (Reactor A) was 25 mm. in internal diameter and 50 cm. in heated length. Reactor B was the same as Reactor A, but filled with 160 grams of '/?-inch Raschig rings to improve the heat transfer. Reactor C was two reactors of Type B, connected in series. Reactor D was 10 mm. in internal diameter and 30 cm. in heated length. Reactor E was two reactors of Type D connected in series. The preheaters were unpacked reactors of type D. Quartz was used for the high-temperature part of the system, since borosilicate glass reactors and preheaters were attacked by hydrogen
thermomeler
thermometer
ocetom dry ice trap
rotameter
era
produc! receiva buporizers
8
BII ond socket joints valve
+
I
Conic sleeve joints Electrial heatng
Figure 1. Schematic dlrawing of experimental equipment
bromide. The vaporizers, however, were constructed from borosilicate glass and lasted through 18 months of experimental work. The vaporizers were borosilicate glass tubes 5 cm. in diameter, 20 cm. long, of about 400-cc. capacity, initially filled with 350 cc. of liquids. The underpart and lower third of the vaporizers were heated by resistance wires. T h e volume of the heated section was about 150 cc. The hleated section of the hydrocarbon vaporizer was filled with the same mixture as the HBrH 2 0 vaporizer. This rserved to lower the boiling point of the aromatic hydrocarbon (the toluene-water-hydrogen bromide azeotrope boils at 84" C. us. 110.5"C. for toluene), and also to protect the silicone grease in the drain valve from slow dissolution in the hot aromatic hydrocarbon. Variation of the concentration of the hydrogen bromide solution and the ratio of vaporization rates made possible the generation of any desired concentrations of the vapor feed components to the reactor. T h e hydrogen bromidewater ratio in the vaporizers did not vary much during a run, since only a small fraction of the vaporizers' contents was used. For the benzene runs a t low concentrations of HBr, one vaporizer was used. I t contained a 47.5 weight 5 solution of H B r in water, in the heated section, and above it a layer of benzene. The feed from this evaporator had the following composition in weight per cent: benzene, 97.5; water, 2.0; HBr, 0.5. Temperature control was by means of Variacs connected in series with the resistance wires and current and voltage meters. The temperature was read by thermometers before and after the reactor. The average temperature a t the outlet was taken as the reactor temperature. Oxygen was introduced through a borosilicate glass porous plug in the center and parallel to the flow of the feed streams upstream of the reactor. Oxygen flow was started only after 5 cc. was collected in the product receiver. Steady state was reached in about 1 minute. The initial non-reacting and unsteady-state operation caused less than 5% error in the determined conversions. T h e dry ice trap plugged easily for the benzene runs, because of the high vapor pressure, high melting point (5.5OC.1, and low conversion of the benzene. Instead,
two alternate schemes were used to recover the benzene from the gaseous effiuent: absorption in a packed column by a mineral wash oil, which was used for most runs, and adsorption in a bed of molecular sieve 13x, which was used for a few runs. For two runs with each recovery system, the total exit gas was collected. The amount of benzene in the gas was within 20% of the value expected from the vapor pressure a t the condenser temperature. For all other runs, the loss of benzene vapor was calculated from the benzene vapor pressure and the volume of the gas, based on the exit gas. High-purity feed materials were used. T h e aromatic hydrocarbons were recirculated after repurification by distillation. Range of Operating Variables in the Reactors. Temperatures, 100" to 450" C.; pressure, atmospheric. The hydrocarbons used were toluene, ethylbenzene, cumene, and benzene. For the runs with two vaporizers, the mole per cent range was Hydrocarbon Oxygen HBr or HC1 Water
5-80% 2.4-50% 2- 15 5-0 1575%
For the benzene runs with one vaporizer, only the oxygen concentration varied in the range of 5 to 25 mole '6. Residence times, 0.2 to 230 seconds, were based on the empty volume of the reactors, a t the reactor temperature. Separation of Products. For the toluene runs, the organic layer was separated from the water layer in the receiver. Benzoic acid was extracted from the organic layer by a NaHCO? solution, and cresols by a NaOH solution. T h e organic layer was then distilled to recover the toluene. The material left was distilled under vacuum to recover benzaldehyde with traces of benzyl bromide. Two isomers of benzyl tolyl ether were separated from the residue by thin layer chromatography. K Ophenol or bromophenols were found. A small amount of phenolic tar was recovered from the cresol fraction. The cresol was a mixture of p and o-cresols. For the benzene runs, the wash oil was distilled. T h e overhead product was combined from the organic layer into the receiver. Phenol was extracted by a KaOH solution. After separation of the water layer, it was acidified. The phenol was extracted by ether, separated by evaporation of the ether, and purified by distillation under vacuum. The organic layer from the KaOH extraction stage was mainly a solution of biphenyl in benzene. The biphenyl was separated by distillation. In a few runs, minute quantities of an unidentified aldehyde ( D S P H derivative-orange needles, with melting point 118"C.) were recovered. Analysis of Products. The products were identified by their melting points, boiling points, and molecular weight determination-by the rise of boiling point of a benzene solution. Phenols and cresols were determined by the boiling point of their bromination products. 0- and p-cresol and 1- and 3-tolyl benzyl ethers were also identified by thin layer chromatography. trans-Stilbene was oxidized to benzaldehyde. Aldehydes and ketones were identified by the melting points of their DNPH (2,4-dinitrophenylhydrazone) derivatives. I n all cases, the products were compared with pure compounds. VOL. 8 N O . 4 DECEMBER 1 9 6 9
437
PhCH?OO* + HBr
The separated products were purified by extraction or distillation and weighed. The amount of benzene lost to degradation products was calculated by a material balance. The major degradation products were CO and carbon, with minor amounts of COP. No CPH2 was detected in the few analyses made of the degradation products. Some very fine carbon was formed, and collected mainly on the reactor and condenser walls. The small amounts in the receiver were easily filtered. Oxidation of Toluene in Presence of HBr
PhCHzOOH
4
+
PhCHzOO*
-
PhCH200*
HzO
+ PhChO 4PhCOOH
(5)
H?O + CO
+ Ph*
7 (6)
degradation products CHrO + PhO* + *C6H, = 0 1
Condensation products are due to the reactivity of the hydroperoxide, which forms cresols from toluene (Achard and Crenne, 1965), and to the activity of the a position of the toluene. At low temperatures, the reaction of the hydroperoxide with toluene forms benzyl alcohol, which is converted in part to benzyl bromide by a reversible reaction with HBr.
PhCHlOOH PhCh,OH
+ PhCHJ+
2PhCh?OH benzyl alcohol
(7)
+ HBr Z PhChzBr + HzO
(8)
The main effect of water is in reversing bromination reactions like Reaction 8. Another effect is the dilution of the reactive mixture. The catalytic activity of hydrogen bromide for condensation is well known (Barnett et al., 1949).
+ PhCHlCHOHPh a P)ICH=CHPh + HzO / stilbene HBr + PhCHeCHOHPh H,O
Similar to the oxidation of toluene promoted by dry hydrogen bromide (Barnett et al., 1949), the important active intermediate is probably benzyl hydroperoxide, which is formed in the usual way.
HBr + O2 + Br* + HOz" Br" + PhCHj- HBr + PhChz* PhCha' + 02' PhCH2OO" benzyl peroxide
+
Degradation products probably follow the decomposition of the peroxide radical to phenyl or phenoxy radicals (Conant and Pratt, 1926; Minkoff and Tipper, 1962, p. 125; Tomura, 1949).
Table I shows typical experimental results from the wide range of experimental parameters. The runs are presented in order of increasing temperature. At low temperature, high residence time, and a low hydrogen bromidetoluene ratio (run l), small amounts of oxidation products, perature, high residence time, and low hydrogen bromidetoluene ratio (run l), small amounts of oxidation products, benzaldehyde, and benzoic acid are formed. Increase of the hydrogen bromide-toluene ratio (run 2) results in increased formation of the condensation product, stilbene, and increased conversion to benzaldehyde. At higher temperatures (runs 3 and 4), the only major product is stilbene. High temperatures, even a t low residence times (runs 7 and 8), result in high amounts of degradation products. A moderately high temperature and a low residence time (run 6) result in small amounts of cresols and benzyl tolyl ethers. T h e degradation products were not of interest to the object of this study and were not analyzed. Barnett et al. (1949) report that the major degradation products are carbon monoxide, ethane, carbon dioxide, and ethylene, which would also be expected for this system. A considerable yield of stilbene can be obtained by this method, and the loss of hydrogen bromide is negligible.
+ PhCHzOH PhCHaOH + PhCHzBr
(4)
Dissociation of the hydroperoxide to benzaldehyde, which can be oxidized to benzoic acid, is (Hock and Lang, 1943) by
+ HzO
PhChaOH
PhCH200H + Br" benzyl hydroperoxide
+
(1) (2)
(3)
(9)
At higher temperatures, cresol and ethers are formed also by the reaction of the hydroperoxide with toluene.
PhCHzOOH
+ PhCHJ-
PhCH20H
+ CHjCsH4OH cresols (10)
PhCHaOH + CH,C,H,OH + PhCHnOCeH4CH3 + Ha0 benzyl tolyl ethers
(11)
Table 1. Oxidation of Toluene in Presence of HBr and HPO
Selectivity of Products from Toluene, Mole % Total Total Amount of Reactants Introduced during Run, Reactor Residence DegradaRun G.-Mole Temp., Time, Benzoic Benull- tramBenzyl tion Reactor Time, C. Sec. Acid dehyde Stilbene bromide Esters Cresoles Products Run Type Min. Toluene H 2 0 HBr O2 b ... .. . Trace Trace 3.9 1 C 450 0.121 0.54 0.045 0.241" 110 222 0.83 6 ... 30.5 ... ... 14.5 2 C 50 0.091 1.43 0.286 0.111 120 23.7 ... Trace 3i85 18.7 . . . Trace Trace 3 C 165 0.273 2.61 0.239 0.250 130 43.5 Trace C
4 5 6 7 8
B E D D
60 60 45 75 90
0.335 1.91 1.72 3.83 5.73
0.64 2.22 0.374 0.803 1.25
0.128 0.130 0.042 0.095 0.14
0.241 0.308 1.2 0.837 1.325
180 200 288 375 415
35.3 4.85 1.2 0.41 0.27
Dry air was source of oxygen for this equipment. 'Not measured. 438
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
...
0.05
...
. .. ...
...
...
...
...
7.6
...
0.58 0.29 0.17
...
Trace
. .. ... ...
0.91 Trace 5.35 1.24 2.2
1.27 0.02 3.03 0.48 0.08
12.2 5.0 40.0 53.8
PhCh?Br + CH3CaH40H+ PhCH?OC6H4CHI + HBr (12) At high temperatures, a tolyl radical can be formed directly, and can react with oxygen to form cresol or decompose to degradation products.
PhCH
+ Br'
-
HBr
+ CHIC6H:%
I
degradation products
CHC6H40*
C H C s H 4 0 H (13)
The decomposition to degradation products in Reaction 13 is analogous to that in Reaction 6. Oxidation of Toluene iin Presence of HCI
+ H?O
Typical results of the oxidation of toluene in the presence of hydrogen chloride and water vapor are presented in Table 11. I n this case 1 he product was colorless, whereas for HBr promotion, the presence of even minute amounts of cresol and ethers, sensitive t o oxidation, caused coloration of the product. No chlorination products were found and only traces of one condensation product (stilbene) were found. At higher temperatures and low residence times (runs 5 and 6), no useful product was formed. At low temperatures (runs 1 to 4), small amounts of oxidation products, benzaldehyde and benzoic acid, were formed; their production increased with increased ratio of hydrogen chloride to toluene. The effect of residence time was less pronounced for this case. Selectivities of to benzaldehyde and S L cto benzoic acid were obtained with an equivalent amount of degradation products. Traces of cresols were found in most runs, apparently as a result of hydroxyliation which was not followed by condensation to ethers, which were promoted by hydrogen bromide. The high vapor pressure of hydrogen chloride makes its recovery from the product more difficult than
that of hydrogen bromide, and an absorption unit following the condensation unit was needed for the higher concentrations of HC1. Oxidation of Ethyl Benzene and Cumene in Presence of HBr H?Oor HCI H?O
+
+
Barnett et al. (1949) report, for the oxidation of ethylbenzene in the presence of hydrogen bromide a t 195OC., yields of about 11'; of acetophenone, 7 ' c of phenol, smaller amounts of bromophenol and benzoic acid, and considerable conversion to degradation products. The addition of water vapor, as shown in Table 111, results in a selectivity of almost 505 of acetophenone, with only about 12': conversion loss to degradation products, a t lower temperatures. The only condensation product obtained was dimethylstilbene. Xo phenols or brominated products were found in this work. Substitution of hydrogen chloride for hydrogen bromide resulted in about 20'c selectivity to a mixture of ethers, with a similar conversion loss to degradation products. For the oxidation of cumene in the presence of hydrogen bromide a t 195"C., Barnett et al. (1949) report yields of 9 . 5 ' ~of phenols, 3% of acetophenone, and large amounts of unidentified products and degradation products. At 120"C., with addition of water for both HBr and HC1 promotion, the major products of the oxidation of cumene were degradation products, despite the low temperatures. The only other product was a mixture of ethers. Phenol and acetone, reported by Barnett et al. (1949), were not obtained in this study. Oxidation of Benzene to Phenol
The experimental results are reported in Tables IV to VI. Tables IV and V summarize the experimental results for runs made with one and two vaporizers, respectively. Table VI singles out runs for which the lowest ratios of biphenyl were obtained. I n each table, the runs are arranged in order of increasing temperature. The accuracy of the measured amounts of reactants
Table II. Oxidation of Toluene in Presence of HCI and H20 (Reactor C)
Run
Total Run Time, Min.
1 2 3 4 5 6
55 150 660 180 105 50
'Total Amounts of Reactants Introduced during Run, G.-Mole Tolueru? H20 HC1 0 2 0.22 0.258 0.21 0.061 1.14 1.34
2.7 2.68 3.33 1.45 5.17 2.44
0.27 0.385 0.61 0.377 0.625 0.63
0.245 1.01 1.03 0.402 0.308 0.67
Reactor Temp.,
Residence Time, Sec.
100 100 102 104 180 260
13 33.2 117 14.4 11.5 3.30
c.
Selectivity of Products from Toluene, Mole i o DegradaBenzalBenzoic tion dehyde acid Stilbene Cresol products 5.0 3.7 3.2 15.2 1.6
...
0.32 1.12 0.76 5.1
Trace 0.24 Trace
... ...
...
...
Trace
Trace Trace 0.29 Trace 0.03
11.7 20.9
...
Not measured. ~
Table 111. Oxidation of Ethyl Benzene and Cumene in Presence of HBr and Ha0 (Reactor C)
Selectivity o f Products, Mole % Total Run Total Amount of .Reactants Introduced During Run, G.-Mole Reactor Residence DegradaTime, EthylTemp., Time, AcetoDimethyltion Min. benzene Cumene H20 HBr HC1 0 2 C. Sec. phenone Acids stilbene Ethers products 245 50 125 95
0.314 0.158 0.144 0.185
1.09 0.733 2.42 1.77
0.212 0.140 0.301 0.41
0.273 0.089 1.12 0.085
130 110 120 120
126 43 28.6 35.5
48.2
... ...
0.21 Trace
...
VOL. 8
... ...
NO.
6.0
... ... ...
... 20.5 0.55 3.9
11.8 22.6 67.5 42.7
4 DECEMBER 1969
439
Table IV. Experimental Results for Runs with One Vaporizer (Reactor A ) "
Run 1 2 3 4 3
6 7 8 9
Amount of Reactants Introduced Measured Total duriv Run' G'-Mole Residence Temp., Run Time, Total Time, 'C. Min. 0, reactants Sec. 150 222 280 280 305 320 330 348 397
359 190 125 155 85 115 115 85 74
2.16 1.27 0.337 1.135 0.420 0.438 0.545 0.335 0.357
8.54 7.65 6.72 7.52 6.80 6.82 6.92 6.72 6.74
Selectiuity o f Products from Benzene, Mole 7; Total Conuersion Yield of Phenol CO, + CO o f Benzene, 57 per Pass, Mole 7; ChHiOH (C6Hi). (+GI o f Benzene Feed of Benzene Feed
17.8 9.03 6.04 6.7 3.88 5.10 5.33 3.65 2.94
13.2 14.75 14.1 12.35 8.57 31.6 28.7 9.27 22
29.3 31.5 43.3 35.7 31.4 35.0 23.7 25.4 29.5
23.2 15.1 1.29 13.85 0.38 6.6 10.85 4.04 5.36
57.5 53.8 42.6 52 63.7 33.4 47.6 65.3 48.4
6.8 4.76 0.56 4.95 0.12 2.31 2.57 1.03 1.58
"Feed composition (except oxygen) for all runs in mole i.c : CsHa, 91.5; H,O, 8.1; HBr, 0.4; total amount o f feed (except oxygen) for each run, 6.38 g.-moles. Table V. Experimental Results for Runs with Two Vaporizers (Reactor A)
Measured Temp.,
Run
C.
10 225 12 268 13 270 14 275 15 280 16 287 18 290 19 295 20 300 22 305 23 318 2ti 330 375 26 27 400-450 a
Total ConYield of Selectiuity of Products version per Amounts o f Reactants Introduced Residence from Benzene, Mole C: Benzene, % Pass, Mole 7; During Run, G.-Mole CO, * CO of Benzerw o f Benzene Time, CsH6 HBr H,O O2 Total Sec. CBH~OH (CsHj)? ("C) Feed Feed
Total Run Time, Min. 245 168 140 106 133 71 66 93 64 167 72 70 111 88
2.28 2.28 2.28 2.28 2.28 2.28 2.28 1.97 2.39@ 2.28 2.28 1.94 2.20 2.28
0.659 0.019 0.66 0.074 0.041 0.028 0.080 0.404 0.033 0.587 0.030 0.350 0.584 0.66
3.97 4.44 3.30 3.86 4.43 3.00 4.19 2.02 2.96 3.29 3.22 1.75 3.52 3.30
0.437 0.368 0.25 0.388 0.774 0.358 0.249 0.237 0.289 1.465 0.138 0.125 0.584 0.592
7.35 7.14 6.49 6.60 7.53 5.67 6.80 4.63 5.68 7.62 5.67 4.17 6.89 6.83
12.00 7.82 7.12 5.26 5.72 4.08 3.09 6.12 3.54 6.80 3.86 4.99 4.46 3.75
19.3 22.5 14.4 20.8 13.4 21.5 37.0 2.19 29.1 4.94 12.5 6.15 18.7 15.5
47.4 20.3 65.4 8.95 21.2 6.83 19.3 41.5 12.5 16.3 7.85 45.2 29.1 29.2
33.3 57.2 19.3 70.2 65.4 71.3 43.7 56.2 58.4 64.3 79.7 48.7 52.2 55.3
18.05 13.4 10.7 16.8 20.2 4.12 7.07 24.6 8.67 40.5 6.53 1.93 14.4 22.5
3.49 3.02 1.54 3.50 2.70 0.89 2.62 0.54 2.53 2.00 0.82 0.12 2.69 3.49
Including 5
