THOMAS H. STRICKLAND and ALAN BELL Research Laboratories, Tennessee Eastman Co., Division of Eastman Kodak Co., Kingsport, Tenn.
Oxidizing Alkylbenzenes with
SO1
Sulfur dioxide can be used as the sole source of oxygen a n d as a catalyst moderator without poisoning the vanadium oxide catalyst
I4
\'i\POR-PHASE: cat.al5tic oxidation of alkylbenzenes with air or oxygen (3. J. 7-9). most investigators have recorded the presence of hot spots in the c a t a l y t bed and have suggested methods for suppressing them. O n e method proposed (3'1\vas the use of small amounts of catalyst moderators, which \\.ere either added to the catalyst prior to oxidation or fed continuously with the material being oxidized. Organic sulfur compounds or sulfur oxides have usual1:- been used for this purpose. Toland considered sulfur dioxide as one of the more efficient of these moderators. particularly \vhen used in combination \vith a vanadium oxide catalyst (7). Sulfur dioxide concentrations of 0.01 10 l.5yo by \veight of the organic compound undergoing oxidation jvere satisfactory. Furthermore. use or sulfur dioxide \vas restricted to those oxidations in ivhich the oxygen-containing gas \vas present in such amounts that the reaction mixture contained a stoichiometric excess of oxygen. Thus. it was assumed that sulfur dioxide was not a reazent in the oxidation, but rather a n inhibitor, or moderating type of poison. to the catalyst system. I n fact. when sulfur dioxide alone \vas passed over a vanadium oxide catalyst. the c a t a l y t was deactivated for appreciable periods of time 17). T h e object of this investigation was to determine if sulfur dioxide could be used as the sole source of oxygen in the vanadium oxide-catalyzed, vapor-phase oxidition of aromatic hydrocarbons such as toluene. o- and p-xylenes. t u t but? lbenzene, ethylbenzene. and butylbenzene. T h e initial investigations were conducted with toluene. Benzoic acid was identified as a product of this reaction. Statisticall) designed experiments Ivere then used to determine the optimum conditions. T h e conditions thus found Lvere also used in the oxidation of other monoalkvlbenzenes a n d
or methylcyclohexane. Statistically designed experiments \\ere again used to study the oxidation of 0- and p-x. lenes. Results Toluene. containing 15% by Jveight of sulfur dioxide. was passed over a vanadium oxide catalyst a t 350' C . a t a rate which gave a contact time of 5 seconds. Among the reaction products were hydrogen sulfide and a trace of benzoic acid. \\'hen the temperature was increased to 380" C.. benzoic acid was obtained in a n 80% yield. based on sulfur dioxide (5). T h e temperature range from 275 ' to 450' C. \vas investigated. Benzoic acid was the predominant product formcd benveen 3.50' and 430' C.. a n d betwc:en 390" and 430' C.. i t \vas the only oxygenated product isolated. Benzaldehyde \\'as the predominant product formed betxveen 275' and 330' C. a n d was the only oxygenated product formed betxvcen 275' a n d 315' C. Neither benzyl alcohol, maleic acid. nor anv organic sulfur compounds were formed
Ivithin the temperature limits investigated. Occasionally, traces of free sulfur \vere observed in a reaction product; however, a t optimum operating conditions. hydrogen sulfide was obtained in quantitative yields. Statistically designed experiments were conducted to determine the optimum conditions for oxidizing toluene to benzoic acid. These experiments iverc: designed so that the response surfaces of three independent variables-temperature, contact time, and mole ratio of hydrocarbon to sulfur dioxide-could be analyzed simultaneously ( I ) . T h e effect of each variable \vas determined a t two levels: temperatures of 330' and 370' C.; contact times of 2.5 a n d 3.5 seconds; and hydrocarbon--sulfur dioxide mole ratios of 3 to 1 a n d 5 to 1. From these experiments. it became obvious that the effect of temperature, particularly a t the higher level, overshadoLved both the effect of contact time a n d hydrocarbon-sulfur dioxide mole ratio (Table Ij. An excess of toluene save the maximum yield of benzoic acid. calculated on the basis of either
This reaction is particularly adaptable for making benzoic acid from toluene. The reactants are readily available and inexpensive. Hydrogen sulfide, the by-product, is readily converted to sulfur dioxide for recycling. The market potential for benzoic acid is excellent. Specifically, the acid i s a potential intermediate for making caprolactam, the precursor o f nylon 6. Thus, it can be catalytically hydrogenated to cyclohexanecarboxylic acid which, according to a recent Belgian patent (582,793)) may b e converted by a one-step process to caprolactam in 80 to 85% yield.
VOL. 53, NO. 1
JANUARY 1961
7
toluene or sulfur dioxide. T h e excess toluene was considered the inert diluent and was recovered unchanged. A r u n was made using the optimum conditions derived from this block of experiments; temperature, 410' C.; contact time. 3 seconds; and a mole ratio of toluene to sulfur dioxide of 3.5 to 1. Benzoic acid was obtained in 757, yield (28% conversion), based on toluene, and 9577, yield (957c conversion), based on sulfur dioxide.
Table I.
T h e catal>st life \cas not determinrd; however, the laborator! -scale equipment \vas operated intermittently for 40 hours without detectable loss of catalyst activity. T h e conditions Lvhich had given the maximum >-ield of benzoic acid from toluene were then used to determine the scope of monoalkylbenzenes Lvhich could be oxidized lvith sulfur dioxide (Table 11). I t was not considered of value to determine the conditions that
Effect of Temperature Overshadowed Effects of Contact Time and Mole Ratio of Hydrocarbon to Sulfur Dioxide Formation of Benzoic
Contact Time, See. 2.5
Mole Ratio Toluene SO? 3 1
5
2.5 3.5
1
3
1
Toluene Basis Tield. Convn.,
Temp.. C. 330 370 330 370 330 370
Yield.
('otivii.,
%
%
c;t
%
1.4 54.6
0.2 9.7 1.0 6.9 0.7 10.0
1.1 38.3 8.0 38.7 5.5 50.3
0.6 33.3 5.3 36.5 4.2 44 2
7.6 27.3 6.9 45.7
I?Hydrocarbon inlet
-6 fi
Borosilicate glass reactor
~
.Id
Sulfur Dioxide ____ Bs-i.
Nitrogen inlet
v-
+Ylf!----
Sulfur dioxide inlet
Thermowell
Brass sleeve Glass beads
Catalyst
Electric heater
To
+
receiver
The lower end of the reactor was connected to a receiving vessel which was vented through an ice-water condenser, a scrubber containing 10% sodium hydroxide solution, a gas-sampling tube, and finally a wet-test meter. Glass beads were used to position the catalyst bed
Table II.
Yield and Conversions of Benzoic Acid from Toluene at 41 0 " Greater Than from Other Monoalkylbenzenes
c. W e r e
Forination of Benzoic id Sulfur Dioxide Bahia Coiivn R Tield % Con711 , C;
Hr drocarhon Bas.
HSdrocarbon Toluene Ethylbenzene Isopropylbenzene Butylbenzene tert-Butylbenzene ~
8
Yield, 7 75.0 31.3 30.8 21.0 5.5
~~~~
INDUSTRIAL AND ENGINEERING CHEMISTRY
27.9 17.2 9.0 7.2 0.7
95.0 52.0 41.0 37.0 12.0
95.0 45.0 41.0 37.0 10.5
Ivould give the maximum yield of benzoic acid from the monoalkylbenzenes other than toluene. \Vhile this investigation \vas in progress, Danforth and Bender ( 2 ) reported the dehydrogenation of methylcyclohexane to toluene using sulfuidioxide and a carbon catalyst. When this experiment Lvith methylcyclohexane and sulfur dioxide !vas repeated using vanadium oxide instead of carbon as the catalyst, benzoic acid was obtained in 26% yield. KO cyclohexanecarboxylic acid was detected in the products from this reaction. Benzaldehyde was often identified in the reaction products from the designed experiments for the oxidation of toluene to benzoic acid. From the data available. it appeared advisable to investiSate temperatures Lvhich kcere lotver than optimum for producing benzoic acid. Therefore, the temperature range of 275' to 330' C. was scanned, using conditions similar to those for the oxidaion of toluene to benzoic acid. Between 310" and 315" C . ? benzaldehyde was formed in 9Oyc yield. based on sulfur dioxide (6). T h e initial investigation into the oxidation of dialkylbenzenes was made with p-xylene under the conditions that were optimum for the oxidation of toluene. Analysis of the reaction products indicated that at least three separate oxidation levels coexisted in the reaction zone: oxidation of a n alkyl group to the corresponding aldehyde ; oxidation of this aldehyde to a carboxylic acid; and oxidation of the second alkyl group to a carboxylic acid. Thus. the products from the oxidation of pxylene included p-tolualdehyde, p-toluic acid, and terephthalic acid. Neither terephthalaldehyde nor terephthalaldehydic acid was identified among the reaction products. LVhen o-xylene was oxidized under the conditions that were optimum for oxidizing toluene. the reaction products included o-toluic acid and o-phthalic acid. o-Tolualdehyde was not identified. Statistically designed experiments were then used in studying the oxidations of o- and p-xylenes. I n both cases, a n increase in the ratio of xylene to sulfur dioxide resulted in a decrease in the yield of the corresponding phthalic acid and an increase in the yield of the corresponding toluic acid (Table 111). The low yields obtained were attributed to the difficulty of removing the oxygenated products from the fixedbed reaction zone. Fluidized- or moving-bed reactors were not investigated.
Experimental Materials. Commercial-grade materials were used without further purification: sulfur dioxide? Matheson CO.; nitrogen, Southern Oxygen Co. ; and
O X I D I Z I N G ALKYLBENZENES
Table Ill.
Results Obtained Under Optimum Conditions for Oxidations of 0 - and p-Xylenes. Ba~iz Conrn..
o-Syictic -___
lieid.
l'roduct
7c
p-Xylene Basis ~~
Tield.
CI ,c
Phthalic acid 2.8 1.7 +Toluic acid 5.0 3.1 Terephthalic acid .. . ... p-Toluic acid ... p-Tolualdehyde ... a Heaction temp.. 110° C . ; inole ratio of
... ...
70
Couvii., 0 ,'O
...
...
... ...
0.2 1.4 1.3
0.1 0.4 0.3
Sulfur Dioxide Basis Tield, Conrn., %
sc
15.8 14.2 2.2
7.5 11.2
11.2 10.1 0.3 1.2 1.6
Colltact
Time, Sec.
5 5 3 3 3
xylene to sulfur dioxide. 3 to 1.
toluene. Cosden Petroleuin C:o. T h e other hydrocarbons Lvere Eastman-grade chemicals. T h e catalyst was 10" vanadium oxide o n alumina (\.-0301 -T. '*-inch pellets) from Harsliaw Chemical Co. Apparatus. T h e liquid reagents \vere metered from modified Mariotte bottles through appropriately sized rotameters. a n d the gsseous reagents were metered through Croxton flolrmeters into the top of a verticall>- mounted borosilicate glass reactor. The reactor was 16 inches long and 25 m m . in outside diameter. A borosilicate glass thermowell. 8 m m . in outside diameter. extended the entire length of the reactor tube. T h e catalyst bed was positioned in the reactor tube by the appropriate arrangement of glass beads having a diameter of 3 m m . T h e reactor was heated with a 750watt electric furnace. This furnace \vas modified \vith booster elements ivhere necessary to maintain a uniform temperature throughout the catalyst bed. A brass sleeve (I s-inch wall thickness) provided the surface for transfer of heat from the furnace to the reactor. The temperature of the furnace \vas controlled by a \Vheelco Capacitrol with Xvliich t\vo thermocouples \rere used simultaneously. O n e thermocouple was located in a thermor~-ellin the brass sleeve. and the other in the thermo\vell of the reactur. The temperature in the reactor \vas recorded by a Leeds and Northrup potentiometer e:iuipped \vith a probc-type thermocouplr located inside the thermowell of the reactor. Experiments. The fo1loJving experimental d a t a are presented for the optimum conditions found for the oxidation of toluene to benzoic acid. toluene to benzaldehyde. and p-xylene to terephthalic acid. Oxidations of Toluene to Benzoic Acid. Toluene (0.1465 mole per hour)) sulfur dioxide (0.0419 inole per hour). and nitrogen (0.005 mole per hour) were passed simultaneousl!. for 6 hours into the reactor containing a vanadium oxide catalyst (30 grams) heated to 410' C. T h e reaction product was dissolved in ethyl ether and extracted
lvith 107@ sodium hydroxide solution. T h e alkaline extract. after acidification n i t h hydrochloric acid. gave 30.1 grams (977, yield. based on sulfur dioxide) of benzoic acid having a melting point of 121-2' C. T h e melting point of this compound mixed with a n authentic sample of benzoic acid was not depressed. .\nal)sis. Calculated for CiH602: C . 68.84; H. 4.96. Found: C, 68.83; H . 4.92. The ethereal solution was dried over anh>drous magnesium sulfate and distilled. T h e toluene recovered (uncorrected boiling point. 109-10' C . ; I#, 1.4974) amounted to 50.7 grams, or 62..5% of the toluene used. Oxidation of Toluene to Benzaldehyde. Toluene (0.244 mole per hour), sulfur dioxide (0.079 mole per hour), and nitrogen (0.0098 mole per hour) were passed simultaneously for 2 hours into the reactor containing a vanadium oxide catalyst (30 grams) heated to 31 5 O C. T h e reaction product, analyzed by gas chromatography, contained 917@ of toluene. 6.4Ye of sulfur dioxide, and 2 . 3 2 ; of benzaldehyde. Benzyl alcohol and benzoic acid \rere not detected. T h e benzaldehyde \+'as further identified by preparing its 2,4-dinitrophenylhydrazone (melting point, 234-6 ' C.). The melting point of a mixture of this compound \\.it11 a n authentic sample of benzaldehyde 2,4-dinitrophenylhydrazone was not depressed. Analysis. Calculated for Cl3H10N4O4: C, 54.55; H. 3.50; S , 19.58. Found: C, 54.67; H, 3.56; S,19.53. Oxidation of p-Xylene to Terephthalic Acid. p-Xylene (0.1445 mole per hour) and sulfur dioxide (0.048 mole per hour) were passed simultaneously for 3.5 hours into the reactor containing a vanadium oxide catalyst (30 grams) at 410' C. T h e reaction mixture was made basic lvith 10% sodium hydroxide solution and extracted with ether. T h e ether extract \vas dried. and its p-tolualdehyde content was determined by means of the standard 2,4-dinitrophenylhydrazine procedure. T h e yield ofp-tolualdehyde was
0.22 gram (1 1 .2ye > ield, based on sulfur dioxide), Thep-tolualdehyde was identified by the preparation of its 2,4dinitrophcn>lhydrazone (melting point, 232-4" C.). The melting point of a mixture of this compound with a n authentic sample of the derivative \vas not depressed. .\nalysis. Calculated for CIdH1J404: C, 55.99; H. 4.02; K. 18.66. Found: C, 56.43; H: 4.10; S. 18.58. T h e unreacted p-xylene (uncorrected boiling point, 136' C. n y , 1.4948); was recovered by distillation, and amounted to 39.6 grams (73..5r;7,) of the p-x)-lene used. T h e alkaline extract \vas acidified with hydrochloric acid and cooled to 10" C.: and the white precipitate that formed was collected by filtration. T h e filtrate was extracted with ether, and the ether Iras then used to wash the white precipitate previously separated. T h c ether \vas evaporated to give 0.20 gram (7.5% yield, based on sulfur dioxide) of p-toluic acid having a melting point of 175-7 C. \Vhen this compound was mixed \vith a n authentic sample of p-toluic acid. the melting point \vas not depressed. The p-toluic acid \vas further identified by its infrared spectrum. T h e ivhite. ether-insoluble precipitate (0.05 gram) \vas identified as rerephthalic acid (2.2Tcyicld! based o n sulfur dioxide) by comparing its infrared spectrum \vith that of a n authentic sample. Acknowledgment
T h e authors wish to express their appreciation to 1%. hZ. Hill for the analysis and interpretation of the statistically designed experiments. and to the analytical laboratory for the analyses of the reaction products. literature Cited (1) Box, G. E. P., jt~ilson,K. B., J . R o j . Statistical Snc. A13, 1-35 (1951). (2) Danforth. J . D., Bender. M. ,J., ISD. ENG.CHEM.46, 1701 (1954). (3) Law, G. H.. Chitwood, H. C. (to
Carbide and Carbon Chemical Corp.), British Patent 518,823 (March 1940). (4) Parks. w.c;.. Yula. K. w.. IND. L N G . CHEM. 33, 891-7 (1941). (5) Strickland, T, H.. Bell. .I. (to Fkstman Kodak Co.). U. S. Patent 2,821,552 (Januarv 1958). (6) Ibid.. 2,928,879 (March 1060). (7) Toland, \Y. G.. J r . (to California Research Corp.). U . S . Patent 2,574,511 (November 1951). (8) Walter, J.. ,J. P m k t . Chem. 107 (1892). (9) Weiss, J . M.. Downs. C. R.. 1x1. ENG.CHEM. 12, 229 (1920). RECEIVED for review May 1960 ACCEPTED October 12. 1960 Division of Organic Chemistry. 136th National Meeting. ACS: Xtlantic City, N. J., September 1959. VOL. 53, NO. 1
JANUARY 1961
9
