M
0
FIGURE 1. APPARATUS FOR VAPOR-PHASE OXIDATION OF XYLENE
Vapor-Phase Catalytic Oxidation of Organic Compounds Production of Toluic Aldehyde and Phthalic Anhydride from Xylene' W. GEORGE PARKS AND CHAUNCEY E. ALLARD hyde, phthalaldehyde, benHE vapor-phase catazene dicarboxylic acids, and lytic oxidation of orRhode Island State College, Kingston, R. I. ganic compounds some toluic acid, benzoic acid, and benzaldehyde. offers a simple method of producing partially oxygenated derivatives. Several procCraver (7) found that o-xylene vapor mixed with air and passed over the oxides of vanadium gave 9.6 parts of phthalic esses such as the oxidation of toluene to benzaldehyde, benzene to maleic acid, and naphthalene to phthalic anhydride are in anhydride, 1.0 part of 0-toluic aldehyde, 1.2 parts of maleic acid, and 6.0 parts of carbon dioxide and water per 100 parts commercialoperation. Although a great many investigations have been carried out on the direct oxidation of benzene and of o-xylene passed over the catalyst. He also found that if toluene (3, 6, 11, 91), relatively little work has been done with this mixture was passed over uranium oxide or molybdenum oxide at 600" C. in the proportion of 7.3 grams of air to 1.0 the xylenes. A major part of the previous work on direct oxidation of both aromatic and aliphatic hydrocarbons involved gram of xylene with a contact time of 0.39 second, a yield of approximately 50 per cent 0-toluic aldehyde resulted without the search for oxidizing media, diluents for reaction control, and selective catalysts, both homogeneous and heteroany appreciable amount of acid production or complete combustion. Molybdenum, vanadium, or tantalum oxide catageneous in their application (1, 2, 6, 22, 26). The purpose of this investigation was to study the effect of various catalysts may be used to form aromatic aldehydes from the xylenes, mesitylene, and p-cumene. lysts upon the partial oxidation of xylene. Previous investigators have found that the oxides of the metals of the fifth Rittman, Byron, and Egloff (19) subjected xylene to temand sixth groups of the Periodic Table were most effectiveas peratures of 650', 725", and 800' C. a t subatmospheric oxidation catalysts. Oxides of these metals and many pressure and pressures of 1, 12, and 18 atmospheres. They others not contained in the fifth and sixth groups were studidentified by distillation toluene, benzene, naphthalene, and ied. Various types of catalyst supports were employed. anthracene. Charlot (6) determined the comparative volOperating conditions, such as time of contact, air-hydroumes of carbon dioxide produced by air oxidation of toluene, diphenylmethane, naphthalene, alcohol, heptane, and xylene carbon ratio, and temperature, were varied over a wide range in the presence of various metallic oxides at 300" to 450" C . with each catalyst. Gibbs and Conover (10) found that a mixture of xylene Buylla and Pertierra (4) observed that o-xylene vapor and vapor and air passed through a tube over a vanadium pentair passed over a vanadium pentoxide catalyst at 450' C. oxide catalyst at 350" to 530" C. produced methylbenzaldeproduced phthalic anhydride. Burgoyne (3) studied the d 0 W combustion Of 0-, m-, and p-xylene. He found the 1 The firat paper in this series appeared in Maroh, 1936, pages 319 to 323.
T
1162
INDUSTRIAL AND ENGINEERING CHEMISTRY
SEPTEMBER, 1939
1163
The reaction chamber consisted of a calorized tube, 1inch (2.54 cm.) i. d., and 24 inches (61 cm.) long, fitted at each end with a cap. Each cap was drilled and threaded to take a $/,-inch pipe. This tube was enclosed by a rectangular welded-steel box, J,7l/z X 71/2 )< 21 inches (19.1 x 19.1 x 53.3 cm.). The steel box was filled with molten lead so that the reaction tube was surrounded on all sides by 3 inches of lead which gave good heat transfer away from the reaction zone. A vaporizing unit, I , into which the xylene was pumped at any desired rate by means of a constantfeed pump (15),was joined t o the 3/4-inch pipe at the inlet end of the reaction tube. This joint was made gastight by means of a packing of asbestos string sealed at the end with Insalute cement. The vaporizer, designed to give thorough mixing of the air and xylene vapor, was wound with Nichrome wire and covered with sheet asbestos to provide a heating unit. Into the perpendicular side arm was inserted the delivery tube from the constant-feed pump, G. The temperature of the vaporizer was regulated by means of a rheostat, P , andmeasured by the thermometer inserted at H . A t the exit end of the reaction tube a Pyrex glass T was inserted into the a/,-inch pipe and made gastight. A thermocouple, L, enclosed in a small-bore Pyrex glass tube, was inserted through the top of this T so that it extended into the catalyst mass located near the center of the reaction tube. The temperature was measured by the pyrometer, K. The vertical end of Apparatus the T extended into a round-bottom flask, M , from which the oxidation products passed into two ice baths, 0, and two waterThe apparatus first used in this investigation consisted of cooled spiral condensers, N . To facilitate changing catalysts, a long tight-fitting Pyrex test tube with numerous small holes a small glass vaporizer connected to a silica reaction tube by blown in its closed end was used. The lip of this tube fitted means of a ground-glass cap. The reaction tube was "4 tightly against the end of the reaction tube so that all vapors had inch (19.1 mm.) in diameter and 24 inches (61 cm.) long, to pass over the catalyst. The heavy lead bath was mounted on wound with Xichrome wire to serve as a heating unit and ina framework made of 3/4-inch iron pipe. Heat necessary to start the reaction was supplied by two parallel rows of gas burnsulated with Alundum cement and asbestos. A series of ers. spiral condensers was connected to the exit end of this tube. The air supplied by a centrifugal pump, A , was passed through Early in the investigation it was found that such a reaction glass wool and then through a purification train containing sodium tube did not afford suitable control of the reaction temperahydroxide, B , sulfuric acid, C , and anhydrous calcium chloride, D , to remove dust, carbon dioxide, ture. In several runs the heat and water. It was then metered of reaction was so excessive by a calibrated flowmeter, E , that the aluminum support was into the xylene vaporizer. The fused together and had to be material used was the xylene fracThe direct vapor-phase catalytic oxidation tion from petroleum; it consisted melted out of the tube. of xylene to produce toluic aldehyde and of all three xylenes with the ortho The temperature control of and meta forms predominating phthalic anhydride has been investigated; the catalytic vapor-phase oxida(93). The xylene was pumped catalysts were employed which varied tion of aromatic compounds from a graduated 50-cc. cylinder, F , through a 0.3-mm. bore capilhas been found to be difficult greatly in chemical nature as well as lary tube into the vaporizer. (8, 16). Many patents have physical structure. Compounds of molybSince the vaporizer unit wasmainbeen granted numerous investained a t a temperature above the denum, vanadium, iron, chromium, tin, tigators on apparatus for temboiling point of xylene, the mazirconium, magnesium, potassium, tungterial was vaporized immediately. perature control (9, I S ) . The At this point the vapor and air reaction to form phthalic anhysten, titanium, and aluminum were used c a m e i n c o n t a c t a n d were dride and toluic aldehyde from singularly and in various combinations. thoroughly mixed as they passed xylene is highly exothermic out of the vaporizer into the reacThey were used either in granular form or tion chamber and through the (Equations 1 and 2 ) and neceson a support of porous aluminum, activated catalyst mass. The desired rate sitates good heat transfer since of xylene flow was obtained by alumina, asbestos fiber, silica gel, pumice, the heat of reaction is greater regulating the length of the stroke than that necessary to mainor Alfrax. The effect of temperature, time of the liquid feed pump (90). The catalyst temperature was meastain the incoming gaseous mixof contact, and air/xylene ratio upon the ured by the thermocouple exture a t the temperature of the type and amount of product was studied. tending into the center of the catalyst : catalyst mass. The exit gases Results on fifteen catalysts which showed passed through a sublimation an appreciable effect are reported. chamber into an ice and salt bath, CeH4(CH& Oz then through t w o water-cooled With a tin vanadate catalyst at 320' C., CHaCsH4 CHO HzO condensers, and finally through a 92.4 kcal. (1) a time of contact of 0.002 minute, and an second ice bath. The length of a run varied from 1 to 5 hours, deair/xylene ratio of 13.4, a yield of 8 per cent pending upon the rate of flow of phthalic anhydride was obtained. With a the xylene, usually until, at least 30 cc. had been passed through vanadium pentoxide catalyst supported on the apparatus. Alfrax at 530" C. and an air/xylene ratio of Analysis 34, 18 per cent phthalic anhydride was obAverago of the values given by Kharasch (14). tained. This catalyst gave yields varying Whenever the drip consisted only of liquids, the water andfrom 45 to 85 per cent phthalic anhydride oil layers were determined by A diagram of the apparatus on the basis of the amount of xylene oxitheir pkiysical boundary in a finally assembled for the vapordized per pass. graduated cylinder. A porphase oxidation of xylene is tion of the oil layer was shaken shown in Figure 1.
oxygen reacted to be in linear proportion to the pressure increase and t o the time, after the initial stage. Alkyl side chains favor the formation of intermediate oxidation products. The energy of activation obtained was 34-41.5 kcal. Maxted ( l 7 ) , in his study of the catalytic properties of tin vanadate with aromatic hydrocarbons, reported phthalic anhydride as the main product in the oxidation of o-xylene. A maximum yield of 59 per cent was obtained with 10 cc. of tin vanadate as catalyst, a catalyst temperature of 290" C., and a space velocity of primary and secondary air of 200 and 600, respectively. Wilken-Jorden (24)) investigating the catalytic oxidation of a mixture of 0-,m-, and p-xylene, obtained small amounts of 0-, m-, and p-toluic aldehyde. Using manganese vanadate as a catalyst with xylenelair ratios of l/0.36 to '/3.a1 and a contact time of 35 to 63 seconds, he found that the oxidation begins a t 350" C. and proceeds slowly to 450" C. where a rapid increase occurs with increase in temperature.
+
(1
+
+
+
INDUSTRIAL AND ENGINEERING CHEMISTRY
1164
TABLE I. EFFECTOF VARIOUS CATALYSTS ON THE VAPOR-PHASE OXIDATIOX OF Catalyst Sn(V0s)i
Sn(VOd4
Fused Vi08
None
Tech. VzOs
None
Sn(V0s)a
Aluminum
Uranium molybdovanadate
None
Uranium molybdovanadate
Aluminum
Ti02
Pumice
Zirconium oxide
Silica gel
Molybdic oxide
Silica gel
Alfrax
Time of Contactb See.
0.0060 0.0051 0,0039 0.0057 0,0067 0.0036 0.0047 0.0042
Air/ Xylene RatioC
Phthalic Toluic Anhydrlde Aldehyde per Pass per Pass
14.8 18.8 20.4 24.0 27.2 27.2 30.6 32.8 22.6 31.7 43.5 64.6 15.6 34.0 59.4 11.0 16.5 20.6 24.3 26.7 34.4 55.7 62.0 42.7 2.7 21.4
%
%
0.6 2.9 3.5 7.1 7.9 7.3 6.6 8.7 4.0 5.7 5.0 2.9 2.9 6.6
3.6
0.6 2.3 2.4 4.5 3.2 3.3 2.4 1.9
.... .. ... ...
... ... .. .. .. ...
...
... ... ... ... ... ,..
.. .. .. ...
.... ..
6.1 10.0 11.6 10.3 9.2
... ...
14.4 13.5 8.2 14.7 15.2 16.0 16.5 18.2 17.5 14.8 10.7 Trrtce
... ... .. .. ..
194 175 225 221 198 222 70 235
450 480 480 480 500 500 500 530 530 530 530 530 530 530 530 530 500 510
17,600 18,780 19,500 26,500 17,700 20,180 24,100 13,100 14,650 17,650 18,490 20,800 23,600 23,720 21,100 24,600 7,700 8,270
0.0034 0.0032 0.0031 0.0023 0.0034 0.0030 0.0025 0.0046 0.0041 0.0034 0.0032 0.0029 0.0025 0.0025 0.0029 0,0025 0.0077 0,0072
0.5 0.5 40.0 39.3 39.0
202 234 I22 56 70
510 510 500 620 460
10.0 16.0 7.1 7.0 11.0 6.0 6.2
1.8 1.2 0.0 0.0 0.8
3 22 242 152
1.25 1.25 5.5 2.0 2.25 2.5 3.0
56.0 60.0 45.5 16.0 15.1 12.4
7.7
36.8 28.8 19.1 8.0 5.3 3.4 0.8
226 138 170 56 136 164 64 86 104 128
30 30 20 20
1.25 1.25 1.5 2.0
48.0 48.0 24.0 13.0
32.0 32.0 16.0 5.0
56 102 130 206
450 480 450 450 450 450 450 550 440 550 450 450 440 450 460 550 460 460
52,760 61,500 8,900 4800 6:480 19,450 40750 24:400 22,300 36,300 22,180 24,100 6,120 17,500 11,700 4,030 5,300 6,260 7,750 5,330 10,100 16,300 25,200
0.0011 0.0001 0.0067 0.0125 0.0092 0,0031 0,0014 0.0025 0.0027 0.0016 0.0027 0.0025 0.0098 0.0034 0.0051 0.0149 0.0113 0.0096 0.0077 0.0113 0,0059 0.0037 0,0024
25 25 25 25 25 25 25 25 40 90 60
2.0 2.0 2.0 1.8 1.5 1.25 2.0 2.0 3.0 1.0 1.0
11.5 17.0 13.0 27.0 22.6 20.0 13.0 12.5 50.0 60.0 50.0
2.0 5.5 3.0 7.6 13.3 10.0 6.0 3.0 5.3 10.0 20.0
200 136 98 74 56
480 500 480 500 470
20,100 14250 1O:OOO 8,100 5,920
0.0030 0.0041 0.0060 0.0074 0.0100
23.9 11.0 10.3 3.8
1.8 1.2 1.1 Trace
1.5 Traoe 1.2 0.0 0.5 1.2 1.0
3.4
Trace
1.1
19,700 21,000 27,400 11,200 3,460 3,420 12,800 21,000 19,000 21,800 23,200
0.0031 0.0029 0.0022 0.0054 0.0170 0.0170 0.0047 0.0029 0.0032 0.0028 0,0026
13.8 23.0 30.8 ' 3.7 2.3 1.9
Trace Trace Trace Trace Trace Trace
18.1 42.6 35.0 35.4 27.4
11.7 13.5 7.4 5.3 4.8
25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 75
2.1 2.0 2.75 2.0 2.0 2.0 2.5 2.0 2.0 2.0 2.0 2.0 1.5 2.75 2.5 2.25 1.5 1.5
13.8 8.0 6.4 5.8 15.0 8.0 5.2 15.0 11.2 11.0 10.0 10.0 9.0 5.5 4.4 7.1 34.6 15.0
5.5 4.2 3.5 2.2 2.0 1.0 0.2 0.0 0.5 23.4 2.0
10 35 35 30
2.0 2.0 1.5 1.5 1.0
10.3 8.5 50.0 50.0 50.0
15 15 15 15 15 15 15
2.0 1.25 3.5 3.0 2.0 2.0 2.5
30 30 40 40 40 40 40
10
1.9 1.5 0.7 0.5 2.2 1.5 0.1
6:4
212 184 196 266 198 170 225 120 136 164
460 450 460 Activated alumina None 136 280 650 102 550 Iron chromate None 70 500 500 26 1.75 9.4 3.7 124 Sn(V0s)a ceric None 510 202 25 2.0 6.5 0.8 oxide 510 7.0 0.2 182 25 2.0 520 25 1.5 8.0 0.3 207 25 2.0 11.0 1.5 220 520 5 Spaoe velocity = liters of gas per hour (at temperature of reaction) per liter of catalyst space. b Time of contact = ratio of volume of catalyst space t o volume of gas passing through catalyst per c Air/xylene ratio = (grams of air per hour)/(grams xylene per hour).
Tungstic oxide
:E FRACTION FROM PETROLEUM XYLES
Air, Xylene Xylene Space Vaporized Recovered Velocity Temp. of Velocity" Catalyst Time Catalyst per Hr. Catalyst per Hr. per Hr. Vol. of Run Support O c. Liters cc. cc. Cc. Hr. 34,100 440 136 10 2.0 50.0 38.0 None 8,500 450 56 1.8 7.5 16 4.0 3,500 350 25 0.7 15 3.0 5.0 17,400 480 102 1.5 15 2.0 12.0 9,350 400 62 0.6 6.4 15 5.0 37,480 300 194 3.7 17.0 10 3.5 8,350 420 54 0.5 4.9 15 4.5 4,430 370 62 0.6 5.1 30 3.5 25,800 320 194 15 7.0 4.0 13.5 29,400 450 194 2.5 13.8 16 2.8 12,300 370 90 0.2 4.8 16 5.0 15,600 350 120 0.0 5.4 16 3.3 22,300 380 165 0.0 16 4.3 5.8 23,900 380 165 1.0 15 4.0 7.5 10,000 1.52 320 Asbestos fiber 30 2.0 14.0 , 5.0 11,700 144 !50 30 2.25 10.5 1.8 15,200 900 30 2.25 12.0 1.3 177 10,600 430 30 2.5 8.0 0.0 ' 136 8,940 116 420 30 2.5 6.0 0.0 510 16,480 30 2.5 7.6 1.2 152 12,800 320 30 3.0 6.7 0.7 152 208 340 14,100 30 3.25 7.7 0.6 Alfrax
VzOs
A
VOL. 31, NO. 9
+
200 217
2.3
27.0 37.7 3.3 1.5 1.9
9.8 8.3
20.8 20.9 24.4 27.5 28.2 31.9 37.6 1.5 3.1 4.9 5.6 7.8 11.5 23.0 ' 1.6 2.9 7.4 21.7
2.6 5.3 4.6 6.2 3.9 2.1 1.3 Trace Trace Trace 1.1 1.5 1.9 2.3
.. .. ..
Tyke 010
...
.... .. ... ... ... ...
... ... ...
4.4 . e *
. * I
...
3.9 4.6 4.5
... ... ... ... ...
.... .. 4.5 4.6 3.5
... ...
0.8
...
... .. . ...
T&e Traoe Trace
...
... Trice Trace
second a t temperature of reaction.
r
with a saturated sodium bisulfite solution for 12 hours (15). The aldehyde was liberated from its addition product with sodium carbonate and extracted with ether. The aldehyde was identified by means of its boiling point and the melting points of its solid derivatives. The water layer was titrated with a standard solution of sodium hydroxide, evaporated to a small volume, and treated with a strong mineral acid, and
the free aromatic acids were extracted with ether. Upon evaporation the crystals were purified by sublimation and identified by their melting point, acid number, and derivatives. A slightly different procedure was required when solid crystals were formed along with the liquid products. T h e liquid portions were removed, and the physical boundary was
SEPTEMBER, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
determined. The oil layer was washed with a large volume of hot water to remove any acid before its aldehyde content was determined. The crystals were washed out of the condenser and flasks with boiling water followed by alcohol. This solution was titrated with standard sodium hydroxide, and the acid was then freed, removed, and identified. The exit gas from the condensing system was analyzed for carbon dioxide, oxygen, and carbon monoxide. The analyses were carried out on all runs, and identifications were run frequently, especially when a new catalyst was being used. Partial oxidation products other than toluic aldehyde and phthalic anhydride were not found in sufficient quantity to be isolated or identified. The oil layer remaining after analysis was distilled to recover the unreacted xylene. The aromatic hydrocarbons used in this investigation were a carefully fractionated xylene cut from petroleum. This xylene fraction had a boiling point of 143' C. and a specific gravity of 0.850, and contained a small quantity of ethylbenzene but no compounds such as pseudo-cumene and mesitylene.
'
CataIysts The materials used as catalysts varied greatly in chemical nature as well as in physical structure. Compounds of molybdenum, vanadium, iron, chromium, tin, zirconium, magnesium, potassium, tungsten, titanium, and aluminum were employed singularly and in various combinations. They were used either in granular form or on a support of porous aluminum, activated alumina, asbestos fiber, silica gel, pumice, or Alfrax. The preparation of catalysts suitable for the partial oxidation of organic compounds is unquestionably one of the most important factors governing the success of such a study. Extreme care must be taken to be sure that the activity of the catalyst is due to its known composition and not to some unknown impurity or variation in the method of preparation. Consequently the procedure for preparing the catalysts used in this investigation is given in detail: URANIUMMOLYBDOVANADATE. One hundred cubic centimeters of 1 N ammonium molybdate solution were mixed with 100 cc. of 1 N ammonium metavanadate solution. To this mixture sufficient 1 N uranium nitrate solution was added to precipitate uranium molybdovanadate completely. The bulky precipitate was washed with water to remove ammonium nitrate, dried a t 100' C . , and broken into small lumps. When we wished to use this material on a support, it was ground to a fine powder and made into a thick paste with water. The support was mixed into the paste and ignited. TIN VANADATE. Tin vanadate was prepared according to the method of Huitema and Brown (1%'). Tin vanadate on asbestos fiber was prepared by mixing into a portion of the moist red-brown precipitate 50 cc. or 5 grams of shredded asbestos fiber. This material was then dried and broken up. Five grams of ammonium metavanadate and 4 grams of stannic chloride produced sufficient tin vanadate to coat the 50 cc. of asbestos. VANADIUM PENTOXIDE ON ALFRAX. Pure white ammonium metavanadate was heated slowly in a large evaporating dish until all ammonia was expelled. Two grams of this material were made into a thin paste with water, 25 cc. of Alfrax were added, and the mixture was stirred and slowly evaporated to dryness. In this manner the Alfrax, besides having a thin coating on the surface, was thoroughly impregnated with the vanadium pentoxide. ZIRCONIUMOXIDEON SILICAGEL. Ten grams of zirconium nitrate were dissolved in warm water, and 60 cc. of 4-16 mesh silica gel added. This mixture was evaporated to dryness over a low flame and then ignited to the oxide. A pure white material resulted. MOLYBDIC OXIDEON SILICAGEL. Ten grams of molybdic acid were dissolved in concentrated ammonium hydroxide and mixed in an evaporating dish with 60 cc. of silica gel. This mixture was heated t o dryness over a low flame and then ignited. TUNGSTIC OXIDE ON ALFRAX. Ten grams of tungstic acid were dissolved in concentrated ammonium hydroxide, mixed with 60 cc. of Alfrax, evaporated to dryness, and ignited. The resulting particles were bright yellow in color.
1165
IRON CHROMATE. This catalyst was prepared according to the method of Meigs (18). ACTIVATED ALUMINA. Granular activated alumina, 8-14 mesh, was used as supplied by the Aluminum Company of America. TITANIUM DIOXIDEON PUMICE.Fifty cubic centimeters of concentrated ammonium hydroxide were added t o 50 cc. of titanium trichloride. Twenty-five grams (60 cc.) of pumice were added, and the mixture was heated strongly to dryness and ignited. TECHNICAL VAN.4DZUM PENTOXIDE.A technical grade of vanadium pentoxide in small round pellets was employed as a catalyst without a support. FUSEDVANADIUM PENTOXIDE. Vanadium pentoxide prepared from ammonium metavanadate was fused in a small crucible. When cool the fused mass was removed and broken into irregular shaped pea-sized particles. TIN VANADATE PROMOTED WITH CERICOXIDE. Eight grams of stannic chloride were dissovled in approximately 200 cc. of water. One gram of ceric oxide was added, and the suspension stirred vigorously while a hot solution containing 10 grams of ammonium metavanadate was added. As the tin vanadate precipitated, the particles of ceric oxide in suspension were enveloped. Twenty cubic centimeters of ground Alfrax were mixed with the precipitate of tin vanadate and ceric oxide to give it more volume. After being washed well with water, the catalyst was dried and heated in a furnace for 2 hours at dull red heat. The catalyst thus prepared was porous, granular, and roughsurfaced, and did not crumble with handling.
Results with Various Catalysts The results obtained with the various catalysts are given in Table I. Examples of the effect of changing controlling factors such as catalyst temperature, time of contact, and quantity of oxygen on the type and quantity of products formed were plotted from these data and are shown in Figures 2, 3, and 4. The first catalyst employed was uranium molybdovanadate on granular aluminum. From a series of preliminary runs with this catalyst it was found that air/xylene ratios of 1 to 5 were favorable for toluic aldehyde formation (Table 11). These ratios correspond to from one to four times the theoretical amount of oxygen required to oxidize xylene to toluic aldehyde. Only traces of acids were found in these runs. TABLE 11. EFFECTOF AIR/XYLENERATIOON TOLUICALDEHYDE FORMATION WITH URANIUMMOLYBDOVANADATE ON ALUMINUM AS CATALYST Ratio, Air/Hydrocarbon 1.5 3.1 4.9 5.5
Toluic Aldehyde, %
Catalyst Vol.
4.5 4.6 3.4
30 20
Trace
cc. 40
30
I n order to employ a larger amount of actual catalyst than could be evaporated onto the aluminum, granular lumps were used. Since more oxygen is required to oxidize xylene to phthalic acid than to the aldehyde, a series of runs was carried out a t higher air/hydrocarbon ratios in a n attempt to find a ratio suitable for acid formation. By using the lead bath for these runs, it was possible to maintain the desired temperature even though a large excess of air was passing through the tube. The effect of increasing the concentration of oxygen is shown in Figure 2. With an air/xylene ratio of 27.5 and a catalyst temperature of 450' C., 6.17 per cent of the xylene vaporized was converted to phthalic anhydride. This ratio of air to xylene is approximately eight times the theoretical ratio (Equation 1). Tin vanadate was used by Maxted (17) for the oxidation of aromatic hydrocarbons and by Huitema and Brown (12) as a catalyst in oxidation of toluene to benzoic acid. These investigators reported it to be an exceptional catalyst in that
INDUSTRIAL AND ENGINEERING CHEMISTRY
1166
it becomes active at the low temperature of 290" C. Figure 3 shows graphically the results of several runs using this catalyst in granular form. It is readily seen that the air/xylene ratio necessary for maximum yield of acid is approximately half that required by the uranium molybdovanadate. At a temperature of 400" C., air/ xylene ratio of 13.4, and catalyst volume equal to 15 cc., 7.9 per cent phthalic anhydride was formed. The activity of this catalyst did not decrease with use after the first few hours. A new sample always gave a slightly higher yield (Table I), but after the first run of 2-4 hours it would decrease sliahtlv and then mainE AIR/HYDROCARBON RATIO tain the same degree of acI
Because of its promise it was studied more intensively than the previous catalysts. The results of several runs are given in Table I. In Figure 4 the effect of temperature on the yield of phthalic acid over a wide range of air/xylene ratios is shown. The curves indicate clearly that a slight increase in the catalyst temperature produces a noticeable increase in the yield of phthalic anhydride over the range studied. This effect was much more pronounced than with the other catalysts investigated. The data in Table I11 show the effect of oxygen concentration upon the percentage of xylene converted to phthalic anhydride and complete combustion. All other variables except the air/xylene ratio were maintained constant, TABLE111. EFFECT OB AIR/XYLENERATIO ON PHTHALIC ANHYDRIDE FORMATION
"
(VzOs on Alfrax,
u
tivity A CREASING OXYGEN cox- of catalyst used 2 months CENTRATION WITH URANIUM was still as active as when MOLYBDOvANADATE A S first prepared. Some diffiCATALYST AT 450" C. culty was encountered in attempting to put the tin vanadate on a support. In this respect it behaved similarly to the uranium molybdovanadate. Asbestos was found to be the only support to which it would adhere satisfactorily. The asbestos offered considerably more resistance than did metallic supports to the flow of the gases. Curve B, Figure 3, shows that the air/xylene ratio suitable for acid production approaches that found for uranium molybdovanadate. The FIGURE 2.
EFFECTOF IN-
NO SUPPORT
TEMR- 380'C.
e
0
4
8
I2
16
20
24
28
AIR/HYOROCARBON
32 36
I
40 4 4 4 8
RATIO
FIGURE 3. EFFECTOF TIN VANADATE AS CATALYST
curve is very sharp and resembles curve A and Figure 2 except for the point of maximum yield. I n Table I data for several runs indicate that with tin vanadate low air/xylene ratios result in toluic aldehyde formation exclusively. As noted with the uranium molybdovanadate catalyst, the ratio favoring toluic aldehyde production ranges from one to three times the theoretical ratio. Zirconium oxide, molybdic oxide, and tungstic oxide on granular Alfrax favored the oxidation of xylene to the aldehyde, even though high air/xylene ratios previously found favorable for phthalic acid production were employed (Table I). This fact was also reported by Craver (7). The yield of aldehyde with these catalysts indicated that the temperature of maximum yield was probably not attained. Further investigation, especially a t higher temperatures, was not carried out because at this time our principle interest was in acid-producing catalysts. The vanadium pentoxide catalyst proved to be very active in oxidizing xylene to either toluic aldehyde or phthalic acid.
VOL. 31, NO. 9
Air/Xylene Ratio
530'
C., time of contact 0.003 minute)
No. of Times Theoretical Ratio
% Phthrllic Anhydride of Xylene Passed
yo Phthalic Anhydride of Xylene Oxidized
% COn Formed
-
These data show that nearly ten times the theoretical air/ xylene ratio is required for maximum yields of phthalic anhydride. With a catalyst temperature of 530' C., a time of contact of 0.003 minute, and an air/xylene ratio of 34.4, 84 per cent of the xylene oxidized per pass is converted to phthalic anhydride. The amount of xylene oxidized completely is relatively small. An increase in the &/xylene ratio above ten times the theoretical decreases the yield of the anhydride. With air/xylene ratios nearly equal to the theoretical ratio, appreciable yields of toluic aldehyde may be obtained with practically no acid production (Table I). Fused vanadium pentoxide used in a granular form without a support gave considerably lower yields of phthalic anhydride than did vanadium pentoxide on Alfrax (Table I). Green (11) states that some investigators have found the fused vanadium pentoxide to be most active. A catalyst of tin vanadate promoted with ceric oxide was prepared which had exceptional physical properties. With an air/xylene ratio of 18 and a catalyst temperature of 500" C. a yield of 11.7 per cent phthalic anhydride was obtained. With the same temperature but an air/xylene ratio of 43, 13.5 per cent phthalic anhydride was formed. This indicated that a point of maximum anhydride production existed between these two air/xylene ratios. Several runs made in a n attempt to find this desired ratio gave increasingly smaller yields, regardless of the air/xylene ratio employed. VELOCITY ON YIELDOF PHTHALIC TABLE IV. EFFECTOF SPACE ANHYDRIDE AND TOLUIC ALDEHYDE Run NO.
1 2
3 4 5
Catalyst T%mp., C. 510 520 500
510 510
Air/ Xylene Ratio 40.0 41.4 40.9 39.4
41.2
Space Velocity 71,730 47,800 30,600 22,200
15,600
%
%
Phtha!ic Toluic Anhydride Aldehyde 2.3 3.8
3.8 3.6 0.6
1.3
1.2
0.8 0.6 0.2
Several experiments were carried out to determine the effect of the space velocity on the yield of phthalic anhydride and toluic aldehyde. The results are given in Table IV and Figure 5. Since the xylene fraction used in this investigation contained some ethylbenzene, it was thought advisable to deter-
SEPTEMBER, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
mine the partial oxidation products. Maxted (17) found benzoic acid to be the main product when tin vanadate catalyst was used. A sample of ethylbenzene was oxidized under conditions that had been found favorable with the xylene fraction. When the vanadium pentoxide catalyst supported on Alfrax a t a temperature of 500" C. and an air/hydrocarbon ratio of 28 were used, a yield of 20 per cent benzoic acid was obtained. No phthalic anhydride was formed during this run. I n view of Maxted's work these results were expected. T o determine whether or not ethylbenzene would undergo similar oxidation in the presence of xylene, a 75 per cent xylene-25 per cent ethylbenzene mixture was prepared. When oxidized under the same conditions as the ethylbenzene, both phthalic anhydride and benzoic acid were formed. Benzoic acid was not produced in quantities sufficient to be identified in any of the investigations on the xylene fraction. Thus, it appears that the quantity of ethylbenzene in this fraction is small. The results of several runs investigating the possibilities of activated alumina as a catalyst are of considerable interest. With 8-14 mesh activated alumina an exceptionally vigorous reaction occurred which resulted in a rapid increase in the catalyst temperature to over 650" C. At this temperature carbon dioxide and water were the only products. I n attempting to control this reaction it was found that a t a cata-
I
I
0
1
4
~
8
~
12
1
16
'
20 24
*
28
1
1
RATIO
'
-
C = 490.C. D 450-C.
l 'AIR/XYLENE
'
32 36 40
"
44
48
~
52
1
56
i'
60
'
64
t1
68
WITH VANADIUX PENTFIGURE 4. EFFECTOF TEMPERATURE OXIDE AS CATALYST
lyst temperature below 350" C. no reaction takes place. At 350" C. or slightly higher, a reaction occurs which results in a rapid increase in the catalyst temperature. It was not found possible to maintain this temperature under 550" C. Only a trace of acid and aldehyde was found at this temperature, most of the xylene being completely oxidized to carbon dioxide and water. It is possible that with good control of the reaction temperatures, partial oxidation products can be obtained. By using a mixture of activated alumina and some relatively inactive catalyst, the reaction temperature might be controlled.
Summary 1. The air/xylene ratio controls the state of partial oxidation in this reaction. With air/xylene ratios of 1.5 to 5.0 toluic aldehyde is produced exclusively; when ratios of 20 to 35 are used, phthalic anhydride was the main product. 2. Although maximum yields of toluic aldehyde are produced when the theoretical air/xylene ratios are used, maximum yields of phthalic anhydride are obtained only when approximately ten times the theoretical ratio is employed. 3. The temperature a t which these reactions take place appears t o be a specific property of the catalyst employed. 4. Certain catalysts such as tin vanadate, vanadium pentoxide, and uranium molybdovanadate are capable of pro-
1167
ducing either toluic aldehyde or phthalic anhydride. Catalysts of zirconium oxide, molybdic oxide, and tungstic oxide are capable of producing only toluic aldehyde even though air/xylene ratios favorable for acid production were investigated. 5. Careful control of the air/xylene ratio and the temperature of the catalyst is essential in this type of reaction. 6. Although specific experiments were not carried out to study the effect of the physical structure of the catalyst, certain observations were made. A catalyst may be active
g4
8'-
I I
L
AIR/XYLENE = 41 SPACE
VELOCITY x
10'
WITH VANADIUM FIGURE 5. EFFECTOF SPACEVELOCITY PENTOXIDE ON ALFRAX
in any form, but the yield of desirable products is greatly increased either by supporting it on a porous carrier or preparing it in an irregularlycshaped granular form. This change in physical structure gives a much greater exposed surface area and affords a greater possibility for active centers and for the adsorption of reacting gases. A catalyst in this form also gives better heat transfer. The'effect of a promoter is due to the change of distortion of the crystal structure. Fused vanadium pentoxide, although active, is not as effective as vanadium pentoxide on Alfrax; also, tin vanadate supported on Alfrax or aluminum is a much more effective catalyst than the same compound in a powdered form. '
I
Literature Cited Bibbs and Lucas, IND.ENG.CHEM.,21, 633 (1929). Bone, Nature, 127, 481 (1931). Roy. SOC.(Lbndon), A161,48 (1937). Burgoyne, PTOC. Buylla and Pertierra, Anales SOC. espaft,fis. quim., 31, 59 (1933). Charlot, Bull. SOC. chim., 53, 572 (1933). Charlot, Compt. rend., 196, 1224 (1933). Craver, British Patent, 189,107 (July 26, 1923) : U. S. Patent 1,636,855 (July 26, 1927). Downs, J . SOC.Chem. I n d . , 45, 188 (1926). Downs, U. S. Patents 1,374,720 (April 20, 1921) and 1,604,739 (Oct. 26, 1927). Gibbs and Conover, British Patent 119,518 (Oct. 1, 1918). Green, "Industrial Catalysis," New York, Macmillan Co., 1928. Huitema and Brown, J . P h y s . Chem., 40, 531 (1936). Jaeger, U. S. Patent, 1,826,548 (Oct. 6, 1932). Kharasch, B u r . Standards J . Research, 2, 359 (1929). King and Sheely, IND. ENG.CHEM.,26, 1151 (1934). Marek and Hahn, "Catalytic Oxidation of Organic Compounds in the Vapor Phase," pp. 383 and 420, New York, Chemical Catalog Co., 1932. Maxted, J . SOC.Chem. I n d . , 47, 101 (1928). Meigs, U. S. Patent 1,486,781 (March 11, 1924). Rittman, Byron, and Egloff, J. IND.ENG. CHEM.,7, 1019 (1915). Tropsch and Mattox, Ibid., 26, 1338 (1934). Weiss and Downs, Ibid., 12, 228 (1920). Weizevich and Frolich, Ibid., 26, 267 (1934). White and Rose, Bur. Standards J . Research, 9, 711 (1932). Wilken-Jorden, J . Chem. Met. Mining SOC.S. Africa, 32, 248 (1932). Yoshikawa and Kiyoski, Bull. I n s t . Phys. Chem. Research (Tokyo), 10, 305 (1931).
PRESENTED before t h e Division of Industrial and Engineering Chemistry a t the 96th Meeting of t h e American Chemioal Society, Milwaukee, Wis.
