catalytic air oxidation of higher aromatics to phthalic anhydride

2880 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 43, No. 12

of increasing the amounts of various contaminants. For these reasons, although no definite numerical relationship between concentration of contaminant and its effect upon corrosion rate was established, it was concluded that addition of small amounts of oxygen, nitrogen, or carbon to zirconium results in only slight lowering of the corrosion resistance of the metal to most media. In the case of hydrochloric acid, however, the effect was quite pronounced. In general, increasing the concentration of the alloying element in zirconium results in a lowering of the corrosion resistance of alloy to the common chemical reagents studied. Nitrogen increases susceptibility of zirconium to corrosion more than equal amounts of oxygen or carbon. Since some segrega- tion was present, especially in the carbon alloys, comparisons of corrosion rates were made on basis of nominal, rather than analytically determined, compositions. Standard zirconium, containing 2 to 3% hafnium, was found to be less resistant to chemical attack a t 35' C. than low-hafnium zirconium, containing less than 0.1% hafnium.

Zirconium and its alloys appeared to be resistant to attack by both 10% sulfuric acid solution and 10% hydrochloric acid, but were readily dissolved by concentrated sulfuric acid. As reported by Taylor ( 4 ) , zirconium was found to be superior to titanium in its corrosion resistance. This superiority was particularly noticeable in hydrochloric acid solutions. An outstand- ing property of zirconium metal, as determined in this investiga- tion, is its resistance to attack by fuming nitric acid.

In most cases a slight gain of weight of samples was noted when zirconium and its alloys were treated with fuming nitric acid. This gain in weight was always accompanied by a pronounced blackening of the surface, probably caused by formation of a thin film of zirconium oxides. Samples of zirconium, autoge- nously welded in an atmosphere of argon, also were equally resistant to action of fuming nitric acid. Zirconium and its alloys were also resistant to both dilute and concentrated nitric acids.

A 1 to 1 hydrochloric acid-nitric acid mixture and a 1 to 1 sulfuric acid-nitric acid mixture appeared to attack zirconium

and its alloys, although somewhat slowly in some instances. However, the 1 to 1 hydrochloric acid-sulfuric acid mixture caused only slight corrosion.

SUMMARY

Chemical corrosion resistance of single specimens of zirconium metal, zirconium alloys, and titanium metal were studied. Where applicable, results obtained are in agreement with those previously reported. Vacuum-arc melting of zirconium metal prior to cold rolling does not materially affect its corrosion resistance, Standard grade zirconium, usually containing about 2.5% hafnium, is less resistant to corrosion than low-hafnium zirconium. Corrosion resistance of zirconium is slightly superior to that of titanium in most media and quite superior in dilute sulfuric acid and in both dilute and concentrated hydrochloric acid. Addition of small percentages of oxygen, nitrogen, and carbon results in a slight decrease in corrosion resistance of low- hafnium zirconium, with the effect being most pronounced in concentrated hydrochloric acid solutions. With the exception of concentrated hydrochloric acid solutions, these zirconium alloys will be acceptable as materials of construction in all media in which zirconium sheet itself is acceptable. Nitrogen alloys of zirconium are less resistant than either carbon or oxygen alloys. In general, increasing the concentration of the alloying material results in a decrease in the corrosion resistance of the alloy. Zirconium and its alloys showed excellent resistance to fuming nitric acid, indicating possible new commercial applications. Zirconium, autogenouslr welded in argon, was equal to zirconium sheet in corrosion resistance.

LITERATURE CITED

(1) Gee, E. A, Golden, L. B., and Lusby, W. E., Jr., IND. ENQ.

(2) Jaffee, R. I., J . Metals, 1, No. 7, 6 (1949). (3) Muterials & Methods, 32, No. 4, 63 (1950). (4) Taylor, Donald F., J . Metals, 42, 639 (1950).

CHEW, 41, 1668 (1949).

RECEIVED March 14, 1951.

Catalytic Air Oxidation of Aromatics to Phthalic Anhydride

J

CORLISS R. IiINNEY AND IRVIKG PINCUS' The Pennsylvania State College, State College, Pa.

The increased demand for phthalic anhydride in recent years has induced manufacturers to look for additional sources of raw material from which it may be made. For this reason, the possibilities of producing this substance by the catalytic air oxidation of certain higher aromatics and coal tar fractions have been surveyed.

From a 200' to 235' C. crude coal tar fraction, 759% yields of high quality phthalic anhydride were produced using a silica-based vanadium pentoxide catalyst. Pure methylnaphthalenes and a methylnaphthalene coal tar cut (235' to 270" C.) gave 28 to 40% yields, apparently free from methyl derivatives. Using an alumina-based catalyst, anthracene and phenanthrene yielded close to 50 and 35%, respectively. A sample of "anthracene salts" gave 24%.

1 Present address, Great Lakes Carbon Corp., Morton Grove, Ill.

Although yields are not so high as with pure naphthalene, phthalic anhydride of satisfactory quality can be produced from other raw material.

HE rapidly expanding demand for phthalicanhydride makes T desirable increasing supplies of raw material from which it may be manufactured. Production from naphthalene is limited by the availability of this hydrocarbon from tar distillation and, to augment this supply, production from o-xylene obtained from petroleum has recently begun (6). Since patent claims have been made that phthalic anhydride appears in the products of the catalytic air oxidation of methylnaphthalene and phenanthrene (d ) , a survey of the possibilities of producing phthalic anhydride from certain higher aromatics and coal tar fractions has been made. A list of substances and fractions oxidized appears in Table I. The materials were selected primarily on the basis of

December 1951 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 288 1

their relatively greater availability. Since the cost of a crude naphthalene or methylnaphthalene cut from coal tar would be expected to approximate that of a creosote oil cut, the current posted price of about 21/* cents per pound for creosote compares favorably with about 61/, cents for naphthalene. Of still greater interest is the possible utilization of anthracene and phenanthrene for which no active market exists a t present. Large quantities of "anthracene salts" containing mostly anthracene, phenanthrene, and carbazole are available at a nominal cost.

TABLE I. SUBSTANCES OXIDIZED Substance Source Properties

Phthalic anhydride Eastman Kodak Co. 129-131O C., melting

Naphthalene Eastman Kodak Co. 79-80° C., melting

1-Methylnaphthalene Reilly Tar and Chem. W& 240.5-243.0' C. 2-Methylnaphthalene Reilly Tar and Chem. 2O range (240" C.) 2.3-Dimethylnaphthalene Edcan Laboratorles Coal tar fraotions By-product coke oven ASTM D 20-30 fractions

point

point

Anthracene

Anthraquinone

Phenanthrene Carbazole

"Anthracene salts"

tar 200-235' C. 235-270° C.

Eastman Kodak Co. 195-200° C., melting point

son Co. point

point

Paragon Div., Mathe- 280-282' C., melting

Reilly Tar and Chem. Eastman Kodak Co. 241-244' C., melting

TheKoonersCo. ASTM D 20-30 frac-

90% minimum

PROCEDURE

Glass apparatus was used, see Figure 1, which was functionally similar to that used by Senseman and Nelson (6) and by Marisic (a). The carburetor and catalyst chambers were, however, sealed together because of the difficulty of maintaining a tight joint between them. The carburetor and reactor were both heated electrically but independently. The temperatures of the substances evaporated in the carburetor were measured by a thermometer inserted directly into the sample. The catalyst

SECONDARY

4lR

I I F""T0f y PRODUCTS

Figure 1. Apparatus Used €or Oxidizing Aromatics

temperature was measured by a chrome-alumel thermocouple placed in a ceramic well which was buried in the catalyst bed, By moving the thermocouple in the well, variations in the temperature of the catalyst .at different points in the bed could be measured. Owing to the exothermicity of the oxidations, the temperature of the bed increased from the front end to a maximum a t about 7 om. and then decreased. The rise in temperature varied with the catalyst used, its activity, the substance being oxidized, and probably other variables. Usually, the rise in temperature did not exceed 50" to 75" C., but withphenanthrene a differential of as much as 120" C. was observed. No attempt was made to control the rise in catalyst temperature, but care was taken to maintain the forward end of the catalyst bed a t constant temperatures, which are the temperatures recorded in Tables I1 and III as the catalyst temperatures.

Primary air before passing through the carburetor was dried over calcium chloride and freed from carbon dioxide by ascarite. The volume of purified air was then measured with a rotameter. Secondary air, purified and metered in the same way, was ad-

70

E60

:eQ

5

L " a

40

M

PO 400 600 am 1000 1200 1400 1600 moo 20

SEMNDARI LIR, MLIMIN.

Figure 2. Oxidation of Phthalic Anhydride in Presence of Catalyst 200

Primary air, 120 ml. per minute

mitted a t the forward end of the reactor through a side tube sealed into the wall. The side tube was bent a t go", parallel to the vapor stream. The end of the tube was closed, but four 1-mm. holes, equally spaced in the circumference of the tube, permitted efficient mixing of the Eecondary air with the hydrocarbon-air mixture coming from the carburetor before it passed through the catalyst bed.

Two commercially available catalysts manufactured by the Davison Chemical Corp. were used. Catalyst 200 was of the German type and was composed of vanadium pentoxide (10%) deposited on silica (65%) and was promoted with potassium sulfate (23%). It was manufactured in the form of 3//16 X a / ie inch cylinders. Catalyst 210 was of the American type and was composed of vanadium pentoxide (7.5%) deposited on alumina (88%). This catalyst was manufactured in '/&oh spheres. Both catalysts were crushed and screened to 10- to 20-mesh size for more effective use in laboratory scale equipment as recommended by the manufacturer. A volume of 33 ml. was used which gave a bed 10 cm. deep in the reactor. Catalyst 200 was heated in a muffle for 2 hours at 427' C. (800" F.) immediately before use, which was also recommended by the manufacturer. Immediate use was found to be essential in establishing the activity of this catalyst.

The bulk densities of the poured 10- to 20-mesh catalysts were 0.761 and 1.234, respectively, for Catalysts 200 and 210. The void spaces in 33 ml. of the poured catalysts were measured with n-heptane and were 22.8 and 20.8 ml., respectively, for the two catalysts. Surface area measurements were made with Russell and Cochran's stearic acid method (4) . Freshly activated Cata- lyst 200 was found to have a surface area of 11.70 square meters per gram and the used Catalyst 200, 11.87 square meters per gram. Fresh Catalyst 210 gave a value of 8.41 square meters per gram and the used, but still active, catalyst 7.78 square meters per gram. All of the above values were averages of two or more determinations.

Reaction gases left the reaction chamber a t about 300' C. and rapidly cooled to room temperature upon passing through the two air condensers arranged in series. Glass wool plugs inserted in the condenser tubes aided the retention of the solid products which were largely caught in the first condenser. Gases from the condensers were passed, by means of a two-way stopcock, either through a water trap for removing last traces of phthalic and maleic anhydrides or through anhydrous calcium chloride into a tared ascarite tube for determining the rate of carbon dioxide production.

The weight of sample passed over the catalyst bed was determined by weighing, to the nearest 0.05 gram, the carburetor with the attached catalyst chamber before and after each run,

2882 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 43, No. 12

TABLE 11. OXIDATIOX DATA CSIXG CATALYST 200 Carbon Primary Secondary

Phthalic Maleic Dioxide % Carburetor Catalyst Sample .4ir. Air Substances and Anhydride, Anhydride. Cornpldte Temperature, Temperature, Weight, Grams Minute Ml./ Minute Ml.)

Certain Conditions Weight % Weight % Combustion c. O c. Naphthalene 90 8 2 .1 25 .1 105-110 375 0.75 120 550 I-Methylnaphthalene 31.4 5 .3 3 9 . 1 125-130 375 0.80 120 550 2 3-Dimethylnaphthalene 40.7 4 . 0 31 .7 160-165 375 0.90 120 550 Coal tar fraction 200-235° C. 7 5 . 0 4.6 32.0 103-110 375 0.90 120 550 Coal tar fraotion: 235-270° C. 28.2 2 . 4 21.7 130-135 375 2.00 120 550

2-Methylnrtphthalene 33.2 4 . 0 35 .8 130-135 385 1 .20 120 550

Anthracene

1 2 3 4

Hours of catalyst use 61.3 3 . 7 2 6 . 5 223-230 425 3 .00 120 1200 27.8 3 2 23.1 . . . . . . 3.05 . . . . . . 2 5 . 5 2 . 1 21.4 . . . . . . 2.70 . . . . . . 2 1 . 4 1 .9 1 9 . 5 . . . . . . 2.70 . . . . . .

Anthraquinone Hours of catalyst use

1 43.9 1 . 5 25 0 290-293 500 2.85 60 1600 2 1 9 . 3 1 . 2 21.5 . . . . . . 1.10 . . . . . . 3 3 . 4 0 . 4 6 . 1 . . . . . . 2:2o . . . . . . 4 2.9 0 . 5 5 .9 . . . . . . 2.40 . . . . . .

Phenanthrene 18 .1 2 . 9 38.9 235-240 400, 410 L 4 5 60 94.5 ... . . . . . . Used catalyst reactivated a t 427' C. 5 .6 0 . 8 17.7 ... 2 . 6 5

Anthraquinone, catalyst used 9 hours with phenanthrene 38.7 1 .5 32.3 290-29.5 400 1 . 7 0 60 1600

The loss in weight was taken a8 the weight of sample oxidized. Temperatures were maintained in the carburetor such that about 1- to 2-gram samples were evaporated in exactly 1 hour, TThich was the length of time of all of the rvperiments recorded in Tables I1 and 111. Usually, the required temperature was established during preliminary tests, but for substances for which vapor pres- sures a t elevated temperatures were available, it was possible to calculate the carburetor temperature to be used. The data in Tables I1 and 111, giving optimum conditions for producing phthalic anhydride, represent over 250 oxidation experiments.

The total weight of solid products caught in the condensers was obtained to the nearest 0.05 gram. Individual yields of phthalic and maleic anhydrides were then determined on the condensate, together with that absorbed in the water trap, by Marisic's method (3') and are given in Tables 11 and TI1 as the per cent of the weight of sample evaporated in the carburetor.

The amount of carbon dioxide produced during 15 minutes of each run was obtained by weighing the ascarite tube to the nearest 0.005 gram. The yield of carbon dioxide was calculated in each case as the per cent of the theoretically complete oxidation to carbon dioxide. In addition, the exit gases from four runs on anthracene were analyzed for carbon monoxide and the total volume measured with a wet test meter. From these data the yield of carbon monoxide was calculated as the theoretical per cent of the carbon available.

Pure phthalic anhydride mas also subjected to oxidation and the results are plotted in Figures 2 and 3. Using this substance, pressure drop measurements across the catalyst bed were made under different rates of ail flow. The change in pressure was measured by means of a differential manometer connected between the secondary air inlet and the exit, T , just beyond the catalyst bed. All of the pressure drop measurements were made a t catalyst temperature of 450' C. The results are given in Fig- ure 4.

DISCUSSION

As a basis for the study of the oxidation of the higher aromatics, listed in Table I, the activity of the catalysts for the production of phthalic anhydride from naphthalene was first established. The maximum yields for the two catalysts a t optimum conditions mill be found in Tables I1 and I11 and it will be seen that the silica-based, potassium sulfate-promoted Catalyst 200 gave a 90% yield a t a catalyst temperature of 375" C., while the alumina-based Catalyst 210 gave a little less than 80% a t a temperature of 400 O C. These results are in accord with actual manufacturing yields reported for these catalysts.

Because it was expected that the search for optimum conditions for the oxidation of the higher aromatics would involve the rela- tive resistance of phthalic anhydride to further oxidation, a 8ys- tematic examination of the oxidation of this substance was also made and the results presented in Figures 2 and 3. Both the catalyst temperature and the space velocity were found to be controlling factors. It is important to note that Catalyst 200 which gave the larger yield of phthalic anhydride from naphthalene was also more effective in further oxidizing phthalic anhydride. Under operating conditions which gave a 90% yield of phthalic anhydride from naphthalene, this catalyst oxidized phthalic anhydride to the extent of over 40%. On the same basis, Catalyst 210 induced the oxidation of phthalic anhydride to the extent of about 20%. These results demonstrate the drsir- ability of controlling the contact time with the catalyst if maximum yields of phthalic anhydride are to be obtained.

The striking regularity of the curves in Figures 2 and 3 suggests the possibility that the change in direction of the curves was due to a change in the character of the flow through the catalyst bed. For this reason, the pressure drop over the two catalyst beds with different rates of flow was determined and the results plotted as shoan in Figure 4. Both catalyst beds occupied the same volume but the void space differed slightly. In spite of this difference the pressure drops across the two catalyst beds mere almost identical. To separate the two curves in the figure, two scales have been employed. There was, however, a noticeable difference in the point a t which turbulent flow began. With both the empty reactor and Catalyst 200, turbulent flow began at a secondary air velocity of 700 ml. per minute. With Catalyst 210, the change occurred at 900 ml. per minute. These values coin- cide almost exactly with the velocities which gave maximum oxidation as shown in Figures 2 and 3. Since turbulent flow vould be expected to give more complete mixing and contact with the catalyst, this probably explains why the oxidation curve? rise to a maximum and then fall off. Naphthalene and its mcthyl derivatives appeared to give maximum yields of phthalic anhydride a t 550 ml. per minute of secondary air, while anthracene and phenanthrene required much higher rates, as shown in Tables I1 and 111. No satisfactory explanation for this difference in require- ment can be offered a t this time.

Having established some of the operating characteristics of the apparatus and catalysts, the remaining oxidations as shown in Tables I1 and I11 were carried out. Both 1- and 2-methylnaphthalene gave markedly poorer yields of phthalic anhydride than naphthalene and similar yields were obtained with both catalysts-about 35%. hlthough Oxidation could yield methyl-

December 1951 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 2883

10

TABLE 111. OX~DATION DATA USIXG CATALYST 210

.

Carbon Primary Secondaty Phthalic Maleic Dioxide % Carburetor Catalyst Sample Air Air,

Substances &nd Anhydride, Anhydride Cornplhe Temterature, Temperature, Weight, Ml.; MI./ Certain Conditions Weight % Weight %' Combustion C. c. Grams Minute Minute

10

Naphthalene 1-Methylnaphthalene 2-Methylnaphthalene 2,3-Dimethylnaphthalene Anthracene

tassium sulfate Anthraquinone

hours with anthraquinone Phenanthrene Carbazole "Anthracene salts"

Catalyst treated with 23% PO.

Naphthalene, catalyst used 10

.

79.5 34 .1 38.5 26.2 49.4

2 .3 20.9

57.8 35.2 0 .0

24 .1

5 . 8 3.4 7 . 0 5 . 6 5 .3

1 . 4 3 .4

6 . 2 7 .6 3 . 7 8 .6

21.2 48.3 47.3 61.8 43.0

6.4 24.7

31.8 43.3 63 .1 37.7

105-110 125-130 130-135 160-165 225-230

290-iS5

105-110 235-240 250-255 235-240

400 425 415 415 475

475

400 425 425 440

. . .

1.10 1 .00 1 .05 0 90 1 .a5

1.80 4.10

1.10 1.20 0 .70 2 .05

120 120 120 120 120

60

120 60 90 60

. . .

550 550 550 550

1200

i ioo 550 945 900 500

Figure 3. Oxidation of Phthalic Anhydride in Presence of Catalyst 210


ated phthalic anhydrides theoretically, the product from both the methylnaphthalenes appeared to be the unsubstituted anhydride as shown by mixed melting point determinations and ultra- violet absorption curves. No reason was found for the fact that Catalyst 200 did not give better yields than 210, but it will be observed that both catalysts convert a much larger percentage of the carbon to carbon dioxide than with naphthalene, and with Catalyst 210 almost half of the carbon is lost in this manner. The yield of maleic anhydride was about 5%, which was not unusual.

2,3-Dimethylnaphthalene gave better yields of phthalic anhydride with Catalyst 200-40.7% as compared with 26.2% for Catalyst 210. As before, the products appeared to be the unmethylated phthalic anhydride. This suggests that the methylated ring was more readily oxidized. However, it is also possible that the methyl groups were eliminated by oxidation to carboxyl groups followed by decarboxylation. In any event, oxidation of the methylated naphthalenes of the type used in this work appears to lead to unmethylated phthalic anhydride. The higher yield with Catalyst 200 was accompanied by a lower conversion to carbon dioxide than with Catalyst 210 which caused a 51.8% conversion to carbon dioxide.

Crude coal tar fractions were oxidized next. The naphthalene cut, boiling from 200" to 235' C., gave a 75% yield of phthalic anhydride with catalyst at 375' C., with accompanying yields of maleic anhydride and carbon dioxide of 4.6 and 32.0%, respectively. While this yield of phthalic anhydride is somewhat less than from pure naphthalene, the economic advantages of using the crude material may very well offset the lower yield. As was expected from the yields obtained from the methylnaphthalenes, the yield from the 235' to 270" C. cut was only 28.2%. Oxida- tions of these coal tar fractions using Catalyst 210 were not

carried out because of the generally lower yields with this catalyst in the naphthalene range.

Oxidations of the higher polynuclear aromatic hydrocarbons, particularly the oxidation of anthracene to anthraquinone, have been investigated @), but apparently nothing has been reported on the oxidation of anthracene to phthalic anhydride. Oxidation with Catalyst 200 gave a maximum yield of 51.3% phthalic anhydride, which is 61.4% of the theory. However, the effective- ness of the catalyst was rapidly impaired and, as shown in Table 11, the yield dropped to 21.4% after only 4 hours of service a t 425" C. A similar behavior wm observed a t both 400" and 450" C., the yields falling from 51.1 to 12.4% and from 46.8 to 14.7%, respectively. Analyses of the product gases for carbon monoxide were also made during the first and third hour runs a t 425' C., and the results, shown in Table I V calculated as the per cent of carbon in the sample, were obtained.

The results given in Table I V show that the retardation of the catalyst affected mainly the yields of phthalic and maleic anhydrides which were approximately halved on using the catalyst for the third hour. The yields of carbon dioxide and monoxide, although lower, remained fairly constant. The production of

TABLE IV. OXIDATIONS OF ANTHRACENE AT 425' C. WITH CATALYST 200

Product

First Third Hour, Hour,

% % Phthalic anhydride 61.4 30.6 Maleic anhydride 6 . 6 3 . 7 Carbon dioxide . 26.6 21.4

6 .2 Total 101.4 61.9

- 6.9 - Carbon monpxide.

I20 I10

0 3100 90 I

B

d g 80 70 2 3 Y

J v1 10 Y)

360 50

40 30

2odO0 4EO 6bo 8 b , l&O I2bO 1200 1200 18bO J1o S E W O A R V 11% YL/MlN

Figure 4. Pressure Drop over Catalyst Bed in Oxidation of Phthalic Anhydride at 450' C.


I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 43, No. 12 2884

carbon monoxide from anthracene was particularly interesting because Marek and Hahn (2) state that no carbon monoxide is formed in the catalytic oxidation of naphthalene.

Since it may be assumed that anthracene would probably be oxidized first to anthraquinone, the oxidation of anthraquinone was also investigated. As expected, the yield of phthalic anhydride rapidly fell Fit'h continued use of the same catalyst. Thus after four successive hour runs, the yield dropped from 43.9 to only 2.970. Consequently, it appears that the retardation of thie catalyst during the oxidation of ant,hracene may be due to the formation of anthraquinone which is responsible for the retarding effect.

On heating the retarded catalyst in air a t 427" C. for 2 hours in a muffle, the original act'ivity was not restored. The bulk density of the retarded catalyst was noticeably smaller than that of the active catalyst, decreasing from 0.761 to 0.703. Likewifie the void space decreased from 22.8 to 20.6 ml. per 33 ml. of poured catalyst. More important perhaps was the fact that the surface area decreased from 11.7 t'o 6.6 square meters per gram, as measured by the adsorption of stearic acid using Russell and Cochran's method (4) . Finally, there was a marked color change when the ret,arded catalyst was exposed to moist air or placed in water. The active catalyst was of a mottled greenish color, which when exposed t o air or treated with water turned a deep bluish green with some light brown particles. The retarded catalyst TYas lighter green than the active catalyst and, when exposed to air or placed in water, it turned a reddish brown with some bluish green particles. On heating the moistened ret'arded catalyst in a muffle for 2 hours a t 427' C., a light green color Tvas produced but again the activity was not restored.

The apparent expansion in volume of the retarded catalyst was unexpected on the basis of the decreased surface area and is con- trary to the statement of Audibert and Raineau (1 ) that the density of a retarded catalyst is always greater than the active form. The assumption that a film of adsorbed anthraquinone covers the active surface does not appear to be likely becausc the catalyst did not, regain its activity on heating. Furt'hermore, the adsorption of anthraquinone would be expected to increase the bulk density, as well as to decrease the void space, because the density of anthraquinone is about twice that of the bulk density of the catalyst.

On the hasis of color, the most likely explanation seems to be that the green active catalyst contains vanadium in the trivalent stat,e and that the retarded catalyst which becomes a reddish brown with water is in the pentavalent state. This is further substantiated by the fact that vanadium pentoxide has a smaller density than the trioxide. Consequentli, it appears that some basic change in the catalyst in contact with anthracene or anthraquinone occurs which cannot' be reversed by heating. These results suggest that the silica-supported catalyst is unsatisfactory for the oxidation of anthracene, unless it can be modified in some way to avoid this loss in activity.

The alumina-based Catalyst 210, on the ot'her hand, did not undergo marked deterioration when used with anthracene or anthraquinone, Table 111. The best yield from anthracene vias 49.4% a t 475" C., which is 59.2% of the theoretical. Anthra- quinone, however, gave only 20.9% with this catalyst. When naphthalene was oxidized with this same catalyst, which had been used for 10 hours wit.h anthraquinone under various conditions, a yield of 57.8% was obtained, This indicates that a IOW yield is inherent with anthraquinone using this catalyst. On the other hand, the activity of this catalyst for naphthalene was somewhat impaired.

The low yield of phthalic anhydride from anthraquinone as compared with anthracene also implied that t'he main course of the oxidation of anthracene to phthalic anhydride was not through anthraquinone as an intermediate, but rather that oxidat ion attacked one of the outside rings first. This is further suggest ed by the mueh higher catalyet temperature and space velocity ob-

served for optimum conversion of anthracene to phthalic anhydride than with naphthalene, as was observed by Senseman and Selson ( 5 ) for optimum conversion of anthracene to ant,hra- quinone. These results Euggest that possibly a larger yield of phthalic anhydride could be obtained from anthracene by raising the temperature still higher and shortening the contact time, but this m-as not at,tenipted. An attempt vias made to promote the activity of this catalyst for anthracene by adding 237, of potassium sulfate, but this destroyed its activity almost completely.

Yields of phthalic anhydride from phenanthrene were somewhat less than from anthracene. Catalyst 200 gave a maximum yield of only 18.1% a t 400" C., but Catalyst 210 gave 35.2% at 425" C. No loss in act'ivity was observed on cont,inued use with either catalyst and no explanation was found for the difference in yield as compared 13-ith those from anthracene. rl larger yield of maleic anhydride was produced but not enough larger to account for the difference in yields. Oxidation to carbon dioxide was also greater with Catalyst 200 and accounted for a part of the difference, but &h Catalyst 210 there was little difference in the amount of carbon dioxide produced. To test the activity of Catalyst 200 after 9 hours of use viith phenanthrene, a run was made with anthraquinone, Table 11. The yield obtained, 38.7%, compares favorably with 43.970 obtained with fresh catalyst and indicabes that a lorn yield is normal from phenanthrene with this catalyst.

Because of the practical difficulties of producing pure anthracene or phenanthrene from coal tar for t'he manufacture of phthalic anhydride, a sample of ant'hracene salts having the distillation characteristics shonn in Table I was investigated. Since Catalyst 210 was not retarded by anthracene and gave a better yield from phenanthrene, it was selected for use. The sample contained 23% carbazole, based on its nitrogen content, and although no pht,halic anhydride would be expected from carbazole, it was first subjected to oxidation to est'ablish its behavior under oxidizing conditions. At 425" C., 15% came through unchanged, 63.1Oj, of the carbon was oxidized to carbon dioxide and 6.4% to maleic anhydride. The fact that some carbazole passed through unchanged suggested the possibility t,hat suitable catalysts and conditions might be found for preparing pure carbazole from anthracene salts by preferentially oxidizing the hydrocarbons.

The maximum yield of phthalic anhydride from the anthracene salts was 24.1% a t 440" C. and a secondary air velocity of 500 ml. per minute. This temperature is midway between the optimum of 475' C. for anthracene and 425" C. for phenanthrene, but the optimum space velocity vias even less than for phenanthrene. The yield of 24%, although low, xas not surprising considering the presence of carbazole and the probable preponderance of phenanthrene over anthracene in the sample of anthracene salts used. It seems likely that the yield could be improved by frac- tionating the salts and particularly by removing the carbazole before oxidation.

ACKNOWLEDGMEKT

This york was conducted as a part of the research program of the Division of Fuel Technology of The Pennsylvania State College. The authors are indebted to the donors of the various samples and catalysts used in t,his invest'igation.

LITERATURE CITED

(1) Audibert, E., and Raineau, A, Compt. rend., 197, 596 (1933). (2) Marek and Hahn, "The Catalytic Oxidation of Organic Com-

pounds in the Vapor Phase," ACS Monograph 61, New York, Chemical Catalog Co., 1932.

(3) hfarisic, M. PI., J . Am. Chem. Soc., 62, 2312 (1940). [4) Russell, A: S., and Cochran, C. N., IKD. ENG. CHEM., 42, 1332

(5) Senseman, C. E., and Nelson, 0. b., Ibid., 15, 521 (1923). (6) Zabel, H. W., Chem. Inds., 64, 573 (1949).

RECEIVED hIay 15, 1951

(1950 j .

catalytic air oxidation of higher aromatics to phthalic anhydride

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