ENGINEERING, DESIGN. AND PROCESS DEVELOPMENT
.
progresses in the’laboratory and design room. We may conclude that a flow sheet is a necessity as an integrated expression of all changes with optimum conditions both chemical and physical for processing from raw material to salable product ( 7 ) . Flow sheets naturally include the equipment needed. Labor costs for chemicals (item 10) have not advanced proportionately as much as they have for many manufactured articles in other fields largely because continuous processing, controlled by instruments, has replaced small batch operations. Also the backbreaking labor of the past decade has been eliminated by mechanical devices in modern processing. We are sure this Subdivision of Chemical Processing will give, in future years, much attention to the reduction of costs in many ways but particularly by saving labor and labor costs through better design. While the Unit Process Symposia are being adsorbed into the broader concept of chemical processing, the unifying aspect of unit processes is too well established in the literature and too useful to be dropped from the thinking of chemical engineers. For example, flow sheets as summations of the unitary changes to which raw materials are subjected to give finished products are firmly established. These unitary changes are either the unit chemical process or the unit physical operation. It is recognized that both these divisions often function simultaneously (10). It has been written ( 1 ) that the “chemical” in “chemical engineering” refers to “unit processes.” The Subdivision of Chemical Processes will sustain and enlarge this assertion. Much improvement has resulted in the past from the classification of chemical change within the framework of the unit process-e.g., oxidation, hydrogenation, pyrolysis. Here similarities become apparent, and general principles are established for guidance of future technology. The history and advantages of, the unit process unitary concept have been described by several authors who have applied this classification of knowledge to chemical change (9, 5, 7 , 9, 11). The Subdivision of Chemical Processes will permanently enlarge this aspect of the parent division of Industrial and Engineering Chemistry and will also permit an extension of the scope of symposia for the various ACS meetings under this division. The majority of papers presented in past unit process symposia
have originated in university research laboratories. However, some of the best presentations have come from industry and from government institutions. It is expected that the change from chemical process symposia to the permanent Subdivision of Chemical Processes will enlarge the contributions from industry as well as from universities. It would be helpful if some of these contributions would report on the following:
1. Investigations of conditions to furnish both the highest and most economical chemical conversions and yields 2. Results of research to indicate the conditions for the close connection required by the chemical change on the part of unit physical operations t o facilitate the chemical change; this is particularly necessary in mixed-phase reactions 3. Researches for conditions to furnish more satisfactory chemical reaction rate (kinetics) 4. Study of catalysts to enhance an otherwise slow reaction rate 5 . Design conditions checked by pilot plants to ensure low cost factory procedures
Literature Cited (1) Groggins, P. H., in “Unit Processes,” Chem. Eng., 58, No. 3, 129 (1951). (2) Groggins, P. H., “Unit Processes in Organic Synthesis,” 4th ed., McGraw-Hill Book Co., New York, 1952. (3) Larian, M. G., “How Batch Unit Processes are Made Continuous,” Chem. Eng., 52, No.5, 114 (1945). (4) Shreve, R. Norris, “Chemical Process Industries,” McGrawHill Book CO., New York, 1945. (5) Shreve, R. Norris, “Chemistry-Key to Better Living,” Unit Processes, p. 60, ACS, Washington 6,D. C., 1951. (6) Shreve, R. Norris, “Classification of Unit Processes,” IND. ENQ.CHEM., 29, 1329 (1937). (7) Shreve, R. Norrjs, in “Encyclopedia Americana,” Chemical Engineering, Americana Gorp., New York, 1954. (8) Shreve, R. Norris, in “Unit Processes,” Chem. Eng., 58, E o . 3, 129 (1951). (9) Shreve, R. Norris, “Unit Processes,” IXD. ENG.CHEM.,35, 263 (1943). (10) Shreve, R. Norris, “Unit Processes in Chemical Processing,” Ibid.,46, 672 (1954). (11) Shreve, R. Norris, “Unit Processes in Retrospect and Prospect,” Ibid.,40, 379 (1948).
Vapor-Phase Air Oxidation of Cyclohexane WILLIAM F. HOOT’
AND
KENNETH A. KOBE, University o f Texas, Austin, Tex.
Vapor-phase air oxidation of cyclohexane was studied with and without catalysts. Without catalysts, aldehydes were the principal products before the end products of carbon dioxide and water. The maximum yield of aldehydes reported a s formaldehyde was 1.4 moles per mole of cyclohexane reacted at 360” C., a residence time of 1.7 seconds, and a feed ratio of 3 moles of air per mole of cyclohexane. Process variables studied were temperature, residence time, oxygen content of feed gases, and surface-to-volume ratio of the reactor. Intermediate products detected were formaldehyde, acetaldehyde, acrolein, pentanal, and cyclohexanone. Use of metallic and metallic oxide catalysts resulted in the end products carbon dioxide and water.
T
HE petrochemical industry produces industrial quantities of oxygenated compounds by direct oxidation of hydrocarbons. Oxidation of cyclohexane is of interest because possible oxidation products include six-carbon aldehydes and ketones. The purpose of this study was possible synthesis of chemicals through vapor-phase oxidation of cyclohexane with air. These studies 1
Present address, Pan American Refining Go., Texas City, Tex.
776
are presented as the vapor-phase air oxidation of cyclohexane using no catalyst, vapor catalysts, and solid catalysts.
Prior Work I n the noncatalytic vapor-phase oxidation, Estradere ( 4 ) passed a mixture of cyclohexane and oxygen (mole ratio 4:l) through a glass tube filled with 3-mm. glass rods. She found
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 4
CHEMICAL PROCESSES that active oxidation began a t 340" C. and that peroxides were formedin a range of temperatures, commencil,g just below 340" C. and disappearing a t temperatures 10' t o 15" higher* Carbon monoxide and dioxide were both formed a t 340' C.; the former increased to 65% of the exit gases a t 400' C. and the latter to 18% at 4000 C. &rdles ( 6 ) heated a mixt,ureof 12% cyclohe+ ane in air in a hard glass tube. A t 320" C. oxygen began t o be consumed, and the amount consumed continued to increase up to 400" C. The flammability limits of cyclohexane in air have been determined t o extend from a lom,er limit of 0.8% cyclehexane to an upper limit of 5.2% CYclohexane at 300" c . ; these limits broaden in range with increasing temperature ( 9 ) .
Figure 1.
a
Continuous flow oxidation unit
The use of hydrogen bromide as a catalyst for the oxidation of cyclohexane was investigated by Nawrocki, Raley, Rust, and Vaughan (9, 10). These investigators reported that cyclohexane in the presence of hydrogen bromide a t 220' C. consumed 40% of the oxygen present and only 8% of the oxygen appeared as carbon oxides and water; cyclohexanone and some diketones were obtained in the products. Oxidation of cyclohexane over solid catalysts has been reported by a number of workers (1, 3, 6, 8, 11-15, 16, 16) under a range of conditions with a variety of catalysts. Oxidation products include small yields of acetaldehyde, acrolein, acetic, pyruvic and maleic acids, peroxides, aldehyde acids, and other unisolated complex compounds. The Milas and Walsh study (8, 15) made use of high ratios of air to cyclohexane over a vanadium pentoxide catalyst. They obtained a 20% yield of maleic acid. Thermodynamic calculations for the formation of these various products show that these oxidation reactions have a high thermodynamic driving force and the problem is to stop before complete oxidation to carbon dioxide and water. All these reactions are highly exot,hermic.
-
Experimental
OLo
Oxidation Unit. Separate streams of cyclohexane and air were passed at constant feed rates through preheaters, then mixed and allowed to react. The oxidation products and unreacted material were determined by analyses and material balances. A diagramatic flow sheet is given in Figure l. Cyclohexane from buret, A , was pumped a t a predetermined constant rate with a variable-speed gear pump, B , metered through a rotometer, C, and fed to the preheater, D. Air or oxygen from a supply of the compressed gas, E, was throttled to constant pressure through a pressure regulator, F, metered through a rotometer, G, and regulated by a needle valve, H , into a preheater. Steam, sometimes used to dilute the air or oxygen, was fed as water from buret J pumped a t a redetermined conmetered through stant rate with variable-speed gear pump orifice meter M , and fed into the air preheater, N . The preheated cyclohexane vapor and air were mixed and allowed t o
{
April 1955
react in the heated reactor, 0. Reaction products were partially condensed in a water-cooled condenser, &, and then frozen out in traps, R, immersed in solid carbon dioxide. Exit gases from the product traps were washed with distilled water in a gas washing bottle, S, and then collected for analysis in a gas sample tube. The reactor and air preheater were made of 1-inch Type 316 stainless steel pipe. The upper section of the reactor, which served as the cyclohexane preheater, and the air preheater each contained a close fitting stainless steel rod on which was machined a square thread screw. The rod was 11 inches long and the square threads were '/4 X '/4 inch to give four threads per inch. in the top Of the rod and Cyclohexane entered through flowed into the upper thread from which it moved downward along the heated wall of the pipe. The reactor chamber b l o w the cyclohexane preheater was 10 inches in length. The air and water preheater was connected to the reactor immediately below the cyclohexane preheater section by a 3/la-inch tube. The preheaters and reactor were immersed in a molten heat transfer salt bath, P , heated by electrical heaters, and agitated by compressed air. The salt bath temperature was maintained constant by a potentiometer-controller. The temperature in the reaction chamber was measured by a movable thermo' couple within a coaxial stainless steel thermowell. A perforated catalyst support plate was welded t o the thermowell tube a t a location adjacent t o the bottom of the heating bath. For safety the entrance to the preheaters and the exit of the reactor were protected by rupture disks, 7'. Cyclohexane Used. The cyclohexane was EastAT man practical grade which was purified by percolation through silica gel and redistillation. The physical properties of the purified cyclohexane were: density a t 25" C., 0.7737 gram per ml.; refractive index a t 25" C., 1.4233; normal boiling point, 80.5' C.; and freezing point, 5.20" C. Purity of the cyclohexane was determined t o be 99.5 mole % by the freezing point method. Analytical Methods. The exit gases were analyzed for carbon dioxide, carbon monoxide, and oxygen. A few analyses were made for hydrogen, but the amount present was never significant, and its analysis was discontinued.
,!,
I 360
io 4bO io
i o
Lo
i o
o!o
TEMPERATURE, *C.
Figure 2.
Dependence of reactor temperature on heat flux from reactor
Moles of airjmole of cyclohexane
= 3.0;
residence time
= 1.4
rec.
Individual aldehydes and ketones were identified in the separate cyclohexane and water layers. Determination of total aldehyde content was made for the noncatalytic runs using a modification of the iodometric method of Walker (14). Separate aldehyde determinations were made on aliquot portions of the condensate cyclohexane layer and the combined condensate water layer and the solution from the gas washing bottle. Acidity in the water condensate layer was determined by titration. Higher boiling distillation fractions in the liquid hydrocarbon product were sep-
INDUSTRIAL AND ENGINEERING CHEMISTRY
777
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT arated using a semimicro Vigreaux column. Benzene in the cyclohexane fraction was estimated by measurement of refractive index. The residence time was calculated on the basis of the volume of feed gases taken a t the average temperature and pressure of the reactor. For the heterogeneous catalytic reactions, the reaction time is given in terms of the space velocity-the volume of feed gases at 0' C. and 760 mm. pressure per unit volume of bulk catalyst per unit of time. The apparent contact time is the reciprocal of this ,space velocity. Process Variables. The process variables in the noncatalytic vapor-phase oxidation are temperature, pressure, residence time, oxygen content of feed gases, and ratio of surface to volume in the reactor. The effect of pressure could not be studied in this experimental equipment, and all experiments were a t atmospheric pressure. In this study, the mixtures of air and cyclohexane which were employed were on the cyclohexane-rich side of the flammability limits, whereas in the work of Milas and Walsh (8, 16)the mixture was on the air-rich side of the flammability limits.
Carbon Balances (Effect of residence time) Moles per Mole of Cyclohexane Reacted Aldehydes as 46%ci Higher Total Cr Cs 65%Ca co COZ- boiling C
Run No.
CI
92
1.232
...... ... . . . .2 ..4.6 7... .39 ...
94
96
97
... ... 0.55'Cl
2.40Ca
......
1.11
... 2.22 . . . . . . . . . 6.66 .........
... ... 0 :5oc,
. . . . . . . . . 2.16Ca . . . . . . ... ... ... .... .. .2. ...9.9. .8....9.6. 0,bi'Cl . . . . . . . . . 2.92Ca 1.37 ......... ... 2.74 . . . . . . .. .. .. .. .. . . .8 ..2.2 0 .'6ZCl . . . . . . . . . 2.67Ca
1.494
0.27 0.11
...
... ... ...
0.06
...... .. .. .. .. .. .. ......
0.99 0.22
... ...
0.06
...... ... . . . . . . ...
1.13 0.31 0.06
...
... ... ...
.. .. .. .. .. .. .. .. .. .. .. ..
1 , l 9 0.33
0.06
. . . . . . . . . ......... .. .. .. .. .. .. .. .. ..
C
Balance,
%
1.67 2.90 7.83
28 48 130
3.39
46
...
2.38 3.49 7.93
...
40 58
132
3.93
... 65
2.99 4.49 10.46
...
50
75
...
174
5.09
... 85
2.95 4.32 9.80
49 72 163
4:8?
81
...
Noncatalytic Vapor-Phase Oxidation Effect of Temperature. The operating temperature was varied while other process variables were held constant: feed ratio 3.0 moles air per mole cyclohexane, residence time 1.4 seconds, and a ratio of reactor surface to reactor volume of 2.1 reciprocal cm. The dependence of the average reactor temperature on the heat flux from a highly exothermic reaction zone to the salt bath is shown in Figure 2. The ordinate of this figure is the yield of aldehydes reported as formaldehyde. Note that high yields of aldehydes corresponds to large differences between the three curves. The average reactor temperature was used as the temperature to express the process variable in the remainder of this investigation. Below the ignition point of the cyclohexane, temperature gradients were most extreme a t the lower reaction temperatures. At 287' C. a temperature difference of 75' C. existed between the reactor thermocouple and the salt bath a t a point 7 inches below the mixing point of the gases. The effect of temperature on the formation of gaseous products is shown in Figure 3. Formation of carbon oxides began a t 360' C., and soon passed through a maximum a t 380' to 400' C. The yields of water and aldehydes were high a t 360' C. and decreased with increasing temperature. The maximum selectivity in formation of total aldehydes reported as forbaldehyde is 21.6 mole % a t 360' C. where 11.1 mole yo of the cyclohexane feed underwent reaction. The aldehyde yield includes both formaldehyde and higher aldehydes. Actually, between 30 and 60% of the aldehydes were estimated to be formaldehyde, as the higher aldehydes condensed out in the cold traps while the formaldehyde was absorbed in the water trap. Qualitative analyses showed positive tests for formaldehyde, acetaldehyde, acrolein, pentanal, and cyclohexanone. Because the analytical method used for quantitative analysis determined the aldehyde groups, the yield was calculated as formaldehyde. Higher boiling fractions of the hydrocarbon product were determined and found to be maximum a t 410' C.; approximately 3 mole % of the cyclohexane dehydrogenated to benzene. Combustion and cracking occurred a t temperatures above 530' C., as hot spots and soot formation were observed. A complete material balance could not be made because of the complex mixture of aldehydes and acids formed in the oxidation reaction. This mixture was so complex that no attempt was made a t separation, but there was qualitative identification of various aldehydes. It is believed that the total yield of aldehyde is indicative of the commercial possibility of the oxidation process. Effect of Residence Time. The effect of residence time was
778
studied with the following fixed process variables: average temperature, 361' C.; feed ratio 3.0 moles air per mole cgclohexane; and a ratio of reactor surface to reactor volume of 2.1 reciprocal cm. The effect of residence time on the formation of gaseous products is shown in Figure 4. At 361' C., the formation of formaldehyde, carbon oxides, and water all showed a maximum a t a residence time of 1.7 seconds where formaldehyde formation was 24.8 mole %. The formation of formaldehyde and water appeared before the formation of carbon oxides was appreciable: At residence time 1.5 seconds, the formation of gaseous products was 1.2 moles of formaldehyde, 2.7 moles of water, 0.3 moles of carbon monoxide, ' and 0.1 mole of carbon dioxide per mole of cyclohexane reacted. During the study of residence time, it was noticed that an induction period was necessary for the reaction. As the residence time decreased, the position of the maximum temperature within the reactor withdrew from the injection point of the reactant gases as shown : Residence Time, Seconds
Distance from Injection Point of Reactant Gases to Position of Maximum Temperature within Reactor, Inches
2.3 1.7 1.5 0.8
4 7 7 9
Using a residence time of less than 0.8 second, no reaction occurred. The time for this induction decreased slightly with increased reaction temperature. The formation of higher boiling fractions in the hydrocarbon product represented 670 of the reacted cyclohexane a t the maximum. Effect of Oxygen Content in Feed Gases. The oxygen content of the feed gases was varied by diluting the air feed with steam. For this study, the following process variables were fixed: residence time, 1.0 second; average reactor temperature, 410' C.; empty reaction chamber. The effect of oxygen content of the feed gases on the formation of gaseous products is shown in Figure 5. The formation of formaldehyde became noticeable a t 0.2 mole of oxygen per mole of cyclohexane in the feed; the formation of carbon dioxide was noticeable a t 0.4 mole and carbon monoxide a t 0.6 mole of oxygen per mole of cyclohexane in the feed. The formation of formaldehyde, carbon dioxide, and carbon monoxide increased with increasing oxygen content of the feed gases. With 1.04 moles of oxygen per mole of cyclo-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 4
$
CHEMICAL PROCESSES
2 8
E
n
n
g a
2 4
K
W
g a
W
fX
20
I W
r W
3
E g
3
16
> . _ W)
W
P
I 2
B
I I-
$
08
3
u 3
::
0 0 LI:
0 4 W 0
W
d
d '340
360
380 AVERAGE
400
420
440
460
REACTOR TEMPERATURE.
480
10
12
14
RESIDENCE TIME,
Figure 3. Effect of temperature on formation of gaseous oxidation products from cyclohexane Moles of air/mole of cyclohexane = 3.0; 1.4 sec.
OB
'C.
residence time =
16 18 SECONDS
22
20
Figure 4. Effect of residence time on formation of gaseous oxidation products from cyclohexane Temp. = 361
' C.;
moles of air/mole of cyclohexane = 3.0
hexane, the formaldehyde formation was 0.288 mole per mole of cyclohexane fed and 34 mole % of the cyclohexane reacted. The effect of the oxygen content of the feed gases on the extent of the reaction is shown in Figure 6. The conversion of the reactants is directly proportional to the oxygen concentration, and the ratio of the oxygen reacted to cyclohexane reacted is constant. At the higher conversion of reactants 3 weight % of the reacted cyclohexane appeared as higher boiling fractions in the condensate. Effect of Surface to Volume Ratio of Reactor. The surface to volume ratio of the reaction chamber was varied by packing the reaction chamber with glass tubing of various diameters, Table I. The void spaces above and below the tubes were filled with glass wool, and a glass wool plug served to mix the preheated reactant gases at the top of the glass tubes.
Table 1.
Surface to Volume Ratio of Reactor Surface-to-Volume Ra.tio,
Cm-1
Packing in Reaction Chamber Empty chamber, 1-inch Schedule 80 pipe
Glass tubes (3), 9.5 mm. o.d., 7.4 mm. i.d., and 22.6 mm. long Glass tubes (7), 8 mm. o.d., 5.9 mm. i.d., and 22.7 cm.long Glass tubes ( l o ) , 6.1 mm. a d . , 3.8 mm. i.d., and 22.7 cm. long Glass tubes (35),3.8 mm. o.d., 2.4 mm. i.d., and 22.7 em. long
2.1
5.9 11.6
MOLES
04
06
OB
10
OXYGEN PER MOLE CYCLOHEXANE IN FEED
Figure 5. Effect of oxygen content on formation of gaseous products Temp. = 410' C.; residence time
= 1.0
sec.
12.2 27.5
The effect of surface-to-volume ratio of the reactor on formation of gaseous products is shown in Figure 7 and on the.conversion of cyclohexane in Figure 8. For this study the fixed process variables were as follows: average reactor temperature, 410' C.; moles air per mole cyclohexane, 3.0; and residence time, 1.4 seconds. The oxidation of cyclohexane is inhibited by an increase in surface-to-volume ratio. Using an empty reactor chamber, 16 mole % of the cyclohexane reacted; using a surface-to-volume ratio of 27.5 cm.-l, only 3.9 mole yo of the cyclohexane reacted. This phase of the study would have been interesting had the packing arrangement been such that the reactor surfaces were broken in three directions rather than two. The formation of formaldehyde, carbon dioxide, carbon monoxide, and water is suppressed with increasing surface-to-volume ratio. April 1955
0 2
0
Oxidation Products Detected. The oxidation products that were detected included cyclohexanone, pentanal, acrolein, acetaldehyde, formaldehyde] carbon dioxide, carbon monoxide, and water. Tests indicated the presence of acids, unsaturated hydrocarbons, peroxides, and cyclohexane oxide. The acid formation never amounted to more than 25 meq. of hydrogen ion formed per 100 grams of cyclohexane oxidized, and the dehydrogenation of Cyclohexane to benzene never amounted to more than 4y0of the cyclohexane oxidized. The following compounds were identified by various tests as odor, color reactions, and the preparation of one or more solid derivations for which the melting point agreed with literature values or with mixed melting points with known samples: formaldehyde, acetaldehyde, acrolein, pentanal, and cyclohexanone. The presence of cyclohexene oxide was suspected. This product was reacted with hydrogen chloride to produce cyclohexene chlorohydrin. However, the cyclohexene chlorohydrin was insufficiently purified for positive identification. The
INDUSTRIAL AND ENGINEERING CHEMISTRY
779
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
1
the aldehyde concentration, which in turn would decrease its autocatalytic effect. This assumes that the rate of the disintegration reaction of peroxides and aldehydes increases more r a p idly than that of the hydrocarbon oxidation reaction. In all cases, the formation of aldehydes precedes the appearance of carbon dioxide and carbon monoxide.
MOLES OXYGEN PER MOLE CYCLOHEXANE I N
Figure 6.
FEED
Effect of oxygen content in feed on conversion of reactants
Residence time = 1 .O sec.; temp. = 4 1 0 ' C.
presence of peroxides was indicated by the liberation of iodine from a potassium iodide solution. Presence of unsaturated hydrocarbons was indicated by the bromide addition test and the Baeyer neutral potassium permanganate test. Unidentified peroxide occurred in the water layer and the cyclohexane layer. Peroxides were present in all distillation fractions from 50" to 180' C. In the higher boiling fractions of the cyclohexane layer there was an unidentified carbonyl compound; its derivative, 2,4-dinitrophenylhydrazone was golden flaky crystals which melted a t 89" to 91" C.
REACTOR
SURFACE T O VOLUME R A T I O , CM-l
Figure 8. Effect of reactor surface-to-volume ratio on conversion of cyclohexane Moles of air/mole of cyclohexane = 3.0; av. reactor temp. = 4 1 0 ' C.; residence time = 1.4 sec.
The oxidation of cyclohexane is inhibited by a decrease in the diameter of the reaction vessel (an increase in the ratio of reactor surface to reactor volume). This is a strong indication that the mechanism is a gas chain oxidation with chain breaking induced a t the walls. This chain breaking could decrease the production of aldehyde autocatalyst which in turn would decrease the total reaction occurring.
Oxidation with Vapor-Phase Catalysts
REACTOR SURFACE TO V O W € RATIO, CM.?
Figure 7. Effect of reactor surface-to-volume ratio on formation of gaseous products Moles of air/mole of cyclohexane = 3.0; av. reactor temp. = 41 0" C.; residence time = 1.4 sec.
Interpretation of Results. There are a number of primary and intermediate reactions that are possible in the vapor-phase oxidation of cyclohexane, and a thorough analysis is difficult. In the temperature region 325' to 450" C., the gradual temperature gradient increase with reaction time shows that the reaction begins to accelerate after an induction period. This can be explained by an autocatalytic course of the reaction that accounts for the decrease of total reaction in the temperature range 450" to 530" C. An aldehyde or peroxide intermediate in this temperature region could be the autocatalyst. It is known that the oxidation stability of aldehydes decreases with increasing temperature above 350" C. Increased temperatures would decrease
780
The catalysts used and their concentration mixed in the cyclohexane feed were: iodine (0.3 wt. yo); nitrogen tetroxide (1.1 wt. %); tetraethyllead (0.2 wt. yo of tetraethyllead in the form of aviation ethyl fluid); aniline (1 vol. %); cyclohexylamine (1 vol. %); isoamyl nitrite ( 1 vol. %); and diethyl ether ( 1 vol. %). The results show that vapor-phase catalysts did not alter the oxidation from the noncatalytic reaction which was already studied. Analyses of the exit gas and distillations of the waterfree condensate show essentially identical results with the noncatalytic oxidation.
Oxidation over Solid Catalysts The solid catalyst investigated were metals and metallic oxides deposited on pumice and silica gel supports. The catalysts used were silver oxide, uranyl vanadate, lead molybdate, molybdic oxide, iron oxide, cobalt oxide, manganese dioxide, and zirconium dioxide on the pumice and silica gel supports. Other catalysts used were platinized silica1 gel, metallic silver on aluminum catalyst, and vanadium pentoxide. Most of these catalysts had been mentioned in patents on vapor-phase oxidations of other hydrocarbons. The standard methods of preparation were used for silver oxide, uranyl vanadate, stannous vanadate, molybdic oxide, lead molybdate, iron oxide, cobalt oxide, manganese dioxide, and zirconium dioxide (7). The catalyst was precipitated on the carrier, washed with water, evaporated to dryness, and dried. The vanadium pentoxide catalyst was the 30-mesh vanadium
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 4
CHEMICAL PROCESSES
pentoxide manufactured by the Coleman and Bell Co. The catalyst of metallic silver on aluminum was prepared by galvanic action. All catalysts were pretreated with the reactants a t 100" t o 150" C. for 30 minutes before use. The process variables were studied over a range' using the different catalysts in an effort to obtain oxidation products other than the end products of reaction-carbon dioxide, carbon monoxide, and water. Feed ratios of 0.5 to 10 moles of air per mole of cyclohexane n-ere used with the silver oxide and vanadium pentoxide catalysts; feed ratios using the other catalysts were 0.8 t o 5 moles of air per mole of cyclohexane. The reaction time was changed in an effort to obtain reaction products other than the end products. The apparent contact times were 0.2 to 12 seconds. Reaction temperatures were varied between 150" and 550" C.
Table II.
Lowest Temperature of Incipient Reaction for Vapor-Phase Oxidation of Cyclohexane
Lowest Temperature of Incipient Reaction, c. Catalyst 240 Vanadium pentoxide 292a Silver oxide 170C Cupric chromite 297 Lead molybdate 294 Uranyl vanadate 330 Molybdic oxide 288 Ferric oxide 336 Vanadium zeolite 207 Cobalt oxide 342 Manganese dioxide 444 Zirconium dioxide 348 Stannous vanadate 340 Shell 105 dehydrogenatic :atalystb a Using water as a diluent, lowest temperature of incipient reaction began st 503' C.; use of 1% cyclohexane in cyclohexane feed lowered temperature to 198' C. I, Analysis: Fez03 = 70%, CrzOa = 30%, CuSOi = 1%, KNOa = 0.5%. c 1% cyclohexanone was added to cyclohexane feed to act as reaction initiator.
.
Products of the vapor-phase air oxidation of cyclohexane ovei these solid Catalysts were the end products of combustion-carbon dioxide and water. Reaction using these catalysts differed in the temperature a t which incipient reaction was noted-Le., using cobalt oxide, reaction occurred a t 207' C whereas with zirconium dioxide reaction occurred a t 444" C The lowest temperature of incipient reaction is shown in Table I1 for the various catalysts. There was no gradient in the degree of reaction with temperature-either the reaction went to the end products of combustion, carbon dioxide and water, or else no reaction occurred. It is probable that secondary oxidation of any primary intermediates occurs in situ on the catalyst surface to form carbon dioxide and water. This investigation showed that, with the catalysts employed, cyclohexane is oxidized to carbon dioxide and water, without desorption of any intermediate products. This work and that of Walsh (8, 15) are companion investigations in the oxidation of cyclohexane. Walsh used a minimum ratio of moles of air per mole of cyclohexane of 40 and the for-
April 1955
mation of maleic anhydride was a maximum a t a ratio of 205. Apparently under these conditions the partial pressure of maleic acid was lowered to such a point that its desorption from the solid vanadium pentoxide catalyst occurred. I n the present work the mixtures were on the cyclohexanerich side of the flammability limit and the maximum ratio of 5 moles of air per mole of cyclohevane was used. Summary
The vapor-phase oxidation of cyclohexane with air was investigated using no catalysts, vapor catalysts, and solid catalysts. Using no catalyst and with a high cyclohexane-air ratio, aldehydes were the most prominent products prior to complete combustion. At 361" C., 24.8 mole yo of the reacted cyclohexane formed aldehydes. As temperature increased, aldehyde formation decreased until a t 500" C. combustion and cracking of cyclohexane were appreciable. The vapor catalysts tried had no effect on the reaction or products. Over metal and metal oxide catalysts the oxidation went completely to carbon dioxide and water without forming intermediate products that could be isolated. Acknowledgment
The authors wish to thank the Humble Oil and Refining Co. for a fellowship held by W F. Hoot. literature Cited (1) Berl, E., Heise, K., and Winnacker, K., 2. physik Chem., A139, 453 (1928). (2) Briand, M., Dumanois, P., and Lafitte, P., Compt. Tend., 197, 322 (1933). (3) Chowdbury, J. K., and Saboor, M . A , , J . I n d i a n Chem. SOC., 14,638 (1937). (4) Estradere, S., Compt. Tend., 196, 674 (1933). (5) Mardles, E. W. J., Trans. Faraday Soc., 27, Pt. 11, 712 (1931). (6) Margolis, L. Ya., and Todes, 0. M., Bull. Acad. Sei. U.S.S.R., Classe sci. Chim., 1947, p. 443. (7) Mellor, J. W., "Comprehensive Treatise on Inorganic and Theoretical Chemistry," Vol. VII, IX, XI, XII, and XIV, Longmans Green, New York, 1922. (8) Milas, N. A,, and Walsh, W. L., J . Am. Chew. SOC.,61, 633 (1939). (9) Nawrocki, P. W., Raley, J. H., Rust, F. F., and Vaughan, W. E., IND.ENG.CHEM.,41,2604 (194Y)# (10) Rust, F. F., Raley, J. H., and Vaughan, W. E., U. S. Patent 2,421,392 (June 3, 1947). (11) Shimose, R., Sei. Paper I n s t . Phys. Chem. Research ( T o k y o ) , 15, 251 (1931). (12) Shmidl, A J., U. S. Patent 2,474,334 (June 28, 1949). (13) Wagner, C. R., U. S. Patent 2,386,372 (Oct. 9, 1945). (14) Walker, J. W., "Formaldehyde," ACS Monograph 98, Reinhold, New York, 1944. (15) Walsh, W. L., Ph.D. Thesis in Chemistry, Mass. Institute of Technology, 1936. (16) Wilken-Jordan, T. J., J . Chpm. Met. M i n i n g SOC.S. Africa,32, 283 (1932). RECEIVED for review July 31, 1954.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTED February 9, 1855.
781
