VAPOR-PHASE NITRATION OF CYCLOHEXANE USING NITROGEN DIOXIDE R0
B E RT0 L E E1 A N
D LY
LE F
.
A LB R IG HT
,
Purdue University, Lafayettc, Ind.
The vapor-phase reaction between cyclohexane and nitrogen dioxide to produce a mixture of nitrocyclohexane, cyclohexanol, cyclohexanone, cyclohexyl nitrite, cyclohexyl nitrate, and dibasic acids was studied using a flow system a t atmospheric pressure. The variables investigated included temperature (200*to residence time (0.5 to 4 minutes), mole ratio of cyclohexane to nitrogen dioxide (1 to 1 up to 6 to 1 ), and severlol different reactors. Product analyses were made using a three-column gas chromatographic
240" C.),
unit supplemented by wet chemical methods. The combination of several analytical methods provided good atom balances for carbon, hydrogen, oxygen, and nitrogen. Conversions of nitrogen dioxide to nitrocyclohexone as high as 16% and yields of nitrocyclohexane based on cyclohexane consumptions of 50% or even higher were obtained. The effects of operating conditions on the kinetics and course of reaction were studied in detail. A reaction mechanism is proposed which accounts for all the maior products obtained.
has in recent years found increased solvent, a reaction intermediate, and a raw material for caprolactam production (5, 24). I t can be prepared from cyclohexane using nitrating agents-e.g., nitrogen dioxide and initric acid-by either liquid-phase or vapor-phase processes (5, 9, 7 7 , 77, 22, 23, 27). Kitration of other hydrocarbons has been reported extensively (20, 23). Nitration reactions are accompanied by competing oxidation reactions which greatly complicate the reaction mechanism. I n general, vapor-phase nitration processes using nitric acid are operated a t fairly high temperatures (400' to 450' C.) and short residence times (0.5 to 10 seconds). Yields are generally low because of considerable breakage of carboncarbon bonds. Nitration of propane using nitrogen dioxide (7, 73) occurs a t relatively low temperatures (200' to 300' C.) and long residence times (1 to 10 minutes). T h e nitration is predominantly at the secondary carbon, and the breakage of carbon-carbon bonds is relatively little. Hence, the speculation that low-temperature nitration of cyclohexane with nitrogen dioxide might result in yields higher than vaporphase nitrations with nitric acid led to this study. Vapor-phase nitration of cyclohexane using nitrogen dioxide apparently has been investigated to only a limited extent; the reaction mechanism is not well understood (8, 25), and no quantitative data were available until recently (9). Numerous products have been identified from various studies (5, 76, 77, 27, 27). Reviewing the reaction mechanisms proposed, the actual nitration step is generally considered to be the combination of a cyclohexyl radical with nitrogen dioxide. Several ITROCYCLOHEXANE
N use as a
1
Present address, Monsanto Co., St. Louis, Mo.
initiation routes have been proposed for the formation of cyclohexyl radicals:
1. Oxygen produced by the dissociation of nitrogen dioxide attacks cyclohexane (25). 2. Nitrogen dioxide and water react to produce nitrous acid and a hydroxyl radical. The hydroxyl radical in turn attacks cyclohexane, producing the cyclohexyl radical and water. 3. Bachman (3,4) and Titov (22) theorized an attack on the paraffin (or in this case cyclohexane) by nitrogen dioxide to form an alkyl (or cyclohexyl) radical and nitrous acid. These initiation routes as well as the intermediate and competing reaction steps were considered in this investigation. The nitration of cyclohexane using nitrogen dioxide was made using statistically designed experiments over the ranges of conditions of probable commercial interest. Apparatus and Operating Procedure
T h e flow system for nitrating cyclohexane is shown schematically in Figure 1. Cyclohexane (99+yo purity obtained from Phillips Petroleum Co.) was metered as a liquid from a feed buret, l , using a stainless steel microbellow pump, 2 , and was vaporized in a preheater, 3, prior to introduction into the reactor, 13. A 10pound cylinder of nitrogen dioxide, 5, was immersed in a 35' C. water bath, 6. T h e lines connecting the nitrogen dioxide to the rest of the system (including the reactor) were electrically heated above 35' C. to prevent condensation of the nitrogen dioxide. The nitrogen dioxide was first passed through a phosphorus pentoxide drying tube, 7, and then metered through a heated Matheson flowmeter, 9. T h e nitrogen dioxide feed system was maintained a t 3 p.s.i.g. by a valve arrangement (valves A , B, and C). T h e materials of construction used for nitrogen dioxide were glass, stainless steel, and Teflon connectors. Cyclohexane and nitrogen dioxide feed streams combined a t the reactor inlet. VOL. 4
NO. 4
OCTOBER 1965
411
K
il 6
U
L
A
d
Figure 1. 1. 2. 3.
9. 10. 1 1. 12. 13.
Cyclohexane feed tube Cyclohexane pump Cyclohexane vaporizer Connector Nitrogen dioxide cylinder Constant temperature bath Phosphorus pentoxide drying column Pressure gage
4. 5. 6.
7. 8.
0' 240
0
bl
L
Y
3
220
i-l
200
0
14. 15. 16.
F:
-
-
44 38
67
-
0.5
la0
1.5
Rerideace Tine, a h
Nitration apparatus
,
0.5
17.
Rotameter Nitrogen cylinder Drying column Molten salt bath Reactor Manometer Condenser Product receiver
1.0
.
1.5
Rer ideacc ' T i m e , m i n o
-=$
&
++
1 B t
0;s
1.0
L 0.5
220 200
1.0
0.5
List of runs with their operating variables
Upper. Glass reactors I and II, S/V = 5.0 cm.-' 6 9 made in glass reactor II l o w e r left. Aluminum reactor I, S / V = 3.65 cm.-' l o w e r ri3ht. Glass reoctor 111, S/V = 2.57 cm.-
412
I&EC PROCESS D E S I G N A N D DEVELOPMENT
oa3
Runs 62 to
'
65
1.0
1.3
Reiidrnce The, win.
1.0
R18idcncc The, rnih
Re8ideace T i o n , nin. Figure 2.
Check valve
t
L
200
3-way ,glass stopcock
L
1.3
Realdeuce Tine, riu.
d 2 220 bl
0 E
2-way glass stopcock
68
53
52
0
Y r)
Sulfuric acid traps Gas sampling tubes Soap bubble meter @ Needle valve
18. 19.
T h e five reactors used during this investigation were :
Renctor
Glass reactor I Glass reactor I1 Glass reactor I11 A1 reactor I A1 reactor I1
Construction 7 connected
O.D., Atfm.
10
U-tubes
Same
Same Coil Coil
10 18 13 13
In all cases, the rea.ctor proper was totally immersed in a large well agitated molten salt bath, 12, providing a n essentially isothermal condition. T h e electrically heated outlet of each reactor extended vertically above the level of salt bath; the outlet line \vas thein bent and extended slightly do\vnward to the product recovery system. T h e bath was heated by a Glas-col heating mantle as well as by immersion heaters. A manometer containing heptacosfluorotributylamine was used to measure the pressure in the reactor and also served as a safety device in case of excess pressure. T h e effluent gases from the reactor \\.ere passed through a 22-inch Graham condenser, 15. T h e condensate \vas collected in a glass receiver, 16, jacketed with cooling water. The uncondensed gases passed through a 250-ml. gas-washing bottle, 17, containing 48% sulfuric acid (to strip off unreacted nitrogen dioxide). Gas sarnpla )\'ere collected in sampling tubes, 18, a t the outlet of the gas-\vashing bottles. T h e flo\v rate of uncondensed gas !vas determined using a soap bubble meter, 19. A run \vas started by first passing cyclohexane through the system a t a desired rate, and then turning on the nitrogen dioxide flow. Generally the system was operated for 150 to 190 minutes, and several samples of condensed organic products were analyzed by gas chromatography as a check for steady state. After the system had reached steady state, the nitrogen products were collected over a 30- to 70-minute period depending on the amount of product. T h e liquid product was drained from the product receiver to a tared separatory funnel. T h e aqueous layer was drained off and the organic product was washed with three or four aliquots of 50 ml. of distilled and demineralized water. T h e liquid samples as well as gas samples and unreacted nitrogen dioxide contained in gas-washing bottles were generally analyzed immediately after their collection, since nitrogen dioxide dissolved in the liquid product was found to react slolvly with some of the products. Product Analysis. The organic products \vex analyzed by gas chromatography using a 3-meter column of Carbowax 2051 on Haloport F at 115-17' C., which separated dissolved gases, 2,4-dimethylpentane (present in small quantities in the cyclohexane feed), cyclohexane, Fvater? cyclohexanone. cyclohexanol plus cyclohexyl nitrite, nitrocyclohexane, and nitrocyclohexene. Cyclohexyl nitrite gave a peak with a retention time identical to that of cyclohexanol; apparently it reacted with column packing material, giving cyclohexanol under the conditions of the gas chromatographic unit (72, 27). A colorimetric determination of cyclohexyl nitrite as described by Ackermann ( 7 ) was made, and cyclohexanol was obtained by difference. The nonvolatiles in the organic product, which \yere the compounds ha.ving a boiling point higher than nitrocyclohexene and which did not appear by gas chromatographic analysis, were determined by pumping do\vn a n aliquot of the organic layer in a vacuum system maintained a t room temperature for 2 days. T h e analysis of several samples of the nonvolatiles showed that they were composed mostly of organic acids, particularly dibasic acids. T h e aqueous layers were very small as compared with the organic layers, and were found to contain little organic material. The aqueous layers were analyzed for weak and strong acids by titrations. The nonvolatiles in aqueous layers were determined in the same manner as those in the organic layers. T h e gaseous products were analyzed on two gas chromatographic columns: Oxygen, nitrogen, nitric oxide, and carbon monoxide were separated on a 4-meter activated Molecular Sieve 13X column a t 0' C . ; nitric oxide, carbon monoxide, nitrous oxide, and carbon dioxide on a 4-meter silica gel column a t 25' C.
I.D., Mm. 8
8 16 11
11
Val., Cc. 735
S / V , Crn.-' 5.0
735 745 735 1425
5 .O 2.57 3.65 3.65
Special Features
4 T.C. wells
... ... ... ...
Cnreacted nitrogen dioxide absorbed in the gas-washing bottles was determined quantitatively by a standard Kjeldahl analysis for nitrogen determination (26).
Results and Discussion
A series of 69 experiments was made to evaluate the effects
of residence time, t , temperature, T , mole ratio of cyclohexane to nitrogen dioxide, R, reactor material of construction, and surface-volume ratio, S / V , on the nitration reaction. T h e first 34 preliminary runs were all operated a t R = 3 to 1. No reaction apparently occurred below 190' C. even a t residence times u p to 4.5 minutes. T h e highest nitrocyclohexane conversion was obtained between 220' and 240' C. Above 260' C., considerable char material formed in the reactor. T h e maximum operable residence times for a given temperature without excessive char material being formed were approximately 30 seconds a t 260' C., 20 seconds a t 280' C., and 10 seconds a t 300' C. T h e last 35 runs together with their opening conditions are tabulated in Figure 2. Generally 98y0 or higher over-all material balances were obtained in all of these runs. Similarly, excellent material balances were obtained for carbon and hydrogen atoms. Oxygen balances were generally high, most values being 105 =k 5%. Nitrogen balances were 80 =t10%. The low nitrogen balances might be the result of incomplete recovery of unreacted nitrogen dioxide. T h e main reason for the discrepancies is the complexity and the variety of compounds in the reaction product. This may be the reason why no previous material balances have been reported in the literature for vapor-phase nitration processes. A typical list of experimental data and sample calculations for a run is presented in Table I. The simplifying assumptions involved in these calculations are indicated. T h e combined experimental error in the determination of nitrocyclohexane conversion was determined from repeat runs and had a standard deviation of 0.306 unit. Hence, the range of 90% confidence limits was 1.07 units in per cent of nitrocyclohexane conversion. The conversions to other components had perhaps slightly high errors, say 5 to 6% on a relative basis, because of their lower concentrations. Effect of Temperature a n d Residence Time. Three levels of temperatures (200°, 220', and 240' C.) and three levels of residence times (0.5, 1.0, and 1.5 minutes) were employed a t mole ratios of 2 to 1 and 4 to 1. In all cases, combinations of high times and high temperatures gave high conversions to nitrocyclohexane (NCH). Figures 3, 4, and 5 indicate the conversions of NO2to NCH. T h e slopes of these curves are typical of other curves for conversion as a function of residence time and temperature. At R = 4 (see Figure 4), the highest nitrocyclohexane conversion was 15.8y0 (runs 54 and 69 a t t = 1.0 minute and T = 240' C . ) , T h e corresponding value a t R = 2 was 13.5% (run 44). The data in Figure 3 were fitted by several empirical equations by the VOL. 4
NO. 4
OCTOBER 1 9 6 5
413
I. Data
2.
1. Date, June 27, 1963 2. Glass reactor 11. V = 735 cc.. St'V = 5.0 cm.-] 3. Cyclohexane feed rate = 81.7 cc.,/hr. = 0.756 gram mole/hr. = 63.7 grams/hr. 4. Nitrogen dioxide feed rate = 0.380 gram mole/hr. = 17.5 grams/hr. 5. Total reactant flow rate = 1.136 gram moles/hr. 6. Mole ratio of cyclohexane to nitrogen dioxide = 1.99 7. Temperatures A. Reactor, 220' C. B. Cyclohexane vaporizer, 129' C. 8. Pressures A. Atmospheric = 748.4 mm. Hg B. Reactor system pressure = 784.9 mm. Hg 9. Times A. Reactor residence time. Assuming plug flow, ideal gas law, and no change in number of moles in the reactor, t = 0.992 minutes B. Leveling-off time = 120 minutes C. Subsequent reaction duration = 60 minutes 10. Liquid product for 60-minute period A. Organic layer = 64.70 grams B. Aqueous layer = 10.00 grams 11 Gaseous product for 60-minute period A. Flow rate = 0.391 cc./sec. B. Total volume = 1408 cc. C. Assuming perfect gas law applies, and correcting for water vapor picked up in the soap bubble meter, the amount of gaseous product = 0.0513 gram mole
11. Product Analysis 1. Organic layer A. Cyclohexyl nitrite determination using Beckman spectrophotometer Concentration of cvclohexvl nitrite = 1.71 weight % Amount of cvclohexvl nitrite = (l,?l%) (64.70 gkams) = 1.11 grams B. Nonvolatiles = 0.56 gram C. Other organics (grams) determined by using Carboivax 20M gas chromatographic column : 2,4-Dirnethylpentane 0.65 Cyclohexane 57.05 Cyclohexanone 0.44 Cyclohexanol and cyclohexyl nitrite 1 52 Cyclohexvl nitrate 0.17 Nitrocyclohexane 4.37 Nitrocyclohexene Trace TYater Trace
Table 1. Experimental Data and Aqueous layer A. Sonvolatiles = 1.76 grams B. Strong acids = 0.0905 gram mole (determined by titration) = 5.70 grams (assuming all is nitric acid) C. Weak acids = 0.0637 gram mole (determined by titration) D. If it was assumed that nonvolatile was composed of adipic acid only, and that weak acids were composed of nitrous acid and adipic acid, the following composition of aqueous layer resulted: Component
Weight, G. 0.79
Hz0 "OB HNO? Adipic acid
3.
co
N2O CO?
5.
i .75
1. ? 6
Gaseous product. \Vhen the analyses on molecular sieve and silica gel gas chromatographic columns were combined, the composition of the gaseous product was Component N" NO
4.
5.70
Weight, G. 0 096 1.230 0.053 0.031 0.188
G. Moles 0.0034 0.0410
0.0019 0,0007
0.0043
Total weight of gaseous product = 1.60 grams Unreacted nitrogen dioxide = 0.0782 gram mole = 3.59 grams Unrecovered cyclohexane. Cyclohexane which was in the noncondensable gaseous stream by virtue of its vapor pressure may be estimated by assuming the validity of Raoult's law for the liquid product and the perfect gas law. Gram moles of unrecovered cyclohexane = 3.54 (lo+) grammole = 0.30 gram
111. Over-all Material Balance Input
Cyclohexane Nitrogen dioxide
G. 63.7 17.5 81.2
output
Organic liquid Aqueous liquid Gases
Nitrogen dioxide Cvclohexane
G. 64.70 10.00
1.60 3.59 0.30
80.10 (continued)
method of least squares using a computer. The following quadratic equation gives a good fit of the data : per cent of NO' conversion to S C H = -1.509237 (lO-')T f 4.99278 (lO-')Tt 6.96852
T 2 - 2.83658t2
where T = temperature, O C., and t = residence time, minutes. A coefficient of linear residence time was also computed, but was eliminated because it was not significantly different 414
IREC P R O C E S S D E S I G N AND DEVELOPMENT
from zero a t a 57, significance level. A strong interaction between temperature and residence time was indicated from the equation. T h e conversions to nitroparaffins normally pass through a maximum a t higher temperatures, because side and secondary reactions are then accentuated (74). Apparently, the temperature range investigated has not reached such a maximum conversion ; however, charring and imminent reactor plugging limited the operable temperatures. Conversions to nonvolatiles and to cyclohexanone were affected by temperature and residence time and were qualita-
Sample Calculations for Run 63
IV. Atom Balances Input CH 2,4-DAMP NOz
G.-Mole 0.750 0,00562 0.380
C 4.500 0.03934
Total 4.535
H
0
N
... 0.7ib'
9.000
0.08992
... 0 . i80
0.760
9.040
0.380
Mole yo of Product
output
1. Organic layer 2,4-DAMP CH Cyclohexanone Cyclohexanol Cyclohexyl nitrate Nitrocyclohexane Cyclohexyl nitrite Adipic acid 2. Aqueous layer H9O
HKO, I-INOZ Adipic acid 3. Gaseous product NI
Nb CO
Na0
4. 5.
COZ
Unrecovered CN Unreacted NOa
0.00646 0.667 0.99445 0,00406 0.00119 0.0338 0.00856 0.00385
0.04522 4.00200 0.02670 0.02436 0.00714 0,2028 0.05136 0.02310
0.10336 8.0040 0,04450 0.04872 0.01309 0.3718 0.09416 0.0385
0.0438 0.0905 0,0397 0.0120
...
0.0876 0.0905 0.0397
...
Oiio
0,
0.00341 0.0410
...
0.00189
o.oOi89
... ...
o .00ii9 0.0338 0.00856
0.0438 0.2715 0.0794 0,0480
0.6405 0.0397
...
4.4820
9.0984
0.1?64 0.7634
0.0is2 0.3012
0.989
1 .oo
1 .00
0.793
T h e amount of 2,4-dimethylpentane \vas calculated on the basis that the technical grade cyclohexane contained 0.75 of 2,4-DMP. This was determined by gas chromamole 7, tography.
V. Results amount of comp. i in product Conversion to component i = NO2 fed (based on nitrogen - dioxide) (m.ole 7 , of comp. i) - _. (conversion of NCH) (mole 7, of XCH) Sitrocyclohexane conversion = 8.897, Cyclohexanone conversion = 1.17Y0 Cyclohexanol conversion = 1.057, Cyclohexyl nitrate conversion = 0.314% Cyclohexyl nitrite conversion = 2.2570 Adipic acid conversion = 3.65% Nitrocyclohexane yield (based on cyclohexane reacted with correction for material balance) (moles of C H reacted to NCH) - _~__ = 42.57, (moles of C H reacted)
tively similar to those conversions to NCH. T h e nonvolatiles were the second largest product obtained, the highest conversion being S.lY0 for run 44 (at 240' C., 1.0 minute, and R = 2). Cyclohexanone conversions were, in all cases, low, the highest being 1.27& T h e effects of temperature and time on conversions to cyclohexanol, cyclohexyl nitrite, and cyclohexyl nitrate were all similar to each other. T h e conversions all increased with time u p to 0.5 minute. At longer times, the conversions continued to increase gradually for 200' C. runs, while those for 240' C. runs began to decrease sharply. T h e
0.326 3.93 0.181 0.067 0.408
o .0Oi396 ...
0.04248
...
...
0.00682 0.0410
0.04lb 0.00189 0,000698 0,00854
... ...
4.19 8.66 3.80
...
.,.
...
0.628 64.1 0.426 0.388 0.114 3.23 0.819 1.325
...
0.00445 0.00406 0.00357 0.0676 0.01712 0.01540
0.00427 0.02124
...
Total Atoms in prod. Atoms in feed
0.120
...
0.000698 0,00427 0.00354 0.0782
...
7.48
Xitrocyclohexane yield (alternative yield value calculated from products converted from cyclohexane) (moles of N C H in product) (moles of NCH cyclohexanone cyclohexanol cyclohexyl nitrate cyclohexyl nitrite nonvolatiles 1 CO 1 COZ) 6 6 = 51.3%
+
+
+ +
+
+
+
Assuming that nitric and nitrous acids were not stable under the reaction conditions and that they were formed from reaction of NO2 with water outside the reactor, total NO2 conversion or per cent of NO2 converted
-
(SO2 reacted) (NO2 fed)
+
(cyclohexyl nitrate nitrocyclohexane 2Nz NO 2N20) nitrite
+
+
+
+ cyclohexyl
conversions obtained a t 220' C. runs were sometimes higher than the 200' and 240' C . runs, and decreased a t long residence times. These trends indicated that these three compounds underwent secondary or side reactions after their formation. T h e highest conversions to cyclohexanol, cyclohexyl nitrite, and cyclohexyl nitrate were 3.2, 3.0, and 0.31%, respectively, which were all obtained at 220' C., 1.0-minute residence, time, and R = 2 or 4. The conversions to gasems products increased gradually with residence time and temperature a t low residence times and temperatures, and rapidly VOL. 4
NO. 4
OCTOBER 1 9 6 5
415
a t higher levels-see Figure 6 for COZ as an example. The maximum conversions of NO2 to each gaseous product were: nitrogen, 11.7%; KO, 61.1%; carbon monoxide, 8.3%; nitrous oxide, 1.55%; carbon dioxide, 8.20%. Yields of nitrocyclohexane from cyclohexane scattered between 30 and 70% without clear-cut trends in regard to temperature or residence time. T h e fraction of cyclohexane that reacted could not be measured accurately, but a yield of 50% or higher was obtained in a number of runs.
Kinetics. The total nitrogen dioxide conversion is a measure of the nitrogen dioxide which reacted, and was used in the determination of the initial over-all rate equation. For all runs, the total nitrogen dioxide conversion apparently increased linearly with residence time up to about 0.5 minute, as indicated, for example, in Figure 7. Beyond that residence time, the total nitrogen dioxide conversion increased more steeply for 240' C. runs than for runs a t lo.&er temperatures, and then started to level off as nitrogen dioxide was depleted.
Gloss
15
-
t ~ 0 . 5 1.0 O.
G I O S S I ~ I I , R = ~e AI I, R:2 0
Glass
b.e
1 ,R . 4
E,R.2
A A A
9
-
1.5 min.
0
-
"
i5
v,
a W >
z
8 IO W
z a X W
I
9 0
a
5
t z
IGlass Reocfar I Glass Reactor IL
I
0
0
0.5
RESIDENCE
1.0
00
1.5
TIME, min.
I
220
240
TEMPERATURE, OC.
Figure 3. Conversion of NOz to nitrocyclohexane using glass reactors I and II and R = 2
Figure 5. Effect of temperature version of NO2 to nitrocyclohexane
# l5
7
i
0 Class Reactor I EZZA GI055 Reoctor P.
on con-
p"
Q v)
a W >
6
z
0
0
g 5 2-
w '0
2
0
a
X W
# 4
9
$" 3
I
8 8 t z
5
$2 I 0
0
RESIDENCE TIME, min. Figure 4. Conversion of NOz to nitrocyclohexane using glass reactors I and II and R = 4 416
I&EC PROCESS DESIGN A N D DEVELOPMENT
RESIDENCE TIME
, min.
Figure 6. Conversion of NO2 to COz using glass reactors I and I I and R = 2
n
GIOSP
Rcoefor
I
Table II. Summary of Probable Reactions in Vapor-Phase Nitration of Cyclohexane Using Nitrogen Dioxide
-
1. Initiation (Reactions of Cyclohexane)
+ +
C~HIZ
0
P
0.5
NO2
.OH
+
+
C6Hii.
+
HNOz (or *OH
NO)
C6H11* $. HzO
1.5
1.0
RESIDENCE TIME, mi?.
Figure 7. Conversion of NO2 to products using glass reactors I and 1 I and R = 2
T h e total nitrogen dioxide conversion was 95% for R = 2 and was 84% for R = 4, both a t 240’ C. and 1-minute residence time. An initial rate equation for the over-all reaction of nitrogen dioxide with cyclohexane (CH) was determined by postulating the following equation:
T h e initial concentrations of NO2 and C H were calculated assuming ideal gas law. The initial reaction rates were obtained from the product of the initial concentration of NO2 and the initial slopes from graphs of total nitrogen dioxide conversion us. residence time (such as Figure 7). Kinetic rate data for propane and methane were previously found to fit a rate equation with nearly unit orders for both hydrocarbon and nitrogen dioxide (24). A similar approximation for the present data introduced a n average error of 2.7y0 in the determination of rate constant, which might be attributed to experimental error. T h e initial nitration rate equation so calculated is:
2. Nitration, Nitrite Formation, Nirrite Decomposition
3. Reactions of Cyclohexoxy Radical
0 - 0 .
4- NO2
+
.OH
f
0
3
0
.
@NOS
0 4
-%- HOOC(CH2)KOOH
0
e o - 0 0
(1 1
T h e activation energy of 33.1 kcal. per gram mole compares favorably with that for cyclohexane oxidation, 27 kcal. per mole (IO), indicating the similarity in the oxidation and nitration reaction mechanisms. If the orders for cyclohexane and nitrogen dioxide axe each unity, as seems probable, the initiation step would presumably be a reaction between a cyclohexane molecule and a nitrogen dioxide molecule, such as the first equation in Table 11. Effect of Cyclohexane-Nitrogen Dioxide Mole Ratio. Four mole ratios were employed: 1, 2, 4, and 6. All the conversions to cyclohexane derivatives would obviously be zero a t a mole ratio of zero, since no cyclohexane would be present. Nitrocyclohexane conversions increased u p to a mole ratio of 2 to 4, then leveled off. Figure 8 indicates part of the results in glass reactors I and 11. Similar but less complete results were obtained a t other operating conditions. A change
(0
+ 0
gram mole cc. sec.
3
+
(0)
+
0.
4
2
0
G
O
H
t o decomposition and oxidation products
4
0
0
-
0
2
0
-!- C~HILOH
4. Other Reactions of Cyclohexyl Radical
-f-
NO
+ *OH -!-
02
-
C~HILNO3
---*
to decompositionand oxidation products
CsHiiOH
(3-00.
VOL. 4
NO. 4 O C T O B E R 1 9 6 5
417
2- 1,oo
9 cn a W >
A 65
.
0.75
8 68
7 Glass Reacior Glass Reactor
1 1.
2
MOLE RATIO
3
,
4
5
6
1
of mole ratio from 2 to 4 in glass reactor I showed a small but significant increase in nitrocyclohexane conversions. This trend was not observed in glass reactor I1 when it was free from charred material deposit on reactor surface. T h e observed increases in nitrocyclohexane conversion were probably caused by changes in reactor surface rather than in the mole ratio. Conversions to cyclohexanone and cyclohexyl nitrate for mole ratios above 2 decreased and began to level off a t higher mole ratios. For mole ratios from 0 to 2, the conversions to cyclohexanone increased sharply; cyclohexyl nitrate conversions, however, increased gradually to maxima a t about R = 2. Figures 9 and 10 are examples (runs in glass reactors I and 11). Conversions to cyclohexyl nitrite and nonvolatiles were affected by mole ratios in a manner similar to the nitrocyclohexane conversions. Figure 11 indicates the conversions to nonvolatiles from runs in glass reactor 11. Nitrogen and carbon monoxide conversions both increased and then leveled off a t higher mole ratios. The conversions to nitric oxide, nitrous oxide, and carbon dioxide all reached maximum values a t a mole ratio between 2 and 4 and then decreased. T h e effect of mole ratio on total nitrogen dioxide conversions was qualitatively the same as on nitrocyclohexane conversion. From a practical standpoint, the results indicated that a mole ratio between 2 and 4 is favorable for higher conversions to valuable cyclohexane derivatives with a minimum of unreacted cyclohexane recycle. Reaction Mechanism. The three initiation routes to produce cyclohexyl radicals listed above are considered in turn.
ROUTE1. This can be eliminated from consideration based on kinetic rate data for dissociation of nitrogen dioxide (78). Under the most strenuous conditions employed in this investigation, the fraction of dissociation of nitrogen dioxide was 0.005, which was too small to account for the conversions of nitrogen dioxide obtained. ROUTE2. Addition of steam in vapor phase nitration of butane (6) was sho\vn to be advantageous for conversion and yield of nitroparaffins, but no increase in the over-all reaction was reported. Since essentially anhydrous reactants were used (as in this investigation), this initiation route is probably not appreciable. ROUTE3. The initiation step proposed by Bachman and Titov-Le., I&EC PROCESS D E S I G N A N D DEVELOPMENT
Figure
9.
68
,
1
3
2
4
MOLE RATIO
CH/~~,
Figure 8. Effect of mole ratio on conversion of NO:, to nitrocyclohexane
418
i
Oo.
T II 5
I
6
CH/~2
Effect of mole ratio on conversion of
NO2 to cyclohexanone
C6H1z
+ NO2
+
C6Hll
+
"02
(2)
is thermodynamically unfeasible. This type of reaction for methane, ethane, and propane nitration involves a positive free energy change of about 21 to 66 kcal. per mole (25). However. considering the initial rate equation obtained and the fact that only cyclohexane and NO2 \vere present initially, Equation 2 or a variety of Equation 2 (such as the production of .OH and S O directly instead of nitrous acid) appears the likely initiation route. There is, however, no direct experimental evidence to prove or disprove this reaction mechanism. If Equation 2 is the initiation step, it is probably much sloiter than the consequent steps involving more reactive free radicals. Utilizing the initial over-all nitration rate equation, Equation 1, the initial KO2 conversion to cyclohexyl radical may be expressed as a function of mole ratio and temperature. The mathematical manipulation entails integration of the initial rate equation, using series expansion for the integrated rate equation and taking limits as the reactor residence time approaches zero. T h e initial concentration of nitrogen diozide may be approximated by the perfect gas law, and a t a given temperature and pressure is inversely proportional to (R l ) , where R is the mole ratio of cyclohexane to nitrogen dioxide. The initial conversion of KO2 to cyclohexyl free 1 ) ; hence, it increases radicals is proportional to R / ( R steeply at loiv mole ratios and gradually levels off for ratios above 1. The derived effect of mole ratio on P I T 0 2 conversion to cyclohexyl radical is in good agreement with the effect of mole ratio on total nitrogen dioxide conversion obtained experimentally. Based on the results of this study and those of previous workers, a set of probable reactions is proposed in Table I1 to account for all the major products reported. Cyclohexyl radicals presumably react with nitrogen dioxide, hydroxyl radical, nitric oxide, cyclohexyl radical, and any available oxygen. In the absence of rate data for the individual competing reaction, the relative concentrations of the reactive species were examined for possible trends. The main reaction of the cyclohexyl radical is probably with nitrogen dioxide (to give nitrocyclohexane and cyclohexyl nitrite), since nitrogen dioxide is present in largest concentration aside from cyclohexane. When the mole ratios, R, are low, the amount of
+
+
cyclohexyl radical formed will be small, since little cyclohexane is present. Although nitrogen dioxide \\.odd be abundant a t lo\v mole ratios, the total amount of nitrocyclohexane and cyclohexyl nitrite produced would remain small, and a large fraction of nitrogen dioxide \vould be unreacted. LYhen the mole ratios are high, a large fraction of the nitrogen dioxide \vi11 react to form cyclohexyl radicals ; however, the nitrogen dioxide concentration would be low, so that the cyclohexyl radicals would tend to react with reactive species other than nitrogen dioxide. The intricate balance would give a nitrocyclohexane conversimon plateau which was experimentally obtained (see Figure 8). At high mole ratios (higher than those investigated here), most of the nitrogen dioxide that reacted ivith cyclohexane Lvould be consumed primarily in the initiation step and 12 itrocyclohexane and cyclohexyl nitrite conversions ivould be expected to be lower. Cyclohexanone presumably results from the reaction between cyclohexoxy radicals (obtained largely by the decomposition of cyclohexyl nitrite) and hydroxyl radicals. Hydroxyl radical formation (from the initiation step), like that for the cyclohexyl radical, \\-auld increaije as the mole ratio is increased. T h e concentration of the cyclohexoxy radical also lvould increase with mole ratio but not to the extent of cyclohexyl radical concentration. Hence, a t intermediate ranges of mole ratios, as the mole ratio increased, larger amounts of the cyclohexoxy radical \vould react with cyclohexane to form cyclohexanol and cyclohexyl radicals rather than react with the hydroxy radical to form cyclohexanone. Cyclohexanone conversion \vould be expected to increase initially with mole ratio, then drop gradually at higher mole ratios (as indicated in Figure 9). Cyclohexyl nitrate conversion us. mole ratio (see Figure 10) also showed a maximum around a mole ratio of 2. An analogous situation probably occurred ; while the concentration of nitrogen dioxide decreased with higher mole ratios, the cyclohexoxy radical and unreacted cyclohexane increased. From the postulated mechanism, cyclohexanol was formed both from the reaction bet\veen cyclohexoxy radicals and cyclohexane and the reaction between cyclohexyl and hydroxy radicals. All these species would be expected to increase with mole ratio, and the experimental results confirm an upward trend as the mole ratio increased. The nonvolatiles which possibly included bicyclohexyl, cyclohexyl ether, and cyclohexyl peroxide, Lvould be expected to increase with mole ratio increase. Cyclohexanone and
cyclohexanol were probably readily oxidized to dibasic acids (which may further react to cyclohexyl adipates) in the presence of an excessive amount of unreacted NO2 (such as would occur a t low mole ratios). At high mole ratios, these oxidation reactions would be less likely to occur. A conversion plateau for nonvolatiles could then logically occur at the higher mole ratios of the present investigation, as was indeed found. In all cases, the experimental results supported the mechanism proposed in Table 11. Effect of Reactor Surface. T h e use of aluminum as a reactor material of construction for nitration processes has not been reported in detail (75). In this study, the product distributions of corresponding runs using glass and aluminum reactors were similar. Excellent reproducibility of results was obtained even after the aluminum reactor was used and exposed to air for a prolonged period of time. Other materials of construction-e.g., carbon steel, stainless steel, and copperwere previously reported to give rather poor reproducibility (2, 73). Glass reactors were reported to exhibit no aging effect (2, 73, 24). However, glass reactors coated with organics or inorganic salts resulted in significant decreases in nitration product yields as compared to untreated glass reactors (7, 74). In the present study, the effect of reactor surface was not noticeable in glass reactor I in runs 35 to 46 and 51 to 55 until glass reactor 11, a duplicate of glass reactor I, was used (glass reactor I was accidentally broken). The earlier runs a t a mole ratio of 2 in glass reactor I1 (runs 62 and 63) duplicated the earlier comparable runs in glass reactor I (runs 35, 36, 37, and 39) within experimental accuracy. At a mole ratio of 4, runs 64 and 66 in glass reactor I1 had a lower conversion to N C H (by about 2 to 3% on an absolute basis) than run 51 in glass reactor I. Later, however, at a mole ratio of 4, the results of run 69 duplicated those of run 54. T h e slight but apparently significant differences in the results from glass reactors I and I1 are not clearly understood. An examination of the sequences of runs indicated that perhaps the condition of the reactor surface had caused the difference in product distribution. Noticeably the glass reactor surface became light brownish from charred material deposit, especially after runs at high temperatures and long residence times. Apparently, with use, the glass reactor surface tended to favor nitrocyclohexane formation. The small, but apparently insignificant, differences in the
666
f
,eo /
0 '
O
MOLE RATIO C H / ~ ~ , Figure 10. Effect of mole ratio on conversion of NO2 to cyclohexyl nitrite
0.5 min.
0 68
62
t
t
1
I
1
I
I
2
3
4
5
6
MOLE RATIO C H / ~ ~ 2 Figure 1 1 . Effect of mole ratio on conversion of NO2 to nonvolatiles VOL. 4
NO, 4 O C T O B E R 1 9 6 5
419
results of runs in glass reactors I and I11 confirmed the relative unimportance of the surface-to-volume ratio of the reactor on nitration reaction a t the levels of S/V investigated (6, 24). Although there are numerous similarities between vapor-phase nitration and oxidation reactions, surface effects for nitrations are considerably less important than for oxidations (79). Conclusions
T h e experimental results support the following initiation step for nitration :
+
c ~ H NOz ~ + ~ C6Hll.
+ H N O Z(or .OH + NO)
T h e combination of high temperature (240’ C.) and long residence time (1 minute) gave high nitrogen dioxide conversions to nitrocyclohexane but also favored char material formation. The optimum mole ratio of cyclohexane to nitrogen dioxide appears to be between 2 and 4. The condition of glass reactor surface has a small but significant effect on nitration product distribution. Conversions of nitrogen dioxide to nitrocyclohexane u p to 16% and yields based on cyclohexane of 50 to 60% indicate that this process might be commercially feasible for producing nitrocyclohexane and other valuable cyclohexane derivatives. Acknowledgment
T h e authors are grateful to J. T. Allen, J. W. Amy, G. B. Bachman, R. 0. Downs, C . R. Hicks, R. H. Rodine, M. L. Whitehouse, and E. M. Winter for their assistance on various aspects of this project. T h e generous financial and technical support furnished by the Commercial Solvents Corp. is gratefully acknowledged. Nomenclature and Dennitions
CH k
= cyclohexane = rate equation constant = constants
m, n NCH = nitrocyclohexane R = mole ratio of cyclohexane to nitrogen dioxide t = residence time, min. T = temperature, “ C . S/V = reactor surface t o volume ratio (Component i),, = initial concentration of comp. i in gram moles per cc. Nitrocyclohexane conversion = (moles of N C H in product)/ (moles of NO2 fed)
420
I & E C PROCESS D E S I G N A N D DEVELOPMENT
Nitrocyclohexane yield = (moles of NCH in product)/(moles of C H reacted) “Component i” conversion = (moles of component i in product)/(moles of NO2 fed) Literature Cited
(1) Ackermann, H. J., Baltrush, H. A., Berges, H. H., Brookover, D. O., Brown, B. B., J . Agr. Food Chem. 6 , 747 (1958). (2) Albright, L. F., Locke, S. A., MacFarlane, D. R., Glahn, G. L., Ind. Eng. Chem. 52, 221 (1960). (3) Bachman, G. B., Addison, L. M., Hewett, J. V., Kohn, L., Millikan, A., J . Org. Chem. 17, 906-13 (1952). (4) Bachman, G. B., Atwood, M. T., Pollack, M., Ibid., 19, 312-23 (1954). (5) Bachman, G. B., Chupp, J. P., Ibid., 21, 655-6 (1956). (6) Bachman, G. B., Hass, H. B., Addison, L. M., Ibid., 17, 914-27 (1952). (7) Bachman, G. B., Hass, H. B., Hewett, J. V., Ibid., 17, 928 (1952). (8) Bachman, G. B., Standish, N. LV., Ibid., 26, 570 (1961). (9) Bonfield, J. H., U. S.Patents 3,133,123; 3,133,124(1964). (10) Emanuel, N. M., Berezin, I. V., Denisov, E. T., T r . Vses. Soueshch. p o Khim. Pererabotke A’ejt. Ugleaodorodou u Poluprod. dlya Sinteza Volokon i Plast. Mass, Baku 1957, 143-56; C.A. 56, 140971’. (11) Geiseler, G., Angew. Chem. 67, 270-3 (1955) ; C.A. 49,95286. (12) Gray, P., Rathbone, P., LVilliams, A , , J . Chem. Sac. 1961, pp. 2620-9. (13) Hass, H. B., Dorsky, J., Hodge, E. B., Ind. Eng. Chem. 33, 1138-43 (1941). (14) Hibshman, H. J., Pierson, E. H., Hass, H. B., Ibid., 32, 427 (1940). (15) Hodge, E. B., Swallen, L. C., U. S.Patent 2,236,905 (1941). (16) Oestmann, M. J., Kahler, J. E., Lutz, G. A., Kircher, J. F., U. S.At. Energy Comm. BMI-1495 (1961) ; C.A.55, 13334d. (17) Quilico, A., Fusco, R., Chim. Znd. (Milan) 30, 135-9 (1948) ; C.A.43, 1018i. ., Jr., LVise, H., J . Chem. Phys. 24, 493 (1956). (19) Semenov, N. N., “Some Problems of Chemical Kinetics and Reactivity,” Vol. 1, pp. 125-6, Pergamon Press, New York, (1958). (20) Titov, A. I., Tetrahedron 19,557-80 (1963). (21) Titov, A. I., Matveeva, M. K., Dokl. Akad. A’auk, U S S R 83, 101-4 (1952) ; C . A . 47,2715b. (22) Titov, A. I., Matveeva, M. K., J . Gen. Chem. U S S R 23, 249 (1 953). (23) Topchiev, A. V., “Nitration of Hydrocarbons and Other Organic Compounds,” pp. 258-66, Pergamon Press, New York, 1959. (24) Topchiev, A. V., Record Chem. Progr. Kresge-Hooker Sci. Lib. 22, 231-46 (1961). (25) Topchiev, A. V., Kaptsov, N. N., Bull. Acad. Sci. U S S R . Diu. Chem. Sci. 1956,883-8 (Eng. transl.) ;( C.A. 51, 8000~’. (26) LVilson, C. L., LVilson, D. LY., ‘ Comprehensive Analytical Chemistry,” pp. 203-11, Elsevier Co., New York, 1962. (27) Yasui, E., Yamada, T,>Kogyo Kagaku Zasshi 6 0 , 56-8 (1957) ; C.A. 53, 4163f. RECEIVED for review June 22, 1964 ACCEPTED January 18, 1965 Division of Industrial and Engineering Chemistry, 150th Meeting; ACS, Atlantic City, N. J., September 1965.
