KINETICS OF DICHROMATE OXIDATION OF o=TOLUIC ACID JOHN C. B I X E L AND DENNIS DRAYER Denver Research Center, Marathon Oil Go., Littleton, Colo. An apparatus and procedure are described for studying the kinetics of the aqueous liquid phase oxidation of o-toluic acid to phthalic acid using sodium dichromate as the oxidizing agent. This reaction was studied a t temperatures of 460°, 500", and 540" F., pressures of 900 to 1050 p.s.i.a., and sodium dichromate to 0-toluic acid feed mole ratios of 1 .O and 1.5. These studies indicate the reaction follows the rate law: r = k [o-toluic acid] [NazCrzO,] [H+]. A possible reaction mechanism has been proposed, from which the above rate equation has been derived.
liquid phase oxidation with sodium dichromate gives high yields (95% or better) of carboxylic acids for a large number of alkyl aromatic hydrocarbons, as shown by Friedman, Fishel, and Shechter (7). Various investigators (2, 3, 6, 7) have reported on the kinetics of chromic acid oxidations in acetic acid solution; however, detailed data are not available on the alkali dichromate oxidation in aqueous media. The aqueous dichromate reaction was thus selected as an important area for kinetic investigation. This paper presents data obtained in a study of the kinetic properties for the oxidation of o-toluic acid (OTA) using sodium dichromate as the oxidizing agent. The oxidation produces phthalic acid (OPA), a compound which has many commercial uses as a n intermediate in the chemical process industries. Oxidation of O T A to OPA requires 1 mole of sodium dichromate. T h e nature of the reaction is:
A
QUEOUS
Preheaters
Stirred
I =f I Reactor Product Cooler Pump BPR= back pressure regulator
v
4- NazCrzO7 COONa (NaOTA)
COONa (NaOPA)
Cr,O,
fH204- NaOH
NaOTA and NaOPA are the sodium salts of OTA and OPA. Sodium hydroxide is formed during the reaction as well as chromic oxide, c1-203a, green solid which is water-insoluble. Dichromate, containing chromium in the plus six valence state, is reduced to chromic oxide with chromium in the plus three state. Experimental
Reactor System. The stirred tank reactor flow unit is schematically illustrated in Figure 1. As a safety precaution, the reactor, preheaters, and product receivers were installed in a high pressure cell constructed of I/*-inch armor plate on three sides. The fourth wall, away from the operator, was of light construction and was designed to blow out in the event of a n explosion. Feed tanks, metering pumps, and all controls were located in a n operating corridor outside the cell. The reactor was a 2-gallon stirred vessel of 316 stainless steel rated a t 1200-p.s.i. maximum operating pressure. I t was equipped with three inlet ports, bottom discharge port, rupture disk port, thermowell, agitator shaft with three impellers, cooling coil, and external electric strip heaters. Aqueous solutions of NaOTA and NazCrz07 were separately metered to the reactor by two positive displacement pumps with variable capacities. Electric heaters were used in some runs to preheat the feed streams. 376
I & E C P R O C E S S D E S I G N A N D DEVELOPMENT
Vent
Drains
Figure 1.
Product
Receivers
Continuous stirred reactor system for o-toluic acid oxidation by dichromate
Coming from the reactor, the product was cooled in a small shell and tube heat exchanger. The product discharge system initially consisted of a diaphragm-operated valve actuated by a pressure controller; however, solid chromic oxide in the reactor effluent clogged this valve and made it inoperable. The method of product recovery finally arrived a t was to collect the product in two 1-gallon high pressure receivers, which were rated a t 1500 p.s.i. In operation, p.roduct was alternately discharged from the two receivers. Nitrogen was used to pressurize the void space above the liquid seal on the receiver dip tubes. As the liquid product accumulated in the receiver, nitrogen was bled out through a back-pressure regulator, thus maintaining a constant reactor system pressure. A liquid-level indicator was used to determine when a receiver was filled and change-over was required. After change-over, the pressure on the full receiver was reduced to atmospheric, and the contents were discharged through the bottom drain valve. Run Procedure. The reactor was first evacuated and the proper amounts of NaOTA solution and NazCrzO, solution were then introduced to fill the reactor with the correct proportions of reactants. The system was pressurized and heated to the desired reaction temperature. After reacting a t temperature for a time equal to the desired contact time, the feed pumps were started a t precalculated rates. Periodic measurements of the product p H determined when steady-state operating conditions had been obtained. At steady-state conditions the product p H remained constant. T h e reactor was operated a t steady state for a t least 1 hour
before samples were taken. Two product samples were saved from each run for analyses. Product Separation and Analyses. Each sample was separated into solid and aqueous phases by vacuum filtration using a Buchner funnel. T h e solid phase was weighed while still wet and analyzed for chromic oxide, sodium dichromate, sodium chromate, OTA, OPA, and water. T h e aqueous phase was weighed and analyzed for sodium dichromate, sodium chromate, OTA, OPA, and water. Gravimetric analysis procedures were used to determine chromic oxide and OTA, while OPA and total Cr(V1) were determined spectrophotometrically.
forms the basis for selection of the postulated mechanism. The experimental measurements are facts, and the mechanism is a model devised to interpret the facts. The proper model is selected by trial and error. Many times work reported in the literature on reactions similar to the one under investigation helps Tvith a judicious choice of the reaction mechanism. Kinetic Model. Of several models tested, the kinetic model giving the best comparison between experimental conversions and calculated conversions utilized the following reaction mechanism:
Sodium hydroxide is formed during the reaction; and as a result, sodium chromate (NazCrOa) is formed a t the expense of sodium dichromate as shown : Na&rzO7
+ 2 NaOH
-t
2 NazCrO4
+ HzO
I t was necessary to determine the relative amounts of dichromate and chromate using the total Cr(V1) analysis. T h e sodium hydroxide titration curve of dichromate gave the fractions of dichromate and chromate us. pH. Feed Materials. Commercial grades of reagents were used without further treatment. T a p water was used throughout except for distilled water used in the analyses. Feed materials used and their purities are :
70Purity
Material OTA N a L k ? 0 72, HzO
98 99
Aqueous feed solutions were mixed to give solutions of 30 weight % NazC1-20~and 25 weight % NaOTA. Solutions of these compositions were used in all of the experimental runs made; however, the proportions of each were varied to vary the reactant mole ratios. T h e NaOTA solution was made by reaction of O T A with a stoichiometric quantity of reagent grade sodium hydroxide. Operating Conditions and Range of Variables Studied
The experimental program was set up to study three temperatures, three reactor holding times, and two reactant feed ratios for a total of 18 experimental runs. The ranges of conditions and operating variables are shown below. Variable T , reaction temperature, F. e, holding time, hr. n, dichromate-OTA feed mole ratio Pressure, p.s.i.a.
460 0.25 1 .O 900
Levels 500 0.50 1.5 1050
540 1.00
The pressure was set to keep the reactor contents in the liquid phase a t the temperature being studied. Runs a t 460' and 500" F. were made a t 900 p.s.i.a., while runs a t 540' F. were made a t 1050 p.s.i.a. All experimental runs were made a t a n ionic strength of approximately 1.5. Kinetic Evaluation
A kinetic study is undertaken for several reasons. From a practical point of view, the chemical engineer is interested in determining a rate equation which describes how operating conditions change the rate of reaction, so he may optimize reactor and process designs. T h e chemist, on the other hand, is mainly interested in determining the reaction mechanism. The mechanism of reaction is defined as the set of simple or elementary reaction steps, such as collisions, taking place between molecules, atoms, or ions that produces the observed over-all reaction. If the reaction mechanism is known, rate equations can be written for the individual reaction steps and combined to give the over-all reaction rate equation. Comparing calculations from the derived rate equation with experimental data then
6+
(HCrzOJ-
+
COONa
acH" -k (HzCrz07)-
(4)
COONa
This mechanism is similar to that of Wiberg and Evans for the chromic acid oxidation of diphenylmethane (6). The rate-controlling step is Reaction 4, and it involves the abstraction of a hydrogen atom from the methyl group of the O T A to form a methylene radical. Reactions similar to 4 have been shown to occur in the autoxidation of p-toluic acid ( 5 ) . The chromium ion products of Reaction 4 are not known, but have been represented here as (H2Cr2Oi) - for simplicity. Further oxidation of the methylene radical, probably through the alcohol and aldehyde to the acid, completes the reaction. I t is not possible to distinguish between these possibilities by the kinetic experiments made in this study. T h e assumption was made that Reactions 1, 2, and 3 were a t equilibrium, and the rate of NaOTA disappearance by Reaction 4 was written showing a first-order dependence on NaOTA, ( C r 2 0 7 -?, ) and hydrogen ion concentration : r = -kq[NaOTA] [HCr207-] = -K&4[NaOTA] [Cri07-2]
x
IH+l (5)
where K3 is the equilibrium constant for 3 and k h is the rate constant for 4. Calculations. The mechanisms were evaluated by calculating rate constants, k , for individual data points. For a rate equation to be considered satisfactory, there should be no trend in k for a given temperature. If no trend was detected, the k values were averaged, and the average k value was used to calculate conversion, x , as a function of time. T h e calculated conversions were then compared to the experimental conversions, and the standard deviation was calculated. The mechanism shown above gave the smallest standard deviation of those tried, and the deviation was within the limits of error as estimated by the technique of Mickley, Sherwood, and Reed ( 4 ) . T h e first step in the calculations was to derive a n expression relating the observed rate constant to the conversion of KaOTA. This derivation was made using the rate Equation 5 and the stoichiometry of the over-all reaction. The over-all reaction is:
+ NazCr2O7
-c
COONa
+ Crz03 4- H20 4- NaOH VOL. 5
NO. 4 O C T O B E R 1 9 6 6
377
+,Na2Cr2074- NaOH
+
NazCrO,
+ iHzO
Over-all,
4-
cxz
$Na2Crz07
-c
COONa
4-
4-
Cr,O,
Na2Cr04 4- $HzO
Results
Since the hydrogen ion concentration was not measured a t the reaction conditions, 1 and 2 were used to eliminate the hydrogen ion concentration from 5 as follows:
K1
=
Kz
=
[HCrO4-lZ [ C ~ Z Q - ~[HzOI I
[ Cr04-2][H+] [HCrOd-]
The concentration of water was considered constant for experimental runs a t the same feed mole ratio, n. Water was formed during the reaction, but this was negligible in comparison to the water initially charged. Finally, a digital computer program was written to perform the calculations.
-
T h e experimental data points are plotted in Figures 2 through 4. The curves shown were calculated using the kinetic model, and the maximum difference between experimental and calculated yield of OPA was 11.3%) which is within the estimated maximum error of 12.8%. Standard deviation of 36 points was 0.0205. The observed rate constants are :
[Cr04-2][ H + ] K11/2[Cr207-2]1/2[H20]1/2
Observed Rate Constant, k, (Gal.)2/(Lb.Mole)2-Hr. 677
F.
Temp., 540 500
or, [H+] =
[Cr04-2]
The Arrhenius constant and activation energy were calculated from the observed rate constants as plotted in Figure 5. These data gave values of 2.94 X 10'5 (gal.)2/(lb. mole)2-
Substituting for [ H + ] in Equation 5:
-k4K3K2K1'I2 [NaOTA] [ C r ~ 0 7 - ~[Hz0]'12 ]~/~
r=
228 53
460
KzK11~2[Crz07-2]1~2[H~0]1/2
Reaction Conditions:
[Cr 04 -2 ]
I
or,
r =
--k [NaOTA] [ Crz07 3/2 [ H z O ] ' ~ ~ [C1-04-~]
I
where k = kJ3KzKl'/2 = observed rate constant. From the stoichiometry of the over-all reaction : [NaOTA] = Co(l
/
n
0.6 Lt-
- x)
Y
If 1
0
Dichromate to 0-Toluic Acid Mol. Ratio
w
-=Calculated
Curves
[Cr04-2] = Cox where Co = concentration of O T A in feed, lb. mole/gal.
HOLDING TIME,B (HR.)
n = dichromate-OTA feed mole ratio
Figure 2.
Effect of holding time on phthalic acid yield
x = conversion of OTA, Ib. mole reacted per lb. mole
fed T h e stoichiometric equations above are not strictly true, since some of the Cr(V1) is present as HCrOa-, HCr207-, etc. However, thermodynamic calculations indicate the equilibria are far toward (CrzO7)+ and (CrOa)-2 a t the reaction conditions. Substituting into Equation 7 :
r=
-kCo3/2(l
- x)(n -
3/z
e I.
l"LReaction Conditions: Tern~erature:500~E
x)3/2[H20]1/2
X
(8) Q
For a continuous stirred tank reactor: r = -
-cox e
w 2. (9)
where 0 = holding time in the reactor, hours. Combining 8 and 9 and solving for k gives the desired relationship between the observed rate constant and the conversion of NaOTA. 378
I&EC PROCESS DESIGN A N D DEVELOPMENT
=1.5
--Calculated
Curves
> '0 0.2 0.4 0.6 0.8 1.0 HOLDING TIME$ (HR.) Figure 3.
Effect of holding time on phthalic acid yield
Discussion
Reaction Conditions: Temperature = 460° F Pressure = 900 psi0
a
-=Calculated
0‘
’
012
I 014 I d6 Oi8 I HOLDING TIME,B (HR.)
I
Figure 4.
Curves
1.b
Effect of holding time on phthalic acid yield
1000 L
One of the biggest problems encountered in oxidation reactions is stopping the oxidation a t the desired product; however, the dichromate oxidation was selective in that no undesirable by-products were detected. Essentially all of the o-toluic acid reacted could be accounted for as phthalic acid in the product. T h e data presented here indicate that the rate of oxidation oi o-toluic acid in aqueous media is proportional to the dichromate (Crz07)-2 concentration. Westheimer, in separate articles (2, 3) on the chromic acid oxidation of benzaldehyde and isopropyl alcohol in acetic acid solution, states that the rate of oxidation is proportional to the concentration of (HCrOd-). However, Wiberg and Evans ( 6 ) , using the same oxidation system as Westheimer, found the oxidation of diphenylmethane, a hydrocarbon, to be proportional to the concentration of total Cr(V1). Furthermore, Wiberg and Evans state that the dichromate ion was found to be about twice as reactive as the acid chromate, (HCrOd-), in their solutions. They conclude that there is a difference in mechanism between the oxidation of alcohols and aldehydes and that of hydrocarbons. T h e data presented in this paper on OTA, a hydrocarbon, appear to support the findings of Wiberg and Evans, although the oxidation systems are slightly different. The mechanism presented here was chosen on the basis that calculations based on this mechanism gave the best correspondence with experimental data, of several mechanisms considered. T h e agreement shown is adequate from a chemical engineering point of view within the range of existing data, and the equations would be useful for reactor design. However, there is no assurance that the chosen mechanism is correct. T h e difficulty is due to the fact that all such postulated mechanisms are merely theories. Better experimental techniques are needed, such as analyzing for reaction intermediates now impossible to isolate. Nomenclature
C, = concentration of OTA in feed, lb. molejgal. k = observed reaction rate constant, (gal.)2/(lb. mole)2-hr. K = reaction equilibrium constant n = dichromate/OTA feed mole ratio r = rate of reaction, lb. mole/hr.-gal. R = gas constant, B.t.u./lb. mole-’ R. T = absolute temperature, ’ R. x = conversion of reactant, lb. mole reacted per lb. mole fed 0 = holding time in reactor, hr. Figure
5.
Relation
of
rate constant temperature
to
reciprocal
hr. and 58,100 B.t.u./lb. mole or 32.3 kcal./gram-mole, respectively, and led to the equation
- 58,100 k
=
2.94 X 10’6e
RT
for the temperature dependence of the observed reaction rate constant,.where R is the gasconstant, 1.987 B.t.u./lb. mole-’ R., and T i s absolute temperature in O R.
literature Cited
(1) Friedman, L., Fishel, D. L., Shechter, H., Division of Petroleum chemistry, 148th Meeting, ACS, Aug. 30-Sept. 4, 1964. ( 2 ) Graham, G. T. E., Westheimer, F. H., J. Am. Chem. SOC. 80, 3030 (1958). (3) Leo, A., Westheimer, F. H., J . Chem. SOC. 74, 4383 (1952). (4) Mickley, H. S., Sherwood, T. K., Reed, C. E., “Applied Mathematics in Chemical Engineering,” 2nd ed., pp. 53-7, McGraw-Hill, New York, 1957. (5) Ravens, D. A. S., Trans. Faraday SOC. 5 5 , 1768 (1959). (6) Wiberg, K. B., Evans, R. S., Tetrahedron 8, 313 (1960). 80,3022 (1958). (7) Wiberg, K. B., Mill, T., J . Am. Chem. SOC. RECEIVED for review October 7, 1965 ACCEPTED June 27, 1966
VOL. 5
NO. 4
OCTOBER 1966
379
