Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 649-653
649
Kinetic Investigations of the Synthesis of Acetic Anhydride by Homogeneous Catalysis Manfred Schrod and Gerhard Luft' Institot fur Chemische Technologie, Technical University of Darmstadt, P 6 100 Darmstadt, West Germany
to give acetic anhydride is reported. A catalyst system, including RhCI3 hydrate, methyl iodide, an organic base, and chromium compounds was used. The synthesis was investigated at CO pressures between 5 and 75 bar and in the temperature range of 165 to 205 'C. The reaction rate is independent of the CO pressure above 15 bar and does not depend on the concentration of methyl acetate. The reaction is first order with respect to rhodium, methyl iodide, and to the base if its concentration is low. With rising concentration of the base the order or reaction decreases. Chromium compounds have less influence on the rate but reduce remarkably the induction period. These results as well as the activation parameters evaluated from the experiments are in good agreement with the assumption that oxidative addition and insertion reaction are the rate-determiningsteps in a catalytic cycle. A kinetic study of the carbonylation of methyl acetate
Introduction Acetic anhydride, an important intermediate component of industrial organic chemistry, is commercially produced by two different methods. The so-called acetic acid process starts from acetic acid to form acetic anhydride via ketene; the second one is a modified acetaldehyde oxidation process. A further method, the pyrolysis of acetone, can no longer compete with others. Because increasing energy costs are involved in the acetic acid process and due to the fact that high temperatures of about 750 "C are necessary to produce ketene, rising prices of the ethylene feedstock, from which acetaldehyde is formed, have increased the production costs of the acetaldehyde oxidation. The increasing costs have encouraged an extensive number of investigations in past years to develop economically more attractive alternative processes. A recent patent assigned to Halcon International (1976) covers a new process for converting t h e raw 'materials, methyl acetate or dimethyl etber, to acetic anhydride. The anhydride is formed by reacting the acetate or the ether with carbon monoxide in the presence of a catalyst. CH3COOCH, CH,0CH3
+ CO + 2CO
-
CH3-C
A
oxidation state from Rh(1) to Rh(II1). In a cycle of reactions, acetic acid is formed, while the initial complex is regenerated. As earlier reported by Hohenschutz (1965), the same results can be obtained using cobalt compounds instead of rhodium, when higher pressures of carbon monoxide are applied. In both cases the rate depends only on the concentration of catalyst and promotor, but not on those of methanol or product. The application of iridium catalysts in the carbonylation of methanol shows other results. Reaction orders are one with respect to iridium, zero for methyl iodide and caFbon monoxide. As the order of methanol is one, Matsumoto et al. (1978) suggest the methanolysis of iridium complex to be the rate-determining step. The role of organic bases OT other heavy metal carbonyls as copromotors is still undefined. This paper considers the influence of all components of the catalyst in the reaction of methyl acetate with carbon monoxide to produce acetic anhydride. Kinetic investigations have been carried out to determine the orders of the reactions, the rate constants, and activation parameters. Based on these data, a reasonable proposal €or the mechanisfn of methyl acetate carbonylation is discussed.
do \O
CHS-C\
/
Experimental Section Materials. RhC13.xH20 ( x = 2.5-3) (Degussa AG), carbon monoxide /99%), methyl iodide (Merck GmbH), Cr(CO)G(Ventron GmbH), trjphenylphosphine, and all other bases used in this wqsk were obtained from commercial sources. Methyl acetate, pyridine, and acetic acid were checked by gas chromatography before use. Chromium compaunds were supplied by Professor Grobe, ZintLInstitute, Technical University of Darmstadt. Apparatus and Procedure (Figure 1). Carbonylations were carried out using a6250-mL autoclave (1)made of Hastelloy C and equipped with magnetic stirrer (8), heating jacket (13) and thermocouple (11). The autoclave was designed to resist temperatures up to 220 OC and pressures up to 250 bar. The reaction rate was measured as follows: the catalyst and the methyl acetate were placed in the autoclave which was heated up to reaction temperature. The carbonylation was then initiated by introducing carbon monoxide, and the pressure was kept constant during the reaction. Samples from the gas and the liquid phase were taken by means of two needle valves
\O
According to the patents of Halcon, pressures of 10 to 100 bar and temperatures of 150 to 200 "C should be applied. The catalyst system may consist of RhC13.xH20, CH31, Cr(CO)6,and an organic compound containing nitrogen or phosphorus. The homogeneous catalyzed process is carried out in the liquid phase. According to reports in ,Chemische Industrie (1980) and Chemical Week (1980), the method was licensed to Eastman Kodak and a plant should start up in 1983. Till now no publication is known on the mechanism of this carbonylation reaction, but one can find analogies to methanol-carbonylation forming acetic acid by the Monsanto process also in the presence of a rhodium catalyst. In the carbonylation of methanol, kinetic measurements, which were carried out by Hjortkjaer and Jensen (1976), show a first-order reaction with respect to rhodium and methyl iodide promotor. This and spectroscopic investigations of the rhodium intermediates, which were carried out by Forster (1979), allow one to assume an oxidative-addition reaction as rate determining. Thus a rhodium complex adds methyl iodide and changes its 0196-4321 I 8 1 l1220-0649$01.25/0
(67). 0
1981 American Chemical Society
650
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981
Table I. Reaction Conditions run no.
T, 'C
1 2 3 4
150 175 175 175 175 175
5 6 7 a
Pyridine. T
var
PCO, CH,COOCH,, bar "ole var 30 30 30 30 30 30
Triphenylphosphine.
00
CH,I, mmol
2206 1640 1640 1640 1640
RhCl,.xH,O, Cr(CO),, mmol mmol
161 161
var 161 161 161 161
var 1640
1.9
var 0.5 0.38 0.38 0.57 0.38
base, mmol
CH,COOH, mmol
total init. vol., mL
2.3 1.1 0.57
3 1a 11.5b 11.5
438
212.5 140
0.45
V U V U
Var 140 140 140 140
var
3.8b 0.95
The amounts are given in mmol.
-
3 501
I
0-
/"--
I
20-
,
> 5
1s
25
5
45
55
65
75 PCOLborl
Figure 2. Dependence of yield after 4 h on CO pressure (see Table I, experiments series 1).
& 7
Figure 1. Apparatus: 1,auklave; 2, CO bomb; 3, pressure reducing valve; 4, valve; 5, manometer; 6, pressure releasing valve; 7, sampling valve; 8, agitator; 9, motor; 10, speed meter; 11, thermocouple; 12, controller; 13, heating; 14, recorder.
Analysis. The samples were quantitatively analyzed by gas chromatography using the method of internal standard (2,3-dimethylbutane; acetone as solvent). The gas chromatograph, Varian 1400, was equipped with a packed column (L= 6 ft, d = in.) of 30% silicon rubber SF 96 on Chromosorb W; carrier gas was nitrogen. After detection of acetone, methyl acetate, and methyl iodide, the temperature was raised from 25 "C to 150 "C. The rates were determined from the slope of the straight line of the concentration vs. time diagram. Results The reaction conditions are listed in Table I. In order to assure the reproducibility of the kinetic data, a number of measurements in standard conditions were tested for several times. The experimental uncertainty was found within a range of *5%. Acetic anhydride was produced with a very high selectivity. Only small amounts (less than 1% by weight) of acetic acid were obtained. Dependence of the Rate on CO Pressure. A t the reported conditions the data obtained are shown in Figure 2. These results showed that CO pressures of more than 15 bar did not influence the rate. At lower pressures the rate decreased linearly. In order to eliminate the influence of CO pressure, the following measurements were carried out at a carbon monoxide pressure of 30 bar. Dependence of the Rate on Catalyst. (a) Concentration of Rhodium and Methyl Iodide. In the further experiments the concentration of RhC13.xH20was varied in the range of 1.36 to 16.3 mM at concentrations of methyl
1.14 m m l
0
1
2
3
Iih
Figure 3. Dependence of acetic anhydride formation on concentration of rhodium,
iodide between 415 and 1662 mM. As might be anticipated, the reaction rate increased with rising concentrations of rhodium (Figure 3) and promotor. The measured rates were plotted vs. the concentrations in a logarithmic scale (Figure 4). From the slope of the straight lines, orders of reactions of 0.92 with respect to rhodium and 0.98 with respect to methyl iodide could be evaluated. (b) Concentration of Base and Cr(CO)+ In order to determine the dependence of the rate of reactions on P(C6H5)3,its concentration was varied between 4.1 and 109 mM. A series of experiments was carried out without Cr(CO)&in a second series, 0.45 mmol of Cr(CO)6 was
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 651 f '9'c
o
15.2 mmol
/
7.6 mmol 3.8 mmol 1.9 mmol m 0,95 mmol e
V-
b
0,57 mmol
3.0-
2,o-
P Figure 4. Rate of acetic anhydride formation vs. concentration of rhodium (see Table I, experiments series 2).
1.0-
Table 11. Dependence of t h e Rate and the Jnduction Period o n t h e Concentration of t h e Base and Cr(CO), -
n b w , mmol kobsd,L2/mo12s,Q kobsd,Lz/molZS,b t h d , min," thd, min,b
0.57 6.5 6.9 69 21
0.95 6.0 6.6 55 20
In the absence of Cr(CO),. Cr( CO),.
1.91 6.2 6.6 27 23
3.82 4.3 4.2 22 8
7.63 3.1 3.7 12 (-4)
15.3 2.2 2.5 11 6
With 0.45 mmol of
added. The results are shown in Figures 5 and 6. Independently of the addition of Cr(CO)6 first-order kinetics were obtained at small concentrations of P(CJ-15),. The order of reaction decreased with rising concentration of the base. From the slope of the rate vs. concentration diagram (Figure 6), at a high level of P(C&5)3 an order of 0.5 was determined. Because of the blockage of the valve it was not possible to make measurements at concentrations of P(C6H& higher than 109 mM. As Table 11shows, the rate of reaction remains practically unchanged if Cr(C0)6is added, but a considerable reduction of the induction period appears. In additional experiments, the concentration of Cr(CO)6 was varied between 1.64 and 10.3 mM. Only in the beginning (45 min) did the rate increase with concentration of chromium. Afterward one curve coincided with the others. The reaction order was calculated to be 0.5. During the induction period active species are formed from RhC13.xHz0 Rh(II1) + CO
+ Hz0
--*
Rh(1) + COZ + 2H+
This reaction step can be revealed by the detection of carbon dioxide in the gas phase. For this purpose a sample was taken after the end of the induction period and analyzed. Using 0.57 mmol of rhodium, a comparable amount of 0.45 mmol of C02 was found. In the absence of RhC13.xHz0 no COz was detectable. (c) Dependence of the Rate on Methyl Acetate Concentration. In nearly all experiments methyl acetate acts as a reactant and solvent. To find out whether the ester participates in the rate-determining step or not, acetic
O
L
Figure 5. Dependence of acetic anhydride formation on concentration of triphenylphosphine (see Table I, experiments series 4).
I
lg io,,
JI
Figure 6. Rate constant of anhydride formation vs. inverse triphenylphosphine concentration.
acid was used as solvent. In these experiments the total volume was kept constant, whereas the ratio between acetic acid and methyl acetate was changed. At given conditions the rate increased only by 15% when the concentration of methyl acetate was increased from 1 to 6 M, which corresponds to a zero-order reaction. Decreasing the ratio of acetic acid to methyl acetate to 1:8 or less, the rate increased. In this range of concentration one can neglect the last influence of the acid by the reaction with the base
652
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981
Table 111. Dependence of Yield, Induction Period, and Rate o n t h e Type of t h e Base
a
ra 7
th$,
base
mol/L h
min
yield: mol %
piperidine pyridine a-picoline 2,6-lutidine 2,4,6-collidine triphenylphosphine
0.95 1.95 2.1 2.25 2.5
30 30 30 15 10
2.4
15
35 69 71 71 80 80
3
3
2'
After 8 h.
2'
1
I
Figure 8. Evaluation of the activation parameters. Arrhenius plot. n+
-20
2.05
2,1
2.15
2.2
2.25
2.3
IdjK-I!
-2.5
-3.G
Figure 7. Dependence of the formation of acetic acid anhydride on the type of the base (temperature 175 "C, CO pressure 30 bar, base 11.45 mmol.
.3.5
-4c
or by the change of solvation of rhodium complexes. (d) Dependence on Type of Base and Metallic Promotor. In order to reveal the role of the base and of the co-promotors, P(C6H5)3was replaced by organic nitrogen-containing compounds. Instead of Cr(CO)6, other chromium compounds like Cr(CO)5C1,[CrC1(NH3)5]Clz, Cr(acacI3, [Cr(py),]C13,CrCl3.6Hz0, [Cr(en)3]C13,Cr((CH3C0)2CBr)3were used. In all experiments the rate was found to be independent of the type of ligands and of the oxidation state of the metal. Even cobalt acetylacetonate gave an identical shape of the curve in the concentration vs. time diagram. Completely different results were obtained by varying the type of the base. Induction period, initial rate, and yield changed remarkably as shown in Table I11 and Figure 7.
The increasing rate might be significantly correlated with basicity, nucleophilic properties, and steric bulk of the nitrogen compounds. Although P(C6H5)3is the weakest base, it gives a high rate and the best yield. This may be attributed to the good ligand and strong nucleophilic properties of phosphorus in this compound. As P(C6H5)3is a rather bulky ligand, these results may also be explained by assuming steric effects as the dominant factor. (e) Dependence on Temperature. The dependence of reaction rate on the temperature is demonstrated in Figures 8 and 9. The preexponential factor (k0)and the activation energy (EA)may be obtained from these figures.
-15
-5,0
-5,s
Figure 9. Evaluation of the activation parameters. Activation enthalpy and entropy (see Table I, experiments series 7).
Activation enthalpy (AH*) and entropy ( A S ) may be calculated from the Eyring diagram (Figure 9). k, = 1.0 X 1014 L2 moF2 h-'
EA = 114 kJ mol-' AS* = -74.0 J mol-' K-' AH* = 10.85 kJ mol-' Low activation enthalpy and negative entropies indicate that oxidative addition will participate in the rate-determining step. Discussion In the synthesis of acetic anhydride by means of a rhodium catalyst two parts are involved: (i) formation of the active catalyst from RhC13.xH,0 while carbon monoxide is oxidized to carbon dioxide and (ii) synthesis of acetic anhydride from methyl acetate, where the rhodium catalyst undergoes a number of steps in a cycle.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 053 R h C $ . x H,O
IRhlCO), 14'
ico.
P%
~
R h I L;iCOI
I
I L; R h L
I L;Rh-COcb,
I/ f
1\
(dl
the dependence of the rate on concentration, steric bulk, and nucleophilic strength of the base. All other steps act very fast, reaction orders being zero with respect to acetic acid anhydride, methyl acetate, and CO. Under reductive elimination, acetyl iodide is generated. It is rather unstable and reacts with acetic ions to acetic anhydride. If L is bound in the complex, CO has to be replaced with L. Otherwise the catalyst will be inactive after three cycles. If both oxidative addition and formation of the acyl complex take part in the rate-determining step and applying the steady-state approximation for the concentration of the intermediate, the rate law can be expressed as d [Rh(c)I dt
CH,COI
+
CH,COO"
% CH,C: ChC,
L' = lo , c O
ot
L = PO,
N -contolnlng Sose
ot
Paj
pyndlne
0 -0 *IO 0
e(r
Figure 10. Reaction mechanism.
The first step, the water-gas shift reaction, was investigated intensively by James and Rempel (1967). They reported that formation of Rh(1) is autocatalytically hastened. They suggested a bridged transition state like Rh(1)....X....Rh(III) where X is CO or C1. In our investigation Cr(II1) was detectable in the beginning of the reaction and an addition of Cr compounds reduced the induction period. Hence chromium may participate by forming the Rh(1) via a chromium and rhodium containing complex similar to that suggested by James. Vallarino (1957) stated that in the water-gas shift reaction complexes like [Rh(CO),XJ (X = C1 or I) were generated to give Rh(P(C6H6),),(CO)Xin the presence of P(C6&)3. P(C&dS
COJ-,H@
Rh(III)-[Rh(CO)2IJ
-~
Rh(P(C&),)z(CO)I Those compounds easily react with methyl iodide under oxidative addition and then rearrange to acetyl complexes. Reductive elimination of acetyl iodide leads to the initial Rh complex. As this work deals only with the kinetics of the acetic anhydride synthesis and stereochemical and spectroscopic investigations are not considered, the initial complex in the cycle may be given as [RhIL,'CO]" (a = 0, 1 , 2 ) where L' may be P(Cd-15),, carbon monoxide, or iodide. Dinuclear complexes may be involved too. I t is also worthy to note that Basolo et al. (1964) or Donek and Wilkinson (1969) showed for manganese and rhodium compounds, that bases can accelerate the formation of acyl complexes. From knowledge of the literature, a reasonable mechanism of the methyl acetate carbonylation explaining our results is proposed (Figure 10). Complex (a), formed from RhC13aH20via [Rh(CO)212]-in the water-gas shift reaction, reacts with methyl iodide to give a Rh(II1)-containing compound (b). The activation energy of 114 kJ/mol, the negative activation entropy of 74 kJ mol-' K-'and reaction orders of 1 with respect to rhodium and methyl iodide show that oxidative addition takes part in the rate-determining step. The following reaction, the migration of methyl group or the less probable insertion of carbon monoxide, is accelerated by P(C6H5), etc. This explains
-
As the complex (c) reacts to acetic acid anhydride in a series of fast steps k_,[Rh(c)] may be ignored. The rate of formation of the complex (c) is thus proportional to the rate of formation of the anhydride and the eq 1 is simplified to d[Rh(c)l klk~[Rh(a)l [CH3Il[LI % (2) dt k-1 + k 2 L I For small concentrations of the base (L), the reaction order with respect to L becomes one, because k2[L] is negligible compared to kland the overall rate constant is hobs = klkz/k.+ which explains the high value of the preexponential factor (ko= 1014L2 moP2 h-l), evaluated from the experiment. With increasing concentrations of L the expression k2[L] can no longer be ignored and the reaction order decreases. It is not clear whether the base stabilizes the catalytic complexes or not. The first spectroscopic investigations gave no hints that the base plays such a role. Further studies in spectroscopy have to be made to get more information on the nature of the intermediates and the role of promotors. Nomenclature a = exponent ki = theoretical rate constants, L2 mol-2 h-' kob = experimental rate constant, L2 mol-2h-' KO = preexponential factor, L2 mol-2 h-' r = rate, mol L-ls-' t = time, s EA = activation energy, k J mol-' AH*= activation enthalpy, kJ mol-' P = pressure, bar AS* = activation entropy, J mol-' K-' T = temperature, K or "C Literature Cited
r-
R. 0. J. Am. Chem. SOC. 1964, 86, 3994. Chem. Week, 1980, 40. Chem. I d . 1980, 32. 275. Donek. I. C.; Wiikinson, G. J. Chem. SOC. A 1969, 2604. Forster, D. Adv. Organomet. Chem. 1979, 77, 257. Halcon International, German Patent, DOS 2610 036, 1976. HJortkjaer, J.; Jensen, 0. R. I d . Eng. Chem. prod. Res. Dev. 1976, 75, 46. Hohenschutz, H.; Hlmmele, W.; von Kutepow, N. Chem. Ing. Tech. 1965, 37, 383. James, B. R.; Rempel, G. L. Chem. Commun. 1967, 158. Matsumoto, T.; Mlzorokl, T.; Ozaki, A. J . CSBI. 1978, 57, 96. Vallarino, L. J . Chem. Soc.1957, 2287. Basolo, F.; Mawby, R. J.; Pearson.
Received for review October 28, 1980 Revised manuscript received May 22,1981 Accepted May 22,1981
