Rhodium Complex Catalyzed Methanol Carbonylation Jes Hjortkjaer. and Vagn Walther Jensen lnstituttet for Kemiindustri, The Technical University of Denmark, Bygning 227, 2800 Lyngby, Denmark
A kinetic study of the rhodium complex catalyzed carbonylation of methanol is reported. The reaction was investigated at CO pressures between 1 and 50 atm and in the temperature range 150-225OC. It was discerned to be zero order with respect to the reactants, and first order with respect to the catalyst and promotor. The activation energy was found to be 14.7 kcal/mol, the values for the activation entropy and enthalpy being in good agreement with the assumption that the oxidative addition is the ratedetermining step in the catalytic cycle.
Introduction The carbonylation of alcohols is in most cases an exoenergetic reaction. For the reaction CH30H
+ CO
-
CH3COOH
+ CH3I
(AGO = -21.242 kcal/mol,
AHo =
- 32.948 kcal/mol)
under standard conditions, which means that the equilibrium is shifted against acetic acid, and that the equilibrium constant decreases with increasing temperature (KO298 = 4 X 1015, KO498 = 6 X lo5). The reaction does not proceed without a catalyst implying that the activation energy is high. The first catalysts used in this process were various acids such as boron trifluoride and phosphoric acid. However, very severe reaction conditions were necessary-in order to get a reasonable rate of reaction-which meant high initial and working costs. Later on various carbonyls of Fe, Co, and Ni with a halogen promotor were used as catalysts for the acetic acid syntheses. Hohenshutz et al. (1966) describe the process used by BASF, where the catalyst is based on cobolt with an iodine promotor. Still reaction conditions are rather drastic (210OC and 500 atm CO pressure). In 1968 Paulik and Roth (1968) reported a new and even better catalyst based on rhodium and iodine. The selectivity is very high (>99%) under reaction conditions (175OC, 1-100 atm CO) and the experimental rate expression (Roth et al., 1968, 1971) suggests reaction orders 1, 1, 0 , and 0 with regard to Rh, I, CH30H, and CO, respectively. No byproducts are found in the liquid phase, not even with 50% hydrogen in the feed. Traces of COn and CH4 are found in the gas phase. The catalytically active complex is formed in situ under reaction conditions, which means that it does not matter in which form rhodium and iodine are added to the reaction mixture. Thus, RhC13,3H20, Rh(1) complexes, Rh(II1) complexes, HI, CH31, and 12 all give the same reaction rates to within 10%. Bromine and chlorine may also be used as promotors, but iodine is the most active. Acetic acid, methanol, methylacetate, water and benzene were used as solvents. A slight increase in reaction rate was observed with increasing polarity of the solvent. Methanol concentration was varied with a factor 3.3 and in this range the rate was found constant within 20%. Reaction rate was independent (within 7%) on CO pressure down to 1 atm. The product was either acetic acid or methylacetate depending on which solvent was used. In order to explain these experimental observations (zero order with respect to reactants and first order with respect to catalyst and promotor) the following mechanism was suggested (Roth et al., 1971) 46
RhL,
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
+
CH3Rh(I)Lm CO
ki
CH3Rh(I)Lm
KZ
a CH3Rh(CO)(I)L, K3
CH3Rh(CO)(I)Lmt.I CH3CO-Rh(I)Lm CH3CO-Rh(I)Lm
K4 + H2O a RhL, + CH3COOH + HI
This mechanism involves an equilibrium reaction between CH30H and HI under formation of CH31, which in the next step is coordinated to RhL, by oxidative addition. RhL, is probably a plane-quadratic Rh(1) complex, possibly Rh(C0)J or (Rh(C0)21)2 which both have been detected experimentally under these conditions (Morris and Tinker, 1973). The next steps in the mechansim are the coordination of CO and the cis-insertion reaction between CH3 and CO under formation of an acyl complex which is hydrolyzed to acetic acid and RhL,. Based on this mechanism and assuming that the oxidative addition of CH3I is rate determining, the following rate expression may be derived 1
r = kl(Ao)Uo)
1
(“20) 1 + (CH3COOH)Io K/’ K’(CH30H) K’(CH3OH) (H2O) 1 1 1 K” = K&3K4(CO) K3K4 K4 where A0 is the total rhodium concentration and I O is the total iodine concentration, The purpose of this paper is to determine reaction orders, rate constants, and activation energies for the rhodium complex catalyzed carbonylation of methanol and to relate these results to the theoretical rate expression and the corresponding mechanism, 1+
+
+-
+-
Experimental Section The kinetic investigations were performed in an isothermal batch reactor by measuring the pressure as a function of time. The pressure drop during any experiment is less than 2% of the total pressure. Limitations to the reaction rate due to transport restrictions between the gas phase and the liquid phase were avoided by magnetic stirring. Above a stirring speed of 750 rpm no increase in reaction rate was observed. The pressure drop converted into molfl. sec is of course only a correct expression for the rate of the
1
r
I
(*I sec
Results 103)
In order to investigate the reproducibility of the kinetic measurements, the rate was measured several times a t three different sets of reaction conditions as shown below.
/
Expt no. Rate (mol/ 1. sec x
0.6;
0.6:
0.::
1
I
0.35 0.35 0.51 2o 0.51 2i
103)
12
8
4
16
It is obvious that the experimental uncertainty is negligible. The following conditions applied to experiments 3, 4, and 26: CO pressure (25OC), 30-50 atm; CHzOH, 3-5 mol/ 1.; Rh, 4.6 X 10-3 mol/l., RhC13.3H20; I , 0.5 mol/l., as HI; solvent, CH3COOH; temperature, 182.6OC. These conditions were considered to be standard, which means that in order to find the dependence of the reaction rate on these variables only one variable in any succeeding experiment is different from these values (apart from changes in solvent due to changes in methanol concentration). Dependence on CO Pressure. Experiments at standard conditions but a t various CO pressures yielded the following results (Figure 1). Expt no. 34 3 4 CO pressure (atm) 0-2 12-24 12-24 Rate (mol/l. sec x lo3) 0-0.62 0.64 0.64
20
Figure 1. Carbonylation rate against CO pressure.
1
o'2
60
r
180
120
( IF sec !LlO3)
b
I
[CH30H](mole/l)
a
4
12
16
20
Figure 2. (a) Carbonylation rate as a function of time for various initial concentrations of methanol: 3.0 mol/l. (I), 8.0 mol/l. (II), 11.4 mol/l. (III), 23.1 mol/l. (IV). (b) Carbonylation rate against methanol concentration.
Table I ~
Max rate (molil. sec Expt no.
Curve no.
3.4
I I1 I11 IV
10
29 5.6
X
lo3)
0.64 0.62 0.51
0.35
Starting concn of CH,OH (mol/l. ) 3.0 8 .O
The CO pressures are the actual pressures at the moment when rate was determined and accordingly not the abovementioned initial pressure. From these experiments it is obvious that the rate is independent of the CO pressure above approximately 2 atm. This has been observed in all experiments performed. Dependence on Methanol Concentration. When the methanol concentration was raised above the standard concentration it became obvious that the reaction rate varied with time. The higher starting concentrations of methanol demanded more time to reach maximum reaction rate as shown in Figure 2a. (See Table I.) The methanol concentration is the total concentration of methanol, ester, and methyl iodide. When the methanol concentration is varied the concentration of water varies simultaneously due to esterification of methanol. In experiments 3 and 4 the water concentration is approximately 5.6 mol/l. (2.6 mol/l. from the aqueous solution of hydrogeniodide and 3.0 mol/l. from the esterification) and in experiment 29 almost the same because the substrate was a mixture of methanol (3.0 mol/ 1.) and ester (8.4 mol/l.). The longer transient course of curve I11 and the lower stationary value of the reaction rate may thus be ascribed to the higher methanol concentration-and to the change in solvent. In experiment 10 (curve 11) the water concentration is essentially higher, namely approximately 2.6 mol/l. (from HI) 8.0 mol/l. (from esterification), while in experiments 5 and 6 (curve IV) the water concentration was only 2.6 mol/ 1. because there was no acid a t the beginning of the reaction. As indicated in the figure the reaction stops abruptly after approximately 2 hr. This phenomenon was observed in all experiments where the reaction was initiated in pure methanol, and furthermore in these experiments the carbonylation was accompanied by the formation of dimethyl ether in considerable amounts and traces of acetaldehyde (identified by melting point and NMR spectrum of the DNPH compound). The dependence of the maximum rate on the total methanol concentration (methanol + ester methyl iodide) in the moment of reaction is shown in Figure 2b. Again it must be emphasized that the concentration of water varies
+
11.4 23.1
carbonylation reaction when no other reaction producing or consuming gasses is occurring a t the same time. The presence of intermediates during the reaction and the product distribution in the liquid phase were investigated by gas-phase chromatography performed on a Perkin-Elmer F11 chromatograph equipped with a packed column: L = 2 m, cl = 2.4 mm, stationary phase: 5% FFAP 1% H:
