Rhodium Complex Catalyzed Methanol Carbonylation. Effects of Medium and Various Additives Jes Hjortkjaer' and Ole Rye Jensen Technical University of Denmark, lnstituttet for Kemiindustri, 2800 Lyngby, Denmark
The sensitivity of the carbonylation reaction to various solvents (including water) and additives has been examined in relation to observed kinetics. It was found that the best medium has a dielectric constant between 11 and 23, and that the polarity of the solvent may be adjusted to the optimal value by adding water. When 0.2 mol/L of phosphoric acid is added, the maximum rate is increased and is maintained even at high methanol concentrations.
Introduction In the first paper of this series (Hjortkjaer et al., 1976) on the carbonylation of methanol, reaction orders, rate constants, and activation energies for the rhodium complex catalyzed carbonylation of methanol were determined. It was found that the reaction was zeroth order with respect to the reactants methanol and carbon monoxide and first order with respect to rhodium and promotor. These results were related to a mechanism requiring that the oxidative addition of CH31 is the rate-determining step. From the kinetic measurements, activation entropy and enthalpy for the oxidative addition of CH31were calculated and found to be in good agreement with the values determined by Uguagliati et al. (1970). The purpose of this paper is to investigate the influence of more indirect reaction parameters such as water concentration, addition of strong acids and different solvents, ligands, and promotors. Experimental Section The kinetic experiments were performed as previously described in an isothermal batch reactor. Transport restrictions between the liquid and gas phase were eliminated by magnetic stirring. The rate was measured by registering the pressure drop as a function of time. The pressure drop is of course only a correct measure of the rate when no other reactions, producing or consuming gases, are running in parallel with the main reaction. In order to detect eventual by-products, all reaction mixtures were analyzed by gas chromatography after the termination of the experiment. Intermediates present during the reaction were detected by taking samples during the reaction. It must be emphasized that these analyses are purely qualitative since they were performed under conditions where very low boiling by-products partly escape. Aldehydes and ketones in the gas phase were detected by leading the gas through a solution of dinitrophenylhydrazine. These DNPH salts were identified by their melting point. COZ in the gas phase was measured quantitatively by weighing BaC03 formed by precipitation with a solution of Ba(OH)2. Results The same set of standard conditions was used as reference for all experiments: CO pressure (25 "C), 40 atm; CH3OH M; iodine concentration, 3 M; Rh concentration, 4.6 X
concentration, 0.5 M; solvent, acetic acid; temperature, 182.6 OC; sort of Rh added, RhCl3, 4.2HzO; iodine, 57% aqueous solution of HI. In all experiments as few parameters as possible have been varied. Iodine and rhodium concentrations were kept constant so that the results could be compared with the measurements under standard conditions. (a) Solvent. With HI as promotor the rate was measured under standard conditions with various solvents as listed in Table I. When HI is used as promotor the water concentration is increased by 2.6 M so that the total water concentration is 2.6 M water from esterification of acetic acid with methanol water from an eventual formation of dimethyl ether (DME). This means that the water concentration is highest at the beginning of experiment no. 9 if complete esterification is assumed. However, experiments 8,10, and 11are comparable because there is no acid present a t the beginning of the reaction and all rates are initial. Gas chromatography on the reaction mixture from experiment no. 10 (with methyl ethyl ketone as solvent) showed severe formation of by-products. With methyl iodide as promotor, the water concentration is decreased by 2.6 M, which means that all water present originates from esterification and DME formation unless water is added. This was done in experiments 5, 12, and 30. (See Table 11.) These experiments indicate that it should be possible to investigate the influence of water on the reaction rate by using CH31 as promotor and dioxane as solvent. (b) The Influence of the Water Concentration on the Reaction Rate. The measurements were made with dioxane as solvent. The water concentration indicated in Table I11 does not include eventual water from esterification or DME formation. If complete esterification is assumed, the contribution of water from this reaction will be approximately 1.5 mol/L. This assumption is, however, hardly fulfilled, implying that the contribution of water from this reaction is less than 1.5 mol/L. The results are listed in Table 111. From Figure 1 it is seen that above a water concentration of approximately 10 M the reaction rate is constant and even falling above 20-25 M. Gas chromatography indicated that the DME formation was more extensive at these high water concentrations. The rates with HI as promotor are higher and comparable with the rates found when acetic acid was used as solvent.
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Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 4, 1977
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Table I
Table IV
Experiment
Solvent
Rate, mol/L-s X 103
9
Acetic acid Benzene Methyl ethyl ketone Dioxane Dioxane
0.64 0.14 0.30
8
10 11
13
Experi- Promot- H2O concn, Conversion, Rate, (rno1L.s) ment or M % x 103 12
CH31
2.6
11
HI
2.6
18
CH31
10.0
28
HI
12.6
4.0 16.4 26.5 33.8 39.8 45.6 51.3 67.5 80.0 83.6 4.5 17.3 36.7 43.0 53.0 66.5 73.0 77.3 8.4 17.5 20.1 28.7 33.7 44.0 58.7 64.5 73.6 77.3 88.2 11.5 21.2 35.9 59.0 70.5 75.0
0.36 0.40
Table I1 Experiment
Solvent
5 6 7 12 30
Benzene 2.6 mol/L H2O Benzene Dioxane Dioxane + 2.6 mol/L HzO Acetic acid 2.6 mol/L HzO
Rate, mol/L.s
+
X
lo3
0 0 0
0.31 0.62
+
Table I11 Experiment Promotor H2O concn, M Rate, mol/L-s X 103 14 15 16
CH31 CH31 CH31 CH31 CH31 CH31 CH31 CH31 CH31 CH31 CH31 HI HI HI HI
12
17 18 19 23 24 25 22 13 28
52 29
0 0.18 0.27 0.32 0.38 0.46 0.45 0.45 0.44 0.415 0.235 0.40 0.62 0.62 0.46
0
0.5 1.5 2.6 5.0 10.0 15.0 17.5 20.0 23.4 27.0 2.6 12.6 10.0 7.6
0.13 0.23 0.31 0.32 0.32 0.31 0.31 0.245 0.15 0.115 0.15 0.29 0.345 0.345 0.28 0.16 0.09 0.04 0.35 0.44 0.46 0.46 0.45 0.415 0.335 0.265 0.185 0.14 0.08 0.44 0.615 0.615 0.35 0.15 0.05
Rale HI
n CH,I
Ol!
a
Water conc
5
10
15
20
(F ) *
25
Figure 1. Carbonylation rate as a function of water concentration with dioxane as solvent and with HI and CHJ as promotors. The dependence of the reaction rate on the concentration was calculated and the results are listed in Table IV. These results are shown in Figure 2. The maximum rate is observed a t less than 50% conversion and a t even lower concentrations a t higher water concentrations. Based on experiment 12 and on the gas chromatographic analyses, concentration profiles have been estimated as a function of the conversion for the various substances present in the reaction mixture. I t was presumed that no DME formation occurred, that the methyl iodide concentration was stationary (ca. 0.5 M), and that methanol was partly esterificated. These presumptions are based on gas chromatography. It was, however, difficult to distinguish between methyl iodide and methyl acetate. The results are shown in Figure 3. 282
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 4, 1977
50
100
Figure 2. The dependence of the reaction rate on the conversion at various water concentrations.
50
Figure 3. Concentration profiles against conversion.
100
Table VI1
Tahle V
Experiment
Acid, concn, M
Rate, (mol/L.s) X 103
Experiment
9 39 40 47 48
None H3P04,0.34 H3P04,0.67 HzSO4,0.20 HC1,0.30
0.64 0.715 0.715 0.27 0.40
34 36 35 38 37
H3P04concn, M
Rate, (mol/L.s) X
0.104 0.25 0.46 0.69 1.14
lo3
0.88 0.63 0.80 0.70 0.71
Table VI Experiment 39 31 32 37
Reaction medium 119 mL of CH3COOH + 3 mL of H3P04 + 18 mL of CH3OH 97.7 mL of CH3COOH 24.3 mL of HzO 18 mL of CHsOH 66.8 mL of CH30H 73.4 mL of CH3COOH + 2.74 g of CC13COOH 61.8 mL of CH30H + 68.4 mL of CH3COOH 10 mL of H3P04 134.2 mL of CH30H + 6 mL of 120.2 mI, of CH30H 20 mL of H3P04
+ + +
+
41 42
+
Rate, (mol/ L-S) x 103 0.715 0.615 0.44 0.71 0.45
'I1
1-(
Methanol conc
5
10
15
20
Figure 4. Reaction rate as a function of methanol concentration with (upper curve) and without the addition of phosphoric acid.
0.64
( c ) T h e Influence of S t r o n g Acids. With acetic acid as solvent and with HI as promotor the reaction rate was measured after addition of various strong mineral acids (see Table
VI. When phosphoric acid is added, the rate is increased by approximately 12%. However, the rate was not increased by further addition of acid. Addition of HzS04 or HC1 decreased the rate. None of the acids gave rise to the formation of byproducts. (d) The Dependence of t h e Reaction Rate on the Methanol Concentration. Assuming that the oxidative addition of CH31to the rhodium complex is the rate-determining step the dependence of the reaction rate should be zeroth order with respect to methanol. However, at higher methanol concentrations it was found that the rate decreased. Since these observations were not immediately intelligible, further investigations have been made with 0.5 M HI as promotor (10 mL aqueous 57% HI). (See Table VI.) The results from experiments 39, 37, and 42-with the methanol concentrations of 3,10, and 20 M, respectively-are shown in Figure 4 together with the corresponding experiments without the addition of H3P04. It is seen in the figure that the addition of H3P04 causes the reaction to be zeroth order with respect to methanol. The decrease in rate seen at the higher methanol concentrations when no H 3 P 0 4 was added might also be due to a change in dielectric constant. The same change may be achieved by adding water, which was done in experiment 31. Based on the works of Gritchfield et al. (1953), LandoltBernstein (19591, and Kilpi et al. (19671, the amount of water added was estimated to correspond to a methanol concentration of approximately 10 M. The rate found in this experiment is comparable to the rate under standard conditions, from which it may be concluded that the high dielectric constant at the higher methanol concentrations does not cause the rate to decrease. In experiment 42 the reaction suddenly stopped after a conversion corresponding to a decrease in methanol concentration of 2.2 M. The arrest was rapid, the rate falling from a
maximum to 0 in 1min. The reaction started again only after addition of further rhodium corresponding to a concentration M. However, the rate corresponded to this conof 4.6 X M, as might be expected if all centration and not to 9.2 X rhodium was still active. Gas chromatography showed that DME formation was very severe a t this high methanol concentration. (e) Addition of H3P04. Under standard conditions, but with the addition of H3P04in various concentrations the rates listed in Table VI1 were found. I t is seen that an increase of the H3PO4 concentration above ca. 0.2 M does not lead to a higher reaction rate. When dioxane was used as solvent no influence of H3P04 was observed. (f) The Influence of Various Additives. When NH41 H3P04 was used as promotor the rate was the same as with HI. However, KI H3P04 gave a lower rate, probably because KI is very slightly soluble in the reaction mixture. Addition of Rh as HRh(CO)(PPh3)3did not affect the rate, nor did the addition of PPh3 in a concentration corresponding to twice the concentration of Rh. It has been reported (Mizoroki et al. (1966)) that the active complex in the Co catalyzed carbonylation is an acetoxy complex. However, addition of sodium acetate in an amount corresponding to the iodine concentration stops the reaction completely. This may be due to the formation of an inact,iveacetoxy complex, which is fairly stable at higher CO pressures (Morris et al., (1973)) or the explanation may simply be that the acetate acts as a buffer preventing the regeneration of CH31. Under standard conditions but with partial pressures of CO, H2, and Nz of 20, 10, and 10 atm, respectively, no change in rate or product distribution was observed. When all of the CO was consumed no further reaction could be observed. (g) By-products i n t h e Gas Phase. In experiments 8,9, and 11the gas phase was analyzed for aldehydes and ketones by precipitates formed with dinitrophynylhydrazine (DNPH), which were identified by the melting point. Only traces of acetone were found, probably originating from the cleaning of the reactor. COa was quantitatively detected by precipitation with Ba(0H)Z. In all experiments an amount corresponding to 3.6 X 10-3 M was found. This may originate from the reduction
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of RhlI1 to Rh1 according to (Cotton and Wilkinson, 1972)
+ H20 + 3CO
Rh(II1)
+
Rh(I)(C0)2
+ CO2 + 2H
Discussion It is very difficult to draw quantitative conclusions regarding the reaction system here investigated. Since so many factors are involved, it is not possible to construct an experiment which specifically shows the influence of only one. This is illustrated as follows. Gas Phase. CH30H CH31
+ HI F! CH3I + H20
(K1 =
1.6 X 105) (1)
+ CH30H s CH3OCH3 +HI (K2 = 9.1 X 2CH3OH i=? CHsOCH3 + H2O (K3 = 63)
(2) (3)
CH,I(g) CH,OH(g) Hug) H,O(g) CH,COOCH,(g)
.It
!t
t
It
lt
CH,I(l) CH,OH(I)
H,O( 1 ) CH,COOCH,( 1)
Liquid Phase. CO + CH,I
CH3COOH
Rh
CH,COOCH, + HI (4) CH,COOH + HI
b (
CH,COI,
+
+ CH3OH
2CH3OH
*
(5)
CH3COOCH3 H20 (acid catalyzed) (6) CH3OCH3 H20 (acid catalyzed) (7)
+
+ ICH3COOH H+ + CH3COOH3P04 2 H+ + H2P04HI
G
H+
(8)
(9) (10)
Thus, the process is not “homogeneous” in the sense that all reactions are occurring in one phase. Methanol may to some degree react with hydrogen iodide in the gas phase forming methyl iodide, which is then absorbed in the liquid phase where it is oxidatively added to the catalytically active rhodium complex. This means that the rate will depend on the distribution between the liquid and the gas phase. The distribution, on the other hand, depends on pressure, temperature, composition, etc. and on whether equilibrium is reached or not. Besides, some iodine is “bound” in an inactive form in reactions 2 and 8, requiring that the concentration of “active iodine” (CH31 and HI) depends on the position of these equilibria. If these things are taken into consideration the rate expression will be altered a little from the one derived previously, namely CH30H(g) +
K’
T--, CH3I(g) + H2O(g)
A + CHJ(1)
CH,COI
284
k
B
CH,COOCH,
+
CH,COOH
HI
+
HI
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 4, 1977
This means that the rate is dependent on the distribution coefficient K Owhich varies with pressure, temperature, and composition of the solvent (solubility of methyliodide). The experimental results are discussed taking the above mentioned facts into consideration. The solvent has a great influence on the reaction rate. In relatively nonpolar solvents as dioxane and benzene no pressure drop is observed when the promotor is methyl iodide. In dioxane the reaction starts when water is added, but not in benzene. When the promotor is an aqueous solution of HI, a pressure drop is observed in both solvents, the rate in dioxane being higher than in benzene. In this case the water concentration of 2.6 M originates from the HI solution. If the water concentration is further increased using dioxane as a solvent, the rate increases until a steady value is reached at a water concentration of ca. 10 M. At very high water concentrations (ca. 22 M) the rate decreases again. This beneficial influence of water may be due to an increase in polarity of the solvent. The rate is highest in dioxane-water mixtures with a dielectric constant between 11and 23. In the standard experiment the dielectric constant is approximately 17, e.g., within this interval. Furthermore, if the dielectric constant is decreased under standard conditions by using CH31as promotor the rate is also decreased (0.51 X (mol/L.s)), but if water is added until the dielectric constant is ca. 17 (experiment 30) the rate is again the same as under standard conditions (experiment 9). From Figure 3 it is seen that the maximum rate in dioxane is reached at a rather high methanol concentration; i.e. the conversion is less than 50%. The acetic acid formed is partly esterified with methanol, which means that the methanol concentration is further decreased. The methyl acetate must be hydrolyzed again to methanol. Since this reaction is catalyzed by strong acids the influence of H3P04was investigated. Although the maximum rate was not increased by addition of H3P04,the rate remained at the maximum value for further 30% conversion. This is in good agreement with the suggestion that the main effect of H3P04is to catalyze the hydrolysis of methyl acetate. This also explains the slight increase in rate observed at low methanol concentration with acetic acid as solvent (experiment 39).
Conclusion The best medium for the reaction is acetic acid. If a 57% aqueous solution of HI is used as promotor the water content is adequate, but if CH3 I is used an equivalent amount of water must be added in order to reach the same reaction rate. The reaction is zero order with respect to methanol over a broad concentration range when a small amount of H3P04 is added.
When dioxane is used as solvent the polarity of the medium may be adjusted to the optimal value by adding water.
Acknowledgment The financial support given by Air Products and Chemicals Inc. is acknowledged. We want to thank A. Bjorkman for useful discussions. Literature Cited Cotton, F. A., Wilkinson. G., "Advanced inorganic Chemistry", Interscience, New York, N.Y., 1972. Gritchfield, F. E.,Gibson, J. A., Hall, J. L., J. Am. Chem. SOC., 7 5 , 1991 (1953).
Hjortkjaer, J., Jensen, V. W., hd. Eng. Chem., Prod. Res. Dev., 15, 46 (1976). Kilpi, S., Lindel, E., Ann. Acad. Sci. Fenn., Ser. All, 136 (1967). Landolt-Bernstein, "Elektrische Eigenschaflen I", Vol. 6, p 750, 1959. Mizoroki, T., Nakayama, M., Chem. SOC.Jpn., 39, 1477 (1966). Morris, D. E., Tinker, H. B., J. Organomet. Chem., 49,C53 (1973). Uguagliati, P., Palazzi, A., Deganello, G., Belluco, V., lnorg. Chem., 9, 724 (1970).
Received for review February 2 , 1977 Accepted July 11,1977
Supplementary Material Available: Reaction conditions a n d thermodynamic data ( 5 pages). Ordering information is given o n any c u r r e n t masthead page.
Steam Deactivation Kinetics of Zeolitic Cracking Catalysts Arthur W. Chester' and William A. Stover Mobil Research and Development Corporation, Research Deparfment, Paulsboro, New Jersey 08066
The rate constants for steam deactivation of three commercial FCC catalysts containing zeolite Y as the active component were determined by laboratory steam treatment for different time periods in the temperature range 1240-1550 O F (100% steam, 0 psig, fluidized bed). The temperature dependence could be empirically represented as the sum of two independent first-order decays: kd(T ) = AM exp(-EMIRT) AZ exp(-EZIRT), representing the independent deactivation of matrix (e.g., loss of porosity) and zeolite (e.g., loss of crystallinity) components. The relative stabilities of the three catalysts differ significantly in different temperature ranges. Increasing temperature as a means of increasing catalyst steam deactivation severity can give misleading estimates of overall catalyst stability, since the relative contributions of the two deactivation mechanisms change with temperature. The application of the results to deactivation in FCC units with different operating modes is discussed.
+
Introduction The stability of cracking catalysts toward deactivation is one of the most important catalyst properties. While selectivity, the yield of desirable vs. undesirable properties, has a profound effect on cracking economics, catalyst stability influences total product yield and operating costs. Catalysts with inadequate stability require excessive fresh makeup rates to attain necessary activity levels for optimum operation. While little information is available on deactivation mechanisms and kinetics for modern zeolitic cracking catalysts, a number of studies on deactivation of amorphous (silica-alumina) catalysts have appeared (see Literature Cited). The primary mode of deactivation involves steam-induced loss of surface area by growth of the ultimate gel particles, resulting also in loss of porosity. While amorphous catalysts deactivate thermally as well as hydrothermally, thermal deactivation is a significantly slower process (John and Mikovsky, 1961). Further, Dobres et al. (1966) have shown that the porosity and surface area distributions of equilibrium catalysts are more similar to steam-aged than thermally aged catalysts. Most of the above work dealt with changes in the physical structures of the catalysts, without relating these to changes in catalytic activity. John and Mikovsky (1961) calculated equilibrium catalyst activities assuming a first-order activity decay with some success. The inclusion of zeolites as the active component of modern cracking catalysts introduces a new aspect to formulations of decay mechanisms. In general, cracking catalysts contain
active zeolite and a less active matrix in various combinations; while each component has unique deactivation characteristics (Letzsch et al., 1976),the components may also influence each other. Letzsch et al. (1976) have shown that, like amorphous catalysts, the zeolite component is more strongly deactivated hydrothermally than thermally. Magee and Blazek (1976) estimate catalyst stability by varying steam treatment severity to match equilibrium catalyst activities and other properties. In view of recent innovations in the design and operation of regenerators in fluid catalytic cracking (FCC) processes (Rheaume et al., 1976) and a trend toward high temperature operation, a more quantitative understanding of cracking catalyst deactivation mechanisms is desirable for design and evaluation of suitable catalysts. In the present study, isothermal kinetic steam aging at relatively low temperatures, combined with the results of high temperature steam treatment, is used to derive a more quantitative picture of the steam deactivation kinetics of three commercial FCC catalysts.
Experimental Section The three catalysts used in this study, designated A, B, and C, are all commercially manufactured catalysts; catalyst properties are listed in Table I. Steam treatments were conducted with 100%steam at atmospheric pressure (0 psig) in cylindrical, 3 in. i.d. alonized Inconel vessels. The catalyst samples (500-1500 g) are added Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 4 , 1977
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