Znd. E n g . C h e m . Res. 1988,27,935-942
935
Ethylene Oxidation on a Supported Liquid-Phase Wacker Catalyst I. Sine Shaw, Joshua S. Dranoff, and John B. Butt* Department of Chemical Engineering, Ipatieff Laboratory, Northwestern University, Evanston, Illinois 60201
The oxidation of ethylene t o acetaldehyde with a supported liquid-phase (SLP) Wacker catalyst has been investigated. The active phase of the catalyst, a chlorinated solution of PdC12, CuC12,and C U ( N O ~ )was ~ , supported on three aluminas of differing texture. The reaction was studied in the temperature range 40-80 "C as a function of the catalyst liquid loading. A unimodal size distribution was found desirable in optimizing overall activity, and maximum activity was obtained for a liquid loading of about 0.1 volume liquid catalyst phase/volume pores. Kinetic studies in excess oxygen suggest that the reaction is approximately first order in ethylene. Catalyst deactivation is probably related to a loss of C1- during the reaction cycle. Acetaldehyde is important as an intermediate in the synthesis of a vast array of products, as described by Smidt et al. (1959). In terms of the Wacker technology, given the palladium-based catalysts of the present study, the pertinent chemistry for the aqueous-phase oxidation of ethylene to acetaldehyde is C2H4 + H2O + PdC12 CH3CHO + Pd + 2HC1 (I) in the presence of an oxidizing agent such as CuClZ, Pd + 2CuC12 PdClz + 2CuC1 (11) and upon further reoxidation 2CuC1 + 1/202 + 2HC1- 2CuC1, + H 2 0 (111) 4
-
Overall, then, we have: CzH4 l/zOz
+
-
CH3CH0
(IV) The kinetics of olefin oxidation in reactions such as this have been investigated by Moiseev et al. (1974), in which the rate correlation for the overall reaction is k [PdC142-] r= [C&bI [ Cl-]"[H30'] with 2 In I3. For the specific kinetics of step I, n = 1. The correlation presented subsequently as eq 5 can be derived from this assuming steps I1 and I11 are both fast and irreversible. As indicated, for set concentrations of palladium and chloride in solution, the reaction is first order in ethylene, but the correlation of eq 1obviously does not state much about the role of oxygen in reaction kinetics. Supported liquid-phase catalysts are materials in which a liquid-phase catalyst is dispersed within a porous support. SLP catalysts have been described by a number of workers (Moravec et al., 1939,1941;Robinson, 1965; Acres et al., 1966; Rony, 1968, 1969; Kenny, 1975, 1978), and there continues to be potential interest in their industrial utilization. Villadsen and Livbjerg (1978) have developed rather extensive mathematical models for molten-phase SO2 oxidation catalysts analagous to the types of SLP materials of interest here. Our present objective is the investigation of the efficiency of SLP catalysts for the Wacker oxidation process. Early suggestions of this appear in the patent literature (Robinson, 1965))although it is not clear that these catalysts actually contain a liquid phase under reaction conditions. More detailed recent work has been reported by Fujimoto et al. (1972,1974) on PdClz as an active SLP Wacker catalyst, but a large number of questions remain. These include retention of the liquid phase, optimal (if any) liquid loadings, catalyst composition
* Author t o whom
correspondence should be addressed.
0888-5885/88/2627-0935$01.50/0
Table I. Texture of the Alumina Supports BET area, total pore material m2/g vol, cm3/g Cyanamid Aero 1000 206 0.48 Harshaw A1-0104 109 0.28 Katalco 83-8096 211 0.83
av pore radius, nm 4.7
5.2 8.1
effects, reaction kinetics, and activity maintenance. Rather peculiar kinetics have been reported by various workers in applications of SLP catalysts to Wacker oxidation (Fujimoto et al., 1972,1974; Komiyama and Inoue, 1968,1974, 1975))in which the reaction rate increases at fiied temperature with an increase in water vapor pressure in the reaction mixture but decreases with increasing temperature with fixed water vapor pressure. This has been interpreted as the result of competition between capillary condensation and liquid-phase evaporation (Katz and Pismen, 1979). From an overall kinetic point of view, however, this competition is reflected in terms of apparent negative activation energies for the reactions involved. Komiyama and Inoue (1968) tried to optimize the distribution of liquid within the support, to localize the liquid catalyst phase within the micropore structure. This is probably a step in the right direction; we are also concerned here with the relationship between pore structure and liquid distribution.
Experimental Section The catalyst preparation procedure is substantially the same as that reported by Desai et al. (1983). In this work, however, three alumina supports, differing in texture, were employed. Data regarding the different textures are given in Table I. The various aluminas were separated into 20132-, 32/ 80-, and 80/120-mesh size ranges and then repeatedly washed in redistilled water to remove fines and 0.1 N HC1 to remove Na+. They were then dried at 85 "C for 6 h and then rehydrated. A mixed aqueous solution of CuCl,, PdC12, and Cu(N03):!was used for impregnation. Several compositions of impregnating solution were employed, but a typical composition was 2.38% CuC1, (as the dihydrate), 0.435% PdC12,and 7.85% C U ( N O ~(as ) ~the trihydrate). The catalysts were impregnated to incipient wetness with this solution, dried at 95 "C for 9 h, then chlorinated with 50 cm3Cl,/(g of catalyst-min) at 25 "C for 15 min. Finally, they were reheated to 130 "C for 20 min to remove excess C12 and then transferred to the reactor system. Such catalysts have been referred to by us previously as "KDS" formulations. An experimental reactor system similar to that reported by Desai et al. (1983) was-employed for reaction measurements; the details are reported by Shaw (1985). Ex0 1988 American Chemical Society
936 Ind. Eng. Chem. Res., Vol. 27, No. 6, 1988
-
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.
acyanamid oharshaw Okatalco
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-
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Figure 2. Comparison of various supports: reaction rate at 76 "C vs CzH4/02,saturator temperature of 55 "C, d, = 0.15 mm. 15.0 0.01
20
saturator
30
temperature
A0
("c 1
Figure 1. Comparison of various supports: reaction rate at 70 "C vs saturator temperature, C2H,/Oz = 2.8, d, = 0.15 mm.
periments were normally run a t low conversions (-5%) of ethylene, with feed compositions typically varying (in the kinetic runs) from 0.15 to 0.70 mol % CzH4and the same range for oxygen, in varying proportions, with the remainder made up of He to an inlet total pressure of 1 atm. Typical catalyst loadings were from 0.05 to 0.2 g. All feeds were saturated upstream of the reactor with water vapor in a presaturator operated at temperatures from 40 to 70 "C. Under such conditions, the influence of extraneous mass- and heat-transfer effects was deemed to be negligible according to well-established criteria (Butt and Weekman, 1974). However, some of the present results indicate a catalytic activity related to the pellet size of support materials, reflective of some sort of transport limitation in spite of the criteria cited. This requires further investigation. Analysis of reactant and product compositions was conducted by GC with a 0.5-m X 0.3-cm Carbosieve-S column at 180 "C using TC detection at 210 "C. Complete separation of ethylene, oxygen, and acetaldehyde was attained under conditions.
Results and Discussion, SLP Aspects Effects of Texture. The pore size distribution and pellet size can be important factors in determining the apparent activity of SLP catalysts (Komiyama and Inoue, 1975; Livbjerg et al., 1974). The three materials investigated here are detailed in Table I, and comparative experimental results for ethylene oxidation are given in Figures 1-3 based on steady-state activity levels (Shaw, 1985). Figure 1shows the observed reaction rate vs saturator temperature. Equilibrium liquid loading on the catalyst particle increases as the saturator temperature is raised. Since the SLP catalyst normally operates at low liquid loading, the catalyst solution is under reaction control and the overall activity increases with increasing catalyst solution in the pellet. Figure 2 depicts the relationship of reaction rate to CzH4/02in the feed. There is an apparent indication of negative order with respect to oxygen in Figure 2; this is the result of the fact that the experiments shown do not have a constant liquid loading. Implications of this with respect to the kinetic model are discussed later. Figure 3 shows that the reaction rate increases with decreasing particle size. This may be the result of a diffusional limit; however, more detailed mod-
A
-
0 katalco
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-?E
cyanamid
o harshaw 10.0
u
O.O
'
0:l
0:2
d3
catalyst 2 z e
0.5
m []:
0.7
Figure 3. Comparison of various supports: reaction rate at 76 "C vs pellet size, saturator temperature of 55 "C, C2H4/02= 2.8.
eling (Glick et al., 1987) indicates that this may be an intrinsic property of the system and not a simple result of diffusional limitation. In any event, smaller particle dimension would appear to increase reaction rate. In all cases, the Aero lo00 was most active, and in the discussion following we shall concentrate on the properties of this material as a liquid-phase support. (The reason for the differences in activity among the supports undoubtedly resides in differences in liquid distribution. This is not observable directly but can be indirectly related to textural properties. Table I indicates that the Aero 1000 alumina has the smallest average pore radius but a relatively large surface area. The pore size distribution is also an important factor; Aero lo00 has a more uniform distribution than the other two materials (Shaw, 1985))with most of the pore volume and surface area contained in pores from 3.5 to 5.5 nm. By contrast, the pore volume of the A1-0104, for example, is essentially bimodal, with major contributions from pores of 1.5-3.5 and 5.0-5.5 nm, while the major contribution to surface area is from the 1.5-3.5-nm pores. Generalizations are dangerous, but here it appears that high area, moderate pore size structures with uniform (or at least unimodal) pore size distributions lead to higher effective activities. The absolute magnitudes of these quantities would no doubt depend on the reaction system and contions of operation, but the trend is in accord with the general simulation results reported by Glick et al. (1987).) Some activity comparisons are of interest. The reaction rates obtained in the present study ranged from 5 X lo4 to 3 X mol of CH,CHO/g of catalyst in the range of
Ind. Eng. Chem. Res., Vol. 27, No. 6,1988 937
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3
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300-
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m
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200-
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[MI Figure 4. Catalytic activity dependence upon PdC12concentration: reaction temperature of 75 “C, saturator temperature of 45 “ C , C2H4/02= 3.0, d, = 0.15 mm. PdCI,
loading,
50-80 OC, wile Katz and Pismen (1979) report rates of 1.3 X to 1.0 X mol of CH,CHO/g of catalyst for a supported Wacker catalyst on activated carbon (BET of 1100 m2/g, mean pore diameter of 1.7 nm) at 100-130 OC. The difference in temperature accounts for a change in rate of about a factor of 10 in view of reported activation energies, so under comparable conditions of temperature, the present results are in reasonable agreement. On the other hand, the present rates are about an order of magnitude larger than the rates measured at identical temperatures by Komiyama and Inoue (1975) with a liquid Wacker catalyst supported on porous glass (pore diameter, 5.0 nm). The major difference is probably the prechlorination step used here, resulting in significant increases in catalyst durability and initial activity. It should also be noted that the catalysts investigated here are entirely selective for the formation of acetaldehyde. Catalyst Composition. There is evidence of a change in the oxidation state of Pd in the SLP catalyst upon exposure to chlorine, since the material changes from dark green to yellow upon exposure, indicative of oxidation of Pd(I1) to Pd(1V) (Cotton and Wilkinson, 1962). However, the catalyst returns to the Pd(I1) state under reaction conditions, as evidenced by the reverse color change. A series of experiments, run with the catalyst after chlorine pretreatment and with constant CuC12 and Cu(NO3), concentrations, was conducted to investigate the effect of PdC12concentration. It had been previously reported that such catalysts in the absence of PdC1, have no oxidation activity (Desai et al., 19831, and this was verified. In Figure 4 are shown the results of such an investigation for the ethylene oxidation reaction at a space velocity of 4500 h-l, C2H4/O2 = 2.88 entrance, and catalyst compositions (concentrations refer to the molarity of the impregnating solution in all cases) of CuC1, = 0.2 M, Cu(NO,),= 0.24 M,and PdC12 from 0.013 to 0.03 M. The rate is linear with [PdC12],in accordance with previous results (Desai et al., 1983; Komiyama and Inoue, 1974, 1975), suggesting some accordance of activity with the oxidation state of the palladium. The amount of CuC12/PdC12has been reported to have no effect on the activity as long as the molar ratio is greater than 2:l. This ratio has been interpreted as that required to maintain the reoxidation of Pd(0) to Pd(I1) in a steady manner, as called for by reaction scheme 11. Present experiments confirm this idea. A series of studies was run
with constant PdC1, (0.026 M) and CU(NO,)~ (0.24 M) and varying CuC1, from 0.1 to 0.3 M,such that the molar ratio of CuC12/PdC12was always >2. No effect of [CuCl,] was found. Certainly if one attempts to use catalysts with very high [PdCl,], this relationship may be altered, but the implementation of this may be limited by the small solubility of PdC12 in aqueous solutions (Shaw, 1985). Finally, there is the question of the influence of Cu(N03)2 on activity of the liquid phase. Lloyd and Rowe (1974) have claimed that addition of the nitrate serves as a promoter of activity, at least as far as CO oxidation is concerned. We have not observed that effect in prior work with CO (Desai et al., 1983) nor is it observable here. Experiments run with variable [Cu(NO,),] and constant concentration of other reactants gave no discernible change in overall catalyst activity. Relation of Liquid Content to Reaction Rate. Rony (1968, 1969) has pointed out that the most efficient SLP catalyst results when only thin films or very small liquid “micropools” are present within the porous structure. Higher loadings result in the blockage of pore passages with liquid, and the entire process is dominated by diffusion in the liquid phase. Livbjerg et al. (1974) have a similar view. Thus, there should be some division between liquid-phase loading and the rate of “conventional” pore diffusion in SLP catalysts that should be optimal. Here, we examine the question of what fraction of catalyst pore volume occupied by liquid, 6, is optimal as far as overall catalyst activity is concerned. Liquid loading is intimately connected with the interaction of water vapor with the support. Previous work for CO oxidation on the KDS catalysts showed a dramatic effect of liquid loading on the rate of reaction. Operation of the catalyst without maintenance of some water vapor pressure level in the feed resulted in complete deactivation; presumbly with a dry feed the initial liquid loading is depleted by evaporation (desorption of water), leading to the formation of Pd(0) which cannot be reoxidized in the absence of a liquid phase. Similar results have been reported by Katz and Pismen (1979). A series of experiments was carried out to investigate the equilibration of water on the surface on the activity of the catalyst. Typical conditions were 0.2 g of catalyst operated at a space velocity of 4500 h-l, with variable water vapor pressure in the feed stream. The results are shown in Figures 5-7. Figure 5 is for reaction with constant CzH4/02 = 3.0 and variable reaction temperatures, Figure 6 for the effect of saturation temperature with the same C2H4/02but fixed reaction temperature, and Figure 7 again for the effect of saturation temperature but with lower C,H4/O2 = 2.0. These are central to the interpretation of kinetics that follows. Figure 5 shows a clear optimum of reaction rate with saturation temperature, with higher reaction temperatures requiring higher saturation temperatures at the optimum. This seems a clear indication of compensation between loss of liquid content at the higher reaction temperature vs increase of moisture content in the feed stream. Figures 6 and 7 indicate no real effect of changing C2H4/O2 on the location of the maximum reaction rate, given the same reaction and saturation temperatures. The important point is that in all cases there is a well-defined maximum in the apparent activity of the catalyst that can be related to a particular water content of the feed mixture. In these studies the maximum occurs when water vapor is at about 20-25% saturation at the reaction temperature. It then remains to relate these experimental observations to an interpretation based on the liquid loading parameter,
938 Ind. Eng. Chem. Res., Vol. 27, No. 6, 1988 SATURATOR TEMP. ) C'( 20 35 45 50 55 60
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0.1 0.2 0.3 014 VAPOR PRESSURE OF THE FEED STREAM, P(atm)
Figure 5. Effect of saturator temperature (feed vapor pressure) on reaction rate a t different reaction temperatures: SV = 4486 h-l, C2H4/02 = 3.0,d, = 0.15 mm. SATURATOR TUIP. (*C 2 0 35 45 50 58 60
70
S5
VAPOR PRESSURE OFTHE FEED STREAH,P(otm)
Figure 7. Effect of saturator temperature (feed vapor pressure) on reaction rate; replication of experiments for reaction temperature of 75 "C, CZH4/02 = 2.0. reaction temp. 'C
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Figure 8. Effect of saturation temperature on liquid loading parameter, 6, a t different reaction temperatures.
I-
2 w K
o .a 0:2 0.3 0.4 VAPOR PRESSURE OF THE FEED STREAM, Pbtm) 0:I
Figure 6. Effect of saturator temDerature (feed vamr Dreeaure) on * reaction rate; replication of experiments for ieaction temperature of 75 OC,C2H,/02 3.0.
6. In the present study, 6 has been determined independently by gravimetry (Shaw, 19851, providing a direct measure as long as separate information on the pore vol-
ume of the support is available. The experimental results of saturation temperature vs 6 are plotted on Figures 8-10. The liquid loading within the catalyst in all cases increases as the water content of the feed stream increases, as might be expected. Also it is indicated that, the higher reaction temperature, the higher the saturation temperature must be to maintain the same value of 6. Apparently, changes in concentration of ethylene and oxygen do not affect the value of 6 for the same reaction temperature.
Ind. Eng. Chem. Res., Vol. 27, No. 6, 1988 939 0.9
-
0.8
-
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0.7-
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1
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2
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d Figure 11. Effect of liquid loading, 6, on reaction rate at different reaction temperatures: CZH4/O2 = 3.0. 1
I
0 RUN I e. 2 A . 3
A
20
30
40
50
60
70
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Figure 9. Replication of effect of saturation temperature on 6: reaction temperature of 75 OC, CZH4/O2 = 3.0. 0.9
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0.6
0.7
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-
Figure 12. Liquid loading effects on reaction rate at 75 "C. Loading expressed as 6 (various saturation temperatures); CZHd/Oz = 3.0.
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saturator temperature ( " c 1 Figure 10. Replication of effect of saturation temperature on 6: reaction temperature of 75 OC, CzH4/OZ = 2.0.
The effect of liquid loading on reaction rate is reflective of the saturation curves of Figures 5-7 and is shown in Figures 11-13. It can be seen that the value of 6 which gives the highest catalyst activity is very close to 0.1, and this value is not affected by experimental conditons. The overall results of these experiments are in general accord with the expectation of Rony (1968) and the simulation results of Glick et al. (1987) and agree with the prior results of Desai et al. (1983) and Komiyama and Inoue (1974), 1975). The optimum value of 6 will depend upon the porous character of the support and the nature of the reaction being carried out, so considerable variation might be expected for other circumstances. Pore Size Distribution. Livbjerg et al. (1974) have discussed the possibility of an optimum pore size distri-
0.5
0:s
017
018
0!9
6 Figure 13. Replication of liquid loading effects on reaction rate at 75 "C. Loading as 6; C2H4/02= 2.0.
bution for maximizing catalyst activity. In the SLP catalyst, one can estimate a rough range of useful pore sizes with a simplified analysis based on the Kelvin equation which gives the relation between the vapor pressure of a liquid and the radius of pores in which condensation occurs:
where P is the vapor pressure of the gas phase (corresponding in this case to the saturator temperature); Pothe vapor pressure corresponding to the reaction temperature, T; V the molar volume of water; y the corresponding
940 Ind. Eng. Chem. Res., Vol. 27, No. 6, 1988 Table 11. Pore Diameters Filled with Condensed Aqueous Phase via Halsey Correlation Approximation feed reaction saturation rk, nm t k , nm r,, nm D,, nm temp "C temp, "C 60 20 0.44 0.57 1.01 2.01 30 0.61 0.64 1.24 2.48 40 0.94 0.74 1.68 3.36 50 1.95 0.94 2.89 5.79 55 3.98 1.19 5.17 10.33 70 20 0.35 0.54 0.89 1.77 30 0.46 0.58 1.04 2.08 40 0.63 0.65 1.28 2.56 50 0.98 0.75 1.74 3.47 60 2.03 0.96 2.99 5.98 1.22 5.35 10.70 65 4.13 20 0.29 0.51 0.80 1.61 80 30 0.37 0.55 0.91 1.83 40 0.47 0.60 1.07 2.15 50 0.66 0.67 1.32 2.65 60 1.02 0.77 1.79 3.58 2.11 0.98 3.09 6.19 70 75 4.29 1.25 5.54 11.10
I
h
- ao
-90
I
surface tension; R the gas constant; and r k the Kelvin radius (a consistent set of units would be P and Po in mmHg, V = 18 cm3/mol, y = 72 dyn/cm (25 "C),R = 8.314 X 107 erg/(mobK), T i n K, and r k in cm). One can correct the Kelvin radius to the actual pore radius, rp, via the Halsey relationship for the thickness of the adsorbed layer, tk, via rp = r k + tk (3)
n
[ C 1 k
- 70
1
Figure 14. Test for reaction order of C2H,, reaction temperatures from 60 to 80 "C, C2H,/02 from 1 to 4.5. Constant 6.
and (4) where 0 is the average thickness of a single layer. The value of u has been given as 0.43 nm for N2,and since we are interested here in an approximate value only, this is used in the following analysis. (This assumes that the reaction rate will not affect the validity of the thermodynamics of the Kelvin approach. This has been verified (Shaw, 1985).) The maximum diameter of pores filled with condensed aqueous phase can be estimated as is shown in Table 11. Comparing Table I1 and Figure 5, it is seen that, when pores larger than ca 3 nm are filed with condensed phase, the catalyst activity begins to decrease. Further, when pores of diameter larger than 10 nm are filled, the catalyst is essentially inactive due to pore filling of the more important small pores. Thus, for smaller pores (
