Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 212-217
212
Most crucial to the practical application of Pd-W03 for automobile exhaust clean-up is the thermal stabilization of WO, supported on yA1203Although the pulsator aged Pd-W03 catalyst maintains excellent saturated HC activity, the potential loss of WO, under operating conditions is not acceptable for toxicological reasons (Clayton and Clayton, 1981). Current efforts are directed to find a thermally stable form of a tungsten oxide which promotes the saturated HC oxidation activity for Pd. Acknowledgment We thank B. Artz and R. Belitz of the Analytical Sciences Department for the XRF analyses, D. Lewis and J. Perry for the pulsator aging and evaluation, W. Watkins for the tungsten oxide volatility data, and W. B. Williamson for providing assitance during the course of this study. We also thank Dr. M. Shelef for his review of the manuscript and helpful suggestions. Registry No. Pd, 7440-05-3; W03, 1314-35-8; carbon monoxide, 630-08-0; nitric oxide, 10102-43-9; propene, 115-07-1;sulfur dioxide, 7446-09-5; propane, 74-98-6.
Literature Cited Adams, K. M.; Gandhi, H. S. US. Patent Application 284759, 1981. Butler, J. W.; Schuetzie, D.; Coivin, A. D.; Korniski, T. J. Dearborn, MI, Jan 1980, EPA Contract 68-02-2787. Clayton, G. D.; Clayton, F. E. “Patty’s Industrial Hygiene and Toxicology”, 2A. 3rd ed.;Wiiey: New York, 1981; pp 1986-95. Gandhi, H. S.;Adams, K. M. US. Patent Application 284762. 1981a. Gandhi, H. S.; Adams, K. M. U S . Patent Application 284763, 1981b. Gandhi, H. S.; Piken, A. G.; Shelef, M.; Deiosh, R. G. SA€ Trans. 1976, 85, 901; SA€ 1978, 760201. Gandhi, H. S.; Piken, A. G.; Stepien, H. K.; Sheief, M.; Deiosh, R . G.; Heyde, M. E. SA€ 1977, 770196. Gandhi, H. S.; Watkins, W. L.; Stepien, H. K. US. Patent 4 192779, 1980. Gandhi, H. S.; Yao, H. C.; Stepien, H. K. ACS Symp. Ser. 1982, 178, 143. Kummer, J. T. Prog. Energy Combust. Sci. 1980, 6 , 177. Meyer, G.; Oosterom, J. F.; Oeveren, W. J. R e d . Trav. Chim. 1959, 7 8 , 417; Chem. Abstr. 1960, 5 4 , 23f. Piummer, H. K., Jr.; Shinozaki, S.;Adams, K. M.; Gandhi, H. S., submitted to J. Mol. Catal. Schiatter, J. C.; Taylor, K. C. J. Catal. 1977, 4 9 , 42. Shelef, M.; Gandhi, H. S. Ind. Eflg Chem. Prod. Res. Dev. 1972, 1 1 , 393. Stepien, H. K.;Williamson, W. B.;Gandhi. H. S. SA€ 1980, 800843. Tittareili, P.; Iannibeiio, A.; Villa, P. L. J. SolM State Chem. 1981, 3 7 , 95. Williamson, W. B.;Stepien, H. K.;Gandhi, H. S. Environ. Sd.Techno/. 1980, 14, 319. Yao, Y. F. Y. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 293.
.
Received for review October 15, 1982 Accepted November 22, 1982
Oxidation of Ethanol and Acetaldehyde over Alumina-Supported Catalysts Robert W. McCabe’ and Patricia J. Mltchell Physical Chemistry Deparlment, General Motors Research Laboratories, Warren, Michigan 48090
The oxidations of ethanol and acetaldehyde were studied in a laboratory flow reactor over alumina-supported catalysts containing 4 wt % Cu-2 wt % Cr, 0.1 wt % Pt, and 4 wt % Mn, respectively. Most experiments were carried out in feedstreams consisting of 0.1 vol % ethanol or 0.025 vol % acetaldehyde and 1% O2 in nitrogen at a space velocity of 5 2 000 (volume feedHvolume catalyst)-‘ h-’ (STP). All three catalysts were found to produce acetaldehyde, carbon monoxide, and carbon dioxide as the major carbon-contalnlng products of ethanol oxidation. CO, was the principal carbon-containing product in the oxidation of acetaldehyde. The steady-state yield of acetaldehyde obtained in the oxidation of ethanol was found to go through a maximum as the temperature was raised over each catalyst. The data suggest that some of the ethanol is oxidized consecutively to acetaldehyde and then to COP over these catalysts. In addition, over Pt, there is also evidence for the direct oxidation of ethanol
to
co,.
Introduction Ethanol-fueled passenger cars produce high emissions of aldehydes (primarily formaldehyde and acetaldehyde) relative to gasoline-fueled cars (Chui, et al., 1979; Goodrich, 1982). Emissions of unburned ethanol are also significant, particularly during cold-start operation where rich airto-fuel ratios are employed to improve driveability (Chui et al., 1979; Bechtold and Pullman, 1980; Bailey and Edwards, 1980). We have undertaken experiments in the laboratory to assess the potential for applying catalytic converters to the control of emissions of oxygenated hydrocarbons from ethanol-fueled cars. The literature contains little information relating to the oxidation of ethanol and acetaldehyde under conditions similar to those in ethanol-vehicle exhaust. Most studies of ethanol oxidation have been undertaken utilizing catalysts and reaction conditions which favor the production of acetaldehyde (Srihari and Viswanath, 1976; Takezawa et al., 1980; Ganguly et al., 1975; Legendre and Cornet, 1972; Iwasawa et al., 1978). Catalysts with high activity 0196-4321/83/1222-0212$01.50/0
for the partial oxidation of alcohols to aldehydes include silver (Thomas, 1970), copper (Walker, 1964), and mixed oxide catalysts, especially iron-molybdate (Santacesaria and Morbidelli, 1981; Edwards et al., 1977; Pernicone et al., 1969). We are aware of only one study which has been undertaken expressly with the objective of examining catalytic activity and selectivity under conditions which favor total oxidation of ethanol to CO, and water (Ismagilov et al., 1979). Catalysts containing copper and chromium on alumina, and manganese on alumina were used in this study. Both of these catalysts have been found (Klimisch, 1968) to be among the most active base metal formulations for the oxidation of hydrocarbons and CO in the exhaust from gasoline vehicles. Data were also obtained for a platinum on alumina catalyst in order to compare the performance of the base metal catalysts with a noble metal catalyst. Experimental Section Reactor Configuration. The reactor consisted of a 2.5 cm 0.d. quartz tube which was placed in a Lindberg tube 1983 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983 213 N2+02 1 AcetaldehydetN2
N2-an t
t
To Vent
Figure 1. Schematic diagram of alcohol/aldehyde oxidation reactor system.
furnace with a single heating zone 30 cm in length. The catalyst consisted of 15 cm3 of 3 mm diameter spherical pellets; 30 cm3 of 3 mm diameter by 3 mm long cylindrical quartz pellets was loaded on top of the catalyst to establish turbulent flow and to improve heat transfer between the reactor walls and the gas stream. Temperatures were measured using a chromel-alumel thermocouple (contained in a 3.0-mm stainless steel sheath) which was positioned a few millimeters below the top of the catalyst bed. The axial temperature gradient through the catalyst bed was at most 25 K and the temperature was found to increase monotonically from the top of the bed to the bottom of the bed. Experiments were undertaken to determine the blank activity of the reactor by substituting quartz beads for the catalyst. The conversion of ethanol was found to increase gradually with temperature in the blank experiment and reached a maximum of 20% at temperatures near 773 K. Conversions of ethanol obtained with catalyst in the reactor were always much higher at corresponding temperatures than those obtained in the blank experiment. Gas Handling and Product Analysis. Figure 1 is a line sketch of the reactor system showing the facilities for gas introduction and product analysis. Ethanol (US1 Absolute, USP Reagent Quality) was introduced into the reactor by bubbling a stream of nitrogen (-2 cm3/s) through a 125-cm3 container of ethanol which was thermostated near room temperature by placing it into a larger container of water. The concentration of ethanol in the full feed to the reactor was controlled at 1000 f 100 ppm by adjusting the nitrogen flow rate through the saturator. Acetaldehyde was supplied from gas mixtures of acetaldehyde in nitrogen (Scott Specialty Chemicals Co.) and was employed at a concentration of 250 f 50 ppm in the full reactor feed in these experiments. Figure 1shows the facilities for analysis of the compositions of the reactor inlet and outlet streams. The gas chromatograph was a Varian Model 1400 which was equipped with a flame ionization detector. The GC contained a stainless steel column (0.236 cm i.d. X 1.83 m length) which was packed with Poropak Q (80-100 mesh). The column was operated isothermally at 428 K, and the detector base was maintained at 493 K. The GC contained a Carle six-port sampling valve with a %cm3sample loop through which effluent gas from the reactor was continuously pumped at a flow rate of -8 cm3/s. Sample injection was accomplished using nitrogen as the carrier gas at a flow rate of 1 cm3/s. The GC was used to measure concentrations of ethanol,. acetaldehyde, and ethylene in the reactor inlet and outlet streams. Small peaks corresponding
to acetic acid, ethyl ether, and other unidentified species were sometimes observed. The output signal from the GC was processed by a Spectra-Physics peak integrator and the concentrations of ethanol, acetaldehyde, and ethylene were calculated from a comparison of peak areas obtained under steady-state reaction conditions with peak areas of calibration gas mixtures of known concentration. The sensitivity of the GC for ethanol, acetaldehyde, and ethylene was found to be on the order of a few parts per million. A portion of the reactor effluent (- 50 cm3/s) was passed through an analyzer train consisting of a flame ionization hydrocarbon analyzer, and nondispersive infrared CO and COz analyzers. The hydrocarbon analyzer was employed only for qualitative identification of transient bursts of hydrocarbons which were sometimes observed over narrow temperature ranges as the catalyst temperature was increased from one steady-state point to the next. The sensitivities of the CO and C02 analyzers were not sufficient to determine CO and C02 levels to accuracies better than &lo%. This, in turn, limited the accuracy of the mass balances in these experiments to only within &lo% since COz was the major carbon-containing reaction product under most reaction conditions. Close control of the temperature of the gases leaving the reactor was necessary in order to prevent losses of products in the transfer tubing leading to the gas chromatograph. Air-cooling of the metal fittings at the reactor outlet was required for experiments carried out at high temperatures in order to prevent decomposition of acetaldehyde and ethanol. The 3 mm 0.d. transfer tubing from the reactor to the GC sampling valve was heated with electrical tape in an attempt to maintain a temperature near 365 K so as to prevent losses of acetaldehyde and ethanol by condensation (particularly in the presence of water produced in the oxidation reactions). Despite these precautions, small losses of ethanol and acetaldehyde may have occurred. The ethanol concentration in the reactor inlet stream was found to be slightly higher than the concentration in the reactor outlet under low-temperature conditions where no reaction was expected. This may have been due, in part, to adsorption of ethanol on the catalyst and support, but we cannot rule out the possibility that condensation or decomposition of ethanol may have occurred on the surfaces of the transfer tubing. Catalysts. Three catalysts were used in this study: (1) 4 w t 70 Cu-2 wt % Cr, (2) 4 wt % Mn, and (3) 0.1 wt % Pt. All of these were supported on 3 mm diameter spherical pellets of alumina (predominantly &phase) obtained from Grace Chemical Co. (surface area = 110 m2/g, density E 0.5 g/cm3). The catalysts were prepared by the incipient wetness impregnation technique from aqueous solution using C U ( N O ~ Cr(N03)3, )~, Mn(NO&, and HzPtCls as the sources of the metals. The catalysts were dried in air at 373 K for 12 h and then heated in air at 675 K for 4 h prior to use in the reactor. Results and Discussion Product Yields from Ethanol Oxidation. Figures 2 through 4 show the yields of the major carbon-containing products obtained as a function of the temperature measured just below the top of the catalyst bed during the steady-state oxidation of ethanol over the Cu-Cr, Pt, and Mn catalysts, respectively. All data were obtained for 0.1 f 0.01 vol % ethanol and 1 vol % O2 in nitrogen at a space velocity of 52 000 (volume feed)(volume catalyst)-' (h)-' (STP). Yields are expressed as the molar conversion of ethanol to each carbon-containing product normalized to the molar concentration of ethanol in the feed. Each figure
214 .-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983 1 .o
m /
rI
100 A1203
0 EL
c
-,,;""-
4wtahCu / 2 w t % C r o n
5 2 . 0 0 0 h Space Velocity I 0 1 vol% Ethanol 1 vol% Oxygen i I
5 2 , 0 0 0 h - l Space Velocity 0 . 1 vol% Ethanol 1 vol% Oxygen
Acetaldehyde
250
350
450
550
650
0 250
750
Temperature ( K ) Figure 2. Product yields and ethanol conversion as a function of catalyst bed temperature for ethanol oxidation over a 4 wt 70Cu-2 wt % Cr on alumina catalyst. -0.1
I
o
0 80 61
Ethanol
[
Conversion
- - - =-
p'
1 Wt% Pt on
I
_-- -
I 1 .o
I
350
450
550
650
750
Temperature ( K ) Figure 5. Ethanol conversions as a function of catalyst bed temperature over 0.1 wt % Pt, 4 wt % Cu/2 wt % Cr, and 4 wt % Mn, all on alumina. V . 7
1 '
5 2 . 0 0 0 h . l Space Velocity 0 1 vol% Ethanol 1 vol% Oxygen
52,000 h Space Velocity 0 1 vol% Ethanol
0 4 1
1 vol% Oxygen
0 21
Acetaldehyde
/
-650 - 750 , . A
0 250
350
450
550
IO
Temperature ( K ) Figure 3. Product yields and ethanol conversion as a function of catalyst bed temperature for ethanol oxidation over a 0.1 wt % Pt on alumina catalyst. 10 4 w t % M n on A 1 2 0 3
0 5 21, 0vol% 0 0 h.l Ethanol Space Velocity 1 vol% Oxygen
I
Ethanol Conversion
250
350
450
I
y"
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/O
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650
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Temperature ( K ) Figure 4. Product yields and ethanol conversion as a function of catalyst bed temperature for ethanol oxidation over a 4 w t % Mn on alumina catalyst.
also shows the fractional conversion of ethanol as a function of temperature. Acetaldehyde, carbon monoxide, and carbon dioxide were the major carbon-containingproducts of ethanol oxidation under steady-statereaction conditions with these catalysts. Mass balances were made by comparing the carbon associated with product CO, COz, and acetaldehyde. with the carbon associated with reacted ethanol. The carbon balances generally accounted for all of the carbon-containing species to within the accuracy of the analytical measurements, indicating that ethylene, acetic acid, ethyl ether, and other unidentified species were present in only small amounts. This is evident in Figures 2-4, where the summation of the yields for CO, COz,and CH,CHO should equal the conversion of ethanol for mass balance closure assuming no other major carbon-containing species. Mass balance closure was generally obtained to within *lo% except at temperatures below -450 K for
250
350
450
550
650
750
Temperature (K) Figure 6. Acetaldehyde yields as a function of catalyst bed temperature over 0.1 wt % Pt, 4 wt % Cu/2 wt % Cr, and 4 wt 70Mn, all on alumina.
the runs made with the Cu-Cr and Mn catalysts and below -390 K for the Pt catalyst. The failure to obtain good mass balance under the low-temperature conditions for these catalysts may have been due to the following causes: (1)losses of ethanol or acetaldehyde by condensation or decomposition on metal surfaces as noted in the previous section, and (2) adsorption of either ethanol or its partial oxidation products on the catalyst. Large adsorption capacities were observed for both ethanol and acetaldehyde on these catalysts and the amounts of these species which were adsorbed were found to be sensitive to temperature. A t low temperatures, adsorption equilibrium was approached slowly (1to 2 h) upon exposure to the feedstream containing ethanol or acetaldehyde. Consequently, steady-state conversions and product yields may not have been obtained at low temperatures with these catalysts. Comparison of Figures 2-4 indicates that similar conversion behavior was observed over all three catalysts as a function of temperature. As temperature was increased, the conversion of ethanol was found to increase sharply and was accompanied by an increase in the yield of acetaldehyde. Acetaldehyde production was found to go through a maximum with increasing temperature over all three catalysts, and the COz production was found to increase sharply near the temperature of the maximum in acetaldehyde yield. Carbon monoxide production was found to increase gradually with increasing temperature. The main differences in ethanol oxidation over the three catalysts were in the temperatures required to effect specific conversions. Differences in the temperature dependences of the catalysts are shown in Figure 5, where ethanol conversion is plotted for all three catalysts as a function of temperature, and in Figure 6, where the acetaldehyde yields are plotted for all three catalysts as a
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983 215
'
O
O
0 . 1 Wt% P l
Acetaldehyde Conversion 1%) 50
/
-
7
!
Ethanol Conversion 5o (%I
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-
250
350
450
550
650
)
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5% 0 2 1%0 2 0 . 5 %0 2
52,000 h . l Space Velocity 0 . 1 vol% Ethanol
750
Temperature ( K ) Figure 7. Acetaldehyde conversions as a function of catalyst bed temperature over 0.1 wt % Pt, 4 wt % Cu/2 wt % Cr, and 4 wt % Mn, all on alumina.
Temperature (K) Figure 8. Ethanol conversions as a function of catalyst bed temperature over the 4 wt % Cu/2 wt 70 Cr catalyst for oxygen feed concentrations of 0.5%, LO%, and 5 % . ~
function of temperature. The effect of temperature is most pronounced in comparing the Pt catalyst with the base metal catalysts. In Figure 5, the temperature of 50% conversion of ethanol is 120-150 K lower over Pt than over the base metals. Similarly, Figure 6 shows the temperature of maximum acetaldehyde production to be 100-150 K lower over Pt than over the base metals. The differences between temperatures required for 50% conversion of ethanol over the Pt and Cu-containing catalysts may be associated with different stabilities of surface ethoxide species formed as intermediates during ethanol oxidation on Pt and Cu. Sexton (1979) found evidence from electron energy loss spectra of ethanol on oxygen pre-covered Cu(100) for an ethoxide intermediate which was stable on the surface to temperatures greater than 370 K. As shown in Figure 5, there is little conversion of ethanol over the Cu-Cr catalyst at temperatures below 370 K. Alkoxide species are less stable on oxygen-coveredPt than Cu, however, and Sexton et al. (1982) have suggested that on oxygen pre-covered Pt(ll1) the surface ethoxide decomposes near 200 K. Product Yields from Acetaldehyde Oxidation. The oxidation of acetaldehyde was examined in the absence of ethanol by reacting feedstreams containing 200-250 ppm acetaldehyde and 1% oxygen in nitrogen at space velocities near 52 000 (volume feed)(volume catalyst)-' (h)-l (STP) over each of the catalysts. Carbon dioxide was the major reaction product over these catalysts. Carbon monoxide was generated at concentrations similar to, or slightly less than, those observed in ethanol oxidation. Acetaldehyde conversions are shown in Figure 7 for all three catalysts as a function of temperature. As for the case of ethanol oxidation, the acetaldehydeconversions shown at the lower temperatures are probably largely apparent conversions due to losses in the transfer lines and/or the failure to obtain equilibrium adsorption of acetaldehyde on the catalyst prior to measuring the conversion. As temperature was increased over each catalyst, the acetaldehyde conversion increased sharply to levels in excess of 90%. The only major differences observed in acetaldehyde conversions over these catalysts were in the temperatures required to effect specific conversions. Pt was found to promote the reaction at lower temperatures than Cu-Cr and Mn, in keeping with the observations for ethanol oxidation. Variation of Oxygen Concentration. The effect of oxygen concentration on ethanol oxidation was examined for the Cu-Cr catalyst under excess-oxygen conditions using feedstreams containing 0.1% ethanol and 0.5 % , 1% , and 5% O2 (0.3% O2 is required for the stoichiometric conversion of 0.1% ethanol to C02and H20). The ethanol
I
Yields
0
I
Ethanol
I
Conversion
I I I I I I
Ethanol Conversion 1%)
i
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i
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I I co
I
I -
co
+Oxygen. Rich-k-OOxygen - Lean+ 0 . 1 % Ethanol, 0.2% 0 2 0.1% Ethanol, 1 % 0 2
Figure 9. Ethanol conversions and product yields in an oxygenconcentration step-change experiment carried out at 595 K catalyst bed temperature.
conversions obtained in these experiments are shown in Figure 8. The only apparent effect of oxygen concentration was to shift the curves for ethanol conversion and acetaldehyde and C02 production to slightly lower temperatures with increasing oxygen concentration. A similar downward shift (-20° maximum) was observed in the peak temperatures of the acetaldehyde yield curves as oxygen concentration was increased. An experiment was also carried out with the Cu-Cr catalyst in a feedstream containing 0.2 f 0.05% oxygen, which is slightly less than the stoichiometric oxygen concentration of 0.3% required for complete oxidation of 0.1% ethanol to COz and H20. After establishing steady-state reaction conditions at 595 K in a feedstream containing 1% O2 and 0.1% ethanol, the oxygen concentration was decreased to 0.2 f 0.05% in stepwise fashion. Conversions were monitored for times in excess of 1 h after changing the oxygen concentration and reestablishing the desired temperature in the reactor. Figure 9 shows the data obtained from this experiment. The switch from oxygen-rich to oxygen-lean conditions resulted in a rapid 5% decrease in ethanol conversion and a near doubling of the acetaldehyde yield. Carbon monoxide production increased by about 45% and carbon dioxide production decreased by 13%. The conversions shown in Figure 9 for 0.2 f 0.05% O2were constant for the period of -2 h over which the products were analyzed. Under oxygen-lean conditions the contributions of reactions such as dehydrogenation and dehydration to ethanol conversion may increase relative to oxidation (Krylov and Fokina, 1968). We estimate that an oxygen concentration of 0.25% would have been necessary to oxidize ethanol to the products observed in the oxygen-leanfeed.
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2 , 1983 XA = 1 - C A / C A , (Curves)
5
10,
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I
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1 O r
I Oxidation D'
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F r a c t i o n a l C o n v e r s i o n of E t h a n o l /Data)
This is within the accuracy of the lean oxygen concentration of 0.2 f 0.05% employed in the experiment. No conclusions can be drawn, therefore, regarding the possible contribution of reactions other than oxidation to the products observed under oxygen-lean conditions, nor is it possible to determine whether or not reduction of the catalyst may have occurred over the period of reaction under oxygen-lean conditions. Catalytic Alcohol Oxidation Mechanisms. Broad overlap was observed in the temperatures over which ethanol was oxidized to acetaldehyde (Figure 6) and acetaldehyde was oxidized to CO, (Figure 7 ) for each of the catalysts of this study. It is likely, therefore, that some of the ethanol in the feed is oxidized in sequential fashion to acetaldehyde and then to CO,. Reaction 1 illustrates the series mechanism showing only the carbon-containing species C,H,OH(g)
2CH,CHO(g) -% 2C02(g)
(1)
We cannot rule out the possibility that the direct oxidation of ethanol to COP (i.e., without the formation of acetaldehyde in the gas phase) may also occur to some extent over these catalysts according to the reaction C2H50H(g)-% 2C02(g)
0
(2)
Ethanol oxidation experiments were carried out over a range of space velocities with the Cu-Cr catalyst to determine the relative importance of reactions l and 2. The space velocity was varied by changing the amount of catalyst in experiments carried out at constant flow rate. The results of the experiments at a catalyst temperature of 518 K and space velocities of 26 OOO, 52 OOO, 104OOO, and 208000 h-' are shown in Figure 10, where the yield of acetaldehyde obtained a t each space velocity is plotted against the corresponding ethanol conversion. A maximum in the acetaldehyde yield was found at a space velocity of 52 000 h-l. We compare our experimental results in Figure 10 to curves showing the theoretical yields of the intermediate B in the first-order series reaction 4 kz A-B-C (3) carried out in a plug-flow reactor. Three curves are shown corresponding to rate constant ratios k , to k , of 0.1, 1.4,
G
Acetaldehyde Oxidation
A,. '460
I
1
1
500
600
700
4 Wt% C"
2 Wf% C I
10-
E
Figure 10. Yield of acetaldehyde as a function of ethanol conversion for a 4 wt % Cu-2 wt % Cr on A1203catalyst in a feed containing 0.1 vol % ethanol and 1%0, (nitrogen balance). Data were obtained at 518 K. Data points (in order of increasing ethanol conversion) were obtained at space velocities of 26 000, 52 000, 104000, 208 000 h-l. The curves were calculated for a first-order series reaction in a plug flow reactor and show yield of the intermediate species B as a function of the conversion of reactant A for various ratios of rate constants k , and k , .
700
IOxidatiom/"
Acetaldehyde 0' Oxidation
0 5l
0
l
400
,
Ethanol Oxidation
J
500
600
700
Temperature (K) Figure 11. Comparison of C02 yields as a function of catalyst bed temperature for acetaldehyde oxidation (dashed lines) and ethanol oxidation (solid lines). For acetaldehyde oxidation the feed contained 0.025 vol % acetaldehyde and 1 vol % oxygen in nitrogen. For ethanol oxidation the feed contained 0.1 vol % ethanol and 1vol O/o oxygen in nitrogen. The space velocity was 52 000 h-l in all experiments.
and 10. The experimental yields of acetaldehyde correspond closely to the theoretical yields of the intermediate species for the general first-order series reaction with k z / k , 1.4. Thus, the space velocity experiments indicate that ethanol oxidation to CO, over the Cu-Cr catalyst follows the series reaction mechanism (reaction 1). The effect of space velocity was not examined in experiments with the Pt and Mn catalysts. However, results from other experiments are presented in Figure 11which suggest that the mechanism of ethanol oxidation over the Pt catalyst is different from that over the Cu-Cr catalyst. Figure 11 shows the yields of C02 obtained from the oxidation of ethanol and the yields of CO, obtained from the oxidation of acetaldehyde plotted as a function of temperature over the three catalysts. Ethanol oxidation to COz is initiated at lower temperatures over Pt than acetaldehyde oxidation to CO,. A reaction mechanism based on the series reaction 1 alone cannot account for this observation. In addition to reaction 1, another reaction pathway (reaction 2) is required for ethanol oxidation to CO, over Pt which does not involve the formation of stable gaseous acetaldehyde. Reaction 2 could involve the formation of gaseous intermediate species other than acetaldehyde or it could involve reaction via adsorbed acetaldehyde or some other adsorbed intermediate species. An alternative mechanism which can be ruled out is the dual-functional mechanism involving dehydration of the ethanol to ethylene by the alumina support and subsequent oxidation of ethylene to C02by the active metal or metal oxide component. Conversion of ethanol to ethylene over the bare alumina support was less than 5% at bed temperatures below 550 K and cannot account for the COP yields over our catalysts. Moreover, in the experiments carried out over the 4 w t % Cu-2 wt % Cr/A1203catalyst at various space velocities, the selectivity for ethylene was found to be less than 1% in all cases and nearly independent of space velocity. A recent study of ethanol oxidation over alumina-supported Cu and Pt catalysts by Ismagilov et al. (1979) provides supporting evidence for the existence of the series
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Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 217-225
reaction (reaction 1) over Cu and for both the series reaction (reaction 1) and the direct oxidation (reaction 2) over Pt. Ismagilov et al. examined the selectivity for ethanol oxidation over Cu and Pt catalysts by varying the space velocity through their reactor. They found that, over Cu, acetaldehyde was produced almost exclusively at high space velocities (Le., low conversions of ethanol) whereas C02was produced almost exclusively at low space velocities (Le., high conversions of ethanol). With the Pt catalyst, Ismagilov et al. found the selectivity for acetaldehyde to be nearly constant (over ethanol conversions ranging from 0.3 to 0.6) while selectivity for C02 increased. They interpreted the selectivity data as indicating that ethanol oxidation proceeds almost exclusively by the series reaction mechanism (reaction 1) over Cu and by a combined series-direct reaction mechanism (reactions 1 and 2) over
217
Acknowledgment The catalysts used in this study were prepared by Dr. Michael D'Aniello, Jr., and Miss Kim Dang. We also wish to thank Mr. R. M. Sinkevitch for helpful suggestions regarding modifications to the reactor system. Registry No. Cu, 7440-50-8; Cr, 7440-47-3; Pt, 7440-06-4; Mn, 7439-96-5; ethanol, 64-17-5;acetaldehyde, 75-07-0;carbon monoxide, 630-08-0; carbon dioxide, 124-38-9.
Literature Cited Bailey, W. H.; Edwards, C. F. Proceedings of the Fourth International Symposium on Alcohol Fuels Technology, Guaruja, Sao Pauio, Brazil, Oct 1980, Paper 6-61. BechtoM, R.; Pullman, J. B. Society of Automotive Engineers, 1980; Paper No. 600260. Chui, G. K.; Anderson, R. D.; Baker, R. E.; Pinto, F. B. Proceedings of the Thlrd International Symposlum on Alcohol Fuel Technology, Asiiomar, CA, May 1979, Paper 11-18, Edwards, J.; Nicoiaidis, J.; Cutlip, M. 6.; Bennett, C. 0.J . Catal. 1977, 5 0 , 24. Ganguiy, N.; Janaklram, K.; Nag. N.; Bhattacharyya, S. J . Appl. Chem. Biotechno/. 1975, 25, 335. Goodrich, R. S. Chem. Eng. Prog. 1982, 78, 29. Ismagiiov, Z . R.; Dobrynkin, N. M.; Popovskii, V. V. React. Kinet. Catal. Lett. 1979, 10, 55. Iwasawa, Y.; Nakano, Y.; Ogasawara, S.J . Chem. Soc.. Faraday Trans 1 1978, 7 4 , 2968. Kiimisch, R. L. "Oxidation of CO and Hydrocarbons Over Supported Transition Metal Oxide Catalyst"; GM Research Publication GMR-842, 1968. Krylov, 0. V.; Fokina, E. A. Repr. Fourth Int. Congr. Catal. Moscow 1968, 3, 1166. Legendre, M.; Cornet, D. J . Catal. 1972, 25, 194. Pernicone, N.; Lazzerin, F.; Liberti, G.; Lanzavecchia, G. J . Catal. 1969, 14, 293. Santacesaria, E.; Morbideili, M. Chem. Eng. Sci. 1981, 3 6 , 909. Sexton, B. A. Surf. Sci. 1979, 88, 299. Sexton, B. A.; Renduiic, K. D.; Hughes, A. E. Surf. Sci. 1982, 121, 181. Srihari, V.; Viswanath, D. S.J . Catal. 1976, 4 3 , 43. Takezawa, N.; Hanamaki, C.; Kobayashi, H. J . Res. Inst. Catal. Hokkaido Univ. 1980, 28(3), 347. Thomas, C. L. "Catalytic Processes and Proven Catalysts"; Academic Press: New York, 1970. Wachs, I. E.; Madix, R. J. Appl. Surf. Sci. 1978, 1 , 303. Walker, J. F. "Formaldehyde"; Reinhold: New York, 1964.
Pt. Our proposed mechanisms are also consistent with recent studies of ethanol oxidation on Cu and Pt single crystals. Wachs and Madix (1978) found acetaldehyde to be the principal carbon-containing product in temperature-programmed reaction (TPR) of adsorbed ethanol with adsorbed atomic oxygen on Cu(ll0). In contrast, Sexton et al. (1982) found CO to be the only carbon-containing gaseous product from the TPR of ethanol with oxygen on a Pt(ll1) surface. Thus, the TPR studies confirm the existence of the first step of the consecutive reaction mechanism (reaction 1) on Cu and suggest that our proposed direct oxidation pathway (reaction 2) in the combined series-direct mechanism on Pt may involve the formation of carbon monoxide as a reaction intermediate. The reaction steps leading to the formation of acetaldehyde from the oxidation of ethanol on Pt have not been elucidated, however, and additional work is needed, both on single crystals and supported catalysts, to identify the reaction intermediates produced in the oxidation of ethanol and to identify the factors controlling selectivity for the production of carbon dioxide and acetaldehyde.
Received f o r review August 9, Revised m a n u s c r i p t received November 22, Accepted November 22,
1982 1982 1982
Catalytic Amination of Aliphatic Alcohols in the Gas and Liquid Phases: Kinetics and Reaction Pathway Alfons Balker,' Walter Caprer, and Wllllam L. Holstein Swiss Federal Instltute of Technology (ETH), Department of Industrial and Engineering Chemistry, 8092 Zurich, Switzerland
The kinetics of the copper-catalyzed amination of long-chain aliphatic alcohols (octanol and decanol) by monomethylamine and dimethylamine have been investigated in both the gas and liquid phases at temperatures between 440 and 540 K. The Individual reactions leadlng to the production of stable intermediates and products are identified. The rate of dehydrogenation of the alcohol determines the overall rate of alcohol conversion to all products. The rate is first order in alcohol in both the gas and liquid phases and inhibited by alcohol, water, and the reactant amine in the gas phase only. The selectivity is determined primarily by the rate of hydrogenation of an adsorbed intermediate and the rate of disproportionation of reactant and product amines. The selectivity of the amination reaction to the desired tertlary amine increases with increasing hydrogen pressure, and first increases and then
decreases with increasing conversion of alcohol.
Introduction The catalytic amination of higher aliphatic alcohols represents an economical way for the synthesis of aliphatic amines. In spite of the industrial importance of this synthesis, to our knowledge, studies on its kinetics and 0196-432118311222-0217$01.50/0
reaction pathway have only been reported for the amination of octanol with ammonia on a molten iron catalyst (Kliger et al., 1975a,b). A variety of different catalyst types for the amination of alcohols has been suggested in the patent literature. As a result of the testing of numerous 0
1983 American Chemical Society
