Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 419-424
419
we have made an attempt to explain the mechanism of the periodic operation effect on the automotive three-way catalyst. We have begun work aimed at this goal.
suitably eliminated, and the surface compositions of reactants are kept suitable to react each other. Then the reaction rate attains the maximum value. The periodic operation effect can be interpreted in terms of the strong adsorption of CO, and this concept has been confirmed in the CO-02 system (Muraki et al., 198513). N 2 0 selectivity is minimum at the period that the NO conversion is maximum. A t the optimum periods, no oxygen atoms are present on the catalyst surface before NO is injected; 0 atoms are produced when NO is injected and are almost scavenged with CO injection. The result is that the N2 formation on the reducing surface is much better than that on the slightly oxidized surface (Muraki et al., 1986). In the periodic operation effect for NO-CO reaction, Pt catalyst, which is the least active, is remarkable, and Rh catalyst, which is the excellent catalyst, is not influenced. In this reaction system, the CO inhibition is strong, and the activities order corresponded to the order of their susceptibilities to CO self-poisonings. Automotive exhaust catalysts in more complex feedstreams show the marked periodic operation effect on the Pt and Pd catalysts (Muraki et al., 1985a; Yokota et al., 1985). This approximately corresponds to the periodic operation effect of our NO/CO cycling reaction (Figure 6), implying that the effect of the injection period of NO gas on the catalytic activities (Figure 11)is indeed governed by the adsorbed CO scavenging process, as previously suspected (Muraki et al., 198513). In addition to HC, we could observe the striking periodic operation effect on the Pd catalyst (Muraki et al., 1985c), since the HC could be chemisorbed strongly onto the noble metals with increasing temperature. The mechanism of periodic operation effect of HC is also interpreted in terms of the strong adsorption of HC. In order to develop a Rh-free three-way catalyst,
Acknowledgment
We are grateful to Professor Y. Murakami and Associate Professor T. Hattori, of Nagoya University, for their helpful suggestions. Registry No. NO, 10102-43-9; CO, 630-08-0; Rh, 7440-16-6; Ru, 7440-18-8; Ir, 7439-88-5; Pd, 7440-05-3; Pt, 7440-06-4. Literature Cited Ashmead, D. R.; Campbell, J. S.; Davies, P.; Farmery, K. SA€ Tech. Pap. Ser. 1974, No. 740249. Bauerle, G. L.; Service, G.; Nobe, K. Ind. Eng. Chem. Prod. Res. Dev. 1972, 1 1 , 54. Canab, R. P.; Winegarden, S. R.; Carison, C. R.; Miles, D. L. SA€ Tech. Pap. Ser. 1978, No. 780205. Hecker, W. C.; Bell, A. T. J . Catal. 1983, 8 4 , 200. John, H.; Volker, R.; Ismail, M. I. Pletlnum Met. Rev. 1978, 2 2 , 92. Kobylinski, T. P.; Taylor, B. W. J . Catal. 1974, 33, 376. Kummer, J. T. f r o g . Energy Combust. Sci. 1980, 6 , 177. Lester, G. R.; Joy, 0.C.; Brennan, J. F. SA€ Tech. Pap. Ser. 1978, No. 780 202. Lorimer, D.; Bell, A. T. J . Catal. 1979, 5 9 , 223. McKee, D. W. J . Catal. 1967, 8 , 240. Muraki, H.; Shinjoh, H.; Sobukawa, H.; Yokota, K.; Fujitani, Y. Ind. Eng. Chem. Prod. Res. h v . , 1985a, 2 4 , 43. Muraki, H.; Sobukawa. H.; Fujitani, Y. Nippon Kagaku Kaishi 1985b, 2 , 176. Muraki, H.; Sobukawa, H.; Fujitanl, Y. Nippon Kagaku Kaishi 1985c, 4 , 532. Muraki, H.; Shinjoh, H.; Fujiani, Y. Ind. Eng. Chem. Prod. Res. Dev. 1988, following paper in this issue. Ohara, H.; Kondoh, S.; Fujiini, Y., J . SOC.Automot. Eng. Jpn. 1978, 16, 9. Schlatter, J. C.; Taylor, K. C. J . Catal. 1977, 4 9 , 42. Shelef, M. Catal. Rev. 1975, 1 1 , 1. Shelef, M.; Gandhi, H. Ind. Eng. Chem. Prod. Res. Dev. 1972, I f , 393. Tauster, S. J.; Murrell, L. L. J . Catal. 1976, 4 1 , 192. Taylor, K. C.; Klimlsch, R. L. J . Catal. 1973, 3 0 , 476. Yokota, K.; Muraki, H.; Fujitani, Y. SA€ Tech. P a p , Ser. 1985, No. 850 129.
Received for review September 4, 1985 Revised manuscript received January 24, 1986 Accepted February I, 1986
Reduction of NO by CO over Alumina-Supported Palladium Catalyst Hldeakl Muraki,' Hirofumi Shlnjoh, and Yoshlyasu Fujltanl Toyota Central Research and Development Laboratories, Inc., Aichi-gun, Aichi-ken 480- 1 1, Japan
The kinetics of NO reduction by CO have been investigated over a Pd/AI,O, catalyst. These studies have been complemented by pulse reaction, transient response method, and TPD. The performance of the Pd catalyst depended strongly on the ratio of NO/CO and the reaction temperature. I t was shown that the N20 selectivity was minimum at the stoichiometric ratio of NO/CO. For NO conversion below 20% at 350 O C , the kinetics for NO reduction and N, and NO , formation over Pd catalysts were first order in NO and inverse first order in CO. The kinetics and product selectivity were found to be in good agreement with a mechanism based on the chemisorption of NO as the rate-limiting step.
Therefore, for its extensive use, it is necessary to find ways to minimize the use of the scarce material or provide a Rh-free catalyst. From the comparison of catalytic behavior between Pt and Pd, TWC performance of Pd is very similar to that of Rh catalyst (Yokota et al., 1985; Muraki et al., 1985). Also, Pd is relatively abundant, domestically available, and significantly less expensive than Pt and Rh. Therefore, we selected Pd as the main component of the Rh-free TWC. The reduction of NO by CO over noble-metal catalysts has been investigated (Lorimer and Bell, 1979; Hecker and Bell, 1983; Dubois et al., 1980; Taylor and Schlatter, 1980;
Introduction
Automotive exhaust catalysts mainly consist of platinum (Pt), palladium (Pd), and rhodium (Rh) because of their high intrinsic activity and durability in automotive exhaust conditions. Of these precious metals, rhodium is used in three-way catalyst (TWC) for its activity to selectively reduce nitric oxide (NO) to nitrogen (N2)with low ammonia (NH,)formation (Shelef and Gandhi, 1972; Kummer, 1980; Schlatter and Taylor, 1977). The standard TWC formulations are those containing both Pt and Rh. The Rh/Pt ratio in TWC with sufficient durability is considerably higher than mine ratio. 0196-432118611225-0419$01.50/0
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1986 American Chemical Soclety
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25,No. 3, 1986
Campbell and White, 1978; Kobylinski and Taylor, 1974) as part of the effort to develop catalysts for the abatement of automotive NO emissions. There have been many investigations of NO reduction by CO over Pt and Rh catalysts, but few about the Pd catalyst under near stoichiometric conditions (Butler and Davis, 1976; Grill and Gonzalez, 1980). In this study we have investigated the mechanism and kinetics of NO reduction by CO and product composition. Pulse reaction and TPD observation were used to identify the catalyst surface states. On the basis of transient response experiments, it was proposed that the reduction of NO is initiated by the chemisorption of NO.
Experimental Section Catalyst. In order to lower the inevitable support effects which affect the intrinsic character of noble-metal catalysts, chemically inactive a-A1203(2-3-mm-diameter sphere; BET surface area 10 m2/g; bulk density 0.79 g/ cm3) was selected as the support. a-A120gwas prepared by calcination of 6-A1203(RhBne-Poulence SCS-79) at 1200 "C for 3 h in air. The catalysts were prepared by impregnation of the a-A1203supports with the aqueous solutions of palladium nitrate. The concentrations of respective solution were adjusted to give 0.006 and 0.24 wt % metal on the finished catalyst. The impregnated spheres were dried overnight a t 110 "C and calcined a t 600 "C for 3 h in air. Activity Measurements. The laboratory reactor system used in this experiment is similar to the previously used system (Muraki et al., 1985). The kinetics of the P d catalyst were measured by a feed of NO/CO mixture in helium. Gas chromatography was used to analyze gases such as NO, N2, NzO, CO, and C02. The stoichiometry number, S , which identifies the redox characteristic of the gas mixture, is defined as follows: S = [NO]/[CO] The nature of the feedstream is reducing, stoichiometric, and oxidizing for S < 1.0, S = 1.0, and S > 1.0, respectively. Pulse Reaction. The apparatus used for the pulse reaction was a conventional pulse reactor, which consisted of a gas chromatograph modified by the introduction of a small reactor between the sample inlet and the analytical column. The reactor consisted of a 6 mm i.d. quartz tube with a 12 mm i.d. bulb-shaped midsection. A quartz fritted disk was fused to the inside of the reactor. Helium gas (44 mL/min), as a carrier, was passed through a Deoxo purifier and a molecular sieve liquid nitrogen trap. Hydrogen was purified by passing it through a Pt catalyst a t 200 "C and a molecular sieve liquid nitrogen trap. Nitric oxide (99.9%) was used without further purification. One milliliter of catalyst set in the reactor was pretreated by (1)increasing the catalyst temperature to 500 "C at a heating rate of 500 "C/h in flowing H, (50 mL/min), (2) reducing the catalyst with H2 a t 500 "C for 1 h, (3) changing the H2 stream to He stream (44 mL/min) for 1 h a t 500 "C, and cooling down to a given temperature. After the pretreatments, 0.25 mL of NO or CO gas was injected into the catalyst. TPD Measurements. The reactor used for the TPD measurements is similar to that used for the pulse reaction. The same reduction was used in the pulse reaction and cooling to room temperature. A gas injection valve was connected in series with the line to the reactor for permitting pulsative injection of the adsorbate. Ten NO or CO pulses were fed to the catalyst every 3 min for 0.5 h. The reactor was heated to 500 "C, and a programmable
100
10
.'" C
0 i
Y
6 50
05
V
v)
0
u
.I
0
z
"
-
n
300
400 Temperature
i°C 1
500
Figure 1. Effect of temperature on conversions, products, and N,O selectivity (5'N20) over the 0.006 wt o/o Pd catalyst at S = 1.0.
s= 1 .o 0 0
50
100
CO Conversion (%)
Figure 2. N 2 0 selectivity vs. CO conversion over the Pd catalyst a t various SV.
power supply was used to control catalyst heating at a linear rate of 40 "C/min. The carrier gas, helium, at atmospheric pressure was flowed to the catalyst at 50 mL/min. The effluent from the reactor was analyzed by the quadrupole mass spectrometer. Transient Response Experiments. NO and CO were injected alternately and periodically into the helium gas stream a t a cycle of 20 s. The periodic concentration change of the NO and CO feedstreams took a square-wave shape, and the time-average concentration of the feedstreams was adjusted to 0.3% NO and 0.3% CO. The hourly space velocity of the gas was kept at 30000. The instantaneous gas composition in the cycled streams delivered to the catalyst was promptly analyzed by the quadrupole mass analyzer. The reactant concentration profiles were measured after NO or CO was injected onto the CO or NO adsorbed surface.
Results Kinetics on the Pd Catalyst. Several experiments using the 0.006 wt % Pd catalyst to examine the effects of the space velocity and NO and CO partial pressure were carried out. It was observed that the catalyst activity underwent a moderate decay in the first 30 min and then slowly approached steady-state performance. The data were taken under steady-state conditions. Figure 1 illustrates the effect of temperature on the conversion of NO and CO, the yield of N2 and NzO, and the selectivity of N2O. The yield of N20 increased with increasing temperature up to 400 "C and then decreased with further increasing temperature. On the other hand, the yield of N2 monotonously increased with increasing temperature. The nitrous oxide selectivity decreased with increasing temperature. An experiment was performed to assess the effect of space velocity on the CO conversion and N 2 0 selectivity. The results are shown in Figure 2. As can be seen, variation of the space velocity over a 17-fold range had no effect on the relation between the CO conversion and the
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
421
10 Pco=0 . 3 %
m
zX N
z L
S
Figure 3. N 2 0 selectivity vs. S at Pco = 0.3%. 10
0.3 0.5
q,=o 3 %
1
2
4
10
9r0, PCOXIO
35OoC
(mole%)
Figure 6. Partial pressure dependence of the rate of for Pd catalyst. 10
-
l
I
I
I
/
J
n
I
Od
10
S
2.0
30
Figure 4. NzO selectivity vs. S at PNo= 0.3%. 401
t
1
1
1
1
1
1
/4
1
sxxz0, L
1 1
0,
z
L
20
Nzformation
1
;AA
0
A0 PNO= Pco=O3 03 % A
0.3 0.5
PNO=0 3 %
' " "
I
1
2
4
10
20
PNO,Pco X I O (mole%)
Figure 7. Partial pressure dependence of the rate of NzO formation for Pd catalyst.
PNOand PCo are the partial pressure of NO and CO, re-
PNO, Pco X 10 (mole%)
Figure 5. Partial pressure dependence of the rate of NO disappearance for Pd catalyst.
N20selectivity. This observation suggests that NzO, once formed, did not undergo further reduction. The effects of the partial pressure of NO and CO were examined by changing the partial pressure of either NO or CO. The results are shown in Figures 3 and 4 as the effects of the S on the selectivity of NzO. When the temperature became higher, 450 and 500 "C, the selectivity to NzO suddenly decreased at S = 1.0. While the temperatures were lower, i.e., 350 and 400 "C,the selectivities to NzO were almost constant (65%) at every S. The dependence of the rate of NO disappearance, rNO, on the partial pressures of NO and CO was determined. To avoid significant variations in the reactant concentration, the reaction temperature was chosen at 350 "C so that the NO and CO conversions were always less than 20%. The results are shown in Figure 5. The data in this figure were used to determine the power-law rate expressions, and rNo was given by ~ N O / ~ N=OPNO/PCO
(1) where k N O is the rate constant of NO disappearance and
spectively. The rate is first order in NO and inverse first order in CO. These results imply that CO is chemisorbed strongly and the catalyst surface is dominated by adsorbed CO, that is to say, the coverage by adsorbed NO is significantly less than that by CO. The dependence of the rates of N2 and N 2 0 formation, rN2and.rNZ0,on the partial pressure of NO and CO were determined as well as r N 0 . The results are shown in Figures 6 and 7. The rates of rNzand rNzowere also given bY ~ . N , / K N= ~ ~ N ~ o / =~ PNN~Oo / ~ C O (11) where k N 2 and k N 2 q are the rate constants of N2 and N20 formation, respectively. It was found that the reaction rates exhibited a negative first-order dependence on the CO partial pressure and a positive first-order dependence on the NO partial pressure. Transient Responses of t h e Cycled Feedstreams. This section will describe two types of response for the periodic NO-CO reaction consisting of NO and CO injection phases. Typical mass spectrometer responses for the cycled feedstream are shown in Figure 8. The results at room temperature are presented in Figure 8a. The concentration changes of CO ( m l e 28) and NO ( m l e 30) feedstreams were square-waves. When the temperature was raised to 200 "C, the products of COz and NzO ( m / e 44) appeared. Figure 8b shows a spectrum at 260 "C. When CO was injected, the spectrum of m l e 44
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
422
-
5
Y
4
6 D
L
2 50°C
1 E t
t
3
5.
Y 1
0
2
U
z
a 1
in
i
A
0 0
-20 ,a
secR T
__ 20 sec-
-2 O s e c -
Ic 31OoC
ibi 260'C
2
Pulse
4
6
8
Number
Figure 10. Effect of pulse number on products for 2.4 wt % Pd catalyst at 250 "C.
Figure 8. Mass spectra for 0.24 wt % Pd catalyst at a cycling period of 20 s. 1
I
I I
I I I I
I
I
0 '
1 1 1
I I I I I
0 Temperature
\ \
C
Figure 9. Effect of temperature on products in the first pulse of NO for 2.4 w t % Pd catalyst.
suddenly increased. However, it moderately increased when NO was injected. The spectra at 310 "C are shown in Figure 8c. The spectrum of N2 ( m / e 28) slightly appeared when NO was injected. These results indicated that rates of both COzand N20 formation in the phase of NO injection were smaller than those of CO injection. These results suggested that the NO injection phase was the rate-limiting step in the process of NO reduction. Pulse Reaction. In order to clearly determine the product distribution of NO/CO reaction, the pulse reaction was carried out at various reaction temperatures. The amount of products, N2 and N20, in the first pulse of NO on the 2.4 wt % Pd catalyst at the given reaction temperature is shown in Figure 9. This figure illustrates the influence of temperature on the dissociation and recombination between the adsorbed NO and dissociated NO on the surface of the reduced Pd catalyst. The N2formation increased with increasing temperature. On the other hand, the N 2 0 formation slightly decreased with increasing temperature, but the amount of N 2 0 was much less than that of N2. Further pulsing experiments of NO were carried out. Figure 10 shows the relation between the amount of N2 and N 2 0 formation and the pulse number at 250 "C. The amount of N2 formation suddenly decreased with increasing pulse number. The amount of NzO formation was very low in the first pulse and increased in the second pulse. After the second pulse, the amount of N20 decreased with further pulses. These results indicated that the N2 formation on the reduced Pd surface was much better than that on the little-oxidized Pd surface. However, the NzO formation on the little-oxidized Pd surface was much better than that on the reduced Pd surface. The steady-state products (vol %) of each pulse of CO and NO in which the CO and NO were alternately pulsed
\
0
100
200
300 4 0 0
500-
500
keep Temperature ( " C ) Figure 11. TPD spectra of CO and COPfollowing CO adsorption on 2.4 wt 70 Pd catalyst. Table I. Product Distribution and N20Selectivity for Pulse Reaction
CO pulse NO pulse temp, "C COP, % NzO, % Nz, % COZ, % NZO, % Nz, % loo -0 1 -0 3 -0 300 42 2 33 13 500 38 -0 2 6 32
SN20
1
0.72 0.15
at 100, 300, and 500 "C were measured. The results are listed in Table I and show that the main product of CO pulsing was only C 0 2 and those of NO pulsing were both N20 and N2. The N20 selectivity at low temperature was larger than that at high temperature. This tendency was similar to the results of the flow reactions. TPD of CO and NO. A TPD spectrum of adsorbed CO is shown in Figure 11. Two distinct CO peaks were observed at 275 and 475 "C. In addition, one COPpeak was observed at 525 "C. The presence of C 0 2 indicated that the disproportionation of CO via the reaction 2CO = C + COz occurred during the desorption. Figure 12 shows the spectra of TPD of NO which adsorbed the saturation coverage at room temperature. Three species were detected during the thermal desorption of adsorbed NO: NO, N2, and N20. NO, N2, and N 2 0 peaks were observed at 190,300,and 250 "C, respectively. The desorption of both N2 and N 2 0 was observed from about 150 "C; that is, the adsorbed NO was dissociated at a low temperature. Discussion The mechanism of NO reduction by CO over Rh and Pt catalysts has been discussed by several authors, but that
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986 423
rates of CO adsorption and desorption are much more rapid than the rate of NO reduction, and hence the surface coverage by CO can be represented by the equilibrium expressions. The second assumption is that reaction 1 is the rate-limiting step in the mechanism. This assumption is supported by the transient response and TPD measurements. Finally, it is assumed that N2 is formed via reaction 5 . This mechanism is similar to that proposed (Hecker and Bell, 1983) over Rh/Si02 catalyst, but the rate-limiting step is not the same. The rate for NO reduction, rNO, can be expressed as r'NO 0
100
200
300 400 500-500 keep
("C) Figure 12. TPD spectra of NO, N2, and N 2 0 following NO adsorption on 2.4 wt 90 Pd catalyst. Temperature
=
jZIPNOev
(111)
where kl is the rate coefficient for reaction 1 and Ov is the fractional surface coverages for vacancies. It is assumed that the surface coverage by CO, is described by a Langmuir isotherm. Imposition of a steady-state balance on the Pd surface sites leads to the following expression for TNO
over Pd catayst has not been discussed. The kinetics for NO reduction and for Nz and N 2 0 formation on Pd catalyst were determined for the conditions where the NO conversion was less than 20% at 350 "C. In all cases, the reaction rates exhibit negative firstorder dependence on CO partial pressure and positive first-order dependence on NO partial pressure. These results suggest that the reduction of NO is rate-limited by the chemisorption of NO and that the surface is dominated by the adsorbed CO. This assumption should be correct since the T P D data of CO indicate that CO desorbs at a relatively high temperature. Of further significance is the observation that the selectivity for N 2 0 formation is independent of gas space velocity and dependent on reactant concentrations and catalyst temperature. These facts suggest that N, and N20are formed via separate pathways. The observed trends in reaction rates, product selectivity, transient response, pulse reaction, and TPD can be discussed in terms of the following mechanism.
+ S +NO, co + s + co, NO, + S N, + 0, NO, + N, N2O + 2s NO, + N, Nz + 0, + S CO, + 0, co*+ 2s NO
-+
--*
(1) (2)
(3) (4)
(5) (6)
Reaction 1 represents the chemisorption of NO, and reaction 2 represents the reversible chemisorption of CO. The T P D study and the NO pulse reaction have shown that a t increasing temperature, NO readily undergoes dissociation to form N and 0 atoms, reaction 3. The formation of N 2 0 is assumed to occur via a LangmuirHinshelwood process, as illustrated by reaction 4. The evidence for the occurrence of this reaction comes from investigations of the TPD and pulse reaction. These studies also suggest that at a relatively low temperature N2 is formed via reaction 5, because N 2 0 selectivity is constant at every S. Reaction 6 describes the removal of adsorbed 0 atoms from the catalyst surface by the reaction with adsorbed CO. Infrared studies for the formation and decomposition of the NCO group suggest that this species is not directly involved in the formation of the reaction products as Dalla Betta and Shelef (1976) suggested. On the basis of the mechanism represented by reactions 1-6, rate expressions can be derived for the kinetics of NO reduction and N 2 0 and N2 formation. Before proceeding, it is desirable to introduce several assumptions. First, the
rNo = ( k i P ~ 0 ) / ( 1+ KzPco)
(IV)
where K 2 is the equilibrium constant associated with reaction 2. Under steady-state reaction conditions, it is assumed that the surface is dominated by adsorbed CO. On the basis of these considerations, it is reasonable to conclude that the second term in the denominator of eq IV is much greater than 1. Under such circumstances, eq IV can be reduced to Equation V is in the form of a power-law rate expression and can be compared directly with the power-law rate expression deduced from experimental results. The chemisorption of NO is known to be reversible, but the surface coverage by NO, is negligible because NO dissociation (reaction 3) is very rapid in this system. Therefore, the interpretation is not affected by the reversibility of NO chemisorption. On the basis of the proposed mechanism, the rates of N 2 0 and N2 formation are given by 0.5124 rNIO = (VI) k4 kgrN0
+
0.5kS rN2= k4 k,rNo
+
where ki represents the rate coefficient for reaction i. Equations VI and VI1 indicate that the dependences of N 2 0 and N2 formation on the partial pressures of NO and CO are identical with those for the overall consumption of NO. Those results are consistent with the experimental observations. As shown in Figures 3 and 4, a t a relatively high temperature the N 2 0 selectivity becomes minimum for a NO/CO ratio near unity and rises sharply for both increasing and decreasing NO/CO ratio. This trend is consistent with the hypothesis that, at the stoichiometric point, the removal of atomic oxygen by reaction 6 accelerates and reaction 5 is superior to reaction 4. Thus, the rate of NO reduction is first order in NO and inverse first order in CO. These results suggest that the reduction of NO is rate-limited by the chemisorption of NO and that the catalyst surface is dominated by the adsorbed CO. Therefore, CO is a major inhibitor in this system. In order to develop a Rh-free three-way catalyst, the inhibition effect of CO must be overcome. We have proposed a way to operate the Rh-free Pd catalyst system
Ind. Eng. Chem. Prod. Res. Dev. 1986, 25,424-430
424
(Muraki et al., 1985): the activity of Pd catalyst is increased under the cycling feed due to the periodic operation effect (Muraki et al., 1986).
Grill, C. M.; Gonzalez, R. D. J. Phys. Chem. 1980, 84, 878. Hecker, W. C.;Bell, A. T. J. Catal. 1983, 84, 200. Kobylinski. T. P.; Taylor, B. W. J. Catal. 1974, 33, 376. Kummer, J. T. Prog. Energy Combust. Sci. 1980, 6, 177. Lorimer, D.; Bell, A. T. J. Catal. 1979, 59, 223. Muraki, H.; Shinjoh, H.; Sobukawa, H.; Yokota, K.; Fujitani, Y. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 43. Muraki, H.; Fujitani, Y. Ind. Eng. Chem. Prod. Res. Dev. 1988, preceding paper in this issue. Schlatter. J. C.; Taylor, K. C. J . Catal. 1977, 49,42. Shelef, M.; Gandhi, H. Ind. €ng. Chem. Prod. Res. Dew. 1972, 7 1 . 393. Taylor, K. C.;Schlatter, J. C. J. Catal. 1980, 63, 53. Yokota, K.; Muraki, H.: Fujitani. Y. Presented at the Society of Automotive Engineers Congress, Detroit, MI, March 1985; paper 850129.
Acknowledgment We are grateful to Professor Y. Murakami and Associate Professor T. Hattori, of Nagoya University, for their helpful suggestions. Registry No. NO, 10102-43-9; CO, 630-08-0; Pd, 7440-05-3. Literature Cited Butler, J. D.: Davis, D. R. J. Chem. SOC., Dalton Trans. 1978, 27, 2249. Campbell, C. T.: White, J. M. Appl. Surf. Sci. 1978, 7 , 347. Dalla Betta, R. A.; Shelef, M. J. Mol. Catal. 1976, 1 , 431. Dubois, L. H.; Hansma, P. K.: Somorjai, G. A. J. Catal. 1980, 65, 318.
Received for review September 9, 1985 Revised manuscript received February 10, 1986 Accepted March 12, 1986
Comparison of Some Solid Catalysts for the Production of Ethanolamines from Ammonia and Ethylene Oxide in the Liquid Phase Lennart Vamling' and Lennart Cider Department of Chemical Reaction Englneering, Chalmers University of Technology, S-4 12 96 Goteborg, Sweden
The ability of different forms of zeolites, such as 13X, 4A, Y, and AW500, to catalyze the formation of mono-, di-, and triethanolamines from ethylene oxide and ammonia in the liquid phase has been examined and compared with that of an organic ion-exchange resin. The ion-exchange resin, Amberlite 200, gives the highest yield of monoethanolamine (MEA), while the zeolite 13X has the highest capacity of producing MEA. Furthermore, it is demonstrated that any contrlbutlons from homogeneous reactions to the total reaction rate are negligible. A fast and highly reproducible method for the direct GC analysis of ethanolamines using fused-silica capillary columns is also presented.
RpNHB-p + HA
Introduction Mono-, di-, and triethanolamines (MEA, DEA, and TEA) are produced by reacting ethylene oxide (EO) with ammonia (NH,) according to NH3 + C2H40 RNH2 NH, EO MEA
CH CHz
CHzCHz
-
'd
RNH2 + C2H40 R2NH MEA EO DEA RzNH C2H40 R,N (111) DEA EO TEA where R represents the group HOCH2CH2-. As already pointed out by Knorr (1899), no reaction occurs by just mixing NH, and EO. Traditionally, water has been added to promote the reactions. The kinetics for this alternative have been studied by Potter and McLaughlin (1947) and by Miki et al. (1966)among others. The water must be separated from the products, an energy-consuming disadvantage. This has led to an increasing interest in anhydrous methods using heterogeneous catalysts. Weibull et al. (1957, 1973) have shown that it is possible to use ion-exchange resins. One plausible reaction mechanism explaining the catalytic effect of these resins is the following, given by Weibull (1970). First, an NH,, MEA, DEA, or TEA molecule (represented by R,NH,, where p = Ck3 and R is the same group as above) is adsorbed a t an acidic site, in this case a sulfonic acid group.
-
0196-4321/86/1225-0424$01.50/0
RPH4-,N**A
reversible adsorption step Second, the oxygen atom in the EO molecule is attracted by the adsorbed molecule.
-+
+
2
+
RPH4.,N..A
C
/7
O*.H4-,RpN..A
reversible adsorption step
This weakens the C-0 bond in the EO molecule and makes it possible for the nitrogen atom in another NH,, MEA, or DEA molecule (R,NH,, where q = 0-2) to make a successful nucleophilic attack, which causes the ring to break. CH CH,
R,NH3-,
+ $7 **H,-,R,N.*A
R,+lH3-,N*.A
+
RpNH3-,
reaction step
This led us to the idea that other sources of acidity could also be used, perhaps ones having higher activity or higher yield of MEA. A potential disadvantage with organic ion-exchange resins is their lack of stability at high temperatures. The requirements of having a high-temperature stability and of containing acidic sites are met by many zeolites. We therefore decided to investigate four different types, one with small cages (4A), one with medium-sized cages (AW500), and two with large cages (13X and Y). For 0
1986 American Chemical Society
