Ethene epoxidation over silver catalysts in the presence of carbon monoxide and hydrogen

Applied Catalysis, 62 (1990) 189-203 Elsevier Science Publishers B.V., Amsterdam -

189 Printed in The Netherlands

Ethene Epoxidation over Silver Catalysts in the Presence of Carbon Monoxide and Hydrogen YOU-SING YONG and NOEL W. CANT* School of Chemistry, Macquarie

University,

NS W 2109 (Australia)

(Received 23 August 1989, revised manuscript received 19 January 1990)

ABSTRACT The epoxidation of ethylene over silver catalysts in the presence of added carbon monoxide and hydrogen has been studied as a possible method for directly using ethylene in streams generated by partial oxidation of methane. When tested separately over a silver sponge catalyst the oxidation of both carbon monoxide and hydrogen are more than one order of magnitude faster than that of ethylene. Measured activation energies are 44,43 and 61 kJ/mol respectively. Under conditions of moderate ethylene conversion (ca. 40% at 239°C) the effect of including carbon monoxide or hydrogen in amounts up to 20% of the ethylene is quite small. The added material is totally oxidised in the initial section of the catalyst bed leaving the remainder available for ethylene epoxidation. Neither ethylene conversion nor ethylene oxide yield is affected significantly. The situation is somewhat different at lower conversions over chlorine moderated catalysts. Added carbon monoxide is then incompletely oxidised and the fraction emerging inhibits ethylene oxidation by direct competition for the silver surface. Conversion is reduced and the selectivity to ethylene oxide is also reduced under ethylene-rich conditions. Under similar conditions hydrogen is also incompletely oxidised but it inhibits less. A particular effect was observed when carbon monoxide was included in the feed for long periods ( > 10 h). Ethylene conversion fell steadily while the selectivity to ethylene oxide rose. The effect was very similar to that noted when experiments were commenced with 1,2-dichloroethane at the ppm level in the feed. It is attributed to small quantities of an impurity in the carbon monoxide since it disappeared if that stream was purified. Keywords: ethylene epoxidation, ethylene oxide, silver, selectivity (ethylene oxide), chlorine.

INTRODUCTION

The pioneering reports of Keller and Bhasin [l] and of Ito et al. [2] have generated much work on the partial oxidation of methane to ethylene and ethane. Many oxide catalysts, in addition to the Li/MgO one originally described, are now known to be effective for co-feed operation with both oxygen and methane present [ 3-6 1. Much of this work has been aimed at the possible utilisation of natural gas at remote locations by conversion to liquid fuels [ 71. However the process also has potential as an alternative to steam cracking of

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190

ethane as the source of ethylene for the petrochemical industry. Two related factors hamper such an application. One is that currently known catalysts work effectively only under methane rich conditions. The fraction of ethylene in the product stream is small (10% by volume at most). Production of a concentrated stream then requires expensive cryogenic separation. It would be preferable to couple partial oxidation of methane to a process capable of stripping ethylene from the dilute stream with return of unreacted methane (and also ethane ) in a recycle loop. The difficulty here is that partial oxidation of methane gives rise to significant amounts of by-products particularly carbon monoxide, carbon dioxide, hydrogen and water. It is obviously desirable that any process for operation in conjunction with methane oxidation should be insensitive to such impurities. If this is not so then the advantage of avoiding separation may be lost. One process which may have application in the above context is ethylene oxide synthesis over silver catalysts. This process can be operated advantageously with some methane, ethane or carbon dioxide in the feed [S]. The desired range of ethylene concentrations for current oxygen-based technology is 20-30% [9]. However the process has been operated with less than 2% ethylene in older air-based plants. There is an enormous literature, and some lingering controversy, concerning the mechanism of ethylene epoxidation over silver [ 10,111. Despite this we know of no studies on the effect of hydrogen or carbon monoxide on the reaction under synthesis conditions. It is clear however that silver is quite a good catalyst for the oxidation of both carbon monoxide and hydrogen at temperatures in the range 50’ C to 150” C [ 12-151. By contrast ethylene oxide synthesis is normally carried out at 230-280°C over silver moderated by adsorbed chlorine generated by decomposition of small amounts of chlorinated hydrocarbons in the feed. One may therefore anticipate a significant effect, by carbon monoxide in particular, on ethylene oxide synthesis. The purpose of this study was to determine the extent of any effect. EXPERIMENTAL

All catalytic measurements were carried out using a single pass flow system operated at near ambient pressure in a manner similar to that given previously [ 161. The basic feed stream comprised ethylene and oxygen in a helium carrier. Hydrogen was added as such, carbon monoxide as a 1.8 or 20% mixture in helium and dichloroethane as a 90 ppm mixture with helium. All helium and hydrogen used were ultrahigh purity grade from CIG (Aust). Oxygen was industrial grade from CIG and ethylene was CP grade from Matheson (New Jersey). The mixtures used were made up by CIG with the carbon monoxide (CP grade) sourced from Matheson. The catalyst used was a high purity silver sponge, made by the method of Kulifay [ 171 and with BET area of 0.38 m*/g as determined by krypton ad-

191

sorption. Previous work had shown that its activity is very reproducible [ 161 and the absence of a support is advantageous when feeding dichloroethane at the ppm level. Catalyst samples (0.3 to 1.5 g of particle size 300 to 600 pm), contained in a U-tube reactor of 4 mm I.D., were reduced in flowing hydrogen prior to use. Treatment for periods of 5 to 12 h at 270°C was quite sufficient to achieve stable initial activity even for samples previously exposed to dichloroethane. Reactant mixtures set up as described above were passed over the catalyst and the exit stream analysed by a three column gas chromatograph. As originally described [ 181 the system separated water, ethylene oxide, carbon dioxide, ethylene and oxygen (in that order in a single analysis) within 15 min. However it could also be used for determination of carbon monoxide by continuing the elution for a total period of 40 min. Hydrogen could be seen as a peak at 0.8 min but was not accurately quantifiable due to a maximum in its response curve. Catalyst activities are reported as the conversion rate of reactants to individual products calculated according to conversion rate/pm01 g ( Ag ) - ’ min- ’ = F,, Yi/n, where F,, is the molar flow rate of all components leaving the reactor, Yi is the mole fraction of product i and ni the number of moles of product per mole of reactant. Most measurements were made under differential conditions (conversion c 15% ) but it was necessary to start some runs with ethylene conversions approaching 50%. Conversion rate is then not a good approximation to the true rate but remains useful for comparing activities under similar conditions and for calculating the selectivity of ethylene oxide formation. This is defined by selectivity =

rate[C,H,+C,H,O] rate[CzHq+C2H40] +rate[C,H,+CO:,(or

H,O)]

The accuracy of the gas chromatograph for determination of carbon dioxide was much greater than for water due to the lower sensitivity for the latter and the tailing nature of its peak. Selectivity was calculated from the ethylene oxide and carbon dioxide analyses alone in all cases except those when large amounts of carbon monoxide were added. The water and ethylene oxide analyses were then used. It was very difficult to assess selectivity when both hydrogen and carbon monoxide were included in the feedstream. However the quantity of ethylene oxide being formed could still be determined accurately. RESULTS AND DISCUSSION

Initial experiments were aimed to show the effect of carbon monoxide on ethylene epoxidation over freshly reduced silver. Approximately 2% carbon monoxide (15Torr, 1 Torr = 133.3 Pa) was added to a stream containing 12% ethylene under conditions such that the initial ethylene conversion was ca. 50%. Results are shown in Fig. 1. It is clear that carbon monoxide oxidation is

Time

on

stream

(hours)

Fig. 1. Conversion and selectivity as a function of time during ethylene oxidation over 0.77 g of silver sponge at 239’ C in the presence of 15 Torr of carbon monoxide. Ethylene pressure of 89 Torr, oxygen pressure of 79 Torr balance helium with total flow-rate of 46 cm3 (STP) /min. (a), ethylene conversion; (b) carbon monoxide conversion; (c), selectivity of ethylene oxide formation.

much faster than ethylene oxidation and also that the catalyst steadily loses activity with time. The fall in ethylene conversion is quite steep during the period 10 to 25 h on stream, At the latter time carbon monoxide conversion is still 100% but thereafter it also falls steeply while ethylene conversion levels out. However throughout the course of the deactivation the selectivity of ethylene oxidation to ethylene oxide rises steadily (from 40% to 80% ). Table 1 summarises these effects together with subsequent measurements in which carbon monoxide was firstly deleted and then readmitted. The latter clearly show that carbon monoxide inhibits ethylene oxidation over the deactivated catalyst. The rate with carbon monoxide present (at 28.8 and 31.2 h) is only one-tenth that in the absence of carbon monoxide (at 29 and 31 h). This rate inhibition is pronounced since the average carbon monoxide pressure along the catalyst bed is less than one-tenth the average ethylene pressure. However it can be seen that inclusion of carbon monoxide does not affect the selectivity to ethylene oxide under these conditions. It is also apparent from the last three entries of Table 1 that the deactivated catalyst does recover slowly when ethylene oxidation is run continuously in the absence of carbon monoxide. However the activity is still only 60% of the initial value after 40 h. The

193 TABLE 1 Rates and selectivity during ethylene oxidation over silver sponge in the presence and absence of CP grade carbon monoxide Conditions as per Fig. 1. Time-onstream (h)

Input CO pressure (Torr)

0.4 28.8

15 15

100 53

29.0

31.0

nil nil

-

31.2 32.1

15 15

56 50

49.2

nil nil nil

-

52.1 74.9

CO conversion (%)

Conversion rates (pm01 (C,H4)g(Ag)-‘min-‘)

Selectivity to ethylene oxide (S)

to C&H,0

to CO, + H,O

47 1.0

90 0.3

34 78

3 4

82 77

0.2 0.2

a4 82

1.8 1.6 10

81 86 74

11 14 1.1 1.0 8 10 28

starting activity could be readily restored in full by hydrogen reduction. Treatment for 12h in a 5% hydrogen/helium mixture at 270°C was quite sufficient. As shown later, the reduction in rate and accompanying enhanced selectivity observed during ethylene oxidation in the presence of carbon monoxide corresponds quite closely to the behaviour induced by known chlorine containing catalyst moderators. The experiments of Fig. 1 were repeated using a carbon monoxide stream from which condensable impurities were removed by passage through two U-tube traps arranged in series and cooled by liquid nitrogen. Results are shown in Fig. 2 for a freshly reduced sample of silver sponge. The conversions of carbon monoxide and ethylene, and the selectivity to ethylene oxide, are now quite stable. This is despite the carbon monoxide pressure, and hence its flow, being almost twice that in the experiments of Fig. 1. Thus we conclude that the deactivation illustrated in Fig. 1 is due to trace quantities of impurities in the carbon monoxide used. Discussion of its likely identity and amount is given later. Similar experiments to those of Fig. 1 were carried out with hydrogen in place of carbon monoxide. Results are shown in Fig. 3. Hydrogen conversion was 100% throughout the 30-h test. Ethylene conversion was similarly constant at approximately 36% but the selectivity to ethylene oxide increased by a small amount over the 12-h test period (from 29% to 36%) No ethane was formed.

194

100

25,

5 Time

10 on

stream

15 (hours)

Fig. 2. Conversion and selectivity as a function of time-on-stream during ethylene oxidation over silver sponge in the presence of carbon monoxide purified by passage through U-tubes cooled to - 196°C (see text). Conditions as for Fig. 1 except for carbon monoxide pressure of 25 Torr. (a), ethylene conversion; (b ) , carbon monoxide conversion; (c ) , selectivity of ethylene oxide formation.

lb

5

0

Time

on stream

(hours)

Fig. 3. Conversion and selectivity as a function of time-on-stream during ethylene oxidation over silver sponge in the presence of 35 Torr of hydrogen. Conditions as for Figs. 1 and 2 except for hydrogen in place of carbon monoxide. (a ) , ethylene conversion; (b ) , hydrogen conversion; (c ) , selectivity to ethylene oxide.

195

It is clear from the foregoing that hydrogen and carbon monoxide are both oxidised much more rapidly than ethylene. The rates of the three reactions were determined separately as a function of temperature under differential conditions. Temperatures were sampled in random order with an intervening treatment of the catalyst with hydrogen during the period of each temperature rise. Results are shown in Fig. 4 as Arrhenius plots. Under the conditions used the oxidation of carbon monoxide and hydrogen have very similar rates and activation energies (44 kJ/mol and 43 kJ/mol respectively). Ethylene oxidation is much slower and exhibits higher activation energy (61 kJ/mol). For rate comparison purposes, each plot was extrapolated to 144” C. The rates of oxidation of ethylene, carbon monoxide and hydrogen were in the ratio 1: 20 : 22. This is indicative only since no adjustment has been made for the somewhat different reductant pressures used because kinetic orders are uncertain at the comparison temperature. Under the conditions used to obtain the data of Figs. 2 and 3, the added carbon monoxide and hydrogen are certainly consumed at the very front section of the catalyst bed leaving the remainder available for conversion of ethylene oxidation. Competition between reactants cannot be observed under such conditions. Tests for inhibition were therefore carried out at lower temperatures to spread the carbon monoxide and hydrogen consumption further along the catalyst bed. Results for reaction at 200°C are shown in Table 2. The in-

lOOOK

(Temperature)

Fig. 4. Arrhenius plots for oxidation over silver sponge. (a), carbon monoxide with carbon monoxide pressure of 15 Torr, oxygen pressure of 78 Torr with balance helium to total flow of 47 cm3 (STP) /min; (b ), hydrogen oxidation with hydrogen pressure of 20 Torr, oxygen pressure of 83 TOR with balance helium and total flow of 43 cm3(STP)/ mm; (c), ethylene oxidation with ethylene pressure of 39 Torr, oxygen pressure of 96 Torr with balance helium and total flow 30 cm3(STP)/min.

196

TABLE 2 Effect of added carbon monoxide and/or hydrogen on the rate of ethylene oxidation over 0.32 g of silver sponge at 200°C with balance helium to total flow rate of 46 cm3 (STP)/min Conversion rates (pmol(C,H,)g(Ag)-’

Conversion (%)

Input pressures (Torr) C,H,

02

CO

Hz

82 86 84 82 85 84

76 76 78 74 75 76

nil 10 nil

nil nil 26 26 nil 26

10

54 54

CO

H,

-

-

100 100

100 100

98

-

98

100

min-‘)

to C,H,O

to COz + H,O

37 21 18 13 8.5 4.6

41 23 a a 9 a

Selectivity to C,H,O (%) 47 47 a a 48 a

These values could not be determined accurately due to interference with the analysis caused by the large amount of water made from hydrogen.

elusion of 10 Torr of carbon monoxide or 26 Torr of hydrogen reduces the rate of formation of ethylene oxide by a factor of approximately two. With both added in these amounts the rate is about one-third of the base value. When the quantity of carbon monoxide added is raised to 54 Torr the rate reduction is a factor of five when hydrogen is absent and ten when it is present. It was not feasible to measure selectivity accurately during this series due to interference by the large amounts of carbon dioxide and/or water produced from carbon monoxide and hydrogen. However as far as could be judged it was fairly constant. In all these experiments, hydrogen and carbon monoxide conversion was very high. Only in those runs using 54 Torr of carbon monoxide did any significant amount of the added material emerge. The conclusion from the foregoing is that ethylene oxidation is only slightly affected if added carbon monoxide or hydrogen is rapidly consumed. If conditions are such that this process is spread throughout the bed then the rate of ethylene oxidation is reduced. The principal cause of the reduced rate must be direct competition between ethylene and added reductants for adsorption sites, or for oxygen adsorbed on the silver surface. Any kinetic effect due to a reduced oxygen pressure must be much less significant since oxidation of carbon monoxide and hydrogen removes at most one-half the oxygen in the experiments of Table 2. That is far too small to explain the five- or ten-fold reduction in the rate of ethylene oxide formation observed. The effect of adding 20 Torr of carbon dioxide on the rate of ethylene oxidation is shown in Fig. 5. The ethylene conversion was almost constant throughout the test (at about 40% ) and approximately that expected for reaction in the absence of additional carbon dioxide. However the distribution between partial and complete oxidation products did change slightly with time.

197

o7

10 Time

on

20 stream

30 (hours)

Fig. 5. The effect of adding 20 Torr of carbon dioxide on the rate of ethylene oxidation over 0.77 g of silver sponge at 239°C using ethylene pressure of 81 Torr, oxygen pressure of 75 Torr with balance helium and total flow of 45 cm3 (STP ) / min. (a), oxidation to carbon dioxide and water; (b), oxidation to ethylene oxide; (c), selectivity to ethylene oxide.

Ethylene oxide formation increased at the expense of total oxidation to carbon dioxide and water. In consequence the selectivity rose from 40 to 48% over the 24-h period of the test. Other groups have also observed that carbon dioxide can increase selectivity and attributed this to adsorbed carbonate species blocking some reaction sites at the surface [ 191. Industrially, ethylene oxide synthesis over silver catalysts is carried out in the presence of chlorine containing moderators in order to maximise selectivity. The effect of including carbon monoxide and hydrogen in the feed under such conditions was investigated in the following way. The silver sponge catalyst was reduced and the reaction started with an input stream in which part of the helium carrier was replaced by a second helium stream containing some dichloroethane (DCE). The dilution was such that the mixtures fed contained ca. 1.4 ppm DCE. Separate sets of experiments were carried out; one with ethylene and oxygen as reactants, a second with carbon monoxide added and a third with hydrogen included. The changes in ethylene conversion with timeon-stream for reaction at 239°C are shown in Fig. 6 and the corresponding changes in conversion of the added carbon monoxide or hydrogen are plotted in Fig. 7. The initial conversion of ethylene is similar in the three experiments as expected for a situation in which oxidation of carbon monoxide and hydro-

Time

on stream

(hours)

Fig. 6. Conversion and selectivity as a function of time during ethylene oxidation over 0.77 g of silver sponge in the presence of 1.4 ppm dichloroethane at 239°C. (a), ethylene pressure 85 Torr, oxygen pressure 80 torr and total flow of 47 cm’ (STP ) / min; (b), as for (a) but with 9.3 Torr of carbon monoxide included in feed; (c), as for (a) but with 33 Torr of hydrogen included in feed.

L

I I

bl a

I-.

I I I I I I I I

i-

c

0

5

10

Time on stream

20

2!

( hours)

Fig. 7. Conversion of carbon monoxide and hydrogen as a function of time during experiments of Fig. 6. (a), carbon monoxide conversion during run plotted as 6(b); (b), hydrogen conversion during run plotted as 6 (c ) .

199

gen is complete. The time course of each reaction is also similar. A steep fall in ethylene conversion commences at about 3 h on stream and plateaus out at 6 h. During this period the selectivity rose steeply to ca. 80% as explained below. Assuming complete uptake of all the chlorine in the DCE fed to that stage the coverage by chlorine is approximately one atom for every four or five surface silver atoms [ 161 at the plateau stage. A corresponding fall in carbon monoxide and hydrogen consumption was observable (Fig. 7) commencing at 5 h and flattening out after 10 h. The delay relative to ethylene oxidation is as expected if the uptake of chlorine is chromatographic. Oxidation of carbon monoxide and hydrogen is sufficiently fast that it will be complete as long as a small fraction of the rear section of the bed remains chlorine free. As shown in Table 3 differences do exist between the three experiments in respect of the behaviour obtained after ten hours operation. In the absence of carbon monoxide or hydrogen, ethylene conversion was ca. 1.5% and the selectivity was very high (ca. 83%). With hydrogen present conversion and selectivity were both somewhat lower (ca. 0.7 and 79% respectively). The reduction in selectivity may be attributable to a lesser coverage by chlorine when hydrogen is present. (Stripping of chlorine was quite rapid in Hz/He at the reaction temperature leading to restoration of the initial rate and selectivity on recommencement of reaction). The rate inhibition by hydrogen is relatively small, especially considering that hydrogen conversion itself was only ca. 3%. Inhibition seems to be less than in the earlier experiments with unmoderated silver (Table 2). Inclusion of hydrogen then halved the rate of oxidation of ethylene under conditions of total hydrogen consumption, The effect of carbon TABLE 3 Effect of added carbon monoxide or hydrogen on the rate of ethylene oxidation over silver with 1.4 ppm dichloroethane included in the feed stream Conditions as per Fig. 6. Chlorine on catalyst

CO(or Hz) pressure (Torr)

Negligible”

nil (nil) 9.3 (nil) nil (33)

Equilibriumb

nil (nil) 9.3 (nil) nil (33)

“Less than 0.5 h on stream. ‘More than 10 h on stream.

CO (or H,) conversion (%)

Conversion rates” (pmol(C,H,)g(Ag)-’ to C,H,O

min-‘)

to CO,+H,O

Selectivity to C&H,0 (a)

monoxide on ethylene oxidation under conditions of continuous DCE moderation was much more pronounced. Ethylene conversion dropped below the limit of detectability ( 2 0.1% ) after 6 h on stream. Two contributory causes may be considered. One is straight competition between ethylene and carbon monoxide under conditions such that > 90% of the input carbon monoxide is emerging unreacted. A second possibility arises because this particular experiment was carried out without scrubbing the carbon monoxide for impurities which could therefore cause deactivation. However, deactivation induced by impurities in the carbon monoxide (Fig. la) required a much longer time scale than that shown in Fig. 6c and is unlikely to be a major contributor to it. It is therefore concluded that the near complete suppression of ethylene oxidation by carbon monoxide on partially chlorine covered silver is a result of competition. An ensemble size effect may be operative since the site requirement for carbon monoxide adsorption is smaller than that for ethylene. The results obtained above suggests that it will be very difficult to obtain high yields of ethylene oxide in the presence of unreacted carbon monoxide. The limitation was further checked under conditions such that the ethylene oxidation rate remained measurable with carbon monoxide present. The quantity of silver was doubled, the ethylene pressure tripled and the temperature raised to 270°C. Table 4 gives results for periodic addition of 49 Torr of carbon monoxide (purified by passage through traps cooled with liquid nitrogen) during reaction over the catalyst after it had been equilibrated with chlorine by prior oxidation of ethylene with 1.9 ppm dichloroethane in the feed. The rate is one order of magnitude lower with carbon monoxide present and the selectivity is reduced from 80% to 50%. Deleterious effects can be anticipated under all conditions such that some carbon monoxide emerges unreacted. (Note that its conversion was ca. 20% during the above tests). The cause of the reduced selectivity to ethylene oxide is unclear. For unpromoted silver selectivity usually increases with oxygen pressure [ 10 ] and this has been attributed to an optimisation of the amount of subsurface oxygen which is believed to favour epoxidation over total oxidation [ 201. Reaction of carbon monoxide with surface oxygen may simultaneously lower the amount of the latter, and that of subsurface oxygen, thus inducing lower selectivity. It may be noted that the ethylene rich conditions used above correspond roughly to those used industrially in oxygen based plants except that the total pressure is lower [ 81. Hence ethylene oxide synthesis involving direct use of the ethylene contained in the product stream from methane coupling is likely to be impractical if the carbon oxide by-product in the stream includes much of the monoxide. A final point concerns the identity and amount of the impurity responsible for the deactivation shown in Fig. 1 when feeding carbon monoxide. The course of reaction is remarkably similar to that observed with 1.4 ppm dichloroethane in the feed (i.e. Fig. 6) except that the time period is longer by a factor of ca. 2.5. This suggests that the feed stream containing carbon monoxide may in-

201 TABLE 4 Effect of carbon monoxide on selectivity of ethylene oxidation over silver sponge equilibrated with 1.9 ppm dichloroethane”

co pressure (Torr) 0

49

0

49

Time-onstream (hours)

% Conversion of

Bate/pmol g (Ag))’ ethylene to

min-’

CO to CO,

Selectivity based on CO,

H,O

-

79 79

80 80

(0.38) (0.41) (0.40)

21 17 16

b b b

45 53 52

5.3 7.6

1.4 3.3

-

79 70

80 88

0.29

(0.35 (0.44) (0.40)

21 17 16

b b b

45 52 52

02

co

C,H,O

26.6 27.0

3.9 4.3

-

5.4 5.9

1.4 1.6

27.3 28.2 28.9

8.0 6.7 6.3

24 20 18

0.31 0.45 0.43

29.6

29.9

3.8 7.3

-

30.1 30.8 31.5

8.0 6.8 6.3

24 19 18

0.48 0.43

CO, ( H,O )

“At 270°C with 1.9 ppm dichloroethane, 245 Torr ethylene, 80 Torr oxygen, balance helium t.o total flow-rate of 46 cm3 (STP) /min over 1.52 g of silver sponge. ‘Not calculable due to interference from the large quantity of carbon dioxide produced by carbon monoxide oxidation.

elude a chlorine-containing impurity in an amount equivalent to about 0.5 ppm DCE. Hydrogen chloride, which readily deposits chlorine on silver catalysts [21], is the simplest possibility. Assuming complete decomposition, the deactivation evident in Fig. 1 could be explained if the 20% carbon monoxide/ helium mixture being used was contaminated with approximately 8 ppm hydrogen chloride. An attempt was made to confirm this using high resolution FTIR with 1 atm (= 101.325 kPa) of the starting 20% CO/He mixture contained in a multipass cell with an effective length of 8 m. The measurements were inconclusive. Whilst blank tests showed that hydrogen chloride at about the anticipated level was detectable they also showed that it interacted quite strongly with water on the walls of the cell leading to considerable uncertainty in the calibration. It is possible that an impurity not containing chlorine is responsible since the behaviour observed following prolonged exposure to carbon monoxide did differ from that produced by DCE in one respect. In the latter situation under ethylene-rich conditions (Table 4) added carbon monoxide lowered selectivity as well as activity whereas only activity was changed in measurements made after long term exposure to carbon monoxide (Fig. 1 and Table 1) .

202 CONCLUSIONS

(i) Under conditions of moderate to high conversion in a flow system ethylene oxide synthesis over unmoderated silver is little affected by added carbon monoxide or hydrogen. The latter are totally consumed in the front section of the catalyst bed leaving the remainder available for ethylene oxidation. This in accord with the relative oxidation rates of the three reductants when tested alone. (ii) A significant reduction in the rate of ethylene oxidation is observed under conditions such that carbon monoxide is incompletely oxidised. Unreacted carbon monoxide inhibits reaction of ethylene through direct competition for surface or adsorbed oxygen. Inhibition increases with carbon monoxide pressure but the selectivity to ethylene oxide versus carbon dioxide and water remains unchanged at 40 to 50%. (iii) The effect of including carbon monoxide in the feed is still more serious with high selectivity silver produced by exposure to dichloroethane. Under ethylene-rich conditions rate inhibition is similar to that with unmoderated silver but the selectivity to ethylene oxide is also lowered (from 80 to 50% ). ACKNOWLEDGEMENTS

This work was funded by a grant from the Australian Research Council. One of us (Y.S. Yong) has been supported by a Macquarie University Research Fellowship.

REFERENCES

8 9 10 11

12 13 14 15

G.E. Keller and M.M. Bhasin, J. Catal., 73 (1982) 9. T. Ito, J-X. Wang, C-H. Lin and J.H. Lunsford, J. Am. Chem. Sot., 107 (1985) 5062. J.S. Lee and S.Y. Oyama, CataI. Rev. Sci. Eng., 30 (1988) 249. IT. Ali Emesh and Y. Amenomiya, J. Phys. Chem., 90 (1986) 4785. K. Otsuka, K. Jinno and A. Morikawa, J. Catal., 100 (1986) 353. J.M. Deboy and RF. Hicks, Ind. Eng. Chem. Res., 27 (1988) 1577. CA. Jones, J.J. Leonard and J.A. Sofranko, Energ. Fuels, 1 (1987) 12. J.N. Cawse, J.P. Henry, M.W. Swartzlander and P.H. Wadia, Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd ed., Wiley-Interscience, New York, 1980, Vol. 9, p. 432. J.C. Zomerjick and M.W. Hall, Catal. Rev. Sci. Eng., 23 (1981) 163. R.A. van Santen and H.P.C.E. Kuipers, Adv. Catal., 35 (1987) 265. E.M. Carter and W.A. Goddard, Surf. Sci., 209 (1989) 243. G.W. Keulks and CC. Chang, J. Phys. Chem., 74 (1970) 2590. N.W. Cant and P.W. Fredrickson. J. Catal., 37 (1974) 531. G. Wurzbacher, J. Catal., 5 (1966) 476. V.E. Ostrovskii and N.N. Dobrovolskii, Proceedings Fourth International Congress on Catalysis, Moscow, 1968, Akademiai Kiado, Budapest, 1971, p. 9, Paper 46.

203 16 17 18 19 20 21

Y.S. Yongand N.W. Cant, Appl. Catal., 48 (1989) 37. S.M. Kulifay, J. Am. Chem. Sot., 83 (1961) 4916. Y.S. Yong, K.J. Soerensen and N.W. Cant, J. Chromatogr., 450 (1988) 399. S.A. Tan, R.B. Grant and R.M. Lambert, Appl. Catal., 31 (1987) 159. R.A. van Santen and C.P.M. de Groat, J. Catal., 98 (1986) 530. P.A. Kilty, N.C. Rol. and W.M.H. Sachtler, in J.W. Hightower (Editor), Proceedings, Fifth International Congress on Catalysis, Palm Beach, 1972, North-Holland, 1973, p. 929.