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Thermooscillations during the complete oxidation of ethylene oxide over a supported silver catalyst
Applied Catalysis, 39 (1988) Ll-L5 Elsevier Science Publishers B.V., Amsterdam -
Ll Printed in The Netherlands
Thermooscillations During the Complete Oxidation of Ethylene Oxide over a Supported Silver Catalyst A. ELIYAS and L. PETROV* Institute of Kinetics and Catalysis, Bulgarian Academy 11, 1113 Sofia (Bulgaria)
of Sciences, Str. Acad. G. Bonchev bl.
(Received 21 January 1988, accepted 17 February 1988)
INTRODUCTION
The complete oxidation of ethylene oxide over a supported Ag/A1203 catalyst has been studied independently and separately from ethene oxidation [ 1,2]. During a steady-state kinetic study in the absence of the industrial promoter of selectivity, ethylene dichloride, autooscillations of the catalyst bed temperature were observed. [ 11. This result is interesting as no oscillations during this reaction have been reported in the literature. According to Stoukides and Vayenas [ 3 1, who studied rate oscillations during propylene oxide oxidation over silver, such oscillations will not be observed during the catalytic oxidation of ethylene, propene and ethylene oxide under the same conditions. The aim of the present letter is to describe briefly the conditions and the form of these thermooscillations, which were observed for the first time during the catalytic oxidation of ethylene oxide. EXPERIMENTAL
Catalyst
The catalyst studied contains about 20 wt.% of silver, supported on a-alumina. Silica was used as a binding compound. The specific surface area of the catalyst is 0.14 m2/g. The catalyst pellets are spherical with a diameter of ca. 6 mm. The silver is supported on the uneven outer geometrical surface of the pellet. The specific surface area of the supported silver is 0.05 m2/g. Other characteristics of the catalyst are listed in refs. 4 and 5. As the overall surface area and the surface morphology of the catalyst are of
0166-9834/88/$03.50
0 1988 Elsevier Science Publishers B.V.
Fig. 1. Morphology of the supported silver catalyst surface at 7400 X magnification. The micrograph was made using a “Superprobe 733” JEOL electron microscope at 25 kV by the SE1 method.
fundamental importance for the reproduction of the oscillation, it is necessary to characterize the surface in detail. The electron spectroscopy for chemical analysis (ESCA) study of the catalyst showed 3-5 eV splitting of the Ag 3d5,2 photoelectron band when the catalyst pellet is in a metal sample holder. This indicates that part of the silver is in the form of islets that have no electric contact with each other or with the sample holder. The rest of the silver is in the form of continuum. The islets and the continuum can both be seen on the micrograph made by an electron microscope Superprobe 733 (JEOL) (Fig. 1). The magnification is 7400 x .
L3
The laboratory flow-circulation reactor [ 41 was charged with 25 g of catalyst, corresponding to 1.25 m2 total silver surface area. Experimental conditions The reaction was studied at a temperature above 300” C, which is of potential interest for the industry as this is a region of high risk. The experiments were conducted at atmospheric pressure and the feed composition was varied as follows: 16.7% 02, 33.3% C2H40, 50.0% Ar (1:2) 25.0% 02, 25.0% C2H40, 50.0% Ar (1:l) 33.3% 02, 16.7% C2H40, 50.0% Ar (2:l) The feed composition is denoted by the ratio oxygen:ethylene oxide. The ethylene oxide contact time varied from 20 s to 2 min. The inlet oxygen and ethylene oxide partial pressures varied from 0.15 to 0.35 atm (15 to 35 kPa) whereas that of argon was practically constant at 0.45 atm (45 kPa). The maximum degree of conversion of ethylene oxide measured under these conditions was about 60%. The highest temperature reached was 370” C. RESULTS AND DISCUSSION
The catalyst bed temperature can be controlled up to 300’ C by a thermoregulator at the indicated contact times. Above this limit the reaction displays a tendency to pass over to an autothermic regime where the temperature is maintained by the heat of the reaction and no additional heating is required. It was under such conditions in the temperature interval 310-335°C that the thermooscillations began when the reagent flow-rates were changed. The form of the oscillations is comparatively simple (see Fig. 2). With some minor exceptions (Fig. 2e) it can be approximated to a regular sinusoid. The period of the oscillations varies from 7 to 38 min, while the amplitude varies from 3% to 19% of the average temperature value. The oscillatory period and the amplitude change with the feed composition and the contact time independently of each other. This is an essential difference from the interconnected change of these quantities, reported in ref. 3, for the rate oscillations during the catalytic oxidation of propylene oxide. In our case oscillations with close period values - 12 to 14 min (Fig. Ba,c,f,g) -had quite a different amplitude: 2.7% to 9.5%. At close amplitude values - 12.2% and 15.2% (Fig. 2d,e) - the period may be quite different: 38 and about 7 min, respectively. The large amplitude oscillations (Fig. 2b,d,e) were observed at the longer contact times of 1 to 2 min. With the average temperature increase from 310 to 355 ‘C the amplitude of the oscillations decreased. The richer the oxygen content of the feed, the smaller is the period. Stoukides and Vayenas [ 31 did not observe ethylene oxide oxidation oscil-
L4
294’C
c
346T 324’C
FZ
\-
70min
8Omin
B
Fig. 2. Form of thermooscillations during ethylene oxide oxidation over a supported silver catalyst; (a) feed composition l:l, ethylene oxide contact time 20 s, period of oscillations 14 min, amplitude 2.7% ofthe average temperature value, (b) 1:2,2 min, 36 min, 19.0%, (c) l:l, 1 min, 12 min, 9.5%, (d) 1:2,1 min, 38 min, 12.2%, (e) 2:1,2 min, about 7 min, up to l&2%, (f) l:l, 20 s, 12 min, 3.3%, (g) l:l, 40 s, 14 min, 5.8%, (h) 2:1,40 s, 9 min, 5.5%, (i) 1:2,40 s, 23 min, 6.4%.
lations, probably because of the low partial pressure ( 10B3 bar) and smaller silver total surface area - 2000 cm2 (compared with 0.15-0.35 atm and 12 500 cm2 in our experiments). A periodic formation of a polymeric surface residue that is followed by its combustion has been proposed [ 31 to explain the oscillations during the oxidation of propylene oxide on silver. The possibility that periodic oxidation-reduction of the silver surface causes the oscillations is rejected. The fact that we observed oscillations during ethylene oxide oxidation shows that the rejection of the latter hypothesis is hasty. In view of the mechanism of ethylene oxide oxidation over silver that we proposed earlier [ 1 ] a third explanation of the oscillations is also possible. The silver surface is periodically covered by molecular adsorbed oxygen, Z02, and then by atomic adsorbed oxygen, ZO. Both forms participate in the complete oxidation of ethylene oxide [ 11. Further research work on this reaction should be focused on discriminating between these three possible oscillation mechanisms during the catalytic oxidation of ethylene oxide.
L5 REFERENCES 1 2 3 4 5
L. Petrov, A. Eliyas, Chr. Maximov and D. Shopov, Appl. Catal., in press. L. Petrov, A. Eliyas, Chr. Maximov and D. Shopov, Appl. Catal., in press. M. Stoukides and C.G. Vayenas, J. Cat& 74 (1982) 266. L. Petrov, A. Eliyas and D. Shopov, Appl. Catal., 18 (1985) 87. L. Petrov, A. Eliyas and D. Shopov, Appl. Catal., 24 (1986) 145.
Ll Printed in The Netherlands
Thermooscillations During the Complete Oxidation of Ethylene Oxide over a Supported Silver Catalyst A. ELIYAS and L. PETROV* Institute of Kinetics and Catalysis, Bulgarian Academy 11, 1113 Sofia (Bulgaria)
of Sciences, Str. Acad. G. Bonchev bl.
(Received 21 January 1988, accepted 17 February 1988)
INTRODUCTION
The complete oxidation of ethylene oxide over a supported Ag/A1203 catalyst has been studied independently and separately from ethene oxidation [ 1,2]. During a steady-state kinetic study in the absence of the industrial promoter of selectivity, ethylene dichloride, autooscillations of the catalyst bed temperature were observed. [ 11. This result is interesting as no oscillations during this reaction have been reported in the literature. According to Stoukides and Vayenas [ 3 1, who studied rate oscillations during propylene oxide oxidation over silver, such oscillations will not be observed during the catalytic oxidation of ethylene, propene and ethylene oxide under the same conditions. The aim of the present letter is to describe briefly the conditions and the form of these thermooscillations, which were observed for the first time during the catalytic oxidation of ethylene oxide. EXPERIMENTAL
Catalyst
The catalyst studied contains about 20 wt.% of silver, supported on a-alumina. Silica was used as a binding compound. The specific surface area of the catalyst is 0.14 m2/g. The catalyst pellets are spherical with a diameter of ca. 6 mm. The silver is supported on the uneven outer geometrical surface of the pellet. The specific surface area of the supported silver is 0.05 m2/g. Other characteristics of the catalyst are listed in refs. 4 and 5. As the overall surface area and the surface morphology of the catalyst are of
0166-9834/88/$03.50
0 1988 Elsevier Science Publishers B.V.
Fig. 1. Morphology of the supported silver catalyst surface at 7400 X magnification. The micrograph was made using a “Superprobe 733” JEOL electron microscope at 25 kV by the SE1 method.
fundamental importance for the reproduction of the oscillation, it is necessary to characterize the surface in detail. The electron spectroscopy for chemical analysis (ESCA) study of the catalyst showed 3-5 eV splitting of the Ag 3d5,2 photoelectron band when the catalyst pellet is in a metal sample holder. This indicates that part of the silver is in the form of islets that have no electric contact with each other or with the sample holder. The rest of the silver is in the form of continuum. The islets and the continuum can both be seen on the micrograph made by an electron microscope Superprobe 733 (JEOL) (Fig. 1). The magnification is 7400 x .
L3
The laboratory flow-circulation reactor [ 41 was charged with 25 g of catalyst, corresponding to 1.25 m2 total silver surface area. Experimental conditions The reaction was studied at a temperature above 300” C, which is of potential interest for the industry as this is a region of high risk. The experiments were conducted at atmospheric pressure and the feed composition was varied as follows: 16.7% 02, 33.3% C2H40, 50.0% Ar (1:2) 25.0% 02, 25.0% C2H40, 50.0% Ar (1:l) 33.3% 02, 16.7% C2H40, 50.0% Ar (2:l) The feed composition is denoted by the ratio oxygen:ethylene oxide. The ethylene oxide contact time varied from 20 s to 2 min. The inlet oxygen and ethylene oxide partial pressures varied from 0.15 to 0.35 atm (15 to 35 kPa) whereas that of argon was practically constant at 0.45 atm (45 kPa). The maximum degree of conversion of ethylene oxide measured under these conditions was about 60%. The highest temperature reached was 370” C. RESULTS AND DISCUSSION
The catalyst bed temperature can be controlled up to 300’ C by a thermoregulator at the indicated contact times. Above this limit the reaction displays a tendency to pass over to an autothermic regime where the temperature is maintained by the heat of the reaction and no additional heating is required. It was under such conditions in the temperature interval 310-335°C that the thermooscillations began when the reagent flow-rates were changed. The form of the oscillations is comparatively simple (see Fig. 2). With some minor exceptions (Fig. 2e) it can be approximated to a regular sinusoid. The period of the oscillations varies from 7 to 38 min, while the amplitude varies from 3% to 19% of the average temperature value. The oscillatory period and the amplitude change with the feed composition and the contact time independently of each other. This is an essential difference from the interconnected change of these quantities, reported in ref. 3, for the rate oscillations during the catalytic oxidation of propylene oxide. In our case oscillations with close period values - 12 to 14 min (Fig. Ba,c,f,g) -had quite a different amplitude: 2.7% to 9.5%. At close amplitude values - 12.2% and 15.2% (Fig. 2d,e) - the period may be quite different: 38 and about 7 min, respectively. The large amplitude oscillations (Fig. 2b,d,e) were observed at the longer contact times of 1 to 2 min. With the average temperature increase from 310 to 355 ‘C the amplitude of the oscillations decreased. The richer the oxygen content of the feed, the smaller is the period. Stoukides and Vayenas [ 31 did not observe ethylene oxide oxidation oscil-
L4
294’C
c
346T 324’C
FZ
\-
70min
8Omin
B
Fig. 2. Form of thermooscillations during ethylene oxide oxidation over a supported silver catalyst; (a) feed composition l:l, ethylene oxide contact time 20 s, period of oscillations 14 min, amplitude 2.7% ofthe average temperature value, (b) 1:2,2 min, 36 min, 19.0%, (c) l:l, 1 min, 12 min, 9.5%, (d) 1:2,1 min, 38 min, 12.2%, (e) 2:1,2 min, about 7 min, up to l&2%, (f) l:l, 20 s, 12 min, 3.3%, (g) l:l, 40 s, 14 min, 5.8%, (h) 2:1,40 s, 9 min, 5.5%, (i) 1:2,40 s, 23 min, 6.4%.
lations, probably because of the low partial pressure ( 10B3 bar) and smaller silver total surface area - 2000 cm2 (compared with 0.15-0.35 atm and 12 500 cm2 in our experiments). A periodic formation of a polymeric surface residue that is followed by its combustion has been proposed [ 31 to explain the oscillations during the oxidation of propylene oxide on silver. The possibility that periodic oxidation-reduction of the silver surface causes the oscillations is rejected. The fact that we observed oscillations during ethylene oxide oxidation shows that the rejection of the latter hypothesis is hasty. In view of the mechanism of ethylene oxide oxidation over silver that we proposed earlier [ 1 ] a third explanation of the oscillations is also possible. The silver surface is periodically covered by molecular adsorbed oxygen, Z02, and then by atomic adsorbed oxygen, ZO. Both forms participate in the complete oxidation of ethylene oxide [ 11. Further research work on this reaction should be focused on discriminating between these three possible oscillation mechanisms during the catalytic oxidation of ethylene oxide.
L5 REFERENCES 1 2 3 4 5
L. Petrov, A. Eliyas, Chr. Maximov and D. Shopov, Appl. Catal., in press. L. Petrov, A. Eliyas, Chr. Maximov and D. Shopov, Appl. Catal., in press. M. Stoukides and C.G. Vayenas, J. Cat& 74 (1982) 266. L. Petrov, A. Eliyas and D. Shopov, Appl. Catal., 18 (1985) 87. L. Petrov, A. Eliyas and D. Shopov, Appl. Catal., 24 (1986) 145.