- Email: [email protected]
Study of the effect of carbon dioxide on the catalytic properties of a silver catalyst in the oxidation of ethylene to ethylene oxide
Applied Catalysis, 10 (1984) 303-312 Elsevier Science Publishers B.V., Amsterdam -Printed
STUDY OF THE EFFECT OF CARBON
IN THE OXIDATION
CATALYST
Beulah GRIFFEayb Chemistry
DIOXIDE
303 in The Netherlands
ON THE CATALYTIC
OF ETHYLENE
TO ETHYLENE
The University,
OXIDE
Whiteknights
aTo whom correspondence should be addressed b Present address: Centro de Quimica, Instituto
(Received
OF A SILVER
, Ernest BLUES and Dereck BRYCE-SMITH
Department,
Cientificas,
PROPERTIES
I.V.I.C.,
11 October
Apartado
Postal
1983, accepted
Park, Reading
Venezolano
1827, Caracas
28 February
RG6 ZAD, U.K.,
de Investigaciones
1010-A, Venezuela.
1984)
ABSTRACT The influence of pretreatment of a silver catalyst with carbon dioxide (CO ) at high temperature in the air oxidation of ethylene (E) to ethylene oxide (E6 ) was studied separately and in conjunction with ethylene dichloride (ED). This study was done by determining the conversion (C) and selectivity (S) of the silver catalyst by gas-chromatography. The experimental results show that the pretreatment of all the silver catalysts investigated with CO at 260°C for 64 h invariably results in some deactivation of the catalysts an 5 usually in an increase in selectivity (S).
INTRODUCTION The air oxygen as catalyst the addition
of ethylene
of water and/or
ing the complete increasing
oxidation
was first reported
oxidation
CO2 to the reacting
of ethylene
the yield of ethylene
Margolis
(E) to ethylene
in a patent to Lefort
the electron
occur on the silver surface when the reactants promoters dioxide
and moderators
are electron
found that similar impurities acceptor Therefore
acceptors,
Cl, I, S, Se, etc.,
impurities
by Lefort. work function
and products
on the electron
ance of the silver catalyst
interesting
with and without
0166-9834/84/$03.00
They
by acceptor
work function.
These
on the silver catalyst.
to study the effect of CO2 on the perform-
in the oxidation
of E to EO.
In the present work a study of the influence of pretreatment with CO2 at high temperature
and when
work function.
work function
the selectivity
which
Thus oxygen and carbon
i.e. they raise the electron
are known to increase
it was considered
changes
are adsorbed,
i.e. they raise the electron
is exerted
of suppress-
and water and thereby
are added to the silver catalyst.
effect
(EO) over silver
gases for the purpose
to carbon dioxide
oxide was described
et al. [2,3] have reported
oxide
(1931) Cl]. In this patent
of the non-promoted
of the catalyst
and some promoted
EDC in the ox.idation of E to EO is presented.
0 1984 Elsevier Science Publishers B.V.
silver catalysts
304 TABLE
1
Catalytic
performance
of a standard
T/"C
unpromoted
silver catalyst
%C
%S
170
18.0
180
25.6
51.4
190
35.9
49.9
53.3
200
50.0
47.0
210
71.8
43.7
220
93.0
38.0
TABLE
2
Catalytic
performance
after CO2 treatment
T/"C
%C
170
13.0
of a standard
silver catalyst
%S 55.3
180
19.7
51.7
190
28.7
49.3
210
55.9
45.3
220
77.6
41.6
EXPERIMENTAL The catalyst
used for the present work was prepared
cribed by Bryce-Smith Promoted ences
according
and Blues [4] and which has been already
silver catalysts
were prepared
[5,6]. The procedure
by the standard
and equipment
method
used were exactly
to a method
des-
reported
[5,6].
outlined
in refer-
as previously
reported
C5,61. The general
procedure
ance of silver catalysts established
pursued
in determining
was as follows:
under standard conditions
then each catalyst
was heated under a slow flow
addition
using the standard
of each catalyst
(170 - 235°C) with EOC (except where
period of 64 h at 260°C. The performance the treatment,
the effect of CO2 on the perform-
the performance
(6.6 ml min-')
of each catalyst
5% E in air mixture,
was stated);
of pure CO2 for a
was redetermined
but without
after
any further
of EDC.
A further deactivation
investigation was reversible
36 h under the standard
was conducted by heating
E/air mixture
to see whether
the catalyst (5%
or not the effect of CO2
at 260°C for a period of 20 to
E).
RESULTS AND DISCUSSION The experimental investigated
results
show that the pretreatment
by CO2 invariably
results
of all the silver catalysts
in some deactivation
of the catalysts
and
I
20
40
60
80
1
100
20
%C
FIGURE
1
FIGURE 2
usually
between
treated;
in an increase
The results catalysts
ide (EDC).
metal
of the effect.
two samples of multipromoted
that the effects
influenced
by the presence
salts and by treatment
as shown by the results quoted
The results obtained
contained: respect moted
and no significant
in Tables
dichlor-
pretreatment effect
on its
1 and 2.
of CO2 pretreatment similar
of the silver
and are additive
of a multipromoted
as can
silver catalyst
which
Na, 680 ppm; K, 200 ppm; Fe, 40 ppm; Ba, 30 ppm; Cl, 180 ppm; with
to the weight
Figure
of alkali and
with ethylene
in the catalyst,
of EDC are qualitatively
1 and 2 with samples
of the silver
of trace amounts
of the catalyst
show that the effects
and the effects
be seen from Figures
(Na, K, Fe, Ba, Ca, Cl)
of CO2 pretreatment
with CO2 has only a minor effect on its activity
catalysts
%S vs %C.
in selectivity.
In the absence of such trace additives
selectivity,
I
100
1 - EDC + C02; 2 - CO2 + EDC.
also indicate
are markedly
alkaline-earth
I
80
(Na, K, Fe, Ba, Ca, Cl) silver catalysts
of E to EO and reversibility
Comparison
silver catalyst
60
%C
CO2 effect on a multipromoted
in the oxidation
40
of silver
1 illustrates
silver catalyst.
almost deactivated
on the so-called
It can be seen that after this treatment
but very high selectivities
After deactivation with E alone,
(5% Ag/A1203).
the effect of C02-pretreatment
of the catalyst
multipro-
the catalyst
iS
are achieved.
by EDC and C02, it was treated
in sequence
5% E in air and air alone, all at 190°C for a period of thirty min-
utes in each experiment. ed., Then the catalyst
No significant
change
in catalyst
was heated under the standard
3 h at 260°C the same temperature
mixture
as in the treatment
performance
was observ-
of air + E (5% E) for
under CO2 (Curve
A
in
Figure
o Multipromoted
plus EDC plus CO2
X Multipromoted
plus CO2 plus EDC
d Li promoted
plus EDC plus CO2
0 Li promoted
plus CO2 plus EDC
/ 40
I 20
701 0
I 60
%C
FIGURE 3
Comparison
catalyst
of the catalytic
in the oxidation
FIGURE 4
Comparison
1).
The catalyst
silver catalysts
ditions
lyst. This conclusion experiments
chloride.
selective
is also in accordance
silver catalyst
formance was as showed
with literature
con-
information
[7] and
that this could also have been removed,
and could account than originally.
the Figure
for the catalyst To establish
the treated
to depend on the EDC already
initially
catalysts
under CO2 and that the effects
and then the perthat
added as chloride
under the standard
mixture
by CO2 of the catalyst
added to the catalyst
that the chlorine
being less
if this was
1. It was then demonstrated
by CO2. EDC and chlorine
by heating
ment with CO2 and therefore heating
1. It may be
had also been added as in-
of 5% E in air at 260°C. The extent of the deactivation
during
Figure
under the foregoing
as EDC was added to the catalyst
in curveeof
of deactivation
thus appears
SC.
20 h under the standard
some chlorine
presume
if not completely
so, a small amount of chlorines
the effects
vs
from the EDC added to be removed from the cata-
at the highest temperatures
are fully reversibly
of E to EO. %S
in curveoin
at high temperatures
One may therefore
at least partially
(Na, K, Fe, Ba,
[5,6].
In this multipromoted organic
are illustrated
the catalyst
also causes the chlorine
previous
in the oxidation
was then heated at 260°C for a further
that heating
silver
of E to EO, treated with CO2 and EDC. %S vs %C.
and the results obtained
concluded
of the lithium promoted
of the effect of CO2 on the multipromoted
Ca, Cl) and Li promoted
mixture
performance
is not removed
before the treat-
from the catalyst
of CO2 and EDC on catalyst
activity
are additive. Figure 2 illustrates
that the effects
was noted that the order of treatment treatment 2 effect on the performance whether
the CO
preceded
of EDC and CO2 are additive,
of the catalyst
or followed
treatment
of the treated catalyst.
although
it
by CO2 and by EDC i.e. by EDC, does have a minor
With the multipromoted
catalyst
307
TABLE 3 Catalytic
performance
of the K promoted
silver catalyst
+ EDC added before treat-
ment with CO2.
T/"C
%C
%S
170
7.0
85.6
190
16.0
81.9
210
32.7
78.8
235
62.0
73.7
TABLE 4 Catalytic
performance
after CO2 treatment
%C
T/"C
of the K promoted
silver catalyst
%S
171
1.8
87.3
190
4.1
81.1
210
9.5
81.1
235
20.3
76.2
TABLE 5 Catalytic
performance
of the Be promoted
silver catalyst
without
EDC before the
CO2 treatment
T/"C
%C
%S
160
12.6
49.9
170
17.7
49.3
180
25.5
46.4
190
37.3
44.3
200
59.4
43.7
220
79.6
34.0
TABLE 6 Catalytic
performance
of the Be promoted
silver catalyst
T/"C
%C
160
12.0
50.8
170
17.6
48.5
190
33.7
42.3
200
55.2
34.7
220
84.1
32.3
%S
after the CO2 treatmen
308 TABLE 7 Catalytic
performance
after 4 h of heating
of the Be-promoted under the mixture
silver catalyst
of E/air
T/"C
SC
160
14.0
64.7
170
18.5
59.6
190
35.4
55.9
57.0
52.3
220
76.7
50.4
(5% E) at 26OOC.
order of treatment
was EDC followed
by CO2.
That the effects of CO2 and EDC are additive tained with samples of the Li promoted These catalysts
with CO2
"/OS
200
the optimum
pretreated
is also shown by the results ob-
catalysts
(266 ppm Li w.r.t.
Ag weight).
were treated with EOC and then CO2 and also in the reverse order
with CO2 and then with EDC and in both cases a very high selectivity achieved
lysts a better performance
was reached with the sample treated
then with EDC than with the sample treated with the former the slightly
the very high selectivity
A third sample of the lithium-promoted a high performance
with silver promoted should be mentioned
first with EDC and then with CO2. Thus of 94% is obtained
catalysts
catalyst
was achieved,
and with the latter
was deactivated
it was inferior
but with EDC
to that obtained
treated with both EDC and CO2. At this stage it
that catalysts
much less EDC to achieve treatment
cata-
first with CO2 and
lower figure of 90.5% is obtained.
only. Although
optimum
treated with CO2 and then with EDC required
performances
than when the reverse order of
was employed.
It is interesting promoted
(> 90%) is
as can be seen from Figure 3. In the case of the lithium-promoted
catalyst
to note that the optimum
order of treatment
is the reverse of that of the multipromoted
of the lithium-
catalyst
(see Figure
4). The effects
of CO2 pretreatment
K w.r.t,
silver weight)
promoted
catalyst
of a potassium-promoted
were essentially
Catalyst
the same as those observed
except that they were less marked
(1,503 ppm of with the Li
as can be deduced
from Tables
3 and 4. A silver catalyst w.r.t.
silver),
of the Be promoted any marked Attempts catalyst
promoted
responded
change
with a group IIA
differently
catalyst
without
in conversion
beryllium
EDC led to a decrease
in Selectivity
effect of CO2 pretreatment
and unusual
result:
without
5 and 6.
it at 260°C for 4 h under the standard
air gave another different
(345 PPm of Be
with CO2. Thus, Pretreatment
as can be seen in Tables
to reverse the adverse
by heating
metal,
to pretreatment
of the Be promoted
mixture
of 5% E in
the S improved markedly
without
309
%C
FIGURE 5
Comparison
of the effect of CO2 on different
promoted
silver catalysts
(treated first with EDC and then with CO2). %S vs %C. any decrease Treatment
in the C as can be seen in Table 7. by CO2 of a Be-promoted
catalyst
ed in a decrease
in C without
the expected
The foregoing
observations
suggest
largely
(the so-called
but gives, together
multipromoted
with the lithium-promoted
by prolonged
The only study similar to Tollefson
treatment
The catalyst
alloy catalyst,
catalyst without
is conditioned
containing
a process
the highest
additives
ation, and maintains
that conversion
selectivities
are affected
to in the literature
in which a silver-alkaline
by flowing
and selectivity
several
is the most deactivated
carbon dioxide
to
to ED within
a relatively
and selectivity
is a patent earth metal
at temperatures
260°C and 350°C. He stated that with such a treatment
its peak conversion
with CO2
with CO2.
to the above referred
[83 who described
with EDC result-
of treatment
silver catalyst)
as can be seen in Figure 5. Silver catalysts a lesser extent
moderated
in S.
that the effects
depend on the nature of the additives.
additives
between
previously
increase
ranging
the catalyst
achieved
short time of oper-
to EO over long periods of
time. Recently
Mikami
et al. [9,10] reported
lysts. When carbon dioxide selectivity
for the following
The adsorption of the metal
ethylene
pulse experiments
on the oxygenated
produced
the changes
ion of the components
by it have been studied in electron
of ethylene
gases such as O2 and CO
surface and the selectivity
with Ag/d-A1203 catalyst
cata-
the epoxidation
pulse was enhanced.
of CO2 on silver metal and the change
surface
[ill has described
acceptor
was adsorbed
oxidation
in electronic
by previous
work function
properties
workers.
which accompany
and of additives.
Margolis
adsorpt-
He stated that
increase the electron work function on the silver 2 of the catalyst, whereas the electron work function
310
Lie8
0
19,
II
O-C:
OTC? 4
FIGURE 6
Different
IV
ates:
kinds of adsorption
I axis of the COP molecule
molecule
interacts
IV carbonate
Ag
4 III
face:
k.
with adsorbed structure
of the CO2 molecule
parallel
on the catalyst
to the metal surface;
oxygen atoms to form mono and bidentate
like structure
3 but adsorbed
sur-
II and III COP carbon-
on to adjacent
silver
atoms. is decreased
by adsorption
of donor gases such as ethylene
CO2 and EO at high temperatures of oxygen and metalloid the surface
activity
to rise. However, More recently ize the adsorbed oxidation
impurities.
Thus,
would be expected
this effect
species
in the presence
of carbon dioxide
on the catalyst
is parallel
spectrum
surface.
to the metal
oxygen atoms to form mono and bidentate Figure 6. They assumed
mode of adsorption carbonate
Taking
into account
the mechanism
proposed
on silver catalyst the formation through
such as II
to a greater
change
of EO is accepted,
it supports
the selectivity
of selectivity
the
two kinds
in Struct-
with adsorbed
as indicated
by Structures
of C of standard
II
[I53 that the primary
to form mono and bidentate pressures
I and II. reported
[10,16,17]
by these authors
and if
that O- species adsorbed
the theory that the adsorption
of the catalysts
can be explained
sites to form Structure
in %S.
information
by Kilty and Sachtler
sites. The slight decrease
Ag'+ active
as indicated
and that at higher CO2 partial
extent as Structures
this valuable
during
indicated
react with E to form C02, and that O2 species contributes
O- increases
provement
and III,
to character-
interacts
by Czanderna
of CO2 is through the O- species
structures
it is adsorbed
impurities.
In one of these modes the
surface
carbonates
as proposed
selectivity
spectroscopy
present on the surface of a silver catalyst of the infrared
of
to that
of these substances
than that of metalloid
ure I, Figure 6. In the second mode, the CO2 molecule
and III,
is similar
to fall off and the reaction
is less marked
interpretation
axis of the CO2 molecule
and that the effects
work function
Force et al. [IL?-141 have used infrared
of E. Their
of adsorption
on the electron
unpromoted
by removing
"combustion"
silver catalyst
as due to adsorption
to of COP
without
im-
of COP on Ag or
I, which will produce a decrease
in %C without
311
In agreement
with the above,
Metcalf
S from 57 to 75% as the partial atm, while
the partial
The present
pressures
study however
CO2 at a high temperature increasing
of the reactants
involved
obtained
surface
temperatures
suggest
used in oxidizing
indicate
that it involves
the silver surface. a decrease
traces of the group when pretreated A partial
IIA metal,
explanation
A possible
is manifested
beryllium,
IA additive
and activity
mechanism
ation of adjacent
dicted
selectivity
a limiting
proposed
sites on the silver surface
previously
of ethylene proposed
which
and that this change alters
in selectivity
and decrease
by CO2 pretreatment
of the catalyst
as in Structure
by Kilty and Sachtler as is proposed
resulting
IV
C16,171, occup-
by US will result
in an increase
by Kilty and Sachtler
in
in the selectivi-
in 1972 C16,171 pre-
of 6/7 or 86% with appropriate
moderation
of the
with chlorine.
the mechanistic
significance
of the very high selectivity
with CO2 treated multipromoted
and lithium
is that these values appear
[16,17] that selectivities
are always
However
it must be said that Sachtler
passing
this limit of monoatomic
with C2H4 can efficiently
recently
Sachtler
oxygen
recombine
i.e. subsurface is the reactant
resulting
silver catalysts
maximum
to form new superoxide
that an interplay
at
theory
of 86%.
the possibility
from the interaction
oxygen must transform
proper.
of 90-94%
Kilty and Sachtler's
et al. [163 admitted
et al. [lo] suggested
is essential,
promoted
to disprove
limited to a theoretical
oxygen,
adsorbed
chemisorbed
a similar change
caused by COP pretreatment
silver atoms on the silver surface
ty. The mechanism
oxygen
on
and
observed.
increase
ion produced
to the mechanism
of the combustion
low conversion
present
in selectivity
from an oxide to a carbonate
in the manner
on to adjacent
obtained
results
of pure silver or silver containing
in the catalyst
for the observed
is adsorbed
Figure 6. According
Finally
IA elements
do not appear to undergo
is converted
is that the carbonate
silver catalyst
of E
but the experimental group
on the silver surface
in activity
a decrease
in the
at the lower
itself.
by an increase
consisting
of the change
its mode of adsorption
the selectivity
in a change
in the presence
used in the pretreatment
Catalysts
with
rather than
with C02.
is that the group alters
resulted
traces of electropositive
in conversion.
catalyst,
is at least semipermanent
has not been determined,
This change
in
gases.
E to ED, but is reversible
and air at the higher temperatures The nature of this change
of the silver catalysts
it as an oxidation
that this treatment
and that this change
an increase
from 0.004 to 0.36
were held constant.
pretreatment
before using
[18] reported
of CO2 was increased
the level of CO2 in the reacting
The results catalyst
and Harriot
pressure
of
of 0;
ions 0;. More
of two kinds of adsorbed Ag atoms to Ag+, while
312
CONCLUSIONS 1.
Heating
of group
silver catalysts
IA metals
ivity and decreases 2.
The effects
promoted 3.
of E to EO, containing
trace amounts
under CO2 at 260°C for 64 h increases
their select-
their activity.
of CO2 pretreatment
silver catalysts
Under optimum
chloride
for the oxidation
as promoters,
and of treatment
with ethylene
dichloride
on
are additive.
conditions,
of a silver catalyst
CO2 pretreatment containing
in conjunction
with ethylene
266 ppm of lithium enables
di-
selectivities
of up to 94% to be achieved. 4.
The effect of CO2 pretreatment
varies markedly 5.
The effect of CO2 pretreatment
catalysts
of the catalysts
at 260°C for 20-30 h, under the standard
air), but appears 6.
of a silver catalyst
Treatment
selectivity
containing
a metal promoter
with the nature of the metal promoter.
to be at least semi-permanent
with CO2 of a pure silver catalyst
or activity
are reversed ethylene/air
at temperatures
by heating mixture
the
(5% E in
< 235°C.
has no significant
effect on the
of the catalyst.
REFERENCES
1 T.E. Lefort, U.S.A. Patent 1,998,878 (1935); and French Patent 729,952 (1931). 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
E. Kh. Enikeev, O.V. Isaev and L. Ya. Margolis, Kinet. Catal., (USSR) (English Trans., ) 1 (1960) 402. L. Ya. Margolis, E. Kh. Enikeev, O.V. Isaev, A.V. Krylova and M. Ya. Kushnerov, Kinet. Catal., (USSR) (English Trans.,) 3 (1962) 153. D. Bryce-Smith and E.T. Blues, British Patent 1,436,185 (1976). B. Griffe, Ph.D. Thesis, Reading University (1978). B. Griffe, E. Blues and D. Bryce-Smith, submitted to Applied Catal., E.T. McBee, H.B. Hass and P.A. Wiseman, Ind. Eng. Chem., 37 (1945) 432. E.L. Tollefson, U.S.A. Patent, 2,686,762 (1954). J. Mikami, Sh. Satoh and H. Kobayashi, J. Catal., 18 (1970) 265. W.M.H. Sachtler, C. Backx and R.A. Van Santen, Catal. Rev. -Sci. Eng., 23 (1981) 127. L. Ya. Margolis, Adv. Catal., 14 (1963) 429. E.L. Force and A.T. Bell, J. Catal., 40 (1975) 356. E.L. Force and A.T. Bell, J. Catal., 38 (1975) 444. E.L. Force and A.T. Bell, J. Catal., 44 (1976) 175. A.W. Czanderna, J. Colloid. Interface Sci., 22 (1966) 482. P.A. Kilty, N.C. Rol and W.M.H. Sachtler in "Proceedings of the Fifth International Congress on Catalysis", Paper 67A, North Holland, Amsterdam (1972). P.A. Kilty and W.M.H. Sachtler, Catal. Rev. -Sci. Eng., 10 (1974) 1. P.L. Metcalf and P. Harriot, I. and E.C. Proc. Des. Develop., 11 (1972) 478.
STUDY OF THE EFFECT OF CARBON
IN THE OXIDATION
CATALYST
Beulah GRIFFEayb Chemistry
DIOXIDE
303 in The Netherlands
ON THE CATALYTIC
OF ETHYLENE
TO ETHYLENE
The University,
OXIDE
Whiteknights
aTo whom correspondence should be addressed b Present address: Centro de Quimica, Instituto
(Received
OF A SILVER
, Ernest BLUES and Dereck BRYCE-SMITH
Department,
Cientificas,
PROPERTIES
I.V.I.C.,
11 October
Apartado
Postal
1983, accepted
Park, Reading
Venezolano
1827, Caracas
28 February
RG6 ZAD, U.K.,
de Investigaciones
1010-A, Venezuela.
1984)
ABSTRACT The influence of pretreatment of a silver catalyst with carbon dioxide (CO ) at high temperature in the air oxidation of ethylene (E) to ethylene oxide (E6 ) was studied separately and in conjunction with ethylene dichloride (ED). This study was done by determining the conversion (C) and selectivity (S) of the silver catalyst by gas-chromatography. The experimental results show that the pretreatment of all the silver catalysts investigated with CO at 260°C for 64 h invariably results in some deactivation of the catalysts an 5 usually in an increase in selectivity (S).
INTRODUCTION The air oxygen as catalyst the addition
of ethylene
of water and/or
ing the complete increasing
oxidation
was first reported
oxidation
CO2 to the reacting
of ethylene
the yield of ethylene
Margolis
(E) to ethylene
in a patent to Lefort
the electron
occur on the silver surface when the reactants promoters dioxide
and moderators
are electron
found that similar impurities acceptor Therefore
acceptors,
Cl, I, S, Se, etc.,
impurities
by Lefort. work function
and products
on the electron
ance of the silver catalyst
interesting
with and without
0166-9834/84/$03.00
They
by acceptor
work function.
These
on the silver catalyst.
to study the effect of CO2 on the perform-
in the oxidation
of E to EO.
In the present work a study of the influence of pretreatment with CO2 at high temperature
and when
work function.
work function
the selectivity
which
Thus oxygen and carbon
i.e. they raise the electron
are known to increase
it was considered
changes
are adsorbed,
i.e. they raise the electron
is exerted
of suppress-
and water and thereby
are added to the silver catalyst.
effect
(EO) over silver
gases for the purpose
to carbon dioxide
oxide was described
et al. [2,3] have reported
oxide
(1931) Cl]. In this patent
of the non-promoted
of the catalyst
and some promoted
EDC in the ox.idation of E to EO is presented.
0 1984 Elsevier Science Publishers B.V.
silver catalysts
304 TABLE
1
Catalytic
performance
of a standard
T/"C
unpromoted
silver catalyst
%C
%S
170
18.0
180
25.6
51.4
190
35.9
49.9
53.3
200
50.0
47.0
210
71.8
43.7
220
93.0
38.0
TABLE
2
Catalytic
performance
after CO2 treatment
T/"C
%C
170
13.0
of a standard
silver catalyst
%S 55.3
180
19.7
51.7
190
28.7
49.3
210
55.9
45.3
220
77.6
41.6
EXPERIMENTAL The catalyst
used for the present work was prepared
cribed by Bryce-Smith Promoted ences
according
and Blues [4] and which has been already
silver catalysts
were prepared
[5,6]. The procedure
by the standard
and equipment
method
used were exactly
to a method
des-
reported
[5,6].
outlined
in refer-
as previously
reported
C5,61. The general
procedure
ance of silver catalysts established
pursued
in determining
was as follows:
under standard conditions
then each catalyst
was heated under a slow flow
addition
using the standard
of each catalyst
(170 - 235°C) with EOC (except where
period of 64 h at 260°C. The performance the treatment,
the effect of CO2 on the perform-
the performance
(6.6 ml min-')
of each catalyst
5% E in air mixture,
was stated);
of pure CO2 for a
was redetermined
but without
after
any further
of EDC.
A further deactivation
investigation was reversible
36 h under the standard
was conducted by heating
E/air mixture
to see whether
the catalyst (5%
or not the effect of CO2
at 260°C for a period of 20 to
E).
RESULTS AND DISCUSSION The experimental investigated
results
show that the pretreatment
by CO2 invariably
results
of all the silver catalysts
in some deactivation
of the catalysts
and
I
20
40
60
80
1
100
20
%C
FIGURE
1
FIGURE 2
usually
between
treated;
in an increase
The results catalysts
ide (EDC).
metal
of the effect.
two samples of multipromoted
that the effects
influenced
by the presence
salts and by treatment
as shown by the results quoted
The results obtained
contained: respect moted
and no significant
in Tables
dichlor-
pretreatment effect
on its
1 and 2.
of CO2 pretreatment similar
of the silver
and are additive
of a multipromoted
as can
silver catalyst
which
Na, 680 ppm; K, 200 ppm; Fe, 40 ppm; Ba, 30 ppm; Cl, 180 ppm; with
to the weight
Figure
of alkali and
with ethylene
in the catalyst,
of EDC are qualitatively
1 and 2 with samples
of the silver
of trace amounts
of the catalyst
show that the effects
and the effects
be seen from Figures
(Na, K, Fe, Ba, Ca, Cl)
of CO2 pretreatment
with CO2 has only a minor effect on its activity
catalysts
%S vs %C.
in selectivity.
In the absence of such trace additives
selectivity,
I
100
1 - EDC + C02; 2 - CO2 + EDC.
also indicate
are markedly
alkaline-earth
I
80
(Na, K, Fe, Ba, Ca, Cl) silver catalysts
of E to EO and reversibility
Comparison
silver catalyst
60
%C
CO2 effect on a multipromoted
in the oxidation
40
of silver
1 illustrates
silver catalyst.
almost deactivated
on the so-called
It can be seen that after this treatment
but very high selectivities
After deactivation with E alone,
(5% Ag/A1203).
the effect of C02-pretreatment
of the catalyst
multipro-
the catalyst
iS
are achieved.
by EDC and C02, it was treated
in sequence
5% E in air and air alone, all at 190°C for a period of thirty min-
utes in each experiment. ed., Then the catalyst
No significant
change
in catalyst
was heated under the standard
3 h at 260°C the same temperature
mixture
as in the treatment
performance
was observ-
of air + E (5% E) for
under CO2 (Curve
A
in
Figure
o Multipromoted
plus EDC plus CO2
X Multipromoted
plus CO2 plus EDC
d Li promoted
plus EDC plus CO2
0 Li promoted
plus CO2 plus EDC
/ 40
I 20
701 0
I 60
%C
FIGURE 3
Comparison
catalyst
of the catalytic
in the oxidation
FIGURE 4
Comparison
1).
The catalyst
silver catalysts
ditions
lyst. This conclusion experiments
chloride.
selective
is also in accordance
silver catalyst
formance was as showed
with literature
con-
information
[7] and
that this could also have been removed,
and could account than originally.
the Figure
for the catalyst To establish
the treated
to depend on the EDC already
initially
catalysts
under CO2 and that the effects
and then the perthat
added as chloride
under the standard
mixture
by CO2 of the catalyst
added to the catalyst
that the chlorine
being less
if this was
1. It was then demonstrated
by CO2. EDC and chlorine
by heating
ment with CO2 and therefore heating
1. It may be
had also been added as in-
of 5% E in air at 260°C. The extent of the deactivation
during
Figure
under the foregoing
as EDC was added to the catalyst
in curveeof
of deactivation
thus appears
SC.
20 h under the standard
some chlorine
presume
if not completely
so, a small amount of chlorines
the effects
vs
from the EDC added to be removed from the cata-
at the highest temperatures
are fully reversibly
of E to EO. %S
in curveoin
at high temperatures
One may therefore
at least partially
(Na, K, Fe, Ba,
[5,6].
In this multipromoted organic
are illustrated
the catalyst
also causes the chlorine
previous
in the oxidation
was then heated at 260°C for a further
that heating
silver
of E to EO, treated with CO2 and EDC. %S vs %C.
and the results obtained
concluded
of the lithium promoted
of the effect of CO2 on the multipromoted
Ca, Cl) and Li promoted
mixture
performance
is not removed
before the treat-
from the catalyst
of CO2 and EDC on catalyst
activity
are additive. Figure 2 illustrates
that the effects
was noted that the order of treatment treatment 2 effect on the performance whether
the CO
preceded
of EDC and CO2 are additive,
of the catalyst
or followed
treatment
of the treated catalyst.
although
it
by CO2 and by EDC i.e. by EDC, does have a minor
With the multipromoted
catalyst
307
TABLE 3 Catalytic
performance
of the K promoted
silver catalyst
+ EDC added before treat-
ment with CO2.
T/"C
%C
%S
170
7.0
85.6
190
16.0
81.9
210
32.7
78.8
235
62.0
73.7
TABLE 4 Catalytic
performance
after CO2 treatment
%C
T/"C
of the K promoted
silver catalyst
%S
171
1.8
87.3
190
4.1
81.1
210
9.5
81.1
235
20.3
76.2
TABLE 5 Catalytic
performance
of the Be promoted
silver catalyst
without
EDC before the
CO2 treatment
T/"C
%C
%S
160
12.6
49.9
170
17.7
49.3
180
25.5
46.4
190
37.3
44.3
200
59.4
43.7
220
79.6
34.0
TABLE 6 Catalytic
performance
of the Be promoted
silver catalyst
T/"C
%C
160
12.0
50.8
170
17.6
48.5
190
33.7
42.3
200
55.2
34.7
220
84.1
32.3
%S
after the CO2 treatmen
308 TABLE 7 Catalytic
performance
after 4 h of heating
of the Be-promoted under the mixture
silver catalyst
of E/air
T/"C
SC
160
14.0
64.7
170
18.5
59.6
190
35.4
55.9
57.0
52.3
220
76.7
50.4
(5% E) at 26OOC.
order of treatment
was EDC followed
by CO2.
That the effects of CO2 and EDC are additive tained with samples of the Li promoted These catalysts
with CO2
"/OS
200
the optimum
pretreated
is also shown by the results ob-
catalysts
(266 ppm Li w.r.t.
Ag weight).
were treated with EOC and then CO2 and also in the reverse order
with CO2 and then with EDC and in both cases a very high selectivity achieved
lysts a better performance
was reached with the sample treated
then with EDC than with the sample treated with the former the slightly
the very high selectivity
A third sample of the lithium-promoted a high performance
with silver promoted should be mentioned
first with EDC and then with CO2. Thus of 94% is obtained
catalysts
catalyst
was achieved,
and with the latter
was deactivated
it was inferior
but with EDC
to that obtained
treated with both EDC and CO2. At this stage it
that catalysts
much less EDC to achieve treatment
cata-
first with CO2 and
lower figure of 90.5% is obtained.
only. Although
optimum
treated with CO2 and then with EDC required
performances
than when the reverse order of
was employed.
It is interesting promoted
(> 90%) is
as can be seen from Figure 3. In the case of the lithium-promoted
catalyst
to note that the optimum
order of treatment
is the reverse of that of the multipromoted
of the lithium-
catalyst
(see Figure
4). The effects
of CO2 pretreatment
K w.r.t,
silver weight)
promoted
catalyst
of a potassium-promoted
were essentially
Catalyst
the same as those observed
except that they were less marked
(1,503 ppm of with the Li
as can be deduced
from Tables
3 and 4. A silver catalyst w.r.t.
silver),
of the Be promoted any marked Attempts catalyst
promoted
responded
change
with a group IIA
differently
catalyst
without
in conversion
beryllium
EDC led to a decrease
in Selectivity
effect of CO2 pretreatment
and unusual
result:
without
5 and 6.
it at 260°C for 4 h under the standard
air gave another different
(345 PPm of Be
with CO2. Thus, Pretreatment
as can be seen in Tables
to reverse the adverse
by heating
metal,
to pretreatment
of the Be promoted
mixture
of 5% E in
the S improved markedly
without
309
%C
FIGURE 5
Comparison
of the effect of CO2 on different
promoted
silver catalysts
(treated first with EDC and then with CO2). %S vs %C. any decrease Treatment
in the C as can be seen in Table 7. by CO2 of a Be-promoted
catalyst
ed in a decrease
in C without
the expected
The foregoing
observations
suggest
largely
(the so-called
but gives, together
multipromoted
with the lithium-promoted
by prolonged
The only study similar to Tollefson
treatment
The catalyst
alloy catalyst,
catalyst without
is conditioned
containing
a process
the highest
additives
ation, and maintains
that conversion
selectivities
are affected
to in the literature
in which a silver-alkaline
by flowing
and selectivity
several
is the most deactivated
carbon dioxide
to
to ED within
a relatively
and selectivity
is a patent earth metal
at temperatures
260°C and 350°C. He stated that with such a treatment
its peak conversion
with CO2
with CO2.
to the above referred
[83 who described
with EDC result-
of treatment
silver catalyst)
as can be seen in Figure 5. Silver catalysts a lesser extent
moderated
in S.
that the effects
depend on the nature of the additives.
additives
between
previously
increase
ranging
the catalyst
achieved
short time of oper-
to EO over long periods of
time. Recently
Mikami
et al. [9,10] reported
lysts. When carbon dioxide selectivity
for the following
The adsorption of the metal
ethylene
pulse experiments
on the oxygenated
produced
the changes
ion of the components
by it have been studied in electron
of ethylene
gases such as O2 and CO
surface and the selectivity
with Ag/d-A1203 catalyst
cata-
the epoxidation
pulse was enhanced.
of CO2 on silver metal and the change
surface
[ill has described
acceptor
was adsorbed
oxidation
in electronic
by previous
work function
properties
workers.
which accompany
and of additives.
Margolis
adsorpt-
He stated that
increase the electron work function on the silver 2 of the catalyst, whereas the electron work function
310
Lie8
0
19,
II
O-C:
OTC? 4
FIGURE 6
Different
IV
ates:
kinds of adsorption
I axis of the COP molecule
molecule
interacts
IV carbonate
Ag
4 III
face:
k.
with adsorbed structure
of the CO2 molecule
parallel
on the catalyst
to the metal surface;
oxygen atoms to form mono and bidentate
like structure
3 but adsorbed
sur-
II and III COP carbon-
on to adjacent
silver
atoms. is decreased
by adsorption
of donor gases such as ethylene
CO2 and EO at high temperatures of oxygen and metalloid the surface
activity
to rise. However, More recently ize the adsorbed oxidation
impurities.
Thus,
would be expected
this effect
species
in the presence
of carbon dioxide
on the catalyst
is parallel
spectrum
surface.
to the metal
oxygen atoms to form mono and bidentate Figure 6. They assumed
mode of adsorption carbonate
Taking
into account
the mechanism
proposed
on silver catalyst the formation through
such as II
to a greater
change
of EO is accepted,
it supports
the selectivity
of selectivity
the
two kinds
in Struct-
with adsorbed
as indicated
by Structures
of C of standard
II
[I53 that the primary
to form mono and bidentate pressures
I and II. reported
[10,16,17]
by these authors
and if
that O- species adsorbed
the theory that the adsorption
of the catalysts
can be explained
sites to form Structure
in %S.
information
by Kilty and Sachtler
sites. The slight decrease
Ag'+ active
as indicated
and that at higher CO2 partial
extent as Structures
this valuable
during
indicated
react with E to form C02, and that O2 species contributes
O- increases
provement
and III,
to character-
interacts
by Czanderna
of CO2 is through the O- species
structures
it is adsorbed
impurities.
In one of these modes the
surface
carbonates
as proposed
selectivity
spectroscopy
present on the surface of a silver catalyst of the infrared
of
to that
of these substances
than that of metalloid
ure I, Figure 6. In the second mode, the CO2 molecule
and III,
is similar
to fall off and the reaction
is less marked
interpretation
axis of the CO2 molecule
and that the effects
work function
Force et al. [IL?-141 have used infrared
of E. Their
of adsorption
on the electron
unpromoted
by removing
"combustion"
silver catalyst
as due to adsorption
to of COP
without
im-
of COP on Ag or
I, which will produce a decrease
in %C without
311
In agreement
with the above,
Metcalf
S from 57 to 75% as the partial atm, while
the partial
The present
pressures
study however
CO2 at a high temperature increasing
of the reactants
involved
obtained
surface
temperatures
suggest
used in oxidizing
indicate
that it involves
the silver surface. a decrease
traces of the group when pretreated A partial
IIA metal,
explanation
A possible
is manifested
beryllium,
IA additive
and activity
mechanism
ation of adjacent
dicted
selectivity
a limiting
proposed
sites on the silver surface
previously
of ethylene proposed
which
and that this change alters
in selectivity
and decrease
by CO2 pretreatment
of the catalyst
as in Structure
by Kilty and Sachtler as is proposed
resulting
IV
C16,171, occup-
by US will result
in an increase
by Kilty and Sachtler
in
in the selectivi-
in 1972 C16,171 pre-
of 6/7 or 86% with appropriate
moderation
of the
with chlorine.
the mechanistic
significance
of the very high selectivity
with CO2 treated multipromoted
and lithium
is that these values appear
[16,17] that selectivities
are always
However
it must be said that Sachtler
passing
this limit of monoatomic
with C2H4 can efficiently
recently
Sachtler
oxygen
recombine
i.e. subsurface is the reactant
resulting
silver catalysts
maximum
to form new superoxide
that an interplay
at
theory
of 86%.
the possibility
from the interaction
oxygen must transform
proper.
of 90-94%
Kilty and Sachtler's
et al. [163 admitted
et al. [lo] suggested
is essential,
promoted
to disprove
limited to a theoretical
oxygen,
adsorbed
chemisorbed
a similar change
caused by COP pretreatment
silver atoms on the silver surface
ty. The mechanism
oxygen
on
and
observed.
increase
ion produced
to the mechanism
of the combustion
low conversion
present
in selectivity
from an oxide to a carbonate
in the manner
on to adjacent
obtained
results
of pure silver or silver containing
in the catalyst
for the observed
is adsorbed
Figure 6. According
Finally
IA elements
do not appear to undergo
is converted
is that the carbonate
silver catalyst
of E
but the experimental group
on the silver surface
in activity
a decrease
in the
at the lower
itself.
by an increase
consisting
of the change
its mode of adsorption
the selectivity
in a change
in the presence
used in the pretreatment
Catalysts
with
rather than
with C02.
is that the group alters
resulted
traces of electropositive
in conversion.
catalyst,
is at least semipermanent
has not been determined,
This change
in
gases.
E to ED, but is reversible
and air at the higher temperatures The nature of this change
of the silver catalysts
it as an oxidation
that this treatment
and that this change
an increase
from 0.004 to 0.36
were held constant.
pretreatment
before using
[18] reported
of CO2 was increased
the level of CO2 in the reacting
The results catalyst
and Harriot
pressure
of
of 0;
ions 0;. More
of two kinds of adsorbed Ag atoms to Ag+, while
312
CONCLUSIONS 1.
Heating
of group
silver catalysts
IA metals
ivity and decreases 2.
The effects
promoted 3.
of E to EO, containing
trace amounts
under CO2 at 260°C for 64 h increases
their select-
their activity.
of CO2 pretreatment
silver catalysts
Under optimum
chloride
for the oxidation
as promoters,
and of treatment
with ethylene
dichloride
on
are additive.
conditions,
of a silver catalyst
CO2 pretreatment containing
in conjunction
with ethylene
266 ppm of lithium enables
di-
selectivities
of up to 94% to be achieved. 4.
The effect of CO2 pretreatment
varies markedly 5.
The effect of CO2 pretreatment
catalysts
of the catalysts
at 260°C for 20-30 h, under the standard
air), but appears 6.
of a silver catalyst
Treatment
selectivity
containing
a metal promoter
with the nature of the metal promoter.
to be at least semi-permanent
with CO2 of a pure silver catalyst
or activity
are reversed ethylene/air
at temperatures
by heating mixture
the
(5% E in
< 235°C.
has no significant
effect on the
of the catalyst.
REFERENCES
1 T.E. Lefort, U.S.A. Patent 1,998,878 (1935); and French Patent 729,952 (1931). 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
E. Kh. Enikeev, O.V. Isaev and L. Ya. Margolis, Kinet. Catal., (USSR) (English Trans., ) 1 (1960) 402. L. Ya. Margolis, E. Kh. Enikeev, O.V. Isaev, A.V. Krylova and M. Ya. Kushnerov, Kinet. Catal., (USSR) (English Trans.,) 3 (1962) 153. D. Bryce-Smith and E.T. Blues, British Patent 1,436,185 (1976). B. Griffe, Ph.D. Thesis, Reading University (1978). B. Griffe, E. Blues and D. Bryce-Smith, submitted to Applied Catal., E.T. McBee, H.B. Hass and P.A. Wiseman, Ind. Eng. Chem., 37 (1945) 432. E.L. Tollefson, U.S.A. Patent, 2,686,762 (1954). J. Mikami, Sh. Satoh and H. Kobayashi, J. Catal., 18 (1970) 265. W.M.H. Sachtler, C. Backx and R.A. Van Santen, Catal. Rev. -Sci. Eng., 23 (1981) 127. L. Ya. Margolis, Adv. Catal., 14 (1963) 429. E.L. Force and A.T. Bell, J. Catal., 40 (1975) 356. E.L. Force and A.T. Bell, J. Catal., 38 (1975) 444. E.L. Force and A.T. Bell, J. Catal., 44 (1976) 175. A.W. Czanderna, J. Colloid. Interface Sci., 22 (1966) 482. P.A. Kilty, N.C. Rol and W.M.H. Sachtler in "Proceedings of the Fifth International Congress on Catalysis", Paper 67A, North Holland, Amsterdam (1972). P.A. Kilty and W.M.H. Sachtler, Catal. Rev. -Sci. Eng., 10 (1974) 1. P.L. Metcalf and P. Harriot, I. and E.C. Proc. Des. Develop., 11 (1972) 478.