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The activity and state of the copper surface in methanol synthesis catalysts
Applied Catalysis, 25 (1986) 101-107 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
101
THE ACTIVITY AND STATE OF THE COPPER SURFACE IN METHANOL SYNTHESIS CATALYSTS G.C. CHINCHEN', K.C. WAUGH2 and D.A. WHAN' ‘Imperial
Chemical
Industries, plc, Agricultural Division, Billingham, Cleveland,
TS23 lLB, U.K. 2 Imperial Chemical Industries, plc, New Science Group, Runcorn Heath, Cheshire, WA7 4QE, U.K.
ABSTRACT The measurement of copper metal surface areas by monitoring nitrous oxide chemisorotion is a well established technique. A frontal chromatographic version of this technique has been developed which is very suitable for in situ measurements and this has enabled the apparent copper areas of various catalysts to be measured after exposures to methanol synthesis gases of different compositions at typical industrial conditions in microreactors cormnonlyused for assessing the methanol synthesis activity of such catalysts. Using such techniques, it has been shown that, first, there is a linear relationship between the methanol synthesis activity of copper/zinc oxide/alumina catalysts and their total copper surface area. Second, that copper supported on other materials has approximately the same turnover number as copper/zinc oxide/alumina catalysts. Third, that under industrial conditions the copper Surface of the catalyst is partially oxidised, to an extent which depends on the composition of the synthesis gas, particularly the CO2/CO ratio. INTRODUCTION Industrial methanol synthesis catalysts are generally based on CuO/ZnO/A1203 or CuO/ZnO/Cr203 compositions and, whilst the process operating at TO - 100 bar
- _
pressure and in the temperature range 230 - 300°C is well established, many aspects of the mechanism of synthesis are still not fully understood and are the subject of continued debate. In particular there is still controversy as to the exact nature of the active centre and more specifically about the chemical state of the individual components of the catalyst, especially the copper under typical methanol synthesis conditions. Under industrial conditions (50 bar, 250°C) using methanol synthesis feeds containing both CO and CO2 over a Cu/ZnO/A1203 catalyst several workers Cl,23 have shown by using labelled carbon oxides that all of methanol is formed from the CO2 component of the feed rather than the CO. The major reaction involved is therefore CO2 + 2H2
+
CH30H + O(s)
0166-9834/86/$03.50
8
1966 Elaevier Science Publishen B.V.
(0
102 where O(s) is a surface
oxygen
atom,
and it is likely
that a role of the CO and H, L
is to scavenge
this surface
oxygen,
by reactions
such as
co + O(s)
+
co*
(2)
H2 + O(s)
+
H20
(3)
The oxidation
state of the catalyst
be controlled
by the relative
believe diagram copper
that the detailed in Figure surface
to an extent
surface
during
rates of reactions
mechanism
be covered
determined
with formate,
by the kinetics
synthesis
(I), (2) and
of methanol
synthesis
1 [3], and it is a consequence
will
methanol
(3). We further
is represented
of this mechanism
formyl,
and relative
will then
methoxy
by the
that the
species
and oxygen
rates of the various
reaction
steps.
FIGURE
1 Mechanism
of methanol
synthesis
on a Cu/ZnO/A1203
catalyst
from a CO,
C02, H2 feed.
The present
work
area and methanol to examine by oxides copper
sets out
synthesis
(i) to examine activity
the same relationship other
component
gas compositions.
than zinc oxide, of these
the relationship
in a series
for catalysts
of Cu/ZnO/A1203
in which
and (iii) to assess
catalysts
at steady
between
catalysts,
the copper
the oxygen
copper
(ii)
is supported
coverage
state for a variety
metal
of the
of synthesis
103
EXPERIMENTAL The apparatus metal
in which
area measurements
synthesis
the methanol
were made
rate measurements
240°C) with
synthesis
synthesis
is shown
gases containing
experimental
rig is totally
The copper
metal
After
(typically
5% H2/N2)
to near ambient
the helium
is then established
or a thermal
frontal
at a steady,
in this way can be calibrated
gas stream
measured
continuously detector.
is shown
by measurements
frontal
a flow of helium (typically
between
flow rate in place of
for N2 and N20 with a mass A typical
in Figure
The
in a H2/N2 mixture
is swept out with
A N20/He
conductivity
chromatogram"
of "reactive
had been reduced
the reactor
temperature.
and the exit gas monitored
spectrometer reactive
at 24O"C,
area
temperature.
PET computers.
by the technique
the catalyst
(50 bar,
the CoPPer
and ambient
by two Commodore
areas were evaluated
[4-61.
and cooled
2 and 6% N20)
controlled
pressure
2. The methanol
conditions
both CO and CO2, whilst
are made at near atmospheric
and the copper
in Figure
industrial
are made under typical
measurements
chromatography"
rate measurements
schematically
line shape
3. Copper
metal
on polycrystalline
- "a
areas
determined
copper
combined
with in situ BET area measurements. Copper
area measurements
to methanol synthesis above.
synthesis
can also be made on catalysts
conditions,
gas from the reactor
Measurement
been exposed of oxygen
of the apparent
to methanol
atom coverage
the copper
has occurred.
by rereducing the CoPPer
by first depressurising
with a stream copper
synthesis
surface
conditions
of the copper The latter
the catalyst
surface
at steady
RESULTS
and sweeping
after
measure
providing
gas mixture
of this technique N20/H2
out the as
the catalyst
a direct
state
have been exposed
and then proceeding
has
of the degree
no sintering
may be assessed
with the H2/N2
area. A full description
surface
area
gives
possibility
c4], and it has been shown that sequential copper
of helium
which
of
and allowed
for
and then redetermining
has been given elsewhere
treatments
do not affect
the
area.
AND DISCUSSION
Information surface
obtained
under methanol
catalysts
is presented
of the Cu/ZnO/A1203 Differing
copper
by varying
in Table
areas
content,
supported supported
used
for a variety
include
on a variety
commercial
of alternative
on a given oxide
and by using catalysts A number
and state of the copper
(50 bar, 240°C)
1. The catalysts
for catalysts
of reaction.
the activity
conditions
type and copper
the copper
to a long period
in this way about synthesis
of differing
which
of materials oxides.
have been achieved
have been subjected
feed gas compositions
was
also used.
Activity/copper
area relationship
The dependence is shown
in Figure
of the methanol
synthesis
4 for all the catalysts
activity
on the copper
used. There
is found
metal
area
to be an extremely
N20
IN He
P Pressure
Pa Compute1 r controlled valve
‘VALVE
OVEN
TO OVEN
P”RGLE
Apparatus for the measurement of methanol synthesis rates and of copper metal areas and total surface areas.
2z
HELIUM
5% N2 IN He
10%
CALIBRATION MIXTURE
FIGWE 2
DIOXIDE
5% ti2 IN. N2
)ti
1w
CARBON
HELIUM
HYDROGEN
SHUTDOWN
Concentration of gas phase s@?CXS
_____-_-
- - -
-
Response of Katharometer
-MSSS
Katharometer +_____-----
spectrometer signal
-
Chati
FIOW swilch in
FIGURE 3
speed
= i En’8 m&i1
Time m minutes
"Reactive frontal chromatogram" for the reaction of N20 with poly-
crystalline copper.
significant
correlation between methanol synthesis activity and copper area
fop these materials and the turnover number: the slope of Figure 4 is essential -1 site-'). The identical for all the catalysts (1.6 x to-* molecules CH30H s conclusions to be drawn are that at1 the copper is active and only the copper surface is implicated in the rate determining step of synthesis; no unique synergy attaches to the copper/zinc oxide combination, supports as diverse as MgO and Si02 have similar effects. Oxygen coverage of copper during methanol synthesis By measuring the apparent copper area after methanol synthesis reaction and comparing it to the area before reaction, the degree of oxygen coverage of the copper surface can be deduced. The fact that this is oxygen coverage and not sintering of the copper is demonstrated by rereducing the catalyst and remeasuri the copper area (Tab'le1).
From the results obtained on both Cu/ZnO/A1203 com-
positions and on a Cu/A1203 material (Table 11, it is clear that the copper is oxidised to between 25% and 40% of a monolayer (50 - 80% of a monolayer of 0~~0) depending on the ethanol
synthesis feed gas composition. Using this data and
some of our previously published data [4], Figure 5 has been assembled which shows that the steady state oxygen coverage of the copper surface under methanol synthesis conditions for both Cu~ZnO~Al~O3 and Cu/A1203 catalysts is essentially a linear function of the CO,/CO ratio in the feed, over the range of values covered. We conclude that CO is therefore the dominant reducing agent for the surface under reaction conditions,
106 Methanol synthesis Actwity/Mol s-1 g-1 xl 06 12.0 ll.OlO.O9.0 8.07.06.0 -
Cu0/2n0/A1203(60:30:10)
SO-
CuOIZnOIA12O3(45:37:18)
0
CuO/MnO
.
CuO/ZnO
30
20
40
Cu Metal Area/m2g-1
FIGURE
4
Methanol
synthesis
activity
as a function
of copper
metal
area.
Oxygen coverage (monolayers) 0.5-
0.4 -
P’ / /
0.3 -
/l /
0.2-
l
’
/ /
l
/ o.i-
CuO/ZnO/Al203(60:30:1o)
l
:’ /
1’
w37:
IO)
A CuO/Al2O3
/
I
I 0
I
1.0
2.0 CO2/CO
FIGURE
5
synthesis
Steady
state
conditions
oxygen
coverage
as a function
of the copper
of the C02/C0
surface
ratio.
ratio
under methanol
107 TABLE
1
Activity
and state of copper
surface
during
Activity
Initial
mol
metal
/s-' g-'
/m2 g-'
methanol
copper
area
synthesis
Re-reduced copper after
area re-
action
Apparent copper after action
/m2 g-' CuO/ZnO/A1203
9.5
x lo+-
33.1
(60:30:10)
6.3 x lO-6
24.9
CuO/ZnO/A1203
2.3 x lo+
CuO/ZnO/A12@3
5.6 x lO-6
(45:37:18)
1.2 x lo-6
3.5
layers
0.3gb
9.0
4.0
0.28'
21.6
11.4
0.24a
2.4
0.40b
12.6
9.0
44.9
42.1
CuO/A1203
4.8 x lo+
19.9
Cu/A1203
4.4 x lo-6
11.7
CuO/MnO
2.7 x lO-6
15.7
CuO/MgO
1.4 x lO+j
9.0
CuQ/MgO
2.6 x lo6
14.9
CuO/ZnO
2.85 x IO+
19.1
CO,
mono-
0.2ga
1.9 x lo6
aFeed
of cu
5.5
11.8 x lO-6
203
area coverage re-
14.0 23.1
CuO/SiO2
cd/Al
Oxygen
He 14, 14, 46, 26
bCO, CO23 H2, C02, He H2,10, 13, 52, 28 'Feed
CO, C02, H2 He 11, 11, 60, 18.
REFERENCES G.C. Chinchen, P.J. Denny, D.G. Parker, G.D. Short, M.S. Spencer, K.C. Waugh and D.A. Whan, ACS Division of Fuel Chemistry, Vo1.29, No.5, p.178 (1984). A. Ya Rozovskii, G. Lim, L.B. Liberov, E.V. Slivinskii, S.M. Loktev, Yu.B. Kagan and A.N. Bashkirov, Kinetics and Catalysis, 18 (1977) 691. M. Bowker and K.C. Waugh, to be published. G.C. Chinchen and K.C. Waugh, J. Catalysis, in press. M. Bowker, J.N.K. Hyland, H.D. Vandervell and K.C. Waugh, Proceedings 8th International Congress on Catalysis, Vol.11 (1984) 35. J.W. Evans, M.S. Wainwright, A.J. Bridgewater and J.D. Young, Applied Catalysis, 7 (1‘983) 75.
101
THE ACTIVITY AND STATE OF THE COPPER SURFACE IN METHANOL SYNTHESIS CATALYSTS G.C. CHINCHEN', K.C. WAUGH2 and D.A. WHAN' ‘Imperial
Chemical
Industries, plc, Agricultural Division, Billingham, Cleveland,
TS23 lLB, U.K. 2 Imperial Chemical Industries, plc, New Science Group, Runcorn Heath, Cheshire, WA7 4QE, U.K.
ABSTRACT The measurement of copper metal surface areas by monitoring nitrous oxide chemisorotion is a well established technique. A frontal chromatographic version of this technique has been developed which is very suitable for in situ measurements and this has enabled the apparent copper areas of various catalysts to be measured after exposures to methanol synthesis gases of different compositions at typical industrial conditions in microreactors cormnonlyused for assessing the methanol synthesis activity of such catalysts. Using such techniques, it has been shown that, first, there is a linear relationship between the methanol synthesis activity of copper/zinc oxide/alumina catalysts and their total copper surface area. Second, that copper supported on other materials has approximately the same turnover number as copper/zinc oxide/alumina catalysts. Third, that under industrial conditions the copper Surface of the catalyst is partially oxidised, to an extent which depends on the composition of the synthesis gas, particularly the CO2/CO ratio. INTRODUCTION Industrial methanol synthesis catalysts are generally based on CuO/ZnO/A1203 or CuO/ZnO/Cr203 compositions and, whilst the process operating at TO - 100 bar
- _
pressure and in the temperature range 230 - 300°C is well established, many aspects of the mechanism of synthesis are still not fully understood and are the subject of continued debate. In particular there is still controversy as to the exact nature of the active centre and more specifically about the chemical state of the individual components of the catalyst, especially the copper under typical methanol synthesis conditions. Under industrial conditions (50 bar, 250°C) using methanol synthesis feeds containing both CO and CO2 over a Cu/ZnO/A1203 catalyst several workers Cl,23 have shown by using labelled carbon oxides that all of methanol is formed from the CO2 component of the feed rather than the CO. The major reaction involved is therefore CO2 + 2H2
+
CH30H + O(s)
0166-9834/86/$03.50
8
1966 Elaevier Science Publishen B.V.
(0
102 where O(s) is a surface
oxygen
atom,
and it is likely
that a role of the CO and H, L
is to scavenge
this surface
oxygen,
by reactions
such as
co + O(s)
+
co*
(2)
H2 + O(s)
+
H20
(3)
The oxidation
state of the catalyst
be controlled
by the relative
believe diagram copper
that the detailed in Figure surface
to an extent
surface
during
rates of reactions
mechanism
be covered
determined
with formate,
by the kinetics
synthesis
(I), (2) and
of methanol
synthesis
1 [3], and it is a consequence
will
methanol
(3). We further
is represented
of this mechanism
formyl,
and relative
will then
methoxy
by the
that the
species
and oxygen
rates of the various
reaction
steps.
FIGURE
1 Mechanism
of methanol
synthesis
on a Cu/ZnO/A1203
catalyst
from a CO,
C02, H2 feed.
The present
work
area and methanol to examine by oxides copper
sets out
synthesis
(i) to examine activity
the same relationship other
component
gas compositions.
than zinc oxide, of these
the relationship
in a series
for catalysts
of Cu/ZnO/A1203
in which
and (iii) to assess
catalysts
at steady
between
catalysts,
the copper
the oxygen
copper
(ii)
is supported
coverage
state for a variety
metal
of the
of synthesis
103
EXPERIMENTAL The apparatus metal
in which
area measurements
synthesis
the methanol
were made
rate measurements
240°C) with
synthesis
synthesis
is shown
gases containing
experimental
rig is totally
The copper
metal
After
(typically
5% H2/N2)
to near ambient
the helium
is then established
or a thermal
frontal
at a steady,
in this way can be calibrated
gas stream
measured
continuously detector.
is shown
by measurements
frontal
a flow of helium (typically
between
flow rate in place of
for N2 and N20 with a mass A typical
in Figure
The
in a H2/N2 mixture
is swept out with
A N20/He
conductivity
chromatogram"
of "reactive
had been reduced
the reactor
temperature.
and the exit gas monitored
spectrometer reactive
at 24O"C,
area
temperature.
PET computers.
by the technique
the catalyst
(50 bar,
the CoPPer
and ambient
by two Commodore
areas were evaluated
[4-61.
and cooled
2 and 6% N20)
controlled
pressure
2. The methanol
conditions
both CO and CO2, whilst
are made at near atmospheric
and the copper
in Figure
industrial
are made under typical
measurements
chromatography"
rate measurements
schematically
line shape
3. Copper
metal
on polycrystalline
- "a
areas
determined
copper
combined
with in situ BET area measurements. Copper
area measurements
to methanol synthesis above.
synthesis
can also be made on catalysts
conditions,
gas from the reactor
Measurement
been exposed of oxygen
of the apparent
to methanol
atom coverage
the copper
has occurred.
by rereducing the CoPPer
by first depressurising
with a stream copper
synthesis
surface
conditions
of the copper The latter
the catalyst
surface
at steady
RESULTS
and sweeping
after
measure
providing
gas mixture
of this technique N20/H2
out the as
the catalyst
a direct
state
have been exposed
and then proceeding
has
of the degree
no sintering
may be assessed
with the H2/N2
area. A full description
surface
area
gives
possibility
c4], and it has been shown that sequential copper
of helium
which
of
and allowed
for
and then redetermining
has been given elsewhere
treatments
do not affect
the
area.
AND DISCUSSION
Information surface
obtained
under methanol
catalysts
is presented
of the Cu/ZnO/A1203 Differing
copper
by varying
in Table
areas
content,
supported supported
used
for a variety
include
on a variety
commercial
of alternative
on a given oxide
and by using catalysts A number
and state of the copper
(50 bar, 240°C)
1. The catalysts
for catalysts
of reaction.
the activity
conditions
type and copper
the copper
to a long period
in this way about synthesis
of differing
which
of materials oxides.
have been achieved
have been subjected
feed gas compositions
was
also used.
Activity/copper
area relationship
The dependence is shown
in Figure
of the methanol
synthesis
4 for all the catalysts
activity
on the copper
used. There
is found
metal
area
to be an extremely
N20
IN He
P Pressure
Pa Compute1 r controlled valve
‘VALVE
OVEN
TO OVEN
P”RGLE
Apparatus for the measurement of methanol synthesis rates and of copper metal areas and total surface areas.
2z
HELIUM
5% N2 IN He
10%
CALIBRATION MIXTURE
FIGWE 2
DIOXIDE
5% ti2 IN. N2
)ti
1w
CARBON
HELIUM
HYDROGEN
SHUTDOWN
Concentration of gas phase s@?CXS
_____-_-
- - -
-
Response of Katharometer
-MSSS
Katharometer +_____-----
spectrometer signal
-
Chati
FIOW swilch in
FIGURE 3
speed
= i En’8 m&i1
Time m minutes
"Reactive frontal chromatogram" for the reaction of N20 with poly-
crystalline copper.
significant
correlation between methanol synthesis activity and copper area
fop these materials and the turnover number: the slope of Figure 4 is essential -1 site-'). The identical for all the catalysts (1.6 x to-* molecules CH30H s conclusions to be drawn are that at1 the copper is active and only the copper surface is implicated in the rate determining step of synthesis; no unique synergy attaches to the copper/zinc oxide combination, supports as diverse as MgO and Si02 have similar effects. Oxygen coverage of copper during methanol synthesis By measuring the apparent copper area after methanol synthesis reaction and comparing it to the area before reaction, the degree of oxygen coverage of the copper surface can be deduced. The fact that this is oxygen coverage and not sintering of the copper is demonstrated by rereducing the catalyst and remeasuri the copper area (Tab'le1).
From the results obtained on both Cu/ZnO/A1203 com-
positions and on a Cu/A1203 material (Table 11, it is clear that the copper is oxidised to between 25% and 40% of a monolayer (50 - 80% of a monolayer of 0~~0) depending on the ethanol
synthesis feed gas composition. Using this data and
some of our previously published data [4], Figure 5 has been assembled which shows that the steady state oxygen coverage of the copper surface under methanol synthesis conditions for both Cu~ZnO~Al~O3 and Cu/A1203 catalysts is essentially a linear function of the CO,/CO ratio in the feed, over the range of values covered. We conclude that CO is therefore the dominant reducing agent for the surface under reaction conditions,
106 Methanol synthesis Actwity/Mol s-1 g-1 xl 06 12.0 ll.OlO.O9.0 8.07.06.0 -
Cu0/2n0/A1203(60:30:10)
SO-
CuOIZnOIA12O3(45:37:18)
0
CuO/MnO
.
CuO/ZnO
30
20
40
Cu Metal Area/m2g-1
FIGURE
4
Methanol
synthesis
activity
as a function
of copper
metal
area.
Oxygen coverage (monolayers) 0.5-
0.4 -
P’ / /
0.3 -
/l /
0.2-
l
’
/ /
l
/ o.i-
CuO/ZnO/Al203(60:30:1o)
l
:’ /
1’
w37:
IO)
A CuO/Al2O3
/
I
I 0
I
1.0
2.0 CO2/CO
FIGURE
5
synthesis
Steady
state
conditions
oxygen
coverage
as a function
of the copper
of the C02/C0
surface
ratio.
ratio
under methanol
107 TABLE
1
Activity
and state of copper
surface
during
Activity
Initial
mol
metal
/s-' g-'
/m2 g-'
methanol
copper
area
synthesis
Re-reduced copper after
area re-
action
Apparent copper after action
/m2 g-' CuO/ZnO/A1203
9.5
x lo+-
33.1
(60:30:10)
6.3 x lO-6
24.9
CuO/ZnO/A1203
2.3 x lo+
CuO/ZnO/A12@3
5.6 x lO-6
(45:37:18)
1.2 x lo-6
3.5
layers
0.3gb
9.0
4.0
0.28'
21.6
11.4
0.24a
2.4
0.40b
12.6
9.0
44.9
42.1
CuO/A1203
4.8 x lo+
19.9
Cu/A1203
4.4 x lo-6
11.7
CuO/MnO
2.7 x lO-6
15.7
CuO/MgO
1.4 x lO+j
9.0
CuQ/MgO
2.6 x lo6
14.9
CuO/ZnO
2.85 x IO+
19.1
CO,
mono-
0.2ga
1.9 x lo6
aFeed
of cu
5.5
11.8 x lO-6
203
area coverage re-
14.0 23.1
CuO/SiO2
cd/Al
Oxygen
He 14, 14, 46, 26
bCO, CO23 H2, C02, He H2,10, 13, 52, 28 'Feed
CO, C02, H2 He 11, 11, 60, 18.
REFERENCES G.C. Chinchen, P.J. Denny, D.G. Parker, G.D. Short, M.S. Spencer, K.C. Waugh and D.A. Whan, ACS Division of Fuel Chemistry, Vo1.29, No.5, p.178 (1984). A. Ya Rozovskii, G. Lim, L.B. Liberov, E.V. Slivinskii, S.M. Loktev, Yu.B. Kagan and A.N. Bashkirov, Kinetics and Catalysis, 18 (1977) 691. M. Bowker and K.C. Waugh, to be published. G.C. Chinchen and K.C. Waugh, J. Catalysis, in press. M. Bowker, J.N.K. Hyland, H.D. Vandervell and K.C. Waugh, Proceedings 8th International Congress on Catalysis, Vol.11 (1984) 35. J.W. Evans, M.S. Wainwright, A.J. Bridgewater and J.D. Young, Applied Catalysis, 7 (1‘983) 75.