Formation of the Co/ZrO2 catalyst active phase in the Fischer-Tropsch synthesis

Article abstract of DOI:10.1007/s11172-010-0295-9

Thermoprogrammed reduction and X-ray powder diffraction were used to study the phase composition and specific features of the activation process of the Co/ZrO₂ catalytic system. Results of the physico-chemical studies were compared with the data on activity and selectivity of catalysts in the Fischer-Tropsch synthesis. Reducing activation of the Co/ZrO₂ catalyst with the syn-gas can be performed directly during the Fischer-Tropsch synthesis, avoiding the high-temperature step of reduction with hydrogen. The Co/ZrO₂ catalytic system can be used for the development of catalysts for the Fischer-Tropsch synthesis, which do not require a separate step of the high-temperature reduction with hydrogen.

Full text of DOI:10.1007/s11172-010-0295-9

Russian Chemical Bulletin, International Edition, Vol. 59, No. 9, pp. 1675—1679, September, 2010
1675
Full Articles
Formation of the Co/ZrO catalyst active phase
2
in the Fischer—Tropsch synthesis
a
b
a,b
and A. N. Loginovab
A. L. Lapidus, D. A. Grigor´ev, M. N. Mikhailov,
aN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
4
7 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5303. Eꢀmail: albert@ioc.ac.ru
b
United Research and Development Center,
str. 2, 55/1 Leninsky prosp., 119333 Moscow, Russian Federation.
Fax: +7 (495) 730 6102. Eꢀmail: mik@ioc.ac.ru
Thermoprogrammed reduction and Xꢀray powder diffraction were used to study the phase
composition and specific features of the activation process of the Co/ZrO catalytic system.
2
Results of the physicoꢀchemical studies were compared with the data on activity and selectivity
of catalysts in the Fischer—Tropsch synthesis. Reducing activation of the Co/ZrO catalyst
2
with the synꢀgas can be performed directly during the Fischer—Tropsch synthesis, avoiding the
highꢀtemperature step of reduction with hydrogen. The Co/ZrO catalytic system can be used
2
for the development of catalysts for the Fischer—Tropsch synthesis, which do not require
a separate step of the highꢀtemperature reduction with hydrogen.
Key words: cobalt catalyst, the Fischer—Tropsch synthesis, zirconia, activation of catalyst.
Classic cobalt Fischer—Tropsch catalysts based on aluꢀ
mina, silica, and alumosilicates possess high activity and
selectivity.1 One of the principal steps in the formation
of active phase of these catalysts is the activation in reꢀ
ducing atmosphere, necessary for the formation of reꢀ
Since formation of the spinel structures is directly reꢀ
lated to the active component—support interaction,
more recently attention of researches was directed
on the development of new catalytic systems with lower
strength of the metal—support interaction. In particuꢀ
,2
3
duced particles of cobalt on the catalyst surface. Recent
lar, the Co/ZrO system belongs to such systems (see
2
4
—7
works
showed that when a precursor of the active
Ref. 10).
component is deposited on a support, formation of difꢀ
ficultꢀtoꢀreduce mixed oxides (spinel structures)8,9 occurs,
that leads to the transformation of considerable amount
of active component into the catalytically inert state.
In the present work, we studied conditions for the forꢀ
mation of active phase in the Co/ZrO catalytic system
2
and their influence on activity and selectivity of the
Fischer—Tropsch catalyst.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1629—1633, September, 2010.
066ꢀ5285/10/5909ꢀ1675 © 2010 Springer Science+Business Media, Inc.
1

1
676
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 9, September, 2010
Lapidus et al.
Experimental
a mixture of two crystalline phases of zirconia, viz., the
tetragonal (tꢀZrO ) and monoclinic (mꢀZrO ). If the calꢀ
2
2
cination temperature is increased, the content of the
monoclinic phase increases, whereas the content of the
tetragonal phase decreases. The ZR1 support calcined at
Support was prepared from zirconia, obtained by calcination
of ZrO(NO ) •6H O (Aldrich) for 4 h at 400 and 800 °C (the
3
2
2
ZR1 and ZR2 supports, respectively). Powdered zirconia conꢀ
taining 10% boehmite, was formed into the cylinderꢀshaped
pellets 2.5 mm in diameter and 4—6 mm in length. The pellets
obtained were calcined for 4 h at 500 °C. Cobalt was deposited
into the catalyst by impregnation of the support with aqueous
solution of cobalt nitrate Co(NO ) •6H O (10 wt.%). After the
4
00 °C contains about 80% tetragonal and 20% monoꢀ
clinic phases, whereas in the case of the ZR2 support subꢀ
jected to the thermal treatment at 800 °C, contribution of
the monoclinic phase increases to 95%. The study of the
formation of zirconia phase from zirconium oxyhydrꢀ
3
2
2
impregnation, the catalyst was calcined for 1 h at 400 °C.
Xꢀray diffraction analysis was performed on a DRONꢀ3
diffractometer at the scanning rate of 0.5 deg min–1, using CuKαꢀ
irradiation and cerium oxide as the internal standard (10 wt.%
with respect to the weight of the sample). The mean diameter of
the crystallites in the samples of calcined catalyst was deterꢀ
mined from the Debye—Scherrer equation.11
1
4
oxide showed that the formation of the tetragonal phase
is observed at the temperatures higher 550 °C, whereas the
monoclinic phase appears only at 1200 °C. To sum up, the
use of zirconium oxonitrate allows us to obtain the crystalꢀ
line phases of zirconia at lower calcination temperature.
Physicoꢀchemical properties of the ZrO2 support
samples are given in Table 1. It is seen that on going from
the tetragonal phase to the monoclinic, the size of crystalꢀ
lites increases, whereas the support specific surface and
the volume of pores decrease. The significant increase in
the size of crystallites upon increase in contribution of the
monoclinic phase has been noted earlier during study of
the reaction of cobalt with the surface of zirconia by elecꢀ
Thermoprogrammed reduction (TPR) of catalysts (250 mg)
was performed on a AutoChem II 2920 instrument (Microꢀ
meritics) in the flow (10 mL min–1) of the 5%H + 95%Ar gas
2
mixture. The linear elevation of temperature was carried out
from 50 to 750 °C at the rate 10 deg min–1
.
The adsorption/desorption isotherm of nitrogen was measꢀ
ured using a ASAP 2010 instrument (Micromeritics). The surface
area of the samples was calculated using the Brunauer—Emꢀ
mett—Teller (BET) equation.12 The size distribution of pores
and their average diameter were determined using the Barrett—Joyꢀ
ner—Halenda method.13
10
tron microscopy and Xꢀray diffraction.
The TPR curves of the 10%Co/ZrO catalysts are
2
shown in Fig. 2. The TPR spectrum of the 10%Co/ZR2
catalyst, prepared based on the monoclinic modification
of zirconia, exhibits three peaks at the temperatures 240,
Microcatalytic experiments were performed in a fixedꢀbed
steel reactor (13 mm in internal diameter), Р = 2 MPa,
H : CO : N2 = 63 : 31.5 : 5.5 (vol.%). Catalysts reduced with
2
3
10, and 510 °C. The first two peaks correspond to the
hydrogen and catalysts without preliminary reducing treatment
were activated in the flow of synꢀgas (the volume rate was
reduction of Co3O4 to CoO, whereas the third peak is
–
1
–1
4,15
1
000 h ). A catalyst was heated to 170 °C at the rate 2 deg min
attributed to the reduction of CoO to metallic cobalt.
and kept at this temperature until stable indices on conversion of
carbon monoxide and selectivity to liquid hydrocarbons and
methane were reached. Then, the temperature was stepꢀwise
elevated by 10 °C until degree of transformation of carbon monꢀ
oxide reached 50—60%. After that, the temperature was stepꢀ
wise elevated by 5 °C until conversion of carbon monoxide
reached 70—80%.
Cobalt oxide Co O on the 10%Co/ZR2 catalyst is reꢀ
3
4
duced in two steps. Apparently, it is due to the fact that
crystallites of Co O of different sizes are present on its
3
4
surface, since the reduction temperature of cobalt(II, III)
oxide strongly depends on the size of its particles.16 It is
The starting mixture and gaseous products of the synthesis
were analyzed by gasꢀadsorption chromatography, using cataroꢀ
meter as a detector, helium as a carrier gas. The first column was
filled with molecular sieves CaA (3 m × 3 mm), a HayeSep was
the second column. The first column was used for the analysis of
I (rel. units)
mꢀZrO2
tꢀZrO2
CO and N (isothermic regime, 80 °C), the second for analysis
2
of CO , CH , and C —C hydrocarbons (temperatureꢀproꢀ
2
4
2
4
–
1
grammed regime, 80—200 °C, 8 deg min ). Composition of the
liquid products of the synthesis was determined by gasꢀliquid
chromatography, using a flameꢀionizing detector and helium as
a carrier gas. A capillary column (50 m) was used for the analysis
with DBꢀPetro 0.5 as a stationary phase (temperatureꢀproꢀ
grammed regime, 50—250 °C, 3 deg min–1).
2
1
Results and Discussion
The Xꢀray patterns of the ZR1 and ZR2 supports are
shown in Fig. 1. As it is seen from the Figure, the calcinaꢀ
tion of zirconium oxonitrate leads to the formation of
2
5
30
35
2θ/deg
Fig. 1. Xꢀray patterns of the ZR1 (1) and ZR2 (2) supports.

Zirconia as support for Fischer—Tropsch catalysts
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 9, September, 2010
1677
Table 1. Physicoꢀchemical properties of the ZrO support samples
Table 2. Degree of reduction of the 10%Co/ZrO2
2
catalysts at different temperatures
2
–1
3
V /cm g
p
–1
Dpav/nm dZrO2/nm
Support
S/m g
T/°С
RDCo (%)
ZR1
ZR2
61.9
32.1
0.19
0.13
10.1
14.2
11.5
41.0
10%Co/ZR1
10%Co/ZR2
Note. S is the surface square, V is the volume of pores, Dpav is the
average diameter of pores, dZrO2 is the average size of ZrO2
350
500
17.3
56.4
28.1
65.6
p
crystallites.
known17 that in the cobalt systems characterized by
a strong metal—support interaction (for example, alumina),
at increased temperatures the reduction of cobalt in the
large Co O crystallites is favored in the first place, whereas
sponding temperature to the total area. The value of RDCo
obtained at 350 °C for the 10%Co/ZR2 (28%) catalyst
indicates that almost all the Co O is reduced to CoO by
this temperature, whereas metallic cobalt is present in
small amounts. At the temperature 500 °C, the degree of
reduction reaches 66% and the most part of cobalt is in the
metallic state. For the 10%Co/ZR1 catalyst, the degree of
reduction at temperatures 350 and 500 °C is lower than for
the 10%Co/ZR2 catalyst.
3
4
3
4
small crystallites are reduced at higher temperatures. Since
zirconia is characterized by a weak interaction with active
component, the Co O crystallites of smaller size are reꢀ
3
4
duced10 in the Co/ZrO system in the lowꢀtemperature
2
region (240 °C), whereas large particles are reduced at
higher temperature (310 °C).
Two broad peaks at the temperatures 300—400 (the
maximum 360 °C) and 400—600 °C (the maximum
The Xꢀray patterns of 10%Co/ZR1 and 10%Co/ZR2
catalysts calcined at 400 °C and reduced at 350 °C are
shown in Fig. 3. The Xꢀray patterns of unreduced samples
contain lines related to the oxide phase Co O . Mean
5
10 °C) are seen on the TPR curve of the 10%Co/ZR1
3
4
catalyst based on the tetragonal modification of zirconia.
The first peak is related to the reduction of cobalt
oxide Co O to CoO and it is displaced to higher temperaꢀ
size of the Co O crystallites, calculated using the
3 4
Debye—Scherrer equation, is about 200 Å. Upon reducꢀ
tion of the catalysts at 350 °C, the lines of the crystalline
Co O phase disappear, obviously, as a result of the fact
3
4
tures by 50 °C as compared to that of the monoclinic modiꢀ
3
4
fication of the catalyst. The second peak corresponds to
the reduction of CoO to metallic cobalt Co (see Refs 15
that the spinel phase of Co O is completely reduced with
3 4
0
the formation of the amorphous to Xꢀrays phase of CoO.
1
9
and 16). In the TPR spectra (see Fig. 2), no highꢀtemꢀ
perature (>600 °C) peaks of reduction are observed, that
indicates the absence of difficultꢀtoꢀreduce cobalt zircoꢀ
nates on the catalyst surface.1
It is known from the literature that for the Co O phase
3 4
to be reduced on the supports of the Al O and SiO type,
2
3
2
the temperatures not lower than 450 °C are required. The
8
XRD data for the 10%Co/ZrO catalysts allow us to draw
2
Table 2 contains the data on the degree of reduction of coꢀ
balt (RDCo) at different temperatures for the 10%Co/ZR1
and 10%Co/ZR2 catalysts, which was evaluated as a ratio
of the area under the TPR curve limited by the correꢀ
a conclusion that the absence of the strong metal—supꢀ
port interaction promotes decrease in the reduction temꢀ
perature of the catalyst.
Analysis of Fig. 2 shows that the reduction process of
cobalt on the 10%Co/ZrO catalysts begins at temperaꢀ
2
tures 200—300 °C, which is close to the temperatures of
the Fischer—Tropsch synthesis. A decrease in the initial
temperature of reduction of cobalt on these catalysts is
also evidenced by the Xꢀray diffraction data, according to
which the Co O phase completely disappears in the
I (rel. units)
2
3
4
samples treated with hydrogen at 350 °C. Therefore, it was
of interest to find whether the 10%Co/ZrO catalysts, that
2
were not subjected to a special step of reducing treatment,
as well as the catalysts reduced at decreased temperaꢀ
ture of 350 °C, possess activity in the Fischer—Tropsch
synthesis. The data on activity and selectivity of the
1
1
0%Co/ZR1 and 10%Co/ZR2 catalysts, preliminary reꢀ
duced with hydrogen at 350 °C (T ) (the volume rate
r
–
1
3
000 h , 1 h), and the 10%Co/ZR1 catalyst treated with
1
00
200
300
400
500
600
700 T/°С
the synꢀgas only during the reaction are given in Tables 3
and 4. It is seen from the Tables that 10%Co/ZR1 and
10%Co/ZR2 samples having different modification of
Fig. 2. The TPR curves of the 10%Co/ZR1 (1) and 10%Co/ZR2 (2)
catalysts.

1
678
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 9, September, 2010
Lapidus et al.
Table 3. Activity and selectivity of the 10%Co/ZrO catalysts in
I (rel. units)
a
2
the Fischer—Tropsch synthesis at the pressure 20 atm and volume
–
1
rate 1000 h
Co O4
3
Catalyst
Tr
T
Converꢀ
YС5+ SС5+ SСH4
sion CO (%) /g m–3 (%)
(%)
/
°С
2
1
1
1
1
0%Co/ZR2 350 250
0%Co/ZR1 350 250
78
78
78
76
80
81
54
57
55
28
27
26
0%Co/ZR1
—
275
Note. T is the temperature of preliminary reduction of catalysts
r
with hydrogen, T is the temperature of reaction, YC5+ is the yield
of hydrocarbons, SC5+ is the selectivity on liquid hydrocarbons,
SCH4 is the selectivity on CH4.
36
38
40
42
44
46
48
2θ/deg
I (rel. units)
b
zirconia support and reduced at 350 °C, show no considꢀ
erable differences. The maximum yield of C5+ hydroꢀ
carbons is reached at the temperature about 250 °C and at
Co O4
3
7
8% conversion of CO. The fractional composition of liquid
hydrocarbons (see Table 4) obtained on these catalysts is
also virtually the same and mainly consists from the fracꢀ
tions C —C (50%) and C —C (40%), that correꢀ
2
5
10
11
18
sponds to the gasoline and diesel fractions.
1
Since no differences between the 10%Co/ZrO catalꢀ
2
ysts, differing in the phase composition of zirconia, were
found, the study of activation of the catalyst with the synꢀ
gas at temperatures of the Fischer—Tropsch synthesis was
performed for the sample of 10%Co/ZR1. As it is seen
from the data in Table 3, the 10%Co/ZR1 catalyst, actiꢀ
vated with the synꢀgas without preliminary treatment with
hydrogen, showed high activity in the Fischer—Tropsch
synthesis and was not inferior in the yield of hydrocarbons
C5+ to the catalysts undergone the special reducing treatꢀ
ment. This catalyst in its activity and selectivity is compaꢀ
36
38
40
42
44
46
48
2θ/deg
Fig. 3. Xꢀray patterns of the 10%Co/ZR2 (a) and 10%Co/ZR1 (b)
catalysts, calcined at 400 °C (1) and reduced at 350 °C (2).
High activity of the catalyst that was not subjected to
reducing treatment with hydrogen described above is apꢀ
parently explained by the fact that on the surface of zircoꢀ
nia, the formation of reduced particles of cobalt is possible
in two ways: as a result of action of hydrogen during highꢀ
temperature reduction of the catalyst and under condiꢀ
tions of the synthesis of hydrocarbons from CO and H2
(see Ref. 22). On the preliminary reduced catalyst, cobalt
is partially present in the form of metal and the number of
active centers of metallic cobalt increases with the temꢀ
perature of reduction. On the sample unreduced with
rable to the classic catalysts of the Fischer—Tropsch
synthesis.2
0,21
In the products of the synthesis obtained
on this catalyst, the content of the gasoline fraction inꢀ
creases to 70%, with the content of normal alkanes being
considerably decreased. At the same time, the content of
olefins and isoꢀalkanes in the reaction products increases.
Such changes of the product composition, obviously, are
due to the increase in the optimum temperature of the
process by 25 °C.
Table 4. Composition of C5+ hydrocarbons, obtained on the 10%Co/ZrO catalysts in the Fischer—Tropsch syntheꢀ
2
sis at the pressure 20 atm and volume rate 1000 h–
1
Catalyst
Тr/°C
Composition (wt.%)
Composition of paraffins (wt.%)
P
Olefins
Hꢀparaffins
Isoparaffins
С —С
С —С
С19+
5
10
11
18
1
1
1
0%Co/ZR2 350
0%Co/ZR1 350
0%Co/ZR1
2
2
8
81
82
66
17
16
26
52
51
70
39
39
27
9
10
3
0.80
0.81
0.74
—
Note. P is the probability of the chain growth α.

Zirconia as support for Fischer—Tropsch catalysts
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 9, September, 2010
1679
0
hydrogen, Co begins to appear only in the process of
10. S. Kittiruangrayab, T. Burakorn, B. Jongsomjit, P. Praserthꢀ
dam, Catal. Lett., 2008, 124, 376.
synthesis and its amount, necessary for the conversion of
CO required to be achieved, is formed at higher temperaꢀ
tures of the process.
The results obtained can be used in the development of
new promising catalysts for the Fischer—Tropsch syntheꢀ
sis, which require no special step of the highꢀtemperature
reduction with hydrogen.
1
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Received April 7, 2010