Study of the Structure of Cobalt-Containing Catalysts Synthesized under Subcritical Conditions

Article abstract of DOI:10.1134/S0023158419050021

Abstract: A physicochemical study of cobalt-containing (10 wt %) silica-supported Fischer–Tropsch catalysts was carried out. The catalysts were obtained under subcritical conditions (T = 200°C, P = 8 MPa) using water (T_c = 374.1°C, P_c = 22.1 MPa) and propanol-2 (T_c = 235.6°C, P_c = 5.8 MPa). The obtained samples were compared with a 10 wt % Co/SiO₂ catalyst prepared by incipient-wetness impregnation. Comparison of the properties of catalysts in the liquid-phase Fischer–Tropsch synthesis showed that the sample prepared in subcritical water was the most active and selective to aliphatic C₆–C₇ hydrocarbons. This sample is characterized by a high surface area (131.7 m²/g), a uniform distribution of particles in the active phase with an average size of 5 nm and higher accessibility of cobalt species for reagents. According to XPS data, the composition of catalyst active phase is mainly represented by two compounds: Co(OH)₂ and Co₃O₄.

Full text of DOI:10.1134/S0023158419050021

ISSN 0023-1584, Kinetics and Catalysis, 2019, Vol. 60, No. 5, pp. 618–626. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Author(s), 2019, published in Kinetika i Kataliz, 2019, Vol. 60, No. 5, pp. 624–632.
Study of the Structure of Cobalt-Containing Catalysts Synthesized
under Subcritical Conditions
M. E. Markovaa, *, A. V. Gavrilenkob, A. A. Stepachevab, V. P. Molchanovb, V. G. Matveevab,
M. G. Sulmanb, c, and E. M. Sulmanb
aTver State University, Tver, 170100 Russia
bTver State Technical University, Tver, 170026 Russia
cNesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow, 119991 Russia
*e-mail: mashulikmarkova@gmail.com
Received September 20, 2018; revised March 29, 2019; accepted April 22, 2019
Abstract—A physicochemical study of cobalt-containing (10 wt %) silica-supported Fischer–Tropsch cata-
lysts was carried out. The catalysts were obtained under subcritical conditions (T = 200°C, P = 8 MPa) using
water (Tc = 374.1°C, Pc = 22.1 MPa) and propanol-2 (Tc = 235.6°C, Pc = 5.8 MPa). The obtained samples
were compared with a 10 wt % Co/SiO2 catalyst prepared by incipient-wetness impregnation. Comparison of
the properties of catalysts in the liquid-phase Fischer–Tropsch synthesis showed that the sample prepared in
subcritical water was the most active and selective to aliphatic C6–C7 hydrocarbons. This sample is charac-
terized by a high surface area (131.7 m2/g), a uniform distribution of particles in the active phase with an aver-
age size of 5 nm and higher accessibility of cobalt species for reagents. According to XPS data, the composi-
tion of catalyst active phase is mainly represented by two compounds: Co(OH)2 and Co3O4.
Keywords: cobalt, silica, subcritical conditions, impregnation, Fischer–Tropsch synthesis
DOI: 10.1134/S0023158419050021
INTRODUCTION
Considerable recent attention is given to the syn-
thesis of supported catalysts using sub- and supercriti-
cal fluids [9]. Due to the unique properties, the super-
critical fluids make it possible to control the size, dis-
persity, structure, and morphology of the obtained
nanoparticles. The low surface tension of supercritical
fluids prevents pores of support from clogging during
synthesis, while the low viscosity accelerates the diffu-
sion of a metal salt solution. Moreover, the solubility
of many compounds increases in supercritical condi-
tions thus expanding the range of metal precursors
used [10]. Supercritical metal deposition is carried out
in several steps: (1) the dissolution of a precursor in a
supercritical fluid; (2) the addition of a porous support
to the resulting solution; (3) the adsorption of the
solution in the pores and on the surface of the support;
(4) the chemical or thermal reduction of the precursor
leading to the formation of the active-phase particles
[11]. Carbon dioxide [8, 9, 12–15] and water [16–18]
are most often used as solvents in the supercritical syn-
thesis of catalysts. In addition, there are data on the
use of methanol [19], ammonia [20], isopropyl alco-
hol [21, 22], cyclohexane [23], and propane [24] as
supercritical solvents. Analysis of literature data
showed that water can be considered the most promis-
ing solvent for sub/supercritical synthesis of catalysts.
Supported metal catalysts are given considerable
attention in various fields of chemical technology.
Such heterogeneous systems are characterized by high
mechanical strength and surface area and make it pos-
sible to decreasing the metal content of a catalyst [1].
Currently, different techniques are used in order to
support metal particles on different materials: incipi-
ent-wetness impregnation [2], colloidal synthesis [3],
microemulsion method [4], sonochemical synthesis
[5], precipitation and co-precipitation [6], polyol
method [7], etc. Physical and physicochemical prop-
erties of the obtained catalysts undergo considerable
change depending on the technique used.
Supported metal catalysts are typically prepared by
impregnation of supports with a solution of a metal
precursor with the subsequent drying, calcination, or
reduction of metallic nanoparticles [2]. However, the
high viscosity and surface tension of the most liquids
used as solvents in the impregnation process lead to a
slow diffusion of metal salts in the pores of support,
and, as a consequence, to the low dispersion of metal
particles and inhomogeneity of the resulting samples
[8]. Furthermore, subsequent drying and thermal
activation may cause significant changes in the struc-
ture of the catalyst leading to a sharp decrease in the
surface area and aggregation of the active phase.
For example, the structure and catalytic properties
of Pd/ZrO2 and Pd(Rd)/TiO2 synthesized in super-
618

STUDY OF THE STRUCTURE OF COBALT-CONTAINING CATALYSTS
critical and subcritical water, respectively, were stud- Catalyst Synthesis by Impregnation
619
ied in [16, 25]. Otsu and Oshima [26] studied the
structure of manganese, lead, and silver oxides sup-
ported on γ-Al2O3 in supercritical water. Sun et al. [27]
synthesized nickel-, cobalt-, and iron-containing cat-
alysts in subcritical ethanol. Yahya et al. [28] studied
the structure and photocatalytic properties of potas-
sium hexatitanate synthesized in super- and subcriti-
cal water. The use of super- and subcritical conditions
for the synthesis of supported catalysts shows that,
despite reaching the critical region, the resulting cata-
lytic systems are characterized by high crystallinity,
nanoscale size and uniform distribution of the active
phase. However, since a sharp change in the properties
(dielectric constant, polarity, etc.) is observed for
polar liquids in the supercritical state, sub- and near-
critical conditions are optimal for the synthesis of
metal-containing composites [17].
The cobalt-containing catalyst was synthesized by
impregnation as follows: 1 g of silica (SiO2) dried to a
constant weight, was impregnated with a mixture con-
sisting of cobalt chloride based on 10 wt % of pure
cobalt, 8 cm3 of ethylene glycol, and 2 cm3 of distilled
water for 15 min. Then, the sample was filtered and
treated with an aqueous solution of sodium bicarbon-
ate with a concentration of 2.76 g/dm3 for 15 min,
washed with distilled water to remove chloride and
carbonate ions and dried in air. The resulting sample
was denoted as Co/SiO2(EG).
The resulting samples were reduced in a hydrogen
flow at 300°C for 3 h to convert metal hydroxides into
oxides with the subsequent reduction.
Physicochemical Study of the Catalysts
Low-temperature nitrogen adsorption. The specific
surface area and porosity were determined using a
Beckman CoulterТМ SA 3100ТМ analyzer (Coulter
Corporation, USA). The preparation of samples was
performed by Beckman CoulterTM SA-PREPTM
device (Coulter Corporation, USA) at a temperature
of –120°C for 60 min. In the analysis, Langmuir,
Brunauer–Emmett–Teller, and t-plot models were
used. The Harkins–Jura equation was used to calcu-
late pore distribution.
In this work, the structure and physicochemical
properties of cobalt nanoparticles deposited on silica,
synthesized in subcritical conditions using water and
propanol-2 as solvents are studied. The obtained
nanosystems are compared with the catalyst prepared
by impregnation.
EXPERIMENTAL
Transmission electron microscopy. Transmission
electron microscopy (TEM) of catalyst samples was
performed to determine the size and distribution of
cobalt-containing particles. The catalyst samples were
prepared by the microslice method with a thickness of
50 nm. The analysis was performed at a voltage of
60 kV using JEOLJEM 1010 microscope (JEOL,
Japan). Microscopic images were obtained using a
Gatan digital camera (Gatan Inc., USA). The analysis
of the microscopic images was performed using Adobe
Photoshop and Scion Image Processing Toolkit.
Synthesis of Cobalt-Containing Catalysts
The following materials were used for the synthesis
of catalysts: cobalt(II) chloride, sodium bicarbonate,
anhydrous silica, propanol-2, ethylene glycol (all
reactants were of chemical purity grade purchased
from Reakhim, Russia), and distilled water.
Catalyst Synthesis in Subcritical Conditions
X-ray photoelectron spectroscopy. To determine
the qualitative composition of the surface and the
valence state of the metal, X-ray photoelectron spec-
troscopy (XPS) of samples was used. The X-ray pho-
toelectron spectra of the samples were obtained on an
ES-2403 photoelectron spectrometer equipped with
PHOIBOS-100-MCD energy analyzer (Specs
GmbH, Germany) and XR-50 X-ray source with a
twin-anode Mg/Al. The AlKα1,2 line was used when
recording the spectra. The radiation power was 250 W.
The survey spectra of the samples were recorded in the
range of 0–1200 eV.
The obtained spectra were analyzed using Casa
XPS software package. To isolate individual states,
model deconvolution of high-resolution spectra was
carried out taking into account such characteristics of
photoelectron sublevels as the binding energy of com-
ponents, the ratio of the areas of component peaks,
and intra-doublet splitting. Shirley background was
chosen as a model background. Minimization was car-
ried out by the Levenberg–Marquardt method.
Synthesis of cobalt-containing catalysts under sub-
critical conditions was carried out in a PARR-4307
high-pressure reactor (Parr Instrument, USA). The
reactor was loaded with 1 g of silica, cobalt chloride
based on 10 wt % of pure cobalt, and 0.1 g of sodium
bicarbonate as a precipitating agent dissolved in
30 cm3 of a solvent (distilled water or propanol-2).
The reactor was sealed and purged with nitrogen to
remove air oxygen. Then the necessary operating pres-
sure of nitrogen (6.0 MPa) controlled by the pressure
gauge and temperature (T = 200°С) were set. The total
pressure in the reactor after heating was 7.5 and 8 MPa
for water and propanol-2, respectively. The process
was carried out with continuous stirring at a rate of
750 rpm. The synthesis time was 15 min. After synthe-
sis, the reaction mixture was cooled to room tempera-
ture, filtered, washed with 15–20 cm3 of the solvent to
remove chloride ions and dried in air. The obtained
samples were designated as Co/SiO2(H2O) and
Co/SiO2(IPA).
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MARKOVA et al.
Diffuse reflectance IR Fourier transform spectros- heating rate of 15°C/min; the temperature of the evap-
copy. Diffuse reflectance IR Fourier transform orator and detector was 260°C; the carrier gas was
helium; and the total helium flow was 30.0 cm3/min.
(DRIFT) spectra were measured using the Protege
460 IR spectrometer (Nicolet, USA) equipped with a
diffuse-reflectance device. Before the analysis, the
samples in the form of 0.25–0.35 mm fraction were
treated in vacuum for 2 h at a temperature of 400°C.
Carbon monoxide was used as a test molecule. Carbon
monoxide was adsorbed at room temperature at an
equilibrium pressure of 1599.9 Pa. Carbon monoxide
desorption was carried out at room temperature in
vacuum. The spectra were measured in the range of
6000–400 cm–1 with a step of 4 cm–1.
RESULTS AND DISCUSSION
Table 1 shows the results of analysis of the specific
surface area of the synthesized samples. The highest
decrease in the specific surface area compared with
the initial silicon oxide is observed for the catalyst
sample synthesized in the subcritical water medium.
In this case, a significant decrease in the surface area
of micropores was also noted. Such a change in the
porous structure is specific for hydrothermal treat-
ment of silica, which leads to its recrystallization and
the disappearance of micropores [33]. A strong
decrease in the micropore surface area for the sample
synthesized in subcritical water medium can also indi-
cate a complete adsorption of the cobalt chloride pre-
cursor in the micropores of the support. For the cata-
lyst synthesized in subcritical propanol-2, a double
decrease in the total surface area and a six times
decrease in the surface area of micropores was also
observed. Due to the polar nature of the solvent used,
the treatment of silicon oxide with subcritical propa-
nol-2 is also likely to lead to support recrystallization,
which affects its porosity. The minimum decrease in
the surface area was observed for the sample synthe-
sized by impregnation. In addition, this cobalt-con-
taining system showed a significant decrease in the
total pore volume compared to the support, which
may be due to the clogging of pores by the solvent
during synthesis.
Analysis of nitrogen adsorption–desorption iso-
therms (Fig. 1) showed that the isotherms of the initial
silica sample (Fig. 1a) and the catalyst synthesized in
subcritical propanol-2 (Fig. 1c) were found to be of
type I characterized the microporous substances with
a weak adsorbate–adsorbent interaction. The iso-
therms of samples synthesized in subcritical water
(Fig. 1b) and by impregnation (Fig. 1d) were found to
belong to type IV with the hysteresis loop of H2 form.
These isotherms are typical for micro-mesoporous
substances, having a wide size distribution of cylindri-
cal pores, with strong adsorbate–adsorbent interac-
tion [34, 35].
Testing of Synthesized Catalysts in Liquid-Phase
Fischer–Tropsch Synthesis
Fischer–Tropsch synthesis in the presence of the
obtained catalytic systems was carried out in a PARR-
4307 steel reactor (Parr Instrument, USA) using n-
dodecane as a solvent. A mixture of CO and H2 in a
volumetric ratio of 1 : 6 was used as syngas. The high
hydrogen content of the gas mixture is due to the need
for additional hydrogenation of olefins and oxygen-
containing compounds formed in the presence of a
cobalt-containing catalyst [29–32]. The process tem-
perature was 200°C, the total pressure in the reactor
was 2.0 MPa, the catalyst weight was 0.1 g, the solvent
volume was 30 cm3.
The liquid phase was analyzed by GCMS using a
GC-2010 gas chromatograph and a GCMS-QP2010S
mass spectrometer (SHIMADZU, Japan). Liquid
phase samples were taken after 3 h of the reaction after
cooling and condensing the liquid products. The anal-
ysis of the liquid phase was carried out under the fol-
lowing conditions: the sample volume, 3 mm3; the ini-
tial temperature of the column 150°C was maintained
for 6 min, then the temperature was increased to
250°C at a heating rate of 15°C/min; injector tempera-
ture, 280°C; carrier gas, helium; helium pressure,
253.5 kPa; total flow rate of helium, 81.5 cm3/min;
linear flow rate of helium, 20.8 cm3/s; a HP-1MS col-
umn with L = 30 m and a film thickness 0.25 μm and
d of 0.25 mm; ion source temperature, 260°C; inter-
face temperature, 280°C; scanning mode from 10 to
800 m/z; scanning speed, 625; and electron impact
ionization.
The analysis of the pore size distribution for the
studied samples (Fig. 2) showed that for the catalysts
synthesized in subcritical conditions, the number of
pores with a size of less than 6 nm decreased compared
with that in the support. This indicates the formation
of the particles of active phase in silica micropores,
although in the case of the catalyst synthesized by
impregnation, the particles of active phase were
formed in the mesopores.
The gas phase samples with a volume of 1 cm3 were
taken after 3 h of the reaction after cooling and con-
densing liquid products. Analysis of the gas phase was
carried out by chromatography using a Crystallux
4000M gas chromatograph (MetaKhrom, Russia),
equipped with a flame ionization and thermal con-
ductivity detectors. To separate the components of the
gas mixture, a Packed column (2.5 m long and 3.0 mm
in diameter) filled with granules of a MN270 polymer
adsorbent (Purolight Inc., UK) with a fraction of
125–250 μm was used. The analysis of gas phase was
Microscopic images of cobalt-containing samples
are shown in Fig. 3. For Co/SiO2(H2O) and
Co/SiO2(IPA) catalysts (Figs. 3a, 3b), the formation
carried out under the following conditions: the initial of small nanoparticles with average diameters of 5 and
temperature of the column (40°C) was maintained for 7 nm, respectively, was observed. In this case, a uni-
4 min, then temperature was increased to 250°C at a form distribution of the particles of active phase
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Table 1. Specific surface area and total pore volume of cobalt-containing catalysts
Specific surface area, m2/g
Total pore volume, dm3/g
Sample
Langmuir model
391
BET model
t-plot
269*
121**
SiO2
389
0.25
0.22
0.23
0.21
127*
5**
Сo/SiO2(Н2О)
Сo/SiO2(IPA)
Сo/SiO2(EG)
128
205
315
132
207
327
179*
25**
274*
53**
* Surface area of mesopores; ** Surface area of micropores.
occurred on the support surface. The highest density observed. The lowest density and irregular distribution
of the active phase particles on the support surface
were detected.
of particle distribution was observed for the sample
synthesized in subcritical water. It indicates a high
content of cobalt on the silica surface. For the catalyst
synthesized by impregnation (Fig. 3b), the formation
Figure 4 shows the survey XPS spectra of the cata-
lyst samples. The analysis of the elemental composi-
of particles with an average diameter of 20 nm was tion of the surface showed that cobalt was most fully
(а)
(b)
180
160
140
120
100
80
60
40
20
160
140
120
100
80
60
40
20
0
0.2
0.4
0.6
Ps/P0
0.8
1.0
0
0.2
0.4
0.6
Ps/P0
0.8
1.0
(c)
(d)
160
140
120
100
80
160
140
120
100
80
60
60
40
40
20
20
0
0.2
0.4
0.6
Ps/P0
0.8
1.0
0
0.2
0.4
0.6
Ps/P0
0.8
1.0
Fig. 1. Nitrogen adsorption–desorption isotherms for the samples: (a) SiO , (b) Co/SiO (H O), (c) Co/SiO (IPA), and
2
2
2
2
(d) Co/SiO (EG).
2
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MARKOVA et al.
Figures 6a–6c show the survey CO DRIFT spectra
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
of the catalysts. Comparison of the spectra showed
that for the samples synthesized by impregnation and
in the subcritical propanol-2 medium, the narrow
absorption band at 3740 cm–1 with a tail extended to
the lower frequencies is observed. This band is
attributed to the vibrations of the isolated Si–OH
groups and OH groups perturbed by the hydrogen
bond. In the same spectral region, broad absorption
bands of alcohol OH groups were registered. During
CO adsorption, especially after prolonged exposure,
bands at 3680 cm–1 appeared in the spectra of the sam-
ples, and the intensity of a band at 3740 cm–1
decreased. The analysis of spectra suggests that the
appearance of a new band is associated with the inter-
action of CO with OH groups on the support surface.
In the spectrum of the sample synthesized in the sub-
critical water medium, a narrow band at 3736 cm–1
and a broad band at ~3669 cm–1 attributed to the oscil-
lations of the isolated Si–OH and OH groups per-
turbed by the hydrogen bond are observed. For this
sample, no changes in this spectral region were
detected during CO adsorption. The analysis of the
high-resolution spectra (Fig. 6d) showed that, for all
samples after CO adsorption, the absorption bands at
2160–2180 cm–1 appeared, characterizing the valence
vibrations of the C=O bond of carbon monoxide mol-
ecule adsorbed in a linear form on Co2+ cations [40].
The positions of these bands almost do not change
during long CO adsorption. When CO desorbed into
the vacuum at room temperature, these bands disap-
peared from the spectrum. Comparison of high-reso-
lution spectra for the studied samples showed that the
sample synthesized in the medium of subcritical water
has the highest accessibility of Co2+ cations for CO
molecules. Thus, the cobalt-containing catalyst syn-
thesized in the medium of subcritical water has better
physical and chemical characteristics compared with
other studied samples.
SiO2
a
b
c
d
Co/SiO2(H2O)
Co/SiO2(IPA)
Co/SiO2(EG)
0
10
20
30
40
50
Dpore, nm
Fig. 2. Pore size distribution for samples (a) SiO , (b)
Co/SiO (H O),
Co/SiO (EG).
2
(c)
Co/SiO (IPA),
and
(d)
2
2
2
2
adsorbed on silica in the subcritical water medium
(11.8 at %). In the case of subcritical propanol-2 and
ethylene glycol, cobalt was found on the surface in
negligible amounts (1.3 and 0.5 at %, respectively),
which is confirmed by the results of TEM analysis.
Analysis of high-resolution spectra [36, 37] for
cobalt (Fig. 5) showed that the surface composition of
Co/SiO2(IPA) and Co/SiO2(EG) samples (Figs. 5b, 5c)
is almost identical and is represented mainly by
cobalt(II) hydroxide (binding energies of 781.0 and
785.5 eV). Cobalt(III) and cobalt(II) oxides (797.1 and
802.9 eV, respectively) were also observed on the sur-
face of these samples. For the Co/SiO2(H2O) sample
(Fig. 5a), the main surface compounds were found to
be cobalt(II) hydroxide (786.2 and 803.5 eV). Also, a
mixed cobalt oxide (Co3O4) with 797.7 eV was
observed on the surface of the sample synthesized in
subcritical water. This compound exhibits catalytic
activity in many reactions (hydrogenation of carbon
oxides, deoxygenation of organic acids, etc.) [38]. The
formation of mixed Co2+/Co3+ oxide can be explained
The samples were tested in liquid-phase Fischer–
Tropsch synthesis, which allows obtaining a wide
range of gaseous, liquid and solid products. In the
by the oxidative effect of the water molecules in a sub- recent years, researchers has focused on the liquid-
critical state [39].
phase process, which makes it possible to control of
100 nm
100 nm
100 nm
(а)
(b)
(c)
Fig. 3. TEM images of cobalt-containing samples: (a) Co/SiO (H O), (b) Co/SiO (IPS), and (c) Co/SiO (EG).
2
2
2
2
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623
(а)
(b)
20
15
10
5
50
40
30
20
10
1000 800
600
400
200
0
1000 800
600
400
200
0
Binding energy, eV
Binding energy, eV
(c)
70
60
50
40
30
20
10
1000 800
600
400
200
0
Binding energy, eV
Fig. 4. Survey X-ray photoelectron spectra of cobalt-containing samples: (a) Co/SiO (H O), (b) Co/SiO (IPA), and (c) Co/SiO (EG).
2
2
2
2
(а)
(b)
50
45
40
35
30
25
Position FWHM % of area
1
2
3
4
Co2p3/2
Co2p1/2
Co2p3/2 sat 786.20
Co2p1/2 sat 803.53
781.67
797.76
3.552
3.391
6.749
4.944
36.681
18.990
30.947
13.381
76
74
72
70
68
66
1
4
2
4
1
3
3
2
Position FWHM % of area
1
2
3
4
Co2p3/2
Co2p1/2
Co2p3/2 sat 785.55
Co2p1/2 sat 802.88
781.01
797.11
3.796
3.624
7.212
5.284
36.884
18.542
31.118
13.456
810 805 800 795 790 785 780 775
Binding energy, eV
810 805 800 795 790 785 780 775
Binding energy, eV
(c)
174
172
170
168
2
166
164
162
160
158
4
1
3
Position FWHM % of area
1
2
3
4
Co2p3/2
Co2p1/2
781.05
797.15
3.291
3.141
6.252
4.580
36.681
18.991
30.947
13.382
Co2p3/2 sat 785.59
Co2p1/2 sat 802.92
810 805 800 795 790 785 780 775
Binding energy, eV
Fig. 5. High-resolution X-ray photoelectron spectra for cobalt-containing samples: (a) Co/SiO (H O), (b) Co/SiO (IPA), and
2
2
2
(c) Co/SiO (EG).
2
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MARKOVA et al.
(а)
(b)
25
20
15
10
5
12
9
1
2
3
4
5
Vacuum, 400°С, 2 h
1
2
3
4
5
Vacuum, 400°С, 2 h
5
1
CO, 12 Torr, 20°С, 10 min
CO, 12 Torr, 20°С, 60 min
CO, 12 Torr, 20°С, 45 h
Vacuum, 400°С, 30 min
CO, 12 Torr, 20°С, 10 min
CO, 12 Torr, 20°С, 60 min
CO, 12 Torr, 20°С, 45 h
Vacuum, 400°С, 30 min
3
4
2
6
5
4
3
1
2
3
0
0
6000 5000 4000 3000 2000 1000
Wavenumber, cm–1
6000 5000 4000 3000 2000 1000
Wavenumber, cm–1
(c)
(d)
12
0.14
4
1
5
1
2
3
4
5
Vacuum, 400°С, 2 h
Co/SiO2(EG) (2167 сm–1
Co/SiO2(IPA) (2164 сm–1
Co/SiO2(H2O) (2180 сm–1
)
)
1
2
3
3
0.12
0.10
0.08
0.06
0.04
0.02
0
CO, 12 Torr, 20°С, 10 min
CO, 12 Torr, 20°С, 60 min
CO, 12 Torr, 20°С, 45 h
Vacuum, 400°С, 30 min
)
9
6
3
0
2
3
2
1
6000 5000 4000 3000 2000 1000
Wavenumber, cm–1
2500 2400 2300 2200 2100 2000 1900 1800
Wavenumber, cm–1
Fig. 6. Survey CO DRIFT spectra of the catalysts: (a) Co/SiO (H O), (b) Co/SiO (IPA), (c) Co/SiO (EG), and (d) high reso-
2
2
2
2
lution CO DRIFT spectrum for cobalt-containing samples.
the chain growth stage due to a decrease in CO solu- in the gas phase was noted, which leads to a decrease
bility, the formation of oxygen-containing products in the selectivity to C5+ hydrocarbons.
and preventing the water-gas shift reaction. Group
Analysis of the liquid phase revealed that the syn-
VIII metals are commonly used as the active sites of
thesized catalysts make it possible to obtain mainly
the Fischer–Tropsch synthesis catalysts. Ruthenium,
linear and cyclic C5–C7 hydrocarbons. When
cobalt, iron, and nickel are the most active. To
Co/SiO2(H2O) and Co/SiO2(IPA) catalysts are used,
a shift in the selectivity to C6–C7 hydrocarbons was
observed. The Co/SiO2(EG) catalyst showed a rather
high selectivity to the aromatic and cyclic hydrocar-
bons. When the Co/SiO2(H2O) catalyst is reused, its
selectivity to C6–C7 hydrocarbons is retained, but
there is a slight increase in the heptadecane content
and the cyclohexane content due to benzene hydroge-
nation. This may be due to the transformation of prod-
ucts adsorbed in the pores of the catalyst. In the case
of reuse of Co/SiO2(IPA), there is a significant
increase in heptadecane content, which indicates a
chain growth reaction. For the catalyst synthesized by
impregnation, an increase in the content of aromatic
and cyclic hydrocarbons was observed, which may be
due to hexane cyclization in the pores of support.
increase the surface area, these metals are deposited
on the porous supports, such as silica gel. Cobalt and
iron catalysts are the most commonly used in industry.
Figure 7 shows the results of catalytic tests.
The obtained samples showed high efficiency in
the process of the production of liquid C5–C7 hydro-
carbons (Fig. 7). Analysis of the gas phase showed the
presence of methane and ethane for all synthesized
samples. However, when using catalysts obtained
under subcritical conditions, the formation of a
smaller amount of C1–C2 alkanes was observed com-
pared with the catalyst obtained by impregnation. Pro-
pane and butane in the gas phase were observed in
trace amounts. The reuse of the studied catalysts
showed that the composition of the gas phase almost
did not change for the Co/SiO2(H2O) and
Co/SiO2(IPS) samples. For the Co/SiO2(H2O) cata-
The calculation of molecular-weight distribution
lyst, an increase in the content of methane and ethane of the Fischer–Tropsch synthesis products showed
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STUDY OF THE STRUCTURE OF COBALT-CONTAINING CATALYSTS
(а)
625
80
60
40
20
0
Hexane
Heptane
Cyclohexane
Benzene
Pentane
(b)
14
12
10
8
6
4
2
Methane
Ethane
0
С3–С4
Fig. 7. Composition of liquid (a) and gas (b) phase of Fischer–Tropsch synthesis.
that in the presence of the catalysts synthesized in the method showed that subcritical synthesis is a promis-
medium of subcritical water and propanol-2 the α
coefficient was found to be 0.77 and 0.68, respectively.
These values indicated the maximum yield of C6
hydrocarbons which correlates well with experimental
and literature data [29, 31, 32]. For the catalyst syn-
thesized by incipient-wetness impregnation, the value
of α was found to be 0.61, that indicates a product shift
toward low-weight hydrocarbons and high selectivity
to methane. Thus, the catalyst synthesized in subcriti-
cal water showed the highest efficiency in the liquid-
phase Fischer–Tropsch synthesis allowing the forma-
tion of С5–С7 liquid hydrocarbons with the highest
yield of C6 hydrocarbons. The results of catalyst testing
correlated well with data on the composition and
structure of the catalysts.
ing alternative to traditional methods, which allows
obtaining the samples with the developed internal sur-
face, uniform distribution, smaller size and higher
accessibility of active phase particles for reagents. Sub-
critical water is the optimal medium for the catalyst
synthesis. The tests of catalysts in the liquid-phase
Fischer–Tropsch synthesis showed that the catalyst
synthesized in subcritical water was the most effective.
ABBREVIATIONS AND NOTATION
Tc
critical temperature
Pc
critical pressure
TEM
BET
XPS
transmission electron microscopy
Brunauer–Emmet–Teller model
X-ray photoelectron spectroscopy
CONCLUSIONS
Comparison of cobalt-containing catalysts synthe-
sized in subcritical conditions and by impregnation
α
chain growth coefficient
KINETICS AND CATALYSIS Vol. 60 No. 5 2019

626
MARKOVA et al.
FUNDING
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The work was supported by the Russian Foundation for
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