Effect of Promoters of Ultrafine Catalysts on the Formation of Carbon Dioxide under Conditions of Fischer–Tropsch Synthesis in a Three-Phase System

Article abstract of DOI:10.1134/S0965544120010090

Abstract: The effect of the properties of modifying additives (Mg and Ca) introduced into nanosized catalytic dispersions on the occurrence of three-phase Fischer–Tropsch synthesis has been studied. It has been found that the introduction of Mg and Ca into the catalytic dispersion composition contributes to a redistribution of oxygen-containing products; namely, it provides a decrease in the yield of the main byproduct—CO₂—and an increase in the yield of alcohols dissolved in water.

Full text of DOI:10.1134/S0965544120010090

ISSN 0965-5441, Petroleum Chemistry, 2020, Vol. 60, No. 1, pp. 75–80. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Neftekhimiya, 2020, Vol. 60, No. 1, pp. 83–88.
Effect of Promoters of Ultrafine Catalysts on the Formation
of Carbon Dioxide under Conditions of Fischer–Tropsch Synthesis
in a Three-Phase System
a,
a
a, b
M. V. Kulikova *, O. S. Dement’eva , and S. I. Norko
a
Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, 119991 Russia
b
Moscow State University, Moscow, 119991 Russia
*
e-mail: m_krylova@ips.ac.ru
Received August 20, 2019; revised August 29, 2019; accepted September 9, 2019
Abstract—The effect of the properties of modifying additives (Mg and Ca) introduced into nanosized catalytic
dispersions on the occurrence of three-phase Fischer–Tropsch synthesis has been studied. It has been found
that the introduction of Mg and Ca into the catalytic dispersion composition contributes to a redistribution
of oxygen-containing products; namely, it provides a decrease in the yield of the main byproduct—CO —and
2
an increase in the yield of alcohols dissolved in water.
Keywords: Fischer–Tropsch synthesis, modified Fe catalysts, nanosized catalysts, three-phase system
DOI: 10.1134/S0965544120010090
INTRODUCTION
dynamic mode of operation of the reactor and can
therefore increase the feedstock conversion per pas-
sage and the catalyst efficiency compared with con-
ventional systems [4, 5].
The ever-tightening environmental requirements
give an impetus to the development of processes for
the production of fuels with a low content of sulfur and
aromatic compounds [1]. Currently, there is a steadily
growing demand for hydrocarbon (HC) fuels with an
ultralow content of heteroatoms (less than 10 ppm sul-
fur and nitrogen [2, 3]). In view of decreasing the
anthropogenic impact on the environment and the
consequences of this impact, particular attention is
Iron-containing catalysts are most advantageous
for Fischer–Tropsch synthesis, because they are inex-
pensive and provide the formation of HCs from syn-
thesis gas with a low Н /СО ratio, which is character-
2
istic of a gas produced from biomass or coal [6, 7]. The
use of nanosized catalysts in slurry reactors leads to a
significant increase in the system efficiency with
respect to target synthesis products to 300–
paid to the development of the so-called CO -neutral
2
processes. Therefore, taking into account the deple-
tion of easily refinable oil reserves, interest in XTL
class processes providing the formation of environ-
mentally friendly synthetic HCs from low-grade raw
materials naturally arises. In particular, Fischer–
Tropsch synthesis is a promising method to produce a
broad range of HCs from coal, natural gas, and bio-
mass.
5
00 g /kg /h [5]; this value is several times higher
C5+ Me
than the efficiency observed in the presence of con-
ventional systems [8]. Despite the above advantages,
nanodispersions have disadvantages characteristic of
supported and micrometer-sized Fe catalysts: they are
prone to deactivation by oxygen-containing reaction
products and the formation of a large amount of CO
2
by the water-gas shift reaction; therefore, a portion of
the carbon is irreversibly converted to a byproduct of
the process [9, 10].
In the case of implementation of the Fischer–
Tropsch process using fixed-bed reactors, the catalyst
activity continuously decreases owing to local over-
heating of the fixed catalyst bed due to the reaction
exothermicity. In slurry reactors, this disadvantage is
eliminated owing to the presence of a dispersion
medium, which efficiently removes heat; however, the
The main methods to hinder the formation of car-
bon dioxide in the presence of Fe catalysts are the
selection of reaction conditions (temperature, pres-
sure, H /CO ratio, partial pressure of water) and the
2
linear flow rates of the feed gas should be high to modification of the catalyst with components that
maintain the microspherical catalyst in suspension. A form a surface that is not prone to the readsorption of
promising approach to implementing the reaction in a water undergoing reaction with adsorbed carbon mon-
gas–liquid–solid system is the use of nanosized dis- oxide to form CO [11–18]. In particular, the authors
2
persions that do not impose restrictions on the hydro- of [12] subjected iron-containing catalysts to chemical
75

7
6
KULIKOVA et al.
modification by depositing a SiO film formed by the additives (Ca and Mg) and suspended in a HC
2
decomposition of tetraethoxysilane on the catalyst medium were studied.
surface. The cited authors showed that the surface
modification did not lead to a decrease in the adsorp-
tion capacity of the catalysts with respect to CO: inhi-
bition of water readsorption and, thus, the water-gas
shift reaction was the result of the formation of a
hydrophobic shell on the active sites, owing to which
The study revealed the effect of the modifying
additive content on the selectivity of the catalytic sus-
pension for carbon dioxide and the target synthesis
products.
EXPERIMENTAL
the formation of CO decreased from 40 to 5% in terms
2
of carbon.
Iron-containing disperse systems for HC synthesis
by the Fischer–Tropsch method were prepared by the
thermal decomposition of iron precursor solutions
The introduction of alkali and alkaline earth metal
additives into Fe catalysts leads to the inhibition of the
oxidation of iron carbide phases, which mediate the
polymerization of carbon-containing moieties to form
HCs, to magnetite, which is the most active phase for
the occurrence of the water-gas shift reaction [13–15].
(
thermolysis).
A calculated amount of iron nitrate was introduced
into a molten dispersion medium (mixture of solid
paraffins of the C –C fraction) from a 40% aqueous
19
32
The authors of [13] found that the promotion of cata- solution at 280°C under stirring in an inert gas stream.
lysts with Mg leads to a decrease in the magnetite con- A number of iron-based samples containing 4.5–
tent in the iron-containing phase of the samples from 12.0 wt % of calcium and magnesium as modifying
4
3.8 to 37.9%; it is this factor to which the cited agents were synthesized. The iron, magnesium, and
authors attributed the decrease in CO selectivity from calcium precursors were nitrates of these metals.
2
Weighed portions of the precursors were calculated for
the zero-valent form of the component; in all cases,
the iron content in the disperse system was 1 wt %; the
amount of the modifying agent was calculated as a
percentage of the Fe content.
50 to 40.8%. It was shown [14] that, under the action
of Ca and Mg, the fraction of carbon-enriched iron
carbides increases; the presence of these carbides con-
tributes to an enhancement of CO adsorption owing to
an increase in the amount of carbon involved in the
chain growth. A study of a set of Fe catalysts with K,
The particle size of the synthesized samples was
Be, Mg, Ca, and Ba additives revealed the inhibitory determined by dynamic light scattering on a Malvern
effect of Ca and Mg in the catalyst composition on the Zetasizer Nano ZS instrument. Sample preparation
catalyst activity in the water-gas shift reaction, was conducted by dissolving 0.01 g of a suspension
whereas other additives contributed to an increase in sample in 10 mL of n-hexane with the addition of 5 wt
the rate of formation of carbon dioxide [15]. A similar % of a surfactant (dioctyl sodium sulfosuccinate).
effect was observed in [16]: the maximum content of
iron carbide species on the catalyst surface was
achieved with an increase in the Mg/Fe ratio to 0.07;
in this case, the fraction of CO converted to HCs
increased with a simultaneous decrease in the amount
of produced carbon dioxide.
X-ray diffraction (XRD) analysis of the
samples was conducted on a Rigaku Rotaflex
D/MAX-RC X-ray diffractometer with a rotating cop-
per anode (wavelength of characteristic K radiation
α
λ = 0.1542 nm). A secondary monochromator (graph-
ite single crystal) was used to minimize the contribu-
tion of the fluorescence emission of iron atoms to the
experimental XRD patterns. X-ray diffraction record-
ing was conducted using a horizontal wide-angle goni-
ometer employing the Bragg–Brentano focusing
scheme in the θ–2θ geometry. Scanning was con-
ducted in a 2θ angular range of 10°–70° at a rate of
The promotion of catalyst systems with Mg, Ca,
and La, along with the inhibition of the formation of
carbon dioxide, contributes to a change in the catalyst
efficiency with respect to unsaturated HCs and oxy-
gen-containing compounds owing to an increase in
the surface basicity during the formation of solid solu-
tions of iron oxides and promoters and, as a conse-
quence, an enhancement of the dissociative adsorp-
tion of CO [17, 18].
2
deg/min in increments of 0.04 deg.
The catalytic activity of the dispersion samples was
determined in a gas flow system with a slurry rector.
The catalyst systems were preactivated under a carbon
monoxide pressure (2 MPa) at a temperature of 360°С
for 12 h. Hydrocarbons were synthesized in a synthesis
Owing to the specific features of formation of the
active phase of nanosized suspensions, compared with
impregnated and coprecipitated catalysts [19, 20],
conventional methods of promotion and modification
of catalyst systems cannot be directly used for disper-
sions intended for the implementation of Fischer–
Tropsch synthesis in a slurry reactor.
gas stream at a flow rate of 10 L/h and a CO : H molar
2
ratio of 1 : 1 under a pressure of 2 MPa in a tempera-
ture range of 260–320°C with an increase in tempera-
ture by 20°C every 12 h. Sampling of the gas and the
liquid for analysis was conducted for each temperature
To explore the problem of targetedly changing the regime 8–12 h after the increase in temperature. The
properties of these systems, nanosized Fe-containing feed synthesis gas and the gaseous synthesis products
catalyst systems modified with alkaline earth metal were analyzed by gas–solid chromatography on a
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EFFECT OF PROMOTERS OF ULTRAFINE CATALYSTS ON THE FORMATION
77
(a)
(b)
Fe3O4
Fe7C3
Fe5C2
Fe3O4
Fe7C3
Fe C
600
600
400
200
0
5
2
1
2% Mg
1
2% Mg
400
7.5% Mg
7.5% Mg
4
.5% Mg
% Mg
200
4.5% Mg
0
0% Mg
20
30
40
50
θ, deg
60
70
20
30
40
50
2θ, deg
60
70
2
Fig. 1. X-ray diffraction patterns of (a) the Fe–(0–12%)Mg and (b) Fe–(0–12%)Ca samples.
Kristallyuks-4000 chromatograph equipped with a method in a temperature range of 260–320°C to
thermal conductivity detector using helium as a carrier determine the phase composition of the active catalyst
gas. To separate CO, CH , and N , a 3 m × 3 mm col- surface formed directly during the CO hydrogenation
4
2
umn packed with CaA molecular sieves was used; CO
reaction. It was shown that the introduction of cal-
cium and magnesium additives contributes to a more
2
and C –C HCs were separated using a HayeSep T-
2
4
vigorous formation of iron carbides Fe C and Fe C
3
packed column. Liquid HCs were analyzed on a
Kristallyuks-4000M chromatograph equipped with a
flame ionization detector and a 50 m × 0.32 mm OV-
5
2
7
(
Figs. 1a, 1b), the presence of which in the catalyst
surface composition inhibits the water-gas shift reac-
tion owing to a decrease in the content of the magne-
tite phase, which is active in this reaction [13, 14].
3
51 capillary column with temperature programming
(
flow rate of helium, hydrogen, and air of 30, 25, and
250 mL/min, respectively). The aqueous phase was
It was found that, in the presence of a magnesium
additive, an increase in the additive content above
analyzed by gas–solid chromatography on a Kristally-
uks-4000M chromatograph equipped with a flame
ionization detector. The flow rate of helium, hydro-
gen, and air was 20, 25, and 250 mL/min, respectively.
A 50 m × 0.32 mm × 0.50 μm HP-FFAP capillary col-
umn (nitroterephthalic acid-modified polyethylene
glycol) was used for analysis. The sample volume was
7
.5% of the amount of the iron-containing phase leads
to a decrease in the amount of the iron carbide phase,
while the effect of the introduction of magnesium is
preserved in the entire concentration range.
The catalytic dispersions were analyzed by
0
7
7
.3 μL. The following temperature schedule was used: dynamic light scattering immediately after formation
0°С (8 min), temperature-programmed heating from by droplet thermolysis. It was shown that, in all the
0 to 110°С at a rate of 10°С/min, temperature-pro-
grammed heating from 110 to 220°С at a rate of
15°С/min, and holding at 220°C for 10 min.
45
40
35
–
To determine the catalyst activity, the following
7.5% Mg
parameters were selected: CO conversion X , % (per-
CO
7.5% Ca
centage ratio of the weight of reacted CO to the weight
30
25
20
of CO fed into the reaction zone); product yield Y ,
N
3
g/m (amount of product N (in grams) formed per
cubic meter of normalized synthesis gas passed
through the catalyst); and product selectivity S , %
15
10
5
N
(
percentage ratio of carbon consumed for the forma-
tion of the product to the total amount of reacted car-
bon).
0
2
4
6
Effective diameter, nm
RESULTS AND DISCUSSION
Samples of catalytic suspensions were studied by
XRD after preactivation in a carbon monoxide stream
and testing in HC synthesis by the Fischer–Tropsch
Fig. 2. Number-average particle size distribution of the
dispersed phase for Fe systems with the different modify-
ing additives.
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7
8
KULIKOVA et al.
Table 1. Effect of the modifying additive on the activity of
catalytic suspensions in HC synthesis by the Fischer–
Tropsch method (T = 300–320°C, XCO = 60%)
In a general case, the introduction of modifiers
into the catalyst system composition led to a signifi-
cant change in the molecular weight distribution of the
reaction products and affected the yield of both gas-
eous and liquid reaction products.
Yield, g/m3
Modifying
additive
P,
g/(kgMe h)
С1 С –С С СО Oxy H O
2
4
5+
2
2
In particular, the introduction of a magnesium
additive in an amount of 4.5 wt % of the iron phase
content led to a decrease in the formation of C –C
–
20
28
47 216
42 128
66 142
48 191 13 62
85 127 22 108
37 154 13 44
45 169 23 67
4
6
8
34
40
83
471
415
656
478
847
373
454
1
4
4
4
7
7
1
1
.5% Mg 13
.5% Ca 18
.5% Mg 30
.5% Ca 13
2% Mg
18
25
30
18
26
29
HCs and a simultaneous increase in the alcohol yield.
The olefin content in the composition of the liquid
HCs increased from 26.4 to 36.1 wt %; in this case, the
molecular weight distribution of the reaction products
was shifted toward the formation of a gasoline fraction
20
25
(
Table 2). A further increase in the magnesium con-
2% Ca
tent to 7.5–12% caused an intensification of the for-
mation of a methane–butane fraction; however, in
this case, the amount of produced carbon dioxide
remained lower than the values recorded for the
cases of introduction of magnesium and calcium addi-
tives, the particle diameter of the dispersed phase was unmodified sample. For these systems, the olefin con-
higher than that in the unpromoted catalyst: 2.1– tent in the product composition achieved 39.4%; in
3
.3 nm versus 0.7 nm (Fig. 2). However, in the pres- addition, 80% of unsaturated compounds were con-
ence of the modifiers, during the Fischer–Tropsch tained in the gasoline fraction of HCs.
synthesis, the dispersed phase particles underwent a
For the calcium-modified systems, a more signifi-
slight coarsening—by 15–20%—versus a fourfold cant change in the formation of carbon dioxide was
increase (up to 3 nm) in the diameter of the metal- observed: in the entire range of activity of the samples,
the selectivity for this product decreased by no less
than 20%; in the region of low conversions, it was 45–
containing particles observed for the unpromoted
sample.
50% (Fig. 3).
The modified Fe catalysts exhibited a low activity
in the secondary water-gas shift reaction. Thus, in all
In the presence of Ca-containing catalysts, the
yield of liquid HCs and alcohols increased by 1.8 and
cases, in the presence of Mg and Ca, the carbon diox- 5.7 times, respectively (compared with the system
ide selectivity decreased owing to the consumption of without additives); the content of unsaturated struc-
oxygen for water and oxygenates soluble in the aque- tures in the composition of C HCs also increased; it
5+
ous phase (mostly С –С alcohols) (Table 1, Fig. 3).
was 38.4 wt % at a Ca content of 7.5% (Tables 1, 2).
1
4
The relative content of alcohols (in % of the total
mass) synthesized in the presence of dispersed iron-
containing systems with additives of modifying agents
in varying concentration is shown in Fig. 4. The intro-
duction of small amounts of modifiers (4.5% Ca and
Mg) contributed to an increase in the C alcohol con-
5
4
3
2
0
0
0
0
–
4.5% Mg
4
+
7.5% Mg
12% Mg
tent, whereas an increase in the additive concentration
led to an intensification of the formation of C alco-
10
2+
hols: their fraction in the composition of oxygen-con-
taining products increased from 75% for the sample
without additives to 80–82%.
0
20
20
40
40
60
80
100
XCO, %
50
40
30
20
–
CONCLUSIONS
4.5% Ca
7.5% Ca
12% Ca
A number of samples of nanosized iron-containing
suspensions with magnesium and calcium additives
prepared by droplet thermolysis have been studied. It
has been found that the introduction of the modifying
additives in an amount of 4.5–12.0 wt % of the iron
10
0
60
80
100
XCO, %
content leads to a decrease in the CO yield by 20–
2
4
0%, while the activity of the catalyst system with
respect to the feedstock conversion is preserved. The
introduction of magnesium and calcium additives
leads to an increase in the content of the gasoline frac-
Fig. 3. Effect of the feedstock conversion on the carbon
dioxide selectivity in the presence of catalytic suspensions
with different modifying additives.
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EFFECT OF PROMOTERS OF ULTRAFINE CATALYSTS ON THE FORMATION
79
Table 2. Effect of the modifying additive on the composition of liquid HCs synthesized by the Fischer–Tropsch method
(T = 300–320°C, XCO = 60%)
Group composition
Fractional composition
С –С
Modifying
additive
α
n-paraffins
isoparaffins
olefins
С –С
С +
5
10
11
18
19
–
30.9
31.9
31.1
32.1
29.2
25.2
33.7
42.8
36.1
35.5
34.7
32.4
35.3
34.0
26.4
32.0
33.4
30.8
38.4
39.4
32.2
62.9
69.2
68.2
59.0
70.0
62.0
71.9
32.0
5.2
2.2
2.3
6.9
2.9
4.0
0.8
0.75
0.74
0.73
0.77
0.70
0.74
0.71
4
4
7
7
.5% Mg
.5% Ca
.5% Mg
.5% Ca
2% Mg
2% Ca
28.6
29.5
34.1
27.0
34.0
27.3
1
1
tion of HCs from 63 to 70%; in all cases, the amount tent is formed (Fe C versus Fe C for the unmodified
5
2
7
3
of unsaturated products increased from 26 to 30–
sample).
4
4 wt %. The introduction of magnesium and calcium
as modifiers provides an increase in the amount of С
alcohols from 75 to 80–82 rel %.
2+
FUNDING
It has been shown that, in all cases, the introduc-
tion of the additives leads to an increase in the particle
diameter of the dispersed phase compared with this
This work was performed at the Topchiev Institute of
Petrochemical Synthesis of the Russian Academy of Sci-
parameter of the unpromoted catalyst: 2.1–3.3 nm ences and supported by the Russian Science Foundation,
versus 0.7 nm; however, during HC synthesis by the project no. 17-73-30046.
Fischer–Tropsch method, the particle size of the dis-
persed phase with modifier additives increased by no
more than 20%.
CONFLICT OF INTEREST
X-ray diffraction analysis has revealed that, in the
presence of magnesium and calcium modifying addi-
tives, during the preactivation of the catalysts with car-
bon monoxide (carbidizing) and exposure to synthesis
One of the authors—M.V. Kulikova—is executive secre-
tary of the Petroleum Chemistry journal. The other authors
declare that there is no conflict of interest regarding the
gas, an iron-containing phase with a high carbon con- publication of this manuscript.
Alcohol composition
100%
9
0%
0%
0%
0%
0%
0%
0%
0%
8
7
6
5
4
3
2
10%
0%
4.5% Mg
4.5% Ca
C1 C2 C3 C4 C5 C6
7.5% Mg
7.5% Ca
12% Mg
12% Ca
–
Fig. 4. Effect of the nature and concentration of the modifying additive on the composition of alcohols synthesized by the
Fischer–Tropsch process (T = 300–320°C, X = 60%).
CO
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KULIKOVA et al.
11. D. B. Bukur, B. Todic, and N. Elbashir, Catal. Today
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Translated by M. Timoshinina
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Vol. 60
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2020