Fischer–Tropsch synthesis with cobalt catalyst and zeolite multibed arrangement

Article abstract of DOI:10.1134/S0965544116030026

The role of zeolite in transformations of hydrocarbons produced from CO and H₂ over a Fischer–Tropsch cobalt catalyst under the conditions of multibed arrangement of the cobalt catalyst and the zeolite has been determined. Hydrocarbon conversion over the HBeta zeolite occurs via the bimolecular mechanism, as evidenced by a low methane yield and a high yield of unsaturated gaseous and liquid hydrocarbons. The conversion over the CaA zeolite obeys the unimolecular mechanism, as evidenced by the formation of increased amounts of methane and saturated gaseous C₂–C₄ hydrocarbons.

Full text of DOI:10.1134/S0965544116030026

ISSN 0965ꢀ5441, Petroleum Chemistry, 2016, Vol. 56, No. 3, pp. 275–280. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © E.Yu. Asalieva, E.V. Kul’chakovskaya, L.V. Sineva, V.Z. Mordkovich, B.M. Bulychev, 2016, published in Neftekhimiya, 2016, Vol. 56, No. 3, pp. 295–300.
Fischer–Tropsch Synthesis with Cobalt Catalyst and Zeolite
Multibed Arrangement
E. Yu. Asalievaa, b, E. V. Kul’chakovskayaa, L. V. Sinevaa, c, V. Z. Mordkovicha, c, and B. M. Bulychevb
a Technological Institute for Superhard and Novel Carbon Materials, Troitsk, Moscow oblast, Russia
b Moscow State University, Moscow, Russia
c OOO INFRA Tekhnologii, Moscow, Russia
eꢀmail: e.asalieva@tisnum.ru
Received October 15, 2015
Abstract—The role of zeolite in transformations of hydrocarbons produced from CO and H2 over a Fischer–
Tropsch cobalt catalyst under the conditions of multibed arrangement of the cobalt catalyst and the zeolite
has been determined. Hydrocarbon conversion over the HBeta zeolite occurs via the bimolecular mechaꢀ
nism, as evidenced by a low methane yield and a high yield of unsaturated gaseous and liquid hydrocarbons.
The conversion over the CaA zeolite obeys the unimolecular mechanism, as evidenced by the formation of
increased amounts of methane and saturated gaseous C2–C4 hydrocarbons.
Keywords: Fischer–Tropsch synthesis, cobalt catalyst, zeolite, skeletal cobalt
DOI: 10.1134/S0965544116030026
Synthetic zeolites are used as catalysts in many
Studying the mechanisms of conversion of hydroꢀ
chemical and petrochemical processes because they carbons formed in the FTS on the acid sites of zeolites
exhibit unique properties, such as acidity and specific is a quite important task, since the preparation and
structure and shape of the cavities and channels. The optimization of these systems requires understanding
Fischer–Tropsch (FTS) synthesis is no exception; this of the processes that occur on the surface. The knowlꢀ
process is a key stage of the technology for the producꢀ edge of the mechanisms will make it possible to design
tion of highꢀquality synthetic oil from syngas (mixture more sophisticated systems that will provide the selecꢀ
of CO and Н2), which is derived from nonpetroleum tive production of synthetic oil with a desired group
carbonaceous raw materials [1–3]. The development and fractional composition. The effect of Hꢀform zeoꢀ
of alternative methods for the production of fuel lites on the composition of the resulting hydrocarbons
hydrocarbons is an important problem in view of the was studied in detail by many investigators [8, 10–11,
limited reserves of oil [4, 5].
13–14]. Lee et al. [15] proposed to use zeolites in the
cationic form for the formation of cobalt clusters of a
certain size; however, the relationship between the
performance of zeolites in this form and the composiꢀ
tion of synthesis products was not studied.
Typically, the active metal used in FTS is cobalt or
iron. It is believed that cobalt catalysts exhibit higher
selectivity and provide the formation of a product that
hardly contains any aromatic and oxygenated hydroꢀ
carbons [6]. Supported cobalt catalysts prepared by
Examination of discrete beds of a cobalt catalyst
impregnating the support with a cobalt salt are most and a zeolite can facilitate the understanding of the
commonly used. A promising alternative is the use of hydrocarbon conversion mechanism and their contriꢀ
skeletal cobalt, which provides additional removal of bution to the product composition and encourage the
heat generated in the reaction owing to intrinsic heat future development of zeoliteꢀcontaining FT cataꢀ
conductivity [7].
lysts. The aim of this study was to determine the effect
of the type of the zeolite as part of a multilayer catalyst
bed containing skeletal cobalt on the composition of
the synthesis products.
In the last decade, the oneꢀstep production of synꢀ
thetic oil from CO and Н2 has attracted the attention
of many scientists around the world [8–11]. It is
known that hydrocarbons formed in the FTS can
undergo transformation in the presence of zeolites or
other solid acids. The introduction of a zeolite into the
catalyst composition provides a significant deviation
EXPERIMENTAL
The test samples were multicomponent beds comꢀ
from the classical Schulz–Flory–Anderson distribuꢀ posed of discrete beds of a cobalt catalyst and a zeolite.
tion and facilitates the production of synthetic oil with The cobalt catalysts to be studied were prepared via
a boiling point below 400
°С
[12–14].
physically mixing an ultrafine aluminum powder to
275

276
ASALIEVA et al.
Total composition of the multicomponent bed
Content, wt %
Sample
cobalt catalyst
Al
zeolite
Co
Al2O3
HBeta/CaA
Co
22
20
20
55
50
50
23
20
20
–/–
10/–
–/10
Co–HBeta
Co–CaA
provide heat removal, utlrafine skeletal cobalt active in respect to space velocity via increasing it to 6000 h–1 in
syngas conversion, and precalcined boehmite (i.e., increments of 1000 h–1 (every 6–12 h) to achieve the
Al2O3). An individual HBeta or CaA zeolite bed sepaꢀ best possible performance at each space velocity of
rated by a silica interlayer of 5 mm thickness was syngas.
loaded to the reactor below the cobalt catalyst; the
Analysis of the feed mixture of syngas and gaseous
beds were. One of the samples (reference) was comꢀ
synthesis products was conducted by gas–solid chroꢀ
posed of the cobalt catalyst only and did not contain
matography using a thermal conductivity detector,
the additional zeolite bed. All powders were diluted
helium as a carrier gas, and temperature programming
with silica in a weight ratio of 1 : 4. The composition of
(
60–200°С). A column with CaA molecular sieves
the multicomponent catalyst beds and the arrangeꢀ
ment of the beds are shown in the table and Fig. 1,
respectively.
was used to separate CO and СН4, and a HayeSepꢀ
packed column was employed to separate СО2 and
С2–С4 hydrocarbons. The rest of the 100% gas by
Hydrocarbons were synthesized in a steel flow weight was hydrogen.
reactor with an inner diameter of 10 mm. Prior to testꢀ
The composition of liquid С5+ hydrocarbons was
ing, the FT catalyst as part of the multicomponent bed
was activated in a hydrogen stream supplied at a space
determined by gas–liquid chromatography using a
flame ionization detector; helium as a carrier gas
(flow rate of 30 mL/min); a 50ꢀm capillary column; a
DBꢀPetro stationary phase; and temperature proꢀ
velocity of 3000 h–1 at 400
°С and 0.1 MPa for 1 h.
After activation, the FT catalyst as part of the mulꢀ
ticomponent bed was conditioned in a stream of synꢀ
gas containing 5 mol % Н2 as an internal standard
(H2/CO molar ratio, 2; space velocity, 1000 h–1; presꢀ
sure, 2 MPa) with a stepwise increase in temperature
gramming (50–270°С; heating rate, 4°C/min).
RESULTS AND DISCUSSION
from 170 to 225°С (by 3–10°Сevery 6 h) to achieve
All the test samples were active in FTS and exhibꢀ
ited a CO conversion of 40–60% (comparison was
conducted at a syngas space velocity of 6000 h–1 and
an optimum temperature for each of the samples: at
269, 274, and 241°C for Co, Co–HBeta, and Co–
CaA, respectively). The reference Co and Co–HBeta
samples exhibited the С5+ hydrocarbon selectivity of
39 and 37%, respectively; the introduction of a sepaꢀ
rate CaA zeolite bed (Co–CaA sample) led to a
decrease in this parameter to 31%. The selectivity for
the main byproduct methane was 32 or 31% in the
presence of the Co or the Co–HBeta sample, respecꢀ
tively, and significantly increased—to 40%—in the
presence of the Co–CaA sample. Considerable differꢀ
ences between the Co–HBeta and Co–CaA zeoliteꢀ
containing samples suggest that the hydrocarbons
formed from syngas on the active sites of the cobalt
catalyst undergo conversion on the acid sites of the
zeolites via different mechanisms. Thus, it is evident
the best possible performance with the given parameꢀ
ters. Catalyst efficiency was calculated as the amount
(in grams) of liquid hydrocarbons produced from the
syngas per kilogram of catalyst per hour. After that, the
synthesis conditions were optimized with respect to
temperature via increasing it by 3–5
°C
and with
(a)
(b)
(c)
Cobalt Catalyst
Cobalt Catalyst
Cobalt Catalyst
Кварц
Silica
Кварц
Silica
Кварц
Silica
HBeta zeolite
CaA zeolite
Кварц
Silica
Кварц
Silica
Fig. 1. Diagram of the multibed arrangement of (a) Co,
(b) Co–HBeta, and (c) Co–CaA samples.
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FISCHER–TROPSCH SYNTHESIS WITH COBALT CATALYST
277
(а)
(b)
Co Co–HBeta Co–CaA
10
9
8
7
6
5
4
3
2
1
6
5
4
3
2
1
0
Co Co–HBeta Co–CaA
0
C H
2
C H
3
C H
4
C H
2
C H
3
C H
4 10
6
8
10
4
6
Fig. 2. Composition of (a) saturated and (b) unsaturated gaseous C –C hydrocarbons.
2
4
than the presence of the CaA zeolite leads to the forꢀ decreased to 2.9 g/m3. In the presence of a discrete
mation of an additional amount of methane.
Co–CaA bed, a substantially lower amount of unsatꢀ
urated hydrocarbons was produced: the yields of ethꢀ
ylene, propylene, and butylene were 0.2, 3.7, and
2.2 g/m3, respectively.
The most commonly assumed mechanisms of
hydrocarbon conversion on zeolite acid sites are the
bimolecular (carbocationic) [16, 17] and unimolecuꢀ
lar (protolytic) mechanisms [18, 19]. Both of them are
chain mechanisms and include three steps: chain iniꢀ
tiation, propagation, and termination. The main
products formed via the bimolecular mechanism are
lower hydrocarbons, mostly unsaturated; in this case,
the production of methane, ethane, and ethylene is
thermodynamically unfavorable, and the main prodꢀ
ucts formed via the unimolecular mechanism are satꢀ
urated hydrocarbons, in particular methane and
ethane [20].
Figure 2 shows the yields of saturated and unsaturꢀ
ated gaseous С2–С4 hydrocarbons formed in the presꢀ
ence of the test discrete beds. The Co sample was charꢀ
acterized by the lowest concentration of ethane, proꢀ
pane, and butane (5.1, 2.5, and 2.7 g/m3, respectively;
Fig. 2a). The introduction of an HBeta zeolite bed
resulted in a slight increase in the yield of saturated
C2–C4 hydrocarbons, up to 5.3 g/m3 of ethane,
2.8 g/m3 of propane, and 3.3 g/m3 of butane. The
introduction of a CaA zeolite bed caused a considerꢀ
able increase in the yield of saturated hydrocarbons:
the ethane yield was 9.8 g/m3, while the yield of proꢀ
pane and butane increased almost threefold to 8.4 and
7.3 g/m3, respectively.
The data are in good agreement with the assumpꢀ
tion of different mechanisms of conversion over the
HBeta and CaA zeolites. The low yield of methane
and the high yield of unsaturated gaseousС2–С4
hydrocarbons in the presence of the HBeta zeolite
suggest that the hydrocarbons undergo transformation
via the bimolecular mechanism. The formation
of larger amounts of methane and saturated gaseous
С2–С4 hydrocarbons in the presence of the CaA zeoꢀ
lite suggests that the conversion of the hydrocarbons
follow the unimolecular mechanism.
The molecular weight distribution of linear satuꢀ
rated С5+ hydrocarbons characterizes the fractional
composition of the resulting hydrocarbons (Fig. 3a).
In the case of the Co sample, the maximum of distriꢀ
bution of alkanes was at a carbon number of 8. The
amounts of
tions were 29.4 and 36.5 wt %, respectively; the
amount of С19+ heavy ꢀalkanes was 10.9 wt %; and
the total ꢀalkane content was 76.8 wt %. The introꢀ
nꢀalkanes of the С5–С10 and С11–С18 fracꢀ
n
n
duction of the HBeta zeolite did not lead to a shift of
the maximum of distribution of linear saturated
hydrocarbons. In this case, the amounts of
nꢀalkanes
of the С5–С10 and С11⎯С18 fractions decreased to
The presence of the Co sample resulted in the forꢀ 24.4 and 29.4 wt %, respectively; the amount of С19+
mation of 0.5 g/m3 of ethylene, 5.4 g/m3 of propylene, heavy
ꢀalkanes slightly decreased to 9.3 wt %; and
and 3.9 g/m3 of butylene (Fig. 2b). The introduction of total
ꢀalkanes made the lowest values of 63.1 wt %.
a separate bed of the HBeta zeolite had no significant Thus, the introduction of the HBeta zeolite bed leads
effect on the formation of unsaturated gaseous С2–С4 to a decrease in the amount of ꢀalkanes. The introꢀ
n
n
n
hydrocarbons: the yields of ethylene and propylene duction of the CaA zeolite resulted in a shift of
were 0.5 and 5.6 g/m3, respectively; the butylene yield the distribution maximum toward lighter hydrocarꢀ
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278
ASALIEVA et al.
(a)
10
9
8
7
6
5
4
3
2
1
0
n
ꢀAlkanes
Co Co–HBeta
Co–CaA
C5 C7 C9 C11 C13 C15 C17 C19 C21 C23 C25 C27 C29
C6 C8 C10 C12 C14 C16 C18 C20 C22 C24 C26 C28
(b)
2.5
2.0
1.5
1.0
0.5
0
Isoalkanes
Co Co–HBeta
Co–CaA
C5 C7 C9 C11 C13 C15 C17 C19 C21 C23 C25 C27 C29
C6 C8 C10 C12 C14 C16 C18 C20 C22 C24 C26 C28
(c)
6
5
4
3
2
1
0
Alkenes
Co Co–HBeta
Co–CaA
C5 C7 C9 C11 C13 C15 C17 C19 C21 C23 C25 C27 C29
C6 C8 C10 C12 C14 C16 C18 C20 C22 C24 C26 C28
Fig. 3. Molecular weight distribution of (a)
nꢀalkanes, (b) isoalkanes, and (c) alkenes.
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FISCHER–TROPSCH SYNTHESIS WITH COBALT CATALYST
279
bons corresponding to carbon number 7. The amounts zeolite had higher molecular weights compared with
of ꢀalkanes of the С5–С10 and С11–С18 fractions the alkenes formed in the presence of the CaA zeolite.
increased to 39.9 and 27.4 wt %, respectively; the
amount of С19+ heavy ꢀalkanes increased to 8.8 wt %;
and total ꢀalkanes made 76.1 wt %. The introduction
of a zeolite bed, regardless of the zeolite type, leads to
a decrease in the amount of ꢀalkanes of the С11–С18
n
Consequently, the introduction of a zeolite, irreꢀ
spective of its type, leads to an increase in the yields of
С5–С10 hydrocarbons and isoalkanes; with the formaꢀ
tion of alkenes being enhanced only in the presence of
the HBeta zeolite. This fact also confirms the assumpꢀ
tion that the conversion of hydrocarbon on zeolite
acid sites occurs via different mechanisms depending
on the zeolite type, either the bimolecular or the uniꢀ
molecular mechanism the presence of the HBeta or
CaA zeolite, respectively.
n
n
n
fraction, whereas the introduction of the CaA zeolite
leads to a significant increase in the amount of satuꢀ
rated normal hydrocarbons of the С5–С10 fraction.
The molecular weight distribution and fractional
composition of the resulting saturated branched
hydrocarbons were significantly different in the presꢀ
ence of the test samples (Fig. 3b). The Co sample was
characterized by a diffused maximum of distribution
corresponding to carbon numbers of 8–12. The
amounts of isoalkanes of the С5–С10 and С11⎯С18 fracꢀ
tions were 3.0 and 5.2 wt %, respectively; the amount
of С19+ heavy isoalkanes was 0.4 wt %; and total isoalꢀ
kanes made 8.6 wt %. The Co–HBeta sample was
characterized by a pronounced distribution maximum
corresponding to carbon number 10. The amount of
С5–С10 isoalkanes increased to 5.8 wt %; the amount
of С11–С18 isoalkanes remained almost unchanged at
a level of 4.9 wt %; С19+ heavy isoalkanes were not
detected in the analysis; and the amount of total isoalꢀ
kanes increased to 10.7 wt %. The molecular weight
distribution of isoalkanes in the presence of the Co–
CaA sample was characterized by the lowest
carbon number of 7. The amount of С5–С10 isoalꢀ
kanes increased to 7.9 wt %; the amounts of С11⎯С18
isoalkanes and С19+ heavy isoalkanes were 5.4 and
0.5 wt %, respectively; and total isoalkanes increased
to 13.8 wt %. Thus, the amount of isoalkanes
increased after the introduction of a zeolite bed
regardless of the zeolite type; the presence of CaA led
to the formation of lighter isoalkanes.
CONCLUSIONS
Thus, based on the foregoing, we can reveal the role
of zeolite in the transformations of hydrocarbons proꢀ
duced over a Fischer–Tropsch cobalt catalyst under
conditions of multibed arrangement of the cobalt catꢀ
alyst and the zeolite.
Hydrocarbons produced from CO and Н2 over
cobalt undergo conversion on zeolite acid sites. In the
presence of the HBeta zeolite, cracking and isomerꢀ
ization occur in accordance with the bimolecular
mechanism, which leads to an increase in the yield of
С5–С10 hydrocarbons and gaseous and liquid alkenes
and isoalkenes; in the presence of the CaA zeolite, the
reactions follow the unimolecular mechanism, which
results in an increased yield of С5–С10 hydrocarbons
and alkanes, particularly methane and С5+ isoalkanes.
ACKNOWLEDGMENTS
This work was supported by the Ministry of Educaꢀ
tion and Science of the Russian Federation, unique
agreement identifier RFMEFI57714X0118 (agreeꢀ
ment no. 14.577.21.0118).
Figure 3c shows the molecular weight distributions
of unsaturated hydrocarbons. The Co sample was
characterized by a distribution maximum correspondꢀ
ing to carbon number 7. The amount of alkenes of the
С5–С10 and С11–С18 fractions were 12.7 and 2.1 wt %,
respectively, and total unsaturated hydrocarbons made
14.8 wt %. The introduction of the HBeta zeolite led
to a shift of the maximum of distribution of unsaturꢀ
ated hydrocarbons to carbon number 8. The amount
of С5–С10 alkenes increased to 23.1 wt %; the amount
of С11–С18 alkenes was 3.2 wt %; and total alkenes
made 26.3 wt %, the highest value among all the test
samples. The introduction of the CaA zeolite led to a
shift of the distribution maximum toward lighter
hydrocarbons corresponding to carbon number 6. The
amounts of alkenes of the С5–С10 and С11–С18 fracꢀ
tions decreased to 9.4 and 0.6 wt %, respectively, and
total alkenes made the lowest quantity of 10 wt %.
Thus, the introduction of the HBeta or CaA zeolite led
to a 1.5ꢀfold increase or about a 1.5ꢀfold decrease in
the amount of alkenes, respectively. It is interesting
that the alkenes produced in the presence of the HBeta
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