Microfibrous entrapped hybrid iron-based catalysts for Fischer–Tropsch synthesis

Article abstract of DOI:10.1016/j.cattod.2016.02.048

Fischer–Tropsch synthesis (FTS) is a highly exothermic reaction, and a FTS reactor must have an efficient heat transfer mechanism in order to maintain a stable reaction temperature. Copper microfibrous entrapped catalyst (Cu MFEC) has demonstrated an excellent intra-bed heat transfer ability for FTS thermal management. The porous structure of Cu MFEC can entrap any pre-manufactured catalyst particles, while its counterpart approaches such as metallic foams or monolith reactor structures are typically based on washcoating process and require the details about catalyst formulation and preparation procedure to proceed. In this investigation, a highly active and stable iron-based FTS (Fe-FTS) catalyst, i.e. iron supported on γ-alumina and promoted with copper and potassium, has been developed. The apparent reaction rate constant of Fe-FTS and its productivity at 27.7% of CO conversion are 167 mmol (H₂ + CO)/g_cat/h and 0.30 gC₅+/g_cat/h, respectively, which are comparable to the most active unsupported Fe-based catalyst. When Fe-FTS was physically mixed with equal mass of mesoporous aluminosilicate (MAS) loaded with 0.3 wt% Pt, a hybrid catalyst was formed. This hybrid catalyst demonstrated an extremely low CO₂ selectivity (27.7 C%) and a very high selectivity (38.1 C%) to liquid products (C₅–C₂₀). The result suggested that FTS and hydrocracking process simultaneously took place within a single reactor. Furthermore, FTS was carried out in a larger tubular reactor (34.0 mm I.D.) packed with Cu MFEC. It demonstrated a radial temperature gradient less than 5 °C and similar reactivity and product selectivity as those obtained in a small FTS reactor (9.5 mm I.D.). While with the same catalyst loading density and the same test conditions, the comparative packed bed (34.0 mm I.D.) reached radial temperature gradient around 54 °C. It was also found that the FTS based Cu MFEC required negligible amount of time to reach its steady state compared with the packed bed. With these attributes and advantages, Cu MFEC approach is a promising alternative to the traditional packed bed for exothermic reactions such as FTS.

Full text of DOI:10.1016/j.cattod.2016.02.048

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Microfibrous entrapped hybrid iron-based catalysts for
Fischer–Tropsch synthesis
Xinquan Chenga, Hongyun Yangb, Bruce J. Tatarchuka,∗
a Center for Microfibrous Materials, Department of Chemical Engineering, 212 Ross Hall, Auburn University, AL 36849, USA
b IntraMicron Inc., 368 Industry Dr., Auburn, AL 36832, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Fischer–Tropsch synthesis (FTS) is a highly exothermic reaction, and a FTS reactor must have an effi-
cient heat transfer mechanism in order to maintain a stable reaction temperature. Copper microfibrous
entrapped catalyst (Cu MFEC) has demonstrated an excellent intra-bed heat transfer ability for FTS ther-
mal management. The porous structure of Cu MFEC can entrap any pre-manufactured catalyst particles,
while its counterpart approaches such as metallic foams or monolith reactor structures are typically
based on washcoating process and require the details about catalyst formulation and preparation pro-
cedure to proceed. In this investigation, a highly active and stable iron-based FTS (Fe-FTS) catalyst,
i.e. iron supported on ␥-alumina and promoted with copper and potassium, has been developed. The
apparent reaction rate constant of Fe-FTS and its productivity at 27.7% of CO conversion are 167 mmol
(H2 + CO)/gcat/h and 0.30 gC5+/gcat/h, respectively, which are comparable to the most active unsupported
Fe-based catalyst. When Fe-FTS was physically mixed with equal mass of mesoporous aluminosili-
cate (MAS) loaded with 0.3 wt% Pt, a hybrid catalyst was formed. This hybrid catalyst demonstrated an
extremely low CO2 selectivity (27.7 C%) and a very high selectivity (38.1 C%) to liquid products (C5–C20).
The result suggested that FTS and hydrocracking process simultaneously took place within a single reac-
tor. Furthermore, FTS was carried out in a larger tubular reactor (34.0 mm I.D.) packed with Cu MFEC. It
demonstrated a radial temperature gradient less than 5 ◦C and similar reactivity and product selectivity
as those obtained in a small FTS reactor (9.5 mm I.D.). While with the same catalyst loading density and
the same test conditions, the comparative packed bed (34.0 mm I.D.) reached radial temperature gradient
around 54 ◦C. It was also found that the FTS based Cu MFEC required negligible amount of time to reach
its steady state compared with the packed bed. With these attributes and advantages, Cu MFEC approach
is a promising alternative to the traditional packed bed for exothermic reactions such as FTS.
© 2016 Elsevier B.V. All rights reserved.
Received 13 December 2015
Received in revised form 25 February 2016
Accepted 28 February 2016
Available online xxx
Keywords:
Microfibrous entrapped catalysts
Fischer–Tropsch synthesis (FTS)
Hydrocracking
Supported Fe-based FTS catalyst
Hybrid catalyst
Mesoporous aluminosilicate
1. Introduction
high temperatures. For the maximum selectivity to desired liquid
products, the FTS reactor should be well controlled at a single sta-
Fischer–Tropsch synthesis (FTS) is a well-known Gas-to-Liquid
technique for converting natural gas or syngas into liquid fuels.
However, FTS is a highly exothermic reaction which causes many
heat transfer issues for FTS intensification since FTS was established
in 1920s [1]. First of all, the reaction temperature is easy to run
away due to extensive heat generation, which can lead to some
severe quality and even safety problems. More importantly, FTS
is a temperature sensitive reaction, and its product distribution
is strongly dependent on the reaction temperature. Undesirable
gaseous products like methane, ethane, etc. will be produced at
ble temperature for entire reactor. In addition, the high reaction
temperature could deactivate the catalysts by sintering its active
metal sites. As a result, an efficient heat transfer is the key to solve
FTS related issues.
In the past, researchers put tremendous efforts to develop var-
ious heat transfer structures for FTS in order to avoid reaction
temperature overshoot and formation of hot spots. The major rep-
resentative works are fluidized reactor and slurry reactor. Both
of them have been successfully used in research and industrial
areas [2–7]. For other types of reactors, including metallic foams
and metal monolith catalyst structures, some successful applica-
tions also existed in their infant stage [8–12]. However, they all
have disadvantages. For instance, fluidized reactors require cata-
lysts with good mechanical strength and complicated downstream
∗
Corresponding author.
E-mail address: tatarbj@auburn.edu (B.J. Tatarchuk).
http://dx.doi.org/10.1016/j.cattod.2016.02.048
0920-5861/© 2016 Elsevier B.V. All rights reserved.
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recovery processes [3]. The serious issues for slurry reactor are
back-mixing of the gas phase, catalyst attrition and separation from
the liquid and wax products [13]. For metallic foams and monolith
reactor structures, catalysts need to be prepared either directly on
the reactor structure or on a washcoating process to load catalytic
components [14,15]. Hence, these two kinds of reactor structures
are not suitable for pre-manufactured catalysts. Through persis-
tent efforts, our group developed a novel catalyst structure called
microfibrous entrapped catalyst (MFEC) structure which is now
commercially available at IntraMicron Inc. MFEC is a structured
catalyst and it consist of a highly porous network structure made
of micron-sized metal, glass, or polymer fibers with small catalyst
particles entrapped inside [16–20]. One of the biggest advantages
of MFEC when compared with metallic foams or monolith reactor
structures is that MFEC can be used for pre-manufactured catalyst
particles. This unique advantage greatly extends the applications
of MFEC on many heterogeneous reactions by using the original
optimized catalyst recipes, instead of seeking a new recipe for
washcoating support [21–23]. Previous efforts have found out that
MFEC demonstrates high void volume, which significantly reduces
pressure drop compared with packed bed. Moreover, intra-particle
heat transfer and mass transfer are enhanced in MFEC structure,
since small particles are used and acceptable uniform particle dis-
tribution are achieved. This unique catalyst structure have been
widely used in various fields. For instance, Yang et al. studied the
effects of microfibrous media on external mass transfer in desul-
furization [24,25]. Zhu et al. discussed the electrical conductivity
of metal microfibrous in fuel cell [26,27]. Gu et al. built a Com-
putational Fluid Dynamics pressure drop model for MFEC filter
[28]. Sheng et al. investigated and simulated the thermal properties
of MFEC and applied to FTS reaction [29–31]. According to Sheng
et al.’s investigation, copper MFEC can significantly enhance intra-
bed heat transfer and maintain a stable reaction temperature for
highly exothermic reaction like FTS.
FTS produces a broad spectrum of products including gas, liquid
and wax [32,33]. In order to maximize amount of hydrocarbons
in the range of liquid fuels, a suitable operational conditions are
required. For example, cobalt based catalyst typically requires
220 ◦C and 20 bar to produce hydrocarbon liquids with the max-
imum selectivity. In order to further enhance hydrocarbon liquid
fraction, paraffinic wax formed during FTS is hydrocracked to form
more liquids. Traditionally, the complicated and expensive process
of hydrocracking is performed downstream in a separate reactor.
However, by physical mixing of an FTS catalyst with an acidic zeolite
component, the FTS and hydrocracking process can simultaneously
occur in a single reactor [34–36]. The common FTS catalysts used in
the industry are cobalt and iron based catalysts [37,38]. Iron-based
catalysts are more favorable to produce long chain hydrocarbons
from coal or biomass (low H2/CO ratio) due to their high water-gas
shift activity, low cost and low methane selectivity [5]. Typically,
unsupported iron catalysts promoted with copper and potassium
are used in the industry. These catalysts provide high activity and
selectivity to hydrocarbons. However, the unsupported Fe-based
catalysts are generally lack of physical strength and durability. In
contrast, alumina-supported Fe-based catalysts although providing
low activity and high methane selectivity, are stronger and more
attrition resistant than unsupported Fe-based catalysts [39].
In the current investigation, a highly active and stable ␥-alumina
supported Fe-based catalyst was prepared and tested for FTS, and
a mesoporous aluminosilicate (MAS) was synthesized as hydroc-
racking catalyst. A hybrid FTS catalyst that physically combined
these two components together, has been developed to be applied
on FTS and hydrocracking reactions simultaneously within a single
reactor. The results revealed that this type of hybrid catalyst can
produce wax-free liquid fuels (C23+ less than 5 wt%), but the C5–C20
selectivity was relative low and the selectivity of light hydrocarbons
(C1–C4) was significantly high. Recently, the addition of 0.3 wt% of
Pt to the MAS for formation of a new hybrid FTS catalyst greatly
enhanced C5–C20 selectivity and reduced the selectivities to CO2
and the light hydrocarbons (C1–C4).
Typical tubular reactors filled of active FTS catalysts will experi-
ence thermal runaway at a large reactor diameter due to inefficient
radial heat transfer through the packed bed. However, the Cu MFEC
structure successfully scaled up the FTS reaction to this larger
degree without losing catalyst activity and major product selec-
tivity, by significantly enhancing heat transfer and maintaining a
stable reaction temperature. In order to demonstrate the benefits
of the MFEC structure, a tubular reactor with a large inside diame-
ter (I.D.) of 34.0 mm I.D. has been designed and tested for FTS using
hybrid FTS catalysts.
2. Experimental
2.1. MFEC structure and catalyst preparation
2.1.1. Fabrication of Cu MFEC
A traditional paper-making technique was developed to pre-
pare MFEC by Auburn University. The detailed process can be
found in the literature [31]. Briefly, micro-sized copper fibers and
cellulose fibers were dispersed in an aqueous suspension. Then,
the resulting mixture was cast into a paper-making model and
formed a preformed sheet by removing the water. A two-sided
sheet can be fabricated by stacking two pieces of performed sheets
together, which were made by copper microfibers with different
diameters. The top layer was made by 12 ␮m diameter copper
microfibers, and the bottom layer was 4 ␮m diameter copper
microfibers (IntraMicron, Auburn, AL, USA). After drying these two
layers, a self-supporting two-sided sheet has been created. Subse-
quently, this preformed sheet was pre-oxidized in a flowing air at
500 ◦C for 30 min in order to remove the bulk of cellulose. Then,
the pre-oxidized sheet was sintered in a flow of hydrogen at 700 ◦C
for 60 min to form fiber-fiber junctions. These junctions can form
a sinter-locked network through the fiber media, which greatly
enhanced heat transfer on this media. The final step was load-
ing catalyst particles with certain sizes into this porous microfiber
media. Actually, after sintering, the top layer of media became very
fluffy, and bottom layer was much compact since it was made by
fibers with smaller diameter. Hence, the catalyst particles can be
easily load into the top layer of this microfiber media, and the bot-
tom layer was aimed to prevent the catalyst particles from leaking
or moving out from the microfiber media.
2.1.2. Preparation of Fe-based FTS catalyst
The Fe-based FTS catalyst was supported on a commercial
grammar-alumina (Alfa Aesar, ␥-Al2O3 #43832). The metal pre-
cursors were iron(III) nitrate nonahydrate (Alfa Aesar, ACS grade,
98.0–101.0 wt%), copper(II) nitrate hemipentahydrate (Alfa Aesar,
ACS grade, 98.0–102.0 wt%), and potassium hydrogen carbonate
(Alfa Aesar, ACS grade, 99.7–100.5 wt% dried basis). In a typi-
cal procedure, ␥-Al2O3 were grounded and sieved to 60–80 mesh
(177–250 ␮m), and then calcined in static air at 750 ◦C for
5 h to remove the hydroxyl groups on the surface. Then, 9.3 g
Al(NO3)3·9H2O, 2.555 g Cu(NO3)3·2.5H2O, and 0.954 g KHCO3 were
dissolved in 36.0 mL deionized water. An excess solution impreg-
nation method was used to load the metal solution on 3.0 g calcined
␥-Al2O3 in three steps [40,41]. In each step, one third of the above
metal solution was added to the support while kept it on a shaker
and dried it slowly in atmosphere. After drying, the sample was
calcined at 300 ◦C for 5 h in static air. Then, the previous steps were
repeated two times in order to load the leftover metal solution. It
must be noted that each time of loading followed by calcination.
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Table 1
3
Finally, an iron-based FTS catalyst with a nominal compositions
of 100Fe/7.5Cu/4K/233Al2O3 (mass basis) was fabricated and was
named as Fe-FTS.
Nitrogen sorption data.
Sample
BET Surface
Area
Pore Volume
(cm3/g)
Average Pore
Diameter
(nm)
(m2/g)
2.1.3. Synthesis of MAS and Pt-MAS
Calcined ␥-Al2O3
Fe-FTS
MAS
216
74
1108
1061
450
0.75
0.25
1.00
0.95
0.27
11.7
10.7
3.5
3.5
0.56
A sol-gel synthetic method was used to synthesize this meso-
porous aluminosilicate (MAS) [42]. Aluminum nitrate nonahydrate
(ANN, J.T. Baker, ACS grade, 98.7 wt%) and tetraethylorthosili-
cate (TEOS, Alfa Aesar, 99.9 wt%) were used as the alumina and
silica source, respectively. Cetyltrimethyl ammonium bromide
(CTAB, Spectrum Chemical MFG. Corp., 98–101 wt%) was used
as the templating agent. Also, ammonium hydroxide (NH4OH,
BDH Chemicals, 30 wt%) and ethyl alcohol (C2H5OH, EMD Chem-
icals Inc., ACS grade, 99.5 wt%) were presented. The molar
composition of the synthesis mixture was as followed: 1.0
TEOS:0.13ANN:40H2O:8C2H5OH:0.25CTAB:10NH4OH. In a typical
synthesis, 4.88 g ANN was dissolved in 46.7 mL C2H5OH and then
mixed with 22.3 mL TEOS. Meanwhile, 9.1 g CTAB, 72.0 mL water,
and 116.8 g NH4OH solution were mixed together under constant
stirring. After 30 min, the TEOS solution was added to the CTAB
mixtures drop by drop while keeping stirring. After all the solution
has been added, the mixture was increased to and kept at 60 ◦C
under stirring for 3 days. The resulting gel was filtered and washed
with an excess amount of water to remove unreacted components.
After drying overnight at 100 ◦C, the resulting sample was calcined
at 540 ◦C for 6 h in order to remove the template.
Pt-MAS
ZSM-5a
a
The sorption data of ZSM-5 was referenced from the literatures [45,46].
An incipient wetness impregnation method has been applied to
prepare 0.3 wt% Pt-MAS by using tetraammineplatinum(II) nitrate
(Alfa Aesar, ACS grade, 99 wt%) as the Pt source. After drying at
120 ◦C for 6 h, the sample was calcined at 500 ◦C for 4 h in static air.
Fig. 1. The Cu MFEC reactor structure (From T1 to T4, the intervals between each
point are 10.0 mm, 2.0 mm and 5.0 mm, respectively. Twall is the outside reactor
temperature).
for the reaction in order to keep the Fe-based FTS catalyst in the
same amount. Before applying the FTS reaction, the catalysts were
pre-reduced in situ at 120 ◦C for 2 h under flow of ultra-high purity
hydrogen (Airgas, HY UHP300, 99.999%), then the temperature was
slowly increased to 280 ◦C for another 10 h, followed by gradu-
ally rose up the temperature to 320 ◦C for another 4 h. Then, the
catalysts were cooled down to 260 ◦C to initiate the FTS reaction.
All the experiments in this paper were carried out at 260 ◦C and
2.0 MPa by using a syngas with H2/CO ratio equals of 2:1. This syn-
gas was directly purchased from Airgas with 1.5 vol% nitrogen as
the internal standard. The outlet stream went sequentially through
a hot trap (150 ◦C) and a cold trap (2 ◦C) to collect heavy hydrocar-
bons and liquid products, respectively. The residual gas products
were analyzed on an online gas chromatograph (GC, Agilent 6890)
equipped with a packed column (Hayesep-DB 100/120). A thermal
conductivity detector (TCD) was used to determine the concentra-
tions of CO, CO2, and C1–C6 using nitrogen as the internal standard.
In the same GC, there was a capillary column (DB-5) connected with
a flame ionization detector (FID) which was used to analyze liquid
and wax products. The collected wax products were dissolved and
diluted in carbon disulfide (Alfa Aesar, ACS grade, >99.9 wt%) and
then shot into the FID.
2.1.4. Preparation of hybrid catalysts
Hybrid catalysts were composed with two parts. One part is
the FTS catalyst mentioned above, Fe-based catalyst (Fe-FTS), with
a nominal compositions of 100Fe/7.5Cu/4K/233Al2O3 (based on
mass). The other part is the hydrocracking catalyst which is ZSM-
5 (Zeolyst, CBV 8014, Si/Al = 80:1), or MAS, or Pt-MAS. In a typical
process, a physical powder mixture of 50 wt% Fe-FTS catalyst and
50 wt% hydrocracking catalyst was pressed into extrudate form,
and then the extrudates were crushed and separated to 60–80 mesh
particles. The resulted hybrid catalysts were named as Fe-FTS/ZSM-
5, Fe-FTS/MAS and Fe-FTS/Pt-MAS for the aforementioned three
hydrocracking catalysts, respectively. It is worth to know that the
ZSM-5 was activated at 540 ◦C for 3 h before using.
2.2. Catalyst characterization
Nitrogen adsorption/desorption isotherms were recorded at
−196 ◦C on a Micromeritics TriStar II 3020 instrument. Before
the measurement, samples were degassed at 200 ◦C under N2
flow overnight. The total surface areas of samples were calcu-
lated via the Brunauer–Emmett–Teller (BET) equation. Average
pore diameter and pore size distribution were calculated by using
Barrett–Joyner–Halenda (BJH) method. The surface morphologies
of Cu MFEC structure before and after catalyst loading were charac-
terized on a JEOL JSM-7000F scanning electron microscope (SEM).
2.3.2. Cu MFEC structure
The Cu MFEC structure was only used in the large reactor
(34.0 mm I.D.). First, the calcined catalyst particles were entrapped
into a sintered copper microfiber media to form a Cu MFEC struc-
ture. Secondly, circular disks with a diameter of 36.0 mm was
punched out from the Cu MFEC to packing into the tubular reactor.
The disk size (36.0 mm) were 6% larger than the inside diameter
of the tubular reactor (34.0 mm), which can offer tremendous con-
tact points between the copper fiber media and the reactor wall.
By doing this, the inside wall heat transfer coefficient increased
significantly when compared with the traditional packed bed. As
shown in Fig. 1, a four-point thermocouple was inserted into the
2.3. FTS reaction measurements
2.3.1. Packed bed
All the FTS experiments in this paper can be separated into two
groups. One tested FTS catalysts in packed beds and the other evalu-
ated these catalysts in the Cu MFEC structure. For packed bed tests,
usually 0.5 g of Fe-FTS catalyst (60–80 mesh) was diluted with cer-
tain amount of glass beads (Alfa Aesar, acid washed, 60–80 mesh).
When it turns to hybrid catalyst, 1.0 g of hybrid catalysts were used
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Fig. 2. N2 sorption data (A) adsorption/desorption isotherms of calcined ␥-Al2O3 and Fe-FTS; (B) pore size distribution patterns of calcined ␥-Al2O3 and Fe-FTS.
Table 2
Performance of different catalysts in both packed bed and Cu MFEC structure.
Reactor sizea
I.D. 9.5 mm
PB
I.D. 34.0 mm
MFEC
Reactor structureb
MFEC
MFEC
PB
Experiment numberc
Catalyst compositiond
SR-1
Fe-FTS
SR-2
Fe-FTS/ZSM-5
SR-3
Fe-FTS/MAS
SR-4
Fe-FTS/Pt-MAS
LR-1
Fe-FTS
LR-2
Fe-FTS
LR-3
Fe-FTS/Pt-MAS
LR-4
Fe-FTS
Voidage, %
43.9
5.7
120
27.7
167
39.0
5.7
120
30.1
181
33.3
5.7
96
33.2
231
33.4
5.7
96
25.7
186
65.6
5.7
60
18.5
143
64.6
3.3
60
24.2
108
44.8
3.3
60
18.1
94
45.3
5.7
60
50.4
284
HSGV, Nl/gFe/h
Time on stream, h
CO conversion, %
Rate constante
Product selectivity, C%
CO2
CH4
C2–C4
C5–C20
41.3
5.2
16.7
29.4
7.4
0.30
0.81
1.0
36.4
9.9
22.9
29.3
1.5
0.28
–
1.9
31.1
16.0
31.9
20.6
0.4
0.22
–
7.0
27.7
12.3
19.9
38.1
2.0
0.32
–
4.1
37.3
7.9
19.2
28.8
6.8
0.21
0.80
1.0
40.3
8.7
22.6
22.8
5.6
0.14
0.76
1.0
24.3
10.8
23.4
39.0
2.5
0.18
–
3.6
44.7
8.3
20.6
24.6
1.8
0.40
0.79
0.8
C21
+
Catalyst productivityf
␣ valueg
H2O/oil mass ratioh
All the experiments in this table were carried out at 260 ◦C and 2.0 MPa by using a syngas with H2/CO ratio equals to 2:1
a
Two kinds of tubular reactor were used, their inside diameter are 9.5 mm and 34.0 mm, respectively.
PB means packed bed and MFEC means copper MFEC structure.
The experiments operated on the small reactor (9.5 mm I.D.) named as SR- and on the large reactor (34.0 mm) named as LR-.
Four kinds of catalysts were used, Fe-FTS is only for Fischer–Tropsch reaction and hybrid catalysts were fabricated from Fe-FTS by physically mixed with ZSM-5, or MAS,
b
c
d
or Pt-MAS.
e
Apparent reaction rate constant means mmol (H2 + CO)/gcat/h. For hybrid catalyst, gcat means per gram of Fe-FTS without counting the equal mass of hydrocracking
catalyst.
f
Catalyst productivity means g C5+/gcat/h and gcat still means the weight of Fe-FTS catalyst for hybrid catalysts.
Calculated by using ASF equation (Figs. S1–S4, Supporting information).
The mass ratio of collected water to oil phase.
g
h
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5
tion. All these facts contributed to this high reaction activity and
high productivity. Apparently, supported Fe-based catalyst owns
many advantages than unsupported ones under a similar reaction
activity and product selectivity [39].
Based on this highly active and stable Fe-based FTS catalyst
(Fe-FTS), three hybrid catalysts were prepared by physically mix-
ing Fe-FTS with ZSM-5, MAS, and Pt-MAS, and were named as
Fe-FTS/ZSM-5, Fe-FTS/MAS, and Fe-FTS/Pt-MAS, respectively. The
corresponding experiments were named as SR-2, SR-3, and SR-4,
respectively. All the experiments (from SR-1 to SR-4) in the small
reactor (9.5 mm I.D.) were operated at the same reaction condi-
tions, and the reaction activities and selectivities of these catalysts
were obtained at similar CO conversion levels of 26–33%. Hence,
the measured values of activity and selectivity for these catalysts
are directly comparable. As shown in Table 2, all the hybrid cata-
lyst demonstrated much different product distributions compared
with that of plain Fe-FTS catalyst. Due to the hydrocracking activity
of ZSM-5 [47], the hybrid catalyst Fe-FTS/ZSM-5 (SR-2 in Table 2)
demonstrated a high selectivity of 32.8 C% to light chain hydro-
carbons (C1–C4) a low selectivity of 1.5 C% to C21+, compared with
those obtained by Fe-FTS at 21.9 C% and 7.4 C%, respectively (SR-1 in
Table 2). Because the decreased selectivity to C21+, the catalyst pro-
duction rate to C5+ slightly decreased from 0.3 to 0.28 gC5+/gcat/h,
even at higher reaction rate and higher CO conversion. In addi-
tion, the liquid product selectivity (C5–C20) in SR-2 was almost the
same as that in SR-1. Therefore, Fe-FTS/ZSM-5 is not an ideal hybrid
catalyst for liquid hydrocarbon production.
Fig. 3. CO conversion vs. time on stream for four experiments carried out in the
small reactor: SR-1 for Fe-FTS; SR-2 for Fe-FTS/ZSM-5; SR-3 for Fe-FTS/MAS; SR-4
for Fe-FTS/Pt-MAS (solid dot for FTS catalyst and empty dot for hybrid catalysts).
reactor from the outside wall of the reactor wall when the reactor
was loaded in the half-way, and then the other half of the bed was
loaded. In that way, the radial temperature distribution profiles can
be measured precisely. For Cu MFEC structures, all the reaction con-
ditions and product analysis system were the same with the above
mentioned packed bed.
3. Results and discussion
According to the literature [46], hydrocracking selectivity to
long chain hydrocarbons can be improved significantly if the pore
size of hydrocracking catalyst can reach mesoporous range. Consid-
ering about this, a mesoporous aluminosilicate (MAS) with a larger
pore size (3.5 nm) was synthesized as the hydrocracking catalyst.
The N2 adsorption/desorption isotherm shown the presence of a
H2 hysteresis loop with an abrupt step around P/P0 = 0.52 in the
desorption branch of MAS (Fig. 4A). This is a typical mesoporous
adsorption and desorption isotherm. As shown in Fig. 3B, MAS has
a narrow pore size distribution, and the average pore diameter is
3.5 nm. Moreover, MAS has a very high surface area (1108 m2/g)
and its pore volume reach of 1.00 cm3/g (Table 1). Therefore, MAS
could be an excellent candidate as a hydrocracking catalyst and the
experimental results confirmed this expectation. Hybrid catalyst
Fe-FTS/MAS (SR-3 in Table 2) significantly rose up the selectivity
of C1–C4 to 47.9 C%, and reduced the C5–C20 selectivity to 20.6
C%. Moreover, the catalyst productivity dropped to 0.22 gC5+/gcat/h.
This is due to the large pore size of MAS can fit in the long chain
hydrocarbons, which greatly increased the chance of heavy prod-
ucts to get hydrocracked to short chain hydrocarbons. These results
also suggest Fe-FTS/MAS is not a suitable catalyst for the selective
production of FTS liquids.
It must be noted that the CO2 selectivity in SR-3 decreased to
31.1 C% which is much less than the CO2 selectivity in SR-1 (41.3
C%). One possible reason is that the acid components of MAS pro-
vided active protons (H+) to terminate the active oxygen (O2−) that
dissociated from CO or H2O, which suppressed the formation of
CO2 [48]. If this assumption is right, the production of H2O should
be increased. In order to prove that, the mass ratio of H2O to liquid
oils has been calculated. The results shown that the mass ratio of
H2O/oil was around 1.0 for Fe-FTS catalyst (SR-1), but it increased to
1.9 when hybrid catalyst Fe-FTS/ZSM-5 was used (SR-2 in Table 2).
The mass ratio further rose up to 7.0 when hybrid catalyst Fe-
FTS/MAS (SR-3 in Table 2) was used. This is an extremely high
H2O/oil mass ratio and the very low C5+ selectivity (21.0 C%) made
a part of the contribution. These results verified our argument that
acid component can suppress water-gas shift reaction activity and
promote water production.
3.1. Catalyst activity and selectivity
3.1.1. Small reactor for catalyst screening
Fe-FTS is an iron-based FTS catalyst that is supported on cal-
cined ␥-alumina and promoted with copper and potassium. The
porous structures of alumina support and Fe-FTS have been mea-
sured by N2 sorption (Fig. 2). As shown in Table 1, after metals
loading, the support’s BET surface area was significantly decreased
from 216 to 74 m2/g, and the pore volume was also drastically
reduced from 0.75 to 0.25 cm3/g due to the very high iron loading
ratio (30 wt%). The average pore diameter was slightly decreased
from 11.7 to 10.7 nm (Table 1). The Fe-FTS catalyst was tested in
a small tubular reactor (9.5 mm I.D.) under a typical FTS reaction
condition (260 ◦C, 2.0 MPa, and H2/CO = 2:1). At a CO conversion
of 27.7% (SR-1 in Table 2), the apparent reaction rate constant of
Fe-FTS was 167 mmol (H2 + CO)/gcat/h and the corresponding pro-
ductivity was 0.30 gC5+/gcat/h. These values are higher than those
obtained on typical supported Fe-based catalysts and comparable
to those achieved on most active unsupported Fe-based catalysts
[43]. In addition, the selectivity to methane was very low (5.2
C%) and a high ␣ value (0.81) was calculated by following the
Anderson-Schulz-Flory distribution (Fig. S1, Supporting informa-
tion). Although the selectivity of CO2 was very high, which is a
common phenomenon for Fe-based FTS catalyst because of the
water-gas shift reaction [44]. Furthermore, the Fe-FTS catalyst was
very stable and no catalyst deactivation was observed during the
120 h of operation (Fig. 3). Therefore, Fe-FTS is a highly active and
stable ␥-alumina supported Fe-based FTS catalyst.
One of the major reasons for this high activity is that the alumina
support was pretreated at 750 ◦C for 5 h before loading metals in
order to remove the hydroxyl groups on the surface. Decreasing
the surface hydroxyl groups can reduce the formation of FeO·Al2O3
surface spinel, which cannot be reduced to the FTS active species
under the typical reduction conditions [39]. Moreover, this Fe-
based FTS catalyst had very high iron loading (30 wt%) and was
promoted by copper and potassium. It is well known that Cu pro-
moter facilitates the dispersion and reduction of iron species, and
potassium promoter enhances the CO adsorption and carburiza-
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Fig. 4. N2 sorption data (A) adsorption/desorption isotherms of MAS and Pt-MAS; (B) pore size distribution patterns of MAS and Pt-MAS.
Because monofunctional cracking catalysts, e.g. MAS usu-
ally produce mostly of short and unsaturated hydrocarbons,
Fe-FTS/MAS is not an ideal hybrid catalyst for the maximum
production of liquid hydrocarbons. In contrast, bifunctional
hydrocracking catalysts can maximize the production of middle
distillates by making a balance between the acid and the metal
functions [49,50]. Typically, noble metals such as Pt or Pd are used
to build bifunctional hydrocracking catalysts due to their higher
hydrogenating activity. This property can improve the yield of mid-
dle distillates as well as reduce the light hydrocarbons formation.
In addition, a strong hydrogenation function can enhance catalyst
stability through suppressing the formation of coke precursors [51].
In light of the aforementioned fundamentals, 0.3 wt% of Pt was
loaded on MAS to form a bifunctional hydrocracking catalyst (Pt-
MAS). The texture property of Pt-MAS was characterized by N2
sorption (Table 1). When compared with MAS, the BET surface area
of Pt-MAS slightly decreased to 1061 m2/g and its pore volume
also slightly reduced to 0.95 cm3/g, but its average pore diameter
almost remained unchanged since Pt loading content was very low.
The hybrid catalyst Fe-FTS/Pt-MAS was tested for FTS performance
(SR-4 in Table 2). The result showed that the C1–C4 selectivity
and CO2 selectivity of Fe-FTS/Pt-MAS were 32.2 C% and 27.7 C%,
respectively, which are much lower than those achieved by Fe-
FTS/MAS, 47.9 C% and 31.1 C%, respectively. Its C5–C20 selectivity
reached 38.1 C%, which is much higher than that of Fe-FTS/MAS
(20.6 C%) and is the highest among all three hybrid catalyst selective
to liquid fraction. Moreover, the catalyst productivity was increased
to 0.32 gC5+/gcat/h even at a slightly lower CO conversion. In a
word, Fe-FTS/Pt-MAS is a good hybrid catalyst formulation for the
maximum liquid hydrocarbon production compared with the
hybrid catalyst with ZSM-5 and MAS. Therefore, the best hybrid
catalyst has been screened out and can be used for further study in
Cu MFEC structure.
3.1.2. Large reactor for Cu MFEC structure
In the catalyst screening step, all the evaluations were per-
formed in a small tubular reactor (9.5 mm I.D.). Since the Cu MFEC
structure can greatly enhance radial heat transfer, a larger reac-
tor could demonstrate this benefit more remarkable. Therefore, a
much larger tubular reactor with 34.0 mm I.D. was used to con-
struct the Cu MFEC reactor. Fig. 5A showed the picture of copper
microfiber media which has a very fluffy structure at the top layer,
hence catalyst particles can easily migrate into the fiber media by
utilizing a shaker and resulting acceptable dispersion (Fig. 5C). After
loading, the catalyst particles were immobilized by the sintered
microfiber network (Fig. 5D), and the compacted bottom layer pre-
vented the loss of catalyst particles. In addition, the Cu MEFC media
were stacked up tightly during the time of packing in order to
enhance axial heat transfer. Therefore, catalysts entrapped in MFEC
structure are immobilized even under a high space velocity.
The first experiment (LR-1 in Table 2) was used to compare with
SR-1 by keeping all the reaction conditions as the same. The results
show that both the apparent reaction rate constant and the catalyst
productivity of LR-1 were decreased when compared with SR-1.
This is mainly because of the CO conversion in LR-1 (18.5%) was
much lower than the CO conversion in SR-1 (27.7%). If the differ-
ences in CO conversion have been corrected, the reaction activity
(apparent rate constant and catalyst productivity) of LR-1 would
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Fig. 5. Images of Cu MFEC structure: (A) Picture of Cu MFEC before catalyst loading; (B) SEM image of Cu MFEC before catalyst loading; (C) Picture of Cu MFEC after catalyst
loading; (D) SEM image of Cu MFEC after catalyst loading.
be higher than the reaction activity of SR-1. In LR-1, the product
selectivities were similar with SR-1. Moreover, ␣value in LR-1 was
0.80 (Fig. S2, Supporting information) which is also similar with
␣ value in SR-1 (0.81). In conclusion, copper microfibrous entrap-
ment enable the high exothermic reaction taking place in a tubular
reactor with a large reactor diameter without compromising the
reaction activity and product selectivity.
operated at a relatively lower temperature than SR-1, which led
to a lower CO conversion. For a comparison purpose, a packed bed
was loaded (LR-4 in Table 2) by using ␣-Al2O3 to dilute the bed to
the same degree as LR-1 and kept all the other reaction conditions
unchanged. It turns out that the CO conversion in LR-4 (50.4%) was
much higher than in LR-1 (18.5%), but with a relatively lower C5–C20
selectivity and higher CO2 selectivity. This is mainly caused by the
very poor heat transfer ability of packed bed, which led to seri-
ous temperature overshooting and increased the CO conversion.
As shown in Fig. 6, the maximal temperature difference between
T1 and T4 for LR-4 reached around 54 ◦C. While the largest temper-
ature difference for all the Cu MFEC structures were less than 5 ◦C.
In addition, it took longer time for the packed bed to reach steady
state. Fig. 7 shows that the time required to reach steady state was
around 24 h for all Cu MFEC structures, and more than 40 h for the
same size packed bed to reach the target CO conversion with 5.0%
variation. Therefore, Cu MFEC structure possessed many obvious
advantages compared with the traditional packed bed.
In order to fairly compare the performance of MFEC with the
catalysts tested in small packed bed, the CO conversion should be
kept at a similar level. Hence, more catalyst was loaded into the
Cu MEFC structure and the space velocity was also decreased. By
holding other reaction conditions unchanged, the CO conversion in
LR-2 was increased to 24.2%, which is comparable to the CO con-
version (27.7%) in SR-1. Compared with LR-1, the apparent reaction
rate constant in LR-2 decreased to 108 mmol (H2 + CO)/gcat/h and
the productivity dropped to 0.14 gC5+/gcat/h. This does not mean
reduced the reaction activity, since more catalyst has been loaded
and lower syngas was pass through the reactor. Meanwhile, the
C5–C20 selectivity reduced to 22.8 C%, and the selectivity of C1–C4
and CO2 slightly increased to 31.3 C% and 40.3 C%, respectively.
Under the same reaction conditions of LR-2, the Cu MFEC
structure entrapped with hybrid catalyst Fe-FTS/Pt-MAS (LR-3 in
Table 2) was tested in order to compare the FTS performance of
SR-4. As a result, both the apparent reaction rate constant and
the catalyst productivity decreased when compared with SR-4.
However, LR-3 had the highest C5–C20 selectivity (39.0 C%) and
the lowest CO2 selectivity (24.3 C%). Meanwhile, the selectivity
of C1–C4 in LR-3 was similar to that in SR-4. Hence, the SR-4 has
been scaled up on a large Cu MFEC reactor (34.0 mm I.D.) without
apparent losing reaction activity and desired product selectivity.
It must be pointed out that, under the same reaction condi-
tions, CO conversion in LR-1 was lower than in SR-1 due to the
impact of Cu MFEC structure. Cu MFEC structure can highly efficient
transfer the reaction heat out of the reactor and avoid tempera-
ture overshoot or the formation of hot spots. This implied that LR-1
3.2. Product distribution
The product distributions of six major experiments were listed
in Fig. 8. As shown in Fig. 8, the hybrid catalyst Fe-FTS/ZSM-5 sig-
nificantly reduced C21+ selectivity and increased the selectivities
to lighter hydrocarbons including CH4 and C2–C4. This is attributed
to the deep hydrocracking activity of ZSM-5. As hybrid catalyst Fe-
FTS/ZSM-5 did, hybrid catalyst Fe-FTS/MAS further increased the
selectivities of light hydrocarbons (CH4 and C2–C4) and greatly
reduced the liquid products (C5–C20) selectivity and wax (C21 + )
selectivity. This is mainly because MAS has a larger pore size
(3.5 nm) than ZSM-5 (0.56 nm), so long chain hydrocarbons can
access the internal surface of MAS and resulted in over hydroc-
racking. Hybrid catalyst Fe-FTS/Pt-MAS with bifunctional catalyst
Pt-MAS increased the production of liquid products by suppress-
ing the formation of C1, C2–C4 and CO2. This bifunctional catalyst
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Fig. 6. Temperature profiles of four experiments in the large reactor: LR-1, LR-2, and LR-3 are Cu MFEC structures; LR-4 is a packed bed diluted with ␣-Al2O3 (T1 to T4 are
radial temperature points started from centerline and the intervals between each point are 10.0 mm, 2.0 mm and 5.0 mm, respectively.).
(Pt-MAS) consisting of noble metal Pt which is responsible for
hydrogenation/dehydrogenation reactions, and the support MAS,
packed bed reactor in SR-4, but the liquid product (C5–C20) selec-
tivity still at a similar level. In short, the MFEC approach enabled
FTS and bifunctional hydrocracking to take place simultaneously in
a single reactor and provide efficient thermal management for both
reactions in a fixed bed reactor with a large reactor diameter.
whose acid sites are responsible for activation of C C and C
H
bonds via a carbocationic mechanism. A proper balance between
acid sites strength and hydrogenation/dehydrogenation activity
could lead to moderate cracking and maximize the formation of
liquid products [50,52].
3.3. Olefin selectivity
LR-1 was carried out in the presence of thermally conduc-
tive copper microfibrous network and at the same experimental
conditions used for SR-1. Fig. 8 shown that LR-1 had a similar prod-
uct distribution pattern with SR-1. This fact confirmed that MFEC
approach can successfully remove the heat generated inside the
34.0 mm I.D reactor and provide a uniform temperature pro-file
as a packed bed reactor of 9.5 mm I.D. (SR-1) did. Similarly, LR-3
was used to compare SR-4. Results shown in Fig. 8 suggest that
they have a similar product distribution pattern except the selec-
tivity of C2–C4 in the MFEC reactor was slightly higher than in the
The olefin content in carbon number fractions for gas phase
products is shown in Fig. 9. It is shown that SR-1 demonstrated
a very high olefin yield because of the relatively low tendency
of secondary reactions for alkali-promoted iron-based catalysts.
Moreover, the olefin content of SR-1 gradually declined with
chain length, which is a typical phenomenon for Fe-based cata-
lyst [38,53,54]. LR-1 demonstrated a similar olefin content with
SR-1, which is another evidence to prove the MFEC approach is
very successful for FTS in larger diameter reactor. Compared with
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Fig. 7. CO conversion vs. time on stream for four experiments operated in the large
reactor: LR-1 and LR-2 for Fe-FTS; LR-3 for Fe-FTS/Pt-MAS (empty dot for Cu MFEC
structure); LR-4 for Fe-FTS (solid dot for packed bed).
Fig. 9. Olefin content of light hydrocarbons for experiments operated in the small
reactor (SR-1 to SR-4, solid dot) and in the large reactor (LR-1 and LR-3, empty dot):
SR-1 for Fe-FTS; SR-2 for Fe-FTS/ZSM-5; SR-3 for Fe-FTS/MAS; SR-4 for Fe-FTS/Pt-
MAS; LR-1 for Fe-FTS with Cu MFEC structure; LR-3 for Fe-FTS/Pt-MAS with Cu MFEC
structure.
plain FTS catalyst in SR-1, the hybrid catalysts (SR-2, SR-3, and SR-4)
demonstrated lower olefin content. This is because the hydrocrack-
ing catalysts provided active protons to terminate the active olefin
components. The olefin content of SR-4 is higher than that of SR-3,
due to the dehydrogenation effect of noble metal Pt. The LR-3 is a
MFEC counterpart of SR-4. The result in Fig. 8 shows that the olefin
content of LR-3 was slightly lower than SR-4. As mentioned above,
the space velocity of LR-3 was decreased in order to increase the CO
conversion to a comparable level with SR-4. Therefore, the lower
olefin content in LR-3 is probably caused by the lower space veloc-
ity of LR-3, which is also consistent with the literature observations
[38,55].
demonstrated FTS and hydrocracking activities simultaneously.
Compared with Fe-FTS, the hybrid catalyst demonstrated low CO2
selectivity of 27.7 C% and high C5–C20 selectivity of 38.1 C%. In addi-
tion, MFEC approach enabled FTS carried out in a larger tubular
reactor (34.0 mm I.D.) without comprising the production rate and
selectivity to desired product. The maximum temperature differ-
ence along the radial direction was less than 5 ◦C for all the MFEC
structures, while the temperature difference for the packed bed
tested under the same condition was around 54 ◦C. Moreover, a
steady state for the Cu MFEC structure can be reached rapidly com-
pared with packed bed of the same size. In conclusion, the Cu MFEC
approach provides a superior thermal management for FTS reaction
and it enables FTS process intensification. When combined with the
hybrid catalyst approach, Cu MFEC approach may provide a unique
opportunity to utilize Gas-to-Liquids economically even at small
scales.
4. Conclusion
The Cu MFEC structure was evaluated for FTS using a highly
active and stable iron-based Fischer–Tropsch synthesis catalyst
(Fe-FTS) and various highly active hybrid FTS catalysts. At a CO
conversion of 27.7%, the apparent reaction rate constant of Fe-FTS
reached 167 mmol (H2 + CO)/gcat/h, and the catalyst productivity
of Fe-FTS was 0.30 gC5+/gcat/h, which are comparable to those of
the most active unsupported Fe-based catalyst. The hybrid cata-
lyst of Fe-FTS and a bifunctional hydrocracking catalyst, i.e. Pt-MAS
Acknowledgement
This work was supported by IntraMicron Inc. under a contact
G00007979.
Fig. 8. Product distribution of the experiments operated both in the small reactor (SR-1 to SR-4) and in the large reactor (LR-1 and LR-3): SR-1 for Fe-FTS; SR-2 for Fe-FTS/ZSM-5;
SR-3 for Fe-FTS/MAS; SR-4 for Fe-FTS/Pt-MAS; LR-1 for Fe-FTS with Cu MFEC structure; LR-3 for Fe-FTS/Pt-MAS with Cu MFEC structure.
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Appendix A. Supplementary data
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in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.02.
048.
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Please cite this article in press as: X. Cheng, et al., Microfibrous entrapped hybrid iron-based catalysts for Fischer–Tropsch synthesis,
Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.048