# Fischer-Tropsch Synthesis over Iron Manganese Catalysts: Effect of Preparation and Operating Conditions on Catalyst Performance

Authors: Ali A. Mirzaei; Samaneh Vahid; Mostafa Feyzi
Journal: Advances in Physical Chemistry
 (2009)

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## Abstract

Iron manganese oxides are prepared using a coprecipitation procedure and studied for the conversion of synthesis gas to light olefins andC5+ hydrocarbons. In particular, the effect of a range of preparation variables such
as [Fe]/[Mn] molar ratios of the precipitation solution, pH of precipitation, temperature of precipitation,
and precipitate aging times  was investigated in detail. The results are interpreted in terms of the structure of
the active catalyst and it has been generally concluded that the calcined catalyst
(at 650C∘ for 6 hours) containing 50%Fe/50%Mn-on
molar basis which is the most active catalyst for the conversion of synthesis gas to light olefins.
The effects of different promoters and supports with loading of  optimum support on the catalytic performance of catalysts are also studied. It was found that the catalyst containing 50%Fe/50%Mn/5 wt.%Al2O3 is an optimum-modified catalyst. The catalytic performance of optimal catalyst has been studied in operation conditions such as a range of reaction temperatures, H2/CO molar feed ratios  and a range of total pressures. Characterization of both precursors and calcined catalysts is carried out by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), BET specific surface area  and thermal analysis methods such as TGA and DSC.


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## Body

## 1. Introduction

Fischer-Tropsch (FT) synthesis is  of
great industrial importance due to the great variety of products obtained such
as paraffins, olefins and alcohols. An
approach to improve the selectivity of the classical Fischer-Tropsch (FT)
process for conversion of synthesis gas to hydrocarbons involves the use of a
bifunctional catalyst system containing a metal catalyst (FT catalyst) combined
with a support. There has been renewed interest in recent years in FT
synthesis, especially for the selective production of petrochemical feedstocks
such as ethylene, propylene, and buthylene (C2–C4 olefins) directly from synthesis gas [1–7]. Compared to other metal catalysts for
Fischer-Tropsch (FT) synthesis, an iron-based catalyst is distinguished by
higher conversion, selectivity to the lower olefins, and flexibility to the
process parameters [8, 9]. However, the use of iron catalyst does not solve the
problem of insufficient selectivity, which represents a general limitation of FT synthesis. Manganese
has been widely used as one of the promoters for FTS on iron catalyst,
particularly in producing low olefins [10–13]. Large
efforts have also been exerted on the individual effect of  manganese
promotion on supported or unsupported iron catalysts [13, 14].
Fe–Mn and Co–Mn catalysts
favor C2–C4 olefins [15, 16]. High C2–C4 selectivity for the iron-rich Fe–Mn solid has been
correlated with the iron manganese oxides phase and two carbide phases, while
the manganese-rich solid has been correlated with two spinel phases and
different carbide phases [11, 17]. The Fe–Mn catalyst, as one of the most important
catalyst systems, has received extensive attention in recent years because of
the higher olefin and middle distillation cut
selectivities which allow their products to be used as a feedstock for the
chemical industry. Therefore, the Fe–Mn catalyst has a promising industrial
application [18–22], and it is well known that higher selectivity
of alkenes can be obtained on Fe/Mn catalysts than on other iron-based
catalysts [23, 24]. The aim of this research work was to investigate the effect
of a range of preparation variables including the precipitate aging time, pH,
temperature of precipitation, and the [Fe]/[Mn] molar ratio of the
precipitation solution of mixed iron manganese oxide catalysts. We also report
further results concerning the effects of different promoters and supports
along with loadings of Al2O3 as an optimum support on
catalytic performance of this catalyst for Fischer-Tropsch synthesis. In
addition, the catalyst structural and morphological was investigated by XRD,
SEM, BET, and thermal analysis methods such as TGA and DSC. Also, the effects
of operation conditions such as H2/CO molar feed ratios, a range of
reaction temperatures and total pressures for
conversion of synthesis gas to light olefins have been studied.


## 2. Experimental

### 2.1. Catalyst Preparation

All the catalysts were prepared using the coprecipitation
procedure. Aqueous solutions ofFe(NO3)3⋅9H2O
(0.5 M) (99%, Merck, Germany) and Mn(NO3)2⋅4H2O (0.5 M)
(99.5%, Merck, Germany) with
different molar ratios were  premixed and the resulting solution heated to 70ºC
in a round-bottomed flask fitted with a condenser. Aqueous Na2CO3 (0.5 M) (99.8%, May & Baker, France) was added dropwise to the mixed nitrate
solution, which was continuously stirred whilst the temperature was maintained
isothermally in the range of 40–80°C. The final
pH achieved was varied between 6.3 and 10.3. This procedure took approximately
10 minutes to complete. The resulting precipitate was then left in this medium
at the required pH and temperature used for the precipitation for times ranging
from 0 to 5 hours. The precipitate was first filtered and then washed several
times with warm distilled water until no further Na+ was observed in
the washings tested by flam atomic absorption. The precipitate was then dried
at 110°C for 16 hours to give a material denoted as the catalyst precursor
which was subsequently calcined in static air in the furnace (650°C, 6 hours)
to give the final catalyst. For preparation of the supported catalysts, the
same amount (20 wt%) of each support such as TiO2 (98%, May &
Baker), SiO2 (98%, Merck), γ-Al2O3 (98%, Merck), MgO (98%, Merck),
and ZSM-5 zeolite (99%, Aldrich, UK) has been added separately to the
mixed solution of iron and manganese nitrates with nominal ratio of Fe/Mn=1/1.
After the test of all these supported catalysts, it was found that Al2O3 is the best support than the others, so the Al2O3 loading
of 5, 10, 15, 20, and 25 wt% based on the total catalyst weight, were used to
obtain the best loading of Al2O3 support. The supported
catalyst was then promoted with different promoters (Li, K, Rb, and Mg) by
adding a small amount (1.5 wt%) of LiNO3 (99.5%, Prolabo, France), KNO3 (99%,
Merck), RbCl (99%, Prolabo), and Mg(NO3)2⋅6H2O (99%,
May & Baker) as separately to the suspension containing
50%Fe/50%Mn/5 wt.%Al2O3.


### 2.2. Catalyst Characterization

#### 2.2.1. X-Ray Diffraction (XRD)

Powder X-ray diffraction (XRD)
measurements were performed using a Bruker Axs  Company,
D8 Advance diffractometer (Germany).
Scans were taken with a2θ stepsize of 0.02 and a counting time
of 1 second using CuKα radiation source generated at 40 kV
and 30 mA. Specimens for XRD were prepared by compaction into a glass-backed
aluminum sample holder. Data was collected over a 2θ
range from 4° to 70° and
phases were identified by matching experimental patterns to entries in the Diffracplus Version 6.0 indexing software.


#### 2.2.2. BET Measurements

Brunauer-Emmett-Teller surface area BET measurements were conducted using a micrometrics
adsorption equipment (Quantachrome instrument, model Nova 2000, USA)
determining nitrogen (99.99% purity) as the analysis gas and the catalyst
samples were slowly heated to 300 for 3 hours under nitrogen atmospheric. Prior
to analysis each precursors and catalyst and after reaction catalysts
measurements specific surface area was evacuated at−196°C for 66 minutes.


#### 2.2.3. Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetriy (DSC)

The TGA and DSC were carried out using
simultaneous thermal analyzer apparatus of Rheometric Scientific Company (STA
1500+ Model, England)
under a flow of dry air. The temperature was raised from25°C to 800°C using a
linear programmer at a heating rate of 10°C/min. The sample weight was between
10 and 20 mg.


#### 2.2.4. Scanning Electron Microscopy (SEM)

The morphology
of catalysts and their precursors was observed by means of an S-360 Oxford Eng
scanning electron microscopy (made in USA). All of the SEM images in this
study are taken at the same magnification of20μm.


### 2.3. Catalyst Testing

The catalyst tests were carried out in a fixed bed
stainless steel microreactor at different operation conditions (Figure1). All
gas lines to the reactor bed were made from 1/4′′ stainless steel tubing.
Three mass flow controllers (Brooks, Model 5850E) equipped with a four-channel
control panel (Brooks 0154) were used to adjust automatically the flow rate of
the inlet gases (CO, H2, and N2 with purity of 99.999%).
The mixed gases passed into the reactor tube, which was placed inside a tubular
furnace (Atbin, Model ATU 150-15) capable of producing temperature up to 1300°C
and controlled by a digital programmable controller (DPC). The reactor tube was
constructed from 1/2′′ stainless steel tubing; internal diameter of 9 mm,
with the catalyst bed situated in the middle of the reactor. The reaction
temperature was controlled by a thermocouple inserted into catalyst bed and
visually monitored by a computer equipped with software. The meshed catalyst
(1.0 g) was held in the middle of the reactor with 110 cm length using quartz
wool. It consists of an electronic back pressure regulator which can control
the total pressure of the desired process using a remote control via the TESCOM
software package integration that improve or modify its efficiency that capable
of working on pressure ranging from atmospheric pressure to 100 bar. The
catalyst was pre-reduced in situ atmospheric pressure in a flowing H2–N2 stream (N2/H2=1, flow rate of each gas=30mL/min) at
400°C for 6 hours before synthesis gas exposure. The FT reactions was carried
out at 260–420°C (P=1–15bar, H2/CO=1/1–3/1,
GHSV=2700–5400h−1). Reactant and product streams were
analyzed online using a gas chromatograph (Varian, Model 3400 Series) equipped
with a 10-port sampling valve (Supelco company, USA, Visi Model),  a sample loop,
and thermal conductivity detector (TCD). The contents of sample loop were
injected automatically into a packed column (Hayesep DB, Altech Company, USA, 1/8′′ OD,
10 meters long, and particle mesh 100/120). Helium was employed as a carrier
gas for optimum sensitivity (flowrate=30mL/min). The calibration was carried
out using various calibration mixtures and pure compounds obtained from American
Matheson Gas Company (USA). GC controlling and collection of all chromatograms
was done via an IF-2000 single channel data interface (TG Co, Tehran, Iran) at windows environment. The results in terms of CO conversion,
selectivity, and yield of products are given at each space velocity. The CO conversion (%) is calculated
according to the normalization method [25]:
(1)CO conversion (%)=(Moles COin)−(Moles COout)Moles  COin×100.
The
selectivities (%) toward the individual components on carbon basis are
calculated according to the same principle [26]:
(2)Selectivity  ofjproduct  (%)=Moles  ofjproduct(Moles  COin)−(Moles  COout)×100.Figure 1
Schematic representation of the reactor in a flow diagram used. (1) Gas cylinders, (2) pressure regulators, (3) needle valves, (4) valves, (5) mass flow controllers (MFCs), (6) digital pressure controllers, (7) pressure Gauges, (8) nonreturn valves, (9) ball valves, (10) tubular furnace, (11) temperature indicators, (12) tubular reactor and catalyst bed, (13) condenser, (14) trap, (15) air pump, (16) silica gel column, (17) gas chromatograph (GC), (18) mixing chamber, (19) (BPR) back pressure regulator (electronically type), (20) control panel (CP).


## 2.1. Catalyst Preparation

All the catalysts were prepared using the coprecipitation
procedure. Aqueous solutions ofFe(NO3)3⋅9H2O
(0.5 M) (99%, Merck, Germany) and Mn(NO3)2⋅4H2O (0.5 M)
(99.5%, Merck, Germany) with
different molar ratios were  premixed and the resulting solution heated to 70ºC
in a round-bottomed flask fitted with a condenser. Aqueous Na2CO3 (0.5 M) (99.8%, May & Baker, France) was added dropwise to the mixed nitrate
solution, which was continuously stirred whilst the temperature was maintained
isothermally in the range of 40–80°C. The final
pH achieved was varied between 6.3 and 10.3. This procedure took approximately
10 minutes to complete. The resulting precipitate was then left in this medium
at the required pH and temperature used for the precipitation for times ranging
from 0 to 5 hours. The precipitate was first filtered and then washed several
times with warm distilled water until no further Na+ was observed in
the washings tested by flam atomic absorption. The precipitate was then dried
at 110°C for 16 hours to give a material denoted as the catalyst precursor
which was subsequently calcined in static air in the furnace (650°C, 6 hours)
to give the final catalyst. For preparation of the supported catalysts, the
same amount (20 wt%) of each support such as TiO2 (98%, May &
Baker), SiO2 (98%, Merck), γ-Al2O3 (98%, Merck), MgO (98%, Merck),
and ZSM-5 zeolite (99%, Aldrich, UK) has been added separately to the
mixed solution of iron and manganese nitrates with nominal ratio of Fe/Mn=1/1.
After the test of all these supported catalysts, it was found that Al2O3 is the best support than the others, so the Al2O3 loading
of 5, 10, 15, 20, and 25 wt% based on the total catalyst weight, were used to
obtain the best loading of Al2O3 support. The supported
catalyst was then promoted with different promoters (Li, K, Rb, and Mg) by
adding a small amount (1.5 wt%) of LiNO3 (99.5%, Prolabo, France), KNO3 (99%,
Merck), RbCl (99%, Prolabo), and Mg(NO3)2⋅6H2O (99%,
May & Baker) as separately to the suspension containing
50%Fe/50%Mn/5 wt.%Al2O3.


## 2.2. Catalyst Characterization

### 2.2.1. X-Ray Diffraction (XRD)

Powder X-ray diffraction (XRD)
measurements were performed using a Bruker Axs  Company,
D8 Advance diffractometer (Germany).
Scans were taken with a2θ stepsize of 0.02 and a counting time
of 1 second using CuKα radiation source generated at 40 kV
and 30 mA. Specimens for XRD were prepared by compaction into a glass-backed
aluminum sample holder. Data was collected over a 2θ
range from 4° to 70° and
phases were identified by matching experimental patterns to entries in the Diffracplus Version 6.0 indexing software.


### 2.2.2. BET Measurements

Brunauer-Emmett-Teller surface area BET measurements were conducted using a micrometrics
adsorption equipment (Quantachrome instrument, model Nova 2000, USA)
determining nitrogen (99.99% purity) as the analysis gas and the catalyst
samples were slowly heated to 300 for 3 hours under nitrogen atmospheric. Prior
to analysis each precursors and catalyst and after reaction catalysts
measurements specific surface area was evacuated at−196°C for 66 minutes.


### 2.2.3. Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetriy (DSC)

The TGA and DSC were carried out using
simultaneous thermal analyzer apparatus of Rheometric Scientific Company (STA
1500+ Model, England)
under a flow of dry air. The temperature was raised from25°C to 800°C using a
linear programmer at a heating rate of 10°C/min. The sample weight was between
10 and 20 mg.


### 2.2.4. Scanning Electron Microscopy (SEM)

The morphology
of catalysts and their precursors was observed by means of an S-360 Oxford Eng
scanning electron microscopy (made in USA). All of the SEM images in this
study are taken at the same magnification of20μm.


## 2.2.1. X-Ray Diffraction (XRD)

Powder X-ray diffraction (XRD)
measurements were performed using a Bruker Axs  Company,
D8 Advance diffractometer (Germany).
Scans were taken with a2θ stepsize of 0.02 and a counting time
of 1 second using CuKα radiation source generated at 40 kV
and 30 mA. Specimens for XRD were prepared by compaction into a glass-backed
aluminum sample holder. Data was collected over a 2θ
range from 4° to 70° and
phases were identified by matching experimental patterns to entries in the Diffracplus Version 6.0 indexing software.


## 2.2.2. BET Measurements

Brunauer-Emmett-Teller surface area BET measurements were conducted using a micrometrics
adsorption equipment (Quantachrome instrument, model Nova 2000, USA)
determining nitrogen (99.99% purity) as the analysis gas and the catalyst
samples were slowly heated to 300 for 3 hours under nitrogen atmospheric. Prior
to analysis each precursors and catalyst and after reaction catalysts
measurements specific surface area was evacuated at−196°C for 66 minutes.


## 2.2.3. Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetriy (DSC)

The TGA and DSC were carried out using
simultaneous thermal analyzer apparatus of Rheometric Scientific Company (STA
1500+ Model, England)
under a flow of dry air. The temperature was raised from25°C to 800°C using a
linear programmer at a heating rate of 10°C/min. The sample weight was between
10 and 20 mg.


## 2.2.4. Scanning Electron Microscopy (SEM)

The morphology
of catalysts and their precursors was observed by means of an S-360 Oxford Eng
scanning electron microscopy (made in USA). All of the SEM images in this
study are taken at the same magnification of20μm.


## 2.3. Catalyst Testing

The catalyst tests were carried out in a fixed bed
stainless steel microreactor at different operation conditions (Figure1). All
gas lines to the reactor bed were made from 1/4′′ stainless steel tubing.
Three mass flow controllers (Brooks, Model 5850E) equipped with a four-channel
control panel (Brooks 0154) were used to adjust automatically the flow rate of
the inlet gases (CO, H2, and N2 with purity of 99.999%).
The mixed gases passed into the reactor tube, which was placed inside a tubular
furnace (Atbin, Model ATU 150-15) capable of producing temperature up to 1300°C
and controlled by a digital programmable controller (DPC). The reactor tube was
constructed from 1/2′′ stainless steel tubing; internal diameter of 9 mm,
with the catalyst bed situated in the middle of the reactor. The reaction
temperature was controlled by a thermocouple inserted into catalyst bed and
visually monitored by a computer equipped with software. The meshed catalyst
(1.0 g) was held in the middle of the reactor with 110 cm length using quartz
wool. It consists of an electronic back pressure regulator which can control
the total pressure of the desired process using a remote control via the TESCOM
software package integration that improve or modify its efficiency that capable
of working on pressure ranging from atmospheric pressure to 100 bar. The
catalyst was pre-reduced in situ atmospheric pressure in a flowing H2–N2 stream (N2/H2=1, flow rate of each gas=30mL/min) at
400°C for 6 hours before synthesis gas exposure. The FT reactions was carried
out at 260–420°C (P=1–15bar, H2/CO=1/1–3/1,
GHSV=2700–5400h−1). Reactant and product streams were
analyzed online using a gas chromatograph (Varian, Model 3400 Series) equipped
with a 10-port sampling valve (Supelco company, USA, Visi Model),  a sample loop,
and thermal conductivity detector (TCD). The contents of sample loop were
injected automatically into a packed column (Hayesep DB, Altech Company, USA, 1/8′′ OD,
10 meters long, and particle mesh 100/120). Helium was employed as a carrier
gas for optimum sensitivity (flowrate=30mL/min). The calibration was carried
out using various calibration mixtures and pure compounds obtained from American
Matheson Gas Company (USA). GC controlling and collection of all chromatograms
was done via an IF-2000 single channel data interface (TG Co, Tehran, Iran) at windows environment. The results in terms of CO conversion,
selectivity, and yield of products are given at each space velocity. The CO conversion (%) is calculated
according to the normalization method [25]:
(1)CO conversion (%)=(Moles COin)−(Moles COout)Moles  COin×100.
The
selectivities (%) toward the individual components on carbon basis are
calculated according to the same principle [26]:
(2)Selectivity  ofjproduct  (%)=Moles  ofjproduct(Moles  COin)−(Moles  COout)×100.Figure 1
Schematic representation of the reactor in a flow diagram used. (1) Gas cylinders, (2) pressure regulators, (3) needle valves, (4) valves, (5) mass flow controllers (MFCs), (6) digital pressure controllers, (7) pressure Gauges, (8) nonreturn valves, (9) ball valves, (10) tubular furnace, (11) temperature indicators, (12) tubular reactor and catalyst bed, (13) condenser, (14) trap, (15) air pump, (16) silica gel column, (17) gas chromatograph (GC), (18) mixing chamber, (19) (BPR) back pressure regulator (electronically type), (20) control panel (CP).


## 3. Results and Discussion

### 3.1. Effect of Preparation Conditions

In this part of
study, we have investigated the effect of a range of iron manganese oxide
catalysts preparation variables at the precursor stage upon the structure of
these materials, and the subsequent influence these structural effects have on
the activity of the final calcined catalysts. The optimum preparation
conditions were identified with respect to the catalytic activity for the
conversion of synthesis gas to light olefins.


#### 3.1.1. Effect of Aging Time

Aging time is
one of the most important factors on the catalytic performance of the catalyst
and it was defined as the time between the formation of precipitate and the
removal of solvent. In our previous study, we demonstrated the importance of
aging time with respect to catalyst activity for oxidation of CO by mixed
copper manganese oxide and mixed copper zinc oxide catalysts [27–35] and for
hydrogenation of CO by mixed of cobalt-iron oxide, cobalt-manganese oxide, and
cobalt-cerium oxide catalysts for Fischer-Tropsch synthesis [36–40]. In all of
these investigations, our results have shown that the aging of the precipitates
obtained by coprecipitation leads to phase changes toward the forms, which are
more stable thermodynamically. In this study, to examine the effect of aging on
the performance of iron manganese oxide catalysts for the hydrogenation of CO,
a series of mixed iron manganese oxide catalysts were prepared by
coprecipitation method ([Fe]/[Mn]=1/1, 70°C, pH=8.3) with a range of aging
times between 0 minute (unaged) and 300 minutes for the precipitate. The
catalysts were prepared by calcination at 650°C for 6 hours and then were
tested for hydrogenation of CO. The effect of aging time on catalytic
performance is shown in Table 1. These results show that there is a
considerable variation in the catalyst performance with respect to aging time
and the sample aged for 3 hours gave the optimal catalytic performance for CO
conversion in FTS. Thus aging time is a parameter of crucial importance in the
preparation of active mixed iron manganese oxide catalysts for the
hydrogenation of CO. Characterization studies were carried out using various
techniques for both the precursors and calcined catalysts. Characterization of
precursors was studied to establish the importance of the structure of the
catalyst precursor in controlling the structure of the final catalyst and
consequently activity. The catalyst precursors which were prepared using
coprecipitation method in different aging times were characterized by thermal
gravimetric analysis (TGA) and the TGA curves of these precursors are displayed
in Figure 2. For these precursors, the  thermogravimetric curves seem to
indicate three-stage decomposition. The first stage is considered to be due to
the removal of adsorbed water (40–110°C) and the
second stage is due to  the decomposition of hydroxyl bimetallic or nitrate
precursor (330–420°C),
respectively. The peak around 550–630°C is due to
the  decomposition of MnCO3 or Fe2CO3(OH)2 to oxides. The TGA curves are involved with total overall  weight loss of
ca. 18–23 wt.%. The
catalyst precursors were also characterized by XRD and similar phases were
identified for these catalyst precursors with preparation ratios (MnCO3 (rhombohedral). The X-ray diffraction patterns for their calcined samples were
similar together, although the relative diffracted intensities from the phases
were slightly different and their patterns are illustrated in Figure 3. The
actual phases identified in these calcined catalysts under the specified
preparation conditions were Mn2O3 (cubic) and Fe2O3 (rhombohedral). Note that during the calcination of the precursors, the
carbonate phases disappeared and oxide phases were formed. Some of these
calcined catalysts were characterized by DSC method and their DSC curves are
presented in Figure 4. These curves show that the
catalysts have high-heat conductivity; this important case for the FTS
catalysts is necessary to conduct the produced heat of reaction. Also, the
absence of peaks on the DSC curves showed that the calcined catalysts have high-heat
stability and it might be a reason why 650°C has been chosen for calcination of
catalysts. In order to identify the changes in the tested catalysts
during the reaction and to detect the phases formed; these catalysts were
characterized by XRD after the test. The actual phases identified in these catalysts
are presented in Table 2. As it shown, all of these tested catalysts have the
MnO and FeO and iron carbide phases, which all of these phases are active in
FTS. MnO is the active phase for production of olefins and iron carbide is the
active phase for hydrogenation of CO. Characterization of both precursors and
calcined catalysts for the series of differently aged samples (before and after
reaction) was carried out using BET surface area and the results are listed in
Table 3.  These results illustrated that
increasing the aging time produced higher surface area materials. The 50%Fe/50%Mn calcined catalyst
aged for 180 minutes was the most active for conversion of synthesis gas to
light olefins. This catalyst has almost a high-specific surface area; it is,
therefore, considered that the higher activity of 50%Fe/50%Mn catalyst may be attributed to its high BET
surface area.Table 1
Effect of aging times on the catalytic performance.
Selectivity (%)Aging time (minute)CO conversion (%)CH4C2H4C3H6C4H8C5+6078.232.024.39.03.47.012087.322.230.110.23.77.318080.221.426.210.03.96.224072.127.423.28.52.46.930069.122.918.19.71.56.1Table 2
Different identified phases in the calcined catalysts with different aging times.
Aging time (minute)Phases0MnO (cubic), FeO (cubic),Fe2C (orthorhombic), CFe2.5 (monoclinic)30MnO (cubic), FeO (cubic),Fe2C (orthorhombic), CFe2.5 (monoclinic)60MnO (cubic), FeO (cubic),Fe2C (orthorhombic), CFe2.5 (monoclinic)120CFe2.5 (monoclinic), MnO (cubic), FeO (cubic)180MnO (cubic), FeO (cubic),CFe2.5 (monoclinic), Fe (cubic)300CFe2.5 (monoclinic), MnO (cubic), FeO (cubic)Table 3
BET results for catalysts containing 50%Fe/50%Mn with different aging times.
Specific surface area (m2/g)Aging time (minute)PrecursorCalcined catalyst (before reaction)Calcined catalyst (after reaction)085.082.379.13089.384.280.56093.286.682.7120107.998.695.6180115.8113.1109.5300119.0115.6112.9Figure 2
TGA curves of the catalyst precursors with different aging times.Figure 3
XRD patterns of the calcined
catalysts with different aging times.Figure 4
DSC curves of
some calcined catalysts with different aging times.


#### 3.1.2. Effect of Precipitation pH

A series of iron manganese oxide catalysts were prepared
by coprecipitation method ([Fe]/[Mn]=1/1, 70°C, 3 hours aging time) with a
range of precipitation pH from 6.3 to 10.3. The catalysts precursors, prepared
using coprecipitation procedure in different pH, were characterized by XRD and
showed the MnCO3 phase as rhombohedral structure. The catalytic
activity for the Fischer-Tropsch synthesis was investigated for the materials
following calcination (650°C, 6 hours) and the effect of precipitation pH on
the catalytic performance is shown in Table 4. It is apparent that during
precursor calcination, carbonate phase leads to the oxide phases and the sample
prepared at pH=8.3 gives the highest activity. These calcined catalysts were
characterized by XRD and their patterns are shown in Figure 5. The actual
phases identified in these catalysts were Mn2O3 (cubic)
and Fe2O3 (rhombohedral). The calcined tested catalysts
prepared at pH=8.3 also were characterized by XRD and different phases
including MnO (cubic), FeO (cubic), and CFe2.5 (monoclinic) were
identified in this catalyst. Note that in the tested catalyst, the oxidic and
carbide phases are formed which both of them are active in FTS [41, 42].Table 4
Effect of precipitation pH on the catalytic performance.
pHCO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+6.384.043.019.68.41.55.37.379.627.919.09.02.77.28.386.322.532.110.84.16.89.381.220.024.110.04.05.910.380.019.022.910.93.26.1Figure 5
XRD patterns of calcined catalysts with different
precipitation pH.


#### 3.1.3. Effect of Solution [Fe]/[Mn] Ratio

Iron manganese
oxide catalysts were prepared by coprecipitation method (70°C, pH=8.3, 3 hours
aging time) with a range of [Fe]/[Mn] solution ratios varying from 100%Fe to
100%Mn and the catalytic performance for the Fischer-Tropsch synthesis was
investigated for the materials following calcination (650°C, 6 hours). The CO
conversion and hydrocarbons selectivity percent present on steady-state
catalytic performance under comparable reaction conditions for the iron-manganese oxide catalysts  with different [Fe]/[Mn] molar ratios are shown in Table 5. The
catalyst precursors prepared using coprecipitation method in different
[Fe]/[Mn] ratios and their calcined catalysts were characterized by XRD. The
catalyst precursor containing 100%Fe-0%Mn was found to be amorphous and the
other precursors showed the MnCO3 phase as rhombohedral structure.
The XRD patterns of the different calcined catalysts with different [Fe]/[Mn]
molar ratios are presented in Figure 6. In the calcined catalyst containing
100%Fe-0%Mn, the Fe2O3 (rhombohedral) phase was observed
and in the calcined catalyst containing 0%Fe-100%Mn, the Mn2O3 (cubic) phase was identified. For the other calcined catalysts, different
phases including Mn2O3 (cubic) and Fe2O3 (rhombohedral) were identified. In order to
identify the changes in the catalyst containing 50%Fe/50%Mn during the reaction
and to detect the phases formed, this catalyst was characterized by XRD after
the test. Its phases were found to be MnO (cubic), FeO (cubic), and CFe2.5 (monoclinic). Characterization of these calcined catalysts was carried out
using the BET surface area measurements and obtained results are presented in
Table 6. The BET results show that the calcined catalysts prepared with a range
of [Fe]/[Mn] solution ratios varying from 100%Fe to 100%Mn have different
specific surface areas. The catalyst with 1/1 [Fe]/[Mn] ratio has a higher
specific surface area than the other catalysts (Table 6), which is one reason
for the enhanced performance of this catalyst [43]. According
to the obtained results (Table 5), since the
catalyst with preparation ratio of 1/1 [Fe]/[Mn] showed the highest selectivity
toward both ethylene and propylene, so the catalyst prepared with this molar
ratio was chosen as the best catalyst for the conversion of synthesis gas to
ethylene and propylene under reaction conditions (P=1atm, T=400°C, H2/CO=2/1).
The specific surface areas (BET) results of the precursors and calcined
catalysts (before and after reaction) for different Fe/Mn molar ratio are given
in Table 6. Monometallic 100%Fe is used as the basis for comparing the
physical characteristics and CO hydrogenation performance and selectivity for
the bimetallic Fe–Mn catalysts. As  shown
in Table 6, the BET surface areas for the catalysts prepared from different
iron/manganese molar ratios are dependent on the Fe/Mn solution ratios.
However, the specific surface area of catalyst precursor and calcined catalysts
before reaction for each molar ratio were found to be nearly similar, the BET
specific surface areas of the catalysts before and after reaction are different
and the specific surface areas of all the calcined catalysts after reaction
were decreased. The BET data for the catalysts containing 50%Fe/50%Mn showed the high-specific
surface area (98.6m2/g for calcined catalyst) this might be a
reason why the 50%Fe/50%Mn
catalyst shows a better catalytic performance than the other catalysts.Table 5
Effect of Fe/Mn molar ratio on the catalytic performance.
Fe/MnCO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+1/044.333.018.67.42.944/159.132.220.18.63.74.92/166.125.531.28.83.94.51/181.220.024.110.04.06.31/280.720.422.69.63.35.71/479.221.321.89.23.77.10/172.023.422.79.64.08.9Table 6
BET results for different molar ratios of Fe/Mn.
Fe/MnSpecific surface area (m2/g)PrecursorCalcined catalyst (before reaction)Calcined catalyst (after reaction)1/065.162.759.34/171.268.266.42/198.196.784.31/1107.998.695.61/2109.8104.7102.11/4111.3108.9106.10/1110.8107.9105.6Figure 6
XRD patterns of calcined catalysts with different Fe/Mn molar ratios.


#### 3.1.4. Effect of Precipitation Temperature

Iron manganese
oxide catalysts were prepared by coprecipitation ([Fe]/[Mn]=1/1, pH=8.3, 3 hours
aging time) with a range of solution temperature from 40 to 80°C. The
catalytic performance of these series catalysts for the conversion of synthesis
gas to light olefins was investigated for the materials following calcination
(650°C, 6 hours). The CO conversion and hydrocarbons selectivity percent
present on steady-state catalytic performance for the iron manganese oxide
catalysts with different solution temperature in reaction conditions (P=1atm, T=400°C, H2/CO=2/1) are shown in Table 7. According to the obtained
results, the percent of CO conversion was partially changed with the change of
precipitation temperature and 70°C is
considered to be practical maximum precipitation temperature and in this study,
it was chosen as the optimum temperature for the catalysts preparation. The XRD
patterns of catalyst precursors prepared by varying the temperature of the
aging solution all showed similar diffraction patterns, the materials were
poorly crystalline and comprised the MnCO3 phase with rhombohedral
structure. The calcined catalysts were characterized by XRD and their patterns
are presented in Figure 7. The XRD patterns of the calcined samples were
similar to each other, although the relative diffracted intensities from the
phases were slightly different. The actual phases identified in these catalysts
under the specified  preparation conditions were Mn2O3 (cubic), Fe2O3 (rhombohedral), and MnO2 (tetragonal).Table 7
Effect of preparation temperature on the catalytic performance.
T(C°)CO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+4052.329.718.58.52.85.45061.529.522.810.13.56.16071.032.527.510.04.15.97081.220.024.110.04.06.38078.125.930.311.14.07.2Figure 7
XRD patterns of calcined
catalysts with different preparation temperatures.


#### 3.1.5. Effect of Different Supports and Support Loadings

Hydrogenation of carbon monoxide is
susceptible to metal support effects, and both specific activity and
selectivity can be markedly influenced by both the metal and the support [44, 45]. In order to study the effect of some supports such as TiO2, SiO2,
Al2O3, MgO, and zeolite into the iron manganese oxide
catalysts ([Fe]/[Mn]=1/1), the same amount (10 wt.%) from each support has been
added separately to a solution containing iron and manganese with above molar
ratio. All of the different supported catalysts were tested for the selectivity
of hydrogenation of CO. The CO conversion and hydrocarbons selectivity of the
catalysts containing different supports are shown in Table 8. As it can be
observed, the catalyst supported by Al2O3 is more active
than the other supported catalysts and shows a high selectivity toward
hydrocarbons. To understand the influence of loading of Al2O3 on the catalytic activity of mixed iron manganese oxide catalysts, a series of
different 50%Fe/50%Mn/wt.%Al2O3 catalysts were prepared by coprecipitation procedure outlined above. The Al2O3 loadings were 5, 10, 20, 25, and 30 wt% based on the total catalyst weight and the catalytic performance results are shown in Table 9. According to these
results, the catalysts loaded with 5 wt.%Al2O3 showed the
optimal catalytic performance for conversion of synthesis gas to light olefins.
This catalyst was characterized by XRD and its patterns on different stages are
shown in Figure 8. The actual phases identified in this catalyst were Mn5O8 (monoclinic), Fe2O3 (rhombohedral), and Mn2O3 (cubic) and its precursors comprised the MnCO3 phase with rhombohedral structure. In the tested
catalyst, different phases including FeO (cubic), MnO (cubic), Fe3O4, and Fe2C (orthorombic) were identified. Characterization of both precursors and calcined-supported catalysts was
carried out using BET surface area measurement and obtained results are
presented in Table 14. In general, the BET results show that the catalyst
precursors containing different supports and the calcined catalysts derived
from these precursors have different specific surface area. However, as shown
in Table 14, the catalyst precursors have higher specific surface areas than
their calcined catalysts. Furthermore, the Al2O3-supported
catalyst has a higher specific surface area than the other supported catalysts
(Table 14), which is one reason for the better catalytic performance of this
catalyst [38]. A detailed SEM study of the precursor calcined and tested
catalysts for the sample containing optimum amount of 5 wt%Al2O3 was also carried out. The SEM images of these catalysts are presented in
Figure 9, and have shown the major differences in their morphology. By addition
of Al2O3 as a support to the catalyst, the particle size
of some grains has slightly increased and the supported catalyst revealed
structural differences, which are probably due to the presence of Al2O3.
This catalyst is comprised of large grains which are embedded in a mixture
consisting of small grains (Figure 9(b)). However, the size of these grains
grew larger by agglomeration in the tested catalyst (Figure 9(c)), which may be
due to sintering after  reactions. This result is in agreement with Galarraga [46], who indicate that high temperature could cause agglomeration of these
small grains, which leads to catalyst deactivation under high temperature. The
agglomeration is also caused by the imhomogenity distribution of metal
precursor (Figure  9(a)).Table 8
Effect of different supports on the catalytic performance.
SupportCO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+TiO261.321.523.68.22.45.8Zeolite70.527.622.39.33.16.3MgO67.836.520.98.42.34.7Al2O380.930.534.712.55.47.1Silica73.229.930.110.44.52.1Table 9
Effect of loading ofAl2O3 on the catalytic performance.
Selectivity (%)Al2O3 (Wt%)CO conversion (%)CH4C2H4C3H6C4H8C5+580.930.534.712.55.42.51075.951.018.47.83.95.01580.057.315.27.54.34.52082.240.627.210.24.25.82567.550.017.58.03.77.83060.949.516.98.63.19.5Figure 8
XRD patterns of the catalyst
containingAl2O3 support in different states (precursor, calcined catalyst
before and after the test).The SEM images of
catalyst containing 50%Fe/50%Mn/5 wt%Al2O3; (a) precursor, (b) calcined catalyst before reaction, and (c) calcined catalyst after reaction.
(a)(b)(c)


#### 3.1.6. Effect of Promoters

Alkali metals have
been used widely as promoters to improve the activity and selectivity of the
catalysts in the CO hydrogenation [47, 48]. To determine the utility of
promoters  on the catalytic performance of mixed iron manganese oxide catalysts, as alkali metals promoters, a small amount (1.5 wt.%) of LiNO3, KNO3, RbCl, and
Mg(NO3)2⋅6H2O were separately introduced to the resulting suspension containing 50%Fe/50%Mn/5 wt%Al2O3.
Then all of these different promoted catalysts were tested at the same reaction
conditions (400°C and feed molar ratio of H2/CO=1/2 at
atmospheric pressure) for the conversion of synthesis gas to light olefins and
the results are shown in Table 10. These results indicate that the addition of
the K, Mg, Li, and Rb promoters into the catalyst texture leads to a decrease
on the selectivity of the catalyst toward light olefins. Thus taking these
results into consideration, the catalyst containing 50%Fe/50%Mn/5 wt.%Al2O3 without any promoter and aged for 3 hours appears to be the optimum-modified catalyst for the conversion of synthesis gas to light olefins.Table 10
Effect of different promoterson the catalytic performance of 50%Fe/50%Mn/5 wt%Al2O3 catalyst.
PromoterCO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+Rb69.048.119.88.52.86.5Mg76.450.717.07.22.14.1Li73.245.918.57.41.711.8K75.546.315.86.01.515.2


### 3.2. Effect of FTS Reaction Conditions

The other
category of factors which have a marked effect on the catalytic performance of
a catalyst is the operating conditions. For optimizing of the reaction conditions
in this study, the effects of operating conditions such asH2/CO
feed molar ratios, reaction temperatures, and reactor total pressures were
examined to investigate the catalyst stability and its performance under
different Fischer-Tropsch operating conditions.


#### 3.2.1. Effect ofH2/CO Molar Feed Ratio

The influence of
theH2/CO molar feed ratio on the steady-state catalytic
performance of the iron manganese oxide catalyst  containing 50%Fe/50%Mn/5 wt.%Al2O3 for the Fischer-Tropsch reaction at 400°C under atmospheric
pressure was investigated and the CO conversion and light olefins products
selectivity percent, present on steady-state catalytic performance, are
presented in Table 11. The results showed that with variation in H2/CO feed ratio from 1/1 to 3/1,
different selectivities with respect to light olefins were obtained.
However, in the case of the H2/CO=1/1, the total selectivity of
light olefins products was higher and the CH4 selectivity was lower
than the other H2/CO
feed ratios under the same temperature and pressure condition. It is also
apparent that, for all of the H2/CO
feed molar ratios, the optimum catalyst shows a high selectivity toward
ethylene. Therefore, the H2/CO=1/1
ratio was chosen as the optimum ratio for conversion of synthesis gas to C2–C4 olefins over iron manganese catalysts.Table 11
Effect of differentH2/CO feed
molar ratios on the catalytic performance of
50%Fe/50%Mn/5 wt%Al2O3 catalyst.
H2/COCO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+1/184.427.436.812.85.410.82/180.930.534.712.55.47.13/197.268.314.17.52.25.9


#### 3.2.2. Effect of Reaction Temperature

The effect
of reaction temperature on the catalytic performance of the 50%Fe/50%Mn/5 wt.%Al2O3 catalyst was studied at a range of temperature between 260–420°C and the
results are presented in Table 12, (P=1atm, H2/CO=1/1). The results show that as the operating
temperature is increased, the CO conversion is increased. In addition, for the
reaction temperature at 360°C, the total selectivity of light olefins products
was higher than the other reaction temperatures under the same reaction
conditions. In general, an increase in the reaction temperature leads to an
increase in the catalytic performance; furthermore, it has shown that the
reaction temperature should not be too low [14]. At low reaction temperatures,
the conversion percentage of CO is too low and so it causes a low-catalytic
performance. On the other hand, increasing the reaction temperature leads to
the formation of large amounts of coke as an unwanted product, as we found in
this work.  Therefore, in this study,
360°C is considered to be the optimum operating temperature because of high-CO
conversion, total selectivity of light olefins products, low CH4, and not formation of coke.Table 12
Effect of different reaction temperatures on
the catalytic performance of 50%Fe/50%Mn/5 wt%Al2O3 catalyst.
T(C°)CO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+28039.220.616.53.10.96.330049.823.517.33.51.85.732067.429.722.15.72.26.934084.427.436.812.85.45.436087.430.538.013.15.56.938087.134.932.29.12.95.340089.441.327.07.93.24.243080.340.524.28.23.75.745078.040.225.18.52.05.1


#### 3.2.3. Effects of Total Pressure

An increase
in total pressure would generally result in condensation of hydrocarbons, which
are normally in the gaseous state at atmospheric pressure. Higher pressures and
higher carbon monoxide conversions would probably lead to saturation of
catalyst pores by liquid reaction products [49]. A different composition of the
liquid phase in catalyst pores at high syngas pressures could affect the rate
of elementary steps and carbon monoxide and hydrogen concentrations. A series
of experiments were carried out for the 50%Fe/50%Mn/5 wt.%Al2O3 catalyst to investigate on the
performance of this catalyst in variation of total pressure in the range of 1–15 bar, at the
optimal conditions of H2/CO=1/1 and 360°C (Table 13). The results
indicate that, at the total pressure of 1 bar, the optimal catalyst showed a
total selectivity of 42.5% with respect to C2–C4 light
olefins and 13.8% produce the C5+ products. It is also
apparent that increasing in total pressure in the ranges of 2–15 bar
significantly increases the C5+ selectivity and leads to
an increase to 43.2% at the pressure of 15 bar. In the other hand, as it can be
seen on Table 13 at the ranges of 1–6 bar total
pressures, no significant decreasing on CO conversion was observed, however,
the light olefins selectivities were increased and the results indicate that at
the total pressure of 6 bar, the optimal catalyst containing
50%Fe/50%Mn/5 wt.%Al2O3 showed the highest total
selectivity of 59.2% with respect to C2–C4 light olefins
and also led to 20.5% total of selectivity toward the C5+ products. The results also indicate that the CO conversion and the total
selectivity with respect to C2–C4 light olefins were
decreased as the total pressures are increased from 6 bar to 15 bar. Hence
because of high CO conversion, low CH4 selectivity, and also higher
total selectivity with respect to C2–C4 olefins at the
total pressure of 6 bar, this pressure was chosen as the optimum pressure.Table 13
Effect of different
total pressures on the catalytic
performance of 50%Fe/50%Mn/5 wt%Al2O3 catalyst.
Selectivity (%)Pressure (bar)CO conversion (%)CH4C2H4C3H6C4H8C5+184.429.829.211.22.113.8284.026.530.711.92.615.6384.025.634.512.53.016.0484.125.137.212.83.217.5584.823.838.913.03.518.7684.019.840.113.25.920.5783.819.731.812.95.727.2882.219.531.212.15.329.6982.819.330.111.05.430.01078.219.028.010.33.534.41178.018.827.710.52.935.61276.618.027.19.22.738.01371.617.224.57.52.139.61468.216.722.27.22.041.71564.216.521.06.71.543.2Table 14
BET results for different supports on Fe/Mn catalysts.
SupportSpecific surface area (m2/g)PrecursorCalcined catalyst (before reaction)Calcined catalyst (after reaction)TiO2112.0109.8105.3Zeolite145.3139.7133.1MgO123.6118.2115.4Al2O3157.5152.3150.7SiO2165.6148.2142.6


## 3.1. Effect of Preparation Conditions

In this part of
study, we have investigated the effect of a range of iron manganese oxide
catalysts preparation variables at the precursor stage upon the structure of
these materials, and the subsequent influence these structural effects have on
the activity of the final calcined catalysts. The optimum preparation
conditions were identified with respect to the catalytic activity for the
conversion of synthesis gas to light olefins.


### 3.1.1. Effect of Aging Time

Aging time is
one of the most important factors on the catalytic performance of the catalyst
and it was defined as the time between the formation of precipitate and the
removal of solvent. In our previous study, we demonstrated the importance of
aging time with respect to catalyst activity for oxidation of CO by mixed
copper manganese oxide and mixed copper zinc oxide catalysts [27–35] and for
hydrogenation of CO by mixed of cobalt-iron oxide, cobalt-manganese oxide, and
cobalt-cerium oxide catalysts for Fischer-Tropsch synthesis [36–40]. In all of
these investigations, our results have shown that the aging of the precipitates
obtained by coprecipitation leads to phase changes toward the forms, which are
more stable thermodynamically. In this study, to examine the effect of aging on
the performance of iron manganese oxide catalysts for the hydrogenation of CO,
a series of mixed iron manganese oxide catalysts were prepared by
coprecipitation method ([Fe]/[Mn]=1/1, 70°C, pH=8.3) with a range of aging
times between 0 minute (unaged) and 300 minutes for the precipitate. The
catalysts were prepared by calcination at 650°C for 6 hours and then were
tested for hydrogenation of CO. The effect of aging time on catalytic
performance is shown in Table 1. These results show that there is a
considerable variation in the catalyst performance with respect to aging time
and the sample aged for 3 hours gave the optimal catalytic performance for CO
conversion in FTS. Thus aging time is a parameter of crucial importance in the
preparation of active mixed iron manganese oxide catalysts for the
hydrogenation of CO. Characterization studies were carried out using various
techniques for both the precursors and calcined catalysts. Characterization of
precursors was studied to establish the importance of the structure of the
catalyst precursor in controlling the structure of the final catalyst and
consequently activity. The catalyst precursors which were prepared using
coprecipitation method in different aging times were characterized by thermal
gravimetric analysis (TGA) and the TGA curves of these precursors are displayed
in Figure 2. For these precursors, the  thermogravimetric curves seem to
indicate three-stage decomposition. The first stage is considered to be due to
the removal of adsorbed water (40–110°C) and the
second stage is due to  the decomposition of hydroxyl bimetallic or nitrate
precursor (330–420°C),
respectively. The peak around 550–630°C is due to
the  decomposition of MnCO3 or Fe2CO3(OH)2 to oxides. The TGA curves are involved with total overall  weight loss of
ca. 18–23 wt.%. The
catalyst precursors were also characterized by XRD and similar phases were
identified for these catalyst precursors with preparation ratios (MnCO3 (rhombohedral). The X-ray diffraction patterns for their calcined samples were
similar together, although the relative diffracted intensities from the phases
were slightly different and their patterns are illustrated in Figure 3. The
actual phases identified in these calcined catalysts under the specified
preparation conditions were Mn2O3 (cubic) and Fe2O3 (rhombohedral). Note that during the calcination of the precursors, the
carbonate phases disappeared and oxide phases were formed. Some of these
calcined catalysts were characterized by DSC method and their DSC curves are
presented in Figure 4. These curves show that the
catalysts have high-heat conductivity; this important case for the FTS
catalysts is necessary to conduct the produced heat of reaction. Also, the
absence of peaks on the DSC curves showed that the calcined catalysts have high-heat
stability and it might be a reason why 650°C has been chosen for calcination of
catalysts. In order to identify the changes in the tested catalysts
during the reaction and to detect the phases formed; these catalysts were
characterized by XRD after the test. The actual phases identified in these catalysts
are presented in Table 2. As it shown, all of these tested catalysts have the
MnO and FeO and iron carbide phases, which all of these phases are active in
FTS. MnO is the active phase for production of olefins and iron carbide is the
active phase for hydrogenation of CO. Characterization of both precursors and
calcined catalysts for the series of differently aged samples (before and after
reaction) was carried out using BET surface area and the results are listed in
Table 3.  These results illustrated that
increasing the aging time produced higher surface area materials. The 50%Fe/50%Mn calcined catalyst
aged for 180 minutes was the most active for conversion of synthesis gas to
light olefins. This catalyst has almost a high-specific surface area; it is,
therefore, considered that the higher activity of 50%Fe/50%Mn catalyst may be attributed to its high BET
surface area.Table 1
Effect of aging times on the catalytic performance.
Selectivity (%)Aging time (minute)CO conversion (%)CH4C2H4C3H6C4H8C5+6078.232.024.39.03.47.012087.322.230.110.23.77.318080.221.426.210.03.96.224072.127.423.28.52.46.930069.122.918.19.71.56.1Table 2
Different identified phases in the calcined catalysts with different aging times.
Aging time (minute)Phases0MnO (cubic), FeO (cubic),Fe2C (orthorhombic), CFe2.5 (monoclinic)30MnO (cubic), FeO (cubic),Fe2C (orthorhombic), CFe2.5 (monoclinic)60MnO (cubic), FeO (cubic),Fe2C (orthorhombic), CFe2.5 (monoclinic)120CFe2.5 (monoclinic), MnO (cubic), FeO (cubic)180MnO (cubic), FeO (cubic),CFe2.5 (monoclinic), Fe (cubic)300CFe2.5 (monoclinic), MnO (cubic), FeO (cubic)Table 3
BET results for catalysts containing 50%Fe/50%Mn with different aging times.
Specific surface area (m2/g)Aging time (minute)PrecursorCalcined catalyst (before reaction)Calcined catalyst (after reaction)085.082.379.13089.384.280.56093.286.682.7120107.998.695.6180115.8113.1109.5300119.0115.6112.9Figure 2
TGA curves of the catalyst precursors with different aging times.Figure 3
XRD patterns of the calcined
catalysts with different aging times.Figure 4
DSC curves of
some calcined catalysts with different aging times.


### 3.1.2. Effect of Precipitation pH

A series of iron manganese oxide catalysts were prepared
by coprecipitation method ([Fe]/[Mn]=1/1, 70°C, 3 hours aging time) with a
range of precipitation pH from 6.3 to 10.3. The catalysts precursors, prepared
using coprecipitation procedure in different pH, were characterized by XRD and
showed the MnCO3 phase as rhombohedral structure. The catalytic
activity for the Fischer-Tropsch synthesis was investigated for the materials
following calcination (650°C, 6 hours) and the effect of precipitation pH on
the catalytic performance is shown in Table 4. It is apparent that during
precursor calcination, carbonate phase leads to the oxide phases and the sample
prepared at pH=8.3 gives the highest activity. These calcined catalysts were
characterized by XRD and their patterns are shown in Figure 5. The actual
phases identified in these catalysts were Mn2O3 (cubic)
and Fe2O3 (rhombohedral). The calcined tested catalysts
prepared at pH=8.3 also were characterized by XRD and different phases
including MnO (cubic), FeO (cubic), and CFe2.5 (monoclinic) were
identified in this catalyst. Note that in the tested catalyst, the oxidic and
carbide phases are formed which both of them are active in FTS [41, 42].Table 4
Effect of precipitation pH on the catalytic performance.
pHCO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+6.384.043.019.68.41.55.37.379.627.919.09.02.77.28.386.322.532.110.84.16.89.381.220.024.110.04.05.910.380.019.022.910.93.26.1Figure 5
XRD patterns of calcined catalysts with different
precipitation pH.


### 3.1.3. Effect of Solution [Fe]/[Mn] Ratio

Iron manganese
oxide catalysts were prepared by coprecipitation method (70°C, pH=8.3, 3 hours
aging time) with a range of [Fe]/[Mn] solution ratios varying from 100%Fe to
100%Mn and the catalytic performance for the Fischer-Tropsch synthesis was
investigated for the materials following calcination (650°C, 6 hours). The CO
conversion and hydrocarbons selectivity percent present on steady-state
catalytic performance under comparable reaction conditions for the iron-manganese oxide catalysts  with different [Fe]/[Mn] molar ratios are shown in Table 5. The
catalyst precursors prepared using coprecipitation method in different
[Fe]/[Mn] ratios and their calcined catalysts were characterized by XRD. The
catalyst precursor containing 100%Fe-0%Mn was found to be amorphous and the
other precursors showed the MnCO3 phase as rhombohedral structure.
The XRD patterns of the different calcined catalysts with different [Fe]/[Mn]
molar ratios are presented in Figure 6. In the calcined catalyst containing
100%Fe-0%Mn, the Fe2O3 (rhombohedral) phase was observed
and in the calcined catalyst containing 0%Fe-100%Mn, the Mn2O3 (cubic) phase was identified. For the other calcined catalysts, different
phases including Mn2O3 (cubic) and Fe2O3 (rhombohedral) were identified. In order to
identify the changes in the catalyst containing 50%Fe/50%Mn during the reaction
and to detect the phases formed, this catalyst was characterized by XRD after
the test. Its phases were found to be MnO (cubic), FeO (cubic), and CFe2.5 (monoclinic). Characterization of these calcined catalysts was carried out
using the BET surface area measurements and obtained results are presented in
Table 6. The BET results show that the calcined catalysts prepared with a range
of [Fe]/[Mn] solution ratios varying from 100%Fe to 100%Mn have different
specific surface areas. The catalyst with 1/1 [Fe]/[Mn] ratio has a higher
specific surface area than the other catalysts (Table 6), which is one reason
for the enhanced performance of this catalyst [43]. According
to the obtained results (Table 5), since the
catalyst with preparation ratio of 1/1 [Fe]/[Mn] showed the highest selectivity
toward both ethylene and propylene, so the catalyst prepared with this molar
ratio was chosen as the best catalyst for the conversion of synthesis gas to
ethylene and propylene under reaction conditions (P=1atm, T=400°C, H2/CO=2/1).
The specific surface areas (BET) results of the precursors and calcined
catalysts (before and after reaction) for different Fe/Mn molar ratio are given
in Table 6. Monometallic 100%Fe is used as the basis for comparing the
physical characteristics and CO hydrogenation performance and selectivity for
the bimetallic Fe–Mn catalysts. As  shown
in Table 6, the BET surface areas for the catalysts prepared from different
iron/manganese molar ratios are dependent on the Fe/Mn solution ratios.
However, the specific surface area of catalyst precursor and calcined catalysts
before reaction for each molar ratio were found to be nearly similar, the BET
specific surface areas of the catalysts before and after reaction are different
and the specific surface areas of all the calcined catalysts after reaction
were decreased. The BET data for the catalysts containing 50%Fe/50%Mn showed the high-specific
surface area (98.6m2/g for calcined catalyst) this might be a
reason why the 50%Fe/50%Mn
catalyst shows a better catalytic performance than the other catalysts.Table 5
Effect of Fe/Mn molar ratio on the catalytic performance.
Fe/MnCO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+1/044.333.018.67.42.944/159.132.220.18.63.74.92/166.125.531.28.83.94.51/181.220.024.110.04.06.31/280.720.422.69.63.35.71/479.221.321.89.23.77.10/172.023.422.79.64.08.9Table 6
BET results for different molar ratios of Fe/Mn.
Fe/MnSpecific surface area (m2/g)PrecursorCalcined catalyst (before reaction)Calcined catalyst (after reaction)1/065.162.759.34/171.268.266.42/198.196.784.31/1107.998.695.61/2109.8104.7102.11/4111.3108.9106.10/1110.8107.9105.6Figure 6
XRD patterns of calcined catalysts with different Fe/Mn molar ratios.


### 3.1.4. Effect of Precipitation Temperature

Iron manganese
oxide catalysts were prepared by coprecipitation ([Fe]/[Mn]=1/1, pH=8.3, 3 hours
aging time) with a range of solution temperature from 40 to 80°C. The
catalytic performance of these series catalysts for the conversion of synthesis
gas to light olefins was investigated for the materials following calcination
(650°C, 6 hours). The CO conversion and hydrocarbons selectivity percent
present on steady-state catalytic performance for the iron manganese oxide
catalysts with different solution temperature in reaction conditions (P=1atm, T=400°C, H2/CO=2/1) are shown in Table 7. According to the obtained
results, the percent of CO conversion was partially changed with the change of
precipitation temperature and 70°C is
considered to be practical maximum precipitation temperature and in this study,
it was chosen as the optimum temperature for the catalysts preparation. The XRD
patterns of catalyst precursors prepared by varying the temperature of the
aging solution all showed similar diffraction patterns, the materials were
poorly crystalline and comprised the MnCO3 phase with rhombohedral
structure. The calcined catalysts were characterized by XRD and their patterns
are presented in Figure 7. The XRD patterns of the calcined samples were
similar to each other, although the relative diffracted intensities from the
phases were slightly different. The actual phases identified in these catalysts
under the specified  preparation conditions were Mn2O3 (cubic), Fe2O3 (rhombohedral), and MnO2 (tetragonal).Table 7
Effect of preparation temperature on the catalytic performance.
T(C°)CO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+4052.329.718.58.52.85.45061.529.522.810.13.56.16071.032.527.510.04.15.97081.220.024.110.04.06.38078.125.930.311.14.07.2Figure 7
XRD patterns of calcined
catalysts with different preparation temperatures.


### 3.1.5. Effect of Different Supports and Support Loadings

Hydrogenation of carbon monoxide is
susceptible to metal support effects, and both specific activity and
selectivity can be markedly influenced by both the metal and the support [44, 45]. In order to study the effect of some supports such as TiO2, SiO2,
Al2O3, MgO, and zeolite into the iron manganese oxide
catalysts ([Fe]/[Mn]=1/1), the same amount (10 wt.%) from each support has been
added separately to a solution containing iron and manganese with above molar
ratio. All of the different supported catalysts were tested for the selectivity
of hydrogenation of CO. The CO conversion and hydrocarbons selectivity of the
catalysts containing different supports are shown in Table 8. As it can be
observed, the catalyst supported by Al2O3 is more active
than the other supported catalysts and shows a high selectivity toward
hydrocarbons. To understand the influence of loading of Al2O3 on the catalytic activity of mixed iron manganese oxide catalysts, a series of
different 50%Fe/50%Mn/wt.%Al2O3 catalysts were prepared by coprecipitation procedure outlined above. The Al2O3 loadings were 5, 10, 20, 25, and 30 wt% based on the total catalyst weight and the catalytic performance results are shown in Table 9. According to these
results, the catalysts loaded with 5 wt.%Al2O3 showed the
optimal catalytic performance for conversion of synthesis gas to light olefins.
This catalyst was characterized by XRD and its patterns on different stages are
shown in Figure 8. The actual phases identified in this catalyst were Mn5O8 (monoclinic), Fe2O3 (rhombohedral), and Mn2O3 (cubic) and its precursors comprised the MnCO3 phase with rhombohedral structure. In the tested
catalyst, different phases including FeO (cubic), MnO (cubic), Fe3O4, and Fe2C (orthorombic) were identified. Characterization of both precursors and calcined-supported catalysts was
carried out using BET surface area measurement and obtained results are
presented in Table 14. In general, the BET results show that the catalyst
precursors containing different supports and the calcined catalysts derived
from these precursors have different specific surface area. However, as shown
in Table 14, the catalyst precursors have higher specific surface areas than
their calcined catalysts. Furthermore, the Al2O3-supported
catalyst has a higher specific surface area than the other supported catalysts
(Table 14), which is one reason for the better catalytic performance of this
catalyst [38]. A detailed SEM study of the precursor calcined and tested
catalysts for the sample containing optimum amount of 5 wt%Al2O3 was also carried out. The SEM images of these catalysts are presented in
Figure 9, and have shown the major differences in their morphology. By addition
of Al2O3 as a support to the catalyst, the particle size
of some grains has slightly increased and the supported catalyst revealed
structural differences, which are probably due to the presence of Al2O3.
This catalyst is comprised of large grains which are embedded in a mixture
consisting of small grains (Figure 9(b)). However, the size of these grains
grew larger by agglomeration in the tested catalyst (Figure 9(c)), which may be
due to sintering after  reactions. This result is in agreement with Galarraga [46], who indicate that high temperature could cause agglomeration of these
small grains, which leads to catalyst deactivation under high temperature. The
agglomeration is also caused by the imhomogenity distribution of metal
precursor (Figure  9(a)).Table 8
Effect of different supports on the catalytic performance.
SupportCO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+TiO261.321.523.68.22.45.8Zeolite70.527.622.39.33.16.3MgO67.836.520.98.42.34.7Al2O380.930.534.712.55.47.1Silica73.229.930.110.44.52.1Table 9
Effect of loading ofAl2O3 on the catalytic performance.
Selectivity (%)Al2O3 (Wt%)CO conversion (%)CH4C2H4C3H6C4H8C5+580.930.534.712.55.42.51075.951.018.47.83.95.01580.057.315.27.54.34.52082.240.627.210.24.25.82567.550.017.58.03.77.83060.949.516.98.63.19.5Figure 8
XRD patterns of the catalyst
containingAl2O3 support in different states (precursor, calcined catalyst
before and after the test).The SEM images of
catalyst containing 50%Fe/50%Mn/5 wt%Al2O3; (a) precursor, (b) calcined catalyst before reaction, and (c) calcined catalyst after reaction.
(a)(b)(c)


### 3.1.6. Effect of Promoters

Alkali metals have
been used widely as promoters to improve the activity and selectivity of the
catalysts in the CO hydrogenation [47, 48]. To determine the utility of
promoters  on the catalytic performance of mixed iron manganese oxide catalysts, as alkali metals promoters, a small amount (1.5 wt.%) of LiNO3, KNO3, RbCl, and
Mg(NO3)2⋅6H2O were separately introduced to the resulting suspension containing 50%Fe/50%Mn/5 wt%Al2O3.
Then all of these different promoted catalysts were tested at the same reaction
conditions (400°C and feed molar ratio of H2/CO=1/2 at
atmospheric pressure) for the conversion of synthesis gas to light olefins and
the results are shown in Table 10. These results indicate that the addition of
the K, Mg, Li, and Rb promoters into the catalyst texture leads to a decrease
on the selectivity of the catalyst toward light olefins. Thus taking these
results into consideration, the catalyst containing 50%Fe/50%Mn/5 wt.%Al2O3 without any promoter and aged for 3 hours appears to be the optimum-modified catalyst for the conversion of synthesis gas to light olefins.Table 10
Effect of different promoterson the catalytic performance of 50%Fe/50%Mn/5 wt%Al2O3 catalyst.
PromoterCO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+Rb69.048.119.88.52.86.5Mg76.450.717.07.22.14.1Li73.245.918.57.41.711.8K75.546.315.86.01.515.2


## 3.1.1. Effect of Aging Time

Aging time is
one of the most important factors on the catalytic performance of the catalyst
and it was defined as the time between the formation of precipitate and the
removal of solvent. In our previous study, we demonstrated the importance of
aging time with respect to catalyst activity for oxidation of CO by mixed
copper manganese oxide and mixed copper zinc oxide catalysts [27–35] and for
hydrogenation of CO by mixed of cobalt-iron oxide, cobalt-manganese oxide, and
cobalt-cerium oxide catalysts for Fischer-Tropsch synthesis [36–40]. In all of
these investigations, our results have shown that the aging of the precipitates
obtained by coprecipitation leads to phase changes toward the forms, which are
more stable thermodynamically. In this study, to examine the effect of aging on
the performance of iron manganese oxide catalysts for the hydrogenation of CO,
a series of mixed iron manganese oxide catalysts were prepared by
coprecipitation method ([Fe]/[Mn]=1/1, 70°C, pH=8.3) with a range of aging
times between 0 minute (unaged) and 300 minutes for the precipitate. The
catalysts were prepared by calcination at 650°C for 6 hours and then were
tested for hydrogenation of CO. The effect of aging time on catalytic
performance is shown in Table 1. These results show that there is a
considerable variation in the catalyst performance with respect to aging time
and the sample aged for 3 hours gave the optimal catalytic performance for CO
conversion in FTS. Thus aging time is a parameter of crucial importance in the
preparation of active mixed iron manganese oxide catalysts for the
hydrogenation of CO. Characterization studies were carried out using various
techniques for both the precursors and calcined catalysts. Characterization of
precursors was studied to establish the importance of the structure of the
catalyst precursor in controlling the structure of the final catalyst and
consequently activity. The catalyst precursors which were prepared using
coprecipitation method in different aging times were characterized by thermal
gravimetric analysis (TGA) and the TGA curves of these precursors are displayed
in Figure 2. For these precursors, the  thermogravimetric curves seem to
indicate three-stage decomposition. The first stage is considered to be due to
the removal of adsorbed water (40–110°C) and the
second stage is due to  the decomposition of hydroxyl bimetallic or nitrate
precursor (330–420°C),
respectively. The peak around 550–630°C is due to
the  decomposition of MnCO3 or Fe2CO3(OH)2 to oxides. The TGA curves are involved with total overall  weight loss of
ca. 18–23 wt.%. The
catalyst precursors were also characterized by XRD and similar phases were
identified for these catalyst precursors with preparation ratios (MnCO3 (rhombohedral). The X-ray diffraction patterns for their calcined samples were
similar together, although the relative diffracted intensities from the phases
were slightly different and their patterns are illustrated in Figure 3. The
actual phases identified in these calcined catalysts under the specified
preparation conditions were Mn2O3 (cubic) and Fe2O3 (rhombohedral). Note that during the calcination of the precursors, the
carbonate phases disappeared and oxide phases were formed. Some of these
calcined catalysts were characterized by DSC method and their DSC curves are
presented in Figure 4. These curves show that the
catalysts have high-heat conductivity; this important case for the FTS
catalysts is necessary to conduct the produced heat of reaction. Also, the
absence of peaks on the DSC curves showed that the calcined catalysts have high-heat
stability and it might be a reason why 650°C has been chosen for calcination of
catalysts. In order to identify the changes in the tested catalysts
during the reaction and to detect the phases formed; these catalysts were
characterized by XRD after the test. The actual phases identified in these catalysts
are presented in Table 2. As it shown, all of these tested catalysts have the
MnO and FeO and iron carbide phases, which all of these phases are active in
FTS. MnO is the active phase for production of olefins and iron carbide is the
active phase for hydrogenation of CO. Characterization of both precursors and
calcined catalysts for the series of differently aged samples (before and after
reaction) was carried out using BET surface area and the results are listed in
Table 3.  These results illustrated that
increasing the aging time produced higher surface area materials. The 50%Fe/50%Mn calcined catalyst
aged for 180 minutes was the most active for conversion of synthesis gas to
light olefins. This catalyst has almost a high-specific surface area; it is,
therefore, considered that the higher activity of 50%Fe/50%Mn catalyst may be attributed to its high BET
surface area.Table 1
Effect of aging times on the catalytic performance.
Selectivity (%)Aging time (minute)CO conversion (%)CH4C2H4C3H6C4H8C5+6078.232.024.39.03.47.012087.322.230.110.23.77.318080.221.426.210.03.96.224072.127.423.28.52.46.930069.122.918.19.71.56.1Table 2
Different identified phases in the calcined catalysts with different aging times.
Aging time (minute)Phases0MnO (cubic), FeO (cubic),Fe2C (orthorhombic), CFe2.5 (monoclinic)30MnO (cubic), FeO (cubic),Fe2C (orthorhombic), CFe2.5 (monoclinic)60MnO (cubic), FeO (cubic),Fe2C (orthorhombic), CFe2.5 (monoclinic)120CFe2.5 (monoclinic), MnO (cubic), FeO (cubic)180MnO (cubic), FeO (cubic),CFe2.5 (monoclinic), Fe (cubic)300CFe2.5 (monoclinic), MnO (cubic), FeO (cubic)Table 3
BET results for catalysts containing 50%Fe/50%Mn with different aging times.
Specific surface area (m2/g)Aging time (minute)PrecursorCalcined catalyst (before reaction)Calcined catalyst (after reaction)085.082.379.13089.384.280.56093.286.682.7120107.998.695.6180115.8113.1109.5300119.0115.6112.9Figure 2
TGA curves of the catalyst precursors with different aging times.Figure 3
XRD patterns of the calcined
catalysts with different aging times.Figure 4
DSC curves of
some calcined catalysts with different aging times.


## 3.1.2. Effect of Precipitation pH

A series of iron manganese oxide catalysts were prepared
by coprecipitation method ([Fe]/[Mn]=1/1, 70°C, 3 hours aging time) with a
range of precipitation pH from 6.3 to 10.3. The catalysts precursors, prepared
using coprecipitation procedure in different pH, were characterized by XRD and
showed the MnCO3 phase as rhombohedral structure. The catalytic
activity for the Fischer-Tropsch synthesis was investigated for the materials
following calcination (650°C, 6 hours) and the effect of precipitation pH on
the catalytic performance is shown in Table 4. It is apparent that during
precursor calcination, carbonate phase leads to the oxide phases and the sample
prepared at pH=8.3 gives the highest activity. These calcined catalysts were
characterized by XRD and their patterns are shown in Figure 5. The actual
phases identified in these catalysts were Mn2O3 (cubic)
and Fe2O3 (rhombohedral). The calcined tested catalysts
prepared at pH=8.3 also were characterized by XRD and different phases
including MnO (cubic), FeO (cubic), and CFe2.5 (monoclinic) were
identified in this catalyst. Note that in the tested catalyst, the oxidic and
carbide phases are formed which both of them are active in FTS [41, 42].Table 4
Effect of precipitation pH on the catalytic performance.
pHCO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+6.384.043.019.68.41.55.37.379.627.919.09.02.77.28.386.322.532.110.84.16.89.381.220.024.110.04.05.910.380.019.022.910.93.26.1Figure 5
XRD patterns of calcined catalysts with different
precipitation pH.


## 3.1.3. Effect of Solution [Fe]/[Mn] Ratio

Iron manganese
oxide catalysts were prepared by coprecipitation method (70°C, pH=8.3, 3 hours
aging time) with a range of [Fe]/[Mn] solution ratios varying from 100%Fe to
100%Mn and the catalytic performance for the Fischer-Tropsch synthesis was
investigated for the materials following calcination (650°C, 6 hours). The CO
conversion and hydrocarbons selectivity percent present on steady-state
catalytic performance under comparable reaction conditions for the iron-manganese oxide catalysts  with different [Fe]/[Mn] molar ratios are shown in Table 5. The
catalyst precursors prepared using coprecipitation method in different
[Fe]/[Mn] ratios and their calcined catalysts were characterized by XRD. The
catalyst precursor containing 100%Fe-0%Mn was found to be amorphous and the
other precursors showed the MnCO3 phase as rhombohedral structure.
The XRD patterns of the different calcined catalysts with different [Fe]/[Mn]
molar ratios are presented in Figure 6. In the calcined catalyst containing
100%Fe-0%Mn, the Fe2O3 (rhombohedral) phase was observed
and in the calcined catalyst containing 0%Fe-100%Mn, the Mn2O3 (cubic) phase was identified. For the other calcined catalysts, different
phases including Mn2O3 (cubic) and Fe2O3 (rhombohedral) were identified. In order to
identify the changes in the catalyst containing 50%Fe/50%Mn during the reaction
and to detect the phases formed, this catalyst was characterized by XRD after
the test. Its phases were found to be MnO (cubic), FeO (cubic), and CFe2.5 (monoclinic). Characterization of these calcined catalysts was carried out
using the BET surface area measurements and obtained results are presented in
Table 6. The BET results show that the calcined catalysts prepared with a range
of [Fe]/[Mn] solution ratios varying from 100%Fe to 100%Mn have different
specific surface areas. The catalyst with 1/1 [Fe]/[Mn] ratio has a higher
specific surface area than the other catalysts (Table 6), which is one reason
for the enhanced performance of this catalyst [43]. According
to the obtained results (Table 5), since the
catalyst with preparation ratio of 1/1 [Fe]/[Mn] showed the highest selectivity
toward both ethylene and propylene, so the catalyst prepared with this molar
ratio was chosen as the best catalyst for the conversion of synthesis gas to
ethylene and propylene under reaction conditions (P=1atm, T=400°C, H2/CO=2/1).
The specific surface areas (BET) results of the precursors and calcined
catalysts (before and after reaction) for different Fe/Mn molar ratio are given
in Table 6. Monometallic 100%Fe is used as the basis for comparing the
physical characteristics and CO hydrogenation performance and selectivity for
the bimetallic Fe–Mn catalysts. As  shown
in Table 6, the BET surface areas for the catalysts prepared from different
iron/manganese molar ratios are dependent on the Fe/Mn solution ratios.
However, the specific surface area of catalyst precursor and calcined catalysts
before reaction for each molar ratio were found to be nearly similar, the BET
specific surface areas of the catalysts before and after reaction are different
and the specific surface areas of all the calcined catalysts after reaction
were decreased. The BET data for the catalysts containing 50%Fe/50%Mn showed the high-specific
surface area (98.6m2/g for calcined catalyst) this might be a
reason why the 50%Fe/50%Mn
catalyst shows a better catalytic performance than the other catalysts.Table 5
Effect of Fe/Mn molar ratio on the catalytic performance.
Fe/MnCO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+1/044.333.018.67.42.944/159.132.220.18.63.74.92/166.125.531.28.83.94.51/181.220.024.110.04.06.31/280.720.422.69.63.35.71/479.221.321.89.23.77.10/172.023.422.79.64.08.9Table 6
BET results for different molar ratios of Fe/Mn.
Fe/MnSpecific surface area (m2/g)PrecursorCalcined catalyst (before reaction)Calcined catalyst (after reaction)1/065.162.759.34/171.268.266.42/198.196.784.31/1107.998.695.61/2109.8104.7102.11/4111.3108.9106.10/1110.8107.9105.6Figure 6
XRD patterns of calcined catalysts with different Fe/Mn molar ratios.


## 3.1.4. Effect of Precipitation Temperature

Iron manganese
oxide catalysts were prepared by coprecipitation ([Fe]/[Mn]=1/1, pH=8.3, 3 hours
aging time) with a range of solution temperature from 40 to 80°C. The
catalytic performance of these series catalysts for the conversion of synthesis
gas to light olefins was investigated for the materials following calcination
(650°C, 6 hours). The CO conversion and hydrocarbons selectivity percent
present on steady-state catalytic performance for the iron manganese oxide
catalysts with different solution temperature in reaction conditions (P=1atm, T=400°C, H2/CO=2/1) are shown in Table 7. According to the obtained
results, the percent of CO conversion was partially changed with the change of
precipitation temperature and 70°C is
considered to be practical maximum precipitation temperature and in this study,
it was chosen as the optimum temperature for the catalysts preparation. The XRD
patterns of catalyst precursors prepared by varying the temperature of the
aging solution all showed similar diffraction patterns, the materials were
poorly crystalline and comprised the MnCO3 phase with rhombohedral
structure. The calcined catalysts were characterized by XRD and their patterns
are presented in Figure 7. The XRD patterns of the calcined samples were
similar to each other, although the relative diffracted intensities from the
phases were slightly different. The actual phases identified in these catalysts
under the specified  preparation conditions were Mn2O3 (cubic), Fe2O3 (rhombohedral), and MnO2 (tetragonal).Table 7
Effect of preparation temperature on the catalytic performance.
T(C°)CO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+4052.329.718.58.52.85.45061.529.522.810.13.56.16071.032.527.510.04.15.97081.220.024.110.04.06.38078.125.930.311.14.07.2Figure 7
XRD patterns of calcined
catalysts with different preparation temperatures.


## 3.1.5. Effect of Different Supports and Support Loadings

Hydrogenation of carbon monoxide is
susceptible to metal support effects, and both specific activity and
selectivity can be markedly influenced by both the metal and the support [44, 45]. In order to study the effect of some supports such as TiO2, SiO2,
Al2O3, MgO, and zeolite into the iron manganese oxide
catalysts ([Fe]/[Mn]=1/1), the same amount (10 wt.%) from each support has been
added separately to a solution containing iron and manganese with above molar
ratio. All of the different supported catalysts were tested for the selectivity
of hydrogenation of CO. The CO conversion and hydrocarbons selectivity of the
catalysts containing different supports are shown in Table 8. As it can be
observed, the catalyst supported by Al2O3 is more active
than the other supported catalysts and shows a high selectivity toward
hydrocarbons. To understand the influence of loading of Al2O3 on the catalytic activity of mixed iron manganese oxide catalysts, a series of
different 50%Fe/50%Mn/wt.%Al2O3 catalysts were prepared by coprecipitation procedure outlined above. The Al2O3 loadings were 5, 10, 20, 25, and 30 wt% based on the total catalyst weight and the catalytic performance results are shown in Table 9. According to these
results, the catalysts loaded with 5 wt.%Al2O3 showed the
optimal catalytic performance for conversion of synthesis gas to light olefins.
This catalyst was characterized by XRD and its patterns on different stages are
shown in Figure 8. The actual phases identified in this catalyst were Mn5O8 (monoclinic), Fe2O3 (rhombohedral), and Mn2O3 (cubic) and its precursors comprised the MnCO3 phase with rhombohedral structure. In the tested
catalyst, different phases including FeO (cubic), MnO (cubic), Fe3O4, and Fe2C (orthorombic) were identified. Characterization of both precursors and calcined-supported catalysts was
carried out using BET surface area measurement and obtained results are
presented in Table 14. In general, the BET results show that the catalyst
precursors containing different supports and the calcined catalysts derived
from these precursors have different specific surface area. However, as shown
in Table 14, the catalyst precursors have higher specific surface areas than
their calcined catalysts. Furthermore, the Al2O3-supported
catalyst has a higher specific surface area than the other supported catalysts
(Table 14), which is one reason for the better catalytic performance of this
catalyst [38]. A detailed SEM study of the precursor calcined and tested
catalysts for the sample containing optimum amount of 5 wt%Al2O3 was also carried out. The SEM images of these catalysts are presented in
Figure 9, and have shown the major differences in their morphology. By addition
of Al2O3 as a support to the catalyst, the particle size
of some grains has slightly increased and the supported catalyst revealed
structural differences, which are probably due to the presence of Al2O3.
This catalyst is comprised of large grains which are embedded in a mixture
consisting of small grains (Figure 9(b)). However, the size of these grains
grew larger by agglomeration in the tested catalyst (Figure 9(c)), which may be
due to sintering after  reactions. This result is in agreement with Galarraga [46], who indicate that high temperature could cause agglomeration of these
small grains, which leads to catalyst deactivation under high temperature. The
agglomeration is also caused by the imhomogenity distribution of metal
precursor (Figure  9(a)).Table 8
Effect of different supports on the catalytic performance.
SupportCO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+TiO261.321.523.68.22.45.8Zeolite70.527.622.39.33.16.3MgO67.836.520.98.42.34.7Al2O380.930.534.712.55.47.1Silica73.229.930.110.44.52.1Table 9
Effect of loading ofAl2O3 on the catalytic performance.
Selectivity (%)Al2O3 (Wt%)CO conversion (%)CH4C2H4C3H6C4H8C5+580.930.534.712.55.42.51075.951.018.47.83.95.01580.057.315.27.54.34.52082.240.627.210.24.25.82567.550.017.58.03.77.83060.949.516.98.63.19.5Figure 8
XRD patterns of the catalyst
containingAl2O3 support in different states (precursor, calcined catalyst
before and after the test).The SEM images of
catalyst containing 50%Fe/50%Mn/5 wt%Al2O3; (a) precursor, (b) calcined catalyst before reaction, and (c) calcined catalyst after reaction.
(a)(b)(c)


## 3.1.6. Effect of Promoters

Alkali metals have
been used widely as promoters to improve the activity and selectivity of the
catalysts in the CO hydrogenation [47, 48]. To determine the utility of
promoters  on the catalytic performance of mixed iron manganese oxide catalysts, as alkali metals promoters, a small amount (1.5 wt.%) of LiNO3, KNO3, RbCl, and
Mg(NO3)2⋅6H2O were separately introduced to the resulting suspension containing 50%Fe/50%Mn/5 wt%Al2O3.
Then all of these different promoted catalysts were tested at the same reaction
conditions (400°C and feed molar ratio of H2/CO=1/2 at
atmospheric pressure) for the conversion of synthesis gas to light olefins and
the results are shown in Table 10. These results indicate that the addition of
the K, Mg, Li, and Rb promoters into the catalyst texture leads to a decrease
on the selectivity of the catalyst toward light olefins. Thus taking these
results into consideration, the catalyst containing 50%Fe/50%Mn/5 wt.%Al2O3 without any promoter and aged for 3 hours appears to be the optimum-modified catalyst for the conversion of synthesis gas to light olefins.Table 10
Effect of different promoterson the catalytic performance of 50%Fe/50%Mn/5 wt%Al2O3 catalyst.
PromoterCO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+Rb69.048.119.88.52.86.5Mg76.450.717.07.22.14.1Li73.245.918.57.41.711.8K75.546.315.86.01.515.2


## 3.2. Effect of FTS Reaction Conditions

The other
category of factors which have a marked effect on the catalytic performance of
a catalyst is the operating conditions. For optimizing of the reaction conditions
in this study, the effects of operating conditions such asH2/CO
feed molar ratios, reaction temperatures, and reactor total pressures were
examined to investigate the catalyst stability and its performance under
different Fischer-Tropsch operating conditions.


### 3.2.1. Effect ofH2/CO Molar Feed Ratio

The influence of
theH2/CO molar feed ratio on the steady-state catalytic
performance of the iron manganese oxide catalyst  containing 50%Fe/50%Mn/5 wt.%Al2O3 for the Fischer-Tropsch reaction at 400°C under atmospheric
pressure was investigated and the CO conversion and light olefins products
selectivity percent, present on steady-state catalytic performance, are
presented in Table 11. The results showed that with variation in H2/CO feed ratio from 1/1 to 3/1,
different selectivities with respect to light olefins were obtained.
However, in the case of the H2/CO=1/1, the total selectivity of
light olefins products was higher and the CH4 selectivity was lower
than the other H2/CO
feed ratios under the same temperature and pressure condition. It is also
apparent that, for all of the H2/CO
feed molar ratios, the optimum catalyst shows a high selectivity toward
ethylene. Therefore, the H2/CO=1/1
ratio was chosen as the optimum ratio for conversion of synthesis gas to C2–C4 olefins over iron manganese catalysts.Table 11
Effect of differentH2/CO feed
molar ratios on the catalytic performance of
50%Fe/50%Mn/5 wt%Al2O3 catalyst.
H2/COCO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+1/184.427.436.812.85.410.82/180.930.534.712.55.47.13/197.268.314.17.52.25.9


### 3.2.2. Effect of Reaction Temperature

The effect
of reaction temperature on the catalytic performance of the 50%Fe/50%Mn/5 wt.%Al2O3 catalyst was studied at a range of temperature between 260–420°C and the
results are presented in Table 12, (P=1atm, H2/CO=1/1). The results show that as the operating
temperature is increased, the CO conversion is increased. In addition, for the
reaction temperature at 360°C, the total selectivity of light olefins products
was higher than the other reaction temperatures under the same reaction
conditions. In general, an increase in the reaction temperature leads to an
increase in the catalytic performance; furthermore, it has shown that the
reaction temperature should not be too low [14]. At low reaction temperatures,
the conversion percentage of CO is too low and so it causes a low-catalytic
performance. On the other hand, increasing the reaction temperature leads to
the formation of large amounts of coke as an unwanted product, as we found in
this work.  Therefore, in this study,
360°C is considered to be the optimum operating temperature because of high-CO
conversion, total selectivity of light olefins products, low CH4, and not formation of coke.Table 12
Effect of different reaction temperatures on
the catalytic performance of 50%Fe/50%Mn/5 wt%Al2O3 catalyst.
T(C°)CO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+28039.220.616.53.10.96.330049.823.517.33.51.85.732067.429.722.15.72.26.934084.427.436.812.85.45.436087.430.538.013.15.56.938087.134.932.29.12.95.340089.441.327.07.93.24.243080.340.524.28.23.75.745078.040.225.18.52.05.1


### 3.2.3. Effects of Total Pressure

An increase
in total pressure would generally result in condensation of hydrocarbons, which
are normally in the gaseous state at atmospheric pressure. Higher pressures and
higher carbon monoxide conversions would probably lead to saturation of
catalyst pores by liquid reaction products [49]. A different composition of the
liquid phase in catalyst pores at high syngas pressures could affect the rate
of elementary steps and carbon monoxide and hydrogen concentrations. A series
of experiments were carried out for the 50%Fe/50%Mn/5 wt.%Al2O3 catalyst to investigate on the
performance of this catalyst in variation of total pressure in the range of 1–15 bar, at the
optimal conditions of H2/CO=1/1 and 360°C (Table 13). The results
indicate that, at the total pressure of 1 bar, the optimal catalyst showed a
total selectivity of 42.5% with respect to C2–C4 light
olefins and 13.8% produce the C5+ products. It is also
apparent that increasing in total pressure in the ranges of 2–15 bar
significantly increases the C5+ selectivity and leads to
an increase to 43.2% at the pressure of 15 bar. In the other hand, as it can be
seen on Table 13 at the ranges of 1–6 bar total
pressures, no significant decreasing on CO conversion was observed, however,
the light olefins selectivities were increased and the results indicate that at
the total pressure of 6 bar, the optimal catalyst containing
50%Fe/50%Mn/5 wt.%Al2O3 showed the highest total
selectivity of 59.2% with respect to C2–C4 light olefins
and also led to 20.5% total of selectivity toward the C5+ products. The results also indicate that the CO conversion and the total
selectivity with respect to C2–C4 light olefins were
decreased as the total pressures are increased from 6 bar to 15 bar. Hence
because of high CO conversion, low CH4 selectivity, and also higher
total selectivity with respect to C2–C4 olefins at the
total pressure of 6 bar, this pressure was chosen as the optimum pressure.Table 13
Effect of different
total pressures on the catalytic
performance of 50%Fe/50%Mn/5 wt%Al2O3 catalyst.
Selectivity (%)Pressure (bar)CO conversion (%)CH4C2H4C3H6C4H8C5+184.429.829.211.22.113.8284.026.530.711.92.615.6384.025.634.512.53.016.0484.125.137.212.83.217.5584.823.838.913.03.518.7684.019.840.113.25.920.5783.819.731.812.95.727.2882.219.531.212.15.329.6982.819.330.111.05.430.01078.219.028.010.33.534.41178.018.827.710.52.935.61276.618.027.19.22.738.01371.617.224.57.52.139.61468.216.722.27.22.041.71564.216.521.06.71.543.2Table 14
BET results for different supports on Fe/Mn catalysts.
SupportSpecific surface area (m2/g)PrecursorCalcined catalyst (before reaction)Calcined catalyst (after reaction)TiO2112.0109.8105.3Zeolite145.3139.7133.1MgO123.6118.2115.4Al2O3157.5152.3150.7SiO2165.6148.2142.6


## 3.2.1. Effect ofH2/CO Molar Feed Ratio

The influence of
theH2/CO molar feed ratio on the steady-state catalytic
performance of the iron manganese oxide catalyst  containing 50%Fe/50%Mn/5 wt.%Al2O3 for the Fischer-Tropsch reaction at 400°C under atmospheric
pressure was investigated and the CO conversion and light olefins products
selectivity percent, present on steady-state catalytic performance, are
presented in Table 11. The results showed that with variation in H2/CO feed ratio from 1/1 to 3/1,
different selectivities with respect to light olefins were obtained.
However, in the case of the H2/CO=1/1, the total selectivity of
light olefins products was higher and the CH4 selectivity was lower
than the other H2/CO
feed ratios under the same temperature and pressure condition. It is also
apparent that, for all of the H2/CO
feed molar ratios, the optimum catalyst shows a high selectivity toward
ethylene. Therefore, the H2/CO=1/1
ratio was chosen as the optimum ratio for conversion of synthesis gas to C2–C4 olefins over iron manganese catalysts.Table 11
Effect of differentH2/CO feed
molar ratios on the catalytic performance of
50%Fe/50%Mn/5 wt%Al2O3 catalyst.
H2/COCO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+1/184.427.436.812.85.410.82/180.930.534.712.55.47.13/197.268.314.17.52.25.9


## 3.2.2. Effect of Reaction Temperature

The effect
of reaction temperature on the catalytic performance of the 50%Fe/50%Mn/5 wt.%Al2O3 catalyst was studied at a range of temperature between 260–420°C and the
results are presented in Table 12, (P=1atm, H2/CO=1/1). The results show that as the operating
temperature is increased, the CO conversion is increased. In addition, for the
reaction temperature at 360°C, the total selectivity of light olefins products
was higher than the other reaction temperatures under the same reaction
conditions. In general, an increase in the reaction temperature leads to an
increase in the catalytic performance; furthermore, it has shown that the
reaction temperature should not be too low [14]. At low reaction temperatures,
the conversion percentage of CO is too low and so it causes a low-catalytic
performance. On the other hand, increasing the reaction temperature leads to
the formation of large amounts of coke as an unwanted product, as we found in
this work.  Therefore, in this study,
360°C is considered to be the optimum operating temperature because of high-CO
conversion, total selectivity of light olefins products, low CH4, and not formation of coke.Table 12
Effect of different reaction temperatures on
the catalytic performance of 50%Fe/50%Mn/5 wt%Al2O3 catalyst.
T(C°)CO conversion (%)Selectivity (%)CH4C2H4C3H6C4H8C5+28039.220.616.53.10.96.330049.823.517.33.51.85.732067.429.722.15.72.26.934084.427.436.812.85.45.436087.430.538.013.15.56.938087.134.932.29.12.95.340089.441.327.07.93.24.243080.340.524.28.23.75.745078.040.225.18.52.05.1


## 3.2.3. Effects of Total Pressure

An increase
in total pressure would generally result in condensation of hydrocarbons, which
are normally in the gaseous state at atmospheric pressure. Higher pressures and
higher carbon monoxide conversions would probably lead to saturation of
catalyst pores by liquid reaction products [49]. A different composition of the
liquid phase in catalyst pores at high syngas pressures could affect the rate
of elementary steps and carbon monoxide and hydrogen concentrations. A series
of experiments were carried out for the 50%Fe/50%Mn/5 wt.%Al2O3 catalyst to investigate on the
performance of this catalyst in variation of total pressure in the range of 1–15 bar, at the
optimal conditions of H2/CO=1/1 and 360°C (Table 13). The results
indicate that, at the total pressure of 1 bar, the optimal catalyst showed a
total selectivity of 42.5% with respect to C2–C4 light
olefins and 13.8% produce the C5+ products. It is also
apparent that increasing in total pressure in the ranges of 2–15 bar
significantly increases the C5+ selectivity and leads to
an increase to 43.2% at the pressure of 15 bar. In the other hand, as it can be
seen on Table 13 at the ranges of 1–6 bar total
pressures, no significant decreasing on CO conversion was observed, however,
the light olefins selectivities were increased and the results indicate that at
the total pressure of 6 bar, the optimal catalyst containing
50%Fe/50%Mn/5 wt.%Al2O3 showed the highest total
selectivity of 59.2% with respect to C2–C4 light olefins
and also led to 20.5% total of selectivity toward the C5+ products. The results also indicate that the CO conversion and the total
selectivity with respect to C2–C4 light olefins were
decreased as the total pressures are increased from 6 bar to 15 bar. Hence
because of high CO conversion, low CH4 selectivity, and also higher
total selectivity with respect to C2–C4 olefins at the
total pressure of 6 bar, this pressure was chosen as the optimum pressure.Table 13
Effect of different
total pressures on the catalytic
performance of 50%Fe/50%Mn/5 wt%Al2O3 catalyst.
Selectivity (%)Pressure (bar)CO conversion (%)CH4C2H4C3H6C4H8C5+184.429.829.211.22.113.8284.026.530.711.92.615.6384.025.634.512.53.016.0484.125.137.212.83.217.5584.823.838.913.03.518.7684.019.840.113.25.920.5783.819.731.812.95.727.2882.219.531.212.15.329.6982.819.330.111.05.430.01078.219.028.010.33.534.41178.018.827.710.52.935.61276.618.027.19.22.738.01371.617.224.57.52.139.61468.216.722.27.22.041.71564.216.521.06.71.543.2Table 14
BET results for different supports on Fe/Mn catalysts.
SupportSpecific surface area (m2/g)PrecursorCalcined catalyst (before reaction)Calcined catalyst (after reaction)TiO2112.0109.8105.3Zeolite145.3139.7133.1MgO123.6118.2115.4Al2O3157.5152.3150.7SiO2165.6148.2142.6


## 4. Conclusions

Many variables in the preparation of the catalyst
during the coprecipitation procedure and the subsequent calcination step are
important in controlling the catalytic performance of iron-manganese mixed
oxide catalysts for conversion of synthesis gas to light olefins. Preparation
conditions for optimum catalytic performance are1/1:[Fe]/[Mn] ratio at pH 8.3
and 70°C for 3 hours aging time, followed by calcination at 650°C for 6 hours.
The optimal-supported catalyst was found to be 50%Fe/50%Mn/5 wt.%Al2O3. The optimal reaction conditions were
found to be 360°C with molar feed ratio of H2/CO=1/1, (GHSV=2700h−1)
under the total pressure of 6 bar. The
characterization of both precursors and calcined catalysts by powder XRD, SEM,
BET specific surface area, and thermal analysis (TGA/DSC) methods showed that
the catalyst precursors are sensitive to the preparation conditions.
Relationships between bulk phases and catalytic performance were complex,
although the catalysts showed X-ray diffraction features which correspond to
amorphous mixed iron manganese oxide phases. The SEM results show that longer
time enhances agglomerate size growth during reaction conditions which may be
due to the formation of iron carbides as active phases for Fischer-Tropsch
synthesis. From the results presented in this study, it is clear that the
precipitation conditions used in the preparation procedure and also the
operation conditions are of crucial importance and control of these parameters
should be incorporated into the design of experimental programmers involving
precipitation as the method of catalyst preparation and design of catalytic
reactor as the method of operation conditions.



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*Source: 151489-2008-11-19.xml*