Journal of Chemistry

Volume 2013 (2013), Article ID 973160, 10 pages

http://dx.doi.org/10.1155/2013/973160

Research Article

Catalytic Performance of Fe-Mn/Si Nanocatalysts for CO Hydrogenation

Mostafa Feyzi, Shirin Nadri, and Mohammad Joshaghani

Faculty of Chemistry, Razi University, P.O. Box 67149, Kermanshah 6714967346, Iran

Received 4 June 2012; Revised 31 July 2012; Accepted 10 August 2012

Academic Editor: Albert Demonceau

Copyright © 2013 Mostafa Feyzi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

A series of (Fe, Mn)/SiO₂ nanocatalysts (, 10, 15, 20, 25, and 30 wt.%) were prepared by sol-gel method and studied for the light olefins production from synthesis gas. It was found that the catalyst containing 20 wt.% (Fe, Mn)/SiO₂ is an optimal nano catalyst for production of C₂–C₄ olefins. Effects of sulfur treatment on the catalyst performance of optimal catalyst have been studied by espousing different volume fractions of H₂S in a fixed bed stainless steel reactor. The results show that the catalyst treated with 6 v% of H₂S had high catalytic performance for C₂–C₄ light olefins production. The best operational conditions were H₂/CO = 3/2 molar feed ratio at 260°C and GHSV = 1100 h⁻¹ under 1 bar total pressure. Characterization of catalysts was carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and surface area measurements.

1. Introduction

Since initial introduction of Fischer-Tropsch synthesis (FTS), increasing concentration has been made for the increasing of advantages as well as drawback reduction of this potentially commercial process. One of the best approaches to improve the selectivity toward viably more important products involves the use of a supported bimetallic catalyst [1]. Using this approach, selective production of petrochemical feed stocks such as ethylene, propylene, and butylenes (C₂–C₄ light olefin) directly from syngas is hoped to be attainable [2, 3]. Due to opinion of high activity and selectivity as well as the cost problems, iron-based catalysts are the catalysts of choice. The other metal partner is typically manganese or cobalt. A high olefin selectivity for Fe-Mn catalysts has been reported due to formation of iron-manganese oxides and carbide phases [3, 4]. Recent studies show that primary α-olefin products of the FTS were subjected to afterward reactions on iron surface [5–8]. Sulfur treatment is the subject of contrast results. Barrault et al. [9] studied the poisoning of cobalt and iron catalysts by sulfur and observed a decrease in catalytic activity and a propensity for forming lower olefins. Similar results are reported by Kitzelmann and Vielstich [10] who used a K₂S to achieve a light poisoning of Fe and Co catalysts. The methane selectivity was reduced by 50% and the lower olefin selectivity was increased by ≈10%. Due to accumulation and deactivating times, most of the researches on sulfur poisoning were done using exaggerated sulfur levels of the syngas. In contrast, there are some recent reviews [11, 12] and reports highlighting the advantageous effects of sulfur on both iron [13, 14] and other active metals [15–20]. It has been demonstrated that a small amount of sulfur species on the catalyst surface could be associated with improved FTS activity and enhanced olefin selectivity. It was also reported that the activity was only slightly lowered by this treatment. Li and Coville have observed a decrease in methanation for Co/TiO₂ catalyst [21, 22]. Anderson et al. found that selectivity toward light hydrocarbons products increased with increasing sulfur content of alkali-promoted iron catalysts and [23–27]. Stenger and Satterfield [26] reported a 60% increase in the activity of a fused magnetite catalyst after exposure to synthesis gas containing H₂S.

Herein, we investigated the role of H₂S-treated catalyst on decreasing of methane and increasing the C₂–C₄ olefins in products. We also reported the optimization process and effects of operational conditions on the catalytic performance of an optimal catalyst. Characterization of catalysts was carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and surface area measurements.

2. Experimental

All chemical reagents and solvents were analytical grade and purchased from Fluka and Merck. The specific surface area, the total pore volume, and the mean pore diameter were measured using a NOVA 2200 instrument. The XRD patterns of the precursor and calcined samples were recorded on a Philips X’ Pert (40 kV, 30 mA) X-ray diffractometer using a Cu Kα radiation source ( Å) and a nickel filter. The TEM investigations were carried out using an H-7500 (120 kV). The morphology of catalyst and precursor was observed by means of S-360 scanning electron microscopy.

2.1. Catalyst Preparation

Fe(NO₃)₃·9H₂O and Mn(NO₃)₂·4H₂O (Fe/Mn molar ratio is 3/1 [28]), tetraethyl orthosilane were dissolved separately in ethanol at 60°C and mixed together. An ethanolic solution of oxalic acid (H₂C₂O₄·2H₂O) (10 wt% excess) was added gradually under constancy stirring (300 rpm) to give transparent monolithic gel. The formation of iron and manganese oxalate was associated with the formation of nitric acid which contributed to the acidity of the medium (pH ). The obtained material containing iron and manganese oxalate was slowly dried at 100°C in oven, powdered, and calcined at 500°C for 6 h in air atmosphere and heating rate of 2°C min⁻¹.

2.2. H₂S Treats the Catalysts

H₂S-treating experiments were performed in a gas circulation system composed of H₂S generator and sulfur treatment reactor parts (Figure 1). The catalyst containing 20 wt.% (Fe, Mn)/SiO₂ was treated with different H₂S volume fraction at 200°C and 1 bar for 5 h. The treated catalysts then were subjected to FTS production of light olefins under same reaction conditions (H₂/CO = 2/1, GHSV = 1000 h⁻¹,  bar at 250°C).

Figure 1: Schematic representation of the H₂S production and treating system. 1: Gas cylinders, 2: pressure regulators, 3: needle valves, 4: mass flow controllers (MFCs), 5: reflux flask, 6: dropping funnel, 7: tank, 8: ball valves, 9: tubular reactor and catalyst bed, 10: tubular Furnace.

2.3. Catalyst Testing

Catalysts were tested in a fixed bed stainless steel reactor at different operational conditions (Figure 2). The meshed catalyst (0.5 g) was diluted with similar granules of quartz beads (1.0 g) and held in the middle of the reactor (30 cm length and internal diameter 6 mm). All catalysts were activated on-line (reduced) for a 10 h period in pure hydrogen (1 bar) at a temperature of 400°C and space velocity of 800 h⁻¹. Reactant and stream products were analyzed on-line using a Varian Star 3600CX gas chromatograph equipped with a thermal conductivity detector (TCD) and a chromosorb column. The heavy hydrocarbon products were analyzed off-line using a Varian CP 3800 gas chromatograph with a Petrocol Tm DH100 fused silica capillary column and a flame ionization detector (FID). The conversion percentage of CO was based on the fraction of CO that formed carbon-containing products according to

where is the number of carbon atoms in product , is the weight percentage of product , and is the percentage of CO in the syngas feed.

Figure 2: Schematic representation of the catalyst performance system.

The selectivity () toward product is based on the total number of carbon atoms in the product and is therefore defined as

3. Results and Discussion

3.1. Catalytic Performance

3.1.1. Effect of Metals Loading

Catalysts of the formula (Fe, Mn)/SiO₂ were prepared with different loadings ( = 5, 10, 15, 20, 25, and 30 wt.% based on the support weight) and tested for the FTS under same reaction conditions (H₂/CO = 2/1, GHSV = 1000 h⁻¹,  bar at 250°C). The CO conversion and products selectivity in the steady state condition (the time required to attain steady-state condition was about 10 h) were summarized in Table 1. According to the results, the catalyst containing 20 wt.% (Fe, Mn)/SiO₂ has the highest selectivity towards C₂–C₄ olefins and the lowest selectivity with respect to methane and CO₂. Therefore, this catalyst was chosen as the optimal catalyst for the conversion of synthesis gas to light olefins. Characterization studies were carried out using various techniques for both the precursors and calcined catalysts. SEM images show a few differences in morphology of precursor and calcined catalysts (Figure 3). The catalyst precursor seems to have relatively larger agglomerations of particles than the calcined catalyst. Characterization studies were carried out using XRD technique for the 20 wt.% (Fe, Mn)/SiO₂ calcined catalysts (Figure 4). The actual identified phases for this catalyst were Fe₃O₄ (cubic), SiO₂ (hexagonal), Mn₂O₃ (cubic), and Fe₂O₃ (rhombohedral). The particle size was determined from the half width of the most intense peak of the diffraction pattern around using the Scherrer equation [29], , where is the mean crystallite diameter, has an assumed value of 0.9, is the X-ray wave length (1.54 Å), and is the width of the diffraction peak at half maximum. The catalysts containing 20 wt.% (Fe,Mn)/SiO₂ have particle sizes of about 35 nm. The catalyst containing 20 wt.% (Fe, Mn)/SiO₂ was characterized with TEM [30] (Figure 5). As shown in Figure 5, the particle sizes are from 25–40 nm. This result conform with obtained results that was studied by using the Scherrer equation.

Table 1: Effect of loading of (Fe-Mn) on the catalytic performance of catalysts.

wt.% (Fe, Mn)
5
10
15
20
25
30

CO conversion (%)
43.7
46.4
52.6
62.8
65.7
60.2

CH₄
31.4
29.1
25.7
24.3
26.3
28.5

46.9
46.7
49
52
52.8
48.3

    C₂H₆
6.8
6.4
6.2
5.7
6.5
7.5

    C₂H₄
11.1
12.8
12.8
14.4
12.7
10.4

    C₃H₈
7.3
6.7
7.0
6.7
7.9
8.6

Product selectivity (%)
    C₃H₆
8.4
7.1
8.3
9.5
8.6
7.5

    C₄H₁₀
6.8
6.9
6.3
7.3
8.0
6.2

    C₄H₈
6.5
6.8
8.4
9.4
9.1
8.1

8.3
9.5
8.7
9.2
10.4
11.1

C₁₀+
6.9
8.7
11.4
8.9
5.1
5.8

CO₂
6.5
6.0
5.2
4.6
5.4
6.3

1.2
1.3
1.5
1.7
1.4
1.2

Olefin/parrafin ratio
C₂
1.6
2
2.1
2.5
2.0
1.4

C₃
1.2
1.1
1.2
1.4
1.1
0.9

C₄
1.0
0.1
1.3
1.3
1.1
1.3

Reaction conditions: H₂/CO = 2/1, GHSV = 1000 h⁻¹, and  bar at 250°C.

(a)

(b)

Figure 3: SEM images of precursor (a) and calcined catalyst containing 20 wt.% (Fe, Mn)/SiO₂.

Figure 4: XRD patterns of the calcined catalyst containing 20 wt.% (Fe, Mn)/SiO₂: • Fe₃O₄ (cubic), Fe₂O₃ (rhombohedral), ▲ Mn₂O₃ (cubic), and ∎ SiO₂ (hexagonal).

Figure 5: TEM image of calcined catalyst containing 20 wt.% (Fe, Mn)/SiO₂.

3.2. Process for H₂S Treatment of the Catalyst

As Figure 1 shows, the system composed of two parts, H₂S generator and sulfur treatment reactor. The H₂S was produced on addition of H₂SO₄ (from dropping funnel 6) to a flask containing Na₂S·H₂O (flask 5) and then was mixed with N₂ and conducted to a stainless steel reactor. The carrier gas containing desired volume fraction of H₂S () was passed over the meshed catalyst (1.0 g) held in the middle of a fixed bed stainless steel reactor (°C and  bar for 5 h). Before H₂S treatment of the catalyst, the system should be exposed to the stream of pure N₂ for 30 min to eliminate the oxygen.

3.2.1. Effect of H₂S Volume Fraction

The catalyst containing 20 wt.% (Fe, Mn)/SiO₂ was treated with different H₂S volume fraction at 200°C and 1 bar for 5 h and then the treated catalysts were subjected to FTS production of light olefins under same reaction conditions (H₂/CO = 2/1, GHSV = 1000 h⁻¹,  bar at 250°C). The results show the highest total selectivity respect to C₂–C₄ light olefins products (C₂–C₄ olefins/C₂–C₄ paraffin = 2.26) as well as the least CH₄ and CO₂ selectivities were achieved using 6/100 H₂S volume fraction (Table 2). Higher volume fractions led to less total CO conversion which is a drawback in the industrially point of view. Thus, the effects of sulfur strongly depend on the S loading, possibly because different catalyst functions are affected by sulfur.

Table 2: Effect of different H₂S volume fractions on the catalytic performance of catalysta.

H₂S volume fractionsb
0.02
0.04
0.06
0.08
0.1
0.12

CO conversion (%)
64.4
65.2
67.4
65.7
58.4
43.5

CH₄
23.4
21.1
18.5
17.6
7.01
17.0

53.5
54.5
56.8
56.5
52.8
53.1

    C₂H₆
5.8
5.7
4.9
6.8
6.7
10.4

    C₂H₄
15.1
15.2
17.7
14.2
13.2
11.3

    C₃H₈
8.3
8.5
8.4
8.9
8.5
9.6

Product selectivity (%)
    C₃H₆
8.4
9.1
9.5
8.6
5.7
6.4

    C₄H₁₀
5.0
5.0
4.1
7.7
9.3
8.1

    C₄H₈
10.9
11.0
12.2
10.3
9.4
7.3

10.2
10.5
10.5
12.0
13
11.0

C₁₀+
9.4
10.5
10.9
10.6
12.7
13.5

CO₂
3.5
3.4
3.3
3.3
4.5
5.4

1.8
1.8
2.3
1.4
1.2
0.9

Olefin/parrafin ratio
C₂
2.6
2.7
3.6
2.1
2.0
1.1

C₃
1.0
1.1
1.1
1.0
0.7
0.7

C₄
0.6
2.2
3.0
1.3
1.0
0.9

aReaction conditions: H₂/CO = 2/1, GHSV = 1000 h⁻¹, and  bar at 250°C.

bH₂S volume fractions = .

A possible reaction proposed by Van der Kraan et al. [27] may be accounted for the advantages of sulfur treatment of iron-based FT catalysts (3):

Partially sulfidation of the catalyst gives the sulfide phases which act as support and hence maintains the dispersion of the iron centers. Vacancies created by the loss of H₂O and sulfur also increase the porosity of the sulfide salt of catalysts.

Surface area of the sulfur-treated and untreated catalysts was determined using N₂ absorption desorption. The results show significant effects of sulfur treatment on porosity and specific surface area of catalyst (Table 3). The results show that specific surface area increases with increasing the H₂S volume fraction until to 6 v%. The data could imply that the S increases the dispersion of the Fe and Mn which might be a reason for the better catalytic performance of the above catalyst [27, 28, 31]. It can be seen from Table 3 that sulfur treatment in excess of 6 v% resulted in a decrease in the specific surface area and CO conversion. According to Table 2, it seems that it is the specific surface area, pore volume, and pore size distribution are dependent to sulfur treatment.

Table 3: N₂ adsorption-desorption measurements of iron-manganese catalyst treated with H₂S.

H₂S volume fractionsa
Specific surface area (m² g⁻¹)
Pore diameter ()
Pore volume (cm³ g⁻¹)

—
205.7
46.4
0.31

2/100
225.6
47.9
0.37

4/100
228.5
49.3
0.39

6/100
233.5
53.2
0.42

8/100
228.3
51.5
0.39

10/100
219.7
50.4
0.38

12/100
213.9
47.9
0.34

aH₂S volume fractions = .

The H₂S-treated (6 v%) catalyst was subjected to XRD characterization before and after catalyst performance test, and the corresponding XRD patterns are presented in Figure 6. Before the performance test, the XRD pattern shows the presence of (hexagonal) phase in addition to Fe₂O₃ (rhombohedral), Mn₂O₃ (cubic), and SiO₂ (hexagonal). These patterns disappear after the performance test and instead another pattern that belongs to Fe₃O₄ (cubic), MnO (cubic), and carbide phases FeC and Fe₂C appears. The results show that the sulfide phase has been removed during the reaction. In addition, metallic iron is rapidly converted to iron carbide during the reaction which may be subjected to further oxidation into Fe₃O₄. It is well known that the iron carbides phases are active for FTS and oxidic species are responsible for production of olefins [32–34].

Figure 6: XRD patterns of the sulfur-treated catalysts (0.6%) before and after test: ◆   (hexagonal), • Fe₃O₄ (cubic), Fe₂O₃ (rhombohedral), ▲ Mn₂O₃ (cubic), MnO (cubic), ∎ SiO₂ (hexagonal), ★ FeC (orthorhombic), and ☆ Fe₂C (hexagonal).

3.3. Effect of Operational Conditions

One of the other major 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 as H₂/CO feed molar ratios, GHSV, reaction temperatures, and reactor total pressures were examined to investigate the catalyst stability and its performance for the light olefins production.

3.3.1. Effect of H₂/CO Molar Feed Ratio

The influence of the H₂/CO molar feed ratio on the steady state catalytic performance of the catalyst treated with 6 v% of H₂S was investigated for the FTS at 250°C, GHSV = 1000 h⁻¹, and atmospheric pressure. The CO conversion and light olefin products selectivity percent are shown in Table 4. The results showed that with variation in H₂/CO molar feed ratios from 1/1 to 3/1, different selectivity with respect to C₂–C₄ light olefins was obtained. Among them, for H₂/Com = 3/2 (GHSV = 1000 h⁻¹), the total selectivity of C₂–C₄ light olefins was the highest while, the CH₄ and CO₂ selectivity was the least. Therefore, the H₂/CO = 3/2 ratio was chosen as the optimum ratio for conversion of the syngas to C₂–C₄ olefins over the 20 wt.% (Fe, Mn)/SiO₂ nanocatalyst treated with 6 v% of H₂S.

Table 4: Effect of different H₂/CO feed ratio on the catalytic performance of catalyst.

H₂/CO molar ratio
1/1
2/1
3/2
3/1

CO conversion (%)
58.7
67.4
70.8
58.3

CH₄
19.8
18.5
16.4
20.4

52.7
56.8
60.9
50.4

    C₂H₆
6.3
4.9
6.5
8.7

    C₂H₄
15.2
17.7
18.2
13.3

    C₃H₈
7.6
8.4
7.4
7.2

Product selectivity (%)
    C₃H₆
8.0
9.5
10.4
7.1

    C₄H₁₀
6.1
4.1
5.3
6.5

    C₄H₈
9.5
12.2
13.1
7.6

12.8
10.5
11.2
12.3

C₁₀+
9.4
10.9
8.5
10.4

CO₂
5.3
3.3
3.0
6.5

1.6
2.3
2.2
1.3

Olefin/parrafin ratio
C₂
2.4
3.6
2.8
1.5

C₃
1.1
1.1
1.4
1.0

C₄
0.5
3.0
2.5
1.2

Reaction conditions: GHSV = 1000 h⁻¹,  bar at 250°C.

3.3.2. Effect of Gas Hourly Space Velocity (GHSV)

To obtain a better understanding of the factors affecting the catalytic performance of 20 wt.% (Fe, Mn)/SiO₂ nanocatalyst treated with 6 v% of H₂S, a series of experiments were carried out at different GHSV from 800 to 1300 h⁻¹ under the reaction conditions (H₂/CO = 3/2,  bar at 250°C), and the results are presented in the Table 5. The CO conversion increased with increasing space velocity and reached a maximum CO conversion of 72% for space velocity of 1100 h⁻¹ and then decreased with further increasing of space velocity. At the same time, methane and CO₂ selectivity decreased till space velocity of 1100 h⁻¹ then increases markedly. Madon and Taylor [35] studied the effect of space velocity on the olefins and paraffins selectivity for Ru catalyst and found that the olefin selectivity increased with increasing space velocity. According to the results in Table 5, at the ranges of 800–1100 h⁻¹, significant increasing on light olefins selectivity was observed. It is apparent that in GHSV = 1100 h−1 the selectivity for C₂–C₄ light olefins was increased. Therefore, in this study, GHSV = 1100 h⁻¹ is considered to be better GHSV at 250°C, because in this GHSV a high CO conversion and total selectivity of light olefins products and low CH₄ and CO₂ selectivity were observed. These results indicate that the GHSV is a parameter of crucial importance on the catalytic performance of iron-manganese catalysts for hydrogenation of CO.

Table 5: Effect of different GHSV on the catalytic performance of catalysta.

GHSV (h⁻¹)
800
900
1000
1100
1200
1300

CO conversion (%)
67.1
68.7
70.8
72.0
65.3
57.3

CH₄
18.7
17.3
16.4
15.3
17.2
19.7

55.2
52.9
60.9
64
57.1
51.5

    C₂H₆
8.8
7.3
6.5
7.4
6.7
7.9

    C₂H₄
13.5
14.5
18.2
18.9
15.7
10.3

    C₃H₈
7.9
6.3
7.4
7.1
7.8
9.8

Product selectivity (%)
    C₃H₆
8.6
8.9
10.4
11.5
10.3
10.2

    C₄H₁₀
6.3
4.6
5.3
4.6
6.4
5.9

    C₄H₈
10.1
11.3
13.1
14.5
10.2
7.4

12.9
13.7
11.2
10.1
12.5
12.9

C₁₀+
8.6
12.1
8.5
8.1
9.2
11.7

CO₂
4.6
4.0
3.0
2.5
4.0
4.2

1.4
1.9
2.2
2.4
1.7
1.2

Olefin/parrafin ratio
C₂
1.5
2.0
2.8
2.6
2.3
1.3

C₃
1.1
1.4
1.4
1.6
1.3
1.0

C₄
0.4
2.5
2.5
3.2
1.6
1.3

Reaction conditions: H₂/CO = 3/2,  bar at 250°C.

3.3.3. Effect of Reaction Temperature

The effect of reaction temperature on the catalytic performance of the 20 wt.% (Fe, Mn)/SiO₂ nanocatalyst treated with 6 v% of H₂S was studied at a range of temperatures between 220–290°C under the same reaction conditions ( bar, H₂/CO = 3/2, and GHSV = 1100 h⁻¹), and the results are presented in the Table 6. The results show that for the reaction temperature at 260°C, the total selectivity of light olefins products was the highest. In addition, the CO conversion increases with increasing the operating temperature. In the same way, it has been reported that at low reaction temperatures, the conversion percentage of CO is low and so it causes a low catalytic performance [9]. On the other hand, increasing the reaction temperature leads to increasing of methane as an unwanted product. Therefore, in this study, 260°C is considered the optimum operating temperature. These results indicate that the reaction temperature is a parameter of crucial importance in the catalytic performance of iron-manganese catalyst for hydrogenation of CO.

Table 6: Effect of different reaction temperature on the catalytic performance of catalyst.

Temperature (°C)
220
230
240
250
260
270
280
290

CO conversion (%)
56.3
61.6
67.9
72.0
73.7
75.2
77.2
79.1

CH₄
14.1
14.9
15.6
15.3
15.3
17.5
21.9
23.8

57.5
57
58
64
65.6
59.5
54.6
51.9

    C₂H₆
9.8
6.6
7.1
7.4
6.5
10.2
11.9
12.6

    C₂H₄
13.1
14.8
16.5
18.9
20.4
11.2
8.6
7.1

    C₃H₈
6.8
6.3
6.8
7.1
6.5
11.0
9.3
8.1

Product selectivity (%)
    C₃H₆
9.3
9.9
10.1
11.5
12.1
9.5
9.7
8.3

    C₄H₁₀
7.4
6.8
5.1
4.6
5.2
7.4
8.3
9.1

    C₄H₈
11.1
12.6
12.9
14.5
14.9
10.2
6.8
6.7

11.2
13.5
12.7
10.1
9.1
12.9
8.4
7.3

C₁₀+
12.4
11.2
10.1
8.1
7.2
14.5
10.2
7.4

CO₂
4.8
3.4
3.1
2.5
2.3
5.6
5.8
9.6

1.4
1.9
2.1
2.4
2.6
1.1
0.9
0.7

Olefin/parrafin ratio
C₂
1.3
2.2
2.3
2.6
3.1
1.0
0.7
0.6

C₃
1.4
1.6
1.5
1.6
1.9
0.9
1.0
1.0

C₄
0.4
2.5
2.5
3.2
2.9
1.4
0.8
0.7

Reaction conditions: H₂/CO = 3/2, GHSV = 1100 h⁻¹, and  bar.

3.3.4. Effects of Total Pressure

A series of experiments were carried out to investigate the performance of the 20 wt.% (Fe, Mn)/SiO₂ nanocatalysts treated with 6 v% of H₂S during variation of total pressure in the range of 1–10 bar, at the optimal reaction conditions of H₂/CO = 3/2, GHSV = 1100 h⁻¹, and 260°C (Table 7). The results indicate that at the total pressure of 1 bar, the optimal catalyst showed a high selectivity respect to C₂−C₄ light olefins. It is also apparent that the C₅–C9 and C₁₀+ selectivities increase with increasing the pressure [36]. The results also indicate that the CO conversion and the total selectivity with respect to C₂−C₄ light olefins decrease with increasing the pressure. Increase in the selectivity of higher molecular weight hydrocarbons of Fe-Mn catalyst upon increasing the pressure can be explained by the increased concentration of α-olefins and readsorption and chain initiation of these primary products on catalyst surface which lead to the ultimate desorption of these α-olefins as larger products.

Table 7: Effect of different total reaction pressure on the catalytic performance of catalyst.

Pressure (bar)
1
2
3
4
5
6
7
8
9
10

CO conversion (%)
73.7
67.5
66.3
66.0
66.0
61.5
62.0
62.3
61.1
58.9

CH₄
15.3
14.7
14.3
14.0
13.7
12.8
11.7
10.6
10.3
10.3

65.5
62.1
57
53.3
50.4
48.5
45.2
39.5
37.2
33.4

    C₂H₆
6.5
5.8
5.1
5.4
5.6
5.4
5.2
5.4
4.7
4.2

    C₂H₄
20.4
18.3
14.2
12.1
8.2
7.4
6.5
6.1
5.8
5.5

    C₃H₈
6.5
4.1
4.7
4.9
5.2
5.2
6.1
5.5
5.2
4.7

Product selectivity (%)
    C₃H₆
12.1
11.7
11.4
10.0
8.7
7.8
6.6
5.9
6.3
5.5

    C₄H₁₀
5.2
5.8
6.3
6.8
6.9
8.4
7.4
6.3
5.8
5.4

    C₄H₈
14.9
16.4
15.3
14.1
15.8
14.3
13.4
10.3
9.4
8.1

9.1
11.3
15.4
16.9
18.7
19.6
21.6
24.7
26.4
27.1

C₁₀+
7.2
9.5
10.9
12.3
13.7
14.3
16.5
20.0
20.6
23.5

CO₂
2.3
2.4
2.4
3.5
3.5
4.8
5.0
5.2
5.5
5.7

2.6
3.0
2.5
2.1
1.9
1.6
1.4
1.3
1.4
1.3

Olefin/parrafin ratio
C₂
3.1
3.2
2.8
2.2
1.5
1.4
1.3
1.1
1.2
1.3

C₃
1.9
2.9
2.4
2.0
1.7
1.5
1.1
1.0
1.2
1.2

C₄
2.9
2.8
2.4
2.1
2.3
1.7
1.6
0.7
1.6
1.5

Reaction conditions: H₂/CO = 3/2, GHSV = 1100 h⁻¹, and °C.

Hence, because of high CO conversion and higher total selectivity with respect to C₂–C₄ olefins at the total pressure of 1 bar, this pressure was chosen as the optimum pressure.

4. Conclusions

In conclusion, it is found that the activity and selectivity of the catalyst are affected by the level of sulfur adsorbed on the catalyst, and the catalyst treated with 6 v% of H₂S showed the best catalytic performance for light olefins production. The operational conditions such as H₂/CO molar feed ratio, gas hourly space velocity (GHSV), reaction temperature, and reaction total pressure were very effective and the optimal operating conditions for production of light olefins were found to be 260°C with molar feed ratio of H₂/CO = 3/2 (GHSV = 1100 h⁻¹) under the total pressure of 1 bar. The optimal nanocatalyst treated with 6 v% of H₂S was found to be superior to the other catalysts in terms of better C₂–C₄ selectivity in the FTS products and higher olefin/paraffin ratio (2.6). In addition, methane formation by using this modified catalyst was suppressed, which caused decreasing of methane selectivity from 24.3 to 15.8% at 260°C with molar feed ratio of H₂/CO = 3/2 (GHSV = 1100 h⁻¹) under the total pressure of 1 bar.

Acknowledgments

The authors thank the Iran National Science Foundation (INSF) for financial support and Razi University Research Council for partial support of this work.

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