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

Abstract

Iron manganese oxides are prepared using a coprecipitation procedure and studied for the conversion of synthesis gas to light olefins and 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 650 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.% is an optimum-modified catalyst. The catalytic performance of optimal catalyst has been studied in operation conditions such as a range of reaction temperatures, /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.

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 ( 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 olefins [15, 16]. High 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 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 /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 of (0.5 M) (99%, Merck, Germany) and (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 (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 . 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 was observed in the washings tested by flam atomic absorption. The precipitate was then dried at for 16 hours to give a material denoted as the catalyst precursor which was subsequently calcined in static air in the furnace (, 6 hours) to give the final catalyst. For preparation of the supported catalysts, the same amount (20 wt%) of each support such as (98%, May & Baker), (98%, Merck), (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 . After the test of all these supported catalysts, it was found that is the best support than the others, so the loading of 5, 10, 15, 20, and 25 wt% based on the total catalyst weight, were used to obtain the best loading of support. The supported catalyst was then promoted with different promoters (Li, K, Rb, and Mg) by adding a small amount (1.5 wt%) of (99.5%, Prolabo, France), (99%, Merck), RbCl (99%, Prolabo), and (99%, May & Baker) as separately to the suspension containing 50%Fe/50%Mn/5 wt.%.

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 a stepsize of 0.02 and a counting time of 1 second using 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 to and phases were identified by matching experimental patterns to entries in the 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 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 from to using a linear programmer at a heating rate of /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 of .

2.3. Catalyst Testing

The catalyst tests were carried out in a fixed bed stainless steel microreactor at different operation conditions (Figure 1). All gas lines to the reactor bed were made from 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, , and 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 and controlled by a digital programmable controller (DPC). The reactor tube was constructed from 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 stream (, flow rate of each ) at for 6 hours before synthesis gas exposure. The FT reactions was carried out at (, , ). 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, OD, 10 meters long, and particle mesh 100/120). Helium was employed as a carrier gas for optimum sensitivity (). 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]: The selectivities (%) toward the individual components on carbon basis are calculated according to the same principle [26]:

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 (, , ) with a range of aging times between 0 minute (unaged) and 300 minutes for the precipitate. The catalysts were prepared by calcination at 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 () and the second stage is due to the decomposition of hydroxyl bimetallic or nitrate precursor (), respectively. The peak around is due to the decomposition of or 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 ( (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 (cubic) and (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 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.

Figure 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 (, , 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 phase as rhombohedral structure. The catalytic activity for the Fischer-Tropsch synthesis was investigated for the materials following calcination (, 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 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 (cubic) and (rhombohedral). The calcined tested catalysts prepared at also were characterized by XRD and different phases including MnO (cubic), FeO (cubic), and (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].

Figure 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 (, , 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 (, 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 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 (rhombohedral) phase was observed and in the calcined catalyst containing 0%Fe-100%Mn, the (cubic) phase was identified. For the other calcined catalysts, different phases including (cubic) and (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 (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 (, , ). 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 (/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.

Figure 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 (, , 3 hours aging time) with a range of solution temperature from 40 to . The catalytic performance of these series catalysts for the conversion of synthesis gas to light olefins was investigated for the materials following calcination (, 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 (, , ) 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 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 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 (cubic), (rhombohedral), and (tetragonal).

Figure 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 , , , MgO, and zeolite into the iron manganese oxide catalysts (), 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 is more active than the other supported catalysts and shows a high selectivity toward hydrocarbons. To understand the influence of loading of on the catalytic activity of mixed iron manganese oxide catalysts, a series of different 50%Fe/50%Mn/wt.% catalysts were prepared by coprecipitation procedure outlined above. The 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.% 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 (monoclinic), (rhombohedral), and (cubic) and its precursors comprised the phase with rhombohedral structure. In the tested catalyst, different phases including FeO (cubic), MnO (cubic), , and (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 -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% 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 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 . 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)).

Figure 8 

XRD patterns of the catalyst containing support in different states (precursor, calcined catalyst before and after the test).

(a)

(b)

(c)

(a)
(b)
(c)

Figure 9 

The SEM images of catalyst containing 50%Fe/50%Mn/5 wt%; (a) precursor, (b) calcined catalyst before reaction, and (c) calcined catalyst after reaction.

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 , , RbCl, and were separately introduced to the resulting suspension containing 50%Fe/50%Mn/5 wt%. Then all of these different promoted catalysts were tested at the same reaction conditions ( and feed molar ratio of 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.% without any promoter and aged for 3 hours appears to be the optimum-modified catalyst for the conversion of synthesis gas to light olefins.

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 as /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 of /CO Molar Feed Ratio

The influence of the /CO molar feed ratio on the steady-state catalytic performance of the iron manganese oxide catalyst containing 50%Fe/50%Mn/5 wt.% for the Fischer-Tropsch reaction at 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 /CO feed ratio from 1/1 to 3/1, different selectivities with respect to light olefins were obtained. However, in the case of the , the total selectivity of light olefins products was higher and the selectivity was lower than the other /CO feed ratios under the same temperature and pressure condition. It is also apparent that, for all of the /CO feed molar ratios, the optimum catalyst shows a high selectivity toward ethylene. Therefore, the ratio was chosen as the optimum ratio for conversion of synthesis gas to olefins over iron manganese catalysts.

3.2.2. Effect of Reaction Temperature

The effect of reaction temperature on the catalytic performance of the 50%Fe/50%Mn/5 wt.% catalyst was studied at a range of temperature between and the results are presented in Table 12, (, ). The results show that as the operating temperature is increased, the CO conversion is increased. In addition, for the reaction temperature at , 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, is considered to be the optimum operating temperature because of high-CO conversion, total selectivity of light olefins products, low , and not formation of coke.

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.% 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 and (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 light olefins and 13.8% produce the products. It is also apparent that increasing in total pressure in the ranges of 2–15 bar significantly increases the 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.% showed the highest total selectivity of 59.2% with respect to light olefins and also led to 20.5% total of selectivity toward the products. The results also indicate that the CO conversion and the total selectivity with respect to light olefins were decreased as the total pressures are increased from 6 bar to 15 bar. Hence because of high CO conversion, low selectivity, and also higher total selectivity with respect to olefins at the total pressure of 6 bar, this pressure was chosen as the optimum pressure.

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 are : ratio at pH 8.3 and for 3 hours aging time, followed by calcination at for 6 hours. The optimal-supported catalyst was found to be 50%Fe/50%Mn/5 wt.%. The optimal reaction conditions were found to be with molar feed ratio of , () 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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