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).
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.
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.
(a)
(b)
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.
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.
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].
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.
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.
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.
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.
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.