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Cobalt–iron nano catalysts supported on TiO2–SiO2: Characterization and catalytic performance in Fischer–Tropsch synthesis
Accepted Manuscript Title: Cobalt-iron nano catalysts supported on TiO2 -SiO2 : characterization and catalytic performance in Fischer-Tropsch synthesis Author: Mostafa Feyzi Nakisa Yaghobi Vahid Eslamimanesh PII: DOI: Reference:
S0025-5408(15)30060-X http://dx.doi.org/doi:10.1016/j.materresbull.2015.07.039 MRB 8347
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Received date: Revised date: Accepted date:
21-1-2015 5-7-2015 24-7-2015
Please cite this article as: Mostafa Feyzi, Nakisa Yaghobi, Vahid Eslamimanesh, Cobalt-iron nano catalysts supported on TiO2-SiO2: characterization and catalytic performance in Fischer-Tropsch synthesis, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2015.07.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Cobalt-iron nano catalysts supported on TiO2-SiO2: characterization and catalytic performance in Fischer-Tropsch synthesis Mostafa Feyzi1 , Nakisa Yaghobi2, Vahid Eslamimanesh2 1
Faculty of Chemistry, Razi University, P. O. Box: +98-67149, Kermanshah, Iran 2
Iran Polymer and Petrochemical Institute, P. O. Box: +98- 14965 Tehran, Iran,
Graphical Abstract
The Co-Fe/TiO2-SiO2 catalysts were prepared. The prepared catalysts were tested for light olefins and and C5-C12 production. The best operational conditions are 250 °C, H2/CO= 1/1 under 5 bar pressure.
Highlights ► The TiO2-SiO2 supported cobalt-iron catalysts were prepared via sol-gel method. ►.The best operational conditions were 250 °C, GHSV=2000 h-1, H2/CO = 1/1 and 5 bar. ► The (Co/Fe)/TiO2-SiO2 is efficient catalyst for light olefins and C5-C12 production.
Abstract
Corresponding Authors Email: [email protected] (M. Feyzi) Tel/Fax:+988334274559,
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A series of Co-Fe catalysts supported on TiO2-SiO2 were prepared by the sol-gel method. This research investigated the effects of (Co/Fe) wt.%, the solution pH, different Co/Fe molar ratio, calcination conditions and different promoters on the catalytic performance of cobalt– iron catalysts for the Fisher–Tropsch synthesis (FTS). It was found that the catalyst containing 35wt.%(Co-Fe)/TiO2-SiO2 (Co/Fe molar ratio is 80/20) promoted with 1.5 wt.% Cu and calcined in air atmosphere at 600 ºC for 7 h with a heating rate of 3 ºC min-1 is an optimal nano catalyst for converting synthesis gas to light olefins and C5-C12 hydrocarbons. The effects of operational conditions such as the H2/CO ratio, gas hourly space velocity (GHSV), different reaction temperature, and reaction pressure were investigated. The results showed that the best operational conditions for optimal nano catalyst are 250 °C, GHSV=2000 h-1, H2/CO molar ratio 1/1 under 5 bar total pressure. Catalysts and precursors were characterized by, X-ray diffraction (XRD), scaning electron microcopy (SEM), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), temperature program reduction (TPR)
and N2
adsorption-desorption measurements.
Keywords: A. surfaces, A. carbides phase, B. sol-gel, C. X-ray diffraction, C. scanning tunneling microscopy
1. Introduction The Fischer-Tropsch synthesis (FTS) entails the conversion of synthesis gas (a mixture of carbon monoxide and hydrogen) to a spectrum of products mainly comprising olefins and paraffins. An approach to improve the selectivity in this process involves the use of a bimetallic catalyst system containing metals catalyst combined with a support [1]. There has been renewed interest in recent years in FTS, especially for the selective production of petrochemical feed stocks such as ethylene, propylene and butylene (C2–C4 olefins) directly from synthesis gas [2,3]. The high temperature (> 350 °C) Fischer-Tropsch process (HTFT) is carried out at slurry
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reactor and produces light olefins. On the other hand, the low temperature (< 250 °C) FischerTropsch process (LTFT) could be carried out at both slurry and fixed-bed reactors and is used for the production of mainly paraffinic heavy hydrocarbons. Due to the thermodynamic and kinetic limitations of the reaction, few catalysts are able to amplify the heavier hydrocarbons fraction. Among them, Ru-based catalysts show higher activity but are very expensive [6-9]. Nickel-based catalysts could be employed but their efficiencies in hydrogenation process which compete with FTS and produce methane as an undesirable by-product [10,11]. Both Cobalt and iron based catalysts individually have advantages and disadvantages properties [12-19]. Therefore effort to couple them in the hope to bring forth a more efficient catalyst having parents’ advantages is of great importance. These bimetal catalysts should be inserted into a support of high porosity to increase the surface area and mechanical stability of the catalysts as well as prohibition of catalyst sintering. In addition to porosity, pore shape and size are critical secondary factors. The best supports are those that are simply manipulate to produce optimum texture properties. Silica [20,21] and γ-alumina [22,23] are good in this regard particularly in sol-gel preparation method [24]. Cobalt and iron are the most important and commercial transition metal catalyst used either as individual monometallic or composed bimetallic systems [25–27]. The bimetallic Co–Fe catalysts are generally highly active mainly due to presence of cobalt. On the other hand, iron is more suitable for poor H2/CO ratio feedstock which makes the performance of the water-gas shift reaction inevitable [28,29]. The purposes of this study is to investigate the influence of catalyst composition and operational conditions on FTS activity and selectivity. Characterization of both precursors and calcined catalysts were carried out by XRD, SEM, TPR, N2 physisorption, TGA and DSC methods.
2. Experimental
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2.1. Typical procedure for preparation of catalysts A series of cobalt-iron catalysts supported on TiO2-SiO2 with different loadings of cobalt-iron has been prepared. The appropriate amount of start materials cobalt nitrate (Co(NO3)2.6H2O) and iron nitrate (Fe(NO3)3.9H2O) were dissolved in ethanol at 50 °C, separately, mixed well with each other (A). The required amounts of tetraethyl orthosilicate (TEOS) as silica source and tetra butoxy titane (Ti(OC4H9)4) as TiO2 source were dissolved in ethanol at 50 °C and then gradually added to the mixed cobalt-iron solution (A) to produce 5, 10, 15, 20, 25, 30, 35, 40 and 45 wt.% of Co/Fe=1/1 (based on TiO2-SiO2 wt.%) respectively. Then, an ethanol solution of oxalic acid (C2H2O4.2H2O, appropriate amount for hydrolysis of TEOS and Ti(OC4H9)4) was added to a mixed solution under constant stirring to obtain a gel form. The gel was dried in an oven (120 ºC, 20 h) to give a material denoted as the catalyst precursor. The promoted catalysts were then prepared by the incipient wetness impregnation method by adding 1.5 wt.% of each promotor to Co–Fe/TiO2-SiO2 precursor. Finally, the obtained samples was dried at 120 °C for 12 h and calcined at desired calcination conditions.
2.2. Catalyst characterization 2.2.1. X-Ray dif fraction (XRD) The XRD patterns of all 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 (λ=1.542 Å) and a nickel filter in the 2θ range of 4°-70°
2.2.2. N2-adsorption-desorption measurements The specific surface area (using BET and BJH methods), the total pore volume and the mean pore diameter were measured using a N2 adsorption-desorption isotherm at liquid nitrogen
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temperature (-196 ºC), using a NOVA 2200 instrument (Quantachrome, USA). Prior to the adsorption-desorption measurements, all the samples were degassed at 110 ºC in a N2 flow for 3 h to remove the moisture and other adsorbates
2.2.3. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) The TGA and DSC were carried out using simultaneous thermal analyzer (Perkin Elmer) under a flow of dry air with a flow rate of 50 ml min-1. The temperature was raised from 20 to 600 ˚C using a linear programmer at a heating rate of 3 ˚C min-1
2.2.4. Scanning electron microscopy (SEM) The morphologies of prepared nanocatalysts and their precursors were observed by means of an S–360 Oxford Eng scanning electron microscope (USA)
2.2.5. Temperature-programmed reduction (TPR) H2-TPR profiles of the unpromoted and promoted nanocatalysts were recorded using a micromeritic TPD–TPR 290 system. The TPR for each sample (50 mg) was performed using a mixture gas of 5%H2/95%Ar (v/v) as the reductant. The sample was heated from 25 to 950 °C at a heating rate of 5 °C min. The flow rate of the mixture gas was 50 ml/min
2.3. Fischer-Tropsch synthesis Catalytic activity test were carried out in a fixed bed stainless steel reactor at different operation conditions (Figure 1). All catalysts were activated (reduced) for 12 h period on line in pure hydrogen (1 bar) at a temperature of 400 ºC at flow rate of 10 ml min-1. Meshed catalyst (0.5 g) diluted with similar granulometry of quartz beads (0.3 g), was held in the middle of the reactor (10 cm length and internal diameter is 6 mm). Reactant and stream products were
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analyzed on line using a Varian gas chromatograph (Star 3600CX) equipped with a thermal conductivity detector (TCD) and a chromosorb column. The heavy hydrocarbon products were off line analyzed using a Varian CP 3800 with a Petrocol Tm DH100 fused silica capillary column and a flame ionization detector (FID). The conversion percentage of CO based on the fraction of CO forming carbon containing products according to below equation:
CO conversion (%)
ni M i 100 M CO
where ni is the number of carbon atoms in product i, Mi is the percentage of product i and MCO is the percentage of CO in the syngas feed. The selectivity (S) of product i, is based on the total number of carbon atoms in the product and therefore is defined as:
S i (%)
ni M i 100 niMi
3. Results and Discussion 3.1.1. Effect of cobalt-iron weight percent The effect of different wt.% of Co/Fe=1/1 (based on the TiO2-SiO2 weight) on the catalytic performance under the same reaction conditions (H2/CO=1/1, gas hourly space velocity (GHSV) =1500 h−1, P=1 bar and T=240˚C) was investigated. As Table 1 shows, the catalyst activity based on CO conversion increases linearly with increasing of Co-Fe wt.% while the selectivity toward undesirable methane and CO2 decreases. After the weight percent of Co-Fe was reached to above 35% methane and CO2 selectivity increases markedly. According to the obtained results, the catalyst containing 35wt.% of Co-Fe presented the best catalytic performance compared to the other tested catalysts. This catalyst showed the highest selectivity
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towards olefinic products (C2-C4) and C5-C12 hydrocarbons and also the lowest selectivity with respect to methane and CO2. Thus, this catalyst was chosen as the optimal catalyst. In FTS, the amount of active metals is affect on catalyst selectivity and activity [30,31]. The specific surface area (BET method) pore volume (BJH method) and pore diameter (BJH method) of the precursors and calcined catalysts (before and after reaction) are given in in Table 2. A relatively high surface area of the precursor is mainly due to solvent evaporation which makes a porous surface of high surface area. The surface area of the calcined catalysts is higher than the corresponding precursors which is mainly due to exit of gaseous caused by phase changing during calcination creating pore on catalyst surface. It can be seen that the increase of (Co-Fe) wt.% results in an increase in the catalyst surface area linearly which is in conformity with catalyst activity based on CO conversion and more than 35wt% were decreased. On the other hand, the pore volume increases with increasing of cobalt-iron ratio to reach a maximum at 35 wt.% (Co-Fe)/ TiO2-SiO2, and then decreases with further increasing of cobalt-iron weight percent which is in agreement with product selectivity trend. The N2 absorption–desorption data are shown that the catalyst was prepared from 35 wt.%(Co-Fe)/TiO2-SiO2, has higher specific surface area, pore volume and pore diameter than the other calcined catalysts and it was the one acceptable reason for better catalytic performance of this catalyst. The 35wt.%(Co-Fe)/TiO2-SiO2 catalyst in different state were characterized by XRD (Figure. 2). The precursor prepared from 35wt.%(Co-Fe)/TiO2-SiO2 was largely found to be amorphous by XRD. The presence of amorphous phase in the XRD pattern of this precursor makes the other phases undetectable, and this is because of iron hydroxid phases. The actual phases were identified for calcined catalyst (calcinations conditions: during 6 h, T=500 ºC at heating rate 2ºC min-1) containing 35wt.% (Co-Fe)/ TiO2-SiO2 under the specified preparation conditions as CoTiO3 (cubic), Fe2O3 (cubic), CoFe2O4 (cubic), Co2SiO4 ( orthorhombic), Fe2SiO4 (cubic) phases.
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In order to identify the phase changes in the 35wt.%(Co-Fe)/TiO2-SiO2 catalyst during the reaction conditions (P=1 bar, H2/CO=1/1, T=240 °C and GHSV=1500 h−1), this catalyst after test was characterized by XRD (Figure 2). The actual phases identified in the XRD pattern was found to be metallic iron and cobalt in form of cubic structure and CoO (cubic), Fe3O4 (cubic), Fe2SiO4 (cubic), and Fe3C (orthorombic) phases. As shown, the tested catalyst has oxidic and iron carbide phases, both of which have been generally accepted to be the most likely active phase for FTS performance [32-36]. One of the affecting parameters on catalytic performance is catalyst particle size. This parameter could be calculated by Scherer-equation [30,31]. The crystallite size of the 35 wt.%(Co-Fe)/TiO2-SiO2 catalyst calculated using Scherer-equation is about 75 nm. It was found that cobalt particle size had a strong impact on cobalt catalyst selectivity; the olefin selectivity correlate with the particle size of catalyst [39-42]. As shown in Figure 3, the TGA/DSC curves of 35 wt.% (Co-Fe)/ TiO2-SiO2 catalyst precursor exhibit a three-step weight loss. The first step at the temperature below 90 °C was attributed to the evaporation of residual moistures in the support and loss of physisorbed waters. The second step at the temperature between 115 and 290 °C was the decomposition of oxalate phases and to remove hydrate and bound waters. The third weight loss occurs above 320 °C corresponds to the full decomposition of all oxalates and the formation of stable TiO2-SiO2, cobalt and iron oxides. DSC measurement was performed in order to provide further evidence for the presence of the various species and evaluates their thermal behavior, as it shown in Figure. 3, the exothermic peak at lower temperature represents the removal of the physically adsorbed material such as physisorbed waters (80-130 oC). The exothermic peak at around 170-290 oC is due to the decomposition of oxalate phases and to remove hydrate and bound waters. The third stage around 300-410 °C is due to the full decomposition of oxalates phases to oxides. SEM images of both precursor and calcined 35 wt.%(Co-Fe)/TiO2-SiO2 catalyst is displayed in figure 4 with irregular shapes. SEM observations have shown differences in morphology of precursor
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and calcined catalysts. The image obtained from catalyst precursor depicts several larger agglomerations of particles (Figure. 4a) and show that this material has a less dense and homogeneous morphology. After the calcination at 500 ºC, 6 h and heating rate of 2 ˚C min-1, the morphological features are different with the precursor sample and shows that the agglomerate size is greatly reduced compared to the precursor (Figure. 4b). It is obvious in this Fig, the crystal sizes were from 60–70 nm. This result confirmed the obtained results studied by using the Scherrer equation.
3.1.2. Effect of pH A series of 35wt.%(Co-Fe)/TiO2-SiO2 catalysts were prepared by sol-gel method with a range of solution pH from 1.0 to 3.5 and tested for Fischer-Tropsch synthesis under the same reaction conditions (H2/CO=1/1, GHSV=1500 h-1, P=1 bar at 240 ºC). The CO conversion and products selectivity percent are shown in Table 3. According to the obtained results, the catalyst containing 35wt.%(Co-Fe)/TiO2-SiO2 was prepared in pH=2.5 has shown the best catalytic performance than the other tested catalysts. This catalyst showed the highest selectivity towards light olefins, C5-C12 hydrocarbons and lowest selectivity with respect to methane and CO2. So, the catalyst prepared in pH=2.5 was chosen as the optimal catalyst for the conversion of synthesis gas to light olefins and C5-C12 hydrocarbons.
3.1.3. Effect of cobalt/iron weight ratio The influence of the cobalt/iron molar ratio on the catalytic performance of cobalt/iron nano catalyst containing 35wt.%(Co-Fe)/TiO2-SiO2 under atmospheric pressure, H2/CO=1/1 and GHSV=1500 h−1 and T=240°C were investigated (Table 4). The results showed that variation of cobalt/iron ratio (90/10-20/80) resulted in different products selectivity. However, for the catalyst containing 35wt.%(Co-Fe)/TiO2-SiO2 with cobalt/iron=80/20, the catalyst
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activity (based on CO conversion), light olefins and C5-C12 hydrocarbons selectivity were higher than the other prepared catalysts. The results are shown that, the selectivity toward undesired products such as methane and CO2 is also lower than that of the other prepared catalysts. Therefore, cobalt/iron=80/20 molar ratio was chosen as the optimum molar ratio for catalyst preparation.
3.1.4. Effect of calcination atmospheres In this part of the study, two samples of precursor containing 35wt.%(Co-Fe)/TiO2-SiO2 with cobalt/iron=80/20 were calcined separately in air and nitrogen atmosphere (500 ºC, 6h at heating rate 2 ºC min-1). The CO conversion and product selectivity percent for these catalysts are shown in Figure. 5 (T=240 ºC, H2/CO=1/1, P=1 atm and GHSV=1500 h-1). The comparison of the results in this Fig, indicated that the catalyst was calcined in air at 500 ºC for 6 h and at heating rate 2 ºC min-1 have the highest CO conversion and highest selectivity to light olefins and C5-C12 hydrocarbons. By taking these results into consideration, the air was a better calcination atmosphere for the 35wt.%(Co-Fe)/TiO2-SiO2 for Fisher-Tropsch synthesis.
3.1.5. Effect of calcination heating rate To consider the effect of calcination heating rate on the catalytic performance, a series of precursors containing 35wt.%(Co-Fe)/TiO2-SiO2 with cobalt/iron=80/20 were calcined (500 ºC for 6 h) at various heating rates in air and tested for FTS. The heating rate varied between 1-5 ºC min-1. The CO conversion and light olefin products selectivity percent are shown in Table 5 (T=240 ºC, H2/CO=1/1, P=1 bar and GHSV=1500 h-1). It can be seen that heating rates up to about 3 ºC min-1 did not exert a major effect on the catalytic performance of the catalyst, while the heating rates in excess of 3 ºC min-1 resulted in a significant decrease in the CO conversion and products selectivity. Therefore, in this study, heating rate 3 ºC min-1 is the optimum heating
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rate for calcination of the 35wt.%(Co-Fe)/TiO2-SiO2 in air atmosphere for light olefins and C5C12 hydrocarbons production.
3.1.6. Effect of calcination time and temperature The influence of a range of calcination time and temperature were examined. At first, 5 samples of precursor containing 35wt.%(Co-Fe)/TiO2-SiO2 were calcined separately in air at 500 ºC with heating rate of 3 ºC min-1 for 5, 7, 8, 9 and 10 h respectively. The CO conversion and products selectivity percent for these catalysts with different calcination time in reaction conditions (P=1 atm, T= 240 ºC, H2/CO = 1/1 and GHSV=1500 h-1) are shown in Table 6. As being seen in this table, the optimum calcinations time is 7 h at 500 ºC for light olefins and C5C12 hydrocarbons production. In order to study the effect of calcination temperature, 6 samples of precursor containing 35wt.%(Co-Fe)/TiO2-SiO2 calcined separately in 7 h (air atmosphere and heating rate of 3 ºC min-1) at 300, 400, 600 and 700 ºC respectively. These calcined samples were tested at the same reaction conditions (P=1 atm, T=240 ºC, H2/CO=1/1 and GHSV=1500 h-1). A comparison of the results in Table 7, for the catalysts prepared at different calcination temperatures indicated that the catalyst was calcined at 600 ºC had the highest CO conversion and highest selectivity with respect to C2-C4 light olefins and C5-C12 hydrocarbons. Therefore, in this study, the best calcination conditions were found to be air atmosphere at 600 ºC for 7 h with a heating rate of 3 ºC min-1.
3.1.7. Effect of promoters Alkali metals [43,44], Zn, Cu, Mn and Ce have been used widely as promoters to improve the activity and selectivity of the catalysts in the CO hydrogenation. Yang et al. [45] studied the conversion of CO and H2 over Fe catalysts promoted by Zn, Cu, Cr, and alkali metals. Impregnated Fe catalysts were less active than co-precipitated catalysts. Zinc was a better
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promoter than Cu or Cr [45,46]. To study the effect of promoters on the catalytic performance of iron–manganese catalyst, a same amount (1.5 wt.%) of Zn, Cu, K and Ce from Zn(NO3).6H2O, Cu(NO3)2.6H2O, KNO3 and Ce(NO3)2.6H2O were separately introduced to the 35wt.%(Co-Fe)/TiO2-SiO2 catalyst. This method confirm with the other reports, which studied by many investigators [46–48]. Then, all of these different promoted catalysts were tested at the same reaction conditions (H2/CO=1/1, GHSV=1500 h-1, P=1 atm at 240 °C) for the conversion of synthesis gas to C2-C4 light olefins and C5-C12 hydrocarbons, the results are shown in Table 8. Comparing the obtained results of Table 8, leads to the conclusion that 35wt.%(Co-Fe)/TiO2-SiO2 catalyst that promoted with 1.5 wt.% of Cu is more active than the other promoted catalysts. Taking these results into consideration, the catalyst that was promoted with Cu, appears to be the optimum modified catalyst for the conversion of synthesis gas to light olefins and C5-C12 hydrocarbon products, this catalyst gave the best catalytic performance. In order to investigate the reduction behavior of the un promoted and Cu-promoted catalysts, characterization of these catalysts was carried out using H2-TPR technique. The H2-TPR was performed and the obtained profiles are illustrated in Fig. 6. All of these calcined catalysts revealed three distinct reduction peaks. In all of these catalysts the first and second peaks appearing in Fig. 6 corresponds to the reduction of Co3O4 to Co° according to the following steps: Co3O4+ H2→3CoO+H2O 3CoO +3H2→3Co°+3H2O These reduction steps can be attributed to the reduction of Co3+ and Co2+ oxide species to metallic cobalt. The third and final peak reduction can be assigned to Fe2O3→ Fe3O4 and Fe3O4→ Fe for un promoted and Cu promoted catalyst [49]. The H2 consumed during different reduction stages were determined by integrating the area of TPR curves. The results are shown that, the H2 adsorption increased from 8.2 to 9.8 ×10−2 (mmol H2/mmol catalyst), for the un
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promoted catalyst and the promoted catalyst that was promoted with Cu respectively. It is concluded that the amount of catalyst reduction is increased from 70.1% to 83.7 mol% by adding the Cu. The fact that iron-cobalt catalyst that was promoted with 1.5 wt.% Cu reducibility is greater than un promoted iron-cobalt catalyst, suggests that the addition of Cupromotes the reduction of iron- cobalt at lower temperatures.
3.2. Effects of operational conditions Operational conditions have noticeable effects on the catalytic performance of the catalyst. To optimize the reaction conditions, the effects of operational conditions such as H2/CO feed molar ratio, GHSV, reaction temperature and reaction total pressure to were examined to evaluate performance of catalyst.
3.2.1. Effect of H2/CO on the catalytic performance The influence of the H2/CO (synthesis gas ratio) on the catalytic performance of 35wt.%(CoFe)/TiO2-SiO2 catalyst promoted with 1.5 wt.% of Cu for the FTS was investigated and catalyst performance, present were shown in Table 9, with the change of H2/CO from 1/3 to 3/1. However, at H2/CO ratio of 1/1, T=240 °C, GHSV=1500 h-1 and p=1 bar, the total selectivity of light olefins, C5-C12 hydrocarbons was higher and CO2 was also lower than other H2/CO ratios. Therefore, H2/CO=1/1 at 240 °C was chosen as the optimum molar feed ratio for converting synthesis gas to C2-C4 light olefins. On one hand, higher H2/CO shortens the residence time, which is unfavorable for chain growth. In this reaction, the effect of H2/CO ratio is remarkable, and in turn the lower H2/CO ratio in the reactor results in a higher C13+ selectivity. From the results, it can also draw the conclusions that the higher H2/CO ratio is preferential for chain termination to produce light hydrocarbons while lower H2/CO ratio is
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preferential for the chain growth and the production of heavy hydrocarbons [50-51]. Therefore, the C13+ selectivity decreased with increasing H2/CO ratio.
3.2.2. Effect of GHSV In order to gain a deep understanding of the factors affecting the catalytic performance of 35wt.%(Co-Fe)/TiO2-SiO2 catalyst promoted with 1.5 wt.% of Cu, a series of experiments were carried out at different GHSV from 1000 to 3000 h−1 under the optimal reaction conditions (H2/CO= 1/1, P= 1 bar, and T= 240◦C). The results were revealed in Table 10. Comparing the obtained results were led to the conclusion that at GHSV=2000 h−1, the selectivity with consideration of C2–C4 light olefins and C5-C12 hydrocarbons were the highest. Therefore, GHSV=2000 h−1 was emphasized to be the optimum one, since at this GHSV, a high CO conversion, high total selectivity of light olefin and C5-C12 hydrocarbon products and also low CH4 selectivity were obtained. These results proved that GHSV is a parameter of crucial importance on the catalytic performance of 35wt.%(Co-Fe)/TiO2-SiO2 nano catalysts promoted with 1.5 wt.% of Cu for CO hydrogenation. The results revealed that the methane selectivity was decreased with increasing the space velocity (2500-3000 h-1).
3.2.3. Effect of reaction temperature To get a full perception of the effect of reaction temperature on the catalytic performance of 35wt.%(Co-Fe)/TiO2-SiO2 nano catalysts promoted with 1.5 wt.% of Cu, temperature range between 220°C-280°C was scrutinized (H2/CO = 1/1, P = 1 bar, and GHSV = 2000 h-1). The effect of reaction temperature on 35wt.%(Co-Fe)/TiO2-SiO2 catalyst were examined in Table 11. It can be realized that the nano catalyst had high activity at 250 °C. In higher temperature CO conversion increased but selectivity to undesirable products such as methane and CO2 were increased too [52]. The CO conversion increased linear with increasing temperature, as shown
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in Table 11, Similar results were also obtained by Schulz and Claeys [53,54]. Both the CO conversion and CO2 formation followed the same trends as the FTS reaction, and almost presented a linear correlation with increasing reaction temperature. Bukur et al. [55] investigated a precipitated iron catalyst in a fixed bed reactor under a variety of process conditions, and have also observed that light olefins selectivity was high at the reaction temperature of around 280 ºC during FTS. Therefore, in this study, 250°C was considered to be the optimum temperature because of the high CO conversion (81.4%), total selectivity to light olefins (47.2%), low CH4 (6.5%) and low CO2 (3.0%). At low reaction temperature (220 ºC), the conversion percentage of CO was low and caused low catalytic activity.
3.2.3. Effects of reaction pressure In commercial process, the FTS reaction usually operates under high pressure. The 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 [56] and it can influences on the catalytic performance. 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 to investigate the performance of 35wt.%(Co-Fe)/TiO2-SiO2 nano catalysts promoted with 1.5 wt.% of Cu during the variation of total pressure in the range of 1-21 bar, at the optimal reaction conditions of H2/CO=1/1 (GHSV=2000) and 250 ºC (Table 12). As shown in Table 12, increasing the reaction pressure after 5 bar has significant influence on the hydrocarbon selectivity. The selectivity to C5-C12 increased slightly, while the selectivity to ethylene, butylenes and propylene decreased. The results indicated that at the total pressure of 5 bar, the 35wt.%(CoFe)/TiO2-SiO2 nano catalysts promoted with 1.5 wt.% of Cu showed high selectivity to C2-C4
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light olefins and C5-C12 hydrocarbons. It was also apparent that increasing in total pressure in the ranges of 5-21 bar increases the heavy hydrocarbon (C5+) selectivity and led to an increase to 47.8% at the pressure of 21 bar. Because of high CO conversion, low CH4 and CO2 selectivity and also higher total selectivity with respect to C2-C4 light olefins C2-C4 light olefins and C5C12 hydrocarbons at the total pressure of 5 bar although in high pressure selectivity to C5-C12 and C13+ were increased.
4. Conclusions The results proved that many variable factors such as cobalt-iron weight percent, pH, cobalt/iron weight ratio, calcination atmospheres, calcination heating rate, calcination time and temperature, different promoters and operational conditions had influence on the catalytic performance of Co-Fe/TiO2-SiO2 nano-catalysts for the production of C2-C4 light olefins and C5-C12 hydrocarbons. The catalytic performance of 35wt.%(Co-Fe)/TiO2-SiO2 nano catalysts promoted with 1.5 wt.% of Cu as an optimal nano catalyst has been studied under different operational conditions including, H2/CO molar ratio, different GHSV, reaction temperatures and total of reaction pressure. The optimal operational conditions were found to be 250 ºC with molar feed ratio of H2/CO=1/1 (GHSV=2000 h-1) under the 5 bar pressure. It was found that, the catalyst reduction increased from 70.1% to 83.7 mol% by adding the Cu as a promoter in the catalyst composition. Characterization of Co-Fe by powder X-ray diffraction and scanning electron microscopy showed that the catalyst precursor are sensitive to the preparation conditions, so that (Co-Fe) wt.% influencing the structure and morphology of the precursors and calcined catalysts and that these parameters should be incorporated into the design of catalyst. Characterization of catalyst precursor using TGA, generally have shown three stage of decomposition. The first weight loss peaks at low temperatures attributed to the evaporation of residual moistures in the support and loss of physisorbed waters, the second weight loss peaks
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at mid temperatures are due to the decomposition of oxalate phases and to remove hydrate and bound waters and the third weight loss peaks at high temperatures attributed to full decomposition of all oxalates and the formation of stable TiO2-SiO2, cobalt and iron oxides. From DSC, the various exothermic transitions, which were, accompanied by different weight losses with changes only in rate of weight loss, were interpreted on the basis of X-ray diffraction and TGA results. The BET data show that there is general increase in specific surface area as a result of increased (Co-Fe) wt.%. This was attributed to the presence of iron oxide in a supersaturated solid within the cobalt oxide. The BET results for the catalyst tested also show that the specific surface area of the calcined catalysts after testing was decreased. Acknowledgements We gratefully acknowledge the Iran National Science Foundation (INSF) for their help and support of this research.
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Figure 1. Schematic representation for the catalyst test system and used reactor
21
Figure 2. XRD patterns of 35wt.%(Co-Fe)/TiO2-SiO2 catalyst in different estate
22
Figure 3. TGA/DSC curves for the 35 wt.% (Co/Fe)/SiO2 catalyst precursor
23
(a)
(b)
Figure 4. The SEM images of catalyst containing 35 wt.%(Co-Fe)/TiO2-SiO2 , (a) precursor and (b) calcined catalyst
24
Figure 5. Effect of different calcination atmosphere on the catalytic performance
25
Figure 6. H2-TPR profiles of un promoted and Cu-promoted 35 wt.% (Co/Fe)/SiO2 catalysts
Table 1. Effect of different wt. % of (Co-Fe) on the catalytic performance of Co–Fe/TiO2SiO2 catalyst
Product selectivity (%)
Wt.%Co- Fe CO conversion (%) CH4 C2H6 C2H4 C3H8 C3H6 C4H10
5 19.3 32.4 9.6 8.3 4.5 6.7 1.4
10 23.2 26.5 8.1 7.6 4.3 5.9 2.4
15 26.1 25.3 7.8 9.0 5.7 8.3 3.9
26
20 31.4 19.3 8.4 10.2 6.9 12.4 5.9
25 34.8 15.8 7.4 11.5 7.8 13.6 6.7
30 38.4 14.5 6.4 12.7 7.5 15.1 6.9
35 41.4 12.2 6.0 13.7 7.2 16.5 5.3
40 40.1 12.9 6.1 11.5 7.3 13.6 6.4
45 38.2 13.9 8.9 10.8 8.3 10.9 5.4
C4H8 C5-C12 C13+ CO2
3.1 9.8 12.5 11.7
4.7 11.2 10.8 11.5
7.5 12.6 9.2 10.7
8.6 12.5 8.2 7.6
9.8 13.5 7.2 6.7
10.7 13.5 6.4 6.3
12.7 14.3 6.1 6.0
11.8 12.4 7.9 10.1
10.1 11.9 13.6 6.2
Table 2. Effect of different wt. % of (Co-Fe) on the N2 adsorption-desorption measurements
(Co-Fe) wt.% 5 10 15 20
S.S.A (m2g-1) a b c 201.8
219.7
205.6 227.3 213.7 234.5 219.4 245.1
a
209.3 26 23 21 20
223.1 228.4 231.6
27
P.D (Å) b
P.V (cm3g-1) b c
c
a
24
21
0.19
0.25
0.22
28 26 31
24 27 30
0.21 0.23 0.26
0.28 0.31 0.39
0.24 0.26 0.31
25 227.9 249.7 234.2 24 33 31 30 239.2 253.5 247.6 27 33 32 35 237.6 261.4 258.9 30 34 34 40 225.9 255.3 245.3 31 37 33 45 219.1 243.1 231.8 29 37 35 S.S.A: specific surface area P.D: pore diameter P.V: pore volume a: precursor, b: catalyst (before reaction), c: catalyst (after reaction)
0.29 0.31 .036 0.34 0.30
0.43 0.47 0.49 0.45 0.42
Table 3. Effect of solution pH on the catalytic performance of 35wt.%(Co-Fe)/TiO2-SiO2 catalyst
Product selectivity (%)
pH CO conversion (%) CH4 C2H6 C2H4 C3H8 C3H6 C4H10
1.0 35.6 14.6 8.7 10.1 8.5 10.4 8.7
1.5 36.5 14.2 7.1 11.1 7.0 13.2 6.4 28
2.0 41.4 12.2 6.0 13.7 7.2 16.5 5.3
2.5 43.7 11.1 5.6 13.9 6.7 16.8 5.4
3.0 40.2 14.2 6.8 12.4 7.5 12.4 5.5
3.5 35.9 15.7 9.2 11.6 8.3 9.9 6.8
0.37 0.42 0.47 0.43 0.36
C4H8 C5-C12 C13+ CO2
11.6 11.7 9.4 6.3
12.0 11.9 8.3 8.8
12.7 14.3 6.1 6.0
11.9 15.1 7.9 5.6
9.8 10.2 11.5 9.7
10.6 12.7 14.9 9.5
Table 4. Effect of different Co/Fe molar ratio on the catalytic performance of 35wt.%(CoFe)/TiO2-SiO2 catalyst
Product selectivity (%)
Co/Fe molar ratio 90/10 80/20 70/30 CO conversion (%) 41.3 49.4 46.3 CH4 15.3 9.6 10.2 C2H6 7.6 6.1 6.4 C2H4 9.1 10.5 11.1 C3H8 7.3 6.2 8.5 C3H6 8.1 14.0 15.1 C4H10 6.7 6.7 7.2
60/40 44.1 11.9 5.3 12.2 7.4 15.5 6.1
29
50/50 43.7 11.1 5.6 13.9 6.7 16.8 5.4
40/60 39.4 12.3 7.8 10.2 7.3 14.5 6.2
30/70 20/80 10/90 38.6 39.6 43.1 15.1 18.2 19.3 6.9 6.2 5.1 11.4 11.7 11.0 5.3 5.1 5.4 13.7 14.2 12.8 6.9 6.1 6.3
C4H8 C5-C12 C13+ CO2
8.4 18.9 10.9 7.7
9.1 21.7 11.4 4.7
9.3 18.1 8.3 5.8
10.3 16.7 8.7 5.9
11.9 15.1 7.9 5.6
10.1 14.5 9.8 7.3
11.2 12.1 8.3 9.1
10.4 10.1 7.4 10.6
8.5 9.9 8.5 13.2
Table 5. Effect of calcination heating rate on the catalytic performance of 35wt.%(CoFe)/TiO2-SiO2 catalyst heating rate (°C min-1) CO conversion (%)
1 43.2
2 49.4 30
3 54.7
4 55.8
5 54.1
Product selectivity (%)
CH4 C2H6 C2H4 C3H8 C3H6 C4H10 C4H8 C5-C12 C13+ CO2
12.4 5.7 13.9 5.6 11.3 7.2 7.8 19.7 10.8 5.6
9.6 6.1 10.5 6.2 14.0 6.7 9.1 21.7 11.4 4.7
8.6 6.1 11.0 5.3 14.7 5.6 9.7 24.8 9.7 4.5
9.8 5.5 10.2 7.8 12.1 7.3 10.0 22.3 10.2 4.8
11.3 6.4 8.6 6.2 11.3 6.2 8.9 21.8 11.9 7.4
Product selectivity (%)
Table 6. Effect of calcination time on the catalytic performance of 35wt.%(Co-Fe)/TiO2SiO2 catalyst calcination time ( h) 5 6 7 8 9 10 CO conversion (%) 46.2 54.7 57.9 55.3 49.2 43.2 CH4 12.1 8.6 8.2 10.2 13.8 14.9 C2H6 7.2 6.1 5.1 5.4 8.6 6.7 C2H4 9.3 11.0 11.9 8.6 7.3 8.3 C3H8 6.4 5.3 5.0 5.4 6.4 6.1 C3H6 10.4 14.7 15.1 11.7 8.9 9.2 31
C4H10 C4H8 C5-C12 C13+ CO2
6.2 8.2 21.1 7.4 11.7
5.6 9.7 24.8 9.7 4.5
4.8 9.6 26.2 10.2 3.9
5.8 9.7 22.4 11.8 9.0
5.2 6.8 20.2 12.7 10.1
6.5 6.4 17.9 13.4 10.6
Table 7. Effect of calcination temperature on the catalytic performance of 35wt.%(CoFe)/TiO2-SiO2 catalyst calcination temperature (°C) 300 400 500 600 700 CO conversion (%) 32.1 45.2 57.9 61.4 56.1
32
Product selectivity (%)
CH4 C2H6 C2H4 C3H8 C3H6 C4H10 C4H8 C5-C12 C13+ CO2
25.8 3.8 6.2 2.9 6.4 4.1 5.2 11.8 6.3 27.5
12.5 4.2 10.7 5.9 14.5 5.9 8.1 21.6 9.3 7.3
8.2 5.1 11.9 5.0 15.1 4.8 9.6 26.2 10.2 3.9
8.0 5.0 12.3 5.1 16.2 4.4 10.1 27.4 8.1 3.4
9.5 6.4 11.1 4.8 13.5 4.7 9.3 21.8 12.6 6.3
Pr od uct
Table 8. Effect of different promoters on the catalytic performance of 35wt.%(Co-Fe)/TiO2SiO2 catalyst promoter K Cu Zn Ce CO conversion (%) 64.2 72.3 66.7 67.6 CH4 7.6 7.1 9.5 10.2 C2H6 6.8 4.6 7.9 6.3 33
C2H4 C3H8 C3H6 C4H10 C4H8 C5-C12 C13+ CO2
15.3 5.4 13.9 4.7 13.9 19.3 8.9 4.2
13.2 4.7 16.8 4.1 10.7 29.5 6.2 3.1
11.5 6.7 12.3 6.1 8.5 17.4 11.6 8.5
13.2 8.6 13.8 7.4 9.1 15.5 12.3 3.6
Product selectivity (%)
Table 9. Effect of H2/CO on the catalytic performance of 35wt.%(Co-Fe)/TiO2-SiO2 catalyst H2/CO 1/3 2/3 1/1 2/1 3/1 CO conversion (%) 64.3 67.5 72.3 70.3 69.5 CH4 8.7 7.6 7.1 8.9 9.8 C2H6 4.1 4.2 4.6 6.8 7.3 C2H4 10.2 12.1 13.2 11.4 10.4 C3H8 3.6 4.8 4.7 5.3 5.9 C3H6 12.3 14.5 16.8 13.7 11.4
34
C4H10 C4H8 C5-C12 C13+ CO2
4.0 7.4 24.3 19.8 5.6
5.2 8.3 27.2 11.8 4.3
4.1 10.7 29.5 6.2 3.1
7.3 10.1 21.4 8.3 6.8
7.7 9.5 18.5 10.9 8.6
Produ ct select ivity (%)
Table 10. Effect of GHSV on the catalytic performance of 35wt.%(Co-Fe)/TiO2-SiO2 catalyst GHSV (h-1) 1000 1500 2000 2500 3000 CO conversion (%) 67.1 72.3 78.4 78.0 74.1 CH4 8.3 7.1 6.5 7.9 8.5 C2H6 3.5 4.6 5.4 6.7 7.8 C2H4 11.2 13.2 15.3 12.4 11.3
35
C3H8 C3H6 C4H10 C4H8 C5-C12 C13+ CO2
5.9 14.3 6.4 8.1 21.4 15.5 5.4
4.7 16.8 4.1 10.7 29.5 6.2 3.1
4.1 17.4 4.3 11.7 26.9 5.4 3.0
6.7 15.9 7.6 10.1 23.7 5.4 3.6
6.9 14.1 8.3 10.0 21.4 6.8 4.9
Product selectivit y (%)
Table 11. Effect of reaction temperature on the catalytic performance of 35wt.%(CoFe)/TiO2-SiO2 catalyst temperature (°C) 220 230 240 250 260 270 280 CO conversion (%) 42.7 61.6 78.4 81.4 82.4 84.5 86.2 CH4 5.4 5.9 6.5 6.5 7.8 10.7 17.6 C2H6 9.8 6.4 5.4 4.1 4.0 6.8 7.5 C2H4 12.7 13.6 15.3 16.4 13.6 10.3 9.4 C3H8 8.9 5.8 4.1 4.0 6.4 7.6 9.0
36
C3H6 C4H10 C4H8 C5-C12 C13+ CO2
13.2 6.3 9.2 20.1 10.9 3.5
15.7 5.2 10.1 25.8 8.3 3.2
17.4 4.3 11.7 26.9 5.4 3.0
18.4 4.2 12.4 26.8 4.2 3.0
14.7 6.3 13.5 25.1 4.0 4.6
12.1 8.9 11.4 21.2 3.4 7.6
11.4 9.3 10.2 14.1 3.0 8.5
Pr od uct sel
Table 12. Effect of reaction total pressure on the catalytic performance of 35wt.%(CoFe)/TiO2-SiO2 catalyst pressure (bar) 1 5 9 13 17 21 CO conversion (%) 81.4 82.6 83.6 84.2 81.7 78.3 CH4 6.5 4.5 4.9 4.9 4.8 4.7 C2H6 4.1 3.1 3.1 3.0 3.0 3.1
37
C2H4 C3H8 C3H6 C4H10 C4H8 C5-C12 C13+ CO2
16.4 4.0 18.4 4.2 12.4 26.8 4.2 3.0
17.1 4.1 19.1 4.4 12.5 27.1 5.5 2.6
38
14.3 5.2 14.1 6.9 11.6 29.5 7.8 2.6
12.2 6.2 11.1 7.3 12.1 31.1 9.3 2.8
11.1 7.0 8.2 7.9 12.1 33.0 10.1 2.8
7.2 7.1 7.1 8.0 12.1 35.1 12.7 2.9
S0025-5408(15)30060-X http://dx.doi.org/doi:10.1016/j.materresbull.2015.07.039 MRB 8347
To appear in:
MRB
Received date: Revised date: Accepted date:
21-1-2015 5-7-2015 24-7-2015
Please cite this article as: Mostafa Feyzi, Nakisa Yaghobi, Vahid Eslamimanesh, Cobalt-iron nano catalysts supported on TiO2-SiO2: characterization and catalytic performance in Fischer-Tropsch synthesis, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2015.07.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Cobalt-iron nano catalysts supported on TiO2-SiO2: characterization and catalytic performance in Fischer-Tropsch synthesis Mostafa Feyzi1 , Nakisa Yaghobi2, Vahid Eslamimanesh2 1
Faculty of Chemistry, Razi University, P. O. Box: +98-67149, Kermanshah, Iran 2
Iran Polymer and Petrochemical Institute, P. O. Box: +98- 14965 Tehran, Iran,
Graphical Abstract
The Co-Fe/TiO2-SiO2 catalysts were prepared. The prepared catalysts were tested for light olefins and and C5-C12 production. The best operational conditions are 250 °C, H2/CO= 1/1 under 5 bar pressure.
Highlights ► The TiO2-SiO2 supported cobalt-iron catalysts were prepared via sol-gel method. ►.The best operational conditions were 250 °C, GHSV=2000 h-1, H2/CO = 1/1 and 5 bar. ► The (Co/Fe)/TiO2-SiO2 is efficient catalyst for light olefins and C5-C12 production.
Abstract
Corresponding Authors Email: [email protected] (M. Feyzi) Tel/Fax:+988334274559,
1
A series of Co-Fe catalysts supported on TiO2-SiO2 were prepared by the sol-gel method. This research investigated the effects of (Co/Fe) wt.%, the solution pH, different Co/Fe molar ratio, calcination conditions and different promoters on the catalytic performance of cobalt– iron catalysts for the Fisher–Tropsch synthesis (FTS). It was found that the catalyst containing 35wt.%(Co-Fe)/TiO2-SiO2 (Co/Fe molar ratio is 80/20) promoted with 1.5 wt.% Cu and calcined in air atmosphere at 600 ºC for 7 h with a heating rate of 3 ºC min-1 is an optimal nano catalyst for converting synthesis gas to light olefins and C5-C12 hydrocarbons. The effects of operational conditions such as the H2/CO ratio, gas hourly space velocity (GHSV), different reaction temperature, and reaction pressure were investigated. The results showed that the best operational conditions for optimal nano catalyst are 250 °C, GHSV=2000 h-1, H2/CO molar ratio 1/1 under 5 bar total pressure. Catalysts and precursors were characterized by, X-ray diffraction (XRD), scaning electron microcopy (SEM), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), temperature program reduction (TPR)
and N2
adsorption-desorption measurements.
Keywords: A. surfaces, A. carbides phase, B. sol-gel, C. X-ray diffraction, C. scanning tunneling microscopy
1. Introduction The Fischer-Tropsch synthesis (FTS) entails the conversion of synthesis gas (a mixture of carbon monoxide and hydrogen) to a spectrum of products mainly comprising olefins and paraffins. An approach to improve the selectivity in this process involves the use of a bimetallic catalyst system containing metals catalyst combined with a support [1]. There has been renewed interest in recent years in FTS, especially for the selective production of petrochemical feed stocks such as ethylene, propylene and butylene (C2–C4 olefins) directly from synthesis gas [2,3]. The high temperature (> 350 °C) Fischer-Tropsch process (HTFT) is carried out at slurry
2
reactor and produces light olefins. On the other hand, the low temperature (< 250 °C) FischerTropsch process (LTFT) could be carried out at both slurry and fixed-bed reactors and is used for the production of mainly paraffinic heavy hydrocarbons. Due to the thermodynamic and kinetic limitations of the reaction, few catalysts are able to amplify the heavier hydrocarbons fraction. Among them, Ru-based catalysts show higher activity but are very expensive [6-9]. Nickel-based catalysts could be employed but their efficiencies in hydrogenation process which compete with FTS and produce methane as an undesirable by-product [10,11]. Both Cobalt and iron based catalysts individually have advantages and disadvantages properties [12-19]. Therefore effort to couple them in the hope to bring forth a more efficient catalyst having parents’ advantages is of great importance. These bimetal catalysts should be inserted into a support of high porosity to increase the surface area and mechanical stability of the catalysts as well as prohibition of catalyst sintering. In addition to porosity, pore shape and size are critical secondary factors. The best supports are those that are simply manipulate to produce optimum texture properties. Silica [20,21] and γ-alumina [22,23] are good in this regard particularly in sol-gel preparation method [24]. Cobalt and iron are the most important and commercial transition metal catalyst used either as individual monometallic or composed bimetallic systems [25–27]. The bimetallic Co–Fe catalysts are generally highly active mainly due to presence of cobalt. On the other hand, iron is more suitable for poor H2/CO ratio feedstock which makes the performance of the water-gas shift reaction inevitable [28,29]. The purposes of this study is to investigate the influence of catalyst composition and operational conditions on FTS activity and selectivity. Characterization of both precursors and calcined catalysts were carried out by XRD, SEM, TPR, N2 physisorption, TGA and DSC methods.
2. Experimental
3
2.1. Typical procedure for preparation of catalysts A series of cobalt-iron catalysts supported on TiO2-SiO2 with different loadings of cobalt-iron has been prepared. The appropriate amount of start materials cobalt nitrate (Co(NO3)2.6H2O) and iron nitrate (Fe(NO3)3.9H2O) were dissolved in ethanol at 50 °C, separately, mixed well with each other (A). The required amounts of tetraethyl orthosilicate (TEOS) as silica source and tetra butoxy titane (Ti(OC4H9)4) as TiO2 source were dissolved in ethanol at 50 °C and then gradually added to the mixed cobalt-iron solution (A) to produce 5, 10, 15, 20, 25, 30, 35, 40 and 45 wt.% of Co/Fe=1/1 (based on TiO2-SiO2 wt.%) respectively. Then, an ethanol solution of oxalic acid (C2H2O4.2H2O, appropriate amount for hydrolysis of TEOS and Ti(OC4H9)4) was added to a mixed solution under constant stirring to obtain a gel form. The gel was dried in an oven (120 ºC, 20 h) to give a material denoted as the catalyst precursor. The promoted catalysts were then prepared by the incipient wetness impregnation method by adding 1.5 wt.% of each promotor to Co–Fe/TiO2-SiO2 precursor. Finally, the obtained samples was dried at 120 °C for 12 h and calcined at desired calcination conditions.
2.2. Catalyst characterization 2.2.1. X-Ray dif fraction (XRD) The XRD patterns of all 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 (λ=1.542 Å) and a nickel filter in the 2θ range of 4°-70°
2.2.2. N2-adsorption-desorption measurements The specific surface area (using BET and BJH methods), the total pore volume and the mean pore diameter were measured using a N2 adsorption-desorption isotherm at liquid nitrogen
4
temperature (-196 ºC), using a NOVA 2200 instrument (Quantachrome, USA). Prior to the adsorption-desorption measurements, all the samples were degassed at 110 ºC in a N2 flow for 3 h to remove the moisture and other adsorbates
2.2.3. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) The TGA and DSC were carried out using simultaneous thermal analyzer (Perkin Elmer) under a flow of dry air with a flow rate of 50 ml min-1. The temperature was raised from 20 to 600 ˚C using a linear programmer at a heating rate of 3 ˚C min-1
2.2.4. Scanning electron microscopy (SEM) The morphologies of prepared nanocatalysts and their precursors were observed by means of an S–360 Oxford Eng scanning electron microscope (USA)
2.2.5. Temperature-programmed reduction (TPR) H2-TPR profiles of the unpromoted and promoted nanocatalysts were recorded using a micromeritic TPD–TPR 290 system. The TPR for each sample (50 mg) was performed using a mixture gas of 5%H2/95%Ar (v/v) as the reductant. The sample was heated from 25 to 950 °C at a heating rate of 5 °C min. The flow rate of the mixture gas was 50 ml/min
2.3. Fischer-Tropsch synthesis Catalytic activity test were carried out in a fixed bed stainless steel reactor at different operation conditions (Figure 1). All catalysts were activated (reduced) for 12 h period on line in pure hydrogen (1 bar) at a temperature of 400 ºC at flow rate of 10 ml min-1. Meshed catalyst (0.5 g) diluted with similar granulometry of quartz beads (0.3 g), was held in the middle of the reactor (10 cm length and internal diameter is 6 mm). Reactant and stream products were
5
analyzed on line using a Varian gas chromatograph (Star 3600CX) equipped with a thermal conductivity detector (TCD) and a chromosorb column. The heavy hydrocarbon products were off line analyzed using a Varian CP 3800 with a Petrocol Tm DH100 fused silica capillary column and a flame ionization detector (FID). The conversion percentage of CO based on the fraction of CO forming carbon containing products according to below equation:
CO conversion (%)
ni M i 100 M CO
where ni is the number of carbon atoms in product i, Mi is the percentage of product i and MCO is the percentage of CO in the syngas feed. The selectivity (S) of product i, is based on the total number of carbon atoms in the product and therefore is defined as:
S i (%)
ni M i 100 niMi
3. Results and Discussion 3.1.1. Effect of cobalt-iron weight percent The effect of different wt.% of Co/Fe=1/1 (based on the TiO2-SiO2 weight) on the catalytic performance under the same reaction conditions (H2/CO=1/1, gas hourly space velocity (GHSV) =1500 h−1, P=1 bar and T=240˚C) was investigated. As Table 1 shows, the catalyst activity based on CO conversion increases linearly with increasing of Co-Fe wt.% while the selectivity toward undesirable methane and CO2 decreases. After the weight percent of Co-Fe was reached to above 35% methane and CO2 selectivity increases markedly. According to the obtained results, the catalyst containing 35wt.% of Co-Fe presented the best catalytic performance compared to the other tested catalysts. This catalyst showed the highest selectivity
6
towards olefinic products (C2-C4) and C5-C12 hydrocarbons and also the lowest selectivity with respect to methane and CO2. Thus, this catalyst was chosen as the optimal catalyst. In FTS, the amount of active metals is affect on catalyst selectivity and activity [30,31]. The specific surface area (BET method) pore volume (BJH method) and pore diameter (BJH method) of the precursors and calcined catalysts (before and after reaction) are given in in Table 2. A relatively high surface area of the precursor is mainly due to solvent evaporation which makes a porous surface of high surface area. The surface area of the calcined catalysts is higher than the corresponding precursors which is mainly due to exit of gaseous caused by phase changing during calcination creating pore on catalyst surface. It can be seen that the increase of (Co-Fe) wt.% results in an increase in the catalyst surface area linearly which is in conformity with catalyst activity based on CO conversion and more than 35wt% were decreased. On the other hand, the pore volume increases with increasing of cobalt-iron ratio to reach a maximum at 35 wt.% (Co-Fe)/ TiO2-SiO2, and then decreases with further increasing of cobalt-iron weight percent which is in agreement with product selectivity trend. The N2 absorption–desorption data are shown that the catalyst was prepared from 35 wt.%(Co-Fe)/TiO2-SiO2, has higher specific surface area, pore volume and pore diameter than the other calcined catalysts and it was the one acceptable reason for better catalytic performance of this catalyst. The 35wt.%(Co-Fe)/TiO2-SiO2 catalyst in different state were characterized by XRD (Figure. 2). The precursor prepared from 35wt.%(Co-Fe)/TiO2-SiO2 was largely found to be amorphous by XRD. The presence of amorphous phase in the XRD pattern of this precursor makes the other phases undetectable, and this is because of iron hydroxid phases. The actual phases were identified for calcined catalyst (calcinations conditions: during 6 h, T=500 ºC at heating rate 2ºC min-1) containing 35wt.% (Co-Fe)/ TiO2-SiO2 under the specified preparation conditions as CoTiO3 (cubic), Fe2O3 (cubic), CoFe2O4 (cubic), Co2SiO4 ( orthorhombic), Fe2SiO4 (cubic) phases.
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In order to identify the phase changes in the 35wt.%(Co-Fe)/TiO2-SiO2 catalyst during the reaction conditions (P=1 bar, H2/CO=1/1, T=240 °C and GHSV=1500 h−1), this catalyst after test was characterized by XRD (Figure 2). The actual phases identified in the XRD pattern was found to be metallic iron and cobalt in form of cubic structure and CoO (cubic), Fe3O4 (cubic), Fe2SiO4 (cubic), and Fe3C (orthorombic) phases. As shown, the tested catalyst has oxidic and iron carbide phases, both of which have been generally accepted to be the most likely active phase for FTS performance [32-36]. One of the affecting parameters on catalytic performance is catalyst particle size. This parameter could be calculated by Scherer-equation [30,31]. The crystallite size of the 35 wt.%(Co-Fe)/TiO2-SiO2 catalyst calculated using Scherer-equation is about 75 nm. It was found that cobalt particle size had a strong impact on cobalt catalyst selectivity; the olefin selectivity correlate with the particle size of catalyst [39-42]. As shown in Figure 3, the TGA/DSC curves of 35 wt.% (Co-Fe)/ TiO2-SiO2 catalyst precursor exhibit a three-step weight loss. The first step at the temperature below 90 °C was attributed to the evaporation of residual moistures in the support and loss of physisorbed waters. The second step at the temperature between 115 and 290 °C was the decomposition of oxalate phases and to remove hydrate and bound waters. The third weight loss occurs above 320 °C corresponds to the full decomposition of all oxalates and the formation of stable TiO2-SiO2, cobalt and iron oxides. DSC measurement was performed in order to provide further evidence for the presence of the various species and evaluates their thermal behavior, as it shown in Figure. 3, the exothermic peak at lower temperature represents the removal of the physically adsorbed material such as physisorbed waters (80-130 oC). The exothermic peak at around 170-290 oC is due to the decomposition of oxalate phases and to remove hydrate and bound waters. The third stage around 300-410 °C is due to the full decomposition of oxalates phases to oxides. SEM images of both precursor and calcined 35 wt.%(Co-Fe)/TiO2-SiO2 catalyst is displayed in figure 4 with irregular shapes. SEM observations have shown differences in morphology of precursor
8
and calcined catalysts. The image obtained from catalyst precursor depicts several larger agglomerations of particles (Figure. 4a) and show that this material has a less dense and homogeneous morphology. After the calcination at 500 ºC, 6 h and heating rate of 2 ˚C min-1, the morphological features are different with the precursor sample and shows that the agglomerate size is greatly reduced compared to the precursor (Figure. 4b). It is obvious in this Fig, the crystal sizes were from 60–70 nm. This result confirmed the obtained results studied by using the Scherrer equation.
3.1.2. Effect of pH A series of 35wt.%(Co-Fe)/TiO2-SiO2 catalysts were prepared by sol-gel method with a range of solution pH from 1.0 to 3.5 and tested for Fischer-Tropsch synthesis under the same reaction conditions (H2/CO=1/1, GHSV=1500 h-1, P=1 bar at 240 ºC). The CO conversion and products selectivity percent are shown in Table 3. According to the obtained results, the catalyst containing 35wt.%(Co-Fe)/TiO2-SiO2 was prepared in pH=2.5 has shown the best catalytic performance than the other tested catalysts. This catalyst showed the highest selectivity towards light olefins, C5-C12 hydrocarbons and lowest selectivity with respect to methane and CO2. So, the catalyst prepared in pH=2.5 was chosen as the optimal catalyst for the conversion of synthesis gas to light olefins and C5-C12 hydrocarbons.
3.1.3. Effect of cobalt/iron weight ratio The influence of the cobalt/iron molar ratio on the catalytic performance of cobalt/iron nano catalyst containing 35wt.%(Co-Fe)/TiO2-SiO2 under atmospheric pressure, H2/CO=1/1 and GHSV=1500 h−1 and T=240°C were investigated (Table 4). The results showed that variation of cobalt/iron ratio (90/10-20/80) resulted in different products selectivity. However, for the catalyst containing 35wt.%(Co-Fe)/TiO2-SiO2 with cobalt/iron=80/20, the catalyst
9
activity (based on CO conversion), light olefins and C5-C12 hydrocarbons selectivity were higher than the other prepared catalysts. The results are shown that, the selectivity toward undesired products such as methane and CO2 is also lower than that of the other prepared catalysts. Therefore, cobalt/iron=80/20 molar ratio was chosen as the optimum molar ratio for catalyst preparation.
3.1.4. Effect of calcination atmospheres In this part of the study, two samples of precursor containing 35wt.%(Co-Fe)/TiO2-SiO2 with cobalt/iron=80/20 were calcined separately in air and nitrogen atmosphere (500 ºC, 6h at heating rate 2 ºC min-1). The CO conversion and product selectivity percent for these catalysts are shown in Figure. 5 (T=240 ºC, H2/CO=1/1, P=1 atm and GHSV=1500 h-1). The comparison of the results in this Fig, indicated that the catalyst was calcined in air at 500 ºC for 6 h and at heating rate 2 ºC min-1 have the highest CO conversion and highest selectivity to light olefins and C5-C12 hydrocarbons. By taking these results into consideration, the air was a better calcination atmosphere for the 35wt.%(Co-Fe)/TiO2-SiO2 for Fisher-Tropsch synthesis.
3.1.5. Effect of calcination heating rate To consider the effect of calcination heating rate on the catalytic performance, a series of precursors containing 35wt.%(Co-Fe)/TiO2-SiO2 with cobalt/iron=80/20 were calcined (500 ºC for 6 h) at various heating rates in air and tested for FTS. The heating rate varied between 1-5 ºC min-1. The CO conversion and light olefin products selectivity percent are shown in Table 5 (T=240 ºC, H2/CO=1/1, P=1 bar and GHSV=1500 h-1). It can be seen that heating rates up to about 3 ºC min-1 did not exert a major effect on the catalytic performance of the catalyst, while the heating rates in excess of 3 ºC min-1 resulted in a significant decrease in the CO conversion and products selectivity. Therefore, in this study, heating rate 3 ºC min-1 is the optimum heating
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rate for calcination of the 35wt.%(Co-Fe)/TiO2-SiO2 in air atmosphere for light olefins and C5C12 hydrocarbons production.
3.1.6. Effect of calcination time and temperature The influence of a range of calcination time and temperature were examined. At first, 5 samples of precursor containing 35wt.%(Co-Fe)/TiO2-SiO2 were calcined separately in air at 500 ºC with heating rate of 3 ºC min-1 for 5, 7, 8, 9 and 10 h respectively. The CO conversion and products selectivity percent for these catalysts with different calcination time in reaction conditions (P=1 atm, T= 240 ºC, H2/CO = 1/1 and GHSV=1500 h-1) are shown in Table 6. As being seen in this table, the optimum calcinations time is 7 h at 500 ºC for light olefins and C5C12 hydrocarbons production. In order to study the effect of calcination temperature, 6 samples of precursor containing 35wt.%(Co-Fe)/TiO2-SiO2 calcined separately in 7 h (air atmosphere and heating rate of 3 ºC min-1) at 300, 400, 600 and 700 ºC respectively. These calcined samples were tested at the same reaction conditions (P=1 atm, T=240 ºC, H2/CO=1/1 and GHSV=1500 h-1). A comparison of the results in Table 7, for the catalysts prepared at different calcination temperatures indicated that the catalyst was calcined at 600 ºC had the highest CO conversion and highest selectivity with respect to C2-C4 light olefins and C5-C12 hydrocarbons. Therefore, in this study, the best calcination conditions were found to be air atmosphere at 600 ºC for 7 h with a heating rate of 3 ºC min-1.
3.1.7. Effect of promoters Alkali metals [43,44], Zn, Cu, Mn and Ce have been used widely as promoters to improve the activity and selectivity of the catalysts in the CO hydrogenation. Yang et al. [45] studied the conversion of CO and H2 over Fe catalysts promoted by Zn, Cu, Cr, and alkali metals. Impregnated Fe catalysts were less active than co-precipitated catalysts. Zinc was a better
11
promoter than Cu or Cr [45,46]. To study the effect of promoters on the catalytic performance of iron–manganese catalyst, a same amount (1.5 wt.%) of Zn, Cu, K and Ce from Zn(NO3).6H2O, Cu(NO3)2.6H2O, KNO3 and Ce(NO3)2.6H2O were separately introduced to the 35wt.%(Co-Fe)/TiO2-SiO2 catalyst. This method confirm with the other reports, which studied by many investigators [46–48]. Then, all of these different promoted catalysts were tested at the same reaction conditions (H2/CO=1/1, GHSV=1500 h-1, P=1 atm at 240 °C) for the conversion of synthesis gas to C2-C4 light olefins and C5-C12 hydrocarbons, the results are shown in Table 8. Comparing the obtained results of Table 8, leads to the conclusion that 35wt.%(Co-Fe)/TiO2-SiO2 catalyst that promoted with 1.5 wt.% of Cu is more active than the other promoted catalysts. Taking these results into consideration, the catalyst that was promoted with Cu, appears to be the optimum modified catalyst for the conversion of synthesis gas to light olefins and C5-C12 hydrocarbon products, this catalyst gave the best catalytic performance. In order to investigate the reduction behavior of the un promoted and Cu-promoted catalysts, characterization of these catalysts was carried out using H2-TPR technique. The H2-TPR was performed and the obtained profiles are illustrated in Fig. 6. All of these calcined catalysts revealed three distinct reduction peaks. In all of these catalysts the first and second peaks appearing in Fig. 6 corresponds to the reduction of Co3O4 to Co° according to the following steps: Co3O4+ H2→3CoO+H2O 3CoO +3H2→3Co°+3H2O These reduction steps can be attributed to the reduction of Co3+ and Co2+ oxide species to metallic cobalt. The third and final peak reduction can be assigned to Fe2O3→ Fe3O4 and Fe3O4→ Fe for un promoted and Cu promoted catalyst [49]. The H2 consumed during different reduction stages were determined by integrating the area of TPR curves. The results are shown that, the H2 adsorption increased from 8.2 to 9.8 ×10−2 (mmol H2/mmol catalyst), for the un
12
promoted catalyst and the promoted catalyst that was promoted with Cu respectively. It is concluded that the amount of catalyst reduction is increased from 70.1% to 83.7 mol% by adding the Cu. The fact that iron-cobalt catalyst that was promoted with 1.5 wt.% Cu reducibility is greater than un promoted iron-cobalt catalyst, suggests that the addition of Cupromotes the reduction of iron- cobalt at lower temperatures.
3.2. Effects of operational conditions Operational conditions have noticeable effects on the catalytic performance of the catalyst. To optimize the reaction conditions, the effects of operational conditions such as H2/CO feed molar ratio, GHSV, reaction temperature and reaction total pressure to were examined to evaluate performance of catalyst.
3.2.1. Effect of H2/CO on the catalytic performance The influence of the H2/CO (synthesis gas ratio) on the catalytic performance of 35wt.%(CoFe)/TiO2-SiO2 catalyst promoted with 1.5 wt.% of Cu for the FTS was investigated and catalyst performance, present were shown in Table 9, with the change of H2/CO from 1/3 to 3/1. However, at H2/CO ratio of 1/1, T=240 °C, GHSV=1500 h-1 and p=1 bar, the total selectivity of light olefins, C5-C12 hydrocarbons was higher and CO2 was also lower than other H2/CO ratios. Therefore, H2/CO=1/1 at 240 °C was chosen as the optimum molar feed ratio for converting synthesis gas to C2-C4 light olefins. On one hand, higher H2/CO shortens the residence time, which is unfavorable for chain growth. In this reaction, the effect of H2/CO ratio is remarkable, and in turn the lower H2/CO ratio in the reactor results in a higher C13+ selectivity. From the results, it can also draw the conclusions that the higher H2/CO ratio is preferential for chain termination to produce light hydrocarbons while lower H2/CO ratio is
13
preferential for the chain growth and the production of heavy hydrocarbons [50-51]. Therefore, the C13+ selectivity decreased with increasing H2/CO ratio.
3.2.2. Effect of GHSV In order to gain a deep understanding of the factors affecting the catalytic performance of 35wt.%(Co-Fe)/TiO2-SiO2 catalyst promoted with 1.5 wt.% of Cu, a series of experiments were carried out at different GHSV from 1000 to 3000 h−1 under the optimal reaction conditions (H2/CO= 1/1, P= 1 bar, and T= 240◦C). The results were revealed in Table 10. Comparing the obtained results were led to the conclusion that at GHSV=2000 h−1, the selectivity with consideration of C2–C4 light olefins and C5-C12 hydrocarbons were the highest. Therefore, GHSV=2000 h−1 was emphasized to be the optimum one, since at this GHSV, a high CO conversion, high total selectivity of light olefin and C5-C12 hydrocarbon products and also low CH4 selectivity were obtained. These results proved that GHSV is a parameter of crucial importance on the catalytic performance of 35wt.%(Co-Fe)/TiO2-SiO2 nano catalysts promoted with 1.5 wt.% of Cu for CO hydrogenation. The results revealed that the methane selectivity was decreased with increasing the space velocity (2500-3000 h-1).
3.2.3. Effect of reaction temperature To get a full perception of the effect of reaction temperature on the catalytic performance of 35wt.%(Co-Fe)/TiO2-SiO2 nano catalysts promoted with 1.5 wt.% of Cu, temperature range between 220°C-280°C was scrutinized (H2/CO = 1/1, P = 1 bar, and GHSV = 2000 h-1). The effect of reaction temperature on 35wt.%(Co-Fe)/TiO2-SiO2 catalyst were examined in Table 11. It can be realized that the nano catalyst had high activity at 250 °C. In higher temperature CO conversion increased but selectivity to undesirable products such as methane and CO2 were increased too [52]. The CO conversion increased linear with increasing temperature, as shown
14
in Table 11, Similar results were also obtained by Schulz and Claeys [53,54]. Both the CO conversion and CO2 formation followed the same trends as the FTS reaction, and almost presented a linear correlation with increasing reaction temperature. Bukur et al. [55] investigated a precipitated iron catalyst in a fixed bed reactor under a variety of process conditions, and have also observed that light olefins selectivity was high at the reaction temperature of around 280 ºC during FTS. Therefore, in this study, 250°C was considered to be the optimum temperature because of the high CO conversion (81.4%), total selectivity to light olefins (47.2%), low CH4 (6.5%) and low CO2 (3.0%). At low reaction temperature (220 ºC), the conversion percentage of CO was low and caused low catalytic activity.
3.2.3. Effects of reaction pressure In commercial process, the FTS reaction usually operates under high pressure. The 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 [56] and it can influences on the catalytic performance. 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 to investigate the performance of 35wt.%(Co-Fe)/TiO2-SiO2 nano catalysts promoted with 1.5 wt.% of Cu during the variation of total pressure in the range of 1-21 bar, at the optimal reaction conditions of H2/CO=1/1 (GHSV=2000) and 250 ºC (Table 12). As shown in Table 12, increasing the reaction pressure after 5 bar has significant influence on the hydrocarbon selectivity. The selectivity to C5-C12 increased slightly, while the selectivity to ethylene, butylenes and propylene decreased. The results indicated that at the total pressure of 5 bar, the 35wt.%(CoFe)/TiO2-SiO2 nano catalysts promoted with 1.5 wt.% of Cu showed high selectivity to C2-C4
15
light olefins and C5-C12 hydrocarbons. It was also apparent that increasing in total pressure in the ranges of 5-21 bar increases the heavy hydrocarbon (C5+) selectivity and led to an increase to 47.8% at the pressure of 21 bar. Because of high CO conversion, low CH4 and CO2 selectivity and also higher total selectivity with respect to C2-C4 light olefins C2-C4 light olefins and C5C12 hydrocarbons at the total pressure of 5 bar although in high pressure selectivity to C5-C12 and C13+ were increased.
4. Conclusions The results proved that many variable factors such as cobalt-iron weight percent, pH, cobalt/iron weight ratio, calcination atmospheres, calcination heating rate, calcination time and temperature, different promoters and operational conditions had influence on the catalytic performance of Co-Fe/TiO2-SiO2 nano-catalysts for the production of C2-C4 light olefins and C5-C12 hydrocarbons. The catalytic performance of 35wt.%(Co-Fe)/TiO2-SiO2 nano catalysts promoted with 1.5 wt.% of Cu as an optimal nano catalyst has been studied under different operational conditions including, H2/CO molar ratio, different GHSV, reaction temperatures and total of reaction pressure. The optimal operational conditions were found to be 250 ºC with molar feed ratio of H2/CO=1/1 (GHSV=2000 h-1) under the 5 bar pressure. It was found that, the catalyst reduction increased from 70.1% to 83.7 mol% by adding the Cu as a promoter in the catalyst composition. Characterization of Co-Fe by powder X-ray diffraction and scanning electron microscopy showed that the catalyst precursor are sensitive to the preparation conditions, so that (Co-Fe) wt.% influencing the structure and morphology of the precursors and calcined catalysts and that these parameters should be incorporated into the design of catalyst. Characterization of catalyst precursor using TGA, generally have shown three stage of decomposition. The first weight loss peaks at low temperatures attributed to the evaporation of residual moistures in the support and loss of physisorbed waters, the second weight loss peaks
16
at mid temperatures are due to the decomposition of oxalate phases and to remove hydrate and bound waters and the third weight loss peaks at high temperatures attributed to full decomposition of all oxalates and the formation of stable TiO2-SiO2, cobalt and iron oxides. From DSC, the various exothermic transitions, which were, accompanied by different weight losses with changes only in rate of weight loss, were interpreted on the basis of X-ray diffraction and TGA results. The BET data show that there is general increase in specific surface area as a result of increased (Co-Fe) wt.%. This was attributed to the presence of iron oxide in a supersaturated solid within the cobalt oxide. The BET results for the catalyst tested also show that the specific surface area of the calcined catalysts after testing was decreased. Acknowledgements We gratefully acknowledge the Iran National Science Foundation (INSF) for their help and support of this research.
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20
Figure 1. Schematic representation for the catalyst test system and used reactor
21
Figure 2. XRD patterns of 35wt.%(Co-Fe)/TiO2-SiO2 catalyst in different estate
22
Figure 3. TGA/DSC curves for the 35 wt.% (Co/Fe)/SiO2 catalyst precursor
23
(a)
(b)
Figure 4. The SEM images of catalyst containing 35 wt.%(Co-Fe)/TiO2-SiO2 , (a) precursor and (b) calcined catalyst
24
Figure 5. Effect of different calcination atmosphere on the catalytic performance
25
Figure 6. H2-TPR profiles of un promoted and Cu-promoted 35 wt.% (Co/Fe)/SiO2 catalysts
Table 1. Effect of different wt. % of (Co-Fe) on the catalytic performance of Co–Fe/TiO2SiO2 catalyst
Product selectivity (%)
Wt.%Co- Fe CO conversion (%) CH4 C2H6 C2H4 C3H8 C3H6 C4H10
5 19.3 32.4 9.6 8.3 4.5 6.7 1.4
10 23.2 26.5 8.1 7.6 4.3 5.9 2.4
15 26.1 25.3 7.8 9.0 5.7 8.3 3.9
26
20 31.4 19.3 8.4 10.2 6.9 12.4 5.9
25 34.8 15.8 7.4 11.5 7.8 13.6 6.7
30 38.4 14.5 6.4 12.7 7.5 15.1 6.9
35 41.4 12.2 6.0 13.7 7.2 16.5 5.3
40 40.1 12.9 6.1 11.5 7.3 13.6 6.4
45 38.2 13.9 8.9 10.8 8.3 10.9 5.4
C4H8 C5-C12 C13+ CO2
3.1 9.8 12.5 11.7
4.7 11.2 10.8 11.5
7.5 12.6 9.2 10.7
8.6 12.5 8.2 7.6
9.8 13.5 7.2 6.7
10.7 13.5 6.4 6.3
12.7 14.3 6.1 6.0
11.8 12.4 7.9 10.1
10.1 11.9 13.6 6.2
Table 2. Effect of different wt. % of (Co-Fe) on the N2 adsorption-desorption measurements
(Co-Fe) wt.% 5 10 15 20
S.S.A (m2g-1) a b c 201.8
219.7
205.6 227.3 213.7 234.5 219.4 245.1
a
209.3 26 23 21 20
223.1 228.4 231.6
27
P.D (Å) b
P.V (cm3g-1) b c
c
a
24
21
0.19
0.25
0.22
28 26 31
24 27 30
0.21 0.23 0.26
0.28 0.31 0.39
0.24 0.26 0.31
25 227.9 249.7 234.2 24 33 31 30 239.2 253.5 247.6 27 33 32 35 237.6 261.4 258.9 30 34 34 40 225.9 255.3 245.3 31 37 33 45 219.1 243.1 231.8 29 37 35 S.S.A: specific surface area P.D: pore diameter P.V: pore volume a: precursor, b: catalyst (before reaction), c: catalyst (after reaction)
0.29 0.31 .036 0.34 0.30
0.43 0.47 0.49 0.45 0.42
Table 3. Effect of solution pH on the catalytic performance of 35wt.%(Co-Fe)/TiO2-SiO2 catalyst
Product selectivity (%)
pH CO conversion (%) CH4 C2H6 C2H4 C3H8 C3H6 C4H10
1.0 35.6 14.6 8.7 10.1 8.5 10.4 8.7
1.5 36.5 14.2 7.1 11.1 7.0 13.2 6.4 28
2.0 41.4 12.2 6.0 13.7 7.2 16.5 5.3
2.5 43.7 11.1 5.6 13.9 6.7 16.8 5.4
3.0 40.2 14.2 6.8 12.4 7.5 12.4 5.5
3.5 35.9 15.7 9.2 11.6 8.3 9.9 6.8
0.37 0.42 0.47 0.43 0.36
C4H8 C5-C12 C13+ CO2
11.6 11.7 9.4 6.3
12.0 11.9 8.3 8.8
12.7 14.3 6.1 6.0
11.9 15.1 7.9 5.6
9.8 10.2 11.5 9.7
10.6 12.7 14.9 9.5
Table 4. Effect of different Co/Fe molar ratio on the catalytic performance of 35wt.%(CoFe)/TiO2-SiO2 catalyst
Product selectivity (%)
Co/Fe molar ratio 90/10 80/20 70/30 CO conversion (%) 41.3 49.4 46.3 CH4 15.3 9.6 10.2 C2H6 7.6 6.1 6.4 C2H4 9.1 10.5 11.1 C3H8 7.3 6.2 8.5 C3H6 8.1 14.0 15.1 C4H10 6.7 6.7 7.2
60/40 44.1 11.9 5.3 12.2 7.4 15.5 6.1
29
50/50 43.7 11.1 5.6 13.9 6.7 16.8 5.4
40/60 39.4 12.3 7.8 10.2 7.3 14.5 6.2
30/70 20/80 10/90 38.6 39.6 43.1 15.1 18.2 19.3 6.9 6.2 5.1 11.4 11.7 11.0 5.3 5.1 5.4 13.7 14.2 12.8 6.9 6.1 6.3
C4H8 C5-C12 C13+ CO2
8.4 18.9 10.9 7.7
9.1 21.7 11.4 4.7
9.3 18.1 8.3 5.8
10.3 16.7 8.7 5.9
11.9 15.1 7.9 5.6
10.1 14.5 9.8 7.3
11.2 12.1 8.3 9.1
10.4 10.1 7.4 10.6
8.5 9.9 8.5 13.2
Table 5. Effect of calcination heating rate on the catalytic performance of 35wt.%(CoFe)/TiO2-SiO2 catalyst heating rate (°C min-1) CO conversion (%)
1 43.2
2 49.4 30
3 54.7
4 55.8
5 54.1
Product selectivity (%)
CH4 C2H6 C2H4 C3H8 C3H6 C4H10 C4H8 C5-C12 C13+ CO2
12.4 5.7 13.9 5.6 11.3 7.2 7.8 19.7 10.8 5.6
9.6 6.1 10.5 6.2 14.0 6.7 9.1 21.7 11.4 4.7
8.6 6.1 11.0 5.3 14.7 5.6 9.7 24.8 9.7 4.5
9.8 5.5 10.2 7.8 12.1 7.3 10.0 22.3 10.2 4.8
11.3 6.4 8.6 6.2 11.3 6.2 8.9 21.8 11.9 7.4
Product selectivity (%)
Table 6. Effect of calcination time on the catalytic performance of 35wt.%(Co-Fe)/TiO2SiO2 catalyst calcination time ( h) 5 6 7 8 9 10 CO conversion (%) 46.2 54.7 57.9 55.3 49.2 43.2 CH4 12.1 8.6 8.2 10.2 13.8 14.9 C2H6 7.2 6.1 5.1 5.4 8.6 6.7 C2H4 9.3 11.0 11.9 8.6 7.3 8.3 C3H8 6.4 5.3 5.0 5.4 6.4 6.1 C3H6 10.4 14.7 15.1 11.7 8.9 9.2 31
C4H10 C4H8 C5-C12 C13+ CO2
6.2 8.2 21.1 7.4 11.7
5.6 9.7 24.8 9.7 4.5
4.8 9.6 26.2 10.2 3.9
5.8 9.7 22.4 11.8 9.0
5.2 6.8 20.2 12.7 10.1
6.5 6.4 17.9 13.4 10.6
Table 7. Effect of calcination temperature on the catalytic performance of 35wt.%(CoFe)/TiO2-SiO2 catalyst calcination temperature (°C) 300 400 500 600 700 CO conversion (%) 32.1 45.2 57.9 61.4 56.1
32
Product selectivity (%)
CH4 C2H6 C2H4 C3H8 C3H6 C4H10 C4H8 C5-C12 C13+ CO2
25.8 3.8 6.2 2.9 6.4 4.1 5.2 11.8 6.3 27.5
12.5 4.2 10.7 5.9 14.5 5.9 8.1 21.6 9.3 7.3
8.2 5.1 11.9 5.0 15.1 4.8 9.6 26.2 10.2 3.9
8.0 5.0 12.3 5.1 16.2 4.4 10.1 27.4 8.1 3.4
9.5 6.4 11.1 4.8 13.5 4.7 9.3 21.8 12.6 6.3
Pr od uct
Table 8. Effect of different promoters on the catalytic performance of 35wt.%(Co-Fe)/TiO2SiO2 catalyst promoter K Cu Zn Ce CO conversion (%) 64.2 72.3 66.7 67.6 CH4 7.6 7.1 9.5 10.2 C2H6 6.8 4.6 7.9 6.3 33
C2H4 C3H8 C3H6 C4H10 C4H8 C5-C12 C13+ CO2
15.3 5.4 13.9 4.7 13.9 19.3 8.9 4.2
13.2 4.7 16.8 4.1 10.7 29.5 6.2 3.1
11.5 6.7 12.3 6.1 8.5 17.4 11.6 8.5
13.2 8.6 13.8 7.4 9.1 15.5 12.3 3.6
Product selectivity (%)
Table 9. Effect of H2/CO on the catalytic performance of 35wt.%(Co-Fe)/TiO2-SiO2 catalyst H2/CO 1/3 2/3 1/1 2/1 3/1 CO conversion (%) 64.3 67.5 72.3 70.3 69.5 CH4 8.7 7.6 7.1 8.9 9.8 C2H6 4.1 4.2 4.6 6.8 7.3 C2H4 10.2 12.1 13.2 11.4 10.4 C3H8 3.6 4.8 4.7 5.3 5.9 C3H6 12.3 14.5 16.8 13.7 11.4
34
C4H10 C4H8 C5-C12 C13+ CO2
4.0 7.4 24.3 19.8 5.6
5.2 8.3 27.2 11.8 4.3
4.1 10.7 29.5 6.2 3.1
7.3 10.1 21.4 8.3 6.8
7.7 9.5 18.5 10.9 8.6
Produ ct select ivity (%)
Table 10. Effect of GHSV on the catalytic performance of 35wt.%(Co-Fe)/TiO2-SiO2 catalyst GHSV (h-1) 1000 1500 2000 2500 3000 CO conversion (%) 67.1 72.3 78.4 78.0 74.1 CH4 8.3 7.1 6.5 7.9 8.5 C2H6 3.5 4.6 5.4 6.7 7.8 C2H4 11.2 13.2 15.3 12.4 11.3
35
C3H8 C3H6 C4H10 C4H8 C5-C12 C13+ CO2
5.9 14.3 6.4 8.1 21.4 15.5 5.4
4.7 16.8 4.1 10.7 29.5 6.2 3.1
4.1 17.4 4.3 11.7 26.9 5.4 3.0
6.7 15.9 7.6 10.1 23.7 5.4 3.6
6.9 14.1 8.3 10.0 21.4 6.8 4.9
Product selectivit y (%)
Table 11. Effect of reaction temperature on the catalytic performance of 35wt.%(CoFe)/TiO2-SiO2 catalyst temperature (°C) 220 230 240 250 260 270 280 CO conversion (%) 42.7 61.6 78.4 81.4 82.4 84.5 86.2 CH4 5.4 5.9 6.5 6.5 7.8 10.7 17.6 C2H6 9.8 6.4 5.4 4.1 4.0 6.8 7.5 C2H4 12.7 13.6 15.3 16.4 13.6 10.3 9.4 C3H8 8.9 5.8 4.1 4.0 6.4 7.6 9.0
36
C3H6 C4H10 C4H8 C5-C12 C13+ CO2
13.2 6.3 9.2 20.1 10.9 3.5
15.7 5.2 10.1 25.8 8.3 3.2
17.4 4.3 11.7 26.9 5.4 3.0
18.4 4.2 12.4 26.8 4.2 3.0
14.7 6.3 13.5 25.1 4.0 4.6
12.1 8.9 11.4 21.2 3.4 7.6
11.4 9.3 10.2 14.1 3.0 8.5
Pr od uct sel
Table 12. Effect of reaction total pressure on the catalytic performance of 35wt.%(CoFe)/TiO2-SiO2 catalyst pressure (bar) 1 5 9 13 17 21 CO conversion (%) 81.4 82.6 83.6 84.2 81.7 78.3 CH4 6.5 4.5 4.9 4.9 4.8 4.7 C2H6 4.1 3.1 3.1 3.0 3.0 3.1
37
C2H4 C3H8 C3H6 C4H10 C4H8 C5-C12 C13+ CO2
16.4 4.0 18.4 4.2 12.4 26.8 4.2 3.0
17.1 4.1 19.1 4.4 12.5 27.1 5.5 2.6
38
14.3 5.2 14.1 6.9 11.6 29.5 7.8 2.6
12.2 6.2 11.1 7.3 12.1 31.1 9.3 2.8
11.1 7.0 8.2 7.9 12.1 33.0 10.1 2.8
7.2 7.1 7.1 8.0 12.1 35.1 12.7 2.9