Synthesis and Activation Study of Iron-Based Fischer Tropsch Catalyst Using Sol-Gel Method

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Synthesis and Activation Study of Iron (Fe) Based Fischer Tropsch (FT) Catalyst Using Sol-gel Method

Muchammad Zainul Anwar, Rachmat Triandi Tjahjanto,* Uswantun Hasanah Chemistry Department, Brawijaya University, Jl. Veteran 65145 Malang

Corresponding author: [email protected] Received 28 May 2019; Accepted 10 December 2019

ABSTRACT

As oil consumption increases from year to year, efforts need to be made to increase energy reserves by developing new renewable energy. One way to develop energy sources is by the synthesis Fischer Tropsch (FT). FT is a synthetic gas conversion reaction (mixture of CO and H₂) into a long chain hydrocarbon mixture. The FT reaction requires a catalyst called the FT catalyst. So far, many studies that examine the effectiveness of catalysts in converting synthesis gas into long chain hydrocarbons, but rarely information about the composition of the phases that exist on the surface of the catalyst. To study about it, we synthesized FT catalysts at various variations of calcination temperature. Fe(NO3)3 as a precursor and Cu(NO3)2 as promoter (20:1) used in this study. The calcination temperature used are 300, 500, and 700°C. Characterization and analysis of catalysts were formed with XRD and SEM-EDX. Calcined catalysts were activated using CO₂ and H₂ gas and then re- characterized with XRD and SEM-EDX. Calcination results the formation of an iron oxide phase, while activation results the formation of iron carbide and zero Fe phases.

Key word: Fischer tropsch catalyst, sol-gel, calcination, activation

INTRODUCTION

The Fischer Tropsch reaction is a conversion reaction of synthestic gas into mixed long chain hydrocarbons [1]. The Fischer Tropsch reaction is a surface polymerization reaction, in which the reaction between reagent, hydrogen (H2), and carbon monoxide (CO) occurs on the surface of the catalyst in situ. The reagent forms a monomer unit, then the monomer will be polymerized to form the product in the form of C1 to C40 hydrocarbons [2]. Previous research has explained that FT reactions require catalysts, carriers, and promoters in their reactions.

The commonly used catalysts are transition metals (group 8-10 elements). Fe, Co, and Ru are metals that have the highest activity in converting synthetic gas (CO + H2) into long chain hydrocarbons according to Venezia et al. [3]. However, Ru is an expensive metal and Co leads to methane formation [3]. Commonly used carriers or supports are silicon (Si), aluminum (Al), and magnesium (Mg). Metals that can be used as promoters, including Cu [4], Ag [4], K [5], and Ru [5]. The most effective promoter is Cu because it can increase the selectivity of C5 +formation and inhibit the formation of methane (CH4) [6].

There are several commonly used methods of synthesis of FT catalysts, including sol- gel, precipitation, and impregnation. The sol-gel method is better at suppressing methane formation [7]. In addition, the sol-gel method is a method that is considered more beneficial because it is easy to regulate the stoichiometry of precursors [8]. The important thing to

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consider in the synthesis of catalysts using the sol-gel method is the concentration of precursors and pH [8]. The concentration of Fe and pH precursors in the synthesis of catalysts has an effect on the catalyst gelation process. In the synthesis of Fe catalyst that has been carried out by Arifin (2017), between all reaction pH, namely 3, 5 and 7, the gel is formed only at pH 5 and 7 at all Fe concentrations. At pH 7, the gel is easier to form and harder in texture than the pH gel 5. The concentration of Fe affects the speed of the gel formation process. The greater the concentration of Fe, the faster the process of gel formation [9].

Activation process aim to activate the catalyst. Catalyst activation that has been used so far is using CO and H2 (synthetic gas).

In addition to synthetic gas, catalyst activation can be carried out with a mixture of CO2

and H2 gas [10]. The reaction is based on the Water Gas Shift (WGS) reaction, namely:

CO2 + H2 ⇄ CO + H2O (1)

Based on the reaction equation (1), when CO2 gas reacts with H2, CO and H2O gas will be formed. CO formed is possible to react with excess H2 to form synthesis gas. Thus, synthesis gas can be converted into long chain hydrocarbons. The equation is as follows:

nCO2 + 3nH2 → (−CH2−)n + 2nH2O (2)

Based on the description above, there are various observations about the effect of the type and amount of promoter addition on the selectivity of the catalyst, as well as the type and amount of gas composition used for activation. However, it has not been found the effect of calcination temperature and activation method on the surface composition of the catalyst specifically. The calcination temperature affects the silicate phase, where the higher the temperature used, it will change the quartz phase and form the trydimite, cristobalite or other silica glass. In addition, observing the composition of the catalyst phase after activation using CO2 gas is a discussion that will be emphasized in this paper.

EXPERIMENT

Chemicals and instrumentations

Chemical compounds used in this research were hydrogen gas, carbon dioxide gas, iron(III) nitrate (Merck), sodium metasilicate (Merck KGaA), nitric acid (Merck 65% purity), ethanol (Merck 96% purity), copper(II) nitrate (Merck), and destillated water. Meawhile, the instruments used during the research were a homemade reactor, analytical balance (Ohaus Pioneer PA214), pH meter (Mettler Toledo), oven (Memmert UL 30), autoclave (Furnace 6000), SEM-EDX (FEI type Inspect S50), and XRD spectrophotometer (PanAnalytical type Expert Pro).

Reaction Procedure

The catalyst was synthesized by the sol gel method. The stages of catalyst synthesis using the sol gel method are preceded by making hydrogels, xerogels, and catalysts. The following is the complete procedure for each of these steps.

Hydrogel synthesis

Making hydrogel was done by taking a solution of 2 mL of 1.0 M nitric acid added to a mixture of 50 mL iron(III) nitrate 0.01 M solution and 2.5 mL copper(II) nitrate 0.01 M.

Then 0.4 M of solution sodium metasilicate was added into the mixture drop by drop while

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stirring with a magnetic stirrer until pH 7 was reached. After that, the solution was aged until the gel was formed.

Xerogel synthesis

The obtained hydrogels were then filtered off using filter paper and washed using 96%

ethanol solution to remove any impurities. Then the gel was dried in the oven at 80 °C for 10 hours. The product was determined as xerogel.

Catalyst synthesis

Xerogel was then calcined in furnace for 4 hours with a temperature variation of 300, 500, 700 ° C. Then, the calcined xerogel was placed in the desiccator for 15 minutes. Each xerogel was then crushed to produce a fine powder. The catalyst was then characterized using XRD to study the effect of contact temperature on the composition of the catalyst phase.

Catalyst activation

The experimental procedure for catalyst activation test begins with weighing 1.0 gram of Fe catalyst, and placed in a clean reactor. On the other hand, the reactor was connected with a series of synthetic gas generating devices, namely hydrogen gas and carbon dioxide gas, as well as heat regulators. The reactor temperature was set to 300 °C, then gas was started flowing into the reactor with 2 different methods, i.e. (i) Methode 1: Hydrogen gas is flowed for 3 hours, then continued with a mixture of hydrogen and carbon dioxide gas for 4 hours. (ii) Methode 2: Hydrogen and carbon dioxide gas were flowed simultaneously for 5 hours. The residual gas was collected in a bottle filled with distillated water. After the gas has been flowed, the temperature was lowered to room temperature, and eventually the gas was stopped at room temperature. The catalyst contained in the next reactor was then taken out, and analyzed using XRD and SEM-EDX.

RESULT AND DISCUSSION Synthesis of catalysts

The catalyst synthesis was carried out with a volume ratio of 20: 1 for 0.1 M iron(III) nitrate precursor and 0.1 M copper(II) nitrate promoter. The mixture is acidified with a solution of nitric acid and added with sodium metasilicate, the mixture forms a hydrogel. The formed hydrogel was washed with 96% ethanol and then dried in an oven to form a brown xerogel as shown in Figure 1.

Figure 1. The result of xerogel synthesized at 80°C

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Xerogel is formed, and then heated using a furnace at a predetermined temperature variation. There is a discoloration of the xerogel before and after calcination. Figure 2 is a synthesized catalyst before activation. Figure 2a is the result of catalyst calcination at 300°C, the image shows a glossy black and brown solid. The calcined catalyst at 500°C (figure 2b), shows a glossy black, green and brown solid, while the calcined catalyst at a temperature of 700°C (figure 2c) is dominantly in the form of light green solids and little brown solids.

Figure 2. Catalyst before activation at calcination temperature 300 °C (a), 500 °C (b), and 700 °C (c)

Activation using a mixture of H2 and CO2 gases causes the catalyst to change color.

While the size and mass of the catalyst did not change significantly. Figure 3 is the product of activation. Activation on 300 ôC causes the catalyst to turn gray and slightly purple. While activating at 500 ôC, the catalyst becomes black, gray, and purple-ish. Activation on 700 ôC causes the color of the catalyst change to purple.

Figure 3. Catalyst after activation at calcination temperature (a) 300 °C, (b) 500

°C, and (c) 700 °C.

The effect of the calcination temperature on the composition of the catalyst phase

Characterization using XRD aims to study the composition of the phases found on the surface of the catalyst. Characterization was carried out on the synthesis catalyst at a predetermined calcination temperature variation. Figure 4 is a diffractograms of the synthesis catalyst. Calcination at 300 and 500 ^oC, the phases that emerge are magnetite (Fe3O4), cristobalite (SiO2), Fe2SiO4. These results correspond to that reported by Mirzaei et al. (2010) [1] which showed the results of calcination resulting in the formation of an iron oxide phase.

Whereas at 700 ^oC, only provide cristobalite and nitratine.

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Figure 4. Diffractogram of catalyst at calcination temperature 300 °C (C-300), 500 °C (C-500), and 700 °C (C-700).

Figure 5. Diffractograms of the catalyst before and after activation at the calcination temperature: (a) 300°C, (b) 500°C, and (c) 700°C.

Effect of activation on the composition of the catalyst phase

The activation process aims to activate the catalyst. The active catalyst is characterized by the formation of zero Fe and iron carbide compounds. Figure 5 is a diffractograms of each catalyst before activation which is overlayed with a catalyst diffractogram after activation. In each image, there was a decrease in the intensity of the diffraction peak after activation compared to the diffractogram before activation. The results of the analysis showed that the C-300 and C-500 catalysts after activation contained Fe, cristobalite, magnetite, Fe3C. After the activation of the C-700 catalyst there is the same phase as before activation, namely

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cristobalite and nitratine. There was no significant phase change at C-700 after being activated, among others, because calcination at a temperature of 700°C formed a hard and stable crystal structure. From the results of the analysis showed that the catalyst that has been active is a C-300 and C-500 catalyst. This is indicated by the formation of the Fe phase. Both catalysts qualify to be used as FT catalysts due to the formation of the Fe3C phase. Iron carbide is the center of catalysts or catalytic sites [10].

Effect of different methods of activation on the composition of the phase on the surface of the catalyst

There are other methods that can be used to activate the catalyst. Figure 6 is a C-500 diffractogram before activation and after activation using two different methods. C-500-1 is a diffractogram for activation of methods 1 and C-500-2 for activation of method 2. Two activated diffractograms have the same pattern, but different diffractogram peaks. As previously explained, the C-500-1 phase that emerges includes Fe, cristobalite, magnetite, FeSi, Fe3C. While the phase that appears on C-500-2 is Fe, Fe2SiO4, and cristobalite. It can be concluded that method 1 is better than method 2 because the method gives rise to the Fe3C phase on the surface of the catalyst.

Figure 6. Diffractograms of the catalyst with different activation method

The following is the result of SEM-EDX from the catalyst before activation (Figure 7), after activation with method 1 (Figure 8), and after activation by method 2 (Figure 9). From the three EDX spectra that produced, each identified Si, O, Na, Fe, and Cu atoms derived from catalyst precursors, including Na2SiO3, Fe(NO3) 3, and Cu(NO3)2. In Figure 6, the spectrum shows the presence of C atoms in the catalyst. This shows that there is a possibility of a bond between Fe and C forming Fe3C. Fe3C is active site of the catalyst in the reaction of the hydrocarbon chain elongation in the Fischer Tropsch synthesis process.

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Figure 7. Results of SEM-EDX catalyst before activation

Figure 8. Results of SEM-EDX catalyst after activation by method 1

Figure 9. Results of SEM-EDX catalyst after activation by method 2

In overall, from the EDX analysis of the catalyst prepared before and after calcination including both method 1 and method 2, still indicates the presence of silica (Si), that may form as oxide of silicone. Thus, the yield calculation and also catalyst synthesized still contained the Si element.

CONCLUSION

The Fischer-tropsch based Fe catalyst with Cu promoter using the sol-gel method was synthesized and activated successfully. The calcination temperature affects the phase that

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appears on the surface of the catalyst. Activation using a mixture of CO2 and H2 gas results the similar phase with activation using a mixture of CO and H2 gas. The best synthesis results were found in catalysts with calcination temperature of 300 and 500°C, because the Fe3C phase was found. The more effective activation was the activation that was preceded by hydrogen gas flow.

CONFLICT OF INTERETS

Authors declare no competing interests.

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