Elsevier

Applied Catalysis A: General

Volume 301, Issue 2, 24 February 2006, Pages 272-283
Applied Catalysis A: General

Characterization of iron-cobalt oxide catalysts: Effect of different supports and promoters upon the structure and morphology of precursors and catalysts

https://doi.org/10.1016/j.apcata.2005.12.022Get rights and content

Abstract

The effect of a range of preparation variables such as the precipitate ageing time and [Fe]/[Co] molar ratio of precipitation solution on the composition and morphology of iron-cobalt oxide catalysts prepared using a co-precipitation method is described and the optimum preparation conditions were identified with respect to the catalyst activity for the Fisher–Tropsch reaction. The effect of different promoters along with loadings of optimum support and promoter on the activity and selectivity of the 40%Fe/60%Co as an optimum molar ratio are studied and it was found that the catalyst containing 40%Fe/60%Co/15 wt%SiO2/1.5 wt%K which aged for 2 h, is an optimum modified catalyst for the conversion of synthesis gas to ethylene and propylene. The results are interpreted in terms of the structure of the active catalyst. Characterization of both precursors and calcined catalysts were carried out using XRD, SEM, BET specific surface area and thermal analysis methods such as TGA and DSC. It was shown that all the different preparation variables influenced the catalyst precursor structure and morphology.

Introduction

The Fischer–Tropsch (FT) reaction entails the conversion of CO and H2 to a spectrum of products mainly comprising alkenes and alkanes [1], [2]. This product spectrum is very broad and consequently many studies have been carried out under FT conditions with the aim of controlling and limiting the product selectivity. The products most desired are those with: (i) a low methane and low oxygenate content, (ii) a high alkene/alkane ratio and (iii) a high wax or C5–C10 content. This control is typically achieved by modification of the catalyst, the reactor and/or the reaction conditions [2]. Catalysts used for the FT process are generally based on iron or cobalt, i.e., Fe/Cu/K [3] oxides or iron and/or cobalt oxide supported on alumina [4] or silica [5].

The Fe-Mn and Co-Mn catalysts, with or without alkaline promoters, are also studied as FT catalysts for production of light olefins from synthesis gas [6], [7]. The high selectivity of Fe-Mn catalysts in C2–C4 has been correlated to the presence of a Fe-Mn spinel oxide. Unfortunately, since carbide phases are formed during the catalytic tests, catalysts have short life times. Cobalt does not carburize as readily as iron under CO/H2 atmosphere and numerous studies have confirmed its efficiency for obtaining light olefins [6]. The protection of the spinel phase under the reaction conditions and the presence of a metallic phase, which does not carburize under test, seem to be essential to produce C2–C4 olefins from CO/H2.

Modification of the traditional FT catalysts (Fe, Co, Ni, Ru) by promoters and supports has provided one means of manipulating the FT products spectrum [8]. Due to the thermodynamic and kinetic limitations of the reaction, few catalysts are able to amplify the C2–C4 hydrocarbons fraction. However, some examples are reported in the literature and these are Fe and/or Co based catalysts on partially reducible oxide supports such as MnO2, V2O5 and TiO2 instead of the conventional inert supports like SiO2 and Al2O3 [9], [10], [11], [12].

Both Co and Fe are typically used when combined with a range of supports and promoters that permit further control over the product spectrum [13], [14]. It has been reported that a mixture of the two most active catalysts, Fe and Co, have generated product streams in the FT reaction richer in olefins than expected from either Fe or Co catalysts [15], [16], [17], [18]. Fe/Co catalyst mixtures, typically supported, have been synthesized by numerous routes [15], [16], [17], [18], [19], [20]. Supports that have been used include alumina [20], silica [21], carbon [22], [23], [24], zirconia [25] and titania [6], [7], [26], [27]. Reports in the patent literature on the synthesis and study of a series of slurry phase unsupported Fe/Co Fischer–Tropsch catalysts have indicated that the addition of small amounts of Co to Fe could influence the Fe catalyst quite dramatically [28]. Indeed, this mixture could have an enhanced activity in comparison to the individual metal oxides [29]. Previous studies have shown that Fe-Co composite catalysts are efficient to produce light olefins from CO/H2 when the cobalt ferrite is preserved under test [30].

The aim of this work was to investigate the effect of a range of preparation variables on structure and morphology of precipitated iron-cobalt catalysts for FT synthesis. These variables include the precipitate ageing time and the [Fe]/[Co] ratio of the precipitation solution of mixed iron-cobalt oxide catalysts. We also report further results concerning the effects of different promoters like Li, K, Rb and Mg and potassium loading and different supports like Al2O3, SiO2, TiO2 and La2O3 and SiO2 loading on structure and morphology of precipitated iron-cobalt catalysts. Different methods such as X-ray diffraction (XRD), scanning electron microscopy (SEM), BET surface area, thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to characterize all of the prepared catalysts. In addition, the effects of some reaction conditions such as H2/CO molar feed ratios and a range of reaction temperatures on structure and morphology of catalysts in conversion of synthesis gas to light olefins have been studied.

Section snippets

Preparation

The catalysts tested in this study were prepared using co-precipitation method. In the co-precipitation method, aqueous solutions of Co(NO3)2·6H2O (0.5 mol/l) and Fe(NO3)3·9H2O (0.5 mol/l) with different molar ratios were pre-mixed and the resulting solution heated to 70 °C in a round-bottomed flask fitted with a condenser. Aqueous Na2CO3 (0.5 mol/l) was added to the mixed nitrate solution with stirring while the temperature was maintained at 70 °C until pH 7.0 was achieved. The resulting

Effect of solution [Fe]/[Co] ratio on catalytic performance

In our previous study [31], we reported the catalytic performance of the iron-cobalt oxide catalysts with different [Fe]/[Co] molar ratios for Fisher–Tropsch synthesis with different molar feed ratios of H2/CO at 450 °C. A plot of CO conversion and ethylene and propylene selectivity percent on steady state catalytic performance for these series catalysts is shown in Fig. 2.

According to the obtained testing results, the catalyst containing 40%Fe/60%Co was chosen as the optimum catalyst for the

Conclusion

Iron/cobalt mixed oxides catalysts have been prepared using co-precipitation procedure by varying the ageing time and [Fe]/[Co] ratio in the precipitation liquor. The effects of different supports and promoters and also loadings of the optimum support and promoter on the morphology of these catalysts have been investigated.

Characterization by powder X-ray diffraction and scanning electron microscopy showed that the catalyst precursors are sensitive to the preparation conditions, so that the

Acknowledgement

We gratefully acknowledge the Research and Technology of Iranian National Petrochemical Company (I.N.P.C.) for helping and supporting this research.

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