Elsevier

Catalysis Communications

Volume 7, Issue 12, December 2006, Pages 1061-1066
Catalysis Communications

Effect of calcium promoter on a precipitated iron–manganese catalyst for Fischer–Tropsch synthesis

https://doi.org/10.1016/j.catcom.2006.05.009Get rights and content

Abstract

A series of precipitated Fe/Mn Fischer–Tropsch synthesis (FTS) catalysts incorporated with calcium promoter were prepared by the combination of co-precipitation and spray-drying technology. The catalysts were characterized by using N2 physisorption, CO2 temperature-programmed desorption and Mössbauer spectroscopy methods. FTS performances of the catalysts were tested in a 1 dm3 continuous stirred tank reactor. It is found that calcium promoter has negligible effect on the textural properties, and the addition of calcium promoter can enhance the surface basicity of the catalyst. An appropriate amount of calcium promoter can promote the reduction and carburization of the catalysts during the reduction and Fischer–Tropsch synthesis (FTS) reaction in syngas, but the excessive addition of calcium promoter will decrease the extent of reduction and carburization. The reaction results indicated that the activities of both FTS and water-gas shift (WGS) decrease with the incorporation of calcium promoter. Calcium promoter can inhibit the hydrogenation ability, suppress the formation of methane, and enhance the selectivities to olefin and higher molecular weight products.

Introduction

Fischer–Tropsch synthesis (FTS) has been used since the early 20th century to produce transportation fuels and chemicals from coal, natural gas and other C-based materials [1]. Iron-based catalysts are usually used for FTS due to their low cost, flexible product distribution [2] and high FTS activity as well as high water-gas shift (WGS) activity [1].

Many metals such as potassium, manganese, copper and calcium, are often used as promoters of the iron-based catalysts to improve their FTS performances (activity, selectivity and stability) [3], but the study of calcium promoter is relatively limited in the previous work. Luo et al. [4] compared the effect of Group II alkali-earth metal on FTS performances over an iron-based catalyst in a continuous stirred tank reactor (CSTR). They found that the addition of calcium promoter has negative effect on the activities of both FTS and WGS. Bukur et al. [5] has reported that both the surface area and the reducibility of the catalysts decrease with the incorporation of CaO promoter, which results in a decrease of catalytic activity over a precipitated Fe/Cu/K/SiO2 catalyst. Guczi et al. [6] has found that CaO is an inert promoter over a supported Fe/MgO catalyst, which normally increases metal dispersion. But CaO promoter segregates to the surface when the catalyst has been calcined in air, leaves small number of active sites, and results in a very low activity. Dry et al. [7] studied the decomposition of carbon monoxide over a fused magnetite FTS catalyst. The catalyst was promoted with K, Na and other promoters such as MgO, Al2O3, CaO and SiO2. The results suggested that the catalyst promoted with CaO has the smallest iron surface area among all the catalysts. They also found that CaO has no chemical promotional effects on the reaction but act only as stabilizers or structural promoters. On the other hand, Dry et al. [8] also found that the surface basicity of the reduced catalysts increased in the order of Ba, Li, Ca, Na, and K.

As an alkalescence promoter, the incorporation of calcium promoter may change the hydrocarbon product selectivity. Especially, introduction the calcium promoter by the combination of co-precipitation and spray-drying technology may change the textural properties and enhance the mechanical intensity of the fresh catalysts, which make the catalysts well adapt to the slurry reactor. Therefore, a systematic study of calcium promoter on FT iron catalyst is necessary. However, these previous studies had not given an exact explanation about the effects of calcium promoter on the active sites and surface basicity of the catalysts, which may subsequently affect catalytic activity and hydrocarbon selectivity.

Recently, iron–manganese catalyst has been attracted an increasing interest due to its high selectivity to low molecular weight alkenes for the FTS [9], [10], [11]. Our group has been carried out a systematically study on the iron–manganese catalysts for slurry and fixed-bed FTS, and some promising potentials have been presented over Fe–Mn catalyst [12], [13]. However, a lot of efforts, such as the investigation of some metallic promoters, are still needed to be performed to improve the performance of the catalyst.

The present work was undertaken to develop a systematic understanding of the effects of calcium promoter on a precipitated Fe–Mn catalysts under industrial operation conditions. Particular attentions were paid to the effect of calcium on the textural properties, surface basicity and the bulk phase structure of the catalyst as prepared, after reduction and during FTS reaction. The FTS and WGS activity, hydrocarbon products distribution and olefin selectivity were systematically investigated and well correlated with the characterization results.

Section snippets

Catalyst preparation

The catalysts used in this study were prepared by the combination of co-precipitated and spray dried method. In brief, a solution containing Fe(NO3)3, Mn(NO3)2, and Ca(NO3)2 in the desired ratio was used in precipitation with NH4OH solution as precipitator at pH = 9.0 ± 0.1 and T = 353 ± 1 K. The precipitate was washed, and then filtered. Required amounts of K2CO3 solution and silica sol were subsequently added into the precipitate and the mixture was re-slurried and spray-dried. The spray-dried powder

Textural properties of the fresh catalysts

Textural properties of the fresh catalysts are summarized in Table 1. It is found that all catalysts possess high surface area (234–265 m2/g). This may be due to the incorporation of SiO2. Our previous study has shown that the addition of SiO2 results in a good dispersion of the active metal and a high surface area [13]. It is also found that there are almost no apparent differences in BET surface area, pore volume and average pore diameter of the fresh catalysts with different calcium content.

Temperature-programmed desorption (TPD)

Conclusion

Promotion of Fe–Mn catalyst with calcium promoter has significant influences on physico-chemical properties of the catalyst, such as the basicity on catalyst surface, as well as the catalytic activity and selectivity during FTS performance. The addition of calcium promoter enhances the surface basicity of the catalysts and has some positive effects on promoting the reduction and carburization of the catalyst. However, excessive addition of calcium promoter can inhibit the reduction and

Acknowledgements

We gratefully acknowledge the financial support from Key Program of National Natural Science Foundation of China (20590361) and National Natural Science Foundation of Shanxi province (2006021014).

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