Effects of sodium and sulfur on catalytic performance of supported iron catalysts for the Fischer–Tropsch synthesis of lower olefins
Graphical abstract
Catalysts promoted with low amounts of sodium and sulfur exhibited higher selectivity to lower olefins and lower methane production. Promoted bulk and α-Al2O3-supported catalysts showed similar selectivities; however, bulk catalysts displayed lower catalytic activity and extensive coke formation.
Introduction
Lower olefins (C2–C4), which are key building blocks of the chemical industry, are traditionally produced from naphtha cracking or fluid catalytic cracking (FCC). High prices of oil and the interest of many countries on reducing their dependence on imported oil have urged research efforts to the development of new processes to produce oil-derived chemicals from alternative sources such as natural gas, coal, or biomass. The interest on these processes is not new and was already an important matter of study after the first oil crisis in the 1970s. During that period, many researchers investigated the use of Fe-based catalysts for the direct production of lower olefins from synthesis gas (syngas).
For this reaction, Iron is the metal of choice not only for its low price and high availability but also for its catalytic properties since the Fischer–Tropsch-to-Olefins reaction (FTO) is carried out at high temperatures [1]. Compared to cobalt, iron has a low methanation activity even when the reaction is performed at temperatures higher than 300 °C, which is necessary to drive product selectivity toward shorter hydrocarbon chains. Additional advantages of iron over cobalt-based catalysts include a higher resistance to contaminants present in syngas, a higher selectivity to olefins and a higher water–gas shift (WGS) activity allowing for the use of CO-rich syngas. Therefore, iron catalysts are especially suitable for the conversion of syngas derived from coal or biomass which contains more contaminants and is rich in CO. Additionally, the products obtained with iron catalysts have a higher olefinic content compared to Co.
Many different elements have been investigated as possible promoters to improve the C2–C4 olefins selectivity such as potassium [2], [3], sodium [4], [5], manganese [6], titanium [7], zinc [8], and vanadium [9]. Sodium has been proposed as an effective promoter to decrease methane selectivity [5], [10], [11], to favor chain growth propagation [5], and to increase the olefin-to-paraffin ratio of the products [4], [11]. It has been reported that sodium, as well as potassium, increase iron carbidization [4], [5] and WGS activity [12]. Ribeiro et al. [4] have suggested that the addition of alkali promoters decreases the C–O bond strength resulting in an increase in the coverage of dissociatively adsorbed CO, which could explain the higher levels of carbidization. Additionally, the high CO coverage could inhibit olefin re-adsorption, thus decreasing the olefin hydrogenation rates. The high coverage of the surface C species also might increase the chain growth probability and the conversion rates [10].
Studies have shown that these beneficial effects are only obtained at low concentrations of sodium. After achieving an optimum loading, further addition of the promoter has a detrimental effect inhibiting reduction and carbidization, decreasing the CO conversion, and shifting the product selectivity to shorter hydrocarbons [4], [13]. Similar effects have been reported for potassium [14]. Ngantsoue-Hoc et al. [12] have shown that not only the promoter loading defines its effectiveness but also the reaction conditions and CO conversion levels.
Studies on sulfur as a promoter are limited as this element has been considered as a poison for Fischer–Tropsch catalysts. However, Bromfield and other researchers have shown that for iron, low concentrations of sulfur may act as a promoter that improves activity, decreases methane selectivity [15], [16], [17], increases the chain growth probability, and enhances olefin selectivity, reduction, and carbidization [18], [19]. These positive effects of the addition of low amounts of sulfur have been observed by co-feeding low amounts of H2S during the reaction and by direct addition of sulfur during catalyst preparation. The promoter effect for sulfur is also highly dependent on promoter concentration, catalyst pretreatment, and reaction conditions [20].
Research on the development of selective catalysts for the production of lower olefins has been mainly focused on bulk or unsupported catalysts with low cost and simple synthesis. These precipitated or fused catalysts can be modified by addition of promoters to increase the selectivity to lower olefins and to decrease methane production. Although catalysts with high selectivity to C2–C4 olefins and to short-chain hydrocarbons (α ∼ 0.4–0.6) have been reported [21], their industrial use has not materialized because of their low mechanical stability [22], [23] under high temperatures and low H2/CO ratios.
Recently, Schulte et al. [24] have reported high selectivities to lower olefins in combination with high activity and low methane production when using iron supported on nitrogen and oxygen functionalized-carbon nanotubes (N–CNT, O–CNT) under the Fischer–Tropsch reaction at 340 °C, 25 bar and a H2/CO ratio of 1.8. Their results underline the importance of the weakly interactive nature of the support to achieve a high catalytic activity.
Most recently, a breakthrough in the selective production of lower olefins from synthesis has been reported [23], [25]. Sodium- and sulfur-promoted iron catalysts prepared using support materials with weak interaction toward iron have displayed high activities and light olefins selectivities with a low methane production and an excellent mechanical and chemical stability [23], [25]. However, still limited information is available about the effect of sodium, sulfur, or a combination of both promoters on supported catalysts studied under the stringent reaction conditions used for the production of lower olefins from CO-rich synthesis gas. Most of the above-mentioned promoter studies have been performed on precipitated or fused catalysts at temperatures lower than 300 °C.
Here, we discuss the effects of sodium and sulfur on iron catalysts supported on α-alumina [23], [26] when the Fischer–Tropsch reaction is carried out at high temperatures (340–350 °C). The catalysts have been prepared using an inert support to improve mechanical stability and to allow a better interaction between the promoters and the iron phase.
Section snippets
Preparation of an unpromoted supported catalyst
A supported catalyst with a nominal loading of 5 wt.% Fe was prepared using the incipient wetness impregnation technique. Catalyst AFe was prepared with a solution of 1.726 g of ammonium iron citrate, C6H8O7·xFe3+·yNH3 (Fluka, purum p.a. 14.5–16 wt.% Fe (Table S1)), in 3.4 ml of demineralized water. For the preparation, 5 g of α-Al2O3 (10 m2 g−1; Pore volume 0.4 ml g−1, AL4196E, BASF) was impregnated with the above-mentioned solution until the support pores were filled. The impregnation steps were
Results and discussion
The iron, sulfur, and sodium loadings of the promoted and unpromoted catalysts are presented in Table 1. The first letter of the sample code corresponds to the type of catalyst: A was assigned to α-alumina-supported samples and B to bulk (or unsupported) catalysts.
The Fe, S, and Na loadings determined with ICP were similar to the nominal loadings. For the S-promoted samples, an S/Fe atomic ratio of 0.009 was expected while Na-promoted catalysts were prepared with a nominal composition of 0.22
Conclusions
The effects of Na and S on Fe-based Fischer–Tropsch catalyst were studied. Sodium promotion of supported iron catalysts resulted in decreased methane selectivity and higher chain growth probability values when the catalysts were tested at 1 bar and 20 bar. However, the addition of sodium had a negative effect on catalytic activity possibly caused by an enhanced extent of carbon deposition as observed from TGA and TEOM experiments.
Modification of α-Al2O3-supported iron catalysts by addition of low
Acknowledgments
Financial support was provided by ACTS-ASPECT (NWO). We thank G. Bonte and A. Chojecki for the catalytic tests at 20 bar performed at Dow Benelux, C. van de Spek and J.D. Meeldijk for the TEM images and T. Zalm for ICP analysis. We would like to thank Prof. E.J.M. Hensen for his contribution in the discussion of the results of in situ Mössbauer spectroscopy experiments.
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