Effect of incorporation manner of Zr on the Co/SBA-15 catalyst for the Fischer–Tropsch synthesis
Graphical abstract
Introduction
The Fischer–Tropsch synthesis (FTS) is a process that converts a mixture of carbon monoxide and hydrogen (also known as syngas) into clean liquid fuels or valuable chemicals [1], [2]. Compared with the refinery diesel, the predominantly linear paraffinic hydrocarbons along with virtually no contaminants such as sulphur, nitrogen and aromatics in the FTS show a higher cetane number and lower pollutant emission level [2]. FTS has sparked renewed interest in utilization of nonpetroleum carbon resources (such as natural gas, coal and biomass) in recent years because of the increasingly stringent environmental regulations and the depletion of light and sweet petroleum reserves.
Extensive researches on the FTS catalysts based on Ru, Co and Fe have been reported [1], [3], [4], [5]. The high cost and low availability of Ru-based catalysts are important concerns limiting their commercial application. Compared to Fe-based catalyst, Co-based catalysts for FTS are usually preferred under carefully selected temperature and H2/CO ratio, and show higher selectivity in linear long-chain paraffin fractions, slower deactivation by water (a by-product of the FTS reaction), less oxygenates and lower CO2 selectivity [6], [7], [8]. Thus, the Co-based catalysts have attracted much attention for the synthesis of wax and liquid-fuel, while the Fe-based catalysts are suitable for the production of alkenes and oxygenate chemicals. Generally, the products of FTS follow the Anderson–Schultz–Flory (ASF) distribution due to the polymerization mechanism. Thus, the liquid fuels (mainly gasoline and diesel fuel) are usually obtained by the catalytic cracking process after FTS [9], [10]. Recently, several studies have succeeded in the direct production of middle isoparaffins (C5–C12) or C10–C20 hydrocarbons with high selectivity [1], [11], [12].
It is generally accepted that the activity of Co-based catalysts for FTS is proportional to the density of surface active Co0. Therefore, Co species should be both highly dispersed and reduced on the catalyst surface [2], [11]. For this reason, Co is commonly deposited on high surface area supports such as SiO2, Al2O3 and TiO2, probably the most extensively used for Co-based FTS catalysts. In the last decades, other alternative supports with mesoporous silicas have also been explored to provide new possibilities for tuning the products selectivity [13], [14]. In particular, the SBA-15 with high surface area and narrow pore diameter distribution allows for high Co dispersion even at high metal loadings and can realize spatial confinement on the metal particle size to restrict the formation of hydrocarbons longer than some characteristic size (say, carbon number n <20) [9]. The addition of promoters, such as noble metals (Ru, Re) [8], [15], [16], transition metal oxides (ZrO2 and MnOx) [3], [17], [18] and some rare earth metal oxides (CaO, La2O3) [18], [19] are capable of modifying the catalyst structure and improving the catalytic performance by enhancing the dispersion and reduction degree of cobalt species. A number of investigations have been focused on Zr-promoted Co catalysts, which exhibited higher CO conversion and C5+ selectivity for FTS than the un-promoted catalysts. Ali et al. [20] reported that the weaker Co–Zr interaction created an active interface that increased activity by favoring CO dissociation. Andreas and Moradi et al. [21], [22] ascribed the increased activity to the easy reduction of cobalt species with the addition of Zr. Rohr et al. [23] concluded that the addition of ZrO2 to Co/Al2O3 catalyst increased the activity and selectivity to heavy hydrocarbons, while reducibility and dispersion have not been improved. Therefore, it is necessary to clarify the influence of Zr on the catalysts textural properties, surface physicochemical properties, reduction degree and catalytic performance for FTS reaction.
In the present contribution, the 15 wt.% Co/SBA-15 catalysts with 5 wt.% Zr promotion were prepared in three different manners to provide the insight into the Zr modification on properties and product distributions of the catalysts. These catalysts were characterized by means of N2 adsorption–desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) as well as temperature programmed reduction (H2-TPR) to elucidate the Co–Zr interaction, dispersion and reducibility of Co species. And the catalytic performances of the catalysts were tested in FTS by using a fixed bed reactor.
Section snippets
Catalyst preparation
The detailed preparation of SBA-15 can be found in elsewhere [9]. Co/SBA-15 catalyst was prepared by incipient wet impregnation of the SBA-15 support with a desired amount of cobalt(II) nitrate dissolved in water in equal with respect to the pore volume of the SBA-15 support. The precursor were dried at 120 °C and calcined at 400 °C for 4 h. The loading amount of Co in the final catalyst was 15 wt.%.
Co/Zr/SBA-15 was prepared by sequentially impregnated method. Typically, the SBA-15 support was
Textural properties of catalysts
The N2 adsorption–desorption isotherms for the SBA-15 and Zr-SBA-15 support presented the hysteresis condensation and evaporation steps characteristic of type H1 at the P/P0 range of 0.65–0.85 according to the IUPAC classification, indicating the periodic mesoporous structure (Fig. 1a) [2]. However, the addition of Co and/or Zr produces a hysteresis characteristic of type H4 at the P/P0 range of 0.4–1.0 without an obvious platform of adsorption saturation (Fig. 1b). The gas uptake at the P/P0
Conclusions
The characterizations revealed that Co3O4 was considered as the main surface cobalt phase, and the BET surface area and pore size were decreased for all the catalysts. The reduction degree of Co species increased from 80% (Co/SBA-15) to 90% (Co/Zr/SBA-15) and 85% (CoZr/SBA-15) due to the formation of somewhat Co-ZrO2 interaction, except for the Co/Zr-SBA-15 catalyst. The interaction between the reaction intermediates and the Lewis acidity of ZrO2 also enhanced the C5+ selectivity by about 10%
Acknowledgments
This work was financially supported by National Natural Science Foundation of China (U1510109), Entrepreneurial and Innovative Leading Talent Project of Qingdao (13-CX-19), the National Basic Research Program of China (973 program, No. 2011CB201502) and Chinese Academy of Sciences (Project No. XDA07070301 and Project No. Y2010022).
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