Characterization of fused Fe–Cu based catalyst for higher alcohols synthesis and DRIFTS investigation of TPSR
Introduction
Higher alcohols synthesis (HAS) from coal or natural gas via syngas has the potential for production of gasoline octane booster and primary materials of chemical industries. The catalytic HAS from syngas was first reported in the 1920's [1]. Up to now, HAS from syngas is still an interesting subject for both industrial application and fundamental research. Several types of catalysts, including (I) the modified methanol catalysts (Zn–Cr and Cu–Zn based catalysts) [2], [3], [4], [5], (II) the modified Fisher–Tropsch (F–T) catalysts (Cu–Co based catalysts) [6], [7], [8], [9], [10], [11], (III) the Mo-based catalysts [12], [13], [14], [15], [16], [17], [18] and (IV) the supported noble metal catalysts (supported Ru, Rh and Pd catalysts) [19], [20], [21], [22], were developed for the conversion of syngas to alcohols. Among these catalyst systems available for HAS from syngas, Cu–Co based catalyst was considered as one of the most promising catalysts [10], [11]. However, the poor stabilization in long-term run and the low total alcohol selectivity from the practical point of view restricted its progress [23]. Using other F–T element such as Fe or Ni to replace Co as active component in the reaction was also studied [24], [25], [26], [27]. These catalysts are usually prepared by co-precipitation method, but the reproducible preparation of a suitable catalyst, by this method, is difficult and all the steps (precipitation temperature, pH value, aging time and temperature, drying and calcination methods, etc.) of preparation of the catalysts should be carefully controlled [28]. In our laboratory, a simple high temperature fusion method to prepare the Fe–Cu based catalyst was developed [29], [30], [31]. The fused Fe–Cu based catalyst via this method showed good results, e.g. 67% C2+OH selectivity at 37% CO conversion obtained over fused Fe1.3Cu1.0V0.04K0.05Mn0.15 catalyst [30], 59% C2+OH selectivity with the space–time yield of higher alcohols of 0.62 g mL− 1 h− 1 over fused Fe1.3Cu1.0Zn0.2K0.05Al0.02 catalyst [31]. In order to find the active species of the fused Fe–Cu based catalyst in HAS from syngas and design catalyst with superior properties, we studied the fused Fe–Cu–Zn–K–Al catalyst in the present work by various techniques, such as X-ray diffraction (XRD), thermal analyses, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of CO adsorption and hydrogen temperature-programmed desorption (H2-TPD). At the same time, DRIFTS was applied for the investigation of temperature-programmed surface reaction (TPSR) of CO and H2.
Section snippets
Preparation of the fused Fe–Cu based catalysts
The purified magnetite, CuO, ZnO, KNO3 and Al2O3 were ground to 300 mesh, respectively, and then they were mixed thoroughly according to the molar ratio of Fe:Cu:Zn:K:Al = 1.3:1.0:0.2:0.05:0.02. After the mixture was fused in an electric furnace at about 1800 K, the products were cooled, crushed and screened to obtain the fused Fe–Cu based samples. The reduced catalyst was prepared via the reduction of the fused catalyst by syngas (1.0 MPa) containing trace of CH4 with n(H2):n(CO):n(CO2) =
Crystalline phase of the fused Fe–Cu based samples
The XRD patterns of the fused Fe–Cu based catalyst before and after reduction are given in Fig. 1. For comparison, the XRD pattern of the mechanically mixed sample is also shown. For the three samples, the diffraction peaks ascribed to Zn, K and Al species are too weak to be identified. Only the diffraction peaks attributed to Cu and Fe species are observed. The mechanically mixed sample exhibits the diffraction peaks of the starting materials of CuO and Fe3O4. Different from the mechanically
Conclusions
In conclusion, the fused Fe–Cu based catalyst is composed of Cu2O, CuFeO2 and CuFe2O4 species. After reduction by syngas, the metallic Fe and Cu are the main components, but minor CuFeO2 and CuFe2O4 species are also present. Over the fused Fe–Cu based catalyst, the dissociative activation of H2 takes place on both Fe and Cu sites. However, the activation of CO occurs not only on the metal sites but also on the metal ion sites. During the activation of CO, the dissociation of CO to C* and O*
Acknowledgments
We are grateful to the Natural Science Foundation of State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences (no. 09-610) for the financial support.
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