Fischer–Tropsch synthesis over Ru promoted Co/γ-Al2O3 catalysts in a CSTR
Introduction
Supported cobalt catalysts are the preferred catalysts for the Fischer–Tropsch reactions to produce long-chain paraffin [1]. Alumina, silica and titania have been the most widely used supports reported and patented during the past two decades [2], [3], [4]. Cobalt catalysts are often used with alumina as support because of its favorable mechanical properties, but have a limited reducibility due to a strong interaction between the support and the cobalt oxides [5], [6]. This can be overcome by promotion with easily reducible metal promoters like Pt, Re or Ru [7]. Ru-promotion improves CO conversion, volumetric productivity, selectivity to C5+ and the ability of generating the catalyst at relatively lower temperatures [8].
Slurry process provides the ability to more readily removing of the heat of reaction, minimizing temperature gradients across the reactor and avoiding localized hot spots [8]. As a result of the improved temperature control, yield losses due to methane formation are reduced and catalyst deactivation due to cocking is decreased [9]. Continuous stirred tank reactor (CSTR) has much greater flexibility than fixed bed and fluidized bed reactors, and can operate in either gasoline or wax mode of operation [9].
The purpose of this study is to examine the effects of Ru as a promoter on the performance of γ-Al2O3 supported Co catalyst in a CSTR reactor. X-ray diffraction (XRD), temperature programmed reduction (TPR), H2-chemisorptions, thermogravimetric analysis (TGA) and BET surface area measurement were used to characterize the prepared catalysts prior to reaction study in a slurry CSTR. The results of this study are reported in this paper.
Section snippets
Catalyst preparation
Catalysts were prepared by incipient wetness co-impregnation of γ-Al2O3 support (Engelhard, AL-3912P, specific surface area 180 m2/g, average particle size 60 μm) with aqueous cobalt (Co(NO3)2 · 6H2O, Merck, purity: >99) and ruthenium(Cl3Ru · xH2O, Merck, purity: 35–40) precursor solutions. The support was first calcined at 500 °C for 10 h at 1.5 °C/min and then cobalt nitrate hexahydrate and ruthenium(III)-chloride-hydrate were dissolved in de-ionized water and the resultant solution was used to
Preparation and calcination
The XRD patterns of dried (16 h, 120 °C) and calcined (2 h, 350 °C) catalysts are shown in Fig. 1(a) and (b). In Fig. 1(a), XRD patterns show the presence of crystallized cobalt nitrate, Co(NO3)2 · 6H2O (JCPDS file number 18-425) and γ-Al2O3 (JCPDS file number 10-425). Fig. 1(b) revealed the presence of crystalline cobalt oxide (Co3O4) (JCPDS file number 9-418) and γ-Al2O3. No XRD peaks of CoAl2O3 and RuO2 were detected for any of the catalyst samples.
Bimetallic effects were enhanced by
Conclusions
The present work was focused on the study of Co/γ-Al2O3 catalysts for the synthesis of high molecular weight hydrocarbons through CO and H2 conversion in the reactor. The addition of Ru to the alumina supported Co catalysts led to improved reducibility and dispersion of cobalt. However, C5+ selectivity of catalyst was not improved. Further addition of Ru to Co/γ-Al2O3 catalysts decreased the activity of catalysts, conversion of CO and reducibility extent. Reduction of pore volume decreased mass
Acknowledgements
The authors acknowledge financial support of this work by the Petrochemical Research & Technology Co. of National Iranian Petrochemical Company and Messrs M. Daftari-Besheli, S. Abedi and S.R. Nokhbeh for their helps.
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