The effect of CO 2 on a cobalt-based catalyst for low temperature Fischer–Tropsch synthesis Y. Yao, X. Liu, D. Hildebrandt ⇑ , D. Glasser Centre of Material and Process Synthesis, School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa article info Article history: Received 5 December 2011 Received in revised form 2 April 2012 Accepted 16 April 2012 Available online 24 April 2012 Keywords: Fischer–Tropsch synthesis Hydrogenation Carbon dioxide Carbon monoxide Fixed bed reactor Cobalt catalyst abstract A series of Fischer–Tropsch synthesis (FTS) experiments, which entailed repeatedly switching between a CO (CO/H 2 /N 2 ) and a CO 2 (CO 2 /H 2 /N 2 ) feed, were conducted in a fixed bed reactor over a cobalt-based cat- alyst. It is worth noting that the effect of the CO 2 on the properties of a cobalt-based catalyst was very small under the reaction conditions we chose. There was no apparent catalyst deactivation at reaction temperatures of 180 °C and 200 °C when we continually alternated between the CO and CO 2 feeds. We observed dramatic changes in the catalyst activity and product selectivity for CO 2 hydrogenation before and after the initial FTS for CO feed at 180 °C. In addition, during the initial CO hydrogenation on the cobalt catalyst, both the olefin and paraffin formation rates suddenly changed from one pseudo-stable state to another. These differences may have been caused by liquid products, whether deposited on the catalyst surface or in the catalyst pores during CO FTS. A mild catalyst deactivation was observed at the operating temperatures of 210 °C and 220 °C, respec- tively. According to the comparison we made between the conversion of the feed gases and the product formation rates for paraffin and olefin, and our speculations concerning possible side reactions, we con- clude that the catalyst deactivation is possibly attributable to the re-oxidation by water. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction The low-temperature (180–250 °C) Fischer–Tropsch synthesis (FTS) over either iron or cobalt catalysts, producing high molecular mass linear waxes, which in turn can be hydro-cracked to produce diesel of exceptionally high quality [1–3]. Cobalt is considered the most suitable metal for the low-temperature FTS of long chain hydrocarbons because its activity and selectivity to linear paraffins are high, and its water–gas shift (WGS) activity is low [1,4,5]. As the cobalt catalysts used in FTS are relatively expensive (compared to the cost of iron), they need to have a high metal dis- persion and long life to be able to offer a good balance between cost and performance [6,7]. This is why catalyst deactivation is a major challenge in cobalt-based FTS [6–8]. The oxidation of cobalt metal to cobalt oxide by the product water, the most abundant byproduct of FTS, has long been believed to be a major cause of the deactivation of supported cobalt FTS catalysts [6,7,9–11]. Owing to the low activity a cobalt catalyst has for WGS, CO 2 is not the major byproduct. Nevertheless, in some cases CO 2 may be a significant component in the syngas obtained from biomass and coal [3,12]. It is therefore necessary to investigate the effect of CO 2 (as an oxidizing agent) on cobalt-based low-temperature FTS. Until recently, the effect of CO 2 on cobalt-based FTS has remained controversial. Some researchers [3,13–15] believe that CO 2 behaves as an inert diluent in the syngas feed at temperatures below 220 °C for FTS over cobalt-based catalysts. Zhang et al. [15] claimed that the catalyst deactivates more rapidly for the conver- sion of CO than for CO 2 , even though the H 2 O/H 2 ratio is at least two times greater for the conversion of CO 2 in cobalt-based FTS. However, Kim et al. [16] concluded that the presence of CO 2 in the feed gas affects the rate of catalytic hydrogenation of CO as well as the product distribution, and that CO 2 acts as a mild oxidiz- ing agent on reduced Co/c-Al 2 O 3 at 220 °C and 20 bar. Riedel and Schaub [17] also found that CO 2 had a negative effect on both the FT reaction rate and deactivation with a catalyst comprising Co–La–Ru–SiO 2 . A cobalt catalyst used with a temperature of 220 °C for FTS may also cause WGS activity and an increase in methanation rates [3]. The technique most commonly applied when studying the effect of CO 2 is the co-feeding of CO 2 in the feed gas during low-temperature FTS [13–19], but relatively little of the published research [13–21] has dealt with the effect of CO 2 on cobalt-based FTS. Furthermore, the chemical utilization of CO 2 as a carbon resource is important from both the economic and environmental standpoints [22]. There have been various attempts to transform carbon dioxide into hydrocarbons, mainly by using catalysts that have been proven to be active in FTS, such as Ni, Ru and Co [23]. Although the need for CO 2 separation before the syngas is used 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.04.045 ⇑ Corresponding author. Tel.: +27 (0)11 7177527; fax: +27 (0)11 7177604. E-mail address: diane.hildebrandt@wits.ac.za (D. Hildebrandt). Chemical Engineering Journal 193–194 (2012) 318–327 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej