Fischer–Tropsch synthesis over un-promoted and Re-promoted g -Al 2 O 3 supported cobalt catalysts with different pore sizes Øyvind Borg a,1 , Nina Hammer a,1 , Sigrid Eri b , Odd Asbjørn Lindva ˚g c , Rune Myrstad c , Edd A. Blekkan a , Magnus Rønning a , Erling Rytter a,b , Anders Holmen a, * a Department of Chemical Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway b StatoilHydro R&D, Research Centre, Postuttak, NO-7005 Trondheim, Norway c SINTEF Materials and Chemistry, NO-7465 Trondheim, Norway 1. Introduction Cobalt is favoured for the synthesis of long-chain hydrocarbons from natural gas-based synthesis gas because of its high activity, high selectivity to linear paraffins, high resistance towards deactivation, and low water–gas shift activity. In order to maximise the exposure of cobalt to gaseous reactants, the metal is normally dispersed on a high surface area support. In some cases, the support interacts strongly with the active phase. Metal– support interactions may leave a fraction of the cobalt chemically inactive after reduction.In order to reduce the amount of non- reduced cobalt, a small amount of a second metal can be introduced into the catalyst system. Rhenium is a frequent choice as reduction promoter. The first application of rhenium in combination with cobalt for Fischer–Tropsch synthesis was reported in 1986 by Mauldin [1]. It is generally accepted that rhenium mainly catalyses the reduction of cobalt species inter- acting with the support. Hilmen et al. [2] concluded that rhenium promotes reduction by hydrogen spillover,and that no direct contact between cobalt and rhenium is necessary in order to obtain the promoting effect. Although the Fischer–Tropsch activity usually increases when Re is present, the increase can often be proportionally correlated with the increase in cobalt dispersion. Thus, the site-time yield is constant [3–6]. While it is well known that Re facilitates the reduction of cobalt oxide, the effect of Re on the product selectivity is not equally clear. Bertole et al. [5] concluded from steady-state isotopic transient kinetics analysis that Re, at a Re/Co weight ratio of 0.1, does not influence the Fischer–Tropsch selectivity. Similar results have been found by others [3,4]. On the other hand, Schanke et al. [7] observed that Re increased the selectivity to long-chain hydro- carbons for cobalt supported on low (15 m 2 /g) and high surface area (190 m 2 /g) alumina.Storsæter et al. [6] investigated the Fischer– Tropsch synthesis behaviour of un-promoted and Re-promoted Catalysis Today 142 (2009) 70–77 A R T I C L E I N F O Article history: Available online 11 February 2009 Keywords: Fischer–Tropsch synthesis Cobalt Rhenium Alumina Pore size Cobalt-time yield Site-time yield C 5+ selectivity A B S T R A C T The effect of rhenium on the Fischer–Tropsch synthesis activity and selectivity of g -Al 2 O 3 supported cobalt catalysts was investigated in fixed-bed reactors at T = 483 K, P = 20 bar, and H 2 /CO = 2.0. Catalysts containing 20 wt.% cobalt and 0 or 0.5 wt.% rhenium were prepared by incipient wetness impregnation of different g -Al 2 O 3 supports with aqueous solutions of cobalt nitrate hexahydrate and for the Re- promoted catalysts, also perrhenic acid. The g -Al 2 O 3 supports had very different pore characteristics. The post-calcination Co 3 O 4 crystallite size was predominantly controlled by the g -Al 2 O 3 support pore diameter. Presence of Re had only a minor effect on the crystallite size. For all catalysts, supported Co 3 O 4 was reduced in two steps to Co 0 with CoO as intermediate species. However, while reduction of Co 3 O 4 to CoO took place in the same temperature range for all catalysts, the reduction temperature of CoO to Co 0 was dependent on the catalyst properties. Large particles present in wide pores were easier to reduce than small particles located in narrow pores. In addition, Re promoted the reduction of CoO. The effect of rhenium as a reduction promoter was less pronounced at increasing pore size and particle size. Re also had a similar positive impact on the cobalt dispersion of the catalysts.Although Re significantly increased the Fischer–Tropsch synthesis cobalt-time yield, it did not modify the site-time yield.The deactivation rates of all the catalysts were also similar up to 100 h on stream. Positive correlations were found between the catalyst pore diameter and the C 5+ selectivity and between the cobalt particle size and the C 5+ selectivity.Re had a consistent positive, albeit small,effect on the C 5+ selectivity. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +47 73 59 41 51; fax: +47 73 59 50 47. E-mail address: anders.holmen@chemeng.ntnu.no (A. Holmen). 1 Present address: StatoilHydro R&D, Research Centre,Postuttak, NO-7005 Trondheim, Norway. Contents lists available at ScienceDirect Catalysis Today j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c a t t o d 0920-5861/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2009.01.012