Fischer-Tropsch synthesis on cobalt catalysts: the effect of water

Edd Anders Blekkan, Øyvind Borg, Vidar Frøseth and Anders Holmen

DOI: 10.1039/b601307b

1. Introduction

Modern GTL (Gas-to-Liquids) technology involves Fischer-Tropsch synthesis for converting natural gas derived synthesis gas to transportation fuels. The Fischer -Tropsch synthesis (FTS) produces a complex mixture of hydrocarbons, consisting of methane, C2+ olefins and paraffins (linear and branched) and oxygenates (mainly alcohols). The product distribution is very dependent on the type of catalyst used and on the reaction conditions. The active catalysts for Fischer-Tropsch synthesis are Fe, Co and Ru. Fe has a high water–gas shift activity and is used when the synthesis gas is produced from coal, i.e. when the water–gas shift reaction is desirable due to low H2/CO ratios in this syngas. Supported cobalt is the preferred catalyst for the Fischer-Tropsch synthesis of long chain paraffins from natural gas due to their high activity and selectivity, low water–gas shift activity and comparatively low price. A key element in improved Fischer-Tropsch processes is the development of active catalysts with high wax selectivity.

The Fischer-Tropsch synthesis follows a polymerization mechanism where a C1 unit is added to the growing chain. A simplified representation of the reaction network is shown in Fig. 1, where the key points are termination by either H -abstraction to give α-olefins or by hydrogenation to give n-paraffins.

The main secondary reactions are hydrogenation and readsorption of primary olefins. However, the reaction network is very complex and involves a large number of reactions. Although the Fischer-Tropsch synthesis has been known since the 1920's the exact mechanism is still a matter of debate. Three mechanisms have been proposed based on different species as the monomer: In the original carbide mechanism proposed by Fischer and Tropsch, CHx is the proposed monomer, in the enol mechanism proposed by Storch et al. oxymethylene (HCOH) is the species responsible for chain growth and in the CO insertion mechanism proposed by Pichler and Schulz the chain growth occurs through the insertion of CO into the metal–methyl bond. The carbide mechanism has been favored for a long time, but a recent study points to the CO insertion route as a more likely mechanism.

Mass transfer effects are very important for the selectivity in the Fischer-Tropsch synthesis. Even though the reactants are in the gas phase, the catalyst pores will be filled with liquid products. Diffusion in the liquid phase is about 3 orders of magnitude slower than in the gas phase and even slow reactions may become diffusion limited. Diffusion limitations may occur through limitation on the arrival of CO to the active points or through the limited removal of reactive products.

In the cobalt-catalyzed Fischer-Tropsch reaction, oxygen is mainly rejected as water and this will generate high partial pressures of water at the reactor exit for fixed-bed reactors. As a consequence of extensive back mixing in slurry reactors, high water concentrations and low reactant concentrations will exist throughout the entire reactor. Water will therefore always be present in varying quantities during the reaction. It has been shown that water affects the activity as well as the selectivity for the Fischer-Tropsch synthesis, but different results have been reported for the effect of water. Even though water apparently influences the activity of various Co catalysts in different ways, water increases the C5+ selectivity and decreases the CH4 selectivity for all Co catalysts.

Different explanations have been proposed to explain this effect, including the influence of water on the adsorbed carbon species on the surface and the reduction of secondary hydrogenation of primary olefins by water, thereby facilitating olefin readsorption and chain initiation.

There are contradictory observations on the effect of water on the performance of Co catalyst using different supports, and an obvious common explanation of the observed effects is not available. The purpose of the present work is to compile and review the studies dealing with the effect of water on cobalt based FTS catalysts. As mentioned, water influences the FT-synthesis in many ways, and we summarize the literature in terms of 3 main areas; deactivation, activity (kinetics) and selectivity. Modern cobalt catalysts are supported, in order to obtain high cobalt surface areas and high catalyst activities. The supports commonly used for cobalt-based Fischer-Tropsch catalysts are metal oxides such as alumina, silica and titania, and the major technology providers offering catalysts or processes use these supports. There are also studies using other supports as well as unsupported cobalt. The different supports used have differing properties, in terms of chemical composition and purity, degree of interaction with the cobalt, and also physical properties (porosity, pore sizes). Here we summarize the results obtained on the various systems, showing that the effect of water is indeed dependent on the support used.

2. Deactivation

Deactivation is a common and important phenomenon in FTS. Deactivation effects of water are recorded on all commonly used supports. The suggested mechanisms include oxidation, sintering and solid state reactions rendering cobalt inactive.

2.1 Unsupported cobalt

Das et al. studied FTS over bulk cobalt in a CSTR, and observed some deactivation at high levels (>20%) of water addition. Bertole et al. studied bulk cobalt in a SSITKA experiment, and found no deactivation using a dry feed, but significant loss of activity when water was added to the feed. Most of this activity could be regained by a treatment in hydrogen (re-reduction), indicating that oxidation of cobalt is responsible for some of the deactivation, but a fraction of the activity was not recoverable this way, and this was attributed to sintering due to the high steam partial...