On the deactivation of cobalt-based Fischer-Tropsch synthesis catalysts

Abstract

The catalytic conversion of synthesis gas, derived from natural gas, into liquid hydrocarbon fuel via the Fischer–Tropsch synthesis (FTS), is currently receiving much attention due to the demand for environmentally friendly liquid fuel and the rising costs of crude oil. From an industrial perspective, both cobalt and iron catalysts have been applied. However, cobalt catalysts are preferred for gas-to-liquid (GTL) processes as they have high activity for FTS, high selectivity to linear hydrocarbons and low activity for the water–gas shift (WGS) reaction. As cobalt is relatively expensive, high catalyst stability is desired. Understanding deactivation is therefore an important objective in the field of cobalt-based FTS and was the main focus of this thesis. In Chapter 3, X-ray adsorption near-edge spectroscopy (XANES) was used to investigate the role of cobalt aluminate formation on the deactivation of Co/Pt/Al2O3 FTS catalysts. These catalysts, which were protected in a wax layer, were removed at various intervals from a 100-barrel/day slurry bubble column reactor, operated at commercially relevant FTS conditions. The amount of cobalt aluminate formed was small and it appeared that its formation was difficult during realistic FTS conditions. Using laboratory CSTR runs it was shown that water does seem to enhance aluminate formation but even at high water partial pressures of 10 bar, =10 wt% cobalt aluminate formed and a reduction was still observed compared to a fresh catalyst. It was proposed that the cobalt aluminate that formed resulted from existing CoO. The results obtained led to the conclusion that cobalt aluminate formation does not significantly influence the deactivation of cobalt-based catalysts during realistic FTS conditions. Following this finding, a review (Chapter 4) was undertaken on the topic of carbon deposition, which was postulated as another potential deactivation mechanism. It was clear that the FTS over cobalt-based catalysts occurred in the presence of an active surface carbidic over layer and in the presence of various hydrocarbon products. However, the conversion of active surface carbidic carbon to other inactive forms (for example bulk carbide, polymeric carbon and graphene) over time could result in deactivation and selectivity loss of the catalyst. Additionally, it is evident that non-desorbing, heavy hydrocarbon wax could lead to pore plugging and deactivation. From the available literature and regeneration patents it did seem that deactivation by carbon deposits is an important deactivation pathway for cobalt-based FTS catalysts under realistic conditions that warranted further study. In order to test the hypothesis that carbon deposition was a potential deactivation mechanism, a study was conducted on Co/Pt/Al2O3 FTS catalysts covered in a wax layer, taken from a 100-barrel/day slurry bubble column reactor operated at commercially relevant FTS conditions for an extended period and is described in Chapter 5. A wax-extraction procedure was developed and applied, enabling characterization of the catalyst by both surface techniques like X-ray photo-electron spectroscopy (XPS) as well as bulk techniques such as transmission electron microscopy (TEM) and temperature programmed (TP) hydrogenation and oxidation. The carbon deposits on the wax extracted catalysts were studied using TP techniques and it was found that there was a slow accumulation of a polymeric type of carbon species on the catalyst during the extended FTS run. This carbon was resistant to hydrogen treatments at temperatures above that used in realistic FTS. High sensitivity, low energy ion scattering (HS-LEIS), energy filtered transmission electron microscopy (EFTEM) and chemisorption analysis of samples containing this resistant polymeric carbon showed that it was dispersed largely over the support as well as on the cobalt phase. A large part of the activity of the catalyst could be recovered by removal of these polymeric carbon deposits and it was thus postulated that these play a role in deactivation of cobalt-based FTS catalysts in extended runs. Understanding the factors that contribute toward carbon deposition is an important step in trying to extend the lifespan of cobalt-based FTS catalysts. In Chapter 6, the impact of temperature and H2/CO ratio on the build-up of carbonaceous species on Co/Pt/Al2O3 catalysts was investigated using both model and realistic FTS tests. The influence of upset conditions on carbon deposition and it subsequent effect on catalyst structure was also investigated. It was found that both temperature and gas composition play important roles in determining the amount and reactivity of carbon deposits on Co/Pt/Al2O3 catalysts. An important factor in determining carbon deposition was the rate of hydrogenation of active carbon compared to the rate of transformation to more stable carbon species. The transformation of active carbon to more stable species occurred faster at higher reaction temperatures and lower H2/CO ratios. Upset conditions resulted in the production of carbon phases (Co2C, encapsulating carbons and filaments) that are detrimental to catalyst activity. Most of the work in the preceding chapters dealt with the study of deactivation using complex industrial catalysts. Chapter 7 discusses some preliminary results of new potential techniques that are able to shed light on the reactivity and morphology of cobalt nanoparticles by using both spherical and planar model catalysts. Spherical model cobalt catalysts were prepared by supporting cobalt nanoparticles on Stöber silica spheres. These were then investigated under different gas environments using in-situ TEM. Secondly, planar model catalysts were prepared by spincoating of preformed cobalt nanoparticles onto silica TEM grids and imaged after thermal treatment. Initial results obtained on the two model systems showed that there is a lot of potential for applying these techniques in future to obtain fundamental information on the reactivity and structure of cobalt FTS catalysts.