The Effect of Potassium on Cobalt-Based Fischer–Tropsch Catalysts with Different Cobalt Particle Sizes

Gavrilović Gavrilović,Blekkan Blekkan,Save Save

doi:10.3390/catal9040351

Gavrilović Gavrilović, Blekkan Blekkan + Show 1 more

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https://doi.org/10.3390/catal9040351

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Abstract

The effect of K on 20%Co/0.5%Re/γ-Al2O3 Fischer–Tropsch catalysts with two different cobalt particle sizes (small, in the range 6–7 nm and medium size, in the range 12–13 nm) was investigated. The catalyst with the smaller cobalt particle size had a lower catalytic activity and C5+ selectivity while selectivities towards CH4 and CO2 were slightly higher than over the catalyst with larger particles. These effects are ascribed to lower hydrogen concentration on the surface as well as the lower reducibility of smaller cobalt particles. Upon potassium addition all samples showed decreased catalytic activity, reported as Site Time Yield (STY), increased C5+ and CO2 selectivities, and a decrease in CH4 selectivity. There was no difference in the effect of potassium between the sample with small cobalt particles compared to the sample with medium size particles). In both cases the specific activity (STY) fell and the C5+ selectivity increased in a similar fashion.

Highlights

Biomass to liquid (BTL) via gasification and integrated Fischer–Tropsch synthesis (FTS) is an attractive process for the production of green diesel and jet fuel [1]
To better understand the nature of the effects reported on the cobalt-based FTS catalysts upon alkali addition, here we report an investigation of the effect of potassium on alumina-supported cobalt catalysts containing small or medium sized cobalt particles
The cobalt particle size was successfully altered without changing the composition of the active material or the physical properties of the support material

Summary

Introduction

Biomass to liquid (BTL) via gasification and integrated Fischer–Tropsch synthesis (FTS) is an attractive process for the production of green diesel and jet fuel [1]. A series of cleaning steps must be applied to fulfil the requirement for pure gas before entering the FTS reactor [4]. The catalyst loss in activity and changes in selectivity that was observed were explained by the hypothesis that potassium is able to move to the cobalt sites responsible for FTS when reactions or the pre-treatment conditions are reached [11,12,13]. Once the potassium reaches the cobalt particles there are very low barriers against transport on the cobalt surface, as calculated using DFT, and the adsorption of K is favourable on all sites, including sites such as the B5 and B6 sites often considered active sites for the FTS [15]

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Conclusion