Activity decline and diffusional changes in a magnesium-promoted cobalt-silica catalyst

Abstract

A Co-Mg/diatomaceous earth catalyst was prepared by slow precipitation. Transmission electron microscopy (TEM) indicated two separate phases: small needles of a Co-Mg phase and the large particles of the diatom fragments. Nitrogen sorption measurements indicated much higherpore volume (0.32 cm 3 g ) for the catalyst than for the control catalyst made by fast precipitation (0.10 cm 3 g ). The catalyst activity in the Fischer-Tropsch synthesis at 155 kPa declined at an unusually fast rate. The decline was accompanied by an increase of the methane selectivity and a decrease of the Schulz-Flory growth factor. Hydrogen treatments temporarily restored the catalyst activity but did not arrest the declines. Increasing the H 2:CO feed ratio for the deactivated catalyst not only restored the high activity but also stabilized the catalyst. An increase of the methane selectivity and a small decrease of the growth factor also accompanied these changes. The changes in catalyst activity and product selectivity correlate well with the anticipated diffusional changes from gas phase diffusion to diffusion through liquids which have been deposited in the pores during the course of the reaction. It is inferred that the diffusional changes cause significant changes in the steady-state concentrations of the reaction intermediates and these, in turn, affect the reaction rate (conversion) and the chain termination (growth factor).