Relation Between Catalyst Properties and Selectivity in Fischer-Tropsch Synthesis

Abstract

Fischer-Tropsch synthesis reaction was investigated on alumina (Al2O3) phases i.e Gamma (γ), Delta (δ), Theta (Θ), and Alpha (α). By using heat treatment, δ-, Θ-, and α- Al2O3 phases were prepared from γ- alumina as the initial phase. In order to study effect of pore size, γ- Al2O3 samples with two different pore size distributions were selected. Hence, the other phases produced from the respective -Al2O3 phase were classified as medium pore (MP) and small pore (SP) series.

Catalysts were prepared with 12, 20 and 30 wt.% Co loading with onestep incipient wetness impregnation method. In the impregnation solution, the mixture of ethylene glycol and deionised water was varied to control Co3O4 particle sizes. For all catalysts, a common precursor i.e., cobalt nitrate hexahydrate was used, and they were calcined under identical conditions of temperature and air flow. All catalyst samples were characterised by standard techniques such as H2-chemisorption, porosity measurements, temperature-programmed reduction, and oxygen titration. Fischer-Tropsch (F-T) synthesis reaction was carried out on all samples by using a fixed-bed reactor at 20 bar, 483 K, and H2/CO=2.1.

For constant cobalt particle sizes, mainly in the range of 8-10 nm, the higher C5+ selectivity was observed for δ– and α–Al2O3 samples compared to γ– and θ–Al2O3,with α–Al2O3 giving the highest selectivity. In addition, MP-series catalysts exhibited higher C5+ selectivity than those of SP-series catalysts. Steady-state isotopic transient kinetic analysis (SSITKA) showed that the amount of CHx surface intermediates followed the same trend as the C5+ selectivity with respect to pore size and alumina phase.

The relation between C5+ selectivity and activity was also studied with different cobalt particle sizes. A maximum in the C5+ selectivity was observed for δ– and θ–Al2O3for a Co particle size of 8-10 nm. In absence of any diffusion limitations, the olefin to paraffin ratio followed the trend of the C5+ selectivity with respect to the Co particle size.

Fischer-Tropsch synthesis reaction was studied with cobalt supported on platelet and fishbone carbon nanofibers which were grown on carbon-felt material. In contrast to the platelet, cobalt catalyst supported on fishbone carbon nanofiber/felt showed better heat and mass flow properties. However, both catalysts deactivated after certain hours under reaction conditions. With the help of transmission electron microscopy, the morphology of alumina phases, cobalt catalysts supported on alumina as well as carbon nanofiber/carbon-felt was studied. The stability of cobalt catalyst supported on α-Al 2O3and fishbone carbon-nanofiber/carbon-felt was compared with carbon nanofiber/felt.