Comparing the deactivation behaviour of Co/CNT and Co/γ-Al2O3 nano catalysts in Fischer-Tropsch synthesis

An extensive study of Fischer-Tropsch (FT) synthesis on cobalt nano particles supported on γ-alumina and carbon nanotubes (CNTs) catalysts is reported. 20 wt% of cobalt is loaded on the supports by impregnation method. The deactivation of the two catalysts was studied at 220°C, 2 MPa and 2.7 L/h feed flow rate using a fixed bed micro-reactor. The calcined fresh and used catalysts were characterized extensively and different sources of catalyst deactivation were identified. Formation of cobalt-support mixed oxides in the form of xCoO·yAl₂O₃ and cobalt aluminates formation were the main sources of the Co/γ-Al₂O₃ catalyst deactivation. However sintering and cluster growth of cobalt nano particles are the main sources of the Co/CNTs catalyst deactivation. In the case of the CO/γ-Al₂O₃ catalyst, after 720 h on stream of continuous FT synthesis the average cobalt nano particles diameter increased from 15.9 to 18.4 nm, whereas, under the same reaction conditions the average cobalt nano particles diameter of the Co/CNTs increased from 11.2 to 17.8 nm. Although, the initial FT activity of the Co/CNTs was 26% higher than that of the CO/γ-Al₂O₃, the FT activity over the Co/CNTs after 720 h on stream decreased by 49% and that over the CO/γ-Al₂O₃ by 32%. For the CO/γ-Al₂O₃ catalyst 6.7% of total activity loss and for the Co/CNTs catalyst 11.6% of total activity loss cannot be recovered after regeneration of the catalyst at the same conditions of the first regeneration step. It is concluded that using CNTs as cobalt catalyst support is beneficial in carbon utilization as compared to γ-Al₂O₃ support, but the Co/CNTs catalyst is more susceptible for deactivation.