oa Fischer-Tropsch Synthesis Over Co/Al2O3 Catalyst-Effect of Pretreatment Procedures

Due to high activity of cobalt metal in syngas conversion to higher hydrocarbons and long chain normal paraffins, supported cobalt catalysts have been widely used for Fischer-Tropsch synthesis (FTS) reaction.

We investigated effect of pretreatment procedures on catalytic performance of 15wt% Co/Al2O3 catalyst prepared by incipient wetness impregnation (IWI). Catalyst was characterized by different methods such as BET, XRD, TPR, TGA, H2-TPD and TPH/TPO-MS. FTS was performed in a fixed-bed reactor at the following conditions: 593 K, 20 bar, H2/CO = 2 and gas space velocities of 3.85 NL/g-cat/h. The catalyst was activated in situ with either hydrogen at 648 K for 12 h, or syngas (CO/H2/He) or 10%CO/Ar at 623 K for 12 h.

BET surface areas of calcined catalyst was 154 m²/g. XRD pattern of calcined catalyst showed existence of cubic Co3O4 in addition to -Al2O3 phase. Metallic cobalt (Co fcc) was found after H2 reduction. Co°(fcc and hcp) and CoxC (x = 2, 3) were found in samples activated in syngas and 10%CO/Ar. Two step reduction of Co3O4 was observed in TPR and TGA experiments. Carbon deposition was observed during TGA using 10%CO/Ar as reducing agent. Degree of reduction was about 83% (H2-TPR following H2 isothermal activation) and dispersion 9.8%. Corrected value of crystallite sizes from combination of chemisorption and H2-TPR following hydrogen isothermal activation was 9.8 nm. TPH/TPO-MS results indicate existence of CoxC and different types of deposited carbon including graphitic carbon after syngas-activation.

CO conversion of 10%CO-activated catalyst was very low (7.4%) whereas the H2-activated catalyst was the most active (CO conversion of 58.8%). CH4 selectivity decreased and C+ selectivity increased, with increasing amount of H2 in the reducing gas. Moreover, the hydrogen reduced catalyst had the highest 1-olefin selectivity (C3-H5 hydrocarbons). These results show that surface carbide sites (created after 10%CO/Ar and syngas pretreatments) favor methane production and secondary hydrogenation of 1-olefins.

In conclusion, H2-activated 15wt% Co/Al2O3 catalyst had the best performance due to high degree of reduction and dispersion.