Effect of pretreatment atmosphere on the activity and selectivity of Co/mesoHZSM-5 for Fischer–Tropsch synthesis

The structure and catalytic performance of bifunctional 10 wt% Co/mesoHZSM-5 catalysts pretreated under different conditions, i.e. in stagnant air, or in a flow of air, N2, or 1 vol% NO/Ar, were investigated for the Fischer–Tropsch synthesis (FTS) under fixed operating conditions of T = 513 K, P = 15 bar, H2/CO = 1. The combination of acid sites and FTS functionality leads to the direct formation of gasoline range hydrocarbons and suppresses the formation of C20+ products. The highest activity, C5–C11 selectivity and lowest CH4 selectivity were obtained for Co/mesoHZSM-5 catalyst pretreated in stagnant air. Pretreatment in gas flow resulted in a lower activity and C5–C11 selectivity, and in a higher CH4 selectivity, in particular for samples pretreated with NO. Characterization shows that this underperformance is due to changes in the Co3O4 particle size distribution and cobalt reducibility, and is related to the cobalt loading relative to the mesopore area. Pretreatment in air or N2 flow increased the number of small Co3O4 particles and increased cobalt reducibility by suppressing the formation of highly dispersed cobalt, e.g. cobalt silicates, in strong interaction with mesoHZSM-5. Pretreatment in a 1 vol% NO/Ar flow significantly increased cobalt dispersion further, decreasing the cobalt reducibility due to the strong interaction between cobalt and mesoHZSM-5. Based on both TEM and in situ DRIFTS studies, the optimum performance of Co/mesoHZSM-5 pretreated in stagnant air could be attributed to a lower fraction of small cobalt particles, known to promote the formation of CH4via hydrogenolysis or direct methanation. Additionally, small cobalt particles are more susceptible to be oxidized under FT conditions, thereby decreasing FT activity and indirectly increasing CH4 selectivity by increasing the H2/CO ratio through the water gas shift reaction.