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.