Mesoporous H-ZSM-5-carbon composites, prepared via tetrapropylammonium hydroxide (TPAOH) post treatment of H-ZSM-5 followed by deposition of pyrolytic carbon, have been used as the support for the preparation of Co-based Fischer-Tropsch catalysts. The resulting catalysts display an improved performance during Fischer-Tropsch synthesis (FTS), with higher activity, higher selectivity towards C5-C9 (gasoline range) hydrocarbons and lower selectivity towards C1 (and C2) than Co/mesoH-ZSM5 (without pyrolytic carbon). This is due to the weaker metal-support interaction caused by the deposited carbon (as revealed by XPS) leading to a higher reducibility of the Co species. Further, the partial deactivation of the Brønsted acid sites by pyrolytic carbon deposition, as was observed by NH₃-TPD, allows the modification of the zeolite acidity. Both the olefin to paraffin (O/P) and the isoparaffin to normal paraffin (I/N) ratios decrease with the increase in the carbon content, opening the door to further tune the catalytic performance in multifunctional FTS operations.

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.