Unravelling acidity–selectivity relationship in the bifunctional process of Fischer-Tropsch synthesis and catalytic cracking

Fischer-Tropsch synthesis (FTS) was carried out over bifunctional model catalysts prepared by physically mixing precipitated iron-based FTS catalysts (P-Fe) and H-ZSM-5. We adjusted the acidity of model catalysts by varying the ratio of P-Fe/H-ZSM-5 and the ratio of Si/Al in the H-ZSM-5. The physical mixing of H-ZSM-5 was adequate to supply the Brønsted acid sites (BAS), which are nonexistent in the P-Fe, to the bifunctional model catalysts without deteriorating the catalytic activity of P-Fe. As a result, we found a linear correlation between the cracking rate and the BAS concentration in the bifunctional process of FTS and cracking for the first time. Furthermore, the model catalysts showed high CO conversion (73–78 %), comparable to that of P-Fe (73 %), and improved C5–C20 selectivity with increased BAS concentration. The highest C5–C20 selectivity obtained in this study (72 wt%) was twice higher than that obtained in the P-Fe (36 wt%).