Effect of Si/Al Ratio on CoZr/MFI Zeolite Composite Catalysts for Direct Isoparaffins Synthesis From Syngas

The effect of acidity on direct isoparaffins synthesis in CO hydrogenation over CoZr/MFI (HZSM-5 and Silicalite-1) zeolite composite catalysts was investigated using zeolites with different Si/A1 ratios by a direct hydrothermal synthesis method. The composite catalysts were characterized by XRD, SEM, H₂-TPR, NH₃-TPD and XPS, and were comparatively investigated for Fischer-Tropsch synthesis in a single fixed-bed reactor under the conditions of 2.0 MPa, 533K, H₂/CO=2.0, 1000h^-1. The effect of the Si/Al ratio on the selectivity of isomers was correlated with the acidity and crystallization of the MFI zeolites, and the Co-support interaction, which strongly affected the Co reducibility, was also explored. The composite catalyst with a Si/Al ratio of 40 exhibited significantly higher selectivity of isoparaffins (23.72 wt% of iso-C_5-11) and lower methane selectivity (14.35 wt%) than other composite catalysts, while the CoZr/Silicalite-1 catalyst showed the largest amount of heavy hydrocarbons (11.92 wt% of C₁₈₊) and the lowest selectivity to isoparaffins (only 2.00 wt% of iso-C_5-11) due to no Al source was introduced in the hydrothermal synthesis. The results indicate that zeolite composite catalyst with low Si/A1 ratio can be used to efficiently realize high selectivity of isoparaffins and low selectivity to methane from syngas.