Synthesis and Characterization of Iron-Substituted ZSM-23 Zeolite Catalysts with Highly Selective Hydroisomerization of n-Hexadecane

A series of Fe-substituted ZSM-23 samples with different Fe/(Fe + Al) ratios were synthesized. Combined characterization by Fourier transform infrared spectroscopic, ESR, and X-ray photoelectron spectrosopic methods confirmed the presence of Fe in the ZSM-23 framework. The crystal size decreased with increasing Fe/(Fe + Al) ratio, and the Fe-substituted ZSM-23 samples exhibited large specific areas and pore volumes. The introduction of Fe in the framework reduced the acid strength compared with the Fe-free counterparts. Compared with the Pt/Fe-free ZSM-23 catalyst, the Pt/Fe-substituted catalysts showed lower activity to the hydroisomerization of n-hexadecane and higher isomerization selectivity. This is attributed to their higher ratios of weak and strong Brønsted acidic sites and smaller crystal size, which improved the product diffusion and suppressed the further product cracking. More 5M-C15 isomers were obtained over the Pt/Fe-substituted catalysts, which was different from the product distribution of Pt/Fe-free ZSM-23 where 2M-C15 isomers were the major isomer products.