Ionic Liquid-Assisted Synthesis of Nanosized ZSM-22 Zeolites with Enhanced Hydroisomerization Performance

Abstract

Advancing the conventional hydrothermal and ionic liquid-assisted synthesis methodologies to produce ZSM-22 zeolites with similar characteristics yet notably distinct defect sites holds substantial significance in elucidating their influence on shape selectivity and hydroisomerization performance. This study investigated the impact of the initial gel ratio and synthesis parameters on the ionic liquid-assisted crystallization process and the crystal size of the ZSM-22 zeolite. The NH₄F feed exhibits a minimal effect on the crystallization process, in contrast to the beneficial effect of improved ionic liquid and decreased water feed on substantially diminishing the crystal size of the resulting zeolites, which exhibit a morphology similar to that of the ZSM-22 zeolite obtained through the conventional synthesis route. Furthermore, the investigation into the crystallization process revealed that both ionic liquids and high temperatures play indispensable roles in accelerating the nucleation and crystal growth of ZSM-22 zeolite. When applied in the n-dodecane hydroisomerization, the nanosized ZSM-22 zeolites prepared by ionic liquid-assisted route exhibit remarkable isomer yield (above 72%), surpassing the conventional and large-size ZSM-22 zeolites. After the product distribution analysis, it is evident that the fewer Si–OH defect sites and the correspondingly strict confinement environment effectively limit the formation of the multibranched intermediates prone to crack, which ultimately improves the hydroisomerization performance, intrinsically reflecting the shape selectivity of the 10-membered ring zeolite by eliminating the interference of the framework defect sites. This work emphasizes the exceptional hydroisomerization performance of the ZSM-22 zeolite, highlighting the significance of its efficient synthesis and wide application.