Crystallization mechanism of hierarchical low silica X zeolite induced by chloride ions and its catalytic implications in low-temperature NH3-selective catalytic reduction

Halide ions are known to influence zeolite crystallization, yet the underlying mechanisms remain insufficiently understood. Herein, we elucidate the role of chloride ions (Cl − ) in the template-free synthesis of hierarchical low silica X (LSX) zeolite nanosheets, providing mechanistic insights into ion-directed crystal growth. Combined comprehensive characterizations and density functional theory (DFT) calculations reveal that Cl − exhibits moderate adsorption strength toward the faujasite ( FAU ) framework compared with other halides. This unique interaction stabilizes the assembly of FAU building units and promotes the formation of hierarchical porous structures. Notably, Cl − preferentially adsorbs on the (111) facet of growing crystals, directing oriented crystal growth along this plane. As these oriented nanosheets assemble, a mesoporous structure emerges, yielding a hierarchical zeolite. The structural advantages of the resulting LSX-Cl zeolite further enable highly dispersed Cu 2+ species with enhanced accessibility, facilitating strong NH 3 coordination at low temperatures and thereby leading to improved activity in NH 3 selective catalytic reduction (SCR). This work not only unravels the halide-driven crystallization mechanism of hierarchical zeolites but also establish an interface-guided design principle for functional zeolite materials.