Engineering hierarchical beta zeolites with isolated metal centers for adjustable catalytic functionality

Abstract

Hierarchical zeolites confine nanoscale metal species that hold great appeal for multifunctional catalysis, although controlled synthesis still remains a challenge. In this paper, hierarchical Beta (hBEA) zeolite confining subnanometric metal species was synthesized with the strategy of cooperative ethylenediamine (en) ligand protection and seed-assisted crystallization in the presence of the unique tetraquaternary ammonium structure-directing agent (SDA). Such a cooperative synthesis environment allowed SDA to direct mesostructure formation while bulk BEA seed maintained framework topology, leading to an interconnected micro-mesoporous architecture with improved textural properties. The en-ligand protection effectively suppressed metal hydroxide precipitation under strongly basic hydrothermal conditions, allowing for the confinement of Pd, Co, Ni, and Cu metal centers within the framework. Characterization by XRD, XPS, and HAADF-STEM confirmed the uniform dispersion of isolated and ultra-small metal nanosites without metallic nanoparticle formation. Building on this design, dimetallic Pd-Co, Pd-Ni, and Pd-Cu@hBEA catalysts were prepared using preformed metal-containing seeds as both structural templates and secondary metal sources, achieving homogeneous co-distribution of both metals. This study demonstrates a versatile and generalizable route for constructing multimetallic hierarchical zeolites capable of stabilizing multiple active centers, offering a robust platform for designing selective and sustainable catalysts for tandem reactions.