Abstract
The engineering of RHO nanozeolite is demonstrated by synthesis from a colloidal precursor suspension using only inorganic structure-directing agents (Na$^+$, Cs$^+$), whereby the particle morphology, Si/Al ratio, cation content, stability, and flexibility are tailored. RHO nanozeolite with a higher Si/Al ratio (2.0) and superior thermal stability (up to 700 °C) compared to previous reports is synthesized. Optimization of the synthesis procedure by introducing additional Si precursors facilitated the targeted improvement in the Si/Al ratio while maintaining the nanosized dimensions of the discrete zeolite crystals with well-defined rhombic dodecahedral morphology. The structural properties of the RHO nanozeolites are characterized by in situ variable-temperature X-ray powder diffraction (XRPD) experiments showing that the nanozeolites possess a single structural phase up to 740 °C; further heating to 760 °C induces a symmetry change from noncentrosymmetric to centrosymmetric associated with a large increase in the anisotropic displacement parameter of the Cs$^+$ extra-framework cations. The structural behavior is unique compared to more siliceous Na$^+$ and Cs$^+$-containing RHO zeolites (Si/Al ≥ 3), which possess a centrosymmetric structure when hydrated. These experiments reveal a delineation, based on the Si/Al ratio and content of the extra-framework cations between the as-synthesized Na$^+$ and Cs$^+$-containing RHO zeolites that possess centrosymmetric or noncentrosymmetric symmetry when hydrated, as well as single or coexisting structural phases, expanding the scope of intelligently designed nanozeolites with tailored properties for precise applications.