Engineering RHO Nanozeolite: Controlling the Particle Morphology, Al and Cation Content, Stability, and Flexibility

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.

Zeolites in Adsorption Processes: State of the Art and Future Prospects

“…These were first described by Shang et al in 2012, after observing remarkably high CO 2 /CH 4 selectivities on low silica exchanged chabazites . Since then, trapdoor phenomena in zeolite adsorption have received a great deal of attention and research effort, due to the large selectivities achievable. − These phenomena are usually susceptible to external stimuli, such as temperature, and are usually triggered by molecules that interact strongly with the extra framework cations present in the zeolite, such as water and carbon dioxide.…”

Role of Al Distribution in CO2 Adsorption Capacity in RHO Zeolites

“…From these observations, we may consider that the distribution of cations within the cage, as a consequence of the framework Al distribution (i.e., the increased presence of Q big species), becomes more favorable for a higher CO 2 adsorption capacity. The higher CO 2 adsorption capacity of 17-RHO compared to our previous report is due to the cooperative effect of low amounts of H 2 O with the Cs + EFCs. , H 2 O is less well removed during pre-treatment for TGA compared to equilibrium adsorption analysis at the same temperature, this behavior is under further investigation. With respect to the mode of adsorption, we have observed previously the formation of both physi- and chemisorbed CO 2 upon adsorption of CO 2 on 20-RHO .…”

“…Previously, we have reported the direct synthesis and characterization of four aluminosilicate Na,Cs-RHO nanosized zeolites prepared from homogeneous clear suspensions with different chemical compositions of highly solubilized Si and Al precursors (samples named 17-RHO, 18-RHO, 20-RHO, and 21-RHO). The syntheses were carried out using the same chemical precursors and synthetic procedure, with slight adjustments to the aging and crystallization times, as previously described. ,, The initial precursor suspensions used for the synthesis of RHO zeolites are summarized in Table S1. We do not impose any post-synthetic treatments or modifications to the Na,Cs-RHO nanosized zeolite samples examined in this work thereby reflecting the final arrangement of the framework atoms and EFCs following the crystallization step.…”

“…The flexible behavior of the different RHO zeolites can be described by the ellipticity (Δ) of the D8Rs (Table ). It is well known that the distortion of the RHO framework, and consequently the ellipticity of the D8Rs, can be tuned by the location of the EFCs and adsorption of guest molecules. , Δ can be numerically calculated by the average of the shortest and longest O–O distances within the cross-section of the D8R . These distances within the crystal structures were measured using VESTA …”

“…The high crystallinity of the samples was confirmed by PXRD analysis; the particle size and shapes of the samples have been described in our previous works. 31,32,34 1).…”