Structural Flexibility of Hydrated RHO Nanosized Zeolite Synthesized via Green Synthesis Approach at Subfreezing Conditions

Understanding the structural flexibility of zeolites under cryogenic conditions is essential for optimizing gas separation and storage performance. This study investigates nanosized RHO zeolite synthesized via green synthesis (without organic structural directing agent) upon hydration and cooling to low temperatures (<273 K) using in situ XRPD, in situ FTIR spectroscopy, and DFT simulations. Template-free synthesis is performed at low temperature (363 K), avoiding calcination or postsynthetic activation, yielding highly crystalline nanosized zeolite with minimal energy consumption and no toxic by-products. Upon hydration at 300 K, nanosized RHO zeolite adopts a two-phase expanded-contracted structure due to distinct water-cation interactions. Upon cooling to 248 K, the hydrated zeolite transitions into a single expanded phase, remaining stable after reheating to 300 K, forming a metastable state. In situ FTIR analysis indicates freezing-induced water molecule rearrangement leads to persistent hydrogen-bonding networks, preventing structural reversion. This metastable state exhibits CO₂ adsorption capacities comparable to conventionally activated RHO zeolite (623 K), achieved through significantly lower energy input. This performance underscores the viability of mild, green chemistry-aligned activation approaches eliminating energy-intensive high-temperature treatments. This novel approach contributes to sustainable separation processes and provides a blueprint for future innovation in porous materials guided by green chemistry principles.