Organic-free, Ultrafast Synthesis of K-CHA Nano-aggregates with Various Morphologies and Their Adsorption Performances

K-Chabazite (K-CHA) zeolites were synthesized with an ultrafast procedure through an eco-friendly and cost-effective “one-pot” method. By solely employing colloidal silica and aluminum hydroxide in the K⁺/Sr²⁺ system without the assistance of seeds, fluorides or organic structure as directing agents, the K-CHA nano-aggregates were successfully synthesized in a few hours. The crystallization behavior of the gels with different SiO₂/Al₂O₃ molar ratios (SARs) was investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray fluorescence (XRF), ²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance (MAS NMR), Fourier-transform infrared spectra (FTIR), UV-Raman characterizations. As the nucleation rates were changed with the SARs, three kinds of K-CHA nano-aggregates, namely, hamburger-, disk- and walnut-shaped, were obtained. A possible formation mechanism of the K-CHA nano-aggregates was proposed. Additionally, the walnut-shaped sample with an SAR of 7 possessed the largest pore volume and specific surface area, resulting in the highest methane adsorption capacity. Furthermore, it exhibited a CH₄/N₂ selectivity of more than 3 under relative pressures ranging from 0 to 0.6. This work offers guidance for modifying the stacking modes of other nano-sized zeolites.