Down-sizing the crystal size of ZK-5 zeolite for its enhanced CH₄ adsorption and CH₄/N₂ separation performances

Down-sizing zeolite crystals to achieve enhanced gas adsorption and separation performances is largely unexplored and underestimated. Nano-sized zeolite can expose more adsorption sites and shorten the diffusion path compared to micro-sized counterparts; however, the synthesis of Nano-sized zeolite is still very challenging. Herein, nano-sized ZK-5 is successfully prepared for the first time via the regulation effects of β-cyclodextrin. The crystal size of ZK-5 can be reduced from micron-size (3 μm) to nano-size (50–100 nm), thus the nano-ZK-5 exhibits a superior surface area (370 cm² g⁻¹) and pore volume (0.22 cm³ g⁻¹) than the micron-sized sample (149 cm² g⁻¹ and 0.07 cm³ g⁻¹), which may be due to the reduction in crystal size, which in turn leads to more micropores being detected. The CH₄ adsorption capacity on nano-sized sample increased by 64% compared to that of micron-sized ZK-5. Notably, the record-high CH₄ adsorption capacity of 1.34 mmol g⁻¹ at 298 K is obtained on the nano-ZK-5 among the commercial zeolites. The equilibrium adsorption selectivity of CH₄/N₂ (20/80, v/v) mixed gas is as high as 4.2 (IAST method) at 1 bar and 298 K. The adsorption kinetics experiments have illustrated the boosted gas diffusion and mass transfer rate. Furthermore, the breakthrough experiments have confirmed the practical feasibility of separating various gas-mixtures, such as CH₄/N₂ (20/80, v/v).