Adjusting gate-opening behavior in a rigid cage-type “molecular trapdoor” metal–organic framework via anion modulation

Exclusive admission of targeted gas molecules by porous materials, i.e., molecular sieving, is highly sought-after to afford highly selective separation but is challenging when gas molecules have similar sizes. Although the "molecular trapdoor" mechanism discovered in zeolites allows for addressing this challenge, the limited adaptability of zeolites constrains the full harnessing of this mechanism. In this study, we have identified a cage-type of stimuli-responsive metal–organic framework (MOF) as a "molecular trapdoor" material through a combined experimental and simulation approach by adjusting its gate-opening behavior via anion modulation. The temperature-dependent gas admission observed does not merely result from the thermal motion of "gate-keeping" anions, but rather emerges from their interaction with guest molecules within the pore. By modifying the "gate-keeping" groups, we achieved the highest selectivity of CO2/N2 (697) and CO2/CH4 (22) at 273 K in ClO4-BTR MOF in this study. Additionally, we have identified a consistent pattern governing the diffusion rate during the gas adsorption process among NO3-BTR, BF4-BTR, and ClO4-BTR MOFs concerning variations in temperature and pressure. This work unveils the fundamental mechanism and enhances our understanding of gas admission in a MOF featuring pore-blocking anions, thus expanding the prospective applications of MOFs as "molecular trapdoor" adsorbents and offering valuable guidance for the strategic design of ionic MOFs with specific gas separation characteristics.