Tuning the gating energy barrier of metal-organic framework for molecular sieving

The role of adsorbent flexibility for adsorptive separation is controversial. Here, we show that the gating action, an unconventional type of flexibility, can be used to achieve molecular sieving. Two isostructural metal-organic frameworks possessing quasi-discrete/gating pores are designed and synthesized, which allow CO2 adsorption from extremely low pressures. Adding amino group on the organic ligand strengthens intra-framework hydrogen bonding, which increases the gating energy barrier to completely block N2 and CH4 with larger sizes, giving record-high CO2/N2 and CO2/CH4 selectivities. C2 and C3 hydrocarbons larger than the pore induce pore opening at significantly different pressures implying C2H4/C2H6 and C3H6/C3H8 molecular sieving. Quantitative mixture breakthrough experiments show CO2/N2 and CO2/CH4 molecular sieving and the first example of switchable C2H4/C2H6 adsorption preference, but reveal C3H6/C3H8 co-adsorption. Computational simulation and in situ single-crystal/powder X-ray diffraction show that the absence and presence of pore opening are responsible for molecular sieving and co-adsorption, respectively.