Ultrathin Covalent Organic Framework Membranes with Tailorable Porous Channels for High-Permeance Helium Separation

Covalent organic frameworks (COFs) are a unique class of porous crystalline materials featuring precisely ordered structures and well-tunable porous channels as well as abundant active functional groups, which enable them as promising membranes for high-performance gas separation. However, construction of thin COF membranes to simultaneously realize high-permeability and high-selectivity He separation has remained a challenge. Here, we report the synthesis of ultrathin (<5 nm thick) porphyrinyl two-dimensional polyimine (Por-2DPI) COF membranes, which comprise single-crystalline domains and highly ordered, vertically aligned one-dimensional (1D) porous channels. We further develop an inner-pore coordination (IPC) strategy to graft bulky phenyl (Ph) and naphthyl (Np) groups on the inner walls of the porous channels. This strategy effectively narrows the intrinsic pore size from ∼2.0 to ∼1.4 nm, which leads to the efficient tailoring of the transmembrane transport toward the high-permeance He separation with high selectivity. Notably, the resultant Por-2DPI-Np COF membrane simultaneously delivers a record-high He permeance of 4323 GPU and a high selectivity of 77.4 (He/CH4), superior to the state-of-the-art reported membranes. The contrast experiments supported by theoretical modeling reveal that the exceptional performance arises from the synergistic effect, where the ultrathin feature and 1D pore arrays ensure high permeance. Meanwhile, the grafted branches impede the diffusion of CH4 more significantly than that of He, resulting in an enhanced molecular sieving effect for high selectivity. Our work provides a general strategy to overcome the permeability–selectivity trade-off and highlights the unprecedented potential of engineered COF membranes for highly efficient helium recovery.