Confined facilitated transport within covalent organic frameworks for propylene/propane membrane separation

• COF-based membranes are utilized for C 3 H 6 /C 3 H 8 separation for the first time. • Large-sized COF pores can achieve maximum contact between Ag + and C 3 H 6 molecules. • High-density Ag + facilitates C 3 H 6 confined transport along nanochannel surface. • Density differences result in asymmetric structure and directional transport. • Confined mechanism with asymmetric structure provide superior separation performance. Energy-efficient membrane technology is sought to replace or integrate with conventional energy- and cost-intensive distillation for precise propylene/propane separation. Covalent organic framework (COF)-based composite membranes (CMs) are promising candidates for the representative difficult propylene/propane separation system. However, the large-size pore of COFs cannot achieve an effective separation of gas pairs with sub-angstrom (0.2 Å) size differences. Here, we design confined facilitated transport nanochannels within COFs as productive fillers to construct CMs. The anchored high-density Ag + carriers facilitate C 3 H 6 monomolecular high-speed transport along the nanochannel surface. Under this confined transport mechanism, the problem of over-sized COF pores can be partially circumvented, and high selectivity obtained. Moreover, the obtained asymmetric configuration of SCOF-Ag/PI CMs results in directional gas transport. As the first work of COF-based membranes for olefin/paraffin separation, SCOF-Ag/PI CMs demonstrate a high propylene permeability of 75.16 barrer and good propylene/propane selectivity of 35.45, out-performing most of the polymer-based membranes and approaching the performance of ZIF-8 membranes. Furthermore, SCOF-Ag/PI CMs exhibit over one-month durability. The excellent separation performance combined with operating stability provides a promising alternative approach toward efficient C 3 H 6 /C 3 H 8 separation.