Hetero-Structured COF Membranes with Graded Pores for Efficient H₂ Purification

Constructing hetero-structured membranes with well-defined pore architectures is a promising strategy to enhance gas separation performance, yet achieving structural continuity across different layers remains challenging. Here, we report a hetero-structured covalent organic framework (COF) membrane featuring graded pores that transition from a microporous selective layer to a mesoporous support. Unlike conventional layer-by-layer assembly, our approach employs a pre-adsorption-induced heteroepitaxial growth strategy, wherein hydrazine monomers are electrostatically pre-organized within the channels of a sulfonated COF scaffold (TpBD-SO₃H). This spatial confinement directs the nucleation and growth of the secondary COF layer (TpHz), creating a chemically integrated and robust interface with a continuous pore size transition from 0.9 nm to 2.5 nm, as evidenced by an intermediate pore size of 1.9 nm arising from partial channel penetration. The resulting membrane exhibits an H₂ permeance of 1854 GPU and an H₂/CO₂ selectivity of 15, surpassing the 2008 Robeson upper bound. This work demonstrates that graded pore engineering offers an effective route to combine high permeability with molecular sieving precision in crystalline membrane design.