Directing Ultramicropore Architecture in Carbon Molecular Sieve Membranes with Lanthanum for High‐Fidelity Gas Separation

A crucial performance ceiling has long limited carbon molecular sieve (CMS) membranes in demanding gas separations, a limitation stemming directly from the inability to precisely dictate their ultramicroporous architecture. Herein, we demonstrate a novel lanthanum (La)‐doping strategy to precisely control the structural evolution of polyimide‐based carbon CMS membranes. Lanthanum incorporation fundamentally alters pyrolysis dynamics by significantly increasing aromatic strand mobility and enabling La complexes to serve as nanoscale Langmuir templates. This entropic‐driven templating effect guides the alignment of carbon strands and optimizes their narrow arrangement, resulting in a markedly narrow pore size distribution for precise gas separation. La‐doped CMS membranes exhibit exceptional molecular sieving capabilities, notably, achieving 100‐fold increase in H 2 /CH 4 selectivity while maintaining high H 2 permeability. Even under realistic operating conditions involving variations in pressure, feed composition, and temperature, the La‐doped CMS membranes retained their high H 2 permeability and selectivity, showing minimal sensitivity to feed composition and pressure and excellent resistance to physical aging. This straightforward, versatile, and scalable metal‐doping approach presents a significant advancement for developing high‐performance CMS membranes, opening new opportunities for challenging industrial gas separations that require precise differentiation of molecules with similar kinetic diameters.