Tailoring Ultramicroporous Structures in Hybrid CMS Membranes Using Cyclodextrins for Enhanced Gas Separation

The development of high-efficiency carbon molecular sieve (CMS) membranes critically relies on precise control of ultramicroporous structures for efficient gas separation. In this study, cyclodextrins (CDs) with distinct molecular cavity sizes (α-, β-, and γ-CD) are employed as molecular-scale regulators to direct the evolution of pore structures in polyimide (PI)-derived CMS membranes through controlled pyrolysis. By systematically varying CD cavity sizes, the packing behavior of PI chains and the subsequent carbonization shrinkage are effectively regulated, leading to differentiated ultramicropore size distributions. This cavity-size-dependent pore engineering strategy enables a clear correlation among the molecular structure of CDs, carbon microstructure evolution, and gas transport behavior. Consequently, resulting hybrid CMS membranes exhibit substantially enhanced separation properties compared with both their precursors and pristine PI-derived CMS membranes, with optimized membranes carbonized at 550 °C going beyond the Robeson upper bounds for CO2/N2, CO2/CH4, H2/N2, and H2/CH4 separations. Particularly, γ-CD-derived CMS membranes achieve a favorable balance between permeability (PCO2 = 12186.6 Barrer, PH2 = 16314.1 Barrer) and selectivity (αCO2/N2 = 35.5, αCO2/CH4 = 41.7, αH2/N2 = 46.6, αH2/CH4 = 54.9), accompanied by stable long-term performance. These findings confirm that cyclodextrins’ incorporation with different cavity sizes provides an efficient approach to tailor the pore architecture of CMS membranes toward enhanced gas separation performance.