Synergy of Tetraalkylammonium and Polyquaternium Enables Zeolite Membranes with Exceptional Gas Separation Performance

Crystalline microporous membranes and films hold great promise for separation and catalysis; but their molecular sieving performance is compromised by boundary defects generated during conventional hydrothermal/solvothermal synthesis. Herein, a dual-ammonium synthesis (DAS) strategy is presented to fabricate high-silica zeolite membranes, where novel polyquaterniums promote the nucleation within interparticle gaps, while a classic tetraalkylammonium facilitates zeolite crystallization. The synergistic interplay between the two ammoniums balances in-plane and out-of-plane growth of the zeolite layer, yielding thin yet defect-scarce zeolite membranes. The generality of the DAS approach is demonstrated by fabricating a couple of high-quality SSZ-13 membranes through the combination of the tetraalkylammonium with various polyquaterniums, introduced either in the gel or seed suspension. Compared to conventional single-ammonium synthesis, the DAS strategy produces thinner and higher-performance zeolite membranes in half the synthesis time. The optimized membrane achieves a CO₂ permeance of 1780 × 10^-9 mol m^-2 s^-1 Pa^-1 and a CO₂/CH₄ selectivity of 460 for an equimolar CO₂/CH₄ mixture, surpassing most reported membranes. This work establishes the DAS strategy as an efficient and reproducible route to produce high-performance zeolite membranes with broad applicability.