Research Progress in Channel-Selective Dealumination of Mordenite for Humid Direct Air Capture of CO 2

Direct air capture (DAC) of CO₂ requires physisorbents that retain their selectivity in the presence of atmospheric water vapour. For zeolites the principal framework-level lever on water affinity is the Si/Al ratio, and the only post-synthetic route to raising it is dealumination. Mordenite (MOR) has become a focus of this effort because its 8-membered-ring (8-MR) side pocket, rather than its 12-membered-ring (12-MR) main channel, governs CO₂ uptake at 400 ppm, so dealumination must be directed at one channel while preserving the other. This paper reviews progress in channel-selective dealumination of MOR and in its evaluation for humid DAC. The underlying principles are summarised first: pore topology and T-site distribution, the evaluation criteria appropriate to trace-concentration humid conditions, the dual role of extra-framework cations as adsorption sites and as protecting agents, and the competing effects that dealumination produces. Four families of channel-selective strategy are then reviewed by category — conventional acid leaching and chelation, size-gated in-situ proton generation, cation site-protection combined with steaming, and silylation capping — together with recent progress in humid CO₂ adsorption performance. The strategies are compared in tabular form, and the characterisation methods appropriate to demonstrating channel selectivity are summarised. Finally, the remaining challenges and prospects for this field are outlined.