Contents S1. Synthesis and Initial Characterisation S2. Crystallographic details of the refined hydrated Na-, (K-and K, Hand nd Cs-MER S3. Structural response to dehydration S4. Adsorption studies S5. In situ laboratory PXRD of M-MER with adsorbed CO2 S6. Crystallographic details of the refined dehydrated solids with adsorbed CO2 S7. CO2/CH4 separation and breakthrough curves S8. Kinetic measurements using the Zero Length Column technique S9. K,H-MER zeolite structural and adsorption results S2 S1. Synthesis and Initial Characterisation Synthesis Colloidal silica, Ludox HS-40 (12.5 g; 40%, suspension in water; Sigma-Aldrich) was added to 35% aqueous solution of tetraethylammonium hydroxide (3.15 g; 35% TEOAH, Sigma-Aldrich) and the resulting mixture was stirred for 1 h. To this mixture, a solution made by dissolving metal Al (0.8 g, 99%, Alfa Aesar) in 3 g TEAOH and KOH (0.6 g, 85%, Fisher Chemicals), which was also mixed for 1 h, was added. The gel formed was continuously stirred for 10 min, transferred to a PTFE-lined stainless-steel autoclave and hydrothermally treated at 423 K under slow rotation (60 rpm). The resultant solid product, collected after 96 h, was

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