Ultramicropore Engineering by Dehydration to Enable Molecular Sieving of H₂ by Calcium Trimesate

The high energy footprint of commodity gas purification and increasing demand for gases require new approaches to gas separation. Kinetic separation of gas mixtures through molecular sieving can enable separation by molecular size or shape exclusion. Physisorbents must exhibit the right pore diameter to enable separation, but the 0.3-0.4 nm range relevant to small gas molecules is hard to control. Herein, dehydration of the ultramicroporous metal-organic framework Ca-trimesate, Ca(HBTC)⋅H₂ O (H₃ BTC=trimesic acid), bnn-1-Ca-H₂ O, affords a narrow pore variant, Ca(HBTC), bnn-1-Ca. Whereas bnn-1-Ca-H₂ O (pore diameter 0.34 nm) exhibits ultra-high CO₂ /N₂ , CO₂ /CH₄ , and C₂ H₂ /C₂ H₄ binary selectivity, bnn-1-Ca (pore diameter 0.31 nm) offers ideal selectivity for H₂ /CO₂ and H₂ /N₂ under cryogenic conditions. Ca-trimesate, the first physisorbent to exhibit H₂ sieving under cryogenic conditions, could be a prototype for a general approach to exert precise control over pore diameter in physisorbents.