Realization of both high hydrogen selectivity and capacity in a guest responsive metal–organic framework

Abstract

Two newly designed semi-flexible tetratopic carboxylate ligands, 5′,5′′′′-(propane-2,2-diyl)bis(2′-methoxy-[1,1′:3′,1′′-terphenyl]-4,4′′-dicarboxylate) (pbtd-OMe⁴⁻) and 5′,5′′′′-(propane-2,2-diyl)bis(2′-ethoxy-[1,1′:3′,1′′-terphenyl]-4,4′′-dicarboxylate) (pbtd-OEt⁴⁻), have been used to connect dicopper paddlewheel building units to afford two isostructural metal–organic frameworks, Cu₂(H₂O)₂(pbtd-OR)·xS (R = Me, PCN-38·xS; R = Et, PCN-39·xS, S represents noncoordinated guest molecules, PCN = porous coordination network) with novel structure and gas sorption properties upon activation. PCN-39 undergoes structural transformations upon guest solvent removal, leading to observation of distinct phases from in situ powder X-ray diffraction measurements, and exhibits selective adsorption of H₂ (up to 2.0 wt%) over CO, CO₂, and N₂, which can be explained by optimized space-filling of the pendant ethoxy group. PCN-38 undergoes no transformation upon activation and exhibits hydrogen uptake up to 2.2 wt%, as well as moderate uptake of other gases. The selective adsorption of hydrogen over other gases highlights the potential application of PCN-39 in industrially important gas separation.