A rare Pb9 cluster-organic framework constructed from a flexible cyclotriphosphazene-functionalized hexacarboxylate exhibiting selective gas separation

Abstract

It is quite challenging to fabricate metal–organic frameworks with permanent porosity using flexible organic ligands owing to their conformation freedom and flexibility making the resultant frameworks very difficult to stabilize. Successful synthesis of such frameworks will enrich the domain of porous compounds for various applications. In this work, we designed a flexible cyclotriphosphazene-functionalized hexacarboxylate, and used it to successfully construct a porous metal–organic framework ZJNU-62 upon solvothermally reacting with Pb²⁺ ions. Single-crystal X-ray diffraction analyses show that ZJNU-62 is composed of nonanuclear Pb₉ clusters connected by organic linkers to form a (4,12)-connected three-dimensional infinite network of Al₃La topology with one-dimensional rectangular channels running along the crystallographic c axis. Although the organic ligand is flexible, the resulting MOF compound ZJNU-62 can sustain the desolvation and exhibit permanent porosity, which has been confirmed by various gas adsorption isotherms. In particular, this compound can selectively adsorb C₂ hydrocarbons and CO₂ from methane under ambient conditions, indicating its promising potential for natural gas purification. The predicted IAST adsorption selectivities are 21.0, 10.9, 13.2 and 5.8 for the equimolar C₂H₂/CH₄, C₂H₄/CH₄, C₂H₆/CH₄ and CO₂/CH₄ gas mixtures at 298 K and 100 kPa, respectively. This work represents a rare example of a Pb-based MOF constructed from a flexible organic ligand exhibiting selective gas adsorption properties.