A Water-Stable Anionic Metal–Organic Framework Constructed from Columnar Zinc-Adeninate Units for Highly Selective Light Hydrocarbon Separation and Efficient Separation of Organic Dyes

Abstract

A metal-organic framework (MOF), {(Me₂NH₂)₂[Zn₆(μ₄-O)(ad)₄(BPDC)₄]}_n (JXNU-4; ad^- = adeninate), with an anionic three-dimensional (3D) framework constructed from one-dimensional (1D) columnar [Zn₆(ad)₄(μ₄-O)]_n secondary building units (SBUs) and 4,4'-biphenyldicarboxylate (BPDC^2-) ligand, was prepared. The anionic 3D framework has 1D square channels with an aperture of about 9.8 Å and exhibits a carboxylate-O-decorated pore environment. The microporous nature of JXNU-4 was established by the N₂ adsorption data, which gives Langmuir and Brumauer-Emmett-Teller surface areas of 1800 and 1250 m² g^-1, respectively. Noticeably, JXNU-4 shows potential as a separation agent for the selective removal of propane and ethane from natural gas with high selectivities of 144 for C₃H₈/CH₄ (5:95) and 14.6 for C₂H₆/CH₄ (5:95), respectively. Most importantly, JXNU-4 shows an aqueous-phase adsorption of a positively charged ion of methylene blue selectively over a negatively charged ion of resorufin, which is pertinent to the anionic nature of the framework, and provides a size-exclusive sieving of methylene blue over other positively charged ions of Janus Green B and ethyl violet, which is relevant to its pore structure, enabling the efficient aqueous-phase separation of organic dyes.