Guest-induced gate-opening in a flexible MOF adsorbent that exhibits benzene/cyclohexane selectivity†

Abstract

We report the synthesis of a 2-fold interpenetrated primitive cubic (pcu) topology network, X-pcu-11-Zn, formulated as [Zn₂(DMTDC)₂(dpb)] (H₂DMTDC = 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid; dpb = 1,4-di(pyridin-4-yl)benzene). Upon removal of solvent molecules from the as-synthesised form, X-pcu-11-Zn-α, transformation from a large-pore “open” phase to a narrow pore phase, X-pcu-11-Zn-β, occurred. The β-phase subsequently exhibited guest-induced switching as evidenced by a 2-step type F-IV² adsorption isotherm for N₂ at 77 K and a 3-step profile with two gate-opening pressures for CO₂ at 195 K. Dynamic vapour sorption studies revealed selective sorption of methanol, ethanol, and CH₃CN over H₂O at 298 K. Furthermore, X-pcu-11-Zn-β selectively adsorbed benzene over cyclohexane concomitant with a gate-opening effect driven by structural transformations. Importantly, the transformations between the guest-free and guest-loaded structures were found to be reversible over six adsorption/desorption cycles. Single-crystal X-ray diffraction analysis of the benzene-loaded phase indicates that selective benzene binding can be attributed to π–π and C–H⋯π aromatic packing interactions.