Hysteretic Gas and Vapor Sorption in Flexible Interpenetrated Lanthanide-Based Metal–Organic Frameworks with Coordinated Molecular Gating via Reversible Single-Crystal-to-Single-Crystal Transformation for Enhanced Selectivity

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Abstract

A series of flexible 3-fold interpenetrated lanthanide-based metal organic frameworks (MOFs) with the formula [Ln(HL)(DMA)2]·DMA·2H2O, where Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Er, DMA = dimethylacetamide, and H4L = 5,5′-(2,3,5,6-tetramethyl-1,4-phenylene)bis(methylene)bis(azanediyl)diisophthalic acid, have been prepared. [Sm(HL)(DMA)2]·DMA·2H2O was studied as an exemplar of the series. The activated Sm(HL)(DMA)2 framework exhibited reversible single-crystal-to-single-crystal (SCSC) structural transformations in response to adsorption and desorption of guest molecules. X-ray single crystal structural analysis showed that activation of [Sm(HL)(DMA)2]·DMA·2H2O by heat treatment to form Sm(HL)(DMA)2 involves closing of 13.8 × 14.8 Å channels with coordinated DMA molecules rotating into the interior of the channels with a change from trans to cis Sm coordination and unit cell volume shrinkage of ∼20%, to a void volume of 3.5%. Solvent exchange studies with CH2Cl2 gave [Sm(HL)(DMA)2]·2.8CH2Cl2 which, at 173 K, had a structure similar to that of trans-[Sm(HL)(DMA)2]·DMA·2H2O. CH2Cl2 vapor sorption on activated cis-[Sm(HL)(DMA)2] results in gate opening, and the fully loaded structure has a similar pore volume to that of trans-[Sm(HL)(DMA)2]·2.8CH2Cl2 structure at 173 K. Solvent exchange and heat treatment studies also provided evidence for intermediate framework structural phases. Structural, thermodynamic, and kinetic aspects of the molecular gating mechanism were studied. The dynamic and structural response of the endothermic gate opening process is driven by the enthalpy of adsorption, entropic effects, and Fickian diffusion along the pores produced during framework structure development thus relating the structure and function of the material. Exceptionally high CO2 selectivity was observed at elevated pressure compared with CH4, H2, O2, and N2 due to molecular gate opening of cis-[Sm(HL)(DMA)2] for CO2 but not for the other gases. The CO2 adsorption induced the structural transformation of cis-[Sm(HL)(DMA)2] to trans-[Sm(HL)(DMA)2], and hysteretic desorption behavior allows capture at high pressure, with storage at lower pressure.

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