Flexible Coordination Network Exhibiting Water Vapor-Induced Reversible Switching between Closed and Open Phases

That physisorbents can reduce the energy footprint of water vapor capture and release has attracted interest because of potential applications such as moisture harvesting, dehumidification, and heat pumps. In this context, sorbents exhibiting an S-shaped single-step water sorption isotherm are desirable, most of which are structurally rigid sorbents that undergo pore-filling at low relative humidity (RH), ideally below 30% RH. Here, we report that a new flexible one-dimensional (1D) coordination network, [Cu(HQS)(TMBP)] (H(2)HQS = 8-hydroxyquinoline-5-sulfonic acid and TMBP = 4,4 '-trimethylenedipyridine), exhibits at least five phases: two as-synthesized open phases, alpha superset of H2O and beta superset of MeOH; an activated closed phase (gamma); CO2 (delta superset of CO2) and C2H2 (epsilon superset of C2H2) loaded phases. The gamma phase underwent a reversible structural transformation to alpha superset of H2O with a stepped sorption profile (Type F-IV) when exposed to water vapor at < 30% RH at 300 K. The hydrolytic stability of [Cu(HQS)(TMBP)] was confirmed by powder X-ray diffraction (PXRD) after immersion in boiling water for 6 months. Temperature-humidity swing cycling measurements demonstrated that working capacity is retained for > 100 cycles and only mild heating (< 323 K) is required for regeneration. Unexpectedly, the kinetics of loading and unloading of [Cu(HQS)(TMBP)] compares favorably with well-studied rigid water sorbents such as Al-fumarate, MOF-303, and CAU-10-H. Furthermore, a polymer composite of [Cu(HQS)(TMBP)] was prepared and its water sorption retained its stepped profile and uptake capacity over multiple cycles.

Automated summary

This study reports a new flexible one-dimensional coordination network that exhibits reversible adsorption properties and good hydrolytic stability. The kinetics of this material compares favorably with well-studied rigid water sorbents. Additionally, a polymer composite of this material retains stepped adsorption profile and high water uptake capacity.