One Step Further: A Flexible Metal-Organic Framework that Functions as a Dual-Purpose Water Vapor Sorbent

We report a water induced phase transformation in a flexible MOF, [Zn₃(OH)₂(btca)₂] (Hbtca = 1H-benzotriazole-5-carboxylic acid), that exhibits a two-step water vapor sorption isotherm associated with water-induced phase transformations. Variable temperature X-ray diffraction studies revealed that the dehydrated phase, LP-β, is almost isostructural with the previously reported solvated phase, LP-α. LP-β reversibly transformed to a partially hydrated phase, NP, at 5% RH, and a fully hydrated phase, LP-γ, at 47% RH. Structural studies reveal that host-guest and guest-guest interactions are involved in the NP, LP-α, and LP-γ phases. The LP-β phase, however, is atypical; molecular modeling studies indicating that it is indeed energetically favorable as a LP structure. To our knowledge, [Zn₃(OH)₂(btca)₂] is only the second sorbent that exhibits water induced LP-NP-LP transformations (after MIL-53) and represents the first regeneration optimized sorbent (ROS) with two steps at RH ranges relevant for both atmospheric water harvesting and dehumidification.