Cooperative Interactions with Water Drive Hysteresis in a Hydrophilic Metal-Organic Framework

Devices that utilize the reversible capture of water vapor provide solutions to water insecurity, increasing energy demand, and sustainability. In all of these applications, it is important to minimize water adsorption-desorption hysteresis. Hysteresis is particularly difficult to avoid for sorbents that bind water strongly, such as those that take up below 10% relative humidity (RH). Even though the theoretical factors that affect hysteresis are understood, understanding the structure-function correlations that dictate the hysteretic behavior in water sorbents remains a challenge. Herein, we synthesize a new hexagonal microporous framework, Ni₂Cl₂BBTQ (H₂BBTQ = 2H,6H-benzo[1,2-d][4,5-d′]bistriazolequinone), to elucidate these principles. Uniquely among its known isoreticular analogues, Ni₂Cl₂BBTQ presents unusually high hysteresis caused by strong wetting seeded by a particularly strong zero-coverage interaction with water. A combination of vibrational spectroscopies and detailed molecular dynamics simulations reveals that this hysteretic behavior is the result of an intricate hydrogen-bonding network, in which the monolayer consists of water simultaneously binding to open nickel sites and hydrogen bonding to quinone sites. This latter hydrogen-bonding interaction does not exist in other isoreticular analogues: it prevents facile water dynamics and drives hysteresis. Our results highlight an important design criterion for water sorbents: in order to drive water uptake in progressively dry conditions, the common strategy of increasing hydrophilicity can cause strong wetting and the formation of superclusters, which lead to undesirable hysteresis. Instead, hysteresis-free water uptake at extremely low humidity is best promoted by decreasing the pore size, rather than increasing hydrophilicity.