Unraveling the Guest-Induced Switchability in the Metal-Organic Framework DUT-13(Zn)

Abstract

A guest-induced flexibility in the framework DUT-13 was investigated in situ to analyze the breathing mechanism upon physisorption of nitrogen (77 K) and n-butane (273 K). The crystal structure of cp phase, solved from PXRD data using the computation-assisted semiempirical approach, shows two times smaller pore volume, compared to the op phase, which is consistent with the corresponding isotherms. The contraction mechanism is mainly based on the conformational isomerism of the benztb4- linker, which transforms from a staggered conformation in op phase to a more eclipsed in cp phase, leading to the contraction of the larger pore. A nearly complete op → cp → op transition was observed in the case of n-butane adsorption at 273 K, while in case of weakly interacting nitrogen molecules a portion of the sample remains in the op phase in the entire pressure range. Apparently, in case of DUT-13 the contraction is crystallite size-dependent, similarly as in a number of other switchable MOFs, which should be investigated more in detail in the future. Methane adsorption at varying temperatures showed a wide hysteresis at the temperatures between 111 K and 140 K. The hysteresis width decreases until it disappears completely at 170 K leading to a reversible isotherm, typical for rigid frameworks. The fact that breathing is observed in a broader temperature range in comparison to DUT‑49 demonstrates that thermodynamics and kinetics favour the DUT-13 contraction. Linker and hinges in DUT-13 are not stiff enough to support the metastable states required for NGA.