Quantification of gas-accessible microporosity in metal-organic framework glasses

Abstract

Metal-organic framework (MOF) glasses are a new class of microporous glass materials with immense potential for applications ranging from gas separation to optics and solid electrolytes. Due to the inherent difficulty to determine the atomistic structure of amorphous glasses, the intrinsic structural porosity of MOF glasses is only poorly understood. In this work, the porosity features of a series of prototypical MOF glass formers from the family of zeolitic imidazolate frameworks (ZIFs) and their corresponding glasses is investigated comprehensively. CO2 gas sorption at 195 K allows to follow the evolution of microporosity when transforming from the crystalline to the glassy state of these materials. Based on these data, the pore volume and the real density of the ZIF glasses is quantified for the first time. Additional hydrocarbon sorption data (n-butane, propane and propylene) together with X-ray total scattering experiments prove that the porosity features (in particular the pore size and the pore limiting diameter) of the ZIF glasses depend on the types of organic linkers present in the glass network. This allows formulating first design principles for a targeted tuning of the intrinsic microporosity of MOF glasses. Importantly, these principles are counterintuitive and contrary to established porosity design concepts for crystalline MOFs but show similarities to strategies previously developed for porous polymers.

Investigating the chemical sensitivity of melting in zeolitic imidazolate frameworks

“…17 ZIF-zni has also recently been reported to undergo melting at 585 °C when heated directly from room temperature. 44 However, in this study, a T g was not reported and it was noted that the resulting glass, denoted a g ZIF-zni, contained zinc oxide. 44 Here, we investigate the thermal response of ZIF-zni in detail to (i) locate its T g and, (ii) determine whether a g ZIF-zni can be prepared without zinc oxide growth.…”

“…44 However, in this study, a T g was not reported and it was noted that the resulting glass, denoted a g ZIF-zni, contained zinc oxide. 44 Here, we investigate the thermal response of ZIF-zni in detail to (i) locate its T g and, (ii) determine whether a g ZIF-zni can be prepared without zinc oxide growth. We then go on to study how the incorporation of different linker chemistry influences melting in zni topology frameworks.…”

“…357 °C for the small endotherm and 585 °C for the melting event). 44 The melting event for ZIF-zni occurred after the first Please do not adjust margins Please do not adjust margins decomposition step in the TGA trace and so was accompanied by ca. 6% mass loss (Fig.…”

Introducing porosity into metal–organic framework glasses

“…NLDFT and Horvath–Kawazoe (HK) are inadequate, and that improvements are required in future statistical models based on CO 2 adsorption to more accurately reproduce the porosity within MOF glasses. 29…”

“…NLDFT and Horvath-Kawazoe (HK) are inadequate, and that improvements are required in future statistical models based on CO 2 adsorption to more accurately reproduce the porosity within MOF glasses. 29 Excitingly, and for the rst time, a g ZIF-62(Zn) is shown to be porous towards larger hydrocarbon gases, such as n-butane, propane, and propylene. The larger kinetic diameter of nbutane (4.3 Å) compared to CO 2 (3.3.…”