Metal-organic frameworks (MOFs) are a rapidly advancing class of modern materials, whose proposed applications in a wide range of applications (e.g. hydrogen storage, CO2 sequestration, fuel cell catalysis, light harvesting, etc.) are based on the ability to design microporous structures combined with organic or inorganic functional properties. Despite impressive advances in development of MOFs, the energetics of their formation and their stability remain poorly understood, hindering the manufacture and commercial deployment. Inelastic neutron scattering (INS) provides a powerful method to understand the thermodynamic driving forces, including entropic factors, for the formation of and stabilities of MOFs. This study explores a family of zeolitic imidazolate frameworks (ZIFs), materials that achieve zeolite-type framework topologies by judicious combination of tetrahedral metal nodes and bridging imidazolate linkers in a stoichiometric ratio 1:2. Specifically, we explore materials based on 2-ethyl- and methylimidazolate linkers with INS. T. Friščić and his group have discovered that mechanochemcial milling of these ZIFs provides a novel route to synthesize and access new materials. They (Nat. Comm. 2013) discovered that Zn(EtIm)2 with the zeolite RHO topology transforms to polymorphs with an analcime (ANA) and β-quartz (QTZ) topology under milling. Katsenis et al. (Nat. Comm. 2015) reported that Zn(MeIm)2 with the sodalite (SOD) topology (ZIF-8) transforms to an amorphous phase before the appearance of polymorphs with the katsenite (KAT) and diamondoid (DIA) topologies. Both series of transformations involve an increase in framework density. Here we present INS spectra of these polymorphs measured on SEQUOIA at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory. INS spectra were collected over the 0–600 meV energy range at 7K. Special attention is given to the low-energy modes that provide insights into many possible and competing pathways governing unique physical mechanisms in these ZIFs.

Inelastic neutron scattering study of zeolitic imidazolate frameworks

2019 Spring National Meeting

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