Defect Identification in Zeolitic Imidazolate Framework Glasses by Solid-State NMR Spectroscopy Using 13CO2 as Probe

Abstract

Melt-quenched glasses from zeolitic imidazolate frameworks (ZIFs), a subset of metal–organic frameworks (MOFs) constructed from imidazolate linkers and divalent metal ions, represent a novel class of porous materials with potential applications in gas separation, optics, and as battery materials. Volumetric adsorption studies in combination with high-pressure ¹³C in situ NMR spectroscopy of CO₂ have emerged as promising tools to investigate the textural properties of porous materials, including ZIFs. However, CO₂ is not inert. It can chemically bind to Lewis basic sites present in the pores, thus changing the identity of CO₂. Here, we use this property to investigate dangling linker defects in crystalline ZIFs and their corresponding glasses or mechanochemically amorphized derivatives before and after exposure to ¹³C-enriched CO₂ at high pressure via solid-state NMR spectroscopy. Dangling linkers in the porous materials are visualized spectroscopically via carboxylation at their non-coordinating N atoms, forming carbamates. We observe that the carboxylation reaction of dangling linkers is much more pronounced in ZIF glasses than in the crystalline parent compounds, substantiating that the glasses feature a considerably higher concentration of such defects. Quantitative ¹³C NMR spectroscopy reveals that approximately 1% of the imidazolate-type linkers are carboxylated in glasses, whereas the amount of the carboxylated linkers is about seven times lower in the pristine ZIFs. These findings offer structural insight into the defects of ZIF glasses and bear significant practical implications for applications ranging from gas separation to catalysis.