Insights into the Mechanochemical Glass Formation of Zeolitic Imidazolate Frameworks

Metal-organic framework (MOF) glasses, known for their potential in gas separation, optics, and solid-state electrolytes, benefit from the processability of their (supercooled) liquid state. Traditionally, MOF glasses are produced by heating MOF crystals to their melting point and then cooling the liquid MOF to room temperature under an inert atmosphere. While effective, this melt-quenching technique requires high energy due to the high temperatures involved. It also limits the scope of new material development by restricting the compositional range to only those combinations of metal ions and linkers that are highly thermally stable. An alternative, mechanical milling at room temperature, has demonstrated its capability to transform MOF crystals into amorphous phases. However, the specific conditions under which these amorphous phases exhibit glass-like behavior remain uncharted. In this study, we explore the mechanochemical amorphization and vitrification of a variety of zeolitic imidazolate frameworks (ZIFs) at room temperature. These ZIFs feature diverse functionalized linkers and different metal ions (Zn2+, Co2+ and Cu2+). Through comprehensive analyses, including X-ray diffraction, calorimetry, and gas sorption studies, we gain valuable insights into the chemical and structural prerequisites and constraints for the mechanochemical vitrification of ZIFs. Our findings demonstrate that ZIFs capable of melting can be successfully converted into glasses through ball-milling. Remarkably, some non-meltable ZIFs can also be vitrified using the ball-milling technique, as highlighted by the preparation of the first Cu2+-based ZIF glass. This research significantly contributes to expanding the currently limited compositional range of MOF glasses. Moreover, it paves the way for integrating thermally sensitive substances into MOF glasses by room temperature mechanochemical vitrification, thus offering new possibilities for innovative applications.