Introduction Metal-organic frameworks (MOFs) exhibit excellent tunability in structure, primarily due to the mutual substitution and doping of central ions and organic ligands. Changes in central ions and organic ligands could impact their glass-forming stability, pore size, catalytic ability, and other physicochemical properties. For example, the MIL-100 metal-organic framework materials, centered around chromium ions, exhibits high stability and catalytic activity in oxidation reactions. However, replacing chromium ions with aluminum ions will lead to decreased stability and catalytic activity in oxidation reactions. Therefore, exploring the mutual combinations of different metal ions and organic ligands is crucial for developing new MOFs. Fe-MOF, characterized by highly controllable pore structure, excellent catalytic performance, and magnetism, is widely applied in the fields of gas adsorption and separation, catalysis, drug delivery, and environmental protection. However, the activity of Fe ions induces a low thermal stability,which consequently leads to the decomposition of most Fe-MOF crystals prior to melting. This impedes the glass transition process through melt-quenching. In this study, a solvent-free method was utilized to prepare a Fe-MOF crystal, which was subsequently transformed into glass state via melt-quenching. The morphological and structural evolution of this Fe-MOF during heating, as well as the mechanism underlying its glass transition, were thoroughly investigated.
Methods In a meticulously controlled environment within a glove box featuring extremely low levels of oxygen and water content(oxygen content ≤0.282 mg·m–3, water content ≤0.3 mg·L–1), the mass of 1.86 g ferrocene and 1.362 g imidazole were weighed,blended together within a polytetrafluoroethylene container and sealed within a sturdy steel outer casing. Subsequently, the reaction vessel was transferred to a high-temperature blast drying oven, following by a controlled reaction process at 150 ℃ for 96 h. After the reaction, the product was washed with dimethylformamide (DMF) until the washing out solution turned transparent. Ultimately the sample was dried within a vacuum drying oven operating at a pressure of –0.1 MPa and a temperature of 80 ℃ for 12 h,permitting the acquisition of the FCIR crystals. To further manipulate the FCIR material into a glassy state, a melt-quenching technique was employed. Approximately 15 mg FCIR crystal were transferred into a specialized platinum crucible, which was then positioned within the Differential Scanning Calorimeter (DSC). The sample undergoes a controlled heating process at 10 ℃·min–1until 500 ℃ under an argon atmosphere flowing at 40 mL·min–1. Subsequently, the sample cooled to room temperature at 20 ℃·min–1.
The thermodynamic behavior characterizations of all the samples were conducted using a Netzsch STA449 F1 instrument.Room temperature powder XRD data (2θ=5° to 50°) were collected with a Bruker D8 Advance diffractometer using Cu Kα(λ=1.540 6 ?) radiation. The surface appearance were acquired by Olympus CX33 optical microscope and Zeiss sigma 500 field emission scanning electron microscope (3 kV). The structural changes were characterized through FT-IR and FT-Raman, which obtained by Bruker INVENIO-S Fourier infrared transform spectrometer with a spectral resolution of 4 cm?1 and Thermo Fisher DXR 2xi spectrometer with a laser of 1 064 nm, respectively.
Results and discussion FCIR go through three processes during the formation of glass: (ⅰ) the terminal imidazole molecules connected to Fe(II) release at 210–301 ℃ with a weight loss of 19.4%, corresponding to the first peak of DSC curve; (ⅱ) the crystal melting at 398–426 ℃ according to the second peak of DSC curve; (ⅲ) the liquid continues to be heated to 500 ℃ and then cooled to room temperature to form glass, which is certified by the absence of the Bragg in the XRD patterns and Tg appeared in the DSC curve. Remarkably, the loss of mass rapidly occurres at 535–575 ℃, which is relative to the thermal decomposition. In addition, the optical and SEM images of FCIR show morphological changes. The release of imidazole disrupts the surface structure of the crystal,leading to the formation of numerous micropores and irregular cracks. The molten sample exhibits a round periphery and the surface shows deformations and wrinkles, indicating that the molten state of FCIR possesses a very high viscosity. By combining FT-IR and FT-Raman spectroscopy, it can be demonstrated that during the process of heating at 210–301 ℃, the terminal imidazole in the Fe-IM6 octahedra disappeared. These findings provide insights into the structural evolution and thermal properties of FCIR at different temperatures, offering important clues for further research on the properties and applications of this material.
Conclusions A Fe-MOF crystal (FCIR) was obtained using a solvent-free method and vitrified through melt-quenching. The release of terminal imidazole led to the transformation of the Fe-IM6 octahedral into Fe-IM4 tetrahedra, which were convinced by FT-IR and FT-Raman spectra. Subsequently, the crystal underwent melting, resulting in a disordered structure at high temperatures, which was retained through rapid cooling to room temperature.