Thermosensitive Structural Changes and Adsorption Properties of Zeolitic Imidazolate Framework-8 (ZIF-8)

Abstract

We compared four types of ZIF-8 with varying sizes and shapes to determine their thermal-structural stability and derive appropriate thermal activation conditions and correlation between structural characteristics and adsorption properties. Under air, the ZIF-8 phase for all the samples was converted completely into the zinc oxide phase above ∼300°C, though thermalgravimetric analysis (TGA) indicated that the original structure was stable to ∼300−350°C. Longer exposures (∼30 d) suggested that thermal activation at ∼200°C was appropriate for the removal of guest and/or solvent molecules under air without structural damage. Despite no noticeable change in X-ray diffraction (XRD) patterns after activation at 250°C under air, the resulting BET surface areas and CO 2 adsorption amounts (at 1 bar and 30°C) of ZIF-8s were reduced to ∼44−54 and ∼72−87%, respectively, as compared to those of appropriately activated ZIF-8s. It appears that after the activation at 250°C under air, some Zn and N atoms were dissociated and converted to ZnOH and NOH, respectively, causing the partial structural damage of ZIF-8s.

Post‐Synthetic Modification of ZIF‐8 Crystals and Films through UV Light Photoirradiation: Impact on the Physicochemical Behavior of the MOF

“…On the other hand, modes at 904, 1041, 1251 and 2200 cm −1 have been associated with bond structures caused by disorders between N and C of the ligand when ZIF‐8 undergoes thermal degradation, but we only observed a band around 2225 cm −1 and a very small shoulder at 1253 cm −1 (Figure ) . Additionally, an acute signal around 1623 cm −1 was present, whose position is similar to the one associated with the C=N vibrational mode of oxyme groups (C=N−OH) in thermally degraded ZIF‐8 . However, in the mZIF‐8 spectrum (Figure ) no signals from N−O stretching mode (strong) were observed which are usually found around 900 cm −1 , while an acute signal emerged at 791 cm −1 that is in the region of bending modes of N−H groups .…”

“…In addition, signals of methyl bending at 1384 cm −1 , CH bending of imidazolate structure at 1145 cm −1 , C=N stretching at 1585 cm −1 and a band series in the region 1350–1500 cm −1 associated with the vibrations of the aromatic ring, were still present . The presence of ZnO that may have been formed is ruled out, because the signals for this oxide around 500 cm −1 were not detected (see Figure S1) . The wide band at 3600–3100 cm −1 and the halo around 1700–1500 cm −1 in mZIF‐8 is compatible with the generation of hydroxyls, while the amplitude of the high‐frequency band suggests that they do not have a defined environment.…”

“…The wide band at 3600–3100 cm −1 and the halo around 1700–1500 cm −1 in mZIF‐8 is compatible with the generation of hydroxyls, while the amplitude of the high‐frequency band suggests that they do not have a defined environment. These spectral characteristics are similar to those of ZIF‐8 subjected to thermal degradation, which were assigned to vibrational modes of Zn‐OH species . On the other hand, modes at 904, 1041, 1251 and 2200 cm −1 have been associated with bond structures caused by disorders between N and C of the ligand when ZIF‐8 undergoes thermal degradation, but we only observed a band around 2225 cm −1 and a very small shoulder at 1253 cm −1 (Figure ) .…”

“…In this line, a controlled thermal annealing of ZIF‐8 crystals in He was carried out which allowed an enhanced CO 2 uptake and CO 2 /N 2 selectivity . The thermal‐structural stability of ZIF‐8 in air was also studied, allowing appropriate conditions that led to modified structural characteristics and adsorption properties . Moreover, an UV approach to photo‐hydroxylate another MOF such as UiO‐66 was reported, even though in this later study water was used as a medium for dispersing the crystals and solid photocatalysts were employed …”

“…On the other hand, the spectroscopic evidence showed marked effects on the chemistry of the MOF, observing modifications due to partial disconnections and disorders of the ZIF‐8 bond structure. Hydroxylation occurs, increasing the number of terminal species of type OH such as Zn‐OH, which is similar to what happened in thermally degraded ZIF‐8 in air, and also nitrogen‐containing terminal species of imine‐type (−C=NH) are generated. It was also showed that these physicochemical changes were reflected in the modified properties of mZIF‐8.…”