2 research outputs found
Thermal Structural Transitions and Carbon Dioxide Adsorption Properties of Zeolitic Imidazolate Framework‑7 (ZIF-7)
As a subset of the
metal–organic frameworks, zeolitic imidazolate
frameworks (ZIFs) have potential use in practical separations as a
result of flexible yet reliable control over their pore sizes along
with their chemical and thermal stabilities. Among many ZIF materials,
we explored the effect of thermal treatments on the ZIF-7 structure,
known for its promising characteristics toward H<sub>2</sub> separations;
the pore sizes of ZIF-7 (0.29 nm) are desirable for molecular sieving,
favoring H<sub>2</sub> (0.289 nm) over CO<sub>2</sub> (0.33 nm). Although
thermogravimetric analysis indicated that ZIF-7 is thermally stabile
up to ∼400 °C, the structural transition of ZIF-7 to an
intermediate phase (as indicated by X-ray analysis) was observed under
air as guest molecules were removed. The transition was further continued
at higher temperatures, eventually leading toward the zinc oxide phase.
Three types of ZIF-7 with differing shapes and sizes (∼100
nm spherical, ∼400 nm rhombic-dodecahedral, and ∼1300
nm rod-shaped) were employed to elucidate (1) thermal structural transitions
while considering kinetically relevant processes and (2) discrepancies
in the N<sub>2</sub> physisorption and CO<sub>2</sub> adsorption isotherms.
The largest rod-shaped ZIF-7 particles showed a delayed thermal structural
transition toward the stable zinc oxide phase. The CO<sub>2</sub> adsorption
behaviors of the three ZIF-7s, despite their identical crystal structures,
suggested minute differences in the pore structures; in particular,
the smaller spherical ZIF-7 particles provided reversible CO<sub>2</sub> adsorption isotherms at ∼30–75 °C, a typical
temperature range of flue gases from coal-fired power plants, in contrast
to the larger rhombic-dodecahedral and rod-shaped ZIF-7 particles,
which exhibited hysteretic CO<sub>2</sub> adsorption/desorption behavior
Thermosensitive Structural Changes and Adsorption Properties of Zeolitic Imidazolate Framework‑8 (ZIF-8)
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<sub>2</sub> 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