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 H2 separations;
the pore sizes of ZIF-7 (0.29 nm) are desirable for molecular sieving,
favoring H2 (0.289 nm) over CO2 (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 N2 physisorption and CO2 adsorption isotherms.
The largest rod-shaped ZIF-7 particles showed a delayed thermal structural
transition toward the stable zinc oxide phase. The CO2 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 CO2 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 CO2 adsorption/desorption behavior.