Thermal Structural Transitions and Carbon Dioxide Adsorption Properties of Zeolitic Imidazolate Framework-7 (ZIF-7)

Abstract

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.

A Review on the Production and Purification of Biomass-Derived Hydrogen Using Emerging Membrane Technologies

“…First, MOF membranes suffer from low hydrothermal stability at high temperatures and in humid environments. Although the MOF and ZIF materials were initially considered to be thermally and chemically stable in various environments [122], recent studies have noted that the stability of these MOF materials and their membranes was not as good as we expected, especially under harsh reaction conditions for long-term operation [141][142][143]. Second, although MOF membranes possess acceptable overall hydrogen separation performance, the gas permeances are still in the range of 10 −8 -10 −6 mol/s·m 2 ·Pa, which is still relatively low compared with industrial demands.…”

Crystal structure of diaqua-catena-poly[diaqua-bis(μ2-5-(4-(1H-1,2,4-triazol-1-yl)phenyl)tetrazol-2-ido-κ2 N:N′)cobalt(II)] dihydrate, C20H24CoN14O4

“…Over the past decades, there has been much interest in the fabrication of coordination polymers (CPs) because of fascinating structural diversities and framework topologies, as well as their potential applications in the fields of ionexchange, adsorption, catalysis, magnetism, and luminescence, and so on [3][4][5][6][7]. Structural design of CPs owing the expected functions is still not easy, and there remains a great challenge to truly understand the factors that affect their formation [8].…”

Crystal structure poly-(μ2-1-(4-(1H-imidazol-1-yl)benzyl)-1H-1,2,4-triazolyl-κ2 N N 1:N 2 N)-(μ3-2,2′-(1,2-phenylene)diacetato-κ5-O 1,O 2:O 2:O 3,O

“…Assemble of coordination polymers (CPs) based on metal and organic building blocks has been expanding in recent years, not only because of fascinating structural diversities and framework topologies, but also due to their potential applications in the elds of gas storage, catalysis, magnetism, luminescence, and so on [1][2][3][4]. The extension of framework topologies or chemical compositions for such materials has been tremendously accelerated because new frameworkforming elements can adopt various coordination geometries to produce some previously unknown framework structures.…”