Bismuth-based metal-organic frameworks (Bi-MOFs) development for CO2 separation

https://doi.org/10.1016/j.micromeso.2021.111548

“The Bi-MOFs synthesized in this study were examined for their ability to separate and adsorb greenhouse gases. Equilibrium gas sorption isotherms were recorded at 293 K on the synthesized Bi-MOFs using CH₄, CO₂, N₂, and SF₆ as adsorbed gases (Fig. 5). The gas uptake capacities at 100 kPa are listed in Table 2. Due to the restricted N₂ diffusion observed on UU-200 at 77 K, the N₂ adsorption isotherms at 293 K were further investigated in order to evaluate the pore size effect. The N₂ sorption capacity of UU-200 at 100 kPa (3.59 cm³ g⁻¹ STP) was found to be comparable to CAU-17 (4.78 cm³ g⁻¹ STP) and SU-101 (2.36 cm³ g⁻¹ STP), despite the noticeably higher N₂-porosity on the latter two MOFs. This indicated that the limited N₂ access observed on UU-200 at 77 K, which was attributed to the presence of narrow pores, could in fact also be related to a mild thermally responsive structural change in the framework. Negative thermal expansions (NTE), i.e. a reduction in the crystallographic unit cell with increasing temperature, has been observed in MOFs such MOF-5 [47], IRMOFs [48], Cu₃BTC₂ [49], and UiO-66(Hf) [50], while positive thermal expansion (PTE) effects, i.e. an increase in the crystallographic unit cell with increasing temperature, has been detected to a lesser degree in framework materials [51]. However, both NTE and PSE have been observed in DMF-solvated DUT-49(Cu) [51,52], where a ∼6.4% reduction in the unit cell volume was observed upon cooling the material from 298 K to 150 K. This phenomenon was attributed to the solidification of the DMF molecules in the pores of the structure resulting in a contraction of the pore volume. Similar observations were made in two DMF-solvated Mn- and Cd-based MOFs [53,54], which both showed low N₂-porosities at 77 K and both PTE and NTE properties, resulting in a slight increase in unit cell volume between 208 and 215 K (corresponding to the melting point of DMF at 212 K). Although no evidence of framework flexibility was observed during the sorption of various gases at 293 K (e.g. CH₄, CO₂, N₂, or SF₆, see Fig. 5 and Table 2). The possibility that UU-200 may undergo a significant enough thermal expansion between 77 K and 293 K for the structure to become N₂-porous at ambient temperatures could explain the conflicting results that were obtained.”

“Fig. 5. CH₄, CO₂, N₂, and SF₆ sorption isotherms recorded at 293 K and 100 kPa for (a) UU-200, (b) CAU-17, (c) CAU-33, and (d) SU-101. The adsorption and desorption branches of the isotherms are indicated by filled and hollow symbols, respectively.”