Powder X-ray diffraction investigation of a NO2-tagged MOF with high affinity to CO2 at mild conditions

oncerns about global warming has prompted research at the academic and industrial level to reduce anthropogenic CO2 emissions. A number of approaches exist for CO2 capture and storage from power plants emissions [1], among which the use of nanoporous adsorbents such as metal-organic frameworks (MOFs) [2]. MOFs’ performances in CO2 adsorption have been optimized also through the use of properly functionalized ligands. As, to the best of our knowledge, the NO2-function is still under-investigated [3], we have recently designed the ligand H2BPZ-NO2 (H2BPZ-NO2 = 3-nitro-4,4’-bispyrazole) and isolated the corresponding Co(II), Cu(II) and Zn(II) MOFs [4]. As disclosed by lab PXRD, irrespective of their metrics these MOFs show 3D (4,4)-connected networks with 1D square (Zn, Co) or rhombic (Cu) channels decorated by BPZ-NO22- ligands, and accounting for an empty volume of ~ 14-35%. As witnessed by TGA and variable-temperature PXRD, while Cu(BPZ-NO2) and Co(BPZ-NO2) are moderately stable (Tdec ~ 583 K and ~ 643 K respectively), Zn(BPZ-NO2) is remarkably robust (Tdec ~ 743 K), without phase change or framework collapse prior to decomposition. If exposed to H2O(v) for 7 days, Zn(BPZ-NO2) begins losing crystallinity, which is nonetheless restored upon suspension in DMF. After thermal activation, Zn(BPZ-NO2) adsorbs 22 wt % CO2 at the rather mild conditions of 298 K and 1 bar, hence ranking among the best performing MOFs under the same experimental conditions [3]. Moreover, at 298 K it shows the significant CO2/N2 selectivity of 25 ( power plants emissions consist of mixtures of N2:CO2 ~ 85:15 v/v ). Overall, these results provide important information to shed light on the chemical and structural properties a host should possess for efficient CO2 adsorption already at rather mild conditions.