Hydrogen storage and carbon dioxide capture in an iron-based sodalite-type metal–organic framework (Fe-BTT) discovered via high-throughput methods

Using high-throughput instrumentation to screen conditions, the reaction between FeCl2 and H3BTT·2HCl (BTT3− = 1,3,5-benzenetristetrazolate) in a mixture of DMF and DMSO was found to afford Fe3[(Fe4Cl)3(BTT)8]2·22DMF·32DMSO·11H2O. This compound adopts a porous three-dimensional framework structure consisting of square [Fe4Cl]7+ units linked via triangular BTT3− bridging ligands to give an anionic 3,8-net. Mössbauer spectroscopy carried out on a DMF-solvated version of the material indicated the framework to contain high-spin Fe2+ with a distribution of local environments and confirmed the presence of extra-framework iron cations. Upon soaking the compound in methanol and heating at 135 °C for 24 h under dynamic vacuum, most of the solvent is removed to yield Fe3[(Fe4Cl)3(BTT)8(MeOH)4]2 (Fe-BTT), a microporous solid with a BET surface area of 2010 m2 g−1 and open Fe2+ coordination sites. Hydrogen adsorption data collected at 77 K show a steep rise in the isotherm, associated with an initial isosteric heat of adsorption of 11.9 kJ mol−1, leading to a total storage capacity of 1.1 wt% and 8.4 g L−1 at 100 bar and 298 K. Powder neutron diffraction experiments performed at 4 K under various D2 loadings enabled identification of ten different adsorption sites, with the strongest binding site residing just 2.17(5) Å from the framework Fe2+ cation. Inelastic neutron scattering spectra are consistent with the strong rotational hindering of the H2 molecules at low loadings, and further reveal the catalytic conversion of ortho-H2 to para-H2 by the paramagnetic iron centers. The exposed Fe2+ cation sites within Fe-BTT also lead to the selective adsorption of CO2 over N2, with isotherms collected at 298 K indicating uptake ratios of 30.7 and 10.8 by weight at 0.1 and 1.0 bar, respectively.