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

Abstract

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.