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 FeCl 2 and H 3BTT·2HCl (BTT 3- = 1,3,5-benzenetristetrazolate) in a mixture of DMF and DMSO was found to afford Fe 3[(Fe 4Cl) 3(BTT) 8] 2·22DMF·32DMSO·11H 2O. This compound adopts a porous three-dimensional framework structure consisting of square [Fe 4Cl] 7+ units linked via triangular BTT 3- 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 Fe 2+ 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 Fe 3[(Fe 4Cl) 3(BTT) 8(MeOH) 4] 2 (Fe-BTT), a microporous solid with a BET surface area of 2010 m 2 g -1 and open Fe 2+ 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 D 2 loadings enabled identification of ten different adsorption sites, with the strongest binding site residing just 2.17(5) Å from the framework Fe 2+ cation. Inelastic neutron scattering spectra are consistent with the strong rotational hindering of the H 2 molecules at low loadings, and further reveal the catalytic conversion of ortho-H 2 to para-H 2 by the paramagnetic iron centers. The exposed Fe 2+ cation sites within Fe-BTT also lead to the selective adsorption of CO 2 over N 2, 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. © The Royal Society of Chemistry 2011.



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