Reversible CO Binding Enables Tunable CO/H2 and CO/N2 Separations in Metal-Organic Frameworks with Exposed Divalent Metal Cations

E. D. Bloch, M. R. Hudson, J. A. Mason, S. Chavan, V. Crocellà, J. D. Howe, K. Lee, A. L. Dzubak, W. L. Queen, J. M. Zadrozny, S. J. Geier, L.-C. Lin, L. Gagliardi, B. Smit, J. B. Neaton, S. Bordiga, C. M. Brown, and J. R. Long, Reversible CO Binding Enables Tunable CO/H2 and CO/N2 Separations in Metal-Organic Frameworks with Exposed Divalent Metal Cations J. Am. Chem. Soc. 136(30), 10752 (2014)


Abstract: Six metal-organic frameworks of the M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate) structure type are demonstrated to bind carbon monoxide reversibly and at high capacity, as required for its efficient separation from gas mixtures. Infrared spectra indicate that, upon coordination of CO to the divalent metal cations lining the pores within these frameworks, the C–O stretching frequency is blue-shifted, consistent with nonclassical metal-CO interactions involving little or no π back-donation. Structure determinations from powder neutron diffraction data reveal M–CO distances ranging from 2.09(2) Å for M = Ni to 2.49(1) Å for M = Zn and M–C–O angles ranging from 161.2(7)° for M = Mg to 176.9(6)° for M = Fe. Significantly, electronic structure calculations employing density functional theory (DFT) resulted in good agreement with the trends apparent in the infrared spectra and crystal structures only upon allowing the extended framework structure to relax in response to CO binding. These results represent the first crystallographically characterized magnesium and zinc carbonyl compounds and the first high-spin manganese(II), iron(II), cobalt(II), and nickel(II) carbonyl species. Adsorption isotherms collected at 25, 35, and 45 °C indicate reversible adsorption, with capacities for the Fe, Co, and Ni frameworks approaching one CO per metal cation site at 1 bar, corresponding to loadings as high as 6.0 mmol/g and 157 cm3/cm3. Consistent with CO binding at the metal cation sites, the six frameworks display (negative) isosteric heats of CO adsorption ranging from 52.7 to 27.2 kJ/mol along the series Ni > Co > Fe > Mg > Mn > Zn, following the Irving-Williams stability order and in good agreement with the results of DFT calculations. The reversible CO binding at high capacity and moderate energy suggests that these frameworks may be of utility for the separation of CO from various industrial gas mixtures, including CO/H2 and CO/N2. Selectivities determined from gas adsorption isotherm data using ideal adsorbed solution theory (IAST) over a range of gas composi-tions at 1 bar and 298 K indicate that all six M2(dobdc) frameworks could potentially be used as solid adsorbents to replace current cryogenic distillation technologies, with the choice of M dictating adsorbent regeneration energy and the level of purity of the resulting gases.

© Berend Smit 2019