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

Six metal-org. frameworks of the M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate) structure type bind carbon monoxide reversibly and at high capacity. IR 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. Structure detns. 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. Electronic structure calcns. employing d. functional theory (DFT) resulted in good agreement with the trends apparent in the IR spectra and crystal structures. These results represent the 1st crystallog. characterized magnesium and zinc carbonyl compds. and the 1st high-spin manganese(II), iron(II), cobalt(II), and nickel(II) carbonyl species. Adsorption isotherms 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 ≤6.0 mmol/g and 157 cm3/cm3. The six frameworks display (neg.) isosteric heats of CO adsorption ranging from 52.7 to 27.2 kJ/mol along the series Ni textgreater Co textgreater Fe textgreater Mg textgreater Mn textgreater Zn, following the Irving-Williams stability order. The reversible CO binding suggests that these frameworks may be of utility for the sepn. of CO from various industrial gas mixts., including CO/H2 and CO/N2. Selectivities detd. from gas adsorption isotherm data using ideal adsorbed soln. theory (IAST) over a range of gas compns. at 1 bar and 298 K indicate that all six M2(dobdc) frameworks could potentially be used as solid adsorbents to replace current cryogenic distn. technologies, with the choice of M dictating adsorbent regeneration energy and the level of purity of the resulting gases.