Mechanism of Preferential Adsorption of SO₂ into Two Microporous Paddle Wheel Frameworks M(bdc)(ted)_0.5

The selective adsorption of a corrosive gas, SO₂, into two microporous pillared paddle-wheel frameworks M­(bdc)­(ted)_0.5 [<i>M</i> = Ni, Zn; bdc =1,4-benzenedicarboxylate; ted = triethylenediamine] is studied by volumetric adsorption measurements and a combination of <i>in situ</i> infrared spectroscopy and <i>ab initio</i> density functional theory (DFT) calculations. The uptake of SO₂ in M­(bdc)­(ted)_0.5 at room temperature is quite significant, 9.97 mol/kg at 1.13 bar. The major adsorbed SO₂ molecules contributing to the isotherm measurements are characterized by stretching bands at 1326 and 1144 cm^–1. Theoretical calculations including van der Waals interactions (based on vdW-DF) suggest that two adsorption configurations are possible for these SO₂ molecules. One geometry involves an SO₂ molecule bonded through its sulfur atom to the oxygen atom of the paddle-wheel building unit and its two oxygen atoms to the C–H groups of the organic linkers by formation of hydrogen bonds. Such a configuration results in a distortion of the benzene rings, which is consistent with the experimentally observed shift of the ring deformation mode. In the other geometry, SO₂ establishes hydrogen bonding with −CH₂ group of the ted linker through its two oxygen atoms simultaneously. The vdW-DF-simulated frequency shifts of the SO₂ stretching bands in these two configurations are similar and in good agreement with spectroscopically measured values of physisorbed SO₂. In addition, the IR spectra reveal the presence of another minor species, characterized by stretching modes at 1242 and 1105 cm^–1 and causing significant perturbations of MOFs vibrational modes (CH_<i>x</i> and carboxylate groups). This species is more strongly bound, requiring a higher temperature (∼150 °C) to remove it than for the main physisorbed species. The adsorption configurations of SO₂ into M­(bdc)­(ted)_0.5 derived by infrared spectroscopy and vdW-DF calculations provide the initial understanding to develop microporous metal organic frameworks materials based on paddlewheel secondary-building units for SO₂ removal in industrial processes