Interaction of intractable gaseous SO2 with new adsorbent—Metal organic frameworks: M-MOF-74 (M = Co, Ni, Zn)

The nature of adsorption between MOF absorbent and SO2 remains a challenging issue due to the difficulty in understanding their interaction force and dynamic behavior on molecular level. Herein, a new series of M-MOF-74 (M = Co, Ni, Zn) with high SO2 adsorption performance were synthesized through an improved solvothermal method. The interaction between M-MOF-74 (M = Co, Ni, Zn) and SO2 was revealed by combining in-situ infrared spectroscopy and density functional theory (DFT) calculations. It was found that Zn-MOF-74 shows excellent SO2 uptakes (8.16 mmol g−1), significantly higher than that of Co-MOF (6.11 mmol/g) and Ni-MOF (6.36 mmol/g). These results were consistent with SO2 binding energies calculated at optimal adsorption sites, which were −11.1 kcal/mol, −12.2 kcal/mol, −18.2 kcal/mol for M-MOF-74 (M = Co, Ni, Zn), respectively. The specific adsorption behavior of Zn-MOF-74 towards SO2 is attributed to its higher negative attraction of electrostatic potential(∼48.2 kcal/mol)and van der Waals pulling forces (∼−2.6 kcal/mol). The higher interaction force between Zn-MOF-74 and SO2 was elucidated not only by their strong dominant dispersion interactions, but also by the formation of hydrogen bonding between SO2 and benzene ring, as demonstrated by independent gradient model (IGM) and infrared analysis. What's more, a unique radial SO2 movement with high speed of 323 p.m.2/ps, which was much higher than that of traditional molecular sieve (∼240 p.m.2/ps) adsorbent, were observed towards Zn-MOF-74 framework wall. Then the pore size of MOF-74 and SO2 motion trajectory was analyzed by Molecular Dynamics. This work gives insight into mechanisms on SO2 adsorption in MOFs.