Density functional study of SO₂ adsorption in HY zeolites

The interaction of SO₂ molecules with active sites of HY zeolite was studied by means of density functional model cluster calculations employing a generalized gradient approximation (GGA) of the exchange-correlation functional. A fragment of the zeolite framework which contains an acid bridging hydroxyl group and an adjacent basic oxygen center was modeled by the cluster H₃SiO(H)AlOSiH₃. The equilibrium geometry structures were determined for various donor-acceptor and hydrogen-bonded conformers of SO₂/HY adsorption complexes. These two adsorption interactions are similarly weak with the hydrogen-bonded form being more stable by about 2 kcal/mol. In this most stable conformer, the interaction of the sulfur atom with the adjacent oxygen center leads to additional stabilization. The assignment of the SO₂/HY complexes to hydrogen bonded species is confirmed by a comparison of experimental and calculated adsorption induced shifts of SO₂ and OH vibrational frequencies.