Theoretical studies of Na+ location in ZSM-5: Model selection for accurate coordination structure and energetics

Location of Na+ in ZSM-5 zeolite has been studied with cluster models and density functional theory of B3LYP. Various cluster models with sizes ranging from 3T to 192T were employed. Their performance in geometry optimization and stability assessment of four probable Na-sites at T1, namely Z6, M7, I2 and I3, were systematically studied. A C-5 type model is constructed by allowing all atoms on the three rings around the Al site to relax during the geometry optimization, and then expanding the region by roughly another three shells of Si atoms, leading to converged prediction of stability sequence of Z6 > I2 > M7 > I3. Based on these results, a general scheme of cluster model construction for reliable prediction of metal cation location in zeolite is proposed.