Role of Germanium in the Formation of Double Four Rings in Zeolites

In recent years, a series of novel zeolites containing double four rings (D4Rs) have been synthesized by introducing germanium into the synthesis mixture. While the structure-directing effects of Ge toward D4R containing structures are well known, little is known about the underlying chemistry. In this work, plane wave density functional theory calculations are used to characterize and rationalize the site preferences, energies, and structural changes occurring when Ge is gradually substituted into the BEC framework up to 25% loadings. The calculations show that the site preference and energies are strongly dictated by the intrinsic flexibility of a given T−O−T linkage (where T = Si or Ge), coupled with its ability to relax to geometries preferred by Ge. Calculations on small molecular fragments are used to explore the geometric variations in Ge-substituted zeolites and provide further insight into the role of Ge in stabilizing D4R units.