Structure, Stability, and Lewis Acidity of Mono and Double Ti, Zr, and Sn Framework Substitutions in BEA Zeolites: A Periodic Density Functional Theory Study

The siting of Ti, Sn, and Zr framework heteroatoms in zeolite BEA and the resulting Lewis acidity were systematically investigated by periodic density functional theory (DFT) calculations. Mono as well as double substitutions were considered. Substitution of Si by Ti, Sn, or Zr in the lattice of BRA zeolite is not random. For all substituted zeolites, the introduction of heteroatoms at the T2 crystallographic position is preferred. Water adsorption was used as a probe for Lewis acidity of lattice sites in these substituted zeolites. Although the Lewis acidity of Sn- and Zr-substituted BEA zeolites is generally quite similar, it is substantially higher than that of Ti BEA. The Lewis acidity of substituted zeolites strongly depends on the crystallographic location of the heteroatoms. For those lattice sites that can be approached from two different directions, interaction with water will be favored from the more accessible direction. Stable structures containing double lattice substitutions at distances below 5.0 angstrom are found in Sn-BEA but not in Ti- or Zr-BEA. It is argued that substituted heteroatoms play an important role during the activation of reactants, with their ability to activate them depending on the type of heteroatom. The presence of paired lattice sites in Sn BEA zeolite substantially enhances Lewis acidity of the zeolites, which dearly distinguishes Sn-BEA from its Ti- and Zr-substituted analogues.