Evaluating the Role of Anharmonic Vibrations in Zeolite Materials

The characterizationof zeolitic materials is often facilitatedby spectroscopic analysis of vibrations, which informs about the bondingcharacter of the substrate and any adsorbents. Computational simulationsaid the interpretation of the spectra but often ignore anharmoniceffects that can affect the spectral characteristics significantly.Here, the impact of anharmonicity is demonstrated with a combinationof dynamical and static simulations applied to the structures formedduring the synthesis of Sn-BEA via solid-state incorporation(SSI): the initial siliceous BEA (Si-& beta;), aluminosilicate BEA(H-& beta;), dealuminated BEA (deAl-& beta;), and Sn-BEA (Sn-& beta;).Heteroatom and defect-containing BEA are shown to have strong anharmonicvibrational contributions, with atomic and elemental resolution highlightingparticularly the prevalence for H atoms (H-& beta;, deAl-& beta;)as well as localization to heteroatoms at defect sites. We simulatethe vibrational spectra of BEA accounting for anharmonic contributionsand observe an improved agreement with experimental data comparedto harmonic methods, particularly at wavenumbers below 1500 cm(-1). The results demonstrate the importance of incorporatinganharmonic effects in simulations of vibrational spectra, with consequencestoward future characterization and application of zeolitic materials.