A vibrational study of the OH and OD bending modes of the Broensted acid sites in zeolites

Deuteration of Brönsted acid sites in zeolites and SAPOs results in in-plane OD bending modes that can be studied with FTi.r. spectroscopy. A band maximum near 870 cm−1 and a shoulder near 890 cm−1 are related to the high-frequency (HF) and low-frequency (LF) OH or OD stretching modes observed for zeolites Y and ZK-5 and for SAPO-17 and SAPO-34. For mordenite. zeolite β, ZSM-5, and SAPO-11, only one type of bending mode is observed near 890 cm−1. Structure type and composition influence the frequencies for these bands. Increasing the sodium content of the samples results in a shift of the fundamentals to lower frequencies. Increasing the
ratio shifts the bands to slightly higher frequencies. The in-plane bending modes are more sensitive to structural differences than are the corresponding stretching modes. The first overtones of the OD bending modes are observed near 1580 cm−1 for the zeolites and near 1540 cm−1 for the SAPOs.

Simulations of the inelastic neutron scattering spectra indicate that the in-plane bending modes are localized modes, whereas the out-of-plane bending modes are coupled to the lattice modes. The in-plane OD bending modes are shifted to higher frequencies due to Fermi resonance with the zeolite TO lattice modes. The observed shift is 80 cm−1 for the zeolites and 110 cm−1 for the SAPOs.