A VIBRATIONAL STUDY OF THE OH AND OD BENDING MODES OF THE BRONSTED ACID SITES IN ZEOLITES

Deuteration of Bronsted 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 beta, 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 Si/Al 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.