PROGRESS TOWARD UNDERSTANDING ZEOLITE-BETA ACIDITY - AN IR AND AL-27 NMR SPECTROSCOPIC STUDY

Zeolite beta was prepared with different Si/Al ratios by a hydrothermal method and dealuminated by leaching with dilute HCl. Samples of beta from the various stages of H-beta synthesis, and after leaching, were characterized by X-ray diffraction and IR and Al-27 EI IAS-NMR (MAS = magic angle spinning) spectroscopies. All the samples exhibited both Lewis and Bronsted acidities. The types of OH groups present, and also their distribution, are greatly affected by sample treatment. Five types of hydroxyl IR absorptions were found: those from strongly acidic bridging hydroxyl groups (3605 cm(-1)), OH groups bonded to extralattice aluminum (3660-3680 cm(-1)), internal SIGH at framework defects (3730 cm(-1)), and terminal SiOH groups (3745 cm(-1)) and a ''very high frequency'' (VHF) absorption (3782 cm(-1)). These bands, except those of the terminal silanols, shifted to different extents upon interaction with probe molecules (pyridine, benzene, hexane) used to evaluate acid strengths. The VHF band is most intense for slightly dealuminated beta and is not observed for borosilicate. The VHF band is associated with (but not solely responsible for) the octahedral Al-27 NMR resonance and is assigned to OH groups connected to aluminum which is leaving the framework (transient-state species). These OH groups are moderately acidic and contribute to beta acidity.