The analysis of the BAS OH band in zeolites

Abstract

The zeolite frameworks ( FER , TON , CHA , and IFR ) with a large diversity of the IR band shapes in the OH stretching region were investigated by combined use of FT-IR spectroscopy and DFT calculations. The analysis of various isolated Brønsted acid sites was performed with a model based on Boltzmann weighted relative energies and OH frequencies obtained by the ω/r correlation method. This approach has recently been used to reveal the non-statistical incorporation of aluminum into the H- MOR framework. The model correctly predicts the band shapes of H- FER and H- TON zeolites in the free OH region of the FT-IR spectra. For H-C HA and H- IFR , however, the mutual BAS interactions cannot be ignored even for high Si/Al ratios, which renders the concept of isolated BAS inadequate. Thus, the periodic calculations of adsorption and catalytic properties of zeotype frameworks should always be carried out with extended models mimicking real heteroatom distribution in these materials. • FER , TON , CHA , and IFR high-silica zeolites with a large diversity of the OH band shapes were investigated. • A combined use of FT-IR spectroscopy and periodic DFT calculations provided rationalization of observed spectral features. • Analysis of 44 different BAS in zeolites was carried out with the ω/r correlation method. • The statistical model of the BAS OH band in high-silica H- CHA was proposed based on empirical FF simulations. • Two mechanisms for the formation of free OH BAS in IFR were identified by analysis of 20 Al pairs at the DFT level.