Bronsted sites of enhanced acidity in zeolites: Experimental modelling

Low-temperature interaction of BF, with acid forms of zeolites is shown to take place in two different ways: as an amphoteric complex (I) and as an acid-base complex (ii). These complexes are observed using FTIR spectroscopy and their assignment is based on theoretical calculations. In the case of BF3/H-ZSM-5 only the weaker bonded complex (II) is observed, whereas in the BF3/H-EMT system both types of complexes are present. Formation of amphoteric complex (I) affects the acid strength of the remaining uncomplexed Bronsted hydroxyls (as shown by low-temperature adsorption of CO on the samples pretreated with BF3). This provides for the first time experimental proof that enhanced Bronsted acidity may result from interaction of acid forms of zeolites with Lewis acids. In contrast, formation of complex (II) does not affect the acidity of the zeolite. The fact that formation of complex (I) was not observed in the case of the BF3/H-ZSM-5 system indicates the significance of Al content and/or topological factors for creation of enhanced acidity in zeolites.