Characterization of Acidic OH Groups in Zeolites of Different Types:  An Interpretation of NH₃-TPD Results in the Light of Confinement Effects

Abstract

Ammonia TPD experiments have been carried out on protonated forms of the zeolites FAU, FER, MFI, and MOR to investigate the acid strength dependence on OH group location and Al content. Effective adsorption energy distributions are derived using a regularization method and corrected to achieve a truer measure of acid strength using nonspecific interaction energies from atomistic Monte Carlo simulations. The correction energies include all interactions not associated with the OH groups and range from 18 to 37 kJ/mol, depending on the region and the zeolite structure. In the FAU structure, we find a bimodal distribution of adsorption energies in both the supercages and the sodalite cages, with the stronger acid sites being more common in the supercages. In contrast, essentially the same acid strength for different regions in MOR is seen, though the acid strength for those sites in the side pockets may be slightly lower. For FER and MFI, the TPD gives unimodal distributions and therefore prohibits distinction between different regions. In agreement with the model of Barthomeuf et al., we see an apparently linear decline (60 kJ/mol) in acid strength beyond a certain aluminum framework density. However, we see a smaller but consistent increase (16 kJ/mol) in acidity before this threshold, despite model predictions that it should be constant.