Spatial Proximities between Brønsted Acid Sites, AlOH Groups, and Residual NH4+ Cations in Zeolites Mordenite and Ferrierite

Abstract

The interaction between Brønsted acid sites (BAS), AlOH groups, and residual NH4+ cations in zeolites mordenite and ferrierite was investigated by solid-state 1H magic-angle spinning and 1H–27Al rotational-echo adiabatic-passage double-resonance (REAPDOR) nuclear magnetic resonance (NMR) techniques in combination with density functional theory (DFT) cluster calculations. Higher temperatures are needed for the full elimination of NH4+ (853 K instead of 723 K compared to other high-silica zeolites). NMR data reveal that residual NH4+ cations are primarily in the vicinity of BAS and other NH4+ in the case of mordenite and AlOH groups in ferrierite materials. DFT calculations for a variety of cluster models with BAS and NH4+ ions embedded in their zeolite environment rationalize the experimental data. It is suggested that NH4+ ions are preferably stabilized within 8-ring pores, explaining the higher temperature required to decompose them.