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 NH 4 + cations in zeolites mordenite and ferrierite was investigated by solid-state 1 H magic-angle spinning and 1 H− 27 Al 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 NH 4 + (853 K instead of 723 K compared to other high-silica zeolites). NMR data reveal that residual NH 4 + cations are primarily in the vicinity of BAS and other NH 4 + in the case of mordenite and AlOH groups in ferrierite materials. DFT calculations for a variety of cluster models with BAS and NH 4 + ions embedded in their zeolite environment rationalize the experimental data. It is suggested that NH 4 + ions are preferably stabilized within 8-ring pores, explaining the higher temperature required to decompose them.

Response of Hydrogen-Bonded Brønsted Acid Sites in ZSM-5 to Adsorption and H/D Exchange of Hydrocarbons

“…Larger alignment angles can also allow hydrogen bonds, but then, they become less likely. The concept was experimentally tested for zeolites ZSM-5, 25 beta, 26 mordenite as well as ferrierite 27 and SSZ-42. 28 Mordenite and ferrierite have a very low content of HB-BAS, and SSZ-42 is the current record holder with about 80% hydrogen bonds.…”