Investigation of solid acidity in zeolites by nuclear magnetic resonance

Abstract

The nature of Bronsted acidity in zeolite catalysts was investigated by several in situ NMR experiments. The proton sites were directly observed by IH-MAS NMR experiments. Dynamics of the zeolite protons were invested using several NMR experiments. Acidic and non-acidic protons were distinguished using [ ]] I double-resonance NMR experiments. This work was the first successful I H- f 27AI NMR investigation of dehydrated zeolites using dipolar dephasing experiments. The protonation equilibria of some very weakly basic probe molecules were investigated by in situ NMR experiments. Acetonitrile was not fully protonated in zeolites, as studied by 13C, 15N, and 1H NMR. At high temperatures, however, the molecule was bent due to interaction with a framework conjugate base site, and a proton was then partially transferred from the zeolite to the base. This partial protonation of the adsorbed acetonitrile gave a relative ordering of zeolite acid strength as follows: HZSM-5 > steamed-HY > Beta > HY > high-silica HY, Mordenite. The proposed structure of adsorbed, partially-protonated acetonitrile was deduced from ab initio were erformed by collaborators. The rate of hydrogen-calculations that were performed by collaborators. The rate of hydrogen-deuterium exchange was investigated by I H-MAS NMR for benzene-d6 in H-zeolites. This rate of exchange was strongly dependent on the strength of benzene adsorption at sodium sites, which explains the decreased H-D exchange rate observed for H,Na-zeolites. Non-framework aluminum in steamed-HY zeolite increased the rate of H-D exchange, suggesting stronger acidity, but the mechanism of this process was not completely determined. Ab initio calculations were used to understand the reaction coordinate of this reaction. Protonation of Hammett indicator molecules in zeolites was investigated by in situ 15N-NMR. p-Nitroaniline was protonated in dehydrated zeolites, but the weaker base, p-nitrotoluene, was not. This contradicts other published reports and suggests that zeolites are not superacidic as previously suspected. The studies of weak base protonation suggest that the mechanism of solid acidity is very different from that in gas or solution phase. Adsorbed bases may interact with several framework sites to lower the transition barrier of protonation. This may explain why solid acids are very good catalysts even though they are not extremely strong Bronsted acids.