Solid State NMR of Acid Sites in Non-Classical Zeolites

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Brønsted acid sites in zeolites have protons located on a bridging oxygen atom between Si and Al, and they are highly active in a wide range of catalytic reactions. Their detection and investigation with respect to the variety of distinct sites is a long-standing subject in fundamental and applied research. Here, a novel geometrical concept was introduced in the initial funding period of this project in terms of an oxygen alignment model, enabling us to predict hydrogen bonding possibilities for Brønsted acid sites. The structural prediction was verified by a range of experimental examples (zeolite Y, ZSM-5, Beta, SSZ-42, SSZ-55, and SSZ-59), where an excellent agreement between the amount of predicted and NMR spectroscopically observed hydrogen bonds was found. The NMR spectroscopic proof of the presence of hydrogen-bonded acid sites is achieved by the analyses of the 1H-27Al heteronuclear dipolar interactions. Their magnitude can clearly distinguish between acid sites near Al atoms and other protic species, such as H2O, NH4+ or SiOH, which can also form hydrogen bonds and show similar 1H chemical shifts as acid sites. The differentiation of Brønsted acid sites within the zeolite framework and extra-framework AlOH is possible by their different 27Al quadrupolar interactions.Future work will deepen and broaden the understanding of the oxygen alignment model regarding its impact on acidic as well as dynamic properties of zeolite acid sites and investigate extra-framework Al moieties by modern 27Al NMR techniques (static wideline and MAS methods). Local proton hopping between neighbored oxygen atoms will be investigated in dependence of mutual oxygen alignment angles. DFT calculations will be used to explore oxygen alignments also with respect to the transition state structure of local proton hopping. Extra-framework Al species are less ordered and their rich diversity will be investigated by using the editing possibilities offered by different 27Al{1H} cross-polarization times in modern, static wideline experiments. Special emphasis will also be given to the further development of the analyses of 1H-27Al dipolar interactions by various double-resonance NMR pulse techniques. These distance sensitive methods will potentially allow to distinguish between isolated, single-site Al catalysts, where additional second Al atoms are distant (> 5Å) from the Brønsted proton, and local Al clustering (two or more Al atoms within 5 Å to the proton). Such a differentiation between single-site and multi-site catalysts should be of particular interest, because a high (local) Al density can alter the transition state energies of catalytic reactions.