Hydrogen Bond Formation of Brønsted Acid Sites in Zeolites

Brønsted
acid sites (BAS) in dry aluminosilicate zeolites,
also known as bridging OH–H groups, may form hydrogen
bonds with other framework oxygen atoms, O, in addition to BAS that
are free of such interactions. These are referred to as perturbed
and unperturbed BAS, respectively. However, hydrogen bonding may occur
only for specific geometric orientations between two framework oxygen
atoms, allowing an OH–H···O interaction.
To evaluate the possibility for such hydrogen bonding, we introduce
an alignment angle κ that describes the angle between the Si–OH–Al bisector and the OH–O direction
of a potential hydrogen-bonding donor–acceptor pair. A total
of 27 zeolite framework topologies were analyzed, and the relevance
of this approach was subsequently demonstrated for H-ZSM-5 with two
different Si/Al ratios. Here, it could be unequivocally shown by advanced
solid-state NMR methods that the debated 1H NMR signal
at ∼6 ppm arises due to the formation of hydrogen-bonded BAS.