Hydrogen Bond Formation of Brønsted Acid Sites in Zeolites

Bronsted acid sites (BAS) in dry aluminosilicate zeolites, also known as bridging OH-H groups, may form hydrogen bonds to other framework oxygen atoms, O, in addition to BAS which are free of such interactions. These are referred to as perturbed and unperturbed BAS, respectively. However, hydrogen bonding may only occur 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 is 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 MAS NMR signal at ~6 ppm arises due to the formation of hydrogen-bonded BAS.