Hydrogen Bonds Dominate Brønsted Acid Sites in Zeolite SSZ-42: A Classification of Their Diversity

The zeolite catalyst SSZ-42 shows a remarkable high abundance (≈80 %) of hydrogen-bonded Brønsted acid sites (BAS), which are deshielded from the ¹ H chemical shift of unperturbed BAS at typically 4 ppm. This is due to their interaction with neighboring oxygen atoms in the zeolite framework when oxygen alignments are suitable. The classification and diversity of hydrogen bonding is assessed by DFT calculations, showing that oval-shaped 6-rings and 5-rings allow for a stronger hydrogen bond to oxygen atoms on the opposite ring side, yielding higher experimental chemical shifts (δ (¹ H)=6.4 ppm), than circular 6-rings (δ(¹ H)=5.2 ppm). Cage-like structures and intra-tetrahedral interactions can also form hydrogen bonds. The alignment of oxygen atoms is expected to impact their role in the stabilization of intermediates in catalytic reactions, such as surface alkoxy groups and possibly transition states.