Disentangling Brønsted Acid Sites and Hydrogen-Bonded Silanol Groups in High-Silica Zeolite H-ZSM-5

Hydrogen-bonded SiOH defects and Brønsted acidic bridging OH groups, defined as Brønsted acid sites (BAS), yield overlapping signals in 1H MAS NMR and FTIR spectroscopies. Here, a clear distinction in the distribution of SiOH defect sites between industrial ZSM-5 zeolites with Si/Al ratios of 14 and 20 as well as 60 is reported. At a high Si/Al = 60 ratio, hydrogen-bonded SiOH groups appear as signals in the 1H MAS NMR spectrum of H-ZSM-5 between 3 and 8 pm, thus severely overlapping with signals caused by unperturbed and hydrogen-bonded BAS. FTIR spectra are similarly affected. The hydrogen-bonded SiOH groups are paired, and they are clearly identified by 1H double-quantum single-quantum (DQ-SQ) MAS NMR correlation spectroscopy. A selective analysis was carried out by 1H{27Al} rotational echo adiabatic pulse double resonance (REAPDOR) to yield H–Al distances for the BAS, whereas the silanols do not significantly contribute to this experiment due to short T2 relaxation times. Spectral overlap of BAS and defect sites is suggested to be of some relevance in particular for zeolites with high Si/Al ratios.