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

Hydrogen-bonded SiOH defects and Bronsted acidic bridging OH groups, defined as Bronsted acid sites (BAS), yield overlapping signals in H-1 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 H-1 double-quantum single-quantum (DQ-SQ) MAS NMR correlation spectroscopy. A selective analysis was carried out by H-1{Al-27} 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 T-2 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.