Acid properties of H-type mordenite studied by solid-state NMR

Acidic properties of H-type mordenite have been studied by solid-state NMR. The framework Si/Al ratios are determined by 29Si MAS NMR spectra, and the contents of framework Al and Brønsted acid sites are determined. 27Al MAS NMR spectra show a presence of extra-framework Al as well as “invisible” Al. The 1H MAS NMR spectra consist of hydrogen-bonded Si–OH, Brønsted acid site, tightly adsorbed water, Al–OH and isolated Si–OH, the amounts of which are evaluated. The average 1H chemical shifts of Brønsted acid sites without proton exchange are roughly estimated, which are in the following order; HM10 < HM15 < HM20. The acid strength of Brønsted acid sites increases with increase in the Si/Al ratio. Trimethylphosphine oxide (TMPO) is adsorbed as a probe molecule from the gas phase to avoid the effect of solvents. A part of the acid sites are sampled by TMPO molecules. The average 31P chemical shift of chemisorbed TMPO increases with increase in the Si/Al ratio, demonstrating again that the acid strength increases with increase in the Si/Al ratio.