Detection of the inhomogeneity of Brønsted acidity in H-mordenite and H-β zeolites: a comparative NMR study using trimethylphosphine and trimethylphosphine oxide as P NMR probes

The Brønsted acidity of H-mordenite (H-MOR) and H-β with similar Si/Al ratios has been characterized by conventional multinuclear solid-state NMR. 1 H/ 27 Al TRAPDOR NMR showed the presence of at least two different types of Brønsted acid sites in H-β. On the other hand, only a single 1 H resonance due to Brønsted acid sites in H-MOR was observed. Solid-state 31 P MAS NMR investigation of H-MOR and H-β zeolites loaded with trimethylphosphine oxide (TMPO) reveals multiple 31 P resonances, resulting from the reaction of TMPO with different types of Brønsted acid sites. The results demonstrate that there is a wide distribution in the strength of the Brønsted acidity in H-MOR and H-β zeolites. When trimethylphosphine was used as a probe molecule, however, only a single 31 P resonance was observed for both samples. Nevertheless, a larger 31 P downfield shift associated with a larger J _P–H value was observed for H-MOR as compared to that of H-β. This suggests that both 31 P chemical shift and J _P–H value might be used to correlate with their average Brønsted acidity, and thus their catalytic activity.