Distinguishing Active Site Identity in Sn-Beta Zeolites Using 31P MAS NMR of Adsorbed Trimethylphosphine Oxide

The identity of the active sites of hydrophobic Lewis acid zeolites was elucidated by solid-state 31P nuclear magnetic resonance (NMR) spectroscopy following adsorption of trimethylphosphine oxide (TMPO) probe molecules. The adsorption of TMPO on these materials resulted in distinct 31P NMR resonances between δiso = 59.9 and 54.9 ppm that correspond to unique chemical environments of the Lewis acidic heteroatoms in the Beta zeolite framework. The 31P NMR resonances were assigned to sites in Sn-Beta zeolites by correlating the variation of 31P NMR spectra during TMPO titration experiments with the corresponding changes in the 119Sn NMR spectra. This method allowed us to establish quantitative relationships between the assignments for each site and the catalytic activity for the glucose isomerization and aldol condensation reactions. The rate of glucose isomerization directly correlated with the combined integrated intensities of 31P MAS NMR resonances at δiso = 55.8 and 54.9 ppm, which amounted to 12–33% of total Sn sites. In contrast, the integrated peak area of a different site at δiso = 58.6 ppm was shown to correlate with aldol condensation activity. The probing method used to identify and quantify distinct active sites within the framework of low-defect Lewis acid zeolites developed in this work is applicable to a wide range of microporous materials, regardless of heteroatom identity