We present a method to identify the active sites of hydrophobic Lewis acid zeolites using 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 = 50 and 65 ppm that correspond to unique chemical environments of the Lewis acidic heteroatoms in the Beta zeolite framework. The 31P NMR resonances were assigned to distinct 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 integrated intensity of a single 31P MAS NMR resonance at δiso = 55.0 ppm, which amounted to 12–32% of total Sn sites. In contrast, the sum of multiple Sn species 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 acidic zeolites developed in this work is applicable to a wide range of microporous materials, regardless of heteroatom identity.

Unraveling active site identity in Lewis acid zeolites using MAS NMR of adsorbed probe molecules

255th National Meeting (2018)

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