TrendsAcidity characterization of heterogeneous catalysts by solid-state NMR spectroscopy using probe molecules

- SSNMR approaches used in probing the acidity of solid acids are summarized.
- The 31P-TMP NMR approach is suitable for identifying acid types.
- 13C-Acetone and 1H-pyridine-d5 techniques are good for determining the acid strength for solid acids with large differences.
- The 31P-TMPO approach is superior in providing more detailed information regarding acid sites.
- Using DFT calculations and experimental observations, correlations between NMR parameters and the acidic strength of solid acids can be obtained.

Characterization of the surface acidic properties of solid acid catalysts is a key issue in heterogeneous catalysis. Important acid features of solid acids, such as their type (Brønsted vs. Lewis acid), distribution and accessibility (internal vs. external sites), concentration (amount), and strength of acid sites are crucial factors dictating their reactivity and selectivity. This short review provides information on different solid-state NMR techniques used for acidity characterization of solid acid catalysts. In particular, different approaches using probe molecules containing a specific nucleus of interest, such as pyridine-d5, 2-13C-acetone, trimethylphosphine, and trimethylphosphine oxide, are compared. Incorporation of valuable information (such as the adsorption structure, deprotonation energy, and NMR parameters) from density functional theory (DFT) calculations can yield explicit correlations between the chemical shift of adsorbed probe molecules and the intrinsic acid strength of solid acids. Methods that combine experimental NMR data with DFT calculations can therefore provide both qualitative and quantitative information on acid sites.