31P NMR Spectroscopy of Adsorbed Probe Molecules as a Tool for the Determination of the Acidity of Molecular Sieve Catalysts (A Review)

The review analyzes the potential of P-31 MAS NMR spectroscopy of adsorbed trisubstituted phosphines and phosphine oxides for the investigation of the acidic properties of molecular sieve catalysts. The main methodological approaches to identifying sites of different types are considered. The assignment of signals in the P-31 MAS NMR spectra of methyl-, ethyl-, n-butyl-, and phenyl-substituted phosphines and their oxides adsorbed on zeolites of different structural types and other molecular sieves to Lewis and Bronsted acid sites is reported, and the potential of P-31 MAS NMR spectroscopy as a tool for the determination of the strength of these sites and their amount is analyzed. It is shown that varying the size of the substituent allows assessment of the localization and accessibility of acid sites in zeolites.