Interrogating the Lewis Acidity of Metal Sites in Beta Zeolites with ¹⁵N Pyridine Adsorption Coupled with MAS NMR Spectroscopy

Abstract

The Lewis acidity of isolated framework metal sites in Beta zeolites was characterized with ¹⁵N isotopically labeled pyridine adsorption coupled with magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The ¹⁵N chemical shift of adsorbed pyridine was found to scale with the acid character of both Lewis (Ti, Hf, Zr, Nb, Ta, and Sn) and Brønsted (B, Ga, and Al) acidic heteroatoms. The ¹⁵N chemical shift showed a linear correlation with Mulliken electronegativity of the metal center in the order Ti < Hf < Zr < Nb < Ta < Sn < H⁺. Theoretical calculations using density functional theory (DFT) showed a strong correlation between experimental ¹⁵N chemical shift and the calculated metal-nitrogen bond dissociation energy, and revealed the importance of active site reorganization when determining adsorption strength. The relationships found between ¹⁵N pyridine chemical shift and intrinsic chemical descriptors of metal framework sites complement adsorption equilibrium data and provide a robust method to characterize, and ultimately optimize, metal-reactant binding and activation for Lewis acid zeolites. Direct ¹⁵N MAS NMR detection protocols applied to the Lewis acid-base adducts allowed the differentiation and quantification of framework metal sites in the presence of extraframework oxides, including highly quadrupolar nuclei that are not amenable for quantification with conventional NMR methods.