Time-Optimized ¹⁵N qNMR Determination of Lewis and Brønsted Site Concentrations and Chemical Bonding of Pyridine at Brønsted Sites in Amorphous Aluminum Hydroxide Fluoride

Time-optimized 15 N quantitative NMR (qNMR) is described for the precise determination of Lewis and Brønsted site concentrations per gram catalyst, enabling a measurement time reduction of more than a factor of 10. The proof-of-principle is demonstrated for amorphous aluminum hydroxide fluorides with their tunable Lewis to Brønsted functionalities and relevance to highly active and selective heterogeneous catalysis using pyridine as a widely accepted probe molecule for Lewis and Brønsted sites. Two different Lewis centers were found, and reliable site concentrations down to the micromole per gram region can be determined. Furthermore, 1 H{ 15 N} REDOR NMR for the acid proton at the Brønsted site yields a distance of the 15 N atom of bonded pyridine to this acid proton of (0.112 ± 0.002) nm, such that fast motion/diffusion of the Brønsted site-bonded pyridine at the surface can be excluded.