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

摘要

Time-optimized ~(15)N quantitative NMR (qNMR) is described for the precise determination of Lewis and Bronsted 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 Bronsted functionalities and relevance to highly active and selective heterogeneous catalysis using pyridine as a widely accepted probe molecule for Lewis and Bronsted sites. Two different Lewis centers were found,, and reliable site concentrations down to the micromole per gram region can be determined. Furthermore, ~1H{~(15)N} REDOR NMR for the acid proton at the Bronsted 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 Bronsted site-bonded pyridine at the surface can be excluded.