A solid acidity scale based on the 31P MAS-NMR shift of chemisorbed triethylphosphine oxide

The change in chemical shift of a chemisorbed probe molecule is an informative tool for the characterization of solid acids. This work demonstrates that the phosphorus-31 isotropic chemical shift of the probe triethylphosphine oxide (TEPO) can be used to identify the acidity of multiple acid sites on a surface. The difference in the phosphorus-31. chemical shift of chemisorbed TEPO with respect to physisorbed TEPO, Delta delta, establishes a scale of solid acidity that can be validated by comparison to enthalpies of calorimetric methods. A linear correspondence is observed for the strong solid acid (silica-gel)(n)(SbCl3)-Cl-V with four acid sites and with enthalpies of -66, -46, -33, and -21 Kcal/mol for interaction of the solid acid with pyridine in a cyclohexane slurry that correlate with Delta delta values of 66, 48, 36, and 22 ppm, respectively. (C) 1999 Academic Press.