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

The change in chemical shift of a chemisorbed probe molecule is an informat ive tool for the characterization of solid acids. This work demonstrates th at the phosphorus-31 isotropic chemical shift of the probe triethylphosphin e oxide (TEPO) can be used to identify the acidity of multiple acid sites o n a surface. The difference in the phosphorus-31. chemical shift of chemiso rbed TEPO with respect to physisorbed TEPO, Delta delta, establishes a scal e of solid acidity that can be validated by comparison to enthalpies of cal orimetric 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 p yridine in a cyclohexane slurry that correlate with Delta delta values of 6 6, 48, 36, and 22 ppm, respectively. (C) 1999 Academic Press.