Measurement of the global acidity of solid acids by P-31 MAS NMR of chemisorbed triethylphosphine oxide

The bonding of triethylphosphine oxide (TEPO) to the acid site of a surface produces a systematic change in the phosphorus-31 isotropic chemical shift that is proportional to the acid strength of the adsorption site. The acid ity of several amorphous solid acids of various strengths, several zeolites (HZSM-5, HY, TS-1, and H-Mordenite), and the crystalline solid 12-tungstop hosphoric acid have been measured. A single parameter scale is developed ut ilizing the chemical shift of TEPO adsorbed on solid acids. Adsorption of T EPO onto Lewis acids demonstrates that this probe measures the global acidi ty of adsorption sites and is insensitive to the Bronsted or Lewis nature o f the site. The significant difference between measuring acidity through th e change in chemical shift of chemisorbed TEPO and those based on different ial techniques is demonstrated for the case of 12-tunstophosphoric acid whe re:a larger acidity is measured by TEPO than by calorimetry. The difference in acidity measured by these two experiments allows for the estimate of th e energy needed to open the 12-tungstophosphoric acid lattice to include py ridine-sized molecules. Simple one pulse MAS NMR techniques are shown to be capable of measuring the global acidity of a wide variety of solid acids.