Measurement of the global acidity of solid acids by 31P 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 acidity of several amorphous solid acids of various strengths, several zeolites (HZSM-5, HY, TS-1, and H-Mordenite), and the crystalline solid 12-tungstophosphoric acid have been measured. A single parameter scale is developed utilizing the chemical shift of TEPO adsorbed on solid acids. Adsorption of TEPO onto Lewis acids demonstrates that this probe measures the global acidity of adsorption sites and is insensitive to the Bronsted or Lewis nature of the site. The significant difference between measuring acidity through the change in chemical shift of chemisorbed TEPO and those based on differential techniques is demonstrated for the case of 12-tunstophosphoric acid where:a larger acidity is measured by TEPO than by calorimetry. The difference in acidity measured by these two experiments allows for the estimate of the energy needed to open the 12-tungstophosphoric acid lattice to include pyridine-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.