CHARACTERIZATION OF ACID SITES IN ZEOLITIC AND OTHER INORGANIC SYSTEMS USING SOLID-STATE P-31 NMR OF THE PROBE MOLECULE TRIMETHYLPHOSPHINE OXIDE

The ability to determine the types and concentrations of acid sites in zeolites and fluid-catalytic cracking systems is important for an inc reased understanding of structure/performance relationships in these m aterials. Currently, a variety of thermal methods exist that allow qua ntitative measurement of the Bronsted acid site concentration. In addi tion, numerous spectroscopic methods using probe molecules are availab le for qualitative and quantitative detection of both Bronsted and Lew is acid sites. In studies utilizing solid-state NMR spectroscopy, prob e molecules containing P-31 nuclei present substantial advantages over probes isotopically enriched with C-13 and N-15 nuclei. These advanta ges include increased sensitivity and chemical shift dispersion. While a number of phosphorus-based experiments have probed the interaction of trimethylphosphine with solid acid catalysts, initial studies of th e more stable trimethylphosphine oxide (TMPO) have only been reported on amorphous silica-alumina surfaces. We now report the successful com pletion of TMPO studies of acid sites in several systems including gam ma-alumina; HY, USY, and dealuminated Y zeolites; and a silica-alumina catalyst with an aluminum concentration of 13%. Comprehensive and con sistent assignments to particular types of sites are made for all reso nance lines in the P-31 MAS NMR signals from TMPO. Based on results fr om dehydroxylated gamma-alumina, new chemical shift assignments are ma de for the TMPO/Lewis acid complex. The assignments of P-31 resonances from molecules not directly associated with nearby Al-27 nuclei (such as crystalline or physisorbed TMPO species) are supported using H-1/P -31/Al-27 triple-resonance NMR methods. The concentrations of Bronsted acid sites from the NMR results are compared with concentrations obta ined from isopropylamine/temperature-programmed-desorption measurement s, and substantial agreement between the methods is found.