Adsorption of Trimethylphosphine Oxide on Silicalite Studied by Solid-State NMR

The adsorption state of trimethylphosphine oxide (TMPO) on siliceous MFI-type zeolite, silicalite, has been studied by solid-state NMR. TMPO was loaded by vapor as well as the solvent methods, and 31P and 1H magic-angle-spinning (MAS) NMR spectra were measured. Most of the 31P signals are located at lower frequency than that of crystalline TMPO, among which the 30-ppm signal is the largest. This is the first case of which the signal at about 30 ppm is dominant. The observation of the 30-ppm signal is independent of the TMPO introduction method, vapor or solvent. The dominant signal at about 30 ppm in the sample prepared by vapor shifts to about 37 ppm for ten months, being accompanied by increase in the spinning sideband intensities. This fact indicates that TMPO molecules move to a more stable site with a smaller space, resulting in suppression of the molecular motion. The temperature increase leads to increase in the fraction of mobile TMPO molecules giving a 31P signal at 33.0 ppm. Comparison with the results of silica nanoparticle and mesoporous silica leads to the conclusion that the signals in the frequency range lower than crystalline TMPO are attributed to TMPO molecules confined in the micropores of the MFI-type framework and not interacting with Brønsted acid sites.