Acid property of MFI-type zeolites probed by trimethylphosphine oxide studied by solid-state NMR

• Acid properties of ZSM-5 type zeolite are studied by solid-state NMR.
• Trimethylphosphine oxide (TMPO) is introduced from the gas phase at 373 K.
• Almost all Brønsted acid sites are probed by TMPO.
• The 31P signals between 90 and 60 ppm are attributed to TMPO on Brønsted acid sites.

Acid properties of ZSM-5 type zeolite are studied by solid-state NMR using trimethylphosphine oxide (TMPO) as a probe molecule. TMPO is introduced from the gas phase at 373 K. The amounts of the loaded TMPO are quantified by the signal intensities of 1H MAS NMR spectra, which are compared with the numbers of Brønsted acid sites estimated by the Si/A ratio of the framework derived from 29Si MAS NMR spectra and the content of fourfold-coordinated Al from 27Al MAS NMR spectra. The use of 373 K for loading the probe molecules increases the loading level of TMPO, and thus almost all the acid sites are probed. The 31P chemical shifts in the range between 90 and 60 ppm are attributed to TMPO adsorbed on Brønsted acid sites, whereas the signals between 60 and 40 ppm are ascribed to TMPO adsorbed on other sites than Brønsted acid sites. The 13C chemical shift of methyl groups in the adsorbed TMPO depends mainly on the confined space as well as on the acid strength.

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