Zeolites are among the most widely used solid acid catalysts because they contain Brønsted and Lewis acid sites, exhibit shape selectivity, and are thermally stable. The current transition from a fossil feedstock, which requires strong Brønsted acidity catalysis, to a renewable feedstock, requiring catalysts with milder Brønsted acidity and/or Lewis acidity, is game changer. In this context, it is of interest to gain an atomic-level insight into the nature, strength and location of zeolite acid sites to fathom their reactivity and catalytic behaviour. it is of interest to gain an atomic-level insight into the nature, strength and location of zeolite acid sites to fathom their reactivity and catalytic behaviour. Trimethylphosphine oxide (TMPO) has been proposed as a reliable probe molecule to study the acid properties of solid acid catalysts, allowing the identification of distinct Brønsted and Lewis acid sites, and the assessment of Brønsted acid strengths. Recently, doubts have been raised regarding the assignment of the 31P NMR resonances of TMPO-loaded zeolites. This work provides an unprecedented insight into the status of TMPO molecules adsorbed in HZSM-5, challenge certain widely accepted 31P NMR spectral assignments, and raise important questions regarding the use of TMPO and other probe molecules to study zeolite Brønsted and Lewis acid sites. It is shown that changes in 31P NMR chemical shifts, usually attributed to differences in acid strength, may also arise from distinct stabilization energies of TMPOH+ ions resulting from confinement effects in ZSM-5 channels. 1H–31P heteronuclear correlation NMR of HZSM-5 samples carefully loaded with varying amounts of TMPO per Al framework site, combined with DFT and AIMD calculations, provide the best description of the TMPO species formed upon interaction with zeolite acid sites.
C. Bornes, M. Fischer, J. A. Amelse, C. F. G. C. Geraldes, J. Rocha, L. Mafra, What Is Being Measured with P‑Bearing NMR Probe Molecules Adsorbed on Zeolites?, J. Am. Chem. Soc.143 (34), 13616 (2021).
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