What Is Being Measured with P-Bearing NMR Probe Molecules Adsorbed on Zeolites?

authors	Bornes, C; Fischer, M; Amelse, JA; Geraldes, CFGC; Rocha, J; Mafra, L
nationality	International
journal	JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
keywords	STATE P-31 NMR; TRIMETHYLPHOSPHINE OXIDE; ACID STRENGTH; QUANTUM-MECHANICS; CHEMICAL-SHIFT; AB-INITIO; H-BETA; SPECTROSCOPY; ADSORPTION; CATALYSIS
abstract	Elucidating the nature, strength, and siting of acid sites in zeolites is fundamental to fathom their reactivity and catalytic behavior. Despite decades of research, this endeavor remains a major challenge. 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 Bronsted and Lewis acid sites and the assessment of Brensted acid strengths. Recently, doubts have been raised regarding the assignment of the P-31 NMR resonances of TMPO-loaded zeolites. Here, it is shown that a judicious control of TMPO loading combined with two-dimensional H-1-P-31 HETCOR solid state NMR, DFT, and ab initio molecular dynamics (AIMD)-based computational modeling provides an unprecedented atomistic description of the host-guest and guest-guest interactions of TMPO molecules confined within HZSM-5 molecular-sized voids. P-31 NMR resonances usually assigned to TMPO molecules interacting with Bronsted sites of different acid strength arise instead from both changes in the probe molecule confinement effects at ZSM-S channel system and the formation of protonated TMPO dimers. Moreover, DFT/AIMD shows that the H-1 and P-31 NMR chemical shifts strongly depend on the siting of the framework aluminum atoms. This work overhauls the current interpretation of NMR spectra, raising important concerns about the widely accepted use of probe molecules for studying acid sites in zeolites.
publisher	AMER CHEMICAL SOC
issn	0002-7863
isbn	1520-5126
year published	2021
volume	143
issue	34
beginning page	13616
ending page	13623
digital object identifier (doi)	10.1021/jacs.1c05014
web of science category	8
subject category	Chemistry, Multidisciplinary
unique article identifier	WOS:000704514200019