Acidity characterization of solid acid catalysts by solid-state 31P NMR of adsorbed phosphorus-containing probe molecules: An update

Yi, Xiaoyan; Zheng, Anmin; Liu, Shang-Bin

doi:10.1016/bs.arnmr.2020.07.004

Cited by 4 publications

(3 citation statements)

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“…In other words, the acid strength should change with varying amounts of probe molecules. 5 However, because P/Al = 0.21, P/Al = 0.38, and P/Al = 0.75 samples correspond to exactly the same synthesis batch, with only different amounts of TMPO (Figure 1), it is clear that different adsorbate loadings strongly affect the spectra and their interpretation.…”

Section: ■ Results and Discussionsupporting

confidence: 68%

“…According to previous interpretations, increasing 31 P NMR chemical shifts are proportional to increasing acid strengths. , Therefore, the observed signal intensity loss in the Brønsted region (δ P > 69 ppm), due to larger amounts of confined TMPO, would mean that the sample with the lowest TMPO content (P/Al = 0.21) should contain a larger number of strong acid sites than the samples with higher TMPO loadings. In other words, the acid strength should change with varying amounts of probe molecules . However, because P/Al = 0.21, P/Al = 0.38, and P/Al = 0.75 samples correspond to exactly the same synthesis batch, with only different amounts of TMPO (Figure ), it is clear that different adsorbate loadings strongly affect the spectra and their interpretation.…”

Section: Resultsmentioning

confidence: 99%

“…31 P NMR chemical shifts of phosphines have been widely used to study the acid properties of solid catalysts. , Trimethylphosphine oxide (TMPO) has been the most widely used probe molecule in solid-state NMR studies of zeolites, − metal–organic frameworks, , heteropolyacids, , etc., due to its high sensitivity and chemical stability and because it is safer than the also used trimethylphosphine . The small kinetic diameter of TMPO, ∼5.5 Å, allows its diffusion in most medium-pore size materials, in contrast with the larger tributylphosphine oxide and triphenylphosphine oxide molecules, commonly used to study the external surface of such materials .…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2021

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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 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. Here, it is shown that a judicious control of TMPO loading combined with two-dimensional 1H–31P 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. 31P NMR resonances usually assigned to TMPO molecules interacting with Brønsted sites of different acid strength arise instead from both changes in the probe molecule confinement effects at ZSM-5 channel system and the formation of protonated TMPO dimers. Moreover, DFT/AIMD shows that the 1H and 31P 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.

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Section: ■ Results and Discussionsupporting

confidence: 68%