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

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.

keywords

STATE P-31 NMR; TRIMETHYLPHOSPHINE OXIDE; ACID STRENGTH; QUANTUM-MECHANICS; CHEMICAL-SHIFT; AB-INITIO; H-BETA; SPECTROSCOPY; ADSORPTION; CATALYSIS

subject category

Chemistry, Multidisciplinary

authors

Bornes, C; Fischer, M; Amelse, JA; Geraldes, CFGC; Rocha, J; Mafra, L

Groups

G1 - Porous Materials and Nanosystems
G4 - Renewable Materials and Circular Economy

Projects

CICECO - Aveiro Institute of Materials (UIDB/50011/2020)

CICECO - Aveiro Institute of Materials (UIDP/50011/2020)

Variable pressure solid-state NMR methods to study gas-surface interactions in nanoporous sorbent materials for industrial post/pre-combustion CO2 capture (PTDC/QEQ-QAN/6373/2014)

Rede Nacional de Ressonância Magnética Nuclear (PTNMR)

Unveiling CO2 chemisorption mechanisms in solid adsorbents via surface-enhanced ex(in)-situ NMR (NMR4CO2)

acknowledgements

C.B. acknowledges FCT for Doctoral Fellowship PD/BD/142849/2018 integrated in the Ph.D. program in NMR applied to chemistry, materials, and biosciences (Grant PD/00065/2013). This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, Grants UIDB/50011/2020 and UIDP/50011/2020, financed by national funds through the FCT/MEC and when appropriate cofinanced by FEDER under the PT2020 Partnership Agreement. We also thank FCT for funding the project PTDC/QEQ-QAN/6373/2014. The NMR spectrometers are part of the National NMR Network (PTNMR) and are partially supported by Infrastructure Project 022161 (cofinanced by FEDER through COMPETE 2020, POCI and PORL and FCT through PIDDAC). This work has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant 865974). M.F. acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG; German Research Foundation), Project 389577027 (FI1800/5-1). This work wassupported by the North-German Supercomputing Alliance (HLRN).