Bornes Carlos, Fischer Michael, Amelse Jeffrey A, Geraldes Carlos F G C, Rocha João, Mafra Luís
CICECO, Aveiro Institute of Materials, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal.
Faculty of Geosciences, University of Bremen, 28359 Bremen, Germany.
J Am Chem Soc. 2021 Sep 1;143(34):13616-13623. doi: 10.1021/jacs.1c05014. Epub 2021 Aug 19.
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 P NMR resonances of TMPO-loaded zeolites. Here, it is shown that a judicious control of TMPO loading combined with two-dimensional H-P 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 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 H and P 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.
阐明沸石中酸位点的性质、强度和位置对于理解其反应活性和催化行为至关重要。尽管经过了数十年的研究，这项工作仍然是一个重大挑战。三甲基氧化膦（TMPO）已被提议作为研究固体酸催化剂酸性的可靠探针分子，可用于识别不同的布朗斯特和路易斯酸位点，并评估布朗斯特酸强度。最近，人们对负载TMPO的沸石的磷核磁共振（P NMR）共振归属提出了质疑。在此表明，通过明智地控制TMPO负载量，并结合二维氢磷异核相关（H-P HETCOR）固体核磁共振、密度泛函理论（DFT）和基于从头算分子动力学（AIMD）的计算建模，能够以前所未有的方式对限制在HZSM-5分子筛孔道内的TMPO分子的主客体和客体间相互作用进行原子尺度的描述。与不同酸强度的布朗斯特位点相互作用的TMPO分子的P NMR共振，实际上源于ZSM-5孔道系统中探针分子受限效应的变化以及质子化TMPO二聚体的形成。此外，DFT/AIMD表明，氢和磷的核磁共振化学位移强烈依赖于骨架铝原子的位置。这项工作彻底改变了目前对核磁共振谱的解释，引发了人们对广泛用于研究沸石酸位点的探针分子的重要关注。
