Elucidating the Germanium Distribution in ITQ-13 Zeolites by Density Functional Theory

ITQ-13 is a medium-pore zeolite that can be prepared in all-silica form and as silicogermanate with Si/Ge ratios as low as 3. Usually synthesised in the presence of fluoride, ITQ-13 is among the very few systems containing fluoride anions in two distinct cage types, cube-like d4r units and [4 ⋅ 56 ] cages. Here, dispersion-corrected density functional theory (DFT) calculations are used to investigate the energetically most favourable Ge distributions for Si/Ge ratios between 55 and 6. The calculations show Ge atoms are incorporated at both the corners of d4r cages and at the basal plane of the [4 ⋅ 56 ] cages, in accordance with 19 F NMR spectroscopy. Two Ge atoms at adjacent corners of [4 ⋅ 56 ] cages are stable at the highest Ge content considered (Si/Ge=6). Such a local environment has not yet been considered in the experimental literature. A calculation of the corresponding 19 F NMR resonance points to overlap with other resonances, which might preclude its clear identification. Additional calculations investigate the variation of the dynamic behaviour of the fluoride anions as a function of the local environment as well as the selective defluorination of the [4 ⋅ 56 ] cages.publishe

Estudo das propriedades de acidez de zeólitos por RMN do estado sólido

Mestrado em QuímicaOs zeólitos têm propriedades únicas, como a porosidade, acidez e estabilidade térmica, desejadas quer na investigação académica, quer nas aplicações industriais. Neste trabalho foram preparadas amostras do zeólito ZSM-5 (Zeolite Socony Mobil–5) com propriedades de acidez distintas, nomeadamente o tipo e a força dos centros ácidos, investigadas recorrendo a experiências multinucleares 1D e 2D de RMN do estado sólido. As moléculas sonda de fósforo apresentam um grande potencial para caracterizar detalhadamente as propriedades de acidez destes materiais, por RMN do estado sólido. Esta tese apresenta novos resultados, mostrando que a interação entre a molécula sonda (TMPO) e os centros ácidos de Brønsted e Lewis não resulta em duas regiões distintas do espectro de RMN de 31P, contrariamente ao que está descrito na literatura. Experiências 2D 1H-31P CP-HETCOR foram utilizadas pela primeira vez em zeólitos com TMPO adsorvida para obter mais informação acerca da interação desta molécula sonda com os centros ácidos. Combinando as experiências 1D e 2D de um conjunto de zeólitos com propriedades distintas, foi possível obter resultados e refutar ou resolver atribuições controversas encontradas na literatura. Neste trabalho foram também utilizados cálculos computacionais DFT, usando o método de cluster, para investigar a interação entre a molécula sonda e os centros ácidos de Brønsted.Zeolites have unique properties, such as porosity, acidity and thermal stability, desired for a wide range of both academic research and industrial applications. In this work, several ZSM-5 zeolite samples (Zeolite Socony Mobil–5) with distinct acidic properties, namely acid type and strength, were prepared and investigated through multinuclear 1D and 2D NMR experiments. Phosphorous containing probes show great potential to achieve a comprehensive characterization of zeolite’s acidic features, via solid-state NMR. New results presented in this thesis reveal that the interaction of TMPO with Brønsted and Lewis acid sites, does not result in two distinct regions in the 31P NMR spectra, in contrast with literature reports. 2D 1H-31P CP-HETCOR experiments were used for the first time in TMPO-loaded zeolites to gain more information on the interaction of TMPO molecules with the zeolite centers. Combining 1D and 2D experiments obtained for a group of ZSM-5 zeolites with tuned acidic properties, provided results that refute or solve certain controversial literature assignments. Cluster-based DFT computational methods were also used to investigate the interaction between TMPO and Brønsted acid sites

What is being measured with p-bearing nmr probe molecules adsorbed on zeolites?

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.publishe

Elucidating the germanium distribution in ITQ-13 zeolites by density functional theory

ITQ-13 is a medium-pore zeolite that can be prepared in all-silica form and as silicogermanate with Si/Ge ratios as low as 3. Usually synthesised in the presence of fluoride, ITQ-13 is among the very few systems containing fluoride anions in two distinct cage types, cube-like d4r units and [4·56] cages. Here, dispersion-corrected density functional theory (DFT) calculations are used to investigate the energetically most favourable Ge distributions for Si/Ge ratios between 55 and 6. The calculations show Ge atoms are incorporated at both the corners of d4r cages and at the basal plane of the [4·56] cages, in accordance with 19F-NMR spectroscopy. Two Ge atoms at adjacent corners of [4·56] cages are stable at the highest Ge content considered (Si/Ge = 6). Such a local environment has not yet been considered in the experimental literature. A calculation of the corresponding 19F-NMR resonance points to overlap with other resonances, which might preclude its clear identification. Additional calculations investigate the variation of the dynamic behaviour of the fluoride anions as a function of the local environment as well as the selective defluorination of the [4·56] cages

Investigating trimethylphosphine oxide interactions with Brønsted and Lewis acid sites in zeolites: a comparative solid-state NMR study of wet-phase and gas-phase adsorption techniques

This study examines the differences in using two primary methods for adsorbing TMPO onto acid catalysts and their influence on the NMR spectra of TMPO-loaded zeolites. The findings show that TMPO molecules interact with both Brønsted and Lewis acid sites, and that the amount of (TMPO)2H+ dimers formed increases with the amount of TMPO added. The use of dichloromethane as a solvent inhibits the formation of dimers and crystalline TMPO but may lead to inaccurate estimations of acid site strength and quantification due to interactions with residual solvent molecules. The gas-phase method is preferred for adsorbing TMPO onto zeolites, but caution is needed to avoid forming a high number of dimers that make interpretation of the NMR spectra challenging.publishe

Elucidating the nature of the external acid sites of ZSM‐5 zeolites using NMR probe molecules

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Raman and Fluorescence Imaging of Polyoxometalate Composite Agarose Films

Composite agarose films were prepared with EuIII containing polyoxometalates, in particular the organic-inorganic hybrid [Eu(W5O18)2(pic)4]13– (Hpic = 3-hydroxypicolinic acid) and a Mo blue EuIII derivative. The latter compound shows interesting EuIII based photoluminescence properties, described for a molybdenum blue compound for the first time. Imaging of optical properties and morphology of the films was explored by Raman and fluorescence measurements in a confocal microscopy system, together with the AFM profile. Raman and fluorescence imaging are powerful tools for the characterization of the films, in particular their chemical composition and homogeneity at the micrometric scale.publishe