Global optimization of extraframework ensembles in zeolites: structural analysis of extraframework aluminum species in MOR and MFI zeolites

Khramenkova, Elena; Venkatraman, Harshini; Soethout, Victor; Pidko, Evgeny A.

doi:10.1039/d2cp03603g

Cited by 7 publications

(4 citation statements)

References 57 publications

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“…Conversions between different Cu monomers, or monomers and dimers, may play an important role in the hydrothermal aging (HTA) of these materials at high temperatures and H 2 O pressures, which is of critical importance for commercial SCR catalysts. Changes in Cu speciation have been observed following HTA treatments. − Our thermodynamic model-predicted depletion of ZCuOH sites at high temperatures is consistent with observations in these studies, however, capturing the microscopic details of speciation changes in HTA environments likely requires accounting for Al extraframework species and Al mobility. − …”

Section: Discussionsupporting

confidence: 82%

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Competition between Mononuclear and Binuclear Copper Sites across Different Zeolite Topologies

Wijerathne,

Sawyer,

Daya

et al. 2024

JACS Au

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A key challenge for metal-exchanged zeolites is the determination of metal cation speciation and nuclearity under synthesis and reaction conditions. Copper-exchanged zeolites, which are widely used in automotive emissions control and potential catalysts for partial methane oxidation, have in particular evidenced a wide variety of Cu structures that are observed to change with exposure conditions, zeolite composition, and topology. Here, we develop predictive models for Cu cation speciation and nuclearity in CHA, MOR, BEA, AFX, and FER zeolite topologies using interatomic potentials, quantum chemical calculations, and Monte Carlo simulations to interrogate this vast configurational and compositional space. Model predictions are used to rationalize experimentally observed differences between Cu-zeolites in a wide-body of literature, including nuclearity populations, structural variations, and methanol per Cu yields. Our results show that both topological features and commonly observed Al-siting biases in MOR zeolites increase the population of binuclear Cu sites, explaining the small population of mononuclear Cu sites observed in these materials relative to other zeolites such as CHA and BEA. Finally, we used a machine learning classification model to determine the preference to form mononuclear or binuclear Cu sites at different Al configurations in 200 zeolites in the international zeolite database. Model results reveal several zeolite topologies at extreme ends of the mononuclear vs binuclear spectrum, highlighting synthetic options for realization of zeolites with strong Cu nuclearity preferences.

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Section: Discussionsupporting

confidence: 82%

“… 138 − 140 Our thermodynamic model-predicted depletion of ZCuOH sites at high temperatures is consistent with observations in these studies, however, capturing the microscopic details of speciation changes in HTA environments likely requires accounting for Al extraframework species and Al mobility. 140 − 143 …”

Section: Discussionmentioning

confidence: 99%

Competition between Mononuclear and Binuclear Copper Sites across Different Zeolite Topologies

Wijerathne,

Sawyer,

Daya

et al. 2024

JACS Au

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“…Thus, at several stages of the study, a limitation has appeared in terms of quality of sampling of the configuration space (Al/H location, water adsorption configurations, cooperative effects, possible dynamic effects on transition state stability). This opens perspectives both on the use of genetic algorithms 65 and ab initio molecular dynamics. 66 Moreover, even more complex systems exist in reality.…”

Section: ■ Conclusionmentioning

confidence: 99%

Impact of Point Defects, External Surfaces Sites, and Si/Al Ratio on Al–O Bonds Hydrolysis Kinetics at the Initiation of Faujasite Dealumination

Jarrin,

de Bruin,

Chizallet

2024

ACS Catal.

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Faujasite is one of the most industrially employed zeolite material, mainly for catalytic applications. Its properties are dependent on its pore network, which can be tuned by dealumination upon steaming. Previous theoretical studies on the subject based on the density functional theory identified plausible dealumination mechanisms for simple bulk structures of faujasite, but to gain a more complete understanding of dealumination, and to be able to compare to experimental situations, theoretical studies must be performed on more realistic and complex systems. In this article, we identify the intermediates and transition states for the first Al–O bond hydrolysis initiating dealumination in the bulk of faujasite for various Si/Al ratios, in the presence of silanol nests defects, and at the external surface of faujasite. We find that Al–O bond breaking is made easier from a kinetic perspective on some sites of the external surface of the material, considering the water adsorbed state as the reference. Moreover, the presence of defects and a decrease of the Si/Al ratio also lower the free energy barriers necessary to dealuminate. These trends are less clear when considering apparent free energy barrier, when the water desorbed state has been taken as a reference. Subtle local effects enter into play explaining the barrier rankings, including a participation of the hydrogen bond network strengthening at the outermost surface of the zeolite, which significantly stabilizes the highest free energy transition state in some cases. These hydrogen bonds make Brønsted–Evans–Polanyi relationships blurrier at the surface than at the bulk sites, suggesting that local effects on the stabilization of the transition states need to be explicitly considered.

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“…Due to the diversity of possible LASs and aluminum species, evaluating the structure and strength of Lewis acidic aluminum remains an active research area. , Both framework and extraframework aluminum species (EFAl) have been suggested as active sites. van Bokhoven et al detected the existence of the threefold coordinated Al species using in situ X-ray absorption near-edge spectroscopy at the Al K-edge.…”

Section: Introductionmentioning

confidence: 99%

Understanding the Hydrocracking of Polycyclic Aromatic Hydrocarbons within FAU Zeolites: Hydrogen Splitting Catalyzed by the Frustrated Lewis Pair

et al. 2023

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To understand the hydrocracking of aromatic rings in the petrochemical industry, the first step, hydrogen splitting on catalytic centers, has been studied based on density functional theory. Using ab initio thermodynamic analysis, the Lewis acid site (LAS) composed of a single extraframework-Al 3+ atom is predicted to be most stable mononuclear extraframework aluminum species during high-temperature calcination. Based on unbiased ab initio molecular dynamics (AIMD) simulations, this LAS is found to be the best catalytic center for hydrogen splitting, where hydrogen can be split spontaneously, resulting in the stable tetrahydroanthracen-1-ylium cation. With the help of AIMD and metadynamics, free energy landscapes characterizing hydrogen splitting processes on five different catalytic centers, viz., single extraframework-Al 3+ atom, two molecular frustrated Lewis pair (FLP) sites, and two ionic FLP sites, have been examined. Each hydrogen splitting occurs via a single-step concerted mechanism. Two molecular FLP sites exhibit higher splitting reactivity with lower free energy barriers. Further charge analysis reveals that the reactivity of FLP sites is strongly affected by the local electric field generated by the hydride acceptor and proton acceptor within FLP sites.

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Global optimization of extraframework ensembles in zeolites: structural analysis of extraframework aluminum species in MOR and MFI zeolites

Abstract: Metal-modified zeolites are versatile catalytic materials with a wide range of industrial applications. Their catalytic behaviour is determined by the nature of the externally introduced cationic species, i.e., its geometry,...

Cited by 7 publications

References 57 publications

Competition between Mononuclear and Binuclear Copper Sites across Different Zeolite Topologies

Competition between Mononuclear and Binuclear Copper Sites across Different Zeolite Topologies

Impact of Point Defects, External Surfaces Sites, and Si/Al Ratio on Al–O Bonds Hydrolysis Kinetics at the Initiation of Faujasite Dealumination

Understanding the Hydrocracking of Polycyclic Aromatic Hydrocarbons within FAU Zeolites: Hydrogen Splitting Catalyzed by the Frustrated Lewis Pair

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