M. Boronat

Computational Modeling of the Mobility, Stability and Al Positioning Ability of Cyclic Cationic Organic Structure-Directing Agents in AEI Zeolite

P. Ferri, P. Cnudde, M. Moliner, V. Van Speybroeck, M. Boronat
JACS Au (Journal of the American Chemical Society)
2025
A1

Abstract 

The stability and mobility of a set of organic structure-directing agents (OSDAs) with different molecular geometries and chargedistribution confined within the pear-like cavities of neutral and Al-containingmodels of AEI zeolites have been investigated by using static densityfunctional theory calculations and ab initio molecular dynamics simulations.The objective is to identify the role of electrostatic interactions between theOSDAs’ positive charge at N+ atoms and the anionic framework AlO4−centers on the preferential stabilization of Al at specific crystallographicpositions, opening the possibility to modulate the Al distribution in AEIzeolites. We find that several classical piperidinium-based OSDAs withdiverse methyl-substituent patterns in the N-containing ring but asymmetrical charge distribution, as well as bulkier nonclassical azoniabicycle-heptane-based OSDAs with the positive chargeasymmetrically located at one side of the molecule, behave similarly. All of them remain almost immobile at the center of the aeicavity along the simulations and always stabilize Al preferentially at the T1 crystallographic position. In contrast, an azabicyclo-octane-based OSDA with the positive charge located outside a cyclo-octane ring lacking substituents exhibits an enhanced mobilitythat includes full rotation within the aei cage and the ability to reach the regions of the cavity not accessible to the other OSDAsinvestigated. As a result, this highly mobile OSDA preferentially stabilizes Al in the T3 site, which might lead to differences incatalyst activity and stability for zeolite samples synthesized using this OSDA.

DOI 

http://dx.doi.org/10.1021/jacsau.5c00094

Mobility and Reactivity of Cu+ Species in Cu-CHA Catalysts under NH3-SCR-NOx Reaction Conditions: Insights from AIMD Simulations

R. Millan, P. Cnudde, V. Van Speybroeck, M. Boronat
JACS Au (Journal of the American Chemical Society)
1 (10), 1778–1787
2021
A1

Abstract 

The mobility of the copper cations acting as active sites for the selective catalytic reduction of nitrogen oxides with ammonia in Cu-CHA catalysts varies with temperature and feed composition. Herein, the migration of [Cu(NH3)2]+ complexes between two adjacent cavities of the chabazite structure, including other reactant molecules (NO, O2, H2O, and NH3), in the initial and final cavities is investigated using ab initio molecular dynamics (AIMD) simulations combined with enhanced sampling techniques to describe hopping events from one cage to the other. We find that such diffusion is only significantly hindered by the presence of excess NH3 or NO in the initial cavity, since both reactants form with [Cu(NH3)2]+ stable intermediates which are too bulky to cross the 8-ring windows connecting the cavities. The presence of O2 modifies strongly the interaction of NO with Cu+. At low temperatures, we observe NO detachment from Cu+ and increased mobility of the [Cu(NH3)2]+ complex, while at high temperatures, NO reacts spontaneously with O2 to form NO2. The present simulations give evidence for recent experimental observations, namely, an NH3 inhibition effect on the SCR reaction at low temperatures, and transport limitations of NO and NH3 at high temperatures. Our first principle simulations mimicking operating conditions support the existence of two different reaction mechanisms operating at low and high temperatures, the former involving dimeric Cu(NH3)2-O2-Cu(NH3)2 species and the latter occurring by direct NO oxidation to NO2 in one single cavity.

Gold Open Access

DOI 

http://dx.doi.org/10.1021/jacsau.1c00337

Theoretical and Spectroscopic Evidence of the Dynamic Nature of Copper Active Sites in Cu-CHA Catalysts under Selective Catalytic Reduction (NH3–SCR–NOx) Conditions

R. Millan, P. Cnudde, A.E.J. Hoffman, C.W. Lopes, P. Concepcion, V. Van Speybroeck, M. Boronat
Journal of Physical Chemistry Letters
11, 23, 10060-10066
2020
A1

Abstract 

The dynamic nature of the copper cations acting as active sites for selective catalytic reduction of nitrogen oxides with ammonia is investigated using a combined theoretical and spectroscopic approach. Ab initio molecular dynamics simulations of Cu-CHA catalysts in contact with reactants and intermediates at realistic operating conditions show that only ammonia is able to release Cu+ and Cu2+ cations from their positions coordinated to the zeolite framework, forming mobile Cu+(NH3)2 and Cu2+(NH3)4 complexes that migrate to the center of the cavity. Herein, we give evidence that such mobilization of copper cations modifies the vibrational fingerprint in the 800–1000 cm–1 region of the IR spectra. Bands associated with the lattice asymmetric T-O-T vibrations are perturbed by the presence of coordinated cations, and allow one to experimentally follow the dynamic reorganization of the active sites at operating conditions.

DOI 

http://dx.doi.org/10.1021/acs.jpclett.0c03020

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