Millán Cabrera, Reisel
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- PublicationComputational study of heterogeneous catalytic systems. Kinetic and structural insights from Density Functional Theory(Universitat Politècnica de València, 2021-02-19) Millán Cabrera, Reisel; Boronat Zaragoza, Mercedes; Instituto Universitario Mixto de Tecnología Química; Agencia Estatal de Investigación[ES] En este trabajo estudiamos dos reacciones catalíticas relevantes para la industria y la localización del anión fluoruro en la zeolita RTH, sintetizada en medio fluoruro. El capítulo 3 es el primer capítulo de resultados, donde se estudia la reducción quimioselectiva del nitroestireno en las superficies Ni(111), Co(111), Cu(111) y Pd(111). El mecanismo generalmente aceptado de esta reacción está basado en el esquema propuesto por Haber en 1898, en el que la reacción puede transcurrir por dos rutas, la directa y la de condensación. En este capítulo exploramos ambas rutas, y observamos que la ruptura de los enlaces N-O y la consecuente formación de enlaces metal-O está más favorecida que la formación de enlaces N-H en las superficies Ni(111) y Co(111), debido al carácter oxofílico de ambos metales. Las etapas más lentas involucran la formación de enlaces N-H. En las superficies de metales nobles como Pt(111) y Pd(111) se observa el comportamiento contrario. La superficie Cu(111) es un caso intermedio comparado con los metales nobles y no nobles. Además, el nitroestireno interactúa con los átomos de Cu de la superficie solo a través de grupo nitro, con lo cual es un candidato ideal para alcanzar selectividades cerca del 100%. Sin embargo, la superficie Cu(111) no es capaz de activar la molécula de H2. En este sentido, proponemos un catalizador bimetálico basado en Cu, dopado con otro metal capaz de activar al H2, tales como el Pd o el Ni. En los capítulos 4 y 5 se ha estudiado la reducción catalítica selectiva de los óxidos de nitrógeno (SCR, en inglés) con amoníaco. Usando métodos de DFT, hemos encontrado rutas para la oxidación de NO a NO2, nitritos y nitratos con energías de activación relativamente bajas. También, hemos encontrado que la reducción de Cu2+ a Cu+ requiere la participación simultánea de NO y NH3. Posteriormente, hemos estudiado la influencia del NH3 en este sistema con métodos de dinámica molecular. El NH3 interacciona fuertemente con el Cu+ de forma que dos moléculas de este gas son suficientes para romper la coordinación del catión Cu+ con los oxígenos del anillo 6r, y formar el complejo lineal [Cu(NH3)2]+. Además, los cationes Cu2+ pueden ser estabilizados fuera de la red mediante la formación del complejo tetraamincobre(II). Debido a la presencia de los cationes Cu+ y Cu2+ coordinados a la red de la zeolita, aparecen bandas en la región entre 800-1000 cm-1 del espectro infrarrojo. El análisis de las frecuencias IR de varios modelos con Cu+ y Cu2+ coordinados al anillo 6r, o formando complejos con amoniaco indica que cuando los cationes Cu+ y Cu2+ están coordinados a los oxígenos del anillo 6r aparecen vibraciones entre 830 y 960 cm-1. Frecuencias en esta zona también se obtienen en los casos en que NO, NO2, O2 y combinaciones de dos de ellos están adsorbidos en Cu+ y Cu2+. Sin embargo, cuando los cationes Cu+ y Cu2+ están fuera del anillo (no hay enlaces entre los cationes de cobre y los oxígenos del anillo 6r) no se obtienen vibraciones de IR en esta región del espectro. Estos resultados indican que con el seguimiento del espectro IR durante la reacción SCR es posible determinar si los cationes Cu+ y Cu2+ están coordinados o no al anillo de 6r en las etapas de oxidación y reducción. Por último, hemos simulado el desplazamiento químico de 19F, δiso,, en la zeolita sintetizada RTH. El análisis del δiso de los distintos modelos utilizados nos ha permitido reconocer la simetría del material sintetizado, el cual pertenece al grupo espacial P1 y la nueva celda unidad ha sido confirmada experimentalmente por difracción de rayos X. Finalmente, hemos asignado la señal experimental que aparece en el espectro de 19F a -67.2_ppm, al F- localizado en un sitio T2, el cual es a su vez la posición más estable. Además, la señal a -71.8 ppm se ha asignado al anión F- localizado en un sitio T4.
- PublicationEffect of Framework Composition and NH3 on the Diffusion of Cu+ in Cu-CHA Catalysts Predicted by Machine-Learning Accelerated Molecular Dynamics(American Chemical Society, 2023-10-18) Millán Cabrera, Reisel; Bello Jurado, Estefanía; Moliner Marin, Manuel; Boronat Zaragoza, Mercedes; Gomez-Bombarelli, Rafael; Instituto Universitario Mixto de Tecnología Química; European Commission; Agencia Estatal de Investigación; Ministerio de Ciencia e Innovación; UNIVERSIDAD POLITECNICA DE VALENCIA; Consejo Superior de Investigaciones Científicas; Ministerio de Ciencia, Innovación y Universidades[EN] Cu-exchanged zeolites rely on mobile solvated Cu+ cations for their catalytic activity, but the role of the framework composition in transport is not fully understood. Ab initio molecular dynamics simulations can provide quantitative atomistic insight but are too computationally expensive to explore large length and time scales or diverse compositions. We report a machine-learning interatomic potential that accurately reproduces ab initio results and effectively generalizes to allow multinanosecond simulations of large supercells and diverse chemical compositions. Biased and unbiased simulations of [Cu(NH3)2]+ mobility show that aluminum pairing in eight-membered rings accelerates local hopping and demonstrate that increased NH3 concentration enhances long-range diffusion. The probability of finding two [Cu(NH3)2]+ complexes in the same cage, which is key for SCR-NOx reaction, increases with Cu content and Al content but does not correlate with the long-range mobility of Cu+. Supporting experimental evidence was obtained from reactivity tests of Cu-CHA catalysts with a controlled chemical composition.
- PublicationA new molecular pathway allows the chemoselective reduction of nitroaromatics on non-noble metal catalysts(Elsevier, 2018-08) Millán Cabrera, Reisel; Liu, Lichen; Boronat Zaragoza, Mercedes; Corma Canós, Avelino; Instituto Universitario Mixto de Tecnología Química; Generalitat Valenciana; MINISTERIO DE ECONOMÍA, INDUSTRIA Y COMPETITIVIDAD[EN] At difference with noble metals, the oxophylic character of non-noble metals strongly facilitates the rupture of the N-O bonds in nitrobenzene, yielding nitrosobenzene as primary reaction intermediate. By combining periodic DFT calculations and kinetic studies, a direct pathway involving successive dissociation of N-O bonds followed by two hydrogenation steps, Ph-NO2 -> Ph-NO -> Ph-N -> Ph-NH -> Ph-NH2, has been found as most favorable on Ni catalysts. The rate determining step of the global process is the hydrogen transfer to adsorbed Ph-N intermediate. The catalyst surface becomes partly oxidized during reaction, which favors the vertical adsorption of the nitroaromatic compounds and enhances selectivity, while total surface oxidation leads to catalyst deactivation. It is proposed that both catalytic activity and selectivity of Ni and, possibly, other non-noble metals can be tuned by controlling the degree of oxidation of the metal surface. (C) 2018 Elsevier Inc. All rights reserved. Keywords
- PublicationImpact of Zeolite Framework Composition and Flexibility on Methanol-To-Olefins Selectivity: Confinement or Diffusion?(John Wiley & Sons, 2020-10-26) Ferri-Vicedo, Pau; Li, Chengeng; Millán Cabrera, Reisel; Martínez Triguero, Luis Joaquín; Moliner Marin, Manuel; Boronat Zaragoza, Mercedes; Corma Canós, Avelino; Instituto Universitario Mixto de Tecnología Química; European Commission; Generalitat Valenciana; China Scholarship Council; European Regional Development Fund; Ministerio de Economía y Competitividad; Agencia Estatal de Investigación[EN] The methanol-to-olefins reaction catalyzed by small-pore cage-based acid zeolites and zeotypes produces a mixture of short chain olefins, whose selectivity to ethene, propene and butene varies with the cavity architecture and with the framework composition. The product distribution of aluminosilicates and silicoaluminophosphates with the CHA and AEI structures (H-SSZ-13, H-SAPO-34, H-SSZ-39 and H-SAPO-18) has been experimentally determined, and the impact of acidity and framework flexibility on the stability of the key cationic intermediates involved in the mechanism and on the diffusion of the olefin products through the8rwindows of the catalysts has been evaluated by means of periodic DFT calculations and ab initio molecular dynamics simulations. The preferential stabilization by confinement of fully methylated hydrocarbon pool intermediates favoring the paring pathway is the main factor controlling the final olefin product distribution.
- PublicationIdentification of Distinct Copper Species in Cu-CHA Samples Using NO as Probe Molecule. A Combined IR Spectroscopic and DFT Study(Springer-Verlag, 2017) Concepción Heydorn, Patricia; Boronat Zaragoza, Mercedes; Millán Cabrera, Reisel; Moliner Marin, Manuel; Corma Canós, Avelino; Instituto Universitario Mixto de Tecnología Química; Generalitat Valenciana; European Research Council; Ministerio de Economía y Competitividad; European Commission[EN] Combining IR spectroscopy of NO adsorption on copper exchanged molecular sieves with the chabazite structure, i.e. Cu-SAPO-34 and Cu-SSZ-13, and theoretical calculations, different types of copper species have been identified. On one hand, [Cu¿OH]+ species can be accurately distinguished, characterized by a ¿NO frequency at 1788¿ 1798 cm¿1 depending on their location in the chabazite structure (6R vs. 8R) and composition (Cu-SAPO-34 vs. Cu-SSZ-13). On the other hand, dimeric copper oxo [Cu¿O¿ Cu]2+ species have been properly identified by means of DFT modelling, that proposes a ¿NO stretching frequency of 1887 cm¿1, which has been confirmed experimentally in the Cu-SAPO-34 sample. Finally the location of isolated Cu2+ ions either in the 6R units or in the 8R positions of the chabazite cavity could be accurately defined according to DFT data, and validated in the experimental IR spectra with IR bands between 1907 and 1950 cm¿1. Regarding to Cu+ species, IR spectroscopy of CO reveals different types of Cu+ species as evidenced by their ability to form mono, di and try carbonyls. The unambiguous differentiation of different types of copper species is of great interest in further identification of active sites for the NH3- SCR reaction.
- PublicationSpectroscopic Evidence and Density Functional Theory (DFT) Analysis of Low-Temperature Oxidation of Cu+ to Cu2+NOx in Cu-CHA Catalysts: Implications for the SCR-NOx Reaction Mechanism(American Chemical Society, 2019-04-05) Moreno-González, Marta; Millán Cabrera, Reisel; Concepción Heydorn, Patricia; Blasco Lanzuela, Teresa; Boronat Zaragoza, Mercedes; Instituto Universitario Mixto de Tecnología Química; Ministerio de Economía y Competitividad; Fundació Bancària Caixa d'Estalvis i Pensions de Barcelona[EN] Despite the intense investigation on the NH3-SCR-NOx reaction mechanism catalyzed by small pore Cu-CHA zeolites, neither the rate-determining step of the process nor the exact nature of the active sites under reaction conditions are clearly established. In this work, in situ EPR and IR techniques combined with DFT calculations are applied to the study of the oxidation half-cycle of the NH3-SCR-NOx reaction on Cu-SSZ-13 and Cu-SAPO-34 catalysts. EPR and IR spectroscopies unambiguously show that Cu+ is oxidized to Cu2+ at room temperature in the presence of the reaction mixture (NO, O-2, and NH3) or NO and O-2, producing adsorbed NO2, nitrites, and nitrates. Several pathways are proposed from DFT calculations to oxidize Cu+ cations placed in the plane of the 6R ring units of SSZ-13 and SAPO-34 to Cu2+, either by NO2 alone or by a mixture of NO and O-2, with activation energy barriers lower than 70 kJ mol(-1). The results reported here demonstrate that a reaction mechanism invoking the formation of nitrate/nitrite intermediates on copper cations attached to the zeolite framework can be operational in the low-temperature region (T < 350 degrees C). Moreover, different intermediates, nitrites versus nitrates, are preferentially stabilized, depending on the catalyst composition, silicoaluminophosphate vs aluminosilicate.
- PublicationMobility and Reactivity of Cu+ Species in Cu-CHA Catalysts under NH3-SCR-NOx Reaction Conditions: Insights from AIMD Simulations(American Chemical Society, 2021-10-25) Millán Cabrera, Reisel; Cnudde, Pieter; Van Speybroeck, Veronique; Boronat Zaragoza, Mercedes; Instituto Universitario Mixto de Tecnología Química; Ghent University; European Commission; Research Foundation Flanders; Agencia Estatal de Investigación; European Regional Development Fund; Ministerio de Ciencia e Innovación; Consejo Superior de Investigaciones Científicas; Ministerio de Economía, Industria y Competitividad[EN] 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, O-2, 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 O-2 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 O-2 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)-O-2-Cu(NH3)(2) species and the latter occurring by direct NO oxidation to NO2 in one single cavity.
- PublicationCombined Spectroscopic and Computational Study of Nitrobenzene Activation on Non-Noble Metals-Based Mono- and Bimetallic Catalysts(MDPI AG, 2021-08) Millán Cabrera, Reisel; Soriano Rodríguez, Mª Dolores; Cerdá-Moreno, Cristina; Boronat Zaragoza, Mercedes; Concepción Heydorn, Patricia; Instituto Universitario Mixto de Tecnología Química; Generalitat Valenciana; European Regional Development Fund; Ministerio Ciencia, Innovación y Universidades; Comisión Interministerial de Ciencia y Tecnología; Ministerio de Economía, Industria y Competitividad[EN] In this paper, substituted anilines are industrially obtained by direct hydrogenation of nitroaromatic compounds with molecular H-2 using metals as catalysts. Previous theoretical studies proposed that the mechanism of the reaction depends on the nature of the metal used as a catalyst, and that rationally designed bimetallic materials might show improved catalytic performance. Herein, we present IR spectroscopic studies of nitrobenzene interactions with monometallic Ni/SiO2, Cu/SiO2 and Pd/SiO2, and with bimetallic CuNi/SiO2 and CuPd/SiO2 catalysts, both in the absence and presence of H-2, combined with density functional theory (DFT) calculations on selected bimetallic NiCu(111) and PdCu(111) models. The results obtained experimentally confirm that the reaction mechanism on non-noble metals such as Ni proceeds through N-O bond dissociation, generating nitrosobenzene intermediates, while, on noble metals, such as Pd, H-attack is necessary to activate the NO bond. Moreover, a bimetallic CuPd/SiO2 catalyst with a Pd enriched surface is prepared that exhibits an enhanced H-2 dissociation ability and a particular reactivity at the boundary between the two metals.
- PublicationCrystalline phase transition in as-synthesized pure silica zeolite RTH containing tetra-alkyl phosphonium as organic structure directing agent(The Royal Society of Chemistry, 2024-01-03) Martinez-Ortigosa, Joaquin; Millán Cabrera, Reisel; Simancas, Jorge; Hernández-Rodríguez, Manuel; Vidal Moya, José Alejandro; Jorda Moret, Jose Luis; Martineau-Corcos, Charlotte; Sarou-Kanian, Vincent; Boronat Zaragoza, Mercedes; Blasco Lanzuela, Teresa; Rey Garcia, Fernando; Instituto Universitario Mixto de Tecnología Química; European Commission; Generalitat Valenciana; Universitat de València; Agencia Estatal de Investigación; European Regional Development Fund; Red Española de Supercomputación; Ministerio de Ciencia e Innovación; Universitat Politècnica de València; Centre National de la Recherche Scientifique, Francia; Conselleria de Innovación, Universidades, Ciencia y Sociedad Digital, Generalitat Valenciana[EN] The choice of structure directing agents (SDAs) in zeolite synthesis significantly impacts the arrangement of active sites, thereby influencing the stabilization of reaction intermediates with profound implications for catalytic applications. Therefore, understanding the distribution of SDAs along with the substitution of heteroatoms for silicon in zeolites is imperative for tailoring optimized materials for specific applications. This study is centered on the synthesis of all-silica RTH type zeolites in the presence of fluoride, utilizing triisopropyl(methyl)phosphonium as the organic SDA (OSDA). Zeolites produced under varying conditions of time and temperature exhibit differences in their X-ray diffractograms, indicating the presence of two distinct crystalline phases. The 19F NMR spectra confirm the presence of fluoride within the small rth cage and exhibit two distinct signals depending on the sample. The 29Si NMR spectra reveal the existence of penta-coordinated F-SiO4 species, resulting in sixteen non-equivalent Si sites. Through ab initio DFT methods, the stabilization energy and 29Si chemical shielding of several models featuring F-SiO4 situated at all crystallographic sites were computed. Comparison with experimental results enabled the identification of the framework position where the five-coordinate silicon is located, which differs between the two crystalline phases of the as-synthesized RTH zeolites. Consequently, the placement of fluoride in either of these two sites within the RTH zeolite can be controlled during the synthesis. It is expected that this methodology can be extended to manipulate the position of trivalent atoms (e.g., Al3+ or B3+), which can affect the catalytic properties of the RTH zeolite.
- PublicationNew insights into fluoride's role in MFI zeolites: unveiling the link between location and synthesis conditions(The Royal Society of Chemistry, 2025-03-11) Martinez-Ortigosa, Joaquin; Millán Cabrera, Reisel; Simancas-Coloma, Jorge; Vidal Moya, José Alejandro; Martineau-Corcos, Charlotte; Rey Garcia, Fernando; Blasco Lanzuela, Teresa; Instituto Universitario Mixto de Tecnología Química; European Commission; Generalitat Valenciana; Agencia Estatal de Investigación; European Regional Development Fund; Universitat Politècnica de València; Conselleria de Innovación, Universidades, Ciencia y Sociedad Digital, Generalitat Valenciana[EN] This study examines the fluoride distribution in pure silica MFI zeolite synthesized in fluoride medium using various tetra-alkyl ammonium and phosphonium cations as organic structure directing agents (OSDAs). The 19F NMR spectra show a resonance at similar to-65 ppm, related with the presence of fluoride atoms in the t-mel cage bonded to the silicon at T9 site, and another signal at similar to-80 ppm. Through DFT calculations, the similar to-80 ppm 19F NMR resonance is attributed to fluoride within the t-mel cage bonded to silicon at the T12 site. Thus, the results reported here reveal that, contrary to previous reports, there is more than one position for fluoride in the zeolite structure. The location of fluoride at T12 is stabilized by using bulky OSDAs and by increasing the concentration of defects in the MFI framework. Moreover, it is shown that the OSDA used in the synthesis determines the content and distribution of siloxy-silanol defects and fluoride in pure silica MFI zeolite. These findings highlight the impact of synthesis parameters in the charge distribution in zeolites, which ultimately governs key structural properties and potential applications of zeolite materials in catalysis and adsorption processes.