A new molecular pathway allows the chemoselective reduction of nitroaromatics on non-noble metal catalysts

[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. KeywordsThis work has been supported by the Spanish Government through "Severo Ochoa Program" (SEV-2016-0683) and by Generalitat Valenciana through AICO/2017/153 Project. Red Espanola de Supercomputacion (RES) and Centre de Calcul de la Universitat de Valencia are gratefully acknowledged for computational resources and technical support. The authors also thank the Microscopy Service of UPV for kind help on measurements. R. M. acknowledges "La Caixa - Severo Ochoa" International PhD Fellowships (call 2015). L. L. thanks ITQ for providing a PhD scolarship.Millán-Cabrera, R.; Liu, L.; Boronat Zaragoza, M.; Corma Canós, A. (2018). A new molecular pathway allows the chemoselective reduction of nitroaromatics on non-noble metal catalysts. Journal of Catalysis. 364:19-30. https://doi.org/10.1016/j.jcat.2018.05.004193036

Spectroscopic 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

"This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Catalysis, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acscatal.8b04717."[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.This work was supported by the Spanish Government through "Severo Ochoa Program" (Nos. SEV 2012-0267; SEV-2016-0683), No. MAT2015-71261-R, and No. CTQ2015-68951-C3-1-R, and by the European Union through No. ERC-AdG-2014-671093 (SynCatMatch). Red Espanola de Supercomputacion (RES) and Centre de Calcul de la Universitat de Valencia are gratefully acknowledged for computational resources and technical support. R.M. acknowledges "La Caixa-Severo Ochoa" International PhD Fellowships (call 2015).Moreno-González, M.; Millán-Cabrera, R.; Concepción Heydorn, P.; Blasco Lanzuela, T.; Boronat Zaragoza, M. (2019). Spectroscopic 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. ACS Catalysis. 9(4):2725-2738. https://doi.org/10.1021/acscatal.8b04717S272527389

Impact of Zeolite Framework Composition and Flexibility on Methanol-To-Olefins Selectivity: Confinement or Diffusion?

This is the peer reviewed version of the following article: P. Ferri, C. Li, R. Millán, J. Martínez-Triguero, M. Moliner, M. Boronat, A. Corma, Angew. Chem. Int. Ed. 2020, 59, 19708, which has been published in final form at https://doi.org/10.1002/anie.202007609. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.[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.This work has been supported by the European Union through ERC-AdG-2014-671093 (SynCatMatch), Spanish Government through "Severo Ochoa" (SEV-2016-0683, MINECO), MAT2017-82288-C2-1-P (AEI/FEDER, UE) and RTI2018-101033-B-I00 (MCIU/AEI/FEDER, UE), and by Generalitat Valenciana through AICO/2019/060. The Electron Microscopy Service of the UPV is acknowledged for their help in sample characterization. Red Espanola de Supercomputacion (RES) and Servei d'Informatica de la Universitat de Valencia (SIUV) are acknowledged for computational resources and technical support. P.F. and R.M. thank ITQ for their contracts. C.L. acknowledges China Scholarship Council (CSC) for a Ph.D fellowship.Ferri-Vicedo, P.; Li, C.; Millán-Cabrera, R.; Martínez-Triguero, J.; Moliner Marin, M.; Boronat Zaragoza, M.; Corma Canós, A. (2020). Impact of Zeolite Framework Composition and Flexibility on Methanol-To-Olefins Selectivity: Confinement or Diffusion?. Angewandte Chemie International Edition. 59(44):19708-19715. https://doi.org/10.1002/anie.202007609S19708197155944Olah, G. A. (2005). Beyond Oil and Gas: The Methanol Economy. Angewandte Chemie International Edition, 44(18), 2636-2639. doi:10.1002/anie.200462121Olah, G. A. (2005). Jenseits von Öl und Gas: die Methanolwirtschaft. 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Extending Graph (Discrete) Derivative Descriptors to N-Tuple Atom-Relations

In the present manuscript, an extension of the previously defined Graph Derivative Indices (GDIs) is discussed. To achieve this objective, the concept of a hypermatrix, conceived from the calculation of the frequencies of triple and quadruple atom relations in a set of connected sub-graphs, is introduced. This set of subgraphs is generated following a predefined criterion, known as the event (S), being in this particular case the connectivity among atoms. The triple and quadruple relations frequency matrices serve as a basis for the computation of triple and quadruple discrete derivative indices, respectively. The GDIs are implemented in a computational program denominated DIVATI (acronym for DIscrete DeriVAtive Type Indices), a module of TOMOCOMD-CARDD program. Shannon‟s entropy-based variability analysis demonstrates that the GDIs show major variability than others indices used in QSAR/QSPR researches. In addition, it can be appreciated when the indices are extended over n-elements from the graph, its quality increases, principally when they are used in a combined way. QSPR modeling of the physicochemical properties Log P and Log K of the 2-furylethylenes derivatives reveals that the GDIs obtained using the tripleand quadruple matrix approaches yield superior performance to the duplex matrix approach. Moreover, the statistical parameters for models obtained with the GDI method are superior to those reported in the literature by using other methods. It can therefore be suggested that the GDI method, seem to be a promissory tool to reckon on in QSAR/QSPR studies, virtual screening of compound datasets and similarity/dissimilarity evaluations

Mobility and reactivity of Cu+ species in Cu-CHA catalysts under NH3-SCR-NOx reaction conditions : insights from AIMD simulations

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

Theoretical and spectroscopic evidence of the dynamic nature of copper active sites in Cu-CHA catalysts under selective catalytic reduction (NH3–SCR–NOx) conditions

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