An alternative catalytic cycle for selective methane oxidation to methanol with Cu clusters in zeolites.

The partial oxidation of methane to methanol catalyzed by Cu-exchanged zeolites involves at present a three-step procedure that requires changing reaction conditions along the catalytic cycle. In this work we present an alternative catalytic cycle for selective methane conversion to methanol using as active species small Cu clusters supported on CHA zeolite. Periodic DFT calculations show that molecular O is easily activated on Cu clusters producing bi-coordinated O atoms able to dissociate homolytically a CH bond from CH and to react with the radical-like non-adsorbed methyl intermediate formed producing methanol, while competitive overoxidation to CO is energetically disfavored.

Effect of Framework Composition and NH on the Diffusion of Cu in Cu-CHA Catalysts Predicted by Machine-Learning Accelerated Molecular Dynamics.

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.

Ag(0) dimers within a thioether-functionalized MOF catalyze the CO to CH hydrogenation reaction.

Ultrasmall silver clusters in reduced state are difficult to synthesize since silver atoms tend to rapidly aggregate into bigger entities. Here, we show that dimers of reduced silver (Ag) are formed within the framework of a metal-organic framework provided with thioether arms in their walls (methioMOF), after reduction with NaBH of the corresponding Ag-methioMOF precursor. The resulting Ag-methioMOF catalyzes the methanation reaction of carbon dioxide (CO to CH hydrogenation reaction) under mild reaction conditions (1 atm CO, 4 atm H, 140 °C), with production rates much higher than Ag on alumina and even comparable to the state-of-the-art Ru on alumina catalyst (Ru-AlO) under these reaction conditions, according to literature results.

Approaching enzymatic catalysis with zeolites or how to select one reaction mechanism competing with others.

Approaching the level of molecular recognition of enzymes with solid catalysts is a challenging goal, achieved in this work for the competing transalkylation and disproportionation of diethylbenzene catalyzed by acid zeolites. The key diaryl intermediates for the two competing reactions only differ in the number of ethyl substituents in the aromatic rings, and therefore finding a selective zeolite able to recognize this subtle difference requires an accurate balance of the stabilization of reaction intermediates and transition states inside the zeolite microporous voids. In this work we present a computational methodology that, by combining a fast high-throughput screeening of all zeolite structures able to stabilize the key intermediates with a more computationally demanding mechanistic study only on the most promising candidates, guides the selection of the zeolite structures to be synthesized.

Influence of the zeolite support on the catalytic properties of confined metal clusters: a periodic DFT study of O dissociation on Cu clusters in CHA.

The catalytic properties of sub-nanometer Cu clusters are modified by interactions with inorganic supports used for their stabilization. In this work, the reactivity towards O dissociation of Cu and Cu clusters confined within the cavities of the CHA zeolite is theoretically investigated by means of periodic DFT calculations. Increasing the Al content in the zeolite framework not only modifies the cluster morphology, but also leads to a decrease in the electronic density available on the supported Cu clusters, which in turn leads to higher activation energies for O dissociation.

BINOL-Containing Chiral Porous Polymers as Platforms for Enantiorecognition.

The enantioselective discrimination of racemic compounds can be achieved through the design and preparation of a new family of chiral conjugated BINOL-porous polymers (CBPPs) from enantiopure ()- or ()-BINOL derivatives and 1,3,5-(4-phenylboronic acid)benzene or 1,3,5-(4-ethynylphenyl)benzene, 1,3,5-triethynyl-2,4,6-trifluorobenzene, and (4-ethynylphenyl)methane as comonomers following Suzuki-Miyaura and Sonogashira-Hagihara carbon-carbon coupling approaches. The obtained CBPPs show high thermal stability, a good specific surface area, and a robust framework and can be applied successfully in the fluorescence recognition of enantiomers of terpenes (limonene and α-pinene) and 1-phenylethylamine. Fluorescence titration of CBPPs-OH in acetonitrile shows that all Sonogashira hosts exhibit a preference for the ()-enantiomer over the ()-enantiomer of 1-phenylethylamine, the selectivity being much higher than that of the corresponding BINOL-based soluble system used as a reference.

Sub-nanometer Copper Clusters as Alternative Catalysts for the Selective Oxidation of Methane to Methanol with Molecular O.

The partial oxidation of methane to methanol with molecular O at mild reaction conditions is a challenging process, which is efficiently catalyzed in nature by enzymes. As an alternative to the extensively studied Cu-exchanged zeolites, small copper clusters composed by just a few atoms appear as potential specific catalysts for this transformation. Following previous work in our group that established that the reactivity of oxygen atoms adsorbed on copper clusters is closely linked to cluster size and morphology, we explore by means of DFT calculations the ability of bidimensional (2D) and three-dimensional (3D) Cu and Cu clusters to oxidize partially methane to methanol.

Direct assessment of confinement effect in zeolite-encapsulated subnanometric metal species.

Subnanometric metal species confined inside the microporous channels/cavities of zeolites have been demonstrated as stable and efficient catalysts. The confinement interaction between the metal species and zeolite framework has been proposed to play the key role for stabilization, though the confinement interaction is elusive to be identified and measured. By combining theoretical calculations, imaging simulation and experimental measurements based on the scanning transmission electron microscopy-integrated differential phase contrast imaging technique, we have studied the location and coordination environment of isolated iridium atoms and clusters confined in zeolite.

The 2D or 3D morphology of sub-nanometer Cu and Cu clusters changes the mechanism of CO oxidation.

The mechanism of the CO oxidation reaction catalysed by planar Cu, three dimensional (3D) Cu, and 3D Cu clusters is theoretically investigated at the B3PW91/Def2TZVP level. All three clusters are able to catalyse the reaction with similar activation energies for the rate determining step, about 16-18 kcal mol, but with remarkable differences in the reaction mechanism depending on cluster morphology. Thus, for 3D Cu and Cu clusters, O dissociation is the first step of the mechanism, followed by two consecutive CO + O reaction steps, the second one being rate determining.

Mobility and Reactivity of Cu Species in Cu-CHA Catalysts under NH-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(NH)] complexes between two adjacent cavities of the chabazite structure, including other reactant molecules (NO, O, HO, and NH), 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 NH or NO in the initial cavity, since both reactants form with [Cu(NH)] stable intermediates which are too bulky to cross the 8-ring windows connecting the cavities.

Combined Spectroscopic and Computational Study of Nitrobenzene Activation on Non-Noble Metals-Based Mono- and Bimetallic Catalysts.

In this paper, substituted anilines are industrially obtained by direct hydrogenation of nitroaromatic compounds with molecular H 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/SiO, Cu/SiO and Pd/SiO and with bimetallic CuNi/SiO and CuPd/SiO catalysts, both in the absence and presence of H, combined with density functional theory (DFT) calculations on selected bimetallic NiCu(111) and PdCu(111) models.

Tailoring Lewis/Brønsted acid properties of MOF nodes hydrothermal and solvothermal synthesis: simple approach with exceptional catalytic implications.

The Lewis/Brønsted catalytic properties of the Metal-Organic Framework (MOF) nodes can be tuned by simply controlling the solvent employed in the synthetic procedure. In this work, we demonstrate that Hf-MOF-808 can be prepared from a material with a higher amount of Brønsted acid sites, modulated hydrothermal synthesis, to a material with a higher proportion of unsaturated Hf Lewis acid sites, modulated solvothermal synthesis. The Lewis/Brønsted acid properties of the resultant metallic clusters have been studied by different characterization techniques, including XAS, FTIR and NMR spectroscopies, combined with a DFT study.

Design and Synthesis of the Active Site Environment in Zeolite Catalysts for Selectively Manipulating Mechanistic Pathways.

By combining kinetics and theoretical calculations, we show here the benefits of going beyond the concept of static localized and defined active sites on solid catalysts, into a system that globally and dynamically considers the active site located in an environment that involves a scaffold structure particularly suited for a target reaction. We demonstrate that such a system is able to direct the reaction through a preferred mechanism when two of them are competing. This is illustrated here for an industrially relevant reaction, the diethylbenzene-benzene transalkylation.

Selective active site placement in Lewis acid zeolites and implications for catalysis of oxygenated compounds.

The selective incorporation of isolated framework Lewis acid sites at specific crystallographic positions in high-silica zeolites was achieved by applying a rationalized post-synthetic grafting methodology. The removal of framework Ge atoms from a Ge-BEC zeolite with low concentrations of Ge in the framework (Si/Ge ∼ 150) followed by grafting allows the synthesis of Sn-BEC zeolites with Sn atoms positionally biased into the double-4-ring (D4R) crystallographic positions of the BEC framework. Spectroscopic characterization using solid-state nuclear magnetic resonance (NMR) coupled with theoretical calculations revealed that Sn atoms preferentially form open Sn sites in the D4R of Sn-BEC.

Hydrogenation of substituted nitroaromatics on non-noble metal catalysts: mechanistic insights to improve selectivity.

The mechanism of nitrobenzene hydrogenation on non-noble metals such as Ni is different from that previously reported for noble metals like Pt. The newly proposed pathway involves the initial dissociation of the two N-O bonds of nitrobenzene (Ph-NO→ Ph-NO → Ph-N), leading to partial oxidation of the catalyst surface, followed by two successive hydrogenation steps (Ph-N → Ph-NH → Ph-NH) that finally produce the functionalized aniline. Due to the oxophilic nature of non-noble metals like Ni, Co or Cu, the hydrogenation of the Ph-N intermediate and the removal of O in the form of water become the most energy demanding steps of the process.

Soluble/MOF-Supported Palladium Single Atoms Catalyze the Ligand-, Additive-, and Solvent-Free Aerobic Oxidation of Benzyl Alcohols to Benzoic Acids.

Metal single-atom catalysts (SACs) promise great rewards in terms of metal atom efficiency. However, the requirement of particular conditions and supports for their synthesis, together with the need of solvents and additives for catalytic implementation, often precludes their use under industrially viable conditions. Here, we show that palladium single atoms are spontaneously formed after dissolving tiny amounts of palladium salts in neat benzyl alcohols, to catalyze their direct aerobic oxidation to benzoic acids without ligands, additives, or solvents.

Theoretical and Spectroscopic Evidence of the Dynamic Nature of Copper Active Sites in Cu-CHA Catalysts under Selective Catalytic Reduction (NH-SCR-NO) 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 Cu cations from their positions coordinated to the zeolite framework, forming mobile Cu(NH) and Cu(NH) 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 region of the IR spectra.

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

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 the 8r windows 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.

Organic-Free Synthesis of Zeolite Y with High Si/Al Ratios: Combined Strategy of In Situ Hydroxyl Radical Assistance and Post-Synthesis Treatment.

Zeolite Y, with a high SiO /Al O ratio (SAR), plays an important role in fluidized catalytic cracking processes. However, in situ synthesis of zeolite Y with high SARs remains a challenge because of kinetic limitations. Here, zeolite Y with an SAR of 6.

Low-Temperature Catalytic NO Reduction with CO by Subnanometric Pt Clusters.

The catalytic subnanometric metal clusters with a few atoms can be regarded as an intermediate state between single atoms and metal nanoparticles (>1 nm). Their molecule-like electronic structures and flexible geometric structures bring rich chemistry and also a different catalytic behavior, in comparison with the single-atom or nanoparticulate counterparts. In this work, by combination of operando IR spectroscopy techniques and electronic structure calculations, we will show a comparative study on Pt catalysts for CO + NO reaction at a very low temperature range (140-200 K).

What Is Measured When Measuring Acidity in Zeolites with Probe Molecules?

Based on theoretical calculations of CO, NH, and pyridine adsorption at different sites in MOR and MFI zeolites, we analyze how confinement effects influence the measurement of acidity based on the interaction of probe molecules with Brönsted acid sites. Weak bases, such as CO, form neutral ZH-CO adducts with a linear configuration that can be distorted by spatial restrictions associated with the dimensions of the pore, leading to weaker interaction, but can also be stabilized by dispersion forces if a tighter fitting with the channel void is allowed. Strong bases such as NH and pyridine are readily protonated on Brönsted acid sites, and the experimentally determined adsorption enthalpies include not only the thermochemistry associated with the proton transfer process itself, but also the stabilization of the Z-BH ion pair formed upon protonation by multiple interactions with the surrounding framework oxygen atoms, leading in some cases to a heterogeneity of acidities within the same zeolite structure.

Base-Controlled Heck, Suzuki, and Sonogashira Reactions Catalyzed by Ligand-Free Platinum or Palladium Single Atom and Sub-Nanometer Clusters.

The assumption that oxidative addition is the key step during the cross-coupling reaction of aryl halides has led to the development of a plethora of increasingly complex metal catalysts, thereby obviating in many cases the exact influence of the base, which is a simple, inexpensive, and necessary reagent for this paramount transformation. Here, a combined experimental and computational study shows that the oxidative addition is not the single kinetically relevant step in different cross-coupling reactions catalyzed by sub-nanometer Pt or Pd species, since the reactivity control is shifted toward subtle changes in the base. The exposed metal atoms in the cluster cooperate to enable an extremely easy oxidative addition of the aryl halide, even chlorides, and allow the base to bifurcate the coupling.

Sub nanometer clusters in catalysis.

Sub nanometer transition metal clusters composed of a small number of atoms exhibit unexpected electronic, optical, magnetic and catalytic properties that often change substantially as a function of cluster atomicity. Several factors influence their unique catalytic behavior, including their discrete electronic structure of molecular-like orbitals and the accessibility of their low-coordinated atoms. In addition, these factors are strongly correlated so that changes in their morphology may provoke large modifications to their electronic structure and vice versa.

Confined Pt Water Clusters in a MOF Catalyze the Low-Temperature Water-Gas Shift Reaction with both CO Oxygen Atoms Coming from Water.

The synthesis and reactivity of single metal atoms in a low-valence state bound to just water, rather than to organic ligands or surfaces, is a major experimental challenge. Herein, we show a gram-scale wet synthesis of Pt stabilized in a confined space by a crystallographically well-defined first water sphere, and with a second coordination sphere linked to a metal-organic framework (MOF) through electrostatic and H-bonding interactions. The role of the water cluster is not only isolating and stabilizing the Pt atoms, but also regulating the charge of the metal and the adsorption of reactants.