Heterogeneous Binding of Dioxygen: EPR and DFT Evidence for Side-On Nickel(II)–Superoxo Adduct with Unprecedented Magnetic Structure Hosted in MFI Zeolite

Implanting Colloidal Nanoparticles into Single-Crystalline Zeolites for Catalytic Dehydration

The encapsulation of functional colloidal nanoparticles (100 nm) into single-crystalline ZSM-5 zeolites, aiming to create uniform core-shell structures, is a highly sought-after yet formidable objective due to significant lattice mismatch and distinct crystallization properties. In this study, we demonstrate the fabrication of a core-shell structured single-crystal zeolite encompassing an Fe3O4 colloidal core via a novel confinement stepwise crystallization methodology. By engineering a confined nanocavity, anchoring nucleation sites, and executing stepwise crystallization, we have successfully encapsulated colloidal nanoparticles (CN) within single-crystal zeolites. These grafted sites, alongside the controlled crystallization process, compel the zeolite seed to nucleate and expand along the Fe3O4 colloidal nanoparticle surface, within a meticulously defined volume (1.5×107≤V≤1.3×108 nm3). Our strategy exhibits versatility and adaptability to an array of zeolites, including but not restricted to ZSM-5, NaA, ZSM-11, and TS-1 with polycrystalline zeolite shell. We highlight the uniformly structured magnetic-nucleus single-crystalline zeolite, which displays pronounced superparamagnetism (14 emu/g) and robust acidity (~0.83 mmol/g). This innovative material has been effectively utilized in a magnetically stabilized bed (MSB) reactor for the dehydration of ethanol, delivering an exceptional conversion rate (98 %), supreme ethylene selectivity (98 %), and superior catalytic endurance (in excess of 100 hours).

Charge Separation in BaTiO3 Nanocrystals: Spontaneous Polarization Versus Point Defect Chemistry

The fate of photogenerated charges within ferroelectric metal oxides is key for photocatalytic applications. The authors study the contributions of i) tetragonal distortion, responsible for spontaneous polarization, and ii) point defects, on charge separation and recombination within BaTiO₃ (BTO) nanocrystals of cubic and tetragonal structure. Electron paramagnetic resonance (EPR) in combination with O₂ photoadsorption experiments show that BTO nanocrystals annealed at 600 °C have a charge separation yield enhanced by a factor > 10 compared to TiO₂ anatase nanocrystals of similar geometries. This demonstrates for the first time the beneficial effect of the BTO perovskite nanocrystal lattice on charge separation. Strikingly, charge separation is considerably hindered within BTO nanoparticles annealed ≥ 600 °C, due to the formation of Ba-O divacancies that act as charge recombination centers. The opposing interplay between tetragonal distortion and annealing-induced defect formation inside the lattice highlights the importance of defect engineering within perovskite nanoparticles.

EDA-NOCV Calculation for Efficient N2 Binding to the Reduced Ni3S8 Complex: Estimation of Ni-N2 Intrinsic Interaction Energies

The binding of the dinitrogen molecule to the metal center is the first and crucial step toward dinitrogen activation. Favorable interaction energies are desired by chemists and biochemists to study model complexes in the laboratory. An electrochemically reduced form of a previously isolated sulfur-bridged Ni₃S₈ complex is inferred to bind N₂ at multiple Ni centers, and this bonded N₂ undergoes reductive protonation to produce hydrazine (N₂H₄) as the product in the presence of a proton donor. Density functional theory (DFT) calculations and quantum theory of atoms in molecules (QTAIM) analysis have been carried out to shed light on the nature of N₂ binding to an anionic trinuclear Ni₃S₈ complex. Additionally, energy decomposition analysis with the combination of natural orbital for chemical valence (EDA-NOCV) analysis has been performed to estimate the pairwise interaction energies between the Ni center and the N₂ molecule under experimental conditions.

Quantitative and Chemically Intuitive Evaluation of the Nature of M-L Bonds in Paramagnetic Compounds: Application of EDA-NOCV Theory to Spin Crossover Complexes

To improve understanding of M-L bonds in 3d transition metal complexes, analysis by energy decomposition analysis and natural orbital for chemical valence model (EDA-NOCV) is desirable as it provides a full, quantitative and chemically intuitive ab initio description of the M-L interactions. In this study, a generally applicable fragmentation and computational protocol was established and validated by using octahedral spin crossover (SCO) complexes, as the transition temperature (T1/2 ) is sensitive to subtle changes in M-L bonding. Specifically, EDA-NOCV analysis of Fe-N bonds in five [FeII (Lazine )2 (NCBH3 )2 ], in both low-spin (LS) and paramagnetic high-spin (HS) states led to: 1) development of a general, widely applicable, corrected M+L6 fragmentation, tested against a family of five LS [FeII (Lazine )3 ](BF4 )2 complexes; this confirmed that three Lazine are stronger ligands (ΔEorb,σ+π =-370 kcal mol-1 ) than 2 Lazine +2 NCBH3 (=-335 kcal mol-1 ), as observed. 2) Analysis of Fe-L bonding on LS→HS, reveals more ionic (ΔEelstat ) and less covalent (ΔEorb ) character (ΔEelstat :ΔEorb 55:45 LS→64:36 HS), mostly due to a big drop in σ (ΔEorb,σ ↓50 %; -310→-145 kcal mol-1 ), and a drop in π contributions (ΔEorb,π ↓90 %; -30→-3 kcal mol-1 ). 3) Strong correlation of observed T1/2 and ΔEorb,σ+π , for both LS and HS families (R2 =0.99 LS, R2 =0.95 HS), but no correlation of T1/2 and ΔΔEorb,σ+π (LS-HS) (R2 =0.11). Overall, this study has established and validated an EDA-NOCV protocol for M-L bonding analysis of any diamagnetic or paramagnetic, homoleptic or heteroleptic, octahedral transition metal complex. This new and widely applicable EDA-NOCV protocol holds great promise as a predictive tool.

EPR Spectroscopy of Iron- and Nickel-Doped [ZnAl]-Layered Double Hydroxides: Modeling Active Sites in Heterogeneous Water Oxidation Catalysts

Iron-doped nickel layered double hydroxides (LDHs) are among the most active heterogeneous water oxidation catalysts. Due to interspin interactions, however, the high density of magnetic centers results in line-broadening in magnetic resonance spectra. As a result, gaining atomic-level insight into the catalytic mechanism via electron paramagnetic resonance (EPR) is not generally possible. To circumvent spin-spin broadening, iron and nickel atoms were doped into nonmagnetic [ZnAl]-LDH materials and the coordination environments of the isolated Fe(III) and Ni(II) sites were characterized. Multifrequency EPR spectroscopy identified two distinct Fe(III) sites (S = 5/2) in [Fe:ZnAl]-LDH. Changes in zero field splitting (ZFS) were induced by dehydration of the material, revealing that one of the Fe(III) sites was solvent-exposed (i.e., at an edge, corner, or defect site). These solvent-exposed sites featured an axial ZFS of 0.21 cm^-1 when hydrated. The ZFS increased dramatically upon dehydration (to -1.5 cm^-1), owing to lower symmetry and a decrease in the coordination number of iron. The ZFS of the other ("inert") Fe(III) site maintained an axial ZFS of 0.19-0.20 cm^-1 under both hydrated and dehydrated conditions. We observed a similar effect in [Ni:ZnAl]-LDH materials; notably, Ni(II) (S = 1) atoms displayed a single, small ZFS (±0.30 cm^-1) in hydrated material, whereas two distinct Ni(II) ZFS values (±0.30 and ±1.1 cm^-1) were observed in the dehydrated samples. Although the magnetically dilute materials were not active catalysts, the identification of model sites in which the coordination environments of iron and nickel were particularly labile (e.g., by simple vacuum drying) is an important step toward identifying sites in which the coordination number may drop spontaneously in water, a probable mechanism of water oxidation in functional materials.

Room temperature O transfer from N2O to CO mediated by the nearest Cd(i) ions in MFI zeolite cavities

The dominant oxidation state of cadmium is +ii. Although extensive investigations into the +ii oxidation state have been carried out, the chemistry of CdI is still largely underdeveloped. Here, we report a new functionality of cadmium created by the zeolite lattice: room temperature O transfer from N2O to CO mediated by the nearest monovalent cadmium ions in MFI zeolite. Thermal activation of CdII ion-exchanged MFI zeolite in vacuo affords the diamagnetic [CdI-CdI]2+ species with a short CdI-CdI σ bond (2.67 Å). This species generates two CdI˙ sites under UV irradiation through homolytic cleavage of the CdI-CdI σ bond, and the thus-formed nearest CdI˙ sites abstract an O atom from N2O to generate the [CdII-Ob-CdII]2+ core, where Ob means bridged oxygen. This bridging atomic oxygen species is transferred to CO at room temperature, through which CO oxidation and regeneration of the CdI-CdI σ bond then proceed. This is the first example pertaining to the reversible redox reactivity of the nearest monovalent cadmium ions toward stable small molecules. In situ spectroscopic characterization captured all the intermediates in the reaction processes, and these data allowed us to calibrate the density-functional-theory cluster calculations, by means of which we were able to show that the charge compensation requirement at the nearest two Al sites arrayed circumferentially in the 10-membered ring of MFI zeolite creates such novel functionalities of cadmium. The unprecedented reactivity of CdI and its origin are discussed.

Structure and reactivity of the first-row d-block metal-superoxo complexes

This review discusses the structure and reactivity of metal-superoxo complexes covering all ten first-row d-block metals from Sc to Zn.

A vicinal effect for promoting catalysis of Pd1/TiO2: supports of atomically dispersed catalysts play more roles than simply serving as ligands

Atomically dispersing metal atoms on supports has been emerging as an effective strategy to maximize the atom utilization of metals for catalysis. However, due to the lack of effective tools to characterize the detailed structure of metal-support interface, the chemical functions of supports in atomically dispersed metal catalysts are hardly elucidated at the molecular level. In this work, an atomically dispersed Pd₁/TiO₂ catalyst with Ti(III) vicinal to Pd is prepared and used to demonstrate the direct involvement of metal atoms on support in the catalysis of dispersed metal atoms. Systematic studies reveal that the Ti(III)-O-Pd interface facilitates the activation of O₂ into superoxide (O₂^-), thus promoting the catalytic oxidation. The catalyst exhibits the highest CO turn-over frequency among ever-reported Pd-based catalysts, and enhanced catalysis in the combustion of harmful volatile organic compound (i.e., toluene) and green-house gas (i.e., methane). The demonstrated direct involvement of metal atoms on oxide support suggests that the real active sites of atomically dispersed metal catalysts can be far beyond isolated metal atoms themselves. Metal atoms on oxide supports in the vicinity serve as another vector to promote the catalysis of atomically dispersed metal catalysts.

Strong Lewis Base Ga4B2O9: Ga-O Connectivity Enhanced Basicity and Its Applications in the Strecker Reaction and Catalytic Conversion of n-Propanol

Heterogeneous solid base catalysis is valuable and promising in chemical industry, however it is insufficiently developed compared to solid acid catalysis due to the lack of satisfied solid base catalysts. To gain the strong basicity, the previous strategy was to basify oxides with alkaline metals to create surficial vacancies or defects, which suffers from the instability under catalytic conditions. Monocomponent basic oxides like MgO are literally stable but deficient in electron-withdrawing ability. Here we prove that a special connectivity of atoms could enhance the Lewis basicity of oxygen in monocomponent solids exemplified by Ga₄B₂O₉. The structure-induced basicity is from the μ₃-O linked exclusively to five-coordinated Ga³⁺. Ga₄B₂O₉ behaved as a durable catalyst with a high yield of 81% in the base-catalyzed synthesis of α-aminonitriles by Strecker reaction. In addition, several monocomponent solid bases were evaluated in the Strecker reaction, and Ga₄B₂O₉ has the largest amount of strong base centers (23.1 μmol/g) and the highest catalytic efficiency. Ga₄B₂O₉ is also applicable in high-temperature solid-gas catalysis, for example, Ga₄B₂O₉ catalyzed efficiently the dehydrogenation of n-propanol, resulting in a high selectivity to propanal (79%). In contrast, the comparison gallium borate, Ga-PKU-1, which is a Brönsted acid, preferred to catalyze the dehydration process to obtain propylene with a selectivity of 94%.

Why do zeolites induce an unprecedented electronic state on exchanged metal ions?

Understanding the exact position and the detailed role of the Al array in zeolites is essential for elucidating the origin of unique properties that can be derived from the metal-ion exchanged in zeolite samples and for designing zeolite materials with high efficiency in catalytic and adsorption processes. In this work, we investigate, for the first time, the important role of the Al array in the reactivity observed on the metal-ion exchanged in zeolites on the basis of the calculation method by utilizing the spontaneous heterolytic cleavage of H₂ observed experimentally on the Zn²⁺-ion exchanged in MFI-type zeolites (Zn²⁺-MFI) as the model reaction. In the case of calculation, two main types of models for considering the Al positions in MFI-type zeolites were adopted: in the first type, the Al atoms with appropriate distances are aligned in the circumferential direction of the straight channel (abbreviated as a circumferentially arrayed Al-Al site); in the second type, the nearest neighbouring Al atoms with appropriate distances are directed toward the straight channel axis (abbreviated as a channel directionally arrayed Al-Al site). Results indicated that the Al-array direction governs the reactivity of Zn²⁺-MFI. The former type of array well explains the experimental fact that spontaneous and irreversible heterolysis of H₂ takes place on Zn²⁺-MFI, even at room temperature, whereas the latter type of array is less reactive; high activation energy is required for the heterolytic cleavage of H₂ (ca. >70 kJ mol^-1). A detailed analysis of the geometric and electronic structures of a series of Zn²⁺-MFI models with various Al-array directions clarified the following facts: the circumferentially arrayed Al-Al site induces an inevitable environment around the Zn²⁺ site, with the simultaneous existence of both a Lewis acid point (coordinatively unsaturated and distorted Zn²⁺) and a Lewis base point (the lattice oxygen atom juxtaposed with exchanged Zn²⁺, which participates in the activation of H₂: O_jL). It is the circumferentially arrayed Al-Al atoms that confer acidic and basic nature on the metal ion and the lattice oxygen atom (O_jL), and ultimately trigger the heterolytic dissociation of H₂, even at 300 K.

Spectroscopic and Theoretical Study of Spin-Dependent Electron Transfer in an Iron(III) Superoxo Complex

It was shown previously (J. Am. Chem. Soc. 2014, 136, 10846) that bubbling of O2 into a solution of Fe(II)(BDPP) (H2BDPP = 2,6-bis[[(S)-2-(diphenylhydroxymethyl)-1-pyrrolidinyl]methyl]pyridine) in tetrahydrofuran at -80 °C generates a high-spin (SFe = (5)/2) iron(III) superoxo adduct, 1. Mössbauer studies revealed that 1 is an exchange-coupled system, [Formula: see text], where SR = (1)/2 is the spin of the superoxo radical, of which the spectra were not well enough resolved to determine whether the coupling was ferromagnetic (S = 3 ground state) or antiferromagnetic (S = 2). The glass-forming 2-methyltetrahydrofuran solvent yields highly resolved Mössbauer spectra from which the following data have been extracted: (i) the ground state of 1 has S = 3 (J < 0); (ii) |J| > 15 cm(-1); (iii) the zero-field-splitting parameters are D = -1.1 cm(-1) and E/D = 0.02; (iv) the major component of the electric-field-gradient tensor is tilted ≈7° relative to the easy axis of magnetization determined by the MS = ±3 and ±2 doublets. The excited-state MS = ±2 doublet yields a narrow parallel-mode electron paramagnetic resonance signal at g = 8.03, which was used to probe the magnetic hyperfine splitting of (17)O-enriched O2. A theoretical model that considers spin-dependent electron transfer for the cases where the doubly occupied π* orbital of the superoxo ligand is either "in" or "out" of the plane defined by the bent Fe-OO moiety correctly predicts that 1 has an S = 3 ground state, in contrast to the density functional theory calculations for 1, which give a ground state with both the wrong spin and orbital configuration. This failure has been traced to a basis set superposition error in the interactions between the superoxo moiety and the adjacent five-membered rings of the BDPP ligand and signals a fundamental problem in the quantum chemistry of O2 activation.

Paramagnetic dioxovanadium(IV) molecules inside the channels of zeolite BEA--EPR screening of VO2 reactivity toward small gas-phase molecules

Interaction of small gas-phase molecules (NO, N2O, O2, CO) with VO2 radicals inside the channels of a dealuminated SiBEA zeolite was investigated by means of electron paramagnetic resonance (EPR), infrared (IR), and mass (QMS) spectroscopies to provide direct insights into the chemistry of a unique paramagnetic state of vanadium - VO2 molecules. A facile way of forming VO2 inside the channels of SiBEA via thermal reduction of VO2(+) precursor cations was shown. Dioxovanadium(IV) was identified based on its unusual EPR signal which, as compared with the typical monooxovanadium(IV) (VO(2+) cation), is featured by rhombic symmetry and a positive Aiso value leading to a hyperfine splitting as large as 32 mT. VO2 molecules exhibit reducing properties transforming N2O and O2 into vanadium intrachannel cage adducts comprising of reactive oxygen species (O(-) and O2(-), respectively). Interaction with CO led to its oxidation to CO2, while paramagnetic NO acted as a scavenger for VO2 radicals producing diamagnetic adducts. The observed reactivity was rationalized in terms of spin-pairing, electron transfer, and oxygen transfer processes. As a result new chemical pathways of vanadium reactivity were demonstrated which were not observed so far either in the homogeneous molecular systems or supported vanadium materials.

Low-temperature reactivity of Zn+ ions confined in ZSM-5 zeolite toward carbon monoxide oxidation: insight from in situ DRIFT and ESR spectroscopy

We report the low-temperature catalytic reactivity of Zn(+) ions confined in ZSM-5 zeolite toward CO oxidation. In situ DRIFT and ESR spectroscopy demonstrated that molecular O2 is readily activated by Zn(+) ion to produce O2(-) species at room temperature (298 K) via facile electron transfer between Zn(+) ion and O2 and that the formation of the active O2(-) species is responsible for the high activity of the ZnZSM-5 catalyst toward CO oxidation.