Catalytic oxygen activation versus autoxidation for industrial applications: a physicochemical approach

Interaction of N2, O2 and H2 Molecules with Superalkalis

Superalkalis (SAs) are exotic clusters having lower ionization energy than alkali atoms, which makes them strong reducing agents. In the quest for the reduction of diatomic molecules (X2) such as N2, O2, and H2 using Møller-Plesset perturbation theory (MP2), we have studied their interaction with typical superalkalis such as FLi2, OLi3, and NLi4 and calculated various parameters of the resulting SA-X2 complexes. We noticed that the SA-O2 complex and its isomers possess strong ionic interaction, which leads to the reduction of O2 to O2 - anion. On the contrary, there are both ionic and covalent interactions in SA-N2 complexes such that the lowest energy isomers are covalently bonded with no charge transfer from SA. Further, the interaction between SA and H2 leads to weakly bound complexes, which results in the adsorption of H2 molecules. The nature of interaction is found to be closely related to the electron affinity of diatomic molecules. These findings might be useful in the study of the activation, reduction, and adsorption of small molecules, which can be further explored for their possible applications.

CO-Assisted Methane Oxidation into Oxygenates over Surface Platinum-Titanium Alloyed Layers

Methane oxidation using molecular oxygen remains a grand challenge in which the obstacle is not only the activation of methane but also the reaction with oxygen, considering the mismatch of the ground spin states. Herein, we report TiO2-supported Pt nanocrystals (Pt/TiO2) with surface Pt-Ti alloyed layers that directly convert methane into oxygenates by using O2 as the oxidant with the assistance of CO. The oxygenate yield reached 749.8 mmol gPt-1 in a H2O aqueous solution over 0.1% Pt/TiO2 under 31 bar of mixed gas (20:5:6 CH4:CO:O2) at 150 °C for 3 h, while the CH3OH selectivity was 62.3%. On the basis of the control experiments and spectroscopic results, we identified the surface Pt-Ti alloy as the active sites. Moreover, CO promoted the dissociation of O2 on the surface of Pt-Ti alloyed layers and the subsequent activation of CH4 to form oxygenated products.

Water co-catalysis in aerobic olefin epoxidation mediated by ruthenium oxo complexes

We report the development of a versatile Ru-porphyrin catalyst system which performs the aerobic epoxidation of aromatic and aliphatic (internal) alkenes under mild conditions, with product yields of up to 95% and turnover numbers (TON) up to 300. Water is shown to play a crucial role in the reaction, significantly increasing catalyst efficiency and substrate scope. Detailed mechanistic investigations employing both computational studies and a range of experimental techniques revealed that water activates the RuVI di-oxo complex for alkene epoxidation via hydrogen bonding, stabilises the RuIV mono-oxo intermediate, and is involved in the regeneration of the RuVI di-oxo complex leading to oxygen atom exchange. Distinct kinetics are obtained in the presence of water, and side reactions involved in catalyst deactivation have been identified.

Endogenous Photosensitizers in Human Skin

Endogenous photosensitizers play a critical role in both beneficial and harmful light-induced transformations in biological systems. Understanding their mode of action is essential for advancing fields such as photomedicine, photoredox catalysis, environmental science, and the development of sun care products. This review offers a comprehensive analysis of endogenous photosensitizers in human skin, investigating the connections between their electronic excitation and the subsequent activation or damage of organic biomolecules. We gather the physicochemical and photochemical properties of key endogenous photosensitizers and examine the relationships between their chemical reactivity, location within the skin, and the primary biochemical events following solar radiation exposure, along with their influence on skin physiology and pathology. An important take-home message of this review is that photosensitization allows visible light and UV-A radiation to have large effects on skin. The analysis presented here unveils potential causes for the continuous increase in global skin cancer cases and emphasizes the limitations of current sun protection approaches.

Spin-Forbidden Addition of Molecular Oxygen to Stable Enol Intermediates-Decarboxylation of 2-Methyl-1-tetralone-2-carboxylic Acid

The deprotonation of an organic substrate is a common preactivation step for the enzymatic cofactorless addition of O2 to this substrate, as it promotes charge-transfer between the two partners, inducing intersystem crossing between the triplet and singlet states involved in the process. Nevertheless, the spin-forbidden addition of O2 to uncharged ligands has also been observed in the laboratory, and the detailed mechanism of how the system circumvents the spin-forbiddenness of the reaction is still unknown. One of these examples is the cofactorless peroxidation of 2-methyl-3,4-dihydro-1-naphthol, which will be studied computationally using single and multi-reference electronic structure calculations. Our results show that the preferred mechanism is that in which O2 picks a proton from the substrate in the triplet state, and subsequently hops to the singlet state in which the product is stable. For this reaction, the formation of the radical pair is associated with a higher barrier than that associated with the intersystem crossing, even though the absence of the negative charge leads to relatively small values of the spin-orbit coupling.

Innovative green oxidation of amines to imines under atmospheric oxygen

In recent years, the development of environmentally benign molecular construction methods has been of great importance, and especially, resource recycling, high atomic efficiency, and low environmental impact are in high demand. From this point of view, attention has also been focused on the development of one-pot synthesis of pharmaceuticals and functional molecules. Imines are excellent synthetic intermediates of these useful molecules, and the environmentally friendly oxidative synthesis of imines from amines has been energetically developed using oxygen (or air), which is abundantly available on the Earth, as an oxidant. This review focuses on the latest innovative and green oxidation systems of amines to imines under atmospheric oxygen, and their application to one-pot/eco-friendly and sustainable synthesis of pharmaceuticals and functional molecules. In particular, catalytic systems that activate molecular oxygen are categorized and described in detail as transition metal catalytic systems, photoirradiated catalytic systems, and organocatalytic systems.

O2 Activation with a Sterically Encumbered, Oxygen-Deficient Polyoxovanadate-Alkoxide Cluster

The isolation of the oxygen-deficient, polyoxovanadate-alkoxide (POV-alkoxide) cluster, [ⁿBu₄N][V₆O₆(OMe)₁₂(MeCN)], and its subsequent reactivity with oxygen (O₂), has demonstrated the utility of these assemblies as molecular models for heterogeneous metal oxide catalysts. However, the mechanism through which this cluster activates and reduces O₂ to generate the oxygenated species is poorly understood. Currently it is speculated that this POV-alkoxide mediates the four-electron O-O bond cleavage through an O₂ bridged dimeric intermediate, a mechanism which is not viable for O₂ reduction at solid-state metal oxide surfaces. Here, we report the successful activation and reduction of O₂ by the calix-functionalized POV-alkoxide cluster, [ⁿBu₄N][(calix)V₆O₆(OMe)₈](MeCN)] (calix = 4-tert-butylcalix[4]arene). The steric hindrance imparted to the open vanadium site by the calix motif eliminates the possibility of cooperative, bimolecular O₂ activation, allowing for a comparison of the reactivity of this system with that of the nonfunctionalized POV-alkoxide described previously. Rigorous characterization of the calix-substituted assembly, enabled by its newfound solubility in organic solvent, reveals that the incorporation of the tetradentate aryloxide ligand into the POV-alkoxide scaffold perturbs the electronic communication between the site-differentiated vanadium(III) ion and the cluster core. Collectively, our results provide insight into the physiochemical factors that are important during the O₂ reduction reaction at oxygen-deficient sites in reduced POV-alkoxide clusters.

Self-Driven Reactive Oxygen Species Generation via Interfacial Oxygen Vacancies on Carbon-Coated TiO2-x with Versatile Applications

The effective activation and utilization of O₂ have always been the focus of scientists because of its wide applications in catalysis, organic synthesis, life and medical science. Here, a novel method for activating O₂ spontaneously via interfacial oxygen vacancies on carbon-coated TiO_2-x to generate reactive oxygen species (ROS) with versatile applications is reported. The interfacial oxygen vacancies can be stabilized by the carbon layer and hold its intrinsic properties for spontaneous oxygen activation without light irradiation, while common surface oxygen vacancies on TiO_2-x are always consumed by the capture of H₂O to form the surface hydroxyls. Thus, O₂ absorbed at the interface of carbon and TiO_2-x can be directly activated into singlet oxygen (¹O₂) or superoxide radicals (·O₂^-), confirmed both experimentally and theoretically. These reactive oxygen species exhibit excellent performance in oxidation reactions and inhibition of MCF-7 cancer cells, providing new insight into the effective utilization of O₂ via oxygen vacancies on metal oxides.

HCl and O2 co-activated bis(8-quinolinolato) oxovanadium(iv) complexes as efficient photoactive species for visible light-driven oxidation of cyclohexane to KA oil

Photoactive species (PA) originating from HCl and O₂ co-activated bis(8-quinolinolato) oxovanadium(iv) can effectively modulate the photocatalytic oxidation of cyclohexane.

Capping experiments reveal multiple surface active sites in CeO2 and their cooperative catalysis

Understanding of surface active sites (SAS) of CeO₂ is crucial to its catalytic applications. In the present study, we have employed capping experiments, DFT calculations, and spectroscopic characterization to study pristine CeO₂ catalyst. We find that multiple SAS coexist on the CeO₂ surface: oxygen vacancies as redox sites and the coordinately unsaturated Ce cations near the oxygen vacancies and the neighboring oxygen ions as Lewis acid–base sites. Dimethylsulfoxide (DMSO), pyridine, and benzoic acid are utilized to cap the redox sites, Lewis acid sites, and base sites, respectively. Selective capping on the redox site does not have much effect on the acid–base catalysis, and vice versa, indicating the distinct surface proximity and independent catalysis of these SAS. We draw attention to a relationship between the well-known redox sites and the surface Lewis acid and Lewis base pairs on CeO₂ surface, which are responsible for driving various heterogeneous catalytic reactions.

Capping with pyridine, benzoic acid, and DMSO in catalytic reactions reveals the locations of surface active sites of CeO₂.

Unraveling reaction networks behind the catalytic oxidation of methane with H2O2 over a mixed-metal MIL-53(Al,Fe) MOF catalyst

Reaction paths underlying the catalytic oxidation of methane with H2O2 over an Fe containing MIL-53(Al) metal-organic framework were studied by periodic DFT calculations. Not only the activation of methane, but the full reaction network was considered, which includes the formation of the active site, the overoxidation of methane to CO2 and the decomposition of H2O2 to H2O and O2. Calculations indicate that the activation barrier for the initial activation of the Fe sites upon reaction with H2O2 is comparable to that of the subsequent C-H activation and also of the reaction steps involved in the undesirable overoxidation processes. The pronounced selectivity of the oxidation reaction over MIL-53(Al,Fe) towards the target mono-oxygenated CH3OH and CH3OOH products is attributed to the limited coordination freedom of the Fe species encapsulated in the extended octahedral [AlO6] structure-forming chains, which effectively prevents the direct overoxidation paths prior to product desorption from the active sites. Importantly, our computational analysis reveals that the active sites for the desired methane oxidation are able to much more efficiently promote the direct catalytic H2O2 decomposition reaction, rendering thus the current combination of the active site and the reactants undesirable for the prospective methane valorization process.

Elucidating light-induced charge accumulation in an artificial analogue of methane monooxygenase enzymes using time-resolved X-ray absorption spectroscopy

We report the use of time-resolved X-ray absorption spectroscopy in the ns-μs time scale to track the light induced two electron transfer processes in a multi-component photocatalytic system, consisting of [Ru(bpy)₃]²⁺/ a diiron(iii,iii) model/triethylamine. EXAFS analysis with DFT calculations confirms the structural configurations of the diiron(iii,iii) and reduced diiron(ii,ii) states.

Cinnamyl alcohol oxidation using supported bimetallic Au–Pd nanoparticles: an investigation of autoxidation and catalysis

In this study, we examine autoxidation and its role on the catalytic aerobic oxidation of cinnamyl alcohol using supported AuPd nanoparticles.

Water as a catalytic switch in the oxidation of aryl alcohols by polymer incarcerated rhodium nanoparticles

Rh nanoparticles that were inactive in toluene, were converted into a powerful catalyst for aryl alcohol oxidation by the presence of water in the reaction media.

Photochemistry and Spectroscopy of Singlet Oxygen in Solvents. Recent Advances which Support the Old Theory

Molecular oxygen is a paramagnetic gas with the triplet O2( ) ground state which exhibits just sluggish chemical reactivity in the absence of radical sources. In contrast, the excited metastable singlet oxygen O2( ) is highly reactive; it can oxygenate organic molecules in a wide range of specific reactions which differ from those of the usual triplet oxygen of the air. This makes the singlet oxygen an attractive reagent for new synthesis and even for medical treatments in photodynamic therapy. As an important intermediate O2( ) has attracted great attention of chemists during half-century studies of its reactivity and spectroscopy, but unusual properties of singlet oxygen makes it difficult to unravel all mysterious features. The semiempirical theory of spin-orbit coupling in dioxygen and in collision complexes of O2 with diamagnetic molecules proposed in 1982 year has explained and predicted many photochemical and spectral properties of dioxygen produced by the dye sensitization in solvents. Recent experiments with direct laser excitation of O2 in solvents provide a complete support of the old theory. The present review scrutinizes the whole story of development and experimental verification of this theory.