Selective Oxidation Of Organic Compounds Research Articles

Recent advances in photothermal catalysis: Coupling hydrogen evolution and organic conversion

Photothermal catalysis, as a green and sustainable technology, has attracted increasing attention in the fields of clean energy and green organic synthesis. Integrating photothermal catalysis with solar-driven hydrogen production and organic synthetic chemistry has become a promising strategy for sustainable energy generation with great research and practical value. In this review, we present an overview of the background of solar-driven photothermal catalytic reactions, introduce the fundamental principles of photocatalysis, thermal catalysis, and photothermal reduction of hydrogen coupled with selective oxidation of organic compounds, and summarize the recent advances in photothermal semiconductor catalysts such as carbon-based materials, titanium-based materials, noble metal composites, and metal sulfides. We also highlight the recent advances in the application of these catalysts in photothermal catalytic water splitting for hydrogen generation and selective oxidation of organic compounds. Finally, we address the challenges and opportunities faced by solar-drive photothermal catalysts in the synergism of photocatalytic hydrogen production and selective oxidation of organic compounds. The aim is to provide guiding insights for the rational design of this dual-function catalytic system, offering a green pathway for the development of clean energy and selective oxidation in organic synthesis.

ZnIn2S4/MOF S-scheme photocatalyst for H2 production and its femtosecond transient absorption mechanism

Photocatalytic water splitting is a popular pathway for H2 evolution, but the slow water oxidation greatly hampers the overall activity. To harness photogenerated holes in an efficient and lucrative way, the water oxidation reaction is replaced by selective oxidation of organic compounds to achieve simultaneous production of H2 and value-added chemicals. Herein, an alternative tactic is reported where an organic compound (benzylamine, BA) not only serves as the precursor for N-benzylidene-benzylamine (NBBA) production but also provides hydrogen sources for H2 evolution, achieving the goal under anhydrous conditions. This process is realized using an S-scheme photocatalyst composed of ZnIn2S4 and the UiO-66-NH2 (U6N) metal-organic framework (MOF). The S-scheme carrier transfer mechanism was validated by in-situ irradiated X-ray photoelectron spectroscopy (ISI-XPS) and femtosecond transient absorption (fs-TA) spectroscopy. With increased carrier efficiency and reinforced redox power endowed by the S-scheme heterojunction, the composite performed better than ZnIn2S4 and MOF. The performance was further ameliorated by Pt-cocatalyst modification, achieving an H2 production rate of 5275 μmol h−1 g−1 as well as BA conversion of 94.3% with 99.3% NBBA selectivity. Mechanistic studies reveal that BA is initially oxidized to carbon-centered radicals and further to imines along with the release of protons. The imine reacts with another BA molecule to form NBBA, while the protons are reduced to H2. This work provides new insights into concurrent photocatalytic H2 production and selective organic oxidation from organic amines using S-scheme photocatalysts.

Operando X-Ray Diffraction Studies of Co Oxide Model Catalysts during Alcohol Oxidation and Oxygen Evolution Reaction

Green energy technology involving hydrogen generation requires developing catalyst materials of commercial viability, good stability and high reactivity for the electrochemical water splitting. Transition-metal oxides such as Co3O4 are among the best precious-metal-free electrode materials for the anodic oxygen evolution reaction (OER) in alkaline electrolysis [1]. Moreover, these spinel-type transition metal oxides are an important class of catalysts for the selective oxidation of organic compounds. Previous in situ and operando surface X-ray diffraction studies of epitaxial Co oxide films by our group showed pronounced changes in the near-surface structure of these catalysts. In particular, a sub-nm CoOx(OH)y skin layer starts forming in the pre-OER potential range which is accompanied by a volume change of the Co3O4 lattice [2]. In a more recent work, where we systematically explored the relationship between the effective thickness of this skin layer and the OER activity, we found clear evidence that the entire skin layer serves as a three-dimensional reactive zone for the OER [3]. Since the skin layer formation already starts several 100 mV negative of the onset of the OER, it is not induced by the catalytic reaction itself, but rather reflects the oxides’ electrochemistry, and may thus be expected also for other electrocatalytic processes, especially other electrocatalytic oxidation reactions which occur in a similar potential range. These oxide materials are not only of interest for OER, but also for the electrochemical oxidation of other compounds, such as alcohols [4]. However, data on the oxides’ structure in the presence of organic compounds does not exist at present. Obtaining such data is of great interest for understanding the mutual influence of oxide-electrolyte interface structure and electrocatalytic activity. In addition, the vast majority of publications on Co oxide oxidation processes, in particular all in situ and operando studies, have been performed up to now at room temperature only, and, thus, the catalytic performance of these materials at higher temperatures remains unknown. As electrolyzers and industrial (electro-) catalytic processes typically operate at elevated temperatures, such data are of considerable interest. Here, we present results of first potential-dependent structural studies of epitaxial Co3O4(111) and CoOOH(001) films electrodeposited on Au(111) single crystal substrates during ethylene glycol oxidation (EG Ox) in 0.1 M NaOH solution and compare those with the behavior found on the same samples in EG-free electrolyte. Ethylene glycol oxidation is a highly suitable reaction for such studies, as it occurs at potentials prior to the onset of the OER [4]. The samples were investigated as a function of electrode potential and temperature by operando surface X-ray diffraction at beamline P23 of PETRA III, DESY. The EG Ox was studied by successively increasing the EG concentration from 0 to 0.1, 0.5 and 2 M, which allows direct quantitative comparison of the potential-dependent structural changes. These studies of the oxides’ structure and electrochemical reactivity were performed at different temperatures in the range of 25 - 70°C. In all cases significant skin layer formation and accompanying changes in the oxide lattice strain were observed. The structural data are correlated with the results of cyclic voltammetry and optical reflectivity measurements that were recorded in parallel with the X-ray diffraction measurements. Together, they provide insights into the potential-dependent evolution of the skin layer thickness and strain and into the stability of the oxides in dependence of EG concentration and temperature. First results indicate that at elevated temperatures larger skin layer thicknesses and strain changes occur. Furthermore, the structural properties recover completely upon cooling back down to room temperature, suggesting that these effects are highly reversible. In the presence of EG, the reversible skin layer formation and changes in the Co3O4 bulk lattice were also observed, even though these structural changes appear to be less pronounced under alcohol oxidation conditions, especially at higher EG concentration. These studies will contribute to an in-depth understanding of the near surface structure of these Co oxide thin films during liquid phase oxidation catalysis, which is a prerequisite for the design of new, abundant and superior catalysts based on transition metal oxides for catalytic selective oxidation processes.We acknowledge financial support by the Deutsche Forschungsgemeinschaft via special research area TRR 247, project B10 (project no. 388390466).[1] A. Bergmann et al., Nature Catalysis 2018, 1, 711 - 719[2] F. Reikowski et al., ACS Catal. 2019, 9, 3811 - 3821[3] T. Wiegmann et al., ACS Catal. 2022 [4] S. N. Sun et al, J. Electrochem. Soc. 2016, 163, H99

H2O2-Based Selective Oxidations Catalyzed by Supported Polyoxometalates: Recent Advances

Polyoxometalates (POMs) are transition metal oxygen anionic clusters that are oxidatively and thermally robust due to their inorganic, metal oxide-like nature. The versatility of their structures and compositions ensures tunable acid and redox properties, solubility, and functionality. The potential of POMs as homogeneous catalysts and building blocks for the construction of heterogeneous selective oxidations catalysts is being intensively investigated. POM catalysts immobilized on solid supports have the clear advantages of easy separation and reuse and, thus, better meet the requests of sustainable chemistry, provided that they are leaching-resistant under the reaction conditions. Here, we give a brief overview of recent advances in the field of liquid-phase selective oxidation of organic compounds using supported POMs and the green oxidant–hydrogen peroxide, with a focus on the critical issues of the catalyst stability and reusability. The scope and limitations of various approaches to POM immobilization are discussed.

Immobilization of Polyoxometalates on Carbon Nanotubes: Tuning Catalyst Activity, Selectivity and Stability in H2O2-Based Oxidations

In recent years, carbon nanotubes (CNTs), including N-doped ones (N-CNTs), have received significant attention as supports for the construction of heterogeneous catalysts. In this work, we summarize our progress in the application of (N)-CNTs for immobilization of anionic metal-oxygen clusters or polyoxometalates (POMs) and use of (N)-CNTs-supported POM as catalysts for liquid-phase selective oxidation of organic compounds with the green oxidant–aqueous hydrogen peroxide. We discuss here the main factors, which favor adsorption of POMs on (N)-CNTs and ensure a quasi-molecular dispersion of POM on the surface and their strong attachment to the support. The effects of the POM nature, N-doping of CNTs, acid additives, and other factors on the POM immobilization process and catalytic activity/selectivity of the (N)-CNTs-immobilized POMs are analyzed. Particular attention is drawn to the critical issue of the catalyst stability and reusability. The scope and limitations of the POM/(N)-CNTs catalysts in H2O2-based selective oxidations are discussed.

9,10-Dihydroanthracene auto-photooxidation efficiently triggered photo-catalytic oxidation of organic compounds by molecular oxygen under visible light

The development of mild and efficient process for the selective oxidation of organic compounds by molecular oxygen (O2) can be one of the key technologies for synthesizing oxygenates. This paper discloses an efficient and mild synthesis protocol for the O2-involved ethylbenzene (EB) photooxidation triggered by 910-dihydroanthracene (DHA) auto- photooxidation in acetone under visible light illumination, which can achieve 87.7 % EB conversion and 99.5 % acetylacetone (ACP) selectivity under ambient conditions. Also, 62.9 % EB conversion and 96.3 % ACP selectivity is obtained in air atmosphere. Furthermore, this protocol has a good adaptability for the photooxidation of other organic substrates such as tetrahydronaphthalene, diphenylmethane, toluene, cyclohexane, cyclohexene, alcohol, methylfuran and thioether to their corresponding oxygenates. A series of control and quenching tests, combined with EPR spectra, suggest that the photo-excited DHA can transfer its photo-electron to O2 to yield a superoxide radical anion (O2⚫−), then DHA is preferentially oxidized to anthraquinone (AQ) by the active O2⚫− owing to its high reactivity. Finally, the in situ generated AQ as an active photo-catalyst can achieve the photooxidation of EB and other organic compounds by O2. The present photo-autoxidation protocol gives a good example for the O2-based selective oxidation of inert hydrocarbons under mild conditions.

Clean Production of 2,3,5-Trimethylhydroquinone Using Peroxidase-Based Catalyst

The selective oxidation of organic compounds to obtain vitamins is one of the key processes in chemistry and biotechnology. The use of toxic compounds such as chromium and manganese oxides for these purposes strongly affects the environment. We propose a novel bio-catalyst supported on magnetically separable nanoparticles based on the immobilized peroxidase. Biocatalysts based on immobilized enzymes received considerable attention due to important applications in syntheses of value-added chemicals, pharmaceuticals and drug intermediates with great catalytic efficiency and high yields of target molecules. In this work, we studied the oxidation of 2,3,6-trimethylphenol to 2,3,5-trimethylhydroquinone (an intermediate of vitamin E) using hydrogen peroxide. The effects of initial substrate concentration, temperature, and pH were studied. The optimum conditions for the process of oxidation 2,3,6-trimethylphenol in the presence of magnetically separable biocatalyst were found: initial substrate concentration – 1.5 mmol/L; temperature – 40 °C; pH – 6.5. The developed biocatalyst showed high activity and selectivity in 2,3,6-trimethylphenol oxidation to 2,3,5-trimethylhydroquinone.

Total Synthesis of Rhodonoids A, B, E, and F, Enabled by Singlet Oxygen Ene Reactions.

Singlet oxygen is a versatile reagent for the selective oxidation of organic compounds under mild reaction conditions. It is frequently invoked in biosynthetic pathways, so it is especially suitable for application in the biomimetic synthesis of natural products. Herein, we show that use of the singlet oxygen ene reaction, combined with [2 + 2] cycloadditions, leads to concise, divergent, and redox-economic total syntheses of several polycyclic members of the rhodonoid family of meroterpenoids.

Prospect of vapor phase catalytic H2O2 production by oxidation of water

Vapor phase catalytic hydrogen peroxide production by oxidation of water is possible by coupling the reaction with oxidation of an organic sacrificial reductant. It is potentially a safer process than direct synthesis from H2 and O2. Based on mechanistic information available mostly for liquid phase catalytic processes, feasible reaction mechanisms for such coupled reactions are proposed based on which desirable catalyst properties are identified. It is found that the surface-adsorbed oxygen bond is an important parameter for identifying desirable catalysts. Thermodynamics can be used to identify the types of organic oxidation reactions that can couple with water oxidation such that H2O2 formation becomes thermodynamically favorable. Reactions such as epoxidation of alkenes and selective oxidation of alkanes to alcohols cannot provide sufficient thermodynamic driving force, whereas oxidation of alcohols to aldehydes and to acids can. Finally, further research is suggested to identify catalytic properties important for H2O2 decomposition and for coupling selective oxidation of organic compounds to oxidation of H2O in order to facilitate development of H2O2 production coupled with selective organic oxidation.

Photocatalytic Selective Oxidation of Organic Compounds in Graphitic Carbon Nitride Systems: A Review

Recent work is reviewed concerning photocatalytic systems derived from graphitic carbon nitride (GCN) for the selective oxidation of various organic compounds including saturated and unsaturated hydrocarbons, alcohols, and organic sulfides. Examples are given for oxidative coupling of a series of organic compounds with the formation of carbon–nitrogen and carbon–carbon bonds as well as cyclization. The properties of various GCN samples were examined, including bulk, mesoporous, layered, and individual materials as well as samples modified with organic compounds, samples doped with metals and nonmetals, and GCN-derived composite materials. The outlook for future research in this field is given at the conclusion.

Synthesis of ceria nanoparticles in pores of SBA-15: Pore size effect and influence of citric acid addition

Abstract The influence of porous structure and addition of citric acid on the formation of ceria particles inside the SBA-15 pores is studied using nitrogen adsorption-desorption at −196 °C, XRD, UV-visible spectroscopy, TEM and TPR-H2 methods. The use of pores of SBA-15 as nanoreactors provides the formation of CeO2 nanoparticles with sizes below 6 nm. According to XRD and TEM results, the addition of citric acid during the impregnation allows obtaining the CeO2 particles with sizes about 3 nm, while the agglomerates of large СеО2 particles are formed on the surface of the SBA-15, when citric acid is not added. Small sizes of ceria particles (3 nm) cause an enhanced reducibility (TPR-H2), high defectiveness of the particles and red shift in the UV-vis adsorption spectra. These properties may improve the catalytic and photocatalytic activity of the CeO2/SBA-15-based catalysts in deep and selective oxidation of organic compounds.

Influence of tetraalkylammonium cations on quality of decatungstate and its photocatalytic property in visible light-triggered selective oxidation of organic compounds by dioxygens

Three decatungstates (DTs) were conveniently synthesized respectively using quaternary ammonium cations of tetramethyl, tetrapropyl and tetrabuthyl as counterions (marked as TMADT, TPADT and TBADT) and their quality, photoluminescence (PL) property and redox capacity were measured by UV–vis, XPS and PL spectra and cyclic voltammograms (CVs), respectively. The photo-catalysis activities of three DTs were checked using visible light-driven selective oxidation of cyclohexane, toluene, ethylbenzene and benzyl alcohol by dioxygens (O2) in acetonitrile (MeCN). The results showed that the photo-catalysis activity of DT anion for these oxidations gradually increases with the shortening of its cationic alkyl chain, which is attributed to the continuous improvement of its quality from TBADT to TMADT. Furthermore, the additives water, 12 M hydrogen chloride (HCl) solution and especially the mixture of both can markedly promote the DTs-photocatalyzed oxidations and such promotion presents an enhancing trend from TBADT to TMADT. which is likely due to the beneficial effects of these additives as follows: i) Water can significantly stabilize the structure of DT anion under light illumination and improve its redox cycling in MeCN; ii) 12 M HCl may obviously improve the oxidative capacity and redox cycling of DT anion and stabilize its photo-excited states. iii) The respective promotion effects of such two additives on photo-catalysis can be well combined and such combined effect is enhanced drastically upon TMADT owing to its hydrophilic feature, thus significantly improving the photo-catalytic efficiency of TMADT under the joint action of water and 12 M HCl.

Synthesis and catalytic property of inorganic-organic hybrid copper vanadate Cu<sub>2</sub>(eIM)<sub>6</sub>[V<sub>4</sub>O<sub>12</sub>] in selective oxidation of sulfides

Polyoxometallates (POMs) a re familiar inorganic building blocks for their rich structural diversities and potential applications in many fields, such as catalysis, magnetism, and energy, etc. As an important subclass of POMs, vanadates are a promising candidate to construct POMs-based hybrid materials due to their unprecedented structural diversity, mixed valency, and variable coordination geometries. Inorganic-organic hybrid vanadates with first-row transition metals show very rich structural diversities. Further, the hybrid vanadates are expected to exhibit improved properties by the synergetic interaction between the metal-organic subunits and the vanadium oxide. Thus, it is essential but remains challenging to control the synthesis of inorganic-organic hybrid vanadates rationally. Recently, our group reported several examples of inorganic-organic hybrid vanadates by controlling the synthetic reaction conditions and further explored their catalytic properties in the oxidation of sulfides, alcohols and olefin, etc. In order to continue our research in the controlling synthesis and catalytic properties of inorganic-organic hybrid vanadates, here we synthesized a hybrid copper vanadate Cu2(eIM)6[V4O12] ( 1 ). In the synthetic procesure, a neutral V6 cluster VIV2VV4O14(eIM)8 (eIM=1-ethylimidazole) which was reported in our earlier research, was selected as vanadium source and reacted with Cu(OAc)2·H2O in the presence of triethylamine by rationally controlling the reaction conditions. Compound 1 was fully characterzized by single-crystal X-ray diffraction (SXRD) and powder X-ray diffraction (PXRD) analyses, FT-IR spectroscopy and element analyses, etc. SXRD analyse shows that the molecular struture of 1 contains a [V4O12]4- anion cluster and two types of copper complex cation, Cu[(eIM)2]2+ and Cu[(eIM)4]2+. All the [V4O12]4- anion clusters were further connected to a two- dimensional (2D) plane network supramolecular structure by four- and six-coordinated copper complexes cations with the eIM ligands. Two types of coordination modes existing in the same tructure is rare in complexes of inorganic-organic hybrid vanadates. Selective oxidation of organic compounds, such as sulfides, alcoholes, aromatic and aliphatic hydrocarbons, etc., is one of the most vital transformations for upgrading raw starting materials into high value added products. Vanadium-containing materials show excellent catalytic performance in selective oxidation of these compounds. Given the importance of sulfoxides and sulfones as intermediates of biologic molecules, chiral auxiliaries and oxo-transfer reagents, the high selective oxidation of sulfides to obtain these compounds is more desirable. As a green oxidant, H2O2 offers significant enviromental and economic benefits over traditional stoichiometric oxidants. First, we selected the oxidation of methyl phenyl sulfide as a model reaction to evaluate the catalytic activities of compound 1 . We evaluated the influencing factors of the ammount of catalyst and oxidant, the reaction temperature and time. Under the optimized conditions, the conversion of methyl phenyl sulfide and the selectivity to corresponding sulfoxide up to 98.5% and 97.5%, respectively. Other aromatic sulfides were also examined and no obvious decrease of the catalytic activity was observed when electron donor or acceptor groups with less steric hindrance were introduced in the monophenyl sulfides (conversion from 87.4% to 96.5%; selectivity toward corresponding sulfoxides from 97.5% to 98.5%). When the reaction conditions were applied to aliphatic sulfides including n -butyl sulfide, 2-chloroethyl ethyl sulfide and dimethyl sulfide, the high conversion (from 92.2% to 98.7%) and selectivity toward corresponding sulfoxides (from 83.5% to 95.5%) were also abtained. We also probed the role of each active center of compound 1 and found that the cooperation between copper complex cations and vanadate anions played some synergistic role in the sulfonation process, which can improve the catalytic performance. As a heterogeneous catalyst, 1 was also selected to examine the long-term stability and after at least 3 runs still maintained its activity without significant changes in its structure which characterized by FT-IR spectra and PXRD analyses, respectively. It is worth pointing out that by controlling the catalytic reaction conditions and using the same oxidant, we can prepare sulfones by various sulfides oxidation with excellent conversion and selectivity up to >99% and 97.5%, respectively.

KMnO4/guanidinium-based sulfonic acid: as an efficient Brønsted acid organocatalyst for the selective oxidation of organic compounds

ABSTRACTKMnO4/guanidinium-based sulfonic acid as a green organocatalytic oxidative system can be used effectively for the selective oxidation of organic compounds in n-hexane as a nonpolar and inert solvent at room temperature in good yields. The use of this approach is described for the oxidation of alkyl arenes, alcohols, and sulfides.

Selective Electrochemical Oxidation of Lactic Acid Using Iridium-Based Catalysts

Catalysts that allow for selective oxidation of organic compounds in solution are useful tools in chemical synthesis, waste remediation, and renewable energy storage schemes. Of these, iridium oxide and iridium-based molecular compounds are frequently employed as catalysts to oxidize substrates ranging from small organic molecules to water. Here, we study the mechanism and evaluate the potential of iridium-based catalysts for small molecule oxidation in wastewater treatment, using lactic acid as a substrate. By investigating the reaction products, kinetic isotope effects, and electrochemical properties of iridium oxide and a molecular analogue on a high surface area antimony-doped tin oxide (ATO) support, we find evidence for a new iridium-catalyzed pathway for lactic acid oxidation. Applying this at concentrations representative of effluent from dairy production processes, we show that iridium oxide can selectively remove small amounts of lactic acid from water by oxidizing it to CO2 with concomitant cat...

Manganese chlorins immobilized on silica as oxidation reaction catalysts

Synthetic strategies that comply with the principles of green chemistry represent a challenge: they will enable chemists to conduct reactions that maximize the yield of products with commercial interest while minimizing by-products formation. The search for catalysts that promote the selective oxidation of organic compounds under mild and environmentally friendly conditions constitutes one of the most important quests of organic chemistry. In this context, metalloporphyrins and analogues are excellent catalysts for oxidative transformations under mild conditions. In fact, their reduced derivatives chlorins are also able to catalyze organic compounds oxidation effectively, although they have been still little explored. In this study, we synthesized two chlorins through porphyrin cycloaddition reactions with 1.3-dipoles and prepared the corresponding manganese chlorins (MnCHL) using adequate manganese(II) salts. These MnCHL were posteriorly immobilized on silica by following the sol–gel process and the resulting solids were characterized by powder X-ray diffraction (PXRD), UVVIS spectroscopy, FTIR, XPS, and EDS. The catalytic activity of the immobilized MnCHL was investigated in the oxidation of cyclooctene, cyclohexene and cyclohexane and the results were compared with the ones obtained under homogeneous conditions.

ChemInform Abstract: Metal‐Free Aerobic Oxidations Mediated by N‐Hydroxyphthalimide. A Concise Review

AbstractReview: 73 refs.

Recent developments in liquid-phase selective oxidation using environmentally benign oxidants and mesoporous metal silicates

This Perspective article surveys recent advances in the synthesis of mesoporous transition-metal-containing silicate materials and their use for the liquid-phase selective oxidation of organic compounds with environmentally friendly oxidants – molecular oxygen, hydrogen peroxide and organic hydroperoxides.

Metal-free aerobic oxidations mediated by N-hydroxyphthalimide. A concise review

Since the beginning of the century, N-hydroxyphthalimide and related compounds have been revealed to be efficient organocatalysts for free-radical processes and have found ample application in promoting the aerobic oxidation of a wide range of organic substrates. When combined with different co-catalysts, they are activated to the corresponding N-oxyl radical species and become able to promote radical chains, involving molecular oxygen, directly or indirectly. Most of the examples reported in the literature describe the use of these N-hydroxy derivatives in the presence of transition-metal complexes. However, eco-friendly standards, including the demand for highly selective transformations, impose the development of metal-free processes, especially for large-scale productions, as in the case of the oxygenation of hydrocarbons. For this reason, many efforts have been devoted in the past decade to the design of new protocols for the activation of N-hydroxy imides in the presence of nonmetal initiators. Herein we provide a concise overview of the most significant and successful examples in this field, with the final aim to furnish a useful instrument for all scientists actively involved in the O2-mediated selective oxidation of organic compounds and looking for environmentally safe alternatives to metal catalysis.

ChemInform Abstract: Update on Selective Oxidation Using Gold

AbstractReview: 174 refs.