Heterogeneously Catalyzed Aerobic Oxidation of Sulfides with a BaRuO3 Nanoperovskite

La1-xSrxFeO3-δ Perovskite Oxide Nanoparticles for Low-Temperature Aerobic Oxidation of Isobutane to tert-Butyl Alcohol

The development of reusable solid catalysts based on naturally abundant metal elements for the liquid-phase selective oxidation of light alkanes under mild conditions to obtain desired oxygenated products, such as alcohols and carbonyl compounds, remains a challenge. In this study, various perovskite oxide nanoparticles were synthesized by a sol-gel method using aspartic acid, and the effects of A- and B-site metal cations on the liquid-phase oxidation of isobutane to tert-butyl alcohol with molecular oxygen as the sole oxidant were investigated. Iron-based perovskite oxides containing Fe4+ such as BaFeO3-δ, SrFeO3-δ, and La1-xSrxFeO3-δ exhibited catalytic performance superior to those of other Fe3+- and Fe2+-based iron oxides and Mn-, Ni-, and Co-based perovskite oxides. The partial substitution of Sr for La in LaFeO3 significantly enhanced the catalytic performance and durability. In particular, the La0.8Sr0.2FeO3-δ catalyst could be recovered by simple filtration and reused several times without an obvious loss of its high catalytic performance, whereas the recovered BaFeO3-δ and SrFeO3-δ catalysts were almost inactive. La0.8Sr0.2FeO3-δ promoted the selective oxidation of isobutane even under mild conditions (60 °C), and the catalytic activity was comparable to that of homogeneous systems, including halogenated metalloporphyrin complexes. On the basis of mechanistic studies, including the effect of Sr substitution in La1-xSrxFeO3-δ on surface redox reactions, the present oxidation proceeds via a radical-mediated oxidation mechanism, and the surface-mixed Fe3+/Fe4+ valence states of La1-xSrxFeO3-δ nanoparticles likely play an important role in promoting C-H activation of isobutane as well as decomposition of tert-butyl hydroperoxide.

Synthesis and catalytic application of nanostructured metal oxides and phosphates

The design and development of new high-performance catalysts is one of the most important and challenging issues to achieve sustainable chemical and energy production. This Feature Article describes the synthesis of nanostructured metal oxides and phosphates mainly based on earth-abundant metals and their thermocatalytic application to selective oxidation and acid-base reactions. A simple and versatile methodology for the control of nanostructures based on crystalline complex oxides and phosphates with diverse structures and compositions is proposed as another approach to catalyst design. Herein, two unique and verstile methods for the synthesis of metal oxide and phosphate nanostructures are introduced; an amino acid-aided method for metal oxides and phosphates and a precursor crystallization method for porous manganese oxides. Nanomaterials based on perovskite oxides, manganese oxides, and metal phosphates can function as effective heterogeneous catalysts for selective aerobic oxidation, biomass conversion, direct methane conversion, one-pot synthesis, acid-base reactions, and water electrolysis. Furthermore, the structure-activity relationship is clarified based on experimental and computational approaches, and the influence of oxygen vacancy formation, concerted activation of molecules, and the redox/acid-base properties of the outermost surface are discussed. The proposed methodology for nanostructure control would be useful not only for the design and understanding of the complexity of metal oxide catalysts, but also for the development of innovative catalysts.

A-site defective La2-xCuO4 perovskite-type oxides for efficient oxidation of cyclohexylbenzene

Phenol production through the oxidation of cyclohexylbenzene (CHB) and the subsequent decomposition of tertiary hydroperoxide has attracted more and more attention. In this study, defective La2-xCuO4 perovskite-type oxide catalysts with tunable A-site deficient structures and abundant surface oxygen vacancies were developed for the liquid phase oxidation of CHB to produce cyclohexylbenzene-1-hydroperoxide (CHBHP). By tuning the amount of A-site La ions in the perovskite structure, more surface oxygen vacancies and Cu+ species were formed in catalysts. The A-site-deficient La1.9CuO4 catalyst achieved significant catalytic efficiency along with a high CHBHP yield of 27.6% at 48.6% CHB conversion under reaction conditions (i.e., 120 °C and 12 h), outperforming those of other transition metal-based catalysts previously reported in the literature. A series of structural characterization methods and catalytic reactions highlighted the crucial roles of surface oxygen vacancies and metal La and Cu ions in the oxidation process. It was revealed that metal ions favored CHB adsorption and activation, while surface oxygen vacancies facilitated the creation of active adsorbed oxygen species. The present study offers an opportunity for the future design of new high-efficiency heterogeneous catalyst systems for CHB oxidation to obtain phenol.

Two 3D Two-Fold Interpenetrated Dia-Like Polyoxometalate-Based Metal-Organic Frameworks: Synthesis and Sulfide Selective Oxidation Activity

Two new three-dimensional (3D) polyoxometalate-based metal-organic frameworks (POMOFs), [M2(btap)4(H2O)4(HPMo10VI Mo2VO40)] (M = Co (1) and Cd (2); btap = 3, 5-bis(1', 2', 4'-triazol-1'-yl)pyridine), have been synthesized under mild hydrothermal conditions and characterized in detail. Single-crystal X-ray diffraction (SXRD) analysis indicates that 1 and 2 are isostructural. In complexes 1 and 2, the metal ion is coordinated with the ligand to form two different left and right helical one-dimensional chains, which are alternately connected in a twisted form to build a two-fold interpenetrated three-dimensional structure, and the polyoxometalate is encapsulated into in the pores generated by the interpenetrating structure. It is noteworthy that 1 and 2, as recyclable catalysts, possess favorable heterogeneous catalytic activity and excellent sulfoxide selectivity in sulfide oxidation reactions, with H2O2 as an oxidant. By reason of the high dispersion of polyoxometalate with good intrinsic activity in the skeleton structure, the title complex has high activity. In addition, no obvious decrease of sulfoxide yield is observed after at least five cycles. These results indicate the excellent catalytic activity and sustainability of 1 and 2.

Nanosized Ti-Based Perovskite Oxides as Acid-Base Bifunctional Catalysts for Cyanosilylation of Carbonyl Compounds

The development of effective solid acid-base bifunctional catalysts remains a challenge because of the difficulty associated with designing and controlling their active sites. In the present study, highly pure perovskite oxide nanoparticles with d⁰-transition-metal cations such as Ti⁴⁺, Zr⁴⁺, and Nb⁵⁺ as B-site elements were successfully synthesized by a sol-gel method using dicarboxylic acids. Moreover, the specific surface area of SrTiO₃ was increased to 46 m² g^-1 by a simple procedure of changing the atmosphere from N₂ to air during calcination of an amorphous precursor. The resultant SrTiO₃ nanoparticles showed the highest catalytic activity for the cyanosilylation of acetophenone with trimethylsilyl cyanide (TMSCN) among the tested catalysts not subjected to a thermal pretreatment. Various aromatic and aliphatic carbonyl compounds were efficiently converted to the corresponding cyanohydrin silyl ethers in good-to-excellent yields. The present system was applicable to a larger-scale reaction of acetophenone with TMSCN (10 mmol scale), in which 2.06 g of the analytically pure corresponding product was isolated. In this case, the reaction rate was 8.4 mmol g^-1 min^-1, which is the highest rate among those reported for heterogeneous catalyst systems that do not involve a pretreatment. Mechanistic studies, including studies of the catalyst effect, Fourier transform infrared spectroscopy, and temperature-programmed desorption measurements using probe molecules such as pyridine, acetophenone, CO₂, and CHCl₃, and the poisoning effect of pyridine and acetic acid toward the cyanosilylation, revealed that moderate-strength acid and base sites present in moderate amounts on SrTiO₃ most likely enable SrTiO₃ to act as a bifunctional acid-base solid catalyst through cooperative activation of carbonyl compounds and TMSCN. This bifunctional catalysis through SrTiO₃ resulted in high catalytic performance even without a heat pretreatment, in sharp contrast to the performance of basic MgO and acidic TiO₂ catalysts.

Synthesis and Aerobic Oxidation Catalysis of Mesoporous Todorokite-Type Manganese Oxide Nanoparticles by Crystallization of Precursors

The pursuit of a high surface area while maintaining high catalytic performance remains a challenge due to a trade-off relationship between these two features in some cases. In this study, mesoporous todorokite-type manganese oxide (OMS-1) nanoparticles with high specific surface areas were synthesized in one step by a new synthesis approach involving crystallization (i.e., solid-state transformation) of a precursor produced by a redox reaction between MnO₄^- and Mn²⁺ reagents. The use of a low-crystallinity precursor with small particles is essential to achieve this solid-state transformation into OMS-1 nanoparticles. The specific surface area reached up to ca. 250 m² g^-1, which is much larger than those (13-185 m² g^-1) for Mg-OMS-1 synthesized by previously reported methods including multistep synthesis or dissolution/precipitation processes. Despite ultrasmall nanoparticles, a linear correlation between the catalytic reaction rates of OMS-1 and the surface areas was observed without a trade-off relationship between particle size and catalytic performance. These OMS-1 nanoparticles exhibited the highest catalytic activity among the Mn-based catalysts tested for the oxidation of benzyl alcohol and thioanisole with molecular oxygen (O₂) as the sole oxidant, including highly active β-MnO₂ nanoparticles. The present OMS-1 nanomaterial could also act as a recyclable heterogeneous catalyst for the aerobic oxidation of various aromatic alcohols and sulfides under mild reaction conditions. The mechanistic studies showed that alcohol oxidation proceeds with oxygen species caused by the solid, and the high surface area of OMS-1 significantly contributes to an enhancement of the catalytic activity for aerobic oxidation.

Hepta-Zr-Incorporated Polyoxometalate Assembly

Driven by the synergistic-directing effect of the lacunary fragments, [B-α-GeW₉O₃₄]^10- and [B-α-GeW₁₁O₃₉]^8-, an unprecedented hepta-Zr-substituted polyoxometalate (POM) assembly K₂Na₆H₁₀(Hpy)₃[SbZr₇O₆(OH)₄(B-α-GeW₉O₃₄)₂(B-α-GeW₁₁O₃₉)₂]·28H₂O (1) was made under hydrothermal condition and structurally characterized. Of which, a unique hepta-Zr cluster, [SbZr₇O₆(OH)₄]¹⁵⁺ core, was built by two trilacunary [B-α-GeW₉O₃₄]^10- fragments and two monolacunary [B-α-GeW₁₁O₃₉]^8- fragments and further arranged in a mode of a vertical cross and formed a pseudo-tetrahedron geometry. Compound 1 features the first Zr₇-cluster-substituted POM. Moreover, 1 is an effective heterogeneous catalyst for the catalytic oxidation of sulfides into the corresponding sulfones with H₂O₂, manifesting distinguished conversion, excellent yield, and desired recyclability.

New Type of Catalyst for Efficient Aerobic Oxidative Desulfurization Based On Tungsten Carbide Synthesized by the Microwave Method

Herein, we present a new type of high-performance catalyst for aerobic oxidation of organosulfur compounds based on tungsten carbide. The synthesis of tungsten carbide was performed via microwave irradiation of the precursors, which makes it possible to obtain a catalyst in just 15 min. The synthesized catalyst was investigated by a variety of physicochemical methods: X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, electron microscopy, and N₂ adsorption/desorption. It was shown that active centers containing tungsten in the transition oxidation state (+4) play a key role in the activation of oxygen. The main factors influencing the conversion of dibenzothiophene (DBT) were investigated. It should be noted that 100% conversion of DBT can be achieved under relatively mild conditions: 120 °C, 3 h, 6 bar, and 0.5% wt catalyst. The catalyst retained its activity for at least six oxidation/regeneration cycles. The simplicity and speed of synthesis of the proposed catalyst in combination with its high activity and stability open broad prospects for its further use both for oxidative desulfurization and for other reactions of aerobic oxidation of organic substrates.

Base-Assisted Aerobic C-H Oxidation of Alkylarenes with a Murdochite-Type Oxide Mg6MnO8 Nanoparticle Catalyst

Heterogeneously catalyzed aerobic oxidative C-H functionalization under mild conditions is a chemical process to obtain desired oxygenated products directly. Nanosized murdochite-type oxide Mg₆MnO₈ (Mg₆MnO₈-MA) was successfully synthesized by the sol-gel method using malic acid. The specific surface area reached up to 104 m² g^-1, which is about 7 times higher than those (2-15 m² g^-1) of Mg₆MnO₈ synthesized by previously reported methods. Mg₆MnO₈-MA exhibited superior catalytic performance to those of other Mn- and Mg-based oxides, including manganese oxides with Mn-O-Mn active sites for the oxidation of fluorene with molecular oxygen (O₂) as the sole oxidant under mild conditions (40 °C). The present catalytic system was applicable to the aerobic oxidation of various substrates. The catalyst could be recovered by simple filtration and reused several times without obvious loss of its high catalytic performance. The correlation between the reactivity and the pK_a of the substrates, basic properties of catalysts, and kinetic isotope effects suggest a basicity-controlled mechanism of hydrogen atom transfer. The ¹⁸O-labeling experiments, kinetics, and mechanistic studies showed that H abstraction of the hydrocarbon proceeds via a mechanism involving O₂ activation. The structure of Mg₆MnO₈ consisting of isolated Mn⁴⁺ species located in a basic MgO matrix plays an important role in the present oxidation.

POM-based porous supramolecular framework for the efficient sulfide-sulfoxide transformation with low molar O/S ratio

The selective oxidation of organic sulfides is a pivotal avenue to prepare sulfoxides that can act as synthetic intermediates of fine chemicals, bioactive molecules, and asymmetric catalysis ligands. To construct...

Flow Electrosynthesis of Sulfoxides, Sulfones, and Sulfoximines without Supporting Electrolytes

An efficient electrochemical flow process for the selective oxidation of sulfides to sulfoxides and sulfones and of sulfoxides to N-cyanosulfoximines has been developed. In total, 69 examples of sulfoxides, sulfones, and N-cyanosulfoximines have been synthesized in good to excellent yields and with high current efficiencies. The synthesis was assisted and facilitated through a supporting electrolyte-free, fully automated electrochemical protocol that highlights the advantages of flow electrolysis.

Sulfoxide and Sulfone Synthesis via Electrochemical Oxidation of Sulfides

The oxidation of diaryl sulfides and aryl alkyl sulfides to the corresponding sulfoxides and sulfones under electrochemical conditions is reported. Sulfoxides are selectively obtained in good yield under a constant current of 5 mA for 10 h in DMF, while sulfones are formed as the major product under a constant current of 10 or 20 mA for 10 h in MeOH. The oxygen of both the sulfoxide and sulfone function is derived from water.

Surface decorated magnetic nanoparticles with Mn-porphyrin as an effective catalyst for oxidation of sulfides

Mn-porphyrin complex was anchored coordinatively to silica-coated surface of magnetic nanoparticles (SMNP). Afterward, a heterogeneous nanocatalyst (Fe3O4@SiO2-MnTCPP) has been characterized by Fourier transform infrared (FT-IR), ultraviolet-visible (UV-vis) spectroscopy, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometry (VSM), thermogravimetric analysis (TGA), and transmission electron microscope (TEM). A thermal stability up to around 350[Formula: see text]C was verified for prepared nanocatalyst based on thermogravimetric analysis. Finally, the catalytic performance of magnetically recoverable Mn-catalyst was exploited in the green oxidation of different sulfides with urea hydrogen peroxide (UHP) in the presence of imidazole as co-catalyst in ethanol under heterogeneous conditions. The eco-friendly property of ethanol strongly induced us to employ it as the reaction solvent in this oxidation system. Complete conversion ([Formula: see text]99) of sulfides to the corresponding sulfoxide or sulfones was obtained for ethyl phenyl sulfide, phenyl vinyl sulfide, diallyl sulfide, thiocyanatoethane, 2-ethyl mercaptoethanol and tetrahydrothiophene. Moreover, the recovered catalysts keep constant conversion yield up to at least three cycles.

Precious Metal-Free LaMnO3 Perovskite Catalyst with an Optimized Nanostructure for Aerobic C-H Bond Activation Reactions: Alkylarene Oxidation and Naphthol Dimerization

In this article, we describe the development of a new aerobic C-H oxidation methodology catalyzed by a precious metal-free LaMnO₃ perovskite catalyst. Molecular oxygen is used as the sole oxidant in this approach, obviating the need for other expensive and/or environmentally hazardous stoichiometric oxidants. The electronic and structural properties of the LaMnO₃ catalysts were systematically optimized, and a reductive pretreatment protocol was proved to be essential for acquiring the observed high catalytic activities. It is demonstrated that this newly developed method was extremely effective for the oxidation of alkylarenes to ketones as well as for the oxidative dimerization of 2-naphthol to 1,1-binaphthyl-2,2-diol (BINOL), a particularly important scaffold for asymmetric catalysis. Detailed spectroscopic and mechanistic studies provided valuable insights into the structural aspects of the active catalyst and the reaction mechanism.

Potassium-incorporated manganese oxide enhances the activity and durability of platinum catalysts for low-temperature CO oxidation

Potassium-incorporated manganese oxide is demonstrated as an efficient support for fabricating highly active and stable Pt catalysts for low-temperature CO oxidation.

Deposition of highly dispersed gold nanoparticles onto metal phosphates by deposition–precipitation with aqueous ammonia

Deposition–precipitation with aqueous ammonia enabled small gold nanoparticles to be deposited onto a series of metal phosphates with high dispersity and density.

Iron phosphate nanoparticle catalyst for direct oxidation of methane into formaldehyde: effect of surface redox and acid–base properties

The surface redox and the weakly basic properties of FePO4 nanoparticles would contribute to the selective CH4 oxidation to HCHO and the suppression of over-oxidation, respectively.

Aerobic oxidative CC bond cleavage of aromatic alkenes by a high valency iron-containing perovskite catalyst

High-valency iron-containing perovskite catalyst BaFeO_3−δ could efficiently promote the additive-free oxidative CC bond cleavage of various aromatic alkenes using O₂ as the sole oxidant.

Template-Free Synthesis of Mesoporous β-MnO2 Nanoparticles: Structure, Formation Mechanism, and Catalytic Properties

Mesoporous β-MnO₂ nanoparticles were synthesized by a template-free low-temperature crystallization of Mn⁴⁺ precursors (low-crystallinity layer-type Mn⁴⁺ oxide, c-distorted H⁺-birnessite) produced by the reaction of MnO₄^- and Mn²⁺. The Mn starting materials, pH of the reaction solution, and calcination temperatures significantly affect the crystal structure, surface area, porous structure, and morphology of the manganese oxides formed. The pH conditions during the precipitation of Mn⁴⁺ precursors are important for controlling the morphology and porous structure of β-MnO₂. Nonrigid aggregates of platelike particles with slitlike pores (β-MnO₂-1 and -2) were obtained from the combinations of NaMnO₄/MnSO₄ and NaMnO₄/Mn(NO₃)₂, respectively. On the other hand, spherelike particles with ink-bottle shaped pores (β-MnO₂-3) were formed in NaMnO₄/Mn(OAc)₂ with pH adjustment (pH 0.8). The specific surface areas for β-MnO₂-1, -2, and -3 were much higher than those for nonporous β-MnO₂ nanorods synthesized using a typical hydrothermal method (β-MnO₂-HT). On the other hand, c-distorted H⁺-birnessite precursors with a high interlayer metal cation (Na⁺ and K⁺) content led to the formation of α-MnO₂ with a 2 × 2 tunnel structure. These mesoporous β-MnO₂ materials acted as effective heterogeneous catalysts for the aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) as a bioplastic monomer and for the transformation of aromatic alcohols to the corresponding aldehydes, where the catalytic activities of β-MnO₂-1, -2, and -3 were approximately 1 order of magnitude higher than that of β-MnO₂-HT. β-MnO₂-3 exhibited higher catalytic activity (especially for larger molecules) than the other β-MnO₂ materials, and this is likely attributed to the nanometer-sized spaces.

Solvent and additive-free efficient aerobic oxidation of alcohols by a perovskite oxide-based heterogeneous catalyst

A perovskite oxide has been utilized for the solvent and additive-free heterogeneous oxidation of various alcohols.

One-pot aerobic oxidative sulfonamidation of aromatic thiols with ammonia by a dual-functional β-MnO2 nanocatalyst

High-surface-area β-MnO2 (β-MnO2-HS) nanoparticles could act as effective heterogeneous catalysts for the one-pot oxidative sulfonamidation of various aromatic and heteroaromatic thiols to the corresponding sulfonamides using molecular oxygen (O2) and ammonia (NH3) as respective oxygen and nitrogen sources, without the need for any additives.