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.

Oxidation Catalysis over Solid-State Keggin-Type Phosphomolybdic Acid with Oxygen Defects

Keggin-type phosphomolybdic acid (PMo₁₂O₄₀), treated with pyridine (Py), forms a crystalline material (PyPMo-HT) following heat treatment under an inert gas flow at ∼420 °C. Although this material is known to have attractive catalytic properties for gas-phase oxidation, the origin of this catalytic activity requires clarification. In this study, we investigated the crystal structure of PyPMo-HT. PyPMo-HT comprises a one-dimensional array of Keggin units and pyridinium cations (HPy), with an HPy/Keggin unit ratio of ∼1.0. Two oxygen atoms were removed from the Keggin unit during crystal structure transformation, which resulted in an electron being localized on the Mo atom in close contact with the adjacent Keggin unit. Upon the introduction of molecular oxygen, electron transfer from this Mo atom resulted in the formation of an electrophilic oxygen species that bridged two Keggin units. The electrophilic oxygen species acted as a catalytically active oxygen species, as confirmed by the selective oxidation of propylene. PyPMo-HT showed excellent catalytic activity for the selective oxidation of methacrolein, with the methacrylic acid yield being superior to that obtained with PMo₁₂O₄₀ and comparable to that obtained with an industrial Keggin-type polyoxometalate (POM) catalyst. The oxidation catalysis observed over PyPMo-HT provides a deeper understanding of POM-based industrial catalytic processes.

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.

β-MnO2 nanoparticles as heterogenous catalysts for aerobic oxidative transformation of alcohols to carbonyl compounds, nitriles, and amides

β-MnO2 nanoparticles exhibit high catalytic performance for the aerobic oxidation of various aromatic, allylic, and heteroaromatic alcohols and one-pot tandem oxidation of alcohols to nitriles and amides in the presence of NH3.

A Perspective on Heterogeneous Catalysts for the Selective Oxidation of Alcohols

Selective oxidation of higher alcohols using heterogeneous catalysts is an important reaction in the synthesis of fine chemicals with added value. Though the process for primary alcohol oxidation is industrially established, there is still a lack of fundamental understanding considering the complexity of the catalysts and their dynamics under reaction conditions, especially when higher alcohols and liquid-phase reaction media are involved. Additionally, new materials should be developed offering higher activity, selectivity, and stability. This can be achieved by unraveling the structure-performance correlations of these catalysts under reaction conditions. In this regard, researchers are encouraged to develop more advanced characterization techniques to address the complex interplay between the solid surface, the dissolved reactants, and the solvent. In this mini-review, we report some of the most important approaches taken in the field and give a perspective on how to tackle the complex challenges for different approaches in alcohol oxidation while providing insight into the remaining challenges.

Nanostructured Manganese Oxides within a Ring-Shaped Polyoxometalate Exhibiting Unusual Oxidation Catalysis

Nanosized manganese oxides have recently received considerable attention for their synthesis, structures, and potential applications. Although various synthetic methods have been developed, precise synthesis of novel nanostructured manganese oxides are still challenging. In this study, using a structurally defined nanosized cavity inside a ring-shaped polyoxometalate, we succeeded in synthesizing two types of discrete 18 and 20 nuclear nanostructured manganese oxides, Mn18 and Mn20, respectively. In particular, Mn18 showed much higher catalytic activity than other manganese oxides for the oxygenation of alkylarenes including electron-deficient ones, and the reaction proceeded through a unique reaction mechanism due to its unusual manganese oxide structure.

Liquid-Phase Cyclohexene Oxidation with O2 over Spray-Flame-Synthesized La1-x Srx CoO3 Perovskite Nanoparticles

La_1−xSr_xCoO₃ (x=0, 0.1, 0.2, 0.3, 0.4) nanoparticles were prepared by spray‐flame synthesis and applied in the liquid‐phase oxidation of cyclohexene with molecular O₂ as oxidant under mild conditions. The catalysts were systematically characterized by state‐of‐the‐art techniques. With increasing Sr content, the concentration of surface oxygen vacancy defects increases, which is beneficial for cyclohexene oxidation, but the surface concentration of less active Co²⁺ was also increased. However, Co²⁺ cations have a superior activity towards peroxide decomposition, which also plays an important role in cyclohexene oxidation. A Sr doping of 20 at. % was found to be the optimum in terms of activity and product selectivity. The catalyst also showed excellent reusability over three catalytic runs; this can be attributed to its highly stable particle size and morphology. Kinetic investigations revealed first‐order reaction kinetics for temperatures between 60 and 100 °C and an apparent activation energy of 68 kJ mol⁻¹ for cyclohexene oxidation. Moreover, the reaction was not affected by the applied O₂ pressure in the range from 10 to 20 bar. In situ attenuated total reflection infrared spectroscopy was used to monitor the conversion of cyclohexene and the formation of reaction products including the key intermediate cyclohex‐2‐ene‐1‐hydroperoxide; spin trap electron paramagnetic resonance spectroscopy provided strong evidence for a radical reaction pathway by identifying the cyclohexenyl alkoxyl radical.

Aerobic oxidation of 5-hydroxymethylfurfural into 2,5-diformylfuran using manganese dioxide with different crystal structures: A comparative study

Aerobic oxidation of 5-Hydroxymethylfurfural (HMF) to 2,5-Diformylfuran (DFF) using O₂ gas represents a sustainable approach for valorization of lignocellulosic compounds. As manganese dioxide (MnO₂) is validated as a useful oxidation catalyst and many crystalline forms of MnO₂ exist, it is critical to explore how the crystalline structures of MnO₂ influence their physical/chemical properties, which, in turn, determine catalytic activities of MnO₂ crystals for HMF oxidation to DFF. In particular, six MnO₂ crystals, α-MnO₂, β-MnO₂, γ-MnO₂, δ-MnO₂, ε-MnO₂, and λ-MnO₂ are prepared and investigated for their catalytic activities for HMF oxidation to DFF. With different morphologies and crystalline structures, these MnO₂ crystals possess very distinct surficial chemistry, redox capabilities, and textural properties, making these MnO₂ exhibit different catalytic activities towards HMF conversion. Especially, β-MnO₂ can produce much higher DFF per surface area than other MnO₂ crystals. β-MnO₂ could achieve the highest C_HMF = 99% and Y_DFF = 97%, which are much higher than the reported values in literature, possibly because the surficial reactivity of β-MnO₂ appears to be highest in comparison to other MnO₂ crystals. Especially, β-MnO₂ could exhibit Y_DFF > 90% over 5 cycles of reusability test, and maintain its crystalline structure, revealing its advantageous feature for aerobic oxidation of HMF to DFF. Through this study, the relationship between morphology, surface chemistry, and catalytic activity of MnO₂ with different crystal forms is elucidated for providing scientific insights into design, application and development of MnO₂-based materials for aerobic oxidation of bio-derived molecules to value-added products.

Water mediated growth of oriented single crystalline SrCO3 nanorod arrays on strontium compounds

Morphology-controlled strontianite nanostructures have attracted interest in various fields, such as electrocatalyst and photocatalysts. Basic additives in aqueous strontium solutions is commonly used in controlling strontianite nanostructures. Here, we show that trace water also serves an important role in forming and structuring vertically oriented strontianite nanorod arrays on strontium compounds. Using in situ Raman spectroscopy, we monitored the structural evolution from hydrated strontium to strontianite nanorods, demonstrating the epitaxial growth by vapor-liquid-solid mechanism. Water molecules cause not only the exsolution of Sr liquid droplets on the surface but also the uptake of airborne CO2 followed by its ionization to CO32-. The existence of intermediate SrHO+-OCO22- phase indicates the interaction of CO32- with SrOH+ in Sr(OH)x(H2O)y cluster to orient strontianite crystals. X-ray diffraction simulation and transmission electron microscopy identify the preferred-orientation plane of the 1D nanostructures as the (002) plane, i.e., the growth along the c-axis. The anisotropic growth habit is found to be affected by the kinetics of carbonation. This study paves the way for designing and developing 1D architecture of alkaline earth metal carbonates by a simple method without external additives at room temperature.

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.

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.

Towards Mechanistic Understanding of Liquid-Phase Cinnamyl Alcohol Oxidation with tert-Butyl Hydroperoxide over Noble-Metal-Free LaCo1-x Fex O3 Perovskites

Noble-metal-free perovskite oxides are promising and well-known catalysts for high-temperature gas-phase oxidation reactions, but their application in selective oxidation reactions in the liquid phase has rarely been studied. We report the liquid-phase oxidation of cinnamyl alcohol over spray-flame synthesized LaCo_1-x Fe_x O₃ perovskite nanoparticles with tert-butyl hydroperoxide (TBHP) as the oxidizing agent under mild reaction conditions. The catalysts were characterized by XRD, BET, EDS and elemental analysis. LaCo_0.8 Fe_0.2 O₃ showed the best catalytic properties indicating a synergistic effect between cobalt and iron. The catalysts were found to be stable against metal leaching as proven by hot filtration, and the observed slight deactivation is presumably due to segregation as determined by EDS. Kinetic studies revealed an apparent activation energy of 63.6 kJ mol^-1 . Combining kinetic findings with TBHP decomposition as well as control experiments revealed a complex reaction network.

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.

Selective cyclohexene oxidation with O2, H2O2 and tert-butyl hydroperoxide over spray-flame synthesized LaCo1−xFexO3 nanoparticles

A series of spray-flame made LaCo_1−xFe_xO₃ nanoparticles showed promising activity for liquid-phase cyclohexene oxidation. Various oxidizing agents, i.e., O₂, H₂O₂ and tert-butyl hydroperoxide, led to different product selectivities.

Strontium Manganese Oxide Getter for Capturing Airborne Cr and S Contaminants in High-Temperature Electrochemical Systems

Traces (ppm to ppb level) of airborne contaminants such as CrO₂(OH)₂ and SO₂ irreversibly degrade the electrochemical activity of air electrodes in high-temperature electrochemical devices such as solid oxide fuel cells by retarding oxygen reduction reactions. The use of getter has been proposed as a cost-effective strategy to mitigate the electrode poisoning. However, owing to the harsh operating conditions (i.e., exposure to heat and moisture), the long-term durability of getter materials remains a considerable challenge. In this study, we report our findings on strontium manganese oxide (SMO) as a robust getter material for cocapture of airborne Cr and S contaminants. The SMO getter with a 3D honeycomb architecture, fabricated via slurry dip-coating, successfully maintains the electrochemical activity of solid oxide cells under the flow of gaseous Cr and S species, validating the getter's capability of capturing traces of Cr and S contaminants. Investigations found that both Sr and Mn cations contribute to the absorption reaction and that the reaction processes are accompanied by morphological elongation in the form of SrSO₄ nanorods and SrCrO₄ whiskers, which favors continued absorption and reaction of incoming S and Cr contaminants. The SMO getter also displays robust stability at high temperatures and in humid environments without phase transformation and hydrolysis. These results demonstrate the feasibility of the use of SMO getter under severe operating conditions representative of high-temperature electrochemical systems.

Catalytic Aerobic Oxidation of C(sp3 )-H Bonds

Oxidation reactions are a key technology to transform hydrocarbons from petroleum feedstock into chemicals of a higher oxidation state, allowing further chemical transformations. These bulk-scale oxidation processes usually employ molecular oxygen as the terminal oxidant as at this scale it is typically the only economically viable oxidant. The produced commodity chemicals possess limited functionality and usually show a high degree of symmetry thereby avoiding selectivity issues. In sharp contrast, in the production of fine chemicals preference is still given to classical oxidants. Considering the strive for greener production processes, the use of O₂ , the most abundant and greenest oxidant, is a logical choice. Given the rich functionality and complexity of fine chemicals, achieving regio/chemoselectivity is a major challenge. This review presents an overview of the most important catalytic systems recently described for aerobic oxidation, and the current insight in their reaction mechanism.

Heterogeneously Catalyzed Aerobic Oxidation of Sulfides with a BaRuO3 Nanoperovskite

A rhombohedral BaRuO₃ nanoperovskite, which was synthesized by the sol-gel method using malic acid, could act as an efficient heterogeneous catalyst for the selective oxidation of various aromatic and aliphatic sulfides with molecular oxygen as the sole oxidant. BaRuO₃ showed much higher catalytic activities than other catalysts, including ruthenium-based perovskite oxides, under mild reaction conditions. The catalyst could be recovered by simple filtration and reused several times without obvious loss of its high catalytic performance. The catalyst effect, ¹⁸O-labeling experiments, and kinetic and mechanistic studies showed that substrate oxidation proceeds with oxygen species caused by the solid. The crystal structure of ruthenium-based oxides is crucial to control the nature of the oxygen atoms and significantly affects their oxygen transfer reactivity. Density functional theory calculations revealed that the face-sharing octahedra in BaRuO₃ likely are possible active sites in the present oxidation in sharp contrast to the corner-sharing octahedra in SrRuO₃, CaRuO₃, and RuO₂. The superior oxygen transfer ability of BaRuO₃ is also applicable to the quantitative conversion of dibenzothiophene into the corresponding sulfone and gram-scale oxidation of 4-methoxy thioanisole, in which 1.20 g (71% yield) of the analytically pure sulfoxide could be isolated.

Liquid-phase oxidation of alkanes with molecular oxygen catalyzed by high valent iron-based perovskite

Hexagonal BaFeO3-δ containing high valent iron species acted as an efficient heterogeneous catalyst for the aerobic oxidation of alkanes without the need for additives. The activity of BaFeO3-δ was much higher than that of typical Fe3+/Fe2+-containing iron oxide-based catalysts, and the recovered catalyst could be reused without significant loss of catalytic performance.

Amino Acid-Aided Synthesis of a Hexagonal SrMnO3 Nanoperovskite Catalyst for Aerobic Oxidation

A simple and efficient synthetic method for preparing high-surface-area perovskites was investigated by focusing on the importance of the formation of an amorphous precursor. Hexagonal SrMnO₃ with high surface area was successfully synthesized by simple calcination of the amorphous precursor prepared using aspartic acid and metal acetates instead of metal nitrates, without pH adjustment. The specific surface area reached up to ca. 50 m² g^-1, which is much larger than that for SrMnO₃ synthesized by previously reported methods. The catalytic activity for heterogeneous liquid-phase aerobic oxidation was significantly improved in comparison with the polymerized complex method, and the present catalytic system was applicable to the oxidation of various substrates.

Heterogeneously-Catalyzed Aerobic Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid with MnO2

A simple non-precious-metal catalyst system based on costeffective and ubiquitously available MnO₂ , NaHCO₃ , and molecular oxygen was used to convert 5-hydroxymethylfurfural (HMF) to 2,5-difurandicarboxylic acid (FDCA) as a bioplastics precursor in 91 % yield. The MnO₂ catalyst could be recovered by simple filtration and reused several times. The present system was also applicable to the aerobic oxidation of other biomass-derived substrates and the gram-scale oxidation of HMF to FDCA, in which 2.36 g (86 % yield) of the analytically pure FDCA could be isolated.