Investigation into Enhanced Catalytic Performance for Epoxidation of Styrene over LaSrCoxFe2–xO6 Double Perovskites: The Role of Singlet Oxygen Species Promoted by the Photothermal Effect

Modern organic transformations: heterogeneous thermocatalysis or photocatalysis?

Organic transformation driven by heterogeneous catalysis is of crucial significance in both fundamental research and modern industrial production of fine chemicals. Thermocatalysis offers excellent applications due to its high activity and excellent scalability, yet still faces significant challenges toward the goals of high efficiency, energy-saving and sustainability. Recently, photocatalysis has emerged as a promising alternative for addressing these issues in a green and economical manner. In practice, the selection of an appropriate catalytic system is a critical factor that can influence the chemical process on multiple levels significantly. In this review, we aim to present a tutorial demonstration about the critical comparison between thermo- and photocatalytic terms for organic transformation. We begin by outlining the basic principles in thermo- and photocatalytic fundamentals, together with summarizing the general advantages and disadvantages of each. Subsequently, given the high sustainability and potentiality exhibited by the photocatalytic process, we present its representative applications including oxidation, reduction, coupling, and cleavage series. The general reaction conditions and activities observed in thermocatalysis for similar reactions are also introduced for comparison. The understanding of reaction mechanisms and the resulting regulations toward activity and selectivity are specifically discussed. Finally, future perspectives of heterogeneous photocatalytic terms for practical applications are elucidated.

Advancing Propylene Epoxidation: the Role of Ethyl Acetate Autoxidation via Cobalt-Nickel Catalyzed C(acyl)─O Bond Scission

The selective autoxidation for the synthesis of valuable oxygenates has provoked keen interest from both academic and industrial sectors. Although the generation of reactive oxygen species via oxygen attack on C─H bonds near ester linkages is well-established, research into aliphatic ester oxidation has primarily focused on combustion, neglecting their potential utility in oxidation processes. Herein, a protocol for producing propylene oxide through the autoxidation of ethyl acetate in tandem with propylene epoxidation is demonstrated. The ethoxy radical, generated by ester C(acyl)─O bond cleavage in situ, subsequently underwent proton-coupled electron transfer with the Co(OAc)(μ-H2O)2Ni, followed by the formation of the peracetic acid optimally suited for the epoxidation reaction. The research not only eliminates the need for co-substrates in the epoxidation process but also fills the application gap in bulk-ester autoxidation, offering insights into the effective utilization of oxy-intermediates in autoxidation reactions.

Proper aggregation of Pt is beneficial for the epoxidation of styrene by O2 over Ptx/γ-Al2O3 catalysts

The dispersion of metal catalysts has multiple effects on catalytic performance, and higher dispersions do not necessarily imply better performance. Herein, we report the epoxidation reaction of styrene over supported platinum catalysts as an example. Compared with the Pt1/γ-Al2O3 catalyst, the Ptn/γ-Al2O3 catalyst with a larger Pt cluster size showed a much better performance. Combining the results of various characterizations and density functional theory calculations, Ptn/γ-Al2O3 was found to be more favorable for oxygen adsorption and activation to generate singlet oxygen species, further promoting the styrene oxidation reaction to styrene oxide in terms of kinetics. In contrast the metallic center of Pt1 in Pt1/γ-Al2O3 was too small to efficiently activate the diatomic oxygen molecule. These insights provide valuable guidance for designing high-performance metal catalysts.

High-Performance Electrocatalysts for Anion-Exchange Membrane Electrolyzers through Acoustic Cavitation

Electrochemical water splitting is a promising technology for the sustainable production of green hydrogen. Large-scale hydrogen production demands efficient electrocatalysts to continuously operate at large current densities. Catalyst deterioration and its peel-off are major concerns at large current densities, resulting in subpar performance. Herein, we utilized acoustic cavitation-assisted electrodeposition to synthesize highly efficient and robust NiFe and NiMn oxyhydroxide catalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. The acoustic cavitation process led to the development of a uniform nanoscale structure, partial amorphization, and the formation of oxygen vacancies, likely as a result of high-strain deformation. The synthesized catalysts demonstrated excellent performance, with very low overpotentials of 285 and 189 mV at 1000 mA/cm2, for OER and HER respectively. The cell configuration required 1.76 V only for achieving 1 A/cm2 and demonstrated negligible deterioration after 24 h of continuous operation. The commercial viability of the developed catalysts was obtained by testing in a 2.5 × 2.5 cm2 anion-exchange membrane (AEM) stack up to a 1.2 A/cm2 current density. The potentials required to reach industry-relevant high current densities of 500 and 1000 mA/cm2 were 2.1 and 2.6 V, respectively. The electrode stability at the electrolyzer scale was investigated by running the stack at current densities from 100 to 1000 mA/cm2 for a total of 100 h, wherein the electrode demonstrated high durability and robustness.

Sulfonated reduced graphene oxide encapsulated perovskite-type ErCoFe oxide nanoparticles for efficient electrochemical water oxidation

Perovskite oxides play a vital role as electrocatalysts in water oxidation due to their flexible and unique electronic structures. In this work, Er-based perovskites ErCo1-xFexO3-δ (x = 0.0, 0.1, 0.3, 0.5, 0.7, and 1.0) denoted as EC, ECF-0.9, ECF-0.7, ECF-0.5, ECF-0.3, and EF, respectively, are synthesized by the sol-gel method. Then, ECF-0.9 is supported on sulfonated reduced graphene oxide (S-rGO) by a hydrothermal method, with weight ratios of 1 : 1 and 3 : 1 of ECF/0.9 to S-rGO (shown as ECF-0.9/S-rGO(50%) and ECF-0.9/S-rGO(75%), respectively). The structural properties and the morphology of the synthesized materials are studied using a series of different techniques. The prepared perovskites are then used as electrode materials for electrochemical water oxidation. ECF-0.9 reveals better activity than pure EF, EC, and other perovskite oxides in terms of overpotential, Tafel slope, electrochemically active surface area (ECSA), and charge transfer resistance (Rct) values. Interestingly, when the optimized perovskite oxide catalyst ECF-0.9 is decorated on S-rGO sheets, the water oxidation activity is significantly improved. ECF-0.9/S-rGO(75%) exhibits superior activity for water oxidation with an overpotential of 290 mV@10 mA cm-2 and a Tafel slope of 41 mV dec-1. Finally, overall water splitting with ECF-0.9/S-rGO(75%) as the anode electrode shows a low electrolysis voltage of 1.60 V, alongside excellent stability for 20 h.

Visible-Light-Induced Oxidation of Styrene by a Polyoxovanadate-Based Carboxylate Derivative

Revealing the design and synthesis of precisely tailored crystalline catalysts for achieving efficient photocatalytic conversion of styrene into high-value-added products remains a challenging task. In this work, a highly stable crystalline polyoxovanadate functionalized by the dl-tartaric acid ligand H2[V4O4(H2O)6(tart)2]·H2O [1, tart = C4H2O6] was successfully synthesized by conventional aqueous solution methods. The photocatalytic performance was evaluated for the photosynthesis of styrene oxide by employing an oxygen source as the oxidant in the visible light (>420 nm) conditions at room temperature with compound 1 as a heterogeneous catalyst. Furthermore, compound 1 manifested accomplished stability and reusability, maintaining a high reaction activity even after three consecutive reaction cycles. This paper not only contributes to broadening the structural diversity of polyoxovanadate but also sheds new light on green catalytic conversion.

Selective Oxidation of p-Cymene over Mesoporous LaCoO3 by Introducing Oxygen Vacancies

The liquid-phase catalytic oxidation of p-cymene to 4-methylacetophenone is an industrially significant reaction. However, the targeted oxidation of a specific C-H bond of p-cymene is extremely difficult due to there being many branched chains in p-cymene. In here, we designed a simple method to synthesize mesoporous LaCoO3 catalysts with rich oxygen vacancy (Oov) sites. The as-prepared mesoporous LaCoO3 after 550 °C calcination (mLaCoO3) exhibits remarkable catalytic activity for solvent-free oxidation of the p-cymene reaction, with a selectivity of over 80.1% selectivity for 4-methylacetophenone and a conversion of 50.2% for p-cymene (120 °C, 3 MPa). Besides, recycling studies have demonstrated that the mLaCoO3 catalysts can be reused ten times in the aerobic oxidation of the p-cymene reaction without significant catalytic activity reduce. The experimental and characterization results indicated that the mesoporous structure of the catalyst is conducive to the generation of surface Oov, which can properly facilitate ion spread during the catalytic process and afford enough O2 for intermediate species, thus is beneficial for the generation of 4-methylacetophenone. This work demonstrates that the selectivity oxide p-cymene with an O2 employing mLaCoO3 catalyst is highly promising for chemical industrial applications.

A novel Ag-loaded 4 Å zeolite as an efficient catalyst for epoxidation of styrene

In this study, a novel Ag-loaded 4 Å zeolite was synthesized through the combined action of strong ultrasound and a high-voltage electrostatic field (the Z-Ag-UE) and its catalytic activity was evaluated in the epoxidation of styrene. The prepared catalysts were characterized using XRD, SEM, XPS, BET, TG, ICP-OES. The results showed that the silver evenly dispersed inside the octahedral 4 Å zeolite structure rather than being attached to the surface of the material like in the impregnation method, and this Ag-loaded 4 Å zeolite had a high surface area, uniform particle size distribution, and excellent high temperature thermal stability. The catalytic performance of the Ag-loaded 4 Å zeolite was investigated by varying the reaction conditions such as the amount of catalyst, temperature, and reaction time. Under optimized conditions, the Ag-loaded 4 Å zeolite showed high selectivity and conversion for the epoxidation of styrene, achieving a conversion rate of up to 98% and a selectivity of 94%. In particular, the catalyst had excellent recyclability and was reused more than fifteen times with the catalytic performance remaining unchanged. This method of loading metal prepared under external field conditions provides a new method and idea for future research in related fields.

Sunlight-driven and gram-scale vanillin production via Mn-defected γ-MnO2 catalyst in aqueous environment

The production of vanillin from biomass offers a sustainable route for synthesizing daily-use chemicals. However, achieving sunlight-driven vanillin synthesis through H2O activation in an aqueous environment poses challenges due to the high barrier of H2O dissociation. In this study, we have successfully developed an efficient approach for gram-scale vanillin synthesis in an aqueous reaction, employing Mn-defected γ-MnO2 as a photocatalyst at room temperature. Density functional theory calculations reveal that the presence of defective Mn species (Mn3+) significantly enhances the adsorption of vanillyl alcohol and H2O onto the surface of the γ-MnO2 catalyst. Hydroxyl radical (˙OH) species are formed through H2O activation with the assistance of sunlight, playing a pivotal role as oxygen-reactive species in the oxidation of vanillyl alcohol into vanillin. The Mn-defected γ-MnO2 catalyst exhibits exceptional performance, achieving up to 93.4% conversion of vanillyl alcohol and 95.7% selectivity of vanillin under sunlight. Notably, even in a laboratory setting during the daytime, the Mn-defected γ-MnO2 catalyst demonstrates significantly higher catalytic performance compared to the dark environment. This work presents a highly effective and promising strategy for low-cost and environmentally benign vanillin synthesis.

One-Dimensional La0.2Sr0.8Cu0.4Co0.6O3-δ Nanostructures for Efficient Oxygen Evolution Reaction

Producing oxygen and hydrogen via the electrolysis of water has the advantages of a simple operation, high efficiency, and environmental friendliness, making it the most promising hydrogen production method. In this study, La0.2Sr0.8Cu0.4Co0.6O3-δ (LSCC) nanofibers were prepared by electrospinning to utilize non-noble perovskite oxides instead of noble metal catalysts for the oxygen evolution reaction, and the performance and electrochemical properties of LSCC nanofibers synthesized at different firing temperatures were evaluated. In an alkaline environment (pH = 14, 6 M KOH), the nanofibers calcined at 650 °C showed an overpotential of 209 mV at a current density of 10 mA cm-2 as well as good long-term stability. Therefore, the prepared LSCC-650 NF catalyst shows excellent potential for electrocatalytic oxygen evolution.

Spontaneous generation of singlet oxygen on microemulsion-derived manganese oxides with rich oxygen vacancies for efficient aerobic oxidation

Developing innovative catalysts for efficiently activating O2 into singlet oxygen (1O2) is a cutting-edge field with the potential to revolutionize green chemical synthesis. Despite its potential, practical implementation remains a significant challenge. In this study, we design a series of nitrogen (N)-doped manganese oxides (Ny-MnO2, where y represents the molar amount of the N precursor used) nanocatalysts using compartmentalized-microemulsion crystallization followed by post-calcination. These nanocatalysts demonstrate the remarkable ability to directly produce 1O2 at room temperature without the external fields. By strategically incorporating defect engineering and interstitial N, the concentration of surface oxygen atoms (Os) in the vicinity of oxygen vacancy (Ov) reaches 51.1% for the N55-MnO2 nanocatalyst. This feature allows the nanocatalyst to expose a substantial number of Ov and interstitial N sites on the surface of N55-MnO2, facilitating effective chemisorption and activation of O2. Verified through electron paramagnetic resonance spectroscopy and reactive oxygen species trapping experiments, the spontaneous generation of 1O2, even in the absence of light, underscores its crucial role in aerobic oxidation. Density functional theory calculations reveal that an increased Ov content and N doping significantly reduce the adsorption energy, thereby promoting chemisorption and excitation of O2. Consequently, the optimized N55-MnO2 nanocatalyst enables room-temperature aerobic oxidation of alcohols with a yield surpassing 99%, representing a 6.7-fold activity enhancement compared to ε-MnO2 without N-doping. Furthermore, N55-MnO2 demonstrates exceptional recyclability for the aerobic oxidative conversion of benzyl alcohol over ten cycles. This study introduces an approach to spontaneously activate O2 for the green synthesis of fine chemicals.

Modulating Charges of Dual Sites in Multivariate Metal-Organic Frameworks for Boosting Selective Aerobic Epoxidation of Alkenes

Selective aerobic epoxidation of alkenes without any additives is of great industrial importance but still challenging because the competitive side reactions including C═C bond cleavage and isomerization are difficult to avoid. Here, we show fabricating Cu(I) single sites in pristine multivariate metal-organic frameworks (known as CuCo-MOF-74) via partial reduction of Cu(II) to Cu(I) ions during solvothermal reaction. Impressively, CuCo-MOF-74 is characteristic with single Cu(I), Cu(II), and Co(II) sites, and they exhibit the substantially enhanced selectivity of styrene oxide up to 87.6% using air as an oxidant at almost complete conversion of styrene, ∼25.8% selectivity increased over Co-MOF-74, as well as good catalytic stability. Contrast experiments and theoretical calculation indicate that Cu(I) sites contribute to the substantially enhanced selectivity of epoxides catalyzed by Co(II) sites. The adsorption of two O₂ molecules on dual Co(II) and Cu(I) sites is favorable, and the projected density of state of the Co-3d orbital is closer to the Fermi level by modulating with Cu(I) sites for promoting the activation of O₂ compared with dual-site Cu(II) and Co(II) and Co(II) and Co(II), thus contributing to the epoxidation of the C═C bond. When other kinds of alkenes are used as substrates, the excellent selectivity of various epoxides is also achieved over CuCo-MOF-74. We also prove the universality of fabricating Cu(I) sites in other MOF-74 with various divalent metal nodes.

Advances in Designing Efficient La-Based Perovskites for the NOx Storage and Reduction Process

To overcome the inherent challenge of NOx reduction in the net oxidizing environment of diesel engine exhaust, the NOx storage and reduction (NSR) concept was proposed in 1995, soon developed and commercialized as a promising DeNOx technique over the past two decades. Years of practice suggest that it is a tailor-made technique for light-duty diesel vehicles, with the advantage of being space saving, cost effective, and efficient in NOx abatement; however, the over-reliance of NSR catalysts on high loadings of Pt has always been the bottleneck for its wide application. There remains fervent interest in searching for efficient, economical, and durable alternatives. To date, La-based perovskites are the most explored promising candidate, showing prominent structural and thermal stability and redox property. The perovskite-type oxide structure enables the coupling of redox and storage centers with homogeneous distribution, which maximizes the contact area for NOx spillover and contributes to efficient NOx storage and reduction. Moreover, the wide range of possible cationic substitutions in perovskite generates great flexibility, yielding various formulations with interesting features desirable for the NSR process. Herein, this review provides an overview of the features and performances of La-based perovskite in NO oxidation, NOx storage, and NOx reduction, and in this way comprehensively evaluates its potential to substitute Pt and further improve the DeNOx efficiency of the current NSR catalyst. The fundamental structure–property relationships are summarized and highlighted to instruct rational catalyst design. The critical research needs and essential aspects in catalyst design, including poisoner resistance and catalyst sustainability, are finally addressed to inspire the future development of perovskite material for practical application.

Activation of molecular oxygen over Mn-doped La2CuO4 perovskite for direct epoxidation of propylene

The synergetic interaction between manganese and copper in LaMn0.5Cu0.5O3 significantly promoted the epoxidation of propylene at lower temperature by converting the active sites from oxygen vacancies to Cu active sites of Cu–O–Mn.