Selective Oxidation Research Articles

Aqueous mixtures of deep eutectic solvents: Tuning the reaction medium and photocatalyst for highly selective amine oxidation

Harvesting light as a source of energy for driving chemical processes is an important aspect of tacking the looming energy crisis. The challenge lies in making the process sustainable while retaining the efficiency and selectivity. In this work, deep eutectic solvents and their aqueous mixtures are used to replace acetonitrile as the reaction medium for the photocatalyzed amine oxidation. It is observed that the use of deep eutectic solvents as reaction media leads to > 90 % conversion but presents issues with efficient workup due to high viscosity. The use of water a cosolvent mitigates the issue. Further modification of the catalyst surface with alizarin enables the use of higher water content (in a 1:1 ratio) and leads to high conversion (>90%) at one-tenth of irradiation intensity. The high yield and selectivity eliminate the need for tedious purification steps to isolate the product. Control experiments indicate cooperative contributions by TiO2, TEMPO and alizarin as mechanistic factors responsible for the enhanced reactivity.

Sustainable production of 2,5-diformylfuran via peroxymonosulfate-triggered mild catalytic oxidation of lignocellulosic biomass

Abstract The relentless depletion of fossil fuels accentuates the urgent necessity for the sustainable synthesis of chemicals from renewable biomass. 5-Hydroxymethylfurfural (HMF), extracted from lignocellulosic biomass, emerges as a beacon of hope for conversion into value-added chemicals. However, the inherent susceptibility of its unsaturated aldehyde groups to excessive oxidation often culminates in undesired reactions, compromising both the yield and specificity of the desired products. Here, we introduce a holistic methodology for the cost-effective and ecologically responsible generation of 2,5-diformylfuran (DFF), through the heterogeneously catalyzed oxidation of HMF utilizing peroxymonosulfate (PMS) under benign conditions. This strategy, characterized by the meticulous enhancement of surface ketone groups via a mixed-salt co-pyrolysis technique, achieves an unparalleled selective activation of PMS, engendering singlet oxygen to catalyze the oxidation of HMF into DFF with a selectivity surpassing 80%. Life cycle assessments underscore a negligible impact on human health, ecosystems, and natural resources, endorsing the holistic utilization of biomass. This integration of pyrolysis for the creation of functional carbonaceous materials within biomass conversion processes significantly enhances sustainability and economic viability, whilst paving new pathways for selective biomass oxidation and the production of high-value chemicals.

Unveiling the Gold Facet Effect in Selective Oxidation of 5-Hydroxymethylfurfural and Hydrogen Production.

Direct oxidation of 5-hydroxymethylfurfural (HMF) to 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), crucial for medical supply production, is hindered by overoxidation. We synthesized gold nanomaterials with distinct single-crystal facets, {111} in octahedra (OC), {100} in nanocubes (NCs), and {110} in rhombic dodecahedra (RD), to investigate the facet-dependent HMF oxidation. The Au RD achieved the spontaneous oxidation of HMF to HMFCA with stoichiometric hydrogen production, maintaining 95% carbon balance, 91% yield, and 98% selectivity. In contrast, Au OC and NCs were inert. The superior performance is due to the absence of a C-H activation energy barrier on the Au(110) facet. Furthermore, gas chromatography and isotope experiments supported that the intermediate is oxidized to produce H2 via H- transfer, rather than H2O via H+ transfer. Oxygen was essential for scavenging electrons, thereby closing the reaction loop. The Au RD exhibited remarkable stability, operating for 240 h without performance degradation, indicating its potential for efficient HMFCA production.

Defective h-BNs-Supported Pd Nanoclusters: An Efficient Photocatalyst for Selective Oxidation of 5-Hydroxymethylfurfural.

5-hydroxymethylfurfural (HMF) is one of the most promising biomass-based chemicals that is used to produce many kinds of important compounds. Especially, the selective conversion of HMF to 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), an important chemical feedstock, has high industrial significance but is technically challenging. In this study, we present a high-performance photocatalyst for selective oxidation of HMF to HMFCA. By integrating an ultrasmall amount of palladium (Pd) nanoclusters (1.12‰ in weight) on defective hexagonal boron nitride nanosheets (Pd/defective h-BN nanosheets (dh-BNs)), outstanding photocatalytic performance can be achieved, resulting in up to a 95% HMF conversion ratio with an 82% HMFCA selectivity. The performance is considerably higher than that of pristine dh-BNs and Pd on defect-free h-BNs. More importantly, this Pd/dh-BNs catalyst maintains a high catalytic activity after eight cycles, demonstrating robust catalytic stability. Density functional theory calculations indicate that Pd/dh-BNs can lower the energy barrier for HMF oxidation and facilitate the desorption of HMFCA, which contributes to the high selectivity catalytic performance. This study not only introduces a promising photocatalyst for sustainable chemical transformations but can also provide valuable insights into the design of advanced photocatalytic material for biorefinery applications.

One-Pot Production of Cinnamonitriles from Lignin β-O-4 Segments Induced by Selective Oxidation of the γ-OH Group.

The construction of N-containing aromatic compounds from lignin is of great importance to expanding the boundary of the biorefinery and meeting the demand for value-added biorefinery. However, it remains a huge challenge due to the complex lignin structure and the incompatible catalysis for C-O/C-C bond cleavage and C-N formation. Herein, sustainable synthesis of cinnamonitrile derivatives from lignin β-O-4 model compounds in the presence of 2,2,6,6-tetramethylpiperidine oxide (TEMPO), (diacetoxyiodo)benzene (BAIB), and a strong base has been achieved in a one-pot, two-step fashion under transition-metal-free conditions. Mechanistic studies suggest that this transformation starts from selective oxidation of Cγ-OH of the β-O-4 model compound, followed by retro-aldol condensation, resulting in the cleavage of the Cα-Cβ bond to afford veratraldehyde. Whereafter, the aldol condensation reaction allows coupling of veratraldehyde with nitriles to provide cinnamonitriles. With this protocol, 3,4-dimethoxycinnamonitrile and 3,4-dimethoxyphenyl-2-phenylacrylonitrile were synthesized from lignin β-O-4 model compounds and showed good antibacterial or antifungal activity, showcasing the application potential of lignin in pharmaceutical synthesis.

Structure matching mechanism of nRu/FeCo2O4 for highly‐selective oxidation of HMF toward FDCA

AbstractThe selective oxidation of 5‐hydroxymethylfurfural (HMF) toward 2,5‐furandicarboxylic acid (FDCA) offers a promising green pathway to obtain monomers for the synthesis of biodegradable plastics. However, developing a high‐selectivity catalyst and understanding the catalytic mechanism are still great challenge. Here, we synthesize a nRu/FeCo2O4 catalyst with Ru nanoparticles loaded on FeCo2O4. The nRu/FeCo2O4 presents excellent HMF oxidation activity with 100% HMF conversion efficiency and 99% FDCA yield under optimized conditions. Density‐functional theory calculations further reveal the structure matching mechanism of nRu/FeCo2O4 for high‐selective oxidation of HMF toward FDCA, that is, Ru loading in FeCo2O4 provides a more suitable structure matching configuration for adsorption of two‐side chains in HMF, which could optimize the adsorption energy and thus increase reactivity. In short, this work provides a promising structure matching strategy for designing dual‐active‐site relay catalyst to oxidize ‐CHO and C‐OH groups in HMF and thus achieve highly‐selective oxidation of HMF toward FDCA.

Synthesis of Benzopyran-Phenylpropanoid Hybrids via Matsuda-Heck-Arylation and Allylic Oxidation.

The synthesis of coumarin- and flavonoid-chalcone hybrids via Pd-catalyzed Heck-type coupling of arene diazonium salts and 8-allylcoumarins and -flavonoids is reported. The β-hydride elimination step proceeds with high regioselectivity if an OMOM-substituent is present at the position C7, adjacent to the allyl group. A selective allylic oxidation of the coupling products was accomplished using DDQ in the presence of silica to furnish the chalcones.

Ru(II)-Catalysed Highly Regioselective C6 C-H/N-H Annulation of Indole-7-carboxamides with Alkynes for the Synthesis of Pyrrolo[3,2-h]isoquinolin-9-ones.

We disclosed an efficient protocol for regioselective C6 C-H/N-H activation/annulation reaction of indole-7-carboxamides with alkynes to synthesize highly substituted pyrrolo[3,2-h]isoquinolin-9-one derivatives. Under optimized reaction conditions, electron-deficient and electron-rich internal alkynes reacted efficiently with various indole-7-carboxamides to deliver desired products in good to excellent yields. The synthetic utility of the product is demonstrated by its selective oxidation to the corresponding isatin derivative. Deuterium insertion and intermolecular competition experiments were also conducted to gain mechanistic insights.

Processing of Oxidized Lead–Zinc Ore by Co-Roasting with Pyrite-Bearing Ore

The world reserves of oxidized lead–zinc ores are large, but their processing faces significant difficulties due to their refractory nature. This paper presents a novel approach to the preparation of refractory oxidized lead ores for flotation. The proposed method is based on the co-roasting of oxidized lead-bearing ores from the Ozernoye polymetallic deposit (Western Transbaikalia, Russia) with fine-grained sulfide lead–zinc ore sourced from the same deposit and the addition of calcium oxide. This method allows for the activation of mineral complexes, the sulfidation of oxidized lead–zinc minerals, and the minimization of the amount of sulfur dioxide gas emitted. Co-roasting oxidized lead–zinc ore with sulfide ore (10–30 wt. pct) at 650–700 °C has been shown to result in the selective oxidation of pyrite and sulfidation of oxidized lead and zinc minerals. The proposed method of processing polymetallic ores is capable of simultaneously involving not only oxidized lead–zinc ores but also refractory sulfide ores, thereby extending the operational lifespan of the mining enterprise and reducing the environmental impact.

Vinylene-Linked Covalent Organic Frameworks for Visible-Light-Promoted Selective Oxidation of Styrene in Water.

Vinylene-linked COFs, as an emerging class of crystalline porous polymers, have been regarded as ideal heterogenous photocatalysts due to their ordered structure, tailored pore size, outstanding stability and fully π-conjugated structure. Unfortunately, their photocatalytic performances are usually impeded by high exciton binding energy and unsatisfactory exciton dissociation efficiency. Herein, the authorsbroke through this dilemma by arrangement of complementary donor-acceptor (D-A) pairs within the COF skeleton to improve charge transfer/separation. Two vinylene-linked COFs (TMT-BT-COF and TMT-TT-COF) are synthesized by Aldol condensation using highly photoactive thienothiophene and benzothiazole groups as donor and electron-deficient triazine units as acceptor. Photochemical/electrochemical studies as well as DFT calculation suggest that these D-A type vinylene-linked COFs endow high charge transfer efficiency and low charge recombination. As a result, both of them demonstrate remarkably catalytic activity in the oxidation of styrene to benzaldehyde with molecular oxygen, with an exceptionally high conversion rate (≥92%) and selectivity (≥90%). Intriguingly, in the presence of NaHCO3, the above COFs could photocatalyze epoxidation styrene in water, and the styrene oxide selectivity reached 53%. This work elucidates the prominent capability of vinylene-linked COFs in the photocatalytic transformation of organic compounds in aqueous media, which may pave a new avenue for their future development.

A Knowledge-Based Molecular Single-Source Precursor Approach to Nickel Chalcogenide Precatalysts for Electrocatalytic Water, Alcohol, and Aldehyde Oxidations.

The development and comprehensive understanding of nickel chalcogenides are critical since they constitute a class of efficient electro(pre)catalysts for the oxygen evolution reaction (OER) and value-added organic oxidations. This study introduces a knowledge-based facile approach to analogous NiE (E = S, Se, Te) phases, originating from molecular β-diketiminato [Ni2E2] complexes and their application for OER and organic oxidations. The recorded activity trends for both target reactions follow the order NiSe > NiS > NiTe. Notably, NiSe displayed efficient performance for both OER and the selective oxidation of benzyl alcohol and 5-hydroxymethylfurfural, exhibiting stability in OER for 11 days under industrially pertinent conditions. Comprehensive analysis, including quasi in situ X-ray absorption and Raman spectroscopy, in combination with several ex situ techniques, revealed a material reconstruction process under alkaline OER conditions, involving chalcogen leaching. While NiS and NiSe experienced full chalcogen leaching and reconstruction into NiIII/IV oxyhydroxide active phases with intercalated potassium ions, the transformation of NiTe is incomplete. This study highlights the structure-activity relationship of a whole series of analogous nickel chalcogenides, directly linking material activity to the availability of active sites for catalysis. Such findings hold great promise for the development of efficient electrocatalysts for a wide range of applications, impacting various industrial processes and sustainable energy solutions.

Molecular-level Modulation of N, S-Co-Doped Mesoporous Carbon Nanospheres for Selective Aqueous Catalytic Oxidation of Ethylbenzene.

Selective oxidation of aromatic alkanes into high value-added products through benzylic C-H bond activation is one of the main reactions in chemical industry. On account of the constantly increasing demand for mass production, efficient, eco-friendly and sustainable catalysts are urgently needed. Herein, we describe a facile and versatile emulsion-assisted interface self-assembly strategy towards molecular-level fabrication of co-doped mesoporous carbon nanospheres with controllable active N and S species. The method enables a high degree of control over nanoparticle sizes, mesoporous nanostructures, contents of heteroatoms and the chemical composition. The optimized catalyst exhibits high catalytic performance of 97 % ethylbenzene conversion and 98 % selectivity to acetophenone. Density functional theory simulations reveal that N, S-co-doping leads to the redistribution of charge and spin densities, introducing more active carbon atoms and realizing aerobic oxidation of ethylbenzene efficiently. This work presents a general strategy for molecular-level design of carbon-based catalysts, and also provides new insight into the influence of heteroatom-doping on catalytic properties.

Forced Dynamic Operation of Propylene Selective Oxidation to Acrolein on Bismuth-Molybdate Structured Catalysts

Forced dynamic operation (FDO) of selective oxidation of propylene to acrolein is evaluated on a structured alumina foam coated with a mixed metal oxide catalyst (bismuth molybdate-based). Reactor experiments examine feed composition modulation as a strategy to increase the yield of acrolein. At lower temperatures, separating the oxidant (O2) and reductant (C3H6) enhances propylene conversion and acrolein selectivity. Variation in the reaction-regeneration switching amplitude and frequency gives a 40% higher cycle-averaged acrolein yield compared to steady-state operation (SSO) for the same overall feed composition. The results suggest that FDO maintains a higher concentration of selective oxygen species during the propylene feed, while pre-oxidation of the catalyst maximizes the amount of selective oxygen species, leading to increased acrolein yield at the beginning of the reaction cycle. Experiments at lower temperature for both commercial promoted and in-house, unpromoted catalysts reveal both catalysts are most active and selective at their highest oxidation states.

Cyanobacterial hydrothermal carbonization carbon as photocatalyst for selective aerobic oxidation of cyclohexane

The exploration of catalyst preparation techniques that incorporate environmental-friendly methodologies has consistently been the focus of scientific inquiry within the field of green synthesis of oxygen-containing organic chemicals. Herein, a metal-free hydrothermal carbonation carbon (HTCC) has been prepared utilizing cyanobacterial biomass produced during a water bloom in a freshwater lake. The HTCC produced is capable of performing photocatalytic selective oxidation of cyclohexane to cyclohexanol and cyclohexanone (KA oil) under ambient conditions. The use of cyanobacterial biomass allows for the efficient preparation of HTCC, which exhibits a conversion rate of cyclohexane that is 4.1 times of HTCC prepared with alternative carbon sources (glucose). The selectivity to KA oil over the samples are almost 100%. Characterizations and analysis reveal that cyanobacteria can lead to self-doping of nitrogen and hydrophobicity of the resulting HTCC, while sulfuric acid etching endow it with more photoactive sp2 hybridized structure units that contribute to a higher photogenerated carrier separation efficiency. The suitable band structure enables the cyanobacterial HTCC to produce superoxide radicals to oxidize cyclohexane. These findings are of great significance for the development of eco-friendly photocatalysts and the utilization of biomass resources.

Application of Laser‐Generated Au‐Ag/Al2O3 Nanoparticle Catalysts for the Selective Catalytic Oxidation of 5‐(Hydroxymethyl) Furfural to 2,5‐Furan‐Dicarboxylic Acid

AbstractConversion of lignocellulosic biomass‐derived 5‐(Hydroxymethyl) furfural (HMF) into 2,5‐Furandicarboxylic acid (FDCA) has a high potential to replace petroleum‐based feedstocks for the production of plastics. In this study, a green synthesis strategy for the selective oxidation of HMF to FDCA is reported by using laser‐generated and specifically designed bimetallic AuAg nanoparticle catalysts supported on Al2O3 using molecular oxygen as oxidant and water as a solvent. Although supported AuAg catalysts are already known as suitable catalysts for this reaction, the optimal reaction conditions (type of base, reaction temperature, oxygen pressure), Au : Ag composition, role of metal‐support effects and origin of the active site still remain under debate. Interestingly, an Au9Ag1/Al2O3 catalyst exhibited the highest activity with a FDCA yield of up to 66 % at 80 % HMF conversion. Hereby, NaHCO3 (instead of NaOH) as a smoother base in 2 : 1 base:HMF ratio, low oxygen pressures of 5 bar, and a mild temperature of 150 °C were found optimal. The yield remained fairly stable on reuse of the catalyst for 3 cycles without hints of catalyst sintering or leaching. The titration of surficial hydroxyls on the AuxAgy/Al2O3 surface showed the highest density of acidic hydroxyls for the most active Au9Ag1/Al2O3 catalyst. The respective hydroxyls potentially originate from metal‐support interactions between the AuAg nanoparticles with the alumina support and act as Lewis acidic sites that oxidize the alcohol moiety of HMF to form FDCA with high selectively. In summary, this study shows that the catalytic activity of AuAg catalysts in HMF oxidation is not just linked to an alloying effect but also to a metal‐support effect where the AuAg nanoparticles appear to directly affect the local acidity of the support.

Selective light-driven methane oxidation to ethanol

Methane (CH4) photocatalytic upgrading to value-added chemicals, especially C2 products, is significant yet challenging due to sluggish energy/mass transfer and insufficient chemical driven-force in single photochemical process. Herein, we realize solar-driven CH4 oxidation to ethanol (C2H5OH) on crystalline carbon nitride (CCN) modified with Cu9S5 and Cu single atoms (Cu9S5/Cu-CCN). The integration of photothermal effect and photocatalysis overcomes CH4-to-C2H5OH conversion bottlenecks, with Cu9S5 as a hotspot to convert solar-energy to heat. In-situ characterizations demonstrate that Cu single atoms play as electron acceptor for O2 reduction to ·OOH/ · OH, while Cu9S5 acts as hole acceptor and site for CH4 adsorption, C − H activation, and C − C coupling. Theoretical calculations demonstrate that Cu9S5/Cu-CCN reduces C − C coupling energy barrier by stabilizing ·CH3 and ·CH2O. Impressively, C2H5OH productivity reaches 549.7 μmol g–1 h–1, with selectivity of 94.8% and apparent quantum efficiency of 0.9% (420 nm). This work provides a sustainable avenue for CH4 conversion to value-added chemcials.

Selective photooxidation of methane to C1 oxygenates by constructing heterojunction photocatalyst with mild oxidation ability

Selective photocatalytic aerobic oxidation of methane to value-added chemicals offers a promising pathway for sustainable chemical industry, yet remains a huge challenge owing to the consecutive overoxidation of primary products. Here, a type II heterojunction were constructed in Ag-AgBr/ZnO to reduce the oxidation potential of stimulated holes and prevent the undesirable CH4 overoxidation side reactions. For photocatalytic oxidation of methane under ambient temperature, the products yield of 1499.6 μmol gcat−1 h−1 with a primary products selectivity of 77.9% was achieved over Ag-AgBr/ZnO, which demonstrate remarkable improvement compared to Ag/ZnO (1089.9 μmol gcat−1 h−1, 40.1%). The superior activity and selectivity result from the promoted charge separation and the redox potential matching with methane activation after introducing AgBr species. Mechanism investigation elucidated that the photo-generated holes transferred from the valence band of ZnO to that of AgBr, which prevent H2O oxidation and enhance the selective generation of •OOH radical.

Synergistic interplay of CuOx and Pd nanodots on TiO2 for efficient and highly selective photocatalytic oxidation of CH4 to oxygenates with O2

Direct photocatalytic methane oxidation to produce liquid oxygenates offers a promising approach for the upgrading of abundant methane under mild conditions, yet it remains a formidable challenge in achieving high reaction rates while maintaining high selectivity. Herein, we report the highly dispersed CuOx and Pd nanodots decorated TiO2 for photocatalytic oxidation of CH4 with O2 at room temperature, which exhibits a remarkable C1 oxygenates production rate of 39.5 mmol·g−1·h−1 with a nearly 100 % selectivity, outperforming most of the state-of-the-art photocatalysts. Both experimental and theoretical studies suggest that the impressive photocatalytic performance is attributed to the synergy of Cu+ species and Pd nanodots. Cu+ species not only promote the interfacial electrons transfer from TiO2 to Pd, but also mediate CH4 oxidation reaction to avoid overoxidation of oxygenates to CO2, while the resulting electron-rich Pd sites boost the production of primary products (CH3OOH and CH3OH) by lowering the reaction energy. This work provides a new pathway for developing highly efficient photocatalysts for the selective conversion of methane to value-added chemicals by designing bimetallic cocatalysts.

Engineering Pt single atom catalysts on titania nanotubes via the photo/electrochemical pathway for a selective ammonia oxidation reaction

Engineering Pt single atom catalysts on titania nanotubes via the photo/electrochemical pathway for a selective ammonia oxidation reaction

Phosphorus Dopants Triggered Enhanced Bulk Polarization and New Active Sites in Hexagonal ZnIn2S4 toward Efficient Photocatalytic Selective Oxidation of Benzyl Alcohol Coupled with H2 Evolution

Phosphorus Dopants Triggered Enhanced Bulk Polarization and New Active Sites in Hexagonal ZnIn2S4 toward Efficient Photocatalytic Selective Oxidation of Benzyl Alcohol Coupled with H2 Evolution