Catalytic Aerobic Oxidation Research Articles

Catalytic Atroposelective Aerobic Oxidation Approaches to Axially Chiral Molecules.

A copper and chiral nitroxide co-catalyzed aerobic enantioselective oxidation process has been developed that allows access to axially chiral molecules. Two complementary atroposelective approaches, oxidative kinetic resolution (OKR) and desymmetrization, were studied using ambient air as the stoichiometric terminal oxidant. OKR of rac-N-arylpyrrole alcohols and rac-biaryl alcohols affords the optically pure compounds with er up to 3.5:96.5 and 5.5:94.5, respectively. Desymmetrization of prochiral diols provides axially chiral biaryl compounds with er up to 99:1.

Continuous catalytic aerobic oxidation of o‑chlorotoluene to o-chlorobenzoic acid under slug flow conditions

Continuous catalytic aerobic oxidation of o‑chlorotoluene to o-chlorobenzoic acid under slug flow conditions

Novel insights into CO2 inhibition with additives in catalytic aerobic oxidation of biomass-derived carbohydrates to formic acid

Formic acid (FA) is considered an ideal energy carrier for hydrogen. Catalytic aerobic oxidation of biomass-derived carbohydrates is a promising method to produce renewable FA. Interestingly, the addition of some low-carbon alcohols or dimethyl sulfoxide remarkably improved FA selectivity in the popular vanadium-based catalytic system. This work unveils an advanced mechanism of the product improvement with additives. Control experiments of vanadium-substituted heteropolyacids (HPA-n) in different redox states and ESR spectroscopy verified that hydroxyl radicals were formed in the re-oxidation of reduced HPA-n. Different-type oxidations and model reactions revealed that the hydroxyl radicals caused overoxidation by oxidizing aldehydic groups to carboxylic groups as the precursors of CO2. The additives scavenged hydroxyl radicals so glucose was oxidized into FA with high selectivity. Furthermore, the hydroxyl radicals were quantified and the effect of isopropanol dosage on glucose oxidation was investigated. The CO2-inhibition effect of isopropanol was also exhibited for various carbohydrates and vicinal diols. Based on the mechanism, catalytic co-oxidation of glucose and methanol was proposed, showing an extremely high FA yield from glucose and high reactivity of methanol. The findings shed light into the mechanism of the product improvement with additives and pave an efficient way to produce a renewable energy carrier.

Synergistic catalysis on atomic Ru-doped manganese oxide for aerobic oxidation of biomass-derived 5-hydroxymethylfurfural

Catalytic oxidation of biomass derived 5-Hydroxymethylfurfural (HMF) feedstock is a promising route to produce value-added chemicals 2, 5-diformylfuran (DFF). Here, we find that incorporating monatomic Ru into the crystalline lattice of α-MnO2 can trigger synergetic catalysis for aerobic oxidation of HMF. Ru species can not only act as active sites for HMF conversion, but also activate surface oxygen species by creating rich oxygen vacancies on the α-MnO2. As result, such catalyst delivers much higher DFF productivity than that of Ru located in the tunnel of support (110 °C, 1.5 MPa, 1 h) and state-of-the-art Ru-based catalysts. Furthermore, the direct conversion of fructose into DFF was realized by combination of Amberlyst-15 and Ru-MnO2 HT catalyst in one-pot via two-step. Characterization and kinetic results indicated that atomically dispersed Ru hosted in the crystalline lattice of support created abundant oxygen vacancies and thus weakened the Mn-O bond, resulting in facile lattice oxygen activation.

Novel chemical looping oxidation of biomass-derived carbohydrates to super-high-yield formic acid using heteropolyacids as oxygen carrier

In recent years, formic acid (FA) shows great potential in fuel cells and hydrogen storage. For FA production from renewable resources, catalytic aerobic oxidation of biomass to FA has been reported. However, the yield of by-product CO2 is fairly high, limiting the FA selectivity and causing greenhouse gas emission. Herein, a novel chemical looping oxidation of biomass-derived carbohydrates to FA was first proposed. A heteropolyacid H8PV5Mo7O40 (HPA-5) was selected as an excellent oxygen carrier. Oxidation of carbohydrates to FA and re-oxidation of reduced HPA-5 (HPA-5red) by O2 were separately performed, by which the FA yield from glucose achieved 95.4%. The extremely high yield of FA is attributed to two reasons. On one hand, the chemical looping process avoids over-oxidation caused by the hydroxyl radicals formed in the re-oxidation of HPA-5red. On the other hand, abundant HPA-5 promotes the glucose oxidation to FA, suppressing the acid-catalyzed decomposition and the oxidation toward saccharic acids. Besides, HPA-5red can be completely re-oxidized by O2 and the chemical looping oxidation has stably performed well in five chemical cycles. This oxidation strategy also shows outstanding FA yields from xylan, cellulose and raw biomass.

Hydroxymethylfurfural oxidation over unsupported Pd-Au alloy catalysts prepared by pulsed laser ablation: Synergistic and compositional effects

Bimetallic nanoparticles are of high interest for electronics, photonics, biomedicine, and heterogeneous catalysis. To reveal the effects of structure and composition of bimetallic nanoparticles on catalytic properties, unsupported particles with clean surfaces and well-defined characteristics should be considered. Herein, colloidal Au, Pd, and Au-Pd catalysts prepared by pulsed laser ablation in liquid are studied in 5-hydroxymethylfurfural (HMF) oxidation. The catalysts are characterized by ELS, UV–vis spectroscopy, XRD, TEM, and SAED methods. The results show the formation of Au1−yPdy alloy particles that remain stable in pure water without the presence of any surfactant and feature higher performance towards formylfuran carboxylic and furandicarboxylic acids as compared with monometallic samples. The results indicate the combined benefits of Pd1−yAuy alloys in aerobic catalytic HMF oxidation due to a synergistic effect comprising the formation of Au-Pd interface in alloy nanoparticles, where the metals activate different moieties of HMF molecule.

Enzymatic Substrate Inhibition in Metal Free Catecholase Activity.

The catecholase activities were routinely modeled using transition metal complexes as catalyst and in some case basic pH were used as a reaction condition. In this article, the catalytic aerobic oxidation of proxy substrate 3,5-di-tert-butylcatechol (DTBC) in methanol using triethylamine/diethylamine as catalyst was demonstrated as a functional mimic of catecholase activity. The kinetic manifestation of DTBC oxidation was explained as enzymatic substrate inhibition pattern in Michaelis-Menten kinetic model. The mechanistic insight of the aerobic oxidation of DTBC was further validated using various spectroscopic techniques and DFT methods.

Biomass-based production of trimellitic and trimesic acids

The production of industrial chemicals with renewable biomass feedstock holds potential to aid the world in pursuing a carbon-neutral society. Trimellitic and trimesic acids are important commodity chemicals in industry that are prepared by the oxidation of petroleum-derived trimethylbenzene. To reduce the dependence on the limited oil source, we develop a potential sustainable alternative towards trimellitic and trimesic acids using biomass-based 2-methyl-2,4-pentandiol (MPD), acrylate and crotonaldehyde as starting materials. The process for trimellitic acid includes dehydration/D-A reaction of MPD and acrylate, flow aromatization over Pd/C catalyst, hydrolysis and catalytic aerobic oxidation (60% overall yield). The challenging regioselectivity issue of D-A reaction is tackled by a matched combination of temperature and deep eutectic solvent ChCl/HCO2H. Crotonaldehyde can also participate in the reaction, followed by Pd/C-catalyzed decarbonylation/dehydrogenation and oxidation to provide trimesic acid in 54% overall yield. Life cycle assessment implies that compared to conventional fossil process, our biomass-based routes present a potential in reducing carbon emissions.

Mechanism-Guided Design of Robust Palladium Catalysts for Selective Aerobic Oxidation of Polyols

The palladium complex [(L1)Pd(μ-OAc)]2[OTf]2 (L1 = neocuproine) is a selective catalyst for the aerobic oxidation of vicinal polyols to α-hydroxyketones, but competitive oxidation of the ligand methyl groups limits the turnover number and necessitates high Pd loadings. Replacement of the neocuproine ligand with 2,2'-biquinoline ligands was investigated as a strategy to improve catalyst performance and explore the relationship between ligand structure and reactivity. Evaluation of [(L2)Pd(μ-OAc)]2[OTf]2 (L2 = 2,2'-biquinoline) as a catalyst for aerobic alcohol oxidation revealed a threefold enhancement in turnover number relative to the neocuproine congener, but a much slower rate. Mechanistic studies indicated that the slow rates observed with L2 were a consequence of precipitation of an insoluble trinuclear palladium species─(L2Pd)3(μ-O)22+─formed during catalysis and characterized by high-resolution electrospray ionization mass spectrometry. Density functional theory was used to predict that a sterically modified biquinoline ligand, L3 = 7,7'-di-tert-butyl-2,2'-biquinoline, would disfavor the formation of the trinuclear (LPd)3(μ-O)22+ species. This design strategy was validated as catalytic aerobic oxidation with [(L3)Pd(μ-OAc)]2[OTf]2 is both robust and rapid, marrying the kinetics of the parent L1-supported system with the high aerobic turnover numbers of the L2-supported system. Changes in ligand structure were also found to modulate regioselectivity in the oxidation of complex glycoside substrates, providing new insights into structure-selectivity relationships with this class of catalysts.

Immobilization of a flavin molecule onto poly(methacrylic acid)s and its application in aerobic oxidation catalysis: effect of polymer stereoregularity.

The isoalloxazine ring system, called flavin, was successfully immobilized on poly(methacrylic acid)s, PMAAs, with different tacticity via post-polymerization modification under suitable conditions. The resulting flavin-containing polymers showed catalytic activity for aerobic oxidation reactions, in which the polymer stereoregularity clearly influenced their catalytic activity.

A facile approach to phenothiazinones via catalytic aerobic oxidation: discovery of an antiproliferative agent.

The production of bioactive pharmaceutical ingredients in a sustainable manner has become essential in the modern academic and industrial community. Herein, we report a chemically robust and sustainable aerobic oxidation for the synthesis of the phenothiazinone framework, using the commercially available TEMPO/HBF4/NaNO2 co-catalytic system under an ambient atmosphere. The reaction is highly efficient with broad scopes and excellent scalability. Preliminary activity screening led to the discovery of compound 3 as a potent antiproliferative agent. The green synthesis of a variety of sulfur containing heterocycles might encourage the pursuit of biologically valuable molecules in the medicinal field.

Study on the catalytic degradation of sodium lignosulfonate to aromatic aldehydes over nano-CuO: Process optimization and reaction kinetics

As one of the few renewable aromatic resources, the research of depolymerization of lignin into high-value chemicals has attracted extensive attention in recent years. Catalytic wet aerobic oxidation (CWAO) is an effective technology to convert lignin like sodium lignosulfonate (SL), a lignin derivative, into aromatic aldehydes such as vanillin and syringaldehyde. However, how to improve the yield of aromatic aldehyde and conversion efficiency is still a challenge, and many operating conditions that significantly affect the yield of these aromatic compounds have rarely been investigated systematically. In this work, we adopted the stirred tank reactor (STR) for the CWAO process with nano-CuO as catalyst to achieve the conversion of SL into vanillin and syringaldehyde. The effect of operating conditions including reaction time, oxygen partial pressure, reaction temperature, SL concentration, rotational speed, catalyst amount, and NaOH concentration on the yield of single phenolic compound was systematically investigated. The results revealed that all these operating conditions exhibit a significant effect on the aromatic aldehyde yield. Therefore, they should be regulated in an optimal value to obtain high yield of these aldehydes. More importantly, the reaction kinetics of the lignin oxidation was explored. This work could provide basic data for the optimization and design of industrial operation of lignin oxidation.

Catalytic aerobic oxidation of lignin-based vanillyl alcohol under base–free conditions over an efficient and reusable LaFeO3 perovskite for vanillin production

Conversion of lignin feedstocks into aromatic chemicals is a highly desirable target for biorefineries, whose depolymerization often requires high temperatures and harsh conditions.

The tail of imidazole regulated the assembly of two robust sandwich-type polyoxotungstate-based open frameworks with efficient visible-white-light-driven catalytic oxidation of sulfides

The tail of imidazole regulated the assembly of fascinating 2Dsqltopology and 3D 2-fold interpenetratedlvtstructure polyoxotungstate-based open frameworks with efficient visible-white-light-driven aerobic catalytic oxidation of sulfides.

Synthesis and Tuning of Gold Nanoparticles Supported on Polymorphic Semicrystalline Nanoporous Polymer for Catalytic Aerobic Oxidation of Alcohols

AbstractThe wide and complex tunability of gold nanoparticles (AuNPs) supported on nanoporous, semicrystalline, and polymorphic polymer matrices consisting of syndiotactic polystyrene (sPS) or multiblock syndiotactic polystyrene‐co‐cis‐1,4 poly(butadiene) (sPSB) is herein described and correlated with the performances in aerobic oxidation of (±)‐1‐phenylethanol. Impregnation or deposition‐impregnation synthetic methods yielded AuNPs of middle size (10–20 nm) on the surface of polymer grains. The co‐precipitation method leads to small AuNPs (4–5 nm) homogenously dispersed in the support with quantitative entrapment of the gold. Thermal or solvent annealing afforded modifications of polymer crystalline phase and AuNPs morphology. The annealing with chloroform/water mixture increases the crystallinity of the nanoporous forms and converts compact γ and β phases into the nanoporous ϵ and δ phases. The thermal treatment of the AuNPs‐sPS(B) catalysts afford defective NPs with enhanced catalytic activity. The solvent‐induced formation of the nanoporous forms improves permeability to solvents and reactants and, in turn, the catalytic properties.

Hemilabile Amine-Functionalized Efficient Azo-Aromatic Cu-Catalysts Inspired by Galactose Oxidase: Impact of Amine Sidearm on Catalytic Aerobic Oxidation of Alcohols.

A series of azo-aromatic copper(II) complexes, [1a-g] and a Cu(I) complex, [1h], with varying amine-functionalized hemilabile pincer-like [HL1-3] and [L1,2], methyl-substituted azo [L3], and imine [L4] ligands, were synthesized and characterized. These complexes were investigated for aerobic oxidation of a variety of aromatic alcohols in the presence of 2.0 mol % precatalysts [1a-g], cobaltocene (2.0 mol %), N-methyl imidazole (NMI) (8.0 mol %), and TEMPOH (2.0 mol %) at room temperature. The Cu(I) complex (1h) acted as a catalyst in the absence of cobaltocene. To understand the mechanism, detailed experimental and theoretical studies have been performed with the representative complex [1a], which has suggested a new kind of mechanism involving a Cu(II)/Cu(I) redox couple. Cobaltocene acts as a reductant to [1a] to generate a Cu(I) complex, which activates dioxygen in the presence of NMI. TEMPOH transfers a hydrogen atom to the activated dioxygen with the generation of TEMPO•, which further participates in α-C-H bond activation in the Cu(II)-alkoxide intermediate in an intermolecular fashion in the catalytic cycle. The amine sidearm in the ligand backbone of the complexes has a significant role in catalytic activity. Complexes with amine sidearms are more effective than complexes without them. Moreover, the aliphatic secondary amine sidearm is more efficient among the amine sidearm than the aromatic secondary amine and tertiary amines. The amine sidearm that remained coordinated to the Cu(II) center is hemilabile, and it facilitates alcohol coordination in the catalytic process. Alcohol coordination was the rate-limiting step, and it was supported by the isotope effect study on benzyl alcohol, substitution effect on the amine moiety of the ligands, and DFT calculation. The hemilabile amine sidearm of the coordinated ligand also acted as a base in deprotonating the alcoholic O-H proton and acted as an acid in releasing H2O2 during the catalysis.

Controllable Synthesis of Mono‐ and Bis‐sulfenylindoles from Indoles and Various Sulfenylation Agents Using KI/SeO2 System

AbstractAn efficient method for selective synthesis of mono‐ and bis‐sulfenylindoles was developed using a novel aerobic catalytic oxidation system KI/SeO2 for sulfenylation of indoles with disulfide/thiols. The selectivity was controlled by varying the amount of the catalytic system and sulfenylation agents. The reaction features easy operation, mild reaction conditions, good atom economy, gram‐scalability, and a metal‐free approach furnishing desired products in up to excellent yields.

Dual-site oxygen activation for enhanced photocatalytic aerobic oxidation by S-scheme Ni2P/Bi3O4Br-OVs heterojunction

Solar-driven activation of molecular oxygen (O2) offers an appealing strategy to realize aerobic oxidation catalysis at room temperature, while the efficiency of photocatalytic O2 activation is severely limited by lacking O2 adsorption sites and poor carrier utilization. Herein, an efficient Ni2P/Bi3O4Br-OVs heterojunction is firstly proposed to overcome the limitation by the dual-site design of oxygen vacancies (OVs) and on Ni2P cocatalyst. By the joint observations from spectroscopic measurements and theoretical simulations, the introduction of OVs and Ni2P on Bi3O4Br nanosheets can not only optimize the light absorption, facilitate the separation and transfer of photogenerated charge carrier through the S-scheme mechanism, but also provide active sites for effective adsorption and activation of oxygen molecules, thereby contributing to highly efficient generation of •O2− radicals. Benefiting from the more oxidative active species of •O2− and h+, the Ni2P/Bi3O4Br-OVs photocatalyst achieves near-unity conversion rate and selectivity in aerobic oxidation of sulfide to sulfoxide, and it also exhibits outstanding photoactivity in the aerobic degradation of phenolic pollutants under visible light. This study showcases a feasible O2 activation approach by defect and heterojunction engineering, and provides a new perspective for designing high-performance photocatalysts in the field of aerobic oxidation catalysis.

Rapid Catalysis for Aerobic Oxidation of Alcohols Based on Nitroxyl-Radical-Free Copper(II) under Ambient Conditions

Rapid Catalysis for Aerobic Oxidation of Alcohols Based on Nitroxyl-Radical-Free Copper(II) under Ambient Conditions

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.