Novel cobalt(Ⅱ) complexes of amino acids-Schiff bases catalyzed aerobic oxidation of various alcohols to ketones and aldehyde

Two cobalt（Ⅱ） complexes 1 and 2 of Schiff bases derived from amino acids were synthesized and used for oxidation of benzyl alcohol with molecular oxygen at different conditions of pH,solvent,temperature and complex/alcohol molar ratio to optimize reaction conditions and to evaluate the catalytic efficiency of new cobalt Schiff base complexes.Under obtained optimum conditions,various alcohols were oxidized to corresponding aldehydes and ketones.

Base-free aerobic oxidation of glycerol on TiO_2-supported bimetallic Au–Pt catalysts

The aerobic oxidation of glycerol provides an economically viable route to glyceraldehyde, dihydroxyacetone and glyceric acid with versatile applications, for which monometallic Pt, Au and Pd and bimetallic Au-Pt, Au- Pd and Pt-Pd catalysts on TiO2 were examined under base-free conditions. Pt exhibited a superior activity relative to Pd, and Au-Pd and Pt-Pd while Au was essentially inactive. The presence of Au on the Au-Pt/TiO2 catalysts led to their higher activities （normalized per Pt atom） in a wide range of Au/Pt atomic ratios （i.e. 1/3-7/1 ）, and the one with the Au/Pt ratio of 3/1 exhibited the highest activity. Such promoting effect is ascribed to the increased electron density on Pt via the electron transfer from Au to Pt, as characterized by the temperature-programmed desorption of CO and infra-red spectroscopy for CO adsorption. Meanwhile, the presence of Au on Au-Pt/TiO2, most like due to the observed electron transfer, changed the product selectivity, and facilitated the oxidation of the secondary hydroxyl groups in glycerol, leading to the favorable formation of dihydroxyacetone over glyceraldehyde and glyceric acid that were derived from the oxidation of the primary hydroxyl groups. The synergetic effect between Au and Pt demonstrates the feasibility in the efficient oxidation of glycerol to the targeted products, for example, by rational tuning of the electronic properties of metal catalysts.

The Variation of Microbial(Methanotroph)Communities in Marine Sediments Due to Aerobic Oxidation of Hydrocarbons

Methanotrophs in marine sediments and overlying water attenuate the emissions of methane into the atmosphere and thus play an important role for the global cycle of this greenhouse gas.However,gas released from natural hydrocarbon seeps are not pure methane but commonly mixed hydrocarbons.Currently,how methanotrophic bacteria behave in the co-presence of methane and heavier hydrocarbons remains unknown.In this paper,the bacteria were cultured aerobically in fresh sediment samples(collected from Bohai Bay in eastern China)at 28℃under the atmospheres of pure methane and methane+ethane+propane mixed gas,respec-tively.The prevailing terrigenous n-alkanes and fatty acids in the original sediment samples varied consistently after incubations,confirming the proceeding of aerobic bacterial activities.The real-time quantitative PCR assay and sequencing of the 16S rRNA and particulate methane monooxygenase(pmoA)genes revealed the changes of microbe communities to a methanotroph-dominating structure after incubations.Particularly,after incubations the family Methylococcaceae(typeⅠmethanotrophs)became dominant with proportions higher than 40%,whereas Methylocystaceae(typeⅡmethanotrophs)nearly disappeared in all incubated samples.More-over,the species of methanotrophs from the samples treated with pure methane were dominated by Methylobacter luteus,whereas Methylobacter whittenburyi took the predominant proportion in the samples treated with mixed gas.The phenomenon suggests that some methanotrophs may also utilize ethane and propane.Collectively,this study may help to gain a better understanding of the ef-fects and contributions of microbial activities in marine hydrocarbon seep ecosystems.

A new approach for the aerobic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid without using transition metal catalysts

The organic compound 2,5-furandicarboxylic acid（FDCA） has been identified by the US Department of Energy（DOE） as a valuable platform chemical for a wide range of industrial applications. Currently, the most popular route for FDCA synthesis is reported to be the oxidation of 5-hydroxymethylfurfural（HMF）by O_2 over the catalysis of noble metals（e.g., Au, Pt, Ru, and Pd）. However, the high costs of noble metal catalysts remain a major barrier for producing FDCA at an industrial scale. Herein, we report a transition metal-free synthesis strategy for the oxidation of HMF to FDCA under O_2 or ambient air. A simple but unprecedented process for the aerobic oxidation of HMF was carried out in organic solvents using only bases as the promoters. According to the high performance liquid chromatography（HPLC） analysis, excellent product yield（91%） was obtained in the presence of NaOH in dimethylformamide（DMF） at room temperature（25 ℃）. A plausible mechanism for the NaOH-promoted aerobic oxidation of HMF in DMF is also outlined in this paper. After the reaction, the sodium salt of FDCA particles were dispersed in the reaction mixture, making it possible for product separation and solvent reuse. The new HMF oxidation approach is expected to be a practical alternative to current ones, which depend on the use of noble metal catalysts.

Rational construction of metal–base synergetic sites on Au/Mg-beta catalyst for selective aerobic oxidation of 5-hydroxymethylfurfural

The selective aerobic oxidation of biomass-derived 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid (FDCA, a potential renewable substitution of fossil-based terephthalic acid to produce polyethylene 2,5-furandicarboxylate plastic) is an appealing transformation for constructing eco-friendly and sustainable chemical processes. Au supported catalysts have showed encouraging performances for this well-received conversion, whose catalytic behavior was greatly affected by the adopted support derived from the existence of metal-support interactions. Herein, a series of Mg-Beta zeolites were hydrothermally synthesized via developed structural reconstruction, which were employed as basic supports for Au catalysts to construct bifunctional catalysts. The relationship between structure (Au particle size, basicity within zeolites and Auδ+ contents) and FDCA yield was concretely established. The conclusion was made that the utilization of Mg-Beta zeolites with strong basicity as the support could not only improve the FDCA yield but also decrease the amount of additional base. Furthermore, the possible reaction mechanism was also proposed via tracking time-dependent variations of corresponding organics and controlled experiment. This work provides some guidance for rationally designing multifunctional catalysts in the view of integrating metal catalysts with metallosilicate zeolites, which was beneficial to the catalytic upgrading of organic compounds with multiple functional groups.

Regulation of Active Oxygen Species by Grain Boundaries to Optimize Reaction Paths toward Aerobic Oxidations

Aerobic oxidation by using molecular oxygen(O_(2))as the oxidant is highly attractive,in which activating O_(2)to reactive oxygen species(ROS)is a prerequisite.Although some progress has been achieved in regulating ROS by heterogeneous catalysts,the strategies to efficiently control ROS in aerobic oxidation are still urgently desired.Herein,grain boundaries(GBs)in metal oxides are discovered to be able to facilely regulate ROS.Impressively,MoO_(3)nanocrystals with high density of GBs(MoO_(3)-600)deliver a mass activity of 83 mmol g^(-1)h^(-1)in aerobic oxidation of benzyl alcohol,7 and 8 times as high as that of MoO_(3)nanoparticles without GBs and Pt/C,respectively.In addition,the selectivity of benzoic acid is 100%during whole reaction process over MoO_(3)-600.Mechanistic studies reveal that the oxygen atoms at GBs in MoO_(3)-600 are highly active to form·OH radicals with the generation of oxygen vacancies,while the oxygen vacancies are replenished by O_(2).The reaction path directly contributes to the excellent catalytic performance.

Highly Dispersed RuOOH Nanoparticles on Silica Spheres:An Efficient Photothermal Catalyst for Selective Aerobic Oxidation of Benzyl Alcohol

Photothermal catalysis represents a promising strategy to utilize the renewable energy source(e.g.,solar energy)to drive chemical reactions more efficiently.Successful and efficient photothermal catalysis relies on the availability of ideal photothermal catalysts,which can provide both large areas of catalytically active surface and strong light absorption power simultaneously.Such duplex requirements of a photothermal catalyst exhibit opposing dependence on the size of the catalyst nanoparticles,i.e.,smaller size is beneficial for achieving higher surface area and more active surface,whereas larger size favors the light absorption in the nanoparticles.In this article,we report the synthesis of ultrafine RuOOH nanoparticles with a size of 2–3 nm uniformly dispersed on the surfaces of silica(SiOx)nanospheres of hundreds of nanometers in size to tackle this challenge of forming an ideal photothermal catalyst.The ultrasmall RuOOH nanoparticles exhibit a large surface area as well as the ability to activate adsorbed molecular oxygen.The SiOx nanospheres exhibit strong surface light scattering resonances to enhance the light absorption power of the small RuOOH nanoparticles anchored on the SiOx surface.Therefore,the RuOOH/SiOx composite particles represent a new class of efficient photothermal catalysts with a photothermal energy conversion efficiency of 92.5%for selective aerobic oxidation of benzyl alcohol to benzylaldehyde under ambient conditions.

Polarization engineering in porous organic polymers for charge separation efficiency and its applications in photocatalytic aerobic oxidations

Photocatalytic aerobic oxidation reactions are largely governed by the efficiency of charge separation and subsequent reactive oxygen species(ROS) generation. Herein, we report a polarization engineering strategy to promote the charge separation and ROS generation efficiency by substituting the benzene unit with furan/thiophene in porous organic polymers(POPs). Benefiting from the extent of local polarization, the thiophene-containing POP(JNU-218) exhibits the best photocatalytic performance in aerobic oxidation reactions, with a yield much higher than those for the furan-containing POP(JNU-217) and the benzenecontaining POP(JNU-216). Experimental studies and theoretical calculations reveal that the increase of local polarization can indeed reduce the exciton binding energy, and therefore facilitate the separation of electron-hole pairs. This work demonstrates a viable strategy to tune charge separation and ROS generation efficiency by modulating the dipole moments of the building blocks in porous polymeric organic semiconductors.

Confined Mn^(2+) enables effective aerobic oxidation catalysis

Effective and mild activation of O_(2) is essential but challenging for aerobic oxidation. In heterogeneous catalysis, high-valence manganese oxide(e.g., +4) is known to be active for the oxidation, whereas divalent MnO is ineffective due to its limited capacity to supply surface oxygen and its thermodynamically unstable structure when binding O_(2) in reaction conditions. Inspired by natural enzymes that rely on divalent Mn^(2+), we discovered that confining Mn^(2+) onto the Mn_(2)O_(3) surface through a dedicated calcination process creates highly active catalysts for the aerobic oxidation of 5-hydroxymethylfurfural, benzyl alcohol, and CO.The Mn_(2)O_(3)-confined Mn^(2+) is undercoordinated and efficiently mediates O_(2) activation, resulting in 2–3 orders of magnitude higher activity than Mn_(2)O_(3) alone. Through low-temperature infrared spectroscopy, we distinguished low-content Mn^(2+) sites at Mn_(2)O_(3) surface, which are difficult to be differentiated by X-ray photoelectron spectroscopy. The combination of in-situ energydispersive X-ray absorption spectroscopy and X-ray diffraction further provides insights into the formation of the newly identified active Mn^(2+) sites. By optimizing the calcination step, we were able to increase the catalytic activity threefold further.The finding offers promising frontiers for exploring active oxidation catalysts by utilizing the confinement of Mn^(2+)and oftenignored calcination skills.

Fe(III)-Catalyzed Aerobic Oxidation of 1,4-Diols

Comprehensive Summary Aerobic oxidation has been catching more and more attention because of its atom economy and environmental friendliness.Oxidation of diols is a challenge due to various oxidative products.Thus,highly selective aerobic oxidation affording specific products is of current interest.In this work,a combination of Fe(NO_(3))_(3)·9H_(2)O/TEMPO/KCl catalysis has been identified as an efficient recipe for the aerobic oxidation of 1,4-diols affordingγ-butyrolactones under mild conditions.The reaction exhibits decent chemo-and regioselectivity of symmetrical and unsymmetrical 1,4-diols.The optically activeγ-lactones may also be prepared from optically active 1,4-diols without erosion of the ee via this method.Furthermore,this approach was successfully applied to synthesize NBP,a commercial drug.

Metal-organic frameworks as photocatalysts for aerobic oxidation reactions

Metal-organic frameworks(MOFs)are emerging as one of the most intriguing classes of heterogeneous photocatalysts owing to their abundant structures,tunable porosity,and versatile functions.The advantages of bottom-up design and reticular synthesis render MOF materials with desired photocatalytic properties for targeted reactions.In this review,we discussed the design and synthesis of MOF-based photocatalysts as well as strategies for enhancing photocatalytic performance.Recent progress on MOFs as platforms for photocatalytic aerobic oxidation reactions was summarized and categorized according to the types of bond formation.We hope this review will give an in-depth insight into MOF-based photocatalytic systems for not only aerobic oxidation reactions but also other organic transformations.A brief outlook on the challenges and opportunities of MOFs as heterogeneous photocatalysts is provided at the end of the review.

(Catecholate)Cu^(I)_(2)-Displayed Porous Organic Polymers as Efficient Heterogeneous Catalysts for the Mild and Selective Aerobic Oxidation of Alcohols

Porous organic polymers(POPs)containing catalytically active sites are of paramount importance for heterogeneous catalysis.However,the catalytically active sites of reported POPs are mostly limited to mononuclear metal species.Herein,we report the reaction between catechol-containing POPs(Cat-POPs)and[CuIMes]n to afford the corresponding Cu^(I)_(2)-CatPOPs with a putative vicinal binuclear(catecholate)Cu^(I)_(2)moiety.The resulting Cu^(I)_(2)-CatPOPs exhibit high Brunauer–Emmett–Teller surface areas,good stability,and excellent catalytic activity toward the aerobic oxidation of a broad range of primary and secondary alcohols under mild conditions,with either 2,2,6,6-tetramethylpiperidinyl-N-oxyl or 9-azabicyclo[3.3.0]nonane-N-oxyl as the cocatalyst.As green aerobic oxidation catalysts,the Cu^(I)_(2)-CatPOPs are much more active than the correspondingmononuclear CuIICatPOPs,where each catecholate moiety only supports one CuII center;CuI-ConPOPs,where the binding sites for CuI is a nonvicinal 1,4-dihydroxybenzene moiety;and the homogeneous analogue(3,6-di-tert-butyl catecholate)Cu^(I)_(2).These results are consistent with a proposed vicinal binuclear Cu^(I)_(2)structure that can efficiently activate molecular oxygen for the aerobic oxidation of alcohols,mechanistically similar to that observed in dicopper-containing oxygenases.Our results demonstrate the facile preparation of POPs with binuclear catalytically active sites that function as green heterogeneous catalysts for efficient oxidation of alcohols.

Screwdriver-like Pd-Ag heterostructures formed via selective deposition of Ag on Pd nanowires as efficient photocatalysts for solvent-free aerobic oxidation of toluene

Heterostructured bimetal nanocrystals with a component having localized surface plasmon resonance(LSPR)property are promising photocatalysts for a series of reactions.In this work,kinetic products of Pd-Ag with a screwdriver-like heterostructure have been successfully fabricated via the selective epitaxial growth of Ag on Pd nanowires(NWs).It was confirmed that the deposition rate(Vdeposition)of Ag is much more sensitive to the temperature,compared to the surface diffusion rate(Vdiffusion)which can be effectively reduced by the binding of poly(vinylpyrrolidone)(PVP)molecules.Then the magnitude of Vdeposition/Vdiffusion has been well tailored for the formation of a kinetic growth environment.The interactions between the components of the as-prepared Pd-Ag heterostructures resulted in intensified LSPR effects.As a result,they gained better photocatalytic performance toward solvent free aerobic oxidation of toluene than Pd NWs,Ag NWs and the mixture of them.Additionally,the Pd-Ag heterostructured nanocrystals exhibited excellent catalytic stability for recycling.This work not only presents an idea for realizing kinetic growth but also supports that LSPR effect is a good tool for improving the photocatalytic activity.

Direct aerobic oxidation of monoalcohol and diols to acetals using tandem Ru@MOF catalysts

The aerobic oxidation of monoalcohols and diols to acetals is an important academic and industrial challenge for the production of fine chemicals and intermediates.The existing methods usually rely on a two-step process in which alcohols are first oxidized to aldehydes over metal catalysts(Ru,Pt,Pd)and then acetalized using acids.Due to the instability of aldehydes,how to avoid over-oxidation to their respective carboxylic acids and esters is a long-standing challenge.For this reason,certain non-conjugated dialdehydes have never been successfully produced from diol oxidation.Hereby we report a Ru@metal-organic framework(MOF)tandem catalyst containing ultra-fine Ru nanoparticles(<2 nm)for direct alcohol to acetal conversion of monoalcohol and diols with noformation of carboxylic acids.Mechanistic study reveals that the presence of Lewis acid sites in the MOF work in concert with Ru active sites to promptly convert aldehydes to acetals thereby effectively suppressing the formation of over-oxidation byproducts.

Au atoms doped in Ti_(3)C_(2)T_(x)MXene:Benefiting recovery of oxygen vacancies towards photocatalytic aerobic oxidation

Photocatalytic aerobic oxidation by using oxygen molecules(O_(2))as green and low-cost oxidants is of great attraction,where the introduction of irradiation has been proved as an efficient strategy to lower reaction temperature as well as promote catalytic performance.Moreover,the oxygen vacancies(OVs)of catalyst are highly active sites to adsorb and activate O_(2)during photocatalytic aerobic oxidation.However,OVs are easily blocked by oxygen atoms from active oxygen species during the catalytic process,leading to the deactivation of catalysis.Herein,a promising catalyst toward photocatalytic aerobic oxidation was successfully developed by recovering the OVs through doping Au atoms into Ti_(3)C_(2)T_(x)MXene(Au/Ti_(3)C_(2)T_(x)).Impressively,Au/Ti_(3)C_(2)T_(x)exhibited remarkable activity under full-spectrum irradiation towards photooxidation of methyl phenyl sulfide(MPS)and methylene blue(MB),attaining a conversion of>90%at room temperature.Moreover,Au/Ti_(3)C_(2)T_(x)also manifested remarkable stability by maintaining>95%initial activity after 10 successive reaction rounds.Further mechanistic studies indicated that the OVs of Au/Ti_(3)C_(2)T_(x)served as the active centers to efficiently adsorb and activate O_(2).More importantly,the doped Au atoms of Au/Ti_(3)C_(2)T_(x)were conducive to the recovery of OVs during photocatalytic process from the results of theoretical and experimental aspects.The recovered OVs of Au/Ti_(3)C_(2)T_(x)continuously and efficiently activated O_(2),directly contributing to the remarkable catalytic activity and stability.

Thermoresponsive block copolymer supported Pt nanocatalysts for base-free aerobic oxidation of 5-hydroxymethyl-2-furfural

A base-free catalytic system for the aerobic oxidation of 5-hydroxymethyl-2-furfural was exploited by using Pt nanoparticles immobilized onto a thermoresponsive poly(acrylamide-co-acrylonitrile)-b-poly(N-vinylimidazole)block copolymer,with an upper critical solution temperature of about 45°C.The Pt nanocatalysts were well-dispersed and highly active for the base-free oxidation of 5-hydroxymethyl-2-furfural by molecular oxygen in water,affording high yields of 2,5-furandicarboxylic acid(up to>99.9%).The imidazole groups in the block copolymer were conducive to the improvement of catalytic performance.Moreover,the catalysts could be easily separated and recovered based on their thermosensitivity by cooling the reaction system below the upper critical solution temperature.Good stability and reusability were observed over these copolymer-immobilized catalysts with no obvious decrease in catalytic activity in the five consecutive cycles.

A silica gel supported cobalt(Ⅱ) Schiff base complex as efficient and recyclable heterogeneous catalyst for the selective aerobic oxidation of alkyl aromatics

A silica gel supported cobalt（lI） Schiff base complex was synthesized and used for the oxidation of alkyl aromatics using molecular oxygen as an oxidant under atmosphere pressure. The catalyst shows a high conversion of alkyl aromatics and selectivity to benzylic ketones, and could be reused at least 5 times without significant loss of catalytic activity.

Non-metallic gold nanoclusters for oxygen activation and aerobic oxidation

In recent decade, Au nanoclusters of atomic precision （AunLm, where L= organic ligand： thiolate andphosphine） have been shown as a new promising nanogold catalyst. The well-defined AunLm catalystspossess unique electronic properties and frameworks, providing an excellent opportunity to correlate theintrinsic catalytic behavior with the cluster＇s framework as well as to study the catalytic mechanismsover gold nanoclusters. In this review, we only demonstrate the important roles of the gold nanoclustersin the oxygen activation （e.g., 302 to 102） and their selective oxidations in the presence of oxygen （e.g., COto C02, sulfides to sulfoxides, alcohol to aldehyde, styrene to styrene epoxide, amines to imines, andglucose to gluconic acid）. The size-specificity （Au25 （1.3 nm）, Au38 （].5 nm）, Au144 （1.9 nm）, etc.）, ligandengineering （e.g., aromatic vs aliphatic）, and doping effects （e.g., copper, silver, palladium, and platinum）are discussed in details. Finally, the proposed reactions＇ mechanism and the relationships of clusters＇structure and activity at the atomic level also are presented.

Direct synthesis of nitriles by Cu/DMEDA/TEMPO-catalyzed aerobic oxidation of primary amines with air

By screening the copper catalysts,ligands,and the reaction conditions,a simple CuCl/DMEDA/TEMPO catalyst system readily available from commercial sources is developed for a direct and selective synthesis of the useful nitriles by an aerobic oxidation reaction of primary amines using air as an advantageous oxidant under mild conditions.