Metal-free Oxidation Research Articles

Cost effective, metal free reduced graphene oxide sheet for high performance electrochemical capacitor application

A simple, cost-effective, and large-scale production of metal-free self-assembled graphene oxide sheets (GOS) with an area of 15 × 15 cm is synthesized using a modified Hummer's method. The reduced graphene oxide sheet (rGOS) was prepared by heat-treating GOS at a range of 300 °C in a hydrogen atmosphere. The XRD results show that GOS has an interlayer distance of 0.79 nm and the rGOS has an interlayer distance of 0.34 nm. The electrochemical analysis of rGOS and GOS was investigated. The porous structure, large-size sheet, and fast transit kinetics of electrochemical analysis and electrons on the interface of electrolyte/electrode contributed to the rGOS showing the high performance of an electrochemical capacitor. The GOS has a specific capacitance value of 54 F/g at 1 A/g, while the reduced GOS has a higher specific capacitance value (469 F/g at 1 A/g), which is 8.6-fold higher than the GOS and has excellent cycle stability. Heat-treated rGOS is regarded as a strong, cost-effective, and scalable electrochemical capacitor material.

STUDY ON THE SYNTHESIS OF UREA-HYDROGEN PEROXIDE AND APPLICATION FOR OXIDATION OF BENZYLIC ALCOHOLS UNDER MICROWAVE ACTIVATION

The Urea-Hydrogen Peroxide complex was synthesized from urea, hydrogen peroxide by using microwave assisted method and characterized by FT-IR. This Urea-Hydrogen Peroxide complex could be employed as an oxidizing agent for metal-free oxidation reaction of benzylic alcohols to carboxylic acids corresponding. This oxidation protocol, carried out under alkaline-water in microwave oven in presence of iodine as a catalyst, was efficient and compatible with a range of benzylic alcohols to afford a direct access to benzoic derivatives in very good yields up to 95.4% and high selectivities.

Fabrication of novel Mn0.43Cd0.57S/ZnCo2O4 p-n heterojunction photocatalyst with efficient charge separation for highly enhanced visible-light-driven hydrogen evolution

The construction of efficient and low-cost photocatalytic systems is a major challenge in the field of photocatalysis. An efficient heterojunction photocatalyst designed for water splitting is also an interesting prospect in energy conversion. In this study, a novel Mn0.43Cd0.57S/ZnCo2O4 p-n heterojunction photocatalyst was prepared via a simple solvothermal method. Mn0.43Cd0.57S was loaded on the surface the of noble metal-free transition metal oxide ZnCo2O4 to prevent the agglomeration of particles, resulting in efficiently improving the photocatalytic performance of the material. Due to the Fermi level balance, the band bending of the two materials constructs a p-n heterojunction, which stimulates the transfer of photogenerated electrons from the CB of Mn0.43Cd0.57S to the CB of ZnCo2O4. The uniformly dispersed p-n heterojunction structure effectively suppresses the recombination of photoinduced charges and preserves stronger redox charges. As a result, the established Mn0.43Cd0.57S/ZnCo2O4 heterojunction showed the best photocatalytic hydrogen production concentration (92.1 mmol g−1 h−1), which was 4.7 times that of the original Mn0.43Cd0.57S (19.4 mmol g−1 h−1) and 54.2 times that of ZnCo2O4 (1.7 mmol g−1 h−1). This strategy will provide new insights into the development of new photocatalysts.

Gram scale, metal-free and selective aerobic oxidation of alcohol and alkyl benzenes using homogeneous recyclable TAIm[MnO4] oxidative ionic liquid under mild conditions: Microwave/ ultrasonic-assisted to carboxylic acid

A homogeneous catalyst-free system has been developed for the selective oxidation of alcohols and alkyl benzenes to the corresponding carbonyl compounds using a recyclable TAIm[MnO4] oxidative ionic liquid. The reactions were performed in the presence of previously reported TAIm[MnO4]/ TAIm[OH] ionic liquids and KMnO4 at ambient temperature under solvent-free conditions. Benzyl alcohols were selectively transferred to benzaldehydes, while under ultrasonic or microwave irradiation, they oxidized to benzoic acid selectively and directly. Scalability of the process also studied at high scale. Also, the ionic liquid could be readily recycled from the aqueous mixture and reused for several consecutive times without any pre-treatment and loss of catalytic activity. In this work, the highly active and toxic KMnO4 salt has been transformed into a mild recyclable oxidant reagent with high selectivity towards alcohol oxidation.

Acetylene/Vinylene-Bridged π-Conjugated Covalent Triazine Polymers for Photocatalytic Aerobic Oxidation Reactions under Visible Light Irradiation.

Solar-driven photocatalytic chemical transformation provides a sustainable strategy to produce valuable feedstock, but designing photocatalysts with high efficiency remains challenging. Herein, two acetylene- or vinylene-bridged π-conjugated covalent triazine polymers, A-CTP-DPA and V-CTP-DPE, were successfully fabricated toward metal-free photocatalytic oxidation under visible light irradiation. Compared to the one without acetylene or vinylene bridge, both resulting polymers exhibited superior activity in photocatalytic selective oxidation of sulfides and oxidative coupling of amines; in particular, A-CTP-DPA delivered an optimal photocatalytic performance. The superior activity was attributed to the broadened spectral response range, effective separation, rapid transportation of photogenerated charge carriers, and abundant active sites for photogenerated electrons due to the existence of the acetylene bridge in the framework. This work highlights the potential of acetylene and vinylene bridges in tuning catalytic efficiency of organic semiconductors, providing a guideline for the design of efficient photocatalysts.

Fabrication of P-Doped Porous Carbon Catalysts, with Inherent N Functionality, from Waste Peanut Shells and Their Application in the Metal-Free Aerobic Oxidation of Alcohols

Based on low-cost, N-containing biomass waste peanut shells and phosphoric acid, the P element was successfully introduced into the carbon skeleton to obtain P-doped porous carbon materials, with inherent N functionality. In comparison to carbon catalysts functionalized with just N or P (NC-800 or PC), the as-prepared P-doped, N-containing porous carbon materials (NPC-T) possessed superior catalytic activity toward the aerobic oxidation of benzyl alcohol (BA). According to these findings, a synergistic effect of N and P species on the catalytic efficiency of carbon materials was observed. The amounts of graphitic N and C3PO species on the surface of NPC-T were shown to be correlated with catalytic activity. In the case of NPC-800, 100% conversion and 99.7% selectivity could be achieved along with a high TOF value (2.49 × 10–3 mol·g–1·h–1). Even when applied to various alcohols, the NPC-800 still showed good catalytic activity. Based upon the mechanistic investigations, both radical and nonradical reaction pathways occurred during the aerobic oxidation of BA over the NPC-800. Furthermore, to facilitate catalyst recycling, the granular P-doped, N-containing carbon (granular NPC-800) was generated, utilizing a kneading heat treatment and extrusion molding technique that did not require the addition of any extra adhesive. This work highlighted the potential high-value utilization of biomass waste in constructing P-doped, N-containing carbon materials and the application as metal-free catalysts in the aerobic selective oxidation reactions.

Direct synthesis of triphenylamine-based ordered mesoporous polymers for metal-free photocatalytic aerobic oxidation

Triphenylamine functionalized ordered mesoporous polymers were synthesized by an evaporation induced self-assembly (EISA) approach and were used as reusable heterogeneous photocatalysts for metal-free photocatalytic aerobic oxidation.

Metal-free aerobic oxidation of benzyl alcohols over the selective N, P dual-doped hollow carbon sphere as the efficient and sustainable heterogeneous catalyst under mild reaction condition

Nitrogen and phosphorous dual-doped carbon materials are one the promising candidate for the ecofriendly and metal-free heterogeneous catalytic systems owing to the abundance activated sites which is caused by doping the heteroatoms in the carbonaceous materials. The N, P-dual-doped carbon hollow sphere (HPNC-700) precursor prepared via template-assisted strategy by using the SiO 2 as the hard template in the presence of glucose, phosphoric acid and melamine as C, P, and N source, respectively. The resultant catalyst was obtained by pyrolyzing process at 700 °C under N 2 atmosphere by adding the NaOH solution to remove the template. According to various physicochemical analysis including FE-SEM, TEM, XRD, XPS, and Raman spectroscopy, the catalyst exhibited the desired and acceptable properties. So, based on these outcomes, the catalytic efficiency of the HPNC-700 was evaluated in the selective aerobic oxidation of benzyl alcohols. As expected, the HPNC-700 represented the acceptable proficiency in the mentioned reaction with high TOF value, full conversion (>99%) and also the notable recyclability. • P, N-dual doped carbon hollow spheres were designed for selective aerobic oxidation of benzyl alcohols. • SiO 2 spheres were used as the hard templet to synthesis the hollow spheres. • The heterogeneous metal-free HPNC-700 catalyst represents the high efficiency toward oxidation reaction at mild condition. • The more defects in the carbonaceous caused the more efficiency in the catalytic performance. • By doping The P and N in the sructure of carbon, the activated sites were developed in the synthesized catalyst.

PIFA-Mediated Tandem Hofmann-Type Rearrangement and Cyclization Reaction of α-Acyl-β-aminoacrylamides: Access to Polysubstituted Oxazol-2(3H)-ones.

An efficient and straightforward synthesis of polysubstituted oxazol-2(3H)-ones has been developed via a tandem Hofmann-type rearrangement and cyclization reaction of various α-acyl-β-aminoacrylamides mediated by phenyl iodine(III) bis(trifluoroacetate) (PIFA) in the presence of trifloroacetic acid (TFA). This novel protocol features readily available starting materials, mild reaction conditions, simple execution, high chemoselectivity, good functional group tolerance, and a metal-free oxidation process.

One-Pot Direct Oxidation of Primary Amines to Carboxylic Acids through Tandem ortho-Naphthoquinone-Catalyzed and TBHP-Promoted Oxidation Sequence.

Biomimetic oxidation of primary amines to carboxylic acids has been developed where the copper-containing amine oxidase (CuAO)-like o-NQ-catalyzed aerobic oxidation was combined with the aldehyde dehydrogenase (ALDH)-like TBHP-mediated imine oxidation protocol. Notably, the current tandem oxidation strategy provides a new mechanistic insight into the imine intermediate and the seemingly simple TBHP-mediated oxidation pathways of imines. The developed metal-free amine oxidation protocol allows the use of molecular oxygen and TBHP, safe forms of oxidant that may appeal to the industrial application.

Hydrogen peroxide based oxidation of hydrazines using HBr catalyst

Azo compounds (RN = NR′) are an important class of organic molecules that find wide application in organic synthesis. Herein, we report an efficient, practical and metal-free oxidation of hydrazines (RNH-NHR’) to azo compounds using 5 mol% HBr and hydrogen peroxide as terminal oxidant. This new method has been demonstrated by 40 examples with excellent yields. In addition, we showcased two examples of the one-pot sequential reactions involving our hydrazine oxidation/hydrolysis/Heck reaction or Cu-catalyzed N-arylation with aryl boronic acid. The distinct advantages of this protocol include metal-free catalysis, waste prevention, and easy operation.

Mechanochemically Prepared Co3O4-CeO2 Catalysts for Complete Benzene Oxidation

Considerable efforts to reduce the harmful emissions of volatile organic compounds (VOCs) have been directed towards the development of highly active and economically viable catalytic materials for complete hydrocarbon oxidation. The present study is focused on the complete benzene oxidation as a probe reaction for VOCs abatement over Co3O4-CeO2 mixed oxides (20, 30, and 40 wt.% of ceria) synthesized by the more sustainable, in terms of less waste, less energy and less hazard, mechanochemical mixing of cerium hydroxide and cobalt hydroxycarbonate precursors. The catalysts were characterized by BET, powder XRD, H2-TPR, UV resonance Raman spectroscopy, and XPS techniques. The mixed oxides exhibited superior catalytic activity in comparison with Co3O4, thus, confirming the promotional role of ceria. The close interaction between Co3O4 and CeO2 phases, induced by mechanochemical treatment, led to strained Co3O4 and CeO2 surface structures. The most significant surface defectiveness was attained for 70 wt.% Co3O4-30 wt.% CeO2. A trend of the highest surface amount of Co3+, Ce3+ and adsorbed oxygen species was evidenced for the sample with this optimal composition. The catalyst exhibited the best performance and 100% benzene conversion was reached at 200 °C (relatively low temperature for noble metal-free oxide catalysts). The catalytic activity at 200 °C was stable without any products of incomplete benzene oxidation. The results showed promising catalytic properties for effective VOCs elimination over low-cost Co3O4-CeO2 mixed oxides synthesized by simple and eco-friendly mechanochemical mixing.

Metal-Free Chemoselective Oxidation of 4-Methylquinolines into Quinoline-4-Carbaldehydes.

A convenient protocol for the synthesis of quinoline-4-carbaldehydes via chemoselective oxidation of 4-methylquinolines using hypervalent iodine(III) reagents as oxidant is described. This method highlights metal-free and mild reaction conditions, nice yield, good functional group tolerance, and high chemoselectivity.

Covalent Triazine Framework as an Efficient Photocatalyst for Regeneration of NAD(P)H and Selective Oxidation of Organic Sulfide.

Covalent triazine frameworks (CTFs), belonging to the super-family of covalent organic frameworks, have attracted significant attention as a new type of photosensitizer due to the superb light-harvesting ability and efficient charge transfer originating from the large surface area. However, the wide optical band gap in CTFs, which is larger than 3.0 eV, hinders the efficient light harvesting in the visible range. To overcome this limitation, we developed the new type CTFs photocatalyst based on the donor-acceptor conjugation scheme by using melamine (M) and 2,6-diaminoanthraquinone (AQ) as monomeric units. The melamine-2,6-diaminoanthraquinone-based covalent triazine frameworks (M-AQ-CTFs) photocatalyst shows the excellent light-harvesting capacity with high molar extinction coefficient, and the suitable optical band gap involving the internal charge transfer character. Combination of M-AQ-CTFs and artificial photosynthetic system including the organometallic rhodium complex, acting as an electron mediator, exhibited the excellent photocatalytic efficiency for the regeneration of the nicotinamide cofactors such as NAD(P)H. In addition, this photocatalyst showed the high photocatalytic efficiency for the metal-free aerobic oxidation of sulfide. This study demonstrates the high potential of CTFs photocatalyst with the donor-acceptor conjugated scheme can be actively used for artificial photosynthesis.

Generation of hydroxyl radicals by metal-free bifunctional electrocatalysts for enhanced organics removal

Low yields of H2O2 and a narrow range of appropriate pH values have been two major drawbacks for electro-Fenton (EF) process. Herein, metal-free electrochemical advanced oxidation processes (EAOPs) were developed with nitrogen and sulfur co-doped electrochemically exfoliated graphene (N, S-EEGr) electrocatalysts, which was confirmed as an outstanding bifunctional catalyst for synchronous generation and activation of H2O2 via (2 + 1) e− consecutive reduction reactions. Specifically, two elements (N, S) in metal-free N, S-EEGr-CF cathode synergize to promote the formation of H2O2 followed by its activation. With N, S-EEGr-CF cathode, phenol of initial 50 mg L−1 could be effectively removed within pH 3–11 and 6.25 mA cm−2, and 100% removal efficiency could be achieved within 15-min even at neutral pH. The pseudo-first-order rate constant for phenol removal in metal-free EAOPs with N,S-EEGr-CF at neutral pH was 10 times higher than that with EF process. Detection of active species, coupled with decay kinetics with specific trapping agents, confirmed that OH was the dominant oxidizing species promoting removal efficiencies of organics (phenol, antibiotics and dyes) at pH 3 and pH 7. In the actual wastewater treatment, the synergistic effect of bifunctional catalyst would also be used for improving the degradation efficiency of organics. Thus, the metal-free EAOPs with N,S-EEGr-CF cathode may serve as an alternative in wastewater treatment with a broadened range of solution pH values and avoiding Fe2+ (catalyst) addition.

Wastewater treatment nexus: Carbon nanomaterials towards potential aquatic ecotoxicity

Carbon nanomaterials (CNMs) provide an effective solution and a novel advancement for wastewater treatment. In this review, a total of 3823 bibliographic records derived from recent 10 years are visualized based on scientometric analysis. The results indicate metal-free CNMs-mediated advanced oxidation processes (AOPs) might be a motive force to develop CNMs application for wastewater treatment; however, corresponding evaluations of aquatic toxicity still lack sufficient attention. Therefore, recent breakthroughs and topical innovations related to prevalent wastewater treatment technologies (i.e., adsorption, catalysis and membrane separation) using three typical dimensional CNMs (nanodiamonds, carbon nanotubes, and graphene-based nanomaterials) are comprehensively summarized in-depth, along with a compendious introduction to some novel techniques (e.g., computational simulation) for identifying reaction mechanisms. Then, current research focusing on CNMs-associated aquatic toxicity is discussed thoroughly, mainly demonstrating: (1) the adverse effects on aquatic organisms should not be overlooked prior to large-scale CNMs application; (2) divergent consequences can be further reduced if the ecological niche of aquatic organisms is emphasized; and (3) further investigations on joint toxicity can provide greater beneficial insight into realistic exposure scenarios. Finally, ongoing challenges and developmental directions of CNMs-based wastewater treatment and evaluation of its aquatic toxicity are pinpointed and shaped in terms of future research.

Efficient Aerobic Oxidation of Organic Molecules by Multistep Electron Transfer.

This Minireview presents recent important homogenous aerobic oxidative reactions which are assisted by electron transfer mediators (ETMs). Compared with direct oxidation by molecular oxygen (O2), the use of a coupled catalyst system with ETMs leads to a lower overall energy barrier via stepwise electron transfer. This cooperative catalytic process significantly facilitates the transport of electrons from the reduced form of the substrate‐selective redox catalyst (SSRCred) to O2, thereby increasing the efficiency of the aerobic oxidation. In this Minireview, we have summarized the advances accomplished in recent years in transition‐metal‐catalyzed as well as metal‐free aerobic oxidations of organic molecules in the presence of ETMs. In addition, the recent progress of photochemical and electrochemical oxidative functionalization using ETMs and O2 as the terminal oxidant is also highlighted. Furthermore, the mechanisms of these transformations are showcased.

The synergistic effect in metal-free graphene oxide coupled graphitic carbon nitride/light/peroxymonosulfate system: Photothermal effect and catalyst stability

The combination of sulfate radical-mediated advanced oxidation process (SA-AOP) and photocatalysis is researched as a highly efficient strategy for pollutant degradation in recent decades. To systematically study the synergistic effect between photocatalysis and peroxymonosulfate (PMS) induced SA-AOP, graphene oxide (GO) coupled graphitic carbon nitride (CN) composites were synthesized as catalyst. About 98% of bisphenol A was decomposed in CNGO/PMS/light system, which was significantly higher than that in CNGO/PMS or CNGO/light system, demonstrating the strong synergistic effect between photocatalysis and SA-AOP. CNGO was deactivated in catalyst/PMS system, but not in catalyst/PMS/light system, showing that the deactivation of CNGO in the presence of sulfate radicals was inhibited by the photocatalysis. The photogenerated electrons may directly reduce the oxidized GO, or react with the oxidative species which may oxidize GO, and thus maintain the activity of CNGO. In addition, the system temperature was elevated due to the light-to-heat conversion on GO. The increased reaction temperature accelerated the degradation reaction due to the improved the charge separation and molecular collision. This work explains the synergy effect between photocatalysis and SA-AOP from different perspectives, and provides a new idea to improve the reusability of carbonaceous catalyst.

Light-triggered elimination of CO2 and absorption of O2 (artificial breathing reaction) in photolysis of 2-(4-nitrophenyl)-1H-indole derivatives.

A new chromophore, 2-(4-nitrophenyl)-1H-indole (NPI), was synthesized as a potential photolabile protecting group. Caged benzoic acids featuring the NPI chromophore were synthesized as model compounds. Benzoic acid was released in moderate yields (~ 40-60%) upon photolysis of the caged benzoic acids without any additional chemical reagents. Interestingly, an aldehyde, 1-(5-(1-formyl-1H-indol-2-yl)-2-nitrophenyl)ethyl benzoate, was isolated in ≈ 20% together with benzoic acid (≈ 40%) in photolysis of a caged benzoic acid, 2-(2-(3-(1-(benzoyloxy)ethyl)-4-nitrophenyl)-1H-indol-1-yl)acetic acid. The functional group, CH2COOH, at the indole nitrogen was transformed into the aldehyde group, CHO, under photolysis conditions in air. The similar photochemical transformation was observed in the photolysis of 2-(2-(4-nitrophenyl)-1H-indol-1-yl)acetic acid, in which the benzoate group is not attached at the nitrophenyl ring. Products analysis, transient absorption spectroscopy, and computational study suggested that intramolecular electron transfer is key for the elimination of CO2 and absorption of O2 for the formation of the aldehyde. The artificial breathing-type reaction can apply to transition metal-free oxidation of amino acids under mild conditions.

Synthesis of Xanthones, Thioxanthones and Acridones by a Metal-Free Photocatalytic Oxidation Using Visible Light and Molecular Oxygen.

9H-Xanthenes, 9H-thioxanthenes and 9,10-dihydroacridines can be easily oxidized to the corresponding xanthones, thioxanthones and acridones, respectively, by a simple photo-oxidation procedure carried out using molecular oxygen as oxidant under the irradiation of visible blue light and in the presence of riboflavin tetraacetate as a metal-free photocatalyst. The obtained yields are high or quantitative.