Metal-free Activation Research Articles

Nonmetallic nitrogen doped hydrothermal carbon driven by ultrasonic mechanochemical action for activating (mono- and dual-) persulfate: Comparison of activation performance and reaction mechanism

A pure non-metallic catalyst, the nitrogen doped hydrothermal carbon (NHCs) which can be comparable to zero-valence-iron, was designed for activation of peroxymonosulfate (PMS) and peroxydisulfate (PDS) assisted by ultrasonic-mechanochemical action. NHCs were prepared via hydrothermal and heat treatments with N sources (melamine derived carbon nitride) and C sources (glucose derived hydrothermal carbon), and the range of preparation temperature of efficacious NHCs was around 400–800 °C. The excess N source (carbon nitride/glucose at 2.00 g/0.50 g) can observably improve the activation ability of NHCs. Especially, the optimal NHCs under ultrasonic driving showed efficient catalytic capability for PMS, 10 mg NHCs could effectively activate 0.75 mmol PMS (the mass ratio at 1:25) for > 90 % removal of contaminant within 10 min (kapp = 0.3653 min−1). Mono- and dual-persulfates (PMS and PDS) with different reactivities led to different reaction mechanisms in metal-free carbocatalysis driven by the physical field. The NHCs particles could change the ultrasonic-field propagation. Afterwards, besides non-radical process (e.g., singlet oxygen (1O2)), 10−2 g/L level NHCs served as the electron shuttle (ES) to place emphasis on electron-transfer process (ETP) in activating PMS while 10−1 g/L level NHCs showed forced adsorption balance and tended to form persistent free radicals (PFRs) in activating PDS due to the ultrasonic mechanochemical action. This study supplements insightful understanding of the differences in metal-free activation, namely, NHCs materials without metal ion pollution toward PMS and PDS activation for water purification.

Metal-Free Activation of Molecular Oxygen by Quaternary Ammonium-Based Ionic Liquid: A Detail Mechanistic Study.

Most oxidation processes in common organic synthesis and chemical biology require transition metal catalysts or metalloenzymes. Herein, we report a detailed mechanistic study of a metal-free oxygen (O2) activation protocol on benzylamine/alcohols using simple quaternary alkylammonium-based ionic liquids to produce products such as amide, aldehyde, imine, and in some cases, even aromatized products. NMR and various control experiments established the product formation and reaction mechanism, which involved the conversion of molecular oxygen into a hydroperoxyl radical via a proton-coupled electron transfer process. Detection of hydrogen peroxide in the reaction medium using colorimetric analysis supported the proposed mechanism of oxygen activation. Furthermore, first-principles calculations using density functional theory (DFT) revealed that reaction coordinates and transition state spin densities have a unique spin conversion of triplet oxygen leading to formation of singlet products via a minimum energy crossing point. In addition to DFT, domain-based local pair natural orbital coupled cluster, (DLPNO-CCSD(T)), and complete active space self-consistent field, CASSCF(20,14) methods complemented the above findings. Partial density of states analysis showed stabilization of π* orbital of oxygen in the presence of ionic liquid, making it susceptible to hydrogen abstraction in a mild, metal-free condition. Inductively coupled plasma atomic emission spectroscopic (ICP-AES) analysis of reactant and ionic liquids clearly showed the absence of any significant transition metal contamination. The current results described the origin of O2 activation within the context of molecular orbital (MO) theory and opened up a new avenue for the use of ionic liquids as inexpensive, multifunctional and high-performance alternative to metal-based catalysts for O2 activation.

Metal-Free Activation of Molecular Oxygen by 9-Fluorenone-Based Porous Organic Polymers for Selective Aerobic Oxidation.

Oxygen activation is a critical step in heterogeneous oxidative processes, particularly in catalytic, electrolytic, and pharmaceutical applications. Among the various catalysts available for photocatalytic O2 activation, homogeneous aryl ketones are at the forefront. To avoid the degradation and deactivation of aryl ketones, 9-fluorenone-based porous organic polymers were designed and regulated by doping them with co-monomers. The obtained heterogeneous photocatalyst showed good performance in O2 activation, and its performance was better than that of homogeneous 9-fluorenone. The obtained heterogeneous photocatalyst showed good reusability. We believe that the presented method and findings represent an important step toward designing catalysts tailored for specific tasks.

Nitrogen-Doped Graphdiyne Nanotubes for Metal-Free Activation of Peroxymonosulfate and Enhanced Degradation of Recalcitrant Heterocyclic Contaminants

Nitrogen-Doped Graphdiyne Nanotubes for Metal-Free Activation of Peroxymonosulfate and Enhanced Degradation of Recalcitrant Heterocyclic Contaminants

Metal-Free Activation of Sulfite by Benzoquinone-Derived Carbon for Efficient Organic Contaminant Degradation: Identification and Regulation of Active Sites

Sulfite [S(IV)]-based advanced oxidation processes (AOPs) driven by carbon materials provide an environmentally friendly, economical route for environmental remediation. However, the active sites and involved radical mechanisms are still controversial due to the complexity of the carbon structure, thus greatly hampering the development of highly efficient systems. Herein, a benzoquinone-derived carbon (BQC)/S(IV) system was developed for organic pollutant degradation in an acidic medium for the first time. The quinone groups and conductivity were confirmed as the key factors for S(IV) activation through cyclic voltammetry, antitheses, specific site masking, and quantitative structure–activity relationship methods. Benefiting from the high content of quinone groups and excellent conductivity, the BQC catalyst exhibited much more effective in activating Na2SO3 for organic pollutant degradation compared to most traditional carbon or even metal catalysts. The chronopotentiometry and in situ Raman results revealed that the formation of C–S(IV)* complexes was the primary step to produce SO3·–, followed by the production of SO4·– via a chain reaction, in which the SO4·– radical was responsible for pollutant degradation. The findings may provide novel insights into reaction mechanisms in S(IV)-AOP systems and pave the way toward highly efficient carbon catalysts to activate S(IV) for the elimination of organic contaminants.

Recent development of stereoselective C-glycosylation via generation of glycosyl radical

C-glycosylation through the formation of glycosyl radical is a very feasible and expedient approach for making of the C–C anomeric bond. In this light, this review article is covering selected literature from 2000 to 2022 on the synthetic development of C-glycosylation involving the generation of glycosyl radical as a crucial intermediate in a stereoselective fashion. The generation of glycosyl radical is controlled by the reaction method applying to the radical precursor. Those methods are categorized in terms of a key source of an initiator such as metal-free activation of radical precursor, photochemical induced approach, and metal-mediated approach including transition metal-mediated, lanthanide, and non-transition metal-mediated approach with brief mechanistic discussion, and stereochemical outcome of resultant C-glycosides in detail. This brief discussion accumulates the stereoselective synthesis of challenging C-linked building blocks in a more facile way via generation of glycosyl radical and those building blocks have a great role as drugs, inhibitors, glycoconjugates in the field of biological and medicinal chemistry.

Metal-free activation of peroxymonosulfate by boron and nitrogen co-doped graphene nanotubes for catalytic oxidation of 4-hydroxybenzoic acid

Recently metal-free catalysts become very promising in environmental catalysis own to the nature of being free of metals for avoiding metal leaching-related secondary contamination. Herein, a series of boron and nitrogen co-doped graphene nanotubes were first synthesised by thermal treatment of urea, boric acid, and polyethene glycol (PEG, 2000). The materials fabricated under varied thermal conditions, e.g., different pyrolysis temperature and retention time, were characterised through advanced physiochemical techniques. The as-prepared materials showed outstanding catalytic activity for degradation of 4-hydroxybenzoic acid (HBA) via peroxymonosulfate (PMS) activation, whereas the catalyst pyrolysed at 1100 ​°C for 6 ​h (BNG-1100-6h) was found to be the best candidate for environmental remediation, thanks to its engineered surface, exposed active sites, and well-tuned functional groups. Based on the optimal carbocatalyst, reaction conditions such as catalyst loading, PMS dosage, solution pH, and reaction temperature were thoroughly investigated to make it a cost-effective catalytic system. A thermal regenerative path was adopted to enhance the catalyst stability and reusability. Quenching tests and electron paramagnetic resonance (EPR) spectroscopic analysis further revealed the dominant role of singlet oxygen (1O2), a non-radical reactive species, in the degradation of HBA. The current research work will not only provide a facile strategy for development of a carbocatalytic system but also open a new perspective for degradation of emerging contaminants such as HBA via a non-radical route.

Glycosyl Vinylogous Carbonates as Glycosyl Donors by Metal-Free Activation.

Synthesis of glycoconjugates employs a glycosylation reaction wherein an electrophile and a nucleophile known as a glycosyl donor and an aglycon, respectively, are involved. Glycosyl donors often contain a leaving group at the anomeric carbon that upon reaction with activator(s) results in a highly reactive electrophilic species reported as an oxycarbenium ion contact pair that will then be attacked by the aglycon. Therefore, identification of the correct glycosyl donor and activation protocol is essential for the synthesis of all glycoconjugates. Recently identified [Au]/[Ag]-catalyzed activation of ethynylcyclohexyl glycosyl carbonates is one such versatile method for the synthesis of glycosides, oligosaccharides, and glycoconjugates. In this work, stable glycosyl vinylogous carbonates were identified to undergo glycosidation in the presence of a sub-stoichiometric quantity of TfOH. The reaction is fast and suitable for donors containing both C2-ethers and C2-esters. Donors positioned with C2-ethers resulted in anomeric mixtures with greater selectivity toward 1,2-cis glycosides, whereas those with C2-esters gave 1,2-trans selective glycosides. The versatility of the method is demonstrated by conducting the glycosylation with more than 25 substrates. Furthermore, the utility of the glycosyl vinylogous carbonate donors is demonstrated with the successful synthesis of the branched pentaarabinofuranoside moiety of the Mycobacterium tuberculosis cell wall.

Interplay of bicarbonate and the oxygen-containing groups of carbon nanotubes dominated the metal-free activation of peroxymonosulfate

Revealing how different physicochemical/texture properties and water matrices affect the carbocatalysis-based oxidation processes (COPs) is critical to develop green remediation methods for wastewater detoxification. However, the interplay of oxygen-containing groups (OCGs) of carbocatalysts and alkalinity on the oxidation behavior of organic pollutants by COPs is rarely documented. In this study, we investigated the mutual effects of OCGs and bicarbonate on the catalytic activation of Peroxymonosulfate (PMS) by carbon nanotubes (CNTs) with respect to the formation of Reactive oxygen species (ROS) and oxidation behavior of ibuprofen. The profiles of the removal kinetics of ibuprofen, degradation products, and product toxicity highly rely on the OCGs and bicarbonate-regulated ROS. The CO groups within CNTs serve as an electron donor to cleave the OO bond of PMS for producing •OH. Increasing the amount of electron-rich CO/COH groups can induce the formation of SO4•−. The electron-rich OCGs deprotonated by bicarbonate promote the PMS activation/decomposition to generate more •OH and/or SO4•−, while the deprotonated electron-deficient OCGs can activate PMS to form 1O2. The HCO3−/CO32− in the bicarbonate-containing CNTs/PMS system can partially quench •OH and SO4•− to produce CO3•−. Despite the diverse ROS controlled by OCGs and bicarbonate, •OH and/or SO4•−-initiated H-atom abstraction reactions are the first steps for the degradation of target contaminant.

Sucrose-derived N-doped carbon xerogels as efficient peroxydisulfate activators for non-radical degradation of organic pollutants

Metal-free activation of peroxydisulfate (PDS) for degrading organic pollutants in water has received increasing attention because it can prevent secondary pollution. However, most of the catalysts that are efficient are derived from non-renewable fossil resources, are very expensive and have complex preparation processes. Also, the emerging non-radical mechanism is still unclear. Herein, 3D sucrose-derived N-doped carbon xerogels (NCXs) were synthesized by a simple and sustainable hydrothermal process and then employed as novel metal-free PDS activators to degrade organic pollutants. The structure, composition and performance of NCXs were regulated by changing the carbonization temperature. The sample carbonized at 900 °C (NCX900) exhibited the best catalytic performance, completely removing bisphenol A in 60 min. Quenching experiments and linear sweep voltammograms demonstrated that PDS was activated mainly through an electron-transfer non-radical mechanism. It was found that graphitic N played a critical role in activating PDS. With this non-radical mechanism, the NCX900/PDS system could adapt well to the wide pH range (3–11) and high Cl− concentration; it selectively oxidized organic pollutants with low ionization potentials. This work provides a sustainable approach to the low-cost and efficient metal-free catalysts for wastewater treatment.

Pyrrolic N-rich biochar without exogenous nitrogen doping as a functional material for bisphenol A removal: Performance and mechanism

Nitrogen configurations doped in carbon matrix serve as the catalytically active sites for the metal-free activation of persulfates (peroxydisulfate PDS, and peroxymonosulfate PMS). Herein, a series of pyrrolic N-rich biochars (BDKx, where x represents the pyrolysis temperature) were facilely prepared by using waste bean dregs as the feedstocks. The resulted BDK900 possessed ultra-high specific surface area (SBET, > 3000 m2 g−1). Accordingly, it exhibited superior catalytic activity in PDS activation, as evidenced by the efficient removal of bisphenol A (BPA, 1358.4 mg g−1) with 0.05 g L−1 BDK900 and 5 mM PDS. Surface-bound radicals and electron-transfer mechanism were discovered to contribute to the BPA oxidation, during which BDK900 played vital roles in PDS activation, BPA accumulation, and regulating electron shuttle from BPA to PDS. Overall, this study sheds new light on the rational design of N-rich biochar and develops a low-cost technology toward remediation of BPA-contaminated wastewater.

Visible light-driven oxidation of arsenite, sulfite and thiazine dyes: A new strategy for using waste to treat waste

Recently, the advanced oxidation processes (AOPs) based on sulfate radical (SO4•⁻) using sulfite has received increasing concerns. Metal-free activation of sulfite is still in urgent demands from the perspective of cleaner production since the reported efficient systems containing sulfite has been relied on the transition metals. In this work, methylene blue (MB⁺), a well-known photosensitizer and thiazine dye contaminant, was utilized to activate sulfite for the efficient photooxidation of arsenite (As(III)) to arsenate (As(V)) under visible light (LED lamps with wavelength 640 nm) and sunlight, respectively. The initial oxidation rates of 5–40 μM As(III) are 28-fold–116-fold as that of the decolorization of 0.27 μM MB⁺ with 50 μM sulfite at pH 7.3. The maximum stoichiometric ratio MB⁺: As(III): sulfite was obtained to be 1 : 142: 247, suggesting an efficient utilization of both MB⁺ and sulfite as waste resources. The electrical energy per order (EE/O) index was calculated to be 1.83 × 10−3 kWh L−1 for the visible light-MB+-sulfite system, which is much less than those reported systems. Oxysulfur radicals (mainly peroxymonosulfate radical, SO5•⁻) produced in this photochemical system are responsible for As(III) oxidation. The other two common thiazine dyes (thionine and Azure B) are also capable of activating sulfite for As(III) oxidation due to the same chromophores. Thus the novel strategy of using waste sulfite and the thiazine dyes is promising for As(III) oxidation under sunlight, with which low-concentration wastes of thiazine dye and sulfite can be utilized to achieve the goal of cleaner production.

Metal-Free Electro-Activated Sulfite Process for As(III) Oxidation in Water Using Graphite Electrodes

Transition-metal-activated sulfite [S(IV)] processes for water decontamination have recently received intense attention in the field of decontamination by advanced oxidation processes (AOPs). However, the drawback with respect to the secondary metal sludge contamination involved in various AOPs has been argued often. In this work, we developed a novel electro-sulfite (ES) process using stable and low-cost graphite electrodes to address that concern. Arsenite [As(III)] was used as the target compound for removal by the ES process because of its wide presence and high toxicity. Parameters, including cell voltage, S(IV) concentration, solution pH, and water matrix, and the mechanisms for reactions on anode and cathode were investigated in electrolytic cells containing one or two compartments, respectively. The results show that the ES process using 1 mM S(IV) and 2 V cell voltage oxidizes 5 μM As(III) at a rate of 0.127 min-1, which is 15-fold higher than mere electrolysis without S(IV) addition (0.008 min-1) at pH 7. Further studies using radical scavengers and electron spin resonance assays demonstrated that oxysulfur radicals (i.e., SO5•- and SO4•-) and HO• are responsible for As(III) oxidation in the ES process. However, HO2• produced via the oxygen reduction reaction in the EO process plays a major role in As(III) oxidation, which explains the lower reaction rate in the absence of S(IV). The effectiveness of the ES process was moreover evidenced by 60-82% As(III) oxidation in field water within 40 min. Overall, this work realizes the metal-free activation of S(IV) and significantly leverages the S(IV)-based water treatment technologies.

Metal-free activation of molecular oxygen by covalent triazine frameworks for selective aerobic oxidation.

Oxygen activation is a critical step in ubiquitous heterogeneous oxidative processes, most prominently in catalysis, electrolysis, and pharmaceutical applications. We present here our findings on metal-free O2 activation on covalent triazine frameworks (CTFs) as an important class of N-rich materials. The O2 activation process was studied for the formation of aldehydes, ketones and imines. A detailed mechanistic study of O2 activation and the role of nitrogen heteroatoms were comprehensively investigated. The electron paramagnetic resonance (EPR) and control experiments provide strong evidence for the reaction mechanism proving the applicability of the CTFs to activate oxygen into superoxide species. This report highlights the importance of a self-templating procedure to introduce N functionalities for the development of metal-free catalytic materials. The presented findings reveal an important step toward the use of CTFs as inexpensive and high-performance alternatives to metal-based materials not only for catalysis but also for biorelated applications dealing with O2 activation.

Insights into the difference in metal-free activation of peroxymonosulfate and peroxydisulfate

Abstract Metal-free activation of peroxymonosulfate (PMS) has been reported to be more efficient than peroxydisulfate (PDS) due to the asymmetrical structure of PMS. However, we report for the first time that the nitrogen-doped porous carbon (NC-900) templated from graphitic carbon nitride exhibits unexpectedly higher catalytic activity in the activation of PDS than PMS, as evidenced by 1.74-fold greater rate constant of NC-900/PDS (0.80 min−1) than that of NC-900/PMS (0.46 min−1). Moreover, NC-900 shows superior reactivity and stability to existing nitrogen-doped nanocarbons in persulfate activation. Experimental and theoretical results revealed that PMS activation involves PMS reduction over the electron-rich graphitic N and PMS oxidation around the electron-deficient carbon atom adjacent to graphitic N in NC-900, in which PMS oxidation is the main reaction rendering singlet oxygen (1O2) the major reactive species. By contrast, PDS reduction over the electron-poor carbon atom dominates PDS activation on NC-900 for hydroxyl radical ( OH) generation. The large amount of OH with stronger oxidation capability than 1O2 contributes to the superiority of NC-900/PDS over NC-900/PMS in organic pollutant degradation. This work provides profound insights into the difference in metal-free PMS and PDS activation and presents fundamental frameworks in the design of advanced carbon-based catalysts toward sustainable environmental remediation.

Metal-Free Activation of C-I Bonds and Perfluoroalkylation of Alkenes with Visible Light Using Phosphine Catalysts.

An efficient metal-free, photomediated iodo perfluoroalkylation under mild conditions was developed. Using catalytic amounts (10 mol %) of phosphines and blue light irradiation, various olefins are transformed into the corresponding addition products within short reaction times. For this purpose, a modular and convenient 3D printed photoreactor was constructed, which is presented as an open source model. The reaction presumably proceeds upon generation of perfluoroalkyl radicals, which are formed by catalyst-induced absorption enhancement.

Metal-Free Activation of N2 by Persistent Carbene Pairs: An Ab Initio Investigation

A metal-free activation process of dinitrogen is reported based on its reaction with two persistent (stable) carbenes. To analyze the reaction mechanism, we first employed methylene as the simplest...

Metal-free activation of persulfates by corn stalk biochar for the degradation of antibiotic norfloxacin: Activation factors and degradation mechanism

This study investigated the degradation of norfloxacin by persulfate activated with corn stalk biochar. The results demonstrate that corn stalk biochar is a green and low-cost carbon material with excellent catalytic activity for the metal-free activation of persulfate to degrade norfloxacin. The removal rate of norfloxacin increased with the increase in the dosage of persulfate, but the increased ratio gradually decreased. With alcohols and phenols as free radical trapping agents, it was determined that norfloxacin degradation by SO- 4· and HO· in the biochar/persulfate system occurred mainly on the surface or boundary layer of biochar. pH has an important effect on norfloxacin degradation in the biochar/persulfate system. Norfloxacin degradation performances under different initial pH conditions are decreased according to the following order: (initial pH 6.5) > (initial pH 3.4) > (initial pH 10.6). The effects of pH on norfloxacin degradation in the biochar/persulfate system was mainly realized through affecting norfloxacin adsorption on corn stalk biochar surface.

Surface Fe(III)/Fe(II) cycle promoted the degradation of atrazine by peroxymonosulfate activation in the presence of hydroxylamine

In this study, we demonstrated that Fe3O4/PMS system in the presence of hydroxylamine (HA) was significantly efficient for atrazine degradation under the near-neutral pH (5.0–6.8) (without buffer). The degradation rate constant of atrazine in Fe3O4/PMS/HA system (0.152 min−1) was 38 times of that (0.004 min−1) in Fe3O4/PMS system and even 4.75 times of that (0.032 min−1) in HA/PMS system. In this Fe3O4/PMS/HA system, the roles of HA were mainly two parts. On one hand, 40% atrazine was decomposed through the activation of peroxymonosulfate (PMS) by HA as a metal-free activator. On the other hand, the addition of HA could highly promote the surface Fe(III)/Fe(II) cycle on the Fe3O4. Meanwhile, the trace dissolved Fe2+ was not the major reason for the atrazine degradation. The reason for different atrazine degradation efficiencies under various aeration conditions was analyzed, which showed that the reduced molecular oxygen was more conducive to accelerate the regeneration of surface Fe(II). Subsequently, the transformation products of HA under different aeration conditions were monitored. What’s more, the reactive species were detected by electron paramagnetic resonance (EPR) and quenching experiments, which revealed that both sulfate radical (SO4•-) and hydroxyl radical (HO•) were responsible for the atrazine degradation, especially sulfate radical. Finally, the reaction mechanism of Fe3O4/PMS/HA system based on the Fe(III)/Fe(II) cycle and the metal-free activation was proposed according to the comprehensive analysis. This study provides an efficient degradation of atrazine organic pollutant in water by the heterogeneous Fenton-like system.

Rational design of an argon-binding superelectrophilic anion

Chemically binding to argon (Ar) at room temperature has remained the privilege of the most reactive electrophiles, all of which are cationic (or even dicationic) in nature. Herein, we report a concept for the rational design of anionic superelectrophiles that are composed of a strong electrophilic center firmly embedded in a negatively charged framework of exceptional stability. To validate our concept, we synthesized the percyano-dodecoborate [B12(CN)12]2-, the electronically most stable dianion ever investigated experimentally. It serves as a precursor for the generation of the monoanion [B12(CN)11]-, which indeed spontaneously binds Ar at 298 K. Our mass spectrometric and spectroscopic studies are accompanied by high-level computational investigations including a bonding analysis of the exceptional B-Ar bond. The detection and characterization of this highly reactive, structurally stable anionic superelectrophile starts another chapter in the metal-free activation of particularly inert compounds and elements.