Metal-free Carbocatalysts Research Articles

Novel Graphitic Oxynitrides as Photocatalysts for Sustainable H2 Production and CO2 Valorization. The Importance of Self-Assembly for Catalytic Activity.

The field of carbocatalysis, often portrayed by paradigmatic graphitic carbonaceous structures, has become a booming topic tailored for multiple applications. To this end, a new metal-free carbocatalyst has been constructed from simple prebiotic monomers such as cyanamide and glyoxal. The resulting material shows an excellent performance as photocatalyst for H2 production and CO2 valorization, thus unveiling its real value to tackle sustainable goals. The unique oxygen-rich carbonaceous structure has been characterized in detail, which is consistent with a graphitic layered network. The described performance in two major societal concerns along with a facile preparation from C1/C2 platforms, makes this type of overlooked oxynitride carbocatalysts promising for real-life environmental endeavors.

Understanding the active sites and associated reaction pathways of metal-free carbocatalysts in persulfate activation and pollutant degradation

Environmental remediation based on peroxomonosulfate- and peroxydisulfate-activation-enabled radical/non-radical oxidation has gained growing interest due to the strong oxidation power, long life of reactive oxygen species, and wide pH adaptability.

Preventing re-emission of malodorous dimethylamine via tunable nonradical oxidation catalyzed by waste mask-derived carbocatalyst

The re-emission of dimethylamine (DMA), as a famous malodorous volatile organic compound (VOC), exhausted from various industries receives emerging concerns. Herein, a metal-free carbocatalyst derived from waste masks was developed to inhibit DMA re-emission via activating peroxymonosulfate (PMS). The optimal inhibition efficiency was around 99%, 85-fold higher than that without any carbocatalysts and PMS. Such enhancement was ascribed to the rapid conversion of DMA (kobs = 0.67 min−1, 4 min) into non-volatile and lower toxic substances (NO3−, NO2− and aliphatic acids) by electron transfer and singlet oxygenation. Experiments combined with correlation analysis and theoretical calculations unveiled that the intensified graphitization degree promoted the electron migration from DMA to the carbocatalyst/PMS* complex due to the enhanced electron transfer rate and elevated redox potential, and PMS preferentially absorbed on the electron-rich C=O and deprotonated COO− groups rather than C-OH and COOH of carbocatalyst and then released SO5− for the generation of 1O2. Based on the theory, tuning the graphitization degree and amount of C=O and COO− groups via a simple annealing strategy triggered a shift from singlet oxygenation to predominantly direct electron transfer pathway, thus achieving optimal suppression efficiency of DMA re-emission. Finally, electrical energy per order (EE/O) analysis demonstrated our technology had a relatively low EE/O value (3.56 kWh/m3) and cost (0.47 $/m3). Considering the high efficiency and low cost of carbocatalytic PMS system endowed with clear mechanism and controllable properties, this work, therefore, pioneers an unparalleled approach for the industrial governance of effluvial VOCs treatment, and provides new insights into the tunable nonradical oxidation mechanism of carbocatalytic PMS activation.

B,F Co-doped carbocatalysts with dual-active-sites for acceptorless dehydrogenation of N-heterocycles under room temperature

Metal-free carbocatalysts have sparked intensive interest in oxidative dehydrogenation. However, efficient metal-free carbocatalysts for non-oxidative dehydrogenation with H2 liberation under mild conditions has rarely been reported. Herein, we successfully fabricated B,F co-doped nitrogen-assembly carbons (BF-NCs), which display outstanding catalytic performance and reaction generality in acceptorless dehydrogenation of N-heterocycles at mild and even at room temperatures, observably outperforming that of undoped NCs or single-doped B-NCs (B-NCs). Specifically, dual-active-sites were achieved in BF-NCs, i.e., the closely-placed graphitic nitrogen (CGN) and the pyridinic N banded BFx. The results of kinetic analysis and DFT directly reveal that the dual-active-sites on carbocatalysts lower the energy barriers for tetrahydroquinoline (THQ) dehydrogenation, thereby accelerating H2 liberation even at room temperature. BF-NC shows promising recyclability and an ability to achieve 98.2% product yield without sacrificing efficacy in a 50-fold scale-up experiment, making the cost-effective and robust carbocatalysts a great contender for H2 liberation in practical industrial application.

Nondissociative activation of O2 for SO2 oxidation on metal-free N-doped carbocatalyst

Oxygen molecules (O2) are generally one of the most important and economic oxidants for a large range of chemical processes. Due to its intrinsically sluggish nature, exploring a suitable catalyst for activation has long been a challenging issue. Based on DFT calculations, we proposed a new mechanism for O2 activation, where O2 could be activated to form an adsorbed peroxide species on a metal-free carbocatalyst, rather than be dissociated to form two adsorbed O atoms as conventional. This kind of activation results in a SO2-O2 coadsorbed intermediate, which separates a high-barrier reaction into two concerted low-barrier (∼ 0.3 eV) reactions. The number of active electrons of the active C sites was proposed to be a descriptor of catalytic activity. Our results indicate a nondissociative mechanism for O2 activation on a metal-free carbocatalyst and provide new insight into SO2 catalytic oxidation at low temperature.

Metal-free carbocatalyst for room temperature acceptorless dehydrogenation of N-heterocycles.

Catalytic dehydrogenation enables reversible hydrogen storage in liquid organics as a critical technology to achieve carbon neutrality. However, oxidant or base-free catalytic dehydrogenation at mild temperatures remains a challenge. Here, we demonstrate a metal-free carbocatalyst, nitrogen-assembly carbons (NCs), for acceptorless dehydrogenation of N-heterocycles even at ambient temperature, showing greater activity than transition metal–based catalysts. Mechanistic studies indicate that the observed catalytic activity of NCs is because of the unique closely placed graphitic nitrogens (CGNs), formed by the assembly of precursors during the carbonization process. The CGN site catalyzes the activation of C─H bonds in N-heterocycles to form labile C─H bonds on catalyst surface. The subsequent facile recombination of this surface hydrogen to desorb H2 allows the NCs to work without any H-acceptor. With reverse transfer hydrogenation of various N-heterocycles demonstrated in this work, these NC catalysts, without precious metals, exhibit great potential for completing the cycle of hydrogen storage.

Metal-free carbocatalysts derived from macroalga biomass (Ulva lactuca) for the activation of peroxymonosulfate toward the remediation of polycyclic aromatic hydrocarbons laden marine sediments and its impacts on microbial community

Potential toxic chemicals, specifically, polycyclic aromatic hydrocarbons (PAHs), are major sediment contaminants. Herein, green seaweed (Ulva lactuca) was used as a feedstock and pyrolyzed at temperature in the range between 300 and 900 °C. The metal-free carbocatalyst (GSBC) for peroxymonosulfate (PMS) activation to degrade PAHs contaminated sediments was studied. The effects of GSBC‒PMS treatment on microbial community abundance was studied as well. The pyrolysis temperature of GSBC preparation affected the PMS activation performance. Results show that GSBC700 exhibited remarkable catalytic characteristics in PAHs degradation by effective activation of PMS. The results also demonstrated that the sulfate radical-carbon-driven advanced oxidation processes (SR-CAOP) reaction achieved 87% and apparent rate constant (kobs) of 6.3 × 10−2 h−1 of total PAHs degradation in 24 h at 3.3 g/L of GSBC, PMS dose of 1 × 10−4 M, and pH 3.0. The degradation of 2-, 3-, 4-, 5-, and 6-ring PAHs was 84, 83, 83, 80, and 89%, respectively. The synergetic effect established between GSBC and PMS enhanced the formation of ROSs, namely, SO4-, HO, and 1O2, which were major species contributing to PAHs degradation. The synergistic effect of π‒π stacking structure and graphitization of GSBC formed electron shuttle, which contributed to PAHs degradation performance. Microbial community structure analyses in the GSBC‒PMS treated sediments showed that the relative abundance of Lactobacillus_rhamnosus species, most of which belonged to the Lactobacillus genus and Firmicutes phylum, which aided in continuing PAHs biodegradation post GSBC‒PMS treatment. Therefore, GSBC can be a promising carbocatalyst produced via biomass-to-biochar conversion as biowaste-to-energy source used in the SR-CAOP-mediated process for sediment remediation.

Graphite oxide catalyzed one-pot synthesis of highly functionalized spirodibenzo[1,4]diazepine derivatives in aqueous ethanol medium

We presented the synthesis of highly functionalized spirodibenzo[1,4]diazepine derivatives using graphite oxide (GO) as a metal-free carbocatalyst via a one-pot three-component strategy.

Nitrogen-modified graphene as a metal-free carbocatalyst for the solvent-free oxidative homo- and heterocoupling of amines

Nitrogen-modified graphene as a metal-free carbocatalyst for the solvent-free oxidative homo- and heterocoupling of amines

Selenium and nitrogen co-doped biochar as a new metal-free catalyst for adsorption of phenol and activation of peroxymonosulfate: Elucidating the enhanced catalytic performance and stability

Coupling of adsorption and advanced oxidation processes triggered by metal-free carbocatalysts is an appealing wastewater purification scheme. However, its practical application is challenging due to the unsatisfactory stability of conventional heteroatom-doped systems. Herein, we innovatively developed a simple and scalable biochemical strategy to synthesize selenium and nitrogen co-doped biochar (Se/N-BC) as a bifunctional catalyst of adsorption-oxidation. The Se/N-BC displays the highest efficiency of phenol (PE) degradation (99.2% of PE was removed within 5 min) with the lowest dosage of catalyst (0.1 g L−1) and peroxymonosulfate (PMS, 0.4 g L−1). More importantly, the Se/N-BC is not only universal in a wide pH range of 3.0–11.0 and complex ionic environment, but also possesses an excellent cycling stability. The Se/N co-doping induces a rapid cycle of adsorption-degradation for PE. The Se/N-BC acts as an "electron transfer bridge", guiding rapid electron transfer from PE to PMS to achieve high-efficient degradation. The Se/N co-doping facilitates the formation of graphitic N and unlocks the potential of adjacent C sites for PMS activation, consequently boost oxidation efficiency. In addition, the oxidation of catalyst is prevented due to the antioxidant properties of Se, which has been a primary concern either to regenerate adsorbate or to enhance degradation performance.

Quinone-amine polymers derived N and O dual doped carbocatalyst for metal-free benzyl alcohol aerobic oxidation

Carbon-based materials are promising environmentally benign heterogeneous catalysts for metal-free catalysis. Aerobic oxidation of benzylic alcohol to aldehydes over carbocatalyst under mild reaction condition is still a great challenge. In the present work, quinone-amine based carbon (QAC) was rationally prepared from the qiunone-amine polymer. The QAC catalyst showed excellent catalytic efficiency for oxidation of benzyl alcohol to benzaldehyde in water. The TOF can reach 29.7 × 10−4 mol∙g-1 h-1 at room temperature under 1 bar O2. To our best knowledge, this is highest activity of metal-free carbocatalyst for the titled reaction. The doped O afforded active sites to enhance activity while doped N offered the radical confinement effect to guarantee the selectivity. The present results demonstrate the QAC catalyst is promising carbon materials for “green catalysis” of aerobic selective oxidation of benzyl alcohol to benzaldehyde.

New functionalization approach synthesis of Sulfur doped, Nitrogen doped and Co-doped porous carbon: Superior metal-free Carbocatalyst for the catalytic oxidation of aqueous organics pollutants

New green synthesis methods of novel commercially applicable carbocatalysts suitable for the catalytic oxidation of organic pollutants, remain a challenge. Herein, Nitrogen doped (N-PC), Sulfur doped (S-PC) and Co-doped (N, S-PC) porous carbon, were synthesized via a new facile environmentally functionalization approach followed by pyrolysis. Thus, Almond Shell (AS) as Bio-sourced material was used for porous carbon (PC) preparation as support. Thereafter, the introduction of Melamine, Diphenyl Disulfide and Thiourea on the PC matrix with high -COCl− and -CCl− groups facilitated the surface functionalization, and allowed us the introduction of high N and S heteroatoms contents in the PC networks surface. It was found that the optimized carbocatalysts could effectively activate PS, and exhibited excellent catalytic performances for the organic pollutants degradation, with an exceptionally low activation energy and fast kinetic reaction. Based on a systematic comparison, it was found that the N-PC exhibited the superior catalytic activity for activating the PS and degrading various organic pollutants. Such superior catalytic activity of the N-PC Carbocatalyst resulted from its high surface area, its low defect degree with the higher content of Graphitic-N. In addition, all experimental and theoretical investigations demonstrated the critical role of graphitic–N for the PS activation. It is expected that the doping process by N will significantly break the chemical inertness of the PC network and will make the N-PC surface more positively charged accelerating hence its interaction with the negatively charged S2O82−. Furthermore, the 1O2 was found to be the predominant reactive oxygen species (ROS) as assessed by radical scavenging tests and EPR experiments. The carbocatalysts were not easily influenced by the water aqueous phase, which give us more obvious advantages over the PS activation.

Ultrasound‐Assisted Synthesis of Nitrogen and Oxygen Containing Heterocycles Using Fluorinated Graphene Oxide as Catalyst: Evaluation of Their Anthelmintic Activities

AbstractSpirooxindole derivatives have been efficiently synthesized via a three‐component reaction consisting of isatin, malononitrile/ pyrazole and various 1,3‐diketones using Fluorinated Graphene Oxide (FGO) as a green and metal free carbocatalyst, under sonication. The main advantages of this synthetic approach lie in its operational simplicity, easy catalyst recovery and recyclability, high yields and short reaction times. The prepared catalyst was characterized by various analytical techniques viz., UV‐Vis, FT‐IR Spectroscopy, TGA, TEM, FE‐SEM and EDX. The in vitro anthelmintic screening of synthesized compounds against a cestode, Raillietina sp. and a nematode parasite, Syphacia obvelata at 200 μg/mL and 800 μg/mL were also carried out which revealed a profound activity, with much reduced morality time of parasite. The SEM study indicated structure alterations in treated parasite, further validating the in vitro anthelmintic property. In addition, molecular docking studies were carried out against β‐tubulin protein to determine binding affinity of the synthesized compounds. All test compound followed the “Lipinski Rule of Five” and in silico pharmacokinetic investigation showed inhibitory activity against CYP enzyme and P‐glycoprotein which may further improve the bio efficacy of compounds.

Persistent free radicals on N-doped hydrochar for degradation of endocrine disrupting compounds

Metal-free carbocatalysts are raising increased interest in advanced oxidation processes (AOPs) for green degradation of aqueous organic pollutants. In the present study, a series of N-doped hydrochars (NHCs) was prepared from pinewood biomass and used in carbocatalysis toward catalytic peroxymonosulfate (PMS) activation to degrade refractory endocrine disrupting compounds. Nitrogen-doping was found to regulate the concentrations and types of persistent free radicals (PFRs) on hydrochar, promoting the electron transfer and oxidative degradation of the contaminants. Mechanistic studies revealed that graphitic N not only shows a strong electron transfer to facilitate PMS activation in a nonradical pathway, but also induces the formation of oxygen-centered PFRs from the structure defects for a radical oxidation pathway. More importantly, the PMS/NHC system demonstrates efficient degradation and satisfactory stability for real water treatment. This study has gained in-depth understanding on the roles of N-dopants in radical and nonradical pathways of biomass-based carbocatalysis, and will provide a promising and environmental-friendly route for efficient degradation of refractory endocrine disrupting compounds for green remediation.

Facile Synthesis of Naphtha-quinoxaline Derivatives from β-lapachone Using Graphene Oxide as Catalyst.

To develop efficient method for the synthesis of naphtha-quinoxaline derivatives via the reaction of β-lapachone with various 1,2-diamines. A mixture of β-lapachone (1mmol), 1,2-diamine (1mmol) and graphene oxide (20mg) in methanol (3mL) was heated at 60°C, under constant stirring for appropriate time. After completion of the reaction, the catalyst was filtered off, washed with ethyl acetate (3x3mL) and the combined filtrate was washed with H2O, dried (anhy. Na2SO4) and concentrated under vacuum. The residue was chromatographed over a column of silica gel eluting with a mixture of hexane and ethyl acetate in different ratios, to afford the desired product. All synthesized compounds were assigned with the help of analytical and 1H, 13C NMR, IR, and mass spectral studies. To establish the catalytic role of GO in the synthesis of naphtha-quinoxaline derivatives, the reaction of β-lapachone with 3,4-diaminotoluene was selected as a model reaction. The catalytic activity of graphene oxide in comparison with other catalysts like acidic resin amberlyst-15 and solid acid catalyst like montmorillonite K-10 were studied. The reaction was also observed in various solvents such as water, acetonitrile, toluene, dichloromethane, ethanol and 1,4-dioxane using GO as a catalyst. Excellent yields were obtained at 60°C in methanol. The efficacy of the present protocol was investigated by the reaction of β- lapachone with other 1,2-diamines. An attractive green metal free carbocatalyst Graphene Oxide (GO) has been successfully utilized for the expedient synthesis of naphtha-quinoxaline derivatives. GO showed high catalytic activity which is attested by the desired products being produced in shorter time. The main advantage of this method is the reusability of the catalyst which makes the procedure sustainable.

Water Promotes the Oxidation of SO2 by O2 over Carbonaceous Aerosols.

Severe haze episodes typically occur with concurrent high relative humidity. Here, the vital role of water in promoting the oxidation of SO2 by O2 on carbonaceous soot surfaces was identified at the atomic level by first-principles calculations. Water molecules can dissociate into surface hydroxyl groups through a self-catalyzed process under ambient conditions. The surface hydroxyl groups, acting as facilitators, can significantly accelerate the conversion of SO2 to SO3 (precursor of particulate sulfate) over soot aerosols by reducing the reaction barriers. Specifically, the hydroxyl groups activate the reactants and stabilize the transition states and products through hydrogen-bonding interactions, making the reactions both thermodynamically and kinetically more favorable at room temperature. The findings indicate that atmospheric humidity plays an important role in enhancing the atmospheric oxidation capacity, thus exacerbating SO2 oxidation and severe haze development. Also, this study unravels a mechanism of surface hydroxyl-assisted O2 and H2O dissociation over metal-free carbocatalysts under normal conditions.

N-doped graphene aerogels as efficient heterogeneous catalytic activators for peroxymonosulfate to remove 2-sec-butyl-4,6-dinitrphenol (DNBP) in aqueous solution

The nitrogen-doped graphene aerogels (NGAs) with interconnected hierarchical porous structures were facilely prepared via hydrothermal process in the mixed aqueous solution of ethylenediamine (EDA) and urea and post-freeze-drying treatment. A series of NGAs samples with different GO:urea:EDA mass ratios were obtained and characterized by Raman, FT-IR, XPS, TEM and SEM. The prepared NGAs were employed as metal-free carbocatalysts in the activation of peroxymonosulfate (PMS) for 2-sec-butyl-4,6-dinitrphenol (DNBP) degradation in aqueous solution. The resultant NGA-4 catalyst with GO:urea:EDA mass ratio of 3:2:4 exhibited the best catalytic activity in PMS activation for oxidative degradation of DNBP among the prepared NGAs samples. The effects of temperature, initial pH value, catalyst concentration and PMS dosage on the degradation of DNBP in NGA-4/PMS system were investigated. The catalytic degradation of DNBP followed the pseudo-first-order kinetic and the activation energy of the degradation reaction was calculated to be 31.3 kJ · mol−1. The quenching experiments clearly demonstrated that SO4·− radicals served as predominant active species in the PMS activation catalytic process. The good applicability made the application of the prepared NGAs materials in PMS activation for removing organic pollutants more practical and feasible.

Graphene oxide: a convenient metal-free carbocatalyst for facilitating amidation of esters with amines

Herein, we have reported a graphene oxide (GO) catalyzed condensation of non-activated esters and amines, that can enable diverse amides to be synthesized from abundant ethyl esters forming only volatile alcohol as a by-product.

Metal-free carbocatalyst for catalytic hydrogenation of N-containing unsaturated compounds

Eco-friendly carbocatalyst is extremely desirable in organic catalysis and wastewater treatment. Herein, we reported a one-pot synchronous hydrothermal process to synthesize a highly efficient N doped holey graphene (NHG) carbocatalyst, in which graphene oxide nanosheets are self-assembled, reduced, chemically etched, and simultaneously doped with nitrogen to form a graphene hydrogel in the presence of H2O2 and NH4OH. Benefiting from the good hydrophilicity, unique hierarchical porous structure, large specific surface area (322.1 m2 g−1), and high N content (9.77 at. %), the resultant NHG carbocatalyst exhibits an excellent catalytic performance for the hydrogenation of N-containing unsaturated compounds. In 4-nitrophenol reduction reaction, the NHG carbocatalyst delivers a turnover frequency of 3.32 × 10−2 min−1. This catalytic activity is superior to that of the commercial Pd/C (5.0 wt%), other previously reported carbocatalysts, and many noble-metal-based catalysts. Furthermore, the molecular structure analysis combined with density functional theory calculations elucidate metal-free catalytic mechanism of hydrogenation reaction for unsaturated chromophore groups in N-containing organic dyes.

N-doped hierarchically porous carbon for highly efficient metal-free catalytic activation of peroxymonosulfate in water: A non-radical mechanism

Metal-free carbo-catalyst has recently emerged as a promising candidate as a substituent for tradition-metal based heterogeneous catalyst for catalytic activation of peroxymonosulfate (PMS). However, most reported carbo-catalysts suffer from low catalytic efficiency and poor stability, thus a high-performance catalyst is urgently desired. In this study, a novel carbo-catalyst (NHPC-800), prepared by using tannic acid and dicyandiamide as renewable carbon/nitrogen feedstocks via a simple pyrolysis route, is reported as an activator of PMS with highly efficient catalytic ability and stability. The as-prepared NHPC-800 possesses as high as 22.4 atom% of nitrogen dopants and a hierarchically porous structure with abundant meso/macropores, accompanied by the abundant edges and wrinkles, which supply sufficient exposed catalytically active centers and fast electrons/mass transportations. Using rhodamine B as a model pollutant, the NHPC-800 shows a highly efficient catalytic ability which is superior to most reported carbo-catalysts and even some state-of-the-art metal catalysts. Based on competitive quenching experiments and electron paramagnetic resonance (EPR) results, a non-radical pathway involving the generation of 1O2 is responsible for the degradation of pollutants. Given that the NHPC-800 shows good recycling performance and strong resistance to adventitious interference such as anions and natural organic matters, we believe NHPC-800 can be a promising candidate for practical applications, and this study can provide inspirations for the further development of highly efficient carbo-catalysts.