Metal-free Heterogeneous Catalyst Research Articles

An Organic Molecular Mimetic Metal-Free Heterogeneous Catalyst for Electrocatalytic Alkyne Semihydrogenation.

The direct construction of metal-free catalysts on conductive substrates for electrocatalytic organic hydrogenation reactions is significant but still unexplored. Here, learning from the homogeneous molecular catalysts, an organic molecular mimetic metal-free heterogeneous catalyst is designed and constructed in situ on a graphite flake electrode via a mild electrochemical oxidation‒reduction relay strategy. The as-prepared -COOH- and -OH-functionalized metal-free catalyst exhibits an electrocatalytic alkyne semihydrogenation performance with a 72% Faradaic efficiency, 99% selectivity and 96% yield of the alkene product, which is comparable to that of noble metal catalysts. The removal of these oxygen-containing groups leads to negligible activity. The experimental and calculation results reveal that the origin of the high activity can be assigned to the -COOH and -OH groups on graphite. A flow electrolytic cell delivers ten grams of hydrogenated products with 81% Faradaic efficiency. This metal-free catalyst is also suitable for gas-phase acetylene semihydrogenation and other electrocatalytic hydrogenation reactions.

Biopolymer-derived structured graphitic carbons as metal-free heterogeneous ozonation catalysts in water

The development of metal-free heterogeneous catalysts for advanced oxidation processes is an important area of research, advancing in sustainability, with potential practical applications in water treatment. In this work, we report the development of defective structured graphitic carbons synthesized from biomass including alginate or chitosan polysaccharides and used as metal-free ozonation catalysts in water. These solids were characterized by several techniques, including powder X-ray diffraction, several spectroscopies (i.e. X-ray photoelectron, Raman or Fourier Transform infrared), elemental combustion analyses, thermogravimetric measurements, and electron microscopic techniques. The catalytic performance of graphitic carbon was examined in the ozonation reaction and the graphitic carbon derived from alginate (G) was found to be the most active catalyst by showing complete degradation in less than 4 h under the operational conditions as oxalic acid (50 mg L−1), catalyst (100 mg L−1), 20 °C, O3 dosage (140 mg h−1) at pH 3. Importantly, this solid retains its activity mostly upon reuse for more than 20 h, an observation that compares favorably with previous reports using graphene-based materials. In addition, activity of partially deactivated catalyst can be recovered by a pyrolysis process associated to the reconstitution of graphitic active sites of the catalyst. Experimental evidence by electron spin resonance together with specific (photo)catalytic experiments is provided to support the role of 1O2 as key intermediate during the oxalic acid degradation in water in the presence of G. This study exemplifies the activity of active graphitic-based solids from biomass precursors as ozonation heterogeneous catalysts in water in the absence of any metal.

Nitrogen-doped metal-free granular activated carbons as economical and easily separable catalysts for peroxymonosulfate and hydrogen peroxide activation to degrade bisphenol A.

The fabrication of low-cost, highly efficient, environmentally friendly, and easily separable metal-free heterogeneous catalysts for environmental remediation remains a challenge. In this study, granular nitrogen-doped highly developed porous carbons with a particle size of 0.25-0.30mm were prepared by preoxidation and subsequent NH3 modification of a commercially available coconut-based activated carbon, and used to activate peroxymonosulphate (KHSO5) or hydrogen peroxide (H2O2) to degrade bisphenol A (BPA). The nitrogen-doped carbon (ACON-950) prepared by NH3 modification at 950°C, with the addition of only 0.15g/L could remove 100% of 50mg/L BPA in 150min, and more than 90% of the removed BPA was due to degradation. The removal rates of total organic carbon of ACON-950/KHSO5 and ACON-950/H2O2 systems reached 60.4% and 66.2% respectively, indicating the excellent catalytic activity of ACON-950. The reaction rate constant was significantly positively correlated with the absolute content of pyridinic N (N-6) and graphitic N (N-Q) and negatively and weakly positively correlated with pyrrolic N (N-5) and defects. Quenching experiments combined with electron paramagnetic resonance demonstrated that singlet oxygen was the dominant reactive oxidative species for BPA degradation. ACON-950 was characterized before and after the degradation reaction using N2 adsorption-desorption analyzer, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The results confirmed the prominent contribution of both the N-6 and N-Q to the catalytic performance of nitrogen-doped carbons. The reusability of ACON-950 and its application in actual water bodies further demonstrated its remarkable potential for the remediation of organic pollutants in wastewater.

A recyclable Cu@C2N nano-catalyst applied in the transformation of alkynes: pH switchable access to ketones and 1,3-diynes

We synthesized a reusable heterogeneous Cu@C2N catalyst for pH switchable conversion of alkynes to ketones and 1,3-diynes with 92% and 95% yield. The catalyst has a broad substrate scope, and the reaction conditions are environmentally-friendly.

Metal-free catalysts with local fluorination regulation for peroxymonosulfate activation: Nearly 100 % singlet oxygen production for selective degradation of aqueous organic pollutants

Generating singlet oxygen (1O2) via activating peroxymonosulfate (PMS) is crucial for the selective removal of organic pollutants in aquatic environments. However, achieving efficient and selective conversion of PMS into 1O2 without consuming energy or metals remains a significant challenge. In this study, graphite phase carbon nitride (g-C3N4) was locally fluorinated via a simple polymerization method. The local fluorinated g-C3N4 (FCN) was used as a metal-free catalyst to activate PMS, enabling highly efficient degradation of 4-chlorophenol (100 %) and detoxification. The FCN/PMS system demonstrates exceptional selectivity for electron-rich pollutants, functions effectively across a broad pH range (2–10), and exhibits remarkable interference resistances to background scavenging ions and natural organic compounds. Furthermore, quenching experiments, probe tests, electron paramagnetic spin resonance measurements, electrochemical characterizations, and theoretical calculations demonstrated that 1O2 was the only reactive species in the catalytic process. Local fluorination regulated the electronic structure of g-C3N4, juxtaposing electron-rich fluorine with electron-deficient carbon dual active sites. This modification alters the electron transfer direction between PMS and g-C3N4, allowing for nearly 100 % selective generation of 1O2via dual electron transfer channels without the need for energy or metals. This study provides a new strategy for designing metal-free heterogeneous catalysts to selectively generate 1O2 in a green way and can facilitate the wider applications of advanced oxidation processes for water purification.

Boron Bifunctional Catalysts for Rapid Degradation of Persistent Organic Pollutants in a Metal-Free Electro-Fenton Process: O2 and H2O2 Activation Process.

Metals usually served as the active sites of the heterogeneous bifunctional electro-Fenton reaction, which faced the challenge of poor stability under acidic or even neutral conditions. Exploring a metal-free heterogeneous bifunctional electro-Fenton catalyst can effectively solve the above problems. In this work, a stable metal-free heterogeneous bifunctional boron-modified porous carbon catalyst (BTA-1000) was synthesized. For the BTA-1000 catalyst, the yield of H2O2 (294 mg/L) significantly increased. The degradation rate of phenol by BTA-1000 (0.242 min-1) increased by an order of magnitude, compared with the porous carbon catalyst (0.0105 min-1). The BTA catalyst could rapidly degrade industrial dye wastewater, and its specific energy consumption was 5.52 kW h kg-1 COD-1, lower than that in previous reports (6.38-7.4 kW h kg-1 COD-1). DFT and XPS revealed that C═O and -BC2O groups jointly promoted the generation of H2O2, and the -BCO2 group played dominant roles in the generation of •OH because the oxygen atom near the electron-giving groups (-BCO2 group) facilitated the formation of hydrogen bond and H2O2 adsorption. This work gained deep insights into the reaction mechanism of the boron-modified porous carbon catalyst, which helped to guide the development of metal-free heterogeneous bifunctional electro-Fenton catalysts.

N, S co-doped porous carbon to accelerate Fe3+/Fe2+ redox cycle for peroxymonosulfate activation

Fenton-like process has important implications for water treatment due to its effective degradation of refractory contaminants. However, sluggish conversion of Fe3+ back to Fe2+ strongly restricts its practical application. Herein, a novel N, S co-doped porous carbon (S-ZCN) derived from ZIF-8 was prepared as a metal-free heterogeneous catalyst to accelerate the redox cycle of Fe3+/Fe2+, leading to more efficient peroxymonosulfate (PMS) activation. With only 20 mg/L S-ZCN added, sulfamethoxazole (SMX) could be removed completely by the S-ZCN/Fe3+/PMS system within 40 min. Reactive oxygen species (ROS) were identified by quenching experiments, electron paramagnetic resonance (EPR), and probe experiments. It was revealed that hydroxyl radical (HO•) was the primary ROS, which contributed 79.2 % to SMX degradation. Verified by Fe concentration detection, chelating agent experiments and the results of XPS and FT-IR spectra, the synergistic effect of N, S co-doping and formation of Fe-Nx sites promoted the electron transfer from sp2-hybridized C to surface-associated Fe3+ to reconstruct Fe2+/PMS system. Based on the detection results of intermediates by Q-TOF analysis, potential degradation pathways for SMX in the S-ZCN/Fe3+/PMS system were proposed. Wide range of pH applicability and strong degradation to various pollutants exhibited the advantages in practical applications. This work provides a novel approach involving metal-free catalysts to break the bottleneck in the field of traditional Fenton-like reaction.

2D redox-active COF with the anthraquinone structure for photocatalytic reduction of uranium

Photocatalysis can be an effective technology for the conversion of U(VI) to U(IV) because it is efficient, facile and free of secondary pollution. In this work, anthraquinone-functionalized covalent organic framework (DQTP) was fabricated by an irreversible tautomerization of 2,6-diaminoanthraquinone (DQ) and 2, 4, 6-triformylphloroglucinol (TP) in the β-ketoenamine structure. The resultant DQTP could be effectively used as a metal-free heterogeneous catalyst for the photoreduction of U(VI) (k = 0.00287 min−1). The pivotal role of anthraquinone moieties as the active sites for U(VI) reduction was evidenced, i.e., to promote charge separation and thus enhance photocatalytic activity, as well as the production of H2O2 followed by the formation of (UO2)O2·2H2O with U(VI) through in-situ reactions. Additionally, free radical capture experiments and spectrometry analysis were conducted to determine the synergistic reduction mechanism of photogenerated electrons, superoxide radicals and hydrogen peroxide. It was reassuring that this work could provide the research basis for the subsequent development of more practical metal-free photocatalysts for the purification of U(VI)-containing wastewater.

Nanographite: A highly active and durable metal-free ozonation catalyst with application in natural waters

The development of metal-free heterogeneous catalysts for advanced oxidation processes has emerged as a key focus for both academia and industry. We report on research that seeks to establish a correlation between the crystallographic, physicochemical, textural, and morphological properties of a series of graphite-based solids and their activities as metal-free ozonation catalysts in aqueous media. Our findings show that nanographite (NG) outperforms traditional graphite-based solids, activated carbon and Co3O4. The enhanced activity of NG is due to defective nanosized graphitic structures with high surface areas. NG exhibits an exceptional activity, durability, and recyclability that surpasses other known catalysts. Our study employs electron spin resonance and selective quenching experiments that reveal the generation of 1O2 during catalytic ozonation. Significantly, NG shows excellent catalytic efficiency even when using natural surface river water or seawater. This study illustrates the importance of the natural properties of graphite-based materials as active and durable ozonation catalysts in water.

Insertion of pillar[5]arene into Tröger's base-derived porous organic polymer for promoted heterogeneous catalytic performance in Knoevenagel condensation and CO2 fixation

Development of new metal-free heterogeneous catalysts has long been the focus of intense research interest. The integration of multifunctional monomers into the skeletons of porous organic polymers (POPs) provides an efficient pathway to achieve this goal. Herein, we rationally designed and successfully prepared a new Tröger's base (TB)-derived POPs by insertion of pillar[5]arene macrocycle as a positively auxiliary group. Combined the both merits of pillar[5]arene macrocycle and TB moiety, the as-prepared polymer was further explored as an effective metal-free heterogeneous catalyst and exhibited promoted catalytic performance in Knoevenagel condensation and CO2 conversion. This work provides a new strategy to fabricate metal-free heterogeneous catalysts based on macrocyclic POPs.

HDBU]Br@P-DD as Porous Organic Polymer-Supported Ionic Liquid Catalysts for Chemical Fixation of CO2 into Cyclic Carbonates

The chemical fixation of CO2 into high value-added cyclic carbonates is of significant potential and sustainability to address the energy and ecological issues. [HDBU]Br@P-DD-m/n, a series of porous organic polymer-supported ionic liquids (PSILs), were fabricated in this work through a one-pot four-component reaction of DMAEMA, DVB, 4-bromobutyric acid, and DBU. The formation of [HDBU]Br and construction of the P-DD polymer network proceeded concurrently with the support of the in situ-formed IL being achieved simultaneously. Characterization of the PSILs by IR, ICP, BET, SEM, and TGA etc. showed that the as-prepared catalysts possessed versatile hierarchical porosity with varying ionic densities and excellent thermostability. The porosity of the copolymer skeleton was found to be affected by the accompanying formation of [HDBU]Br. The PSILs were then evaluated as catalysts for the cycloaddition reaction of CO2 with epoxides, among which [HDBU]Br@P-DD-4/1 showed the optimal performance under atmospheric pressure and solvent-/cocatalyst-free conditions (95% yield and 99% selectivity for CO2 cycloaddition with epichlorohydrin). By correlating the performance and structures of the catalysts, the synergistic effect of the tertiary amine, [HDBU]Br, and pore distribution was demonstrated. In addition, the catalyst was found to be applicable in synthesizing a range of differently substituted carbonates in good to excellent yields. Finally, the catalyst could be recovered readily and good catalytic activity was still observed after use for 6 runs. The work not only provided an active, reusable, and metal-free heterogeneous catalyst for the fixation of CO2 into cyclic carbonates but also developed a facile one-step multicomponent strategy for constructing PSIL catalysts, which would be promising to find wide applications in catalysis and organic synthesis field.

F,O,S-Codoped Graphitic Carbon Nitride as an Efficient Photocatalyst for the Synthesis of Benzoxazoles and Benzimidazoles

Graphitic carbon nitride (g-C3N4) is a metal-free heterogeneous catalyst that has attracted attention because of its good chemical stability, cost-effectiveness, and environmental friendliness. In the work presented herein, F,O,S-codoped carbon nitride was synthesized by heating a mixture of melamine cyanurate and trifluoromethanesulfonic acid at 550 °C for 50 min. The obtained product was characterized by transmission electron microscopy, infrared spectroscopy, X-ray powder diffraction, CHNS elemental analysis, total combustion-ion chromatography, X-ray photoelectron spectroscopy, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, and UV-Vis spectroscopy. Results point to an F,O,S-codoped g-C3N4. The material was applied as a photocatalyst for the formation of benzoxazoles and benzimidazoles by condensation–aromatization of 2-aminophenol or 1,2-phenylenediamine with suitable aldehydes (viz. benzaldehyde, 4-chlorobenzaldehyde, 2-naphthaldehyde, 2-hydroxybenzaldehyde, and 2-methoxybenzaldehyde), obtaining yields of up to 90% in 15 min under visible light irradiation, with good selectivity and reusability. Thus, the reported findings suggest that this F,O,S-codoped g-C3N4 may hold promise as a metal-free photocatalyst for the rapid synthesis of 2-arylbenzoxazoles and 2-arylbenzimidazoles.

Sulfur anchored on N-doped porous carbon as metal-free peroxymonosulfate activator for tetracycline hydrochloride degradation: Nonradical pathway mechanism, performance and biotoxicity

Dual heteroatoms doping is an efficient method to improve catalytic activity of carbon materials. In this research, N, S co-doped porous carbon material (S-NPC) was prepared by direct thermal reduction method of metal organic frameworks and proved to be an excellent metal-free heterogeneous Fenton-like catalyst for peroxymonosulfate (PMS) activation. Tetracycline hydrochloride (TC) could be completely degraded in a short time of 20 min with 20 mg S-NPC and 10 mg PMS at 25 °C, attributing that N, S co-doping provided more defects (ID/IG = 0.998) to produce more active sites for PMS activation. Based on the results from radical identification, scavenging tests, density functional theory (DFT) and electrochemical analysis, non-radical pathway based on singlet oxygen (1O2) and electron transfer was proposed to be the dominant way in the S-NPC/PMS system and the contribution rate was about 95.872 %. Three possible pathways of TC degradation were proposed by high resolution liquid chromatography-mass spectrometry (LC-MS) and the toxicity of intermediate products was lower than TC. This study provided a novel strategy to degrade pollutants by preparing efficient co-doping metal-free Fenton-like catalyst.

Nanocarbon shells with self-inherent N, P derived from chlorella pyrenoidosa for aqueous catalytic ozonation: nonradical-dominated mechanisms

Nanocarbon shells with self-inherent N, P derived from waste chlorella pyrenoidosa (CP) as N/P/C precursors were prepared for the first time using a one-step pyrolysis method to remove ibuprofen (IBU) for catalytic ozonation. The CP at 1000 °C (NPC1000) had superior catalytic ozonation performance over MnO2, Al2O3, and g-C3N4. The catalytic ozonation process follows a nonradical oxidation mechanism via surface adsorbed atomic oxygen (*Oad), singlet oxygen (1O2) and electron transfer pathway. The result indicated that graphitic N, pyridinic N and C3PO functional groups were the major active sites based on the correlation between material structure and catalytic performance. Furthermore, the catalytic activity has little impact on the removal of IBU wastewater containing inorganic anions and humic acid (HA). This study opens new applications for microalgae-derived biocarbon materials and provides a new insight into the mechanism of metal-free heterogeneous ozone catalyst.

Functional Porous Ionic Polymers as Efficient Heterogeneous Catalysts for the Chemical Fixation of CO2 under Mild Conditions.

The development of efficient and metal-free heterogeneous catalysts for the chemical fixation of CO2 into value-added products is still a challenge. Herein, we reported two kinds of polar group (−COOH, −OH)-functionalized porous ionic polymers (PIPs) that were constructed from the corresponding phosphonium salt monomers (v-PBC and v-PBH) using a solvothermal radical polymerization method. The resulting PIPs (POP-PBC and POP-PBH) can be used as efficient bifunctional heterogeneous catalysts in the cycloaddition reaction of CO2 with epoxides under relatively low temperature, ambient pressure, and metal-free conditions without any additives. It was found that the catalytic activities of the POP-PBC and POP-PBH were comparable with the homogeneous catalysts of Me-PBC and PBH and were higher than that of the POP-PPh3-COOH that was synthesized through a post-modification method, indicating the importance of the high concentration catalytic active sites in the heterogeneous catalysts. Reaction under low CO2 concentration conditions showed that the activity of the POP-PBC (with a conversion of 53.8% and a selectivity of 99.0%) was higher than that of the POP-PBH (with a conversion of 32.3% and a selectivity of 99.0%), verifying the promoting effect of the polar group (−COOH group) in the porous framework. The POP-PBC can also be recycled at least five times without a significant loss of catalytic activity, indicating the high stability and robustness of the PIPs-based heterogeneous catalysts.

N-hydroxyphthalimide anchored on hexagonal boron nitride as a metal-free heterogeneous catalyst for deep oxidative desulfurization

A metal-free N-hydroxyphthalimide/hexagonal boron nitride (NHPI/h-BN) catalytic system was developed for deep oxidative desulfurization (ODS) of fuel oils. Detailed experiments find that the heterogenization process of loading NHPI on h-BN not only benefits to the dispersion and utilization of NHPI, but also can significantly promote the catalytic performance. By employing NHPI/h-BN as the catalyst, azodiisobutyronitrile (AIBN) as the metal-free initiator, a 95% conversion of dibenzothiophene (DBT) can be acquired under the reaction conditions of 120 °C and atmospheric pressure with molecular oxygen (O2) as oxidant. Moreover, the heterogenization is convenient for the regeneration of the catalyst with >94% DBT conversion after being recycled seven times. Characterizations illustrate that the promoted catalytic activity along with the regenerability originate from the interactions between NHPI and h-BN. The catalytic mechanism study shows that molecular oxygen is readily activated by the NHPI/h-BN to form a superoxide radical (O2•−), which oxidize DBT to DBTO2 for desulfurization.

In Situ Growth of Nitrogen-Doped Carbon Nanotubes Based on Hierarchical Ni@C Microspheres for High Efficiency Bisphenol A Removal through Peroxymonosulfate Activation.

N-doped carbon nanotubes (NCNTs) are promising metal-free heterogeneous catalysts toward peroxymonosulfate (PMS) activation in advanced oxidation processes for wastewater remediation. However, conventional CNTs always suffer from serious agglomeration and low N content, which renders their design synthesis as an important topic in the related field. With hierarchical Ni@C microspheres as a nutritious platform, we have successfully induced in situ growth of NCNTs on their surface by feeding melamine under high-temperature inert atmospheres. These as-grown NCNTs with a small diameter (ca. 20 nm) are firmly rooted in Ni@C microspheres and present loose accumulation on their surface, and their relative content can be tailored easily by manipulating the mass ratio of melamine to Ni@C microspheres. The investigation on bisphenol A (BPA) removal reveals that the loading amount of NCNTs affects the catalytic performance greatly, and the optimum ratio of melamine to Ni@C microspheres is 5.0 because the corresponding MNC-5.0 possesses sufficient surface N sites and moderate electron transfer, resulting in powerful PMS activation and sufficient utilization of reactive oxidative species (ROS). MNC-5.0 also addresses its advantages as compared with other NCNTs from post treatment and spontaneous growth strategies. The primary ROS responsible for BPA degradation are identified as hydroxyl radical, sulfate radical, superoxide radical, and singlet oxygen through quenching experiments and electron paramagnetic resonance, and the corresponding catalytic mechanism is also put forward based on these results.

Hypercrosslinked Ionic Polymers with High Ionic Content for Efficient Conversion of Carbon Dioxide into Cyclic Carbonates

The effective conversion of carbon dioxide (CO2) into cyclic carbonates requires porous materials with high ionic content and large specific surface area. Herein, we developed a new systematic post-synthetic modification strategy for synthesizing imidazolium-based hypercrosslinked ionic polymers (HIPs) with high ionic content (up to 2.1 mmol g−1) and large specific surface area (385 m2 g−1) from porous hypercrosslinked polymers (HCPs) through addition reaction and quaternization. The obtained HIPs were efficient in CO2 capture and conversion. Under the synergistic effect of high ionic content, large specific surface area, and plentiful micro/mesoporosity, the metal-free catalyst [HCP-CH2-Im][Cl]-1 exhibited quantitative selectivities, high catalytic yields, and good substrate compatibility for the conversion of CO2 into cyclic carbonates at atmospheric pressure (0.1 MPa) in a shorter reaction time in the absence of cocatalysts, solvents, and additives. High catalytic yields (styrene oxide, 120 °C, 8 h, 94% yield; 100 °C, 20 h, 93% yield) can be achieved by appropriately extending the reaction times at low temperature, and the reaction times are shorter than other porous materials under the same conditions. This work provides a new strategy for synthesizing an efficient metal-free heterogeneous catalyst with high ionic content and a large specific surface area from HCPs for the conversion of CO2 into cyclic carbonates. It also demonstrates that the ionic content and specific surface area must be coordinated to obtain high catalytic activity for CO2 cycloaddition reaction.

Entropy-stabilized metal oxide nanoparticles supported on reduced graphene oxide as a highly active heterogeneous catalyst for selective and solvent-free oxidation of toluene: a combined experimental and numerical investigation

Noble metal-free heterogeneous catalysts are highly desired for selective and solvent-free oxidation reactions. However, their practical application has been greatly restricted by their moderate activity.

Origins of Electron-Transfer Regime in Persulfate-Based Nonradical Oxidation Processes.

Persulfate-based nonradical oxidation processes (PS-NOPs) are appealing in wastewater purification due to their high efficiency and selectivity for removing trace organic contaminants in complicated water matrices. In this review, we showcased the recent progresses of state-of-the-art strategies in the nonradical electron-transfer regimes in PS-NOPs, including design of metal and metal-free heterogeneous catalysts, in situ/operando characterization/analytical techniques, and insights into the origins of electron-transfer mechanisms. In a typical electron-transfer process (ETP), persulfate is activated by a catalyst to form surface activated complexes, which directly or indirectly interact with target pollutants to finalize the oxidation. We discussed different analytical techniques on the fundamentals and tactics for accurate analysis of ETP. Moreover, we demonstrated the challenges and proposed future research strategies for ETP-based systems, such as computation-enabled molecular-level investigations, rational design of catalysts, and real-scenario applications in the complicated water environment. Overall, this review dedicates to sharpening the understanding of ETP in PS-NOPs and presenting promising applications in remediation technology and green chemistry.