Carbon nitride for the selective oxidation of aromatic alcohols in water under visible light

Stabilizing BiVO4 Photoanode in Bicarbonate Electrolyte for Efficient Photoelectrocatalytic Alcohol Oxidation

In order to expand the application of bismuth vanadate (BiVO4) to the field of photoelectrochemistry, researchers have explored the potential of BiVO4 in catalyzing or degrading organic substances, potentially presenting a green and eco-friendly solution. A study was conducted to investigate the impact of electrolytes on the photocatalysis of benzyl alcohol by BiVO4. The research discovered that, in an acetonitrile electrolyte (pH 9) with sodium bicarbonate, BiVO4 catalyzed benzyl alcohol by introducing saturated V5+. This innovation addressed the issue of benzyl alcohol being susceptible to catalysis in an alkaline setting, as V5+ was prone to dissolution in pH 9 on BiVO4. The concern of the photocorrosion of BiVO4 was mitigated through two approaches. Firstly, the incorporation of a non-aqueous medium inhibited the formation of active material intermediates, reducing the susceptibility of the electrode surface to photocorrosion. Secondly, the presence of saturated V5+ further deterred the leaching of V5+. Concurrently, the production of carbonate radicals by bicarbonate played a vital role in catalyzing benzyl alcohol. The results show that, in this system, BiVO4 has the potential to oxidize benzyl alcohol by photocatalysis.

Multifunctional carbon nitride nanoarchitectures for catalysis

Catalysis is at the heart of modern-day chemical and pharmaceutical industries, and there is an urgent demand to develop metal-free, high surface area, and efficient catalysts in a scalable, reproducible and economic manner. Amongst the ever-expanding two-dimensional materials family, carbon nitride (CN) has emerged as the most researched material for catalytic applications due to its unique molecular structure with tunable visible range band gap, surface defects, basic sites, and nitrogen functionalities. These properties also endow it with anchoring capability with a large number of catalytically active sites and provide opportunities for doping, hybridization, sensitization, etc. To make considerable progress in the use of CN as a highly effective catalyst for various applications, it is critical to have an in-depth understanding of its synthesis, structure and surface sites. The present review provides an overview of the recent advances in synthetic approaches of CN, its physicochemical properties, and band gap engineering, with a focus on its exclusive usage in a variety of catalytic reactions, including hydrogen evolution reactions, overall water splitting, water oxidation, CO2 reduction, nitrogen reduction reactions, pollutant degradation, and organocatalysis. While the structural design and band gap engineering of catalysts are elaborated, the surface chemistry is dealt with in detail to demonstrate efficient catalytic performances. Burning challenges in catalytic design and future outlook are elucidated.

Uranyl nitrate as a recyclable homogeneous photocatalyst for selective cross-coupling of N-substituted amines and indoles

A homogeneous photocatalytic recyclable system for the selective radical-radical cross-coupling of N-substituted amines and indoles has been established. This system could conduct in water or acetonitrile, featuring the reuse of uranyl nitrate as the recyclable photocatalyst via a simple extraction. With this mild strategy in hand, good to excellent yields of cross-coupling products could be achieved even under the irradiation of sunlight, including 26 natural product derivatives and 16 natural product inspired re-engineered compounds. A radical-radical cross-coupling mechanism was newly proposed based on experimental evidence and reported literature. This strategy has been also applied to a gram scale synthesis to demonstrate its practical utility.

Organocatalysis with carbon nitrides

Carbon nitrides, a distinguished class of metal-free catalytic materials, have presented a good potential for chemical transformations and are expected to become prominent materials for organocatalysis. This is largely possible due to their low cost, exceptional thermal and chemical stability, non-toxicity, ease of functionalization, porosity development, etc. Especially, the carbon nitrides with increased porosity and nitrogen contents are more versatile than their bulk counterparts for catalysis. These N-rich carbon nitrides are discussed in the earlier parts of the review. Later, the review highlights the role of such carbon nitride materials for the various organic catalytic reactions including Knoevenagel condensation, oxidation, hydrogenation, esterification, transesterification, cycloaddition, and hydrolysis. The recently emerging concepts in carbon nitride-based organocatalysis have been given special attention. In each of the sections, the structure-property relationship of the materials was discussed and related to their catalysis action. Relevant comparisons with other catalytic materials are also discussed to realize their real potential value. The perspective, challenges, and future directions are also discussed. The overall objective of this review is to provide up-to-date information on new developments in carbon nitride-based organic catalysis reactions that could see them rising as prominent catalytic materials in the future.

Temperature- and pH-Responsive Polymeric Photocatalysts for Enhanced Control and Recovery

The emergence of heterogeneous photocatalysis has facilitated redox reactions with high efficiency, without compromising the recyclability of the photocatalyst. Recently, stimuli-responsive heterogeneous photocatalytic materials have emerged as a powerful synthetic tool, with simple and rapid recovery, as well as an enhanced dynamic control over reactions. Stimuli-responsive polymers are often inexpensive and easy to produce. They can be switched from an active "on" state to an inert "off" state in response to external stimuli, allowing the production of photocatalyst with adaptability, recyclability, and orthogonal control on different chemical reactions. Despite this versatility, the application of artificial smart material in the field of heterogeneous photocatalysis has not yet been maximized. In this Minireview, we will examine the recent developments of this emerging class of stimuli-responsive heterogeneous photocatalytic systems. We will discuss the synthesis route of appending photoactive components into different triggerable systems and, in particular, the controlled activation and recovery of the materials.

Catalytic oxidation of polystyrene to aromatic oxygenates over a graphitic carbon nitride catalyst

The continuous increase in manufacturing coupled with the difficulty of recycling of plastic products has generated huge amounts of waste plastics. Most of the existing chemical recycling and upcycling methods suffer from harsh conditions and poor product selectivity. Here we demonstrate a photocatalytic method to oxidize polystyrene to aromatic oxygenates under visible light irradiation using heterogeneous graphitic carbon nitride catalysts. Benzoic acid, acetophenone, and benzaldehyde are the dominant products in the liquid phase when the conversion of polystyrene reaches >90% at 150 °C. For the transformation of 0.5 g polystyrene plastic waste, 0.36 g of the aromatic oxygenates is obtained. The reaction mechanism is also investigated with various characterization methods and procedes via polystyrene activation to form hydroxyl and carbonyl groups over its backbone via C-H bond oxidation which is followed by oxidative bond breakage via C-C activation and further oxidation processes to aromatic oxygenates.

Identification of the Structure of Triethanolamine Oxygenation Products in Carbon Nitride Photocatalysis

Triethanolamine (TEOA) is one of the most commonly used sacrificial agents in photocatalysis. Due to its more complex structure compared to, for example, ethanol, and its sacrificial role in photocatalysis, it gives a mixture of products. The structures of these molecules are not usually analyzed. Herein, we obtain and isolate the products of TEOA and N‐tert‐butyl diethanolamine oxygenation under photocatalytic conditions with ≈15 % yield, and followingly characterized them by NMR and mass spectroscopy. The reaction is mediated by potassium poly(heptazine imide) (K‐PHI) in the presence of O₂ and affords formyl esters of β‐hydroxyethylene formamides from the corresponding ethanolamines.

Visible-Light Photoredox Catalysis in Water

The use of water in organic synthesis draws attention to its green chemistry features and its unique ability to unveil unconventional reactivities. Herein, literature about the use of water as a reaction medium under visible-light photocatalytic conditions is summarized in order to highlight challenges and opportunities. Accordingly, this Synopsis has been divided into four different sections focused on (1) the unconventional role of water in photocatalytic reactions, (2) in-/on-water reactions, (3) water-soluble photocatalysts, and (4) photomicellar catalytic systems.

TiO2 nanoparticles decorated with Co-Schiff base-g-C3N4 as an efficient photocatalyst for one-pot visible light-assisted synthesis of benzimidazoles

In this study, a novel heterogeneous visible light-driven nanocatalyst was produced via the complexation of Co(ii) with g-C₃N₄-imine-functionalized TiO₂ nanoparticles. It was characterized using different techniques such as Fourier-transform infrared (FT-IR), energy-dispersive X-ray spectrum (EDS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), thermogravimetric analysis (TGA), ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The catalyst promoted several different transformations in a one-pot reaction sequence: aerobic photooxidation of benzylic alcohols to aldehydes and then the tandem synthesis of benzimidazoles through the dehydrogenative coupling of primary benzylic alcohols and aromatic diamines. The photocatalyst proved to be highly active, robust, selective, and recyclable under organic reaction conditions and provided affordable products with good to high yields. The results proposed that the improved photoactivity predominantly benefits from the synergistic effects of the heterojunction of Co-carbon nitride on TiO₂ nanoparticles. Moreover, this protocol provides standard conditions avoiding undesirable additives and limitations of oxidation methods, and may help to develop a new strategy for the development of photocatalysis based organic transformations.

The Co-g-C₃N₄-imine/TiO₂ nanohybrid promotes different transformations in a one-pot reaction sequence: aerobic photooxidation of benzylic alcohols to aldehydes, and then the tandem synthesis of benzimidazoles.

Graphene-based materials for metronidazole degradation: A comprehensive review

Due to its cytotoxic effect, metronidazole (MNZ) is a drug commonly used to treat bacterial, protozoal, and microaerophilic bacterial infections. After consumption, it undergoes a series of metamorphic reactions that lead to the degradation of oxidized, acetylated, and hydrolyzed metabolites in the environment. To eliminate such pollutants, due to their high potential, adsorption and photocatalysis extensive processes are used in which graphene can be used to improve efficiency. This review analyses the use of graphene as an absorbent and catalyst with a focus on absorption and photocatalytic degradation of MNZ by graphene-based materials (GBMs). The parameters affecting the adsorption, and photocatalytic degradation of MNZ are investigated and discussed. Besides, the basic mechanisms occurring in these processes are summarized and analyzed. This work provides a theoretical framework that can direct future research in the field of MNZ removal from aqueous solutions.

Selective oxidation of aromatic alcohols in the presence of C3N4 photocatalysts derived from the polycondensation of melamine, cyanuric and barbituric acids

A set of C3N4 samples has been prepared by using melamine, cyanuric acid and barbituric acid as the precursors. The materials were subjected both to physical and chemical characterization and were used as photocatalysts for the selective oxidation of aromatic alcohols in water suspension under UV and visible irradiation. The photoactivity of the materials versus the partial oxidation of four substituted benzyl alcohols was investigated. The type and position of the substituents in the aromatic molecule influenced conversion and selectivity to the corresponding aldehyde. The presence of barbituric and cyanuric acids in the preparation method has changed the graphitic-C3N4 structure, and therefore both the characteristics of the material and the ability of light to activate the surface of the photocatalyst. The most active material prepared in the presence of melamine and cyanuric acid showed a remarkable selectivity towards the aldehyde even under visible irradiation.

Heterogeneous photocatalytic anaerobic oxidation of alcohols to ketones by Pt-mediated hole oxidation

We report a platinum nanocluster/graphitic carbon nitride (Pt/g-C₃N₄) composite solid catalyst with a photocatalytic anaerobic oxidation function for highly active and selective transformation of alcohols to ketones. The desirable products were successfully obtained in good to excellent yields from various functionalized alcohols at room temperature, including unactivated alcohols. Mechanistic studies indicated that the reaction could proceed through a Pt-mediated hole oxidation initiating an α-alcohol radical intermediate followed by a two-electron oxidation pathway. The merit of this strategy offers a general approach towards green and sustainable organic synthetic chemistry.

Recent Advances in Heterogeneous Photo-Driven Oxidation of Organic Molecules by Reactive Oxygen Species

The photo-driven oxidation of organic molecules into corresponding high-value-added products has become a promising method in chemical synthesis. This strategy can drive thermodynamically non-spontaneous reactions and achieve challenging thermocatalytic processes under ambient conditions. Reactive oxygen species (ROS) are not only significant intermediates for producing target products via photoinduced oxidation reactions but also contribute to the creation of sustainable chemical processes. Here, the latest advances in heterogeneous photo-driven oxidation reactions involving ROS are summarized. The major types of ROS and their generation are introduced, and the behaviors of various ROS involved in photo-driven processes are reviewed in terms of the formation of different bonds. Emphasis is placed on unraveling the reaction mechanisms of ROS and establishing strategies for their regulation, and the remaining challenges and perspectives are summarized and analyzed. This Review is expected to provide an in-depth understanding of the mechanisms of ROS involved in photo-driven oxidation processes as an important foundation for the design of efficient catalysts. Clarifying the role of ROS in oxidation reactions has important scientific significance for improving the atomic and energy efficiency of reactions in practical applications.

Aqueous solution photocatalytic synthesis of p-anisaldehyde by using graphite-like carbon nitride photocatalysts obtained via the hard-templating route

Graphite-like carbon nitride (GCN)-based materials were developed via the hard-templating route, using dicyandiamide as the GCN precursor and silica templates. That resulted in urchin-like GCN (GCN-UL), 3D ordered macroporous GCN (GCN-OM) and mesoporous GCN (GCN-MP). The introduction of silica templates during GCN synthesis produced physical defects on its surface, as confirmed by SEM analysis, increasing their specific surface area. A high amount of nitrogen vacancies is present in modified catalysts (revealed by XPS measurements), which can be related to an increase in the reactive sites available to catalyse redox reactions. The textural and morphological modifications induced in GCN an enhanced light absorption capacity and reduced electron/hole recombination rate, contributing to its improved photocatalytic performance. In the photocatalytic conversion of p-anisyl alcohol to p-anisaldehyde in deoxygenated aqueous solutions under UV-LED irradiation, the GCN-UL was the best photocatalyst reaching 60% yield at 64% conversion for p-anisaldehyde production after 240 min of reaction. Under oxygenated conditions (air), the process efficiency was increased to 79% yield at 92% conversion only after 90 min reaction. The GCN-based photocatalyst kept its performance when using visible-LED radiation under air atmosphere. Trapping of photogenerated holes and radicals by selective scavengers showed that under deoxygenated conditions, holes played the primary role in the p-anisaldehyde synthesis. Under oxygenated conditions, the process is governed by the effect of reactive oxygen species, namely superoxide radicals, with a significant contribution from holes.

Visible-light photooxidation in water by 1O2-generating supramolecular hydrogels

An aqueous photocatalytic system exploits photophysical properties arising from the formation of supramolecular hydrogels, with properties and assembly modulated by the amino acids appended to an organic chromophore.

Enhanced Photocatalytic Activity of Aerogel Composed of Crooked Carbon Nitride Nanolayers with Nitrogen Vacancies

Self-supported aerogel composed of carbon nitride nanolayers can act as a bifunctional photocatalyst and show enhanced photoreduction and photooxidation performance due to the large surface areas and nitrogen vacancies. The carbon nitride aerogel can catalyze hydrogen evolution at a rate of nearly 4.2 mmol h^-1 g^-1 and oxidize benzyl alcohols with a high conversion efficiency and selectivity under milder conditions. Note that the activity of carbon nitride aerogel for photochemical alcohol oxidation shows outstanding performance compared with carbon nitride based photocatalysts. Both density functional theory and experimental results demonstrate that the introduction of nitrogen vacancies within the carbon nitride aerogel contributes to the formation of a crooked structure and enhanced adsorption of oxygen compared with a bulk sample.

Surface-Doped Graphitic Carbon Nitride Catalyzed Photooxidation of Olefins and Dienes: Chemical Evidence for Electron Transfer and Singlet Oxygen Mechanisms

A new photocatalytic reactivity of carbon-nanodot-doped graphitic carbon nitride (CD-C3N4) with alkenes and dienes, has been disclosed. We have shown that CD-C3N4 photosensitizes the oxidation of unsaturated substrates in a variety of solvents according to two competing mechanisms: the energy transfer via singlet oxygen (1O2) and/or the electron transfer via superoxide (O·−2). The singlet oxygen, derived by the CD-C3N4 photosensitized process, reacts with alkenes to form allylic hydroperoxides (ene products) whereas with dienes, endoperoxides. When the electron transfer mechanism operates, cleavage products are formed, derived from the corresponding dioxetanes. Which of the two mechanisms will prevail depends on solvent polarity and the particular substrate. The photocatalyst remains stable under the photooxidation conditions, unlike the most conventional photosensitizers, while the heterogeneous nature of CD-C3N4 overcomes usual solubility problems.

Dual-Responsive Photocatalytic Polymer Nanogels

Selective activation of photocatalysts under constant light conditions has recently been targeted to produce multi-responsive systems. However, controlled activation, with easy recovery of the photocatalysts, induced by external stimuli remains a major challenge. Mimicking the responsiveness of biological systems to multiple triggers can offer a promising solution. Herein, we report dual-responsive polymer photocatalysts in the form of nanogels consisting of a cross-linked poly-N-isopropylacrylamide nanogel, copolymerised with a photocatalytically active monomer. The dual-responsive polymer nanogels undergo a stark decrease in diameter with increasing temperature, which shields the photocatalytic sites, decreasing the activity. Temperature-dependent photocatalytic formation of NAD+ in water demonstrates the ability to switch photocatalysis on and off. Moreover, the photocatalysed syntheses of several fine chemicals were carried out to demonstrate the utility of the designed material.

One-step green conversion of benzyl bromide to aldehydes on NaOH-modified g-C3N4 with dioxygen under LED visible light

NaOH-modified g-C₃N₄ facilitates a green and sustainable route for the oxidation of benzyl bromide to benzaldehyde with 99% conversion.

Catalytic Organic Reactions in Water toward Sustainable Society

Traditional organic synthesis relies heavily on organic solvents for a multitude of tasks, including dissolving the components and facilitating chemical reactions, because many reagents and reactive species are incompatible or immiscible with water. Given that they are used in vast quantities as compared to reactants, solvents have been the focus of environmental concerns. Along with reducing the environmental impact of organic synthesis, the use of water as a reaction medium also benefits chemical processes by simplifying operations, allowing mild reaction conditions, and sometimes delivering unforeseen reactivities and selectivities. After the "watershed" in organic synthesis revealed the importance of water, the development of water-compatible catalysts has flourished, triggering a quantum leap in water-centered organic synthesis. Given that organic compounds are typically practically insoluble in water, simple extractive workup can readily separate a water-soluble homogeneous catalyst as an aqueous solution from a product that is soluble in organic solvents. In contrast, the use of heterogeneous catalysts facilitates catalyst recycling by allowing simple centrifugation and filtration methods to be used. This Review addresses advances over the past decade in catalytic reactions using water as a reaction medium.

Recent advances in functional mesoporous graphitic carbon nitride (mpg-C3N4) polymers

Mesoporous micro-/nanostructures acting as supports for catalysts or used directly in catalysis reactions generally show fascinating performances that could lead to great potential for application. In the past few decades, extensive efforts have been devoted to the exploration and enrichment of graphitic carbon nitride (g-C₃N₄) based research. Especially, mesoporous g-C₃N₄ (mpg-C₃N₄) with controllable porosity and electronic/atomic structure can bring to bear unique physicochemical properties and has been widely applied in the fields of photocatalysis, adsorbents, sensors and chemical templates. However, a comprehensive summary on mpg-C₃N₄ micro/nanostructures is less reported and there is an urgent need to further promote the development of function-oriented mpg-C₃N₄-based materials. Herein, we will overview the significant advances in functional mpg-C₃N₄ polymers, including general synthesis strategies and growth mechanisms, modifications of electronic/atomic structures and interfacial properties (such as exfoliation, doping and hybridizing), as well as their current applications. Finally, several emerging issues and perspectives are also proposed.

Solvothermal synthesis of CdIn2S4 photocatalyst for selective photosynthesis of organic aromatic compounds under visible light

Ternary chalcogenide semiconductor, cadmium indium sulfide (CdIn₂S₄), was prepared by a simple solvothermal method using ethylene glycol as a solvent, as well as indium chloride tetrahydrate (InCl₃^.4H₂O), cadmium nitrate tetrahydrate [Cd(NO₃)₂^.4H₂O], and thiacetamide (TAA) as precursors. The resulted sample was subject to a series of characterizations. It is the first time to use CdIn₂S₄ sample as a visible light-driven photocatalyst for simultaneous selective redox transformation of organic aromatic compounds. The results indicate that the as-synthesized CdIn₂S₄ photocatalyst not only has excellent photocatalytic performance compared with pure In₂S₃ and CdS for the selective oxidation of aromatic alcohols in an oxygen environment, but also shows high photocatalytic redox activities under nitrogen atmosphere. A possible mechanism for the photocatalytic redox reaction in the coupled system was proposed. It is hoped that our current work could extend the applications of CdIn₂S₄ photocatalyst and provide new insights for selective transformations of organic compounds.

Decorating CoP and Pt Nanoparticles on Graphitic Carbon Nitride Nanosheets to Promote Overall Water Splitting by Conjugated Polymers

The splitting of water into H₂ and O₂ using solar energy is one of the key steps in artificial photosynthesis for the future production of renewable energy. Here, we show the first use of CoP and Pt nanoparticles as dual co-catalysts to modify graphitic carbon nitride (g-C₃ N₄ ) polymer to achieve overall water splitting under visible light irradiation. Our findings demonstrate that loading dual co-catalysts on delaminated g-C₃ N₄ imparts surface redox sites on the g-C₃ N₄ nanosheets that can not only promote catalytic kinetics but also promote charge separation and migration in the soft interface, thus improving the photocatalytic efficiency for overall water splitting. This robust, abundant, and stable photocatalyst based on covalent organic frameworks is demonstrated to hold great promise by forming heterojunctions with CoP and Pt for catalyzing the direct splitting of water into stoichiometric H₂ and O₂ using energy from photons.

Solar-Driven Reduction of Aqueous Protons Coupled to Selective Alcohol Oxidation with a Carbon Nitride-Molecular Ni Catalyst System

Solar water-splitting represents an important strategy toward production of the storable and renewable fuel hydrogen. The water oxidation half-reaction typically proceeds with poor efficiency and produces the unprofitable and often damaging product, O2. Herein, we demonstrate an alternative approach and couple solar H2 generation with value-added organic substrate oxidation. Solar irradiation of a cyanamide surface-functionalized melon-type carbon nitride ((NCN)CNx) and a molecular nickel(II) bis(diphosphine) H2-evolution catalyst (NiP) enabled the production of H2 with concomitant selective oxidation of benzylic alcohols to aldehydes in high yield under purely aqueous conditions, at room temperature and ambient pressure. This one-pot system maintained its activity over 24 h, generating products in 1:1 stoichiometry, separated in the gas and solution phases. The (NCN)CNx-NiP system showed an activity of 763 μmol (g CNx)(-1) h(-1) toward H2 and aldehyde production, a Ni-based turnover frequency of 76 h(-1), and an external quantum efficiency of 15% (λ = 360 ± 10 nm). This precious metal-free and nontoxic photocatalytic system displays better performance than an analogous system containing platinum instead of NiP. Transient absorption spectroscopy revealed that the photoactivity of (NCN)CNx is due to efficient substrate oxidation of the material, which outweighs possible charge recombination compared to the nonfunctionalized melon-type carbon nitride. Photoexcited (NCN)CNx in the presence of an organic substrate can accumulate ultralong-lived "trapped electrons", which allow for fuel generation in the dark. The artificial photosynthetic system thereby catalyzes a closed redox cycle showing 100% atom economy and generates two value-added products, a solar chemical, and solar fuel.