Polymeric carbon nitride (C3N4) as heterogeneous photocatalyst for selective oxidation of alcohols to aldehydes

Visible-Light Photocatalytic Highly Selective Oxidation of Alcohols into Carbonyl Compounds by CsPbBr3 Perovskite

Conversion of alcohols into corresponding carbonyl compounds through an oxidation reaction with high conversion and selectivity simultaneously under mild conditions still remains a great challenge. Herein, a cost-effective and highly efficient photocatalytic protocol for selective oxidation of alcohols was developed using CsPbBr3 perovskite as a heterogeneous photocatalyst, which afforded aldehydes/ketones exclusively with a yield of 99% at ambient temperature under an air atmosphere. Moreover, the photocatalyst can be recycled at least 5 times without a significant decrease in catalytic activity. The detailed reaction mechanism was investigated by a series of quenching experiments, including Stern-Volmer experiments and electron paramagnetic resonance spectroscopy analysis as well as DFT calculations.

Nanosponge-C3N4 composites as photocatalysts for selective partial alcohol oxidation in aqueous suspension

A set of four composite materials was prepared, consisting of a nanosponge matrix based on β-cyclodextrin in which carbon nitride was dispersed. The materials were characterized by the presence of diverse cross-linker units joining the cyclodextrin moieties, in order to vary the absorption/release abilities of the matrix. The composites were characterized and used as photocatalysts in aqueous medium under UV, visible and natural solar irradiation for the photodegradation of 4-nitrophenol, and for the selective partial oxidation of 5-hydroxymethylfurfural and veratryl alcohol to the corresponding aldehydes. The nanosponge-C₃N₄ composites showed higher activity than the pristine semiconductor, which can probably be attributed to the synergic effect of the nanosponge, capable of increasing the substrate concentration near the surface of the photocatalyst.

Waste Biomass Selective and Sustainable Photooxidation to High-Added-Value Products: A Review

Researchers worldwide seek to develop convenient, green, and ecological production processes to synthesize chemical products with high added value. In this sense, lignocellulosic biomass photocatalysis is an excellent process for obtaining various outcomes for the industry. One issue of biomass transformation via heterogeneous catalysis into valuable chemicals is the selection of an adequate catalyst that ensures high conversion and selectivity at low costs. Titanium oxide (TiO2), is widely used for several applications, including photocatalytic biomass degradation, depolymerization, and transformation. Graphite carbon nitride (g-C3N4) is a metal-free polymeric semiconductor with high oxidation and temperature resistance and there is a recent interest in developing this catalyst. Both catalysts are amenable to industrial production, relatively easy to dope, and suited for solar light absorption. Recent investigations also show the advantages of using heterojunctions, for biomass derivates production, due to their better solar spectrum absorption properties and, thus, higher efficiency, conversion, and selectivity over a broader spectrum. This work summarizes recent studies that maximize selectivity and conversion of biomass using photocatalysts based on TiO2 and g-C3N4 as supports, as well as the advantages of using metals, heterojunctions, and macromolecules in converting cellulose and lignin. The results presented show that heterogeneous photocatalysis is an interesting technology for obtaining several chemicals of industrial use, especially when using TiO2 and g-C3N4 doped with metals, heterojunctions, and macromolecules because these modified catalysts permit higher conversion and selectivity, milder reaction conditions, and reduced cost due to solar light utilization. In order to apply these technologies, it is essential to adopt government policies that promote the use of photocatalysts in the industry, in addition to encouraging active collaboration between photooxidation research groups and companies that process lignocellulosic biomass.

Photocatalyst-free light driven dehydrogenation of alcohols into carbonyl compounds under mild conditions

Herein, we reported a photocatalyst-free, facile and eco-friendly method for conducting dehydrogenation of alcohols to corresponding aldehydes or ketones with high selectivity under mild conditions. The methodology exhibited outstanding tolerance with electron-donating and electron-withdrawing groups and afforded series of aldehydes or ketones in considerable yields. Furthermore, the plausible mechanism was investigated by control experiments and DFT calculations. The advantages of readily accessible, atomic economy and green reaction conditions for the present method will endow it with prospective application in chemical synthesis.

Environment Friendly g-C3N4-Based Catalysts and Their Recent Strategy in Organic Transformations

Organic molecules synthesized in an environmentally friendly manner have excellent therapeutic potential. The entire preparation technique was examined in the existence of a light source, implying that light has been replaced by heating and the usage of dangerous chemicals has decreased, resulting in less pollution of the environment. The advantages of these nanocarbon catalysts include high efficiency, environmentally friendly synthesis, eco-friendly, inexpensive, and non-corrodible. In organic transformations, solid metal base/metal-free catalysts produce better results. Here, the metal-free semiconductor g-C₃N₄ was used to demonstrate the catalytic behavior of organic conversions. g-C₃N₄ is a two-dimensional material and a p‑type semiconductor to enhance the photocatalytic activity. The excellent properties of g-C₃N₄ sheet lead to the support of metals to form metal-organic frameworks. Most of the reactions gained positive response under visible light irradiation. This review will inspire readers in widen the applications of g-C₃N₄ based catalyst in various organic transformation reactions.

High specific surface CeO2-NPs doped loose porous C3N4for enhanced photocatalytic oxidation ability

Porous C₃N₄(PCN) is favored by researchers because it has more surface active sites, higher specific surface area and stronger light absorption ability than traditional g-C₃N₄. In this study, cerium dioxide nanoparticles (CeO₂-NPs) with mixed valence state of Ce³⁺and Ce⁴⁺were doped into the PCN framework by a two-step method. The results indicate that CeO₂-NPs are highly dispersed in the PCN framework, which leads to a narrower band gap, a wider range of the light response and an improved the separation efficiency of photogenerated charge in PCN. Moreover, the specific surface area (145.69 m²g^-1) of CeO₂-NPs doped PCN is a 25.5% enhancement than that of PCN (116.13 m²g^-1). In the experiment of photocatalytic selective oxidation of benzyl alcohol, CeO₂-NPs doped porous C₃N₄exhibits excellent photocatalytic activity, especially Ce-PCN-30. The conversion rate of benzyl alcohol reaches 74.9% using Ce-PCN-30 as photocatalyst by 8 h of illumination, which is 25.7% higher than that of pure porous C₃N₄. Additionally, CeO₂-NPs doped porous C₃N₄also exhibits better photocatalytic efficiency for other aromatic alcohols.

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.

Characteristics of transmission light in tetracycline hydrochloride polluted wastewater and the response of g-C3N4 under different transmission spectral range during the photodegradation process

The purification efficiency of the contaminants in the process of photocatalysis is influenced by the co-function of catalytic activity of materials, aquatic environment conditions and characteristics of transmission light. Here, tetracycline hydrochloride (TC-HCl) was introduced as the target pollutant, and the effects of different depths and TC-HCl concentrations on the transmission light intensity and spectral distribution were explored. The results show that incident light decreases with the increase of depth and pollutant concentration. The increase of depth influences the irradiance greatly, however, increase of concentration mainly lead to the narrow of transmission spectral range in the underwater field. The coupling relationship among pollutants, transmission spectral characteristics and photocatalytic reaction efficiency was discussed. Results show that the reduction of the underwater spectral range will reduce the effective response area of the material significantly, which directly leads to the reduction of pollutant removal efficiency in the degradation process. Aiming at different aquatic environment, photocatalytic materials with appropriate response spectral range should be selected to improve the light absorption ability, so that the removal efficiency can be improved significantly.

Hydrazone-Linked Heptazine Polymeric Carbon Nitrides for Synergistic Visible-Light-Driven Catalysis

Heptazine-based conjugated polymeric carbon nitrides (PCNs) are promising metal-free photocatalysts, yet their synthesis is challenging due to the electron-deficiency and insolubility of heptazine units. Indeed, heptazine-containing polymers have only been prepared through nucleophilic substitution with amines by using toxic cyameluric chloride as the starting material. Herein, we report the novel and environmentally friendly method for preparing heptazine-based mesoporous PCNs with hydrazone links formed through a simple Schiff base condensation of melem-NH₂ and aldehydes. Unlike cyameluric chloride, melem-NH₂ is non-toxic, stable, and can be readily obtained from melem and hydrazine in solution. We demonstrate that the hydrazone linkages and the heptazine units synergistically enhance the photocatalytic activity of PCNs in visible-light-driven aerobic oxidation of benzyl alcohol to benzaldehyde. In particular, the polymer constructed from melem-NH₂ and p-phthalaldehyde shows 17 times more activity than graphitic carbon nitride (g-C₃ N₄ ).

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.

Facile sonochemical synthesis of Ag2O-guar gum nanocomposite as a visible light photocatalyst for the organic transformation reactions

Silver Oxide (Ag₂O)-Guar gum nanocomposite was fabricated via a simple sonochemical co-precipitation method. The obtained photocatalyst was characterized with various techniques such as X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, UV-vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, scanning electron microscopy and transmission electron microscopy along with energy dispersion X-ray spectroscopy. The findings have demonstrated that Ag₂O nanoparticles are spherical of 5-20 nm and were dispersed on the surface of polysaccharide guar gum to form Ag₂O-guar gum nanocomposite. The as-synthesized nanocomposite was enacted as a competent photocatalyst for the reduction of nitrobenzene and oxidation of benzyl alchohol. The conversion efficiency for the reduction of nitrobenzene was 96 % with the addition of sodium borohydride, and the conversion of benzyl alcohol was 98 %. The highly efficient photocatalytic activity was due to the exceedingly dispersed Ag₂O-guar gum nanocomposite where effective separation rate of energy driven electron-hole pairs and stronger light absorption occurs. The possible mechanism of the reactions was implicated in understanding the active species involved in the photocatalytic study.

Anaerobic Alcohol Conversion to Carbonyl Compounds over Nanoscaled Rh-Doped SrTiO3 under Visible Light

Photocatalytic oxidation of organic compounds on semiconductors provides a mild approach for organic synthesis and solar energy utilization. Herein, we identify the key points for the photocatalytic oxidation over Pt-loaded Rh-doped strontium titanate allowing the conversion of alcohols efficiently and selectively to aldehydes and ketones under anaerobic conditions and visible light with an apparent quantum efficiency of pure benzyl alcohol oxidation at 420 nm of ≤49.5%. Mechanistic investigations suggest that thermodynamically the controlled valence band edge position via Rh doping provides a suitable oxidation ability of photogenerated holes, avoiding the powerful hydroxyl radical intermediates prone to overoxidation resulting in high selectivity. Kinetically, oxygen vacancies induced by Rh³⁺ substitution in the SrTiO₃ lattice not only favor the dissociative adsorption of alcohols yielding alkoxy species but also induce the weakening of the α-C-H bond facilitating its cleavage by the photogenerated holes. Pt nanoparticles deposited as a cocatalyst contribute to the final hydrogen evolution.

Photocatalytic Hydrogen Evolution via Water Splitting: A Short Review

Photocatalytic H2 generation via water splitting is increasingly gaining attention as a viable alternative for improving the performance of H2 production for solar energy conversion. Many methods were developed to enhance photocatalyst efficiency, primarily by modifying its morphology, crystallization, and electrical properties. Here, we summarize recent achievements in the synthesis and application of various photocatalysts. The rational design of novel photocatalysts was achieved using various strategies, and the applications of novel materials for H2 production are displayed herein. Meanwhile, the challenges and prospects for the future development of H2-producing photocatalysts are also summarized.