3D cubic mesoporous C3N4 with tunable pore diameters derived from KIT-6 and their application in base catalyzed Knoevenagel reaction

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.

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.

Fabrication of branched gold copper nanoalloy doped mesoporous graphitic carbon nitride hybrid membrane for surface-enhanced Raman spectroscopy analysis of carcinogens

Carcinogens in food samples show great potential threat to human health due to their wide distribution and high carcinogenicity. In this work, branched AuCu nanoalloy doped mesoporous graphitic carbon nitride hybrid membrane (mpg-C₃N₄/AuCu) was fabricated for SERS analysis of carcinogens including benzidine and zearalenone in food. The AuCu was in-situ grown on mpg-C₃N₄ to form mpg-C₃N₄/AuCu composites. The as-fabricated mpg-C₃N₄/AuCu membrane can effectively combined synergistic effect of localized surface plasmon resonance properties of branched AuCu nanoalloy and semiconductor characteristics of mpg-C₃N₄. The limit of detection for crystal violet is 1.0 ng/L with enhancement factor of 3.7 × 10⁸. The mechanism of high SERS activity of mpg-C₃N₄/AuCu membrane was investigated by density functional theory simulations. The mpg-C₃N₄/AuCu membrane was used for direct determination of benzidine, and indirect determination of zearalenone with 3,3',5,5'-tetramethylbenzidine as markers in food. The limits of detection of SERS method were 0.14 and 0.03 μg/L for benzidine and zearalenone, respectively. It provides a new strategy for design and fabrication of high-quality SERS substrates for carcinogens analysis.

Template-free preparation of carbon nitride hollow spheres with adjustable sizes for photocatalytic hydrogen generation

Carbon nitride hollow spheres (CNHS) with adjustable sizes were successfully fabricated via a template-free supramolecular pre-assembly strategy, in which melamine-cyanuric acid (MCA) hollow spheres were constructed through hydrogen bonds. A feasible formation mechanism was proposed, which coupled an inside-out Ostwald ripening with the supramolecular pre-assembly process. Interestingly, the sizes of MCA could be manipulated by changing the pre-assembly temperature. Consequently, the sizes of CNHS were adjustable. The optimal CNHS exhibited excellent photocatalytic hydrogen evolution rate (98.6 μmol/h) in the visible-light region, which was approximately 11 times higher than that of bulk carbon nitride calcined by melamine. The significantly improved performance was due to the contributions including: the unique architectures with remarkable light absorption ability, high electrical conductivity, relatively narrowed band gap, fast charge separation. This work provides a facile template-free supramolecular pre-assembly strategy to fabricate carbon nitride hollow spheres with adjustable sizes for the first time.

An Overview of Metal-free Sustainable Nitrogen-based Catalytic Knoevenagel Condensation Reaction ‎

Knoevenagel condensation reaction counts as a vital condensation in organic chemistry due to the synthesis of valuable intermediates, ‎heterocycles, and fine chemicals from commercially available reactants through forming new C=C...

Magnetic solid-phase extraction of pyrethroid and neonicotinoid insecticides separately in environmental water samples based on alkaline or acidic group-functionalized mesoporous silica

In this work, amino- or carboxyl-functionalized magnetic KIT-6 have been synthesized separately. The two nanocomposites were successfully used to enrich pyrethroids and neonicotinoids insecticides from environmental water samples, respectively.

Graphitic Carbon Nitride as a Platform for the Synthesis of Silver Nanoclusters

Graphitic carbon nitride (CN) synthetized by the thermal polycondensation of melamine at 550 °C for 4 h was further exfoliated by heating at 500 °C for 3 h. Silver cations were adsorbed on the exfoliated graphitic carbon nitride (CNE) and then reduced by sodium borohydride forming silver nanoclusters (NCs) with a size of less than 1 nm. The NCs were located on the CNE surface and did not change the CNE properties except for its pore size distribution and thereby specific surface area (SSA). The Ag NCs were able to collect the photoinduced electrons of CNE and thus reduce their recombination with the holes. It was also documented by the increase in the CNE photocatalytic activity in terms of the degradation of antibiotic Ofloxacin. This study demonstrates the ability of CNE to serve as a platform for a simple and fast synthesis of Ag NCs without any stabilizing compounds.

A review of the recent progress on heterogeneous catalysts for Knoevenagel condensation

One of the most crucial attributes of synthetic organic chemistry is to design organic reactions under the facets of green chemistry for the sustainable production of chemicals. Thus, due to the intensified environmental and safety concern, the need for new technologies for conducting chemical transformation has grown. In this regard, there is enormous interest in the use of heterogeneous catalysts as they generally avoid the generation of waste, require fewer toxic reagents, as well as entail easier separation and recycling of the catalyst. α,β-Unsaturated acids have been widely used in various industrial applications and have been identified as one of the most promising chemicals obtained via the Knoevenagel condensation reaction. This review aims to discuss the most pertinent heterogeneous catalytic systems such as zeolites, mesoporous silica, ionic liquids, metal oxides, and graphitic carbon nitride-based catalysts in the Knoevenagel reaction. Ultimately, this review focuses not only on the catalyst but also provides an overall idea and guide for the preparation of new catalysts with outstanding properties by looking at the chemical and engineering aspects such as the reaction conditions and the mechanisms.

A Comprehensive Study of Coke Deposits on a Pt-Sn/SBA-16 Catalyst during the Dehydrogenation of Propane

Catalytic propane dehydrogenation is an attractive method to produce propylene while avoiding the issues of its traditional synthesis via naphtha steam cracking of naphtha. In this contribution, a series of Pt-Sn/SBA-16 catalysts were synthesized and evaluated for this purpose. Bimetallic Pt-Sn catalysts were more active than catalysts containing only Pt. The catalyst with the best performance was assessed at different reaction times of 0, 60, 180, and 300 min. The evolution of coke deposits was also studied. Thermogravimetric analysis demonstrated the presence of two types of coke on the catalyst surface at low and high temperature, respectively. Raman results showed an increased coke’s crystal size from 60 to 180 min on stream, and from 180 to 300 min under reaction, Raman suggested a reduction in the crystal size of coke. Also transmission electron microscopy confirmed a more evident agglomeration of metallic particles with reaction times higher than 180 min. These results are consistent with the phenomena called “coke migration” and the cause is often explained by coke movement near the particle to the support; it can also be explained due to sintering of the metallic particle, which we propose as a more suitable explanation.

CoO-3D ordered mesoporous carbon nitride (CoO@mpgCN) composite as peroxymonosulfate activator for the degradation of sulfamethoxazole in water

A facile impregnation method was used to fabricate a hybrid CoO-3D ordered mesoporous carbon nitride (CoO@mpgCN) catalyst that effectively activated peroxymonosulfate (PMS) for the degradation of pharmaceutical chemical, exemplified by antibiotic sulfamethoxazole (SMX) in aqueous solutions. The CoO@mpgCN/PMS system exhibited high catalytic reactivity and SMX removal efficiency over a wide pH range with an observed rate constant (k_obs) of 0.314 min^-1. Furthermore, CoO@mpgCN was stable with consistently high degree of SMX degradation without having cobalt dissolution and loss of catalytic activity for at least five consecutive cycles. The significant catalysis performance of CoO@mpgCN was due to its uniformly distributed mesopores, large specific surface area, and high electron transfer ability at the active CoO sites. Both quenching experiments and electron paramagnetic resonance (EPR) analysis verified the yield, in abundance, of highly active species, specifically SO₄^- and OH from the CoO@mpgCN activation of PMS, primarily. Hence, SMX degradation followed a radical chain reaction mechanism. The result of this study revealed a novel prospective of CoO@mpgCN composite as PMS activator for the remediation of recalcitrant pollutants in water.

Nanostructured Carbon Nitrides for CO2 Capture and Conversion

Carbon nitride (CN), a 2D material composed of only carbon (C) and nitrogen (N), which are linked by strong covalent bonds, has been used as a metal-devoid and visible-light-active photocatalyst owing to its magnificent optoelectronic and physicochemical properties including suitable bandgap, adjustable energy-band positions, tailor-made surface functionalities, low cost, metal-free nature, and high thermal, chemical, and mechanical stabilities. CN-based materials possess a lot of advantages over conventional metal-based inorganic photocatalysts including ease of synthesis and processing, versatile functionalization or doping, flexibility for surface engineering, low cost, sustainability, and recyclability without any leaching of toxic metals from photocorrosion. Carbon nitrides and their hybrid materials have emerged as attractive candidates for CO₂ capture and its reduction into clean and green low-carbon fuels and valuable chemical feedstock by using sustainable and intermittent renewable energy sources of sunlight and electricity through the heterogeneous photo(electro)catalysis. Here, the latest research results in this field are summarized, including implementation of novel functionalized nanostructured CNs and their hybrid heterostructures in meeting the stringent requirements to raise the efficiency of the CO₂ reduction process by using state-of-the-art photocatalysis, electrocatalysis, photoelectrocatalysis, and feedstock reactions. The research in this field is primarily focused on advancement in the synthesis of nanostructured and functionalized CN-based hybrid heterostructured materials. More importantly, the recent past has seen a surge in studies focusing significantly on exploring the mechanism of their application perspectives, which include the behavior of the materials for the absorption of light, charge separation, and pathways for the transport of CO₂ during the reduction process.

Tunable Effect of the Calcination of the Silanol Groups of KIT-6 and SBA-15 Mesoporous Materials

The calcination process is a crucial step during SBA-15 and KIT-6 synthesis. It is used to completely remove the organic template and condense silanol groups, and it allows the determination of the textural and physical properties of these materials, depending on the adopted conditions. Moreover, calcination influences the number of silanols available on the surface of the material. The concentration of silanols is important if these materials were synthesized for use in adsorption or functionalization. To understand and optimize the silanol groups of SBA-15 and KIT-6, in this study, the temperature and time calcination parameters were varied. The experiments were performed at 300, 400, and 500 °C for 300, 400, and 500 min. The results show that the ideal temperature to preserve the silanol groups is 300 °C, but to optimize the textural properties, it is better to calcine these molecular sieves at 400 °C. A calcination for 10 h did not give better results than a calcination for 5 h, demonstrating that the former duration is excessive for use.

Synthesis of polyimide-modified carbon nanotubes as catalyst for organic pollutant degradation via production of singlet oxygen with peroxymonosulfate without light irradiation

Polyimide-modified carbon nanotubes (PI/CNTs) were synthesized via a solvent-free thermal method and used as a metal-free catalyst to activate peroxymonosulfate for organic contaminant degradation without light irradiation. The characterization results suggested that PI was loaded onto the surface of CNTs. The catalytic ability of the PI/CNTs was strongly correlated with the content of PI in the catalysts. The PI/CNTs (22% of PI) showed the highest catalytic efficiency for organic pollutant degradation at room temperature. The degradation efficiency of acid orange 7 (AO7) dye was significantly enhanced to 98.9% within 15 min, compared to the efficiency of 2.2% exhibited by pure PI. The radical quenching tests and electron paramagnetic resonance spectrometry proved that singlet oxygen, instead of hydroxyl radicals or sulfate radicals, played a dominant role during the catalytic oxidation of AO7. The influences of operation parameters including temperature and catalyst amount were investigated. The PI/CNTs metal-free catalyst exhibited high catalytic activity under a broad range of pH values. The recycling study of four repeated reactions demonstrated good stability of the PI/CNTs. This work provided a promising metal-free catalyst for degradation of organic pollutants in aqueous solutions, contributing to the development of green materials for sustainable remediation.

Simultaneous extraction of permethrin diastereomers and deltamethrin in environmental water samples based on aperture regulated magnetic mesoporous silica

The aperture of KIT-6 can influence the recoveries of magnetic solid phase extraction.

Synthesis of mesoporous carbon with tunable pore size for supercapacitors

Mesoporous carbon (MC) has wide applications, including in drug delivery, catalysis, absorption, energy storage/conversion, etc.