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The interest in using carbon nitrides (CN) for CO2 conversion has stimulated extensive research on CN synthesis. Herein, we report the synthesis of two novel CN materials using low-cost commercially available precursors at low temperatures in a short duration of time. Two CN materials, one derived from 5-amino tetrazole (named 4NZ-CN) and the other derived from 3, 5-diamino-1, 2, 4-triazole (named 3NZ-CN) precursors, are prepared by refluxing these precursors for 2 h at 100 °C. 4NZ-CN and 3NZ-CN catalysts show higher surface areas (55.80 and 52.00 m2 g-1) and more basic sites (10.05 and 5.65 mmol g-1) than the conventional graphitic carbon nitride (g-C3N4) derived from melamine, for which the corresponding values are 9.20 m2 g-1 and 0.62 mmol g-1, respectively. In addition, both CN exhibit a 3-fold higher catalytic activity for CO2 cycloaddition to epoxides than g-C3N4. The structure-activity relationship was ascertained using a combination of experimental and computational studies, and a catalytic mechanism was proposed. This work provides a facile strategy for the synthesis of novel CN materials at relatively low temperatures, and the developed catalysts show remarkable performance in the conversion of CO2 to value-added chemicals.
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Single-atom catalysts (SACs) are an emerging class of materials, leveraging maximum atom utilization and distinctive structural and electronic properties to bridge heterogeneous and homogeneous catalysis. Direct imaging methods, such as aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, are commonly applied to confirm the atomic dispersion of active sites. However, interpretations of data from these techniques can be challenging due to simultaneous contributions to intensity from impurities introduced during synthesis processes, as well as any variation in position relative to the focal plane of the electron beam. To address this matter, this paper presents a comprehensive study on two representative SACs containing isolated nickel or copper atoms. Spectroscopic techniques, including X-ray absorption spectroscopy, were employed to prove the high metal dispersion of the catalytic atoms. Employing scanning transmission electron microscopy imaging combined with single-atom-sensitive electron energy loss spectroscopy, we scrutinized thin specimens of the catalysts to provide an unambiguous chemical identification of the observed single-atom species and thereby distinguish impurities from active sites at the single-atom level. Overall, the study underscores the complexity of SACs characterization and establishes the importance of the use of spectroscopy in tandem with imaging at atomic resolution to fully and reliably characterize single-atom catalysts.
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Invited for this month's cover is the group of Gianvito Vilé at the Politecnico di Milano. The ChemSusChem cover image depicts in an artistic manner the concept of ligand entrapping of isolated metals to design single-atom catalysts. The Research Article itself is available at 10.1002/cssc.202301529.
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Catalytic reactions involving molecular hydrogen are at the heart of many transformations in the chemical industry. Classically, hydrogenations are carried out on Pd, Pt, Ru or Ni catalysts. However, the use of supported Au catalysts has garnered attention in recent years owing to their exceptional selectivity in hydrogenation reactions. This is despite the limited understanding of the physicochemical aspects of hydrogen activation and reaction on Au surfaces. A rational design of new improved catalysts relies on making better use of the hydrogenating properties of Au. This Review analyses the strategies utilized to improve hydrogen-Au interactions, from addressing the importance of the Au particle size to exploring alternative mechanisms for H2 dissociation on Au cations and Au-ligand interfaces. These insights hold the potential to drive future applications of Au catalysis.
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Cross-coupling reactions are of great importance in chemistry due to their ability to facilitate the construction of complex organic molecules. Among these reactions, the Ullmann-type C-O coupling between phenols and aryl halides is particularly noteworthy and useful for preparing diarylethers. However, this reaction typically relies on homogeneous catalysts that rapidly deactivate under harsh reaction conditions. In this study, we introduce a novel heterogeneous catalyst for the Ullmann-type C-O coupling reaction, comprised of isolated Cu atoms chelated to a tetraethylenepentamine-pyrrole ligand that is immobilized on graphite nanoplatelets. The catalytic study reveals the recyclability of the material, and demonstrates the crucial role of the pyrrole linker in stabilizing the Cu sites. The work expands the potential of single-atom catalyst nanoarchitectures and underscores the significance of ligands in stabilizing metals in cationic forms, providing a novel, tailored catalyst for cross-coupling chemistries.
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In recent years, the "Escape-from-Flatland" trend has prompted the synthetic community to develop a set of cross-coupling strategies to introduce sp3-carbon-based fragments in organic compounds. This study presents a novel nickel-catalyzed electrochemical methodology for reductive cross-electrophile coupling. The method enables C(sp2)-C(sp3) linkages using inexpensive amine-derived radical precursors and aryl iodides. The use of electrochemistry as a power source reduces waste and avoids chemical reductants, making this approach a more sustainable alternative to traditional cross-coupling methods.
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Efficient catalytic methods for the trifluoromethylation of (hetero)arenes are of particular importance in organic and pharmaceutical manufacturing. However, many existing protocols rely on toxic reagents and expensive or sterically hindered homogeneous catalysts. One promising alternative to conduct this transformation involves the use of carbon nitride, a non-toxic photocatalyst prepared from inexpensive precursors. Nonetheless, there is still little understanding regarding the interplay between physicochemical features of this photocatalyst and the corresponding effects on the reaction rate. In this work, we elucidate the role of carbon nitride nanostructuring on the catalytic performance, understanding the effect of surface area and band gap tuning via metal insertion. Our findings provide new insights into the structure-function relationships of the catalyst, which we exploit to design a continuous-flow process that maximizes catalyst-light interaction, facilitates catalyst reusability, and enables intensified reaction scale-up. This is particularly significant given that photocatalyzed batch protocols often face challenges during industrial exploitation. Finally, we extrapolate the rapid and simplified continuous-flow method to the synthesis of a variety of functionalized heteroaromatics, which have numerous applications in the pharmaceutical and fine chemical industries.
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Single-atom catalysts hold the potential to significantly impact the chemical sector, pushing the boundaries of catalysis in new, uncharted directions. These materials, featuring isolated metal species ligated on solid supports, can exist in many coordination environments, all of which have shown important functions in specific transformations. Their emergence has also provided exciting opportunities for mimicking metalloenzymes and bridging the gap between homogeneous and heterogeneous catalysis. This Review outlines the impressive progress made in recent years regarding the use of single-atom catalysts in organic synthesis. We also illustrate potential knowledge gaps in the search for more sustainable, earth-abundant single-atom catalysts for synthetic applications.
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The development of efficient catalysts is a highly necessary but challenging task within the field of environmental water remediation. Single-atom catalysts are promising nanomaterials within this respect, but in-depth studies encompassing this class of catalysts remain elusive. In this work, we systematically study the degradation of gemfibrozil, a persistent pollutant, on a series of carbon nitride photocatalysts, investigating both the effect of (i) catalyst textural properties and (ii) metal single atoms on the contaminant degradation. Tests in the absence of the catalyst result in negligible degradation rates, confirming the stability of the contaminant when dispersed in water. Then, photocatalytic tests at optimal pH, solvent, and wavelength reveal a correlation between the support surface area and the degradation. This points to the role of carbon nitride surface nanostructure on gemfibrozil degradation. In particular, the use of silver on mesoporous carbon nitride single-atom catalyst (Ag@mpgC3N4) leads to an unprecedented degradation of gemfibrozil (>90% within 60 min). The possible degradation intermediates and products were identified by mass spectrometry and were inert by cytotoxicity evaluation. We anticipate that, with further refinement and customization, the carbon nitride catalysts reported herein may find broad applications for light-driven degradation of other contaminants of emerging concern.
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The global warming crisis has sparked a series of environmentally cautious trends in chemistry, allowing us to rethink the way we conduct our synthesis, and to incorporate more earth-abundant materials in our catalyst design. "Single-atom catalysis" has recently appeared on the catalytic spectrum, and has truly merged the benefits that homogeneous and heterogeneous analogues have to offer. Further still, the possibility to activate these catalysts by means of a suitable electric potential could pave the way for a true integration of diverse synthetic methodologies and renewable electricity. Despite their esteemed benefits, single-atom electrocatalysts are still limited to the energy sector (hydrogen evolution reaction, oxygen reduction, etc.) and numerous examples in the literature still invoke the use of precious metals (Pd, Pt, Ir, etc.). Additionally, batch electroreactors are employed, which limit the intensification of such processes. It is of paramount importance that the field continues to grow in a more sustainable direction, seeking new ventures into the space of organic electrosynthesis and flow electroreactor technologies. In this piece, we discuss some of the progress being made with earth abundant homogeneous and heterogeneous electrocatalysts and flow electrochemistry, within the context of organic electrosynthesis, and highlight the prospects of alternatively utilizing single-atom catalysts for such applications.
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Azetidinium salts are important motifs in organic synthesis but are difficult to obtain due to extremely long synthetic protocols. Herein, a rapid continuous-flow process for the on-demand synthesis of azetidinium salts is described. In particular, the nucleophilic addition of secondary amines and the subsequent intramolecular N-cyclization have been investigated in batch and continuous-flow modes, exploring the effects of solvent type, temperature, reaction time, and amine substituent on the synthesis of azetidinium salts. This has enabled us to quickly identify optimal reaction conditions and obtain microkinetic parameters, verifying that the use of a flow reactor leads to a reduction of the activation energy for the epichlorohydrin aminolysis due to the better control of mass and heat transfer during reaction. This confirms the key role of continuous-flow technologies to affect the kinetics of a reaction and make synthetic protocols ultrarapid and more efficient.
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Because of their peculiar nitrogen-rich structure, carbon nitrides are convenient polydentate ligands for designing single atom-dispersed photocatalysts. However, the relation between catalysts' textural properties and their photophysical-photocatalytic properties is rarely elaborated. Herein, we report the preparation and characterization of a series of single-atom heterogeneous catalysts featuring highly dispersed Ag and Cu species on mesoporous graphitic C3N4. We show that adjustment of materials textural properties and therefore metal single-atom coordination mode enables ligand-to-metal charge transfer (LMCT) or ligand-to-metal-to-ligand charge transfer (LMLCT), properties that were long speculated in single-atom catalysis but never observed. We employ the developed materials in the degradation of organic pollutants under irradiation with visible light. Kinetic investigations under flow conditions show that single atoms of Ag and Cu decrease the number of toxic organic fragmentation products while leading to a higher selectivity toward full degradation. The results correlate with the selected mode of charge transfer in the designed photocatalysts and provide a new understanding of how the local environment of a single-atom catalyst affects the surface structure and reactivity. The concepts can be exploited further to rationally design and optimize other single-atom materials.
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Controlling the structure sensitivity of catalyzed reactions over metals is central to developing atom-efficient chemical processes. Approaching the minimum ensemble size, the properties enter a non-scalable regime in which each atom counts. Almost all trends in this ultra-small frontier derive from surface science approaches using model systems, because of both synthetic and analytical challenges. Exploiting the unique coordination chemistry of carbon nitride, we discriminate through experiments and simulations the interplay between the geometry, electronic structure, and reactivity of palladium atoms, dimers, and trimers. Catalytic tests evidence application-dependent requirements of the active ensemble. In the semi-hydrogenation of alkynes, the nuclearity primarily impacts activity, whereas the selectivity and stability are affected in Suzuki coupling. This powerful approach will provide practical insights into the design of heterogeneous catalysts comprising well-defined numbers of atoms.
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Palladium-catalysed cross-coupling reactions, central tools in fine-chemical synthesis, predominantly employ soluble metal complexes despite recognized challenges with product purification and catalyst reusability1-3. Attempts to tether these homogeneous catalysts on insoluble carriers have been thwarted by suboptimal stability, which leads to a progressively worsening performance due to metal leaching or clustering4. The alternative application of supported Pd nanoparticles has faced limitations because of insufficient activity under the mild conditions required to avoid thermal degradation of the substrates or products. Single-atom heterogeneous catalysts lie at the frontier5-18. Here, we show that the Pd atoms anchored on exfoliated graphitic carbon nitride (Pd-ECN) capture the advantages of both worlds, as they comprise a solid catalyst that matches the high chemoselectivity and broad functional group tolerance of state-of-the-art homogeneous catalysts for Suzuki couplings, and also demonstrate a robust stability in flow. The adaptive coordination environment within the macroheterocycles of ECN facilitates each catalytic step. The findings illustrate the exciting opportunities presented by nanostructuring single atoms in solid hosts for catalytic processes that remain difficult to heterogenize.
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Indium oxide catalyzes acetylene hydrogenation with high selectivity to ethylene (>85 %); even with a large excess of the alkene. Inâ situ characterization reveals the formation of oxygen vacancies under reaction conditions, while an in depth theoretical analysis links the surface reduction with the creation of well-defined vacancies and surrounding In3 O5 ensembles, which are considered responsible for this outstanding catalytic function. This behavior, which differs from that of other common reducible oxides, originates from the presence of four crystallographically inequivalent oxygen sites in the indium oxide surface. These resulting ensembles are 1)â stable against deactivation, 2)â homogeneously and densely distributed, and 3)â spatially isolated and confined against transport; thereby broadening the scope of oxides in hydrogenation catalysis.
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Herein, we present an approach to create a hybrid between single-atom-dispersed silver and a carbon nitride polymer. Silver tricyanomethanide (AgTCM) is used as a reactive comonomer during templated carbon nitride synthesis to introduce both negative charges and silver atoms/ions to the system. The successful introduction of the extra electron density under the formation of a delocalized joint electronic system is proven by photoluminescence measurements, X-ray photoelectron spectroscopy investigations, and measurements of surface ζ-potential. At the same time, the principal structure of the carbon nitride network is not disturbed, as shown by solid-state nuclear magnetic resonance spectroscopy and electrochemical impedance spectroscopy analysis. The synthesis also results in an improvement of the visible light absorption and the development of higher surface area in the final products. The atom-dispersed AgTCM-doped carbon nitride shows an enhanced performance in the selective hydrogenation of alkynes in comparison with the performance of other conventional Ag-based materials prepared by spray deposition and impregnation-reduction methods, here exemplified with 1-hexyne.
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We report the preparation and hydrogenation performance of a single-site palladium catalyst that was obtained by the anchoring of Pdâ atoms into the cavities of mesoporous polymeric graphitic carbon nitride. The characterization of the material confirmed the atomic dispersion of the palladium phase throughout the sample. The catalyst was applied for three-phase hydrogenations of alkynes and nitroarenes in a continuous-flow reactor, showing its high activity and product selectivity in comparison with benchmark catalysts based on nanoparticles. Density functional theory calculations provided fundamental insights into the material structure and attributed the high catalyst activity and selectivity to the facile hydrogen activation and hydrocarbon adsorption on atomically dispersed Pd sites.
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Paladio/química , Catálisis , Hidrogenación , Microscopía Electrónica de Transmisión de RastreoRESUMEN
Herein, we report the preparation of small and narrowly distributed (2.1 ± 0.5 nm) Ag nanoparticles supported on passivated silica, where the surface OH groups are replaced by OSiMe3 functionalities. This synthetic method involves the grafting of silver(I) bis(trimethylsilyl)amide ([AgN(SiMe3)2]4) on silica partially dehydroxylated at 700 °C, followed by a thermal treatment of the grafted complex under H2. The catalytic performance of this material was investigated in the semi-hydrogenation of propyne and 1-hexyne and compared with that of 2.0 ± 0.3 nm Ag nanoparticles supported on silica. Whilst surface passivation slightly decreases the activity in both reactions (by a factor 2-3), probably as a result of the decreased alkyne adsorption properties or the presence of less accessible active sites on the passivated support, the AgNP@SiO2 catalysts demonstrate a remarkable selectivity for the production of alkenes.
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The determination of structure-performance relationships of ceria in heterogeneous reactions is enabled by the control of the crystal shape and morphology. Whereas the (100) surface, predominantly exposed in nanocubes, is optimal for CO oxidation, the (111) surface, prevalent in conventional polyhedral CeO2 particles, dominates in C2H2 hydrogenation. This result is attributed to the different oxygen vacancy chemistry on these facets. In contrast to oxidations, hydrogenations on CeO2 are favored over low-vacancy surfaces owing to the key role of oxygen on the stabilization of reactive intermediates. The catalytic behavior after ageing at high temperature confirms the inverse face sensitivity of the two reaction families.
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We report on the excellent stereo and chemoselectivity of nanosized silver and gold catalysts in the three-phase hydrogenation of acetylenic compounds under flow chemistry conditions. The materials featuring metal nanoparticles in the range of 2-21 nm were prepared by spray deposition or incipient wetness impregnation of silver nitrate and sol immobilisation of gold chloride on different carriers (Al2O3, SiO2, TiO2, and carbon), followed by activation in various atmospheres. The samples were characterised by ICP-OES, N2 sorption, XPS, HAADF-STEM, and HRTEM, and evaluated in a continuous-flow flooded-bed micro-reactor. Both metals display optimal activities for particles below 5 nm, enabling stable operation at T = 373 K and P = 10 bar. While the performance of the silver catalysts is less influenced by the support, the gold nanoparticles exhibit significant activity only when deposited on TiO2, likely due to the strong metal-support interaction. Hydrogenations of functionalised alkynes reveal that silver and gold match, and in some cases exceed, the selectivity of benchmark palladium-based catalysts. Furthermore, in contrast to Pd, the Ag and Au samples require no modifiers, which brings fundamental and practical simplifications for their understanding and large scale manufacture. Therefore, these materials could be advantageously used for the continuous production of olefinic intermediates in the fine chemical and pharmaceutical industries.