Functionalized Metal-Free Carbon Nanosphere Catalyst for the Selective C–N Bond Formation under Open-Air Conditions
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Functionalized Metal-Free Carbon Nanosphere Catalyst for the Selective C–N Bond Formation under Open-Air Conditions
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ACS Omega

Cite this: ACS Omega 2024, 9, 33, 35676–35685
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https://doi.org/10.1021/acsomega.4c03987
Published August 8, 2024

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Abstract

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A versatile shape-controlled carbon nanomaterial that can efficiently catalyze the selective C–N coupling reactions under metal-free and open-air conditions was developed by applying N-doping and KOH activation strategies in candle soot (ANCS). The TEM and elemental mapping results showed the formation of sphere-shaped carbon particles as well as the uniform distribution of nitrogen species in the carbon framework. KOH activation enhanced the specific surface area of carbon, whereas N-doping enriched the electron-deficient nature by introducing functional N-based pyrrolic/graphitic structures in the carbon framework. The synergistic effect of N-doping and KOH activation significantly improved the catalytic efficiency of the carbon catalyst (ANCS), giving a 96% conversion of o-phenylenediamine (OPD) with a good selectivity to 2-phenylbenzimidazole (97%). In contrast, the pristine carbon exhibited very low activity (48% conversion of the OPD and 36% selectivity to 2-phenylbenzimidazole). Besides, the ANCS nanomaterial provided a facile catalytic approach for the homo- and cross-C–N condensation of various aromatic amines and diamines to produce diverse functional imines and benzimidazoles at mild conditions. This work provided promising insights into developing advanced, metal-free carbon-based catalysts for selective C–N coupling reactions to produce valuable drug motifs.

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1. Introduction

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Transition metals are the essential components of numerous industrially used heterogeneous catalysts. (1) However, their expensive nature and rapid catalytic deactivation impose several practical limitations. Thus, developing efficient metal-free catalysts is a primary goal of the catalysis community. Carbon-based materials have received significant attention for heterogeneous catalysis because of the unique properties of carbon, such as high abundance, low cost, remarkable hydrothermal stability, tunable porosity, and high specific surface area. (2,3) Another vital advantage of carbon materials is their facile functionalization either by acid or base molecules or by elements based on the target catalytic application, while tuning their porous framework to achieve enhanced diffusion properties. Thus, the fine engineering of the structure and porosity of carbon could provide potential strategies for developing multifunctional carbon-based materials with selective catalytic properties for the chemical industry.
The carbon–nitrogen (C–N) coupling is a pivotal chemical reaction for producing diverse nitrogenous organic compounds for the pharma industry. (4−12) In particular, benzimidazole-based compounds are widely used drugs for various medical treatments (Figure 1). The coupling of benzylamine with o-phenylenediamine (OPD) gives 2-phenylbenzimidazole. (8) However, this reaction typically requires a strong oxidant (TBHP, H2O2, or molecular oxygen) to accelerate the oxidative dehydrogenation of benzylamine to the benzaldehyde intermediate, which then reacts with OPD to give 2-phenylbenzimidazole. A similar mechanism can also be expected in the other C–N condensation of aromatic amines. Using air as the oxidant for the selective C–N condensation with an efficient metal-free heterogeneous catalyst has great promise in terms of both economic and eco-friendly points of view. (12−14)

Figure 1

Figure 1. Benzimidazole-based drug molecules.

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Several types of carbon materials, such as carbon black, mesoporous carbon, graphene, carbon nanofibers, and carbon nanotubes, have been developed for various applications, including heterogeneous catalysis. (15,16) In recent years, candle soot (CS) nanospheres have emerged as versatile carbon nanomaterials because of their well-controlled particle size/morphology, high specific surface area, tunable porosity, and efficient functionalization. (17,18) The CS nanospheres can be efficiently synthesized by using a facile combustion approach of the candles. The morphology of carbon nanospheres allows for efficient functionalization, expanding their applications in heterogeneous catalysis, adsorption, gas separation, energy storage, and drug delivery. (19) The waving flake curvature structure of the carbon nanospheres provides more active edge sites and dangling bonds for heterogeneous catalytic applications. (20) Another advantage of carbon nanospheres is their interconnected graphitic network, which can control the mass diffusion properties of the reactants, intermediates, and products to achieve higher yields of the desired products at mild reaction conditions. The CS nanospheres can exhibit good dispersion in various organic solvents and reagents because of their flexible carbon structure, enabling them to be promising catalytic materials for various organic chemical reactions.
KOH activation and N-doping are two fundamental strategies widely investigated to develop functional carbon materials. These strategies can tune the porous framework of carbon while introducing functional groups and defect sites in its structure. On the one hand, the redox reactions of carbon with KOH facilitate the intercalation of metallic K between the carbon lattices. (21) The subsequent removal of K results in increased porosity and a higher specific surface area. The KOH concentration and the activation temperature determine the reactions between carbon and KOH, and the consequent porous structure and surface area. On the other hand, replacing some carbon atoms with N dopants in the carbon framework creates electron-deficient carbon as the inserted N pulls the electron density from the adjacent carbon because of its higher electronegativity than the carbon. (22−25) The electron-deficient carbon, acting as the Lewis acid site, can interact with nucleophilic molecules like benzylamine to accelerate the C–N coupling reactions. (2,26) The inserted N species can also act as Lewis basic sites. Thus, N-doped carbon materials can be used for both acid- and base-catalyzed reactions. Moreover, N-doping can introduce various N-heterocycles, such as pyridinic-N, pyrrolic-N, graphitic-N (or quaternary-N), and pyridinic-N-oxide, depending on the reagents used, preparation method, and postsynthesis activation strategy. The unique features of bonding and coordination configurations of N-doped heterocyclic structures can selectively influence the activity of carbon catalysts for specific catalytic applications. The nucleophilic nature of the nitrogen of these species can enhance the interaction of the carbon catalyst with the electron-deficient intermediates (e.g., benzaldehyde) for achieving higher reaction rates in the C–N condensation reactions. (27) These promising benefits motivated us to develop a functional CS nanomaterial. In this work, we used a simple combustion process to prepare well-defined CS nanospheres with homogeneous particle sizes and morphology. The CS nanospheres are effectively functionalized by treatment with melamine (nitrogen source) and then KOH activation. Another advantage of carbon nanospheres is their interconnected graphitic network, which can control mass diffusion properties of the reactants, intermediates, and products to achieve higher yields of the desired products at mild reaction conditions. The developed carbon materials were characterized by using various analytical techniques to elucidate their properties in comparison to those of the pristine CS material. The ANCS catalyst was found to be highly active for the C–N condensation reactions to synthesize diverse imines and benzimidazoles without using any external oxidizing reagent, highlighting the eco-friendly and economic aspects of this work for organic chemistry.

2. Experimental Section

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A detailed procedure of the synthesis, characterization, and catalytic activity of the carbon materials is provided in the Supporting Information.

3. Results and Discussion

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3.1. Characterization Studies

The TEM analysis is carried out to elucidate the morphology and particle size of the carbon materials (Figure 2). The CS sample contains well-defined spherical particles (particle size is 70–85 nm), which are interlinked to form aggregates with large voids. (28) The ANCS sample also contains spherical-shaped carbon particles with a particle size of 50–75 nm. The STEM-EDX analysis confirmed the uniform dispersion of carbon, oxygen, and nitrogen species in the ANCS catalyst (Figure 3). Especially, the uniform incorporation of nitrogen can provide N-based functional groups in the carbon framework, which could play a crucial role in the C–N coupling reactions. All the carbon materials showed type IV isotherms (Figure S1, Supporting Information). The CS and ANCS materials showed a typical H3-type hysteresis loop, which represents slit-shaped voids in the aggregates of carbon spheres. In contrast, the ACS material showed an H4-type hysteresis loop corresponding to narrow slit-shaped voids. The presence of voids (Figure 2) formed by interlinked aggregates of spherical particles could be the reason for the observed hysteresis loops. The BET surface area of the CS nanomaterial was found to be 276 m2/g (Table S1, Supporting Information). The activation of the CS nanomaterial with KOH significantly increased the BET surface area (2050 m2/g for ACS). However, the N-doping, followed by KOH activation, led to a decrease in BET surface area (1596 m2/g for ANCS), which could be due to the aggregation of the particles and/or structural changes in the carbon framework. The mean pore volume and pore diameter of the carbon materials were found to be in the range of 0.8–1.8 cc/g and 2.8–12 nm, respectively (Table S1, Supporting Information).

Figure 2

Figure 2. TEM images of the CS and ANCS materials.

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Figure 3

Figure 3. STEM-EDAX elemental mapping images of the ANCS catalyst.

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The powder XRD data showed a broad diffraction peak at 2 theta of 25.2° in the CS sample, which belongs to the (002) plane of the graphene sheets (Figure 4a). (29,30) The broadness of the graphitic (002) plane was increased in ACS and ANCS samples, which is due to the increased defects along with variation in the interlayer stacking of the graphene sheets due to the KOH activation and N-doping. The ACS and ANCS samples show an additional broad XRD peak at 2 theta of 43.7°, corresponding to the (100) plane of the graphitic structure. The Raman spectra of all the samples showed two bands at 1349 and 1589 cm–1, which belong to the defective (D) and graphitic (G) nature of the carbon, respectively (Figure 4b). (31,32) The defective D band can denote disordered carbon, structural defects, and/or the defunctionalization of oxygen-based species in carbon materials. The ID/IG ratio represents the relative number of defect sites in the carbon framework. The ANCS sample contains a higher ratio of ID/IG (1.53) in comparison to the ACS (1.26) and CS (0.86) samples. This is due to the replacement of carbon atoms in the graphitic structure with nitrogen atoms, forming nitrogen-based functional groups with disordered carbon and structural defects in the ANCS sample, which was elucidated by FT-IR, EPR, and XPS analyses in the following sections.

Figure 4

Figure 4. (a) Powder XRD patterns and (b) Raman spectra of candle soot (CS), activated candle soot (ACS), and activated N-doped candle soot (ANCS) catalysts.

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The FT-IR analysis was conducted to identify the functional groups in the carbon materials (Figure 5a). All the samples exhibit a broad peak in the range 3300–3500 cm–1 belonging to functional hydroxyl (−OH) and/or amine (−NH) groups. The pure CS sample shows several peaks belonging to −C–O (1238 cm–1), −C=C (1596 cm–1), −COOH/–C=O (1730 cm–1), and −OH (3418 cm–1) groups. (26,33) These peaks are not found in the ACS sample, which is due to the removal of oxygen-containing groups by KOH activation. In the place of oxygen functional groups, various N-based functional groups (−C–N; 1144 cm–1, −C=N; 1587 cm–1, and −NH; 3449 cm–1) were noticed in the ANCS sample, indicating the N-doping in the carbon structure, in line with the Raman studies (Figure 4b). An FT-IR peak corresponding to the aromatic methyl group (−CH; 593 cm–1) is observed only in the case of the ANCS sample. (26,33) The EPR analysis showed that the ACS and ANCS catalysts exhibit broader EPR signals and lower g-values as compared to the pure CS catalyst (Figure S2, Supporting Information). This indicates the perturbations in the electronic and surface structure of the carbon framework after the KOH and N-doping treatments. (34,35) A decrease in the g-value of the ACS and ANCS catalysts compared to the pure CS catalyst indicates a decrease in the degree of oxidation of the carbon in ACS and ANCS catalysts. The increased width of the EPR peaks reveals an increased BET surface area of ACS and ANCS catalysts compared to the CS catalyst (Table S1, Supporting Information). (35) Though the EPR analysis is used to estimate the defect density in the carbon materials, it must be indicated that the EPR technique cannot detect all defects present in the carbon framework. (34)

Figure 5

Figure 5. (a) FT-IR spectra and (b) C 1s XPS, (c) N 1s XPS, and (d) O 1s XPS of candle soot (CS), activated candle soot (ACS), and activated N-doped candle soot (ANCS) catalysts.

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The XPS analysis was conducted to elucidate the nature of carbon, nitrogen, and oxygen species in the carbon catalysts. The C 1s XP spectrum of pure CS shows three major peaks at the binding energies (BEs) of 284.2, 285.7, and 289.6 eV, corresponding to the carbon species of C=C, C–O, and O–C=O groups, respectively (Figure 5b). (30) The close observation of Figure 5b reveals that the major C 1s peak (284.2 eV) belonging to C=C in the CS sample is shifted to higher BEs in the ACS and ANCS samples, whereas the C 1s peaks of the C–O and O–C=O groups are shifted to lower BEs. The ANCS sample exhibits an additional peak at 292.2 eV, corresponding to carbon species in the N–C=O group, confirming the N-doping in the carbon structure. The N 1s XP spectrum of the ANCS sample shows two peaks at 399.7 and 402.3 eV, which belong to pyrrolic and graphitic nitrogen, respectively (Figure 5c). (26,36) The pyrrolic nitrogen is dominant in the ANCS sample. The CS sample shows three O 1s XPS peaks at 530.8, 532.6, and 534.9 eV, corresponding to oxygen in the C=O, C–O, and O=C–OH groups, respectively (Figure 5d). (26) The ACS sample does not contain the third O 1s peak (O=C–OH group), indicating the selective defunctionalization of acidic groups in carbon by KOH (base). The exposure of the KOH/CS mixture to the temperature of 900 °C promotes the reaction of the KOH base with the oxygen-containing acidic functional groups of CS, resulting in the removal of O=C–OH groups from the carbon matrix. (37,38) Applying higher temperatures (900 °C) after the KOH treatment can also remove the oxygen-containing groups from the carbon matrix in the form of oxygen, carbon dioxide, and water. (39) However, the ANCS sample exhibited the third O 1s peak, corresponding to the O=C–OH group (Figure 5d). This indicates that KOH activation is not very effective for N-doped CS material to defunctionalize oxygen species. The reason is that the inserted N species in the carbon matrix can exhibit a strong electrostatic interaction with the O=C–OH groups, which are very difficult to defunctionalize by the KOH treatment. (40,41) The elemental compositions of carbon, nitrogen, oxygen, and hydrogen in the ANCS catalyst, estimated by CHNO analysis, were found to be 95.78, 1.31, 2.46, and 0.45 wt %, respectively. However, a very low N amount (0.39 wt %) was found by the XPS analysis as it is a surface-sensitive technique and cannot detect the total nitrogen amount in the ANCS catalyst.
The catalyst’s acid sites (Bro̷nsted and Lewis), especially Lewis acid sites, play a crucial role in the C–N coupling reactions. (11,12) The electron-deficient carbon in the developed carbon catalysts can act as the Lewis acid sites (LA), and the hydroxyl and carboxyl species are the Bro̷nsted acid sites (BA). The pyridine-adsorbed FT-IR analysis of carbon catalysts is conducted to estimate the nature of the acid sites (Figure 6). The noticed FT-IR peaks in the range of 1408–1467, 1471–1521, 1524–1554, 1556–1619, and 1645–1699 cm–1 are assigned to LA, LA + BA, BA, LA, and BA, respectively (Figure 6). (11,42) The CS sample contains only one peak at 1556–1619 cm–1, corresponding to LA sites. In contrast, the ACS and ANCS samples contain both BA and LA sites’ peaks, but their intensity is lower in the ACS sample compared to the ANCS sample. It indicates the presence of more Lewis and Bro̷nsted acid sites in the ANCS catalyst.

Figure 6

Figure 6. Pyridine-adsorbed FT-IR spectra of the CS, ACS, and ANCS catalysts.

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4. Catalytic Activity Studies

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4.1. C–N Coupling of OPD with Benzylamine Using Carbon Nanomaterials

The catalytic efficiency of the developed carbon materials was investigated for the synthesis of 2-phenylbenzimidazole via cascade C–N coupling of OPD with benzylamine using a 1.5:2.5 molar ratio of OPD and benzylamine in toluene solvent at a 4 h reaction time and 100 °C reaction temperature under open-air conditions. In the blank case (i.e., without the catalyst), no noticeable conversion of OPD was found (entry 1, Table 1). The observed OPD conversion and product selectivity over the developed carbon nanomaterials (entries 2–4, Table 1) at the same reaction conditions employed for the blank test indicate their catalytic efficacy in the C–N condensation reaction under metal-free and open-air conditions. Pure CS material gave 48% conversion of the OPD, but very low selectivity to 2-phenylbenzimidazole (36%) was achieved (entry 2, Table 1). In the case of the ACS catalyst, both the level of conversion of OPD (76%) and 2-phenylbenzimidazole (82%) was significantly increased (entry 3, Table 1). The best results were found with the ANCS catalyst, which gave 89% conversion with 90% 2-phenylbenzimidazole selectivity (entry 4, Table 1). With the increase of reaction time from 4 to 5 h, the ANCS catalyst gave 96% conversion of OPD and 97% 2-phenylbenzimidazole selectivity (entry 5, Table 2). With 5 and 10 wt % of the ANCS catalyst, low conversion/selectivity was obtained (entries 6 and 7, Table 1), respectively. Under the inert atmospheric conditions (i.e., N2), the level of the OPD conversion was very low (entry 8, Table 1), indicating the necessity of air for the C–N coupling reaction.
Table 1. C–N Coupling of Benzylamine and OPD Using Various Carbon-Based Catalystsa
s. no.catalystOPD conversion (%)benzimidazole selectivity (%)other products
1blanknegligible  
2CS483664
3ACS768218
4ANCS899010
5bANCS96973
6cANCS583961
7dANCS828119
8eANCS86139
a

1.5 mmol o-phenylenediamine (OPD), 2.5 mmol benzylamine, 15 wt % catalyst with respect to OPD, 100 °C, 4 h, toluene (1 mL), and open-air conditions.

b

5 h.

c

5 wt % catalyst.

d

10 wt % catalyst.

e

Inert atmospheric condition (N2 gas).

Table 2. Substrate Scope in the C–N Coupling of Benzylamine and OPD Using the ANCS Catalysta
a

1.5 mmol OPD, 2.5 mmol benzylamine, 15 wt % catalyst with respect to OPD, 100 °C, 5 h, toluene (1 mL), and open-air conditions.

b

OPD conversion.

c

Benzimidazole selectivity.

4.2. Kinetic and Hot-Filtration Studies in the C–N Coupling of Benzylamine and OPD Using the ANCS Catalyst

The effect of reaction temperature from 1 to 5 h in the coupling of benzylamine and OPD using the ANCS catalyst was conducted at 100 °C in toluene solvent (Figure 7a). Both the conversion of OPD and product selectivity were rapidly increased with the increase of reaction time up to 3 h. This is due to the availability of high concentrations of reactants until 3 h, accelerating the forward reaction toward benzimidazole formation. After 3 h of reaction time, the reaction faces equilibrium limitations, resulting in a trivial increase in OPD conversion. The maximum conversion of OPD (96%) with 97% selectivity to benzimidazole was obtained at 5 h. The hot-filtration test was performed to elucidate the stability of the ANCS catalyst in the C–N condensation of the OPD and benzylamine (Figure 7b). Initially, the reaction was conducted for 2 h, and then the ANCS catalyst was removed from the reaction mixture by centrifugation. The reaction was continued for another 3 h using the obtained catalyst-free filtrate at the same reaction conditions. The conversion of OPD remains the same almost after removing the catalyst. Hence, the leaching of the active sites has not occurred, and the reaction is truly heterogeneous, emphasizing the stability of the ANCS catalyst.

Figure 7

Figure 7. (a) Kinetic studies (reaction conditions: 1.5 mmol of o-phenylenediamine (OPD), 2.5 mmol of benzylamine, 15 wt % catalyst with respect to the OPD, 100 °C, 1 mL of toluene, and open-air conditions) and (b) hot-filtration test using the ANCS catalyst (reaction conditions: 1.5 mmol of o-phenylenediamine (OPD), 2.5 mmol of benzylamine, 15 wt % catalyst with respect to the OPD, 100 °C, 1 mL of toluene, and open-air conditions).

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4.3. C–N Coupling of Diverse Benzylamines and OPDs Using the ANCS catalyst

The versatile catalytic efficacy of the ANCS material was demonstrated for the C–N coupling of various substituted benzylamines and OPDs (Table 2). The cross-coupling of model compounds, such as benzylamine and OPD, gave 96% conversion with 97% selectivity to benzimidazole (entry 1, Table 2). With this reference, the ANCS catalyst showed reasonably good activity in the C–N condensation of para-substituted benzylamines with OPD (entries 2 and 3, Table 2). In the case of C–N condensation of ortho- and meta-substituted benzylamines with OPD, low conversions/selectivities were obtained irrespective of the substituents’ nature (entries 4–6, Table 2). It indicates that the steric hindrance over the electronic effects of the substituents is a dominating factor in the condensation of substituted benzylamines with OPD over the ANCS catalyst. Optimum conversions/selectivities were obtained when both OPD and benzylamine contained the substituents at meta and para positions, respectively (entries 7 and 8, Table 2). In the case of ortho-chloro benzylamine and meta-methyl OPD (entry 9, Table 2), 74% conversion of OPD with 92% selectivity to benzimidazole was obtained at 5 h, which is due to the steric hindrance of the substituents. Optimum selectivity to benzimidazole (99%) was obtained when the para-substituted benzylamines reacted with the meta-substituted OPD (entries 10 and 11, Table 2). Overall, the ANCS catalyst showed great potential for the condensation of various benzylamines and OPDs to produce functional benzimidazoles at metal-free and open-air conditions.

4.4. Benzylamine Homocoupling Using the ANCS Catalyst

To further understand the catalytic role of ANCS nanomaterial in the C–N condensation reactions under open-air conditions, we conducted the homocoupling of various benzylamines, which is a pivotal reaction to obtain valuable imines for the pharma industry. (43) The reaction conditions, including temperature, time, and catalyst amount, were optimized to achieve maximum conversion/selectivity in benzylamine homocoupling. The probable reaction mechanism is presented in the Supporting Information (Figure S3). (43) The complete conversion of benzylamine with 99% selectivity to imine was obtained at 100 °C for 5 h in an open-air atmosphere with toluene as the solvent (entry 1, Table 3), whereas only a 2% conversion of benzylamine was noticed in the case of a blank test (i.e., without the catalyst). This emphasizes the catalytic role of the ANCS nanomaterial in benzylamine homocoupling under open-air conditions. Irrespective of the position and nature of the substituents on benzylamine, optimum conversions and/or selectivity were obtained over the ANCS catalyst (entries 2–7, Table 3). Unlike the C–N condensation of benzylamine with an OPD (Table 2), the effect of steric hindrance of the substituents is not observed in the case of benzylamine homocoupling.
Table 3. Benzylamine Homocoupling Using the ANCS Catalysta
a

1 mmol benzylamine, 10 wt % catalyst, 100 °C, 5 h, toluene (1 mL), and open-air conditions.

b

Benzylamine conversion.

c

Imine selectivity.

The probable mechanism for the C–N coupling of benzylamine with OPD is shown in Figure 8. (43−45) Here, the oxidation of benzylamine using atmospheric air takes place to produce a benzaldehyde intermediate. Then, one of the NH2 groups of OPD, which acts as a nucleophile, reacts with benzaldehyde to give an imine intermediate with water as a byproduct. The subsequent intramolecular C–N condensation, followed by the release of water, gives the desired benzimidazole product. Several controlled reactions between benzylamine and OPD were conducted to estimate the formation of the benzaldehyde intermediate. About 2–3% selectivity of benzaldehyde was noticed, indicating that benzaldehyde reacts with the OPD as soon as it forms from benzylamine. (44,45) However, the identification of benzaldehyde supports the proposed mechanism in Figure 8. In this probable mechanism, the formation of benzaldehyde from benzylamine and the subsequent activation of benzaldehyde to couple with the OPD can be accelerated by the acid and defect sites of the carbon catalyst. Zhao et al. found that the defects and high BET surface area can contribute to the adsorption and activation of oxygen over a carbon catalyst, which is crucial for the oxidative coupling of amines. (43) Raman studies showed that the ANCS catalyst contains a higher number of defect sites due to the synergistic effect of N-doping and KOH activation (Figure 4b). Naturally, the defect sites are electron-deficient, thus they can activate benzylamine and intermediates (benzaldehyde) to facilitate the forward reaction steps in the C–N coupling of benzylamine with OPD. XPS studies provided a piece of evidence for the presence of pyrrolic-N and graphitic-N species in the ANCS catalyst (Figure 5c). Both pyrrolic-N and graphitic-N species are found to provide active sites for the oxidative C–N condensation of amines. (2,26) Moreover, the π electrons of pyrrolic-N atoms can enhance the interaction of the ANCS catalyst with the electron-deficient intermediates (e.g., benzaldehyde and imine), accelerating the reaction toward benzimidazole formation. (27) The pyridine-adsorbed FT-IR studies indicated that the ANCS catalyst contains strong FT-IR peaks belonging to the Lewis acid sites (Figure 6). The C–N coupling reactions are catalyzed by the Lewis acid sites, explaining the excellent activity of the ANCS catalyst. Therefore, it is concluded that the higher concentration of defect sites, functional N-heterocyclic structures, and higher amount of Lewis acid sites are the reasons for the excellent catalytic activity of the ANCS nanomaterial in the selective C–N condensation reactions under metal-free and open-air conditions. The literature comparison revealed (Table S2, Supporting Information) that the ANCS catalyst gave optimum yields of 2-phenylbenzimidazole (94%) at milder reaction conditions without using any external oxidant (entry 4, Table S2, Supporting Information). In contrast, the literature reports used homogeneous, expensive, and metal-based catalysts, higher reaction temperatures (100–130 °C), longer reaction times (5–12 h), and molecular oxygen as the oxidant (entries 1–3, Table S2, Supporting Information). This indicates the significance of the ANCS catalyst, which is active for the C–N coupling reaction under open-air conditions at 100 °C and 5 h (entry 4, Table S2, Supporting Information).

Figure 8

Figure 8. Schematic representation of coupling of benzylamine with an OPD to form benzimidazole over the ANCS catalyst.

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5. Conclusions

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We successfully developed an efficient metal-free carbon catalyst for the selective C–N coupling of amines under open-air conditions. The ANCS catalyst prepared by N-doping and KOH activation showed excellent activity in selective C–N bond formation, which was attributed to more defects, functional N-based groups, and Lewis acid sites. The ANCS catalyst contains well-defined carbon nanospheres, and the nitrogen species are uniformly distributed in the carbon framework. The ANCS catalyst does not show any leaching of the active sites as confirmed by a hot-filtration test, indicating its practical application for the C–N condensation reactions under metal-free and air conditions. The broad application of the ANCS catalyst is showcased for the synthesis of diverse benzimidazoles and imines under mild conditions.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c03987.

  • Experimental section; N2 adsorption–desorption isotherms; BET surface area, pore volume, and pore diameter; EPR spectra; the probable reaction mechanism for homocoupling of benzylamine; and the comparison of the activity of the ANCS catalyst with that of the literature reports for the C–N coupling of OPD with benzylamine (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Kumar Krishan - Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Telangana 502284, India
    • Bhattu Swapna - Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Telangana 502284, India
    • Ankit Kumar Chourasia - Department of Chemical Engineering, Indian Institute of Technology Hyderabad, Kandi, Telangana 502284, India
    • Chandra S. Sharma - Department of Chemical Engineering, Indian Institute of Technology Hyderabad, Kandi, Telangana 502284, India;  Orcidhttps://orcid.org/0000-0003-3821-1471
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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B.S. is thankful to the PMRF and MHRD for providing fellowship. P.S. acknowledges the funding support from the SERB-CRG (CRG/2022/005932).

References

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    A review. The development of cost-effective, efficient, and novel catalytic systems is always an important topic for heterogeneous catalysis from academia and industrial points of view. Heteroatom-doped carbon materials have gained more and more attention as effective heterogeneous catalysts to replace metal-based catalysts, because of their excellent physicochem. properties, outstanding structure characteristics, environmental compatibility, low cost, inexhaustible resources, and low energy consumption. Doping of heteroatoms can tailor the properties of carbons for different utilizations of interest. In comparison to pure carbon catalysts, these catalysts demonstrate superior catalytic activity in many org. reactions. This review highlights the most recent progress in synthetic strategies to fabricate metal-free heteroatom-doped carbon catalysts including single and multiple heteroatom-doped carbons and the catalytic applications of these fascinating materials in various org. transformations such as oxidn., hydrogenation, hydrochlorination, dehydrogenation, etc.
  3. 3
    Wang, W.; Ma, L.; Wang, H.; Jiang, X.; He, Z.-H.; Wang, K.; Yang, Y.; Li, L.; Liu, Z.-T. Tridoped Mesoporous Carbon as a Metal-Free Catalyst for Ammoxidation of (Hetero)Aromatic Alcohols to Nitriles. ACS Appl. Nano Mater. 2023, 6 (16), 15193– 15203,  DOI: 10.1021/acsanm.3c02877
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    Wu, C.; Bu, J.; Wang, W.; Shen, H.; Cao, Y.; Zhang, H. Imine Synthesis by Benzylamine Self-Coupling Catalyzed by Cerium-Doped MnO2 under Mild Conditions. Ind. Eng. Chem. Res. 2022, 61 (16), 5442– 5452,  DOI: 10.1021/acs.iecr.2c00311
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    Zhang, M.; Wu, S.; Bian, L.; Cao, Q.; Fang, W. One-Pot Synthesis of Pd-Promoted Ce–Ni Mixed Oxides as Efficient Catalysts for Imine Production from the Direct N-Alkylation of Amine with Alcohol. Catal. Sci. Technol. 2019, 9 (2), 286– 301,  DOI: 10.1039/C8CY01857J
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    He, L.; Lou, X.; Ni, J.; Liu, Y.; Cao, Y.; He, H.; Fan, K. Efficient and Clean Gold-Catalyzed One-Pot Selective N-Alkylation of Amines with Alcohols. Chem. – Eur. J. 2010, 16 (47), 13965– 13969,  DOI: 10.1002/chem.201001848
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    Efficient and clean Gold-catalyzed one-pot selective N-alkylation of amines with alcohols
    He, Lin; Lou, Xia-Bing; Ni, Ji; Liu, Yong-Mei; Cao, Yong; He, He-Yong; Fan, Kang-Nian
    Chemistry - A European Journal (2010), 16 (47), 13965-13969, S13965/1-S13965/8CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A convenient synthesis of secondary amines via gold-catalyzed N-alkylation of primary amines with alcs. is described. Using Au/TiO2-VS (very small Au nanoparticles, ca. 1.8 nm) as catalyst, a series of secondary amines were obtained in excellent yields. This catalyst system provides a versatile and environmentally benign protocol for the economic synthesis of amines.
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    He, L.; Qian, Y.; Ding, R.; Liu, Y.; He, H.; Fan, K.; Cao, Y. Highly Efficient Heterogeneous Gold-catalyzed Direct Synthesis of Tertiary and Secondary Amines from Alcohols and Urea. ChemSusChem 2012, 5 (4), 621– 624,  DOI: 10.1002/cssc.201100581
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    Mavvaji, M.; Akkoc, S. Recent Advances in the Application of Heterogeneous Catalysts for the Synthesis of Benzimidazole Derivatives. Coord. Chem. Rev. 2024, 505, 215714  DOI: 10.1016/j.ccr.2024.215714
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    Kalbande, P. N.; Singh, N.; Swapna, B.; Umbarkar, S.; Sudarsanam, P. One-Pot Synthesized Efficient Molybdenum-niobium-Oxide Nanocatalyst for Selective C-O and C-N Coupling Reactions at Mild Conditions. Catal. Commun. 2023, 183, 106766  DOI: 10.1016/j.catcom.2023.106766
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    One-pot synthesized efficient molybdenum-niobium-oxide nanocatalyst for selective C-O and C-N coupling reactions at mild conditions
    Kalbande, Pavan Narayan; Singh, Nittan; Swapna, Bhattu; Umbarkar, Shubhangi; Sudarsanam, Putla
    Catalysis Communications (2023), 183 (), 106766CODEN: CCAOAC; ISSN:1566-7367. (Elsevier B.V.)
    An efficient molybdenum-niobium-oxide nanomaterial was synthesized by a one-pot hydrothermal method for selective C-O (glycerol ketalization) and C-N coupling (benzylamine oxidn.) reactions. The catalytically favorable properties, such as defective metal sites, truncated surfaces, and uniform metal dispersion in the MoO3-Nb2O5 nanorods, calcined at 500°C (MoNb OPS-5), were confirmed by Raman, HR-TEM, and STEM-EDX, resp. Because of improved Lewis/Bronsted acidic strength, the MoNb OPS-5 catalyst showed higher activity in glycerol ketalization and benzylamine oxidn. at mild conditions, giving superior selectivity to solketal (97%) and dibenzylimine (99%), resp. The MoNb OPS-5 catalyst showed high structural stability and considerable good reusability efficacy.
  10. 10
    Marakatti, V. S.; Sarma, S. Ch.; Joseph, B.; Banerjee, D.; Peter, S. C. Synthetically Tuned Atomic Ordering in PdCu Nanoparticles with Enhanced Catalytic Activity toward Solvent-Free Benzylamine Oxidation. ACS Appl. Mater. Interfaces 2017, 9 (4), 3602– 3615,  DOI: 10.1021/acsami.6b12253
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    Synthetically Tuned Atomic Ordering in PdCu Nanoparticles with Enhanced Catalytic Activity toward Solvent-Free Benzylamine Oxidation
    Marakatti, Vijaykumar S.; Sarma, Saurav Ch.; Joseph, Boby; Banerjee, Dipanjan; Peter, Sebastian C.
    ACS Applied Materials & Interfaces (2017), 9 (4), 3602-3615CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Synthesis of ordered compds. with nano size is of particular interest for tuning the surface properties with enhanced activity and selectivity toward various important industrial catalytic processes. In this work, we synthesized ordered PdCu nanoparticles as highly efficient catalyst for the solvent-free aerobic oxidn. of benzylamine. The PdxCu1-x catalysts with different chem. compns. (x = 0, 0.25, 0.4, 0.5, 0.6, 0.75, 1) were prepd. by polyol method using NaBH4 as a reducing agent and were well-characterized by X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy, XPS, transmission electron microscopy (TEM) energy-dispersive anal. of X-rays, and X-ray absorption fine structure. The effect of different metal concns. of Pd and Cu on the formation of PdxCu1-x nanoparticles was investigated. The XRD and TEM confirmed the formation of ordered PdCu intermetallic phase with body-centered cubic (BCC) structure for the synthetic compn. of Pd/Cu = 1:1. For compns. x = 0, 0.25, 0.75, and 1, PdxCu1-x alloy with face-centered cubic (FCC) structure was obsd., whereas mixed phase of BCC and FCC was obsd. for x = 0.4 and 0.6. The use of strong reducing agent (NaBH4) was essential to synthesize PdCu ordered phase compared to weak reducing agents such as oleylamine and ascorbic acid. The PdCu nanocatalyst with ordered structure (BCC) showed excellent catalytic activity compared to PdxCu1-x alloy nanoparticles with FCC structure. The at. ordering in the PdCu intermetallic was the driving force for the enhancement in the catalytic activity with high benzylamine conversion of 94.0% and dibenzylimine selectivity of 92.2% compared to its monometallic and alloy counterparts. Moreover, ordered PdCu alloy showed good recyclability and activity toward the oxidn. of different amines.
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    Arun Kumar, M.; Kamali, M.; Putla, S. B.; Subha, P.; Sudarsanam, P. Nb2O5/Ce1–xNbxO2−δ Nanorod Catalyst for Selective Oxidative Coupling of Aromatic Alcohols and Amines. ACS Appl. Nano Mater. 2024, 7 (6), 5899– 5911,  DOI: 10.1021/acsanm.3c05593
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    Singh, N.; Putla, S. B.; Pratap Singh, C.; Kalbande, P. N.; Choudhary, P.; Krishnamurty, S.; Krishnan, V.; Bhatte, K.; Sudarsanam, P. Shape-Controlled MoO3/MnOx Nanocatalyst for the Selective Synthesis of 2-Phenylquinoxaline Drug Motifs. ACS Appl. Nano Mater. 2023, 6 (24), 23442– 23453,  DOI: 10.1021/acsanm.3c04820
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    Ge, C.; Sang, X.; Yao, W.; Zhang, L.; Wang, D. Unsymmetrical Indazolyl-Pyridinyl-Triazole Ligand-Promoted Highly Active Iridium Complexes Supported on Hydrotalcite and Its Catalytic Application in Water. Green Chem. 2018, 20 (8), 1805– 1812,  DOI: 10.1039/C7GC02892J
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    Unsymmetrical indazolyl-pyridinyl-triazole ligand-promoted highly active iridium complexes supported on hydrotalcite and its catalytic application in water
    Ge, Chenyang; Sang, Xinxin; Yao, Wei; Zhang, Liang; Wang, Dawei
    Green Chemistry (2018), 20 (8), 1805-1812CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
    Herein, an indazolyl-pyridinyl-triazole ligand was synthesized and its Ir complex supported on hydrotalcite was characterized via x-ray power diffraction (XRD), XPS, energy dispersive X-ray (EDX) spectroscopy and TEM. This new heterogeneous catalyst bearing the unsym. indazolyl-pyridinyl-triazole ligand exhibits high catalytic activity in H2O. Both functionalized amines and imines were obtained from the challenging selective reaction of benzylamines with arylamines through transfer hydrogenation and dehydrogenation under clean conditions. In particular, this catalyst system showed good recovery performance in H2O. Mechanistic studies showed that this transformation occurs via amine dehydrogenation, hydrolysis and condensation processes. The direct capture of the reaction intermediate provides sufficient proof for this process.
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    Al-Hmoud, L.; Jones, C. W. Reaction Pathways over Copper and Cerium Oxide Catalysts for Direct Synthesis of Imines from Amines under Aerobic Conditions. J. Catal. 2013, 301, 116– 124,  DOI: 10.1016/j.jcat.2013.01.027
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    Reaction pathways over copper and cerium oxide catalysts for direct synthesis of imines from amines under aerobic conditions
    Al-Hmoud, Linda; Jones, Christopher W.
    Journal of Catalysis (2013), 301 (), 116-124CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
    Copper(II) oxide, cerium oxide, and several copper(II) oxide supported on cerium oxide catalysts are explored in the aerobic oxidative homocoupling of benzylamine to form N-benzylidenebenzylamine in DMSO at 110 °C. Although both CuO and CeO2 alone catalyze the reaction, CuO-CeO2 catalysts most efficiently catalyze the reaction, providing higher rates (per g Cu) due to the presence of both copper and ceria species in the reactor. Catalysts with lower copper loadings and increased ceria content reduce the product yield, as ceria domains can catalyze the decompn. of the desired product as well. The amine conversion occurs with a significant induction period, assocd. with the putative formation of an initial benzylimine intermediate in the case of catalysis with CeO2 alone or from slow copper solubilization in cases where supported or unsupported copper catalysts are used, followed by rapid conversion to the N-benzylidenebenzylamine product. Copper leaching studies clearly demonstrate that catalysis using copper-contg. catalysts is primarily assocd. with turnover by sol. copper species. Expts. targeted at elucidating the reaction pathway suggests that copper oxide domains promote the coupling of the initial intermediate, benzylimine, with benzylamine to produce the N-benzylidenebenzylamine product (path A), with a max. prodn. rate of 888 μmol/m2 h (13.3 mmol/gCu h) over the pure CuO catalyst or 22.9 μmol/m2 h (26.1 mmol/gCu h) over the CuOCeO2 catalyst. But cerium oxide domains probably primarily convert the benzylimine to benzaldehyde, followed by condensation of the benzaldehyde with benzylamine in a rapid step to yield the N-benzylidenebenzylamine product (path B), with a max. amine prodn. rate of 2.74 μmol/m2 h. Both ceria and copper(II) oxide domains promote the initial benzylimine formation at comparable rates. Although the CuO-CeO2 catalyst leaches ∼11% of the copper during the reaction, and these sol. copper species are largely responsible for the catalytic turnover, the recovered solid can be recycled until the copper is depleted, catalyzing the reaction with an identical rate per g Cu in a second cycle, after calcination. The copper-ceria family of catalysts offers an alternative, potentially lower cost compn. for the target oxidative homocoupling reaction than previously studied precious metal catalysts, although copper leaching is a distinct drawback to the catalyst compn.
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    Huang, S.; Zhao, Z.; Wei, Z.; Wang, M.; Chen, Y.; Wang, X.; Shao, F.; Zhong, X.; Li, X.; Wang, J. Targeted Regulation of the Selectivity of Cascade Synthesis towards Imines/Secondary Amines by Carbon-Coated Co-Based Catalysts. Green Chem. 2022, 24 (18), 6945– 6954,  DOI: 10.1039/D2GC02161G
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    Targeted regulation of the selectivity of cascade synthesis towards imines/secondary amines by carbon-coated Co-based catalysts
    Huang, Songtao; Zhao, Zijiang; Wei, Zhongzhe; Wang, Mingxuan; Chen, Yi; Wang, Xiaosa; Shao, Fangjun; Zhong, Xing; Li, Xiaonian; Wang, Jianguo
    Green Chemistry (2022), 24 (18), 6945-6954CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
    Imines and secondary amines have attracted widespread attention in the fields of medicine owing to their unique unit structures and biol. activities. However, the design of catalysts for the targeted syntheses of imines and secondary amines presents challenges due to the uncontrollable cascade reaction of nitrobenzene (NB) and benzaldehyde (BA). Herein, we report that carbon-coated Co and PtCo catalysts (Co/SiO2@CN and PtCo/SiO2@CN) realize the directional synthesis of imines and secondary amines under the same conditions, resp., through the synergistic action of the active components. The superior catalytic performance benefited from the integrated geometric and electronic effects. Specifically, the uniformly coated nitrogen-doped carbon drives the adsorption ability of NB more strongly than BA and inhibits the formation of benzyl alc. For Co/SiO2@CN, Co-Nx and metallic Co synergistically promote the highly selective synthesis of imines with much higher conversion (97%) and selectivity (98%) than most Co-based catalysts at 60°C. For PtCo/SiO2@CN, the electron transfer between Pt and Co promotes the hydrogen spillover ability and enables it to afford 100% BA conversion and 100% secondary amine selectivity, reaching the TOF value of 296 h-1. This synthesis strategy provides an advanced concept for designing chemoselective catalysts, which has important implications for both scientific research and industrial applications.
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    Liu, X.; Dai, L. Carbon-Based Metal-Free Catalysts. Nat. Rev. Mater. 2016, 1 (11), 16064,  DOI: 10.1038/natrevmats.2016.64
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    Carbon-based metal-free catalysts
    Liu, Xien; Dai, Liming
    Nature Reviews Materials (2016), 1 (11), 16064CODEN: NRMADL; ISSN:2058-8437. (Nature Publishing Group)
    Metals and metal oxides are widely used as catalysts for materials prodn., clean energy generation and storage, and many other important industrial processes. However, metal-based catalysts suffer from high cost, low selectivity, poor durability, susceptibility to gas poisoning and have a detrimental environmental impact. In 2009, a new class of catalyst based on earth-abundant carbon materials was discovered as an efficient, low-cost, metal-free alternative to platinum for oxygen redn. in fuel cells. Since then, tremendous progress has been made, and carbon-based metal-free catalysts have been demonstrated to be effective for an increasing no. of catalytic processes. This Review provides a crit. overview of this rapidly developing field, including the mol. design of efficient carbon-based metal-free catalysts, with special emphasis on heteroatom-doped carbon nanotubes and graphene. We also discuss recent advances in the development of carbon-based metal-free catalysts for clean energy conversion and storage, environmental protection and important industrial prodn., and outline the key challenges and future opportunities in this exciting field.
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    Pahra, S.; Sangabathula, O.; Sharma, C. S.; Devi, P. A Noble Metal-Free Candle Soot Derived Carbon Electrocatalyst for Simultaneous H2 Generation and Wastewater Treatment. J. Phys. Chem. Solids 2023, 173, 111106  DOI: 10.1016/j.jpcs.2022.111106
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    Chourasia, A. K.; Shavez, M.; Naik, K. M.; Bongu, C.; Sharma, C. S. Candle Soot Nanoparticles versus Multiwalled Carbon Nanotubes as a High-Performance Cathode Catalyst for Li–CO2Mars Batteries for Mars Exploration. ACS Appl. Energy Mater. 2023, 6 (1), 378– 386,  DOI: 10.1021/acsaem.2c03285
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    Candle Soot Nanoparticles versus Multiwalled Carbon Nanotubes as a High-Performance Cathode Catalyst for Li-CO2Mars Batteries for Mars Exploration
    Chourasia, Ankit K.; Shavez, Mohd; Naik, Keerti M.; Bongu, Chandrasekhar; Sharma, Chandra S.
    ACS Applied Energy Materials (2023), 6 (1), 378-386CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
    Increased CO2 emissions on the earth causing global warming and climate change have provided a thrust to explore Li-CO2 battery chem., where CO2 is used as an energy carrier. In addn., the occurrence of CO2 as a major natural abundant gas in the Martian atm. opens the possibility of using Li-CO2 batteries for interplanetary Mars missions. In this work, we aim to investigate facile and inexpensive candle soot carbon nanoparticles as a cathode catalyst against com. available multiwalled carbon nanotubes (MWCNTs) for stable and high-performance Li-CO2 batteries for Mars exploration. The unique interconnected morphol. and higher surface area of candle soot nanoparticles facilitate better reversibility (more than 80 cycles) compared to MWCNTs even at a high c.d. of 200 mA g-1 with a cutoff capacity of 500 mAh g-1. The full discharge capacity for candle soot nanoparticles was measured to be 5318 mAh g-1 with a coulombic efficiency of 42% as compared to 16% for MWCNTs. The rate capability studies were performed to establish the ability to operate the system reversibly at different current densities in a simulated Martian atm. The outcome of this study paves the way toward developing a candle soot cathode-based practicable Li-CO2 battery for utilization on Mars.
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    Zhang, P.; Qiao, Z.-A.; Dai, S. Recent Advances in Carbon Nanospheres: Synthetic Routes and Applications. Chem. Commun. 2015, 51 (45), 9246– 9256,  DOI: 10.1039/C5CC01759A
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    Recent advances in carbon nanospheres: synthetic routes and applications
    Zhang, Pengfei; Qiao, Zhen-An; Dai, Sheng
    Chemical Communications (Cambridge, United Kingdom) (2015), 51 (45), 9246-9256CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    A review. Carbon-based materials are the most popular material types in both fundamental research and industrial applications, partly because of their well-controlled nano-morphologies. In the past two decades, we have witnessed a no. of breakthroughs in carbon research: fullerenes, carbon nanotubes, and more recently graphene. Nowadays, carbon nanospheres are attracting more and more attention worldwide due to their excellent performance in various fields: drug delivery, heterogeneous catalysis, encapsulation of support and electrode materials. Actually, spherical carbon is an old material, whereas controlling carbon spheres in the nanometer range is a recent story. In the past 5 years, it has become possible to precisely control the particle size, surface area, pore size, chem. compn., and dispersity of carbon nanospheres. Toward this end, a no. of synthetic strategies are emerging, such as hydrothermal carbonization of biomass-based resources, extended Stober synthesis, and org.-org. self-assembly via different binding methods. In this feature article, we summarize recent routes for carbon nanospheres and briefly touch on their applications to shed light on the potential of this field. Throughout this article, a special emphasis is placed on the possible modulation of spherical structures at the nanoscale, and we wish to inspire many more designs and applications of carbon nanostructures in the near future.
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    Nieto-Márquez, A.; Romero, R.; Romero, A.; Valverde, J. L. Carbon Nanospheres: Synthesis, Physicochemical Properties and Applications. J. Mater. Chem. 2011, 21 (6), 1664– 1672,  DOI: 10.1039/C0JM01350A
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    Carbon nanospheres: synthesis, physicochemical properties and applications
    Nieto-Marquez, Antonio; Romero, Rubi; Romero, Amaya; Valverde, Jose Luis
    Journal of Materials Chemistry (2011), 21 (6), 1664-1672CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    The discovery of carbon nanostructures, essentially carbon nanotubes (CNT) and carbon nanofibres (CNF) has led to a big effort devoted to their synthesis, characterization, surface modification and use. Indeed, these structures have encountered application in a wide range of technol. fields, such as adsorption, catalysis, hydrogen storage or electronics. Apart from the filamentous arrange of graphene sheets conducting to CNT or CNF, carbon can bond in other different ways to create structures with dissimilar properties. The pairing of pentagonal and heptagonal carbon rings can result in the formation of carbon nanospheres (CNS). This novel nanostructure has only now started to attract significant research activity. In its spherical arrangement, the graphite sheets are not closed shells but rather waving flakes that follow the curvature of the sphere, creating many open edges at the surface. Contrary to the chem. inert C60, the unclosed graphitic flakes provide reactive "dangling bonds" that are proposed to enhance surface reactions, establishing CNS as good candidates for catalytic and adsorption applications. Despite the embryonic stage of the field and the existing data being too scattered, this work is aimed to provide a comprehensive review of the existing literature related to CNS, exploring the different prepn. routes employed, the crit. characterization results as well as the applications studied so far.
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    Wang, J.; Kaskel, S. KOH Activation of Carbon-Based Materials for Energy Storage. J. Mater. Chem. 2012, 22 (45), 23710,  DOI: 10.1039/c2jm34066f
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    KOH activation of carbon-based materials for energy storage
    Wang, Jiacheng; Kaskel, Stefan
    Journal of Materials Chemistry (2012), 22 (45), 23710-23725CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    Because of their availability, adjustable microstructure, varieties of forms, and large sp. surface area, porous carbon materials are of increasing interest for use in hydrogen storage adsorbents and electrode materials in supercapacitors and lithium-sulfur cells from the viewpoint of social sustainability and environmental friendliness. Therefore, much effort has been made to synthesize and tailor the microstructures of porous carbon materials via various activation procedures (phys. and chem. activation). In particular, the chem. activation of various carbon sources using KOH as the activating reagent is very promising because of its lower activation temp. and higher yields, and well-defined micropore size distribution and ultrahigh sp. surface area up to 3000 m2 g-1 of the resulting porous carbons. In this feature article, recent research progress since 2007 is covered on the synthesis of KOH-activated carbons for hydrogen and elec. energy storage (supercapacitors and lithium-sulfur batteries). The textural properties and surface chem. of KOH-activated carbons depend on not only the synthesis parameters, but also different carbon sources employed including fossil/biomass-derived materials, synthetic org. polymers, and various nanostructured carbons (e.g. carbon nanotubes, carbon nanofibers, carbon aerogels, carbide-derived carbons, graphene, etc.). Following the introduction to KOH activation mechanisms and processing technologies, the characteristics and performance of KOH-activated carbons as well as their relations are summarized and discussed through the extensive anal. of the literature based on different energy storage systems.
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    Li, M.; Xu, F.; Li, H.; Wang, Y. Nitrogen-Doped Porous Carbon Materials: Promising Catalysts or Catalyst Supports for Heterogeneous Hydrogenation and Oxidation. Catal. Sci. Technol. 2016, 6 (11), 3670– 3693,  DOI: 10.1039/C6CY00544F
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    Nitrogen-doped porous carbon materials: promising catalysts or catalyst supports for heterogeneous hydrogenation and oxidation
    Li, Mingming; Xu, Fan; Li, Haoran; Wang, Yong
    Catalysis Science & Technology (2016), 6 (11), 3670-3693CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
    A review. Developing novel and efficient catalysts is a crit. step in common heterogeneous hydrogenation and oxidn. reactions. Despite the frequent study of metal oxide-supported catalysts, porous carbon materials have also emerged as valuable potential catalysts. However, due to their highly microporous structures and inferior structural functionalities, traditional activated carbons (ACs) have become increasingly less popular for industrial applications. To deal with the disadvantages of ACs, tremendous efforts were made to develop novel nitrogen-doped porous carbon (NPC) materials with novel features such as highly porous structures and abundant structural nitrogen heteroatom decoration. As catalysts or catalysts supports, NPC materials showed superior activities in many applications covering a wide range of heterogeneous hydrogenation and oxidn. reactions. In this contribution, the authors review the fabrication methods for NPC materials used in heterogeneous hydrogenations and oxidns. and highlight the intrinsic catalytic mechanisms along with the catalyst design strategies.
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    Fiorio, J. L.; Garcia, M. A. S.; Gothe, M. L.; Galvan, D.; Troise, P. C.; Conte-Junior, C. A.; Vidinha, P.; Camargo, P. H. C.; Rossi, L. M. Recent Advances in the Use of Nitrogen-Doped Carbon Materials for the Design of Noble Metal Catalysts. Coord. Chem. Rev. 2023, 481, 215053  DOI: 10.1016/j.ccr.2023.215053
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    Recent advances in the use of nitrogen-doped carbon materials for the design of noble metal catalysts
    Fiorio, Jhonatan Luiz; Garcia, Marco A. S.; Gothe, Maite Lippel; Galvan, Diego; Troise, Paula Castellani; Conte-Junior, Carlos A.; Vidinha, Pedro; Camargo, Pedro H. C.; Rossi, Liane M.
    Coordination Chemistry Reviews (2023), 481 (), 215053CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    A review. Noble metals nanoparticles (NPs) and single atoms (SAs) supported on nitrogen-doped carbon (NC) materials display remarkable activity and selectivity in a wide variety of reactions, spanning hydrogenations, oxidns., Fischer-Tropsch synthesis, and Suzuki coupling. Due to the unique interaction between the NC structure and the anchored metal center, both phys. and chem. properties of the catalysts can be finely tuned. Moreover, the precise control of the coordination environment in the host support can pave the way to designing efficient noble metal catalysts with optimized active centers. This approach opens avenues for improving stability, selectivity, and catalytic activity. This review covers the recent progress in the field of catalysis by noble metals supported on N-doped carbon materials. An overview of various catalytic systems based on Au, Ag, Pd, Pt, Ru, Rh is discussed, and structure-performance relations in catalysis are described based on theor. and exptl. investigations for different classes of metals and reactions. Finally, challenges and perspectives for engineering heterogeneous catalysts based on noble metals embedded in N-doped carbon materials are described to tackle challenges regarding activity and selectivity.
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    Kaare, K.; Yu, E.; Volperts, A.; Dobele, G.; Zhurinsh, A.; Dyck, A.; Niaura, G.; Tamasauskaite-Tamasiunaite, L.; Norkus, E.; Andrulevičius, M.; Danilson, M.; Kruusenberg, I. Highly Active Wood-Derived Nitrogen-Doped Carbon Catalyst for the Oxygen Reduction Reaction. ACS Omega 2020, 5 (37), 23578– 23587,  DOI: 10.1021/acsomega.0c01974
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    Shi, Z.; Yang, W.; Gu, Y.; Liao, T.; Sun, Z. Metal-Nitrogen-Doped Carbon Materials as Highly Efficient Catalysts: Progress and Rational Design. Adv. Sci. 2020, 7 (15), 2001069  DOI: 10.1002/advs.202001069
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    Metal-Nitrogen-Doped Carbon Materials as Highly Efficient Catalysts: Progress and Rational Design
    Shi, Zhangsheng; Yang, Wenqing; Gu, Yuantong; Liao, Ting; Sun, Ziqi
    Advanced Science (Weinheim, Germany) (2020), 7 (15), 2001069CODEN: ASDCCF; ISSN:2198-3844. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. As a typical class of single-atom catalysts (SACs) possessing prominent advantages of high reactivity, high selectivity, high stability, and maximized at. utilization, emerging metal-nitrogen-doped carbon (M-N-C) materials, wherein dispersive metal atoms are coordinated to nitrogen atoms doped in carbon nanomaterials, have presented a high promise to replace the conventional metal or metal oxides-based catalysts. In this work, recent progress in M-N-C-based materials achieved in both theor. and exptl. investigations is summarized and general principles for novel catalysts design from electronic structure modulating are provided. Firstly, the applications and mechanisms on the advantages and challenges of M-N-C-based materials for a variety of sustainable fuel generation and bioinspired reactions, including the oxygen redn. reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), carbon dioxide redn. reaction (CO2RR), nitrogen redn. reaction (NRR), and nanozyme reactions are reviewed. Then, strategies toward enhancing the catalytic performance by engineering the nature of metal ion centers, coordinative environment of active centers, carbon support, and their synergistic cooperation, are proposed. Finally, prospects for the rational design of next generation high-performance M-N-C-based catalysts are outlined. It is expected that this work will provide insights into high-performance catalysts innovation for sustainable and environmental technologies.
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    Wang, K.; Jiang, P.; Yang, M.; Ma, P.; Qin, J.; Huang, X.; Ma, L.; Li, R. Metal-Free Nitrogen-Doped Carbon Nanosheets: A Catalyst for the Direct Synthesis of Imines under Mild Conditions. Green Chem. 2019, 21 (9), 2448– 2461,  DOI: 10.1039/C9GC00908F
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    Metal-free nitrogen -doped carbon nanosheets: a catalyst for the direct synthesis of imines under mild conditions
    Wang, Kaizhi; Jiang, Pengbo; Yang, Ming; Ma, Ping; Qin, Jiaheng; Huang, Xiaokang; Ma, Lei; Li, Rong
    Green Chemistry (2019), 21 (9), 2448-2461CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
    Herein, a highly stable, porous, multifunctional and metal-free catalyst was developed, which exhibited significant catalytic performance in the oxidn. of amines and transfer hydrogenation of nitriles under mild conditions; this could be attributed to the presence of numerous active sites and their outstanding BET surface area. The obtained results showed that most of the yields of imines exceeded 90%, and the cycling performance of the catalyst could be at least seven runs without any decay in the reaction activity, which could be comparable to those of metal catalysts. Subsequently, a kinetic study has demonstrated that the apparent activation energy for the direct synthesis of imines from amines is 67.39 kJ mol-1, which has been performed to testify that the catalytic performances are rational. Via catalyst characterizations and exptl. data, graphitic-N has been proven to be the active site of the catalyst. Hence, this study is beneficial to comprehend the mechanism of action of a metal-free N-doped carbon catalyst in the formation of imines.
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    Pyridinic-N-Doped Graphene Paper from Perforated Graphene Oxide for Efficient Oxygen Reduction
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    ACS Omega (2018), 3 (5), 5522-5530CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
    We report a simple approach to fabricate a pyridinic-N-doped graphene film (N-pGF) without high-temp. heat treatment from perforated graphene oxide (pGO). pGO is produced by a short etching treatment with hydrogen peroxide. GO perforation predominated in a short etching time (∼1 h), inducing larger holes and defects compared to pristine GO. The pGO is advantageous to the formation of a pyridinic N-doped graphene because of strong NH3 adsorption on vacancies with oxygen functional groups during the nitrogen-doping process, and the pyridinic-N-doped graphene exhibits good electrocatalytic activity for oxygen redn. reaction (ORR). Using rotating-disk electrode measurements, we confirm that N-pGF undergoes a four-electron-transfer process during the ORR in alk. and acidic media by possessing sufficient diffusion pathways and readily available ORR active sites for efficient mass transport. A comparison between Pt/N-pGF and com. Pt/C shows that Pt/N-pGF has superior performance, based on its more pos. onset potential and higher limiting diffusion current at -0.5 V.
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    Activated Microporous Carbon Nanospheres for Use in Supercapacitors
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    Microporous carbon spheres (MCSs) fabricated using the extended Stober method have unique features, such as a narrow particle size distribution and a variety of applications. Here, we report the synthesis of nanosized MCSs using a modified version of the extended Stober method at an ambient condition, which is a simple, quick, and scalable process. The activation of nanosized MCSs using CO2 is conducted to increase the surface area of MCSs. The activated microporous carbon nanospheres (AMCNSs) have a sp. surface area of 3259 m2/g and a mean diam. of about 52 nm. To the best of our knowledge, this is the smallest reported particle size for carbon spheres with a very high sp. surface area. In addn., AMCNSs have a hierarchical structure, which is beneficial for mass transport. The AMCNSs are evaluated for application in supercapacitors. The results show that a capacitance of 225 F/g at a c.d. of 0.5 A/g in a 6 M KOH aq. electrolyte can be achieved, indicating that AMCNSs have a potential for supercapacitor applications.
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    Assessing the structural properties of graphitic and non-graphitic carbons by Raman spectroscopy
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    Carbon (2020), 161 (), 359-372CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
    We study the transformation from mol. to cryst. of (non-)graphitic carbons synthesized from org. precursors by heat treatment. Easy assessment of structural properties resulting from heat treatment protocols is mandatory for industrial process monitoring. Raman spectroscopy, in particular, the Raman lineshape anal. of G and D mode, offers quick assessment of the av. sheet size of such carbons. We validate this method by performing Raman, WAXS, and EPR measurements of series of resin and pitch-based carbons synthesized. The crystallite sizes of the WAXS anal. for the individual samples are related to corresponding positions and linewidths of G and D Raman modes and show excellent agreement between expt. and modeling from large sizes down to 4 nm. The theor. master curves are independent of the precursor used in the synthesis, in contrast to models for the intensity ratio of D and G band vs. size. The latter are not universally valid and differ for each class of precursors. For sizes <4 nm, our lineshape model fails as it is based on the band structure and phonon dispersions of ideal graphene. Thus, 4 nm corresponds to the fundamental transition from mol. to cryst. character for non-graphitic carbons.
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    Towards understanding the chemical reactions between KOH and oxygen-containing groups during KOH-catalyzed pyrolysis of biomass
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    Energy (Oxford, United Kingdom) (2022), 245 (), 123286CODEN: ENEYDS; ISSN:0360-5442. (Elsevier Ltd.)
    In this study, the effect of KOH/biomass ratios and pyrolysis temps. on products distribution, the evolution of oxygen-contg. groups in pyrolytic tri-state products and the possible chem. reactions between oxygen-contg. groups and KOH were investigated. The results indicated that KOH can react with oxygen-contg. groups in biomass to facilitate gas generation at lower ratios (≤1:2) or higher temp. (∼800°C). When at higher ratios (>1:2) and 600°C, KOH has an inhibitory effect on gas prodn. With a significant decreased in the oxygen-contg. species and acids, the phenols and hydrocarbons became the main species in bio-oil. Moreover, KOH facilitates methoxyphenol conversion and has high selectivity for alkyl phenol generation. For biochar, at lower ratios (<1:2), K can attack the oxygen sites of phenolic hydroxyl groups, forming dangling bonds and promoting the formation of arom. rings with anhydride groups. However, at higher ratios (>1:2), the oxygen-contg. groups on the surface of biochar gradually changed from anhydride groups to quinone groups, and finally form a biochar with large arom. ring and rich in quinone groups. As analyzed above, a possible chem. reaction between KOH and oxygen-contg. groups during KOH-catalyzed pyrolysis of biomass was proposed.
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    El-Hendawy, A.-N. A. Variation in the FTIR Spectra of a Biomass under Impregnation, Carbonization and Oxidation Conditions. J. Anal Appl. Pyrolysis 2006, 75 (2), 159– 166,  DOI: 10.1016/j.jaap.2005.05.004
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    Variation in the FTIR spectra of a biomass under impregnation, carbonization and oxidation conditions
    El-Hendawy, Abdel-Nasser A.
    Journal of Analytical and Applied Pyrolysis (2006), 75 (2), 159-166CODEN: JAAPDD; ISSN:0165-2370. (Elsevier B.V.)
    The FTIR spectra of untreated date pits (DP) and their H3PO4-preimpregnated sample together with their chars as well as samples subjected to different activation schemes were detd. These activated carbons were prepd. using in the activation stage phosphoric acid, one- and two-step steam treatment, or potassium hydroxide. This series of activated carbons were oxidized with HNO3 and their surface modifications were detected by using FTIR spectra in the course of oxidn. conditions. The presence of cellulose and hemicellulose, lignin and protein constituents were detected in the raw material. Significant changes were obsd. in the chem. structure of the date pits under carbonization where arom. structures were developed and accompanied by losing most of aliph. CH species in the intermediate char product. However, char-IR spectrum revealed the presence of trace amts. of uncarbonized material. Whereas, activated carbons possessed arom. C=C bonds and oxygen groups. The chem. structure of the activated carbon were found to be influenced markedly depending on the scheme of activation followed. Increasing the activation temp. led to a complete destruction of C=O and C-O species simultaneously by promoted polyarom. structures. The detectable FTIR for the oxidized activated carbons confirmed the presence of large amt. of oxygen functionalities of acidic nature appearing as carbonyl, phenolic, hydroxylic, carboxylic and lactonic species groups on the carbon surface. Phosphoric acid was found to have a prominent effect on the botanical structure of the raw material. Its mechanism was based on initiation of the bond cleavage leading to dehydration and elimination reactions that release volatile products. This was followed by aromaticity and formation of a stronger crosslinked solid of new biopolymer involving phosphate esters.
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    Salinas-Torres, D.; Navlani-García, M.; Mori, K.; Kuwahara, Y.; Yamashita, H. Nitrogen-Doped Carbon Materials as a Promising Platform toward the Efficient Catalysis for Hydrogen Generation. Appl. Catal. A Gen. 2019, 571, 25– 41,  DOI: 10.1016/j.apcata.2018.11.034
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    Applied Catalysis, A: General (2019), 571 (), 25-41CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
    Nitrogen-doped carbon materials have emerged as versatile candidates with outstanding features as both support of metal-based catalysts and metal-free catalytic systems. Their use has been traditionally linked to electrochem. applications, but the no. of applications in which these materials show their virtues has been exponentially increasing in the last decade. Particularly, catalysts based on metal nanoparticles supported on nitrogen-doped carbon materials have shown to be attractive for important catalytic reactions as they display enhanced performances endowed by well-dispersed, stabilized and electronically promoted nanoparticles as well as by the modified acid-base properties of the support. The present review is focused on the most recent breakthroughs achieved by these catalytic systems in the hydrogen prodn. from some of the most representative hydrogen carrier mols. by highlighting the effect of the incorporation of nitrogen in the support matrix.
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    Zheng, Y.; Wang, J.; Li, D.; Liu, C.; Lu, Y.; Lin, X.; Zheng, Z. Insight into the KOH/KMnO4 Activation Mechanism of Oxygen-Enriched Hierarchical Porous Biochar Derived from Biomass Waste by in-Situ Pyrolysis for Methylene Blue Enhanced Adsorption. J. Anal Appl. Pyrolysis 2021, 158, 105269  DOI: 10.1016/j.jaap.2021.105269
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    Journal of Analytical and Applied Pyrolysis (2021), 158 (), 105269CODEN: JAAPDD; ISSN:0165-2370. (Elsevier B.V.)
    A high-performance hierarchical porous carbon enriched in oxygen-contg. functional groups was prepd. by using pine as the raw material, KOH and KMnO4 as a green activator via cost-effective one-step or two-step in-situ pyrolysis processes. The adsorption performance, kinetics and thermodn. of activated biochar were evaluated using methylene blue (MB) as a model pollutant. Biochar structure was investigated by X-ray diffraction (XRD), Raman spectroscopy, Fourier transform IR (FTIR) spectroscopy, XPS, N2 adsorption/desorption isotherms (BET) and SEM (SEM) to explore its morphol., phys., and chem. properties. A possible adsorption mechanism and pathway were also proposed. The results showed that a two-step process, higher activation (KOH and KMnO4) to biomass ratio led to excellent adsorption performance. AC-KOH-1-1 and AC-KMnO4-1-3 presented the max. equil. adsorption capacities of 637.5 mg/g and 439.5 mg/g, resp. The adsorption isotherm fit well with the Freundlich isotherm model and pseudo-second-order kinetic model, which exhibited better monolayer adsorption and chem. adsorption control. In addn., the adsorption was dominated by pore diffusion and π-π stacking interactions for KOH activation due to the well-developed pore vol. (0.04990 mg/L), pore channel (18.65 nm), aromatized structure (66.03%) and surface defective structure. However, the adsorption capacity was controlled by hydrogen bonding, electrostatic adsorption interactions, cation exchange and the donor-receptor-effect for AC-KMnO4 due to it having the highest proportion of O- and N-contg. functional groups. Therefore, this cost-efficient biochar prepn. technol. for the efficient removal of org. pollutants has great potential for practical application.
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    Ji, N.; Yin, J.; Rong, Y.; Li, H.; Yu, Z.; Lei, Y.; Wang, S.; Diao, X. More than a Support: The Unique Role of Nb2O5 in Supported Metal Catalysts for Lignin Hydrodeoxygenation. Catal. Sci. Technol. 2022, 12 (12), 3751– 3766,  DOI: 10.1039/D2CY00245K
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    Catalysis Science & Technology (2022), 12 (12), 3751-3766CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
    A review. Nb2O5-supported metal materials are a vital class of green catalysts with excellent lignin hydrodeoxygenation activity and reusability in an aq. catalytic system, in which the Nb2O5 support plays a pivotal role because of its nontoxic nature, strong acidity and remarkable stability in water. This review summarizes the recent advances in Nb2O5-supported metal catalysts for lignin hydrodeoxygenation, esp. the special role of Nb2O5. In particular, the review first presents an overview of the structure, acid and redox properties of Nb2O5. It then focuses on the unique contribution of Nb2O5 in hydrodeoxygenation of lignin model compds. based on comparison with other common supports, including its promotion of C-O cleavage and enhancement of suitability and reusability in the aq. phase. Strategies for the modification of Nb2O5 and the conversion of real lignin or its oil to aroms. over Nb2O5-supported metal catalysts are further summarized. Finally, the article attempts to highlight the remaining challenges and provide some outlooks for the future development of Nb2O5-supported metal catalysts. This review will provide valuable insights for developing advanced green catalysts for lignin valorization and other challenging environmental catalytic processes.
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    Zhao, J.-X.; Wang, W.; Jiao, Z.-F.; Guo, X.-Y. A Highly Efficient Defective Carbon Catalyst for Oxidative Coupling of Amines. Catal. Commun. 2023, 177, 106652  DOI: 10.1016/j.catcom.2023.106652
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    A review. Thus in this review, focused on the oxidative reactions of benzylamines including a brief description of the reaction mechanisms for the synthesis of N-heterocyclic compds. based on the catalytic system applied. In this respect, the related studies from 2012 to 2019, have been categorized and classified in five main sections.

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  • Abstract

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    Figure 1

    Figure 1. Benzimidazole-based drug molecules.

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    Figure 2

    Figure 2. TEM images of the CS and ANCS materials.

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    Figure 3

    Figure 3. STEM-EDAX elemental mapping images of the ANCS catalyst.

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    Figure 4

    Figure 4. (a) Powder XRD patterns and (b) Raman spectra of candle soot (CS), activated candle soot (ACS), and activated N-doped candle soot (ANCS) catalysts.

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    Figure 5

    Figure 5. (a) FT-IR spectra and (b) C 1s XPS, (c) N 1s XPS, and (d) O 1s XPS of candle soot (CS), activated candle soot (ACS), and activated N-doped candle soot (ANCS) catalysts.

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    Figure 6

    Figure 6. Pyridine-adsorbed FT-IR spectra of the CS, ACS, and ANCS catalysts.

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    Figure 7

    Figure 7. (a) Kinetic studies (reaction conditions: 1.5 mmol of o-phenylenediamine (OPD), 2.5 mmol of benzylamine, 15 wt % catalyst with respect to the OPD, 100 °C, 1 mL of toluene, and open-air conditions) and (b) hot-filtration test using the ANCS catalyst (reaction conditions: 1.5 mmol of o-phenylenediamine (OPD), 2.5 mmol of benzylamine, 15 wt % catalyst with respect to the OPD, 100 °C, 1 mL of toluene, and open-air conditions).

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    Figure 8

    Figure 8. Schematic representation of coupling of benzylamine with an OPD to form benzimidazole over the ANCS catalyst.

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      Liu, Lichen; Corma, Avelino
      Chemical Reviews (Washington, DC, United States) (2018), 118 (10), 4981-5079CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It was shown in the literature that many factors including the particle size, shape, chem. compn., metal-support interaction, and metal-reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relations at the mol. level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles are discussed. Also, the authors will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidn., selective oxidn., selective hydrogenation, org. reactions, electrocatalytic, and photocatalytic reactions. The authors will compare the results obtained from different systems and try to give a picture on how different types of metal species work in different reactions and give perspectives on the future directions toward better understanding of the catalytic behavior of different metal entities (single atoms, nanoclusters, and nanoparticles) in a unifying manner.
    2. 2
      Rangraz, Y.; Heravi, M. M. Recent Advances in Metal-Free Heteroatom-Doped Carbon Heterogeneous Catalysts. RSC Adv. 2021, 11 (38), 23725– 23778,  DOI: 10.1039/D1RA03446D
      2
      Recent advances in metal-free heteroatom-doped carbon heterogonous catalysts
      Rangraz, Yalda; Heravi, Majid M.
      RSC Advances (2021), 11 (38), 23725-23778CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      A review. The development of cost-effective, efficient, and novel catalytic systems is always an important topic for heterogeneous catalysis from academia and industrial points of view. Heteroatom-doped carbon materials have gained more and more attention as effective heterogeneous catalysts to replace metal-based catalysts, because of their excellent physicochem. properties, outstanding structure characteristics, environmental compatibility, low cost, inexhaustible resources, and low energy consumption. Doping of heteroatoms can tailor the properties of carbons for different utilizations of interest. In comparison to pure carbon catalysts, these catalysts demonstrate superior catalytic activity in many org. reactions. This review highlights the most recent progress in synthetic strategies to fabricate metal-free heteroatom-doped carbon catalysts including single and multiple heteroatom-doped carbons and the catalytic applications of these fascinating materials in various org. transformations such as oxidn., hydrogenation, hydrochlorination, dehydrogenation, etc.
    3. 3
      Wang, W.; Ma, L.; Wang, H.; Jiang, X.; He, Z.-H.; Wang, K.; Yang, Y.; Li, L.; Liu, Z.-T. Tridoped Mesoporous Carbon as a Metal-Free Catalyst for Ammoxidation of (Hetero)Aromatic Alcohols to Nitriles. ACS Appl. Nano Mater. 2023, 6 (16), 15193– 15203,  DOI: 10.1021/acsanm.3c02877
      There is no corresponding record for this reference.
    4. 4
      Wu, C.; Bu, J.; Wang, W.; Shen, H.; Cao, Y.; Zhang, H. Imine Synthesis by Benzylamine Self-Coupling Catalyzed by Cerium-Doped MnO2 under Mild Conditions. Ind. Eng. Chem. Res. 2022, 61 (16), 5442– 5452,  DOI: 10.1021/acs.iecr.2c00311
      There is no corresponding record for this reference.
    5. 5
      Zhang, M.; Wu, S.; Bian, L.; Cao, Q.; Fang, W. One-Pot Synthesis of Pd-Promoted Ce–Ni Mixed Oxides as Efficient Catalysts for Imine Production from the Direct N-Alkylation of Amine with Alcohol. Catal. Sci. Technol. 2019, 9 (2), 286– 301,  DOI: 10.1039/C8CY01857J
      There is no corresponding record for this reference.
    6. 6
      He, L.; Lou, X.; Ni, J.; Liu, Y.; Cao, Y.; He, H.; Fan, K. Efficient and Clean Gold-Catalyzed One-Pot Selective N-Alkylation of Amines with Alcohols. Chem. – Eur. J. 2010, 16 (47), 13965– 13969,  DOI: 10.1002/chem.201001848
      6
      Efficient and clean Gold-catalyzed one-pot selective N-alkylation of amines with alcohols
      He, Lin; Lou, Xia-Bing; Ni, Ji; Liu, Yong-Mei; Cao, Yong; He, He-Yong; Fan, Kang-Nian
      Chemistry - A European Journal (2010), 16 (47), 13965-13969, S13965/1-S13965/8CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A convenient synthesis of secondary amines via gold-catalyzed N-alkylation of primary amines with alcs. is described. Using Au/TiO2-VS (very small Au nanoparticles, ca. 1.8 nm) as catalyst, a series of secondary amines were obtained in excellent yields. This catalyst system provides a versatile and environmentally benign protocol for the economic synthesis of amines.
    7. 7
      He, L.; Qian, Y.; Ding, R.; Liu, Y.; He, H.; Fan, K.; Cao, Y. Highly Efficient Heterogeneous Gold-catalyzed Direct Synthesis of Tertiary and Secondary Amines from Alcohols and Urea. ChemSusChem 2012, 5 (4), 621– 624,  DOI: 10.1002/cssc.201100581
      There is no corresponding record for this reference.
    8. 8
      Mavvaji, M.; Akkoc, S. Recent Advances in the Application of Heterogeneous Catalysts for the Synthesis of Benzimidazole Derivatives. Coord. Chem. Rev. 2024, 505, 215714  DOI: 10.1016/j.ccr.2024.215714
      There is no corresponding record for this reference.
    9. 9
      Kalbande, P. N.; Singh, N.; Swapna, B.; Umbarkar, S.; Sudarsanam, P. One-Pot Synthesized Efficient Molybdenum-niobium-Oxide Nanocatalyst for Selective C-O and C-N Coupling Reactions at Mild Conditions. Catal. Commun. 2023, 183, 106766  DOI: 10.1016/j.catcom.2023.106766
      9
      One-pot synthesized efficient molybdenum-niobium-oxide nanocatalyst for selective C-O and C-N coupling reactions at mild conditions
      Kalbande, Pavan Narayan; Singh, Nittan; Swapna, Bhattu; Umbarkar, Shubhangi; Sudarsanam, Putla
      Catalysis Communications (2023), 183 (), 106766CODEN: CCAOAC; ISSN:1566-7367. (Elsevier B.V.)
      An efficient molybdenum-niobium-oxide nanomaterial was synthesized by a one-pot hydrothermal method for selective C-O (glycerol ketalization) and C-N coupling (benzylamine oxidn.) reactions. The catalytically favorable properties, such as defective metal sites, truncated surfaces, and uniform metal dispersion in the MoO3-Nb2O5 nanorods, calcined at 500°C (MoNb OPS-5), were confirmed by Raman, HR-TEM, and STEM-EDX, resp. Because of improved Lewis/Bronsted acidic strength, the MoNb OPS-5 catalyst showed higher activity in glycerol ketalization and benzylamine oxidn. at mild conditions, giving superior selectivity to solketal (97%) and dibenzylimine (99%), resp. The MoNb OPS-5 catalyst showed high structural stability and considerable good reusability efficacy.
    10. 10
      Marakatti, V. S.; Sarma, S. Ch.; Joseph, B.; Banerjee, D.; Peter, S. C. Synthetically Tuned Atomic Ordering in PdCu Nanoparticles with Enhanced Catalytic Activity toward Solvent-Free Benzylamine Oxidation. ACS Appl. Mater. Interfaces 2017, 9 (4), 3602– 3615,  DOI: 10.1021/acsami.6b12253
      10
      Synthetically Tuned Atomic Ordering in PdCu Nanoparticles with Enhanced Catalytic Activity toward Solvent-Free Benzylamine Oxidation
      Marakatti, Vijaykumar S.; Sarma, Saurav Ch.; Joseph, Boby; Banerjee, Dipanjan; Peter, Sebastian C.
      ACS Applied Materials & Interfaces (2017), 9 (4), 3602-3615CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Synthesis of ordered compds. with nano size is of particular interest for tuning the surface properties with enhanced activity and selectivity toward various important industrial catalytic processes. In this work, we synthesized ordered PdCu nanoparticles as highly efficient catalyst for the solvent-free aerobic oxidn. of benzylamine. The PdxCu1-x catalysts with different chem. compns. (x = 0, 0.25, 0.4, 0.5, 0.6, 0.75, 1) were prepd. by polyol method using NaBH4 as a reducing agent and were well-characterized by X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy, XPS, transmission electron microscopy (TEM) energy-dispersive anal. of X-rays, and X-ray absorption fine structure. The effect of different metal concns. of Pd and Cu on the formation of PdxCu1-x nanoparticles was investigated. The XRD and TEM confirmed the formation of ordered PdCu intermetallic phase with body-centered cubic (BCC) structure for the synthetic compn. of Pd/Cu = 1:1. For compns. x = 0, 0.25, 0.75, and 1, PdxCu1-x alloy with face-centered cubic (FCC) structure was obsd., whereas mixed phase of BCC and FCC was obsd. for x = 0.4 and 0.6. The use of strong reducing agent (NaBH4) was essential to synthesize PdCu ordered phase compared to weak reducing agents such as oleylamine and ascorbic acid. The PdCu nanocatalyst with ordered structure (BCC) showed excellent catalytic activity compared to PdxCu1-x alloy nanoparticles with FCC structure. The at. ordering in the PdCu intermetallic was the driving force for the enhancement in the catalytic activity with high benzylamine conversion of 94.0% and dibenzylimine selectivity of 92.2% compared to its monometallic and alloy counterparts. Moreover, ordered PdCu alloy showed good recyclability and activity toward the oxidn. of different amines.
    11. 11
      Arun Kumar, M.; Kamali, M.; Putla, S. B.; Subha, P.; Sudarsanam, P. Nb2O5/Ce1–xNbxO2−δ Nanorod Catalyst for Selective Oxidative Coupling of Aromatic Alcohols and Amines. ACS Appl. Nano Mater. 2024, 7 (6), 5899– 5911,  DOI: 10.1021/acsanm.3c05593
      There is no corresponding record for this reference.
    12. 12
      Singh, N.; Putla, S. B.; Pratap Singh, C.; Kalbande, P. N.; Choudhary, P.; Krishnamurty, S.; Krishnan, V.; Bhatte, K.; Sudarsanam, P. Shape-Controlled MoO3/MnOx Nanocatalyst for the Selective Synthesis of 2-Phenylquinoxaline Drug Motifs. ACS Appl. Nano Mater. 2023, 6 (24), 23442– 23453,  DOI: 10.1021/acsanm.3c04820
      There is no corresponding record for this reference.
    13. 13
      Ge, C.; Sang, X.; Yao, W.; Zhang, L.; Wang, D. Unsymmetrical Indazolyl-Pyridinyl-Triazole Ligand-Promoted Highly Active Iridium Complexes Supported on Hydrotalcite and Its Catalytic Application in Water. Green Chem. 2018, 20 (8), 1805– 1812,  DOI: 10.1039/C7GC02892J
      13
      Unsymmetrical indazolyl-pyridinyl-triazole ligand-promoted highly active iridium complexes supported on hydrotalcite and its catalytic application in water
      Ge, Chenyang; Sang, Xinxin; Yao, Wei; Zhang, Liang; Wang, Dawei
      Green Chemistry (2018), 20 (8), 1805-1812CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      Herein, an indazolyl-pyridinyl-triazole ligand was synthesized and its Ir complex supported on hydrotalcite was characterized via x-ray power diffraction (XRD), XPS, energy dispersive X-ray (EDX) spectroscopy and TEM. This new heterogeneous catalyst bearing the unsym. indazolyl-pyridinyl-triazole ligand exhibits high catalytic activity in H2O. Both functionalized amines and imines were obtained from the challenging selective reaction of benzylamines with arylamines through transfer hydrogenation and dehydrogenation under clean conditions. In particular, this catalyst system showed good recovery performance in H2O. Mechanistic studies showed that this transformation occurs via amine dehydrogenation, hydrolysis and condensation processes. The direct capture of the reaction intermediate provides sufficient proof for this process.
    14. 14
      Al-Hmoud, L.; Jones, C. W. Reaction Pathways over Copper and Cerium Oxide Catalysts for Direct Synthesis of Imines from Amines under Aerobic Conditions. J. Catal. 2013, 301, 116– 124,  DOI: 10.1016/j.jcat.2013.01.027
      14
      Reaction pathways over copper and cerium oxide catalysts for direct synthesis of imines from amines under aerobic conditions
      Al-Hmoud, Linda; Jones, Christopher W.
      Journal of Catalysis (2013), 301 (), 116-124CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      Copper(II) oxide, cerium oxide, and several copper(II) oxide supported on cerium oxide catalysts are explored in the aerobic oxidative homocoupling of benzylamine to form N-benzylidenebenzylamine in DMSO at 110 °C. Although both CuO and CeO2 alone catalyze the reaction, CuO-CeO2 catalysts most efficiently catalyze the reaction, providing higher rates (per g Cu) due to the presence of both copper and ceria species in the reactor. Catalysts with lower copper loadings and increased ceria content reduce the product yield, as ceria domains can catalyze the decompn. of the desired product as well. The amine conversion occurs with a significant induction period, assocd. with the putative formation of an initial benzylimine intermediate in the case of catalysis with CeO2 alone or from slow copper solubilization in cases where supported or unsupported copper catalysts are used, followed by rapid conversion to the N-benzylidenebenzylamine product. Copper leaching studies clearly demonstrate that catalysis using copper-contg. catalysts is primarily assocd. with turnover by sol. copper species. Expts. targeted at elucidating the reaction pathway suggests that copper oxide domains promote the coupling of the initial intermediate, benzylimine, with benzylamine to produce the N-benzylidenebenzylamine product (path A), with a max. prodn. rate of 888 μmol/m2 h (13.3 mmol/gCu h) over the pure CuO catalyst or 22.9 μmol/m2 h (26.1 mmol/gCu h) over the CuOCeO2 catalyst. But cerium oxide domains probably primarily convert the benzylimine to benzaldehyde, followed by condensation of the benzaldehyde with benzylamine in a rapid step to yield the N-benzylidenebenzylamine product (path B), with a max. amine prodn. rate of 2.74 μmol/m2 h. Both ceria and copper(II) oxide domains promote the initial benzylimine formation at comparable rates. Although the CuO-CeO2 catalyst leaches ∼11% of the copper during the reaction, and these sol. copper species are largely responsible for the catalytic turnover, the recovered solid can be recycled until the copper is depleted, catalyzing the reaction with an identical rate per g Cu in a second cycle, after calcination. The copper-ceria family of catalysts offers an alternative, potentially lower cost compn. for the target oxidative homocoupling reaction than previously studied precious metal catalysts, although copper leaching is a distinct drawback to the catalyst compn.
    15. 15
      Huang, S.; Zhao, Z.; Wei, Z.; Wang, M.; Chen, Y.; Wang, X.; Shao, F.; Zhong, X.; Li, X.; Wang, J. Targeted Regulation of the Selectivity of Cascade Synthesis towards Imines/Secondary Amines by Carbon-Coated Co-Based Catalysts. Green Chem. 2022, 24 (18), 6945– 6954,  DOI: 10.1039/D2GC02161G
      15
      Targeted regulation of the selectivity of cascade synthesis towards imines/secondary amines by carbon-coated Co-based catalysts
      Huang, Songtao; Zhao, Zijiang; Wei, Zhongzhe; Wang, Mingxuan; Chen, Yi; Wang, Xiaosa; Shao, Fangjun; Zhong, Xing; Li, Xiaonian; Wang, Jianguo
      Green Chemistry (2022), 24 (18), 6945-6954CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      Imines and secondary amines have attracted widespread attention in the fields of medicine owing to their unique unit structures and biol. activities. However, the design of catalysts for the targeted syntheses of imines and secondary amines presents challenges due to the uncontrollable cascade reaction of nitrobenzene (NB) and benzaldehyde (BA). Herein, we report that carbon-coated Co and PtCo catalysts (Co/SiO2@CN and PtCo/SiO2@CN) realize the directional synthesis of imines and secondary amines under the same conditions, resp., through the synergistic action of the active components. The superior catalytic performance benefited from the integrated geometric and electronic effects. Specifically, the uniformly coated nitrogen-doped carbon drives the adsorption ability of NB more strongly than BA and inhibits the formation of benzyl alc. For Co/SiO2@CN, Co-Nx and metallic Co synergistically promote the highly selective synthesis of imines with much higher conversion (97%) and selectivity (98%) than most Co-based catalysts at 60°C. For PtCo/SiO2@CN, the electron transfer between Pt and Co promotes the hydrogen spillover ability and enables it to afford 100% BA conversion and 100% secondary amine selectivity, reaching the TOF value of 296 h-1. This synthesis strategy provides an advanced concept for designing chemoselective catalysts, which has important implications for both scientific research and industrial applications.
    16. 16
      Liu, X.; Dai, L. Carbon-Based Metal-Free Catalysts. Nat. Rev. Mater. 2016, 1 (11), 16064,  DOI: 10.1038/natrevmats.2016.64
      16
      Carbon-based metal-free catalysts
      Liu, Xien; Dai, Liming
      Nature Reviews Materials (2016), 1 (11), 16064CODEN: NRMADL; ISSN:2058-8437. (Nature Publishing Group)
      Metals and metal oxides are widely used as catalysts for materials prodn., clean energy generation and storage, and many other important industrial processes. However, metal-based catalysts suffer from high cost, low selectivity, poor durability, susceptibility to gas poisoning and have a detrimental environmental impact. In 2009, a new class of catalyst based on earth-abundant carbon materials was discovered as an efficient, low-cost, metal-free alternative to platinum for oxygen redn. in fuel cells. Since then, tremendous progress has been made, and carbon-based metal-free catalysts have been demonstrated to be effective for an increasing no. of catalytic processes. This Review provides a crit. overview of this rapidly developing field, including the mol. design of efficient carbon-based metal-free catalysts, with special emphasis on heteroatom-doped carbon nanotubes and graphene. We also discuss recent advances in the development of carbon-based metal-free catalysts for clean energy conversion and storage, environmental protection and important industrial prodn., and outline the key challenges and future opportunities in this exciting field.
    17. 17
      Pahra, S.; Sangabathula, O.; Sharma, C. S.; Devi, P. A Noble Metal-Free Candle Soot Derived Carbon Electrocatalyst for Simultaneous H2 Generation and Wastewater Treatment. J. Phys. Chem. Solids 2023, 173, 111106  DOI: 10.1016/j.jpcs.2022.111106
      There is no corresponding record for this reference.
    18. 18
      Chourasia, A. K.; Shavez, M.; Naik, K. M.; Bongu, C.; Sharma, C. S. Candle Soot Nanoparticles versus Multiwalled Carbon Nanotubes as a High-Performance Cathode Catalyst for Li–CO2Mars Batteries for Mars Exploration. ACS Appl. Energy Mater. 2023, 6 (1), 378– 386,  DOI: 10.1021/acsaem.2c03285
      18
      Candle Soot Nanoparticles versus Multiwalled Carbon Nanotubes as a High-Performance Cathode Catalyst for Li-CO2Mars Batteries for Mars Exploration
      Chourasia, Ankit K.; Shavez, Mohd; Naik, Keerti M.; Bongu, Chandrasekhar; Sharma, Chandra S.
      ACS Applied Energy Materials (2023), 6 (1), 378-386CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
      Increased CO2 emissions on the earth causing global warming and climate change have provided a thrust to explore Li-CO2 battery chem., where CO2 is used as an energy carrier. In addn., the occurrence of CO2 as a major natural abundant gas in the Martian atm. opens the possibility of using Li-CO2 batteries for interplanetary Mars missions. In this work, we aim to investigate facile and inexpensive candle soot carbon nanoparticles as a cathode catalyst against com. available multiwalled carbon nanotubes (MWCNTs) for stable and high-performance Li-CO2 batteries for Mars exploration. The unique interconnected morphol. and higher surface area of candle soot nanoparticles facilitate better reversibility (more than 80 cycles) compared to MWCNTs even at a high c.d. of 200 mA g-1 with a cutoff capacity of 500 mAh g-1. The full discharge capacity for candle soot nanoparticles was measured to be 5318 mAh g-1 with a coulombic efficiency of 42% as compared to 16% for MWCNTs. The rate capability studies were performed to establish the ability to operate the system reversibly at different current densities in a simulated Martian atm. The outcome of this study paves the way toward developing a candle soot cathode-based practicable Li-CO2 battery for utilization on Mars.
    19. 19
      Zhang, P.; Qiao, Z.-A.; Dai, S. Recent Advances in Carbon Nanospheres: Synthetic Routes and Applications. Chem. Commun. 2015, 51 (45), 9246– 9256,  DOI: 10.1039/C5CC01759A
      19
      Recent advances in carbon nanospheres: synthetic routes and applications
      Zhang, Pengfei; Qiao, Zhen-An; Dai, Sheng
      Chemical Communications (Cambridge, United Kingdom) (2015), 51 (45), 9246-9256CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      A review. Carbon-based materials are the most popular material types in both fundamental research and industrial applications, partly because of their well-controlled nano-morphologies. In the past two decades, we have witnessed a no. of breakthroughs in carbon research: fullerenes, carbon nanotubes, and more recently graphene. Nowadays, carbon nanospheres are attracting more and more attention worldwide due to their excellent performance in various fields: drug delivery, heterogeneous catalysis, encapsulation of support and electrode materials. Actually, spherical carbon is an old material, whereas controlling carbon spheres in the nanometer range is a recent story. In the past 5 years, it has become possible to precisely control the particle size, surface area, pore size, chem. compn., and dispersity of carbon nanospheres. Toward this end, a no. of synthetic strategies are emerging, such as hydrothermal carbonization of biomass-based resources, extended Stober synthesis, and org.-org. self-assembly via different binding methods. In this feature article, we summarize recent routes for carbon nanospheres and briefly touch on their applications to shed light on the potential of this field. Throughout this article, a special emphasis is placed on the possible modulation of spherical structures at the nanoscale, and we wish to inspire many more designs and applications of carbon nanostructures in the near future.
    20. 20
      Nieto-Márquez, A.; Romero, R.; Romero, A.; Valverde, J. L. Carbon Nanospheres: Synthesis, Physicochemical Properties and Applications. J. Mater. Chem. 2011, 21 (6), 1664– 1672,  DOI: 10.1039/C0JM01350A
      20
      Carbon nanospheres: synthesis, physicochemical properties and applications
      Nieto-Marquez, Antonio; Romero, Rubi; Romero, Amaya; Valverde, Jose Luis
      Journal of Materials Chemistry (2011), 21 (6), 1664-1672CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
      The discovery of carbon nanostructures, essentially carbon nanotubes (CNT) and carbon nanofibres (CNF) has led to a big effort devoted to their synthesis, characterization, surface modification and use. Indeed, these structures have encountered application in a wide range of technol. fields, such as adsorption, catalysis, hydrogen storage or electronics. Apart from the filamentous arrange of graphene sheets conducting to CNT or CNF, carbon can bond in other different ways to create structures with dissimilar properties. The pairing of pentagonal and heptagonal carbon rings can result in the formation of carbon nanospheres (CNS). This novel nanostructure has only now started to attract significant research activity. In its spherical arrangement, the graphite sheets are not closed shells but rather waving flakes that follow the curvature of the sphere, creating many open edges at the surface. Contrary to the chem. inert C60, the unclosed graphitic flakes provide reactive "dangling bonds" that are proposed to enhance surface reactions, establishing CNS as good candidates for catalytic and adsorption applications. Despite the embryonic stage of the field and the existing data being too scattered, this work is aimed to provide a comprehensive review of the existing literature related to CNS, exploring the different prepn. routes employed, the crit. characterization results as well as the applications studied so far.
    21. 21
      Wang, J.; Kaskel, S. KOH Activation of Carbon-Based Materials for Energy Storage. J. Mater. Chem. 2012, 22 (45), 23710,  DOI: 10.1039/c2jm34066f
      21
      KOH activation of carbon-based materials for energy storage
      Wang, Jiacheng; Kaskel, Stefan
      Journal of Materials Chemistry (2012), 22 (45), 23710-23725CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
      Because of their availability, adjustable microstructure, varieties of forms, and large sp. surface area, porous carbon materials are of increasing interest for use in hydrogen storage adsorbents and electrode materials in supercapacitors and lithium-sulfur cells from the viewpoint of social sustainability and environmental friendliness. Therefore, much effort has been made to synthesize and tailor the microstructures of porous carbon materials via various activation procedures (phys. and chem. activation). In particular, the chem. activation of various carbon sources using KOH as the activating reagent is very promising because of its lower activation temp. and higher yields, and well-defined micropore size distribution and ultrahigh sp. surface area up to 3000 m2 g-1 of the resulting porous carbons. In this feature article, recent research progress since 2007 is covered on the synthesis of KOH-activated carbons for hydrogen and elec. energy storage (supercapacitors and lithium-sulfur batteries). The textural properties and surface chem. of KOH-activated carbons depend on not only the synthesis parameters, but also different carbon sources employed including fossil/biomass-derived materials, synthetic org. polymers, and various nanostructured carbons (e.g. carbon nanotubes, carbon nanofibers, carbon aerogels, carbide-derived carbons, graphene, etc.). Following the introduction to KOH activation mechanisms and processing technologies, the characteristics and performance of KOH-activated carbons as well as their relations are summarized and discussed through the extensive anal. of the literature based on different energy storage systems.
    22. 22
      Li, M.; Xu, F.; Li, H.; Wang, Y. Nitrogen-Doped Porous Carbon Materials: Promising Catalysts or Catalyst Supports for Heterogeneous Hydrogenation and Oxidation. Catal. Sci. Technol. 2016, 6 (11), 3670– 3693,  DOI: 10.1039/C6CY00544F
      22
      Nitrogen-doped porous carbon materials: promising catalysts or catalyst supports for heterogeneous hydrogenation and oxidation
      Li, Mingming; Xu, Fan; Li, Haoran; Wang, Yong
      Catalysis Science & Technology (2016), 6 (11), 3670-3693CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      A review. Developing novel and efficient catalysts is a crit. step in common heterogeneous hydrogenation and oxidn. reactions. Despite the frequent study of metal oxide-supported catalysts, porous carbon materials have also emerged as valuable potential catalysts. However, due to their highly microporous structures and inferior structural functionalities, traditional activated carbons (ACs) have become increasingly less popular for industrial applications. To deal with the disadvantages of ACs, tremendous efforts were made to develop novel nitrogen-doped porous carbon (NPC) materials with novel features such as highly porous structures and abundant structural nitrogen heteroatom decoration. As catalysts or catalysts supports, NPC materials showed superior activities in many applications covering a wide range of heterogeneous hydrogenation and oxidn. reactions. In this contribution, the authors review the fabrication methods for NPC materials used in heterogeneous hydrogenations and oxidns. and highlight the intrinsic catalytic mechanisms along with the catalyst design strategies.
    23. 23
      Fiorio, J. L.; Garcia, M. A. S.; Gothe, M. L.; Galvan, D.; Troise, P. C.; Conte-Junior, C. A.; Vidinha, P.; Camargo, P. H. C.; Rossi, L. M. Recent Advances in the Use of Nitrogen-Doped Carbon Materials for the Design of Noble Metal Catalysts. Coord. Chem. Rev. 2023, 481, 215053  DOI: 10.1016/j.ccr.2023.215053
      23
      Recent advances in the use of nitrogen-doped carbon materials for the design of noble metal catalysts
      Fiorio, Jhonatan Luiz; Garcia, Marco A. S.; Gothe, Maite Lippel; Galvan, Diego; Troise, Paula Castellani; Conte-Junior, Carlos A.; Vidinha, Pedro; Camargo, Pedro H. C.; Rossi, Liane M.
      Coordination Chemistry Reviews (2023), 481 (), 215053CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      A review. Noble metals nanoparticles (NPs) and single atoms (SAs) supported on nitrogen-doped carbon (NC) materials display remarkable activity and selectivity in a wide variety of reactions, spanning hydrogenations, oxidns., Fischer-Tropsch synthesis, and Suzuki coupling. Due to the unique interaction between the NC structure and the anchored metal center, both phys. and chem. properties of the catalysts can be finely tuned. Moreover, the precise control of the coordination environment in the host support can pave the way to designing efficient noble metal catalysts with optimized active centers. This approach opens avenues for improving stability, selectivity, and catalytic activity. This review covers the recent progress in the field of catalysis by noble metals supported on N-doped carbon materials. An overview of various catalytic systems based on Au, Ag, Pd, Pt, Ru, Rh is discussed, and structure-performance relations in catalysis are described based on theor. and exptl. investigations for different classes of metals and reactions. Finally, challenges and perspectives for engineering heterogeneous catalysts based on noble metals embedded in N-doped carbon materials are described to tackle challenges regarding activity and selectivity.
    24. 24
      Kaare, K.; Yu, E.; Volperts, A.; Dobele, G.; Zhurinsh, A.; Dyck, A.; Niaura, G.; Tamasauskaite-Tamasiunaite, L.; Norkus, E.; Andrulevičius, M.; Danilson, M.; Kruusenberg, I. Highly Active Wood-Derived Nitrogen-Doped Carbon Catalyst for the Oxygen Reduction Reaction. ACS Omega 2020, 5 (37), 23578– 23587,  DOI: 10.1021/acsomega.0c01974
      There is no corresponding record for this reference.
    25. 25
      Shi, Z.; Yang, W.; Gu, Y.; Liao, T.; Sun, Z. Metal-Nitrogen-Doped Carbon Materials as Highly Efficient Catalysts: Progress and Rational Design. Adv. Sci. 2020, 7 (15), 2001069  DOI: 10.1002/advs.202001069
      25
      Metal-Nitrogen-Doped Carbon Materials as Highly Efficient Catalysts: Progress and Rational Design
      Shi, Zhangsheng; Yang, Wenqing; Gu, Yuantong; Liao, Ting; Sun, Ziqi
      Advanced Science (Weinheim, Germany) (2020), 7 (15), 2001069CODEN: ASDCCF; ISSN:2198-3844. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. As a typical class of single-atom catalysts (SACs) possessing prominent advantages of high reactivity, high selectivity, high stability, and maximized at. utilization, emerging metal-nitrogen-doped carbon (M-N-C) materials, wherein dispersive metal atoms are coordinated to nitrogen atoms doped in carbon nanomaterials, have presented a high promise to replace the conventional metal or metal oxides-based catalysts. In this work, recent progress in M-N-C-based materials achieved in both theor. and exptl. investigations is summarized and general principles for novel catalysts design from electronic structure modulating are provided. Firstly, the applications and mechanisms on the advantages and challenges of M-N-C-based materials for a variety of sustainable fuel generation and bioinspired reactions, including the oxygen redn. reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), carbon dioxide redn. reaction (CO2RR), nitrogen redn. reaction (NRR), and nanozyme reactions are reviewed. Then, strategies toward enhancing the catalytic performance by engineering the nature of metal ion centers, coordinative environment of active centers, carbon support, and their synergistic cooperation, are proposed. Finally, prospects for the rational design of next generation high-performance M-N-C-based catalysts are outlined. It is expected that this work will provide insights into high-performance catalysts innovation for sustainable and environmental technologies.
    26. 26
      Wang, K.; Jiang, P.; Yang, M.; Ma, P.; Qin, J.; Huang, X.; Ma, L.; Li, R. Metal-Free Nitrogen-Doped Carbon Nanosheets: A Catalyst for the Direct Synthesis of Imines under Mild Conditions. Green Chem. 2019, 21 (9), 2448– 2461,  DOI: 10.1039/C9GC00908F
      26
      Metal-free nitrogen -doped carbon nanosheets: a catalyst for the direct synthesis of imines under mild conditions
      Wang, Kaizhi; Jiang, Pengbo; Yang, Ming; Ma, Ping; Qin, Jiaheng; Huang, Xiaokang; Ma, Lei; Li, Rong
      Green Chemistry (2019), 21 (9), 2448-2461CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      Herein, a highly stable, porous, multifunctional and metal-free catalyst was developed, which exhibited significant catalytic performance in the oxidn. of amines and transfer hydrogenation of nitriles under mild conditions; this could be attributed to the presence of numerous active sites and their outstanding BET surface area. The obtained results showed that most of the yields of imines exceeded 90%, and the cycling performance of the catalyst could be at least seven runs without any decay in the reaction activity, which could be comparable to those of metal catalysts. Subsequently, a kinetic study has demonstrated that the apparent activation energy for the direct synthesis of imines from amines is 67.39 kJ mol-1, which has been performed to testify that the catalytic performances are rational. Via catalyst characterizations and exptl. data, graphitic-N has been proven to be the active site of the catalyst. Hence, this study is beneficial to comprehend the mechanism of action of a metal-free N-doped carbon catalyst in the formation of imines.
    27. 27
      Bang, G. S.; Shim, G. W.; Shin, G. H.; Jung, D. Y.; Park, H.; Hong, W. G.; Choi, J.; Lee, J.; Choi, S.-Y. Pyridinic-N-Doped Graphene Paper from Perforated Graphene Oxide for Efficient Oxygen Reduction. ACS Omega 2018, 3 (5), 5522– 5530,  DOI: 10.1021/acsomega.8b00400
      27
      Pyridinic-N-Doped Graphene Paper from Perforated Graphene Oxide for Efficient Oxygen Reduction
      Bang, Gyeong Sook; Shim, Gi Woong; Shin, Gwang Hyuk; Jung, Dae Yool; Park, Hamin; Hong, Won G.; Choi, Jinseong; Lee, Jaeseung; Choi, Sung-Yool
      ACS Omega (2018), 3 (5), 5522-5530CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
      We report a simple approach to fabricate a pyridinic-N-doped graphene film (N-pGF) without high-temp. heat treatment from perforated graphene oxide (pGO). pGO is produced by a short etching treatment with hydrogen peroxide. GO perforation predominated in a short etching time (∼1 h), inducing larger holes and defects compared to pristine GO. The pGO is advantageous to the formation of a pyridinic N-doped graphene because of strong NH3 adsorption on vacancies with oxygen functional groups during the nitrogen-doping process, and the pyridinic-N-doped graphene exhibits good electrocatalytic activity for oxygen redn. reaction (ORR). Using rotating-disk electrode measurements, we confirm that N-pGF undergoes a four-electron-transfer process during the ORR in alk. and acidic media by possessing sufficient diffusion pathways and readily available ORR active sites for efficient mass transport. A comparison between Pt/N-pGF and com. Pt/C shows that Pt/N-pGF has superior performance, based on its more pos. onset potential and higher limiting diffusion current at -0.5 V.
    28. 28
      Kakunuri, M.; Sharma, C. S. Candle Soot Derived Fractal-like Carbon Nanoparticles Network as High-Rate Lithium Ion Battery Anode Material. Electrochim. Acta 2015, 180, 353– 359,  DOI: 10.1016/j.electacta.2015.08.124
      28
      Candle Soot derived Fractal-like Carbon Nanoparticles Network as High-Rate Lithium Ion Battery Anode Material
      Kakunuri, Manohar; Sharma, Chandra S.
      Electrochimica Acta (2015), 180 (), 353-359CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      The authors report a facile and inexpensive approach to synthesize fractal-like interconnected network of C nanoparticles from candle soot and its direct application as anode material for high-rate Li ion batteries used for elec. vehicles. At low charge/discharge rate (0.5 C), an initial discharge capacity is 1997 mAh/g with moderate 30% coulombic efficiency that increased to 91% after 10 cycles. More importantly, at very high charge/discharge rate (10 C), reversible capacity was stabilized at 170 mAh/g even after 1000 cycles. This remarkable electrochem. performance may be ascribed to unique morphol. of these hard C nanoparticles that reduces the diffusion length and also allows fast adsorption/desorption of Li ions on their surface.
    29. 29
      Tsai, C.-Y.; Tai, H.-C.; Su, C.-A.; Chiang, L.-M.; Li, Y.-Y. Activated Microporous Carbon Nanospheres for Use in Supercapacitors. ACS Appl. Nano Mater. 2020, 3 (10), 10380– 10388,  DOI: 10.1021/acsanm.0c02291
      29
      Activated Microporous Carbon Nanospheres for Use in Supercapacitors
      Tsai, Cheng-Yen; Tai, Hung-Chun; Su, Chien-An; Chiang, Li-Ming; Li, Yuan-Yao
      ACS Applied Nano Materials (2020), 3 (10), 10380-10388CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
      Microporous carbon spheres (MCSs) fabricated using the extended Stober method have unique features, such as a narrow particle size distribution and a variety of applications. Here, we report the synthesis of nanosized MCSs using a modified version of the extended Stober method at an ambient condition, which is a simple, quick, and scalable process. The activation of nanosized MCSs using CO2 is conducted to increase the surface area of MCSs. The activated microporous carbon nanospheres (AMCNSs) have a sp. surface area of 3259 m2/g and a mean diam. of about 52 nm. To the best of our knowledge, this is the smallest reported particle size for carbon spheres with a very high sp. surface area. In addn., AMCNSs have a hierarchical structure, which is beneficial for mass transport. The AMCNSs are evaluated for application in supercapacitors. The results show that a capacitance of 225 F/g at a c.d. of 0.5 A/g in a 6 M KOH aq. electrolyte can be achieved, indicating that AMCNSs have a potential for supercapacitor applications.
    30. 30
      Luo, H.; Lv, Y.; Tian, S.; Li, G.; Dai, W. Metal-Free Nitrogen-Doped Porous Carbons for Nitriles and Amides Synthesis from 1,2-Diols via Oxidative Cleavage of C–C Bonds. ACS Catal. 2023, 13 (22), 14996– 15006,  DOI: 10.1021/acscatal.3c03584
      There is no corresponding record for this reference.
    31. 31
      Zhou, Q.; Guo, X.; Song, C.; Zhao, Z. Defect-Enriched N,O-Codoped Nanodiamond/Carbon Nanotube Catalysts for Styrene Production via Dehydrogenation of Ethylbenzene. ACS Appl. Nano Mater. 2019, 2 (4), 2152– 2159,  DOI: 10.1021/acsanm.9b00124
      There is no corresponding record for this reference.
    32. 32
      Li, X.; Luo, W.; Zhu, K.; Chen, Y.; Huang, Y.; Jin, C.; Qiu, R.; Luo, S.; Guan, G.; Yan, K. Electronic Modulation of S and N Co-Implanted Carbon as Fenton-like Photocatalysts for Water Remediation. Chem. Eng. J. 2023, 474, 146016  DOI: 10.1016/j.cej.2023.146016
      There is no corresponding record for this reference.
    33. 33
      Várhegyi, G.; Szabó, P.; Till, F.; Zelei, B.; Antal, M. J.; Dai, X. TG, TG-MS, and FTIR Characterization of High-Yield Biomass Charcoals. Energy Fuels 1998, 12 (5), 969– 974,  DOI: 10.1021/ef9800359
      There is no corresponding record for this reference.
    34. 34
      Schuepfer, D. B.; Badaczewski, F.; Guerra-Castro, J. M.; Hofmann, D. M.; Heiliger, C.; Smarsly, B.; Klar, P. J. Assessing the Structural Properties of Graphitic and Non-Graphitic Carbons by Raman Spectroscopy. Carbon 2020, 161, 359– 372,  DOI: 10.1016/j.carbon.2019.12.094
      34
      Assessing the structural properties of graphitic and non-graphitic carbons by Raman spectroscopy
      Schuepfer, Dominique B.; Badaczewski, Felix; Guerra-Castro, Juan Manuel; Hofmann, Detlev M.; Heiliger, Christian; Smarsly, Bernd; Klar, Peter J.
      Carbon (2020), 161 (), 359-372CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
      We study the transformation from mol. to cryst. of (non-)graphitic carbons synthesized from org. precursors by heat treatment. Easy assessment of structural properties resulting from heat treatment protocols is mandatory for industrial process monitoring. Raman spectroscopy, in particular, the Raman lineshape anal. of G and D mode, offers quick assessment of the av. sheet size of such carbons. We validate this method by performing Raman, WAXS, and EPR measurements of series of resin and pitch-based carbons synthesized. The crystallite sizes of the WAXS anal. for the individual samples are related to corresponding positions and linewidths of G and D Raman modes and show excellent agreement between expt. and modeling from large sizes down to 4 nm. The theor. master curves are independent of the precursor used in the synthesis, in contrast to models for the intensity ratio of D and G band vs. size. The latter are not universally valid and differ for each class of precursors. For sizes <4 nm, our lineshape model fails as it is based on the band structure and phonon dispersions of ideal graphene. Thus, 4 nm corresponds to the fundamental transition from mol. to cryst. character for non-graphitic carbons.
    35. 35
      Ottaviani, M. F.; Mazzeo, R. EPR Characterization of Graphitized and Activated Micro- and Meso-porous Carbons. Microporous Mesoporous Mater. 2011, 141, 61– 68,  DOI: 10.1016/j.micromeso.2010.10.049
      There is no corresponding record for this reference.
    36. 36
      Yang, M.; Guo, D.; Zhang, T.; Liu, G.; Wu, N.; Qin, A.; Liu, X.; Mi, H. Controlled Synthesis of Ultrafine β-Mo2C Nanoparticles Encapsulated in N-Doped Porous Carbon for Boosting Lithium Storage Kinetics. ACS Omega 2021, 6 (44), 29609– 29617,  DOI: 10.1021/acsomega.1c03888
      There is no corresponding record for this reference.
    37. 37
      Vashchynskyi, V.; Okhay, O.; Boychuk, T. Chemical Activation of Apricot Pit-Derived Carbon Sorbents for the Effective Removal of Dyes in Environmental Remediation. C (Basel) 2023, 9 (4), 93,  DOI: 10.3390/c9040093
      There is no corresponding record for this reference.
    38. 38
      Hu, M.; Ye, Z.; Zhang, Q.; Xue, Q.; Li, Z.; Wang, J.; Pan, Z. Towards Understanding the Chemical Reactions between KOH and Oxygen-Containing Groups during KOH-Catalyzed Pyrolysis of Biomass. Energy 2022, 245, 123286  DOI: 10.1016/j.energy.2022.123286
      38
      Towards understanding the chemical reactions between KOH and oxygen-containing groups during KOH-catalyzed pyrolysis of biomass
      Hu, Mian; Ye, Zhiheng; Zhang, Qi; Xue, Qiping; Li, Zhibin; Wang, Junliang; Pan, Zhiyan
      Energy (Oxford, United Kingdom) (2022), 245 (), 123286CODEN: ENEYDS; ISSN:0360-5442. (Elsevier Ltd.)
      In this study, the effect of KOH/biomass ratios and pyrolysis temps. on products distribution, the evolution of oxygen-contg. groups in pyrolytic tri-state products and the possible chem. reactions between oxygen-contg. groups and KOH were investigated. The results indicated that KOH can react with oxygen-contg. groups in biomass to facilitate gas generation at lower ratios (≤1:2) or higher temp. (∼800°C). When at higher ratios (>1:2) and 600°C, KOH has an inhibitory effect on gas prodn. With a significant decreased in the oxygen-contg. species and acids, the phenols and hydrocarbons became the main species in bio-oil. Moreover, KOH facilitates methoxyphenol conversion and has high selectivity for alkyl phenol generation. For biochar, at lower ratios (<1:2), K can attack the oxygen sites of phenolic hydroxyl groups, forming dangling bonds and promoting the formation of arom. rings with anhydride groups. However, at higher ratios (>1:2), the oxygen-contg. groups on the surface of biochar gradually changed from anhydride groups to quinone groups, and finally form a biochar with large arom. ring and rich in quinone groups. As analyzed above, a possible chem. reaction between KOH and oxygen-contg. groups during KOH-catalyzed pyrolysis of biomass was proposed.
    39. 39
      El-Hendawy, A.-N. A. Variation in the FTIR Spectra of a Biomass under Impregnation, Carbonization and Oxidation Conditions. J. Anal Appl. Pyrolysis 2006, 75 (2), 159– 166,  DOI: 10.1016/j.jaap.2005.05.004
      39
      Variation in the FTIR spectra of a biomass under impregnation, carbonization and oxidation conditions
      El-Hendawy, Abdel-Nasser A.
      Journal of Analytical and Applied Pyrolysis (2006), 75 (2), 159-166CODEN: JAAPDD; ISSN:0165-2370. (Elsevier B.V.)
      The FTIR spectra of untreated date pits (DP) and their H3PO4-preimpregnated sample together with their chars as well as samples subjected to different activation schemes were detd. These activated carbons were prepd. using in the activation stage phosphoric acid, one- and two-step steam treatment, or potassium hydroxide. This series of activated carbons were oxidized with HNO3 and their surface modifications were detected by using FTIR spectra in the course of oxidn. conditions. The presence of cellulose and hemicellulose, lignin and protein constituents were detected in the raw material. Significant changes were obsd. in the chem. structure of the date pits under carbonization where arom. structures were developed and accompanied by losing most of aliph. CH species in the intermediate char product. However, char-IR spectrum revealed the presence of trace amts. of uncarbonized material. Whereas, activated carbons possessed arom. C=C bonds and oxygen groups. The chem. structure of the activated carbon were found to be influenced markedly depending on the scheme of activation followed. Increasing the activation temp. led to a complete destruction of C=O and C-O species simultaneously by promoted polyarom. structures. The detectable FTIR for the oxidized activated carbons confirmed the presence of large amt. of oxygen functionalities of acidic nature appearing as carbonyl, phenolic, hydroxylic, carboxylic and lactonic species groups on the carbon surface. Phosphoric acid was found to have a prominent effect on the botanical structure of the raw material. Its mechanism was based on initiation of the bond cleavage leading to dehydration and elimination reactions that release volatile products. This was followed by aromaticity and formation of a stronger crosslinked solid of new biopolymer involving phosphate esters.
    40. 40
      Salinas-Torres, D.; Navlani-García, M.; Mori, K.; Kuwahara, Y.; Yamashita, H. Nitrogen-Doped Carbon Materials as a Promising Platform toward the Efficient Catalysis for Hydrogen Generation. Appl. Catal. A Gen. 2019, 571, 25– 41,  DOI: 10.1016/j.apcata.2018.11.034
      40
      Nitrogen-doped carbon materials as a promising platform toward the efficient catalysis for hydrogen generation
      Salinas-Torres, David; Navlani-Garcia, Miriam; Mori, Kohsuke; Kuwahara, Yasutaka; Yamashita, Hiromi
      Applied Catalysis, A: General (2019), 571 (), 25-41CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
      Nitrogen-doped carbon materials have emerged as versatile candidates with outstanding features as both support of metal-based catalysts and metal-free catalytic systems. Their use has been traditionally linked to electrochem. applications, but the no. of applications in which these materials show their virtues has been exponentially increasing in the last decade. Particularly, catalysts based on metal nanoparticles supported on nitrogen-doped carbon materials have shown to be attractive for important catalytic reactions as they display enhanced performances endowed by well-dispersed, stabilized and electronically promoted nanoparticles as well as by the modified acid-base properties of the support. The present review is focused on the most recent breakthroughs achieved by these catalytic systems in the hydrogen prodn. from some of the most representative hydrogen carrier mols. by highlighting the effect of the incorporation of nitrogen in the support matrix.
    41. 41
      Zheng, Y.; Wang, J.; Li, D.; Liu, C.; Lu, Y.; Lin, X.; Zheng, Z. Insight into the KOH/KMnO4 Activation Mechanism of Oxygen-Enriched Hierarchical Porous Biochar Derived from Biomass Waste by in-Situ Pyrolysis for Methylene Blue Enhanced Adsorption. J. Anal Appl. Pyrolysis 2021, 158, 105269  DOI: 10.1016/j.jaap.2021.105269
      41
      Insight into the KOH/KMnO4 activation mechanism of oxygen-enriched hierarchical porous biochar derived from biomass waste by in-situ pyrolysis for methylene blue enhanced adsorption
      Zheng, Yunwu; Wang, Jida; Li, Donghua; Liu, Can; Lu, Yi; Lin, Xu; Zheng, Zhifeng
      Journal of Analytical and Applied Pyrolysis (2021), 158 (), 105269CODEN: JAAPDD; ISSN:0165-2370. (Elsevier B.V.)
      A high-performance hierarchical porous carbon enriched in oxygen-contg. functional groups was prepd. by using pine as the raw material, KOH and KMnO4 as a green activator via cost-effective one-step or two-step in-situ pyrolysis processes. The adsorption performance, kinetics and thermodn. of activated biochar were evaluated using methylene blue (MB) as a model pollutant. Biochar structure was investigated by X-ray diffraction (XRD), Raman spectroscopy, Fourier transform IR (FTIR) spectroscopy, XPS, N2 adsorption/desorption isotherms (BET) and SEM (SEM) to explore its morphol., phys., and chem. properties. A possible adsorption mechanism and pathway were also proposed. The results showed that a two-step process, higher activation (KOH and KMnO4) to biomass ratio led to excellent adsorption performance. AC-KOH-1-1 and AC-KMnO4-1-3 presented the max. equil. adsorption capacities of 637.5 mg/g and 439.5 mg/g, resp. The adsorption isotherm fit well with the Freundlich isotherm model and pseudo-second-order kinetic model, which exhibited better monolayer adsorption and chem. adsorption control. In addn., the adsorption was dominated by pore diffusion and π-π stacking interactions for KOH activation due to the well-developed pore vol. (0.04990 mg/L), pore channel (18.65 nm), aromatized structure (66.03%) and surface defective structure. However, the adsorption capacity was controlled by hydrogen bonding, electrostatic adsorption interactions, cation exchange and the donor-receptor-effect for AC-KMnO4 due to it having the highest proportion of O- and N-contg. functional groups. Therefore, this cost-efficient biochar prepn. technol. for the efficient removal of org. pollutants has great potential for practical application.
    42. 42
      Ji, N.; Yin, J.; Rong, Y.; Li, H.; Yu, Z.; Lei, Y.; Wang, S.; Diao, X. More than a Support: The Unique Role of Nb2O5 in Supported Metal Catalysts for Lignin Hydrodeoxygenation. Catal. Sci. Technol. 2022, 12 (12), 3751– 3766,  DOI: 10.1039/D2CY00245K
      42
      More than a support: the unique role of Nb2O5 in supported metal catalysts for lignin hydrodeoxygenation
      Ji, Na; Yin, Jianyu; Rong, Yue; Li, Hanyang; Yu, Zhihao; Lei, Yaxuan; Wang, Shurong; Diao, Xinyong
      Catalysis Science & Technology (2022), 12 (12), 3751-3766CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      A review. Nb2O5-supported metal materials are a vital class of green catalysts with excellent lignin hydrodeoxygenation activity and reusability in an aq. catalytic system, in which the Nb2O5 support plays a pivotal role because of its nontoxic nature, strong acidity and remarkable stability in water. This review summarizes the recent advances in Nb2O5-supported metal catalysts for lignin hydrodeoxygenation, esp. the special role of Nb2O5. In particular, the review first presents an overview of the structure, acid and redox properties of Nb2O5. It then focuses on the unique contribution of Nb2O5 in hydrodeoxygenation of lignin model compds. based on comparison with other common supports, including its promotion of C-O cleavage and enhancement of suitability and reusability in the aq. phase. Strategies for the modification of Nb2O5 and the conversion of real lignin or its oil to aroms. over Nb2O5-supported metal catalysts are further summarized. Finally, the article attempts to highlight the remaining challenges and provide some outlooks for the future development of Nb2O5-supported metal catalysts. This review will provide valuable insights for developing advanced green catalysts for lignin valorization and other challenging environmental catalytic processes.
    43. 43
      Zhao, J.-X.; Wang, W.; Jiao, Z.-F.; Guo, X.-Y. A Highly Efficient Defective Carbon Catalyst for Oxidative Coupling of Amines. Catal. Commun. 2023, 177, 106652  DOI: 10.1016/j.catcom.2023.106652
      43
      A highly efficient defective carbon catalyst for oxidative coupling of amines
      Zhao, Ji-Xiao; Wang, Wen; Jiao, Zhi-Feng; Guo, Xiang-Yun
      Catalysis Communications (2023), 177 (), 106652CODEN: CCAOAC; ISSN:1566-7367. (Elsevier B.V.)
      Porous carbons were prepd. from tea peels, and employed as the catalyst for oxidative coupling of benzylamine. An optimal tea peel carbon (TPC-2-800) shows large sp. surface area of 1715 m2 g-1 and excellent catalytic performance for benzylamine oxidn. At 70°C and ambient air conditions, the yield of imine can get to 99%. Further investigations suggest the outstanding performance can be attributed to the high sp. surface area and rich edge defects, which help the adsorption and dissocn. of mol. oxygen. This work provides a ref. for the application of biomass carbon materials in aerobic oxidn. reactions.
    44. 44
      Li, F.; Dai, X.; Qi, W. Primary Amine Coupling on Nanocarbon Catalysts: Reaction Mechanism and Kinetics via Fluorescence Probe Analysis. Green Energy Environ. 2020, 5 (4), 453– 460,  DOI: 10.1016/j.gee.2020.09.008
      There is no corresponding record for this reference.
    45. 45
      Tashrifi, Z.; Khanaposhtani, M. M.; Larijani, B.; Mahdavi, M. Recent Advances in the Oxidative Conversion of Benzylamines. Tetrahedron 2021, 84, 131990  DOI: 10.1016/j.tet.2021.131990
      45
      Recent advances in the oxidative conversion of benzylamines
      Tashrifi, Zahra; Khanaposhtani, Mohammad Mohammadi; Larijani, Bagher; Mahdavi, Mohammad
      Tetrahedron (2021), 84 (), 131990CODEN: TETRAB; ISSN:0040-4020. (Elsevier Ltd.)
      A review. Thus in this review, focused on the oxidative reactions of benzylamines including a brief description of the reaction mechanisms for the synthesis of N-heterocyclic compds. based on the catalytic system applied. In this respect, the related studies from 2012 to 2019, have been categorized and classified in five main sections.

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    • Experimental section; N2 adsorption–desorption isotherms; BET surface area, pore volume, and pore diameter; EPR spectra; the probable reaction mechanism for homocoupling of benzylamine; and the comparison of the activity of the ANCS catalyst with that of the literature reports for the C–N coupling of OPD with benzylamine (PDF)


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