Iron single-atom anchored N-doped carbon: an efficient catalyst for one-pot, scaled-up pentazolate synthesis

Abstract

The discovery of ambient stable pentazolate salts has paved way for a new frontier in the chemistry of pentazoles, which represent a distinctive category of energetic materials. One challenge impeding their practical applications lies in achieving the scale-up of pentazolate synthesis via a facile and low-cost route. An efficient and recyclable iron single-atom catalyst with FeN₄ sites (Fe₁@NC-700) has been first developed using a pyrolysis-milling strategy for scaled-up pentazolate synthesis and achieves significant yields (38.4 g, 15% yield for NaN₅; 77.6 g, 26% yield for CoN₅) via a three-step, one-pot process with a low iron dosage (0.67 mol%). According to experimental and theoretical calculation results, the exceptional catalytic performance of Fe₁@NC-700 can be attributed to its stable FeN₄ enzyme-like catalytic active sites, which facilitate the formation of a Fe(IV)=O intermediate, and it maintains its catalytic activity even under weakly acidic conditions and after five cycles.

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Introduction

In 2017, some of us first synthesized ambient stable pentazolate (cyclo-N₅⁻) salts, which have attracted great interest in the field of high-energy-density materials (HEDMs) owing to their potential high energy and unique energy release mode.¹ In the last five years, sodium pentazolate [NaN₅(H₂O)]·2H₂O (NaN₅) has been used as a raw material for the synthesis of various cyclo-N₅⁻ complexes, including pentazolate metallic and non-metallic salts, co-crystals and 3D metal-inorganic frameworks, via metathesis reactions (Fig. 1).² Furthermore, cyclo-N₅⁻, as a rare and preferred ligand for building topology architectures, provides new possibilities in coordination chemistry.²ⁱ Therefore, the use of NaN₅ as a basic raw material for all N₅⁻ based energetic materials is a fundamental of pentazolate chemistry.

Fig. 1 Synthesis of various pentazolate complexes from NaN₅.

At present, NaN₅3 is synthesized via the C–N bond cleavage of arylpentazole 2, which is produced from arylamine hydrochloride (2,6-dimethyl-4-aminophenol hydrochloride, 1) through a diazotization and cyclization reaction (Scheme 1). The yield of arylpentazole 2 is about 60%, while the yield of the C–N bond cleavage step is poor (∼10%); thus, the total yield is only ∼6% with respect to arylamine hydrochloride 1. In the C–N bond cleavage step, a tedious work-up procedure and excessive iron catalyst and oxidant usage are the norm, thereby making this approach unsuitable for large-scale synthesis.^1a Hence, it is appealing and desirable to improve the synthetic process of NaN₅ for the achievement of its efficient scaled-up synthesis.

Scheme 1 Current issues and potential solutions in pentazolate synthesis.

Hang's group revealed the mechanism of the oxidative cleavage of the C–N bond of 2: Fe(Gly)₂ reacts with mCPBA to form a Fe(IV)=O intermediate, which can realize the oxidative C–N bond cleavage of 2 to form NaN₅. DFT calculations indicate that the Fe(IV)=O intermediate plays a crucial role in the reaction, since it greatly reduces the reaction activation energy.³ It is well known that some iron-based enzymes with a FeN₄ structure (such as cytochrome P450 or hemin) can activate oxygen to form Fe(IV)=O species for the oxidation of organic substrates under mild conditions.⁴ According to the bionic theory, a series of N-doped carbon-supported iron single-atom catalysts with enzyme-like FeN_x sites have been developed for selective oxidation reactions.⁵ Hence, an N-doped carbon-supported iron single-atom catalyst with FeN_x sites could exhibit good performance in the C–N bond cleavage of 2 for the effective preparation of NaN₅.

In addition, the unstable compound 2 must be purified using acetone washing at −60 °C in the original preparation process, which greatly reduces the yield of 2 owing to the decomposition and dissolution of 2 in acetone. The separation and purification of NaN₅ requires column chromatography, which is costly and inefficient, and makes the process difficult to scale up. According to these results, we propose three strategies for the upgrading of this synthetic process: (1) an iron single-atom catalyst with highly active FeN_x sites could be applied instead of Fe(Gly)₂ for the selective oxidative cleavage of the C–N bond to improve the activity of Fe(IV)=O. (2) The synthesis of 3 could be directly completed via a one-pot, three-step method, which would avoid the decomposition of 2 during the work-up process. (3) Pentazolate ions could be converted into cobalt pentazolate precipitates (CoN₅) via a coordination-precipitation strategy, which can be separated by simple filtration and transformed into other pentazolate salts through metathesis reactions.⁶

Along this line, we have developed a pyrolysis-milling approach for the synthesis of N-doped carbon-supported iron single-atom catalysts (Fe₁@NC-x), which display much better performance than Fe(Gly)₂ and hemin owing to their high activity and the stability of enzyme-like FeN₄ single-atom sites. These materials can also achieve a one-pot, three-step process for the scaled-up synthesis of NaN₅ (38.4 g) and CoN₅ (77.6 g), providing several advantages, including a much lower dosage (0.67 mol%) of the recyclable iron catalyst (>5 runs), a simplified work-up procedure and higher pentazolate yields (Fig. 2). To the best of our knowledge, this is the first example of single-atom catalysis for scaled-up pentazolate synthesis.

Fig. 2 One-pot, scaled-up pentazolate synthesis over an iron single-atom catalyst.

Results and discussion

An iron single-atom catalyst supported on N-doped carbon material was prepared using a pyrolysis-milling strategy (Fig. 3a). A mixture of melamine and carbon black was calcined at x °C (x = 700, 800 and 900) under a nitrogen atmosphere to prepare a nitrogen-doped carbon material (NC-x). Then, Fe₁@NC-x was prepared by ball-grinding NC-x with FeSO₄·7H₂O. As shown in Fig. 3b–d, no iron nanoparticles are observed for any Fe₁@NC-x catalysts. There are only two broad peaks at 25° and 43°, which can be attributed to the (002) and (100) crystal planes of graphitic carbon, respectively (Fig. S1†).⁷ These results suggest that no highly crystalline Fe-containing species exist in these Fe₁@NC-x catalysts.

Fig. 3 (a) Synthesis route of Fe₁@NC-x; HRTEM images of Fe₁@NC-700 (b), Fe₁@NC-800 (c), Fe₁@NC-900 (d); AC-HAADF-STEM images of Fe₁@NC-700 (e), Fe₁@NC-800 (f), and Fe₁@NC-900 (g).

According to Raman spectra (Fig. S2†), the D (defect) and G (graphite) bands of Fe₁@NC-x were observed at about 1336 cm⁻¹ and 1582 cm⁻¹, respectively. The I_D/I_G value of Fe₁@NC-700 is higher than that of the other two catalysts, indicating that Fe₁@NC-700 has the most defects.⁸ The Fe contents of Fe₁@NC-700, Fe₁@NC-800 and Fe₁@NC-900 are 0.82 wt%, 0.73 wt% and 0.65 wt%, respectively (Table S1†). The BET surface area and aperture of the Fe₁@NC-x catalysts are shown in Fig. S3† and Table S2.† Fe₁@NC-700 has the highest BET surface area (68.41 m² g⁻¹) and largest average aperture (38.56 nm) among these catalysts, which is consistent with Raman results.⁹

The surface chemical states of these catalysts were measured using X-ray photoelectron spectroscopy (XPS). The N 1s spectrum of Fe₁@NC-x could be deconvoluted into four peaks centered at binding energies of 398.4, 399.4, 400.5 and 403.2 eV, which were assigned to pyridinic N, Fe–N, pyrrole N and N-oxide, respectively (Fig. S4†).¹⁰ The N content of the three catalysts was calculated by semi-quantitative analysis of XPS results (Table S3†). Among the three catalysts, Fe₁@NC-700 has the highest N content, which further explains the reason for its high defect degree and corresponds to Raman and BET characterization data.¹¹ The defects on the carbon material are of great significance for capturing N species, which can anchor more iron species to generate FeN_x sites. Moreover, defects enlarge the surface area of the catalysts, which is beneficial to expose more active sites and thus increase interaction between the catalyst and reactive species. Furthermore, Fe₁@NC-700 has the highest Fe–N content, which has a positive correlation with the number of active Fe sites, further elucidating the reason for its high catalytic activity.¹²

Aberration-corrected high annular angle dark-field scanning TEM (AC-HAADF-STEM) and X-ray absorption fine structure (XAFS) spectroscopy were also employed to study the Fe configurations of Fe@NC-x catalysts at the atomic level. As shown in Fig. 3e–g, isolated bright spots (marked by red circles) show good dispersion in these catalysts, proving that iron species exist in these catalysts as single atomic iron sites.

The energy absorption edge of Fe₁@NC-700 lies between the energy absorption boundaries of Fe foil and Fe₂O₃, indicating that the Fe valence state is between 0 and +3 (Fig. 4a).¹³ Fourier-transformed k³-weighted extended X-ray absorption fine structure (FT-EXAFS) spectra are shown in Fig. 4b. Fe₁@NC-700 exhibits only a main peak at 1.6 Å, which is attributed to the Fe–N bond, further confirming that the Fe species of Fe₁@NC-700 mainly exist in the form of single Fe atom sites.¹⁴ According to the results of fitting using the program FEFF6, the coordination number of the single Fe atom is about 3.9 (Table S4†), and the fitting curve of the FeN₄ sites is basically consistent with this catalyst curve (Fig. 4c); thus, iron single-atom sites should mainly be FeN₄ coordination structures.¹⁵

Fig. 4 (a and b) XANES and FT EXAFS spectra at the Fe K-edge of Fe₁@NC-700, Fe foil, FePc and Fe₂O₃. (c) Corresponding fitting of the EXAFS spectrum of Fe₁@NC-700 in R space.

Fig. 5 (a) One-pot, three-step, scaled-up pentazolate synthesis over Fe₁@NC-700. (b) Single-crystal structure of CoN₅. (c) Recyclability of Fe₁@NC-700. (d) EPR spectra of Fe₁@NC-700, hemin and FeCl₂ in the presence of excess mCPBA at 77 K.

With these iron catalysts in hand, their catalytic activities were studied in a one-pot three-step process for the synthesis of NaN₅ (Table 1), whose structure was confirmed using MS, IR and ion chromatography (Fig. S5–S7 and S11a†). Fe(Gly)₂ could not achieve this three-step, one-pot process (entry 1), since its Fe ions would dissociate from the ligand under acidic conditions. Hemin, with its stable FeN₄ sites, could achieve the selective oxidation and cleavage of C–N bonds to synthesize NaN₅ under acidic conditions, with a yield of 10%. When FeCl₂ was used as the catalyst, no desired product was obtained, indicating that the Fe ions without N coordination have no catalytic activity for the reaction (entry 3). The yields of Fe₁@NC-x are much higher than those of Fe(Gly)₂, hemin and FeCl₂ (entries 1–6), since their Fe sites are anchored into NC to form stable FeN₄ single-atom sites, which exhibit excellent catalytic oxidation performance.¹⁶ In addition to the FeN₄ sites, the porous structure of Fe₁@NC-700 promotes the reaction to a certain extent, because it is beneficial for the interaction between reactants (or intermediates) and catalysts.

Table 1 Synthesis of NaN₅via a one-pot process over different iron catalysts^a

Entry	Catalyst	Catalyst (g)	mCPBA^b (g)	Yield^c (%)
a Reaction conditions: step 1: 87 mmol 2,6-dimethyl-4-aminophenol hydrochloride (15.1 g), 96 mmol HCl, 90 mmol NaNO2, THF, −5 °C, 0.5 h; step 2: 90 mmol NaN3, MeOH, −40 °C, 2 h; step 3: catalyst, mCPBA, MeCN, −15 °C, 12 h. b m-Chloroperbenzoic acid. c Yields were calculated based on arylamine hydrochloride 1.
1	Fe(Gly)2	42	33	0
2	Hemin	56	33	10
3	FeCl2	4	33	0
4	Fe1@NC-800	4	33	16
5	Fe1@NC-900	4	33	14
6	Fe1@NC-700	4	33	18
7	Fe1@NC-700	2	33	13
8	Fe1@NC-700	3	33	16
9	Fe1@NC-700	5	33	18
10	Fe1@NC-700	4	16.5	10
11	Fe1@NC-700	4	24.8	16
12	Fe1@NC-700	4	41.3	18
13	Fe1@NC-700-2	4	33	17
14	Co1@NC-700	4	33	1
15	NC-700	4	33	0
16	—	4	33	0

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Among the three Fe₁@NC-x catalysts, Fe₁@NC-700 exhibits the best performance because it has the most iron single-atom sites (entries 4–6). The dosages of Fe₁@NC-700 and mCPBA were also optimized, and the combination of 4 g Fe₁@NC-700 and 33 g mCPBA in the 87 mmol scale reaction was the best option (entries 6–12). Fe₁@NC-700-2 and Co@NC-700 were synthesized via the same preparation methods using FeCl₃ and CoSO₄·7H₂O as metal precursors. Fe₁@NC-700-2 displays similar catalytic activity to Fe₁@NC-700 (entries 6 vs. 13), while only trace NaN₅ can be obtained in the case of Co₁@NC-700 (entry 14), since it cannot form a Co(IV)=O intermediate. Blank experiments verify that the iron sites are the catalytically active center of this reaction (entries 15 vs. 16).

To further highlight the potential of this catalyst, we increased the reaction scale 20 times (1.74 mol, 302 g of 1). Fe₁@NC-700 maintained good catalytic activity, and the yield of NaN₅ reached 15% (Fig. 5a). Nevertheless, the work-up procedure for the separation and purification of NaN₅ is tedious, including multiple room-temperature vacuum rotary evaporations for water removal and column chromatography, which results in an ultra-long post-treatment time, the use of excessive organic solvents, high energy consumption, and loss and decomposition of NaN₅ (Table 2, entry 2).

Table 2 Comparison of different synthetic routes for scaled-up pentazolate synthesis^a

Entry	Route	Purification	Cat. Fe (g)	Solvent^b (L)	PT^c (h)	Yield^d (%)	Purity^e (%)
a Reaction conditions: step 1: 1.74 mol 2,6-dimethyl-4-aminophenol hydrochloride (302 g), 1.92 mol HCl, 1.80 mol NaNO2, THF, −5 °C, 1 h; step 2: 1.80 mol NaN3, MeOH, −40 °C, 2 h; step 3: catalyst, 660 g mCPBA, MeCN, −15 °C, 12 h. b Post-processing solvent usage. c Post-processing time. d Yields were calculated based on arylamine hydrochloride 1. e Product purity was determined using ion chromatography.
1	Two-pot	Column chromatography	Fe(Gly)2 (840)	33.0	26	7 (NaN5)	70
2	One-pot	Column chromatography	Fe1@NC-700 (80)	31.0	20	15 (NaN5)	90
3	One-pot	Precipitation and filtration	Fe1@NC-700 (80)	6.5	7	26 (CoN5)	98

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To eliminate these issues, a coordination-precipitation strategy has been developed. Organic acid salts in the reaction solution are first converted into organic acids by adjusting the pH and further removed by extraction. Then, pentazolate anions (N₅⁻) cooperate with Co²⁺ salts to yield CoN₅ precipitate ([Co(H₂O)₄(N₅)₂]·4H₂O), which can be separated by simple filtration (Fig. 5b, S8–S10, S11b; Tables S5 and S6†). Furthermore, CoN₅ can replace NaN₅ as the basic raw material for other pentazolate salts.⁶ A 26% yield of CoN₅ (77.6 g) was afforded through the coordination-precipitation strategy.

To verify the stability and recyclability of Fe₁@NC-700, the scaled-up synthesis of CoN₅ was chosen as the model reaction. The substrate (arylamine hydrochloride) was completely converted into pentazolate and other by-products. Fe₁@NC-700 should be able to adsorb some by-products generated during the reaction, such as phenols, mCPBA and m-chlorobenzoic acid. After the reaction was completed, the weight of the filtered recovered catalyst increased by 9% after drying, which confirmed our hypothesis. Therefore, ethanol washing is necessary to remove adsorbates during the catalyst recovery process. However, pentazolate salts have good water solubility, so they should not be adsorbed on the catalyst. As shown in Fig. 5c, Fe₁@NC-700 could still maintain high catalytic activity after five runs. A comparison of different synthetic routes for the scaled-up pentazolate synthesis is provided in Table 2. Compared to the other two routes, the one-pot CoN₅ synthetic route has a higher yield and purity and shorter post-processing time and uses less Fe catalyst and post-processing solvent.

To confirm the Fe(IV)=O intermediate in this process, a low-temperature (77 K) electron paramagnetic resonance (EPR) experiment was carried out. Rhombic signals were observed after the reaction of hemin and Fe₁@NC-700 with excess mCPBA, thereby demonstrating the existence of a Fe(IV)=O intermediate.¹⁷ The EPR singlet intensity of Fe₁@NC-700 is higher than that of hemin, suggesting that Fe₁@NC-700 is more conducive to the generation of Fe(IV)=O than hemin. No singlet was detected in the case of FeCl₂, which means that it failed to react with mCPBA to afford Fe(IV)=O. This can explain why FeCl₂ has no catalytic performance in this reaction.

The Gibbs free energy profiles for the reaction of iron catalysts (hemin, Fe(Gly)₂ and FeN₄) with mCPBA are also illustrated in Fig. 6a and S12–S14.† The energy barrier for the reaction of FeN₄ (the catalytically active site of Fe₁@NC-700) with mCPBA to generate Fe(IV)=O is the lowest (3.49 kcal mol⁻¹). This further proves that the formation of Fe(IV)=O over Fe₁@NC-700 is easier than for other catalysts (hemin and Fe(Gly)₂). Fig. 6b shows the free energy diagram for the selective oxidative cleavage of the C–N bond. According to the Gibbs free energy profiles of these two reactions, the energy barriers of iron-catalyzed reactions for NaN₅ synthesis follow the order Fe₁@NC-700 (6.23 kcal mol⁻¹) < Fe(Gly)₂ (9.85 kcal mol⁻¹) < hemin (11.33 kcal mol⁻¹).

Fig. 6 (a) Gibbs free energy profiles for the reaction of iron catalysts with mCPBA. (b) Gibbs free energy profiles for the reaction of arylpentazole 2 with Fe(IV)=O intermediates. Silvery, grey, dark blue, red, green and light-blue balls represent hydrogen, carbon, nitrogen, oxygen, chlorine and iron atoms, respectively.

According to above presented results, the superior catalytic performance of Fe₁@NC-700 can be attributed to following aspects. (1) It has stable FeN₄ enzyme-like catalytic active sites, which promote the formation of a Fe(IV)=O intermediate and maintain catalytic activity even under weakly acidic conditions and after multiple cycles. (2) It has a higher surface area with more defects and the FeN₄ sites than Fe₁@NC-800 and Fe₁@NC-900, all of which can boost its catalytic activity.

Conclusions

In summary, an iron single-atom catalyst (Fe₁@NC-700) with the FeN₄ sites has been fabricated using a pyrolysis-milling strategy and can achieve scaled-up pentazolate synthesis (38.4 g, 15% yield for NaN₅; 77.6 g, 26% yield for CoN₅) via a three-step, one-pot process with a 0.67 mol% iron dosage. The superior catalytic performance of Fe₁@NC-700 can be attributed to two key factors. (1) It possesses stable FeN₄ enzyme-like catalytic active sites as evidenced by EPR and DFT calculation results, which promote the formation of an Fe(IV)=O intermediate and maintain catalytic activity even under weakly acidic conditions and after five cycles. (2) It has a higher surface area and more defects and the FeN₄ sites than Fe₁@NC-800 and Fe₁@NC-900 based on BET, Raman, ICP and XPS results, all of which can boost its catalytic activity. This innovative work integrates single-atom catalysis with the synthesis of energetic materials, offering a novel approach for the scalable production of pentazolates and broadening the application scope of single-atom catalysis.

Data availability

Data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22275090 and 22105102) and Young Elite Scientist Sponsorship Program by CAST (YESS20210074).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta03312d

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