Two-step synthesis of amino-methyl-N-phenylcarbamate from toluidine: new preparative method and mechanism

Abstract

Amino-methyl-N-phenylcarbamate (TMC) is an important organic intermediate. In this study, a new two-step route, i.e., methoxycarbonylation and a direct amination reaction, was designed for synthesis of TMC. During this process, p-toluidine first reacted with dimethyl carbonate to form metholylcarbamate (MTCM), and then MTCM further converted to TMC via the direct amination reaction. It was found that in the methoxycarbonylation process, the Zn(OAc)₂ catalyst was a better choice and p-toluidine was almost completely converted into MTCM, corresponding to 99% p-toluidine conversion and 95.4% MTCM yield. As for the direct amination reaction, the Fe–V/ZSM-5 bimetallic catalyst was selected and a high yield of total TMC was obtained at 69.6%. The catalyst characterization results showed that the synthesized bimetallic catalysts retained the structural features of ZSM-5, while the abundance of metal ions on the surface promoted the reaction.

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1. Introduction

Carbamate compounds are an important chemical intermediates, which can be used in the production of dyes,^1,2 polymer materials,^3,4 electronics^5,6 plastics,^7,8etc. Among them, amino-methyl-N-phenylcarbamate (TMC), being as a kind of aromatic carbamate compounds, is firstly found in the synthesis of toluene dicarbamate (TDC) by using toluene diamine and dimethyl carbonate.⁹ TDC is an important raw material for the synthesis of toluene diisocyanate (TDI), which is widely used in the polyurethane industry.^9–11

Currently, there are several ways to produce TMC. One is to use 2,4-diaminotoluene, methyl chloroformate and triethylamine to obtain TMC with a yield of 17.9%.¹² It is simple but has a low yield. Another process is to convert the amino group on 2-methyl-5-nitroaniline to a carbamate group using dimethyl carbonate (DMC), and then carry out the hydrogenation reaction to obtain TMC. The yield of this method is 97%.¹³ However, the hydrogenation reaction in the process requires high pressure and uses nitro compounds generated from the nitration reaction as feedstock. The harsh conditions and formation of regioisomers limit applicability. Therefore, developing a green and safety synthesis method for TMC remains a critical research challenge.

Recently, Li¹⁴ found that synthesis of phenylcarbamate (a simple aromatic carbamate compound) from DMC and aniline via the methoxycarbonylation process could be achieved using a loaded ZrO₂ catalyst. Peng¹⁵ also investigated the reaction over Zn(OAc)₂/SiO₂ catalyst with 86.9% yield of phenylcarbamate. In addition, Nie¹⁶ explored the synthesis of toluidine by direct amination of toluene with hydroxylamine hydrochloride over vanadium catalysts under mild conditions. In recent years, Liu¹⁷ and Falk¹⁸ studied the amination reaction of aromatic hydrocarbons using a novel organic hydroxylamine reagent and achieved results on a variety of substrates. Considering these, a new two-step process was designed for the synthesis of TMC (see Fig. 1), i.e., toluidine (take p-toluidine for example) first reacted with DMC to form metholylcarbamate (MTCM) via the methoxycarbonylation process (reaction I), and then MTCM further converted to TMC with amine reagent by the direct amination reaction (reaction II). Even more interesting is that the initial material of toluidine can also be produced by direct amination of toluene, instead of selective hydrogenation of nitrotoluene.¹⁶

Fig. 1 Reaction process for synthesizing TMC.

The objective of this study was to investigate the two-step reaction (see Fig. 1). Various reaction conditions were optimized including catalyst, reaction temperature time, etc. Moreover, the synthesized non-homogeneous catalysts were prepared and characterized. And a high yield of TMC was achieved in the present work.

2. Experimental

2.1 Materials and reagents

All reagents were of analytical reagent grade and used without further purification. Dimethyl carbonate, p-toluidine, zinc acetate, hydroxylamine sulfate, hydroxylamine-O-sulfonic acid, tert-butyl hydroxycarbamate, p-anisoyl chloride and trifluoromethanesulfonic acid were purchased from Aladdin Industrial Co. (China). Benzoyl chloride, 4-nitrobenzoyl chloride, nosyl chloride, p-anisylsulfonyl chloride, sodium metavanadate, ammonium metavanadate, sodium molybdate, ammonium molybdate and ferrous acetate were purchased from Shanghai Macklin Biochemical Technology Co. (China). Triethylamine, dichloromethane, sodium chloride, sodium bicarbonate, sodium sulfate, ferrous sulfate, ferrous chloride, ferric chloride and ferric nitrate were purchased from Tianjin Komiou Chemical Reagent Co., Ltd. (China).

2.2 Amine reagent preparation

Some of the amination reagents were prepared as follows.¹⁷tert-Butyl N-hydroxycarbamate (1 equiv.) was first dissolved in dichloromethane. After cooling the solution to 0 °C, triethylamine (1 equiv.) was added and stirred for 30 min. The corresponding formyl chloride (1.05 equiv.) was subsequently added. The solution was brought to room temperature and stirred for 12 h. At the end of the reaction, the organic phase was washed with deionized water, saturated sodium bicarbonate solution and saturated saline. The organic phases were combined and dried under reduced pressure to remove water. The solid obtained after treatment was dissolved in a certain amount of ether and stirred while cooling. After the solution was cooled to 0 °C, trifluoromethanesulfonic acid was added dropwise and reacted for 2 h. After that, the solid was separated by centrifugation and washed with ether. A series of protonated aminating reagents were obtained after drying.

2.3 Catalyst preparation

A series of Fe/ZSM-5 (MCM-41, Ts-1, γ-Al₂O₃, SBA-15, CeO₂) catalysts were prepared using the impregnation method. 3 g of ZSM-5 molecular sieves, 15 mL of deionized water and an appropriate amount of Fe(NO₃)₃·9H₂O were placed in a beaker and reacted at 60 °C for 3 h. Thereafter, water was removed at 70 °C on a rotary evaporator. The solid obtained was sufficiently dried and placed in a muffle furnace and calcined at 550 °C for 5 h. Finally, Fe/ZSM-5 (MCM-41, Ts-1, γ-Al₂O₃, SBA-15, CeO₂) catalysts were obtained by hydrogen reduction at 700 °C for 5 h in a tube furnace.

The Fe–V(Mo)/ZSM-5 catalyst was prepared as follows: 3 g of ZSM-5 molecular sieves, 20 mL of deionized water, and equal amounts of ammonium metavanadate (or ammonium molybdate) and oxalic acid were placed in a beaker. After stirring and dissolving, Fe(NO₃)₃·9H₂O was added and the reaction was carried out at 60 °C for 4 h. After vacuum drying, the catalyst was calcined and reduced with hydrogen under the same conditions as those used for the preparation of Fe/ZSM-5 to produce the desired catalyst.

2.4 Characterization methods

The X-ray diffraction (XRD) patterns were measured on a Rigaku SmartLab SE X-ray powder diffractometer using Cu Kα radiation at 40 kV and 40 mA. Data were collected in the 2θ range from 5° to 90° at a scan rate of 6° min⁻¹. The BET data were obtained with a ASAP 2460 M+C specific surface and porosity analyzer. X-ray photoelectron spectroscopy (XPS) experiments were performed on a K-Alpha spectrometer from Thermo Scientific. The content of metal components in the samples was determined using an Agilent 5110 ICP-OES inductively coupled plasma emission spectrometer. FT-IR analysis was carried out using a Shimadzu-IR Tracer 100 FT-IR spectrometer equipped with a MIR TGS detector. The surface structure and micromorphology of the catalysts were observed using a scanning electron microscope – ZEISS Gemini SEM 300. For morphological characterization, a transmission electron microscope (TEM, FEI Talos F200S) was employed.

2.5 Catalytic activity evaluation and product analysis

Reaction I. The methoxycarbonylation of p-toluidine was carried out in a 100 mL high-pressure reactor. 0.092 g of catalyst, 3.19 g of p-toluidine and 50 mL of DMC were added in the reactor, sealed and replaced with N₂ for 3 times. After the reaction, the reaction solution was filtered and then distilled on vacuum to MTCM. The concentrations of organic components were analyzed by high performance liquid chromatography (Waters e2695). Chromatographic conditions were as follows. A Turner C18 (200 mm × 4.6 mm 5 μm) chromatographic column and a UV 2998 PDA detector were used for analysis. The detection wavelength was 232 nm and CH₃OH/H₂O (70/30, V/V) was used as the mobile phase. The conversion of p-toluidine and the yield of MTCM were calculated by the following eqn (1) and (2).where m₀ is the mass of p-toluidine in the reactant, g; m₁ is the mass of p-toluidine in the products, g; M_Toluidine is the molar mass of p-toluidine, g; m_MTCM is the mass of the MTCM component in the products, g; and M_MTCM is the molar mass of MTCM, g.

Reaction II. MTCM, the catalyst (Fe–V/ZSM-5), solvent (HFIP) and NH₂ source were added to a 50 mL triple-necked flask equipped with a stirring and condensation reflux unit, and stirred at 40 °C for 2 h. After the reaction, the reaction system was cooled down to room temperature, and neutralized with 30% NaOH solution to pH = 7–8. After that, the organic phase was extracted with dichloromethane. The samples were analyzed by high performance liquid chromatography and detection conditions were the same as for reaction I. The conversion of MTCM and the yield of 2,4-TMC and 3,4-TMC were calculated by the following eqn (3)–(5).where m_MTCM is the mass of MTCM in the reactant, g; m_MTCM′ is the mass of the MTCM component in the products, g; m_2,4-TMC and m_3,4-TMC are the masses of component 2,4-TMC and 3,4-TMC in the products, g; M_MTCM is the molar mass of MTCM, g; M_2,4-TMC and M_3,4-TMC are the molar masses of component 2,4-TMC and 3,4-TMC.

3. Results and discussion

3.1 Reaction I

In the methanocarbonylation process, a series of Lewis acid catalysts was investigated, and the results are shown in Table 1. As can be seen from the data, the catalysts Zn(NO₃)₂·6H₂O, Bi(OAc)₃ and Pb(OAc)₂ show no catalytic activity. If Mn(OAc)₂·4H₂O was employed, a little of MTCM can be obtained (entry 4). Furthermore, when Zn(OAc)₂·2H₂O and Zn(OAc)₂ were used as the catalyst, MTCM yield greatly improved to 76.2% and 87.3%, respectively. The catalytic activity of Zn(OAc)₂·2H₂O is not as high as that of Zn(OAc)₂. This may be due to the crystal water in Zn(OAc)₂·2H₂O, leading to easy hydrolysis of DMC. This not only reduces the amount of feedstock involved in the reaction, but also converts Zn(OAc)₂ to inactive ZnO species during the hydrolysis.^19,20 Therefore, the Zn(OAc)₂ catalyst was chosen for the rest of the experiments.

Table 1 Catalytic effect of different catalysts^a

Entry	Catalyst	Conversion (%)	Yield (%)
a Reaction conditions: p-toluidine (0.03 mol), DMC (50 mL), catalyst (0.5 mmol), 160 °C, 6 h.
1	Zn(NO3)2·6H2O	0	0
2	Bi(OAc)3	0	0
3	Pb(OAc)2	0	0
4	Mn(OAc)2·4H2O	6.8	3.6
5	Zn(OAc)2·2H2O	80.7	76.2
6	Zn(OAc)2	91.9	87.3

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Furthermore, catalyst amount, solvent, reaction temperature and time were optimized as shown in Fig. 2. MTCM yield first increases, and then remains around 90% with increasing catalyst amount (Fig. 2a). Thus, 0.5 mmol of the Zn (OAc)₂ catalyst was used for further optimization. As for DMC addition, a similar trend was observed, and better results can be achieved at 50 mL of DMC (Fig. 2b). For the variation of MTCM versus temperature (Fig. 2c), a bell shaped curve is observed. High yield of MTCM is obtained at 140 °C. However, the yield is reduced obviously with the temperature above 180 °C. It is probably due to the carbon on the carbonyl group replacing the hydrogen atom of the amino group on the benzene ring, producing N-methylates and generating the noncondensable gas CO₂.²¹ For the influence of the reaction time, the yield of MTCM can be improved at 4 h (Fig. 2d). The optimum reaction conditions were 0.5 mmol of Zn(OAc)₂, 50 mL of DMC, conducted at 140 °C for 4 h. Under these conditions, p-toluidine was almost completely converted into MTCM, corresponding to 99% p-toluidine conversion and 95.4% MTCM yield.

Fig. 2 Effects of catalyst amount (a), DMC amount (b), temperature (c) and time (d) on methanocarbonylation. Reaction conditions: (a) p-toluidine, 0.03 mol; DMC, 50 mL; 160 °C, 6 h. (b) p-toluidine, 0.03 mol; Zn(OAc)₂, 0.5 mmol; 160 °C, 6 h. (c) p-Toluidine, 0.03 mol; Zn(OAc)₂, 0.5 mmol; DMC, 50 mL; 6 h. (d) p-Toluidine, 0.03 mol; Zn(OAc)₂, 0.5 mmol; DMC, 50 mL; 140 °C.

3.2 Reaction II

3.2.1 Catalytic activity of homogeneous catalysts in amination reactions. In the amination process (see Fig. 3), the above-prepared MTCM was used as a raw material, and various parameters were investigated in homogeneous system. These results are displayed in Fig. 4.

Fig. 3 Reaction process and amination reagents.

Fig. 4 Effects of different amine reagents (I), catalysts (II), solvents (III), amount of amine reagent (IV), temperature (V) and time (VI) on one-step amination. Reaction conditions: (I) amine reagent, 1.6 mmol; FeSO₄·7H₂O, 0.2 mmol; HAc : H₂O (3 : 1), 0.2 M; 60 °C, 4 h. (II) amine reagent f, 1.6 mmol; catalyst, 0.2 mmol; HAc : H₂O (3 : 1), 0.2 M; 60 °C, 4 h. (III) amine reagent f, 1.6 mmol; catalyst, 0.2 mmol; HFIP, 0.2 M; 60 °C, 4 h. (IV) Fe(OAc)₂, 0.2 mmol; HFIP, 0.2 M; 60 °C, 4 h. (V) amine reagent f, 1.8 mmol; Fe(OAc)₂, 0.2 mmol; HFIP, 0.2 M; 4 h. (VI) amine reagent f, 1.8 mmol; Fe(OAc)₂, 0.2 mmol; HFIP, 0.2 M; 40 °C. The amount of MTCM was 1 mmol in all cases.

Initially, a series of amination reagents and catalysts were screened. As depicted in Fig. 4I, the MTCM yield depended greatly on the amination reagents. When organic hydroxylamine salts were used (amine reagent d–j), the MTCM yield is much higher that of inorganic hydroxylamine salts (amine reagent a–c). The inorganic hydroxylamine salts possess easy hydrolysis, causing the amount involved in the reaction becomes less, reducing the yield.²² Among organic hydroxylamines, reagent f has the best reactivity. So the organic hydroxylamine reagent f, i.e., (NbzONH₃)⁺(OTf)⁻, was selected as the provider of the amino group. Concerning the catalyst (Fig. 4II), the activity of ferrous salts is superior to other compounds of vanadium, molybdenum, and trivalent iron, which is similar to the findings of some researchers.^23,24 Specifically, the Fe²⁺ species facilitate the coupling between the substrate and amino radicals (·NH₂) generated from the amination agent, thereby promoting C–N bond formation. Hence, the optimum homogeneous catalyst is Fe(OAc)₂ for reaction II.

Next, the influence of solvent and the amount of amine reagent f was studied as indicated in Fig. 4III and IV. For the influence of solvent, different polar solvents were found in good yields, with HFIP showing the best activity, attributed to its possession of high polarity, low nucleophilicity, and strong hydrogen bond-donating ability.^25–27 With increasing the amount of amine reagent f, MTCM yield first increases, and then decreases at 55%. So 2 mmol of amine reagent f is enough for the present amination.

Furthermore, the effects of reaction temperature and time were studied. As shown in Fig. 4V and VI, the MTCM yield first increases, reaches a maximum, and then decreases with increasing temperature or time. Based on the comprehensive experimental evidence, both elevated reaction temperatures and prolonged reaction durations were found to adversely affect the results. The optimum reaction temperature and time are 60 °C and 2 h, respectively. The total yield of TMC under these conditions could reach 62.9%.

3.2.2 Catalytic activity of nonhomogeneous catalysts containing Fe active species. Based on the above results, homogeneous ferrous salts possess excellent reactivity. However, it has disadvantages such as separation difficulties and difficult recovery. Therefore, nonhomogeneous catalysts were employed, i.e., Fe active species were loaded onto zeolite with a more uniform pore structure and larger specific surface area. The structural properties, Fe loading and catalytic performance of different carrier catalysts are listed in Table 2. It shows that the Fe/ZSM-5 catalyst has the best catalytic activity owing to its large specific surface area and pore volume.

Table 2 Texture properties of catalysts with different supports and their effects on reaction performance^a

Catalyst	Specific surface area^b (m^2 g^−1)	Pore volume^b (cm^3 g^−1)	Pore diameter^b (nm)	Conversion (%)	Yield (%)
					2,4-TMC	3,4-TMC
a Reaction conditions: MTCM (1 mmol), (NbzONH3)^+(OTf)^− (1.8 mmol), catalyst (0.1 g), HFIP (0.2 M), stirred at 40 °C under air, 2 h. b The texture properties was determined by BET.
Fe/SBA-15	379	0.98	8.74	61.1	39.9	3.0
Fe/MCM-41	182	0.18	6.07	66.2	42.4	3.4
Fe/γ-Al2O3	92.8	0.49	17.67	55.9	34.2	3.2
Fe/Ts-1	338	0.13	10.96	59.3	36.2	3.8
Fe/CeO2	4.67	0.022	13.55	52.4	33.3	3.3
Fe/ZSM-5	310	0.074	7.31	68.6	45.5	3.6

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Additionally, V and Mo species have been known to have the ability to catalyze the amination of aromatics.¹⁶ Therefore, loading of V or Mo on Fe/ZSM-5 was considered to explore the catalyst structure and its effect on the amination reaction. Table 3 shows the structural properties, metal loading and catalytic performance of different catalysts. After doping with V and Mo, the MTCM yields were slightly increased to 69.67% and 64.00%, respectively.

Table 3 Texture properties of different catalysts and their effects on reaction performance^a

Catalyst	Specific surface area^b (m^2 g^−1)	Pore volume^b (cm^3 g^−1)	Pore diameter^b (nm)	Conversion (%)	Yield (%)
					2,4-TMC	3,4-TMC
a Reaction conditions: MTCM (1 mmol), (NbzONH3)^+(OTf)^− (1.8 mmol), catalyst (0.1 g), HFIP (0.2 M), stirred at 40 °C under air, 2 h. b The texture properties was determined by BET.
ZSM-5	364	0.076	6.22	0	0	0
Fe/ZSM-5	310	0.074	7.31	68.6	45.5	3.6
Fe–V/ZSM-5	269	0.072	7.44	79.4	62.4	7.3
Fe–Mo/ZSM-5	253	0.073	7.82	76.2	58.1	6.0

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3.2.3 Characterization of catalysts. Fig. 5a shows the N₂ adsorption–desorption curves for the ZSM-5, Fe/ZSM-5, Fe–V/ZSM-5 and Fe–Mo/ZSM-5 catalysts. It can be seen that the four isotherms are roughly similar and capillary condensation of the mesopores are found. The N₂ adsorption–desorption isotherm exhibited an H4-type hysteresis loop in the relative pressure (P/P₀) range of 0.4–0.9, which is characteristic of mesoporous materials with slit-shaped pores. At higher relative pressures (P/P₀ > 0.9), a pronounced increase in adsorption capacity was observed, attributable to the formation of interparticle voids between zeolite crystallites.²⁸ These properties indicate that the synthesized catalysts have the special structure of zeolite molecular sieves.^29,30Fig. 5b shows that the four catalysts show broad bands corresponding to the stretching and deformation vibrations of O–H bonds at wavelengths of ∼3443 and 1640 cm⁻¹. Meanwhile, the absorption bands at 1236 cm⁻¹ and 1095 cm⁻¹ are associated with the asymmetric stretching of Si–O bonds, the peak detected at 807 cm⁻¹ is attributed to the symmetric stretching of Si–O–Si, and the absorption bands at 548 cm⁻¹ and 442 cm⁻¹ absorption bands at 548 cm⁻¹ and 442 cm⁻¹ are attributed to asymmetrically stretched Si–O–T and T–O (tetrahedral) bending vibrations of internal tetrahedral AlO₄ and SiO₄, respectively.^31,32 These indicate that the ZSM-5-based catalysts have a typical MFI (mobile-type 5) molecular sieve structure in all bands. Fig. 5c shows the XRD patterns of the four catalysts. The peaks detected at 2θ = 7.98, 8.91, 14.0, 14.8, 15.9, 20.6, 23.1, 23.9, 24.6, and 30.0° corresponded to the ZSM-5 molecular sieve's (101), (200), (102), (301), (302), (103), (501), (303), (403), and (503) planes, showing their characteristic MFI structures (JCPDS card No. 49-0657). The (110), (200), and (211) planes of Fe were detected at 2θ = 44.6, 65.0, and 82.3° (JCPDS card No. 06-0696), indicating that Fe formed crystalline material on ZSM-5 molecular sieves.³³ But the characteristic peaks of V and Mo are not detected, suggesting that the two metal elements are highly dispersed in the amorphous state in the catalysts.

Fig. 5 N₂ adsorption–desorption isotherms (a); FTIR spectra (b) and XRD patterns (c) of the synthesized catalysts.

Scanning electron microscopy (SEM) analysis reveals the intricate crystalline morphology of the catalyst (Fig. 6). The micrographs demonstrate that the catalyst particles are uniformly covered with irregularly shaped crystals. Following the incorporation of Fe as the active metal, a noticeable increase in particle size and packing density is observed on the catalyst surface. This phenomenon is presumably attributed to the formation of aggregated larger particles during the calcination and reduction processes, resulting from the relatively high metal loading.

Fig. 6 SEM-micrographs of ZSM-5 (a), Fe/ZSM-5 (b), Fe–V/ZSM-5 (c) and Fe–Mo/ZSM-5 (d) catalysts.

Subsequent introduction of the second active metal further enhances the surface roughness of the catalyst, a finding that correlates well with the XRD patterns and N₂ adsorption–desorption results. Importantly, the presence of sufficient active metal species on the catalyst surface is identified as a critical factor governing the efficiency of the amination reaction.

Fig. 7 presents a systematic investigation of the microstructural characteristics of the Fe–V bimetallic modified ZSM-5 catalyst through transmission electron microscopy (TEM). The images reveal that the catalyst support possesses a well-ordered mesoporous–microporous composite structure, with uniform pore channels measuring 5–8 nm in diameter and exhibiting continuous, well-defined pore walls. This observation confirms that the zeolite framework maintains its structural integrity without significant collapse or disordering during the metal loading process.

Fig. 7 TEM-micrographs of Fe–V/ZSM-5 (a), HRTEM (b and c), HAADF (d) and EDS-mapping (e–h).

Notably, spherical nanoparticles with diameters ranging from 10 to 30 nm are observed on the catalyst surface. High-resolution TEM analysis shows lattice fringes with a spacing of 0.25 nm, corresponding to the (110) plane of α-Fe, which confirms the presence of iron-based active phases.

Fig. 7(e–h) display the EDS elemental mapping results, which provide insights into the spatial distribution of the metallic components. The Fe signals exhibit a dispersed distribution throughout the zeolite matrix, with localized regions of higher concentration. Meanwhile, the V distribution overlaps significantly with Fe, showing enhanced intensity at the zeolite lattice nodes. This suggests that vanadium species may coordinate with framework aluminum sites in ZSM-5, forming stable bimetallic active centers.

Fig. 8 shows the XPS spectra of the three loaded catalysts. The characteristic peaks of Fe can be detected on all the samples, and the characteristic peaks of V 2p and Mo 3d are detected on the V and Mo doped ZSM-5 catalyst respectively. Fig. 8a shows the survey scan of the different catalysts. Fig. 8b shows the Fe 2p spectra of different Fe 2p profiles of the loaded catalysts, and similarly, the regional signal of Fe⁰ was detected near 707.4 eV. The signals detected at 710.1 eV and 713.7 eV belong to the Fe 2p^3/2 orbitals, while the Fe²⁺ and Fe³⁺ located near 725 eV belong to the same Fe 2p^1/2 orbitals.^33–35Fig. 8c shows the V 2p mapping of Fe-V/ZSM-5, with signals detected in the region of V⁴⁺ near 516.3 eV as well as 523.7 eV.³⁶Fig. 8d shows the Mo 3d mapping of Fe–Mo/ZSM-5, with signals detected near 227.7 and 230.8 eV, 229.3 and 232.5 eV, 232.4 and 235.5 eV, and 233.7 and 236.7 eV, respectively, where the regional signals of Mo⁰, Mo⁴⁺, Mo⁵⁺, and Mo⁶⁺ were detected.³⁷ The XPS spectra indicates that all the surfaces of the prepared catalysts are rich in metal ions, which effectively promote the reaction.

Fig. 8 XPS spectra of the as-prepared catalysts. Survey scan (a), Fe 2p (b), V 2p (c) and Mo 3d (d).

In summary, comprehensive characterization of the heterogeneous catalyst confirms the successful immobilization of active Fe and V species on the ZSM-5 support without compromising the zeolite's structural integrity. Furthermore, the catalyst exhibits abundant accessible metal ions on its surface and likely contains stabilized active centers within the framework, which collectively facilitate efficient catalytic performance in the amination reaction.

3.2.4 Reusability of catalysts. Reuse experiments were conducted for the Fe–V/ZSM-5 catalyst. From the experimental data in Fig. 9, it can be seen that the catalyst did not show a substantial decrease in yield at the second use, but from the third use onwards, the catalytic activity decreased rapidly thereafter, dropping to a total yield of 13.63% by the fifth use.

Fig. 9 Experiments on the reuse of catalysts.

In order to investigate the reason of catalyst deactivation, ICP tests were carried out on the catalysts before and after use, where 0.0488 g of catalyst was dissolved using 25 mL of strong acid, and the elemental content of the sample was measured by diluting it 100 times (Table 4). As can be seen from the data in the table, there was no significant decrease in the metal content of Fe and V after the first use, while there was a very large decrease in the metal content of the catalyst after the third use. It is presumed that the dissolution of metals occurred. In the amination reaction system, hexafluoroisopropanol is an acidic solvent, while the amination reagent generates p-nitrobenzoic acid after participating in the reaction, which strengthens the acidity of the system and leads to the dissolution of Fe and V active species on the catalyst into the solution. And with the increase of the number of experiments, the amount of catalyst recovered is less and less.

Table 4 Elemental content of Fe-V/ZSM-5 catalysts (%)

Number of times used	Fe	V
1	20.7	5.07
2	19.1	4.74
3	14.5	3.31
4	4.26	0.96
5	3.12	0.77

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To enhance the reusability of the catalysts, the roasting temperature during catalyst preparation was changed, and an increase in temperature would result in a tighter bonding of the catalysts. Therefore, the reaction performance of the catalysts at different roasting temperatures was investigated, and the specific experimental data are shown in Fig. 10. It showed that when the roasting temperature was increased to 600 °C and 650 °C, the total yield decreased to a certain extent with the increase in the number of times of use, until the fourth use was found to have a higher yield than that of the roasting temperature of 550 °C, which indicates that the increase in the roasting temperature enhances the reusability of the catalysts but decreases the catalytic activity of the catalysts catalytic activity.

Fig. 10 Reuse experiments of catalysts prepared at different roasting temperatures.

4. Conclusion

In summary, a new two-step method for synthesis of TMC was successfully realized in this study. As for the methanocarbonylation process (step I), the optimum reaction conditions were determined as follows. The Zn(OAc)₂ catalyst amount was 0.5 mmol, the DMC solvent consumption was 50 mL, carried out at 140 °C for 4 h. And p-toluidine was almost completely converted into MTCM, corresponding to 99% p-toluidine conversion and 95.4% MTCM yield. For the direct amination process (step II), organic hydroxylamine reagent (NbzONH₃)⁺(OTf)⁻ was selected as a better choice for providing amino group. In the homogeneous reaction system, the suitable reaction temperature and time are 60 °C and 2 h, respectively. The total yield of TMC could reach 62.9% with Fe(OAc)₂ as the catalyst. Meanwhile, in the nonhomogeneous reaction system, the Fe–V/ZSM-5 bimetallic catalyst was chosen and characterized by XRD, FT-IR, XPS, etc. The total TMC yield would reach 69.6%.

Author contributions

Xiangyu Wen: writing – review & editing, writing – original draft, investigation, and conceptualization. Qiusheng Yang: validation and data curation. Ming Li: formal analysis. Xiaoshu Ding: writing – review & editing. Peng Zhai: writing – review & editing. Yao Lu: writing – review & editing. Xinqiang Zhao: formal analysis. Yanji Wang: formal analysis. Dongsheng Zhang: writing – review & editing, supervision, resources, and formal analysis.

Conflicts of interest

There are no conflicts to declare.

Data availability

Supplementary information: The SI includes the original drawing data of each image in the article. See DOI: https://doi.org/10.1039/D5RE00246J.

The data supporting this article has been included as part of the SI.

Acknowledgements

This work was supported by the Hebei Innovation Capability Improvement Plan Project (25364002D), the Central Guidance Local Science and Technology Development Fund of Hebei Province (254Z1402G) and the National Natural Science Foundation of China (21878069, U20A20152).

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