Synthesis of Amines by Oxidative Coupling of Benzylamine over a Vanadium-Nitrogen Co-doped Porous Carbon Catalyst

Abstract

Synthesis of imine compounds via benzylamine oxidative coupling has become one of the most ideal methods due to its high atom economy and environmental friendliness. The key is to develop high performance non-noble metal-based heterogeneous catalysts. In this work, a vanadium-nitrogen co-doped porous carbon (V-N-C) catalyst was prepared via high temperature pyrolysis (900 ℃ for 2 h in an inert atmosphere) combined with acid leaching (1 mol·L-1 HCl solution at 120 ℃ for 12 h) approach by using biomass chitosan as the sacrificial template, vanadium acetylacetonate as the source of metal vanadium, and ZnCl2 as the pore-forming agent. Various characterzations techniques including a high-angle annular dark-field scanning transmission electron microscopy (HAADF -STEM) investigation were used to analyze the composition, structure, vanadium species size, content, and other physical and chemical properties of the catalyst, and its catalytic performance was evaluated in the oxidative coupling reaction of benzylamine. The characterization results showed that the specific surface area of the V-N-C catalyst was as high as 1 470 m2·g-1, the pore volume was 1.06 cm3·g-1, and the mass fraction of the vanadium species was 0.19% that were highly dispersed on the support likely in the form of single atoms (VNx). In the oxidative self-coupling reaction of benzylamine to the imine (reaction conditions: toluene as solvent, 110 ℃, 1.01×105 Pa O2, 12h), the developed V-N-C exhibited excellent activity (99%), exclusive selectivity (99%), outperforming the homogeneous VO(acac)2 and heterogeneous V2O5 catalysts. Moreover, V-N-C was repeatedly used 9 times without any decay in reactivity and stability. Furthermore, V-N-C presented excellent universality for a series of substrates containing different functional groups. Mechanism studies indicated that the reaction steps were involved in the initial formation of benzylamine and H2O2 intermediates by activating benzylamine and oxygen molecules, respectively, on the

VNx and defect sites of V-N-C, then benzylamine and benzylamine condensed to release an NH3 molecule to generate the target product imine.

Keywords: selective catalytic oxidation; heterogeneous catalysis; vanadium-nitrogen co-doped porous carbon; oxidative coupling

0 Intiroduction

Imines are heterocyclic compounds containing a C=N double bond structure and are used as common intermediates in organic reactions such as cyclization, redox, addition, and condensation [1]. Imine ligands containing heteroatoms (oxygen, sulfur, phosphorus, etc.) can form metal complexes with metal ions and are applied in the synthesis of a range of biologically and pharmacologically active compounds, heterocycles, or natural products [2-4]. Conventional imines are mainly obtained by condensation reactions of primary amines and carbonyl compounds in the presence of homogeneous Lewis acid catalysts [5-6], which has the disadvantage that the reaction process requires the addition of dehydrating agents and the use of unstable aldehyde raw materials to improve the reaction efficiency, resulting in low substrate utilization, complex operation and environmental pollution [7].

The synthesis of imines by oxidative coupling of amines in the presence of oxidants is a green route. Many catalytic systems have been reported in recent years, including noble metal catalysts [8-11] and photocatalysts [12-14], but most of the above reactions suffer from low yields or selectivity, relatively harsh reaction conditions, and limited applicability of substrates. Therefore, from a green and sustainable view of chemistry, there is an urgent need to develop a new green and efficient transition metal catalyst.

Transition metal (M) and heteroatomic nitrogen co-doped porous carbon (M-N-C) materials are a promising class of catalysts with promising applications, whose porous carbon skeleton structure facilitates the adsorption/diffusion of reactants and products, thus enhancing the catalytic activity [15-18]. And the lone pair electrons of heteroatom N can trap metal centers and form stable metal active sites, thus catalyzing a variety of organic reactions efficiently [19-20]. Chitosan, as renewable biomass, contains amino and hydroxyl functional groups in its structure, which have strong coordination ability and can effectively chelate metal ions. Chitosan-metal complex-derived materials not only partially inherit the characteristics of carrier materials but also are expected to retain the highly dispersed properties of metal centers, which are ideal precursors for the preparation of transition metal-based and nitrogen co-doped carbon materials [21-23]. In addition, the introduction of the porogenic agent ZnCl2 during the carbonization treatment of chitosan can effectively reduce the production of tar [10] and also promote the cross-linking reaction between C, H, and O cross-linking reaction, which helps to form a porous structure and increase the specific surface area of the material [24], thus improving the catalytic activity and selectivity. Using biomass chitosan as a sacrificial template, VO(acac)₂ as a metallic vanadium source, and ZnCl₂ as an activating porogenic agent, a high specific surface area vanadium-nitrogen co-doped porous V-N-C catalyst with the high specific surface area was synthesized by carbon heat treatment combined with pickling. The catalysts were investigated in the presence of O² as the oxidizing agent and toluene as the solvent. O² is the oxidizing agent and toluene is the solvent for the oxidative coupling reaction of benzylamine and its derivatives. The catalytic performance of the catalyst was investigated in the oxidative coupling reaction of benzylamine and its derivatives with O² as the oxidant and toluene as the solvent, and the possible reaction mechanism was initially speculated.

1 Experimental part

1.1 Main reagents

Chitosan was purchased from Sinopharm Chemical Reagent Company; vanadium acetylacetonate and zinc chloride were purchased from Shanghai Marel Chemical Technology Co. The reagents were purchased from Shanghai Aladdin Company; o-aminothiophenol, 2-aminothiophenol, p-methoxyaniline, p-chloroaniline and p-methylaniline were purchased from Beijing Inokai Company. All the above reagents were analytically pure, and high-purity N2 (99.999%) was purchased from Shanghai Datong Gas Co.

1.2 Catalyst characterization

The crystal structure of the catalyst was analyzed by X-ray diffractometer (XRD, D8 Adance type) with the following conditions: working voltage of 40 kV, current of 40 mA, radiation source of Cu target Kα rays, a wavelength of 0.154 2 nm, 2θ range of 2°~80°. The samples were analyzed by Raman spectroscopy (RenishawRM 10000, 532 nm) for the degree of graphitization and defects. The morphology of the samples was characterized using a scanning electron microscope (SEM, Model S-4800, 5 kV) and a transmission electron microscope (TEM, JEM-2100F, 200 kV) equipped with an E-1030 gold spray unit. The size and dispersion of metal species in the catalysts were probed using a spherical aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM, FEI Themis Z). A physisorption instrument (Micromeritics ASAP Model 2020) was used to determine the nitrogen adsorption-desorption isotherm of the catalyst at -196 °C. The sample elemental valence states were obtained using an X-ray photoelectron spectrometer (XPS, Thermo ESCALAB 250XI) with the following characterization conditions: the radiation source was Al Kα rays (1 486.6 eV), the system vacuum

The inductively coupled plasma emission spectrometer (ICP-AES, iCAP 7400, Thermo, USA) was used to determine the metal content was determined by inductively coupled plasma emission spectrometry (ICP-AES, iCAP 7400, Thermo, USA).

1.3 Catalyst preparation

V-N-C catalysts were prepared by dissolving 2 g of chitosan in 120 mL of 0.5% acetic acid solution, adding 6.5 g of zinc chloride and 20 mg of vanadium acetylacetonate, stirring and mixing, and reacting under reflux in a water bath at 80 ℃ for 8 h. The samples were then transferred to a freeze dryer for drying for 12 h. The dried samples were ground and placed in a corundum boat, placed in a tube furnace, and carbonized under N2 atmosphere at a temperature of 3 ℃-min-1 for 2 h. The samples were then transferred to a round bottom flask containing 50 mL of 1 mol-L-1 HCl solution and acid-washed for 12 h at 120 ℃ in an oil bath to remove unstable metals or metal oxides, cooled and then extracted and washed in a vacuum oven. The black solid was obtained by filtering and washing, and drying at 120 ℃ for 12 h in a vacuum oven. The preparation schematic is shown in Figure 1. Preparation of nitrogen co-doped porous carbon material (N-C): similar to the above mentioned method, except that no addition was made in the synthesis. The black solids were prepared using the similar method as above, except that the metal vanadium source was not added in the synthesis.The black solid is N-C.

Fig. 1 Schematic illustration of the synthesis process of V-N-C catalyst

1.4 Benzylamine oxidation coupling reaction

Weigh 30 mg of V-N-C catalyst, measure 3 mL of toluene and 0.5 mmol of benzylamine, and add them sequentially to a 10 mL Schlenk tube with an oxygen bag attached. Before the reaction, the air in the tube was replaced with oxygen (repeated 3 times), and then the reaction tube was placed in an oil bath at 110 °C to start timing the reaction. Samples were taken at regular intervals and the liquid-phase mixture was quantified on a gas chromatograph (Agilent GC-7890B) equipped with a flame ionization detector and a capillary column (HP -5, 30 m×0.32 mm×0.25 µm), using biphenyl as internal standard. Analytical conditions: the carrier gas was nitrogen, the column flow rate was 1 mL-min-1, and the injection port and detector temperature were The columns were programmed to warm up to an initial temperature of 60 ℃, and the column was operated at 10 ℃. The initial temperature was 60 ℃, and the column was ramped up to 200 ℃ at a rate of 10 ℃-min-1.

The column was ramped up to 200 ℃ at a rate of 10 ℃-min-1 and maintained for 4 min. The conversion, selectivity, and conversion frequency (TOF) were calculated by The equations for conversion= (n0-ns)/n0×100%, Selectivity=na/(na+nb)×100%, TOF=ns/(nct), where n0 is the initial amount of the substrate benzylamine, ns is the initial amount of the substrate benzylamine, and ns is the initial amount of the substrate benzylamine. amount of substance, ns is the amount of substance of the substrate benzylamine, Na is the product imine is the amount of substance of the substrate benzylamine, nb is the amount of substance of the by-product, NC is the amount of catalyst NC is the amount of catalyst substance, and t is the reaction time. The obtained products were analyzed by gas chromatography-mass spectrometry (GC-MS). The obtained products were confirmed by gas chromatography-mass spectrometry (GC-MS, Agilent 7890N-5975).

2 Results and Discussion

2.1 Catalyst characterization

Figure 2a shows the XRD patterns of V-N-C and N-C. Two distinct diffraction peaks appear at 23.5° and 43.0°, which are attributed to the (002) and (101) crystal planes of amorphous carbon [25-26], indicating that both V-N-C and N-C are partially graphitized C-based materials. In addition, the characteristic diffraction peaks of the V species are not found in the figures, indicating a high dispersion and small particle size of the V species in the catalysts. As shown in Figs. 2b and 2c, the N2 adsorption amounts of the samples V-N-C and N-C were both large, and they both had microporous/mesoporous composite structures with specific surface areas of 1 470 and 1 000 m2-g-1 and total pore volumes of 1.06 and 0.54 cm3-g-1, respectively, and the high specific surface areas and porous structures were helpful for the adsorption and diffusion of the reactants. The mass fraction of metal V in V-N-C was measured by ICP-AES as 0.19%. As shown in Fig. 2d, two strong peaks were observed in the Raman spectra of the samples, which were located in the D-band at 1,350 cm-1 and the G-band at 1,598 cm-1 with ID/IG of 0.99, indicating that the high-temperature carbonization treatment and acid rinsing not only formed a certain amount of graphitic carbon structure on the surface of the samples but also produced a large number of defects [27-29]. Figure 2e shows the high-resolution N1sXPS spectrum of V -N -C. The four photoelectron peaks were attributed to pyridine nitrogen (398.3 eV), pyrrole nitrogen (399.3 eV), graphite nitrogen (400.7 eV), and oxidized nitrogen (402.7 eV), indicating that the N atoms in the chitosan molecule were transformed into different nitrogen species distributed in the carbon skeleton after the high-temperature carbonization treatment. Figure 2f shows the high-resolution V2p XPS spectrum with a weak photoelectron peak, indicating the relatively low V content in the sample, and the results are consistent with XRD and ICP-AES. As shown in Fig. 3a, the V-N-C sample showed irregular blocky structure and also did not show obvious lattice stripes and V nanoparticles (Figs. 3b, 3c). Isolated bright spots below 0.5 nm are observed from Fig. 3d (marked with red circles) scattered in the porous carbon network, so we speculate that V species in V-N-C may be highly dispersed in the carbon skeleton as single atomic VNx [30]. in the carbon skeleton [30].

Fig.2 (a) XRD patterns, (b) N2 adsorption-desorption isotherms, (c) pore size distributions, (d) Raman spectra of V-N-C and N-C;(e) N1s and (f) V2p XPS spectra of V-N-C

Fig.3 (a) SEM image, (b) TEM image, (c) HRTEM image, (d) aberration-corrected HAADF-STEM image of V-N-C

2.2 Catalyst reaction performance

2.2.1 Catalytic effect of different catalysts

We have evaluated the catalytic performance of the V-N-C catalysts by selecting the benzylamine coupling reaction with O2 as the oxidant as the probe reaction. As seen in Table 1, almost no products were produced without catalyst or with only carrier N-C (Table 1, Entry 2, 5), indicating that V-based catalysts are indispensable in the reaction. Compared with the conventional catalyst V2O5 and the homogeneous catalyst VO(acac)2 (Entry 3, 4), the V-N-C catalyst yielded up to 99% conversion of benzylamine oxide and 99% selectivity of the target product imine (Entry 1). In addition, we also investigated the reaction effect of V-N-C in a solvent free system and found that the conversion of benzylamine was still up to 94% and the selectivity of the target product imine was 99% by appropriately extending the reaction time (Entry 6). The above results indicate that the single-atom dispersed V-N-C catalyst has very high catalytic activity in the benzylamine oxidation self-coupling reaction. Moreover, the V-N-C catalysts exhibited the highest TOF values compared with the reported catalysts (Table 2, Entry 1-5), which may be may be related to its highly dispersed VNx center, which further indicates its This may be related to its highly dispersed VNx center and further indicates its excellent catalytic activity.

2.2.2 Substrate generality

As shown in Table 3, the range of substrate applicability of the V-N-C catalysts was As shown in Table 3, the range of substrates for V-N-C catalysts was extended. The benzylamine derivatives containing different functional groups can to the corresponding imines with high conversion and selectivity (Table 3. Entry 1~9). For catalytic oxidative coupling reactions of heteroatomic amines also have high activity (Entry 10~12). In addition, it is also possible to promote inert aliphatic amines (Entry 13) as well as the oxidative coupling of secondary amines (Entry 14).

2.2.3 Cross-coupling reactions of benzylamine with different amines

We also evaluated the cross-coupling of benzylamine with different amines by V-N-C in the in the synthesis of asymmetric imine reactions. As shown in Table 4, the The conversion rates were almost 100% and the selectivity was high (Table 4. Entry 1~4). In addition, the activity of V-N-C in catalyzing the reaction of benzylamine with cyclic or straight-chain aliphatic amines with 81% and 83% activity, respectively, and the selectivity of the target products The selectivity of the target products was 84% and 73%, respectively (Entry 5, 6). Evaluation results further demonstrated the good generality of V-N-C in amine-amine oxidative coupling reactions. The results further demonstrate the good generality of V-N-C in amine-amine oxidation coupling reactions.

2.2.4 Thermal filtration experiments and reproducibility

As shown in Fig. 4a, a hot filtration experiment was used to determine the stability of the active component in V-N-C. When the reaction proceeded to 90 min with rapid removal of the V-N-C catalyst by hot filtration and further extension of the reaction time, it was found that the conversion of benzylamine no longer increased and was stable at about 49%, indicating that there was no problem of dissolution of any active V species during use and the oxidative coupling reaction carried out on the V-N-C catalyst was fully multiphase catalytic. In addition, the reusability of V-N-C was examined as shown in Figure 4b, and the catalyst was recycled nine times without any degradation in catalytic performance, further indicating that V-N-C has excellent reusability. To further verify the stability of the recycled catalysts, XRD and HAADF-STEM analyses were performed on the used catalysts (Figure 5). It was found that no significant changes occurred in the used V-N-C compared to the fresh catalyst, both in material structure and vanadium species size, confirming the good stability of the V-N-C catalyst under the catalytic conditions investigated.

2.2.5 Preliminary investigation of the reaction mechanism

Combining the experimental results and literature reports [35-37], we speculated the possible reaction mechanism for the synthesis of imines from benzylamine over the V-N-C catalyst, as shown in Figure 6. In the early stage of the reaction, benzylamine is converted to benzylimine 1b by activation at the VNx site [30], and oxygen molecules are activated at the defective site to form the intermediate H2O2 [35]. At this point, benzylimine 1b can be obtained as imine 1c through 2 pathways. path A: 1b condenses with another molecule 1a to remove NH3 to produce the target product 1c; path B: 1b interacts with water to produce benzaldehyde, which condenses with another molecule 1a to remove H2O to produce the target product 1c. we used a colorimetric method [4] to verify the intermediate H2O2 produced in the reaction, and since H2O2 can convert peroxidase (POD), subsequently N,N-diethyl-p-phenylenediamine sulfate (DPD) was oxidized by POD to a cation (DPD+), and DPD+ showed 2 absorption peaks at 510 and 551 nm in the ultraviolet-visible (UV-Vis) absorption spectrum. The results are shown in Figure 7.

The intensity of both absorption peaks increased significantly with the increase of reaction time, which fully indicated that H2O2 was involved in the oxidative coupling process of benzylamine.

Table 1 Catalytic properties of various catalysts for oxidative self⁃coupling of benzylamine

a Reaction conditions: catalyst (30 mg), benzylamine (0.5 mmol), toluene (3 mL), O2 pressure (1.01×105 Pa), 110 ℃ , 12 h; b Catalyst (60 mg), benzylamine (15 mmol), O2 balloon (1.01×105 Pa), 110 ℃ , 16 h.

Table 2Comparison of the oxidative self⁃coupling reaction activity of benzylamine over different catalysts

Table 3 Self-coupling reaction of various amines to imines catalyzed by V-N-C^a

a Reaction conditions: amine (0.5 mmol), V-N-C (30 mg), toluene (3 mL), O2 pressure (1.01×105 Pa),110 ℃, 12 h.

Table 4 Cross-coupling reaction of 1a with various amines catalyzed by V-N-C^a

a Reaction conditions: 1a (0.125 mmol), amine 2 (0.375 mmol), V -N – C (30 mg), toluene (3 mL), O² pressure (1.01×10⁵ Pa), 110 ℃ , 12 h; b Conversion and selectivity were based on 1a and confirmed by GC-MS.

Fig.4 (a) Thermal filtration test and (b) reusability for the benzylamine oxidative self-coupling of V-N-C catalyst

Fig.5 (a) XRD pattern and (b) aberration-corrected HAADF-STEM image of spent V-N-C catalyst

To verify that benzylimine is an intermediate of the reaction, we replaced benzylimine with N-benzylidene methylamine, which was added to V-N-C and benzylamine. The reaction was carried out as shown in Figure 8, and within only 1 h, 74% of N-benzylidene benzylidenemethylamine was converted to the final product, further indicating that benzylimine was the the most likely reaction intermediate. In addition, since the V-N-C catalyzed benzyl oxidation of benzylamine, no aldehydes were detected in the synthesis of imine, which basically ruled out The possibility of pathway B was largely ruled out.

Fig.6 Possible reaction mechanism of benzylamine to imine catalyzed by V-N-C catalyst

Fig.7 UV-Vis absorption spectra of the DPD/POD reagent after reaction with H2O2 over V-N-C

Fig.8 Oxidative coupling reaction of benzylamine and N-benzylidenemethyl amine over V-N-C catalyst

3 Conclusion

Chitosan was used as the precursor, vanadium acetylacetonate as the metal source, and zinc chloride as the pore-forming agent. ZnCl2 as the pore-forming agent, a simple heat treatment combined with pickling was used to A high specific surface area vanadium-nitrogen co-doped porous carbon (V-N-C) catalyst was prepared. catalyst with high specific surface area and vanadium-nitrogen co-doped porous carbon (V-N-C). and cross-coupling into asymmetric imines. This may be attributed to the fact that the V-N-C catalysts have been used in the synthesis of symmetric imines by oxidative coupling of benzylamine and asymmetric imines by cross coupling. This may be due to the fact that the V species in V-N-C are nearly monoatomic and highly dispersed on the carbon carrier. The VNx active center is fully exposed, which makes the catalytic performance significantly better than that of the multiphase reaction. The catalytic performance is significantly better than that of the multiphase V2O5 and homogeneous VO(acac)2 catalysts. In addition, the catalytic performance of V-N-C did not decrease after nine cycles of application. without any degradation, indicating that it has excellent stability and has potential The catalytic performance of V-N-C did not decrease after 9 cycles, indicating its excellent stability and potential application.