Facile and clean transformation for synthesizing secondary arylamines through one-pot reductive amination of aniline, using aldehyde catalyzed by the supported nickel and poly(vinyl sulfonic acid) on mesoporous carbon CMK-3 (Ni/PVSA/CMK-3) as a novel acid-metal bi-functional heterogeneous catalyst. Sodium borohydride was used as the source of hydrogen for the reduction of imine. The reaction was performed at room temperature, in a short reaction time, without any by-products. Various characterization techniques including FT-IR, XRD, TG, BET, SEM, TEM, DRS-UV and AAS were employed to reveal the relationship between catalyst nature and catalytic performance. Reaction results demonstrate that the optimized Ni/PVSA/CMK-3 catalyst shows comparable catalytic performance thanks to the nickel metals and the acidic nature of polymer in mesopore channels of CMK-3. This method has several advantages such as, eco-friendly (used water as solvent), moderate to high yields, simple work-up procedure and catalyst filtered easily and reused without obvious loss of activity.

1. Introduction

Nowadays, amines are privileged in industry that have found prevalent applications as intermediates for pharmaceuticals, biologically active compounds, rubber, solvents, fine chemicals, dyes, herbicides, and in the manufacture of detergents and plastics. Reductive amination demonstrate one of the most versatile and convenient methods of amine synthesis. This reaction has two steps including formation of an imine during reaction between primary amine and a carbonyl substrate, and reduction of the imine with adequate hydride source. There are two detached approaches for the reductive amination: the direct approach, which uses the in situ-generated imine, and the indirect approach, which uses the prior isolated imine. The former approach has several advantages such as one-pot procedure, increasing yields, having simple setup, easily separated from the product, being stable and compatible reagents, and the mild reaction conditions.

To this end, over the past decades, researchers have been reported several studies on reductive amination reaction with several different catalysts, which among them, heterogeneous catalysis are prominent than homogenous catalysis owing to separate and recover capabilities. Moreover, it has been proven that accomplish this reaction needs two character including metallic and acidic; consequently, bi-functional heterogeneous catalysts are useful in this reaction.

Recently, several metal nanoparticles acted as a hydride transfer such as Pt, Ni, Cu, and Pd. Despite of the fact that an effective control of particle size and a uniform distribution of nanoparticles in catalytic applications are generally predicted, nanoparticles typically accumulate together in bulk-like materials that hardly reduce selectivity and the activity of catalysts. To overcome with this problem, mesoporous silica, zeolites, polymers or macromolecular organic ligands have been used in order to immobilize metal nanoparticles in their pores. Mesoporous silica materials and zeolites have excellent order and surface area than other materials like polymer and etc.; subsequently, they are sufficient for catalysis approaches.

Lately, mesoporous silica used for producing mesoporous carbon (CMK-n) as hard template. These materials contain several benefits compared to mesoporous silica and zeolites for instance, high mechanical stability, high thermal stability in nitrogen atmosphere, superb stability in strong acids and bases, and other engrossing properties such as narrow pore size distributions, high surface areas, and ordered frameworks. In addition, mesoporous carbon materials have hydrophobic nature on their surfaces and it helps to embed nanopolymers in their pores.

In our previous studies, it asserts that when polymer embedded into mesoporous materials, they have perfect function. Owing to the fact that they have small particles and subsequently having high surface areas. Moreover, polymer nanoparticles fix in porous and they could not leach from their supporters. Accordingly, in this work, we will introduce a novel heterogeneous organic hybrid catalyst based on a carbon mesoporous material. In this circumstance, mesoporous carbon CMK-3 replicating from mesoporous silica SBA-15 was prepared and used as suitable support for Nickel nanoparticle/poly vinyl sulfonic acid/CMK-3 (Ni/PVSA/CMK-3). Furthermore, the catalyst was used effectively for the one-pot reductive amination of amine compounds using aldehyde in the presence of a small amount of NaBH₄ as a mild reducing agent and two sort of solvent containing water and acetonitrile at room temperature without any by-products.

2. Experimental method

2.1. Catalyst characterization

The samples have been analyzed by FT-IR spectroscopy (using a PerkinElmer 65 in KBr matrix in the range of 4000-400 cm^-1). The thermal gravimetric analysis (TGA) data were obtained by a Setaram Labsys TG (STA) in a temperature range of 30-650 ^-C and heating rate 10 ^-C min^-1 in nitrogen atmosphere. The X-ray powder diffraction (XRD) of the catalyst was carried out on a Bruker D8Advance X-ray diffractometer using nickel filtered Cu Kα radiation at 40 kV and 20 mA. The BET specific surface areas and BJH pore size distribution of the samples were determined by adsorption-desorption of nitrogen at liquid nitrogen temperature, using a Series BEL SORP 18. For the measurement of nickel, a Perkin Elmer AAnalyst 300 atomic absorption spectrophotometer was used. The slit width, linear range and wave length for Ni were 0.2 nm, 2 and ppm232 nm, respectively. Scanning electron microscope (SEM) studies were performed on Philips, XL30, SE detector. Transmission electron microscope (TEM) observations were performed on a JEOL JEM.2011 electron microscope at an accelerating voltage of 200.00 kV using EX24093JGT detector in order to obtain information on the size of nickel nanoparticles and the DRS UV-vis spectra were recorded with JASCO spectrometer, V-670 from 190 to 2700 nm. Moreover, X-ray photoelectron spectra (XPS) was recorded on ESCA SSX-100 (Shimadzu) using a non-monochromatized Mg Kα X-ray as the excitation source. The products were characterized by ¹H NMR and ¹³C NMR spectra (Bruker DRX-500 Avance spectrometer at 500.13 and 125.47 MHz, respectively). Melting points were measured on an Electrothermal 9100 apparatus and they were uncorrected. All the products were known compounds and they were characterized by FT-IR, ¹H NMR and ¹³C NMR. All melting points are compared satisfactorily with those reported in the literature.

2.2. Catalyst preparation

The employed mesoporous carbon (CMK-3) was synthesized following the method reported by Ryoo using SBA-15 as template.

2.2.1. Preparation of SBA-15

Mesoporous silica SBA-15 was prepared using block copolymer Pluronic P123 (EO₂₀PO₇₀EO₂₀) template as a structure directing agent and tetraethylorthosilicate (TEOS) as the silica precursor through the addition of H₃PO₄ by novel method as described in the literature. In a general synthesis, Pluronic P123 (2 g) was dissolved at room temperature in deionized water (75.4 mL) and H₃PO₄ (4.2 mL, 85%), after that TEOS (4.6 mL) was added to the solution and synthesis was fulfilled by stirring at 35 ^-C for 24 h in sealed Teflon breakers, and it was consequently placed at 100 ^-C for 24 h. Afterwards, the solution was filtered, washed with deionized water, and lastly dried at 95 ^-C for 12 h in air. Template removal was accomplished by calcination in air using two successive steps; first heating at 250 ^-C for 3 h and then at 550 ^-C for 4 h.

2.2.2. Preparation of CMK-3

Mesoporous carbon CMK-3 was prepared using mesoporous silica SBA-15 as template and sucrose as the carbon precursor. 1.0 g SBA-15 was added to 5 mL aqueous solution containing 1.25 g (3.65 mmol) sucrose and 0.14 g (1.42 mmol) of H₂SO₄ (98%). The resulting mixture was heated in an oven at 100 ^-C for 6 h and next 160 ^-C for another 6 h. In order to obtain entirely polymerized sucrose inside the pores of the SBA-15 template, 5 mL aqueous solution containing 0.8 g (2.33 mmol) sucrose and 0.09 g (0.917 mmol) of H₂SO₄ were added again, and the mixture was subjected to the thermal treatment described above one more time. Then, it was carbonized under nitrogen gas flow at 900 ^-C for 6 h with a heating rate of 5 ^-C min^-1. Finally, the resulting solid was washed with 1 M NaOH solution (50 vol. % ethanol-50 vol. % H₂O) twice to remove the silica template, filtered, washed with ethanol until pH = 7, and dried at 100 ^-C for 4 h.

2.2.3. Preparation of Poly(vinyl sulfonic acid)/CMK-3

2.2.4. Preparation of Ni nanoparticle-poly(vinyl sulfonic acid)/CMK-3

At first, Vinylsulfonic acid sodium was converted into its acidic form using the ion exchange resin (Amberjet 1200 H, 2 equiv. L^-1, Aldrich). Ni/PVSA/CMK-3 was synthesized as follows: in the first place, 1 mL aqueous solution of NiCl₂.6H₂O (0.5 M) was added to the obtained PVSA/CMK-3 (0.1 g) together with 3 mL of H₂O. The mixture was heated for 5 h at 353 K. Next, the solution of NaBH₄ [0.057 g (1.5 mmol)] dissolved in 5 mL methanol was added to the mixture drop by drop in 20-30 min. Then, the solution was stirred for 3 h. After that, adding the same amount of NaBH₄ was repeated and again the mixture was stirred for 3 h. Consequently, the solution was filtered and washed sequentially with deionized water and methanol to remove excess NaBH₄ and NiCl₂, and was dried in room temperature to yield Ni/PVSA/CMK-3. The Ni content of the catalyst was estimated by decomposing. Known amount of the catalyst by perchloric acid, nitric acid, fluoric acid, hydrochloric acid, and the Ni content was estimated by atomic absorption spectrometer. The Ni content of Ni/PVSA/CMK-3 estimated by atomic absorption spectrometer was 2.1 mmol g⁻¹.

2.3. General procedure for one-pot reductive amination of aldehydes.

A mixture of Aniline (2 mmol) and benzaldehyde (2 mmol) in water or acetonitrile (3 mL) was placed in a round bottom flask and stirred for 1 min at room temperature. Afterward, to the resulting mixture, Ni/PVSA/CMK-3 (0.04 g) and NaBH₄ (6 mmol) were added and the mixture was stirred at room temperature until TLC showed the complete disappearance of the benzaldehyde. Then, the reaction mixture was quenched with water (10 mL) and the product was extracted with diethylether (2 – 10 mL). After they finished, the organic phase was dried over anhydrous Na₂SO₄, filtered and concentrated. In the end, the products were obtained very pure just by extract with diethylether in the majority of the reactions. The product was identified with a melting point, FT-IR spectroscopy techniques, ¹HNMR and ¹³CNMR.

3. Results and discussion

3.1. Catalyst characterization

Figure 1 shows the FTIR spectra of CMK-3 (a), PVSA/CMK-3 (b) and Ni/PVSA/CMK-3 (c). A broad band at around 3380-3470 cm⁻¹ was observed in all samples. The O-H stretching vibration of the adsorbed water molecules mainly caused it. Moreover, in the CMK-3 spectrum, there are not any signals belong to organic bonds, resulting from the complete carbonization of sucrose (Fig. 1a). The presence of a new absorption bands at 1041 and 1186 cm^-1 attributed to the S=O group of PVS, affirming the existence of the grafted PVSA chains on the CMk-3. In addition, the band at about 1650 cm^-1 is attributed to adsorbed water, which is similar to related reports[]. The presence of peaks at around 2940 cm⁻¹ and 1450 cm⁻¹ correspond to the aliphatic C-H stretching and bending in PVSA/CMK-3, respectively (Fig. 2b). The appearance of the above bands shows that PVSA has been attached into mesoporous of CMK-3 and the synthesis of PVSA/CMK-3 has been successful.

The profiles of thermogravimetric analysis of PVSA/CMK-3 and Ni/PVSA/CMK-3 under nitrogen atmosphere are shown in Fig. 2. The degradation of Poly(vinyl-sulfonic acid) commences at 150C and this stage continues to a little less than 300C. The next stage involves only a little degradation and occurs over the temperature range of 300 to 500C. These evidence are shown Poly(vinyl-sulfonic acid) cannot tolerant the temperature due to polymers are not protect by any supporter. The TGA curves of PVSA/CMK-3 shows a small mass loss (around 5%, w/w) in the temperature range of 100-330 -C, which is apparently associated with degradation of SO₂ and ethylene from PVSA (Fig. 2). At temperatures above 330 -C, PVSA shows one main stage of degradation. The mass loss for PVSA in the second step is equal to 11.5% (w/w) which correspond to the degradation of the methane. In light of the difference between the PVSA and PVSA/CMK-3 curves, it is clear that PVSA/CMK-3 has higher thermal stability and slower degradation rate than PVSAP. Hence, after hybridization, the thermal stability is enhanced significantly that is beneficial for the catalyst application. In addition, Ni/PVSA/CMK-3 shows two separate weight loss steps that are almost similar to the PVSA/CMK-3. The only difference is temperature between 330 and 445 C, which Ni/PVSA/CMK-3 shows slower degradation rate than PVSA/CMK-3 in these range. It asserts that the hybrid Ni/PVSA/CMK-3 had higher thermal stability than PVSA/CMK-3. It may be related to the presence of Nickel nanoparticles in the composite structure. Consequently, it is proper thermal stability is boosted after hybridization because of intense the catalyst application.

Figure 1 shows the powder XRD patterns of SBA-15, CMK-3, PVSA/CMK-3 and Ni/PVSA/CMK-3. The low angle diffraction pattern of SBA-15 shows three reflections at 2Ï´ values from 0.5 to 2^° including one strong peak at (100) and two weak peaks at (110) and (200), which corresponds to the well-known ordered arrangement of SBA-15 in the space group p6mm of 2-D hexagonal symmetry. The silica SBA-15 used as template to synthesis CMK-3. As can be seen, the XRD pattern of CMK-3 show three diffraction peaks at 2Ï´ = 1.04^°, 1.79^° and 2.05^° (Fig. 3b). It could be marked to (100), (110) and (200) diffractions of the 2D hexagonal space group p6mm, which is compatible with previous articles.

After polymerization by poly (vinyl sulfonic acid), the X-ray diffraction of PVSA/CMK-3 shows the same pattern with CMK-3. This evidence indicates that the structure of the CMK-3 was retained after the polymerization (Fig. 3c). Albeit, the intensity of the characteristic reflection peaks of the PVSA/CMK-3 is found to be diminished (Fig. 1b). Composite contains less CMK-3 due to the dilution of the carbon material by PVSA; subsequently, this dilution can be responsible for a decrease in the peak intensity. By the way, the XRD patterns of CMK-3 and PVSA/CMK-3 are almost similar to SBA-15, which it shows CMK-3 is a accurate replica of the mesoporous silica SBA-15 and the polymerization process does not damage the structure of CMK-3. After immobilize nickel in the PVSA/CMK-3, Ni peak cannot be seen in XRD since the homogeneity of Ni particles in the Ni/PVSA/CMK-3, and it lonely shows an amorphous pattern at 2θ values of about 44Ëš (Fig. 3, inside). In order to demonstrate the existence of Ni nanoparticles in the Ni/PVSA/CMK-3 catalyst was exposed to temperature (400ËšC). Meanwhile, amorphous Ni changed to crystalline and appear a peak with low intensity at 2θ = 44.29Ëš, which can be attributed to the small size of nickel nanoparticles and the plane (111) of fcc nickel. Eventually, after immobilize the nickel nanoparticles on composite, structure has not changed and it is represented a successful synthesis of the catalyst.

The specific surface area, pore volume and the pore size of the CMK-3, PVSA/CMK-3 and Ni/PVSA/CMK-3 samples are summarized in Table 1. All samples exhibit a type IV adsorption isotherm with an H1 hysteresis loop by capillary condensation at relative pressure around 0.3-0.7 (Fig. 4). It is clear in table 1 that the PVSA/CMK-3 and Ni/PVSA/CMK-3 exhibits a smaller specific surface area, and pore volume in comparison to those of pure CMK-3. Thanks to the successful incorporation of the poly(vinyl sulfonic acid) into the mesoporous carbon. As can be seen, pore diameter increases in the PVSA/CMK-3 and Ni/PVSA/CMK-3 in comparison to CMK-3. This evidence shows the incorporation and growth of hyperbranched polymers and consequently produces the pressure (physical pressure on the wall of the channels) inside the CMK-3 mesoporous. By adding Ni nanoparticles into the PVSA/CMK-3, the specific surface area and pore volume decrease, asserting that nickel nanoparticles are located inside the pores of the CMK-3. In spite of the fact that there are significant decreases in the pore volume and surface area, the pores of Ni/PVSA/CMK-3 were not blocked by deposition of the hyperbranched homopolymer and nickel nanoparticles. Moreover, the BJH pore size distribution curves of the PVSA/CMK-3 and Ni/PVSA/CMK-3 are exhibited a narrow pore size distribution (Fig. 5). It clarifies that the homopolymer and nickel nanoparticles are satisfactory distributed on the channels of the Ni/PVSA/CMK-3. This result is agreement with TEM analysis and shows the effective role of the hyperbranched polymer to entrap and uniformly disperse nickel nanoparticles.

<Figure 4>, <Table 5>, <Figure 5>

Fig. 6 gave the scanning electron microscopy (SEM) photographs of CMK-3 and PVSA/CMK-3 and Ni/PVSA/CMK-3. All the SEM images are shown rod-like morphology, which is attributed to carbon mesoporous. Although, virtually no significant differences observe in surface morphology between CMK-3 and PVSA/CMK-3, it is obvious that after hybridization the surface of CMK-3 is become coarser; indicating the most of polymerization of PVSA occurred in the pores of CMK-3, which was also supported by the decrease in surface area and pore volume as shown in Table 1. In addition, by immobilizing Ni nanoparticles, several spherical beads are seen on the mesoporous carbon. However, most of them are incorporated inside the carbon mesoporous structure, which is not observable in the SEM images. It is necessary to mention that after loading nickel nanoparticles on the surface of CMK-3, the structure of the mesoporous carbon is remained. Moreover, XRD analysis and TEM images confirmed this claim.

The PVSA/CMK-3 and Ni/PVSA/CMK-3 were inspected by means of TEM micrographs technique (Fig. 7). The ordered hexagonal p6mm mesostructure of PVSA/CMK-3 and Ni/PVSA/CMK- 3 can be seen, indicating after polymerization and incorporation of PVSA and Ni nanoparticles, the ordered structure of mesoporous carbon is retained. Additionally, the places with darker contrast could be assigned to the presence of Pd particles with different distribution (Fig. 7c-d). As can be seen, the small dark spots could be ascribed to nickel nanoparticles with ∼X nm average diameter, presumably located into the mesoporous channels. On the other hand, larger dark spots are shown in fig. 7 c-d, which are corresponded to Ni nanoparticles agglomerate on the external surface with average diameter of ∼5-10 nm.

Fig. 8 shows the DRS-UV of PVSA/CMK-3 and Ni/PVSA/CMK-3. previous reports were proven that DRS-UV of the cationic nickel have only d-d transitions peaks including ³T_1g(P)←³A_2g (F) (368 nm) and ³T_1g (F)←³A_2g (F) (576 nm), which these two peak do not show in Ni/PVSA/CMK-3. Moreover, the DRS-UV of Ni/PVSA/CMK-3 shows feature bands around 205 nm and 330 nm, which are attributed to the presence of Ni nanoparticles in these samples. By comparing these data, it can be found that cationic nickels are converted to the nickel nanoparticles by reduction of NaBH₄.

3.2. Catalytic activity

Synthesized nanocomposite was characterized by different methods in the former section. This section is introduced the application of this bi-functional catalyst to the reductive amination reaction. During two decade, enormous investigation devoted to develop environmental friendly synthesis. Since, using water as a reaction medium in transition metal-catalyzed processes is one of the most essential goal of sustainable chemistry. Water is nontoxic solvent, readily available, an inexpensive, nontoxic solvent and non-inflammable. It provides privilege over organic solvents from an environmental and an economic aspect. Accordingly, the effect of several parameters on the one-pot tandem reductive amination of aldehydes with aniline over Ni/PVSA/CMK-3 as acid-metal bifunctional catalyst was perused in water at room temperature and the outcome are as follows:

At the first monitoring of experiments, diverse amounts of NiCl2.6H2O were tested to identify the effect of nickel nanoparticles concentration on the reductive amination reaction. Hence, the amount of NiCl2.6H2O to prepare Ni/PVSA/CMK-3 was changed from 1 mmol/g to 15 mmol/g and then measured by the Atomic Absorption spectroscopy technique (AAS) which are shown in Table 2. It is clear that the activity of catalytic steadily improved by increasing NiCl2.6H2O form 1 mmol/g to 5 mmol/g. According to the catalytic reaction mechanism, nickel nanoparticle mediated electron transfer from BH4- ion to the imine intermediates (Scheme 1). Subsequently, the amounts of H- sites on the catalyst surface are grown by increasing nickel nanoparticles. Thus, larger amount of hydrides can be transferred to the imine groups through the catalyst. On the other hand, by further increasing the amount of NiCl₂.6H₂O (more than 5 mmol/g), the catalytic activity was diminished, which can be attributed to after a certain amount of nickel chloride increases, a larger amount of nanoparticles is loaded on the surface of the CMK-3 that may have caused the mesopore channels to narrow. In Fact, the nanoparticle size will increase by increasing the amount of NiCl₂.6H₂O. Therefore, in some places, the pore size will narrow and it is able to lessen the rate of reactants diffusion into the porous. In one word, lower performance of the catalyst produced with higher NiCl₂.6H₂O concentration will be anticipated. Despite of this fact, it does not mean the pores are throughout clogged. According to these results, the catalyst provided by 5 mmol/g NiCl₂.6H₂O presented the best catalytic activity.

To identify the effect of NaBH₄ amount (as a hydride donor) on the reductive amination the reaction was carried out using various amounts of NaBH₄ in the presence of Ni/PVSA/CMK-3 as catalyst. As shown in Table 3, the yield was increased by increasing the amount of NaBH₄ (until 6 mmol). The excess values did not have any effect on the reaction. Therefore, 6 mmol NaBH₄ was the best value to perform reductive amination reaction.

The influence of the solvent on catalytic activity was investigated in the reductive amination reaction using Ni-PVSA/CMK-3 catalyst and NaBH₄ as hydride donor, at room temperature. The results are gathered in Table 4. Four vital factor acts to fulfil reductive amination reaction including dielectric constant, dipole moment, solubility in NaBH₄, hydrophobic effect, protic and aprotic solvent effect.

The results revealed that the reaction time in ethanol solvent is slow due to NaBH₄ hardly solving in ethanol and the reaction rate is tardy. In addition, the reaction rate in water solvent is slow because although dielectric constant and solubility of water in NaBH₄ is high, carbon mesoporous CMK-3 have hydrophobic nature. It causes substances and catalyst cannot have perfect interaction together. The hydrophobic nature of acetonitrile and oxolane are higher than other solvent that presented above; thus, these two solvent have more similarity to hydrophobic nature of CMK-3. Moreover, dipole moment of acetonitrile is higher than other solvent. Thanks to this feature, the reaction rate increase. Whereas the methanol solvent has mediate circumstance of dielectric constant, solubility in NaBH₄, and hydrophobic effect aspect, the reaction time diminish. It is noteworthy to mention that the combination of all these factors together cause this process. Regarding these situation, water and acetonitrile were finally selected as the solvent for the reaction because of their environmental friendly and highly efficient, respectively; and all other optimization and reaction separately accomplished by these two solvent.

The effect of the amount of catalyst was defined for reductive amination reaction (Table 5). Due to the fact, the catalyst synthesized is worthy, it is decided that the amount of catalyst optimize by decreasing down to the 0.04 g, nevertheless the reaction time were increased. However, reducing the amount of catalysts until 0.02 g was not sufficient. Since, the quantity of 0.04 g for both solvent was found to be the best weight of catalyst.

The reusability of the catalyst was studied by using Ni/PVSA/CMK-3 in water and acetonitrile solvent (Chart 1). After each cycle, the catalyst was filtered off, washed with water (10 mL) and ethanol (3 mL – 5 mL). After that, catalyst dried at 60 ËšC and reused in the reductive amination reaction with a fresh reaction substances. It might be noted that after each run, a slight amount of the catalysts were lost in the filtration process. Herein, to overcome this problem, after each experiment the amount of remaining catalyst was specified and the molar ratio of the reactants was adjusted according to the remaining amount of the catalyst. The catalyst was reused up to 5 times. The catalyst that react in acetonitrile solvent have serious loss activity. In further investigation, it recognized that the catalyst used in acetonitrile solvent was somewhat destroyed. It can be attributed to the interaction between acetonitrile as a solvent and PVSA/CMK-3 composite. In other cases, not only the reusability of the catalyst that performed in water was adequate, but also the catalyst exhibit high stability in this status. This result obtained by SEM and XRD characterization, which can be seen in Figure 9 and 10. As shown in SEM images of reused catalyst in water as reaction solvent is well retained, which is very essential for the catalyst applications. Similarly, the XRD pattern shows a diffraction peak at low angle (1.04°). It display that the catalyst structure remain. Because of this fact that the reusability in the heterogeneous catalysts is fundamental, water in reductive amination reaction chosen as a compatible solvent.

The catalytic activity of the Ni/PVSA/CMK-3 in the reductive amination was compared with CMK-3, PVSA/CMK-3, and without a catalyst. The results are available in Table 6. The consequences affirm the significance role of the acid-metal heterogeneous catalyst in sort of reaction. As shown, the reaction dose not fulfil up to 5% without catalyst. There is the important issue that NaBH₄ function as a mild hydride donor agent, which is incapable reagent for reducing imine groups solely. In a similar manner, this result obtained by using CMK-3 due to the fact that mesoporous carbon CMK-3 does not have any active sites to carry out the reaction. By using the PVSA/CMK-3, with improve acidic feature of the mesoporous carbon the carbonyl group activated and thus the yield moderately increased to 40% and 35% in acetonitrile and water solvent, respectively. In addition, using Ni/PVSA/CMK-3, the reaction efficiency was increased to 97% in 35 and 63 min in acetonitrile and water solvent, respectively; Because of the role of nickel nanoparticles as species to transfer hydride ions from NaBH₄ to imine groups.

The interesting point in catalyst investigation is heterogeneous nature. In this regard, the catalyst was separated from the reaction mixture at approximately 50% conversion of the starting substances by filtration and then centrifugation. The reaction progress in the filtrate circumstance was monitored (data not shown). No further reductive amination reaction occurred even at addition times, representing that the nature of reaction process is heterogeneous and there is not any progress for the reaction in homogeneous phase.