Introduction

Metal nano oxides have attracted a lot of attention in recent years due to their unique properties, which include a large surface area. Ferrites are those magnetic materials whose main component is "iron oxide" and for this reason they have favorable magnetic properties1,2. Ferrites are ceramics that are considered as magnetic materials. Ferrites, like ferromagnetic materials, have structures in the magnetic domain and the residual ring. The crystal structure of spinel is one of the ternary structures of materials that have the general formula AB2O4, where A and B can be cations of different metals. In the spinel crystal lattice structure, divalent ions A (such as Co2+) are located in tetrahedral cavities and trivalent ions B (such as Fe3+) are located in octahedral cavities3. Cobalt ferrite has received much attention in recent years as a catalyst substrate4,5. By reducing the diameter of these magnetic particles to nano dimensions, the available external surface area for surface modification increases6. After surface modification, these particles are easily spread inside the solvent and create a stable diffusion. Silica is a very suitable coating for the modification of magnetic nanoparticles. Due to its richness in hydroxy functional groups for subsequent modifications, as well as its high thermal and mechanical stability7.

Mesoporous silicates due to having appropriate characteristics such as chemical and thermal stability, having the appropriate shape and degree of porosity, as well as having a surface that can specifically react with organic and inorganic groups, in the fields of protection of catalysis, removal of heavy metals from water and many other fields such as surface adsorption and chemical separation8,9. MCM-41 is an inorganic substance. The pore size of MCM-41 changes from 20 to 100 Å during synthesis and each gram of this material has an area of about 1000–1300 m210. Due to the easier synthesis of MCM-41 compared to other substances, this substance has been used the most11. For easy separation and better use of MCM-41, it is better to cover it with magnetic compound. For effective applications of silicate mesoporous structures in many basic processes, it is necessary to modify the surface of these solids by functional groups. Silica mesoporous have many silanol groups on their surface. These surface groups can be converted into other groups by chemical reactions. By attaching different functional groups and organic structures on the surface, mesoporous surface properties such as hydrophilicity and hydrophobicity and the ability to bind to guest molecules can be controlled12. Modifying mesoporous materials gives various properties to these materials so that these materials find various applications in fields such as catalysis of chemical reactions, separation processes and making chemical sensors. The presence of free silanol groups on the surface of mesoporous silica materials shows that by functionalizing these porous silica materials, active and suitable sites can be created to perform reactions on these materials13. It is possible to prevent the separation of catalytically active species from the support surface by creating covalency. Incorporation of metal centers in the mesoporous silica materials is necessary to make the materials as potential catalysts.

Tetrazoles are acyclic compounds that consist of a five-membered ring containing four nitrogens and one carbon14. Due to having a low energy occupied molecular orbital (HOMO), tetrazoles resist the strongest oxidizing agents15. These compounds are acidic and their pKa is similar to their corresponding carboxylic acids16. These compounds have various applications in the synthesis of other organic substances, pharmaceutical and biological industries as well as military industries17. Due to having high potential energy, low sensitivity to impact and friction, and having high explosion heat, tetrazoles have a good potential to be used in gas and explosives producers. These compounds are suitable substitutes for explosives due to their lower toxicity, less soot production and greater strength. Among the medicinal compounds containing tetrazole ring, we can mention antibacterial, antimicrobial, antiviral, antidiabetic, anti-schizophrenic, antihypertensive and cyclooxygenase-2 inhibitor18,19,20,21. Also, tetrazoles are used as propellants in rocket and jet engines in the aerospace industry22,23. Due to having several ring coordination centers and high ring electron density, tetrazoles are used in the synthesis of complex compounds and also as protective insulators for metals24. The synthesis of tetrazole ring is a vital step in organic and medicinal chemistry, and various methods have been provided for the synthesis of these compounds25,26,27. The results of the conversion of nitril to tetrazole over various heterogeneous and homogeneous catalysts are summarized in Table 1. The non-reusability of the homogeneous catalyst, the low tetrazole yields, and toxic reaction conditions hindered their use for the synthesis of tetrazole. Despite the verity of studies on the use of catalysts for the direct synthesis of tetrazole, there is no paper has focused on the use of CoFe2O4/MCM-41-based heterogenous catalysts for the synthesis of tetrazole. Therefore, it is highly desirable to progress an efficient catalyst for the selective synthesis of tetrazole under mild reaction conditions in a benign environmental solvent.

Table 1 Catalytic performance of different heterogeneous and homogeneous catalysts for conversion of nitrile to tetrazole via sodium azide.
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In this paper, we demonstrated that MCM-41/PA/Cu alloys deposited on CoFe2O4, with post-functionalization modification method and could be used for the synthesis of tetrazole in an aqueous medium, which was more suitable than other reported catalysts. A Comparison of methods in synthesis of heterogeneous catalysts are summarized in Table 2. The reason for the design of CoFe2O4/MCM-41/PA/Cu catalyst is as follows: (1) The high Lewis acidity of the catalyst can facilitate the adsorption of substrate. (2) Using magnetic properties for easier separation. (3) MCM-41 protects the magnetic cores from environmental factors, and therefore, recoverable magnetic nano catalysts can be used in relatively harsh reaction conditions (strong acidic conditions, hard oxide etc.). (4) Catalyst aggregation and clumping are effectively prevented by the outer layer of porous silica. (5) Porous silica creates a passageway for substrates to enter and products to exit. (6) Using pores of MCM-41 to attach more active center. (7) The synthesis method was very convenient and safe. To elucidate the significant activity of CoFe2O4/MCM-41/PA/Cu catalyst, CoFe2O4/PA/Cu catalyst was prepared, and catalytic performance was compared with that of the CoFe2O4/MCM-41/PA/Cu catalyst.

Table 2 A Comparison of methods in synthesis of heterogeneous catalysts.
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Material and methods

As shown in Fig. 1, the CoFe2O4/MCM-41/PA/Cu catalyst (CF/M/PA/C denotes nanoalloy with CF (CoFe2O4), M (MCM-41), N-phenyl anthranilic acid (PA), and Cu (C)) was successfully fabricated. Catalyst preparation, and product analysis are given in the Supporting Information.

Figure 1
figure 1

Schematic illustration of the synthesis of the CF/M/PA/C denotes nanoalloy with CF (CoFe2O4), M (MCM-41), N-phenyl anthranilic acid (PA), and Cu (C).

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Results and discussion

Catalyst characterization

XRD analysis

Figure 2 presents the XRD patterns of the MCM-41, CoFe2O4, CoFe2O4/MCM-41 and CoFe2O4/MCM-41/PA/Cu catalyst. The XRD profiles of all samples show the characteristic diffraction peaks at round 18.1, 31.2, 36.5, 44.0, 54.2, 57.9, and 63.1° assigned to the (111), (220), (311), (400), (422), (511), and (440) planes of the spinel ferrite type CoFe2O4 (JCPDS No. 22-1086), and no impurity peaks were present. A broad peak at around 20–30° was representative for amorphous MCM-41 in the core shell structure. In the diffraction curve of CoFe2O4/MCM-41 and CoFe2O4/MCM-41/PA/Cu, the characteristic diffraction peaks were the same as CoFe2O4, which is a confirmation of the preservation of the phase of this nanoparticle during the functionalization stages. It can be concluded that silica is amorphous and the crystalline structure of CoFe2O4 magnetic nanoparticles is maintained after silica coating. Crystallite size was determined by the Debye Scherrer's equation for maximum of an observed peak (Table 3). The low XRD profiles of CoFe2O4/MCM-41 and CoFe2O4/MCM-41/PA/Cu show the characteristic diffraction peaks of CoFe2O4 and MCM-41.

Figure 2
figure 2

XRD patterns (a) normal and (b) low angle of the MCM-41, CoFe2O4, CF/M, and CF/M/PA/C denotes nanoalloy with CF (CoFe2O4), M (MCM-41), N-phenyl anthranilic acid (PA), and Cu (C).

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Table 3 Structural and magnetic properties of the samples synthesized in this studya.
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FE-SEM, EDX, and HRTEM analysis

The surface morphology and elemental distribution studies of the MCM-41, CoFe2O4, CoFe2O4/MCM-41 and CoFe2O4/MCM-41/PA/Cu were analyzed by FE-SEM micrographs (Fig. 3), high-resolution HRTEM images (Fig. 4), and EDS elemental mapping (Fig. 4). The FE-SEM images of MCM-41 showed that the particles have a spherical shape with an average diameter of 140–180 nm. The well-ordering and uniformity of the particles with the smooth surface can be observed in this Figure. The FE-SEM images of CoFe2O4 shows several tiny CoFe2O4 particles. That the nanoparticles are slightly agglomerated well-distributed spherical particles and an average diameter of 15–20 nm. The FE-SEM images of CoFe2O4/MCM-41 and CoFe2O4/MCM-41/PA/Cu confirm the formation of CoFe2O4/MCM-41 composites due to the presence of particles of MCM-41 and nanoparticles of CoFe2O4 in the Figure and rough surface with an average particles size of 15–20 nm and 20–25 nm respectively (Table 3). The histogram curve for the size of the CoFe2O4/MCM-41/PA/Cu were further investigated using SEM (Fig. 3). From this histogram, the particle size of the CoFe2O4/MCM-41/PA/Cu sample is estimated to be 20–25 nm.

Figure 3
figure 3

SEM micrographs, and particle size distributions of (a,b) CoFe2O4, (c,d) MCM-41, (e,f) CF/M, and (g,i) CF/M/PA/C denotes nanoalloy with CF (CoFe2O4), M (MCM-41), N-phenyl anthranilic acid (PA), and Cu (C).

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Figure 4
figure 4

HRTEM micrographs (a–c), the SAED pattern (d), elemental mapping (e–l), and EDX (m, n) of CF/M/PA/C denotes nanoalloy with CF (CoFe2O4), M (MCM-41), N-phenyl anthranilic acid (PA), and Cu (C).

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According to HRTEM image the CoFe2O4/MCM-41/PA/Cu exhibited nearly spherical morphology with black core (CoFe2O4) and a gray shell (MCM-41), thus suggesting that the CoFe2O4 magnetite are also homogeneously dispersed inside the MCM-41 silica. HRTEM images of the CoFe2O4/MCM-41/PA/Cu show pore wall region. In the SAED (selected area electron diffraction) pattern, circularly arranged diffraction spots are clearly seen, indicating the single-phase polycrystalline structure of CoFe2O4/MCM-41/PA/Cu. Table 3 show the particles size of CoFe2O4/MCM-41/PA/Cu determined with HRTEM. The EDS spectra and elemental mapping were performed to map the presence of elementals and determine as a quantitative elemental analysis. For CoFe2O4/MCM-41/PA/Cu shows the Si, Fe, Co, N, C, O and Cu elements.

Magnetic properties

The magnetic field dependence of magnetization of CoFe2O4, CoFe2O4/MCM-41 and CoFe2O4/MCM-41/PA/Cu were measured by a vibrating sample magnetometer (VSM) and the results are presented in Fig. 5. When the applied magnetic field is increased to 15,000 Oe, the magnetization of the samples would approach saturation. The ferromagnetic behavior of the core–shell composites being approved with significant hysteresis loops in the M–H curve. The ferromagnetic behavior confirms that samples can be separated with an applied magnetic field or a magnet. The values of saturation magnetization (Ms), remnant magnetization (Mr), and coercivity (Hc), are given in Table 3. CoFe2O4, CoFe2O4/MCM-41 and CoFe2O4/MCM-41/PA/Cu composites possessed a typical ferromagnetic hysteresis, with a saturation magnetization of 49.41 emu/g and 26.52 emu/g, 13.81 emu/g respectively. The CoFe2O4 has higher magnetic saturation than those of CoFe2O4/MCM-41 and CoFe2O4/MCM-41/PA/Cu, mainly attributing to the non-magnetic coating layer on the surface due to quenching of the surface moment.

Figure 5
figure 5

Magnetization curves of CoFe2O4, CF/M, and CF/M/PA/C denotes nanoalloy with CF (CoFe2O4), M (MCM-41), N-phenyl anthranilic acid (PA), and Cu (C).

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N2 adsorption–desorption isotherms

The results of N2 adsorption–desorption isotherms and the BJH pore size distribution of the MCM-41, CoFe2O4/MCM-41 and CoFe2O4/MCM-41/PA/Cu catalyst is illustrated in Figs. 6 and 7 and Table 4. For All the as-synthesized samples, the isotherm curves could be attributed to a type IV adsorption isotherm with a narrow hysteresis loop, as per the IUPAC nomenclature, the prepared composite still preserved mesoporous structure. These results consistently identify type IV isotherms with H1 hysteresis loops for the ordered silicas45. Table 4 shows the values of the surface area got by the Brunauer –Emmett –Teller (BET) and t-plot method, pore sizes and pore volumes of MCM-41, CoFe2O4/MCM-41 and CoFe2O4/MCM-41/PA/Cu. The MCM-41 shows high surface area by BET and t-plot of 1030.6 m2/g and 954.3 m2/g respectively, mean pore size of 6.95 nm, and pore volume of 1.79 cm3/g. The CoFe2O4/MCM-41 shows surface area by BET and t-plot of 444.7 m2/g and 404.4 m2/g respectively, mean pore size of 6.89 nm, and pore volume of 0.875 cm3/g. The CoFe2O4/MCM-41/PA/Cu shows surface area by BET and t-plot of 437.2 m2/g and 395.4 m2/g respectively, mean pore size of 6.60 nm, and pore volume of 0.721 cm3/g. The MCM-41 has higher pore diameter, pore volume, and surface area, than those of CoFe2O4/MCM-41 and CoFe2O4/MCM-41/PA/Cu. These results are the indication to the fact that high surface area of MCM-41 could enable the uniform distribution of CoFe2O4 and PA/Cu and block the pores of MCM-41 but the immobilized group still reveal typical mesoporous structure, which would ensure efficient distribution of catalytic site. Also, the BJH pore size distribution was between 1 and 2 nm for the samples.

Figure 6
figure 6

N2 adsorption –desorption isotherms of MCM-41, CF/M, and CF/M/PA/C denotes nanoalloy with CF (CoFe2O4), M (MCM-41), N-phenyl anthranilic acid (PA), and Cu (C).

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Figure 7
figure 7

BJH pore width distribution of MCM-41, CF/M, and CF/M/PA/C denotes nanoalloy with CF (CoFe2O4), M (MCM-41), N-phenyl anthranilic acid (PA), and Cu (C).

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Table 4 Textural properties of the samples synthesized in this studya.
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FT-IR analysis

FT-IR spectrum related to MCM-41, CoFe2O4, CoFe2O4/MCM-41, PA and CoFe2O4/MCM-41/PA, and CoFe2O4/MCM-41/PA/Cu catalyst can be seen in Fig. 8. The sharp bands associated with OH stretching vibrations of surface hydroxyl groups and water presented at 3200 and 3700 cm−1. A medium band in the region of 1623–1650 cm−1 is mainly attributed to the H–O–H bending motion. From Fig. 1, the signals observed at round 462 cm−1 can be correlated with the rocking motion of bridging oxygens band in Si–O–Si. In addition, the symmetric and asymmetric stretching vibration of Si–O–Si bonds can be correlated with the 808 cm−1 and 1078 cm−1 bands respectively which is common to all the spectra. The extra bands at 960–970 cm−1 are attributed to incorporation of metal into the skeleton of mesoporous MCM-41 sample. This band is mainly attributed to the vibration mode of Si–OH, but the intensity of the band increases and is shifted to lower wavenumber when metals are attached. It indicates the incorporation of MCM-41 silica onto the magnetite nanoparticles. In all curves, the signals observed at round 587 cm−1 can be assigned to the stretching modes of Fe–O bond. The special bands in the FT-IR of the PA at around 1326 cm−1, 1442 cm−1, and 2859 cm−1 can be attributed to the stretching vibration of C–N of amine, the C–H bending, and the CH2 symmetric stretching vibration respectively. After modification of the PA, the specific bands are observed in the FT-IR of the CoFe2O4/MCM-41/PA. The presence of Lewis and Brønsted acid sites indicating the acidic characteristics of catalysts was determined with DRIFT spectra by Şimşek and et al.46. In this paper, the Lewis acid site of catalysts was identified with the FT-IR. The FT-IR spectrum of the CoFe2O4/MCM-41/PA/C sample (Fig. 8) shows special bands of the FT-IR of the PA, which has been shifted to a lower wavenumber, showing that PA ligand are coordinated to Lewis’s acid site (Cu (II)).

Figure 8
figure 8

FT-IR spectra of MCM-41, CoFe2O4 CF/M, PA, CF/M/PA and CF/M/PA/C denotes nanoalloy with CF (CoFe2O4), M (MCM-41), N-phenyl anthranilic acid (PA), and Cu (C).

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Catalyst evaluation

The catalyst was investigated in the role of the catalysts for the one-pot conversion of nitrile to tetrazole with 4-chlorobenzonitrile (1 mmol), catalyst (100 mg), NaN3 (1.2 mmol), at 100 °C in an aqueous medium, and the results are listed in Table 5. In the presence of the MCM-41 and CoFe2O4 support without any metal loading, nitrile was not converted (Table 5, entry 1 and 2). The nitrile conversion over the CoFe2O4/MCM-41 was 30% (Table 5, entry 3), which indicated the important of composite. When the nitrile conversion reaction was performed over the CoFe2O4/MCM-41/PA product was moderate (60%) but the tetrazole selectivity was 100% (Table 5, entry 4). The nitrile conversion over the CoFe2O4/MCM-41/PA/Cu catalyst was high (84%; Table 5, entry 5), which indicated the important role of the Cu metal sites for the production of tetrazole, the high Lewis acidity of the catalyst can facilitate the adsorption of substrate. To elucidate the significant activity of CoFe2O4/MCM-41/PA/Cu catalyst, CoFe2O4/PA/Cu catalyst was prepared, and catalytic performance was compared with that of the CoFe2O4/PA/Cu catalyst. According to Table 5, CoFe2O4/MCM-41/PA/Cu catalyst has very high activity compared to CoFe2O4/PA/Cu.

Table 5 4-Chlorobenzonitrile conversion, product selectivity, yields, TON, and TOF over several catalystsa.
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The catalytic efficiency of the designed CoFe2O4/MCM-41/PA/Cu composite was examined in the synthesis of tetrazole. To test the activity of the catalyst the reaction of 4-chlorobenzonitrile (1 mmol), with NaN3 (1.2 mmol) was selected as model substrates and the influences of some parameters on the synthesis of tetrazole including; temperatures, amount of catalyst, and solvents were investigated using CoFe2O4/MCM-41/PA/Cu as the catalyst during one hour (Fig. 9). The reaction was screened with various solvents such as EtOH, DMSO, DMF, dioxane, acetonitrile, EtOH/water, toluene, and water were tested. The use of a protic polar solvent will rise the reaction rate. To develop an environmentally benign, the use of nontoxic and inexpensive solvents should be considered. In this work, in water the highest yield is observed. The other solvents, had lower yields.

Figure 9
figure 9

5-(4-Chlorophenyl))-1H-tetrazole yield at different (a) solvent (b) catalyst amount, and (c) temperatures during one hour.

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We have also studied the effect of catalyst weights (0, 30, 50, 70 and 100 mg). The results displayed that the progress of the yield was raised from 60 (30 mg) to 65 (50 mg) then to 78 (70 mg) and 88% (100 mg). The results displays that the best yield was achieved at 100 mg of the catalyst and the product yield was increased with small amount after 100 mg.

We have also carried out the reaction in different temperature. The results displayed that the progress of the yield was raised from 60 (40 °C) to 65 (60 °C) then to 77 (80 °C) and 88% (100 °C). The best result was obtained at 100 °C.

After optimization of the reaction conditions, we extended the scope of the reaction for various derivatives of nitrile compounds. Accordingly, both electron-donating and electron withdrawing substituents have been employed for this reaction. The conversion could also be very successfully carried out at very short time. These results strongly confirm that this method is applicable for green and fast conversion of a large variety of derivatives that is an excellent advantage in the green chemistry world (Table 6).

Table 6 Synthesis of 5-substituted 1H-tetrazoles derivatives in the presence of CF/M/PA/C.
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The source of the catalysis in the formation of 1H-tetrazoles is the coordination of the nitrile substrate to the Lewis acidic copper. The dominant factor influencing [2 + 3] cycloaddition is coordination of Cu2+ to the nitrile, and. Subsequent nucleophilic attack by azide, followed by hydrolysis produces tetrazole as the end product47 (Fig. 10).

Figure 10
figure 10

Plausible Reaction Pathway for the Direct Conversion of over the CoFe2O4/MCM-41/PA/Cu Catalyst.

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Catalyst reusability

The reusability of the designed catalyst was studied for the one-pot conversion of nitrile to tetrazole with 4-chlorobenzonitrile (1 mmol), catalyst (100 mg), NaN3 (1.2 mmol), at 100 °C in an aqueous medium and the findings are given in Fig. 11. For this, after the reaction was completed, the spent catalyst was removed after each run through applying an external magnet, then washed with ethyl acetate and used for the next run. The catalyst showed nearly the same performance compared with the first run even after six consecutive reaction runs in terms of the conversion of nitrile. The stable performance could be related to the negligible leach in the Cu loading. In another study a leaching test was performed to show the nature of catalyst in the reaction process. To do this, the reaction was stopped after about 50% of the reaction process was completed, then the catalyst was removed using an external magnet. The catalyst-free mixture was then allowed to continue for 20 min. In this case no further conversion was observed indicating that the catalyst operates in a heterogeneous manner.

Figure 11
figure 11

Reusability of the CoFe2O4/MCM-41/PA/Cu catalyst. Reaction conditions: 4-chlorobenzonitrile (1 mmol), NaN3, (1.2 mmol), catalyst (100 mg) and water (3mL) at 100 °C.

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Conclusion

The present work successfully developed a novel catalyst with core –shell structure (CoFe2O4/MCM-41/PA/Cu) for the one-pot the conversion of nitrile to tetrazole that has powerful catalytic activity, high separation efficiency and good reusability. The incorporation of CoFe2O4 could not only impart the catalyst with a strong magnetism, but also tune its acidity to promote tetrazole production. A high yield was achieved from nitrile conversion in an aqueous medium. The as- catalyst could be reused at least six times by an external magnetic field without substantial change in catalytic activity. This proposed strategy exposes some advantages over those available towards tetrazole production from nitrile, such as the nonuse of hazardous metal or dual catalysts, fine catalyst recyclability with magnetic separation, and the implementation of a green and sustainable route.