Copper-decorated core–shell structured ordered mesoporous containing cobalt ferrite nanoparticles as high-performance heterogeneous catalyst toward synthesis of tetrazole

The present work describes the synthesis of copper immobilization on CoFe2O4/MCM-41 as a catalyst, which is created by attaching copper and ligand (N-phenyl anthranilic acid (PA)) on the surface of CoFe2O4/MCM-41 (CoFe2O4/MCM-41/PA/Cu). The synthesized CoFe2O4/MCM-41 support and immobilized copper were identified by FTIR, TEM, VSM, SEM XRD, and nitrogen adsorption–desorption analysis. The results showed that MCM-41 silica was coated with magnetite nanoparticles and copper was successfully immobilized on this structure. The catalytic performance of synthesized catalyst was tested in the synthesis of tetrazole. It was shown that the solid catalyst exhibited a strong magnetic response and showed good catalytic activity in the synthesis of tetrazole. The catalytic test showed that copper supported on CoFe2O4/MCM-41 hybrid showed much better catalytic activity than copper supported on CoFe2O4, indicating that MCM-41 plays an important role in CoFe2O4/MCM-41 hybrid for the synthesis of tetrazole. Separation of the solid catalyst from the reaction mixture was easily performed by external magnetism without apparent mass loss. And the catalyst could be reused six times for the synthesis of heterogeneous tetrazole.

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 controlled 12 .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 materials 13 .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 carbon 14 .Due to having a low energy occupied molecular orbital (HOMO), tetrazoles resist the strongest oxidizing agents 15 .These compounds are acidic and their pK a is similar to their corresponding carboxylic acids 16 .These compounds have various applications in the synthesis of other organic substances, pharmaceutical and biological industries as well as military industries 17 .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 inhibitor [18][19][20][21] .Also, tetrazoles are used as propellants in rocket and jet engines in the aerospace industry 22,23 .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 metals 24 .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 compounds [25][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 CoFe 2 O 4 /MCM-41based 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.
In this paper, we demonstrated that MCM-41/PA/Cu alloys deposited on CoFe 2 O 4 , 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 CoFe 2 O 4 /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.

Results and discussion
Catalyst characterization.XRD analysis.Figure 2 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 wellordering and uniformity of the particles with the smooth surface can be observed in this Figure  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.  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 silicas 45 .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, CoFe 2 O 4 /MCM-41 and CoFe 2 O 4 /MCM-41/PA/ Cu.The MCM-41 shows high surface area by BET and t-plot of 1030.6 m 2 /g and 954.3 m 2 /g respectively, mean pore size of 6.95 nm, and pore volume of 1.79 cm 3 /g.The CoFe 2 O 4 /MCM-41 shows surface area by BET and t-plot of 444.7 m 2 /g and 404.4 m 2 /g respectively, mean pore size of 6.89 nm, and pore volume of 0.875 cm 3 /g.The CoFe 2 O 4 /MCM-41/PA/Cu shows surface area by BET and t-plot of 437.2 m 2 /g and 395.4 m 2 /g respectively, mean pore size of 6.60 nm, and pore volume of 0.721 cm 3 /g.The MCM-41 has higher pore diameter, pore volume, and surface area, than those of CoFe 2 O 4 /MCM-41 and CoFe 2 O 4 /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 CoFe 2 O 4 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.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 CH 2 symmetric stretching vibration respectively.After modification of the PA, the specific bands are observed in the FT-IR of the CoFe 2 O 4 /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 CoFe 2 O 4 /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)).
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), NaN 3 (1.2mmol), at 100 °C in an aqueous medium, and the results are listed in   www.nature.com/scientificreports/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.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).
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 Cu 2+ to the nitrile, and.Subsequent nucleophilic attack by azide, followed by hydrolysis produces tetrazole as the end product 47 (Fig. 10).

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), NaN 3 (1.2mmol), 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.

Conclusion
The present work successfully developed a novel catalyst with core -shell structure (CoFe 2 O 4 /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 CoFe 2 O 4 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.
CoFe 2 O 4 , CoFe 2 O 4 /MCM-41 and CoFe 2 O 4 /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 CoFe 2 O 4 has higher magnetic saturation than those of CoFe 2 O 4 / MCM-41 and CoFe 2 O 4 /MCM-41/PA/Cu, mainly attributing to the non-magnetic coating layer on the surface due to quenching of the surface moment.N 2 adsorption-desorption isotherms.The results of N 2 adsorption-desorption isotherms and the BJH pore size distribution of the MCM-41, CoFe 2 O 4 /MCM-41 and CoFe 2 O 4 /MCM-41/PA/Cu catalyst is illustrated in Figs. 6 and 7 and Table

Figure 10 .
Figure 10.Plausible Reaction Pathway for the Direct Conversion of over the CoFe 2 O 4 /MCM-41/PA/Cu Catalyst.
To elucidate the significant activity of CoFe 2 O 4 /MCM-41/PA/Cu catalyst, CoFe 2 O 4 /PA/Cu catalyst was prepared, and catalytic performance was compared with that of the CoFe 2 O 4 /MCM-41/PA/Cu catalyst.

Table 1 .
. The FE-SEM images of CoFe 2 O 4 shows several tiny CoFe 2 O 4 particles.That the nanoparticles are slightly agglomerated welldistributed spherical particles and an average diameter of 15-20 nm.The FE-SEM images of CoFe 2 O 4 /MCM-41 and CoFe 2 O 4 /MCM-41/PA/Cu confirm the formation of CoFe 2 O 4 /MCM-41 composites due to the presence of Catalytic performance of different heterogeneous and homogeneous catalysts for conversion of nitrile to tetrazole via sodium azide.particles of MCM-41 and nanoparticles of CoFe 2 O 4 in the Figure and rough surface with an average particles size of 15-20 nm and 20-25 nm respectively (Table3).The histogram curve for the size of the CoFe 2 O 4 /MCM-41/PA/Cu were further investigated using SEM (Fig.3).From this histogram, the particle size of the CoFe 2 O 4 / MCM-41/PA/Cu sample is estimated to be 20-25 nm.According to HRTEM image the CoFe 2 O 4 /MCM-41/PA/Cu exhibited nearly spherical morphology with black core (CoFe 2 O 4 ) and a gray shell (MCM-41), thus suggesting that the CoFe 2 O 4 magnetite are also homogeneously dispersed inside the MCM-41 silica.HRTEM images of the CoFe 2 O 4 /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 CoFe 2 O 4 /MCM-41/PA/Cu.Table 3 show the particles size of CoFe 2 O 4 /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 CoFe 2 O 4 / MCM-41/PA/Cu shows the Si, Fe, Co, N, C, O and Cu elements.Magnetic properties.The magnetic field dependence of magnetization of CoFe 2 O 4 , CoFe 2 O 4 /MCM-41 and CoFe 2 O 4 /MCM-41/PA/Cu were measured by a vibrating sample magnetometer (VSM) and the results are presented in Fig.

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

Table 3 .
Structural and magnetic properties of the samples synthesized in this study a .a Here, CF/M//PA/C denotes catalysts with CoFe 2 O 4 (CF), MCM-41 (M), N-phenyl anthranilic acid (PA), and Cu (C).b Measured using maximum of an observed peak and Scherrer equation.c Measured using SEM.d Measured using HRTEM.e Here, H c , M r , and M s denote the values of coercivity, remnant magnetization, saturation magnetization respectively.

Table 4 .
Textural properties of the samples synthesized in this study a .a Here, S BET , S t-Plot , d avg , and V total denote the Brunauer -Emmet -Teller specific surface area, surface area by t-plot, average pore size, and total pore volume respectively.