Development of novel and green NiFe2O4/geopolymer nanocatalyst based on bentonite for synthesis of imidazole heterocycles by ultrasonic irradiations

Geopolymers as aluminosilicate inorganic polymers and eco-friendly building materials which can be used as substrate for different kinds of composite. In this research, according to the fabrication of geopolymer based on bentonite as a substrate and embedment of NiFe2O4 nanoparticles in the construction of this polymer, the synthesis of a new magnetic nanocomposite (NiFe2O4/geopolymer) was investigated for the first time. In order to describe its chemistry and morphology features, different analyses such as Fourier transform infrared spectroscopy, field-emission scanning electron microscopy and transmission electron microscopy images, Brunauer–Emmet–Teller adsorption–desorption isotherm, X-ray diffraction pattern, energy-dispersive X-ray analysis, thermogravimetric analysis, and vibrating-sample magnetometer analysis were used. The application of this novel nanocatalyst was studied for one-pot three-component condensation reaction of substituted imidazole derivatives by accelerated ultrasonic irradiations. Compared to the other conventional catalysts which were used for the synthesis of imidazole derivatives, the green synthesis method for fabrication of this heterogeneous and magnetic nanocatalyst, its high thermal stability, being eco-friendly, noticeable efficiency and easy reusability have become privileges to be superior.


Results and discussion
For the first time, the synthesis of NiFe 2 O 4 nanocomposite based on geopolymer substrate and NiFe 2 O 4 nanoparticles was introduced as a new magnetic nanocatalyst under the mild conditions. According to the Fig. 1, the fabrication of this new nanocatalyst was conducted in two synthesis steps; the first step included the preparation and synthesis of NiFe 2 O 4 nanoparticles by coprecipitation method and the second step by intercalation method consisted of the in situ preparation of geopolymer based on bentonite clay and the addition of NiFe 2 O 4 spinels to the synthetic solution of geopolymer. The formation of hydrogen bonds between hydroxyl groups of geopolymer and hydroxyl groups of synthetic NiFe 2 O 4 nanoparticles were conducted and then, the synthetic magnetic nanoparticles were intercalated into the porous structure of geopolymer. Various detection techniques including Fourier-transform infrared (FT-IR) spectroscopy for characterizing the functional groups, energy dispersive X-ray (EDX) analysis to identify the elemental composition, using field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) images for observing the morphology and size of the NiFe 2 O 4 /geopolymer nanocomposite, Brunauer-Emmet-Teller (BET) adsorption-desorption isotherm to indicate the surface area, pore volume and pore size of geopolymer and NiFe 2 O 4 /geopolymer nanocomposite, X-ray diffraction (XRD) pattern for determining the crystalline phase of bentonite clay, synthetic geopolymer and NiFe 2 O 4 /geopolymer nanocomposite, thermogravimetric (TG) analysis for the evaluation of thermal behavior and stability and vibrating-sample magnetometer (VSM) analysis for the characterization of magnetic properties of synthetic NiFe 2 O 4 /geopolymer nanocomposite are discussed respectively. characterization of the nife 2 o 4 /geopolymer nanocomposite. FT-IR analysis. As could be seen in Fig. 2, the formation of the NiFe 2 O 4 /geopolymer nanocomposite was confirmed by FT-IR spectroscopy technique. Figure 2a illustrated the FT-IR spectrum of NiFe 2 O 4 nanoparticles. The two absorption bands of octahedral complex around 427 cm −1 and tetrahedral complex around 587 cm −1 were observed in the spectrum 47 50 . Figure 2c as a FT-IR spectrum of geopolymer, has conceded the fabrication of polymer based on bentonite clay. According to this spectrum, the band amplification and high transmittance percentage of OH groups have been increased; also, an observed strong absorbance band around 1,430 cm −1 51,52 was confirmed the existence of C-O-C asymmetric vibration mode of CO 3 2ion which has been determined in all spectra of geopolymer samples. A broad band at range of 900-1,000 cm −1 (986 cm −1 ) was attributed to the Si-O-Si stretching vibration mode 53 . Besides, due to the formation of geopolymer structure, the Si-O-Si stretching vibration mode has shifted to the lower wavenumbers 53 . A broad band Fig. 2d illustrated the FT-IR spectrum of synthetic nanocomposite. According to the FT-IR spectrum of synthetic nanocomposite, the strong Si-O-Si stretching vibration mode (990 cm −1 ) was assigned in the related spectrum (Fig. 2d). A considerable reduction in amplification of O-H stretching band which was observed is due to the interaction between magnetic NiFe 2 O 4 nanoparticles and OH groups of geopolymer. As mentioned before, two absorbance bands at 578 cm −1 and 475 cm −1 were characterized as bending vibration modes of Al-O-Si and Si-O-Si. Apart from these mentioned bands, as well, the absorption bands of octahedral and tetrahedral complexes of NiFe 2 O 4 nanoparticles were also determined at 427 cm −1 and 587 cm −1 . Conforming to these observations, it can be concluded that the absorbance bands of NiFe 2 O 4 nanoparticles have overlapped with the mentioned absorption bands of synthetic geopolymer. Based on the obtained results from EDX spectrum of designed NiFe 2 O 4 /geopolymer nanocomposite, the presence of iron, nickel and oxygen peaks was related to the structure of synthetic magnetic NiFe 2 O 4 nanoparticles. As well as, alongside of oxygen peak, the aluminum and silicon peaks were attributed to the fabricated and inorganic substrate, geopolymer.
Contexture characterization of geopolymer and NiFe 2 O 4 /geopolymer nanocomposite. The BET analysis of geopolymer and NiFe 2 O 4 /geopolymer nanocomposite was evaluated by nitrogen gas adsorption. As could be seen, the nitrogen adsorption tubular isotherm of NiFe 2 O 4 /geopolymer nanocomposite is indicated in (Fig. 3b). According to the obtained results (Table was attached (Table S2, entries 1-17). As can be seen the catalytic activity of synthetic nanocomposite was evaluated by three kinds of method including room temperature, reflux and ultrasonic condition (Table S2, entries 1-5). It was noteworthy that the best result was observed by ultrasonic bath and using of 0.03 g nanocatalyst (Table S2, entry 5). After the method determination, the best type of solvent for the reaction was analyzed. Based on the obtained results, the highest yield percentage was observed in ethanol, the green and non-toxic solvent (Table S2, entry 5). In a close and precise investigation, the catalytic activity of bentonite, synthetic substrate (geopolymer) and NiFe 2 O 4 magnetic nanoparticles was evaluated before the value optimization of catalyst (Table S2, entries [11][12][13]. In comparison to bentonite which did not show any catalytic activity, the activity of geopolymer was 20%, also, the activity of NiFe 2 O 4 nanoparticles was reported 38%. By combination of geopolymer and NiFe 2 O 4 nanoparticles and introducing a new magnetic nanocomposite, the catalytic activity of synthetic nanocomposite was increased up to 65%. The loading of NiFe 2 O 4 nanoparticles in the surface of geopolymer and the existed hydroxyl groups in the structure of geopolymer caused to increase the efficiency of the nanoparticles by the distribution of magnetic NiFe 2 O 4 nanoparticles and preventing the aggregation of the nanoparticles. Also, their   Table 1. Mechanism study of the NiFe2O 4 /geopolymer nanocatalyst in synthesis process of imidazole derivatives. The outline of the proposed mechanism for synthesis of imidazole derivatives was indicated in Fig. 7. According to   . Intermediate (V) was formed by release of water and the cyclization process was carried out by intermolecular nucleophilic attack. Eventually, the 2,4,5-trisubstituted imidazole product (VI) was fabricated by dehydration process and released of catalyst. After the reaction completion, the magnetic nanocatalyst was separated by the external magnet from the reaction ambient and could be utilized for several runs.
Evaluation of catalyst reusability for several runs. One of the most important aspect of a catalyst is its recovery and reusability. For this purpose, first, the NiFe 2 O 4 /geopolymer nanocatalyst was recovered and separated from the reaction ambient by an external magnet. Then, the nanocatalyst washed with ethanol and dried at 70 °C for an overnight. After the separation process, the nanocatalyst was reused with the same amount in the model reaction. The recycle-ability of synthetic nanocatalyst was substantial and after 8 runs (Fig. S1), no significant reduction was observed in its catalytic activity. In addition, the stability of recycled nanocatalyst was confirmed by the FT-IR and EDX analyses ( Fig. S2 and S3).
experimental General. In this study, the bentonite nanoclay (Bentonite Aldrich 682659) was applied as a primary source for the synthesis of a geopolymer. Also, all chemicals such as nickel nitrate, iron nitrate (III), sodium hydroxide, sodium silicate, and other solvents and reagents were analytical grade and purchased from Merck and Aldrich company. Melting points were measured on an Electrothermal 9,100 apparatus and are uncorrected. FT-IR spectra were recorded on a Shimadzu IR-470 spectrometer by the method of KBr pellets. 1 H NMR and 13 C NMR spectra were recorded with a Bruker DRX-500 Avance spectrometer at 500 and 125 MHz, respectively. FE-SEM images were taken with KYKY-EM3200. TEM images were prepared by ZEISS device (EM10C-100 kV model, Germany) and the physical absorption of gas molecules on the surface area of geopolymer and the synthetic magnetic NiFe 2 O 4 /geopolymer nanocomposite were carried out by BET technique (micromeritics ASAP 2020). EDX analysis was recorded with a Numerix DXP-X10P. XRD patterns of the solid powders were carried out using a JEOL JDX-8030 (30 kV, 20 mA). TG analysis was taken by Bahr-STA 504 instrument under the air atmosphere, VSM analysis was carried out by LBKFB model-magnetic kavir, as well as, the fabrication and catalytic application of synthetic nanocomposite was evaluated by Elmasonic device, S model (60 H). The identification of products was accomplished by comparison of their spectroscopic and analytical data with those of authentic samples. General procedure for the synthesis of 2,4,5-trisubstituted imidazole derivatives (4a-q). Taking into account the ultrasonic condition, the reaction of a mixture consists of benzil (0.8 mmol), several types of substituted aldehyde (0.8 mmol), ammonium acetate (2.0 mmol) and NiFe 2 O 4 /geopolymer nanocomposite as catalyst (0.03 g) was carried out in the ethanol as a green solvent. The progression of the reaction was monitored by TLC in appropriate times. According to the reaction accomplishment, the separation of catalyst was conducted by an external magnet, also, 2,4,5-trisubstituted imidazole derivatives were attained by recrystallization process in ethanol. The synthesis of All the known product were approved by the comparison of their melting points with those of authentic literature samples (Table 1) and in some case the 1 H NMR and 13 C NMR spectra were taken (supplementary information file).

conclusions
In summary, according to the substantial aspects of sonochemistry, using this technique can be leaded to generate catalysts with high efficiency. In this work, a new magnetic nanocomposite according to geopolymer and NiFe 2 O 4 nanoparticles was fabricated under the ultrasonic condition. The characterization and features of NiFe 2 O 4 /geopolymer as a new magnetic nanocomposite were carried out by wide range of spectroscopic techniques such as FT-IR, EDX, FE-SEM, TEM, BET, XRD, TGA, VSM analysis. According to the obtained results, NiFe 2 O 4 /geopolymer nanocomposite was categorized as mesoporous compounds. The volume and size of pores of synthetic nanocomposite were increased due to the presence and addition of magnetic and cubic NiFe 2 O 4 nanoparticles during the in situ preparation of geopolymer; as well as, their presence has induced a magnetic property to this synthetic nanocomposite. Its thermal stability was substantial due to the presence of polymeric structure of geopolymer. The catalytic activity of NiFe 2 O 4 /geopolymer nanocomposite was evaluated by one pot three condensation reaction of 2,4,5-triaryl-1H-imidazoles. According to the time of reaction and considerable isolated organic products, this new nanocomposite demonstrated fundamental catalytic activity. In spite of reaction accomplishment by ultrasonic irradiations, the chemically and mechanically stability of proposed nanocatalyst was considerable and its reusability was determined by various analyses including FT-IR and EDX.

Supporting information
Additional supporting information including the table of BET analysis, 1 H and 13 C NMR of the products, the table of optimizing of the reaction conditions, spectroscopic characterization data of the FT-IR spectrum and the EDX analysis of recycled nanocatalyst and diagram of reusability of nanocatalyst can be found in the online version of this article at the publisher's web site.