Introduction

Green chemistry is one of the most important research activities for chemists, both in the laboratory and industry. Therefore, many efficient, eco-friendly and clean synthetic strategies have been developed for the synthesis of biologically and industrially active molecules [1,2,3,4,5]. Meanwhile, metal-catalyzed multi-component reaction is one of the significant areas of green chemistry research. Transition metal-catalyzed click synthesis of triazoles is a powerful method for the synthesis of diverse complex molecules. Triazoles derivatives have developing application in medicinal chemistry and biological activities [6,7,8,9,10]. They also have numerous industrial applications as florescent whiteners, dyestuffs, photo-stabilizers of polymers, and optical brightening agents [11, 12]. Forasmuch as copper-catalyzed click reaction is one of the best methods for the synthesis of 1,2,3-triazoles [13, 14], numerous homogeneous copper catalysts have been reported [15, 16]. Most of these successful methods suffer from non-reusability of the catalysts, and the usage of toxic and/or expensive ligands [17,18,19,20]. To overcome these problems, many researchers have focused their efforts on copper-based heterogeneous systems [17, 21, 22].

Natural biopolymers are the attractive subjects for the design of bio-supported catalysts due to their eco-friendly, low cost and non-toxic properties [23,24,25,26,27,28,29,30]. Peanut shell as an agro-industrial waste containing considerable fraction of the biodegradable lignocellulosic waste [31] is discarded in the environment or burned about 13.7 million tons per year [32, 33]. This promising natural and renewable raw material consists of a combination of lignin, cellulose, proteins and hemicellulose biopolymers (Fig. 1) [34, 35]. There are many polar functional groups such as hydroxyl, methoxy and carboxyl groups on the surface of peanut shell. Therefore, peanut shell is an attractive candidate as a natural, renewable, non-toxic and very low, or no cost environmentally friendly support for metal nanoparticles.

Fig. 1
figure 1

(This figure was designed by authors and it has been taken by authors)

Composition of peanut shell

Full size image

These days, the application of ultrasonic technology has reported for organic compounds synthesis, emulsification, extraction, nanoparticle formation, and degassing [36,37,38,39]. Sonication method has important advantages such as high efficiency, selectivity and yield, economic performance, short reaction time, and low environmental pollution [20, 40,41,42].

Herein, in continuation of our research toward the development of nano bio-based catalytic systems [43,44,45], the synthesis of copper oxide supported on peanut shell (Cu2O@PS) as a heterogeneous nano-biocatalyst and its catalytic activity for the click synthesis of triazoles in EtOH-H2O as green solvent under ultrasonic irradiation is reported.

Results and discussion

Synthesis and characterizations of the catalyst

The preparation of the Cu2O@PS nanocomposite is described schematically in Scheme 1. The Cu2O@PS nanocomposite obtained by the reaction of peanut shell powder with copper acetate in water at 70 °C for 5 h. The catalyst was centrifuged and washed with water, ethanol, and acetone then dried in the oven at 70 °C.

Scheme 1
scheme 1

(The source of this diagram is taken from “http://nolinsteel.com/peanuts/” and the used softwares are Chemdraw and Paint. The Scheme was designed by authors)

Schematic diagram of catalyst preparation

Full size image

The Cu2O@PS nanocomposite was characterized by FT-IR, thermogravimetric analysis (TGA), atomic absorption spectroscopy (AAS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) analysis and X-ray diffraction (XRD) measurements. The FT-IR spectrum of peanut shell and Cu2O@PS are shown in Fig. 2. The band around 3400 cm−1 was ascribed to the mixed stretching vibration absorption band of amino and hydroxyl groups. The bands at 2950 cm−1 were assigned to aliphatic C–H, mainly CH2 stretching. The small band obtained at 1738 cm−1 was assigned to the carbonyl groups stretching vibration [29, 46]. As can be seen in the FT-IR spectrum of Cu2O@PS, the presence of characteristic bands of PS in the 1738 and 3500 cm−1 regions clearly confirms the existence of PS in the final catalyst. Also, the band shift from 1738 cm−1 (in PS) to 1727 cm−1 (in Cu2O@PS) reveals the coordination of copper to peanut shell [46].

Fig. 2
figure 2

FT-IR spectra of peanut Shell (A), and Cu2O@PS (B)

Full size image

Thermogravimetric analysis was further used to study the composition of the catalyst (Fig. 3). The TGA curve of the catalyst shows a weight loss at ~ 100 °C that is associated with the release of physically adsorbed water. The weight loss above ~ 230 °C (and continued to ~ 600 °C) is related to the decomposition of PS and organic groups on the Cu2O@PS. Thermal analysis showed that the catalyst has good thermal stability up to 230 °C. Also, the copper content on the Cu2O@PS nanocomposite was measured 0.28 mmol g−1 by atomic absorption spectroscopy.

Fig. 3
figure 3

Thermal gravimetric analysis of Cu2O@PS

Full size image

Morphologies of fresh peanut shell and the Cu2O@PS nanocomposite were determined by SEM. The fresh PS is basically smooth (Fig. 4a). The SEM images of Cu2O@PS show the formation of spherical particles in size around 30–40 nm on the surface of the peanut shell (Fig. 4b).

Fig. 4
figure 4

FESEM images of PS (a), and Cu2O@PS (b)

Full size image

In addition, comparing the EDS analysis of Ps and Cu2O@PS clearly shows the presence of Cu, C, O, and N elements in this composite and demonstrate that copper was anchored to the PS (Fig. 5a and b). In the XRD pattern of the Cu2O@PS nanocomposite, the diffractions at 2θ = 36.4°, 42.5°, 61.4°, and 73.4° can be assigned to the (111), (200), (220) and (311) lattice planes of Cu2O, in accordance with Cu2O standard data (JCPDS card NO. 05–0667) (Fig. 6).

Fig. 5
figure 5

Energy dispersive X-ray spectroscopy (EDS) of PS (a), and Cu2O@PS (b)

Full size image
Fig. 6
figure 6

XRD pattern of PS and Cu2O@PS nanocomposite (a), Cu2O (b)

Full size image

Catalytic studies

The catalytic activity of Cu2O@PS nanocomposite was investigated in the click synthesis of 1,2,3-triazoles. The reaction of phenyl acetylene 1, 4-nitrobenzyl bromide 2 and sodium azide (NaN3) was chosen as a model reaction under different conditions. As the first experiment, K2CO3 was used as a base in the presence of 1 mol% of catalyst, and this reaction was tested by employing various solvents such as toluene, MeCN, MeOH, EtOH, H2O, EtOH-H2O, and MeOH-H2O at 50 °C under ultrasonic irradiation for 45 min (Table 1, entries 1–7). A superior yield was obtained when EtOH-H2O (1:1) was used as the solvent (Table 1, entry 6). Then, different base were screened in the model reaction. A moderate yields were obtained with Cs2CO3 and KOt-Bu (entries 8 and 9) and the reaction proceed in fairly good yields in the presence of NaOH and KOH (entries 10 and 11). Also, some experiments were carried out at different temperatures, and finally 50 °C was chose as optimum reaction temperature (entries 6, 12–14). Effect of the catalyst loading was also investigated under the optimum reaction conditions. It was found that, when the amount of catalyst increased from 0.5 and 1 to 2 mol%, the yield of product changed from 65 and 91% to 92%, respectively. So, 1 mol% of catalyst is sufficient to promote this reaction (entries 6, 15–16). When this reaction was carried out without catalyst, the yield of the product was trace (entry 17). To delineate the role of ultrasound, the reaction was investigated without ultrasonic irradiation at 50 °C in various solvents. In all reactions, the result obtained by the use of ultrasound irradiation leads to a higher yield (entries 18–20). Finally, when this reaction was carried out with PS or Cu2O as catalyst, the yield of the product was trace and 78% yield, respectively (entries 21 and 22).

Table 1 Screening of the reaction conditions
figure a
Full size table

To explore the scope of the click reaction various benzyl bromides and aryl acetylenes, containing both electron donating and electron withdrawing functionalities were screened in optimized reaction conditions and high isolated yields were obtained (Table 2). Under the same reaction conditions benzyl chlorides provided target products in good yields (Table 2, entries 12–14).

Table 2 Click synthesis of 1H-1,2,3-triazoles
figure b
Full size table

Observation of great potential activity of Cu2O@PS nano-biocatalyst in the Click reaction of benzyl bromides and aryl acetylenes encouraged us to investigate the Click reaction of aryl acetylenes 1 with phenacyl bromides 4 and sodium azide in the same reaction conditions. As can be seen from Table 3, the Click reaction of aryl acetylenes and phenacyl bromides contain electron withdrawing or donation groups provide 1H-1, 2, 3-triazol-ethan-1-one derivatives 5 in good isolated yields in the presence of 1 mol% of catalyst in EtOH-H2O under ultrasound irradiation at 50 °C.

Table 3 Synthesis of 1H-1, 2, 3-triazol-ethan-1-one derivatives by Click reaction
figure c
Full size table

Then, we examined the heterogeneous nature of the catalyst. Firstly, to assess the copper leaching of the catalyst, we performed hot filtration test for the click reaction of 4-nitro benzyl bromide 2, phenylacetylene 1 and NaN3. The reaction was stopped after ~ 50% of the reaction time. Hot filtrate was transferred to another flask containing base and H2O-EtOH at 50 °C. Upon further heating of the catalyst-free solution for 1.5 h, no considerable progress was observed by GC analysis (Fig. 7a). Moreover, atomic absorption spectroscopy (AAS) of the same reaction solution at the midpoint of completion indicated that no significant quantities of copper were lost to the reaction medium during the process. Furthermore, the reusability of catalyst was investigated in the reaction of 4-nitro benzyl bromide, phenylacetylene, and NaN3. The catalyst could be reused successively five times without significant loss of activity (Fig. 7b). Moreover, atomic absorption spectroscopy revealed that the loading of copper was 0.27 mmol g−1 after five runs and there was no significant change in the copper content of the recovered catalyst. All results confirm the reaction occurs mainly via a heterogeneous pathway. The SEM micrographs of reused catalyst after five times reveal that the reused catalyst has a similar texture with fresh catalyst (see Additional file 1).

Fig. 7
figure 7

Hot filtration test observed by GC analysis (a), the reusability of the catalyst (b)

Full size image

Based on literature reports [61], a possible mechanism for click catalytic synthesis of triazole is proposed in Scheme 2. Synthesis of triazole proceeds through the formation of copper acetylide (A). The coordination of organic azide (B) (formed in situ by the reaction of organic bromide with NaN3) to the copper acetylide, followed by the Huisgen 1, 3-dipolar cycloaddition reaction of (A) and (B) give the complex (C). Subsequently, the desired triazole was obtained by copper-acidic hydrogen exchange followed by regeneration of the catalyst for the next use in the catalytic cycle. It is notable; functional groups such as hydroxyl, amine, methoxy and carboxyl groups on the surface of peanut shell have good potential to coordinate with copper nanoparticles.

Scheme 2
scheme 2

(The sources of Graphical abstract are internet “https://pngtree.com/freepng/vector-arrow-earth_520723.html”, “http://aisphysicalscience.pbworks.com/w/page/1623001/29%20Cu%20-%20Copper” and “https://pngtree.com/so/peanut-shells” and the used softwares are Chemdraw and Paint. The Scheme was designed by authors)

The proposed mechanism of the reaction. The Scheme was designed by authors

Full size image

Reports in Table 4, compares the efficiency of Cu2O@PS nanocomposite with some other heterogeneous copper catalysts in literature in the Click reaction of phenylacetylene, phenacyl bromides and NaN3. Table 4 shows that although all of methods have good efficiency, the present catalyst affords some advantages such as biodegradability using green nano bio-support for immobilization of copper, reasonable reaction time and low temperature which are all energy and time-consuming processes.

Table 4 Comparison of efficiency of various heterogeneous catalysts for triazole synthesis
Full size table

Experimental

Material and measurements

All chemicals were purchased from Merck, Aldrich or Fluka were used without further purification. IR spectra were recorded on a Shimadzu FT-IR-470 FT-IR spectrophotometer. EDS characterization was performed using an electron microscopy Oxford Instrument Company, Germany. Field emission scanning electron microscopy (FESEM) was performed using a ZEISS instrument, SIGMA VP model, Germany. The NMR spectra were recorded on a Brukerdrx-300Avance spectrometer. The concentration of Cu was estimated using a Shimadzu AA-680 flame atomic absorption spectrophotometer. Diffraction data were collected on a STOE STADI P with scintillation detector, secondary monochromator and Cu-Ka1 radiation (λ = 1. 5406 Å). Gas chromatography was performed on a Trace GC ultra from the Thermo Company equipped with FID detector and Rtx®-1 capillary column. Melting points of products were measured with an Electrothermal 9100 apparatus and are uncorrected. Thermogravimetric analysis (TGA) was done by D-32609 Hullhorst. The peanut shell was obtained from Astaneh Ashrafiyeh Township located in 37° 16′ latitude and 49° 56′ longitude in north of Iran.

Preparation of Cu2O@Peanut shell

Crushed peanut shells were ground in a ball mill to a fine powder. A mixture of peanut shell powder (1 g) and copper acetate (0.1 g) was stirred in de-ionized water (30 ml) at 70 °C for 5 h. The catalyst was then centrifuged and washed with water, ethanol, and acetone and dried in the oven at 70 °C to obtain Cu2O@Peanut shell.

General procedure for Click reactions

A mixture of Cu2O@PS (1 mol% of Cu, 40 mg), K2CO3 (2 mmol), aryl bromide (1.0 mmol), phenyl acetylene (1.2 mmol), and NaN3 (1.2 mmol) in H2O-EtOH (3 ml, 1:1) was sonicated at 50 °C for an appropriate time. After completion of the reaction monitored by TLC (EtOAc:n-hexane (1:3), the catalyst was separated and the filtrate was extracted with Chloroform (2 × 2 ml). The organic solvents were removed under vacuum and the pure product was obtained by recrystallization with CHCl3:n-hexane (1:3). All of the Click products are known compound and were reported previously.

Conclusions

In summary, Cu2O@Peanut shell nano-biocomposite was synthesized and used as an effective heterogeneous catalyst in a one-pot Huisgen 1,3-dipolar cycloaddition reaction under ultrasonic irradiation in EtOH-H2O as a green solvent for the synthesis of 1,2,3-triazole derivatives. The reusability of the catalyst is high and the catalyst can be reused five times without a significant decrease in its catalytic activity. Notable features of this catalytic reaction are bio-degradable and bio-renewable polymeric support, compatibility with a wide range of substrate, mild reaction conditions, high atom economy, good-yields of the products, ligand-free, leaching-free and eco-friendliness characteristics of the catalyst.