Introduction

In recent decades, preparation of porous materials has been a very interesting research field [1]. Apart from zeolites which have natural counterparts [2], metal–organic frameworks [3], porous silicas such as SBA-15 [4] and MCM-41 [5], and covalent organic frameworks [6] have been the subjects of extensive research. A wide variety of methods have been developed for preparing porous metals, including the template method, magnetron sputtering, electrodeposition, and the dealloying process [7]. Porous metals have been used in electrochemical sensors, electrochemical energy systems [8], as well as for electrochemical catalysis and surface-enhanced Raman scattering (SERS) [9].

Dealloying is a very reliable method for preparing porous metals such as gold, in which Ag-Au alloys immersed in nitric acid are dealloyed under free corrosion conditions [10]. The sequential elemental dealloying approach has also been used for the fabrication of porous metal oxides, whose sensors have been exploited for electronic listening [11]. Copper dealloying has also been applied with binary alloys like Mn–Cu [12],Cu–Zr [13], Mg-Cu [14], Al-Cu [15], Ti-Cu [16], and also ternary alloys such as Al-Cu-Mg [17], Ti–Cu–Ag [18] to prepare porous copper. The porosity is dependent on such conditions as potentiostatic dealloying and free corrosion.

N-substituted 1,2,3-triazoles have been prepared through click synthesis in the presence of a Cu (I) catalyst [19]. In addition, a microwave-assisted click chemistry synthesis of 1,4-disubstituted 1,2,3-triazoles via a copper (I)-catalyzed three-component reaction [20] as well as, a synthesis of 1,4-disubstituted 1,2,3-triazoles from in situ generated azides [21], as well as a regioselective synthesis of 1,4,5‐trisubstituted 1,2,3‐triazoles via copper‐catalyzed azide–alkyne cycloaddition has been reported [22]. Furthermore, multicomponent click synthesis of 1,2,3-triazoles from epoxides has been demonstrated to be catalyzed by copper nanoparticles on activated carbon [23]. Selective catalytic reduction of nitro compounds into their amine counterparts is of great value in organic synthesis [24]. The reduction of (i) p-nitrophenol and (ii) hexacyanoferrate (III) by borohydride ions is used as catalytic model reactions for evaluating the catalytic activity of nanocatalysts. It is worth noting that, no reaction should take place in the absence of nanoparticles [25]. Several catalysts have been utilized for this transformation including Raney nickel [26], molybdenum hexacarbonyl and DBU under microwave irradiation [27], carbon monoxide and water in the presence of the PdCl2/Fe/I2 system [28], surface modification of TiO2 nanoparticles [29], zeolite‐supported copper nanoparticles with 2‐propanol as a sustainable reducing agent [30], graphene-ZnO–Au nanocomposites [31], magnetic core–shell nanocatalyst [32], palladium supported on magnetic nanoparticles modified with carbon quantum dots [33], COF-Supported Co/Co(OH)2 nanoparticles [34], hydrazine-Fe3O4 [35], Fe3O4@sepiolite-Pd2+ [36], and supported gold nanoparticles [37]. Furthermore, the reduction of carbonyl compounds to the corresponding alcohols is considered an important transformation in the synthesis of biologically active compounds [38], with several catalysts such as Ni nanoparticles [39], zeolite supported Cu nanoparticles [40], Raney Ni–Al alloy and Al powder [41] and ball milling with NaBH4 [42] having been proposed for this purpose. Given the importance of these transformations in organic synthesis, there is a need for easier, simpler and more efficient methods that overcome the disadvantages. Considering the great versatility of porous and nano-sized copper[43], we decided to utilize the synergic effect of its high surface area and its splendid activity in the catalysis of organic reactions including azide-alkyne cycloaddition reaction, reduction of nitrobenzenes to anilines, and reduction of aromatic aldehydes to benzyl alcohols [44].

Results and discussion

In the first step, nanofibrous copper (0) was prepared via controlled etching of Cu–Zn alloy by NaOH solution. The etching creates porous sites in the metal texture, accessible by the reactants, and increases the surface area; thus, increasing the reaction rate by providing a greater catalyst- reactant interface. Terminal atoms of interfaces in heterogeneous catalysts have been demonstrated to have higher energy, resulting in a stronger interaction between the catalyst surface and reactant molecules [45]. To verify the structure of the nanocatalyst, several characterization techniques were used including SEM, TEM, XRD, BET and EDS analyses. The SEM images of the nanofibrous copper and the Cu–Zn alloy were recorded to allow a comparison of the surface structure of the metal to be made (Fig. 1). The Cu metal has a very smooth and solid texture before etching. After dealloying of Cu–Zn alloy, the Cu nanofibrous framework stays and the Zn is etched. Accordingly, the porous structure is formed, which is apparent in SEM images. Comparing the Cu–Zn alloy and nanostructured Cu, the difference in the surface texture can be easily detected. It can be seen that nanostructured Cu has a porous structure resembling that of a woven fabric, and can provide ample space for the starting materials to accumulate. The porous nature of the copper catalyst was also examined through transmission electron microscopy (TEM). As can be vividly seen in the TEM image, the metal particles are very transparent, indicating their porous nature (Fig. 2).

Fig. 1
figure 1

The SEM images of Cu–Zn alloy before etching (upper; Zn = 92.3 wt%, Cu = 7.7 wt%) and the structure of nanofibrous copper (0) (lower; Zn = 8.8 wt%, Cu = 91.2 wt%)

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Fig. 2
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The TEM images of nanofibrous copper (0)

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The BET test was also conducted to evaluate the gas adsorption capacity of the porous catalyst (Fig. 3). According to the BET analysis, the surface area was calculated to be 8.9 m2/g. It can be concluded from the BET and SEM analyses that the copper texture changed into a fibrous structure form, which can prove more beneficial in providing a greater surface area than solid metal as the catalyst in the chemical reactions.

Fig. 3
figure 3

BET analysis of nanofibrous Cu (0)

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In the next step, the XRD pattern of the nanoprous copper was recorded (Fig. 4). According to the Debye‐Scherrer equation, the crystallite size was calculated to be 98.7 nm for the peak at 2θ = 43.4°.

Fig. 4
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The XRD pattern of nanofibrous copper (0)

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It should also be pointed out that the dealloyed content was considerable according to the EDS analysis (Fig. 5). Before dealloying the Cu–Zn alloy had 92.3% of Zn content compared to 8.8%. after dealloying.

Fig. 5
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The EDS analysis of a Cu–Zn alloy, and b Nanofibrous copper (after dealloying)

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After the characterization of the nanofibrous copper, it was decided to utilize this catalyst in the one-pot preparation of 1,2,3-triazole derivatives via azide-alkyne cycloaddition reaction. The reaction was conducted with a mixture of phenyl acetylene (1 mmol), sodium azide (1 mmol), epoxide/alkyl chloride (1 mmol), nano-porous copper (0.05 g) and distilled water (5 mL) under reflux conditions (Scheme 1). The reaction was found to be completed after 6 h, and the corresponding product namely 2-phenyl-2-(4-phenyl-1H-1,2,3-triazol-1-yl) ethan-1-ol was isolated.

Scheme 1
scheme 1

Model reaction for the one-pot preparation of 1,2,3-triazole derivatives. Conditions: 80 °C, 6 h

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Delighted with this result, we decided to prepare the similar products using the same method. As a result, a series of the 1,2,3-triazole derivatives were prepared (Table 1). According to the results, it was found out that the sole product in the ring opening of the epoxides, is the product which was prepared by the nucleophilic attack to the less hindered position of the epoxide ring except for styrene oxide.

Table 1 One-pot preparation of 1,2,3-triazole derivatives
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After the satisfactory results obtained from the synthesis of 1,2,3-triazole derivatives, we tried to broaden the scope of catalytic activity of the catalyst; thus the reduction reactions were selected as model reactions. In reduction reactions, metal catalysts are utilized to enhance the reaction rate. Metals facilitate the electron and hydrogen transfer on their surface. Given the greater surface area provided by the nanofibrous Cu, it was considered a logical choice for selection as a good catalyst in this reaction. Accordingly, the reduction of nitrobenzenes to anilines was chosen as a benchmark. As a model reaction, nitrobenzene (1 mmol), sodium borohydride (2 mmol), nano-porous copper (0.01 g) and ethanol (5 mL) were heated at reflux conditions. Surprisingly, after 30 min, the reaction was complete and aniline was obtained. It should be noted that the reaction produced negligible yields after 2 h in the absence of the catalyst. Afterwards, a series of nitrobenzenes were converted into their corresponding aniline derivatives (Table 2).

Table 2 Reduction of nitrobenzene derivatives into anilines, in the presence of nanofibrous Cu (0)
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In a similar vein, nanofibrous Cu was used to prepare benzyl alcohols from aromatic aldehyde derivatives (Table 3). Easy post-reaction recovery separation and catalyst reusability are among the great advantages of heterogeneous catalysts. In this regard, after completion of nitrobenzene reduction reaction, the catalyst was removed by simple filtration, washed with ethanol (5 mL) and then reused seven times without significant loss of activity (Fig. 6).

Table 3 Reduction of aromatic aldehydes into benzyl alcohols in the presence of nanofibrous Cu (0) under reflux conditions
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Fig. 6
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The reusability of catalyst in nitrobenzene reduction, Conditions: 80 °C, 30 min

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To assess the stability of the catalyst [47], The conversion versus the reaction time was plotted for the nitrobenzene reduction reaction in the presence of the catalyst. It was obvious that the deactivation of the catalyst occurs in the following runs due to the change in the apparent rate constant in the consecutive experimental runs (Fig. 7).

Fig. 7
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Assessment of the stability of the catalyst as an indication of the progress of deactivation (e.g. the graph was drawn by the comparison of UV–visible absorption spectra for the reduction of 4-nitrophenol to 4-aminophenol. the disappearance of the absorbance of the reactant at λmax = 400 nm was used as the benchmark)

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To demonstrate the superiority of the catalytic performance of nanofibrous copper over the reported catalysts, the reduction of nitrobenzene to aniline was regarded as a representative example (Table 4). While in the majority of cases (except for entry 4), comparative yields of the desired product were obtained following the nanofibrous copper-catalyzed procedure, the reported procedures required long reaction time periods (entry 1), or high catalyst loading (entries 1–4). These results clearly demonstrate that nanofibrous copper is an equally or more efficient catalyst for this reaction.

Table 4 Comparison of nanofibrous copper (0) with reported catalysts in the reduction of nitrobenzene to aniline
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Experimental

General

All commercially available chemicals were purchased from Sigma-Aldrich (USA) and Merck (Germany) companies and used without further purification. Products were characterized by their physical constant and comparison with authentic samples. Reaction monitoring was accomplished by TLC on silica gel polygram SILG/UV 254 plates. The IR spectra were recorded on a BOMEM MB-Series 1998 FT-IR spectrophotometer using KBr pellets for the samples and the catalyst in the range of 4000–400 cm−1. 1H and 13C NMR spectra were recorded in DMSO-d6 on a Bruker 250 MHz spectrometer using TMS as the internal standard. Thermal stability of the supported catalyst was examined by BÄHR, SPA 503 Thermo-gravimetric Analyzer (TGA) at heating rate of 10 °C min−1 over the temperature range of 40–950 °C. TEM images were recorded by Zeiss-EM10C-100 kV. SEM images were recorded by ZEISS-FESEM-Sigma VP. XRD analysis was recorded by Panalytical X’ Pert Pro. BET analysis was performed by Quantachrome.

General procedure for the preparation of nanofibrous Copper

In a 250 mL round-bottom flask, 0.5 g of Cu–Zn alloy was mixed with 0.5 g NaOH and 100 mL of distilled water at room temperature for 24 h. Afterwards, the mixture was filtered and washed with distilled water for several times. The nanofibrous copper precipitate was dried at 60 °C for 24 h.

Typical procedure for the preparation of 1,2,3-triazole derivatives

A mixture of phenyl acetylene (1 mmol), sodium azide (1 mmol), epoxide/alkyl chloride (1 mmol), nano-porous copper (0.05 g, 80 mol%) and distilled water (5 mL) was heated at reflux conditions. Completion of the reaction was indicated by TLC [acetone/n-hexane (3:10)], following which the catalyst was filtered, and the product was extracted using ethyl acetate. The organic layer was concentrated, and the product was obtained by evaporating the solvent in a rotary evaporator. Further product purifications can be done via recrystallization in an ethanol–water mixture or in the case of oily substances via column chromatography.

Typical procedure for the reduction of nitrobenzenes to anilines

A mixture of nitrobenzene derivative (1 mmol), sodium borohydride (2 mmol), nano-porous copper (0.01 g, 15 mol%) and ethanol (5 mL) was heated at reflux conditions. After completion of the reaction, as indicated by TLC [EtOAc/n-hexane (1:4) or GC, the catalyst was filtered and the product was obtained after evaporation of ethanol. Further product purifications can be done via recrystallization in an ethanol–water mixture or in the case of oily substances via column chromatography.

Typical procedure for the reduction of aromatic aldehydes into benzyl alcohols

A mixture of aromatic aldehyde (1 mmol), sodium borohydride (2 mmol), nano-porous copper (0.01 g, 15 mol%) and ethanol (5 mL) was heated at reflux conditions. Completion of the reaction was monitored by TLC [ethyl acetate/n-hexane (1:4)] or GC, following which the catalyst was filtered, and the product was obtained after evaporation of ethanol. Further product purifications can be done via recrystallization in an ethanol–water mixture or in the case of oily substances via column chromatography.

2-Phenyl-2-(4-phenyl-1H-1,2,3-triazol-1-yl)ethanol (Table 1, Entry 1): Pale yellow solid; 1HNMR(400 MHz, CDCl3) δ 8.22 (s, 1H;NCH = C), 7.31–7.78 (m, 10 H; ArH), 4.71–4.84, 4.21–4.34 (2 m, 2H; CH2), 3.72 (dd, 3JH,H = 8.7 Hz, 3 J = 4.7 Hz, 1H; CHCH2), 3.36 (wide peak 1H, OH); 13C NMR (100 MHz, CDCl3) δ 147.8 (NCCH), 138.1, 131.9, 129.9, 129.8, 129.5, 128.9, 128.2, 126.4, 121.6 (NCCH), 67.6 (CHCH2), 64.7 (CH2); IR (neat, cm−1) ν 699, 765, 1050, 1073, 1124, 1275, 1379, 1456, 1497, 2939, 3032, 3087, 3123, 3478.

Conclusion

Nanofibrous copper was prepared using a simple and straightforward method that involved the dealloying of Cu–Zn alloy in an alkaline solution. This nanoporous catalyst proved useful in azide-alkyne cycloaddition reactions, reduction of nitrobenzenes to anilines, and reduction of aromatic aldehydes to benzyl alcohols. Given the satisfactory results obtained from these reactions, it can serve as an alternative nanostructured catalyst for similar organic reactions as well as functional group transformations. This catalyst showed very good results including high product yield, short reaction time and recyclability.