1 Introduction

The various metal/metal oxide loaded mesoporous silica nanostructures have attracted several attentions during the past decades especially in the field of heterogeneous catalysis [1, 2]. In addition, the application of mesoporous silica structure is familiar in various scientific fields such as drug delivery, drug release [3, 4], gene delivery [5], biosensing [6], and as adsorption media for removing of environmental pollutants [7, 8]. The wide range of mesoporous silica nanostructures with various morphologies, sizes, pore structures and pore sizes could be synthesized [9]. In addition, thanks to the mechanical, thermal and chemical stability, controllable synthesis, high adsorption capacity, facilitated diffusion of substances as well as the high surface area, mesoporous silica nanomaterials are known as one of the best catalyst supports. In addition to the various favorable features due to the presence of mesoporous channels, further improvement of mass transfer could be expected for hollow structures. Moreover, the enrichment of reactants in the cavities as well as the arrangement of active sites interior the hollow structure, the enhancement of the catalytic activity could be resulted [10,11,12]. In some cases, due to the specific role of the hollow catalyst, the whole structure could be regarded as a nanoreactor [13]. On the other hand, by restriction of the catalyst active sites into an outer shell of mesoporous silica, the chemical stability, dispersibility and also the more tolerance toward the unfavorable oxidation and aggregation phenomena could be resulted [14,15,16,17]. The synthesis of silica mesoporous structure could be done by applying the modified Stöber method in the presence of cetyltrimethylammonium bromide (CTAB) as a cationic surfactant [18, 19].

The hollow silica nanostructures could be synthesized using both hard and soft templates [9]. Carbon microspheres (CMS) could be introducing as suitable hard (solid) templates for synthesis of hollow silica structures [20]. The surface of carbon spheres is rich of hydroxyl and carboxyl functional groups and thus it is favorably hydrophilic [21]. In addition, the high interactions between CMS surface with metal cations could be induced by the presence of these functional groups [21]. Moreover, the removal of CMSs is facile and efficient by calcination under the air atmosphere. By applying CMSs as template, metal oxide hollow shells also could be synthesized by the one pot synthesis procedure using CMS as sacrificial template. To this reason, a solution containing certain metal salt and the carbohydrate precursor is treated under the optimized hydrothermal conditions. The product of the hydrothermal process is CMSs with outer hydrophilic shell occupied with the cations of the metal. The hollow metal oxide shells are achieved during the removal of carbon cores under the air atmosphere calcination process [22]. Several metal oxide hollow shells such as Fe2O3, MgO, CeO2, NiO, Co3O4 and CuO were reported to be prepared by this method [22].

1,2,3-Triazole compounds are known as valuable chemicals in various fields such as biologically active pharmaceutical, artificial receptors, photographic material, dyes, organic semiconductor, electroluminescent material, and corrosion inhibitors [23]. As a common way for synthesis of triazole compounds, the Click reaction could be accelerated by copper species as catalyst [24,25,26,27,28]. Although Cu(I) species are known as common catalysts for Click reaction, some reports were focused on the Cu(0) nanoparticles [29, 30], Cu(II) compounds [31, 32] and even Cu based bimetallic catalysts [33]. The Cu catalysis of Click reaction was also reported using various nanostructures such as CuO hollow nanostructures [34], CuO nanowires [35] and nanoporous Cu structures [36]. However, novel Cu catalysts by using various substrates such as MOFs [37, 38], activated carbon [39, 40], zeolite [41], agarose [42], and etc. have been reported. Specially, using silica as support, several CuO/SiO2 catalysts were reported for applications in several gas phase and liquid phase important reactions such as the production of fatty alcohols from natural oil [43], methanol conversion and hydrogen production [44], hydrogenation of reactants to alcohols [45, 46], gas phase synthesis of ethanol via syngas [47], and etc.

Following our previous researches in the field of heterogeneous catalysis of the main organic reactions [27, 48,49,50,51,52,53,54,55,56,57,58,59], the catalyst based on CuO mesoporous silica was prepared for catalyzing of 1,2,3-triazole production via the cycloaddition reaction. The unique structural feature of catalyst were characterized via several techniques and some catalytic aspects such as reactivity enhancement, recyclability and also the heterogeneous action of CuO based catalyst have been studied.

2 Experimental

2.1 Materials and Apparatus

The phase analysis of catalyst was studied using XRD with a Bruker D8 ADVANCE diffractometer. NMR spectra were obtained applying Bruker AMX 300 MHz instruments. FE-SEM images were recorded using a TESCAN, Model: MIRA3 scanning electron microscope operating at an acceleration voltage of 30.0 kV. Elemental compositions were determined with an SC7620 energy-dispersive X-ray analysis (EDX) presenting a 133 eV resolution at 20 kV. Transmission electron microscopy (TEM) was performed with a Leo 912 AB (120 kV) microscope (Zeiss, Germany). All compounds were obtained from Merck in analytical grade and were used without any further purification.

2.2 Synthesis of Cu Included Carbon Microspheres

Here, for synthesis of CuO–HMSS, the Cu included carbon microspheres (Cu–CMS) was prepared at first via an optimized one step hydrothermal procedure. Practically, a 6 ml aliquot of solution of 1 M of Cu(NO3)2 was added to 30 ml solution of 0.5 M of glucose. The resultant mixture was reached to the volume of 90 ml by adding the doubled distilled water and was transferred into a Teflon lined stainless steel autoclave vessel. By adjusting a heating program, the precursor mixture was heated up from room temperature to 190 °C along 5 h and was kept at 190 °C for 24 h. Finally, the mixture was cooled to room temperature (25 °C). The resulting particles was filtered and washed three times with double distilled water and finally dried at 60 °C overnight (sample is denoted as Cu–CMS).

2.3 Synthesis of SiO2 Coated Cu–CMS

For synthesis of silica shell over the Cu–CMS, a modified Stöber method was used. Briefly, 0.5 g of as-synthesized Cu–CMS was well dispersed into the 25 ml of double distilled water. 15 ml of ethanol, 0.07 g of CTAB, 0.25 ml of 0.25% solution of NH4OH and 120 µl of TEOS were added and the resultant mixture was sonicated for 30 min and then stirred for 6 h at room temperature. Finally, the product was filtered and washed three times with double distilled water and ethanol and finally dried at 60 °C overnight.

2.4 Synthesis of CuO–HMSS Catalyst

The SiO2 coated Cu–CMS sample is converted to CuO–HMSS as a final product via two steps of calcination under Ar and air atmospheres, respectively. Along calcination step, by elimination of CMS template and CTAB as organic constituents, the mesoporous hollow structure of Cu containing silica is obtained. However, the oxidation of Cu species to CuO as well as the annealing of SiO2 shell are performed during the calcination procedure.

2.5 Synthesis of 1,2,3-Triazole Compounds

Cautions: because of the heat and shock sensitivity of sodium azide, the required caution should be exercised for prevention of explosion.

1 mmol of phenylacetylene, 1 mmol of alkyl halide/epoxide, 1.2 mmol of sodium azide, 0.02 g of CuO–HMSS and 3 ml of water were delivered into the round bottle flask and the mixture of reactants was heated at 80 °C until the reaction completeness. The CuO–HMSS catalyst was separated via centrifugation and was dried after washing with ethylacetate to be applied in the next runs. Here, some water insoluble or sticky products could be dissolve into the ethylacetate. Thus, this ethylacetate portion was added to the final extract of the reaction mixture to attain the complete recovery of product. After separation of CuO–HMSS catalyst, the product was extracted from the reaction mixture by using ethylacetate. Finally, the product was purified via the recrystallization procedure. All the products were confirmed by the spectroscopic method using 1H and 13C NMR. (See Supporting Information, Spectral data).

2.5.1 1-(4-Methylbenzyl)-4-phenyl-1H-1,2,3-triazole: (Table 2, Entry 2)

1H NMR (DMSO-d6, 300 MHz, δ, ppm): 2.29 (s, 3H), 5.60 (s, 2H), 7.18–7.30 (m, 4H), 7.30–7.47 (m, 3H), 7.84–7.87 (m, 2H), 8.62 (s, 1H). 13C NMR (75 MHz, DMSO-d6, δ, ppm): 21.1, 53.3, 121.7, 125.6, 128.3, 129.3, 129.8, 131.4, 133.5, 137.9.

2.5.2 1-(4-Bromobenzyl)-4-phenyl-1H-1,2,3-triazole: (Table 2, Entry 3)

1H NMR (DMSO-d6, 300 MHz, δ, ppm): 5.66 (s, 2H), 7.20–7.35 (m, 3H), 7.41 (t, 2H, J = 9.0 Hz), 7.56 (d, 2H, J = 9.0 Hz), 7.81 (d, 2H, J = 6.0 Hz), 8.82 (s, 1H). 13C NMR (75 MHz, DMSO-d6, δ, ppm): 52.6, 121.8, 122.3, 125.6, 128.4, 129.4, 129.8, 130.8, 131.1, 132.1, 135.6, 147.0.

2.5.3 1-(4-Nitrobenzyl)-4-phenyl-1H-1,2,3-triazole: (Table 2, Entry 4)

1H NMR (DMSO-d6, 300 MHz, δ, ppm): 5.86 (s, 1H), 7.31–7.36 (m, 1H), 7.45 (t, 2H, J = 7.2 Hz), 7.58 (d, 2H, J = 8.7 Hz), 7.81 (d, 2H, J = 9.0 Hz), 8.25 (d, 2H, J = 9.0 Hz), 8.71 (s, 1H). 13C NMR (75 MHz, DMSO-d6, δ, ppm): 52.6, 122.4, 124.4, 125.5, 128.4, 129.3, 130.0, 143.8, 147.7.

3 Results and Discussion

The Cu included carbon microsphere (Cu–CMS) was synthesized at first via the one step hydrothermal procedure. Then, a protective shell of SiO2 in the presence of CTAB (via the modified Stöber method) is formed over the Cu–CMSs. Finally, by elimination of CMSs and CTAB residuals (as organic constituents) via the air calcination, the novel hallow mesoporous silica particles—containing cavities including CuO species (CuO–HMSS)—was resulted. The above mentioned steps were presented in Scheme 1.

Scheme 1
scheme 1

Different steps of preparation of CuO–MSS catalyst using Cu–CMS template

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Figure 1 represents the SEM micrographs of Cu–CMS (micrograph a) and CuO–HMSS (micrograph b, c). The regular solid spheres of Cu–CMS with the smooth surfaces are observable in Fig. 1a. In the case of CuO–HMSS, due to the application of CMSs as a hard template for synthesis of HMSS structures, the approximately uniform size distribution of particles with average diameter around 1.3 µm is achievable. In contrast to Cu–CMS, the surface of CuO–HMSS particles is appeared to become roughened which could be referred to the SiO2 shell morphology. Additionally, large cavities on some particles in the image (specified by arrows) are remarkable. However, due to improper position of other particles in the image, some cavities could not be observed.

Fig. 1
figure 1

SEM image of a Cu–CMS, b, c CuO–MSS with two magnifications. The large cavities on CuO–MSS particles were signified by using the arrows

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For better clarification of the structure of CuO–HMSS, the TEM micrograph was also studied. Figure 2a, b shows the corresponding TEM images with two magnifications. Similar to SEM micrographs, the approximately narrow size distribution of spherical particles with diameter around 1.3 µm is observable. However, some cavities are observable which are specified by circles in the Fig. 2a. Additionally, the mesoporous structure of SiO2 could be clearly distinguished from the magnified image represented in Fig. 2b.

Fig. 2
figure 2

TEM monographs of CuO–MSS nanostructures with two magnifications (a, b). The round circles in a, remark the cavities interior the CuO–MSS nanostructures. The mesoporous structure also could be concluded from the magnified image (b)

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Using the BET analysis (the adsorption–desorption isotherm in Fig. S1) of CuO–HMSS catalyst, the BET surface area of 141.8 m2 g−1 was obtained. The investigable type IV isotherm as well as the hysteresis shape could be referred to the presence of mesoporous structure. Additionally, the adsorption average pore width was obtained to be 3.0 nm. Thus according to nitrogen adsorption studies as well as TEM analysis, it can be concluded that, the silica structure is mesoporous in nature (See Supporting Information, Fig. S1).

Figure 3 represents the XRD spectra of CMS template (spectrum a), Cu–CMS before any calcination process (spectrum b) and the final product CuO–HMSS after the calcination and removal of CMS template. The characteristic peaks of Cu species are obvious in diffractograms b and c. According to JCPDS #00-004-0836, the diffraction peaks at 2θ values of 43.3, 50.4 and 74.1 in diffractogram b, could be referred to (111), (200) and (220) planes of Cu(0), respectively. The reduction of metal salt and the stabilization of Cu(0) species is done by reductive nature of glucose precursor which is occurred during the hydrothermal production of CMS. The diffraction peaks in diffractogram c could be attributed to CuO, according to JCPDS #00-045-0937. Herein, during the calcination under the air atmosphere, the conversion of zero-valent Cu species to CuO could be resulted. It means that after the calcination step, the CuO is the dominant form of copper in the catalyst.

Fig. 3
figure 3

XRD spectra of (a) CMS, (b) Cu–CMS, and (c) CuO–MSS samples

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The elemental composition of catalyst was also characterized via the EDX technique. According to ICP analysis, the weight percentage of 9.75% was obtained for Cu as the active component of the catalyst (See Supporting Information, Fig. S2).

Additionally, according to elemental mapping data in Fig. 4, the dispersion of Cu species is approximately homogeneous over the particles and no aggregated Cu species was distinguished. In addition, the presence of SiO2 shell could be resulted from the association of Si and O atoms over the selected particle.

Fig. 4
figure 4

Elemental map of CuO–MSS, a for O, b for Si, c for Cu, d the corresponding SEM area

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The catalytic performance of the CuO–HMSS was examined for the synthesis of 1,4-disubstituted-1,2,3 triazole. In order to achieve the most suitable conditions for the designed protocol, reaction between benzyl bromide, phenylacetylene and sodium azide was chosen as a model reaction and was studied considering various parameters such as the solvent and the amount of catalyst (Table 1). As shown in Table 1, H2O was found to be the most effective solvent which provides the fast reaction rate and also the high product yield (Table 1, entry 6). In the presence of other solvents such as CH3CN, THF, DMF, EtOH and H2O/EtOH, the longer reaction completion times and the lower yields were obtained (Table 1, entry 1–5). During our optimization studies, various amounts of catalyst were tested and results showed that the reaction proceeds with excellent yield when the amount of 0.02 g of the catalyst was utilized (Table 1, entry 6). The further increase in the amount of CuO–HMSS catalyst just had negligible effects on the efficiency of the reaction (Table 1, entry 7). In the absence of any catalyst no progress in reaction was observed. Furthermore, for catalyst values below 0.02 g, the lower yields and the longer reaction times have been recorded.

Table 1 Reaction conditions for optimization of click reaction catalyzed by CuO–HMSS
figure b
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As shown in Table 2, under the optimized conditions, we investigated the reaction of epoxides or alkyl or benzyl halides with sodium azide and phenylacetylene. The results of Table 2 indicate that cycloaddition reaction with various benzyl halides bearing either both electron-donating and with-drawing substituents were completed in a short time with excellent yields (Table 2, entry 1–5). However, in the case of electron-donating group, the rate of reaction was increased (Table 2, entry 2). Alkyl halides such as 1-chlorohexane, 2-bromoethyl benzene, 3-bromo-2-methylprop-1-ene and 1-bromo-2-methylpropane were also converted in high yields to the desired products. However, in comparison with benzyl halides, longer reaction times were required (Table 2, entry 6–9). Next, we explored the synthesis of symmetrical bis-triazoles starting from the 1,2-dibenzyl bromide, by the successive 1,3-dipolar cycloaddition reactions with phenylacetylene. The satisfactory yields were also obtained for synthesis of symmetrical bis-triazoles (Table 2, entry 10). Then the reactivity of substituted epoxides with sodium azide and phenylacetylene was studied. The aliphatic epoxides react relatively fast and provided the corresponding products with high yields. Other epoxides such as 2-((4-allyl-2-methoxyphenoxy)methyl)oxirane and 7-oxabicyclo[4.1.0]heptane were also react with sodium azide and phenylacetylene to produce corresponding products with the yields of 80 and 85%, respectively (Table 2, entry 11–16).

Table 2 The yields and reaction completeness time for CuO–HMSS catalyzed Click reaction
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The recyclability of CuO–HMSS catalyst was also studied. For this reason, after the termination of each synthesis run, the CuO–HMSS catalyst was separated, washed and dried to be applied in the next synthesis run. This procedure was repeated successively for seven runs of synthesis. Figure 5 shows the SEM of CuO–HMSS catalyst after the seven runs of triazole synthesis reaction. It shows that the catalyst framework was preserved during the seven runs of reaction. In addition, according to ICP data, the Cu content of recycled catalyst (after seven runs) was determined to be 9.35%. Conclusively, just the loss of 4.5% of Cu as active ingredient after seven runs of reaction was recognized. This remarkable stability could be referred to the unique structure of the catalysts which is composed of a protective thick shell of mesoporous silica. As stated before, due to the mesoporous nature of SiO2 as well as the presence of cavities in catalyst structure, the mass transfer to active sites is facilitated regardless of the thickness of the silica shell.

Fig. 5
figure 5

The SEM image of CuO–MSS catalyst after recycling for seven runs of cycloaddition reaction

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The CuO–HMSS catalyst performance during the successive cycles was represented in Fig. 6. Clearly, the loss of reactivity in the presence of catalyst is favorably low even at 7th run of reaction.

Fig. 6
figure 6

Recyclability of CuO–MSS catalyst for the model reaction of phenylacetylene with benzyl bromide and sodium azide

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To approve the heterogeneous nature of the Cu–HMSS for catalysis of cycloaddition reaction, the hot filtration test was performed. Briefly, after 50% progress of reaction, the catalyst was separated from the reactants vessel and then was allowed to proceed for excess 1 h. Here, in the absence of any CuO–HMSS catalyst, no progress in the reaction was observed. According to this observation, it could be resulted that the tested Click reaction over the CuO–HMSS catalyst, was proceeds via the heterogeneous route (Supporting Information, Fig. S3).

4 Conclusion

Here, the novel heterogeneous nanocatalyst based on CuO loaded mesoporous SiO2 was prepared for synthesis of 1,2,3 triazoles via the Click reaction in water. The favorable structural features such as microporosity, presence of cavities in catalyst spheres and the excellent dispersion of CuO species were characterized. In addition, the high chemical and mechanical stability was obtained for CuO–HMSS catalyst under the optimized reaction condition. Conclusively, the CuO–HMSS catalyst could be suggested for catalyzing of cycloaddition reaction for preparation of 1,2,3-triazole compounds under the satisfactory mild condition.