Synthesis of a magnetic-based yolk-shell nano-reactor: A new class of monofunctional catalyst by Cu0-nanoparticles and its application as a highly effective and green catalyst for A3 coupling reaction

Zeinab Elahimehr; Firouzeh Nemati; Ali Elhampour

doi:10.1016/j.arabjc.2018.11.011

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Original article

13 (

); 3372-3382

doi:

10.1016/j.arabjc.2018.11.011

Synthesis of a magnetic-based yolk-shell nano-reactor: A new class of monofunctional catalyst by Cu⁰-nanoparticles and its application as a highly effective and green catalyst for A³ coupling reaction

Zeinab Elahimehr, Firouzeh Nemati^⁎, Ali Elhampour

Department of Chemistry, Semnan University, Semnan, Iran

⁎Corresponding author. fnemati@semnan.ac.ir (Firouzeh Nemati)

Received: 2018-9-3, Accepted: 2018-11-11, Published: 2018-01-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Abstract

A novel and yolk/shell nanoreactor catalyst (H-Fe₃O₄@h-Cu⁰@m-SiO₂) was designed and synthesized with a hollow magnetite core encapsulated in a mesoporous silica shell which Cu⁰-nanoparticles were decorated in the interior cavity shell in order to enhance the catalytic activity. The morphology, structure and physicochemical properties of the yolk/shell nanoreactor were then fully characterized by using high resolution transmission electron microscopy (HRTEM), field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction patterns (XRD), Fourier transform infrared spectroscopy (FT-IR). The N₂ absorption-desorption isotherms indicate a Brunauer–Emmett–Teller (BET) specific surface area of 287.36 m²/g and a pore width of 2–50 nm. Applied as a catalyst for three-component A³ coupling reaction of alkynes, aldehydes and amines under the optimized condition. Interestingly, the characterization results indicate that (H-Fe₃O₄@h-Cu⁰@m-SiO₂) has great catalytic performance, which could be related to unique hollow morphology with an active core contains Cu⁰-nanoparticles, protective mesoporous shell and hollow void. This aspect of morphology leads to favoring the transfer of the reactants and products. Reusability up to 10 times without any obvious decline in catalytic activity and short reaction time are other advantageous of this green nano-reactor.

Keywords

Hallow sphere nanoparticles

Porous shell

Yolk/shell nanoreactor

Coupling reaction

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1 Introduction

Hollow nanoparticles with functionalized interior space, specially core@void@shell structures that frequently called yolk-shell nanoreactors (YSN), have been known as an ideal candidate for catalyst applications due to their unique properties, including low density, large surface area, free void space between the core and porous shell and excellent loading capacity in the void space.

The yolk-shell nanostructures (YSNs), which consists catalyst species inside the cavity supplemented with a porous nano shell are a promising skeleton for the nanoreactor, which can tackle the issues of conventional nanoparticle-based catalyst. Among them nanoparticle-based nanoreactors have attracted considerable attention due to both scientific and application perspectives. Avoiding the aggregation, greater selectivity, easy recovery from the effluent, high stability, and eventually avoiding to significant decreases in surface area and catalytic activity are reasons of this interesting researches (Lee et al., 2014; Priebe and Fromm, 2015; Yang et al., 2012; Zhou et al., 2017 ).

Accessible pore channels of the mesoporous materials within high surface area and hollow void is another important factor to effect the catalytic performances of a yolk/shell nanoreactor catalyst which are beneficial for the diffusion and direction of transportation the starting materials and products. Functionalization of mesoporous silica nanomaterials has been proved because of good chemo and thermal stability as a shielding factor to protect the active sites from the harsh reaction conditions, and facilitating efficient transportations of starting materials and products for promoting various organic transformations under the green condition (Long et al., 2017; Yue et al., 2017; Zhou et al., 2017 ).

The most important feature in many catalyst systems is recovery and recycle after reaction completion without a meaningful reduction of the catalytic ability. So, the magnetic separation could be a good preference, which also is facilitated the elimination of the long time separation procedures such as filtration or centrifugation. Recently, excellent properties of hollow magnetic microspheres (H-F₃O₄) have attracted great interest because of their magnetic condensation, low density, high specific surface, large surface permeability, thermal stability and so. Accordingly, among different yolk-shell nanoparticles, YSNs that contains a magnetic core encapsulated in hollow mesoporous shell, due to high stability, biocompatibility, and significant magnetic strength of iron oxides, are in demand catalyst supports (Do et al., 2017; Hussain and Das, 2017; Liu, 2018; Nabid et al., 2014, 2015 ).

Recently, employing metal nanoparticles (NPs) as a benign and competitive alternative to common catalysis, due to high surface-to-volume ratio, has significantly increased (Baran, 2018b, 2018a; Baran et al., 2018; Nasrollahzadeh et al., 2015b, 2015b ). However, the main difficulty in the use of metal NPs such as Co, Fe and Cu comes from their susceptibility toward leach, sinter, and oxidation during catalyst reaction, which are resulted the catalytic activity was lost. Therefore, to fully exploit the interior space of the hollow nanoreactor, the selectivity functionalized interior cavity by encapsulating catalytic active species (metal NPs) by a porous shell, is the most vital component which allows chemical transformations to happen inside the area of the protected cavity in environmentally acceptable conditions. Recently, different yolk/shell nano-catalysts based on Cu⁰-nanoparticles in interior cavity of nanoreactor were reported, which have the potential to be used in a wide range of application because more desirable compared to the precious metals, owing to important advantages such as high natural abundance and low cost of copper NPs (Borah et al., 2014; Do et al., 2017; Hao et al., 2016; Nabid et al., 2014; Zhang et al., 2016 ).

Three-component reaction of terminal alkynes, amines and aldehydes is one of the most powerful and useful synthetic tools for the synthesis of propargylamine derivatives. The propargylamines have played a remarkable role as starting materials for the manufacture various of chemicals such as agricultural chemicals, polymers, natural products such as β-lactams, herbicides, fungicides, and pharmaceutical products for the cure of some diseases such as Parkinson’s and Alzheimer’s disease (Hussain and Das, 2017; Mirhosseyni et al., 2016). In view of the importance of the propargylamines and in continuous of our pursuits for designing new composites (Elhampour et al., 2018; Elhampour and Nemati, 2017; Rangraz et al., 2018a, 2018b; Sabaqian et al., 2017, 2018; Shahamat et al.,) here in, we reported a magnetic yolk/shell nano-reactor, which consists significant impact of Cu⁰-nanoparticles were used as activity center for synthesize of propargylamines with excellent yields, reducing reaction times and also minimize the undesired products with very excellent reusability under solvent-less condition.

2 Experimental section

2.1

2.1 Materials

Iron (III) chloride hexahydrate (FeCl₃ 6H₂O), Sodium acetate (NaOAc·3H₂O), Ethylene glycol (EG), Polyvinylpyrrolidine (PVP), Tetraethyl orthosilicate (TEOS), (3-aminopropyl) triethoxysilane (APTES), Copper(II) acetate (Cu(CH₃COO)₂), Sodium borohydride (NaBH₄), Cetyl trimethylammonium bromide (CTAB), Sodium carbonate (Na₂CO₃), Acetic acid (CH₃COOH) and Ammonia (NH₃) with analytical grade reagents, bought from Sigma-Aldrich. No additional purification was done.

2.2

2.2 Synthesis of hollow-Fe₃O₄ (H- Fe₃O₄)

Hollow-Fe₃O₄ particles were synthesized according to our earlier report. Typically, ethylene glycol (EG, 40 mL), PVP (1.0 g), ferric chloride hexahydrate (FeCl₃·6H₂O, 5 mmol) and then 30 mmol of sodium acetate (NaOAc) were added and stirred for 15 min. Thereafter, the obtained solution was transferred into a Teflon-lined stainless steel autoclave (100 cm³). The autoclave was heated to 200 °C and remained at this temperature for 12 h. After cooling the autoclave to the room temperature, the hollow-Fe₃O₄ spheres obtained as black solid. Ethanol and deionized water were used for washing the hollow-Fe₃O₄ particles and then were dried at 80 °C in an oven vacuum overnight (Mirhosseyni et al., 2016).

2.3

2.3 Synthesis of H-Fe₃O₄@SiO₂

1 g of H-Fe₃O₄ particles were dispersed in a mixture of ethanol (60 mL, 99%), water (20 mL) and concentrated ammonium hydroxide (1.5 mL, 28%). The ultrasonic vibration in a water bath was used for homogenizing of the suspension. Then, a solution of tetra ethyl orthosilicate (TEOS) (0.5 mL) in ethanol (10 mL) was added to the mixture drop by drop under continuous stirring for 12 h. Finally, the resulting silica-coated H-Fe₃O₄ were collected by magnetic separation and washed several times with ethanol to remove black silica nano-particles. Finally, the obtained H-Fe₃O₄@SiO₂ was dried in an oven overnight (Nemati et al., 2012).

2.4

2.4 Preparation of H-Fe₃O₄@SiO₂ functionalized (3-aminopropyl) triethoxy silane (H-Fe₃O₄@SiO₂-NH₂)

0.2 g of prepared H-Fe₃O₄@SiO₂ magnetic nanoparticles was dispersed in 40 mL of anhydrous ethanol under ultrasonic. After 15 min, 5 mL of (3-aminopropyl) triethoxy silane (APTES) was added. The mixture was heated at 80 °C for 24 h under N₂ atmosphere. Then, the functionalized MNPs were separated and washed several times with ethanol eventually and dried at 60 °C. This product was be referred as H-Fe₃O₄@SiO₂-NH₂ (Zhu et al., 2014b).

2.5

2.5 Synthesis of H-Fe₃O₄@SiO₂-Cu⁰

In 15 mL water, copper acetate Cu(OAc)₂ (0.0254 mmol) was dissolved and added to 0.5 g of H-Fe₃O₄@SiO₂-NH₂ magnetic nanoparticles (MNPs) under vigorous stirring for 6 h, drop by drop. In next step, the solvent was evaporated by a rotary evaporator, which 0.3 g of composite was obtained. Then, it was dispersed in 50 mL of water. The aqueous solution of NaBH₄ (134 mg, 3.54 mmol) was slowly added to the suspension of the above composite. The suspension was stirred for 15 min under nitrogen gas atmosphere. During this stage, Cu (II) was converted into Cu (0) nanoparticles (Cu⁰ NPs). After magnetic separation, the obtained H-Fe₃O₄@SiO₂-NH₂-Cu⁰ particles were washed with deionized water several times to remove impurities and then rested in an oven for 12 h (Borah et al., 2014).

2.6

2.6 Synthesis of H-Fe₃O₄@SiO₂-Cu⁰@SiO₂

0.1 g H-Fe₃O₄@SiO₂-Cu⁰ particles that obtained from the previous step were dispersed in 20 mL water and was applied to ultrasonic vibration for 15 min (part A). 150 mg of cetyltrimethyl ammonium bromide (CTAB) and NH₃·H₂O (28%, 0.55 mL) were poured into the mixture of 30 mL ethanol and 30 mL H₂O and then stirred at 30 °C in water bath (part B). After that, the part A and part B were mixed together and sonicated for 2 h. Next, TEOS (0.25 mL) was quickly added into the mixture, and was stirred further for 6 h. Finally, H-Fe₃O₄@SiO₂-Cu⁰@SiO₂ particles were separated from the suspension magnetically and dried in an oven at 70 °C overnight (Deng et al., 2008).

2.7

2.7 Synthesis of H-Fe₃O₄@hollow-Cu⁰@SiO₂ and H-Fe₃O₄@h-Cu⁰@m-SiO₂ yolk-shell nano-reactor

The H-Fe₃O₄@SiO₂-Cu⁰@SiO₂ core/shell particles were dispersed in 10 mL of deionized water. Then, 100 mg Na₂CO₃ was added into the above suspension during vigorous stirring. After that, it was stirred at 50 °C for 10 h, to remove the inner SiO₂ layer. Finally, the as prepared H-Fe₃O₄@h-Cu⁰@SiO₂ yolk/shell catalyst were collected by using a permanent magnet, washed with ethanol and water, and dried in an oven at 70 °C overnight. In order to synthesis of mesoporous and porous silica shell, the purified H-Fe₃O₄@h-Cu⁰@SiO₂ microspheres were re-dispersed in 100 mL ethanol until the formation of a black solution. Removing the template (CTAB) was done using addition of 1 mL glacial acetic acid and refluxing the mixture at 80 °C for 12 h. The removal procedure was repeated for two times. The H-Fe₃O₄@hollow-Cu⁰@mesoporous-SiO₂ (named H-Fe₃O₄@h-Cu⁰@m-SiO₂) yolk-shell nano-reactor was collected magnetically and washed with deionized water (Zhang et al., 2016; Zhu et al., 2014a).

2.8

2.8 Typical procedure for the synthesis of propargylamine derivatives

In a flask, the mixture of arylaldehyde (1 mmol), amine (1.2 mmol), alkyne (1.5 mmol) and nano-reactor (0.006 g) as a catalyst were added and stirred at 110 °C for a suitable time. Monitoring of the progress of reaction was done by TLC (n-hexane: ethyl acetate, 2:8). When the reaction was ended, the mixture was cooled and diluted with hot ethanol and stirred for some minutes. Afterwards, the catalyst was separated using a permanent magnet, washed with ethanol, and dried in the oven. Then, it re-used for a sequential run under the same reaction condition. The resulting remainder was purified by a short column chromatography on silica gel to acquire the desired product in high purity. All the products are known compounds and the spectral data and melting points were identical to those reported in the literature.

3 Result and discussion

3.1

3.1 Preparation of nano-reactor

Scheme 1 illustrates the five stages of the preparation of novel nano-reactor as catalyst. Briefly, H-Fe₃O₄ particles were assembled according to our recently reported procedure (Mirhosseyni et al., 2016) and the surface of hollow magnetic particles were covered with SiO₂ layer by a sol-gel method (Nemati et al., 2012). Boosting the interaction with Cu nano-particles through the coordination ligand with amino groups was achieved by modification of H-Fe₃O₄@SiO₂ micro-spheres with APTES (Zhu et al., 2014b). The Cu⁰ nanoparticles were immobilized on the exterior of H-Fe₃O₄@SiO₂-NH₂ and was covered by SiO₂ layers by means of the Zhang’s procedure using CTAB as template (Borah et al., 2014; Deng et al., 2008). Chemical etching of inner SiO₂ layer with Na₂CO₃ solution followed by extraction of template (Zhang et al., 2016; Zhu et al., 2014a), resulted H-Fe₃O₄@h-Cu⁰@m-SiO₂ yolk-shell nano-reactor as described in experimental section.

Schematic representation for preparation of H-Fe3O4@h-Cu0@mSiO2 yolk-shell nano-reactor.

3.2

3.2 Characterization of magnetic H-Fe₃O₄@h-Cu⁰@mSiO₂ yolk-shell nanoreactor

3.2.1

3.2.1 FT-IR spectra

The Fourier transform infrared spectroscopy spectra has been accomplished to check out the structure of the stages of synthesized H-Fe₃O₄@h-Cu⁰@mSiO₂ nano-reactor. As shown in the Fig. S1a the broad peak over the range of 3470 cm⁻¹ correspond to the OH stretching vibration that are available on the exterior of magnetic H-Fe₃O₄ particles. The absorption band observed at 850 cm⁻¹ is related to the stretching vibration of Fe-O-Fe in the spinel octahedral structure of H-Fe₃O₄, which shows decrease in intensity after encapsulation of silica on the magnetic particles (Liu, 2018; Mirhosseyni et al., 2016; Xiong et al., 2012 ). For all the silica-based materials, the strong and weak absorption bands at 1088 and 1009 cm⁻¹ were appeared, which arise from Si-O-Si asymmetrical stretching vibration and Si-O symmetrical stretching vibration, respectively (Fig. S1b–e) (Do et al., 2017; Zhou et al., 2017). Meantime, functionalization of particles with APTES was confirmed with the absorbance bands at 2976, 2939, 1568 and 1465 cm¹, which indicated the stretching modes of the -CH₂ and –NH₂ groups of APTES (Fig. S1c) (Khatiri et al., 2012; Zhou et al., 2017). The bands at 2976 and 1100 cm⁻¹ are correspond to aliphatic C-H and C-N bonds stretching vibrations in structure of CTAB, respectively (Fig. S1d), which were almost disappeared upon extraction (Fig. S1e). It proved nearly all the CTAB was extracted from the silica shell.

3.2.2

3.2.2 XRD patterns

The phase and combination of as-synthesized sample were thoroughly investigated by applying the XRD patterns. The crystalline cubic spinel structure of H-Fe₃O₄ particles was confirmed by reflections peaks at (2 2 0), (4 0 0), (4 2 2), (5 1 1), (4 4 0), and (5 3 3) (Fig. 1a) (Liu, 2018; Mirhosseyni et al., 2016). The wide reflection peak at 2θ = 20–30° was confirmed that the silica layer was successfully covered on the surface of magnetic particle (Fig. 1a) (Nemati and Saeedirad, 2013). The additional peak at 2θ = 50.3° which is assigned to the (2 0 0) indices of the metallic Cu. The metal Cu nanoparticles peaks at 43.3° and 73.99° overlap with the 43.2° and 74.3° peaks of H-Fe₃O₄. These results confirm the formation of copper nanocrystals only in zero oxidation state on the exterior of H- Fe₃O₄@SiO₂-NH₂-Cu⁰ NPs (Fig. 1b) (Borah et al., 2014; Do et al., 2017; Tang et al., 2014 ). As well as, the XRD pattern of H-Fe₃O₄@h-Cu⁰@mSiO₂ yolk-shell nano-reactor shows the same peaks, which coincides with H- Fe₃O₄@SiO₂-NH₂-Cu⁰ NPs and obviously not changed after coating with mesoporous silica. These results verify that the peak positions and the crystalline structures of H-Fe₃O₄ nanoparticles during the procedure of nano-reactor synthesis were kept unchanged. By applying the Scherrer's equation (D = kλ/βcosθ) the average crystalline size of the H-Fe₃O₄ particles is estimated to be 33 nm.

XRD patterns of H-Fe3O4 (a), H- Fe3O4@SiO2-NH2-Cu0 (b), H-Fe3O4@h-Cu0@mSiO2 (c).

3.2.3

3.2.3 Thermo gravimetric analysis

The thermogravimetric analysis (TGA), DTA and DTG analysis of H-Fe₃O₄@h-Cu⁰@mSiO₂ yolk/shell nanoreactor catalyst were evaluated and depicted in Fig. 2 and Fig. S2, respectively. TGA analysis was carried out at temperatures ranging from 30 to 800 °C under air flow. Three stages of mass loss were observed in the thermogram of nano-reactor. The initial lose in the range of 38–125 °C is related to the adsorbed water or organic solvents in the surface of nano-reactor. The second mass loss was occurred between 310 and 530 °C, which could be correspond to the remaining CTAB in the structure of composite. The third stage (530–740 °C) is due to dihydroxylation of silanol groups, which be done in the temperatures higher than 500 °C.

TGA and DTA diagrams of H-Fe3O4@h-Cu0@mSiO2.

Inaddition, the synthesized nano-reactor exhibits a considerable thermal stability at high temperature.

3.2.4

3.2.4 FE-SEM-EDX and TEM analysis

Fig. 3 shows the typical images of FE-SEM of H- Fe₃O₄@SiO₂-NH₂-Cu⁰ NPs and H-Fe₃O₄@h-Cu⁰@mSiO₂ Yolk-Shell nano-reactor. The FE-SEM images show that the materials have a clean spherical shape, well dispersed and uniform in morphology. Furthermore, the FE-SEM of H-Fe₃O₄@h-Cu⁰@mSiO₂ yolk-shell nano-reactor declared its mono disperse nanostructures. The energy dispersive X-ray spectroscopy (EDX) analysis of the H-Fe₃O₄@h-Cu⁰@mSiO₂ yolk-shell nano-reactor has been proved the presence of expected elements Fe, Si, Cu and O in synthesized nano-reactor (Fig. S3). The amount of Cu in the inner cavity shell of final catalyst was measured using ICP-AAS and it was 0.035%.

FE-SEM images of (a) H- Fe3O4@SiO2-NH2-Cu0 NPs, (b) H-Fe3O4@h-Cu0@mSiO2 yolk-shell nanoreactor.

The high resolution transmission electron microscopy (HR-TEM) obviously appears three distinct sections. The black cores of microspheres of H-Fe₃O₄, the small black spheres of Cu NPs and the gray mesoporous channels of SiO₂ which are aligned perpendicular to the microspheres surface of H-Fe₃O₄ as shell. The HR-TEM also displays the Cu nanoparticles dispersed in the inner surface of mesoporous silica shell (Fig. 4).

HR-TEM images of H-Fe3O4@h-Cu0@mSiO2 yolk-shell nano-reactor.

3.2.5

3.2.5 BET and BJH analysis

Fig. 5 exhibits the N₂ adsorption–desorption isotherm and the corresponding BJH pore-size distribution curve of the H-Fe₃O₄@h-Cu⁰@mSiO₂ yolk/shell nanoreactor. As can be seen in Fig. 5, a typical type IV isotherm with high relative pressures (P/P₀ < 0.1) was present for as-prepared nanoreactor, which is characteristic of mesoporous material with uniform and desirable pores.

Nitrogen adsorption-desorption isotherm of yolk/shell H-Fe3O4@h-Cu0@mSiO2 (A) and the corresponding pore-size distribution curves isotherm (B).

The BET surface area, pore diameter and pore volume of H-Fe₃O₄@h-Cu⁰@mSiO₂ yolk/shell nanoreactor were 287.36 m²/g, 4.9817 nm and 0.3579 cm³/g, respectively. These data further prove that the mesoporous structure is dominant in synthesized nanoreactor.

3.3

3.3 Catalytic activity

Having synthesized and characterized of H-Fe₃O₄@h-Cu⁰@mSiO₂ yolk/shell nanoreactor, at first, its catalytic activity was compared with H-Fe₃O₄@SiO₂-NH₂-Cu⁰, H-Fe₃O₄@SiO₂-Cu⁰@CTAB-SiO₂ and H-Fe₃O₄@h-Cu⁰@CTAB-SiO₂ core/shell nanoparticles. So, they were used as a heterogeneous magnetic catalysts in one-pot, three component reaction of benzaldehyde, morpholine, and phenylacetylene, which was selected as the model reaction for synthesis of propargylamines (Table 1, entries 2–5). As shown in Table 1, the yield of the corresponding product is negligible for the A³ coupling reaction by using H-Fe₃O₄@SiO₂-Cu⁰@CTAB-SiO₂ and H-Fe₃O₄@h-Cu⁰@CTAB-SiO₂, due of the presence of silica shell and CTAB, which has led to decrease in the activity of zero copper nanoparticles. The H-Fe₃O₄@SiO₂-NH₂-Cu⁰ shows lower yield than the H-Fe₃O₄@h-Cu⁰@mSiO₂ nanoreactor. This observation can be due to the stronger and more effective interaction between zero copper nanoparticles as a predominant catalytic active site with reactants in the internal cavity of the magnetic H-Fe₃O₄@h-Cu⁰@mSiO₂ yolk/shell nanoreactor.

Table 1 Optimization of the reaction of three-component reaction catalyzed by H-Fe₃O₄@h-Cu⁰@mSiO₂^a

Entry	Catalyst (g)	Condition	Yield (%)^b
1	–	Solvent free/110 °C	Trace
2	H-Fe₃O₄@SiO₂-NH₂-Cu⁰(0.006)	Solvent free/110 °C	85
3	H-Fe₃O₄@SiO₂-Cu⁰@CTAB-SiO₂(0.006)	Solvent free/110 °C	Trace
4	H-Fe₃O₄@h-Cu⁰@CTAB-SiO₂(0.006)	Solvent free/110 °C	Trace
5	H-Fe₃O₄@h-Cu⁰@mSiO₂ (0.01)	Solvent free/25 °C	Trace
6	H-Fe₃O₄@h-Cu⁰@mSiO₂ (0.01)	Solvent free/80 °C	60
7	H-Fe₃O₄@h-Cu⁰@mSiO₂ (0.01)	Solvent free/100 °C	90
8	H-Fe₃O₄@h-Cu⁰@mSiO₂ (0.01)	Solvent free/110 °C	98
9	H-Fe₃O₄@h-Cu⁰@mSiO₂ (0.01)	Solvent free/130 °C	98
10	H-Fe₃O₄@h-Cu⁰@mSiO₂ (0.006)	Solvent free/110 °C	98
11	H-Fe₃O₄@h-Cu⁰@mSiO₂ (0.003)	Solvent free/110 °C	70
12^b	H-Fe₃O₄@h-Cu⁰@mSiO₂ (0.006)	Toluene^c/110 °C	60
13^b	H-Fe₃O₄@h-Cu⁰@mSiO₂ (0.006)	DMF^c/110 °C	70

a Reaction conditions: benzaldehyde (1 mmol), morpholine (1.2 mmol), phenylacetylene (1.5 mmol); at ambient atmosphere. Time of the reactions for entries 1–11: 15 min, for entries 12 and 13: 1 h.

b Isolated yield.

c Solvent (3 mL).

In the next step, after optimization of appropriate catalyst, in order to obtain the best suitable conditions for synthesis of target products by A³ coupling reaction, we have optimized the reaction variables by studying the effect of reaction parameters including solvent, temperature, and the amount of catalyst for the aforementioned reaction as shown in Table 1, entries 5–11. The results established that most favorable reaction condition was achieved by using 6 mg catalyst (0.03 mol %) in the solvent-less condition at 110 °C. The ratio of aldehyde: amine: alkyne as substrates was optimized to 1:1.2:1.5.

Having the optimized reaction condition for examining the scope of our methodology, we have applied the procedure to various substrates for this catalytic system. For this purpose, various type of substituents arylaldehydes (electron-donating and electron-withdrawing), alkynes and secondary amines were employed and the results are out came in Table 2. The results indicate arylaldehydes bearing different functional groups such as -Br, -Cl, -Me, -OMe, and –OH, were able to produce the corresponding A³ coupling products. However, the transformation with electron-donating or electron-withdrawing substitutions on arylaldehyde derivatives has been observed with no difference in reactivity.

Table 2 Synthesis of propargylamine derivatives using H-Fe₃O₄@h-Cu⁰@mSiO₂ as nano-reactor.^a

Entry	Aldehyde	Amine	Product	Time (min)	Yield (%)^b
1	C₆H₅-CHO	Morpholine	4a	15	98
2	3-HO-C₆H₄-CHO	Morpholine	4b	25	83
3	4-Me-C₆H₄-CHO	Morpholine	4c	35	92
4	4-HO-3-OMe-C₆H₄-CHO	Morpholine	4d	45	90
5	2-HO-C₆H₄-CHO	Morpholine	4e	25	85
6	2-Me-C₆H₄-CHO	Morpholine	4g	35	93
7	4-MeO-C₆H₄-CHO	Morpholine	4g	35	90
8	Thiophene-2-carbaldehyde	Morpholine	4 h	30	94
9	4-F-C₆H₄-CHO	Morpholine	4i	30	93
10	2-F-C₆H₄-CHO	Morpholine	4j	30	90
11	4-Br-C₆H₄-CHO	Morpholine	4k	30	93
12	4-Cl-C₆H₄-CHO	Morpholine	4L	30	92
13	4-CHO-C₆H₄-CHO	Pipiridine	4m	30	95
14	2-naphtaldehyde	Piperidine	4n	30	94
15	C₆H₄-CHO	Piperidine	4o	20	96
16	3-OHC₆H₄-CHO	Piperidine	4p	25	81
17	4-HO-3-Me-C₆H₄-CHO	Piperidine	4q	40	89
18	4-MeO-C₆H₄-CHO	Piperidine	4r	35	86
19	4-FC₆H₄-CHO	Piperidine	4s	30	91
20	Thiophene-2-carbaldehyde	Piperidine	4t	30	93
21	C₆H₄-CHO	Morpholine	4u	20	97
22	C₆H₄-CHO	Piperidine	4v	20	95
23	2-HO-C₆H₄-CHO	Morpholine	4w	25	80
24	C₆H₅-CHO	Diethyl amine	4x	30	81

For entries 1–20 and 24, R = Ph and for entries 21–23, R = -C(CH₃)₂OH.

a Reaction conditions: arylaldehyde (1 mmol), amine (1.2 mmol), alkyne (1.5 mmol), H-Fe₃O₄@h-Cu⁰@mSiO₂ (0.006 g), under solvent-free condition at 110 °C at ambient atmosphere.

b Isolated yield.

A comparative study has been made for the catalyst performance of the H-Fe₃O₄@h-Cu⁰@mSiO₂ yolk/shell nanoreactor with the other reported copper-metal containing catalyst for the synthesis of propargylamine derivatives (Table 3).

Table 3 Comparison of catalytic activity of H-Fe₃O₄@h-Cu⁰@mSiO₂ with reported copper content catalyst for synthesis of propargylamine 4a.

Entry	Catalyst	Condition	Time (h)	Yield (%)	Ref
1	Cu@PMO-1L^a	CHCl₃/60 °C	24	96	Gholinejad et al. (2015)
3	Cu/Al	Toluene/100 °C	22	94	Dulle et al. (2013)
3	CuNPs@TiO₂	Solvent free/70 °C	7	91	Albaladejo et al. (2012)
4	Cu NPs	Solvent free/80 °C	7	90	Shouli, et al. (2017)
5	Cu@MOF-5-C^b	Toluene/110 °C	6	90	Cheng et al. (2017)
6	CuO/GNS^c	CH₃CN/82 °C	5	89	Gopiraman et al. (2015)
7	SiO₂@Cu	Toluene/110 °C	5	94	Guo et al. (2013)
8	Cu-MPTA-l^d	CH₂Cl₂/25 °C	24	93	Salam et al. (2013)
9	Cu-MCM-41	Solvent free/110 °C	2/30	93	Abdollahi-Alibeik and Moaddeli (2014)
10	Cu/G	Toluene/110 °C	4	96	Frindy et al. (2016)
11	H-Fe₃O₄@h-Cu⁰@mSiO₂	Solvent free/110 °C	15 min	98	This work

a Copper supported on periodic mesoporous organosilica (PMO) ionic liquid.

b Metal-organic-frameworks (MOFs)-Prous carbon materials.

c CuO nanoparticles (CuO NPs)/graphene nanosheet (GNS).

d Mesoporous poly-triallylamine (MPTA-1).

Incomparable superiorities of our protocol quite clear in terms of reaction time and yield for the synthesis of propargylamines via A³ coupling reaction compared to other reported catalysts. Therewith, by using this yolk/shell nanoreactor as catalyst, the reaction could progress under benign reaction condition, i.e. solvent-free and low amount of catalyst with maximum efficiency and activity of copper-metal.

3.4

3.4 Reaction mechanism

A proposed mechanism route for the three component synthesis of propargylamines, which catalyzed in the internal cavity of the H-Fe₃O₄@h-Cu⁰@mSiO₂ yolk/shell nanoreactor is illustrated in Scheme 2.. In the first stage, the formation of Cu-acetylide intermediate take place from activation of the terminal C—H bond of acetylenic substrate by zero copper nanoparticles in the internal cavity of nanoreactor. In the second step, the iminium ion is produced from the reaction of aldehyde and secondary amine, which permitted the reaction with Cu-acetyline to provide the desired propargylamine and regenerate the active h-Fe₃O₄@h-Cu⁰@mSiO₂ yolk/shell nanoreactor for further reaction.

Reaction mechanism for the synthesis of propargylamines.

3.5

3.5 Reusability of the catalyst

Stability, recovered simply and reusability of a magnetic catalyst are one the most important basic parameter for sustainability of yolk/shell nanoreactor. So, the recyclability of h-Fe₃O₄@h-Cu⁰@mSiO₂ catalyst was examined for the model reaction and the results show that very acceptable outcomes. Upon completion of the reaction, hot ethanol was added to the reaction mixture and the catalyst was easily separated using an external magnetic and extensively washed (see experimental section). The results determined that, the yield of the corresponding product was saved to 96%, even after 10 runs (Run 1: 98%; Run 2: 98%; Run 3: 98%; Run 4: 97%; Run 5: 97%; Run 6: 97%; Run 7: 96%; Run 8: 96% Run 9: 96%; Run 10: 98%;).

From where, the H-Fe₃O₄@SiO₂-NH₂-Cu⁰ shows moderate catalytic efficiency toward A³ coupling reaction (Table 1, entry 2), we examined the recyclability of it too. However, with the increasing of run to 3 cycle in model reaction, losing of catalytic effect could be observed and the yield of the corresponding product decreased to 51%. This diminishing mainly could be due to leaching of copper nanoparticles. So, the Cu amounts, which loaded in H-Fe₃O₄@SiO₂-NH₂-Cu⁰ was determined before and after three runs by AAS. They were 0.04% and 0.01%, respectively.

On the other hand, h-Fe₃O₄@h-Cu⁰@mSiO₂ could serve its catalytic performance even after 10 runs. It is quite clear that the porous shell of silica is responsible for fixation and stability of copper nanoparticles. The content of Cu deceased from 0.035% to 0.030% after 10 cycles. So, we make sure the principle reason for the deactivation of catalyst is leaching of active sites, which is copper nanoparticles.

Interestingly, comparing the XRD pattern of fresh and recovered Fe₃O₄@h-Cu⁰@mSiO₂ yolk/shell nanoreactor catalyst displayed that the phase composition and crystalline structure of the prepared nano reactor remained unchanged after using ten times in the three-component A³-coupling reaction and by applying the Scherrer's equation (D = kλ/βcosθ) the average crystalline size of the H-Fe₃O₄ particles is estimated to be 33 nm (Fig. 6).

XRD pattern of Fe3O4@h-Cu0@mSiO2 (a) before use and (b) after reused 10 runs.

4 Conclusion

In summary, a novel and efficient magnetic catalyst, H-Fe₃O₄@h-Cu⁰@mSiO₂ yolk/shell nanoreactor was designed and prepared with Fe₃O₄ hollow spheres with mesoporous and hollow silica shell that zero copper nanoparticles are concentrated in the shell inner cavity via a facile hydrothermal method. H-Fe₃O₄@h-Cu⁰@mSiO₂ yolk/shell nanoreactor was successfully applied as superior and magnetic catalyst for pushing three-component A³-coupling reaction of alkyne, amine and aldehyde for synthesis of c propargylamines under benign condition. The magnetic H-Fe₃O₄@h-Cu⁰@mSiO₂ yolk/shell nanoreactor catalyst could promote A³ coupling reaction to obtain up to 98% yield of the coupling product under optimized condition, and most importantly, this yolk/shell nanoreactor could be easily separated and reused up and it offers 96% yield of the desired product even after using ten times of the catalyst, and it is worth mention that the loss of copper metal in the inner cavity was negligible. The result revealed that the magnetic H-Fe₃O₄@h-Cu⁰@mSiO₂ yolk/shell nanoreactor catalyst is premiere over to other previously reported catalyst in terms of yield, reaction times, and recyclability.

Acknowledgements

The authors gratefully acknowledge the Semnan university research council for the financial of this project.

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Appendix A

Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2018.11.011.

Appendix A

Supplementary material

The following are the Supplementary data to this article:

Supplementary data 1

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[1] Abdollahi-Alibeik M., Moaddeli A., . Copper modified spherical MCM-41 nano particles: an efficient catalyst for the three-component coupling of aldehydes, amines and alkynes in solvent-free conditions. Rsc Adv.. 2014;4:39759-39766.
[Google Scholar]

[2] Albaladejo M.J., Alonso F., Moglie Y., Yus M., . Three-component coupling of aldehydes, amines, and alkynes catalyzed by oxidized copper nanoparticles on titania. Eur. J. Org. Chem.. 2012;2012:3093-3104.
[Google Scholar]

[3] Baran N.Y., Baran T., Menteş A., . Production of novel palladium nanocatalyst stabilized with sustainable chitosan/cellulose composite and its catalytic performance in Suzuki-Miyaura coupling reactions. Carbohydr. Polym.. 2018;181:596-604.
[Google Scholar]

[4] Baran T., . Pd (0) nanocatalyst stabilized on a novel agar/pectin composite and its catalytic activity in the synthesis of biphenyl compounds by Suzuki-Miyaura cross coupling reaction and reduction of o-nitroaniline. Carbohydr. Polym.. 2018;195:45-52.
[Google Scholar]

[5] Baran T., . Ultrasound-accelerated synthesis of biphenyl compounds using novel Pd (0) nanoparticles immobilized on bio-composite. Ultrason. Sonochem.. 2018;45:231-237.
[Google Scholar]

[6] Borah B.J., Borah S.J., Saikia L., Dutta D.K., . Efficient three-component coupling reactions catalyzed by Cu 0-nanoparticles stabilized on modified montmorillonite. Catal. Sci. Technol.. 2014;4:1047-1054.
[Google Scholar]

[7] Cheng S., Shang N., Feng C., Gao S., Wang C., Wang Z., . Efficient multicomponent synthesis of propargylamines catalyzed by copper nanoparticles supported on metal-organic framework derived nanoporous carbon. Catal. Commun.. 2017;89:91-95.
[Google Scholar]

[8] Deng Y., Qi D., Deng C., Zhang X., Zhao D., . Superparamagnetic high-magnetization microspheres with an Fe3O4@ SiO2 core and perpendicularly aligned mesoporous SiO₂ shell for removal of microcystins. J. Am. Chem. Soc.. 2008;130:28-29.
[Google Scholar]

[9] Do Q.C., Kim D.-G., Ko S.-O., . Nonsacrificial template synthesis of magnetic-based yolk-shell nanostructures for the removal of acetaminophen in fenton-like systems. ACS Appl. Mat. Inter.. 2017;9:28508-28518.
[Google Scholar]

[10] Dulle J., Thirunavukkarasu K., Mittelmeijer-Hazeleger M.C., Andreeva D.V., Shiju N.R., Rothenberg G., . Efficient three-component coupling catalysed by mesoporous copper–aluminum based nanocomposites. Green Chem.. 2013;15:1238-1243.
[Google Scholar]

[11] Elhampour A., Mirhosseyni M.S., Nemati F., . Palladium nanoparticles supported on modified hollow Fe3O4@ TiO2: preparation, characterization, and catalytic activity in Suzuki cross-coupling reactions. J. Chin. Chem. Soc.. 2018;65:875-882.
[Google Scholar]

[12] Elhampour A., Nemati F., . Palladium nanoparticles supported on modified hollow-Fe₃O₄@ TiO₂: catalytic activity in Heck and Sonogashira cross coupling reactions. Org. Prep. Proced. Int.. 2017;49:443-458.
[Google Scholar]

[13] Frindy S., El Kadib A., Lahcini M., Primo A., García H., . Copper nanoparticles supported on graphene as an efficient catalyst for A 3 coupling of benzaldehydes. Catal. Sci. Technol.. 2016;6:4306-4317.
[Google Scholar]

[14] Gholinejad M., Karimi B., Aminianfar A., Khorasani M., . One-pot preparation of propargylamines catalyzed by heterogeneous copper catalyst supported on periodic mesoporous organosilica with ionic liquid framework. ChemPlusChem.. 2015;80:1573-1579.
[Google Scholar]

[15] Gopiraman M., Deng D., Ganesh Babu S., Hayashi T., Karvembu R., Kim I.S., . Sustainable and versatile CuO/GNS nanocatalyst for highly efficient base free coupling reactions. ACS Sustain. Chem. Eng.. 2015;3:2478-2488.
[Google Scholar]

[16] Guo H., Liu X., Xie Q., Wang L., Peng D.-L., Branco P.S., Gawande M.B., . Disproportionation route to monodispersed copper nanoparticles for the catalytic synthesis of propargylamines. RSC Adv.. 2013;3:19812-19815.
[Google Scholar]

[17] Hao P., Ren J., Yang L., Qin Z., Lin J., Li Z., . Direct and generalized synthesis of carbon-based yolk–shell nanocomposites from metal-oleate precursor. Chem. Eng. J.. 2016;283:1295-1304.
[Google Scholar]

[18] Hussain N., Das M.R., . Magnetically recoverable graphene-based nanocomposite material as an efficient catalyst for the synthesis of propargylamines via A³ coupling reaction. New J. Chem.. 2017;41:12756-12766.
[Google Scholar]

[19] Khatiri, R., Reyhani, A., Mortazavi, S., Hossainalipour, M., 2012. Preparation and characterization of Fe3O4/SiO2/APTES core-shell nanoparticles. In: Proceedings of the 4th International Conference on Nanostructures (ICNS4), Kish Island IR Iran, pp. 12–14.

[20] Lee J., Kim S.M., Lee I.S., . Functionalization of hollow nanoparticles for nanoreactor applications. Nano Today. 2014;9:631-667.
[Google Scholar]

[21] Liu X., . Preparation of porous hollow Fe ₃O₄/P(GMA–DVB–St) microspheres and application for lipase immobilization. Bioprocess. Biosyst. Eng.. 2018;41:771-779.
[Google Scholar]

[22] Long Y., Liu Y., Zhao Z., Luo S., Wu W., Wu L., Ma J., . Distinctive morphology effects of porous-spherical/yolk-shell/hollow Pd-nitrogen-doped-carbon spheres catalyst for catalytic reduction of 4-nitrophenol. J. Colloid Interface Sci.. 2017;496:465-473.
[Google Scholar]

[23] Mirhosseyni M.S., Nemati F., Elhampour A., . Hollow Fe₃O₄@DA-SO₃H: an efficient and reusable heterogeneous nano-magnetic acid catalyst for synthesis of dihydropyridine and dioxodecahydroacridine derivatives. J. Iran Chem. Soc.. 2016;4:791-801.
[Google Scholar]

[24] Nabid M.R., Bide Y., Abuali M., . Copper core silver shell nanoparticle–yolk/shell Fe₃O₄@ chitosan-derived carbon nanoparticle composite as an efficient catalyst for catalytic epoxidation in water. RSC Adv.. 2014;4:35844-35851.
[Google Scholar]

[25] Nabid M.R., Bide Y., Habibi Z., . Synthesis of a yolk/shell Fe₃O₄@ poly (ionic liquid) s-derived nitrogen doped graphitic porous carbon materials and its application as support for nickel catalysts. RSC Adv.. 2015;5:2258-2265.
[Google Scholar]

[26] Nasrollahzadeh M., Azarian A., Maham M., Ehsani A., . Synthesis of Au/Pd bimetallic nanoparticles and their application in the Suzuki coupling reaction. J. Industr. Eng. Chem.. 2015;21:746-748.
[Google Scholar]

[27] Nasrollahzadeh M., Sajadi S.M., Babaei F., Maham M., . Euphorbia helioscopia Linn as a green source for synthesis of silver nanoparticles and their optical and catalytic properties. J. Colloid Interface Sci.. 2015;450:374-380.
[Google Scholar]

[28] Nemati F., Heravi M.M., Rad R.S., . Nano- Fe₃O₄ encapsulated-silica particles bearing Sulfonic acid groups as a magnetically separable catalyst for highly efficient knoevenagel condensation and michael addition reactions of aromatic aldehydes with 1, 3-cyclic diketones. Chin. J. Catal.. 2012;33:1825-1831.
[Google Scholar]

[29] Nemati F., Saeedirad R., . Nano- Fe₃O₄ encapsulated-silica particles bearing sulfonic acid groups as a magnetically separable catalyst for green and efficient synthesis of functionalized pyrimido [4, 5-b] quinolines and indeno fused pyrido [2, 3-d] pyrimidines in water. Chin. Chem. Lett.. 2013;24:370-372.
[Google Scholar]

[30] Priebe M., Fromm K.M., . Nanorattles or yolk-shell nanoparticles—what are they, how are they made, and what are they good for? Chem.-A Eur. J.. 2015;21:3854-3874.
[Google Scholar]

[31] Rangraz Y., Nemati F., Elhampour A., . Diphenyl diselenide immobilized on magnetic nanoparticles: a novel and retrievable heterogeneous catalyst in the oxidation of aldehydes under mild and green conditions. J. Colloid Interface Sci.. 2018;509:485-494.
[Google Scholar]

[32] Rangraz Y., Nemati F., Elhampour A., . Magnetic chitosan composite as a green support for anchoring diphenyl diselenide as a biocatalyst for the oxidation of sulfides. Inter. J. Biol. Macromol.. 2018;117:820-830.
[Google Scholar]

[33] Sabaqian S., Nemati F., Nahzomi H.T., Heravi M.M., . Palladium acetate supported on amidoxime-functionalized magnetic cellulose: synthesis, DFT study and application in Suzuki reaction. Carbohydr. Polym.. 2017;177:165-177.
[Google Scholar]

[34] Sabaqian S., Nemati F., Nahzomi H.T., Heravi M.M., . Silver (I) dithiocarbamate on modified magnetic cellulose: synthesis, density functional theory study and application. Carbohydr. Polym.. 2018;184:221-230.
[Google Scholar]

[35] Salam N., Kundu S.K., Roy A.S., Mondal P., Roy S., Bhaumik A., Islam S.M., . Cu-grafted mesoporous organic polymer: a new recyclable nanocatalyst for multi-component, N-arylation and S-arylation reactions. Catal. Sci. Technol.. 2013;3:3303-3316.
[Google Scholar]

[36] Shahamat, Z., Nemati, F., Elhampour, A. Facile template-free route to fabricate core–shell Fe3O4@PANI-SO3H urchin-like nanoparticles as reusable catalyst for Hantzsch reaction: change morphology upon sulfonation. Res. Chem. Intermed. 44, 6649–6666.

[37] Shouli A., Menati S., Sayyahi S., . Copper (II) chelate-bonded magnetite nanoparticles: a new magnetically retrievable catalyst for the synthesis of propargylamines. C. R Chim.. 2017;20:765-772.
[Google Scholar]

[38] Tang M., Zhang S., Li X., Pang X., Qiu H., . Fabrication of magnetically recyclable Fe₃O₄@Cu nanocomposites with high catalytic performance for the reduction of organic dyes and 4-nitrophenol. Mater. Chem. Phys.. 2014;148:639-647.
[Google Scholar]

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Synthesis of a magnetic-based yolk-shell nano-reactor: A new class of monofunctional catalyst by Cu⁰-nanoparticles and its application as a highly effective and green catalyst for A³ coupling reaction

Abstract