Preparation of a novel Fe 3 O 2 magnetic core shell nanocatalyst for Knoevenagel reaction in aqueous medium

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Preparation of a novel Fe

3

O

4

@SiO

2

@propyl@DBU

magnetic core–shell nanocatalyst for Knoevenagel

reaction in aqueous medium

Xiaoyu Zhang, Xiaoyan He & Sanhu Zhao

To cite this article: Xiaoyu Zhang, Xiaoyan He & Sanhu Zhao (2021) Preparation of a novel Fe3O4@SiO2@propyl@DBU magnetic core–shell nanocatalyst for Knoevenagel reaction in aqueous medium, Green Chemistry Letters and Reviews, 14:1, 85-98, DOI: 10.1080/17518253.2020.1862312

To link to this article: https://doi.org/10.1080/17518253.2020.1862312

© 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group

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Preparation of a novel Fe

3

O

4

@SiO

2

@propyl@DBU magnetic core

–shell

nanocatalyst for Knoevenagel reaction in aqueous medium

Xiaoyu Zhanga, Xiaoyan Heaand Sanhu Zhaoa,b

a

School Chemistry and Material Science, Shanxi Normal University, Linfen, People’s Republic of China;bDepartment of Chemistry, Xinzhou Teachers University, Xinzhou, People’s Republic of China

ABSTRACT

Magnetic nano catalysts are widely used in various organic reactions due to their characteristics of magnetic separation and recycling. Magnetic Fe3O4@SiO2 nano catalyst modified by 1,

8-diazabicyclo [5.4.0]-7-undecene (DBU) was prepared and applied in the Knoevenagel reaction. The results showed that the catalyst has an excellent effect on the Knoevenagel reaction, which not only improves the reaction rate and yield, but also it can be easily separated from the reaction mixture driven by external magnetic force. Furthermore, the catalyst can be recycled 10 cycles with no loose of the catalytic activity.

ARTICLE HISTORY

Received 25 August 2020 Accepted 3 December 2020

KEYWORDS

Magnetic nano catalyst; recycle; DBU; Knoevenagel reaction

1. Introduction

Knoevenegal reaction is the condensation reaction of a reactive methylene compounds with aldehydes or ketones under weak base conditions, which resulting in α, β- unsaturated carbonyl compounds (1). In recent years, to improve the Knoevenegal reaction, protic ionic liquids (2), mesoporous materials (3), amine-functionalized ionic liquid-based periodic mesoporous organosilica (4), carbamic acid ammonium salt (5), zeolite (6), MOFs (7– 13), melamine-derived graphitic carbon nitride (14), mol-ybdenum carbide (15), amines functionalized C60 (16),

quinine(17), polyoxometalate (POM) cluster organic fra-meworks (18–20), Pickering emulsion (21), Fe3O4@ZIF-8

(22) and nanohybrid material (23) have been reported as catalysts to catalyze this reaction. Although these catalysts have good catalytic effect, most of them suffer from harsh recovery conditions, low yield, long reaction time or poor

reuse effect. Even to satisfy the need of certain catalysts, some environmentally harmful solvents are often used. Therefore, in line with the concept of green chemistry, it is necessary to develop a green catalyst with high efficiency, easy recovery, and high reuse rate.

In recent years, magnetic nanoparticles have been widely studied as catalysts used for various reactions (24, 25). Among them, Maleki’s group developed a series of magnetic nano catalysts, such as SiO2

-sup-ported Fe3O4 (26, 27), palladium-loaded magnetic

material(28), copper-loaded magnetic agar (Cu2O/

Agar@Fe3O4) (29), cellulose-based magnetic

nanocom-posite (30), biological macromolecule chitosan magnetic nanometer catalyst (31), chromium-based magnetic composite nano catalyst (32) and the Fe3O4@SiO2

-OSO3H catalyst (33), which were used in the synthesis

of heterocyclic compounds, α-aminonitriles and the

© 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

CONTACT Sanhu Zhao sanhuzhao@163.com School Chemistry and Material Science, Shanxi Normal University, Linfen, Shanxi 041001, People’s Republic of China; Department of Chemistry, Xinzhou Teachers University, Xinzhou, Shanxi 034000, People’s Republic of China

Supplemental data for this article can be accessed athttps://doi.org/10.1080/17518253.2020.1862312. 2021, VOL. 14, NO. 1, 85–98

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oxidation reaction of alcohols, excellent catalytic effects were obtained. In addition, Zhang’s group developed the molybdenum-supported graphene oxid/Fe3O4 for

the construction of spiro-oxindole dihydropyridines (34). Gao’s group demonstrated the synthesis of cyclo-hexanone derivatives catalyzed by a magnetic metal organic framework material (35). All these catalysts not only have the advantages of easy separation and excel-lent catalytic effect, but also most of the magnetic nano catalysts are supported by Fe3O4and carry organic

mol-ecules on the surface of the support. The spherical mag-netic nanoparticles have a high specific surface, organic molecules can be loaded onto the body surface as much as possible to provide more active sites (36–39). Although some typical magnetic nanoparticle Fe3O4

combining with organic catalyst have been developed and can efficiently catalyze the C–C bond coupling reac-tions (40–42), the development of magnetic nano cata-lysts still needs further exploration.

DBU, an organic base with steric hindrance, can efficiently catalyze the Knoevenegal reaction, but the reuse and separation of DBU is difficult (43). To address the above questions and to continue our research inter-ests (44,45), a green and efficient Knoevenegal reaction catalyst, Fe3O4@SiO2modified by DBU (Scheme 1), was

developed and used in the Knoevenegal reaction, the high reaction yield and short reaction times were obtained. Since the catalyst itself was supported by mag-netic Fe3O4, it is easy to be separated and retains its

orig-inal activity after 10 times of cyclic tests. 2. Materials and methods

2.1. Materials and products

All chemical reagents used are analytically pure and have not been further purified. For the NMR spectra of Knoeve-nagel reaction products, see Supplementary Materials.

2.2. Methods

Transmission electron microscopy (TEM) images were obtained from a FEI TECNAI G20 instrument. The X-ray

diffraction (XRD) images were obtained by using an XRD-6100 device with Cu Kα radiation (λ = 0.15418 nm). The infrared spectra (FTIR) were obtained from IR Affinit-IS Fourier Transform Spectrometer by dis-persing samples in KBr disks. The thermogravimetric analysis images (TG) were obtained from the thermogra-vimetric analyzer STA 449F5 operating at 220 V and 16 A in the atmosphere of nitrogen. Scanning electron micro-scope (SEM) images were performed on FESEM FEI Quanta 400 FEG. The vibrating sample magnetometer (VSM) pattern was carried out with SQUID-VSM (America Quantum Design). Brunner-Emmet-Teller (BET) analysis was recorded by the automatic specific surface area and porosity analyzer TriStar II 3020 (America Micromeritics). 1H NMR (400 MHz) and 13C NMR (101 MHz) spectra were recorded on a Bruker Avance DPX 400 spectrometer using DMSO-d6 or

CDCl3 as solvent. The elemental analyses were

per-formed on a Vario EL Elemental Analyzer.

2.3. Preparation of the magnetic nano catalyst As shown in Scheme 2, Nano Fe3O4 and Fe3O4@SiO2

were prepared by the method of chemical coprecipi-tation. Owing to the outer layer of Fe3O4@SiO2provides

a suitable graft site for subsequent functionalization, 3-chloropropyl trimethoxysilane was selected to condense with Fe3O4@SiO2in toluene under nitrogen protection

and the product 2 was obtained. Then, under nitrogen protection, the quaternization of DBU by product 2 was conducted in toluene and the final magnetic nano-catalyst 3 was achieved.

2.3.1. Synthesis of magnetic nanoparticles Fe3O4 Magnetic Fe3O4particles were prepared by the method

of coprecipitation (46). FeCl3·6H2O (8.1 g, 30 mmol) and

FeCl2·4H2O (3.4 g, 17 mmol) were added into a 250 mL

round bottom flask containing 100 mL distilled water. Under the protection of nitrogen, the reaction mixture was stirred at 30°C. When all ingredients have dissolved, 13.7 mL NH3·H2O was quickly added, and the color of

solution changed from orange to black rapidly. The solid product was collected and washed with deionized water until the solution remains neutral. Then it was washed repeatedly with acetone and ethanol in turn, the residual solvent was removed by vacuum method and the black crystalline particles Fe3O4were obtained.

2.3.2. Synthesis of magnetic nanoparticles Fe3O4@SiO2

The surface of Fe3O4was coated with a layer of silica by

sol–gel method (47,48). With the ultrasound assist, the magnetic nanoparticle Fe3O4 was dispersed into Scheme 1.Magnetic nano catalyst Fe3O4@SiO2@propyl@DBU.

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150 mL ethanol and 30 mL water, the mixture was stirred for 30 min. Then the concentrated ammonia water and ethyl n-silicate were added, and the reaction continued to stir at room temperature for 24 h. The resulting magnetic solids were collected by external magnets and washed to neutral by using deionized water, then it was washed with ethanol and dried by rotary evaporation to obtain a gray solid powder. 2.3.3. Synthesis of alkyl modified magnetic nanoparticle

Under the ultrasonic assist, the grey solid powder of Fe

3-O4@SiO2 (3.0 g) was dispersed in toluene (50 mL), and

under the protection of nitrogen, 3-chloropropyl tri-methoxysilane (3.0 g) was added, the reaction mixture was stirred at 130°C for 24 h. The resulting magnetic solids were collected by external magnets and washed to neutral with deionized water. Then it was washed with ethanol and dried by rotary evaporation to obtain a brown solid powder Fe3O4@SiO2@propylchloride.

2.3.4. Synthesis of DBU modified magnetic nanoparticles (MNPs)

The brown solid powder Fe3O4@SiO2@propylchloride

(2.0 g) was dispersed in toluene (50 mL) with the ultra-sound assist, then DBU (2.0 g) was added. Under the pro-tection of N2, the reaction was reflux for 24 h at 130°C.

The resulting magnetic nanoparticles were collected by external magnets and washed to neutral with deionized water. The crude product was further purified by ethanol

and the target magnetic nano-catalyst MNPs was obtained after vacuum drying.

2.4. General procedure for Knoevenagel reaction P-chlorobenzaldehyde (0.7 g, 5 mmol) was added the 50 mL round bottomflask containing ethyl cyanoacetate (0.6 mL, 6 mmol), K2CO3(1.0 g) and MNPs (0.3 g) mixed

in the complex solvent (6 mL H2O + 2 mL PEG-400),

then the reaction mixture was stirred at 40°C and TLC was used to monitor the reaction. when the substrate was completely consumed or the reaction is in equili-brium, 50 mL water was added, and the MNPs was sep-arated from the reaction system using a permanent magnet, the solid crude product was collected and recrystallized with 90% ethanol, and the almost pure target product was obtained. All desired products were characterized by melting point determination and NMR spectroscopy, the spectral data of all products are listed as follows. Ethyl-3-(4-chlorophenyl)-2-cyanoacrylate (Table 3, Entry 1) White crystal.1H NMR (400 MHz, CDCl3)δ 8.18 (s, 1H, CH=), 7.92 (d, J = 8.4 Hz, 2H, ArH), 7.46 (d, J = 8.4 Hz, 2H, ArH), 4.38 (q, J = 7.1 Hz, 2H, CH2), 1.39 (t, J = 7.1 Hz, 3H, CH3). 13C NMR (101 MHz, CDCl3) δ 162.15, 153.27, 139.50, 132.18, 129.89, 129.63, 115.22, 103.53, 62.83, 14.13. Ethyl-2-cyano-3-(4-nitrophenyl)acrylate (Table 3, Entry 2).

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Yellow solid.1H NMR (400 MHz, CDCl3)δ 8.36 (d, J =

8.8 Hz, 2H, ArH), 8.33 (s, H, CH=), 8.16 (d, J = 8.8 Hz, 2H, ArH), 4.45 (q, J = 7.1 Hz, 2H, CH2), 1.44 (t, J = 7.1 Hz, 3H,

CH3).

Ethyl-2-cyano-3-phenylacrylate (Table 3, Entry 3). White crystal.1H NMR (400 MHz, CDCl3)δ 8.26 (s, 1H, CH=), 8.00 (m, 2H, ArH), 7.51 (m, 3H, ArH), 4.41 (q, J = 7.1 Hz, 2H, CH2), 1.41 (t, J = 7.1 Hz, 3H, CH3). Ethyl-2-cyano-3-(2, 4-dichlorophenyl)acrylate (Table 3, Entry 4). White solid; 1H NMR (400 MHz, CDCl3)δ 8.63 (s, 1H, CH=), 8.23 (d, J = 8.6 Hz, 1H, ArH), 7.55 (d, J = 1.9 Hz, 1H, ArH), 7.42 (d, J = 8.6 Hz, 1H, ArH), 7.29 (s, 2H), 4.43 (q, J = 7.1 Hz, 2H, CH2), 1.43 (t, J = 7.1 Hz, 3H, CH3). Ethyl-2-cyano-3-(4-cyanophenyl)acrylate (Table 3, Entry 5) White crystal;1H NMR (400 MHz, CDCl3)δ 8.25 (s, 1H, CH=), 8.07 (d, J = 8.4 Hz, 2H, ArH), 7.80 (d, J = 8.3 Hz, 2H, ArH), 4.42 (q, J = 6.8 Hz, 2H, CH2), 1.42 (t, J = 7.5 Hz, 3H, CH3). Ethyl-2-cyano-3-(4-hydroxyphenyl)acrylate (Table 3, Entry 8)

Yellow solid.1H NMR (400 MHz, DMSO-d6)δ 10.83 (s,

1H, OH), 8.24 (s, 1H, CH=), 8.00 (d, J = 8.3 Hz, 2H, ArH), 6.95 (d, J = 8.3 Hz, 2H, ArH), 4.29 (q, J = 7.0 Hz, 2H, CH2), 1.29 (t, J = 7.0 Hz, 3H, CH3). 13C NMR (101 MHz, DMSO-d6) δ 163.32, 163.03, 155.10, 134.40, 122.95, 116.88, 116.81, 97.51, 62.38, 14.48. Ethyl-2-cyano-3-(4-phenylphenyl)acrylate (Table 3, Entry 9)

Green solid. 1H NMR (400 MHz, DMSO-d6) δ 8.45 (s,

1H, CH=), 8.17 (d, J = 8.0 Hz, 2H, ArH), 7.93 (d, J = 8.0 Hz, 2H, ArH), 7.80 (d, J = 7.6 Hz, 2H, ArH), 7.54–7.45 (m, 3H, ArH), 4.34 (q, J = 7.0 Hz, 2H, CH2), 1.32 (t, J = 7.0 Hz, 3H, CH3). 13C NMR (101 MHz, DMSO-d6) δ 162.34, 154.87, 145.13, 138.91, 132.03, 130.78, 129.57, 129.11, 127.80, 127.43,116.29, 102.40, 62.80, 14.30. Anal. calcd for C18H15NO2: C, 77.96; H, 5.45; N, 5.05. found: C, 77.98; H, 5.49. N, 4.98.

Ethyl-2-cyano-3-(4-fluorophenyl)acrylate (Table 3, Entry 10) White solid. 1H NMR (400 MHz, CDCl3)δ 8.22 (s, 1H, CH=), 8.05 (t, J = 7.7 Hz, 2H, ArH), 7.21 (t, J = 7.7 Hz, 2H, ArH), 4.39 (q, J = 7.1 Hz, 2H, CH2), 1.41 (t, J = 7.1 Hz, 3H, CH3). Diethyl-3, 3’-(1, 4-phenylene)-bis(2-cyanoacrylate) (Table 3, Entry 11)

White crystalline solid. 1H-NMR (400 MHz, CDCl3) δ

8.27 (s, 2H, CH=), 8.11 (s, 4H, ArH), 4.42 (q, J = 7.1 Hz, 4H, CH2), 1.42 (t, J = 7.1 Hz, 6H, CH3).

Ethyl-2-cyano-3-(2, 4-dimethoxyphenyl)acrylate (Table 3, Entry 12)

Yellow crystal.1H NMR (400 MHz, DMSO-d6)δ 8.52 (s,

1H, CH=), 8.22 (d, J = 8.9 Hz, 1H, ArH), 6.77 (d, J = 9.0 Hz, 1H, ArH), 6.73 (s, 1H, ArH), 4.29 (q, J = 7.0 Hz, 2H, CH2), 3.91 (d, J = 11.2 Hz, 6H. OCH3), 1.29 (t, J = 7.0 Hz, 3H, CH3). 13C NMR (151 MHz, DMSO-d6) δ 166.07, 163.12, 161.79, 148.15, 130.48, 117.00, 113.16, 107.74, 98.76, 97.81, 62.41, 56.78, 56.40, 14.48. Ethyl-2-cyano-3-(4-dimethylaminephenyl)acrylate (Table 3, Entry 13)

Yellow solid.1H NMR (400 MHz, DMSO-d6) δ 8.11 (s,

1H,CH=), 7.96 (d, J = 8.6 Hz, 2H,ArH), 6.84 (d, J = 8.6 Hz, 2H,ArH), 4.26 (q, J = 7.0 Hz, 2H,CH2), 3.09 (s, 6H,CH3),

1.28 (t, J = 7.0 Hz, 3H,CH3). 13C NMR (151 MHz,

DMSO-d6) δ 169.85, 163.90, 154.57, 154.14, 134.21, 118.72,

117.98, 112.12, 111.47, 92.45, 61.87, 14.57.

Ethyl 2-cyano-3-anthracenylacrylate (Table 3, Entry 14)

Yellow solid.1H NMR (400 MHz, DMSO-d6) δ 9.41 (s,

1H,CH=), 8.89 (s, 1H, ArH), 8.27 (d, J = 7.9 Hz, 2H, ArH), 8.60 (d, J = 8.2 Hz, 2H, ArH), 7.70 (t, J = 7.3 Hz, 4H, ArH), 4.49 (q, J = 6.9 Hz, 2H, CH2), 1.45 (t, J = 6.9 Hz, 3H, CH3). 13 C NMR (151 MHz, DMSO-d6) δ 161.17, 155.79, 130.80, 130.44, 129.43, 128.67, 127.78, 126.44, 125.33, 123.82, 114.82, 113.90, 62.98, 14.45. 2-Amino-4-(1-cyano-2-methoxy-2-oxoethyl)-4H-chromene-3-carboxylat (Table 3, Entry 15)

White solid.1H NMR (400 MHz, CDCl3) δ 7.31 (s, 1H, ArH), 7.10 (d, J = 8.4 Hz, 3H, ArH), 4.74 (s, 1H, CH), 4.27 (d, J = 6.2 Hz, 4H, 2OCH2), 4.00 (s, 1H, CH), 1.32 (d, J = 8.6 Hz, 6H, 2CH3). 13C NMR (101 MHz, CDCl3) δ 168.09, 165.18, 16247, 150.17, 129.03, 128.17, 124.68, 120.07, 116.63, 115.26, 73.29, 62.72, 59.94, 46.81, 45.41, 37.27, 13.69. Ethyl 2-cyano-3-(4-ammoniaphenyl)acrylate (Table 3, Entry 16)

Yellow solid.1H NMR (400 MHz, DMSO-d6) δ 8.03 (s,

1H, CH=), 7.84 (d, J = 8.3 Hz, 2H, ArH), 6.73 (s, 2H, ArH), 6.66 (d, J = 8.3 Hz, 2H, ArH), 4.25 (q, J = 7.0 Hz, 2H, CH2),

1.27 (t, J = 7.0 Hz, 3H, CH3). 13C NMR (151 MHz,

DMSO-d6) δ 164.01, 155.51, 154.83, 134.88, 118.82, 118.03,

113.97, 91.67, 61.82, 14.58.

Ethyl 2-cyano-3-(furan-2-yl) acrylate (Table 3, Entry 17)

Light yellow needle crystals.1H NMR (400 MHz, CDCl3)

δ 8.04 (s, 1H, =CH), 7.77 (s, 1H, -Fur-H), 7.40 (s, 1H, -Fur-H), 6.69 (s, 1H, -Fur-H), 4.38 (q, J = 7.1 Hz, 2H, CH2), 1.40 (t, J

= 7.1 Hz, 3H, CH3).

Ethyl 2-cyano-3-(N-ethylcarbazol-3-yl) acrylate (Table 3, Entry 18)

Yellow crystals.1H NMR (400 MHz, CDCl3)δ 8.70 (s, 1H,

=CH), 8.39 (s, 1H, ArH), 8.17 (dd, J = 21.2, 8.1 Hz, 2H, ArH), 7.43 (dt, J = 28.3, 21.0 Hz, 4H, ArH), 4.40 (td, J = 14.2,

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7.0 Hz, 4H, 2CH2), 1.46 (dd, J = 16.4, 7.3 Hz, 6H, 2CH3).13C

NMR (101 MHz, CDCl3)δ 163.66, 155.91, 142.76, 140.58,

129.04, 126.95, 125.38, 123.52, 122.78, 122.57, 120.95, 120.56, 117.02, 109.24, 109.29, 97.53, 62.30, 37.95, 14.34, 13.90. Anal. calcd for C20H18N2O2: C 75.45, N

8.80, H 5.70; found C 75.49, N 8.75, H 5.65.

2-(4-chlorobenzylidene) malononitrile (Table 3, Entry 19)

White solid.1H NMR (400 MHz, CDCl3) δ 7.88 (d, J =

8.2 Hz, 2H, ArH), 7.77 (s, 1H, =CH), 7.54 (d, J = 8.3 Hz, 2H, ArH).

2-(4-nitrobenzylidene) malononitrile (Table 3, Entry 20)

Yellow solid.1H NMR (400 MHz, CDCl3)δ 8.42 (d, J =

8.6 Hz, 2H, ArH), 8.10 (d, J = 8.6 Hz, 2H, ArH), 7.92 (s, 1H, =CH).

(4-methoxybenzylidene) malononitrile (Table 3, Entry 21)

Light yellow crystals.1H NMR (400 MHz, CDCl3)δ 7.92

(d, J = 6.6 Hz, 2H, ArH), 7.67 (s, 1H, =CH), 7.03 (d, J = 6.6 Hz, 2H, ArH), 3.93 (s, 3H, CH3).

3. Results and discussion

3.1. Structure characterization

The morphologies of Fe3O4@SiO2@propyl@DBU and Fe

3-O4@SiO2 nonoparticales were analyzed by TEM. As

shown inFigure 1, two kinds of particles are spherical, and the diameters of them are between 10 and 20 nm. Combined with XRD analysis, it can be preliminarily confirmed that the SiO2, DBU and alkyl group are

coated on the outside of the dark nano-Fe3O4cores.

3.2. XRD analysis

The composition and structure of magnetic nanoparti-cles Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2@propylchloride

and Fe3O4@SiO2@propyl@DBU were confirmed by

X-ray diffraction (XRD). As shown in the Figure 2, the board peak at 18°–28° indicate that an amorphous silicon sell was formed around Fe3O4, the diffraction

peaks 220, 311, 400, 422, 511 and 440 at 30.3°, 35.68°, 43.34°, 53.08°, 57.22° and 62.7° respectively represent the standard Fe3O4 diffraction card (JCPDS 89-0688)

(49). All diffraction peaks of magnetic nanoparticles conform to the peaks of standard Fe3O4, indicates that

the surface modification of Fe3O4 does not cause

phase transition. Additionally, the particle size of all magnetic nanoparticles can be calculated according to the Debye-Scherrer equation (D = kλ/βcosθ, where D is the crystal diameter, K is 0.89,λ is the wavelength of X-ray, θ is the Braggs angle in radians, and β is the full width at half maximum of the peak in radians). As show inFigure 2, for the particle of Fe3O4, 2θ = 35.678,

β = 0.598, D = 16.3 nm. the particle of Fe3O4@SiO2, 2θ

= 35.64, β = 0.754, D = 12.9 nm. the particle of Fe3O

4-@SiO2@propylchloride, 2θ = 35.58, β = 0.696, D =

14.0 nm. And for the particle of Fe3O4@SiO

2-@propyl@DBU, 2θ = 35.76, β = 0.834, D = 11.7 nm. The diameter values of the magnetic nanoparticles are con-sistent with those obtained by TEM analysis.

3.3. FTIR analysis

In order to better make certain the functional groups and the structure of the magnetic nanoparticles, the FTIR spectroscopy was also performed. As shown in Figure 3, the peak at 572 cm−1 is the characteristic peak of Fe-O bond stretching vibration, the broad peak at 1068 cm−1is the characteristic peak of the Si-O bond stretching vibration. The characteristic peaks at 2897cm−1and 2927cm−1 in curve (3) and curve (4) are associated with C–H bond stretching vibrations, suggesting that alkyl loads are on magnetic nanoparti-cles. The peak at 1639 cm−1in curve (4) is the character-istic peak of C–N bond stretching vibration, which

Figure 1. TEM images of magnetic nanoparticles. (A) Fe3O 4-@SiO2, (B) Fe3O4@SiO2@propyl@DBU.

Figure 2. XRD patterns of nanoparticles. (1) Fe3O4, (2) Fe3O 4-@SiO2, (3) Fe3O4@SiO2@propylchloride, (4) Fe3O4@SiO2@propyl@DBU.

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means the DBU molecule is attached to the magnetic nanoparticles.

3.4. TG analysis

To further investigate the structure, composition and thermal stability of the magnetic nanoparticles, the TG analysis was performed in the temperature range from 25°C to 1000°C. As can be seen from Figure 4, Curve (1) is the mass loss diagram of Fe3O4, from

which we can see that Fe3O4 has two obvious stages

of mass loss. The first stage is from 25°C to 200°C with mass loss of 3.29%, possibly due to the evapor-ation of physically adsorbed water or residual solvent. The second stage is from 200°C to 700°C, with mass loss of 4.41%, which may be caused by hydroxyl group decomposition on the surface of

Fe3O4. Adding the minimal loss of 0.06% above 700°

C, the final mass loss is 7.76%. Curve (2) is the mass loss diagram of Fe3O4@SiO2. It can be seen from the

figure that Fe3O4@SiO2 has two obvious mass loss

stages. The first stage is from 25°C to 200°C with mass loss of 6.85%, which may be due to the evapor-ation of water or residual solvent physically adsorbed. The second stage is from 200°C to 600°C, the mass loss is 4.45%, which may be due to the silicon shell decomposition. There is a minimal loss of 0.03% above 600°C, and the final mass loss is 11.33%. Curve (3) is the mass loss diagram of Fe3O4@SiO2

modified by chloropropyl group. It can be seen from the Figure 4 that there are three stages of mass loss. The first stage is from 25 ° C to 120 ° C, with mass loss of 2.43%. which may be attributed to the residue water or other solvent on the surface of mag-netic nanoparticle. The second stage was from 120°C to 540°C with mass loss of 6.55%, probably due to the decomposition of surface chloropropyl group and residual coupling agent. The third stage is from 540° C to 750°C with mass loss of 1.88%, which may be due to the decomposition of silicon shell. Adding the minimal loss of 0.59% above 750°C, the final mass loss is 11.45%. Curve (4) is the catalyst modified by DBU. It can be seen from the figure that there are three obvious mass loss stages. The first stage is from 25°C to 120°C, with mass loss of 1.7%, which may be due to the evaporation of water or residual solvent physically adsorbed. The second stage was from 120°C to 600°C, with mass loss of 9.02%, probably due to the cracking of DBU and chloropropyl group on the surface. The third stage is from 600°C to 1000°C, with mass loss of 2.36%, which may be due to the decomposition of silicon shell, and the final mass loss of 13.08%. Through TG analysis and comparison of the mass loss curves of the four kinds of magnetic nanoparticles, we may confirm that every step of prep-aration of magnetic nano catalyst is successful, and the DBU and alkyl group are indeed loaded on the surface of Fe3O4@SiO2 particles.

Combined with elemental analysis, it also performed to determine the amount of N in the catalyst and it was found to be 0.4215%. The loading of the DBU can be calculate and the value is about 2.29%.

3.5. VSM analysis

The magnetic measurement was carried out in the applied magneticfield at room temperature. The mag-netic characteristics were measured by sweeping the magnetic field from −20000 to +20000 Oe using VSM and the results were shown in the Figure 5. Not only Figure 3. FTIR spectra of nanoparticles (1) Fe3O4, (2) Fe3O

4-@SiO2, (3) Fe3O4@SiO2@propylchloride, (4) Fe3O4@SiO2@propyl@DBU.

Figure 4.TG curves of nanoparticles. (1) Fe3O4, (2) Fe3O4@SiO2, (3) Fe3O4@SiO2@propylchloride, (4) Fe3O4@SiO2@propyl@DBU.

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the magnetic nano catalyst has good paramagnetism, but also it can be dispersed well in water. Under the external magnetic force, the dispersed nanoparticles can be aggregated very quickly. All of this provides a basis for the application and recovery of magnetic nano catalyst in water reaction.

3.6. SEM analysis

The morphology of catalyst was analyzed by scanning electron microscope (SEM). It can be seen from Figure 6that the average size of the nanoparticles is between 10 and 15 nm, the shape of the catalyst particles is approximately spherical, and the agglomeration effect is produced due to their magnetic effect. The

agglomeration effect makes the magnetic nano catalyst easily separated.

3.7. BET analysis

The N2adsorption–desorption isotherm is as shown in

Figure 7, the apparent hysteresis loop in Figure 7 indi-cates that the catalyst belongs to mesoporous material. The surface area, pore volume and pore diameter of the catalyst were determined by BET test and the BET data is as shown in Table 1. According to the pore size and surface area of the catalyst, combined with the catalytic effect experiment, the nano catalyst can effectively cat-alyze the Knoevenagel reaction. The reason may be a comprehensive effect of ionic microenvironment, nan-ometer size, micropore effect and weak alkali environment.

Figure 5.VSM image of Fe3O4@SiO2@propyl@DBU particles.

Figure 6.SEM images of Fe3O4@SiO2@propyl@DBU particles.

Figure 7.N2adsorption-desorption and pore size distribution curve.

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3.8. Application of magnetic nanocatalyst in Knoevenagel reaction

In order to evaluate the catalytic activity of magnetic nano particle Fe3O4@SiO2@propyl@DBU (MNPs), the

Knoevenagel reaction of p-chlorobenzaldehyde with ethyl cyanoacetate was selected as model reaction, and all kinds of reaction conditions were investigated, the experimental results are summarized inTable 2. As can be seen from Table 2, various factors influencing the reaction such as reaction temperature, base, solvents and amount of catalyst were investigated and opti-mized. Firstly, Fe3O4@SiO2@propyl@DBU (MNPs) was

directly applied to the solvent-free Knovenagel reaction of p-chlorobenzaldehyde and ethyl cyanoacetate. Unfor-tunately, TLC showed no target product after 12 h (Table 2, entry 1), which may be caused by less contact between the MNPs and the reaction substrate in the solid–liquid two-phase state. Based on the good dis-persion of MNPs in water, we added 2 mL H2O to the

reaction system and found that the target product was obtained with a yield of 45% after 6 h (Table 2, entry 2). Since water can prompt the interaction of catalyst with reaction substrate, the amphiphilic PEG-400 might be better for the reaction. Indeed, when 2 mL PEG-400 was added to the aqueous reaction system, the target product was obtained with 50% yield after 3 h (Table 2, entry 3). Although the reaction time is greatly shor-tened, the yield is not ideal. Therefore, 1.0 g inorganic weak base K2CO3was added into the reaction system.

To our surprise, only after 30 min, the target product was obtained with 88% yield (Table 2, entry 5). At the same time, we also noticed that under the same reaction conditions, if there was no catalyst MNPs, only 70% yield of product was obtained after 40 min (Table 2, entry 11),

and K2CO3alone does not facilitate the reaction (Table 2,

entry 6). In order to further evaluate the catalytic effect of MNPs, various reaction systems were used for the Knovenagel reaction between p-chlorobenzaldehyde and ethyl cyanoacetate (Table 2, entries 12–24). The results showed that the MNPs had an obvious catalytic effect, which not only accelerate the reaction rate, but also greatly improve the yield. In addition, other com-ponents such as K2CO3, H2O and PEG-400 can cooperate

with the catalyst MNPs, which can effectively promote the Knovenagel reaction. In subsequent studies, organic base DABCO was used in the model reaction, but only the target product with moderate yield was obtained (Table 2, entries 12 and 13). In order to Table 1.The results of BET surface area analysis.

Parameters Fe3O4@SiO2@propyl@DBU

Surface area

Single point surface area at P/Po = 0.294306215

6.2670 m2/g

BET surface area 6.6029 m2/g

Langmuir surface area 10.2029m2/g t-Plot external surface area 7.2890 m2/g

Pore volume

Single point adsorption total pore volume of pores less than 181.2138 nm diameter at P/Po = 0.989213975

0.0202327 cm3/g

BJH Adsorption cumulative volume of pores between 1.7000 and 300.0000 nm diameter

0.020561 cm3/g

BJH Desorption cumulative volume of pores between 1.7000 and 300.0000 nm diameter

0.021049 cm3/g

Pore size Adsorption average pore diameter (4 V/A by BET)

12.25931 nm BJH Adsorption average pore

diameter (4 V/A)

11.4992 nm BJH Desorption average pore

diameter (4 V/A)

8.2881 nm

Table 2.Optimization of the reaction conditions.a

Entry solvent–catalyst system

Temp.

(oC) (min)Time Yield(%)

1 MNPs (0.5 g) 30 720 ND 2 MNPs (0.5 g) + H2O (2 mL) 30 360 45 3 MNPs (0.5 g) + H2O (2 mL) + PEG-400(2 mL) 30 180 50 4 H2O (2 mL) + PEG-400(2 mL) 30 720 ND 5 MNPs (0.5 g) + H2O (2 mL) + PEG-400 (2 mL) + K2CO3(1.0 g) 30 30 88 6 K2CO3(1.0 g) 30 40 ND 7 K2CO3(1.0 g + MNPs (0.5 g) 30 720 ND 8 K2CO3(1.0 g) + MNPs (0.5 g) + H2O (2 mL) 30 40 78 9 K2CO3(1.0 g) + NPs (0.5 g) + PEG-400(2 mL) 30 40 80 10 K2CO3(1.0 g) + H2O (2 mL) 30 40 67 11 K2CO3(1.0 g) + H2O(2 mL) + PEG-400(2 mL) 30 40 70 12 MNPs (0.5 g) + H2O (2 mL) + DABCO (1.0 g) 30 20 58 13 MNPs (0.5 g) + H2O(2 mL) + PEG-400(2 mL) + DABCO (1.0 g) 30 20 60 14 MNPs (0.5 g) + H2O(2 mL) + PEG-400(2 mL) + K2CO3(1.0 g) 30 30 88 15 MNPs (0.5 g) + H2O(2 mL) + PEG-400(2 mL) + K2CO3(1.0 g) 40 20 98 16 MNPs (0.5 g) + H2O(2 mL) + PEG-400(2 mL) + K2CO3(1.0 g) 50 20 98 17 MNPs (0.5 g) + H2O(2 mL) + PEG-400(2 mL) + K2CO3(1.0 g) 60 20 95 18 MNPs (0.5 g) + H2O(4 mL) + PEG-400(2 mL) + K2CO3(1.0 g) 40 20 98 19 MNPs (0.5 g) + H2O(6 mL) + PEG-400(2 mL) + K2CO3(1.0 g) 40 15 98 20 MNPs (0.5 g) + H2O(8 mL) + PEG-400(2 mL) + K2CO3(1.0 g) 40 20 98 21 MNPs (0.1 g) + H2O(6 mL) + PEG-400(2 mL) + K2CO3(1.0 g) 40 30 ND 22 MNPs (0.3 g) + H2O(6 mL) + PEG-400(2 mL) + K2CO3(1.0 g) 40 15 100 23 MNPs (1.0 g) + H2O(6 mL) + PEG-400(2 mL) + K2CO3(1.0 g) 40 15 98 24 MNPs (1.5 g) + H2O(6 mL) + PEG-400(2 mL) + K2CO3(1.0 g) 40 15 98 a

Reaction conditions: p-chlorbenzaldehyde(5 mmol) and ethyl cyanoacetate (6 mmol).

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further optimize the catalytic effect of MNPs, the reac-tion temperature (Table 2, entries 14–17), the amount of catalyst (Table 2, entries 21–24) and the ratio of water to PEG-400 were further explored. It was found that the target product was obtained with 100% yield after 15 min when the reaction temperature is 40°C and the solvent catalyst system is MNPs (0.3 g) + H2O

(6 mL) + PEG-400 (2 mL) + K2CO3(1.0 g).

To further evaluate the scope and limitation of the catalyst MNPs on different substrates, all kinds of aro-matic aldehydes were selected to react with ethyl cya-noacetate under the optimized conditions. As can be seen from Table 3, for various aromatic aldehydes attached to electron-withdrawing or electron-donating groups, the target product can be obtained almost at a quantitative yield within 1–40 min, and the reaction of aldehydes attached to electron-withdrawing groups is generally faster than that of aldehydes attached to electron-donating groups. Interestingly, the fused ring compound 9-anthraldehyde produced its target product at a 96% yield after 15 min, and for the ter-ephthalaldehyde, both aldehyde groups undergo the

Knovenagel reaction, 99% yield was achieved after 15 min. However, for the 4-dimethylaminobenzaldehyde and 4-aminobenzaldehyde attached to the strong elec-tron-donating groups, there is a great difference in the reaction yield. The reason may be due to the instability of p-aminobenzaldehyde, as TLC analysis showed that there were multiple by-products produced during the reaction. It is particularly important to note that the reac-tion of 2-hydroxybenzaldehyde with ethyl cyanoacetate did not result in the normal Knovenagel product, Instead, a heterocyclic compound was obtained, which was consistent with the results reported in reference (61). To our delight, heteroaryl aldehydes also under-went Knoevenagel reaction to give the corresponding adduct with a good yield (Table 3, entries 17, 18). To broaden the scope of the Knoevenagel reaction cata-lyzed by MNPs, we also carried out the MNPs induced Knoevenagel reaction of aldehydes with other active methylene compounds. When the malononitrile was selected as activated methylene compound, excellent yields were obtained (Table 3, entries19–20), unfortu-nately, when ethyl acetoacetate was used in the

Table 3.Knoevenagel reaction of aldehydes with active methylene.a

Entry RCHO Product Time mp (°C) (lit. mp) Yield (%)b

1 15 93.5 (92–94) (50) 100 2 1 169.2 (168–169) (51) 100 3 20 51.8 (50–52) (52) 100 4 15 82.0 (81–82) (53) 96 5 5 168.7 (168–169) (54) 100 6 20 127.6 (126–128) (55) 100 7 30 88.3 (86–88) (56) 98 (Continued)

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Table 3.Continued.

Entry RCHO Product Time mp (°C) (lit. mp) Yield (%)b

8 20 171.4 (170–171) (56) 100 9 40 121.8 99 10 30 97.4 (94–96) (56) 98 11c 15 199.8 (199–200) ( 57) 99 12 20 140.5 (138–139) (58) 98 13 20 125.3 (125–126) (59) 98 14 15 183.2 (184) (60) 96 15 10 138.7 (139) (61) 99 16 30 162.5 (162–163) (62) 36 17 15 90.5(91–93) (63) 98 18 20 142.0 98 19 15 162.5(162–163) (64) 100 20 5 160–162(161.5–162) (64) 100 21 15 112–114(113.5–114) (64) 98

aGeneral reaction conditions: [HyEtDBU]Br (3 g), H

2O (1 mL), DABCO (10 mmol), aldehyde (10 mmol), ethyl cyanoacetate (12 mmol). b

Refers to work-up yield.

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Knoevenagel reaction catalyzed by MNPs, almost no product was detected. In conclusion, magnetic nanoca-talysts have excellent catalytic activity to the Knovenagel reaction of various aromatic aldehydes with ethyl cya-noacetate and malononitrile.

Based on the excellent catalytic effect of magnetic nano-catalyst MNPs, its recyclability was further investi-gated, and the results are summarized in Table 4. Firstly, p-chlorobenzaldehyde (0.7 g, 5 mmol) were added to the solution of ethyl cyanoacetate (0.6 mL, 6 mmol), MNPs (0.3 g) and K2CO3(1.0 g) in the

compo-site solvent PEG400–H2O (6 mL H2O + 2 mL PEG400),

then the reaction mixture was stirred at 40°C and the reaction progress was monitored by thin layer chrom-atography (TLC). After 15 min, TLC showed that p-chlor-obenzaldehyde was consumed. When 50 mL water was added to the reaction mixture, a large number of floccu-lent solid products appeared. After the MNPs was separ-ated from the reaction system by using the permanent magnet, the solid crude product was collected and recrystallized with 90% ethanol, and almost pure target product was obtained with quantitative yield. The separated MNPs were washed with ethyl acetate (10 mL × 2) and then used in the next cycle. It can be seen from Table 4 that MNPs can be recycled, and there is almost no change in product yield and reaction time after 10 cycles.

4. Conclusion

In summary, organic base DBU was immobilized on the surface of silica-coated Fe3O4 nanoparticle to afford a

functional magnetic nano catalyst Fe3O4@SiO

2-@propyl@DBU, which was used in the Knovenagel reac-tion of numerous aromatic aldehydes with ethyl cyanoacetate. The results showed that the nano catalyst MNPs have high catalytic activity, for most aldehydes,

the reaction products were obtained at a nearly quanti-tative yield within 1–40 min. In addition to this, the cat-alyst MNPs can be easily recovered under external magneticfield, and reused for 10 runs with almost no loss of activity.

Acknowledgements

We are grateful for thefinancial support for this work from the Fund for ‘Shanxi 1331 Project’ Key Subjects Construction (2017-2021) and the Natural Science Foundation of Shanxi Pro-vince (No.201601D102015).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Funding

We are grateful for thefinancial support for this work from the Fund for ‘Shanxi 1331 Project’ Key Subjects Construction (2017–2021) and the Natural Science Foundation of Shanxi Province [grant number 201601D102015].

Notes on contributors

Xiaoyu Zhangis a graduate student at Shanxi Normal Univer-sity, supervised by professor Sanhu Zhao, who is currently studying green chemistry.

Xiaoyan Heis a graduate student at Shanxi Normal University, supervised by professor Yongsheng Qiao and Sanhu Zhao, who is currently studying synthetic chemistry.

Sanhu Zhaoreceived his MSc degree in organic chemistry at the Shanxi University of Taiyuan in 2005. His PhD work was in the field of green chemistry at the Shanxi University of Taiyuan, graduation in 2015. In 2012, he became a professor in chemistry in Xinzhou Teachers University. His interest is focused on green chemistry, organic synthesis, teaching and learning innovations.

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