Nano Ag-doped magnetic-Fe3O4@mesoporous TiO2 core–shell hollow spheres: synthesis and enhanced catalytic activity in A3 and KA2 coupling reactions

Monatsh Chem

DOI 10.1007/s00706-017-1948-2

ORIGINAL PAPER

Nano Ag-doped magnetic-Fe₃O₄@mesoporous TiO₂core–shell

hollow spheres: synthesis and enhanced catalytic activity in A³

and KA²coupling reactions

Ali Elhampour¹

Friuozeh Nemati¹Majid M. Heravi²

•

Received: 24 November 2016 / Accepted: 27 February 2017

Ó Springer-Verlag Wien 2017

Abstract Nano Ag-doped Fe₃O₄@mesoporous TiO₂core–

shell hollow spheres (h-Fe₃O₄@m-TiO₂/Ag) were efﬁ-

ciently prepared. They were fully characterized by FT-IR,

FE-SEM, EDX, ICP, TGA, XRD, and BET methods. In

addition, their magnetic properties were determined by

means of the VSM method. Also, the q_shellof the prepared

hollow Fe₃O₄micro-particles was calculated, being found

just slightly lighter than that of solid sphere-like Fe₃O₄.

The catalytic activity of this novel catalyst system was

successfully examined as a novel catalyst system in A³

coupling reactions (aldehyde ? amine ? acetylene). This

highly active magnetic catalyst was easily separated using

an external magnet and re-used in ﬁve consecutive runs in

the model reaction without appreciable loss of its catalytic

activity.

Graphical abstract

Keywords Nanocatalyst Á Hollow sphere Á Silver catalyst Á

Three-component reaction Á Heterogeneous catalyst

Introduction

In recent years, many attempts have been made on the

synthesis of low or non-toxic nanomaterials, especially

those of magnetic metals, alloys, and metal oxides. Among

them, iron oxide-based magnetic materials have received

much attention due to their low cost, high resistance to

corrosion, and being relatively environmentally benign

[1, 2]. In three recent decades, they have fruitfully been

used in catalysis of a plethora of organic transformation,

owing to their high speciﬁc surface areas as well as

excellent stability, ﬂexibility, simple recovery, and effec-

tive reusability [3–6]. Therefore, numerous methods for the

preparation of iron oxide-based magnetic nanoparticles

have been achieved and reported [7–14]. In this line, the

preparation of various magnetite Fe-nanostructures with

different morphologies including monodisperse nanoparti-

cles, microspheres, nano-rods, nano-belts, nano-wires

nano-tubes, and nano-ﬂowers has been achieved [15–18].

Hollow structure nano-particles have particularly been

Electronic supplementary material The online version of this

article (doi:10.1007/s00706-017-1948-2) contains supplementary

material, which is available to authorized users.

& Friuozeh Nemati

fnemati@semnan.ac.ir

& Majid M. Heravi

mmh1331@yahoo.com

1

Department of Chemistry, Semnan University, Semnan, Iran

2

Department of Chemistry, School of Science, Alzahra

University, Vanak, Tehran, Iran

123

A. Elhampour et al.

found enhancing several organic transformations [19–25].

Fe-nanoparticles have been found acting as highly effective

hallow magnetic spheres [8, 26–30]. In particular, hollow

magnetic iron oxide nanoparticles have stirred up the

interest of organic synthetic chemists being examined as

catalyst due to their low material density as well as strong

magnetic response as well as simple recovery by an

external magnet [31]. Recently, several strategies have

been developed to prepare hollow magnetite spheres such

as the nanoscale Kirkendall effect, Ostwald ripening,

molten salt corrosion, reverse micelle transport, and layer-

by-layer assembly [32–36]. These approaches were mainly

achieved, based on the use of well-established template

method, such as hard template, soft template, and sacriﬁ-

cial template [37–39]. Thus, the template method has been

the method of choice for the successful preparation of a

wide array of hollow structures.

catalysts as well as improving atom economy is over-

growing, overshadowing and compensating the use of

catalysts [58, 63–83]. Herein, we wish to report the

preparation of novel, recoverable, and recyclable Ag

nanoparticles supported on magnetic hollow-Fe₃O₄cores

with a mesoporous TiO₂shell to efﬁciently catalyze A³

coupling reaction under mild reactions conditions to yield

propargylamine derivatives in excellent yields.

Results and discussion

Synthesis of magnetic-Fe₃O₄@m-TiO₂/Ag hollow

sphere

The route leading to magnetic-Fe₃O₄@m-TiO₂/Ag hollow

spheres is illustrated in Scheme 1. The magnetized catalyst

was prepared from relatively inexpensive commercially

available materials and fully characterized by FT-IR, FE-

SEM, TEM, XRD, TGA, BET, N₂adsorbtion–desorbtion,

and VSM techniques. A pathway to obtain the hollow

microspheres was suggested as follows: the ethylene glycol

used plays a dual role as a solvent and a reducer during the

solvothermal process. It can also provide -OH groups

which coordinate with Fe^3?. The Fe₃O₄nuclei are gener-

ated by changing of pH (via the addition of NaAc) at

elevated temperature; they quickly grow up to become

small nanoparticles and aggregate to form the micro-

spheres, owing to their high surface energy [84]. According

to Scheme 1, a compact TiO₂layer was directly immobi-

lized on the generated hollow-Fe₃O₄microspheres by a

sol–gel method.

Hollow-structured porous spheres, featured with hollow

cores and porous shells, exhibit low density, well-deﬁned

interior voids, high speciﬁc surface area [40–44].

Titanium dioxide has recently been employed as an

essential shell, due to its unique surface properties, high

photo catalytic activity, low cost, and non-toxicity [45]. In

this regard, different nano TiO₂structures have been

designed, i.e., core–shell, mesoporous, and hollow sphere

structures [46–48]. As could be expected, the inclusion of a

magnetic Fe₃O₄core into TiO₂nano spheres gives them the

advantage of simple recovery and thus recyclability as well

avoiding their undesirable aggregation [49]. In the past few

years, an increasing number of multicomponent reactions

(MCRs) have been developed as a powerful tool for the

synthesis of complex molecules through an assembly of

several simple starting materials in a one-pot fashion.

Nowadays, the advantages of MCRs are well established

[50]. Among the different MCRs, the A³coupling reaction

(aldehyde, alkyne, and amine) is a method of choice for the

synthesis of high-value propargylamine derivatives [51].

Propargylamines are frequently used as synthetically ﬂex-

ible key intermediates [52–54] in the synthesis of natural

products and biologically active compounds as well as

nitrogen-containing biologically active molecules [55].

Propargylamines are conventionally synthesized by a cat-

alyzed A³coupling reaction using transition metals such as

copper [56–58], iridium [59], nickel [60], and iron [61].

Due to its cost, silver has scarcely been used as catalyst

in organic transformations. As a matter of fact its merits,

such as high catalytic activity towards coupling reactions,

cycloadditions, sigmatropic rearrangements, cycloisomer-

izations, and nitrene transfer reactions have largely been

overlooked [62]. It might be mentioned that silver is

absolutely non-toxic and easy to handle. Nowadays, the

development of environmentally friendly catalyzed MCRs

enjoying the easy recovery and effective recyclability of

Characterization of magnetic Fe₃O₄@m-TiO₂/Ag

hollow spheres

FT-IR Spectra

The FT-IR spectra of the h-Fe₃O₄, h-Fe₃O₄@m-TiO₂, and

h-Fe₃O₄@m-TiO₂(h = hollow) doped with Ag nanopar-

ticles in the range of 4000–500 cm^-1were recorded and

presented in Fig. 1. In all these spectra, two main absorp-

tion bands at around 450 and 560 cm^-1are observed.

These bands can be attributed to the intrinsic stretching

vibrations of Fe–O and Ti–O in the sample. Other char-

acteristic peaks at 1398 and 1633 cm^-1are associated with

the presence of stretching vibrations of Ti–O and Fe–O–Ti

bonds [87].

X-Ray Diffraction Spectra

The X-ray diffraction (XRD) patterns of the as-synthesized

microspheres are shown in Fig. 2a, b, showing the XRD

patterns of the nano-Fe₃O₄and hollow Fe₃O₄micro-

spheres. The strong diffraction peaks at 2h = 30.1°, 35.5°,

123

Nano Ag-doped magnetic-Fe₃O₄@mesoporous TiO₂core–shell hollow spheres: synthesis and…

Scheme 1

1. EtOH

deionized H₂O

NH₃.H₂O

2. Autoclave

160 °C, 20 h

1. Autoclave

200 °C, 10 h

2. Vacum oven

80 °C, overnight

TBOT

EtOH

Acetonitrile

NH₃.H₂O

h-Fe₃O₄

h-Fe₃O₄@TiO₂

PVP

NaAC

FeCl_3.6H₂O

Ethylene glycol

h-Fe₃O₄@mTiO₂

1. Ag(NH₃)₂NO₃

PVP/EtOH

Reflux,70 °C, 4 h

2. dried

50 °C, 12 h

h-Fe₃O₄@mTiO₂/Ag

structure of Ag (JCPDS card No. 04-0783) that conﬁrmed

the successful preparation of crystalline h-Fe₃O₄@m-TiO₂/

Ag microspheres. The broadening of Ag diffraction peaks

conﬁrmed that the size of Ag particles is in the nano-meter

range.

Thermo-gravimetric analysis (TGA)

The temperature-dependent changes can be veriﬁed by the

determination of mass loss employing thermo-gravimetric

analysis (TGA). The TGA of h-Fe₃O₄, h-Fe₃O₄@m-TiO₂,

and h-Fe₃O₄@m-TiO₂/Ag microspheres are shown in

Fig. 3. The initial loss region of samples (up to 150 °C) is

due to the evaporation of the absorbed water and solvent.

By increasing the temperature, a drastic weight loss for

h-Fe₃O₄, h-Fe₃O₄@m-TiO₂was observed, owing to the

decomposition of adsorbed organic groups on their sur-

faces. The removal of organic components starts at around

200 °C and is virtually completed at 450 °C. Beyond this

temperature, the mass loss stays constant. The h-Fe_3-

O₄@m-TiO₂/Ag nano-composite shows lower weight lose

(nearly 8%), because it doped with Ag nano-particles and

has low porosity for absorbing solvent. Thus, it can be

concluded that as-prepared magnetic composite is sufﬁ-

ciently stable under the reaction conditions including the

reaction temperature.

Fig. 1 The FT-IR spectra of a h-Fe₃O₄, b h-Fe₃O₄@m-TiO₂, and c

h-Fe₃O₄@m-TiO₂/Ag

43.2°, 52.7°, 57.1°, and 62.7° can be indexed to the (220),

(311), (400), (422), (511), and (440) reﬂections. They are

typical in the structure of Fe₃O₄(JCPDS no. 19-629).

Compared with the nano-Fe₃O₄particles, the peaks of

h-Fe₃O₄are narrower, indicating the formation of micro

size particles. The h-Fe₃O₄@m-TiO₂microspheres showed

additional peaks, which can be assigned to anatase TiO₂

(JCPDS card No. 01-075-2545). The average crystal size of

TiO₂was measured being about 28 nm. In the XRD pattern

of h-Fe₃O₄@m-TiO₂/Ag microspheres, Fig. 2d shows the

characteristic peaks of h-Fe₃O₄(labeled as F) and m-TiO₂

(labeled as M), indicating that the h-Fe₃O₄@m-TiO₂

structure remained unchanged. The characteristic peaks at

2h of 37.9°, 44.1°, 64.3°, and 77.2° (labeled as A) could be

attributed to XRD spectra of the face-centered-cubic

Vibrating sample magnetometer (VSM)

The magnetic hysteresis loops of samples were conven-

tionally studied by vibrating sample magnetometer (VSM).

The magnetization curve for nano magnetic Fe₃O₄hollow

sphere, h-Fe₃O₄@m-TiO₂, and h-Fe₃O₄@m-TiO₂/Ag is

123

A. Elhampour et al.

Fig. 2 XRD pattern of a nano-

Fe₃O₄, b h-Fe₃O₄, c

h-Fe₃O₄@m-TiO₂, and d

h-Fe₃O₄@m-TiO₂/Ag

Fig. 3 TGA analysis of a

h-Fe₃O₄, b h-Fe₃O₄@m-TiO₂,

and c h-Fe₃O₄@m-TiO₂/Ag

Field emission scanning electron microscopy (FE-SEM)

For the full structural characterization of magnetic Fe₃O₄

hollow sphere products, FE-SEM and TEM images were

measured at different stages during the course of its prepa-

ration. Figure 5a shows the SEM image of the h-Fe₃O₄

microspheres with diameters of 300–400 nm. The surface of

the hollow spheres is rough and composed of closely packed

Fe₃O₄nano-particles with the well-distributed size ranging

from 30 to 40 nm. The EDX characterization demonstrated

that the hollow spheres were pure. Next, the Fe₃O₄hollow

spheres were coated with a layer of TiO₂through a

hydrothermal process. As it can be clearly seen, the TiO₂

layer formed via tiny particles is closely shelled on the

surface of Fe₃O₄hollow sphere. Moreover, the EDX pattern

depicted in Fig. 4. The maximum saturation magnetization

(Ms) values of h-Fe₃O₄@m-TiO₂and h-Fe₃O₄@m-TiO₂/Ag

were found to be 39.26 and 33.83 emu/g, respectively. It is

clear that the Ms value of the corresponding nano magnetic

Fe₃O₄hollow sphere (70.66 emu/g) is signiﬁcantly higher

than that of the h-Fe₃O₄@m-TiO₂, which may be due to the

contribution of the non-magnetic material. Compared with

h-Fe₃O₄@m-TiO₂, the h-Fe₃O₄@m-TiO₂/Ag shows

a

slightly smaller Ms value which is attributed to the immo-

bilization of Ag nano-particles on the surface of

h-Fe₃O₄@m-TiO₂microspheres. Notably, having all these

complex features, the catalyst can be easily separated from

the reaction mixture which actually is a suspension system

with the aid of an external magnetic force.

123

Nano Ag-doped magnetic-Fe₃O₄@mesoporous TiO₂core–shell hollow spheres: synthesis and…

Fig. 4 The magnetization

curves of a h-Fe₃O₄, b

h-Fe₃O₄@m-TiO₂, and c

h-Fe₃O₄@m-TiO₂/Ag

of Fe₃O₄@m-TiO₂hollow spheres conﬁrmed the existence

of Ti with 5.1% (by mass) in it (Fig. 5b). Finally, in order to

prepare the h-Fe₃O₄@m-TiO₂/Ag core shell structure, Ag

ions adsorbed on the Fe₃O₄@m-TiO₂hollow spheres through

an in situ wet chemistry. The FE-SEM images of the Ag-

doped Fe₃O₄@m-TiO₂hollow spheres are shown in Fig. 5c.

As it can be seen, the surfaces of the porous TiO₂shell are

covered with medial quantities of small particles. Further-

more, the corresponding EDX pattern conﬁrmed that the

prepared composite contains four elements (O, Fe, Ti, and

Ag) and reveals the content of *14.7 and 2.2 wt% for the

TiO₂shell and Ag nano-particles of the h-Fe₃O₄@m-TiO₂/

Ag. Inductively coupled plasma (ICP) results also indicated

that the loading capacity of the Ag on the support is 2.1 wt%.

sphere, the pore structures of the h-Fe₃O₄@m-TiO₂and

h-Fe₃O₄@m-TiO₂/Ag were measured by N₂adsorption–

desorption analyses. As shown in Fig. 7a, the isotherms

were identiﬁed as type IV according to the IUPAC clas-

siﬁcation, which is characteristic of mesopore structure.

Measured by BET model, the h-Fe₃O₄@m-TiO₂and

h-Fe₃O₄@m-TiO₂/Ag had surface areas of 66.5 and

57.1 m²/g and pore volume of 0.142 and 0.139 m³/g.

Moreover, their corresponding pore size distributions were

evaluated using the Barrett–Joyner–Halenda (BJH) model

and centered at 22 and 19.8 nm for h-Fe₃O₄@m-TiO₂and

h-Fe₃O₄@m-TiO₂/Ag (Fig. 7b). Considering the average

pore size and the diameter of Ag NPs, the insertion of Ag

into the pores is possible that can help to prevent the loss of

Ag nano-particles in practical applications, especially when

they are used as catalysts.

Transmission electron microscopy (TEM)

Other detailed morphological and structural features of the

nano magnetic hollow sphere were also examined by TEM.

From the TEM image (Fig. 6), the magnetic cores as black

spheres have average size of about 300 nm, and the tita-

nium mesoporous shell in gray color has an average

thickness of about 40 nm. These results disclosed that the

spherical shape of the microspheres was retained even after

etching with NH₃–H₂O. The Ag nano-particles could be

observed with an average diameter of about 2 nm which

are homogeneously immobilized on the surface of Fe_3-

O₄@m-TiO₂hollow spheres. Although a relatively high Ag

loading of 2.1% was used, no evidence for Ag NPs

agglomeration was observed.

Determination of the physical property of the shell

in hollow sphere

Determination of the physical property of shell in hollow

sphere particles is important since it is a factor which directly

affects functionality of the desired catalyst. Thus, we deter-

mined the shell density using speciﬁc surface area and

shell thickness in accordance with our earlier report [88].

Evolution of the Catalytic activity of h-Fe₃O₄@

m-TiO₂/Ag

Having prepared and fully characterized h-Fe₃O₄@

m-TiO₂/Ag, its activity as a heterogeneous magnetically

catalyst was evaluated in the A³and KA²coupling reac-

tions. To ﬁnd the optimized reaction conditions, the three-

component reaction of benzaldehyde, piperidine, and

N₂adsorption–desorption isotherms and BJH distribution

To investigate the effect of Ag doped on the surface area

and pore size distribution of the nano magnetic hollow

123

A. Elhampour et al.

Fig. 5 The FEG-SEM–EDS analysis of a h-Fe₃O₄, b h-Fe₃O₄@m-TiO₂, and c h-Fe₃O₄@m-TiO₂/Ag

phenylacetylene as the model reaction was selected which

gave the corresponding propargylamine 4a (Scheme 2). In

this regard, various reaction parameters such as solvent,

temperature, and the amount of catalyst were tested. In

addition, with particular reference to the importance of

green chemistry, the model reaction was also conducted

under solvent-free conditions (Table 1).

Delightfully, the best result was achieved when 1.0

equiv of benzaldehyde, 1.2 equiv of piperidine, 1.5 equiv

of phenylacetylene were reacted in the presence of 0.01 g

catalyst (2.1 mol % Ag) at 100 °C in solvent-free condi-

tions (Table 1, entry 6).

Securing the optimal reaction conditions, the substrate

scope of the reaction for the synthesis of propargylamine

derivatives was investigated using differently substituted

benzaldehydes, aliphatic aldehydes, and aryl acetylenes

bearing electron-donating substituents as well as electron-

withdrawing groups. The results are depicted in Table 2

(4a–4k). All the used arylaldehydes with either electron-

donating or electron-withdrawing substituent afforded the

corresponding products in good yields and short reaction

As it can be seen in Table 1, in the absence of catalyst, a

low yield of the products was obtained (entry 1). To

illustrate the essential requirement of Ag for this reaction,

the model reaction was conducted with h-Fe₃O₄and

h-Fe₃O₄@mTiO₂. The results showed that the reaction

produced a lower yield in the speciﬁed time (entries 2

and 3).

123

Nano Ag-doped magnetic-Fe₃O₄@mesoporous TiO₂core–shell hollow spheres: synthesis and…

Fig. 6 The TEM image of

h-Fe₃O₄@m-TiO₂/Ag

Fig. 7 a N₂adsorption–desorption isotherms h-Fe₃O₄@m-TiO₂and h-Fe₃O₄@m-TiO₂/Ag. b BJH pore size distribution curves for h-Fe₃O₄@m-

TiO₂and h-Fe₃O₄@m-TiO₂/Ag

Notably, the reaction failed to occur using aromatic

amines. In addition, in order to envisage the scope of the

catalytic activity of h-Fe₃O₄@m-TiO₂/Ag magnetic com-

posite, the optimized reaction conditions were tested for

the three-component coupling of ketones, secondary ami-

nes and arylacetylenes (KA²coupling, Table 3).

Scheme 2

O

Reaction conditions

N

Ph

H

N

Ph

H

Ph

Generally, ketoimines are considered to be less reactive

towards nucleophilic additions than aldimines due to steric

hindrance and electronic effects. Therefore, the in situ

generation of ketoimines including the direct alkyne addi-

tion reaction still remains a signiﬁcant challenge [89]. We

optimized the reaction conditions for KA²coupling reaction

of cyclohexanone, piperidine, and phenylacetylene. A

times. Moreover, as shown in Table 2, the synthesis of

propargylamine derivatives also worked well with aliphatic

aldehydes (Table 2, 4m–4p). In addition, this strategy

successfully worked in the coupling of various amines such

as piperidine, morpholine and pyrrolidine.

123

A. Elhampour et al.

Table 1 Optimization of the reaction of three-component reaction catalyzed by h-Fe₃O₄@m-TiO₂/Ag

Entry

Catalyst (g)

Condition

Time/min

Yield/%^a

1

None

Solvent free/100 °C

Solvent free/25 °C

Solvent free/60 °C

Solvent free/100 °C

Solvent free/120 °C

Solvent free/100 °C

Toluene/100 °C

60

300

60

50

60

90

Trace

45

2

h-Fe₃O₄

3

h-Fe₃O₄@m-TiO₂

4

h-Fe₃O₄@m-TiO₂/Ag (0.01)

h-Fe₃O₄@m-TiO₂/Ag(0.01)

h-Fe₃O₄@m-TiO₂/Ag (0.01)

h-Fe₃O₄@m-TiO₂/Ag (0.015)

h-Fe₃O₄@m-TiO₂/Ag (0.020)

h-Fe₃O₄@m-TiO₂/Ag (0.01)

5

6

97

7

96

8

84

9

96

10^b

11^b

12^b

13^b

73

Water/100 °C

Ethanol/Reﬂux

Trace

43

DMSO/100 °C

50

Reaction conditions: benzaldehyde (1 mmol), piperidine (1.2 mmol), phenylacetylene (1.5 mmol)

a

Isolated yield

solvent (3 cm³)

b

Table 2 Three-component reaction of aldehydes, secondary amines, and terminal alkynes catalyzed by h-Fe₃O₄@m-TiO₂/Ag

R³

R²

O

N

H

N

h-Fe₃O

₄@m-TiO₂/Ag (0.01 g)

R²

R³

H

R¹

Ar

R¹

solvent free / 100 ^o

C

Ar

Aldehyde

Amine

Alkyne

Product

Time/min

Yield/%^a

C₆H₅–CHO

Piperidine

PhC:CH

4a

4b

4c

4d

4e

4f

97

60

65

60

50

75

90

55

65

50

45

50

40

60

90

91

95

93

90

85

91

90

82

90

86

90

83

92

90

4-BrC₆H₄–CHO

4-MeC₆H₄–CHO

Thiophene-2-carbaldehyde

Terephthalaldehyde

1-Naphthaldehyde

C₆H₅–CHO

q-MeC₆H₄–C:CH

q-FC₆H₄–C:CH

PhC:CH

4g

4h

4i

C₆H₅–CHO

Morpholine

Pyrrolidine

Piperazine

Pyrrolidine

Morpholine

Piperidine

Morpholine

C₆H₅–CHO

PhC:CH

4j

C₆H₅–CHO

PhC:CH

4k

4l

Cyclohexanecarbaldehyde

Butyraldehyde

HCHO

PhC:CH

4m

4n

4o

4p

q-MeC₆H₄–C:CH

q-FC₆H₄–C:CH

PhC:CH

HCHO

Reaction conditions: aldehydes (1 mmol), amines (1.2 mmol), alkynes (1.5 mmol), h-Fe₃O₄@m-TiO₂/Ag (0.010 g) under solvent-free condition

at 100 °C

a

Isolated yield

123

Nano Ag-doped magnetic-Fe₃O₄@mesoporous TiO₂core–shell hollow spheres: synthesis and…

Table 3 Three-component reaction of aliphatic ketones, secondary amines, and terminal alkynes catalyzed by h-Fe₃O₄@m-TiO₂/Ag

R³

R⁴

O

N

H

N

e

h-F

₃O

₄@m-TiO₂/Ag (0.01 g)

R³

R⁴

R⁵

or

1

2

R⁵

solvent free / 100 ^o

C

or

1

2

Ketone

Amine

Alkyne

Product

Time/min

Yield/%^a

Cyclopentanone

Cyclohexanone

Piperidine

Morpholine

Pyrrolidine

Piperidine

Morpholine

PhC:CH

5a

5b

5c

5d

5e

5f

140

150

160

190

140

160

87

81

80

85

80

90

80

q-MeC₆H₄–C:CH

PhC:CH

q-FC₆H₄–C:CH

5g

Reaction conditions: ketones (1.5 mmol), amines (1 mmol), alkynes (1.5 mmol), h-Fe₃O₄@m-TiO₂/Ag (0.010 g), solvent-free at 100 °C

a

Isolated yield

1.5:1:1.5 mixture of the aforementioned compounds with

0.01 g magnetic composite was stirred under solvent-free

condition at 100 °C. The desired quaternary carbon-con-

taining propargylamine 5d was obtained in 85% yield. The

substrate scope of the KA²coupling reaction was evaluated

using cyclic ketones (cyclohexanone and cyclopentanone),

secondary amines, and alkynes under optimal conditions.

The results are shown in Table 3. It might be noted that

aromatic ketones are virtually unreactive which had been

realized, previously [90, 91]. Table 4 shows clearly the

merits of this protocol for the synthesis of propargylamine

4a in comparison with other catalysts.

simple and almost quantitative recovery of the magnetized

catalyst in pure form to be re-used in several further runs

without appreciable loss in its catalytic activity (see

below).

Recycling and leaching of the catalyst

The recovery and recyclability of the catalyst in catalytic

reactions are important and vital issues. Most importantly,

these aspects and qualities make the method economically

feasible, thus, industrially proﬁtable as well as environ-

mentally sustainable. To study these important features and

constancy of the activity of this novel used catalyst system,

the catalyzed three-component reaction involving ben-

zaldehyde, piperidine, and phenylacetylene was selected as

The novel catalyst system has some advantages such as

giving the selected three-component reaction in a short

reaction time, providing excellent yields, and offering

Table 4 Comparison of catalytic activity of h-Fe₃O₄@m-TiO₂/Ag with reported silver catalyst in the catalyzed synthesis of propargylamine 4a

Entry

Catalyst

Condition (solvent/temperature)

Time/h

Yield/ %^b

References

1

2

3

4

5

6

7

8

9

a

PS–NHC–Ag(I)^a

ZnO-IL/Ag^b

Ag-CIN-1^c

Solvent free/50 °C

H₂O/reﬂux

5

3

92

98

86

97

86

92

70

80

[92]

[93]

H₂O/40 °C

12

24

1

[94]

Ag–G^d

CH₂Cl₂/60 °C

Solvent free/100 °C

PEG/100 °C

CH₂Cl₂/60 °C

H₂O/reﬂux

[95]

h-Fe₃O₄@m-TiO₂/Ag

Nano Ag

This work

[96]

20

5

Silver(I) complexes

AgI

[97]

14

10

[98]

Nano Ag₂O

CHCl₃/r.t.

[99]

Polystyrene-supported N-heterocyclic carbine-Ag(I)

IL: 1-(3-trimethoxysilylpropyl)-3-methylimidazolium

Silver grafted porous covalent imine network

Silver-graphene

b

c

d

123

A. Elhampour et al.

a model reaction and conducted under optimal reaction

conditions including the use of optimized quantity of the

catalyst. The progress of this reaction was monitored by

TLC (using n- hexane:ethyl acetate 80:20 as eluent). After

completion the reaction, hot ethanol was added to the

reaction mixture and the catalyst was easily separated using

an external magnet. It was washed with ethanol and dried

at 70 °C. There was no appreciable loss in weight of the

used catalyst. We examined the model reaction to another

important feature. The progress of the reaction was moni-

tored as in initial model reaction. Before the completion of

the reaction, the catalyst was excluded using an external

magnet and allowed the reaction to proceed. After a while

the reaction was stopped. No increase in the yield was

observed showing the negligible amount of leaching. This

experiment is referred to as hot method and is commonly

used to prove that a minimum amount of leaching is

involved in a certain catalyst system. Then, the recovered

catalyst was reused in the selected model reaction under

optimized reaction conditions using fresh substrates.

This reaction was conducted at least ﬁve times, per-

forming the similar precautions which were taken for

model reaction to check the possible leaching and to make

sure the nature of heterogeneous catalysis involved in the

reaction. The yields obtained when the recovered catalyst

system was re-used were quite reasonable showing no

signiﬁcant loss of activity (Fig. 8). The tiny decrease

observed in the yield, due to the gradual loss of the catalyst

during washing up process, is well expected.

The magnetic Fe₃O₄@m-TiO₂/Ag hollow sphere catalyst

exhibited high catalytic activity in the synthesis of

propargylamine by A³and KA²coupling reactions. Fur-

thermore, and most importantly, this catalyst could be

easily separated and re-used up to ﬁve times without sig-

niﬁcant loss of activity and selectivity. Easy recoverability

and reusability of the catalyst without any further puriﬁ-

cation nominate it as a potential candidate for being tried in

different organic transformations and particularly worth-

while being tried in various catalyzed MCRs. We also

believe that this facile preparative strategy can be exten-

ded, in particular applied to the preparation of super

paramagnetic catalyst with hollow sphere structure having

qualities expected for both.

Experimental

All chemicals, including FeCl₃Á 6H₂O, NH₄Ac, ethylene

glycol, ethanol, acetonitrile, NH₃Á H₂O, tetrabutyl orthoti-

tanate (TBOT), and polyvinylpyrrolidone (PVP) in analytical

grade, were purchased from Sigma-Aldrich, and used without

further puriﬁcation. The progress of the reaction was moni-

tored by TLC on commercial aluminum-backed plates of

silica gel 60 F254 and visualized using ultraviolet light.

Melting points were determined in open capillaries using an

1

Electrothermal 9100. H NMR and ¹³C NMR spectra were

recorded using Bruker spectrometer at 400, 300, and 100,

75 MHz. Magnetic h-Fe₃O₄@m-TiO₂/Ag characterized by

FT-IR spectra which were obtained from KBr pellets in the

range of 400–4000 cm^-1using a Shimadzu 8400 s spec-

trometer. X-ray diffraction (XRD) was detected by Philips

Conclusion

˚

using Cu-Ka radiation of wavelength 1.54 A; scanning elec-

In conclusion, we designed and prepared Ag-doped Fe₃O₄

hollow spheres with mesoporous TiO₂shells via a facile

solvothermal method. The phase and structure of the cat-

alyst were fully characterized by XRD, FT-IR, FE-SEM,

EDX, TGA, and VSM measurements. The existence of

porous shells was clearly and convincingly revealed by N₂

adsorbtion–desorbtion, BET, and TEM characterizations.

tron microscopy, FE-SEM-EDX, analysis was performed

using Tescanvega II XMU Digital Scanning Microscope.

Samples were coated with gold at 10 mA for 2 min prior to

analysis. Transmission Electron Microscope (TEM) images

were taken with a CM30300Kv ﬁeld emission transmission

electron microscope. The magnetic properties of the catalyst

were conﬁrmed using a vibrating sample magnetometer

(VSM, Lakeshore7407) at room temperature. BET surface

areas were acquired on a Beckman Coulter SA3100 Surface

Area Analyser. The elemental analysis of the catalyst was

carried out on simultaneous ICP-Optical Emission spec-

trometer (Spectro Genesis—Metek, Germany). Thermo-

gravimetric analysis (TGA) was obtained from a LINSEIS

modeleSTSPT 16,000thermal analyzerunder air atmosphere

at a heating rate of 5 °C min^-1

.

Synthesis of nano magnetic Fe₃O₄hollow sphere

Hollow magnetic particles were synthesized through a

solvothermal reaction [84]. Brieﬂy, PVP (1.0 g) was

Fig. 8 Reusability of the h-Fe₃O₄@m-TiO₂/Ag catalyst

123

Nano Ag-doped magnetic-Fe₃O₄@mesoporous TiO₂core–shell hollow spheres: synthesis and…

dissolved in 40 cm³of ethylene glycol (EG), followed by

the addition of 1.35 g FeCl₃Á6H₂O (5 mmol) and 2.46 g

CH₃COONa (30 mmol). The resulting mixture was stirred

vigorously for 15 min, and was then transferred into a

Teﬂon-lined stainless steel autoclave (100 cm³). Next, the

autoclave was heated at 200 °C for 10 h. After this time,

the autoclave was cooled to room temperature. Upon

cooling, a black residue was formed which was washed

several times with ethanol and deionized water and dried at

80 °C in a vacuum oven overnight.

onto the surfaces of h-Fe₃O₄@m-TiO₂via the electrostatic

attraction. This dispersion was added into 15 cm³of

ethanol contained PVP (0.1 g) and heated at 70 °C for 4 h.

The precipitated solid was collected by the aid of an

external magnet, washed several times with ethanol and

deionized water, and dried at 50 °C for 12 h.

Synthesis of 1-(1,3-diphenylprop-2-ynyl)piperidine

(4a): general procedure

To a mixture of an appropriate benzaldehyde (1.0 mmol),

piperidine (1.2 mmol), and a suitable phenylacetylene

(1.5 mmol), 100 mg magnetic h-Fe₃O₄@m-TiO₂/Ag was

added. The mixture was heated at 100 °C for 1 h (the

progress of reaction was monitored by TLC). After com-

pletion of the reaction, the mixture was cooled to room

temperature and diluted with 10 cm³hot ethanol. Then, the

catalyst was separated by the aid of an external magnet

from the cooled mixture, washed with acetone, dried in

oven, and re-used for a consecutive run under the similar

reaction conditions. After separation of catalyst, the mix-

ture was concentrated and the resulting residue was

puriﬁed by column chromatography on silica gel using a

short column to afford the desired product in excellent

yield (97%).

Synthesis of h-Fe₃O₄@TiO₂core shell

The Fe₃O₄@TiO₂hollow spheres were synthesized by

direct coating of a layer of TiO₂on the Fe₃O₄through sol–

gel method, hydrolyzing tetrabutyl orthotitanate (TBOT) in

a mixture of EtOH and CH₃CN [85]. Thus, 50 mg of the

as-synthesized hollow Fe₃O₄was dispersed in a mixed

solvent containing 90 cm³of ethanol and 30 cm³of ace-

tonitrile with the aid of ultrasound. Then, 0.5 cm³of NH₃–

H₂O was added into the mixture. After that, 1 cm³of

TBOT was added to the above suspension under stirring for

1.5 h. The above core shell was collected by an external

magnet and washed three times with ethanol.

Preparation of h-Fe₃O₄@mesopouros TiO₂core

shells via solvothermal treatment of h-Fe₃O₄@TiO₂

core shell

Acknowledgements The authors gratefully acknowledge the Semnan

University Science and Technology Park, Research Council and Vice-

chancellor for Education and Postgraduate student for ﬁnancial sup-

port of this work.

The mesoporous TiO₂shells were prepared by

a

solvothermal treatment of the already obtained Fe₃O₄@-

TiO₂hollow spheres in a mixed solvent of ethanol and

deionized water, with NH₃ÁH₂O as a porosity modiﬁer [85].

Brieﬂy, 50 mg of the as-prepared Fe₃O₄@TiO₂hollow

spheres was dispersed in 40 cm³of ethanol and 20 cm³of

deionized water, and later a certain amount of NH₃ÁH₂O

(3 cm³) was added to the above suspension. The mixture

was transferred to a Teﬂon-lined stainless steel autoclave

which was then sealed (100 cm³capacity). The sealed

autoclave was kept at 160 °C for 20 h. Then, the autoclave

was cooled to room temperature, and the desired product

(h-Fe₃O₄@mesopouros TiO₂core shell) was collected

from the reaction medium with the help of an external

magnet and washed with ethanol and water several times.

Compliance with ethical standards

Conﬂict of interest The authors declare that they have no conﬂict of

interest.

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Article Doi

Nano Ag-doped magnetic-Fe3O4@mesoporous TiO2 core–shell hollow spheres: synthesis and enhanced catalytic activity in A3 and KA2 coupling reactions

DOI: 10.1007/s00706-017-1948-2

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Article abstract of DOI:10.1007/s00706-017-1948-2

Full text of DOI:10.1007/s00706-017-1948-2

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