Zirconium(IV)-modified silica@magnetic nanocomposites: Fabrication, characterization and application as efficient, selective and reusable nanocatalysts for Friedel-Crafts, Knoevenagel and Pechmann condensation reactions

Article abstract of DOI:10.1016/j.catcom.2013.02.016

A highly efficient and magnetically retrievable zirconium(IV) based nanocatalyst has been synthesized by covalent grafting of 3-hydroxy-2-methyl-1, 4-naphthoquinone onto amine functionalized silica coated magnetic nanoparticles followed by complexation with ZrOCl_2. The structural and magnetic properties of obtained nanocatalyst are identified by TEM, SEM, XRD, EDS, VSM and FT-IR analyses. The potential of resultant organic-inorganic hybrid nanocatalyst was effectively evaluated for the range of organic transformations such as Friedel-Crafts, Knoevenagel and Pechmann condensation reactions. The magnetically recoverable nano-hybrid catalyst presented high TON and chemoselectivity with the ease of reusability for multiple cycles without any appreciable loss in its catalytic activity.

Full text of DOI:10.1016/j.catcom.2013.02.016

Catalysis Communications 35 (2013) 110–114
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
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Short Communication
Zirconium(IV)-modified silica@magnetic nanocomposites: Fabrication,
characterization and application as efficient, selective and reusable
nanocatalysts for Friedel–Crafts, Knoevenagel and Pechmann
condensation reactions
⁎
R.K. Sharma , Yukti Monga, Aditi Puri
Department of Chemistry, University of Delhi, Delhi, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 January 2013
Received in revised form 15 February 2013
Accepted 15 February 2013
Available online 22 February 2013
A highly efficient and magnetically retrievable zirconium(IV) based nanocatalyst has been synthesized by
covalent grafting of 3-hydroxy-2-methyl-1,4-naphthoquinone onto amine functionalized silica coated
magnetic nanoparticles followed by complexation with ZrOCl2. The structural and magnetic properties of
obtained nanocatalyst are identified by TEM, SEM, XRD, EDS, VSM and FT-IR analyses. The potential of resultant
organic–inorganic hybrid nanocatalyst was effectively evaluated for the range of organic transformations such as
Friedel–Crafts, Knoevenagel and Pechmann condensation reactions. The magnetically recoverable nano-hybrid
catalyst presented high TON and chemoselectivity with the ease of reusability for multiple cycles without any ap-
preciable loss in its catalytic activity.
Keywords:
Zirconium
Magnetically retrievable
Grafting
© 2013 Elsevier B.V. All rights reserved.
Silica
Nano-hybrid
1. Introduction
catalytic development, recently, we have reported the importance of zir-
conium modified silica gel as a heterogeneous catalyst for the synthesis of
With the commencement of the modern era, a shift in emphasis in
green chemistry is apparent with the desire to develop more environ-
mental friendly routes to a myriad of materials. This shift has become
more evident with the growth of nanotechnology [1]. In this respect,
the development of marvelous ‘superparamagnetic nanoparticles’ cre-
ates remarkable buzz in the field of green chemistry as their use pro-
vides easy magnetic recovery from the reaction product [2,3]. But
unfortunately, magnetic nanoparticles are prone to aggregate, which
tend to limit their applications. To avoid this, formation of passive coat-
ing of inert material on the surface of iron oxide nanoparticles is neces-
sary. Among various inorganic supports for stabilization, silica is very
promising because of its good thermal and mechanical stability, easy
availability and inexpensiveness, and simple covalent modification
with organometallic moieties [4,5].
The importance of zirconium as a homogeneous catalyst has al-
ready been cited in the number of biologically and pharmaceutically
significant organic transformations [6,7]. However these homoge-
neous catalysts endow with the metal contamination of products
which may be harmful for human health as well as for environment.
To meet the criteria of renewable and environmentally conscious
various biologically important molecules [8]. Continuing our aim to ex-
plore novel heterogeneous catalysts [9–12], recently we have moved
with the upbringing nanotechnology using magnetically recoverable cat-
alytic systems [13], and this time we report the novel silica based
superparamagnetic zirconium(IV) nanocatalyst (Zr(IV)-HMNQ@ASMPs)
for the synthesis of various industrially significant compounds via
Friedel–Crafts, Knoevenagel condensation and Pechmann condensation
reactions. The biological, pharmaceutical, medicinal, and therapeutic
properties of bis-indolyl methanes [14], α,β-unsaturated carbonyl com-
pounds [15] and coumarin derivatives [16] have been well established
in the literature. Though, there are many synthetic routes to obtain
these derivatives, but no one such reagent is commercialized to date.
Also, the developed methods have several drawbacks such as unsatisfac-
tory yields, stringent reaction conditions, use of toxic organic reagents and
prolonged reaction time. Therefore, improvement of catalytic route in
terms of environmentally benign and economical approach is highly
desirable.
2. Experimental
2.1. General remarks
⁎
Corresponding author at: Department of Chemistry, University of Delhi, Delhi-110007,
X-ray diffraction (XRD) was performed using a Bruker (D8 Ad-
vance). Transmission electron microscopy (TEM) images were acquired
India. Tel./fax: +91 11 27666250.
E-mail address: rksharmagreenchem@hotmail.com (R.K. Sharma).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.02.016

R.K. Sharma et al. / Catalysis Communications 35 (2013) 110–114
111
using a Jeol, 2010, at 300 kV. Energy dispersive spectroscopy (EDS)
analysis was performed using an adjacent Oxford (INCA) system.
The quantitative content of zirconium was determined by ICP-MS
Agilent 75003 (G3272A). Scanning electron microscope (SEM) im-
ages were taken by Gemini Ultra55 (Zeiss). Magnetization curve
was obtained by vibrating sample magnetometer (VSM) (EV-9,
Microsense). FTIR spectra were recorded on Perkin Elmer Spectrum
2000. The derived products were analyzed using Agilent gas chroma-
tography (6850GC) HP-5MS column (30 m×0.25 mm×0.25 μm),
5 mol% phenylmethylpolysiloxane, flow gas He.
2.2. Preparation of catalyst and its applications
Scheme 1. Sequence of events in the preparation of Zr(IV)-HMNQ@ASMPs.
2.2.1. Synthesis of amine-functionalized silica coated magnetic
nanocomposites
For the synthesis of the magnetic nanoparticles (MNPs) co-
precipitation method was used [17]. Coating of these MNPs by silica
was performed via sol–gel approach [18]. Silica encapsulated mag-
netic nanoparticles (SMPs) were functionalized according to the
reported procedure [19] using 3-aminopropyltriethoxysilane to af-
ford aminopropylated SMPs (ASMPs).
concentrated to give products. The structure elucidations of the prod-
ucts were confirmed by GC–MS.
3. Results and discussion
2.2.2. Synthesis of Zr(IV)-HMNQ complex grafted silica coated magnetic
nanocatalyst
3.1. Shape and morphology of the nanoparticles
3-Hydroxy-2-methyl-1,4-naphthoquinone (HMNQ) was pre-
pared according to the reported procedure [20]. HMNQ (2 mmol)
and ASMPs (2 g) were then stirred in absolute ethanol and refluxed
for 3 h. The grafted ASMPs (2 g) were stirred with solution of
4 mmol of ZrOCl2 in acetone for 4 h. The resulted Zr(IV)-HMNQ@
ASMPs were separated magnetically, washed with water and dried
under vacuum (Scheme 1).
The size and morphology of the prepared catalyst were character-
ized by TEM (Fig. 1). MNPs prepared by the chemical co-precipitation
are quasi spherical with an average of 7–10 nm as shown in Fig. 1(a).
From HR-TEM as shown in Fig. 1(b), the average interfringe distance
of MNPs was measured to be ∼0.3 nm which, corresponds to (2 2 0)
plane of inverse spinel structured Fe3O4. The nanoparticles depicted in
Fig. 1(c) after silica encapsulation step have a discrete core/shell struc-
ture, and their uniform magnetic core is surrounded by a 2–3 nm thick
silica shell. Fig. 1(d) reveals the grafting of organic polymer (APTES)
onto the surface of silica coated nanoparticles.
The SEM images of nanoparticles before (a) and after (b) encapsula-
tion are shown in Fig. 2. The smooth surface of the magnetic core parti-
cles was roughened due to deposition of silica coating to magnetic
nanoparticles, which formed uniform and continuous shell around
them. Separate silica aggregates are not observed indicating that
precipitation of primary silica nanoparticles was performed only on
the surface of the functionalized core particles.
2.2.3. Experimental procedure for transformations
The obtained nanocatalyst was investigated for a number of im-
portant organic transformations such as Friedel–Crafts (Table 1),
Knoevenagel condensation (Table 2) and Pechmann condensation
(Table 3) reactions. Aliquots for reaction monitoring for each of the re-
action were removed at regular time intervals and analyzed by GC. After
completion of reaction, the catalyst was separated using external mag-
net (see ESI-S1). Rest of the solution was taken out with pipette and
extracted with ethyl acetate, washed with brine solution, dried and
Table 1
Friedel–Craft reaction of aldehydes with indoles.
Entry
1.
R1
H
R2
H
Conv.a (%)
Select.b (%)
100
TON(TOF)c
495(990)
99
*99, 98, 98, 97, 96d
2.
3.
4.
5.
6.
7.
8.
H
H
H
2-Me
2-Me
2-Me
2-Me
4-OCH3
4-CH3
4-Cl
96
99
94
96
95
97
95
99
96
94
99
95
99
97
480(960)
495(990)
470(940)
480(960)
475(950)
485(970)
475(950)
H
4-OCH3
4-CH3
4-Cl
Reaction conditions: catalyst (20 mg), aldehyde (1 mmol), indoles (2 mmol) at R.T., time 30 min.
a
Conversion=[(initial moles of substrate−final moles of substrate)/initial moles of substrate]×100.
b
Selectivity(product)=[desired product%/(total GC peak area% for all the products%)]×100.
c
TON=number of moles of product per mole of catalyst/TOF=TON per hour.
Recycling experiment.
d

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R.K. Sharma et al. / Catalysis Communications 35 (2013) 110–114
Table 2
Knoevenagel-condensation of aldehydes with ethylcyanoacetate.
Entry
1.
R1
H
Conv.a (%)
Select.b (%)
100
TON(TOF)c
495(1500)
99
*99, 98, 98, 97, 97d
2.
3.
4.
5.
6.
7.
3-NO2
4-OH
4-OCH3
4-Cl
4-CH3
95
97
93
96
97
99
99
96
99
97
96
475(1439)
485(1469)
465(1409)
480(1454)
485(1469)
495(1500)
Cinnamaldehyde
100
Reaction conditions: catalyst (20 mg), aldehyde (1 mmol), ethylcyanoacetate (2 mmol) at R.T., time 20 min.
a
Conversion=[(initial moles of substrate−final moles of substrate)/initial moles of substrate]×100.
b
Selectivity(product)=[desired product%/(total GC peak area% for all the products%)]×100.
c
TON=number of moles of product per mole of catalyst/TOF=TON per hour.
Recycling experiment.
d
3.2. Phase and structural characterization of the nanoparticles
support matrices at 588 cm−1 which is assigned to Fe\O stretching vi-
bration [22]. After silica coating of MNPs (SMPs), significant reduction
of the intensity of the Fe\O vibration bands is observed, while a
broad band appears at 1090 cm−1 is assigned to the Si\O stretching vi-
brations. On functionalization, no major changes are observed, however
an extra band at 2926 cm−1 appears for \CH2 stretching of the
silylating agent [23]. In case of catalyst, the IR band related to the an-
chored complex is observed at 1649 cm−1 due to C_N stretching vi-
bration. On metallation, C_N frequency decreased to lower wave
number (1637 cm−1) indicative for formation of metal–ligand bond.
X-ray diffraction measurement was performed to investigate the in-
fluence of modifications on the structure of the magnetic core surface.
XRD patterns of the native MNPs revealed that the positions and the rel-
ative intensities of the diffraction peaks matched well with the standard
XRD data of JCPDS card number (19-0629) for Fe3O4 crystal with a cubic
spinel structure (see ESI-S2). The XRD pattern for SMPs showed a weak
broad band at 2θ=20o–24o which is due to the amorphous silane shell
formed around the magnetic core. The broadening of XRD peaks in
SMPs is attributed to the decrease in crystalline size of the nanoparticles
[21]. Also, there is no any other appreciable shift in the peak positions,
which indicates the structural stability of magnetite nanoparticles.
The presence of well resolved peak of zirconium is observed in the
EDX spectrum (see ESI-S3) of the Zr(IV)-HMNQ@ASMPs, which con-
firms the immobilization of zirconium on the HMNQ grafted ASMPs.
The zirconium content in catalyst was found to be 0.14 mmol/g using
ICP-MS.
3.3. Magnetic properties
The magnetization curves of coated, uncoated and zirconium(IV)
complex grafted nanocatalyst showed no hysteresis opening with
complete reversibility at room temperature. The coercivity and rem-
anence were not observed in any of the curves which confirmed the
superparamagnetism of nanoparticles. Fig. S5 (see ESI) showed the
decrease in saturation magnetism from 53.45 emu/g of MNPs to
15.06 emu/g of Zr(IV)-HMNQ@ASMPs. The deviation between
The FT-IR spectroscopy is employed to monitor the immobilization
process by comparing the precursor and modified surfaces (see
ESI-S4). The spectra of the MNPs showed characteristic of the magnetite
Table 3
Pechmann-condensation of phenols with β-keto ester.
Entry
1.
R1
R2
Conv.a(%)
Select.b (%)
100
TON(TOF)c
495(2981)
3-OH; 5-OH
OEt
99
*99, 98, 97, 97, 96d
2.
3.
4.
5.
6.
2-OH; 3-OH
3-OH
3-Me
3-NH2
3-OCH3
OEt
OEt
OEt
OEt
OEt
98
99
96
98
97
99
96
97
95
99
490(2951)
495(2981)
480(2891)
490(2951)
485(2921)
Reaction conditions: catalyst (20 mg), phenol (1 mmol), β-keto ester (1 mmol) at 110 °C, time 10 min.
a
Conversion=[(initial moles of substrate−final moles of substrate)/initial moles of substrate]×100.
b
Selectivity(product)=[desired product%/(total GC peak area% for all the products%)]×100.
c
TON=number of moles of product per mole of catalyst/TOF=TON per hour.
Recycling experiment.
d

R.K. Sharma et al. / Catalysis Communications 35 (2013) 110–114
113
Fig. 1. TEM micrographs of (a) MNPs (b) HR-TEM image (c) SMPs and (d) ASMPs.
them is attributed to the increased mass of diamagnetic silica over
MNPs and grafting of metal–ligand complex over ASMPs [24].
condensation of coumarins showed a drastic change in the reaction
time period and product yield with increasing temperature (Fig. S7).
Whereas, the other two reactions were not showed significant changes,
hence these reactions were optimized at room temperature. As can be
seen from Tables 1, 2 and 3, the catalytic system worked exceedingly
well in all the three cases with a wide range of substrates.
3.4. Catalytic studies
To investigate the catalytic activity of organic–inorganic hybrid ma-
terial different reactions were studied. The reactions did not proceed in
the absence of catalyst or only with ASMPs as catalyst. Whereas,
Zr(IV)-HMNQ@ASMPs served as an efficient catalyst for the reactions
with highest conversion and selectivity. This indicated that the reaction
is catalyzed by Zr(IV). The use of metal salt (ZrOCl2) as homogeneous
catalyst showed quite poor activity pointing out the importance of the
HMNQ ring (Table S2). In this case, HMNQ with strong σ donating abil-
ity of nitrogen and oxygen donor acted as a co-catalyst and also an-
chored the Zr(IV) in a better way. In addition, this homogeneous
catalyst decomposed after the reaction and could not be reused. To ob-
tain best catalytic activity, reaction conditions were optimized using dif-
ferent solvents and solvent-free conditions (Fig. S6). Interestingly,
under solvent free condition, maximum conversion was obtained in
all three cases. Hence, all the reactions were carried in solvent free con-
ditions. As far as effect of temperature is concern, only Pechmann
3.5. Catalyst stability and reusability
To analyze the heterogeneity of catalyst, the reactions were car-
ried out using Zr(IV)-HMNQ@ASMPs as catalyst under the conditions
indicated in Tables 1, 2, and 3 and the solid Zr(IV) nanocatalyst was
removed magnetically at about half the specified reaction time peri-
od. Then, the filtrate was allowed to react under the same conditions.
After separation of catalyst, no further conversion of the reactant was
found. The ICP-MS analysis of the post reaction mixture showed no
detectable (b0.01 ppm) zirconium, which further signifies the stabil-
ity and heterogeneity of prepared nanocatalyst.
For testing the recoverability, the separated catalyst has been
reused for five consecutive reaction cycles in all the three cases. The
conversion of the reactant after five cycles was almost constant and
Fig. 2. SEM images of (a) MNPs and (b) SMPs.

114
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catalyst was observed. Comparison of the TEM image of fresh and re-
covered catalysts depicts that the structural and morphological prop-
erties of the anchored complex remain unaltered (Fig. S8).
4. Conclusions
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In conclusion, we have developed an environmentally benign
protocol for the synthesis of industrially significant product under
fast, simple, effective and environmentally benign conditions. Mild
reaction conditions, high activity and selectivity, economic viability,
easy preparation and reusability of the catalyst make the present
protocol a greener alternative.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2013.02.016.
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