Clay encapsulated ZnO nanoparticles as efficient catalysts for N-benzylation of amines

Article abstract of DOI:10.1016/j.catcom.2011.08.026

ZnO nanoparticles encapsulated in K10-clay (K10-ZnO) are synthesised and characterised by UV-DRS, Emission spectra, powder XRD, SEM and HRTEM analyses. The constrained space and also the polar active sites in the clay support stabilise zinc oxide nanoparticles by preventing that aggregation and consequently no extra-capping agent is required. The synthesised ZnO nanoparticles are used for the efficient N-benzylation of anilines and the reusability of the catalyst is also studied. A suitable mechanism is proposed for this transformation.

Full text of DOI:10.1016/j.catcom.2011.08.026

Catalysis Communications 16 (2011) 15–19
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Clay encapsulated ZnO nanoparticles as efficient catalysts for N-benzylation
of amines
⁎
Amarajothi Dhakshinamoorthy, Pitchai Visuvamithiran, Vairaperumal Tharmaraj, Kasi Pitchumani
School of Chemistry, Madurai Kamaraj University, Madurai-625021, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 June 2011
Received in revised form 17 August 2011
Accepted 26 August 2011
Available online 5 September 2011
ZnO nanoparticles encapsulated in K10-clay (K10–ZnO) are synthesised and characterised by UV-DRS, Emis-
sion spectra, powder XRD, SEM and HRTEM analyses. The constrained space and also the polar active sites in
the clay support stabilise zinc oxide nanoparticles by preventing that aggregation and consequently no extra-
capping agent is required. The synthesised ZnO nanoparticles are used for the efficient N-benzylation of an-
ilines and the reusability of the catalyst is also studied. A suitable mechanism is proposed for this
transformation.
Keywords:
Nano ZnO
© 2011 Elsevier B.V. All rights reserved.
K10-clay
N-benzylation
Heterogeneous catalysis
1. Introduction
compound, not only because of its increased surface area, but also due
to the changes in surface properties such as surface defects [19].
Zinc oxide is a direct wide band gap wurtzite-type semiconductor
with a band gap energy of 3.37 eV and free excitation energy of
60 meV at room temperature [1]. Its high specific surface area makes it
a potential candidate for a wide range of applications [2,3] with proper-
ties that are not present in solution or gas phase. Zinc oxide is non-toxic
and compatible with skin, making it a suitable additive for textiles and
surfaces that come in to contact with humans. It also has other wide
ranging applications as sensors [4], in solar cells [5], cell labelling [6],
etc. It is a well-known catalytic material in a large number of industrial
processes. As a heterogeneous catalyst, it has a high catalytic activity, is
non-toxic, insoluble in polar as well as non-polar solvents and also
cheap. On the other hand, mixed-ZnO and its metals (i.e. zinc ferrite)
have been used as photocatalysts, readily decompose halogen com-
pounds and the photocatalytic activity increases as the size of ZnO parti-
cles is reduced to nanometer levels [7–10]. ZnO catalyses wide range of
organic reactions efficiently. Reactions such as Friedel–Craft's acylation
[11], dehydration of aldoximes into nitriles [12], nucleophilic ring open-
ing of epoxides by amines to synthesise β-aminoalcohols [13], micro-
wave assisted preparation of cyclic ureas [14], N-formylation of amines
[15] and other organic transformations [16] are efficiently catalysed by
ZnO. The specific chemical, surface and nanostructured properties of
zinc oxide make it a suitable candidate for catalysis and gas sensing ap-
plications [17,18]. When used as a catalyst, the activity of nano ZnO is
expected to be enhanced with respect reference to the commercial
A general problem of metal oxide nanoparticles is their tendency to
undergo agglomeration, which increases their particle size and there-
fore, dramatically reduces their catalytic activity [20,21]. Two general
strategies have been developed to stabilise the particle size of metal
oxide nanoparticles by using either suitable solid surfaces or appropri-
ate ligands [22]. New possibilities for zinc oxide catalysis can be devel-
oped when the ZnO nanoparticles can be stabilised by encapsulating
them inside the clay interlayer to combine the unique catalytic prop-
erties of ZnO nanoparticles with shape-selectivity. This prompted us
to develop ZnO nanoparticles encapsulated in K10-montmorillonite
clay. N-benzylation of amines with alkyl halides is an important syn-
thetic method to obtain mono- and di-N-benzylated products. How-
ever, this reaction in the presence of a base requires longer reaction
time, higher temperature and affords lower yields of desired product
[23]. Inorganic bases like NaHCO3, [24] K2CO3, [25] etc., have also
been employed in N-benzylation of amines.
In the present study, we report catalytic activity of nano ZnO–K10-
clay in benzylation of aromatic amines and the salient features are
presented below. It is relevant to note that unlike other heteroge-
neous metal oxide catalysts, nano ZnO does not generate any poison-
ous waste end-products after the reaction is completed.
2. Experimental
2.1. Preparation of Zn2+-exchanged K10-montmorillonite clay
A sample of 1 g of the appropriate clay was stirred with 25 mL of
⁎
Corresponding author. Tel: +91 452 2456614; fax: +91 452 2459181.
E-mail address: pit12399@yahoo.com (K. Pitchumani).
1 M zinc nitrate solution for three days. The solution was filtered
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.08.026

16
A. Dhakshinamoorthy et al. / Catalysis Communications 16 (2011) 15–19
400000
1.2
1.0
0.8
0.6
0.4
0.2
0.0
b
a
(ii)
350000
300000
250000
200000
(i)
(iii)
200
300
400
500
600
700
800
400
420
440
460
480
500
520
540
Wavelength (nm)
Wavelength (nm)
Fig. 1. (a) UV-DRS spectra of (i) bulk ZnO, (ii) nano-ZnO powder and (iii) nano-ZnO encapsulated K10-montmorillonite clay (b) emission spectra of nano-ZnO encapsulated K10-
montmorillonite clay.
and washed repeatedly with water to remove excess of zinc nitrate. It
was then dried overnight in a hot air oven at 90°C.
cooled solution is extracted with dichloromethane (10 mL), filtered,
dried with anhydrous sodium sulphate and the solvent is removed
to give the product.
The reaction mixture, after completion, is analysed by GC and the
products are characterised by GC–MS. GC analysis is performed using
a Shimadzu 17A model gas chromatography with capillary column
(ZB-5, 30 m×0.25 mm inside diameter (i. d.), 0.25 μm film thickness),
FID detector, high purity nitrogen as carrier gas, in the temperature
range of 100–250°C (10°C/min). 0.5 μL sample was injected for the
analysis from the reaction mixture after workup procedure.
2.2. Preparation of ZnO nanoparticles entrapped in K10-montmorillonite
clay
Zn2+-supported on K10-clay (500 mg) is suspended in 5 mL of
1 M sodium hydroxide solution and the mixture is stirred at room
temperature for 30 min, filtered, washed continuously with distilled
water until the mother liquor was neutral. The solid was then dried
and calcined at 250 °C for 3 h and then stored. It is characterised by
UV-DRS spectra recorded by using JASCO-UV–Vis Spectrophotometer
(V-550), the reflectance spheres are barium sulphate coated. Fluores-
cence measurements were made on a Fluoromax-4 Spectrofluorome-
ter (HORIBA JOBIN YVON) with the excitation slit set at 2.0 nm band
pass and emission at 4.0 nm band pass in 1 cm×1 cm quartz cell,
powder XRD (JOEL Model JDX 8030 X-ray diffractometer using Cu
Kα radiation), scanning electron microscopy attached with an energy
dispersive X-ray spectrometer (SEM-EDS Hitachi, S-3400 N), high
resolution TEM (HRTEM, Philips CM12).
3. Results and discussion
3.1. Morphological analysis of the materials
The obtained nano ZnO–K10-montmorillonite clay composite is
characterised by powder XRD, UV-diffuse reflectance and emission
spectra, SEM and EDX. Particle size is determined by HRTEM analysis.
The UV–Vis, spectra for ZnO–K10-montmorillonite nanohybrid is
given in Fig. 1a. The freshly prepared ZnO-montmorillonite clay exhibits
a new band at around 355 nm which is an indication of the formation of
zinc oxide nanoparticles. Also, the samples exhibit a similar behaviour
in absorption on storage, suggesting a stable zinc electronic configura-
tion due to the partial electrostatic interaction between the guest ZnO
and the host montmorillonite clay. In the fluorescence spectra, an emis-
sion peak appears at 429 nm (λexc=365 nm), which is attributed to the
2.3. Procedure for N-benzylation of amines
To a solution of amines (0.5 mmol) in 3 mL of water/acetonitrile
(1:1), benzyl chloride (0.5 mmol) is added followed by clay
entrapped ZnO nanoparticles (50 mg). This heterogeneous mixture
is heated with stirring for about 12 h at 70 °C and then cooled. The
140
140
120
100
80
a
b
120
100
80
60
40
20
0
60
40
20
0
0
10
20
30
40
50
60
70
80
0
10
20
30
40
50
60
70
80
2 theta (degree)
2 theta (degree)
Fig. 2. Powder XRD patterns of a) pure K10-montmorillonite clay b) nano ZnO encapsulated K10-montmorillonite clay.

A. Dhakshinamoorthy et al. / Catalysis Communications 16 (2011) 15–19
17
Fig. 3. SEM images of ZnO encapsulated montmorillonite at different magnifications.
as prepared ZnO–montmorillonite nanohybrids (Fig. 1b) and this clear-
ly confirms the presence of the ZnO nanoparticles.
isotherm and XRD pattern, aniline was found to adsorbed readily
into the montmorillonite interlayer being perpendicularly oriented
to silicate sheet (with an increase in d001 spacing from 1.26 nm to
1.48 nm, corresponding to on amount of aniline ~11.7 mmol g−1).
This increase in basal spacing will not only facilitate addition of ben-
zyl chloride, but also ensure closer proximity between the two layers.
As shown in Table 1 the effect of solvents and temperature are
studied in the reaction of aniline as a model compound with benzyl
chloride under various reaction conditions. The reaction is also stud-
ied with various conventional catalysts such as zinc powder, ZnO
bulk and ZnO nano. With nano ZnO-clay composite (entries 3–10),
the percentage conversion is high in acetonitrile–water mixture. In-
creasing the temperature (70 °C for 12 h) results in a higher yield of
di-N-benzylated products. In pure methanol, only 52% conversion
is noticed. However, in methanol–water mixture, a very significant
increase in percentage conversion is obtained. Similarly, in solid
state reactions also, the conversion is good, but both mono- and di-
benzylated products are formed in equal amounts. The yield is very
low, when only water is used as the solvent. In the absence of a cata-
lyst, the reaction is slow at room temperature (entry 1). The effect of
various forms of Zn as catalyst using bulk ZnO, ZnO nano and zinc
powder are also studied (entries 13–17). The observed results clearly
indicate that mono-N-benzylated product is the major product at
room temperature while on the other hand yield of di-N-benzylated
product increases at higher temperature.
ZnO encapsulated on K10-montmorillonite clay is subjected to
powder XRD analysis to reveal the crystalline nature of support as
well as to examine the presence of ZnO. Zincite appears to be the pre-
dominant ZnO species [JCPDS data file (36–1451)], which has main
diffraction peaks with maximum intensities at 35.2 (101), 46.7
(102) and 51.5 (110). Also, the lower intensity of the peaks (marked
as *) in nano ZnO (Fig. 2) may be attributed to the deeper penetration
of ZnO onto the lamellar structures of clay.
SEM analysis of ZnO nanocomposites encapsulated in montmoril-
lonite shows the surface morphology of the clay which is found to be
present in the form of granules (Fig. 3). The ZnO content of these clay-
entrapped nanoparticles is found to be 3.65 wt.%, using Energy Dis-
persive X-ray Analysis (EDX, a technique used in conjunction with
SEM).
HRTEM images of ZnO encapsulated montmorillonite clay at dif-
ferent magnifications clearly show that the ZnO particles are in the
range of 12 nm (Fig. 4) and most of the particles are embedded in
the layered sheets of the clay.
3.2. Benzylation of amines
In the present study, ZnO nanoparticles encapsulated on montmo-
rillonite are used as catalysts in benzylation of aniline and its deriva-
tives (Scheme 1). Since the oxyanions of ZnO are mildly basic and the
Zn cation on the polar surface is mildly acidic, this combination will
result in an acid-base bifunctional catalytic system. In addition, the
layered structure of montmorillonite, acting as a polar support, is
expected to provide closer proximity between the reagents. It is per-
tinent to note here that, in a recent study [26] using adsorption
Monosubstituted product is obtained when methanol:water is
used as a solvent in 2:1 ratio and 1:2 ratio for 12 h (Table 2, entry
6) and 8 h (entry 16) respectively. When the reaction is carried out
in aqueous medium, only monosubstituted product is obtained
when the reaction time is 6 h (entry 18) and 12 h (entry 20). If the re-
action time is 8 h (entry 19), the selectivity is reduced to 76%.
Fig. 4. (a–b) HRTEM images of highly dispersed ZnO nanoparticles embedded in clay matrix. (c) HRTEM image of a selected portion of ZnO nanoparticles on clay matrix.

18
A. Dhakshinamoorthy et al. / Catalysis Communications 16 (2011) 15–19
Table 2
Effect of medium and temperature on mono/di; selectivity in N-benzylation of aniline.
Entry
Medium
Time
(h)
T °C
Percentage
conversion
Mono
Di
Ratio of
mono:di
1
2
3
4
CH3OH
6 h
8 h
12 h
6 h
8 h
12 h
6 h
8 h
2 h
4 h
6 h
8 h
10 h
12 h
6 h
8 h
12 h
6 h
8 h
12 h
70
80
RT
70
80
RT
70
80
RT
RT
RT
RT
RT
RT
70
80
RT
70
80
RT
100.0
100.0
95.2
93.7
96.5
49.2
84.5
71.0
93.4
96.8
98.2
100.0
55.2
88.3
78.0
53.0
74.7
61.6
85.6
37.1
57.5
48.4
42.3
50.2
53.1
49.2
34.8
39.0
50.0
53.1
54.6
52.3
42.1
82.5
65.5
53.0
70.2
61.6
77.7
37.1
42.5
51.6
52.9
43.5
43.4
–
1.34:1
0.93:1
0.80:1
1.10:1
1.20:1
–
Scheme 1. Benzylation of amines.
CH3OH:H2O
(2:1)
5
6a
7
CH3OH:H2O
(1:1)
49.7
32.0
43.4
43.7
43.6
47.7
13.1
05.8
12.5
–
0.70:1
1.21:1
1.15:1
1.20:1
1.25:1
1.10:1
3.21:1
14.20:1
5.20:1
–
Benzylation studies are also extended to other aromatic amines
8
9
and the observed results are given in Table 3, which shows
that di-N-benzylated product predominates in aniline as well as in
p-toluidine. In a similar manner, benzylamine also gives di-N-
benzylated product as the major one than its mono benzylated
product. Another interesting feature of this protocol is the absence
of ring benzylated products. m-Chloroaniline, on the other hand,
yields mono N-benzylated product as the predominant product. Al-
iphatic amine such as N-butylamine yields exclusively the di-N-
benzylated product. N-Methylaniline reacts in a facile manner to af-
ford the corresponding tertiary amine in very good yield. Aromatic
secondary amines like indole, 1,2,3,4-tetrahydrocarbazole and im-
idazole fail to give the corresponding N-benzylated products at
room temperature as well as at 70 °C.
10
11
12
13
14
15
16a
17
18a
19
20a
CH3OH:H2O
(1:2)
04.5
–
15.60:1
–
H2O
07.9
–
9.80:1
–
Substrate (0.5 mmol), benzyl chloride (0.5 mmol), ZnO nano-K10-clay (50 mg); per-
centage conversion is determined by GC.
a
Only monobenzylation is observed.
The reusability of the clay entrapped ZnO nanoparticles is also
studied for the benzylation of N-methylaniline. After the reaction,
the solution is filtered, washed with dichloromethane and dried at
60 °C. The catalyst is reused directly without further purification
and is used for two consecutive runs with only a marginal decrease
in percentage yield. The reusability of the clay entrapped ZnO catalyst
was also tested for the benzylation of aniline up to five successive
runs, with only marginal decrease in catalytic activity (Table 4).
Zn polar surface] to facilitate the formation of benzylated products.
It is also likely that ZnCl2, generated in situ by the reaction of ZnO
with HCl, may also catalyse the reaction.
4. Conclusions
K10-montmorillonite clay nano ZnO is synthesised and charac-
terised by spectroscopic and microscopic techniques. They are used
in the N-benzylation of anilines to give mono- and di-benzylated
products in good yield. Secondary amine is also converted to the cor-
responding N-benzylated product in a facile manner, indicating clear-
ly that it is a new protocol for the synthesis of secondary and tertiary
amines over conventional catalysts. Other significant advantages of
the present study include stabilisation of nano ZnO in clay layers
3.3. Mechanism of benzylation of amines
To account for these observations, the following mechanism may
be visualised (Scheme 2). Oxygen atom of clay encapsulated ZnO
nanoparticles [ZnO (0001−)\O polar surface] polarises C\Cl bond
in benzyl chloride which is then attacked by aniline [brought in
close proximity as it is attracted by the zinc cations of ZnO (0001)\
Table 1
Optimization of the reaction conditions for the N-benzylation of aniline using clay entrapped ZnO nanoparticles as catalyst.
Entry
Medium
Time
(h)
T °C
Catalyst
Percentage
conversiona
Mono
Di
Ratio
mono:di
1
2
CH3CN
CH3OH
H2O
H2O
CH3OH
CH3OH
CH3OH
10
24
12
24
10
12
24
10
24
12
6
6
8
6
5
8
5
RT
RT
RT
RT
RT
RT
RT
RT
RT
70
70
70
80
80
80
80
80
–
14.0
–
11.0
–
03.0
–
3.6:1
–
K10-clay
3b
4b
5
K10–nanoZnO
K10–nanoZnO
K10–nanoZnO
K10–nanoZnO
K10-clay
K10–nanoZnO
K10–nanoZnO
K10–nanoZnO
K10–nanoZnO
K10–nanoZnO
Bulk ZnO
37.1
43.0
52.0
95.0
–
37.1
43.0
47.0
42.1
–
–
–
–
–
05.0
52.9
–
9.4:1
0.8:1
–
6
7
8
9
10
11
12
13
14
15b
16
17
CH3OH:H2Oc
CH3CN:H2Oc
CH3CN:H2Oc
CH3OH:H2Od
CH3OH:H2Oe
CH3OH:H2Oc
CH3OH:H2O
CH3OH:H2Oc
CH3CNH2Oc
CH3CN:H2Oc
88.0
76.0
89.0
72.5
69.5
65.6
98.6
30.8
54.1
40.1
73.0
47.0
39.0
57.0
28.2
51.6
71.6
30.8
50.6
35.9
15.0
29.0
50.0
15.5
37.9
14.0
27.0
–
408:1
1.62:1
0.78:1
3.68:1
1:1.34
3.67:1
2.65:1
–
Nano ZnO
Powder Zn powder
Bulk ZnO (Bulk)
Powder Zn
03.5
04.2
14.4:1
8.54:1
50 mg of catalyst was used for all reactions.
a
Determined by GC.
Only monobenzylation is observed.
Solvent ratio is 1:1.
Ratio of aniline:benzyl chloride is 2:1.
b
c
d
e
Ratio of aniline:benzyl chloride is 1:2.

A. Dhakshinamoorthy et al. / Catalysis Communications 16 (2011) 15–19
19
Table 3
N-benzylation of substituted anilines with ZnO nanoparticles encapsulated in clay
matrix.
Substrate
Conversiona
%
Percentage of products (% yield)a
mono- di-
89
NH2
NHCH2C6H5
N(CH2C6H5)2
(39)
(50)
96
NHCH2C6H5
N(CH2C6H5)2
NH2
(32)
(64)
Cl
Cl
Cl
95
NHCH2C6H5
N(CH2C6H5)2
NH2
CH3
(73)
(22)
Scheme 2. Plausible mechanism for N-benzylation of aniline catalysed by nano ZnO/
clay composite.
CH3
CH3
79
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CH2N(CH2C6H5)2
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Table 4
Reusability of N-benzylation of aniline using clay entrapped ZnO nanoparticles as
catalyst.
Run
First
89
Second
87
Third
85
Fourth
83
Fifth
82
Yield (%)
Aniline (0.5 mmol), benzyl chloride (0.5 mmol), ZnO nano-K10 clay (50 mg), water:
acetonitrile (1.5:1.5 mL), 12 h, 70 °C. After completion of the reaction, catalyst is
filtered and washed thrice with dichloromethane, air dried and reused for successive
runs percentage conversion is determined by GC.