Submicron ZnO raspberries as effective catalysts for Fries rearrangement

Article abstract of DOI:10.1039/c5ra03448e

o-Xylene-in-water emulsion droplets have been exploited as soft templates for the synthesis of submicrometer-sized raspberry-like ZnO morphologies. The optical properties, structure and morphology of the microstructures have been characterised by absorption and fluorescence spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and selected area electron diffraction analysis. It has been found that the microstructures could act as an environmentally benign and cost-effective alternative to conventional Lewis acid catalysts for the illustrious Fries rearrangement of o-methylphenyl acetate to 3-methyl-4-hydroxyacetophenone, employed as the model reaction and which has been probed by NMR spectroscopic studies.

Full text of DOI:10.1039/c5ra03448e

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Submicron ZnO raspberries as effective catalysts
for Fries rearrangement
Cite this: RSC Adv., 2015, 5, 41780
Mohammed Ali, Hasimur Rahaman, Sudip Kumar Pal, Nikita Kar
and Sujit Kumar Ghosh*
o-Xylene-in-water emulsion droplets have been exploited as soft templates for the synthesis of
submicrometer-sized raspberry-like ZnO morphologies. The optical properties, structure and
morphology of the microstructures have been characterised by absorption and fluorescence
spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission
electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and selected area electron
diffraction analysis. It has been found that the microstructures could act as an environmentally benign
and cost-effective alternative to conventional Lewis acid catalysts for the illustrious Fries rearrangement
of o-methylphenyl acetate to 3-methyl-4-hydroxyacetophenone, employed as the model reaction and
which has been probed by NMR spectroscopic studies.
Received 25th February 2015
Accepted 1st May 2015
DOI: 10.1039/c5ra03448e
www.rsc.org/advances
properties by morphological tuneability provides new chemical
insights from an old material. In addition to these physico-
1
. Introduction
8
In recent years, self-assembly of nanoscale objects has pio- chemical properties, numerous environmental advantages,
neered a facile strategy to design micrometer-sized hollow such as, non-toxicity, corrosion resistance, recyclability and
structures that have attracted increasing attention because of easy disposal of the material have encouraged its application as
their specic morphology, fascinating properties and wide greener catalysts in a diverse range of catalytic and photo-
8
applications in physics, chemistry, biology and materials catalytic reactions. In the early 1900s, Fries and co-worker
1
science. Amongst the three different approaches currently reacted phenolic esters of acetic and chloroacetic acid with
explored to effect ordering of nanoparticles by self-assembly aluminum chloride and isolated a mixture of o- and p-acetyl-
processes, liquid–liquid interfaces offer an important alterna- and chloroacetyl phenols and in general, the conversion of
tive scaffold for the organisation of nanoscale substances into phenolic esters to the corresponding o- and/or p-substituted
2
higher-order assemblies. Although catalysis is a mature eld, it phenolic ketones and aldehydes, in the presence of Lewis or
possesses a vital role in chemical-based industries, accounting Brønsted acids has been recognized as the Fries rearrange-
3
9
for about a quarter of the world's gross domestic product. The ment. Different types of Fries rearrangement have been
eld of nanocatalysis has undergone an exponential growth reported so far in the literature using varieties of catalyst

4
10
during the past decade. This is due to the fact that particles in materials. For example, Paul and colleague has shown that
the nanometer size regime inherently provide a high surface-to- zinc powder efficiently catalyzes the selective Fries rearrange-
5
volume ratio, making them attractive candidates for catalysis.
ment of acetylated phenols under microwave heating or with
The application of transition metal oxides in the nanometer size conventional heating in the presence of N,N-dimethylforma-
regime in catalysis has become fascinating because they mide and in some cases, it was seen that different products were
manipulate the surface activity of the metal and hence, the obtained using microwave heating and conventional heating.
11
catalysis at the nanoscale, imperting activity and selectivity to Krishnamurthy and co-authors reported a convenient way to
6
heterogeneous catalysis. The benet of using heterogeneous carry out the Fries rearrangement of phenolic esters to
catalysis is the facile recovery and recyclability of the catalyst hydroxyphenyl ketones by reuxing in nitrobenzene in the
materials and thus, meet the modern requirements for green presence of Naon-H, a solid superacidic resin sulfonic acid
7
12
catalysts. Amongst the transition metal oxides, zinc oxide catalyst. Belokon and group reported TiCl
ZnO) at the nanoscale dimension have attracted immense rearrangement via regioselective acylation of p-hydroxy [2.2]
interest due to its direct wide band gap (3.37 eV), high exciton para-cyclophane to o-acylhydroxy [2.2] para-cyclophanes.
-catalysed Fries
4
(
13
binding energy (60 meV) and the opportunity to vary its Dupont and colleagues have optimized the Fries rearrange-
ment with methanesulfonic acid to give p-hydroxyacetophenone
with high conversion and selectivity. However, the Fries rear-
rangement of aryl esters, a special case of Friedel–Cra
Department of Chemistry, Assam University, Silchar-788011, India. E-mail: sujit.
kumar.ghosh@aus.ac.in
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acylation provides an important route for the synthesis of microstructures were cut with an Ultracut E Reichert-Jung
aromatic hydroxyl aryl ketones which are important interme- microtome prepared at room temperature using a diamond
diates for several pharmaceuticals, such as, paracetamol, sal- knife (Drukker) and dropped on a silicon wafer by using a
butamol etc. However, the use of aluminium chloride can lead loop. Fourier transform infrared (FTIR) spectra were recorded
to violations of several principles of green chemistry through in the form of pressed KBr pellets in the range of 400–4000
ꢁ1
the release of hazardous substances in the environment. While cm on a Shimadzu-FTIR Prestige-21 spectrophotometer. The
the present state-of-the-art literature reveals that ZnO at the powder X-ray diffraction patterns were obtained using a D8
bulk or at the nanoscale has been employed as greener catalysts ADVANCE BROKERaxs X-ray Diffractometer with CuK
a
radia-
ꢀ
ꢂ
in Friedel–Cras acylation, Knoevenagel condensation, Bigi- tion (l ¼ 1.4506 A); data were collected at a scan rate of 0.5
ꢁ
1
ꢂ
nelli reaction, Hantzsch condensation, phospha-Michael addi- min in the range of 10–80 . The NMR spectra were recorded
tion, Beckmann rearrangement and so on
14
and Fries in BRUKER AVANCE II 400 FT-NMR spectrometer.
rearrangement has been carried out using a wide variety of
heterogeneous media or catalysts viz., scandium tri-
15
16
17
2.2. Synthesis of zinc oxide microstructures

uoromethanesulfonate, sulfated zirconia, zeolite, electron
18
beam irradiation within a cyclodextrin inclusion complex,
Zinc oxide microstructures have been synthesized in a one pot
19
20
microwave irradiation, ionic liquids, etc.; with the advance- reaction by hydrolysis of zinc acetate dihydrate as precursor
ment of nanotechnology, therefore, it seems quite appealing to using water and o-xylene as the solvent media. An amount of
design the landscape of a greener chemical approach for the 0.057 g of zinc acetate dihydrate was dissolved in water/o-xylene
Fries rearrangement. In this article, we have reported a so- mixture (9 : 1 v/v) in a double-necked round bottom ask by
ꢂ
templated strategy for the synthesis of microstructured zinc reuxing the mixture in a water bath at 45 C for 45 min. Aer
oxide assemblies and exploited the particles as viable alterna- dissolving the precursor zinc acetate, potassium hydroxide
tives to conventional Lewis acid catalysts in the Fries rear- solution (0.5 g KOH dissolved in 13.5 mL of water) was added to
rangement of o-methylphenyl acetate to 3-methyl-4- the reaction mixture. The reaction mixture was, then, kept on
hydroxyacetophenone employed as the model reaction.
reuxing for another 15 min; aer that, the temperature was
increased to 60 C and the reuxing was continued for another
ꢂ
1
h. Reuxing of the reaction mixture at higher temperature
2
. Experimental
facilitates to achieve smaller size and monodispersity of the
constituent ZnO particles in the microstructures. Then, the
2.1. Reagents and instruments
All the reagents used were of analytical reagent grade. Zinc water bath was removed and the mixture was allowed to stir for
acetate dihydrate [Zn(OOCCH ) $2H O] (Sigma-Aldrich), potas- 12 h at room temperature. The zinc oxide particles so obtained
3
2
2
sium hydroxide (S. D. Fine Chemicals, India), hydrochloric acid were retrieved by centrifugation at 10 000 rpm for 5 min and
Qualigens' Fine Chemicals, India) were used as received. stored in the dark for 3 days before using as catalysts in the
(
o-Xylene (Sisco Research Laboratories, India) and were used corresponding organic reaction.
without further purication. Double distilled water was used
throughout the course of the investigation. The temperature
2
.3. Synthesis of the o-methylphenyl acetate
was 298 ꢀ 1 K for all experiments.
Absorption spectrum was recorded in a Lambda-750 digital Synthesis of o-methylphenyl acetate has been carried out by
2
1
UV-vis NIR spectrophotometer (PerkinElmer, England) using following the procedure described in Vogel's text book. An
the solid sample accessories. Fluorescence spectrum was aliquot of 5 mmol of o-cresol was taken in a 50 mL double-
measured with
a PerkinElmer LS-45 spectrouorimeter necked round bottom ask, dissolved in 25 mL dichloro-
equipped with a pulsed xenon lamp and a photomultiplier methane and the reaction mixture stirred for 15 min followed
tube with R-928 spectral response. The spectrouorimeter was by the addition of 7.5 mmol of triethylamine. Then, acetic
linked to a personal computer and utilized the FL WinLab anhydride (10 mmol) was added dropwise and the reaction
soware package for data collection and processing. Scanning mixture was stirred for 12 h at room temperature. The
electron microscopic (SEM) images were recorded by using a completion of the reaction was monitored by thin layer chro-
JEOL (JSM-6360) Zeiss 1530 Gemini instrument equipped with matographic technique. Aer completion of the reaction, the
a eld emission cathode with a lateral resolution of approxi- post reaction mixture was ltered through a sintered funnel
mately 3 nm and acceleration voltage 3 kV aer sputtering the and the residue was washed with ethyl acetate (2 ꢃ 5 mL). The
sample on a silicon wafer (approx. 6 nm). Energy dispersive combined ethyl acetate was transferred to a separating funnel,
X-ray (EDX) analysis was performed on a LEO 1530 eld washed with water (3 ꢃ 10 mL) and dried using activated
4
emission scanning electron microscope using an X-ray MgSO . The solvent was removed in a rotary evaporator at
detector. Transmission electron microscopy (TEM) and room temperature under reduced pressure. The crude product
selected area electron diffraction (SAED) was carried out on a was puried by column chromatography over silica gel
JEOL JEM 2100 (JEOL, Japan) microscope operated at 200 kV. (60–120 mesh) using ethyl acetate–petroleum ether as eluent
An epoxy resin (EpoFix kit, Struers) was used to prepare to afford the desired ketone. A schematic presentation of the
specimens for cross-sectional microtomed TEM imaging. synthesis of o-methylphenyl acetate has been illustrated in
Ultrathin sections, 50–70 nm in thickness, of the Scheme 1.
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Scheme 1 Schematic presentation of the synthesis of o-methylphenyl
acetate.
3. Results and discussion
Fig. 2 (a–c) Representative scanning electron microscopic images
with increased magnification; (d) microtomed transmission electron
micrograph: (e) selected area electron diffraction pattern and
The ZnO microstructures have been synthesised in an one-pot
reaction by alkaline hydrolysis of zinc acetate dihydrate
employing o-xylene-in-water emulsion droplets as the so
templates. Fig. 1 shows the absorption and photoluminescence
spectra of the ZnO raspberries in the solid state. The absorption
(f) energy dispersive X-ray spectrum of the ZnO microstructures.
spectrum of the as-prepared particles shows two maxima, one at micrograph (panel a) shows ZnO nanobuilding units are self-
54 nm (4.88 eV) and the other at 294 nm (4.22 eV) that arise due assembled into nearly spherical submicrometer-sized aggre-
to the excitonic transitions between the trapped energy states gates resulting in raspberries-like appearance with average
2
22
situated within the connement region. The optical band edge diameter ca. 0.7 mm. Careful inspection (panel b) recognises
band gap 4.4 eV) bears the characteristic electronic transition that the assemblies possess well-dened hollow structures and
(
energy taking place to promote the electrons to the energy states there is, typically, one hole on the surface of each microsphere
in the conduction band from the valence band, including, that appears due to adequate stabilisation of the nanoparticles
24
excitonic effects. The uorescence spectrum (lex ꢄ 254 nm) of at interfaces with a high degree of organizational selectivity.
ZnO microstructures shows an UV emission band at 372 nm Inset shows the photograph of raspberries showing the
3.33 eV) that arises due to the radiative recombination of a hole morphological resemblance with the particles. At higher reso-
in the valence band and an electron in the conduction band lution (panel c), self-assembly of the nanobuilding units on the
excitonic emission) and a broad emission in the visible that interface as constructional base and roughened surface of the
could be attributed to the presence of multiple surface defects assemblies are apparent. The microtommed transmission
(
(
23
in the microstructures. Therefore, it seems that the formation electron micrograph of the cross-section (panel d) of the
of microstructures controls the various aspects of the recom- assemblies exhibit hollow interior of the microstructures
bination process and favours the radiative transitions from the bearing spherical cavity, with the thickness of the shell ranging
oxygen vacancies/trap levels in the band gap to give a broad from 40 to 100 nm. The corresponding selected area electron
visible luminescence for the assemblies.
diffraction pattern (panel e) of the as-prepared raspberries
The morphology, composition and crystallinity of the as- reveals the appearance of polycrystalline-like diffraction, which
prepared ZnO assemblies synthesised at the droplet interface is consistent with reections (100), (002), (101), (102) and (110),
are described in Fig. 2. Low resolution scanning electron corresponding to the hexagonal wurtzite phase of ZnO particles,
indicating that the microstructure is an ordered assembly of
small nanocrystal sub-units without crystallographic orienta-
25
tion. The EDX spectrum (panel f) of the hollow microstruc-
tures indicates the presence of Zn elements in the composites.
The signals of C, O and Si elements come from the as-prepared
ZnO, acetate counterions adsorbed onto the particle surface and
the silicon wafer employed for the SEM measurement.
The crystallinity and phase of the as-synthesised ZnO
microstructures is shown in Fig. 3. The strong reections shown
in the pattern could be indexed to the pure hexagonal phase of
4
ꢀ
Zn with a space group of C6v and cell constants a ¼ 3.25 A, and c
ꢀ
¼
5.21 A (JCPDS card no. 76-0704), which suggests that the
product comprises ZnO nanocrystals with the wurtzite
26
structure.
Fig. 4 shows the FTIR spectrum of the as-prepared ZnO
assemblies formed at the interface of emulsion droplets. The
ꢁ
1
presence of a sharp peak at 458 cm could be assigned to the
Fig. 1 Absorption and fluorescence spectra of ZnO microstructures in
the solid state.
27
stretching vibration of Zn–O bonds in the composites. In
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30
assembly of the nanocrystals. During the evolution, the ZnO
clusters become, mainly, conned at the vicinity of the interface
of the emulsion droplets and subsequently, the evolution of
shell-like structure follows. When two ZnO building blocks
come together, the capillary forces between them strengthen
the agglomerate by van der Waals' forces. The monomers in the
inner part of the droplet would diffuse to the surface due to the
concentration gradient, resulting in hollow structures, and
simultaneously such nanostructures would aggregate and fuse
together to form nanometer- or micrometer-sized hollow
31
spheres. The resulting naked hollow spheres with accessibility
of both interior and exterior having roughened surface because
of their low effective density and high specic surface area
possess the potential for promising applications, especially, in
catalysis.
The as-prepared ZnO raspberries were employed as Lewis
acid catalysts for the Fries rearrangement of aryl ester to
p-hydroxyaryl ketone. The synthesis of the starting organic
compound has been described in the Experimental section. In
the catalytic reaction, 40 mg of the catalysts was dispersed with
Fig. 3 X-Ray diffractogram of the as-prepared ZnO raspberries.
3
.5 mL of 0.1 mM o-methylphenyl acetate solution and the
ꢂ
mixture was hydrolysed at 55 C for 5 h under stirring condition.
The completion of the reaction was tested by thin the catalytic
reaction, 40 mg of the catalysts was dispersed with 3.5 mL of
0
.1 mM o-methylphenyl acetate solution and the mixture was
ꢂ
hydrolysed at 55 C for 5 h under stirring condition. The
completion of the reaction was tested by thin layer chromato-
graphic technique and subsequently, worked up to achieve the
desired product. A schematic presentation of the catalytic
protocol has been described in Scheme 2.
1
13
Fig. 5 shows the H and C NMR spectra of the rearrange-
1
ment of the ester in the presence of the catalysts. H NMR
spectra (panel A) of the ester does not exhibit any characteristic
NMR peak around alcoholic region, whereas, a well dened
peak at d 3.481 is seen for the product that conrms the
32
formation of o-hydroxy ketone upon rearrangement. More-
Fig. 4 FTIR spectrum of ZnO raspberries formed at the water/o-
xylene interface.
1
3
over, in the C NMR spectra (panel B), the presence of a peak at
d 169 for esteric carbon for the reactant is not seen in the
product which further authenticates the rearrangement reac-
ꢁ
1
addition, a weak absorption at around 1267 cm corresponds tion under the experimental condition. A detailed analysis of
to C–O stretching frequency of acetate counterions adsorbed the NMR spectra could be enunciated as follows:
28
onto the metal oxide surface. The absence of any other
dominant peak indicates that the particles are merely naked
and emphasizes the aptability of the particles for catalytic
applications.
A plausible mechanism of the evolution of ZnO microstruc-
tures by alkaline hydrolysis of zinc acetate dihydrate in an
immiscible liquid pair could be enunciated as follows.
The formation of the assembly was not seen in water (boiling
3
.1. o-Methylphenyl acetate
1
ꢂ
6
H (300 Hz, CDCl ; Me Si at 25 C, ppm) d ¼ 2.19 (s, 3H, H ), 2.28
3
4
4
1
2
5
(3H, H ), 3.481 (s, 1H, H ), 7.01 (1H, H ), 7.13 (1H, H ), 7.24 (1H,
ꢂ
ꢂ
point 100 C) or o-xylene (boiling point 144.4 C) alone; this
indicates oil-in-water emulsion droplets provide the restricted
environment at the liquid–liquid interface in harnessing into
29
raspberry-like assemblies. It is now well established in the
literature that a liquid–liquid interface is a non-homogeneous
2
region, having a thickness of the order of a few nanometers.
The interface between two immiscible liquids, thus, offers an
Scheme 2 Catalytic protocol for Fries rearrangement in the presence
important scaffold for the chemical manipulation and self- of ZnO raspberries as catalysts.
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1
13
Fig. 5 (A) H and (B) C NMR spectra of o-methylphenyl acetate (a) before and (b) after Fries rearrangement in the presence of ZnO raspberries
as catalysts.
3
13
1
9
4
5
H ); C NMR, d ¼ 16.16 (C ), 20.81 (C ), 121.91 (C ), 126.09 (C ),
It is well-established in the literature that Fries rearrange-
ment is an organic reaction used to convert aryl ester to o- and/
or p-hydroxyaryl-ketones using a Lewis acid catalyst involving
the migration of an acyl group of phenolic ester to benzene
ring. In the present reaction, ZnO assemblies act as alternative
to conventional Lewis acid catalysts. Therefore, the mechanism
begins with coordination of the ester to the Lewis acid, followed
by a rearrangement which generates an electrophilic acylium
6
2
7
3
8
1
26.97 (C ), 130.13 (C ), 131.16 (C ), 149.49 (C ), 169.28 (C ).
3
.2. p-Hydroxyaryl ketones
33
1
ꢂ
1
H (300 Hz, CDCl
3
; Me
4
Si at 25 C, ppm) d ¼ 2.078 (s, 3H, H ),
6
2
3
2
.196 (s, 3H, H ), 6.913 (m, 1H, H ), 7.047 (1H, H ), 7.127 (1H,
4
5
13
9
6
H ), 7.142 (s, 1H, H ); C NMR, d ¼ 16.16 (C ), 20.81 (C ), 121.91
2
3
8
7
4
(
(
C ), 126.09 (C ), 126.97 (C ), 130.13 (C ), 131.16 (C ), 149.49
1
5
C ), 169.28 (C ).
The proposed methodology is capable of providing the
desired product in good yields (92–97%). At the end of the
reaction, p-hydroxyaryl ketones and ZnO microstructures remain
in the reaction mixture. In this reaction, submicrometer-sized
ZnO assemblies act as heterogeneous catalysts which are
inorganic materials. Therefore, the catalysts were separated
from the reaction mixture by centrifugation or natural sedi-
mentation at the end of the reaction and subsequent drying in
air. It is observed that the assemblies could be reused up to
seven cycles of operations without any apparent loss of activity
and the catalytic turnover frequency is ca. 24.14 per gm of the
catalysts. A histogram showing the yield of product for each
cycle is presented in Fig. 6. Therefore, it is evident that the as-
prepared ZnO raspberries could be projected as catalysts for
Fries rearrangement of phenolic esters to p-hydroxyphenyl
carbonyl compounds with industrially favourable turnover
frequency.
Fig. 6 Histogram showing the yield of product in different cycles of
the reusability of the catalysts.
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7
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8
9
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Scheme 3 Schematic representation of the reaction mechanism in
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1
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1
1
1
1
1
1
1
2
2
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selectively, at the para position, via an electrophilic aromatic
substitution. Deprotonation regenerates aromaticity and
Brønsted acid work-up regenerate the Lewis acid catalyst
provided with the p-hydroxyaryl ketonic product. A plausible
mechanism of the reaction in the presence of ZnO as catalysts is
depicted in Scheme 3.
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34
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In conclusion, the present methodology based on the formation
of o-xylene-in-water emulsion droplets could offer a viable
platform for the synthesis of submicrometer-sized ZnO assem-
blies. These hollow and template-free assemblies could be
envisaged as greener catalysts in Fries rearrangement for the
conversion of phenolic ester to p-hydroxyaryl carbonyl
compound with high turnover frequency. It is anticipated that
the strategy of preparing hollow ZnO assemblies could be
utilized as a landscape to the self-assembly of nanobuilding
units of other transition metal oxides at the uid interfaces to
explore the application of these light-weight materials in
nanotechnology.
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Acknowledgements
We gratefully acknowledge nancial support from DST, New
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