Rice husk derived nanosilica supported Cu(II) complex: An efficient heterogeneous catalyst for oxidation of alcohols using TBHP

Article abstract of DOI:10.1039/c5ra21148d

We have developed a heterogenized catalyst of Cu(ii) complex immobilized onto nanosilica obtained from rice husk (RH), through coordination of imine, generated via Schiff-base condensation between 3-aminopropyltriethoxysilane (APTES) functionalized nanosilica and acetamide. The synthesized catalysts were characterized by FT-IR, XRD, BET surface area measurements, ESR, SEM-EDX and ICP-AES. The synthesized catalyst showed excellent activity towards controlled oxidation of different primary and secondary alcohols in the presence of tert-butyl hydrogen peroxide (TBHP). Furthermore, the catalysts could be recovered by simple filtrations and can be reused upto many consecutive cycles without loss in activity.

Full text of DOI:10.1039/c5ra21148d

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Page 1 of 18
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Rice Husk Derived Nanosilica Supported Cu (II) Complex: An Efficient Heterogeneous
Catalyst For Oxidation Of Alcohols Using TBHP
a
a
a
ab
a
Nibedita Gogoi , Smarika Dutta , Tahsina Begum , Utpal Bora * and Pradip K. Gogoi *
Department of Chemistry, Dibrugarh University, Dibrugarh 786004, Assam, India.
Department of Chemical Sciences, Tezpur University, Napaam, Tezpur 784028, Assam,
India.
a
b
CH3
O
C
O Si(CH ) N
O
NH2
Cl
3
3
Cu
Cl
Rice Husk
Nanosilica
Cu@imine-SiO2
OH
O
Cu@imine-SiO2
R1 R2 acetonitrile
TBHP, 50 °C
R1 R2

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Rice Husk Derived Nanosilica Supported Cu (II) Complex: An Efficient Heterogeneous
Catalyst For Oxidation Of Alcohols Using TBHP
a
Nibedita Gogoi , Tahshina Begum , Smarika Dutta , Utpal Bora * and Pradip K. Gogoi *
a
a
ab
a
a
Department of Chemistry, Dibrugarh University, Dibrugarh 786004, Assam, India.
b
Department of Chemical Sciences, Tezpur University,Napaam, Tezpur 784028, Assam,
India.
Corresponding Author e-mail:dr.pradip54@gmail.com, utbora@yahoo.co.in
Abstract: We have developed a heterogenized catalyst of Cu(II) complex
immobilized onto nanosilica obtained from rice husk (RH), through coordination of imine,
generated via Schiff-base condensation between 3-aminopropyltriethoxysilane (APTES)
functionalized nanosilica and acetamide. The synthesized catalysts were characterized by
FT-IR, XRD, BET surface area measurements, ESR, SEM-EDX and ICP-AES. The
synthesized catalyst showed excellent activity towards controlled oxidation of different
primary and secondary alcohols in the presence of tert-butyl hydrogen peroxide (TBHP).
Furthermore, the catalysts could be recovered by simple filtrations and can be reused upto
many consecutive cycles without loss in activity.
Keywords: Nanosilica, Rice Husk, Alcohol Oxidation, TBHP, Cu@imine-nanoSiO
Introduction:
The oxidation of alcohols into the corresponding aldehydes or ketones is one of the
2
1
.
most significant functional group transformations in organic synthesis that has got several
industrial and academic applications. Traditionally, stoichiometric oxidants such as
[1]
manganese and chromium oxides are necessary to mediate this key conversion, sometimes
producing almost the same amount of oxidant-derived waste. Moreover, various reports were
[2, 3, 4]
available for the transition metal catalyzed oxidation reactions of alcohols such as Fe
[
5]
[6,7]
Mo , Ru , Pt , Pd,
[8]
[9,10,11]
[12,13]
[14]
[15]
, polyoxometalates , Mn , Os etc., using
[16]
Vanadium
molecular oxygen or H
2
O or TBHP as oxidizing agent. Nevertheless, a homogeneous
2
catalytic system suffers from a number of drawbacks like separation of the catalyst from the
product and rarely the catalyst is reused. Therefore, the development of catalytic system
combining high catalytic effectiveness with easy recovery and reuse of the catalytic species is
a topic of prime current interest. Solid supported transition metal catalysts have found
immense interest in recent years for the development of new synthetic methodologies due to
its high catalytic activity and reusability. Moreover, both copper(II) as well as copper(I)
[17,18]
complexes
have been shown to be effective under homogeneous or heterogeneous

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[
19,20,21]
condition for the oxidation of alcohols
.Thus, efforts are being made to transform
active homogeneous copper catalysts into effective heterogeneous catalysts by adopting
[22,23] [24]
, clay , SBA-
various immobilization strategies utilizing solid supports like silica
[25,26]
[27]
and zeolite etc. Due to some specific characteristics of silica like low cost, wide
15/16
accessibility, high thermal stability, and high surface reactivity, it is well known as one of the
best supports for grafting of homogeneous metal compounds through coordinative linkage.
Silica is one of the most valuable inorganic multipurpose compounds and naturally abundant
oxides which are mainly made by synthetic means for its technological applications. In
nature, silica is generally found in agro waste like rice husk (RH), which therefore can
provide an alternating pathway to extract silica. It is well known that the silica content of the
[28]
rice husk ash (RHA) can be as high as 90–97% .The surface of silica contains Silanol (Si-
O-H) and Siloxane (Si-O-Si) groups which are the main keys of activity for any process
[29]
taking place on the surface and which are considered to be non-reactive . The silica
obtained from rice husk ash (RHA) provides an abundant and cheap alternative source of
silica which is useful for many applications particularly as a support for heterogeneous
[
30, 31]
catalysis
.
In the present work, we have used rice husk (RH) derived nanosilica having high
[
32,33,34,35,36]
surface area and high reactivity prepared by reported method
. This nanosilica
shows strong and a large number of acidic sites in comparison with commercial silica,
making it a good support material to prepare copper immobilized catalyst to carry out
oxidation of alcohols.
2. Experimental Section:
2.1 General Information:
All chemicals were obtained commercially and used as received without further
drying or purification. The characterization of silica supported materials can be carried out
only by some limited techniques such as FT-IR, ESR, XRD, SEM-EDX, ICP-AES, Brunner
Emmet Teller (BET) surface area analysis only. This is because of the insolubility of the
silica supported metal complexes in all common organic solvents.
X-ray diffraction pattern (XRD) of silica and nanosilica were recorded on a Bruker
0
AXS D8 Advance Diffractometer with Cu-K (1.541A radiation). FT-IR spectra were
recorded in KBr pellets on a Shimadzu IR prestige-21 FT-IR spectrophotometer (200–4000
−1
2
cm ). ESR spectra of the catalysts were recorded at liquid N temperature at SAIF, IIT
Bombay. The Cu loading in the catalyst were determined using inductively coupled plasma-
atomic emission spectroscopy (ICP-AES) analysis on a Thermo Electron IRIS Intrepid II

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XSP DUO. GC –MS was performed on an Agilent Technologies GC system 7820 coupled
with a mass detector 5975 and SHRXI–5MS column (15 m length, 0.25 mm inner diameter
1
and 0.25 micron film thickness). H spectra were recorded in CDCl
3
using TMS as an internal
standard on a BruckerAvance 400MHz spectrometer. The specific surface areas were
calculated following BET procedure. CuCl2.2H O is from Arora Matthy Ltd., acetamide and
t-butyl hydroperoxide (TBHP) from MERCK, hydrogen peroxide (H O ), APTES (3-
2
2
2
aminopropyltriethoxysilane) were purchased from Sigma Aldrich.
2.2 Methods
2.2.1 Synthesis of materials: To prepare the silica supported catalysts, first we have
[
32,33,34,35,36]
prepared the nanosilica from rice husk (RH)
.The method of extraction of
nanosilica from rice husk involves mainly four steps which are as described below:
(i) Washing and drying: Firstly, the RH is washed with water in order to remove
impurities like dust, sand etc. associated with it. It is then dried in oven at 110ºC for
2
4 hours. The dried RH is refluxed with 1M HCl for about 90-100 minutes under
stirring condition. This RH is then decanted and washed with warm distilled water to
make it acid free followed by heating in oven at 110ºC for 20 hours.
(
ii) Heating: A known quantity of RH which come across the above treatment is heated at
00ºC for 2.5 hours after which white coloured rice husk ash (RHA) is obtained.
7
(iii) Extraction of silica: For this purpose, 10 g RHA is stirred with 50 mL NaOH (2.5 M)
solution in a beaker and heated at 70-80ºC for 3 hours followed by filtration. The
residue so obtained is washed with 20 mL boiled water. After then, 10M H
2 4
SO added
to make its pH equal to 2 followed by addition of NH OH to make its pH equal to 8.
4
The precipitate is allowed to dry in oven after which white coloured silica is obtained.
(iv) Extraction of nanosilica: At last, the extracted silica is refluxed with 6 M HCl for 4
hours and washed to make it acid free. To this, 2.5 M NaOH is added and stirred for 1
2 4
hour. 10 M H SO is added to this solution to make the pH of the solution equal to 8.
The precipitate so obtained is washed with warm water and dried in oven for 2 days to
get white coloured nanosilica.
Here we would like to highlight some images of rice husk materials involved in the
preparation of nanosilica (fig 1.).

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(
a)
(b)
Fig.1: (a) Rice Husk (RH), (b) Acid treated rice husk, (c) RH on burning in presence of air,
(
(c)
(d)
(e)
(f)
ο
d) RH when burnt in a furnace at 700 C, (e) Silica, (f) Nanosilica
2.2.2 Synthesis of Cu@imine-nanoSiO catalyst:
2
Step I: Synthesis of APTES functionalized silica gel: APTES@nanoSiO
2
ο
3
.8 g of nanosilica (dried at 120 C for 2h) was added to 100 mL of dry toluene
followed by addition of 2.0 mmol of APTES. The resultant mixture was refluxed under N
2
atmosphere for 6h and was allowed to cool. The solids were separated by filtration and
washed repeatedly through Soxhlet extraction with toluene. The resulting material was then
ο
dried at 120 C for 24 h to obtain APTES immobilized silica.
Step II: Synthesis of imine functionalized silica gel: imine@nanoSiO
2
In a suspension of 2.3 g of APTES@nanoSiO in 60 mL ethanol, 2.12 mmol of
2
acetamide was added in small portions and the reaction mixture was refluxed for 6 h. The
solids were separated from the solvent by filtration and washed repeatedly through Soxhlet
extraction with ethanol and acetone. The resulting acetamide grafted silica was dried at 120
οC for 24 h and designated as imine@nanoSiO
Step III: Immobilization of CuCl .2H O on to imine@nanoSiO
2 2
g of imine@nanoSiO was added to a solution of 60 mg CuCl .2H O in 40 mL of
.
2
2
2
2 2
: Cu@imine-nanoSiO
2
2
acetone and the reaction mixture was stirred at room temperature for 24 h. The resulting
solution was filtered off and the solids were washed several times through Soxhlet extraction
[37]
with acetone. The material was designated as Cu@imine-nanoSiO2 . The reaction involved
was given in the Scheme 1.
2
.2.3 Typical procedure for oxidation of different primary and secondary alcohol using
copper complex:
A 50 mL round-bottom flask was charged with a mixture of alcohol (1 mmol),
catalyst (15 mg, Cu 0.26 mol %), oxidant and solvent (4 mL). The mixture was stirred at
0οC temperature for the required time. After completion, the catalyst was subjected to
5
centrifugation and the residual solid after filtration was washed with the same solvent (4 mL)
for three times. The filtrate was diluted with water (10 mL) and extracted with ether (3×10
mL).

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O
OH
EtO
OH + EtO Si(CH ) NH
OH
O
Toluene
O Si(CH ) NH + H C C NH
2
2
3
2
2 3
2
3
reflux, N atm.
2
EtO
O
6
h
nanoSiO2 APTES
APTES@nanoSiO2
ethanol, reflux,
h
6
CH3
CH3
C
O
O Si(CH ) N
O
O Si(CH ) N
CuCl .2H O
2
C
2
NH2
NH2
3
3
2 3
acetone, r.t., 24 h
O
O
Cu
Cl
Cl
imine@nanoSiO2
Cu@imine-nanoSiO2
Scheme 1. Preparation of the supported Schiff base Cu(II) complex
The resultant organic phases was washed with brine (2×10 mL) and dried over
anhydrous Na SO , filtered and evaporated under reduced pressure. The residue was
2
4
subjected to silica gel column chromatography (ethyl acetate-hexane, 2:8) to obtain the
desired products. For recycling experiments, residue catalyst after filtration was washed with
excess water (3×5 mL) and diethyl ether (3×5 mL) in sequence. After drying the recovered
catalyst at 100 °C overnight in an oven, the residual catalyst was subjected to subsequent run
of the oxidation reaction by charging with the required amount of substrates (alcohol),
oxidant and solvent, without further addition of catalyst. The isolated products were
1
confirmed by comparing their FT-IR, H NMR and mass spectral data with that of reported
samples.
3
3
3
. Results and discussion:
.1 Characterization of the materials:
.1.1 XRD of silica and nanosilica: Figure 2 shows XRD analysis of silica obtained from
RH, in which sharp peaks appear indicating the crystalline nature of silica. Again, Figure 3
ο
shows XRD pattern of nanosilica extracted from silica calcinated at 700 C. The sample is
ο
amorphous as a broad diffraction peak around 2
θ
̴
21.8 was obtained. These results agree
[32]
with those obtained in the previously reported studies .Similarly, Figure 4 represents the
XRD of Cu@imine-nanoSiO which shows some sharp peaks similar to silica showing the
crystallinity of the silica framework in presence of Cu(II)imine complex. Though there is
similarity in the XRD profiles of SiO and Cu@imine-nanoSiO , there intensity of the peaks
differs as observed from Fig. 2 and Fig. 4, respectively. This change in the peak intensity of
nanosilica after immobilization has altered the nano-SiO surface resulting the modification
2
2
2
2

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of its crystallinity. However, it is observed that the peak at 21.8° has higher intensity in case
of Cu@imine-nanoSiO compared SiO
2
2
.
3
.1.2 FT-IR study:
From the FT-IR spectra of silica, nanosilica and silica based materials, it is observed
that the FT-IR spectra of silica and nanosilica (Fig. 5, 6) are almost similar. A band at 1626
-
1
cm which corresponds to the OH bending vibration of water molecule present in the silica.
-
1
The large sharp peak at 1103.33 cm is due to the structural siloxane framework which is the
-1
asymmetric stretching frequency of Si-O-Si and 797.60 cm is also due to symmetric
-1
stretching of the same. The band at 467.76 cm is due to the bending frequency of Si-O-Si.
-
1
A large broad band at 3455.62 cm is due to OH stretching frequency.
-
1
When nanosilica is functionalized by APTES, a new band appears at 1540 cm due
to NH
2
bending mode, indicating successful grafting of APTES on to nanosilica (Fig. 7b).
-1
Interestingly, a new band at 1631 cm appears due to C=N stretching of imine generated
through Schiff base condensation between amine group of APTES@nanoSiO and C=O
group of acetamide (Fig. 7c). In FT-IR spectra of Cu@imine-nanoSiO , a band at 451 cm
2
-1
2
-1
and 326 cm due to ѵCu-N and ѵCu-Cl respectively (Fig. 8d), confirming that Cu is
coordinated through nitrogen and chlorine.
1000
21.8
800 10.58
600
400
200
10
20
30
Twotheta(Degree)
40
50
60
70
80
Fig.2: XRD of silica
Fig. 3: XRD of nanosilica
Fig 4 : XRD of Cu@imine-nanoSiO
2
3.1.3 BET analysis for surface area measurement:

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The BET analysis for surface area measurements of the produced silica based
materials are shown in Table 1. From our study, the surface area of nanosilica is found to be
greater than that of silica and also the surface area of silica based materials decreased
accordingly. The subsequent decrease is attributed to the successful immobilization of metal
complex.
Fig.5: FT-IR spectra of silica
Fig. 6: FT-IR spectra of nanosilica.
Fig.7: FT-IR spectra of (a) nanosilica (b)
APTES@nanoSiO (c) imine@nanoSiO
nanoSiO
2
2
(d) Cu@imine-
2
Table 1:
BET measurement of silica and silica based materials
2
SBET (m /g)
Entry
Materials
Silica
1
2
3
148.12
Nanosilica
189.97
APTES@nanoSiO2
175.45

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4
5
imine@nanoSiO
2
157.05
145.85
Cu@imine-nanoSiO2
3
.1.4 ESR, ICP-AES and SEM-EDX analysis:
The ESR spectra of the catalysts (Fig. 8) recorded at liquid nitrogen temperature in
magnetically dilute form showed distinct signal for Cu(II) species showing four hyperfine
lines with g-values of 2.19 (g ) and 2.04 (g ) indicating tetragonal distortion as expected.
║
┴
Fig. 8: ESR spectra of Cu@imine-nanoSiO2
Considering the results of the ICP-AES and SEM-EDX observations, we are able to
determine the metal attachment on the surface of solid supports. The amount of metal content
in the Cu@imine-nanoSiO
.26 mol% per 15 mg of the catalyst. A comparison of EDX analysis (fig.9a, 9b) of
nanosilica with Cu@imine-nanoSiO confirms the presence of copper to the catalyst, which
indicating the formation of metal complexes with the anchored ligand The SEM images (Fig.
2
0a, 10b) of nanosilica and the immobilized copper on nanosilica (Cu@imine-nanoSiO )
2
catalyst is determined by ICP-AES analysis and is found to be
0
2
1
clearly indicates the changes in the morphologies of the catalyst after introduction of metals
were observed by SEM images.
4
4
. Cu@imine-nanoSiO catalyzed alcohol oxidations:
2
.1Catalyst screening and solvent optimization
To investigate the effectiveness of the copper complex in oxidation reaction, 1-
phenylethanol was chosen as a model substrate. Reactions were performed under different
reaction conditions and results are summarized in Table 2. Initially, we carried out the
oxidation reaction by treating 1-phenylethanol (1 mmol) with copper 0.26 mol% using H
2 2
O
or TBHP as oxidant in various solvents at 50 °C. Generally, solvent plays a significant role in

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the conversion and product distribution of alcohol oxidation reaction. In order to investigate
the effect of solvent for oxidation reaction, different solvents such as H O, MeOH, THF,
2
CH Cl , toluene, acetonitrile and solvent-free condition were chosen for oxidation of 1-
2
2
phenylethanol (Table 2, entries 1-10 ).
Fig. 9a: SEM-EDX of nanosilica
Fig. 9b : SEM-EDX of Cu@imine-nanoSiO
2
Fig.10a: SEM image of nanosilica
Fig.10b: SEM image of Cu@imine-nanoSiO
2
On study the effect of solvents, we have observed that, acetonitrile is the most suitable
solvent for the reaction than the other solvents (Table 2, entry 9). Acetonitrile is polar and
have a very high dielectric constant with a polarity index of 5.8. So, it may readily dissolve
TBHP along with other organic substrates (reactants). This facilitates the adsorption of
reactants on the active site of the catalyst and increases the effectiveness of the conversion
and selectivity. Although, water has a high polarity index of 9.0, the conversion of 1-
phenylethanol is lower than that of acetonitrile. It is due to its poor solubility in the reaction
[
38,39,40]
medium
0).
. In solvent-free condition, the yield of product is trace amount (Table 2, entry
1
The effect of catalyst amount on oxidation of 1-phenyl ethanol was also investigated.
The results shows that with decreasing of the amount of catalyst to 10 mg (0.18 mol%) the
yield of product decreased, while increasing amount of catalyst to 20 mg (0.35 mol%) did not
show any considerable effect on the reactivity (Table 2, entries 11, 12). Again, it was seen
that the reaction gives lower yield at room temperature (Table 2, entry15). During the course

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2 2
of our study, H O is found to be less effective than TBHP for oxidation reaction (Table 2,
entries 1-6). Again, we found that 0.3 mL of TBHP was sufficient for the effective
conversion. However, the reaction did not proceed in the absence of TBHP (Table 2, entry
16).
After the investigation of the effects of different parameters for the oxidation of 1-
phenylethanol, as a standard substrate, the best conditions were chosen and various alcohols
were oxidized in the presence of a catalytic amount of Cu@imine-nanoSiO and TBHP as an
2
oxidant at 50 °C in acetonitrile. Further study was performed to enhance the scopes of
oxidation reactions in different substituted primary and secondary alcohols.
Table 2:
Optimization of oxidation of 1-phenylethanol
a
OH
CH3
O
Cu@imine-nanoSiO2
CH3
Solvent, oxidant,50° C
b
Entry
Solvent
Oxidant
Catalyst (mg) Time (min) Yield (%)
1
2
3
4
5
6
7
8
9
H
2
O
H
H
H
H
H
H
2
2
2
2
2
2
O
O
O
O
O
O
2
2
2
2
2
2
15
15
15
15
15
15
15
15
15
15
10
20
15
15
15
15
180
180
180
180
180
180
90
33
41
MeOH
CH Cl
THF
Toluene
CH CN
2
2
53
55
60
3
80
H O
2
TBHP
TBHP
TBHP
H O
65
MeOH
60
76
CH CN
3
35
96
10
11
12
13
14
15
16
-
180
45
trace
90
2
2
CH
CH
CH
CH
CH
CH
3
3
3
3
3
3
CN
CN
CN
CN
CN
CN
TBHP
TBHP
TBHP
TBHP
TBHP
-
40
96
c
50
80
d
40
97
e
150
180
60
-
a
Reaction conditions: 1-phenyl ethanol (1 mmol), oxidant (0.3 mL), solvent (4 mL),
Cu@imine-nanoSiO (15 mg, 0.26 mol% Cu) ca. 50 ºC in air unless otherwise noted.
2
b
c
d
Isolated yield. TBHP (0.1 mL) TBHP (0.5 mL). r.t (5h).
e

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Both the primary and secondary alcohols on oxidation gave satisfactory to good yields
of the corresponding aldehydes and ketones, although a significant variation in yields was
noticed. The results are summarized in Table 3. It has been seen from Table 3 that, in general
secondary alcohol was also found to be more reactive toward oxidation under the same
reaction condition.
Table 3
a
2
Oxidation reactions of various alcohols using TBHP and Cu@imine-nanoSiO catalyst
OH
O
Cu@imine-nanoSiO2
R1 R acetonitrile
2
R1 R2
TBHP, 50 °C
b
Entry
Alcohol
Product
Time
min)
Yield(%)
(
1
35
35
30
30
96
98
97
97
2
3
4
5
30
95
6
7
8
5
67
69
5
OH
10
69
80
O2N
9
30
OH
CH3
O
1
0
20
75
CH3
OH
OH
1
1
1
2
25
15
78
90
CH (CH ) CH OH
3
2 6
2
a
Reaction conditions: alcohol (1 mmol), oxidant (0.3 mL), solvent (4 mL), Cu@imine-
b
nanoSiO (15 mg, 0.26 mol% Cu) ca. 50 ºC in air unless otherwise noted. Isolated yield.
2
Formation of product was confirmed by comparing FT-IR; 1H & 13C NMR spectra, GC-MS
and melting point measurement.

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2
Both electron withdrawing and electron donating substituents, such as p-Cl, p-NO , p-
Br, p-MeO, p-Me in the benzene ring gave the desired products in 69–98% yield (Table 3,
entries 1–8). Primary benzyl alcohols with electron donating and withdrawing groups gave
products in good yields (Table 3, entries 6-8). Primary benzyl alcohols bearing halogen group
was also successfully oxidized to benzaldehyde with satisfactory yields (Table 3, entry 7).
4.2 Reuse of the catalysts:
Reusability of a catalyst is an important and attractive criterion for eco-friendliness,
efficiency and industrial relevance. Therefore, we studied the reusability of the catalyst up to
th
5
cycle using 1-phenylethanol as model substrate (Table 4). The reusability tests were done
by simple filtration of the reaction mixture. For this purpose, after completion of the reaction,
the catalysts were separated from the reaction mixture by centrifuging and then the catalysts
were dried in an oven. After drying, it was again used for alcohol oxidation following the
same procedure as mentioned above. It was found that the yield of the product goes on
decreasing as the number of cycle increases. The slight decreasing trend of the product
2
formation was due to physical loss of the nanosilica supported Cu@imine-nanoSiO complex
during separation and purification. Again, it was observed that, the reaction time goes on
increasing with increase of number of cycle as shown in the table 4.
Table 4:
Recycling of the catalyst for the oxidation reaction
No. of cycle
Time (min)
% yield
96
1
2
3
4
5
35
45
50
65
65
94
90
88
85
Finally, a probable mechanism of the alcohol oxidation reaction has been proposed which is
as given below in the scheme 2

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H
H
H
R
OH
R
OH
t-BuOOH
t-BuOO
CuII
CuI
t-BuOOH
t-BuO
SiO2
SiO
2
t-BuOH
O
R
H
CuII + t-BuOOH
I
+
Cu + t-BuOO + H
CuII + t-BuO + H O
I
+
Cu + t-BuOOH + H
2
Scheme 2: Plausible mechanism of the alcohol oxidation
th
after 5 use
Fig. 11: SEM-EDX spectra Cu@imine-nanoSiO
2
th
after 5 cycle
Fig.12: FT-IR spectra of Cu@imine-nanoSiO
2
To gain better insight into how our catalytic system changes during the reaction, we
have also studied the catalytic evolution by means of SEM and energy dispersive X-ray

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th
analysis (EDX). Interestingly, inspection of the SEM image of a sample of catalyst after 5
cycle from the alcohol oxidation reaction indicates the immobilization of copper metal onto
nanosilica based materials i.e. the recovered catalyst showed almost the similar SEM image
like the fresh catalyst. Furthermore, on the basis of the results of the EDX spectra, Cu seems
to be present on the surface of solid support. Moreover, the IR spectra of the used catalyst
also indicates no shift in any major peak, regarding to Cu-N and Cu-Cl bands.
Table:5 Comparative results for the oxidation of alcohols using reported conditions with the present
method
Entry
Metal Catalyst
Condition
Yield/
conversion
reference
[38]
1
2
CuAl
CuAl
2
O
4
-CCM and
O -MCM.
Acetonitrile, oxidants, 65-99 % yield
time: 8 h, temp 80 °C.
2
4
Cu/HT (7.3 mol%),
.
p-xylene, Ar, time: 4-
4 h, temp 130-150 °C,
72-99%
conversion
[41]
2
3
silica-supported palladium Molecular O
2
, K
2
CO
3
,
catalysts bearing N–N, N– time: 2.5- 15 h, 90 °C.
40-96% yield
[42]
S and N–O chelating
ligands
4
5
Cu
3
(BTC)
2
TEMPO, Na
acetonitrile, 22 h, 75
C, oxygen atmosphere
2
CO
3
,
2-89 % yield
67-98 % yield
[43]
°
Cu@imine-nanoSiO
2
TBHP, Acetonitrile,
time: 5-35 min, 50 °C
Present work
4.3 Metal leaching test
To verify the leaching of metal during the cycles, a metal leaching test was
performed. The alcohol oxidation of 1-phenylethanol was started in the usual way. After
completion of the reaction, the reaction mixture was subjected to centrifugation. The leach
metal in the filtrate was determined by ICP- AES analysis and found to be 0.25 ppm Cu.
These results suggest the strong coordination and stability of the metal framework which
precludes the metal leaching during the reaction
2
In addition, the catalytic performance of Cu@imine-nanoSiO was compared with
some other reported ligand and metal systems (Table 5). The present procedure is effective
for the oxidation of alcohols in a short reaction time and at low temperature so is applicable
to a wide variety of functional groups
Conclusion:

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In conclusion, we have developed a novel copper-based heterogeneous catalyst
by immobilizing CuCl .2H O onto imine-functionalized nanosilica coordination. From
2
2
the environmental point of view, the extraction of silica from RH is safe, cost effective and
environment friendly. Considering easy availability of RH worldwide, the production of
nanosilica from RH can be utilized as low cost technology in fine chemical synthesis. The
synthesized catalyst exhibited excellent activity for the selective and rapid oxidation of
benzylic alcohols to the corresponding aldehydes and ketones. The reaction time is very fast
and no over-oxidized product was observed. The catalytic system could be reused
successfully upto fifth cycle without significant loss of activity or yield or extension of the
reaction time. This methodology will find interest in both academic as well as industry due to
its cost-effective process involving heterogeneous nanosilica.
Acknowledgements: The authors acknowledge to SAIF-Kochi, SAIF-IIT Bombay and
CSIR-NEIST, Jorhat for carryout various instrumental analysis.
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