Hybrid composite of CuO with g -C 3N 4 as a photoactive catalyst: an efficient approach for the oxidation of alcohols

Article abstract of DOI:10.1007/s12039-018-1581-6

Abstract: An eco-friendly method for the oxidative transformation of alcohols to their corresponding carbonyl compounds by using g-C ₃N ₄@ CuO as a photoactive heterogeneous catalyst has been developed. The catalyst was characterized

Full text of DOI:10.1007/s12039-018-1581-6

J. Chem. Sci.
(2019) 131:4
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-018-1581-6
REGULAR ARTICLE
Hybrid composite of CuO with g-C N as a photoactive catalyst:
3
4
an efficient approach for the oxidation of alcohols
∗
∗
TASKIA RAHMAN, GEETIKA BORAH and PRADIP K GOGOI
Department of Chemistry, Dibrugarh University, Dibrugarh, Assam 786 004, India
E-mail: geetikachem@yahoo.co.in; dr.pradip54@gmail.com
MS received 14 August 2018; revised 12 November 2018; accepted 12 November 2018
Abstract. An eco-friendly method for the oxidative transformation of alcohols to their corresponding carbonyl
compoundsbyusingg-C N4@CuOasaphotoactiveheterogeneouscatalysthasbeendeveloped. Thecatalystwas
3
characterized by SEM-EDX, TEM, BET surface area measurements, powder XRD, FTIR, photoluminescence
and UV-Vis spectroscopy. It was found to be very effective for the conversion of both primary and secondary
alcohols into aldehydes and ketones with excellent yields using tert-butyl hydrogen peroxide (TBHP) as oxidant
at room temperature in the presence of visible light in aqueous medium. The catalyst can be separated from the
reaction mixture by simple centrifugation and reused up to five cycles without significant loss in activity.
Keywords. Nanocomposite; graphitic carbon nitride; alcohol oxidation; copper oxide; photocatalyst.
1
. Introduction
range15 due to wide bandgap. However, using TiO2
supported Au nanocomposites have shown excellent
16
The use of photocatalyst in the presence of visible photoactivity in alcohol oxidation and dye degrada-
light energy for chemical transformation is an active tion in the presence of visible light indicating the role of
area of research today.1 Photocatalysis is considered structure and properties of support material. Recently,
to be an environmentally benign protocol with appli- g-C N has been highly explored as photocatalyst due
–3
3
4
cations in hydrogen production,4 dye degradation,
,5
5,6
to its unique stability, low cost, low bandgap (2.7eV),
◦
water splitting, etc. The controlled oxidation of alco- excellent heat endurance (as high as 600 C) and high
hols into corresponding aldehydes or ketones is one resistance to chemicals such as strong acids and strong
of the most important organic syntheses with applica- bases. The presence of metal in g-C N enhances
17
3
4
tions as intermediates in pharmaceutical industries or the photocatalytic activity by narrowing the band gap
in bulk/fine chemical industries. Traditionally, in the and expanding photo absorption into visible range.18,19
oxidation of alcohols to get aldehydes and ketones, tran- Although g-C N has a better capacity for utilization of
3
4
7
sition metals such as Cr(VI) and Mn(II) were widely sunlight compared to other photocatalysts due to lower
used, which are toxic, corrosive as well as expensive. bandgap, it suffers from a high recombination rate of
Several reports on both homogeneous and heteroge- electron–hole pairs. However, the coupling of g-C N4
3
8
9
10
neous catalysis based on metals such as Ag, Fe, Pd,
with appropriate semiconductors can enhance its pho-
11
12
Au, and V have shown excellent catalytic activ- tocatalytic activity by lowering the recombination rate.
ity in alcohol oxidation in the presence of molecular There are very few reports on the use of g-C N as
3
4
O , air, H O or TBHP as oxidizing agents. However, a photocatalyst in oxidative transformations. Recently
2
2
2
in recent times, heterogeneous catalysts have gained researchers have demonstrated benzyl alcohol conver-
20
much importance due to its easy separation, selectiv- sion to benzaldehydes using CdS/g-C N , Ru-g-C3
3
4
ity, stability and reusability.13 In recent times, among N , MIL-125-Ag/g-C N and BiVO /g-C N cat-
21
22
23
4
3
4
4
3
4
various photocatalysts, TiO (anatase) showed excel- alysts. In 2016, Verma and coworkers have reported
2
14
24
lent results but it has two major constraints like the selective oxidation of alcohols as well as alco-
low quantum efficiency and limited photo absorption hol esterification by photoactive VO@g-C N . Good
25
3
4
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-018-1581-6) contains
supplementary material, which is available to authorized users.
0123456789().: V,-vol

4
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(2019) 131:4
mechanical stability, rich nitrogen content, wideband 2.2 Preparation of g-C N @CuO
3
4
semiconductor and chemical inertness provide a good
prospect for using carbon nitride as a catalyst sup-
Two grams of urea is taken (as it is cheap, easily available
compound and an active molecular precursor under thermal
treatment27 in a mortar pestle and 10 mg of cupric acetate is
added into it and grounded in a pestle to make it into a fine
19
port. However, the use of expensive, toxic metals and
harsh reaction conditions demand the need for more
eco-friendly catalyst systems which urged us to use
cheaper, low cost, environment-friendly CuO. More-
over, it is an important p-type semiconductor which has
found many diverse applications such as in gas sensors,
catalysis, batteries, high-temperature superconductors,
◦
powder and kept in a muffle furnace at 400 C for 1 h. After
28,29
cooling, the catalyst was grounded into fine powder.
2.3 Photocatalytic experiment
26
solar energy conversion and field emission emitters.
In a typical procedure, a mixture of alcohol (1 mmol),
As part of our ongoing efforts to develop solidly sup- catalyst (3 mg), oxidant (0.3 mL) and solvent (5 mL) was
added in the 50 mL round-bottomed flask surrounded by cir-
ported transition metal photocatalysts, we report here a
culating water jacket for reaction to prevent thermal oxidation
facile method for the synthesis of g-C N @CuO and its
3
4
and the mixture was stirred for the required time in pres-
ence of visible light using 100 W LED light (400–700 nm)
which is equivalent to 0.16 sun (distance of light source from
application in the oxidation of alcohol to corresponding
aldehydes and ketones. To the best of our knowledge,
this is the first example of a photoactive g-C N @CuO
3
4
16
reaction flask is 28 cm). The progress of the reactions was
catalyst for the oxidative transformation of alcohol. As
the reaction proceeds under mild conditions and the cat-
alyst are recoverable, it provides an environmentally
benign alternative to the existing protocols.
monitored by TLC. After completion of the reaction, the cat-
alyst was recovered by simple filtration and the filtrate was
poured into water. The organic product was extracted with
ethylacetate(3x10mL). Thecombinedorganicfractionswere
dried over anhydrous sodium sulphate and concentrated. The
residue was purified by column chromatography using sil-
ica gel (60–120 mesh) with ethyl acetate/hexane as eluent to
obtain the desired products.
2
. Experimental
3
. Results and Discussion
2.1 Materials and methods
3
.1 Characterization of the catalyst
Urea (Merck), cupric acetate (Loba Chemie), TBHP (Merck)
and all alcohols were obtained from Alfa Aesar. The chem-
icals were used as received without any further purifi-
The FTIR spectrum of g-C N (Figure S1, SI) shows
cation or drying. The 1HNMR spectra of the products similarities with the previous reports. In the IR spec-
3
4
30
were recorded in CDCl3 at 400MHz spectrometer. X-ray trum, g-C N exhibits characteristic IR bands due to
3
4
diffraction pattern (XRD) of g-C N4 and g-C N4@CuO
3
3
were recorded on a Bruker AXS D8 Diffractometer with
Cu-Kα(1.541Å radiation). FTIR spectra were recorded in KBr
pelletsonaShimadzuIRprestige-21FTIRspectrophotometer
−
1
(
400−4000 cm ). The solid UV-Vis spectra (200–800 nm)
were recorded on JESCO750 spectrophotometer. The Photo-
luminescence (PL) spectra were recorded on a Fluoromax-4
spectrofluorometer with an excitation wavelength of 350 nm.
The Cu loading in the catalyst were determined using induc-
tively coupled plasma-atomic emission spectroscopy (ICP-
AES) analysis on a Thermo Electron IRIS INTERPID II XSP
1
13
DUO. H and C spectra were recorded in CDCl3 using
TMS as an internal standard on a BRUKER AVANCE II
400 MHz spectrometer. The SEM images were obtained with
a Quanta 200 FEG Scanning electron microscope, EDX spec-
trawererecordedonthesameinstrument attachedtotheSEM.
TEM analyses were performed on Jeol/ JEM 2100 instrument
equipped with a high-resolution CCD camera and an acceler-
ating voltage of 200 KV. X-ray photoelectron spectra (XPS)
were recorded on an XPS-AES Module, Model: PHI 5000
Versa Prob II. Specific surface areas were recorded following
BET procedure.
Figure 1. Solid UV-Vis spectra of (a) g-C N4 (black) (b)
3
g-C N4@ CuO (blue).
3

J. Chem. Sci.
(2019) 131:4
Page 3 of 9
4
at 1636 cm 1 and 1499 cm
−
−1
and due to spectroscopy (Figure 1). It was seen that in g-C N a
ν
C=N
3
4
−1
aromatic ν
8
at 1327 and 1271 cm . The peak at sharp increase of light absorption occurs at wavelength
C=N
−1
10 cm could be assigned to the characteristic out- 450 nm which can be assigned to its intrinsic band gap of
of-plane bending vibrational modes of s-triazine units. 2.7eV which is consistent with earlier reports.31 How-
−
1
The broad absorption band at 3174 cm corresponds ever, in the spectrum of g-C N @CuO, it was observed
3
4
to ν . After the introduction of CuO, no noticeable that there is a stronger light absorption in both ultravi-
N−H
change in the main characteristic bands of g-C N could olet at 327 nm and in the visible region around 736 nm
3
4
be observed, indicating that there is non-covalently which can be attributed to the presence of CuO in the
coupled interaction between g-C N and CuO in the sample.
3
4
composite. The optical properties of both pure g-C N
From the powder X-ray diffraction (PXRD) pattern
3
4
and g-C N @CuO were determined by solid UV-Vis of g-C N (Figure 2), two broad diffraction peaks at
3
4
3
4
◦
◦
2
θ values of 10.6 and 26.8 which are indexed to
(
001) and (002) planes were obtained. As in the case
24
of VO@ g-C N no XRD peak was obtained for VO,
3
4
in XRD of g-C N @CuO due to the amorphous nature
3
4
of g-C N and very low loading of CuO, a signal for
3
4
CuO could not be detected. However, the presence of
Figure 2. XRD patterns of (i) g-C N4 and (ii) g-C N4@
3
3
CuO.
Figure 4. XPS spectra of Cu 2p and O1s level (inset).
Figure 5. PL spectra (excitation wavelength 350 nm) of (a)
g-C N4 and (b) g-C N4@CuO.
Figure 3. TEM image of g-C N4@CuO with SAED pat-
tern.
3
3
3

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J. Chem. Sci.
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Table 1. Surface area and pore size distribution of the nanomaterials.
−1
3
−1
Entry
Catalyst
g-C N4
Surface area (m2g
)
Pore volume (cm g
)
Pore radius (nm)
1
2
12
44
0.013
0.073
1.668
1.864
3
g-C N4@CuO
3
CuO was confirmed from SEM-EDX and XPS analyses. Table 2. Optimization of oxidation of 1-phenyl ethanol
a
using g-C N4@CuO .
The TEM analysis (Figure 3) shows the sheet formation
of g-C N and SAED pattern (in the inset of Figure 3)
3
OH
O
3
4
indicates a diffused ring pattern which suggests the
amorphous nature of the material and also confirms the
catalyst
CH3
CH3
solvent, oxidant,
17
nanosized structure of g-C N . The presence of cop-
3
4
visible light
per is confirmed by SEM-EDX and ICP-AES analysis of
the catalyst. The SEM images of g-C N [Figure S3(a), Entry
Solvent
Oxidant Catalyst(mg) Yield (%)b
3
4
Supplementary Information] and g-C N @CuO [Figure
S4(b), Supplementary Information] clearly indicates the
changes in the morphology of the catalyst after the intro-
3
4
1
2
3
4
5
6
7
8
9
1
1
CH3CN
H2O
CH3OH
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
O2
TBHP
TBHP
TBHP
TBHP
TBHP
5
5
5
5
3
2
2
3
–
3
3
3
40
45
30
60
60
50
35
Trace
–
30
duction of metal in g-C N . The amount of metal content
3
4
H O
2
in g-C N @ CuO catalyst is found to be 0.0029 mmol
H O
3
4
2
of Cu in 0.003 g of the catalyst.
H2O
H2O
–
H2O
H2O
CH3CN
X-ray photoelectron spectroscopy (XPS) is a quan-
titative spectroscopic method that determined the ele-
mental composition, kinetic energy, empirical formula,
chemical state, binding energy, and electronic-state of
c
c
d
0
1
20
20
the elements that present within a material. Here XPS 12
2
H O
analysis was performed to check the oxidation state of
Bold indicates optimised condition
copper in the catalyst as shown in Figure 4. The two
peaks at 929.37eV and 931.6eV can be assigned to
Cu2p3/2 and 949.14eV is for Cu2p1/2 of CuO which is in
a
Reaction Conditions: 1-phenyl ethanol (1 mmol), catalyst
2–5 mg), solvent (5 mL) Oxidant (3 mL), time 2 h, 100W
(
LED light
32–35
b
c
d
good agreement with the previous literature reports.
The gap between Cu2p3/2 and Cu2p1/2 is 19.8eV which
3
4
Isolated yield, g-C N as catalyst, in dark condition.
25
is in agreement of standard value of 20.0eV for CuO.
be favourable for adsorbing reactant molecules on the
Moreover, thepresenceoftwosatellitepeaksat940.8eV
and 960.5eV also indicate the existence of CuO on
36
active sites of the catalyst. We have reported ear-
lier the photocatalytic oxidation of the alcohol with
3
3
g-C N sheets. Furthermore, the peak at 528.4eV also
3
4
16
34
TiO supported gold as a heterogeneous catalyst with
indicates the presence of CuO.
2
a band gap of 2.9eV and TiO (anatase) has a band
Figure 5 shows steady-state photoluminescence (PL)
spectra for g-C N and g-C N @CuO. The g-C N
2
gap of 3.2eV. In the present work, we have used
3
4
3
4
3
4
g-C N @CuOas a photocatalyst where g-C N has a
shows a strong PL emission peak at 432 nm with an
excitation wavelength of 350 nm. It was seen that the
intensity of PL of g-C N @CuO decreases as com-
3
4
3
4
band gap of 2.8eV and g-C N @CuO has a band gap of
3
4
2
.3eV obtained from Tauc plot (Figure S2, Supplemen-
3
4
tary Information). The lower band gap of g-C N @CuO
increases the photocatalytic activity of the catalyst.
pared to g-C N which is due to the decrease in the
3
4
3
4
−
+
recombination rate of electron–hole (e −h ) pair lead-
ing to better charge separation on insertion of metal in
1
6,17
the support.
This explains the better photocatalytic
3.2 Photocatalytic activity
oxidation of alcohol by g-C N @CuO compared to bare
3
4
g-C N . The BET analysis of g-C N and g-C N @CuO Initially, we have chosen 1-phenyl ethanol as the model
3
4
3
4
3
4
provides key information regarding change occurring substrate with 100 W LED as a light source to perform
in the surface of the catalyst. The surface areas of optimisation studies (Table 2). We have investigated the
2
−1
g-C N and g-C N @CuO were found to be 12 m g
reaction with different solvents such as water, acetoni-
3
4
3
4
2
−1
and 44 m g respectively (Table 1). Undoubtedly, the trile and methanol and the amount of catalyst on the
higher surface area of the catalyst, g-C N @CuO would oxidation of alcohol. On studying the effects of solvent
3
4

J. Chem. Sci.
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4
Table 3. CuO@g-C N4 catalysed oxidation of alcohol.
3
OH
O
g-C N @CuO
3
4
R1
R2
H
2
O, TBHP
R1
R2
visible light
Entry Substrate
Product
Time (h) Yield (%)a TON TOF
1
2
4
96
95
343
339
86
75
4.5
3
4
4
96
94
343
336
86
75
4.5
5
6
7
8
9
4
5
90
93
91
90
85
80
70
88
321
332
325
321
304
286
250
314
80
66
65
80
61
71
50
70
5
4
5
10
11
12
4
5
4.5
Reaction conditions: Alcohol (1 mmol); g-C N4@CuO(3 mg); TBHP
3
(
0.3 mL) H2O (5 mL); 100W LED light, room temperature, TON= mole
a
of product/mole of Cu, TOF=TON/time, Isolated yields.
we have observed water is the most suitable solvent 90–96% yield (Table 3, entry 1–5). Benzyl alcohol and
with 3 mg of the catalyst (0.28 mol%) gives compar- its derivatives were also efficiently oxidized to corre-
atively high conversion in 2 h (Table 2, entry 5). The sponding aldehydes with 88–93% yield (Table 3, entry
use of g-C N (Table 2, entry 10 and 11) instead of 6–12). However, there is no such notable differences in
3
4
g-C N @CuO decreases the yield of the product.
product yield with electron donating (–OMe, –Cl, etc.)
3
4
During the course of our photocatalytic study, we and an electron withdrawing groups (–NO , –OH, etc.)
2
haveexaminedtheperformanceofvariousoxidantssuch in the benzene ring (Table 3, entry 6–12). Moreover,
asH O , TBHP, O -atmosphere. OutoftheseTBHPpro- the position of the substituent in the benzene ring did
2
2
2
vides an effective conversion of the oxidation reaction. not bring any appreciable difference in product yield
TBHP is considered to be a non-toxic and environ- (Table 3, entry 9 and 12).
mentally benign oxidant and generates tert-butanol as
37–39
co-product which is easily separated by distillation.
In our next move, we have examined the catalytic activ-
ity of g-C N @CuO to a wide range of primary and Reusability of the catalyst was checked by using 1-
3
.3 Reusability test
3
4
secondary alcohols to evaluate the scope and limita- phenyl ethanol as the substrate. After the completion
tions of the current protocol. The oxidation of secondary of the reaction, the catalyst was recovered by centrifu-
alcohols such as 1-phenyl ethanol, its derivatives and gation and washed 2–3 times with distilled water and
diphenylmethanol gave the corresponding ketones in dried in an oven for 1 h and reused for next cycle. The

4
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J. Chem. Sci.
(2019) 131:4
Scheme 1. Plausible mechanism for g-C N4@CuOcatalyzed oxidation of alcohols.
3
Table 4. Comparative study on reported conditions of Cu along with metals like V, Bi, Cd and Ag
catalyzed the oxidation of alcohols to corresponding carbonyl compounds with the present protocol.
Entry
1
Metal catalyst
Condition
Yield/Conversion (%)
72–99% conversion
Reference
40
Cu/HT (7.3 mol%)
p-xylene, Ar atmosphere,
time: 4–24 h,
temp:130 − 150 C
◦
2
3
4
Cu@imine-nano SiO2 TBHP, acetonitrile, time:
55–98% yield
41
42
20
◦
5–35 min, 50 C
Cu(bpy)2 /bentonite TBHP, time: 8 h reflux, N2
42–57% conversion
+
atmosphere
Benzotrifluoride, 300W Xe
CdS/g-C N4
3
arc lamp, time:6 h, temp:
◦
6
0 C
5
6
Ru/g-C N4
MIL-125/Ag/g-C N4 ethanol, visible-light
UV LED (390 nm), H2O, r.t
73% conversion
12–92% conversion
21
22
3
3
irradiation (>400 nm),
O2-atm; 6 h.
methanol, time : 16
7
BiVO4/g-C N4
79%
23
3
◦
h),temp(25 − 27 C), light
source (250 W medium
pressure visible Hg Lamp),
oxidant (molecular O2)
CH3CN, H2O2 40W domestic
bulb, r.t, time:1–2.5h
8
9
VO@ g-C N4
85–98% yield
70–96% yield
24
3
g-C N4@CuO
H2O, TBHP, r.t, time: 4–5 h,
visible light
Present work
3

J. Chem. Sci.
(2019) 131:4
Page 7 of 9
4
Table 5. Comparative study with benzyl alcohol in terms of yield/conversion and TON.
Entry
1
Catalyst
Reaction conditions
Yield/conversion (%) TON
Ref.
23
BiVO4/g-C N4
Methanol, time: 16 (h),
81.3 (conv%)
9.2
3
◦
temp(25 − 27 C), light
source (250 W medium
pressure visible Hg Lamp),
oxidant (molecular O2)
2
Au-TiO2
H2O : CH3CN, TBHP, visible
86
74.13
100
16
43
light
◦
3
4
Copper(II) complex H2O, TBHP, temp:60 C
100 (conv%)
93
g-C N4@CuO
H2O, TBHP, room temp time:
–5 h, visible light
332 Present work
3
4
catalyst can be reused up to five consecutive cycles with temperature with broad functional group tolerance. The
no profound loss in yield of product and no apparent loss yield and TON of our present protocol is compared with
in activity (Figure S5, Supplementary Information). To other similar catalyst (Table 5).
investigate catalyst leaching we have carried out ICP-
th
AES analysis of the filtrate after 5 catalytic cycle. From
ICP-AES analysis of the filtrate after the fifth cycle, we 4. Conclusions
have found that there is a negligible amount Cu in the
filtrate (0.91 ppm) suggesting that CuO was held tightly We have developed an eco-friendly, novel protocol for
to the g-C N facilitating the recycling of the catalyst.
oxidation of alcohols into corresponding aldehydes and
The proposed mechanism of photocatalytic oxidation ketones using nanocomposite g-C N @CuO. The cata-
of alcohol by g-C N @CuO is shown in Scheme 1. lyst showed an excellent catalytic activity for a variety
3
4
3
4
3
4
The CuO particles loaded on g-C N could facilitate of alcohols at room temperature under visible light in
3
4
the separation of the electron–hole pair and promote the the aqueous medium. The catalyst was easily recovered
6
interfacial electron transfer process. The photogener- and reused for five cycles without losing its catalytic
ated electron of g-C N nanosheets might help in the activity.
3
4
decomposition of TBHP which leads to the formation of
a highly reactive species t-BuOO. This species seems to Supplementary Information (SI)
be responsible for the photocatalytic oxidation of alco-
hols to corresponding aldehydes and ketones.
The preparation of g-C N4 characterization of the catalyst is
available at www.ias.ac.in/chemsci.
3
Acknowledgements
3.4 Hot filtration test
The authors acknowledge the analytical services provided by
STIC Kochi, SAIF IIT Madras, SAIF NEHU Shillong, SAIF
IIT Kanpur, and BIT Bengaluru. The authors are also grateful
to UGC, New Delhi, India for the financial support under
the scheme SAP-DRS-I (2016-2021) and the Department of
Science and Technology for financial assistance under the
DST-FIST program.
The heterogeneity of g-C N @CuO catalyst was con-
3
4
firmed by hot filtration test. The reaction was carried
◦
out using 1-phenyl ethanol as substrate at 60 C under
similar conditions. The catalyst was filtered off by
centrifugal precipitation after 20 min and half of the
contents of the vessel were allowed to react for 4 h. The
percentage conversion as determined by GC showed
constancy at 30% even after 4 h. A parallel reaction
without hot filtration of the catalyst showed 96% con-
version. These results affirmed non-leaching processes
during the reactions. (Figure S6, Supplementary Infor-
mation).
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