Zinc Oxide/Graphene Oxide as a Robust Active Catalyst for Direct Oxidative Synthesis of Nitriles from Alcohols in Water

Article abstract of DOI:10.1007/s10562-021-03779-2

In this work, without using any linker or chemical modification of graphene oxide, a zinc oxide immobilized graphene oxide-based catalyst was used for the direct aerobic oxidative conversion of alcohols to the nitriles in water. In the first step, graphene oxide was prepared and then zinc ions were electrostatically adsorbed onto the surface of graphene oxide. In the following step, zinc oxide nanoparticles were generated via in-situ growth in presence of NaOH. It was illustrated that graphene oxide layers can control the size of in-situ generated zinc oxide nanoparticles. Various aromatic/aliphatic/heteroaromatic primary alcohols converted to the nitriles in high yields under O₂ balloon with ZnO/GO catalyst. This catalyst can be used for 7 successful consecutive runs without significant loss of activity. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-021-03779-2

Catalysis Letters
https://doi.org/10.1007/s10562-021-03779-2
Zinc Oxide/Graphene Oxide as a Robust Active Catalyst for Direct
Oxidative Synthesis of Nitriles from Alcohols in Water
Iraj Sarvi1 · Ehsan Zahedi1
Received: 2 July 2021 / Accepted: 18 August 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
In this work, without using any linker or chemical modifcation of graphene oxide, a zinc oxide immobilized graphene
oxide-based catalyst was used for the direct aerobic oxidative conversion of alcohols to the nitriles in water. In the frst step,
graphene oxide was prepared and then zinc ions were electrostatically adsorbed onto the surface of graphene oxide. In the fol-
lowing step, zinc oxide nanoparticles were generated via in-situ growth in presence of NaOH. It was illustrated that graphene
oxide layers can control the size of in-situ generated zinc oxide nanoparticles. Various aromatic/aliphatic/heteroaromatic
primary alcohols converted to the nitriles in high yields under O2 balloon with ZnO/GO catalyst. This catalyst can be used
for 7 successful consecutive runs without signifcant loss of activity.
Graphic Abstract
Keywords Graphene oxide · Zinc oxide · Nitriles · Oxidative synthesis of nitriles
1 Introduction
Cyano group containing compounds were found in various
important fne chemicals and intermediates such as dyes,
pharmaceuticals, agrochemicals, natural materials, biologi-
cal molecules and polymers [1–4]. Nitrile groups can be
converted to the diferent valuable functional groups like
amide, amine, imine, oxime aldehyde, ketone, carboxylic
* Iraj Sarvi
i.sarvi@gmail.com
1
Chemistry Department, Islamic Azad University, Shahrood
Branch, Shahrood, Iran
Vol.:(0123456789)
1 3

I. Sarvi, E. Zahedi
acid, ester [5–7], and many heterocycles such as 1,2,3-tria-
zoles, tetrazoles, oxazoles, and thiazoles [8–10]. Various
methodologies for the formation of nitriles have been devel-
oped using diferent starting materials and reagents [11].
Nucleophilic substitution reactions through Sandmeyer [12]
Rosenmund–von Braun [13] and Letts reactions [14] need
toxic cyanide ion. Also, radical cleavage, dehydration of
amides or aldoximes, formal acid/nitrile exchange, arene
C–H functionalization with transition-metal-catalysts and
electrophilic CN transfer strategies reported which involve
additional steps, high temperature/pressure and high cost,
with waste formation caused by stoichiometric reagents
[15–19]. A range of synthetic routes use azide, hydroxy-
lamine and ammonia as alternative sources of toxic cyanide
for preparation of nitriles from aldehydes [20], amines [21],
azides [22], oximes [23], aryl halides [24], and arenes [25].
The aerobic ammoxidation of alcohols or amination/dehy-
dration reactions with aqueous ammonia were also reported
for the synthesis of nitriles as an atom-economical method in
which only water and hydrogen are produced as by-products
[26, 27]. In this context, heterogeneous catalysts have an
important role because of both the ease with which the com-
ponents of the reaction can be separated, and the reusability
of the catalysts [28–31]. Several heterogeneous catalysts
based on ruthenium hydroxide [32], Copper [33], iron [34],
gold [35] magnetic lanthanum hydroxide [36] platinum [37]
or silver [38] have been reported. Recently, carbon based
catalysts were investigated for the aerobic oxidative syn-
thesis of nitriles. Among these, N-doped carbon supported
cobalt-oxide or iron-oxide [39], metal-free N-doped meso-
structure carbon [40], cobalt anchored N-doped mesoporous
carbon [41], and cobalt nanoparticles on ZIF-8 derived
porous N-doped carbon [42] iron modifed N-doped carbon
on fbrous silica nanospheres [43], iron/copper bimetals
on nitrogen-containing polymers [44] could be mentioned.
Also, Ni–Cu bimetal and Ni NPs loaded aluminum oxide
catalysts [45, 46] were used for direct oxidative synthesis of
nitriles have been reported. Among diferent supports, 2D
materials are more economical and environmental-benign
choices for catalytic systems. Environment-friendly boron
nitrides [47], carbon nitrides [48], metal nitrides [49], gra-
phene [50] metal oxides [51], and polymers [52] are 2D
supports that were used for organic transformations. 2D gra-
phene and their oxidized form called graphene oxide due to
high planar surface area, thermal conductivity, mechanical
strength, and electron mobility have been used in catalytic
applications, storage of energy, biomedicine, and sensors
[53]. Metallic nanoparticle-graphene-based nanocatalysts
were reviewed, recently [54]. In this work, we prepared zinc
oxide immobilized graphene oxide for aerobic direct oxida-
tion of alcohols to the aromatic and aliphatic nitriles for the
frst time. Previously, ZnO/CH3COCl catalyst was reported
for dehydration of diferent aldoximes into corresponding
nitriles [55]. Recently, cyanation of aryl iodides with for-
mamide as the CN source and zinc phosphine as the catalyst
was reported [56]. Also, a hybrid graphene oxide/zinc oxide/
chloroprene rubber system was prepared and their mechani-
cal properties were studied [57]. Graphene oxide/zinc oxide
nanocomposite and reduced graphene oxide/zinc oxide nan-
oparticles were used for the synthesis of 1H-tetrazoles and
3-substituted indoles [58, 59]. In this report, without using
any linker or chemical modifcation of graphene oxide, zinc
oxide nanoparticles immobilized graphene oxide was syn-
thesized which was the highly active catalyst for the aerobic
oxidative synthesis of nitriles from primary alcohols in pres-
ence of ammonia in water (Scheme 1).
2 Experimental
All materials and solvents were purchased from Merck and
Aldrich suppliers and used as received.
The nitrile products were characterized by 1HNMR and
13CNMR in CDCl3 using a Bruker DRX-300 AVANCE
spectrometer (300 MHz). For characterization of catalyst
structure transmission electron microscopy (TEM) on Leo
912AB microscope and scanning electron microscope
(SEM) on Leo 1450VP microscope were taken. The chemi-
cal structure of the catalyst was studied by fourier-trans-
form infrared spectroscopy (FT-IR) using a Nicolet Fourier
spectrophotometer. The inductively coupled plasma optical
emission spectrometry (ICP-OES) analyses using an Avio
200 ICP Optical Emission Spectrometer was measured the
content of zinc on fresh and reused catalyst. X-ray powder
difraction (XRD) analysis was performed for crystalline
structure study in the 2teta range of 15–80°.
2.1 Synthesis of Zinc Oxide/Graphene Oxide
Catalyst (ZnO/GO)
Graphene oxide (GO) was synthesized using modifed Hum-
mer’s method [60]. To the mixture of graphite (1 g) and
concentrated H2SO4 (25 mL) in an ice-water bath KMnO4
(3 g) was added and continuously stirred at 5 °C for 10 min.
This suspension was stirred at 30 °C for 30 min and then
sonicated for 20 min. Distilled water (150 mL) was added
and the stirring-sonication process was repeated for 5 times
which was followed by addition of the hydrogen peroxide
(15 mL) under sonication for 60 min to exfoliate achieved
graphite oxide. Black precipitate was then collected via cen-
trifuging at 10,000 rpm (30 min). Achieved GO was washed
with HCl (2 M) and distilled water to be neutralized and
dried at 50 °C.
For the generation of ZnO nanoparticles on GO, an
aqueous solution of Zinc acetate (1 mmol/mL) was added
to the aqueous dispersion of GO (6 mg/mL) under stirring
1 3

Zinc Oxide/Graphene Oxide as a Robust Active Catalyst for Direct Oxidative Synthesis of Nitriles…
HO2C
HO2C
Zn2+
OH
OH
Zn2+
HO2C
OH
HO2C
OH
Zn2+
CO2H
CO2H
Zn2+
Zinc acetate
2+
O
ZnO
Zn2+
OH
OH
Adsorption
O
O
Zn2+
GO
OH
OH
HO2C
HO2C
OH
OH
HO2C
CO2H
O
ZnO
OH
O
OH
OH
ZnO
OH
Aromatic
HO2C
OH
Aromatic
Aliphatic
CO2H
ZnO
Aliphatic
CN
ZnO
Aq. NH3
O2
Heterocyclic
Heterocyclic
ZnOO
ZnO
OH
O
ZnO
OH
ZnO/GO
Scheme 1. Illustration of catalyst preparation pathway
for 30 min. The resultant mixture was sonicated for 20 min
and then stirred for 1 h. to obtain a suspension. In the next
step, the homogenous suspension was heated for 1 h (90 °C).
Then, the pH of suspension was adjusted at 10 with an aque-
ous solution of NaOH (1 wt.%) and stirred for 2 h at 90 °C.
After fltration and washing of solid product, the sample was
heated at 160 °C for 3 h.
chromatography (hexane/ethyl acetate, 10:2), and the desired
nitrile product was characterized by 1HNMR and 13CNMR.
3 Results and Discussion
The ligand-free zinc oxide immobilized graphene oxide
catalyst was prepared in a simple method. The graphite was
oxidized to graphene oxide and then zinc ions electrostati-
cally adsorped onto the surface of graphene oxide. Then,
zinc oxide nanoparticles were generated in-situ in presence
of NaOH as depicted in Scheme 1.
2.2 General Procedure for Synthesis of Nitriles
from Alcohols
In a round bottom fask, alcohols (1 mmol), NH3 (2 mmol),
and water (2 mL) were mixed. Then ZnO/GO catalyst
(3 mol%) was added and the resultant mixture was stirred
at 90 °C under an O2 balloon. progress of the reaction was
monitored by thin-layer chromatography (TLC). After the
completion of the reaction, the catalyst was centrifuged and
washed with water and chloroform before drying at 60 °C
in the oven. The mixture of reaction was purifed by column
Chemical structure of catalyst was studied with CHNS
analysis, and FTIR analysis. Based on CHNS analysis results
carbon, hydrogen, nitrogen, and sulfur contents of GO are
49.8%, 4.9%, 0%, and 0%, respectively (Table 1, entry 1).
The absence of any sulfur element in the GO composition
confrms that organosulfate species which were produced
in the synthesis procedure of GO due to using sulfuric acid
were removed completely in the work-up step. Content of
1 3

I. Sarvi, E. Zahedi
Fig. 1a, the peak from 3000 to 3700 cm−1 is attributed to the
hydroxyl stretching vibration. Also, characteristic absorption
peaks of GO correspond to the carboxyl C = O, aromatic
C = C, epoxy C–O, alkoxy C–O, and = C–H groups were
appeared at 1715 cm−1, 1620 cm−1, 1040 cm−1, 1055 cm−1,
and 786 cm−1, respectively. The absorption peaks of oxygen-
containing functionalities confrm the successful oxidization
of graphite into GO. The FT-IR spectrum of the ZnO/GO
catalyst shows the characteristic peaks of GO (Fig. 2b). In
addition, a peak at 450 cm−1 can be attributed to the E2 mode
of hexagonal ZnO [57].
Table 1 CHNS Elemental composition study of GO and ZnO/GO
Entry
Element
C (%)
H (%)
S (%)
N (%)
1
2
GO
ZnO/GO
49.8
32.2
4.9
2.1
0
0
0
0
Based on CHNS analysis
the oxygen element calculated 45.3% for GO. This is an
evidence for successful oxidization of graphite to graphene
oxide. ICP-OES analysis revealed that the content of zinc
on ZnO/GO catalyst is 25 wt.%. The content of carbon, and
hydrogen was decreased to 32.2% and 2.1% for ZnO/GO
catalyst which is due to the generation of ZnO species on
the catalyst surface (Table 1, entry 2).
The XRD analysis for crystalline structure studies of
ZnO, GO and ZnO/GO catalyst are shown in Fig. 2. In
Fig. 2a for the ZnO NPs, the nine characteristic peaks at
2θ=31.8° (100), 34.5° (002), 36.3° (101), 47.5° (102), 56.6°
(110), 62.9° (103), 66.5° (200), 68.0° (112), and 69.1° (201)
can be attributed to a hexagonal wurtzite zinc oxide structure
To characterize the chemical structure of GO and Zn/
GO the FT-IR spectrum was taken (Fig. 1). As shown in
Fig. 1 FTIR spectrum of (a)
GO, and (b) ZnO/GO catalyst
a
=C-H
C-O
C-OH
C=C
C-H
C=O
b
ZnO
3900
3400
2900
2400
1900
1400
900
400
-1
Wavenumber (cm )
Fig. 2 XRD curves of (a)
ZnO, (b) GO, and (c) ZnO/GO
catalyst
(100)
(101)
GO
(001)
JCPDS card No. 36-1451
(002)
(201)
(112)
(110)
c
(102)
(103)
(200)
b
a
10
20
30
40
2θ
50
60
70
1 3

Zinc Oxide/Graphene Oxide as a Robust Active Catalyst for Direct Oxidative Synthesis of Nitriles…
based on JCPDS card No. 36-1451. In Fig. 2b for the GO
support, a characteristic peak at 2θ = 12.5° (001) is seen
which confrm the successful synthesis of GO from graph-
ite. The XRD pattern of ZnO/GO catalyst show all the nine
characteristic peaks of zinc oxide. The disappearance of the
GO characteristic peak in this curve reveals that the GO lay-
ers exfoliated due to the growth of ZnO NPs between layers
[57]. The crystal sizes of ZnO in ZnO/GO is about 23 nm
based on Debye–Scherrer formula for 36.3° (101) peak.
The morphology of GO and ZnO/GO catalyst was studied
by TEM and SEM analysis (Fig. 3). The GO sheets with
wavy shapes and folded edges are shown in TEM images of
GO and ZnO/GO (Fig. 3a and b). Dispersion of ZnO NPs
onto the surface of ZnO/GO catalyst is illustrated in Fig. 3b.
The uniform small size of ZnO NPs confrms that the growth
of ZnO NPs was between GO layers and the size of ZnO NPs
was controlled with GO sheets so that the particles in the
outer layer have a larger size than the inner nanoparticles.
The size of ZnO NPs is about 5–10 nm.
in 4 h with 97% of nitrile product yield (Table 2, entry 4,
3% b). This showed that ZnO/GO catalysis was efective
for the production of benzonitrile. For the best amount of
catalyst 1, 2.5, 5, and 15 mol% of ZnO/GO was used and the
high yield of product was achieved in the range of 50–97%
(Table 2, entries 5–8). The high yield of benzonitrile was
observed when the reaction temperature was in the range
of 90–100 °C. Further decrease of reaction temperature led
to the lower yield of the product. The optimum temperature
was selected to be 90 °C (Table 2, entries 9–11). Polar sol-
vents (water, DMF, and ethanol) have higher yield than non-
polar solvents (toluene and n-hexane). Among them, water
was a safe and green choice of a solvent for this reaction
(Table 2, entries 12–15). To prove the role of GO in the reac-
tion process, ZnO nanoparticles were synthesized and used
as catalyst in the model reaction under optimized reaction
conditions (Table 2, entry 16). With 5 mol% of ZnO only
35% yield of fnal product was achieved after 24 h. Prob-
ably, aggregation of ZnO nanoparticles during the reaction
process and large size of synthesized nanoparticles led to
the lower activity of ZnO nanoparticles. This control experi-
ment confrm that GO support was efective on the catalyst
efciency. The model reaction was also carried out in the
absence of NH3. The benzaldehyde was produced as fnal
product and no other product was detected. In these studies
only benzaldehyde b was produced as byproduct.
After morphological and chemical characterization of the
catalyst, the efciency of ZnO/GO catalyst was studied in
the synthesis of various nitriles from primary alcohols via an
aerobic oxidative method. The conversion of benzyl alcohol
to the benzonitrile in water at 100 °C was studied as a model
reaction. Ammonia (2 mmol) as a nitrogen source under the
oxygen atmosphere via an O2 balloon was used for this con-
version. The efect of catalyst on reaction process, amount of
catalyst, solvent, and reaction temperature were optimized to
achieve a high yield of the fnal product a (Table 2). With the
catalyst-free condition and also with GO (0.1 g) the reaction
did not proceed and no benzonitrile product was observed,
even after 24 h (Table 2, entries 1 and 2). Therefore, the
presence of a catalyst is necessary for this conversion. Zinc
acetate was used as catalyst which was efective on reaction
progress but even after 24 h benzonitrile was determined
with only 20% of yield (Table 2, entry 3). When 10 mol%
of ZnO/GO catalyst was used the reaction was completed
The efciency of ZnO–GO catalyst in the synthesis of
various nitriles was also investigated. The substrate scope
of primary alcohols is collected in Table 3. With optimized
reaction conditions, aromatic, aliphatic, and hetero aromatic
nitriles were synthesized. After completion of the reactions
only nitriles were produced as a product but in earlier stages
of all reactions corresponding aldehydes of used alcohols
were detected by TLC similar to the previous reports [61].
Benzyl alcohols with electron-donating and electron-with-
drawing groups were produced the corresponding benzoni-
triles in high yields (93–97%), (Table 3 entries 1–7). Based
Fig. 3 TEM image of a GO, b
ZnO/GO
1 3

I. Sarvi, E. Zahedi
Table 2 Evaluation of optimum reaction conditions
Entry
Catalyst
Mol% of catalyst
Solvent
T (°C)
Time (h)
Yield a (%)a
1
2
3
4
5
6
7
8
–
GO
–
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
DMF
100
100
100
100
100
100
100
100
90
24
24
24
4
4
4
–
–
0.1 (g)
Zinc acetate
ZnO/GO
ZnO/GO
ZnO/GO
ZnO/GO
ZnO/GO
ZnO/GO
ZnO/GO
ZnO/GO
ZnO/GO
ZnO/GO
ZnO/GO
ZnO/GO
GO
10
10
15
5
2.5
1
5
5
5
5
5
5
5
5
20
97
90
97
85
50
97
75
40
98
98
50
55
–
8
10
4
4
4
4
9b
10
11
12
13
14
15
16
70
50
90
Refux
refux
100
90
EtOH
Hexane
Toluene
Water
4
15
15
24
Reaction condition: benzyl alcohol (1.0 mmol), catalyst (1–15 mol%), Aq. NH3 (2.0 mmol), and solvent (2.0 ml) under an O2 balloon at
50–100 °C
aIsolated yield
bSelected as the optimized reaction conditions
on Table 3 results the position of substituents was not very
efective in the product yields (Table 3 entries 2–4). Het-
erocycles nitriles and aliphatic nitriles were also produced
successfully via this procedure (Table 3 entries 8–10).
Detection of aldehyde in the earlier stage of the reaction
confrms that in the mechanism of the reaction, alcohol was
converted to aldehyde via the aerobic oxidative dehydro-
genation at the frst step (Scheme 2, A). In the next step,
the dehydrative condensation of an aldehyde with ammonia
led to the production of an imine intermediate which was
oxidized and dehydrogenated to the nitrile (Scheme 2, B,
C) [62].
with fresh substrates and solvent. The recovered catalyst was
active for 7 consecutive runs (Fig. 4). The ICP-OES analysis
showed that only 0.1 wt.% of Zn species was leached during
recycle studies.
The results of ZnO/GO catalyst was compared with other
reported catalysts (Table 4). Table 4 demonstrate that ZnO/
GO is an active and efective catalyst in terms of reaction
time or yield in green reaction media.
4 Conclusion
The ability of the ZnO/GO catalyst to be reused in the
model reaction under optimized reaction conditions was also
investigated. The model reaction was carried out and the
progress of the reaction was monitored by TLC. To com-
pare the recovery performance of the catalyst in a low con-
version rate, after 50 min the catalyst was fltered of. The
yield of the produced product after 50 min was about 40%.
After each catalytic run, the catalyst was centrifuged and
then washed with water and ethyl acetate before drying in
an oven at 60 °C. This catalyst was used for the next run
Ligand-free zinc oxide immobilized graphene oxide was
active in the direct aerobic oxidative conversion of aro-
matic/aliphatic/heteroaromatic alcohols to the corresponding
nitriles. The layers of graphene oxide can control the size of
zinc oxide nanoparticles when they were generated in-situ.
The green water solvent can be used as the reaction medium
and the catalyst is stable in water at refux condition. The
ZnO/GO catalyst was highly active for the conversion of
1 3

Zinc Oxide/Graphene Oxide as a Robust Active Catalyst for Direct Oxidative Synthesis of Nitriles…
Table 3 Substrate scopea
Entry
1
Substrate
Product
Time (h)
4
Yield (%)b
97
2
3
6
5
93
93
4
5
6
7
4
5
5
5
95
95
96
97
8
6
90
9
6
90
50
10
10
aReaction conditions: primary alcohol (1.0 mmol), Aq. NH3 (2.0 equiv.), ZnO/GO (5 mol%), water (2.0 ml) under an O2 balloon at 90 °C
bIsolated yield
1 3

I. Sarvi, E. Zahedi
Scheme 2. Possible mechanism of ZnO/GO catalyzed oxidative synthesis of nitriles
Fig. 4 Recycling study of ZnO/
GO catalyzed synthesis of ben-
zonitrile (after 40 min)
50
45
40
35
30
25
20
15
10
5
40
40
40
39
38
36
36
0
1
2
3
4
5
6
7
Run
Table 4 Comparison of the ZnO/GO results with other reported catalysts
Catalyst
Oxidant
Cat. amount (mol%)
Solvent
Temp (°C)
Time (h)
Yield (%)
Refs
Ag/N-CS-1
Co@NC
Fe/Cu
ZnO/CH3COCl
Zn(OAc)2/ligand
Grubbs catalyst
ZnO/GO
O2 Balloon
O2 (0.1 MPa)
O2 (5 bar)
(From aldoximes)
(From aryl iodides)
O2 (1 atm)
3
4
2
EtOH
H2O
t-Amyl alcohol
refux
50
130
80
140
110
90
2
12
24
94
93
99
[38]
[42]
[44]
[55]
[56]
[63]
This work
3:3 mmol
20/10
2
5
–
20 min 92
24
4
NMP
Toluene
H2O
81
78
97
O2 balloon
4
1 3

Zinc Oxide/Graphene Oxide as a Robust Active Catalyst for Direct Oxidative Synthesis of Nitriles…
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