A silver NP-dispersed water extract of fly ash as a green and efficient medium for oxidant-free dehydrogenation of benzyl alcohols

Article abstract of DOI:10.1039/c7ra12225j

Herein, a green, efficient, and new catalytic system for dehydrogenative oxidation of benzyl alcohols using Ag nanoparticles (NPs) dispersed in water extract of fly ash (WEFA) has been developed. Various characterization techniques were performed to authenticate the formation of Ag@WEFA. The as-prepared Ag NPs (10-20 nm) were found to be dispersed in WEFA, as indicated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images. With Ag@WEFA, a variety of substituted benzyl alcohols were efficiently converted to carbonyl compounds in high yields. All the reactions were deliberately carried out without using any ligand or hazardous organic solvent. This catalytic system involving WEFA is a genuinely new concept. It is, therefore, expected to attract attention from researchers working in the areas of sustainable chemistry.

Full text of DOI:10.1039/c7ra12225j

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A silver NP-dispersed water extract of fly ash as
a green and efficient medium for oxidant-free
dehydrogenation of benzyl alcohols†
Cite this: RSC Adv., 2018, 8, 1313
*
Bishal Bhuyan, Arijita Paul, Meghali Devi and Siddhartha Sankar Dhar
Herein, a green, efficient, and new catalytic system for dehydrogenative oxidation of benzyl alcohols using
Ag nanoparticles (NPs) dispersed in water extract of fly ash (WEFA) has been developed. Various
characterization techniques were performed to authenticate the formation of Ag@WEFA. The as-
prepared Ag NPs (10–20 nm) were found to be dispersed in WEFA, as indicated by transmission electron
microscopy (TEM) and scanning electron microscopy (SEM) images. With Ag@WEFA, a variety of
substituted benzyl alcohols were efficiently converted to carbonyl compounds in high yields. All the
reactions were deliberately carried out without using any ligand or hazardous organic solvent. This
catalytic system involving WEFA is a genuinely new concept. It is, therefore, expected to attract attention
from researchers working in the areas of sustainable chemistry.
Received 8th November 2017
Accepted 12th December 2017
DOI: 10.1039/c7ra12225j
rsc.li/rsc-advances
peroxide would be more attractive. Therefore, oxidative trans-
formation through an oxidant-free dehydrogenative route is of
great signicance.14,15 Dehydrogenative oxidation with the
1. Introduction
The increasing amount of environmental concerns has led
researchers worldwide to develop cleaner and greener reactions
for organic transformations. Development of efficient, atom-
economical, and selective reactions, which can be performed
under safe and mild conditions, has merged out to be one of the
prominent subjects in synthetic organic chemistry.
release of hydrogen gas in the absence of any oxidants must be
a powerful route from the viewpoint of atom economy. To date,
several nanoparticle-based catalytic systems have been reported
to exhibit signicant applications in these transformations.16,17
Hosseini-Sarvari et al. prepared nanosized Ag/ZnO catalysts for
oxidant-free dehydrogenation of primary and secondary benzyl
alcohols to the corresponding aldehydes and ketones under
atmospheric pressure.14 However, majority of these catalytic
protocols have to be carried out under reux conditions in
organic solvents such as toluene. Moreover, they possess some
unavoidable limitations such as troublesome catalyst recovery,
requirement of acid or base additives, solvents, and involve-
ment of high cost. Importantly, there is a serious need for
a greener reaction medium involving lesser impact on envi-
ronment and having features that are attractive from an
industry viewpoint. From green chemistry perspective, it must
be very important to develop a new catalytic system that can be
used in a greener solvent such as water. In this context, use of
water extract of natural feedstock in any organic transformation
is of enormous signicance as this methodology would elimi-
nate any harsh acids or bases, harmful organic solvents, and
other potentially harmful external reagents.18–21
The oxidation of alcohols is one of the fundamental trans-
formations in synthetic organic chemistry.1,2 The oxidation
products are recognized to be essential intermediates in the
manufacture of agrochemicals, ne chemicals, pharmaceuti-
cals, and high-value commodity chemicals.3 With the
increasing concern for economic and environmental accept-
ability, researchers across the world are putting signicant
efforts to accomplish this oxidation reaction with oxygen or
hydrogen peroxides.4,5 Several excellent catalysts have been
developed for environmentally benign oxidation of alcohols to
carbonyl compounds. There are reports on the use of Au,6 Pd,7
Pt,8 Co,9 Ru,10 and Mn11 among transition metal catalysts and
TEMPO12 and mesoporous carbon nitride (mpg-C3N4)13 from
the family of metal-free catalysts for these transformations
using suitable oxidants such as H2O2 or molecular oxygen.
However, from the safety and environmental point of view, the
development of more atom economic catalytic systems that run
efficiently without the use of molecular oxygen or hydrogen
Utilization of waste in important organic transformations
has been regarded as a signicant approach among chemists.
Fly ash, a coal combustion residue, has been reported to
contain oxides of Si, Al, Ca, Fe, Mg, K, and Na as predominant
constituents with trace amounts of B, Mo, Se, Sr, Mn, Ti, etc.
Numerous organic transformations such as Claisen–Schmidt
condensation,22 Knoevenagel condensation,23 Beckmann
Department of Chemistry, National Institute of Technology, Silchar, Silchar-788010,
Assam, India. E-mail: ssd_iitg@hotmail.com; Fax: +91-3842-224797; Tel: +91-3842-
242915
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra12225j
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rearrangement,24 chlorination,25 etc. have been achieved by
modication of y ash as a catalyst. Additionally, silver-based
systems have been oen used for oxidation of alcohols.26,27
Ivanova and co-workers designed Ag/SiO2 and utilized it for the
oxidant-free dehydrogenation of ethanol.28 Moreover, Mitsu-
dome et al. successfully fabricated Ag/hydrotalcite catalysts for
non-oxidant dehydrogenation of secondary alcohols.29 On the
basis of these discoveries, we became interested in the design of
highly compatible and stable Ag-based catalysts dispersed in
waste (water extract of y ash) and their application in efficient
and atom-economical reactions under safe and mild condi-
tions. In the present study, we report an eco-friendly synthetic
route to Ag@WEFA for its high catalytic performance in dehy-
drogenation of alcohols to the corresponding carbonyl
compounds. Exhaustive literature survey reveals that NPs
dispersed in WEFA have never been reported for any organic
transformations. The salient features of the protocol lies in the
use of mild environmentally friendly conditions and high yield
of the desired products. In addition, the re-use of the catalyst by
a very simple method has also been demonstrated.
2.3 Preparation of WEFA (water extract of y ash)
WEFA was prepared by adding 50 mL of distilled water to 5 g of
y ash. It was followed by stirring for 10 min and ltration. The
ltrate was named as WEFA (Fig. 1).
2.4 Preparation of Ag NP-dispersed WEFA (Ag@WEFA)
For Ag@WEFA, 0.5 g of as-prepared Ag NPs was gradually
poured into 20 mL WEFA followed by ultrasonication (Selec-TC-
533, PCi-Analytics made) for 15 min.
2.5 Dehydrogenation of benzyl alcohols using Ag@WEFA
A mixture of benzyl alcohol (1 mmol) and Ag@WEFA (3 mL) was
ꢀ
taken and reuxed at 70 C for an appropriate time. The prog-
ress of the reaction was monitored by thin layer chromatog-
raphy (TLC). Aer completion of the reaction, the mixture was
diluted with 20 mL of water and extracted with 20 mL of
diethylether. The combined organic layer was washed with
brine and dried over Na2SO4 and evaporated in a rotary evapo-
rator under reduced pressure. The resulting product was puri-
ed by column chromatography to afford the pure product. All
compounds are known and have been characterized by
comparison of their physical and spectroscopic data with those
of authentic samples. A gas burette with a H2 sensor was used to
conrm the evolution of hydrogen gas from the reaction. Yield
percent of the isolated products was calculated using the
following equation.
2. Experimental
At rst, we planned to design a bio-molecule-assisted synthetic
route to Ag NPs taking into consideration their stability and
efficiency. Leaves of Paederia foetida Linn. were obtained from
the National Institute of Technology (NIT) campus, Silchar,
Assam, and subjected to washing and drying at ambient
temperature for further use.
Yield% ¼ (actual yield)/(theoretical yield) ꢂ 100
2.1 Materials and physical measurements
Silver nitrate (AgNO3) and alcohols were purchased from Sigma
Aldrich. Fly ash was obtained from Farakka super thermal
power plant situated in West Bengal, India. Double distilled
water was used throughout the experiment. FTIR spectra were
obtained using a KBr matrix via a Bruker 3000 Hyperion
Microscope with the Vertex 80 FT-IR system. XRD measure-
ments were carried out using a Bruker AXS D8-Advance powder
3. Results and discussions
Previously, Bayat31 and his co-researchers had designed
magnetically retrievable Ag NPs deposited on the Fe3O4@SiO2
surface and studied their efficacy in oxidant-free dehydrogena-
tion of alcohols. Ag NPs were prepared via a chemical route, and
tetraethoxysilane (TEOS) was used as a silica source. In addi-
tion, stringent and tedious experimental steps were performed
for the synthesis of the catalyst. In contrast, our present meth-
odology summarizes the design of a neat catalytic system
comprising biosynthesized Ag NPs and water extract of y ash.
The presence of SiO2 in high percentages in y ash also facili-
tates prevention of agglomeration of Ag NPs and provides a high
surface area to stabilize the Ag NPs. Interestingly, Ag@WEFA
accelerates the dehydrogenative oxidation reaction to about
three folds under mild optimized conditions. It is noteworthy to
mention that all the reactions have been efficiently carried out
˚
X-ray diffractometer with Cu-Ka radiation (l ¼ 1.5418 A) at
a scan speed of 2ꢀ minꢁ1. Transmission electron microscopy
images were obtained using a JEOL, JEM 2100 equipment.
Scanning electron microscopy images were obtained using
a JEOL JSM-7600F FEG-SEM instrument equipped with EDS. 1H
NMR spectra of the compounds were obtained using a Bruker
AV III at 500 MHz.
2.2 Preparation of Ag NPs
Ag NPs were prepared with slight modication of our previously
reported method.30 Leaf extract of Paederia foetida Linn. (15 mL)
was mixed with 85 mL aqueous solution of (10ꢁ3 M) AgNO3, and
the mixture was stirred at room temperature for about 8 h. The
mixture changed from initial light yellow to dark brownish; this
indicated the formation of colloidal silver. The obtained
suspension was centrifuged at 10 000 rpm, followed by oven
drying to achieve solid Ag NPs. The reduction of AgI to Ag0 in the
process was accomplished by the leaf extract.
Fig. 1 Preparation of WEFA.
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Fig. 2 (a) FTIR spectra and (b) XRD patterns of the original fly ash.
without using any ligand, binding agents (TEOS or CPTES) or UV-vis spectra, which provide evidence for the formation of Ag
hazardous organic solvent.
nanoparticles. Appearance of a distinct peak around 443 nm
clearly indicates SPR of Ag nanoparticles. A rm increase in the
intensity of SPR with time is observed, which indicates an
increase in the yield of the nanoparticles. In the XRD studies,
the formation and identication of phases of the synthesized
metal nanoparticles were investigated. The XRD patterns of Ag
NPs (Fig. 4(b)) shows peaks at 2q equal to 38.19ꢀ, 44.27ꢀ, 64.79ꢀ,
77.70ꢀ, and 81.59ꢀ, which corresponds to (111), (200), (220),
(311), and (222) planes of the face-centered cubic silver system
(space group Fm3m, JCPDS le no. 89-3722).
Fig. 5 (TEM image) of the as-prepared Ag NPs shows that the
particles formed are poly-disperse and consistently spherical
with a size range of 10–20 nm. The high resolution transmission
electron microscopy (HRTEM) image (Fig. 5(b)) shows that the
inter-planar spacing between two adjacent planes is 0.23 nm,
which corresponds to the (111) plane of face-centered cubic
silver. Selected area electron diffraction (SAED) patterns
conrm poly-crystalline nature and shows good concentric
rings displaying various planes of face-centered cubic silver
(Fig. 5(c)).
3.1 Characterization of the original y ash
Fourier Transform Infra-Red (FT-IR) spectroscopic analysis was
performed to recognize the various functional groups attached to
the original y ash (Fig. 2(a)). The presence of symmetric Si–O–Si
stretching and O–Si–O bending vibrations is observed to be at
about 793 cmꢁ1 and 476 cmꢁ1; the absorption bands at
1077 cmꢁ1 and 560 cmꢁ1 ascertain the presence of asymmetric
Si–O–Al and Si–O–Si stretching frequencies, respectively.32
Furthermore, a broad absorption peak at 3453 cmꢁ1 corresponds
to the presence of hydroxyl groups of silanols (–Si–OH).33,34 X-ray
diffraction (XRD) patterns (Fig. 2(b)) conrmed the presence of
quartz, mullite, and hematite phases in the original y ash.32 The
peaks are in good agreement with hexagonal quartz (JCPDS card
no. 85-1054), orthorhombic mullite (JCPDS card no. 88-0107),
and hematite (JCPDS card no. 88-2359). In the SEM images
(Fig. 3) of original y ash, the presence of predominantly
spherical particles with smooth surface is evident.
3.2 Characterization of the as-prepared Ag NPs
3.3 Characterization of Ag@WEFA
The visible formation of the Ag NPs was conrmed by We have performed HR-TEM, SEM, and energy dispersive X-ray
ultraviolet-visible (UV-vis) and XRD analysis. Fig. 4(a) shows the spectroscopy (EDS) analysis to authenticate the formation of
Fig. 3 (a) SEM images of the original fly ash and (b) magnified view of the fly ash.
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Fig. 4 (a) UV-vis spectra of the as-prepared Ag NPs and (b) XRD patterns of Ag NPs.
Fig. 5 (a) TEM images of the as-prepared Ag NPs, (b) HR-TEM image, and (c) SAED patterns.
Ag@WEFA. The uniform dispersion of 10–20 nm Ag NPs in Ag, Si, and O in large proportions along with trace amounts of
WEFA was observed in the TEM image (Fig. 6(a)). The dark Fe, Ti, Ca, Mg, K, Na, and Al. This is the rst ever report on the
patches indicate Ag NPs, whereas grey coloured materials in the successful synthesis of NPs dispersed in a water extract of waste-
vicinity of Ag NPs indicate the presence of WEFA. The SEM material. It can be claimed to be a very clean and green catalytic
image (Fig. 6(b)) also shows good dispersion of Ag NPs in WEFA system-cum-reaction medium for any transformation.
and is consistent with the TEM ndings. It must be emphasized
that EDS characterization of Ag@WEFA is of critical importance
to know the exact composition. Fig. 6(c) shows the presence of
3.4 Catalytic efficiency of Ag@WEFA towards oxidant-free
dehydrogenation of benzyl alcohols
The efficiency of the reaction medium (Ag@WEFA) was exam-
ined for dehydrogenation of benzyl alcohols to their corre-
sponding carbonyl compounds. To optimize the reaction
conditions, 4-methylbenzyl alcohol was taken as the model
substrate, and the amount of Ag@WEFA, reaction time, and
reaction temperature were studied. From the results of Table 1,
Table 1 Optimization of the amount of Ag@WEFA, reaction time, and
temperature
Sl no.
Ag@WEFA (mL)
Time (h)
Temp (ꢀC)
Yield (%)
1
2
3
4
5
6
8
5
3
3
2
2
1.5
2.5
3
2.75
5
110
70
70
90
70
97
96
98
95
91
45
Fig. 6 (a) HR-TEM image, (b) SEM image, and (c) EDS spectra of
Ag@WEFA.
7
RT
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we found that 3 mL of Ag@WEFA at a reaction time of 3 h (at 70 entry 2). A further increase in the amount of Ag@WEFA to 8 mL
ꢀC) procured best results (Table 1, entry 3). The desired product with a reaction temperature of 110 ꢀC provided an excellent
was obtained in very high yield (98%) with high purity. Even yield of 97% of the desired product in 1.5 h (Table 1, entry 1). A
5 mL of Ag@WEFA furnished the reaction in a 96% yield at decrease the amount of Ag@WEFA to 2 mL provided a good
a reaction time of 2.5 h under identical conditions (Table 1, 91% yield in 5 h (Table 1, entry 5). At room temperature (RT),
a negligible 45% yield was obtained in 7 h (Table 1, entry 6).
Aer exploring a wide range of reaction conditions, it was noted
that 3 mL of Ag@WEFA at a reaction temperature of 70 ꢀC and
reaction time of 3 h was optimal for carrying out the reactions. It
Table 2 Catalytic dehydrogenation of benzyl alcohols in the presence
of Ag@WEFAa
is noteworthy to mention that the amount of Ag dispersed
per mL of WEFA is 0.025 g.
Entry
1
Substrate
Product
Time
4
Yield (%)
96
To explore the scope of this newly designed catalytic system,
reactions of a series of benzyl alcohols were conducted under the
optimized reaction conditions. Results of the dehydrogenative
oxidation of benzyl alcohols in Ag@WEFA are summarized in
Table 2. Various substituted benzyl alcohols bearing electron-
withdrawing and electron-donating groups showed an excel-
lent reactivity and delivered the desired products in a very good
to excellent yield within very short reaction time (Table 2, entries
1–13). Hydrogen gas evolved during the reaction was detected
and measured using a gas burette tted with a H2 detector.
Conclusively, it was observed that p-substituted electron-
donating groups give more yield than the electron-withdrawing
groups. The ability of this newly designed catalytic system to
operate for a wide range of substrates under these low optimized
conditions is quite interesting. Therefore, it can be ascertained
that our new reaction medium involving Ag@WEFA promises to
be an efficient and neat catalytic tool for numerous industrially
important organic transformations. The present method offers
numerous advantages including high yield, short duration,
benign catalytic system etc. Furthermore, facile nature of the
protocol without use of high temperature, solvent, and special
handling procedures is noteworthy. This neat catalytic system
2
3
3
5
98
94
4
5
6
7
93
92
97
4.5
4
7
8
3.5
5.5
96
92
9
6
90
96
10
6.5
11
12
7
9
5
93
92
95
13
a
Scheme 1 A plausible mechanism for oxidant-free dehydrogenation
of benzyl alcohols using Ag@WEFA.
Reaction condition: alcohol (1 mmol), Ag@WEFA (3 mL), reaction
temperature 70 ꢀC, and N2 atmosphere.
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Table 3 Comparison of the literature reported catalysts with present catalyst
Entry
Catalyst
Solvent
Temp (ꢀC)
Time (h)
Yield (%)
Ref.
1
2
3
4
5
6
7
Ru/AlO(OH)
PVP-Ru
Ag/HT
Ag/Al2O3
Fe3O4@SiO2–Ag
Ag/ZnO
Toluene
Water
80
Reux
130
100
Reux
100
8
99
97
99
82
98
98
98
35
36
29
26
31
14
24
10
24
24
8
pXylene
Toluene
Toluene
Toluene
WEFA
Ag@WEFA
Reux
3
Present work
thus provides tremendous scope for superior activity with denitely adds to the potency of the protocol. To the best of our
excellent yields as compared to the existing protocols. knowledge, this is the most greener and efficient catalytic tool
We have denitely observed a dramatic acceleration of the for oxidant-free dehydrogenation of benzyl alcohols reported to
rate of dehydrogenation reaction in Ag@WEFA; however, the date. Hence, the current study on Ag@WEFA will denitely open
mechanism is not fully understandable. However, as previously new avenues for utilizing these waste materials as neat, robust,
mentioned, y ash contains oxides of SiO2 along with trace and sustainable alternatives for numerous other organic
amounts of K2O and Na2O. Therefore, water extract of y ash transformations in laboratory as well as in batch scale in near
contains Si–OH groups, which interact with benzyl alcohol via future.
hydrogen bond interaction. Sequentially, the cleavage of C–H
bond takes place on Ag NPs over the surface of WEFA. The
proton abstraction from Si–OH groups in WEFA takes place to
Conflicts of interest
produce H2 adsorbed on the Ag NPs. In the next step, the silica
surface of WEFA adsorbs acetaldehyde produced by hydrogen
bonds. Finally, H2 and benzaldehyde molecules are desorbed
There are no conicts of interest to declare.
Acknowledgements
from the Ag@WEFA surface; this regenerates the active sites of
our Ag@WEFA catalytic system (Scheme 1).31
The authors thank STIC Cochin, SAIF, IIT Bombay, and SAIF,
The recyclability of Ag@WEFA was found to be an addi-
Gauhati University for analysis. BB acknowledges Director, NIT
tional attractive feature of this protocol. We reused Ag@WEFA
Silchar, for providing institutional fellowship.
upto 5th cycle without much signicant loss in its efficiency
(Table S1 in ESI†). For the recyclability test, 4 methylbenzyl
alcohol was taken as a model substrate; aer completion of the References
reaction, products were extracted with diethylether, and
1 V. R. Choudhary, A. Dhar, P. Jana, R. Jha and B. S. Uphade,
Ag@WEFA separated from the product was washed with more
Green Chem., 2005, 7, 768–770.
diethylether and reused. The retention of the components
2 A. F. Lee, C. V. Ellis, J. N. Naughton, M. A. Newton,
was evident from the TEM image of the spent catalyst
C. M. A. Parlett and K. Wilson, J. Am. Chem. Soc., 2011,
(Fig. S1 in ESI†).
Table 3 provides a comparison between our method and
some of the previously reported methods for the dehydroge-
nation of benzyl alcohols using various catalysts under different
reaction conditions. Ru/AlO(OH) and Fe3O4@SiO2–Ag seem to
be efficient catalysts as compared to others. However, Ru-based
catalysts are known to be costly, and the preparation of Fe3-
O4@SiO2–Ag undergoes a complex reaction workup, requires
use of binding agents, etc. Taking into consideration all these
factors, our present protocol may be considered as a better,
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