Transfer hydrogenation and hydration of aromatic aldehydes and nitriles using heterogeneous NiO nanofibers as a catalyst

Article abstract of DOI:10.1039/C8NJ03433H

A simple and efficient hydrogen transfer reaction of aldehydes and hydration of nitriles using nickel oxide nanofibers (NiO NFs) as a heterogeneous catalyst is reported. NiO NFs prepared by electrospinning technique was cubic (confirmed by XRD) with an average diameter of 80 nm (obtained from HR-TEM) and utilized as a nanocatalyst for heterogeneous transfer hydrogenation of aromatic aldehydes and hydration of aromatic nitriles. All the reaction products produced with minimum reaction time and maximum yield were confirmed using GC-MS with NIST library. Furthermore, heterogeneity of the catalyst was confirmed with ICP-MS analysis. The as-prepared catalyst was reused for six cycles and was found to be efficient. Hence, the present catalytic synthesis of alcohols and amides may be an economically viable process.

Full text of DOI:10.1039/C8NJ03433H

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Transfer hydrogenation and hydration of aromatic
aldehydes and nitriles using heterogeneous NiO
nanofibers as a catalyst†
Cite this:DOI: 10.1039/c8nj03433h
S. Thenmozhi and K. Kadirvelu
*
A simple and efficient hydrogen transfer reaction of aldehydes and hydration of nitriles using nickel oxide
nanofibers (NiO NFs) as a heterogeneous catalyst is reported. NiO NFs prepared by electrospinning
technique was cubic (confirmed by XRD) with an average diameter of 80 nm (obtained from HR-TEM) and
utilized as a nanocatalyst for heterogeneous transfer hydrogenation of aromatic aldehydes and hydration
of aromatic nitriles. All the reaction products produced with minimum reaction time and maximum yield
were confirmed using GC-MS with NIST library. Furthermore, heterogeneity of the catalyst was confirmed
with ICP-MS analysis. The as-prepared catalyst was reused for six cycles and was found to be efficient.
Hence, the present catalytic synthesis of alcohols and amides may be an economically viable process.
Received 10th July 2018,
Accepted 1st August 2018
DOI: 10.1039/c8nj03433h
rsc.li/njc
to our knowledge, starting from the original Meerwein–Ponndorf–
Verley (MPV) reduction, till date, the transition metal catalyst plays
1. Introduction
Aromatic alcohols and amides are functional groups present in a dominant role in the transfer hydrogenation process.7,8
most of the organic precursors that are mainly used for the Particularly, nickel-based catalysts have been proven to be highly
synthesis of pharmaceuticals, agrochemicals, bio-active molecules efficient towards hydrogenation reactions.9 However, the use of
and fine chemicals.1 The direct and single step synthesis of these large quantities of this catalyst keeps this method away from being
compounds in a simple manner is hydrogenation/reduction of the environmentally benign. In this regard, nanoscale materials as
corresponding carbonyls.2 Hydrogenation could be performed catalysts have attracted researchers over other materials.
either by directly using hydrogen gas (e.g., flow reactor method)
The conversion of nitriles to amides was traditionally well
or by using a hydrogen donor, such as alcohols, formates and known, for which large quantity of KOH as active surface was
NH3BH3. Kappe et al. reported an exclusive review on the used to reduce the reaction time.10 Further developments were
heterogeneous catalytic direct hydrogenation of carbonyl, nitrile, reported on hydration of nitrile using different types of catalytic
nitro and unsaturated compounds in a detailed manner using a systems by various researchers.11 Pillai et al. elaborated the
hydrogen gas flow reactor method.3 In addition, Metin et al. mechanism for hydration of nitriles using heterogeneous MnO2
utilized ammonia borane as a hydrogen source instead of direct catalyst. The group reported that factors such as surface nature,
hydrogen and CoPd nanoparticles as a catalyst for the hydro- polarizability, surrounding ligand and oxidation state of the
genation of aromatic nitro, nitrile and carbonyl compounds.4 metal ion play a major role in this process.12 With this back-
Though the direct hydrogenation reactions were well developed ground, we prepared simple and efficient NiO nanofibers using
for this purpose, they have some drawbacks, such as use of electrospinning technique and used them as heterogeneous
expensive (noble) metal catalysts, safety hazards, and use of nanocatalysts for the transfer hydrogenation of carbonyl and
pressurised H2.5 Hence, a green, safe and cost effective alternate hydration of nitrile compounds.
for the reduction reaction is of major interest among researchers,
due to which an eco-friendly transfer hydrogenation (TH) or
borrowing hydrogen method have been developed.6 According
2. Experimental section
2.1. General
Defence Research and Development Organisation-Bharathiar University Centre for
Life Sciences, Bharathiar University Campus, Coimbatore – 641 046, India.
Analar (AR) grade ultrapure poly(vinyl pyrrolidone) (PVP), nickel
acetate, organic substrates of aldehydes and nitriles were purchased
from Sigma Aldrich. Solvents, such as isopropyl alcohol (IPA),
E-mail: kadirvelukrishna@yahoo.com; Fax: +91-0422-2425459;
Tel: +91-0422-2428156
† Electronic supplementary information (ESI) available: TGA curve of composite
ethylacetate and dichloromethane were purchased from Merck
Millipore. Nanofibers were prepared using E-spin Nano (Physics
nanofibers, porosity measurement of NiO NFs and GC-MS spectra of synthesised
products. See DOI: 10.1039/c8nj03433h
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Equipment Pvt. Ltd, India). A small portion of the nanofibers solution was filtered to remove the catalyst and the organic
were placed on a carbon tape for EDAX analysis and coated with products were extracted with (3 Â 5 mL) ethyl acetate (EA). All
gold for SEM imaging (Quanta 200, ICON analytical). HR-TEM the separated products were dissolved in ethyl acetate for
images and selected area electron diffraction (SAED) patterns GC-MS analysis. The removed catalyst was washed with water
were recorded for a dispersion of nanofibers in ethanol on and EA for the next cycle.
the copper grid. Simultaneous Thermal Analyser (STA, Perkin
Elmer, Germany) was employed for the thermal analysis of
nanofibers. For this analysis, a small quantity (5 mg) of fibers
was taken with alumina in a sample holder. Chemical nature of
the fibers was obtained using Nicolet Avatar FT-IR and powder
XRD (PAN Analytical using Cu Ka radiation) instruments. Synthesis
of all the hydrogenated organic products was confirmed by GC-MS
(Perkin Elmer, Germany, with NIST Library) equipped with capillary
column (Restek, 0.3 mm diameter & 30 m length) and mass
analyser. Heterogeneity of the nanocatalyst during the transfer
hydrogenation process was confirmed by inductively coupled
plasma with mass spectroscopy (ICP-MS, Thermo scientific
company). For ICP-MS analysis, a small portion of the organic
product was acid-digested with nitric and sulphuric acid (1 : 1
volume ratio) for 5 hours and finely diluted for the trace level
detection or leaching of metal during the reduction process. All the
analytical experiments were precisely performed in triplicate.
2.4. Hydration of aromatic nitriles
Aromatic nitriles (0.1 mM) were taken in a round bottom flask
and 5 mL IPA, 1 mM potassium hydroxide and 3.7 mg NiO NFs
were added (Scheme 2). Constant temperature of 40 1C was
maintained throughout the reaction with continuous stirring.
At 25 min, turbidity appeared in the reaction mixture and
immediately, white precipitate settled at the bottom of the flask.
The reaction mixture was collected with water and centrifuged
to remove the nanocatalyst. Then, the solution was extracted
with ethyl acetate for GC-MS analysis and pure solid products
were crystallised from the solution.
3. Results and discussion
3.1. Preparation and characterisation of nanofibers
Preparation of direct metal oxide nanofibers using electro-
spinning technique has some difficulties, such as low viscous
nature of the precursor and high surface tension due to the
presence of metal ions. Hence, the polymer (PVP) as template
material was chosen to prepare metal–polymer composite
nanofibers. The as-prepared electrospun metal–polymer composite
nanofibers were calcinated at 500 1C (TGA given in ESI†) to remove
the polymeric template and produce pure NiO NFs. The composite
and pure NiO NFs were irradiated with IR to get the functional
transformation upon calcination, as presented in Fig. 1(a). As
expected, the FT-IR spectrum of the metal–polymer composite
nanofibers showed sharp peaks at 3510 cmÀ1 (O–H str),
1620 cmÀ1 (CQO str), and 1188 cmÀ1 (C–N str) corresponding
to PVP,13 while a single sharp peak at 520 cmÀ1 in the FT-IR
spectrum of pure NiO NFs correspond to Ni–O bond stretching.
These results confirmed that the polymer template was completely
removed from composite NFs and produced pure NiO NFs. The
composite and calcinated nanofibers were then subjected to
powder X-ray analysis and the results are presented in Fig. 1(b).
A broad spectrum at 201 corresponding to PVP was noticed in
metal–polymer composite nanofibers. In the XRD pattern of
calcinated fibers, sharp peaks at 2y equal to 37.31, 43.51, and
62.91 corresponding to 111, 200, and 220 planes were noticed,
which correspond to pure cubic NiO NFs.14 Lack of peaks at 201
2.2. Preparation of metal oxide nanofibers
Based on our previous report on PVP,13 a homogenous mixture
of 15 wt% PVP in dimethyl formamide (DMF) and 0.1 mM
nickel acetate in ethanol (1 : 1 volume ratio) was taken in a
hypodermic syringe to prepare metal–polymer composite nano-
fibers. One end of a high voltage power supply was connected to
the syringe and the other was connected to the collector
surrounded by an aluminium foil. Formation of ‘‘Taylore cone’’
was noticed at the optimised flow rate of 0.6 mL hÀ1, high
voltage of 12 kV and distance of 11 cm from tip to collector.
Electrospinning was carried out for 8 h to obtain smooth and
thick metal–polymer composite nanofibers. Thus, the obtained
composite nanofibers were separated from the aluminium foil
and calcinated at high temperature to obtain phase pure metal
oxide nanofibers (NFs).
2.3. Transfer hydrogenation of aromatic aldehydes
Initially, 1 mM of substrate was added into the solution of IPA
(5 mL) containing 2 mM of KOH and loaded with NiO NFs
(3.7 mg, 5 mM) (Scheme 1). The reaction mixture was refluxed
at 60 1C in a round bottom flask and monitored by thin layer
chromatography (TLC). The molar ratio of the substrate and the
catalyst to IPA was optimised for controlled release of hydrogen
by constant volume addition method. After completion, the
Scheme 2 Synthesis of aromatic amides from aromatic nitriles using NiO
NFs as catalyst.
Scheme 1 NiO NFs catalysed transfer hydrogenation of aromatic aldehydes.
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Fig. 1 (a) FT-IR curves and (b) powder-XRD patterns of composite and pure NiO NFs.
and except cubic phase proved that the obtained NiO nanofibers ESM). A small portion of pure NiO NFs were then subjected to high
were free of polymer and had good phase purity. resolution transmission electron microscopy imaging (Fig. 2(c)). The
The SEM image of the composite nanofibers (Fig. 2(a)) show SAED pattern, present in Fig. 2(c), shows bright spots rather than
smooth surfaces, with average single fiber diameter of 140 nm. Upon circles, ascribed to pure NiO NFs. Hence, the calcinated NFs were
calcination, the smooth composite nanofibers turned into rough highly crystalline and the d-spacing values further confirmed the
NiO nanofibers (Fig. 2(b)). Then, the average diameter of single fiber cubic phase. The EDAX spectrum revealed that NiO NFs possess
was decreased to 80 nm, which was confirmed from SEM and HR- 68% of Ni and 32% of oxygen in the calcinated product. The
TEM results. The change in pore structure from micropores to advantage of the proposed method for the preparation of NFs was
mesopores on calcination, due to the removal of PVP content in also proved from the EDAX spectrum as no trace carbon impurities
the nanofibers, was confirmed by porosity measurements (given in were present in the final NiO NFs.
Fig. 2 (a) SEM image of composite NFs, (b) HR-TEM image, (c) SAED pattern and (d) EDAX spectrum of NiO NFs.
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3.2. Catalytic activity
3.2.1. Transfer hydrogenation of aromatic aldehydes. Having
successfully prepared pure NiO nanofibers as catalyst, we inves-
tigated its efficiency towards transfer hydrogenation of aromatic
aldehydes. Initially, 4-bromobenzaldehyde was selected as a
model substrate with different solvents, such as methanol,
ethanol, IPA and 1-butanol (Table 1, entries 15–17) to under-
stand the solvent effect on hydrogenation process. Among the
solvents investigated, IPA provided better yield than others in
the present catalytic system. Then, molar ratio of solvent to
substrate and catalyst was screened by constant volume addition
of IPA. From these screening tests, 5 mL of IPA was chosen for
further study. Furthermore, reaction conditions were optimised at
various time, temperature, base and catalyst loading.
Table 1 details the progress of the reaction under different
environmental conditions and their corresponding yield of the
product. From the first two entries of Table 1, it is well understood
that there is no progress in the reaction in absence of catalyst or
base. Presence of strong hydroxyl base (KOH) accelerates the reaction
towards rapid formation of reduced product than weak carbonate
base (Table 1, entries 8, 12 and 13). Furthermore, temperature of the
reaction was varied from 25 1C to 70 1C (Table 1, entries 3–8) and the
optimum thermal energy required for the conversion was found to
be 60 1C (Table 1, entry 7). The minimum quantity of the catalyst
required for the completion of the reaction was verified from low to
high catalyst loading (Table 1, entries 8–11) and 3.7 mg (0.005 mol%)
of NiO NFs was chosen as optimum. With these optimised
conditions, we further extended the scope of TH to other aromatic
aldehydes to verify the efficacy of our NiO nanocatalyst.
Benzaldehydes substituted with various electron donating and with-
drawing groups were treated with the above conditions and as
expected, these substrates produced the yield of above 90%. How-
ever, significant difference in the yield was found for three different
halide-substituted benzaldehyde derivatives due to their position
(ortho-steric effect), size and electronegativity values (Fig. 3,
Fig. 3 Hydrogenated products using NiO nanofibers as a heterogeneous
catalyst. Note: substrate (1 mM), catalyst (3.7 mg), KOH (2 mM) in IPA (5 mL)
at 60 1C. TOF in (per hour).
entries 1a–g). In case of alkyl substituted benzaldehyde, methyl
group (+I effect) provides more electron density than the other
groups on the carbonyl group, which hampers the yield (Fig. 3(1h
and 1i)). The derivative with mono and di-substituted alkoxy
(–OCH3) groups afforded higher yield (Fig. 3(1g–i)) due to their
electron withdrawing (ÀI effect) nature. The excellent yield for
various substitutions could be explained by the unique properties,
such as narrow diameter and more active sites per unit volume of
NiO NFs catalyst. GC-MS spectra of the converted products are
given in electronic ESI.†
3.2.2. Hydration of aromatic nitriles. The solid product
obtained from the initial investigation of hydration with benzo-
nitrile (BN) was analysed with GC-MS to confirm the expected
product. The analytical results revealed that benzamide was the
major product obtained in the above conversion (given in ESI†).
Furthermore, the scope of the reaction was extended to chloride
and fluoride substituted benzonitrile, both of which are difficult to
react. Interestingly, both chloro- and fluoro-substituted benzo-
nitriles afforded higher yield of corresponding amides (Fig. 4,
2b–f) without any change in their positions. Additionally, 2-nitro-
benzonitrile, an electron withdrawing group-substituted BN, also
yield 98% of 2-nitrobenzamide (Fig. 4, 2g) without any change in
the position of the nitro group. This conversion showed the
selectivity of the catalyst towards hydration of cyano group in
the bi-functional molecule. This valuable feature extends the
potential of the present catalyst for major pharmaceutical
applications as this catalyst can hydrate the nitrile group with-
out affecting the nitro moiety to enable further conversions.
Table 1 Optimisation of conditions for TH of 4-bromobenzaldehyde with
IPA
Catalyst
quantity (mg) (1C)
Temperature Time
GC
S. no. Base
(min) yield (%)
1
2
3
4
5
6
7
8
—
3.7
—
60
60
25
30
40
50
60
70
60
60
60
60
60
60
60
60
60
60
60
60
30
30
30
20
20
60
20
20
30
30
20
60
60
60
—
—
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
3.7
3.7
3.7
3.7
3.7
3.7
1.9
5.5
7.4
25
30
50
68
96
95
45
96
99
25
30
50
10a
20b
45c
9
10
11
12
13
14
15
16
17
Na2CO3 3.7
K2CO3
NaOH
KOH
KOH
KOH
3.7
3.7
3.7
3.7
3.7
3.3. Proposed mechanism for transfer hydrogenation and
hydration of aromatic aldehydes and nitriles
a
Transfer hydrogenation process in homogeneous catalysis occurs
by different pathways such as, direct hydrogen transfer, hydride
Note: substrate (1 mM), KOH (2 mM) in IPA (5 mL). Reaction in
b
c
methanol. Reaction in ethanol. Reaction in 1-butanol.
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(reactant and solvent) molecules and thus provides more proximity
for rapid product formation.
3.4. Heterogeneity and leach proof test
To predict the nature of catalytic process in TH, the reaction
was stopped in between and the catalyst was removed by
centrifugation. Then, the reaction was continued further with-
out the catalyst and was monitored by GC-MS. The regular
reaction progress with respect to substrate and product is
presented in Fig. 5(a). After the removal of catalyst at 10 min
(substrate 1*), there was no progress in the reaction, which was
confirmed by GC-MS. This result evidenced that the catalyst
followed a heterogeneous path. Furthermore, catalyst leaching
was examined with small quantity of product using ICP-MS. To
our delight, there was no trace Ni detected in the final product.
Most of the heterogeneous catalyst usually failed this test due
to the metal leaching into the solution during processing, thus
adding impurities to the product.8a This is the first report on
leach-free transfer hydrogenation of aldehydes and hydration
of nitriles using NiO NFs as a heterogeneous catalyst.
Fig. 4 Hydrated products with reaction time and GC-MS yield (isolated
yields in parenthesis). Note: substrate (0.1 mM), catalyst (3.7 mg), KOH
(1 mM) in IPA (5 mL) at 40 1C. TOF in (per hour).
transfer or ionic pathway. Particularly, in transition metal catalysed
reactions, the most preferable mechanism is the hydride route
(monohydride and dihydride) due to faster ligand exchange. The
monohydride route is further divided into inner-sphere or outer-
sphere mechanism based on the type of catalyst used in the
process.6a The dihydride mechanism was reported by Yus et al.
using Ni nanoparticles as the catalyst in TH process.9a Similarly,
both the direct hydrogen and hydride transfer path was also
followed in heterogeneous TH process, which was detailed in a
review article by Gilkey and Xu.15 The ligand exchange in the
homogeneous method and surface adsorption in the hetero-
geneous process leads to the formation of product by either of
the two routes. In this study, NiO NFs provided more active
surface for the adsorption of hydrogen from IPA, thus enhancing
the activity through direct hydrogen transfer method by the
formation of six membered intermediate between IPA, substrate
and catalyst. In the hydration process, nucleophilic attack by
water (from IPA and KOH) on the electron deficient carbon of
–CRN (in benzonitrile) followed by tautomerisation produced
hydrated products. The key role of the catalyst in this process was
to afford more active surface, which could adsorb the substrate
3.5. Reusability test
Reusability of a catalyst is a major concern in large scale
applications and makes the process environmentally more
viable. Hence, the catalyst removed from the first cycle was
utilized further for six times to prove its maximum efficiency.
As expected, the catalyst was unaffected by solvent treatment
and gave excellent yield of 95% (by GC-MS) even in the sixth
run, similar to the fresh catalyst (Fig. 5(b)) without any changes
in the properties.
3.6. Advantages over other methods
The present catalytic system affords product yield of above 90% with
minimum reaction time of 15–60 min. Moreover, the catalyst is
obtained using very simple preparation procedures and exhibits
good reusability of six cycles with high efficiency. Recently, a similar
type of transfer hydrogenation was reported by Ley et al. using flow
chemistry method, which required high pressure and difficult
Fig. 5 (a) Reaction progress monitored by GC-MS (b) re-usability test.
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heterocyclic nitriles with nanoferrite-Ru(OH) as the heterogeneous
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In comparison to the abovementioned reports, a single-step rapid
transfer hydrogenation of aldehydes and hydration of nitriles using
NiO nanofibers as a heterogeneous catalyst was superior with
advantages of mild reaction conditions with minimum catalyst
loading. In addition, easy separation reusability of catalyst and were
the added advantages of this method for scaling.
4. Conclusion
The electrospun pure-phase cubic NiO nanofibers were prepared
and studied for its catalytic efficiency towards transfer hydrogenation
of aromatic aldehydes and hydration of aromatic nitriles. All the
catalytic reactions proceeded well with minimum time (15–60 min)
and maximum yield (above 90%). The major advantage of this
method is non-leaching of the metal in the final product and
reusability of the catalyst for six cycles. The use of green solvents,
absence of by-products and mild reaction conditions make the
catalytic system prepared in this present work advantageous over
the existing catalytic systems. Hence, the present catalytic system
could be upscaled for various pharmaceutical and fine chemical
applications.
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Conflicts of interest
Authors declare no conflict of interest.
Acknowledgements
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The authors are thankful to Defence Research and Development
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