Heterogenization of cobalt nanoparticles on hollow carbon capsules: Lab-in-capsule for catalytic transfer hydrogenation of carbonyl compounds

Article abstract of DOI:10.1016/j.mcat.2018.01.036

Incorporation of cobalt nanoparticles (Co NPs) in porous iron oxide nanospheres (Fe₃O₄ NSs) templated, glucose derived hollow carbon capsules (HCCs), with an objective to achieve activity and stability simultaneously, facilitates higher catalytic activity of Co NPs in transfer hydrogenation of ketones and aldehydes. A variety of ketones and aldehydes are hydrogenated successfully with excellent yields and high turnover number (TON). This system constitutes one of the most general, heterogeneous, highly stable catalyst, which does not require additives for activation and employs mild reaction conditions. Other significant advantages are low Co content (0.38 mol%) for a catalytic hydrogenation reaction, functional-group tolerance, inexpensive, environmentally benign nature and reusability.

Full text of DOI:10.1016/j.mcat.2018.01.036

MolecularCatalysis448(2018)153–161
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Heterogenization of cobalt nanoparticles on hollow carbon capsules: Lab-in-
capsule for catalytic transfer hydrogenation of carbonyl compounds
⁎
⁎
Basuvaraj Suresh Kumara, Arlin Jose Amalia,b, , Kasi Pitchumania,b,
a
Department of Natural Products Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai, 625021, Tamil Nadu, India
Centre for Green Chemistry Processes, School of Chemistry, Madurai Kamaraj University, Madurai-21, Tamil Nadu, India
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Hollow carbon
Cobalt nanoparticles
Carbonyl compounds
Transfer hydrogenation
Heterogeneous catalysis
Incorporation of cobalt nanoparticles (Co NPs) in porous iron oxide nanospheres (Fe3O4 NSs) templated, glucose
derived hollow carbon capsules (HCCs), with an objective to achieve activity and stability simultaneously, fa-
cilitates higher catalytic activity of Co NPs in transfer hydrogenation of ketones and aldehydes. A variety of
ketones and aldehydes are hydrogenated successfully with excellent yields and high turnover number (TON).
This system constitutes one of the most general, heterogeneous, highly stable catalyst, which does not require
additives for activation and employs mild reaction conditions. Other significant advantages are low Co content
(0.38 mol%) for a catalytic hydrogenation reaction, functional-group tolerance, inexpensive, environmentally
benign nature and reusability.
1. Introduction
hydrogen and temperature for realizing high catalytic activity. On the
other hand, use of hydrogen donor reagents in transfer hydrogenation
Catalysis is a key technology for achieving environmentally benign
processes in chemical, pharmaceutical and materials industries [1,2].
Consequently, in the search towards newer catalysts, greater focus is
devoted to metal or metal oxide nanoparticles [3,4], which are am-
phidextrous combining advantages of both homogeneous and hetero-
geneous catalysts. In this context, though noble metals are studied ex-
tensively [5–9], first row transition metals, which are earth abundant,
easily available and inexpensive attract less attention as catalysts. Re-
cently, Fe, Cu, Zn and Co emerged as highly effective, soluble and
promising catalysts in various organic reactions [10–13]. Among these
base metals, cobalt is the first base metal catalyst made from a non-
precious metal properties closely matching those of platinum [14–16]
and cobalt catalysts are identified recently as the best candidates for
converting biomass to value added products [17], syngas to clean liquid
fuels because of lower deactivation rates, low water-gas shift activity
and high chain growth probability [18–20].
However, despite their above advantages and utility, for hydro-
genation reactions only very few cobalt based catalysts have been re-
ported. For example, Zhang’s and Rosler’s groups have reported PNP-
stabilized homogeneous Co(II) complexes as catalysts for transfer hy-
drogenation of C]O and C]N bonds [21–23]. Beller and co-workers
have developed cobalt-based nanocatalysts by pyrolysis of cobalt(II)
acetate phenanthroline complex for hydrogenation process [1,24–26].
However, these heterogeneous Co catalysts require very high pressure
reactions can avoid the use of autoclaves, high pressure hydrogen, high
temperature and thus are highly relevant for industrial applications
[27,28]. In a recent study Long et al., have used Co containing metal
organic frameworks as the sacrificial template for preparing Co@C-N as
heterogeneous catalyst for catalytic transfer hydrogenation using iso-
propanol as hydrogen transfer agent [29]. However, the synthesis of
catalyst requires high temperature. Hence, significant improvements
are therefore desirable to realize more sustainable, environmentally
friendly, economical, heterogeneous Co catalysts involving use of hy-
drogen donor reagents for transfer hydrogenation reactions. Herein, we
report a “cheap and simple” non-precious metal, ie., Co NPs as het-
erogeneous catalysts for transfer hydrogenation of carbonyl com-
pounds. The catalyst was prepared by incorporation of Co NPs in porous
Fe3O4 NPs templated, glucose derived hollow carbon capsules (HCCs),
to achieve efficiency, reusability, high catalytic activity and stability.
HCCs have strong encapsulation ability, controllable permeability,
surface functionality, low density and excellent chemical and thermal
stabilities. Their tailored structure can find many potential applications
in energy storage, e.g., in lithium batteries [30], and also as catalyst
supports in heterogeneous catalysis. They provide means to stabilize
catalytically active species via confinement within their cavity, thereby
mitigating deactivation pathways, such as metal sintering or leaching
[31]. In addition, the shell subunits shielding the catalytically active
species can exert molecular discrimination, based on either chemical
⁎
Corresponding authors at: Department of Natural Products Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai, 625021, Tamil Nadu, India
E-mail addresses: arlinjose.chem@mkuniversity.org (A.J. Amali), pitchumani.chem@mkuniversity.org (K. Pitchumani).
https://doi.org/10.1016/j.mcat.2018.01.036
Received 23 November 2017; Received in revised form 25 January 2018; Accepted 30 January 2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

B.S. Kumar et al.
MolecularCatalysis448(2018)153–161
precipitated black products were collected by an external magnet and
washed several times with ethanol. Finally, the black colored product
was dried in vacuum for 24 h at 60 °C.
2.3. Synthesis of carbon@Fe3O4 nanospheres (C@Fe3O4)
Carbon@Fe3O4 were synthesized by hydrothermal carbonization of
glucose in presence of Fe3O4. The synthesized Fe3O4 solid spheres
(100 mg) were dispersed in water (10 mL) containing glucose (1.6 gm)
by ultrasonication. The mixture was transferred into a teflon lined
stainless steel autoclave, sealed, and heated at 160 °C for 10 h, and then
cooled at room temperature. The precipitated black solid was collected
from the solution by an external magnet and washed several times with
ethanol. Finally, the black coloured product was dried in vacuum for
24 h at 60 °C to afford C@Fe3O4.
Scheme 1. Schematic representation for the synthesis of Co NPs@HCCs.
compatibility or molecular sieving, which pave the way towards se-
lective catalytic transformations. Also, presence of a confined en-
vironment may enhance the catalytic activity due to local concentration
of substrates [32,33].
2.4. Synthesis of hollow carbon capsules (HCCs)
2. Experimental
The as synthesized C@Fe3O4 nanospheres (1 gm) were dispersed in
water (10 mL) to which HCl (40 mL, 3 N) solution was added and al-
lowed to stir for 5 h. After completion of the reaction, the reaction
mixture was centrifuged (3000 rpm, 15 min), the residue was collected,
washed and dried (60 °C, 5 h) to obtain hollow carbon capsules (HCCs).
2.1. Synthesis of cobalt nanoparticles (Co NPs)
Co NPs were synthesized by adding an aqueous solution of NaBH4
into an ice cold aqueous solution of CoCl2·6H2O and PVP. In a typical
procedure, an aqueous ice-cold solution (8 mL) containing PVP
(10.5 mg) and CoCl2·6H2O (0.1 mmol, 23.7 mg) was stirred (10 min)
under nitrogen atmosphere. Subsequently, an aqueous ice-cold solution
(3 mL) containing NaBH4 (10 mg) was added dropwise. The reaction
was allowed to continue in ice-cold conditions for 3 h to afford Co NPs.
2.5. Incorporation of Co NPs on hollow carbon capsules (Co NPs@HCCs)
Co NPs@HCCs were prepared by deposition method. Typically,
HCCs (300 mg) were well dispersed in ethylene glycol (100 mL) under
magnetic stirring. To this, a solution containing Co NPs (11 mL) was
added dropwise (30 min) under vigorous stirring. The above prepared
reaction mixture was stirred for 2 h at 80 °C. After completion of the
reaction, the Co NPs incorporated HCCs (Co NPs@HCCs) were sepa-
rated by centrifugation (3000 rpm, 10 min), washed three times with
water and acetone and dried in vacuum to obtain Co NPs@HCCs.
2.2. Hydrothermal synthesis of Fe3O4 nanospheres (Fe3O4 NSs)
Iron (III) chloride hexahydrate (1 mmol, 0.270 gm) and urea
(9 mmol, 0.540 gm) were added into ethylene glycol (10 mL) under
magnetic stirring. The resultant solution was transferred into a teflon
lined stainless steel autoclave, sealed, and heated to 200 °C for 12 h. The
Fig. 1. TEM images of Fe3O4 NSs (a,b); C@Fe3O4 NSs (c,d) and HCCs (e,f).
154

B.S. Kumar et al.
MolecularCatalysis448(2018)153–161
Fig. 2. TEM images of Co NPs (a), Co NPs@HCCs (c & e) and particle size distribution graph of image c (d); SAED pattern of Co NPs (b) & Co NPs@HCCs (f).
2.6. General procedure for transfer hydrogenation of carbonyl compounds
using Co NPs@HCCs
the solvent was evaporated and the residue was subjected to column
chromatography for further purification. The purified compounds were
characterized by 1H NMR spectroscopy using CDCl3 as solvent and TMS
as internal standard. The NMR spectral details are given in supporting
information (Fig. S6-S21).
Co NPs@HCCs (40 mg, 0.38 mol% of Co), carbonyl compounds
(2 mmol), isopropanol (4 mL) and KOH (0.5 mmol) were taken in a
Schlenk tube with a Teflon stopcock, sealed and heated at 90 °C for the
required time with constant stirring. After completion of the reaction,
the catalyst was separated by centrifugation and the reaction mixture
was decanted. It was washed with water and extracted with EtOAc and
3. Results and discussion
In this study, composed of hollow carbon capsules (HCCs)
155

B.S. Kumar et al.
MolecularCatalysis448(2018)153–161
Fig. 3. Powder XRD (a) and XPS patterns of Co NPs (b).
Fig. 4. PXRD of Co NPs, HCCs and Co NPs@HCCs (a), FT-IR spectral analysis of HCCs (b), Raman spectrum of HCCs (c) and N2 adsorption desorption isotherm at 77 K of HCCs (d).
incorporated with Co NPs are true “Lab-in-Capsule” catalysts. Co NPs
were synthesized by ice-cold reduction of Co2+ using NaBH4 and they
show magnetic behaviour (Fig. S1, Supporting Information). This “Lab-
in-Capsule” fabrication process is a four step procedure, and each step is
well controlled, scalable and reproducible (Scheme 1). Initially, ferrite
nanospheres (Fe3O4 NSs) were synthesized by a robust hydrothermal
reaction based on high temperature reduction of Fe3+ ions with ethy-
lene glycol and urea. Next, a thin layer of carbon shell was coated on
the surface of the Fe3O4 NSs through carbonization of glucose under
hydrothermal conditions [34,35] and subsequent hard Fe3O4 NSs
template etching was carried out using HCl to furnish HCCs (Scheme 1).
Finally, the pre-synthesized Co NPs were incorporated onto the HCCs
by deposition method under reflux conditions to result in a Co NPs@
HCCs nanocomposite (Scheme 1). The presence of functional groups
such as −COOH, −OH and eC]O in HCCs can be beneficially used to
immobilize and stabilize a catalytically active Co NPs [35,36].
The crystallinity and phase composition of synthesized nano-
particles as ferrite nanospheres (Fe3O4 NSs) was confirmed by PXRD
analysis (Fig. S2, supporting information) and it show porous spherical
morphology with particle sizes of 150–200 nm (Fig. 1a,b). It is observed
that the prepared C@Fe3O4 NSs upon carbonization of glucose on Fe3O4
NSs also remain spherical and consist of Fe3O4 core with carbon-shell.
156

B.S. Kumar et al.
MolecularCatalysis448(2018)153–161
Table 1
Optimization of reaction conditions in Co NPs@HCCs catalyzed transfer hydrogenation reaction.a
Entry
Catalyst
Co loading (mmol)
Base
i-PrOH (mL)
Time (h)
Yield (%)b
1
–
–
–
–
–
KOH
KOH
KOH
–
KOH
TEA
6
6
6
6
6
6
6
6
6
6
6
6
8
4
4
4
4
4
4
4
4
4
24
24
24
24
16
16
16
16
5
18
18
18
18
18
18
18
18
28
5
Nil
Nil
Nil
Nil
95
Nil
Nil
10
38
98
75
34
75
98
10
30
65
96
35
96
80
98
2c
3
HCCs
4
5
6
7
8
9
10
11d
12e
13
14
15f
16g
17h
18
19
20
21
22
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
Co NPs@HCCs
0.2
0.2
0.2
0.2
0.2
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.05
1
K2CO3
Cs2CO3
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
1
0.5
0.5
16
5
8
a
Reaction conditions: 4-Chloroacetophenone (2 mmol), catalyst (40 mg), base (0.5 mmol), solvent (i-PrOH), temp (90 °C).
b
c
d
e
f
Isolated yield.
1 mmol of KOH.
0.25 mmol of KOH.
0.12 mmol of KOH.
Reaction at RT.
Reaction at 60 °C.
Reaction at 80 °C.
g
h
The particle size is 180–250 nm and the thickness of the carbon shell
deeper insight into the oxidation state of Co NPs in the HCCs supported
catalyst. Two characteristic peaks in XPS analysis located at 780.6 eV
and 796.7 eV in the high-resolution Co 2p spectrum are attributed to
the metallic cobalt (Fig. 3b) [39]. The weak satellites peaks observed at
around 786.2 eV and 802.5 eV are due to surface oxidation of Co NPs
[40].
TGA analysis reveals a small weight loss from room temperature to
150 °C corresponding to dehydration followed by another weight loss
from 250 to 400 °C due to decomposition of carbon. The above ob-
servations clearly show that the HCCs are stable upto 250 °C in air (Fig.
S4, Supporting Information). The bands at 3600-3250 (broad), 2950,
1699 and 1612 cm−1 in FT-IR spectra correspond to eOeH, eCeH, C]
O and C]C stretching frequencies respectively, confirming their pre-
sence of functional groups in HCCs (Fig. 4b). The Raman spectrum of
HCCs shows the ID/IG value of carbon matrix as 0.70 indicating the
amorphous nature (Fig. 4c) [41,42]. N2 adsorption desorption isotherm
at 77 K for HCCs shows the surface area as 30.8 m2g−1 (Fig. 4d). In-
ductively coupled plasma-optical emission spectrometric (ICP-OES)
analyses show that the HCCs contain 1.12 wt.% of Co (Fig. S5, Sup-
porting Information).
formed from the carbonization of glucose is about 20–50 nm (Fig. 1c,d).
Followed by etching with HCl, it is observed that the prepared HCCs
shows strong contrast between the dark edge and the center of the
spherical particles, evidencing their hollow structure (Fig. 1e,f). The
diameter of the hollow carbon spheres is about 180–250 nm, and the
wall thickness is about 20–50 nm. The energy-dispersive X-ray spec-
troscopic (EDX) point analyses of randomly chosen HCCs display uni-
form compositions (Fig. S3a & S3b, Supporting Information). The
analyses reveal that the elemental composition of carbon is 81 wt% and
oxygen is 19 wt% on HCCs. This result suggests clearly that all the
Fe3O4 are removed on C@Fe3O4 NSs. TEM images of the pre-synthe-
sized Co NPs shows spherical morphology (Fig. 2a) and it is deposited
uniformly in the HCCs. The mean particle size is found to be as 11 nm
from particle size distribution analysis (Fig. 2d). SAED patterns of
synthesized Co NPs and deposited Co NPs (Co NPS@HCCs) suggest the
amorphous nature of Co NPs (Fig. 2b and f).
The crystallinity and phase composition of the Co NPs are further
investigated by powder X-ray diffraction (PXRD), which reveals that
there is no diffraction in the XRD spectrum, reminiscent of the XRD
feature for the amorphous Co metal, giving further evidence for the
amorphous phase of these NPs (Fig. 3a) [37]. Studies show that Co NPs
formed by atom-by-atom growth arrangement at a low reaction tem-
perature will result in the amorphous phase.[38] PXRD pattern of the
HCCs exhibits typical broad reflections (2θ = 10–30°), attributable to
carbon (002) reflections. Incorporation of amorphous Co NPs on HCCs
shows a similar pattern in PXRD and also there is no change in the
phases (Fig. 4a). XPS measurements are also carried out to obtain a
The thoroughly characterized Co NPs@HCCs were subsequently
tested in the transfer hydrogenation of 4-chloroacetophenone to 4-
chlorophenylethanol as a model process (Table 1). Reactions were
performed at 90 °C using 2-propanol as both solvent and reductant
under different reaction conditions. Blank runs (without catalysts and
only HCCs) showed no reactivity (Table 1, Entries 1–3). To our surprise
Co NPs@HCCs were highly active for this transformation which de-
monstrated that Co NPs were essential. Similarly, base was also needed
157

B.S. Kumar et al.
MolecularCatalysis448(2018)153–161
Table 2
Table 2 (continued)
Substrate scope in Co NPs@HCCs catalyzed transfer hydrogenation of carbonyl com-
pounds.a
Entry
20
Reactant
Product
Yieldd
Nil
TON
Nil
Entry
1
Reactant
Product
Yieldd
95
TON
250
a
Reaction conditions: Substrate (2 mmol), Co NPs@HCCs (40 mg, 0.38 mol%), sol-
vent (i-PrOH, 4 mL), KOH (0.5 mmol), temp. (90 °C), time (18 h).
2b
3c
4
85
50
95
21
b
Reaction in fresh Co NPs (5 mg, 4.2 mol%).
Reaction in reused (1st cycle) Co NPs from b.
c
d
12
Isolated yield.
250
for product formation, as the yield dropped in its absence (Table 1,
Entry 4). Among the various inorganic and organic bases tested, KOH
gave excellent yields (Table 1, Entry 5). The concentration of KOH also
influenced the activity of the reaction and 0.5 mmol of KOH was op-
timum. Increasing the quantity of 2-propanol to 8 mL resulted in a re-
duced yield of 4-chlorophenylethanol and 4 mL of 2-propanol was
found to be optimum for this reaction (Table 1, Entries 9–14). Tem-
perature was also found to be an important parameter, with yield im-
proving from room temperature to 80 °C (Table 1, Entries 15–17).
Variations in the amount of catalyst and reaction time indicated that
0.1 mmol of catalyst and 18 h were optimum (Table 1, Entries 18–22).
Based on the observed results, the optimized reaction conditions as
2 mmol of substrate, 4 mL of 2-propanol, 0.5 mmol of base, 40 mg of Co
NPs@HCCs as catalyst in 90 °C for 18 h. The use of hydrogen donor
reagents in transfer hydrogenation reactions avoids the use of auto-
claves, high pressure hydrogen and are thus highly relevant for in-
dustrial applications [27,28]. High concentration of low coordination
sites on surface and short range ordering of Co atoms in amorphous Co
NPs are responsible for the high catalytic activity in the transfer hy-
drogenation of carbonyl compounds.[43,44]
5
96
252
6
7
98
98
257
257
8
9
92
78
239
205
10
88
231
11
12
86
80
226
213
With the optimized reaction conditions in hand (Table 1, Entry 14),
the substrate scope for the transfer hydrogenation in the present study
was explored (Table 2). Aryl ketones such as acetophenone, 1-acet-
ylnaphthalene, 2-acetylnaphthalene, 1-acetylbiphenyl and 4-chlor-
oacetophenone gave excellent yields (Table 2, Entries 1 and 4–7). On
the other hand 3,4-dichloroacetophenone and 2-chloroacetophenone
gave slightly lower yields and this may be due to the electronic effects
in the substrate (Table 2, Entries 9–10). Decreased yield with 3,4-di-
methoxy and 3-methoxyacetophenones compared to 4-methox-
yacetophenone, was attributed to decreased resonance interactions
(Table 2, Entries 11–13). In addition, hydrogenation of benzophenone
derivatives were also studied which gave significant yields (Table 2,
Entries 14–15). Studies were extended to aldehydes also (Table 2, En-
tries 16–18) which gave lower yields, when compared to ketones. Re-
action with only Co NPs (Table 2, entries 2–3) for fresh and first recycle
gave 85% and 50% of products. When these results of Co NPs were
compared with Co NPs@HCCs, Co NPs showed low yield and TON. This
may be due to the instability of Co NPs, which may undergo agglom-
eration in the absence of stabilizers. The system is also selective and not
efficient for hydrogenation of nitroaromatic compounds and nitriles. To
confirm selective reduction of carbonyl groups by Co@HCCs, reduction
carried out simultaneously with nitrobenzene, benzonitrile, 4-chlor-
oacetophenone in same reaction vessel under the optimized conditions
and the reaction was followed by GC and 4-chlorophenylethanol was
selectively formed with good yield. Absence of aniline and benzylamine
as products (Table 2, Entries 19–20), demonstrates interesting se-
lectivity towards only ketones and aldehydes in the present catalytic
system.
13
14
83
82
214
215
15
16
82
85
215
223
17
18
78
72
205
189
19
Nil
Nil
Since both stability as well as recyclability is important parameters
for applications of heterogeneous catalysts, the catalytic materials Co
158

B.S. Kumar et al.
MolecularCatalysis448(2018)153–161
Fig. 5. (a) Recycling of catalyst in transfer hydrogenation of 4-chloroactophenone using Co NPs@HCCs; (b) & (c) TEM and SAED patterns of reused Co NPs@HCCs.
of 1-(4-chlorophenyl)ethanol was observed as 58%. These results
clearly demonstrate that Co NPs@HCCs are truly heterogeneous in
nature.
Table 3
Sheldon's hot filtration test for the Co NPs@HCCs catalyzed transfer hydrogenation re-
action.
To account for the observed catalytic activity by Co NPs@HCCs, the
following tentative mechanism, involving Co NPs hydride intermediate
is proposed (Scheme 2), by analogy with a previously reported system
[46]. Initial binding of isopropanolate ion (I) to Co NPs and subsequent
β-elimination yield Co NPs hydride species (II). 1,2-Insertion of the
carbonyl substrate gives intermediate (III) which then undergoes ex-
change with isopropanol affording the alcohol end product. Compared
to organic bases, alkali metal hydroxides readily give alkoxides. An
alkoxide, as the conjugate base, has more nucleophilic character than
the corresponding alcohol and readily enters into the catalytic cycle by
binding to Co NPs to form intermediate I (Scheme 2).
Our catalytic system is also compared with various reported het-
erogeneous transition metal catalysts and also other Co containing
catalytic systems for transfer hydrogenation reactions of carbonyl
compounds (Table 4). Compared to them, our catalytic system is ad-
vantageous in terms of easy catalyst preparation, simpler procedure for
environmental benign organic synthesis, use of non-noble metals,
higher activity, lower amount of Co (0.38 mol%) for catalytic transfer
hydrogenation reaction and can be reusable.
Catalyst
Yield (%)a
Co NPs@HCCs
9 h
56
(9 + 9) h
58
Reused catalyst
96
Reaction Conditions: 4-chloroacetophenone (2 mmol), i-PrOH (4 mL), KOH (0.5 mmol),
catalyst (40 mg, 0.38 mol%), temp (90 °C).
a
Isolated yield.
NPs@HCCs are tested in the transfer hydrogenation of 4-chlor-
oacetophenone upto 5 times (Fig. 5a). The desired product 4-chlor-
ophenylethanol is obtained in 73% yield after the fifth cycle. Though,
however considerable loss of catalytic activity is observed in Co NPs@
HCCs. These observations show that the present Co NPs@HCCs system
is still very efficient and can be effectively recycled. The TEM images of
the Co NPs@HCCs after fifth cycle show that the Co NPs are still intact.
When compared to the fresh catalyst, the XRD pattern, TEM images and
corresponding SAED pattern of the reused catalyst (Fig. 5b–c) show
slightly crystalline behaviour, which also indicates marginal reduction
in catalytic activity in the transfer hydrogenation reaction of 4-chlor-
oacetophenone due to the formation of crystalline Co NPs [45]. Hence,
the observed decrease in catalytic activity after reuse can be construed
as evidence that the reported catalytic activity is mainly due to amor-
phous Co NPs.
To provide additional evidence for a heterogeneous mechanism, the
reactivity of species in the supernatant mixture was studied by
Sheldon's hot filtration test (Table 3). First, Co NPs@HCCs were sub-
jected to the regular catalytic conditions, in the standard reaction of 4-
chloroacetophenone. After continuing the reaction under optimized
conditions for about 9 h, the catalyst was filtered under hot conditions
from the reaction mixture with 56% formation of product 1-(4-chlor-
ophenyl)ethanol. After removal of the catalyst, the filtrate was sub-
jected to the reaction conditions for an additional 9 h. After 18 h, yield
4. Conclusion
In summary, we have developed an efficient selective non-noble
metal based catalyst, Co NPs incorporated in porous Fe3O4 NSs tem-
plated, glucose derived HCCs. This inexpensive, environmental friendly
and easily handled Co catalyst facilitates the conversion of ketones and
aldehydes to alcohols with excellent yields. This catalytic system con-
stitutes one of the most general, heterogeneous, stable catalysts which
does not require additives for activation and proceeds readily under
mild reaction conditions (does not use gaseous hydrogen). Other ad-
vantages include low Co content (0.38 mol%) for a catalytic hydro-
genation reaction, functional-group tolerance, selectivity and can be
159

B.S. Kumar et al.
MolecularCatalysis448(2018)153–161
Scheme 2. Proposed mechanism for the transfer hydrogenation of ketones by Co NPs@HCCs in isopropyl alcohol.
Table 4
Comparison with previously reported metal based catalytic systems for transfer hydrogenation of carbonyl compounds.
S. No Catalyst
Metal loading (mol%) H2-Source
Solvent
Base
Time (h) Temp (°C) Yield (%) TON
Ref.
1
2
3
4
Co(II)-(PNPCY)-(CH2SiMe3)
PN3-5P-Co(II)Chlorido complexes 0.25–3
2
–
i-PrOH-THF
2-Methyl-2-butanol NaOBu
–
24
24
80
RT
92–99
92–99
60–99
93–99
4–95
69–88
65–99
87–99
89–99
46–99
31–99
65–99
80–99
83–98
84
46–50
368–400 [23]
15–25
9–10
3–73
[22]
20 bar H2
15–30 bar H2
–
–
Co(OAc)2·4H2O-L1
Co@C-N-900-15 h
Ru@FeCSNPs
Ru/Fe@SiO2
Ir@CN
4
10
1.3
3.9
0.5–1
2
EtOH
i-PrOH
i-PrOH
i-PrOH
H2O
i-PrOH
H2O
H2O
–
–
15–20
12–60
24
0.5–2
18
7–16
2–10
0.5–24
2.5–10
48
12–48
3–24
85
100–130
80–100
85–100
80–100
100
80
40
40
120
130
25
RT
82
83
RT
80
70
90
[24]
[29]
[46]
[47]
5
KOH
KOH
NaOH
NaOH
–
6a
7
–
18–22
130–198 [48]
HCOOH
–
HCOONa
HCOONa
–
–
8
9
RuII −SiO2-NH2
Ru-TsDPEN/F(M)
RhTsDPEN/Cp*
Au/TiO2
44–50
89–99
46–99
40–127
32–49
8–10
21–25
80
14–19
28–39
19–26
17–21
[49]
[50]
[51]
[52]
[53]
[54]
[55]
[56]
[57]
[58]
[59]
[60]
1
10
11
12
13
14
15
16
17
18
19
20
1.0
0.78
2
–
i-PrOH
EtOH
K2CO3
–
–
Pyridine
KOH
KOH
NaBHEt3 15
KOH
–
[(dcype)Ni(COD)]
Pd/Urea-MCF
Pt/C
Co3(CO)9CCl
Co(II)HMA
CoCl2
NiHMA
Cu NPs/Y-zeolie
Co NPs@HCCs
10
HCOOH/Et3N EtOAc
H2-0.1 MPa
0.4
1.05
5
0.5–2.5
3.5
4.7
0.38
Cyclohexnae
–
–
–
–
–
–
i-PrOH
i-PrOH
DCM
i-PrOH
i-PrOH
i-PrOH
5
71–96
71–98
67–90
78–98
72–98
1.5–4
1
18
KOH
189–257 This work
a
Microwave condition; CSNPs- coreshell nanoparticles; Cp*-pentamethylcyclopentadiene; TsDPEN = 4-methylphenylsulfonyl-1,2-diphenylethylenediamine; MCF-siliceous mesocel-
lular foam; Co@CeN = Metal organic framework derived carbon nano composites; L1 = 1,10-phenanthroline-α-Al2O3; HMA = hexagonal mesoporous aluminophosphate molecular
sieves; NiHMA = Ni incorporated hexagonal mesoporous aluminosilicate; HCCs = hollow carbon capsules.
reused several times with marginal loss of its activity, which makes this
system an attractive alternative pathway for hydrogenation reactions.
This may open the way towards the design of nanoparticle incorporated
structures with an adept matrix which can function as encapsulation,
protection, and delivery agents, find applications as diverse as drug
delivery vehicles, sensors, biomarkers, anodes or cathodes for lithium
ion batteries etc..
thanks DST, New Delhi, India for DST-INSPIRE Faculty Fellowship. KP
acknowledges financial support provided by CSIR, New Delhi, India.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.mcat.2018.01.036.
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