Cobalt-entrenched N-, O-, and S-tridoped carbons as efficient multifunctional sustainable catalysts for base-free selective oxidative esterification of alcohols

Green Chemistry
PAPER
Cite this: Green Chem., 2018, 20,
3542
Received 27th April 2018,
Accepted 17th June 2018
DOI: 10.1039/c8gc01333k
rsc.li/greenchem
Cobalt-entrenched N-, O-, and S-tridoped carbons
as efficient multifunctional sustainable catalysts for
base-free selective oxidative esterification
of alcohols†
Devaki Nandan, Giorgio Zoppellaro, Ivo Medřík, Claudia Aparicio,
Pawan Kumar, Martin Petr, Ondřej Tomanec, Manoj B. Gawande, *
Rajender S. Varma and Radek Zbořil *
We report the synthesis of sustainable and reusable non-noble transition-metal (cobalt) nanocatalysts
containing N-, O-, and S-tridoped carbon nanotube (Co@NOSC) composites. The expensive and benign
carrageenan served as the source of carbon, oxygen, and sulfur, whereas urea served as the nitrogen
source. The material was prepared via direct mixing of precursors and freeze-drying followed by carbon-
ization under nitrogen at 900 °C. Co@NOSC catalysts comprising a Co inner core and outer electron-rich
heteroatom-doped carbon shell were thoroughly characterized using various techniques, namely, TEM,
HRTEM, STEM elemental mapping, XPS, BET, and ICP-MS. The utility of the Co@NOSC catalyst was
explored for base-free selective oxidative esterification of alcohols to the corresponding esters under
mild reaction conditions; excellent conversions (up to 97%) and selectivities (up to 99%) were discerned.
Furthermore, the substrate scope was explored for the cross-esterification of benzyl alcohol with long-
chain alcohols (up to 98%) and lactonization of diols (up to 68%). The heterogeneous nature and stability
of the catalyst facilitated by its ease of separation for long-term performance and recycling studies
showed that the catalyst was robust and remained active even after six recycling experiments.
EPR measurements were performed to deduce the reaction mechanism in the presence of POBN
(α-(4-pyridyl-1-oxide)-N-tert-butylnitrone) as a spin-trapping agent, which confirmed the formation of
•
CH2OH radicals and H•
radicals, wherein the solvent plays an active role in a nonconventional manner.
A plausible mechanism was proposed for the oxidative esterification of alcohols on the basis of EPR
findings. The presence of a cobalt core along with cobalt oxide and the electron-rich N-, O-, and
S-doped carbon shell displayed synergistic effects to afford good to excellent yields of products.
Introduction
Nanomaterials have garnered incredible attention due to their
unique and versatile properties and numerous applications in
various fields including energy conversion, chemical engineer-
ing, environmental and biological applications, and sustain-
able catalysis.1–6
The advancement in the assembly of nano-
structured materials has been observed due to modern nano-
technological developments, predominantly because of nano-
scale features as well as tunable properties such as compo-
sitions, shapes, size, surface area, porosity, and accessible
active sites.7–9
In recent years, it has been reported that a
variety of noble-metal nanoparticle-based catalysts can provide
active sites for efficient catalysis and thus, they have been
widely investigated.10,11
The progress in the sustainable devel-
opment of heteroatom-doped carbon-based supports has been
outstanding due to their semiconductive nature, and the elec-
tronic density can be tuned by the amount of heteroatom
doping.12–15
Due to the presence of lone pairs, heteroatoms
contribute additional electrons to the π-conjugated system,
which in turn increases the charge density on the carbon
sheets. Moreover, it has been reported that such doping with
heteroatoms can transform the carbon sheets into semi-
conductors by shifting the Fermi level from the Dirac point.16
†Electronic supplementary information (ESI) available: The experimental
details, TEM, low angle XRD pattern and N2 adsorption and desorption isotherm
of the catalyst and control samples. STEM elemental mapping images of the
Co@NOSC catalyst. The XPS survey scan and HR-XPS of fresh and reused cata-
lyst. The EPR spectrum along with the theoretical calculations and DFT model
and comparative performance of the Co@NOSC catalyst with prior reported art.
See DOI: 10.1039/c8gc01333k
Regional Centre of Advanced Technologies and Materials, Department of Physical
Chemistry, Faculty of Science, Palacký University Olomouc, Šlechtitelů 27, 783 71
Olomouc, Czech Republic. E-mail: manoj.gawande@upol.cz, radek.zboril@upol.cz
3542 | Green Chem., 2018, 20, 3542–3556 This journal is © The Royal Society of Chemistry 2018
Publishedon21June2018.DownloadedbyUniversityofAlbertaon7/12/20196:21:29AM.
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The presence of various heteroatoms provides more accessible
active sites on the surface of carbon materials and controls the
overall hydrophobicity of the carbon sheets.17,18
The presence
of electron-rich heteroatoms and the associated functional
groups such as carboxylic and hydroxyl groups on the surface
of doped carbon species facilitates strong interactions between
nanosheets and metal nanoparticles for realizing metal-func-
tionalized catalysts.19
Furthermore, heteroatom doping has
been shown to ensure prominent advantages in terms of stabi-
lizing the metal nanoparticles.20,21
The oxidation of alcohols to aldehydes, acids, esters, and
acetals is a common and widely used reaction in organic trans-
formations with catalysis.22–28
The direct synthesis of esters via
selective oxidation of alcohols is a challenging and useful reac-
tion because of the widespread applications of esters in the
fields of fine chemicals, cosmetics and polymer indus-
tries.20,26,29,30
For this purpose, various noble-metal-based
homogeneous catalysts have been identified;28,31,32
however,
the use of expensive noble metals, the homogeneous nature of
the reaction, and the tedious separation processes render
them as impractical choices. Sustainability tenets dictate the
use of inexpensive, non-noble metal-based, and reusable
heterogeneous catalysts for the selective conversion of alcohols
to esters.20,33
Transition metal nanoparticles supported on
active supports are becoming favorable choices to replace
noble-metal-based catalysts; the syntheses of esters from alco-
hols by using supported metal nanoparticles have been
attempted.20,33
Multistep preparations, poor stability, and
leaching from the surface of the support materials are some of
the aspects that need to be addressed besides the use of bases,
which are not environmentally friendly.26,33
In recent years,
nitrogen-doped carbons with electron-rich surfaces have
emerged as active supporting materials as well as promoters
that catalyze specific reactions;34–40
cobalt-based catalysts are
inexpensive, noble-metal-free, and recyclable sustainable cata-
lysts for the aerobic oxidation of alcohols and the selective oxi-
dation of alcohols to esters.20,33,35,41
Most of these procedures
require the use of environmentally compromising additives
and bases for the aerobic oxidation of alcohols.17,19,21,33
Metallic cobalt nanoparticles activated by nitrogen doping
have displayed improved activity under base-free con-
ditions,20,41
but the multistep and long procedures with expen-
sive reagents limit their wider applications. Consequently,
there is an urgent need to design a concise and all-encompass-
ing multifunctional benign catalyst comprising basic as well as
metallic sites in a single step. Herein, we demonstrate the one-
step synthesis of an eﬃcient and multifunctional sustainable
catalytic system based on nitrogen-, oxygen-, and sulfur-rich
carbon-nanotube-coated cobalt nanocomposites (Co@NOSC).
The assembly requires inexpensive and renewable carragee-
nan, which provides carbon, oxygen, and sulfur; urea is used
as a nitrogen source. The Co@NOSC catalyst has been pre-
pared via direct pyrolysis of a cobalt salt, urea, and bio-source
carrageenan mixture (Scheme 1), and it is eﬀectively used for
the base-free selective oxidation of alcohols to esters under
mild conditions.
The simple method entails the biodegradable carrageenan
biopolymer, which serves as a precursor for carbon, oxygen as
well as sulfur, and it possesses gelling properties, which facili-
tate the entrapment of nitrogen-rich urea in the ensuing gel;
the distribution of urea in the gel network allows flexible
amounts of nitrogen doping. The presence of diverse types of
basic nitrogens (pyridinic) along with oxygen and sulfur on
the carbon shell enables the promotion of base-free reactions.
Furthermore, the presence of hydrophilic oxygen, sulfur, and
nitrogen on hydrophobic carbons enables the interaction
between the catalysts and the reactants.
Experimental
Synthesis of Co@NOSC
A homogeneous cobalt-containing precursor solution was pre-
pared by dissolving carrageenan (3 g) in 80 mL water at 80 °C.
Urea (16 g) was dissolved separately in 40 mL water, and this
solution was slowly added to the carrageenan solution under
Scheme 1 A schematic representation of the preparation of the Co@NOSC catalyst.
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vigorous stirring, followed by the addition of 2 g of cobalt
nitrate hexahydrate (Co(NO3)2·6H2O) in 20 mL water. This
mixture was stirred at 80 °C until the volume was reduced to
100 mL. This ensuing mixture was immediately cooled in a
deep freezer to obtain a solid gel, which was then freeze-dried
to obtain a pink powder. The material was transferred into a
crucible and heated up to 500 °C at a rate of 1 °C min−1
under
N2 atmosphere, and it was maintained at this temperature
for 2 h. The temperature was then increased to 900 °C at a
rate of 2 °C min−1
under N2 atmosphere and maintained for
2 h. The resulting black solid sample was washed with boiling
water at 100 °C (1500 mg of the catalyst in 45 mL water in a
closed 2-dram vial) for 3 h followed by methanol washing at
65 °C for 3 h to remove the weakly bound moieties. Due to the
ease of oxidation of surface-bound metallic cobalt,36
the
samples were heated at 400 °C under H2 for 1 h, and they were
vacuum preserved under N2 prior to use. The synthesized mag-
netic material was identified as cobalt in the nitrogen-, oxygen-,
and sulfur-doped carbon nanotube composite Co@NOSC; the
cobalt content of Co@NOSC, as confirmed by ICP-MS analysis,
was found to be 20.89 wt%. The corresponding reference
sample NOSC was prepared by following the aforementioned
procedure without the addition of cobalt salt.
Synthesis of urea–cobalt (Co@NC)
The urea–cobalt composite was synthesized by a similar pro-
cedure without using the carrageenan-incorporated gel (only
urea and cobalt salt were used as precursors). In a typical syn-
thesis, 19 g urea was dissolved in 40 mL water followed by the
addition of 2 g Co(NO3)2·6H2O in 20 mL water. This mixture
was stirred at 80 °C until the volume was reduced to 20 mL.
Since urea did not undergo gelling, for comparison purposes,
20 mL volume was cooled in a deep freezer to obtain a solidi-
fied sample, which was then freeze-dried. The ensuing pink
powder was transferred to a crucible, which was heated to
500 °C at a rate of 1 °C min−1
under N2 atmosphere, and it
was maintained at that temperature for 2 h. Subsequently, the
temperature was increased to 900 °C at a rate of 2 °C min−1
under N2 atmosphere, and it was maintained for 2 h. The
synthesized magnetic material was identified as cobalt in the
nitrogen-doped carbon nanotube composite Co@NC; the
cobalt content of Co@NC, as confirmed by ICP-MS analysis,
was found to be 60.59 wt%.
Synthesis of carrageenan–cobalt (Co@OSC)
For the synthesis of the carrageenan–cobalt composite, a
similar approach was used by maintaining the stoichiometric
ratio of the precursors except that urea was not added in the
synthesis step. Briefly, 1.5 g of carrageenan was dissolved in
40 mL water followed by the addition of 1 g Co(NO3)2·6H2O in
5 mL water. This mixture was stirred at 80 °C until the volume
was reduced to 35 mL. This mixture was immediately cooled to
initiate gel formation. The solid gel was then freeze-dried,
and the pink powder was transferred to a crucible, which
was heated to 500 °C at a rate of 1 °C min−1
under N2 atmo-
sphere, and it was maintained at this temperature for 2 h. This
was followed by increasing the temperature up to 900 °C at a
rate of 2 °C min−1
under the N2 atmosphere; this temperature
was maintained for 2 h. The synthesized magnetic material
was identified as cobalt in the oxygen- and sulfur-doped
carbon composite Co@OSC; the cobalt content of Co@OSC, as
confirmed by ICP-MS analysis, was found to be 37.39 wt%.
Catalytic reactions
The oxidative esterification of various types of alcohols was
carried out in a 10 mL 2-dram vial sealed with a
polytetrafluoroethylene/silicone cap (PTFE). The reaction vessel
was charged with 0.5 mmol of alcohol and 60 mg of the cata-
lyst. The 2-dram vial was degassed and purged several times
with pure oxygen, and the balloon filled with pure O2 was
inserted with a needle after the addition of alcohol and metha-
nol. The 2-dram vial was transferred into a preheated oil bath
at 60 °C and then heated to the desired temperature with con-
tinued stirring. After the completion of the reaction, the cata-
lyst was separated by using an external magnet followed by
washing and centrifugation. The conversion and yield of pro-
ducts were determined by FID-based GC. The recyclability of
Co@NOSC was further investigated under identical reaction
conditions (0.5 mmol of benzyl alcohol, 60 mg of the catalyst,
5 mL of methanol, 24 h, 60 °C). For the recycling studies, after
the separation of the reaction catalyst via an external magnet/
centrifuge, it was washed several times with water and ethanol.
The reusability was assessed by reducing the catalyst using H2
flow at 400 °C for 1 h and then using it in the next run (at least
up to the 6th
cycle).
Results and discussion
Synthesis and structural analysis of Co@NOSC, Co@NC, and
Co@OSC catalysts
The formation of nitrogen-, oxygen-, and sulfur-rich (up to
1.7% N, 21.9% O, and 1.1% S, based on XPS) carbon resulted
in a cage-like structure, wherein cobalt ions interacting with
the urea molecules were trapped in the carrageenan gel
(Scheme 1). The freeze-drying of the mixture ensured that the
cobalt-interacted urea was entombed in the carrageenan gel.
During the carbonization process, the urea molecules started
to condense with the evolution of ammonia, which was
restricted due to the presence of more hydrophilic groups in
carrageenan; this phenomenon was observed by Shi et al.,42
wherein higher yields of carbon nitride were obtained in the
presence of silica because of its hydrophilic nature. The high-
temperature carbonization at 900 °C facilitated the formation
of a cobalt nanotube/nanoparticle core having a nitrogen-,
oxygen-, and sulfur-rich carbon shell. For comparison pur-
poses, we prepared control materials using the combination of
carrageenan cobalt (Co@OSC), urea cobalt (Co@NC), and urea
carrageenan (NOSC); these materials were synthesized by a
procedure similar to that of Co@NOSC.
The synthesized materials are well characterized using
various analytical techniques. The phase compositions and
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crystalline structures of the samples are analyzed using
powder X-ray diffraction (PXRD) (Fig. 1A). The PXRD pattern of
a freshly synthesized Co@NOSC catalyst shows sharp diffrac-
tion lines (2θ) at 51.81° and 60.59°, corresponding to the inter-
layer spacing (d) of 0.2048 and 0.1770 nm, thus indicating the
presence of metallic cobalt with a cubic structure (space group
Fm3m).37,43
The additional diffraction lines at 17.97°, 34.77°,
and 61.26° with d-spacings of 0.5743, 0.2993, and 0.1756 nm,
respectively, correspond to the crystal planes (111), (311), and
(440), and they can be ascribed to the presence of cubic cobalt
sulfide (Co9S8, space group Fm3m).44
The presence of Co9S8 is
due to the transformation of some cobalt species into cobalt
sulfide by reaction with the sulfur present in carrageenan
during heating at elevated temperatures. Careful refinement of
the XRD patterns allows the calculation of the crystalline
weight percentage of the individual cobalt domains as metallic
cobalt (∼76.1%), Co9S8 (23.2%) and CoO (0.7%) (Fig. S1†).
The presence of graphitic N-, O-, and S-tridoped carbon
(with the hexagonal space group P63/mmc) was confirmed by
the diffraction line at 2θ = 30.6° with a d-spacing of 0.3384 nm.
The average crystallite sizes were calculated with Scherrer’s
equation, and they were found to be 42.46 nm for elemental
cobalt and 40 nm for Co9S8. The recycled Co@NOSC catalyst
also showed a similar pattern, and the results revealed that
there were no changes in the phase structure during the reac-
tion and the washing step (Fig. 1A(b) and (c)). Furthermore,
the presence of specific D and G bands in the Raman spectra
of the Co@NOSC catalyst proved the graphitic nature of the
carbon material (Fig. 1B). The G band originated from the in-
plane vibration of sp2
carbons, whereas the D band originated
from the out-of-plane vibrations of sp3
carbons, thus repre-
senting defects in the graphene structure. The intensity ratio
of the D and G band (ID/IG) was found to be 1.21, which rep-
resented abundant defects due to the doping of heteroatoms
in the conjugated network.45,46
The morphology of Co@NOSC was thoroughly investigated
using TEM and HRTEM. The TEM image of the Co@NOSC
material showed irregular nanotubes and nanospheres of
different shapes with a clearly observable Co core surrounded
by a graphenic N-, O-, and S-tridoped carbon shell (Fig. 2a and
b, Fig. S2†). The average diameter of the core was found to be
in the range of ∼10–15 nm, whereas the thickness of the shell
was in the range of ∼12–15 nm (Fig. 2c). The fine structure of
the material analyzed by HRTEM showed that the material
contained highly integrated nanostructures of layered N-, O-,
and S-tridoped carbon-coated cobalt nanotubes. The HRTEM
image at the 20 nm scale shows various crystallite fringes; e.g.,
at 2 nm, it clearly shows interlayer spacings of 0.29 nm,
0.20 nm, and 0.33 nm ascribed to the (311) plane of Co9S8,
(111) plane of metallic cobalt, and (002) plane of NOS-doped
carbon, respectively (Fig. 2d–f). The presence of cobalt in the
inner core and heteroatom-doped carbon outer shell was
further confirmed by the STEM elemental mapping of the
Co@NOSC catalyst (Fig. 3 and Fig. S3†). Interestingly, it was
observed that the heteroatoms N, O, and S were uniformly dis-
tributed in the carbon shell, thus confirming the efficiency
and even doping throughout the material. The XRD pattern of
the control samples Co@OSC and Co@NC was compared with
that of Co@NOSC (Fig. S4b†). The XRD pattern of Co@OSC
(Fig. S4 and S5†) was similar to that of Co@NOSC, which con-
tained a higher amount of Co9S8 along with metallic cobalt
and CoS; however, in the case of Co@NC (Fig. S4 and S6†), in
addition to metallic cobalt with a cubic structure (space group
Fm3m), some other phases such as cobalt oxide (space group
Fm3m), cobalt nitride (space group F4ˉ3m) and cobalt with hex-
agonal structure (P63/mmc) were observed. The TEM result of
Co@OSC (Fig. S7a–f†) showed that the inner cobalt core was
surrounded by a carbon shell but in Co@NC (Fig. S7g–l†), less
coating was observed due to a greater loss of ammonia during
the condensation of urea; this supported the special inter-
Fig. 1 (A) Powder X-ray diffraction (PXRD) patterns of (a) fresh, (b) H2-treated, and (c) recycled Co@NOSC catalyst samples. (B) Raman spectrum of
Co@NOSC.
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action (Scheme 1) in the case of Co@NOSC, which was further
supported by its catalytic activity and stability (ICP-MS
analysis).
X-ray photoelectron spectroscopy (XPS) revealed the pres-
ence of oxygen, nitrogen, and sulfur along with carbon in the
material (Fig. S8†). The elemental compositions of nitrogen,
oxygen, sulfur, carbon, and cobalt in the material were found
to be 1.7, 21.9, 1.1, 71.6, and 3.7 at%, respectively, which
showed the significant doping of diﬀerent heteroatoms in the
carbon sheets.
The high resolution XPS spectra of the catalyst in the C 1s
region can be deconvoluted into four peak components with
binding energies of 284.6, 285.5, 286.9, and 288.5 eV corres-
ponding to CvC–C, C–O/C–N, CvO, and OvC–O types of
carbons, thus showing the graphitic nature of the carbon
material34
(Fig. S9a†). The appearance of the specific N 1s
Fig. 2 TEM images of the Co@NOSC catalyst sample at the (a) 500 nm, (b) 100 nm, (c) 20 nm, (e) 20 nm scale; (d) and (f) selected areas with a
2 nm scale bar showing the d-spacing.
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peak component at 398.5, 400.7, and 401.6 eV confirms the
presence of pyridinic, pyrrolic, and graphitic nitrogens in the
NOSC-doped materials (Fig. 4a). The three peak components
at 529.7, 531.6, and 533.11 eV in the O 1s spectra of the
material, originating due to the presence of oxygen on the
surface of cobalt (CvO and C–O), reveal that graphitic carbon
is doped with oxygen (Fig. 4b). The high-resolution S 2p peaks
are also deconvoluted, mainly into two peak components
associated with C–S–C (164.6/163.0) in the case of a fresh
sample; for reused samples, a slight shift in this peak com-
ponent can be seen (163.6/162.4 eV) along with that of the
C–SOx–C (169.8/168.7 eV) species in both samples (Fig. 4c and
ESI Fig. S7†).34
The HR-XPS result of the catalyst in the Co 2p
region shows the presence of cobalt oxide (3.7 wt%) (Fig. 4d)
on the surface of the materials. Thus, the doping of all hetero-
atoms along with that of cobalt is confirmed by XPS; elemental
mapping shows the presence of multifunctionalities of the
Co@NOSC catalyst. Furthermore, all these functionalities are
intact in the reused catalyst at least up to the 6th
cycle after
reaction, and the elemental compositions of nitrogen, oxygen,
sulfur, carbon, and cobalt in the material are 3.1, 25.9, 1.0,
62.7, and 7.4 at%, respectively, according to XPS analysis
(Fig. S8–S10†). The BET surface area analysis indicates that the
Co@NOSC catalyst displays a high surface area of 383.09 m2
g−1
Fig. 3 (a) HRTEM image, (b) HAADF image, (c–l) STEM elemental mapping images of Co@NOSC showing C, N, O, S, and Co.
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when compared to Co@NC (45.90 m2
g−1
) and Co@OSC
(369.90 m2
g−1
). The type IV nitrogen adsorption–desorption
isotherms with a H4-shaped hysteresis loop observed for
Co@NOSC and Co@OSC confirm the micro–mesoporous
nature of these materials, whereas the H2 type hysteresis loop
observed for Co@NC confirms the mesoporous nature of the
material (Fig. S11A†).47
The higher uptake at low pressure is
associated with the filling of micropores in the case of
Co@NOSC and Co@OSC as compared to that for Co@NC;47
this is further supported by the micropore surface area calcu-
lated by the T-plot method (Table S1†), which confirms the
purely mesoporous nature of Co@NC, whereas Co@NOSC
(micropore surface area of 347.82 m2
g−1
) and Co@OSC (micro-
pore surface area of 274.80 m2
g−1
) are hierarchical porous
materials. Furthermore, the above-mentioned observations are
also supported by the respective pore size distribution results
(Fig. S11B†); the contribution of mesopores is greater for
Co@NC as compared to those for Co@NOSC and Co@OSC.
Thus, from the above-mentioned discussion, it is apparent that
Co@NOSC and Co@OSC are hierarchical porous materials
(microporous as well as mesoporous, whereas Co@NC is meso-
porous). The presence of mesoporous nature of the Co@NOSC,
Co@OSC, and Co@NC materials is further evidenced by low-
angle XRD patterns (Fig. S4A†). One broad peak is observed for
all three materials, signifying that the average pore-center to
pore-center correlation length confirms the mesoporous nature
of the materials having hierarchical pores.48,49
Oxidative esterification of alcohols over the Co@NOSC catalyst
The synthesized and well-characterized Co@NOSC catalyst was
tested for the selective one-step oxidation of alcohols to esters
without employing any base. Benzyl alcohol was chosen as a
model substrate for the reaction with methanol, which served as
a solvent as well as a reagent, and O2 gas was used as the term-
inal oxidant under mild and base-free conditions. As can be
seen from Table 1, the oxidative esterification reaction did not
proceed in the absence of the catalyst or O2 or in the presence of
the Co@NOSC sample heated at 550 °C (Table 1, entries 1–4). To
compare the catalytic activity, we tested all three catalytic com-
ponents, namely, Co@NC, Co@OSC, and Co@NOSC having
cobalt contents of 60.59 wt%, 37.39 wt%, and 20.89 wt%,
respectively. A small amount of benzyl alcohol was oxidized to
methyl benzoate (conversion 15%, selectivity 16%) over
Co@OSC and Co@NC (conversion 50%, selectivity 74%) with a
higher amount of cobalt content (Table 1, entries 5 and 6).
These results indicated that the Co nanoparticle-based catalyst
Fig. 4 XPS spectra of the Co@NOSC catalyst: (a) N 1s, (b) O 1s, (c) S 2p, and (d) Co 2p regions.
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without doped carbon could not promote oxidative esterification
of alcohol in the absence of base at low temperature. However,
the Co@NOSC catalyst (having nitrogen, oxygen, sulfur, and
carbon along with cobalt) exhibited excellent conversion (97%)
of benzyl alcohol under similar conditions with more than 98%
selectivity for methyl benzoate (Table 1, entry 16). These findings
clearly showed that the synthesized Co@NOSC catalyst consider-
ably promoted such a reaction to yield methyl benzoate in the
absence of any base. The catalytic activity of the present catalyst
can be due to its specific structure of an inner metallic core and
outer electron-rich heteroatom-doped carbon (nitrogen, oxygen,
and sulfur). To optimize the ideal reaction conditions, initially,
various reaction parameters such as the amount of catalyst and
reaction time and the amount of methanol that plays an active
role (acts as a co-catalyst; confirmed by EPR, discussed in
mechanism) were appraised to achieve conversion as high as
97% (Table 1, entries 7–16). It was found that for 25 mg of cata-
lyst, up to 95% conversion can be achieved in 48 h, whereas
60 mg catalyst afforded 97% conversion in only 24 h (Table 1,
entries 8 and 16).
For the first 17 h, the reaction proceeded very quickly with
less selectivity (87–93%) but after this period, the reaction
became slow, and the selectivity increased to more than 98%
(Table 1, entries 14–16). Furthermore, after increasing the
amount of the catalyst weight up to 60 mg, 97% conversion
and >98% selectivity of methyl benzoate were obtained after
24 h. The initial fast reaction rate may be because of numerous
freely available (pyridinic nitrogen and metallic cobalt along
with oxygen and sulfur) active sites. It is also discussed in the
mechanism section that both metallic cobalt and heteroatoms,
especially nitrogen, were synergistically playing active roles for
higher conversion. We observed that both hydrogen free rad-
icals and H2O2 were formed during the reaction, due to which
the pyridinic nitrogen was protonated; also, the oxidation of
surface-coated cobalt may have occurred. This could be the
reason for the slow reaction rate after 7 h. Due to the basic
nature of nitrogen, it also played an active role in increasing
the selectivity of methyl benzoate. From the entry 5, we can
observe that the Co@OSC catalyst had a smaller amount of
metallic cobalt (Fig. S5†), and the absence of nitrogen yielded
much less selectivity (∼16%) for methyl benzoate. Moreover,
Co@NC having a higher amount of metallic cobalt and
(Fig. S6†) less nitrogen doping exhibited 50% conversion with
74% selectivity towards methyl benzoate.
From these optimization parameters, it was apparent that
60 mg (0.21 mmol cobalt) of the catalyst with 5 mL methanol
and an O2 balloon was sufficient to achieve the conversion of
benzyl alcohol as high as 97% with more than 98% selectivity
in 24 h at 60 °C. The strong electronic interactions and the
synergistic effects between the N-, O-, and S-tridoped carbon
and cobalt certainly helped improve the catalytic activity com-
pared to the activities of their individual components such as
Co@OSC, Co@NC, NOSC, and Co@NOSC calcined at 550 °C.
With the optimized conditions in hand, the versatility of
the catalyst was ascertained by using various substituted
benzyl alcohols, bi-benzyl alcohols, N-heterocyclic alcohols,
S-heterocyclic alcohols, and aliphatic alcohol substrates for
aerobic oxidative esterification (Table 2). Table 2 shows that
for 4-halo (–Cl, –Br) and 3-halo (–F) substituted benzyl alco-
hols, more than 97% conversion and up to 98% selectivity
could be achieved (Table 2, entries 2–4); other substituted
(–CH3, –OMe, –CF3, and –NO2) benzyl alcohol derivatives were
also explored (Table 2, entries 5–11). Notably, in the case of
–CH3 and OMe, excellent performance of the catalyst with
Table 1 Optimization of the oxidative esterification of benzyl alcohola
Entry Catalyst (mg)
Cobalt mmol/benzyl alcohol
mmol (molar ratio) Time (h) Conversionb
(%) Selectivityb
(%)
1 NOSC (60)c
0/0.50 24 2 10
2 Co@NOSC (60)d
0.21/0.50 24 — —
3 No catalyst 00/0.50 24 — —
4 Co@NOSC-550 °C calcined (80) — 24 NR —
5 Co@OSC (60) 0.38/0.50 24 15 16
6 Co@NC (60) 0.61/0.50 24 50 74
7 Co@NOSC (25) 0.08/0.50 24 72 93
8 Co@NOSC (25) 0.08/0.50 48 95 97
9 Co@NOSC (50) 0.16/0.50 24 93 97
10 Co@NOSC (50) 0.16/0.50 48 99 99
11 Co@NOSC (60) 0.21/0.50 24 97 >98
12 Co@NOSC (60) 2 mL MeOH 0.21/0.50 24 70 87
13 Co@NOSC (60) 4 mL MeOH 0.21/0.50 24 90 98
14 Co@NOSC (60) 0.21/0.50 7 60 87
15 Co@NOSC (60) 0.21/0.50 17 91 93
16 Co@NOSC (60) 0.21/0.50 24 97 >98
a
Reaction conditions: benzyl alcohol (0.5 mmol), methanol (5 mL), O2 balloon. b
Determined by GC. NR = No reaction. c
Catalyst: Prepared by car-
rageenan and urea calcined at 900 °C. d
Reaction conditions (a
) without O2 under N2.
View Article Online

high conversion (97%) and selectivity (>98%) towards the
corresponding methyl ester was observed. Importantly, –NO2-
substituted benzyl alcohol exhibited excellent activity (>90%
conversion and selectivity) compared to previously reported
noble-metal-based homogeneous catalysts;22
this may be due
to the presence of more hydrophilic heteroatoms on the
surface of the catalyst. However, –CF3-substituted benzyl
alcohol could afford only 58% conversion; this may be
ascribed to the presence of more electronegative fluorine
atoms, which may restrict the interaction of alcoholic groups
with the catalyst surface (Table 2, entry 10). We chose para-,
meta-, and ortho-isomers of methoxybenzyl alcohols to study
the effect of steric hindrance on the reaction; the reactivity
order was found to be para > meta > ortho (Table 2, entries
Table 2 Substrate scope of oxidative esterification reactions with methanola
Entry Substrate Product Conversionb
(%) Selectivityb,c
(%)
1 97 >98
2 97 99
3 97 98
4 97 99
5 96 93
6 96 98
7 84/95d
98/98d
8 78/90d
64/95d
9 62/73d
58/61d
10 58/60d
>98/98d
11 90 >98
12 89/98d
>98/99d
13 78/98 95/98
14 66/80d
78/86d
15 90 98
16 36/44d
2/5d
a
Reaction conditions: benzyl alcohol (0.5 mmol), methanol (5 mL), O2 balloon, 60 mg catalyst (0.21 mmol cobalt). b
Determined by GC. c
Side
product is only the corresponding aldehyde. d
Reaction time 48 h.
View Article Online

6–8). The moderate conversion (78%) and selectivity (64%)
values for ortho –OMe-substituted methyl benzoate demon-
strated the blocking effect of this functional group on the
esterification reaction of the corresponding alcohols (entry
8).20
In contrast to poor yields (20%) and selectivity (∼21%)
reported in previous reports, even with a Pd-based catalyst,22
excellent conversion (96%) and selectivity (>98%) were exhibi-
ted by 4-methoxybenzyl alcohol in our study; this was further
supported by ortho –CH3-substituted benzyl alcohol, where
73% conversion with 61% selectivity (Table 2, entry 9) was dis-
cerned. Heterocyclic alcohols, namely, furfuryl alcohol and
2–pyridine methanol could be oxidized to corresponding
esters with good conversion (78–89%) and selectivity 95–98%
(Table 2, entries 12 and 13); for attaining maximum conver-
sions for some of these reactants, the reaction time was
increased up to 48 h (Table 2, entries 12 and 13). Methyl cinna-
mate and dimethyl terephthalate are two important chemicals
widely used in polymer industries,50,51
and they can be pre-
pared from cinnamyl alcohol (80% conversion with 86%
selectivity) and 1,4-benzenedimethanol (90% conversion with
98% selectivity), respectively (Table 2, entries 14 and 15).
Furthermore, the lignin-derived bio-molecule, namely, vanillyl
alcohol can be used as a substrate to acquire significant pro-
ducts such as vanillin (48% selectivity) and methyl vanillate
(5% selectivity) (Table 2, entry 16), which are applied in foods,
beverages, cosmetics, and drugs.52
Thus, the use of novel and
inexpensive carbon, oxygen, sulfur (carrageenan), and nitrogen
(urea) precursors for efficient catalyst preparation is supported
by superior/comparative performance when compared with
previously reported results (Table S2†).
Aerobic esterifications of benzyl alcohol with other aliphatic
alcohols including ethanol, propanol, butanol, and pentanol
were also performed (Table 3). It was observed that linear alco-
hols gave good conversion results relative to branched chain
alcohols, which was due to the +I effect of the methyl group;
this decreased the –C–H cleavage. Furthermore, we explored
this Co@NOSC catalytic system for the oxidative lactonization
of diols (Table 4); lactonization of diols is an important reac-
tion because of the potential of lactones in biological appli-
cations as well as in polymer industries.53
The versatility of our
catalytic system became apparent during the lactonization of
diols as good conversion (66–68%) and selectivity (91–92%)
were observed.
The reusability of a catalyst is a key factor for its sustainable
applicability in heterogeneous catalysis. We examined the
stability and the reusability of the Co@NOSC catalyst by reco-
vering the used catalyst from the reaction, and we also studied
the compositional integrity via characterization. The catalyst
recycling was accomplished by washing with water and
ethanol, and heat treatment at 400 °C under hydrogen for 1 h.
Heating the catalyst at 400 °C under H2 for 1 h after each step
showed no significant loss in catalytic performance for the
reuse experiments (Fig. 5). The ICP-MS result of the filtrate
showed no significant leaching (0.015 wt%), confirming the
stable nature of the catalyst. The XRD spectra of the
Co@NOSC catalyst after six recycling processes did not show
any significant visible change, which revealed that the catalyst
retained its crystalline nature even after its sixth reuse
(Fig. 1c). Owing to the oxidizing nature of cobalt nanoparticles,
the presence of cobalt oxide was observed in the XPS analysis
of the reused catalyst, and we also observed slight shifting of
the Co 2p3/2 peak from the binding energy value of 778 eV to
779 eV (Fig. S10d†). No detectable change in the binding ener-
gies of C 1s and N 1s was discerned except for the slight shift-
ing of the pyridine nitrogen peak from 401.0 eV to 398.8 eV,
thus reaffirming no discernible change in the chemical
nature of the carbon shell (Fig. S9 and S10a, c†). However, a
slight shift in the pyridine nitrogen peak can be explained by
the protonation of the pyridinic nitrogen during the reaction.
From the TEM images of the reused catalyst after the 6th
cycle,
we observed that cobalt metal was still intact in the inner core,
Table 4 Co@NOSC-catalyzed lactonization of diolsa
Entry ROH Product Conversionb
(%) Selectivityb
(%)
1 68 92
2 65 91
3 66 92
a
Reaction conditions: diols (0.1 mmol), toluene 3 mL, 80 mg
(0.28 mmol cobalt) catalyst, O2 balloon, temperature (100 °C), reaction
time (22 h). b
Determined by GC.
Table 3 Co@NOSC-catalyzed cross-esterification of benzyl alcohol
with long chain alcoholsa
Entry ROH Product Conversionb
(%) Selectivityb
(%)
1c
98 85
2c
40 50
3d
30 75
4e
85 55
a
Reaction conditions: benzyl alcohol (0.05 mmol), ROH (2 mL), 35 mg
(0.11 mmol cobalt) catalyst, O2 balloon. b
Determined by GC.
c
Temperature, 70 °C. d
Temperature, 100 °C. e
Temperature, 110 °C;
reaction time, 22 h.
View Article Online

and it was surrounded by the heteroatom-rich carbon shell
(Fig. S12†).
The oxidation reaction of benzyl alcohol in the presence of
the cobalt-catalyst was further probed by the EPR technique in
conjunction with spin-trapping experiments using α-(4-pyridyl-
1-oxide)-N-tert-butylnitrone (POBN) as the spin-trapping agent.
POBN was initially dissolved in MeOH in the presence of the
substrate benzyl alcohol, and the solution was warmed for
30 min at 60 °C. The experiment revealed that clear degra-
dation of the POBN radical precursor into a nitroxide spin-
active species does not occur in these solvents within the time
frame and temperature. In the presence of the catalyst and
after stirring the reaction mixture for 30 min at 60 °C under
air, the solution EPR spectrum (T = 218 K) revealed clear pres-
ence of trapped radical species (Fig. 6a and Fig. S13†), which
confirmed the formation of radical intermediates formed
during the oxidation reaction of benzyl alcohol. Simulation of
the observed EPR resonance (Fig. 6a, red line, Sim) indicated
that the resonance signal may arise from the formation of the
POBN-CH2OH radical adduct.54
Fig. 6c shows the optimized
structure of the POBN-CH2OH radical obtained by DFT/UBP86/
6-31G* (gas-phase) with the drawn spin density distribution.
The observed solution EPR resonance was well reproduced
under the spin-Hamiltonian framework by the third-order per-
turbation theory (WinEPR SimFonia v. 1.25) using the follow-
ing spin-Hamiltonian parameters for the nitroxide radical:
hyperfine coupling constants (hfcc), namely, aN of 15.00 G, aH
of 3.30 G for β-hydrogen, giso of 2.0052(2), Lorentzian/Gaussian
ratio of 0.20 with line-width function (a + bm + cm2
), which
included the tumbling effects on nitrogen (N–O•
moiety is
shown in Fig. 6c) with parameters a = 1.99, b = −0.24 and c =
0.20. These estimated hfcc parameters were in fair agreement
with the isotropic Fermi contact coupling values obtained in
the gas phase (DFT/UB3LYP/6-31G*(d,p)) for the POBN-CH2OH
radical adduct, which were 13.06 G for aN and 2.86 G for
β-hydrogen aH (Table S4†). Similar EPR spectrum and spin-
Hamiltonian parameters (aN of 15.25 G and aH of 3.75 G) were
obtained by Halpern and co-authors for the PBN-CH2OH
radical adduct (Fig. S14†).55
Pou et al. also obtained a similar
EPR spectrum by dissolving 4-POBN in oxygen-saturated water-
buffer solution in the presence of Me2SO4 and H2O2, which
gave the corresponding POBN-CH2OH radical adduct with aN
of 14.45 G and aH of 2.25 G.56
Therefore, we can infer that
MeOH molecules play an active role in controlling the fate of
the radical species generated during catalysis. When H2O2 is
formed and converted to 2OH•
radicals, OH•
radicals interact
rapidly with the solvent molecules (MeOH), producing
•
CH2OH and H2O. To further validate this hypothesis, spin-
trapping experiments were performed with the sole presence
of (i) the catalyst, (ii) the spin-trapping agent POBN, and (iii)
the substrate benzyl alcohol by following the same experi-
mental conditions as reported above but without the addition
of MeOH. No radical species were successfully observed/
trapped by POBN under these conditions, as inferred from the
featureless EPR spectrum of the solution (Fig. S15†). We also
tested an alternative scenario in which only a small amount of
MeOH was added to the mixture containing the catalytic
system and the aromatic substrate, thus considering it as a
limiting reactant. Upon the addition of few microliters of
MeOH (20 μL) to the mixture composed of the catalyst, POBN,
and benzyl alcohol (2 mL of benzyl alcohol) and after 30 min
of heating at 60 °C under air, POBN again trapped radical
species in the solution. However, the observed EPR signal was
found to be much more complex compared to the previous
experimental signal, which was obtained when excess MeOH
was added to the reaction mixture. The observed EPR signal is
shown in Fig. 6b (T = 243 K, please see Fig. S14† also) together
with the EPR spectrum simulation (Fig. 6b, red line, Sim). Two
radical species contributed to the observed signal: (1)
POBN-CH2OH radical (entrapment of •
CH2OH) and (2)
POBN-H radical (entrapment of •
H). The simulated EPR spec-
trum by the third-order perturbation theory was obtained
using the following spin-Hamiltonian parameters: (2)
(POBN-H) (aN) of 16.20 G, 2H (aH) of 8.10 G, Lorentzian/
Gaussian ratio of 0.22 and line-width function (a + bm + cm2
)
with parameters a = 1.99, b = −0.20 and c = 0.18 and giso of
2.0052(2). (1) (POBN-CH2OH) (aN) of 14.35 G, 1H (aH) of 3.0 G,
Lorentzian/Gaussian ratio of 0.23 and coefficients a = 1.30, b =
−0.25, and c = 0.20 with giso of 2.0052(3). Similar spin-
Hamiltonian parameters for the POBN-H radical were obtained
by Aurian-Blajeni et al. upon irradiation of aqueous suspen-
sions of tungsten oxide in the presence of MeOH using POBN
as a trapping agent (POBN-H radical, aN of 16.6 G and aH of
10.25 G).57
From the spectrum simulation shown in Fig. 6c,
the relative weights of the two trapped spin components,
namely, •
H and •
CH2OH were calculated as percentages with
respect to the total EPR double-integrated signal, and they
were estimated to be 45% and 55%, respectively; thus, these
radicals appeared to form together, and about 1 : 1 ratio was
obtained from the catalytic decomposition of CH3OH → •
H +
•
CH2OH. The contributing resonances of POBN-CH2OH and
Fig. 5 Recyclability of the catalyst by treatment with hydrogen at
400 °C for 1 h.
View Article Online

POBN-H in the spectrum simulation shown in Fig. 6b are expli-
citly given in the ESI as Fig. S16.† Fig. 6d shows the optimized
structure of the POBN-H radical species obtained by DFT/
UBP86/6-31G* (with the spin density distribution, gas-phase)
together with the calculated isotropic Fermi contact coupling
values (DFT/UB3LYP/6-31G*(d,p)). It was noted here that the
theoretical calculation (gas-phase) poorly reproduced the
experimentally determined values of hfcc; thus, the interaction
between POBN-H and the substrate/solvent benzyl alcohol in
the medium seemed strong enough to produce a large modu-
lation in the hyperfine terms of the radical molecule.
Therefore, on the basis of the EPR results, product analysis
and previously reported studies, we propose a possible mecha-
nism for the oxidative esterification of benzyl alcohols by the
Co@NOSC catalyst (Fig. 7).10,20,58–60
The metallic cobalt core
of the Co@NOSC catalyst serves as a source of electrons,
whereas the carbon shell provides active sites, which aid the
binding of the substrate to the surface and facilitate electron
transfer reactions. At the onset of the catalysis, it is presumed
that molecular triplet oxygen is formed quickly; after accepting
one electron from the metal catalyst, it is converted into the
superoxide anion radical (O2
•−
) (Step I). In the next step, this
reactive superoxide anion radical abstracts a hydrogen radical
(•
H) from the surface-adsorbed molecules of methanol and
transforms into a hydroperoxide radical (•
HO2), whereas
methanol forms a •
CH2OH radical intermediate (Step II).
These •
CH2OH radicals are eﬀectively trapped by POBN,
forming the POBN-CH2OH radical adduct, as detected in the
EPR analysis. The •
HO2 radical further abstracts one •
H from
•
CH2OH and forms hydrogen peroxide together with an alde-
hyde (CH2vO) (Step III). H2O2 in the presence of the catalyst
can be dissociated again into •
OH (Step IV). In the subsequent
Fig. 6 X-band (9.17 GHz) EPR spectrum of the trapped radical species. Panel (a) shows the solution spectrum (T = 218 K) of the supernatant col-
lected from the catalyst/benzyl alcohol/POBN/MeOH mixture together with its EPR simulation (red line). Simulation parameters are given in the
main text. Panel (b) shows the solution spectrum (T = 243 K) of the supernatant collected from the catalyst/benzyl alcohol/POBN/MeOH mixture in
which MeOH has been added as the limiting agent; its EPR simulation is also shown (red line). The spin-Hamiltonian parameters are given in the
main text. Experimental parameters: 100 kHz modulation frequency, 30 ms time constant, 2.0 G modulation width, 5.0–6.0 mW of applied micro-
wave power, 1 min sweep time. Scans accumulated and averaged are shown in (a) 21 and in (b) 11. Panel (c) shows the optimized structures (gas-
phase) of the POBN-CH2OH radical and (d) the POBN-H radical (DFT/UBP86/6-31G*, energy −764.43954 a.u. for (c) and energy −649.91719 a.u. for
(d)), where the spin-density isosurface was drawn at 0.002 IsoVal. Fermi contact terms (hﬀc) obtained for the optimized structures (〈S2〉 = 0.7500, c;
〈S2〉 = 0.7500, d) were calculated by the single-point DFT theory (UB3LYP/6-31G*(d,p), gas-phase).
View Article Online

step, •
OH radicals attack benzyl alcohol to form benzaldehyde
(Step V). The formation of benzaldehyde as a stable molecule
and the sole side product is supported by our GC analysis.
This can be seen from Table 1 (entries 15–17), where we can
observe that the initial lower selectivity for the ester (87% in
7 h) increases with time to more than 98%. In the subsequent
step, because of the presence of more heteroatoms on the cata-
lyst’s surface, the benzaldehyde reacts with methanol (present
in excess) to form a reactive hemiketal intermediate, which
transforms into methyl benzoate via abstraction of hydrogen
by the •
OH radical (Step VI).
Conclusions
We have demonstrated a simple and environmentally friendly
synthesis of Co@NOSC using inexpensive carrageenan and urea
with non-precious cobalt, which affords a stable and reusable
catalyst with high surface area. The prepared material shows
excellent catalytic performance towards base-free selective oxi-
dative esterification of alcohols. The superiority of the catalyst
can be ascribed to the synergistic effects of metallic cobalt and
electron-rich N-, O-, and S-doped carbon, which result in base-
free catalytic activity. Sustainable features of the catalyst com-
prise the ease of synthesis and the use of renewable raw
materials such as carrageenan as a source of carbon and
oxygen. We believe that the catalytic performance of the catalyst
can be accredited to its specific structure of an inner metallic
core and outer electron-rich heteroatom-doped carbon (nitro-
gen, oxygen, and sulfur). The catalyst is very stable and robust
and can be recycled several times without any significant loss of
its activity. Moreover, the simple base-free approach coupled
with non-hazardous oxidants may show promise for additional
selective transformations of other organic compounds in high
yield. This interesting protocol certainly opens the door to
further material development by using renewable resources
such as carrageenan; it is a rich source of sulfur and has
specific gelling properties for specific target applications in cat-
alysis, electrocatalysis, and photocatalysis.
Conflicts of interest
There are no conflicts to declare.
Fig. 7 A possible reaction mechanism for the oxidative esterification of benzyl alcohols in the presence of the Co@NOSC catalyst.
View Article Online

Acknowledgements
The authors gratefully acknowledge the support from the
Ministry of Education, Youth and Sports of the Czech Republic
under Project No. LO1305, the support by the Operational
Programme Research, Development and Education – European
Regional Development Fund, Project No. CZ.02.1.01/0.0/0.0/
16_019/0000754 of the Ministry of Education, Youth and
Sports of the Czech Republic, and the assistance provided by
the Research Infrastructure NanoEnviCz supported by the
Ministry of Education, Youth and Sports of the Czech Republic
under Project No. LM2015073. The work was also partly
funded by the Internal Grant of the Palacký University
Olomouc, Czech Republic (Project No. IGA_PrF_2017_007).
The authors thank to V. Ranc for Raman analysis,
Ms. J. Stráská and Ms. P. Bazgerová for SEM/TEM analysis, and
D. Milde for ICP-MS analysis.
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