Nitrogen doped carbon spheres with wrinkled cages for the selective oxidation of 5-hydroxymethylfurfural to 5-formyl-2-furancarboxylic acid

Article abstract of DOI:10.1039/d0cc07856e

Nitrogen doped carbon spheres with wrinkled cages (NCSWCs), which were used for the first time as metal-free catalysts, exhibited high catalytic activity and selectivity in the oxidation of 5-hydroxymethylfurfural (HMF) to 5-formyl-2-furancarboxylic acid

Full text of DOI:10.1039/d0cc07856e

Volume 57
Number 16
25 February 2021
Pages 1965–2100
ChemComm
Chemical Communications
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ISSN 1359-7345
COMMUNICATION
Vivek Polshettiwar, Hua Tan et al.
Nitrogen doped carbon spheres with wrinkled cages for
the selective oxidation of 5-hydroxymethylfurfural to
5
-formyl-2-furancarboxylic acid

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Nitrogen doped carbon spheres with wrinkled
cages for the selective oxidation of
Cite this: Chem. Commun., 2021,
7, 2005
5-hydroxymethylfurfural to
5
Received 2nd December 2020,
Accepted 21st January 2021
5-formyl-2-furancarboxylic acid†
a
a
a
b
a
cd
Jiaping Zhu,
Weibing Liu,
Chaojian Yao, Ayan Maity, Jielai Xu, Tong Zhan,
Mingtai Sun,
DOI: 10.1039/d0cc07856e
c
c
b
Suhua Wang, Vivek Polshettiwar * and
a
rsc.li/chemcomm
Hua Tan
*
9
–15
Nitrogen doped carbon spheres with wrinkled cages (NCSWCs), including HMF oxidation to DFF or FDCA.
The doped
which were used for the first time as metal-free catalysts, exhibited nitrogen atoms can modulate the surface properties of the
high catalytic activity and selectivity in the oxidation of 5-hydro- carbon materials and also improve their electronic features,
xymethylfurfural (HMF) to 5-formyl-2-furancarboxylic acid (FFCA) under which thus leads to the enhancement of their catalytic perfor-
base-free conditions using tert-butyl hydroperoxide (TBHP) as the mance as metal-free catalysts. On the other hand, N-doped
oxidant. The mechanistic studies found that this reaction was catalyzed hollow porous carbon spheres (NHCSs), which contain a com-
by the synergy between NCSWCs and TBHP. The density functional bined micro/mesoporous shell and macroporous cage, have
theory (DFT) calculations further suggested that the hydroperoxyl radi- drawn more and more research interest in supercapacitors and
1
6,17
cals from TBHP adsorbed on the carbon atoms adjacent to the graphitic rechargeable batteries.
However, NHCSs have been seldom
N atoms are the active sites.
used as metal-free catalysts for organic catalytic reactions in the
literature.
HMF, as one of the important platform chemicals, can be oxidized
NHCSs are synthesized using the hard templating method,
to a series of value-added chemicals, such as 2,5-diformylfuran soft templating method, modified St o¨ ber method, hydrothermal
1
6
(
2
DFF), 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), FFCA, carbonization method, and so on. Among these methods, the
1
,5-furancarboxylic acid (FDCA), etc. However, most studies were hard templating method is more facile and controllable. The only
focused on the selective oxidation of HMF to DFF or FDCA due to drawback for this method is poor availability of the hard tem-
18
their versatility as monomers and intermediates for pharmaceu- plates with tunable size, shape, and morphology. Dendritic
2
,3
ticals, ligands, and other applications. FFCA, which has pro- fibrous nanosilica (DFNS) possesses special fibrous morphology,
mising applications in fuels, chemical intermediates, and drugs, which leads to high surface area and enhanced accessibility
etc., has not yet received widespread attention, although several compared with mesoporous silica and can be synthesized on a
studies have been reported on the production of FFCA by the large scale and their sizes, surface areas, and fiber densities can
4
–8
19,20
oxidation of HMF over metal based catalysts.
be controlled.
Thus, in this work, DFNS was used as the hard
Recently, considering the economic cost and environmental template to synthesize NCSWCs first via the hydrothermal carbo-
friendliness, N-doped carbon materials (NCs) have been widely nization using glucose as the carbon source, and then pyrolysis
0
0
used as metal-free catalysts for various chemical reactions, with 5,5 -diamino-3,3 -bis(1H-1,2,4-triazole) (DABT) as the nitro-
gen source, followed by template etching. The obtained NCSWCs
were used as metal-free catalysts in the oxidation of HMF to FFAC
under base-free conditions.
The DFNS used in the synthesis are DFNS-1, 2, 3, and 4,
which have different sphere sizes and fibrous morphologies
with varying fiber densities, as shown in Fig. S1 (ESI†). Their
adsorption and desorption isotherms in Fig. S2 (ESI†) indicated
that DFNS is a typical mesoporous material and its detailed
textural properties are summarized in Table S1 (ESI†). The TEM
images of the obtained NCSWCs shown in Fig. 1(a–d) and
Fig. S3 (ESI†) indicated that the sizes of NCSWC-1-800,
NCSWC-2-800, NCSWC-3-800, or NCSWC-4-800 are similar to
a
College of Chemistry, Guangdong University of Petrochemical Technology,
Maoming 525000, P. R. China. E-mail: huatan@gdupt.edu.cn
b
Department of Chemical Sciences, Tata Institute of Fundamental Research (TIFR),
Dr Homi Bhabha Road, Mumbai-400005, India. E-mail: vivekpol@tifr.res.in
c
College of Environmental Science and Engineering, Guangdong University of
Petrochemical Technology, Maoming 525000, P. R. China
d
Guangdong Provincial Key Laboratory of Petrochemical Pollution Process and
Control, Maoming 525000, P. R. China
†
Electronic supplementary information (ESI) available: Experimental, TEM, N
2
sorption isotherms, textural properties and chemical compositions, the relation-
ship between catalytic activity and different N species, XRD, Raman, XPS N 1s
spectra, and catalytic stability tests. See DOI: 10.1039/d0cc07856e
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the (002) peak increased with the increase of the pyrolysis
temperature from 600 1C to 950 1C, which suggests that the
content of graphitic carbon increased from NCSWC-4-600, to
NCSWC-4-800, and then to NCSWC-4-950. The Raman spectra in
Fig. S7 (ESI†) show that the NCSWCs exhibited two strong peaks
ꢀ
1
ꢀ1
at 1588 cm (G band) and 1349 cm (D band). The intensity
ratio of I /I reduced as the pyrolysis temperature increased,
D
G
suggesting an improved crystalline structure in NCSWCs with
fewer defect sites as the pyrolysis temperature rises.
Elemental analysis on NCSWCs-800 listed in Table S2 (ESI†)
confirms that the N contents in NCSWC-1-800, NCSWC-2-800,
NCSWC-3-800, and NCSWC-4-800 are 13.69 wt%, 16.34 wt%,
14.92 wt%, and 15.21 wt%, respectively. The STEM-EDX ele-
mental mapping results in Fig. 1(f–h) also reveal that the
element N is homogenously distributed in the whole NCSWC-
4
6
-800. We also prepared NCSWC by pyrolysis with melamine at
00 1C and 800 1C for comparison, which were designated as
2
1
NCSWC-4-600-M and NCSWC-4-800-M, respectively. It can be
seen from Table S2 (ESI†) that the N contents are 17.09 wt%
and 7.12 wt% in NCSWC-4-600-M and NCSWC-4-800-M, respec-
tively, while the N contents can reach up to 26.18 wt% and
1
5.21 wt% in NCSWC-4-600 and NCSWC-4-800, respectively,
and even when the pyrolysis temperature reached 950 1C,
.77 wt% of N content can be still retained in NCSWC-4-950.
In addition, compared with NCs prepared by the post treatment
method with urea or NH as the nitrogen sources, the N
6
Fig. 1 TEM images of NCSWC-1-800 (a), NCSWC-2-800 (b), NCSWC-3-
8
3
9
,22
00 (c), and NCSWC-4-800 (d) and EDX mapping for C (f), N (g) and O contents in the NCSWCs are also much higher.
(
h) of the STEM image (e) of NCSWC-4-800.
XPS quantitative analysis also indicates that N with high
contents exists in NCSWCs-800, as shown in Table S2 (ESI†).
those of the corresponding DFNS and the diameters of their Their high resolution XPS N 1s spectra in Fig. S8 (ESI†) can
cages were approximately 20–30 nm, 40–50 nm, 40–50 nm, and be curve-fitted into three components located at around
3
00–350 nm, respectively. Moreover, different morphologies of 398.6 ꢁ 0.1 eV, 400.1 ꢁ 0.1 eV, and 401.1 ꢁ 0.1 eV, which
the wrinkled cages in NCSWCs-800 suggested that the carbon correspond to pyridinic N, pyrrolic N, and graphitic N,
2
3
precursor can be diffused well within the inner part of DFNS. respectively. The relative peak areas of different N species
These results demonstrated that this synthetic method allowed are listed in Table S3 (ESI†). When the pyrolysis temperature is
the preparation of NCSWCs with different sphere/cage sizes 600 1C, the most abundant N species is pyridinic N in NCSWC-
and morphologies of the wrinkled cages by using DFNS with 4-600, whose relative peak area is 56.43%, with the increase of
different size and fiber density as the hard template, which the pyrolysis temperature, the relative peak areas of pyridinic N
normally cannot be achieved by using the solid spheres of and pyrrolic N decrease, but that of graphitic N rapidly
silica.
All the NCSWCs-800 exhibits a typical IV-type curve with a to 51.98% in NCSWC-4-950, respectively. As for NCSWCs-4-M,
hysteresis loop and a steep uptake (P/P 40.97) in Fig. S4 the relative peak areas of graphitic N are only 14.94% and
ESI†), indicating the coexistence of mesopores and macro- 20.64% in NCSWC-4-600-M and NCSWC-4-800-M, respectively.
increases from 9.84% to 42.32% in NCSWC-4-800, and then
0
(
pores. The detailed textural parameters of the NCSWCs-800 in Besides, the contents of graphitic N and pyridinic N are also
Table S2 (ESI†) show that the BET surface areas of NCSWCs-800 listed in Table S3 (ESI†).
2
ꢀ1
2
ꢀ1
are in the range from 742.4 cm
g
to 965.6 cm
g
, and
The catalytic performance of NCSWCs-800 was evaluated in
notably, NCSWC-4-800 possessed the largest mesopore and HMF oxidation using TBHP as the oxidant under base-free
macropore volumes among all the NCSWCs-800, which are conditions. It can be seen from Table 1 that FFCA was produced
probably derived from its huge wrinkled cages. The Barrett– as the main product on all the NCSWCs-800 with high selectivity
Joyner–Halenda (BJH) pore size distributions in Fig. S4 (ESI†) (B90%), but HMF conversion varied on different NCSWCs, from
show that NCSWCs-800, like DFNS, also contains dual pores, the lowest 79.1% on NCSWC-1-800, to 85.4% on NCSWC-3-800,
small and narrow pores, whose sizes are located at 2–3.5 nm, 86.7% on NCSWC-2-800, and to the highest of 92.3% on NCSWC-
and the large and broad pores, whose sizes are larger than 4-800. Combined with the characterization results, the highest
1
0 nm. The XRD patterns of NCSWCs in Fig. S6 (ESI†) show two catalytic activity of NCSWC-4-800 could be attributed to its
peaks at around 24.61 and 43.71, which are attributed to the largest mesopore and macropore volumes. To examine the roles
002) and (100) planes of graphite, respectively. The intensity of of NCSWCs-800 and TBHP in this reaction, HMF oxidation was
(
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Table 1 Catalytic activity of various catalysts in HMF oxidation
Selectivity (%)
DFF
Samples
HMF Conv. (%)
HMFCA
FFCA
FDCA
FFCA yield (%)
NCSWC-1-800
NCSWC-2-800
NCSWC-3-800
NCSWC-4-800
NCSWC-4-600
NCSWC-4-950
NCSWC-4-800-M
NMC-800
79.1
86.7
85.4
92.3
67.2
75.7
62.5
58.6
32.3
38.7
8.3
7.2
7.9
1.1
1.2
1.6
1.4
1.4
1.1
1.0
1.7
0
89.2
90.1
88.6
90.5
74.7
83.3
67.3
61.8
0
1.1
1.2
1.5
1.9
0.9
1.6
1.3
0
70.6
78.1
75.7
83.5
50.2
63.1
42.1
36.2
0
5.8
22.1
13.1
29.6
35.5
7.2
No catalyst
0
0
a
NCSWC-4-800
88.7
2.1
8.6
3.3
ꢀ
1
Reaction conditions: 10 mL HMF aqueous solution (0.05 mol L ), 50 mg catalyst, 70 wt% TBHP aqueous solution (TBHP/HMF = 4, molar ratio),
a
2
reaction temperature = 100 1C, reaction time = 8 h. 1 MPa O instead of TBHP.
first conducted using TBHP as the oxidant without catalysts, the and NCSWC-4-800 in Fig. S9 (ESI†), while there is no correlation
HMF conversion can reach 32.3%, but only DFF as the desired between FFCA yield and the content of pyridinic N species. Since
oxidation product with very low selectivity (7.2%) was obtained, the textural properties and morphology of these NCSWCs are
as shown in Table 1. HMF oxidation was also conducted by using similar as shown in Table S2 (ESI†), it can be concluded that the
1
2
MPa O to replace TBHP over NCSWC-4-800, and 38.7% HMF graphitic nitrogen species play a vital role in NCSWCs for HMF
conversion can also be achieved, with DFF, as the main product, oxidation to FFCA. In addition, NMC-800 was also prepared as
as well as the other desired HMF oxidation products, such as the reference catalyst and used for this reaction under identical
HMFCA, and FFCA, obtained without producing any degradation conditions. As shown in Table S2, S3 and S10 (ESI†), NMC-800 is
products. Furthermore, if TBHP was added in this reaction with a mesoporous material with a high surface area and large
the TBHP/HMF molar ratio of 2, the HMF conversion was mesopore volume and its graphitic N content is 5.96%, which
increased to 65.9% and FFCA became the main product with is higher than that in NCSWC-4-950. However, the FFCA yield
the yield of 38.5%. When the TBHP/HMF molar ratio reached 6, over NMC-800 is even lower than that over NCSWC-4-800-M. It is
although full HMF conversion can be achieved, the FFCA yield is suggested that the wrinkled cages in NCSWCs are much more
only 73.4%, because degradation products were produced. These beneficial for HMF oxidation than the common N-doped meso-
results suggested that the HMF oxidation to FFCA in this work porous carbon.
was catalyzed by the synergy between NCSWCs and TBHP.
We also performed DFT calculations on the reaction path-
Furthermore, since different types of the doped N atoms have way of HMF oxidation to FDCA. HMF oxidation is initiated by
a distinct influence on the catalytic performance of the carbon- the cleavage of the O–O bond of TBHP to produce hydroxyl
ꢂ
ꢂ
25
based catalysts, NCSWCs with different contents of graphitic and ( OH) and tert-butoxy ((H
3
C)
3
CO ) radicals. However, it is
ꢂ
ꢂ
pyridinic N species, such as NCSWC-4-800-M, NCSWC-4-600, found that the OH radicals can react with TBHP to form OOH
NCSWC-4-950, and NCSWC-4-800, were used as the catalysts radicals, which are more preferred to be adsorbed on carbon
FFCA yield is atoms adjacent to the graphitic N atoms of NCSWCs, as shown
linearly proportional to the content of graphitic nitrogen species in Fig. 2. Since OOH radicals are not stable when HMF
9
,10,24
for this reaction to identify the active sites.
ꢂ
ꢂ
in the order of NCSWC-4-800-M, NCSWC-4-600, NCSWC-4-950, molecules are approaching to NCSWCs, the OOH radical
Fig. 2 DFT calculations on the reaction pathway of HMF oxidation to FDCA over NCSWC-4-800 under base-free conditions by using TBHP as the
oxidant.
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ꢂ
ꢂ
cleaves into O and OH radicals adsorbing on carbon atoms
JZ and HT gratefully acknowledge the financial support
ꢂ
adjacent to the graphitic N atoms. The OH radicals on from the research fund (518047 and 519015) from Guangdong
NCSWCs oxidize the –OH group of the HMF molecule to form University of Petrochemical Technology and the Natural Science
ꢂ
CQO and H O, while the O radicals attack the –CH – to Foundation of Guangdong Province (2018A030307053), China.
2
2
ꢂ
produce QCH– and OH radicals, and then DFF is obtained VP acknowledges the department of atomic energy (DAE),
in the reaction pathway of TS1. Alternatively, the O radicals government of India (R&D-TFR-RTI4003) for funding.
ꢂ
alone attack the –HCQO to form HMFCA in the reaction
pathway of TS2. However, the energy barrier (E
a
) of TS1 is
Conflicts of interest
1.24 eV, lower than that of TS2, indicating that the reaction
pathway TS1 was kinetically favoured. Our experimental results
also confirmed that the reaction pathway for HMF oxidation
was HMF-DFF-FFCA because the yield of HMFCA was very low.
There are no conflicts to declare.
Notes and references
ꢂ
Subsequently, the O radicals adsorbed on carbon atoms adja-
1
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to FDCA; if the TBHP/HMF molar ratio is larger than 4, then
5
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ꢂ
ꢂ
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7
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ꢂ
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and FFCA selectivity remained almost unchanged during
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5
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1
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14
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2
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2008 | Chem. Commun., 2021, 57, 2005ꢀ2008
This journal is The Royal Society of Chemistry 2021