A metal-free, high nitrogen-doped nanoporous graphitic carbon catalyst for an effective aerobic HMF-to-FDCA conversion

Article abstract of DOI:10.1039/c6gc02118b

We report a metal-free catalysis of the aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acids (FDCA), employing zeolitic-imidazole framework (ZIF-8) derived, nitrogen-doped nanoporous carbon (denoted as NNC) as an effective heterogeneous catalyst. The effect of high graphitic nitrogen loading in the NNC on the catalytic production of FDCA was demonstrated and discussed.

Full text of DOI:10.1039/c6gc02118b

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DOI: 10.1039/C6GC02118B.
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A Metal-Free, High Nitrogen-Doped Nanoporous Graphitic Carbon
Catalyst for an Effective Aerobic HMF-to-FDCA Conversion
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
b
Chi Van Nguyen, Yu-Te Liao, Ting-Cih Kang, Jeffrey E. Chen, Takuya Yoshikawa, Yuta
b
b
a
Nakasaka, Takao Masuda, and Kevin C.-W. Wu *
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report a metal-free catalysis of aerobic oxidation of 5- systems, could cause not only difficult purification of products
2
3
hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acids but also serious environmental pollution. For economic cost
FDCA), employing zeolitic-imidazole framework (ZIF-8) derived, and green chemistry, the development of metal-free and
(
nitrogen-doped nanoporous carbon (denoted as NNC) as the effective heterogeneous catalyst for the aerobic oxidation of
effective heterogeneous catalyst. The effect of high graphitic HMF to FDCA is extremely important.
nitrogen loading in the NNC on the catalytic production of FDCA
was demonstrated and discussed.
Nitrogen-doped carbon materials (NCs) have attracted
2
4-26
27-29
enormous interest in electrochemistry
and catalysis
,
owing to its easy preparation process, low cost, and
outstanding catalytic performance. NCs are commonly
synthesized through the post-treatment of previously
The sustainable production of transportation fuels and
value added chemicals from renewable biomass resources has
1
-3
3
0
attracted great attention due to diminishing fossil resources.
3
prepared porous carbon with a nitrogen source (e.g. NH ),
Highly valued chemical components such as 5-
hydroxymethylfurfural (HMF) and its derivative furan
but this typically involve the drawbacks of lower nitrogen
concentration and non-uniform nitrogen distribution.
Recently, the synthesis of metal-organic frameworks (MOFs)
inspires new approaches for making NCs. By using nitrogen
compounds
(i.e.
2,5-dimethylfuran
(DMF),
2,5-
furandicarboxylic acid (FDCA), and γ-valerolactone (GVL))
converted from lignocellulosic biomass, have been
(N)-containing organic ligands (i.e. imidazoles), zeolitic
4
-9
demonstrated through various catalytic processes. Among
these biochemicals, 2,5-furandicarboxylic acid (FDCA) has been
regarded as an important bio-based monomer for synthesizing
imidazole frameworks (ZIFs) with uniform and high N loading
3
1
have been synthesized. For example, ZIF-8 can be formed
using zinc ions as metal nodes and 2-methylimidazole as
organic ligands, and its preparation can be easily done in
1
0-14
valuable polymers,
PEF), which is considered to be a promising alternative to the
most abundantly used polymer, polyethylene terephthalate
especially polyethylene furanoate
3
2, 33
(
aqueous or organic solvent systems.
Like other MOFs
materials, ZIF-8 exhibits high specific surface area and well-
defined morphology. In addition, after careful thermal
treatments, one can easily remove metal ions and carbonize
the framework, while retaining nitrogen in the framework to
result in nitrogen-containing nanoporous carbon (NNC)
(PET), for bottle production.
FDCA has been commonly oxidized from HMF by using
1
diverse catalysts, including stoichiometric oxidants,
5
1
homogeneous metal salts, enzyme, and heterogeneous
6
17
1
8-21
22
or metal oxide catalysts. Among them, noble metal
3
4-36
metal
nanoparticles.
Unlike conventional NC using post-
catalysts (i.e. Pd, Au, Pt, and Ru) have been widely used as
treatment to introduce nitrogen into carbon framework, ZIF-8
derived NNC materials directly implement nitrogen during
synthesis, thus enabling the encapsulation of nitrogen into
carbon framework with high uniformality and quantity. We
and others have previously applied functional NNC materials
2
3
efficient catalysts in various catalytic systems. However, the
high costs of employing noble metal catalysts greatly inhibit
development in industrial applications. Additionally, the
leakage of metal ions from metal-based catalysis into reaction
3
7
for catalytic reactions
3
and capacitive deionization of
8
wastewater. Since many nitrogen-doped carbon materials
have so far been reported as efficient catalysts for selective
3
9
oxidation of hydrocarbon such as benzylic alcohol,
4
0
41
42
ethylbenzene, xanthene, and Cumene, we propose that
functional NNC materials could also be an alternative
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promising catalyst to replace noble metal catalysts in the 70-95%) in these NNC samples, which has been regarded as
oxidation of HMF to FDCA.
electrochemically active sites for various electrochemical
3
Herein, we report the first metal-free catalysis of HMF-to- applications, such as supercapacitors and Li ion batteries.
5, 36
FDCA conversion using ZIF-8-derived NNC nanoparticles as the However, we found that the amount of quaternary-N
solid catalysts. Typically, ZIF-8 with controllable particle sizes increased from 4% to 25% as calcination temperature
o
O and 2-methylimidazole in increased from 600 to 900 C, respectively. Quaternary-N (N-
can be prepared from Zn(NO
3
)
2
·6H
2
methanol at room temperature, as modified from a previous Q) specie is regarded as graphitic N, and can serve as a
3
6
report. The prepared ZIF-8 samples were then calcined under catalytic active site for aerobic oxidation owing to enhanced
o
2
N atmosphere at various temperatures from 600 to 900 C to reaction rate. The detailed mechanism has been explained
2
2
optimize the corresponding specific surface area, pore size, previously, where sp N-O adducts or oxygen radicals are
and graphitic carbon framework. Residual zinc was later formed by the adsorption of molecular oxygen on graphitic N
removed with concentrated HCl prior to an activation process or on atomic nitrogen and adjacent carbon, thus becoming
o
at 150 C in vacuum for 5 hrs. The final samples were named active sites for oxidation.
27, 39
NNC-X, where X represents the carbonization temperatures of
o
00, 700, 800, and 900 C.
Conversion of HMF to FDCA was conducted using various
6
catalysts. As shown in Fig. 1a and Table S3, no FDCA
The crystalline structure and surface morphology of the production was observed when Zn(NO ) ·6H O, 2-
3
2
2
synthesized parent ZIF-8 and NNC materials were methylimidazole, activated carbon (AC), graphite, ZnO, and
characterized with X-ray diffraction (XRD) and scanning ZIF-8 were used as catalysts, despite the 100% conversion of
electron microscopy (SEM). The XRD patterns of the HMF to other products. We suggest that this was owing to
synthesized parent ZIF-8 and its corresponding NNC degradation of HMF in the basic environment of our reaction
nanoparticles were in good agreement with those that were system. In contrast, FDCA was produced in the presence of the
3
6
previously reported in literatures (Fig. S1-S2†). A broad peak NNC catalysts. For instance, FDCA yields of 11%, 35%, 72%, and
o
at approximately 2θ = 24 was observed and its intensity 80% were observed with NNC-600, NNC-700, NNC-800, and
increased with increasing calcination temperature, which NNC-900 as the catalyst, respectively. Surprisingly, FDCA yield
4
3
indicates the content of graphene in the NNC samples.
increased significantly corresponding to the increase of
Additionally, we did not observe any diffraction peaks for Zn or calcination temperature on NNC samples. Because it has been
ZnO due to minimal amounts of residual Zn and ZnO in the known that the graphitic nitrogen plays a main role for aerobic
2
7
samples. Another evidence is provided by inductively coupled oxidation, here we analyzed the configuration of nitrogen
plasma mass spectrometry (ICP-MS) where the amount of Zn species in the NNC-X samples.
species was measured to be only around 3-8wt% (Table S1†).
The nanoparticular morphology of the prepared ZIF-8 samples
(with an average diameter of around 150-200 nm) is well
remained even after calcination at different temperatures and
washing with HCl acid, as shown in SEM images (Fig. S3-S4†).
The amounts of nitrogen and the porous properties of the
synthesized NNC materials were determined by elemental
analysis (EA) and nitrogen adsorption/desorption isotherms,
respectively. Because our catalysts do not contain any metal,
these two properties are important to the catalytic activity of
the samples. As shown in Table S1†, the nitrogen content
decreased from 29.4, 27.7, 21.6 to 15.3wt% as the calcination
o
temperature increased from 600, 700, 800 to 900 C,
respectively. This phenomenon can be explained by the
collapse of the nitrogen-containing 2-imidazole groups, thus
4
4
resulting in the reduction of nitrogen content.
This
phenomenon is also suggested to be the reason for the
increased specific surface area of the NNC samples upon
2
-1
carbonization (from 600 to 800 m g as the calcination
o
temperature increased from 600 to 900 C, respectively) with
the same mircopor size of 0.6 nm (Table. S2 and Fig S5-S8†).
The bonding configurations of N atom in the NNC samples
Figure 1: a) The effect of different catalysts on the HMF-to-FDCA conversion. The
were examined with X-ray photoelectron microscopy (XPS). As conversions were 100% for all cases after reaction for 48 hr. b) The relationship
between the yield of FDCA and the N-Q atomic percentage on the NNC-X samples.
shown in Fig. S9 and Table S1†, the N-1s peak can be
Reaction condition: 0.63 mmol HMF in 10 mL water, 0.2 g catalysts, the molar ratio
deconvoluted to three peaks representing pyridinic-N (N-6) at
-
1
o
2 3
HMF/K CO of 1:3, 100 mL·min oxygen at ambient pressure, T=80 C, time =48 hr.
3
98.4±0.2 eV, pyrrolic-N (N-5) at 399.8±0.2 eV, and
4
5
quaternary-N (N-Q) at 401.1 ±0.2 eV. The pyridinic-N and
pyrrolic-N parts (i.e. N-5 + N-6) were predominant (i.e. around
2
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The FDCA yields versus three types of N-1s were plotted in
Fig. 1b and Fig. S10†. It is recognized that the FDCA yield is
proportional to the amount of the graphitic nitrogen (N-Q) in
NNC carbon framework, but there are no connections
between N-6 or N-5 and FDCA production. Consequently, we
can conclude that graphitic nitrogen is the active site for
catalyzing the HMF-to-FDCA conversion. Similar to various
aerobic oxidation where N-doped carbons were used as the
2
7, 39
catalyst,
we suppose that the mechanism of the aerobic
oxidation is also through a radical process, where the N-Q sites
can help generate oxygen radicals for oxidation. To further
prove this hypothesis, we added 1,4-benzoquinone as a radical
scavenger in the reaction system and found that the
production of FDCA was greatly inhibited with the presence of
such an anti-radical agent in the system (Fig. S11†)
The reaction route of HMF-to-FDCA conversion using our
metal-free catalysts is studied. In general, as shown in Fig. 2a
.
,
two favourable pathways for aerobic oxidation of HMF to
FDCA with the presence of heterogeneous catalysts have been
2
3, 46, 47
reported.
According to the pathway, FDCA can be
Figure 2: a) possible pathway of converting HMF to FDCA. Reaction condition: 0.63
obtained from FFCA (5-formyl-2-furancarboxylic acid)
2 3
mmol HMF in 10 mL water, 0.2 g NNC-900 catalyst, the molar ratio HMF/K CO of 1:3,
-
1
o
generated from HMF with either DFF (2,5-diformylfuran) or 100 mL·min oxygen at ambient pressure, reaction temperature of 80 C; b) The kinetic
HMFCA (5-hydroxymethyl-2-furancarboxylic acid) as the reaction study.
intermediates. Here, the kinetics of the HMF-to-FDCA
conversion using the NNC-900 catalyst was studied. As shown
One of the most important advantages of using
in Fig. 2b and Fig. S12†, it can be seen that when HMF was heterogeneous catalysts is its recoverability and reusability.
consumed, some intermediates including HMFCA and FFCA The recycle test of the NNC-900 catalyst was thus studied. The
o
were generated and peaked in amounts after eight hours of aerobic HMF-to-FDCA oxidation was carried at 80 C using 0.2
reaction. As reaction time was further increased, these g NNC-900 catalyst, in the molar ratio K CO /HMF of 1:3 (i.e.
3
2
-1
intermediates were then consumed to produce FDCA. It is 0.63 mmol HMF). A flow rate of 100 mL·min oxygen gas was
worth noting that no DFF intermediate was observed, which purged into the mixture reaction with stir for 48 hr. After each
indicate that the reaction pathway in this study would be HMF- run, the catalyst was separated from the reaction mixture by
to-HMFCA-to-FFCA-to-FDCA, as shown in Fig. 2a. We also centrifugation, and washed several times with deionized water
found that the consumption of HMF was accelerated as the and methanol. The sample was then dried under vacuum at
o
reaction temperature was increased, and a 100% consumption 150 C for 5 h prior to subsequent experiments. As shown in
o
of HMF could be achieved within 4 hours at 80 C (Fig. 2b). This Fig. 3, it is found that the catalytic activity of NNC-900 could be
result indicated that the HMF-to-HMFCA conversion is the fast reused several times without signifcantly losing activity (i.e.
step. In contrast, the consumption of HMFCA and FFCA were the yield of FDCA was 80% initially and 70% after the fourth
relatively slow, resulting in a long reaction time (i.e. 48 hours) run). The spent NNC-900 catalyst was characterized by XRD
for the final production of FDCA. These results were in good and XPS, and we found that there was no difference between
agreement with a previous report that proposed the FFCA-to- the crystalline structures but with slightly decreased N-Q
FDCA conversion as the rate-limiting step of HMF-to-FDCA amounts (Fig. S13 and S14† and Table S1†). It is strongly
4
8
o
conversion. A higher reaction temperature over 80 C was confirmed that the N-Q configuration of NNC catalysts played
not preferred due to a unstable structure of HMF at such a an important role for aerobic oxidation of HMF to FDCA.
4
8
high temperature and basic environment.
Although nitrogen-doped carbon has been used in HMF
2
7
oxidation, we conclude that the NNC samples prepared by
our method would exhibit higher amounts of the N-Q structure
in the carbon framework, resulting in enhanced catalytic
performance. The FDCA yield obtained here using a metal-free
catalyst is compatible with those previous results using metal-
containing catalysts (Table S4). Additionally, the effect of the
base used in this study (K
yields was studied. As shown in the Fig. S15, the total yields of
all products increased when the amount of K CO was
decreased. Additionally, the highest yield of FDCA was not
resulted from the highest amount of K CO (i.e. HMF/K CO
CO = 1/3. Thus, we
2 3
CO ) on the product distribution and
2
3
2
3
2
3
=
1
/1) but an optimal ratio of HMF/K
2
3
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conclude that an optimized molar ratio of HMF/base is
necessary and important.
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Figure 3: Recycle test of the NNC-900 catalyst. Reaction conditions: 0.63 mmol HMF in
-1
1
2 3
0 mL water, 0.2 g NNC-900 catalyst, the molar ratio HMF/K CO of 1:3, 100 mL·min
o
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materials with low nitrogen-loading density, the ZIF-8 derived
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loading amount of nitrogen, especially for the formation of
1
2
2
2
9
0
1
2
1
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o
graphitic nitrogen (N-Q) after calcination at 900 C. The
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be useful for catalyzing aerobic oxidation of HMF to FDCA with
o
a maximum yield of 80% at 80 C and ambient pressure. The
pathway of HMF-to-FDCA conversion is also estimated, where 23 Z. Zhang and K. Deng, ACS catal., 2015, 5, 6529-6544.
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would have much potential in not only biomass conversion but
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The authors would like to thank the Ministry of Science and
Technology (MOST), Taiwan (104-2628-E-002-008-MY3, 105-
2221-E-002-227-MY3, and 105-2218-E-155-007) and the
National Taiwan University (105R7706) for the funding support.
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DOI: 10.1039/C6GC02118B
GraphicalꢀAbstractꢀ
AꢀMetal-Free,ꢀHighꢀNitrogen-DopedꢀNanoporousꢀ
GraphiticꢀCarbonꢀCatalystꢀforꢀanꢀEffectiveꢀAerobicꢀHMF-
to-FDCAꢀConversionꢀ
Chiꢀ Vanꢀ Nguyen,ꢀ Yu-Teꢀ Liao,ꢀ Ting-Cihꢀ Kang,ꢀ Jeffreyꢀ E.ꢀ
Chen,ꢀTakuyaꢀYoshikawa,ꢀYutaꢀNakasaka,ꢀTakaoꢀMasuda,ꢀ
andꢀKevinꢀC.-W.ꢀWu*ꢀꢀ
Nanoporousꢀ carbonꢀ nanoparticlesꢀ withꢀ highꢀ graphiticꢀ
nitrogenꢀamountsꢀwereꢀsynthesizedꢀandꢀusedꢀasꢀaꢀmetal-
freeꢀcatalystꢀforꢀeffectiveꢀHMF-to-FDCAꢀconversion.ꢀꢀ