Acidic ion functionalized N-doped hollow carbon for esterification of levulinic acid

Article abstract of DOI:10.1039/c9nj04752b

Acidic ion functionalized N-doped hollow carbon (NHC-[C₄N][SO₃CF₃]) has been successfully synthesized by quaternization of N-doped hollow carbon (NHC) with 1,4-butanesultone, followed by ion exchange with trifluoromethanesulfonic acid. The catalyst was characterized by Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), acid-base titration techniques and other methods. A hollow spherical catalyst with regular morphology, high acid density (2.72 mmol g^-1) and good stability was obtained. Various characterizations showed that NHC-[C₄N][SO₃CF₃] possesses abundant nanopores (3.41 nm), large Brunauer-Emmett-Teller (BET) surface area (154 m² g^-1) and strong and controllable Br?nsted acid sites. The as-prepared NHC-[C₄N][SO₃CF₃] was then used for the acid-catalyzed esterification of levulinic acid with ethanol. At the optimum conditions, the highest conversion of levulinic acid reached 94.17% and high conversion was maintained after recycling four times. Further investigation of the catalytic activity in the esterification of different aliphatic and aromatic alcohols with levulinate was carried out, showing good results.

Full text of DOI:10.1039/c9nj04752b

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DOI: 10.1039/C9NJ04752B
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Acidic ion functionalized N-doped hollow carbon for esterification
of levulinic acid
Received 00th January 20xx,
Accepted 00th January 20xx
Qifang Zhang, Pingping Jiang*, Zhixin Nie, Pingbo Zhang
Acidic ion functionalized N-doped hollow carbon (NHC-[C4N][SO3CF3]) has been successfully synthesized by quaternization
of N-doped hollow carbon (NHC) with 1,4-butanesultone, followed by ion exchange with trifluoromethanesulonic acid. The
catalyst was characterized by fourier transform infrared (FT-IR), scanning electron microscopy (SEM), acid−base titration
techniques and other methods. And a hollow spherical catalyst with regular morphology, high acid density (2.72 mmol/g)
and good stability was obtained. Various characterizations showed that NHC-[C4N][SO3CF3] possesses abundant nanopores
(3.41 nm), large Brunauer–Emmett–Teller (BET) surface areas (154 m2·g-1) and strong and controllable Brønsted acid sites.
The as-prepared NHC-[C4N][SO3CF3] was then used for the acid-catalyzed esterification of levulinic acid with ethanol. At
the optimum conditions, the highest conversion of levulinic acid reached 94.17% and high conversion was maintained
after four recycles. Further investigation of the catalytic activity in the esterification of different aliphatic and aromatic
alcohols with levulinate was carried out, showing good results.
DOI: 10.1039/x0xx00000x
KEYWORDS: Catalyst preparation; Esterification ； N-doped hollow carbon ； Acidic ion ； Levulinic acid
Conventional esterification catalysts, such as concentrated
sulfuric acid, phosphoric acid, p-toluenesulfonic acid and other
homogeneous acid catalysts suffer from many problems
including side reactions, corrosion of equipment, difficulty in
separation and reuse 16,17, which does not meet the principle
of "green and sustainable chemistry". Therefore, it has
become a top priority to seek and explore green and efficient
catalysts to replace traditional catalysts. In recent years, due to
1. Introduction
Due to gradual depletion of fossil resources and the increasing
demand for resources, the transformation and utilization of
1,2
renewable resources have become the focus of global
.
Biomass is regarded as one of the renewable resources to
replace fossil fuels. Researchers pay more and more attention
to the high value-added chemicals and platform compounds
prepared by biomass 3. Among them, levulinate esters are
the
recyclability
and
environmental
friendliness,
heterogeneous solid acid catalysts such as sulfonated metal
very important and valuable platform compounds 4, which can
18-20
21
oxides
,
functionalized organometallic skeleton
,
,
7,8
undergo hydrolysis 5,6, reductive amination
and other
22
23-25
sulfonated SBA-15
,
sulfonated carbon catalysts
reactions. Therefore, levulinate esters are widely used in
perfumes, plasticizers, biofuels, etc 9,10. Especially, as a short-
chain fatty ester, ethyl levulinate (EL) has been used as a new
hybrid biofuel of diesel and gasoline because of its similar
properties to biodiesel, and it is considered to be one of the
most stable second-generation biofuels 11-13. EL can also be used
as a chemical raw material in the perfume industry and a
substrate for various organic condensation and addition
reactions 14,15. Since LA is readily hydrolyzed from biomass,
more and more attention has been paid to the green process
of esterification of LA with ethanol. Compared with cellulose
hydrolysis, EL prepared by esterification can obtain higher
conversion and yield at lower temperature and short time.
26
sulphonated polystryne catalysts
and functionalized ionic
27
liquids
have received extensive attention. Among them,
carbon materials are widely used in adsorption, catalysis and
electrochemistry because of their many advantages, such as
low density, high thermal stability, low cost and non-toxicity
28,29
.
Owing to the high acid density, sulfonic acid
functionalized carbon materials can be used in esterification,
transesterification and hydration as effective solid acid
catalysts. However, the bulk sulfonated carbon-based solid
acid catalysts have very small BET surface area, resulting in low
acidity and poor acid site accessibility, thereby limiting its
catalytic activity 30. These defects can be adjusted by designing
the morphology and pore structure of the carbon-based solid
acid, which can improve the mass transfer rate and increase
the contact between the reactants and active sites. In addition,
the sulfonation of active groups plays an important role in the
performance of catalysts. Sulfonation reagents such as
concentrated sulfuric acid can reduce the stability of catalysts,
and lead to the random distribution and non-differential
a. Key Laboratory of Synthetic and Biological Colloids, School of Chemistry and
Materials Engineering, Jiangnan University,1800 Lihu Street, Wuxi 214122,China.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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formation of acid active sites on the surface of catalysts. of trifluoromethanesulonic acid at room tempVeirewatAurtricele Ownilitnhe
However, selective deposition of -SO3H on the edge of carbon vigorous stirring for 8 h. The final sample was centrifuged and
material with large specific surface area can improve the washed several times with dichloromethane, and dried
accessibility of large volume molecular reactants to acidic overnight in an oven at 60 °C, named as NHC-[C4N][SO3CF3].
DOI: 10.1039/C9NJ04752B
sites.
2.3 Catalyst characterization
Herein, we propose a simple and efficient strategy to
introduce strong Brønsted acid sites into N-doped hollow
carbon materials. Carbon spheres with porous and regular
shell-core structure were prepared by classical StÖber method,
and then high acidity and controllable solid acid catalyst was
obtained by quaternization and ion exchange treatment. The
catalyst was used for esterification of levulinic acid with
ethanol, showing excellent catalytic activity and reusability.
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FT-IR spectra were recorded on a VECTOR22 type FT-IR
spectroscopy analyzer in the 4000 ～ 500cm−1 region. Raman
Spectra were obtained on a inVia micro-confocal Raman
spectrometer. XRD spectra were measured by Bruker D8-
Advance X-ray diffractometer (CuKα, λ=0.15418 nm, tube
voltage 40 kV. Tube current 40 mA, scan angle 10°～90°, scan
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rate
4 (°)/min, scan step length 0.02°). Nitrogen (N2)
adsorption isotherms and Brunauer–Emmett–Teller (BET)
surface areas were measured by using a ASAP2020 MP
automatic surface area and microporous physical adsorption
instrument, and the samples were degassed at 150 °C for 6 h
before analysis. The pore size distribution was calculated using
BJH method. XPS was performed on American Thermo Fisher
Scientific X-ray photoelectron spectroscopy analysis. TEM
images were obtained by using a JEOL JEM-2010 plus
transmission electron microscope. The microstructures and
element distribution of the samples were collected on Japan
Hitachi Co., Ltd. S-4800 Field Emission Scanning Electron
Microscope (SEM) and Energy Spectrometer (EDS).
The acid-exchange capacity was determined by dispersing
catalysts in aqueous solution of NaCl through a typical acid-
base titration with NaOH solution 32. The measurement
procedure was as follows: 0.15 g of catalyst was added to 20
mL NaCl solution (0.1 mol/L), stirred at room temperature for
24 h, and separated to obtain filtrate. Then the filtrate was
titrated to neutral with 0.01 mol/L NaOH solution and the
volume of NaOH solution consumed was recorded to calculate
the total acid density.
2. Experimental
2.1 Materials
Tetraethoxysilane (TEOS), formaldehyde, melamine, 1,4-
butane sultone, trifluoromethanesulonic acid (HSO3CF3) were
purchased from Adamas, levulinic acid was purchased from
Macklin，resorcinol, ammonia, anhydrous methanol, toluene,
dichloromethane, anhydrous ethanol, N-pentanol, Octanol,
Benzyl alcohol, 1,2-propanediol were purchased from
Sinopharm Chemical Reagent Co., Ltd. All of the reagents were
used without further purification.
2.2 Catalyst Preparation
Synthesis of core-shell SiO2@resorcinol-formaldehyde
nanospheres (SiO2@RF). Core-shell SiO2@RF nanospheres
were prepared by two-step StÖber method 31. In a typical
synthesis, 10% ammonia solution (20 mL) was slowly added to
150 mL of anhydrous ethanol, then tetraethoxysilane (TEOS,
3.4 mL) was added dropwise, stirring at room temperature for
1 h, 0.6 g of resorcinol and 0.6 mL of formaldehyde solution
were added under stirring next. Afterwards, the brown
solution was transferred to a 200 mL polytetrafluoroethylene
high temperature reactor and reacted in an oven at 100 °C for
12 h. After cooling to room temperature, orange solid was
separated by centrifugation and dried overnight at 80 °C.
Synthesis of N-doped hollow carbon spheres (NHC). 0.5 g of
SiO2@RF and melamine (0.3 g) were dispersed in 3 mL of
anhydrous methanol, stirred for several hours until methanol
was completely evaporated at room temperature. The
prepared power melamine/SiO2@RF was calcined at 600 °C
for 1 h under nitrogen stream in the tube furnace, and the
heating rate was 5 °C/min. Then the product was etched with
10 wt% HF to remove SiO2 totally, washed with deionized
water to neutrality. The black power was dried at 70 °C and
recorded as NHC.
2.4 Catalytic Reaction Procedure.
Esterification of levulinic acid with ethanol was carried out in a
round-bottomed flask equipped with a reflux condenser, a
magnetic stirrer and an oil bath. Typically, 1 g of levulinic acid,
7.5 mL of ethanol and 0.05 g of catalyst were added into a 25
mL flask, and a small amount of the solution was taken to
determine the initial acid value of the system, then heated to
70 °C and the reaction time was recorded. Then the acid value
of the system was determined every hour until it remained
unchanged. And the acid value of the solution was determined
according to GB/T5530-2005.
3. Results and discussion
3.1 FT-IR
Synthesis of acidic ion functionalized N-doped hollow carbon
(NHC-[C4N][SO3CF3]). The above 0.3 g of NHC was dispersed
into 20 mL of toluene containing 0.1 g of 1,4-butane sultone,
stirring at room temperature for 1 h. After stirring at 115 °C for
8 h under refluxing, the mixture was cooled to ambient
temperature, the resultant product was collected by
centrifugation and dried at 60 °C. Afterwards, the sample was
treated with a mixture containing 25 mL of toluene and 1 mL
Fig. 1 shows the FT-IR spectra of the NHC and NHC-
[C4N][SO3CF3]. It can be seen from Fig.1 that the two samples
exhibit at 3436 cm-1, which assigns to –OH stretching vibration
peaks. And the stretching vibration peaks of -CH2, C=C and C-N
appear at 2919, 1630 and1401 cm-1 33, respectively. While the
peaks appearing at 1260 cm-1 is associated with C-F stretching
vibration peak. Moreover, around 1179 and 1034 cm-1 are
attributed to symmetric and asymmetric stretching vibration
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of S=O, which are from the -SO3CF3 group 34,35. The peaks of N and N+-H are derived from the NHC structure and N+-C is
the C-F bond and the S=O bond indicated that the strong acid derived from the nitrogen atom in the NHC quaternized by 1,4-
ion is successfully introduced to the NHC carrier. Compared butane sultone. Similarly, the signals of S 2p associated with S
with the fresh catalyst, the catalyst after four recycles not only 2p1/2 (168.1 eV) and S 2p3/2 (169.4 eV) can also be clearly
View Article Online
DOI: 10.1039/C9NJ04752B
has the characteristic peaks of the carrier skeleton, but also observed (Fig. 2d) 39
the characteristic peaks of the active group,indicating that the 3.3 Raman
active group of the catalyst is still stable after reusing for four
.
The Raman spectra of NHC and NHC-[C4N][SO3CF3] are
depicted in Fig. 3. The both samples show two broad signal
peaks at around 1331 and 1574 cm-1 corresponding to the D-
band and the G-band, respectively 40. The D-band is attributed
to the SP3 hybridized graphite carbon atoms, which reflects the
defects and disordered structures in the carbon material. The
G-band belongs to the SP2 carbon atoms 29,41. The degree of
graphitization of the sample is usually expressed by the
intensity ratio of ID/IG 42. NHC-[C4N][SO3CF3] exhibits higher
intensity ratio (ID/IG=1.043) as compared with 0.982 of pure
NHC. This result reflects the lower degree of graphitization or
disordered carbonaceous structure of the catalyst, originating
from the introduction of heteroatoms and indicating the active
group is successfully grated onto the NHC.
cycles.
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NHC
NHC-[C4N][SO3CF3]
NHC-[C4N][SO3CF3]-used
C-H
C-N
C=C
S=O
OH
C-F
4000 3500 3000 2500 2000 1500 1000 500
-1
Wavenumber (cm )
Fig. 1. FT-IR spectra of the NHC, NHC-[C4N][SO3CF3] and used fourth times
catalyst.
G
D
3.2 XPS
Fig. 2 exhibits the XPS spectra of NHC-[C4N][SO3CF3]. As shown
in Fig. 2a, there are six obvious signal peaks in the XPS spectra,
corresponding to O 1s (532 eV), N 1s (399.7 eV), C 1s (283.7
eV) in the NHC skeleton 36 and F 1s (687.3 eV), S 2s (232.8 eV),
and S 2p (167.5 eV) from -[C4N][SO3CF3]. Among them, C 1s
can be deconvoluted and fitted into four peaks C-C (284.6 eV),
C-N (286.1 eV), C-S (287.7 eV), C-F (291.9 eV), which is due to
graft the -[C4N][SO3CF3] group onto the NHC support (Fig. 2b)
37. In addition, the three peaks obtained by the decomposition
of N 1s appear at 398.7 eV, 400.5 eV, and 401.4 eV, and which
can be attributed to C-N, N+-H, and N+-C (Fig. 2c) 38. Wherein C-
NHC
NHC-[C N][SO CF ]
4
3
3
1000 1200 1400 1600 1800 2000 2200
Raman shift (cm-1)
Fig. 3. Raman spectra of the NHC and NHC-[C4N][SO3CF3].
C 1s
NHC
(a)
(b)
NHC-[C N][SO CF ]
C-C
4
3
3
O 1s
CKLL
N 1s
OKLL
F 1s
C-N
C-S
C-F
S 2s
S 2p
1200 1000 800 600 400 200
0
296 294 292 290 288 286 284 282 280
Binding energy (eV)
Binding energy (eV)
(c)
(d)
N-C
S 2p3/2
N+-C
N+-H
S 2p1/2
174 172 170 168 166 164 162 160
408 406 404 402 400 398 396 394
Binding energy (eV)
Fig. 2. XPS spectra of survey of the NHC and NHC-[C4N][SO3CF3] (a), C 1s (b), N 1s (c), S 2p (d) of the NHC-[C4N][SO3CF3].
Binding energy (eV)
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3.4 Morphology
introduction of active group -[C4N][SO3CF3], which largely
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occupies and blocks the nanopores of caDtaOlyI:s1t0..1In03a9d/Cd9itNiJo0n4,7t5h2Be
total acidity of the samples measured by acid-base titration is
also exhibited in Table 1. The catalyst has a high acidity of 2.72
mmol/g, which plays an important role in the high catalytic
activity in the esterification.
Fig. 4 shows the SEM and TEM images of NHC and NHC-
[C4N][SO3CF3]. It can be seen from the Fig. 4a and d that the
prepared samples possess monodispersed, regular, uniform
and spherical nanostructure with no impurity on the surface.
Moreover, the core-shell structure of the samples can be
observed obviously from TEM images (Fig. 4b,c,e and f). And
the carrier NHC presents hollow nanospheres with outer
diameter about 250 nm and shell thickness around 20 nm.
After grafting the -[C4N][SO3CF3] group, the catalyst maintains
the same outer diameter as the carrier, but the shell thickness
increases by 5 nm. This interesting phenomena implies that
the -[C4N][SO3CF3] group is successfully introduced to the
surface of catalyst, resulting in thickening of the shell. In
addition, Fig. S1 exhibits the SEM images of the fourth time
used catalyst. As shown in Fig. S1, the used catalyst still
maintains a regular spherical structure, but a small amount of
Table 1. Physicochemical properties of NHC and NHC-[C4N][SO3CF3].
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Vd
b
Total aciditya
(mmol·g-1)
SBET
Dc
(nm)
sample
(cm3·g-
(m2·g-1)
1
)
NHC
-
608
154
4.91
3.41
2.12
0.75
NHC-[C4N][SO3CF3]
2.72
a Acid density measured by acid-base titration.
Fig. 4. SEM images of NHC (a) and NHC-[C4N][SO3CF3] (d),TEM images of NHC (b,c), NHC-[C4N][SO3CF3] (e,f).
b The data were obtained from N2 desorption results.
catalyst is broken. Generally speaking, the original spherical
morphology of the catalyst can be maintained after four
recycles. Moreover, the EDS images of the catalyst NHC-
[C4N][SO3CF3] are shown in Fig. S2. As can be seen that the
contents of C, N, O, S and F element are more evenly and
distributed on the catalyst.
c Average pore size.
d Total pore volume.
3.7 Catalytic Performance
Since the esterification of levulinic acid with ethanol is a
reversible reaction, the reaction should be shifted to the right
in order to improve the yield of ethyl levulinic acid. To this
end, increasing the amount of raw material levulinic acid or
ethanol is an effective means. However, considering that
ethanol is cheaper, easier to obtain and recycle than levulinic
acid, excessive ethanol is used to react. And the effects of
reaction time, catalyst dosage, molar ratio of acid to alcohol
and reaction temperature on the conversion of levulinic acid
3.6 Porosity Properties
The Fig. S3 shows the nitrogen adsorption/desorption
isotherms and pore size distribution of the samples, and the
corresponding data are shown in Table 1. The nitrogen
adsorption/desorption isotherms of the carrier and the
catalyst belong to the type IV isotherms, which is due to the
condensation accumulation of nitrogen in the pores of the
material and belongs to the mesoporous material 43. And a H4
type hysteresis loop appears in both samples at relative
pressures in the range of 0.45-0.95. Fig. S3b further illustrates
that the samples belong to mesoporous material with pore
size of 4.91 and 3.41 nm, respectively. It can be seen from
Table 1 that the BET surface area of the carrier NHC is 608
m2·g-1, while decreased to 154 m2·g-1 after grafting acidic
group. Compared with the NHC, the decreased BET surface
area, pore volume, and pore size can be originated from the
are demonstrated in the support information(Fig. S4-S7).
100
80
60
40
20
0
1
2
Cycling times
3
4
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Fig. 5. Effect of cycling times on the conversion of levulinic acid. Reaction
conditions: 1 g levulinic acid, 7.5 mL ethanol, 0.05 g catalyst , 7 h and 70 °C.
conditions (Fig. 6). It can be seen from Table 2 that the
prepared catalyst has high activity for the following reactions.
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DOI: 10.1039/C9NJ04752B
Also an interesting phenomenon is found: as the carbon chain
grows, the conversion of levulinic acid gradually decreases.
This may be attributed to the large molecule of long carbon
chain alcohols, resulting in inadequate access to the pores of
the catalyst and insufficiently contact with the catalyst.
Moreover, the conversion of levulinic acid with dihydric
alcohol is lower than that of monoalcohols, which is due to the
larger steric hindrance of dihydric alcohol.
The recyclability of the catalyst is an important factor in
affecting the performance of the catalyst. The stability of the
catalyst NHC-[C4N][SO3CF3] was tested under the optimal
experimental conditions. After the end of the reaction, the
catalyst was separated by centrifugation and washed with
ethanol several times, dried overnight in an oven at 80 °C, and
used for the next reaction. The results are exhibited in Fig. 5.
After cycling for four times, the conversion of levulinic acid can
still reach more than 80%, indicating that the catalyst has good
catalytic activity andreusability. The surface acid amount of
the catalyst was determined to be 1.33 mmol/g after the fourth
run, which may be due to the partial shedding of the active
group during many experiments and washing processes,
resulting in a decrease in catalytic activity.
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4. Conclusion
In this work, a strong acid ion functionalized N-doped carbon-
based solid acid catalyst with perfect unique hollow spherical
morphology was successfully prepared and studied the activity
in the probe reaction of levulinic acid with ethanol, as well as
the esterification reaction of levulinic acid and polyols,
alcohols with different chain lengths and aromatic alcohols,
which exhibited good catalytic activity. The unique hollow and
porous spherical structure contributes greatly to the excellent
performance of the catalyst, which provides the catalyst with
larger BET surface area (154 m2·g-1) and more acidic sites (2.72
mmol/g).
The prepared solid acid catalyst was used for the esterification
reaction of levulinic acid with ethanol. The optimum
conditions for the reaction are as follow: molar ratio of
levulinic acid to ethanol was 1:15, reaction temperature was
70 °C, the catalyst dosage was 5 wt.% of levulinic acid, the
Fig. 6. Catalytic esterification of levulinic acid with alcohols.(R: aliphatics or
phenyl; n:1-8; x: 1,2)
After the characterization and activity test of the catalyst was
completed, we next tested the esterification reaction of
levulinic acid with different alcohols to investigate the effects
of catalyst in the esterification of polyols, alcohols with
different chain lengths and aromatic alcohols in the same
Table 2. Esterification of levulinic acid with different alcohols.
Conversion（%
）
Entry
Acid
Alcohol
MeOH
Product
O
O
O
O
O
96.48
OH
OH
OH
OH
OMe
O
O
O
O
O
O
O
2
3
4
94.17
OH
O
O
OH
O
84.71
83.37
O
O
O
OH
O
O
O
O
OH
O
O
5
6
79.08
89.97
OH
OH
O
OH
OH
O
O
O
O
O
O
O
Reaction conditions: acid to alcohol molar ratio was 1:15, 0.05 g catalyst, 7 h and 70 °C
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24 S. Na, Z. Minhua, D. Xiuqin and W. Lingtao, RSC VAiedwv.A,rt2ic0le1O9n,lin9e,
conversion of levulinic acid reached 94.17%. And the
conversion of levulinic acid reached 83.57% after reusing for
four cycles.
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DOI: 10.1039/C9NJ04752B
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work was financially supported by the Fundamental
Research Funds for the Central Universities (JUSRP51623A),
the International Joint Research Laboratory for Biomass
Conversion Technology at Jiangnan University.
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View Article Online
DOI: 10.1039/C9NJ04752B
Graphical Abstract：Unique hollow carbon spheres with high acidity were used for efficient catalytic synthesis of
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ethyl levulinate with good reusability.
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