Efficient biomass transformations catalyzed by graphene-like nanoporous carbons functionalized with strong acid ionic liquids and sulfonic groups

Article abstract of DOI:10.1039/c4gc01052c

Strong acid ionic liquids and sulfonic group bifunctional graphene-like nanoporous carbons (GNC-SO₃H-ILs) have been synthesized by treating nitrogen containing graphene-like nanoporous carbons (GNCs) with 1,3-propanesultone, ion exchanging with HSO₃CF₃ or H₂SO₄. Introducing nitrogen is important for grafting strong acid ionic liquids and sulfonic group in GNCs, which were synthesized from carbonization of a mixture of dicyandiamide or melamine and glucose. GNC-SO₃H-ILs possess abundant nanopores, nanosheet structure, good dispersion and controlled acidity. By themselves, they are capable of enhancing the fast diffusion of reactants and products, while increasing the exposure degree of acidic sites in GNC-SO₃H-ILs throughout various reactions. The above characteristics resulted in their much improved catalytic activity in biomass transformations such as the production of biodiesel and depolymerization of crystalline cellulose into sugars, which was even comparable to those of homogeneous ionic liquid and mineral acids.

Full text of DOI:10.1039/c4gc01052c

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Efficient biomass transformations catalyzed by
graphene-like nanoporous carbons functionalized
with strong acid ionic liquids and sulfonic groups†
Cite this: DOI: 10.1039/c4gc01052c
Fujian Liu,a,b Weiping Kong,a Liang Wang,b Xianfeng Yi,c Iman Noshadi,d
Anmin Zheng*c and Chenze Qi*a
Strong acid ionic liquids and sulfonic group bifunctional graphene-like nanoporous carbons (GNC-SO3H-ILs)
have been synthesized by treating nitrogen containing graphene-like nanoporous carbons (GNCs) with
1,3-propanesultone, ion exchanging with HSO3CF3 or H2SO4. Introducing nitrogen is important for graft-
ing strong acid ionic liquids and sulfonic group in GNCs, which were synthesized from carbonization of a
mixture of dicyandiamide or melamine and glucose. GNC-SO3H-ILs possess abundant nanopores,
nanosheet structure, good dispersion and controlled acidity. By themselves, they are capable of enhan-
cing the fast diffusion of reactants and products, while increasing the exposure degree of acidic sites in
GNC-SO3H-ILs throughout various reactions. The above characteristics resulted in their much improved
catalytic activity in biomass transformations such as the production of biodiesel and depolymerization of
crystalline cellulose into sugars, which was even comparable to those of homogeneous ionic liquid and
mineral acids.
Received 7th June 2014,
Accepted 11th September 2014
DOI: 10.1039/c4gc01052c
www.rsc.org/greenchem
severe conditions.7,9 Surprisingly, crystalline cellulose easily
transformed into sugars in the presence of alkylmethylimida-
1. Introduction
Great effort has been made to develop efficient and cost zolium ionic liquid solvents because of their unique ability to
effective ways to transform biomass into biofuels, due to the break down the crystalline cellulose into soluble polymer
low cost, excellent renewability and environmental friendliness chains. However, the high cost of ionic liquids and the reac-
of biomass when compared with conventional fossil energy.1–9 tion mixtures’ acute recyclability create a demand for new cata-
Biomass may therefore be the most important feedstock for lysts with extremely high effectiveness. Researchers have
replacing conventional fossil energy in the future. Generally, discovered that acids, bases or enzyme catalysts were active
biofuels were typically produced by two routes: (i) towards when producing biomass transformations.1,2,7,10–16 Acid cata-
transesterification to biodiesel and (ii) depolymerization of lysts were highly efficient and low in cost for biomass trans-
crystalline cellulose into sugars and fine chemicals.1,2,7,10–15 formation, which could catalyze the transformation of low-
Notably, biodiesel production was focused on employing low quality and low cost biomass into biofuels under rather mild
cost and low quality plant oil or waste oil as the feedstocks, conditions. The replacement of mineral acids with solid acids
typically held under green and mild conditions.16 Alternatively, in terms of biomass transformation has received considerable
the depolymerization of cellulose was mostly held under attention regarding green and sustainable chemistry.10–19
Among various heterogeneous acid catalysts, carbon based
solid acids showed controlled wettability, good thermal and
chemical stabilities and superior catalytic activity throughout
aKey Laboratory of Alternative Technologies for Fine Chemicals Process of Zhejiang
Province, Department of Chemistry, Shaoxing University, Shaoxing, 312000, China.
various reactions, combining the advantages of the solid acids
E-mail: qichenze@usx.edu.cn
bDepartment of Chemistry, Zhejiang University (XiXi Campus), Hangzhou, 310028,
with both organic and inorganic frameworks.3,20–24 Graphene
based nanomaterials act as novel carbon materials. Materials
China
cWuhan Center for Magnetic Resonance, State Key Laboratory of Magnetic Resonance
and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics,
Chinese Academy of Sciences, Wuhan 430071, China.
scientists, chemists and physicist recognize that these
materials possess excellent thermal and mechanical stabilities,
unique nanosheet structure, controlled wettability and very
good dispersion in various systems.25–30 These are the ideal
candidates for solid acidic catalysts or catalyst supports
because they are capable of largely reducing the limitation of
E-mail: zhenganm@wipm.ac.cn
dPolymer Program, Institute of Materials Science and Department of Chemistry,
University of Connecticut, Storrs, CT 06269, USA
†Electronic supplementary information (ESI) available. See DOI: 10.1039/c4gc01052c
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mass transfer, increase the exposure degree of active sites applications when exploring biofuel through green and sus-
and enhance the recyclability of the catalysts in various tainable processes.
reactions.31–33 Due to the frequency of graphene nanomaterial
layers sticking together, it is imperative to create pillars
between the layers, maximizing the surface area and accessibil-
2. Experimental details
2.1 Preparation of catalysts
ity during various reactions.31–33 Still, the synthesis of gra-
phene based nanomaterials with large BET surface areas and
abundant nanopores through facile and cost-effective routes
2.1.1 Synthesis of GNCs. GNCs were synthesized from car-
remains a challenge.34–38 Abundant nanopores in graphene- bonization of the mixture of melamine and glucose with
based nanomaterials must be created to increase the degree of various weight ratios under N2 conditions. Typically, 0.1 g of
active sites, which contribute importantly to their application
during heterogeneous catalysis.31
glucose and 4.0 g of melamine were mixed together. After
grinding the mixture for 10 minutes, the sample was trans-
We report here the successful synthesis of novel acid ionic ferred into the tube furnace under flowing N2 conditions. The
liquids and sulfonic group bifunctional graphene-like nano-
temperature of the tube furnace experienced the following
porous carbons (GNC-SO3H-ILs), which were synthesized by heating processes: from room temperature to 600 °C (2.5 °C
treating nitrogen containing graphene-like nanoporous min−1), from 600 °C to 800 °C (1.67 °C min−1), and remained
carbons (GNCs) with 1,3-propanesultone. The anions were
at 800 °C for 1 h. After cooling the furnace to room tempera-
exchanged with strong acids of H2SO4 or HSO3CF3. The intro- ture, GNCs with monolithic morphology were obtained. More-
duction of nitrogen atoms into GNCs forms strong acid ionic over, GNCs could also be synthesized from carbonization of
liquids and sulfonic groups through the quaternary ammoni-
zation reaction. The GNC supports could be easily synthesized weight ratios under similar conditions.
the mixture of dicyandiamide and glucose using similar
using a one-step carbonization mixture. This mixture should
consist of dicyandiamide/melamine and glucose, using no
2.1.2 Synthesis of strong acid ionic liquids and sulfonic
group bifunctional GNCs (GNC-SO3H-ILs). GNC-SO3H-ILs
additional templates (as shown in Scheme 1). The resulting were synthesized from quaternary ammonization of GNCs with
bifunctional carbon based solid acids showed controlled 1,3-propanesultone, and anion exchange with strong acids of
acidity, unique nanosheet structure, abundant nanopores and
H2SO4 or HSO3CF3. As a typical run for the synthesis of GNC-
good stability. Interestingly, GNC-SO3H-ILs showed signifi- [C3N][SO3CF3], 1.2 g of GNC were added into a mixture con-
cantly improved catalytic activity and good recyclability in taining 20 mL of toluene and 0.3 g of 1,3-propanesultone.
biomass transformation, including transesterification to bio-
After heating the reaction mixture at 110 °C for 24 h under
diesel and depolymerization of crystalline cellulose into sugars refluxing, the reaction mixture was cooled down to room temp-
and 5-hydroymethylfurfural (HMF). This process was better erature (25 °C). The sample was then treated with a solution
than those of acidic resin of Amberlyst 15, sulfonic group func-
containing 25 mL of toluene and 3–5 mL HSO3CF3 for 24 h at
tionalized mesoporous SBA-15, and heteropolyacid of room temperature under vigorous stirring. GNC-[C3N][SO3CF3]
H3PW12O40, which was even comparable to those of homo- could be obtained by filtering, washing with a large amount of
geneous acid ionic liquids and mineral acids. The preparation
CH2Cl2 to remove the residual HSO3CF3, and then drying it at
of GNC-SO3H-ILs fosters a facile and cost-effective method to 60 °C for 12 h. This procedure was repeated twice. GNC-[C3N]-
synthesize highly efficient graphene based solid acids for [SO3CF3] was then obtained, where [SO3CF3] stands for the
biomass transformation, which is crucial for their various
treating acid of trifluoromethanesulfonic acid and C3 stands
Scheme 1 The procedures for synthesizing GNC-[C3N][SO3CF3].
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for 1,3-propanesultone. The detailed synthetic procedures of of Avicel, 100 mg of crystalline cellulose of Avicel was dissolved
GNC-[C3N][SO3CF3] are also shown in Scheme 1. For compari- in a mixture containing 1.5 g of [C4mim]Cl ionic liquid and
son, [C3vim][SO3CF3] (“C3” stands for 1,3-propanesultone, 0.5 mL of DMSO at 100 °C for 3–5 h under vigorous stirring
“vim” stands for vinyl imidazole and “[SO3CF3]” stands for the until a clear solution was obtained, followed by an additional
treating acid of HSO3CF3) and SBA-15-SO3H (S/Si = 0.2, “SBA” 20 mg of GNC-[C3N][SO3CF3]. Then, 600 μL of water was slowly
stands for ordered mesoporous silica) were synthesized accord- introduced into the reaction mixture while the reaction temp-
ing to the literature.8,39
erature remained at 100 °C. At different time intervals,
samples were withdrawn, weighed, quenched immediately
with cold water, and centrifuged at 14 800 rpm for 5 min to
2.2 Catalytic reactions
2.2.1 Transesterification. Transesterification of tripalmitin remove catalysts and unreacted cellulose, giving the reaction
with methanol was executed as follows: 0.84 g (1.04 mmol) of mixture. Various acid catalysts such as Amberlyst 15, HCl and
tripalmitin was melted into a flask equipped with a condenser [C3vim][SO3CF3] were also used for catalyzing depolymeriza-
and a magnetic stirrer at 65 °C. Then, 0.10 g of catalyst was tion of Avicel, which used the same number of acidic sites as
quickly added under vigorous stirring to form a homogeneous in GNC-[C3N][SO3CF3].
mixture. 2.47 mL (61 mmol) of methanol was then added;
Meanwhile, the isolated cellulose was thoroughly washed
after further stirring the mixture for 14 h at 65 °C under reflux- with water, and recovered through centrifugation. The amount
ing, the reaction was complete; the catalysts could be separ- of cellulose isolated was determined by weight.
ated by centrifugation. The molar ratio of tripalmitin/
2.2.4 Testing total reducing sugar (TRS). TRS value was
methanol was 1/60 and the mass ratio of catalyst/tripalmitin usually used for evaluating the depolymerization degree of
was 0.119. The product was methyl palmitate with selectivity crystalline cellulose catalyzed by various catalysts, which was
near 100%, which was analyzed by gas chromatography of tested through the DNS method.7,9,14 As a typical run, a
Agilent 7890 with a flame ionization detector (FID). The mixture containing 0.5 mL of 3,5-dinitrosalicylic acid (DNS)
column was a HP-INNOWax capillary column (30 m); the reagent and 0.5 mL of reaction mixture was heated for 5 min
initial temperature was 100 °C, the ramping rate was 20 °C at 100 °C and then cooled to room temperature. Then, 4 mL
min−1, and the final temperature was 280 °C; the temperature of deionized water was added to dilute the mixture. The color
of the FID detector was 350 °C. In this reaction, the concen- intensity of the mixture was measured on a NanoDrop 2000
tration of methyl palmitate was calculated through the internal UV-spectrophotometer at 540 nm. The concentration of the
standard (dodecane) method, establishing the standard curve.
total reducing sugars was calculated based on a standard curve
Transesterification of sunflower oil with methanol was exe- obtained with glucose.
cuted as follows: 1.0 g of sunflower oil (1.16 mmol) was added
The concentrations of glucose and cellobiose in the reac-
into a flask equipped with a condenser and a magnetic stirrer. tion mixture were measured using the Shimadzu LC-20A HPLC
The temperature was rapidly increased to 65 °C. Then, 3.5 mL system based on the standard curve method, which was
of methanol (86.3 mmol) and 0.15 g of catalyst were quickly equipped with a SCR-101N column using extra-pure water
added under vigorous stirring. The reaction occurred for 18 h. during the mobile phase at a flow rate of 0.5 mL min−1. The
The molar ratio of sunflower oil to methanol was 1/75 and the column’s temperature was set to 55 °C. In the meantime, a
mass ratio of catalyst to tripalmitin was 0.1. The products were refraction index was used for the detection of sugars in the
mainly methyl palmitate (C16:0), methyl stearate (C18:0), methyl water. The concentration of 5-hydroxymethylfurfural (HMF)
oleate (C18:1), methyl linoleate (C18:2), and methyl heptacosane was also measured using a Shimadzu LC-20A HPLC system
(C27:0). The quantification of these products was analyzed by based on the standard curve method. It was equipped with a
using an Agilent GC/MS instrument (Agilent 6890N/5975I) CAPCELL PAK C18 column using methanol and water (metha-
with a programmable split/splitless injector.
nol–water = 80 : 20) during the mobile phase, at a flow rate of
2.2.2 Catalysts regeneration. 1.0 g (1.16 mmol) of sun- 0.7 mL min−1, and the column’s temperature was set to 50 °C.
flower oil was melted into a flask equipped with a condenser At the same time, an ultraviolet detector with the wavelength
and a magnetic stirrer at 65 °C. Then, 0.15 g of catalyst was number at 254 nm was used for the detection of HMF in the
quickly added under vigorous stirring to form a homogeneous reaction mixture.
mixture; then 3.5 mL (86.3 mmol) of methanol was added;
after stirring the mixture for 18 h at 65 °C under reflux, the
reaction was complete. Centrifugation was then carried out for
catalyst separation. The catalyst was used for the next run after
washing the sample with a large amount of CH2Cl2, which
3. Results and discussion
3.1 Structural characterization
removed the absorbed reactants and products, and drying the Fig. 1A shows XRD patterns of GNC and GNC-[C3N][SO3CF3].
sample under vacuum conditions.
Notably, GNC exhibits two broad peaks centered at 26.1 and
2.2.3 Depolymerization of crystalline cellulose into sugars 43.2°. These are associated with the graphite structure, which
and HMF. The depolymerization of crystalline cellulose from was extremely close to the peaks of graphene based nanoma-
Avicel was also chosen as the model reaction for evaluating the terials with a nanosheet structure, reported previously.31,37
activities of various catalysts. As a typical run for depolymerization This result indicated that graphene-like structures with several
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Fig. 1 (A) XRD patterns and (B) Raman spectra of (a) GNC and (b) GNC-[C3N][SO3CF3].
layers was formed in the GNC. After grafting the strong XRD results. Compared with GNC, GNC-[C3N][SO3CF3] showed
acid ionic liquids and the sulfonic group onto the network of a decreased intensity of the Raman spectrum, which could be
GNC, the sample of GNC-[C3N][SO3CF3] was given, which attributed to the introduction of acidic groups into the
showed almost no change in XRD patterns compared to the sample. It is also noteworthy that the position and shape of
GNC support. The above results indicated that the graphene- each peak in the Raman spectrum does not shift after the
like nanosheet structure was well maintained in GNC-[C3N]- introduction of acidic groups into GNC. This indicates that the
[SO3CF3].
In addition, the graphene-like structures of GNC and GNC- GNC-[C3N][SO3CF3], in good agreement with XRD results.
[C3N][SO3CF3] have also been confirmed by Raman spectra. It Table 1 presents the parameters of various acid catalysts.
graphene-like nanosheet structure was well maintained in
is well known that Raman spectroscopy is a powerful approach GNC-[C3N][SO3CF3] showed the content of acidic sites at
for determining the properties of graphene based nanomater- 1.25 mmol g−1, lower than those of various acid catalysts
ials such as the number and orientation of layers, the quality including SBA-15-SO3H (1.98 mmol
g
−1), H3PW12O40
and types of edges and the effects of perturbations involving (3.5 mmol g−1), Amberlyst 15 (4.3 mmol g−1) and H2SO4
the structure strain, doping, disorder and functional (10.2 mmol g−1). Alternatively, the BET surface area of GNC-
groups.40,41 Fig. 1B shows the Raman spectra of GNC and [C3N][SO3CF3] (184 m2 −1, Fig. S1†) was higher than that of
g
GNC-[C3N][SO3CF3]. It was noted that a series of peaks at acidic resin of Amberlyst 15 (45 m2 g−1). Compared to the GNC
approximately 1378, 1591, 2639 and 2822 cm−1 were clearly support (945 m2 g−1), the decreased surface area in GNC-[C3N]
observed in these samples, which could be attributed to the [SO3CF3] should result from the grafting of acid active sites,
signals of D, G, 2D and S3 bands.26 The broadened D-band which largely increases the weight of the GNC network. Pre-
indicated the presence of sp3 carbon atoms with defects and viously, similar results have also been reported.39
disorders at the edges and the boundaries of the graphene
In order to obtain the relationship between the catalyst
domains. The sharp G-band indicated the presence of sp2 structure and reactive activity, the surface morphology of the
carbon atoms in a graphitic 2D hexagonal lattice, confirming GNC-[C3N][SO3CF3] was analyzed in detail by the multiple
the formation of graphitic carbon, in good agreement with image techniques (TEM, HR-TEM and AFM images).
Table 1 The textural and acidic parameters of various catalysts
Acid contentsa
N contenta
SBET
Vp
b
Samples
(mmol g−1
)
(mmol g−1
)
(m2 g−1
)
(cm3 g−1
)
Dp c (nm)
GNC
—
1.82
1.57
1.62
—
—
—
945
184
173
814
4.2
45
3.1
0.59
0.55
1.4
—
0.31
—
—
3.8 & 33.7
3.6 & 31.2
3.4 & 30.2
7.5
—
40
—
—
GNC-[C3N][SO3CF3]
GNC-[C3N][SO3CF3]d
SBA-15-SO3H
1.25
1.22
1.98
3.50
4.30
2.96
10.20
e
H3PW12O40
Amberlyst 15
[C3vim][SO3CF3]e
—
—
—
—
e
H2SO4
a The acid contents were measured by CHNS elemental analysis. b Pore volumes (Vp) were estimated at relative pressure p/p0 ≈ 0.98. c Average pore
diameters were estimated from the BJH model. d GNC-[C3N][SO3CF3] after being recycled five times in transesterification of sunflower oil with
methanol. e The acid contents of H3PW12O40, [C3vim][SO3CF3] and H2SO4 were calculated from the molecular formula.
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Fig. 2 (a, b) TEM images, (c–f) high-resolved TEM images, and (g) AFM image and corresponding thickness analysis of GNC-[C3N][SO3CF3].
Typically, TEM images indicate a typical nanosheet structure planes. When the XRD and Raman aforementioned were
with three dimensional network clusters in GNC-[C3N]- combined with TEM and HR-TEM, the uniform graphitized
[SO3CF3], exhibiting abundant nanoporous and hollow struc- networks were determined. In the meantime, the AFM image
tures (Fig. 2a, b and S2†). Furthermore, highly-resolved TEM of GNC-[C3N][SO3CF3] further affirms its nanosheet structure
images were also used to confirm the graphene structure with (Fig. 2g). The thickness of layers ranged from 1.7 to 3.5 nm,
crumpled and entangled character (Fig. 2c–f and S1†), exhi- indicating the stick had several graphene layers in GNC-[C3N]-
biting several nanocrystallites with well-defined lattice [SO3CF3]. These results confirmed that the graphene-like
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nanoporous carbon network was formed in GNC-[C3N]- C–C and C–N in the GNC network, and C–O and C–F in the
[SO3CF3].
strongly acidic ionic liquids and sulfonic group.8 In the mean-
Fig. 3 shows the X-ray photoelectron spectroscopy (XPS) time, the N1s spectrum could be fitted and deconvoluted into
spectra of GNC-[C3N][SO3CF3], which exhibits the signals of two peaks, centered at approximately 398.4, 400.4 and 401.8
S
2p (168.8 eV), C1s (284.8 eV), N1s (402.1 eV), O1s (532.4 eV) and eV, which should be assigned to the C–N, N+–C3 ionic liquid
F1s (686.8 eV). Notably, the broad C1s spectrum could be fitted group and the protonated structure of GNC-[N+H][SO3CF3] in
and deconvoluted into four peaks, centered at approximately GNC-[C3N][SO3CF3].8 Similarly, the O1s associated with O–S
284.8, 286.1, 287.0 and 292.3 eV, which should be assigned to (531.9 eV) and O–SCF3 (533.1 eV) could also be observed.8
Fig. 3 XPS spectra of (a) S2p, (b) C1s, (c) N1s, (d) O1s and (e) survey of GNC-[C3N][SO3CF3].
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The above results confirm that strongly acidic ionic liquids 3.2 Catalytic reactions
and sulfonic groups have been successfully grafted onto the
network of GNCs, in good agreement with FT-IR and elemental
analysis results (Fig. S3† & Table 1).
3.2.1 Towards transesterification to biodiesel. Fig. 5 shows
the dependences of catalytic activities on the time in transes-
terification of tripalmitin with methanol catalyzed by various
samples. Notably, GNC-[C3N][SO3CF3] showed much better cat-
alytic activity than those of H3PW12O40, SBA-15-SO3H and
Amberlyst 15. For instance, the yield of biodiesel (methyl pal-
mitate) catalyzed by GNC-[C3N][SO3CF3] was up to 78.8% after
10 h of reaction, much higher than those of H3PW12O40
(51.6%), SBA-15-SO3H (50.5%) and Amberlyst 15 (39.7%).
Increasing the reaction time up to 14 h, the yield of methyl pal-
mitate catalyzed by GNC-[C3N][SO3CF3] was increased up to
88.5%. This was even comparable to that of the homogeneous
acid ionic liquid of [C3vim][SO3CF3] (89.8%). In contrast,
H3PW12O40, SBA-15-SO3H and Amberlyst 15 still gave relatively
lower biodiesel yields at 64.1, 60.8 and 53.6%, respectively.
Apart from tripalmitin, GNC-[C3N][SO3CF3] also showed
good catalytic activity for the transformation of plant oils such
as sunflower oil into biodiesel. Table 2 presents catalytic data
on transesterification of sunflower oil with methanol for the
production of biodiesel over various catalysts. Interestingly,
Furthermore, the acidity properties of GNC-[C3N][SO3CF3]
were also characterized by the 31P NMR probe techniques
involving the adsorbed trimethylphosphine (TMP) and tri-
methylphosphine oxygen (TMPO). It is noteworthy that the 31P
NMR probe technique is a sensitive and reliable technique to
determine the acid type (Brønsted or Lewis acid) and acid
strength of solid acid catalysts.42–44 As shown in Fig. 4a, using
TMP as a probe molecule, the 31P resonances at −3.2 ppm
were assigned to the protonated adducts of [(CH3)3P-H]+, and
attributed to the reaction of TMP with the Brønsted acidic
protons. In contrast, almost no resonances were observed in
the range of −20 to −60 ppm due to the interaction with Lewis
acid sites.42,43 Therefore, it is indicative that no Lewis acid was
formed over GNC-[C3N][SO3CF3]. As aforementioned in the
Introduction, solid-state 31P MAS NMR of adsorbed TMPO is
an efficient technique for acidity characterization of solid acid
catalysts such as zeolites, sulfated mesoporous metal oxides
and heteropolyacids.42,43 Interestingly, GNC-[C3N][SO3CF3]
showed multiple 31P resonance peaks at approximately 45, 48,
51.2, 52.8, 54.5, 56.6 and 63.1 ppm, indicating the presence of
various Brønsted acid sites with different acid strengths in
GNC-[C3N][SO3CF3] (Fig. 4b). The weakly acidic sites (45 &
48 ppm) may be attributed to the presence of the carboxylic or
hydroxyl group in GNC-[C3N][SO3CF3]. While the relatively
strongly acidic sites (51.2, 52.8, 54.5, 56.6 and 63.1 ppm)
should be attributed to the strong acid ionic liquids, sulfonic
group and protonated structure of GNC-[N+H][SO3CF3] located
in the outer and inner surfaces of GNC-[C3N][SO3CF3]. The
GNC-[N+H][SO3CF3] structure should be formed from protona-
tion of the N atom in GNC by HSO3CF3. It is noteworthy that
the acidity of the –SO3H group is stronger than GNC-[N+H]
[SO3CF3], and the –SO3H group has a better catalytic role for
its activity. The strongly acidic sites in GNC-[C3N][SO3CF3]
showed a similar acid strength to that of HY zeolites (55 and
65 ppm) and bifunctional carbon–silica nano composited
solid acid (40–70 ppm).44,45
after 18 h of reaction, the yields of biodiesel including C16:0
,
C18:0, C18:1, C18:2 and C27:0 catalyzed by GNC-[C3N][SO3CF3]
were almost as high as inside the pure acidic ionic liquid of
[C3vim][SO3CF3] (89.1, 94.8, 95.2, 93.1, and 86.7% in homo-
geneous vs. 88.5, 93.6, 94.7, 92.3 and 81.4% in heterogeneous),
which was evencomparable to that of H2SO4 (87.4, 94.1, 95.9,
94.4 and 82.1%). Therefore, it is clear that the high catalytic
reactivity has been well retained. As presented in Table 2, the
activity of GNC-[C3N][SO3CF3] was much higher than those of
heteropolyacid of H3PW12O40 (58.9, 59.2, 54.3, 60.1 and
50.9%), Amberlyst 15 (54.8, 50.2, 48.6, 50.5 and 26.4%) and
SBA-15-SO3H (60.9, 56.5, 55.4, 52.6 and 45.7%). The excellent
Fig. 5 Dependences of catalytic activities on the time in transesterifica-
tion of tripalmitin with methanol catalyzed by (a) [C3vim][SO3CF3] (the
same number of acidic sites as that of GNC-[C3N][SO3CF3]), (b) GNC-
[C3N][SO3CF3], (c) H3PW12O40, (d) SBA-15-SO3H and (e) Amberlyst 15.
Reaction conditions: 0.84 g tripalmitin; 2.47 mL methanol; 0.1 g catalyst;
reaction temperature, 65 °C; reaction time, 14 h.
Fig. 4 Room temperature 31P MAS NMR spectra of adsorbed (a) TMP
acquired with proton decoupling, and (b) TMPO of GNC-[C3N][SO3CF3].
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Table 2 Catalytic data on transesterification of sunflower oil with methanol over various samplesa
Conversion of esters (%)
Methyl palmitate
Methyl stearate
Methyl oleate
Methyl linoleate
Methyl heptacosane
(C27:0)
Catalysts
(C16:0
)
(C18:0
)
(C18:1
)
(C18:2
)
[C3vim][SO3CF3]b
H2SO4
GNC-[C3N][SO3CF3]
GNC-[C3N][SO3CF3]d
H3PW12O40
89.1
87.4
88.5
84.3
58.9
54.8
60.9
94.8
94.1
93.6
89.1
59.2
50.2
56.5
95.2
95.9
94.7
92.3
54.3
48.6
55.4
93.1
94.4
92.3
90.7
60.1
50.5
52.6
86.7
82.1
81.4
78.3
50.9
26.4
45.7
c
Amberlyst 15
SBA-15-SO3H
a 1.0 g sunflower oil; 2.47 mL methanol; 0.15 g catalyst; reaction temperature, 65 °C; reaction time, 18 h. The catalytic activity of the catalysts was
characterized quantitatively by the conversion of fatty acid methyl esters (FAME, Y %) which was calculated as follows: Yield = (MD/MT) × 100%,
where MD and MT are the numbers of moles of each FAME produced and expected, respectively. In this section, MT is the number of moles of
FAME catalyzed by 0.2 g of H2SO4 for 24 h, which was performed at 65 °C with the same content of feedstock as that of GNC-[C3N][SO3CF3].
b Imidazole based acidic ionic liquid synthesized using the same procedure as that of GNC-[C3N][SO3CF3]. c The same number of acidic sites as
that of GNC-[C3N][SO3CF3]. d The 5th cycle in transesterification of sunflower oil with methanol.
catalytic activity found in GNC-[C3N][SO3CF3] for biodiesel pro-
duction could be attributed to its controlled acidity, good dis-
persion, abundant and hierarchical nanoporosity, and unique
nanosheet structure. By themselves, they are capable of enhan-
cing the fast diffusion of reactants and products, increasing
the exposure degree of catalytically active sites, in processes
of various reactions. Compared to GNC-[C3N][SO3CF3], the low
BET surface areas of H3PW12O40 and Amberlyst 15 resulted
in their low exposure degrees and easy poison of active sites,
further leading to their low catalytic activities. Although
SBA-15-SO3H has large BET surface areas, its functional group
could be easily occluded in the silica wall.46 Moreover, the
accessibility of reactants to catalytically active sites located into
the mesopores of SBA-15-SO3H is usually difficult in compari-
son to two dimensional nanosheet structures of GNC-[C3N]-
[SO3CF3].27 These factors resulted in a lower catalytic activity of
SBA-15-SO3H. The above results prove that GNC-[C3N][SO3CF3]
Fig. 6 Dependences of catalytic activities on the time in depolymeriza-
tion of Avicel into sugars catalyzed by (a) GNC-[C3N][SO3CF3], (b) HCl,
could be used as a highly efficient solid acid for biodiesel
production.
(c) [C3vim][SO3CF3] and (d) Amberlyst 15 (b, c, d used the same number
of acidic sites as those in GNC-[C3N][SO3CF3]). Reaction conditions:
0.1 g, Avicel; 1.5 g [C4mim]Cl ionic liquid, and 0.5 mL DMSO mixed sol-
vents; 20 mg catalyst; reaction temperature, 100 °C; reaction time, 5 h.
The total reducing sugars were monitored using the 3,5-dinitrosalicylic
acid (DNS) reagent.
More importantly, GNC-[C3N][SO3CF3] showed very good
recyclability for catalyzing biodiesel production. For example,
in the reaction of sunflower oil with methanol, even after
being recycled five times, the yields of biodiesel catalyzed by
GNC-[C3N][SO3CF3] were still up to 86.2, 90.8, 92.1, 89.4 and
79.5% (Table 2), comparable to fresh GNC-[C3N][SO3CF3] (88.5, than that of Amberlyst 15, one of the most efficient commer-
93.6, 94.7, 92.3 and 81.4%, Table 2).
cial acidic resins, comparable to those of homogeneous HCl
3.2.2 Depolymerization of crystalline cellulose into bio- and [C3vim][SO3CF3] (the same number of active sites as in
fuels. To further decrease the cost of biofuels, much effort has GNC-[C3N][SO3CF3]). For example, the yields of total reducing
been made to develop green and cost-effective ways to produce sugars, mono- and disaccharides catalyzed by GNC-[C3N]-
renewable biofuels from cellulosic biomass, in recent years. [SO3CF3] was 94.6% after 5 h of incubation, higher than those
The depolymerization of crystalline cellulose (Avicel) into of Amberlyst 15 (40.5%), HCl (87.1%) and [C3vim][SO3CF3]
sugars, catalyzed by various acid catalysts, has also been inves- (84.9%). Similar results could also be obtained from HPLC
tigated in this work, and was achieved in the presence of 1-n- data (as shown in Table 3); the yields of the main products
butyl-3-methylimidazolium and DMSO mixed solvents.
including glucose (44.1%), cellobiose (32.7%) and HMF
Fig. 6 shows the dependences of catalytic activities on (11.6%) catalyzed by GNC-[C3N][SO3CF3] were much higher
the time in depolymerization of Avicel catalyzed by GNC-[C3N]- than that of Amberlyst 15 (14.3, 17.5 and 3.2%), which were
[SO3CF3], Amberlyst 15, HCl and [C3vim][SO3CF3]. Notably, comparable to those of homogeneous HCl (39.8, 20.4 and
GNC-[C3N][SO3CF3] showed significantly better catalytic activity 10.7%) and [C3vim][SO3CF3] (26.3, 34.5 and 10.1%). Presumably,
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Table 3 Yields of sugars and dehydration products in the depolymeri-
zation of Avicel catalyzed by various solid acidsa
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