Sulfonated polyaniline as a solid organocatalyst for dehydration of fructose into 5-hydroxymethylfurfural

Article abstract of DOI:10.1039/c6gc03604j

The rehydration of 5-hydroxymethylfurfural (HMF), an important bio-based chemical building block, to levulinic acid (LA) and formic acid (FA) over Br?nsted acid catalysts is the key block to the effective production of HMF from hexose. In this work, we develop a novel acidic solid organocatalyst, sulfonated polyaniline (SPAN), for the effective dehydration of fructose into HMF in the low-boiling water/1,4-dioxane cosolvent. The highest HMF yield of 71% is obtained from fructose with complete restriction of HMF rehydration to LA. We demonstrate that hydrogen bonds form between the ring-attached sulfonic acid group and the quinoid imine nitrogen as a result of internal doping, which confines the Br?nsted acidity of the SPAN catalyst. The H-bonded sulfonic acid species is active for fructose-to-HMF dehydration and complete suppression on HMF rehydration. The chemical bonding of sulfonic acid groups on the backbone of the PAN chain allows stable recyclability of the polymer catalyst. This work highlights the potential importance of confining Br?nsted acidity on a solid organocatalyst via H-bonding for transforming renewable carbohydrates into fine chemicals.

Full text of DOI:10.1039/c6gc03604j

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Sulfonated Polyaniline as Solid Organocatalyst for Dehydration of
Fructose into 5-Hydroxymethylfurfural†
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
a
a
a
Jinhang Dai , Liangfang Zhu* , Dianyong Tang , Xing Fu , Jinqiang Tang , Xiawei Guo and
a
Changwei Hu*
DOI: 10.1039/x0xx00000x
The rehydration of 5-hydroxymethylfurfural (HMF), an important bio-based chemical building block, to levulinic acid (LA)
and formic acid (FA) over Brønsted acid catalysts is the key block to the effective production of HMF from hexose. In this
work, we develop a novel acidic solid organocatalyst, sulfonated polyaniline (SPAN), for the effective dehydration of
fructose into HMF in low-boiling water/1,4-dioxane cosolvent. A highest HMF yield of 71% is obtained from fructose with
complete restriction of HMF rehydration to LA. We demonstrate that hydrogen bonds form between the ring-attached
sulfonic acid group and the quinoid imine nitrogen as a result of internal doping, which confines the Brønsted acidity of the
SPAN catalyst. The H-bonded sulfonic acid species is active for fructose-to-HMF dehydration and the complete suppression
on HMF rehydration. The chemical bonding of sulfonic acid group on the backbone of PAN chain allows stable recyclability
of the polymer catalyst. This work highlights the potential importance of confining Brønsted acidity on solid organocatalyst
via H-bond for transforming renewable carbohdyrates into fine chemicals.
www.rsc.org/
rehydration could be partially or fully inhibited by continuously
1-23
extracting HMF from the aqueous phase to organic phase,
2
Introduction
2
4
continuously removing water under reduced pressure, or
2
stabilizing HMF with an appropriate organic solvent.
In recent years, the acid-catalyzed dehydration of biomass-
4-28
derived hexose to 5-hydroxymethylfurfural (HMF),
a
Inhibition of HMF rehydration via rational catalyst design is a
more promising strategy because it is of low energy-
consumption compared to process control. Xiao and Qi
potentially promising platform molecule, has been of great
interest because several kinds of fine chemicals, liquid fuels,
and monomers for polymeric materials could be synthesized
2
9, 30
1
,
2
groups
reported the isolation of water molecules from the
from HMF.
developing efficient solid acid catalysts, typically inorganic
Remarkable progress has been achieved on
acid sites by increasing the hydrophobicity of the acid catalysts.
However, these existed catalysts were highly efficient in high-
boiling solvent containing dimethyl sulfoxide (DMSO), which
increases the separation energy-consumption and limits the
practical application of these catalysts. In this context,
effective solid acid or base catalysts that are workable in low-
boiling solvent are yet to be developed to meet both the
3
-9
solid acids, to convert hexose into HMF. However, the acid
sites in inorganic solid acids are prone to be occupied by water
molecules or leached under experimental conditions, leading
to deactivation of the inorganic acid catalysts. Organic
1
0-15
Brønsted solid acids, typical sulfonated polymers,
have
shown good activity and stability for hexose dehydration as
the sulfonic acid groups are chemically linked on the chain of
the polymer catalysts. Nevertheless, the rehydration of HMF to
levulinic acid (LA) and formic acid (FA) over Brønsted acid
catalysts is still the key block to the effective production of
HMF from hexose because of the low selectivity to HMF and
3
1, 32
selectivity and recyclability issues.
Polyaniline (PAN) is a basic polymeric material composed of
benzenoid amine (-NHC -) and quinoid diimine (-N=C =N-)
6
H
4
6 4
H
structure units. Because of its facile synthesis and good
stability, PAN has found a variety of catalytic applications as a
nanoscale support for metal nanoparticles or complexes in
1
6-20
the difficulty in separation of LA from HMF.
3
3, 34
Suzuki coupling or redox reactions.
However, the intrinsic
So far, several effective strategies have been developed to
suppress the rehydration of HMF. For example, the HMF
basicity of PAN has not yet been well utilized for catalytic
reaction. In our previous work, formyl-modified PAN was used
as a solid organic base for catalytic dehydration of fructose to
HMF, wherein the rehydration of HMF was thoroughly
a.
Key Laboratory of Green Chemistry and Technology
Ministry of Education, College of Chemistry
Research Institute for New Materials Technology
Chongqing University of Arts and Sciences, Chongqing 402160 (PR China)
35
b.
suppressed due to the absence of any acid site. However,
this typical base catalyst was only active in DMSO. Therefore,
further tailoring of PAN is needed to improve its catalytic
† E-mail: changweihu@scu.edu.cn and zhulf@scu.edu.cn; Fax: +86 28 85411105;
Tel: +86 28 85411105
Electronic Supplementary Information (ESI) available: results of elemental analysis performance for hexose dehydration in low-boiling solvent.
and BET; spectra of XPS and ATR-IR; images of TEM; and activity results of control
experiments. See DOI: 10.1039/x0xx00000x
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Scheme 1. Schematic illustration of the synthesis of SPAN catalyst for
dehydration of fructose into HMF.
Herein, we report that sulfonated polyaniline (SPAN) with
mutually reactive amine/imine and sulfonic acid group in its
polymer chain could be used as novel acidic solid
organocatalyst for the selective dehydration of fructose to
HMF in low-boiling water/1,4-dioxane cosolvent (Scheme 1).
The confinement of the Brønsted acidity of SPAN contributes
to a highest HMF yield of 71% from fructose with no LA
formation. The structure, morphology, and physiochemical
Fig. 1 SEM images of the SPAN catalysts synthesized by varying the molar ratio of
APS/monomers as (a) 1/3, (b) 3/3, (c) 6/3, (d) 9/3, (e) 11/3, and (f) 12/3; (g)
properties of the SPAN catalysts are characterized by means of Schematic diagram for the fabrication of SPAN catalysts.
elemental analysis, X-ray diffraction (XRD), X-ray
photoelectron spectroscopy (XPS), FTIR, SEM, TEM, and BET. SPAN-3/3) to nanorods (SPAN-6/3, SPAN-9/3, and SPAN-11/3),
The stability of HMF over SPAN is studied by in situ attenuated then to irregular nanogranules (SPAN-12/3), together with the
total reflection infrared spectroscopy (ATR-IR). The internal decrease of the aspect ratio (L/D) with increasing nAPS/nmonomers
.
3
8, 39
and external doping of SPAN are deliberately distinguished and According to Kaner’s work,
correlated with the recyclability of the solid organocatalysts. A favors the generation of uniform PAN nanofibers. This is
possible reaction mechanism for this SPAN-catalyzed consistent with our work when nAPS/nmonomer is below 1/1.
the fast polymerization of AN
dehydration reaction is proposed.
However, the morphology and aspect ratio variation with
excess APS in this work might be resulted from the change in
oxidation rate due to the existence of SAN. Combined with the
results of elemental analysis, we infer that the monomer of
Results and discussion
acidic SAN tends to copolymerize with basic AN at higher [APS].
One possible explanation is that the presence of electron-
Catalyst preparation and characterization
SPAN was synthesized by fast oxidative copolymerization of withdrawing sulfonic acid group on SAN hinders the
aniline (AN) and metanilic acid (SAN) in aqueous ammonium polymerization due to the larger steric hindrance of sulfonic
o
36
40,
persulfate (APS) at 5 C for 24 h. The as-prepared SPAN-x/y acid group and smaller electron density of the amino group,
4
1
catalysts were named according to the amount of APS used,
especially when [APS] is low. We suppose that the
wherein x/y represents the molar ratio of APS/(AN+SAN) surfactant-like SAN-anilinium salts form in the early stage of
ranging from 1/3 to 12/3.
the polymerization through the acid-base interactions
The elemental analysis (Table S1†) of the as-prepared solid between AN and SAN monomers in aqueous solution, the
catalysts reveals that the S/N molar ratio increases with assembly of which generates micelles that act as the nucleus
increasing nAPS/n(AN+SAN) from 1/3 to 11/3 and then decreases for the subsequent polymerization (Fig. 1g). Therefore, the
when nAPS/n(AN+SAN) is enlarged to 12/3. The S/N molar ratio is SAN-anilinium salts have enough time to grow along the
indicative of the doping level of SAN in the as-prepared SPAN direction of a rigid molecular chain to form regular nanofibers
catalysts, therefore, the highest doping level is 23.8% obtained at lower nAPS/nmonomers (1/3~3/3); the faster copolymerization
on SPAN-11/3 under the preparation conditions used. The XRD speed at higher nAPS/nmonomers (6/3~11/3) favors both the
patterns (Fig. S1†) show typical amorphous structure with a growth along and cross the polymer chain and leads to a
o
single broad diffraction peak centred at 2θ of ca. 23.9 except smaller aspect ratio of the catalysts. When nAPS/nmonomers is
SPAN-3/3 and SPAN-6/3. The appearance of diffraction peak beyond 12/3, the over-oxidation results in the formation of
o
42
centred at larger 2θ of ca. 25.8 and smaller crystallographic irregular nanogranules.
spacing (d) indicate the obvious presence of external dopants
The molecular structure of the SPAN catalysts were analyzed
on SPAN-3/3 and SPAN-6/3. The BET results (Table S2†) by FTIR (Fig. 2a). For fresh SPAN-1/3, the characteristic IR
3
7
-1
reveal that all these as-prepared SPAN catalysts have surface bands centred at 1575 and 1502 cm , respectively, are
2
-1
g with pore width of 10~13 Å.
areas of about 20~36 m
·
assigned to C=C stretching vibrations of quinoid and
4
3
-1
The SEM (Fig. 1a-f) and TEM (Fig. S2†) images of the benzezenoid rings. The band at 1300 cm corresponds to the
catalysts indicate the formation of nanofibers (SPAN-1/3,
C-N stretching vibration of the secondary aromatic amine, the
2
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Table 1. Binding energies and percentage of each component in total S 2p and N 1s from XPS spectra.
Binding energies (eV) (component content, %)
Entry
Cat.
Peak
S 2p
Component 1
ꢀ
Component 2
167.5/168.6 (28%)
167.5/168.6 (23%)
167.4/168.6 (31%)
167.4/168.6 (38%)
167.5/168.6 (40%)
167.5/168.7 (27%)
399.4 (34%)
Component 3
168.0/169.1 (72%)
168.0/169.1 (60%)
167.9/169.0 (18%)
167.9/169.1 (24%)
168.0/169.1 (25%)
168.0/169.1 (24%)
400.0 (66%)
Component 4
1
2
3
4
5
6
7
8
9
SPANꢀ1/3
SPANꢀ3/3
SPANꢀ6/3
SPANꢀ9/3
SPANꢀ11/3
SPANꢀ12/3
SPANꢀ1/3
SPANꢀ3/3
SPANꢀ6/3
SPANꢀ9/3
SPANꢀ11/3
SPANꢀ12/3
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
167.1/168.2 (17%)
167.0/168.2 (51%)
166.9/168.1 (38%)
167.0/168.2 (35%)
166.9/168.0 (49%)
ꢀ
398.7 (13%)
398.6 (13%)
398.7 (16%)
398.7 (13%)
398.5 (20%)
399.3 (61%)
400.4 (15%)
401.6 (11%)
401.6 (10%)
401.6 (8%)
401.6 (7%)
401.7 (7%)
399.4 (60%)
400.3 (17%)
N 1s
1
1
1
0
1
2
399.4 (58%)
400.4 (18%)
399.4 (56%)
400.4 (24%)
399.4 (53%)
400.4 (20%)
-
1
1
125~1200 cm is significantly broader for catalysts SPAN-3/3
and SPAN-6/3 than for other catalyst samples. On account that
the actual S/N molar ratio increases with increasing the APS
amount, we assume that the interaction level of sulfonic acid
4
5
with imine/amine nitrogen decreases.
That is, the
copolymerization of SAN with AN lowers the interaction level
between sulfonic acid group and the imine/amine nitrogen on
the PAN chain.
The surface composition of SPAN-x/y catalysts were then
analyzed by XPS. Six peaks (three doublets) are resolvable for
the S 2p spectra of all catalysts except SPAN-1/3 (Fig. S3†). The
doublet with lowest binding energy (BE) is from the sulfur in
the ammonium sulfonate that formed from the salification
between the sulfonic acid group and the imine and/or amine
4
6
nitrogen. However, the two doublets with higher BEs are
lower than that reported as the sulfur in free sulfonic acid
4
6
group, therefore, they are tentatively ascribed to the sulfur
in the sulfonic acid groups hydrogen-bonded (H-bonded) with
benzenoid amine (-NH-) and quinoid imine (-N=) nitrogen to
4
7, 48
form stable six-member ring structure.
A theoretical
simulation of selected fragments in SPAN at M06-2x/def2-
tzvpp level confirms that the ring-attached sulfonic acid group
interacts with the benzenoid amine and quinoid imine
nitrogen via H-bond, among which the interaction with the
Fig. 2 FT-IR spectra of (a) fresh, and (b) used SPAN catalysts.
49
latter is thermodynamically favored (Fig. S4†). That is, H-
bonded sulfonic acid species and ammonium sulfonates
coexist on the solid SPAN catalysts, wherein the former species
is inclined to form between the ring-attached sulfonic acid
group and the imine nitrogen as a result of internal doping. In
combination with the FTIR results, we assume that the
monomer of SAN copolymerizes with AN at higher
nAPS/nmonomers, thereby inserting into the backbone of the
polymer catalysts and internally doping the imine/amine
-
1
band at 1230 cm is related to the protonated C-N group,
whereas the broad and overlapping band at around
-1
1
125~1200 cm is originated from the -N=quinoid=N-
(
electron-like band) stretching modes and is often related to
3
the doped structure. The appearance of IR bands at 1036,
7
-1
6
93 and 619 cm , assigning to the S=O, S-O and C-S stretching
vibrations, respectively, is indicative of the existence of -SO
3
groups in SPAN-1/3.
3
H
6, 44
That is, the FTIR spectrum describes
nitrogen via H-bonding. Otherwise, at lower nAPS/nmonomers
,
the formation of sulfonic acid-doped PAN. It is of interest to
most of the SAN externally dopes the polymer catalysts,
resulting in the formation of ammonium sulfonates (Fig. S5†).
The properly deconvoluted XPS spectra of N 1s (Fig. S6†) verify
-1
note that an obvious upshift from 1575 cm (SPAN-1/3) to
-1
1
594 cm (SPAN-12/3) was observed in the FTIR spectra of
SPAN-x/y catalysts with increasing nAPS/nmonomers, which might
be indicative of the peroxidation of SPAN and accompanying
4
decrease in doping level. As a support, the IR band at
the existence of quinoid imine (398.6
±0.1 eV), benzenoid
+
amine (399.4 0.1 eV), polarized quinoid imine (-N =,
·
±
2
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a
Table 2. Conversion of fructose over various catalysts in water/1,4ꢀdioxane.
b
V
water/V1,4ꢀ
dioxane
Yield of products (mol%)
HMF FA LA
No
1
Cat.
ꢀ
Conv.
6
FF
ꢀ
5/95
ꢀ
ꢀ
ꢀ
3
9
2
SPANꢀ1/3
5/95
5/95
5/95
5/95
5/95
5/95
93
1
ꢀ
<1
(
ꢀ)
5
9
2
(2)
4
(2)
5
(7)
4
(4)
3
(4)
4
1
(<1)
3
4
5
6
7
SPANꢀ3/3
SPANꢀ6/3
SPANꢀ9/3
SPANꢀ11/3
SPANꢀ12/3
98
100
98
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
(
(
(
(
(
40)
5
48)
6
51)
7
69)
6
56)
44
71
57
45
9
2(2)
2(2)
2(2)
2(2)
5
Fig. 3 Linear dependence of HMF yield on the S/N molar ratio of fresh and used
SPAN catalysts.
1
99
catalysts. However, no LA was detected in the case of SPAN-
8
100
1
1/3, while LA formed over Amberlyst-15 regardless of the
8
9
SPANꢀ11/3
SPANꢀ11/3
SPANꢀ11/3
SPANꢀ11/3
0/100
10/90
15/85
20/80
100
98
83
ꢀ
ꢀ
ꢀ
ꢀ
5
1
<1
<1
presence or absence of water in solvents (Table S3†). The
inhibition role of SPAN-11/3 on HMF rehydration was
observed in all cases by varying the catalyst amount as well as
the reaction temperature and time (Figs. S7~9†). The higher
acid density on Amberlyst-15 might contribute to the HMF
rehydration, but it was excluded because the same amount of
benzenesulfonic acid (BSA, 0.032 mmol) as highly-dispersed
acid catalyst in solution still led to the formation of LA (6%)
with HMF yield of 60% (Table S4†, entry 1). Obviously, the
extra existence of base sites in SPAN is deemed to be
responsible for its high activity and selectivity towards HMF
formation. In control experiments, the HMF yield was
increased to 63% with LA yield decreased to 3% when BSA and
diphenylamine (0.032 mmol) were both added (Table S4†,
entry 2); whereas, the HMF yield was increased to 67% with no
LA formed when equimolar SAN bearing adjacent sulfonic acid
group and amine group in a single catalyst was used (Table S4†,
entry 3). Furthermore, the addition of diphenylamine (0.032
mmol) into the Amberlyst-15 catalyzed fructose dehydration
resulted in the increase of HMF yield to 63% and decrease of
LA yield to 5% (Table S4†, entry 4). These results strongly
demonstrate that the acid-base interactions between sulfonic
acid group and amine nitrogen in solution account for the
inhibition on HMF rehydration.
5
5
7
1
1
0
1
68
Amberlystꢀ
1
2
c
0/100
100
25
22
4
8
1
5
Amberlystꢀ
1
1
3
4
c
5/95
5/95
100
27
46
3
27
ꢀ
12
ꢀ
5
ꢀ
1
5
d
DSꢀPAN
a
Reaction conditions: 0.25 mmol of fructose, 1 mL of water/1,4-dioxane solvent
o
(
V/V = 5/95), 15 mg of catalyst (if not specified), 140 C, 3 h, under N
2
b
atmosphere. The values in brackets represent the yield for the second cycling.
c
SPAN-1/3 was not reused because it was homogeneously dispersed in solvents.
5
mg of Amberlyst-15 (nSO3H = 0.032 mmol, equal to that on SPAN-11/3). Further
activity results of Amberlyst-15 in water/1,4-dioxane containing higher water
d
contents are shown in Table S3†. DS-PAN is intrinsic polyaniline without acid
doping.
+
+
4
00.3
±
±
0.1 eV), and protonated amine/imine (-NH -/-NH =,
0.1 eV) in the solid SPAN catalyst as the result of both
4
01.6
internal and external doping. The percentage of each
component in total S 2p and N 1s are listed in Table 1, from
which we find that the sulfonic acid group exist mainly in the
form of H-bonded species rather than ammonium sulfonates
in the solid SPAN catalysts, especially on SPAN-11/3 catalyst.
Catalytic activity of SPAN catalysts
A blank experiment without catalyst did not lead to HMF
formation (Table 2, entry 1). Significantly, all of the SPAN
catalysts were active for fructose dehydration in water/1,4-
Origin of the activity to HMF formation and inhibition on HMF
rehydration
o
In the heterogeneous water/1,4-dioxane catalytic system, we
suppose that the acid-base interactions between sulfonic acid
group and imine/amine nitrogen on SPAN catalysts are
retained owing to the stability role of water on H-bonded
dioxane (V/V = 5/95) at 140 C for 3 h (Table 2, entries 2-7).
The yield of HMF increased from 39% to 71% with increasing
nAPS/n(AN+SAN) used from 1/3 to 11/3, and then declined to 68%
with further increasing nAPS/n(AN+SAN) to 12/3. In all cases, no LA
was detected, illustrating the complete inhibition of HMF
rehydration by these polymer catalysts. The reactivity of LA, if
formed, with SPAN was excluded since LA was stable over
SPAN. Only small amount of by-products, i.e., FA (< 5%) and
furfural (< 2%), were detected, both of which might come from
species and ammonium sulfonates. Notably,
a linear
correlation between the S/N molar ratio and the yield of HMF
was obtained (Fig. 3), which demonstrates that the presence
of sulfonic acid group contributes to the catalytic activity. As a
support, an intrinsic polyaniline (DS-PAN) catalyst bearing no
acidic functional group gave out an extremely low HMF yield of
5
0, 51
the direct decomposition of fructose.
3
% (Table 2, entry 14), thus indicating that base sites
In addition, we found that the catalytic performance of
SPAN-11/3 (Table 2, entries 8-11) was higher than Amberlyst-
(secondary amine and imine nitrogen) in SPAN are not the
main active component for fructose dehydration in water/1,4-
dioxane. A highest HMF yield of 71% was obtained due to the
limit of the doping level.
1
5 (Table 2, entries 12 and 13), a commercial sulfonated resin
bearing sulfonic acid group in the backbone, under identical
experimental conditions with the same amount of sulfonic acid
group on catalysts (0.032 mmol). The existence of proper
amount of water in solvent enhanced HMF yields for both
Meanwhile, we found that the HMF yield decreased when
more amount (0.096 mmol) of diphenylamine or aniline was
4
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Fig. 4 Recyclability of SPAN-11/3 catalyst for fructose dehydration. Reaction
conditions: 0.25 mmol of fructose, 15 mg of SPAN, 1 mL of water/1,4-dioxane
o
solvent (V/V = 5/95), 140 C, 3 h, N
2
.
initially located in the solid SPAN catalysts as external dopant
i.e., SAN). Therefore, this leaching should be responsible for
(
the loss in activity. For SPAN-11/3, the HMF yield decreased
slightly from 71% to 69% owing to the less existence of
externally doped sulfonic acid species. Notably, the IR bands at
Scheme 2. Proposed mechanism for the SPAN-catalyzed dehydration of fructose
to HMF.
-1
1
575 cm in the FTIR spectra of all the used SAPN-x/y catalysts
-1
added to BSA (Table S4†, entry 5-6), which illustrates that the were all downshifted to 1594 cm (characteristic IR band for
salification of sulfonic acid group with amine group led to the fresh SPAN-11/3) (Fig. 2b), therefore supporting the decrease
decrease in activity, as proposed by G. Sampath et al.
5
2
in interaction level between sulfonic acid group and the
Therefore, we assume that the H-bonded sulfonic acid group is amine/imine nitrogen due to the removing of externally doped
the main catalytic active site for fructose dehydration. A sulfonic acid species from the fresh catalysts. The linear
solvent effect was observed (Table S5†), wherein proꢀc relationship between the HMF yield and the S/N molar ratio of
solvents (methanol and ethanol) gave out higher HMF yields used SPAN catalysts is also exhibited in Fig. 3, where the slope
than polar aprotic or weakly polar solvent (hexane, acetonitrile, for used catalysts is higher than that of fresh catalysts, further
ethyl ether, and chloroform) as the latter stabilize the H- indicating that the remained H-bonded sulfonic acid species on
5
3
bonded species to a less extent.
SPAN is the main active site for fructose dehydration. Finally,
We further investigated the influence of SPAN on the the SPAN-11/3 catalyst showed stable catalytic activity for
stability of HMF by recording the in situ ATR-IR of HMF-water- HMF formation and inhibition of HMF rehydration for at least
o
1
,4-dioxane mixture in the presence of SPAN-11/3 at 95 C. five runs with HMF yield no less than 65% (Fig. 4). The SEM
The characteristic IR bands of HMF kept almost invariable image of the used catalyst shows no change in morphology (Fig.
within 3 h (Fig. S10†), which suggests that HMF is stable when S12). Moreover, no decrease in S/N molar ratio was observed
mixing with SPAN catalyst. In contrast, the ATR-IR spectra in after the fifth catalytic run (Table S1†), which demonstrates
the presence of Amberlyst-15 gave out obvious red shift from the recyclability of SPAN-11/3 for fructose dehydration.
-
1
1
346 to 1340 cm (hydroxymethyl O-H stretching vibration in
Extension of substrate
HMF) accompanying with the appearance of IR band centred
25,
-
1
at 1731 cm (carboxylic C=O stretching vibration) (Fig. S11†).
5
We found that the polymeric SPAN-11/3 catalyst was active for
This result reveals the interaction of hydroxymethyl -OH in the conversion of inulin and sucrose, giving out HMF yields of
4
HMF with the free sulfonic acid group in Amberlyst-15, which 59% and 41% (Table S6†), respecꢀvely, in low-boiling
further result in the subsequent rehydration of HMF to LA (6%) water/1,4-dioxane cosolvent. However, only 11% of HMF yield
and FA (8%) (Table S4†, entry 7). Therefore, the inhibition role was obtained when glucose was used as the substrate,
of SPAN on HMF rehydration is deemed to be derived from the suggesting that this polymer catalyst is less active for glucose-
stabilization of hydroxymethyl -OH in HMF by the confined to-fructose isomerization. Future study will be focused on the
Brønsted acid site on SPAN. A plausible catalytic mechanism maximization of HMF yield from more available glucose.
for the SPAN-catalysed fructose dehydration in water/1,4-
dioxane is proposed and shown in Scheme 2.
Conclusions
Recyclability of SPAN
In conclusion, we have developed a novel acidic solid
All of the SPAN-x/y catalysts were used for recycling
organocatalyst, sulfonated polyaniline (SPAN), for effective
experiments. The results shown in Table 1 reveal different
dehydration of fructose into HMF in low-boiling water/1,4-
degree of activity decrease on these solid catalysts. No obvious
dioxane cosolvent. A highest HMF yield of 71% is obtained
loss in catalyst weight was observed after the first catalytic run,
from fructose with no LA formation. We demonstrate that
whereas the elemental analysis of the used solid catalysts
hydrogen bonds form between the ring-attached sulfonic acid
showed the loss of sulfur contents (Table S1†). It is reasonable
group and the quinoid imine nitrogen as a result of internal
to assume that the leached sulfur components might be
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Journal Name
doping, which confines the Brønsted acidity of the SPAN the desorption branches of the isotherms. The XRD patterns
catalyst. The H-bonded sulfonic acid species is active for were recorded with a DX-1000 X-ray diffractometer equipped
fructose-to-HMF dehydration and the complete suppression with Cu-Kα radiation. Data were collected over a 2θ range of
o
o
on HMF rehydration. The chemical bonding of sulfonic acid 5~75 with a step of 0.0544 using continuous scanning mode.
group on the backbone of polyaniline chain allows stable The XPS spectra were collected on a XSAM800 spectrometer
recyclability of the polymer catalyst for at least five runs (KRATOS) with Al Kα radiation (1486.6 eV) operating at 12 kV
without obvious activity loss. This work highlights the use of and 12 mA. The BE value was calibrated with C1s at 284.6 eV
novel solid organocatalyst for transforming biomass-derived and was estimated to be accurate within 0.2 eV. ATR-IR
o
starting materials to fine chemicals and the potential spectra were collected at 95 C on the ReactIR iC10 system
importance of acid-base interactions on solid catalyst in (Mettler Toledo) equipped with a liquid nitrogen cooled
confining the Brønsted acidity for improving the product mercury cadmium telluride (MCT) detector with a resolution of
-
1
selectivity.
4 cm . The FT-IR was performed on a Nicolet Nexus 6700
−1
spectrometer in the wavenumber range of 4000~400 cm
−
with a spectral resolution of 4 cm using KBr pellets.
1
Experimental section
Catalytic reactions
Materials
All catalytic reactions were carried out in pressurized tubes. In
Fructose (99%), inulin (95%), HMF (98%), furfural (98%), FA, a typical reaction procedure, a 15 mL reaction tube was
,4-dioxane (ACS grade), and SAN (98%) were purchased from charged with 45 mg (0.25 mmol) of fructose, 15 mg of SPAN, 1
1
J&K chemical co. ltd. LA (98%) was purchased from Sigma mL of solvent (water/1,4-dioxane) in turn, followed by being
Aldrich. Glucose, sucrose, aniline, methanol, ethanol, hexane, sealed and heated in a temperature-controlled oil bath with
o
acetonitrile, chloroform, and APS were of analytical grade magnetic stirring at 140 C for required reaction time. After
obtained commercially from Kolong Chemical Company reaction, the mixture was cooled gradually to room
(Chengdu, China) and used without further purification. Aniline temperature. The solid catalysts were separated from the
was distilled before use. Deionized (DI) water (resistance>18.2 resulting mixture by filtration. For substrate extension, glucose,
-
1
M
Ω
cm ) was used in all experiments.
inulin, and sucrose (0.25 mmol based on monosacchride),
respectively, were used as the substrate for catalytic reaction.
Preparation of SPAN
Analysis of products
In a typical run for the synthesis of SPAN, 2.7 mmol of freshly
distilled AN and 0.9 mmol of SAN were dissolved in 40 mL of The quantitative analysis of fructose, glucose, HMF, LA, FA and
water, followed by the addition of 20 mL of aqueous solution FF was performed by HPLC (Dionex, UItiMate 3000 Series)
of APS (0.06~0.72 M) in one portion. The polymerization was equipped with a dionex PG-3000 pump, an aminex column
o
o
carried out at 5 C for 24 h without stirring. The resulting black HPX-87 column (Bio-Rad) (50 C), a shodex 101 Refractive
o
precipitates were washed with DI water until the filtrate Index detector (35 C) and a Variable Wavelength detector.
became transparent, followed by being dried on vacuum at 80 0.005 M of aqueous H SO solution was used as mobile phase
2
4
o
−1
C overnight.
at a flow rate of 0.6 mL·min . The conversion of fructose and
the yields of products were calculated based on external
standard curves, which was constructed with authentic
Preparation of DS-PAN
Intrinsic polyaniline (DS-PAN) was prepared with the same samples. Conversion of fructose and yields of products were
procedure to SPAN, except that 3.6 mmol of AN and 0.9 mmol defined as follows.
of H
followed by being treated with 0.1 M NH
4
SO that doped on the polymer chains. The resulting brown 100%
2
SO
4
were used as monomer and acid source, respectively,
Conversion of fructose (mol%) =
3
2
·H O to remove
(moles of fructose reacted)/(moles of starting fructose) ×
H
2
precipitates were washed with DI water until the pH value of
the filtrate became neutral, then they were dried on vacuum
o
at 80 C overnight.
Yield of products (HMF, LA, FA, FF) (mol%) =
(moles of products formed)/(moles of starting fructose) ×
100%
Characterization of SPAN
Elemental analysis was conducted on Thermo Finnigan (FLASH,
Acknowledgements
1
112 SERIES) instrument. The SEM images were obtained
through low vacuum SEM (FEI Inspect F) instrument. The SPAN This work was supported by the National Natural Science
samples were coated with gold prior to SEM observation. N
Foundation of China (No. 21536007), the Special Research Fund
2
physisorption isotherms of the catalysts were measured at 77 for the Doctoral Program of Higher Education (No.
K on a Micromeritics Tristar II 3020 analyzer . The surface areas 20120181130014) of China, and the Basal Research Fund of the
were determined by the Brunauer-Emmett-Teller (BET) Central University (2016SCU04B06). The characterization from
equation. The pore volumes and average pore diameters were the Analytical and Testing Center of Sichuan University was
greatly appreciated.
determined by the Barrett-Joyner-Halenda (BJH) method from
6
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ARTICLE
2
7. A. S. Amarasekara, L. D. Williams and C. C. Ebede,
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Green Chemistry
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DOI: 10.1039/C6GC03604J
Sulfonated Polyaniline as Solid Organocatalyst for Dehydration
of Carbohydrates into 5-Hydroxymethylfurfural
a
a
b
a
a
Jinhang Dai , Liangfang Zhu* , Dianyong Tang , Xing Fu , Jinqiang Tang , Xiawei
a
a
Guo and Changwei Hu*
a
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College
of Chemistry, Sichuan University, Chengdu, Sichuan 610064, P. R. China.
b
Research Institute for New Materials Technology, Chongqing University of Arts and Sciences,
Chongqing 402160, P.R. China
Fax: +86 28 85411105; Tel: +86 28 85411105.
E-mail: changweihu@scu.edu.cn or zhulf@scu.edu.cn
Sulfonated polyaniline with mutually reactive sulfonic acid group and amine/imine
represents an organocatalyst for effective production of 5-hydroxymethylfurfural
(
HMF) from carbohydrates in low-boiling solvent with complete restriction of HMF
rehydration.