Formyl-modified polyaniline for the catalytic dehydration of fructose to 5-hydroxymethylfurfural

Article abstract of DOI:10.1002/cssc.201600503

We report an unprecedented solid organic-base catalyst, formyl-modified polyaniline (FS-PAN), for the dehydration of fructose to 5-hydroxymethylfurfural (HM with a high yield of 90.4 mol%. We demonstrate that the nitrogen atoms incorporated between the phenyl rings in the backbone of the polyaniline chain contribute to the basicity of the catalyst. The grafting of electron-withdrawing formyl groups to the imine nitrogen atoms leads to a significant increase of basicity of the polymer catalyst owing to the greater localization of electrons at the amide nitrogen atom formed. A linear dependence of the yield of HMF on the grafting level of formyl groups in FS-PAN indicates that the amide acts as the active phase. A possible reaction mechanism for this organic-base-catalyzed dehydration reaction is proposed. The side-reaction of HMF rehydration is inhibited thoroughly, and the condensation of any reaction intermediates to undesirable oligomers is restrained by this base catalyst. This organic-base catalyst can be recycled completely without loss of activity. This research highlights the first application of a highly effective and stable solid base catalyst for the transformation of renewable carbohydrates into fine chemicals.

Full text of DOI:10.1002/cssc.201600503

DOI: 10.1002/cssc.201600503
Full Papers
Formyl-Modified Polyaniline for the Catalytic Dehydration
of Fructose to 5-Hydroxymethylfurfural
Liangfang Zhu,[a] Jinhang Dai,[a] Mingyang Liu,[a] Dianyong Tang,[b] Shuqing Liu,[a] and
Changwei Hu*[a]
We report an unprecedented solid organic-base catalyst,
formyl-modified polyaniline (FS-PAN), for the dehydration of
fructose to 5-hydroxymethylfurfural (HMF) with a high yield of
90.4 mol%. We demonstrate that the nitrogen atoms incorpo-
rated between the phenyl rings in the backbone of the polyan-
iline chain contribute to the basicity of the catalyst. The graft-
ing of electron-withdrawing formyl groups to the imine nitro-
gen atoms leads to a significant increase of basicity of the
polymer catalyst owing to the greater localization of electrons
at the amide nitrogen atom formed. A linear dependence of
the yield of HMF on the grafting level of formyl groups in FS-
PAN indicates that the amide acts as the active phase. A possi-
ble reaction mechanism for this organic-base-catalyzed dehy-
dration reaction is proposed. The side-reaction of HMF rehy-
dration is inhibited thoroughly, and the condensation of any
reaction intermediates to undesirable oligomers is restrained
by this base catalyst. This organic-base catalyst can be recycled
completely without loss of activity. This research highlights the
first application of a highly effective and stable solid base cata-
lyst for the transformation of renewable carbohydrates into
fine chemicals.
Introduction
Renewable biomass-derived sugars are good alternatives to di-
minishing fossil resources for the synthesis of bulk chemicals
and transportation fuels.[1] The development of effective meth-
ods to convert naturally available C6 sugars to useful and poly-
functional molecules, such as 5-hydroxymethylfurfural (HMF), is
of great significance for biorefinery.[2]
lectivity and satisfactory stability for the dehydration of hexose
are still limited.
The use of solid base catalysts for hexose dehydration may
result in cleaner reactions because of the lack of acid sites.
However, strongly alkaline conditions should be avoided as
they promote the degradation of the monosaccharide to more
than 50 different byproducts.[10] Although organic-base cata-
lysts (i.e., amines) have been achieved the glucose-to-fructose
isomerization with high yields and selectivities,[11] few examples
have focused on the dehydration of fructose to HMF. The de-
velopment of effective solid base catalysts for fructose dehy-
dration will open up a new avenue to the highly selective syn-
thesis of HMF from biomass-derived saccharides.
As a versatile and key platform chemical, HMF is produced
mainly through the acid-catalyzed dehydration of C6 sugars.[3]
Recent efforts have been devoted to the development of solid
acid catalysts to meet the recyclability issue.[4] However, the
Brønsted acid sites in solid acids are prone to deactivation or
leaching by water formed in the dehydration step, and these
problems can be limited by using Lewis acid catalysts instead
of Brønsted acids.[5] However, HMF can be further rehydrated
to levulinic acid (LA) and formic acid (FA) by the acid cataly-
sis;[6] thus, the process requires the continuous extraction of
HMF from the reaction solution by an organic solvent,[7] the
continuous extraction of water under reduced pressure,[8] or
the stabilization of HMF by an appropriate solvent to increase
the selectivity to HMF.[9] Successful catalysts that allow high se-
Polyaniline is the first conducting polymer in which a nitro-
gen atom is incorporated between the phenyl rings in the
backbone of the polymer chain. The electronic properties of
polyaniline can be adjusted through acid/base doping or oxi-
dation/reduction treatments,[12] which result in significant dif-
ferences to its properties. Polyaniline is a hybrid of benzenoid
amine (ÀNHC6H4À) and quinoid diimine (ÀN=C6H4=NÀ) struc-
tures (Scheme S1),[13] whereas H2SO4-doped polyaniline (S-PAN,
Scheme 1a) is conductive owing to the delocalization of p-
electrons through the “protonated quinone imine” structure.[14]
The imine nitrogen atoms are protonated preferentially be-
cause the imine nitrogen atom is more basic than the amine
nitrogen atom.[15]
[a] Dr. L. Zhu, J. Dai, M. Liu, S. Liu, Prof. C. Hu
Key Laboratory of Green Chemistry and Technology
Ministry of Education, College of Chemistry
Sichuan University
Chengdu, Sichuan 610064 (PR China)
E-mail: changweihu@scu.edu.cn
[b] Dr. D. Tang
Herein, we report that the grafting of formyl groups onto
the quinoid imine nitrogen atoms of S-PAN to endow the poly-
mer catalyst (i.e., formyl-modified polyaniline, abbreviated as
FS-PAN, Scheme 1b) with a significantly higher basicity, which
results in a high HMF yield of 90.4 mol% for the catalytic dehy-
Research Institute for New Materials Technology
Chongqing University of Arts and Sciences
Chongqing 402160 (PR China)
Supporting Information for this article can be found under:
http://dx.doi.org/10.1002/cssc.201600503.
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Scheme 1. The structures of (a) S-PAN and (b) FS-PAN. The red arrows indi-
cate the direction of electron flow in the polymer catalyst. The presence of
benzenoid amine units in FS-PAN is omitted but presented in Scheme S2.[13]
dration of fructose in DMSO. The structure, activity, and recy-
clability of this solid base catalyst are evaluated. We show that
the solid organic-base catalyst is highly selective and stable
during the dehydration process. A possible reaction mecha-
nism for this organic-base-catalyzed dehydration reaction is
proposed. The side-reaction of HMF rehydration to LA is sup-
pressed thoroughly, the condensation of any reaction inter-
mediates to undesirable oligomers is limited, and fructose is
converted completely in this base-catalyzed system.
Figure 1. SEM images of (a) S-PAN and (b) FS-PAN (inset: SEM images, for
which the catalysts were sonicated); (c) XRD patterns of S-PAN and FS-PAN;
(d) FTIR spectra of S-PAN and FS-PAN.
tion of the benzenoid structure.[18] It is generally accepted that
the relative peak intensity I1570/I1483 is indicative of the extent of
oxidation in polyaniline.[12a] The band at n˜ ꢀ1293 cmÀ1 origi-
nates from the CÀN stretching of secondary aromatic amines,
whereas the band at n˜ ꢀ1236 cmÀ1 is related to protonated CÀ
N groups. The band at n˜ ꢀ1116 cmÀ1 corresponds to the CÀH
aromatic in-plane bending and is often related to the doped
structure; therefore, the relative peak intensity I1116/I1293 reflects
the doping level of polyaniline. Moreover, the band at
n˜ ꢀ792 cmÀ1 originates from the CÀH out-of-plane bending in
1,4-disubstituted ring structures and indicates the occurrence
of the para coupling of aniline during the polymerization. In
comparison with the S-PAN spectrum, we found clear shifts
Results and Discussion
Catalyst preparation and characterization
We synthesized the catalyst precursor S-PAN through the oxi-
dative polymerization of aniline with ammonium persulfate
(APS) in dilute aqueous H2SO4 solution at 58C.[16] The oligomers
and any possible byproducts formed in the polymerization
process were removed by repeated washing with water, meth-
anol, and diethyl ether. To prepare the FS-PAN catalyst, S-PAN
was reacted with formaldehyde solution (40 wt%, nHCHO/nAN
=
24) at 1408C for 4 h, and the product was washed with water
and dried at 1008C overnight.
The morphologies of the S-PAN precursor and FS-PAN cata-
lyst were characterized by SEM (Figure 1a and b). Both of
them are irregular aggregates of polyaniline nanoparticles
(ꢀ450 nm) with diameters of 100 to 300 mm. The BET surface
areas of S-PAN and FS-PAN were determined to be 21.6 and
25.5 m2 gÀ1, respectively, and their average pore diameters
were 1.4 and 1.3 nm (Table S1).[13] The XRD pattern of S-PAN
shows the typical amorphous structure of doped polyaniline
with two diffraction peaks centered at 2qꢀ21 and 25.58,
whereas a broad diffraction peak centered at 2qꢀ248 for FS-
PAN corresponds to a dedoped (deprotonated) polyaniline
structure (Figure 1c).[17] These results indicate the occurrence
of deprotonation during the modification of S-PAN with form-
aldehyde. The elemental analysis revealed that the doping
level of H2SO4 in S-PAN is approximately 18.0%, whereas nearly
no sulfur atoms exist in the backbone of the FS-PAN catalyst
(Table S2);[13] this confirms the complete removal of H2SO4
from S-PAN after the formaldehyde treatment.
from n˜ ꢀ1293 to 1330, 1483 to 1504, and 1570 to 1606 cmÀ1
,
respectively, in the FTIR spectrum of FS-PAN. These upshifts
can be explained by the occurrence of deprotonation,[19] as in-
dicated by the XRD and element analysis results. Furthermore,
the significant decrease of I1116/I1330 compared with I1116/I1293 for
S-PAN confirms the dedoping of FS-PAN from S-PAN by formal-
dehyde. Furthermore, a decrease in I1606/I1504 compared with
I1570/I1483 suggests the reduction of quinoid to benzenoid struc-
tures. That is, in addition to deprotonation, the modification of
S-PAN with formaldehyde also leads to the further reduction of
polyaniline. Notably, we observed two new infrared bands cen-
tered at n˜ =1676 and 1256 cmÀ1; the former band corresponds
to the amide C=O stretching and suggests that formyl groups
have been introduced to the FS-PAN catalyst. The band at n˜ =
1256 cmÀ1 could not be assigned to the stretching of protonat-
ed CÀN groups of the deprotonated catalyst; therefore, it is ra-
tionally related to a new form of CÀN stretching, probably
amide CÀN stretching. That is, the new formyl groups in FS-
PAN are most probably grafted onto the quinoid imine nitro-
gen atom during the formaldehyde treatment.
We used FTIR spectroscopy to characterize the structures of
the catalysts (Figure 1d). For S-PAN, the band at n˜ =1570 cmÀ1
corresponds to the C=C ring stretching vibration of the qui-
none units, whereas the band at n˜ =1483 cmÀ1 with a shoulder
at n˜ =1380 cmÀ1 is assigned to the C=C ring stretching vibra-
The S-PAN and FS-PAN were further characterized by X-ray
photoelectron spectroscopy (XPS). The benzenoid amine, qui-
noid imine, protonated imine, polarized quinoid imine, and
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formyl-grafted amine structures in polyaniline can be differenti-
ated quantitatively in the deconvoluted N1s XPS spectrum. For
S-PAN (Figure 2a), the N1s XPS spectrum is deconvoluted to
We subsequently used ESR spectroscopy to study the elec-
tron spins in S-PAN and FS-PAN (Figure S2, curves a and g).[13]
The obtained spectra can be described with simple peak-to-
peak linewidths (DHpp) and average g values, as listed in
Table 1. For the untreated S-PAN (Entry 1), we observe two
lines with average g values of approximately 2.0025, which are
assigned to typical p radicals in conjugated carbon systems.
Table 1. Results from ESR and FTIR spectroscopy of FS-PAN catalysts ob-
tained through the treatment of S-PAN with formaldehyde for different
times.[a]
Entry Treatment time [h]
g
DHpp [G] I1116/I1330 I1606/I1504 I1676/I1330
1
2
3
4
5
6
7
8
0
2.0025 3.9
2.0031 2.5
2.0032 3.1
2.0035 15.4
2.0035 15.4
2.0035 16.1
2.0035 16.1
2.0029 13.8
1.29
1.05
1.00
0.97
0.96
0.96
0.96
0.71
0.85
0.62
0.60
0.60
0.57
0.54
0.48
0.87
0
0.25
0.5
1.0
2.0
3.0
4.0
0.34
0.62
0.72
0.72
0.72
0.73
0
[b]
–
Figure 2. Deconvoluted N1s XPS spectra of (a) S-PAN and (b) FS-PAN; decon-
voluted C1s XPS spectra of (c) S-PAN and (d) FS-PAN.
[a] Treatment conditions: S-PAN (1 g), formaldehyde solution (20 mL,
40 wt%, nHCHO/nAN =24), stirring, 1408C. [b] D-PAN obtained by treating S-
PAN with aqueous ammonia.
three components at binding energies (BEs) of 399.3, 400.9,
and 402.8 eV, which are assigned to benzenoid groups (ÀNHÀ),
+
+
[20]
C
radical cations (ÀN =), and cations (ÀN H=), respectively.
This result is consistent with the unpaired spin delocalized pri-
marily on the N-incorporated phenyl rings.[22] For FS-PAN
(Entry 7), the g value increases to 2.0035, a typical value for
heteroatom systems,[23] which indicates that the spins become
more localized at the nitrogen sites.[24] In addition, we also ob-
served the broadening of DHpp from 3.9 G for S-PAN to 16.1 G
for FS-PAN, which qualitatively supports the greater localiza-
tion of charge carriers at the amide nitrogen atoms of FS-
PAN.[25] As a result, the basicity of the FS-PAN catalyst is en-
hanced owing to the higher electron density at the amide ni-
trogen atom.
The surface contents of these three kinds of nitrogen species
were evaluated to be 70.7 (399.3 eV), 25.6 (400.9 eV), and 3.7%
(402.8 eV), and these results indicate the existence of polarons/
bipolarons owing to the protonation of quinoid imines. As
a consequence, the peak at BE=398.2 eV for imine nitrogen
atoms is not observed. For FS-PAN (Figure 2b), only two com-
ponents at BEs of 399.3 and 400.2 eV are resolvable. The two
peaks at BE>400.9 eV are not resolvable, which supports the
complete deprotonation of FS-PAN and is also consistent with
the structure proposed on the basis of the FTIR results. The
new peak at a BE of 400.2 eV is assigned to the amide nitrogen
atoms originating from the grafting of the formyl groups to
the imine nitrogen atoms. Theoretical simulations at the M06-
2x/def2-tzvpp level support the above assignment (Fig-
ure S1).[13] Furthermore, the surface contents of the benzenoid
amine nitrogen and amide nitrogen atoms of approximately
86.7 and 13.3%, respectively, indicate that the grafting level of
formyl groups is approximately 13.3%. The C1s spectra of S-
PAN and FS-PAN were also deconvoluted. For S-PAN (Fig-
ure 2c), the main peak at a BE of 284.6 eV is assigned to the
CÀC/CÀH groups present in the polymer in addition to the
standard substance. The peaks at BEs of 285.9 and 288.5 eV are
attributed to CÀN and C=N+H groups, respectively. The shake-
up satellite peak at BE=291.2 eV is attributed to CÀN groups
in polymer.[21] For FS-PAN (Figure 2d), three components at
BE=284.6, 285.9, and 287.7 eV are resolvable, and the latter
one is assigned to the C=O groups that have been introduced
through formaldehyde modification. Therefore, both the N1s
and the C1s XPS results confirm the grafting of formyl groups
to the imine nitrogen atoms of FS-PAN.
We also studied the FTIR spectra of the FS-PAN catalysts pre-
pared by treating S-PAN with formaldehyde for different times
to get detailed information on their structural variations. The
results are presented in Figure S3,[13] and the characteristic
peak intensities are listed in Table 1. The values of I1116/I1330
,
I1606/I1504, and I1676/I1330 represent the doping level, oxidation
level, and grafting level of the FS-PAN catalysts, respectively.
Within 1 h, I1116/I1330 and I1606/I1504 decrease rapidly with time,
and this reflects a fast deprotonation from S-PAN and the re-
duction of the quinone moieties to form phenyl rings. After
1 h, I1116/I1330 does not vary, but I1606/I1504 still displays a slight
decreasing tendency, which indicates the ongoing reduction of
the quinone structure owing to the excess of formaldehyde in
the system. Accordingly, I1676/I1330 exhibits an increasing tenden-
cy from 0 to 1 h; the value reaches a maximum value at 1 h
and then remains almost invariable over 1 to 4 h. This intensity
variation reflects the fast grafting of the formyl groups within
the first 1 h. That is, the reaction of formaldehyde with S-PAN
starts with the removal of the dopant (sulfuric acid), followed
by the grafting of formyl groups to the imine nitrogen atoms
and the gradual quinoid to benzenoid reduction.
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The ESR spectra of these FS-PAN catalysts obtained
with different modification times were also studied
to reveal the electronic characters of the catalysts
during the formaldehyde treatment (Figure S2).[13]
The g values and DHpp values obtained from the ESR
spectra are also listed in Table 1; the g values of
these FS-PAN catalysts increase significantly within
1 h (Entries 1 to 4) and then remain unchanged from
1 to 4 h (Entries 5 to 7). The increase of the g value
within 1 h reveals the fast localization of electrons at
the nitrogen sites, which may not be caused by the
reduction of polyaniline because the occurrence of
reduction after 1 h does not lead to a further increase
of the g value. As we could not determine whether
the localization of electrons at the nitrogen sites is in-
duced by the deprotonation of the dopant or graft-
ing of the formyl group, we prepared a deprotonated
polyaniline (D-PAN) to solve this problem by treating
S-PAN with excess aqueous ammonia. The FTIR and
ESR spectra of D-PAN are shown in Figures S4 and
Table 2. Catalytic dehydration of C6 sugars to HMF over polyaniline-based catalyst-
s.[a]
Entry Catalyst Conversion [mol%] YHMF [mol%] YDFF [mol%] YLA [mol%] YFA [mol%]
1
2
S-PAN 100
FS-PAN 100
44.9
90.4[b]
6.4
42.9
52.3
36.4
21.9
84.5
84.6
64.2
7.2
0.4
–
0.5
0.5
–
9.2
–
–
–
–
–
–
–
–
12.1
3.5
4.7
27.2
9.5
1.4
1.3
6.6
9.4
9.1
3[c] FS-PAN 89.7
4[d] FS-PAN 100
5[e] FS-PAN 100
6
7
D-PAN[f] 100
P-PAN[g] 100
–
8[h] FS-PAN 100
9[i] FS-PAN 100
10[j] FS-PAN 100
1.9
1.4
1.0
–
[a] Reaction conditions: fructose (45 mg), catalyst (30 mg), DMSO (1 mL), 1408C, 4 h.
[b] TON=5.3 based on the amide nitrogen atoms. [c] Glucose as the substrate.
[d] Sucrose as the substrate. [e] Inulin as the substrate. [f] D-PAN was prepared by
treating S-PAN with aqueous ammonia for deprotonation. [g] P-PAN was prepared
by treating D-PAN with phenylhydrazine to reduce the quinoid imine to benzenoid
amine. [h] Acetone as the additive (nacetone/nfructose =3:1). [i] DHA as the additive (nDHA
fructose =3:1). [j] Glycerol as the additive (nglycerol/nfructose =3:1).
/
n
S5,[13] respectively. The lower I1116/I1330 ratio compared with that
of S-PAN is indicative of the successful deprotonation of D-
PAN. The g value and DHpp for D-PAN are 2.0029 and 13.8 G, re-
spectively (Table 1, Entry 8). The smaller g value of D-PAN than
that of FS-PAN demonstrates that the localization of charge
carriers at the nitrogen atoms of the FS-PAN catalyst is not in-
duced mainly by the deprotonation of S-PAN but may be
caused by the grafting of formyl groups to the imine nitrogen
atoms. We observed the gradual broadening of DHpp from 3.9
to 16.1 G during the formaldehyde treatment from 0 to 4 h;
the initial narrowing of DHpp within 0.5 h may be derived from
the promotion of the mobility of electrons in the polymer
chain backbone because of the coexistence of dopant and
electron-withdrawing formyl groups.
Figure 3. Images of the products from the catalytic dehydration of fructose
over (a) S-PAN and (b) FS-PAN.
Activity comparison of S-PAN and FS-PAN
occur owing to the presence of a Brønsted acid, even in the
well-known polar aprotic solvent DMSO that is frequently used
to stabilize HMF.[9a]
The catalytic performances of S-PAN and FS-PAN for fructose
dehydration were studied in DMSO at 1408C for 4 h (Table 2,
Entries 1 and 2). Over the S-PAN catalyst precursor, an HMF
yield of 44.9 mol% was obtained, and the formation of by-
products such as FA (12.1 mol%), LA (9.2 mol%), 2,5-formylfur-
an (DFF, 7.2 mol%), and furfural (0.3 mol%) was observed. The
resulting mixture was dark (Figure 3a). The simultaneous for-
mation of FA and LA indicates the occurrence of HMF rehydra-
tion in the dehydration system. The formation of DFF, the oxi-
dized product from HMF, reveals the oxidative nature of S-PAN,
as is consistent with the FTIR spectroscopy results. The appear-
Significantly, if FS-PAN was used as the catalyst, a clear
orange mixture (Figure 3b) afforded a higher HMF yield of
90.4 mol%. The yield of DFF decreased to 0.4 mol%, which im-
plies that the oxidative ability of the FS-PAN catalyst is reduced
owing to the reduction of a quinoid imine to a benzenoid
amine structure during formaldehyde modification. FA
(3.5 mol%) was the main byproduct detected. However, no LA
was produced; therefore, the rehydration of HMF is restrained
thoroughly in this base-catalyzed system, and the formed FA
ance of a small amount of furfural is indicative of the direct CÀ may be produced directly from the decomposition of fruc-
C cleavage of fructose in DMSO.[26] The carbon balance was
evaluated to be 62.0%. As fructose was completely converted,
the carbon imbalance indicates the formation of insoluble
humins or soluble oligomers. The low selectivity (44.9%) of
HMF in this S-PAN-catalyzed system may be attributed to the
presence of sulfuric acid that has been doped in the polymer
chain. That is, several side-reactions, such as the rehydration of
HMF and the condensation of any reaction intermediates,
tose.[27] The carbon balance increased from 62.0% (for S-PAN)
to 91.6% for this organic-base catalyst. However, this FS-PAN
catalyst exhibits low activity for the catalytic dehydration of
glucose to HMF (Table 1, Entry 3), possibly because of the low
catalytic activity for glucose-to-fructose isomerization in DMSO.
For sucrose or inulin as the substrate, HMF yields of 42.9 and
52.3 mol%, respectively, were obtained on the basis of the
conversion of the monosaccharide (Table 1, Entries 4 and 5);
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therefore, the FS-PAN catalyst is active for the dehydration of
fructose units under the reaction conditions applied.
Identification of the active phase for fructose dehydration
We are interested in the high activity and selectivity of the FS-
PAN catalyst. To discern whether the catalytic activity origi-
nates from the deprotonation of the Brønsted acid, the reduc-
tion of the quinoid diimine, or the grafting of formyl groups
on FS-PAN, we prepared a reductive form of polyaniline (i.e., P-
PAN) by treating D-PAN with phenylhydrazine for activity com-
parison. The lower I1596/I1494 ratio in the FTIR spectrum of P-PAN
reveals the transformation of the quinoid imine structure to
the benzenoid amine structure in P-PAN (Figure S6).[13] The
yields of HMF over D-PAN and P-PAN were 36.4 and
21.9 mol%, respectively (Table 2, Entries 6 and 7). Hence, for
the four polyaniline-based catalysts, the activity order towards
fructose dehydration is FS-PAN>S-PAN>D-PAN>P-PAN. Con-
sidering the structures of these catalysts, we tentatively infer
that the high activity of FS-PAN is not derived from the dedop-
ing or reduction of the polyaniline backbone but may originate
from the grafting of formyl groups to the imine nitrogen atom.
Specifically, the activity of the formed amide nitrogen atom is
higher than that of the imine nitrogen atom, and the latter is
more active than the amine nitrogen atom.
Figure 4. Yields of HMF over FS-PAN catalysts prepared by varying (a) the
ratio of formaldehyde to aniline monomer (nHCHO/nAN) and (b) the treatment
time with formaldehyde. (c) Linear dependence of HMF yield on the grafting
level of formyl groups (expressed as I1676/I1330) in FS-PAN catalysts.
ing formyl groups at the amide nitrogen atom. A more basic
FS-PAN catalyst may favor the interaction between fructose
and the catalyst to initiate the dehydration reaction.
To illustrate the above point, in control experiments, we syn-
thesized a series of FS-PAN catalysts by varying the amount of
formaldehyde used to modify the S-PAN precursor. The FTIR
spectra of the as-synthesized FS-PAN catalysts are plotted in
Figure S7.[13] The I1676/I1330 ration, representative of the grafting
level of formyl groups in FS-PAN, increases significantly as the
molar ratio of formaldehyde to aniline monomer (nHCHO/nAN) in-
creases, reaches a maximum at nHCHO/nAN =24, and then re-
mains unchanged with further increase of nHCHO/nAN to 36. Cor-
respondingly, the yield of HMF exhibits the same variation ten-
dency as the nHCHO/nAN ratio changes (Figure 4a); this reveals
a close correlation between the catalytic activity and the graft-
ing level of formyl groups in the FS-PAN catalysts. Similarly, the
activity of another series of FS-PAN catalysts prepared by vary-
ing the time of formaldehyde modification is presented in Fig-
ure 4b. The yield of HMF increased significantly as the treat-
ment time increased from 0 to 1 h and then remained invaria-
ble after 1 h. This activity variation is consistent with the
change of the grafting level in the FS-PAN catalysts (as illustrat-
ed in Table 1). Notably, the positive linear dependence of the
yield of HMF on the grafting level of formyl groups (expressed
as I1676/I1330 from the FTIR spectra) for all of the as-synthesized
FS-PAN catalysts is summarized in Figure 4c, which indicates
that a higher grafting level of formyl groups in FS-PAN allows
a higher HMF yield. The results reveal strongly that the amide
formed by the grafting of formyl groups is the active phase for
the dehydration of fructose to HMF. The maximum turnover
number (TON) was determined to be 5.3 relative to the amide
nitrogen atoms. From these results in combination with the
ESR analysis results, it is reasonable to assume that the high
activity of FS-PAN is caused by the enhanced basicity of the
polymer catalyst owing to the presence of electron-withdraw-
Possible reaction mechanism for the organic-base-catalyzed
dehydration reaction
To date, much effort has been devoted to reveal the mecha-
nism for the dehydration of fructose to HMF catalyzed by
Lewis or Brønsted Acids. However, the catalytic system without
H+ (or metal) ions has not been studied fully. In a preliminary
study on the mechanism of the FS-PAN-catalyzed dehydration
reaction, we investigated the interactions between the FS-PAN
catalyst and fructose by introducing additives (acetone, 1,3-di-
hydroxyacetone, or glycerol) with structures similar to those of
acyclic or cyclic fructose (Figure S8).[13] The competitive interac-
tion of FS-PAN with fructose and additive molecules will influ-
ence the dehydration of fructose to HMF. If acetone or 1,3-di-
hydroxyacetone was added, a slight decrease in HMF yield was
observed (Table 2, Entries 8 and 9); therefore, the catalyst inter-
acts weakly with the carbonyl groups in acetone and 1,3-dihy-
droxyacetone. This result also suggests that fructose exists
mainly in the form of cyclic furanose in DMSO, typically a/b-
furanose, rather than the acyclic form.[9c] In contrast, the intro-
duction of glycerol decreases the yield of HMF to 64.2 mol%
(Table 2, Entry 10), which is indicative of the strong interaction
of the hydroxy groups in fructose with FS-PAN. Therefore, we
can assume rationally that the dehydration of fructose may be
initiated by the interaction of the hydroxy groups in a/b-fura-
nose with the amide structure in the FS-PAN catalyst.
A possible reaction mechanism for this organic-base-cata-
lyzed dehydration reaction is proposed in Scheme 2; the 2-hy-
droxy oxygen atom with lone electron pairs in fructose reacts
with the electrophilic amide carbon atom in FS-PAN to form in-
termediate 1, followed by the release of one H2O molecule
with the aid of the lone electron pair of the amide nitrogen
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compared with that of the fresh FS-PAN catalyst indicates the
dissociation of formyl groups from the polymer catalyst. Fur-
thermore, a thermogravimetric (TG) study revealed that the FS-
PAN catalyst begins to decompose at elevated temperature
(up to ~4008C, Figure S11).[13] Consequently, this polymeric FS-
PAN catalyst is completely recyclable in four successive runs,
and the HMF yield was least 85.8 mol% (Figure 5). The FTIR
spectra of fresh and used FS-PAN catalysts show no clear differ-
ences (Figure S12).[13] These results demonstrate the high activ-
ity and stability of the solid organic-base catalyst in the pres-
ence of water.
Scheme 2. Proposed mechanism for the FS-PAN-catalyzed dehydration of
fructose to HMF.
atom. Then, the formed enol intermediate 2 is converted into
an aldehyde through keto–enol tautomerism and releases an-
other H2O molecule with the aid of the amide nitrogen atom.
Subsequently, the amide nitrogen atom acts as a base to cap-
ture a proton connected to C5 of fructose to form intermedi-
ate 3. Finally, a third H2O molecule is released to produce HMF.
Further investigations on the capture of the intermediates are
underway.
Figure 5. Recycling of FS-PAN catalyst for the catalytic dehydration of fruc-
tose to HMF. Reaction conditions: fructose (45 mg), catalyst (30 mg), DMSO
(1 mL), 1408C, 4 h.
Effect of reaction solvent and temperature
Conclusions
We studied the influence of the reaction solvent on the forma-
tion of HMF (Table S3). The results show that the reaction in N-
methylpyrrolidone (NMP) produced a high HMF yield of
67.1 mol%. However, the dehydration of fructose in 1,4-diox-
ane, THF, and methyl isobutyl ketone (MIBK) resulted in lower
HMF yields of 15.9–20.5 mol%. Possibly, NMP favors the disso-
lution of fructose and may promote further the interaction be-
tween fructose and the organic-base catalyst.
We have demonstrated that the nitrogen heteroatoms incor-
porated between the phenyl rings in the backbone of the
polyaniline chain contribute to the basicity of the catalyst. The
delocalization of p electrons in H2SO4-doped polyaniline (S-
PAN) results in lower electron density at the nitrogen atoms
and lower basicity of the catalyst, which leads to a lower yield
of 5-hydroxymethylfurfural (HMF, 44.9 mol%) from fructose de-
hydration in DMSO. However, the grafting of electron-with-
drawing formyl groups to the quinone imine nitrogen atoms
leads to greater localization of electrons at the amide nitrogen
atom formed and significantly higher basicity of the formyl-
modified polyaniline (FS-PAN) catalyst. As a result, the FS-PAN
catalyst exhibits high catalytic activity and selectivity for HMF
The effect of reaction temperature on the dehydration reac-
tion was studied in the range 120 to 1608C. The results plotted
in Figure S9 reveal that the HMF yield increases significantly
from 120 to 1408C to reach a maximum value of 90.4 mol% at
1408C and then decreases to 84.9 mol% as the reaction tem-
perature increases further to 1608C. The yield of the byproduct
FA does not change clearly with the variation in reaction tem-
perature; therefore, the slight decrease in HMF yield at elevat-
ed temperature indicates the aggregation of HMF to undesira-
ble oligomers. In all cases, the absence of LA highlights the in-
hibition of HMF rehydration by the organic-base catalyst in
various solvents and at various temperatures.
formation with complete conversion of fructose [YieldHMF
=
90.4 mol%, turnover number (TON)=5.3]. In addition, we have
illustrated a positive linear dependence of the dehydration ac-
tivity of FS-PAN on the grafting level of formyl groups. Notably,
the rehydration of HMF to levulinic acid (LA) and the conden-
sation of any reaction intermediates to undesirable oligomers
are effectively inhibited in this base-catalyzed reaction system.
The thermostability of FS-PAN allows its reuse for at least four
runs without a clear decrease of activity. This research high-
lights the application of a highly selective and recyclable solid
base catalyst for the transformation of renewable carbohy-
drates into fine chemicals.
Recycling of the organic-base catalyst
The thermostability of the FS-PAN catalyst was studied by re-
cording the FTIR spectra of the catalysts after calcination at dif-
ferent temperatures (140, 220, and 3008C; Figure S10).[13] No
clear differences were observed in the FTIR spectra of the fresh
catalyst and the catalysts calcined at 140 and 2208C, whereas
the clear decrease of the intensity of the infrared band at n˜ =
1676 cmÀ1 in the spectrum of the catalyst calcined at 3008C
&
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a Thermo Finnigan (FLASH, 1112 SERIES) instrument. The FTIR spec-
tra of samples as KBr disks were recorded with a Nicolet Nexus
6700 spectrometer with a spectral resolution of 4 cmÀ1 in the
wavenumber range 500–4000 cmÀ1 at room temperature. The XPS
spectra were recorded with a XSAM800 spectrometer (KRATOS)
with AlKa radiation (1486.6 eV) operating at 12 kV and 12 mA. The
binding energy (BE) was calibrated with C1s at 284.8 eV and was
estimated to be accurate within 0.2 eV. A linear background was
subtracted from all spectra. The ESR spectra were recorded at am-
bient temperature with a Bruker BioSpin ESR spectrometer. Ther-
mogravimetric/differential thermogravimetric (TG/DTG) analysis
was performed with a Netzsch TG209 instrument at a heating rate
Experimental Section
Materials
Formaldehyde (40 wt%), H2SO4 (98%), aqueous ammonia (37 wt%)
and analytical reagent (AR) grade aniline, APS, nitrobenzene, meth-
anol, and diethyl ether were purchased from Kolong Chemical
Company (Chengdu, China). 1,3-Dihydroxy acetone (98%), AR-
grade furfural, acetone, phenylhydrazine, and glycerol were pur-
chased from J&K Chemical Co. Ltd. DMSO (AR grade) was obtained
from Fuyu Fine Chemical Co., Ltd. (Tianjin, China). d-Fructose [bulk
reagent (BR) grade] was purchased from REGAL Co. Ltd. FA (98%)
was purchased from Sigma–Aldrich. Glycerol (spectrophotometric
grade) was purchased from Alfa Aesar. LA (98%) was purchased
from Acros Organics. DFF (98%) was purchased from Tokyo Chemi-
cal Industry. Aniline was distilled before use. All of the other chemi-
cal reagents were used as purchased without further purification.
Deionized (DI) water (resistance>18.2 MWcmÀ1) was used in all ex-
periments.
of 10 KminÀ1 with a nitrogen flow of 20 mLminÀ1
.
Catalytic dehydration of fructose
All catalytic reactions were performed in pressure tubes heated in
a temperature-controlled oil bath with magnetic stirring. In a typical
reaction procedure, a 35 mL reaction tube was charged with fruc-
tose (45 mg), FS-PAN (30 mg), and DMSO (1 mL) in turn. The mix-
ture was heated to 1408C and incubated for 4 h. After the comple-
tion of the reaction, the mixture was cooled gradually to room
temperature. The organic-base catalyst was separated from the re-
sulting mixture by filtration.
Catalyst preparation
The catalyst precursor S-PAN was synthesized through the oxida-
tive polymerization of aniline in aqueous H2SO4 solution at 58C. In
a typical synthesis, freshly distilled aniline (0.22m) was dissolved in
aqueous H2SO4 solution (0.78m, 200 mL). Then, a solution of APS
(0.78m, 80 mL) was added slowly over 1 h to the above solution
with the reaction temperature maintained below 58C, as the poly-
merization of aniline is strongly exothermic. After the complete ad-
dition, the mixture was stirred for another 30 min. The resultant
precipitate (S-PAN) was separated by filtration, washed consecu-
tively with water, methanol, and diethyl ether to remove the oligo-
mers and any possible byproducts, and then dried at 1008C over-
night. To prepare the FS-PAN catalyst, S-PAN (1 g) was reacted with
formaldehyde solution (20 mL, nHCHO/nAN =24) at 1408C for 4 h, and
the product was washed with water three times and dried at
1008C overnight. The deprotonated polyaniline catalyst (D-PAN)
was prepared by treating S-PAN with excess aqueous ammonia at
room temperature. The reductive polyaniline catalyst (P-PAN) was
In control experiments, additive compounds (acetone, DHA, or
glycerol) were added together with fructose before the dehydra-
tion reaction. The molar ratio of the additive compound to fructose
was 3:1, and the other reaction conditions were kept unchanged.
Product Analysis
HMF, DFF, and furfural from the liquid products were analyzed
quantitatively by GC (FILI, GC-9700) with an Innowax capillary
column (30 mꢂ0.25 mm), a flame ionization detector (FID), and
a ZB-2020 integrator. Typical conditions for GC analysis were as fol-
lows: injector temperature 2608C, detector temperature 2708C, ni-
trobenzene as the internal standard. Byproducts such as FA and LA
were analyzed quantitatively by HPLC (Dionex, UItiMate 3000
Series) with a Dionex PG-3000 pump, an Aminex HPX-87 column
(Bio-Rad) (508C), a Shodex 101 refractive index detector (358C) and
a variable-wavelength detector with H2SO4 (5 mm) as the mobile
phase at a flow rate of 0.6 mLminÀ1. The yields of the products
were calculated from external-standard curves constructed with
authentic samples. The conversion of fructose, product yields, and
the carbon balance were defined as follows: conversion of fructose
(mol%)=(moles of fructose reacted)/(moles of starting fructose)ꢂ
100%; yield of HMF (mol%)=(moles of HMF formed)/(moles of
starting fructose)ꢂ100%; yield of byproduct (FA, LA, DFF, and fur-
fural; mol%)=(moles of byproduct formed)/(moles of starting fruc-
tose)ꢂ100%; carbon balance (%)=(moles of carbon atoms in all
products)/(moles of carbon atoms in starting fructose)ꢂ100%.
prepared by treating D-PAN with phenylhydrazine (Ph, nPh/nAN
1:1) at 1408C for 4 h.
=
To study the influence of formaldehyde treatment on the catalytic
activity, two series of FS-PAN catalysts were prepared as follows:
(1) FS-PANx catalysts (x represents the treatment time) were pre-
pared by varying the treatment time with HCHO with nHCHO/nAN
kept at 24; (2) FS-PANy catalysts (y represents the molar ratio of
HCHO to AN) were prepared by varying the amount of HCHO used
(nHCHO/nAN =1–36).
Catalyst characterization
The SEM images were recorded with a low-vacuum SEM (FEI In-
spect F) instrument. The catalyst samples were coated with gold
before SEM observation. The N2 physisorption isotherms of the cat-
alysts were measured at À1968C with a Micromeritics ASAP-2020
analyzer. The surface areas were determined with the BET equation.
The pore volumes and average pore diameters were determined
by the Barrett–Joyner–Halenda (BJH) method from the desorption
branches of the isotherms. The XRD patterns were collected with
a LTD DX-1000 CSC diffraction instrument operating at 40 kV and
25 mA with nickel-filtered CuKa radiation (l=1.54056 ꢁ). The data
were collected in the 2q range 5–708 with a step of 0.05448 in con-
tinuous-scanning mode. Element analysis was performed with
Acknowledgements
We thank the Natural Science Foundation of China (No.
21536007), the Special Research Fund for the Doctoral Program
of Higher Education (No. 20120181130014) of China, the Science
and Technology Program of Sichuan (No. 2013JY0015), and the
Basal Research Fund of the Central University (2016SCU04B06)
for financial support.
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Received: April 18, 2016
Revised: May 28, 2016
Published online on && &&, 0000
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L. Zhu, J. Dai, M. Liu, D. Tang, S. Liu,
C. Hu*
&& – &&
Formyl-Modified Polyaniline for the
Catalytic Dehydration of Fructose to
5-Hydroxymethylfurfural
Out of the FS-PAN: An unprecedented
solid organic-base catalyst, formyl-
modified polyaniline (FS-PAN), is devel-
oped for the highly selective dehydra-
tion of fructose to 5-hydroxymethylfur-
fural (HMF). The side-reaction of HMF
rehydration to levulinic acid is inhibited
thoroughly, and the condensation of re-
action intermediates to undesirable olig-
omers is restrained.
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