Sulfonated Porous Polymeric Nanofibers as an Efficient Solid Acid Catalyst for the Production of 5-Hydroxymethylfurfural from Biomass

Article abstract of DOI:10.1002/cctc.201500709

Sustainable supply of energy is one of the biggest challenges today. The conversion of energy from any abundant and renewable resources would be an ideal solution for this ever increasing demand of sustainable energy. Biomass provides a potential energy alternative through the platform chemical 5-hydroxymethylfurfural (HMF), which is considered as a sustainable source for liquid fuels and commodity chemicals. Herein, we report the synthesis of a nanoporous polytriphenylamine (PPTPA-1) having high surface area (1437 m² g^-1) by a simple one-step oxidative polymerization pathway. Upon sulfonation of PPTPA-1, the sulfonated polymer SPPTPA-1 showed very high surface acidity and it has been successfully employed as a solid acid catalyst for direct conversion of sugar to HMF. Both PPTPA-1 and SPPTPA-1 materials have distinct nanofiber morphologies and they are characterized thoroughly by using powder XRD, FTIR spectroscopy, ¹³C solid state magic-angle-spinning NMR spectrometry, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, and N₂ sorption techniques. We have optimized the HMF yields by using different carbohydrate sources and estimated the recycling efficiency of the catalyst.

Full text of DOI:10.1002/cctc.201500709

DOI: 10.1002/cctc.201500709
Full Papers
Sulfonated Porous Polymeric Nanofibers as an Efficient
Solid Acid Catalyst for the Production of
5-Hydroxymethylfurfural from Biomass
Sujan Mondal,[a] John Mondal,*[b] and Asim Bhaumik*[a]
Sustainable supply of energy is one of the biggest challenges
today. The conversion of energy from any abundant and re-
newable resources would be an ideal solution for this ever in-
creasing demand of sustainable energy. Biomass provides a po-
tential energy alternative through the platform chemical 5-hy-
droxymethylfurfural (HMF), which is considered as a sustainable
source for liquid fuels and commodity chemicals. Herein, we
report the synthesis of a nanoporous polytriphenylamine
(PPTPA-1) having high surface area (1437 m2 gÀ1) by a simple
one-step oxidative polymerization pathway. Upon sulfonation
of PPTPA-1, the sulfonated polymer SPPTPA-1 showed very
high surface acidity and it has been successfully employed as
a solid acid catalyst for direct conversion of sugar to HMF. Both
PPTPA-1 and SPPTPA-1 materials have distinct nanofiber mor-
phologies and they are characterized thoroughly by using
powder XRD, FTIR spectroscopy, 13C solid state magic-angle-
spinning NMR spectrometry, field-emission scanning electron
microscopy, high-resolution transmission electron microscopy,
and N2 sorption techniques. We have optimized the HMF
yields by using different carbohydrate sources and estimated
the recycling efficiency of the catalyst.
Introduction
Over a period of several centuries we, the people of the world,
are modernized, globalized, and civilized. Energy is an issue
that touches every person on the planet, because it is one of
the primary needs of our existence. With the increase in the
world population the gap between the supply and demand for
energy is growing day by day.[1] On the other hand the fossil
fuels are not renewable and hence their reserves are limited.
Thus, our energy reserves are declining and this may cause
a big problem in future if the issue remain unresolved for
long.[2] Scientists have developed an alternative resource “bio-
mass” to a viable fuel as an energy alternative.[3] Biomass-de-
rived platform chemical 5-hydroxymethylfurfural (HMF) is now
in widespread use.[4] HMF is an attractive molecule; it is a versa-
tile and key intermediate of many industrially important chemi-
cals and liquid fuels. For example, 2,5-furandicarboxylic acid,
2,5-bis(hydroxymethyl)furan and 2,5-bis(hydroxymethyl)tetrahy-
drofuran are used as polyester building blocks.[5] The rehydrat-
ed products of HMF, levulinic acid and formic acid, are also val-
uable chemicals. Levulinic acid is a precursor of liquid fuel g-
valerolactone,[6] 2,5-dimethylfuran another very demanding
biofuel having very high energy density and insoluble in
water[7] can be directly synthesized from HMF by means of hy-
drogenesis or catalytic reduction.[8] With respect to the uncon-
trolled CO2 emission, people are more concerned about the
global warming, and because of the rise of oil pricing research-
ers are showing more interest in the synthesis of biofuels by
biomass-conversion processes.[9]
In the presence of an acid catalyst, carbohydrate sugars get
converted mainly into HMF through dehydration.[10] Both Lewis
acids as well as liquid mineral acids can catalyze these reac-
tions. Many research groups have reported solid acid catalysts
like zeolites,[10] metal chlorides,[11] phosphates/phosphonates/
graphene oxides,[12] and sulfuric acid/g-valerolactone[13] for the
successful synthesis of HMF or other catalytic reactions. Highly
corrosive and strong mineral acids like HCl, H2SO4, and H3PO4
are often employed as catalyst for the HMF synthesis.[14] Solid
acid catalysts have significant advantages over mineral acids
because they are heterogeneous in nature, thus they are easily
separable from the reaction mixture, nontoxic, noncorrosive in
nature, recyclable, and diminishing other environmental prob-
lems associated with mineral acids. High surface area and the
presence of micropores/mesopores are the most crucial param-
eters for the efficiency of these solid acid catalysts. A wide
range of strategies have been developed to design porous ma-
terials, which do not involve the use of templates or structure-
directing agents.[15] In this context, porous organic polymers
(POPs) are very demanding as one can tune the pore aperture
and reactive functional groups, which are present as the build-
ing blocks of the respective POPs.[16] A wide range of related
porous polymeric materials such as porous aromatic frame-
works,[17] conjugated microporous polymers,[18] porous organic
[a] S. Mondal, Prof. Dr. A. Bhaumik
Department of Materials Science
Indian Association for the Cultivation of Science
2A and 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700 032 (India)
E-mail: msab@iacs.res.in
[b] Dr. J. Mondal
Inorganic and Physical Chemistry Division
CSIR-Indian Institute of Chemical Technology (IICT)
Uppal Road, Hyderabad 500007 (India)
E-mail: johncuchem@gmail.com
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500709.
ChemCatChem 2015, 7, 3570 – 3578
3570
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
frameworks/materials,[19] covalent organic frameworks,[20] etc.
have been synthesized through controlled polymerization/
polycondensation of reactive monomers recently, and these
materials have found useful applications as means of gas stor-
age, sensing, and for catalysis.[21]
nylamine material PPTPA-1. Yield of the polymer network is
quantitative as observed with the other reported porous or-
ganic polymers. Owing to the highly cross-linked structure, this
PPTPA-1 polymer is insoluble in the common organic solvents.
The thermal stability of the polymer has been examined by
thermogravimetry analysis (TGA, see Figure 3), which suggests
that the polymer is thermally stable as a result of their highly
cross-linked robust aromatic framework. In the presence of
Lewis acid FeIII, the tertiary N atoms bound to the phenyl rings
of the triphenylamine (TPA) monomer are activated. For the
second TPA molecule, which is not bound to FeIII, the para po-
sitions of the benzene rings are electron-rich. Thus, electrophil-
ic para substitution and oxidative polymerization proceed
smoothly in the presence of FeCl3 and TPA monomers self-
polymerize into two-dimensional frameworks with a fibrous
morphology. Sulfonation of the aromatic ring inside the
PPTPA-1 polymer in the presence of chlorosulfonic acid leads
to the formation of the SPPTPA-1 polymer. From CHN analysis,
the PPTPA-1 polymer contains 86.87 wt% carbon (C), 5.15 wt%
hydrogen (H), and 5.18 wt% nitrogen (N). After sulfonation,
the sufonated material showed 38.94 wt% carbon (C),
3.76 wt% hydrogen (H), 2.18 wt% nitrogen (N), and 9.78 wt%
sulfur (S) in the SPPTPA-1. Further, as estimated through the
acid–base titration, the SPPTPA-1 material has an acid strength
of 4.30 mmolgÀ1. Owing to such high acidity, SPPTPA-1 could
be employed as solid acid catalyst for the dehydration of car-
bohydrate biomass.
Ionic liquids have been used widely as a solvent for the syn-
thesis of HMF over several acid catalysts.[22] Various biphasic
systems, such as water–methyl isobutyl ketone, water–butanol,
and water–THF have also been used as a solvent to improve
the HMF yield by carbohydrate dehydration.[23] Although there
are few reports on hyperbranched poly(triphenylamine)s syn-
thesized through Grignard reaction or Pd/Ni-catalyzed CÀC or
CÀN coupling reactions,[24] the scope for the sulfonation of this
aromatic network to design high-surface-area solid acid cata-
lyst and its application in biomass energy have not been ex-
plored. Herein, we first report the synthesis of a highly porous
polytriphenylamine (PPTPA-1) by a simple one-step oxidative
self-polycondensation of triphenylamine. PPTPA-1 has been
sulfonated very efficiently to the sulfonated polymer SPPTPA-
1 having very high surface acidity. We have performed the de-
hydration of carbohydrates/biomass in high-boiling solvents
such as DMSO, N-methyl-2-pyrrolidone (NMP), and N,N-dime-
thylacetamide (DMA) with ten weight percent of LiCl under mi-
crowave irradiation. HMF production from carbohydrates (fruc-
tose, sucrose) and biomass such as sugarcane bagasse over
this novel solid acid catalyst SPPTPA-1 is very efficient and eco-
nomically friendly. High surface area, strong surface acidity
along with good thermal stability of this sulfonated porous
polymer could be responsible for its high catalytic efficiency in
the HMF production.
Porosity and nanostructure
Wide-angle powder X-ray diffraction (WAXRD) analysis of
PPTPA-1 (Figure 1Aa) displayed two broad peaks, which could
Results and Discussion
Synthesis of porous organic polymer
One-step oxidative polymerization process (Scheme 1) of tri-
phenylamine in dichloroethane solvent in the presence of
FeCl3 catalyst has been adopted to develop porous polytriphe-
Figure 1. A) WAPXRD patterns and B) Nitrogen adsorption/desorption iso-
therms of a) PPTPA-1 and b) SPPTPA-1.
be attributed to the mostly amorphous framework structure
composed of individual TPA units polymerized in a random
fashion and could also be matched with the diffractions of the
carbon planes (002) and (101), respectively.[25] These broad-
and weak-intensity peaks could be assigned to the formation
of some regular-ordered pore structure resulting from the
polymerization of monomer units through homocoupling. On
the other hand, the WAXRD spectrum of SPPTPA-1 (Figure 1Ab)
displays a single mild hump between 2q value 10–308. After
sulfonation, the disappearance of the weak-intensity peaks
Scheme 1. Synthesis of PPTPA-1 and SPPTPA-1.
ChemCatChem 2015, 7, 3570 – 3578
www.chemcatchem.org
3571
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
could signify lowering of ordered nanostructure. However, the
adsorption/desorption isotherm data (see below) suggested
the retention of the porous architecture of PPTPA-1. Porosity
and surface area of the materials were investigated by the N2
adsorption–desorption analysis at 77 K. N2 adsorption/desorp-
tion isotherms of PPTPA-1 and SPPTPA-1 are shown in Fig-
ure 1B. The adsorption isotherms demonstrate high N2 uptake
at very low relative pressure (Figure 1Ba) suggesting the mate-
rials are mainly microporous in nature. At high P/P0 of N2 de-
sorption, the profile shows a broad hysteresis loop correspond-
ing to the presence of mesopores. The presence of interparti-
cle mesopores or cages can be correlated with the existence of
a large hysteresis loop in the high-pressure region, for which
irreversible gas uptake can take place. The hysteresis loops in
the isotherms could be attributed to the outcome of the swel-
ling of polymeric network upon the gas adsorption when the
polymer–solvent interaction is much greater than the poly-
mer–polymer interaction. Weber et al. have nicely explained
that restricted filling of pre-existing pores is also one of the im-
portant factor for this hysteresis in microporous organic poly-
mer.[25c] The increase in solvation pressure plays a crucial role
for the elastic expansion of restricted pores to allow diffusion
of gas molecules. The nature of the solvation pressure de-
pends on the nature of the pore size. The contraction in
narrow pore size (pore size 0.3 nm) at relatively low pressure
can reduce the throat size down to sizes, inhibiting the uptake
of gas. However, with the increase of P/P0, solvation pressure
increases causing flexible expansion of restricted access pores
(nominally closed pores) resulting in the observed hysteresis
loop in the desorption isotherm of the porous organic poly-
mer. Brunauer–Emmett–Teller (BET) surface area of the PPTPA-
1 is 1437 m2 gÀ1 with total pore volume of 0.881 cm3 gÀ1. On
the other hand, adsorption–desorption isotherms of SPPTPA-
1 displayed the typical type I isotherm (Figure 1Bb) corre-
sponding to the presence of micropores in this sulfonated
polymer. Upon sulfonation of the PPTPA-1 material, the BET
surface area is decreased to 737 m2 gÀ1 with a total pore
volume of 0.338 cm3 gÀ1. The decrease in the BET surface area
and pore volume suggested that aromatic rings are successful-
ly functionalized with the sulfonic acid functional groups
which hinder the N2-adsorption inside the nanoporous channel
of PPTPA-1 polymer. The corresponding pore size distribution
of the material reveals a hierarchical pore system with two dif-
ferent types of pores. The pore size distribution (Figure 2A)
showed a major peak centred at 1.57 nm together with a very
weak peak at 4.13 nm suggesting the PPTPA-1 is mostly micro-
pores in nature. The pore width is also decreased in sulfonated
material SPPTPA-1. Its pore size distribution was obtained by
using the non-local density functional theory model, which
suggested the presence of the peak pore of a dimension
1.30 nm together with a minor peak at 1.86 nm (Figure 2B).
Figure 2. Non-local density functional theory pore size distributions of
A) PPTPA-1 and B) SPPTPA-1.
Figure 3. TGA and DTA plots of PPTPA-1 (black lines) and SPPTPA-1 (red
lines). Solid lines represent the TGA and dotted lines represent the DTA pro-
file of the materials.
up to 558C could be attributed to the intercalated and ad-
sorbed water molecules by the material. The PPTPA-1 polymer
is thermally stable in nitrogen atmosphere up to 5158C. After
5158C, thermal cleavages of the porous PPTPA-1 framework
starts. On the other hand, for the sulfonated material the first
weight loss occurs through the loss of physically adsorbed
water molecules present at the pore surface followed by
strongly adsorbed water molecules having a noncovalent inter-
action with free sulfonic acid groups.[26] Further, weight loss in
the temperature range 160–2708C could be attributed to the
loss of free sulfonic acid groups followed by the cleavage of
the organic framework beyond 5158C. High thermal stability of
this nanoporous polymer could be attributed to the higher hy-
perbranched cross-linking and the rigid polymeric networks in
the molecular skeleton.
Electron microscopic analyses
The morphologies of the PPTPA-1 and SPPTPA-1 were investi-
gated by field-emission scanning electron microscopic (FESEM)
analysis. FESEM images of the samples shown in Figure 4 re-
vealed that the PPTPA-1 and its sulfonated materials both have
nanofiber-like morphology. They have infinite length and fold-
ing with other nearest fibres.
Thermal stability
The thermal stability of the materials (PPTPA-1 and SPPTPA-1)
was examined by TGA under N2 flow. In Figure 3, the TGA plot
of PPTPA-1 polymer is shown, in which a first 2.7% weight loss
ChemCatChem 2015, 7, 3570 – 3578
www.chemcatchem.org
3572
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
Figure 4. FESEM images of A,B) PPTPA-1 and C,D) SPPTPA-1 nanofibers.
From these FESEM images, the fibrous nature for both
PPTPA-1 and SPPTPA-1 are clear. For SPPTPA-1, the diameter of
the nanofiber is to some extent thicker than for PPTPA-1. High-
resolution images of these nanofibers are given by HRTEM
analysis. The representative HRTEM images (Figure 5A, B) clear-
ly show that each nanofiber with quite uniform thickness in
PPTPA-1 is homogenously distributed. However, the pore diam-
eter of each nanofiber is not uniform, it ranges from 11.4 nm
to 73.0 nm. HRTEM images of SPPTPA-1 material (Figure 5C–F)
reveal that the nanofiber-like morphology remains unaltered
after sulfonation of the aromatic rings of PPTPA-1 material.
Electrophilic para substitution followed by oxidative polymeri-
zation with head-to-tail coupling proceed smoothly in the
presence of FeCl3 and the self-polymerization of monomer TPA
molecule leads to the growth of the polymer in 1D direction,
resulting in the formation of nanorods. Triphenylamine mole-
cules also behaved as the capping agents for FeIII ions that can
be selectively adsorbed onto the lateral surfaces of monomer,
and self-assembly of the monomer in 1D direction takes place
causing complete inhibition of the lateral growth of the poly-
mer.
Figure 5. HRTEM images of A,B) PPTPA-1 and C–F) SPPTPA-1 nanofibers at
different magnifications.
As a result, growth of the polymer continues along the
c axis in the direction of 001 plane leading to the develop-
ment of nanofiber-like morphology. No template, surfactant,
and structure-directing agents are needed to develop this
nanofiber-like morphology. Secondary overgrowth, which leads
to the generation of agglomerated particles, is suppressed
during the course of the reaction. At the initial stage of poly-
merization the formation of nanofibers by rapid polymerization
of triphenylamine monomers in their surrounding area is en-
couraged by the initiator molecules through “rapidly-mixed re-
actions”. It can be assumed that secondary growth of amine
polymer will be minimized because of the total consumption
of the initiator during the formation of nanofibers and the
poor availability of reactants.[27]
Figure 6. FTIR spectra of PPTPA-1(black) and SPPTPA-1 (red). The right spec-
trum shows the magnification of the region from 600 to 1400 cmÀ1
.
Spectroscopic studies
In Figure 6 the FTIR spectra of the PPTPA-1 and SPPTPA-1 are
shown using KBr pellet as a reference. In these spectra, the
peak at approximately 1273 cmÀ1 could be attributed to the
CÀN stretching of the tertiary amines and 815 cmÀ1 could be
assigned to a CÀH out-of plane bending of the para-disubsti-
tuted benzene rings. This result suggess that the coupling of
ChemCatChem 2015, 7, 3570 – 3578
www.chemcatchem.org
3573
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
the TPA monomer has taken place.[28] On the other hand the
FTIR spectrum of SPPTPA-1 showed some additional bands
along with all the peaks of PPTPA-1, suggesting the presence
of main polytriphenylamine framework in the sulfonated mate-
rial. The peaks at 1005, 1031, and 1058 cmÀ1 could be assigned
to the asymmetric O=S=O stretching vibrations of ÀSO3H
groups.[26] The band at approximately 991 cmÀ1 could be re-
sponsible for O=S=O symmetric stretching frequency and
744 cmÀ1 for the characteristic SÀO stretching vibration.
XPS analysis
The XPS survey spectrum of the sulfonated material SPPTPA-
1 is provided in Figure 7C. It demonstrates four peaks centered
at approximately 168.6, 284.8, 399.1, and 532.8 eV. These can
be assigned to the S2p, C1s, N1s, and O1s, respectively. In the
XPS spectrum of S2p region (Figure 7D), the binding energy
peak centered at 168.7 eV is observed, which could be attribut-
ed to the S2p of the ÀSO3H groups and thus this XPS result
confirms the presence of the ÀSO3H groups in SPPTPA-1.[29]
13C CP MAS NMR studies
Catalytic conversion of carbohydrates to HMF over
SPPTPA-1
Information on the chemical environment of different carbon
nuclei of PPTPA-1 and SPPTPA-1 materials has been investigat-
ed employing the 13C cross-polarization magic-angle spinning
(CP MAS) NMR analysis, as shown in Figure 7. The
Catalytic performance of newly developed SPPTPA-1 catalyst in
biomass conversion reaction has been examined for the con-
version of fructose to HMF under microwave conditions. Our
study begun with the reaction of fructose (25 mg) with
SPPTPA-1 (5 mg) catalyst in the presence of DMSO (2 mL)
under microwave reaction conditions at 1208C. Catalytic con-
version could be observed and the isolated yield of HMF was
90.3% after 20 min (Table 1, entry 1). Then we increased the re-
Table 1. Microwave-assisted dehydration of carbohydrates to HMF
catalyzed by SPPTPA-1.[a]
Entry Substrates
Solvent
T
t
HMF yield TON
[8C] [min] [%]
1
2
3
4
5
6
7
8
9
10
fructose
fructose
fructose
fructose
fructose
sucrose
sucrose
sucrose
sucrose
sucrose
DMSO
DMSO
DMSO
NMP
120 20
130 20
140 20
140 20
90.3
91.6
94.6
86.7
95.9
51.4
49.5
31.4
46.0
56.6
12.4
16.4
17.1
18.8
5.7
5.83
5.92
6.11
5.60
6.20
3.32
3.20
2.03
3.00
3.66
–
–
–
–
–
–
–
–
–
DMA–LiCl (10%) 140 20
DMSO 140 10
DMA–LiCl (10%) 140 10
DMA–LiCl (10%) 140 20
NMP
NMP
140 10
140 20
140 20
140 30
140 35
140 60
Figure 7. 13C CP MAS NMR spectra of A) PPTPA-1 and B) SPPTPA-1. C) XPS
survey spectrum of SPPTPA-1 and D) S2p XPS spectrum of SPPTPA-1.
11[b] sugarcane bagasse DMSO
12[b] sugarcane bagasse DMSO
13[b] sugarcane bagasse DMSO
14[b] sugarcane bagasse DMSO
15[b] sugarcane bagasse DMA–LiCl (10%) 140 20
16[b] sugarcane bagasse DMA–LiCl (10%) 140 30
17[b] sugarcane bagasse NMP
18[b] sugarcane bagasse NMP
19[b] sugarcane bagasse NMP
PPTPA-1 polymer displayed
a
strong signal at d=
127.27 ppm, which could be assigned to the C2 and C3 carbon
of the benzene rings, together with two signals at d=135.23
and 147.13 ppm for C4 and C1 carbon atoms, respectively.
After sulfonation, the SPPTPA-1 material displayed two strong
signals at d=127.7 and at d=138.16 ppm, attributable to the
nonsubstituted benzene carbon and the benzene carbon that
is directly connected to the ÀSO3H group, respectively. The ad-
ditional signal at d=150.6 ppm could be attributed to the C1
carbon atoms (Figure 7). The signal for C4 carbon might be
overlapped within these peaks. The presence of these aromatic
carbons deduced from the positive shift after sulfonation sup-
port the existence of polymeric frameworks of TPA in PPTPA-
1 and ÀSO3H groups in SPPTPA-1.
4.5
140 20
140 30
140 60
12.9
14.8
19.8
[a] Solvent (2 mL); T=temperature; microwave assisted heating condi-
tions; SPPTPA-1 (5 mg catalyst loading); TON=Turnover number (moles
of substrate converted per mole of active sites). [b] HMF yields are report-
ed as weight percentages.
action temperature from 1208C to 130 and 1408C under other-
wise the same reaction conditions, and the yield of HMF was
improved from 91.4 to 94.6% (Table 1, entries 2 and 3). UV/Vis
absorption band of HMF was calibrated by using standard
HMF solution of 99% purity and was quantitatively correlated
with that obtained from the different reaction mixtures. A
good correlation in the peak intensity with the concentration
ChemCatChem 2015, 7, 3570 – 3578
www.chemcatchem.org
3574
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
of HMF is observed in the UV/Vis spectroscopic analysis. In Fig-
ure S1 in the Supporting Information, the UV/Vis absorption
spectrum of pure HMF solution is shown. It displays a distinct
peak at 284 nm with corresponding molar extinction coeffi-
cient (e) value of 1.66”104 mÀ1 cmÀ1. We have also checked the
catalytic activity of SPPTPA-1 for dehydration of fructose in
NMP and DMA–LiCl (10%) under identical reaction conditions
(Table 1, entries 4 and 5). For biomass conversion processes,
ionic liquids are employed as attractive solvents owing to their
beneficial effect in improving the HMF yield.[30] We have used
less expensive DMA–LiCl solvent for our present study to opti-
mize the catalytic efficiency of SPPTPA-1. These ionic liquids
are a salt, formed through pairing of a cation with an anion,
have high polarity, water solubility, and negligible vapor pres-
sure.[31] Carbohydrates can polymerize and form hydrogen
bonds through their hydroxyl groups in the aqueous environ-
ment, which makes them reluctant towards dissolution. The
charged constituents of the ionic liquid form electron donor–
electron acceptor complexes with the hydroxyl groups of car-
bohydrates and prevent the network of hydrogen bonds and
thus help to keep the carbohydrates dissolved.[32] Under the
comparable reaction conditions, 86.6% and 95.9% HMF yields
are observed for fructose dehydration in NMP and DMA–LiCl
solvents, respectively. Good HMF yields in NMP and DMSO
could be attributed to their good microwave absorbing ability,
avoidance of side reactions and catalytic ability in glucopyra-
nose to fructofuranose conversion at higher temperature. By
using SPPTPA-1 catalyst, we next examined the scope of su-
crose in biomass conversion under identical reaction condi-
tions. If sucrose was treated at 1408C in DMSO for 10 min,
then 51.4% HMF product yield was obtained (Table 1, entry 6).
Microwave irradiation of sucrose in DMA–LiCl (10%) solvent
with SPPTPA-1 catalyst for 10 and 20 min delivered a HMF
product yield of 49.5% and 31.4%, respectively (Table 1, en-
tries 7 and 8). Similarly, sucrose could be transformed into cor-
responding HMF in moderate yields employing NMP as a sol-
vent (Table 1, entries 9 and 10).[33] On the other hand, for sug-
arcane bagasse conversion to HMF takes place in multiple
steps. First sugarcane bagasse is converted to monosacchar-
ides through hydrolysis, then isomerization of glucopyranose
to fructofuranose occurs, and finally fructofuranose is hydro-
lyzed to HMF. Here NMP solvent delivered the greater yield of
corresponding HMF than DMSO and DMA–LiCl. For sugarcane
bagasse we have achieved 18.8% product yield of HMF in
DMSO after 60 min microwave irradiation at 1408C tempera-
ture (Table 1, entry 14). We have also conducted the biomass
conversion of sugarcane bagasse in DMA–LiCl (10%), and
a very poor corresponding HMF yield of 4.5% was observed
(Table 1, entry 16). Under these reaction conditions for biomass
conversion of sugarcane bagasse, NMP was observed as the
best solvent. In Figure 8, the reaction profiles for biomass con-
version of fructose and sucrose over SPPTPA-1 catalyst under
identical reaction conditions (DMSO, 1408C and microwave
condition) are shown. The reaction time was varied from 2 to
30 min under microwave conditions. For fructose dehydration,
the yield of HMF was first increased with increasing the reac-
tion time from 2 to 20 min giving 64.6% to 94.6% HMF. After
Figure 8. A) Reaction profiles of SPPTPA-1 catalyst in biomass conversion of
fructose and sucrose against time. Reaction conditions: fructose or sucrose
(25 mg), SPPTPA-1 catalyst (5 mg), T=1408C, microwave irradiation, DMSO
(2 mL). B) Dehydration of fructose over SPPTPA-1 catalyst at variable catalyst
loading under microwave irradiation. Reaction conditions: fructose or su-
crose (25 mg), T=1408C, microwave irradiation, DMSO (2 mL), t=20 min
(fructose) and 10 min (sucrose). C) Kinetic profile for fructose conversion to
HMF in the recycling process. Reaction conditions: fructose (25 mg), SPPTPA-
1 (5 mg), DMSO (2 mL), T=1408C, t=20 min
that, from 20 to 25 min reaction time the HMF yield decreased
to 88.0% from 94.6%. On the other hand for sucrose dehydra-
tion the percentage of HMF is increased up to 10 min giving
maximum 51.4% of HMF yield and then sucrose conversion is
decreased with further increasing reaction time as shown in
Figure 8A. No improvement in biomass conversion to the cor-
responding HMF has been observed upon prolonged reaction
time. To explain the decrease in HMF yield with the increase of
reaction time, it may be assumed that further rehydration of
HMF to levulinic acid and formic acid as the side products
occurs. Besides, self-polymerization of HMF molecules or cross-
polymerization of HMF with fructose also arise delivering
humins.[34]
The efficiency of SPPTPA-1 in biomass conversion reaction
was examined using the conversion of fructose to HMF using
DMSO solvent under microwave conditions. All the dehydra-
tion reactions are performed at 1408C for 20 min using 2 mL
of DMSO solvent. As shown in Figure 8B, for a fixed amount of
fructose (25 mg) the dehydration product yield for HMF in-
ChemCatChem 2015, 7, 3570 – 3578
www.chemcatchem.org
3575
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
creased from 87.2 to 99.0% with the increase in catalyst load-
ing from 1 to 10 mg. For the high catalyst loading, the HMF
yield continued to improve, suggesting that the rate of fruc-
tose dehydration becomes faster with higher catalyst loading.
120 (H) and sulfonated silica for the catalytic conversion of
fructose and sucrose to the corresponding HMF. The synthesis
of those above mentioned catalysts is provided in the Support-
ing Information. A comparison study was conducted by con-
sidering catalytic conversion of both fructose and sucrose
using 5 mg of SPPTPA-1 catalyst in DMSO under optimized re-
action conditions. In that case we achieved 94.6 and 51.4%
HMF product yields, respectively, whereas under comparable
reaction conditions but with 5 mg of each catalyst Amberlite
IR-120 (H) and sulfonated mesoporous silica, we obtained 41.6
and 70.4% HMF yield from fructose, respectively (Table 2, en-
Catalyst reusability
To evaluate the recyclability and reusability of SPPTPA-1 catalyst
we performed catalytic biomass conversion for fructose
(25 mg) under optimized reaction conditions. After the com-
pletion of the reaction, the catalyst was recovered by centrifu-
gation. The recovered catalyst was washed with methanol,
ethyl acetate, and acetone followed by drying in an oven at
1208C for approximately 5 h before use in the next catalytic
run. No addition of acid, base, and special reagent, and no spe-
cial treatment (such as calcinations at high temperature) are
desirable to reactivate the recovered catalyst. The kinetic pro-
file for fructose conversion to HMF in the recycling process is
provided in the Figure 8C. The rate of conversion of fructose
into corresponding HMF did not show any appreciable change
during the progress of the reaction through the four catalytic
cycles. The results signify that our sulfonated SPPTPA-1 catalyst
could be recycled for four successive catalytic cycles without
remarkable loss of catalytic activity. At the 4th catalytic cycle
the yield of HMF for fructose conversion was declined by 5%
compared to the 1st catalytic cycle. This fact can be anticipat-
ed by the partial blocking of accessible catalytic active sites
(ÀSO3H group) inside the porous organic polymer PPTPA-
1 owing to the formation of polymeric product humines
during the course of catalytic reaction.
Table 2. Comparative study of the catalytic efficiency of SPPTPA-1 and
the other sulfonated materials in the microwave-assisted dehydration of
carbohydrates to HMF.[a]
Entry Substrates Catalyst
Catalyst amount
[mg]
T
HMF yield
[min] [%]
1
2
3
4
5
6
7
8
9
10
fructose
fructose
sucrose
sucrose
fructose
fructose
sucrose
sucrose
fructose
sucrose
amberlite IR-120(H)
5
20
20
10
10
20
20
10
10
20
10
41.6
60.5
9.7
21.4
70.4
92.2
23.2
40.5
94.6
51.4
amberlite IR-120(H) 25
amberlite IR-120(H)
amberlite IR-120(H) 25
sulfonated silica
sulfonated silica
sulfonated silica
sulfonated silica
SPPTPA-1
5
5
25
5
25
5
SPPTPA-1
5
[a] Substrate amount=25 mg; solvent=DMSO (2 mL); T=1408C; micro-
wave=assisted heating conditions.
To understand the heterogeneous nature of our catalyst we
have performed hot filtration test for the biomass conversion
of fructose to HMF under optimized reaction conditions. This
experimental result undoubtedly proved that no leaching of
surface organic functional group took place during the course
of catalytic reaction. The reaction was stopped after 10 min
and the catalyst removed by centrifugation from the hot reac-
tion mixture. After 10 min of the reaction, 86.1% yield for fruc-
tose to the corresponding HMF was achieved. Then the same
catalytic reaction was performed with the filtrate for additional
20 min. No improvement in HMF yield was observed beyond
86.1%. This experimental result demonstrates that our catalyst
is heterogeneous in nature and catalytically active surface
functional groups are strongly anchored with the polymer or-
ganic backbone unit in such a way that no leaching takes
place. Acid–base titration method was employed to measure
the surface acidity in fresh SPPTPA-1 catalyst before and after
the catalytic reactions. Acidity measurement of the reused cat-
alyst revealed that the total surface acidity was decreased
slightly to 3.87 mmolgÀ1 after the 4th catalytic cycle. This de-
crease is within the experimental error of chemical analysis. As
the acid strength of the reused catalyst is still comparable to
the fresh one, this observation clearly signifies that no leaching
of surface acidic functional groups takes place during the
course of the reaction. To investigate the catalytic efficiency of
SPPTPA-1 we compared the catalytic activity of SPPTPA-1 cata-
lyst with those of two other sulfonated catalysts Amberlite IR-
tries 1 and 5), and 9.7% and 23.2% HMF from sucrose (Table 2,
entries 3 and 7), respectively. From these experimental results
we can conclude that a high surface area and large pore
volume of the porous organic polymer allow easy diffusion of
organic substrates into their nanoporous channel assisting ef-
fective interaction with the catalytic active centers which re-
sults the greater catalytic performance requiring shorter reac-
tion time compared with the other conventional catalysts.[34]
Characterization of the reused catalyst
The reused SPPTPA-1 catalyst was characterized through N2
sorption and SEM analyses for investigation of any noticeable
change in SPPTPA-1 after the catalytic reactions. After the 4th
catalytic cycle, N2 adsorption–desorption analysis of the used
SPPTPA-1 revealed the BET surface area 631 m2 gÀ1 with the
corresponding pore volume of 0.256 cm3 gÀ1. Substantial de-
creases in BET surface area and pore volume of the reused cat-
alyst compared with fresh SPPTPA-1 signifies clogging of nano-
porous channel which hinders the adsorption of nitrogen into
SPPTPA-1. This clogging may arise from the formation of poly-
meric product humines during the course of catalytic reaction.
The FESEM images of the reused SPPTPA-1 catalyst (Figure S4)
suggest that the morphology of the SPPTPA-1 material has
been unaltered. The above characterizations of reused SPPTPA-
ChemCatChem 2015, 7, 3570 – 3578
www.chemcatchem.org
3576
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
estimated acid strength of sulfonated porous polymer SPPTPA-
1 imply that the SPPTPA-1 catalyst is thermally and mechanical-
ly very stable during the catalytic reaction conditions.
1 was 4.30 mmolgÀ1
.
Conclusion
HMF synthesis
We have fabricated a new porous polymer polytriphenylamine
through a one-step oxidative polymerization process using an-
hydrous FeCl3 as the oxidizing agent. Upon sulfonation the sul-
fonated polymer SPPTPA-1 shows very high surface acidity.
Both polytriphenylamine and its sulfonated polymer possess
typical nanofiber morphologies and have very high BET surface
areas. Sulfonated porous polytriphenylamine showed excellent
catalytic activity for the conversion of carbohydrates and bio-
mass to the value added platform chemical 5-hydroxymethyl-
furfural. Design and synthesis of environment-friendly and eco-
nomically viable highly porous material reported herein could
be employed as an efficient acid catalyst for the large scale
and sustainable direct conversion of biomass to HMF in future.
For the dehydration of carbohydrates into HMF, a microwave tube
equipped with a stir bar was charged with the desired amount of
carbohydrate, catalyst, and solvent (2 mL). The loaded and sealed
microwave tube was then placed in a microwave reactor and the
reaction was performed for the desired time at 413 K.[37] Then the
reaction mixture was allowed to cool to RT, then centrifuged and
filtered. The filtered products were analyzed by UV/Vis spectropho-
tometry. Sugarcane bagasse was collected from local Indian
market, crushed into powder, and dried in an oven at 373 K before
use.
Determination of HMF yields
To determine the HMF yield from the reaction mixture we have
employed UV/Vis spectrophotometric technique.[38] Presence of
strong distinct peak at 284 nm with the progress of time was esti-
mated quantitatively to evaluate the HMF yield. Upon completion
the HMF conversion reaction the reaction mixture was centrifuged
and the supernatant liquid was used for UV analysis. Measuring the
absorbance value at 284 nm and using this molar extinction coeffi-
cient value, the HMF yield of all the reactions were calculated. The
percentage of HMF yields for all the reaction products are tabulat-
ed in Table 1. Initially a standard HMF solution of 99% purity is
used for calibration. After a good correction the reaction mixtures
were analyzed by the UV/Vis spectrophotometer. This analysis was
repeated and showed only Æ3 wt% error.
Experimental Section
Synthesis of porous polymer PPTPA-1
Porous polytriphenylamine material PPTPA-1 was synthesized by
one-step oxidative polymerization process. We have slightly modi-
fied the standard synthetic pathway for oxidative polymerization
as reported in the literature[28,35] to achieve almost complete poly-
merization (Scheme 1). In a typical synthesis, a mixture of TPA
(2 mmol, 491 mg) and anhydrous FeCl3 (6 mmol, 973 mg) was first
taken in a 250 mL round-bottom flask and then dry dichloroethane
(30 mL) was poured into that flask and the mixture was stirred for
20 h under N2 atmosphere with constant heating at 353 K. Then
the blue colored precipitate was poured into acetone and the de-
posited polymer product was filtered and washed successively
with acetone, THF, and methanol. To remove iron completely from
the polymer, the product was further washed with methanol for
three days in a soxhlet apparatus. The final product was dried
under high vacuum and characterized by FTIR and 13C CP MAS
NMR spectrometry.
Reusability of the catalyst
SPPTPA-1 catalyst was recovered after the reaction and thoroughly
washed with methanol, ethyl acetate, and acetone, successively.
Then the catalyst was dried for 5 h at 393 K. The recyclability of
the SPPTPA-1 catalyst was determined by using dehydration of
fructose in DMSO solvent as a representative reaction. Following
this method SPPTPA-1 catalyst was reused for further four times
and HMF yield was calculated for every time.
Synthesis of sulfonated porous polymer SPPTPA-1
In a typical synthesis procedure of SPPTPA-1, a mixture of PPTPA-
1 (300 mg) polymer and DCM (30 mL) were taken in a round
bottom flask and placed in an ice bath. Chlorosulfonic acid (3 mL)
was then added to the mixture over 20 min under continuous stir-
ring and then the stirring was continued for three days at RT.[36] Fi-
nally the bluish-black product was poured into ice, the solid was
filtered and substantially washed with distilled water and dried to
obtain sulfonated porous polymer SPPTPA-1.
Hot filtration test
To evaluate the heterogeneous nature of SPPTPA-1, we conducted
hot filtration test considering fructose dehydration into HMF as
a model reaction. In a typical catalytic procedure, the microwave
tube containing fructose (25 mg), SPPTPA-1 (5 mg) along with
DMSO (2 mL) was placed in a microwave reactor and the reaction
was performed for 10 min at 413 K. After 10 min the reaction mix-
ture, was cooled down and centrifuged to separate the catalyst. At
that time 86.1% HMF yield was achieved. Then the reaction was
further run with the filtrate for additional 20 min but no improve-
ment in the HMF yield beyond 86.1% was observed. This experi-
mental result clearly demonstrates that no leaching of sulfonic acid
takes place during the course of the reaction and SPPTPA-1is truly
heterogeneous in nature.
Surface acidity measurement
To measure the surface acidity we estimated the acid strength of
sulfonated material SPPTPA-1 through acid–base titration with
excess NaOH solution. Before titration, SPPTPA-1 (100 mg) was
mixed in water (50 mL) along with of standardized sodium hydrox-
ide solution (40 mL) at RT, and the mixture was stirred overnight.[26]
Then the mixture was filtered and the excess NaOH was titrated
with 0.1n oxalic acid solution using phenolphthalein indicator. The
ChemCatChem 2015, 7, 3570 – 3578
www.chemcatchem.org
3577
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
[17] a) H. Ren, T. Ben, E. Wang, X. Jing, M. Xue, B. Liu, Y. Cui, S. Qiu, G. Zhu,
Chem. Commun. 2010, 46, 291–293; b) D. E. Demirocak, M. K. Ram, S. S.
Srinivasan, D. Y. Goswami, E. K. Stefanakos, J. Mater. Chem. A 2013, 1,
13800–13806.
[18] a) S. Ren, R. Dawson, A. Laybourn, J. Jiang, Y. Khimyak, D. J. Adams, A. I.
Cooper, Polym. Chem. 2012, 3, 928–934; b) H. Bildirir, J. P. Parakno-
witsch, A. Thomas, Chem. Eur. J. 2014, 20, 9543–9548.
[19] a) J. Weber, Q. Su, M. Antonietti, A. Thomas, Macromol. Rapid Commun.
2007, 28, 1871–1876; b) A. Bhunia, I. Boldog, A. Moeller, C. Janiak, J.
Mater. Chem. A 2013, 1, 14990–14999; c) L. Y. Li, Z. L. Chen, H. Zhong,
R. H. Wang, Chem. Eur. J. 2014, 20, 3050–3060.
Acknowledgements
We thank the Department of Science and Technology, New Delhi,
Government of India for research funding. S.M. wishes to thank
CSIR, New Delhi for his junior research fellowship (09/080(0881)/
2013-EMR-I). J.M. thanks Department of Science & Technology,
New Delhi, Government of India for his DST-INSPIRE Research
project grant in CSIR-IICT, Hyderabad.
[20] J. F. Van Humbeck, T. M. McDonald, X. F. Jing, B. M. Wiers, G. S. Zhu, J. R.
Long, J. Am. Chem. Soc. 2014, 136, 2432–2440.
Keywords: acidity · biomass · carbohydrates · nanostructures ·
[21] J. Liu, K. K. Yee, K. K. W. Lo, K. Y. Zhang, W. P. To, C. M. Che, Z. T. Xu, J.
Am. Chem. Soc. 2014, 136, 2818–2824.
polymers
[22] a) J. Long, B. Guo, X. Li, Y. Jiang, F. Wang, S. C. Tsang, L. Wang, K. M. K.
Yu, Green Chem. 2011, 13, 2334–2338; b) I. J. Kuo, N. Suzuki, Y. Yamau-
chi, K. C. W. Wu, RSC Adv. 2013, 3, 2028–2034.
[23] a) V. V. Ordomsky, J. van der Schaaf, J. C. Schouten, T. A. Nijhuis, ChemSu-
sChem 2012, 5, 1812–1819; b) J. N. Chheda, J. A. Dumesic, Catal. Today
2007, 123, 59–70; c) F. L. Yang, Q. S. Liu, M. Yue, X. F. Bai, Y. G. Du,
Chem. Commun. 2011, 47, 4469–4471; d) E. Nikolla, Y. Romµn-Leshkov,
M. Moliner, M. E. Davis, ACS Catal. 2011, 1, 408–410.
[24] a) S. Tanaka, T. Iso, Y. Doke, Chem. Commun. 1997, 2063–2064; b) M.
Jikei, R. Mori, S. Kawauchi, M. Kakimoto, Y. Taniguchi, Poly. J. 2002, 34,
550; c) G. Kim, F. Basarir, T. H. Yoon, Synth. Met. 2011, 161, 2092–2096;
d) T. Ben, K. Shi, Y. Cui, C. Pei, Y. Zuo, H. Guo, D. Zhang, J. Xu, F. Deng, Z.
Tian, S. Qiu, J. Mater. Chem. 2011, 21, 18208–18214.
[25] a) L. Hu, G. Zhao, X. Tang, Z. Wua, J. Xu, L. Lin, S. Liu, Bioresour. Technol.
2013, 148, 50–507; b) P. Ling, Q. Hao, J. Lei, H. Ju, J. Mater. Chem. B
2015, 3, 1335–1341; c) J. Jeromenok, J. Weber, Langmuir 2013, 29,
12982–12989.
[26] M. Pramanik, M. Nandi, H. Uyama, A. Bhaumik, Green Chem. 2012, 14,
2273–2281.
[27] J. Huang, R. B. Kaner, Chem. Commun. 2006, 367–376.
[28] W. Ni, J. Cheng, G. Li, X. Gu, L. Huang, Q. Guan, D. Yuan, B. Wang, RSC
Adv. 2015, 5, 9221–9227.
[29] X. Tian, F. Su, X. S. Zhao, Green Chem. 2008, 10, 951–956.
[30] A. Pinkert, K. N. Marsh, S. S. Pang, M. P. Staiger, Chem. Rev. 2009, 109,
6712–6728.
[31] A. A. Rosatella, L. C. Branco, C. A. M. Afonso, Green Chem. 2009, 11,
1406–1413.
[32] A. Pinkert, K. N. Marsh, S. S. Pang, Ind. Eng. Chem. Res. 2010, 49, 11121–
11130.
[33] a) D. Dallinger, C. O. Kappe, Chem. Rev. 2007, 107, 2563; b) X. Tong, Y. L,
ChemSusChem 2010, 3, 350.
[34] a) C. Sievers, I. Musin, T. Marzialetti, M. B. V. Olarte, P. K. Agrawal, C. W.
Jones, ChemSusChem 2009, 2, 665–671; b) A. M. Taher, D. M. Cates, Car-
bohydr. Res. 1974, 34, 249–261; c) Y.-D. Chiang, S. Dutta, C.-T. Chen, Y.-T.
Huang, K.-S. Lin, J. C. S. Wu, N. Suzuki, Y. Yamauchi, K. C.-W. Wu, Chem-
SusChem 2015, 8, 789–794; d) M. B. Gawande, R. Zboril, V. Malgras Y.
Yamauchi, J. Mater. Chem. A 2015, 3, 8241–8245.
[35] a) C. Takahashi, S. Moriya, N. Fugono, H. C. Lee, H. Sato, Synth. Met.
2002, 129, 123–128; b) A. Iwan, D. Sek, Prog. Polym. Sci. 2011, 36,
1277–1325.
[36] a) W. Lu, D. Yuan, J. Sculley, D. Zhao, R. Krishna, H. C. Zhou, J. Am. Chem.
Soc. 2011, 133, 18126–18129; b) N. K. Mal, A. Bhaumik, M. Matsukata,
M. Fujiwara, Ind. Eng. Chem. Res. 2006, 45, 7748–7751.
[37] a) A. Dutta, D. Gupta, A. K. Patra, B. Saha, A. Bhaumik, ChemSusChem
2014, 7, 925–933.
[38] a) A. Dutta, A. K. Patra, S. Dutta, B. Saha, A. Bhaumik, J. Mater. Chem.
2012, 22, 14094–14100; b) S. Dutta, A. K. Patra, B. S. Rana, A. K. Sinha,
B. Saha, A. Bhaumik, Appl. Catal. A 2012, 435–436, 197–203; c) S.
Dutta, S. De, A. K. Patra, M. Sasidharan, A. Bhaumik, B. Saha, Appl. Catal.
A 2011, 409–410, 133–139.
[1] G. J. Kramer, M. Haigh, Nature 2009, 462, 568–569.
[2] D. Tilman, R. Socolow, J. A. Foley, J. Hill, E. Larson, L. Lynd, S. Pacala, J.
Reilly, T. Searchinger, C. Somerville, R. Williams, Science 2009, 325, 270–
271.
[3] A. A. Rosatella, S. P. Simeonov, R. F. M. Frade, C. A. M. Afonso, Green
Chem. 2011, 13, 754–793.
[4] a) R. J. van Putten, J. C. V. D. Waal, E. D. Jong, C. B. Rasrendra, H. J.
Heeres, J. G. D. Vries, Chem. Rev. 2013, 113, 1499–1597; b) J. Z. Chen,
Y. Y. Guo, J. Y. Chen, L. Song, L. M. Chen, ChemCatChem 2014, 6, 3174–
3181.
[5] a) M. Balakrishnan, E. R. Sacia, A. T. Bell, Green Chem. 2012, 14, 1626–
1634; b) J. N. Chheda, G. W. Huber, J. A. Dumesic, Angew. Chem. Int. Ed.
2007, 46, 7164–7183; Angew. Chem. 2007, 119, 7298–7318; c) G. W.
Huber, J. N. Chheda, C. J. Barrett, J. A. Dumesic, Science 2005, 308,
1446–1450; d) J. Y. Cai, H. Ma, J. J. Zhang, Q. Song, Z. T. Du, Y. Z. Huang,
J. Xu, Chem. Eur. J. 2013, 19, 14215–14223.
[6] a) J. C. Serrano-Ruiz, J. A. Dumesic, Energy Environ. Sci. 2011, 4, 83–99;
b) E. I. Gürbüz, D. M. Alonso, J. Q. Bond, J. A. Dumesic, ChemSusChem
2011, 4, 357–361.
[7] Y. Romµn-Leshkov, C. J. Barrett, Z. Y. Liu, J. A. Dumesic, Nature 2007, 447,
982–985.
[8] G. Y. Li, N. Li, Z. Q. Wang, C. Z. Li, A. Q. Wang, X. D. Wang, Y. Cong, T.
Zhang, ChemSusChem 2012, 5, 1958–1966.
[9] a) S. Dutta, S. De, B. Saha, ChemPlusChem 2012, 77, 259–272; b) R. M.
Baum, Chem. Eng. News 2009, 87, 5; c) A. K. Sengupta, M. German, Envi-
ron. Sci. Technol. 2015, 49, 12–13; d) J. Jae, G. A. Tompsett, A. J. Foster,
K. D. Hammond, S. M. Auerbach, R. F. Lobo, G. W. Huber, J. Catal. 2011,
279, 257–268.
[10] a) V. V. Ordomsky, J. van der Schaaf, J. C. Schouten, T. A. Nijhuis, J. Catal.
2012, 287, 68–75; b) M. Watanabe, Y. Aizawa, T. Iida, R. Nishimura, H. In-
omata, Appl. Catal. A 2005, 295, 150–156; c) S. Dutta, S. De, M. I. Alam,
M. M. Abu-Omar, B. Saha, J. Catal. 2012, 288, 8–15; d) R. Weingarten, A.
Rodriguez-Beuerman, F. Cao, J. S. Luterbacher, D. M. Alonso, J. A. Dume-
sic, G. W. Huber, ChemCatChem 2014, 6, 2229–2234; e) P. Srinivasu, D.
Venkanna, M. L. Kantam, J. Tang, S. K. Bhargava, A. Aldalbahi, K. C. W.
Wu, Y. Yamauchi, ChemCatChem 2015, 7, 747–751.
[11] a) J. Liu, Y. Tang, K. Wu, C. Bi, Q. Cui, Carbohydr. Res. 2012, 350, 20–24;
b) D. H. K. Jackson, D. Wang, J. M. R. Gallo, A. J. Crisci, S. L. Scott, J. A.
Dumesic, T. F. Kuech, Chem. Mater. 2013, 25, 3844–3851.
[12] a) Y. Romµn-Leshkov, J. N. Chheda, J. A. Dumesic, Science 2006, 312,
1933–1937; b) H. L. Wang, Q. Q. Kong, Y. X. Wang, T. S. Deng, C. M.
Chen, X. L. Hou, Y. L. Zhu, ChemCatChem 2014, 6, 728–732.
[13] L. Qi, Y. F. Mui, S. W. Lo, M. Y. Lui, G. R. Akien, I. T. Horvath, ACS Catal.
2014, 4, 1470–1477.
[14] a) M. Bicker, J. Hirth, H. Vogel, Green Chem. 2003, 5, 280–284; b) J. N.
Chheda, Y. Romµn-Leshkov, J. A. Dumesic, Green Chem. 2007, 9, 342–
350.
[15] S. A. Corr, D. P. Shoemaker, E. S. Toberer, R. Seshadri, J. Mater. Chem.
2010, 20, 1413–1422.
[16] a) A. Modak, M. Nandi, J. Mondal, A. Bhaumik, Chem. Commun. 2012,
48, 248–250; b) W. Lu, Z. W. Wei, D. Q. Yuan, J. Tian, S. Fordham, H. C.
Zhou, Chem. Mater. 2014, 26, 4589–4597; c) L. Bromberg, X. Su, T. A.
Hatton, Chem. Mater. 2014, 26, 6257–6264.
Received: June 19, 2015
Published online on September 17, 2015
ChemCatChem 2015, 7, 3570 – 3578
www.chemcatchem.org
3578
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim