Production of furfural from xylose and corn stover catalyzed by a novel porous carbon solid acid in γ-valerolactone

Article abstract of DOI:10.1039/c7ra03995f

A resorcinol-formaldehyde resin carbon (RFC) catalyst with a well-developed, ordered, mesoporous framework was prepared using a soft template method at room temperature. The carbon was sulfonated in water using sulfanilic acid under mild atmospheric conditions. The sulfonated RFC (S-RFC) was characterized by N₂ adsorption-desorption, elemental analysis, TEM, XPS, and FT-IR. It was determined that S-RFC is an efficient solid acid catalyst for furfural production from xylose and corn stover in γ-valerolactone (GVL). The effects of reaction time, reaction temperature, catalyst loading, substrate dosage and water concentration were investigated. 80% furfural yield and 100% xylose conversion were obtained from xylose at 170 °C in 15 min with 0.5 g catalyst. Comparatively, 68.6% furfural yield was achieved from corn stover at 200 °C in 100 min when using 0.6 g catalyst. Since there was no discernable decrease in furfural yield after multiple conversions utilizing the same catalyst, the recyclability of the catalyst is considered good.

Full text of DOI:10.1039/c7ra03995f

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Production of furfural from xylose and corn stover
catalyzed by a novel porous carbon solid acid in
g-valerolactone†
Cite this: RSC Adv., 2017, 7, 29916
a
a
Yuanshuai Zhu,a Wenzhi Li,
Yijuan Lu,a Tingwei Zhang,
Hasan Jameel,b
*
Hou-min Changb and Longlong Mac
A resorcinol-formaldehyde resin carbon (RFC) catalyst with a well-developed, ordered, mesoporous
framework was prepared using a soft template method at room temperature. The carbon was sulfonated
in water using sulfanilic acid under mild atmospheric conditions. The sulfonated RFC (S-RFC) was
characterized by N2 adsorption–desorption, elemental analysis, TEM, XPS, and FT-IR. It was determined
that S-RFC is an efficient solid acid catalyst for furfural production from xylose and corn stover in g-
valerolactone (GVL). The effects of reaction time, reaction temperature, catalyst loading, substrate
dosage and water concentration were investigated. 80% furfural yield and 100% xylose conversion were
obtained from xylose at 170 ꢀC in 15 min with 0.5 g catalyst. Comparatively, 68.6% furfural yield was
achieved from corn stover at 200 ꢀC in 100 min when using 0.6 g catalyst. Since there was no
discernable decrease in furfural yield after multiple conversions utilizing the same catalyst, the
recyclability of the catalyst is considered good.
Received 8th April 2017
Accepted 3rd June 2017
DOI: 10.1039/c7ra03995f
rsc.li/rsc-advances
uses in polymer, rubber, and pharmaceutical applications –
attracted the interest of many researchers. Ultimately, it is
Introduction
agreed that the production and utilization of furfural would be
benecial for mitigating the energy and environment crisis and
increasing protability of a biorenery economical prots.
For the rst time in 1921, Quaker Oats Company commer-
cially produced furfural from xylose and xylan using H2SO4 as
catalyst.19 Since then improvements in furfural production have
been made using homogenous catalysts such as mineral acids,20
organic acids,21 and Lewis acids22–24 as well as ionic liquid.25
Despite the improvements, the poor recyclability of homoge-
nous catalysts and equipment corrosion they cause hinder their
use. To overcome these obstacles, an increasing number of
greener, heterogeneous catalysts have been developed.
Under the circumstance, multifarious zeolites were explored
and gained more and more popularity in biomass conversion
including SBA-15,26 functionalized MCM-41 (ref. 27) and Sn-
MMT28 as well as SAPO-34.29 But hydrothermal stability of
zeolites still needs to be improved. Besides, commercial ion
exchange resins (Amberlyst-70, Amberlyst-15 and Naon) were
also investigated in detail.30–32 Nevertheless, the large-scale
industrial applications of ion exchange resin are hindered by
low porosity, high price and the poor endurance against high
temperature. Moreover, recent years has witnessed a fast
development of sugar catalyst, which was typically prepared
through a two-step process.33–36 These catalysts were expected to
be promising in biomass conversion because they are inex-
pensive, a source of ample and renewable carbon source as well
as good acid density. However, the further development is still
This research is driven by the growing environmental concerns
associated with burning fossil fuels and the efforts to develop
a sustainable and reliable energy source to ensure both the
development of society and a greener planet.1 Lignocellulosic
biomass is an abundant, natural, and renewable resource that
has enormous applications in producing higher-value chem-
icals2,3 and biofuels.4–6 Lignocellulosic biomass is comprised of
three primary components – cellulose, hemicellulose, and
lignin – where each component has distinct characteristics and
properties that allows it to be tailored to a unique purpose.7
Furfural is a versatile chemical derived from pentosane-rich
agricultural and forestry residues such as corn cobs, corn stover,
saw dust, and straw. Despite the reported concerns in regards to
the production of potential biofuels and fuel additives, such as
2-methylfuran,8,9 g-valerolactone10–12 and long chain hydrocar-
bons,13 using furfural as a feedstock, conversion of furfural into
valuable chemicals, such as dicarboxylic acid,14–16 furfuryl
alcohol17 and tetrahydrofurfuryl alcohol18 – which has broad
aDepartment of Thermal Science and Energy Engineering, University of Science and
Technology of China, Laboratory of Basic Research in Biomass Conversion and
Utilization, Hefei 230026, P. R. China. E-mail: liwenzhi@ustc.edu.cn
bDepartment of Forest Biomaterials, North Carolina State University, Raleigh, NC
27695-8005, USA
cCAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion,
Chinese Academy of Sciences, Guangzhou 510640, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra03995f
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faced with some challenges, such as high sulfonation temper- formaldehyde solution (37 wt%) was added into a 50 ml round-
ature and low porosity. bottle ask under vigorous agitation at room temperature. Aer
Sulfonated ordered mesoporous carbons (S-OMCs) was the anhydrous Na2CO3 dissolved completely, 11.0 g resorcinol
a good alternative by virtue of well-developed porosity with high was put into the alkaline solution for pre-polymerization. Aer
specic surface area and symmetrical pore distribution, which 70 min, the solution was poured into a mixture containing 8.0 g
provides the appropriate support for catalytic conversion of F127, 50.0 g ethanol and 40.0 g distilled water under stirring.
sugars into chemicals. Many investigations suggested that S- Aer 10 min, 5.0 ml HCl (2 M) was added into the hybrid to
OMCs performed well in biodiesel production37,38 which was accelerate the polymerization process. Aer agitation for about
usually prepared via a nanocasting method39,40 followed by 2.2–2.5 min, a turbid mixture appeared which suggested the
sulfonation with H2SO4. It's worth noting that the environmental occurrence of phase separation. The hybrid was further agitated
and safety issues associated with the use of HF to remove the for 60 min and kept standing overnight to form the sedimentary
hard templates cannot be ignored. Meanwhile, sulfonation with polymer gel. Finally, the gel was natural withering for 12 h then
concentrated H2SO4 was inefficient and could cause damage to aged at 80 ꢀC for one day. The resulting so, sticky, orange
the mesoporous framework. Recently, an easier, safer and milder polymer was calcined under a N2 atmosphere at 350 ꢀC for 2.5 h
sulfonation technique was proposed with sulfanilic acid,41 which to remove the template (F127) and then at 500 ꢀC for 4 h to
can sulfonate single wall carbon nanotubes under moderate accomplish the carbonization at a heating rate of 1 ꢀC minꢁ1 to
atmospheric conditions effectively. Inspired by their work, we obtain the ordered mesoporous carbon (RFC).
prepared a resorcinol-formaldehyde resin carbon (RFC) using
a scaled-up experiment based on previous work42 with some Sulfonation of RFC
minor modications, followed by mild sulfonation using sulfa-
nilic acid as sulfonation reagent in water to obtain a novel
porous carbon solid acid catalyst (S-RFC). During the whole
Sulfonation of RFC was conducted following the reported
procedures41 with slight modications. 3 g RFC and 12 g sul-
fanilic acid were put into a 500 ml round-bottom ask con-
process of catalyst preparation, HF and H2SO4 were avoided,
taining 300 ml distilled water. The mixture was heated to 80 ꢀC
which is more efficient and environmental friendly over other
then 6 g isoamyl nitrite was added once the sulfanilic acid was
similar catalyst. To the best of our knowledge, the novel catalyst
completely dissolved. The mixture was agitated vigorously
(S-RFC) has not been reported for use in biomass conversion.
overnight at 80 ꢀC and then cooled to room temperature for
In this study, a novel porous carbon solid acid was prepared
ltration. The lter cake was washed with distilled water,
ethanol and acetone repeatedly till the ltrate was clear. The
catalyst was collected and sonicated in DMF (50 ml) for 20 min
to eliminate any residual substrates. The black solid powder
was isolated by ltration and swilled with acetone several times
and then dried in vacuum oven. The obtained samples were
and characterized by various instruments for its physical and
chemical properties. The catalyst was utilized to produce
furfural from xylose and corn stover. The inuences of reaction
temperature, reaction time, and catalyst loading were investi-
gated. Bio-based GVL was used as solvent due to its superior
properties in biomass utilization.43,44 More importantly, GVL
donated as S-RFC.
can be derived from biomass by a integrated process.45,46
Catalyst characterization
Experimental section
Materials
Both RFC and S-RFC were characterized by FT-IR (Nicolet 8700
(KBr disks)), BET (Tristar II 3020), XPS (Thermo Scientic
ESCALAB 250Xi), TEM (JEM 2010 electron microscope) and
D-(+)-xylose (98%), furfural (99%), 5-HMF (99%), sulfanilic acid
elemental analysis (EA, Elementar model Vario EL III). Since
(AR, 99.5%) and isoamyl nitrite (95%) were all purchased from
sulfur existed in the single form of –SO3H, per XPS, the amount
Aladdin. Pluronic F127 (Mav ¼ 12 600, EO106PO70EO106) was
of –SO3H was determined by the mole of sulfur present within
supplied by Sigma-Aldrich. Hydrochloric acid (HCl, 36–38%),
the carbon support and calculated by elemental analysis.
ethanol (AR), acetone (AR), N,N-dimethylformamide (DMF, AR),
resorcinol (AR, 99%), formaldehyde solution (AR, 37 wt%) and
anhydrous NaCO3 (AR) were obtained from Sinopharm Chem-
Typical procedure for furfural production from xylose and
corn stover
ical Reagent Co., Ltd. g-Valerolactone (95%) was provided by
Experiments for furfural production from xylose and corn stover
LangFang Hawk Technology and Development Co., Ltd. Corn
were implemented in a 100 ml mechanical stirring zirconium
stover was gathered from a cropland located in AnHui Province
autoclave equipped with a PID temperature controller. Different
and cut into pieces, then ushed with running water to remove
proportions of catalyst, substrate, and solvent were charged into
the reactor. The sealed reactor was heated to a given tempera-
ture and maintained for different time duration with a rotation
of 500 rpm. The reaction ended by immersing the autoclave in
a water bath to cool the reaction mixture to room temperature
any impurities. Aer drying to constant-weight at 80 ꢀC, the
corn stover was ground into uniform particles and sieved by a 40
mesh screen.
Synthesis of RFC
rapidly. Aerwards, the reaction mixture was ltered and the
RFC was prepared per the reported method42 with slight ltrate was preserved in a refrigerator for subsequent analysis
modications. In this procedure, 0.056 g Na2CO3 and 11.3 g by means of HPLC.
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uniform distribution of honeycomb-shaped hexagonal pores
and stripe-like channels. In addition, Fig. S1† also illustrates
the mesoporous structure was well preserved aer sulfonation
because no serious collapse and deconstruction of pores and
channels was discovered between image A and image B, proving
that the sulfonation technique was mild and efficient, which
was consistent with the BET results. Besides, these well-
preserved pore channels could provide abundant reaction pla-
ces during reactions, which promotes the diffusion and
adsorption of xylose molecules.
Table 2 shows the elemental composition of RFC and S-RFC.
A trace amount of nitrogen was incorporated into RFC, most
likely during calcination since it was calcined in a N2 atmo-
sphere. This trace amount of nitrogen was enriched to some
extent in the sulfonated RFC. It has been demonstrated that
sulfonation of carbon with sulfanilic acid and isoamyl nitrite
result in the direct incorporation of aromatic ring to the Sp2
carbon of amorphous carbon, and the presence of the trace
amount of nitrogen must be due to the surface contamination.41
Sulfonation also resulted in an incorporation of sulfur, as ex-
pected, and the concomitant increase in oxygen and hydrogen
contents as can be seen in Table 2.
In FT-IR spectrum of RFC and S-RFC (Fig. S2†), the charac-
teristic peaks at 1124, 1185, and 1220 cmꢁ1 can be ascribed to
SO3H groups. The bands at 1035 and 1011 cmꢁ1 represented
S]O asymmetric and symmetric stretching vibrations respec-
tively.47 This conrms that –SO3H groups had been successfully
graed into the carbonaceous material,48 which was in good
agreement with results of elemental analysis and XPS. In
addition, the small band at 3470 cmꢁ1 in RFC is most likely
attributed to incomplete carbonization. The stronger absorp-
tion at 3430 cmꢁ1 is due to the OH stretching of the sulfonic
acid.
Fig. S3† shows S 2p XPS spectrum of S-RFC, which was
carried out to establish the valence state of sulfur. The single
peak at 168.7 eV illustrates the existence of SO3H species in S-
RFC,49 and all the sulfur atoms were conned to –SO3H groups.
Therefore, the density of SO3H was estimated based on sulfur
content in elemental analysis.
HPLC analysis of furfural, HMF and xylose
The concentration of products (furfural and HMF) in the ltrate
was determined using a high-performance liquid chromato-
graph (Waters 515 pump) equipped with a Waters Symmetry®-
C18 column and an Ultraviolet Detector (Waters 2489) at
a wavelength of 280 nm. A mixture of methanol/water with
a volume ratio of 2/3 was used as eluent owing at 0.4 ml minꢁ1
and the temperature of column oven was set as 30 ꢀC. The
concentration of xylose was determined separately using high-
performance liquid chromatograph (Waters 515 pump) that
was equipped with a Waters XBridgeTM Amide column and
a refractive index detector (Waters 2414). Both the column oven
and the detector were kept at 45 ꢀC. Acetonitrile/distilled water
(3/2, v/v) served as mobile phase and was run at 0.4 ml minꢁ1
Pre-constructed calibration curves were used for quantitative
analysis and xylose conversion, furfural yield, furfural selectivity
and HMF yield were calculated as shown below:
.
Xylose conversion ¼ (1 ꢁ moles of xylose remained/
moles of starting xylose) ꢂ 100%
Furfural yield (from xylose) ¼ (moles of furfural produced/
moles of starting xylose) ꢂ 100%
Furfural selectivity ¼ (furfural yield/xylose conversion) ꢂ 100%
Furfural yield (from corn stover) ¼ (moles of furfural produced/
moles of starting xylan in corn stover) ꢂ 100%
HMF yield (from corn stover) ¼ (moles of HMF produced/
moles of starting glucan in corn stover) ꢂ 100%
Results and discussion
Characterization of the carbon solid acid catalyst
Physical properties of RFC and S-RFC are listed in Table 1. RFC
presented a well-developed porous structure with specic surface
area, pore volume, and average pore diameter of 604 m2 gꢁ1, 0.72
cm3 gꢁ1, and 4.78 nm respectively. Aer sulfonation, different
levels of decline occured to various parameters but S-RFC still
maintained an excellent porous framework and its BET param-
eters were 530 m2 gꢁ1, 0.56 cm3 gꢁ1, and 4.2 nm respectively. The
graed SO3H, occupying some pore space, might be the reason
for this decrease. It was concluded because of the modest decline
in these parameters that sulfonation does not cause a serious
collapse or deconstruction of the pore structure.
Effect of reaction temperature and residence time on xylose
conversion, furfural yield and furfural selectivity
Xylose dehydration was conducted in a well-sealed autoclave
equipped with a stirrer and a PID temperature controller
capable of achieving various temperatures (150 ꢀC, 160 ꢀC,
ꢀ
ꢀ
170 C, and 180 C) within a time duration from 5 to 30 min
with 0.5 g catalyst, 0.8 g xylose, and 32 ml GVL. Fig. 1 illustrated
that reaction temperature and residence time play a vital role on
xylose conversion, furfural yield and furfural selectivity. The
effect of temperature was also signicant. As you can see from
Fig. 1B, xylose conversion reached 95% at a reaction tempera-
ture of 150 ꢀC within 15 minutes. The time required for
complete conversion of xylose at 150 ꢀC, 160 ꢀC, 170 ꢀC and
180 ꢀC was 20 min, 15 min, 10 min and 5 min, respectively. The
furfural yield and furfural selectivity follow identical trends to
the conversion of xylose as seen in Fig. 1A and C. Both furfural
Mesopores were clearly visible in the TEM images (Fig. S1 in
ESI†). The ordered mesoporous structure was conrmed by the
Table 1 Textural parameters of RFC and S-RFC
Entry
SBET (m2 gꢁ1
)
Va (m3 gꢁ1
)
Db (nm)
RFC
S-RFC
604
530
0.72
0.56
4.78
4.2
a
Pore volume. b Average pore diameter.
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Table 2 Empirical formula of RFC and S-RFC based on elemental analysis data
Elemental composition (%)
C
H
O
N
S
SO3H aciditya (mmol H+ per g)
RFC
S-RFC
84.47
73.02
3.53
3.91
13.788
20.168
0.035
0.20
—
2.754
—
0.86
a
Calculated based on elemental analysis.
yield and furfural selectivity increase with residence time and
reach a maximum at a given temperature at 15 min. Aer
15 min, both yield and selectivity declines at higher tempera-
tures (170 and 180 ꢀC) but stabilize at lower temperatures (150
Effect of various acid catalysts on furfural production
The S-RFC was also experimentally compared with three
homogenous acid (HCl, H2SO4, PTSA$H2O) and three solid acid
catalyst (PTSA-POM, Amberlyst-15, H-Beta) at 170 ꢀC for 15 min
in GVL and the result was depicted in Table 5. Compared with
homogeneous acid (HCl, H2SO4, PTSA), S-RFC showed superior
catalytic performance and easier recyclability, affording
a furfural yield of 80% from xylose. The high propensity of
furfural towards to polymerization in strong acid system might
be the cause for the inferior performance of mineral strong
acids.50 Among the tested heterogeneous acid, S-RFC also per-
formed better in view of its superior porous structures and good
acidity. Overall, S-RFC is an efficient heterogeneous catalyst for
furfural formation in a green solvent GVL.
Compared with the conventional sulfonation method (i.e.,
boiling in concentrated H2SO4), the sulfonation with benzene-
sulfonic acid radical generated by the diazo-reaction between
sulfanilic acid and isoamyl nitrite can be carried out under
relatively moderate reaction conditions, which effectively avoids
the collapse of pore channels of RFC. In this study, the SO3H
was graed into S-RFC in water at 80 ꢀC under atmospheric
environment. Aer sulfonation, the pore channels were found
to be well preserved as shown in TEM images (Fig S1†) and the
well-preserved pore channels could provide abundant reaction
places during reactions, which promotes the diffusion and
adsorption of xylose molecules. Therefore, S-RFC exhibited
better furfural yield and selectivity for the conversion of xylose
among the test acid catalysts.
In addition, this work also demonstrates some advantages
compared with similar works. Firstly, nontoxic GVL is used as
green solvent and GVL itself can be produced from furfural.6 It
is clear that S-RFC exhibits a comparable, even superior,
performance to homogenous catalysis with better recyclability
properties.20–25 In addition, 80% furfural yield is achieved from
xylose in 15 min, which is comparable with other heterogeneous
catalyst.27–32,47,50 Although 93% furfural is obtained from xylose
catalyzed by modied MCM-41,27 the furfural yield sharply
declined to less than 50% aer rst run with catalyst recycling.
Besides, 80% furfural can be obtained using Naon NR 50 in
a biphase system,32 which is a comparable result with S-RFC,
but the reaction time is longer (1 h) requiring microwave irra-
diation and an extra addition of cocatalyst (NaCl). Moreover, SC-
CCA47 shows a good furfural yield from xylose and corn stalk
while the catalyst preparation need isolated 4-BDS as sulfona-
tion reagent.
ꢀ
and 160 C). Noteworthy, the maximum furfural yield of 80%
was obtained at 170 ꢀC for 15 min. In addition, the lower
furfural yields at 150, 160 and 180 ꢀC compared to that of 170 ꢀC
suggested that side and yield loss reactions compete more
favourably at these conditions. Previous studies have proposed
three main yield loss reactions limiting the nal furfal yield as
illustrated in Scheme 1,50–52 including fragmentation, resin-
ication andꢀ condensation. Specically, lower temperatures
ꢀ
(150 C, 160 C) decelerate the conversion of reactants, which
possibly benets the condensation between xylose (or inter-
mediates) and formed furfural molecules, resulting in
decreased furfural yield. However, although higher temperature
(180 ꢀC) promoted xylose conversion, fragmentation and res-
inication of furfural into resinous products were enhanced as
well possibly because of the stronger molecular thermal
motion,48 which also explained the observation that the reaction
ꢀ
mixture produced a deeper color at 180 C compared with the
lower temperatures as seen in Fig. 1A.
Controlled experiments using no catalyst, RFC were also
carried out using the optimal condition for xylose conversion to
furfural (170 ꢀC, 15 min). The results are illustrated in Table 3.
Without the presence of the catalyst, only a trace amount of
furfural was formed. Similar result was also obtained using RFC
as a catalyst, demonstrating the importance of the –SO3H
bearing catalyst for furfural production. Also, shown in Table 3
is the signicance of keeping the reaction system under anhy-
drate conditions because an addition of 4 ml of water in the
system signicantly decreased the yield of furfural.
In order to make clear the effects of water on furfural
formation, experiments with different water concentrations
were conducted at 170 ꢀC for 15 min. The results are shown in
Table 4. The furfural selectivity was found to decrease from
80.8% to 39.4% as the water concentration increased from
1 ml to 4 ml, which veried that water has a negative impact
on furfural selectivity. On the other hand, prolonging reaction
time to 30 min in 32 ml GVL containing 6 ml H2O resulted in
the highest furfural yield and selectivity of 63% and 65%,
respectively, which are still lower than those in pure GVL (80%
and 80.8%). Thus, it can be concluded that the lower furfural
yield was caused by lower furfural selectivity instead of reac-
tion rates.
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Scheme 1 Conversion of xylose to furfural and plausible reactions in
the process.
Table 3 Controlled experimentsa
Substrate
Catalyst
Solvent
Furfural yield (%)
0.8 g xylose No catalyst 32 ml GVL
0.8 g xylose 0.5 g RFC 32 ml GVL
0.8 g xylose 0.5 g S-RFC 32 ml GVL
Trace
Trace
80.0 ꢃ 2.1
0.8 g xylose 0.5 g S-RFC 32 ml GVL + 4 ml H2O 69.0 ꢃ 1.9
a
170 ꢀC, 15 min for all experiments, 25 min heating-up time.
trace amount of furfural was obtained in the controlled exper-
iments using no catalyst (Table 3). Increasing catalyst dosage
had a positive impact on furfural production when the loading
was less than 62.5% (0.5 g). Furfural yield decreased once the
catalyst loading increased beyond 62.5%, which is probably due
to superuous and accessible acid sites allowed for more
furfural degradation reactions (fragmentation, resinication
and condensation),50–52 resulting in the generation of humins or
other unknown products as illustrated in Scheme 1.
Reusability of catalyst
To understand the recyclability of the solid acid, three kinds of
catalyst were synthesized by changing the formula among RFC,
sulfanilic acid and isoamyl nitrite with a mass ratio of 1 : 2 : 2
(S-RFC-1), 1 : 4 : 2 (S-RFC-2) and 1 : 4 : 4 (S-RFC-3) in the
process of sulfonation. Then, a four-cycle experiment was con-
ducted consecutively for each catalyst. Aer each experiment,
the catalyst was separated by ltration, followed by wash with
water and acetone and drying in oven for next run.
The recycling experiments were carried out at 170 ꢀC in
10 min rather than 15 min since the difference in furfural yield
between these time periods was small (78% vs. 80%) as shown in
Fig. 1. Also, 0.6 g S-RFC was used instead of 0.5 g to minimize the
effect caused by catalyst loss aer each run. The results are
shown in Fig. 3. The yield of furfural declined slightly in each
recycling run, which might be attributed to the leaching of acid
sites, but the yield did not change signicantly between the four
runs. Specically, S-RFC-2 (1 : 4 : 2) gave the best yield during all
Fig. 1 Effect of temperature and time on furfural production from
xylose. Reaction conditions: 0.8 g xylose, 0.5 g S-RFC, 32 ml GVL,
25 min heating-up time.
Effect of catalyst loading
The effect of catalyst loading on furfural yield was also investi-
gated with a catalyst loading (mass ratio of catalyst/xylose)
between 37.5–87.5% (Fig. 2). The catalyst dosage was of great
importance for the conversion of xylose into furfural since only
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Table 4 Effect of water addition on furfural productiona
Time (min)
Solvent
Xylose conversion (%)
Furfural yield (%)
Furfural selectivity (%)
15
15
15
15
15
15
20
25
30
40
32 ml GVL
99.0 ꢃ 0.5
99.0 ꢃ 0.5
98.0 ꢃ 0.5
90.5 ꢃ 1.0
68.9 ꢃ 1.5
61.2 ꢃ 2.0
94.0 ꢃ 1.0
96.0 ꢃ 0.7
97.0 ꢃ 0.5
99.0 ꢃ 0.5
80.0 ꢃ 2.1
78.5 ꢃ 2.0
69.0 ꢃ 1.9
54.0 ꢃ 1.2
32.0 ꢃ 1.4
24.1 ꢃ 1.1
59.0 ꢃ 1.5
62.4 ꢃ 1.7
63.0 ꢃ 1.4
60.0 ꢃ 1.6
80.8 ꢃ 1.7
79.3 ꢃ 1.6
70.4 ꢃ 1.5
59.3 ꢃ 1.0
46.4 ꢃ 1.0
39.4 ꢃ 0.5
62.76 ꢃ 0.9
65.0 ꢃ 1.3
65.0 ꢃ 1.1
60.6 ꢃ 1.3
32 ml GVL + 2 ml H2O
32 ml GVL + 4 ml H2O
32 ml GVL + 6 ml H2O
32 ml GVL + 8 ml H2O
32 ml GVL + 10 ml H2O
32 ml GVL + 6 ml H2O
32 ml GVL + 6 ml H2O
32 ml GVL + 6 ml H2O
32 ml GVL + 6 ml H2O
a
Reaction conditions: 170 ꢀC, 0.5 g xylose, 0.8 g xylose, 25 min heating-up time.
Table 5 Furfural production from xylose using various catalystsa
Xylose conversion
Entry
Catalyst
Furfural yield (%)
(%)
1
2
3
4
5
6
7
PTSA-POM
Amberlyst-15
H-Beta
72.5 ꢃ 1.5
64.8 ꢃ 1.5
78.0 ꢃ 1.2
74.0 ꢃ 1.6
73.0 ꢃ 2.2
68.0 ꢃ 1.9
80.0 ꢃ 2.1
99% ꢃ 0.5
99% ꢃ 0.5
99% ꢃ 0.5
99% ꢃ 0.5
99% ꢃ 0.5
98% ꢃ 0.5
99% ꢃ 0.5
PTSA$H2Ob
c
H2SO4
HClb
S-RFC
a
Reaction conditions: 0.5 g different acid catalyst, 0.8 g xylose, 32 ml
GVL, 170 ꢀC, 15 min reaction time, 25 min heating-up time.
b
0.215 mmol HCl and PTSA$H2O (corresponding to 0.25 g S-RFC).
0.1075 mmol H2SO4 (corresponding to 0.25 g S-RFC).
c
Fig. 3 Reusability of S-RFC-x (x ¼ 1, 2, 3). Reaction conditions: 0.6 g
catalyst, 0.8 g xylose, 32 ml GVL, 170 ꢀC, 10 min.
0.6 g S-RFC in 32 ml GVL at 170 ꢀC for a shorter reaction time of
5 min. The result was shown in ESI (Table S1†). The furfural yield
decreased slightly in former four runs and reduced remarkably
at 5th run, indicating that the S-RFC deactivated seriously aer
4th reusability. No obvious mass addition was observed for
reused catalyst aer recycling experiments, suggesting that the
deposits is not the main cause for the deactivation of S-RFC. In
addition, the sulfur content of reused catalyst was measured by
an Elementar (EA, Elementar model Vario EL III), which was
shown in ESI (Table S2†). It is observed that the S content
decreases gradually in former four runs and remarkably
decreases at run 5, suggesting the partial leaching of SO3H aer
reaction, which also explains the deactivation of S-RFC. Simi-
larly, some other acid catalysts50,53 also suffered from the deac-
tivation and the regeneration was attempted. Regeneration of
reused S-RFC was attempted by removal of deposits or replen-
ishing the S lost with a new sulfonation cycle. However, the
activity of deactivated S-RFC cannot be recovered by these two
ways (Table S1†). Currently, the S-RFC can only be reused for four
runs with a furfural yield of above 70%. The stability and
regeneration of S-RFC still needs to be studied in a further step.
Fig. 2 Effect of catalyst loading (mass ratio of catalyst/xylose) on
furfural production from xylose. Reaction conditions: 0.8 g xylose,
32 ml GVL, 170 ꢀC, 15 min, 500 rpm, 25 min heating-up time.
runs, which was chose as optical formula and applied in all other
experiments. However, the differences among the three catalysts
in terms of furfural yield and recyclability are relatively small.
To further understand the deactivation of S-RFC, a ve-cycle
consecutive recycling was conducted with 0.8 g xylose and
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Inuence of substrate concentration on furfural production
Experiments with different substrate concentration were con-
ducted using S-RFC under optimized reaction conditions. The
results are shown in Table 6. 67.4% furfural yield was obtained
when 4.8% xylose was employed and only 18% furfural yield was
achieved when 9.6% corn stover was added. The results
demonstrated that the yield loss reactions were more tend to
happen with the increase of substrate concentration. As illus-
trated in Scheme 1, we suppose that the condensation between
furfural and substrate or intermediates may be the main cause
for the decreasing furfural yield. When higher concentration of
substrate was employed, more reactants and intermediates
would exist in reaction medium, which enlarged the collision
between formed furfural and xylose or intermediates. And as
a result, more humins and other by-products were formed,
leading to lower furfural yield.
Production of furfural from corn stover
Given the availability and diversity of raw material, corn stover
was chosen as a substrate to produce furfural in a one-pot
reaction. The reaction was carried out in a well-sealed,
mechanically stirred autoclave running at different tempera-
tures (180 ꢀC, 190 ꢀC, 200 ꢀC, 210 ꢀC) with time ranging from 20–
140 min with a mixture of 0.6 g catalyst, 0.8 g corn stover and
32 ml GVL. The corn stover used for experiments contained
31.6% glucan and 20.5% xylan in accordance with our previous
work.54 This data was used to calculate the yields of HMF and
furfural. Furfural yield exhibited an unambiguous response to
residence time and reaction temperature (Fig. 4). A comparison
of Fig. 4A with Fig. 1A clearly demonstrates the need of pro-
longing time and increasing temperature for the conversion of
corn stover directly to furfural. The highest furfural yield
(68.6%) was obtained at 200 ꢀC for 100 min as contrasted to
Fig. 4 Furfural production from corn stover. Reaction conditions:
0.6 g S-RFC, 0.8 g corn stover, 32 ml GVL, 30 min heating-up time.
ꢀ
high xylose conversion at 170 C with 15 min. The prolonged
time and increased temperature are expected as xylan in corn
stover needs to be hydrolyzed to xylose before it can be con-
Despite, a lower yield of 68.6% for direct conversion of corn
verted to furfural. The lower yield of furfural (68.6% from corn stover to furfural, the results indicate that furfural is relative
stover versus 80% for xylose) is also expected as the higher stable even at 200 ꢀC for 100 min, demonstrating the efficacy of
temperature and prolonged time cause some decomposition of GVL as a solvent for furfural production.
furfural as shown in Fig. 1A and 4A.
Table 6 Effects of substrate concentration on furfural productiona
Substrate (wt%)
Catalyst
Reaction conditions
Furfural yield (%)
2.4 wt% xylose
3.6 wt% xylose
4.8 wt% xylose
6.0 wt% xylose
7.2 wt% xylose
9.6 wt% xylose
2.4 wt% corn stover
4.8 wt% corn stover
7.2 wt% corn stover
9.6 wt% corn stover
12 wt% corn stover
0.5 g S-RFC
0.5 g S-RFC
0.5 g S-RFC
0.5 g S-RFC
0.5 g S-RFC
0.5 g S-RFC
0.6 g S-RFC
0.6 g S-RFC
0.6 g S-RFC
0.6 g S-RFC
0.6 g S-RFC
170 ꢀC, 15 min, 32 ml GVL
170 ꢀC, 15 min, 32 ml GVL
170 ꢀC, 15 min, 32 ml GVL
170 ꢀC, 15 min, 32 ml GVL
170 ꢀC, 15 min, 32 ml GVL
170 ꢀC, 15 min, 32 ml GVL
200 ꢀC, 100 min, 32 ml GVL
200 ꢀC, 100 min, 32 ml GVL
200 ꢀC, 100 min, 32 ml GVL
200 ꢀC, 100 min, 32 ml GVL
200 ꢀC, 100 min, 32 ml GVL
80.0 ꢃ 2.1
72.0 ꢃ 1.8
67.4 ꢃ 1.5
55.0 ꢃ 1.1
40.0 ꢃ 1.2
20.0 ꢃ 1.0
68.6 ꢃ 1.6
40.0 ꢃ 1.3
26.2 ꢃ 1.4
18.0 ꢃ 1.2
13.3 ꢃ 0.8
a
25 min heating-up time.
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Table 7 Effect of water on furfural yielda
by calcining resorcinol-formaldehyde resin. The catalyst
exhibited good acid density, specic surface area, pore volume,
Entry Substrate
Solvent
Furfural yield (%)
and average pore diameter of 0.86 mmol gꢁ1 SO3H, 530 m2 gꢁ1
,
0.56 cm3 gꢁ1, and 4.2 nm respectively. The resulting catalyst was
used to convert xylose and corn stover into furfural in GVL. High
furfural yield was obtained from xylose (80%) with a reaction
time of 15 min at 170 ꢀC and corn stover (68.6%) conversion
under similar reaction conditions. Aer analyzing the recycla-
bility of the catalyst across four consecutive runs, it was
concluded that the catalyst retained its activity. In addition to
the applications investigated in this paper, the catalyst also
displays an enormous potential to be used in other acid-
catalyzed reactions and industrial applications.
1
2
0.8 g corn stover 32 ml GVL
68.6 ꢃ 1.8
0.8 g corn stover 32 ml GVL + 4 ml H2O 61.5 ꢃ 1.3
a
200 ꢀC, 100 min, 0.6 g S-RFC, 25 min heating-up time.
Compared to similar catalytic system, an obvious difference
is noted regarding different types of biomasses. For example,
60% furfural is gained from corn stalk but only 29% furfural is
produced from pinewood.23,47 Interestingly, 83.5% furfural is
obtained from corn stalk which is a higher yield than the
furfural produced from xylose (80.4%). The differences in yield
are ascribed to the conversion of cellulose50 need to explain this
a bit more. However, the phenomenon did not occur in our
previous study.47 In present work, the acid density played an
important role on the direct conversion of biomass to furfural
because the acid density of S-RFC (SO3H 0.86 mmol gꢁ1) and SC-
CCA47 (SO3H 1.14 mmol gꢁ1) is lower than the density of PTSA-
POM50 (SO3H 2.3 mmol gꢁ1). 68.6% furfural yield from corn
stover is acceptable, but the critical point is how to separate
a catalyst from the biomass residue. Introducing magnetism is
a promising technique, and the doping of iron into S-RFC is
possible in the fabrication process of resorcinol and formalde-
hyde.55 Improving acid density and introducing magnetism are
good areas for future work to improve S-RFC.
Inuences of water on furfural production from corn stover
were also studied and the results are illustrated in Table 7. As
seen with the dehydration of xylose, the addition of 11% of water
(4 ml water in 32 ml of GVL) reduced furfural yield from corn
stover as well. The presence of water accelerates furfural degra-
dation and promotes side reactions, resulting in lower furfural
selectivity. Compared to experiments without water, a much
darker reaction mixture is observed in the water/GVL system.
No scaling nor sticky agglomerated residue was observed
aer any of the reactions, allowing it to be concluded that GVL
is a good solvent for corn stover dehydration into furfural.
Furthermore, furfural (boiling point: 162 ꢀC) can be isolated
from GVL (boiling point: 208 ꢀC) by distillation, which is an
extra advantage over other efficient solvents, such as DMSO
Acknowledgements
This study was nancially supported by the State Key Program
of National Natural Science Foundation of China (51536009),
the National Key Technology R&D Program of China (No.
2015BAD15B06) and the international technology cooperation
plan of Anhui (No. 1503062030)
Notes and references
1 X. Li and P. Wang, ACS Catal., 2016, 6, 7621–7640.
2 J. Xu, X. Xie, J. Wang and J. Jiang, Green Chem., 2016, 18,
3124–3138.
3 G. Kabir and B. H. Hameed, Renewable Sustainable Energy
Rev., 2017, 70, 945–967.
4 G. Kumar, S. Shobana, W. H. Chen, Q. V. Bach, S. H. Kim,
A. E. Atabani and J. S. Chang, Green Chem., 2017, 19, 44–67.
5 K. Wu, Y. WU, Y. Chen, H. Chen, J. Wang and M. Yang,
ChemSusChem, 2016, 9, 1355–1385.
6 A. W. Bhutto, K. Qureshi, R. Abro, K. Harijan, Z. Zhao,
A. A. Bazmi, T. Abbas and G. Yu, RSC Adv., 2016, 6, 32140–
32170.
´
7 C. Sanchez, L. Serrano, M. A. Andres and J. Labidi, Ind. Crops
Prod., 2013, 42, 513–519.
8 M. J. Gilkey, P. Panagiotopoulou, A. V. Mironenko,
G. R. Jenness, D. G. Vlachos and B. Xu, ACS Catal., 2015, 5,
3988–3994.
9 S. Srivastava, G. C. Jadeja and J. Parikh, RSC Adv., 2016, 6,
1649–1658.
ꢀ
(boiling point: 189 C).
A small amount of HMF was also measured during the
dehydration of corn stover, which is less than 5% yield, indi-
cating that a few hexoses was converted (Fig. 4B). The acid
strength of the catalytic system was not strong enough to
penetrate and hydrolyze crystalline cellulose. In addition, no
Lewis acid was present in the system to isomerize the hexoses to
fructose, which is an intermediate in the conversion of hexoses
to HMF. Ultimately the yield of HMF was too small to be of
commercial signicance.
´
10 L. Bui, H. Luo, W. R. Gunther and Y. Roman-Leshkov, Angew.
Chem., Int. Ed., 2013, 52, 8022–8025.
11 W. Wu, Y. Xu, S. Chang, J. Deng and Y. Fu, ChemCatChem,
2016, 8, 3375–3380.
12 B. Hernandez, J. Iglesias, G. Morales, M. Paniagua, C. Lopez-
Aguado, J. L. G. Fierro, P. Wolf, I. Hermans and J. A. Melero,
Green Chem., 2016, 18, 5777–5781.
´
´
13 T. Wang, K. Li, Q. Liu, Q. Zhang, S. Qiu, J. Long, L. Chen,
L. Ma and Q. Zhang, Appl. Energy, 2014, 136, 775–780.
´
14 N. Alonso-Fagundez, I. Agirrezabal-Telleria, P. L. Arias,
J. L. G. Fierro, R. Mariscal and M. L. Granados, RSC Adv.,
2014, 4, 54960–54972.
Conclusions
In conclusion, we have prepared a novel, effective, and stable 15 X. Li, X. Lan and T. Wang, Catal. Today, 2016, 276, 97–104.
solid acid catalyst via the mild sulfonation of carbon produced 16 X. Li, B. Ho and Y. Zhang, Green Chem., 2016, 18, 2976–2980.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 29916–29924 | 29923

View Article Online
RSC Advances
Paper
´
17 J. M. Tukacsa, M. Bohusa, G. Dibob and L. T. Mika, RSC Adv., 36 J. Zou, D. Cao, W. Tao, S. Zhang, L. Cui, F. Zeng and W. Cai,
2017, 7, 3331–3335. RSC Adv., 2016, 6, 49528–49536.
18 Y. Yang, J. Ma, X. Jia, Z. Du, Y. Duan and J. Xu, RSC Adv., 37 M. Zhang, A. Sun, Y. Meng, L. Wang, H. Jiang and G. Li,
2016, 6, 51221–51228. Microporous Mesoporous Mater., 2015, 204, 210–217.
19 H. J. Brownlee and C. S. Miner, Ind. Eng. Chem., 1948, 40, 38 R. Liu, X. Wang, X. Zhao and P. Feng, Carbon, 2008, 46, 1664–
201–204. 1669.
20 V. Choudhary, S. I. Sandler and D. G. Vlachos, ACS Catal., 39 R. Ryoo, S. H. Joo and S. Jun, J. Phys. Chem. B, 1999, 103,
2012, 2, 2022–2028. 7743–7746.
21 F. Delbecq, Y. Wang and C. Len, J. Mol. Catal. A: Chem., 2016, 40 J. Feng, W. Song, L. Sun and L. Xu, RSC Adv., 2016, 6, 110337–
423, 520–525. 110343.
22 S. Le Guenic, F. Delbecq, C. Ceballos and C. Len, J. Mol. 41 B. K. Price, J. M. Tour and J. Am, Chem. Soc., 2006, 128,
Catal. A: Chem., 2015, 410, 1–7. 12899–12904.
23 Y. Yang, C. W. Hu and M. M. Abu-Omar, ChemSusChem, 42 J. Xu, A. Wang and T. Zhang, Carbon, 2012, 50, 1807–1816.
2012, 5, 405–410.
43 S. G. Wettstein, D. M. Alonso, Y. Chong and J. A. Dumesic,
24 K. R. Enslow and A. T. Bell, Catal. Sci. Technol., 2015, 5, 2839–
2847.
25 A. V. Carvalhoab, A. M. da Costa Lopes and R. Bogel-Łukasik,
RSC Adv., 2015, 5, 47153–47164.
Energy Environ. Sci., 2012, 5, 8199–8203.
¨
¨
44 E. I. Gurbuz, J. M. R. Gallo, D. M. Alonso, S. G. Wettstein,
W. Y. Lim and J. A. Dumesic, Angew. Chem., Int. Ed., 2013,
52, 1270–1274.
¨
26 I. Agirrezabal-Telleria, J. Requies, M. B. Guemez and 45 D. M. Alonso, J. M. R. Gallo, M. A. Mellmer, S. G. Wettstein
P. L. Arias, Appl. Catal., B, 2014, 145, 34–42. and J. A. Dumesic, Catal. Sci. Technol., 2013, 3, 927–931.
27 S. Kaiprommarat, S. Kongparakul, P. Reubroycharoen, 46 M. Chia and J. A. Dumesic, Chem. Commun., 2011, 47, 12233–
G. Guan and C. Samart, Fuel, 2016, 174, 189–196. 12235.
28 H. Li, J. Ren, L. Zhong, R. Sun and L. Liang, Bioresour. 47 T. Zhang, W. Li, Z. Xu, Q. Liu, Q. Ma, H. Jameel, H. M. Chang
Technol., 2015, 176, 242–248. and L. Ma, Bioresour. Technol., 2016, 209, 108–114.
29 S. M. Bruce, Z. Zong, A. Chatzidimitriou, L. E. Avci, 48 W. Zhang, H. Tao, B. Zhang, J. Ren, G. Lu and Y. Wang,
J. Q. Bond, M. A. Carreon and S. G. Wettstein, J. Mol.
Catal. A: Chem., 2016, 422, 18–22.
30 X. Hu, R. J. Westerhof, D. Dong, L. Wu and C. Z. Li, ACS
Sustainable Chem. Eng., 2014, 2, 2562–2575.
31 W. Jeon, C. Ban, J. E. Kim, H. C. Woo and D. H. Kim, J. Mol.
Catal. A: Chem., 2016, 423, 264–269.
32 S. Le Guenic, D. Gergela, C. Ceballos, F. Delbecq and C. Len,
Molecules, 2016, 21, 1102.
33 M. Toda, A. Takagaki, M. Okamura, J. N. Kondo, S. Hayashi,
K. Domen and M. Hara, Nature, 2005, 438, 178.
34 L. Yan, N. Liu, Y. Wang, H. Machida and X. Qi, Bioresour.
Technol., 2014, 173, 462–466.
Carbon, 2011, 49, 1811–1820.
49 Q. Shu, J. Gao, Z. Nawaz, Y. Liao, D. Wang and J. Wang, Appl.
Energy, 2010, 87, 2589–2596.
50 Z. Xu, W. Li, Z. Du, H. Wu, H. Jameel, H. M. Chang and
L. Ma, Bioresour. Technol., 2015, 198, 764–771.
51 L. Zhang, G. Xi, J. Zhang, H. Yu and X. Wang, Bioresour.
Technol., 2017, 224, 656–661.
52 A. S. Dias, M. Pillinger and A. A. Valente, Microporous
Mesoporous Mater., 2006, 94, 214–225.
53 L. Zhang, G. Xi, K. Yu, H. Yu and X. Wang, Ind. Crops Prod.,
2017, 98, 68–75.
54 Q. Liu, W. Li, Q. Ma, S. An, M. Li, H. Jameel and H. M. Chang,
Bioresour. Technol., 2016, 211, 435–442.
´
´
35 C. de la Calle, J. M. Fraile, E. Garcıa-Bordeje, E. Pires and
´
L. Roldan, Catal. Sci. Technol., 2015, 5, 2897–2903.
55 T. M. Zhang, D. L. Zhao, L. Yin and Z. M. Shen, J. Alloys
Compd., 2010, 508, 147–151.
29924 | RSC Adv., 2017, 7, 29916–29924
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