Efficient catalytic conversion of corn stalk and xylose into furfural over sulfonated graphene in γ-valerolactone

Article abstract of DOI:10.1039/c9ra01411j

Sulfonated graphene (SG) was prepared and employed to convert corn stalk and xylose into furfural. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) were used to characterize SG. The effects of reaction time, temperature, substrate loading, catalyst dosage and solvents on the reaction were researched and optimized. SG exhibited high catalytic activity in the conversion of xylose and corn stalk to furfural. A fairly high furfural yield of 96% was achieved at 150 °C from xylose and a 71.9% furfural yield was obtained when using a 10.7 ratio (mass ratio: xylose to SG) at 140 °C. While a 48% furfural yield was obtained from corn stalk (based on the starting combined moles of xylan and glucan in corn stalk; yield was >100%, if based on only xylan) using a substrate loading (corn stalk to catalyst mass ratio) of 2.14 and a 19% 5-hydroxymethylfurfural (5-HMF) yield was obtained. What's more, a 43.9% yield of furfural was obtained in only 20 min. In addition, the reusability of SG was also investigated and shown to have good stability for xylose dehydration.

Full text of DOI:10.1039/c9ra01411j

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Efficient catalytic conversion of corn stalk and
xylose into furfural over sulfonated graphene in g-
valerolactone†
Cite this: RSC Adv., 2019, 9, 10569
a
b
b
Jianru Ma,ab Wenzhi Li,
Shengnan Guan,a Qiying Liu,
Qingqing Li,
*
a
a
a
b
*
Chaofeng Zhu, Tao Yang, Ajibola Temitope Ogunbiyi and Longlong Ma
Sulfonated graphene (SG) was prepared and employed to convert corn stalk and xylose into furfural.
Transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron
spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) were used to characterize SG.
The effects of reaction time, temperature, substrate loading, catalyst dosage and solvents on the
reaction were researched and optimized. SG exhibited high catalytic activity in the conversion of xylose
and corn stalk to furfural. A fairly high furfural yield of 96% was achieved at 150 ꢀC from xylose and
a 71.9% furfural yield was obtained when using a 10.7 ratio (mass ratio: xylose to SG) at 140 ꢀC. While
a 48% furfural yield was obtained from corn stalk (based on the starting combined moles of xylan and
glucan in corn stalk; yield was >100%, if based on only xylan) using a substrate loading (corn stalk to
catalyst mass ratio) of 2.14 and a 19% 5-hydroxymethylfurfural (5-HMF) yield was obtained. What's more,
a 43.9% yield of furfural was obtained in only 20 min. In addition, the reusability of SG was also
investigated and shown to have good stability for xylose dehydration.
Received 24th February 2019
Accepted 28th March 2019
DOI: 10.1039/c9ra01411j
rsc.li/rsc-advances
Mineral acids (H2SO4 or HCl) are currently the important
catalysts for commercial production of furfural.12,13 Recently, some
1. Introduction
efficient homogeneous catalysts were used for the dehydration of
xylose to furfural in single-phase or biphasic system. FeCl3$6H2O
was used for the production of furfural from xylose in aqueous
environment through hydrolysis to produce a strong acidic
medium.14 CrCl3 as a bifunctional catalyst, combined with ionic
liquid, was found to produce furfural from xylan with a 63% yield
In recent years and with the rapid development of the world's
economy, the consumption of fossil based resources has been
excessively increasing, which has contributed to serious envi-
ronment pollution and global warming. Consequently, renew-
able resources, one of which is biomass, have attracted great
attention as promising candidates for producing high value-
added chemicals and biofuels1 with minimal adverse environ-
ment effects. Studies have shown that more than 300 kinds
of chemicals can be used as raw materials for the production
of chemical products, and some of them are
considered as promising biochemical substances by the US
Department of Energy and other scholars, such as furfural,
5-hydroxymethylfurfural (5-HMF), levulinic acid, etc.2 Furfural is
mainly derived from hemicelluloses and can be used to produce
many useful chemicals, such as furfural alcohol,3–5 tetrahy-
drofurfuryl alcohol,6,7 g-valerolactone (GVL)8,9 and 2-methyl-
furan, tetrahydrofuran.10,11
with the aid of microwaves at 200 C.15 AlCl3$6H2O was used as
ꢀ
catalyst to prepare furfural from xylose with a 75% furfural yield in
a water–tetrahydrofuran biphasic medium.16 However, commer-
cial production of furfural still faces greater challenges and diffi-
culties, as this process comes with great issues such as
environment pollution and equipment's erosive damage caused by
homogeneous acid catalysts.17 Although the use of homogeneous
catalysis is an efficient method to produce furfural, in order to
recycle catalysts and avoid toxic homogenous acids and equip-
ment corrosion problem, heterogeneous catalytic systems have
been received greater attention, partly because it allows for the
catalysts reusability. Solid acid catalysts have the most potential for
furfural production. SO42ꢁ/ZrO2–Al2O3/SBA-15 solid acid catalyst
was prepared for the dehydration of xylose in toluene–water
biphasic system with a 53.4% furfural yield at 160 ꢀC.18 Dias et al.
used sulfonic acid-anchored MCM-41 to produce furfural from D-
xylose.19 Also, Gurbuz and co-workers studied various solid acid
catalysts to convert hemicellulose into furfural.20
aLaboratory of Basic Research in Biomass Conversion and Utilization, Department of
Thermal Science and Energy Engineering, University of Science and Technology of
China, Hefei 230026, PR China. E-mail: liwenzhi@ustc.edu.cn
bCAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion,
Chinese Academy of Sciences, Guangzhou 510640, PR China. E-mail: mall@ms.giec.
ac.cn
Among these heterogeneous catalysts, sulfonated carbon-
based solid acid materials have attracted great attention due
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra01411j
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to their high chemical activity, high thermal stability, high 10 min, which have been reported barely for efficient produc-
specic surface area and low production costs.21–23 These tion of furfural at such less reaction time in previous articles.
sulfonated carbon-based materials can be prepared by The catalyst was synthesized and characterized by a variety of
sulfonating activated carbon in sulfuric acid or pyrolyzing equipment to conrm its structure. The reaction temperature,
organics to form carbon materials and treating with acid reaction time, various solvent and dosage of catalyst and
subsequently and these materials can process higher acid substrate were investigated in the process of reaction of
density aer sulfonation. Pang and co-workers placed the acti- conversion xylose and corn stalk into furfural. In addition, the
vated carbon in concentrated sulfuric acid at 250 ꢀC under N2 to stability and reusability of SG were also investigated.
form a sulfonated carbon solid acid.24 The sucrose was used as
carbon source and added SBA-15 hard template to form meso-
2. Materials and methods
2.1 Materials
porous carbon, followed by sulfonation with SO3/H2SO4.25 In
our previous work, sulfonated carbon catalyst (SC-CCA), where
the carbon source was from sucrose, was employed to catalyze
xylose and corn stalk into furfural in GVL.26 Although solid acid
catalysts largely decrease environmental pollution and equip-
ment corrosion, the product yield is low and there are high
reaction temperature and long residence time during the reac-
tion process. Therefore, in order to improve furfural yield and
enhance the reaction efficiency, graphene-based solid acid
catalyst was employed to produce furfural from xylose and corn
stalk in this work.
D-Xylose (98%), furfural (99%), sulfanilic acid (AR, 99.5%),
sodium nitrite (AR, 99%), 5-HMF (99%) and g-butyrolactone
(AR, GBL) were purchased from Aladdin Industrial Inc.
(Shanghai, China). Graphite powder (SP), hydrazine hydrate
(AR, 85%), sulfuric acid (AR, 95–98%), hydrochloric acid (AR,
36–38%), 1,4-dioxane (AR), dimethylsulfoxide (AR, DMSO),
dimethylformamide (AR, DMF) and other reagents were
purchased from Sinopharm Chemical Reagent Co. Ltd
(Shanghai, China). g-Valerolactone (95%, GVL) was bought
from Langfang Hawk Technology and Development. Co., Ltd
(Langfang, Hebei, China). All reagents were directly used as
received without further treatment. The corn stalk used in this
study came from Mengcheng, Anhui, China. The corn stalk was
washed with water and then dried in oven. The corn stalk was
ground into particles to a size of 40 mesh and then further dried
to constant weight.
Graphene-based materials have attracted great attention in
recent years due to its unique properties such as thermal
conductivity, electrical, optical and catalytic properties.
Sulfonated graphene has been applied in the hydrolysis of ethyl
acetate,27 the dehydration of fructose to HMF,28 and catalytic
conversion of levulinic acid to GVL.29 In addition, different
solvents have been used to convert xylose and corn stalk to
furfural, and in order to increase the yield of furfural in
a greener way, we need to choose an optimal solvent solution.
Water, the inexpensive and eco-friendly solvent, has been used
2.2 Graphite oxide (GO) preparation
for furfural production. However, the yield of furfural was fairly Graphite oxide was prepared by a slightly modied Hummer's
low because of the degradation of furfural in water.30 GVL method.33 Graphite powder (3.0 g) was added into a mixture of
derived from biomass has been used for furfural production. concentrated 12 mL H2SO4, 2.5 g P2O5, and 2.5 g K2S2O8. The
Using GVL as a solvent will decrease the activation energy solution was kept at 80 ꢀC and agitated for 4.5 h by using an oil-
barrier of xylose dehydration and increase the rampart of bath. The solution was then cooled down to the room temper-
furfural degradation, therefore, the dehydration rate of xylose ature and diluted with 0.5 L double distilled water and le
will increase, in contrast, the degradation rate of furfural will overnight. Finally, the product was washed with double distilled
decrease.20,30 Zhang and co-workers dehydrated xylose to water several times to remove the residual acid. The product
produce furfural in GVL by using FeCl3$6H2O as catalyst with was dried at ambient temperature.
a 86.5% furfural yield at 170 ꢀC.31 Solid acid catalyst was used to
The pre-oxidized graphite powder (1.0 g) was put into a cold
dehydrate xylose and corn stalk into furfural by using GVL as (0 ꢀC) concentrated H2SO4 (27.5 mL) solution and stirred.
solvent with a 78.5% furfural yield from xylose at 170 ꢀC.26 GVL KMnO4 (3.5 g) was then added gradually under stirring while
is a nontoxic and green solvent and the product furfural is easily the temperature of the mixture was maintained below 20 ꢀC.
ꢀ
separated from GVL by distillation. Therefore, GVL was used as Successively, the mixture was stirred at 35 C for 2 h and then
the solvent to produce furfural in this study.
carefully diluted with double distilled water (115 mL). This lead
Diazonium of sulfanilic acid and sodium nitrite formed in to the release of a large amount of heat and kept the tempera-
situ has been used in this work to prepare sulfonated graphene32 ture at 50 ꢀC. Aer adding all of the 58 mL double distilled
and we contributed to the efficient furfural production from water, the solution was stirred for 2 h and then additional water
corn stalk and xylose. For the conversion of corn stalk, the (230 mL) was added and stirred for a while. Then 5 mL of 30%
furfural yield would exceed 100% if calculated by adopting the H2O2 was put into mixture in drop, and the color of solution
starting moles of xylan in corn stalk because of extra furfural changed into brilliant yellow along with bubbling. The mixture
production from glucan in corn stalk, consequently, combined was washed with 1 : 10 HCl aqueous solution (500 mL) three
moles of starting xylan and glucan in corn stalk were used to times by centrifugation at 4500 rpm for 5 min to remove metal
calculate furfural yield in this work. A 48% furfural yield was ions, and then washed with double distilled water until neutral
obtained from corn stalk at in 50 min, and what's more and no chloride ion detected by AgNO3 aqueous solution. The
a furfural yield of 40% was obtained from corn stalk just in product was collected by lyophilization.
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The samples were analyzed by high performance liquid
chromatography (HPLC, LC-2010AHT, SHIMADZU) equipped
with a C18 column (ZORBAX, Eclipse XDB-C18, Agilent) and an
UV detector (SPD-10A, SHIMADZU) at 280 nm. A mixture solu-
tion of methanol and water (2/3 v/v) was used as the mobile
phase (0.4 mL minꢁ1), and the column temperature was
2.3 Sulfonated graphene preparation
Graphite oxide (500 mg) was added into 250 mL double distilled
water and exfoliated to graphene oxide by ultrasonication of
graphite oxide dispersion (2 mg mLꢁ1). Then, a 5% sodium
carbonate solution was added so as to make the PH of the
mixture achieve 9–10. Finally, 10 mL 64% hydrazine hydrate was
put into the suspension and the solution was allowed to reux for
24 hours at 95 ꢀC. The mixture was cooled to ambient tempera-
ture and ltered through Millipore membrane (0.45 mm) and
washed with double distilled water and methanol. The product,
reduced graphene oxide (RGO), was freeze-dried and collected.
RGO (245 mg) was added into 40 mL water and allowed to
sonicate for 1 h. Then, 2.108 g sodium nitrite and 1.757 g sul-
fanilic acid were added into the resulting mixture, where the
diazonium would be formed. The mixture was stirred for 24 h at
room temperature to achieve sulfonated graphene. The solution
was ltered through Millipore membrane and washed with
100 mL 1 M HCl, double distilled water and 200 mL acetone
repeatedly; this process has proved to be specically efficient in
the isolation of SG at high purity.34–36 The black power was
acquired by lyophilization for 24 h.
ꢀ
maintained at 30 C. Furfural and HMF yields for the various
products have been calculated on a molar basis as follows:
furfural yield (from xylose) ¼ (moles of furfural produced/moles
of starting xylose) ꢂ 100%
furfural yield (from corn stalk) ¼ (moles of furfural produced/
moles of starting xylan and glucan in corn stalk) ꢂ 100%
furfural yield (from fructose) ¼ (moles of furfural produced/moles
of starting fructose) ꢂ 100%
furfural yield (from glucose) ¼ (moles of furfural produced/moles
of starting glucose) ꢂ 100%
furfural yield (from cellulose) ¼ (moles of furfural produced/
moles of starting cellulose) ꢂ 100%
2.4 Catalyst characterization
HMF yield (from corn stalk) ¼ (moles of HMF produced/moles
of starting glucan in corn stalk) ꢂ 100%
The catalyst materials were characterized using SEM from an
XL-30 ESEM instrument (FEI Company, USA) and TEM from
a JEM-2100F instrument (JEOL, Japan). The sample was
dispersed in the alcohol with the aid of ultrasound, and then
dropped onto a copper grid until it became dried under warm
light. FTIR spectra were recorded from samples using a Thermo
Scientic Nicolet iS-50 instrument with a KBr disk over a range
of 400–4000 cmꢁ1. The XPS measurement was carried from an
ESCALAB 250 instrument that employs monochromatic Al Ka
radiation (1486.6 eV).
3. Results and discussion
3.1 Characterization of catalyst
SEM and TEM revealed the morphologies of RGO, SG and used
SG (the 5th recycling SG) as shown in Fig. S1.† It can be seen
from SEM and TEM images of the catalysts that RGO and SG
exhibit a curled and layered structure. Aer ves reactions, the
morphology of SG barely changed while the acid density
decreased by elemental analysis as shown in Table 1. The
thermal stability of SG was tested by using thermꢁo1gravimetric
analysis (TGA) with a heating rate of 10 ꢀC min under N2
atmosphere as shown in Fig. S2.† The mass losses of 15.06%
below 130 ꢀC could be attributed to volatiles' desorption, mainly
moisture. The sample was then steady until 190 ꢀC at which
point the TGA spectrum for SG clearly showed a weight loss
corresponding to the gradual disappearance of the graed
sulfonic acid groups.32
2.5 Catalytic conversion of xylose
The catalytic reaction took place in a 15 mL sealed thick-walled
glass tube with 28 mg of the catalyst, 60 mg xylose and 10 mL
solvent under magnetic stirring. The tube was heated at
a specic temperature and sustained for different reserved
times in an oil bath. Aer the reaction, the reactor was cooled to
room temperature, and the reaction mixture was ltered,
collected, and stored in a refrigerator for subsequent analysis.
2.6 Catalytic conversion of corn stalk
The FT-IR spectra of GO, RGO, and SG presented different
functional groups on the graphene sheet as shown in
Fig. S3.† The pristine graphite oxide reveal the presence of C–
O, C–OH, C]O, carbonyl moieties stretching vibration at
The compositional analysis had been summarized in previous
articles by our research group.17 There are mainly 20.5% xylan,
31.6% glucan and 22.8% lignin in raw corn stalk and some
other minor components such as glycan, extractive, ash etc. The
catalytic reaction was performed in a 25 mL stainless-steel
autoclave. In a typical process, 75 mg corn stalk was dispersed
10 mL GVL and mixed with 35 mg catalyst. The reactor was
Table 1 The elemental analysis of SG and used SGa catalysts
Entry
N (%)
C (%)
S (%)
SO3H density (mmol gꢁ1
)
ꢀ
heated up to 190 C for 50 min at 600 rpm. At the end of the
SG
Used SG
1.42
2.03
51.50
59.24
6.565
4.325
2.05
1.35
reaction, the reactor was immediately cooled with cold water,
and the reaction mixture was ltered, collected, and stored in
refrigerator for subsequent analysis.
a
The 5th recycling SG catalyst.
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1060, 1230 (epoxy), 1384, 1630 and 1732 cmꢁ1 (carboxylic acid 288.9 eV are ascribed to C]O and O–C]O bonds, respectively.
and carbonyl moieties) respectively.37,38 Aer reduction with Aer reduction by hydrazine hydrate, the RGO exhibited similar
hydrazine hydrate, peaks at 1060, 1230, 1398, 1630 cmꢁ1 valence states for the carbon element to the GO, but the peak
almost disappeared while peak at 1570 cmꢁ1 appeared indi- intensity of some oxygen-containing groups (such as C–O, C]
cating the dehydration and reduction of O–H, C–O, and C]O O) are much lower than those in GO (Fig. 1B), meaning that
and appearance of N–H.37 The sulfonation of RGO to SG a large amount of oxygen functional groups were shed during
peaks at 616, 1002, 1030, 1121, and 1175 cmꢁ1 conrmed the the reduction process. In addition to those oxygen-containing,
existence of SO3H with catalyst17 and aer ve reaction runs the C–N peak at ꢃ285.8 eV appeared in RGO due to the reduc-
with SG catalyst, the peaks of SO3H decayed due to SO3H tion of hydrazine hydrate.39 SG has the same carbon valence
falling from catalyst. The sulfur element was uniformly state as RGO aer sulfonation with sodium nitrite and sulfanilic
dispersed on the surface of SG revealed in EDS mapping acid in situ (Fig. 1C). The two peaks at 232.1 and 168.1 eV cor-
(Fig. S4†).
responding to S 2s and S 2p, respectively, suggest that the
X-ray photoelectron spectroscopy (XPS) was used to carry out sulfonic acid group successfully gras onto the surface of
the surface characterization and trace the evolution of the material (Fig. 1E). The S 2p binding energy (168.1 eV), which is
structural properties of the materials as shown in Fig. 1. The slightly lower than the anticipated value of sulfonic acid (168.8
surface oxygen functional groups and distribution of GO were eV) can be explained by electron transfers from graphene to
veried by deconvoluting the C 1s peak into its relative sulfonic acid groups (Fig. 1D), and the small peak at ꢃ400 eV is
components (Fig. 1A). The C–C bond at ꢃ284.6 eV and C–O ascribed to N 1s due to the use of hydrazine hydrate to reduce
(including epoxide and hydroxyl) bond at ꢃ286.8 eV formed two graphene oxide to graphene. This view can be conrmed by the
main peaks in GO respectively. The peaks at ꢃ287.6 and small peaks in the XPS spectrum of RGO.32
Fig. 1 High resolution XPS spectrum of C 1s in GO (A), RGO (B) and SG (C). (D) High resolution of S 2p spectrum of SG. (E) Survey scan spectra of SG.
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48% at 190 ꢀC in 50 minutes. The variation of furfural yield with
time was similar to that of temperature as shown in Fig. 2B.
When residence time was excessively increased, the yield of
furfural would decrease due to condensation and degradation of
products. However, it is worth noting that a yield of 40% was
achieved in only 10 min at 190 ꢀC for conversion of corn stalk into
furfural. The effects of temperature and time on 5-HMF
production from corn stalk are also investigated (Fig. S5†) and
3.2 Effects of temperature and time on the yield of furfural
The effects of reaction time and temperature on the production
of furfural from corn stalk and xylose were studied, and the result
was shown in Fig. 2. The study found that temperature and time
had a positive inuence on the furfural yield. As shown in Fig. 2A,
when the running temperature of xylose was increased from 130
to 140 ꢀC, the yield of furfural increased sharply. When the
temperature was 150 ꢀC, the yield of furfural reached a maximum
ꢀ
a 19% 5-HMF yield was gained at 180 C in 50 min and more
ꢀ
of 96% in 40 minutes. The furfural yield at 140 C (92.3%) was
details are summarized in the ESI.† A desirable furfural yield was
achieved in such a short reaction time, which indicates that the
sulfonated graphene catalyst exhibits superior catalytic activity
and high selectivity and well dispersed in solvent.
Controlled experiments were conducted and the results were
shown in Table 2. There was no furfural production in the
absence of catalyst. This result is the same as using RGO catalyst,
which further illustrates the importance of acidity in the
production of furfural from corn stalk. The effects of other
catalysts in previous articles on furfural yield were also listed as
shown in Table 2. A 43.9% furfural yield was obtained when
using SG catalyst in only 20 min and the furfural production
slightly lower than that at 150 ꢀC. Therefore, considering energy
conservation, xylose conversion was carried out at 140 ꢀC for
other experiments, the result of which was impressive in terms of
the conversion to xylose into furfural in such low temperature
compared to using other solid acids. As the reaction temperature
continued to rise, the yield of furfural slowly decreased, because
an increase in temperature causes the polymerization of the
small furfural molecule.26,40 For corn stalk, the yield of furfural
increased slowly with increasing temperature (130–190 ꢀC),
which might be due to the initial hydrolysis of cellulose and
hemicellulose into hexoses and pentoses, followed by the dehy-
dration of the latter to form furfural. The yield of furfural reached
Fig. 2 Effects of temperature (A) and time (B) on furfural production from xylose and corn stalk. Reaction conditions: for xylose: 28 mg catalyst,
60 mg xylose, 10 mL GVL, for corn stalk: 35 mg catalyst, 75 mg corn stalk, 10 mL GVL. (A) Reaction time: 40 min and 50 min for xylose and corn
stalk. (B) Reaction temperature: 140 ꢀC and 190 ꢀC for xylose and corn stalk.
Table 2 Furfural production from biomass using diverse catalysta
Furfural production relative ratec
Catalyst
Catalyst mass (mg)
Reaction conditions
Substrate
Furfural yield (%)
(minꢁ1 mgꢁ1
)
References
No catalyst
RGO
SG
SC-CaCt-700
SC-CCA
S-RFC
SPTPA
FeCl3
0
190 ꢀC, 20 min
190 ꢀC, 20 min
190 ꢀC, 20 min
200 ꢀC, 100 min
200 ꢀC, 100 min
200 ꢀC, 100 min
175 ꢀC, 30 min
185 ꢀC, 100 min
Corn stalk
Corn stalk
Corn stalk
Corn stalk
Corn stalk
Corn stalk
Corn cob
Corn cob
Trace
Trace
43.9/100.3b
93b
0
0
This work
This work
This work
17
26
41
42
31
40
40
45
300
600
30
9.6
0.125
0.021
0.002
0.001
0.082
0.083
60.6b
68.6b
73.9b
79.6b
a
Reaction solvent: GVL. b Furfural yield was calculated only from xylan part of corn stalk and corn cob. c Furfural production relative rate ¼ furfural
yield/(reaction time ꢂ catalyst mass).
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Fig. 3 Plausible reaction pathway for the transformation of hexoses into furfural.
relative rate was up to 0.125 minꢁ1 mgꢁ1, which can prove that mg) and corn stalk (75 mg) was each kept constant, which was
SG exhibits high performance in the process of catalytic conver- used to explore the changes of yields by changing the amount of
sion of corn stalk into furfural compared to other acidic catalysts, catalyst as shown in Fig. 4A. Furfural yield went up gradually
such as SC-CaCt-700, SC-CCA, S-RFC, SPTPA, and FeCl3.
with an increase in the catalyst dosage; 92.3% and 43.9%
The mechanism for the conversion of glucose and fructose into furfural yields were obtained from xylose in 40 min and corn
furfural has been studied in recent years and it has been found that stalk in 20 min, where the mass ratios of catalyst to substrate
hexoses could also be converted to furfural under an acid system,43,44 were 7 : 15 and 4 : 6, respectively. It is noteworthy that since
which could be explained as shown in Fig. 3. Glucose was converted there was almost no difference in furfural yield of conversion
to fructose through isomerization by N-doped graphene on Lewis corn stalk when using a mass ratio of 7 : 15 (SG/corn stalk)
acid sites45 and then fructose was converted to arabinose by decar- compared to ratio of 4 : 6, therefore, a mass ratio of 7 : 15 was
bonylation. Finally, furfural was obtained by the dehydration of adopted to execute corn stalk conversion in other experiments
arabinose. In order to conrm if the hexoses and cellulose could be in this work. Furfural yield decreased slowly or remained
converted to furfural, some additional experiments were carried out, constant (Fig. 4A), when the catalyst loading continued to
and in addition, the convertibility of the by-product, 5-HMF, to increase, which is because side reactions like condensation,
furfural (see Table 3). Cellulose, glucose and fructose can all be polymerization and resinication occurred under excessive
converted to furfural under the same reaction system. Therefore, the acidic medium (provided by SG).20,40 Theꢀ5-HMF yield reached
cellulose fraction of corn stalk should also be involved when 8% when using a 2 : 6 mass ratio at 190 C in 20 min and the
calculating the yield of furfural. It was reported that 5-HMF was also yield also decreased as the catalyst loading further increased as
converted to furfural by decarbonylation;46 however, there was no shown in Fig. S5C.†
furfural found in our reaction system by using 5-HMF as substrate.
Similarly, catalyst dose was also xed to inspect the change
This shows that different reaction systems give diverse possible in furfural yield by changing substrate loading. As shown in
pathways for the transformation of hexoses into furfural and the Fig. 4B (substrate loading: mass ratio of corn stalk to catalyst),
mechanism needs to be further explored.
furfural yield gradually decreased as the substrate loading
increased. This might be ascribed to the occurrence of
condensation and polymerization reactions among products,
intermediates and reactants, as higher substrate dose increases
the probability of molecular collision in reactions.47,48 The
highest furfural yield was obtained from xylose and corn stalk at
the substrate loading of 2.14 respectively. It was worth noting
that a 71.9% furfural yield was obtained at a substrate loading
of 10.7 within 40 minutes at 140 ꢀC, which was a high value for
furfural production under such a substrate loading. For the
corn stalk, a 31.6% furfural yield was obtained with a substrate
loading of 4.28, which was a relatively higher value for conver-
sion of corn stalk into furfural. The effect of substrate loading
on 5-HMF production was similar to that on furfural produc-
tion. A 15.1% 5-HMF yield was obtained with a 2.14 substrate
loading and the yield declined as substrate loading increased
(Fig. S5D†). More humins were formed as the corn stalk dosage
increased and deposited on the surface of catalyst, which
caused catalyst deactivation and further hindered furfural
production.
3.3 Effects of catalyst and substrate loading on the yield of
furfural
The effects of catalyst dosage and substrate loading on furfural
yield were studied in this work (Fig. 4). The amount of xylose (60
Table 3 Effects of diverse substrate on furfural production by using
sulfonated graphene catalysta
Furfural yield
(%)
Substrate
Solvent
Reaction conditions
Fructose
Glucose
Cellulose
Fructose
Glucose
Cellulose
HMF
GVL
GVL
GVL
GVL
GVL
GVL
GVL
190 ꢀC, 50 min
190 ꢀC, 50 min
190 ꢀC, 50 min
170 ꢀC, 40 min
170 ꢀC, 40 min
170 ꢀC, 40 min
170 ꢀC, 40 min
17.8
22.4
23.2
18.3
23.3
25.5
Trace
a
35 mg catalyst, 75 mg substrate, 10 mL GVL.
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Fig. 4 Effects of catalyst loading (A) and substrate dose (B) on furfural production from xylose and corn stalk. Reaction conditions: 140 ꢀC,
40 min, 60 mg xylose, 10 mL GVL for xylose and 190 ꢀC, 20 min, 75 mg corn stalk, 10 mL GVL for corn stalk in (A). 140 ꢀC, 40 min, 28 mg catalyst,
10 mL GVL for xylose and 190 ꢀC, 50 min, 35 mg catalyst, 10 mL GVL for corn stalk in (B). Catalyst loading ¼ mass ratio of catalyst to xylose or corn
stalk; substrate loading ¼ mass ratio of xylose or corn stalk to catalyst.
propanol, cellulose pyrolysis to glucose.30 Although GVL favors
3.4 Effects of various solvents on furfural production
the production of furfural, water, as a solvent, is greener, cheaper
While the use of catalyst in the production of furfural from
and abundant in the world. Therefore, experiments were con-
xylose and corn stalk is very crucial, the solvents system also
ducted at 50, 100, 150, 200, 250 minutes at 190 ꢀC to observe the
plays a very vital role in the reaction process. Solvents will not
change in furfural yield from corn stalk in the aqueous phase
only affect the product species, but also affect the reaction
kinetics to change the yield of furfural by changing the stability
(Fig. 6), and at the same time, a low 5-HMF yield was received.
When the time was extended, furfural yield reached a maximum
of acidic protons relative to the protonated transition state.30,49
of 16.5% at 150 min and then began to slowly decrease. The yield
Under the same reaction conditions, water and different
organic solvents were applied as reaction solvents, and the results
of furfural obtained in water was much lower than that in GVL
due to the stronger polarity of water which caused polymerization
are shown in Fig. 5. When using GVL as solvent, the highest
between furans.50 Although low yields of furfural and 5-HMF were
furfural yields were achieved from xylose and corn stalk in these
obtained from water, water is a very important solvent for
solvents involved in my experiments, respectively. GBL gave
sustainable development from an industrial perspective. There-
a relatively higher yield close to the result acquired from GVL,
fore, the problem of low yield of furfural needs to be further
while 1,4-dioxane gave a medium yield. A extremely low yield was
obtained when using DMSO, DMF and water as solvents and
looked into and solved by researchers.
there was no 5-HMF formed when DMF and 1,4-dioxane were
used as solvents (Fig. S5E†). GVL is a promising solvent, which
3.5 Catalyst reusability
not only promotes the production of furfural, but also benets Reusability and solvent-stability of a catalyst are very signicant
other reactions such as dipropylene glycol's dehydration to factors for industry to assess the sustainability and protability
Fig. 5 Furfural production from xylose (140 ꢀC, 40 min, 60 mg xylose,
28 mg catalyst) and corn stalk (190 ꢀC, 50 min, 75 mg corn stalk, 35 mg Fig. 6 Conversion of corn stalk in pure water (10 mL) with 75 mg corn
catalyst) in various solvents.
stalk and 35 mg catalyst at 190 ꢀC.
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of a process plant. To test the stability of the SG catalyst, ve humins were generated due to the long reaction time, which
consecutive, recycling experimental runs were conducted and deposited on the catalyst, leading to a decrease in catalytic
the results are shown in Table 4. Typically, 28 mg SG and 60 mg activity. Therefore, it is necessary to cut down the reaction time
xylose were added into thick-walled glass and then 10 mL GVL to enhance catalyst reusability. In addition, the loss of sulfonic
was added as solvent with magnetic stirring (Entry 1). The acid groups from SG, which causes a decrease in the hydrolysis
reaction temperature was held at 140 ꢀC by oil bath heating. At capacity of the cellulose and hemicellulose fractions of corn
the end of reaction, the mixture was separated through a Milli- stalk, would further affect the yield of furfural. Hence, the
pore membrane. The product was collected and determined by accumulated corn stalk residues, the decrease in acid density
HPLC and SG catalyst was washed with water and acetone and humins deposition on the SG, responsible for catalyst
repeatedly. The recycled SG was dried and used for next cycle.
deactivation, are the main causes of low the furfural production
The results in the Table 4 showed that the yield of furfural from corn stalk.
gradually decreased during ve runs. Aer the h run, the
yield of furfural reached 70%, indicating strong acid density
and desirable solvent-stability of SG. FT-IR and elemental
4. Conclusions
analysis tests on the 5th recycling catalyst were carried out to Sulfonated graphene was successfully prepared and high cata-
investigate the reasons for the decrease in furfural yield. The lytic performance and acid density were observed during
acid density decreased and the FT-IR signal intensities of conversion of corn stalk and xylose into furfural. A 48% furfural
sulfonic acid group weakened, thus slight SO3H was leached yield from corn stalk was obtained, and more importantly, 40%
during the ve runs, which was the reason for the decrease of yield obtained just in 10 min when a 2.14 mass ratio (corn/SG)
furfural yield.
was used. A 96% furfural yield from xylose was obtained at
In addition, the SG reusability for conversion of corn stalk 150 ꢀC, what's more, a furfural yield of 71.9% was achieved
was also studied. As we know, the reusability of catalysts is poor when using a xylose/SG mass ratio of 10.7 at 140 ꢀC. In addition,
for the conversion of corn stalk, because of more humins GVL, GBL and 1,4-dioxane also gave good performances in
formed, which will deposit on catalysts.17 On the other hand, furfural production. Therefore, SG catalyst is a promising
the catalysts are hardly separated from the corn stalk residues material for the production of furfural.
aer reaction, thus, the catalyst and residues were collected
together aer each run and then washed with water, acetone
and GVL repeatedly, which was directly used as catalyst for the
Conflicts of interest
next run. The results are shown in Table 4. First, 35 mg catalyst There are no conicts to declare.
and 75 mg corn stalk were used in a stainless-steel autoclave at
190 ꢀC for 50 min in the rst set of cycles (Entry 2). The rst run
gave a yield of 46%; however, fairly low yields of ꢃ10% were
Acknowledgements
obtained from second run to h run. Then the second set of This work is nancially supported by the National Natural
cyclic tests was performed by using 100 mg catalyst and 50 mg Science Foundation of China (51536009 and 51576199), the
corn stalk at 190 ꢀC for 50 min. The rst run gave a 31.3% yield, Natural Science Foundation of Guangdong Province
25% yield at 2nd run and the yields were maintained at around (2017A030308010) and the Local Innovative and Research
15% for other runs as shown in Entry 3. Unlike the second set of Teams Project of Guangdong Pearl River Talents Program
cycles, the reaction time of corn stalk conversion was kept at (2017BT01N092).
15 min in the third set of cycles. The yields of furfural decreased
from 39.6% to 12.1% slowly in the ve runs as shown in Entry 4.
These results indicated that the yield of furfural decreased
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