Conversion of wheat straw into formic acid in NaVO3-H2SO4 aqueous solution with molecular oxygen

Article abstract of DOI:10.1039/c4gc01440e

Formic acid (FA) is an important material widely used in the chemical industry and in processes involved in energy storage and transformation. In this work, production of FA from wheat straw, an abundant lignocellulosic biomass, in NaVO₃-H₂SO₄ aqueous solution with molecular oxygen (O₂) was studied. The conversion showed high efficiencies: 47% (based on carbon, 75.2% based on mass) of FA and 7.3% (based on carbon, 7.6% based on mass) of acetic acid were obtained. The products mainly come from polysaccharides (hemicellulose and cellulose) in wheat straw. Acetic acid is derived from the acetyl groups in (1) the hemicellulose structure or (2) the deep hydrolysis product, levulinic acid. The removal of each lignocellulosic component in wheat straw follows the order: hemicellulose, lignin and cellulose. The catalytic system shows good reusability, and multiple feedings led to a high concentration of FA, which is beneficial to reduce FA-water separation cost.

Full text of DOI:10.1039/c4gc01440e

Green Chemistry
View Article Online
View Journal
PAPER
Conversion of wheat straw into formic acid in
NaVO –H SO aqueous solution with molecular
3
2
4
Cite this: DOI: 10.1039/c4gc01440e
oxygen†
a
b
a
a
c
Muge Niu, Yucui Hou, Shuhang Ren, Weize Wu* and Kenneth N. Marsh*
Formic acid (FA) is an important material widely used in the chemical industry and in processes involved in
energy storage and transformation. In this work, production of FA from wheat straw, an abundant ligno-
3 2 4 2
cellulosic biomass, in NaVO –H SO aqueous solution with molecular oxygen (O ) was studied. The con-
version showed high efficiencies: 47% (based on carbon, 75.2% based on mass) of FA and 7.3% (based on
carbon, 7.6% based on mass) of acetic acid were obtained. The products mainly come from polysacchar-
ides (hemicellulose and cellulose) in wheat straw. Acetic acid is derived from the acetyl groups in (1) the
hemicellulose structure or (2) the deep hydrolysis product, levulinic acid. The removal of each ligno-
cellulosic component in wheat straw follows the order: hemicellulose, lignin and cellulose. The catalytic
system shows good reusability, and multiple feedings led to a high concentration of FA, which is beneficial
to reduce FA–water separation cost.
Received 28th July 2014,
Accepted 12th September 2014
DOI: 10.1039/c4gc01440e
www.rsc.org/greenchem
catalyst. The pentavalent vanadium was found to be the active
component of the catalyst. Monosaccharides could be fully
1
. Introduction
Formic acid (FA), the simplest carboxylic acid, is an important converted and the FA yields could reach ≈50% (based on
1
1
material widely used in industry. Besides the traditional use in carbon, 7 h at 70 °C). Fu et al. studied glucose conversion to
chemical, agricultural, pharmaceutical, textile and rubber FA using the same catalyst at elevated temperatures from 100
industries, FA recently has been recognized as a form of hydro- to 170 °C. The FA yield from glucose (55% in 3 h at 100 °C)
gen storage as it is easily degraded to hydrogen at room temp- was comparable to the results of Wasserscheid et al. The
1
–6
erature by catalytic conversion. Furthermore, FA can be used efficiency was low when cellulose, the highly polymerized
7
–9
in fuel cells to efficiently generate electricity. Conversion of biomass, was transformed. Cellulose was minimally converted
1
2
biomass, the most abundant renewable resource to FA could and the FA yield was very low (7% in 24 h at 90 °C and 3% in
1
1
be considered as a “green” energy transformation from a low 9 h at 170 °C ). When organic acid (p-toluene sulfonic acid,
1
2
11
grade energy form to a higher grade form.
TSA ) and inorganic acid (HCl ) were chosen to accelerate
Recently, FA production from biomass in hot water with the cellulose degradation, the FA yield was raised to 19% and
molecular oxygen (O ) as an oxidant using a vanadium(V)-con- 34%, respectively. Subsequently, an alternative heteropoly acid
2
taining homogeneous catalyst has been demonstrated to be a
promising process.
H
8
PV
This conversion has a high selectivity as an additive, and a 28% yield of FA was obtained.
to FA. The conversion is efficient when the bio-based mono- In our previous work, the readily-available NaVO was used
5 7
Mo O40 was synthesized and used as a catalyst with TSA
1
0–14
13
3
1
0
2 4
saccharides are used as substrates. Wasserscheid et al. first as the catalyst to convert cellulose to yield FA in H SO
1
4
studied biomass conversion under hydrothermal conditions aqueous solution. When dissolved in acidic solution with pH
+
from 60 to 90 °C using the heteropoly acid H PV Mo O as a less than 1, NaVO exists in the form of VO , which shows a
5
2
10 40
3
2
high activity to catalyze the conversion to yield FA. The FA
yield could reach ≈60% in 2 h at 160 °C. In NaVO –H SO
3
2
4
a
aqueous solution, cellulose can be relatively easily converted
and FA can be obtained in high yield compared with the use of
V-containing heteropoly acid aqueous solution.
From the viewpoint of industrial applications the direct
transformation of raw materials to chemicals is preferable.
Due to the high content of polysaccharides, raw lignocellulosic
materials, which are abundant and widely distributed, have
State Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, Beijing 100029, China. E-mail: wzwu@mail.buct.edu.cn;
Fax: +86 10 64427603
b
Department of Chemistry, Taiyuan Normal University, Taiyuan 030031, China
c
Center for Energy, School of Mechanical and Chemical Engineering, The University
of Western Australia, Crawley, WA 6009, Australia. E-mail: ken.marsh@uwa.edu.au
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4gc01440e
This journal is © The Royal Society of Chemistry 2014
Green Chem.

View Article Online
Paper
Green Chemistry
great potential to produce FA. So far, the conversion of raw mass of H
2
SO
4
was diluted with distilled water and mixed with
lignocellulosic materials to FA is inefficient, and related the NaVO solution. The mixed solution was then diluted to a
3
1
2
research is limited.
known volume with distilled water to make the mass fractions
Wheat straw, the second largest biomass feedstock in the listed above.
1
5
3
world, is an abundant agriculture residue with low commer-
cial value. In this work, conversion of wheat straw in the made of Hastelloy alloy (HC 276) containing a magnetic
highly efficient NaVO –H SO aqueous solution with O along stirrer. In a typical procedure, a known mass of the substrate
with the effects of various reaction parameters on the con- and 6.0 cm of NaVO –H SO aqueous solution were loaded
The conversion was carried out in a 25 cm batch reactor
3
2
4
2
3
3
2
4
version were studied. The results showed that the conversion into the reactor. The reactor was then sealed and purged
of wheat straw could yield 47% FA and 7.3% acetic acid. The with O . After that, O was charged into the reactor to a
contributions of each component of wheat straw and the desired pressure. Next, the reactor was put into a furnace with
2
2
reason for acetic acid formation were investigated in detail. a heating rate of 24 to 30 °C min− and stirred at 1000 r min
1
−1
.
Then the removal order of each component of wheat straw was The pressure and the temperature of the reactor were moni-
1
3
studied by C CPMAS NMR. Lastly, the reusability of the cata- tored using a pressure transducer with an uncertainty of
lytic solution was investigated, and the result demonstrates ±0.025 MPa and a thermocouple with an uncertainty of
good reusability through several runs.
±0.5 °C, respectively. The reaction time was recorded from the
time when the desired reaction temperature was reached
(
about 5 min after being put into the furnace). After the
required reaction time, the reactor was quenched in a cold
water bath, and when the reactor reached room temperature,
the gases in the reactor were released and the liquid mixture
was filtered. The residues were washed with distilled water and
dried in an oven at 60 °C for 24 h before further analysis. The
liquid samples were analyzed by HPLC.
2
. Experimental section
2
.1 Materials and analysis method
Wheat straw was obtained from Xinjiang county, Shanxi pro-
vince. It was dried at 60 °C for 24 h and milled into powder
(<40 mesh) before use. The elemental analysis of wheat straw
was carried out through a CHNS elemental analyzer (Vario EL
cube, Germany). The components (water extracts, cellulose,
hemicellulose, lignin and ash) of wheat straw were analyzed
2.3 Analysis of products
1
6
according to the Van Soest method. The analysis results are The liquid samples were analyzed by high-performance liquid
shown in Tables 1 and 2. Microcrystalline cellulose (96%), chromatography (HPLC, Waters 2695, USA) with a Shodex SH
xylan and sodium metavanadate (NaVO
from Aladdin Reagent Inc. (Shanghai, China). Sulfuric acid detector (Waters 4110, USA) was employed to analyze other
H SO , 98%) was purchased from Beijing Modern Oriental products. The column oven temperature was 55 °C and the
3
, AR) were purchased 1011 column (Shodex, Japan). A differential refractive index
(
2
4
Fine Chemical Co., Ltd (Beijing, China). Oxygen (O
and nitrogen (N
, 99.999%) were supplied by Beijing Haipu centration of 0.01 mol dm− and a flow rate of 0.5 cm min
Gases Co., Ltd (Beijing, China). All reagents were of analytical The yields of products were calculated on a carbon base,
2
2 4
, 99.995%) mobile phase was diluted H SO aqueous solution with a con-
3
3
−1
2
.
grade and were used without further purification.
unless otherwise specified. The conversion of wheat straw was
calculated by the difference in solid mass before and after the
1
3
2
.2 Conversion of substrates
reaction. The C CPMAS NMR investigations of the original
wheat straw and the residues after conversion were performed
on a Bruker AV300 NMR spectrometer at a resonance frequency
of 75.47 MHz. Chemical shifts were referenced relative to TMS.
A series of NaVO –H SO aqueous solutions with different
mass fractions (NaVO : 0.35%, H SO : 0%, 0.7%, 1%, 1.5%,
2
3
2
4
3
2
4
% and 2.5%) were prepared before the conversion. In a
typical procedure, a known amount of NaVO3 was first dis-
solved in distilled water by ultrasonic heating. Then, a known
2.4 Recycle of the catalytic system
After completion of the conversion, the solid was separated by
filtration. The liquid products were extracted with diethyl
ether. Then the diethyl ether dissolved in the liquid was swept
Table 1 The elemental analysis of wheat straw as mass fraction w
Element
00w
C
H
O
N
S
2
by N at 50 °C in 10 min. Water was added to the recovered
1
41.73
5.434
52.115
0.556
0.165 liquid system to fit the original volume before the next run.
a
Table 2 The lignocellulose component of wheat straw as mass fraction w
Component
Extraction
27.4
Cellulose
38.2
Hemicellulose
29.3
Lignin
4.1
Ash
1.0
100w
a
The components of wheat straw were analyzed according to the Van Soest method.
Green Chem.
This journal is © The Royal Society of Chemistry 2014

View Article Online
Green Chemistry
Paper
and temperatures. The results are shown in Fig. 1a–1c. The
conversion was efficient, and wheat straw could be totally con-
verted in 2 min at 160 °C with FA as the main product. In
3
. Results and discussion
3
.1 Conversion of wheat straw
3
.1.1 Effect of temperature on the conversion and pro- addition, a considerable amount of acetic acid was formed.
ducts. The conversion of wheat straw in NaVO –H SO Both the FA yield and acetic acid yield increased with tempera-
was carried out at different times ture when using a shorter reaction time. After 5 min at 160 °C,
7% of FA and 7.3% of acetic acid were formed. The decrease
3
2
4
aqueous solution with O
2
4
in the amount of FA formed at 160 °C and 170 °C after pro-
longed reaction time was probably due to the degradation of
FA at high temperature (the stability of FA at different tempera-
tures is shown in Fig. S1 in the ESI†).
3.1.2 Effect of H
2 4 2
SO concentration and O pressure on the
products. The acid concentration in the catalytic system has a
1
7
great influence on lignocellulose hydrolysis. The effect of
SO concentration on the yields of FA and acetic acid is
shown in Fig. 2. When there was no H SO , the FA yield was
low. When H SO was added, the FA yield increased signifi-
H
2
4
2
4
2
4
cantly. The increase of FA yield is due to the accelerated rate of
hydrolysis to form more monosaccharides, which act as pre-
1
4
cursors to FA. Compared with FA, the change of acetic acid
yield was insensitive to the H SO concentration. With no
SO added, the acetic acid yield was nearly the same value
as that with small amounts of H SO . Acetic acid can be
2
4
H
2
4
2
4
formed by the removal of acetyl groups in hemicellulose (the
source of acetic acid was further studied in section 3.2). With
the H SO concentration further increasing, the deep hydro-
2
4
lysis of polysaccharides was accelerated and levulinic acid was
1
8
largely formed. When levulinic acid was used as the sub-
strate a high selectivity of acetic acid (25%) was observed.
Thus, additional acetic acid could also possibly be obtained by
levulinic acid oxidation. At the same time, the FA yield decreased.
We studied the stability of FA at the corresponding H SO con-
2
4
centration to examine whether the decrease in FA yield was
due to the degradation of FA. Results show that when the
H SO mass fraction was less than 1%, the FA yield decreased
2
4
2 4
to 89%, and when the H SO mass fraction was further
Fig. 1 The effect of temperature on (a) conversion of wheat straw,
b) FA yield and (c) acetic acid yield at different reaction times. Con-
ditions: wheat straw, 0.05 g; H SO , mass fraction w = 2%; NaVO , w =
, 3 MPa.
(
Fig. 2 The effect of H
Conditions: wheat straw, 0.05 g; NaVO
3 MPa; temperature, 160 °C; time, 5 min.
2 4
SO mass fraction on FA and acetic acid yields.
3
2
4
3
3 2 2
, w = 0.35%; H O, 6.0 cm ; O ,
3
0
2 2
.35%; H O, 6.0 cm ; O
This journal is © The Royal Society of Chemistry 2014
Green Chem.

View Article Online
Paper
Green Chemistry
increased the FA yield did not show a significant decrease sions above, we have concluded possible reaction pathways for
Fig. S2†). Therefore, the decrease in FA yield at high H SO the wheat straw conversion in NaVO –H SO aqueous solution
(
2
4
3
2
4
concentrations may result from more substrates being trans- with O
formed in deep hydrolysis rather than by catalytic oxidation. small molecular substances under an acidic system. Mono-
FA and acetic acid are oxidative products in the catalytic saccharides (C5 sugars and C6 sugars) were formed (glucose
system. Hence, O pressure could influence the product distri- and xylose were observed in 1 min of reaction at 140 °C,
bution significantly. The effect of O pressure on the yields of 150 °C and 160 °C with a yield of not more than 1%). On the
FA and acetic acid is shown in Fig. 3. When the O pressure other hand, NaVO dissolved in H SO aqueous solution (pH <
2
(see Fig. 4). Wheat straw was first decomposed to
2
2
2
3
2
4
+
19,20
was increased, the catalytic oxidation to FA was accelerated, so 1) formed a stable vanadium species, VO
2
.
Then, based
1
4
the FA yield increased. When O
FA became unstable and was overoxidized, resulting in a solved VO2 formed in acidic aqueous solution (mass fraction
decrease in FA yield (Fig. S3†). At low O pressure, the pro- of H SO of 2% in aqueous solution, pH < 1). This transform-
duction rate of acetic acid was low. An increase in the O ation showed high selectivity to yield FA. The reduced
pressure increased the oxidation rate, which led to an increase vanadium species were reoxidized to VO2 by molecular O2.
of acetic acid yield. At high O pressure, the catalytic oxidation The products (mainly formic acid) could be further oxidized to
from monosaccharides to FA also accelerated and had a com- CO , due to consecutive oxidation reactions. Deep hydrolysis
2
pressure was raised to 5 MPa, on our previous study, these sugars were catalyzed by dis-
+
2
2
4
2
+
2
2
petitive relationship with deep hydrolysis, which led to acetic could also occur to produce levulinic acid, which was detected
acid formation (via levulinic acid by oxidation, as described in in very low yields (from 0.8% to 1.3%). Levulinic acid was
the last paragraph). Therefore, the acetic acid yield decreased.
further oxidized to acetic acid. Acetic acid could also be
3
.1.3 Catalytic pathways of wheat straw conversion in obtained from hemicellulose by the removal of its acetyl
NaVO
3
–H
2
SO
4
aqueous solution with O
2
. Based on the discus- groups. This is discussed in the following section.
3.2 The sources of products in wheat straw conversion
As shown above, two main products—FA and acetic acid—were
formed after the conversion. In Table 3, experiments with
various lignocellulosic components as substrates were carried
out to give the contribution of each component to the pro-
ducts. For convenience, the yields mentioned in this section
were based on mass. The contribution of each component to
the product was calculated using the following formula:
Contribution ¼ yield of product
ꢀ
mass fraction of the component in
wheat straw
The mass fraction of each component was based on the
data in Table 2.
The conversion of wheat straw in the catalytic system is
Fig. 3 The effect of O
2
pressure on FA and acetic acid yields. Con-
ditions: wheat straw, 0.05 g; H
2
SO
4
, w = 2%; NaVO
3
, w = 0.35%; H
2
O, listed for comparison (Table 3, entry 1). The neutral detergent
3
6
.0 cm ; temperature, 160 °C; time, 5 min.
fiber (NDF, obtained after a neutral detergent wash of wheat
+
Fig. 4 Proposed reaction pathways for the wheat straw conversion into FA and acetic acid and the role of VO
with O
2
in NaVO
3
–H
2
SO
4
aqueous solution
2
.
Green Chem.
This journal is © The Royal Society of Chemistry 2014

View Article Online
Green Chemistry
Paper
Table 3 Conversions of wheat straw and other materials, yields of FA and acetic acid, and contributions of each component to FA and acetic acid
Conversion
w/%
FA yield
w/%
Acetic acid
yield w/%
Contribution
to FA w/%
Contribution to
acetic acid w/%
Entry
Substrate
1
2
3
4
5
6
Wheat straw
NDF
ADF
Lignin
Cellulose
Xylan
100
100
100
88.0
97.6
80.6
82.4
96.8
75.2
101.4
91.8
16.5
94.6
85.6
—
7.6
9.7
5.7
4.5
1.9
4.2
1.8
7.2
75.2
73.6
39.7
0.7
36.1
25.1
—
7.6
7.0
2.5
0.2
0.7
1.2
1.8
7.2
a
7
Wheat straw
Wheat straw
b
8
30.3
30.3
3
a
2 4 3 2 2 2
Conditions: substrate, 0.05 g; H SO , w = 2%; NaVO , w = 0.35%; H O, 6.0 cm ; O : 3 MPa; temperature, 160 °C; time, 5 min. No O (replaced by
N ) or NaVO . No NaVO .
2 3 3
b
1
6
straw using the Van Soest method ), mainly containing the This conversion turns out to be hydrolysis. Acetic acid was
structured components: cellulose, hemicellulose and lignin, formed, but the yield was low (1.8%). When O was added, the
2
gave contributions of 73.6% and 7.0% to FA and acetic acid, yield of acetic acid was raised to 7.2% (Table 3, entry 8), which
respectively (Table 3, entry 2). These contributions are very was very close to that of catalytic conversion (7.3%, Table 3,
close to those of the raw material, wheat straw (75.2% and entry 1). O
.6%, Table 3, entry 1). This indicates that most of the FA and significantly without the help of the catalyst. In section 3.1.2,
acetic acid are obtained from the structured components in oxidation of levulinic acid, which comes from deep hydrolysis
wheat straw: cellulose, hemicellulose and lignin (see Fig. 4). of cellulose, can also produce acetic acid. Thus, we draw the
Acidic detergent fiber (ADF, obtained after an acidic deter- conclusion that the acetic acid formed in catalytic conversion
2
pressure accelerates the removal of acetyl groups
7
1
6
gent wash of wheat straw using the Van Soest method ), of wheat straw is (1) from the bond breaking of acetyl groups,
which mainly contains cellulose and lignin, gave contributions which are distributed in the hemicellulose structure, and (2)
of 39.7% and 2.5% to FA and acetic acid, respectively (Table 3, from levulinic acid in deep hydrolysis of polysaccharides (cel-
entry 3). This result also shows that the contributions of lulose and hemicellulose).
hemicellulose to FA and acetic acid are 33.9% (73.6% down to
3
.3 The solid degradation in wheat straw conversion
3
9.7%) and 4.5% (7.0% down to 2.5%), respectively.
Lignin produces very limited FA and acetic acid under the Lignocellulose is composed of three major components: cellu-
same conditions, and the contributions to FA and acetic acid lose, hemicellulose and lignin. Cellulose consists of linear
are very small (0.7% and 0.2%, respectively, Table 3, entry 4). polysaccharides with β-1,4 linkages of D-glucopyranose mono-
Thus, it can be concluded that the FA and acetic acid are mers. Compared with cellulose, hemicellulose is a more
mainly formed from the structured polysaccharides: cellulose branched sugar polymer of various monosaccharides, among
and hemicellulose.
which hydroxyl groups are often substituted with acetyl groups
The purchased microcrystalline cellulose and xylan were or carboxylic groups. Lignin is an amorphous polymer consist-
also used as substrates (Table 3, entries 5 and 6). These two ing of methoxylated phenylpropane structures. Lignin and
polysaccharides cannot be fully converted, and the contri- hemicellulose combine with each other to fill the space
butions to FA and acetic acid are relatively low compared with between cellulosic fibers. Each of them has a unique chemical
the corresponding components in wheat straw. The purchased structure. The behaviors of lignocellulosic components in the
polysaccharides (microcrystalline cellulose and xylan) are quite catalytic conversion need to be better understood. To investi-
1
3
different in structure than the corresponding components in gate the degradation of the three components, the C CPMAS
lignocellulosic materials. Purchased materials have higher NMR spectra of residues after different reaction times were
crystallinities and are more difficult to convert.
studied (see Fig. 5).
According to our recent work, FA was formed through a
In Fig. 5, (a) is for wheat straw, the lignocellulosic material;
1
4
catalytic conversion of polysaccharides. Where does the con- (b), (c) and (d) are for residues after 5 min, 20 min and
siderable amount of acetic acid come from? The contribution 40 min, respectively (in order to facilitate the NMR measure-
of hemicellulose to acetic acid (4.5%) is more than that of ment, 1 g of wheat straw and a longer reaction time were
cellulose (2.3%). As a polymer of sugars, hemicellulose has a employed). The signals and the corresponding carbons in
2
3–25
similar structure to cellulose, except for the non-ignorable wheat straw are shown as follows.
The region between
21
amount of acetyl groups linked in the side chains. In the 60 ppm and 110 ppm is specific to carbons of cellulose. The
hydrolysis process of lignocellulosic materials, acetic acid signals at 20 ppm and 173 ppm are assigned to the methyl and
comes from the hydrolysis of the acetyl groups attached to the carbonyl carbons in the acetyl group attached to hemi-
2
2
hemicellulose. Thus, the acetic acid formation is most likely cellulose, respectively. The region between 120 ppm and
due to the breaking of bonds to the acetyl groups. An exper- 160 ppm is specific to aromatic carbons of lignin. The signal
iment without O or NaVO was carried out (Table 3, entry 7). at 55 ppm is assigned to the methoxyl carbon in lignin. The
2
3
This journal is © The Royal Society of Chemistry 2014
Green Chem.

View Article Online
Paper
Green Chemistry
1
3
Fig. 5
SO
C CPMAS NMR spectra of (a) wheat straw and residues after reaction of (b) 5 min, (c) 20 min, and (d) 40 min. Conditions: wheat straw, 1 g;
3
H
2
4
, w = 2%; NaVO
3
, w = 0.35%; H
2
O, 12.0 cm ; O
2
, 3 MPa; temperature, 160 °C.
signals at 20 ppm and 173 ppm disappear in 5 min (see Fig. 5a
and 5b), indicating that the acetyl groups in hemicellulose
degrade fast in the transformation. Although the signals of cel-
lulose and hemicellulose are highly overlapped, the splits of the
signals at 72 ppm and 63 ppm suggest the disappearance of the
corresponding signals, which are assigned to carbons of hemi-
2
4,26
cellulose.
Compared with the degradation rate of hemi-
cellulose, the degradation rate of lignin is slower, as shown by
the gradual weakening of the signals of the aromatic carbons
(
(
between 120 ppm and 160 ppm) and methoxyl carbon
56 ppm) at longer reaction times (see Fig. 5a, 5b and 5c). The
signals assigned to cellulose remain unchanged in the whole
process. It suggests that cellulose degradation is the slowest
transformation. The relative decrease in signal at 83 ppm (C-4
of amorphous cellulose) compared with that at 88 ppm (C-4 of
crystalline cellulose) shows that the amorphous cellulose de-
gradation is faster than the crystalline cellulose. This confirms Fig. 6 The reuse of the catalytic solution. Conditions: wheat straw,
3
0
.05 g; H
2
SO
4
, w = 2%; NaVO
3
, w = 0.35%; H
2
O, 6.0 cm ; O
2
, 3 MPa;
the known fact that amorphous cellulose has a more disordered
structure and is easier to convert than crystalline cellulose.
It is interesting to compare this catalytic conversion with
hydrolysis pretreatment, in which lignin is the last component
temperature, 160 °C; time, 5 min.
1
7,27
remaining.
the conversions of these two reactions (see Table 3, entries
and 7), we can conclude that it is the oxidation that makes
Considering the structure of lignocellulose and
1
the wheat straw conversion easier by following the hemi-
cellulose–lignin–cellulose removal order.
3
.4 The reuse of the catalytic system
The reusability of NaVO –H SO aqueous solution in wheat
3
2
4
straw conversion was studied, and the result is shown in
Fig. 6. After four runs, the yields of FA and acetic acid were
maintained without a significant decrease.
Although the conversion is efficient (the yield of FA is
7
5.2% based on mass), the concentration of FA in aqueous
solution after reaction is low (mass fraction of 0.63%). The sep-
aration of FA from an aqueous solution with such a low con-
centration is difficult. From the viewpoint of industrial
application, increasing the concentration of FA in aqueous
Fig. 7 The FA mass fraction after multiple feed experiments. Con-
ditions: wheat straw (each time), 0.05 g; H SO , w = 2%; NaVO
, 3 MPa; temperature, 160 °C; time, 5 min.
2
4
3
,
3
2 2
solution is of great importance for reducing the energy con- w = 0.35%; H O, 6.0 cm ; O
Green Chem.
This journal is © The Royal Society of Chemistry 2014

View Article Online
Green Chemistry
Paper
sumption of separation. Thus, the amount of substrate needs
to be increased. However, like most lignocelluloses, wheat
6 T. Zell, B. Butschke, Y. Ben-David and D. Milstein, Chem. –
Eur. J., 2013, 19, 8068–8072.
7 M. Weber, J. T. Wang, S. Wasmus and R. F. Savinell,
J. Electrochem. Soc., 1996, 143, 158–160.
8 C. Rice, S. Ha, R. I. Masel, P. Waszczuk, A. Wieckowski and
T. Barnard, J. Power Sources, 2002, 111, 83–89.
9 N. V. Rees and R. G. Compton, J. Solid State Electrochem.,
2011, 15, 2095–2100.
2
8
straw has a very fluffy structure. As a consequence, the
amount of feedstock in one pot is limited. Based on the
results of the catalytic system reusability, adding wheat straw
repeatedly to the used catalytic system is considered to be the
most suitable method to increase FA mass fraction. FA mass
fraction after each feed is shown in Fig. 7. After 5 feed times
(0.05 g of wheat straw each time), FA mass fraction in aqueous 10 R. Wölfel, N. Taccardi, A. Bösmann and P. Wasserscheid,
solution is raised from 0.63% to 2.84%.
Green Chem., 2011, 13, 2759–2763.
11 J. Li, D. Ding, L. Deng, Q. Guo and Y. Fu, ChemSusChem,
2012, 5, 1313–1318.
4
. Conclusions
12 J. Albert, R. Wölfel, A. Bösmann and P. Wasserscheid,
Energy Environ. Sci., 2012, 5, 7956–7962.
The conversion of wheat straw in NaVO –H SO aqueous solu-
3
2
4
1
1
1
1
3 J. Albert, D. Lüders, A. Bösmann, D. M. Guldi and
P. Wasserscheid, Green Chem., 2014, 16, 226–237.
4 W. Wang, M. Niu, Y. Hou, W. Wu, Z. Liu, Q. Liu, S. Ren and
K. N. Marsh, Green Chem., 2014, 16, 2614–2618.
5 F. Talebnia, D. Karakashev and I. Angelidaki, Bioresour.
Technol., 2010, 101, 4744–4753.
tion with O
2
produces a high yield of FA (47%) and acetic acid
7.3%). These products mainly come from polysaccharides
hemicellulose and cellulose). Acetic acid is formed mainly
(
(
from the acetyl groups that are part of the hemicellulose struc-
ture or from the deep hydrolysis product, levulinic acid. The
removal of each lignocellulosic component during the con-
version follows the order of hemicellulose, lignin and cellu-
lose. Based on the demonstrated reusability of the catalytic
system, multiple feedings of wheat straw can achieve high
mass fractions of FA, which is beneficial to reduce the FA–
water separation costs.
6 H. K. Goering and P. J. V. Soest, Forage fiber analysis
(
USDA-ARS Agriculture Handbook 379), Government Printing
Office, Washington, DC, 1970.
1
1
7 Y. Sun and J. Cheng, Bioresour. Technol., 2002, 83, 1–11.
8 C. Chang, P. Cen and X. Ma, Bioresour. Technol., 2007, 98,
1448–1453.
1
2
9 D. C. Crans, J. J. Smee, E. Gaidamauskas and L. Yang,
Chem. Rev., 2004, 104, 849–902.
0 A. Sadoc, S. Messaoudi, E. Furet, R. Gautier, E. L. Fur,
L. l. Pollès and J.-Y. Pivan, Inorg. Chem., 2007, 46,
Acknowledgements
The authors thank Professors Zhenyu Liu and Qingya Liu for
valuable suggestions and discussion. This work was financially
supported by the Natural Science Foundation of China
4835–4843.
21 F. M. Gírio, C. Fonseca, F. Carvalheiro, L. C. Duarte,
S. Marques and R. Bogel-Łukasik, Bioresour. Technol., 2010,
(
(
21076138), the National Basic Research Program of China
2011CB201303) and the Program for New Century Excellent
101, 4775–4800.
2
2
2
2 S. Gámez, J. J. González-Cabriales, J. A. Ramírez, G. Garrote
and M. Vázquez, J. Food Eng., 2006, 74, 78–88.
3 H. Wikberg and S. L. Maunu, Carbohydr. Polym., 2004, 58,
Talents in University (NCET-08-0710).
461–466.
References
4 G. Gilardi, L. Abis and A. E. G. Cass, Enzyme Microb.
Technol., 1995, 17, 268–275.
1
A. Boddien, D. Mellmann, F. Gartner, R. Jackstell,
H. Jungle, P. J. Dyson, G. Laurenczy, R. Ludwig and 25 S. L. Maunu, Prog. Nucl. Magn. Reson. Spectrosc., 2002, 40,
M. Beller, Science, 2011, 333, 1733–1736.
151–174.
2
3
4
Y. Himeda, Eur. J. Inorg. Chem., 2007, 3927–3941.
Y. Himeda, Green Chem., 2009, 11, 2018–2022.
26 A. M. Gil and C. P. Neto, Annu. Rep. NMR Spectrosc., 1999,
37, 75–117.
R. Tanaka, M. Yamashita and K. Nozaki, J. Am. Chem. Soc., 27 M. J. Taherzadeh and K. Karimi, Bioresources, 2007, 2, 472–
009, 131, 14168–14169. 499.
Z. Wang, J. Yan, Y. Ping, H. Wang, W. Zheng and Q. Jiang, 28 S. Mani, L. G. Tabil and S. Sokhansanj, Biomass Bioenergy,
Angew. Chem., Int. Ed., 2013, 52, 4406–4409. 2004, 27, 339–352.
2
5
This journal is © The Royal Society of Chemistry 2014
Green Chem.