Bio-oil upgrading at ambient pressure and temperature using zero valent metals

Article abstract of DOI:10.1039/c2gc35426h

As a liquid product from the fast pyrolysis of renewable biomass, bio-oil has a great potential use as a fuel or high-value chemicals resource, but it is accompanied by inherent drawbacks, such as strong corrosiveness and chemical instability. A facile me

Full text of DOI:10.1039/c2gc35426h

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PAPER
Bio-oil upgrading at ambient pressure and temperature using zero valent
metals†
Wu-Jun Liu, Xue-Song Zhang, Yan-Chao Qv, Hong Jiang* and Han-Qing Yu
Received 27th December 2011, Accepted 14th May 2012
DOI: 10.1039/c2gc35426h
As a liquid product from the fast pyrolysis of renewable biomass, bio-oil has a great potential use as a
fuel or high-value chemicals resource, but it is accompanied by inherent drawbacks, such as strong
corrosiveness and chemical instability. A facile method for bio-oil upgrading by zero valent metals
(Al, Fe, Mg, and Zn) at ambient temperature and pressure was reported for the first time in this work.
The chemical features of the raw and upgraded bio-oils were analyzed by Nuclear Magnetic Resonance
(NMR) and GC-MS. The results show that the zero valent Zn favorably demonstrated improvement of the
quality of bio-oil. In the upgraded bio-oil, the CvO compounds are reduced from 9.8 to 3.1 mol%, and
the pH value was elevated from 3.53 to 4.85, which significantly increased the chemical stability and
decreased the corrosiveness of bio-oil. The possible formation mechanism of 13 newly formed
compounds in the upgraded bio-oil was explored, and most of them were found to be derived from
hydrogenation of CvO compounds. This work provides an economical and environmentally friendly
approach for bio-oil upgrading.
compounds with unsaturated bonds (e.g., CvC and CvO
Introduction
bonds) in bio-oil, which are active for polymerization and con-
densation reactions.6 To improve the quality of bio-oil and make
The declining of fossil fuel resources, increasing demands of
energy and resources, and growing concerns over global climate
change make it imperative to develop new economical and
environmentally friendly processes for the conversion of
renewable biomass to liquid fuels and chemicals.1 As a liquid
product from the fast pyrolysis of renewable biomass, bio-oil has
a great potential to be used as fuel or high-value chemicals
resource.1b,c,2
Bio-oil is a multi-component mixture of different types of
compounds (e.g., water, hydrocarbons, oxygenated compounds,
and pyrolytic lignin) derived primarily from thermal decompo-
sition reactions of three key biomass building blocks, i.e., lignin,
cellulose, and hemicellulose.3 Some of the compounds are
closely related to the undesirable properties of bio-oil (e.g.,
strong corrosiveness, low heating value, and chemical instabil-
ity), which are the inherent drawbacks limiting its widely use.2
For example, the strong corrosiveness is due to the presence of
carboxylic acids (e.g., acetic acid) in bio-oil;4 the low heating
value is attributed to the large amounts of oxygenated com-
pounds present in the bio-oil;2,5 and the chemical instability is
mainly because of the presence of aldehydes and other
it more suitable to use as a fuel and chemical resource, an
upgrading process is needed.
The techniques for bio-oil upgrading include hydrogena-
tion,6b,7 catalytic cracking,8 and steam reforming.9 Among these,
hydrogenation is the most widely used method for bio-oil
upgrading, which mainly involves hydro-deoxygenation and
hydro-esterification processes.7b,c,10 The hydro-deoxygenation
process can convert the aldehydes and other compounds with
unsaturated bonds into stable compounds (e.g., hydrocarbons
and alcohols), as well as reduce the oxygen content, thus increas-
ing the heating value and chemical stability of bio-oil. The
hydro-esterification process can significantly reduce the organic
acid contents by esterification reactions, thus decreasing the cor-
rosiveness of the bio-oil.
These processes, however, often require complicated equip-
ment, noble metal catalysts (e.g., Pd, Pt and Ru), and high
pressure and temperature.10,11 Furthermore, the hydrogenation
process is often halted by catalyst deactivation and reactor clog-
ging at a high temperature.4a,11a In order to avoid these pro-
blems, more effective methods should be established to upgrade
bio-oil at ambient temperature and pressure.
Some zero valent metals (ZVMs), e.g., Fe, Zn, Al, and Mg,
are strong reductants widely used in organic reactions. For
example, zero valent Fe and Zn are the commonly used reduc-
tants in reducing nitro compounds into amines,12 while in the
Clemmensen reduction, the zero valent Zn is a key reductant in
the conversion of carbonyl groups (e.g., ketones and aldehydes)
Department of Chemistry, University of Science and Technology of
China, Hefei 230026, China. E-mail: jhong@ustc.edu.cn;
Fax: +86-551-3607482
†Electronic supplementary information (ESI) available: Additional
experimental descriptions, Tables S1 and S2, and Fig. S1 and S2 are pro-
vided. See DOI: 10.1039/c2gc35426h
2226 | Green Chem., 2012, 14, 2226–2233
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Table 1 Hydrogenation of model compound of bio-oil by ZVMs under different conditions
Entry
Solventa
ZVMs
Acid
Time (min)
Benzaldehyde conversion (%)
Benzalcohol selectivity (%)
Toluene selectivity (%)
1
2
3
4
5
6
THF
THF
THF
THF
CH3OH
CH2Cl2
Al
Fe
Mg
Zn
Zn
Zn
CH3COOH
CH3COOH
CH3COOH
CH3COOH
CH3COOH
CH3COOH
180b
180b
30c
60
60
60
Trace
Traced
11
95
96
Trace
Trace
87
>99
>99
>99
Trace
Trace
13
<1
<1
95
<1
a The solvent is mixture of the listed organic solvent and water with a ratio of 3 : 1 (m/m). b Because the reaction between zero valent Fe, Al and
CH3COOH is very slow. c The reaction between zero valent Mg and CH3COOH is very fast, and the reaction is complete within 30 min. d Means not
detectable.
into methylene groups.13 These reactions are usually conducted
in acidic conditions at ambient temperature and pressure, with a
high product yield and selectivity.
In bio-oil, there are many ketones and aldehydes, which are
mainly responsible for the instability of bio-oil, and organic
acids, which are mainly responsible for the corrosiveness of bio-
oil. When the ZVM is added into bio-oil, it can react with the
organic acids to generate active atomic hydrogen.14 The formed
hydrogen can actively react with the ketones and aldehydes
in situ to form the hydrogenation products (e.g., alcohols and
alkanes), which can significantly improve the stability and
increase the heating value of the bio-oil. Meanwhile, the con-
sumption of organic acids in this process can reduce the corro-
siveness of bio-oil. Therefore, the use of ZVM for bio-oil
upgrading can effectively improve the quality of bio-oil. More
importantly, compared to the conventional hydrogenation
process, this process can be conducted at ambient temperature
and pressure, and no catalyst and additional hydrogen gas are
needed. However, to the best of our knowledge, there is no
report about the use of ZVMs for bio-oil upgrading till now.
In this work, ZVMs were used for bio-oil upgrading. The
main objective of this work is to investigate the feasibility and
mechanism involved in such a process. Zero-valent Zn was
selected as a representative metal. The changes of chemical com-
position of bio-oil before and after upgrading were compared
and analyzed in detail using NMR and GC-MS to elucidate the
effect of the ZVMs on the upgrading process. Our results show
that the quality of bio-oil could be significantly improved by
adding zero valent Zn into it. This study provides a multipurpose
method for bio-oil upgrading, which can simultaneously reduce
the corrosiveness, increase the chemical stability and heating
value of the bio-oil.
Scheme 1 A proposed mechanism for the Zn-mediated hydrogenation
of benzaldehyde.
bio-oil to investigate the hydrogenation using ZVMs at ambient
temperature and pressure without addition of hydrogen gas and
catalyst. Acetic acid, a common carboxyl acid abundant in the
bio-oil, was chosen to create the acidic conditions of bio-oil.
Four ZVMs (i.e., Al, Fe, Mg, and Zn) were tested in this work,
and the results are presented in Table 1. It can be seen that the
zero-valent Zn worked well in the hydrogenation of benzal-
dehyde. The conversion of benzaldehyde was higher than 95%
under different reaction conditions (entries 4–6), and the benz-
aldehyde is almost stoichiometrically converted to benzalcohol
(with a selectivity higher than 99%). The results, both in benz-
aldehyde conversion and benzalcohol selectivity, are comparable,
even better than other results reported using hydrogen gas for
hydrogenation with catalysts at high temperature or pressure.16
The mechanism of Zn-involving hydrogenation is proposed as
follows: Zn first adsorbs the hydrated proton ionized by the acid,
then the electron of Zn is transferred to the proton, and the
proton changes into atomic hydrogen, which is a strong reducing
radical with a half-life of 0.3 s. The active atomic hydrogen
quickly transfers its electron to another acceptor (the carbonyl
group of benzaldehyde) and itself bind to the acceptor to form
the hydrogenation product17 (Scheme 1). The benzaldehyde
cannot be hydrogenated to benzylalcohol and/or toluene in the
presence of zero-valent Al and Fe even after 180 min (entries 1
Results and discussion
Hydrogenation of model compound of bio-oil
Benzaldehyde, a typical carbonyl compound with high reactivity
and chemical instability,15 was selected as a model compound of
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Fig. 1 The conversion of benzaldehyde, and yields of benzalcohol and
toluene at different reaction times.
and 2). One possible explanation is that the active hydrogen gen-
erated from the reactions of zero-valent Al and Fe with acetic
acid is in very small amounts, which is not enough to hydrogen-
ate the benzaldehyde. For the zero-valent Mg (entry 3), due to
its high activity, it can quickly react with the acetic acid to gener-
ate hydrogen gas (H2) instead of active atomic hydrogen. While
compared with the in situ formed active hydrogen, H2 is very
inactive and cannot hydrogenate benzaldehyde at ambient temp-
erature and pressure without a catalyst.15
The conversion of benzaldehyde and the yields of benzalcohol
and toluene at different reaction times is shown in Fig. 1. The
hydrogenation of benzaldehyde by zero valent Zn was a very
rapid process, and more than 85% of the benzaldehyde could be
converted within 20 min, accompanied by almost stoichiometric
formation of benzalcohol. While for toluene, the deoxygenation
product of benzaldehyde, its yield was still lower than 1% after
10 h reaction, indicating that the deoxygenation cannot occur in
the zero-valent Zn and acetic acid system. From the above analy-
sis, it is concluded that among four ZVMs, zero-valent Zn shows
the most favorable performance in the hydrogenation of a bio-oil
model compound, and the process is very fast and highly selec-
tive. Therefore, in the subsequent upgrading of real bio-oil, zero-
valent Zn was chosen as the model ZVM.
Fig. 2 Images of the raw and upgraded bio-oil samples.
significantly reducing the corrosiveness of bio-oil. The above
results confirm that the quality of bio-oil, e.g., heating value,
stability, and corrosiveness, was significantly improved by the
upgrading by zero-valent Zn.
The Zn concentration in the upgraded bio-oil was 69.0 mg
kg−1 after reaction. A calculation was carried out on the basis of
the Zn mass balance in the entire reaction (eqn (1))
Total analysis of the bio-oils before and after upgrading
Fig. 2 shows the images of the bio-oil samples before and after
upgrading. The raw bio-oil was a black opaque liquid, while
after upgrading, the bio-oil became a transparent reddish brown
liquid. The main properties and elemental compositions of the
bio-oils are given in Table 2. The oxygen content of the
upgraded bio-oil was lower than that of the raw bio-oil, while its
carbon and hydrogen contents were higher than those of the raw
bio-oil. The heating value of the upgraded bio-oil was 13.4 MJ
kg−1, slightly higher than that of the raw bio-oil (12.5 MJ kg−1).
The pyrolytic lignin, one of the main factors influencing the
thermal stability and homogeneity of bio-oil,18 was decreased
from 5.83 to 4.62 wt% after upgrading by zero-valent Zn. The
pH value, a main parameter reflecting the acidity of a solution,
was 4.85 in the upgraded bio-oil, much higher than that in the
raw bio-oil (3.53), indicating that the acid content of bio-oil was
decreased with the upgrading of zero-valent Zn, thus
Znadd ¼ Znbioꢀoil þ Znsolid residue
ð1Þ
where Znadd is the amount of Zn dosed into the reaction (0.5 g
Zn to 10.0 g raw bio-oil); Znbio-oil (69.0 mg kg−1) is the Zn con-
centration in the bio-oil after upgrading; Znsolid residue is the Zn
content in the solid residue after reaction.
The results show that more than 99.8% of the added Zn
remained in the solid phase, which can be easily recycled and
reused. However, compared with the Zn concentration in the raw
bio-oil, the Zn concentration in the upgraded bio-oil is still high.
Considering that the Zn2+ cation in the bio-oil is usually
2228 | Green Chem., 2012, 14, 2226–2233
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hydrophilic and that the bio-oil is easily phase-separated, a phase
separation process in the upgraded bio-oil was carried out, and
the Zn concentration in the organic phase, the main part of the
bio-oil to be used as a fuel or chemicals source, was 2.6 mg
kg−1 only, indicating that the pollution of Zn in the final bio-oil
could be avoided well.
indicating the decrease in aldehyde, ketones, and carboxylic
acids, which are mainly responsible for the chemical instability
and corrosiveness of bio-oil. The content of C atoms in the C–O
group of the upgraded bio-oil was 44.3 mol%, significantly
higher than that of the raw bio-oil (36.8 mol%), suggesting the
increase in alcohols, which are the reduction products of CvO
compounds. Compared to the CvO compounds (e.g., aldehyde
and ketones), which are usually chemically instable and easily
polymerized and oxidized, the C–O compounds (e.g., alcohols)
are more stable.
Comparison between the NMR spectra of the raw and upgraded
bio-oils
The chemical features of the raw and upgraded bio-oils were
further investigated by 13C-NMR and the results are provided in
Table 3, while the typical 13C-NMR spectra of the bio-oils are
shown in Fig. S2.† The assignment of peaks to carbon atoms in
different chemical environments is based on previous reports for
bio-oils.19 The most significant trend is the decrease in CvO
group (170–215 ppm of chemical shift), which is accompanied
mostly by an increase in aliphatic C–O groups (50–90 ppm of
chemical shift) for the upgraded bio-oil. For example, the
content of C atoms in the CvO group of raw bio-oil is 9.8 mol
%, much higher than that of the upgraded bio-oil (3.1 mol%),
Changes of organic components in the upgrading process
The organic components of raw and upgraded bio-oils were ana-
lyzed by GC-MS, and their GC-MS chromatograms are shown
in Fig. 3. A significant change in upgraded bio-oil is that 13
peaks (i.e., S1–S13 in chromatogram of upgraded bio-oil) were
newly formed in the upgrading process. The peaks in the chro-
matograms were identified, and compound assignments are pre-
sented in Table 4. Acetic acid (peak 1), which was the most
abundant compound in the raw bio-oil, was significantly
decreased in the upgraded bio-oil. Other carboxylic acids, such
as 2-hydroxypropanoic acid (peak 4), and 2,3-dihydroxypropa-
noic acid (peak 7) were also greatly decreased. The decrease in
carboxylic acids can result in the increase in pH value of bio-oil,
Table 2 Comparison among the elemental compositions and other
properties of the raw and upgraded bio-oils
Raw bio-oil
Upgraded bio-oil
Table 3 13C-NMR chemical shift assignment ranges and carbon
contents of the raw and upgraded bio-oils
C (wt%)
34.1
9.0
0.2
50.6
n.d.a
12.5
5.83
46.3
3.53
36.4
9.5
0.2
48.4
69.0 (2.6)b
13.4
4.62
42.5
4.85
H (wt%)
N (wt%)
O (wt%)
Zn (mg kg−1
Contenta
Chemical
)
Heating value (MJ kg−1 c
)
shift range
(ppm)
Raw
bio-oil bio-oil
Upgraded
Type of carbon atom
Pyrolytic lignin (wt%)
Water content (wt%)
pH
0–50
50–90
90–120
C–C (aliphatic)
C–O (alcohols and ethers)
C–C (aromatics) and CvC (olefinces)
13.9
36.8
26.8
12.7
16.9
44.3
23.5
12.2
3.1
a Not detected. b 69.0 mg kg−1 is the Zn concentration in the total
upgraded bio-oil, while 2.6 mg kg−1 is the Zn concentration in the
organic phase of the upgraded bio-oil after a phase separation process.
c Heating-value (MJ kg−1) = (3.55C2 − 232C − 2230H + 51.2C × H +
131N + 20 600)/1000.5a
120–150 C–O (phenols and aromatic ethers)
170–215 CvO (ketones, aldehydes, and carboxyls) 9.8
a mol% of total carbon.
Fig. 3 GC-MS chromatograms of the raw and upgraded bio-oils.
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Table 4 Main components of the raw and upgraded bio-oils
Content (area%)
Raw bio-oil
Peak no.
R.T. (min)
Molecular formula
Compounds assignmenta
Upgraded bio-oil
1
2
3
4
5
6
7
8
3.989
4.852
5.681
6.025
8.082
8.674
9.102
C2H4O2
C3H6O2
C4H8O3
C3H6O3
C4H8O2
C4H6O2
C3H6O4
C5H4O
C6H10O3
C6H8O3
C5H6O2
C4H4O2
C6H10O2
C5H6O2
C6H8O
C5H6O2
C6H6O
—
Acetic acid
12.2
13.7
3.4
1.7
3.1
2.2
1.7
11.3
0.6
1.1
0.3
3.9
0.3
0.6
1.1
1.0
2.1
1.4
2.7
0.4
3.9
0.6
1.4
1.6
4.7
0.6
0.3
3.1
4.7
0.6
0.3
0.3
0.8
0.8
1.7
0.6
0.5
1.0
0.5
1.2
0.4
0.5
0.5
3.7
10.2
2.6
0.5
2.9
0.3
0.7
2.8
1.0
0.3
0.3
4.1
n.d.b
0.7
1.4
1.1
2.3
1.3
2.6
0.5
5.1
0.5
1.6
1.6
4.9
0.4
0.3
3.3
4.5
0.4
n.d.
n.d
0.7
0.6
n.d
n.d
0.7
0.8
0.8
1.0
1.6
n.d.
0.3
4-Hydroxybutan-2-one
Ethyl-2-hydroxyacetate
2-Hydroxypropanoic acid
1-Hydroxybutan-2-one
Succinaldehyde
2,3-Dihydroxypropanoic acid
Furfural
3-Oxobutyl acetate
2,5-Dimethoxyfuran
3-Methylfuran-2(5H)-one
Furan-2(3H)-one
3-Methylpentane-2,4-dione
2-Methoxyfuran
2,5-Dimethylfuran
4-Methylfuran-2(5H)-one
Phenol
Cannot be identified
Cannot be identified
1,3-Dimethoxycyclopentane
3-Methylcyclopentane-1,2-dione
5-Isopropyldihydrofuran-3(2H)-one
o-Cresol
p-Cresol
2-Methoxyphenol
4-Hydroxy-4-methyltetrahydro-2H-pyran-2-one
2-Methylbenzene-1,3-diol
4-Ethylphenol
2-Methoxy-4-methylphenol
4-Methyl-1,3-dioxolan-2-one
Cannot be identified
Cannot be identified
4-Ethyl-2-methoxyphenol
2,6-Dimethoxyphenol
4-Hydroxy-3-methoxybenzaldehyde
2-Hydroxy-6-methylcyclohex-3-enecarboxylic acid
Cannot be identified
1-(4-Hydroxy-3-methoxyphenyl)propan-2-one
Cannot be identified
1-(2,3,4-Trihydroxyphenyl)ethanone
2-Ethoxy-6-(methoxymethyl)phenol
4-Hydroxy-3,5-dimethoxybenzaldehyde
3-(4-Hydroxy-2-methoxyphenyl)acrylaldehyde
10.763
12.322
13.510
13.952
14.140
14.935
15.288
16.515
16.914
17.169
17.532
17.993
18.302
19.093
19.947
20.225
21.064
21.595
22.356
22.853
24.560
25.502
25.742
26.987
27.873
28.499
30.192
32.534
32.694
34.313
35.209
35.991
36.434
39.851
40.116
41.923
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
—
C6H8O2
C7H14O2
C7H12O2
C7H8O
C7H8O
C7H8O2
C6H10O3
C8H10O2
C8H10
O
C8H10O2
C4H6O3
—
—
C9H12O2
C8H10O2
C8H8O3
C8H12O3
—
C10H12O3
—
C8H8O4
C10H14O3
C9H10O3
C10H10O3
a The newly formed compounds in the upgraded bio-oil are not shown. b n.d. means less than 0.3 area%.
thus reducing the corrosiveness of bio-oil. Another notable
change is the decrease in aldehydes in the upgraded bio-oil. Fur-
fural (peak 8), the third most abundant compound in the raw
bio-oil (11.3%), was decreased to 2.8% in the upgraded bio-oil.
Other aldehydes, like succinaldehyde (peak 6), 4-hydroxy-3-
methoxybenzaldehyde (peak 35), and 4-hydroxy-3,5-dimethoxy-
benzaldehyde (peak 42), were also significantly decreased in the
upgraded bio-oil.
The identification of the 13 newly formed compounds and
their possible formation routes are presented in Table 5. Com-
pounds S1–S4 and S12 are alcohols derived from the hydrogen-
ation of aldehydes and ketones. The compounds S5–S7 are
esters, possibly deriving from the esterification of carboxylic
acid and alcohols. Other newly formed compounds (S8–S11 and
S13), which are all phenolic compounds, are considered as the
fragments from the pyrolytic lignin.20 PL is the oligomer gener-
ated from the thermal cracking of lignin, which is an amorphous
polymer consisting of methoxylated phenylpropane structures.21
In the presence of active hydrogen generated from the reaction
of Zn and carboxylic acids, the methoxylated phenylpropane
structures of the PL were broken, and some aryl ether
bonds were hydrogenated to form aryl hydroxyl bonds,
which are the main functional groups in the newly formed
compounds (S8–S13). Furthermore, the Zn(II)-carboxyl acid
complexes formed in situ might act as a catalyst in the hydrogen-
ation of aryl ether bonds to aryl hydroxyl compounds.22 The
formation of compounds S8–S13, which are the reduced
fragments from PL, is also confirmed by the results in Table 1
that the content of PL in the upgraded bio-oil is lower than that
in the raw bio-oil.
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Table 5 The newly formed compounds in the upgrading process and their possible formation pathways
Peak no.
Compounds assignment
Area (%)
0.5
Possible formation pathway
S1
S2
0.7
S3
S4
0.4
3.1
S5
S6
1.0
1.4
S7
S8
0.8
0.4
Fragment from the pyrolytic lignin
Fragment from the pyrolytic lignin
S9
0.8
0.3
S10
Fragment from the pyrolytic lignin
Fragment from the pyrolytic lignin
S11
S12
0.6
0.3
S13
0.9
Fragment from the pyrolytic lignin
without catalyst and additional hydrogen gas was proposed and
tested. Four ZVMs (i.e., Al, Fe, Mg, and Zn) were tested using
benzaldehyde as a model compound of bio-oil, and the results
shows that the zero-valent Zn could almost stoichiometrically
Conclusions
A facile method for bio-oil upgrading by zero valent metals
(Al, Fe, Mg, and Zn) at ambient temperature and pressure
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Bio-oil upgrading
convert benzaldehyde into benzalcohol with a selectivity higher
than 99%. In a real bio-oil system, zero-valent Zn also
demonstrated improvement of the bio-oil quality. The formation
mechanism of the new compounds in the upgraded bio-oil was
proposed. The compounds S1–S4 and S12 were derived from
the hydrogenation of aldehydes and ketones; the compounds
S5–S7 were possibly derived from the esterification of carboxyl
acid and alcohols; and S8–S11 and S13 were considered to be
fragments from the PL. The results from this work provide
a new approach to produce liquid fuels and chemicals from
renewable biomass.
Typical procedure for the bio-oil upgrading. Zero-valent zinc
powder (0.50 g), raw bio-oil (10.0 g), and tetrahydrofuran (THF,
2.0 g) were put into a 100 mL round bottomed flask, then the
mixture was vigorously stirred (600 rpm) with a magnetic stirrer
at ambient pressure and temperature. After reaction for 300 min,
the mixture was filtered with a membrane filter (0.45 μm) and
stored in a refrigerator for further analysis.
Product analysis. The hydrogen (H), carbon (C), nitrogen (N)
and oxygen (O) contents of the raw and upgraded bio-oils were
determined using an elemental analyzer (VARIO EL III, Elemen-
tar Inc., Germany). The Zn contents in the raw and upgraded
bio-oils were analyzed by inductively coupled plasma-atomic
emission spectrometry (ICP-AES, Optima 7300 DV, Perkin
Elmer Co., USA). The pH values of the raw and upgraded bio-
oils were measured with a pH meter (PHS-25, China). The pyro-
lytic lignin (PL) of the bio-oils are determined as described by
Scholze and Meier.23 Generally, 100 g of bio-oil was added
dropwise to 1000 mL of ice-cooled NaCl aqueous solution
(0.1 mol L−1) with stirring in a magnetic stirrer. Thereafter, the
formed PL was separated by pumping filtration, resuspended in
ice-cooled water, and stirred for about 240 min to wash out water
soluble components. Finally, the mixture was filtered and the wet
PL was vacuum dried at 303 K. The dried PL was weighed to
determine its yield.
Experimental section
Materials
Benzaldehyde was used as a model compound for the unstable
components of the bio-oil. Four types of ZVMs, i.e., Fe, Zn, Al,
and Mg, used in this study were in powder form with particle
size smaller than 0.12 mm. All reagents used in this work
(except specifically mentioned) were purchased from Sinopharm
Chemical Reagent Co., Shanghai, China, and were used without
further purification. The bio-oil was obtained from fast pyrolysis
of rice husk in a fluidized-bed reactor under nitrogen flux. Bio-
oil collected from the condensation section was selected as real
matrix for upgrading experiments without any pretreatment
(hereafter named as raw bio-oil).
The 13C NMR spectra of the bio-oil were recorded using a
400 MHz nuclear magnetic resonance (NMR) spectrometer (AV
400, Bruker Inc., Switzerland) at room temperature. The NMR
experiments were quantitative, and the samples were prepared by
dissolving accurately weighed ∼50 mg of bio-oil in dimethyl
sulfoxide-d6 (Sigma-Aldrich Co., Shanghai, China). In spectral
analysis, the integration values of given peaks were compared to
the total area of all peaks, resulting in mol% units.
As for the GC-MS analysis, the bio-oils were separated into
two fractions by dichloromethane (CH2Cl2) extraction. The com-
pounds in the CH2Cl2 soluble fraction were analyzed by gas
chromatography–mass spectrometry (GC-MS, 7890A GC/5975C
MS with a HP-5MS capillary column, 30 m × 0.25 mm ×
0.25 μm, Agilent Inc., USA). High purity He gas was employed
as carrier gas at a constant flow rate of 1.0 mL min−1, and a split
of the carrier gas (1 : 20) was used. The temperature of the GC
injector was 523 K. The GC oven temperature was programmed
from 313 to 453 K at 4 K min−1 and then to 523 K at 10 K
min−1, and the final temperature was kept for 5 min.
Typical procedure for the hydrogenation of model compound of
bio-oil
ZVMs powder (5.0 mM) was placed in a 25 mL reaction tube,
followed by the addition of benzaldehyde (2.0 mM, 0.212 g),
acetic acid (10.0 mM, 0.580 g), organic solvent (3.0 g), and
water (1.0 g). Then the mixture was vigorously stirred (600 rpm
with a magnetic stirrer) at ambient pressure and temperature.
After reaction, the mixture was filtered with a membrane filter
(0.45 μm). The conversion and product selectivity were period-
ically determined by gas chromatography (Model 1690, Kexiao
Co., Hangzhou, China) equipped with a capillary column
(1.0 μm, 50 m, and 0.25 mm) and a flame ionization detector
using naphthalene (C10H8) as an internal standard. The injection
and detector temperatures were both 523 K, and a heating
program of column temperature from 333 to 503 K with a rate of
8 K per minute was applied. The reactant and products (i.e., ben-
zaldehyde, benzalcohol, and toluene) were identified by compar-
ing the retention time with that of the pure components. The
conversion of reactant and selectivity of products were calculated
as eqn (2) and (3), respectively.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (50978242), National 863 Program
(2009AA06Z305), and the Fundamental Research Funds for the
Central Universities (WK2060190007, J1030412).
Cr0 ꢀ Crt
Conversion ð% Þ ¼
ꢁ 100%
ð2Þ
Cr0
Cpt
Selectivity ð% Þ ¼
ꢁ 100%
ð3Þ
Notes and references
Cr0 ꢀ Crt
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where Cr0 is the initial concentration (mmol) of the reactant, Crt
is the concentration (mmol) of reactant at time t (min), and Cpt is
the concentration (mmol) of the product at time t (min).
2232 | Green Chem., 2012, 14, 2226–2233
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