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Efficient hydro-liquefaction of woody biomass over ionic liquid nickel based
catalyst
Article in Industrial Crops and Products · March 2018
DOI: 10.1016/j.indcrop.2018.01.033
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Industrial Crops & Products 113 (2018) 157–166
Contents lists available at ScienceDirect
Industrial Crops & Products
journal homepage: www.elsevier.com/locate/indcrop
Efficient hydro-liquefaction of woody biomass over ionic liquid nickel based
catalyst
Qingyin Lia,b, Dong Liub, Linhua Songc, Xulian Houd, Chongchong Wua, Zifeng Yanb,
T
⁎
a
Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive, NW, Calgary, Alberta T2N 1N4, Canada
State Key Laboratory of Heavy Oil Processing, PetroChina Key Laboratory of Catalysis, China University of Petroleum, Qingdao 266580, China
c
College of Science, China University of Petroleum, Qingdao 266580, China
d
China Petroleum Engineering Co., Ltd., Beijing Company, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Hydro-liquefaction
Bio-oil
Ionic liquid
Biomass
Sawdust
The effective hydro-liquefaction process of sawdust is proposed over different ionic liquid Ni-based catalysts at
320 °C. This study investigated the relationship between ionic liquid composition and catalytic performance. The
maximum sawdust conversion of 83.11% and liquid yield of 52.38% were achieved over the catalytic system
including nickel chloride and 1-butyl-3-methyllimidazolium bromide ([Bmim]Br), suggesting that the anion in
ionic liquid played an vital role on the sawdust hydro-liquefaction. The defined impact index was provided to
further evaluate the liquefaction performance using ionic liquid Ni-based catalyst. The largest impact index
obtained from [Bmim]Br/NiCl2 was in agreement with its excellent catalytic liquefaction performance. The
analysis of ionic liquid Ni-based catalyst indicated the coordination interaction between ionic liquid and nickel
chloride, which would be favorable to feedstock conversion and improve the bio-oil quality.
According to the component distribution, the introduced ionic liquid Ni-based catalytic system was beneficial
to sawdust cracking, resulting in the increased amount of smaller compounds. Additionally, the oil compositions
highly depended on the employed type of ionic liquid.
1. Introduction
To solve energy crisis and environmental deterioration caused by
unrestrained exploitation of fossil energies, biomass, as one of the most
abundant and renewable resources, has gained growing attentions
(Zacher et al., 2014; Zhang et al., 2012). The lignocellulosic biomass
can be converted into bio-fuels through main thermo-chemical technologies including gasification, pyrolysis, and liquefaction (Demirbas
and Balat, 2006; Bridgwater, 2012; Liu et al., 2012). Recently, the
biomass liquefaction in supercritical ethanol has been comprehensively
investigated (Xu and Etcheverry, 2008; Chumpoo and Prasassarakich,
2010). The studies exhibited some potential advantages such as excellent solubility, low corrosivity, and hydrogen donation ability (Brand
et al., 2013).
Many researches concerning biomass liquefaction in supercritical
ethanol mainly focused on effect of parameters on the reaction behavior
of raw materials. According to the previous literature, the utilized
catalyst was one of most significant variables to enhance bio-oil yield
and improve quality, and thus many efforts were made to explore the
catalytic performance on the biomass conversion (Perego and Bianchi,
2010; Alonso et al., 2010). Due to the negligible vapor pressure, high
⁎
thermally stability, and strong dissolution ability, ionic liquid has been
widely employed in liquefaction of biomss for bio-oil or valuable chemicals (Pârvulescu and Hardacre, 2007; Zhao et al., 2007). Lu et al.
found that an acidic ionic liquid 1-carboxypropyl-3-methyl imidazolium chloride was considered as an effective catalyst for the conversion
of carbohydrates into 5-hydroxymethylfurfural (Hu et al., 2013). Li
et al. reported that the concentrated fructose afforded 5-hydroxymethylfurfural with a high yield in the combination of ionic liquid
and microwave irradiation without catalyst (Li et al., 2011). Lu and his
co-workers reported that the acidic ionic liquid [Bsmim]HSO4 exhibited
a good performance on the liquefaction of sawdust (Lu et al., 2013). In
particular, with the addition of ionic liquid, the hydrogen structure in
lignocellulose would be disrupted, and then interaction among the
biomass components were highly weakened, resulting in the improved
degradation of raw material (Mäki-Arvela et al., 2010).
Additionally, efficient conversion of lignocellulosic biomass was
carried out in the ionic liquid-metal ion system. Zhang et al. have reported that the addition of AlCl3 in the 1-butyl-3-methylimidazolium
chloride facilitated the conversion of woody biomass into furfural
(Zhang et al., 2013). Chinnappan et al. demonstrated that sucrose and
glucose could be converted effectively to 5-hydroxymethylfurfural in
Corresponding author.
E-mail address: zfyancat@upc.edu.cn (Z. Yan).
https://doi.org/10.1016/j.indcrop.2018.01.033
Received 30 June 2017; Received in revised form 9 December 2017; Accepted 13 January 2018
0926-6690/ © 2018 Published by Elsevier B.V.
Industrial Crops & Products 113 (2018) 157–166
Q. Li et al.
As illustrated in Table 1, the chemical compositions of tested sample
were determined based on Van Soet method (Carrier et al., 2011). The
elemental components were measured via a Vario EL III elemental
analyzer. The oxygen content was evaluated from mass balance closure
without regard to inorganics contained in the feedstock. The Higher
Heating Value was estimated from the elemental results, and calculated
by the formula described in the literature (Huang et al., 2013). Additionally, the elemental compositions and HHV were analyzed based
on a dry and ahs-free basis.
the presence of pyridinium based dicationic ionic liquid ([C10(EPy)2]
2Br−) and chromium chloride (Chinnappan et al., 2015). In view of
biomass hydro-liquefaction, Ni-based catalyst is suggested to enhance
raw material conversion and highly improve the bio-oil quality (Grilc
et al., 2014). Therefore, the Ni-based catalyst (NiCl2) in the ionic liquid
was utilized as the catalytic system for biomass hydro-liquefaction. On
the one hand, the metal chloride had a coordination interaction with
ionic liquid, and higher catalytic activity was expected, which could
promote the biomass conversion effectively (Hines et al., 2008). On the
other hand, the presence of ionic liquid could destroy the network
structure of raw material, and then the biomass reaction could be
highly improved.
To further evaluate the liquefaction behavior from ionic liquid Nibased catalyst, a novel impact index was defined with tetralin employed
as the chemical probe. Generally, tetralin was considered as the excellent hydrogen donor for biomass conversion (Beauchet et al., 2011).
In the closed reaction system, the ionic liquid in the catalytic system
would promote the feedstock conversion, and increase the amount of
produced intermediates. Therefore, these fragments could greatly affect
the hydrogen-donating ability of tetralin. On the other hand, the presence of ionic liquid had an influence on the solvent effect, and thus
altered the liquefaction performance of lignocellulose, which definitely
affect the tetralin conversion during the process. According to the above
discussion, a new impact index was put forward to explore the impact
of ionic liquid Ni-based catalyst on the lignocellulose liquefaction.
According to the previous work, sawdust and its three sub-components hydro-liquefaction were investigated with [Bmim]Cl and NiCl2 as
the catalytic system (Liu et al., 2015). However, the influence of ionic
liquid in the catalytic system on the liquefaction behavior was seldom
reported. Besides, the formed transition metal complex between nickel
chloride and ionic liquid should be described in detail. The aim of this
work was to determine the catalytic performance of various ionic liquids in nickel chloride on the liquefaction performance of sawdust.
Additionally, the defined new index was used as a reference to further
evaluate the ionic liquid Ni-based catalyst influence in the reaction
system. The chemical composition of bio-oil derived from the optimal
condition were analyzed by Fourier transform infrared (FTIR), gas
chromatography-mass spectrometry (GC–MS), elemental analysis (EA),
and Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS). The ionic liquid nickel catalyst was characterized by FTIR, 1H
nuclear magnetic resonance (1H NMR) and electron spray ionizationmass spectrometry (ESI-MS), respectively.
HHV (MJ/kg) = 338.2 wt.%(C) + 1442.8(wt.%(H) − wt.%(O)/8)
(1)
2.2. Experimental procedures and product separation
In each catalytic run, 1 g sawdust, 10 mL ethanol, 300 μg/g Ni-based
catalyst (NiCl2) and a certain amount of ionic liquid was placed into the
autoclave, and then the ethanol solution was stirred to make it mixed
evenly. The reactor was purged with hydrogen and then elevated to
4.0 MPa initial pressure. Subsequently, it was heated up to the required
temperature and maintained for the desired time. Finally, the reaction
was quenched immediately with cooling water. The procedure for separation of liquefaction products was described previously (Liu et al.,
2015). To explore the product distributions, the yields of bio-oil and
conversion were defined based on the Eqs. (2)–(3). Especially, the obtained raw oil surely contained the employed catalyst. However, the
remaining ionic liquid was not taken into consideration in the mass
balance due to its small amount. It should be noted that the gaseous
yield included yield of volatile components, produced water and gas.
Yield of bio-oil =
Conversion =
Weight of bio-oil
× 100%
Weight of sawdust
(2)
Weight of sawdust − Weight of residue
× 100%
Weight of sawdust
(3)
All the product yields were calculated on the tested sample. Beside,
each experiment was duplicated three times under identical conditions
to ensure the accuracy of data. The results were dispersed within 4%
standard derivation and the calculated mean value was analyzed to
investigate the catalytic performance on the sawdust hydro-liquefaction.
2.3. Characterizations
2. Experimental section
The analysis of nickel species in ionic liquid was performed through
FTIR, ESI-MS, and 1H NMR, respectively. The 1H NMR spectrum was
acquired from a Bruker Avance III 500 MHz NMR spectrometer with
500 MHz resonance frequency. ESI-MS analysis was conducted using an
Agilent 6300 mass spectrometry.
The functional group distribution was determined using a Nicolet
6700 FT-IR spectrometer. The chemical composition of bio-oil was
characterized via a GC–MS system from ThermoFisher with a DB-35MS
column (30 m × 0.25 mm × 0.25 μm). The dominant components detected in the liquid product were identified by a NIST mass spectral
database.
The compositional analysis of bio-oils was conducted using a 9.4 T
FT-ICR MS instrument (Bruker Apex-Ultra) equipped with ESI. The
negative mode was operated with the source voltage of 3.0 KV and the
2.1. Material and methods
The sawdust was obtained from wood processing industry in
Qingdao, China. The raw material was firstly washed and dried in an
oven at 378 K overnight. Subsequently, the desired sample with 60
meshes was achieved through the pretreatment of crushing and sieving.
The ionic liquids including [Bmim]Br, [Bmim]Cl, and [Emim]Cl were
synthesized according to the reported procedures (Burrell et al., 2007;
Holbrey et al., 2001). Additionally, [Bmim]BF4 was supplied from
Sigma-Aldrich without further purification. All other chemicals were
purchased from Sinopharm Chemical Reagent Co., Ltd. The lab-made
high-pressure autoclave was depicted in Supplementary Fig. S1.
Table 1
Chemical and elemental compositions of sawdust.
Elemental composition/wt.%
C
47.68
a
H
6.30
Chemical composition/wt.%
N
0.45
a
O
45.57
Cellulose
48.27
Caculated by difference.
158
Hemicellulose
19.50
Lignin
19.80
Extractives
11.40
Ash
0.97
Industrial Crops & Products 113 (2018) 157–166
Q. Li et al.
Fig. 1. Effect of ionic liquid types on the bio-oil yield (a) and conversion
(b).
initial and end voltage of capillary column of 3.5 KV and −320 KV.
Each sample solution was infused at 150 μL/h.
better catalytic performance compared with BF4 based ionic liquid due
to nucleophile of Br anion. As reported from previous literatures, the
ionic reactions including nucleophilic, electrophilic, and elimination
take place in the closed system as the main conversion process (Durak
and Aysu, 2014; Kabyemela et al., 1997). During the liquefaction process, Br anion could attack the hydrogen bond from raw material,
leading to its depolymerization and decomposition. In addition, it is
worth nothing that the BF4 was considered as a non-coordinating anion,
which was unable to affect biomass dissolution as a reaction medium
(Swatloski et al., 2002). On the other hand, BF4 based ionic liquid was
unable to have a coordination interaction with NiCl2, which weakened
sawdust conversion significantly compared with other catalytic systems.
Additionally, the catalytic performance of [Bmim]Cl and [Emim]Cl
was much similar with regard to conversion and bio-oil yield. It was
noted that the presence of alkyl group in imidazolium had no noticeable
improvement on the sawdust conversion. Besides, the better reaction
behavior in Br based ionic liquid could be observed than that in Cl
based ionic liquid, as bromide ion was a better leaving group and
3. Results and discussions
3.1. Effect of ionic liquid types
To obtain efficient ionic liquid Ni based catalyst, the influence of
ionic liquid type on the bio-oil yield and sawdust conversion at 320 °C
was investigated, which was depicted in Fig. 1.
The bio-oil yield and conversion from sawdust hydro-liquefaction in
[Bmim]Br/NiCl2 were higher than the ones in the presence of other
catalytic system, whereas the catalytic activity of NiCl2 in [Bmim]BF4
was lowest, resulting in the poor liquefaction behavior. The highest
conversion and bio-oil yield from sawdust liquefaction at 10 min was up
to 75.55% and 52.38% with [Bmim]Br catalytic system, while the obtained conversion and bio-oil yield in [Bmim]BF4 were 72.37% and
48.54%, indicating that the anion played an important role on the
sawdust liquefaction. It was suggested that Br based ionic liquid showed
159
Industrial Crops & Products 113 (2018) 157–166
Q. Li et al.
Moreover, the obtained oil yield was higher than that with ionic liquid
dosage over 1.0 wt.%. Therefore, it was acceptable that the excessive
ionic liquid dosage for sawdust liquefaction may accelerate the degradation of bio-oil into volatile compounds, resulting in the decreased
oil yield.
As shown in Fig. 2b, the conversion profiles of sawdust liquefaction
with different ionic liquid amounts showed a similar tendency and even
overlapped. With the prolonged time, the conversion of sawdust was
firstly elevated, and then declined gradually. The optimal reaction behavior could be achieved at 30 min. Clearly, when the amount of ionic
liquid was increased, the conversion was enhanced due to its high
catalytic performance. Since the ionic liquid dosage exceeded 1.0 wt.%,
the obtained conversion was weakened extensively. The possible reason
may be that the excessive ionic liquid favored the occurrence of undesired reactions, and thus accelerated the production of solid residue,
resulting in the poor conversion rate.
exhibited stronger nucleophile capacity than chloride ion, which was
consisted with the obtained results from Ryu et al. (Ryu et al., 2012).
The residence time duration may define the product distribution
and sawdust conversion. Clearly, the maximum bio-oil yield could be
achieved at the shortest time. In other words, the shorter reaction time
was beneficial for the formation of bio-oil products. Since the time was
prolonged, the produced oil compounds were prone to be decomposed
into gaseous fraction, and furthermore the degraded intermediates were
convert to solid residue through repolymerization and condensation,
resulting in the decreased bio-oil yield (Yang et al., 2004). On the other
hand, as shown in Fig. 1b, the conversion was firstly increased, and
then showed a declined trend. It may be explained that the undesired
reaction was occurred during the longer time, and sawdust conversion
may be reached the saturation point. Therefore, the polymerization
reaction for some small intermediates and products was enhanced,
which improved the formation of solid residue, and the sawdust conversion was highly prevented with the increased time.
3.3. Impact index of ionic liquid Ni-based catalyst
3.2. Effect of ionic liquid dosage
As a chemical probe, tetralin was employed to develop a deeper
understanding of liquefaction performance over ionic liquid nickel
based catalyst. According to tetralin conversion derived from different
conditions, a novel impact index was provided to evaluate the influence
of ionic liquid nickel based catalyst.
The influence of ionic liquid dosage on hydro-liquefaction of sawdust was investigated at 320 °C and 30 min in [Bmim]Br/NiCl2. The
experimental results were displayed in Fig. 2.
In Fig. 2a, as the amount of ionic liquid was 0.5 and 1 wt.%, the biooil yield presented had a similar tendency, which decreased with prolonged time, and thus the optimized liquid yield reached 50.47% and
58.58%, respectively. In addition, it was indicated that formation of
bio-oil was improved when the ionic liquid content increased from 0.5
to 1.0 wt.%. However, when the ionic liquid amount was increased to
2.0 wt.%, the bio-oil yield was initially increased from 30.41% to
40.45%, and then decreased to 35.85% continually. It should be noted
that the similar changed trend occurred in the absence of ionic liquid.
Impact index =
αi − αo
αo
(4)
Where αo is the tetralin conversion over nickel-based catalyst and αi is
the corresponding tetralin conversion from various ionic liquid nickel
based catalyst treatment. Clearly, the larger impact index indicated a
prominent role played by the specific catalytic system, and thus the
liquefaction behavior of sawdust was greatly improved.
Fig. 3 illustrates the impact index obtained from different ionic liquid nickel based catalyst treatment at 320 °C and 30 min. The impact
index from [Bmim]BF4 condition was much lower, whereas the higher
value was achieved from [Bmim]Br treatment. Additionally, it should
be noted that the similar index was observed with [Bmim]Cl and
[Emim]Cl catalytic systems. In particular, the changed tendency of biooil yield and conversion was similar to that of impact index under the
identical conditions.
The [Bmim]Br was interacted with NiCl2, and the formed complex
would facilitate the feedstock conversion, and favor the thermal
cracking of large fragments and stabilization via free radicals.
Additionally, the dominant reactions including ionic and free radicals
reaction took place in the closed system. The introduction of ionic liquid would affect the solvent polarity, and enhance the interaction
between solvent and raw feedstock, resulting in the improved liquefaction behavior. On the other hand, due to the destroyed hydrogen
Fig. 2. Effect of ionic liquid dosage on the bio-oil yield (a) and conversion (b).
Fig. 3. Impact index obtained from different ionic liquid Ni-based catalyst treatment.
160
Industrial Crops & Products 113 (2018) 157–166
Q. Li et al.
Fig. 6. Positive ions ESI–MS spectrum of [Bmim]Cl/NiCl2.
Fig. 4. FTIR spectra of [Bmim]Cl and [Bmim]Cl/NiCl2.
bond structure from ionic liquid treatment, the liquefaction degree was
highly enhanced, leading to the increase of active intermediates. Then,
these produced fractions would be dissolved and stabilized well in the
reaction medium, and thus the oil yield and conversion were highly
improved.
However, as to its limited coordination capability, [Bmim]BF4 exhibited a poor influence on the sawdust liquefaction. The solvent effect
would not be enhanced significantly in the presence of [Bmim]BF4.
Moreover, the conversion behavior was weakened due to the less-disturbed subcomponent structures.
In the cased of [Bmim]Cl and [Emim]Cl conditions, the similar
impact index implied the resembled liquefaction degree of raw material. The observation was consistent with above discussion, suggesting
that the major contributor on the sawdust conversion was attributed to
the anions in ionic liquid.
Fig. 7. FTIR spectra of sawdust and bio-oils produced from sawdust liquefaction in different catalysts.
3.4. Characterization of ionic liquid nickel based catalyst
imidazole ring with Cl. It should be pointed out that the band was
disappeared and shifted to a higher frequency (from 3074 to
3101 cm−1) with addition of nickel dichloride. The observation may be
due to the formed new species [Bmim][NiCl4], as the interaction of
CeH with [NiCl4]− was much lower than that between CeH and Cl.
The formation of Ni-containing ionic liquid was investigated by
FTIR and 1H NMR. As illustrated in Fig. 4, the spectrum of [Bmim]Cl/
NiCl2 was totally different from that of [Bmim]Cl, indicating the strong
interaction between ionic liquid and NiCl2. The broad adsorption peak
at 3000–3100 cm−1 belonged to the interaction of CeH in the
Fig. 5. 1H NMR spectra of (a) [Bmim]Cl/NiCl2 and (b) [Bmim]Cl.
161
Industrial Crops & Products 113 (2018) 157–166
Q. Li et al.
Table 2
Elemental compositions of bio-oil from sawdust conversion in different catalysts.
Samples
Sawdust
NiCl2
[Bmim]Br
[Bmim]Cl
[Bmim]BF4
[Emim]Cl
a
Elemental analysis/wt.%
HHV/MJ/kg
C
H
N
Oa
H/C
O/C
47.68
63.70
64.28
63.06
63.57
63.12
6.30
7.35
7.86
7.58
7.51
7.63
0.45
0.24
0.49
0.51
0.35
0.43
45.57
28.71
27.37
28.85
28.57
28.82
1.59
1.38
1.47
1.44
1.42
1.45
0.72
0.34
0.32
0.34
0.34
0.34
17.00
26.97
28.14
27.06
27.18
27.16
Calculated by difference.
Fig. 8. Classification of the oil compositions categorized by functional groups from
sawdust hydro-liquefaction with different catalysts.
Fig. 9. Oxygenated class distribution for bio-oil derived from sawdust conversion in NiCl2
and [Bmim]Br/NiCl2.
Fig. 10. Double bond equivalents distribution for (a) O5, (b) O6 and (c) O7 compounds
detected in bio-oil derived from sawdust conversion in NiCl2 and [Bmim]Br/NiCl2.
Additionally, the obtained results were supported by 1H NMR
spectra from Fig. 5. A series of sharp peaks corresponding to [Bmim]Cl
became broad, and transferred to higher chemical shifts, suggesting
that NiCl2 was highly associated with ionic liquid.
As shown in Fig. 6, the formed complex from the NiCl2 in the ionic
liquid was further determined by ESI–MS. Under the ESI positive ion
mode, the predominant peak in the spectrum with m/z 617.0 was assigned to ([Bmim]3[NiCl4])+ fragment. It could be confirmed the coordination from ionic liquid [Bmim]Cl and NiCl2, and the intermediate
complex [NiCln](n−2)−was indeed produced in the catalytic system.
This result was consistent with previous literature (Tao et al., 2012),
and the formed complex could weaken the glycosidic bond through
reacting with oxygen atom, which facilitated the lignocellulosic
component depolymerization and further degradation.
3.5. Analysis of bio-oil
3.5.1. FTIR
The FTIR spectra of sawdust and bio-oils obtained from different
ionic liquid Ni based catalyst were recorded to determine the functional
groups. The FTIR spectra are presented in Fig. 7.
The oil product showed the similar compositions regardless of the
presence of ionic liquid. Due to the identical FTIR spectra from bio-oils,
the type of ionic liquid in catalytic system have no significant influence
on the chemical compositions of oil.
The peak at 3430 cm−1 represents the typical of OeH stretching
162
Industrial Crops & Products 113 (2018) 157–166
Q. Li et al.
3.5.3. GC–MS
The chemical compositions of bio-oils derived from sawdust hydroliquefaction with different catalysts were identified by GC–MS. The
comparison of the observed compounds was illustrated in
Supplementary Table S1. The dominant components in the bio-oils with
relative content higher than 0.5% were determined. Additionally, the
percentage value was defined as the peak area of detected compound
out of the entire peaks.
To better understand effect of ionic liquid types on the oil compositional distributions, the obtained chemical components were further
categorized to different groups according to the functional groups,
which included esters, phenols, carboxylic acids, ketones, aldehydes,
alcohols, esters, and sugar derivatives.
In Fig. 8, the most abundant species from sawdust liquefaction were
ethyl esters regardless of the employed catalyst. Generally, the produced acids from degradation of carbohydrate may undergo esterification and substitution reactions to generate the esters and derivatives. Additionally, these ester species could highly weak the acidity,
corrosiveness, and instability of liquid products owing to the transformation of acid compounds. It was found that the higher ester content
was found in the oil with NiCl2 than those from ionic liquid Ni based
catalyst, implying that these ester compounds may further degrade to
new species. The ester compositions have a promising potential to be a
constituent of chemicals, solvents, food, and fuels.
In addition to ester compounds, a certain amount of aldehydes and
ketones were detected from sawdust liquefaction. The major aliphatic
ketones were formed via complex reactions of active intermediates,
such as decomposition, dehydration, isomerization, and aldol reactions
(Tao et al., 2013). Furthermore, the produced ester compounds would
be converted to the small ketones. Clearly, the generation of ketones
was enhanced with ionic liquid Ni-based catalyst. The ketones are
considered as a kind of important platform molecules that was widely
applied in many fields such as pharmaceuticals, pesticides, fuels, and
resins.
According to the aromatic structure, the dominant components were
phenolic compounds and derivatives from thermal cleavage of lignin,
which included phenol, phenol, 4-ethyl-2-methoxyl, and phenol, 2methoxy-4-propenyl (Kim et al., 2014). On the other hand, some
complex aromatic compounds with high molecular weight were unable
to be detected via GC–MS due to its non-volatile characteristic. The
phenols contents from different ionic liquid Ni-based catalysts were
similar, which was attributed to insufficient conversion of lignin component. The phenolic compounds mainly derived from lignin decomposition can be utilized as raw materials for the preparation of phenolic
resin such as phenol-formaldehyde, and as aromatic hydrocarbon for
fuels and solvents (Bu et al., 2012). In addition, small acids and alcohols
such as 5-hexen-3-ol, 2, 3-dimethyl and 2-hydroxypropanoic acid and
could also be found in the bio-oil, which may be produced from degradation of cellulose and hemicellulose. Additionally, the amount of
carboxylic acids and alcohols from ionic liquid Ni-based catalyst were
significantly higher than that with Ni-based catalyst, indicating that
cellulose and hemicellulose conversion were improved.
Especially, the sugar derivatives were only found in the oil with
ionic liquid Ni-based catalyst. Such results were explained that the ionic
liquid Ni-based catalyst showed a better catalytic performance on the
sawdust conversion, resulting in the improved conversion degree, and
thus some small compounds could be detected,
Fig. 11. Carbon number distributions of bio-oil obtained from hydro-liquefaction of
sawdust in different catalysts.
vibration, indicating the presence of alcohols and phenols. The bands
between 3000 and 2800 cm−1 could be attributed to the CeH
stretching from methoxyl group. Additionally, these peaks appeared at
1379 and 1468 cm−1 correspond to methyl and methylene bending
vibration, suggesting that certain amounts of alkyl groups are included
in the bio-oil (Nazari et al., 2015). The absorbance of these peaks is
much greater in the oils compared with that of raw material. It could be
explained that the hydrocracking and deoxygenation were enhanced,
leading to the formation of long chain compounds. The peak attributed
to the C]O stretching vibration was observed at 1739 cm−1, indicating
the presence of ketones, aldehydes and esters in the liquid products.
The characteristic peak of carbonyl group was enhanced dramatically
compared with that of sawdust, indicating that the dehydration reaction took place during the process. The peaks at 1614 cm−1 and
1516 cm−1 are contributed to the aromatic skeleton vibration, whereas
the adsorption was disappeared in the sawdust, suggesting the degradation of lignin component (Sun et al., 2011). The peaks between
1300 and 1000 cm−1 are assigned to the CeO stretching vibration,
which further confirmed that esters, phenols and alcohols were contained in the bio-oil.
3.5.2. Elemental analysis
The elemental compositions and heating values of bio-oils obtained
in different catalyst are revealed in Table 2. The carbon and hydrogen
content are increased significantly, whereas the oxygen content is much
lower in comparison to that in the feedstock, leading to higher heating
values. In addition, the H/C molar ratios of oils are lower than that of
raw material, indicating the presence of aromatic compounds in the
liquid product. The highest H/C ratio was achieved from [Bmim]Br/
NiCl2 treatment, indicating that hydrogenation reaction was highly
improved. With regards to NiCl2, the treated bio-crude contained lower
H/C molar ratio than that of the oil from other catalytic systems, suggesting that few saturated compounds were produced in Ni based catalyst. It was confirmed that catalytic performance of ionic liquid Ni
based catalyst was superior to that of Ni-based catalyst in terms of
sawdust hydro-liquefaction.
On the other hand, the O/C molar ratio in the bio-oil was significantly lower than that of raw material, suggesting that the deoxygenation of degraded compounds occurred. It is worth nothing that the
lowest O/C ratio was obtained from [Bmim]Br/NiCl2, which was ascribed to the enhanced dehydration and decarboxylation reaction, resulting in the formation of water and gaseous products. Especially, the
analysis result of heating values was consistent with the explanation.
The heating values of bio-oil from catalytic treatment were higher than
that of sawdust, and the highest heating value was obtained in the oil
with [Bmim]Br/NiCl2
3.5.4. FT-ICR MS
Heteroatom class analysis of bio-oils derived from sawdust conversion treated from NiCl2 and [Bmim]Br/NiCl2 was displayed in Fig. 9.
The relative abundance for individual class was estimated through peak
magnitudes for each oxygenated species divided by the summed magnitudes of all assigned components.
The most abundance classes in the bio-oil from Ni-based catalyst
were concentrated in the O6-O9 species, whereas the centered
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Fig. 12. Reaction mechanism of lignocellulose conversion over ionic liquid Ni-based catalyst.
amount would react with the cracked intermediates. Therefore, these
components could be stablized well, and the undesired reactions including condensation and polymerization was highly prohibited. Additionally, the presence of ionic liquid would destroy the chemical
structure of raw material to a certain extent, accelerating the decomposition rate. The produced fragments were prone to be converted into
smaller compounds.
As shown in Fig. 11, the carbon number distribution of bio-oils from
different catalytic system was presented. These results indicated that oil
composition produced from [Bmin]Br/NiCl2 shifted to lower carbon
numbers than that over NiCl2. In the case of ionic liquid Ni-based catalyst, the dominant species were the compounds with carbon number of
18 and 19, whereas the main components in the sample from NiCl2
treatment exhibited relatively higher carbon number region 22–23. It
was suggested that the ionic liquid Ni-based catalyst might promote
compounds from [Bmim]Br/NiCl2 systems was dominated by O6 and O7
oxygenates. The relative content of higher oxygen-containing compounds from ionic liquid nickel-based catalyst was much lower than
that from conventional condition. These results show that sawdust
conversion was promoted over [Bmim]Br/NiCl2 catalytic system. The
produced large fragments would further decompose into the smaller
components. On the other hand, the interaction between oxygenated
compounds and active radicals were greatly enhanced, and thus these
fractions were transferred to the compounds with low oxygen number.
In terms of O5-O7 classes, the double bond equivalents distributions
observed from oil product are shown in Fig. 10. Clearly, the DBE distribution from [Bmin]Br/NiCl2 system generally shifted to lower value
in comparision to that from Ni-based catalyst. Especially, the difference
was significant in the both lower and larger DBE ranges. With the assistance of ionic liquid Ni-based catalyst, the active radicals with high
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Q. Li et al.
China (21176259).
sawdust thermally cracking, and thus the fragments with higher weight
further converted into smaller ones. In addition, the presence of more
active radicals would inhibit the second reactions, resulting in the increased small species yield.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at https://doi.org/10.1016/j.indcrop.2018.01.033.
3.6. Reaction mechanism
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Nickel chloride in ionic liquid was considered as an effective catalyst for the hydro-liquefaction of sawdust. The anion in ionic liquid
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[Bmim]Br/NiCl2 showed an excellent catalytic influence on the biomass
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Acknowledgments
This work has been financially supported by the Key Joint
Foundation of PetroChina and Natural Science Foundation of China
(No. U1362202), the PetroChina key programs on oil refinery catalysts
(2010E-1908, 2010E-1903) and National Natural Science Foundation of
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