Influence of solvent on upgrading of phenolic compounds in pyrolysis bio-oil

Article abstract of DOI:10.1039/c4ra10891d

Solvents play a pivotal role in many chemical reactions as well as in upgrading of pyrolysis bio-oil. Herein we report a systematic study of the effects of solvents such as water, methanol, ethanol, propanol, butanol, acetone, ethyl acetate, tetrahydrofur

Full text of DOI:10.1039/c4ra10891d

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Influence of solvent on upgrading of phenolic
compounds in pyrolysis bio-oil
Cite this: RSC Adv., 2014, 4, 49924
ab
Gang Feng,ab Zhen Liu,ab Ping Chenab and Hui Lou
*
Solvents play a pivotal role in many chemical reactions as well as in upgrading of pyrolysis bio-oil. Herein we
report a systematic study of the effects of solvents such as water, methanol, ethanol, propanol, butanol,
acetone, ethyl acetate, tetrahydrofuran and hexane on hydrogenation of a lignin-derived component of
pyrolysis bio-oil, phenol, over activated carbon supported palladium catalysts. The solvents were
classified into four groups so as to better understand the effect of them. For hydrogen bond donor–
hydrogen bond acceptor (HBD–HBA) solvents and hydrogen bond acceptor (HBA) solvents, the
conversion of phenol decreased with increasing polarity/polarizability p*. Phenol was converted
completely in hexane or water at 250 ꢀC. However, in methanol or ethanol, it was converted partially.
Synergistic action of multiple factors had an effect on the hydrogenation of phenol. To convert phenol
better, water and hexane as solvents were excellent in upgrading of pyrolysis bio-oil.
Received 22nd August 2014
Accepted 30th September 2014
DOI: 10.1039/c4ra10891d
www.rsc.org/advances
Ni catalysts and reported that in water main product was
cyclopentanone by the rearrangement of furfural and furfuryl
1. Introduction
alcohol, while in alcohols, the furan ring rearrangement did not
proceed.
Pyrolysis bio-oil is considered to be a promising alternative
liquid transportation fuel. It contains many components such
as water, acids, alcohols, ethers, ketones, esters, aldehydes,
phenols, saccharides, hydrocarbons, etc.1 Bio-oil cannot be
directly used as a transportation fuel without prior upgrading.
Usually hydrotreating is used in upgrading pyrolysis bio-oil to
improve the quality.2–5
Selective hydrogenation of phenol to cyclohexanone, an
important chemical feedstock, attracted more attention for its
unique efficiency in gas phase11–13 or liquid phase,14,15 especially
in green solvent water.16–18 However, the role of solvent is not
extensively studied and is not clear.
Herein we report a systematic study of solvent effects on the
hydrogenation of phenol, a typical lignin-derived component of
bio-oil, over activated carbon supported palladium catalyst.
Because phenolic compounds are present with relatively big
amount, the hydrogenation of phenol as a model is important
guidance for upgrading pyrolysis bio-oil. Taking into account
components of pyrolysis bio-oil, in this work we selected four
groups of solvents such as water, hydrogen bond donor–
hydrogen bond acceptor (HBD–HBA) solvents (methanol,
ethanol, propanol and butanol), hydrogen bond acceptor (HBA)
solvents (acetone, ethyl acetate and tetrahydrofuran), and
hydrocarbon solvents (hexane). The relationships of reactants,
products, solvents and catalysts are also investigated to under-
stand the catalytic process. Results described herein may give
researchers some references in the selection of solvents
appropriate for upgrading of pyrolysis bio-oil and producing
valuable chemicals.
In hydrogenation reactions the solvent plays a pivotal role
and have dramatic effect on rate and selectivity.6,7 Zhang et al.8
investigated hydrogenation of cinnamaldehyde using Pd/C
catalyst, and reported that the main products were hydro-
cinnamaldehyde, 3-phenyl propanol, and propyl benzene in
non-polar solvents, while in polar solvents besides the products
above additional cinnamaldehyde diacetal and hydro-
cinnamaldehyde diacetal were formed. Bennett et al.9 studied
hydrogenation of 2-pentyne over Pd/Al2O3 and reported that the
reaction in heptane was faster than in isopropanol due to
increased substrate solubility. And the rate was intermediate in
the solvent of 50/50 heptane/isopropanol, but both cis/trans
pentene ratio and pentene/pentane ratio were the biggest. Akpa
et al.7 studied solvent effects on hydrogenation of 2-butanone
over Ru/SiO2 catalyst and showed that the hydrogenation reac-
tion had dramatically higher initial rate in water than in iso-
propyl alcohol, methanol, and heptane. Hronec et al.10 studied
hydrogenation of furfural and furfuryl alcohol over Pt, Ru and
2. Experimental
2.1. Materials
aInstitute of Catalysis, Department of Chemistry, Zhejiang University, Hangzhou
310028, China. E-mail: hx215@zju.edu.cn; Fax: +86 571 88273283; Tel: +86 571
88273283
Pd/C (5%) catalyst was purchased from Aladdin. All of the
bKey Laboratory of Applied Chemistry of Zhejiang Province, Hangzhou 310028, China analytical reagent grade chemicals were purchased from
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Table 1 Hydrogenation conversion of phenol over Pd/C at different
temperature
commercial suppliers: phenol (Aladdin); methanol, ethanol,
propanol, ethyl acetate and tetrahydrofuran (Chengdu Kelong
Chemical Reagent Company); butanol, acetone and hexane
(Sinopharm Chemical Reagent Co., Ltd). H2 (99.999%) was
supplied by Hangzhou Jingong Specialty Gases Co., Ltd.
Phenol conversion at different
temperature/%
Solvent
100 ꢀC
120 ꢀ
C
150 ꢀ
C
Water
Methanol
Ethanol
Propanol
Butanol
Acetone
Ethyl acetate
Tetrahydrofuran
Hexane
17.4
2.8
3.7
10.2
11.2
—
—
—
—
46.5
3.8
5.8
17.5
18.8
4.9
31.2
8.4
87.0
6.2
9.7
35.3
38.5
6.8
64.8
12.1
2.2. Phenol hydrogenation
The hydrogenation of phenol over Pd/C was carried out in
various solvents in a 25 mL stainless steel autoclave. In a typical
procedure for hydrogenation, phenol (0.250 g, 0.00266 mol), Pd/
C (0.010 g) and solvent (10 mL) were added into the autoclave.
Then, the autoclave was pressured to 1.0 MPa with hydrogen
aer being purged with hydrogen for 10 times to remove the air,
and was heated to react at object temperature for 1 h with
stirring at 800 rpm. Aer cooling to ambient temperature in ice
water, products were analyzed on a gas chromatography (GC,
Agilent 6820) with a ame ionization detector (FID) and a DB-
WAX capillary column (30 m ꢁ 0.32 mm ꢁ 0.25 mm). Identi-
cation of the products was performed on a gas chromatography-
mass spectrometry (GC-MS, Agilent 6890/5973).
76.7
100
3.2. The effect of solvents
Solvents play a crucial role in many chemical reactions. Because
of complicated physical and chemical properties of solvents, it
is very difficult to evaluate all of solvents by single criterion. The
solvents used here are hence classied into four groups: (1)
water; (2) hydrogen bond donor–hydrogen bond acceptor
(HBD–HBA) solvents, such as methanol (MeOH), ethanol
(EtOH), propanol (PrOH) and butanol (BuOH); (3) hydrogen
bond acceptor (HBA) solvents, such as acetone (Ace), ethyl
acetate (EA) and tetrahydrofuran (THF); (4) hydrocarbon
solvents, such as hexane (Hex). Some parameters of solvents
such as HBD ability a,20 HBA ability b,20 polarity/polarizability
p* (ref. 20) and polarity ENT (ref. 21) are listed in Table 3.
From Table 3 it can be seen that water shows the highest
polarity and displays the highest HBD ability. Hexane shows the
lowest polarity and display no HBD ability and HBA ability. The
HBD–HBA solvents show higher polarity and display higher
HBD ability. The HBA solvents show lower polarity and HBA
ability than that of HBD–HBA solvents, and display almost no
HBD ability. These properties of solvents are correlated with the
catalytic activity in the hydrogenation of phenol.
2.3. Cyclohexanone hydrogenation
The hydrogenation of cyclohexanone on Pd/C was carried out as
the hydrogenation of phenol.
2.4. Infrared (IR) spectroscopy
Fourier transform infrared (FTIR) spectra were recorded on a
spectrometer (Bruker Vertex 70) in the range of 4000–650 cmꢂ1
with a spectral resolution of 4 cmꢂ1. Prior to measurement, a
droplet of the sample was placed between two CaF2 thin wafers.
Spectra were recorded in air with an average of 64 scans.
3. Results and discussion
3.1. Phenol hydrogenation at different conditions
3.2.1 Water. Results summarized in Tables 1 and 2 show
that solvents play a crucial role in hydrogenation of phenol.
Water is an excellent solvent for this reaction.
The hydrogenation of phenol over Pd/C at 120 ꢀC and 150 ꢀC are
summarized in Table 1. Obviously, higher temperature led to
higher conversion of phenol. For example, the conversion was
100% in hexane at 150 ꢀC while it was 76.7% at 120 ꢀC. At lower
temperature the cyclohexanol was formed only in water and
hexane. In order to study the reaction mechanism, higher
reaction temperature and hydrogen pressure and longer reac-
tion time were employed, and results were summarized in Table
2. At 250 ꢀC phenol was converted completely in water, ethyl
acetate, tetrahydrofuran and hexane, meanwhile only part of
phenol was converted to cyclohexanone and cyclohexanol
except in water, tetrahydrofuran and hexane. From the results
in Tables 1 and 2 it was derived that phenol was hydrogenated
to cyclohexanone rstly and then to cyclohexanol by the
subsequent hydrogenation. Scheme 1 shows the reaction
pathway for hydrogenation of phenol on Pd/C at different
temperature and it is in accord with the mechanism of hydro-
genation on supported Pd catalysts.13,19
The solvent may affect the adsorption strength of reactants
and products on the catalyst surface. Because of the immisci-
bility with water cyclohexanone formed during hydrogenation
of phenol may not quickly desorb from the catalyst surface and
hence may be further hydrogenated to cyclohexanol. On the
contrary, cyclohexanone could desorb easily for good solubility
in other solvents, as a result further hydrogenation to cyclo-
hexanol is hindered. Density functional theory (DFT) calcula-
tion results show that water can reduce the binding energy (20–
25%) between phloroglucinol molecule and Pt(111) and Pd(111)
surfaces in comparison to that in gas phase.22 It is plausible that
the binding energy between phenol and metal surface decreases
because of the formation of hydrogen bond between phenol and
water. Moreover, water can lower the activation barrier as well
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Table 2 Solvent effects on the hydrogenation of phenol over Pd/C at 250 ꢀ
C
Selectivity
C]O/%
Selectivity
C–OH/%
Selectivity
others/%
Solvent
Phenol conversion/%
Water
Methanol
Ethanol
Propanol
Butanol
Acetone
Ethyl acetate
Tetrahydrofuran
Hexane
100
28.6
69.3
96.0
97.4
79.6
100
100
100
5.7
77.2
63.2
57.6
48.6
38.6
1.8
94.3
11.9
27.8
34.2
43.9
57.9
68.2
92.0
84.9
0
10.9
9.0
8.2
7.5
3.5
30.0
0
8.0
15.1
0
molecule, especially methanol and ethanol, on the same active
site may cause the drop in phenol conversion.13
The binding energy of phenol–methanol (265 ꢃ 8 meV) is
higher than that of phenol–water (243 ꢃ 5 meV).27 This means
the interaction of phenol–methanol is stronger than that of
phenol–water. Therefore hydrogenation of phenol is more
difficult in methanol than in water.
Scheme 1 Reaction pathway for hydrogenation of phenol on Pd/C at
different temperature.
To better discern inhibiting effect of methanol, the hydro-
genation of phenol on Pd/C was ꢀperformed in methanol at
hydrogen pressure 4.0 MPa at 250 C for 4 h with stirring, and
then the catalyst was re-used in water for the same reaction
under the same condition. The conversion was 44.8% in water.
However the conversion was 100% if catalyst was used rstly in
water.
The inhibiting effect of ethanol was also examined by
hydrogenation of phenol in mixed solvents of ethanol with
water or hexane, as shown in Fig. 1. In the mixed solvents, an
increasing percentage of ethanol in water or hexane led to a
decrease in phenol conversion and cyclohexanol selectivity,
whereas the selectivity of cyclohexanone and other by-products
yield increased.
On the other hand, the hydrogenation rate of phenol may
also depend on the activation itself. Karthick et al.28 calculated
the excitation energies from HOMO (highest occupied molec-
ular orbital) to LUMO (lowest unoccupied molecular orbital) for
2,4,6-tris(dimethylaminomethyl)phenol in water, ethanol, and
methanol by TD-DFT (time-dependent density functional
as increase the proton diffusion coefficient.7,23 As a conse-
quence, water promotes the hydrogenation of phenol.
3.2.2 HBD–HBA solvents. As summarized in Table 1, the
conversion is highest in butanol, followed by propanol, ethanol
ꢀ
and methanol among HBD–HBA solvents at 150 C. The same
order is also present at 120 ꢀC. Obviously methanol and ethanol
inhibit hydrogenation of both phenol and cyclohexanone, also
as seen in Table 2. That may result from strong interactions
between methanol or ethanol and phenol and cyclohexanone.
Hydrogen solubility in the solvent may play a crucial role in
catalytic reaction.6 The solubility of hydrogen in alcohols
increases with increasing carbon chain length.24 This is one of
reasons that the conversion of phenol increases from methanol
to butanol. Hydrogen solubility increases with increasing
temperature and pressure in alcohols.25 Correspondingly, the
conversion of phenol is higher with increasing temperature and
pressure in alcohols, as shown in Tables 1 and 2. The result that
methanol effectively inhibits the hydrogenation of aromatic
compounds is in accord with a previous work.26 On the other
hand, the competitive adsorption between phenol and alcohol
Table 3 Some parameters of solvents and phenol
Solvent
HBD ability a
HBA ability b
Polarity/polarizability p*
Polarity ETN
Water
Methanol
Ethanol
Propanol
Butanol
Acetone
Ethyl acetate
Tetrahydrofuran
Hexane
1.17
0.98
0.86
0.84
0.84
0.08
0
0
0
1.65
0.47
0.66
0.75
0.90
0.84
0.43
0.45
0.55
0.00
0.30
1.09
0.60
0.54
0.52
0.47
0.71
0.55
0.58
ꢂ0.04
0.72
1.000
0.762
0.654
0.617
0.586
0.355
0.228
0.207
0.009
—
Phenol
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Fig. 2 Hydrogenation conversion of phenol versus solvent polarity/
polarizability p*.
Fig. 1 Hydrogenation conversion of phenol versus ethanol volume
percent.
theory) method with B3LYP/6-311++G (d, p) basis set using PCM
(polarizable continuum model), and the values of excitation
energies were 3.8562, 3.8596 and 3.8612 eV, respectively.
Accordingly, it is plausible that the excitation energies of phenol
in water, ethanol and methanol have the same order as
water < ethanol < methanol. As a result, the order of reaction is
water > ethanol > methanol, this is in accord with the activity
order of hydrogenation as listed in Tables 1 and 2.
3.2.3 HBA solvents. As given in Table 1, among HBA
solvents, the conversion is higher in ethyl acetate (64.8%) but
very low in tetrahydrofuran (12.1%) and acetone (6.8%) at
ꢀ
150 C. A similar result with the same order is also present at
ꢀ
120 C.
For HBA solvents, the solvent molecule can supply lone pair
electrons by oxygen atoms. The interaction between metal and
lone pair electrons blocks the active site.29 As a result, the
hydrogenation activity decreases by competitive adsorption
between reactants and solvents on the metal surface. At the
same time, the interaction between HBA solvents and phenol
(HBD substance with a ¼ 1.65)20 suppresses the adsorption of
Fig. 3 Hydrogenation conversion of phenol versus hydrogen bond
donor (HBD) ability a.
phenol onto the metal surface. Therefore the activity decreases of solvent polarity/polarizability p* and HBD ability a are shown
in HBA solvents. in Fig. 2 and 3, respectively. In HBD–HBA and HBA solvents, the
3.2.4 Hydrocarbon solvents. In hexane the conversionꢀis conversion is decreasing with the increase of polarity/polariz-
highest among all of solvents and can come to 100% at 150 C ability p* and HBD capability a.
ꢀ
and 250 C.
Phenol is immiscible with hexane. The interaction between
phenol and hexane is obviously weak. Meanwhile the adsorp-
tion of hexane molecule on metal active site is also weak. As a
result, the hydrogenation would not be inhibited in hexane.
3.4. Cyclohexanone hydrogenation
To investigate solvent effects on products, the hydrogenation of
cyclohexanone in various solvents was carried out at 150 ꢀC, and
results were shown in Fig. 4. Obvious variation is seen in
different solvents, and conversions are low in all of solvents
except water. This means the hydrogenation is more difficult for
3.3. Correlation of conversion with solvent parameters
To explain the dramatic inuence of solvents, the relationship cyclohexanone relative to phenol. The results indicate the
between the hydrogenation conversion of phenol and sol- discussion above is reasonable.
vatochromic parameters such as HBD capability a, HBA capa-
Fourier transform infrared (FTIR) spectra are used to discern
bility b, polarity/polarizability p* and polarity ETN are the interaction between cyclohexanone and solvents. FTIR
investigated. For each group of solvents, plots of hydrogenation spectra of cyclohexanone in various solvents are shown in Fig. 5.
conversion of phenol as a function of the empirical parameter For pure cyclohexanone C]O bond stretching vibration band is
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Table 4 Various kinds of interactions in hydrogenation of phenol
Interaction (C ¼ catalyst, P ¼ product,
R ¼ reactant, S ¼ solvent)
Solvent
S–R
S–C
S–P
C–R
C–P
P–R
Sum
Water
HBD–HBA
HBA
+
+
ꢂ
+
+
+
+
+
+
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
3+3ꢂ
2+4ꢂ
2+4ꢂ
4+2ꢂ
ꢂ
ꢂ
+
ꢂ
ꢂ
+
Hydrocarbon
+
at 1715 cmꢂ1. In water and HBD–HBA solvents C]O stretching
shis to lower wavenumbers because of the formation of
hydrogen bonding. On the contrary, there are no evident shis
for the C]O band when cyclohexanone is in HBA and hydro-
carbon solvents. It is reasonable that the interaction between
cyclohexanone and solvents is weaker for HBA and hydrocarbon
solvents in contrast to water and HBD–HBA solvents.
Fig. 4 Solvent effects on the conversion of cyclohexanone hydro-
genation at 150 ꢀC.
3.5. Inuencing mechanism of reaction
From the discussion above, it can be concluded that chemical
reactions are inuenced by multiple factors, as shown in
Scheme 2. If the promotion factor is bigger than the inhibition
one, the reaction will accelerate. On the contrary, the reaction
will decelerate.
Various kinds of interactions in hydrogenation of phenol are
listed in Table 4. The promotion is biggest in hydrocarbon
solvents followed by water. On the contrary, in HBD–HBA and
HBA solvents the inhibition is major. As a result, the solvents
order of conversion is hydrocarbon solvents > water > HBD–HBA
solvents ꢄ HBA solvents.
4. Conclusion
Chemical reactions are inuenced by multiple factors. When
promotion interaction dominates, the reaction can occur easily
and vice versa. The solvatochromic parameters of solvents such
as HBD capability a, HBA capability b, polarity/polarizability
p* and polarity ETN may have a correlation with the hydrogena-
tion conversion of phenol over activated carbon supported
palladium catalysts. Water and hydrocarbons are the excellent
solvents for the hydrogenation of phenol. In HBD–HBA and
HBA solvents, the phenol conversion was decreasing with
increasing solvent polarity/polarizability p* and HBD capability a.
Fig. 5 FTIR spectra of cyclohexanone in various solvents.
Acknowledgements
We thank the nancial support of National Key Basic Research
Program of China (2013CB228104).
Notes and references
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