Hydrodeoxygenation of lignin-derived phenols into alkanes over carbon nanotube supported Ru catalysts in biphasic systems

Article abstract of DOI:10.1039/c4gc01992j

Phenolic compounds derived from lignin are important feedstocks for the sustainable production of alkane fuels with C6-C9 carbons. Hydrodeoxygenation (HDO) is the main chemical process to remove oxygen-containing functionalities. Here, we have reported the HDO of phenols in a biphasic H₂O/n-dodecane system. A series of supported Ru catalysts were prepared, characterized and explored for the reaction among which Ru/CNT showed the highest catalytic activity towards the production of alkanes. The model reaction with eugenol achieved a high conversion (>99%) and a high alkane selectivity (98%), which was much higher than the results from the monophasic system (56.5% yield of alkanes in H₂O). The reaction conditions including reaction temperature, hydrogen pressure and the ratio of H₂O/n-C₁₂H₂₆ were optimized. The kinetic experiments revealed that eugenol was first hydrogenated to 4-propyl-guaiacol, and then deoxygenated into 4-propyl-cyclohexanol which was the main detected intermediate of the reaction. After that, 4-propyl-cyclohexanol was dehydrated and hydrogenated into propylcyclohexane. Moreover, various phenols and dimeric lignin model compounds were also successfully converted into alkanes in the biphasic systems. The construction of the biphasic solvent-Ru/CNT catalyst system highlights an efficient route for the conversion of lignin-derived phenolic compounds to biofuels.

Full text of DOI:10.1039/c4gc01992j

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Hydrodeoxygenation of lignin-derived phenols
into alkanes over carbon nanotube supported
Ru catalysts in biphasic systems†
Cite this: DOI: 10.1039/c4gc01992j
Meng-Yuan Chen,‡a Yao-Bing Huang,‡b Huan Pang,a Xin-Xin Liua and Yao Fu*a
Phenolic compounds derived from lignin are important feedstocks for the sustainable production of
alkane fuels with C6–C9 carbons. Hydrodeoxygenation (HDO) is the main chemical process to remove
oxygen-containing functionalities. Here, we have reported the HDO of phenols in a biphasic H2O/
n-dodecane system. A series of supported Ru catalysts were prepared, characterized and explored for the
reaction among which Ru/CNT showed the highest catalytic activity towards the production of alkanes.
The model reaction with eugenol achieved a high conversion (>99%) and a high alkane selectivity (98%),
which was much higher than the results from the monophasic system (56.5% yield of alkanes in H2O). The
reaction conditions including reaction temperature, hydrogen pressure and the ratio of H2O/n-C12H26
were optimized. The kinetic experiments revealed that eugenol was first hydrogenated to 4-propyl-guaia-
col, and then deoxygenated into 4-propyl-cyclohexanol which was the main detected intermediate
of the reaction. After that, 4-propyl-cyclohexanol was dehydrated and hydrogenated into propylcyclo-
hexane. Moreover, various phenols and dimeric lignin model compounds were also successfully con-
verted into alkanes in the biphasic systems. The construction of the biphasic solvent-Ru/CNT catalyst
system highlights an efficient route for the conversion of lignin-derived phenolic compounds to biofuels.
Received 14th October 2014,
Accepted 24th December 2014
DOI: 10.1039/c4gc01992j
www.rsc.org/greenchem
which showed good activities towards this conversion.6
However, these catalysts may cause sulfur contamination
1 Introduction
Lignocellulosic biofuels are considered as promising alterna- in the product and fast deactivation of the catalysts.7 Thus,
tives to the traditional fossil fuels and are gaining increasing non-sulfided catalyst systems are required for the HDO of the
interest all over the world.1 Among the main components of phenolic mixture.8
lignocelluloses, lignin is a biopolymer consisting of phenolic
Currently, the reported non-sulfided catalyst systems for the
units with a mass fraction of 15–30 wt%, but with a relatively phenol HDO processes can be divided into two types: (1)
higher energy density than cellulose and hemicellulose.2 mixed catalyst systems with a transition-metal based catalyst
However, due to the complex structure and high oxygen and an acidic catalyst which are responsible for the hydrogen-
content of lignin, the depolymerized monomers are a mixture ation and dehydration procedures,9 respectively (e.g. Pd/
10
of phenols, and difficult to be directly used as chemicals or C-H3PO4 and RANEY® Ni-Nafion/SiO211); (2) bifunctional
fuels.3 To address this problem, hydrodeoxygenation (HDO) catalystswhichcombinetheactivehydrogenatingsitesandacidic
was reported as an effective method for the upgrade of this sites into one catalyst. Examples of such types of catalysts
phenolic mixture into conventional transport alkane fuels.4,5 are Ni/HZSM-5,12 Ru/HZSM-513 among others.14 These systems
Previous research on the HDO of phenols was mainly concen- constitute significant advances of the HDO of phenols, all of
trated on the supported sulfide CoMo and NiMo catalysts which were conducted in a monophasic system such as water
or n-decane.9–15 To our knowledge, the HDO of phenols into
alkanes has not yet been performed in a biphasic system, an
alternative solvent system to the traditional monophasic
aCollaborative Innovation Center of Chemistry for Energy Materials, Anhui Province
Key Laboratory for Biomass Clean Energy and Department of Chemistry, University
media for the production of alkanes from biomass. Thus,
we have focused on the exploration of HDO of phenols into
alkanes in biphasic systems.
Actually, biphasic systems have already been proposed for
the conversion of biomass derived compounds into various
chemicals and showed remarkable advantages over monophasic
of Science and Technology of China, 230026 Hefei, Anhui, China.
E-mail: fuyao@ustc.edu.cn
bCollege of Chemical Engineering, Nanjing Forestry University, 210037 Nanjing,
China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4gc01992j
‡These authors equally contributed to the work.
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systems.16 For example, recent work from Resasco17 and used for X-ray photoelectron spectroscopy (XPS) of the sup-
Dumesic18 research groups indicated that biphasic reaction ported Ru catalysts.
systems showed significant advantages in protecting the
2.3 Catalyst preparation
products from further degradation by extracting the products
produced from the monophasic solvent, simplifying the separ-
ation steps to achieve the final products, minimizing the side
reactions and increasing the overall yield. Moreover, many
biomass-refining processes such as the refining of bio-oil
are actually biphasic systems which contain up to 30–40%
water.17a Rather than separating out the by-products which are
hydrophilic through multiple steps, it would be better to carry
out sequential reactions in a biphasic system which avoided
complicated purification.
Herein, we report the HDO of phenols into cycloalkanes in
the biphasic system water/n-dodecane over heterogeneous
Ru catalysts supported on carbon nanotubes (CNT), a new
material with high surface area, high mechanical strength and
good chemical stability.19 The catalyst was well characterized
and tested in the HDO of eugenol. Different reaction con-
ditions were evaluated to obtain the highest product yield. The
catalyst was also applied to the HDO of other phenols and
dimeric lignin model compounds to yield alkanes. A kinetic
study was also carried out to gain preliminarily insight into
the reaction mechanism.
All of the supported Ru catalysts in this study were prepared
by the wetness impregnation methods. For 5 wt% Ru/CNT,
ruthenium(III) trichloride (RuCl3·3H2O, 0.1422 g) was dissolved
in 10 ml water, then the aqueous solution was added to a solu-
tion (30 ml H2O) containing 1.0 g CNT. The mixture was
stirred at room temperature for 12 h. After impregnation, the
mixture was transferred to an oven and dried at 120 °C for
12 h to afford the catalyst precursor. The reduction conditions
for the precursor are as follows: the temperature was raised
from room temperature to 250 °C at the rate of 5 °C min−1 and
kept at 250 °C for 4 h; the reduction gas stream was a
mixture of H2 and N2 (the flow rates were 20 ml min−1 and
100 ml min−1, respectively). After cooling to room temperature,
the resulting black powder was collected. The preparation
procedures for Ru/AC, Ru/ZrO2 and Ru/CeO2 were similar to
that for Ru/CNT.
2.4 Catalyst test
The HDO reaction was carried out in a 25 ml Parr reactor
equipped with a magnetic stirrer. In a typical experiment,
eugenol (164 mg, 1.0 mmol), the catalyst and solvents (6 ml
n-dodecane and 6 ml water) were added to the reactor. After
purging the reactor with hydrogen 3 times, it was sealed and
5.0 MPa hydrogen pressure was maintained at ambient tempe-
rature. Reactions were conducted at the corresponding tempe-
ratures. After the reaction was complete, the reactor was
cooled and the organic layer was collected and analyzed using
a gas chromatograph (GC) and a gas chromatograph-mass
spectrometer (GC-MS).
2 Experimental
2.1 Materials
Multi-walled carbon nanotubes (MWCNTs, diam: 10–20 nm,
length: 5–15 μm), 5 wt% Pt/C, AC, eugenol, di-p-tolyl ether,
tert-butylcyclohexane and benzyl phenyl ether were supplied
by TCI. 5 wt% Pd/C was supplied by Sigma Aldrich. 5 wt%
Rh/C, 5 wt% Ru/C, RuCl3·3H2O (Ru: 37 wt%), ZrO2, CeO2,
4-propylphenol and n-dodecane were supplied by Aladdin
Industrial Inc. 4-n-Propylguaiacol was prepared according to a
previous reported method.3a
3 Results and discussion
3.1 Characterization of the catalysts
Fig. 1 shows the X-ray powder diffraction (XRD) analysis of
the Ru based catalysts with different supports. No obvious
2.2 Catalyst characterization
A micromeritics ASAP 2020 analyzer (Tristar II 3020M) was
used to record nitrogen adsorption/desorption and CO adsorp-
tion isotherms. The surface area was determined through
the Barrett–Emmet–Taller (BET) method. The Barret–Joyner–
Halenda method was used to determine the average pore size
and pore volume. X-ray power diffraction (XRD) patterns of
Ru/CNT, Ru/C, Ru/ZrO2 and Ru/CeO2 were recorded on an
X’pert (PANalytical) diffractometer at 40 kV and 40 mA.
Transmission electron microscopy (TEM) microphoto-
graphs were acquired on a JEOL-2010 electron microscope.
The samples were suspended in methanol. Scanning electron
microscopy (SEM) microphotographs were obtained using a
SIRION 200 electron microscope. Scanning transmission elec-
tron microscopy (STEM) microphotographs and element maps
were recorded on a JEM-2100F electron microscope. A Thermo
Scientific Escalab 250-X-ray photoelectron spectrometer was Fig. 1 XRD patterns of the supported Ru catalysts.
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Fig. 2 (a) SEM micrograph of 5%Ru/CNT; (b) TEM micrograph of 5%Ru/
CNT.
Fig. 4 XPS spectra in the Ru 3p region for Ru/CNT.
Table 1 Hydrodeoxygenation of eugenol over various catalystsa
Fig. 3 (a) STEM micrograph of 5%Ru/CNT; (b) elemental mapping of
Ru; (c) elemental mapping of C.
Yield/%
Entry Catalyst Solvent
Conv./%
100
A
B
C
D
1
Pt/C
H2O
3.5 18.6 45.7 15.7
diffraction peaks of Ru species were observed. The XRD
pattern of Ru/CNT was similar to that from the previously
reported work.20
2
3
4
5
6
7
8
9
10
11
12
13
14
H2O–n-C12H26 100
H2O 100
H2O–n-C12H26 100
H2O 100
H2O–n-C12H26 100
H2O 100
H2O–n-C12H26 100
Ru/CNT H2O 100
H2O–n-C12H26 100
0.2
0.5
0.1
9.7
5.8 17.3 59.2
Pd/C
Rh/C
Ru/C
5.2 93.9
4.3 95.3
7.6 58.4
0.2
0.1
1.5
5.9
Fig. 2 shows the scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) images of Ru/CNT
catalysts. The structure and morphology of the catalyst can be
clearly seen from the pictures, much of the CNT’s structure
remained unchanged during the catalyst’s preparation pro-
cedure. The Ru particles were well distributed on the CNT with
a mean size of ∼5 nm. Scanning transmission electron
microscopy (STEM) and the corresponding elemental mapping
analysis of the catalyst were also carried out, and the
Ru particles can be seen clearly from the STEM images
in Fig. 3a. The elemental mapping analysis of Ru and C
(Fig. 3b–3c) of the same section of Ru/CNT gave a direct vision
of the distribution of Ru particles.
1.9 12.9 78.2
59.5 3.1 0.3
21.5 26.2 35.2
56.5
94
29.2
45.7
—
—
—
—
2.1
1.0
0.4
—
Ru/ZrO2 H2O
100
2.4 24.7
5.5 40.0
0.1
H2O–n-C12H26 100
0.2
0.2
0.2
Ru/CeO2 H2O
100
7.4 12.5 47.0
12.7 60.5 25.6
H2O–n-C12H26 100
a Reaction conditions: eugenol (1 mmol), catalysts (50 mg), n-C12H26
H2O = 6/6 ml, 5 MPa H2, 220 °C, 3 h.
–
3.2 Hydrodeoxygenation of eugenol
X-ray photoelectron spectroscopy (XPS) characterization of The initial experiments were carried out to investigate the cata-
Ru metals in the Ru/CNT catalyst is shown in Fig. 4. The Ru 3p lysts’ activities towards the HDO of eugenol in monophasic
pattern can be divided into three peaks which can be attribu- and biphasic systems. A variety of carbon-supported noble
ted to the different oxidative states of the Ru species Ru0 metal catalysts Ru/C, Pt/C, Pd/C and Rh/C were tested for the
(462.1 eV), Ru4+ (464.2 eV), and Ru4+ (hydrate) (466.8 eV).21 catalytic activities and the final products were extracted and
The ratio between metallic Ru and oxidative state Ru species analyzed by GC-MS. The products after the reaction were pro-
(Ru4+ and Ru4+ (hydrate)) was about 1.01. The high valance pylcyclohexane (A), 4-propyl-cyclohexanol (B), 2-methoxy-
Ru species might be further reduced when the catalyst was 4-propyl-cyclohexanol (C) and 2-methoxy-4-propylphenol (D)
subjected to the HDO reaction under reductive conditions at (Table 1, entry 1). However, the selectivity of the target product
the specific temperature. The BET analysis of Ru/CNT and alkane for Pt/C was only 3.5%, together with a lot of ring
other Ru catalysts is also presented in the ESI.† The surface hydrogenation products. When a biphasic system was used,
area of Ru/CNT was 201.6 m2 g−1 with a pore volume and pore the products were still a mixture with an even lower alkane
size of 0.72 m3 g−1 and 138.6 Å, respectively. The characteri- yield (Table 1, entry 2). For other supported metal catalysts
zation of the other catalysts was also carried out and presented Pd/C and Rh/C, the main products were the ring hydrogen-
in the ESI.†
ation products C, the oxygen-containing functional groups
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were difficult to be cleaved (Table 1, entries 3–6). When the directly affect the catalytic activity; (3) specific adsorption pro-
Ru/C catalyst was used, the alkane yield increased to 59.5%, perties mainly due to their peculiar morphology, the role of
the highest yield of the reactions catalyzed by the selected cata- defects and opening/closing of the tubes. Thus, the difference
lysts, which implied that Ru is more active for the cleavage of in catalytic performances between Ru/CNT and other Ru based
the C–O bonds (Table 1, entry 7). However, the total product catalysts in the biphasic system may be due to the unique
yield extracted from the aqueous solution was only about 60%, structure of the CNT that promoted the contact of the catalysts
and the other products cannot be collected and detected, and substrates. All of the above advantages of Ru/CNT may
which might be attributed to the decomposition of the formed lead to a superior activity in the HDO reactions in biphasic
alkanes under the current reaction conditions. Similar results systems.
can also be found in the other reported studies.14b,15a It is
noteworthy that when the biphasic system H2O/n-C12H26 was
3.3 Effect of different organic solvents
used, the total detected products were up to 83% which was
higher than that from the aqueous system (Table 1, entry 8).
The possible reason for this phenomenon was that the pro-
ducts could be quickly moved into the organic phase after its
production. Meantime, the raw materials would be converted
into target products in quantity in the aqueous phase, leading
to an obvious increase in alkane yields.
According to the results in Table 1, the organic solvent had a
significant impact on the product distribution and selectivity.
Therefore, several organic solvents such as methyl isobutyl
ketone (MIBK), γ-valerolactone (GVL), cyclohexane (C6H12),
n-octane (n-C8H18), n-tetradecane (n-C14H30) and n-dodecane
(n-C12H26) were used for the HDO of eugenol under the
specific conditions. Fig. 5 shows the propylcyclohexane yields
with different organic solvents. Obviously, the biphasic
systems with oxygen-free alkane solvents such as cyclohexane,
n-octane, n-tetradecane and n-dodecane led to higher alkane
yields over the aqueous system. In contrast, the use of oxygen-
containing organic solvents such as methyl isobutyl
ketone and γ-valerolactone as the organic phase for the
biphasic systems gave lower propylcyclohexane yields. Besides,
the oxygen-containing organic solvents were not stable under
the HDO reaction conditions and reacted together with the
eugenol. Thus, long-chain alkanes would be better candidates
for the biphasic solvent systems.
In order to further improve the product yield, we next
turned to the Ru based catalysts with different supports which
might also be a critical factor in determining the product yield
and selectivity. We were excited to find that an almost quanti-
tative yield of alkanes (98%, including 94% propylcyclohexane
and 4% propylcyclopentane) was achieved when the biphasic
solvent system H2O/n-C12H26 was used in the Ru/CNT catalyzed
HDO reaction under the specific reaction conditions. In con-
trast, the alkane product propylcyclohexane obtained in the
monophasic system (H2O) was only 56.5%. The addition of the
organic solvent n-C12H26 to the aqueous system increased
the collectable alkane products to 98%. When the reaction was
carried out in pure n-dodecane, the yield of propylcyclohexane
was only 4% (Fig. 6). When the HDO reaction was carried out
in pure n-dodecane, the yield of propylcyclohexane was only
4% and the main product was 4-propyl-cyclohexanol, the main
intermediate of the reaction. This result revealed that the
aromatic ring could be hydrogenated in the organic solvent
over the hydrogenating site but the deoxygenation step (dehy-
dration) was difficult to proceed in the absence of acids. Pre-
vious work on the HDO of phenols in water showed that the
H2O could generate the in situ H+ for the dehydration of the
oxygen-containing groups at the evaluated temperature.22
Thus, we reasoned that water is a key factor for the HDO of
lignin derived phenols. The above results implied that bipha-
sic systems did have the advantages in protecting the products
and increasing the product yield over monophasic systems.
Also, for comparison, Ru/ZrO2 and Ru/CeO2 were prepared and
used in the model reaction, but showed poor catalytic activities
in catalyzing the eugenol into alkanes. The biphasic systems
with these two catalysts still gave higher product yields than
aqueous systems which further confirmed that the biphasic
systems could protect the organic molecules from decompo-
sition and increased the product yield.
3.4 Effect of the ratio of organic solvent/water
To better understand the influence of the organic phase on
the product distribution, further studies were carried out to
investigate the effect of the ratio of organic solvent/water.
Fig. 6 presents the product distribution when using different
ratios of organic solvent/water. The reaction carried out in
monophasic water gave 56.5% yield of propylcyclohexane. The
addition of about 4% (v/v) organic solvent n-dodecane to water
increased the alkane yield to 82%. The reaction reached a
maximum alkane yield of 94% when the ratio was 1 : 1. Further
increasing the organic solvent ratio led to a decrease of the
As reported in the previous literature,23 the advantages of
CNT supports lie in the following aspects: (1) the mesoporosity
of CNT allows significant decreases in mass-transfer
Fig. 5 Effect of different organic solvents. Reaction conditions:
eugenol (1 mmol), catalysts (50 mg), organic solvent–water = 6/6 ml,
limitations; (2) specific metal–support interactions which can 5 MPa H2, 220 °C, 3 h.
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Fig. 8 Effect of the reaction pressure. Reaction conditions: eugenol
(1 mmol), catalysts (50 mg), n-C12H26–H2O = 6/6 ml, 220 °C, 3 h.
Fig. 6 Effect of the dosage of organic solvent, the total volume of the
solvent: 12 ml. Reaction conditions: eugenol (1 mmol), catalysts (50 mg),
5 MPa H2, 220 °C, 3 h.
revealed that the desired reaction temperature could effectively
accelerate the HDO process and minimize the decomposition
of the alkane to afford the highest yield of propylcyclohexane.
yield. It is worth noting that the yield of propylcyclohexane was
only 4% when the reaction was carried out in pure organic
phase, and the major products were 2-methoxy-4-propyl-cyclo-
hexanol and 4-propyl-cyclohexanol, which indicated that water
is essential for these reactions to afford alkanes. A possible
explanation for the above results was that the H+ ion generated
from water under the hydrothermal conditions could act
as the acid that helps with the deoxygenation of the oxygen-
containing groups.
3.6 Effect of the reaction pressure
In addition to the reaction temperature, the reaction pressure
was another important parameter to establish suitable con-
ditions for the efficient conversion of eugenol. Fig. 8 shows
the reaction product distribution under different reaction
pressures. The conversions of eugenol were >99% under all the
investigated pressures from 1.0 MPa to 6.0 MPa. The reaction
conducted under 1 MPa H2 afforded 2-methoxy-4-propyl-
phenol (22.6%), 2-methoxy-4-propyl-cyclohexanol (12.7%) and
4-propyl-cyclohexanol (42.4%), the yield of propylcyclohexane
was just 5.4%. Further increasing the reaction pressure from
2.0 MPa to 5.0 MPa led to an increase of the propylcyclohexane
yield (from 31.4% to 94%). However, higher pressure (6.0 MPa)
would lead to the cleavage of C–C bonds of propylcyclohexane
and result in a decreased propylcyclohexane yield (90%).
3.5 Effect of the reaction temperature and hydrogen pressure
Fig. 7 shows the HDO results at different reaction tempera-
tures. When the reaction was carried out at a lower reaction
temperature (160–200 °C), 4-propyl-cyclohexanol was the main
product (91%) and only a small amount of the alkane was
formed. Then, increasing the reaction temperature led to a
sharp increase of the alkane yield, and 4-propyl-cyclohexanol
decreased to a negligible level, which indicated that 4-propyl-
cyclohexanol might be the intermediate in the reaction.
Increasing the reaction temperature was favorable for the deoxy-
genation step to remove the oxygen-containing groups. The
model reaction reached a maximum yield of 94% at 220 °C.
Further increasing the temperature led to a decrease of the
propylcyclohexane yield which may be attributed to the
decomposition of the alkanes to small molecules that cannot
be detected. The experiments on the reaction temperature
3.7 Hydrodeoxygenation of other phenolic compounds
To investigate the scope of the current catalytic system in the
biphasic system, a series of lignin-derived phenolic com-
pounds (including monomers and dimers) were evaluated
under the optimized conditions (Table 2). Hydrodeoxygenation
of lignin-derived phenolic monomers containing six to nine
carbon atoms such as anisole, guaiacol, catechol, 4-methylphe-
nol, 4-ethylguaiacol and 4-propylphenol gave moderate to good
yields of alkanes (Table 2, entries 1–12). Under the optimized
conditions, high selectivities (>90%) towards alkanes were
achieved for most of the cases explored, at the full conversion
of phenols. For phenols with lower reactivities, the alkane
selectivities were lower (Table 2, entries 6, 7 and 12). We
further investigated the HDO of more complicated dimeric
lignin model compounds. According to the reported work by
Lercher, Zhao et al. β-O-4, α-O-4, and 4-O-5 linkages are the
most common types of C–O bonds in hardwood lignin.24 All of
these three linkages could be converted to the corresponding
alkanes effectively, as shown in Table 2, entries 13–17.
Besides, the phenols separated from crude bio-oil were also
investigated in the biphasic systems (see ESI†). The crude bio-
oil was obtained by flash pyrolysis of rice husk at 550–600 °C
Fig. 7 Effect of the reaction temperature. Reaction conditions: eugenol
(1 mmol), catalysts (50 mg), n-C12H26–H2O = 6/6 ml, 5 MPa H2, 3 h.
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Table 2 Hydrodeoxygenation of other lignin-derived phenolic com-
pounds over Ru/CNT in a biphasic system
Entrya
1
Substrates
GC yield (%)
91
92
91
2
3
4
Fig. 9 Recycle of the catalyst and the XPS analysis of the used catalyst.
Reaction conditions: eugenol (1 mmol), catalysts (50 mg), n-C12H26
–
H2O = 6/6 ml, 5.0 MPa H2, 220 °C, 3 h.
90
phenols and alkanes is also presented in the ESI.† The appli-
cation of the biphasic system to these phenols further demon-
strated that the biphasic system with the Ru/CNT catalyst has
great potential for application in the upgrading of bio-oil.
5
6
92
85
7
86
3.8 Recyclability of the catalyst
The recyclability of the catalyst was an important parameter
for the heterogeneous catalysis. Thus, the Ru/CNT catalyst was
collected after the reaction and washed with dodecane; after
that, the catalyst was directly used for the next run. The
product yields are listed in Fig. 9. The catalyst was used five
times and it maintained good activity, with only a slight
decrease in the product yield. The XPS analysis of the used
catalysts showed that Ru(IV) and Ru(IV)(hydrate) particles were
further reduced to Ru0 particles under the reductive reaction
conditions.
8
9
96
94
10
11
12
94
90
80
3.9 Mechanism
In order to gain preliminary insight into the reaction mecha-
nism, the reaction was traced and analyzed at different reaction
times (Fig. 10). The intermediates 4-propylcyclohexanol and
2-methoxy-4-propyl-cyclohexanol were formed as the reaction
started at the first 10 min, the propylcyclohexane yield was
only 3.1%. Then, 2-methoxy-4-propyl-cyclohexanol decreased
13
14
15
89
64
44
95
90
gradually while 4-propylcyclohexanol increased with
a
maximum yield of 74.3% at 30 min which was decreased until
the end of the reaction. The alkane product propylcyclohexane
16
17
61
53
48
a Reaction conditions: substance (1 mmol), catalysts (50 mg),
n-C12 26–H2O = 6/6 ml, 5.0 MPa H2, 220 °C, 3 h.
H
according to the previous work and phenols were separated
from the crude bio-oil by a modified glycerol-assisted distilla-
tion technology.25 The separated phenols were treated with a
NaHCO3 solution, extracted with hexane and evaporated. The
mixture of phenols was then subjected to the HDO procedure
in biphasic systems. About 25 wt% alkanes were obtained after
the HDO reaction. The GC-MS analysis of the extracted
Fig. 10 Effect of the reaction time. Reaction conditions: eugenol
(1 mmol), catalysts (50 mg), n-C12H26–H2O = 6/6 ml, 5 MPa H2, 220 °C.
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Fig. 11 Proposed reaction pathway.
increased along with the reaction time and reached the
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reaction mechanism was proposed, similar to the work
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In conclusion, the hydrodeoxygenation of lignin-derived phe-
nolic monomers and dimers into alkane fuels has been per-
formed with Ru/CNT in the n-dodecane/water biphasic system.
Under the optimized conditions, >99% conversion of eugenol
with a high alkane selectivity (98%, including 94% propylcyclo-
hexane and 4% propylcyclopentane) was achieved. Biphasic
systems showed superior advantages over monophasic systems
in the HDO reactions. Besides, the unique structure of CNT
helped in improving the selectivity towards propylcyclohexane.
The kinetic experiments revealed that eugenol was converted
into propylcyclohexane through the reaction intermediates
2-methoxy-4-propyl-cyclohexanol and 4-propyl-cyclohexanol.
The current research emphasized an efficient biphasic catalyst
system for transforming lignin-derived phenolic compounds
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