Hydrodeoxygenation of phenols as lignin models under acid-free conditions with carbon-supported platinum catalysts

Article abstract of DOI:10.1039/c1cc14859a

Carbon-supported Pt catalysts are highly active and reusable for the aqueous-phase hydrodeoxygenation of phenols as lignin models without adding any acids. It is suggested that Pt/carbon facilitates the hydrogenation of phenols and the hydrogenolysis of the resulting cyclohexanols. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc14859a

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COMMUNICATION
Hydrodeoxygenation of phenols as lignin models under acid-free
conditions with carbon-supported platinum catalystsw
Hidetoshi Ohta, Hirokazu Kobayashi, Kenji Hara and Atsushi Fukuoka*
Received 6th August 2011, Accepted 27th September 2011
DOI: 10.1039/c1cc14859a
Carbon-supported Pt catalysts are highly active and reusable
for the aqueous-phase hydrodeoxygenation of phenols as lignin
models without adding any acids. It is suggested that Pt/carbon
facilitates the hydrogenation of phenols and the hydrogenolysis
of the resulting cyclohexanols.
Scheme 1 Reaction pathway for the hydrodeoxygenation of phenols.
Bio-oils derived from lignocellulosic biomass have attracted
significant attention as a feedstock for the renewable produc-
tion of biofuels that are a promising alternative to fossil fuels.1
Lignin is the second most abundant natural organic polymer
consisting of methoxylated phenylpropane units, and is a rich
source of phenolic bio-oils.2,3 However, the bio-oils are highly
oxygenated, thus leading to undesirable properties and chemical
instability. Therefore, upgrading is necessary for the utilization
of bio-oils.1,3 Hydrodeoxygenation with supported CoMo and
NiMo sulfides is the most common process for upgrading such
bio-oils into transportation fuels.4 However, these systems
suffer from coke formation, sulfur contamination, and water-
induced catalyst deactivation.5 For that reason, non-sulfided
catalyst systems have been developed for the hydrodeoxygenation
of phenolic bio-oils.6
catalysts (Scheme 1). Here, the deoxygenation of phenols
includes the hydrogenation of aromatic rings and the sub-
sequent hydrogenolysis of the C–O bond of cyclohexanols.
A series of carbon-supported Pt catalysts (2 wt% Pt loading)
were prepared by a typical impregnation method using an
aqueous solution of H2PtCl6 with different types of carbon
materials such as AC(N) and AC(W) (activated carbons from
Norit and Wako), CMK-3 (mesoporous carbon),9 MWCNT
(multi-walled carbon nanotube, TCI), and BP2000 (carbon
black, Cabot). The catalysts were characterized by N2 adsorp-
tion, XRD, and TEM analyses. The characterization data are
summarized in Table S1 and Fig. S1 and S2 in ESI.w
The catalytic performances of the carbon-supported Pt
catalysts were examined in the hydrodeoxygenation of 4-propyl-
phenol (1) in water at 280 1C under 4 MPa H2 (initial pressure at
RT) (Table 1). The Pt/AC(N) catalyst, which was prepared from
an easily available AC(N) support, showed remarkably high
catalytic activity to give propylcyclohexane (2) in 97% yield
(entry 1, Fig. S3 in ESIw). Pt/CMK-3 and Pt/MWCNT also
afforded 2 in slightly lower yields (entries 2 and 3). Pt/AC(W)
gave 2 in 73% yield and 4-propylcyclohexanol (3) in 16% yield
(entry 4), suggesting the reaction pathway as shown in Scheme 1.
By contrast, Pt/BP2000 provided markedly lower conversion of 1
(16%) and lower yield of 2 (9%) (entry 5). Oxide-supported
Pt/ZrO2, Pt/TiO2, and Pt/CeO2 were moderately effective for this
reaction, affording 2 in good yields (entries 6–8). In these cases,
propylbenzene (5) was also obtained in 3–10% yield probably via
the direct hydrogenolysis of 1. In sharp contrast, Pt/g-Al2O3 gave
no reaction due to the structural transformation of g-Al2O3 into
boehmite [AlO(OH)] during the reaction (entry 9, Fig. S4 in
ESIw). Besides, Rh, Ru, and Pd catalysts supported on AC(N)
were also used (entries 10–12), but these catalysts gave lower
yields of 2 than the Pt/AC(N) catalyst.10 With the highly active
Pt/AC(N) catalyst, a reaction of 1 on a gram-scale was carried
out with a high substrate to catalyst molar ratio (S/C) of 1000
at 280 1C for 2 h, which afforded 2 in 499% yield (entry 13).
In the large-scale reaction, an excellent yield (499%) of 2 was
Recently, Kou and Lercher et al. reported the aqueous-phase
hydrodeoxygenation of phenolic bio-oils into cycloalkanes by
using Pd/C with a liquid acid catalyst (H3PO4).7 The reaction
consists of metal-catalyzed hydrogenation of the aromatic ring/
cycloalkene and acid-catalyzed dehydration of cycloalkanol to
cycloalkene. However, the addition of an acid catalyst is necessary
and the recovery of the acid from the reaction mixture is difficult.
The combination of RANEYs Ni and a solid acid (Nafion/SiO2) has
also been reported,8 but the thermal stability of Nafion is not
sufficient at high temperature (typically 300 1C). Accordingly, the
development of new heterogeneous catalysts with high activity and
durability is still important for the hydrodeoxygenation reaction.
In the present study, we found that readily available carbon-
supported Pt catalysts showed high activity and durability in
the hydrodeoxygenation of phenols without adding any acid
Catalysis Research Center, Hokkaido University, Kita 21 Nishi 10,
Kita-ku, Sapporo, Hokkaido 001-0021, Japan.
E-mail: fukuoka@cat.hokudai.ac.jp; Fax: +81-11-706-9139;
Tel: +81-11-706-9140
w Electronic supplementary information (ESI) available: Detailed
preparation, catalytic reaction, characterization methods including
XRD patterns, TEM images and N2 adsorption results in this work.
See DOI: 10.1039/c1cc14859a
c
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Chem. Commun., 2011, 47, 12209–12211 12209

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Table 1 Hydrodeoxygenation of 4-propylphenol (1) with supported
metal catalystsa
Table 2 Hydrodeoxygenation of bio-derived phenolic oils with the
Pt/AC(N) catalysta
Yieldb (%)
MeOH
2
Entry Substrate
Conversion (%)
100
Yield (%)
16
78
66
65
5
5
6
2
3
4
5
Entry Catalyst
Conversion (%)
1
2
3
4
5
6
7
8
Pt/AC(N)b
Pt/CMK-3
100
100
97
92
o1
0
0
0
o1
o1
0
0
0
0
0
0
0
2
0
0
0
0
0
0
0
1
10
3
8
0
17
18
100
100
2
2
Pt/MWCNTc 100
92
73
9
66
77
83
0
83
55
22
499
499
499
1
16
1
Pt/AC(W)d
Pt/BP2000e
100
16
100
100
100
0
100
100
84
100
100
100
f
Pt/ZrO2g
Pt/TiO2
Pt/CeO2
o1
2
h
o1
0
o1
o1
57
0
i
9
Pt/g-Al2O3
19
100
97
58
67
10
12
3
2
10
11
12
13j,k
14j,l
15j,m
Rh/AC(N)b
Ru/AC(N)b
Pd/AC(N)b
Pt/AC(N)b
Pt/AC(N)b
Pt/AC(N)b
0
0
20c
6
o1
a
Reaction conditions: substrate (2.0 mmol), 2 wt% Pt/AC(N) (98 mg,
S/C = 200), water (40 ml), initial H2 pressure at RT = 4 MPa, 280 1C,
6 h, stirred at 600 rpm. Yield based on carbon. 6 (10 mmol, 1.66 g),
2 wt% Pt/AC(N) (488 mg, S/C = 200), without water, 300 1C, 24 h.
0
0
0
0
0
b
c
a
Reaction conditions: 4-propylphenol 1 (2.0 mmol, 272 mg), catalyst
(2 wt% metal, S/C = 200), water (40 ml), initial H2 pressure at RT =
b
4 MPa, 280 1C, 1 h, stirred at 600 rpm. Activated carbon, SX Ultra
(Norit). Multi-walled carbon nanotube (TCI). Activated carbon
c
d
e
(Wako). Carbon black Black Pearls 2000 (Cabot). JRC-ZRO-2.
f
g
h
i
P-25, Degussa. JRC-CEO-2. JRC-ALO-2. 10 mmol (1.36 g) of 1
j
k
l
m
was used (S/C = 1000). 280 1C, 2 h. 240 1C, 10 h. Without water,
280 1C, 4 h.
also obtained even at 240 1C with a longer reaction time of 10 h
(entry 14). Furthermore, Pt/AC(N) also catalyzed the conversion
of 1 into 2 without water-solvent (entry 15), which shows that the
catalyst is applicable both in water and under the solvent-free
conditions.
The hydrodeoxygenation of guaiacols and syringols was also
studied with the Pt/AC(N) catalyst in water solvent (Table 2).
Under the conditions (4 MPa H2, 280 1C, 6 h), 4-propylguaiacol
(6), 4-allylguaiacol (7), and 4-acetonylguaiacol (8) were effi-
ciently converted to propylcyclohexane 2 in 65–78% yields
(entries 16–18, Fig. S5, ESIw). A derivative of syringol, 4-allyl-
2,6-dimethoxyphenol (9), also afforded 2 in 58% yield (entry 19).
In these reactions, the methoxy group was hydrolyzed to give
methanol in 2–3% yields.7,8 The low yields of methanol might
be due to its evaporation or decomposition. In addition, propyl-
cyclopentane (10) with a shorter carbon-chain (C8) was obtained
in 5–10% yields possibly via skeletal isomerization and hydro-
genolysis of cycloalkane.7,8 Without the solvent, the reaction of 6
gave the hydrodeoxygenation products 2 and 10 in good yields at
300 1C (entry 20).11 The formation of small amounts of 1 and 5
from 6 suggests the aromatic C–O bond cleavages under the
solvent-free conditions.12 These results indicate that this catalyst
system is applicable to upgrading various phenolic bio-oils into
cycloalkanes.
Fig. 1 Reuse experiments in the hydrodeoxygenation of 4-propyl-
phenol (1) to propylcyclohexane (2) by Pt/AC(N) in water (a) and
under solvent-free conditions (b). Reaction conditions: see entries 13
and 15 in Table 1.
subjected to the reuse experiments to give 2 in 499% yields
(Fig. 1a and Table S2 in ESIw). The pH of the aqueous solution
after the first reaction was neutral (pH = 7.0), showing no
leaching of acid components from the Pt/AC(N) catalyst. The
experiments under the solvent-free conditions also demon-
strated the reusability of the Pt/AC(N) catalyst (Fig. 1b and
Table S2 in ESIw). The XRD patterns of the recovered Pt
catalysts showed that the Pt nanoparticles still retained their
small sizes (o2 nm) (Fig. S6 in ESIw), thus indicating that the
catalysts were durable under the catalytic conditions.
To elucidate the reaction pathway from 1 to 2, the hydro-
deoxygenation of intermediate
3 was also investigated
(Table 3).13 Without catalyst, 3 was dehydrated to form
4-propylcyclohexene (11) in 11% yield (entry 21), suggesting
that the dehydration was catalyzed by in situ generated protons
(pKw = 11.2 at 280 1C).14–16 Accordingly, the addition of a base
into the reaction completely suppressed the formation of 11
(entry 22). The Pt/carbon catalysts in Table 3 greatly promoted
The reusability of the Pt/AC(N) catalyst was tested for the
aqueous-phase reaction of 4-propylphenol (1) at 280 1C with
the S/C ratio of 1000. After the first reaction (Table 1, entry 13),
Pt/AC(N) was recovered by centrifugation, which was successively
c
12210 Chem. Commun., 2011, 47, 12209–12211
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Table 3 Hydrodeoxygenation of 4-propylcyclohexanol (3) in watera
4 (a) E. Furimsky, Appl. Catal., A, 2000, 199, 147; (b) M. J. Girgis
and B. C. Gates, Ind. Eng. Chem. Res., 1991, 30, 2021.
5 (a) E. Furimsky and F. E. Massoth, Catal. Today, 1999, 52, 381;
(b) A. Centeno, E. Laurent and B. Delmon, J. Catal., 1995,
154, 288; (c) E. Laurent and B. Delmon, J. Catal., 1994,
146, 281; (d) E. Laurent and B. Delmon, Ind. Eng. Chem. Res.,
1993, 32, 2516; (e) A. Vuori, A. Helenius and J. B.-S. Bredenberg,
Appl. Catal., 1989, 52, 41.
6 (a) W. Wang, Y. Yang, H. Luo, T. Hu and W. Liu, React. Kinet.,
Mech. Catal., 2011, 102, 207; (b) N. Yan, Y. Yuan, R. Dykeman,
Y. Kou and P. J. Dyson, Angew. Chem., Int. Ed., 2010, 49, 5549;
(c) S. Echeandia, P. L. Arias, V. L. Barrio, B. Pawelec and J. L.
G. Fierro, Appl. Catal., B, 2010, 101, 1; (d) D.-Y. Hong,
S. J. Miller, P. K. Agrawal and C. W. Jones, Chem. Commun.,
2010, 46, 1038; (e) V. A. Yakovlev, S. A. Khromova,
O. V. Sherstyuk, V. O. Dundich, D. Y. Ermakov, V. M.
Novopashina, M. Y. Lebedev, O. Bulavchenko and V. N. Parmon,
Catal. Today, 2009, 144, 362; (f) A. Gutierrez, R. K. Kaila,
M. L. Honkela, R. Slioor and A. O. I. Krause, Catal. Today, 2009,
147, 239; (g) S. Ramanathan and S. T. Oyama, J. Phys. Chem., 1995,
99, 16365.
Yield (%)
11
2
Entry
Catalyst
Conversion (%)
21
22
23
24
25
26
27
28
No catalyst
No catalystb
Pt/AC(N)
Pt/CMK-3
Pt/MWCNT
Pt/AC(W)
Pt/BP2000
Pt/AC(N)b
15
2
100
99
97
64
100
66
11
0
0
0
0
0
0
0
0
0
95
82
87
55
84
58
7 (a) C. Zhao, Y. Kou, A. A. Lemonidou, X. Li and J. A. Lercher,
Angew. Chem., Int. Ed., 2009, 48, 3987; (b) C. Zhao, J. He, A. A.
Lemonidou, X. Li and J. A. Lercher, J. Catal., 2011, 280, 8.
8 C. Zhao, Y. Kou, A. A. Lemonidou, X. Li and J. A. Lercher,
Chem. Commun., 2010, 46, 412.
a
Reaction conditions: 4-propylcyclohexanol (2.0 mmol), Pt catalyst
(2 wt% Pt, 98 mg, S/C = 200), water (40 ml), initial H2 pressure at RT =
b
4 MPa, 280 1C, 1 h, stirred at 600 rpm. CaCO3 (4.0 mmol), pH = 10.
9 S. Jun, S. H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu,
T. Ohsuna and O. Terasaki, J. Am. Chem. Soc., 2000, 122,
10712.
10 In the case of Rh/AC(N) (Table 1, entry 10), propylcyclopentane
(o1%) was formed, but no other products were observed on
GC of the liquid and gas phases. In the reaction of Ru/AC(N)
(entry 11), propylcyclopentane (11%), 2-methylpropylcyclopentane
(8%), and some unidentified products were detected in the liquid
phase, where the total carbon yield in the liquid phase was 86%.
Methane (3%) was observed in the gas phase.
11 In this reaction, 1 (1%), 3 (o1%), 5 (o1%), 2-methoxy-4-
propylcyclohexanol (o1%), 2-methylpropylcyclo-pentane (1%),
methane (3%), and minor unidentified products were observed.
12 (a) T. Nimmanwudipong, R. C. Runnebaum, D. E. Block and
B. C. Gates, Catal. Lett., 2011, 141, 779; (b) X. Zhu, L. L. Lobban,
R. G. Mallinson and D. E. Resasco, J. Catal., 2011, 281, 21.
13 When the reaction of 3 was performed with Pt/AC(N) under 1 atm
of He atmosphere, the dehydrogenation of 3 gave a mixture of
4-propylphenol 1 (15%) and 4-propylcyclohexanone 4 (10%)
together with propylcyclohexane 2 (2%), 3 (57%), and propyl-
benzene 5 (1%). Thus, the reaction of 3 was carried out under
pressurized H2 to evaluate the hydrogenolysis ability of the Pt/
carbon catalysts.
14 (a) N. Akiya and P. E. Savage, Chem. Rev., 2002, 102, 2725;
(b) P. E. Savage, Chem. Rev., 1999, 99, 603; (c) W. L. Marshall and
E. U. Franck, J. Phys. Chem. Ref. Data, 1981, 10, 295.
15 (a) N. Akiya and P. E. Savage, Ind. Eng. Chem. Res., 2001,
40, 1822; (b) B. Kuhlmann, E. M. Arnett and M. Siskin, J. Org.
Chem., 1994, 59, 3098.
the conversion of 3 to 2, and among the tested catalysts,
Pt/AC(N) afforded the highest yield of 2 (entry 23, 95%). Even
in the presence of a base, the reaction by Pt/AC(N) gave 2 in
good yield (entry 28). The promotional effect is probably due to
the enhancement of C–O bond hydrogenolysis by Pt/carbon.17
In summary, we have developed a highly efficient carbon-
supported Pt catalyst for the hydrodeoxygenation of phenols
under acid-free conditions. The Pt/AC(N) catalyst is prepared
from the readily available AC(N) support and is reusable
without loss of catalytic activity. The detailed mechanism
and the practical application to lignin are now under study.
This work was financially supported by JSPS KAKENHI
(20226016).
Notes and references
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17 NH3-TPD profiles of Pt/AC(N) and AC(N) showed no peak,
suggesting that Pt/AC(N) and AC(N) had no acidity (Fig. S7 in
ESIw). Therefore, acid-catalysis of Pt/AC(N) in the reaction of 3
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 12209–12211 12211