Hydrodeoxygenation and coupling of aqueous phenolics over bifunctional zeolite-supported metal catalysts

Article abstract of DOI:10.1039/b918209h

Pt supported on HY zeolite is successfully used as a bifunctional catalyst for phenol hydrodeoxygenation in a fixed-bed configuration at elevated hydrogen pressures, leading to hydrogenation-hydrogenolysis ring-coupling reactions producing hydrocarbons, some with enhanced molecular weight.

Full text of DOI:10.1039/b918209h

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Hydrodeoxygenation and coupling of aqueous phenolics over bifunctional
zeolite-supported metal catalysts
Do-Young Hong,a Stephen J. Miller,b Pradeep K. Agrawala and Christopher W. Jones*a
Received (in Cambridge, UK) 7th September 2009, Accepted 30th November 2009
First published as an Advance Article on the web 16th December 2009
DOI: 10.1039/b918209h
Pt supported on HY zeolite is successfully used as a bifunctional
catalyst for phenol hydrodeoxygenation in a fixed-bed con-
figuration at elevated hydrogen pressures, leading to hydrogenation–
hydrogenolysis ring-coupling reactions producing hydrocarbons,
some with enhanced molecular weight.
molecular weight and thus of moderate value as a fuel. To this
end, an important goal in biomass upgrading is often molecular
weight enhancement, producing molecules of appropriate
mass for use as gasoline or diesel fuels.20–22
Here we report the use of bifunctional, zeolite-supported
metal catalysts for the combined hydrodeoxygenation–
hydrogenation–coupling of phenol, selected as a representative
bio-oil model compound, in a fixed-bed reactor at elevated
hydrogen pressures up to 4 MPa. Pt/HY catalysts are shown
to be efficient at hydrodeoxygenation and ring saturation, with
some ring-coupling producing 12 carbon products.
Bio-oils obtained from fast pyrolysis of biomass have potential
as feedstocks for chemical production1–3 and as clean renewable
liquid fuels.4–6 However, bio-oils must be upgraded if they are
to be used as replacements or supplements for typical diesel
or gasoline transportation fuels. This is because bio-oils
inherently have some non-ideal properties, including low
heating values, high viscosity, incomplete volatility, and chemical
instability, due to the large oxygen content (35–40 wt%) of the
liquid.7,8 Upgrading of bio-oils to conventional transport fuel
requires significant deoxygenation, which can be accomplished
by two main routes: hydrodeoxygenation and zeolite upgrading.8,9
Especially hydrodeoxygenation has been deemed to be an
effective method for bio-oil upgrading.9
The supported platinum catalyst was prepared by conven-
tional impregnation using tetraamine platinum(II) nitrate
(Pt(NH3)4(NO3)2). After impregnation, the sample was dried
at 373 K for 12 h, then calcined at 773 K for 4 h. The catalyst
was pretreated in a hydrogen flow (50 cm3 minÀ1) at 773 K and
atmospheric pressure for 1 h and cooled to the selected
reaction temperature under hydrogen flow. The hydro-
deoxygenation of aqueous phenol was conducted in a fixed-bed
flow reactor at a temperature of 473–523 K and at a pressure
of 4 MPa. The products were analyzed by on-line gas
chromatography with an FID detector using an HP-1ms
column for hydrocarbon analysis and a TCD detector using
an HP-PLOT S column for analysis of hydrogen and light
hydrocarbon products.
Most hydrodeoxygenation studies have focused on sulfided
CoMo and NiMo-based catalysts7,10,11 that are industrial
hydrotreating catalysts developed for removal of sulfur,
nitrogen and oxygen from petrochemical feedstocks. However,
these catalysts may be less suitable for hydrodeoxygenation of
bio-oil because of the desire to prevent addition of sulfur to the
inherently low sulfur biomass-derived feed, fast deactivation
of the catalysts and poisoning by water molecules.12
Therefore, non-sulfided catalysts, such as Pt/SiO2–Al2O3,13
vanadium nitride,14 Pd/C,15 Mo–Ni/g-Al2O3,16 and Ni–Cu/
Al2O3,17 have been evaluated for the hydrodeoxygenation of
the bio-oils.
To gain insight into the reaction pathway for the hydro-
deoxygenation of aqueous phenol over the bifunctional zeolite
catalyst, we varied the space velocity (WHSV) by conducting
numerous experiments at different contact times. Fig. 1 shows
a plot of monocyclic products selectivity versus phenol
conversion. To adjust the phenol conversion, we carried out
A significant fraction of fast pyrolysis and other bio-oils is a
mixture of phenol compounds. In some cases, up to 20–30%
of the organic phase produced by fast pyrolysis can be
composed of phenolic species.9,18,19 In addition, if the saccharide
fraction of lignocellulosic biomass is used for production of
alcohols, often the phenol-rich lignin fraction is available at
low cost as a feedstock for other uses. This lignin can be
pyrolyzed or otherwise broken down into a mixture of
phenolic species and used as a feedstock for transportation
fuel production.8 However, most of the phenolic fragments
will be 6–9 carbons in mass, and after deoxygenation and
ring saturation, the hydrocarbons formed will be of lower
a School of Chemical & Biomolecular Engineering, Georgia Institute of
Technology, 311 Ferst Drive NW, Atlanta, GA 30332, USA.
E-mail: cjones@chbe.gatech.edu; Fax: +1 404 894 2866;
Tel: +1 404 385 1683
Fig. 1 Plot of monocyclic products selectivity versus phenol conversion.
Reaction conditions: temperature, 473 K; H2 pressure, 4 MPa; catalyst
loading, 100 mg; catalyst, 0.5 wt% Pt/HY (SiO2/Al2O3 = 12); WHSV,
5–20 hÀ1; H2O content, 5 wt%.
b Chevron Energy Technology Company, 100 Chevron Way, 10-3436,
Richmond, CA 94802, USA
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8 reactions with various space velocities (5–20 hÀ1), with all
other conditions fixed. The data shown in Fig. 1 represent the
activity and selectivity after 2 hours on-stream; at this time
there was a pseudo-steady state, and the catalyst activity was
relatively stable. Each point in Fig. 1 represents a different run.
It can be seen that cyclohexane is the predominant product
with a selectivity of more than 60% even at low phenol
conversions. The selectivity to cyclohexane increased almost
linearly with increasing phenol conversion. At low phenol
conversion (B13%), cyclohexanol and cyclohexane were the
most abundant products, with small quantities of cyclohexene
(B4.5%) and benzene (B1.5%) also observed. The selectivity
to cyclohexanol steeply decreased with increasing phenol
conversion, with a significant drop around 25% phenol
conversion. On the other hand, the selectivity to cyclo-
hexanone sharply increased with conversion along with the
decline of cyclohexanol selectivity, consistent with less efficient
hydrogenation, after which the selectivity to this product also
gradually decreased with conversion.
Scheme 1 Reaction pathway of phenol to monocyclics in the catalytic
hydrogenation treatment.
A series of bicyclic products formed from the coupling of
two phenol-derived ring species was also observed. Specifically,
cyclohexyl phenol, cyclohexyl cyclohexanone and bicyclohexyl
were routinely observed (Fig. 2). At low phenol conversion,
cyclohexyl phenols were predominantly produced among the
bicyclic products and the selectivity to this product gradually
decreased with an increase in phenol conversion. The selectivity
to 2-cyclohexyl cyclohexanone initially increased with increasing
phenol conversion, and then it decreased at higher levels of
conversion. The formation of bicyclohexyl increased as the
selectivity of cyclohexyl phenol and 2-cyclohexyl cyclohexanone
decreased, suggesting these compounds are consumed to
produce bicyclohexyl. At high phenol conversion, a small
amount of methyl cyclopentyl cyclohexane was found. A trace
of tricyclic compounds was also observed.
Traces of cyclohexenol, which is known to be an intermediate
product of phenol hydrogenation,23 were only detected at
phenol conversions less than 10%. These results suggest that
phenol is hydrogenated to cyclohexenol as an intermediate
product in the initial steps, and then this species is rapidly
converted to cyclohexanol or cyclohexanone. Under various
reaction conditions using different catalysts, some authors
have suggested that phenol is hydrogenated to cyclohexanone
as an intermediate, and then cyclohexanol and/or cyclohexene
is formed, subsequently.24–26 Others have proposed direct
pathways to cyclohexanol without passing through the cyclo-
hexanone intermediate.15,23 Under our reaction conditions,
the formation of cyclohexanol is more significant than
cyclohexanone, suggesting two competing pathways for the
conversion of cyclohexenol to cyclohexanone and cyclohexanol.23
However, hydrogen mass transfer limitation may influence the
observed product distributions. Under the conditions used,
3-phase, trickle bed conditions may exist at lower conversions
and high space velocities, whereas a single gas/vapour phase
should be present at higher conversions (>70%) and lower
space velocities, based on preliminary Aspen Hysis simulations
using the Peng–Robinson equation of state. In cases where
a liquid phase is present, the reaction pathway may be
misconstrued due to hydrogen transport limitations, and
further studies addressing this issue are warranted.
From the plots of mono- and bicyclic products selectivity
versus phenol conversion as shown in Fig. 1 and 2, we can
deduce that the formation of o- and p-cyclohexyl phenol is
dominant in the presence of cyclohexanol as an alkylating
agent. The intervention of the cyclohexyl cation provides a
reasonable pathway for formation of the bicyclics as well by
the known electrophilic aromatic substitution of phenol.27–29
The formation of 2-cyclohexyl cyclohexanone was well correlated
with the formation of cyclohexanone. Based on literature
reports, we suggest that the 2-cyclohexyl cyclohexanone is
produced from cyclohexanone by aldol condensation over the
acidic zeolite.30,31 Taking these observations into account, we
Despite the uncertainties described above, over the course of
the reaction, the selectivity to cyclohexene did not change with
conversion even though there was a significant change in the
selectivity to cyclohexanol and cyclohexanone. This result
suggests that cyclohexene may be formed from both cyclo-
hexanol and cyclohexanone. Interestingly, the selectivity to
benzene was fixed over the range of phenol conversions, as
shown in Fig. 1. This suggests that phenol is converted to
benzene by a different pathway, perhaps via direct hydro-
genolysis to benzene. In addition, a small amount of methyl
cyclopentane, which is an isomerization product of cyclo-
hexane, was consistently observed. Taking the overall results
summarized in Fig. 1 into account, one can begin to consider
the reaction pathway for the hydrodeoxygenation of aqueous
phenol over Pt/HY catalysts as shown in Scheme 1.
Fig. 2 Selectivity to bicyclic products versus phenol conversion.
Reaction conditions: temperature, 473 K; H2 pressure, 4 MPa; catalyst
loading, 100 mg; catalyst, 0.5 wt% Pt/HY (SiO2/Al2O3 = 12); WHSV,
5–20 hÀ1; H2O content, 5 wt%.
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Chem. Commun., 2010, 46, 1038–1040 | 1039

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structure on the yield of bicyclics suggests that it is possible to
tune this selectivity further.
In conclusion, we report an efficient non-sulfided catalyst
for the hydrodeoxygenation–hydrogenation–coupling of
phenol in a fixed-bed reactor at elevated hydrogen pressure.
The zeolite-supported Pt catalyst acts as a bifunctional
catalyst, with hydrogenation and acid functions yielding high
activity and selectivity to monocyclics as well as production of
useful bicyclics. The suggested reaction pathway may be
relevant to not only aqueous phenol conversion but also more
complex phenolic mixtures, such as fast pyrolysis bio-oils.
This work was supported by Chevron through the
Georgia Tech Strategic Energy Institute. Do-Young Hong
acknowledges the Korea Research Foundation Grant
(KRF-2007-357-D00055).
Scheme 2 Reaction pathway of phenol to bicyclics in the catalytic
hydrogenation treatment.
propose the reaction pathway for bicyclic formation, as shown
in Scheme 2.
The conversion of phenol was carried out over a metal-free
HY zeolite as well as Pt/Al2O3 and Pt/SiO2 catalysts (Table 1).
The HY zeolite was not an effective catalyst and the conversion
of phenol was negligible under the conditions used. In the case
of Pt/Al2O3 and Pt/SiO2 catalysts, the main products were
cyclohexanol and further reactions, such as hydrogenolysis
and coupling, were not observed. This result implies that the
formation of cyclohexane from phenol under these conditions
requires bifunctional catalysis under the conditions used, i.e.,
the presence of acidic protons for dehydration and a metallic
function for hydrogenation was needed.
Notes and references
1 G. W. Huber, J. N. Chheda, C. J. Barrett and J. A. Dumesic,
Science, 2005, 308, 1446.
2 A. K. Sarma and D. Konwer, Energy Fuels, 2005, 19, 1755.
3 A. V. Bridgwater, Catal. Today, 1996, 29, 285.
4 A. V. Bridgwater, J. Anal. Appl. Pyrol., 1999, 51, 3.
5 D. Mohan, C. U. Pittman and P. H. Steele, Energy Fuels, 2006, 20,
848.
6 A. V. Bridgwater and G. V. C. Peacocke, Renewable Sustainable
Energy Rev., 2000, 4, 1.
7 E. Furimsky, Appl. Catal., A, 2000, 199, 147.
8 S. Czernik and A. V. Bridgwater, Energy Fuels, 2004, 18, 590.
9 G. W. Huber, S. Iborra and A. Corma, Chem. Rev., 2006, 106,
4044.
10 D. C. Elliott, D. Beckman, A. V. Bridgwater, J. P. Diebold,
S. B. Gevert and Y. Solantausta, Energy Fuels, 1991, 5, 399.
11 D. C. Elliott, Energy Fuels, 2007, 21, 1792.
12 E. Laurent and B. Delmon, J. Catal., 1994, 146, 281.
13 Y. H. E. Sheu, R. G. Anthony and E. J. Soltes, Fuel Process.
Technol., 1988, 19, 31.
14 S. Ramanathan and S. T. Oyama, J. Phys. Chem., 1995, 99,
16365.
15 C. Zhao, Y. Kou, A. A. Lemonidou, X. B. Li and J. A. Lercher,
Angew. Chem., Int. Ed., 2009, 48, 3987.
16 Y. Xu, T. J. Wang, L. L. Ma, Q. Zhang and L. Wang, Biomass
Bioenergy, 2009, 33, 1030.
17 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.
18 S. Vitolo, M. Seggiani, P. Frediani, G. Ambrosini and L. Politi,
Fuel, 1999, 78, 1147.
19 M. Stocker, Angew. Chem., Int. Ed., 2008, 47, 9200.
20 T. Hoskins, MS thesis, Georgia Institute of Technology, 2008.
21 G. W. Huber, J. N. Chheda, C. J. Barrett and J. A. Dumesic,
Science, 2005, 308, 1446.
We also explored the effect of the zeolite support in the
hydrodeoxygenation of phenol, as described by the data in
Table 1. The conversion of phenol did not depend on the
topology of the zeolite used as the support for the Pt species
under these conditions, and all catalysts displayed high activity
and selectivity to cyclohexane even at high space velocity
(20 hÀ1). The yields of cyclohexane (B95%) achieved were
quite good in all cases compared to the reported results with
the most active sulfided CoMo catalysts, which produced
33.8% benzene and 3.6% cyclohexane from phenol at 673 K
in a batch reactor.11 As expected, the formation of coupling
products, which may give useful, higher molecular weight
hydrocarbons for liquid fuels, was strongly influenced by the
zeolite structure. The selectivity to bicyclics with HY and Hb
zeolite-supported catalyst (B4%) was higher than over the
HZSM-5 (B1%) catalyst, consistent with catalysis in the
micropores and expected shape-selective effects. Even though
these initial studies only produced small quantities of the cyclic
products, the observation of the important effect of zeolite
22 D. A. Simonetti and J. A. Dumesic, ChemSusChem, 2008, 1, 725.
23 H. A. Smith and B. L. Stump, J. Am. Chem. Soc., 1961, 83, 2739.
24 A. K. Talukdar, K. G. Bhattacharyya and S. Sivasanker, Appl.
Catal., A, 1993, 96, 229.
25 S. Velu, M. P. Kapoor, S. Inagaki and K. Suzuki, Appl. Catal., A,
2003, 245, 317.
26 A. Pinheiro, D. Hudebine, N. Dupassieux and C. Geantet, Energy
Fuels, 2009, 23, 1007.
27 R. K. M. R. Kallury, T. T. Tidwell, D. G. B. Boocock and D. H.
L. Chow, Can. J. Chem., 1984, 62, 2540.
Table
1 Hydrodeoxygenation of phenol on zeolite-supported
platinum catalystsa
Selectivity (%)
CyHxd CyHxole Bicyclics Tricyclics
PhOH conv. (%)c
Catalystb
HY
Pt/HY
Pt/Hb
0.8f
99.8f
—
—
0.0
0.0
0.0
—
—
93.7
94.7
89.7
2.93 93.38
1.39 94.62
4.0
4.1
1.4
0.0
0.0
0.1
0.4
0.0
0.0
0.0
100.0f
Pt/HZSM-5 96.8f
28 R. Anand, T. Daniel, R. J. Lahoti, K. V. Srinivasan and B. S. Rao,
Catal. Lett., 2002, 81, 241–246.
Pt/Al2O3
Pt/SiO2
99.8g
99.7g
29 R. Anand, K. U. Gore and B. S. Rao, Catal. Lett., 2002,
81, 33.
a
Reaction conditions: H2 pressure, 4.0 MPa; catalyst loading, 100 mg;
b
WHSV, 20 hÀ1; H2O content, 10 wt%. Pt loading is 1.0 wt%.
c
30 T. Xu, E. J. Munson and J. F. Haw, J. Am. Chem. Soc., 1994, 116,
1962.
d
Conversion of phenol. Cyclohexane. Cyclohexanol. Reaction
e
f
31 J. Huang, W. Long, P. K. Agrawal and C. W. Jones, J. Phys.
Chem. C, 2009, 113, 16702.
g
temperature is 523 K. Reaction temperature is 473 K.
ꢀc
This journal is The Royal Society of Chemistry 2010
1040 | Chem. Commun., 2010, 46, 1038–1040