Catalytic roles of metals and supports on hydrodeoxygenation of lignin monomer guaiacol

Article abstract of DOI:10.1016/j.catcom.2011.10.011

Hydrodeoxygenation of the lignin monomer guaiacol was performed on the bifunctional catalysts of noble-metals supported on acidic matrices. The catalytic roles of metal nanoparticles and acidic supports were elucidated using the acid-site-measurement-dependent catalysis results, which demonstrated that metals were responsible for the hydrogenation of aromatic rings while metal-deposited acidic supports were indispensable to the deoxygenation of oxygenates. Among various combinations of metals and supports, Rh/SiO ₂-Al₂O₃ and Ru/SiO₂-Al ₂O₃ exhibited the highest cyclohexane yields.

Full text of DOI:10.1016/j.catcom.2011.10.011

Catalysis Communications 17 (2012) 54–58
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Catalytic roles of metals and supports on hydrodeoxygenation of lignin
monomer guaiacol
Cho Rim Lee a,b, Ji Sun Yoon a,c, Young-Woong Suh d, Jae-Wook Choi a, Jeong-Myeong Ha a,
,
⁎
Dong Jin Suh a, Young-Kwon Park b
a
Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
Department of Energy and Environmental System Engineering, University of Seoul, Seoul 130-743, Republic of Korea
Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 120-749, Republic of Korea
b
c
d
Department of Chemical Engineering, Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 August 2011
Received in revised form 22 September 2011
Accepted 11 October 2011
Available online 17 October 2011
Hydrodeoxygenation of the lignin monomer guaiacol was performed on the bifunctional catalysts of noble-metals
supported on acidic matrices. The catalytic roles of metal nanoparticles and acidic supports were elucidated using
the acid-site-measurement-dependent catalysis results, which demonstrated that metals were responsible for the
hydrogenation of aromatic rings while metal-deposited acidic supports were indispensable to the deoxygenation
of oxygenates. Among various combinations of metals and supports, Rh/SiO2-Al2O3 and Ru/SiO2-Al2O3 exhibited
the highest cyclohexane yields.
Keywords:
Hydrodeoxygenation
Guaiacol
© 2011 Elsevier B.V. All rights reserved.
Lignin
Rh/SiO2-Al2O3
Catalyst
Biomass
1. Introduction
processes is required. In addition, further chemical processes can convert
the lignin fragments to other valuable chemicals. The catalytic hydroge-
The depletion of fossil fuels creates the research opportunities on
sustainable energy and chemical resource development. While various
energy sources, such as solar, wind, and biofuel have been suggested,
biomass, including corn, sugarcane, grass, wood, and algae, remains
the only resource for both energy and chemical production [1]. Biomass
is composed of complex polymers, such as cellulose, hemicellulose, and
lignin, which are valuable carbon sources being capable of replacing
coal, natural gas, and petroleum in the chemical industry. Of these bio-
polymers, the polysaccharides of cellulose and hemicellulose have been
investigated extensively as raw materials to produce alcohols, organic
acids, and furfurals because of their easier processibility of fermentation
and catalytic decomposition [1,2]. In contrast, complex lignin has been
regarded as a waste byproduct of the lignocellulosic biomass conversion
processes despite the fact that it is composed of valuable aromatic
monomers [3,4].
nation of lignin fragments has been investigated, with guaiacol and its
derivatives frequently selected as lignin model compounds for the
hydrodeoxygenation of lignin fragments [6]. Mo-based catalysts
[7–10], transition-metal-based catalysts [11], and zeolites [12] have
been investigated to prepare hydrodeoxygenated aromatic compounds
from guaiacol. With these catalysts, the formation of catechol and
phenol was observed, thus confirming demethylation or deoxygenation
of aromatic lignin monomers [10,12,13]. In comparison, noble metal
catalysts fully hydrogenated the aromatic rings of lignin monomers
and then hydrodeoxygenated methoxy and hydroxyl groups to obtain
saturated deoxygenated hydrocarbons [14–16]. The acidity of the sup-
port also affects the reaction pathways and product distributions [13].
Although the catalytic conversion of lignin model compounds has
been studied and while many reports on this topic have been published,
catalytic roles of each heterogeneous catalyst components and the in-
terplay between these components have yet to be elucidated so that
highly active and selective catalysts can be designed. In this study, we
investigate the hydrodeoxygenation of the lignin monomer guaiacol
using bifunctional catalysts composed of metal nanoparticles and solid
acids. Guaiacol, a possible derivative of lignin's coniferyl alcohol fragment,
is a major component of biomass pyrolysis oil [17]. It contains hydroxyl
and methoxy groups, which are frequently found in various lignin mono-
mers [1]. Its hydrodeoxygenated products, cyclohexanol, cyclohexanone,
In search of new sustainable carbon and energy sources, research
efforts to enable the use of lignin as a valuable chemical resource
have recently increased sharply [5]. Because of its complex structure,
depolymerization of lignin by means of catalytic and thermochemical
⁎
Corresponding author. Tel.: +82 2 958 5837; fax: +82 2 958 5209.
E-mail address: jmha@kist.re.kr (J.-M. Ha).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.10.011

C.R. Lee et al. / Catalysis Communications 17 (2012) 54–58
55
and cyclohexane in this study, can be blended with petroleum as sustain-
able fuel sources [18]. The roles of catalyst components are elucidated
using various combinations of metals and supports, which allows better
understanding the catalytic hydrodeoxygenation of lignin fragments.
microscopy (TEM) and N2-physisorption, which was discussed in the
supplementary information.
2.4. Guaiacol hydrodeoxygenation and product analysis
The catalysis was performed in a 160-mL stainless steel batch reactor.
The supported catalyst (0.4 g, 3 wt.% Rh and 5 wt.% Pt, Pd, and Ru) was
charged into the reactor and guaiacol or 2-methoxycyclohexanol
(7.5 wt.%) dissolved in n-decane (40 mL) was injected. The reactor was
filled with H2 (40 bar at room temperature) and the reaction mixture
was maintained under stirring at 1000 rpm. The reaction mixture was
heated to 250 °C at a rate of ~6.7 °C/min and the isothermal reaction
was allowed to proceed for 1 h. After the reaction, the reactor was cooled
to room temperature using cooling water. The liquid phase product dis-
tributions were obtained using a GC (Younglin, Acme 6000E) equipped
with an HP-5 capillary column (60 m×0.25 mm×0.25 μm) and a flame
ionization detector (FID). They were identified with a GC-MS (Agilent
Technologies, 7890A, HP-5 capillary column, 60 m×0.25 mm×0.25 μm).
2. Experimental
2.1. Materials
All chemicals were used without further purification unless indicated
otherwise. Guaiacol (2-methoxyphenol, 98.0%) was purchased from TCI
(Tokyo, Japan). Phenol (99%), cyclohexanone (99.8%), cyclohexanol
(99%), cyclohexene (99%, inhibitor-free), ( )-trans-1,2-cyclohexanediol
(96%), n-decane (99%), chloroplatinic acid hexahydrate (H2PtCl6·6H2O),
rhodium (III) chloride hydrate (RhCl3·xH2O), palladium(II) chloride
(PdCl2), and silica-alumina catalyst support (grade 135) were purchased
from Aldrich (Milwaukee, WI, USA). 2-Methoxycyclohexanol (97%),
ruthenium (III) chloride (RuCl3, anhydrous), and alumina (γ-phase)
were purchased from Alfa (Ward Hill, MA, USA). Vulcan XC-72R carbon
black was supplied by Cabot (Boston, MA, USA) and treated with nitric
acid prior to use [18]. Helium, hydrogen, nitrogen, oxygen/nitrogen (1%
v/v), and ammonia/helium (5% v/v) were purchased from Shinyang
Sanso (Seoul, Korea).
3. Results and discussion
3.1. Hydrodeoxygenation of guaiacol with various catalysts
Guaiacol, a lignin monomer model compound, was converted to a
fully hydrogenated liquid fuel, cyclohexane, using bifunctional catalysts
of the noble metals Pt, Rh, Pd, and Ru supported on acidic matrices of
Al2O3 (Al), SiO2-Al2O3 (SiAl), and nitric-acid-treated carbon black
(NAC) (Fig. 1). The metal particle size and the support surface area
were measured to confirm that the metal nanoparticles were distributed
on the high surface area matrices (see Supplementary information). All
of these catalysts except Pt/SiAl (50% guaiacol conversion) and Pd/NAC
(54%) exhibited the complete conversion (100%) of guaiacol and 14–
60% yields of cyclohexane under the reaction conditions described in
the experimental section. The incomplete conversion of guaiacol asso-
ciated with Pt/SiAl and Pd/NAC is attributed to the presence of fewer
active sites as measured by CO-chemisorption (Supplementary in-
formation). Detailed TEM, CO-chemisorption, and XRD results to con-
firm the formation of metal nanoparticle catalysts are discussed in the
supplementary information. Other hydrogenated intermediates of 2-
methoxycyclohexanol, cyclohexanol, and cyclohexanone, were also
obtained. Among the possible products, the GC-MS analysis did not ex-
hibit the formation of 2-methoxycyclohexanone, which has been
obtained from phenol with Rh-based catalysts [14]. However, the for-
mation of 2-methoxycyclohexanol was noted. Cyclohexanone was
reported to be obtained directly from phenol and cyclohexanol was
expected to be a short-lived intermediate [8–11], but cyclohexanol
was one of major products produced with Al- and NAC-supported
2.2. Catalyst preparation
All the supported catalysts were prepared by an incipient-wetness
impregnation method. Prior to impregnation, the support powders
were sieved to obtain particles smaller than 75 μm and dried in air
at 105 °C for 24 h. The impregnated catalysts were calcined in a He
flow at 400 °C for 5 h, reduced in a H2 flow at 350 °C for 2 h, and
then passivated with a 1% O2/N2 flow at room temperature for 2 h
to be stored under ambient conditions.
2.3. Catalyst characterization
NH3-temperature programmed desorption (NH3-TPD) results were
recorded using a BELCAT-B temperature programming unit (Belcat,
Osaka, Japan) equipped with a thermal conductivity detector (TCD).
The catalyst powder (~50 mg) was reduced in a H2 flow at 350 °C for
30 min, and then purged using a He flow for 10 min. The sample was
cooled to room temperature and passivated with NH3 in equilibrium
with a 5% NH3/He flow (50 mL/min). While the sample was heated to
900 °C at a rate of 5 °C/min under a He flow (30 mL/min), desorbed
NH3 was detected with TCD. The catalysts were also characterized
with CO-chemisorption, X-ray diffraction (XRD), transmission electron
Fig. 1. Product distributions of the hydrodeoxygenation of guaiacol with Pt, Rh, Pd, and Ru supported on Al, SiAl, and NAC. The reactions were performed on 0.4 g of 5 wt.% (Pt, Pd,
and Ru) and 3 wt.% (Rh) catalysts. The conversions of guaiacol were 100% for all of the catalysts except Pt/SiAl (50% conversion of guaiacol) and Pd/NAC (54%).

56
C.R. Lee et al. / Catalysis Communications 17 (2012) 54–58
catalysts in this study. These observations combined with the absence
of phenol (Fig. 1) indicate that the reaction pathway in Fig. 1 may be
different from the pathway of “guaiacol → phenol → hydrodeoxyge-
nated products” reported in the literature [7,9,11,12,14]. Because
phenol, which is converted to cyclohexanone (0–6% yield), was not
obtained, cyclohexanone may be obtained by the inter-conversion
between cyclohexanone and cyclohexanol [7]. The other possible
product of cyclohexene, which can be expected to form during the
hydrogenation of aromatic compounds [19], was not observed with
GC-MS, either, because cyclohexene can easily be hydrogenated to
cyclohexane with noble-metal catalysts in a high-pressure H2
environment.
The product distributions were more dependent on the acid sup-
ports than they were on the noble metals. Among the catalysts used
in this study, the SiAl-supported catalysts exhibited the highest cyclo-
hexane yields of 17–60%. Compared to the SiAl-supported catalysts, the
Al- and NAC-supported catalysts exhibited lower yields of cyclohexane
with higher yields of cyclohexanol and 2-methoxycyclohexanol. The for-
mation of 2-methoxycyclohexanol was more significant with NAC-
supported catalysts, indicating their lower hydrodeoxygenation activity.
The importance of acid supports was also confirmed when the active
sites of metals were quantitized. Rh/Al ([CO]/[Metal]=0.959) has ~10
times the number of active sites compared to Rh/SiAl (0.0988) (Table
S2 in supplementary information), but Rh/SiAl exhibited much better
hydrodeoxygenation activity. Unlike Rh catalysts, Pt/Al and Pd/Al,
exhibiting better hydrodeoxygenation activity, have ~4 times the
number of active sites compared to Pt/NAC and Pd/NAC (Table S2
in supplementary information). These observations indicate that
the hydrodeoxygenation activity is less dependent on the number
of metal's active sites.
sites of weak and strong acidities at ~150 and ~500 °C, respectively
[20,21], which may affect the catalytic activity. The strength and nature
of the acidities require the further investigation to clarify the detailed
roles of solid acids.
3.3. Bifunctional catalysis on noble metals supported on acidic matrices
Based on the observation that the production of fully hydrodeoxyge-
nated cyclohexane depended on the acidity of the supports (Figs. 1 and
2), the catalytic roles of metal nanoparticles and acidic supports were in-
vestigated using the reaction results on various catalysts (Fig. 3). First,
noble-metal-free SiAl was used to catalyze the hydrogenolysis of guaiacol
(Fig. 3(a)), which exhibited the low conversion of guaiacol (29%) and the
negligible yields of 2-methoxycylclohexanol (0.3%) and cyclohexane
(0.2%), respectively. Second, Rh/ZrO2 containing negligible acidity
(Fig. 2) and nonporous structure (Supplementary information) was
3.2. NH3-TPD for catalyst acidity measurements
The acid supports were characterized using an acid site measure-
ment by NH3-TPD (Fig. 2). The use of solid acid catalysts initiates
the hydrodeoxygenation of guaiacol [15] whereas the deoxygenation
of 2-methoxycyclohexanol is not likely occur without solid acids [16].
The acidities of supported Rh catalysts, measured at 100–850 °C, were in
the order of Rh/SiAl (1.85 mmol NH3/g catalyst)>Rh/Al (1.19)>Rh/
NAC (0.53)>Rh/ZrO2 (0.13), directly affecting the yields of fully hydro-
deoxygenated cyclohexane (SiAl (57%)>Al (25)>NAC (20)>ZrO2 (1))
and 2-methoxycyclohexanol (SiAl (0%)bAl (9)bNAC (41)bZrO2 (89)).
These observations serve as evidence that (i) the hydrodeoxygenation
occurred on the acid sites of the supports, and (ii) the amount of remain-
ing 2-methoxycyclohexanol with a hydrogenated cyclohexyl ring in-
creased as the acidity decreased. We also observed two different acid
Fig. 3. (a) Product distributions of the conversion of guaiacol with Rh/SiAl at T1 in (b),
Rh/SiAl when fully processed, Rh/ZrO2, a mixture of Rh/ZrO2 and SiAl, and the conver-
sion of 2-methoxycyclohexanol on SiAl at 250 °C and 40 bar with 1000 rpm. (b) Heated
pressure change of the guaiacol conversion system on Rh/SiAl and SiAl.
(c) Temperature-dependent product distributions with the conversion of guaiacol at
40 bar at 1000 rpm.
Fig. 2. NH3-temperature programmed desorption of 3 wt.% Rh supported on SiAl
(1.85 mmol NH3/g catalyst), Al (1.19), NAC (0.53), and ZrO2 (0.13).

C.R. Lee et al. / Catalysis Communications 17 (2012) 54–58
57
Fig. 4. Proposed reaction pathway of guaiacol to cyclohexane with bifunctional catalysts.
used instead of acidic supports, which yielded 1% cyclohexane and 89% 2-
methoxycyclohexanol although the conversion of guaiacol was high at
99% (Fig. 3(a)). These observations indicate that the metal nanoparticles
catalyze the hydrogenation of aromatic rings but that acid sites are re-
quired to hydrodeoxygenate guaiacol further. We also performed the
hydrogenation of guaiacol on Rh/SiAl under heating from room tem-
perature to 250 °C (Fig. 3(b)) and observed that the reaction pressure
increased with the temperature. It started to decrease at ~58 °C before
increasing at ~108 °C (T1 in Fig. 3(b)). The decreasing pressure indi-
cates the consumption of H2 by the hydrogenation of guaiacol at 58–
108 °C, which fully converted guaiacol and yielded 91, 2, 0.1, and 0.3%
of 2-methoxycyclohexanol, cyclohexanone, cyclohexanol, and cyclo-
hexane, respectively, when the heating was stopped at ~108 °C (T1 in
Fig. 3(b)). A decrease in the pressure, or H2 consumption, was also
observed when the hydrodeoxygenation of guaiacol-converted 2-
methoxycyclohexanol was isothermally performed at 250 °C (T2 in
Fig. 3(b)). In addition to these results, the product yields depending
on the reaction temperature were observed, and cyclohexane was
obtained at temperatures above the range of 220 – 250 °C (Fig. 3(c)).
The findings outlined above suggest that the hydrodeoxygenation of
guaiacol occurs through a combination of hydrogenation of the aromatic
ring on metal at 58 – 108 °C and deoxygenation of the oxygenates with
metal-deposited acid catalysts at a temperature above 250 °C. In order
to confirm this hypothesis, we performed the hydrodeoxygenation of
guaiacol (7.5 wt.% in 40 mL n-decane) with Rh/ZrO2 (0.4 g, 3 wt.% Rh)
and a mixture of Rh/ZrO2 (0.4 g, 3 wt.% Rh) and noble-metal-free SiAl
(0.4 g) at 250 °C under 40 bar of pressure (Fig. 3(a)). Because nonporous
ZrO2 exhibited fewer acid sites and lower surface area (Supplementary
information) compared to the other supports of Al, SiAl, and NAC, the
catalysis on Rh/ZrO2 exhibited the full hydrogenation of the aromatic
ring of guaiacol but a negligible amount of deoxygenation, yielding
89% 2-methoxycyclohexanol and 6% hydrodeoxygenated products of
cyclohexanol, cyclohexanone, and cyclohexane. In contrast to the poor
hydrodeoxygenation activity of Rh/ZrO2, the mechanically mixed SiAl
and Rh/ZrO2 produced a 20% yield of 2-methoxycyclohexanol and an
18% combined yield of hydrodeoxygenated products. In addition, the
hydrodeoxygenation of 2-methoxycyclohexanol (7.5 wt.% in 40 mL n-
decane) attempted with noble-metal-free SiAl (0.4 g) exhibited a 79%
conversion of 2-methoxycyclohexanol and a 15% yield of cyclohexane,
both of which are similar to the results obtained with a mixture of Rh/
ZrO2 and SiAl. These observations indicate that the hydrogenated
product, 2-methoxycyclohexanol, obtained with Rh/ZrO2 diffused
to the surface of SiAl and was further deoxygenated on SiAl. The
yield of cyclohexane with a mixture of Rh/ZrO2 and SiAl was, however,
lower than that with Rh/SiAl, which indicates that the close proximity of
SiAl to hydrogen-adsorbed metals accelerates the hydrodeoxygenation
process.
temperature, we did not observe the formation of phenol despite the
fact that it has been frequently reported to form via the demethylation
and dehydration of guaiacol [7]. Because of the excellent hydrogenation
activity of noble metal nanoparticles, the reaction pathway (guaiacol →
2-methoxycyclohexanol → hydrodeoxygenated products) suggested in
Fig. 4 is very different from the well-known pathway (guaiacol → phenol
→ hydrodeoxygenated products) [7,9,11,12,14].
4. Conclusions
The catalytic hydrodeoxygenation of guaiacol occurred when using bi-
functional catalysts of metal nanoparticles supported on acidic matrices,
constituting the hydrogenation of aromatic rings on metal catalysts and
the deoxygenation on metal-deposited acidic supports. The hydrodeoxy-
genation of guaiacol required acid catalysts whose acidity appears to de-
termine the degree of deoxygenation. It was also observed that the
hydrogenation of aromatic rings occurred following the deoxygenation
process when the reaction system was heated. Because various lignin
monomers of phenolic compounds share similar molecular structures
with guaiacol, the results here pertaining to the hydrodeoxygenation of
guaiacol may be applicable to the conversion of other lignin monomers
and dimers, such as eugenol, coniferyl alcohol, or aryl-ether derivatives
[1]. Understanding the catalytic roles of bifunctional catalyst components
will assist with the design of highly active catalysts for the conversion of
lignin fragments.
Acknowledgments
This work was supported by the National Research Foundation of
Korea (NRF) grant funded by the Korean government (MEST) (No.
2010–0028850).
Appendix A. Supplementary data
Supplementary data to this article can be found online at doi:10.
1016/j.catcom.2011.10.011.
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