One-pot catalytic reaction to produce high-carbon-number dimeric deoxygenated hydrocarbons from lignin-derived monophenyl vanillin using Al2O3-cogelled Ru nanoparticles

Article abstract of DOI:10.1016/j.apcata.2016.07.012

Al₂O₃-cogelled Ru nanoparticle (Ru@Al) catalyst was prepared by a one-pot in-situ alumina gelation method using a PVP-stabilized Ru colloid solution. The Ru@Al catalyst exhibited excellent catalytic activity during the liquid-phase hydrodeoxygenation of vanillin, demonstrating 100% conversion, as well as significantly higher yields of fully deoxygenated compounds compared to other conventional alumina-supported Ru catalysts. We also observed better selectivity to deoxygenated dimers with the Ru@Al catalyst. The improved catalytic selectivity was attributed to the hypothesized three-dimensional structures of Al₂O₃ surrounding the Ru nanoparticles, which improved the two-step reaction, containing the dimerization of the phenolic compounds and the hydrodeoxygenation of phenolic dimers to produce deoxygenated high-carbon-number hydrocarbons.

Full text of DOI:10.1016/j.apcata.2016.07.012

Applied Catalysis A: General 524 (2016) 243–250
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
One-pot catalytic reaction to produce high-carbon-number dimeric
deoxygenated hydrocarbons from lignin-derived monophenyl vanillin
using Al2O3-cogelled Ru nanoparticles
Indri Yatia,b,1, Adid Adep Dwiatmokoa,c,1, Ji Sun Yoona, Jae-Wook Choia,
Dong Jin Suha,c,d, Jungho Jaea,b, Jeong-Myeong Haa,b,d,∗
a Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea
b Department of Clean Energy and Chemical Engineering, Korea University of Science and Technology, Daejeon 34113, Republic of Korea
c Department of Green Process and System Engineering, Korea University of Science and Technology, Daejeon 34113, Republic of Korea
d Green School (Graduate School of Energy and Environment), Korea University, Seoul 02841, 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 4 April 2016
Received in revised form 15 July 2016
Accepted 17 July 2016
Available online 18 July 2016
Al2O3-cogelled Ru nanoparticle (Ru@Al) catalyst was prepared by a one-pot in-situ alumina gelation
method using a PVP-stabilized Ru colloid solution. The Ru@Al catalyst exhibited excellent catalytic activ-
ity during the liquid-phase hydrodeoxygenation of vanillin, demonstrating 100% conversion, as well as
significantly higher yields of fully deoxygenated compounds compared to other conventional alumina-
supported Ru catalysts. We also observed better selectivity to deoxygenated dimers with the Ru@Al
catalyst. The improved catalytic selectivity was attributed to the hypothesized three-dimensional struc-
tures of Al2O3 surrounding the Ru nanoparticles, which improved the two-step reaction, containing the
dimerization of the phenolic compounds and the hydrodeoxygenation of phenolic dimers to produce
deoxygenated high-carbon-number hydrocarbons.
Keywords:
Vanillin
Ruthenium
Alumina
© 2016 Elsevier B.V. All rights reserved.
Hydrodeoxygenation
Condensation
1. Introduction
Because the formation of monomeric deoxygenated hydrocarbons
from lignin-derived small-molecule phenolic compounds has been
The production of hydrocarbon fuels from biomass is a next
major challenge in the effort to develop advanced biofuels, and
the thermolysis of lignocellulosic biomass appears to be a practi-
cal method to produce bio-oils, precursors of petroleum, to replace
hydrocarbon fuels [1–3]. In order to produce petroleum-like hydro-
carbon fuels, the oxygen atoms in bio-oils must be removed, which
has been achieved by catalytic hydrodeoxygenation (HDO) [4–6].
Along with HDO, the production of high-carbon-number hydro-
carbons has been studied in order to produce high-energy-density
fuels to be blended with diesel fuels [7,8]. Alkylation [9], aldol con-
densation [7,10], and other condensation processes [8] have been
suggested in the literature.
observed when bifunctional catalysts of metals supported on solid
acids were used [11–19], a new catalyst design was required
to produce high-carbon-number deoxygenated hydrocarbons and
prevent the formation of oligomers.
In our previous study, we prepared sintering-resistant Pt
nanoparticles, which were strongly interacted with alumina
nanorods (Pt@Al2O3), by a simple preparation method, with which
a highly active water gas shift reaction was performed without
significant sintering of the Pt nanoparticles [20]. With a simi-
lar approach, herein we report that a prepared Al2O3-cogelled
Ru nanoparticle (Ru@Al) catalyst exhibits interesting performance
during the hydrodeoxygenation of vanillin at 40 bar and 270 ◦C in
an aqueous solution. Vanillin, 4-hydroxy-3-methoxybenzaldehyde,
was selected as a small-molecule phenolic compound to repre-
sent lignin-derived small molecules in this study. It contains three
oxygenated functional groups: aldehyde, ether, and hydroxyl [21].
The HDO of vanillin has been reported to produce vanillin alco-
hol, p-creosol, and guaiacol as major products using supported
noble-metal catalysts of Pd, Ru, and Au [21–25]. Moreover, we
previously reported that the activity of the Ru catalyst on the
HDO of lignin monomer was higher than those of the Pd and
For the production of high-carbon-number hydrocarbons dur-
ing the HDO of small phenolic compounds, the condensation of
small molecules on acid catalysts is investigated in this study.
∗
Corresponding author at: Clean Energy Research Center, Korea Institute of Sci-
ence and Technology, Seoul 02792, Republic of Korea.
E-mail address: jmha@kist.re.kr (J.-M. Ha).
These authors equally contribute to this work.
1
http://dx.doi.org/10.1016/j.apcata.2016.07.012
0926-860X/© 2016 Elsevier B.V. All rights reserved.

244
I. Yati et al. / Applied Catalysis A: General 524 (2016) 243–250
Pt catalysts [11,26]. In this study, we observe that the catal-
ysis using Ru@Al is more selective toward the formation of
dimeric hydrocarbon compared to other conventionally impreg-
nated alumina-supported Ru catalysts (Ru/Al2O3) which are more
selective toward monomeric hydrocarbons. The reaction pathway
is suggested for the hydrodeoxygenation of vanillin, and the for-
mation of dimeric hydrocarbons on Ru@Al was discussed based on
the structures of the catalysts. The observations noted with Ru@Al
may apply to the production of high-carbon-number hydrocarbon
fuels from lignocellulose.
Tecnai G2 F20 instrument at 200 kV. A SEM analysis with EDX
mapping was performed using an FE-SEM Hitachi S-4200 instru-
ment operating at 30 kV. N2-Physisorption results were obtained
using a Micromeritics ASAP 2020 instrument. NH3-temperature-
programmed desorption/mass spectroscopy (NH3-TPD/MS), H2-
TPD/MS, and CO-chemisorption results were obtained using a
BELCAT-B catalyst analyzer (BEL Japan, Inc.) equipped with a ther-
mal conductivity detector (TCD) and a mass spectrometer (MS)
with an appropriate pretreatment (Table S1). X-ray photoelec-
tron spectroscopy (XPS) was performed using a PHI 5000 Versa
Probe (Ulvac-PHI) with a monochromatized microfocused Al X-ray
source. The binding energy was calibrated with C 1s at 284.6 eV.
2. Experimental
2.1. Materials
2.4. Catalysis
All chemicals were used without further purification. Ruthe-
nium(III) chloride hydrate (RuCl3·nH2O), polyvinylpyrrolidone
(PVP), aluminum tri-sec-butoxide (Al[OCH(CH3)C2H5]3, 97%), and
vanillin were purchased from Aldrich (Milwaukee, WI, USA). Ethy-
lene glycol was purchased from Junsei Chemical (Tokyo, Japan).
␥-Al2O3 was purchased from Alfa Aesar (Ward Hill, MA, USA).
Deionized water (18.2 Mꢀ cm) was prepared using an aqua-
MAX Ultra 370 water purification system (Young Lin Instrument,
Anyang, Korea).
The catalytic hydrodeoxygenation of vanillin was performed in
a stainless steel autoclave reactor (∼100 mL). In a typical reaction
procedure, vanillin (1.8 mmol), water (30 mL), and a solid cata-
lyst (50 mg) were added to the reactor. After flushing with N2
gas, the reactor was pressurized to 40 bar with H2 at room tem-
perature. The reaction was performed at 270 ◦C for 2 h with an
agitation rate of 800 rpm. After the reaction, the reactor was cooled
to room temperature and the mixture was extracted with ethyl
acetate. The catalyst powder was removed and the liquid product
was collected. The extracted products were analyzed using a gas
chromatograph–mass spectrometer combination (GC/MS, Agilent
7890A with 5975C inert MS XLD) with a HP-5 capillary column
(60 m × 0.25 mm × 250 ␮m). The conversion of vanillin, product
selectivity, product yield, and the oxygen removal are defined as
follows:
2.2. Preparation of catalysts
PVP-stabilized Ru nanoparticles were prepared by modifying
a method described in the literature [27]. RuCl3·nH2O (0.0285 g,
0.14 mmol) dissolved in DI water (0.61 mL) was added to PVP
(0.252 g) dissolved in ethylene glycol (19 mL). The mixture was then
heated for 1 h at 190 ◦C to produce PVP-stabilized Ru NPs. Ru@Al
was prepared by polymerizing alumina precursors in the presence
of PVP-stabilized Ru NPs. Al[OCH(CH3)C2H5]3 (3.29 mL,13 mmol)
dissolved in ethanol (130 mL) was added to a previously prepared
colloidal solution of Ru nanoparticles and the mixture was stirred
for two days at room temperature. The resulting mixture was cen-
trifuged with an addition of acetone, washed with acetone, and
dried in air for 16 h at 100 ◦C. The mixture was further calcined in
air for 2 h at 400 ◦C, reduced under a H2 flow for 4 h at 400 ◦C, and
then passivated with a 0.5% O2/N2 (v/v) flow for 30 min at room
temperature to be stored under ambient conditions. Alumina pre-
pared in the absence of Ru (FreeAl) was synthesized with the same
procedure described for the synthesis of Ru@Al, only without the
addition of RuCl3·nH2O. Alumina aerogel (AeroAl) was prepared
following the method described in our previous work [8]. Ru NPs
supported on FreeAl, AeroAl, and ␥-Al2O3 (Ru/FreeAl, Ru/AeroAl,
and Ru/␥-Al2O3, respectively) were prepared by adding alumina
as a support (0.658 g) to a previously prepared colloidal solution
of PVP-stabilized Ru nanoparticles, with the mixture then stirred
for two days at room temperature. The produced mixture was cen-
trifuged with the addition of acetone, washed with acetone, and
dried in air for 16 h at 100 ◦C. The mixture was further calcined in air
for 2 h at 400 ◦C, reduced under a H2 flow for 4 h at 400 ◦C, and then
passivated with a 0.5% O2/N2 (v/v) flow for 30 min at room temper-
ature to be stored under ambient conditions. The metal loadings of
all catalysts prepared in this study were 2 wt%.
Conversionofvanillin(mol%) = (C0 − Cf )/(C0) × 100
Selectivitytoproducts(mol%) = (Ci)/(C0 − Cf ) × 100
Yieldofproducts(mol%) = (Ci)/(C0) × 100
Oxygenremoval = [(Yieldof0-Os) + 2/3 × (Yieldof1-Os)
+ 1/3 × (Yieldof2-Os)]
where C0 and Cf are the initial and final quantities (mol) of vanillin,
respectively, and Ci is the quantity (mol) of the identified product.
In addition, 0-Os, 1-Os, and 2-Os denote molecules containing no
oxygen atom, one oxygen atom, and two oxygen atoms, respec-
tively.
3. Results and discussion
3.1. TEM, SEM, and XRD
Ru nanoparticles cogelled with alumina (Ru@Al) were pre-
pared using an in-situ alumina gelation method in the presence
of Ru colloids stabilized with PVP (Figs. 1(a) and S1(a)). Other
alumina-supported Ru catalysts were synthesized by a modified
impregnation method exhibiting Ru nanoparticles clearly exposed
on alumina supports: Ru/FreeAl (Figs. 1 and S1(b)), Ru/AeroAl
(Figs. 1 and S1(c)) and Ru/␥-Al2O3 (Figs. 1 and S1(d)) using FreeAl
(alumina prepared by the same method used for the synthesis or
Ru@Al but in the absence of Ru), AeroAl (alumina aerogel), and
␥-Al2O3 (commercially available ␥-alumina) as supports, respec-
tively. Bright-field TEM (BF-TEM) did not clearly observe the Ru
particles (Fig. 1(a)), but HAADF-STEM exhibited the formation
of Ru particles on the alumina support (Fig. S1(a)). Based on
these electron micrographs, it is hypothesized that numerous Ru
2.3. Characterization of catalysts
The powder X-ray diffraction (XRD) results were obtained
using a Rigaku Miniflex-II X-ray diffractometer (Tokyo, Japan)
equipped with Cu K␣ave (ꢁ = 0.15418 nm) at 30 kV and 15 mA. High-
resolution transmission electron microscopy (HR-TEM) images
and high-angle annular dark-field scanning transmission elec-
tron microscopy (HAADF-STEM) images were obtained using a

I. Yati et al. / Applied Catalysis A: General 524 (2016) 243–250
245
Fig. 1. BF-TEM images of (a) Ru@Al, (b) Ru/FreeAl, (c) Ru/AeroAl, (d) Ru/␥-Al2O3, and (e) FreeAl.

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I. Yati et al. / Applied Catalysis A: General 524 (2016) 243–250
Fig. 2. XPS results of alumina-supported Ru catalysts.
nanoparticles in the Ru@Al may be partially or fully wrapped inside
the space between the alumina support particles because the alu-
mina in Ru@Al was formed by a sol-gel process in the presence of
PVP-stabilized Ru nanoparticles [20].
for each catalyst prepared with different methods not affecting the
catalytic activity (Fig. 2). Because the Ru 3d3/2 peak was overlapped
with the strong C 1 s peak, Ru 3d5/2 was studied in an effort to
understand the electronic states of Ru. The formation of Ru0 was
confirmed for all catalysts, and cationic Ru peaks were not clearly
observed. In addition, negligible differences in the Al 2p exhibit-
ing Al3+ peaks were observed except for Ru/AeroAl, whose Al3+
was slightly more cationic with higher binding energy by ∼0.2 eV.
The O 1s peaks did not exhibit a significant difference between the
catalysts.
SEM observations with EDX mapping exhibited that the cata-
lysts in this study were composed of only Ru, Al, and O (Fig. S2).
The elemental compositions on the catalyst surface indicated that
the fractions of Ru were 2.2–2.6 wt% for Ru@Al, Ru/FreeAl, and
Ru/AeroAl, not significantly different from the introductory loading
of Ru (2 wt%), whereas Ru/␥-Al2O3 exhibited 5.0 wt% Ru (Table S2).
Because the alumina supports of Ru@Al, Ru/FreeAl, and Ru/AeroAl
are nanometer-scaled structures compared to the thick nonporous
alumina of Ru/␥-Al2O3, the alumina of Ru/␥-Al2O3 appeared at
lower amounts in the EDX measurements and the superficial load-
ing of Ru for Ru/␥-Al2O3 increased. The XRD results for Ru@Al,
Ru/FreeAl, Ru/AeroAl, and Ru/␥-Al2O3 confirmed the formation of
metallic hexagonal Ru particles (Fig. S3). Note that Ru/␥-Al2O3
exhibited ␥-Al2O3 [28] and boehmite crystal structures [29], and
the alumina supports of other catalysts were amorphous.
3.4. CO-chemisorption and H2-TPD
The Ru dispersions of catalysts were measured using CO-
chemisorption, which exhibited [CO]/[Ru] = 0.162–0.266 (mol/mol)
(Table S3). Ru@Al and Ru/␥-Al2O3 exhibited lower [CO]/[Ru] values
of 0.162 and 0.164, respectively, while Ru/FreeAl and Ru/AeroAl
exhibited higher corresponding [CO]/[Ru] values of 0.266 and
0.264. In addition, we attempted H2-TPD to understand the bond-
ing of H2 or H on the catalyst surface (Fig. S5). Ru@Al and Ru/FreeAl
exhibited similar desorption behaviors, indicating major desorp-
tion curves at 250–600 ◦C or higher. Ru@AeroAl exhibited a strong
desorption peak at 400–500 ◦C. Ru/␥-Al2O3 exhibited a broad des-
orption peak at 350–600 ◦C. These observations indicate that the
adsorption of H atoms on the Ru surface was weaker for Ru@Al
and Ru/FreeAl and stronger for Ru/AeroAl and Ru/␥-Al2O3. Based
on these observations, the hydrogenation activities of Ru@Al and
Ru/FreeAl may be different from those of Ru/AeroAl and Ru/␥-Al2O3
affecting the catalytic hydrogenation and hydrodeoxygenation
activity.
3.2. N2-Physisorption
The N2-physisorption results also demonstrated the forma-
tion of possible interparticle spaces between alumina particles
for Ru@Al (Table 1 and Fig. S4). BJH and t-plot analyses indi-
cate that Ru@Al did not contain micropores, instead creating only
mesopores of approximately 4 nm. As previously observed for
alumina-supported Pt catalysts [20], the alumina particles must
be nonporous, and the observed mesopores appeared to be cre-
ated by the interparticle space. Other catalysts exhibited similar
BET surface areas ranging from 262 to 322 m2/g, except for Ru/␥-
Al2O3 (61 m2/g). Ru/AeroAl had a pore width of 5–17 nm, but
FreeAl, Ru/FreeAl, and Ru/␥-Al2O3 did not exhibit well-defined
mesoporous structures. From the t-plot analyses, a large micropore
surface area was observed for FreeAl and Ru/FreeAl, whereas a neg-
ligible micropore surface area was observed for Ru@Al. Ru/AeroAl
and Ru/␥-Al2O3 exhibited a smaller micropore surface area com-
pared to those of Ru@Al and FreeAl.
3.5. NH3-TPD/MS
In the range of 100–900 ◦C from the NH3-TPD results (Fig. S6),
the formation of water, carbon monoxide, and carbon dioxide was
observed as well as desorbed ammonia. The desorption of water
at a high temperature can be attributed to the dehydration of the
alumina surface, and carbon monoxide and carbon dioxide may
form from the residual carbon on the catalyst surface. Based on
these observations, we measured the acid concentrations using
the TCD curve at 100–400 ◦C (Table 1), which exhibited ammonia
and a small shoulder of carbon monoxide at 300 ◦C. The quan-
tities of acid sites of Ru@Al and Ru/FreeAl were nearly identical
3.3. XPS
The XPS results of the prepared catalysts indicate that the elec-
tronic structures of Ru and Al2O3 were not significantly different

I. Yati et al. / Applied Catalysis A: General 524 (2016) 243–250
247
Table 1
BET surface areas, pore volumes, and acid site concentrations of supported Ru catalysts.
Catalyst
BET surface area (m2/g)
Pore volume (cm3/g)a
Micropore surface area (m2/g)b
Acid site concnetration (mmol/g)c
Ru@Al, fresh
Ru@Al, spent
Ru/FreeAl, fresh
Ru/AeroAl, fresh
Ru/␥- Al2O3, fresh
FreeAl, fresh
262
82
284
322
61
0.3
0.2
0.2
1.3
0.2
0.2
∼0
∼0
256
15
27
207
0.56
0.27
0.56
0.50
0.19
–
237
a
Pore volumes were calculated using the BJH desorption pore size distributions measured by N2-physisorption.
Measured by a t-plot analysis using the N2-physisorption results.
b
c
Quantity of acid sites measured using NH3 desorbed at 100–400 ◦C using the NH3-TPD/MS results.
reaction using Ru/FreeAl also exhibited a good oxygen removal
(61.7%), although it produced less 0-Os (39.8% vs. 51.4% for Ru@Al)
and more 1-Os (32.9% vs. 1.7% for Ru@Al). The other alumina-
supported Ru catalysts, in this case Ru/␥-Al2O3 and Ru/AeroAl,
exhibited lower oxygen removals of 15.8% for Ru/␥-Al2O3 and 24.9%
for Ru/AeroAl, without producing 0-Os and 1-Os. The Ru-free FreeAl
exhibited very low catalytic activity, with oxygen removal of 17.1%,
and it did not produce 0-Os or 1-Os. The reactions using FreeAl and
Ru/␥-Al2O3 produced large quantities of oligomers with the poor
hydrodeoxygenation of vanillin.
The effects of reaction temperature on the catalysis results were
also investigated using Ru@Al. at 210–300 ◦C (Fig. S8). With increas-
ing reaction temperature from 210 to 270 ◦C, the selectivities to
0-Os and oligomers increased from 15.0 to 55.2% and 20.4–37.5%,
respectively, while those of 1-Os and 2-Os decreased from 31.5 to
4.4% and 31.6–2.9%, respectively. The decreasing activity at 300 ◦C
was probably attributed to the formation of non-condensable gases
through catalytic cracking of vanillin or the formation of catalyst-
poisoning char at high temperature (Fig. S8(a)). It should be noted
that Ru@Al produced significantly larger quantity of dimeric deoxy-
genates than monomeric ones at all temperature range (Fig. S8(b)).
3.7. Reaction pathway
The different activities of the catalysts were further stud-
ied by devising the probable reaction pathways (Fig. 4), which
indicated that improved condensation activity is required to
produce dimeric hydrocarbons. As reported in the literature
[22,23], vanillin alcohol (2) and p-creosol (3) are formed by
the conversion of vanillin. The conversion of vanillin (1) to
p-creosol occurred through (i) the hydrogenation of vanillin
Fig. 3. (a) Catalysis results and (b) product distribution of the HDO of vanillin using
Ru@Al, Ru/FreeAl, Ru/AeroAl, and Ru/␥-Al2O3. The reaction was performed with
vanillin (1.8 mmol), water (30 mL) and a catalyst (50 mg) at a H2 pressure of 40 bar
(measured at room temperature) and at 270 ◦C for 2 h.
to vanillin alcohol and (ii) the hydrogenolysis of the
C O
bond. Monomeric oxygenates can be hydrodeoxygenated to
cycloalkanes, including cyclohexane and methylcyclohexane. In
addition to the monomeric compounds, dimeric compounds were
formed by the condensation of the reactants and intermediates
[30]. Vanillin alcohol and p-creosol were condensed to form
5-(4-hydroxy-3-methoxybenzyl)-2-methoxy-4-methylphenol
(0.56 mmol/g for both), whereas Ru/AeroAl exhibited a slightly
lower value (0.50 mmol/g), with Ru/␥-Al2O3 exhibiting the lowest
value (0.19 mmol/g).
3.6. Catalysis
(9)
and
2-(4-hydroxy-3-methoxybenzyl)-6-methoxy-4-
The hydrodeoxygenation (HDO) of vanillin was performed
using the alumina-supported Ru catalysts prepared in this
study (Fig. 3 and Table S4), with Ru@Al exhibiting good con-
densation and hydrodeoxygenation activities. The compounds
containing no oxygen atoms (0-Os), those containing only
one oxygen atom (1-Os), those containing two oxygen atoms
(2-Os), and high-molecular-weight oligomers were produced
from the three-oxygen-containing vanillin (Fig. S7). The HDO
using Ru@Al produced the largest quantity of 0-Os (51.4%),
including 1-cyclohexylmethyl-2-methyl-cyclohexane (19.0%) and
1,1ꢀ-methylenebis(cyclohexane) (7.1%), which can be obtained by
the condensation of two phenolic monomers, as well as methyl-
cyclohexane (12.8%), exhibiting an oxygen removal of 52.5%. The
methylphenol (11), which were further hydrogenated
to
form
1-(cyclohexylmethyl)-2-methylcyclohexane
(10)
and
hydrogenolyzed
to form 1-(cyclohexylmethyl)-3-
methylcyclohexane (12), respectively. The condensation of vanillin
alcohol and guaiacol formed 4-(3-hydroxy-4-methoxybenzyl)-
2-methoxyphenol (7), which was further hydrogenated and
hydrogenolyzed to form 1,1ꢀ-methylenebis(cyclohexane) (8).
These reaction pathways indicated that the production of
deoxygenated monomeric hydrocarbons must arise because
of hydrogenation and hydrogenolysis with less condensation,
whereas the production of deoxygenated dimeric hydrocar-
bons requires condensation in addition to hydrogenation and
hydrogenolysis. From these catalysis results, Ru@Al and Ru/FreeAl

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I. Yati et al. / Applied Catalysis A: General 524 (2016) 243–250
Fig. 4. Reaction pathways for the HDO of vanillin.
produced a significantly large quantities of dimeric deoxygenates,
while FreeAl, Ru/␥-Al2O3, and Ru/AeroAl preferentially produced
2-Os and oligomers without 0-Os and 1-Os. Ru@Al exhibited
more dimeric 0-Os, including 1,1ꢀ-methylenebis(cyclohexane) and
1-(cyclohexylmethyl)cyclohexane, compared to Ru/FreeAl.
adjustment of the catalytic activity by the size-dependent morphol-
ogy change.
As discussed above, the generally accepted descriptors for the
supported noble metal catalysts did not determine the deoxygena-
tion activity in this study. Another factor to consider with regard
to the catalytic activity of Ru@Al, the morphology of the supports,
was introduced. As depicted in Fig. 3(b) for the fully deoxygenated
products, the HDO of vanillin on Ru@Al preferentially produced
dimers (33.0%) rather than monomers (18.4%). In contrast, the reac-
tion on Ru/FreeAl produced monomers (33.1%) instead of dimers
(6.7%). The observed selectivity can be attributed to the different
structures of the catalysts and thus the adjustment of the reaction
pathway (Figs. 4 and 5). Because Ru@Al was prepared by mixing
Ru colloids with alumina precursors, the nanoparticles can be sur-
rounded by Al2O3 nanostructures, as previously suggested [20,32].
Based on the reaction pathway of Fig. 4, the improved conden-
sation must be attributed to the preferred reaction on alumina
before the reactants were hydrogenated or hydrodeoxygenated
on Ru (Fig. 5(b)). Because the hydrogen-adsorbing Ru nanoparti-
cles were surrounded three-dimensionally by Al2O3, the phenolic
monomer reactants were adsorbed onto the Al2O3 surface before
they could reach the hydrogen-adsorbed Ru, after which they
were dimerized prior to the reaction with the hydrogen atoms.
The dimeric compounds were further oligomerized or diffused
to adjacent Ru surfaces, becoming hydrodeoxygenated to dimeric
0-Os. The reaction on two control catalysts, Ru/FreeAl and Ru-
free FreeAl, was performed to confirm the reaction results. The
reactants on Ru/FreeAl were quickly hydrodeoxygenated before
they were dimerized because Ru particles appeared to be exposed
on the bulk surface of FreeAl, improving the hydrogen-involved
reaction of the phenolic reactants (Fig. 5(a)); the formation of
deoxygenated dimers was not preferred, and p-creosol and guaiacol
were subsequently converted to methylcyclohexane (4) and cyclo-
hexane (6), respectively. FreeAl without Ru particles cannot initiate
hydrogenation because hydrogen atoms cannot adsorb onto the
Based on these observations, the structures of the catalysts were
studied in more depth in an effort to understand how the condensa-
tion and hydrodeoxygenation activities of Ru@Al were improved.
The generally known descriptors including the metal dispersion,
the pore structure, and the acidity were investigated in this study
(Fig. S9). First, the acidity of the catalysts was investigated using
NH3-TPD/MS (Table 1 and Fig. S6) because the HDO activity of
bifunctional catalysts may depend on their acidity [11]. Although
highly active Ru@Al and Ru/FreeAl exhibited more acid sites com-
pared to Ru/␥-Al2O3 and Ru/AeroAl, the deoxygenation activity was
not fully dependent on the quantity of the acid sites (Fig. S9(a)).
Second, the pore structure of the catalysts was investigated,
indicating that the BET surface area did not determine the catalytic
deoxygenation activity (Fig. S9(b)). Ru/AeroAl with the largest BET
surface area and well-developed mesopores exhibited a smaller
oxygen removal, while Ru/FreeAl and Ru@Al with smaller BET sur-
face areas exhibited greater oxygen removals.
Third, the quantity of active-surface Ru atoms measured by
CO-chemisorption (Table S3) was correlated with the catalytic
activity (Fig. S9(c)). Active Ru@Al and poor Ru/␥-Al2O3 exhibited
the fewest surface Ru atoms, while poor Ru/AeroAl and active
Ru/FreeAl exhibited large quantities of surface Ru atoms. These
observations indicate that the quantity of Ru active sites did not
determine the hydrogenation or hydrodeoxygenation activity. Note
that the XRD results of supported Ru catalysts exhibited the diffrac-
tions of metallic hexagonal Ru but no other Ru derivatives (Fig. S3),
while the diameters of the Ru nanoparticles, as measured by the
Scherrer equation using Ru(101) at 2␪ = 44◦, were similar at 10.8,
11.7, 12.8, and 11.9 nm for Ru@Al, Ru/FreeAl, Ru/AeroAl and Ru/␥-
Al2O3, respectively [31]. These observations exclude the possible

I. Yati et al. / Applied Catalysis A: General 524 (2016) 243–250
249
Fig. 5. Suggested catalysis on (a) Ru@Al and (b) Ru/FreeAl.
catalyst surface. Instead, phenolic compounds can be dimerized or
oligomerized on the alumina surface to produce the oxygenated
aromatic dimers and oligomers.
the alumina surface before the hydrogen-involved reaction on the
Ru surface occurred. The observations in this study can be used for
the production of high-energy-density fuels during the upgrading
of biomass-thermolysis oils.
The important synergy of Ru and alumina during the catalysis on
Ru@Al was further observed with the reaction using spent Ru@Al
(Fig. S10 and Table S5), which indicates that the Al2O3 structure
is important to determine the catalytic activity of Ru@Al. While
significant deactivation was observed for the spent Ru@Al exhibit-
ing poor deoxygenation activity, the sintering of Ru nanoparticles
was not observed, with [CO]/[Ru] = 0.161 (mol/mol) for the spent
Ru@Al compared to 0.162 (mol/mol) for the fresh Ru@Al. In con-
trast to the stable Ru nanoparticles, the transformation of alumina
was distinct. The XRD of the spent Ru@Al exhibited the forma-
tion of boehmite from the amorphous alumina of fresh Ru@Al (Fig.
S11). The formation of the bulk crystal structure from amorphous
structure requires the transition of Al and O atoms to build an
ordered structure leading to the significant transformation of alu-
mina. The N2-physisorption exhibited a decreased BET surface area
for spent Ru@Al because of the significant change in the nanos-
tructures, which is most likely attributed to the phase transition
of amorphous alumina to boehmite (Fig. S12). The NH3-TPD/MS
exhibited an approximately 50% decrease of the acid concentrations
for the spent Ru@Al, which, however, cannot explain the almost
complete loss of deactivation activity (Fig. S13). These observations
indicate that the nanostructure of Al2O3 affects the catalytic con-
densation and deoxygenation activity as suggested in Fig. 5, while
the transformation of the alumina nanostructure deactivates the
Ru@Al catalyst.
Acknowledgements
This work was supported by the National Research Council of
Science & Technology (NST) grant by the Korea government (MSIP)
(No. CAP-11-04-KIST). This work was also supported by the Korea
Institute of Science and Technology (KIST) institutional program
(2E26550).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2016.07.
012.
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