Hydrodeoxygenation of phenol as a model compound for bio-oil on non-noble bimetallic nickel-based catalysts

Article abstract of DOI:10.1002/cctc.201402011

Ni/HZSM-5 catalysts for the hydrodeoxygenation of bio-oil were modified with Cu and Co. A first test series with phenol as a model revealed that the presence of Ni is essential for high activity. However, modification with Cu deteriorated the catalytic performance significantly whereas an admixture of Co increased the activity and selectivity toward the target hydrocarbons benzene and cyclohexane. Characterization studies performed by using TEM, X-ray photoelectron spectroscopy, and XRD techniques elucidated detrimental effects of the second metals: both Cu and Co form alloys with Ni; however, Cu caused a loss of active sites as well as tended to segregate at the surface whereas Co made the particles smaller and strongly stabilized the active Ni sites, that is, Ni dispersion increased. A Ni-Co/HZSM-5 catalyst (10 wt% of each metal content) showed 99% hydrocarbon selectivity at complete phenol conversion. Additional studies using intermediate products such as benzene, cyclohexene, and C ₆ oxygenates as feeds highlight the effect of the catalyst composition on key reaction steps.

Full text of DOI:10.1002/cctc.201402011

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DOI: 10.1002/cctc.201402011
Hydrodeoxygenation of Phenol as a Model Compound for
Bio-oil on Non-noble Bimetallic Nickel-based Catalysts
Thuan Minh Huynh,*[a, b] Udo Armbruster,[a] Marga-Martina Pohl,[a] Matthias Schneider,[a]
Joerg Radnik,[a] Dang-Lanh Hoang,[a] Binh Minh Quoc Phan,[b] Duc Anh Nguyen,[b] and
Andreas Martin*[a]
Ni/HZSM-5 catalysts for the hydrodeoxygenation of bio-oil
were modified with Cu and Co. A first test series with phenol
as a model revealed that the presence of Ni is essential for
high activity. However, modification with Cu deteriorated the
catalytic performance significantly whereas an admixture of Co
increased the activity and selectivity toward the target hydro-
carbons benzene and cyclohexane. Characterization studies
performed by using TEM, X-ray photoelectron spectroscopy,
and XRD techniques elucidated detrimental effects of the
second metals: both Cu and Co form alloys with Ni; however,
Cu caused a loss of active sites as well as tended to segregate
at the surface whereas Co made the particles smaller and
strongly stabilized the active Ni sites, that is, Ni dispersion in-
creased. A NiꢀCo/HZSM-5 catalyst (10 wt% of each metal con-
tent) showed 99% hydrocarbon selectivity at complete phenol
conversion. Additional studies using intermediate products
such as benzene, cyclohexene, and C6 oxygenates as feeds
highlight the effect of the catalyst composition on key reaction
steps.
Introduction
The production of liquid fuels from biomass for partially replac-
ing fossil fuels has attracted significant attention in many
countries all over the world. Fast pyrolysis technology was first
developed in late 1970s and is considered to be a promising
method for converting biomass to liquid products (so-called
bio-oil) because it gives high liquid oil yield (up to 80% based
on dry feed).[1] However, bio-oil is typically viscous and acidic
and has a low heating value because of high water and
oxygen content; hence, it is unsuitable for direct use as a fuel
component.[2] Upgrading of bio-oil via oxygen removal is thus
necessary to stabilize and improve its quality. There are two
main upgrading pathways to convert bio-oil into liquid fuels:
catalytic cracking and catalytic hydrotreating.[3] Compared with
catalytic cracking, hydrodeoxygenation (HDO) appears to be
a more promising process because it affords higher liquid oil
yield. Moreover, HDO can be used to upgrade bio-oil for direct
blending with fossil fuels such as gasoline and diesel as well as
to provide a cofeed for conventional refineries.[4]
hydrotreating catalysts. HDS catalysts (binary sulfides of CoMo
or NiMo supported on g-Al2O3), which were reported by Lau-
rent and Delmon[5] and Romero et al.,[4a,6] quickly deactivate
owing to the loss of sulfur during the reaction and carbon
deposition as well as the presence of water in bio-oil. Support-
ed noble metal catalysts (Pt, Pd, Ru, and Rh) show higher activ-
ity than conventional HDS catalysts; however, the application
of these catalysts is limited owing to their high cost and limit-
ed resources.[4a,6b,7] In recent years, supported non-noble mon-
ometallic catalysts such as Fe/SiO2, Co/SiO2, and Ni-based ma-
terials have been used in HDO studies.[8] Although Ni-based
catalysts showed higher activity than other catalysts, coke dep-
osition was still a problem for bio-oil upgrading. For instance,
Zhang et al. observed that 36 wt% of residue such as coke was
deposited on the surface after hydrotreatment.[8e] Zhao et al.
observed the deactivation of Ni catalysts owing to sintering
during the hydrothermal treatment and recycling process as
well as partly Ni leaching from the catalyst into water.[8c]
For the HDO catalyst development, many researchers have
mainly investigated two types of catalysts that have been used
in refineries for a long time as hydrodesulfurization (HDS) and
Bimetallic catalysts often show properties different from
those of the corresponding monometallic catalysts.[9] There-
fore, bimetallic catalysts have also attracted more attention
from researchers in recent years. For instance, Ardiyanti et al.
stated that bimetallic NiꢀCu catalysts on d-Al2O3 improve the
product properties of upgraded fast pyrolysis oil.[10] Bykova
et al. used the Ni-based sol–gel catalysts for guaiacol upgrad-
ing and proposed that the bimetallic catalyst (36.5%Ni2.3%Cu/
ZrO2–SiO2–La2O3) was most promising for the HDO of this
model compound.[11] Do et al. observed that bimetallic cata-
lysts (NiꢀPt/g-Al2O3 and CoꢀPt/g-Al2O3) show high conversion
and change the product selectivity compared to monometallic
catalysts (Pt/g-Al2O3) in the vapor phase HDO of meta-cresol.[12a]
[a] T. M. Huynh, Dr. U. Armbruster, Dr. M.-M. Pohl, Dr. M. Schneider,
Dr. J. Radnik, Dr. D.-L. Hoang, Dr. A. Martin
Leibniz Institute for Catalysis at the University of Rostock
Albert Einstein Str. 29a, 18059 Rostock (Germany)
Fax: (+49)3811281-51246
E-mail: thuan.huynh@catalysis.de
andreas.martin@catalysis.de
[b] T. M. Huynh, B. M. Q. Phan, Dr. D. A. Nguyen
Vietnam Petroleum Institute
173 Trung Kinh, Yen Hoa, Cau Giay, Hanoi (Vietnam)
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Other groups recently reported on the HDO of model com-
pounds such as guaiacol or meta-cresol and real bio-oil on bi-
metallic catalysts such as PtꢀFe/C, NiꢀFe/SiO2, and NiꢀFe/
Al2O3.[12b–d] The results revealed that bimetallic catalysts slightly
improved the product properties of the upgraded bio-oil[12d] as
well as significantly changed the product distribution than did
monometallic catalysts.[12b,c]
denominated as xNi, yCu, zCo, and xNiyCu or xNizCo, in which
x, y, and z are the rounded content (wt%) of Ni, Cu, and Co, re-
spectively, as determined from inductively coupled plasma op-
tical emission spectrometry (ICP–OES) analysis. N2 adsorption
studies revealed a steady decline in adsorption capacities with
the increase in metal content, for example, for the Ni-contain-
ing catalysts 4Ni, 12Ni, and 21Ni. This finding could be attribut-
ed to pore blocking by the increasing proportion of loaded
metal species, which is confirmed by the results of Gayubo
et al.[15] In contrast, the adsorbed N2 quantity for mono- and bi-
metallic catalysts with 20 wt% loading was the same; that is,
no differences in adsorption capacities were observed irrespec-
tive of the metal or metal combination used. Thus, the differ-
ences in catalytic performances must be related to reasons
other than pore size effects because the micropore volume is
similar in all these samples.
Herein, Cu and Co are used as modifiers for Ni, which is
more active in hydrogenation, to develop supported bimetallic
catalysts using nonsulfided, non-noble metals to overcome
some indicated drawbacks. Cu can help to reduce nickel oxide
at lower temperature.[13] Co as a promoter decreases particle
size and increases metal dispersion.[14] To increase the catalyst
efficiency, the use of a hydrothermally stable, high-surface area
support is beneficial. HZSM-5 is widely used as a catalyst or
support in the petroleum industry owing to its outstanding
properties such as well-defined structure and highly active acid
sites. Moreover, HZSM-5 impregnated with Ni shows higher hy-
drothermal stability than does neat HZSM-5.[15]
XRD
Two series of bimetallic catalysts–NiꢀCu/HZSM-5 and NiꢀCo/
HZSM-5–with a different ratio of metals were prepared, charac-
terized, and tested in the HDO of phenol as a model com-
pound for bio-oil to demonstrate the deoxygenation efficiency
as well as to understand the fundamental effect of the second
metal on bimetallic catalysts. The catalytic performance (activi-
ty, product distribution, and coke deposition) and solid-state
properties of monometallic and bimetallic catalysts are dis-
cussed herein.
The XRD patterns of the pre-reduced monometallic catalysts
21Ni, 21Cu, and 19Co are shown in Figure 1. The relative inten-
Results and Discussion
Characterization of the catalysts
Elemental analysis and physisorption
The results of elemental analysis (metal content) and N2 physi-
sorption are summarized in Table 1. The catalysts have been
Figure 1. XRD patterns of the monometallic reference catalysts.
Table 1. N2 physisorption results and metal content of the parent HZSM-
5 support and prepared catalysts.
sities of Cu and Ni reflections are similar. However, the intensi-
ties of Co reflections are much lower because Co atoms have
a larger mass attenuation coefficient and thus the absorption
of X-rays is high.
Catalyst
Metal content [wt%]
N2 adsorbed volume
[cm3 gꢀ1 at STP][a]
Ni
Cu
Co
HZSM-5 support
Monometallic catalyst
4Ni
12Ni
21Ni
94.5
In the XRD patterns of the calcined and reduced 11Ni10Cu
and 4Ni14Cu bimetallic catalysts (Figure 2), NiO and CuO
phases were detected in the calcined catalysts whereas a Niꢀ
Cu alloy and metallic Cu were detected in both the catalysts
after the reduction. Furthermore, the broad reflections in the
range of 7–308 demonstrate the well-dispersed HZSM-5 struc-
ture. Notably, the formation of a NiꢀCu alloy occurred during
the reduction at 6008C in H2 flow. This finding is in good
agreement with a previous report from Sinfelt,[16] who suggest-
ed that Ni and Cu were completely miscible and form a NiꢀCu
alloy at high temperature (4508C) during the preparation pro-
cess but Cu tends to segregate.
3.8
12.3
21.4
–
–
–
–
20.9
–
–
–
–
–
75.2
73.4
65.4
64.7
63.2
21Cu
19Co
–
18.7
Bimetallic catalyst
17Ni2Cu
16Ni4Cu
11Ni10Cu
4Ni14Cu
16Ni2Co
16Ni4Co
10Ni10Co
5Ni14Co
16.7
15.8
10.9
4.1
15.5
16.2
9.6
2
–
–
–
63.0
65.3
63.3
63.1
67.0
63.7
65.4
63.5
4.3
9.5
14.3
–
–
–
1.9
3.6
9.5
13.7
4.5
–
The XRD analysis of the calcined 10Ni10Co and 5Ni14Co cat-
alysts was also performed (Figure 3a). The presence of NiO,
[a] STP=standard temperature and pressure (1 bar, 08C).
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Co3O4, and/or NiCo2O4 spinel was observed in the XRD patterns
of the calcined samples. In the case of the reduced NiꢀCo/
HZSM-5 catalysts, it was difficult to identify the state of metal
sites because the observed reflections did not exactly match
with Ni or Co reflections. Nevertheless, the highly resolved
XRD pattern in Figure 3b shows that the largest reflection
shifted from the right-hand side (near metallic Ni reflection) to
the left-hand side (near metallic Co reflection) with the in-
crease in the Co/Ni ratio. Thus, Ni and Co could coexist; alloys
with a different composition of Ni and Co could be formed;
and in the case of the Co-rich samples, Ni could be embedded
in larger Co areas as a result of the former spinel phase. For in-
stance, the discussed reflection for the reduced 10Ni10Co cata-
lyst was broad, small, and shifted toward the Co position.
Moreover, the main reflections in 10Ni10Co and 5Ni14Co sam-
ples were smaller than the reflections in 21Ni, 16Ni2Co, and
16Ni4Co samples, which points toward a higher dispersion of
the metal particles in 10Ni10Co and 5Ni14Co samples than in
other samples.
Temperature-programmed reduction
The temperature-programmed reduction (TPR) profiles of mon-
ometallic (21Ni and 21Cu) and bimetallic (16Ni4Cu, 11Ni10Cu,
and 5Ni15Cu) precursors are shown in Figure 4. The broad re-
Figure 2. XRD patterns of the NiꢀCu/HZSM-5 bimetallic catalysts:
a) 11Ni10Cu and b) 4Ni14Cu.
Figure 4. TPR profiles of Ni, Cu, and NiꢀCu precursors supported on
HZSM-5.
duction profile of the 21Ni sample consists of two peaks at
low temperature (350–4508C) and high temperature (500–
6008C). There are two possible explanations for this observa-
tion. First, nickel oxide could be reduced in two steps as
shown in Equations (1) and (2), and this finding is confirmed,
for example, by Hadjiivanov et al.[17] Second, different interac-
tion strengths of Ni2+ species with HZSM-5 depending on their
location outside or inside the zeolite framework may result in
different reducibilities. This observation is in good agreement
with a previous report from Maia et al.[18] Similar to the 21Ni
sample, the TPR profile of the 21Cu sample also shows a low-
temperature peak (200–2408C) and a high-temperature peak
(280–3308C). Both the peaks of the 21Cu sample were located
at lower temperatures compared with those of the 21Ni
Figure 3. XRD patterns of the NiꢀCo/HZSM-5 bimetallic catalysts: a) calcined
catalysts and b) reduced catalysts.
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sample. This finding indicates that copper oxides are more
easily reduced than nickel oxides.
(250–3008C) was observed in NiꢀCo bimetallic samples. This
peak could be due to the reduction of the NiCo2O4 spinel,
which was present in the calcined precursors, as confirmed by
XRD analysis (Figure 3a). This peak is mainly visible in samples
with a higher Co content (e.g., 10Ni10Co) compared with
those containing less Co (e.g., 16Ni4Co). This result is in good
agreement with previous reports,[22] which suggested that the
NiCo2O4 spinel was more easily reduced as compared with
Co3O4, NiO, and CoO. On the basis of these TPR results, the re-
duction temperature for the activation of NiꢀCo bimetallic cat-
alysts was set to 6508C.
2 NiO þ H2 ! Ni2O þ H2O
Ni2O þ H2 ! 2 Ni þ H2O
ð1Þ
ð2Þ
In contrast, supported NiꢀCu precursors could be reduced
at a much lower temperature than the 21Ni sample (Figure 4).
In the absence of Cu, nickel oxide was reduced stepwise at ap-
proximately 400 and 5508C; however, it was mostly reduced at
lower temperature (200–4008C) with at least three peaks in
the case of NiꢀCu precursors, which is likely to be due to syn-
ergistic interaction between the oxide phases and even physi-
cal mixing of NiO and CuO.[13,19] In addition, the re-
The H2 consumption during TPR and the theoretical values
are summarized in Table 2. The total amount of H2 consumed
duction profiles of NiꢀCu samples showed different
Table 2. Quantitative TPR results for the selected precursors supported on HZSM-5.
Precursor Metal content [mmolgꢀ1 H2 consumption [mmolgꢀ1
intensities at high and low reduction temperatures,
which can be related to a different Ni/Cu ratio. There-
fore, the presence of Cu in NiꢀCu bimetallic catalysts
strongly promotes Ni reduction. The reduction tem-
perature for the activation of NiꢀCu bimetallic cata-
lysts was set to 6008C.
]
]
Ni
Cu
Co
Theoretical[a] Experimental Exp./theor. ratio [%]
21Ni
3.64
–
–
–
–
–
3.64
3.36
3.36
2.96
3.29
3.57
3.78
3.87
4.23
3.02
3.11
3.08
3.00
3.16
3.44
3.56
3.74
3.67
82
93
92
101
96
96
94
97
87
16Ni4Cu 2.68
11Ni10Cu 1.85
4Ni14Cu 0.70
0.68
1.51
2.25
3.30
–
The TPR profiles of monometallic (21Ni and 19Co)
samples and those of NiꢀCo/HZSM-5 precursor series
are shown in Figure 5. For the 19Co catalyst, at least
three peaks were recorded at low (300–4508C) and
high (6508C) temperatures. This result can be attrib-
uted to the multiple oxidation states of Co and is in
good agreement with reports from Wang et al.[20] and
Zhao et al.;[21] the latter group detected CoO on the
Co/SBA-15 catalyst after the reduction at 6008C in H2
flow. In comparison, the effect of the addition of Co
21Cu
–
–
16Ni4Co 2.76
10Ni10Co 1.64
5Ni14Co 0.77
0.61
1.61
2.32
3.17
–
–
19Co
–
[a] These theoretical values were calculated on the basis of the total reduction of NiO
to Ni, of CuO to Cu, and of Co3O4 to Co.
during the TPR of 21Ni and 19Co was significantly lower than
to Ni on reducibility was not as strong as that of Cu in NiꢀCu
bimetallic precursors. Although the reduction profile was still
broad, complicated, and consisted of at least three peaks, the
temperature maximum was slightly decreased than in 21Ni
and 19Co samples. In addition, a new peak at low temperature
the theoretical value (82 and 87%, respectively), whereas the
total amount of 21Cu was close to the theoretical value. This
result indicates that nickel oxide and cobalt oxide are more dif-
ficult to reduce completely than copper oxide, which in turn
explains the beneficial effect of
Cu on Ni reducibility. In addition,
bimetallic precursors (NiꢀCu and
NiꢀCo) showed H2 uptake values
similar to theoretical values,
which reveals that the addition
of Cu and Co promotes the re-
duction of nickel oxide, probably
owing to the formation of
a spinel phase in the latter case.
In these studies, complicated
and broad peaks were observed
in the reduction profiles of mon-
ometallic and bimetallic catalysts
owing to the multiple oxidation
states of the metals and different
interaction between metals and
the HZSM-5 support. In addition
to the oxidation state of the
concerned metal, its position in
Figure 5. TPR profiles of Ni, Co, and NiꢀCo precursors supported on HZSM-5.
the lattice has to be considered
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(extra- versus intraframework) as well as the formation of new
structures such as NiCo2O4 spinel. More importantly, the addi-
tion of Cu or Co promotes the reduction of nickel oxide, which
leads to a shift of reduction peaks to lower temperatures, and
the effect of Cu is significantly stronger than that of Co.
Co than for the corresponding NiꢀCu samples; however, after
the reduction, this content remained stable. For the 5Ni14Co
sample, even with the lowest amount of Ni (Table 3), an in-
crease in the surface amount of Ni was observed whereas the
surface Co/Si ratio was the same for both the NiꢀCo samples.
From the above results, it is evident that the addition of Co
to Ni/HZSM-5 strongly stabilizes the Ni sites at the surface
whereas the addition of Cu shows the reverse effect.
X-ray photoelectron spectroscopy
The elemental compositions in the near-surface region of both
the calcined and reduced precursors (21Ni, 16Ni4Cu, 11Ni10Cu,
10Ni10Co, and 5Ni14Co) are listed in Table 3.
TEM
To gain more insight into the morphology, particle size, ele-
mental composition, and dispersion of metal species on the
support, the reduced 21Ni, 19Co, and 10Ni10Co samples were
selected for TEM analysis. In general, a broad distribution of
particle size and different shapes were observed (Figure 6). In
the 21Ni catalyst (Figure 6a), large particles of diameter 20–
30 nm represented the largest fraction whereas small particles
of diameter less than 2 nm were detected by spherical aberra-
tion-corrected high-angle annular dark field (HAADF) imaging.
A few larger particles of diameter up to 80 nm were also ob-
served. In the case of the 19Co catalyst (Figure 6c), most of
the particles were found in the size range of 8–10 nm. These
particles were accompanied by a netlike structure consisting of
particles less than 3 nm in diameter and some large particles.
In the case of the 10Ni10Co catalyst, mostly small (3–5 nm)-
and medium (10–20 nm)-sized particles were observed (Fig-
ure 6b). The 1:1 mixture of Ni and Co (Figure 6b) forms parti-
cle sizes in between the boundaries set by the pure Ni and Co
systems.
Table 3. Change in the elemental composition of the selected catalysts.
Sample
21Ni
Ni/Si ratio
Cu(Co)/Si ratio
calcined
reduced
16Ni4Cu
0.24
0.11
–
–
calcined
reduced
11Ni10Cu
calcined
reduced
10Ni10Co
calcined
reduced
5Ni14Co
calcined
reduced
0.14
0.04
0.04
0.03
0.13
0.02
0.03
0.06
0.06
0.04
0.04
0.06
0.05
0.07
0.19
0.19
The surface Ni/Si ratio of the reduced 21Ni precursor ac-
counts for 46% of the initial Ni/Si ratio of the calcined 21Ni
sample. This result could be attributed to the sintering or mi-
gration of Ni into the pores (bulk) during the reduction. The
addition of Cu or Co to Ni/HZSM-5 to form the bimetallic cata-
lyst showed a different effect on the fraction of Ni active sites
in the near-surface region. In the case of NiꢀCu/HZSM-5 cata-
lysts, the surface Ni/Si ratio decreased substantially to 29 and
15% for 16Ni4Cu and 11Ni10Cu samples, respectively, after the
reduction in comparison with that of the 21Ni catalyst (46%).
This result suggested that adding Cu does not stabilize Ni
active sites at the surface. In contrast, the surface Cu/Si ratio
was only slightly decreased for
One question that could not be answered through these
TEM images is the elemental composition in the case of
10Ni10Co. Because Ni and Co are neighbors in the periodic
table, neither differences in the HAADF images nor differences
in the lattices at high resolution are expected. For that reason,
energy-dispersive X-ray (EDX) mapping of all three samples
was performed (Figure 7). Notably, the support material de-
composed during EDX measurement. Most zeolites tend to
suffer damage by the strong and fast beam, in some cases at
200 kV within minutes. The images obtained before and after
EDX mapping are shown in Figure 8. During mapping, the
samples were scanned by an electron beam of approximately
16Ni4Cu after the reduction but
significantly
increased
for
11Ni10Cu, which leads to Cu en-
richment in the near-surface
region. These results are in
agreement with
Qiwu et al.[23]
a report of
In the presence of Co, the Ni
content in the near-surface
region is much more stable
during the reduction. In general,
for the calcined samples a lower
Figure 6. STEM–HAADF images of the reduced catalysts: a) 21Ni, b) 10Ni10Co, and c) 19Co. Arrows indicate small
Ni/Si ratio was observed for Niꢀ particles with weak contrast.
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IR spectra of adsorbed pyridine
To investigate the acid proper-
ties of some selected reduced
samples, the IR spectra of ad-
sorbed pyridine was used; the
results are presented in Figure 9
and Table 4. The spectra in
Figure 9 show two bands at
1544 and 1451 cmꢀ1 that are as-
signed to Brønsted and Lewis
acid sites, respectively. The total
acidity of some HZSM-5-support-
ed catalysts is also listed in
Table 4.
Figure 7. EDX mappings of the reduced catalysts: a) 21Ni, b) 10Ni10Co, and c) 19Co. Red: Si, green: Ni, blue: Co.
Scale bars=50 nm.
Figure 8. STEM-HAADF images of the same region of reduced catalyst
10Ni10Co before and after EDX mapping.
0.17 nm for 100–150 sweeps and every minute an image was
taken for drift control.
The HZSM-5 support collapsed owing to the loss of long-
range order (crystallinity) by the electron bombardment; how-
ever, the metal particles remained in the same position. They
seem to recrystallize under the measurement conditions be-
cause an increase in the size of the smallest particles and
a change in contrast for the larger particles were detectable. It
is possible that the loss of focus of the smallest particles oc-
curred and the contours of the large particles were blurred
owing to the collapsed support.
Figure 9. IR spectra of adsorbed pyridine on the HZSM-5 support and select-
ed monometallic and bimetallic catalysts at 1508C. L-Py=band for pyridine
bound to Lewis sites; PyH+ =band for pyridine bound to Brønsted sites.
Table 4. Acidity data for the support and selected monometallic and bi-
metallic catalysts.
Sample
Acidity [mmolgꢀ1
]
Brønsted
Lewis
Total
More importantly, EDX mapping gave information on the
distribution of different elements, as shown in Figure 7. More-
over, different particle sizes and netlike structures are clearly
visible in this figure. Furthermore, these EDX measurements
showed that the Ni/Co ratio was independent of the particle
size as well as of the position inside one metal particle (not
shown here). The color mix of metal particles in Figure 7b indi-
cates that the homogeneous NiꢀCo alloy with the same ratio
was found.
HZSM-5
21Ni
10Ni10Co
5Ni14Co
19Co
451
243
162
166
165
168
345
509
527
460
619
588
672
693
624
In comparison to neat HZSM-5, a significant decrease in the
overall Brønsted acidity was observed after impregnation with
a metal. This finding is consistent with previous suggestion[18]
that metal species (e.g., Ni2+) act as exchangeable cations and
replace protons at Brønsted sites. In contrast, overall Lewis
acidity increases and significantly surpasses the remaining
Brønsted acidity. However, the total acidity (pyridine amount)
remains the same (588–693 mmolgꢀ1). Although the formation
These results reveal that a homogeneous NiꢀCo alloy was
formed under the given preparation conditions and its compo-
sition was independent of the particle size. Moreover, the for-
mation of the NiꢀCo alloy led to a reduced metal particle size
on this material. Thus, the NiꢀCo alloy significantly increases Ni
dispersion compared with the 21Ni catalyst.
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of Lewis acid sites is not fully understood, this kind of acidity
could be generated via the dehydration of protonated oxide
bridges after high-temperature treatments in the calcination or
reduction steps.[24]
of Ni with Cu, the conversion decreased steadily. The exchange
of Ni with Cu (an inactive site) results in a decrease in Ni
amount (an active site) in the bimetallic NiꢀCu/HZSM-5 cata-
lyst, which leads to a decrease in activity. Moreover, the addi-
tion of Cu to Ni/HZSM-5 significantly decreased the number of
Ni active sites at the surface, as indicated by X-ray photoelec-
tron spectroscopy (XPS) results (Table 3).
In general, it has to be considered that all the catalysts de-
rived from such precursors offer metal sites after the reduction
as well as a combination of Brønsted and predominant Lewis
acid sites. Such a combination is preferable for the HDO reac-
tion because acid sites may cleave CꢀO bonds through proto-
nation and metal sites can activate H2 and reduce aromatic
rings.
The above results suggest that the 21Ni catalyst is highly
active in the HDO of phenol whereas the 21Cu catalyst is
highly inactive. The combination of metal and acid sites is
needed to efficiently eliminate oxygen from phenol. Tuning of
the metal sites may provide additional control of the hydroge-
nation severity (either attacking substituents or aromatic ring
systems) and selectivity. Therefore, the substitution of Ni with
Cu strongly affects both catalytic activity and selectivity. Such
an effect was reported, for example, by Sinfelt et al.[25] The ac-
tivity of NiꢀCu alloys decreases substantially over the entire
range of compositions for the hydrogenolysis of ethane to
methane, whereas the effect is little for the dehydrogenation
of cyclohexane with the same catalysts. Herein, the formation
of the NiꢀCu alloy was observed in both 11Ni10Cu and
4Ni14Cu catalysts and metallic Cu was detected as well
(Figure 2).
Activity test results
Phenol HDO on NiꢀCu/HZSM-5 series
The catalytic test results for bimetallic NiꢀCu/HZSM-5 and
monometallic catalysts (Ni/HZSM-5 and Cu/HZSM-5) as well as
for the pure support (HZSM-5) are presented in Figure 10. The
test with the support showed that the conversion of phenol
was less than 10% after 2 h at 2508C. The supported monome-
tallic 4Ni, 12Ni, and 21Ni catalysts showed higher phenol con-
versions of 14, 82, and 98%, respectively. With the last catalyst,
selectivity toward deoxygenated products reached 98%, which
included approximately 88% for cyclohexane and 8% for ben-
zene. In contrast, the 21Cu catalyst demonstrated extremely
less activity, which reached 2% phenol conversion, and thus
Cu sites are likely to be inactive for this reaction.
To clarify the effect of an admixture of Cu to Ni/HZSM-5 on
the catalytic performance, one more experiment was per-
formed. One catalyst was prepared by physical mixing of two
reduced monometallic catalysts (21Ni and 21Cu) in a weight
ratio of 1:1 (denoted as 21Ni+21Cu). In this case, the overall
weight fractions of Ni, Cu, and HZSM-5 were similar to those of
the 11Ni10Cu catalyst. However, the NiꢀCu alloy was present
in the 11Ni10Cu catalyst but not in the 21Ni+21Cu catalyst, as
With the substitution of only 2 wt% of Ni (related to catalyst
weight) with Cu (17Ni2Cu), the conversion decreased substan-
tially from 98 to approximately 50%. With further substitution
Figure 10. Phenol HDO on the parent HZSM-5 support, various monometallic catalysts, and NiꢀCu/HZSM-5 samples. Reaction conditions: 5.3 mmol of phenol,
10 g of H2O, 0.025 g of the catalyst, P=50 bar at room temperature, T=2508C, t=2 h.
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confirmed by the above characterization results. Surprisingly,
the conversion and selectivity toward deoxygenated products
obtained with the 21Ni+21Cu catalyst were much higher than
those obtained with the 11Ni10Cu catalyst and were compara-
ble with those obtained with the 12Ni catalyst (Figure 10). The
presence of a significant amount of pure Ni particles on the
surface of 21Ni+21Cu is sufficient to introduce high activity in
such a catalyst. Cu species themselves proved to be inactive
(see above) and cannot really interact with Ni particles.
HZSM-5, was tested in phenol HDO under the same reaction
conditions. The catalytic performance results in terms of
phenol conversion and product distribution are shown in
Figure 11. The 19Co catalyst showed low activity for this reac-
tion because conversion was less than 10%. Surprisingly, with
the substitution of 5 wt% of Co (related to catalyst weight)
with Ni (5Ni14Co), the conversion increased substantially to
96% and selectivity toward deoxygenated products reached
approximately 96%. Conversion and HDO selectivity obtained
with the 5Ni14Co sample are much higher than those obtained
with 4Ni and 12Ni samples (whereas the addition of Cu to cat-
alysts with a similar Ni content led to a substantial decrease in
activity), which could be due to the higher number of active
sites at the surface of 5Ni14Co, as indicated by XPS results (see
Table 3). Furthermore, for the catalyst with a Ni/Co ratio close
to unity (10Ni10Co), the conversion was complete and the se-
lectivity toward deoxygenated products achieved was the
highest observed (99.3%). These results are probably due to
the formation of a NiCo2O4 spinel phase in the catalyst precur-
sors, which leads to a higher dispersion of metal species in the
case of 5Ni14Co and 10Ni10Co than in the samples containing
less Co. As a proof, the addition of small amounts of Co to Ni
(16Ni2Co) reveals results similar to those obtained with the
monometallic 21Ni sample. The addition of larger amounts of
Co (16Ni4Co) but not in a ratio promoting a spinel formation
further lowers the activity, because the formation of a spinel
phase does not occur at all or does not occur to a sufficient
extent.
The product distribution indicates that the selectivity toward
the deoxygenated products of bimetallic NiꢀCu/HZSM-5 cata-
lysts was lower than that of the monometallic Ni catalyst. In
addition, benzene selectivity was lower in bimetallic NiꢀCu/
HZSM-5 catalysts, which could be due to the fact that separate
Cu sites may be involved in the consecutive reaction steps
after phenol has been activated by Ni sites first. There could
also be an effect of the NiꢀCu alloy that is active for aromatic
hydrogenation.[10] Furthermore, the significant amount of Cu at
the surface possibly affects the amount and performance of
active Ni sites. For example, Ene et al. reported that the high
strength of the interaction of Cu with benzene and phenol
was observed by using FTIR and EPR.[26] Our results reveal that
the admixture of Cu to bimetallic NiꢀCu/HZSM-5 catalysts sig-
nificantly lowers the conversion and selectivity toward hydro-
carbons in phenol HDO, which can be considered as a negative
synergistic effect.
Phenol HDO on NiꢀCo/HZSM-5 series
Moreover, little differences are observed in the product dis-
tribution, and 10Ni10Co showed the best selectivity toward hy-
drocarbons. In comparison with the 21Ni catalyst, the
The series of NiꢀCo/HZSM-5 bimetallic catalysts with different
Ni and Co loading, as well as the monometallic catalyst Co/
Figure 11. Phenol HDO on the parent HZSM-5 support, monometallic 21Ni and 19Co catalysts, and NiꢀCo bimetallic samples. Reaction conditions: 5.3 mmol
of phenol, 10 g of H2O, 0.025 g of the catalyst, P=50 bar at room temperature, T=2508C, t=2 h.
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10Ni10Co catalyst performed better in terms of cyclohexane
to occur. These sites were not provided by these three cata-
selectivity. Notably, the NiꢀCo catalysts outperformed the Niꢀ lysts, and therefore the reaction stopped at the ketone–alcohol
Cu catalysts in terms of activity and deoxygenation selectivity.
Assuming that phenol hydrogenation is the rate-determin-
ing step in the HDO process,[8c] the above-discussed effects of
the substitution of Co with Ni on solid-state properties as de-
termined from TPR, XRD, XPS, and TEM analysis could explain
the observed catalytic performances. The 21Ni sample is highly
active because it possesses the highest Ni content among all
catalysts. By adding Cu or Co, the Ni amount decreases; how-
ever, Co is more effective in forming highly dispersed alloys
with Ni. Although the total amount of highly active Ni decreas-
es, it is compensated by an increase in dispersion. By genera-
tion of small metallic Ni particles in an alloy, the activation and
insertion of H2 into phenol could be significantly accelerated.
Moreover, the formation of the NiꢀCo alloy with an appropri-
ate Ni/Co ratio contributes to an improved catalytic per-
formance. In contrast, selectivity could be controlled by the ad-
sorption mode of phenol on the surface. If the adsorption
would occur via the hydroxyl group on Brønsted sites, the
weakening of the OꢀC bond would make it more susceptible
to H2 attack, for example, by the H2 that was dissociated earlier
on surface metal sites. If the adsorption would occur via the ar-
omatic ring, possibly on Lewis sites, the reduction to cyclohex-
anol would be the first step, leading to another pathway via
cyclohexene.
stage. The effect of both the sites on the product distribution
can be seen in the work of Hong et al.,[7a] which uses Pt doped
on different acidic supports under typical conditions of 2508C
and 40 bar (1 bar=100 kPa) pressure at room temperature. In
contrast, Ohta et al. reported on a highly active acid-free Pt/AC
catalyst used for the HDO of 4-propylphenol at higher temper-
ature (2808C) and 40 bar pressure at room temperature.[7b] As
expected, hydrogenation is catalyzed by Pt; however, the sub-
sequent dehydration of the formed cyclohexanol also occurs.
The authors claim an in situ generation of protons in hot com-
pressed water (pKw 11.2 at 2808C), which catalyze the dehydra-
tion step.
HDO of the selected intermediate products
To gain more insight into the effect of the catalysts on the in-
dividual reaction steps, intermediate products cyclohexanol,
cyclohexanone, cyclohexene, and benzene were tested under
the same reaction conditions as used in phenol HDO. The re-
sults of the HDO of cyclohexanol on supported 21Ni,
10Ni10Co, and 19Co catalysts are presented in Figure 13a.
Conversion on the two Ni-containing catalysts was complete
and only slightly less than that on the Ni-free catalyst. This
result correlates well with the overall acidities of these materi-
als, properties that mostly govern the catalytic activity. Because
of the high acidities provided by the support, all three catalysts
led to high deoxygenation selectivities through dehydration.
The product distribution also reveals that the elimination of
oxygen attached to saturated rings is easier than that from
phenol (cf. Figure 11).
To understand the effect of metallic Ni or NiꢀCo alloy sites
on the catalytic performance, pure Ni and two NiꢀCo alloys
with different Ni/Co ratios (50:50 and 25:75) were prepared
without support and tested in phenol HDO. These materials
should be considered as nonacidic. The catalytic activities of
pure Ni and the NiꢀCo alloy (50:50) were comparable and
found to be slightly higher than the catalytic activity of the
NiꢀCo alloy (25:75) (Figure 12). These results indicate that the
NiꢀCo alloy itself can catalyze the hydrogenation of phenol to
cyclohexanone/cyclohexanol in the same way as pure Ni. Fur-
thermore, only cyclohexanone and cyclohexanol were obtained
in the product mixture, which confirmed that phenol was hy-
drogenated but HDO required both metal sites and acid sites
Nevertheless, the extent of the hydrogenation of cyclohex-
ene to cyclohexane on these catalysts was significantly differ-
ent and revealed the hydrogenation activity of the metal sites.
In the case of 19Co, cyclohexene accounted for 35% in the
product mixture, whereas it was converted completely to cy-
clohexane on both 21Ni and 10Ni10Co catalysts.
In the following experiments, conversion of other intermedi-
ates was studied only on 21Ni and 10Ni10Co catalysts. In the
case of cyclohexanone feed, both catalysts showed similar cat-
alytic performance in terms of conversion and selectivity. Of
the hydrogenation products, cyclohexane was predominant
with more than 90% selectivity, followed by cyclohexanol and
methylcyclopentane. The formation of methylcyclopentane
points to the effect of the available Brønsted sites, which leads
to the isomerization of the carbon skeleton (Figure 13b). The
conversion of cyclohexanone (requiring hydrogenation to cy-
clohexanol and hydrogenolysis) is most likely governed by the
metal sites that activate H2.
Cyclohexene was almost completely converted (conversion
>95%) to cyclohexane with more than 99% selectivity with
use of the Ni-containing catalysts (Figure 13c), and a small
amount of benzene was detected owing to the dehydrogena-
tion of cyclohexene. This finding is not surprising as it is
known that Ni can act as both a hydrogenation and a dehydro-
genation catalyst.
Figure 12. Phenol HDO on pure Ni and the NiꢀCo alloy. Reaction conditions:
5.3 mmol of phenol, 10 g of H2O, 0.02 g of the catalyst, P=50 bar at room
temperature, T=2508C, t=2 h.
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in
the
hydrodeoxygenation
(HDO) of phenol as a model
compound for bio-oil in batch
experiments.
The
experimental
results
reveal that Ni/HZSM-5 is more
active than monometallic Cu/
HZSM-5 and Co/HZSM-5 for
phenol HDO, most likely owing
to its outstanding capability of
H2 activation and dissociation.
Cu/HZSM-5 and Co/HZSM-5
show poor performance. For bi-
metallic catalysts, the addition of
Cu to Ni/HZSM-5 strongly pro-
motes Ni reducibility and leads
to the formation of the NiꢀCu
alloy but generates separate Cu
species with larger particle size;
these effects finally lower con-
version and selectivity toward
deoxygenated products. In con-
trast, the addition of Co to Ni/
HZSM-5 within an optimum Ni/
Co ratio (1) favors the formation
of a spinel phase after the calci-
nation step in the precursor
Figure 13. HDO of the selected intermediate products: a) cyclohexanol, b) cyclohexanone, c) cyclohexene, and
d) benzene. Reaction conditions: 5.3 mmol of the reactant, 10 g of H2O, 0.025 g of the catalyst, P=50 bar at room
temperature, T=2508C, t=2 h.
In the benzene conversion under HDO conditions, the
10Ni10Co catalyst showed higher activity than the 21Ni cata-
lyst (Figure 13d). These results help us understand why the se-
lectivity toward cyclohexane in phenol HDO with the
10Ni10Co catalyst was higher than with the 21Ni catalyst
(Figure 11). The bimetallic catalyst is more powerful and can
attack the benzene ring and thus promotes the final steps
toward cyclohexane at a higher reaction rate. This catalyst pro-
vides an excellent balance of acid and metal sites.
Coke deposition
To evaluate coke deposition on the catalysts, the carbon con-
tent of the used catalysts was measured with a CHN analyzer
after 2 h of reaction time. As shown in Figure 14, the monome-
Figure 14. Coke deposition after phenol HDO.
tallic catalyst 21Ni showed more coke deposition in compari-
son with any other catalysts of this series. The formation of the
alloy decreases the particle size, which possibly also helps to
lower the rate of coke deposition.[14,27] In addition, the stabiliza-
tion of Ni active sites by the presence of Co could affect the
coke deposition. Last but not least, the considerably lower
Brønsted acidity of Co-containing catalysts may also suppress
the coke deposition (Table 4).[28]
compounds and (2) leads to finely dispersed Ni domains after
the reduction. This fact not only increases the catalytic activity
but also changes the distribution of deoxygenated products
because some key hydrogenation steps in the reaction se-
quence were accelerated. The addition of Co also suppresses
the coke deposition as compared with monometallic catalysts.
In summary, we have developed an outstanding bimetallic
catalyst (10Ni10Co/HZSM-5) for phenol HDO in the aqueous
phase. These findings have left open the option for bio-oil up-
grading on such a new “alloy” material supported on a zeolite
under typical hydrothermal conditions.
Conclusions
Herein, various Ni-, Cu-, or Co-containing HZSM-5-supported
catalysts and two series of bimetallic catalysts (NiꢀCo/HZSM-5
and NiꢀCu/HZSM-5) were prepared, characterized, and tested
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where.[29] The calcined catalyst samples were powdered and placed
into the reactor and heated in N2 flow at 3008C for 30 min before
the TPR experiment. Optimum sample weights were estimated by
using Monti–Baiker equation.[30] After cooling the sample to 508C
in N2 flow, the TPR experiment was performed in 5.03% H2/He
(flow rate: 50 mLminꢀ1; temperature program: hold at 508C for
20 min, increase to 6508C at a rate of 10 Kminꢀ1, and hold at
6508C for 40 min). H2 consumed was monitored with a calibrated
thermal conductivity detector (TCD) (GOW-MAC Instruments).
Experimental Section
Catalyst preparation
All the catalysts used herein were prepared by using the incipient
wetness impregnation method. HZSM-5 (PZ-2/25H with SiO2/
Al2O3 =34, Zeochem AG, Switzerland) was calcined in air at 4508C
for 2 h to remove physisorbed water. Ni(NO3)2·6H2O (Merck), Cu-
(NO3)2·3H2O (Merck), and Co(NO3)2·6H2O (Fluka) were used as Ni,
Cu, and Co precursors, respectively. The calculated amounts of the
precursor (or a mixture of two precursors for bimetallic catalysts)
were dissolved in a known amount of deionized water and then
impregnated in the support under stirring for 16 h at RT. A rotary
evaporator was used to remove water. The samples were ground
and dried overnight at 1108C and then calcined in air at 5508C for
4 h (heating rate: 10 Kminꢀ1) to obtain the calcined catalyst. Before
the reaction test, the calcined catalyst powder was pre-reduced in
a tubular quartz reactor by heating from RT to the desired temper-
ature (depending on the catalyst; heating rate: 10 Kminꢀ1) in H2
flow (flow rate: 100 mLminꢀ1). The final temperature was main-
tained for 4 h. After cooling to RT, the reduced sample was trans-
ferred to the autoclave.
The XPS spectra were recorded with a VG Scientific ESCALAB 220-
iXL spectrometer using monochromatic AlKa radiation (E=
1486.6 eV). The binding energies were referenced to C1s at
284.8 eV. The areas of the peak were determined after background
subtraction and fitting with Gaussian–Lorentzian curves. From
these areas, the amount of each component in the near-surface
region was calculated by division by the element-specific Scofield
factor and the transmission function of the spectrometer.
TEM was used to gain more information on metal sites, their dis-
persion, and their particle size on the catalysts. The measurements
were performed on a JEM-ARM200F microscope (JEOL) operating
at 200 kV, which was aberration corrected by a CESCOR (CEOS, Ger-
many) for the scanning TEM applications. The microscope was
equipped with a JED-2300 (JEOL) energy-dispersive X-ray spec-
trometer for chemical analysis. HAADF combined with EDX imag-
ing was operated with a spot size of 0.16 nm and a 40 mm con-
denser aperture. The HAADF detector was operated with a spot
size of 0.13 nm and a 40 mm condenser aperture. For the bright-
field (BF) scanning TEM images, annular BF with a beam stopper
and a 3 mm BF aperture was used. The sample was ground and
deposited on a holey carbon-supported Cu grid and transferred to
the microscope.
Pure Ni and the NiꢀCo alloy were also prepared by using the same
method but without impregnation on the support.
Catalyst characterization
The metal content of the calcined samples was determined from
ICP–OES analysis (715-ES, Varian, USA). The sample (ꢁ10 mg) was
digested with aqua regia (8 mL) and hydrofluoric acid (2 mL). The
solution was filled up to 100 mL and analyzed by using ICP–OES.
The data analysis was performed with the Varian 715 ES software.
Elemental analysis of the spent catalyst was performed with a True-
space CHNS analyzer (Leco Instrument, Ltd.)
The N2 physisorption measurements were performed at ꢀ1968C
with a Micromeritics ASAP 2010 apparatus. Before measurements,
the samples were degassed in vacuum at 2008C for 4 h.
For acidity characterization by applying the IR technique, pyridine
was used as a probe molecule. The measurements in the transmis-
sion mode were performed on a Bruker Tensor 27 FTIR spectrome-
ter equipped with a heatable and evacuable homemade reaction
cell with CaF2 windows connected to a gas dosing and evacuation
system. The sample powders were pressed into self-supporting
wafers (diameter: 20 mm; weight: 50 mg). Before pyridine adsorp-
tion, the samples were pretreated by heating in 5% H2/He up to
4008C for 10 min, subsequent cooling to RT, and evacuation. Pyri-
dine was adsorbed at RT until saturation. Then, the reaction cell
was evacuated for removing physisorbed pyridine. The desorption
of pyridine was followed by heating the sample in vacuum up to
3008C and recording spectra every 50 K.
Catalytic activity measurement
In a typical test, phenol (0.5 g, 5.3 mmol), water (10 g), and a cata-
lyst (0.025 g) were loaded into an autoclave (25 mL volume, Parr
Instruments). The reactor was flushed with Ar and then with H2 to
remove air. Then, the autoclave was pressurized with H2 to 50 bar
at RT and heated to 2508C. The start time was recorded when the
required reaction temperature was reached, and the stirring speed
was set to 650 rpm. After the completion of the reaction, the auto-
clave was cooled to RT and the gas was analyzed by using an on-
line gas chromatograph (HP 5890) equipped with both a flame ion-
ization detector (PoraPLOT Q column, 25 mꢂ0.53 mmꢂ0.20 mm)
and a TCD (HP-PLOT Molsieve column, 25 mꢂ0.53 mm) using Ar as
a carrier gas. The liquid products (organic and aqueous phases)
were analyzed with another gas chromatograph (Shimadzu GC
17A) equipped with an autosampler and a flame ionization detec-
tor (CP-FFAP column, 25 mꢂ0.32 mm) using He as a carrier gas.
Mesitylene and 1,4-dioxane were used as the internal standards for
the quantification of organic and aqueous phases, respectively.
The XRD measurements used for the phase composition study
were performed with a theta/theta X-ray diffractometer (X’Pert
PRO, PANalytical, The Netherlands) using CuKa radiation (l=
1.5418 ꢁ) and operating at 40 kV and 40 mA and equipped with
a X’Celerator RTMS detector. The samples were scanned in the 2q
range of 5 to 658 or 5 to 808. The phase analysis was performed
with the program suite WinXPOW (STOE&Cie GmbH), which includ-
ed the powder diffraction file data based on the International
Centre of Diffraction Data.
Conversion and selectivity were calculated on the basis of the
number of carbon moles defined as follows. Carbon balances were
calculated from the detected products in gas and liquid phases
and reached 90%ꢂ3%. Missing carbon was mostly due to depos-
its on the surface of the catalysts and some unknown minor peaks
in chromatograms.
The TPR experiments were performed to investigate the reducibili-
ty of the catalyst as well as to determine the optimum tempera-
ture to reduce the metal oxide completely. The TPR experiments
were performed in a homemade setup described in detail else-
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D molesstarting material
Conversion ð%Þ ¼
ꢃ 100
ð3Þ
ð4Þ
molesstarting material
molesC atoms, product i
Selectivity ð%Þ ¼
ꢃ 100
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Received: February 5, 2014
Published online on && &&, 0000
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ChemCatChem 0000, 00, 1 – 13
&12
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ÞÞ
These are not the final page numbers!

FULL PAPERS
It is not good for metal to be alone:
Homogeneous NiꢀCo alloy supported
on HZSM-5 is highly active for the hy-
drodeoxygenation of phenol as a model
compound for bio-oil in terms of activi-
ty, hydrogenation of aromatic rings, and
coke resistance as compared with
monometallic and bimetallic NiꢀCu cat-
alysts.
T. M. Huynh,* U. Armbruster, M.-M. Pohl,
M. Schneider, J. Radnik, D.-L. Hoang,
B. M. Q. Phan, D. A. Nguyen, A. Martin*
&& – &&
Hydrodeoxygenation of Phenol as
a Model Compound for Bio-oil on
Non-noble Bimetallic Nickel-based
Catalysts
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 13
&13
&
ÞÞ
These are not the final page numbers!