Metallic nickel supported on mesoporous silica as catalyst for hydrodeoxygenation: effect of pore size and structure

Article abstract of DOI:10.1007/s11164-018-3377-1

Catalytic hydrodeoxygenation (HDO) of anisole, a methoxy-rich lignin-derived bio-oil model compound, was carried out over a series of Ni-containing (5, 10, 20, and 30?wt%) catalysts with commercial silica and ordered mesoporous silica SBA-15 as support. Both supports and catalysts were characterized by N₂ adsorption–desorption isotherms, X-ray diffraction, CO chemisorption, and transmission electron microscopy (TEM). Catalytic reaction was performed at 250?°C and 10?bar H₂ pressure. Depending on the catalyst support used and the content of active metal, the catalytic activity and product distribution changed drastically. Increase of the nickel loading resulted in increased anisole conversion and C₆ hydrocarbon (benzene and cyclohexane) yield. However, loading more Ni than 20?wt% resulted in a decrease of both conversion and C₆ yield due to agglomeration of Ni particles. In addition, Ni/SBA-15 samples exhibited much stronger catalytic activity and selectivity toward C₆ hydrocarbon products compared with Ni/silica catalysts. The differences in catalytic activity among these catalysts can be attributed to the effect of the pore size and pore structure of mesoporous SBA-15. SBA-15 can accommodate more Ni species inside channels than conventional silica due to its high pore volume with uniform pore structure, leading to high HDO catalytic activity.

Full text of DOI:10.1007/s11164-018-3377-1

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3377-1
Metallic nickel supported on mesoporous silica
as catalyst for hydrodeoxygenation: effect of pore size
and structure
Min Su Jang1 Tuan Ngoc Phan1 Im Sik Chung2,3 In-Gu Lee4
•
•
•
•
Young-Kwon Park5 Chang Hyun Ko1
•
Received: 2 September 2016 / Accepted: 24 November 2016
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract Catalytic hydrodeoxygenation (HDO) of anisole, a methoxy-rich lignin-
derived bio-oil model compound, was carried out over a series of Ni-containing (5,
10, 20, and 30 wt%) catalysts with commercial silica and ordered mesoporous silica
SBA-15 as support. Both supports and catalysts were characterized by N2 adsorp-
tion–desorption isotherms, X-ray diffraction, CO chemisorption, and transmission
electron microscopy (TEM). Catalytic reaction was performed at 250 °C and 10 bar
H2 pressure. Depending on the catalyst support used and the content of active metal,
the catalytic activity and product distribution changed drastically. Increase of the
nickel loading resulted in increased anisole conversion and C6 hydrocarbon (ben-
zene and cyclohexane) yield. However, loading more Ni than 20 wt% resulted in a
decrease of both conversion and C6 yield due to agglomeration of Ni particles. In
Min Su Jang and Tuan Ngoc Phan have contributed equally to this work.
&
&
&
1
In-Gu Lee
samwe04@kier.re.kr
Young-Kwon Park
catalyca@uos.ac.kr
Chang Hyun Ko
chko@jnu.ac.kr
School of Chemical Engineering, Chonnam National University, 77, Yongbong-ro, Buk-gu,
Gwangju 61186, Republic of Korea
2
Polymer and Nanomaterial Research Part, Korea Research Institute of Bioscience and
Biotechnology, 125 Gwahak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea
3
Nanobiotechnology, University of Science and Technology, 217 Gajeong-ro, Yuseong-gu,
Daejeon 34113, Republic of Korea
4
Biomass and Waste Energy Laboratory, Korea Institute of Energy Research, 152 Gajeong-ro,
Yuseong-gu, Daejeon 34129, Republic of Korea
5
School of Environmental Engineering, University of Seoul, Seoul 02504, Republic of Korea
123

M. S. Jang et al.
addition, Ni/SBA-15 samples exhibited much stronger catalytic activity and
selectivity toward C6 hydrocarbon products compared with Ni/silica catalysts. The
differences in catalytic activity among these catalysts can be attributed to the effect
of the pore size and pore structure of mesoporous SBA-15. SBA-15 can accom-
modate more Ni species inside channels than conventional silica due to its high pore
volume with uniform pore structure, leading to high HDO catalytic activity.
Keywords Hydrodeoxygenation Á Supported Ni catalysts Á Mesoporous
silica Á SBA-15 Á Anisole
Introduction
As a result of consumption of fossil fuels such as oil, coal, and natural gas, global
warming and environmental pollution have become one of the most important issues
in human history. The drive towards use of alternative and renewable energy source
has gradually become a general tendency worldwide [1–3]. Among renewable
energy sources, biomass has high utilization potential, not only for energy but also
as transportation fuel or for chemical production. Many studies have reported
investigations on processes for transformation of biomass into liquid-phase biofuel.
Fast pyrolysis is a simple and effective method to convert high-molecular-weight
biomass to low-molecular-weight bio-oil. However, bio-oil produced from fast
pyrolysis contains high oxygen concentration, limiting its application on industrial
scale [2, 4, 5]. The best strategy for removing these oxygen-containing compounds
is catalytic hydrodeoxygenation (HDO) [2, 6–8].
The HDO process takes place during treatment of bio-oil by hydrogen in the
temperature range from 250 to 450 °C and high pressure (20–300 bar), so that
oxygen can be removed in the form of water [9, 10]. However, under these
conditions, complete hydrogenation of the aromatic ring of substituted phenols is
inevitable, leading to high hydrogen consumption [10]. Therefore, it is necessary to
design a highly active catalyst for the HDO reaction to allow direct deoxygenation
of lignin model compounds to aromatic hydrocarbons under mild reaction
conditions. A remarkable number of catalysts have been investigated. Industrial
sulfide CoMo and NiMo catalysts have been widely studied for hydrotreatment of
model compounds [11–15]. Other catalysts based on transition metals [16, 17],
supported noble metal [18–21], and metallic phosphides [22, 23] have been
demonstrated, exhibiting comparable catalytic activity for HDO. Nickel-supported
catalysts seem to be promising because of their low cost, stability, and high activity
[5].
Recently, due to facile diffusion of reactants into pores, mesoporous materials
have attracted considerable attention from many researchers [24–26]. Ru-promoted
Co/mesoporous silica SBA-15 was reported to be a remarkable promising catalyst
for many kinds of reaction, including Fischer–Tropsch synthesis, dry reforming, and
ethane dehydrogenation [27–29]. Owing to their regular pore structure and
inertness, mesoporous silica materials have been used as supports for catalysts for
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Metallic nickel supported on mesoporous silica as…
HDO of biomass-derived compounds [5, 30]. However, studies on the role of the
mesoporous structure of the support for hydrotreatment of lignin model compounds
are limited.
In the work presented herein, ordered mesoporous silica SBA-15 with high
specific surface area and sharp pore size distribution was synthesized. The obtained
mesoporous material was utilized as support for nickel catalysts to investigate their
catalytic activity for HDO of a lignin model compound (anisole). A series of Ni-
containing (5, 10, 20, and 30 wt%) catalysts were prepared and used in the HDO
reaction at 250 °C for 2 h to understand the role of the active metal content. In
addition to mesoporous silica, the activity of Ni on commercial silica (Ni/silica)
catalyst was also studied to examine the effect of the mesopore structure of the
support on the catalytic activity in the HDO reaction.
Experimental
Preparation of catalysts
SBA-15 was synthesized according to a preparation method described in literature
[31]. The typical method was as follows: P-123 (8 g, Aldrich) was first dissolved in
mixed solution of 320 g 2 M HCl and 60 g deionized water under vigorous stirring
until completely dissolved. Then, tetraethylorthosilicate (17 g, Aldrich) was added
to this solution, followed by stirring for 5 min. The solution was kept at 35 °C for
20 h, followed by posttreatment at 100 °C for 24 h. The resulting white powder was
filtered and dried at 100 °C overnight. Finally, the dried powder was calcined at
550 °C for 2.5 h to obtain mesoporous silica SBA-15.
Nickel was incorporated on the surface of each silica support (conventional silica
and SBA-15) by impregnation method using aqueous solution of nickel nitrate
hexahydrate [Ni(NO3)2Á6H2O, Aldrich]. An exact amount of Ni precursor was
impregnated into SBA-15 to obtain various Ni loadings (5, 10, 20, and 30 wt%).
The samples were calcined in air flow at 500 °C for 3 h, followed by reduction in
hydrogen flow at 400 °C within 3 h. Finally, the sample was passivated in oxygen
environment at room temperature for 30 min before HDO reaction.
Characterization of catalysts
High-angle X-ray diffraction (D/MAX Ultima III, Cu Ka, Rigaku) and low-angle
XRD (X/Pert PRO Multi Purpose, Cu Ka) analyses were used to characterize the
phase structure of the pure supports and the Ni-supported catalysts. N2 adsorption–
desorption isotherms were collected at 77 K using a Micromeritics ASAP2020. The
specific surface area was calculated according to the Brunauer-Emmett-Teller
(BET) method. The pore size distribution was identified from the desorption
isotherm using the Barrett–Joyner–Halenda (BJH) method. The Ni dispersion and
particle size were calculated from the adsorbed amount of CO, measured by CO
chemisorption at 50 °C. All samples were reduced at 400 °C for 3 h in 10% H2 gas
flow before treating with 10% CO ? 90% He gas mixture. The morphology of Ni
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M. S. Jang et al.
crystallites was observed by transmission electron microscopy (TEM, CM200). The
Ni loading on the catalysts was measured by inductively coupled plasma optical
emission spectroscopy (ICP-OES, OPTIMA8300, PerkinElmer, USA).
Catalytic test
Hydrodeoxygenation of anisole was conducted in a 100-mL autoclave-type batch
reactor. Typically, 1.0 g catalyst and 40 mL 10 wt% anisole solution were added
into the reactor. Dodecane (Aldrich) was used as solvent for the anisole solution.
Nitrogen gas was purged into the reactor to form an inert environment, then
hydrogen gas was filled into the reactor to initial hydrogen pressure of 10 bar. The
reactor was heated to 250 °C and maintained at this temperature for 2 h. The reactor
was cooled naturally to room temperature after completion of the HDO process. The
solid product of the HDO reaction was filtered and washed with toluene and acetone
several times. The liquid product was analyzed by gas chromatography (GC,
Agilent Technology 6890) using an HP-INNOWAX column and flame ionization
detector (FID).
Results and discussion
Catalyst properties
Supported nickel catalysts were prepared using various metal loading contents and
different supports for HDO of anisole to understand the mechanism of this complex
reaction. The N2 adsorption–desorption isotherms and pore size distribution of all
samples are displayed in Fig. 1. According to the International Union of Pure and
Applied Chemistry (IUPAC) classification, both supports and catalysts possessed
type IV sorption isotherms, as seen in Fig. 1a, b, which is associated with porous
materials. However, while the hysteresis loop of the conventional silica-based
catalysts was type H2 with narrow neck and wider body (ink-bottle pores), the Ni
catalyst supported on SBA-15 showed type H1 hysteresis, indicating a narrow
distribution of relatively uniform (cylindrical-like) pores [32]. For all samples,
loading of active metal onto both supports resulted in an evident drop of both
specific surface area and total pore volume, indicating deposition of metal
nanoparticles within the channels of the supports. The porous structure of both silica
supports was maintained after incorporation of nickel.
Figure 1c, d shows the corresponding pore size distributions of the silica supports
and catalysts. The Ni/SBA-15 catalysts and SBA-15 support exhibited uniform
pores with sharp pore size distribution. The appearance of a small peak at 4 nm in
these cases when a large amount of Ni was loaded may be due to presence of Ni
species in the pores, resulting in a lower proportion of pores of intermediate size
(6–7 nm). On the other hand, as can be observed, the silica support and Ni/silica
catalysts displayed a wide range of pore size with average of 6–8 nm, meaning that
the conventional silica had only a small portion of mesopores.
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Metallic nickel supported on mesoporous silica as…
Fig. 1 N2 adsorption–desorption isotherms of a Ni/SiO2 and b Ni/SBA-15 catalysts; corresponding pore
size distribution of c Ni/SiO2 and d Ni/SBA-15 catalysts
The textural properties of the supports and corresponding supported catalysts are
summarized in Table 1, which also lists the Ni loadings identified by ICP-OES. The
actual Ni content values are slightly lower than the designed values. In spite of the
similar average pore size (6–8 nm), the surface area of the catalysts supported on
SBA-15 was much higher than for those supported on silica. Although the active
component was loaded up to 20 wt%, the physical properties of the silica and
mesoporous silica support did not change much compared with the pristine silica
and SBA-15.
Figure 2 displays the XRD patterns of the supported nickel catalysts. The broad
peak, observed in all patterns and centered at 2h of 23°, is attributed to amorphous
silica. In the case of the calcined supported nickel catalysts (Fig. 2a, c), four peaks
appearing at 2h = 37.1°, 43.3°, 62.9°, and 76.5° can be matched with (111), (200),
(220), and (311) crystal planes of monoclinic nickel(II) oxide (NiO), respectively, in
Joint Committee on Powder Diffraction Standards (JCPDS) card no. 47-1049. After
reduction at 400 °C, the XRD patterns (Fig. 2b, d) of the reduced Ni/silica and Ni/
SBA-15 samples showed peaks at 2h of 45.5°, 51.9°, and 76.3°, respectively,
corresponding to lattice plane (111), (200), and (220) of metallic nickel (JCPDS
04-0850). No peaks corresponding to NiO were found in the XRD patterns of the
reduced samples, demonstrating that NiO species in the calcined catalysts was
completely reduced to active metallic Ni particles. In addition, with increasing
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M. S. Jang et al.
Table 1 Physicochemical properties of supports and supported metal catalysts
Catalyst Ni
loading
(wt%)
Specific
surface area
(m2/g)
Total pore
volume
(cm3/g)
Average
pore size
(nm)
CO
uptake
(lg/mol)
Dispersion Ni
(%)
particle
size (nm)
SiO2
5Ni/
–
510
421
0.79
0.63
7.1
6.2
–
–
–
3.2
60
10.9
9.2
SiO2
10Ni/
SiO2
9.1
391
352
0.55
0.52
6.2
6.2
159
65
10.2
2.0
9.9
20Ni/
SiO2
18.7
49.4
SBA-15
–
767
747
1.05
1.05
6.2
6.2
–
–
–
5Ni/
SBA-
15
4.0
82
12.1
8.4
10Ni/
SBA-
15
7.9
654
574
0.91
0.77
6.2
6.2
220
67
16.3
2.4
6.2
42
20Ni/
SBA-
15
16.2
nickel loading from 5 to 20 wt%, the sharpness and intensity of the peaks increased,
indicating presence of larger metallic particles.
The low-angle X-ray diffraction patterns (0.75–2.5°) of the pristine mesoporous
silica SBA-15 support and corresponding SBA-15-based catalysts are shown in
Fig. 2e. Three well-resolved peaks attributed to (100), (110), and (200) planes of
pristine SBA-15 are clearly observed for all samples, indicating that the typical one-
dimensional hexagonal mesopore structure of SBA-15 was retained after nickel
incorporation. This is in good agreement with data collected from N2 adsorption–
desorption measurements.
TEM images of reduced Ni-supported catalysts are shown in Fig. 3, revealing Ni
particles with sphere-like morphology. For the Ni/silica catalysts, Ni species were
overlapped and incorporated with others in an irregular way. In contrast, the Ni/
SBA-15 samples exhibited uniform long channels, and a large portion of the Ni
particles located in cylinder-like pores. Generally, with increasing Ni loading, the
Ni particles tended to agglomerate and form larger particles. When the Ni content
was increased to 20 wt%, the overwhelming majority of Ni species were located
outside the channels of the mesoporous SBA-15.
The dispersion and average Ni particle size estimated from CO chemisorption
measurements are reported in Table 1. These results indicate that increasing the Ni
content from 5 to 10 wt% led to an increase of CO uptake, because the amount of
metal active sites was increased. However, further increase of the Ni loading to
20 wt% resulted in a drastic decrease in the amount of CO adsorbed, which is
attributed to aggregation of Ni particles. Particle aggregation results in not only a
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Metallic nickel supported on mesoporous silica as…
Fig. 2 XRD patterns of a nickel oxide supported on silica, b metallic nickel supported on silica, c nickel
oxide supported on mesoporous SBA-15, and d metallic nickel supported on mesoporous silica SBA-15;
e Low-angle XRD patterns of Ni/SBA-15 with different nickel loadings from 5 to 20 wt%
lower number of active sites but also larger particle diameter, in accordance with the
XRD patterns and TEM images. Among the studied catalysts, 10 wt% Ni/SBA-15
exhibited the best metal dispersion and smallest Ni particle size. In summary, the
data from TEM images and CO chemisorption measurements indicate that the
distribution of active sites is strongly influenced by the pore size and pore structure
of the support.
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M. S. Jang et al.
Fig. 3 TEM images of a Ni/silica, b 5Ni/silica, c 10Ni/silica, d 5Ni/SBA-15, e 10Ni/SBA-15, and
f 20Ni/SBA-15
Catalytic activity
We evaluated and compared the catalytic activity of the Ni catalysts supported on
two types of silica (silica and SBA-15) in HDO of anisole (methoxy-rich model
compound). Since the investigated catalysts had almost the same nickel loading, it is
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Metallic nickel supported on mesoporous silica as…
expected that any activity disparity can mainly be attributed to differences in the
pore size and pore structure of the support. Figure 4 presents the conversion of
anisole over these two catalysts with different nickel contents. The supported Ni
catalysts showed quite good conversion of anisole ([ 50% for 10 wt% nickel
loading). Metallic nickel is considered to be promising in terms of active sites for
hydrodeoxygenation of lignin-derived compounds [5, 9]. At low Ni content
(5 wt%), the silica- and SBA-15-based catalysts exhibited the same level of anisole
conversion. However, there was a clear difference when a larger amount of nickel
was incorporated into the channels of the support. The conversion over the Ni/SBA-
15 catalyst drastically increased with increasing Ni content up to 20 wt%, being
much higher than that over Ni/silica. This could be due to the uniformity of the pore
structure and high pore volume of the mesoporous SBA-15 support. Higher total
pore volume allows incorporation of more nickel particles deep inside pores,
resulting in a uniform distribution of active sites over the support. In the case of the
conventional silica support, some portion of the Ni particles had to be located on the
external surface of the support because the pores were almost occupied due to the
high Ni content. When the Ni loading was increased to 30 wt%, a decrease of
anisole conversion appeared, which can be attributed to agglomeration of Ni
particles for such high Ni content. Agglomeration of Ni particles, decreasing the
active sites, was also observed for the 20 wt% Ni catalysts, and became much
stronger for the 30 wt% Ni catalysts.
The product distributions of HDO of anisole using various Ni contents are shown
in Fig. 5. The main identified products were saturated compounds such as
cyclohexane, methoxycyclohexane, and cyclohexanol, directly deoxygenated
Fig. 4 Anisole conversion over supported nickel catalysts. Reactions carried out using 1.0 g catalyst at
250 °C for 2 h
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M. S. Jang et al.
Fig. 5 Product distributions of a Ni/SiO2 and b Ni/SBA-15 catalysts. Reactions carried out using 1.0 g
catalyst at 250 °C for 2 h
benzene, along with intermediates such as cyclohexene, cyclohexanone, n-hexane,
and phenol. As seen in Fig. 5, the product distribution changed significantly
depending on the nickel loading and kind of silica support used. Anisole was mainly
converted to cyclohexanol when using the 5 wt% Ni/silica catalyst, while most of
123

Metallic nickel supported on mesoporous silica as…
the anisole was converted to methoxycyclohexane when using 5 wt% Ni/SBA-15,
indicating that hydrogenation (HYD) was the dominant reaction pathway at low Ni
loading. With increasing nickel loading, this trend shifted toward the direct
deoxygenation (DDO) route with a drastic increase of the yield toward the
unsaturated product (benzene) over both supports (conventional silica and SBA-15).
In particular, the Ni/SiO2 catalysts directly deoxygenated anisole to form benzene
without hydrogenation. In contrast, the Ni/SBA-15 catalysts enhanced the DDO
route and saturation of the aromatic ring via HYD to form cyclohexane
simultaneously.
From the viewpoint of hydrodeoxygenation, the yield of hydrocarbons such as
cyclohexane and benzene is an important indicator to investigate the degree of HDO
reaction. Therefore, the yield of C6 hydrocarbons is shown in Fig. 6, calculated as
the sum of benzene and cyclohexane yields, to compare the influence of the silica
and mesoporous silica supports. The yield of C6 products increased suddenly as the
level of nickel loading was increased. Furthermore, the type of support also affected
the C6 hydrocarbon yield. In case of using mesoporous SBA-15 as support, the yield
of C6 hydrocarbon was much higher (approximately twice) than for the silica
support. This result can mainly be attributed to the differences in the mesoporous
structure and pore size distribution of the supports, demonstrating that the
mesoporous structure of the support controls the HDO reaction route. In spite of
the fact that the particle diameter of the 20 wt% Ni/SBA-15 catalyst was larger than
that of 10 wt% Ni/SBA-15 due to agglomeration of Ni particles, this catalyst
exhibited the strongest catalytic activity for HDO of anisole and the highest yield of
Fig. 6 C6 hydrocarbon yield versus kind of catalyst. Reactions carried out using 1.0 g catalyst at 250 °C
for 2 h
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M. S. Jang et al.
C6 hydrocarbons. Ni loading of 30 wt% resulted in not only decreased anisole
conversion but also decreased C6 hydrocarbon yield. Thus, 20 wt% can be
considered to be the maximum Ni loaded onto the support. Selection of a
suitable active site support system and optimization of the content of active sites for
specific reaction conditions are necessary steps for further application of such
catalysts on industry scale.
Conclusions
Nickel nanoparticles supported on silica and ordered mesoporous silica SBA-15
were prepared as catalysts for HDO of lignin model compound anisole. High nickel
loading decreased the surface area and pore volume of the silica supports, but did
not affect their mesoporous structure. The catalytic activity of the catalysts for HDO
reaction of anisole depended drastically on the level of nickel content and the kind
of support used. High nickel incorporation resulted in strong catalytic activity and
selectivity toward directly deoxygenated product, indicating that nickel has
promising active sites for catalytic upgrading of biomass. Moreover, the
mesoporous structure of the SBA-15 support facilitated diffusion of Ni species
into the pores, promoting these active sites, which led to higher anisole conversion
and C6 product yield.
Acknowledgements This work was conducted under the framework of the Research and Development
Program of the Korea Institute of Energy Research (KIER) (B8-2426-1). This work was also supported by
the National Research Council of Science & Technology (NST) grant by the Korea government (MSIP)
No. CAP-16-05-KIMM.
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