Nickel-promoted mesoporous ZSM5 for carbon monoxide methanation

Article abstract of DOI:10.1039/c5ra11661a

Nickel-promoted mesoporous ZSM5 (Ni/mZSM5) was prepared for CO methanation. XRD, NMR and SEM analysis confirmed the structural stability of Ni/mZSM5 with coffin type morphology. The nitrogen physisorption and pyrrole adsorbed FTIR analyses indicated the presence of micro-mesoporosity and a moderate amount of basic sites on both mZSM5 and Ni/mZSM5. At 623 K, Ni/mZSM5 showed a high rate of CO conversion (141.6 μmol CO g-cat^-1 s^-1) and 92% CH₄ yield. Ni/mZSM5 showed better catalytic performance than Ni/MSN (82.4 μmol CO g-cat^-1 s^-1, 82% CH₄ yield), Ni/HZSM5 (29.0 μmol CO g-cat^-1 s^-1, 54.5% CH₄ yield), and Ni/γ-Al₂O₃ (14.5 μmol CO g-cat^-1 s^-1, 38.6% CH₄ yield). It is noteworthy that the superior catalytic performance of Ni/mZSM5 could be attributed to the presence of both micro-mesoporosity and basicity, which led to a synergistic effect of Ni metal active sites and the mZSM5 support. In situ FTIR spectroscopy showed that CO and H₂ may be adsorbed on Ni metal followed by spillover to form adsorbed CO and adsorbed H on the mZSM5 surface. Then, two possible mechanisms for CO methanation were proposed. In the first mechanism, the adsorbed CO may be reacted with H₂ to form CH₄ and H₂O. In the second mechanism, the adsorbed H may be reacted with CO to form CH₄ and CO₂. However, in this case, the former is the predominant pathway as the methanation reaction is favored by inhibition of the water-gas shift reaction.

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DOI: 10.1039/C5RA11661A.
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Nickel-promoted mesoporous ZSM5 for carbon
monoxide methanation
Cite this: DOI: 10.1039/x0xx00000x
a
a,b*
c,d
a
d
L. P. Teh, S. Triwahyono, A. A. Jalil, C. R. Mamat, S. M. Sidik, N. A. A.
d
e
f
Fatah, R. R. Mukti, and T. Shishido,
Received 00th June 2015,
Accepted 00th June 2015
Nickel-promoted mesoporous ZSM5 (Ni/mZSM5) was prepared for CO methanation. XRD,
NMR and SEM analysis confirmed the structural stability of Ni/mZSM5 with coffin type
morphology. The nitrogen physisorption and pyrrole adsorbed FTIR analyses indicated the
presence of micro–mesoporosity and a moderate amount of basic sites on both mZSM5 and
Ni/mZSM5. At 623 K, Ni/mZSM5 showed a high rate of CO conversion (141.6 μmol CO/ g-
cat s) and 92% CH
μmol CO/ g-cat s, 82% CH
Ni/γ-Al (14.5 μmol CO/ g-cat s, 38.6% CH
4
4
yield. Ni/mZSM5 showed better catalytic performance than Ni/MSN (82.4
yield), Ni/HZSM5 (29.0 μmol CO/ g-cat s, 54.5% CH yield), and
yield). It is noteworthy that the superior
4
4
2
O
3
catalytic performance of Ni/mZSM5 could be attributed to the presence of both micro–
mesoporosity and basicity, which led to a synergistic effect of Ni metal active sites and the
2
mZSM5 support. In situ FTIR spectroscopy showed that CO and H may be adsorbed on Ni
metal followed by spillover to form adsorbed CO and adsorbed H on the mZSM5 surface.
Then, two possible mechanisms for CO methanation were proposed. In the first mechanism,
DOI: 10.1039/x0xx00000x
www.rsc.org/
the adsorbed CO may be reacted with H
2
to form CH
4
and H
2
O. In the second mechanism, the
adsorbed H may be reacted with CO to form CH
4
and CO
2
. However, in this case, the former is
the predominant pathway as the methanation reaction is favored by inhibition of the water–gas
shift reaction.
In previous reports, catalytic performances for CO methanation
have been mostly investigated on various supports, such as silica,
Introduction
4
-10
alumina, and mesoporous material.
plasma prepared Ni/SiO on CO methanation. It gave about 82%
CO conversion at 673 K. Guo et al. studied the effect of ZrO in
Yan et al. reported the use of
2
Methanation of carbon oxides (CO and/or CO ), also known as the
5
2
Sabatier reaction, has been an indispensable reaction for producing
1
methane. The catalytic conversion of syngas (H
2
+ CO) into
2
6
Ni/Al
2 3
O for CO methanation. 100% CO conversion was obtained
methane (so-called synthetic natural gas, SNG) is currently of utmost
importance owing to the requirements of environmental regulations.
Due to the abundance of carbon monoxide released into the
atmosphere, methanation of CO has attracted increasing attention for
effectively mitigating CO buildup and recycling the carbon
at 623 K. On the other hand, Liu et al. studied the influence of V
2
7
O
3
in the catalytic performance of Ni/Al
2
O
3
for CO methanation. At
yield.
6
73 K, it showed nearly 100% CO conversion and 89% CH
4
Moreover, Zhang et al. reported that 10 wt% Ni-MCM-41 exhibited
2
excellent activity and stability in the CO methanation with 95.7%
resource. Besides, the catalytic methanation of CO is also
8
CH
4
yield at 623 K. Besides, Gao et al. prepared the high surface
2
potentially an effective method of reducing the content of CO in H -
area Ni supported on barium hexaaluminate (Ni/BHA) for improved
rich reformate gas mixture, which is normally used in fuel cell
9
3
CO methanation compared with the conventional Ni/BHA. It gave
applications.
1
00% CO conversion and 95.7% CH
4
yield at 673 K. In addition, Jia
et al. reported the improved CO methanation with the use of nickel
a
10
Department of Chemistry, Faculty of Science, Universiti Teknologi supported on the perovskite oxide CaTiO
3
(Ni/CTO). At 673 K, it
Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia.
showed 100% CO conversion and 84% CH yield. Nevertheless,
4
b
Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi they are seldom supported on zeolites. For many catalytic reactions,
Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia.
structure and activity were greatly influenced by the nature of the
c
Institute of Hydrogen Economy, Universiti Teknologi Malaysia, 81310 UTM
Johor Bahru, Johor, Malaysia.
Department of Chemical Engineering, Faculty of Chemical Engineering,
11-14
support material.
d
Zeolites have proven to be suitable for a variety of applications
in industrial heterogeneous catalysis, separation, and adsorption
processes. Zeolite ZSM5 is a crystalline aluminosilicate with an MFI
Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia.
e
Division of Inorganic and Physical Chemistry, Faculty of Mathematics and
Natural Sciences, Institut Teknologi Bandung, Jl Ganesha No 10, Bandung structure. It possesses both acidic and basic sites. The bridging OH
4
0132, Indonesia.
groups, the trigonally coordinated and extra framework aluminum
f
15
Department of Applied Chemistry, Graduate School of Urban contributed to the acidity. While, the basicity is due to the basic
Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa,
Hachioji, Tokyo 192-0397, Japan.
framework oxygen atoms bearing the negative charge. The negative
charge on the oxygen atoms is enhanced as the electropositive
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character of the nonframework compensating cations increases.16 following molar composition of CTAB:EG:NH
OH:H
O
=
4
2
The extraordinary catalytic performance of zeolite catalysts is due to 0.0032:0.2:0.2:0.1. After vigorous stirring for about 30 min at 353
their crystalline frameworks and topological channel structures.17 K, 1.2 mmol of tetraethyl orthosilicate and 1 mmol of 3-aminopropyl
However, the relatively small individual micropores in zeolites cause triethoxysilane were added to the clear mixture to give a white
diffusion limitations and significantly influence the transportation to suspension solution. This solution was then stirred for another 2 h,
and from the active site, severely limiting their application in and the sample was collected by centrifugation at 3000 rpm. The
industry. Moreover, deactivation caused by coke formation is also a synthesized MSN was dried at 333 K and calcined at 823 K for 3 h.
severe problem that routinely arises in catalytic applications
The 5 wt% Ni-promoted supports were prepared by the wet
catalyzed by zeolites. Therefore, mesoporous zeolites possessing impregnation method over mZSM5, HZSM5, γ-Al , and MSN
micro–mesoporosity are urgently needed as an effective solution to supports. The aqueous nickel nitrate (Ni(NO ·6H O) was
overcome these drawbacks. impregnated on the support at 353 K, and was then dried in an oven
1
8
2
O
3
3
)
2
2
A large number of supported metal catalysts have been reported at 383 K overnight before calcination in air at 823 K for 3 h.
to be active for CO methanation. Various transition metals like Ni,
Co, Rh, Ru, Pd, Pt, and so on have been investigated over different Characterization
1
9-20
supports.
However, some noble metals such as Rh and Ru are
not economical for large-scale production of methane due to their The crystalline structure of the catalyst was studied by X-ray
high cost. Therefore, the use of nickel-based catalysts is preferred diffraction (XRD) recorded on a Bruker Advance D8 X-ray powder
from the commercial standpoint because of their low cost and wide diffractometer (40 kV, 40 mA) using Cu Kα radiation source in the
availability. It should be noted that the catalytic performance of the range of 2θ = 2–80° with a scan rate of 0.1° continuously. The
nickel-based catalysts depends not only on the active nickel metal nitrogen physisorption analysis of the catalysts was carried out by
sites but also on the chemical and physical properties of the using a Beckman Coulter SA 3100. Prior to the measurement, the
supporting materials.
catalyst was put into a sample tube holder, followed by evacuation at
In our previous work, we prepared mesoporous ZSM5 573 K for 1 h. Then, adsorption of nitrogen was carried out at 77 K.
mZSM5) by the dual templating method and tailored the zeolite Surface area, pore size distributions and pore volumes were
(
2
1
properties by varying the aging time. In the present work, we determined from the sorption isotherms using a non-local density
prepared nickel-promoted mesoporous ZSM5 (Ni/mZSM5) for CO functional theory (NLDFT) method. MAS NMR spectra were
2
7
methanation. The correlation of their physicochemical properties obtained using a Bruker Avance 400 MHz spectrometer. Al MAS
with the catalytic performances is presented and discussed. For NMR spectra were obtained at 104.2 MHz using pulse length of 1.9
2
9
comparison purposes, we also studied different types of supports μs, spin rate of 7 kHz, and relaxation time delay of 2 s. Si MAS
such as commercial HZSM5, γ-Al , and mesostructured silica NMR spectra were recorded at a frequency of 79.4 MHz using a 4 μs
2
O
3
nanoparticles (MSN). Moreover, in situ FTIR spectroscopy of CO radio frequency pulses, a recycle delay of 60 s and spinning rate of 7
methanation using mZSM5 and Ni/mZSM5 catalyst was also kHz using a 4 mm zirconia sample rotor. The surface morphology of
performed in order to provide deeper insight into the reaction the samples was performed using scanning electron microcopy
mechanism. The high activity of structurally stable Ni/mZSM5 for (JEOL JSM-6390LV) working at 15 kV. In the characterization of
CO methanation was strongly determined by the presence of both the basic properties, pyrrole has been used as a probe molecule. The
micro–mesoporosity and basicity, which led to a synergistic effect CO methanation was also performed by in situ FTIR spectroscopy to
between Ni metal active sites and the mZSM5 support.
study the surface species formed during the reaction. All the
measurements were performed on an Agilent Cary 640 FTIR
spectrometer equipped with a high-temperature stainless steel cell
Experimental
Catalyst preparation
2
with CaF windows. Prior to the measurements, all samples were
activated at 673 K for 1 h. For pyrrole adsorption, the activated
catalyst was exposed to 4 Torr of pyrrole at room temperature for 5
min, followed by outgassing at room temperature, 323, 373, 423, and
The mesoporous ZSM5 was prepared by dual templating method
using tetrapropylammonium bromide (TPA-Br) as micropore
directing agent and benzalkonium chloride as mesopore directing
4
73 K for 5 min. All spectra were recorded at room temperature. For
CO+H adsorption studies, the sample was activated at 673 K for 1 h
followed by flowing under H stream (10 ml/min) at 673 K for 1 h.
The formation of surface species during the CO methanation was
carried out by introducing a mixture of CO (20 Torr) and H (160
Torr) to the catalyst at room temperature, followed by heating to
23, 373, 423, 473, 523, 573, and 623 K. For the interaction of
2
agent. The starting parameters are Si/Al = 22.90, H
TPA-Br/Si = 0.17, benzalkonium chloride/Si = 0.06, and NaOH/Si =
.15. Firstly, the mixture of benzalkonium chloride,
tetrapropylammonium bromide (TPA-Br), sodium hydroxide
NaOH), and distilled water (H O) was homogeneously mixed at
room temperature under vigorous stirring for 5 min. Then,
aluminium hydroxide, Al(OH) and tetraethyl orthosilicate (TEOS),
Si(OC were added and homogeneously mixed at room
2
O/Si = 18.30,
2
0
2
(
2
3
H
H
2
with pre-adsorbed CO and interaction of CO with pre-adsorbed
studies, the sample was activated using the same procedure as
3
2
2
5 4
H )
above. Firstly, the activated sample was heated in the presence of CO
or H at 623 K for 1 h in order to adsorb CO or H on the catalyst
surface. Then, the interaction of pre-adsorbed CO or H samples with
or CO gas was done by exposing 160 Torr of H or 20 Torr of
temperature under vigorous stirring for 3 h. After that, the mixture
was transferred into autoclave and maintained at 423 K for 0.5 day.
The product was washed, filtered, and drying at 383 K for 3 h. The
as-synthesized catalyst was calcined at 823 K for 3 h. The prepared
catalyst was denoted as mZSM5.
2
2
2
H
2
2
CO, respectively, at room temperature and subsequent heating to 623
K with the increment of 50 K from room temperature. All spectra
were recorded at room temperature.
A commercial HZSM5 (Zeolyst International) with Si/Al
atomic ratio of 23 was used as a catalyst support. A commercial γ-
Al
modification, HZSM5 and γ-Al
prepared by the sol–gel method according to a report by Aziz et al.
In brief, cetyltrimethylammonium bromide (CTAB), ethylene glycol Carbon monoxide methanation was carried out in a fixed-bed quartz
EG), and NH
OH solution were dissolved in water with the reactor at temperature range of 423–673 K. Initially, 0.2 g of catalyst
2
O
3
(Sigma-Aldrich) was used as a catalyst support. Prior to
Catalytic testing
2
O
3
was treated at 823 K. MSN was
2
2
(
4
2
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were treated in an oxygen stream for 1 h followed by a hydrogen as (100), (110), and (200) reflections of a hexagonal ordered
stream for 3 h at 773 K and cooled down to the desired reaction mesostructure (P6mm), which is typical for MCM-41 type
temperature in a hydrogen stream. When the temperature became materials.22,26 The intensity of peaks was decreased, which may be
stable, a mixture of H
gas hourly space velocity (GHSV) and H
2
and CO was fed into the reactor at a specific because the ordered MSN support structure was slightly disturbed by
/CO mass ratio. The the presence of Ni. The presence of metal crystallites on the catalysts
2
composition of the outlet gases was analyzed by an on-line 6090 N was characterized using wide-angle XRD (30–70°), as shown in the
Agilent gas chromatograph equipped with a TCD detector. The inset figure of Fig. 1. The characteristic diffraction peaks of the NiO
moisture trap was installed at the outlet gas line of the reactor to particles at 2θ = 37.1°, 43.2°, and 62.7° were observed for
2
7
prevent moisture from entering the GC. The CO conversion, CH
4
Ni/mZSM5, Ni/HZSM5, and Ni/MSN. However, no diffraction
. This may
be due to the superposition of the NiO particles’ diffraction peaks
with γ-Al peaks or because NiO particles are too small to be
detected by XRD.
and CO selectivity, CH and CO yield, and rate of CO conversion peaks of the NiO particles were observed for Ni/γ-Al O
2
4
2
2 3
were calculated according to the following equations:
2
O
3
MCH4  MCO2
(1)
XCO (%) 
100
MCO  MCH4  MCO2
1
00
100
7
00
700
MCH4
100
* *
*
*
(2)
(3)
SCH4 (%) 
SCo2 (%) 
*
*
MCH4  MCO2
3
0
50
70
30
50
70
MCo2
100
MCH4  MCO2
mZSM5
HZSM5
XCO SCH4
(
4)
YCH4 (%) 
YCO2 (%) 
100
Ni/mZSM5
Ni/HZSM5
XCO SCO2
(5)
(6)
0
20 40 60 80
0
20 40 60 80
100
1
00
100
100
2000
nCO
1 1
Rateof CO conversion (μmol CO g - cat s ) 
*
*
Wcat s
*
3
0
3
50
70
30
50
70
where, XCO is the conversion of carbon monoxide (%), SCH and SCO2
4
is the selectivity of CH
4
(%) and CO
(%) and CO (%), respectively; M is a mole
4 2
of the CO, CH or CO . The rate of CO conversion was expressed in
2
(%), respectively, YCH and
4
γ-Al
2
O
Y
CO is the yield of CH
4
2
2
Ni/γ-Al O3
2
areal rate form. The rate of CO conversion is reported as moles of
CO converted (μmol CO) divided by the weight of the catalyst (g-
cat) and time (s).
MSN
Ni/MSN
10
0
20 40 60 80
θ[º]
0
2
4
6
8
Results and discussion
Physical properties of the catalysts
2
2θ [º]
Fig. 1 XRD patterns of mZSM5, HZSM5, γ-Al
, MSN, catalysts; the inset shows NiO (*) peaks.
and Ni-promoted catalysts. The typical diffraction peaks of MFI-
2 3
O , MSN, and Ni-promoted
2 3
Fig. 1 shows the XRD patterns of mZSM5, HZSM5, γ-Al O
2
3
type zeolite (ZSM5) are at 2θ = 7–10° and 22–25°. After the
introduction of Ni, the intensity of peaks of mZSM5 did not change
much while that of commercial HZSM5 was slightly decreased. This NLDFT pore size distribution of mZSM5, HZSM5, γ-Al
O
2 3
Fig. 2 shows the N
2
adsorption–desorption isotherms and
, MSN,
indicated some minor structural degradation of commercial HZSM5. and Ni-promoted catalysts. For mZSM5-based catalysts (mZSM5and
Moreover, mZSM5 possessed higher crystallinity as the intensity of Ni/mZSM5), all isotherms were type IV adsorption isotherms with
peaks of mZSM5 was higher than that of commercial HZSM5. An type H1 hysteresis loops, which is typically exhibited by uniform
identical XRD pattern for the predominantly γ-phase Al
observed for Al -based catalysts. The XRD patterns showed no uptake at low relative pressure indicated the presence of
additional signals other than that of γ-phase Al at 2θ = 37.0°, microporosity. In addition, an increased uptake at relative pressures
peaks were slightly of P/P
= 0.2–0.4 was due to the presence of mesoporosity. The first
increased upon the introduction of Ni. The slight increase in step at a relative pressure of 0.2–0.4 was due to the presence of
crystallinity may be due to the elimination of distorted aluminum intraparticle pores, while the second step at P/P
= 0.9–1.0 was due to
2 3
O was mesoporous material according to the IUPAC classification. A sharp
2
O
3
2
O
3
2
4-25
4
5.7°, and 66.6°.
However, the γ-Al
2
O
3
0
0
2
8
sites, leading to a more ordered framework structure of γ-Al
addition, it also indicated that nickel may interact with γ-Al
2
O
3
3
. In the presence of interparticle pores. These results confirm the
2
O . In permanence of the mesoporous phase in parallel with the
the case of MSN-based catalysts, there are three well-resolved Bragg microporous phase in mZSM5. Besides, it is noteworthy that the
diffraction peaks at 2θ = 2.4°, 4.0°, and 4.4°, which can be indexed second step at higher partial pressure was slightly decreased for
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Ni/mZSM5, which could be attributed to the fact that Ni particles smaller observed pore size after the introduction of Ni onto HZSM5.
blocked some of the interparticle pores of mZSM5. On the contrary, For Al -based catalysts, the pore size distribution was centered at
commercial HZSM5 demonstrated a type I isotherm with type H4 9.4 nm. Only a slight decrease in the pore volume was observed on
hysteresis loops, which is usually exhibited by microporous solids.29 Ni/γ-Al
For MSN-based catalysts, bimodal pore size
No obvious changes were observed upon the introduction of Ni. For distribution of 3.7 and 43.0 nm was observed. A marked decrease in
Al -based catalysts (γ-Al and Ni/γ-Al ), all isotherms were pore volume was observed on Ni/MSN.
type IV adsorption isotherms (according to the IUPAC The summary data on surface areas and total pore volumes of
2
O
3
2
O
3
.
a
2
O
3
2
O
3
2 3
O
classification) with type H1 hysteresis loops, which is characteristic all catalysts are listed in Table 1. In all cases, it can be seen that the
of mesoporous materials, broad pore size distribution, and uniform surface area and total pore volume decreased considerably after the
3
0-31
cylindrical shape.
No significant difference was noticed for Ni/γ- introduction of Ni, suggesting that a portion of the Ni particles were
. Moreover, MSN-based dispersed in the pores of the supports.
Al with respect to the bare γ-Al O
2
O
3
2 3
catalysts (MSN and Ni/MSN) exhibited a type IV isotherm with a
type H1 hysteresis loop, confirming a typical adsorption profile for a Table 1 Textural properties of mZSM5, HZSM5, γ-Al2O3, MSN, and Ni-
promoted catalysts.
mesostructured material. The filling of intraparticle and interparticle
pores was observed at P/Po = 0.2–0.4 and 0.9–1.0, respectively. The
Catalysts
Surface area
Total pore volume
decrease of the step at high partial pressure could be attributed to the
fact that the Ni particles blocked the interparticle pores of MSNs.
The pore size distribution of all catalysts was calculated by the
non-local density functional theory (NLDFT) method. Significantly,
narrow pore size distributions in the range of 3–6 nm were observed
for mZSM5. It is noteworthy that mZSM5 has both micropores and
mesopores and the amount of mesopores is higher than in
commercial HZSM5. With Ni–metal loading, the pore size of the
mZSM5 shifted towards a higher occurrence of micropores and
slightly higher mesopore size. For HZSM5, an obvious decrease of
the pore volume at a pore size of 2.4 nm led to the evolution of
2
-1
3
-1
(
m g )
733
477
389
367
198
184
965
769
(cm g )
0.248
0.203
0.222
0.199
0.531
0.485
1.573
0.867
mZSM5
Ni/mZSM5
HZSM5
Ni/HZSM5
2 3
γ-Al O
Ni/γ-Al O
2 3
MSN
Ni/MSN
27Al MAS NMR and 29Si MAS NMR offer a strong and
effective tool for characterizing the structure of zeolite. In general,
species with different structures or different chemical environments
of the aluminum and silicon atoms will have different chemical
Pore size [nm]
Pore size [nm]
0
2
4
6
8
10
0
2
4
6
8
10
0
0
0
0
0
.4
.3
.2
.1
0.4
0.3
0.2
0.1
0
mZSM5
HZSM5
3
2
1
20
40
60
320
240
160
80
2
7
29
32
shifts in their Al MAS NMR and Si MAS NMR spectra. Fig.
2
7
29
3A and B show the Al MAS NMR and Si MAS NMR spectra of
2
7
all catalysts, respectively. The Al MAS NMR was carried out to
detect the presence of tetrahedral coordinated atoms (in the
framework sites) and octahedral coordinated aluminum atoms
(possibly as extra-framework aluminum, EFAL). As shown in Fig.
3
A, three signals were observed for mZSM5: one signal at 61 ppm
Ni/HZSM5
Ni/mZSM5
and two signals at around 0 ppm. A sharp resonance at 61 ppm
corresponds to the tetrahedrally coordinated aluminum in the
framework structure. This demonstrated that most of the aluminum
atoms are incorporated into the zeolite framework. Additionally, two
resonance signals were observed around 0 ppm, corresponding to the
octahedral aluminum species in a highly symmetric environment and
distorted octahedral aluminum species. For Ni/mZSM5, three signals
were observed. A sharp signal at 59.5 ppm can be assigned to
tetrahedral framework aluminum species. In addition, two octahedral
aluminum species can be detected, both with an isotropic shift
around 0 ppm, one type in a highly symmetric environment and one
80
0
0
0
0
0.2 0.4 0.6 0.8
1
0
0
0.2 0.4 0.6 0.8
1
10 20 30 40 50
10 20 30 40 50
MSN
0
0
0
0
0
.4
2.5
2
γ-Al O
2
3
1
750
400
050
6
5
3
1
80
10
40
70
0
.3
33
more distorted. As compared with mZSM5, the intensity of the
1
signal at around 0 ppm increased obviously may be due to the
occurrence of dealumination during the calcination at 823 K, which
1
1
0
0
.5
1
34
.2
then increased the extra-framework aluminum species. On the
other hand, only two signals were observed for HZSM5, at 56.5 and
0 ppm, which are attributed to tetrahedral and octahedral aluminum
species, respectively. For Ni/HZSM5, two signals were observed at
55.6 and 0 ppm, corresponding to tetrahedral and octahedral
7
3
00
50
0
Ni/γ-Al O
2
3
Ni/MSN
.1
.5
aluminum species, respectively. For Al
and Ni/γ-Al ), two signals centered at 71.5 and 11 ppm were
observed and can be assigned to tetrahedrally coordinated Al and
2 3 2 3
O -based catalysts (γ-Al O
2
O
3
0
0.2 0.4 0.6 0.8
Relative pressure [P/Po]
1
0
0.2 0.4 0.6 0.8
Relative pressure [P/Pο]
1
3
5
octahedrally coordinated Al, respectively. In Fig. 3B, only a
dominant signal was observed at –106 ppm, which is assigned to the
4
crystallographically equivalent site of (≡SiO) Si for both mZSM5
and Ni/mZSM5. No significance difference was observed upon the
Fig. 2 N
of mZSM5, HZSM5, γ-Al
2
adsorption-desorption isotherms and NLDFT pore size distribution
, MSN, and Ni-promoted catalysts.
3
2
2 3
O
introduction of Ni. For HZSM5, a dominant signal was observed at –
4
| RSC Adv., 2015, 00, 1-9
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1
04.4 ppm. Additionally, two shoulder peaks appeared at –98 and – Intrinsic basicity of the catalysts
9
3 2
3.5 ppm, indicating the formation of (≡SiO) Si and (≡SiO) Si,
respectively. For MSN and Ni/MSN, three signals at –102, –93.5, Infrared spectroscopy with probe molecules is commonly used for
3
8
and –84.5 ppm were observed, which can be assigned to (≡SiO)
4
Si, surface acidity and basicity characterizations. In the present work,
pyrrole was used as a probe molecule for basicity characterization.
Fig. 5 shows the FTIR spectra of pyrrole adsorbed on activated
catalysts in the N-H stretching region. For all catalysts, the main
3
2
(≡SiO)
3
2
Si, and (≡SiO) Si, respectively.
A
B
-
1
broad band situated in the region of 4000–2800 cm can be assigned
to the N-H stretching vibrations of chemisorbed pyrrole (C NH)
interacting with the basic sites of framework oxygen atoms. The H-
donor property of pyrrole allows the formation of C NH-O
bridges with basic oxygen. For zeolite-based catalysts, it also
interacts via an aromatic system with the nonframework cations.
4
H
4
mZSM5
mZSM5
4
H
4
Ni/mZSM5
Ni/mZSM5
HZSM5
HZSM5
39
Both interactions happen simultaneously and influence each other.
-
1
A sharp band at 3478 cm was observed for mZSM5, which is
attributed to the perturbed N–H stretch of pyrrole molecules
interacting with the surface of basic sites. Furthermore, the band at
139 cm is attributed to the pyrrole in a liquid phase with medium
strength, while the band at 2940 cm is assigned to a fundamental
aliphatic v(CH) vibration. An obvious reduction in the intensity of
these peaks was observed for Ni/mZSM5. As a comparison with
mZSM5-based catalysts, commercial HZSM5-based catalysts
showed lower peak intensity, which indicates the lower basicity of
2 3
the catalysts. In addition, Al O -based catalysts showed two bands at
560 and 3380 cm . The band at 3560 cm is attributed to surface
Ni/HZSM5
Ni/HZSM5
γ-Al
2
O
3
MSN
-
1
3
Ni/γ-Al
2
O
3
-1
Ni/MSN
4
0
1
50 100 50
0
-50 -50 -75 -100 -125 -150
2
7
29Si shift [ppm]
Al shift [ppm]
27
29
Fig. 3 (A) Al MAS NMR and (B) Si MAS NMR spectra of all catalysts.
-
1
-l
3
hydroxyls interacting with the pyrrole ring, while the band appearing
20
1
6
Fig. 4 shows the SEM images of mZSM5 and Ni/mZSM5. As
illustrated in the images, mZSM5 possessed a smooth surface with a
typical coffin-type morphology. Similarly, Ni/mZSM5 also had a
coffin-type morphology but the surface was covered with some Ni
metal. Xin et al. observed that the parent ZSM-5 had a smooth
surface with typical coffin shape and uniform crystallite size of 1.5–
1
1
6
2
8
4
0
12
8
1.0
1.0
4
0
2
73 323 373 423 473
273 323 373 423 473
Outgassing temperature [K]
Outgassing temperature [K]
3
6
2
.5 μm. In addition, Zhou et al. reported the synthesis of
mesoporous ZSM-5 zeolite crystals by conventional hydrothermal
treatment under stirring. Without stirring, conventional MFI
morphology was quite smooth and no mesopores or growth steps on
crystal surfaces were observed. On the contrary, rough and
moustache-like surfaces were observed with stirring. The authors
mZSM5
HZSM5
3
7
Ni/mZSM5
Ni/HZSM5
proposed that this mesoporous ZSM-5 crystal contains
a
microporous coffin-shaped core crystal wrapped by a mesoporous
shell composed of uniformly aligned zeolite nanocrystals.
4000 3600 3200 2800 4000 3600 3200 2800
5
4
3
2
1
0
0
0
0
0
0
16
12
8
1.0
1.0
A
B
4
0
2
73 323 373 423 473
273 323 373 423 473
Outgassing temperature [K]
Outgassing temperature [K]
MSN
γ-Al O
2
3
Ni/MSN
C
D
Ni/γ-Al O
2
3
4
000 3600 3200 2800 4000 3600 3200 2800
-1
-1
Wavenumber [cm ]
Wavenumber [cm ]
Fig. 5 FTIR spectra of pyrrole adsorbed on activated catalysts at room
temperature, followed by heating in the vacuum at room temperature; the
x
inset shows the relative intensity I /I473, where x = room temperature, 323 K,
Fig. 4 SEM images of (A) mZSM5, (B) Ni/mZSM5, (C) closed up single 373 K, 423 K, and 473 K. Basic sites (●) of mZSM5, HZSM5, γ-Al O , and
2
3
particle of mZSM5, and (D) closed up single particle of Ni/mZSM5.
MSN ; Basic sites (○) of Ni-promoted catalysts.
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-
l
at 3380 cm is due to the N-H vibration of adsorbed species forming gave 29.0 μmol CO/ g-cat s and 54.5% CH
4
yield, and Ni/γ-Al
yield at 623 K. Besides,
at 3530 cm indicates the position of a pyrrole N–H band in the gas Ni-promoted ZSM5 (Ni/ZSM5) gave 37.5 μmol CO/ g-cat s and 60.3
2 3
O
4
1
intermolecular bonds. In the cases of MSN and Ni/MSN, the band gave 14.5 μmol CO/ g-cat s and 38.6% CH
4
-
1
-
1
phase and the band at 3430 cm indicates the physisorbed pyrrole in % CH
a liquid-like state, where the N-H group interacts with the π-system small amount of CO
of another pyrrole molecule. The band at 3467 cm is attributed to the reaction temperatures studied. These results showed that the
4
yield (inset figure in Fig. 6). As shown in Fig. 6C, only a
(< 10%) was produced for all catalysts under
2
4
2
-1
the perturbed N–H stretch of pyrrole molecules interacting with the existence of Ni metal sites inhibited the water–gas shift reaction of
2
2
surface of basic sites. The IR bands of adsorbed pyrrole on the Ni-
promoted catalysts seem to be less intense than those of the low CO
corresponding supports. This suggests a decrease of the basicity as a suggestion.
result of the introduction of Ni. These results showed that Ni- It is noteworthy that our results are up to par with previous
promoted catalysts contained fewer available sites for the adsorption reported literature reviews (Table 2). Derekaya and Yasar et al.
of pyrrole than the corresponding supports. This may be because the reported CO methanation over NaY-zeolite in which Ni/ZrO /NaY
Ni metal sites block some of the pyrrole adsorption sites of the appeared to be the most active catalyst with 100% conversion at 548
CO to CO
2
and favored the methanation reaction. The presence of
yield for all Ni-promoted catalysts corroborates this
2
2
4
4
supports, leading to a decrease in CO adsorption on the catalysts.
K. In addition, Ding et al. reported the high activity of Ni/Al
CeO with 91.6% CO conversion, 92% CH selectivity, and 84%
CH yield at 623 K. Moreover, Variava et al. studied carbon-
nanotube supported catalysts for CH
CO methanation was used to examine the catalytic activity of results, 13 wt% Ni/MWNT achieved the highest activity with ~95%
mZSM5, HZSM5, γ-Al , MSN, and Ni-promoted catalysts in the CO conversion, ~85% CH selectivity, and ~81% CH yield at 623
temperature range of 423–673 K (Fig. 6A–C). For mZSM5, high K. Shinde et al. reported the implementation of 23 wt% Ni/TiO for
production. They studied the sonication and conventional
yield of 42.9%. The low methanation activity at ˂ impregnation methods, and the former showed higher activity for
2 3
O -
2
4
4
5
Catalytic performance
4
4
6
4
production. Based on their
2
O
3
4
4
2
4
7
methanation activity was only observed at high temperature (≥ 723 CH
4
2
1
K), with a CH
23 K for bare mZSM5 may be due to the absence of metal sites, CH
which is crucial for the adsorption of CO and H
4
7
4
formation, with ~99% CO conversion, 88% CH
yield at 593 K.
4
selectivity, and
2
followed by 87% CH
4
spillover toward the support. However, without the support, Ni metal
was also inactive in the CO methanation, which may be due to metal
sintering. Li et al. reported that without SiO
occurred, which led to low catalytic activity in CO
2
, sintering of Ni
180
100
80
100
80
60
40
20
0
A
B
C
4
3
2
methanation.
1
80
50
120
100
80
100
80
60
40
20
0
1
1
50
20
Previously, we have reported the application of mZSM5 for both
acid and base-catalyzed reactions, in which bare mZSM5-0.5D
appeared to be the best catalyst for CO methanation. In this report,
the presence of metal active sites such as Ni markedly enhanced the
intrinsic properties of mZSM5 towards CO methanation. This
indicated that metallic nickel is necessary for CO methanation over
the studied system.
Fig. 6A shows the rate of CO conversion for all catalysts as a
function of the reaction temperatures. The activity of all Ni-
promoted catalysts showed an obvious increase with increasing
temperature. It is noteworthy that the catalytic performance of
Ni/mZSM5 was superior compared to that of other Ni-promoted
1
6
0
9
6
0
0
6
4
2
0
0
0
0
40
20
0
90
30
0
60
30
0
400
500
600
700
400
500
600
700
400
500
600
700
Temperature [K]
Temperature [K]
Temperature [K]
Fig. 6 (A) Rate of CO conversion, (B) Yield of CH
as a function of the reaction temperature at GHSV = 13,500 ml/g h and
/CO = 8/1. mZSM5 (□), Ni/mZSM5 (■), HZSM5 (◊), Ni/HZSM5 (♦), γ-
(∆), Ni/γ-Al (▲), MSN (○), and Ni/MSN (●). The inset shows
4 2
, and (C) Yield of CO
2 3
catalysts (Ni/MSN, Ni/HZSM5, and Ni/γ-Al O ); it presents a
significant catalytic activity (rate of CO conversion = 141.6 μmol
CO/ g-cat s) at 623 K. Additionally, Ni/mZSM5 exhibited the
H
Al
2
O
2 3
2 3
O
highest yield of CH
6.1% at 673 K, as demonstrated in Fig. 6B. Meanwhile, Ni/MSN
only gave 82.4 μmol CO/ g-cat s and 82% CH yield, Ni/HZSM5
4
of 92.0% at 623 K, which increased notably to catalytic performance of Ni/mZSM5 and Ni/ZSM5 at 623 K. Ni/mZSM5 ( ),
9
and Ni/ZSM5 ( ).
4
Table 2 Comparison study of Ni-promoted catalysts.
Catalytic performance [%] Reaction conditions
Catalyst
Reference
CO conversion
CH
4
selectivity
CH
4
yield
92
54
39
82
-
Temperature [K]
Pressure [MPa]
Ni/mZSM5
Ni/HZSM5
100
59
40
87
100
91.6
~95
~99
92
93
96
95
-
92
~85
88
623
623
623
623
548
623
623
613
0.1
0.1
0.1
0.1
-
0.1
0.1
0.1
This study
This study
This study
This study
[44]
Ni/γ-Al
Ni/MSN
Ni/ZrO /NaY
Ni/Al -CeO
Ni/MWNT
Ni/TiO
2
O
3
2
2
O
3
2
84
[45]
[46]
[47]
~81
87
2
6
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Recently, we reported a study of mesoporous ZSM5 having on mZSM5, which was assigned to atomic hydrogen. It can be
both intrinsic acidic and basic sites for cracking and methanation and suggested that bare mZSM5 has a low ability to adsorb and
we concluded that the co-existence of micro–mesoporosity with the dissociate molecular hydrogen to atomic hydrogen. From our
presence of inter- and intra-particle pores and dual intrinsic acidic– previous results, it is known that high methanation activity over
basic sites is vital for acid-catalyzed and base-catalyzed reactions. In mZSM5 only happens at high temperature (at 723 K). Therefore, in
the present work, we focus on base-catalyzed CO methanation the present study, Ni metal was introduced to mZSM5 support to
reaction for methane production. Fig. 7 shows the relationship of the convert gaseous CO and H
2
to adsorbed species on mZSM5 support,
basic sites with the catalytic activity at 623 K. Conversion of carbon which allowed high interaction between the two reactants and
-
1
monoxide to methane is essentially catalyzed by the support over the lowered the reaction temperature. A band was observed at 1850 cm
-
1
basic sites and therefore the presence of these basic sites is a key and shifted to 1810 cm at higher temperature, indicating the
0
0
point in CO methanation to produce methane. With bare support, the presence of adsorbed carbonyls on Ni sites (Ni -CO) on the
presence of basic sites did not show any significance effect on the Ni/mZSM5.51-52 The band at 1850 cm shifted to 1810 cm upon
catalytic performance (rate of CO conversion and yield of CH and temperature increase, which is likely caused by the destabilization of
CO ). However, the catalytic activity is enhanced in the presence of Ni -CO species. This may also be due to the CO desorption from
Ni metal active sites and thus a synergistic effect of Ni metal active more labile adsorption on Ni sites. Moreover, the evolution of the
-1
-1
4
0
2
-
1
sites and mZSM5 support could be claimed to occur. Results from adsorption band at 1625 cm is attributed to the presence of atomic
pyrrole adsorbed FTIR (Fig. 5) showed that the concentration of hydrogen. Furthermore, the formation of adsorbed carbonate species
basic sites in Ni/mZSM5 is higher than in Ni/HZSM5 and Ni/γ- was only observed on Ni/mZSM5, as evidenced by the adsorption
Al O catalysts but lower than in Ni/MSN. Notably, an optimum bands at 1510 and 1360 cm . At 623 K, Ni/mZSM5 showed a fully
2 3
amount of basic sites is needed to obtain a high yield of methane. diminished in gaseous CO bands, a notable depletion in Ni -CO
These results are in accordance with other studies reported in the bands, and progressive formation of carbonate species. At this
-
1 53
0
3
1,48-49
literature.
temperature, the system’s energy starts to be high enough for it to
dissociate and then hydrogenate or to hydrogenate CO directly until
methane formation.
1
1
60
20
100
100
80
60
40
20
0
A
B
C
8
6
4
2
0
0
0
0
0
A
B
2110 1625 1510
2170
1810
2
110
2
170
1625
i
i
1360
8
4
0
0
0
0
1
2
3
4
5
0
1
2
3
4
5
0
1
2
3
4
5
Basicity peak intensity
Basicity peak intensity
Basicity peak intensity
Fig. 7 Relationship of basicity-catalytic activity at 623 K (A-C). mZSM5
(
□), Ni/mZSM5 (■), HZSM5 (◊), Ni/HZSM5 (♦), γ-Al
2
O
3
(∆), Ni/γ-Al
2
O
3
b
b
(▲), MSN (○), and Ni/MSN (●). Empty symbol in (A) is multiple 5 times the
original value.
2500 2100 1700 1300 2500 2100 1700 1300
C
D
2340
1
625
510
360
1
625
810 1510
1
1
Mechanistic investigation of CO methanation
1780
i
i
1360
1
The nature of the active sites and reaction mechanisms for CO
methanation has been a longstanding topic in heterogeneous
catalysis. There is a lot of controversy regarding these issues. Two
possible mechanisms for CO methanation have been proposed, that
is, a direct CO dissociation mechanism and a hydrogen-assisted CO
5
0
dissociation mechanism.
The FTIR adsorption spectra of CO+H
and Ni/mZSM5, the interaction of H with pre-adsorbed CO, and the
interaction of CO with pre-adsorbed H on Ni/mZSM5 are presented
in Fig. 8. The blank reaction (without catalyst) of CO+H showed no
significant peak, which showed that adsorbed species is needed for
the methanation reaction. Furthermore, the adsorption of CO+H on
2
adsorption on mZSM5
2
2
2
a
a
2
500 2100 1700 1300 2500 2100 1700 1300
2
-1
-1
Wavenumber [cm ]
Wavenumber [cm ]
Ni showed no IR adsorption peak as the experiment could not
proceed because the Ni pellet became black after being reduced by
the hydrogen flow. As mentioned earlier, CO methanation on Ni was
negligible, indicating that a methanation reaction was probably not
Fig. 8 Evaluation of the FTIR spectra of adsorbed gases (CO+ H
mZSM5, (B) Ni/mZSM5, (C) interaction of H with pre-adsorbed CO
Ni/mZSM5, and (D) interaction of CO with pre-adsorbed H Ni/mZSM5. The
2
) on (A)
2
2
taking place on the Ni surface. For in situ FTIR spectroscopy of samples were heating up to (b) room temperature, (c) 323 K, (d) 373 K, (e)
-
1
CO+H
2
(Fig. 8A and B), the adsorption bands at 2170 and 2110 cm
423 K, (f) 473 K, (g) 523 K, (h) 573 K, and (i) 623 K. (a) pre-adsorbed CO
(
for C) or pre-adsorbed H
2
(for D) on Ni/mZSM5 at room temperature.
were observed for both mZSM5 and Ni/mZSM5 catalysts, which can
be assigned to the gaseous CO. A band at 1625 cm was observed
-
1
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In order to clarify the predominant reaction pathway for CO monoxide methanation on a Ru(0001) surface based on a density
2
methanation over Ni/mZSM5, the interaction of H
adsorbed CO and interaction of CO with pre-adsorbed H
examined by in situ FTIR (Fig. 8C and D). In the study of the intermediate from CO dissociation, resulting in active C and CH
interaction of H with pre-adsorbed CO, four adsorption bands were species, respectively. The active C and CH species subsequently
observed at 1780, 1625, 1510 and 1360 cm . The band at 1780 cm , undergo stepwise hydrogenation to CH
2
with pre- functional theory (DFT) study. Their result showed that the reaction
2
was pathway for CO methanation proceeds via either a COH or a CHO
2
-
1
-1
4
. Zhen et al. studied CO
and proposed that CO was
at 623 K. No obvious changes of the other three adsorption bands dissociated on Ru species surfaces to form carbon species (COads
were observed. On the other hand, in the study of the interaction of and oxygen species (Oads) and then reacted with activated H on Ni
2
0
which was assigned to Ni -CO species, was significantly decreased methanation on Ni-Ru/γ-Al
2
O
3
2
)
5
5
2
CO with pre-adsorbed H , the results showed one additional centers to form methane and water. However, the role of Ni and Ru
-
1
adsorption band at 2340 cm , which was assigned to gaseous CO
CO species may result from the interaction of the adsorbed CO with reaction can also proceed through
the oxide surface of mZSM5 or as a consequence of the water–gas dissociation mechanism (formate route), which has been proposed in
shift reaction and accumulation on the support. With increasing the literature.
temperature, reduction of the adsorption bands of Ni -CO and atomic formate species suggested that it is not a possible route for CO
2
.
species was not discussed in detail. Besides, the methanation
2
a
hydrogen-assisted CO
5
6-57
However, in the present work, the absence of
0
-
1
hydrogen at 1810 and 1625 cm , respectively, were observed. methanation over mZSM5-based catalyst.
Moreover, a methanation reaction can occur on the Ni sites as well
0
as on the mZSM5 support. However, in this case, the methanation
Conclusions
sites on the mZSM5 support are more active as compared to the ones
0
on the Ni sites. From these results, we can propose two possible
mechanisms for CO methanation. In the first mechanism, the
A
(
structurally stable nickel-promoted mesoporous ZSM5
Ni/mZSM5) was prepared for CO methanation. Ni/mZSM5 was
found to be highly active for CO methanation, with a high rate of CO
conversion (141.6 μmol CO/g-cat s) and 92% CH yield at a
adsorbed CO species may be reacted with H
In the second mechanism, the adsorbed H may be reacted with CO to
form CH and CO . However, in this case, the former is the
2 4 2
to form CH and H O.
4
4
2
relatively low temperature of 623 K. Ni/mZSM5 showed superior
predominant pathway as the methanation reaction is favored by
inhibition of the water–gas shift reaction. Therefore, a plausible
reaction mechanism of CO methanation over Ni/mZSM5 is shown in
Scheme 1.
catalytic performance compared to Ni/MSN, Ni/HZSM5, and Ni/γ-
Al O . In this case, the presence of Ni, micro-mesoporosity, and
2 3
basicity is of crucial significance. Additionally, the introduction of
Ni provokes a synergistic effect between the metal active sites and
2
the mZSM5 support. Based on in situ FTIR studies, CO and H may
CO + H2
CH + CO + H O
be adsorbed on Ni metal followed by migration onto the mZSM5
surface to form adsorbed CO and adsorbed H. Then, two possible
mechanisms for CO methanation were proposed. In the first
4
2
2
CH + H O
H2
4
2
O
CH + CO
4 2
CO
mechanism, the adsorbed CO may be reacted with H
and H O. In the second mechanism, the adsorbed H may be reacted
with CO to form CH and CO . However, in this case, the former is
2 4
to form CH
2
C H H
O
O
C
O
4
2
the predominant pathway as the methanation reaction is favored by
the inhibition of the water–gas shift reaction.
C
H
H
Ni0
O
O
O
O
mZSM5
Acknowledgements
This work was supported by the Universiti Teknologi Malaysia
through Research University Grant No. 06H17 and MyPhd
Scholarship (Teh Lee Peng) from the Ministry of Higher Education,
Malaysia.
Scheme
Ni/mZSM5.
1 Plausible reaction mechanism of CO methanation over
Under the experimental conditions used in the present work, the
0
presence of metal carbonyl (Ni -CO) was observed. This may References
suggest that the route to methane formation was formed via metal
carbonyl. Unfortunately, in this experiment, the CH
x
vibration bands
in the 2800–3000 cm region were not detected for these catalysts.
Chen et al. studied the reaction mechanism of Si–Ni/SiO catalyst by
temperature-programmed reaction FTIR (TPR-FTIR) and
1
2
3
4
5
6
7
P. Sabatier and J.B. Senderens, Compt. Rend. Acad. Sci., 1902, 134,
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1
5
2
S-T. Zhang, H. Yan, M. Wei, D. G. Evans, and X. Duan, RSC Adv.,
2014, 4, 30241.
5
1
temperature-programmed desorption FTIR (TPD-FTIR). From
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linear-and bridge-type species; 2) the bridge-type CO was easily
H. Yoshida, K. Watanabe, N. Iwasa, S-I Fujita, and M. Arai, Appl.
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silicide; and 3) the linearly adsorbed CO and the C
quickly reacted with dissociated hydrogen (H ) to form the CH
Zarfl et al. reported on the DRIFTS study of commercial Ni/γ-Al O
3
s
(adsorbed carbon) and CO
2
on the surface of nickel
2
2759.
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