Study of structural and catalytic properties of Ni catalysts prepared from inorganic complex precursor for Fischer-Tropsch synthesis

Article abstract of DOI:10.1016/j.molstruc.2017.05.027

The silica- and alumina- supported Ni catalysts synthesized by thermal decomposition of inorganic precursors were evaluated for Fischer–Tropsch synthesis (FTS); the structural properties and performance of the catalysts were compared to those of samples constructed via impregnation method. The results revealed that the synthesized catalysts have higher catalytic activity comparison to those prepared via the conventional impregnation method. The effect of the preparation method on the structural properties shows that synthesizing the catalyst through inorganic precursor route is more appropriate. Characterization of catalysts is carried out using inductively coupled plasma (ICP), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and BET specific surface area.

Full text of DOI:10.1016/j.molstruc.2017.05.027

Accepted Manuscript
Study of structural and catalytic properties of Ni catalysts prepared from inorganic
complex precursor for Fischer-Tropsch synthesis
Sania Saheli, Ali Reza Rezvani, Azim Malekzadeh
PII:
S0022-2860(17)30610-5
10.1016/j.molstruc.2017.05.027
MOLSTR 23768
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 10 December 2016
Revised Date: 25 April 2017
Accepted Date: 10 May 2017
Please cite this article as: S. Saheli, A.R. Rezvani, A. Malekzadeh, Study of structural and catalytic
properties of Ni catalysts prepared from inorganic complex precursor for Fischer-Tropsch synthesis,
Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.05.027.
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ACCEPTED MANUSCRIPT
Graphical abstract

ACCEPTED MANUSCRIPT
Study of structural and catalytic properties of Ni catalysts prepared from inorganic
complex precursor for Fischer-Tropsch synthesis
Sania Saheli a, Ali Reza Rezvani a,*, Azim Malekzadeh b
a Department of Chemistry, University of Sistan and Baluchestan, P. O. Box 98135-674, Zahedan, Iran
b School of Chemistry, Damghan University (DU), Damghan, Iran
*E-mail address: Ali@hamoon.usb.ac.ir
Abstract
The silica- and alumina- supported Ni catalysts synthesized by thermal decomposition of
inorganic precursors were evaluated for Fischer–Tropsch synthesis (FTS); the structural
properties and performance of the catalysts were compared to those of samples constructed via
impregnation method. The results revealed that the synthesized catalysts have higher catalytic
activity comparison to those prepared via the conventional impregnation method. The effect of
the preparation method on the structural properties shows that synthesizing the catalyst through
inorganic precursor route is more appropriate. Characterization of catalysts is carried out using
inductively coupled plasma (ICP), powder X-ray diffraction (XRD), scanning electron
microscopy (SEM), and BET specific surface area.
Keywords: Coordination polymers; Inorganic precursor; Impregnation; Fischer-Tropsch
synthesis; Nickel catalyst; Synthesis route.
1. Introduction
In recent years, the Fischer–Tropsch synthesis (FTS) has received considerable attention from
academic institutions and governments alike, due to environmental contamination, rapidly rising
price of crude oil and subtractive oil reserves [1‒3]. The FTS is a viable route for the production
of clean fuels from biomass-derived syngas, natural gas, or coal. In this process, highly important
factor is a catalyst, which is selected based on the key role in product distribution [4, 5].
Group VIII transition metals are highly able to catalyze the FTS. This performance for FTS is
defined by the nature of CO and H2 adsorption on the group VIII transition metals [6]; moreover,
these metals demonstrate some acting in the C‒C coupling reaction during hydrogenation of
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carbon monoxide. Among the group VIII transition metals which are catalyzed the FTS; the
more active metals are iron, cobalt, nickel and ruthenium [7‒10]. However, iron and cobalt are
rather considered due to selectivity and cost [11]. In the FTS, nickel used less owing to methane
selectivity while in many of the catalytic researches, based on iron or cobalt catalysts, nickel has
improved the catalyst performance [12, 13]. In recent study carried out by Enger et al. [14], it
was shown that the Ni might be capable to generate different productions and methane in not
only product of FTS. Moreover, some factors such as support, CO/H2 ratio and particles size of
catalyst can affect to FTS productions. Apart from these factors, production procedure of the
catalyst strongly affects the outcomes of FTS. Most of the catalysts for FTS manufactured by co-
precipitation, sol-gel, impregnation and fusion methods [15‒20] while use of the inorganic
precursor complexes as novel technique for preparing materials create some benefit such as
lower particle sizes and the homogeneous dispersion of metals on the support surface which
increase catalyst performance [21].
To date, coordination polymers or metal–organic frameworks showing various novel structures
and intriguing properties in the field of catalyst [22‒24], however, the role of these types of
combinations as catalyst precursor for FTS have not been explored. Recently, through the self-
assembly reaction of H3btc and Ni(NO3)2, we have successfully isolated a new coordination
polymer with interesting geometry, [Ni(H2btc)(OH)(H2O)2] [25]. Considering the importance of
FTS and the unique ability of metal‒Organig frameworks to create special topology, in this
work, the inorganic precursors [Ni(H2btc)(OH)(H2O)2]/SiO2 and [Ni(H2btc)(OH)(H2O)2]/Al2O3
are used to prepare Ni catalysts, furthermore, evaluated the catalytic performance of these
catalysts and compared with reference catalysts which are produced by impregnation method.
Also, respective studies on the physicochemical characteristics performed by X-ray diffraction,
Brunauer-Emmett-Teller and scanning electron microscope.
2. Experimental
2.1. Materials
All the used chemicals were regent grade and used without further purification, purchased from
Aldrich.
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2.2. Synthesis of complex
Ni(NO3)2 .6H2O (0.290 g , 1.0 mmol), 1,3,5-benzenetricarboxylic acid (0.105 g , 0.5 mmol),
NaOH (0.060 g , 1.5 mmol) were mixed in 15 ml distilled water at the room temperature. The
mixture was transferred to Parr-Teflon lined stainless steel vessel and was heated at 130 °C for 3
days. After cooling to room temperature, air-stable blue crystals were obtained. Yield: 63%
Anal. Calc. for C9H10NiO9: C, 33.69; H, 3.14. Found: C, 33.31; H, 3.19%. IR (cm-1): 3498-3279
(m, br), 2920(w), 1710(s), 1617(s), 1570(s), 1385(s), 1365(s), 749(m), 743(m).
2.3. Synthesis of [Ni(H2btc)(OH)(H2O)2]/SiO2 precursor
The catalyst precursor was prepared from mixing an aqueous solution of [Ni(H2btc)(OH)(H2O)2]
(3.20 g, 10.0 mmol, 90% Wt) and SiO2 (0.32 g, 10% Wt), the mixture was stirred and evaporated
at 30 °C to dryness.
2.4. Synthesis of silica-supported nickel catalyst, Ni/SiO2
The nickel- silicon catalyst was obtained through calcinations of [Ni(H2btc)(OH)(H2O)2]/SiO2
precursor at 700 °C at the atmosphere of static air in the electric furnace and maintaining in this
temperature for 6 h.
2.5. Synthesis of [Ni(H2btc)(OH)(H2O)2]/Al2O3 precursor
The catalyst precursor was prepared from mixing an aqueous solution of [Ni(H2btc)(OH)(H2O)2]
(3.20 g, 10.0 mmol, 90% Wt) and Al2O3 (0.32 g , 10% Wt), the mixture was stirred and
evaporated at 30 °C to dryness.
2.6. Synthesis of alumina-supported nickel catalyst, Ni/Al2O3
The nickel- alumina catalyst was obtained through calcinations of [Ni(H2btc)(OH)(H2O)2]/Al2O3
precursor at 700 °C at the atmosphere of static air in the electric furnace and maintaining in this
temperature for 6 h.
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2.7. Preparation of reference catalysts
For comparison, the silica- or alumina- supported Ni reference catalysts were constructed by
impregnation method. In first, the support (silica or alumina) was impregnated with an aqueous
solution of nickel nitrate. Then, the suspension was stirred for 10 h at 30 °C. The resulting
mixture was filtered and the obtained precipitate dried at 120 °C. Finally, the precursor catalyst
was calcined at 600 °C for 5 h.
2.8. Sample characterization
Fourier transform infrared (FT-IR) spectra were measured on JASCO 460 spectrophotometer
1
with KBr pellets in the 4000–400 cm− range. Elemental analyses were performed on Leco,
CHNS-932 elemental analyzer. The crystal structure of [Ni(H2btc)(OH)(H2O)2] was obtained via
the single-crystal X-ray diffraction technique. Metallic content was determined by ICP in an
OPTIMA 2000 DV ICP Optical Emission spectrometer. X-ray diffraction data of the complex
was collected on a STOE-IPDS 2T diffractometer using a graphite monochromated Mo Kα
radiation (λ=0.71073 Å). X-ray powder analyses were conducted using a Bruker D8 advance by
considering 2θ range from 10° to 70°. The morphology of the catalysts was observed using a
MIRA3 TESCAN FE SEM operating at 30 kV. The BET surface areas of the samples were
measured by N2 physisorption using a ASAP 2020 apparatus. Each catalyst sample was degassed
under nitrogen atmosphere at 350 °C for 4 h.
2.9. Catalytic activity measurements
Catalytic performance tests for the Fischer-Tropsch reaction were conducted in a fixed bed micro
reactor operating at the atmospheric pressure. For each run 0.8 g catalyst was maintained in the
reactor by using quartz wool. All catalysts were reduced in situ at the atmospheric pressure in
hydrogen/nitrogen mixture (50% vol. H2) at 400 °C for 8 h before disposal of synthesis gas.
Thereafter, the N2/H2 was replaced by the current of H2/CO (2:1). The Fischer-Tropsch reaction
was carried out in the range of 280-400 °C. The product stream was analyzed by a GC on a FI
detector equipped with a methanizer. The catalytic performance was evaluated in terms two
parameters:
(I) CO conversion % = {[(Mols of COin) - (Mols of COout)] / Mols of COin} × 100
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(II) Selectivity of j product (%) = {Mols of j product / [(Mols of COin) - (Mols of COout)]} × 100
3. Results and discussion
3.1. Complex characterization
The reaction of Ni(NO3)2 .6H2O with 1,3,5-benzenetricarboxylic acid and NaOH in the aqueous
solution under the hydrothermal condition led to formation of the air-stable complex
[Ni(H2btc)(OH)(H2O)2] 1. This compound was characterized by elemental analysis, infrared
spectroscopy and single crystal X-Ray diffraction.
X-ray crystallographic data determine that complex 1 crystallizes in the monoclinic space group
P21/n (Table 1). The asymmetric unit consists of one Ni(II) ion, one H2btc‒ ligand, two coordinated
water molecules and one OH‒ ion. The title complex is coordination polymer which each metal
center is conjoined to two neighboring ones through two µ-oxo bridges. The coordination geometry
of Ni(II) ion is a distorted square pyramidal. The Ni(II) ion is coordinated by two oxygen atoms
from carboxyl (O3) and carboxylate (O1) groups of H3btc ligand, three oxygen atoms of two water
molecules (O8, O7) and one hydroxyl ion (O9) (Fig.1). Among three functional groups of H3btc
ligand only one carboxyl group is uncoordinated whereas the other carboxyl and carboxylate groups
are connected to unidentate coordination mode to Ni(II).
<Place Fig. 1 here>
<Place Table 1 here>
The FT-IR spectrum of 1 (Fig.2) shows the broad band at 3498–3279 cm‒1 which is attributed to
the anti-symmetric and symmetric stretching vibrations of coordinated water molecules. The band
appearing around 1710 cm‒1 is assigned to ν(C=O) of the carboxyl groups and is confirmed that
carboxyl groups are not fully deprotonated. The intense band at 1617 cm‒1 indicates that the
carboxyl group is deprotonated. The bands observed at 1246 and 1244 cm‒1 are ascribed to ν(C‒O)
vibrations. In the desired complex, the presence of carboxylate group is reflected by IR spectrum in
bands of the anti-symmetric, νa(COO), and symmetric, νs(COO), stretching vibrations at1570 and
1385 cm–1, respectively. The frequency difference between anti-symmetric and symmetric
stretching vibrations of the of the carboxylate group is ∆ν = 185 which is suggested monodentate
coordination mode of the carboxylate group to the Ni(II) ion. The C‒H stretching vibrations of the
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benzene ring of H3Btc ligand emerge at 3105 and 3070 cm–1 [26]. The two C‒H in-plane bending
modes observe at 1187 and 1181 cm–1. The band at 890 cm–1 can be assigned to C‒H out-of-plane
bending mode [27, 28].
<Place Fig. 2 here>
3.2. Catalysts characterization
The FT-IR spectra of both catalysts prepared by thermal decomposition of inorganic precursor
complex are shown in Fig.3. In the IR spectrum of Ni/SiO2, the broad bonds at 3420 cm‒1 is
assigned to OH stretching vibration of physically adsorbed water and the H‒OH bending
vibration is appeared at 1625 cm‒1. The intensive band at 1107 cm‒1 is assigned to Si‒O‒Si
anti-symmetric stretching vibration and the band observed at 804 cm‒1 are imputed to the
symmetric stretching vibration of the Si‒O‒Si. The OSi‒O bending vibration emerges at 535
cm‒1. The band at 594 cm‒1 is attributed to Si‒O bending mode [29‒31]. Investigating of the
FT‒IR spectrum of alumina-supported nickel catalyst show that the bands centered at 3427 cm‒1
and 1644 cm‒1 are belonged to OH stretching vibration and H‒OH bending mode of physically
adsorbed water, respectively. The peak corresponding to 1154 cm‒1 is assigned to Al‒O‒Al
anti-symmetric bending mode. The vibration bands at 850, 676 and 533 cm‒1 are attributable to
bending and stretching modes of AlO6 [32].
<Place Fig. 3 here>
The composition of the synthesized and reference catalysts determined by ICP analysis. The
loading of Ni in the catalysts prepared by thermal decomposition of inorganic precursor
supported on silica and alumina are 60.2 and 58.6, respectively. The loading of Ni in the
reference catalysts are 52.4 % and 51.9 % for supporting silica and alumina, respectively.
Characterization of Ni catalysts prepared by thermal decomposition of inorganic precursors were
carried out by using X-ray diffraction, and the obtained X-ray diffractograms of the silica- and
alumina- supported Ni catalysts are depicted in Fig. 4. The XRD pattern of the nickel- silica
calcined catalyst synthesized from inorganic precursor contains diffraction peaks at 2Ɵ = 38.7,
48.7, 58.6, 69.1°, which are assigned to diffraction of NiO2. Characteristic peaks of SiO2 at 2Ɵ=
25.9, 35.6 are observed for calcined catalyst. The nickel- alumina synthesized catalysts displays
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the characteristic XRD peaks of NiO2 at2Ɵ= 38.7, 48.7, 58.6, 69.1°. The diffraction peaks at
2Ɵ= 35.3, 46.4 are attributed to Al2O3. The diffractograms of reference catalysts prepared by
impregnation include diffraction peaks at 2Ɵ = 37.2, 43.2, 62.8°, which are allocated to NiO
phase (Fig. 5). Comparison of the X-ray diffraction peaks broadening in the synthesized catalysts
and the reference samples display that crystallite size is smaller for the constructed catalysts. The
average particle sizes were determined by Scherrer’s equation. The average crystallite sizes of
synthesized catalysts supported on SiO2 and Al2O3 are 58.1 and 54.3 nm, respectively, while the
average sizes of reference catalysts are 84.6 and 73.5 nm for silica and alumina supported,
respectively. These results show that the fabricated catalysts lead to the improvement of the
scattering of the metal and simplify producing smaller particles than the reference catalysts.
<Place Fig. 4 here>
<Place Fig. 5 here>
The morphological properties of the catalysts were determined using scanning electron microscopy
(SEM). The SEM micrographs of the catalysts prepared by thermal decomposition of inorganic
precursors and impregnation method are displayed in Fig. 6 and Fig. 7, respectively. The
morphological features of the synthesized catalysts and reference catalysts are different. The
electron micrographs obtained from Ni/SiO2 and Ni/Al2O3 catalysts synthesized through inorganic
precursor path have smaller particles in comparison with Ni/SiO2 and Ni/Al2O3 reference catalysts.
Moreover, the grade of agglomeration fabricated catalysts is lower than reference catalysts which
are shown that preparing catalysts via inorganic precursor is more appropriate route toward
impregnation method.
<Place Fig. 6 here>
<Place Fig. 7 here>
The BET results of the catalysts constructed by thermal decomposition of inorganic precursors and
the reference catalysts created by impregnation are presented in Table 2. As observed from Table 2,
both synthesized catalysts have higher surface area than the reference catalysts. The usage of the
acceptable procurement procedure (thermal decomposition of inorganic precursor complex) lead to
enhancement in the specific surface areas of the these catalysts, which is a persuasive evidence for
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the improvement in the dispersion of the active oxide phase and the increment in catalytic
performance of these synthesized catalysts.
<Place Table 2 here>
3.3. Catalytic performance
The catalytic performance of the fabricated and reference catalysts were evaluated at atmospheric
pressure in the temperature range 280-400 ℃ with H2/CO ratio of 2.
Generally, an increment in the reaction temperature leads to an increasing in the catalytic activity,
moreover, it has been obvious that the reaction temperature should not be too low [33]. The CO
conversion is negligible at low reaction temperature which is afforded to low catalytic performance.
The enhancing temperature promotes CO dissociation, and provides more surface C atoms for
forming hydrocarbons. On the other hand, with raising reaction temperature, methane formation
increases due to the abundant surface H atoms [34]. Apart from the reaction temperature, the
preparation method of catalyst remarkably affects the CO conversion and hydrocarbon selectivity.
As observed from Fig. 8, the CO conversion percentages of Ni catalysts prepared by thermal
decomposition of inorganic precursor and impregnation have increased with increment temperature
from 280 °C to 400 °C. The CO conversion is higher for Ni-SiO2 and –Al2O3 catalysts synthesized
through inorganic precursor approach compared to reference catalysts produced from impregnation
method in all studied temperature. The catalytic activity of calcined catalysts was obtained by
different procedures increased with enhancing temperature from 280 to 400 °C (Fig. 9). According
to obtained results, the constructed Ni-Al2O3 catalyst has higher selectivity to light olefins products
and lower methane selectivity than the Ni-SiO2 catalyst. On the other side, both of the reference
catalysts have lower selectivity to light olefins products and higher methane selectivity compared to
the synthesized catalysts. It can be seen that, for Ni-SiO2 sample, the optimal operating temperature
equals 340 °C given the high selectivity of light olefins products and low methane selectivity
whereas for the Ni-Al2O3 sample the optimal temperature is 360 °C.
.
In general, the fabricated Ni catalysts have premier catalytic performance in comparison with the
findings achieved for the reference Ni-catalysts prepared by impregnation method. These results
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due to the synthetic route used for the synthesized Ni catalyst allows it having high BET specific
surface area, small particle size, low degree agglomeration and good active phase dispersion.
Conclusion
The thermolysis of [Ni(H2btc)(OH)(H2O)2]/SiO2 and [Ni(H2btc)(OH)(H2O)2]/Al2O3 as inorganic
precursors was used for construction of Ni/ SiO2 and Ni/Al2O3 catalyst. The reference catalysts
were prepared through impregnation method. The SEM micrographs of samples display that
synthesized catalysts have lower particle sizes and agglomeration in comparison with reference
catalysts. These results are in good agreement with the BET outcomes; it means that the catalysts
prepared by thermal decomposition of inorganic precursors have higher surface area than reference
catalysts. Activity of the catalysts in FTS was investigated at 280-400 °C with H2/CO ratio of 2 and
specified that the fabricated catalysts are more active than impregnation reference samples.
Furthermore, it can be concluded that preparing catalysts by inorganic precursor is more suitable
procedure toward impregnation method.
Acknowledgements
The financial support from the Iranian National Science Foundation (Grant No. 95834583) is
gratefully acknowledged.
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Table 1
Crystallography data for [Ni(H2btc)(OH)(H2O)2].
Complex
1
Empirical formula
Formula Weight
C9H10NiO9
320.86
Crystal system
Space group
Monoclinic
P21/n
Crystal size (mm)
0.09 × 0.09 × 0.40
a[Å]
b[Å]
c[Å]
β[°]
Volume/Å3
Temperature [K]
6.8658(14)
18.849(4)
8.5608(17)
92.98(3)
1106.4(4)
293(2)
Z
4
Dcalcd.[g cm–3]
1.926
F(000)
656
Reflections collected /unique
wR2
R
4287/1937
0.0920
0.0429
Table 2
BET results of synthesized and reference catalysts.
synthesized Sample
Ni/SiO2
BET surface area (m2/g) reference Sample
BET surface area (m2/g)
Ni/SiO2
98
30
Ni/Al2O3
Ni/Al2O3
110
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Fig.1. The coordination environment of Ni(II) ion in complex 1.
70
60
50
40
30
20
10
0
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber cm‒1
Fig. 2. The FT-IR spectrum of complex 1 prepared under hydrothermal condition.
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80
70
60
50
40
30
20
10
a
4000
3400
2800
2200
1600
1000
400
Wavenumber cm‒1
50
40
30
20
10
b
4000
3400
2800
2200
1600
1000
400
Wavenumber cm-1
Fig. 3. The FT-IR spectra of synthesized Ni catalysts supported on SiO2 (Fig.3a.) and Al2O3 (Fig. 3b.).
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a
b
10
20
30
40
50
60
70
2 Tetha
Fig. 4. The XRD patterns of Ni /SiO2 (Fig.4a.) and Ni /Al2O3 (Fig. 4b.) synthesized catalysts prepared by
thermal decomposition of inorganic precursor.
a
b
10
20
30
40
2 Tetha
50
60
70
Fig. 5. The XRD patterns of Ni /SiO2 (Fig.5a.) and Ni /Al2O3 (Fig. 5b.) reference catalysts prepared by
impregnation method.
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Fig. 6. SEM micrographs of synthesized catalysts a) Ni /SiO2 and b) Ni /Al2O3.
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Fig.7. SEM micrographs of reference catalysts a) Ni /SiO2 and b) Ni /Al2O3.
100
80
60
40
20
0
400
380
360
Temperature°C
340
320
300
280
Ni/SiO2 synthesized Ni /Al2O3 synthesized Ni /SiO2 reference Ni /Al2O3 reference
Fig. 8. Effect of reaction temperature on CO conversion percentages of synthesized and reference
catalysts.
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50
40
30
20
10
0
a
280
300
320
340
360
380
400
Temperature ℃
CH4
C2H4
C3H6
C4H8
50
b
40
30
20
10
0
280
300
320
340
Temperature ℃
360
380
400
CH4
C2H4
C3H6
C4H8
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50
40
30
20
10
0
c
280
300
320
340
360
380
400
Temperature ℃
CH4
C2H4
C3H6
C4H8
50
40
30
20
10
0
d
280
300
320
340
360
380
400
Temperature ℃
CH4
C2H4
C3H6
C4H8
Fig. 9. Effect of different reaction temperatures on the catalytic performance of synthesized
(a: Ni /SiO2, b: Ni /Al2O3) and reference(c: Ni /SiO2, d: Ni /Al2O3) catalysts.
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ACCEPTED MANUSCRIPT
Highlights
ꢀ The fabricated catalysts are more active than impregnation reference samples.
ꢀ The Ni catalysts were synthesized through new inorganic precursor.
ꢀ The synthesized catalysts have higher selectivity to light olefins than reference
conventional samples.