Silica-supported iron oxide nanoparticles: unexpected catalytic activity in hydrogenation of phenylacetylene

Article abstract of DOI:10.1016/j.mencom.2017.09.028

Hydrogenation of phenylacetylene in liquid phase (100–150 °C, 6–20 bar H₂) over FeO_x / SiO₂ catalysts prepared by thermal decomposition of ammonium trioxalatoferrate affords styrene with selectivities 70–80% depending on t

Full text of DOI:10.1016/j.mencom.2017.09.028

Mendeleev
Communications
Mendeleev Commun., 2017, 27, 512–514
Silica-supported iron oxide nanoparticles: unexpected
catalytic activity in hydrogenation of phenylacetylene
Anastasiya A. Shesterkina,*a Elena V. Shuvalova,a Elena A. Redina,a
Olga A. Kirichenko,a Olga P. Tkachenko,a Igor V. Mishina and Leonid M. Kustova,b
a N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. Fax: +7 499 135 5328; e-mail: anastasiia.strelkova@mail.ru
b Department of Chemistry, M. V. Lomonosov Moscow State University, 119991 Moscow,
Russian Federation
DOI: 10.1016/j.mencom.2017.09.028
Styrene
Ethylbenzene
Hydrogenation of phenylacetylene in liquid phase (100–150°C,
6–20 bar H2) over FeOx/SiO2 catalysts prepared by thermal
decomposition of ammonium trioxalatoferrate affords styrene
with selectivities 70–80% depending on temperature and
pressure.
H2
H2
PhCºCH
PhCºCH
Fe2+
Fe3+
FeOx
SiO2
100–150 °C, 6–20 bar H2, EtOH
The development of efficient and environmentally safe catalytic
systems is a topical problem of up-to-date catalysis. The
abundance, low cost and low toxicity of iron oxides make their use
in catalysis highly sustainable and preferable as compared to the
commonly used Pd, Pt, Ru, Rh, and Ni catalysts of hydrogenation.
Generally, iron-based catalysts have been applied at high
temperatures and high pressures for large-scale fuel and platform
chemical production (the Haber–Bosch process, the water–gas
shift and the Fischer–Tropsch reactions) as well as for organic
synthesis.1–4 However, only a handful of iron catalysts have
been developed for selective hydrogenation under relatively mild
conditions.2–8 Recently, Fe0 nanoparticles of 1–5 nm in diameter
were reported to catalyze hydrogenation of olefins and alkynes
in organic solvents, while no hydrogenation of aromatic ring was
observed.9–10 A limitation of the use of such Fe0 nanoparticles
resides in their sensitivity towards oxidation, the complicated
preparation process and impossibility of separation even mag-
netically. The Fe0 nanoparticles were synthesized from specific
expensive reagents under water- and oxygen-free conditions.
The core-shell iron (iron oxide) materials prepared in water-
containing solutions using common Fe salt and reducing agent,
as well as the commercial iron core-shell nanoparticle materials
are more stable to water and air.11 The powder of these materials
comprising the chains of larger particles consisting of Fe0 core
(diameter of 44 nm) and a Fe oxide shell (thickness of 6 nm)
can be magnetically separated from the reaction medium. Such
catalysts are active and strictly selective for alkene and alkyne
hydrogenation in the presence of carbonyl and aromatic groups,
suggesting that the occurrence of an iron oxide shell is not an
obstacle to the catalytic activity at 80°C and 40 bar H2.
can be efficiently separated and used in a continuous-flow
process instead of periodic batch process (cf. ref. 13). However,
the preparation of Fe3O4(6 nm)/alumina catalyst13 is very complex,
and its activity depends on the nature of silica support. The
supported Fe0 nanoparticles have been prepared using common
procedure by reduction of silica-deposited Fe precursor at 500°C
in a hydrogen flow.14,15 These 50%Fe/SiO2 materials are active
and selective for the partial hydrogenation of phenylacetylene
(PA) at 60°C and H2 pressure 10 bar, while the hydrogenation
rates are low, perhaps, due to the large (~17 nm) particle size.
Previously,16–18 we prepared materials comprising iron oxide
nanoparticles on the alumina and silica supports by means of low
temperature decomposition of ammonium trioxalatoferrate salt
in air.19 Herein, we tested them in liquid-phase hydrogenation of
PA,† which is commonly used as a model reaction in the studies
on the catalytic activity of materials in hydrogenation of alkynes
under mild conditions due to its commercial importance.10,14,20,21
Hydrogenation of PA over the FeOx/HS samples started at
100–105 °C giving styrene and ethylbenzene at the 99% carbon
balance. The complete conversion occurred at 105°C in 2–5 h
depending on the temperature of preliminary calcination of the
†
The FeOx/SiO2 materials were prepared by incipient wetness im-
pregnation of a silica support with a saturated aqueous solution of
(NH4)3[Fe(C2O4)3]·3H2O (pure, 98%, Acros Organics). The granulated
commercial silica supports with a high specific (HS) surface area (S =
= 300 m2 g–1, average pore width 12 nm, Russia) and a low specific (LS)
surface area (S = 105 m2 g–1, average pore width 36 nm, Khimmed,
Russia) were used. The impregnated samples were dried at 60°C in an oven
and then calcined at 250–500°C in air to decompose the precursor.20–22
The reference sample a-Fe2O3 with SBET = 90 m2 g–1 was prepared by
decomposition of (NH4)3[Fe(C2O4)3]·3H2O at 500°C in air, its phase
composition was confirmed with XRD analysis.
The samples were characterized by TEM, XRD analysis and DRIFT
spectroscopy of adsorbed CO using the described procedures.22,23 The
samples morphology was studied using a Hitachi HT7700 transmission
electron microscope. Images were acquired in the bright-field TEM mode
at the 100 kV accelerating voltage. Before measurements, the samples
were deposited on 3 mm carbon-coated copper grids from isopropanol
suspension. Target-oriented approach was used for the optimization of
the analytical measurements.25
In view of the fact that smaller Fe0 nanoparticles exhibited
the catalytic activity at lower temperatures and H2 pressures
(25°C and 10–20 bar),9,10 it seems important to find the way
of their stabilization via deposition on the surface of a shaped
support. The modification of Fe nanoparticles with a silicone
polymer affords a nanocomposite containing particles with a size
of 2–3 nm with the Fe0–FeOx core-shell structure.12 These hybrid
systems modified with metal nanoparticles seem promising for
catalysis. Heterogeneous catalysts in the form of shaped bodies
© 2017 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
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Mendeleev Commun., 2017, 27, 512–514
100
80
60
40
20
0
100
80
60
40
20
0
(a)
(b)
100
80
60
40
20
0
3
1
3
4
2
2
3
1
4
1
4
2
0
50 100 150 200 250
Time/min
0
25
50
XPA (%)
75
100
0
100
200
300
Figure 1 (a) Time dependence of the PA conversion and (b) styrene
selectivity vs. PA conversion over (1) a-Fe2O3 and the FeOx/HS sample
Time/min
calcined at (2) 250, (3) 400 and (4) 500°C. Reaction conditions: C0 PA
=
Figure 3 Time dependence of (1, 3) PA conversion and (2, 4) styrene
selectivity over the FeOx/HS sample calcined at 350°C. Reaction conditions:
= 0.13 m; nPA :nFe = 12; T = 105°C; pH = 13 bar.
2
C0 PA = 0.15 m; nPA :nFe = 10; T = 105°C; pH = 20 (1, 2) or 13 bar (3, 4).
2
FeOx /HS-350
20 bar H2
FeOx /
HS-250
100
3.00
2.00
1.00
0.00
3
80
60
40
20
0
1
4
FeOx /
HS-400
2
FeOx /
HS-350
FeOx /
HS-500
α-Fe2O3
Figure 2 Initial rate of PA hydrogenation for the FeOx/HS sample calcined
at 250, 350, 400 and 500°C and a-Fe2O3. Reaction conditions: C0PA = 0.13 m;
0
200
400
600
Time/min
T = 105°C; pH = 13 bar.
2
Figure 4 Time dependence of the (1, 3) PA conversion and (2, 4) selectivity
to styrene over the samples (1, 2) FeOx/HS calcined at 350°C and (3, 4)
FeOx/LS reduced in H2 at 350°C. Reaction conditions: C0 PA = 0.13 m;
sample [Figure 1(a)]. The selectivity to styrene depended on
the PA conversion as well as on the calcination temperature of
the samples [Figure 1(b)]. The best selectivity reached 80% for
75% PA conversion at pH = 13 bar over the sample calcined
at 250°C, whereas it decr2eased to 60–65% when approaching
the full PA conversion.
nPA :nFe = 15; T = 110°C; pH = 13 bar.
2
under reaction conditions, which results in S-shaped conversion
curves for the samples calcined at 350°C.
The highest initial rates of PA hydrogenation were observed for
the material calcined at 250°C (Figure 2). The relatively low initial
rate for the samples calcined at 350°C seems to be strange,
however, these results were highly reproducible. The catalytic
activity of the bulk a-Fe2O3 sample was very low as compared
with the silica-supported iron oxide materials [Figures 1(a), 2].
The H2 pressure affected the catalytic activity which con-
siderably increased at 20 bar as compared with that at 13 bar,
for example, over the FeOx/HS sample calcined at 350°C
(Figures 2, 3). It should be mentioned that induction period was
observed when the reaction was performed at lower H2 pressure
of 6 bar. This feature demonstrates the generation of the active
sites that performed the hydrogenation of a triple bond only in
direct contact with the reaction mixture and H2. The induction
period disappeared at the higher H2 pressure of 13–20 bar. The
sample calcined at 350°C seems to contain two iron oxide phases.
The first one is Fe3O4 as the product of thermal decomposition of
ammonium trioxalatoferrate at 250°C, and another one is Fe2O3,
the product of its oxidation at higher temperatures.16–19 These
phases are likely to exhibit different rates of initial reduction
Apparently, the sample surface is reduced in situ before the
reaction starts. It should be pointed out that preliminary reduction
to Fe3O4 in a hydrogen flow at 350°C (conditions based on
data from ref. 18) resulted in a more active catalyst FeOx/LS
(LS is low surface) compared to FeOx/HS calcined at 350°C
with a close selectivity (67 and 60%, respectively) at the complete
PA conversion (Figure 4).
The obtained data revealed that hydrogen consumptions
occurred only until the complete PA conversion to styrene and
ethylbenzene. No further styrene hydrogenation was observed
(Figure 4), perhaps, due to very weak adsorption of a double
bond on the surface sites of the samples. This fact allows us to
suppose that PA hydrogenation to styrene and to ethylbenzene
proceeded as parallel reactions on different surface sites.
The main results obtained in a mini-autoclave are summarized
in Table 1. The PA conversion was similar for the samples
on both supports at 110°C, while at 150°C it was higher for the
sample prepared using the HS silica. The selectivity to styrene
decreased with the reaction temperature. It should be mentioned
Table 1 Hydrogenation of PA over the FeOx/SiO2 samples and the SiO2
supports calcined at 350°C.a
The catalytic properties of FeOx/SiO2 samples were studied in the
liquid phase hydrogenation in the batch mode using a lab-constructed
mini-autoclave glass reactor (5 ml), as well as a larger autoclave (100 ml)
fitted with a glass reactor and a probe valve. The reaction conditions
were as follows: the 0.13–0.15 m PA solution in ethanol, the H2 pressure
6–20 bar, 100–150°C, the molar ratio PA:Fe = 5–15. The reaction
products were identified by GC analysis24,25 with undecane as an internal
standard (0.19 m) that was introduced in the initial reaction mixture.
Only styrene and ethylbenzene were formed as the reaction products. The
time dependence of the PA conversion and selectivity to styrene was
studied. The catalytic activity was evaluated by the initial rate of PA
hydrogenation referred to the molar amount of iron.
Sample
T/°C
t/h
XPA (%) Sstyrene (%) Sethylbenzene (%)
FeOx/HS
110
150
4
2
13
97
76
41
24
59
HS-SiO2
FeOx/LS
150
2
6
81
13
110
150
4
3
10
16
87
59
13
41
LS-SiO2
150
3
5
83
12
aReaction conditions: mini-autoclave, CPA = 0.15 m, nPA :nFe = 5; CC
=
20H42
= 0.19 m; pH = 10 bar.
2
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Mendeleev Commun., 2017, 27, 512–514
that some PA hydrogenation proceeded over the Fe-free supports
at 150°C when an incomplete C balance (94–95%) indicated
formation of some other products in such a process.
and environmentally safe procedure. These nanoparticles catalyze
hydrogenation of the triple CºC bond in PA in the liquid phase
under relatively mild conditions. The catalyst herein obtained is
superior to its analogues reported previously.9–11,14,15
The TEM images of the FeOx/HS samples calcined in air at
250–350°C (Figure S1, see Online Supplementary Materials)
provide evidence for numerous isolated nanoparticles of the size
1–4 nm that are clearly recognized as dark features with almost a
spherical or semispherical shape. In addition to isolated nano-
particles, their aggregates of the size 10–20 nm can be found in
the images of the FeOx/HS sample after calcination. Only slight
changes in the particle size were observed when the calcination
temperature of the sample was raised from 250 up to 400°C. In
the TEM images of the FeOx/LS sample calcined in air at 350°C,
the seldom crystallites of the size 10–20 nm were observed in
addition to the above mentioned nanoparticles and aggregates
(Figure S1). The silica-supported nanoparticles in all samples,
even calcined in air at 500°C, were XRD amorphous, while the
diffraction pattern of the bulk iron oxide sample exhibited the
reflections of the a-Fe2O3 phase with corresponding intensities.
The a-Fe2O3 sample consisted mostly of the isolated nano-
particles of the size 10–20 nm.
Electron microscopy characterization was performed in the
Department of Structural Studies of the N. D. Zelinsky Institute
of Organic Chemistry, Russian Academy of Sciences.
This work was supported by the Russian Science Foundation
(project no. 14-50-00126).
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2017.09.028.
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2353
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Figure 5 DRIFT spectra recorded after CO adsorption for 17 h on the
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Received: 14th December 2016; Com. 16/5124
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