Iridium supported on MgF2-MgO as catalyst for toluene hydrogenation

Article abstract of DOI:10.1016/j.catcom.2011.11.006

The use of mixed magnesium oxo-fluoride support, obtained by one-step sol-gel method, for iridium active phase allowed obtaining new catalysts of high activities in the hydrogenation of toluene. The Ir/MgF₂-MgO catalyst was a hundred times more active than that of the iridium system supported on MgF₂. The effect of the MgO content and catalyst pre-treatment conditions appeared to have a clear influence on the activity of tested iridium catalysts. The highest activity, expressed as TOF, s^{- 1}, was obtained for the catalyst activated at 500 °C supported on magnesium oxo-fluoride containing 60 mol.% MgO.

Full text of DOI:10.1016/j.catcom.2011.11.006

Catalysis Communications 18 (2012) 1–4
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Iridium supported on MgF –MgO as catalyst for toluene hydrogenation
2
Michał Zieliński ⁎, Maria Wojciechowska
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60–780 Poznań, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The use of mixed magnesium oxo-fluoride support, obtained by one-step sol–gel method, for iridium active
Received 29 September 2011
Received in revised form 31 October 2011
Accepted 3 November 2011
Available online 15 November 2011
phase allowed obtaining new catalysts of high activities in the hydrogenation of toluene. The Ir/MgF
2
–MgO
catalyst was a hundred times more active than that of the iridium system supported on MgF . The effect of
2
the MgO content and catalyst pre-treatment conditions appeared to have a clear influence on the activity
of tested iridium catalysts. The highest activity, expressed as TOF, s− , was obtained for the catalyst activated
at 500 °C supported on magnesium oxo-fluoride containing 60 mol.% MgO.
1
Keywords:
Iridium catalyst
©
2011 Elsevier B.V. All rights reserved.
Sol–gel synthesis
Magnesium fluoride
Magnesium oxide
Toluene hydrogenation
1
. Introduction
catalyst supports or catalysts as such [17–19]. The advantages of
this method are high purity of the materials obtained, possibility of
control of particle size and porous structure of the products, relatively
low temperature of the process (b100 °C) and relatively low cost.
In this work the catalytic properties of iridium supported on mag-
nesium oxo-fluorides have been used to obtain new toluene hydroge-
nation catalysts. To the best of our knowledge, no data are available
Great significance of the processes of aromatic hydrocarbon
hydrogenation is mainly related to their use for improvement of
fuel quality [1] and has been a basic step of synthesis of many organic
compounds [2,3]. Among the metallic phases highly active and often
used in hydrogenation reaction is platinum [4]. A metal cheaper than
platinum [5] but of limited application in catalysis is iridium. On in-
dustrial scale it is used for production of acetic acid (Cativa™ pro-
cess) [6,7], and in the processes of hydrodesulphurization [8–10].
Lower cost of iridium and its greater resistance to deactivation
caused by impurities present in the raw material processed makes
it interesting for catalytic applications.
2
on investigation of Ir/MgF –MgO catalysts in toluene hydrogenation
reaction. Activity of the new catalysts has been compared with iridi-
um catalysts supported on pure MgF or MgO.
2
2. Experimental
Recently, much interest has been devoted to the use of binary sys-
2 2
2.1. Synthesis of MgF –MgO, MgF and MgO samples
2 2 2 3 2 2 3 2 2 3
tems (SiO –TiO , Al O –SiO , Al O –TiO , MgO–Al O ) [11–15] as cat-
alytic supports because of their high performance, usually better than
that of single-component systems. Surface properties of binary sup-
ports strongly depend on the method of their preparation. The most
often used methods are based on mechanical stirring of components
Two MgF –MgO samples of different MgO content were synthe-
2
sized by the sol–gel method from magnesium methoxide and an
aqueous solution of hydrofluoric acid. 120 cm3 of 0.5 M solution of
freshly synthesized magnesium methoxide (from magnesium turn-
ings for Grignard synthesis, MERCK) in methanol was added drop-
[
11], co-precipitation [14] or sol–gel process [11–13,15]. In this
paper the mixed MgF –MgO systems, pure MgF and MgO obtained
by sol–gel method have been tested as potential supports of iridium
catalysts for toluene hydrogenation. The samples of MgF –MgO with
3
− 1
2
2
wise (20 cm ∙h ) at room temperature under intense stirring to
an aqueous solution of hydrofluoride (40%, POCH – Polish Chemicals
Reagents). The amount of hydrofluoride solution was chosen to en-
sure 30 and 60 mol% MgO in the samples. The resulting dense gels
2
different content of MgO and pure MgO have been prepared by one-
step sol–gel method in the reaction of magnesium methoxide dis-
solved in methanol with hydrofluoric acid [16]. The sol–gel processes
have become an important and useful method for the synthesis of
of MgF
2 2
–Mg(OH) were subjected to ageing for 40 h at RT, and
then to drying at 80 °C for 3 h. The dried samples were calcined for
4 h at 400 °C. The MgF
x is the mol% of MgO.
2
–MgO samples were labelled as xMO, where
The MgF
Mg(OCH and anhydrous HF (48.8% HF in methanol, Aldrich) in a
way analogous to the above described synthesis of MgF –MgO, but
2
support was obtained by the sol–gel method from
⁎
Corresponding author at: Faculty of Chemistry, Adam Mickiewicz University in
3 2
)
Poznań, Grunwaldzka 6, 60–780 Poznań (Poland). Fax: +48 61 829 1505.
E-mail address: mardok@amu.edu.pl (M. Zieliński).
2
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.11.006

2
M. Zieliński, M. Wojciechowska / Catalysis Communications 18 (2012) 1–4
under strict anhydrous conditions. The dried sample was calcined for
2.4. Catalyst characterisation
4
h at 400 °C – denoted as MF.
MgO was obtained by the sol–gel method by hydrolysis of magne-
2.4.1. Surface area
sium methoxide (120 cm3 of 0.5 M solution) in water and treated
Surface area was determined by the low temperature (−196 °C)
nitrogen adsorption carried out on ASAP 2010 analyzer (Micromeri-
tics GmbH). Specific surface area was determined by using the
Brunauer-Emmet-Teller (BET) method.
similarly to MgF
2
–MgO. The dried sample was calcined for 4 h at
4
00 °C – denoted as MO.
2
.2. Synthesis of iridium catalysts
2.4.2. The determination of metal dispersion by hydrogen chemisorption
The reduced samples were placed in an ASAP 2010 C sorptometer
Iridium was deposited on MgF
conventional impregnation using methanolic solution of Ir
2
, MgO and MgF
2
–MgO supports by
(CO)12
The content of Ir was 1 wt.%. The catalysts were dried at 80 °C for
4 h – fresh catalysts. Prior to the determination of metal dispersion
by hydrogen chemisorption measurement, the samples were reduced
with H at 400 °C for 2 h — denoted as Ir/support-4 and reduced with
at 500 °C for 2 h — denoted as Ir/support-5.
and were evacuated for 15 min at room temperature and then at
00 °C for 60 min, followed by additional reduction in hydrogen
4
.
4
3
−1
flow (40 cm ⋅min ) at 400 °C and evacuated again for 120 min at
2
4
00 °C. Chemisorption of hydrogen was carried out at 35 °C and the
isotherms were determined using 5 different pressures in the range
of 6–40 kPa. After the measurement carried out at the first set of pres-
sures was completed, the catalyst was evacuated at 35 °C for 30 min
to remove reversibly adsorbed hydrogen and the same procedure
was repeated. The difference between adsorbed hydrogen extrapolat-
ed to zero pressure value for two isotherms equals to the amount of
hydrogen irreversibly bound. Metallic dispersions were calculated
by the ratio of irreversible uptake to the total metal content assuming
an H:Ir=1 surface stoichiometry [20,21].
2
H
2
2
.3. Catalytic test — hydrogenation of toluene
Toluene hydrogenation was performed at atmospheric pressure
using a fixed-bed flow reactor and H
2
as carrier gas. Fresh catalyst,
after drying (0.025 g) was placed in the reactor and reduced for 2 h
3
−1
in a flow (100 cm ⋅min ) of pure hydrogen (99.99% purchased
3
−1
from Messer) at the range of 300–550 °C. The H
2
(50 cm ⋅min
)
3
. Results and discussion
passed through a saturator filled with toluene. The concentration of
toluene in feed was stable and was 0.75μmol⋅cm . The catalysts
were heated at the rate of 10 °C∙min
ried out at 50–225 °C over the same catalyst. The reaction products
were analyzed on a gas chromatograph equipped with a capillary
column RESTEK - MXT - 1. The catalytic activity was presented as
apparent rate calculated by following Equation:
−
3
−
1
2
Iridium catalysts were obtained with the use of MgF , MgO and
and catalytic tests were car-
mixed magnesium fluoride–magnesium oxide supports. The magne-
sium oxo-fluoride supports were synthesized by the one-step sol–
gel method [16] in contrast to the described in literature two-step
2
method of preparation of MgO–MgF support, based on hydrolysis
of magnesium methanolate in the first stage and the treatment of
the hydroxide obtained with hydrogen fluoride in the second stage
[22]. In the one-step method, magnesium methanolate was treated
by hydrofluoric acid of the concentration adjusted to obtain samples
containing 0, 30, 60 and 100 mol% of MgO. The amount of magnesium
ꢀ
ꢁ
FYC
N
molTl
rt ¼
mol min
Ir
2
oxide introduced to MgF was estimated on the basis of the intensity
3
−1
where F is the total flow rate of feed (cm ⋅min ); Y the fractional
of the XRD peaks (not included) assigned to relevant crystalline
phases [16]. Parameters characterising the support and catalysts are
given in Table 1.
Hydrogenation of toluene to methylcyclohexane was performed
in a gas-phase in the temperature range 50–225 °C. The first parame-
ters to be optimized were the content of MgO in the support and
3
conversion; C the concentration of toluene in the feed (molTl⋅cm )
and N the iridium content (molIr) in the sample.
was calculated by dividing the
number of molecules converted per second by the number of active
−
1
Turnover frequency - TOF, s
2
iridium atoms measured by H adsorption.
Table 1
Characterisation of the support and catalysts.
Supports and catalysts[a]
Pre-treatment condition
MgO content in supports[b]
mol.%
,
BET surface area[c],
m ⋅g
H
chemis.[d],
Average Ir crystallite size[e]
nm
,
2
2
−1
−1
μmol⋅gIr
MF
calcined at 400 °C, 4 h
calcined at 400 °C, 4 h
calcined at 400 °C, 4 h
calcined at 400 °C, 4 h
0
32
–
–
3
6
0MO
0MO
33.3
59.9
100
0
33.3
59.9
100
0
144
208
152
31
158
252
149
23
–
–
–
–
MO
Ir/MF-4
–
–
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
reduction at 400 °C, 2 h
reduction at 400 °C, 2 h
reduction at 400 °C, 2 h
reduction at 400 °C, 2 h
reduction at 500 °C, 2 h
reduction at 500 °C, 2 h
reduction at 500 °C, 2 h
reduction at 500 °C, 2 h
1121
1821
2783
2299
667
1429
2085
1362
2.58
1.59
1.04
1.26
4.33
2.02
1.39
2.12
Ir/30MO-4
Ir/60MO-4
Ir/MO-4
Ir/MF-5
Ir/30MO-5
Ir/60MO-5
Ir/MO-5
33.3
59.9
100
131
193
122
[
[
[
a] See Experimental Section for nomenclature.
b] MgO content was estimated from XRD signals intensity [16].
c] The Brunauer-Emmet-Teller surface areas were determined by N adsorption at −196 °C using a Micromeritics ASAP2010 sorptometer. The conditions of measurements are
2
specified in Experimental Section.
[
[
d] The amount of irreversibly chemisorbed hydrogen.
e] Mean size of Ir particles calculated from the amount of irreversibly chemisorbed hydrogen. The conditions of measurements are specified in Experimental Section.

M. Zieliński, M. Wojciechowska / Catalysis Communications 18 (2012) 1–4
3
temperature of toluene hydrogenation. Results of the catalytic stud-
ies are given in Fig. 1. The MgF , MgO and magnesium oxo-fluoride
2
supports did not show any activity in the gas phase toluene hydroge-
nation, while the reduced Ir-catalysts were active in this process. In
all catalytic tests the only product was methylcyclohexane.
Fig. 1 illustrates the effects of reaction temperature and MgO
content in support on the activity of iridium catalyst. Prior to the
reaction, the catalysts were activated at 500 °C. This temperature
was chosen to avoid decomposition of the active phase precursor,
4
i.e. tetrairidium dodecacarbonyl–Ir (CO)12. Our earlier studies of
iridium catalysts supported on magnesium fluoride (obtained in a
reaction of basic magnesium carbonate with an aqueous solution
of hydrofluoric acid) have shown that the process of decarbonyla-
tion of Ir
Alexeev and Gates the process of decarbonylation of Ir
supported on Al [24] or MgO [25] is complete at 300 °C. That is
4
(CO)12 is complete at ~475 °C [23], while according to
4
(CO)12
2 3
O
why the plot in Fig. 1 presents the activities of the catalysts activated
in hydrogen flow at 500 °C.
Fig. 2. The effect of activation temperature on apparent rate of toluene hydrogenation
reaction on Ir/60MO catalysts at 125 °C.
All catalysts exhibit a maximum of activity as a function of the
reaction temperature. For the catalysts supported on pure MgF
2
and
MgF –MgO the temperature of the maximum activity was 125 °C.
2
For the catalyst supported on MgO (Ir/MO-5) the temperature of
the maximum activity was shifted towards higher temperatures, i.e.
400 °C the catalyst activity rapidly increases, which can be explained
by full decomposition of the iridium precursor Ir (CO)12. Further in-
4
to 150 °C. The hydrogenation activities of Ir/MgF
were much higher than that of Ir/MO-5 and the lowest activity
exhibited the Ir/MF-5 catalyst. The introduction of MgO to MgF
has a great effect on the Ir/MgF –MgO catalysts activity in toluene
hydrogenation reaction. The activity optimum was observed for
the MgO content of about 60 mol% for Ir/60MO-5. The hydrogenation
activity of the catalyst with 33.3 mol.% (Ir/30MO-5) was comparable
with that of Ir/MO-5 and was about 25% lower than that of Ir/60MO-
2
–MgO systems
crease in temperature, to 500 °C, did not cause significant changes in
the catalyst activity, while the activation at 550 °C was observed
to cause a decrease in the catalyst activity. This decrease was inter-
preted as related to changes in the specific surface area of the
catalysts and dispersion of the active phase (Table 1).
As follows from the data presented, the activity of iridium cata-
2
lysts is first of all dependent on the content of MgO in the MgF –
MgO support and the temperature of reduction of the catalysts stud-
ied. On the basis of the low-temperature nitrogen adsorption data,
see Table 1, the introduction of MgO to magnesium fluoride was
found to significantly affect the surface area of the support. The sur-
2
2
5
. The activity of Ir/MF-5 catalyst was almost two orders of magni-
tude lower.
Besides the optimum temperature for the reaction of hydrogena-
tion, another important parameter is the temperature of catalyst
activation. A decrease in the activation temperature at high degree
of conversion allows first of all to save the energy needed for the
catalyst activation. Fig. 2 presents the Ir/60MO catalyst activity ver-
sus the temperature of its activation; with increasing activation tem-
perature the activity of this catalysts increases. After the activation at
face area of the systems MgF
2
–MgO changed from 144 to even
2
−1
208m ⋅g
for 30MO-4 and 60MO-4, respectively, and was almost
five to seven times greater than that of pure magnesium fluoride
2
− 1
(32m ⋅g ). All the supports obtained were mesoporous materials
of the mean pore diameter of 4–12 nm. After the introduction of
the active phase, the specific surface area of the catalysts supported
on MgF
ported on MgF
2
or MgO slightly decreased, while that of the catalysts sup-
–MgO increased. As the above changes in the specific
2
surface area were small, it can be concluded that the impregnation of
the support with the iridium precursor and its later thermal treat-
ment in hydrogen at 400 °C did not have great influence on the
porous structure of the catalysts. Greater changes in the surface
area and in particular for the catalysts supported on MgF
recorded after the catalysts activation at 500 °C. MgF has almost
chemically inert surface, high hardness and good thermal stability
up to ~500 °C, and already at 500 °C crystallisation of MgF proceeds
and surface area decreases [26]. Introduction of MgO to MgF delays
2
, were
2
2
2
the process of crystallisation, which allows to obtain supports of
greater surface area and greater thermal resistance.
Another parameter influencing the activity of iridium catalysts in
hydrogenation of toluene is the dispersion of the metal. The disper-
2
sion and size of crystallites of iridium in the Ir/MgF –MgO catalysts
were characterised on the basis of the hydrogen chemisorption iso-
therms at 35 °C (Table 1.). The use of magnesium oxo-fluoride as a
support provided catalysts of very good dispersion of iridium, reach-
ing even 100% in the sample Ir/60MO-4. The dispersion of iridium
was found to depend on the content of MgO in the support. With in-
2
creasing surface area of the MgF –MgO supports the dispersion of
iridium increases, which is in agreement with literature data [27].
With increasing temperature of the catalysts reduction, the active
phase dispersion decreased, which was the most pronounced for
Ir/MF-5 and Ir/MO-5 catalyst and the least pronounced for the
Fig. 1. The effect of reaction temperature and MgO content in the magnesium oxo-
fluoride support on apparent rate of toluene hydrogenation reaction by iridium catalyst
activated at 500 °C.

4
M. Zieliński, M. Wojciechowska / Catalysis Communications 18 (2012) 1–4
2
catalysts supported on MgF –MgO. A similar influence of the temper-
Acknowledgments
ature of catalysts reduction on the size of iridium crystallites was
reported in [28] concerning the Ir/Al systems.
O
2 3
The financial support of the Polish Committee for Scientific Research
In our study we have employed hydrogen chemisorption, because the
amount of chemisorbed hydrogen enables determination of not only the
average size of metallic particles, but also the number of iridium surface
(grants N N204 141339 and N N204 214140) is gratefully acknowledged.
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1
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−
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