Avoiding rhenium loss in non-hydrolytic synthesis of highly active Re-Si-Al olefin metathesis catalysts

Article abstract of DOI:10.1016/j.catcom.2014.09.024

Rhenium oxide-based catalysts are highly active olefin metathesis catalysts. However, sublimation of rhenium heptoxide during the calcination step can lead to the loss of rhenium. We show that rhenium losses in Re-Si-Al mixed oxide catalysts synthesized by non-hydrolytic sol-gel can be avoided providing that strictly anhydrous conditions are kept in all the preparation steps and during storage. Re-Si-Al mixed oxides with various Re₂O₇ loadings and SiO₂/Al₂O₃ ratios were prepared, without any loss of rhenium. Catalysts were amorphous, exhibited good mesoporous textures and acidity and performed outstandingly in the metathesis of ethene and trans-2-butene to propene.

Full text of DOI:10.1016/j.catcom.2014.09.024

Catalysis Communications 58 (2015) 183–186
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Avoiding rhenium loss in non-hydrolytic synthesis of highly active
Re–Si–Al olefin metathesis catalysts
a
b
b
c,
a,
Karim Bouchmella , Mariana Stoyanova , Uwe Rodemerck , Damien P. Debecker ⁎, P. Hubert Mutin ⁎⁎
a
Institut Charles Gerhardt, UMR 5253, CNRS-UM2-ENSCM-UM1, Université Monpellier 2, cc 1701, Montpellier 34095, France
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany
Institute of Condensed Matter and Nanoscience–MOlecules, Solids and reactiviTy (IMCN/MOST), Université catholique de Louvain, Croix du Sud 2 box L7.05.17, 1348 Louvain-La-Neuve, Belgium
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 August 2014
Received in revised form 10 September 2014
Accepted 12 September 2014
Available online 22 September 2014
Rhenium oxide-based catalysts are highly active olefin metathesis catalysts. However, sublimation of rhenium
heptoxide during the calcination step can lead to the loss of rhenium. We show that rhenium losses in Re–Si–
Al mixed oxide catalysts synthesized by non-hydrolytic sol–gel can be avoided providing that strictly anhydrous
conditions are kept in all the preparation steps and during storage. Re–Si–Al mixed oxides with various Re
2 7
O
loadings and SiO /Al ratios were prepared, without any loss of rhenium. Catalysts were amorphous, exhibited
2
2 3
O
good mesoporous textures and acidity and performed outstandingly in the metathesis of ethene and trans-2-
butene to propene.
Keywords:
Alkene metathesis
Rhenium
© 2014 Elsevier B.V. All rights reserved.
Mesoporous mixed oxides
Non-aqueous sol–gel
Silica–alumina
1
. Introduction
Olefin metathesis is an important reaction for several sectors of the
regeneration steps, and has been ascribed to the volatility of rhenium
heptoxide, which sublimes at 523 K [14,15]. Several authors have re-
ported rhenium loss during the catalyst preparation, independent of
the support and the preparation method (wet impregnation, incipient
wetness and thermal spreading) [14,16–20]. According to the literature,
the loss of rhenium is favored in the presence of water [16,21–23], at
high Re loadings [17] and at high temperatures [19,22].
There are very few examples of rhenium-based metathesis catalysts
obtained by a sol–gel method [24,25]. Non-hydrolytic sol–gel (NHSG)
[26,27] is emerging as a powerful method to prepare tailored heteroge-
neous catalysts [28] and especially mesoporous mixed oxides [29]. Re-
cently we have reported the NHSG synthesis of highly active Re–Si–Al
chemical industry (organic synthesis, polymerization chemistry,
petro-chemistry) [1]. For the metathesis of gaseous light alkenes, the
most successful catalysts are those based on W, Re and Mo oxides
[
2–4]. Even though Re is expensive, rhenium-based catalysts are attrac-
tive because they are highly active and selective at low temperature [1,
]. Re /Al was even used industrially for the production of
propene from ethene and 2-butene in the liquid phase at 35 °C and
5
2
O
7
2 3
O
6
0 bar (meta-4 process) [1].
Rhenium-based metathesis catalysts are generally prepared by in-
cipient wetness impregnation of an oxide support, such as TiO
2
, Al
[6–9]. Their activity depends on the nature of the support:
/SiO –Al catalysts are known to be more active than
[10], while ReO /SiO catalysts are virtually inactive [11,
2]. A specific problem encountered with rhenium-based catalysts is
2
O
3
,
metathesis catalysts [30], by reaction at 110 °C of ReCl
5
, SiCl
4 3
, AlCl
i
or SiO
thus, ReO
Re /Al
2
–Al
2
O
3
with a stoichiometric amount of Pr O, followed by drying and calcina-
2
x
2
2
O
3
tion in ambient air. As anticipated from the particularly low Tammann
temperature of rhenium oxide, migration of Re species during the calci-
nation led to highly dispersed Re surface species. However, rhenium
losses occurred during calcination of the samples with a low Al content,
which nevertheless exhibited very interesting textures. No loss of rheni-
um during calcination was observed for the samples with a high Al con-
tent, but these materials showed only moderate specific surface areas.
Recently, we observed that the texture of Re–Si–Al oxide catalysts
made by NHSG changed after exposure to moisture. This observation
and the literature on the loss of rhenium, prompted us to re-
investigate the NHSG synthesis of Re–Si–Al catalysts. In the present
2
O
7
2
O
3
x
2
1
linked to the volatility of rhenium [13], which, conjugated with the
high price of rhenium has hindered industrial application of these cata-
lysts [4]. Rhenium loss is often observed during the calcination and/or
⁎
Correspondence to: D. Debecker, Institute of Condensed Matter and Nanoscience–
MOlecules, Solids and reactiviTy (IMCN/MOST), Université catholique de Louvain, Croix
du Sud 2 box L7.05.17, 1348 Louvain-La-Neuve, Belgium. Tel.: +32 10 47 3665;
fax: +32 10 47 3649.
⁎⁎
Corresponding author. Tel.: +33 4 6714 4943; fax: +33 4 6714 3852.
work, Re–Si–Al mixed oxides with Re
2 7
O loadings of 2, 10 or 20 wt.%
E-mail addresses: damien.debecker@uclouvain.be (D.P. Debecker),
hubert.mutin@univ-montp2.fr (P. Hubert Mutin).
and SiO /Al ratios of 17 or 0.3 (Table 1) were prepared by non-
2
2 3
O
http://dx.doi.org/10.1016/j.catcom.2014.09.024
1566-7367/© 2014 Elsevier B.V. All rights reserved.

184
K. Bouchmella et al. / Catalysis Communications 58 (2015) 183–186
Table 1
Composition of the samples.
Sample
Experimental compositiona (nominal)
Re/(Re + Si + Al)
%
Re
(wt.%)
2
O
7
SiO
(wt.%)
2
Al
(wt.%)
2
O
3
2 2 3
SiO /Al O
Re2SA17
0.5 (0.5)
2.9 (2.9)
5.3 (5.6)
0.5 (0.4)
2.3 (2.4)
5.4 (5.5)
2.1 (2.0)
10.8 (10.8)
18.7 (19.6)
2.1 (1.9)
9.7 (10.0)
20.8 (20.1)
92.2 (92.1)
84.7 (84.3)
76.7 (75.6)
20.0 (23.4)
20.2 (21.1)
18.3 (17.4)
5.6 (5.9)
4.5 (4.9)
4.6 (4.8)
78.0 (74.7)
70.1 (68.9)
60.9 (62.5)
16.4 (15.6)
18.8 (17.2)
16.8 (15.7)
0.26 (0.31)
0.29 (0.31)
0.30 (0.28)
Re10SA17
Re20SA17
Re2SA0.3
Re10SA0.3
Re20SA0.3
a
Determined from ICP-AES results. Nominal compositions (under brackets) are calculated on the basis of the amount of each precursor introduced in each preparation.
hydrolytic sol–gel; anhydrous conditions were maintained in all the
preparation steps (synthesis, washing, drying and calcination) and dur-
ing the storage of the materials.
total micropore volume of the fresh samples was estimated using t-
plot analysis. The acidity was evaluated by temperature programmed
desorption of ammonia (NH
910 apparatus with a thermal conductivity detector. Samples
(100 mg) were loaded in the glove box and then pretreated in helium
3
-TPD) on a Micromeritics AutoChem
2
2
. Experimental section
−
1
at 500 °C for 60 min (ramp 10 °C min ). Adsorption of NH
3
(5 vol.%
in helium; flow rate 30 mL min ) was done at 100 °C for 45 min.
Physisorbed NH was removed by purging with helium at 100 °C for
1 h (flow rate 30 mL min ). The TPD measurement was conducted
−
1
2
.1. Preparation of the catalysts
3
−
1
In order to avoid water, all the manipulations were done under a dry
−
1
argon atmosphere using classical glovebox and Schlenk techniques, the
reaction was performed in glass tubes sealed under vacuum, the calcina-
tion was performed under a dry air atmosphere, and the catalysts were
stored in a glovebox under dry argon prior to use. The catalysts were pre-
pared by reaction of the chloride precursors with a stoichiometric
by heating the sample from 100 to 500 °C at a 10 °C min rate.
2.3. Catalytic test
Although industrial feeds comprise both cis- and trans-2-butene, the
cross-metathesis of ethene and pure trans-2-butene to propene was se-
lected as a model reaction to allow better comparison between catalysts
with different cis–trans isomerization activities. Indeed, trans-2-butene
usually reacts faster than cis-2-butene on Re-catalysts. The activity of
the catalysts was measured in a multi-channel apparatus, which allows
fully automated control of gas flows and of three temperature zones
(gas pre-heating, reactor, and post reactor lines equipped with 16-
port valve) along with reactor switching and product sampling [31].
All catalysts were sieved in the 200–315 μm granulometric fraction.
The catalysts (100 mg) were introduced in straight quartz reactors
(4 mm i.d.). Prior to reaction, the catalysts were activated for 2 h at
i
i
amount of diisopropyl ether ( Pr
2
O): the number of moles of Pr
2
O was
O was equal to the
total number of Cl groups in the precursors. SiCl (99.9%), AlCl
i
i
calculated so that the number of Pr groups in Pr
2
4
3
(
99.9%), and ReCl
5
(99.5%) were purchased from Alfa Aesar and used as
i
received. Pr
wire. CH Cl
pared in 1.5 g quantities in 80 mL sealed glass tubes. The chloride precur-
2
O (Aldrich, 99%) was dried by distillation over sodium
2
2 2
was dried by distillation over CaCl . The catalysts were pre-
i
2 2 2
sors were introduced first, then Pr O and finally CH Cl (10 mL). The
tube was sealed and heated at 110 °C for 4 days under autogenous pres-
sure (ca. 0.7 MPa). After cooling down to room temperature, the tube
was opened in a glovebox, washed under a dry argon atmosphere with
−
1
−1
2 2
CH Cl , dried at 20 °C under vacuum (10 Pa) for 1 h and then for 4 h at
550 °C (temperature ramp of 5 K min ) in dry N
in each reactor). Afterwards the system was cooled down to the reac-
tion temperature (40 °C) under the same N flow. The reaction was car-
ried out at 40 °C in a trans-2-butene: ethene: N (45:45:10 vol.%) total
flow of 8 ml min ; trans-2-butene (99.00%), ethene (99.95%), and N
(99.999%) (Linde) were further purified over Molsieve 3A (Roth) filled
columns. N was additionally purified by an oxygen filter (Oxysorb-
2
(8 mL min flow
1
20 °C. The resulting xerogel was crushed in a mortar in a glovebox
−
1
and calcined in a tube furnace under dry air (50 mL min ) for 5 h at
2
−
1
5
00 °C (heating rate 10 °C min ), leading to a white powder.
The samples are labeled RexSAy, where x represents the Re
ing in wt.% (x = 2, 10 and 20) and y is the Si/Al molar ratio (y = 17 and
2
−
1
O
2 7
load-
2
0
.3) (Table 1).
2
glass, Linde). The composition of the reaction gas was analyzed online
by an Agilent 6890 gas chromatograph. The separation of hydrocarbons
was performed on a HP-AL/M column (30 m length, 0.53 mm i.d.,
0.15 μm film thickness), using a temperature ramp between 95 and
2
.2. Characterization of the catalysts
The elemental analysis (Re, Si and Al) of the samples was carried
out at the Service Central de Microanalyse of the Centre National de
la Recherche Scientifique (CNRS) in Vernaison (France), using Induc-
tively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES). The
atomic percentages of Re, Si, Al and Cl were obtained by energy
dispersive X-ray spectroscopy (EDX), using an X-Max Silicon Drift
Detector mounted on a FEI Quanta FEG 200 scanning electron micro-
scope. The values given for each sample correspond to the average of
three measurements. Powder X-ray diffraction (XRD) diffractograms
were obtained with a Philips X-Pert Pro II diffractometer using the Kα
radiation of Cu (λ = 1.5418 Å). The 2θ range was recorded between
2
140 °C and FID detection. Each analysis took about 6.5 min. N , used
as an internal standard, was analyzed on a HP Plot-Q column with
TCD detection. The experiments were carried out using a slight over-
pressure to prevent contamination. The specific activity was defined
as the number of moles of propene produced per gram of catalyst and
per hour. The apparent turnover frequency (TOF) is defined as the num-
ber of moles of propene produced per mole of Re (considering the total
Re content, even if only part of the Re is actually active in the reaction)
and per second.
−
1
1
0° and 80° with rate of 0.02° s . N
2
physisorption experiments
3. Results and discussion
were performed at 77 K on a Micromeritics Tristar sorptometer. The
samples were outgassed at 150 °C under vacuum (2 Pa) overnight.
The specific surface area was determined via the BET method in the
3.1. Synthesis and characterization of the materials
0
.05–0.30 P/P
0
range. The pore size distribution was derived from the
Elemental analysis by ICP-AES showed that the experimental
composition of the catalysts was very close to the nominal one
desorption branch using the BJH method. The average pore diameter
is calculated as (4 × Pore Volume / BET specific surface area). The
2 2 3 2 7
(Table 1), independent of the SiO /Al O ratio or the Re O loading.

K. Bouchmella et al. / Catalysis Communications 58 (2015) 183–186
185
1
1
200
000
1200
a
b
Re2SA17
Re10SA17
Re20SA17
Re2SA0.3
Re10SA0.3
Re20SA0.3
1000
800
600
400
200
0
8
6
4
2
00
00
00
00
0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure
Fig. 1. N -physisorption isotherms of the catalysts with (a) Si/Al ≈ 17 and (b) Si/Al ≈ 0.3.
2
In particular the Re
of elemental analysis) to the targeted one, which means that no Re was
lost during the calcination step, even for the alumina-poor formulations
2
O
7
content was identical (within the uncertainty
presence of weak acidic sites. The thermograms of the materials con-
taining 10 and 20 wt.% of Re showed a NH desorption peak above
450 °C, indicating the presence of stronger acidic sites for these samples,
particularly for the alumina-poor materials.
O
2 7
3
(
2 2 3
SiO /Al O ~ 17) or at high Re loadings. This finding confirms the im-
portance of avoiding the hydration of Re surface species in order to
avoid Re volatilization. The extreme volatility of hydrated Re surface
3
The amount of NH molecules desorbed per gram of sample at the
end of the TPD experiment is given in Table 3, together with the acid
species has previously been noted for Re/SiO
2
catalysts [16,21].
site density, which takes into account the specific surface area of the
As shown in Fig. 1, the nitrogen physisorption isotherms of the sam-
ples were typical of mesoporous solids. All of the samples exhibited high
materials. The amount of desorbed NH
loading, as reported previously by Räty and Pakkanen for Re catalysts
supported on γ-Al [32]. The Re20SA0.3 sample, with a high Re load-
ing and a high alumina content was the most acidic sample, in terms of
the amount of NH desorbed and the density of acid sites.
3
roughly increased with the Re
2
−1
specific surface areas (from 420 to 730 m g ) and pore volumes (from
2 3
O
3
−1
0
.5 to 1.8 cm g ), along with negligible microporosity (Table 2).
Table 2 shows that both V and D decreased when the Re loading
p
p
3
increased. The highest specific surface areas were obtained for the
Re10SA17 and Re20SA17 samples with low alumina content. The
Re10SA0.3 and Re20SA0.3 samples with a high alumina content showed
3.2. Catalytic activity
somewhat lower specific surface areas. Remarkably, the V
p
and D
p
The calcined samples were tested in the cross-metathesis of ethene
and trans-2-butene to propene at 40 °C (Fig. 2). The by-products detect-
ed were 1-butene, 1-pentene, trans-2-pentene, cis-2-pentene and
hexenes. Fig. 2 shows that the deactivation was relatively slow. EDX
analysis of the spent catalysts showed that whatever the composition
no loss of rhenium occurred during activation and reaction (composi-
tions available as supplementary material, Table S1).
values found for Re10SA0.3 are two to three times higher than those re-
ported previously for a catalyst with a similar composition which was
washed and calcined under ambient conditions (the catalyst was la-
beled “Re3SiAl0.3” in our previous paper but it corresponds to
Re10SA0.3 in the present labeling system) [30].
Regardless of their composition, all the materials appeared amor-
phous to X-ray diffraction (diffractograms available as supplementary
material, Figure S1), indicating the absence of crystalline rhenium
oxide domains larger than about 5 nm.
As shown in Fig. 2, the specific activity was highly dependent on the
composition of the materials. The activity of the alumina-rich samples
increased with the Re
poor samples reached a maximum for the sample with 10 wt.% Re
loading. For a given Re loading, the specific activity of the Al-rich
2 7
O loading, whereas the activity of alumina-
The metathesis activity of rhenium oxide supported on silica–alumi-
na catalysts is known to be strongly influenced by the acidity of the cat-
2 7
O
2 7
O
alysts. Temperature programmed desorption of ammonia (NH
3
-TPD)
samples is significantly higher than that of the Al-poor samples, the
(
thermograms available as supplementary material, Figure S2) showed
most active samples being Re20SA0.3 and Re10SA0.3 (Table 4).
a broad desorption peak centered at about 200 °C, indicating the
Table 3
Acidity measured by NH
Table 2
3
-TPD (100–500 °C).
Textural properties of the samples: specific surface area (SBET), total pore volume (V
P
) and
average pore diameter (D ).
P
Sample
Acidity
Sample
S
BET (m2 g−1
)
V
p
(m3 g−1
)
D
p
(nm)
NH
3
desorbed (mmol g−1
)
Density of acid sites (nm−2
)
Re2SA17
480
730
630
540
530
420
1.8
1.0
0.9
1.6
1.0
0.5
15
5
6
12
8
4
Re2SA17
0.5
2.2
2.5
2.5
2.3
2.8
0.6
1.8
2.4
2.8
2.6
4.0
Re10SA17
Re20SA17
Re2SA0.3
Re10SA0.3
Re20SA0.3
Re10SA17
Re20SA17
Re2SA0.3
Re10SA0.3
Re20SA0.3

186
K. Bouchmella et al. / Catalysis Communications 58 (2015) 183–186
Table 4
carried out under strictly anhydrous conditions, Re sublimation during
calcination of alumina-poor formulations was suppressed, even at
very high rhenium loading. In addition, under these conditions Re–Si–
Al materials with very high specific surface areas could be obtained
whatever the Si/Al ratio or Re loading. This allowed us to obtain highly
active mixed oxide metathesis catalysts, showing unprecedented spe-
cific activities.
Specific activity and apparent turnover frequency (taken at 10–14 min on stream) for the
different RexSAy samples.
Sample
Specific activity
“TOF”
−
1
− 1)
(10 s−1
−3
(mmol g
h
)
Re2SA17
3
56
47
12
87
10
35
17
38
60
33
Re10SA17
Re20SA17
Re2SA0.3
Re10SA0.3
Re20SA0.3
Acknowledgments
101
The authors from ICGM acknowledge the Ministère de l'Enseignement
Supérieur et de la Recherche and the Centre National de la Recherche
Scientifique (CNRS) in France for financial support.
1
1
10
00
Re20SA0.3
Appendix A. Supplementary data
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
Re10SA0.3
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.09.024.
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Re20SA17
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−
1
−1
with an excellent specific activity of 87 mmol g
h
and a high
−
3
−1
“TOF” value of 60 · 10
s
, nearly twice higher than the values re-
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(
[
2
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3
−1
grafted onto silica (TOF = 250 · 10
ometallic chemistry approach [33].
s
at 20 °C) by a surface organ-
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[
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4
. Conclusions
The non-hydrolytic sol–gel method offers a very simple route to
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[
[
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highly active mesoporous Re–Si–Al metathesis catalysts, starting from
commercially available chloride precursors. We demonstrate here that
when the whole preparation procedure as well as the storage was
2062–2063.