One-pot direct synthesis route to self-assembled highly ordered Zn-decorated mesoporous aluminium oxide toward efficient and sustainable metathesis heterogeneous catalyst design

Article abstract of DOI:10.1039/c5ra12057h

Hexagonally well-organized ZnCl₂-OMA materials have been successfully synthesized by a one-pot approach using a sol-gel method at ambient temperature and pressure. These materials were prepared through simultaneous self-assembly process with F127 as directing agent combined with in situ impregnation of ZnCl₂. The new approach was found to be compatible with various common aluminium precursors and carboxylic acids used as self-assembly interfacial protectors. The obtained ZnCl₂-OMA materials were fully characterized using XRD, N₂ adsorption-desorption, TEM, SEM, EDX, XPS and ¹H and ²⁷Al MAS-NMR techniques. These materials were compared with the zinc chloride-doped organized mesoporous alumina (ZnCl₂-OMA) supports prepared through a conventional two steps process that includes OMA synthesis and then post-synthesis incorporation of ZnCl₂. Thus, the one-pot synthesized materials were found to exhibit improved properties compared to the conventional ones, particularly larger BET surface area. The synthesized ZnCl₂-OMA materials were then used as catalytic supports for methyltrioxorhenium (MTO), showing enhanced catalytic performance for methyl oleate self-metathesis, demonstrating better activity and selectivity towards desired metathesis products. These features are highly beneficial for large-scale materials synthesis through fast, simple, easy and low-cost introduction of functionalities on mesoporous materials surface without multi-step procedures.

Full text of DOI:10.1039/c5ra12057h

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One-pot direct synthesis route to self-assembled highly ordered
Zn-decorated mesoporous aluminum oxide toward efficient and
sustainable metathesis heterogeneous catalyst design
Received 00th January 20xx,
Accepted 00th January 20xx
Abdelnasser Abidli*ab
DOI: 10.1039/x0xx00000x
Hexagonally well-organized ZnCl2-OMA materials have been successfully synthesized by a one-pot approach using a sol-gel
method at ambient temperature and pressure. These materials were prepared through simultaneous self-assembly
process with F127 as directing agent combined with in situ impregnation of ZnCl2. The new approach was found to be
compatible with various common aluminum precursors and carboxylic acids used as self-assembly interfacial protectors.
The obtained ZnCl2-OMA materials were fully characterized using XRD, N2 adsorption-desorption, TEM, SEM, EDX, XPS and
1H and 27Al MAS-NMR techniques. These materials were compared with the zinc chloride-doped organized mesoporous
alumina (ZnCl2-OMA) supports prepared through a conventional two steps process that includes OMA synthesis and then
post-synthesis incorporation of ZnCl2. Thus, the one-pot synthesized materials were found to exhibit improved properties
compared to the conventional ones, particularly larger BET surface area. The synthesized ZnCl2-OMA materials were then
used as catalytic supports for methyltrioxorhenium (MTO), showing enhanced catalytic performance for methyl oleate
self-metathesis, demonstrating better activity and selectivity towards desired metathesis products. These features are
highly beneficial for large-scale materials synthesis through fast, simple, easy and low-cost introduction of functionalities
on mesoporous materials surface without multi-step procedures.
www.rsc.org/
improved acid-base properties,14-16 as well as providing
enhanced catalytic performance usually through surface
synergetic interactions with those species.17 However,
Introduction
Metal oxide-based supported catalysts are considered as an
2
conventionally, the modification of the catalysts supports is
conventionally performed via a post-synthesis methodology,
through a challenging multistep process. These procedures are
usually laborious as well as energy and time consuming.
Moreover, these features render the synthesis process costly.
In addition, the use of several separation and treatment steps
increase both the use and the generation of possible toxic
solvents, compounds and gasses. Furthermore, such long
processes may hinder the control over the materials synthesis
and alter their properties (e.g. textural, morphological,
chemical, etc.) as well as the synthesis yield. These drawbacks
present enormous limitations towards large scale synthesis of
such modified materials and catalysts, as well as their broad
application.
Therefore, the most suitable route for the modification and
functionalization of these catalyst supports is to perform an in
situ incorporation of the desired components, prior (or during)
to the formation of the host materials. These one-step
procedures will highly increase the efficiency of the synthesis
processes by offering a variety of advantages including faster
synthesis by shortening the process from several days to few
hours or minutes.18-22 These one-step approaches are more
cost-effective and leads to higher reaction yields.23 This
renders the introduction of desired functionalities easier and
simpler with limited pore blocking phenomena.24 Moreover,
important class of heterogeneous catalysts.1, Among these
metal oxides, porous systems represent the major share of this
family, more particularly mesoporous materials which
gathered enormous attention for their wide application
spectrum in catalysis.3 These materials were also
advantageously used for other applications including energy
conversion and storage, gas sensing as well as adsorption and
separation applications.4-7 Mesoporous aluminum oxide (e.g.
γ-Al2O3) is one of the most commonly used catalytic supports.8
Moreover, a variety of active species including organics,
metals, organometallics and metal oxides were grafted on
alumina surface generating highly robust heterogeneous
catalysts.9-11 Furthermore, for heterogeneous catalytic
conversions, alumina supports are usually modified with such
species before incorporation of the active components. These
species usually offers better stability to the alumina matrix and
the catalyst,12 increased dispersion of active components,13
a.Department of Soil Sciences and Agri-Food Engineering, Laval University. G1V
0A6, Quebec City, Quebec, Canada. E-mail: abdelnasser.abidli.1@ulaval.ca; Tel:
+1-581-777-0860; Fax: +1-418-656-3723
b.Centre in Green Chemistry and Catalysis (CGCC), H3A 0B8, Montreal, Quebec,
Canada.
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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this methodology offers better control over the materials to be reached. Detailed characterizations and comparison with
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25, 26
synthesis, uniformity, purity and homogeneity.22,
Thus, alumina-based supports prepared viDaOI:t1h0e.103c9o/Cn5vReAn1t2i0o5n7aHl
allowing access to catalytic supports with better properties approach and their catalytic performance are provided. This
that are key factor for the enhancement of their catalytic successful methodology is a step forward towards an easy and
performance.18, 27 Actually, several reports showed that such efficient scalability. Also provide a rapid access route for such
synthesized catalysts always exhibit significantly better desired materials, especially with the recent attention
catalytic properties compared to those prepared via the gathered around zinc chloride impregnated on alumina and
conventional multistep process.18, 27, 28
alumina-silica materials for other applications such as methyl
Therefore, in view of these promising features, we decided to chloride synthesis37, 38 and catalytic alkylation reactions.39-42 As
adopt this methodology for our study on modified mesoporous well as other zinc chloride-modified materials including
alumina-based metathesis catalysts. Previously, we have mesoporous carbon nitride (ZnCl2/mp-C3N4) and doped
reported a successful synthesis of a highly active metathesis graphitic carbon nitride (Zn-g-C3N4) for other catalytic
catalysts using methyltrioxorhenium (MTO) supported on applications (e.g. cycloaddition and transesterification).43,
44
functionalized mesoporous alumina. First, through several Therefore, in addition to the synthesis of modified-alumina,
optimizations we showed that ZnCl2 is the most suitable metal this method represents a promising approach to prepare
halide for alumina modification, offering the highest various functionalized mesoporous materials for broad
metathesis performance among several other metal halides applications.
used for the study.29-31 The MTO supported ZnCl2-modified
Al2O3 catalyst was found to exhibit higher metathesis activity
compared to the unmodified alumina.29-31 This catalyst showed
high performance for both methyl oleate and triolein self-
metathesis.29-31 However, this catalyst displayed relatively slow
kinetics and unsatisfactory selectivity towards desired
metathesis products. Therefore, we replaced the traditional
alumina support having wormhole-like mesostructure by a
hexagonally ordered mesoporous alumina (OMA). Thus, we
have recently reported an enhanced performance of the MTO-
based catalysts for methyl oleate self-metathesis using the
ZnCl2-modified OMA supports.32 The OMA-based catalysts
were successfully tested for their activity for methyl oleate
self-metathesis. In fact, metathesis reaction in one of the most
interesting catalytic route providing access to a variety of
valuable monomers via C=C bond formation.33, 34 On the other
hand, methyl oleate is chosen as a model molecule having
both long carbon chain containing C=C bond which will be
subjected to metathesis catalysis conditions, and also the ester
functional groups to evaluate the MTO-based catalyst
tolerance for functional groups. In addition, methyl oleate is a
bulky molecule which is suitable to evaluate the kinetics and
molecular diffusion inside the mesoporous network. In the
meantime, this renewable substrate originated form vegetable
oils represents an interesting feedstock for the synthesis of
value-added compounds via valorization through metathesis
pathway.35, 36
Experimental
One-pot synthesis of zinc chloride (ZnCl2)-modified ordered
mesoporous alumina (OMA)
Well-organized ZnCl2-doped hexagonal mesoporous alumina
materials denoted as ZnCl2-OMA were prepared by a facile
one-pot synthesis through a combined sol-gel process and in
situ impregnation method. All the reagents were used as
received without further purification. Various aluminum
precursors were used including Al(OBus)3, Al(OPri)3,
Al(OBut)3 and Al(NO3)3·9H2O, with distinct carboxylic acids
(citric, tartaric, fumaric, oxalic, malonic, maleic and acetic acid)
in order to generalize this methodology and to verify the
reproducibility of the newly designed synthesis approach. For
a typical synthesis of these modified ordered mesoporous
materials; Pluronic F127 (2.17 g) was dissolved in anhydrous
ethanol (20 mL) under magnetic stirring at room temperature.
Then, the investigated carboxylic acid (2.6 mmol) was added,
while the pH of the solution was adjusted below 2 by adding
hydrochloric acid (1.0-1.5 mL), giving a clearer solution.
Subsequently, the investigated aluminum-precursor (10 mmol)
was added under violent magnetic stirring. Simultaneously,
zinc chloride (170 mg, 1.25 mmol) was dissolved in ethanol (10
mL) by applying a 30 s vortex mixing. The dissolved zinc
chloride was then slowly added dropwise to the above
Pluronic F127-containing solution under gentle stirring. The
resulting mixture was then covered with polyethylene film,
and continuously stirred for an additional 5 h at room
temperature to homogenize the content. Afterwards, the
mixture was transferred for aging into a drying oven at 60 °C,
allowing slow and compete evaporation of solvents within 48h.
Finally, the resulting dried solid was annealed at 400 °C with a
slow ramping rate (1 °C/min), and calcined for 4 h in air to
achieve the final product (ZnCl2-OMA materials) with
optimized Al : Zn molar ratio of 8 : 1.
Therefore, in this study, in order to improve furthermore the
MTO-based catalyst design and performance, we developed a
new direct one-pot self-assembly strategy for the synthesis of
ZnCl2-modified OMA supports. Instead of the traditional
methodology that we usually adopted which consist on
preparing the OMA supports then
a
post-synthetic
functionalization with ZnCl2 in a laborious process. Thus, we
are aiming to improve the synthesis process trough a simpler,
faster, easier and cost-effective process. The new synthesis
protocol is expected to offer the desired materials with
enhanced chemical, morphological, and textural properties
which may likely lead to better metathesis catalytic
performance. In addition, higher synthesis yields are expected
Catalysts preparation and metathesis reaction
3 wt.% of methyltrioxorhenium (MTO) was impregnated on
the prepared ZnCl2-OMA catalytic supports. Various supports
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those synthesized using different aluminum precursors and loop according to IUPAC classification. This results correlate
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DOI: 10.1039/C5RA12057H
carboxylic acids were used, and numerous tests were with the capillary condensation of nitrogen inside the confined
performed for each catalyst composition (support). The mesoporous solids, indicating that all samples have organized
evaluation of the catalyst was carried out on a batch reactor. mesoporous network with uniform and well-defined cylinder-
After impregnation of MTO (6.2 mg) on ZnCl2-OMA supports like pore geometry.45, 46
(200 mg) in hexane under N2 atmosphere at 45°C, the
prepared 3 wt.% MTO/ZnCl2-OMA catalyst was loaded in the
glass reactor. Afterwards, 750 mg of methyl oleate in 2 mL of
hexane was injected. The reacting mixture was heated to the
desired temperature (45°C) with stirring under atmospheric
pressure. After reaction was complete, the reactor was cooled,
and the catalyst was removed by centrifugation. The methyl
oleate conversion as well as reaction products identification
and quantification were conducted through GC and GC-MS
analysis using highly pure commercial standards.
Characterization of the prepared materials and surface properties
The textural properties of calcined samples including the BET
specific surface area, total pore volume and BJH pore diameter
distribution have been measured by nitrogen adsorption-
desorption using a volumetric adsorption analyzer (Model
Autosorb-1, Quantachrome Instruments). The hexagonal
crystalline structure of the calcined zinc chloride-modified
alumina samples were characterized by powder X-ray
diffraction (XRD). Small-angle X-ray scattering (SAXS) patterns
were recorded using Ultima III Rigaku monochromatic
diffractometer (Model D/MAX-2200). Transmission electron
microscopy (TEM, JEM-1230 electron microscope) was
performed in order to examine the structure of the materials'
porous network. The samples microstructures and
morphologies were observed using
a scanning electron
microscope (SEM, JEOL model JSM-840A), while the elemental
composition and the Al : Zn atomic ratio in the synthesized
materials was determined using energy dispersive X-ray
spectrometry (EDX, JEOL model JSM-840A). X-ray
photoelectron spectroscopy (XPS, AXIS ULTRA from Kratos
Analytical) measurements were conducted to identify the
materials surface species and their oxidation/coordination
states, as well as to confirm the elemental composition and
the Al : Zn atomic ratios analyzed by EDX. The catalyst
framework was investigated by proton solid-state NMR
spectroscopy (1H MAS NMR), while the form, coordination
state and environment of aluminum, as well as the surface and
intrinsic Lewis acidic site concentrations were studied by
means of aluminum solid-state magic-angle spinning NMR
spectroscopy measurements (27Al MAS NMR). See also S1 in
the ESI for more details on characterization experiments and
instrumentations.
Fig. 1 (A) Nitrogen adsorption-desorption isotherms and (B) pore size
distributions of the prepared ZnCl2-OMA samples. All samples were calcined
at 400 °C.
Table 1 summarizes the Brunauer-Emmet-Teller (BET) surface
areas (SBET), total pore volumes (PV) average pore size (dBJH
)
data obtained by N2 adsorption-desorption at 77 K illustrating
the textural properties and porosity parameters of the
synthesized ZnCl2-OMA materials. Relatively large BET surface
are and pore volume were obtained, up to 358 m2 g-1 and 0.67
cm3 g-1, respectively. In addition, nearly all the samples
displayed uniform narrow pore size distribution with a mean
diameter from 3.4 to 6.5 nm. However, some analyzed
samples present some secondary porosity (Table 1). These
results suggest that the presence of ZnCl2 did not hamper the
formation of the desired hexagonally ordered mesoporous
materials.
Results and discussion
Characterization of the alumina-based materials
Nitrogen adsorption-desorption measurements
As mentioned above, the one-pot synthesis was performed
Fig. 1A shows the N2 adsorption-desorption isotherms of the using distinct aluminum precursors and carboxylic acids in
prepared ZnCl2-OMA samples. All obtained isotherms exhibit a order to generalize the synthesis procedure and verify it's
typical IV-type curve accompanied with a H1-type hysteresis reproducibly.
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Table 1 Textural properties and porosity parameters obtained by N2 adsorption-desorption at 77 K for the synthesized zinc chDloOriId: 1e0-m.10o3d9if/ieCd5RoAr1g2a0n5iz7eHd
mesoporous alumina (ZnCl2-OMA) materials using different aluminum precursors and carboxylic acids
a
b
ZnCl2-OMA
samples
Acid
Aluminum precursor
Synthesis
approach
SBET
PV
dBJHc (nm)
(m2 g-1)
(cm3 g-1)
ZnCl2-OMA-1
ZnCl2-OMA-2
ZnCl2-OMA-3
ZnCl2-OMA-4
ZnCl2-OMA-5
ZnCl2-OMA-6
ZnCl2-OMA-7
ZnCl2-OMA-8
ZnCl2-OMA-9
ZnCl2-OMA-10
ZnCl2-OMA-11
ZnCl2-OMA-12
ZnCl2-OMA-13
ZnCl2-OMA-14
ZnCl2-OMA-15
ZnCl2-OMA-16
ZnCl2-OMA-17
ZnCl2-OMA-18
Tartaric
Fumaric
Citric
Al(OBus)3
Al(OBus)3
Al(OBus)3
Al(OBus)3
Al(OBus)3
Al(OBus)3
Al(OPri)3
Al(OPri)3
Al(OPri)3
Al(OPri)3
Al(OPri)3
Al(OPri)3
One-pot
Two-stepe
One-pot
203
154
299
214
307
316
282
ND
238
ND
252
ND
234
ND
181
ND
199
ND
159
ND
219
190
232
241
358
239
307
257
224
ND
201
139
226
222
183
ND
0.30
0.26
0.41
0.31
0.42
0.44
0.36
ND
0.27
ND
0.27
ND
0.46
ND
0.23
ND
0.32
ND
0.34
ND
0.38
0.39
0.47
0.38
0.67
0.41
0.44
0.44
0.41
ND
0.39
0.40
0.41
0.46
0.33
ND
3.8 and 5.6
3.8
3.8
3.8
3.8
3.8
3.8
ND
3.4
Two-stepe
One-pot
Two-stepe
One-pot
Oxalic
Two-stepe
One-pot
Maleic
Malonic
Oxalic
Two-stepe
One-pot
ND
3.4
ND
Two-stepe
One-pot
3.8 and 6.5
ND
Two-stepe
One-pot
Malonic
Acetic
3.4
ND
3.8 and 4.9
ND
3.8 and 6.5
ND
Two-stepe
One-pot
Two-stepe
One-pot
Maleic
Tartaric
Citric
Two-stepe
One-pot
3.8 et 4.9
5.7
Two-stepe
One-pot
7.7
Two-stepe
One-pot
3.8 and 4.9
3.8 and 5.6
4.9
t
Citric
Al(OBu )3
Two-stepe
One-pot
t
Tartaric
Malonic
Citric
3.8
5.6
3.8
ND
Al(OBu )3
Two-stepe
One-pot
t
Al(OBu )3
Two-stepe
One-pot
Al(NO3)3•9H2O
Al(NO3)3•9H2O
Al(NO3)3•9H2O
3.8 and 4.9
3.8 and 6.5
3.8 et 5.6
3.8
Two-stepe
One-pot
Tartaric
Malonic
Two-stepe
One-pot
3.8
ND
Two-stepe
aBET specific surface area (m2 g-1). bBJH pore volume (cm3 g-1) determined at P/P0 = 0.997. cBJH average pore diameter (nm). dAl:Zn atomic ratio of 8:1 determined by
EDX analysis. ND: not determined. eBET-BHJ results obtained previously.32
47
Therefore, we used the same precursors (Al(OBus)3, Al(OPri)3,
On the other hand, the investigated aluminum precursors
Al(OBut)3 and Al(NO3)3·9H2O,) and acids (citric, tartaric, proved also similar chelation abilities of their metallic centers
fumaric, oxalic, malonic, maleic and acetic acid) that we have (Al) with both the templating agent (F127) fragments and the
previously used for the synthesis of catalytic ZnCl2-OMA added carboxylic acids used as organic-inorganic interfacial
materials through conventional two-steps process.32
protectors assisting the ordered self-assembly process. This
As depicted in Fig. 1 and Table 1, we successfully prepared kind of organized assembly is tailored by means of several
various ZnCl2-OMA samples using different carboxylic acids, interfacial interactions including covalent and hydrogen
while no significant difference was observed on their textural bonding as well as van der Waals interactions, which plays a
properties and their porosity parameters. Furthermore, the pivotal role in the mesophase construction.48, 49
targeted organized ZnCl2-OMA materials were successfully In comparison with the materials synthesized via the
obtained upon the use of different aluminum precursors as conventional two-steps process,32 as depicted in Table 1,
illustrated in Table 1 and Fig. 2 with no significant shift. This almost all the newly prepared ZnCl2-OMA samples (one-pot)
similarity is ascribed to the identical behavior of the possess larger BET surface area. As we reported previously, we
investigated precursors and acids, suggesting that the different believe that the lower BET surface area obtained through the
carboxylic acids tested exhibit similar coordination abilities two-steps synthesis is primarily ascribed to the additional
with the metal centers (Al) via monodentate or bridging calcination step during the ZnCl2 impregnation, as well as the
bidentate modes during the mesophase formation pathway.32, filling of added ZnCl2 inside the already formed mesopores.32
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These troublesome effects were advantageously avoided when respectively.50,
adopting the one-pot synthesis process were in situ mesoporous network of the analyzed ZnCl2-OMA samples with
impregnation of ZnCl2 is occurred with no need for subsequent hexagonal arrays without any lattice shrinkage.
annealing step that may alter the materials surface.
ARTICLE
51
These results suggest the well-ordered
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Fig. 2 (A) Nitrogen adsorption-desorption isotherms and (B) BJH pore size
distributions curves of the prepared ZnCl2-OMA sample. All samples were
calcined at 400 °C.
Meanwhile, both synthesis procedures led to ZnCl2-OMA
materials with relatively similar total pore volume and pore
size distributions (Table 1), suggesting that independently from
Fig. 3 Small-angle X-ray scattering (SAXS) patterns of zinc chloride-modified
ordered mesoporous alumina samples. All samples were calcined at 400 °C.
the synthesis process, similar porosity features can be
As illustrated from the BET results, similarly, the XRD analysis
revealed no significant shift in the XRD patterns when using
distinct carboxylic acids (Fig. 3i) or different aluminum
precursors (Fig. 3ii). It is worth to note that the obtained XRD
analysis results were similar to those obtained for the ZnCl2-
OMA samples prepared using the conventional two-steps
process where the 2-D hexagonal p6mm structure was also
detected for the prepared samples.32 Furthermore, no
crystalline alumina phase was formed at 400 °C, as shown form
the wide angle XRD patterns recorded for the calcined ZnCl2-
OMA samples (Fig. S1 in the ESI). These results indicate the
formation of an amorphous network. More interestingly, the
data obtained is a clear evidence for the absence of any
obtained using the same aluminum precursors and carboxylic
acids having identical ligands and carbon chain length,
respectively.
X-ray diffraction (XRD) analysis
The prepared ZnCl2-OMA materials calcined at 400 °C were
analyzed by powder X-ray diffraction to verify their mesoscopic
order. Fig. 3 shows the small-angle X-ray scattering (SAXS)
patterns of the calcined ZnCl2-OMA samples. The patterns
exhibit a major sharp and well-resolved peak at around a 2θ
value of 1.0 ° indexed as the (100) Bragg reflection of the 2-D
hexagonal p6mm structure (space group), with two minor
peaks observed at around 2θ = 1.8 and 2.1 ° those
corresponding to the higher order (110) and (200) reflections,
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additional Zn-containing crystalline phases (e.g. ZnAl2O4, ZnO), well-ordered hexagonal arrangement of tubulaVriewmAertsicolepOonrliense
which are usually obtained in this range of calcination along the [001] direction, and the alignDmOeIn: 1t0.o10f3u9n/Cif5oRrAm120a5n7Hd
temperatures.52, 53
Morphology and structure of ZnCl2-OMA: TEM and SEM analysis
non-interconnected cylindrical mesopores along the [110]
direction are observed in all analyzed ZnCl2-OMA samples.
In addition, the diameter of the cylindrical mesopores as
observed from the TEM micrographs was around 5 nm in
consistency with the obtained N2 adsorption-desorption data.
The observed organized structure of the materials prepared
through one-pot process was similar to that observed for the
ZnCl2-OMA samples prepared using the conventional two-
steps process.32 The TEM data confirm again that the presence
of ZnCl2 during the mesophase formation does not alter the
organized assembly of alumina during the templating and the
calcination process, and preserves the organized alumina
framework. Furthermore, the micrographs displayed in Fig. 4
indicate the absence of any aggregation of Zn species/phases.
After organic molecules (surfactant and carboxylic acid)
removal at 400 °C, the samples were also analyzed by
transmission electron microscopy (TEM). The observed images
are represented in Fig. 4 along the 001 and 110 directions for
the obtained ZnCl2-OMA materials. In a good agreement with
the results illustrated from the SAXS and N2 adsorption-
desorption analyses (Fig. 1-3), the TEM micrographs (Fig. 4)
confirm the presence of hexagonal ordering in the prepared
mesoporous ZnCl2-OMA materials. Furthermore, as evidenced
by SAXS and N2 adsorption-desorption measurements, highly
ordered 2-D hexagonal mesostructure was successfully
obtained upon the use of different aluminum precursors and
diverse carboxylic acids as depicted in Fig. 4. The extremely
Fig. 4 TEM images of the zinc chloride-modified ordered mesoporous alumina materials synthesized using Al(OBus)3 with different carboxylic acids: (a) citric,
(c) tartaric, (d) malonic, (e, f) maleic, (g) fumaric and (i) oxalic acid, viewed along the [110] orientation, and (b) citric, (h) fumaric and (j) acetic acid, viewed
along the [001] orientation. And using different aluminum precursors; Al(OPri)3 for (k) fumaric and (n) tartaric acid; Al(NO3)3·9H2O for (l) citric and
Al(OBut)3 for (m) citric and (o) tartaric acid, viewed along the [110] orientation. All samples were calcined at 400 °C.
To probe into the morphological features of the synthesized particle size distribution varying from 20 to 50 μm with gaps
ZnCl2-OMA materials, the prepared samples were examined by between crystals (Fig. 5a-d). However, a relatively smooth
SEM. Fig. 5a-d shows plane-view SEM images of the ZnCl2- particles surface is obtained, suggesting the excellent
OMA samples. The mesoporous ZnCl2-OMA microparticles incorporation of ZnCl2. In our previous work, the ZnCl2-OMA
exhibit a non-uniform three-dimensional (3D) architecture materials prepared using the conventional two-steps process
ranging from cuboid-like to plate-like shaped morphologies have smaller microparticles with less regular shape and
resulting from irregular micelles aggregation during the EISA morphology, which was ascribed to the mechano-chemical
process. Thus, the observed microparticles have a non-uniform dislocation effect occurred during the ZnCl2 impregnation
6 | J. Name., 2012, 00, 1-3
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step.32 Unlike the two-steps conventional approach, the newly Additionally, the Au, Pd and C signals were detected (Fig. 5e-
View Article Online
DOI: 10.1039/C5RA12057H
designed one-pot process led to efficient and homogeneous h), which is due to residuals of the Au/Pd and C films used for
incorporation of ZnCl2 particle in the OMA matrix. However, the specimens preparation for analysis. For all analyzed
we believe that the presence of ZnCl2 in the surfactant micelle samples, the Zn/Cl ratios calculated from the EDX data higher
before thermal treatment is among the reasons that led to than 1/2 (Table S1 in the ESI) that corresponds to the
relatively larger particles (compared to two-steps process) composition on ZnCl2, indicating the presence of further Zn-Cl
where ZnCl2 is entirely embedded in OMA phase.
species.
X-ray photoelectron spectroscopy (XPS) measurements
To precisely determine the elemental composition of the
ZnCl2-OMA as well as the electronic state and environment of
the detected elements (Al, O, Zn and Cl) and the bonding
configurations of the related species, X-ray photoelectron
spectroscopy (XPS) measurements were conducted. The XPS
spectra obtained are displayed in Fig. 6 and were carefully
studied. The survey spectrum (Fig. 6a) exhibits four main peaks
centered at around 75 eV, 200 eV, 530 eV and 1020 eV, which
are assigned to Al, Cl, O and Zn species, respectively. These
elements are attributed to ZnCl2 and Al2O3 phase. Similar
compositions were found in the analyzed samples prepared
using different carboxylic acids and aluminum precursors
without any significant effect of these experimental conditions
(Fig. S2 in the ESI). The measured Al/Zn atomic ratios values
from the XPS data are found to be around 8 (Table S2 in the
ESI). These values are close to the desired and optimized
nominal Al/Zn ratio of 8 for the synthesized ZnCl2-OMA
samples. Moreover, in all analyzed samples, the calculated Cl :
Zn ratios from the survey spectra obtained were always found
to be lower than the nominal 2 : 1 atomic ratio for the
incorporated ZnCl2, where higher Zn amounts were detected
compared to Cl (Cl/Zn ~ 0.61-0.68), similarly to the data
obtained by EDX measurements (Table S1 in the ESI). These
results suggest that in addition to ZnCl2 and Zn-Cl species, the
samples contain further Zn-containing species like bridging Al-
O-Zn-O-Al or Zn(OH)2 species. However, thermal
decomposition of zinc chloride may partially occur at higher
annealing temperatures.32 The decovolution of each element
spectrum is then necessary to investigate the exact species
formed on the surface and in the framework of the prepared
ZnCl2-OMA materials.
Fig. 5 Representative SEM images (a-d) and energy dispersive X-ray (EDX)
spectra (e-h) obtained for ZnCl2-modified OMA microparticles prepared
using different aluminum precursors and carboxylic acids: (a, e) Al(OBus)3-
oxalic acid; (b, f) Al(OPri)3-tartaric acid; (c, g) Al(OBut)3-fumaric acid and (d,
h) Al(NO3)3·9H2O-citric acid. All samples were calcined at 400 °C.
The Al 2p spectrum displayed in Fig. 6b revealed the presence
Furthermore, as shown from previous analysis data (XRD, BET
and TEM), the use of various carboxylic acids and aluminum
precursors did not result in any significant morphology
changes (Fig. 5a-d).
of one component, the peak at 74.4 eV corresponds to Al
55
present in the Al2O3 phase.54,
Subsequently, the O 1s
spectrum (Fig. 6c) can be deconvoluted into two components.
Two peaks with similar intensities are observed at 531.2 eV
and 532.5 eV, which are attributed to bulk oxygen present in
the Al2O3 lattice (Al-O-Al) and higher concentration hydroxyl
groups (Al-O-H) present in the Al2O3 materials surface,
respectively.55-58 Unlike the conventionally prepared ZnCl2-
OMA materials,32 the absence of a third peak at higher BE
indicates that no molecular H2O is physisorbed at this
temperature.58 These binding energy values are slightly higher
compared to those we reported for the pure alumina
materials.32 This could be due to the presence of ZnCl2 and its
interactions with the species present in the alumina phase,
which leads to a slight change in the electronic environment of
these elements. The hydroxyl groups peak at 532.5 eV may
Elemental analysis: energy-dispersive X-ray spectroscopy (EDX)
Details on the chemical composition of the ZnCl2-OMA
materials are provided. Thus, the successful one-pot
incorporation of ZnCl2 with mesoporous organized alumina is
further evidenced by EDX analysis. As depicted in Fig. 5e-h
displaying the EDX spectra obtained for the prepared ZnCl2-
OMA samples, O and Al sharp signals were detected along with
Zn and Cl signals which belong to Al2O3 and ZnCl2 species,
respectively. These elements were detected for all the
analyzed samples synthesized using various carboxylic acids
and aluminum precursors. These results prove again the
efficiency of the in situ incorporation of ZnCl2 into OMA.
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also correspond to the Zn(OH)2 species that results from ZnCl2 species like bridging Al-O-Zn-O-Al or Zn(OH)2 specVieiesw,AtrhticeleZOnnClinle2
interaction with alumina surface.59 The O 1s XPS data obtained / Zn-Cl species are likely bonded to the alumina surface
DOI: 10.1039/C5RA12057H
previously for the two steps-prepared ZnCl2-OMA materials through interactions with surface hydroxyl groups. With the
showed a lower intensity (amount) of Al-O-H peak compared presence of Zn precursors in the reaction mixture before the
to that of Al-O-Al.32 On the other hand, herein, the two peak formation of the alumina phase, we logically suspected the
intensities are quite equal. This could be due to elimination on formation of other phases besides alumina. Zinc aluminate
the second calcination step which helps avoiding further (ZnAl2O4) and zinc oxide (ZnO) are the most likely phases to be
dehydroxylation of alumina surface.
obtained under such similar conditions. Fortunately, the XPS
The zinc species bonding configurations were also measurements are clear evidence that no ZnAl2O4 phase was
investigated. The Zn 2p3/2 spectrum (Fig. 6d) shows one formed along with the ZnCl2-modified OMA phase. If ZnAl2O4
component with a sharp peak at 1022.1 eV, which could be phase was present, the XPS analysis would show a single O 1s
assigned to distinct Zn2+ species including Zn-O, ZnCl2 or component associated with a peak at 531.4 eV, while Zn 2 p3/2
Zn(OH)2.32, 60, 61 We then studied the Cl- spectrum (Fig. 6e).
and Zn 2 p1/2 peaks would appear at 1044.9 eV and 1021.5-
64
1021.9 eV, respectively.63,
Similarly, no evidence of
formation of zinc oxide phase (ZnO) was found, which is
characterized by Zn 2 p3/2 and Zn 2 p1/2 peaks at 1021.5 eV and
1044.6 eV.64 It is worth to mention that even if zinc chloride
thermal decomposition is possible, the resulted Zn is likely
converted into the species cited above, while no trace of
elemental Zn were found in the analyzed samples which can
result in two Zn 2 p3/2 and Zn 2 p1/2 peaks at lower BE values;
1021.4 eV and 1044.5 eV, respectively, as reported
previously.64
Solid-state NMR spectroscopy analysis
After performing several characterizations that evidenced the
formation of hexagonally organized mesoporous alumina
materials doped with zinc chloride species, further
characterization must be conducted to investigate the nature
of interactions and intimate contact occurring on the alumina-
1
zinc chloride interface. Therefore, we performed H MAS NMR
experiments to evaluate the zinc chloride bonding with
alumina surface. Fig. 7 shows the 1H MAS NMR spectra of
several ZnCl2-OMA samples in which three broad resonance
peaks were displayed. The first peak at 1.8 ppm is attributed to
the protons of the basic terminal hydroxyl groups present on
the ZnCl2-OMA surface.65 The second peak at 3.9 ppm is
assigned to the protons of the acidic bridging hydroxyl groups
also present on the ZnCl2-OMA surface.65 Both basic and acidic
66
groups present moderate strength.65, However, unlike the
XPS measurement which showed no evidence of the presence
of physisorbed water on the ZnCl2-OMA surface (O 1s
spectrum, Fig. 6c), the 1H MAS NMR analysis revealed the
presence of moisture trace on the analyzed samples.
Therefore, the third peak at 4.2 ppm corresponds to the
hydrogen-bonded water which is physisorbed on the ZnCl2-
OMA surface.65, 67 The use of various aluminum precursors and
diverse carboxylic acids for the synthesis of the ZnCl2-OMA
Fig. 6 XPS analysis of ZnCl2-OMA sample synthesized using malonic acid with materials led to no significant effect on the surface
Al(OBus)3 aluminum precursor: (a) survey spectrum and (b) Al 2p, (c) O 1s,
(d) Zn 2p3/2 and (e) Cl 2p3/2-Cl 2p1/2 deconvoluted spectra. All shifts for the
samples were corrected by normalization of the C 1s binding energy to
285.0 eV.
composition as confirmed by 1H MAS NMR analysis (Fig. 7, see
also Fig. S3-S5 in the ESI). Furthermore, unlike the two step
impregnation of ZnCl2 that we have recently reported
demonstrating a selective interaction of ZnCl2 with basic
terminal hydroxyl groups leading to the formation of various
Al-O-Zn-Cl species,32 the in situ incorporation of ZnCl2 did not
lead to total neutralization of the basic terminal hydroxyl
groups (at 1.8 ppm). These results do not exclude the usual
This spectrum can be deconvoluted into two components; a
well-defined peak at 198.9 eV and a smaller peak at 200.5 eV
correspond to Zn-Cl 2p3/2 and Zn-Cl 2p1/2 species,
respectively.32, 60, 62 In addition to the possible Zn-containing
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preferential interaction of the ZnCl2 with basic terminal intermediate phase,69,
ARTICLE
70
which will be then converted to
View Article Online
DOI: 10.1039/C5RA12057H
hydroxyl groups, which may lead to a loss in Brønsted alumina phase after annealing. Afterwards, the calcined ZnCl2-
67, 68
acidity.29-32,
However, this may suggest that the OMA samples were analyzed. The 27Al MAS NMR spectra
change of Al species
elimination of a second calcination step at 400 °C may enhance displayed in Fig. 8b-e show
a
the concentration of the surface hydroxyl groups, as a result of coordination. We then observed the rise of one sharp and two
avoiding further dehydroxylation of alumina surface under broad resonance peaks (Fig. 8b-e). The major well-defined
such high thermal treatment conditions. On the other hand, resonance peak centered at 3 ppm is attributed to the
71
the elimination of this second calcination step may be the octahedral Al3+ species in the alumina phase (AlO6).70,
reason behind the detection of the physisorbed water trace in Subsequently, the two broad minor resonance peaks centred
almost all analyzed samples (Fig. 7, see also Fig. S3-S5 in the at 32 and 63 ppm (Fig. 8b-e) correspond to the penta- and
ESI). Ultimately, combination of XPS and 1H MAS NMR data tetrahedral Al3+ species in the alumina phase (AlO5, AlO4),
suggests that various Al-O-Zn-Cl/Al-O-Zn-O-Al surface species respectively.70 As reported recently in our work, this
can be formed through interaction of ZnCl2/Zn-Cl/Zn conversion of the octahedral Al3+ species upon calcination to
components with either basic terminal hydroxyl groups, acidic pentahedral and tetrahedral Al3+ species is attributed to the
bridging hydroxyl groups or both.
partial substitution of oxygen ions by hydroxyl groups in
octahedral Al centers during the calcination process.14, 32, 72 All
27Al MAS NMR spectra for the samples prepared using
different carboxylic acids and diverse aluminum precursors
show approximately the same profile regardless of the
experimental conditions (Fig. 8, see also Fig. S6-S8 in the ESI).
It is worth to mention that unlike alumina phase, the 27Al MAS
NMR spectra of the ZnAl2O4 spinel obtained under similar
thermal conditions (300-500 °C) exhibits a single peak around
0 ppm which is characteristic of Al3+ ions occupying AlO6 sites,
as reported previously.52
1
Fig. 7 H MAS NMR spectra of ZnCl2-OMA samples prepared using citric acid
with different aluminum precursors. Indicating (a) basic terminal hydroxyl
groups (b) acidic bridging hydroxyl groups, and (c) hydrogen bonded water
physisorbed on alumina surface. All samples were calcined at 400 °C.
27Al MAS NMR experiments were conducted to investigate the
Al environment and the changes related to the in situ ZnCl2
incorporation. Fig.
8 shows the various coordination
geometries detected for Al species in the analyzed as-prepared
(uncalcined) and calcined ZnCl2-OMA samples. A single sharp
resonance peak was observed at around 0 ppm for the as-
prepared sample (Fig. 8a). This suggests that the uncalcined
sample mainly contains octahedral Al3+ species. This peak can
be associated with the presence of the AlOOH boehmite
Fig. 8 27Al MAS NMR spectra of ZnCl2-OMA samples.
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In comparison, the ZnCl2-OMA samples prepared using the two Catalytic performance of the ZnCl2-OMA-based catalysts
View Article Online
The newly synthesized 3 wt.% MTO/ZnClD2-OOI:M10A.10c3a9t/aCly5RstAs12w05e7rHe
step process resulted in AlO5 and AlO6 resonance peaks with
relatively similar intensities.32 Herein, we observe that the AlO5
resonance peaks have lower intensities compared to those of
the AlO6 resonance peaks (Fig. 8, see also Fig. S6-S8 in the ESI),
indicating the lower number of AlO5 sites. This suggests that
the in the one-pot synthesized ZnCl2-OMA materials the zinc
chloride was preferably doped on these sites. This suggestion
is in a good agreement with the earlier speculation about the
distribution of the Al3+ species in the alumina matrix; where it
has been proposed that AlO6 sites are found in the bulk of the
alumina inorganic framework walls, while the AlO4 and AlO5
sites are likely on the surface.71 A similar phenomenon was
recently observed for the Cu-Al2O3 system.73
evaluated for their catalytic metathesis activity. The self-
metathesis reaction was carried out as a model reaction, one
of the representative Re-catalyzed reactions (Scheme 2). Also,
we have chosen methyl oleate as the model substrates to
verify both; the catalyst tolerance for functionalized molecules
as well as the mesoporous network for bulky molecules
diffusion abilities (Scheme 2). The experiments were carried
out under similar conditions of temperature (45 °C) and
pressure (atmospheric pressure) during 90 min of reaction
time, to those we previously have optimized for the same class
of catalyst and reaction.30-32
In order to highlight the advantages of the newly prepared
ZnCl2-OMA materials as catalytic supports for MTO, here we
present an extended comparison with other alumina-based
One-pot formation of the ZnCl2-doped OMA
In our previous work we showed that the organized self- catalytic support those we have previously synthesized and
assembly of the alumina mesophase in governed by the used for the design of the MTO metathesis catalyst (Table 2).
complexation effect of the interfacial protectors (carboxylic Several one-pot prepared supports were used and numerous
acids).32 Briefly, the uniform aggregation of a liquid-crystal experiments were performed for each supports in ordered to
phase of surfactant micelles during the EISA process occurs investigate their performance and verify the reproducibility of
upon complexation of bifunctional carboxylic acids the process. The data reported in Table 2 (Entry 1, 3, 5 and 7)
(carboxylate groups) with Al cations on one side, as well as are average values for several metathesis experiments
with the nonionic surfactant (F127) via van der Waals forces conducted for similar catalysts supported over ZnCl2-OMA
and hydrogen bonding on the other side (different carboxylate supports prepared with the same aluminum precursor and
group). Subsequently, and based on the characterizations carboxylic acid.
data, particularly XPS and MAS NMR, herein we propose a
O
similar mechanism for the formation of well-dispersed zinc
species within the OMA channels via a one-pot process. The
O
HCl is required to adjust the solution pH; however the strong
methyl oleate
interaction of Cl- with Al sites can lead the organized assembly
to collapse during the sol-gel process. Thus, carboxylic acids
3 wt.% MTO/
play a major role to prevent this phenomenon by interaction
of their carboxylate fragments with the Al centers during the
self-assembly process though monodentate or bridging
ZnCl2-OMA
O
O
(3) methyl elaidate
bidentate modes. Meanwhile, the added zinc precursor
(1) trans/cis-9-octadecene
migrates insides the inner hydrophobic core of the F127
+
O
surfactant micelles during the evaporation-induced self-
O
O
assembly process.74, 75 Therefore, the removal of the organic
O
(2) trans/cis-dimethyl-9-octadecene-1,18-dioate
molecules via calcination at high temperatures lead to
simultaneous mesoporous channels formation and grafting of Scheme 2 Methyl oleate self-metathesis observed products
zinc species on formed alumina surface hydroxyl groups as
1
demonstrated by H MAS NMR analysis, while maintaining the
well-ordered internal mesopore architecture. This tentative
formation process is illustrated in Scheme 1.
The other alumina supports used for comparison include the
zinc chloride-promoted (Table 2, Entry 8) and unpromoted
(Table 2, Entry 9) conventional wormhole-like alumina that we
have reported earlier;29-31 as well as the well-ordered
hexagonal ZnCl2-OMA prepared via two steps process that we
have also recently reported (Table 2, Entry 2, 4 and 6).32 In
addition, the newly one-pot prepared ZnCl2-OMA-based
heterogeneous metathesis catalyst performance (Table 2,
Entry 1, 3, 5 and 7) has been compared to that of the
homogeneous commercially available and widely used 2nd
generation Grubbs catalyst (Table 2, Entry 10).
Briefly, the addition of zinc chloride to alumina supports was
found to offer a great catalytic promoting effect for the
metathesis reaction.29-31 The unmodified wormhole-like
Scheme 1 Schematic representation of the one-pot evaporation induced
self-assembly process for the formation of ZnCl2 embedded in well-
organized mesoporous alumina (OMA) materials.
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alumina supports showed the weakest metathesis ordered cylindrical mesopores of these supports. VCieowmArptiaclreeOdnlitnoe
performance among all tested catalysts, while their promoted the disordered and interconnected poresDoOfI:t1h0e.10w3o9/rCm5hRAo1le20-l5i7kHe
counterparts showed much more enhanced catalytic mesoporous alumina supports where molecular diffusion
performance (Table 2, Entry 8).29-31 Afterwards, we recently limitations and blocking effects may occurs. All these features
showed that the zinc chloride-promoted hexagonally were also provided using similar one-pot synthesized ZnCl2-
organized mesoporous alumina (ZnCl2-OMA) materials are the OMA supports, which offered also better activity than the
most suitable support of MTO catalysts showing relatively MTO-based catalysts supported over zinc chloride-modified
better methyl oleate conversions.32 In all cases methyl oleate and unmodified wormhole-like alumina (Table 2).
self-metathesis leads to the formation of both desired The catalytic performance of the successfully prepared ZnCl2-
metathesis products including 9-octadecene and dimethyl-9- OMA supports via a one-pot synthesis process studied in this
octadecene-1,18-dioate (Scheme 2) as well as undesired work was also compared to the performance of the ZnCl2-
product (methyl elaidate, Scheme 2). However, using the OMA-based catalysts previously prepared through two steps
ordered alumina supports, improved selectivity was reached process. Interestingly the one-pot prepared ZnCl2-OMA
towards the desired metathesis products (Table 2, Entry 2, 4 materials were found to be the best catalytic support for MTO.
and 6). More interestingly, we demonstrated that the new Metathesis reaction data displayed in Table 2 (Entry 1, 3, 5 and
catalyst design based on organized mesoporous network has 7) shows higher catalytic performance including; methyl oleate
an enormous effect on improving the reaction rate for the conversion (up to 93.6%), desired metathesis products yield
conversion of such bulky molecule (methyl oleate).32 This (up to 73.7 %) and selectivity (up to 3.7), using these catalytic
enhancement is attributed to the absence of diffusion supports. For the structure-activity correlation, we believe that
limitations of substrates and metathesis products within the this enhanced performance may arise from various factors.
Table 2 Catalytic performance for methyl oleate self-metathesis over heterogeneous 3 wt.% MTO-based catalysts supported on organized
mesoporous alumina (ZnCl2-OMA) prepared via two-steps or one-pot process. And comparison with metathesis performance of 3 wt.%
MTO/ZnCl2-meso-Al2O3 (wormhole-like), MTO/meso-Al2O3 (wormhole-like) and commercial homogeneous 2nd generation Grubbs catalyst.
Metathesis products (%)c
DMP
(%)d
Se
References
Catalysta
Conv.
(%)b
Entry
(1)
trans
(1)
(2)
(2)
(3)
cis
trans
cis
1
2
3
4
5
6
7
8
9
3 wt.% MTO/ZnCl2-OMA-1 (Al(OBus)3 + citric, op)
3 wt.% MTO/ZnCl2-OMA-1 (Al(OBus)3 + citric, ts)
3 wt.% MTO/ZnCl2-OMA-2 (Al(OBus)3 +tartaric, op)
3 wt.% MTO/ZnCl2-OMA-2 (Al(OBus)3 +tartaric, ts)
3 wt.% MTO/ZnCl2-OMA-3 (Al(OPri)3 + tartaric, op)
3 wt.% MTO/ZnCl2-OMA-3 (Al(OPri)3 + tartaric, ts)
3 wt.% MTO/ZnCl2-OMA-4 (Al(OBut)3 + tartaric, op)
3 wt.% MTO/ZnCl2-meso-Al2O3 (ts)
80,2
78.2
93,6
88.2
86,7
87.4
79,8
86.7
2.4
13,7
15.5
48,8
29.3
34,8
31.8
14,1
24.4
0.2
12,7
14.3
17,1
8.5
9,4
8.9
13,2
8.3
0.3
3,9
14,9
9.1
5,4
12.9
9,1
14.1
13,6
16.2
0.2
6,9
4.3
2,4
4.6
2,5
2.4
6,4
3.6
0.3
4,2
32,0
35.0
19,9
32.9
30,9
30.2
32,5
34.3
1.4
48,2
43.2
73,7
55.3
55,8
57.2
47,3
52.4
1.0
1.51
1.23
3.70
1.68
1.81
1.89
1.46
1.53
0.71
1.22
This work
Abidli et al.28
This work
Abidli et al.28
This work
Abidli et al.28
This work
Pillai et al.25-27
Pillai et al.25-27
Pillai et al.25-27
3 wt.% MTO/meso-Al2O3
2nd generation Grubbs catalyst
10
89.6
20,1
21,0
40,4
49,2
aThe investigated 3 wt.% MTO/ZnCl2-OMA catalysts supports were prepared using different aluminum precursors and carboxylic acids, while meso-Al2O3
represents the wormhole-like mesoporous alumina. bConv.: conversion, expressed as Conv. (%) = ([methyl oleate]0 - [methyl oleate]t)/[methyl oleate]0 × 100.
cThe numbers appearing for the metathesis products 1, 2, and 3 (Scheme 1) are expressed in terms of individual product yield (yieldi), defined as: yieldi (%) =
[producti]/([methyl oleate]0 - [methyl oleate]t) × 100, where [producti] is the molar concentration of the product i = 1 to 3, and [methyl oleate]0 and [methyl
d
oleate]t are the molar concentrations of methyl oleate initially and at a given time t, respectively. DMP: desired metathesis products (1 + 2) yield, which is
the summation of the individuals yields of 1 and 2. eThe selectivity (S) is defined as the yield of all desired products over the yields of all undesired products,
which represents; S = Yield (1 and 2)/Yield of (3). op: ZnCl2-OMA supports prepared through one-pot process. ts: ZnCl2-OMA supports prepared through two-
steps process.
Firstly, the newly one pot-prepared ZnCl2-OMA supports area favors better dispersion of supported MTO species, which
exhibits larger BET surface area (Table 1) compared to the two improves the concentration and availability of metathesis
steps-prepared ZnCl2-OMA supports. The lower BET surface active species and intermediates allowing more efficient use of
area of these later is due to the subsequent calcination step exposed supported MTO molecules. Secondly, unlike the two
and pore filling with zinc chloride upon post-modification, steps synthesis process, avoiding a second calcination step
which alters their textural properties. The BET specific surface during the one-pot preparation process will reduce the
area was dramatically dropped upon incorporation of zinc dehydroxylation of the alumina surface as illustrated above in
chloride.32 On the other hand, during the one pot synthesis of XPS and MAS NMR sections. Along with this, we showed that
ZnCl2-OMA supports this additional annealing step was zinc chloride is incorporated on the alumina via interactions
avoided with in situ impregnation of zinc chloride. This is in a with surface hydroxyl groups. As
a result, inhibited
good agreement with several studies showing that supported dehydroxylation may allow better incorporation of Zn active
catalysts' activity is often proportional to the specific surface intermediates which are crucial for the MTO metathesis
area of these catalysts.76-83 Furthermore, higher BET surface catalytic action, displaying a key synergetic effect. Finally, we
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believe that the single-step approach allow simultaneous zinc exhibited higher BET surface area compared to the
View Article Online
DOI: 10.1039/C5RA12057H
species deposition and mesopores walls formation, releasing conventionally two steps-synthesized materials. Outstanding
more homogeneous and well-dispersed zinc species on the arrangement of the mesopores was observed by TEM
OMA surface. Moreover, this simultaneous process may result micrographs. Not surprisingly, these materials with such
in a confinement effect of ordered mesochannels of OMA enhanced features exhibited better catalytic performance for
inhibiting Zn species aggregation and growth. This methyl oleate self-metathesis when used as support for the
agglomeration phenomenon may likely occur during the two rhenium-based catalyst. Avoiding the second calcination step,
steps synthesis which restricts Zn species interactions with which is required during the conventional synthesis, led to a
alumina surface and limits their availability for the catalytic limited dehydroxylation phenomena. Thus, materials with
process. It is also worth to mention that the separate wet enhanced hydroxyl group concentration and availability on the
impregnation method during the two-steps synthesis was alumina surface were obtained, which was a key factor for an
found to alter the textural and morphological properties of the effective grafting of zinc chloride and methyltrioxorhenium
OMA (mechano-chemical effect), along with formation of zinc species. Therefore,
a better catalytic performance was
chloride aggregates, as revealed by SEM images.32
reached. This efficient synthesis may open-up further catalytic
These results are very exciting showing higher performance, applications of these well ordered supports. On the other
even better than those obtained with the traditional highly hand, rather than modified-alumina, the successful one-pot
active 2nd generation Grubbs catalyst (Table 2, Entry 10), with strategy could be extended and then stimulate further
slightly better methyl oleate conversion (93.6% vs. 89.6%), but attempts to prepare other functionallized materials that are
exhibiting much more selectivity towards desired metathesis conventionally prepared through laborious multistep
products (73.7% vs. 49.2%). It is worth to mention that we processes. Especially, doped or modified functionalized
previously showed that the OMA-supported MTO catalyst was mesoporous materials that are widely used in catalysis. This
found to exhibit fast kinetics,32 comparable to that observed successful methodology is a step forward towards an easy and
over the Grubbs catalyst,31 with the possibility of recycling and efficient scalability.
reuse for further metathesis runs.
In addition to the enhanced catalytic activity of the MTO-based
metathesis catalysts using the newly prepared ZnCl2-OMA
Acknowledgements
materials, this one-pot synthesis process presents further
advantages. Economically, compared to the two-steps process,
this process allowed a reduced use of solvents. Also, it reduces
energy consumption through elimination of additional
evaporation and calcination steps, while reducing enormously
the preparation time. More interestingly, using the one-pot
process we reached higher synthesis yields, obtaining higher
amounts of ZnCl2-OMA materials, almost 75% higher in weight
than those obtained using the two steps process. This was
mainly attributed to limitation of weight loss during the two
subsequent calcination processes.
Financial support from the Natural Sciences and Engineering
Research Council of Canada (NSERC) and Canadian Foundation
for Innovation (CFI) is greatly appreciated. A. Abidli would like
to thank Prof. Khaled Belkacemi for the N2 adsorption-
desorption analysis and for providing his laboratory facility at
Laval University where the materials/catalysts synthesis and
the metathesis experiments were conducted. A. Abidli
acknowledges Alain Adnot, Pierre Audet, Prof. Safia Hamoudi,
Ronan Corcuff, André Ferland and Richard Janvier for the XPS,
MAS NMR, XRD, GC-MS, SEM and TEM analyses, respectively.
A great appreciation is dedicated to Dr K. T. Venkateswara Rao
for his valuable suggestions and advices.
Conclusions
Notes and references
In summary, we have developed a novel straightforward one-
pot sol-gel synthesis approach for zinc-doped well-organized
mesoporous alumina preparation. In situ zinc chloride
incorporation was successfully achieved along with the
formation of ordered alumina mesoporous network. Based on
XPS data, zinc chloride was impregnated via interaction with
surface hydroxyl groups. Furthermore, the XPS measurements
suggested that Al-O-Zn-Cl may be the major surface species
formed upon these interactions. Moreover, in order to
establish an efficient low-cost production process, this rapid
and sustainable methodology was found to be suitable even
when using inexpensive aluminum precursors such as
aluminum nitrate nonahydrate. The use of various aluminum
precursors and carboxylic acid did not result in any significant
difference between the prepared materials due to the similar
self-assembly pathway during the sol-gel process.
Furthermore, the materials prepared via the one-pot route
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Graphical Abstract
TOC abstract: Zn-modified well-ordered mesoporous alumina successfully synthesized via one-pot process and
evaluated as support for rhenium-based catalyst for methyl oleate self-metathesis
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