Mesoporous RuO2/TiO2 composites prepared by cyclodextrin-assisted colloidal self-assembly: Towards efficient catalysts for the hydrogenation of methyl oleate

Article abstract of DOI:10.1039/c5ra27161d

Mesoporous RuO₂/TiO₂ composites were prepared using a template-directed colloidal self-assembly approach combined with a cyclodextrin (CD)-assisted aqueous impregnation method. The supramolecular assemblies formed between the randomly methylated β-cyclodextrin (RaMeβ-CD) and the block copolymer P123 acted as a template for the formation of a highly porous TiO₂ network over which uniform dispersion of ruthenium nanoparticles was achieved. By combining dynamic light scattering, X-ray diffraction, N₂-adsorption, temperature-programmed reduction, field-emission scanning electron microscopy and high-resolution transmission electron microscopy, we show that CD-based assemblies provide a versatile and easily accessible toolbox with different functionalities for generating metal-supported catalysts with controlled pore architecture and uniform metal distribution. The performance of these supported catalysts was evaluated in the liquid phase hydrogenation of methyl oleate (MO, C18:1) to methyl stearate (MS, C18:0). Control of ruthenium dispersion into the large pores of RaMeβ-CD-P123-templated TiO₂ material enhanced catalyst activity and selectivity for the hydrogenation of the internal C = C bond and permitted catalyst separation and reuse without loss of activity. Our findings highlight the pivotal role played by the CD-based assemblies on the performance of supported ruthenium catalysts.

Full text of DOI:10.1039/c5ra27161d

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PAPER
Mesoporous RuO2/TiO2 composites prepared by
cyclodextrin-assisted colloidal self-assembly:
towards efficient catalysts for the hydrogenation of
methyl oleate†
Cite this: RSC Adv., 2016, 6, 14570
a
Rudina Bleta, Sebastien Noel,a Ahmed Addad,b Anne Ponchela and Eric Monfliera
*
´
¨
Mesoporous RuO2/TiO2 composites were prepared using a template-directed colloidal self-assembly
approach combined with a cyclodextrin (CD)-assisted aqueous impregnation method. The supramolecular
assemblies formed between the randomly methylated b-cyclodextrin (RaMeb-CD) and the block
copolymer P123 acted as a template for the formation of a highly porous TiO2 network over which uniform
dispersion of ruthenium nanoparticles was achieved. By combining dynamic light scattering, X-ray
diffraction, N2-adsorption, temperature-programmed reduction, field-emission scanning electron
microscopy and high-resolution transmission electron microscopy, we show that CD-based assemblies
provide a versatile and easily accessible toolbox with different functionalities for generating metal-
supported catalysts with controlled pore architecture and uniform metal distribution. The performance of
these supported catalysts was evaluated in the liquid phase hydrogenation of methyl oleate (MO, C18:1) to
methyl stearate (MS, C18:0). Control of ruthenium dispersion into the large pores of RaMeb-CD-P123-
templated TiO2 material enhanced catalyst activity and selectivity for the hydrogenation of the internal
C]C bond and permitted catalyst separation and reuse without loss of activity. Our findings highlight the
pivotal role played by the CD-based assemblies on the performance of supported ruthenium catalysts.
Received 18th December 2015
Accepted 25th January 2016
DOI: 10.1039/c5ra27161d
www.rsc.org/advances
purpose, the design of nanostructured support materials with
high specic surface area and tailored porosity is of crucial
importance for enhancing the catalyst effectiveness.19,20
1. Introduction
Creating self-assembled nanostructured materials with well-
dened architectures and enhanced catalytic performance is of
central importance for applications in the eld of heterogeneous
catalysis.1–6 Template-directed colloidal self-assembly, based on
the spontaneous organisation of colloidal nanoparticles around
an organic template by non-covalent interactions, provides
a simple and versatile strategy for building robust materials with
hierarchical order at multiple length scales.7–12 Metal nano-
particles can be uniformly dispersed within these materials
providing nanostructured composites with tunable properties.
The importance of metal–oxide interfaces has long been recog-
nised and different possibilities have been proposed for
improving the catalyst performance including the porosity of the
support, the size and distribution of metal nanoparticles, the
metal–support interactions,13–17 as well as the surface character-
istics of nanoparticles through chemical modication.18 For this
Recently, our group has explored the possibility of using
cyclodextrin (CD)-based supramolecular assemblies as template
or structure directing agents for the elaboration of inorganic
materials with a large variety of morphologies, porosities and
crystal phase compositions.21–25 A particular feature of CDs is
their ability to form, in association with polymers, supramolec-
ular assemblies with many different kinds of architectures such
as for instance, spherical or ellipsoidal micelles, vesicles, elon-
gated polypseudorotaxanes and three-dimensional hydrogels.26–29
Interestingly, silica oligomers23 or colloidal nanoparticles
prepared by sol–gel process21,22,25 may easily inltrate within these
assemblies and replicate their original structure. Moreover, CDs
have also the unusual property to form adducts with a large
number of metal salts enabling their uniform dispersion over
solid supports.30–35 We aim to take advantage of the experience
that our group has acquired on the elaboration of nanostructured
titanium dioxide (TiO2) in order to use this material as support
for uniform dispersion of ruthenium nanoparticles. Indeed,
besides its interest as efficient photocatalyst for fundamentals
and industrial applications,36–39 titania has also proved to be an
important support in heterogeneous catalysis due to its high
surface area, chemical and thermal stability, as well as ability to
a
´
Univ. Artois, CNRS, Centrale Lille, ENSCL, Univ. Lille, UMR 8181, Unite de Catalyse
et Chimie du Solide (UCCS), F-62300 Lens, France. E-mail: rudina.bleta@univ-artois.
fr
bUniv. Lille, CNRS, INRA, ENSCL, UMR 8207
Transformations, F-59000 Lille, France
–
´ ´
UMET – Unite Materiaux et
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra27161d
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strongly interact with various metal nanoparticles.40–44 Moreover, room temperature for 24 h. A xerogel was recovered aer drying
ruthenium has been identied as a promising catalyst for appli- the sol at 60 ꢀC for 72 h aer which time it was calcined in air at
cations in the hydrogenation of biomass-derived compounds such 500 ꢀC for 2 h using a heating ramp of 5 ꢀC minꢁ1. In a second
as levulinic acid,45 furanic molecules,46,47 arabinoic acid,48 xylose,44 step, 800 mg of the calcined titania materials, prepared without
and fatty esters49–51 under mild conditions.
or with the supramolecular template (RaMeb-CD/P123 molar
Herein, the TiO2 support is synthesized using the recently ratio ¼ 4.5), were impregnated with an aqueous solution of
developed RaMeb-CD-P123 directed colloidal self-assembly ruthenium(III) nitrosyl nitrate prepared without or with b-CD
approach24,25 while the native b-CD is employed as dispersing (total volume 6 mL, 0.0098 mol Lꢁ1 b-CD, b-CD/Ru molar ratio ¼
ꢀ
agent to improve dispersion of metal nanoparticles over the solid 0.09). The suspension was maintained under stirring at 75 C
support. Since the efficiency of the nal catalyst depends on until the water was completely evaporated,44 then dried at 120
various factors including the characteristics of the support,52 the ꢀC (24 h) and calcined at 400 C (4 h) under air ow. The ob-
ꢀ
size and composition of individual metal nanoparticles53,54 as tained catalysts were identied according to the following
well as the metal–support interactions,1,13,14 control of the prep- notation: RuxTiO2-sg or RuxTiO2-ns where x indicates the
aration process is essential. For this purpose, we have under- weight percentage of ruthenium in the TiO2 support, while sg
taken a detailed investigation of the effect of both the and ns refer to conventional sol–gel and nanostructured TiO2
supramolecular template (RaMeb-CD-P123 assemblies) and the respectively. If the native b-cyclodextrin is used as dispersing
dispersing agent (native b-CD) on the characteristics of the agent, b-CD is also added in the sample notation. Thus, Ru2.5b-
heterogeneous Ru/TiO2 catalysts. Finally, the efficiency of these CDTiO2-ns indicates a catalyst prepared with 2.5% Ru sup-
latter was evaluated in the liquid phase hydrogenation of methyl ported on a nanostructured TiO2 material with assistance of b-
oleate (MO, C18:1) to methyl stearate (MS, C18:0) under mild CD, while Ru2.5TiO2-sg indicates a catalyst prepared also with
conditions (50 ꢀC, 40 bar H2).55–57
2.5% Ru, but supported on a conventional sol–gel TiO2 support
without assistance of b-CD. In some experiments, two
commercial TiO2 were also used as supports, i.e. pure anatase
and a mixture of anatase (80%) and rutile (20%) (P25).
2. Experimental
2.1 Chemicals
Randomly methylated b-cyclodextrin (denoted RaMeb-CD, average
degree of molar substitution (DS) 1.8 and average molar weight
(Mw) 1310 g molꢁ1) was provided by Wacker Chemie GmbH while
2.3 Characterisation methods
Dynamic light scattering (DLS) measurements were performed
at 25 ꢀC with a Malvern Zeta Nanosizer instrument equipped
with a 4 mW He–Ne laser operating at 633 nm and using
a backscattering detection system (scattering angle q ¼ 173ꢀ).
Powder X-ray diffraction data were collected on a Siemens
D5000 X-ray diffractometer in a Bragg–Brentano conguration
with a Cu Ka radiation source. Scans were run over the angular
domains 5ꢀ < 2q < 80ꢀ with a step size of 0.02ꢀ and a step time of
2 s. Crystalline phases were identied by comparing the
experimental diffraction patterns to Joint Committee on Powder
Diffraction Standards (JCPDS) les. The RuO2 crystallite sizes
were estimated from the line broadening of the (110) diffraction
peak using the Scherrer equation,60 i.e. D ¼ Kl/(b cos q), where K
is the shape factor (taken as 0.9 in this study considering that
the native b-cyclodextrin (denoted b-CD, average Mw 1135 g molꢁ1
)
`
was a gi from Roquette Freres. PEO-PPO-PEO triblock copolymer
[PEO poly(ethylene oxide) and PPO poly(propylene oxide)], deno-
ted Pluronic P123 (average composition PEO20PPO70PEO20, Mw
5800 g molꢁ1), titanium isopropoxyde, Ti(OiPr)4 (Mw 284.3 g
molꢁ1, d 0.96 g cmꢁ3), nitric acid (HNO3, 68%), and methyl oleate
(Mw 296.49 g molꢁ1) were purchased from Sigma Aldrich. Ruth-
enium(III) nitrosyl nitrate (Ru(NO)(NO3)3, 1.5% Ru, Mw 318.10 g
molꢁ1) was procured from STREM Chemicals. All chemicals were
used as received without further purication.
2.2 Synthesis of TiO2 materials and RuO2/TiO2 composites
Titanium dioxide nanoparticles were synthesised according to the particles are spherical), l is the X-ray radiation wavelength
a previously reported sol–gel method.58 Briey, 30 mL (0.1 mol) (1.54056 A for Cu Ka), b is the full width at half-maximum
˚
of Ti(OiPr)4 was rst dissolved in 27 mL (0.35 mol) of iso- (fwhm) and q is the Bragg angle. Nitrogen adsorption–desorp-
ꢀ
propanol. Then, 160 mL of hot distilled water was added rapidly tion isotherms were collected at ꢁ196 C using an adsorption
at 85 ꢀC under vigorous stirring (hydrolysis ratio h ¼ [H2O]/[Ti] analyser Micromeritics Tristar 3020. Prior to analysis, 200–400
ꢀ
¼ 88). Aer 15 min, 1.3 mL of nitric acid ([HNO3]/[Ti] ¼ 0.2) was mg samples were outgassed at 320 C overnight to remove the
added dropwise to peptise the viscous precipitate and the species adsorbed on the surface. From N2 adsorption
mixture was maintained under reux at 85 ꢀC for 16 h. The nal isotherms, specic surface areas were determined by the BET
product was a stable translucent suspension of TiO2 nano- method61 and pore size distributions were calculated using the
particles crystallised in anatase (70%) and brookite (30%). The NLDFT (nonlocal density functional theory) model62 assuming
concentration of titanium in the sol was 0.5 mol Lꢁ1, as deter- a cylindrical pore structure. The relative errors were estimated
mined by weight loss on ignition at 1000 ꢀC for 2 h.59 Subse- to be the following: SBET, 5%; pore volume (pv) (DFT), 5%; pore
quently, Pluronic P123 (5 wt%, P123/Ti molar ratio ¼ 0.017) and size (ps) (DFT), 20%. Temperature programmed reduction
RaMeb-CD (50 mg mLꢁ1, RaMeb-CD/Ti molar ratio ¼ 0.076) (TPR) measurements were performed using a Micromeritics
were added successively to 100 mL of the above titania sol. The AutoChem 2920 chemisorption analyser equipped with
mixture was stirred for 3 h, and then allowed to equilibrate at a thermal conductivity detector (TCD) for monitoring the H2
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consumption. Prior to analysis, 30–40 mg samples were placed
in a U-tube quartz reactor and outgassed under argon ow at
120 ꢀC for 2 h. Aer cooling to 25 ꢀC, the sample was reheated at
500 ꢀC with a heating rate of 10 ꢀC minꢁ1 and the RuO2
reduction was performed in a 5% H2/Ar (v/v) ow with a ow
rate of 10 mL minꢁ1. Field emission scanning electron
microscopy (FE-SEM) observations were performed to examine
the morphology of the support materials using a FEG Hitachi S-
4700 eld-emission microscope operating at 5 kV. Before
imaging, samples were covered with a thin layer of carbon to
reduce the accumulation of charges at high magnication. High
resolution transmission electron microscopy (HR-TEM) obser-
vations were performed on a Tecnai microscope operating at an
accelerating voltage of 200 kV. The RuO2/TiO2 powder was
deposited directly on the surface of a carbon coated copper grid.
Fig. 1 Schematic illustration of the template-directed synthesis of
mesoporous Ru/TiO2 catalysts where RaMeb-CD-P123 assemblies act
as supramolecular template, the native b-CD as dispersing agent and
TiO2 colloids as building blocks for the construction of a nano-
structured framework.
2.4 Catalytic tests
The performance of the heterogeneous catalysts was evaluated
in the liquid phase hydrogenation of methyl oleate (CH3-
(CH2)7CH]CH(CH2)7CO2CH3, C18:1) to methyl stearate
(CH3(CH2)16CO2CH3, C18:0).56,57 Before catalytic testing, the
RuO2/TiO2 composite was introduced in a U-tube quartz reactor
and reduced at 400 ꢀC for 4 h under H2 atmosphere with
a heating rate of 2 ꢀC minꢁ1 and a ow rate of 30 mL minꢁ1. The
catalytic tests were carried out using a 50 mL cylindrical stain-
less steel autoclave reactor equipped with a magnetic drive
stirring and a thermostatic bath for the temperature control. In
a typical experiment, 80.8 mg supported catalyst (0.02 mmol Ru)
was introduced to a solution containing 593 mg methyl oleate
(MO/Ru molar ratio ¼ 100) dissolved in 10 mL of heptane. The
mixture was purged three times with nitrogen at room
temperature and then, pressurized with H2 (40 bar). The reac-
tions were run at a constant temperature of 50 ꢀC for 2 h using
a stirring speed of 750 rpm. Finally, the reaction products were
analysed using a Varian 3900 gas chromatograph (GC) equipped
with a CP-Sil-5B column (15 m ꢂ 0.25 mm ꢂ 0.25 mm) and
a ame ionisation detector (FID). The injector and detector
temperatures were 250 ꢀC. Conversion and selectivity were
quantied based on the relative GC-areas referred to an external
standard (dodecane) calibrated to the corresponding pure
compounds. Each catalytic run was performed in duplicate and
the values reported are the averages between two runs, with an
uncertainty equal or less than 5%. For the recycling tests, the
catalyst was recovered aer each ꢀrun by ltration, washed with
heptane three times, dried at 70 C for 2 h and reduced under
Ti molar ratio ¼ 88) as building blocks. Aer removal of the
supramolecular template by calcination at 500 ꢀC, a nano-
structured and highly porous material was recovered. Next, the
porous support was impregnated with a solution of ruth-
enium(III) nitrosyl nitrate and native b-CD (Ru/b-CD molar ratio
11), followed by drying at 120 ꢀC and calcination at 400 ꢀC under
air-ow. In this synthetic procedure, the RaMeb-CD acts as
micelle expander21,22 providing an enlargement of the pore
diameter of the TiO2 support, while the native b-CD behaves as
dispersing agent30 ensuring a uniform repartition of ruthenium
nanoparticles over the TiO2 support. Finally, the performance of
these supported catalysts was evaluated in the liquid phase
hydrogenation of methyl oleate (MO, C18:1) to methyl stearate
ꢀ
(MS, C18:0) under mild conditions (50 C, 40 bar H2).
We rst examined the impact of RaMeb-CD-P123 assemblies
on the structural and textural characteristics of the support
material. The sol–gel TiO2 material (TiO2-sg, no template used)
(Fig. 2A) presents very low pore volume and surface area (0.03
cm3 gꢁ1 and 21 m2 gꢁ1 respectively, Table 1) due to the small
size of the crystallites in the titania sol and their high level of
aggregation which facilitates the contact between neighbouring
grains, thus resulting in a very rapid conversion to rutile (XRD
Fig. 2B) and a drastic decrease of the pore volume.25 The
isotherm and pore size distribution of nanostructured RaMeb-
CD-P123-templated TiO2 (TiO2-ns) (Fig. 2A) indicate that the
supramolecular assemblies remarkably improve the textural
characteristics of the support material. Indeed, a dramatic
increase in the pore size (from 5.3 to 16.3 nm) and pore volume
(from 0.03 to 0.36 cm3 gꢁ1) is noticed together with a strong
ꢀ
H2 ow at 400 C for another 4 h.
increase in the specic surface area (from 21 to 80 m2 gꢁ1
)
3. Results and discussion
(Table 1). Moreover, from the XRD patterns shown in Fig. 2B, it
can be seen that the RaMeb-CD-based assemblies make the
titania material more stable against the anatase-to-rutile and
brookite-to-rutile phase transformation, as evidenced by the
strong decrease in the intensity of the (110) peak at 2q ¼ 27.4ꢀ
assigned to the rutile phase (R) (JCPDS card no. 00-034-0180).25
Finally, from the representative FE-SEM images, it is clear that
3.1 Preparation and characterisation of RuO2/TiO2
composites
The synthetic procedure employed for the preparation of the
mesoporous RuO2/TiO2 composites is schematically illustrated
in Fig. 1. First, the support material consisting of mesoporous
TiO2 was prepared using the RaMeb-CD-P123 assemblies as
template and the sol–gel synthesized TiO2 nanoparticles (H2O/
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Fig. 2 N2-adsorption isotherms and corresponding pore size distri-
butions (inset) (A) and XRD patterns (B) of the sol–gel titania materials
prepared without template (a) and with RaMeb-CD-P123 assemblies
(b). FE-SEM images of TiO2-sg (C) and TiO2-ns (D) showing the effect
of the supramolecular template on the porosity of the resulting
material. Samples were calcined at 500 ꢀC.
Fig. 3 Hydrodynamic radius distributions (RH) of the scattered inten-
sity (A) for the native b-CD solution (0.016 M) prepared without or with
Ru(III) nitrosyl nitrate (b-CD/Ru molar ratio ¼ 1). XRD patterns of the
corresponding xerogels dried at 60 ꢀC (B). RH distributions of the native
b-CD solution (0.016 M), TiO2 sol (0.5 M), and b-CD/TiO2 mixtures (b-
CD/Ti molar ratio ¼ 0.032) (C). XRD patterns of the corresponding
xerogels dried at 60 ꢀC (D).
the RaMeb-CD-based assemblies also play a key role in
restructuring the titania framework. Thus, while the TiO2-sg
material is made up predominantly of shapeless and aggregated
particles (Fig. 2C), the RaMeb-CD-P123-templated material
(TiO2-ns) is mainly comprised of ne and rounded grains
randomly packed into a highly porous framework (Fig. 2D).
Then, the interactions between Ru(III) nitrosyl nitrate and
native b-CD were investigated using dynamic light scattering
(DLS) combined with X-ray diffraction (XRD) measurements.
Fig. 3A displays the DLS plots of native b-CD (0.016 M) before (a)
and aer equimolar addition of ruthenium salt (Ru/b-CD molar
ratio ¼ 1) (b). As expected, b-CD presents a bimodal size
distribution plot with two populations centred at 70 and 380 nm
associated with the formation of aggregates. Interestingly, aer
addition of ruthenium nitrosyl nitrate, these aggregates
undergo partial disintegration, as evidenced by the shi of the
hydrodynamic radius to 22 and 130 nm respectively. Further
evidence for such interaction was provided from XRD
measurements performed on b-CD and Ru/b-CD assemblies
ꢀ
dried at 60 C (Fig. 3B). Thus, the numerous diffraction lines
characteristic of the cage-type crystalline microstructure of b-
CD disappear upon addition of ruthenium nitrosyl nitrate
indicating disruption of the b-CD microstructure. In analogy
with the experiment described above, DLS measurements were
also performed on the b-CD/TiO2 suspensions (Fig. 3C). From
the size distribution plots of the TiO2 sols (0.5 M) prepared
without and with the native b-CD (b-CD/Ti molar ratio ¼ 0.032),
the decrease in the apparent hydrodynamic radius from 110 to
85 nm suggests rupture of TiO2 aggregates due to the adsorp-
tion of the cyclic molecule onto the TiO2 surface.24 Moreover,
Table 1 Characteristics of the TiO2 materials and supported RuO2/TiO2 catalysts after thermal treatment at 400 ꢀC
SBETa (m2 gꢁ1
)
Vcum (cm3 gꢁ1
)
PSc (nm)
RuO2 sized (nm)
b
Sample
TiO2 supports
TiO2-sg
TiO2-ns
21 ꢃ 1
80 ꢃ 5
0.030 ꢃ 0.002
0.360 ꢃ 0.018
5.3 ꢃ 1.1
—
—
16.3 ꢃ 3.3
RuO2-TiO2 catalysts
Ru2.5TiO2-sg
Ru2.5b-CDTiO2-sg
Ru2.5TiO2-ns
8 ꢃ 0
0.015 ꢃ 0.001
0.045 ꢃ 0.002
0.265 ꢃ 0.012
0.314 ꢃ 0.016
6.2 ꢃ 1.2
5.9 ꢃ 1.2
16.5 ꢃ 3.3
16.8 ꢃ 3.3
15.8 ꢃ 1.6
11.5 ꢃ 1.2
9.8 ꢃ 1.0
9.2 ꢃ 0.9
16 ꢃ 1
51 ꢃ 3
75 ꢃ 4
Ru2.5b-CDTiO2-ns
a
b
c
Specic surface area determined in the relative pressure range 0.1–0.25. Cumulative pore volume. Average pore size resulting from NLDFT
calculations. d RuO2 crystallite size determined using the Scherrer equation.
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the disappearance of the most intense XRD reections (at 9.0ꢀ
and 12.5ꢀ) of the corresponding TiO2 and b-CD/TiO2 xerogels
ꢀ
dried at 60 C conrms the breakdown of the cage-type struc-
ture of b-CD aer interaction with titania surface (Fig. 3D).
Taken together, our results provide evidence of the fact that
b-CD presents a strong affinity for both the mesoporous TiO2
support and the Ru(III) nitrosyl nitrate. Such behaviour is likely
to alter the degree of dispersion of the ruthenium nanoparticles
over the support and, by consequence, affect the performance of
the nal catalyst.
On the bases of the results obtained above, we focused the
continuation of our study on the preparation and character-
ization of a series of Ru/TiO2 catalysts using both conventional
sol–gel TiO2 (TiO2-sg) and nanostructured RaMeb-CD-P123-
templated TiO2 (TiO2-ns) supports. The ruthenium nitrosyl
nitrate impregnating solution was prepared without or with
assistance of the native b-CD (Ru/b-CD ¼ 11).
Fig. 5 N2-adsorption isotherms (A) and corresponding pore size
distribution plots (B) of RuO2/TiO2 composites (2.5 wt% Ru) supported
over conventional sol–gel TiO2 (TiO2-sg) and nanostructured RaMeb-
CD-P123-templated TiO2 (TiO2-ns) prepared without or with assis-
tance of native b-CD.
XRD patterns of the different catalysts are presented in
Fig. 4. Similarly to the undoped TiO2-ns support (Fig. 2B),
Ru2.5TiO2-ns and Ru2.5b-CDTiO2-ns catalysts contain only
anatase and brookite implying that no anatase-to-rutile or
brookite-to-rutile phase transformation occurs during the
second calcination at 400 ꢀC. On the other hand, three distinct
crystalline phases (anatase, brookite and rutile) are clearly
observed for the Ru2.5TiO2-sg and Ru2.5b-CDTiO2-sg catalysts.
Furthermore, for all catalysts, in addition to the peaks of the
support material, other reexion lines are clearly identied at 2q
¼ 27.8ꢀ and 34.9ꢀ which could be indexed to the (110) and (101)
planes respectively of the tetragonal RuO2 (JCPDS 00-043-1027).
Note that for the TiO2-sg supported catalysts, the (110) and (101)
planes of RuO2 and rutile-TiO2 overlap due to the lattice
matching between these two phases both of which adopt
a tetragonal structure. It is also worthy to mention that both the
support and the dispersing agent affect the size of RuO2 crys-
tallites. Thus, for the Ru2.5 and Ru2.5b-CD catalysts deposited
over TiO2-ns, the crystallite sizes determined from the line
broadening of the (110) diffraction peak are 9.8 nm and 9.2 nm
respectively, while the average sizes obtained for the catalysts
deposited over TiO2-sg are beyond 10 nm (Table 1).
To further investigate the impact of RuO2 nanoparticles on
the textural characteristics of the catalysts, N2-adsorption
analyses were carried out. From the adsorption isotherms
(Fig. 5A) and corresponding pore size distributions (Fig. 5B), it
can be noticed that, whatever the type of the support (TiO2-sg or
TiO2-ns), the textural characteristic are only slightly modied
aer deposition of RuO2 nanoparticles, indicating that no pore
blocking occurs. For instance, the surface area, pore volume
and pore size of the Ru2.5b-CDTiO2-ns catalyst (75 m2 gꢁ1, 0.31
cm3 gꢁ1 and 16.8 nm respectively) are very close to those of the
undoped TiO2-ns support (80 m2 gꢁ1, 0.36 cm3 gꢁ1 and 16.3 nm
respectively) (Table 1). In a somewhat different way, the Ru2.5b-
CDTiO2-sg catalyst presents a higher pore volume compared to
the TiO2-sg support (0.045 cm3 gꢁ1 vs. 0.030 cm3 gꢁ1) as well as
a second population of mesopores centred at 14.8 nm. This is
an indication that the native b-CD has also a benecial effect in
restructuring the titania framework during the impregnation
step due to the ability of this cyclic molecule to adsorb onto the
surface of the oxide nanoparticles, as mentioned earlier.
To probe the reducibility of mesoporous RuO2/TiO2 compos-
ites, temperature programmed reduction (TPR) measurements
were also carried out. The TPR proles of the different supported
catalysts are shown in Fig. 6.
As expected, the non-doped TiO2 material shows a very weak
signal and no reduction peak indicating that the support alone
can hardly react with H2. In contrast, the Ruꢀ2.5TiO2-sg catalyst
displays a very broad peak centred at ꢄ320 C assigned to the
reduction of Ru4+ to metallic Ru0.63 Such a high reduction
temperature may result from the formation of RuO2 aggregates
which interact strongly with the rutile phase (major polymorph
in the TiO2-sg support) and which are difficult to reduce. On the
Fig. 4 XRD patterns of RuO2/TiO2 composites (2.5 wt% Ru) supported
over conventional sol–gel TiO2 (TiO2-sg) and nanostructured RaMeb-
CD-P123-templated TiO2 (TiO2-ns). Both composites were prepared
without or with assistance of native b-CD as dispersing agent. R in the
figure denotes rutile-TiO2.
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performed on two selected catalysts, i.e. Ru2.5b-CDTiO2-ns and
Ru2.5b-CDTiO2-sg.
From the TEM image shown in Fig. 7a, it can be noticed that
the surface of the RaMeb-CD-P123-templated TiO2 support is
decorated with RuO2 nanoparticles which can be directly
visualised by the bright spots resulting from the difference in Z-
contrast between the two oxides (Fig. 7b). Moreover, EDS
elemental mapping on different regions of the TEM micrograph
(Fig. 7c–f) allows to further differentiate the two phases, i.e. TiO2
(white regions) and RuO2 nanoparticles (dark regions).
TiO2 nanoparticles in this sample are identied as a mixture
of tetragonal anatase (ꢄ15 nm) and orthorhombic brookite (ꢄ10
nm) (Fig. 7g and h). Interestingly, mesopores of 15–20 nm
diameter, resulting from the self-assembly of these nanoparticles
around the supramolecular template, can also be clearly visual-
ised in this micrograph (pink circles), in agreement with N2-
adsorption analyses. On the other hand, RuO2 nanoparticles are
more or less spherical and approximately 10–12 nm in size
(Fig. 7i), in agreement with XRD results. Conversely, the Ru2.5b-
CDTiO2-sg catalyst shows mainly a dense and less porous
network (Fig. 7j) made-up of anatase (ꢄ35%), brookite (ꢄ26%)
and rutile (ꢄ39%) and over which RuO2 particles tend to form
elongated heterogeneous structures (Fig. 7k–n). Such unusual
epitaxial growth of RuO2 nanoparticles over rutile-TiO2 was also
observed by Xiang et al.64 in a previous study and was suggested
to result from the structural matching between these two phases
which actually share the same type of lattice symmetry (tetrag-
Fig. 6 H2-TPR profiles of RuO2/TiO2 composites (2.5 wt% Ru) sup-
ported over conventional sol–gel TiO2 (TiO2-sg) and nanostructured
RaMeb-CD-P123-templated TiO2 (TiO2-ns) without or with assistance
of b-CD. The TPR profile of non-doped TiO2-ns support is added for
comparison.
other hand, the Ru2.5b-CDTiO2-sg catalyst displays two
detached peaks at ꢄ320 ꢀC and ꢄ210 ꢀC suggesting that, at
least, two different ruthenium species coexist in this sample.
Thus, the higher temperature may be attributed to the reduc-
tion of RuO2 aggregates, in strong interaction with the TiO2-sg
support, which exist also in the Ru2.5TiO2-sg catalyst, while the
lower one may be assigned to the reduction of small and more
easily reducible RuO2 species in weak interaction with the
support and formed only in the presence of b-CD. This result
suggests that, by locating at the metal–support interface, the
cyclodextrin should reduce the contact points between TiO2 and
RuO2 particles, thus preventing the incorporation of these latter
into the titania support during the calcination process. Note
that the catalysts supported over TiO2-ns globally display lower
reduction temperatures, i.e. in the range of 180–230 ꢀC,
compared to those supported over TiO2-sg materials (210–320
ꢀC). This is an indication that the RaMeb-CD-P123-templated
support promotes the formation of small and readily reduc-
ible RuO2 nanoparticles. Moreover, as RuO2 has different lattice
parameters from TiO2-anatase and TiO2-brookite (the major
polymorphs in the TiO2-ns support), the metal–support inter-
actions in these catalysts should be weaker compared to those
between RuO2 and TiO2-sg support. Taken together, our results
provide evidence of the fact that small nano-sized and well-
dispersed RuO2 particles could be obtained using nano-
structured TiO2 prepared by template-directed colloidal self-
assembly as support and native b-CD as dispersing agent.
Further evidence for the key role of RaMeb-CD-based assem-
blies on the dispersion of RuO2 nanoparticles was provided by
high resolution transmission electron microscopy (HR-TEM)
combined with high angle annular dark-eld scanning trans-
mission electron microscopy (HAADF-STEM). Observations were
˚
onal) and have very similar lattice parameters (a ¼ 4.59 A, c ¼
˚
˚
˚
2.96 A for rutile-TiO2 and a ¼ 4.49 A, c ¼ 3.10 A for RuO2).
3.2 Catalytic performance
Methyl esters of vegetable oils, derived from polyunsaturated
fatty acids, can be transformed through partial or complete
hydrogenation into saturated fatty acids with improved physical
properties (e.g. high melting point and oxidative stability).55,65
Methyl stearate (MS, C18:0) is an important saturated oil ob-
tained from catalytic hydrogenation of unsaturated methyl
oleate (MO, C18:1) and widely used as a feedstock for catalytic
hydrogenolysis into its corresponding saturated fatty alcohol
(stearyl alcohol, C18:0).56,66 Ru-based catalysts have been shown
to be effective for low temperature hydrogenation of carboxylic
acid esters, especially for selective hydrogenation of the olenic
C]C bond without affecting the carbonyl C]O bond. In the
present study, we evaluated the performance of the different
supported Ru/TiO2 catalysts in the liquid phase hydrogenation
of methyl oleate to methyl stearate.
Prior to catalytic tests, RuO2/TiO2 composites were pre-
treated under hydrogen ow at 400 ꢀC for 4 h to reduce the
RuO2 in metallic Ru. TEM analyses conrmed that Ru(0)/TiO2-
sg was made-up predominantly of aggregated particles
dispersed in a dense and less porous network (TiO2-sg), while
Ru(0)/TiO2-ns catalyst was mainly comprised of small and well-
dispersed nanoparticles dispersed in a porous and nano-
structured network (TiO2-ns) (Fig. S1, ESI†). This is in agree-
ment with what was observed on non-reduced RuO2/TiO2-sg
and RuO2/TiO2-ns composites (Fig. 7).
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Fig. 8 Comparison of the catalytic activity (a) and selectivity (b) of
different supported catalysts in the hydrogenation of methyl oleate
(C18:1) to methyl stearate (C18:0). Reaction conditions: Ru (0.02
mmol), methyl oleate (MO/Ru molar ratio ¼ 100), H2 (40 bar), solvent
(heptane, 10 mL), stirring rate (750 rpm), temperature (50 ꢀC), reaction
time (2 h).
and TiO2-ns) as well as with the Ru2.5 catalyst supported over
commercial TiO2 (Ru2.5TiO2-A and Ru2.5TiO2-P25) are added for
comparison. Depending on the preparation method, clear
differences can be observed in both the catalytic activity (Fig. 8a)
and selectivity (Fig. 8b). The control TiO2-sg and TiO2-ns supports
show almost no activity in the hydrogenation of methyl oleate.
On the other hand, all supported Ru/TiO2 catalysts are selective
in the hydrogenation of the olenic C]C bond producing the
fully saturated ester, i.e. methyl stearate (C18:0) as the main
¨
product and the methyl elaıdate (C18:1, trans-9) as the only
byproduct. Moreover, it is worth noting that the catalytic
performance strongly differs depending on the type of the
support (TiO2-sg or TiO2-ns) and the impregnation method (with
or without assistance of native b-CD). Thus, when Ru is deposited
over nanostructured RaMeb-CD-P123-templated TiO2, the cata-
lytic activity increases sharply from 36% (Ru2.5/TiO2-sg) to 82%
(Ru2.5/TiO2-ns). Interestingly, further improvement in the
conversion efficiency is noticed when the native b-CD is used as
dispersing agent. Thus, the conversion rate increases by ꢄ14%
(from 36% to 50%) for the Ru2.5 catalyst deposited over TiO2-sg
and by ꢄ12% (from 82% to 94%) for the same catalyst deposited
over TiO2-ns support. Note that among the four supported cata-
lysts prepared using colloidal self-assembly approach, Ru2.5b-
CDTiO2-ns is the fastest one, achieving almost full MO conver-
sion (ꢄ94%) in 2 h. Moreover, it is interesting to notice that the
activity of this catalyst is almost 3-fold higher than that of
ruthenium deposited over commercial anatase-TiO2 (Ru2.5TiO2-
A) and P25-TiO2 (Ru2.5TiO2-P25).
Fig. 7 TEM image (a) and HAADF-STEM images (b–d) of the Ru2.5b-
CDTiO2-ns catalyst showing uniform dispersion of RuO2 nanoparticles
over the mesoporous RaMeb-CD-P123-templated TiO2 material; EDS
spectra with the electron beam focused first on a white region (TiO2)
(e), and then in a dark region (RuO2) (f) of the catalyst images. HR-TEM
micrographs of the Ru2.5b-CDTiO2-ns catalyst indicating uniform
dispersion of TiO2 nanoparticles crystallised in anatase and brookite (g
and h), as well as RuO2 nanoparticles (5–10 nm) (i) TEM and HR-TEM
images of the Ru2.5b-CDTiO2-sg catalyst composed mostly of a non-
porous support material showing TiO2 and RuO2 nanoparticle
aggregation (j–n).
The results obtained in catalysis aer 2 h of reaction (40 bar
H2, 50 ꢀC) with the four catalysts, prepared using either
conventional sol–gel or nanostructured TiO2 supports, with the
assistance or not of the native b-CD (Ru/b-CD ¼ 11), are shown in
Fig. 8. The results obtained with the non-doped supports (TiO2-sg
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Similarly to the catalytic activity, clear differences can be consecutive cycles, indicating that the supported catalyst exhibits
observed also in the selectivity of the supported catalysts. Thus, high stability under the employed reaction conditions (Fig. 9).
the increase in the methyl oleate conversion when moving from
We expect that the high surface area and large pore volume
Ru2.5TiO2-sg to Ru2.5TiO2-ns leads also to a dramatic increase generated by the template-directed self-assembly of titania colloids
in the selectivity for the methyl stearate (from 47% to 80%). should allow for a good dispersion of nanosized ruthenium
However, although a benecial effect of b-CD is noticed for the particles and, therefore, effectively prevent aggregation while
Ru2.5TiO2-sg catalyst (MS selectivity increases from 47% to improving the chemical stability of the catalyst. Moreover, the
ꢀ
67%), no clear difference exists between the Ru2.5TiO2-ns thermal treatment at 400 C should allow for a stable xation of
catalysts prepared without or with assistance of b-CD (MS ruthenium species over the TiO2 material, preventing leaching
selectivity is the same ꢄ80%).
from the matrix to the reaction media during the catalytic process.
Over the investigated mesoporous Ru/TiO2 catalysts, the Hence, the high efficacy of this catalyst combined with ease of
catalytic performance agrees well with the textural characteristics recovery and reuse without loss of conversion and selectivity
of the support and the degree of dispersion of the ruthenium makes the template-directed colloidal self-assembly a simple and
species. Indeed, the highest performance of the Ru2.5b-CDTiO2- versatile approach for the synthesis of supported catalysts with
ns catalyst may result from a combined effect of improved high performance in liquid phase hydrogenation of methyl oleate.
textural characteristics of the support and uniform dispersion of
ruthenium nanoparticles. Thus, the high surface area of the
4. Conclusions
RaMeb-CD-P123-templated TiO2 material (80 m2 gꢁ1) should
provide a high level of dispersion of ruthenium nanoparticles, In conclusion, we have demonstrated that the approach
facilitating the contact of methyl oleate with the catalyst surface combining the template-directed colloidal self-assembly with
during the hydrogenation reaction. On the other hand, the high a cyclodextrin-assisted impregnation method allows to prepare
pore volume (0.36 cm3 gꢁ1) should allow for more reactant highly active and selective Ru/TiO2 catalysts for the hydroge-
molecules to be adsorbed to the internal surface of the pores, nation of methyl oleate (MO, C18:1) to methyl stearate (MS,
thus improving the diffusion of reactants and products toward C18:0) under mild conditions (50 ꢀC, 40 bar H2). The supra-
the active phase during the catalytic process. Finally, the use of molecular assemblies formed between RaMeb-CD and block
native b-CD as dispersing agent may allow for reducing the copolymer P123 promoted the formation of a nanostructured
interparticle aggregation thanks to the ability of this cyclodextrin and highly porous TiO2 support material over which uniform
to interact with both RuNO(NO3)3 and TiO2 nanoparticles.30,32,33 dispersion of ruthenium species was achieved, without or with
Overall, our results reveal a dual role played by the CD-based the assistance of the native b-cyclodextrin. The high activity of
assemblies: an enhancement of the porosity and surface area of these catalysts was attributed to the ability of the supramolec-
the TiO2 material and an improvement of the dispersion of Ru ular CD-based assemblies to generate TiO2 materials with
nanoparticles over the support, parameters that appear to be improved structural and textural characteristics and interact in
essential for enhancing the efficacy of the catalyst in the liquid a specic way with both the TiO2 support and Ru species, thus
phase hydrogenation of methyl oleate.
improving the metal–support interactions.
Moreover, as leaching and deactivation are common
Overall, our results revealed that the CD-based assemblies
phenomena encountered in any liquid phase hydrogenation provide a versatile and easily accessible toolbox with different
reaction, the ease of recovery and reusability of the catalyst are functionalities for generating metal-supported catalysts with
important features that need to be considered. For this purpose, controlled pore architecture and uniform metal distribution.
we have investigated the recyclability of the most efficient cata- The approach combining the use of two different cyclodextrins
lyst, i.e. Ru2.5b-CDTiO2-ns, in three successive runs. Interest- with different functionalities is versatile and may be generalised
ingly, no loss of activity and selectivity was observed during the to other monometallic or bimetallic catalysts supported over
simple or mixed oxides prepared by sol–gel process.
Acknowledgements
The Chevreul Institute (FR 2638), the European Regional
Development Fund (ERDF), the Conseil Regional du Nord-Pas
`
de Calais, the CNRS and the Ministere de l'Enseignement
´
Superieur et de la Recherche are acknowledged for supporting
and funding this work. We thank Laurence Burylo (UCCS,
University of Lille) for technical assistance in XRD measure-
ments, Antonio Da Costa (UCCS, University of Artois) and Nora
Djelal (UCCS, University of Lille) for their help with FE-SEM
¨
analyses, as well as Loıc Brunet of the BICeL-Campus Lille 1
Fig. 9 Reusability tests of the Ru2.5b-CDTiO2-ns catalyst. Reaction
conditions: methyl oleate/ruthenium molar ratio (MO/Ru ¼ 100), H2
(40 bar), solvent (heptane, 10 mL), stirring rate (750 rpm), temperature
(50 ꢀC), reaction time (2 h).
Facility for access to instruments and technical advices. We are
indebted to the Research Federation FRABio (Univ. Lille, CNRS,
FR 3688, FRABio, Biochimie Structurale et Fonctionnelle des
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´
´
Assemblages Biomoleculaires) for providing the scientic and 21 R. Bleta, C. Machut, B. Leger, E. Monier and A. Ponchel,
technical environment conducive to achieving this work. We Macromolecules, 2013, 46, 5672–5683.
`
´
kindly acknowledge Roquette Freres for generous gi of the 22 R. Bleta, C. Machut, B. Leger, E. Monier and A. Ponchel, J.
native b-CD.
Inclusion Phenom. Macrocyclic Chem., 2014, 80, 323–335.
´
23 R. Bleta, S. Menuel, B. Leger, A. Da Costa, E. Monier and
A. Ponchel, RSC Adv., 2014, 4, 8200–8208.
24 R. Bleta, A. Lannoy, C. Machut, E. Monier and A. Ponchel,
Langmuir, 2014, 30, 11812–11822.
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