An efficient route to prepare highly dispersed metallic copper nanoparticles on ordered mesoporous silica with outstanding activity for hydrogenation reactions

Article abstract of DOI:10.1039/c5cy00428d

Copper nanoparticles at relatively high metal loading of 10 wt.% were successfully synthesized via incipient wetness impregnation by using polyether-functionalized ordered mesoporous silica as organic-inorganic hybrid supports. The effect of functionalized triblock copolymer Pluronic P123 (0, 25 and 50 wt.% of total triblock copolymer in gel) on the structural and catalytic properties of copper nanocomposite materials was specifically investigated. The oxide forms of materials were systematically characterized by nitrogen physisorption, SAXS, WAXS, TEM, EDXS, DR UV-vis and TPR, while the metallic forms were analysed by N₂O chemisorption, WAXS and TEM. The results indicated that the use of mesoporous silica hybrids leads to highly dispersed supported copper nanoparticles (dispersions in the range of 43-57%) displaying excellent activity in the hydrogenation of cinnamaldehyde. The average particle size was shown to decrease from 2.3 to 1.7 nm with the increase in the amount of functionalized triblock copolymer from 0 to 50 wt.%. Under standard test conditions, all the nanosized copper catalysts showed high catalytic activity but selectivity towards the saturated aldehyde. Further improvements in activity and selectivity towards the unsaturated alcohol were achieved by increasing the hydrogen pressure and applying a two-step reduction-oxidation pre-treatment, respectively. The structure and controllable properties of nanoscale copper materials developed herein, both oxidic and metallic forms, make them very attractive candidates for both fundamental research and practical catalytic applications.

Full text of DOI:10.1039/c5cy00428d

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An efficient route to prepare highly dispersed
metallic copper nanoparticles on ordered mesoporous
silica with outstanding activity for hydrogenation
reactions
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
a
b
a
a
Constantin Rudolf , Fatima Abi-Ghaida , Brindusa Dragoi , Adrian Ungureanu *,
b
a
Ahmad Mehdi *, Emil Dumitriu ,
DOI: 10.1039/x0xx00000x
www.rsc.org/
Copper nanoparticles at relatively high metal loading of 10 wt. % were successfully synthesized
via incipient wetness impregnation by using polyether-functionalized ordered mesoporous silica
as organic-inorganic hybrid supports. The effect of functionalized triblock copolymer Pluronic
P123 (0, 25 and 50 wt. % of total triblock copolymer in gel) on the structural and catalytic
properties of copper nanocomposite materials was specifically investigated. The oxide forms of
materials were systematically characterized by nitrogen physisorption, SAXS, WAXS, TEM,
2
EDXS, DR UV-Vis and TPR, while the metallic forms were analysed by N O chemisorption,
WAXS and TEM. The results indicated that the use of mesoporous silica hybrids leads to highly
dispersed supported copper nanoparticles (dispersions in the range 43-57 %) displaying
excellent activity in the hydrogenation of cinnamaldehyde. The average particle size was shown
to decrease from 2.3 to 1.7 nm with the increase in the amount of functionalized triblock
copolymer from 0 to 50 wt. %. Under standard test conditions, all the nanosized copper catalysts
showed high catalytic activity but selectivity towards the saturated aldehyde. Further
improvements in activity and selectivity towards the unsaturated alcohol were achieved by
increasing the hydrogen pressure and applying a two-step reduction-oxidation pre-treatment,
respectively. The structure and controllable properties of nanoscale copper materials developed
herein, both oxidic and metallic forms, make them very attractive candidates for both
fundamental research and practical catalytic applications.
Introduction
oxidation state, and chemical/physical environment play key
roles in determining the performance of supported NPs. For
2
In the field of heterogeneous catalysis, the unique electronic instance, one of the most interesting metals is copper, which is
structure and the high active surface of metal (oxide) catalytically inactive in bulk form during hydrogenation
nanoparticles (NPs) determine the selectivity and activity of reactions but becomes active when the particle size decreases to
catalysts. An extensive array of research was effectuated for the 10 nanometers or bellow.3-9 As an example, commercial
development of new heterogeneous catalysts with high methanol synthesis catalysts typically consist of
performances and thus with great applicative potential in fine nanoparticles (about 5-10 nm in size) spaced by smaller ZnO
Cu
6
,7
chemistry, petrochemistry, pollution prevention/control and crystallites.
However, copper NPs are difficult to be
1
other important fields. For these applications, catalysts are stabilized, especially at high loading of copper on silica
usually formulated as metal (oxide) NPs deposited on inorganic supports, due to the high mobility of copper species, resulting
or organic porous supports. Striking different catalytic in significant sintering, even at low temperatures, and formation
properties have been reported for NPs as compared with the of undesired large agglomerates.9
bulk counterparts. Particle size, geometry, composition,
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Therefore, attempts have been made to disperse copper under the cost-effective, nanosized copper-based catalysts have great
the form of small particles by using mesoporous silica SBA-15 potential to replace the expensive noble metal-base catalysts,
as supports based on some properties such as high they were seldom investigated in cinnamaldehyde
8, 30, 31
hydrothermal stability, high surface area, and two-dimensional hydrogenation.
cylindrical pores.
For example, Guttierez et al. recently
Even though various strategies can be reported that the selectivity levels achieved over Cu/MCM-41
envisioned to obtain high dispersion and enhanced stability of catalysts (20% metal loading, Cu NPs of 3.8 nm in size) are
9
-11
1
1,
12
copper NPs, such as deposition precipitation,
higher than those over noble metal-based catalysts (Au and Pt),
incipient wetness under the same reaction conditions.
1
1
11, 13
8
precipitation,
wet impregnation,
9
impregnation (IWI), and IWI followed by mild drying IWI-
4,
7
10
MD,
microwave irradiation assisted impregnation and Experimental
8
chemical reduction, the impregnation routes remain simpler,
less expensive and environmentally friendly. Moreover, Synthesis of bis-silylated P123
functionalization of mesoporous surface by chelating groups
like –COOH is often required to obtain a high dispersion of Cu Bis-silylated P123 (Si-P123) was synthesized according to
1
6
NPs via impregnation with aqueous solution of copper Grandsire et al. as follows: a solution of 10 g (1.72 mmol) of
1
3, 14
nitrate.
surfactant micelle in the as-synthesized SBA-15 can be = propyleneoxide; molecular weight of 5800; BASF Corp.) in
employed to entrap hydrophobic precursors of copper acetyl CH Cl (200 mL) was dried with magnesium sulphate and
acetonate, resulting also in high dispersion of Cu NPs, yet at filtered. CH
Alternatively, the hydrophobic core of the P123 Pluronic P123 (EO20PO70EO20, where EO = ethyleneoxide, PO
2
2
1
5
Cl
was removed and polymer was heated under
2
2
o
less than 1 wt. % copper loading. From these recent studies, it vacuum at 100 C for 12 h. Dry P123 was then silylated with 3-
is evident that the functionalization of mesoporous silica indeed isocyanatopropyltriethoxysilane (ICPTES). To a solution of
offers specific tailoring of the surface chemical properties of P123 (0.1 M) in dry THF (tetrahydrofuran,100 mL), an excess
support which improves the interaction between copper of ICPTES (4 equiv.) and Et
precursors and pore surfaces, finally leading to high dispersions added at room temperature. The mixture was stirred at reflux
of copper NPs stabilized within the pores of support.
for 72 h under argon. After removal of THF and Et N, the
N (triethylamine, 2 equiv.) were
3
3
Herein, we report a new route to prepare highly dispersed obtained crude was washed with pentane four times and dried,
metallic copper nanoparticles on mesoporous silica at relatively giving rise to 9.39 g (1.50 mmol, 87%) of Si-P123 as colourless
high copper loading of 10 wt. %, based on the functionalization oil.
of mesoporous silica supports with polyether groups. First, a
functionalized bis-silylated triblock copolymer Pluronic P123 Synthesis of functionalized supports and catalysts
(
Si-P123) containing triethoxysilyl moieties at the chain ends
1
6
was prepared, according to a previously reported procedure.
For this study a series of supports were obtained based on a
1
6
The functionalized triblock copolymer had thereafter two major previous reported method. The difference between them is the
roles: (i) as secondary structure direction agent (SDA) for the amount of Si-P123 used. All the supports were denoted as Mx,
two-dimensional hexagonal SBA-15-like mesostructure where “M” stands for mesoporous support and “x” stands for
containing functional polyether chains covalently bound to the the mass percentage of Si-P123 used in gel (x, wt. % = mSi-
silica framework where the primary SDA was the P123/(mSi-P123+mP123). The M0 support was prepared according to
unfunctionalized P123 triblock copolymer, and (ii) as the following procedure. 8.2 g (1.36 mmol) of block copolymer
dispersing agent for the copper precursors impregnated as P123 was solubilized in 300 mL of an aqueous HCl solution (
aqueous solutions into the support porosity. The polyether- pH = 1.5) in an Erlenmeyer flask. After 2-3 h of stirring, the
functionalized mesoporous silica with controlled contents of solution was poured into another flask containing 18.7 g (89.78
covalently bound P123 presented enhanced capacity to stabilize mmol) of tetraethoxysilane at room temperature. The reaction
copper precursors, thus explaining the high dispersion of medium was left for 2 h and 30 min at room temperature under
copper (oxide) NPs achieved after surfactant removal by regular magnetic stirring to give rise to a microemulsion; the
calcination under air, and reduction under hydrogen. As a solution appeared perfectly clear and colorless. The flask was
result, the metal forms of catalysts manifested outstanding immersed in an oil bath at 60 °C and after 2 min, 150 mg (3.6
performances in cinnamaldehyde hydrogenation. This reaction mmol) of NaF was added under stirring to allow hydrolysis-
3
2
is very interesting from a fundamental point of view (control of polycondensation. A white precipitate formed in a few
3, 17
the chemoselectivity) and it has been extensively studied.
minutes. The reaction mixture was left under stirring for 72 h at
Likewise, cinnamaldehyde hydrogenation is industrially 60 °C, the resulting powder was filtered off and the unbound
relevant because the products of partial or total hydrogenation surfactant was removed by Soxhlet extraction over ethanol for
o
are being used in flavour and perfume industry, cosmetics, 24h and then was dried at 120 C under vacuum to give 5.6 g of
pharmaceutics etc. Consequently, a wide variety of efficient M0 as white powder. For the synthesis of M25 and M50
noble metal-based catalysts were reported for the supports, the same procedure was followed, apart from the
hydrogenation of cinnamaldehyde, amongst which are amounts of Si-P123 and P123 used, as shown in Table 1.
1
8-23
24, 25
26-28
29
platinum,
iridium,
ruthenium,
and rhodium. Albeit
2
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-1
Table 1. Amounts of bisilylated P123 and P123 used for supports synthesis
H
2
containing flow was stabilized (40 mL min , 5 vol. % H
in
2
Ar) and the TPR was performed from 50 to 500 °C with a
temperature ramp of 5 °C min .
Sample
-
1
M0
M25
M50
SDA
The dispersion of metallic copper (D ), average copper
m P123 (g)
8.20
0.00
6.15
2.05
4.10
4.10
Cu
particle size (d ) and active surface area (S ) were determined
Cu
Cu
m Si-P123 (g)
using a nitrous oxide chemisorption method using specific
3
4-37
algorithms.
The experiments were conducted on a Chembet
A certain amount of support was put in contact with the
corresponding aqueous copper nitrate precursor solution. The
metal precursor/silica composites were gently dried under air at
Pulsar TPR/TPD from Quantachrome. The first reduction was
carried out under the same conditions used for TPR, up to 350
°
C. After the reduction step, the gas was switched to Ar and the
2
5 ± 1 °C for 5 days. The powders were sequentially submitted
reactor was cooled down to 50 °C, kept for 30 min, and then the
o
to calcination under stagnant air in a muffle oven at 500 C for
6
−1
gas was switched to N O (40 mL min ) for 30 min in order to
2
-
1
h (heating ramp of 1.5 °C min ) to obtain the oxide forms of
oxidize the surface zero-valent Cu to Cu O. The sample was
2
catalysts and burn off the organic components. To avoid the
again flushed with pure Ar for 30 min, and then a second TPR
run was performed.
3
3
“
container effect” throughout calcination, the same open
container and similar sample depths were used. After
calcination, the solids were stored under ambient conditions in
sealed flasks without special precautions. The samples were
labeled as Cu /Mx.
Hydrogenation of trans-cinnamaldehyde
1
0
The liquid phase hydrogenation of cinnamaldehyde was carried
out at atmospheric pressure in a round-bottom threeneck glass
reactor equipped with a bubbler, magnetic stirrer and a reflux
Catalyst characterization
condenser. Prior to reaction all the samples were activated by
The thermogravimetric (TG) measurements were performed on
a computer coupled Q-derivatograph (MOM). This analysis
was performed for the supports to observe the difference
between the loading degree with Si-P123 and were carried out
in the temperature range between 20 and 600 °C with a heating
rate of 10 °C min . All measurements were performed under
stagnant air.
Nitrogen physisorption measurements were carried out at −196
o
reduction under flowing H at 350 C for 10 h with a heating
2
o
-1
ramp of 1.5 C min . The following reaction conditions were
applied: 0.265 g of catalyst, 1.0 mL of trans-cinnamaldehyde,
2
1
5.0 mL of propylene carbonate as solvent, hydrogen flow of
.0 L h , reaction temperature of 150 °C, and agitation speed
−1
–
1
of 900 rpm.
Cu /M0-c and Cu /M50 samples were also tested at high
1
0
10
pressure. The catalytic reaction was carried out in a Parr batch
reactor under the following conditions: 0.265 g of catalyst, 1.0
mL of trans-cinnamaldehyde, 40 mL of 2-propanol as solvent,
°
C on an Autosorb MP-1 instrument from Quantachrome.
Surface areas and pore volumes were calculated from the
corresponding isotherms using conventional algorithms.
The SAXS and WAXS experiments were made on ID02 at 12.4
keV with d = 1.5m. The samples were inserted between two
Kapton glasses. All measurements were dark subtracted and the
background was measured with a double empty Kapton glass.
All measurements were normalized by the transmission
coefficient and by the acquisition time.
1
0 bar of H , reaction temperature of 130 °C, and agitation
2
speed of 750 rpm. At certain periods, aliquots of reaction
mixture were withdrawn and analyzed with gas-
a
chromatograph (HP 5890 equipped with a DB-5 capillary
column (25 m × 0.20 mm × 0.33 μm) and FID detector). The
products were identified from the retention time of the pure
compounds and also verified with a GC-MS (Agilent 6890N
system equipped with an Agilent 5973 MSD detector and a DB-
Diffuse reflectance UV-visible (DR UV-vis) spectra were
recorded on a Shimadzu UV-2450 spectrometer equipped with
an integrating sphere unit (ISR-2200). The spectra were
5-ms column). For the quantitative analyses, the FID response
factors for each compound were taken into consideration. The
selectivity towards the products and the total conversion were
collected in the range between 200–800 nm by using BaSO as
4
the reflectance standard.
Transmission Electron Microscopy (TEM) analyses were
performed with a JEOL 1200 EXII.
Energy X-Ray dispersive spectroscopy (EDXS) analyses were
performed with an environmental scanning electronic
microscope Quanta 200 coupled with an Oxford INCA
analyzer.
38
calculated as described in a previous paper. For the recycling
tests, Cu /M50 catalyst was recovered from the reaction
1
0
mixture by centrifugation, washed with fresh 2-propanol and
ethanol, dried, calcined and reactivated, before being tested
under high-pressure conditions.
Results and discussion
Temperature programmed reduction and chemisorption
experiments were performed on a Chembet Pulsar TPR/TPD Physico-chemical characterization
from Quantachrome. Amounts of about 30 mg of calcined
sample were inserted in a U-shape microreactor. Before each The TG analysis for the uncalcined supports (Figure 1)
TPR run, the catalyst was activated at 500 °C for 1 h under a displayed three steps of weight loss. The first loss in weight
-
1
flow of simulated air (40 mL min ). After cooling to 50 °C, the corresponds to the removal of weakly adsorbed water
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molecules, while the second weight loss relates to the water within the mesopores (vide infra). The sample Cu /M50
1
0
derived from the dehydroxilation of the surface sylanols and it displays the lowest intensity of SAXS peaks indicative of a less
o
39,
is observed only for the M0 support between 130 and 180 C.
ordered mesostructure at short- and long-range as compared
4
0
The third weight loss corresponds to the P123 decomposition with Cu /M0 and Cu /M25 samples, though the quality of the
and differs from one support to another. For M25 it was sample is high enough.
detected from 180 C, while for M50 it starts at 150 C. Also
1
0
10
4
1
o
o
M0 support shows the decomposition of the surfactant from
o
230
C. The difference between the decomposition
temperatures may came from the different loadings with Si-
P123. High loadings conduct to lower decomposition
temperatures suggesting that not all of the Si-P123 is integrated
Figure 2. SAXS patterns for the oxide form of materials: a-Cu10/M0, b-Cu10/M25,
c-Cu10/M50
Figure 1. TG curves for the supports: a-M0, b-M25 and c-M50
within the silica matrix. The supports show a final weight loss
o
after the TG analysis up to 500 C as follows: M0 – 11 wt. %,
M25 – 17 wt. % and M50 – 25 wt. % in line with the gradual
increase in the amount of surfactant bound on silica framework.
Small angle X-ray scattering (SAXS) results for the calcined
Cu-based materials are shown in Figure 2. All patterns display
intense peaks accompanied by two less intense peaks, which
were indexed as (100), (110) and (200) reflections which
correspond to highly ordered hexagonal 2D materials with a
4
2, 43
p6mm symmetry.
The results demonstrate the maintenance
of pore symmetry of the parent supports and a good textural
uniformity of these materials after impregnation and
calcination. No significant shifts of the q values were observed
along the series, indicating similar unit cell parameters of the
obtained mesostructured composites, irrespective of the amount
of Si-P123 used to prepare the functionalized supports.
However, a clear variation in the intensity of peaks can be
noticed. The most intense peaks were observed for Cu /M25
Figure 3. WAXS patterns for the oxide and metal forms of materials: a-Cu-
1
0
/M0(red), b-Cu10/M0(calc), c-Cu10/M25(red), d-Cu10/M25(calc), e-Cu-
/M50(red), f-Cu10/M50(calc)
10
Wide angle X-ray scattering (WAXS) was performed on the
calcined (calc) and reduced (red) samples of Cu /M0, Cu-
1
0
1
0
sample (Figure 2,b) suggesting the highest degree of
mesostructure ordering together with a high dispersion of CuO
NPs into the channels of the ordered mesoporous SBA-15-like
10/
M25 and Cu /M50 (Figure 3). The WAXS patterns for
10
Cu /M0, both oxide and metal forms, display small and
1
0
0
broadened peaks attributed to CuO (Figure 3,b) and Cu phases
Figure 3,a), respectively, indicating that the size of
4
4
silica. The intensity peaks decreases for Cu /M0 sample
1
0
(
(
Figure 2, a), which can be tentatively interpreted by the
correspoding crystallites are small but detectable. In opposite,
the samples Cu /M25 and Cu /M50 show hardly
distinguishable diffraction peaks clearly indicating the presence
increase in the extent of filling of the main mesopores with
CuO NPs when compared with Cu /M25, taking also into
1
0
10
1
0
account the TEM results showing oxide particles confined
4
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ARTICLE
of only finely dispersed oxide and metallic crystallites with an presence of more surfactant covalently bound to silica
average particle size below the detection limit of apparatus (less framework, occluding thus the mesopores and the micropores
4
5
than ~3 nm). This provides evidence that highly dispersed of support. This may explain the significant drop of
2
-1
copper (oxide) nanoparticles suported on mesoporous silica at microporous area from 266 to 98 and 25 m g as the amount
relatively high metal loading of 10 wt. % can be successfully of Si-P123 increases from 0 to 25 and 50 wt. %, respectively.
synthesized via simple incipient wetness impregnation method At the same time, the pore volume and pore size are similar for
3
-1
by using polyether-functionalized mesoporous silica as M0 and M25 supports (around 1.8 cm g and 9.7 nm) and they
3
-1
supports. Likiwise, the results obviously show that the decrease for M50 sample (1.43 cm g and 9.2 nm). On the
fabricated catalysts are resistant against particle sintering upon other side, all the textural properties decline when M0 support
the calcination and reduction processes.
is subjected to calcination due to the dehydroxylation of the
silica surface and shrinkage of the mesostructure. For example,
2
-1
the specific surface area decreases from 996 to 719 m
whereas the pore diameter decreases from 9.7 to 9.3 nm.
g
All the oxide-loaded mesoporous silicas show lower specific
areas than the parent uncalcined supports due to the thermal
treatment upon calcination and as a result of loading the pores
with phases of copper oxide. It is interesting to observe that
after calcination, the microporous surface area of Cu /M25 and
1
0
Cu /M50 increases as compared with the parent supports (for
1
0
2
-1
example from 25 to 113 m
g
for M50), which can be
associated with the removal of the micropore-occluded
surfactant. The pore diameter for all oxide forms of materials
was found to be in a small range from 9.1 to 9.3 nm.
Table 2. Textural properties for M0-c, M0, M25, and M50 supports and
oxide form of materials
Figure 4.
2
N physisorption isotherms for the oxide form of materials (left) and
NL-DFT pore size distribution (right) for the oxide forms of materials: a-Cu10/M0-
c, b-Cu10/M0, c-Cu10/M25, d-Cu10/M50
S
BET
S
μ
V
p
V
μ
D
p
(nm)
Sample
2
-1
2
-1
3
-1
3 -1
(
m g )
(m g )
(cm g )
(cm g )
The textural characteristics of the functionalized supports and
oxide-loaded samples were evaluated from the nitrogen
M0-c
719
582
996
855
917
684
646
590
173
116
266
244
98
1.25
1.07
1.74
0.97
1.84
0.98
1.43
0.98
0.080
0.051
0.127
0.116
0.036
0.081
0.001
0.051
9.3
9.3
9.7
9.3
9.7
9.2
9.2
9.1
adsorption-desorption isotherms. Figure
4
displays the
Cu /M0-c
10
isotherms and the pore size distributions for the oxide form of
catalysts, whereas Table 2 centralizes the textural properties of
both supports and catalysts. Characteristic IUPAC type IV
isotherms alongside H1 hysteresis loops can be noticed for all
samples. Also, from NL-DFT pore size distribution, a narrow
distribution of the pores was observed according to their
diameter. These features point towards ordered SBA-15-like
mesostructures having narrow size distributions of the
cylindrical mesopores, and indicate that the mesoporous
structure of the supports was retained after the deposition of
M0
Cu10/M0
M25
Cu10/M25
M50
174
25
Cu10/M50
113
S
BET – specific surface area, Sμ – micropore surface area, V
V – micropore volume, D – pore diameter
p
– pore volume,
4
6
μ
p
copper oxide, which is in good agreement with the SAXS
analysis. It is worthy of note that the hysteresis loops show no
forced closures on the desorption branches of the isotherms,
suggesting the absence of cavitation effects resulting from the
partial blockage of primary mesopores with metal oxide NPs,
which usually occurs when particles are mainly confined whitin
Transmission electron microscopy (TEM) images recorded at
low-resolution was performed for both oxide-loaded samples
and the reduced catalysts (Figure 5). As a first remark, all the
solids display hexagonal mesoporous structure with a high
long-range order and narrow pore size distribution, typical for
4
the mesopores of the support.
SBA-15-like materials, confirming SAXS and N physisorption
2
Data displayed in Table 2 illustrate the net effect of Si-P123
content on the textural properties of supports. Thus, uncalcined
M0 support prepared without Si-P123 shows the highest
results. The silica grains appeared free from large aggregates on
the external surface even after the reduction process,
confirming thus WAXS data. In the case of Cu10/M0 (calc) and
Cu /M0 (red) small copper particles were observed confined
2
-1
specific surface area along the series (996 m g ). With the
gradual increase in the amount of bis-silylated surfactant the
1
0
2
-1
inside the silica channels, though some particles appearing
larger than the pore diameter can be also observed. However,
specific area slightly decreases to 917 m g for M25 and then
2
-1
significantly decreases to 646 m g for M50, in line with the
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Cu /M25 materials display no distinguishable particles, even charge transfer of the type Ligand-to-Metal Charge Transfer
1
0
after the reduction process, clearly indicating that the particles (LMCT) which is characteristic of highly dispersed copper
2+
2-
47
are indeed highly dispersed. The TEM images recorded on (isolated) Cu -O in octahedral coordination environment.
Cu /M50 samples are qualitatively very similar to those of The Cu /M0 (a) displays a continuous absorption in the 300 –
1
0
10
Cu /M25 and for the sake of brevity are not shown here. 650 nm range which can be related to the presence of linear
1
0
2
+
Anyway, the TEM results are in excellent agreement with the oligonuclear Cu clusters (for example, species like (Cu-O-
corresponding WAXS analyses.
2
+ 48
Cu) ). The maximum observed for Cu /M25 (b) and
10
Cu /M50 (c) located between 600 and 800 nm, seen also in
1
0
Cu /M0 curve, was assigned to the d–d transition band of the
1
2
0
+
Cu ions into a pseudo-octahedral environment or charge-
transfer transitions from other lower-symmetry Cu species (e.g.,
4
7, 49, 50
square planar CuO , 1D Cu chains).
The spectroscopy
x
results represent therefore a confirmation on the presence of
highly dispersed CuO clusters of low nuclearity for Cu /M25
1
0
and Cu /M50 materials and of larger CuO particles for
1
0
Cu /M0.
1
0
The reducibility of the oxide-loaded materials was analysed via
TPR and the registered curves are illustrated in Figure 7. First
of all, the experimental hydrogen consumption was close to that
theoretically calculated taking into account the elemental
analysis of the calcined samples, and therefore the TPR profiles
reflect the total reduction of CuO to zero-valent metallic
copper. As stated in literature, the reduction temperature of
copper oxide depends on the particle size and also on the
dispersion, lower temperatures being associated with higher
5
1,52
dispersions.
The profiles for Cu /M0, Cu /M25 and
10 10
Figure 5. TEM images for (A) Cu10/M0 (calc), (B) Cu10/M0 (red), (C) Cu10/M25 Cu10/M50 (curves b, c and d) display reduction maxima in a
(
calc) and (D) Cu10/M25 (red)
o
narrow temperature range of 250-260 C, signifying the
reduction of copper cations in highly dispersed CuO particles.
In contrast, the TPR profile of Cu /M0-c shows a maximum of
1
0
o
reduction temperature shifted at ~275 C, suggesting the lowest
dispersion of CuO along the series.
Figure 6. DR UV-Vis spectra for the oxide form of materials: a-Cu10/M0, b-
Cu10/M25, c-Cu10/M50
Figure 7. TPR profiles of oxide forms of materials: a-Cu10/M0-c, b-Cu10/M0,
c-Cu10/M25, d-Cu10/M50
Cu /M0, Cu /M25 and Cu /M50 in their oxidized forms were
1
0
10
10
also investigated by diffused reflectance UV-Vis spectroscopy.
All the recorded spectra (Figure 6) display a absorption
maximum between 230 – 270 nm which can be assigned to
The results presented so far highlight the positive role of
polyether chains in stabilizing Cu ions upon impregnation,
most likely by coordination with oxygen atoms in the
2+
6
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5CY00428D
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Journal Name
Catalysis Science & Technology
ARTICLE
hydrophillic ethylene oxide –CH -O-CH - groups of data obtained over copper-based mesoporous materials, our
2
2
polyetheric surfactants. A similar stabilizing mechanism in the catalysts show very high dispersions and high surface areas of
case of CuO/SBA-15 materials was recently proposed by Chen catalytic active sites. For instance, at similar copper loading
1
4
2+
et al. involving the coordination of Cu ions with carboxylate (10.9 vs 10.6 %), Cu /M50 displays a much higher dispersion
1
0
−
COO groups. By controlling the content of the functionalized than Cu /SBA-15 prepared over mesoporous SBA-15
1
0
surfactant in synthesis gel, novel organic-inorganic SBA-15- functionalized with –COOH groups (57.5 vs 37 %) in line with
2
-1
13
like hybrids containing occluded P123 triblock polymer the higher copper surface area (42.4 vs 25 m Cu
g cat). Such a
covalently bound to the silica framework can be designed, comparison is particularly interesting to emphasize the
creating unique hydrophilic microenvironments between the effectiveness of our approach to fabricate by a simple method
silica wall and template aggregates, which can be further highly dispersed and stable nanosized copper nanoparticles
exploited as confined spaces to directly infiltrate hydrophilic supported on mesoporous host structures.
metal precursor species for a rapid functionalization. Upon
Table 3. EDXS and N
2
O chemisorption data for copper catalysts
surfactant removal by calcination, decomposition of copper
precursors to stabilized copper oxide takes place, resulting in
intra-porous copper species highly dispersed on the surface of
the SBA-15-like materials. On the other hand, taking into
account that another type of interaction may occur between
copper precursors with silica walls through Si-OH silanol
groups to stabilize the oxide phases, the density of these groups
may play also a crucial role. Indeed, our results show that as
compared with the uncalcined M0 support, which presumably
has high density of silanol groups (the surfactant was removed
by extraction with ethanol), the use of calcined M0 as support
EDXS
Cu
N
2
O chemisorption
Sample
D
Cu
d
(nm)
Cu
S
(m Cu
Cu
S
Cu
(wt. %)
2
-1
2
-1
(%)
g
Cu
)
(m Cu
g
cat
)
Cu10/M0-c
Cu10/M0
9.9
8.3
12.0
56
5.6
10.0
11.4
10.9
43.7
47.7
57.5
2.3
2.1
1.7
296
322
389
29.6
36.8
42.4
Cu10/M25
Cu10/M50
DCu - dispersion degree, dCu – average size of Cu particles, SCu – Cu surface
0
area
conducted to poorly dispersed CuO (and Cu ) particles (vide
infra) because of the dehydroxylation of silanols resulting in
less stabilizing surface groups, and favouring thus serious CuO
aggregation. Indeed, previous studies suggested that a high
copper dispersion on SBA-15 mesoporous silica is facilitated
Catalytic performance in hydrogenation of cinnamaldehyde
The catalytic performance of synthesized materials was
evaluated in the liquid phase hydrogenation of cinnamaldehyde
4
3, 53
when the surface is rich in silanol groups.
For example,
(
CNA) with the purpose of investigating the activity and
Wang et al. showed that abundant silanol groups in as-prepared
SBA-15 silica favors the enhancement of host–guest
interactions, leading to a higher dispersion level of the guest
selectivity in relation with the physico-chemical properties of
the samples. As seen in Scheme 1, the hydrogenation proceeds
in two major directions, the hydrogenation of C=C and C=O
double bonds, with the formation of hydrocinnamaldehyde
5
3
copper species, when compared with calcined SBA-15.
To evaluate copper dispersion, the titration of superficial
(
HCNA) and cinnamyl alcohol (CNOL), respectively. These
copper atoms with N O was performed. Table 3 illustrates the
2
two main compounds can be totally hydrogenated leading to the
formation of hydrocinnamyl alcohol (HCNOL).
metal loading degrees, dispersion degrees, Cu surface areas,
and Cu average particle sizes for all catalysts. EDXS analysis
confirms the loading degree, which is close to the calculated
ones within experimental errors (10 wt. %). As expected,
2
-1
Cu /M50 has the highest surface area (389 m g
Cu) and
Cu) and further for
g Cu), thus confirming the positive effect
1
0
Cu
2
-1
decreases for Cu /M25 (322
m
g
Cu
1
0
2
-1
Cu /M0 (296 m
Cu
1
0
of the bis-silylated surfactant used. The dispersion degree and
the average Cu particle size follow the same pattern as Cu
surface area, for Cu /M50 high dispersion (57,5 %) conducts
1
0
to a low particle size (1,7 nm), also for Cu /M25 decreasing
1
0
with 10 % the dispersion leads to higher particle size (2,1 nm).
The catalysts prepared without using Si-P123 displays a
dispersion degree of 43,7 % and 2,3 nm particle size, the lowest
dispersion and the highest particle size among the samples
prepared with uncalcined (extracted) supports. However, if the
support is subjected to calcination prior impregnation, the
dispersion degree falls with a factor of five to 8,3 %, while the
average particle size increases with a factor of 6 (12 nm),
Scheme 1. Reaction pathways for the hydrogenation of cinnamaldehyde
The progression of CNA conversion vs time under atmospheric
pressure (Patm) as well as 10 bar (P10) is illustrated in Figure
8. It can be observed that all catalysts derived from uncalcined
(
extracted) supports are highly active in hydrogenation, being
able to completely convert the substrate in a maximum of 300
min of reaction. In opposite, the catalyst obtained on a calcined
2
-1
conducting to a low Cu surface area (56,1 m
g
Cu). It is
Cu
finally worthy to note that as compared with relevant literature
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ARTICLE
Journal Name
M0 support manifested a very low catalytic activity (conversion outstanding catalytic activity in hydrogenation but low
of ~3 mol % in 360 min of reaction). The results are very selectivity towards CNOL, this is in agreement with other
interesting considering the mild reaction conditions, especially studies on copper-based catalysts. In addition, analysis of our
the atmospheric pressure. Under these conditions, the catalytic data as compared with literature highlights that copper
activity follows the order: Cu /M0-c << Cu /M0 < Cu /M25 supported on functionalized ordered mesoporous silica
1
0
10
10
2
-
<
Cu /M50, in fair agreement with the evolution of S (m g developed in this study are among the most active copper based
10 Cu
1
cat) (see Table 3). Indeed, Cu /M0-c showed a negligible catalysts, under similar reaction conditions of temperature and
10
catalytic activity due to the low capacity of poorly dispersed pressure. For instance, in 2011 Volpe et al. obtained a
copper to activate the hydrogen molecules. However, examples conversion of ~40 mol % on a Cu/MCM-48 (2,5 wt. % Cu)
from literature show that in order to have enhanced sample after 300 min when the reaction was conducted at 10
3
0
chemisorption capacity and hence superior intrinsic activity in atm. Also in 2012, the same team proved a decrease in
0
the hydrogenation of CNA, the number of low-coordinated Cu
conversion to ~25 mol % after 220 min with another Cu/MCM-
3
1
54
sites must be high, which can be achieved only for highly 48 (4,1 wt. % Cu). Marchi et al. with a Cu/SiO (8,1 wt. %)
2
3
8, 54-56
dispersed copper nanoparticles.
after 480 min arrived to a conversion of cinnamaldehyde of 58
Further improvement in the catalytic activity was made by mol % also in high pressure reaction conditions. In comparison,
increasing the hydrogen pressure. Thus, two samples with our catalysts completely converted the cinnamaldehyde after
extreme catalytic activity were tested under 10 bar H2, 300 min where the reaction was conducted under atmospheric
Cu /M0-c and Cu /M50, respectively. As expected, increasing pressure. Increasing the pressure of the reaction (10 bar), total
1
0
10
the pressure from atmospheric pressure to 10 bar concludes to conversion is achieved after 45 min. To carry out a
an enhanced catalytic activity, from 240 min of total conversion straightforward comparison with the results from literature, the
to 45 min of total conversion for Cu /M50 and from 3 mol % activity of Cu /M50 was also calculated under the form of
1
0
10
after 360 min to ~ 30 mol % after the same period for Cu /M0- turnover frequency (TOF, in moles of CNA converted per mole
1
0
-
3
-1
c.
of surface copper per unit time). A TOF value of 23·10 s was
obtained under 10 bar H , which was much higher than those
2
-
3
-1
values previously reported for Cu/MCM-48 (1·10 s ),
Cu/MCM-41 (2·10 s ), Cu/CeO (7.4·10 s ), and even for
unsupported Cu NPs (8·10
-
3
-1
-3 -1
2
-
3
-1 8
s ). Furthermore, under
atmospheric pressure, Cu /M50 demonstrated a high TOF
1
0
-
3
-1
value of 3·10 s , thus confirming the oustanding activity of
our catalysts.
Table 4. Selectivities for all the tested samples at 30 mol % isoconversion
Selectivity (mol %)
Run
Sample
HCNA
-
54
82
77
CNOL
-
43
12
13
HCNOL
1
2
3
4
5
6
Cu10/M0-c-Patm
Cu10/M0-c-P10
Cu10/M0-Patm
Cu10/M25-Patm
Cu10/M50-Patm
Cu10/M50-P10
-
3
6
10
10
15
Figure 8. Catalytic activity of copper catalysts: a-Cu10/M50-P10, b-Cu10/M50-
Patm, c-Cu10/M25-Patm, d-Cu10/M0-Patm, e-Cu10/M0-calc-P10 and f-Cu10/M0-c-
Patm
73
62
17
23
Patm - reaction was conducted at atmospheric pressure
The selectivities achieved for all tested samples are displayed in P10 – reaction was conducted at 10 atm hydrogen pressure
Table 4, as calculated at a CNA isoconversion of 30 mol %.
Exception shows Cu /M0-c-Patm (run 1), for which the In order to further enhance chemoselectivity of catalysts
1
0
selectivity was not calculated due to the low conversion level. It towards unsaturated alcohol, the metal form of Cu10/M50
can be observed that at atmospheric pressure, high selectivities catalyst was subjected to an oxidation treatment for 5 min with
o
59
towards saturated aldehyde HCNA (> 70 %) are obtained (runs synthetic air at 300 C. A TPR curve was registered before
-5). With the increase in the hydrogen pressure to 10 atm, the and after the oxidation treatment. The corresponding profiles
3
selectivities are still towards HCNA, but with a slight increase are displayed in Figure 9, noticing that the curve (a) is very
in the selectivity towards unsaturated alcohol CNOL from 17 to similar to that reported in Figure 7,d. It is clear that the oxidized
2
3 mol % for Cu10/M50 catalyst (run 5 vs run 6), as also sample shows a different reduction behaviour, characterized by
o
reported in other studies dealing with the effect of hydrogen an important peak with maximum at around 185 C. This can be
0
pressure on the catalytic performances of the supported metals explained by the fact that a part of surface Cu was oxidized to
5
7, 58
60
in the hydrogenation of α,β – unsaturated aldehydes.
It can be concluded that irrespective of pressure, finely temperature peak is attributed to the reduction of Cu to Cu , in
Cu O upon the applied treatment. Therefore, this low-
2
+
0
6
1, 62
dispersed metallic copper NPs on mesoporous silica manifest agreement with previous studies.
8
| J. Name., 2012, 00, 1-3
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ARTICLE
Figure 9. TPR profiles for Cu10/M50 catalysts: a- before oxidation, b- after
Figure 11. Recycling tests for Cu10/M50 catalyst (CNA conversion after 45 min of
oxidation.
reaction; selectivities at 30 mol % isoconversion)
rd
The catalytic performance of the material prepared is this way after the 3 run (Figure 11), in line with the minor decrease in
was compared with that of Cu /M50 catalyst without the the dispersion degree of copper measured for the corresponding
1
0
oxidation treatment. The catalytic results obtained under spent catalyst, as compared with the fresh sample (56.1 vs 57.5
atmospheric pressure are displayed in Figure 10 for Cu /M50 %). These results demonstrate the possibility to reuse the spent
1
0
without oxidation (A) and after oxidation treatment (B). It can catalysts as well as the good stability of copper nanoparticles
be observed that the selectivity profiles are totally different. synthesized on polyether-functionalized ordered mesoporous
The original catalyst shows selectivity towards HCNA higher silica after repeating calcination-reduction cycles.
than 60 mol % at all conversion levels, whereas the selectivity
to CNOL is the utmost of ~30 mol % at low conversion levels Conclusions
(
~ 20 mol %) and continuously decreases to null with the
Polyether-functionalized SBA-15-like mesoporous materials
were successfully used as host structures to prepare highly
dispersed copper nanoparticles below 2 nm in size, at high
metal loading of 10 wt. %, with outstanding dispersion degrees
of up to 57.5 %. A strong dependence of the amount of Si-P123
used for the support synthesis on the morpho-structural
properties of materials was demonstrated by various techniques.
Increasing the amount of functionalized surfactant up to 50 wt.
% leads to an increase in copper active surface area, a decrease
in the average particle, in line with enhanced dispersion
degrees. The copper supported catalysts manifest excellent
activity in the cinnamaldehyde hydrogenation reaction and high
selectivity towards saturated aldehyde (>70 mol %), under
atmospheric pressure conditions. However, improvement in
both activity and selectivity towards the unsaturated alcohol (~
increase in the conversion. In contrast, the oxidized Cu /M50
1
0
catalyst shows higher selectivity to CNOL of ~ 60 mol % at ~
0 mol % conversion, yet the reaction rate is strongly
5
diminished (conversion of ~ 50 mol % after 1600 min of
reaction). These interesting results can be easily rationalized by
considering that upon oxidative treatment, Cu O decorating
metallic Cu NPs is obtained, generating cooperative metal-
electron deficient sites (Cu /Cu ) that could activate the
molecular hydrogen (Cu ) and carbonyl group of unsaturated
aldehyde (Cu ), respectively and improve chemoselectivity, at
the expense of decreasing activity owing to the reduction of the
number of metal active sites.
2
0
0
+
0
+
6
3-66
60 mol %) was attained by increasing the hydrogen pressure
and by applying a two-step reduction-oxidation pre-treatment,
0
+
to obtain cooperative metal-electron deficient Cu /Cu sites. In
addition, the catalysts were reusable, with no significant
changes in catalytic performance and the degree of copper
dispersion after three catalytic runs.
Figure 10. Catalytic activity and selectivity profiles for Cu10/M50 catalysts: A- before Acknowledgements
oxidation, B- after oxidation for 5 min. Selectivity is plotted vs conversion.
This work was partially supported by two grants of the
Romanian National Authority for Scientific Research, CNCS-
Finnaly, the reusability of Cu /M50 material was evaluated UEFISCDI (project numbers PN-II-RU-TE-2012-3-0403 and
1
0
PN-II-ID-PCE-2011-3-0868).
upon three catalytic runs. An essentially constant
chemoselectivity and a slight decrease in activity were observed
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