"Imprinting" catalytically active pd nanoparticles onto ionic-liquid-modified Al2O3 supports

Full text of DOI:10.1002/cctc.201300123

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DOI: 10.1002/cctc.201300123
“Imprinting” Catalytically Active Pd Nanoparticles onto
Ionic-Liquid-Modified Al2O3 Supports
Leandro Luza,[a] Aitor Gual,[a] Dario Eberhardt,[b] Sꢀrgio R. Teixeira,[b] Sandra S. X. Chiaro,[c]
and Jairton Dupont*[a]
A new sputtering chamber that allows the constant mixing of
Introduction
the solid support during sputtering by using an electro-mag-
netic oscillator was developed for the generation of metal
nanoparticles (top down approach) uniformly distributed over
the solid supports. By using this new chamber, small and well-
distributed Pd nanoparticles (Pd-NPs) of 2.6 or 4.3 nm were
produced over Al2O3 and new imidazolium ionic liquids cova-
lently supported on Al2O3 by simple sputtering from a Pd foil.
The Pd-NPs uniformly distributed over the solid supports dis-
play comparable catalytic performances in the hydrogenation
of 1,3-cyclohexadiene and 1,3-cyclooctadiene to that achieved
by using a catalyst prepared by conventional chemical meth-
ods (bottom up approach). Mechanistic and labelling studies
show that the hydrogenation of 1,3-cyclohexadiene catalysed
by Pd-NPs occurs via meta-stable p-allyl intermediates, charac-
teristic of homogeneous-like catalytically active sites, and dis-
proportionation through the outer-sphere mechanism. The
ratio of hydrogenation to disproportionation is dependent on
the Pd-NP size, and the disproportionation products are more
pronounced with small NPs because of the higher affinity of
dienes for this size of particle. Therefore, the surface structural
features of small Pd-NPs facilitate the arrangement of diene
molecules in the correct geometry for the transfer of H from
the donor to the acceptor sites, typical of poly-metallic catalyt-
ically active sites. It is proposed that the reaction of 1,3-cyclo-
hexadiene under H2 can be used to probe the homogeneous/
heterogeneous nature of supported metal NPs. It is demon-
strated for the first time that highly active and selective nano-
catalysts were obtained by using the sputtering-deposition
technique (top down approach) and this opens a new window
of opportunity for the preparation of size-controlled metal NPs
with clean surfaces.
Transition metal nanoparticles (M-NPs) are currently used as ef-
fective catalysts for hydrogenation processes in which homo-
geneous and traditional heterogeneous catalysts are less effi-
cient.[1,2] Modern soluble M-NPs are commonly synthesised by
chemical methods, which involve the reduction of metallic
salts or the decomposition of organometallic complexes in the
presence of stabilising agents.[1–5] Recently, the use of sputter-
ing-deposition techniques was revealed as an interesting alter-
native for the synthesis of soluble M-NPs with clean surfaces
under mild conditions with easy control of their size and shape
by appropriate tuning of the sputtering conditions.[6–15] This
approach allows the rapid synthesis of M-NP catalysts by the
deposition of metals on support surfaces or the fast generation
of M-NPs in solution by using low volatility liquids such as sili-
cones, triglycerides, and ionic liquids (ILs). This method has
been widely applied for the formation of thin (nano-sized)
films onto supports;[16] however, it is limited as it is extremely
difficult to prepare uniformly distributed M-NPs on solid
catalytic supports.[17]
Among various types of agents for stabilising soluble M-NPs,
ILs[18] are extensively employed because of their high efficiency
in hydrogenation processes.[4] Recently, the use of supported
ionic liquid phases (SILPs) is receiving growing interest as the
IL surface area is increased relative to its volume and the sub-
strate can readily diffuse to the catalysts, overcoming the
mass-transfer limitations produced by the high viscosity of the
IL.[19–22] The SILP system uses much smaller amounts of IL than
those required under liquid–liquid bi-phasic conditions, and
this behaviour is of crucial importance for reactions that in-
volve gases with poor solubility in ILs (e.g., H2 and CO). There-
fore, catalysis that uses a SILP as a support combines the posi-
tive effects of ILs on the catalytic performance with the high
stability of the catalysts, good diffusion of the reactants and
easy product separation displayed by heterogeneous catalysts.
The most widely used ILs in SILP applications are imidazolium
ILs, which combine the high solvation power of polar species
with their weak coordination strength, making them suitable
for reactions with electrophilic catalysts.[21,22] Silica is the most
widely used support for the immobilisation of M-NPs owing to
its high surface area and the presence of OH, which facilitates
the anchoring of stabilising agents, whereas the use of other
supports has been seldom reported.[23] To the best of our
knowledge, the use of Al2O3-based supports, which offer high
[a] L. Luza, Dr. A. Gual, Prof. Dr. J. Dupont
Institute of Chemistry, UFRGS
Avenida Bento GonÅalves, 9500
Porto Alegre 91501-970 RS (Brazil)
Fax: (+55)5133087304
E-mail: jairton.dupont@ufrgs.br
[b] D. Eberhardt, Prof. Dr. S. R. Teixeira
Institute of Physics, UFRGS
Avenida Bento GonÅalves, 9500
Porto Alegre 91501-970 RS (Brazil)
[c] Dr. S. S. X. Chiaro
Petrobras, CENPES-PETROBRAS
Rio de Janeiro 21941-915 RJ (Brazil)
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stability at high pH, in a SILP system has not been explored
until now.
We report the synthesis and characterisation of imidazolium
ILs covalently immobilised on Al2O3 surfaces. Pd-NPs were uni-
formly distributed on these supports by using a sputtering-
deposition technique. Furthermore, the hydrogenation of
1,3-cyclohexadiene was used to probe the surface properties
of these new SILP Pd-NPs. The hydrogenation of 1,3-cyclohexa-
diene was chosen as a benchmark reaction as it is a structure-
sensitive reaction[24] and its selectivity (hydrogenation vs. dis-
proportionation) can be used as a fingerprint for the homoge-
neous or heterogeneous nature of the Pd-NPs surface. More-
over, the partial hydrogenation of p-conjugated systems, such
as 1,3-dienes, is a process of growing interest because of the
formation of high-added-value alkenes conventionally used as
building blocks in organic synthesis.[25]
Scheme 1. Synthesis of M1–M4.
Results and Discussion
To obtain uniformly distributed M-NPs in both solid and liquid
substrates, a new sputtering chamber has been developed by
our group. This new chamber contains an electro-magnetic os-
cillator (with variable controlled frequency), which allows the
constant movement of the conical flask that contains the
support (Figure 1).[26,27]
Figure 2. 13C CP-MAS NMR spectra of a) M1, b) M2, c) M3 and d) M4.
136 ppm were assigned to the three imidazolium C atoms
(Figure 2). Thus, these NMR signals confirmed the presence of
the unaltered imidazolium ILs in M1–M4. No signals attributed
to the methoxy leaving groups (65 ppm) of the silane function
were observed, which confirms that the trimethoxysilane
group effectively reacted with the hydroxyl groups of the
Al2O3 surface under our reaction conditions.
Figure 1. Schematic representation of the sputtering-deposition chamber
used for the immobilisation of Pd-NPs onto Al2O3 supports.
Synthesis and characterisation of the modified Al2O3
supports
FTIR analysis of M1–M4 showed C=N stretching vibration
bands of the imidazolium rings and CÀH stretching vibration
bands of the alkyl groups at 1635 and 2950 cmÀ1, respectively
(Figure 3). Thus, these IR signals also confirm the presence of
the unaltered imidazolium ILs in M1–M4. Furthermore, the
The M1 support was synthesised by the reaction of 1-methyl-
3-(trimethoxysilylpropyl)imidazolium chloride (1) with the hy-
droxyl groups of the Al2O3 surface and further ion-exchange
treatment according to modified literature procedures[28] was
performed to obtain the M2–M4 supports that contained
À
FTIR spectra of M2–M4 display signals attributed to NTf2
À
À
À
NTf2À, PF6 and BF4 counter-ions, respectively (Scheme 1).
13C cross-polarisation magic angle spinning (CP-MAS) NMR
spectra displayed four sets of upfield broad peaks (10, 25, 36
and 52 ppm), which could be assigned to the propylene and
methylene functions, whereas the resonances at 123 and
(1058, 1145, 1220 and 1350 cmÀ1), PF6 (741 and 841 cmÀ1) and
BF4 (763, 1039 and 1061 cmÀ1), respectively, which confirm
À
the efficiency of the ClÀ-exchange procedure.[29,30]
N2 physisorption analysis revealed type-IV-like isotherms
with pore diameters of 6.6–7.1 nm for the unmodified Al2O3
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À
nounced for the supports that contained PF6 and
À
BF4 (40 and 50% T3, respectively), which decom-
À
posed more easily than NTf2 (10% T3; Figure 4).
Therefore, M1 and M2 should be appropriate sup-
ports for the deposition of Pd-NPs by sputtering as
the IL is much more uniformly distributed on their
surfaces than on the surfaces of M3 and M4.
Sputtering deposition of Pd-NPs onto Al2O3-based
supports
Al2O3, M1 and M2 were selected as suitable materials
for the deposition of Pd-NPs by the sputtering tech-
nique at 100 W for 1.5 min. These supported catalysts
were analysed by using inductively coupled plasma
optical emission spectroscopy (ICP-OES), which deter-
mined Pd contents of 0.11Æ0.01, 0.10Æ0.01 and
0.12Æ0.02 wt.%, respectively. TEM analysis revealed
the formation of small NPs (2.6Æ1.3, 2.7Æ1.1 and
4.3Æ0.6 nm, respectively) uniformly distributed on
the supports (Figure 5).
Figure 3. FTIR spectra of a) M1, b) M2, c) M3 and d) M4.
Table 1. Characterisation of the supports.
It is clear that similar Pd-NP sizes were obtained on
Al2O3 and M1, whereas M2 displayed larger NPs. This
behaviour suggests that the size of the NPs is con-
Support
SBET
[m2 gÀ1
Pore volume
[cm3 gÀ1
Pore diameter
[nm][a]
Organic content
[b]
]
]
[mmolIL gÀ1
]
Al2O3
M1
M2
M3
M4
195
134
127
125
80
0.47
0.31
0.28
0.30
0.20
7.1
6.9
6.6
7.0
7.1
–
0.46
0.36
0.39
0.38
[a] Determined by using Tristar 3020 Micromeritics equipment. The specif-
ic surface areas were determined by the BET multipoint method and the
average pore size was obtained by BJH method; [b] Calculated on the
basis of the nitrogen content, which was determined by elemental analy-
sis by using a CHN Perkin–Elmer M CHNS/O Analyzer, model 2400.
and M1–M4 (Table 1). The M1–M4 supports displayed lower
surface areas (SBET) and pore volumes than unmodified Al2O3,
which indicate that the ILs filled the pores. Similar behaviour
was observed for the ILs covalently supported on SiO2.[31] Ele-
mental analysis revealed that M2–M4 contained slightly lower
amounts of IL than M1, which suggested that the IL was re-
moved from these supports during the ClÀ-exchange step
(Table 1).
The effect of ClÀ exchange on the support structure was in-
vestigated by 29Si CP-MAS NMR analysis (Figure 4). The spec-
trum of M1 displays typical signals of Si atoms attached to one
C atom and to the Al2O3 surface at À46 and À55 ppm as-
signed to the T1 (20%) and T2 (80%) species, respectively.[32]
After ClÀ exchange, a new signal appeared at À65 ppm in the
spectra of M2–M4, which was attributed to the formation of
T3 and suggests that re-arrangement processes operate under
the reaction conditions of the ClÀ-exchange step. This behav-
iour could be ascribed to the formation of HF by the hydrolysis
of the F-based anions.[33] This behaviour was much more pro-
Figure 4. 29Si CP-MAS NMR spectra of a) M1, b) M2, c) M3 and d) M4.
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yield Pd*/M2 (Figure 5). Pd*/M2 was tested in the hy-
drogenation of 1,3-cyclohexadiene and produced
a slightly lower activity and higher benzene selectivi-
ty than those of Pd/M2 (Table 2, entry 3 vs. 6). How-
ever, analysis of the reaction kinetics revealed that
Pd*/M2 displayed an incubation period probably
caused by the less clean NP surfaces obtained by
conventional methods (Figure 7). The substrate scope
was investigated by performing the hydrogenation of
1,3-cyclooctadiene catalysed by Pd/M2 at 408C,
which produced exclusively cyclooctene with a turn-
over frequency (TOF) of up to 5.9 sÀ1 (Figure 8).
For comparison, Pd(acac)2 and Pd-NPs dissolved in
BMI·NTf2 were also tested as catalysts for the same
reaction (IL/Pd ratio=15, 1,3-cyclohexadiene/Pd
ratio=4500, 4 bar H2, 408C) and lower 1,3-cyclohexa-
diene conversions were obtained (up to 10% after
24 h for the homogeneous system and up to 75%
after 3 h for the IL-soluble NPs). These results demon-
strate that the true active species are Pd-NPs and
Figure 5. TEM images and histograms of a) Pd/Al2O3, b) Pd/M1, c) Pd/M2 and d) Pd*/M2.
trolled by the nature of the anions, and larger NPs are ob-
tained by increasing the non-polar domains of the anions, akin
to M-NPs prepared in pure ILs.[13] Indeed, the Pd-NPs dispersed
in pure BMI·NTf2 (BMI=1-butyl-3-methylimidazolium) by sput-
tering under the same reaction conditions and Pd*/M2 syn-
thesised by the H2 reduction of Pd(acac)2 (acac=acetylaceton-
ate) displayed similar sizes (4.3 and 5.1 nm, respectively).
their immobilisation onto Al2O3 improved their catalytic activity
with no effect on their product selectivity.
Catalysis of the partial hydrogenation of 1,3-dienes by
PD-NPs
These new nano-catalysts prepared by sputtering techniques
were first tested in the selective hydrogenation of 1,3-cyclo-
hexadiene under 4 bar of H2 at 408C as this reaction is com-
monly used as a benchmark test for the evaluation of the ac-
tivity and selectivity of Pd-NPs used as hydrogenation
catalysts.[34–39]
The catalysts synthesised by sputtering had quite high activ-
ities (ca. 15.8–20.1 sÀ1; Table 2, entries 1–5). These catalysts
formed cyclohexene (4) with high selectivity (up to 95%) to-
gether with significant amounts of benzene (3; ca. 5–11%) as
a by-product (which originates from the hydrogen dispropor-
tionation of the diene; Table 2, entries 1–3; Figures 6 and 7),
which is not commonly reported in the literature as attention
is usually only paid to the partial hydrogenation selectivity (cy-
clohexene vs. cyclohexane formation rate).[34–39] This behaviour
was much less pronounced for Pd-NPs supported on M2, prob-
ably because of their larger sizes compared to those supported
on Al2O3 and M1 (Table 2, entries 1–3 and Figures 6 and 7).
This is a strong indication of the higher diene affinity for the
smaller NP surfaces, which may allow competition between
the hydrogenation and disproportionation pathways.[40,41] The
catalytic activity was affected by the reaction temperature,
with activities of up to 20.1 sÀ1 obtained at 608C with small
variations in the product selectivity (Table 2, entries 3–5).
For comparison, Pd-NPs of similar sizes supported on M2
were synthesised by H2 reduction of Pd(acac)2 precursors to
Figure 6. Hydrogenation of 1,3-cyclohexadiene: t [s] vs. selectivity [%] for
~
&
cyclohexene catalysed by Pd/Al2O3 ( ) and Pd/M1 ( ) and t [s] vs. 1,3-cyclo-
~
&
hexadiene conversion [%] catalysed by Pd/Al2O3 ( ) and Pd/M1 ( ).
Figure 7. Hydrogenation of 1,3-cyclohexadiene: t [s] vs. selectivity [%] for
~
&
cyclohexene with Pd/M2 ( ) and with Pd*/M2 ( ) and t [s] vs. 1,3-cyclohexa-
~
&
diene conversion [%] catalysed by Pd/M2 ( ) and with Pd*/M2 ( ).
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Table 2. Hydrogenation of 1,3-cyclohexadiene by Pd-NPs.
Entry[a,b]
Catalyst
Conversion
[%] (t [h])
3
[%]
4
[%]
5
[%]
TOF
[c,d]
[sÀ1
]
1
2
3
4[e]
5[f]
6[g]
Pd/Al2O3
Pd/M1
Pd/M2
Pd/M2
Pd/M2
Pd*/M2
100 (0.13)
100 (0.17)
100 (0.75)
100 (0.75)
100 (0.50)
100 (0.75)
11
11
5
6
6
87
88
95
92
93
90
2
1
0
2
1
1
19.1
18.2
15.8
13.2
20.1
14.2
9
[a] Reaction conditions: Pd (0.19 mmol), 1,3-cyclohexadiene/Pd=4500,
CH2Cl2 (10 mL), H2 (4 bar), 408C; [b] Conversion and selectivity deter-
mined by GC analysis; [c] TOF=mol 1,3-cyclohexadiene converted/(mol
Pd surfaceꢂtime); [d] Calculated from the slope of plots of TON vs. t at
low substrate conversions;[42] [e] 208C; [f] 608C; [g] Synthesised by H2
reduction of Pd(acac)2.
Figure 9. Hydrogenation of 1,3-cyclohexadiene with Pd/M2: t [s] vs. conver-
~
&
sion [%] for cyclohexene with H2 ( ) and with D2 ( ).
Figure 8. Hydrogenation of 1,3-cyclooctadiene catalysed by Pd/M2: t [s] vs.
Scheme 2. Proposed mechanism for the Pd-NP-catalysed hydrogenation of
1,3-cyclohexadiene.
~
selectivity [%] for cyclooctene ( ) and t [s] vs. 1,3-cyclooctadiene
~
conversion [%] ( ).
The 2H NMR spectra performed with relaxation delays of
1–10 s displayed two broad signals at 1.95 and 1.56 ppm as-
signed to 6 and 7 together with a signal attributed to 8 at
1.37 ppm (Figure 10).
Mechanistic studies
Mechanistic studies were performed to elucidate the reaction
pathways involved in the hydrogenation and disproportiona-
tion of 1,3-cyclohexadiene catalysed by Pd/M2.
First, the reaction kinetics from using H2 and D2 were com-
pared, and it was found that a lower activity was obtained
with D2 (TOFs of up to 15.8 and 11.5 sÀ1 for H2 and D2, respec-
tively), which indicates a small kinetic isotopic effect under the
reaction conditions employed (Figure 9) probably as a result of
the involvement of HÀ/DÀ transfer in two steps of the
hydrogenation pathway (Scheme 2).
By analysing the integral ratio between these two signals,
we estimated a 6/7 ratio of 3.5:1. The 1H NMR spectra, also
performed with relaxation delays of 1–10 s, displayed signals
attributed to 3 (7.30 ppm) and 8 (1.37 ppm) together with
three broad signals at 5.62, 1.94 and 1.55 ppm assigned to
a mixture of 4, 6 and 7 (Figure 11). Analysis of the integral
ratios between the signals at 1.94 and 1.55 ppm with respect
to the signal at 5.62 ppm gave values of 1.44 and 1.61, which
confirms the formation of two [D2]cyclohexene isomers (6 and
7). Quantification of the reaction products by using GC analysis
Furthermore, a higher formation of 3 (up to 8%) and lower
selectivity to 4 (up to 90%) were obtained by using D2 as a re-
actant (Figure 9). GC–MS analysis of the reaction products re-
vealed the formation of [D2]cyclohexene (6 and 7) and
[D4]cyclohexane (8) and no incorporation of D in the benzene
1
2
together with analysis of the H and H NMR integral ratios and
their comparison with a standard sample of non-deuterated
cyclohexene allowed us to estimate the composition of the
product distribution as 8% of 3, 8% of 4, 64% of 6, 18% of 7
and 2% of 8.
ring. NMR analysis (1H, H, 13C, COSY and HSQC) confirmed the
2
formation of 8 and that there was no incorporation of D in the
benzene ring.
The 13C NMR spectra displayed four upfield signals attributed
to the cyclohexene structural isomers (6 and 7; 126.9, 126.8
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Figure 12. 13C NMR spectra of the reaction products obtained in the Pd/M2-
catalysed reduction of 1,3-cyclohexadiene with D2.
Figure 10. 2H NMR spectra of the reaction products obtained in the Pd/M2-
catalysed reduction of 1,3-cyclohexadiene with D2.
Scheme 3. Proposed mechanism for the Pd-NP-catalysed disproportionation
of 1,3-cyclohexadiene.
tected in the disproportionation of 1,3-cyclohexadiene per-
formed by using Pd/M2 pre-treated with D2. Interestingly, the
activities found for the catalyst with or without pre-treatment
with H2 were similar (TOFs of up to 1.75 and 1.62 hÀ1, respec-
tively), which suggests that H2 is not directly involved in the re-
action mechanism (Figure 13). However, the catalyst pre-treat-
ed with D2 displayed a lower activity (TOF of up to 1.17 hÀ1),
which indicates the presence of kinetic isotopic effects. As
there is no incorporation of D in the products, the isotopic
effect may be ascribed to the changes in the NP structure in-
Figure 11. 1H NMR spectra of the reaction products obtained in the Pd/M2-
catalysed reduction of 1,3-cyclohexadiene with D2.
and 126.7 ppm) and 3 (128.0 ppm; Figure 12). Signals attrib-
uted to the C atoms labelled with D with coupling constants
of 20 Hz were observed at 24.5, 24.4 and 21.8 ppm together
with signals attributed to non-deuterated C atoms at 24.8, 22.3
and 22.2 ppm. These observations strongly suggest the occur-
rence of two distinct pathways: one for the reduction that in-
volves the species that results from H2 and a second H transfer
of the 1,3-cyclohexadiene (disproportionation). The H (from H2)
for the reduction of the 1,3-cyclohexadiene ring probably in-
volves the formation of meta-stable p-allyl intermediates
(Scheme 2). This type of pathway that involves p-allyl inter-
mediates is typically observed in the hydrogenation of 1,3-
dienes by mono-metallic Pd catalyst precursors.[43,44]
The disproportionation of the diene most probably occurs
by a concerted step in which one H atom is transferred from
one diene to another (outer-sphere-like mechanism)[45] as
equal amounts of non-deuterated 3 and 4 (Scheme 3) are ob-
served under D2. Moreover, non-deuterated 3 and 4 were de-
Figure 13. Disproportionation of 1,3-cyclohexadiene with Pd/M2: t [h] vs.
~
&
*
conversion [%] with H2 ( ), without H2 ( ) and with D2 ( ).
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rett–Joyner–Halenda (BJH) method. Solid-state 13C and 29Si NMR
measurements of M1–M4 were performed by using a Bruker 400
spectrometer at the Universidade Nova de Lisboa. FTIR spectra
were obtained by using a ABB FTLA 2000 instrument with a resolu-
tion of 4 cmÀ1 with 128 cumulative scans. TEM samples were pre-
pared by the slow evaporation of a drop of each colloidal solution
deposited under an Ar atmosphere onto a holey C-covered Cu
grid. The TEM experiments were performed by using a JEOL-JEM
1200ExII electron microscope operating at 120 kV. GC analyses
were performed by using an Agilent Technologies GC System 6820
with a DB-17 column (oven temperature 408C). GC–MS analyses
were performed by using a Shimadzu QP50 instrument with a Rtx-
5 MS column (oven temperature 408C) that employed an ionising
duced by the absorption of H2 or D2. The presence of such
species may induce structural changes that may or may not fa-
cilitate the arrangement of diene molecules in the correct ge-
ometry for the transfer of H from the donor to the acceptor
sites. Indeed, such structural changes were suggested several
years ago,[46] and more recently it has been demonstrated that
H2 and HÀ species on the surface of small M-NPs strongly
affect catalytic reactions.[47]
Conclusions
2
voltage of 17 eV.[49] 1H, H, 13C NMR, COSY and HSQC analysis of the
The synthesis and characterisation of imidazolium ILs covalent-
ly supported on Al2O3 are reported. Immobilisation of Pd-NPs
on these supports can be rapidly and easily accomplished by
simple sputtering from a Pd foil by using our new sputtering
chamber. This chamber allows the constant mixing of the solid
support during sputtering by using an electro-magnetic oscilla-
tor. The nano-catalysts prepared by this technique are uniform-
ly distributed over the solid supports and display comparable
catalytic performance in the hydrogenation of 1,3-cyclohexa-
diene and 1,3-cyclooctadiene to those achieved by using a simi-
lar catalyst prepared by using conventional methods. It has
been demonstrated for the first time that highly active and se-
lective nano-catalysts were obtained by using the sputtering-
deposition technique and this opens a new window of oppor-
tunity for the preparation of size-controlled metal NPs with
clean surfaces. Mechanistic studies show two different path-
ways: the hydrogenation and disproportionation of 1,3-cyclo-
hexadiene catalysed by Pd/M2 occur via meta-stable p-allyl in-
termediates—typical of homogeneous-like catalytically active
sites—and disproportionation through the outer-sphere mech-
anism—characteristic of poly-metallic catalytically active sites.
Therefore, it is reasonable to propose that the reaction of
1,3-cyclohexadiene under H2 can be used to probe the homo-
geneous/heterogeneous nature of supported metal NPs. More-
over, the appearance of significant amounts of disproportiona-
tion products (benzene nucleus) on the hydrogenation of cy-
clohexadienes is a strong indication of the presence of small
Pd-NPs. Indeed, the ratio of hydrogenation to disproportiona-
tion is dependent on the Pd-NP size, and disproportionation
products are more pronounced with small NPs because of the
higher affinity of dienes for this size of particle.
samples obtained by D2 reduction of 1,3-cyclohexadiene catalysed
by Pd/M2 were performed by using a Varian 400 MHz spectrome-
ter at the CNANO/UFRGS. The incorporation of D in the reaction
products was quantified by comparing the 1H NMR spectra with
these obtained from a standard sample (relaxation delay=1–10 s)
and by performing 2H NMR experiments (relaxation delay 10 s).[50]
Synthesis of M1–M4
Compound 1 (1.0 g, 3.56 mmol) was dissolved in dry CH3CN
(25 mL) and added to dry Al2O3 (5.0 g). The suspension was kept at
908C under Ar and vigorously stirred for 72 h. The IL-functionalised
Al2O3 was washed, centrifuged and dried to yield M1. Based on the
amount of 1 on the M1 support, an excess (1.2 equiv) of LiNTf2,
KPF6 or NaBF4 was dissolved in de-ionised water (25 mL) and
added to M1 (1.0 g) to exchange the ions. The suspensions were
vigorously stirred for 48 h. The mixtures were washed, centrifuged
and dried to yield M2, M3, and M4.
Sputtering deposition of Pd-NPs
As a general procedure, Al2O3, M1 or M2 (1.0 g) was placed in
a conical Al flask and placed inside the vacuum chamber. The
chamber was closed and its pressure lowered to a base pressure of
4 mbar, and the support was evacuated for 4 h. Then the vacuum
chamber was placed under a sputtering working pressure of
4 mbar by adding Ar flow. The supports were continuously homo-
genised by revolving the Al flask at a frequency of 24 Hz. The Pd
was sputtered onto the revolving support at 100 W power for
1.5 min. After the deposition, the chamber was vented with N2,
and the grey powder was recovered and stored under Ar for their
further characterisation and application.
Experimental Section
General methods
All syntheses were performed by using standard Schlenk tech-
niques under Ar. CH3CN and CH2Cl2 were purified according to
standard procedures.[48] The Al2O3 used in this study was provided
by Petrobras. The IL 1 was prepared according to a procedure de-
scribed previously.[28] Elemental analysis of the ILs immobilised on
the support surfaces was carried out by using a CHN Perkin–Elmer
M CHNS/O Analyzer, model 2400. The N2 isotherms of the supports,
previously de-gassed at 1008C under vacuum for 3 h, were ob-
tained by using a Tristar 3020 Micromeritics instrument. The specif-
ic surface areas were determined by using the BET multi-point
method, and the average pore size was obtained by using the Bar-
Synthesis of Pd-NPs by H2 reduction of Pd(acac)2
As a general procedure, M2 (1.0 g) and the appropriate amount of
a solution of Pd(acac)2 in CH3OH (5 mL) were added to a Fischer–
Porter reactor. The reactor was pressurised with 4 bar of H2 and
warmed to 758C. After 5 min, the solution became black, which
confirmed the formation of Pd-NPs. The reaction was maintained
under these conditions for 3 h to ensure the total reduction of the
Pd precursor before the reactor was cooled and de-pressurised.
After centrifugation of the reaction mixture, Pd*/M2 was isolated
by simple decantation.
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nanoparticles · supported catalysts
· hydrogenation · palladium ·
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Received: February 14, 2013
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FULL PAPERS
Going against tradition: Uniformly dis-
tributed Pd nanoparticles on imidazo-
lium-ionic-liquid-modified Al2O3 surfaces
are prepared by a top down approach
by using a new sputtering chamber. The
hydrogenation of 1,3-cyclohexadiene is
used to probe the surface properties of
these new Pd nanoparticles.
L. Luza, A. Gual, D. Eberhardt,
S. R. Teixeira, S. S. X. Chiaro, J. Dupont*
&& – &&
“Imprinting” Catalytically Active Pd
Nanoparticles onto Ionic-Liquid-
Modified Al2O3 Supports
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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9
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These are not the final page numbers!