Sputtering-deposition of Ru nanoparticles onto Al2O3 modified with imidazolium ionic liquids: Synthesis, characterisation and catalysis

Article abstract of DOI:10.1039/c4dt03039g

Well-distributed Ru nanoparticles (Ru-NPs) were produced over Al₂O₃ supports modified with covalently anchored imidazolium ionic liquids (ILs) containing different anions and cation lateral alkyl chain lengths by simple sputtering from a Ru foil. These Ru-NPs were active catalysts for the hydrogenation of benzene. Furthermore, depending on the nature of the IL used to modify the support (hydrophilic or hydrophobic), different catalytic behaviours were observed. Turnover numbers (TON) as high as 27000 with a turnover frequency (TOF) of 2.73 s^-1 were achieved with Ru-NPs of 6.4 nm supported in Al₂O₃ modified with an IL containing the N(SO₂CF₃)₂^- anion, whereas higher initial cyclohexene selectivities (ca. 20% at 1% benzene conversion) were attained for Ru-NPs of 6.6 nm in the case where Cl^- and BF₄^- anions were used. Such observations strongly suggest that thin layers of ILs surround the NP surface, modifying the reactivity of these catalytic systems. These findings open a new window of opportunity in the development of size-controlled Ru-NPs with tuneable reactivity. This journal is

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ARTICLE TYPE
Sputtering-deposition of Ru nanoparticles onto Al2O3 modified with
imidazolium ionic liquids: Synthesis, characterisation and catalysis
Lucas Foppa,a Leandro Luza,aAitor Gual, a Daniel E. Weibel,a Dario Eberhardt,b Sérgio R. Teixeiraband
Jairton Dupont a,c*
5
Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Wellꢀdistributed Ru nanoparticles (RuꢀNPs) were produced over Al2O3 supports modified with covalently
anchored imidazolium ionic liquids (ILs) containing different anions and cation lateral alkyl chain lengths
by simple sputtering from a Ru foil. These RuꢀNPs were active catalysts for the hydrogenation of
10 benzene. Furthermore, depending on the nature of the IL used to modify the support (hydrophilic or
hydrophobic), different catalytic behaviours were observed. Turnover numbers (TON) as high as 27,000
with a turnover frequency (TOF) of 2.73 sꢀ1 were achieved with RuꢀNPs of 6.4 nm supported in Al2O3
modified with an IL containing the N(SO2CF3)2− anion, whereas higher initial cyclohexene selectivities
−
(ca. 20% at 1% benzene conversion) were attained for RuꢀNPs of 6.6 nm in the case where Cl− and BF4
15 anions were used. Such observations highly suggest that thin layers of IL surround the NPs surface,
modifying the reactivity of these catalytic systems. These findings open a new window of opportunity in
the development of sizeꢀcontrolled RuꢀNPs with tuneable reactivity.
since the ILs surface area is increased relative to its volume and
the substrate can readily diffuse to the catalysts, overcoming the
50 mass transfer limitations produced by the high viscosity of the
IL.21, 22 The SILP system uses much smaller amounts of IL than
those required under liquid/liquid biphasic conditions, and this
behaviour is of crucial importance for reactions involving gases
with poor solubility in IL (for example H2 and CO). Therefore,
55 catalysis using SILP as a support combines the positive effects of
IL 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
60 combine the high solvation power of polar species with their
weak coordination strength, making them suitable for reactions
with electrophilic catalysts.23,22 Silica is the most widely used
support for the immobilisation of MꢀNPs due to their high surface
area and the presence of OH, which facilitate the anchoring of
65 stabilising agents, whereas the use of other supports has scarcely
been reported.24 To the best of our knowledge, the use of alumina
(Al2O3)ꢀbased supports in an SILP system, which offer higher
stability at higher pH, has been scarcely explored until now.14
Here, we report the synthesis and characterisation of a series of
70 imidazolium ILs covalently immobilised onto Al2O3 surfaces.
RuꢀNPs were uniformly distributed on these supports by a
sputteringꢀdeposition technique. Furthermore, the hydrogenation
of benzene was used to probe the surface properties of these new
SILP RuꢀNPs. The hydrogenation of benzene was chosen as a
75 benchmark reaction since it is a structureꢀsensitive reaction.4
Moreover, the partial hydrogenation of aromatics, such as
benzene, is of interest for the treatment of diesel to obtain lowꢀ
aromatic content diesel fuels, the hydrogenation of aromatic
Introduction
Transition metal nanoparticles (MꢀNPs) are currently used as
20 effective catalysts for hydrogenation processes in which
homogeneous and traditional heterogeneous catalysts are less
efficient.1,2,3,4 Modern soluble MꢀNPs are commonly synthesised
by chemical methods, which involve the reduction of metallic
salts or the decomposition of organometallic complexes in the
25 presence of stabilising agents.1,2,5,6,7,8 Recently, the use of
sputteringꢀdeposition techniques was revealed as an interesting
alternative for the synthesis of soluble MꢀNPs with cleaner
surfaces under mild conditions and easier control of their size and
shape by appropriate tuning of sputtering conditions.9,
10,11,12,13,14,15
30
This approach allows the rapid synthesis of MꢀNPs
catalysts by deposition of metals on support surfaces or the fast
generation of MꢀNPs in solution using low volatility liquids such
as silicones, triglycerides, and ILs. This method is widely applied
for the formation of thin (nanosized) films onto supports16;
35 however, it is limited since it is extremely difficult to prepare
uniformly distributed MꢀNPs on solid catalytic supports.17
Recently, our group reported the synthesis of PdꢀNPs by
sputteringꢀdeposition of Pdꢀfoil using Al2O3 and SiO2 modified
with imidazolium ILs as supports.14, 18 These NPs were revealed
40 as an active catalyst for the hydrogenation of dienes, and
displayed improved activities and selectivities than those
obtained using PdꢀNPs synthesised by conventional
decomposition of organometallic precursors.
Among the various types of agents for stabilising soluble Mꢀ
45 NPs, ILs19 are extensively employed owing to their higher
efficiency in hydrogenation processes.6,20 Recently, the use of
supported ionic liquid phase (SILP) is receiving growing interest
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−
polymers to produce new materials and the production of fine 55 BF4 (763, 1039 and 1061 cm−1), respectively, thus confirming
chemicals.4
the efficiency of the Cl− exchange procedure26,27
.
Table 1. Characterisation of the supports.
Results and Discussion
Synthesis and characterisation of the Al2O3 modified supports
Support
SBET
/
Pore
Volume
/cm2 gꢀ1[a]
0.47
0.31
0.28
0.30
0.20
0.34
0.33
Pore
Organic
m2gꢀ1[a]
Diameter /
nm[a]
8.0
Content[b]
/
5
The M1ꢀM4 hybrid alumina supports have been prepared as
already described14 and their physicochemical properties were
included in order to compare the data with supports B1ꢀB4. It is
expected that the replacement of the methyl group in M1-M4 for
the nꢀbutyl side chain (B1-B4) will increase the nonꢀpolar
mmol IL gꢀ1
ꢀ
Al2O3
M1
M2
M3
M4
B1
B2
B3
B4
195
134
127
125
80
143
147
139
107
6.9
6.6
7.0
7.1
7.0
6.9
7.5
6.7
0.46
0.36
0.39
0.38
0.38
0.27
0.31
0.35
10 domains of the hybrid materials.
Thus, the M1 and B1 supports were synthesised by respectively
reacting
1ꢀmethylꢀ3ꢀ(trimethoxysilylpropyl)ꢀimidazolium
0.33
0.23
chloride (1) and 1ꢀnꢀbutylꢀ3ꢀ(trimethoxysilylpropyl)ꢀimidazolium
chloride (2) with the hydroxyl groups of the alumina surfaces and
15 their further ionꢀexchange treatment by modified literature
procedures25 was carried out in order to obtain the M2ꢀM4 and
[a] Determined using a Tristar 3020 Micromeritics equipment. The
60 specific 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 determined by elemental analysis using a
CHN Perkin Elmer M CHNS/O Analyzer, model 2400.
−
−
−
B2ꢀB4 supports containing counterꢀions NTf2 , PF6 and BF4 ,
respectively (Scheme 1).
65 Nitrogen physisorption analysis revealed typeꢀIVꢀlike isotherms
with pore diameters of 6.6 to 8.0 nm for the unmodified Al2O3
and M1ꢀM4 and B1ꢀB4 supports (Table 1). The M1ꢀM4 and B1ꢀ
B4 supports displayed lower surface areas and pore volumes than
the unmodified Al2O3, indicating that the ILs were filling the
70 pores. Similar behaviour was observed for the ILs covalently
supported on SiO2.28
20
25
Elemental analysis revealed that M2ꢀM4 and B2ꢀB4 supports
displayed slightly lower amounts of IL than the M1 and B1
supports, which suggests that IL was removed from these
75 supports during the Cl− exchange step (Table 1). A comparison of
the data displayed between the supports reveals that B1ꢀB4 (with
a nꢀbutyl group) supports displayed higher surface areas and pore
volumes with slightly larger pore diameters than M1ꢀM4 (with a
methyl group) supports, which could be directly related to the
80 fact that B1ꢀB4 supports contained lower IL amounts than M1ꢀ
M4 supports.
30
Scheme 1. Synthesis of M1–M4 and B1–B4 supports.
13C CPꢀMAS NMR of the supports M1ꢀM4 displayed four sets of
upꢀfield broad peaks (10, 25, 36 and 52 ppm) which could be
assigned to the propylene and methylene functions, whereas the
resonances at 123 and 136 ppm were assigned to the three
The effect of the Cl− exchange on the M1ꢀM4 and B1-B4 support
structure was investigated by 29Si CPꢀMAS NMR analysis
(Figure 3). The spectra of M1 and B1 displayed the typical
85 signals of silicon atoms attached to one carbon atom and to the
alumina surface at −46 and −55 ppm assigned to the T1 (20% and
60%, respectively) and T2 (80% and 40%, respectively)
species.29 After the Cl− exchange, a new signal at −65 ppm
appeared on the M2ꢀM4 and B2-B4 supports, which was
90 attributed to the formation of T3 species, suggesting that
rearrangement processes were operating under the reaction
conditions of the Cl− exchange step. This behaviour could be
ascribed to the formation of hydrogen fluoride by hydrolysis of
the fluorineꢀbased anions.30 This effect was much more
35 imidazolium carbon atoms (Figure 1). 13C CPꢀMAS NMR of the
supports B1ꢀB4 displayed seven sets of upꢀfield broad peaks (11,
12, 19, 25, 32, 49 and 52 ppm) which could be assigned to the
propylene and butylene functions, whereas the resonances at 123
and 136 ppm were assigned to the three imidazolium carbon
40 atoms (Figure 1). Thus, these NMR signals confirmed the
presence of the unaltered imidazolium ILs in M1-M4 and B1-B4
supports. It should be noted that no signal attributed to the
methoxy leaving groups (65 ppm) of the silane function was
observed, confirming that the trimethoxyꢀsilane group effectively
45 reacted with the hydroxyl groups of the Al2O3 surface under our
reaction conditions.
FTꢀIR analysis of M1ꢀM4 and B1ꢀB4 supports showed C=N
stretching vibration bands of the imidazolium rings and C−H
stretching vibrations bands of the alkyl groups at 1635 and 2950
50 cm−1, respectively (Figure 2). Therefore, these IR signals also
confirmed the presence of the unaltered imidazolium ILs in M1ꢀ
M4 and B1ꢀB4 supports. Furthermore, FTꢀIR analysis of the
−
−
95 pronounced for the supports containing the anions PF6 and BF4
(30ꢀ40% and 50% of T3, respectively), which decomposed more
−
easily than NTf2 (10% of T3) (Figure 3). A comparison of the
data displayed between the supports reveals that B1ꢀB4 (with an
nꢀbutyl group) displayed more uniform distributions of the IL
100 onto the Al2O3 than M1-M4 (with a methyl group). This effect
could be directly ascribed to the higher steric hindrance produced
by the nꢀbutyl group compared with the methyl group.
−
supports M1ꢀM4 and B1ꢀB4 displayed signals attributed to NTf2
−
(1058, 1145, 1220 and 1350 cm−1), PF6 (741 and 841 cm−1) and
2
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5
10 Figure 1.13C CPꢀMAS NMR spectra of (a) M1, (b) M2, (c) M3 and (d) M4, (f) B1, (g) B2, (h) B3 and (i) B4. 13C NMR (a)ꢀ(d) were previously described
in ref. 14.
15
20
25
30
Figure 2. FTꢀIR spectra of (a) M1, (b) M2, (c) M3 and (d) M4, (e) B1, (f) B2, (g) B3 and (h) B4. FTꢀIR (a)ꢀ(d) were previously described in ref. 14.
35
40
45
50 Figure 3.29Si CPꢀMAS NMR spectra of (a) M1, (b) M2, (c) M3 and (d) M4,(e) B1, (f) B2, (g) B3 and (h) B4. 29Si NMR (a)ꢀ(d) were previously described
in ref. 14.
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M1-M4 and B1-B4 supports stability was also tested by means of
TGA (thermogravimetric analysis) (Figures S1 and S2 on
Supporting Information). IL decomposition temperature was
observed in the range between 300ꢀ450°C, which makes these
This behaviour suggested that the metal nanoparticle size
obtained under similar sputtering conditions is mainly governed
by the nature of the support and by the amount of metal deposited
(sputtering deposition conditions) onto the support, and no
5
materials suitable for a wide range of applications in catalysis. 60 significant influence of the IL lateral alkyl chain or of the nature
Similar decomposition temperatures were observed for the case
supported ionic liquids synthesised by impregnation of
imidazolium salts onto Al2O3 supports.31
of the counterꢀanion was observed. This is in contrast to that
which was observed in net ILs in which the nature of anion also
plays an important role on the size of MꢀNPs prepared by
sputtering deposition.10
Therefore, M2 and B1ꢀB4 were selected as suitable supports for
10 the deposition of RuꢀNPs by the sputtering technique in order to
study the effect of the structure and distribution of the ILs
covalently anchored onto the Al2O3 on the RuꢀNPs physical and
catalytic properties.
65 Table 2. Characterisation of the RuꢀNPs supported onto Al2O3, M2 and
B1ꢀB4.
15 Sputtering deposition of Ru-NPs onto Al2O3-based supports
70
RuꢀNPs were obtained on the aboveꢀmentioned supports by
magnetronꢀsputtering using a power of 145 W for 10 min. The
catalysts were then characterised. Metal content determination by
XꢀRay Fluorescence (XRF) analysis revealed values of Ru
20 content between 0.8 and 1.5% (w/w) (see Table 2). DarkꢀField
Transmission Electron Microscopy (DFꢀTEM) revealed that
small nanoparticles (ca. 4.8ꢀ7.7 nm) with narrow size
distributions (ca. 1.3ꢀ1.8 nm) were obtained (see Table 2 and
Figure 4). It should be noted that the catalyst with the lowest
25 metal content (Ru/Al2O3) also displayed the smallest nanoparticle
mean diameter (4.8 nm) and the catalyst containing the highest
metal concentration (Ru/B3) displayed the largest one (7.7 nm),
whereas the rest of the catalysts bearing metal contents between
1.0 and 1.3% (w/w) displayed nanoparticles with similar sizes
30 (6.4ꢀ6.6 nm).
RuꢀNPs
NPs Φ [a]/ nm
4.8 ± 1.3
Ru content [b]/ %(w/w)
Ru/Al2O3
Ru/B1
0.8 ± 0.1
1.1 ±0.1
6.6 ± 1.5
6.6 ± 1.4
6.4 ± 1.4
7.7 ± 1.8
6.6 ± 1.4
Ru/M2
Ru/B2
Ru/B3
Ru/B4
1.0 ±0.1
1.3 ± 0.1
1.5 ±0.2
1.0 ±0.1
[a] Determined by measuring at least 300 nm NPs of the darkꢀfield TEM
micrographs; [b] Determined by XꢀRay Fluorescence Analysis (XRF).
75
80
35
40
45
50
55
85
90
95
100
Figure 4. Dark Field Transmission Electron Microscopy of the catalysts Ru/Al2O3 (a), Ru/B1 (b), Ru/M2 (c), Ru/B2 (d), Ru/B3 (e) and Ru/B4 (f).
4
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For comparative purposes, RuꢀNPs supported onto Al2O3 with
were tested in the hydrogenation of benzene under 4 bar of H2 at
higher metal content (ca. 3% (w/w)) were synthesised by 55 75°C, since this reaction is commonly used as a benchmark test
magnetronꢀsputtering and characterised by XꢀRay Diffraction
(XRD) and XꢀRay Photoelectron Spectroscopy (XPS). Analysis
of the selected region of the XRD patterns of Al2O3 and of the
RuꢀNPs immobilised onto Al2O3 revealed the appearance of a
for evaluation of the activity and selectivity of RuꢀNPs.4
5
Table 3 displays the activities obtained in terms of TON and
TOF. The following order of activity was observed: Ru/B2 >
Ru/B3 > Ru/M2 ≈ Ru/Al2O3 > Ru/B4 > Ru/B1. (Table 3, Entries
signal at 44o due to the presence of Ru(0)ꢀhcp NPs, although the 60 1ꢀ6). Catalyst Ru/B2, whose support contains the covalently
−
metal content was probably not high enough to provide a clear
10 peak even for high counts.
Additionally, samples with a metal content of 3% (w/w) prepared
by magnetronꢀsputtering were compared to catalysts with similar
anchored IL with the anion NTf2 and an nꢀbutyl chain attached
to the imidazolium ring of the cation, afforded the highest
activity. With this catalyst, TONs of 27000 (moles benzene
(BEN)/mol surface Ru) were achieved at high conversions (ca.
metal content obtained by hydrogen reduction of Ru(Meꢀ 65 100%). Compared to the nonꢀmodified support, Ru/B2 as well as
allyl)2(COD) (general procedure described in Supporting
15 Information) using XPS. First, the quantification of the survey
XPS spectra shows that the Ru/Al ratio is more than six times
higher when the RuꢀNPs were synthesised by magnetronꢀ
sputtering (0.2) compared to the colloidal method (0.03).
Ru/B3 displayed higher turnover frequencies (Table 3, Entries 1,
4 and 5). Additionally, Ru/B2 displays a slightly higher activity
with respect to Ru/M2, in which the imidazolium cation of the IL
contains a methyl side chain (Table 3, Entries 3 and 4). These
70 TOF values are much higher than those already described in the
literature for similar systems using RuꢀNPs synthesized from
organometallic precursors, the nanoparticles being dispersed in
pure BMI.PF6 IL during reaction (ca. 0.0056 sꢀ1 at 75°C and 4 bar
H2).4, 32
20
75 Table 3. RuꢀNPs catalysed hydrogenation of benzene.
25
80
Entry[a]
Catalyst
TON
TOF/sꢀ]
BEN
Conv./ % [b]
25.2
[b,c]
[b,d]
30
1
2
Ru/Al2O3
Ru/B1
9160
2160
9160
9600
9260
4000
980
2.55
0.59
2.55
2.73
2.62
1.09
0.28
Figure 5.Selected region of the XPS spectra of the RuꢀNPs immobilised
onto Al2O3: synthesised by reduction of Ru(Meꢀallyl)2(COD) (a), and
synthesised by magnetronꢀsputtering (b).
6.7
24.6
34.8
44.5
11.3
2.7
3
Ru/M2
4
Ru/B2
Furthermore, Ru 3d and C 1s XPS spectra were obtained and
35 analysed (see Figure 5). C 1s and Ru 3d peaks strongly overlap
and they were not sufficiently well resolved under our
experimental conditions. However, the relative quantification of
both contributions shown in Figure 5 can be determined by peak
fittings. The fitting shows that the C/Ru ratio decreased from 5.0
40 when the RuꢀNPs were synthesised by reduction of Ru(Meꢀ
5
Ru/B3
6
7[e]
Ru/B4
Ru/Al2O3 ꢀ BMI.NTf2
8[e]
Ru/Al2O3 ꢀ BMI.PF6
Ru/Al2O3ꢀ BMI.BF4
1.5
11.4
550
4150
0.12
1.14
9[e]
[a] Reaction conditions: benzene (BEN) (2.64 g, 34 mmol), Ruꢀcatalyst
allyl)2(COD) to 0.7 when they were prepared by magnetronꢀ 85 (50 mg), 75°C and 4 bar of H2. Conversion determined by GCꢀFID; [b]
Calculated for 1 h reaction time; [c] TON=converted moles of benzene
sputtering. These results, together with the information obtained
(BEN)/moles of Ru on the surface of the NP; 33 [d] TOF obtained from
from elemental analysis, demonstrate the benefit of the
TON vs. time slope; [e] Imidazolium ILs (ca. 0.3 mmol IL g cat. ꢀ1) were
magnetronꢀsputtering method to allow the deposition of a much
45 higher concentration of RuꢀNPs in the surface region of the
support, in contrast to the catalysts synthesised by conventional
chemical methods. From Figure 5, it is also possible to observe a
higher surface oxidation of the RuꢀNPs. Data suggest that about
64% and 24% of the RuꢀNPs were oxidised after preparation by
50 magnetronꢀsputtering and by the reduction of Ru(Meꢀ
allyl)2(COD), respectively.
dispersed by simple impregnation onto the Ru/Al2O3 catalyst.
90
Cyclohexene (CHE) selectivity as a function of the benzene
(BEN) conversion was also monitored during the experiments
(See Figure 6). Even if selectivity values attained are not as high
as those reported for similar supported Ru catalysts modified with
95 more hydrophilic dycianamideꢀcontaining ILs (70% at very low
conversion ca. 3%),34, 35 our experiment show that common ILs
covalently bonded to the support can significantly increase the
amount of cyclohexene obtained.
Ru-NPs catalysed the hydrogenation of benzene
These novel nanocatalysts prepared by the sputtering technique
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It was observed that the catalysts whose supports contain Cl− and
BF4 as the counterꢀion of the supported IL (Ru/B1 and Ru/B4, 60 8), whereas similar values of TOF were obtained in the case of
compared to Ru/B2 and Ru/B3, (Table 3, Entries 2 vs. 7 and 4 vs.
−
respectively) produced much higher selectivities towards the
partially hydrogenated product (up to 20% at low benzene
conversions) compared to the other SILP catalysts or the
Ru/Al2O3 catalyst, despite being less active.
Ru/Al2O3 impregnated with BMI.BF4 and Ru/B4 (Table 3,
Entries 6 vs. 9). It should be noted that B4 support has the less
uniform IL distribution onto the Al2O3 according to 29Si NMR.
This is evidence that the IL distribution achieved with the method
65 described in this work is of fundamental importance for the
performance of the catalysts.
5
10
15
20
Conclusions
In conclusion, a new series of Al2O3 supports modified with
covalently anchored imidazolium ILs were synthesised.13C NMR
70 and IR revealed not only that the structure of the IL was stable in
the Al2O3 support but also that the anionꢀexchange process was
accomplished with success. N2ꢀphysisorption analysis showed
that the surface area and the pore diameter were reduced by the
presence of the IL, which suggests that the IL is filling the pores.
75 Characterisation of the obtained nanocatalysts by TEM analysis
revealed the formation of small and wellꢀdispersed nanoparticles.
XRD analysis confirmed that the nanoparticles possessed the
Ru(0)ꢀhcp structure, and XPS results suggested that the RuꢀNPs
were preferentially deposited onto the near surfaces of the
80 support. These nanocatalysts were tested for the hydrogenation of
benzene, and it was observed that activity was higher for those
supports containing more hydrophobic (apolar) ILs (TOF up to
2.73 sꢀ1). Such catalysts produced TONs as high as 27000.
Additionally, the cyclohexene selectivity as a function of the
85 benzene conversion was monitored and it was observed that the
selectivity (up to 20% at 1% benzene conversion) was higher for
catalysts whose supports contain a more hydrophilic (polar) IL.
Finally, it should be remarked that the use of Al2O3 supports
modified with covalently anchored ILs is more suitable for
90 catalytic applications with respect to simple impregnation with
ILs, since much lower activities are observed for the latter case,
in which IL is not well dispersed onto the support. The catalytic
behaviours observed highly suggest that, by using these
procedures, it is possible to obtain nanocatalysts with thin layers
95 of ILs surrounding/interacting with the nanoparticle surface,
which modifies the performance of these catalysts.
Figure 6. Cyclohexene (CHE) selectivity vs. benzene (BEN) conversion:
25 Ru/Al2O3 (ꢀ●ꢀ), Ru/B1 (ꢀ○ꢀ). Ru/M2 (ꢀ▲ꢀ), Ru/B2 (ꢀ∆ꢀ), Ru/B3 (ꢀ■ꢀ) and
Ru/B4 (ꢀ□ꢀ).
This difference can be ascribed to the effect of the nature of the
IL on their supports, which is governed by (i) the nature of the
lateral alkyl chain of the imidazolium cation (extension of nonꢀ
30 polar domains) and (ii) the charge separation between the cation
and the anion. The polar character of the IL increases as the
charge separation in the ionic pair is greater, i.e. weaker hydrogen
bond between the imidazolium ring and the anion exist. Thus, in
order to evaluate the polarity, it is possible to compare relative
35 strengths of the interactions between the cation and the anion.
Experimental and theoretical studies36ꢀ40 found the following
order of intrinsic hydrogen bond strength with 1ꢀnꢀbutylꢀ3ꢀ
−
methylimidazolium according to the anion: Cl− > CF3CO2− >BF4
>PF6− >NTf2− >BPh4 . This is indeed the inverse of the activity
−
40 order observed in our experiments. Less polar hydrophobic ILꢀ
modified supports yielded more active catalysts probably due to
its affinity to the apolar substract (benzene). The increase in
activity observed when the side alkyl chain of imidazolium
changes from methyl to nꢀbutyl is in accordance with this the
45 polarity effect on the catalytic behaviour.
Concerning the selectivity, an inverse order is observed, i.e. more
polar hydrophilic IL display higher selectivities towards the
olefin. It is known that ILs enhance the cyclohexene formation
under biphasic conditions due to its lower solubility with respect
50 to benzene32 as well as by binding to the metal surface34, 35. The
results presented herein suggest that these effects can be achieved
by using SILP systems with a hydrophilic character.
In order to study the effect of the distinct IL distribution onto the
Al2O3 on the catalytic performance, experiments were carried out
55 using the Ru/Al2O3 catalyst simply impregnated with common 1ꢀ
nꢀbutylꢀ3ꢀmethylꢀimidazolium ILs. Interestingly, results revealed
that much lower activities were achieved with the addition of
both BMI.NTf2 and BMI.PF6 to the nonꢀmodified support
Experimental Part
General Methods
100 All syntheses were performed using standard Schlenk techniques
under argon atmosphere. CH3CN and CH2Cl2 were purified by
standard procedures.41 The Al2O3 used in this study was provided
by
Petrobras.
Bis(2ꢀmethylallyl)(1,5ꢀ
(Ru(Meꢀallyl)2(COD)) was
cyclooctadiene)ruthenium(II)
105 purchased from SigmaꢀAldrich. The IL 1ꢀmethylꢀ3ꢀ
(trimethoxysilylpropyl)ꢀimidazolium chloride (1) and 1ꢀnꢀbutylꢀ
3ꢀ(trimethoxysilylpropyl)ꢀimidazolium chloride (2) was prepared
by a previously described procedure.25 H2 (> 99.999%) was
purchased from WhiteꢀMartins. Elemental analysis of the ILs
110 immobilised on the support surfaces was carried out on a CHN
Perkin Elmer M CHNS/O Analyzer, model 2400. The N2
isotherms of the supports, previously degassed at 100°C under
vacuum for 3 h, were obtained using a Tristar 3020 Micromeritics
6
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DOI: 10.1039/C4DT03039G
equipment. The specific surface areas were determined by the
further characterisation and application.
BET multipoint method and the average pore size was obtained 60 Details about the chamber containing an electroꢀmagnetic
by BJH method. Solidꢀstate 13C and 29Si NMR measurements for
the supports were performed on a Bruker 400 spectrometer at the
Universidade Nova de Lisboa. The infrared spectra were obtained
on an ABB FTLA 2000 with a resolution of 4 cm−1, with 128
oscillator (with variable controlled frequency) which allows the
constant movement of the conical flask are described
elsewhere.12, 44
5
cumulative scans. TEM samples were prepared by the slow 65 Hydrogenation of benzene:
evaporation of a drop of each colloidal solution deposited under
an argon atmosphere onto a holey carbonꢀcovered copper grid.
2.64 g of benzene (BEN) (34 mmol) were added to a Fischerꢀ
10 The Dark Field TEM experiments were performed with a JEOL –
JEM 1200ExII electron microscope operating at 200 kV. The Ru
Porter reactor containing the 50 mg of the Ruꢀnanocatalyst. After
that, the reactor was pressurized with 4 bar of H2 at the desired
content was determined by Xꢀray fluorescence (XRF) carried out 70 temperature. Samples were taken from the reaction mixture at
using a Shimadzu XRFꢀ1800 sequential Xꢀray fluorescence
spectrometer. Samples were prepared in KBr and calibration was
15 performed using bromine as an internal standard. Xꢀray
diffraction (XRD) analyses were carried out using a Philips
X’Pert MPD diffractometer with BraggꢀBrentano geometry using
a graphite curvedꢀcrystal with the Cu Kα Xꢀray radiation (1,5406
Å). Xꢀray photoelectron spectroscopy (XPS) experiments were
20 carried out in a conventional electron spectrometer (Omicron)
equipped with a high performance hemispherical energy analyser
with a seven channeltron detector. Mg Xꢀray source (Mg Kα1,2
=1253.6 eV) was used as excitation source and the Al 2p signal at
74.5 eV was taken as reference peak.42, 43 Analysis of the Ru, C,
25 Al and Cl envelopes was performed and peakꢀfitted after
subtraction of a Shirley background using Gaussian–Lorenzian
peak shapes obtained from the Casa XPS software package.
regular intervals. The conversion and selectivity was determined
by GCꢀFID analysis using an Agilent Technologies GC System
6820 with a DBꢀ17 column (oven temperature 40°C).
Notes and references
75 aInstitute of Chemistry, UFRGS, Avenida Bento Gonçalves, 9500, Porto
Alegre 91501ꢀ970 RS, Brazil. Fax: (+55) 5133087304; Eꢀmail:
jairton.dupont@ufrgs.brAddress.
bInstitute of Physics, UFRGS, Avenida Bento Gonçalves, 9500, Porto
Alegre 91501ꢀ970 RS, Brazil.
80 c School of Chemistry, University of Nottingham, University Park,
Nottingham, NG7 2RD, UK
Jairton.dupont@nottingham.ac.uk
† Electronic Supplementary Information (ESI) available: [Figure 1S and
2S show TGA analysis profiles for supports and description of a general
85 procedure for the synthesis of supported RuꢀNPs from Ru(Meꢀ
allyl)2(COD)]. See DOI: 10.1039/b000000x/
‡ Footnotes should appear here. These might include comments relevant
to but not central to the matter under discussion, limited experimental and
spectral data, and crystallographic data.
Synthesis of the covalently supported ILs M1–M4 and B1-B4:
30
90
1ꢀmethylꢀ3ꢀ(trimethoxysilylpropyl)ꢀimidazolium chloride (1) or
1ꢀnꢀbutylꢀ3ꢀ(trimethoxysilylpropyl)ꢀimidazolium chloride (2)
(3.56 mmol) were dissolved in dry CH3CN (25 mL) and added to
5.0 g of dried alumina. The suspension was kept at 90°C under
35 argon and vigorously stirred for 72 h. The ILꢀfunctionalised
alumina was washed, centrifuged, and dried to yield the support
M1 and B1, respectively. Based on the amount of IL on the M1
and B1 support, an excess (1.2 eq.) of LiNTf2, KPF6, and NaBF4
salts was dissolved in deionised water (25 mL) and added to the
40 M1 or B1 (1.0 g) in order to exchange the ions. The suspensions
were vigorously stirred for 48 h. The mixtures were washed,
centrifuged, and dried to yield the supports M2ꢀM4 and B2ꢀB4.
The removal of chlorides from the support was checked by
analysing the Clꢀ content of the washing fractions by titration with
45 AgNO3.
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DOI: 10.1039/C4DT03039G
Ruthenium nanoparticles obtained by sputtering deposition from bulk
metal onto an ionicꢀliquid modified alumina are highly active catalysts for
the hydrogenation of benzene.
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