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N.C. Antonels et al. / Journal of Molecular Catalysis A: Chemical 421 (2016) 156–160
Table 1
upon immobilization on the silica 100 support when compared to
results obtained for silica 60. The immobilization of G6-RuDEN on
silica 100 has no significant effect on the Ru NP diameter, which
maintained an average diameter of 2.2 nm. The physical properties
of the support material are therefore of great importance, especially
with regards to the pore size. Most likely, the larger average pore
size of silica 100 facilitates the penetration of the RuDENs into the
support and therefore allows for better RuDEN dispersion.
The average Ru NP size for each of the RuDENs, RuDENs supported on silica 60 and
silica 100 and the percentage change in size upon immobilization.
Support
Generation
Catalyst name Ru particle
Change in
diameterb
%
diametera
nm
None (dendrimer) G4
G4-RuDEN
G5-RuDEN
G6-RuDEN
G4-RuSil60
G5-RuSil60
G6-RuSil60
1.2 0.1
1.4 0.1
2.2 0.3
2.4 0.3
2.1 0.3
2.1 0.3
–
–
–
+100
+50
0
+17
+7
0
G5
G6
The changes in the physical characteristics such as surface area,
pore volume and pore diameter were investigated upon coating
with the ionic liquid. The uncoated catalyst was prepared with
a metal loading of 0.08–0.15 wt% Ru as confirmed by ICP-OES.
BET analysis of the G6-RuSil60 uncoated catalyst indicates a sur-
Silica 60
G4
G5
G6
G4
G5
G6
Silica 100
G4-RuSil100 1.4 0.2
G5-RuSil100 1.5 0.2
G6-RuSil100 2.2 0.2
face area (ABET) of 462.1 m2 g−1, an average pore volume (Vpore
)
a
Data derived from TEM studies.
Increase in particle size compared to pure RuDEN in absence of support.
of 0.82 cm3 g−1 and an average pore diameter (dpore) of 5.6 nm.
Changes were observed for the surface area when comparing the
change in the physical characteristics upon coating with an ionic
liquid is consistent with literature and confirms the presence of the
ionic liquid on the solid catalyst surface for the G6-RuSCILL cata-
lysts [13]. The physical characteristics of the coated and uncoated
catalysts prior to use in the reaction are listed in Table 2. Addition-
ally, BET data obtained for the G6-RuSCILL catalysts with an ionic
liquid loading, ␣ = 0.2, prior to and upon completion of the reac-
tion are listed to account for possible ionic liquid leaching under
reaction conditions.
b
Teflon liner, operated in batch mode and stirred at 1200 rpm.
Depending on the reaction, the conditions were set at 75 ◦C, 90 ◦C
and 110 ◦C and a H2 pressure of 10 bar. The catalyst (0.08–0.1 wt%
Ru, 0.2–0.3 g, 1.84 mol Ru) was transferred to the autoclave.
The autoclave was charged with cyclohexane (30 mL) and toluene
(0.34 mL, 0.2947 g, 3198.9 mol) with n-decane as the internal
standard (0.34 mL). The reactor was purged thrice at ambient
temperature with H2 gas, depressurized and heated to 90 ◦C and
allowed to stir at this temperature for 1 h to allow catalyst acti-
vation. The desired temperature, either 75, 90 or 110 ◦C, was set
and the reactor pressurized to a H2 pressure of 10 bar to com-
mence reaction. Samples of the reaction mixture were periodically
taken and analyzed by gas chromatography on a Shimadzu GC-2010
equipped with a 30 m Restek Rtx®-5 capillary column.
Comparison of the BET results for the pre- and post-run G6-
RuSCILL catalyst indicates that there was no significant change in
the average pore volume values. Since the used SCILL catalysts have
been carefully dried in vacuum to remove any accumulated com-
pounds, except the ionic liquid, it can be assumed that the chosen
ionic liquids are suitable for use in the preparation of the Ru-SCILL
tion conditions.
3. Results and discussion
The nanoparticles prepared from the Gi-PAMAM-OH dendrimer
are referred to as Gi-RuDEN respectively with i = 4, 5, 6. HRTEM
analysis of the RuDENs prior to immobilization was conducted and
the results have been previously published [21]. An average particle
size of 1.2 0.1 nm, 1.4 0.1 nm and 2.2 0.3 nm was observed for
the G4-RuDEN, G5-RuDEN and G6-RuDEN catalysts respectively. A
series of immobilized RuDEN catalysts were prepared using silica
60 or silica 100 as catalyst support and are referred to as Gi-RuSil60
and Gi-RuSil100 respectively. The average sizes of the nanoparticles
are summarized in Table 1 (images and histograms of which can be
found in the Supporting Information).
A 100% and 50% Ru NP size increase was observed for the G4-
RuDEN and the G5-RuDEN respectively upon immobilization on
silica 60. The increased dendrimer generation is therefore benefi-
cial for the stabilization of the Ru NP during the immobilization on
silica. The narrow Ru NP size distribution is maintained and, more
interestingly, the sintering of the G4- and G5-RuDENs is minimized
available Ru/Al2O3 was used as benchmark catalyst. As depicted
in Fig. 1, the selectivity toward the intermediate methylcyclohex-
ene (MCHe) was low at all conversion levels and declined with
increasing conversion, following the generally known trends from
literature [21]. The yield for MCHe reached 2.6% at maximum
(see Table S1 in supporting information). Due to the high load-
ing of 5 wt% Ru in this catalyst, the consecutive reaction of MCHe
to MCHa is facilitated and lower metal loading would be ben-
eficial. A 0.15 wt% Ru/Al2O3 catalyst was prepared by incipient
wetness impregnation and evaluated under identical conditions.
As expected, the maximum yield improved by a factor of two to
5.5% MCHe.
Deposition of the dendrimer particles on Al2O3 proved more
difficult than on silica since it required a proper adjustment of the
isoelectric point of Al2O3. As a result, it was more convenient to
deposit the dendrimer derived metal nanoparticles (MNPs) onto
Table 2
Surface area, average pore volume and average pore diameter for the uncoated and coated G6-RuSil60 catalysts prior to and after reaction.
Ionic liquid
Loading ␣ vol%
Surface area ABET m2 g−1
Pore volume Vpore mL g−1
Pore diameter dpore nm
a
–
−
462.1
286.5
258.0
275.5
314.5
232.7
203.4
308.3
259.2
267.5
0.82
0.64
0.56
0.58
0.64
0.54
0.56
0.57
0.51
0.56
5.6
6.4
6.2
6.0
5.9
6.8
7.9
5.4
5.6
5.9
[BMIM][BF4]
10
20
20b
10
[BMIM][PF6]
[BMIM][NTf2]
20
20b
10
20
20b
a
Native, uncoated G6-RuSil60.
b
Post-catalytic run, reaction conditions: p (H2) = 10 bar, G6-RuSil60, 3.19 mmol of toluene, 0.00184 mmol Ru and 30 mL of cyclohexane.