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N.C. Antonels, R. Meijboom / Catalysis Communications 57 (2014) 148–152
Table 1
Typical surface area, average pore volume and average pore diameter changes for the uncoated and coated G5-RuSil60 catalysts.
a
b
Catalyst
Surface area, SBET (m2·g−1
)
Total pore volume, VT (cm3·g−1
)
Average pore diameter, Dp (Å)b
Uncoated G5-RuSil60
462.1
323.5
344.9
357.5
342.1
347.8
0.82
0.66
0.65
0.65
0.63
0.67
56.2
58.4
55.7
53.6
55
10% [BMIM][BF4] G5-RuSCILL
10% [BMIM][PF6] G5-RuSCILL
10% [BMIM][NTf2] G5-RuSCILL
10% [EMIM][EtS] G5-RuSCILL
10% [EMIM][OcS] G5-RuSCILL
56.7
a
Determined by BET analysis.
Determined by BJH analysis.
b
Post-catalytic BET N2 physisorption analysis confirms maintained inter-
action of the ionic liquid with the catalyst surface.
RuSil60. In this case, the formation of isopulegol is rapid enough to
exceed formation of citronellal. The result suggests that the dendrimer
stabiliser decomposes at 130 °C when the solvent and substrate are
present. This results in a more exposed nanoparticle surface, allowing
for an increase in the conversion of citral albeit with the formation of
isopulegol. This was not observed for G5-RuSil60 and G6-RuSil60 possi-
bly as result of a more stable dendrimer architecture. Additionally, stud-
ies into factors affecting the selectivities observed in the hydrogenation
of α,β-unsaturated compounds reveal that basic promoters such as so-
dium hydroxide increase selectivity towards the saturated aldehyde
[26]. This helps further explain why the higher generation dendrimers
with increased tertiary amines, which can act as basic promoters, help
promote C_C double bond hydrogenation.
3.2. Catalytic evaluations
3.2.1. Evaluation of RuSil60 catalysts in the hydrogenation of citral
Brief investigations into the effects of temperature and pressure on
the activity of the catalyst were conducted using cyclohexane as the
solvent. The conversion of citral at these conditions is illustrated in
Fig. 1(a). A minimal 5–6% increase in conversion upon increasing the
pressure from 10 bar to 30 bar was observed. The selectivity observed
in each reaction was towards citronellal exclusively.
A high initial conversion of citral was observed with a levelling-off of
the citral conversion during the course of the reaction. Inhibition of the
reaction was observed after the higher initial activity. The decrease in
conversion with time can be explained by an increase in the inhibitory
decarbonylation reaction [12,23–25]. The carbon monoxide that forms
competitively adsorbs to active sites on the nanoparticle surface. The
overall kinetic behaviour resulting from the decay in catalytic activity
was seen for all citral hydrogenation reactions concerning the evalua-
tion of RuSil60. The highest conversion was observed for the
G4-RuSil60 catalyst for reactions conducted at 90 °C (Fig. 1c). The
lower conversion observed for the higher generations can be
explained by the possible increase in steric hindrance that inhibits
adsorption of citral. An increase in temperature from 90 to 130 °C
caused no significant increase in conversion except for G4-RuSil60.
This marked increase in conversion at 130 °C for G4-RuSil60 is how-
ever ascribed to the formation of unwanted side products.
3.2.2. Evaluation of RuSCILL catalysts in the hydrogenation of citral
The use of ionic liquid as a catalyst coating can influence the product
selectivity observed when evaluating different catalysts however, in this
study only the activity was affected since selectivity towards citronellal
was maintained. RuSCILL catalysts with different ionic liquids are pre-
sented with focus on the effect of the ionic liquid on the catalyst activity.
The reactions were run at 90 °C to help minimise formation of ionic
liquid decomposition products which could further modify the catalyst
surface [27]. When comparing the results of the RuSCILL catalysts to that
of the RuSil60 catalyst (Table 2), both increases and decreases in the
conversion of citral were observed upon coating with the various ionic
liquids. A decrease in the conversion was observed compared to the un-
coated catalyst when using the ionic liquids [EMIM][EtS], [EMIM][OcS],
[BMIM][PF6] and [BMIM][BF4] as coatings with an α-value of 0.1 as seen
for entries 4–7.
A significant increase in the conversion of citral to 50% was observed
when using the ionic liquid [BMIM][NTf2] as a catalyst coating with an
α-value of 0.1. This suggests that the [BMIM][NTf2] ionic liquid coating
minimises the effect of decarbonylation. The increase in activity is
most likely competitive displacement of CO by the ionic liquid. The
[NTf2] ionic liquids are known to displace CO showing strong ligand-
like interactions with the surface of the nanoparticle [27]. This would
suggest that the ionic liquid has a negative effect on the activity of the
When considering the selectivity results obtained (Fig. 2), there is a
clear indication that the increased conversion of citral is a result of the
increased formation of the side product isopulegol, formed by the
cyclisation of citronellal [12,25]. The overall percentage of citronellal
in the reaction mixture does not change significantly when utilising
G5-RuSil60 and G6-RuSil60, which indicates that the formation of citro-
nellal is rapid enough to compensate for the intrinsic rate of formation
of isopulegol. Increase in temperature to 130 °C causes a dramatic
increase in the selectivity towards isopulegol when evaluating G4-
Fig. 1. Conversion of citral illustrated for a) pressure dependant conversion of citral at 120 min and 240 min for a H2 pressure of 10 bar and 30 bar using the G4-RuSil60 catalyst, and
conversion of citral using the various RuSil60 catalysts and at various temperatures illustrated by the b) time-resolved conversion of citral at 110 °C and c) end of run conversion of citral
at 90 °C, 110 °C and 130 °C.