Exploring Opportunities for Platinum Nanoparticles Encapsulated in Porous Liquids as Hydrogenation Catalysts

Article abstract of DOI:10.1002/chem.201905288

The unusual combination of characteristics observed for porous liquids, which are typically associated with either porous solids or liquids, has led to considerable interest in this new class of materials. However, these porous liquids have so far only been investigated for their ability to separate and store gases. Herein, the catalytic capability of Pt nanoparticles encapsulated within a Type I porous liquid (Pt@HS-SiO₂ PL) is explored for the hydrogenation of several alkenes and nitroarenes under mild conditions (T=40 °C, P_H2=1 atm). The different intermediates in the porous liquid synthesis (i.e., the initial Pt@HS-SiO₂, the organosilane-functionalized intermediate, and the final porous liquid) are employed as catalysts in order to understand the effect of each component of the porous liquid on the catalysis. For the hydrogenation of 1-decene, the Pt@HS-SiO₂ PL catalyst in ethanol has the fastest reaction rate if normalized with respect to the concentration of Pt. The reaction rate slows if the reaction is completed in a “neat” porous liquid system, probably because of the high viscosity of the system. These systems may find application in cascade reactions, in particular, for those with mutually incompatible catalysts.

Full text of DOI:10.1002/chem.201905288

A Journal of
Accepted Article
Title: Exploring Opportunities for Platinum Nanoparticles Encapsulated
in Porous Liquids as Hydrogenation Catalysts
Authors: Ellen Brigitte Hemming, Anthony F. Masters, and Thomas
Maschmeyer
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.201905288
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10.1002/chem.201905288
Exploring Opportunities for Platinum Nanoparticles Encapsulated in Porous
Liquids as Hydrogenation Catalysts
a
a
a
Ellen B. Hemming, Anthony F. Masters and Thomas Maschmeyer*
a
Laboratory of Advanced Catalysis for Sustainability, School of Chemistry, The University of Sydney,
Sydney 2006, Australia. Email: thomas.maschmeyer@sydney.edu.au.
Abstract
The unusual combination of characteristics observed for porous liquids, which are typically
associated with either porous solids or liquids, has led to considerable interest in this new class
of materials. However, these porous liquids have so far only been investigated for their ability to
separate and store gases. Herein, we explore the catalytic capability of Pt nanoparticles
encapsulated within a Type I porous liquid (Pt@HS-SiO PL) for the hydrogenation of several
2
alkenes and nitroarenes under mild conditions (T = 40 °C, PH = 1 atm). To understand the effect
2
of each component of the porous liquid on the catalysis, the different intermediates in the porous
liquid synthesis (i.e. the initial Pt@HS-SiO , the organosilane-functionalised intermediate and
2
final porous liquid) were employed as catalysts. For the hydrogenation of 1-decene, the
Pt@HS-SiO PL catalyst in ethanol had the fastest reaction rate when normalised with respect to
2
the concentration of Pt. The reaction rate slowed when the reaction was completed in a “neat”
porous liquid system, which was most likely due to the high viscosity of the system. These
systems may find application in cascade reactions, in particular, those with mutually
incompatible catalysts.
Introduction
The idea that well-defined permanent porosity could, through careful design, be introduced
[
1]
into a liquid was first suggested in 2007 by O’Reilly et al. They proposed three possible
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Chemistry - A European Journal
10.1002/chem.201905288
categories of porous liquids: neat porous liquids composed of discrete molecular hosts (Type I),
discrete molecular hosts dissolved in a bulky solvent (Type II), and liquids comprising porous
[
1-2]
frameworks dispersed in a sterically hindered solvent (Type III).
Porosity is a characteristic
typically associated with solid materials and so it was hypothesised that these porous liquids
could potentially possess unique combinations of properties, such as the size-selective sorption
[1-2]
properties of a porous solid and the fluidity of a liquid.
However, it was not until 2015 that
these systems became a reality with the first reports of the successful synthesis of liquids
[
3-4]
containing permanent pores.
Since then examples of porous liquids of all three classes have
Despite the advancements made within this field, to date these systems have
been reported in the literature only with respect to their ability to capture, separate and store
[
5-9]
been reported.
[
3-6, 8, 10]
gases.
Conversley, the unique combinations of properties means that the potential
[
2, 11-12]
applications for these systems are extremely broad.
Recently, we reported the first successful encapsulation of metal nanoparticles within a Type
[13]
I porous liquid. This porous liquid consists of hollow silica spheres (diameter ca 16 nm) that
[
4]
are functionalised with a large corona-canopy (Figure 1). The corona-canopy consists of a
positively charged organosilane corona moiety ionically bound to a negatively charged
poly(ethylene glycol)-based canopy (PEGS). The 2 nm pores that permeate through the silica
shell prevent the movement of either the corona or the canopy into the cavity, thus, maintaining
[
4]
the intrinsic porosity. Our method for encapsulating Pt nanoparticles within the porous liquid,
involved first encapsulating the Pt nanoparticles (ca 1.2 nm) in thiol-functionalised hollow silica
[13]
spheres (HS-SiO ) before corona-canopy functionalisation.
The presence of the thiol
2
functional groups was found to be crucial to the successful encapsulation and then retention of
[13]
the nanoparticles during corona-canopy functionalisation. The present work was undertaken to
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explore the possible application of these systems as catalysts, with the intention of understanding
the underlying principles behind catalysis in porous liquids.
Figure 1. Schematic of Pt nanoparticles encapsulated in a Type I porous liquid.
Previous research has firmly established the enhanced capacity of these liquids to
dissolve/absorb gas, inherently enabled by the presence of cavities, which become enriched with
[
3-4, 10]
gas.
This cavity enrichment should lead to an increased concentration of gas around the
catalyst and, hence, might in some cases improve the rates of reactions for the porous liquid as
[
11]
compared to a conventional, molecular one. Therefore, it is expected that the application of
porous liquids as catalysts will be most advantageous when applied to reactions involving
gaseous reagents. As porous liquids have not previously been employed as catalysts, a simple
catalytic reaction, preferably with at least one gaseous reagent, was required in order to advance
our understanding of the catalytic capability of these systems. Hydrogenation reactions are one
of the most studied and most important synthetic transformations at both the laboratory and
[
14-15]
industrial scales.
These reactions have been reported with an extensive range of both
heterogeneous and homogeneous transition metal-based catalysts using a variety of hydrogen
[
16]
donors, including hydrogen gas.
Herein, we report on our investigation of the catalytic
applicability of Pt nanoparticles encapsulated in a Type I porous liquid for hydrogenation
reactions under mild conditions.
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Results and Discussion
Hydrogenation of 1-decene
The hydrogenation of 1-decene to decane was selected as the initial reaction to study the
catalytic activity of the porous liquid system due to its simplicity. This test reaction was
completed under benign batch reaction conditions (T = 40 °C, PH = 1 atm). As porous liquids
2
have not previously been studied as catalysts, we first needed to understand how each component
of the shell-corona-canopy influences the catalytic activity of this Type I porous liquid. This was
investigated by testing the catalytic activity of the three stages of the porous liquid synthesis: the
initial Pt nanoparticles encapsulated in hollow silica sphere (Pt@HS–SiO ), the organosilane-
2
functionalised intermediate (OS–(Pt@HS–SiO )), and the resulting porous liquid (Pt@HS–
2
SiO PL). The amounts of each catalyst used were standardised, from the TGA traces (Figure
2
S1), to the mass of inorganics (e.g. Pt@SiO ) present in each sample rather than the amount of
2
Pt. This ensured that comparable numbers of hollow silica spheres were present in each reaction.
The Pt@HS–SiO PL is liquid-like. However, as both Pt@HS–SiO and OS–(Pt@HS–SiO )
2
2
2
are solids they require a solvent. To enable direct comparison of reactivity between all catalysts,
all three catalysts were studied using ethanol as the solvent. However, as ethanol is small enough
to diffuse through the pores in the silica shell and fill the cavities, in this system the reactants
must diffuse through pores that are (at least partially) occupied by solvent. But it is this intrinsic
porosity that provides the unique properties of these systems. So in addition to reactions in
ethanol, the Pt@HS–SiO PL catalyst was also studied using poly(ethylene glycol) sulfonate-
2
based potassium salt (KPEGS) as a non-conventional solvent, in which the pores and cavity of
the functionalised SiO spheres are free of solvent. As the PEGS anion forms the canopy, and
2
thus is the main component of the porous liquid, the addition of KPEGS will not affect the
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porosity of the system. Moreover, due to the small scale of the reaction with the porous liquid
catalyst, an additional solvent was required in this case to ensure efficient mixing. Since the
addition of KPEGS does not affect the intrinsic porosity, this system could be considered as
comparable to a “neat” porous liquid. Figure 2 shows the reaction profiles for the conversions of
1
-decene for the four catalytic systems.
100
50
Pt@HS-SiO2
OS-(Pt@HS-SiO2)
Pt@HS-SiO PL (EtOH)
2
Pt@HS-SiO PL (KPEGS)
2
0
0
100
200
Time (min)
300
Figure 2. Conversion of 1-decene for Pt@HS–SiO ( ), OS–(Pt@HS–SiO ) ( ), Pt@HS–SiO
2
2
2
PL(EtOH, ) and Pt@HS–SiO PL(KPEGS, ) at 40 °C and 1 atm H2.
2
From the reaction profiles (Figure 2) it can be seen that the rates of reaction for each
catalytic system decreased in the order Pt@HS–SiO ˃ OS–(Pt@HS–SiO ) ˃ Pt@HS–SiO
2
2
2
PL(EtOH) ≥ Pt@HS–SiO PL(KPEGS). For the Pt@HS–SiO catalyst, a strong linear
2
2
correlation was observed when ln(decene) was plotted as a function of time (Figure S2),
indicating that for this catalyst the kinetics of this reaction are first order with respect to decene.
However, hydrogenation reactions catalysed by supported metal nanoparticles typically follow
[
17-19]
zero order kinetics with respect to the substrate.
The first order kinetics observed for
Pt@HS–SiO suggest that under these conditions the hydrogenation of decene is diffusion
2
[
20]
controlled. The permeability of the shell in core-shell nanoreactors is known to significantly
influence the kinetics of a reaction, impacting on the diffusion of reagents and products into and
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[
21-22]
out of the cavity and, hence, to and from the active catalyst.
Conversely, the reaction
profiles (Figure 2) for the OS–(Pt@HS–SiO ), Pt@HS–SiO PL(EtOH) and Pt@HS–SiO
2
2
2
PL(KPEGS) catalytic systems all initially display a linear correlation for the conversion of
decene as a function of time, signifying zero order kinetics with respect to decene for these
systems. At high decene conversions (˃80 %) the kinetics change so that the decene conversion
as a function of time is well reproduced by assuming kinetics that are first order with respect to
decene (Figure S2). The initial zero order kinetics can be attributed to the saturation of the
catalytic surface by the reagents and implies that for these systems the initial rate was not
[
18]
affected by the mass transfer of decene to the catalyst.
However, as the concentration of
decene decreases over the course of the reaction, the reaction eventually becomes diffusion
controlled, leading to a shift in the kinetics from zero order to first order with respect to
[
23]
decene. The change in the kinetics of the reactions, with respect to decene, from first order for
Pt@HS-SiO to zero order upon functionalisation with the corona-canopy, is unlikely to be due
2
to an improvement in the diffusion of the reagents to the catalyst as it is expected that
functionalisation will slow the rate of diffusion into the cavity. Rather it is more probable that the
difference in the kinetics of the catalytic systems is due to a change in the structure of the metal
nanoparticles themselves. During the functionalisation of the silica spheres by the corona-canopy
the Pt nanoparticles were found to agglomerate, increasing from approximately 0.84 to 1.2
[13]
nm. It has been well established that the shape/morphology, size and environment can have a
[
24-25]
considerable effect on the catalytic reactivity of metal nanoparticles.
Therefore, it is
probable that the change in the kinetics is due to a combination of factors, which would require
further in-depth study beyond the scope of the current paper.
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The initial pseudo zero order rate constants (k ) for the OS–(Pt@HS–SiO ), Pt@HS–SiO
2
0
th
2
PL(EtOH) and Pt@HS–SiO PL(KPEGS) catalytic systems were calculated from the reaction
2
profiles (Figure 2). The pseudo first order rate constants (k ) for each catalytic system were
1
st
calculated by modelling the data from Figure 2, assuming pseudo-first order kinetics (Table 1).
However, these rate constants are calculated independently of the Pt concentration and, thus, do
not take into account the different amounts of Pt in each catalyst. This is significant because, as
previously stated, the amount of each catalyst used was normalised to the mass of silica not Pt.
From ICP-OES analysis the Pt@HS–SiO PL sample had considerably lower concentrations of
2
Pt than either Pt@HS–SiO or OS–(Pt@HS–SiO ) (Table S1). Therefore, the rate constants
2
2
cannot be used to compare the catalytic activities of the different catalytic systems.
Table 1. Catalytic activity of the different catalyst systems for the hydrogenation of 1-decene.
k0th
k1st
TOFapp
a
Entry
Catalyst
Solvent
−1 b
−1 c
−1 d
(
mmol min )
(min )
0.0896
(h )
1
2
3
Pt@HS–SiO2
EtOH
EtOH
EtOH
—
2006
930
OS–(Pt@HS–SiO2)
0.0120
0.0071
0.0025
0.0682
0.0431
0.0087
Pt@HS–SiO PL
2062
768
2
4
Pt@HS–SiO PL
KPEGS
2
a
Conditions: 5 mg Pt@HS-SiO (or equiv.), 1-decene (0.5 mmol), 40 °C, 1 atm H .
2
2
b
Approximated from the initial linear sections of the reaction profiles in Figure 2
c
Approximated by modelling decene consumption with DynaFit4 assuming pseudo-first order reaction kinetics
[
26]
(
d
i.e. –d[decene]/dt = k[decene]).
−1 −1
Apparent turnover frequency; moldecene molPt h , calculated at roughly 25–30% conversion.
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A common method used to compare the catalytic activity of different catalysts is the
turnover frequency (TOF). However, the TOF assumes zero order kinetics with respect to the
[27-28]
reactants and varies with changing reaction conditions.
Despite the ambiguity of this term in
this instance it does provide a means to compare the different catalytic systems. Here, the
apparent turnover frequency (TOF ) is defined as the number or moles of decene converted per
app
mole of Pt per hour, calculated at a decene conversion of about 90%. From the TOFapp the order
of reactivity follows the trend Pt@HS–SiO PL(EtOH) ˃ Pt@HS–SiO ˃ OS–(Pt@HS–SiO ) ˃
2
2
2
−
1
Pt@HS–SiO PL(KPEGS) (Table 1). The TOF for Pt@HS–SiO was 2006 h (Table 1, entry
2
app
2
−
1
1
). This decreased to 930 h when the silica spheres were functionalised with the organosilane
Table 1, entry 2). The catalytic activities of polymer-supported catalysts are known to be
adversely affected when diffusional processes hinder the movement of reagents to the catalyst
(
[
29]
thus resulting in a reduction in the rate of reaction. Therefore, this decrease is not unexpected
as the reagents have to diffuse across the large charged corona-layer (~2 nm) to reach the active
catalyst. Furthermore, it could reasonably be expected that the addition of the canopy to form the
porous liquid would result in a further decrease in the rate of reaction. Conversely, the opposite
−
1
occurred with the TOFapp actually increasing upon the addition of the canopy to 2062 h (Table
, entry 3). This could possibly be attributed to the improved dispersion of the porous liquid in
ethanol compared to the Pt@HS–SiO and OS–(Pt@HS–SiO ). From the transmission electron
1
2
2
microscopy (TEM) images (Figure S3) the porous liquid exists as discrete spheres, while the
silica spheres in both Pt@HS–SiO and OS–(Pt@HS–SiO ) typically aggregate forming large
2
2
clusters of spheres. It is expected that it would be difficult for reagents to diffuse into the cavities
of the spheres in the middle of the clusters if similar behaviour occurs in solution. Conversely,
when the catalytic system exists as discrete spheres, the reagents can diffuse into the cavities
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from all directions, making the Pt nanoparticles more accessible and, hence, improving the rate
of reaction.
When the reaction is completed in ethanol the solvent facilitates the diffusion of reagents
into the cavities. We hypothesised that, for the “neat” porous liquid, when a liquid with
molecules small enough to diffuse through the pores of the silica shells is added, the molecules
will preferentially diffuse into the cavities as it is easier to displace gas than to dissolve in the
corona-canopy, where it would need to break interactions between the chains. This could be
expected to lead to an increase in the concentration of reactants around the catalyst and, hence,
increase the rate of reaction. However, the TOFapp was found to decrease by more than three-
−1
fifths to 768 h (Table 1, entry 4) for the Pt@HS–SiO PL catalyst when KPEGS was employed
2
as the solvent instead of ethanol. This decrease in the TOFapp is likely (at least partly) due to the
high viscosity of the KPEGS-based system. This phenomenon is known to occur for reactions in
[
30-31]
viscous media, with the rate of reaction becoming diffusion dependent.
Additionally, it has
*
previously been shown that the dipolarity/polarizability (π ) of the solvent used has a significant
*
effect on the kinetics of hydrogenation reactions, with solvents with higher π improving the
[
32]
adsorption of non-polar reagents on to the catalytic surface leading to faster rates of reactions.
For the “neat” porous liquid catalytic system the cavities are most likely to be occupied by
decene and decane rather than the solvent molecules, KPEGS, which is too large to diffuse into
*
the cavities. Therefore, the rate of reaction observed for this system is not influenced by the π of
the solvent as the species around the catalyst are non-polar and, hence, a slower reaction rate is
*
observed when compared to the reactions completed in ethanol (π = 0.51), where the solvent can
[
32]
diffuse into the cavities, interact with the Pt nanoparticles and alter the rate of reaction. While
the expected increases in catalytic activity in the “neat” porous liquid was not observed, this may
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be due to the mild conditions employed. It possible that under more forcing conditions, for
example higher temperatures and H pressures, a more significant improvement of the catalytic
2
[
33-35]
activity would be observed for the neat porous liquid, because of a decrease in viscosity.
Another important factor in the application of a catalyst, particularly industrially, is its
recyclability. In this regard, a number of properties of this Type I porous liquid, including its
non-volatility and thermal stability, means that this system could potentially be recycled by a
simple distillation of the products. To test this hypothesis, the Pt@HS–SiO PL(KPEGS) system
2
was recycled, for the hydrogenation of 1-decene, by drying the reaction mixture under vacuum at
4
0 °C overnight between each reaction. It was found that this catalytic system could be used at
least three times with no loss in the catalytic activity (Table 2).
Table 2. Recyclability of the Pt@HS–SiO PL(KPEGS) system for the hydrogenation of
2
a
1
-decene.
st
nd
rd
Cycle
1
2
3
Conversion of
9
7.8
99.3
98.4
b
1
-decene (%)
a
Conditions: 5 mg Pt@HS–SiO (or equiv.), 1-decene (0.5 mmol), 40 °C, 1 atm H for 5 h.
2
2
b
Conversion were determined by GC.
Hydrogenation of other substrates
The reaction protocol was further extended to include other substrates to explore the scope of
these catalytic systems for hydrogenation reactions (Table 3). For all substrates studied, the
activity of the different catalytic systems followed the general trend observed for 1-decene, with
the slowest rate occurring for the Pt@HS–SiO PL(KPEGS) system. There was complete
2
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selectivity to the desired hydrogenation product for 1-decene and 4-methylcyclohexene with the
four catalytic systems (Table 3, entries 1–8). However, for the hydrogenation of nitrobenzene,
side products were detected, to varying degrees, for all catalytic systems except for Pt@HS–SiO2
(
Table 3, entries 10–12). This was most significant for the Pt@HS–SiO PL(KPEGS) system
2
where, despite a conversion of 46%, the selectivity towards aniline, the desired product, was only
5
1% (Table 3, entry 12). These side products included N-phenylhydroxylamine, nitrosobenzene
[
36]
and azoxybenzene, all of which are known intermediates of the hydrogenation of nitroarenes.
The presence of these intermediates is therefore, a result of the slower reaction rates observed for
the OS–(Pt@HS–SiO ) and Pt@HS–SiO PL catalysts.
2
2
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a
Table 3. Hydrogenation of various substrates by the different catalytic systems.
b
Entry
Substrate
Catalyst
Solvent Time (h)
Product
Yield (%)
c
(Selectivity, %)
1
2
3
4
5
6
7
8
9
Pt@HS–SiO2
EtOH
EtOH
0.5
1.5
1.5
5.0
1.0
7.0
7.0
24
90.4
OS–(Pt@HS–SiO2)
94.2
Pt@HS–SiO PL
EtOH
90.8
2
Pt@HS–SiO PL
KPEGS
EtOH
88.4
2
Pt@HS–SiO2
94.9
OS–(Pt@HS–SiO2)
EtOH
94.2
Pt@HS–SiO PL
EtOH
85.9
2
Pt@HS–SiO PL
KPEGS
EtOH
81.6
2
Pt@HS–SiO2
3.0
24
89.7
1
1
1
1
1
0
1
2
3
4
5
OS–(Pt@HS–SiO2)
EtOH
87.4 (88)
95.9 (93)
23.7 (51)
92.7
Pt@HS–SiO PL
EtOH
24
2
Pt@HS–SiO PL
KPEGS
EtOH
24
2
Pt@HS–SiO2
3.0
24
Pt@HS–SiO PL
EtOH
26.2 (52)
8.5
2
1
Pt@HS–SiO PL
KPEGS
24
2
a
Conditions: 5 mg Pt@HS–SiO (or equiv.), 1-decene (0.5 mmol), 40 °C, 1 atm H .
Yields were determined by GC.
Selectivity relative to converted substrate quoted towards the product illustrated, calculated when additional
2
2
b
c
products were detected by GC. In all other instances, the selectivity was 100%.
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One of the major difficulties often encountered during the hydrogenation of
halonitrobenzenes is a poor selectivity towards the reduction of the nitro-group, with
[
36-37]
dehalogenation often occurring as an undesirable side reaction.
For the reduction of
4
-chloronitrobenzene, both the Pt@HS–SiO and Pt@HS–SiO PL showed complete selectivity
2
2
towards reduction of the nitro-group, with no dehalogenation products observed (Table 3, entries
3–15). When this reaction was completed by the Pt@HS–SiO PL catalyst in ethanol, however,
1
2
the selectivity towards 4-chloroaniline was only 52%, with two additional products detected (See
supporting information). The formation of these side products is most likely due to a reaction
[
38]
between an intermediate and ethanol.
Significantly, when the Pt@HS–SiO PL(KPEGS)
2
system was utilised for the same reaction no side products were detected (Table 3, entry 15).
Therefore, completing this reaction in a “neat” porous liquid system improved the selectivity as
there was no ethanol with which the intermediates could react. Additionally, all hydrogenation
reactions were completed at 40 °C, which is well below the melting point of
4
-chloronitrobenzene (ca 84 °C). Despite the higher melting point and absence of a conventional
solvent, the Pt@HS–SiO PL(KPEGS) system was still capable of catalysing the hydrogenation
2
of 4-chloronitrobenzene. The PEGS-based canopy was capable of dissolving
4
-chloronitrobenzene, enabling the diffusion of the reagent to the catalyst.
The application of a “neat” porous liquid system could therefore, enable a range of catalytic
reactions to be completed at lower temperatures without the addition of a conventional solvent,
potentially reducing undesirable side reactions and improving selectivity. While the Type I
porous liquids reported so far have all been based around a hollow silica sphere, the conceivable
[4-5]
scope of these systems is vast, particularly with respect to potential catalysts.
These systems
could be readily tailored in terms of the catalyst, shell material and corona-canopy and so it
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should be possible to design systems for specific reactions. Clearly also, although beyond the
scope of the present paper, different porous liquids could be easily combined to catalyse
[
39-40]
sequential reactions, perhaps even combining otherwise incompatible catalysts.
We have
operated only at moderate pressures. However, given the enhanced solubility of gases in porous
[
3-4, 10]
liquids, additional scope exists for reactions with gaseous reagents.
Conclusions
In summary, we have demonstrated for the first time the applicability of porous liquids as
catalysts. In this study we investigated the hydrogenation of several different reagents under mild
conditions. The Pt@HS-SiO PL catalyst in ethanol was found to be most active, with respect to
2
the concentration of platinum, for the hydrogenation of 1-decene compared to Pt@HS–SiO and
2
OS–(Pt@HS–SiO ). However, the reaction rate slowed when completed in a “neat” porous liquid
2
system due to the high viscosity of the system. For the hydrogenation of 4-chloronitrobenzene,
the “neat” porous liquid system was found to have complete selectivity towards 4-chloroaniline,
which was a significant improvement compared to the 52% selectivity for the same reaction
completed in ethanol. Additionally, it was found that the “neat” porous liquid system could be
recycled at least twice with no loss in catalytic activity for the hydrogenation of 1-decene.
This study provides the first step to improving our understanding of the utilisation of porous
liquids for a range of applications. However, further research is required to fully explore the
catalytic applicability of these systems.
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Experimental
Materials
Dodecane, mesitylene, nitrobenzene (all Merck), poly(ethylene glycol) 4-nonylphenyl 3-
sulfopropyl ether potassium salt (KPEGS), 4-methylcyclohexene (both Sigma Aldrich), Celite
5
45, n-heptane (both VWR), 4-chloronitrobenzene (Fluka), 1-decene (Alfa Aesar), ethanol
(UChem) and hydrogen gas (High purity, BOC) were used as received.
Preparation of Pt-based catalysts
[
13]
The Pt-based catalysts were prepared and characterised as previously reported.
General procedure for the catalytic hydrogenation reactions
Typically, a mixture of the catalyst (5 mg Pt@HS–SiO , 6 mg OS–(Pt@HS–SiO ) or 50 mg
2
2
Pt@HS–SiO PL) in ethanol (5 mL) was flushed with H and then stirred under an atmosphere of
2
2
H at 40 °C for 1 hour to activate the catalysts prior to the reaction. The substrate (0.5 mmol) and
2
internal standard (20 µL) were added and the mixture was stirred at 40 °C under an atmosphere
of H (balloon) for the desired length of time. The workup was conducted in air. For the solid
2
catalysts (Pt@HS–SiO and OS–(Pt@HS–SiO )) the reaction mixture was centrifuged to remove
2
2
the catalyst. For the reactions utilising Pt@HS–SiO PL as the catalyst, the reaction mixture was
2
passed through a plug of Celite to remove the porous liquid.
For reactions completed in the “neat” porous liquid, a mixture of Pt@HS–SiO PL (50 mg)
2
in KPEGS (0.45 g) was dried under vacuum at 40 °C overnight before backfilling with H . The
2
catalyst was activated by stirring the solution at 40 °C under an atmosphere of H for 1 hour
2
prior to use. The substrate (0.5 mmol) and standard (20 µL) were added and the solution was
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Chemistry - A European Journal
10.1002/chem.201905288
stirred at 40 °C under an atmosphere of H (balloon) for the desired length of time. The reaction
2
mixture was then dispersed in ethanol (~5 mL) in air and a sample was passed through a plug of
Celite to remove the catalyst. An external standard was used for reactions involving 1-decene
and 4-chloronitrobenzene.
The reaction mixtures were analysed by either Gas Chromatography-Flame Ionisation
Detector (GC-FID, Shimadzu GC 2010 Plus Gas Chromatograph) or Gas Chromatography-Mass
Spectrometry (GC-MS, Shimadzu GCMS-QP2010 Gas Chromatograph). The standards used
were as follows; mesitylene for 1-decene, n-heptane for 4-methylcyclohexene, and dodecane for
both nitrobenzene and 4-chloronitrobenzene.
Acknowledgments
The authors acknowledge the facilities and the scientific and technical assistance of the
Australian Microscopy & Microanalysis Research Facility at the Australian Centre for
Microscopy & Microanalysis at The University of Sydney. E.B.H. acknowledges the receipt of
an Australian Government Research Training Program Stipend scholarship.
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Chemistry - A European Journal
10.1002/chem.201905288
Entry for the Table of Contents
Porous liquids for catalysts: Platinum nanoparticles encapsulated within a Type I porous liquid were successfully employed
for the first time as hydrogenation catalysts. In addition, this work explored the effect of each component of the corona-
canopy on the catalytic activity.
Keywords: Heterogeneous catalysis, Hydrogenation, Mesoporous material, Platinum, Porous liquids
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