Catalysis Science & Technology
Page 2 of 11
DOI: 10.1039/C8CY01346B
genations in the presence of water and air came from the re-
search group of Moores, who published several seminal stud-
ies on catalytic hydrogenations and oxidations using Fe/Fe-oxide
core shell NPs dispersed in solvents such as water and ethanol,
both in conventional reactors and under flow conditions.13–16 It
is surprising, however, that the catalytic application of Fe NPs in
alternative solvents such as ionic liquids (ILs) has not been ex-
plored in any great detail, despite a number of synthetic protocols
ing liquid cells with X-ray permeable windows, our group has
shown that in situ XAS studies can be used to follow catalyst
speciation in their reaction solutions.2
7,28
Since the recyclabil-
ity of Fe NP catalysts in alternative solvents depends on inter-
actions with said solvent under reaction conditions, it is imper-
ative to characterize these systems in liquid conditions in or-
der to have a better understanding of the catalytic process as a
whole. For example, in a recent study, Kimura and colleagues
synthesized zerovalent Fe NPs in P[8,8,8,8][Anion] ([Anion]=
formate, bis(trifluoromethylsulfonyl)imide) ILs by laser ablation
of Fe foils.29 XAS results showed that the formate anion could
slow down oxidation of the zerovalent Fe NPs compared to the
bis(triflimide) anion system. However, no catalysis was attempted
with these NPs.
1
7,18
for the fabrication of Fe NPs in ILs.
Many routes to Fe and
FeOx NPs in ILs involve decomposition of organometallic com-
plexes containing iron(0), such as Fe(CO) or Fe(COT)2 (COT
5
1
9,20
=
1,3,5,7-cyclooctatetraene), in IL media.
In a representa-
tive example, Leal et al. decomposed Fe(COT)2 in 1-N-butyl-3-
methylimidazolium bis(trifluoromethanesulfonyl)imide IL in the
◦
presence of 5 bar H at 75 C to generate fairly uniform but cat-
2
This study compares the catalytic behaviour and longevity
21
alytically inactive 5.3±1.6 nm superparamagnetic FeO NPs. To
the best of our knowledge, there are no existing studies compar-
ing and contrasting the catalytic activity and post-catalytic fate
of the Fe NPs in protic solvents vis-a-vis ionic liquids. One of the
few studies that looked at Fe NPs as potential hydrogenation cat-
alysts in IL media used nitrile-functionalized bis(triflimide) imi-
of Fe NPs in two different solvent types.
Two differ-
ent sizes of polyvinylpyrrolidone (PVP) capped Fe@FexOy
NPs were synthesized by the reduction of Fe salts with
NaBH , and their catalytic activities were studied in ethanol
4
for the hydrogenation of alkenes.
In contrast, Fe NPs
were also synthesized in two tetraalkylphosphonium ionic liq-
uids: tri(hexyl)tetradecylphosphonium chloride, P[6,6,6,14]Cl,
a representative halide IL that has proved to be an ex-
cellent ’solvent-cum-stabilizer’ for catalytically active Pd and
dazolium ILs, with a methyl substituent on the imidazolium C to
2
prevent proton abstraction by the Grignard reagent used for re-
duction of the Fe precursor, as a solvent as well as a NP-stabilizer
2
2
for Fe NP catalyzed hydrogenations. These Fe NPs could selec-
tively generate (Z)-alkenes from alkynes. However, this protocol
necessitated the use of a task-specific IL and heptane as a co-
solvent, as well as strict anaerobic conditions.
Ru NPs, and tri(hexyl)tetradecylphosphonium bis(triflimide),
30
P[6,6,6,14]NTf , a halide-free IL.
Ru NPs have been shown
2
to remain highly active hydrogenation catalysts in the latter IL
31
for eight consecutive reaction cycles by Luska et al. These ILs
There are certain obvious reasons why the fabrication of Fe NPs
in solvents other than water or volatile organics merits a detailed
investigation. While iron oxide NPs in water are cheap and easy
to make, they also suffer from challenges such as larger over-
possess intrinsic nanoparticle-stabilizing abilities due to anion ad-
sorption on NP surfaces, unlike solvents such as ethanol, where
an external stabilizer (such as PVP) is necessary to ensure stable
NP dispersions. All NPs were then examined as catalysts in the hy-
drogenation of representative alkenes with molecular hydrogen.
The changes in the speciation of the Fe NPs before and after hy-
drogenation reactions was monitored by in situ Fe K-edge XANES
spectra. In IL solvents, oxidative degeneration of Fe NPs occurs
via a mechanism that does not involve the formation of a pro-
tective oxide coating on the nanoparticle surfaces, which appears
to limit their use as stabilizing solvents for catalytically active Fe
NPs.
1
4,15
all sizes and inhomogeneous size distributions.
In addition,
speciation studies performed on these catalytic systems are most
commonly of the ex situ variety, such as XRD and XPS of iso-
lated catalytic samples. These studies are incapable of monitoring
changes in the oxidation states of Fe atoms on the NP surfaces,
in real time, in the reaction medium. It is to be noted that the
behavior of metal NPs in ILs can differ significantly when com-
pared to water and other protic solvents; we recently showed
that Ni NPs in tetraalkylphosphonium halide ILs upon exposure
to oxygen form a chloronickellate complex rather than a nickel
oxide coating on the NP surfaces, which has been previously re-
2 Experimental Section
2
3,24
2.1 Materials and methods
ported to happen in other solvents.
Similarly, Kessler et al.
reported that adjustment of reaction parameters, such as tem-
perature, time and heating methods, have a profound influence
on the oxidation state of the metal in metal NPs generated via
the heating of metal salts in tetraalkylphosphonium acetates.25 It
is important, therefore, to use techniques such as X-ray absorp-
tion spectroscopy (XAS) which can give us valuable information
about the speciation of transition metals in solution. XAS is a
powerful tool to probe the oxidation states and coordination en-
vironments of elements in materials with short-range order such
as nanostructures. XAS places few experimental constraints on
the samples; for example, it is possible to follow speciation and
oxidation state changes in liquid solutions using XAS.26 By us-
Unless otherwise mentioned, all chemicals were used as received.
Iron(III) chloride and Iron(III) acetylacetonate (Fe(acac) ) were
3
purchased from Fisher Scientific. LiAlH4 (2.0 M in THF) was
purchased from Sigma Aldrich. Fe(II) sulfate heptahydrate
(FeSO .7H O), 1-octene, octane, 2-norbornene and norbornane
4
2
were purchased from Sigma-Aldrich. PVP (M. W. 58,000 g/mol)
was purchased from Alfa Aesar. Sodium borohydride, methanol
and ethanol (HPLC grade) were purchased from Fisher Scientific.
Eighteen MΩ cm Milli-Q water (Millipore, Bedford, MA) was used
for the synthesis of Fe@FexOy NPs in the mixture of methanol and
water. The tri(hexyl)tetradecylphosphonium (P[6,6,6,14]X; X= -
Cl or -NTf ) ILs were purchased from Cytec Industries Ltd., and
2
2
|
1–10