Optimization of transition metal nanoparticle-phosphonium ionic liquid composite catalytic systems for deep hydrogenation and hydrodeoxygenation reactions

Article abstract of DOI:10.1039/c4gc01716a

A variety of metal nanoparticle (NP)/tetraalkylphosphonium ionic liquid (IL) composite systems were evaluated as potential catalysts for the deep hydrogenation of aromatic molecules. Particles were synthesized by reducing appropriate metal salts by LiBH₄ in a variety of ILs. Gold NPs were used as probes to investigate the effect of both chain lengths of the alkyl substituents on the phosphonium cation and the nature of anions, on the stability of NPs dispersed in the ILs. The presence of three medium-to-long alkyl chains (such as hexyl) along with one long alkyl chain (such as tetradecyl) in the IL, coupled with highly coordinating anions (such as halides, or to a smaller extent, bis-triflimides) produced the most stable dispersions. These ILs also showed maximum resistance to heat-induced sintering; for example, TEM studies of Pt NPs heated under hydrogen to 120 °C showed only moderate sintering in trihexyl(tetradecyl)phosphonium chloride and bis(triflimide) ILs. Finally, olefinic hydrogenations, aromatic hydrogenations, and hydrodeoxygenation of phenol were carried out with Ru, Pt, Rh and PtRh NPs using hydrogen at elevated pressures. From preliminary studies, Ru NPs dispersed in trihexyl(tetradecyl)phosphonium chloride emerged as the catalyst system of choice. The presence of borate Lewis-acid by-products in the reaction medium (from the borohydride reduction step) allowed for partial phenol hydrodeoxygenation.

Full text of DOI:10.1039/c4gc01716a

View Article Online
View Journal
Green
Chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: A. Banerjee and
R. W. J. Scott, Green Chem., 2014, DOI: 10.1039/C4GC01716A.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/greenchem

Page 1 of 9
Green Chemistry
View Article Online
DOI: 10.1039/C4GC01716A
Green Chemistry
RSCPublishing
ARTICLE
Optimization of transition metal nanoparticle-
phosphonium IL composite catalytic systems for deep
hydrogenation and hydrodeoxygenation reactions
Cite this: DOI: 10.1039/x0xx00000x
Abhinandan Banerjeea and Robert W. J. Scotta
Received 00th January 2012,
Accepted 00th January 2012
A variety of metal nanoparticle (NP)/tetraalkylphosphonium ionic liquid (IL) composite
systems were evaluated as potential catalysts for the deep hydrogenation of aromatic
molecules. Particles were synthesized by reducing appropriate metal salts by LiBH4 in a
variety of ILs. Gold NPs were used as probes to investigate the effect of both chain lengths
of the alkyl substituents on the phosphonium cation and the nature of anions, on the stability
of NPs dispersed in the ILs. The presence of three mediumꢀtoꢀlong alkyl chains (such as
hexyl) along with one long alkyl chain (such as tetradecyl) in the IL, coupled with highly
coordinating anions (such as halides, or to a smaller extent, bisꢀtriflimides) produced the
most stable dispersions. These ILs also showed maximum resistance to heatꢀinduced
sintering; for example, TEM studies of Pt NPs heated under hydrogen to 120oC showed only
moderate sintering in trihexyl(tetradecyl)phosphonium chloride and bis(triflimide) ILs.
Finally, olefinic hydrogenations, aromatic hydrogenations, and hydrodeoxygenation of
phenol were carried out with Ru, Pt, Rh and PtRh NPs using hydrogen at elevated pressures.
From preliminary studies, Ru NPs dispersed in trihexyl(tetradecyl)phosphonium chloride
emerged as the catalyst system of choice. The presence of borate LewisꢀAcid byꢀproducts in
the reaction medium (from the borohydride reduction step) allowed for partial phenol
hydrodeoxygenation.
DOI: 10.1039/x0xx00000x
www.rsc.org/
environmental importance.15ꢀ17 Several groups, including our
own, have shown that NP dispersions of catalytically active
Introduction
In recent years, ionic liquids (ILs) have been used
extensively for the stabilization of metal nanoparticles (NPs)
and/or as reaction media for metal NPꢀcatalysed reactions.1ꢀ5
Imidazolium ILs, in particular, have been studied extensively
for their remarkable ability to serve as reaction media and/or
stabilizers for such NPꢀcatalysed transformations.6ꢀ10 However,
the imidazolium ILs offer challenges such as baseꢀinduced
deprotonation and consequent carbene formation,11 hydrolysis
of anions such as PF6 and BF4 to generate corrosive acids,12
variations in the efficiency of NP stabilization induced by the
presence of trace amounts of impurities such as unreacted IL
precursors,8,13 and comparatively higher melting points of
imidazolium halides, which are not roomꢀtemperature ILs.14 It
is surprising, therefore, that tetraalkylphosphonium ILs, which
are stable under highly basic conditions, commercially
available at high levels of purity, less sensitive to moisture and
oxygen than their imidazolium counterparts, and largely present
as liquids at room temperature even when combined with
highly coordinating anions such as halides, have not been
investigated more extensively as potential replacements for
imidazolium ILs in catalytic reactions of industrial and
precious metals in tetraalkylphosphonium ILs are effective and
recyclable catalysts for diverse classes of reactions such as
oxidations,18 selective hydrogenations,19,20 and CꢀC cross
coupling reactions.21,22
One class of reactions that have remained relatively
unexplored in the context of metal NP/tetraalkylphosphonium
IL composite catalysts is deep hydrogenations, or
hydrogenations of aromatic compounds.23 This is a key reaction
in processes such as hydroꢀrefining of heavy oil, production of
petrochemicals, synthesis of pharmaceutical products, and
generation of biofuel from nonꢀedible lignin.24 Synthesis of
biofuels from lignin includes both hydrogenation as well as
hydrogenolysis steps,25ꢀ27 and it is expected that under suitably
tuned reaction conditions, a metal NP/IL composite catalyst can
perform both reactions simultaneously. NPs of precious metals
such as Ir, Rh and Ru have the advantage of high activity for
the hydrogenation of aromatic compounds under mild reaction
conditions, and their dispersions in functionalized imidazolium
ILs have been used in several studies for hydrogenation of
mononuclear aromatic species.4,28ꢀ30 Dyson and coꢀworkers,
-
-
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 00, 1-8 | 1

Green Chemistry
Page 2 of 9
View Article Online
ARTICLE
Green Chemistry
DOI: 10.1039/C4GC01716A
notably, have applied these systems as catalysts in reactions bestow upon it greater NP stabilizing abilities and the presence
such as hydrodeoxygenation (HDO) of phenol to cyclohexane, of one alkyl chain longer than the other three (and the resultant
and regioselective hydrogenation of toluene to methyl asymmetry in the molecular structure of the IL) is crucial for
cyclohexene in imidazolium ILs.31ꢀ33 Other protocols also exist the stability of the NPs in the ILs.4,18ꢀ21
for similar conversions, using metal or metal oxide catalysts in
different IL media, but to the best of our knowledge,
tetraalkylphosphonium ILs have not been used for metal NPꢀ
catalysed aromatic hydrogenations, despite their NPꢀstabilizing
abilities and chemical inertness to a variety of reagents.34,35
In this work, we show the synthesis and stabilization of
catalytically active precious metal NPs in a variety of
tetraalkylphosphonium ILs. Initial studies concerning the
correlation between structural aspects of the ILs (such as
polarizability of anions, lengths of alkyl substituents, etc.) and
their
NPꢀstabilizing
abilities
show
that
trihexyl(tetradecyl)phosphonium chloride (P[6,6,6,14]Cl and
bis(triflimide) (P[6,6,6,14]NTf2) were promising candidates for
catalytic NP/IL nanocomposite fabrication when compared to
‘shortꢀchain’ (P[4,4,4,4]Cl), ‘mediumꢀchain’ (P[8,8,8,8]Br),
and poorlyꢀcoordinatingꢀanionꢀcontaining (P[6,6,6,14]N(CN)2)
ILs. Ru, Rh, Pt, and RhPt NPs in P[6,6,6,14]Cl were active
catalysts for simple hydrogenations of allylic alcohols.
However, the Ru NP/P[6,6,6,14]Cl IL composite proved to be
the most effective system for deep hydrogenations of aromatic
compounds such as toluene and phenol (a ligninꢀanalogue) at
elevated temperatures and high hydrogen pressures. Minimal
NP growth was seen to occur even at higher temperatures under
hydrogen atmospheres in P[6,6,6,14]Cl or P[6,6,6,14]NTf2 ILs
compared to other tetraalkylphosphonium ILs. Interestingly,
significant conversion of phenol to the HDO products was seen
even in the absence of an added Lewis acid. Further
investigations showed the presence of residual borates in the
ꢀ
reaction medium (byꢀproducts from the initial BH4 reduction)
facilitated the conversion of phenol to Cꢀ6 hydrocarbons.36,37
This study shows that metal NP/IL composites hold promise for
the potential conversion of lignin to C6ꢀalkanes.
Figure 1. UV-Visible spectra of Au NPs in various representative ILs recorded
immediately after synthesis (light grey solid line), after three days (deep grey
dashed line), and after heating under N2 at 150oC for 1 h(black dotted line).
Results and discussion
UV-Vis spectroscopic study of Au NP/IL nanocomposites
The presence of highly coordinating anions (such as halides
and bis(triflimide)s) are necessary for the IL to be a good NP
stabilizer.20 It is evident, therefore, that P[6,6,6,14]Cl (and to a
smaller extent, P[4,4,4,14]Cl) satisfy the criteria for being
efficient NPꢀstabilizers, and could potentially be used in
catalytic nanocomposites for hydrogenations. TEM images (Fig
S1, ESI) support these conclusions: in P[4,4,4,14]Cl before heat
treatment, the average particle size is 4.5± 0.6 nm; after heat
treatment, there is an average particle size growth of ~ 7 nm,
with an average final particle size of 11.3 ± 7.1 nm. Similarly,
in P[6,6,6,14]Cl, before heatꢀtreatment, the average particle size
is 4.1 ± 0.6 nm; after heatꢀtreatment, a bimodal distribution of
particle sizes is observed, with mean particle diameters of 3.5 ±
0.6 nm (which corresponds to the asꢀsynthesized particle sizes)
and 12.2 ± 3.5 nm (which corresponds to the sintered NP sizes).
Although even the longerꢀchained, asymmetric ILs failed to
Au NPs were synthesized by reducing HAuCl4 by LiBH4 in
a variety of ILs. Following the synthesis, the stability of the Au
NPs over time and after heating under nitrogen at 150oC for an
hour was monitored by UVꢀVis spectroscopy, as shown in
Figure 1. Other than the Au NP/P[4,4,4,4]Cl IL system, all
systems show an initial Au plasmon band in the 520ꢀ600 nm
range. It is evident that the Au NPs show maximum resistance
to coalescence in P[6,6,6,14]Cl and P[4,4,4,14]Cl, given the
absence of any tremendous shifts in the plasmon band. Au NPs
were unstable to aggregation over 3 days in P[4,4,4,4]Cl, and
P[4,4,4,8]Cl ILs; this is seen by the shift and dampening of the
plasmon band to much higher wavelengths, which is typically
due to dipoleꢀdipole interactions between aggregated
particles.38 This observation is in line with previous literature:
namely, longer alkyl chains in a tetraalkylphosphonium IL
2 | Green Chem., 2014, 00, 1-8
This journal is © The Royal Society of Chemistry 2014

Page 3 of 9
Green Chemistry
Green Chemistry
View Article Online
ARTICLE
DOI: 10.1039/C4GC01716A
protect Au NPs completely from heatꢀinduced sintering (as
The Pt NPs grew in size from 4.7 ± 1.2 nm to 12.2 ± 6.1 nm
indicated by a blue shift of the Au NP plasmon bands after upon heating under hydrogen in P[4,4,4,8]Cl. On the other
heating), it was noted that no visible NP precipitation occurred hand, in P[6,6,6,14]Cl, both the initial size of the Pt NPs and
in any of these systems.
the increase in Pt NP size was much smaller (<2 nm). The only
nonꢀhalide IL tested, P[6,6,6,14]NTf2, showed a moderate
increase in particle size upon exposure to hydrogen under heat
Pt NPs in ILs after hydrogen treatment
It is known that presence of hydrogen at elevated (from 5.2 ± 1.2 nm to 9.2 ± 2.1 nm). Other nonꢀhalide ILs such
temperatures can lead to the formation of surface metal as P[6,6,6,14]N(CN)2 showed spontaneous NP aggregation and
hydrides in Pt NPs. This could potentially destabilize the NPs, precipitation at much lower temperatures (ca. 800C). On the
leading to their agglomeration and precipitation as other basis of these observations, we selected P[6,6,6,14]Cl (as a
stabilizing species get displaced. Thus it was essential for us to representative halide IL) and P[6,6,6,14]NTf2 (as
study the a system under hydrogenation reaction conditions in representative nonꢀhalide IL) for subsequent experiments.
the absence of substrates.39,40 Pt NPs were synthesized in the
a
Hydrogenation of 2-methylbute-3-en-2-ol by NP/IL composites
ILs and exposed to 1 atm hydrogen at 150oC for 12h to study
their stability. TEM images of Pt NPs in P[4,4,4,8]Cl and
The hydrogenation of 2ꢀmethylbuteꢀ3ꢀenꢀ2ꢀol to 2ꢀ
P[6,6,6,14]Cl before and after hydrogen treatment are shown in methylbutanꢀ2ꢀol was selected as an initial test reaction, using
Figure 2.
Pt, Ru, Rh, and Pt/Rh bimetallic NPs. At 30 bar hydrogen
pressure and at moderately elevated temperatures (> 600C),
high conversions were the norm with almost all the NPs under
investigation, with conversions ranging from 88ꢀ97% (Table
1). Both P[6,6,6,14]Cl and P[6,6,6,14]NTf2 proved to be
capable of metal NP stabilization under reaction conditions.
Table 1 summarizes the results.
Table 1 Hydrogenation of 2ꢀmethylbuteꢀ3ꢀenꢀ2ꢀol catalysed by 10 mM
NP/IL composites*
Entry
IL
NP system
Conversion#
1
2
P[6,6,6,14]Cl
P[6,6,6,14]Cl
Pt
Pt/Rh
92%
90%
3
4
5
P[6,6,6,14]Cl
P[6,6,6,14]NTf2
P[6,6,6,14]NTf2
Ru
Rh
Pt
97%
90%
88%
* Conditions: substrate:catalyst = 215; 30 bar Hydrogen; 700C; 24 h.
#% conversion derived from GCꢀFID analysis (see Experimental).
Deep hydrogenation of toluene by NP/IL composites
The deep hydrogenation of toluene to methylcyclohexane
requires a high pressure of hydrogen and an efficient catalyst to
go to completion, owing to the aromatic stability of the benzene
ring.41 It is, therefore, a suitable trial reaction for testing the
efficacy of metal NP/IL nanocomposite hydrogenation
catalysts. The results obtained during the course of these trials
Figure 2. TEM images of Pt NPs in P[4,4,4,8]Cl (before heating: A, after heating:
B), P[6,6,6,14]Cl (before heating: C, after heating: D), and P[6,6,6,14]NTf2 (before
heating: E, after heating: F). Average sizes of NPs are as follows: (A): 4.7 ± 1.2
nm, (B): 12.2 ± 6.1 nm, (C): 2.1 ± 0.4 nm, (D): 3.8 ± 0.9 nm, (E): 5.2 ± 1.2 nm, and
(F): 9.2 ± 2.1 nm. Heating was performed under 1 atm hydrogen at 1500C for
12h.
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 00, 1-8 | 3

Green Chemistry
Page 4 of 9
View Article Online
ARTICLE
Green Chemistry
DOI: 10.1039/C4GC01716A
have been summarized in Table 2, for one halideꢀcontaining HDO of phenol by NP/IL composites.
and one halideꢀfree IL. It can be seen that Pt NPs and PtꢀRh
We selected 10mM Ru NP/P[6,6,6,14]Cl as our composite
catalyst of choice for the deep hydrogenation of phenol under
elevated hydrogen pressures. UVꢀVis spectroscopy suggests
full reduction of the RuCl3 precursor to Ru NPs (Figure S8, SI);
and TEM images (Fig S9, ESI) indicated the average particle
size was 2.9 ± 1.2 nm. The hydrogenation of phenol and
substituted phenols have been carried out over group VIII
metals by others; hydrogenation of phenol produces two
important compounds, cyclohexanone and cyclohexanol, both
of which are utilized in the manufacture of a large number of
industrial products.48ꢀ53 A tentative pathway for phenol HDO
has been depicted in Figure 3.
NPs were unsuccessful in catalysing this reaction. When
Rh2(OAc)6 was used as a precursor for the Rh NPs, poor
conversions were obtained; use of Ru NPs led to higher yields
and TONs of ca. 70. In all cases, methylcyclohexane was the
only product; no partially hydrogenated products could be
detected. This is in agreement with previous work by Dupont
and coꢀworkers, who used Ru NPs in ILs such as 1ꢀbutylꢀ3ꢀ
methylimidazolium and 1ꢀdecylꢀ3ꢀmethylimidazolium Nꢀ
bistriflimides (ꢀNTf2) and tetrafluoroborates to give TONs of
ca. 170 after 18 hours of reaction.42 Similarly, a Ruꢀcluster
catalyst, used by Welton and coꢀworkers for hydrogenation of
toluene in 1ꢀbutylꢀ3ꢀmethylimidazolium tetrafluoroborate, gave
an average TON of 240.43The presence of halides, which are
known to inhibit catalytic activities of nanoparticles via activeꢀ
site blocking, could be responsible for reduced yields. For
example, Ir nanoclusters have been shown by others to be
poisoned by Clꢀ for benzene hydrogenation, but remained active
for hydrogenation of simple olefinic moieties.44 However, this
would not explain why Ru NPs in P[6,6,6,14]NTf2 (a nonꢀ
halide IL) show similar catalytic activities as the P[6,6,6,14]Cl
IL in this study. We are not certain why the deep hydrogenation
activity of NPs in tetraalkylphosphonium ILs is lower than for
other classes of ILs; possibilities for reduced activity include
growth or aggregation of the NP catalysts, polydispersity of
catalytic sites in this system and/or presence of different
catalytic sites on NP surfaces.45ꢀ47 Moderate NP growth for the
Ru NP system after catalysis is documented in the next section.
Figure 3. Reaction pathway for phenol HDO by Ru NP in IL.
Phenol was found to be highly soluble in the NP/IL
composite solution, and showed moderate conversions under
specified
reaction
conditions.
Conversions,
product
distributions, etc. of various trials have been summarized in
1
Table 3; H and 13C NMR spectra can be found in the ESI.
Higher temperatures and greater H2 pressures led to a greater
degree of conversion of phenol. Cyclohexanone was not present
in any of our product isolates, indicating that it is rapidly
converted to cyclohexanol; this is similar to results obtained by
some researchers, while others have identified it as a key
intermediate, possibly indicating that the nature of the catalyst
determines the relative rates of the various steps.54ꢀ56 It was
surprising, however, that instead of cyclohexanol, the expected
hydrogenation product of phenol, a mixture of cyclohexanol,
cyclohexene and cyclohexane was isolated. In the presence of
a Lewis Acid that had been added deliberately to the reaction
mixture to catalyse the dehydration of the alcohol, this would
be in accordance with previous reports: in fact, steps leading to
each of these products has been summarised in Figure 3.57ꢀ59
However, our system did not contain any intentionallyꢀadded
Lewis Acids. In the following section, we identify the species
responsible for this serendipitous dehydration.
Table 2 Hydrogenation of toluene catalysed by 10mM MNP/IL composites.*
Entry
IL
NP system
Conversion#
1
2
3
4
5
P[6,6,6,14]NTf2
P[6,6,6,14]NTf2
P[6,6,6,14]NTf2
P[6,6,6,14]Cl
P[6,6,6,14]Cl
P[6,6,6,14]Cl
P[6,6,6,14]Cl
Ru
Pt
Rh
Ru
Pt
22%
~2%
5%
34%
~2%
4%
6
Pt/Rh
Ru
In a representative experiment, the TON for the phenol
hydrogenation/HDO was ~65 after 24 h at 1200C under 25 bar
hydrogen. This TON is modest compared to those reported by
Yan et al. in their polymeric IL systems, as well as in the Lewis
Acidꢀfunctionalized ILs,31,32,53,59 (TONs of the order of 200ꢀ
500) but comparable with the TONs reported for other systems
such as Pt nanowires in water (TON= 50), Pd/C/lanthanide
triflates in dichloromethane and ILs (TON= 10) and Rh NPs in
ILs (TON= 100).60ꢀ63 The involvement of the hydroxyl group
attached to the ring in the reaction mechanism has previously
7†
30%
* Conditions: substrate:catalyst = 188; 30 bar Hydrogen; 1200C; 24 h.
† Reaction performed in a Parr reactor equipped with a mechanical stirrer.
# % conversion derived from GCꢀFID analysis (see Experimental).
4 | Green Chem., 2014, 00, 1-8
This journal is © The Royal Society of Chemistry 2014

Page 5 of 9
Green Chemistry
Green Chemistry
View Article Online
ARTICLE
DOI: 10.1039/C4GC01716A
species in the reaction medium were Lewis Acid catalysts.69
Similarly, Riisager and coꢀworkers showed that boric acid, in
the presence of salts such as NaCl, is an efficient catalyst for
the dehydration of fructose to 5ꢀhydroxymethylfurfural.70 Boric
acid, being cheap, nonꢀtoxic, and less corrosive than traditional
dehydrating agents (such as concentrated sulphuric acid), is a
more benign option for acidꢀcatalysed dehydrations.71 Others
have demonstrated the conversion of chitin into a nitrogenꢀ
containing furan derivative under optimized conditions by
using boric acid as a dehydrating agent.72 However, it was still
essential for us to rule out other species present in the reaction
mixture as possible catalysts. Therefore, control experiments
were devised to eliminate the IL itself as the Lewis acid
responsible for the dehydration. Details of these control
experiments have been summarized in Figure 4. The IL itself
been found to diminish catalytic activity via activeꢀsite
blocking,64ꢀ67 which might account for reduced TONs. Ru has
long since been known for its ability to form cyclohexene
selectively during processes such as benzene hydrogenation
using Ru NPs.67,68 Possible phaseꢀseparation of cyclohexene
during the reaction is a potential cause for incomplete
conversion of cyclohexene to cyclohexane. Heatꢀinduced
changes in particle morphology and/or chemical nature under
high H2 pressures during the course of the reaction could also
account for this. Two backꢀtoꢀback catalytic cycles were carried
out in order to investigate the recyclability of the system (entry
6); it was seen that the conversion dropped slightly upon
recycling with cyclohexene becoming the major product. TEM
images (see SI, Fig. S10) of Ru NPs in P[6,6,6,14]Cl after one
catalytic cycle show slight growth of the average NP size to 4.8
± 2.0 nm. The presence of a very small population of larger
NPs can also be seen in these studies, which may be sintered
Ru NPs or the IL itself.
showed no dehydration activity (control experiment
A in Fig.
5), while Ru NPs generated in the IL by hydrazine reduction
also did not convert cyclohexanol to C6 hydrocarbons (control
experiment
converted cyclohexanol to cyclohexane and cyclohexene
(control experiment ). Addition of sodium tetraborate
(100mM) to the IL generated ca. 15% C6 hydrocarbons from
cyclohexanol. Higher conversions ca. 35%) for the
B). Ru NPs prepared by borohydride reduction
C
Table 3 HDO of phenol catalysed by 10mM RuNP/IL composites.*
(
dehydration of cyclohexanol were seen when lithium
borohydride was added to a neat IL sample, followed by
exposure to air and quenching, thereby generating a soluble
borate species (control experiment
D). An additional control
experiment, in which the NP/IL composite was used in the
absence of H2 to estimate the effect of unreacted borohydride
on the product distribution pattern, led to less than 1%
conversion, with traces of the alkene as the only identifiable
product. Further confirmation of the presence of borate species
was obtained by 11B NMR spectroscopy (see ESI), which
indicated the presence of unreacted borohydride in a freshly
prepared unquenched Ru NP/IL composite. Upon being
quenched with acidified methanol or by prolonged exposure to
air, the borohydride peak vanished, and a new peak appeared,
which could be assigned to a borate species.73,74 Thus, one of
the advantages of synthesizing metal NPs in ILs via
borohydride reduction is the generation of Lewis Acidic borate
byꢀproducts in the reaction mixture that can catalyse subsequent
reaction steps in a tandem fashion.
Entry T(OC) PH2 (bar)
Conversion#
A
B
C
1
2
3
120
120
110
135
65
25
25
22
25
20
23%
28%
20%
20%
10%
15%
27% 52%
38% 48%
60%
62% 20%
77% 11%
41%
21%
14%
37%
18%
12%
15%
3%
4
5
6$
110
25
44%
*Conditions: substrate:catalyst = 230; 24 h; all reactions performed in a Parr
reactor equipped with a mechanical stirrer.
$ Recyclability test: RuNP/IL composite recovered after removal of reactants
and products via vacuum stripping used for a second catalytic cycle.
Conclusions
A new composite system, transition metal NPs dispersed in
tetraalkylphosphonium ILs, was investigated for hydrogenation
and phenol HDO catalysis at elevated temperatures and
pressures. The following conclusions were drawn from the
study:
1. Tetraalkylphosphonium ILs bearing three mediumꢀtoꢀlong
alkyl chains (such as hexyl) along with one long alkyl chain
(such as tetradecyl) in the cation, along with a highly
coordinating anion (such as halides, or to a smaller extent, bisꢀ
triflimides) are able to offer maximum stabilization for metal
NPs.
# % conversion and selectivities of A, B, and C derived from GCꢀFID
analysis (see Experimental).
HDO of phenol by MNP/IL composites: role of borates
The generation of cyclohexane and cyclohexene products
suggested that the initially generated cyclohexanol underwent
dehydration promoted by a Lewis Acid entity present in the
reaction mixture. A similar observation was made in the past by
Kobayashi and coworkers using polymerꢀstabilized AuPd
catalysts in water, who showed that advantageous borate
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 00, 1-8 | 5

Green Chemistry
Page 6 of 9
View Article Online
ARTICLE
Green Chemistry
DOI: 10.1039/C4GC01716A
2. Ru NPs in P[6,6,6,14]Cl and P[6,6,6,14]NTf2 can catalyse Tetrachloroauric acid, HAuCl4.4H2O and potassium
the hydrogenation of aliphatic as well as aromatic molecules at tetrachloropalladate, K2PdCl4, (both 99.9%, metals basis) were
high temperatures under high hydrogen pressures; yields and all obtained from Alfa Aesar. Other metal precursors, such as
TONs are higher for the aliphatic substrate.
rhodium
acetylacetonate
([CH3COCH=C(Oꢀ)CH3]3Rh),
ruthenium chloride (RuCl3.xH2O), sodium hexachlororhodate
(Na3RhCl6), platinum acetylacetonate ([CH3COCH=C(Oꢀ
)CH3]2Pt), and rhodium acetate ([Rh(CH3COO)2]2•2H2O) were
purchased from Sigma Aldrich and used without purification.
All metal salts were stored under vacuum and flushed with
nitrogen after every use. Commercial samples of all the
tetraalkylphosphonium ILs mentioned in this work were
generously supplied by Cytec Industries Ltd. Commercial
samples of liquid ILs were dried under vacuum at 70oC for 5ꢀ6
hours with stirring before use. P[4,4,4,14]Cl, which was a solid
at roomꢀtemperature, was melted via heating, and used without
preceding purification steps, while P[4,4,4,4]Cl, which was
obtained as a solution in toluene, was heated under vacuum for
6 hours at 700C for solvent removal. Deuterated solvents were
purchased from Cambridge Isotope Laboratories. 18 MΩcm
MilliꢀQ water (Millipore, Bedford, MA) was used throughout.
UVꢀVis spectra were obtained using a Varian Cary 50 Bio UVꢀ
Vis spectrophotometer with a scan range of 200ꢀ800 nm and an
1
optical path length of 1.0 cm. H and 13C NMR spectra were
obtained using a Bruker 500 MHz Avance NMR spectrometer;
chemical shifts were referenced to the residual protons of the
deuterated solvent. TEM analyses of the NPs in IL were
conducted using a Philips 410 microscope operating at 100 kV,
with magnifications ranging from 55,000 to 145,000x (see scale
bars in individual TEM images). The samples in IL were
prepared by ultrasonication of a 5% solution of the NP/IL
solution in THF followed by dropꢀwise addition onto a carbonꢀ
coated copper TEM grid (Electron Microscopy Sciences,
Hatfield, PA). To determine average particle diameters, a
minimum of 100 particles from each sample were measured
from several TEM images using the ImageJ program.
Conversion and selectivity for the catalytic reactions were
obtained by gasꢀ chromatography (GC) using a flame ionization
detector (FID, Agilent Technologies 7890A) and a HPꢀInnowax
capillary column.
Figure 4. Summary of control experiments performed to evaluate the role of
borohydride side products present within the composite catalyst matrix in phenol HDO
product distribution. Reaction conditions common to all reactions were as follows: 24
bar Hydrogen pressure; IL P[6,6,6,14]Cl; 1200C, 24 hours. A: in neat P[6,6,6,14]Cl, B: in
P[6,6,6,14]Cl containing 10mM Ru NPs synthesized via hydrazine reduction; C: in
P[6,6,6,14]Cl containing 10mM Ru NPs synthesized via borohydride reduction; D: in
P[6,6,6,14]Cl containing ca. 150mM soluble borate generated from lithium
borohydride.
3. Ru NPs in P[6,6,6,14]Cl can catalyse the HDO of phenol at
110ꢀ1250C under 20ꢀ25 bar hydrogen pressures, with moderate
yields.
Synthesis of NPs in ILs
4. The product distribution pattern for the nanocompositeꢀ
catalysed phenol HDO indicates that the residual borates
present within the catalyst as a byꢀproduct of the borohydride
reduction step actually serve as a Lewis acid catalyst leading to
dehydration of cyclohexanol, thus generating C6 hydrocarbons.
It is expected that further research into metal NP/IL
nanocomposites, with or without coꢀcatalytic additives, would
offer greater insight into the chemistry of these promising
catalytic materials.
In a representative synthesis of 5mM Au NPs in IL, 20 mg
of HAuCl4 was added to
a
10mL sample of the
trinbutyl(noctyl)phosphonium chloride (P[4,4,4,8]Cl) IL at
600C, and vigorously stirred. To this solution, a stoichiometric
excess of LiBH4 reagent (1.0 mL, 2.0M in THF) was injected
dropꢀwise over a period of five minutes. A brisk effervescence
followed, and the entire solution turned wineꢀred, indicating NP
formation. After the addition of LiBH4, volatile impurities were
removed by vacuumꢀstripping the system at 700C. The Au NP
solution thus obtained was stored under nitrogen in a capped
vial until use. For the synthesis of Pt and Ru NPs, platinum
acetylacetonate and ruthenium chloride, respectively, were used
as precursors. Rh NPs, on the other hand, were difficult to
synthesize, possibly owing to the limited solubility of most of
our Rh precursors of choice. Rhodium acetate had a very high
Experimental Section
General Information
All chemicals except for the ones listed below were
purchased from Sigma Aldrich and used as received.
6 | Green Chem., 2014, 00, 1-8
This journal is © The Royal Society of Chemistry 2014

Page 7 of 9
Green Chemistry
Green Chemistry
View ArticAle ROnTliInCeLE
DOI: 10.1039/C4GC01716A
degree of solubility in our ILs, and subsequently, we used that Notes and references
a Department of Chemistry,
University of Saskatchewan, Saskatoon,
SK S7N 5C9, Canada,
as our precursor of choice. For the synthesis of coꢀreduced Ptꢀ
Rh bimetallic NPs, both precursors were simultaneously
dissolved in the IL, and reduced at the same time. Generally,
after NP synthesis, excess LiBH4 was quenched with acetone,
and volatiles were subsequently removed by vacuumꢀstripping
at 700C.
eꢀmail: robert.scott@usask.ca,
Phone: 1ꢀ306ꢀ966ꢀ4730
†
Electronic Supplementary Information (ESI) available: [NMR spectra
Stability Tests for NP/IL composites
of phenol HDO products, 11B NMR spectra of borohydride to borate
As mentioned previously, the NPs were subjected to conversion, TEM images of Au NPs in various ILs before and after
conditions similar to those they would experience during the heating, TEM images and UVꢀVisible spectra of Ru NPs before and after
course of reactions in order to evaluate their stability under catalysis]. See DOI: 10.1039/b000000x/
such conditions. Briefly, Au NPs formed in shortꢀalkylꢀchained,
mediumꢀalkylꢀchained, and longꢀalkylꢀchained ILs were
examined by UVꢀVis spectroscopy (after dilution with MeOH
to ~0.1mM) directly after synthesis, after 3 days of storage, and
after being heated to ~ 1000C under nitrogen for a period of 15
minutes to a half hour. Rh and Pt NPs, similarly, were heated
under hydrogen at 1500C for 12 hours, and their TEM images
recorded before and after the heat treatment. The results from
these experiments assisted us in selecting an optimal metal
NP/IL catalytic composite for high pressure hydrogenations.
1
2
3
4
M. Ali, A. Gual, G. Ebeling and J. Dupont, ChemCatChem, 2014, 6,
2224.
J. Dupont and M. R. Meneghetti, Curr. Opin. Colloid Interface Sci.
2013, 18, 54.
,
G. S. Fonseca, A. P. Umpierre, P. F. Fichtner, S. R. Teixeira and J.
Dupont, Chem. Eur. J., 2003, , 3263.
9
A. Banerjee, R. Theron and R. W. J. Scott, Chem. Commun., 2013,
49, 3227.
5
6
P. Migowski and J. Dupont, Chem. Eur. J., 2007, 13, 32.
P. Dash, N. A. Dehm and R. W. J. Scott, J. Mol. Catal. A: Chem.
,
General Procedure for Hydrogenation Reactions
2008, 286, 114.
Hydrogenation of organic substrate reactions were carried
out in hermetically sealed stainlessꢀsteel Parr highꢀpressure
reactors. For the hydrogenations of 2ꢀmethylbuteꢀ3ꢀenꢀ2ꢀol and
toluene (except for one experiment; see Table 2), a Parr 4790
highꢀpressure static reactor (without stirring facilities) was
used. For phenol HDO, a dynamic Parr reactor equipped with a
temperature control system, a mechanical stirrer, and a pressure
meter (Parr 4560) was selected. In a general procedure, 10 mL
of the NP solution in IL was mixed with the precursor,
transferred to the reaction chamber inside the reactor, flushed
with hydrogen at moderate pressures to ensure removal of
dissolved oxygen, and heated to the requisite temperature under
a high pressure of hydrogen inside the sealed reactor (typically,
30 bar). After reaction, the mixture was allowed to cool to
ambient temperatures, the hydrogen pressure was released, and
the reaction mixture was transferred to a Schlenk flask and
vacuum stripped to remove any products formed and/or
unreacted starting materials. The products extracted were
7
8
P. Dash and R. W. J. Scott, Chem. Commun., 2009, 812.
M. T. Kessler, M. K. Hentschel, C. Heinrichs, S. Roitsch and M. H.
Prechtl, RSC Adv., 2014, , 14149.
L. Foppa, J. Dupont and C. W. Scheeren, RSC Adv., 2014,
4
9
4, 16583.
10. L. Luza, A. Gual, C. P. Rambor, D. Eberhardt, S. R. Teixeira, F.
Bernardi, D. L. Baptista and J. Dupont, Phys. Chem. Chem. Phys.,
2014, advance article. DOI: 10.1039/C4CP03063J
11 L. S. Ott, M. L. Cline, M. Deetlefs, K. R. Seddon and R. G. Finke, J.
Am. Chem. Soc., 2005, 127, 5758.
12 R. P. Swatloski, J. D. Holbrey and R. D. Rogers, Green Chem., 2003,
5, 361.
13 L. L. Lazarus, C. T. Riche, N. Malmstadt and R. L. Brutchey,
Langmuir, 2012, 28, 15987.
14 M. Atilhan, J. Jacquemin, D. Rooney, M. Khraisheh and S. Aparicio,
Ind. Eng. Chem. Res., 2013, 52, 16774.
15 R. E. Del Sesto, C. Corley, A. Robertson and J. S. Wilkes, J.
Organomet. Chem., 2005, 690, 2536.
16 K. J. Fraser and D. R. MacFarlane, Aust. J. Chem., 2009, 62, 309.
17 C. J. Bradaric, A. Downard, C. Kennedy, A. J. Robertson and Y.
1
subsequently characterized by H NMR, 13C NMR, and GCꢀ
FID. Conversion and selectivity were obtained from GCꢀFID.
For GCꢀFID analysis, 100µL of a neat extract was mixed with
enough ethyl acetate to make a final volume of 10mL. 1mL of
this solution was then taken in a GCꢀvial, and subjected to
analysis. The percentage conversion and product selectivities
were calculated from two identical measurements, using the
GCꢀFID peak areas and calibration curves for each species.
Zhou, Green Chem., 2003, 5, 143.
18 A. Maclennan, A. Banerjee and R. W. Scott, Catal. Today, 2013,
207, 170.
19 K. L. Luska and A. Moores, Green Chem., 2012, 14, 1736.
20 A. Banerjee, R. Theron and R. W. J. Scott, ChemSusChem, 2012,
109.
5,
21 H. A. Kalviri and F. M. Kerton, Green Chem., 2011, 13, 681.
22 V. Ermolaev, D. Arkhipova, L. S. Nigmatullina, I. K. Rizvanov, V.
Milyukov and O. Sinyashin, Russ. Chem. Bull., 2013, 62, 657.
Acknowledgements
The authors acknowledge financial assistance from the National Sciences and
Engineering Research Council of Canada (NSERC) in the form of an NSERC
Engage grant. In addition, we thank Al Robertson and Jeffrey Dyck Cytec
Industries, Inc. for providing samples of the ILs surveyed as well as
providing valuable input into the project design.
23 S.ꢀC. Qi, L. Zhang, X.ꢀY. Wei, J.ꢀi. Hayashi, Z.ꢀM. Zong and L.ꢀL.
Guo, RSC Adv., 2014, 4, 17105.
24 S. P. Srivastava and J. Hancsók, Fuels and Fuel-Additives, John
Wiley and Sons, Hoboken, New Jersey, 2014.
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 00, 1-8 | 7

Green Chemistry
Page 8 of 9
View Article Online
ARTICLE
Green Chemistry
DOI: 10.1039/C4GC01716A
25 T. Q. Hu, C.ꢀL. Lee, B. R. James and S. J. Rettig, Can. J. Chem.
,
56 A. L. Jongerius, R. Jastrzebski, P. C. Bruijnincx and B. M.
Weckhuysen, J. Catal., 2012, 285, 315.
1997, 75, 1234.
26 E. Furimsky, Appl. Cat. A: Gen., 2000, 199, 147.
27 S. W. Eachus and C. W. Dence, Holzforschung, 1975, 29, 41.
57 N. Yan, C. Zhao, P.J. Dyson, C. Wang, L.ꢀT. Liu and Y. Kou,
ChemSusChem, 2008, 1, 626.
28 J. Zhong, J. Chen and L. Chen, Catal. Sci. Technol., 2014, advance 58 C. Zhao, Y. Kou, A. A. Lemonidou, X. Li and J. A. Lercher, Chem.
article. DOI: 10.1039/C4CY00583J
29 X.ꢀD. Mu, J.ꢀQ. Meng, Z.ꢀC. Li and Y. Kou, J. Am. Chem. Soc.
2005, 127, 9694.
Commun., 2010, 46, 412.
,
59 C. Zhao, Y. Kou, A. A. Lemonidou, X. Li and J. A. Lercher, Angew.
Chem. Int. Ed., 2009, 48, 3987.
30 A. P. Umpierre, G. Machado, G. H. Fecher, J. Morais and J. Dupont, 60 Z. Guo, L. Hu, H.ꢀH. Yu, X. Cao and H. Gu, RSC Adv., 2012,
Adv. Synth. Catal., 2005, 347, 1404. 3477.
31 J. Chen, J. Huang, L. Chen, L. Ma, T. Wang and U. I. Zakai, 61 T. Yu, J. Wang, X. Li, X. Cao and H. Gu, ChemCatChem, 2013,
ChemCatChem, 2013, , 1598. 2852.
32 N. Yan, Y. Yuan, R. Dykeman, Y. Kou and P. J. Dyson, Angew. 62 A. L. Maksimov, S. N. Kuklin, Y. S. Kardasheva and E. A.
2,
5,
5
Chem. Int. Ed., 2010, 49, 5549.
Karakhanov, Petrol. Chem., 2013, 53, 157.
33 F. Schwab, M. Lucas and P. Claus, Angew. Chem. Int. Ed., 2011, 50
,
63 J. Y. Shin, D. J. Jung and S.ꢀG. Lee, ACS Catal., 2013, 3, 525.
10453.
64 D. K. Lee, S. J. Ahn and D. S. Kim, 9th International Symposium on
Catalyst Deactivation, Lexington, KY, USA, 2001.
34 T. Ramnial, D. D. Ino and J. A. C. Clyburne, Chem. Commun., 2005,
325.
65 A. Popov, E. Kondratieva, L. Mariey, J. M. Goupil, J. El Fallah, J.ꢀP.
Gilson, A. Travert and F. Maugé, J. Catal., 2013, 297, 176.
35 T. Ramnial, S. A. Taylor, M. L. Bender, B. Gorodetsky, P. T. K. Lee,
D. A. Dickie, B. M. McCollum, C. C. Pye, C. J. Walsby and J. A. C. 66 E. Furimsky, Catal. Rev., 1983, 25, 421.
Clyburne, J. Org. Chem., 2008, 73, 801.
67 M. F. F. Rodrigues and A. J. G. Cobo, Catal. Today, 2010, 149, 321.
36 T. Ståhlberg, S. RodriguezꢀRodriguez, P. Fristrup and A. Riisager, 68 C. Fan, Y.ꢀA. Zhu, X.ꢀG. Zhou and Z.ꢀP. Liu, Catal. Today, 2011,
Chem. Eur. J., 2011, 17, 1369.
160, 234.
37 E. A. Khokhlova, V. V. Kachala and V. P. Ananikov, ChemSusChem
,
69 W.ꢀJ. Yoo, H. Miyamura and S. Kobayashi, J. Am. Chem. Soc., 2011,
133, 3095.
2012,
5, 783.
38 S. Link and M. A. ElꢀSayed, J. Phys. Chem. B, 1999, 103, 4212.
70 T. Ståhlberg, S. RodriguezꢀRodriguez, P. Fristrup and A. Riisager,
Chem. Eur. J., 2011, 17, 1456.
39 S. Kishore, J. A. Nelson, J. H. Adair and P. C. Eklund, J. Alloys
Compd., 2005, 389, 234.
71 M. Walia, U. Sharma, V. K. Agnihotri and B. Singh, RSC Adv., 2014,
40 T. Hawa and M. Zachariah, J. Chem. Phys., 2004, 121, 9043.
4, 14414.
41 P. A. Rautanen, J. R. Aittamaa and A. O. I. Krause, Ind. Eng. Chem. 72 X. Chen, S. L. Chew, F. M. Kerton and N. Yan, Green Chem., 2014,
Res., 2000, 39, 4032. 16, 2204.
42 M. H. G. Prechtl, M. Scariot, J. D. Scholten, G. Machado, S. R. 73 T. P. Onak; H. Landesman; R. E. Williams J. Phys. Chem. 1959 63,
,
Teixeira and J. Dupont, Inorg. Chem., 2008, 47, 8995.
43 P. Dyson, D. Ellis and D. Parker, Chem. Commun., 1999, 25.
44 E. Bayram, M. Zahmakıran, S. Özkar and R. G. Finke, Langmuir
2010, 26, 12455.
1533.
74 C. D. Good; D.M. Ritter J. Am. Chem. Soc., 1962, 84, 1162.
,
,
45 E.E. Finney and R. G. Finke, Inorg. Chim. Acta., 2006, 359, 2879.
46 Y. Xia, Y. Xiong, B. Lim and S. E. Skrabalak, Angew. Chem. Int. Ed.
2009, 48, 60.
47 A. R. Tao, S. Habas, and P. Yang, Small, 2008,
48 N. Mahata, K. Raghavan, V. Vishwanathan, C. Park and M. Keane,
Phys. Chem. Chem. Phys., 2001, , 2712.
4, 310.
3
49 J.ꢀF. Zhu, G.ꢀH. Tao, H.ꢀY. Liu, L. He, Q.ꢀH. Sun and H.ꢀC. Liu,
Green Chem., 2014, 16, 2664.
50 Y. Wang, J. Yao, H. Li, D. Su and M. Antonietti, J. Am. Chem. Soc.
,
,
,
2011, 133, 2362.
51 H. Liu, T. Jiang, B. Han, S. Liang and Y. Zhou, Science, 2009, 326
1250.
52 A. Chen, Y. Li, J. Chen, G. Zhao, L. Ma and Y. Yu, ChemPlusChem
2013, 78, 1370.
53 H. Xu, K. Wang, H. Zhang, L. Hao, J. Xu and Z. Liu, Catal. Sci.
Technol., 2014, , 2658.
4
54 H. Ohta, H. Kobayashi, K. Hara and A. Fukuoka, Chem. Commun.
,
2011, 47, 12209.
55 C. Zhao and J. A. Lercher, ChemCatChem, 2012,
8 | Green Chem., 2014, 00, 1-8
4, 64.
This journal is © The Royal Society of Chemistry 2014

Page 9 of 9
Green Chemistry
View Article Online
DOI: 10.1039/C4GC01716A
Optimization of transition metal nanoparticle-phosphonium ionic liquid
composite catalytic systems for deep hydrogenation and hydrodeoxygenation
reactions
Abhinandan Banerjee and Robert W. J. Scott
TOC Graphic:
Single-sentence abstract: "Stable metal nanoparticles in tetraalkylphosphonium ionic liquids can
catalyze hydrogenations, as well as phenol hydrodeoxygenation, owing to presence of adventitious
borates"