Solubility adjustable nanoparticles stabilized by a novel PVP based family: Synthesis, characterization and catalytic properties

Article abstract of DOI:10.1039/b905625d

Systematic fabrication of nanoparticle stabilizers can substantially modify the properties of prepared nanoparticles; here the synthesis of solubility adjustable nanoparticles is achieved by employing a family of polarity modulated stabilizers. The Royal

Full text of DOI:10.1039/b905625d

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Solubility adjustable nanoparticles stabilized by a novel PVP based
family: synthesis, characterization and catalytic propertiesw
Ning Yan, Jia-guang Zhang, Yinyin Tong, Siyu Yao, Chaoxian Xiao,
Zichen Li and Yuan Kou*
Received (in Cambridge, UK) 20th March 2009, Accepted 11th May 2009
First published as an Advance Article on the web 12th June 2009
DOI: 10.1039/b905625d
Systematic fabrication of nanoparticle stabilizers can substantially
modify the properties of prepared nanoparticles; here the syn-
thesis of solubility adjustable nanoparticles is achieved by
employing a family of polarity modulated stabilizers.
identical conditions. Thus, a series of poly-3-ethyl-1-vinyl-2-
pyrrolidone (C2-PVP), poly-3-butyl-1-vinyl-2-pyrrolidone
(C4-PVP), poly-3-hexyl-1-vinyl-2-pyrrolidone (C6-PVP) and
poly-3-octyl-1-vinyl-2-pyrrolidone (C8-PVP) was obtained.
Their molecular weights were found to be approximately
14–19 kDa and their structures verified by elemental analysis,
NMR and IR techniques (see ESIw for data and spectra).
Their decomposition temperatures under both inert and
oxidative atmosphere are higher than 350 1C, as demonstrated
by TGA analysis (see ESI,w Fig. S3). These four polymers,
together with commercially available PVP, formed a family of
MNPs stabilizers (hereinafter referred as Cn-PVP) with similar
capability of protecting MNPs but distinctive solubility behaviors.
To investigate the possibility of using alkyl modified PVP as
MNP stabilizers and the behaviors of the prepared MNPs,
soluble Rh MNPs protected by Cn-PVP were prepared using
RhCl3 as the metal precursor and H2 as the reducing agent (see
ESIw for synthetic details). The micrographs of the Rh MNPs are
shown in Fig. 1. Relatively narrow unimodal size distributions
with a diameter centered at approximately 3 nm are observed for
PVP, C2-PVP, C4-PVP and C6-PVP protected Rh MNPs. For
C8-PVP protected Rh MNPs the size distribution is slightly wider
and the average diameter is approximately 1 nm larger. The Rh
nanoparticles were further characterized by FT-IR spectroscopy
using CO as the probe. For all the five samples, three peaks in the
region of 1980–2080 cmꢀ1 were observed (see ESI,w Fig. S6),
Metal nanoparticles (MNPs), due to their promising optical,
electronic, magnetic and catalytic properties, have received
intense and continuous interest over the last few years.1 Recent
advances have already led to the success in controlled synthesis of
MNPs with different size, morphology, structure and com-
position.2 It is then highly desirable that the resulting MNPs
can be dissolved in a given solvent for application purposes.
Although there are several excellent examples of hydrophilic–
hydrophobic bifunctional stabilizers that can work in both
aqueous and organic media,3 in most cases MNPs protected by
a certain stabilizer are only soluble in a limited number of
solvents. Thus the lack of the tunable solubility of the MNPs
has become an obstacle for their applications.3e Poly-1-vinyl-2-
pyrrolidone (PVP), for example, is one of the most powerful
stabilizers for MNP preparation. The applications of the PVP
stabilized MNPs, however, are largely limited to water4 and
alcohol5 media due to the insoluble nature of PVP in non-protic
solvents. Previously, we developed a novel polymer by copoly-
merization of PVP monomer and an ionic monomer which
became applicable in ionic liquids (ILs).6 Nevertheless, a versatile
strategy for the application of PVP type stabilizers is still lacking.
We report herein the development of a polarity adjustable PVP
family by molecular engineering of PVP to expand the range of
solvents used for the preparation and application of MNPs.
N-Vinyl-2-pyrrolidone (NVP), the monomer of PVP, can be
functionalized first by deprotonation at position 3 by a strong
base to form a carbanion followed by a nucleophilic reaction.7
Reaction of NVP with lithium diisopropylamide (LDA) and
then with alkyl bromide gave two products, 3-alkyl-1-vinyl-2-
pyrrolidone and 3,3-dialkyl-1-vinyl-2-pyrrolidone (see Scheme 1).
Free radical polymerization of 3-alkyl-1-vinyl-2-pyrrolidone
yielded the corresponding polymer whereas 3,3-dialkyl-1-
vinyl-2-pyrrolidone was resistant to polymerization under
which can be attributed to Rh(CO)2 (B2072 and B2000 cmꢀ1
)
and Rh–CO (2017 cmꢀ1), respectively.8 The similarity of their
adsorption curves indicates the surface coordination properties
of the five nanoparticles are similar.
The current work focuses on the systematic solubility
modulation of the stabilizer so that reaction media can be
chosen rationally. As such, the solubility of the MNPs-Cn-PVP
as a function of aliphatic chain length was of particular
interest. The solubility of various Rh MNPs in thirteen
PKU Green Chemistry Centre, Beijing National Laboratory for
Molecular Sciences, College of Chemistry and Molecular Engineering,
Peking University, Beijing 100871, China.
E-mail: yuankou@pku.edu.cn; Fax: +86-10-62751708;
Tel: +86-10-62757792
w Electronic supplementary information (ESI) available: Detailed
synthetic procedures and characterization of the Cn-PVP and
Cn-PVP protected MNPs, as well as instrumentation and catalysis.
See DOI: 10.1039/b905625d
Scheme 1 Synthetic strategy for Cn-PVP.z
Chem. Commun., 2009, 4423–4425 | 4423
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Fig. 2 Solubility of Rh MNPs stabilized by the PVP family in
thirteen solvents (from top to bottom: Rh-PVP, Rh-C2-PVP,
Rh-C4-PVP, Rh-C6-PVP and Rh-C8-PVP; Rh 5 mM, Rh : Cn-PVP =
1 : 10, pictures taken after one month storing).
MNPs (Fig. 1(f)). The turnover frequency (TOF) of Rh-PVP
towards toluene hydrogenation was 322 hꢀ1, similar to that
of Rh MNPs stabilized by PVP-IL copolymers.6a Rh-Cn-PVP
(n = 2, 4, 6) exhibited much higher activity, with TOF values
approximately twice that of Rh-PVP MNPs. Considering that
the TEM and IR analysis of the Rh MNPs were similar, the
different activity may arise from the decreased polarity of
the Rh-Cn-PVP which makes toluene more accessible to the
surface of the MNPs. The TOF value of Rh-C8-PVP MNPs
decreased slightly with respect to Rh-Cn-PVP (n = 2, 4, 6),
which could be due, at least in part, to the increased diameter
of the Rh-C8-NPs or to the lower solubility of Rh-C8-PVP in
ethanol compared to the others (vide supra).
Fig. 1 TEM micrographs and size distributions of Rh MNPs stabilized
by PVP and Cn-PVP: (a) Rh-PVP; (b) Rh-C2-PVP; (c) Rh-C4-PVP;
(d) Rh-C6-PVP; (e) Rh-C8-PVP. (f) TOF of Rh stabilized by the
PVP family towards toluene hydrogenation (80 1C, 3 MPa H2, ethanol
10 mL, toluene 1 mL, Rh 0.05 mmol).
common solvents, ranging in polarity from water to hexane,
was examined (see Fig. 2). Rh-PVP was only soluble in water,
ethanol and partially soluble in acetic acid and acetonitrile,
being completely insoluble in all the other solvents. As the
length of the side chain increases, the Rh-Cn-PVP gradually
became more nonpolar and insoluble in water. However, their
solubility in less polar solvents is greatly improved; for
example, Rh-C2-PVP dissolved in THF and halogenated
alkanes. Rh-C6-PVP exhibited the best solubility behavior
and was soluble in 10 solvents, including hexane. It is inter-
esting to note that the solubility behavior of the Cn-PVP
family protected Rh MNPs in acetonitrile and acetone
did not obey monotonic trends. Rh-PVP can not be dissolved
in either of the two solvents. As the side alkyl chain
length increases, Rh-Cn-PVP became first soluble and then
insoluble in both cases. The partition coefficients of the
various Rh-Cn-PVP MNPs were also tested using hexane
and water (see ESI,w Fig. S4). Rh-PVP was completely dis-
solved in the bottom aqueous layer while Rh-C8-PVP was
found exclusively in the hexane phase, displaying a dramatic
solubility change in the series of Rh-Cn-PVP MNPs.
TEM and CO adsorption analysis, together with the solubility
tests and the catalytic activity of the Rh-Cn-PVP, suggest that
(a) the solubility of the MNPs prepared by employing Cn-PVP
as stabilizer is adjustable, enabling optional choice of a solvent
as the reaction media; (b) other properties of the MNPs (at
least for Rh), such as the size, the chemical environment and
the catalytic activity are similar.
In a demonstration of the versatility of Cn-PVP based
MNPs, other metals and reducing agents, as well as solvents
with different polarity, were used (see ESI,w Fig. S7 for TEM
pictures and EDX analysis). Not surprisingly, Ru, Pt and Pd
(entries 1–3, Table 1) MNPs were successfully obtained
by classical methods employing EtOH or NaBH4 as the
reductant. Further utilizing the tunable solubility of the novel
Cn-PVP stabilizers, LiAlH4 was employed for the preparation
of Ag MNPs in dioxane (entry 4, Table 1). Co MNPs were
readily prepared by thermal decomposition of Co2(CO)8 in the
presence of C8-PVP in toluene or squalene (entries 5 and 6,
Table 1). From this it is clear that functionalizing PVP with an
Arene hydrogenation was considered to be an ideal model
reaction to evaluate the catalytic activity of various Rh MNPs.
Since ethanol is the only solvent that can dissolve (or partially
dissolve) all five Rh-Cn-PVPs, toluene hydrogenation was
carried out in ethanol at 80 1C and 3 MPa H2 by Rh-Cn-PVP
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Table 1 MNPs prepared by different Cn-PVP and synthetic strategy in various solvents
Entry
Metal
Precursor
Stabilizer
Solvent
Prep. method
Size/nm
1
2
3
4
5
6
Ru
Pd
Pt
Ag
Co
Co
RuCl3
PdCl2
H2PtCl6
AgOAc
Co2(CO)8
Co2(CO)8
C2-PVP
C4-PVP
C4-PVP
C6-PVP
C8-PVP
C8-PVP
EtOH–H2O
EtOH
EtOH
Dioxane
Toluene
Squalane
NaBH4 as reductant
EtOH as reductant
NaBH4 as reductant
LiAlH4 as reductant
Thermal decomp.
Thermal decomp.
B5–7a
5.0 ꢂ 1.2
2.3 ꢂ 0.3
4.0 ꢂ 1.1
15–30a
15–30a
a
MNPs were found to stick to each other and as a result the size distribution was not determined.
alkyl chain not only endows MNPs with adjustable solubility
but also expands the range of possible reagents (both reduc-
tants and metal precursors) and as a result the scope of PVP
based MNPs formation and catalysis.
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Recently, several Fischer–Tropsch (F–T) processes based on
soluble Ru and Co MNPs have been established.9 However,
these researches were conducted in water and ILs, which are
not the conventional solvents applied in the slurry reactor for
F–T reaction. A quasi-homogeneous process that uses high
boiling point alkanes as solvent, as currently practised in
industry, has not yet been developed. With this in mind, we
tested the F–T synthesis activity over Co MNPs derived from
the thermal decomposition of Co2(CO)8 in squalane and
stabilized by C8-PVP. Co-C8-PVP exhibited F–T reaction
activity at temperatures higher than 170 1C with the TOF
value increasing with temperature. The reaction of syngas
(20 atm, H2–CO 2 : 1) over the Co-C8-PVP at 200 1C for
12 h gave an average TOF value of 1.3, of the same order of
magnitude as the value observed for supported cobalt catalysts
and higher than that of Co MNPs in ILs (see ESI,w Table S1).
However, the MNPs were not stable and after reaction, Co
MNPs were partially precipitated and Co black was found in
the autoclave. Nevertheless, this is the first example of a F–T
process catalyzed by soluble Co MNPs in paraffin solvents,
demonstrating that MNPs stabilized by the hydrophobic
C8-PVP can be applied in nonpolar solvents for catalysis.
In summary, we have developed a general strategy for the
preparation of solubility adjustable MNPs by selecting appro-
priate Cn-PVP as the stabilizer. We believe the current
work should broaden the applications of MNPs protected by
PVP-like polymers. For example, they can be, in principle,
applied as catalysts for novel biphasic reactions. Moreover,
the modified PVP family for MNPs preparation demonstrates
that there is great potential to improve quasi-homogeneous
catalysis by tailoring the stabilizers. Further examples involving
steric, electronic modifications and bi-functional stabilizers are
currently under development in this laboratory.
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This work was supported by the National Science Founda-
tion of China (Projects 20773005 and 20533010). The authors
thank Mr Ryan Dykeman for useful discussions.
Notes and references
z All organic synthesis was handled with standard Schlenk techniques.
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Chem. Commun., 2009, 4423–4425 | 4425