Gold nanoparticles incarcerated in nanoporous syndiotactic polystyrene matrices as new and efficient catalysts for alcohol oxidations

Article abstract of DOI:10.1002/chem.201101034

The controlled synthesis of gold nanoparticles (AuNPs), incarcerated in a semicrystalline nanoporous polymer matrix that consisted of a syndiotactic polystyrene-co-cis-1,4-polybutadiene multi-block copolymer is described. This catalyst was successfully te

Full text of DOI:10.1002/chem.201101034

FULL PAPER
DOI: 10.1002/chem.201101034
Gold Nanoparticles Incarcerated in Nanoporous Syndiotactic Polystyrene
Matrices as New and Efficient Catalysts for Alcohol Oxidations
Antonio Buonerba,[a] Cinzia Cuomo,[a] Sheila Ortega Sꢀnchez,[a] Patrizia Canton,[b] and
Alfonso Grassi*[a]
Abstract: The controlled synthesis of
gold nanoparticles (AuNPs), incarcer-
ated in a semicrystalline nanoporous
polymer matrix that consisted of a syn-
responding kinetic plots were among
the highest found for polymer-incarcer-
ated AuNPs. Similar values in terms of
reactivity and selectivity were found in
the oxidation of primary alcohols, such
as cinnamyl alcohol and 2-thiophene-
methanol, and secondary alcohols, such
as indanol and a-tetralol. The remark-
able catalytic properties of this system
were attributed to the formation, under
these reaction conditions, of the nano-
porous e crystalline form of syndiotac-
tic polystyrene, which ensures facile
and selective accessibility for the sub-
strates to the gold catalyst incarcerated
in the polymer matrix. Moreover, the
polymeric crystalline domains pro-
duced reversible physical cross-links
that resulted in reduced gold leaching
and also allowed the recovery and
reuse of the catalyst. A comparison of
catalytic performance between AuNPs
and annealed AuNPs suggested that
multiple twinned defective nanoparti-
cles of about 9 nm in diameter consti-
tuted the active catalyst in these oxida-
tion reactions.
diotactic
polystyrene-co-cis-1,4-poly-
butadiene multi-block copolymer is de-
scribed. This catalyst was successfully
tested in the oxidation of primary and
secondary alcohols, in which we used
dioxygen as the oxidant under mild
conditions. Accordingly, (Æ)-1-phenyl-
ACHTUNGTRENNUNGethanol was oxidised to acetophenone
in high yields (96%) in 1 h, at 358C,
whereas benzyl alcohol was quantita-
tively oxidised to benzaldehyde with a
selectivity of 96% in 6 h. The specific
rate constants calculated from the cor-
Keywords: alcohol
oxidation
·
·
gold heterogeneous catalysis
·
nanoparticles · nanoporous polymer
matrix
Introduction
of the main targets in gold catalysis is to stabilise the AuNPs
to prevent the aggregation of the nanoparticles without de-
activating the active sites. For this purpose the AuNPs are
typically supported on carbon or metal oxides, such as TiO2,
CeO2, Al2O3 and SiO2.[5,6,12]
Relatively less studied are polymeric supports, even
though their use could undoubtedly lead to a number of ad-
vantages:[13] The hydrophilic and -phobic properties of the
polymer matrix can be tuned by appropriate functionalisa-
tion of the surface of the polymer particles; the presence of
functional groups that contain donor atoms can covalently
bind the metal nanoparticles and reduce their tendency to
coalesce or leach from the support; and, last, the polymer
morphology can control the accessibility to the catalytic
metal nanoparticles that are embedded in the polymer
matrix by the substrates.
Gold nanoparticles (AuNPs) with controlled dimensions and
a narrow dimensional distribution have found application in
nanoscience[1a] and nanotechnology,[1] because of their pecu-
liar electronic[2] and optical[3] properties. Remarkably, this
“noble metal” was converted into a highly active redox cata-
lyst when the particle size was reduced to 1–10 nm.[4] Ac-
tually, the AuNP-catalysed oxidation of alkanes,[5] alcohols,[6]
and olefins to the corresponding carbonyl[7] compounds and
epoxides;[8] the direct synthesis of hydrogen peroxide;[9] and
the oxidation of carbon monoxide to carbon dioxide[10]
under mild conditions and in the presence of air or dioxygen
as the oxidising agent,[11] have been recently reported. One
[a] Dr. A. Buonerba, Dr. C. Cuomo, Dr. S. Ortega Sꢀnchez,
Prof. A. Grassi
Tsukuda et al. extensively studied the catalytic perfor-
mance of AuNPs that were stabilised by poly(N-vinyl-2-pyr-
rolidone).[14] Recently, Kobayashi et al. reported the con-
trolled synthesis of AuNPs of about 1 nm in diameter by the
Dipartimento di Chimica e Biologia
and NANO MATES Research Centre
for NANOMAterials and nanoTEchnology
Universitꢁ degli Studi di Salerno
Via Ponte don Melillo, 84084 Fisciano (Italy)
Fax : (+39)089969824
reduction of [AuClACHTUNRGTNEUNG(PPh3)] with NaBH4 in the presence of
stereoirregular random copolymers of styrene with para-sub-
stituted styrene monomers, which had pendant epoxy and
alkoxy groups.[15] Notably, this incarcerated gold catalyst
(PI-AuNPs) was successfully used in multiple alcohol oxida-
tions with high efficiency.
E-mail: agrassi@unisa.it
[b] Dr. P. Canton
Dipartimento di Scienze Molecolari e Nanosistemi
Universitꢁ Caꢂ Foscari Venezia
Via Torino 155/B, 30170 Venezia-Mestre (Italy)
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709

Table 1. Synthesis of AuNPs incarcerated in sPS and sPSB matrices.
We extensively studied the synthesis and properties of
syndiotactic polystyrene and copolymers of syndiotactic
polystyrene with conjugated 1,3-dienes.[16] Syndiotactic poly-
styrene is a semicrystalline thermoplastic polymer character-
ised by a Tg value of 1058C and a melting temperature of
2708C; it is thermally stable and not reactive with strong ox-
idising chemicals and/or strong Brønsted–Lowry and Lewis
acids or bases.[17a] Five crystalline forms, denoted as a, b, g,
d and e, were described for this polymer.[17a,b] It is notewor-
thy that the last two crystalline forms were characterised by
the presence of nanopores and -channels, respectively,
where the diffusion of low-molecular-weight molecules was
higher than in the amorphous phase, and thus, clathrate and
intercalated co-crystalline forms were produced.[17]
Catalyst[a] Polymer Thermal
Solvent[b]
Crystal- AuNP
matrix
annealing
linity[c]
[%]
diameter
[nm]
1-d
2-d
2(t)-b
2(t)-d
2(t)-e
sPS
–
–
–
–
–
58
23
21
–
6.1
4.6
9.5
9.5
9.5
sPS87B
sPS87B
sPS87B
sPS87B
1708C, 5 h
1708C, 5 h H2O/toluene
1708C, 5 h H2O/CHCl3
31
[a] The symbol (t) stands for annealed samples; the Greek symbol indi-
cates the crystalline form of the sPS domain. [b] Further treatment of the
annealed catalyst with a mixture of solvents to effect transformation of
the crystalline polymer phase. [c] Relative crystallinity of the sPS fraction
estimated by differential scanning calorimetry (DSC), assuming a DHm
reference value of 53.2 JgÀ1[17d] for highly crystalline sPS.
We recently synthesised multi-block copolymers compris-
ing segments of syndiotactic polystyrene and cis-1,4-poly-
butadiene (sPSB). The sPSB samples, which contained buta-
diene with a molar fraction higher than 0.15 were soluble in
chloroform at room temperature.[16] When the average sty-
rene block lengths were longer than nine units, the copoly-
mers showed crystalline behaviour that was analogous to
that observed for syndiotactic polystyrene. The sPSBs could
be, thus, considered as novel examples of thermoplastic elas-
tomers, in which the crystalline polystyrene domains acted
as reversible physical cross-links.[16a]
The wide-angle powder X-ray diffraction (WAXD) spec-
trum of 1-d exhibited two patterns: one showing signals at
2q=8.6 and 9.98, which is diagnostic of the d co-crystalline
form of syndiotactic polystyrene with THF,[17] and an addi-
tional pattern that exhibited characteristic broad signals at
2q=38.1, 44.4, 64.6 and 77.88, which were attributed to the
(111), (200), (220) and (311) planes of the fcc crystalline lat-
tice of nanocrystalline gold (Figure 2).[18] An average parti-
cle diameter of 6.1 nm was calculated by the Scherrer
method.[19] The TEM micrograph of 1-d showed AuNPs, ap-
parently of tens of nanometers in diameter, homogeneously
incarcerated in the polymer matrix (Figure 3a). High-resolu-
tion transmission electron microscopy (HRTEM) analysis,
however, demonstrated that these AuNPs consisted of ag-
gregates of primary particles (with diameters of a few nano-
meters), which shared crystalline faces (Figure 3b).
The sPSBs were more soluble in organic solvents than
syndiotactic polystyrene; permitted milder conditions for
the synthetic procedure. Copolymers with a styrene weight
fraction of 87%, namely, sPS87B, were more appropriate and
were thus utilised to prepare samples that had a gold weight
fraction of 2% (2-d). The Scherrer method suggested an
average diameter of 4.6 for the AuNPs. TEM analysis,
which was carried out in several regions, gave an average di-
ameter value for the AuNPs that was in agreement with the
value predicted by the Scherrer method (Figure 3c and d); a
limited number of very small AuNPs (of about 2 nm or less)
were also detected in 2-d.
Results and Discussion
We initially obtained AuNPs incarcerated in a syndiotactic
polystyrene matrix by swelling the polymer in THF
(0.5 wt%) heated at reflux, followed by the addition of
HAuCl4 (0.6 mm) at room temperature and treatment with
sodium triethylborohydride (B/Au molar ratio of 8) at 608C.
The overall process produced a red slurry. Fast coagulation
of the polymer in methanol yielded a colourless filtrate and
a red polymer (1-d; Figure 1 and Table 1). The gold loading
in 1-d was assessed by atomic absorption spectroscopy
(AAS) and a value of 2.0 wt% was found. Thus, the entrap-
ment of the gold precursor in the syndiotactic polystyrene
polymer matrix was quantitative and this yielded a hetero-
geneous catalyst that contained AuNPs in a predetermined
concentration.
Figure 1. Synthesis of the gold catalysts (Table 1).
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Gold Nanoparticle Catalysts for Alcohol Oxidations
FULL PAPER
Figure 2. WAXD profiles showing the reflections of the crystalline syn-
diotactic polystyrene domains and those of the AuNPs in a) 1-d THF
clathrate form; b) 2-d THF clathrate form; c) 2(t)-b; d) 2(t)-d; and
e) 2(t)-e.
In the synthetic route we followed, the polystyrene matrix
strongly stabilised the AuNPs, probably through a strong in-
teraction with the aromatic moieties of the polymer chains,
without requiring additional ligands or stabilisers, as previ-
ously showed by Kobayashi et al. who used a similar poly-
mer matrix.[15] Subsequently, sample 2-d was heated for 5 h
at 1708C to test the thermal stability of these catalytic sys-
tems and, as a result, was converted into 2(t)-b. Therefore,
annealing effected a crystalline-phase change (from the d to
the b form),[17] while the AuNP dimension increased from
4.6 to 9.5 nm (Table 1 and Figures 2 and 3c and f).
Remarkably, the morphology of the single-crystal AuNPs
changed from cuboctahedral in 2-d (Figure 3d) into twinned
and multiple-twinned nanoparticles (MTPs) in 2(t)-b (Fig-
ure 3 f), whereas isolated single-crystal AuNPs with icosahe-
dral morphology were also observed. The SAED pattern of
these nanoparticles is typical of that of crystalline gold (see
Figure 3d).
AuNPs incarcerated in the sPSB polymer matrix were
tested in the model reaction of the aerobic oxidation of (Æ)-
1-phenylethanol to acetophenone, under the reaction condi-
tions reported in reference [15] (1.25 mmol of 1-phenyletha-
nol, 258C, oxygen fed at atmospheric pressure, with water
or a mixture of water and chloroform 1:1 v/v as the solvent).
In a preliminary screening, the oxidation reaction was car-
ried out with KOH as the co-catalyst and water as the sol-
vent to preserve the crystalline phase of the polymer matrix.
The catalyst 2-d was more active than 1-d (compare en-
tries 1 and 2 in Table 2) as was partly expected by the spher-
ical morphology of isolated AuNPs found in 2-d (versus the
nanoparticle aggregates of 1-d).
Figure 3. a) TEM micrograph of 1-d; b) HRTEM micrograph of 1-d;
c) TEM micrograph with the AuNP diameter distribution in 2-d;
d) HRTEM and selected-area electron diffraction (SAED) micrographs
of a typical cuboctahedral AuNP found in 2-d; e) TEM micrograph with
the AuNP diameter distribution in 2(t)-e; and f) HRTEM micrograph of
a typical defective AuNP found in 2(t)-e.
The oxidation rate of 2(t)-b (Table 2, entry 3) was higher
than that of 2-d, despite the fact that the average particle
size was greater and the permeability of the crystalline poly-
mer phase was lower[17] in 2(t)-b than in 2-d (see below),
which showed that the annealing process enhanced the cata-
lytic performance of AuNPs. Subsequently, sample 2(t)-d
(Table 2, entry 4) was tested under the same conditions to
definitively confirm this finding and the role of the porous
polymer phase, and it was found that the highest oxidation
rates were achieved in water.
Chloroform was introduced into the reaction mixture to
improve the diffusion of oxygen and the swelling of the
polymer phase. Surprisingly, the conversion of 1-phenyletha-
nol to acetophenone, in a solvent mixture of H2O and
CHCl3 (1:1 v/v), was quantitative in 5 h (Table 2, entry 7).
The kinetic profile of the 2(t)-catalysed oxidation of 1-phe-
nylethanol, under these conditions and varying concentra-
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711

A. Grassi et al.
Table 2. Oxidation of 1-phenylethanol, catalysed by AuNPs incarcerated
in sPS and sPSB polymer matrices.[a]
conditions, the oxidation rate was higher than that previous-
ly reported for PI-AuNPs by Kobayashi et al.[15]
Despite the presence of simple physical cross-links, the
catalyst 2(t) was repeatedly used at least six times without
any observable loss of activity (>97% in 5 h). The induc-
tively coupled plasma optical emission spectrometry (ICP-
OES) analysis of both the reaction solution and the catalyst
at the end of the oxidation runs confirmed that there was no
loss of gold from the polymer matrix. It was found that reac-
tivation of the catalyst by thermal annealing or washing with
acids or bases after each catalytic run was superfluous.[6,15a]
Similarly, the oxidation of benzyl alcohol at 358C, with
oxygen at atmospheric pressure as the oxidant, proceeded
smoothly and exhibited high efficiency and selectivity
(Table 3, entry 1, and Figure 4c and d). A conversion higher
Catalyst
Solvent
KOH/alcohol
molar ratio
t
Yield[b]
[%]
[h]
1
2
3
4
5
6
7
8
9
1-d
2-d
H2O
H2O
H2O
H2O
1
1
1
1
1
1
1
0.3
0.5
3
24
24
24
24
7
24
5
24
24
24
1
30
57
74
84
83
70
>99
68
88
>99
96
2(t)-b
2(t)-d
2(t)-e
2-d
2(t)-e
2(t)-e
2(t)-e
2(t)-e
2(t)-e
H2O
H2O/CHCl3
H2O/CHCl3
H2O/CHCl3
H2O/CHCl3
H2O/CHCl3
H2O/CHCl3
10
Table 3. Alcohol oxidation catalysed by 2(t)-e.[a]
11[c]
1
Substrate
Product[b]
t
Yield[b] Selec-
[a] Reaction conditions: (Æ)-1-phenylethanol (1.25 mmol), solvent
(15 mL), T=258C, PO2 =1 bar. [b] Determined by GC analysis using ani-
sole (1.25 mmol) as the internal standard. [c] T=358C.
[h] [%]
tivity[b]
[%]
1
benzyl alcohol
benzaldehyde
cinnamaldehyde
6
6
7
1
6
>99
>99
76
>99
86
30
29
–
–
97
95
2[c] cinnamyl alcohol
3
4
5
6
7
8
9
10
thiophene-2-methanol 2-formylthiophene
1-indanol
a-tetralol
furfuryl alcohol
geraniol
1-butanol
1-hexanol
cyclohexanol
>99
>99
>99
>99
>99
–
tions of KOH (Table 2, entries 7–10), showed that the KOH/
alcohol molar ratio of 1:1 was optimal, and that after a par-
tial induction time of about 30 min the reaction followed
pseudo-first-order kinetics with k=(1.10Æ0.10) hÀ1 (Fig-
ure 4a and b). This was one of the highest rate constant
values observed for this reaction carried out at ambient tem-
perature, and catalysed by polymer-incarcerated AuNPs.
The increase of the reaction temperature to 358C resulted
in the quantitative oxidation of 1-phenylethanol to aceto-
phenone in only 60 min (Table 2, entry 11). Under these
1-indanone
a-tetralone
furfural
geranial
–
6
21
24
24
24
–
–
>99
cyclohexanone
10
[a] Reaction conditions: alcohol=0.507 mmol, catalyst=0.200 g, sol-
vent=H2O/CHCl3 (1:1, total volume 6 mL), KOH=0.507 mmol, T=
358C, PO2 =1 bar. [b] Determined by NMR spectroscopy or GC-MS
analysis when using anisole as the internal standard. [c] T=08C.
than 99 mol% with a selectivity of 97% in benzaldehyde
was attained in 6 h. Initial reaction rates were higher (a con-
version of about 84% was obtained in 2 h). At longer reac-
tion times (>24 h), further oxidation of benzaldehyde was
not observed. The reaction followed a pseudo-first-order
rate law with a rate constant value of (0.55Æ0.04) hÀ1 (cal-
culated for a reaction time of 6 h). The performance and se-
lectivity of 2(t) in benzyl alcohol oxidations was remarkable
relative to other polymer-supported AuNP[14,15] catalysts, the
use of which led to partial or complete overoxidation to
benzoic acid. The mechanism of the overoxidation of benzyl
alcohol to benzoic acid or benzylbenzoate via the formation
of the gem-benzyldiol ArCH(OH)2 or the benzyl hemiacetal
ArCH(OH)OBn intermediates, respectively, was thoroughly
investigated.[5a,20] The reaction temperature, the solvent, the
presence of a Brønsted–Lowry/Lewis base and the support
were among the experimental conditions that significantly
influenced the selectivity of the reaction. The source of high
selectivity found with 2(t)-e could be explained by taking
two significant factors into account: 1) the mild conditions
employed (i.e., a reaction temperature of 358C) hampered
the oxidation of the diol or the hemiacetal produced by the
reaction of benzaldehyde with benzyl alcohol under basic
Figure 4. a) Oxidation of 1-phenylethanol catalysed by 2(t)-e with varia-
&
tion of the KOH/1-phenylethanol molar ratio: 0.3 ( ; Table 2, entry 8),
~
!
*
0.5
( ; Table 2, entry 9), 1.0 ( ; Table 2, entry 7), 3.0 ( ; Table 2,
entry 10). b) Plot of ln([A]/[A]0) versus time ([A] is the molar concentra-
tion of 1-phenylethanol). c) Oxidation of benzyl alcohol catalysed by
2(t)-e (experimental conditions are those given in entry 1, Table 3).
d) Plot of ln([A]/[A]0) versus time ([A] is the molar concentration of
benzyl alcohol).
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Gold Nanoparticle Catalysts for Alcohol Oxidations
FULL PAPER
conditions; and 2) the hydrophilic nature of the diol and the
hemiacetal disfavoured their repartition in the fully hydro-
phobic nanoporous polymer phase swelled in chloroform,
thus inhibiting their further oxidation.
diffusion of the alcohol toward the gold catalyst, a process
which was probably favoured by the presence of the nano-
channels of the polymer host phase. Notably, the kinetic
plots reported by Kobayashi et al.[15c], and in Figure 4,
showed an induction time that needs to be further investi-
gated in detail.[21]
SEM analysis of 2(t)-b showed a compact polymer phase
after annealing at 1708C, whereas for 2(t)-e (which resulted
from the catalytic run in chloroform/water), it showed a
higher specific surface area and the formation of a micropo-
rous polymer phase (Figure 6).
The crystalline phase of sPS87B in 2(t) was thoroughly in-
vestigated by WAXD analysis after carrying out catalytic ex-
periments in water/chloroform to investigate the beneficial
role of the polymer host matrix and of chloroform in the
catalytic performance of 2(t) (Figure 2). The b crystalline
form of syndiotactic polystyrene, which resulted from an-
nealing at 1708C, was transformed in this solvent mixture
into a crystalline phase that contained the nanoporous e
crystalline form 2(t)-e. The crystalline index of the polymer
phase was estimated by DSC after calculating the heat of
melting of the syndiotactic polystyrene domains at 243–
2478C (Figure 5) and using the limit value of 53.2 JgÀ1 for
Figure 6. SEM micrographs of a) 2(t)-b and b) 2(t)-e.
A variety of primary and secondary alcohols were
screened (Table 3) to get a deeper insight into the catalytic
performance of 2(t). Cinnamyl alcohol and thiophene-2-
methanol were oxidised with high selectivity in 6 h (Table 3,
entries 2 and 3). Benzylic alcohols, such as 1-indanol and
1,2,3,4-tetrahydro-1-naphthol (a-tetralol) were quantitative-
ly and readily oxidised to the corresponding ketones in 1
and 6 h, respectively (Table 3, entries 4 and 5), whereas
linear and cyclic aliphatic alcohols were slowly oxidised, if
at all, under the same conditions (Table 3, entries 8–10).
Figure 5. DSC traces of a) 2-d, b) 2(t)-b and c) 2(t)-e.
the heat of melting of syndiotactic polystyrene.[17d] The crys-
tallinity index increased from about 20% in 2-d and 2(t)-b
to about 31% in 2(t)-e after the catalytic run (Table 1).
Two aliquots of 2(t) were treated with toluene/water and
chloroform/water to definitively confirm the role of the
polymer host matrix; as a result, 2(t)-d and 2(t)-e were pro-
duced, respectively, in which the AuNPs had the same ther-
mal history and thus the same morphology. The results of
the oxidation of 1-phenylethanol in water (Table 2, entries 4
and 5) unequivocally showed that the polymer phase of 2(t)-
e was responsible for the highest catalytic activity. Two
pieces of experimental evidence further confirmed this con-
clusion: The oxidation of 1-phenylethanol in water catalysed
by preformed 2(t)-e was more extensive than that catalysed
by 2(t)-d (Table 2, entries 4 and 5). The oxidation of 1-
phenyl ethanol with PI-AuNPs, as reported by Kobayashi
et al., was zero order with respect to the concentration of 1-
phenylethanol (the process was diffusion-controlled).[15c] The
access of the substrate to the catalytic nanoparticles embed-
ded in the polymer phase occurred only by slow diffusion
through the polystyrenic amorphous phase. In our case, the
reaction was first order with respect to the molar concentra-
tion of 1-phenylethanol, in agreement with the performance
of AuNPs supported on metal oxides. This suggested a fast
Conclusion
We have reported the controlled synthesis of naked AuNPs
incarcerated in a fully hydrocarbon polymer matrix, which
were characterised by a high Tg value and contained reversi-
ble high-melting physical cross-links of crystalline polymer
domains. The cross-links still permit to the substrates to be
accessible to the AuNPs and impart good stability toward
gold leaching. The catalyst 2(t) was highly efficient in suc-
cessive oxidations of 1-phenylethanol under mild conditions.
Primary alcohols, such as benzyl alcohol, cinnamyl alcohol
and thiophene-2-methanol, yielded the corresponding alde-
hydes in high yields and with high selectivity. Similar results
were obtained in the oxidation of secondary alcohols, such
as 1-indanol and a-tetralol. The formation of the nanopo-
rous e crystalline phase of the polymer matrix was proposed
to rationalise catalyst performance. Additionally, WAXD
and HRTEM analyses of 2(t) after six catalytic runs of 1-
phenylethanol oxidation revealed the presence of multi-
twinned and defective AuNPs of about 9 nm in diameter.
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713

A. Grassi et al.
sPS87B (4.90 g) was added. The mixture was stirred for 3 d at room tem-
perature and then heated to reflux for 1 h to complete the swelling of the
polymer. HAuCl4 (0.200 g; 5.08ꢄ10À4 mol) was added at room tempera-
ture and the resulting slurry was kept under agitation for 24 h, and was
subsequently heated to reflux for 1 h. A solution of sodium triethylboro-
hydride (4.1 mL, 1m) in THF was added at 408C, producing a rapid
change of the colour from pale yellow to red. The polymer rapidly pre-
cipitated with the addition of methanol, was recovered by filtration,
washed with fresh methanol and dried in vacuo at room temperature.
Thus, the improved catalytic performance of 2(t) could also
be ascribed to this AuNP morphology. It is noteworthy that,
recently, Hutchings et al. showed that multi-twinned gold
nanoparticles in Au–Pd/C samples, with predominantly
icosa- and decahedral structures, exhibited higher catalytic
activity than cuboctahedral single/double twinned particles
of Au–Pd/TiO2 samples in toluene oxidations.[5a] The find-
ings reported herein seem to support this hypothesis. More-
over, the thermoplastic nanoporous matrix we described
could allow us to successfully tackle the challenging problem
of the selective oxidation of alcohols.
Preparation of 2(t)-b, 2(t)-d and 2(t)-e: Samples of 2 (2.0 g) were an-
nealed on a hot plate at 1708C for 5 h to effect the transition from the
sPS-d form to the b form (2(t)-b). Samples of 2(t)-b (0.5 g) were stirred
in a mixture of toluene and water (15 mL, 1:1 v/v) for 24 h to effect the
transition from the sPS-b form into the d form (2(t)-d). Samples of 2(t)-b
(0.5 g) were stirred in a mixture of chloroform and water (15 mL, 1:1 v/v)
for 24 h to yield 2(t)-e. The crystalline phase of polymers was analysed by
powder WAXD.
Experimental Section
Determination of the gold loading in 1 and 2: The sample (50 mg) was
acid digested in a Kjeldahl flask by treatment with concentrated H2SO4
(2.5 mL, 98 wt%) at 2508C for 30 min and then with H2O2 (4.0 mL,
35 wt%) at room temperature. The resulting solution was heated at
2508C until to produce a clear colourless solution. Aqua regia (1.5 mL)
was added at room temperature and the solution was diluted with an
aqueous solution of HCl (10 vol%) to a final volume of 10.0 mL. The re-
sulting solution was analysed by AAS or ICP. Calibration was performed
by analysing seven standard solutions of AuIII prepared by progressive di-
lution of a standard solution for AAS ((1.000Æ0.002) gLÀ1 in water with
2 wt% of HCl) with water and an aqueous solution of HCl (10 wt%).
General: The manipulation of air- and moisture-sensitive compounds was
performed under a nitrogen atmosphere using standard Schlenk tech-
niques and an MBraun glovebox. TetrachloroauricACTHNUGRTNEUNG(III) acid, sodium trie-
thylborohydride (1m in THF), (Æ)-1-phenylethanol, benzyl alcohol, cin-
namyl alcohol, 2-thiophenemethanol, 1-indanol, a-tetralol, furfuryl alco-
hol, geraniol, 1-butanol, 1-hexanol, cyclohexanol, anisole, potassium hy-
droxide, sulfuric acid and hydrogen peroxide were purchased from
Sigma–Aldrich and used as received. Oxygen was purchased from Riv-
oira and used as received. CDCl3, and [D2]1,1,2,2-tetrachloethane
(TCDE) were purchased from Euriso–Top and used as received. The
Oxidation of alcohols catalysed by 2: A 50 mL round-bottomed two-
necked flask equipped with a magnetic stirrer bar was charged with
KOH (0.070 g, 1.25ꢄ10À3 mol), H2O (7.5 mL), alcohol (1.25ꢄ10À3 mol),
CHCl3 (7.5 mL), anisole (0.135 mL, 1.25ꢄ10À3 mol) and the catalyst
(0.500 g, alcohol/Au molar ratio = 25). The mixture was stirred at 358C
and O2 was supplied at atmospheric pressure. An aliquot of the reaction
mixture was precipitated in CD3OD or CD3CN and the filtrate was ana-
lysed by 1H NMR spectroscopy and GC-MS. The reaction was terminated
at the end of the run with an excess of methanol. The polymer was recov-
ered by filtration and the filtrate analysed by GC-MS.
goldACHTUNGTRENNUNG
(2 wt%) used in AAS and ICP-OES analyses was purchased from Carlo
Erba and used as received. Syndiotactic polystyrene (sPS) and sPSB
multi-block copolymers were synthesised according to literature proce-
dures.[16]
Instrumentation: NMR spectra were recorded by using
a Bruker
AVANCE 400 spectrometer (400 MHz for 1H and 100 MHz for 13C).
WAXD spectra were obtained, in reflection, with an automatic Bruker
D8 powder diffractometer using nickel-filtered CuKa radiation. TEM
analyses were carried out with a JEOL (JEM 3010) electron microscope
operating at 300 kV, with a point-to-point resolution of 0.17 nm (at
Scherzer defocus). Specimens for TEM analysis were sonicated in 2-prop-
anol, or dissolved in chloroform, and then transferred (10 mL) onto a
copper grid covered with a lacey carbon film supplied from Assing. SEM
analyses were carried out with a scanning electron microscope (JEOL).
AAS analysis was performed on a PerkinElmer AAnalyst 100 spectro-
photometer using an Au hollow cathode lamp (Perkin–Elmer). ICP-OES
was performed on a Perkin–Elmer Optima 7000 DV instrument. GC
analyses were carried out with a Focus GC spectrometer (Thermo Elec-
tron Corporation) equipped with a FAMEWAX column (Crossbond
PEG, 30 m, 0.32 mm ID) and a FID detector, and a GC-MS 7890A/
5975C spectrometer (Agilent Technologies) equipped with an OPTIMA
17MS column (diphenylpolysiloxane/dimethylpolysiloxane, 1:1, 30 m,
0.25 mm ID) and a mass-selective detector. Thermal analyses were car-
ried out on a TA Instrument DSC 2920 calorimeter (heating rate=
108CminÀ1).
Acknowledgements
Financial support from the Ministero dell’Istruzione, dell’Universitꢁ e
della Ricerca (MIUR) is gratefully acknowledged. We are also grateful
to Dr. Patrizia Oliva, Dr. Patrizia Iannece and Dr. Ivano Immediata for
technical assistance.
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714
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Received: April 5, 2011
Published online: December 9, 2011
Chem. Eur. J. 2012, 18, 709 – 715
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