Pd-nanoparticles supported onto functionalized poly(lactic acid)-based stereocomplexes for partial alkyne hydrogenation

Article abstract of DOI:10.1016/j.apcata.2013.09.053

Bipyridine-functionalized poly(lactic acid) (PLA)-based stereocomplexes were employed to stabilize Pd-nanoparticles (NPs). The well defined heterogeneous catalysts were suitable to catalyze the partial hydrogenation reaction (i.e. p(H₂) = 3 bar

Full text of DOI:10.1016/j.apcata.2013.09.053

Applied Catalysis A: General 469 (2014) 132–138
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Pd-nanoparticles supported onto functionalized poly(lactic
acid)-based stereocomplexes for partial alkyne hydrogenation
Giorgio Petruccia, Werner Oberhauserb,∗, Mattia Bartolia, Guido Giachia,
Marco Frediania,∗, Elisa Passagliac, Laura Capozzolid, Luca Rosia
a Dipartimento di Chimica, Università di Firenze, via della Lastruccia 13, 50019 Sesto Fiorentino, Firenze, Italy
b Istituto di Chimica dei Composti OrganoMetallici (ICCOM)-CNR, Via Madonna del Piano 10, 50019 Sesto Fiorentino, Firenze, Italy
c ICCOM-CNR, UOS Pisa, Area della Ricerca, via Moruzzi 1, 56124 Pisa, Italy
d Centro di Microscopie Elettroniche “Laura Bonzi” (Ce.M.E.)-ICCOM, Area di Ricerca CNR di Firenze, via Madonna del Piano 10, 50019 Sesto Fiorentino,
Firenze Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 July 2013
Received in revised form 6 September 2013
Accepted 26 September 2013
Available online 6 October 2013
Bipyridine-functionalized poly(lactic acid) (PLA)-based stereocomplexes were employed to stabilize Pd-
nanoparticles (NPs). The well defined heterogeneous catalysts were suitable to catalyze the partial
hydrogenation reaction (i.e. p(H2) = 3 bar, 25 and 60 ◦C) of phenylacetylene and diphenylacetylene in THF
to give styrene and cis-stilbene with a chemoselectivity of 96% and 91%, respectively. Since the polymer
support revealed stable under real catalytic conditions, stabilizing also efficiently the Pd-NPs in the course
of the catalytic reaction, the heterogeneous catalytic system was easily recyclable. The catalytic activity
as well as the chemoselectivity of the supported catalyst proved to be comparable in four consecutive
catalytic cycles even by recovering the catalyst in air atmosphere after each cycle.
Keywords:
Polyesters
Stereocomplexes
Alkyne hydrogenation
Palladium nanoparticles
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
of the latter heterogeneous catalyst. Hence other heteroge-
neous catalysts have been developed such as carbon-based
The catalytic partial hydrogenation of alkynes to alkenes is
of special relevance in bulk, and fine chemical industries [1,2].
This latter catalytic process is most conveniently performed
with the Lindlar catalyst (i.e. palladium nanoparticles (NPs)
supported onto quinoline-promoted CaCO3 which is partially
poisoned with lead [3]). A further development of the latter
catalyst using BaSO4 as support and quinoline as modifier gave
results which were found superior to that obtained by Lindlar’s
catalyst in terms of reproducibility and ease of preparation [4].
The presence of an organic surface modifier [5–8], which is mainly
involved in the rearrangement process of the Pd-(NPs) [9] revealed
mandatory for obtaining high chemo- and regioselectivity in
the partial hydrogenation of terminal and internal alkynes to
terminal olefins and internal cis-olefins, respectively, avoiding
hence the over-hydrogenation of the substrate giving alkanes.
The organic modifier has always to be added to the catalytic
reaction solution after each catalytic cycle and consequently
separated from the catalytic solution, which is a clear drawback
nanomaterials [10–13], inorganic materials [14–17], func-
tional organic polymers [18,19] (e.g. poly(ethylenimine) [20,21],
polyaniline [22], poly(ethylene glycol) [23], poly(methacryalate)
[24], poly(N-vinyl-2-pyrrolidone) [25]), resins [26–28], porous
organic polymers [29,30] and organic-inorganic composite
materials [31,32].
Since functional polymer-based catalytic systems generally
exhibit important advantages over traditional catalysts such as:
(i) the possible control of the particle shape exerted by func-
tional groups located in the polymer chain; (ii) the improvement
of catalytic selectivity; (iii) the stabilization of NPs by suppres-
sion of their aggregation [18,19,33]. In this context we propose
bipyridine-functionalized poly(lactic acid) (PLA)-based stereocom-
plexes [34–39] as heterogeneous polymer support for Pd-(NPs),
which efficiently catalyzed the partial hydrogenation of pheny-
lacetylene and diphenylacetylene to styrene and cis-stilbene as
major organic compounds. The effect of the Pd-(NPs)-localization
in different polymer environments on the catalysts’ performance
was studied.
Stereocomplexation of PLA (i.e. interaction between PLA
chains with opposite stereoconfiguration) which is based on the
CH3. . .O(carbonyl oxygen atom) hydrogen bonds as proved by
IR-spectroscopy [40] confers high mechanical performance [41],
thermal stability [42] and hydrolysis resistance [43] to PLA.
∗
Corresponding authors. Tel.: +39 55 5225284/+39 55 4573459;
fax: +39 55 5225203/+39 55 4573531.
E-mail addresses: werner.oberhauser@iccom.cnr.it (W. Oberhauser),
marco.frediani@unifi.it (M. Frediani).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.09.053

G. Petrucci et al. / Applied Catalysis A: General 469 (2014) 132–138
133
2. Experimental
temperature for 1 h; Yield = 980 mg (98%) (La) and 960 mg (96%)
(Lb).
2.1. Materials
2.2.4. Synthesis of Pd(OAc)2(l-L1)
In a Schlenk tube was dissolved l-L1 (400.0 mg, 0.042 mmol)
in deareated CH2Cl2 (10.0 mL). To this latter solution was added
Pd(OAc)2 (9.90 mg, 0.044 mmol) under nitrogen and the resulting
solution was allowed to stir for 3 h, followed by its concentra-
tion to dryness. The crude product was washed with diethyl ether
(2 × 10.0 mL) and dried at room temperature giving a yellow pow-
der; Yield = 395.0 mg (97%); Pd-content: 0.85 wt%. NMR data are
reported in Appendices A and B.
Pd(OAc)2 (OAc—acetate), l- or d-lactide, Sn(Oct)2 (Oct—2-
hexylhexanoate) 4,4ꢀ-dihydroxymethyl-2,2ꢀ-bipyridine were pur-
chased from Aldrich. l- And d-lactide were sublimed before
utilization and stored thereafter at 4 ◦C under a nitrogen atmo-
sphere. Solvents such as n-hexane, CHCl3, and HPLC-grade THF
were purchased from Aldrich, while CH2Cl2 was purchased from
J.T. Baker. All solvents were used without further purification.
2.2. Catalyst preparation
2.2.5. Synthesis of Pd(OAc)2La/b in THF
2.2.1. Synthesis of l-L1 and d-L1
In a round-bottom flask were suspended La and Lb (300 mg)
in THF (8.0 mL). To the latter suspensions was added Pd(OAc)2
(6.96 mg, 0.031 mmol) and the resulting yellow suspensions were
stirred under nitrogen for 5 h. Afterwards, the reaction mixtures
were centrifuged and the supernatant decanted. The obtained
solids were washed with THF (2 × 10.0 mL) and centrifuged in order
to remove quantitatively uncoordinated Pd(OAc)2. Yield = 295.0 mg
(96%) (Pd(OAc)2La) and 290.0 mg (94%) (Pd(OAc)2Lb).
A
Schlenk tube was successively charged with l- or d-
lactide (5.0 g, 34.715 mmol), 4,4ꢀ-dihydroxymethyl-2,2ꢀ-bipyridine
(134.0 mg, 0.620 mmol) and Sn(Oct)2 (55.9 mg, 0.181 mmol), fol-
lowed by heating the resulting solid reaction mixture at 135 ◦C for
3 h under nitrogen. Afterwards, the reaction mixture was allowed to
cool to room temperature and the sublimed lactide was removed
mechanically. The crude reaction product was then dissolved in
CHCl3 (20.0 mL) and upon addition of n-hexane (50.0 mL) to the
latter solution the product precipitated as white powder which
was separated from solution by filtration and dried at 30 ◦C for
24 h. Yield (l-L1) = 4.728 g, (92%); yield (d-L1) = 4.695 g, (91%). Mn
(l-L1) = 9500 g mol−1 (1H NMR), 9700 g mol−1 (GPC-RI), polydis-
persity index (PDI) = 1.00; Mn (d-L1) = 9500 g mol−1 (1H NMR),
10,000 g mol−1 (GPC-RI), PDI = 1.04. NMR data are reported in
Appendices A and B.
ꢀ
2.2.6. Synthesis of Pd(OAc)2L(a/b) in CH2Cl2
La and Lb (15.0 mg) were suspended separately in a Schlenk
tube in deareated CH2Cl2 (3.0 mL). To the latter suspensions was
added Pd(OAc)2 (1.0 mg) and the resulting yellow suspensions were
stirred under nitrogen for 3 h. Afterwards, the suspensions were
concentrated to dryness and the obtained yellow solids were sus-
pended in CD2Cl2 in order to carry out 1H NMR spectroscopic
measurements.
2.2.2. Synthesis of l-L2 and d-L2
A Schlenk tube was successively charged with a (1:1) mixture
of l- and d-lactide (1.1 g, 7.359 mmol), 4,4ꢀ-dihydroxymethyl-
2,2ꢀ-bipyridine (247.0 g, 1.143 mmol) and Sn(Oct)2 (72.2 mg,
0.234 mmol) in a dry box under nitrogen. Afterwards, the solid reac-
tion mixture was heated at 135 ◦C for 3 h under nitrogen, followed
by cooling to room temperature. The crude reaction product was
dissolved in CHCl3 (5.0 mL) and on addition of n-hexane (25.0 mL)
an oily product precipitated which was washed with n-hexane
(2 × 10.0 mL) and dried under vacuum. Yield: 0.950 g (73%). The
degree of polymerization, determined by 1H NMR spectroscopy,
was found to be 12. The above isolated product was dissolved in
toluene (10.0 mL) and the obtained solution was divided into two
portions. To each portion was added either l-lactide or d-lactide
(5.0 g, 35.0 mmol), and Sn(Oct)2 (60.2 mg, 0.195 mmol). The result-
ing reaction mixtures were refluxed under a nitrogen atmosphere
for 24 h. Afterwards, the latter reaction mixtures were allowed
to cool to room temperature and the products were precipitated
and washed with n-hexane (10.0 mL). The obtained crude products
were again dissolved in CH2Cl2 (10.0 mL) and reprecipitated with
n-hexane (40.0 mL) giving off-white powders which were sepa-
rated by filtration and dried under vacuum at room temperature for
24 h. Yield (l-L2) = 4.895 g, (89%); yield (d-L2) = 4.917 g, (90%). Mn (l-
L2) = 11,200 g mol−1 (1H NMR), 10,320 g mol−1 (GPC-RI), PDI = 1.15;
Mn (d-L2) = 12,300 g mol−1 (1H NMR), 10,250 g mol−1 (GPC-RI),
PDI = 1.33. NMR data are reported in Appendices A and B.
ꢀ
2.2.7. Synthesis of Pd@La/b and Pd@L(a/b)
In an autoclave Pd(OAc)2La/b (400.0 mg) was suspended in
deareated THF (20.0 mL) at room temperature. The autoclave
was then pressurized with dihydrogen (15.0 bar) and stirred at
room temperature for 12 and 16 h. Afterwards, the dihydrogen
pressure was released and the obtained black suspension was
transferred into a Schlenk tube, where the solvent was completely
removed. The obtained gray powder was washed with diethyl ether
(2 × 10.0 mL) and then dried under vacuum giving Pd@La/b. An iden-
ꢀ
tical synthesis procedure using CH2Cl2 instead of THF gave Pd@La/b
.
ꢀ
(ICP-OES)-analyses of Pd@La/b and Pd@L(a/b) showed a Pd-content
of 0.48 wt%.
2.3. Catalyst characterization
1H and 13C{ H} NMR data were obtained with a Bruker Avance
1
DRX-300 spectrometer at 300.13 and 75.47 MHz, respectively.
CD2Cl2 used for NMR experiments was dried over molecular sieves
and chemical shifts are reported in ppm (ı).
(GPC)-analyses were per formed with a GPC system, equipped
with a Waters Binary HPLC 1525 pump, a manual injector with a
six-way valve and a 200 ␮L loop, three in-series connected Shodex
KF-802.5, KF-803 and KF-804 columns (length: 300 mm each, inner
diameter: 8.0 mm, 24,500 theoretical plates, exclusion limit for PS
up to 400 000 g mol−1; a RI detector (Optilab T-rEXTM, Wyatt Tech-
nology). HPLC-grade THF with a water content of maximal 0.1 vol%
was used as eluent at a constant flux of 1.0 mL min−1, keeping the
columns at 30.0 ◦C by a thermostat. The GPC system was calibrated
using polystyrene (PS) as standard.
2.2.3. Synthesis of La and Lb
Both (PLA)-based stereocomplexes were synthesized as follows:
l-L1 or l-L2 (500 mg) were separately dissolved in CH2Cl2 (8.0 mL).
To these latter solutions were added d-L1 and d-L2 (500 mg),
respectively, dissolved in CH2Cl2 (8.0 mL) at room temperature. The
obtained clear solutions were successively stirred for 5 min and
the solvent completely evaporated by means of a vacuum pump.
The obtained white-off solids were dried under vacuum at room
(XRPD)-spectra were acquired on a X’Pert PRO (PANalytical)
powder diffractometer in a 2ꢀ range from 5.0◦ to 60.0◦ with a step
size of 0.1050◦ and a counting time of 428.9 s, using Cu K␣ radiation
˚
(ꢁ = 1.541874 A).

134
G. Petrucci et al. / Applied Catalysis A: General 469 (2014) 132–138
ꢀ
2.5. Catalyst recycling experiments with Pd@Lb in air
atmosphere
Table 1
(DSC)-data for l-L1, l-L2, La, Lb, Pd@La and Pd@Lb.
Sample
Tg (◦C)
Tc (◦C)/ꢂH(J g−1
)
Tm (◦C)/ꢂH(J g−1
)
ꢀ
Pd@Lb (18.5 mg, 0.834 ␮mol) was introduced into a teflonated
l-L1
l-L2
La
49.7
51.0
n.d.
n.d.
n.d.
n.d.
111.5/−40.4§
118.4/−39.6§
101.4/−42.6
95.5/−34.3
145.4; 157.6/40.1
148.6; 156.8/39.6
223.5/60.7*
steel autoclave (80.0 mL) along with the THF (10 mL) solution of
phenylacetyelene or diphenylacetylene (4.0 mmol) in air atmo-
sphere. The autoclave was then successively sealed, heated to 25
or 60 ◦C and pressurized with dihydrogen (3.0 bar). After a reac-
tion time of 1 h, the autoclave was successively cooled to 5 ◦C by
means of a water–ice bath, dihydrogen vented off and the content
of the autoclave centrifuged in order to separate the solid supported
catalyst from the liquid. The clear catalyst solution was analyzed
by GC. The recovered solid was then suspended in THF (10.0 mL),
the resulting suspension centrifuged and the supernatant removed
in order to quantitatively remove the organic substrate and/or
product. Afterwards, the recovered solid was placed again in the
autoclave followed by the addition of a THF (10 mL) solution of
the appropriate substrate (4.0 mmol) in air atmosphere. The further
proceeding for the next catalytic run corresponds to that reported
above.
Lb
217.7/69.7*
224.4/93.4
216.5/68.7
Pd@La
Pd@Lb
181.0/−88.6
142.2/−68.6
§
Cold crystallization.
1st heating.
*
Table 2
(TG)-data for l-L1, l-L2, La, Lb, Pd@La and Pd@Lb.
Sample
T (◦C) (5% weight loss)
T (◦C) (peak)
Residue (wt%)
l-L1
l-L2
La
215.1
217.3
214.9
203.5
293.8
253.7
261.8
265.8
273.1
267.5
357.9
295.1
3.2
2.6
2.3
2.2
4.7
3.3
Lb
Pd@La
Pd@Lb
(ICP-OES)-analyses were carried out on an ICP apparatus of the
type Perkin Elmer Optima 2000 OES DV.
3. Results and discussion
(TEM)-measurements were carried out on a TEM PHILIPS CM
12, equipped with an OLYMPUS Megaview G2 camera and using
an accelerating voltage of 100 kV. Samples for (TEM)-analyses were
prepared by depositing a drop of an ethanol suspension of the sam-
ple on a holey and lacey film on the carbon support of 200 mesh Cu
grids, followed by the evaporation of the solvent.
As shown in Scheme 1a, two different types of PLA-
based macroligands (i.e. l/d-L1 and l/d-L2) were synthesized
by a Sn(Oct)2-catalyzed ring opening polymerization (ROP) of
l/d-lactide [44], using 4,4ꢀ-dihydroxymethyl-2,2ꢀ-bipyridine as ini-
tiator. The macroligands l/d-L1 are build up by isotactic PLA which
is characterized by a molar mass (Mn) of 9500 g mol−1, while l/d-
L2 exhibited atactic PLA (i.e. ca. 12 lactide units as determined by
1H NMR spectroscopy), directly linked to the bipyridine functional
moiety. This latter PLA spacer is directly linked to a block of isotac-
tic PLA with a polymer length comparable to that found in l/d-L1.
The successful covalent anchoring of the bipyridine functionality
to the PLA polymer chain was proved by acquiring 1H-NMR spec-
tra of l/d-L1 and l/d-L2 which showed for the benzyl hydrogen
atoms a singlet centered at 5.30 ppm, whereas the same hydrogen
atoms showed for 4,4ꢀ-dihydroxymethyl-2,2ꢀ-bipyridine a 1H NMR
doublet at 4.85 ppm (J = 4.8 Hz)) [45,46].
(DSC)-analyses were carried out under an atmosphere of nitro-
gen with a Perkin Elmer instrument calibrated with In and Pd. For
the (DSC)-analyses of l-L1 and l-L2 the following heating program
was applied: 1st heating: 30.0 to 200.0 ◦C at 20.0 ◦C min−1 holding
the latter temperature for 1.0 min, followed by cooling the sam-
ple to 30.0 ◦C and holding the latter temperature for 5.0 min; 2nd
heating: from 30.0 to 200.0 ◦C at a heating rate of 20.0 ◦C min−1
.
For the (DSC)-analyses of La, Lb, Pd@La, and Pd@Lb the follow-
ing heating program was applied: 1st heating: 30.0 to 235.0 ◦C at
20.0 ◦C min−1 holding the latter temperature for 1.0 min, followed
by cooling the sample to 30.0 ◦C and holding the latter temperature
for 5.0 min; 2nd heating: from 30.0 to 235.0 ◦C at a heating rate of
20.0 ◦C min−1. Relevant (DSC)-data are summarized in Table 1.
(TG)-analyses were performed with a Seiko TG/DTA 7200 instru-
ment in the temperature interval from 30 to 700 ◦C with a heating
rate of 10 ◦C min−1 under a nitrogen atmosphere. Relevant (TG)-
data are compiled in Table 2.
By mixing CH2Cl2 solutions of macroligands of opposite
stereoisomerism (Scheme 1b) in an 1:1 molar ratio and by com-
pletely evaporating the solvent of the stereocomplex solution, La
and Lb were obtained as off-white solids with 98% yield.
X-ray powder diffraction (XRPD)-spectra of La and Lb exclu-
sively showed the Bragg reflections for (PLA)-based stereocom-
plexes centered at 2ꢀ: 11.8◦ (1 1 0), 20.5◦ (3 0 0) + (0 3 0) + (1 2 1)
and 23.6◦ (2 2 0). Importantly the broad hump at 32.5◦ (2ꢀ), which
corresponds to the (2 3 1) and (1 0 3) reflections, is detectable only
when the solvent slowly evaporates from stereocomplex solution
(Fig. 1, traces a and b) [34–37,39].
2.4. Catalytic partial hydrogenation of phenylacetylene and
diphenylacetylene in THF
ꢀ
Pd@La/b and Pd@L(a/b) (0.834 ␮mol of Pd) were introduced
In accordance with the (XRPD) results, (DSC)-analyses carried
out on isolated La/b (Table 1) showed the absence of an exother-
mic peak at ca 160 ◦C which is characteristic for the melting point
of isotactic PLA [36]. This experimental finding proves that both
stereoisomers of PLA are involved in the stereocomplex forma-
tion. As a result, melting points of 223◦ and 218◦ for La and Lb,
respectively, were found, as far as the first heating is concerned
[38].
into a teflonated stainless steel autoclave (80.0 mL) which was
then sealed and evacuated. Afterwards, deareated THF (10.0 mL)
solutions of the substrates (4.0 mmol) were introduced into the
autoclave by suction. The autoclave was then charged with dihy-
drogen (3 bar) and heated at the desired temperature with stirring.
After the desired reaction time the autoclave was cooled to 5 ◦C
by means of a water-ice bath, dihydrogen vented and the catalytic
solution analyzed by GC and GC-MS.
(GC)-analyses were performed with a Shimadzu 2010 gas chro-
matograph equipped with a flame ionization detector and a 30 m
(0.25 mm i.d., 0.25 ␮m film thickness) VF-WAXms capillary col-
umn, while (GC-MS)-analyses were carried out with a Shimadzu
QP 5000 apparatus, equipped with a 30 m (0.32 mm i.d., 0.50 ␮m
film thickness) CP-WAX 52 CB WCOT-fused silica column.
The reaction of a suspension of La and Lb in THF with Pd(OAc)2
gave the corresponding Pd-complexes (i.e. Pd(OAc)2La/b) as pale
yellow solids (Scheme 1c) which were washed several times with
fresh THF in order to remove uncoordinated Pd(OAc)2. La and Lb
proved to be insoluble in THF and CH2Cl2 even at 60 ◦C. Importantly,
the latter stereocomplexes showed good swelling properties in
CH2Cl2, which was exploited to study the coordination of Pd(OAc)2

G. Petrucci et al. / Applied Catalysis A: General 469 (2014) 132–138
135
Scheme 1. Syntheses of PLA-based macroligands, stereocomplexes, and stereocomplex-stabilized Pd-(NPs).
to La and Lb by 1H NMR spectroscopy. To this purpose, La/b, sus-
respectively (ꢀScheme 1c). Accordingly, 1H NMR spectra of Pd@La/b
and Pd@L(a/b) acquired in CD2Cl2 after a reduction reaction last-
ing 12 h showed for the bipyridine hydrogen atoms chemical shifts
which are typically found for uncoordinated La and Lb (Appendices
A and B), confirming hence the successful reduction of coordinated
Pd(OAc)2 to La/Lb-supported NPs.
ꢀ
pended in CH2Cl2, reacted with Pd(OAc)2 giving Pd(OAc)2L(a/b)
which was directly subjected to 1H NMR measurements carried out
in CD2Cl2. For comparison reason, Pd(OAc)2(l-L1), which is com-
pletely soluble in CD2Cl2, was synthesized. The corresponding 1H
NMR spectra are compiled in Fig. 2.
As shown in Fig. 2, the 1H NMR signals assigned to the bipyri-
dine moiety shifted upon coordination to Pd(OAc)2 (Fig. 2, trace b
vs. a; d vs. c and f vs. e), confirming the successful coordination of
(XRPD)-measurements carried out on isolated Pd@La/b con-
firmed the stability of the stereocomplex under reductive
experimental conditions (Fig. 1, traces c/d vs. a/b), while (TEM)-
ꢀ
Pd(OAc)2 to the bipyridine units of La and Lb. Moreover, the broad
images of Pd@La/b and Pd@L(a/b) proved the presence of Pd-(NPs)
ꢀ
1H NMR signals for coordinated bipyridine in Pd(OAc)2L(a/b) con-
in the latter samples (Fig. 3).
firmed the anisotropic chemical environment in swollen La and Lb.
From a comparison of the (TEM)-histograms (Fig. 3) emerged
1
A
13C{ H} NMR spectrum of isolated Pd(OAc)2(l-L1) acquired in
that the reduction of Pd(OAc)2La/b, carried out in CH2Cl2, led to
ꢀ
ꢀ
CD2Cl2 proved the symmetrical coordination of Pd(OAc)2 to the
bipyridine moiety, showing for both coordinated acetate moieties
a singlet at 23.0 (i.e CH3COO) and 177.7 ppm (i.e. CH3COO).
THF and CH2Cl2 suspensions of Pd(OAc)2L(a/b) were treated with
the same average Pd-(NP) size in Pd@La and Pd@Lb (Fig. 3c/d).
This experimental fact is rationalized by the good swelling prop-
erties of La/Lb in CH2Cl2 forming gels [34,47], while La/Lb do not
dihydrogen (15 bar) in an autoclave at room temperature for 12 h,
ꢀ
yielding Pd@La/b and Pd@L(a/b) (i.e. Pd-(NPs) stabilized by La/b),
Fig. 2. 1H NMR spectra acquired in CD2Cl2 in the chemical shift range from
ꢀ
7.06 to 8.89 ppm. L-L1 (a), Pd(OAc)2(l-L1) (b), La (c), Pd(OAc)2La (d), Lb (e) and
ꢀ
Fig. 1. (XRPD)-traces of La (a), Lb (b), Pd@La (c) and Pd@Lb (d).
Pd(OAc)2Lb (f).

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G. Petrucci et al. / Applied Catalysis A: General 469 (2014) 132–138
ꢀ
ꢀ
Fig. 3. (TEM)-images and histograms of Pd@La (a), Pd@Lb (b), Pd@La (c) and Pd@Lb (d).

G. Petrucci et al. / Applied Catalysis A: General 469 (2014) 132–138
137
swell in THF. As a consequence, the reduction of Pd(OAc)2La (i.e. La
contains no amorphous PLA-spacer) in THF led to Pd@La which is
characterized by the biggest Pd-(NP) average size combined with
the largest Pd-(NP) distribution among the cases studied. In fact, a
rigid polymer structure without swelling properties in a given sol-
vent slows down the kinetic of the Pd(II) reduction, leading thus to
the formation of large Pd-(NPs) particles size [33].
Importantly, the presence of Pd-(NPs) anchored onto La/b
increased significantly the thermal stability of the latter stereo-
complexes as proved by (TG)-analyses carried out on Pd@La/b and
on La/b (Table 2). As a result, the onset degradation temperature of
Pd@La/b was 50–80 ◦Chighercomparedto that of La/b and the fastest
decomposition rate observed at T (peak) followed the same trend
(i.e. 357.9 (Pd@La) vs. 273.1 ◦C (La) and 295.1 (Pd@Lb) vs. 267.5 ◦C
(Lb) (Table 2). Comparable experimental results were found for
poly(ethylene glycol)-stabilized Pd-(NPs), provided the palladium
Scheme 2. La vs. Lb-based Pd-catalysis.
by the substrates. This latter experimental fact, corroborates the
importance of the nano-environment of a functionalized polymer
phase for the substrate migration and thus for the catalytic activity
of supported Pd-(NPs) [19,27] (Scheme 2).
ꢀ
Analogous catalytic reactions carried out with Pd@La and
ꢀ
was finely dispersed throughout the polymer matrix [46].
ꢀ
Pd@La/b and Pd@L(a/b) were applied to catalyze the partial
Pd@Lb obtained after 12 and 16 h of dihydrogen reduction, gave
hydrogenation of phenylacetylene (A) and diphenylacetylene (B)
in THF to give styrene and cis-stilbene as the major organic
compounds. In order to ensure the absence of gas-liquid dif-
fusion limitations during the catalytic reactions, stirrer velocity
(500–1000 rpm) and dihydrogen pressure (3–8 bar) were screened.
As a result, we found almost identical substrate conversions and
chemoslectivities in the latter range of experimental conditions.
The experimental results compiled in Table 3 are thus carried out
with a stirrer velocity of 700 rpm and a dihydrogen pressure of
3 bar.
comparable catalytic results in terms of catalytic activity and
chemo- or regioselectivity. This experimental fact is indicating that
the amount of residual Pd(II) located on the Pd-(NPs), if any, is
negligible for the observed catalytic outcome and that Pd(0) is the
effective catalytic species.
Since phenylacteylene is known to be structure insensitive
regarding hydrogenation reactions with Pd-(NPs) in the size range
from 2.5 to 5.6 nm [48], the significantly lower chemoselectivity for
ꢀ
ꢀ
phenylacetylene hydrogenation of Pd@La/a compared to Pd@Lb/b
is due to the hindered access of phenylacetylene to Pd-(NPs) in
La (Scheme 2), fostering hence the hydrogenation of Pd-surface-
adsorbed styrene to ethyl benzene instead of replacing adsorbed
styrene by phenylacetylene [11]. (XRPD)-analyses carried out on
recovered Pd@La and Pd@Lb after diphenylacetylene hydrogena-
tion at 60 ◦C for 1 h confirmed the excellent stability of the organic
support under real hydrogenation reaction conditions (Fig. 4).
The Pd-leaching into the THF solution of Pd@La and Pd@Lb-
catalyzed diphenylacetylene hydrogenation reactions carried out
at 60 ◦C for 1 and 2 h was followed by (ICP-OES)-analyses, carried
out on the catalytic THF solutions. As a result, the Pd@Lb-catalyzed
reactions showed slightly higher values for the Pd leaching com-
pared to Pd@La (i.e. Pd@La: 0.8 ppm (1 h), 2.8 ppm (2 h); Pd@Lb:
The TOF-values reported in Table 3 refer to the substrate-
accessible amount of supported palladium, which was deter-
mined by (TEM)-measurements, applying the following equation:
D = 0.9/d, where D is the palladium dispersion and d is the mean
diameter in nm of the Pd-(NPs) [11,16].
A screening of the catalysts’ performance exhibited for Lb-
ꢀ
based catalysts (i.e. Pd@Lb and Pd@Lb ) a significantly higher
catalytic activity compared to the La counterparts, regardless
of the substrate and reaction temperature employed (Table 3,
entries 3/4 vs. 1/2 and 9/10 vs. 7/8). The same trend applies to the
chemoselectivity of styrene formation and to a lesseꢀr extent to the
ꢀ
formation of cis-stilbene. Since Pd@La and Pd@Lb , which were
obtained by an identical reduction procedure in CH2Cl2, show an
1.8 ppm (1 h), and 3.8 ppm (2 h)).
ꢀ
Catalyst recycling experiments with Pd@Lb were carried out
identical palladium distribution (D) of 0.289 it is evident that the
ꢀ
significantly higher substrate conversion found for Pd@Lb com-
ꢀ
at 25 and 60 ◦C by preparing the catalyst suspension in air atmo-
sphere. As a result, the catalytic activity as well as the chemo- and
stereoselectivity of the partial hydrogenation reactions dropped
pared to Pd@La is due to an easier access of the Pd-(NPs), which
are embedded in the amorphous (i.e. atactic) polymer matrix of Lb,
Table 3
ꢀ
Partial hydrogenation of selected alkynes by Pd@La/b and Pd@L(a/b) a
.
Entry
Conv. (%)/TOFb
Sel(olefin)(%)/Sel(cis-isomer) (%)
2 h
Catalyst
Substrate
1 h
2 h
1 h
1
2
3
Pd@La
A
A
A
A
A
A
B
B
B
B
B
B
11/3612
14/2340
41/9065
44/6560
42/6264
40/5965
28/9195
29/4847
76/11,335
86/12,826
83/12,380
78/11,633
17/2790
28/2340
67/7407
62/4623
–
89
88
96
95
94
94
90/91
91/91
90/94
90/94
90/94
89/93
87
85
95
95
–
ꢀ
Pd@La
Pd@Lb
ꢀ
4
Pd@Lb
ꢀ
Pd@Lb
ꢀ
5c
6d
7e
Pd@Lb
–
–
Pd@La
39/6404
n.d.
100/n.d.
n.d.
–
–
92/92
n.d.
85/95
n.d.
–
ꢀ
8e
Pd@La
9e
Pd@Lb
ꢀ
10e
11c,e
12d,e
Pd@Lb
ꢀ
Pd@Lb
ꢀ
Pd@Lb
–
aPd (0.834 ␮mol); substrate: A (phenylacetylene); B (diphenylacetylene) (4.0 mmol); THF (10.0 mL); p(H2) (3 bar); T (25 ◦C). Catalytic suspension prepared under nitrogen.
b
TOF defined as mmol(product) × (mmol(Pd) × h)−1, with mmol(Pd) = mmol of substrate-accessible Pd.
c
1st catalytic cycle with catalytic suspension prepared in air atmosphere.
d
4th catalytic cycle with catalytic suspension prepared in air atmosphere.
e
T (60 ◦C).

138
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4. Conclusions
We showed that: (i) Functionalized (PLA)-based stereocom-
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Appendix A. Supplementary data
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