Palladium nanoparticles supported on hyperbranched aramids: Synthesis, characterization, and some applications in the hydrogenation of unsaturated substrates

Article abstract of DOI:10.1021/ma034245i

Hyperbranched (HB) aromatic polyamides (aramids), synthesized from A₂ + B₃ reagents (A = p-phenylenediamine; B = trimesic acid), have been used as polymeric supports for palladium(0) nanoparticles. The chemical-physical properties of

Full text of DOI:10.1021/ma034245i

4
294
Macromolecules 2003, 36, 4294-4301
Palladium Nanoparticles Supported on Hyperbranched Aramids:
Synthesis, Characterization, and Some Applications in the
Hydrogenation of Unsaturated Substrates
†
†
‡
,§,⊥
Da n iela Ta bu a n i, Or ietta Mon ticelli, An d r ea Ch in ca r in i, Cla u d io Bia n ch in i,*
§
§
,†,|
F r a n cesco Vizza , Sim on etta Mon eti, a n d Sa ver io Ru sso*
Department of Chemistry and Industrial Chemistry, Genoa University and
INSTM Genoa research unit, via Dodecaneso 31, 16146 Genova, Italy, INFM Genoa unit,
via Dodecaneso 33, 16146 Genova, Italy, and Institute of Chemistry of Organometallic Compounds,
ICCOM-CNR, Via J . Nardi 39, 50132 Firenze, Italy
Received February 26, 2003; Revised Manuscript Received April 24, 2003
ABSTRACT: Hyperbranched (HB) aromatic polyamides (aramids), synthesized from A
A ) p-phenylenediamine; B ) trimesic acid), have been used as polymeric supports for palladium(0)
nanoparticles. The chemical-physical properties of these aramids, denoted as p P DT, have been compared
to those of the polymer p ABZAIA obtained from the AB monomer 5-(4-aminobenzamido)isophthalic
2
+ B
3
reagents
(
2
acid. The two HB polymers show structural differences in the shape of the macromolecule as well as the
nature of the end groups. The p P DT materials exhibit a less regular structure with both amino and
carboxy terminal groups, while the p ABZAIA polymer contains a single amine focal unit per molecule
0
and carboxy end groups. HB aramid-supported Pd nanoparticles were obtained by reduction of polymer-
supported PdCl
hyperbranched structure, were investigated to gain insight into the nature of the interactions occurring
between PdCl and the support. The nature of the interactions between the polymeric matrix and PdCl
2 4
with an aqueous solution of NaBH . Some model compounds, mimicking the polymeric
2
2
was investigated by means of FT IR and XPS techniques. XPS spectroscopy, performed on both polymers
and model molecules, indicated that the interaction between the support and the metallic precursor
involves the terminal amino groups. The particle dimensions and the metallic dispersion were determined
by TEM analysis. This study showed that the dimensions and dispersion of the metal clusters on p ABZAIA
0
are smaller than those on p P DT. A preliminary study of the catalytic performance of a specific P d /
p P DT catalyst in the hydrogenation of relevant unsaturated substrates (benzene, benzylideneacetone,
phenylacetylene, diphenylacetylene, and quinoline) has been carried out in either nonpolar or polar
solvents. In most cases, the catalyst proved to be efficient, selective, and recyclable, yet the nature of the
solvent affected remarkably the catalysis outcome in terms of both activity and selectivity.
In tr od u ction
and Kevlar.3,5 However, the amido nitrogen atoms in
these materials are very weak nucleophiles, and the
number of terminal amino groups is too low for binding
an acceptable amount of metal. In this respect, dendritic
polyamides may be a valid alternative to conventional
polymeric supports as they exhibit high chemical and
thermal stability as well as excellent functional-group
availability/accessibility. In a recent paper, Gr o¨ hn et al.
showed that dendrimers can be used as polymeric
The first example of a metal phase supported on a
polymeric matrix dates back to 1956 when Akaburi and
1
co-workers employed a palladium catalyst dispersed on
a silk fibroin fiber to obtain optically active amines and
amino acids by hydrogenation of appropriate organic
precursors. Since then an impressive amount of work
2
has been carried out, especially as regards the use of
noble metals supported on polymers as catalysts for
hydrogenation reactions.3 It is now apparent that the
catalytic activity of such materials is influenced by the
nature of both the polymer matrix and the reaction
solvent.
The generally low thermal and chemical stability and
the poor mechanical properties constitute the major
drawbacks of polymeric supports, including polyamides
10
templates to anchor metal nanoclusters. These au-
thors described a system, comprising poly(amidoamine)
-8
(
PAMAM) molecules dispersed in a hydrophilic cross-
linked polymer, in which both the nanoparticles size and
the metal loading could be adjusted by simply varying
the degree of polymerization of the polymer.
Among dendritic polyamides, the hyperbranched ones
constitute a unique class for their easy one-step prepa-
ration that can allow for an effective scale-up of the
utilization processes. To the best of our knowledge, no
report concerning the use of hyperbranched polyamides
9
introduced first by Izumi in 1959. In polyamides, which
can interact with the metal through the nitrogen atoms,
these drawbacks can be partially overcome using ma-
terials, containing arylamido groups, such as Nomex
(aramids) as support material for metal phases has been
published so far.
†
Genoa University and INSTM Genoa research unit.
INFM Genoa unit.
Institute of Chemistry of Organometallic Compounds, ICCOM-
In this paper is reported a study in which aramids,
obtained by either A2 + B3 reagents or AB2 monomers,
are used as support material for palladium nano-
particles, ultimately generating robust and recyclable
hydrogenation catalysts.
‡
§
CNR.
⊥
E-mail: bianchin@fi.cnr.it.
E-mail: russo@chimica.unige.it.
|
1
0.1021/ma034245i CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/23/2003

Macromolecules, Vol. 36, No. 12, 2003
Palladium Nanoparticles 4295
Sch em e 1. AB
2
Mon om er a n d Str u ctu r e of th e HB
p ABZAIA Ar a m id
Sch em e 2. Syn th esis a n d Str u ctu r e of th e (A
p P DT Ar a m id s
2
3
+ B )
F igu r e 1. Sketch of the model molecule M1,3,5.
ylic acid chloride and p-aminotoluene according to a general
1
5
synthetic procedure.
P r ep a r a tion of Ar a m id -Su p p or ted P d 0 Clu ster s. Poly-
5
mer powders were stirred in an aqueous solution of acetic acid
containing PdCl
for 48 h. The polymer/PdCl
impregnation process, a change in color of both polymer and
solution occurred. The powders were filtered, washed several
times with deionized water, and dried overnight at 60 °C.
Palladium reduction was carried out using an aqueous solution
2
(Aldrich, 0.01 wt %) at room temperature
ratio was 10/1 (w/w). During the
Ta ble 1. P olym er iza tion Con d ition s Em p loyed in th e
Syn th esis of th e p P DT Sa m p les
2
sample code
[NH2]/[COOH]
Tp (°C)
tp (min)
p P DT1
p P DT2
p P DT3
p P DT4
p P DT5
1/2
2/3
2/3
1/1
2/3
80
80
80
80
80
90
90
60
90
90
4
of NaBH (0.1 M) at 50 °C for 2 h. After reduction, the powders
were washed extensively to remove the excess of the reducing
agent and dried at 60 °C under vacuum for at least 24 h. The
same procedure for impregnation and reduction was followed
for the model molecules.
Ta ble 2. Ch a r a cter iza tion Da ta of th e HB P olym er s
Descr ibed in Th is P a p er
Ch a r a ct er iza t ion of t h e H B Ar a m id -Su p p or t ed P d 0
Clu ster s. TEM measurements were performed with a high-
resolution transmission electron microscope (J EOL 2010). The
polymer/Pd powders were suspended in 2-propanol and a drop
of the resultant mixture was deposited on a carbon grid.
XPS analyses have been performed using a PHI 5602
Multitechnique instrument, featuring a charge neutralizer and
a monochromated Al X-ray source (1486.6 eV) set at 350 W
anode power. The data were acquired in multiregion mode,
with 23.50 eV pass energy, takeoff angle of 45 deg, and 0.05
eV energy resolution.
-3
sample
code
tp
[η]a
Mw × 10
(g‚mol
Tg
(°C)
(dL‚g-1)
-1
)
(min) solubility
p P DT1
p P DT2
p P DT3
P P DT4
p ABZAIA
90
90
60
90
120
total
total
total
total
total
0.10
0.29
0.17
0.40
0.36
4.25
23.8
153
165
100.2
153
a
In H2SO4 95-97% at 25 °C.
Samples were prepared by pressing the polymer/Pd powders
onto a thick indium film, then they were set into a pre-
Exp er im en ta l Section
P olym er Syn th esis. The hyperbranched (HB) aramid
p olyABZAIA was obtained following a previously described
procedure, which involves the direct polymerization of the
-7
pumping chamber (=10 Torr) for 10 min and then trans-
-9
ferred in the main chamber (=10 Torr) for analysis. PHI
MultiPak software was used for data treatment and analysis.
Because of the intrinsic insulating nature of the samples
and the need of a charge neutralization device, all XPS spectra
have been software shifted to set the main carbon peak binding
energy to 284.7 eV. Curve fits have been performed without
smoothing, using Gaussian templates and Shirley background
subtraction.
AB
ABZAIA) (Scheme 1).11,12
The procedure used to synthesize the HB aramids p P DT
from the A + B reagents (A ) p-phenylenediamine; B )
trimesic acid) (Scheme 2) is given elsewhere.
2
-type monomer 5-(4-aminobenzamido)isophthalic acid
(
2
3
1
3
As shown in Table 1, different p P DT polymers could be
prepared by varying the reactant ratio as well as the poly-
merization time.
The FT IR spectra were recorded with a Bruker IFS66
spectrometer on KBr pellets.
Polymer characterization data, including viscosity, solubility
in organic solvents (such as DMF, DMSO, NMP), glass
transition temperature, and GPC measurements, are provided
The palladium content in the polymeric support was deter-
mined by inductively coupled plasma atomic emission spec-
troscopy (ICP-AES) with a J obin Yvon (series J Y24) instru-
ment at a sensitivity level of 500 ppb. Each sample (20-50
mg) was treated in a microwave heated digestion bomb
(Milestone, MLS-200) with a mixture of 4 mL of a NaOCl
solution (6-14% free chlorine Riedel-de Ha e¨ n) and 2 mL of 2
M NaOH and heated to boiling temperature for a few minutes.
The resulting solutions were analyzed after increasing the
volume to 50 mL in a volumetric flask.
1
1-14
in Table 2.
Porosity measurements were performed on two samples of
hyperbranched aramids (see later) by an ASAP 2010 Mi-
cromeritics apparatus.
The AB monomer ABZAIA and benzene-1,3,5-tricarboxylic
2
acid tris-p-tolylamide (M1,3,5) (Figure 1) were used as model
molecules. M1,3,5 was prepared from 1,3,5-benzenetricarbox-

4
296 Tabuani et al.
Hyd r ogen a tion Rea ction s w ith P d /p P DT. All reactions
Macromolecules, Vol. 36, No. 12, 2003
0
and manipulations were routinely performed under a nitrogen
or argon atmosphere by using standard Schlenk techniques.
n-Pentane was distilled under nitrogen from LiAlH , while
4
reagent grade MeOH was used as received. Quinoline was
distilled in vacuo and stored under argon. Batch reactions
under a controlled pressure of gas were performed with a
stainless steel Parr 4565 reactor (100 mL) equipped with a
Parr 4842 temperature and pressure controller and a paddle
stirrer. GC analyses of the reaction mixtures were performed
on a Shimadzu GC-14 A gas chromatograph equipped with a
flame ionization detector and a 30 m (0.25 mm i.d., 0.25 µm
film thickness) SPB-1 Supelco fused silica capillary column.
GC/MS analyses were performed on a Shimadzu QP 5000
apparatus equipped with an identical capillary column. As a
general procedure, a 100 mL Parr autoclave was charged with
0
1
6 mg of a specific catalyst, P d /p P DT5, containing 2.02 wt
%
Pd, (determined by ICP-AES) the desired amount of
substrate, 30 mL of solvent (either n-pentane or MeOH), and
(30 bar). The ensemble was heated to the established
H
2
F igu r e 2. Structural units of the A
obtained from p-phenylenediamine (A) and trimesic acid (B)
T, L, and D denote terminal, linear and dendritic units,
2
+ B
3
HB aramids
temperature and then stirred (1500 rpm) for the desired time,
after which the vessel was cooled to ambient temperature and
depressurized. The liquid contents were analyzed by GC and
GC/MS. Above 1500 rpm, the rates were independent of the
agitation speed at all the temperatures studied, thus indicating
the absence of mass transport limitations. The stability of the
catalyst against leaching from the support was tested as
follows. (i) After a catalytic run, the catalyst was separated
by filtration from the liquid phase under nitrogen, washed with
the solvent and then reused for a second, identical run. After
the liquid phase was analyzed by GC, the solvent was removed
under vacuum and the residue was analyzed by ICP-AES. No
trace of metal was detected by ICP-AES. (ii) To refrain from
filtering the solid catalyst after each catalytic run, several
reactions were carried out in a 100 mL Parr reactor fitted with
a dip pipe with a sintered (2 µm) metal piece at its dipping
end. Upon completion of the reaction, the solution was forced
out through the sintered dip pipe by applying a nitrogen
pressure of ca. 2 bar at the gas inlet valve of the reactor, thus
retaining the catalyst in the reactor under nitrogen. The
catalyst was washed with the appropriate solvent (3 × 20 mL).
After a sample of the filtrate was analyzed by GC, most of the
solvent was distilled out and the residue was analyzed by ICP-
AES. A fresh solution of the substrate to be hydrogenated was
then loaded through a thin Teflon pipe connected to the
reactor. The reactor was then pressurized with hydrogen to
(
respectively). See ref 13.
F igu r e 3. FT IR spectra of p P DT2 alone, after impregnation
0
with PdCl
2
and after reduction to Pd .
3
0 bar, heated to the appropriate temperature and then stirred
for the desired time.
Ta ble 3. P or osity Da ta of p ABZAIA a n d p P DT3 HB
P olym er s
Resu lts a n d Discu ssion
surface
average pores
diameter (nm)
area (m ‚g-1)
2
Ch a r a cter iza tion of th e Hyp er br a n ch ed Ar a -
m id s. The molar masses of the p P DT polymers, deter-
mined using a SEC apparatus equipped with a single-
capillary viscosimetry and MALS detectors, are in-
variably smaller than those of the AB2 systems in
consequence of the short reaction time required to avoid
the gel point and obtain a soluble material. Consistently,
p ABZAIA is featured by a greater intrinsic viscosity
than any p P DT samples.
1H NMR and FT IR analyses provided valuable
information on the influence of the polymerization
conditions on the structure of p P DT, especially in terms
of degree of branching and structural units concentra-
tion.13 The six structural units from the (A2 + B3)
system, as determined by H NMR measurements, are
illustrated in Figure 2.
As shown in Table 1, both the polymer structure and
the material properties can be modified by varying the
reactant ratio and the polymerization time as well. The
possibility to finely tune the properties of the polymer
matrix represents a considerable advantage over in-
sample code
p ABZAIA
p P DT3
30
10
40
13
organic materials for the exploitation of aramids as
catalyst support.
An important characteristic of any catalyst support
is the porosity, which can influence the accessibility of
the substrate to the active sites. Surface areas and
average pore dimension distribution for p ABZAIA and
p P DT3 are reported in Table 3. Both systems exhibit
large surface areas, while the porosity of p ABZAIA is
higher than that of p P DT3.
The above measurements have not been extended to
the other pPDT samples (pPDT 1, pPDT 2, and pPDT
4), as the main purpose of this specific investigation was
confined to a simple comparison of the two classes of
materials.
1
The elevated porosity and the good thermal stability
(>400 °C) for the p P DT materials), combined with a
large number of accessible functional groups, forecast

Macromolecules, Vol. 36, No. 12, 2003
Palladium Nanoparticles 4297
F igu r e 4. Variation of the XPS signal with time in the Pd 3d region (the appearance of a new signal at low binding energies is
indicative of palladium reduction).
a great potential of these HB aramids as support
materials for metal phases.
Ch a r a cter iza tion of Ar a m id -Su p p or ted P d Cl2.
Palladium dichloride was supported on either p P DT or
p ABZAIA polymers by a standard procedure, which
involves the stirring of an aqueous solution of PdCl2 and
acetic acid in the presence of the solid polymer at room
temperature for 48 h.5
A useful spectroscopic technique to study polymer-
supported metal compounds is FT IR spectroscopy. The
exploitation of this technique has allowed to study the
interaction between palladium and some oligomeric
aramids featured by a structure similar to the present
5
HB polymers, in terms of the sequence of CO and NH.
The FT IR spectra recorded on p P DT2 and p ABZAIA
at 120 °C before and after impregnation with PdCl2 did
not show any difference except for the shape of some
-1
bands above 3300 cm in the region of the N-H stretch.
-
1
Figure 3 shows the IR spectra in the 4000-2000 cm
region for p P DT2 alone, after impregnation with PdCl2
II
0
and after Pd reduction to Pd (vide infra).
The observed spectral changes were clearly consistent
with an interaction between PdCl2 and the terminal
groups. Since both the line shape and position of the
-
1
band due to the carboxy groups (at ca. 1700 cm ) did
not change upon PdCl2 adsorption, it is very likely that
only the amino groups are involved in the bonding
interaction to palladium(II). The occurrence of nitrogen-
palladium bonding interactions has been also reported
in model compounds with oligomeric aramids of Kevlar
type.5
XPS spectroscopy is an important technique for the
characterization of polymer-supported metal com-
pounds, especially to study the interactions between the
2
F igu r e 5. TEM micrograph of P d Cl /p P DT2 after XPS
analysis.
indicated by the appearance of a shoulder at low BE
vertical line in Figure 4), due to the progressive
(
II
0
reduction of Pd (337.5 and 342.8 eV) to Pd (336.0 and
3
41.3 eV).
A TEM micrograph of P d Cl2/p P DT2, acquired after
a long period of XPS analysis, confirmed unambiguously
metal and the polymeric matrix as well as determine
the oxidation state of the anchored metal.16-19 Also, XPS
the formation of metal clusters (Figure 5). This draw-
spectroscopy can allow one to distinguish between
interacting and noninteracting groups in polymers
back of the XPS technique has some precedents1
8,19
and
containing different types of nitrogen atoms.1
7-19
was overcome by us using analysis times shorter than
2
0 min.
In the present case, XPS spectroscopy was used to
study the polymer/metal interactions before and after
reduction. As shown in Figure 4, the exposure of a
P d Cl2/p P DT2 sample to X-rays for a long time affected
the palladium signal. In particular, the shape of the Pd
Figure 6 shows the N 1s signal peak of the (A2 + B3)
polymer p P DT2. Since this peak can be well fitted by
a single Gaussian curve, any distinction between amino
and amido groups in the polymer would be only specu-
lation.
3
d signal showed a dependence on the analysis time,

4
298 Tabuani et al.
Macromolecules, Vol. 36, No. 12, 2003
F igu r e 6. N 1s signal fitting for a fresh sample of p P DT.
Ta ble 4. Th eor etica l a n d XP S Atom ic P er cen ta ges of th e
Mod el Molecu les ABZAIA a n d M1,3,5
Ta ble 5. P er cen ta ge of Den d r itic, Lin ea r , a n d Ter m in a l
Un its a s Obta in ed fr om 1H NMR An a lysis of Va r iou s
p P DT Sa m p les
ABZAIA
theoretical
M1,3,5
theoretical
p P DT1
p P DT2
p P DT3
p P DT4
atom
XPS
XPS
AA′B(L)
A′2B(L)
A′B2(T)
A2A′(D)
A′3(D)
6.1
45.4
30.2
0
12.2
6.1
9.2
41.7
27.0
0
15.4
6.7
15.6
36.4
23.6
4.4
9.8
10.2
25.8
23.5
12.1
10.6
6.6
C
O
N
68.3
21.3
9.0
70.0
21.3
8.2
83.4
8.3
8.3
82.5
8.3
9.1
Analogously, a comparison of the XPS signal of all
the elements contained in the HB polymer (i.e. C, O,
N), before and after impregnation of PdCl2, did not
reveal any difference in both line shape and position.
AA′2(D)
21.4
Ta ble 6. P er cen ta ge of Am in o Gr ou p s a n d P d (a s Weigh t
P er cen tb a n d Atom ic Con cen tr a tion ) of HB P olym er s
a n d Mod el Molecu les
c
Our results are at variance with those reported by
Michalska et al. who observed a shift of the pyridine
nitrogen and carboxy oxygen to higher BE upon analysis
of Pd complexes formed on heterocyclic polyamides.
This fact is likely due to the presence of weaker
interactions in our systems.
Another route to investigate metal/polymer interac-
tions is to study the amount of metal retained by
different polymers and model molecules. To test the
reliability of the elemental analysis data obtained from
the XPS experiments, the monomer ABZAIA and the
model molecule M1,3,5 were studied by XPS spectros-
copy. As shown in Table 4, the experimental analyses
for C, O, and N were in good agreement with the
theoretical elemental data for all the substrates inves-
tigated.
percentage of
amino end groups
Pd concn
(wt %)b
Pd atomic
concn (%)
c
sample code
12.2a
0.23
0.42
0.51
1.11
0.15
0.68
0
1.87
3.36
4.06
8.53
1.20
5.28
0
II
18,19
P d Cl2/p P DT1
P d Cl2/p P DT2
P d Cl2/p P DT3
P d Cl2/p P DT4
P d Cl2/p ABZAIA
P d Cl2/ABZAIA
P d Cl2 + M 1,3,5
15.9a
a
30.2
57.8a
33.3
0
a
From 1H NMR analysis, as the total of A units referred to A
+ A′ + B - substituted benzene rings. From XPS analysis.
From XPS analysis, considering the atomic weights of C, N, O,
b
c
and Pd.
molecule M1,3,5, devoid of amino groups, did not retain
any palladium. This result is a further confirmation that
the amido groups do not interact with the metal. This
For HB aramids possessing different amounts of
functional groups, the retention of palladium compounds
is controlled by the chemical structure of the polymer.
Since the present p P DT polymers have been prepared
using different experimental conditions, the percentages
of D, L, and T units (Figure 2) (hence, the concentra-
tions of the functional groups in each polymeric product)
behavior, found also in the case of oligomeric aramids,5
may be ascribed, as previously mentioned, to the strong
electronic stabilization induced by the aromatic ring. It
is worth noticing that the metal content in the AB2
polymer was invariably lower than that retained by any
A2 + B3) sample, which reflects the higher number of
amino groups in the latter.
Plotting the percentage of palladium vs the percent-
age of NH2 groups for the various p P DT samples as
well as model compounds, a linear correlation was
obtained (Figure 7), while an opposite trend was found
for the -COOH units; i.e., by increasing the carboxy
group percentage, the amount of retained palladium
decreased.
Upon exchange with PdCl2, the polymers were no
longer soluble in any solvent. As previously observed
by J o et al. for poly(vinylpyridine) matrixes, this effect
(
1
vary from one sample to another, as determined by H
NMR spectroscopy (Table 5).
Consistent with the existence of NH2-Pd interactions,
as suggested by the FT IR analysis, a correlation
between the percentage of the amino terminal groups
II
and the amount of Pd in each HB polymer was
observed. As shown in Table 6, the palladium(II) content
is proportional to the NH2 concentration in both p P DT
and model compounds. In accord with the importance
of the NH2 groups for binding palladium(II) ions, the
2
0

Macromolecules, Vol. 36, No. 12, 2003
Palladium Nanoparticles 4299
F igu r e 7. Percentage of Pd retained by various p P DT polymers and model compounds as a function of the percentage of amino
groups.
Ta ble 7. Meta llic Disp er sion (MD) a n d d va of p P DT2 a n d
p ABZAIA
Sch em e 3. Hyd r ogen a tion Rea ction s of Va r iou s
Un sa tu r a ted Su bstr a tes w ith p P DT2
sample
code
metallic
dispersion (%)
dvab
(nm)
average
diameter (nm)
a
p P DT2
p ABZAIA
18.7
25.0
6.0
4.5
3.5
2.5
a
Calculated as MD ) N(s)m/N(t)m. N(t)m: total number of
metallic atoms. N(s)m: number of metallic atoms on the surface.
b
Calculated as dva ) 6∑(Vi)/∑(Ai); Vi and Ai: volume and area of
a single cluster.
may be due to polymer cross-linking promoted by the
supported metal species.
Ch ar acter ization of Ar am id-Su ppor ted P d Nan o-
p a r ticles. Palladium nanoparticles on either p P DT or
p ABZAIA were obtained by reduction of supported
PdCl2 with an aqueous solution of NaBH4 at 50 °C for
2
h. High-resolution TEM was employed to determine
the dimensions and dispersion of the metallic particles.
Indeed, both features are important in determining any
catalytic activity as a high metallic dispersion (i.e., low
dimensions of the metal particles) generally corre-
sponds to a high number of metal sites on the catalyst
surface.
prepared at long polymerization time, confirmed the
relationship between the dimensions of the metal
clusters and the number of binding groups on the
polymer surface.
Finally, it is worth mentioning that the cluster
dimensions, in terms of average diameter, observed in
the present HB aramids (Table 7) are similar to those
obtainable using inorganic supports for palladium cata-
TEM micrographs showed that the concentration of
0
the palladium clusters on P d /p P DT2 is quite high,
0
higher than that of P d /p ABZAIA (Figures 8 and 9,
2
1
22
respectively). In either case, metal particles lower than
nm were observed.
The diameter was calculated directly from TEM
micrographs, averaging on a large number of Pd par-
ticles.
Table 7 summarizes the results obtained from the
TEM measurements.
The differences between the two systems are probably
due to the larger concentration of the terminal amino
groups (responsible for the interaction with the metal)
in p P DT than in p ABZAIA.
lysts (1.1-1.3 nm on C and 3.5 nm on Al2O3) as well
1
as those reported for other polymeric supports (less than
2
3
4
nm on nylon and between 2 and 4 nm on other
8
resins).
Ca ta lytic Hyd r ogen a tion Rea ction s. The hydro-
genation of the unsaturated substrates shown in Scheme
was performed in solvents with remarkably different
3
polarity such as n-pentane and MeOH at relatively high
hydrogen pressure (30 bar) in standard autoclaves. In
this preliminary study of the catalytic potential of
palladium clusters supported on HB aramids, was
0
employed a specific material, P d /p P DT5, with a pal-
A perusal of p P DT materials containing a large
concentration of amino groups such as p P DT3,
ladium content of 2.02 wt % (determined by ICP-AES).
In Scheme 3 are also shown all the hydrogenation

4
300 Tabuani et al.
Macromolecules, Vol. 36, No. 12, 2003
0
0
F igu r e 9. (a) TEM micrograph of P d /p ABZAIA. (b) Histo-
F igu r e 8. (a) TEM micrograph of P d /p P DT2. (b) Histogram
of the relative population as a function of the diameter of the
Pd particles.
gram of the relative population as a function of the diameter
of the Pd particles.
by an appropriate choice of the solvent. In fact, we
noticed a remarkable solvent effect: MeOH completely
inhibited the hydrogenation of benzene to cyclohexane
products observed under the present experimental
conditions. These are detailed in Table 8 together with
the results obtained.
The substrates investigated represent a variety of
carbon-carbon and carbon-heteroatom unsaturations,
spanning from aromatic, olefinic, and acetylenic C-C
bonds to ketonic C-O bonds and to aromatic C-N
bonds.
(
entries 1 and 2), yet it allowed for the selective
reduction of phenylacetylene to ethylbenzene (entry 6)
and of quinoline to 1,2,3,4-tetrahydroquinoline (entry
1
2). Surprisingly, quinoline was not hydrogenated at all
in n-pentane (entry 11). A positive influence of MeOH
on the catalytic activity was also observed for benzyl-
ideneacetone (entry 4).
Reactions at 80 °C were performed for phenylacet-
ylene (entries 7 and 8) to show that the hydrogenation
of the triple bond can be stopped selectively in MeOH
to give styrene.
Phenylacetylene was hydrogenated in n-pentane to
a mixture of ethylbenzene and ethylcyclohexane (entry
5), whereas diphenylacetylene was selectively reduced
All reactions were reproducible with deviations lower
than 1%, and the catalyst was fully recyclable, in terms
of both activity and selectivity, for three consecutive
runs using the procedures described in the Experimen-
tal Section.
0
For all substrates, p P DT5/P d behaved as an effec-
tive catalyst with fairly good activities and excellent
selectivities. These latter could be remarkably improved

Macromolecules, Vol. 36, No. 12, 2003
Palladium Nanoparticles 4301
Ta ble 8. Hyd r ogen a tion of Va r iou s Un sa tu r a ted Su bstr a tes w ith P d /p P DT5a
0
entry
T (°C); time (h)
solvent
substrate
substrate/M
TOF,b,c productd
1
2
3
4
5
6
7
8
9
1
1
1
1
100; 1
100; 1
100; 1
100; 1
100; 1
100; 1
80; 1
n-pentane
MeOH
n-pentane
MeOH
n-pentane
MeOH
n-pentane
MeOH
n-pentane
n-pentane
n-pentane
MeOH
benzene
benzene
866
866
165
165
165
165
165
165
165
303, CY
0
143, BA
165, BA
145, EB; 20, EC
165, EB
benzylideneacetone
benzylideneacetone
phenylacetylene
phenylacetylene
phenylacetylene
phenylacetylene
diphenylacetylene
diphenylacetylene
quinoline
75, EB; 8, EC
110, ST; 55, EB
140, DPE
27, DPE
0
80; 1
100; 1
100; 6
130; 1
130; 1
130; 8
0
1
2
3
165
1650
1650
1650
1
quinoline
1,2,3,4-tetrahydroquinoline
348, THQ
MeOH
0
a
0
b
Experimental conditions: P d /p P DT5, 2.02 wt % Pd; 30 bar H2; 30 mL of solvent; 1500 rpm. Average values over at least three
runs. c Mole product: (mol Pd)
DPE ) 1,2-diphenylacetylene, and THQ ) 1,2,3,4-tetrahydroquinoline.
-1
h
-1 d
.
CY ) cyclohexane, BA ) benzylacetone, ST ) styrene, EB ) ethylbenzene, EC ) ethylcyclohexane,
1
Refer en ces a n d Notes
to 1,2-diphenylethane (entry 9), which seems to reflect
steric effects. Indeed, the selective and quantitative
formation of 1,2-diphenylethane was observed also by
increasing the reaction time to 6 h (entry 10).
(1) Akaburi, S.; Sakurai, S.; Izumi, Y.; Fugi, Y. Nature (London)
1956, 178, 323.
(
2) Gates, B. C. Chemistry and Chemical Engineering of Catalitic
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Studies are currently under way in our laboratories
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activity and selectivity of the hydrogenation reactions
(
0
(4) Galvagno, S.; Donato, A.; Neri, G.; Pietropaolo, D.; Staiti, P.
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(
(
(
(
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1
Con clu sion s
1
For the first time hyperbranched polyamides with
terminal amino and carboxy functional groups have
been used as support materials for metal nanoparticles.
The preparation of these innovative materials involves
anchoring of PdCl2 to the support prior to reduction with
NaBH4 in water. Although XPS and IR results are
2
(9) Izumi, Y. Bull. Chem. Soc. J pn. 1959, 32, 936.
(10) Gr o¨ hn, F.; Kim, G.; Bauer, B. J .; Amis, E. J . Macromolecules
2
001, 34, 2179.
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28, 13.
(
1
preliminary, these techniques allowed us to assess the
most appropriate class of polymers to be used as catalyst
support. XPS spectroscopy (together with IR) has shown
that the interaction between the polymer matrices and
the PdCl2 precursor involves exclusively the NH2 groups.
Therefore, pPDT polymers turned out to be good can-
didates for catalytic supports. Furthermore, by means
of XPS analysis, it was possible to determine the
oxidation state of the anchored metal. TEM spectroscopy
has revealed that the cluster dimensions and the metal
dispersion are greatly influenced the nature of the
polymeric support. In particular, the number of amino
groups has been found to control the size of the Pd
clusters as well as the metal loading.
The aramid-supported palladium materials proved to
be effective and robust catalysts for the selective hy-
drogenation of various unsaturated substrates, span-
ning from arenes, to R,â-unsaturated ketones, to alkynes,
to aromatic heterocycles. Notably, it was observed that
the polarity of the solvent (either MeOH or n-pentane)
can control either the activity or the selectivity of the
hydrogenation reactions.
(
12) Monticelli, O.; Mendichi, R.; Bisbano, S.; Mariani, A.; Russo,
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(13) Monticelli, O.; Mariani, A.; Voit, B.; Komber, H.; Mendichi,
R.; Pitto, V.; Tabuani, D.; Russo, S. High Perform. Polym.
2
001, 13, S45.
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2001, 34, 5487.
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VCH: Weinheim, Germany, 1999; p 451.
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1
77, 149.
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(
(
(
19) Michalska, Z. M.; Strzelec, K.; Sobczak, J . W. J . Mol. Catal.
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Ack n ow led gm en t. C.B. thanks the CNR for financ-
ing a CNR-FONACIT (Venezuela) scientific agreement
and the EC for Contract 2002-00196.
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MA034245I