146
B.L. Albuquerque et al. / Journal of Catalysis 340 (2016) 144–153
ture) (0.375 mmol). The formation of the Pd0NPs is visually charac-
terized by a colour change from yellow to dark brown.
2.6. Catalytic activity studies
In a Schlenk tube, 10 mL of the colloidal Pd0NPs (0.015 mmol)
and the substrate (1.5 mmol) were introduced. The mixture is
strongly stirred with a magnetic bar, covered with a septum and
degasified over vacuum. After the bubble evolution ceases, a bal-
loon filled with H2 is attached in the septum and the degasification
procedure is repeated at least 2 times. For kinetic experiments, one
aliquot of the reaction mixture ( 1.0 mL) is removed at a desired
time with a syringe and the organic compounds are extracted with
diethyl ether for GC analysis (Carrier gas He, Isobaric at 60 kPa, Col-
2.5. Characterization of Pd0NP colloids
The Pd0NPs@stabilizer suspensions were characterized by TEM,
scattering techniques and zeta potential values. For the TEM anal-
ysis, four drops of the aqueous colloidal NPs were deposited on a
400-square mesh copper grid with carbon film and dried naturally.
The micrographs were acquired using a JEOL JEM-1011 electron
microscope operating at 100 kV. At least 140 particles were consid-
ered to plot a histogram of the NP size distribution, and the average
particle size was obtained by a Gaussian fit of the size distribution.
The dynamic light scattering (DLS) experiments and zeta potential
were carried out using a DelsaNano C instrument (Beckmann Coul-
ter). The aqueous suspensions of Pd0NP were analysed at 25 °C,
after the temperature equilibration, about 10 min after the cell
was placed in the DLS apparatus. SAXS experiments were per-
formed on the SAXS1 beam line of the Brazilian Synchrotron Light
Laboratory (LNLS – Campinas, SP, Brazil) and at the Institut de Phy-
sique in Université de Rennes1, France. The system configuration
for the experiments carried out in LNLS (Brazil) was made as in
our recent publications [32–34]. The solutions were loaded into a
temperature-controlled vacuum flow-through cell composed of
two mica windows separated by 1 mm, normal to the beam [35].
For the experiments carried out on the SAXS1 beam line, the colli-
mated beam (k = 1.55 Å) crossed the sample through an evacuated
flight tube and was scattered to a Pilatus 300 K 2D detector (Dec-
tris) and a 2D CCDmarCCD 165 detector, respectively. The incident
beam was detected at two different sample-to-detector distances
on the SAXS1 beam line, 500 mm (silver behenate was used for
sample-to-detector distance calibration), to achieve the scattering
vector range q (from 0.1 to 5 nmꢀ1). The experiments carried out in
the bench-top configuration, X-ray patterns were collected with a
Pilatus 300k (Dektris, Grenoble, France), mounted on a micro-
source X-ray generator GeniX 3D (Xenocs, Sassenage, France) oper-
umn CP-Chirasil-Dex-CB – 30 m ꢁ 0.25 mm ꢁ 0.25
lm, oven tem-
perature varies with substrate, injector at 250 °C and FID
Detector at 250 °C).
3. Results and discussion
3.1. Synthesis and stability of Pd0NPs
The Pd0NPs catalytic systems (Fig. 1) were prepared in the pres-
ence of various protective agents, such as HEA16Cl surfactant, the
N-dodecylated PEI (M6PEI) or HEA16Cl/PEI self-assemblies with
different ratios. The HEA16Cl was synthesized according to a pro-
cedure reported in the literature [31]. M6PEI was prepared from
an adapted procedure based on a stepwise aldimine formation–re-
duction in methanol [40], using dodecyl aldehyde as the alkylating
agent of PEI (800 Da) in a 6:1 ratio (Scheme 1). The 1H NMR anal-
ysis, based on the ratio of integrals of ACH2A signals from the
polymer backbone and those of ACH2A signal from the alkyl chain,
confirmed the complete conversion of primary amines into sec-
ondary, thus providing a new polymer M6PEI with a 6:1 Dode-
cyl:PEI ratio. This method proved to be very straightforward in
the selective alkylation of primary amines and could be applied
for a variety of commercially available aldehydes.
In a first set of experiments, the formation of polymer–surfac-
tant complexes was characterized in water. The steady state fluo-
rescence spectroscopy was chosen as an alternative to surface
tension experiments [27]. Pyrene is commonly used as a probe
since its fluorescence emission spectrum is sensitive to the polarity
of the medium [41,42].
The cmc value of M6PEI and the cac value between M6PEI and
HEA16Cl were extracted from the graphs in Fig. 2a and b, respec-
tively. While the commercial PEI does not exhibit hydrophobic
domains and self-aggregation [27], we showed that the synthe-
sized M6PEI bearing an average of six lipophilic alkyl chains with
12 carbon atoms self-aggregates into micelles above critical micel-
lar concentrations (cmc) of 0.084 mmol Lꢀ1 in monomer units or
0.002 mmol Lꢀ1 using the average molecular weight determined
by 1H NMR. The aggregation behaviour of M6PEI and HEA16Cl
was determined at 0.089 mmol Lꢀ1 that is 13.8 times lower than
the cmc of HEA16Cl (1.23 mmol Lꢀ1) [31]. For the cac determina-
tion the concentration of M6PEI was 0.058 mmol Lꢀ1 (monomer
units), which is lower than its cmc. These investigations demon-
strated that the mixed aggregation occurs as also previously
ating at 30 watts. The monochromatic CuK
k = 1.541 Å. The diffraction patterns were therefore recorded for
reciprocal spacing q = 4
a radiation is
p
⁄ sinh/k in a range of repetitive distances
from 0.15 nmꢀ1 (4180 nm) to 7.7 nmꢀ1 (80 nm). The samples were
loaded in quartz capillary tubes (£ = 1.1 mm) and the tubes placed
in a homemade sample holder. For both experimental set-ups the
2D images were found to be isotropic and they were normalized
using the FIT2D software developed by Hammersley [36]. Also,
the resulting I (q) vs. q scattering curves were corrected by subtrac-
tion of the scattering from the pure solvent and then placed on an
absolute scale using water as the standard. The log (q) vs. log qꢀ1
scattering profile of the Pd0NPs could be fitted using the Beaucage
Power Law [37] and Core–Shell models [38]. The fitting procedures
and other analysis were performed using the SASfit software devel-
oped by Kohlbrecher and available free of charge [39], which
makes use of the least squares fitting approach to minimize the
chi squared (
v
2) parameter.
C10H21
H 25
C12
NH2
O
NH
N
NaBH4
rt, 12 h
N
+
N
H
MeOH, THF, rt, 24 h
n
C10H21
N
N
N
H
N
H
n
n
Scheme 1. Synthesis of M6PEI by reductive alkylation in MeOH/THF.