F. Liguori, P. Barbaro / Journal of Catalysis 311 (2014) 212–220
213
measurements were performed on a FEI Quanta 200 microscope
operating at 25 keV accelerating voltage in the low-vacuum mode
(1 torr) and equipped with an EDAX Energy Dispersive X-ray Spec-
trometer (EDS). X-ray maps were acquired on the same instrument
using a 512 Â 400 matrix, 100
ls dwell time and 25 keV accelerat-
ing voltage. TEM (Transmission Electron Microscopy) analyses
were performed using a CM12 PHILIPS instrument at 120 keV
accelerating voltage. The sample preparation was carried out by
dispersing the grinded resin into about 1 mL of ethanol and treat-
ing the solution in an ultrasonic bath for 30 min. Successively, a
drop of solution was deposited onto a carbon coated Cu TEM grid
and the solvent left to evaporate. XRD (X-ray Diffraction) patterns
were recorded with a PANanalytical XPERT PRO powder diffrac-
Fig. 1. Sketch of composition (left) and representative SEM image (1600 magni-
fications, right) of MonoBor.
tometer, employing CuKa radiation (k = 1.54187 Å), a parabolic
MPD-mirror and a solid state detector (PIXcel). The samples were
prepared on a silicon wafer (zero background) that was rotating
(0.5 rotations per second) during acquisition. All XRD patterns
were acquired at room temperature in a 2h range from 30° to
55°, applying a step size of 0.0263° and a counting time of
295.29 s. TGA analyses were performed with a Seiko EXSTAR TG/
DTA 7200 thermogravimetric analyzer. The metal content in the
supported catalysts was determined by Atomic Absorption Spec-
trometry (AAS) using a ANALYST200 spectrometer. Each sample
(50–100 mg) was treated in a microwave-heated digestion bomb
(Milestone, MLS-200, 20 min @ 220 °C) with concentrated HNO3
(1.5 mL), 98% H2SO4 (2 mL), and 0.5 mL of H2O2 30%. After filtra-
tion, the solutions were analyzed. The content of metal leached
in the solutions recovered after catalysis was determined by Induc-
tively Coupled Plasma Atomic Emission Spectroscopy (ICP-OES)
with a Varian 720ES instrument at a sensitivity of 500 ppb. The
solutions were analyzed directly after 1:5 dilution in 0.1 M hydro-
chloric acid. GC-analyses were performed on a Shimadzu GC 2010
gas chromatograph equipped with flame ionization detector and a
ammonium polymers prepared by amine treatment [46] or by
copolymerization of 2-(acryloyloxy)ethyl trimethylammonium
chloride and PEGDA [47].
We recently reported the synthesis and characterization of a
macroporous polymeric monolith, hereinafter called MonoBor
(Fig. 1), in which cation-exchange tetraphenylborate anions are
incorporated in
matrix [48]. A striking feature of this material is its reproducible,
isotropic microstructure based on a ‘rigid’ skeleton of ca. 6
thickness forming a narrow size distribution of interconnected
flow-through pores of 10 m size, corresponding to a BET surface
a highly cross-linked styrene–divinylbenzene
l
m
l
area of 9.72 m2 gÀ1, which guarantees a very low flow resistance
and a high mechanical stability. The chemical and thermal resis-
tance of the polymer was demonstrated up to 350 °C. The monolith
was easily obtained in situ within the walls of a commercial glass
column resistant up to 1200 psi pressure, so as to provide a mono-
lithic mesoreactor (i.d. 3 mm, length 25 mm) perfectly suited for
use in synthetic applications under continuous flow [49]. The
exchange ability of the monolith was demonstrated and the
anchoring of cationic metal complexes was successfully achieved
to give a uniform metal distribution within the solid support.
Herein we describe the immobilization of palladium nanoparti-
cles onto MonoBor and the use of the corresponding monolithic
reactor (Pd@MonoBor) in the catalytic partial hydrogenation reac-
tion of alkynes under continuous flow. Choice of palladium NPs
was motivated by their known versatility as catalysts, so as to be
often used to investigate the effect of the support on the catalyst
efficiency [50], and by the method we recently developed for the
effective and environmentally friendly immobilization of PdNPs
onto monolithic supports [15]. The hydrogenation reaction of
substituted alkynes was selected because of its high significance
as probe of catalyst chemo-, stereo- and regio-selectivity, and
because it is the technology currently adopted for the large-scale
production of several important fine-chemicals, including pharma-
ceuticals, fragrances and food ingredients [51].
30 m (0.25 mm ID., 0.25
ID, 0.25 m FT) VF-WAXms fused silica capillary column. GC–MS
analyses were performed on Shimadzu QP5000 apparatus
lm FT) SPB-1 Supelco or a 30 m (0.25 mm
l
a
equipped with a SPB-1 Supelco fused silica capillary column or
on a Shimadzu GC–MS 2010 apparatus equipped with a 30.0 m
(0.32 mm ID) WCOT-Fused silica CP-Wax 52 CB capillary column.
Reactions under a controlled pressure of hydrogen were performed
using a H2 generator Parker H2PEM-260. Reactions under continu-
ous flow were carried out using a reactor system constructed at
Istituto di Chimica dei Composti OrganoMetallici, Sesto Fiorentino
(Italy). The system was designed to allow for a simultaneous flow
of substrate solution and hydrogen gas (up to 40 bar pressure). The
reactor was completely inert, as all wet parts were made of PEEK,
PFA or PFTE. A constant flow of substrate solution was regulated by
an AlltechÒ model 426 HPLC pump in PEEK. A constant flow of
hydrogen gas was adjusted, and its pressure monitored, by a
BRONKHORST flow controller and pressure meter, respectively.
The concurrent flows of gas and liquid were driven through a
T-shaped PEEK mixer to ensure efficient gas dispersion. The mixed
hydrogen-substrate solution stream was introduced in the reactor
through a 6-port Rheodyne switching valve in PEEK. The system
was equipped with a temperature controller. At the outlet of the
reactor, the product solution was collected for GC analysis and
the excess amount of the hydrogen gas released to the atmospheric
pressure. The reaction products were unequivocally identified by
the GC retention times, mass and NMR spectra of those of authen-
tic specimens.
2. Experimental section
2.1. 1. Materials and methods
All reaction and manipulations were performed under nitrogen
by using standard Schlenk techniques, unless otherwise stated.
Tetrahydrofurane was distilled from sodium-benzophenone prior
of use. The MonoBor monolith was prepared as previously
described into a commercial Omnifit Labware Glass Column (3.0
i.d. Â 25 length mm) equipped with 0.2
l
m PE frit to ensure an
2.2. Synthesis of Pd@MonoBor
optimum flow distribution [48]. All the other reagents were
commercial products and were used as received without further
purification. Palladium on activated charcoal (5 wt.%) was obtained
from Aldrich. ESEM (Environmental Scanning Electron Microscopy)
A solution of Pd(NO3)2 in THF (0.025 M) was flowed through a
MonoBor monolithic column (3.0 i.d. Â 25 length mm, ca. 30 mg
dry material) at 0.5 mL minÀ1 for 3 h using an HPLC pump and an