F. Liguori et al. / Applied Catalysis A: General 488 (2014) 58–65
59
catalytic membranes containing Pd species [37,38], and their use
in hydrogenation reactions of various organic substrates in the
liquid phase [39] (PVA, polyvinyl alcohol; POGE, polyoxyethy-
lene/polyglyceryl ether). It was shown that the addition of zirconia
greatly improves the mechanical and chemical resistance of PVA
membranes in hydrophilic solvents, hence, their potential for use
under catalytic conditions over prolonged reaction times.
Herein we report on the preparation of PdNP embedded into the
parent pellets and their use in catalytic partial hydrogenation reac-
tion of unsaturated C-C bonds, both under batch and continuous
flow conditions.
particles. X-ray diffraction (XRD) spectra were recorded with a
PANanalytical XPERT PRO powder diffractometer, employing CuK␣
˚
radiation (ꢀ = 1.54187 A), a parabolic MPD-mirror and a solid state
detector (PIXcel). The samples were subjected to measurement
without grinding and prepared on a silicon wafer (zero background)
that was rotating (0.5 rotations per second) during spectra acqui-
sition. All XRD spectra were acquired at room temperature in a 2ꢁ
range from 4 to 95◦, applying a step size of 0.0263◦ and a counting
time of 77.5 s.
2.4. Hydrogenation reactions
2. Experimental
Reactions under a controlled pressure of hydrogen were per-
formed using a non-metallic Büchi Miniclave® (50 mL internal
volume) equipped with a pressure controller and an orbital stir-
rer set at 150 rpm rate and a H2 generator Parker H2PEM-260.
Catalytic flow hydrogenations were carried out using a home-
made continuous-flow reactor system constructed at Istituto di
Chimica dei Composti Organo Metallici, Firenze (Italy). The sys-
tem was designed to allow for a simultaneous flow of substrate
solution and hydrogen gas through a reactor tube containing the
heterogeneous catalyst. The reactor was completely inert, as all
wet parts were made of PEEK, PFA or PFTE. The flow of the sub-
strate solution was regulated by an Alltech® model 426 HPLC
pump in PEEK. A constant flow of hydrogen gas was adjusted by
a flow controller BRONKHORST HI-TEC model F200CV-002-RGD-
11-V-MFC. The hydrogen pressure in the reactor was monitored
by a BRONKHORST HI-TEC P502C-AGD-11-V-6K0R-EPC meter. 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 mod. 9060 switching valve in PEEK. The solid cat-
alyst was packed into a commercial 3 mm inner diameter Omnifit®
glass column, equipped with 10 m PE frits at the entrance of the
catalyst bed to ensure an optimum flow distribution. 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.
2.1. General information
under nitrogen atmosphere by using standard Schlenk techniques,
unless otherwise stated. The hybrid zirconia/PVA materials con-
taining palladium oxide were prepared as previously described
[37,38]. All the other chemicals were reagent grade, commercial
products and were used as received without further purifica-
tion.
2.2. Catalyst preparation
In
a
ganic/polymeric beads NKZPDB-5 type (1.01 g, 1.7 0.2 mm
diameter, composition in weight ratio to 80% saponified PVA nor-
malized to 1: ZrO2 0.14, PdO 0.21, polyoxyethylene polyglycerol
ether 0.19) [37,38] were introduced into a round bottom flask
equipped with a lateral stopcock containing nitrogen-degassed
water (40 mL). The suspension was cooled to 0 ◦C and an excess
of NaBH4 (365 mg, 9.6 mmol) was added in portions under a
stream of nitrogen. The solution was stirred with an orbital-stirrer
under nitrogen at 0 ◦C for 30 min, and then at room temperature
for 24 h. After that time, the water solution was removed by
decantation, and the pellets were carefully washed with degassed
water (3 × 30 mL) and methanol (3 × 30 mL), then dried under a
stream of nitrogen overnight. The brow-grey pellets thus obtained
(Pd@NKZPDB-5) were stored under nitrogen before being used
in catalytic hydrogenation reactions. For the purpose of evaluate
the metal loading in Pd@NKZPDB-5, the pellets were dried under
vacuum overnight and analyzed by ICP-OES to give a typical bulk
0.10% (w/w) Pd content.
2.4.1. Hydrogenation reactions by Pd@NKZPDB pellets under
batch conditions
In
a typical experiment, Pd@NKZPDB -1 pellets (440 mg,
0.03 wt% Pd, 1.1 × 10−3 mmol Pd), were placed under nitrogen
into a metal-free autoclave. A degassed solution of substrate
in methanol (0.05 M, 21.0 mL) was transferred under nitrogen
via a Teflon tube into the autoclave. Nitrogen was replaced
by hydrogen with three cycles pressurization/depressurization.
The autoclave was finally charged with the desired pressure of
hydrogen and stirred at 150 rpm using a orbital stirrer. After
the desired time, the reactor was depressurized and the solu-
tion was completely removed under a stream of hydrogen using
a gas-tight syringe. A sample of this solution was analyzed
by GC and GC-MS for product identification and determination
and by ICP-OES for metal leaching. For recycling experiments,
a fresh solution of the substrate was then transferred under
hydrogen via a gas-tight syringe into the reactor containing
the catalyst recovered. The autoclave was again charged with
hydrogen, stirred at 150 rpm and, after the desired time, the
mixture was treated as described above. The same recycling pro-
cedure was used in the subsequent hydrogenation cycles. After
use in catalysis, the solid catalyst was washed with methanol
(3 × 10 mL) and diethyl ether (3 × 10 mL), dried in a stream of
nitrogen. The reaction products were unequivocally identified by
the GC retention times and mass spectra of those of authentic
specimens.
In an analogous procedure, the hybrid inorganic/polymeric
beads NKZPDB-1 type, featuring a ca. 0.8 mm diameter ZrO2 core
covered with a film of ca. 300 m hybrid material (1.2 0.2 mm
diameter, composition in weight ratio to 80% saponified PVA nor-
malized to 1: ZrO2 0.10, PdO 0.21, polyoxythylene polyglycerol
ether 0.19), was treated as above to give the same Pd content in
the film (EDS) and a bulk 0.03% (w/w) Pd loading (ICP-OES).
2.3. Catalyst characterization
Environmental Scanning Electron Microscopy measurements
(ESEM) were performed on a FEI Quanta 200 microscope operat-
ing at 25 keV accelerating voltage in the low-vacuum mode (1 torr)
and equipped with an EDAX Energy Dispersive X-ray Spectrometer
(EDS). Samples for Transmission Electron Microscopy (TEM) anal-
yses were prepared by inclusion of the membranes into Struers
EpoFix® epoxy resin, followed by lapping. The samples were then
cut with a RMC MT-XL ultramicrotome to give a film thickness of
60 nm. TEM measurements were carried out using a CM12 PHILIPS
instrument at 120 keV accelerating voltage. Statistical nanoparti-
cle size distribution analysis was typically carried out on 300–400