M.I. Burrueco et al. / Applied Catalysis A: General 485 (2014) 196–201
197
Pd catalyst
Base
N,N-dimethylformamide suspension containing the amount
required to ensure that the final catalysts would include 0.75% of
Pd by weight. After the suspension was stirred at room temperature
for 24 h, the solvent was evaporated to dryness and the residual
solid dried in a stove at 120 ◦C. The resulting solid, named HT-Pd(II),
was reduced to Pd(0) with the hydrocarbon cyclohexene as hydro-
gen donor. To this end, an amount of 1 g of pre-catalyst was refluxed
with 10 mL of cyclohexene at 83 ◦C for 1 h. Under these conditions,
cyclohexene was converted into benzene and hydrogen released
in the reaction reduced impregnated Pd2+ to Pd0 (see Scheme 2).
After the mixture was cooled, the catalyst, labelled HT-Pd(0), was
filtered off and washed with cyclohexane and methanol.
B(OH)2
+
X
X = Halogen or Triflate
Scheme 1. General process for Suzuki cross-coupling reaction.
83 ºC
Pd0/hydrotalcite
+
Pd2+/hydrotalcite
+
Scheme 2. Reduction of Pd2+ with cyclohexene as hydrogen donor.
brucite-like layers to be positively charged. Restoring electroneu-
trality requires the insertion of an appropriate anion [19] in
The parent hydrotalcite-like compound is the natural mineral
hydrotalcite, of formula Mg6Al2(OH)2CO3·4H2O. Magnesium ion
can be exchanged both with trivalent cations and with diva-
2.2. Characterization of the catalysts
The Mg/Al ratio in the hydrotalcite and the amount of Pd
deposited on it were determined by inductively coupled plasma
mass spectrometry (ICP-MS) on an ELAN DRC-E Perkin Elmer
ICP-MS instrument under standard conditions. BET surface areas
were calculated from nitrogen adsorption–desorption isotherms
obtained at −196 ◦C on a Micromeritics ASAP 2010 instrument.
Samples were outgassed in vacuo at 100 ◦C for 12 h prior to use. All
solids [HT, HT-Pd(II) and HT-Pd(0)] were checked for crystallinity
by X-ray diffraction (XRD) analysis. Powder patterns were recorded
on a Siemens D-5000 diffractometer using CuK␣ radiation over the
range 5–80◦. Particle size and external morphology of the sam-
ples were examined with a JEOL JEM2010 transmission electron
microscope (TEM). X-ray photoelectron spectroscopy (XPS) mea-
surements were made on an Escalab 210 spectrophotometer, using
pellets 13 mm in diameter that were obtained by pressing at a low
pressure. Because of the sample dimensions, experiments were
conducted in the large-area XPS (LAXPS) mode. A double-anode X-
ray gun at an average power of 100 W (10 kV × 10 mA) was used for
this purpose. Vacuum in the main chamber was always better than
6 × 10−9 mbar. Raman spectra for the solids were acquired with a
Renishaw Raman instrument (InVia Raman Microscope) equipped
with a Leica microscope furnished with various lenses, monochro-
mators and filters, and a CCD. Spectra were obtained by excitation
with green laser light (532 nm) from 500 to 800 cm−1. A total of
128 scans per spectrum were performed in order to improve the
signal-to-noise ratio.
lent ones [21,22]. The general formula of hydrotalcites is thus
m–
[M(II)1–xM(III)x(OH)2]x+[Ax/m
]
·nH2O, where M(II) and M(III) are a
divalent and trivalent metal, respectively, at octahedral positions of
Mg2+ in brucite-like layers and A is the interlayer anion—which can
vary widely in nature and be either inorganic or organic. x, which is
defined as the ratio M(II)/[M(II) + M(III)], typically ranges from 0.17
to 0.33, which corresponds to an M(II)/M(III) ratio of 2–4 [9].
2. Experimental
2.1. Preparation of the catalyst
The hydrotalcite used was prepared by using a coprecipitation
method described elsewhere [23]. In a typical synthetic run, a
solution containing 0.3 mol of Mg(NO3)2·6H2O and 0.15 mol of
Al(NO3)3·9H2O in 250 mL of de-ionized water was used (Mg/Al = 2).
This solution was slowly dropped over 500 mL of a Na2CO3 solution
at pH 10 at 60 ◦C under vigorous stirring. The pH was kept constant
by adding appropriate volumes of 1 M NaOH during precipitation.
The suspension thus obtained was kept at 80 ◦C for 24 h, after
which the solid was filtered and washed with 2 L of de-ionized
water. The hydrotalcite thus prepared was ion-exchanged with
carbonate to remove nitrate ions intercalated between layers. The
procedure involved suspending the solids in a solution containing
0.345 g of Na2CO3 in 50 mL of bidistilled, de-ionized water per
gram of hydrotalcite at 100 ◦C for 2 h. Then, each solid was filtered
off in vacuo and washed with 200 mL of bidistilled, de-ionized
water. The new hydrotalcite thus obtained was subjected to fur-
ther ion-exchange under the same conditions. The resulting Mg/Al
solid, named HT, was used to support Pd2+ by deposition from an
2.3. Suzuki reaction
The Suzuki cross coupling reaction was conducted in a 25 mL
two-neck flask at 100 ◦C containing 1.5 mmol of phenylboronic
acid, 0.99 mmol of arylhalide, 5 mL of water, 1.98 mmol of potas-
sium carbonate, 75 mg of sodium dodecylsulphate and 0.02 mmol
(2.13 mg) of Pd. The system was stirred throughout the process.
The resulting products were identified from their retention times
by GC/MS analysis.
Mg-Al-(OH)x
layers
3. Results and discussion
3.1. Characterization of catalysts
The elemental analysis performed by using ICP methodology
revealed that the Mg/Al ratio in both the starting hydrotalcite, Pd(II)
pre-catalyst and Pd(0) catalyst was 2.03, and also that the Pd con-
tent of the latter two was 0.6 wt% (see Table 1). Therefore, solid HT
had an Mg/Al ratio identical with the theoretical value. Also, such a
metal ratio was preserved in the pre-catalyst and catalyst obtained
from the hydrotalcite. These results suggest that Mg and Al cations
were virtually completely incorporated into the solid phase.
Hydrotalcite HT has a surface area of about 78 m2/g. When pal-
ladium is loaded into this hydrotalcite, to obtain solid HT-Pd(II), a
Hydrogen
bonds
Water and carbonate ions
Hydrogen
Oxygen
Carbon
Magnesium or Aluminium
Fig. 1. Structure of hydrotalcite.