GC using a 30 m crosslinked phenylmethylsilicone capillary
column. Besides unreacted substrate, the only products
observed were a-bromopropiophenone, 2-hydroxyethyl-2-
phenylpropionate and 2-phenyl-3-methyldioxine.
supported zeolites in C–C coupling reactions, we performed
the Suzuki reaction of bromobenzene and phenylboronic acid
in toluene at reflux temperature in the presence of palladium
supported on KX and on HBeta. While in the first case the
reaction forms biphenyl in 87% yield at 24 h, under the same
conditions but using HBeta the conversion of bromobenzene
was only 10%. The above result indicates that the most appro-
priate supported palladium catalyst for the Suzuki requires
soft palladium atoms rather than hard ones.
Electrochemistry and XPS measurements
Zeolite-modified electrodes were prepared, as previously
1
6,17
described,
by transferring a few microlitres (typically 50
ml) of a dispersion of the zeolite (10 mg) in acetone (5 ml) to
the surface of a freshly polished glassy carbon electrode
In summary, the results obtained for the rearrangement of
the cyclic ethylene acetal of 2-bromopropiophenone show that
the product distribution obtained depends in a consistent man-
ner on the nature of the sites. Each type of site, either Br o¨ nsted
or hard and soft Lewis acid sites, gives specifically a distinctive
product. Particularly important is the finding that the soft-
ness/hardness nature of the Lewis sites can be assessed by sim-
ple measurement of the product distribution. Furthermore,
given the current interest on Pd catalyzed reactions supported
on inorganic solids, our data show that a wide range
in the tuning of the electron density on the palladium can be
achieved through modification of the support composition,
this leading to a variation of its softness/hardness and as a
consequence in its catalytic activity.
2
GCE; BAS MF 2012; geometrical area 0.07 cm ). One drop
(
of a solution of the Paraloid B72 (1%) in acetone was then
ꢃ2
added. The coatings examined contained 0.2–1.5 mg cm of
the dry zeolite.
PdCl
literature.
2
-attached electrodes were prepared as described in the
1
8,19
The sample (1–2 mg) was powdered in an agate
mortar and pestle. The lower end of the electrode was gently
rubbed over the powder and finally cleaned with a tissue paper.
Linear potential scan voltammograms and differential pulse
voltammograms were performed with a Metrohm E506 polar-
ecord. A standard three-electrode arrangement was used, with
a platinum auxiliary electrode, a saturated calomel reference
electrode separated from the solution using a capillary salt
bridge and a GCE electrode. Electrochemical measurements
4
were conducted in water using 0.10 M LiClO (Aldrich) as sup-
Experimental
porting electrolyte. All electrochemical measurements were
performed at room temperature (298 K) in well-deaerated
solutions under an atmosphere of argon.
XPS measurements were carried out at room temperature
with a concentric hemispherical analyzer operated in the con-
Materials and methods
NaX was a commercial sample (Aldrich 13X). KX and CsX
were obtained from NaX by three consecutive room tempera-
ture ion exchanges using KAcO and CsAcO aqueous solutions
of increasing concentration (0.3, 0.4, and 0.5 M) and a solid–
liquid ratio of 1:10. HBeta was prepared by hydrothermal crys-
tallization using tetrapropylammonium as template, followed
by calcination in air flow as reported. The Si/Al ratio of
HBeta determined by chemical analysis was 12.5. NaBeta
was prepared from HBeta by ion exchange using a 0.6 M aqu-
stant pass energy mode (50 eV). A Mg Ka X-ray source
ꢃ9
(
hu ¼ 1253.6 eV) was used. A vacuum of ca. 5 ꢁ 10 Torr
was always attained in the analysis chamber during XPS
recording. Charging effects were calibrated by the C(1s) line
at 284.6 eV.
1
4
3
eous solution of NaHCO . All-silicon Beta zeolite was also
obtained by hydrothermal crystallization under dynamic
conditions, using aerosil as silicon source and tetrapropyl-
ammonium bromide as template at 140 C as reported.
Acknowledgements
ꢂ
14
Financial support of the Spanish DGES (project number MAT
2
003-01226) and Generalidad Valenciana (grupos 03-020) is
gratefully acknowledged. AP thanks the Spanish Ministry of
Education for a scholarship.
Natural sepiolite was obtained from TOLSA (Vallecas,
Spain) and was free from carbonate minerals. Potassium and
cesium sepiolites were obtained by autoclaving natural sepio-
ꢂ
lite at 200 C with a 1 M aqueous solution of NaOH for
6
1
5
h. The resulting Na-sepiolite was thoroughly washed with
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7
8
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2
4
176–4211.
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0
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1
Test reactions were carried out by stirring magnetically a sus-
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ꢂ
1
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3
64
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 3 6 1 – 3 6 5