Oxidation of Terminal Olefins by Dioxygen
FULL PAPER
Ϫ The HZ zeolite used in our experiments was obtained by con-
trolled thermal decomposition of the NH4Z ammonium form (cal-
cination with oxidation in the presence of dioxygen). This is a two-
step method: preparation of the NH4Z form from the NaZ form
by exchange, then calcination from NH4Z to HZ (500Ϫ600°C).
This decomposition is accompanied by aluminum extraction of the
zeolite structure (AlIV Ǟ AlVI) and deblocks the pores: adsorption
of benzene goes from 1% (NaZ form) to around 6% (HZ form).
could correspond to the appearance of the maximum around 375
nm, was only observed with the LZY82 zeolite (vide supra).
Characterization of Pd0/Zeolite Samples by Electron Microscopy:
The exchanged zeolite samples, calcined and reduced, were exam-
ined by electron microscopy at high resolution using a J.E.M. 120
apparatus (JEOL) allowing the palladium particles on the zeolite
support to be observed by transmission.
The histograms of the calcined and reduced samples using a lin-
Faujasite Y has a very open structure associated with a very large
volume; two Y zeolites were used:
ear increase in temperature (13°C/h) to 300Ϫ450°C showed a rela-
˚
tively homogeneous particle size distribution (ca. 20Ϫ30 A). On
Ϫ The LZY82 zeolite (Union Carbide) (SiO2/Al2O3 ratio ϭ
5.6Ϫ6.5)
the other hand, all the Pd0/HZ samples resulting from calcination
at 600°C with a slightly higher temperature gradient (22°C/h), then
reduction at 13°C/h to 350°C had a heterogeneous particle disper-
Ϫ The LZY52 zeolite (Linde) (SiO2/Al2O3 ratio ϭ 4.75).
˚
sion. The majority of particles showed a diameter close to 30 A;
3
4
Preparation of Zeolite Samples. Ϫ Exchange of the Zeolite
(Mordenite or Faujasite): The conventional method for introducing
palladium(II) into zeolites was used[12]. For all samples, about 5%
(w/w) of the palladium complex was added. A solution (pH ϭ 11.8)
of [Pd(NH3)4]Cl2 was passed three times through an Amberlite
anion exchanger; the absence of ClϪ ions from the resulting solu-
tion was checked by silver nitrate. The ClϪ ions are exchanged by
OHϪ giving [Pd(NH3)4]2ϩ(OHϪ)2 dissolved in water. Following the
third exchange, the base was added dropwise onto the zeolite sus-
pended in water and maintained for 20 h at ambient temperature
with slight stirring. After filtration, the product was carefully
washed with distilled water and dried for 12 h at 110°C.
˚
however, a few very large clusters were observed in the 10 Ϫ10 A
range, proving that partial coalescence occurred with certain par-
ticles.
Analysis of Pd/Zeolite Samples by X-ray Photoelectron Spec-
troscopy: Palladium is characterized by the 3d3/2 and 3d5/2 peaks,
which correspond to the 3d electron emission having a J spin-orbit
coupling of either 3/2 or 5/2 (J ϭ L ϩ S with L ϭ 2). The kinetic
energy of this electron can be obtained from the spectra and the
energy of the bond is calculated by using, as internal reference, the
C1s line of carbon whose bond energy is 285 eV.
E1 (Pd 3d3/2 or 5/2) ϭ 285 ϩ Ec (C1s) Ϫ Ec (Pd 3d3/2 or
)
5/2
The catalysts prepared by ion exchange from the
To estimate the palladium, a sensitivity factor of 10 was used for
[Pd(NH3)4](OH)2 complex contain a large amount of ammonia the 3d5/2 line with respect to C1s. This value was taken from litera-
which must be eliminated under conditions that avoid pal-
ladium(II) ion reduction which would result in the migration and
clustering of the metallic atoms thus formed.
ture data. For the spectrometer used, a correction factor had to be
determined from a defined compound. The Pd/Si ratios measured
by X-ray Photoelectron Spectroscopy or by chemical analysis were
very similar for the two types of zeolites. Moreover, XPS allowed
the characterization of the nitrogen remaining after thermal treat-
ment. The bond energies for N1s (401.5Ϫ403 eV) showed that it was
not NH3 but perhaps a form of oxidized nitrogen[14]. These species
cannot be held responsible for the difference in the selectivity found
at different stages of thermal treatment for both zeolites.
Thermal Treatment in Dioxygen: Following the work of Bergeret
et al.[13], we adopted a calcination procedure in which the tempera-
ture was very slowly increased (13°C/h) with a continuous flow of
dioxygen (ca. 5 1/h) through the zeolite (ca. 1 g), spread on a thin
layer of a glass frit in a vertical tubular reactor of transparent silica.
The final calcination temperature (300Ϫ350°C) was maintained for
15 h.
Catalyst Tests: In a routine experiment, the palladium-based pre-
cursor was introduced into a Schlenk tube at a concentration equiv-
alent to 0.02 mol·lϪ1 if all the palladium were in solution with the
cocatalysts, followed by addition of the solvent (absolute ethanol)
and the olefin. After cold degassing, dioxygen was introduced at
atmospheric pressure and the reaction tube was plunged into a
thermostated bath. Adsorption of oxygen was measured during the
reaction time (from 1 to 24 h). The systems were analysed by gas
chromatography.
Thermal Treatment in Dihydrogen: Fast reduction was carried out
by placing around 1 g per sample, already treated with dioxygen,
on a thin layer of a glass frit in the same double-walled reactor
after purging the tube under hydrogen (ca. 5 l/h); the precursor was
reheated (13°C/h) to 300Ϫ450°C; this temperature was maintained
for 15 h making it possible to obtain similar samples for the prod-
ucts treated under comparable conditions.
Characterization of PdII/Zeolite Samples by UV/Vis Spec-
trometry: The coordination of Pd2ϩ ions was followed by UV/Vis
spectrometry in diffuse reflexion with a Beckmann 5270 spec-
trometer equipped with a double monochromator. Barium sulfate
was used as reference.
At the end of the reaction, an internal reference (ortho-dichloro-
benzene, for example) was added to estimate the ketones formed.
Gas Chromatography and Tandem Gas ChromatographyϪMass
Spectrometry (GC-MS)
A UV/Vis spectroscopic study of the exchanged zeolite, non-cal-
Gas Chromatography: The reaction mixtures were analysed by
cined (dried at 110°C for 20 h), showed that the tetraammine com- chromatography: column length 3 m, diameter 2.16 mm of Chro-
plex was introduced into the zeolite without any modification. This
is shown by the absorption at 1540 nm characteristic of the
mosorb WAW-DMCS treated by FFAP (10 or 20%); injected vol-
ume: 0.3 µl. Analytical conditions: chromatography with flame ion-
ization detector: Girdel model 330; Delsi Enica 10 integrator; vec-
tor gas: N2 (40 ml·minϪ1); hydrogen (20 ml·minϪ1); air (300
ml·minϪ1). Column temperature: 65 or 100°C; injector tempera-
ture: 180°C; detector temperature: 190°C.
PdϪNH3 system[11]
.
Following thermal treatment in the presence of dioxygen, the
catalyst took on an orange-rose colour which, upon contact with
air, progressively changed to rose-beige due to absorbed water. This
colour change probably indicates the formation of two species. One
could be the tetrahedral “Pd(O)3(H2O)” with C3v symmetry of the
Pd(O)3 group[11]. The other, the planar species [Pd(H2O)4]2ϩ, which
Capillary Columns: Capillary columns allow more efficient sep-
arations with similar compounds such as olefin isomers. Analyses
were carried out with a silica capillary column (length: 50 m; diam-
Eur. J. Org. Chem. 1998, 1901Ϫ1906
1905