Journal of Molecular Catalysis, 48 (1988)
343 - 341
PREPARATION OF DIVALENT Pd(I1) SPECIES ON SEPIOLITE AND ITS CATALYSIS OF OLEFIN DIMERIZATIONS KIYOTOMI KANEDA, TADAO KIRIYAMA, TADASHI HIRAOKA, and TOSHINOBU IMANAKA Department of Chemical Engineering, Toyonaka, Osaka 560 (Japan)
Faculty of Engineering
Science, Osaka University,
(Received December 3, 1987; accepted April 8, 1988)
Summary Sepiolite-bound Pd(I1) species are prepared from the treatment of sepiolite with Pd(C$I&. XPS spectra of the Pd complexes show the formation of divalent Pd species on sepiolite. The Pd complexes are stable in the atmosphere and show high activity for olefin dimerization.
Introduction Homogeneous palladium complexes exhibit a unique and versatile reactivity for organic reactions and play an important role in organic synthesis as catalysts. In order to overcome the problem of separation of homogeneous metal complexes, we have continuously studied the heterogenization of soluble Pd complexes using polystyrene [ 1, 21 and cellulose supports [ 31. We have chosen sepiolite as an inorganic support to chemically bind the Pd(CsH,), complex. Sepiolite is a hydrated magnesium silicate of fibrous morphology; the open channels (5.7 A X 11.0 A) extend longitudinally in the direction of the fiber axis, and zeolitic, structural, and bound waters exist in the channels. The palladium complex formed on sepiolite is a divalent Pd species and shows catalytic activity for dimerization and codimerization of olefins. Here, we wish to report the preparation and the catalysis of the sepiolite-bound Pd( II) complex.
Experimental The general procedure for preparation of the sepiolite-bound Pd(I1) complexes is as follows. In order to remove zeolitic water, sepiolite, provided by Takeda Chemical Industry, was heated at 150 oC/lO-3 torr for 2 h . Pd(C&), complex was synthesized from the reaction of Pd,Cl,(C$-I,), complex with C3HSMgCl in diethyl ether at -70°C. After filtration, the 0304-5102/88/$3.50
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ether was evaporated at -60 “C. The resulting mixture was extracted with pentane, followed by filtration at -40 “C. Evaporation of the pentane gave Pd(C+H,), in 40% yield based on Pd&!l,(C$Is),. The structure was assigned by ‘H NMR [ 51. To sepiolite was added the pentane solution of Pd(C&), and the resulting mixture stirred at -20 “C. After 1 h, the sepiolite was washed with dried pentane until the filtrate became colorless, cu. three cycles. All the above procedures were carried out under argon atmosphere. The sepiolite-bound Pd complexes are stable at room temperature in atmospheric conditions. Treatment of the Pd complexes with Hz gave a mixture of propylene and propane, cu. 70% yield based on Pd metal, accompanied by a color change from white to grey. Under ethylene atmosphere, the Pd complexes yielded C5 olefins. These results indicate that one ally1 moiety remains on Pd in the sepiolite-bound Pd complexes.
Results and discussion Figure 1 shows the XPS spectra of the Pd complexes (Pd 0.62 wt%). The Pd complexes were sensitive to the X-ray radiation. After 30 min radiation, binding energies of Pd 3d are shifted to values lower than those after 15 min radiation, due to Pd reduction. After 15 min radiation, the binding energy of Pd 5/2 can be observed at 337.5 eV, assignable to divalent Pd species . The 336.5 eV peak is due to monovalent Pd( I) species derived from reduction of the Pd(I1) species during the X-ray radiation. In order to confirm chemically the formation of the divalent Pd species on sepiolite, dimerization of styrene as a typical Pd( II)-catalyzed reaction [ 7,8] was carried out in benzene at 60 “C. The Pd complex exhibits high activity in the styrene dimerization to give exclusively (E)-1,3-diphenyl-1-butene. After the Ph I
Styrene/Pd = 1600
Activity: 250 mol of styrene (g-atom Pd)-’ h-’
dimerization, the Pd complex was easily separated.from the reaction mixture in the atmosphere. The used Pd complex did not show catalytic activity at 60 “C. Increasing the reaction temperature to 80 “C yielded the dimer at a low rate. In the Pd complexes both before and after the dimerization, the ESR signal at g = 1.996 due to monovalent Pd(1) species  could be observed at room temperature: the signal intensity for the used Pd complex was about ten times stronger than for the fresh Pd complex (Fig. 2). This result shows that the active Pd(I1) species for the dimerization, e.g. Pdhydride, is unstable, which might lead to the formation of inactive Pd(1) species. The phenomenon of this Pd reduction to Pd(1) species could be also observed in the XPS spectra (Fig. 1. c,d).
Binding energy (ev) Fig. 1. Pd 3d 512 and 312 XPS styrene dimerization. A: Fresh after 30 min X-ray radiation; C: ered catalyst after 30min X-ray 6, 335.1 eV.
spectra for sepiolite-bound Pd complex before and after catalyst after 15 min X-ray radiation; B: fresh catalyst recovered catalyst after 15 min X-ray radiation; D: recovradiation. 1, 341.7; 2, 340.7; 3, 337.5; 4, 336.4; 5, 335.8;
The fresh Pd complex is active also for ethylene dimerization and codimerization of ethylene and styrene. (E)-2-butene is the main homodimer of ethylene. All codimers are derived from the addition of ethylene to the CYposition of styrene; the codimers are 3-phenyl-1-butene, 2-phenyl-2-butene and 2-phenyl-1-butene. The dimerization and codimerization can be explained by the divalent Pd-hydride mechanism . The Pd-hydride species may be generated from the following reaction:
It is notable that other Pd complexes prepared from the complexation of Pd(C,H& with A1203, MgO, SiOz and NaY zeolite showed no activity for the dimerization. The Pd binding energies of these complexes showed values lower than those of the sepiolite-bound Pd complexes by 0.5 to 1.0 eV in
g = 1.996
7.1 x 100
5.0 x 10
Fig. 2. ESR spectra of sepiolite-bound Pd complex before and after styrene dimerization. Field, 3580 * 150 G. The spectra were recorded at room temperature. A: Fresh catalyst before styrene dimerization, B : recovered catalyst after styrene dimerization.
Pd3d XPS spectra. Ryndin et al. have prepared the Pd catalyst from the treatment of SiOz with Pd(C$I& species, but did not succeed in detecting Pd( II) species [lo]. At present, we do not know why only sepiolite can form Pd(I1) species from Pd(C$&), complex. However, we suggest perhaps the Pd(C,H,), complex reacts with bound water on magnesium to form -Mg-O-Pd”(C&) species. Details of the ligand environment around Pd are not yet clear. Shapeselective reactions using the present sepiolite-bound Pd complex are under investigation.
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