A R T I C L E S
Mori et al.
alcohols.7 However, they often require high catalyst concentra-
tions, an excess of ligands or bases, and high oxygen pressures.
In addition, the use of homogeneous metal catalysts on an
industrial scale is limited by practical problems due to the
difficulties in recovering the expensive catalyst metals and
ligands from the reaction mixture. In this context, the current
research has been focused on the development of efficient
heterogeneous catalysts possessing superior activity as well as
high reusability.8 Although some progress has been achieved
by heterogeneous Ru catalysts,9 only a few heterogeneous Pd
ones are available to date, for example, Pd on activated carbon,10
Pd on pumice,11 Pd-hydrotalcite,12 Pd on TiO2,13 and polymer-
supported Pd.14 Unfortunately, these heterogeneous Pd systems
suffer from low catalytic activities and a limited substrate scope.
biomaterials, adsorbents, and ion exchangers. However, few
excellent applications as catalysts or catalyst supports have
emerged so far.15 We have recently disclosed a new strategy
for the design of high-performance heterogeneous catalysts
utilizing the hydroxyapatite as a macroligand for catalytically
active centers.16 The introduction of transition metal cations,
such as Ru and Pd, into the apatite framework could generate
stable monomeric phosphate complexes, which exhibited promi-
nent catalytic performances for various oxidation reactions using
molecular oxygen16a-d and for carbon-carbon bond-forming
reactions.16d,g Herein, we present the synthesis and characteriza-
tion of the palladium-grafted hydroxyapatite and its evaluation
as a heterogeneous catalyst for the oxidation of alcohols by use
of molecular oxygen. The in situ generated Pd nanocluster on
the hydroxyapatite surface has proven to be effective as a
catalytically active species for selective and environmentally
benign alcohol oxidation. We also mention the interesting
aspects such as substrate scope and recycling of the catalyst as
well as size effect of the formed Pd nanoclusters on the catalytic
activities.
3-
Hydroxyapatites possess Ca2+ sites surrounded by PO4
tetrahedra parallel to the hexagonal axis, which have attracted
considerable interest in view of their potential usefulness as
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Results and Discussion
Catalyst Preparation and Characterization. The choice of
hydroxyapatites as catalyst supports is motivated by the fol-
lowing advantages:16 (i) well-defined monomeric active species
can be immobilized on their surface, on the basis of high ion-
exchange ability and adsorption capacity; (ii) nonporous struc-
ture can help to overcome the problems toward mass transfer
limitation; and (iii) weak acid-base properties prohibit side
reactions induced by the support itself. To obtain a highly active
heterogeneous Pd catalyst for aerobic alcohol oxidation, we
synthesized two types of hydroxyapatite-supported Pd catalysts
designed with strict compositional control of the hydroxyapa-
tites.
Hydroxyapatites [HAP; Ca10-Z(HPO4)Z(PO4)6-Z(OH)2-Z
(0 e Z e 1; 1.5 e Ca/P e 1.67)] were synthesized from Ca-
(NO3)2‚4H2O and (NH4)2HPO4 by the precipitation method
according to a literature procedure.17 Selecting an appropriate
Ca/P molar ratios in the preparation sequence yielded the
stoichiometric hydroxyapatite Ca10(PO4)6(OH)2 (HAP-0; Z )
0, Ca/P ) 1.67) and the nonstoichiometric Ca-deficient hy-
droxyapatite Ca9(HPO4)(PO4)5(OH) (HAP-1; Z ) 1, Ca/P )
1.50), respectively. The crystallographic structure of the Ca-
deficient hydroxyapatite is identical to that of the stoichiometric
one, and its charge deficiency due to the lack of Ca2+ in the
lattice is compensated by introduction of H+ into the PO4
3-
ion and removal of OH- in the parent unit cell.15 The prepared
HAPs (HAP-0 and HAP-1) were characterized by powder X-ray
diffraction (XRD), infrared spectroscopy (IR), and elemental
analysis.
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10658 J. AM. CHEM. SOC. VOL. 126, NO. 34, 2004