Treatment of a stoichiometric hydroxyapatite (HAP), Ca10(PO4)6(OH)2, with PdCl2(PhCN)2 gives a new type of palladium-grafted hydroxyapatite. Analysis by means of powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX), IR, and Pd K-edge X-ray absorption fine structure (XAFS) proves that a monomeric PdCl2 species is chemisorbed on the HAP surface, which is readily transformed into Pd nanoclusters with a narrow size distribution in the presence of alcohol. Nanoclustered Pd0 species can effectively promote the alcohol oxidation under an atmospheric O2 pressure, giving a remarkably high turnover number (TON) of up to 236,000 with an excellent turnover frequency (TOF) of approximately 9800 h(-1) for a 250-mmol-scale oxidation of 1-phenylethanol under solvent-free conditions. In addition to advantages such as a simple workup procedure and the ability to recycle the catalyst, the present Pd catalyst does not require additives to complete the catalytic cycle. The diameters of the generated Pd nanoclusters can be controlled upon acting on the alcohol substrates used. Oxidation of alcohols is proposed to occur primarily on low-coordination sites within a regular arrangement of the Pd nanocluster by performing calculations on the palladium crystallites.

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The interaction of metals with ligands is the key factor in the design of catalysts and much effort has been devoted to the rational control of metal-ligand interactions in order to exploit catalytic properties. Quite sophisticated heterogeneous catalysts have been produced by controlling the size and shape of active metal species, and by screening and altering the composition of the supports.[1] The supports can be considered as "macro ligands" for supported active metals, and the fine-tuning of the interactions between active metal species and supports is the most important factor through which high catalytic performance can be attained. Despite many intrinsic advantages of heterogeneous catalysts over homogeneous ones, such as their durability at high temperatures and reusability, the fine-tuning of metal-ligand interactions in heterogeneous catalysts is more difficult than in homogeneous catalysts, and remains a challenging objective.Our research group has recently reported that silver nanoparticles (AgNPs) on a basic support of hydrotalcite (Ag/HT) catalyzed the chemoselective reductions of nitrostyrenes [2] and epoxides [3,4] to the corresponding anilines and alkenes when using alcohols or CO/H 2 O as a reducing reagent while retaining the reducible C=C bonds. During the reductions, polar species of hydrides and protons were formed in situ at the interface of AgNPs/HT through a cooperative effect between the AgNPs and basic sites (BS) of HT, which were then exclusively active for the reduction of the polar functional groups (Figure 1). However, the use of H 2 instead of alcohols or CO/H 2 O in our Ag catalyst system caused reductions of both the polar groups (nitro and epoxide) and the nonpolar C=C bonds. This nonselective reduction was due to the formation of nonpolar hydrogen species through the homolytic cleavage of H 2 at the AgNPs surface, which is active for C = C bond reduction (Figure 2 a).We envisioned that AgNPs covered with a basic material (BM), namely, the core-shell nanocomposite AgNPs@BM, would be a reasonable structure for performing the above complete chemoselective reductions (Figure 2 b). The AgNPs@BM structure can maximize the interface area of the AgNPs-BM, while minimizing the area of the bare AgNPs. This property would enable the exclusive formation of the heterolytically cleaved hydrogen species through a concerted effect between AgNPs and basic sites of BM that suppresses the unfavorable formation of homolytically cleaved hydrogen species on the bare AgNPs. The resulting Ag hydride and proton species would lead to complete chemoselective reduction of polar functionalities while retaining the C=C bonds.

Achieving precise control of active species on solid surfaces is one of the most important goals in the development of highly functionalized heterogeneous catalysts. The treatment of hydroxyapatites with PdCl(2)(PhCN)(2) gives two new types of hydroxyapatite-bound Pd complexes. Using the stoichiometric hydroxyapatite, Ca(10)(PO(4))(6)(OH)(2), we found that monomeric PdCl(2) species can be grafted on its surface, which are easily transformed into Pd(0) particles with narrow size distribution in the presence of alcohols. Such metallic Pd species can effectively promote alcohol oxidation using molecular oxygen and are shown to give a remarkably high TON of up to 236 000. Another monomeric Pd(II) phosphate complex can be generated at a Ca-deficient site of the nonstoichiometric hydroxyapatite, Ca(9)(HPO(4))(PO(4))(5)(OH), affording a catalyst with Pd(II) structure and high activity for the Heck and Suzuki reactions. To the best of our knowledge, the PdHAP are one of the most active heterogeneous catalysts for both alcohol oxidation under an atmospheric O(2)() pressure and the Heck reaction reported to date. These Pd catalysts are recyclable in the above organic reactions. Our approach to catalyst preparation based on the control of Ca/P ratios of hydroxyapatites represents a particularly attractive method for the nanoscale design of catalysts.

Hydrotalcite-supported gold nanoparticles (Au/HT) were found to be a highly efficient heterogeneous catalyst for the aerobic oxidation of alcohols under mild reaction conditions (40 8C, in air). This catalyst system does not require any additives and is applicable to a wide range of alcohols, including less reactive cyclohexanol derivatives. This Au/HT catalyst could also function in the oxidation of 1-phenylethanol under neat conditions; the turnover number (TON) and turnover frequency (TOF) reached 200,000 and 8,300 h À1 , respectively. These values are among the highest values compared to those of other reported catalyst systems at high conversion. Moreover, the Au/HT can be recovered by simple filtration and reused without any loss of its activity and selectivity.

Hydrotalcite-supported Pt nanoparticles catalyze the direct transformation of furfural to 1,2-pentanediol in a high yield of 73% under additive-free conditions. The Pt nanoparticle catalyst is easily recoverable and reusable while maintaining high activity and selectivity.

We report a facile synthesis of new core-Au/shell-CeO2 nanoparticles (Au@CeO2) using a redox-coprecipitation method, where the Au nanoparticles and the nanoporous shell of CeO2 are simultaneously formed in one step. The Au@CeO2 catalyst enables the highly selective semihydrogenation of various alkynes at ambient temperature under additive-free conditions. The core-shell structure plays a crucial role in providing the excellent selectivity for alkenes through the selective dissociation of H2 in a heterolytic manner by maximizing interfacial sites between the core-Au and the shell-CeO2.

Additive free: Hydrotalcite‐supported silver nanoparticles act as a highly efficient heterogeneous catalyst for the dehydrogenation of diverse alcohols (see picture) in the absence of any acceptors such as molecular oxygen, hydrogen peroxide, or unsaturated organic compounds. The solid Ag catalyst was readily reusable without any loss of activity and selectivity.