Controlled Synthesis of Hydroxyapatite-Supported Palladium Complexes as Highly Efficient Heterogeneous Catalysts

Abstract

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.

Selective Oxidation of Alcohols to Carbonyl Compounds and Carboxylic Acids with Platinum Group Metal Catalysts

“…The first test achieved 93% conversion, which after two recycles showed no loss in activity. This result demonstrates that the selective oxidation reaction is achievable in high yields under mild conditions with a catalyst loading that is even lower than the Pd loading used by Kaneda for oxidations in organic solvents [36] or that by Sheldon [9,14,15] for oxidations in water.…”

“…A very recent second breakthrough was found by Kaneda et al, who achieved very high turnover numbers without a secondary metal by immobilising Pd on a hydroxyapatite. [36] This system is also active for the oxidation of usually poorly reactive aliphatic alcohols, of which fewer examples exist in heterogeneous catalysis. Heterogeneous catalysts offer obvious advantages with respect to catalyst recycling, although very recently a number of water-soluble homogeneous catalyst systems have been developed that allow facile catalyst recycling via simple separation of the catalyst containing aqueous phase from the organic phase.…”

Synthesis of thermally stable metal substituted hydroxy apatites for the selective oxidation of light paraffins

“…Opre et al [22,23] also studied the selective oxidation of alcohols with oxygen on cobalt promoted ruthenium-hydroxyapatite. Earlier, hydroxyapatites were used as supports in a many studies [24][25][26][27][28]. Recently, our group has also successfully used cobalt, iridium and platinum hydroxyapatites as catalysts for solvent-free Knoevenagel condensation reactions of aldehydes with esters, resulting in good yields [29,30].…”

Self-Assembled Complexes of Non-cross-linked Amphiphilic Polymeric Ligands with Inorganic Species: Highly Active and Reusable Solid-Phase Polymeric Catalysts

“…The catalytic activity of PdAS for the heterogeneous Suzuki-Miyaura reaction [76][77][78][79][80][81][82][83][84][85][86][87][88][89] was investigated (Table 5). Since "water" is the most safe, non-toxic, and easily available solvent, numerous attempts have been made in utilizing it to the organic reactions.…”

“…[110][111][112][113] Although many efforts to prepare solid-phase catalysts for the Heck reaction have been made, homogeneous catalytic systems have advantages on catalytic activity. [114][115][116][117][118][119][120][121][122] In fact, it was known that designing recyclable system for the Heck reaction was more challenging than that for the Suzuki-Miyaura reaction. These catalysts were less stable under the Heck reaction condition, and thus often decompose physically or chemically.…”