Takayoshi Hara

Magnetically recoverable heterogeneous catalyst: Palladium nanocluster supported on hydroxyapatite-encapsulated γ-Fe2O3 nanocrystallites for highly efficient dehalogenation with molecular hydrogen

The dechlorination of various organochlorides using atmospheric molecular hydrogen (H2) can be efficiently catalysed by magnetically separable Pd nanoclusters supported on the surface of hydroxyapatite-encapsulated γ-Fe2O3 (PdHAP-γ-Fe2O3). For instance, dechlorination of chlorobenzene gave benzene with an excellent turnover frequency (TOF) of 2500 h−1 in the presence of 1 atm of H2. This PdHAP-γ-Fe2O3 catalyst can be recovered briefly using an external magnetic field and reused at least three times without loss of its high catalytic activity.

Highly efficient dehydrogenation of indolines to indoles using hydroxyapatite-bound Pd catalyst

A hydroxyapatite-bound palladium catalyst was found to be effective for the dehydrogenation of various types of indolines to give the corresponding indoles. Moreover, the catalyst was readily recovered from the reaction mixture, and could be reused without any loss of its catalytic activity.

Selective Photocatalytic Oxidation of Alcohols to Aldehydes in Water by TiO2 Partially Coated with WO3

Semiconductor TiO(2) particles loaded with WO(3) (WO(3)/TiO(2)), synthesized by impregnation of tungstic acid followed by calcination, were used for photocatalytic oxidation of alcohols in water with molecular oxygen under irradiation at λ>350 nm. The WO(3)/TiO(2) catalysts promote selective oxidation of alcohols to aldehydes and show higher catalytic activity than pure TiO(2). In particular, a catalyst loading 7.6 wt % WO(3) led to higher aldehyde selectivity than previously reported photocatalytic systems. The high aldehyde selectivity arises because subsequent photocatalytic decomposition of the formed aldehyde is suppressed on the catalyst. The TiO(2) surface of the catalyst, which is active for oxidation, is partially coated by the WO(3) layer, which leads to a decrease in the amount of formed aldehyde adsorbed on the TiO(2) surface. This suppresses subsequent decomposition of the aldehyde on the TiO(2) surface and results in high aldehyde selectivity. The WO(3)/TiO(2) catalyst can selectively oxidize various aromatic alcohols and is reusable without loss of catalytic activity or selectivity.

Highly efficient and selective hydrogenation of unsaturated carbonyl compounds using Ni–Sn alloy catalysts

Inexpensive Ni–Sn-based alloy catalysts, both bulk and supported, exhibited high selectivity in the hydrogenation of a wide range of unsaturated carbonyl compounds and produced unsaturated alcohols almost exclusively. For the bulk Ni–Sn alloy catalysts, a relatively high reaction temperature of 453 K was required to achieve an efficient hydrogenation of CO rather than CC. The catalyst that consisted of the Ni–Sn alloy dispersed on TiO2 allowed a remarkable reduction of the reaction temperature to 383 K. Both the Ni3Sn2 and Ni3Sn alloy phases were found to be responsible for the enhancement of the chemoselectivity. The Ni–Sn alloy catalysts were reusable without any significant loss of selectivity.

Creation of highly stable monomeric Pd(II) species in an anion-exchangeable hydroxy double salt interlayer: Application to aerobic alcohol oxidation under an air atmosphere

An active Pd(II) catalyst supported on the Ni–Zn mixed basic salt (NiZn), which is classified by the anion-exchangeable layered hydroxy double salts, was synthesized by simple intercalation of the anionic Pd(II) hydroxyl complex. The divalent Pd species in the interlayer of NiZn maintained their original monomeric structure during the aerobic alcohol oxidation, due to the strong electrostatic interaction between the NiZn host and anionic Pd(II) species. This catalyst could be reused without any loss of the catalytic activity and selectivity in the aerobic alcohol oxidation.

Creation of monomeric La complexes on apatite surfaces and their application as heterogeneous catalysts for Michael reactions

Using a cation-exchange method, an equimolar substitution of La3+ for Ca2+ occurred by the treatment of stoichiometric hydroxyapatite (HAP: Ca10(PO4)6(OH)2) with an aqueous solution of La(OTf)3, affording a monomeric hydroxyapatite-bound La complex (LaHAP). Physicochemical characterization by means of XRD, XPS, IR, and La K-edge XAFS analyses proved that a monomeric La3+ phosphate complex was generated on its surface. Such monomeric La3+ species function as an efficient heterogeneous catalyst for the Michael reaction of 1,3-dicarbonyls with enones under aqueous or solvent-free conditions. The work-up procedure is straightforward and the spent catalyst could be recycled without any loss of the catalytic activity. Further application to an asymmetric version was also investigated using various apatite catalysts modified with chiral organic ligands. Enantioselectivity was found to depend on the chiral ligand, solvent, and rare earth metal triflate precursor (RE(OTf)3) for the reaction of methyl 1-oxoindan-2-carboxylate with methyl vinyl ketone. Under optimized reaction conditions, a monomeric fluoroapatite-bound La complex catalyst modified with (R,R)-tartaric acid (TA-LaFAP) provided the Michael adduct quantitatively in up to 60% ee.

Highly efficient dehalogenation using hydroxyapatite-supported palladium nanocluster catalyst with molecular hydrogen

A hydroxyapatite-supported Pd nanocluster (PdHAP) efficiently catalysed the dehalogenation of various organic halides in the presence of atmospheric molecular hydrogen; chlorobenzene gave benzene in an excellent TON of 10000 for 10 h. Moreover, the dehalogenation of halophenols in aqueous conditions was also achieved.

Catalytic investigations of carbon–carbon bond-forming reactions by a hydroxyapatite-bound palladium complex

A new type of hydroxyapatite-bound palladium complex (PdHAP-1) was synthesized by treatment of a nonstoichiometric Ca-deficient hydroxyapatite, Ca9(HPO4)(PO4)5(OH), with PdCl2(PhCN)2 in acetone solution. Characterization by means of physicochemical methods revealed that a monomeric PdII phosphate complex could be generated at a Ca-deficient site, which displayed outstanding catalytic activities for the Mizoroki–Heck reaction and Suzuki–Miyaura coupling reaction of aryl bromides. The remarkably high catalytic activity of the hydroxyapatite catalyst is ascribed to the exceptionally robust monomeric Pd structure, in which Pd is surrounded by anionic phosphate ligands, as confirmed by XAFS analysis. It is also proven that, upon adjustment of the solvent system, the PdHAP-1 was able to catalyze the Suzuki–Miyaura coupling of activated aryl chlorides in the presence of TBAB. Under such conditions, the in situ generated Pd nanocluster on the surface of hydroxyapatite was effective as a catalytically active species.

Efficient deprotection of N-benzyloxycarbonyl group from amino acids by hydroxyapatite-bound Pd catalyst in the presence of molecular hydrogen

Hydroxyapatite-bound Pd catalyst was found to be highly effective for the deprotection of N-benzyloxycarbonyl group from amino acids in the presence of molecular hydrogen. The catalyst was also applicable to the hydrogenolysis of a sterically encumbered core-Z-protected poly(amido amine) dendrimer, affording the dendritic amino compound in 99% yield.

Preparation of clay-supported Sn catalysts and application to Baeyer–Villiger oxidation

Clay-intercalated Sn catalysts were prepared by a conventional cation-exchange method and used for the Baeyer–Villiger oxidation of various ketones with hydrogen peroxide as an oxidant. The intercalation of monomeric Sn species into the clay interlayer was monitored by solid-state 7Li MAS NMR. Solid-state 119Sn MAS NMR and Sn K-edge XAFS analysis revealed that an isolated Sn species, such as [SnIV(OH)x(H2O)5−x](4−x)+ (x = 0–3), was formed in the clay interlayers. Our clay-intercalated Sn catalysts showed extremely high performance in Bayer–Villiger oxidation and were also reusable without any significant loss of activity or selectivity.