Kazu Okumura

Toluene combustion over palladium supported on various metal oxide supports

Abstract Metal–support interaction in the catalytic combustion of toluene was studied using metal oxides with different acid–base properties as supports for Pd. The catalytic performance was correlated with XPS data and the reaction order for oxygen. These studies revealed that the affinity for oxygen of Pd surface changed according to the acid–base character of metal oxide over MgO, Al2O3, SiO2, SnO2, Nb2O5, and WO3. However, ZrO2 exhibited exceptional character in that metal Pd was unusually stabilized, which was derived from the weak interaction between Pd and support surface. The reason for high toluene combustion activity of Pd/ZrO2 was ascribed to the stabilization of metal Pd on ZrO2.

Support effect of zeolite on the methane combustion activity of palladium

Abstract Catalytic combustion of methane was carried out over Pd loaded on MFI and MOR zeolites. The activity and durability of the catalysts in the methane combustion was dependent on the Al concentration, the kind of cation and the structure of zeolite supports. Especially, Pd loaded on MOR was more active than that on MFI. The catalytic performance of Pd was correlated with the measurement of reaction order, activation energy and local structure of Pd determined by Pd K-edge EXAFS. A significant difference in the state of Pd was observed between MFI and MOR, where the Pd metal and PdO existed during the reaction over MOR and MFI, respectively. The high activity of Pd/MOR for methane combustion was ascribed to the generation of a mixture of Pd metal and PdO during the reaction.

Prominent catalytic activity of Ga-containing MCM-41 in the Friedel–Crafts alkylation

Ga-containing MCM-41 were prepared by addition of Ga to the synthesis gel (Ga-MCM-41) or by impregnation of Ga onto MCM-41 (Ga/MCM-41), and their structure, acid properties and catalytic properties were studied. In the as-synthesized form of Ga-MCM-41, the introduced Ga was incorporated in the MCM-41 network structure, which was confirmed from the EXAFS and the 71Ga NMR. The transformation to the H-form caused the partial elimination of Ga from the MCM-41 network. The nature of acid sites generated on Ga was studied by IR and NH3 temperature programmed desorption methods. Lewis acid sites were dominant independent of the preparation methods as evidenced by the IR of adsorbed pyridine. Ga-containing MCM-41 exhibited prominent activity in the liquid phase reaction between benzyl chloride and benzene (Friedel–Crafts alkylation) under such a low temperature as 313 K. The highest yield of diphenylmethane was obtained on the Ga impregnated MCM-41. Ga2O3 located outside the MCM-41 network was considered to bear active species for the reaction.

Hydrodeoxygenation of Vicinal OH Groups over Heterogeneous Rhenium Catalyst Promoted by Palladium and Ceria Support

Heterogeneous ReOx-Pd/CeO2 catalyst showed excellent performance for simultaneous hydrodeoxygenation of vicinal OH groups. High yield (>99%), turnover frequency (300 h(-1)), and turnover number (10,000) are achieved in the reaction of 1,4-anhydroerythritol to tetrahydrofuran. This catalyst can be applied to sugar alcohols, and mono-alcohols and diols are obtained in high yields (≥85%) from substrates with even and odd numbers of OH groups, respectively. The high catalytic performance of ReOx-Pd/CeO2 can be assigned to rhenium species with +4 or +5 valence state, and the formation of this species is promoted by H2/Pd and the ceria support.

Support Effect of Metal Oxide on Rh Catalysts in the CH4-CO2 Reforming Reaction

Rhodium supported on various metal oxides was studied in the dry reforming reaction using methane and carbon dioxide, in order to reveal the effect of support on the catalytic activity of Rh. The TOF (turnover frequency) based on exposed Rh decreased in the following sequence of support: TiO2 > La2O3 = CeO2 > ZrO2 = MgO = SiO2 = MCM-41 > γ-Al2O3. The different TOF could be correlated with the stabilized species of Rh atom. It was concluded that the supports influenced the electronic state of Rh atoms, and that Rh metal catalyzed this reaction as a prominent active species.

Energy-Dispersive XAFS Studies on the Spontaneous Dispersion of PdO and the Formation of Stable Pd Clusters in Zeolites

Spontaneous dispersion and clustering processes of Pd were measured by means of the energy-dispersive EXAFS method. The spontaneous dispersion of bulky metal Pd into highly dispersed PdO was directly observed on the H-type zeolite in the atmosphere of O2. In contrast to H-type zeolites, simple oxidation of the agglomerated Pd was observed on Na-ZSM-5. The structural change of Pd was followed in the atmosphere of hydrogen. The clustering processes of metal Pd depended on the kind of zeolite, and these were categorized into three groups. The first group, i.e., Na-ZSM-5 and H-beta, showed monotonic agglomeration of metal Pd by increasing the reduction temperature. The second group consisted of H-ZSM-5 and H-mordenite where the formation of Pd6 clusters was found. On these zeolites, the generation of Pd6 clusters was reversibly observed upon the repetition of reduction and oxidation treatments. The third group consisted of H-Y and USY zeolites where the formation of Pd13 clusters was observed. From these findings, it was concluded that the crystal structure and acid sites of zeolites had profound influences on the dynamic behavior and the genesis of Pd clusters with various structures.

Selective Hydrogenation of Lactic Acid to 1,2-Propanediol over Highly Active Ruthenium–Molybdenum Oxide Catalysts

Modification of Ru/C with a small amount of MoOx (RuMoOx /C) enhanced the catalytic activity in the hydrogenation of L-lactic acid to form 1,2-propanediol and maintained high selectivity. The turnover frequency based on the amount of Ru over the optimized RuMoOx /C catalyst (Mo/Ru molar ratio=1:16) was 114 h(-1) at 393 K, which was about 4 times higher than that over Ru/C. The same effect of MoOx was obtained over RuMoOx /SiO2 , although RuMoOx /SiO2 showed slightly lower activity than that of RuMoOx /C. RuMoOx /C achieved a high yield of 95 % in 18 h at 393 K and was applicable to various carboxylic acids to provide the corresponding alcohols in high yields. Modification with MoOx also brought about suppression of racemization and (S)-1,2-propanediol was obtained in high enantiomeric excess at 353 K. Based on kinetic analysis and characterization data, such as XRD, TEM, CO adsorption by a volumetric method, FTIR spectroscopy, and X-ray absorption spectroscopy, for RuMoOx /C and RuMoOx /SiO2 , the catalyst structure and reaction mechanism are proposed.

Highly Dispersed Pd Species Active in the Suzuki–Miyaura Reaction

Structures of Pd/zeolites immersed in solvents were measured by in situ X-ray absorption fine structure (XAFS). Systematic studies revealed that the selection of an appropriate support (USY-zeolite), thermal treatment temperature of USY, solvent (o-xylene), H(2) partial pressure (6%), and the use of a Pd amine complex affect the structure of Pd. As a result, we found that monomeric Pd can be obtained in the USY support with H(2) bubbling in o-xylene. The structural properties of Pd correlate well with its catalytic performance in the Suzuki-Miyaura coupling reactions; a very high TON of up to 11,000,000 was obtained over the monomeric Pd.

Hydrothermal synthesis and catalysis of Nb2O5–WOx nanofiber crystal

Nb2O5–WOx synthesized by a hydrothermal method having a long nano-crystalline structure with ca. 10 nm diameter exhibited a high catalytic activity in the Friedel–Crafts alkylations and acylations when it was calcined in N2.

Hydrogenation of dicarboxylic acids to diols over Re–Pd catalysts

A Re–Pd/SiO2 (Re/Pd = 8) catalyst was applied to hydrogenation of dicarboxylic acids (succinic acid, glutaric acid and adipic acid) to diols. In the hydrogenation of dicarboxylic acids, ex situ liquid-phase (in only 1,4-dioxane solvent) reduced Re–Pd/SiO2 showed much higher activity than in situ liquid-phase (in the mixture of dicarboxylic acid and 1,4-dioxane) and gas-phase reduced ones, in which the in situ liquid-phase reduced catalyst has been reported to show good activity in the hydrogenation of monocarboxylic acids. High diol yields (71–89%) were achieved in the hydrogenation of dicarboxylic acids on the ex situ liquid-phase reduced catalyst at 413 K. Lactones and hydroxycarboxylic acids were first formed as intermediates in the reaction of C4–C5 and ≥C6 dicarboxylic acids, respectively. Characterization using XRD, XPS and XAS indicates that ex situ liquid-phase reduced catalysts with high activity contains comparable amounts of Re0 and Ren+ species, both of which have been reported to be necessary for good performance. The amount of Ren+ species on the in situ liquid-phase reduced catalysts is much larger than that of surface Re0 species. This result suggests that the presence of dicarboxylic acids suppresses the reduction of Re species to Re0 on the calcined catalysts while that of monocarboxylic acids does not, which leads to the low activity in the hydrogenation of dicarboxylic acids on in situ liquid-phase reduced catalysts.