Development of platinum-group-metal (PGM)-free catalysts for green chemical synthesis is an important topic in synthetic catalysis, because PGM will soon be in short supply in the near future. Our group has paid attention to the catalytic functions of Ag clusters. Here, we summarize our recent work on size- and support-specific catalysis of Ag clusters for green organic synthesis. Ag clusters supported on Al₂O₃ act as effective heterogeneous catalysts for (1) oxidant-free dehydrogenation of alcohols to carbonyl compounds, (2) coupling of alcohols with amines to form amides and H₂, (3) N-alkylation of anilines with alcohols, (4) C–C cross-coupling reaction of alcohols, (5) selective hydrogenation of nitroaromatics, and (6) direct synthesis of N-substituted anilines from nitroaromatics and alcohols. To establish a catalyst design concept, effects of Ag particle size and acid-base character of support oxides are investigated. The structure-activity relationships for all reactions show similar tendencies; metallic Ag clusters with smaller size and acid-base bifunctional nature of the support oxide are preferable. We propose that cooperation between coordinatively unsaturated Ag sites of Ag clusters and acid-base pair sites at the metal-support interface is a key concept for the design of Ag cluster catalysts for the above reactions.

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The isomerization of n-hexadecane over Pt–WO₃ catalysts supported on TiO₂–SiO₂ synthesized by glycothermal reaction with various Si/Ti molar ratios was examined. The catalyst performance depended on Si/Ti molar ratio and WO₃ loading. The characterization of the catalysts by XRD, XAFS, UV-vis and so on revealed that with increasing the WO₃ loading, the structure of surface W species changed from monomeric species to polytungstate species, which is considered to significantly affect the isomerization selectivity of the catalysts.

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The NO storage-reduction properties of 1 wt% Pt–10 wt% Li₂O/TiO₂–Al₂O₃ catalysts prepared by the impregnation and sol-gel methods were investigated in the presence and absence of SO₂. The surface area and NO storage amount of catalysts were smaller for the Pt–Li₂O/TiO₂–Al₂O₃ catalyst than for the Pt–Li₂O/Al₂O₃ catalyst. However, the Pt–Li₂O/TiO₂–Al₂O₃ sample with mixed oxide support achieved high NO removal efficiency for a long period, although the reduction of NO sorption amount by sulfur poisoning was comparable for all catalysts. These results revealed that the mixed oxide support in the catalyst is a key component for continuous NO removal. The preparation method affected the NO storage amount and the crystalline phases of catalysts. The strong diffraction peak of Li₂TiO₃ phase was observed in the XRD pattern of sample prepared by the impregnation method. The formation of Li₂TiO₃ weakened the basicity of the catalyst, resulting in decreased NO storage capacity under a SO₂-free atmosphere and the enhancement of tolerance to sulfur poisoning.

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Mo-SBA-15 (mesoporous silica SBA-15 containing molybdenum oxides) was hydrothemally synthesized with H₂MoO₄, P123 as the template and tetraethylorthosilicate (TEOS). Mo species of precursor solutions and final products were characterized by Raman spectroscopy. Mo-P123 complexes are formed in solution by reaction of MoO₂Cl₂ dissolved in concentrated HCl and P123, but MoO₂Cl₂ does not react with TEOS. N₂ adsorption-desorption analysis and small angle X-ray diffraction reveal that the SBA-15 framework is formed in Mo-SBA-15 synthesized. This is explained that the Mo-P123 complexes act to form the SBA-15 framework as an effective template in the hydrothermal aging. The formed molybdenum oxides are expected to be dispersed in the SBA-15 channels and their structures are affected by the volume of concentrated HCl. UV-Vis spectroscopy indicates that the tetrahedrally coordinated mono-molybdates dominate in Mo-SBA-15 when 20 mL of concentrated HCl (HCl/H₂MoO₄ weight ratio = 16.0) is added.

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The use of pressurized oxygen greatly enhanced catalytic activity for oxidative esterification of propionaldehyde to methyl propionate with methanol using heavy-metal-free Pd/C and Pd/Al₂O₃. For example, the conversion of propionaldehyde, the selectivity to methyl propionate, and the yield of the propionate using 5 % Pd/Al₂O₃ were 99.9, 62.7, and 62.6 %, respectively, at 1.5 MPa of oxygen gas and 333 K. This result demonstrated that Pb doping of Pd catalysts was not needed to achieve comparable activity, which runs contrary to previous reports, in which it had been suggested that loading of Pd catalysts using a heavy metal such as Pb was needed for the great activity such as 93.2, 76.8, and 71.6 % of the conversion of propionaldehyde, the selectivity to methyl propionate, and the yield of the propionate, respectively, at 0.3 MPa of oxygen gas and 353 K using 5 % Pd/Al₂O₃ doped with 5 % Pb.

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Recently developed solid bases of MgO and CaO covered with Al₂O₃ were applied to the alkylation of malonate with several types of alkyl halides. Diethyl malonate was converted into mono-alkylated and/or di-alkylated malonate by these solid bases without solvent. Prepared solid bases showed higher activity than unmodified MgO and CaO. Al₂O₃/MgO gave mono-alkylated malonates with high selectivity. Al₂O₃/CaO gave di-alkylated malonates selectively. Al₂O₃/CaO also catalyzed alkylation of mono-alkylated malonates. MgO and CaO covered with Al₂O₃ are useful solid bases for the alkylation of activated methylene groups. The difference in product selectivity was attributed to the base strength of uncovered MgO and CaO.

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The direct conversion reaction of ethanol to propylene was investigated using lanthanum- and magnesium-co-modified H-ZSM-5 catalyst with Si/Al₂ ratio of 80. Propylene selectivity of 30 % was obtained at 0.1 MPa, 550 °C. The ethylene and propylene selectivities were 44 % and 27 %, respectively, even after 50.2 h. The rapid increase in ethylene formation was also depressed, possibly because the catalyst modifications prevented reduction of performance by dealumination and coking. The deactivation of lanthanum- and magnesium-co-modified H-ZSM-5 with Si/Al₂ ratio of 80 was slower than that of catalyst with Si/Al₂ ratio of 150. Because more acid sites are left even if dealumination proceeds, higher effects on the propylene production and ethylene restraint might be maintained with Si/Al₂ ratio of 80.

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Energy requirements for extracting 1-MJ equivalent of hydrocarbons from dehydrated algae cake (water content: 70 %) of Botryococcus braunii was estimated for four feasible hydrocarbon extraction processes for biofuel production from microalgae; hexane extraction, hexane extraction with hydrothermal pretreatment, supercritical carbon dioxide extraction, and DME extraction. Input energy per 1 MJ-equivalent hydrocarbon extraction under the assumed reported conditions was 0.73-1.83 MJ/MJ-equivalent hydrocarbon for the 4 extraction processes, and the input energy required in the optimum case amounted to more than 70 % of the recovered energy. Consideration of methods to reduce the energy required for extraction in each process revealed that the following studies were important: research on increasing the extraction rate of wet microalgae, device design to minimize losses of extraction media, and increased efficiencies of the heat recovery and power recovery equipment.

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The synthesis of MFI-type silicalite was investigated using quartz, the most chemically stable of the silicon oxides and silicates, in order to expand the sources of silica available for silicalite production. It was found that MFI-type silicalite could be synthesized from quartz thermally treated in very concentrated aqueous sodium hydroxide solution. Moreover, the effect of magnesium ions, which are included in various kinds of silicates, was examined. Magnesium ions depressed the synthesis of MFI-type silicalite; however, magnesium hydroxide could be easily separated from the aqueous solution of the treated quartz. Accordingly, we conclude that the removal of magnesium is needed for the synthesis of MFI-type silicalite from quartz.

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