Takashi Toyao

TiO2-Supported Re as a General and Chemoselective Heterogeneous Catalyst for Hydrogenation of Carboxylic Acids to Alcohols

TiO2 -supported Re, Re/TiO2 , was found to promote selective hydrogenation of carboxylic acids having aromatic and aliphatic moieties to the corresponding alcohols. Re/TiO2 showed superior results compared to other transition-metal-loaded TiO2 and supported Re catalysts for selective hydrogenation of 3-phenylpropionic acid. 3-phenylpropanol was produced in 97 % yield under mild conditions (5 MPa H2 at 140 °C). Contrary to typical heterogeneous catalysts, Re/TiO2 does not lead to the formation of dearomatized byproducts. The catalyst is recyclable and shows a wide substrate scope in the synthesis of alcohols (22 examples; up to 97 % isolated yield).

Rhenium-Loaded TiO2: A Highly Versatile and Chemoselective Catalyst for the Hydrogenation of Carboxylic Acid Derivatives and the N-Methylation of Amines Using H2 and CO2

Herein, we report a heterogeneous TiO2 -supported Re catalyst (Re/TiO2 ) that promotes various selective hydrogenation reactions, which includes the hydrogenation of esters to alcohols, the hydrogenation of amides to amines, and the N-methylation of amines, by using H2 and CO2 . Initially, Re/TiO2 was evaluated in the context of the selective hydrogenation of 3-phenylpropionic acid methyl ester to afford 3-phenylpropanol (pH2 =5 MPa, T=180 °C), which revealed a superior performance over other catalysts that we tested in this study. In contrast to other typical heterogeneous catalysts, hydrogenation reactions with Re/TiO2 did not produce dearomatized byproducts. DFT studies suggested that the high selectivity for the formation of alcohols in favor of the hydrogenation of aromatic rings is ascribed to the higher affinity of Re towards the COOCH3 group than to the benzene ring. Moreover, Re/TiO2 showed a wide substrate scope for the hydrogenation reaction (19 examples). Subsequently, this Re/TiO2 catalyst was applied to the hydrogenation of amides, the N-methylation of amines, and the N-alkylation of amines with carboxylic acids or esters.

Synthesis of 2,5-disubstituted pyrroles via dehydrogenative condensation of secondary alcohols and 1,2-amino alcohols by supported platinum catalysts

Direct synthesis of 2,5-disubstituted pyrroles has been achieved via acceptorless dehydrogenative heterocyclization of 1,2-aminoalcohols and secondary alcohols by utilizing a heterogeneous carbon-supported Pt catalyst (Pt/C). The optimized method gave 92% yield of 2-ethyl-5-phenyl-1H-pyrrole as a desired product from 2-amino-1-butanol and 1-phenylethanol in the presence of 0.1 mol% of Pt/C and 1.1 equiv. of KOtBu. It has been revealed that Pt/C demonstrates superior catalytic activity to a number of catalysts tested in this study including other transition metal-loaded carbon and various metal-oxide-supported Pt catalysts. In addition, the turnover number (TON) obtained in the present system was found to be higher than those obtained in previously-reported catalytic systems. It is significant that the Pt/C catalyst could be recycled as a heterogeneous catalyst without significant loss in the activity and showed a wide substrate scope for the 2,5-disubstituted pyrrole forming process.

Supported rhenium nanoparticle catalysts for acceptorless dehydrogenation of alcohols: structure–activity relationship and mechanistic studies

Al2O3-supported Re with different oxidation states and Re0 metal nanoparticles on various supports are prepared, characterized and tested for the dehydrogenation of 2-octanol. The activity of Re/Al2O3 increases with the fraction of metallic Re. The activity of metallic Re depends on the support oxides, and the support with moderate electronegativity (Al2O3) gives the highest turnover frequency (TOF) per surface Re0 site. Re/Al2O3 is effective for acceptorless dehydrogenation of various aliphatic secondary alcohols to ketones. The kinetic isotope effects on the dehydrogenation of 2-propanol show that dissociation of the α-C–H bond of 2-propanol is the rate-limiting step. The IR study of the reaction of gas phase 2-propanol over the Re/Al2O3 surface shows that the acid–base pair site of Al2O3 is responsible for the O–H dissociation of 2-propanol. The structural requirements are discussed on the basis of the mechanistic results.

Heterogeneous catalysts for the cyclization of dicarboxylic acids to cyclic anhydrides as monomers for bioplastic production

Cyclic anhydrides, key intermediates of carbon-neutral and biodegradable polyesters, are currently produced from biomass-derived dicarboxylic acids by a high-cost multistep process. We present a new high-yielding process for the direct intramolecular dehydration of dicarboxylic acids using a reusable heterogeneous Lewis acid catalyst, Nb2O5·nH2O. Various dicarboxylic acids, which can be produced by a biorefinery process, are transformed into the corresponding cyclic anhydrides as monomers for polyester production. This method is suitable for the production of renewable polyesters in a biorefinery process.

Fe3O4@HKUST-1 and Pd/Fe3O4@HKUST-1 as magnetically recyclable catalysts prepared via conversion from a Cu-based ceramic

Nanocomposites obtained by integrating iron oxide magnetic nanoparticles (Fe3O4) into a metal–organic framework (HKUST-1 or Cu3(BTC)2, BTC = 1,3,5-benzenetricarboxylate) are synthesized through conversion from a composite of a Cu-based ceramic material and Fe3O4. In situ small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) measurements reveal that the presence of Fe3O4 leads to the fast conversion and synthesis of HKUST-1 with small particle sizes. The prepared MOF composite (Fe3O4@HKUST-1) is found to catalyze the one-pot sequential deacetalization–Knoevenagel condensation reaction as a magnetically collectable and recyclable catalyst. In addition, Pd nanoparticles are also incorporated into the material (Pd/Fe3O4@HKUST-1) by addition of a Pd colloidal solution during the conversion of the precursor composite to HKUST-1. The resulting Pd/Fe3O4@HKUST-1 can be utilized for hydrogenation of 1-octene in the liquid phase.

Hydrodeoxygenation of Fatty Acids, Triglycerides, and Ketones to Liquid Alkanes by a Pt–MoOx/TiO2 Catalyst

Various supported metal catalysts are screened for hydrogenation of lauric acid and 2‐octanone as model reactions for the transformation of biomass‐derived oxygenates to liquid alkanes (biofuels) in a batch reactor under solvent‐free conditions. Among the catalysts tested, Pt and MoOx co‐loaded on TiO2 (Pt–MoOx/TiO2) shows the highest yields of n‐alkanes for both of the reactions. Pt–MoOx/TiO2 selectively catalyzes the hydrodeoxygenation of various fatty acids and triglycerides to n‐alkanes without C−C bond cleavage under 50 bar H2 and shows higher turnover numbers than the catalysts in the literature. Pt–MoOx/TiO2 is effective also for the hydrodeoxygenation of various ketones to the corresponding alkanes. In situ IR study of the reaction of adsorbed acetone under H2 suggests that the high activity of Pt–MoOx/TiO2 is attributed to the cooperation between Pt and Lewis acid sites of the MoOx/TiO2 support.

Oxidant-free Dehydrogenation of Glycerol to Lactic Acid by Heterogeneous Platinum Catalysts

Among various supported metal nanoparticle catalysts, Pt‐loaded carbon (Pt/C) catalysts, including a commercial one, showed the highest yield for selective dehydrogenation of glycerol to lactic acid under oxidant‐free, solvent‐free, and alkaline conditions. For the reaction with Pt/C, the effects of conditions (type of basic additives, the amount of water, atmosphere, reactor) on the catalytic activity and selectivity were studied. Combined with a reaction pathway in the literature and our result of control reactions, we proposed a strategy for decreasing the selectivity of byproducts. The optimized conditions gave a high turnover number of 69 000. To clarify the structural factors affecting the catalytic activity, we show the effect of electronic states of various transition metals on the activity as well as the effect of Pt particle size on the turnover frequency per surface Pt atom.

Acceptorless dehydrogenative coupling reactions with alcohols over heterogeneous catalysts

Heterogeneous catalysis for the one-pot synthesis of added-value chemicals is a growing area in green chemistry. Among various types of organic transformations that are accessible by this approach, acceptorless dehydrogenative coupling (ADC) reactions have been established as efficient processes that generate various classes of organic compounds via the formation of C–O, C–N, C–S, C–C, and CC bonds. In this review, we summarize recent developments on the one-pot synthesis of organic compounds that are driven by the dehydrogenation of alcohols promoted by supported transition-metal catalysts in the absence of hydrogen acceptors. A major feature of the design of such catalysts is the cooperation between the metal sites and the acid and/or base sites on the metal–oxide supports. Recent examples for the organometallic catalysis of ADC reactions of alcohols are presented and their efficiency is put into contrast with that of related reactions carried out using conventional supported transition-metal catalysts. Finally, aspects pertaining to the mechanism and catalyst design of these new ADC reactions are discussed in the context of directions for future developments in this area.

Direct Synthesis of Lactams from Keto Acids, Nitriles, and H2 by Heterogeneous Pt Catalysts

We report herein the first general catalytic system for the direct synthesis of N‐substituted γ‐ and δ‐lactams by reductive amination/cyclization of keto acids (including levulinic acid) with nitriles and H2 under mild conditions (7 bar H2, 110 °C, solvent free). The most effective catalyst, Pt and MoOx co‐loaded TiO2 (Pt‐MoOx/TiO2), shows a wide substrate scope, high turnover number (TON), and good reusability.