Wataru Onodera

Sustainable heterogeneous platinum catalyst for direct methylation of secondary amines by carbon dioxide and hydrogen.

Pt and MoO(x) co-loaded TiO2 is found to be highly effective for direct methylation of aliphatic and aromatic secondary amines by CO2 and H2 under solvent-free conditions. This is the first additive-free and reusable heterogeneous catalytic system with acceptable turnover number.

Selective hydrogenation of levulinic acid to valeric acid and valeric biofuels by a Pt/HMFI catalyst

We describe one-pot high-yield catalytic pathways for the conversion of levulinic acid (LA) to valeric acid (VA) or valeric acid esters (so-called valeric biofuels) under relatively mild conditions (2 or 8 bar H2, 200 °C). A thorough screening study reveals that a HMFI zeolite-supported Pt metal cluster (Pt/HMFI) with an average cluster size of 1.9 nm shows the highest yield of VA (99%) under solvent-free conditions. The use of ethanol or methanol as solvent changes the selectivity, resulting in 81–84% yields of ethyl valerate (EV) or methyl valerate (MV). Pt/HMFI is also effective for selective formation of valeric acid esters from γVL in alcohols under H2. Kinetics, in situ infrared (IR), and acidity–activity relationship studies show a cooperative mechanism of Pt and Bronsted acid sites of HMFI. VA formation from LA can be driven by Pt-catalyzed hydrogenation of LA to γVL, which undergoes proton-assisted ring-opening by HMFI, followed by Pt-catalyzed hydrogenation. Valeric ester formation from LA is driven by esterification of LA to levulinic ester, which is hydrogenated by Pt.

Hydrodeoxygenation of fatty acids and triglycerides by Pt-loaded Nb2O5 catalysts

Platinum nanoparticles loaded onto various supports have been studied for the selective hydrogenation of lauric acid to n-dodecane. The activity depends on the support material and pre-reduction temperature. Pt/Nb2O5 reduced at 300 °C gives the highest activity. Pt/Nb2O5 shows higher activity than various Nb2O5-supported transition metals (Ir, Re, Ru, Pd, Cu, Ni). Under solvent-free conditions Pt/Nb2O5 is effective for the hydrodeoxygenation of lauric, capric, palmitic, myristic, oleic, and stearic acids under 8 bar H2 at 180–250 °C, which gives high yields (88–100%) of linear alkanes with the same chain length as the starting compound. Tristearin is also converted to give 93% yield of n-octadecane. Pt/Nb2O5 shows more than 60 times higher turnover number (TON) than the previously reported catalysts for the hydrogenation of stearic acid to n-octadecane. Mechanistic study shows a consecutive reaction pathway in which lauric acid is hydrogenated to 1-dodecanol, which undergoes esterification with lauric acid as well as hydrogenation to n-dodecane. The ester undergoes hydrogenolysis to give the alcohol, which is hydrogenated to the alkane. Infrared (IR) study of acetic acid adsorption on Nb2O5 indicates that Lewis acid–base interaction of Nb cation and carbonyl oxygen, which suggests a possible role of Nb2O5 as an activation site of carbonyl groups during hydrodeoxygenation.

Amidation of Carboxylic Acids with Amines by Nb2O5 as a Reusable Lewis Acid Catalyst

Among 28 types of heterogeneous and homogenous catalysts tested, Nb2O5 shows the highest yield for direct amidation of n‐dodecanoic acid with a less reactive amine (aniline). The catalytic amidation by Nb2O5 is applicable to a wide range of carboxylic acids and amines with various functional groups, and the catalyst is reusable. A comparison of the results of the catalytic study and an infrared study of the acetic acid adsorbed on the catalyst suggests that activation of the carbonyl group of the carboxylic acid by Lewis acid sites on Nb2O5 is responsible for the high activity of the Nb2O5 catalyst. Kinetic studies show that Lewis acid sites on Nb2O5 are more water‐tolerant than conventional Lewis acidic oxides (Al2O3, TiO2). In comparison with the state‐of‐the‐art homogeneous Lewis acid catalyst for amidation (ZrCl4), Nb2O5 undergoes fewer negative effects from basic additives in the solution, which indicates that Nb2O5 is a more base‐tolerant Lewis acid catalyst than the homogeneous Lewis acid catalyst.

Hydrodeoxygenation of sulfoxides to sulfides by a Pt and MoOx co-loaded TiO2 catalyst

Supported metal nanoparticle catalysts were studied for the hydrodeoxygenation of sulfoxides to sulfides under solvent-free and mild conditions (50–155 °C, 1 or 7 atm H2). The catalytic activity for the model reaction of diphenyl sulfoxide depended on the type of metals, support materials and co-loaded oxides of transition metals (V, Nb, Mo, W, Re). Pt and MoOx co-loaded TiO2 (Pt–MoOx/TiO2) showed the highest activity. Pt–MoOx/TiO2 was reusable after the reaction and was effective for the reduction of various sulfoxides and showed a higher turnover number (TON) than previously reported catalysts. Using Pt–MoOx/TiO2, benzylphenylsulfone was reduced by H2 to give phenylbenzyl sulfide via benzylphenyl sulfoxides, which represented the first example of catalytic conversion of a sulfone to a sulfide by H2. Characterization studies of Pt–MoOx/TiO2 show that the surface of TiO2 is covered by small (or thin layer) Mo oxide species with exposed Mo cations as Lewis acid sites, and 4–5 nm sized Pt metal nanoparticles are supported on the Mo oxide-covered TiO2.

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.

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.

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.