New and efficient catalysts for enantioselective borohydride reduction of ketones and imines

Optically active aldiminato cobalt(II) complexes have been found to catalyze the enantioselective reduction of ketones with sodium borohydride affording the corresponding optically active secondary alcohols in high chemical yields with high enantioselectivities. The enantioselective borohydride reduction is also applicable to not only C=O bonds in aromatic ketones but also to C=N bonds in aromatic imines.     

New trends in the kinetics of hydroformylation of vinyl acetate using Rh complex catalysts

The kinetics of hydroformylation of vinyl acetate using [Rh(CO)₂Cl]₂ complex catalyst has been investigated at 80 °C. The trends are quite different from those observed for the HRh(CO)(PPh₃)₃-catalyzed systems. The dependence of the rate on P(H₂) and P(CO) was found to be linear, whereas the dependence of rate on vinyl acetate concentration was found to be first order, followed by substrate-inhibited kinetics at higher olefin concentrations. The rate dependence on the catalyst concentration was found to be fractional order. A rate equation has been proposed and kinetic parameters evaluated.     

Nano-structured Pd-Sn catalysts for alcohol electro-oxidation in alkaline medium

Active self-supported Pd₁Sn_x catalysts are synthesized by the sacrificial support method. Their catalytic activities towards C₁ to C₃ alcohol oxidation are determined in alkaline medium. The comparison of the activities allows discarding a first base-catalyzed deprotonation reaction of alcohol as oxidation rate-determining step, speaking up for mechanisms involving a C-adsorption of carbon bearing an alcohol function. It is also shown that desorption of dihydroxyacetone is the glycerol oxidation-limiting step at low potentials.     

Synthesis of five-membered N-heterocycles using Rh based metal catalysts

Abstract

One of the highly emerging and an important aspect of organic chemistry is the metal catalyzed synthesis of heterocycles. The methodologies used earlier for their synthesis were less approachable to the organic chemist because of their high cost, highly specified instrumentation, and inconvenient methods. For both the stereoselective and regioselective synthesis of five-membered nitrogen containing heterocycles, cyclic reactions that are Rh-catalyzed have known to be very efficient. The present review covers the varied applications of Rh as a catalyst and its importance in the formation of five-membered nitrogen containing heterocycles. The fascinating research that has been done in this area is enclosed in this review.     

Reduction of ester groupings in porphyrins with sodium borohydride

Reduction of the corresponding acyl derivatives by means of sodium borohydride [1, 2] is usually employed to obtain porphyrins with -hydroxyalkyl substituents.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, p. 860, June, 1973.     

Covalently immobilized porphyrins as photooxidation catalysts

Porphyrins covalently linked to aminomethylated Merrifield polymers, by chlorosulfonation activation of the porphyrin nucleus, are able to generate singlet oxygen with an efficiency which is related to the spacer between porphyrin and the polymer backbone. Juglone and ascaridole are efficiently produced in the presence of these supported catalysts.     

Investigation of Pt–Ru nanoparticle catalysts for low temperature methanol electro-oxidation

Pt–Ru nanoparticle-based methanol electro-oxidation catalysts with high concentration of metallic phase on carbon black have been synthesised by a low-temperature colloidal preparation route. Amorphous Pt–Ru oxide nanoparticles were deposited on carbon and subsequently reduced in hydrogen stream at different temperatures to obtain crystalline phases with tailored particle size. The electro-catalytic activity for methanol oxidation was investigated in half-cell from 30 to 60 °C. The results were interpreted in terms of particle size, crystallographic structure, degree of alloying and carbon monoxide adsorption properties. The best performance was achieved for the catalyst with intermediate particle size in the investigated range. Furthermore, it is observed that the optimal properties for these catalysts depend on the operating temperature.     

Sterically crowded cyclopropanation catalysts. Syn-selectivity using rhodium(III)porphyrins

Rhodium(III)porphyrins catalyze the decomposition of ethyl diazoacetate and the transfer of ethoxy-carbonylcarbene to alkenes to form cyclopropanes in moderate to high yields. When compared with other catalysts a large syn-selectivity was observed on reaction with cis-alkenes. This selectivity increased with the size of the substituents, and suggested a preferential direction of approach of the alkene towards a rhodium—carbene complex.     

Granulated rhodium catalysts of sodium borohydride hydrolysis for generators of high-purity hydrogen

Sodium borohydride hydrolysis in a flow reactor with turbulent mixing of reactants in a catalytic bed by the evolving hydrogen bubbles was studied. The stability of catalytic systems decreases in the order 1% Rh/Sibunit > 1% Rh/TiO₂ > 1% Rh/γ-Al₂O₃. The decrease in the hydrogen generation rate is caused by the formation of a metaborate film on the catalyst surface, by the loss of the active component, and by disintegration of support granules and their removal with the flow of the spent liquid. High granule strength and macroporous structure of 1% Rh/Sibunit ensure stable generation of hydrogen.     

The function of ruthenium oxides in Pt-Ru catalysts for methanol electro-oxidation at low temperatures

The electrocatalytic activities of different binary Pt-Ru(ox) catalysts have been investigated in half-cell experiments by cyclic voltammetry and stationary current–potential measurements. The materials have been prepared using a modification of the Adams method. X-ray analytical methods (X-ray diffraction, XRD, and energy dispersive X-ray spectroscopy, EDX) as well as thermogravimetric analysis (TGA) have been used to characterize the composition and the catalysts' content of the crystalline phases, and their surface areas have been determined by the BET method. It is found that the composition of the catalyst is strongly influenced by the synthesis temperature, which is varied between 400 and 600 °C. In contrast, the particle size of the metallic phases of the catalysts is not significantly affected for synthesis temperatures below 600 °C, as investigated by transmission electron microscopy. Synthesis temperatures of 500 °C favor the formation of crystalline RuO₂ phases, whereas at synthesis temperatures below 500 °C the presence of metallic alloy and of hydrous oxides was derived by the combined XRD and EDX measurements. The stationary current–potential curves show a correlation with the different synthesis temperatures. It can be concluded that both the presence of an alloyed metallic Pt-Ru phase as well as the presence of amorphous hydrated Ru oxides are favorable for the electrocatalytic oxidation of methanol.Dedicated to Prof. Wolf Vielstich on the occasion of his 80th birthday in recognition of his numerous contributions to interfacial electrochemistry     

Synergistic enhancement of formic acid electro-oxidation on PtxCuy co-electrodeposited binary catalysts

A propitious binary catalyst composed of Pt and Cu which were electrodeposited simultaneously onto a glassy carbon (GC) substrate was recommended for the formic acid (FA) electro-oxidation reaction (FAOR); the principal anodic reaction in the direct FA fuel cells (DFAFCs). The simultaneous co-electrodeposition of Pt and Cu in the catalyst provided an opportunity to tune the geometric functionality of the catalyst to resist the adsorption of poisoning CO at the Pt surface that represented the major impediment for DFAFCs marketing. The catalytic activity of the catalyst toward FAOR was significantly influenced by the (Pt⁴⁺/Cu²⁺) molar ratio of the electrolyte during electrodeposition, which also affected the surface coverage of Pt and Cu in the catalyst. Interestingly, with a molar (Pt⁴⁺/Cu²⁺) ratio of (1:4), the catalyst sustained superior (3.58 compared to 0.65 obtained at the pristine Pt/GC catalyst) activity for FAOR, concurrently with up to four-times (0.73 compared to 0.18 obtained at the pristine Pt/GC catalyst) improvement in the catalytic tolerance against CO poisoning. This associated, surprisingly, a negative shift of ca. 336 mV in the onset potential of FAOR, in an indication for the competitiveness of the catalyst to minimize superfluous polarizations in DFAFCs. Furthermore, it offered a better (ended up with 20% loss in the activity) stability for continuous (1 h) electrolysis than pristine Pt/GC catalyst (the loss reached 35%). The impedance and CO stripping measurements together excluded the electronic element but confirmed the geometrical influence in the catalytic enhancement.     

Ru effect on the catalytic performance of Pd@Ru/C catalysts for methanol electro-oxidation

Pd@Ru bimetallic nanoparticles deposited on carbon black electro-catalysts have been fabricated by microwave-assisted polyol reduction method and investigated for methanol electro-oxidation(MEO). The structure and electro-catalytic properties of the as-prepared catalysts were characterized by XRD, SEM, TEM and cyclic voltammetry(CV) techniques. The results showed that the introduction of Ru element(2-10 wt%) into Pd 20 wt%/C(hereafter, denoted as Pd/C) produced a series of core-shell structured binary catalysts. Pd@Ru 5 wt%/C(hereafter, denoted as Pd@Ru5/C) catalyst displayed the highest catalytic activity towards MEO. And the mass activity of Pd@Ru5/C electrode catalyst at E =-0.038 V(vs. Hg/HgO) was 1.42 times higher than that of Pd/C electrode catalyst. In addition, the relationship between the catalytic stability for MEO on Pd@Ru/C catalysts and the value of Jbp/Jfp(the ratio of MEO peak current density in the negative scan and positive scan) were also investigated. The result demonstrated that Pd@Ru5/C offering the smallest value of Jbp/Jfpdisplayed the best stable catalytic performance.     

Rh/nylon catalysts for partial hydrogenation of benzene to cyclohexene

Hydrogenation of benzene on Rh/nylon produced cyclohexene with a high selectivity. The influence of contact time, pretreatment of catalyst and rhodium salt precursors have been studied. Samples prepared from Rh(III) were found more selective than catalysts obtained from Rh(I). It is suggested that electron deficient metal sites are responsible for cycloolefin formation.

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Rh/ . , . , Rh(III) , , Rh(I). , .

     

Resistance to sulfur poisoning of Rh supported catalysts

Sulfur thiotolerance by thiophene during toluene hydrogenation on rhodium supported catalysts prepared on different supports and metal precursors have been studied. The catalysts were characterized by H₂ chemisorption, TEM, TPD of ammonia and XPS. The turnover frequency of the reaction does not change significantly with the carrier but in the presence of 10 ppm of thiophene a drastic decrease in the activity is observed. The initial deactivation constant follows the order Rh/SiO₂>Rh/TiO₂>Rh/Al₂O₃.     

Ethanol electro-oxidation on catalysts with TiO2 coated carbon nanotubes as support

Pt–TiO₂/CNTs electrocatalysts for direct ethanol fuel cells (DEFCs) were prepared by sol–gel and ethylene glycol reduction method. XRD and TEM showed that the size of the Pt particles on TiO₂/CNTs is 3.5–4 nm and with narrow particle size distribution. HRTEM revealed that a thin layer of uniform amorphous TiO₂ on CNTs was formed and the faces of the Pt crystal on Pt–TiO₂/CNTs catalysts were quite “rough” and “rounded” and some grain bounders and/or twins also appeared. The electrochemical studies using cyclic voltammetry (CV), chronoamperometry and CO stripping voltammetry indicate that Pt–TiO₂/CNTs catalysts have higher electro-catalytic activity and CO-tolerance for ethanol oxidation than Pt/C (20 wt% Pt, E-TEK) and Pt/CNTs catalyst in acid. The Pt/TiO₂ molar ratio was also optimized and proved that 1:1 was the best Pt/TiO₂ molar ratio.     

Rh and Pt catalysts activated by UHV treatments

The H₂–D₂ exchange activity of Rh powder was enhanced by high-temperature UHV treatment accompanied with an enrichment of the high-index faces of Rh. The thermal decomposition of H₂PtCl₆·6H₂O under UHV condition resulted in an extremely active Pt powder catalyst.

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H₂–D₂ Rh , - Rh - . H₂PtCl₆·6H₂O Pt .

     

Synthesis of sterically encumbered porphyrins as catalysts for shape‐selective oxidations

A general method is described for the synthesis of sterically encumbered porphyrins whose shielding superstructure can take on the enzymatic role of substrate discrimination. This method is based on an improved synthesis of pyrroles substituted with a 2,6‐dibromophenyl group, followed by a Suzuki cross‐coupling to replace the Br with aryl groups. Porphyrins assembled from such pyrrole units have a barrel shape with the metal center completely fenced by four β‐substituted terphenyl shielding wings. The Fe and Mn porphyrins prove to be excellent catalysts for regioselective epoxidation of alkenes.     

On the nature of activation of anchored rhodium catalysts by borohydride

The nature of the active polymer anchored rhodium catalysts used for hydrogenation has been deduced to be a mono- or dihydridorhodium(III) species.