Colloidal Rhodium Research Articles

Effect of adenosine monophosphate on visible-light driven nicotinamide mononucleotide reduction in a system of water-soluble zinc porphyrin and colloidal rhodium nanoparticles

Effect of adenosine monophosphate on visible-light driven nicotinamide mononucleotide reduction in a system of water-soluble zinc porphyrin and colloidal rhodium nanoparticles dispersed with polyvinylpyrrolidone was clarified.

Visible-light-induced enzymatic reactions using an NADH regeneration system of water-soluble zinc porphyrin and homogeneous colloidal rhodium nanoparticles

Visible-light-driven molecular conversion using the NADH regeneration system of triethanol amine, water-soluble zinc porphyrin, colloidal Rh nanoparticles and NAD+ in the presence of various dehydrogenases was developed.

Synthesis, characterization and hydrogen sensing properties of nanosized colloidal rhodium oxides prepared by Pulsed Laser Ablation in water

Nanosized colloidal rhodium oxide particles in water were obtained by using a nanosecond pulsed excitation, starting from a high purity rhodium target. The as-synthesized colloids show an almost spherical shape and size in the range between 2 and 12 nm. Characterization results indicate that colloidal particles are composed mainly of oxidized RhxOy phases, whose content increase after annealing at 200 °C in air and decrease under hydrogen treatment in mild conditions, suggesting a reversible behavior. The hydrogen sensing characteristics of thin films deposited by drop casting from the colloidal solution onto alumina sensor substrates provided with platinum interdigitated electrodes, were also investigated. The developed conductometric hydrogen sensor shows high response to low concentration of the target gas in air at low operating temperature and full reversibility with fast dynamics. Further, a good selectivity towards hydrogen was demonstrated, testing other simple molecules such as CO, CO2, NO2 and O2. To our knowledge, this is the first report on the use of nanosized rhodium/rhodium oxide colloids in conductometric gas sensors.

Enantioselective hydrogen transfer hydrogenation on rhodium colloid systems with optically active stabilizers

Hydrogen transfer hydrogenation of acetophenone and methyl benzoylformate with 2-propanol was studied on colloidal systems obtained by reduction of rhodium complexes in the presence of optically active compounds: chiral diamines, quaternary salt (4S,5S)-(–)-N 1,N 4-dibenzylene-2,3-dihydroxy-N 1,N 1,N 4,N 4-tetramethylbutane-1,4-diammonium dichloride and (8S,9R)-(–)-cinchonidine. The increase in the molar ratio modifier/Rh leads to the increase in the enantioneric excess (ee) of the reaction products. The largest ee [43.8% of (R)-1-phenylethanol and 58.2% of methyl ester of (R)-mandelic acid] were achieved for the ratios (8S,9R)-(–)-cinchonadine: Rh = 9: 1 and 3: 1, respectively. The catalyst was characterized by the high-resolution transmission electron microscopy, X-ray diffraction analysis, and thermal analysis.

Rh(0) Nanoparticles: Synthesis, Structure and Catalytic Application in Suzuki–Miyaura Reaction and Hydrogenation of Benzene

Rhodium colloids have been prepared by chemical reduction of an aqueous solution of rhodium trichloride with different aliphatic alcohols (from methanol to pentanol) in the presence of polyvinylpyrrolidone (PVP) as the stabilizing agent. The obtained Rh(0) nanoparticles have been characterized structurally by means of X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy. Catalytic activity of the synthesized Rh/PVP colloids has been evidenced in the Suzuki–Miyaura coupling reaction of phenylboronic acid with bromobenzaldehyde. The choice of the reducing agent and reduction conditions enabled to obtain Rh(0) colloids showing various mean nanoparticle diameters (ranging from 3.8 to 6.0 nm) and different nanoparticle morphologies. These two factors play the decisive role from point of view of catalytic activity of the presented systems. The most active Rh/PVP catalyst, prepared using ethanol as the reducing agent, have also been applied in hydrogenation of benzene to cyclohexane, showing very high activity in this process.

New ammonium surfactant-stabilized rhodium(0) colloidal suspensions: Influence of novel counter-anions on physico-chemical and catalytic properties

Novel anionic species, such as hydrogen carbonate (HCO(3)(−)), fluoride (F(−)), triflate (CF(3)SO(3)(−)), tetrafluoroborate (BF(4)(−)) and chloride (Cl(−)) were investigated as new partners of water soluble N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl) ammonium salts, used as a protective agent of rhodium colloids. The effect of the surfactant polar head on the micellar behavior, size and morphology of the nanospecies was studied by adapted physico-chemical experiments (surface tension measurements, dynamic light scattering, thermogravimetric and TEM analyses) and discussed in terms of strong or weak stabilization of the growing nanoparticles surface. Finally, the influence of the nanoenvironment generated by the surfactant with various counter-anions was evaluated via the hydrogenation of aromatics.

N-Donor ligands based on bipyridine and ionic liquids: an efficient partnership to stabilize rhodium colloids. Focus on oxygen-containing compounds hydrogenation

Polynitrogen ligands and/in ionic liquids (ILs) are described as a pertinent tandem to efficiently stabilize rhodium nanoparticles (NPs) in the size range of 2.0 nm for catalytic applications. Several N-donor ligands based on bipyridine skeleton were used as extra protective agents in [BMI][PF(6)] and compared in the hydrogenation of functionalized aromatic compounds at 80 °C and under 40 bar H(2). The nature of the bipyridine derivative and its influence on the coordination mode on the particle surface were proposed to explain the observed different kinetic properties. The hydrogenation of various oxygen-containing arenes was investigated and original results were described in the reduction of anisole and cresols as model lignin compounds, providing a significant ratio of ketone derivatives. A first explanation based on possible reaction routes is proposed to justify the formed products.

Rh–RhSxnanoparticles grafted on functionalized carbon nanotubes as catalyst for the oxygenreduction reaction

Rhodium–rhodium sulfide nanoparticles supported on multi-walled carbon nanotubes (CNTs) were synthesized via a multi-step colloid route. The CNTs were first exposed to nitric acid to generate oxygen-containing functional groups, and then treated with thionyl chloride to generate acyl chloride groups. The grafting of thiol groups was subsequently carried out by reaction with 4-aminothiophenol. Colloidal rhodium nanoparticles were synthesized using rhodium chloride as metal source, sodium citrate as stabilizer, and sodium borohydride as reducing agent. The immobilization of the generated colloidal rhodium nanoparticles was achieved by adding the thiolated CNTs to the colloidal suspension. All these steps were monitored by X-ray photoelectron spectroscopy, which disclosed the presence of rhodium sulfide, whereas metallic rhodium was detected by X-ray diffraction, suggesting that the nanoparticles probably consist of a metallic Rh core covered by a sulfide layer. Scanning and transmission electron microscopy studies showed that the diameter of the catalyst particles was about 7 nm even at high Rh loadings. Rotating disc electrode measurements and cyclic voltammetry were employed to test the electrocatalytic activity in the oxygen reduction reaction in hydrochloric acid. Among all the synthesized catalysts with different rhodium loadings (4.3–21.9%), the 16.1% rhodium catalyst was found to be the most active catalyst. In comparison to the commercial E-TEK Pt/C catalyst, the 16.1% catalyst displayed a higher electrochemical stability in the highly corrosive electrolyte, as determined by stability tests with frequent current interruptions.

Synthesis of phthalocyanine stabilized rhodium nanoparticles and their application in biosensing of cytochrome c

A single step synthesis route is described for the preparation of rhodium nanoparticles using a cobalt aminophthalocyanine macrocyclic complex as a stabilizer. The results of nanoparticles characterization using electronic absorption, Raman and X-ray spectroscopes as well as transmission electron microscopy are reported. Rhodium nanoparticle modified electrode behavior as examined by cyclic and differential pulse voltammetry is also provided. The nanoparticles were found to be well dispersed and stabilized throughout the macromolecular matrix. TEM studies showed that they have an average diameter of 3 to 5 nm with spherical shape. The colloidal rhodium was then used for electrochemical sensing of cytochrome c using glassy carbon electrode. The results showed that the colloidal rhodium nanoparticles enhanced the electron transfer process between cytochrome c and the electrode. Differential pulse voltammetric measurements of cytochrome c at the colloidal rhodium nanoparticles modified glassy carbon electrode showed a linear relationship with the oxidation peak currents in the concentration range of 100 nM to 3 μM of cytochrome c.

Amine-tethered N-heterocyclic carbene complexes of rhodium(I)

A new family of rhodium-diene based complexes has been developed that incorporates an N-heterocyclic carbene ligand with an N-donor tether. The ligand is denoted Mes[CNH] 2 (where Mes[CNH] is 2,4,6-Me3C6H2NC3H2NCH2CH2NH-2,4,6-Me3C6H2) and Mes[CN] for the amido form. The synthesis of the Mes[CNH] ligand involves reaction of N-mesitylimidazole with 2-chloroethyl-N-mesitylamine under melt conditions, followed by deprotonation with KN(SiMe3)2. The reaction of Mes[CNH] with [(diene)RhCl]2 results in the formation of the monodentate complexes, Mes[CNH]Rh(diene)Cl (where diene = 1,5-cyclooctadiene (COD): 3a; diene = 2,5-norbornadiene (NBD): 3b). Bidentate variants could be isolated as either a neutral species, Mes[CN]Rh(diene) 4a–4b, via deprotonation, or an ionic analogue such as [Mes[CNH]Rh(diene)]BF4 5a–5b by reaction with NaBF4. Compounds 4–5 are the first examples of rhodium compounds that contained a bidentate NHC ligand with a pendant amino or amido donor. Complexes 3–5 were characterized fully and the solid-state single crystal X-ray structures of 3a, 4a, and 5b are discussed. The utility of these complexes as catalyst precursors for hydrogenation reactions was examined and it was determined that these systems are not significantly more active than colloidal rhodium when parallel reactions were run. Various methods of transfer hydrogenations were also investigated with 3a, which did not yield an appreciable conversion of either benzophenone or N-benzylideneaniline as substrates.Key words: rhodium, N-heterocyclic carbene, catalysis, hydrogenation.

Hydroformylation with Dendritic‐Polymer‐Stabilized Rhodium Colloids as Catalyst Precursors

AbstractHydroformylation of 1‐hexene with rhodium colloids stabilized by poly(ethylene imine) amides with a hyperbranched polyamine core and a lipophilic periphery is reported. The catalyst selectivity for aldehydes increases with increasing temperature or pressure. The ratio of linear to branched aldehydes decreases with increasing temperature. At 80 °C and 90 atm, 1‐hexene is converted completely with 96% selectivity for aldehydes, with a ratio of linear/branched aldehydes of 58:42. Turnover numbers up to 3 × 104 (calculated with respect to the entire amount of Rh present) were observed. The results indicate that the catalyst precursor colloids serve as a reservoir for the formation of soluble Rh(I) complexes as the active species.magnified image

Evidence of colloidal rhodium formation during the biphasic hydroformylation of 1-octene with thermoregulated phase-transfer phosphine rhodium(I) catalyst

A thermoregulated phase-transfer (TRPT) Rh(I) complex catalyst A prepared from Rh(acac)(CO)2 and a thermoregulated ligand CH3(OCH2CH2)mPPh2 (Mw = 918) was applied to the biphasic hydroformylation of 1-octene, and a high activity with an aldehyde yield of 97.5% was demonstrated. After three recycling steps, the aldehyde yield gradually decreased. Transmission electron microscopy (TEM) revealed that after the first cycle Rh colloids were generated in situ in the aqueous phase, and in subsequent runs Ostwald ripening occurred. An independently prepared colloidal Rh(0) TRPT catalyst D also exhibited high hydroformylation activity under identical experimental conditions, and after two times of recycling an activity decrease was also observed. It is suggested that in situ from Rh(acac)(CO)2 colloidal Rh particles are generated, which demonstrate thermomorphic behaviour and a high hydroformylation activity. Subsequently, agglomeration processes result in an activity decay, as observed in the TRPT Rh(I) complex catalyst system. Copyright © 2005 John Wiley & Sons, Ltd.

Hydrogenation of olefins in aqueous phase, catalyzed by polymer-protected rhodium colloids: kinetic study

Highly water-soluble polymer such as PVP was used to stabilize colloidal suspensions of active rhodium particles and to perform catalytic hydrogenation of oct-1-ene in a two liquid phase system. The effects of various parameters on the stability and activity of these nanoparticles under more or less severe conditions have been investigated. Experiments show that PVP–Rh colloids can be reused twice or more at 50°C and 0.3 MPa without losing activity. Much higher turnover than with usual biphasic catalysis were performed. The MET micrographs of the catalytic phase of the PVP–Rh system, before and after hydrogenation shows two nanometer particles containing rhodium oxide and metallic rhodium. IR and NMR spectroscopy analysis of the colloids shows geminal and terminal Rh–CO species with a preponderance of the geminal ones. The kinetics of biphasic hydrogenation and simultaneous isomerisation of oct-1-ene have been studied in the presence of ethanol cosolvent, using low concentrations of PVP–Rh colloids. The rate was found to be first order with respect to catalyst and olefin concentrations and hydrogen pressure. A kinetic model was fitted to the observed data, and was found to predict the rates with a good agreement. The activation energy was found to be as low as 3.4 kcal/mol suggesting the reaction to be limited by diffusion of oct-1-ene through the protective polymer layer.

Stabilized rhodium(0) nanoparticles: a reusable hydrogenation catalyst for arene derivatives in a biphasic water-liquid system.

A colloidal system based on an aqueous suspension of rhodium(o) nanoparticles proved to be an efficient catalyst for the hydrogenation of arene derivatives under biphasic conditions. The rhodium nanoparticles (2-2.5 nm) were synthesized by the reduction of RhCl3 x 3H2O with sodium borohydride and were stabilized by highly water-soluble N-alkyl-N-(2-hydroxyethyl)ammonium salts (HEA-Cn). These surfactant molecules were characterized by measurements of the surface tension and the aqueous dispersions with rhodium were observed by transmission electron cryomicroscopy. The catalytic system is efficient under ultramild conditions, namely room temperature and 1 atm H2 pressure. The aqueous phase which contains the protected rhodium(0) colloids can be reused without significant loss of activity. The microheterogeneous behavior of this catalytic system was confirmed on a mercury poisoning experiment.

Selective photocatalytic transfer-hydrogenation to 1,5-cyclooctadiene with light transition metal modified rhodium colloid catalyst

Effects of additive were examined for the photocatalytic transfer hydrogenation from 2-propanol to 1,5-cyclooctadiene (1,5-COD) with using acetone and Rh 4(CO) 12 as a catalyst precursor. Photo-irradiation easily transformed Rh 4(CO) 12 to colloidal dispersion in the presence of acetone. Primary particle size of colloid was ca. 2 nm and it formed secondary particles through aggregation. With colloidal rhodium catalyst, the selectivity of cyclooctene (COE) was 86% and the ratio of hydrogenation rate R (=cyclooctadiene to cyclooctene/cyclooctene to cyclooctane) was 4.8. Addition of Fe(NO 3) 3·9H 2O enhanced the performance of selective transfer hydrogenation. When Fe salt was used, selectivity reached to 96% and R value decreased to 1.1. The addition of Fe salt did not change primary particle size, but the formation of secondary particle was diminished. Other salt of light transition metal, Ni(NO 3) 2·6H 2O and Cu(NO 3) 2·3H 2O, also changed the rate of transfer hydrogenation and R value. These changes were attributable to the change of affinity between surface and substrates causing modification with metal cation.

Evolution of the Rhodium Colloid Supported on Graphite Studied by Atomic Force Microscopy in the Tapping Mode

Rhodium colloid stabilized by trioctylamine and deposited on a highly oriented pyrolitic graphite was studied by atomic force microscopy in the tapping mode. Most of the intact colloid particles are found to be agglomerated via two-dimensional islands or sphere-shaped clusters attached to the steps of the graphite surface. No particle sintering occurs after thermal treatments in vacuo at 100−550 °C. Oxidation at 550 °C leads to sintering of the particles contacting with the carbon support but not within spherical 3D-aggregates. In addition, AFM images show the gasification trails following the Rh particles on graphite. Data indicate a strong colloid−support interaction under oxidative conditions.

Carbonylation of methanol catalyzed by polymer-protected rhodium colloid

Poly( N-vinyl-2-pyrrolidone)(PVP) protected rhodium colloid (Rh-PVP) was used as a catalyst for methanol carbonylation under elevated temperature (140°C) and high pressure (5.4 MPa). The activity of Rh-PVP colloid catalyst increased while recycled for 6 times totally lasting 56.5 h. During the reaction, the catalyst was still in a colloidal state verified by TEM observation. XPS and IR studies of the catalyst demonstrated that colloidal rhodium metal particles partially changed to [Rh(CO) 2I 2] − under the reaction conditions. The rhodium ions in the reaction mixture amounted to 29% of the total charged rhodium after 13 h carbonylation. An additional XPS investigation of the catalyst was carried out; the results showed that oxidative addition of methyl iodide to rhodium colloid caused the transformation of rhodium species from Rh 0 to Rh 1+.

X-ray absorption and X-ray photoelectron spectroscopy of a rhodium colloid.

Transition metal colloids are potential precursors of heterogeneous catalysts with application to selective chemical reactions. Sample preparation techniques are described. Experimental details are given of the characterization of these often air-sensitive particles by X-ray photoelectron and X-ray absorption spectroscopy. First results obtained with both techniques for a Rh-colloid show that the metal is mainly present in the zerovalent chemical state. But the spectra indicate further chemical states of Rh which can be assigned to the outermost metal atoms of the colloid interacting with organic ligands or to the educt Rh-halides.

Reaction conducted under rather severe conditions for a colloidal catalyst ‐ hydroformylation of propylene catalyzed by polymer‐protected rhodium colloids

AbstractHydroformylation of propylene has been conducted by a polymer‐protected rhodium colloid catalyst with an activity of 130 mol butanal / g atom Rh·h. The neat colloid catalyst is stable enough for repeated use for 7 times lasting ∼50 h under the reaction condition (temperature: 363 K, pressure: 40 bar) with a total TON of 5,065 cycle / atom Rh.

Stabilizing structure of tertiary amine-protected rhodium colloid dispersions in chloroform

Tertiary amine-protected rhodium colloids have been prepared in chloroform by phase transfer of a rhodium(III) salt from an aqueous to the organic phase followed by reduction with hydrogen. The rhodium colloid dispersions, thus prepared, present a narrow size distribution and are stable. The structure of the protecting amine molecules on the surface of the colloids was investigated by NMR spectroscopy, transmission electron microscopy (TEM), and hydrodynamic radius measurements according to the Taylor dispersion method. Carbon-13 NMR spectroscopy can provide some information on the conformation of the amine molecules on the surface. The size of the entire colloid including the amine layer surrounding the metal surface, obtained by the Taylor dispersion method, is considered to be fairly consistent with the size of the naked metal particles obtained by TEM and the size of the adsorbed layer, and thus supports the conformational information.