Silica-supported Ruthenium Catalysts Research Articles

Alkaline Modified Mesoporous Silica Supported Ruthenium Catalyst for Improved α-Amino Acid Synthesis.

Amino acids are a class of compounds with wide-ranging applications. The synthesis of amino acids from biomass-derived α-keto acids and ammonia is a sustainable way but the unstable primary imine intermediates (R-C=NH) easily form oligomers. Herein, targeting this problem, alkaline modified mesoporous silica was employed as a support for ruthenium (Ru/M-MCM-41), which could be used as a bifunctional catalyst in the reductive amination of α-keto acids to synthesize α-amino acids. The incorporation of Sr improved the dispersion of Ru nanoparticles and enhanced metal-support interactions via electron transfer from Sr to Ru, and the active Ru sites could efficiently hydrogenate primary imine intermediates to α-amino acids, thus prohibiting the formation of oligomers. Moreover, the Sr-dopant introduces base sites that could catalyze the hydrolysis of oligomers back to primary imine intermediates and finally hydrogenated to α-amino acids. As a result, >99% yield of glycine was achieved from glyoxylic acid over Ru/Sr-MCM-41, which is nearly three times that achieved over Ru/MCM-41 (32.2%).

Scalable and continuous access to pure cyclic polymers enabled by 'quarantined' heterogeneous catalysts.

Cyclic polymers are topologically interesting and envisioned as a lubricant material. However, scalable synthesis of pure cyclic polymers remains elusive. The most straightforward way is to recover a used catalyst after the synthesis of cyclic polymers and reuse it. Unfortunately, this is demanding because of the catalyst's vulnerability and inseparability from polymers, which reduce the practicality of the process. Here we develop a continuous circular process, where polymerization, polymer separation and catalyst recovery happen in situ, to dispense a pure cyclic polymer after bulk ring-expansion metathesis polymerization of cyclopentene. It is enabled by introducing silica-supported ruthenium catalysts and newly designed glassware. Different depolymerization kinetics of the cyclic polymer from its linear analogue are also discussed. This process minimizes manual labour, maximizes the security of vulnerable catalysts and guarantees the purity of cyclic polymers, thereby showcasing a prototype of a scalable access to cyclic polymers with increased turnovers (≥415,000) of precious catalysts.

Surface hydroxyl dependent adsorption of ruthenium on SiO2(0 0 1) – Understanding metal–support interaction

To understand the metal–support interaction of oxide supported transition metal catalysts, we computed the adsorption structures and energies of ruthenium on the idealized SiO2(001) surfaces with full (OH, 100%), partial (OH, 50%) surface hydroxyl and without surface hydroxyl (OH, 0%) at different ruthenium loading. On the fully (up to 9Ru) and partially (up to 6Ru) hydroxylated surfaces, ruthenium adsorption prefers surface O-H oxidative addition and forms regularly distributed Ru(H)2 species. After the reductive and re-combinative desorption of H2, the adsorbed ruthenium atoms prefer structures with aggregated 2Ru dimeric units on the fully hydroxylated surface, while structures of remote and dispersed singly adsorbed Ru atoms on the partially hydroxylated surface. On the hydroxyl free surface (up to 6Ru), the adsorbed ruthenium atoms prefer three-dimensional aggregated structures, which become more stable with the increase of ruthenium loading. This indicates the surface hydroxyl dependent ruthenium adsorption on SiO2 (001), and this finding is important for understanding the structure, stability and activity of silica supported ruthenium catalysts and catalysis.

C–H Bromination of 6-Phenylpurines with a Silica-Supported Ruthenium Catalyst

C–H Bromination of 6-Phenylpurines with a Silica-Supported Ruthenium Catalyst

Efficient hydrogenolysis of cellulose into sorbitol catalyzed by a bifunctional catalyst

A sulfonic acid-functionalized silica-supported ruthenium catalyst (Ru/SiO2–SO3H) was employed for the hydrogenolysis of cellulose in one pot in neutral water medium. Ru/SiO2–SO3H is a bifunctional catalyst containing both a Brønsted acidic site and a metal site (Ru). Compared with the mechanical mixture of a silica-supported Brønsted acid (SiO2–SO3H) and a silica-supported Ru catalyst (Ru/SiO2), the bifunctional catalyst showed much higher yield of sorbitol, which could reach up to 61.2% when the reaction was performed for 10 h at 150 °C. Through the characterization by XPS and pyridine-adsorbed FT-IR, the existence of the interaction between sulfonic groups and Ru nanoparticles in the Ru/SiO2–SO3H catalyst was observed. The sulfonic acid groups and metal sites in the adjacent position were important for enhancing the yield of sorbitol. In addition, the present catalyst can be reused five times with only a slight decrease in yield of sorbitol in the consecutive recycles.

Synthesis and Hydrodesulfurization Properties of Noble Metal Phosphides: Ruthenium and Palladium

Silica-supported ruthenium and palladium phosphide catalysts (Ru2P, RuP, Pd3P, Pd5P2) were investigated for the hydrodesulfurization (HDS) of dibenzothiophene (DBT). The Ru and Pd phosphide catalysts were prepared by temperature-programmed reduction of hypophosphite-based precursors consisting of uncalcined or calcined Ru/SiO2 or Pd/SiO2 impregnated with ammonium hyposphosphite (NH4H2PO2). The Ru2P/SiO2 and RuP/SiO2 catalysts prepared from uncalcined precursors had smaller average crystallite sizes, higher CO chemisorption capacities, and higher HDS activities than the catalysts prepared from the calcined precursors, while the effect of preparation method on catalytic properties was less clear for the Pd3P/SiO2 and Pd5P2/SiO2 catalysts. Following HDS testing at 673 K, X-ray diffraction analysis revealed that the Pd5P2/SiO2 catalysts decomposed to give Pd3P on the silica support, while the other phosphides exhibited good stability during the testing period. At temperatures at which high DBT conversion was observed (>598 K), the Ru and Pd phosphide catalysts were less active than sulfided Ru/SiO2 and Pd/SiO2 catalysts prepared from the uncalcined metal precursors.

An Attempt to Activate Silica Supported Ruthenium Catalyst Abbreviating Reduction Procedure; Evaluation of Oxidation State of Ruthenium Species by Hydrogenation of 1,3-butadiene

An Attempt to Activate Silica Supported Ruthenium Catalyst Abbreviating Reduction Procedure; Evaluation of Oxidation State of Ruthenium Species by Hydrogenation of 1,3-butadiene

Hydrogenation of bisphenol A – Using a mesoporous silica based nano ruthenium catalyst Ru/MCM-41 and water as the solvent

A green approach to the hydrogenation of bisphenol A (BPA) including the selection of catalysts and solvents was demonstrated in this report. The catalyst preparation was also using a simple and green method for synthesizing a silica-supported ruthenium catalyst (Ru/MCM-41) in supercritical carbon dioxide. The characterizations of the Ru/MCM-41 nanocomposites were preformed by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). The nanoparticles have an average size of 3.4 nm which is smaller than the pore size of MCM-41. The hydrogenation of BPA was carried out in a hydrogen pressure of 50 bars and a temperature range from 75 to 85 °C with different catalysts and solvents combinations. It was found that using the synthesized catalyst Ru/MCM-41 in water medium could achieve the highest efficiency and durability for the hydrogenation of BPA.

ChemInform Abstract: NMR Study of the Influence of Adsorbed Sulfur on Hydrogen Chemisorption on Supported Ruthenium Catalysts.

Abstract NMR of hydrogen chemisorbed on silica-supported ruthenium catalysts was used to study the dissociation of H 2 S and its effect on subsequent hydrogen adsorption. A mixture of H 2 S/He was used to sulfide the reduced catalyst. The hydrogen coverages resulting from the dissociation of H 2 S as measured by NMR yielded estimates of sulfur coverages in good agreement with the results of X-ray fluorescence analysis of sulfur on the samples. The H 2 S dissociation reaction was found to be enhanced on higher dispersion catalysts suggesting an important role for defect-like sites. At sulfur coverages greater than 0.25 sulfur atoms per surface ruthenium atom, no hydrogen from the dissociation of H 2 S could be detected remaining on the surface. On the presulfided catalysts dosed to saturation with additional hydrogen, the NMR spectra revealed the presence of two species of hydrogen: a mobile species and a less mobile species. At sulfur coverages greater than 0.5 no detectable hydrogen could be adsorbed on the surface when exposed to 30 Torr of H 2 . The ratio of weakly bound hydrogen to the strongly bound hydrogen at saturation coverage decreased with increasing sulfur coverage. The small change in the magnitude of the NMR lineshift of hydrogen-on-metal with sulfur coverage suggested that the adsorbed sulfur does not induce a strong electronic effect on the chemisorption of hydrogen.

ChemInform Abstract: The Effect of Chlorine on Hydrogen Chemisorption by Silica-Supported Ru Catalysts: A Proton NMR Study.

Silica-supported ruthenium catalysts prepared from the RuCl3 · 3H2O precursor contain residual chloride after reduction in flowing hydrogen. The amount of this chlorine was determined by X-ray fluorescence spectroscopy to be less than one monolayer on the ruthenium surfaces. Both volumetric hydrogen chemisorption and proton NMR show suppression of hydrogen chemisorption capacity associated with the presence of chlorine contamination, which can be removed effectively with hot water elution. The variations of the hydrogen chemisorption capacity with chlorine coverage suggests that chlorine would completely block and poison the Ru surfaces for hydrogen chemisorption at a Cl coverage of about 76%. The effect of residual chloride on the proton NMR resonance position and the spin-lattice relaxation time for the chemisorbed hydrogen indicates an electronic interaction between Cl adspecies and surface Ru, which results in the weakening of the H-Rus chemisorptive bond at the Ru surfaces. This reduced H-Rus interaction may be the cause for an increase in the ratio of the reversibly adsorbed hydrogen over the irreversibly adsorbed hydrogen with increasing Cl coverage. It is proposed that the mechanism for suppression of hydrogen chemisorption by chlorine involves both physical site blocking of surface Ru atoms and shortranged electronic interactions between Cl adatoms and neighboring surface Ru atoms.

A Mesoporous Silica-Supported Ruthenium Catalyst for Metathesis

A Mesoporous Silica-Supported Ruthenium Catalyst for Metathesis

Comparing Strengths of Surface Interactions for Reactants and Solvents in Porous Catalysts Using Two-Dimensional NMR Relaxation Correlations

Two-dimensional nuclear magnetic resonance (NMR) relaxation time correlation measurements have been used to observe the behavior of liquids inside porous catalyst pellets; in particular, liquids of relevance to the hydrogenation of 2-butanone over a silica-supported ruthenium catalyst (Ru/SiO2). The behavior of 2-butanone is studied and compared to that of water and 2-propanol, which are used as solvents in this hydrogenation reaction. From the ratio of NMR relaxation times, T1/T2, for the liquids confined in the pores, it is possible to infer the relative strengths of the surface interaction for each liquid. Water is seen to have the strongest surface interaction, and 2-butanone has the weakest surface interaction. These results are supported by displacement experiments, in which one liquid replaces the other over time within the pore space of the catalyst. For comparison, the behavior of the same liquids in an alumina-supported palladium catalyst (Pd/Al2O3) was also studied. The variation in the strengt...

Homologation of ethylene without metathesis on silica supported ruthenium catalyst

A 1:1 mixture of 12C2-C2H4 (H2C=CH2) and 13C2-C2H4 (H213C=13CH2) was contacted on Ru/SiO2at 200oC. Propylene (homologation) was observed without 13C1-C2H4 formation (metathesis reaction). This might at least be corroborating evidence for discrimination of active sites in regard with the homologation and the metathesis.

Thermolysis of Ru(acac) 3 supported on silica and alumina

Precursors of alumina- and silica-supported ruthenium catalysts were prepared by pore volume impregnation of the dehydrated supports with a benzene solution of Ru(acac) 3. The prepared samples were characterized by diffuse reflectance FTIR and TG/DTG–DTA in order to discriminate Ru(acac) 3 which interacts with the support surface. Weight loss, experimentally observed upon thermolysis of parent and supported Ru(acac) 3, was compared with that calculated using the proposed reaction stoichiometry. Conditions for activating silica- and alumina-supported Ru(acac) 3 in an oxidizing atmosphere with complete removal of acetylacetonate ligands from the catalyst precursor are recommended.

Isotopic tracing experiments in syngas production from methane on Ru/Al 2O 3 and Ru/SiO 2

The catalytic production of syngas from methane was investigated over alumina- and silica-supported ruthenium catalysts. It is shown that the support is involved in reactions with reactants and products somewhat modifying the catalytic behaviour. From isotopic tracing experiments it is concluded that the syngas production reaction follows the same indirect pathway over the two catalysts, i.e. the sequence of total oxidation of methane, followed by reforming of the unconverted methane with carbon dioxide and steam to syngas. Recent literature is analysed in the light of our results.

Optimization of the Volumetric Hydrogen Chemisorption Technique for Dispersions of Ru/SiO 2 Catalysts

Metal dispersions of silica-supported ruthenium catalysts determined by the standard volumetric chemisorption technique were found to be inaccurate due to irreversible spillover of hydrogen to the support. Direct evidence was obtained via in situ 1H NMR for an irreversibly bound component of the hydrogen that migrated from the metal to the support on a time scale of tens of minutes or longer at room temperature. It was also shown in this work that hydrogen saturated the surface of the metal particles on a shorter time scale, on the order of minutes. Based on 1H NMR, optimum conditions for the volumetric chemisorption method were determined in order to alleviate the interference of spillover with measurements of hydrogen uptake on the metal. Ruthenium dispersions obtained from the optimized volumetric chemisorption technique were more accurate and agreed well with the results of the 1H NMR method.

ChemInform Abstract: Improved Activity of a Silica Supported Ruthenium Catalyst by Carbon Monoxide Pretreatment.

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Improved activity of a silica supported ruthenium catalyst by carbon monoxide pretreatment

Abstract Silica supported ruthenium catalysts have been prepared by incipient wetness impregnation with RuCl3 (aq). After drying and reduction these catalysts were additionally treated with carbon monoxide or CO/H2 at 250°C. Due to this treatment the catalyst's carbon monoxide chemisorption capacity significantly increased compared to normal reduction. At the same time the intrinsic reactivity for carbon monoxide dissociation is enhanced. These effects are attributed to the structural changes of the catalysts. A model, based on TPR, XPS, IR and TEM measurements, is proposed. The carbon monoxide treatment results in a catalyst that is better reducible and has larger metal ensembles.

Interaction of Hydrogen with Supported Ru Catalysts: High Pressure in Situ NMR Studies

An in situ 1H nuclear magnetic resonance (NMR) technique has been employed to study the interaction of hydrogen with silica-supported ruthenium catalysts in the temperature range 300-473 K and at pressures of 10 −5-760 Torr. In addition to an upfield NMR peak (called α) occurring at ∼−60 ppm and observed at low pressures ( P < 100 Torr), we have noted the emergence of a second peak (called β) occurring at ∼−30 ppm at elevated pressures ( P > 100 Torr). These two hydrogen populations represent adsorbed hydrogen interacting with Ru metal particles, as indicated by the observed Knight shifts. The α hydrogen species exists in the form of less mobile hydrogen species α 1, or a mobile hydrogen species α M with heats of adsorption determined by microcalorimetry to be about 70 kJ/mol and 40 kJ/mol, respectively. The heat of adsorption of the β species was determined to be approximately 10 kJ/mol. Hydrogen adsorption isotherms were obtained from the NMR peak intensities calibrated by a gadolinium-doped lanthanum hydride internal reference. A higher dispersion of metal particles correlated with a larger amount of the weakly adsorbed β hydrogen species. The H/Ru surface ratio was observed to be in excess of 5 at elevated pressures (>500 Torr). The high H/Ru surface was due to the adsorption of large amounts of β hydrogen.

Homologation of olefins with methane on transition metals

Alkylation of olefins using methane has been realized on transition metal catalysts. The main problem is to get methane dissociatively adsorbed together with an olefin. This is due to the difficult activation of the strong tetrahedral C-H bonds of methane. To react methane with an olefin, a reaction sequence is used consisting of three steps. First methane is dissociatively adsorbed between 600 and 800 K on a reduced transition metal catalyst. After cooling, an olefin is coadsorbed at a temperature between 300 and 400 K. Upon subsequent hydrogenation carbon-carbon bond formation occurs. The mechanism appears to be related to that occurring in the Fischer-Tropsch reaction. After hydrogenation the reaction cycle can be repeated. Methane addition to ethene, propene, and acetylene is demonstrated to occur using silica-supported ruthenium and cobalt catalysts. With {sup 13}CH{sub 4} it was shown that propane and butane are formed both by self-homologation of ethene and propene, respectively, as well as by methane incorporation. Carbon scrambling according to the metathesis reaction is very slow. 86 refs., 9 figs., 3 tabs.