Silica-supported Ru Catalysts Research Articles

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.

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.

Metal Particle Growth during Glucose Hydrogenation over Ru/SiO2 Evaluated by X-ray Absorption Spectroscopy and Electron Microscopy

Biorenewable resources such as carbohydrates are considered alternative feedstocks for oxygenated chemicals. This work investigates the stability of silica-supported Ru catalysts in the aqueous phase conversion of glucose to sorbitol. In situ X-ray absorption spectroscopy at the Ru K edge revealed that air-exposed silica-supported Ru was in an oxidized state but was subsequently reduced in aqueous solutions saturated with 40 bar H(2) at 373 K. Furthermore, exposure to aqueous phase conditions resulted in the sintering of Ru particles on the silica surface. However, the presence of glucose in the aqueous phase stabilized the growth of the Ru particles. Batchwise hydrogenation of glucose at 373 K and 80 bar H(2) over a Ru/SiO(2) (2.67 wt %) catalyst is nearly 100% selective to sugar alcohol with an average turnover frequency of 0.21 +/- 0.04 s(-1). The hydrogenation reaction was not mass transfer limited according to the Madon-Boudart criterion.

Direct conversion of methane to higher hydrocarbons via an oxygen free, low-temperature route

The direct conversion of methane to higher hydrocarbons over a silica-supported Ru catalyst has been investigated via an oxygen free, two-step route. The reaction consists of decomposition of methane over a 3% silica-supp orted Ru catalyst at temperatures\((T_{CH_4 } )\) between 400 and 800 K to produce surface carbonaceous species followed by rehydrogenation of these species to higher hydrocarbons at\(T_{H_2 } \) of 368 K. It was found that the Ru/SiO2 catalyst exhibits a trend similar to that for single-crystal Ru catalysts. However, the temperature at which a maximum in ethane selectivity occurs shifts toward a higher temperature. It was also found that the ethane yield can be optimized by changing the surface carbon coverage. Under optimum conditions a net ethane yield of about 13–15% has been realized. For this two-step reaction sequence, only a few reaction cycles could be operated without intermediate high temperature rehydrogenation and without significant loss in ethane yield. This is attributed to large amounts of inactive carbon that could not be hydrogenated at 368 K. Higher methane partial pressures were found to be desirable for this reaction. The activity of the catalyst could also be maintained at total pressures up to 10 atm.

The effect of chlorine on hydrogen chemisorption by silica-supported Ru catalysts: A proton NMR study

Silica-supported ruthenium catalysts prepared from the RuCl 3 · 3H 2O 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-Ru s chemisorptive bond at the Ru surfaces. This reduced H-Ru s 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.

The role of alkali metals as promoters in the methanation and Fischer-Tropsch reaction: An in situ study

The effect of alkali metals as catalytic modifiers on the methanation and Fischer-Tropsch reaction over a series of silica-supported Ru catalysts has been studied. When hydrocarbon product distributions were compared at constant CO conversion or at constant CH 4 formation turnover frequencies, rather than at constant temperature, differences in selectivity were small for the series of catalysts studied. The temperature-programmed desorption of CO from these catalysts showed the presence of three desorption maxima, centered at 100, 220, and 450 °C, respectively. The relative intensity of the maximum centered at 220 °C was observed to increase slightly on the catalysts modified by the addition of alkali metals. However, this slight increase in binding energy did not appear to promote the formation of surface carbon. The high-temperature maximum centered at 450 °C was assigned to CO formed by the reaction between surface carbon and H 2O from the support. An “ in situ” infrared study of the CO surface species present on the catalyst surface during reaction showed only one band centered at 2030 cm −1 on supported Ru. This was in contrast to the two bands centered at 2020 and 1950 cm −1, which were observed for the K- and Cs-promoted catalysts. The results of this study suggest a depression in the rate of hydrogenation of the surface carbon species as a result of the addition of the alkali metal modifier. This is explained by invoking site blocking by the alkali metal adatoms rather than by an electronic effect.

The coadsorption of CO and H 2 on silica-supported Ru catalysts

The coadsorption of CO and H 2 on silica-supported Ru catalysts

Methanation and Fischer-Tropsch studies on potassium-promoted silica-supported Ru catalysts

The role of potassium as a promoter in the methanation and Fischer-Tropsch reaction has been studied. When the potassium content of a series of silica-supported Ru catalysts was increased, turnover numbers for both methane and hydrocarbon formation decreased. Significant increases in the selectivity for higher hydrocarbon and olefin formation occurred as the potassium concentration was increased. The results of a temperature-programmed desorption study showed that the binding sites of CO on Ru were significantly changed due to the presence of the potassium. When the potassium concentration was increased, the high-temperature desorption maxima centered at 370 °C increased relative to the low-temperature desorption maxima at 200 °C. This result is interpreted in terms of the stronger CRu bond resulting from the presence of the potassium. The results of the temperature-programmed desorption study were verified using in situ infrared spectroscopy. As the potassium content of the RuSiO 2 catalysts was increased, additional CO infrared bands assigned to more strongly bound CO were observed. An in situ infrared temperature-programmed reaction study for the potassium-promoted catalyst showed that the more weakly bound CO desorbed without undergoing reaction to form methane.

Evidence for [formula omitted] isotope effects on fischer-tropsch synthesis over supported ruthenium catalysts

The effects of using D 2 rather than H 2 during Fischer-Tropsch synthesis were investigated using alumina- and silica-supported Ru catalysts. For the alumina-supported catalysts, the rate of CD 4 formation was 1.4 to 1.6 times faster than the formation of CH 4. A noticeable isotope effect was also observed for higher molecular weight products. The magnitude of the isotope effects observed using the silica-supported catalyst was much smaller than that found using the alumina-supported catalysts. The formation of olefins relative to paraffins was found to be higher when H 2 rather than D 2 was used, independent of the catalyst support. The observed isotope effects are explained in terms of a mechanism for CO hydrogenation and are shown to arise from a complex combination of the kinetic and equilibrium isotope effects associated with elementary processes occurring on the catalyst surface.

Synthesis of hydrocarbons from CO and H 2 over silica-supported Ru: Reaction rate measurements and infrared spectra of adsorbed species

The synthesis of hydrocarbons from CO and H 2 was studied on a silica-supported Ru catalyst, and the species present on the catalyst surface were characterized by infrared spectroscopy. Initial rates of methane formation were correlated by the expression N CH 4 = 5.6 × 10 6 exp (−24,000/RT) P H 2 1.5 P CO 0.6 . The synthesis of ethane, propylene, and propane, the principle products observed in addition to methane, was favored at high CO partial pressures, low H 2 CO ratios, and low temperatures. The primary feature observed in the infrared spectra was a band at 2030 cm −1, associated with chemisorbed CO. Neither the position nor intensity of this band was affected by the CO partial pressure or the H 2 CO ratio. The CO band intensity did decrease with increasing temperature due to a decrease in CO surface coverage. Bands were also observed at 2950, 2910, and 2845 cm −1 and were assigned to hydrocarbon structures surrounded by chemisorbed CO. These structures could be removed from the catalyst surface by hydrogenation, but do not appear to be intermediates in the synthesis of stable products. Reduction of the catalyst following steady-state reaction revealed that the working catalyst maintains a reservoir of carbon several Ru monolayers in magnitude. Carbon deposition mechanisms and the role of carbon in the synthesis of hydrocarbons are discussed.

Infrared study of the reactions between NO and CO and NO and H 2 on a silica-supported Ru catalyst

Observations of the interactions of NO with CO and H 2 on a silica-supported Ru catalyst have been performed using infrared spectroscopy. The formation of an isocyanate species was found when NO reacted with preadsorbed CO. When NO and CO interacted with the surface simultaneously, N 2O and CO 2 were formed in addition to the isocyanate species. Mechanisms for the formation of the isocyanate structure, N 2O, and CO 2 are proposed and discussed. Results of the interaction of NO with hydrogen (as D 2) showed that adsorbed NO reacted with D 2 to produce species containing ND and OD bonds.

An infrared study of NO and CO adsorption on a silica-supported Ru catalyst

Infrared spectra were obtained for NO and CO adsorbed on reduced and oxidized Ru supported on silica. Adsorption of NO on a fully reduced sample produced bands at 1810 and 1860 cm −1. With prolonged catalyst exposure to NO, the latter band shifted to higher frequencies. Spectra of NO adsorbed on an oxidized sample showed only a single band at 1880 cm −1. Both the band at 1810 cm −1 and that occurring between 1860 and 1880 cm −1 are assigned to NO adsorbed as NO +. The low frequency band is associated with adsorption on partially oxidized Ru sites and the high frequency band with adsorption on more fully oxidized sites. On a fully reduced sample, CO adsorption produced a single band at 2040 cm −1 which is assigned to linearly bonded CO. Oxidation of Ru and subsequent adsorption of CO produced bands at 2130 and 2070 cm −1. This pair of bands is assigned to the symmetric and asymmetric stretches of two CO molecules bonded to a single oxidized Ru site.