P. Ramírez de la Piscina

Reactions of propene on supported molybdenum and tungsten oxides

Abstract Alumina-supported tungsten and molybdenum oxide catalysts with a wide range of MO3 loadings were studied in the metathesis of propene. The catalysts were activated by several procedures in order to correlate the oxidation state of Mo or W, as characterized by XPS, and the acidic character, as determined by IR of adsorbed pyridine, with the main metathesis reaction and side olefin reactions. The catalytic activity of the samples under standard conditions was found to be invariant with the pretreatment (air or He). This leads to the conclusion that the activation of catalysts takes place under the propene atmosphere during the first minutes of reaction. XPS studies of fresh and spent catalysts showed that the active sites for metathesis seem to be W(VI) or Mo(VI) species. Both MoO 3 Al 2 O 3 and WO 3 Al 2 O 3 catalysts are active in parallel reactions such as polymerization, isomerization and cracking. The product distribution for the metathesis reaction differed greatly from that expected. Thus, for all catalysts the ethene/butene ratio is lower than 1 and isobutene was the main component of the C4 fraction, its proportion increasing with the metal oxide content. Studies by IR of adsorbed pyridine showed the Bronsted acid sites generated on MoO3 or WO3 in samples with high metal oxide content to be responsible for the lateral acid-catalyzed reactions.

CO/CO2 hydrogenation and ethylene hydroformylation over silica-supported PdZn catalysts

Silica-supported PdZn catalysts have been studied in CO and CO2 hydrogenation and in ethylene hydroformylation. The dilution of surface Pd by Zn lowers the hydrogenating capability of the catalysts and favours the production of higher hydrocarbons in CO hydrogenation. The catalyst with a molar ratio Pd : Zn = 3 showed an enhanced ability to insert CO into an M–alkyl bond; this catalyst produced higher oxygenates in the CO hydrogenation and was the most active in all reactions studied.

Modification of the surface acidity of γ-alumina

Preparation of the samples of nonprotonated gamma-alumina and protonated -gamma-alumina (for the latter, the addition of standardized solutions of HNO/sub 3/ at different concentrations). The quantitative acid site content was determined by uv spectrophotometry using cyclohexane solutions of pyridine or quinoline as titrants. As soon as the adsorption of titrant upon the solid remained constant formation of a monolayer of base upon the acid sites with equal or lower pKs was assumed. A mathematical expression was used to calculated the results. 10 references, 2 figures.

Selective synthesis of alcohols from syngas and hydroformylation of ethylene over supported cluster-derived cobalt catalysts

Hydroformylation of ethylene and CO hydrogenation were studied over cobalt-based catalysts derived from reaction of Co2(CO)8 with ZnO, MgO and La2O3 supports. At 433 K a similar activity sequence was reached for both reactions: Co/ ZnO > Co/La2O3 > Co/MgO. This confirms the deep analogy between hydroformylation and CO hydrogenation into alcohols. In the CO hydrogenation the selectivity towards alcohol mixture (C1-C3) was found to be near 100% at 433 K for a conversion of 6% over the Co/ZnO catalyst; this catalyst showed oxo selectivity higher than 98% in the hydroformylation of ethylene. Magnetic experiments showed that no metallic cobalt particles were formed at 433 K. It is suggested that the active site for the step that is common to both reactions is related to the surface homonuclear Co2+/[Co(CO)4]− ion-pairing species.

FTIR study of the interaction of CO and CO2 with silica-supported PtSn alloy

Abstract The interaction between CO 2 and the silica-supported PtSn alloy gave rise to an infrared band at 2339 cm −1 which was assigned to the coordination of this molecule to the bimetallic phase. The coadsorption of CO and CO 2 indicates that both compete for similar coordination sites on the supported PtSn alloy. A CO singleton vibration frequency at ca. 2054 cm −1 was determined for both Pt/SiO 2 and PtSn/SiO 2 catalysts. A dilution effect caused by tin in the silica-supported PtSn alloy is proposed. The presence of isolated Pt sites in the particles of this alloy determine their unique capability to coordinate the CO 2 molecule in the active sites.

Simple kinetic-thermometric determination of submicrogram quantities of ruthenium based on its catalytic effect on the Ce(IV)-As(III) reaction

Abstract The study of different variables that affect the catalytic action of ruthenium on the Ce(IV)-As(III) reaction is carried out in order to select the best analytical conditions for its thermometric determination. In the final solution 0.7–41 μg l −1 of Ru(III) or 0.1–2.5 μg l −1 of Ru(VIII) were determined, with very few interferences. The method was applied to determine ruthenium in the K 4 [Ru(CN) 3 ] · 3H 2 O complex and the results are compared with those obtained from a spectrophotometric method.

Bimetallic Pt-M (M=Ga, In, Ti) silica-supported catalysts for isobutane de dehydrogenation

Silica-supported Pt-M (M=Ga, In, Tl) bimetallic catalysts have been prepared and tested for the isobutane dehydrogenation reaction. PtGa and PtIn catalysts showed very high selectivity to isobutene. Under the reaction conditions selected, the 1Pt1Ga(1) catalyst also exhibited very good stability over a period of 70 h on-stream operation. The catalysts were characterized, by transmission electron microscopy, powder X-ray diffraction and photoelectron spectroscopy. The observation of PtM alloyed phases by one or more of these techniques, with dispersion depending on M, Pt loading and Pt/M ratio, appears to be responsible for the catalytic behavior of these systems.

Kinetic-thermometric study of hydrogen peroxide decomposition in basic media catalyzed by Mn(II): Influence of several organic ligands

Abstract A kinetic-thermometric study of the influence of several organic ligands [hydrazine, tiron and triethylenetetramine (TETA)] in the hydrogen peroxide decomposition reaction catalyzed by Mn(II) in basic media is presented. All of these organic ligands form chelates with Mn(II). The reaction rates were determined from the temperature-time curves. The limit of sensitivity obtained was 0.8 μg ml −1 for the reaction catalyzed by hydrazine-Mn(II) complex, 80 ng ml −1 for the reaction catalyzed by the tiron-Mn(II) complex and 0.1 ng ml −1 for the reaction catalyzed by the TETA-Mn(II) complex. We have optimized this last reaction to determine Mn(II) based on the catalytic effect of the [(TETA)-Mn(II) - (OH)] + complex. The linear response range is for 0.1–13 ng ml −1 with a relative standard deviation of 2.5% for 4.5 ng ml −1 of Mn(II). It is known that a lot of metal ions catalyze this reaction, but only Fe(III) interferes at lower levels.

Bimetallic Pd–Zn silica-supported catalyst for CO hydrogenation. In situ DRIFT study

Abstract We report the behaviour of a silica-supported Pd–Zn catalyst in the hydrogenation of CO (total pressure=20 bar, T=548–598 K). Unusual high selectivity to higher hydrocarbons and higher oxygenated products (ethanol and acetaldehyde) was observed. An in-situ DRIFT study under reaction conditions, with analysis by mass spectrometry of products evolved, was carried out. The presence of surface acetate species is related to the production of acetaldehyde, which is proposed to be the primary C2-oxygenated product.

Relationship between surface properties of PtSn–SiO2 catalysts and their catalytic performance for the CO2 and propylene reaction to yield hydroxybutanoic acid†

The catalytic coupling of CO 2 and propylene has been studied on a silica-supported platinum-tin catalyst containing the well-defined PtSn phase. Microcalorimetry of adsorption has been applied to investigate the interaction of the reactants with the surface of the catalyst. When the adsorption properties of the bimetallic catalyst are compared with those of monometallic counterparts, significant differences in the strength and number of adsorption sites concerning CO 2 and propylene are evidenced. The adsorption properties of the bimetallic catalyst have been related to its catalytic behaviour.