van Jhmc Jos Wolput

Hydrogenation of carbon monoxide over rhodium/silica catalysts promoted with molybdenum oxide and thorium oxide

The promotion of silica-supported rhodium catalysts in the hydrogenation of carbon monoxide by molybdenum oxide and thorium oxide has been examined. Temperature programmed reduction studies indicated the formation of rhodium molybdates, while no evidence was found for the formation of such mixed oxides in the thorium oxide-promoted catalysts. Hydrogen and carbon monoxide chemisorption were suppressed by the presence of molybdenum oxide, pointing to a coverage of the rhodium particles by this promoter oxide. The catalysts with MO: Rh ratios exceeding one even exhibited an almost complete suppression of the rhodium chemisorption capacity. In the thorium oxide-promoted catalysts the chemisorption of hydrogen and carbon monoxide were not suppressed. Infrared spectroscopy of adsorbed carbon monoxide showed that molybdenum oxide completely suppressed bridge-bonded and linearly bonded carbon monoxide, as well as the gemdicarbonyl species. Thorium oxide addition resulted in a minor decrease of the linearly bonded carbon monoxide, while the bridge-bonded carbon monoxide was suppressed to a greater extent. The IR spectra of the thorium oxide-promoted catalysts also exhibited a broad absorption band between 1300 and 1750 cm-‘, which is thought to be due to carbon monoxide bonded with the carbon atom to the metal and with the oxygen atom to the promoter ion. Carbon monoxide hydrogenation was greatly enhanced by the presence of both molybdenum oxide and thorium oxide. Thorium oxide-promoted catalysts had a high selectivity to C,-oxygenates, while the molybdenum oxide-promoted catalysts exhibited a high methanol selectivity. Ethylene addition to a working catalyst showed that the carbon monoxide insertion reaction, which is thought to be responsible for the formation of oxygenates, was not enhanced by molybdenum oxide, nor by thorium oxide. The ethylene addition experiments indicated that the role of the promoter is to enhance carbon monoxide dissociation. The results can be understood by assuming that side-bonded carbon monoxide, with its weakened C-O bond, is responsible for the higher carbon monoxide dissociation activity.

Preparation and characterization of vanadium oxide promoted rhodium catalysts

Abstract The location of the promoter element in rhodium on alumina and silica catalysts promoted by vanadium oxide has been studied by various techniques. Our results prove that an intimate contact between the active component, rhodium, and the promoter, vanadium oxide, is present in most catalysts studied. For the silica-supported systems, temperature programmed reduction and diffuse reflectance infrared spectroscopy pointed to the formation of a mixed oxide (RhVO 4 ) during calcination. Reduction of this oxide phase resulted in a vanadium oxide layer on top of the metal particle, as could be concluded from carbon monoxide chemisorption experiments. CO chemisorption was suppressed in the Rh/V 2 O 3 /SiO 2 catalysts, while transmission electron microscopy showed that the rhodium particle size was not influenced by the addition of vanadium oxide. This indicates that the suppression of CO chemisorption is not due to a decrease of metal particle size, but due to covering of the metal particle. Infrared spectroscopy showed that the amount of linearly bonded and bridge-bonded CO was almost completely suppressed, while the amount of gem-dicarbonyl species remained unaffected. No suppression of hydrogen chemisorption was observed. From this and TPD experiments it could be concluded that hydrogen adsorption occurs both on the exposed Rh atoms, as well as on the vanadium oxide patches partly covering the surface Rh atoms, pointing to the formation of hydrogen bronzes. Temperature programmed reduction experiments showed that RhVO4 was not formed in Rh/V 2 O 3 /Al 2 O 3 during calcination. For V/Rh 2 O 3 and V 2 O 3 particles exist separately on the support, due to the strong interaction between V 2 O 5 and Al 2 O 3 . Only for catalysts with a V/Rh value around 7.0 (near-monolayer of vanadium oxide on alumina), oxidation at 898 K resulted in the formation of RhVO 4 . For these catalysts, Rh2O3 might be positioned on top of the vanadium oxide layer after calcination. Almost all adsorbed CO was present in the form of the gem-dicarbonyl species and only a minor suppression of CO adsorption was observed.

An in situ Fourier transform infrared study of zeolitic vibrations : dehydration, deammoniation, and reammoniation of ion-exchanged Y zeolites

Abstract Dehydration, deammoniation, and reammoniation of ion-exchanged Y zeolites are studied in situ withFTi.r. spectroscopy in the range 40–4000 cm−1. This broad spectroscopic range enables the monitoring of the cationic vibrations, the zeolitic lattice modes, the bending and stretching modes of ammonium ions, and adsorbed water molecules and the stretching modes of the acidic hydroxyls under experimental conditions. Dehydration of zeolites with monovalent cations does not lead to significant migration of these cations and only minor changes are observed for the lattice modes. Dehydration of zeolites with divalent cations results in a migration of these cations into the hexagonal prisms, giving rise to large changes for the zeolitic lattice modes. Deammoniation results in the formation of acidic hydroxyls by protonation of the lattice, which strongly affects the zeolitic lattice modes. The extent of the changes for the lattice modes is related to the amount of protons present. The observed changes for the lattice modes can be explained by an increase in electrostatic interactions between the cations and the zeolitic framework, by deformations of the flexible zeolitic lattice, and by coupling of the lattice modes with the in-plane and out-of-plane bending modes of the acidic bridging hydroxyl groups.

Fourier transform infrared and inelastic neutron scattering study of HY zeolites

Abstract A combination of FT i.r. and INS spectroscopy is used in a vibrational study of the bending and stretching vibrations of the acidic hydroxyl groups of Y zeolites. The influence of the number of acidic Bro¨nsted sites and theSi/Al ratio is discussed. Out-of-plane hydroxyl bending modes are assigned to vibrations centered around 419 cm −1 and in-plane hydroxyl bending modes are assigned to vibrations centered around 1089 cm −1 . Upon dealumination, these bands are shifted by approximately 30 cm −1 to lower values. The less intense bands at 319,470,565,765, and 1130 cm −1 are assigned to proton-coupled framework vibrations. Upon dealumination, the mode at 319 cm −1 is shifted to lower frequencies and the modes at 565 and 1130 cm −1 are shifted to higher frequencies.

A vibrational study of the OH and OD bending modes of the Brönsted acid sites in zeolites

Abstract Deuteration of Bronsted acid sites in zeolites and SAPOs results in in-plane OD bending modes that can be studied with FT i.r. spectroscopy. A band maximum near 870 cm −1 and a shoulder near 890 cm −1 are related to the high-frequency (HF) and low-frequency (LF) OH or OD stretching modes observed for zeolites Y and ZK-5 and for SAPO-17 and SAPO-34. For mordenite. zeolite β, ZSM-5, and SAPO-11, only one type of bending mode is observed near 890 cm −1 . Structure type and composition influence the frequencies for these bands. Increasing the sodium content of the samples results in a shift of the fundamentals to lower frequencies. Increasing the Si AI ratio shifts the bands to slightly higher frequencies. The in-plane bending modes are more sensitive to structural differences than are the corresponding stretching modes. The first overtones of the OD bending modes are observed near 1580 cm −1 for the zeolites and near 1540 cm −1 for the SAPOs. Simulations of the inelastic neutron scattering spectra indicate that the in-plane bending modes are localized modes, whereas the out-of-plane bending modes are coupled to the lattice modes. The in-plane OD bending modes are shifted to higher frequencies due to Fermi resonance with the zeolite TO lattice modes. The observed shift is 80 cm −1 for the zeolites and 110 cm −1 for the SAPOs.

The mechanism of the copper ion catalyzed autoxidation of cysteine in alkaline medium

Abstract Quantitative e.s.r. measurements carried out during the copper catalysed alkaline autoxidation of cysteine show that the Cu(II)-dicysteine complex represents almost the total amount of copper. Only a small fraction ( The kinetics of the catalytic reaction obeys the rate expression The results cannot be explained by a simple Cu(II)/Cu(I) redox mechanism. Instead, a reaction model is proposed, which is based upon the involvement of a thiyl- and a superoxo-activated Cu(II) complex, respectively. These two types of complexes are operative in a chainlike propagation cycle.

Accumulation and reactions of H2O2 during the copper ion catalysed autoxidation of cysteine in alkaline medium

The stoichiometry of the copper catalysed alkaline autoxidation of cysteine has been investigated. Products of this oxidation reaction are cystine, H{2}O{2} and H{2}O. No oxygen containing sulfur acids are produced as long as cysteine is present in the reaction liquid.The H{2}O{2} generated reacts with cysteine selectively to form cystine and H{2}O with r{H}{2}{O}{2} = k{H}{2}{O}{2} [CyS][H{2}O{2}] (k{H}{2}{O}{2} = 0.17 1 mol}-{}1{ s}-{}1{).The reaction between H{2}O{2} and cysteine is not catalysed by the copper complex as present during the catalytic autoxidation reaction. On the other hand, experiments carried out in the absence of oxygen show a marked catalytic effect ascribed to the presence of a Cu(I) complex.Accumulation curves of H{2}O{2} have been measured during the copper catalysed autoxidation of cysteine. Information about the rate of production of H{2}O{2} at the catalytic site has been obtained by making use of these accumulation curves together with the kinetic data obtained for the reaction of H{2}O{2} with cysteine. It is concluded that apart from a two electron reduction of dioxygen to H{2}O{2} a four electron reduction to H{2}O should also be taken into account.The selectivity of the oxidation reaction justified a reconsideration of the free thiyl radical mechanism proposed in the literature to occur during the copper catalysed autoxidation of cysteine.

Infrared spectroscopic study of the bending modes of the Brønsted acidic sites in deuterated zeolites

Abstract Deuteration of Bronsted acidic sites in zeolites gives observable infrared in-plane OD bending modes. For zeolite Y and ZK-5, a maximum near 870 cm −1 with a shoulder near 890 cm −1 is observed. These signals are related to, respectively, the high-frequency and the low-frequency stretching modes. Mordenite, zeolite β and ZSM-5 possess a single mode near 890 cm −1 . The zeolite structure type and composition influences the position of the modes strongly. A large anharmonicity parameter, x OD δδ (−80 cm −1 ), is found due to Fermi resonance with the zeolite lattice modes.

The IR transmission windows of hydrogen bonded complexes in zeolites: A new interpretation of IR data of acetonitrile and water adsorption on zeolitic Broensted sites

Abstract The ∼2800, ∼2400 and ∼1700 cm−1 infrared OH bands recently found upon adsorption of many basic molecules on Bronsted sites of zeolites, including acetonitrile and water, are suggested to be analogous to the (A,B,C) triplet of medium-strong molecular X-OH‥Y complexes in vapours, liquids and solids. IR studies of CD3CN and CCI3CN adsorptions on H-ZSM5, H-FeSil, H/D-ZSM5 and H/D-FeSil zeolites show these bands to be in a good agreement with the resonance theory of the (A,B,C) triplet, developed for molecular H-complexes. A new interpretation of IR data of water adsorption on zeolites, using this theory and results of ab initio calculations, suggests the water complex with the bridging OH group of zeolites to be “neutral”, the one water OH group interacting by hydrogen bonds both with the bridging OH group and with the nearest AlOSi oxygen and the other OH group being free.

Extraction of copper(II) and nickel(II) by long-chain aliphatic dioximes

Abstract In the search for a selective nickel extractant symmetric dialkyl dioximes have been studied in the extraction of copper and nickel ions from ammoniaca