Iron Catalysis in Organic Synthesis

Phthalocyanine Metal Complexes in Catalysis

The article discusses oxidation catalysis by substitutional cation doping of binary oxides. Substitutional cation doping is not the only possibility. One can imagine that replacing some anions with other anions may also be beneficial. There is evidence that the presence of small amounts of halogen in the feed or on the oxide surface improves its catalytic activity. It is very likely that doped oxide catalysts have been used before the concept was formulated explicitly. Most oxide catalysts have low levels of impurities that may be substitutional dopants. If they segregate at the surface, they can affect the catalytic activity without our knowledge even though their net concentration is very low. It is also possible that the 'as-prepared' catalyst is a doped oxide that, under reducing reaction conditions, is converted to very small metallic dopant clusters supported on the host oxide. The physical and chemical properties of such clusters are different from those of a bulk metal, and it is difficult to distinguish them from a doped oxide.

Catalysis by doped oxides.

A comprehensive review of recent advances in the field of oxygen reduction electrocatalysis utilizing nonprecious metal (NPM) catalysts is presented Progress in the synthesis and characterization of pyrolyzed catalysts, based primarily on the transition metals Fe and Co with sources of N and C, is summarized Several synthetic strategies to improve the catalytic activity for the oxygen reduction reaction (ORR) are highlighted Recent work to explain the active-site structures and the ORR mechanism on pyrolyzed NPM catalysts is discussed Additionally, the recent application of Cu-based catalysts for the ORR is reviewed Suggestions and direction for future research to develop and understand NPM catalysts with enhanced ORR activity are provided

Nonprecious Metal Catalysts for Oxygen Reduction in Heterogeneous Aqueous Systems

As a result of the adaptation of life to an aerobic environment, nature has evolved a panoply of metalloproteins for oxidative metabolism and protection against reactive oxygen species. Despite the diverse structures and functions of these proteins, they share common mechanistic grounds. An open-shell transition metal like iron or copper is employed to interact with O2 and its derived intermediates such as hydrogen peroxide to afford a variety of metal–oxygen intermediates. These reactive intermediates, including metal-superoxo, -(hydro)peroxo, and high-valent metal–oxo species, are the basis for the various biological functions of O2-utilizing metalloproteins. Collectively, these processes are called oxygen activation. Much of our understanding of the reactivity of these reactive intermediates has come from the study of heme-containing proteins and related metalloporphyrin compounds. These studies not only have deepened our understanding of various functions of heme proteins, such as O2 storage and trans...

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

The oxidation state and local geometry of tellurium in MoVTeNbO catalysts used in the ammoxidation or oxidation of propane were characterized by X-ray absorption, Mossbauer, and X-ray photoelectron (XPS) spectroscopies. The results obtained by Mossbauer, and XPS spectroscopies showed that the catalysts contained Te(IV) in the bulk and mainly Te(VI) at the surface. X-ray absorption fine structure (EXAFS) measurements allowed to determine that the tellurite entities corresponded to TeO4E trigonal bipyramid in the hexagonal phase and to TeO3E somewhat distorted trigonal pyramid in the orthorombic one. The completely determined environment appeared to correspond in both phases to crystallographic sites in hexagonal channels. These results allowed to determine the stoichiometries of the two phases which are TeM3O10 for the hexagonal phase and Te2M20O57 for the orthorombic phase (M=Mo, V, Nb). 125Te Mossbauer isomer shift and pre-peak surface of the X-ray absorption spectra of the Te LIII-edge have been correlated and contributions of the Te(5s) and Te(2p3/2) to the structure have been analyzed.

Characterization of tellurium in MoVTeNbO catalysts for propane oxidation or ammoxidation

Ni–Nb–O catalysts with various Nb contents (0–19%) have been studied as catalysts for the oxidative dehydrogenation of ethane. An optimum ethane conversion of 33% with 78% of selectivity to ethylene was obtained for a Ni–Nb–O catalyst containing 15% of Nb at 350 °C with a weight to flow W/F ratio of 0.54 g s mL−1. The catalysts have been characterized using various techniques allowing to show that the efficiency of the catalysts was related to a surface interaction between the NiO active phase and an amorphous Nb2O5 phase. Because ammonium oxalato niobate was used as niobium precursor, oxalate ions were present; concomitantly to the surface interaction described, the thermal decomposition of these oxalate ions induced a reduction in NiO oxygen over-stoichiometry which also influences the catalytic properties. It was demonstrated that only a small percent of Nb was necessary to have these effects and the optimal activity observed for 15% of Nb had to be related to another process which corresponds to the decrease in the active phase particle size induced by the amorphous Nb2O5 phase; this phase prevents the sintering of the active phase particles during the thermal transformation of the catalysts precursors. It was also shown that the thermal decomposition of oxalates limits NiO crystal growth. Above 15% of Nb the amorphous phase reacted with the nickel oxide to form an inactive mixed niobium and nickel oxide phase NiNb2O6. The study of the catalytic properties over long periods of time showed that the Ni–Nb–O catalysts underwent a slow but systematic deactivation. This deactivation has been attributed both to the continuous reduction of the NiO oxygen over-stoichiometry in the reaction conditions and to the formation of the NiNb2O6 phase thermodynamically stable at the catalytic reaction temperature. Attempt to regenerate the catalysts has failed.

Ni-Nb-O catalysts for ethane oxidative dehydrogenation

Study of cesium or cesium-transition metal-substituted Keggin-type phosphomolybdic acid as isobutane oxidation catalysts. I. Structural characterization

Synergetic effect between phases in MoVTe(Sb)NbO catalysts used for the oxidation of propane into acrylic acid

The acid–base properties of fresh and used niobium-zirconium mixed oxide catalysts, used for the dehydration of glycerol to acrolein, have been characterized by various techniques. These techniques include ammonia thermo-programmed desorption (TPD), infrared spectroscopy of absorbed pyridine, and adsorption microcalorimetry of ammonia and sulfur dioxide. Relationships between the catalytic properties and the acid–base properties of fresh catalysts have been investigated. The most efficient catalysts were shown to be those for which the zirconia support had a better niobium oxide species covering, but no specific relationship could be established between the acid–base, and the selectivity to acrolein. Characterization of the used catalysts showed that the acidic properties of the catalysts had changed considerably with the time spent on stream. No further strong acid sites, and only weak or very weak acid sites were detected. A linear relationship between the total quantity of remaining acidic sites and the rate of glycerol conversion was determined, taking into account an intrinsic activity of the stronger sites, which is more than ten times that of the weaker ones. The deactivation of the catalysts as a function of time on stream has been related to the formation of cyclic molecules, produced by the reaction of acrolein with by-products resulting from the decomposition of hydroxyacetone and also possibly acrolein and glycerol.

Jean-Marc M. Millet

Papers

Characterization of tellurium in MoVTeNbO catalysts for propane oxidation or ammoxidation

Ni-Nb-O catalysts for ethane oxidative dehydrogenation

Study of cesium or cesium-transition metal-substituted Keggin-type phosphomolybdic acid as isobutane oxidation catalysts. I. Structural characterization

Synergetic effect between phases in MoVTe(Sb)NbO catalysts used for the oxidation of propane into acrylic acid

Acid-base properties of niobium-zirconium mixed oxide catalysts for glycerol dehydration by calorimetric and catalytic investigation