Niobium phosphates as new highly selective catalysts for the oxidative dehydrogenation of ethane

Several niobium phosphate phases have been prepared, fully characterized and tested as catalysts for the selective oxidation of ethane to ethylene. Three distinct niobium phosphate catalysts were prepared, and each was comprised predominantly of a different bulk phase, namely Nb(2)P(4)O(15), NbOPO(4) and Nb(1.91)P(2.82)O(12). All of the niobium phosphate catalysts showed high selectivity towards ethylene, but the best catalyst was Nb(1.91)P(2.82)O(12), which was produced from the reduction of niobium oxide phosphate (NbOPO(4)) by hydrogen. It was particularly selective for ethylene, giving ca. 95% selectivity at 5% conversion, decreasing to ca. 90% at 15% conversion, and only produced low levels of carbon oxides. It was also determined that the only primary product from ethane oxidation over this catalyst was ethylene. Catalyst activity also increased with time-on-line, and this behaviour was ascribed to an increase of the concentration of the Nb(1.91)P(2.82)O(12) phase, as partially transformed NbOPO(4), formed during preparation, was converted to Nb(1.91)P(2.82)O(12) during use. Catalysts with predominant phases of Nb(2)P(4)O(15) and NbOPO(4) also showed appreciable activity and selectivities to ethylene with values around 75% and 85% respectively at 5% ethane conversion. The presence of phosphorous is required to achieve high ethylene selectivity, as orthorhombic and monoclinic Nb(2)O(5) catalysts showed similar activity, but displayed selectivities to ethylene that were <20% under the same reaction conditions. To the best of our knowledge, this is the first time that niobium phosphates have been shown to be highly selective catalysts for the oxidation of ethane to ethylene, and demonstrates that they are worthy candidates for further study.     

Phosphated ceria,selective catalysts for oxidative dehydrogenation of isobutane

The oxidative dehydrogenation (ODH) of isobutane over pure ceria and phosphated ceria catalysts, containing two different amounts of phosphorus, was examined at temperatures ranging from 450 to 610 °C. The catalysts were characterized using nitrogen adsorption, DRX, SEM, EDX, XPS and TPR techniques. Adding phosphorus to ceria and increasing the phosphorus content results in a modification of the physicochemical characteristics of the catalyst, the redox ability of the catalytic material being strongly diminished. At the same time, by adding phosphorus to ceria and increasing the phosphorus content, a decrease of the catalytic activity accompanied by an important increase of the selectivity for isobutene, mainly at the expense of carbon oxides, was observed. A compensation effect in catalysis was also observed for the isobutane conversion on this series of catalysts.     

The selective oxidative dehydrogenation of ethane over hydrothermally synthesised MoVTeNb catalysts

Mo-V-Te-Nb metal oxide catalysts prepared by hydrothermal synthesis and heat-treated in N2 at high temperatures (600-700 degrees C) show high activity and selectivity for the oxidative dehydrogenation of ethane to ethene. Yields of ethene of 75% have been obtained at 400 degrees C on the best catalysts.     

Single-chain polymers as homogeneous oxidative photoredox catalysts

Single-chain polymer nanoparticles (SCNPs) are emerging as versatile catalytic platforms that provide excellent control over solubility. The confined nature of SCNPs can improve the rate of catalysis. While significant headway has been made in thermally-induced transition-metal catalysis with SCNPs, light-activated SCNP catalysts have received little attention. We are developing triarylpyrylium tetrafluoroborate (TPT)-functionalized SCNPs as oxidative photocatalysts. Herein, we comprehensively study the impact of light source on both SCNP compaction and TPT absorbance through gel-permeation chromatography and UV/Vis spectroscopy. We observe that compaction is expedited using light sources that excite the photocatalyst (e.g., blue LEDs), which is attributed to the ability of TPT to dimerize sytrenics under similar photoredox conditions. The resultant metal-free SCNP photocatalysts enable the oxidation of benzyl alcohols in good yields. The SCNP is further investigated for the amidation of 4-bromobenzaldehyde, wherein it affords higher yields of the benzamide product compared to both small-molecule and unfolded polymer controls. We attribute the combined results to the colocalization of the TPT photoredox catalyst and pyrene electron relay within the SCNP, which likely aids in single-electron transfer processes. The scope of amidation reactions was also extended to other aryl aldehydes, wherein deactivated substrates afforded the highest yield of the desired amide.     

Bulky diarylammonium arenesulfonates as selective esterification catalysts

More environmentally benign alternatives to current chemical processes, especially large-scale, fundamental reactions such as ester condensations, are highly desirable for many reactions. Bulky diarylammonium pentafluorobenzenesulfonates and tosylates serve as extremely active dehydration catalysts for the ester condensation reaction of carboxylic acids with equimolar amounts of sterically demanding alcohols and acid-sensitive alcohols. Typically, the esterification reaction is performed in heptane by heating at 80 degrees C in the presence of 1 mol % of the catalyst without removing water. Esterification with primary alcohols proceeds without solvents even at room temperature. Furthermore, 4-(N-mesitylamino)polystyrene resin-bound pentafluorobenzenesulfonate can be recycled more than 10 times without activity loss.     

Clays as selective catalysts in organic synthesis

Suitably modified smectite clays can be very selective catalysts for a wide range of organic reactions. While it has long been known that such materials can act as Bronsted and Lewis acids, it has been shown recently that they are also effective Diels-Alder catalysts. A selection of illustrative reactions is given which emphasises their wide range of use, their selectivity, and the ease of work-up after reaction. In each case, mechanistic information is presented, e.g., on the site of reaction (whether interlayer or surface), rate determining steps, etc. The regiochemical consequences of the restricted reaction space are stressed.Based on material presented at the Fourth International Symposium on Inclusion Phenomena and the Third International Symposium on Cyclodextrins, Lancaster, U.K., 20–25 July 1986.     

Pd-Ga intermetallic compounds as highly selective semihydrogenation catalysts

The intermetallic compounds Pd(3)Ga(7), PdGa, and Pd(2)Ga are found to be highly selective semihydrogenation catalysts for acetylene outperforming established systems. The stability of the crystal and electronic structure under reaction conditions allows the direct relation of structural and catalytic properties and a knowledge-based development of new intermetallic catalyst systems. In the crystal structure of PdGa palladium is exclusively surrounded by gallium atoms. The alteration of the Pd coordination in PdGa leads to a strong modification of the electronic structure around the Fermi level in comparison to elemental Pd. Electronic modification and isolation of active sites causes the excellent catalytic semihydrogenation properties.     

Determination of the thiol redox state of organisms: new oxidative stress indicators

This study describes a new methodology by which the concentrations of non-protein (NP) thiols glutathione (GSH), cysteine (CSH), N-acetylcysteine (AcCSH), and protein (P) thiols (PSH), as well as the contribution of these components to symmetric and mixed disulfides (NPSSR, NPSSC, NPSSCAc, PSSR, PSSC, PSSCAc, PSSP) can reliably be measured. The methodology consists of a strict sequence of methods which are applied to every sample. Free thiols at any given state of the procedure are measured by Ellmans assay, the CSH fraction is measured by its unique response in the ninhydrin assay, AcCSH is selectively measured with ninhydrin after enzymatic deacylation, proteins are separated from non-protein thiols/disulfides by precipitation with trichloroacetic or perchloric acid, disulfides are reduced into free thiols with borohydride, mixed disulfides between a protein and a non-protein component are measured by extracting the non-protein thiol from the protein pellet after borohydride treatment, and protein thiols/disulfides are measured after resolubilization of the protein pellet.When this method was applied to animal and fungal tissue, new molecular indicators of the thiol redox state of living cells were identified. The findings of the present study clearly show that the new parameters are very sensitive indicators of redox state, while at the same time the traditional parameters GSH and GSSG often remain constant even upon dramatic changes in the overall redox state of biological tissue. Therefore, unbiased assessment of the redox state also requires explicit measurement of its most sensitive thiol indicators.Electronic Supplementary Material Supplementary material is available in the online version of this article at . A link in the frame on the left on that page takes you directly to the supplementary material.     

Natural calcium minerals as catalysts of oxidative conversion of methane

The studies of the influence of the kind of natural calcium mineral and the kind and amount of sodium promoter on the conversion degree of methane and oxygen, selectivity and yield of the oxidative conversion of methane have shown the usability of limestone as a basic component of the catalyst for this process.     

Superhydrophobic materials as efficient catalysts for hydrocarbon selective oxidation

A new type of superhydrophobic material, FP-Co-SiO(2) was prepared with organic groups immobilized on the surface of the SiO(2)-based nanocomposite. This material showed much higher catalytic activity for selective oxidation of hydrocarbons than an equivalent hydrophilic catalyst.     

Novel vanadium phosphate phases as catalysts for selective oxidation

In our effort to induce novel modifications in the structure of some important vanadium phosphate phases used as selective oxidation catalysts, it has been observed that metal ions such as Zn²⁺, Ni²⁺, Pd²⁺can be incorporated into the vanadyl hydrogen phosphate VOHPO₄0.5H₂O phase in very different ways depending upon the medium of preparation. It has been found that the metal ions are either substituted into the lattice with retention of structure of the parent compound or intercalated between the layers of a new mixed-valent phase. These new metal-incorporated phases are catalytically active and the palladium incorporated compound in particular displays shape selective catalysis for different oxidation and reduction reactions. In another approach, the preparation of VOHPO₄0.5H₂O) has been modified to give a novel crystalline phase containing mixed-valentvanadium and having NH₃ species bound to the lattice. This phase could be a potential catalyst for ammoxidation reactions. In addition, novel mesostructured vanadium phosphate phases have been prepared using a long-chain amine as the templating agent involving a ligand templating mechanism of formation.     

Structured polyphenylenes as carriers of palladium nanoparticles used as selective hydrogenation catalysts

Structured polyphenylenes that are used as matrices for immobilization of palladium nanoparticles are synthesized through the cyclocondensation of acetylaromatic compounds followed by structuring at different temperatures and, as a result, different crosslink densities. The relationship between the structure and structuring temperature of the polymers is investigated. It is shown that the sizes of palladium nanoparticles immobilized in polyphenylene matrices depend on the conditions of polymer structuring. The resulting catalytic systems are examined for the selective hydrogenation of triple bonds in acetylene alcohols.     

Partially selective semiconductor redox electrodes

Redox electrodes made of passivated metals of the subgroups IV to VI of the periodic system with a surface coating of n-conducting metal oxides, generated by oxidizing agents or anodic oxidation, are well suited for measuring the concentration of oxidizing agents. Neither oxygen nor reducing agents affect the measurements. There is a correlation between the mean conductivity of the passive layer and its selectivity. The aged n-conducting passive layers permit reproducible potential settings, and the occurring changes in potential are greater than what is to be expected according to the Nernst equation. The potentiometric measuring of nitrosyl ions and chlorine is used as an example to demonstrate the selectivity of these electrodes.     

Multifunctional design of single-atom catalysts for multistep reactions

With the merits of high atom utilization, low cost, unique and tunable microstructures, as well as the particular catalytic behaviors, single-atom catalysts(SACs) have gained worldwide interest and achieved great advancements in heterogeneous catalysis recently. However, catalyzing an intricate multistep reaction is usually challenging for one single-atom center, in which case multiple catalytic sites are needed. In this review, the experimental and computational advances in the construction str...     

Preparation and redox properties of N,N,N-1,3,5-trialkylated flavin derivatives and their activity as redox catalysts

Eight different flavin derivatives have been synthesized and the electronic effects of substituents in various positions on the flavin redox chemistry were investigated. The redox potentials of the flavins, determined by cyclic voltammetry, correlated with their efficiency as catalysts in the H2O2 oxidation of methyl p-tolyl sulfide. Introduction of electron-withdrawing groups increased the stability of the reduced catalyst precursor.     

Highly selective oxidative cross-coupling of 2-naphthol derivatives with chiral copper(I)-bisoxazoline catalysts

The asymmetric oxidative coupling reaction of 3-hydroxy-2-naphthoate and 2-naphthol derivatives with the CuCl-(S)-(−)-2,2′-isopropylidenebis(4-phenyl-2-oxazoline) catalyst under an O₂ atmosphere was carried out. The reaction proceeded in a highly cross-coupling selective manner (?99.7%) with a moderate enantioselectivity of up to 65%.     

Pd cluster nanowires as highly efficient catalysts for selective hydrogenation reactions

Palladium is a key catalyst invaluable to many industrial processes and fine-chemical synthesis. Although recent progress has allowed the synthesis of Pd nanoparticles with various shapes by using different techniques, the facile synthesis of Pd nanocrystals and turning them into a highly active, selective, and stable catalyst systems still remain challenging. Herein, we report the highly selective one-pot synthesis of monodisperse Pd cluster nanowires in aqueous solution; these consist of interconnected nanoparticles and may serve as highly active catalysts because of the enrichment of high index facets on the surface, including {443}, {331}, and {221} steps. For the first time, carbon nanotube and γ-Al(2)O(3) immobilized Pd cluster nanowires showed highly enhanced catalytic performance in the liquid-phase selective hydrogenation of cinnamaldehyde and gas-phase hydrogenation of 1,3butadiene relative to immobilized Pd icosahedra and nanocubes, as well as commercial Pd catalysts.     

Pretreatment-induced redox processes in molybdena-alumina metathesis catalysts

The ESR investigation of MoO₃/Al₂O₃ metathesis catalysts was carried out and the results interpreted in relation to the previous XPS study [1]. The increase of the Mo(V) signal upon treatment with argon was attributed to the oxidation of ESR-silent Mo(IV) centers. The enhancement of Mo(V) signal upon exposure to propene at 303 K was assigned to the reduction of Mo(VI). This revised version was published online in June 2006 with corrections to the Cover Date.