Chromium incorporation study

Cr-ZSM-5 catalysts prepared by solid-state reaction from different chromium precursors (acetate, chloride, nitrate, sulphate and ammonium dichromate) were studied in the selective ammoxidation of ethylene to acetonitrile. Cr-ZSM-5 catalysts were characterized by chemical analysis, X-ray powder diffraction, FTIR (1500-400 cm 1), N2 physisorption (BET), 27A1 MAS NMR, UV-Visible spectroscopy, NH3-TPD and H2-TPR. For all samples, UV-Visible spectroscopy and H2-TPR results confirmed that both Cr(VI) ions and Cr(III) oxide coexist. TPD of ammonia showed that from the chromium incorporation, it results strong Lewis acid sites formation at the detriment of the initial Bronsted acid sites. The catalyst issued from chromium chloride showed higher activity and selectivity toward acetonitrile. This activity can be assigned to the nature of chromium species formed using this precursor. In general, C r6+ species seem to play a key role in the ammoxidation reaction but Cr203 oxide enhances the deep oxidation. 

Chromium Incorporation Basic Concepts and the YAlOi-YCrOi Case Study 1269... 

Chromium Incorporation Basic CorKepts and the YAlOr-YCrOs Case Study 1273... 

Liu L, Li H (2006) A comparative study on catalytic performances of chromium incorporated and supported mesoporous MSU-x catalysts for the oxidehydrogenation of ethane to ethylene with carbon dioxide. Catal Today 115 235-241... 

During the course of their studies on the chromium-mediated [6 + 2]- and [6 + 2 + 2]-reactions, the Rigby group uncovered a new four-component [6 + 2 + 2 + 2]-cycloaddition.167 When the two terminal alkynes are connected by three methylene units, an anticipated [6 + 2 + 2]-cycloaddition occurs in a moderate yield. Surprisingly, when the alkynes are tethered by four or five methylenes, a third alkyne is incorporated in an overall [6 + 2 + 2 + 2]-process (Scheme 71). The authors propose that the [6 + 2 + 2 + 2]-cycloaddition products arise from a [3 + 2] reaction of an alkyne with the initially formed [6 + 2 + 2]-reaction products (Scheme 72). These reactions greatly increase structural complexity by stereoselec-tively converting four achiral components to a pentacyclic product with six contiguous stereocenters. 

Tons of spent abrasives result each year from the removal of old paint from bridges. Because the spent abrasives contain metals from the paint, some spent abrasives may be considered hazardous by the Toxicity Characteristic (TC) criteria. Incorporation of the spent blasting abrasives in nonstructural concrete offers an opportunity to recycle the spent abrasives while immobilizing potentially leachable metals. Solvents that are less aggressive will mean that abrasives will be necessary in some applications. Study results indicated that considerable quantities of these spent abrasives can be solidified/stabilized while reducing the leachability of cadmium, chromium, and lead and producing compressive strengths over 6895 kN/m2 (1000 psi) (Brabrand and Loehr, 1993). 

Melanin granules are secreted by melanocytes in the hair papilla and distributed to keratin in the hair cortex and inner layers of the hair sheath during normal development. Melanogenesis is subject to hormonal control and has been the focus of intensive genetic studies. Two main forms of melanin exist in human skin—eumelanin and phaeomelanin, both of which are derived from tyrosine through the action of tyrosinase (a cupro-enzyme) and possibly other key enzymes (with nickel, chromium, iron, and manganese as cofactors). Tyrosine is converted to dihydroxyphenylalanine and, via a series of intermediate steps, to indole-5,6-quinone, which polymerizes to eumelanin. Phaeomelanins are produced by a similar mechanism but with the incorporation of sulfur (as cysteine) by a nonenzymatic step in the oxidation process. 

Aluminophosphates (A1P04) were discovered in 198248 and a large amount of research has been directed towards the incorporation of various elements into the framework of these molecular sieves 49 A particular area of study is the oxidation of primary and secondary alcohols to the corresponding carbonyl compounds, which are useful synthetic intermediates. Traditionally, alcohol transformations are performed with stoichiometric chromium(VI) reagents.50 However, due to environmental problems associated with chromium-containing effluent, attention has focused on the use of chromium in conjunction with oxidizing agents such as tert-butyl hydroperoxide.51 Sheldon and co-workers... 

Direct evidence for a combination of catalytic fluorination and chlorination [4] was obtained from radiotracer studies in which fluorinated chromia catalysts were labelled with the short-lived (t /2 = 110 min) / + emitting isotope fluorine-18 [11]. Using this isotope it was possible to probe the interactions between HF and various fluorinated chromia catalysts more directly than had been possible hitherto. Three types of surface F-containing species were differentiated, weakly adsorbed HF which was easily removed by an inert gas flow, non-labile F, believed to be bound directly to surface Crin, and catalytically active F which could be incorporated into the organic products [12]. The controversy between dismutation (concerted F-for-Cl and Cl-for-F transfers) and non-concerted halogen exchange processes has been resolved more recently and the evidence is described later in the chapter. What is clear from this early work however, is the importance of aluminium and chromium(III) oxides as catalyst precursors. Fluorination of the surfaces of these oxides is slow (cf [12]) and although there are many references to alu-... 

Chromium. So-called stainless steels contain in their alloys elements soeh as Cr, Ni, Co, and Mo) that eventually become incorporated into the passive oxide films. Cr(III), as shown by EXAFS studies with hydrous ferric acxie and goethite (18), forms inner-sphere surface complexes in the form of... 

Cuadrado and co-workers have reported the synthesis of chromium-containing organosilicon dendrimers.334 These dendrimers, 282, have chromium tricarbonyl units incorporated pendent to the terminal aromatic rings.334 Synthesized via the reaction of the silane dendrimer precursor with chromium hexacarbonyl, complete complexation was not possible due to steric hindrance at higher generations. Electrochemical studies showed that the oxidation of the chromium atoms occurred reversibly in the absence of a nucleophilic species and that the chromium tricarbonyl units behaved as isolated redox centers. 

Chromium diffusion and reduction in soil microsites. The X-ray microprobe is useful for in-situ studies of redox reactions within soil columns. T. Tokunaga (LBNL) has pioneered this approach for study of heterogeneous reactions at small scales in soils and has developed mini-soil columns with X-ray transparent windows that incorporate electrodes for redox potential measurement (Fig. 24). 

Accelerated ageing of the catalysts on line was performed by heating to 800°C for 120 h. This treatment caused the catalyst activities to drastically decrease. The best catalyst was still the chromium-promoted system but conversion decreased to 40% at 450°C. The performance of the vanadium catalyst after ageing was poor and the authors attribute this to the melting point of vanadium (690°C), which caused loss of the promoter from the catalyst. It is also evident that the BET surface area of the vanadium aged catalyst decreased by 84% from the initial value. However, these studies indicate that the incorporation of other metals with platinum may have a beneficial effect for complete oxidation activity. 

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