By Chromium Catalysts

Appreciable quantities are also obtained as a by-product in the manufacture of hydrogen from naphtha-gaseous hydrocarbons. In this process the gaseous hydrocarbon and superheated steam under a pressure of about 10 atmospheres and at a temperature of 1000 K are passed over a nickel-chromium catalyst. Carbon monoxide and hydrogen are produced ... 

Union Carbide Corp. also uses a siUca-supported chromium catalyst in their extremely low cost Unipol gas-phase linear low density ethylene copolymer process, which revolutionized the industry when it was introduced in 1977 (86—88). The productivity of this catalyst is 10 —10 kg polymer/kg transition metal contained in the catalyst. By 1990, the capacity of Unipol linear low density polyethylene reactors was sufficient to supply 25% of the world s total demand for polyethylene. 

Fig. 1. Examples of the kinetic curves during ethylene polymerization by chromium oxide catalysts. Support—SiOs temperature—80°C polymerization at constant ethylene pressure in perfect mixing reactor. Curve 1—catalyst reduced by CO at 300°C. Curve 2— catalyst activated in vacuum (400°C) polymerization in the case of (1) and (2) in solvent (heptane) ethylene pressure 10 kg/cm2 02 content in ethylene 1 ppm, HsO 3 ppm. Curves 3, 4, 5, 6—catalyst activated in vacuum (400°C) polymerization without solvent ethylene pressure 19 (curve 3), 13 (curve 4), 4 (curve 5), and 2 (curve 6) kg/cm2 02 content in ethylene 1 ppm, HsO = 12 ppm.

The change of shape of the kinetic curves with monomer and inhibitor concentration at ethylene polymerization by chromium oxide catalysts may be satisfactory described 115) by the kinetic model based on reactions (8)-(14). 

The multi-component procedure is also effective for the chromium-catalyzed addition of organic halides to aldehydes (the Nozaki-Hiyama-Kishi reaction) [73]. The active Cr(II) species is recycled by redox interaction with Mn powder as the stoichiometric co-reductant in the presence of MesSiCl (Scheme 34), which mainly liberates the chromium catalyst from the alkoxide adduct. The chemo- and diastereo-selective addition reaction is performed with a variety of organic halides and alkenyl triflates. In the case of crotyl bromide, the addition is highly stereoconvergent, i.e., the respective anti-... 

The initial rate of polymerization of methyl methacrylate initiated by chromium allyls (12) in toluene showed identical dependences on monomer and catalyst concentrations, as Zr(benzyl)4 initiated polymerization of styrene. Some data for the monomer dependence are shown in Fig. 14. 

The mechanism proposed for the polymerization of styrene initiated by Zr (benzyl) 4 differs from that proposed for the polymerization of methyl methacrylate initiated by chromium allyls (12, 44)- In these papers it was considered that the concentration of complex (I) was comparable to the catalyst concentration, a fact which now seems unlikely in view of the... 

Chromium zeolites are recognised to possess, at least at the laboratory scale, notable catalytic properties like in ethylene polymerization, oxidation of hydrocarbons, cracking of cumene, disproportionation of n-heptane, and thermolysis of H20 [ 1 ]. Several factors may have an effect on the catalytic activity of the chromium catalysts, such as the oxidation state, the structure (amorphous or crystalline, mono/di-chromate or polychromates, oxides, etc.) and the interaction of the chromium species with the support which depends essentially on the catalysts preparation method. They are ruled principally by several parameters such as the metal loading, the support characteristics, and the nature of the post-treatment (calcination, reduction, etc.). The nature of metal precursor is a parameter which can affect the predominance of chromium species in zeolite. In the case of solid-state exchange, the exchange process initially takes place at the solid- solid interface between the precursor salt and zeolite grains, and the success of the exchange depends on the type of interactions developed [2]. The aim of this work is to study the effect of the chromium precursor on the physicochemical properties of chromium loaded ZSM-5 catalysts and their catalytic performance in ethylene ammoxidation to acetonitrile. 

Hitachi Cable Ltd. (35) has claimed that dehydrogenation catalysts, exemplified by chromium oxide—zinc oxide, iron oxide, zinc oxide, and aluminum oxide—manganese oxide inhibit drip and reduce flammability of a polyolefin mainly flame retarded with ATH or magnesium hydroxide. Proprietary grades of ATH and Mg(OH)2 are on the market which contain small amounts of other metal oxides to increase char, possibly by this mechanism. 

Recently, a multistep synthesis of ( )-deplancheine was developed by Rosen-mund and Casutt (120), starting from tryptamine and coumalic ester. In the key step of this approach, 1,4-addition of hydrogen to diene 179 was achieved with full stereoselectivity by means of hydrogen in the presence of toluenetricarbonyl-chromium catalyst. 

Stoichiometric hexavalent chromium oxidants have been used for a variety of oxidation reactions but, due to environmental problems of chromium-containing waste, catalytic versions with soluble chromium catalysts have been developed, for example by Muzart ° using mainly TBHP as oxygen source. For instance, Muzart and coworkers... 

When controlled nitridation of surface layers is required, as for example in the modification of the chemical properties of the surface of a support, the atomic layer deposition (ALD) technique can be applied." This technique is based upon repeated separate saturating reactions of at least two different reactants with the surface, which leads to the controlled build-up of thin films via reaction of the second component with the chemisorbed residues of the first reactant. Aluminium nitride surfaces have been prepared on both alumina and silica supports by this method wherein reaction cycles of trimethylaluminium and ammonia have been performed with the respective supports, retaining their high surface areas." This method has been applied to the modification of the support composition for chromium catalysts supported on alumina." ... 

Puurunen et al. have investigated the effect of surface nitridation of y-AI2O3 supports by the atomic layer deposition process on the activity of chromium catalysts for isobutane dehydrogenation. Nitridation was observed to suppress activity and it was argued that oxide ions were more active for the dissociation of isobutane. 

Another property of the iron-defective molybdate is the presence of Mo=0 double bonds on the surface. The hydrogen-abstracting capacity of the catalyst is closely related to Mo6 contained in the Mo=0 as is shown in Sect. 3. There the role of iron is also discussed. It is, however, interesting to note here that pure iron oxides accelerate combustion and that a W03—Fe2(W04)3 catalyst is practically inactive [254], Replacement of iron by chromium is possible but leads to a lower activity [253]. Baussart et al. [46] prepared stoichiometric NiMo04 which showed selective behaviour towards formaldehyde in a pulsed column below 375°C. 

Numerous catalysts have been investigated chromium catalysts 286-289 on various substrates (used in the vapor phase and activated by traces of various compounds), copper carbon oxysulfide,293-295 catalysts based... 

Therefore, in such heterogeneous polymerizations, almost all industrial catalysts are supported, for example on silica, whereas the typical Ziegler s titanium catalysts are by definition supported on magnesium chloride. These catalysts are adsorbed at the surface or incorporated into the crystal structure of the support. Other catalysts, such as Phillips chromium catalysts, can be coupled at the support surface by a chemical bond. 

Raney nickel catalysts, unpromoted or doped with molybdenum or chromium, were prepared from the precursor alloys of the type Ni A13. The structure and phase composition of the catalysts have been deternfmetl. Hydrogenation of valeronitri le at 90°C and 1.6 MPa in cyclohexane was performed to evaluate catalyst activities and the relative amounts of amines formed. Doping catalysts by chromium improved reaction rates and yields of primary amine, whereas molybdenum addition was ineffective. 

The principal elements deriving from the construction materials of exhaust system and catalyst can are iron, nickel, chromium, and copper. Iron is the major component of the debris retained by the catalyst nickel and chromium are usually components used to fabricate high-temperature materials for thermal reactors incorporated in some systems upstream of the catalyst. Copper may originate in engine bearings or in the copper lines used for air injection. As it is known that metals often cause deterioration of the high catalytic activity of platinum, all of them must be regarded as potential poisons. 

Gibson and workers [27] at BP have reported a high-throughput approach directed towards the optimization of chromium polyethylene catalysts. The study aimed to improve the performance of a previously discovered chromium catalyst. In this study, the ca. 200-member hemi-salen ligand library was complexed in situ to a soluble chromium precursor [p-tolylCrCl2(thf)3], followed by addition of 180 equiv. of MAO, and exposure to 1 atm of ethylene for 15 min. Relative activities were determined from the polyethylene yields, and a new highly active chromium catalyst was uncovered. The Coates group has reported a clever approach to the... 

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