Zirconium reactions

It is convenient to consider reactor accidents alongside weapon explosions so that the release of fission products can be compared, but the mode of dispersion is quite different. The configuration and thermal capacity of power reactors are such that bomb-like explosions are not possible. In the Chernobyl accident, nuclear overheating, a steam explosion and steam/zirconium reactions all contributed to the disruption of the reactor (U.S.S.R. State Committee, 1986), but the longdistance environmental effects were due to the subsequent releases of fission products from the damaged reactor. 

Additional analysis may have to take into account another mechanism of cladding damage, i.e. the acceleration of the steam-zirconium reaction at temperatures above 800°C. At 1000°C and above, the heat release from zirconium oxidation in steam becomes commensurate with the residual heat release in the fuel. 

It should be noted that stainless steel also undergoes a very similar reaction wifli steam to produce hydrogen and release energy. The stoichionoetry of the stainless steel reaction with water is identical to the reaction above, except that Zr is replaced with Fe. However, the energy released from a stainless steel and steam reaction is about lOx lower than the zirconium reaction and hence generally less dangerous (Knief, 2008). 

Figure Bl.25.9(a) shows the positive SIMS spectrum of a silica-supported zirconium oxide catalyst precursor, freshly prepared by a condensation reaction between zirconium ethoxide and the hydroxyl groups of the support [17]. Note the simultaneous occurrence of single ions (Ff, Si, Zr and molecular ions (SiO, SiOFf, ZrO, ZrOFf, ZrtK. Also, the isotope pattern of zirconium is clearly visible. Isotopes are important in the identification of peaks, because all peak intensity ratios must agree with the natural abundance. In addition to the peaks expected from zirconia on silica mounted on an indium foil, the spectrum in figure Bl. 25.9(a)...

Other catalysts which may be used in the Friedel - Crafts alkylation reaction include ferric chloride, antimony pentachloride, zirconium tetrachloride, boron trifluoride, zinc chloride and hydrogen fluoride but these are generally not so effective in academic laboratories. The alkylating agents include alkyl halides, alcohols and olefines. 

The equilibrium is more favorable to acetone at higher temperatures. At 325°C 97% conversion is theoretically possible. The kinetics of the reaction has been studied (23). A large number of catalysts have been investigated, including copper, silver, platinum, and palladium metals, as well as sulfides of transition metals of groups 4, 5, and 6 of the periodic table. These catalysts are made with inert supports and are used at 400—600°C (24). Lower temperature reactions (315—482°C) have been successhiUy conducted using 2inc oxide-zirconium oxide combinations (25), and combinations of copper-chromium oxide and of copper and silicon dioxide (26). 

Dialkylaminoethyl acryhc esters are readily prepared by transesterification of the corresponding dialkylaminoethanol (102,103). Catalysts include strong acids and tetraalkyl titanates for higher alkyl esters and titanates, sodium phenoxides, magnesium alkoxides, and dialkyitin oxides, as well as titanium and zirconium chelates, for the preparation of functional esters. Because of loss of catalyst activity during the reaction, incremental or continuous additions may be required to maintain an adequate reaction rate. 

Tin reacts completely with fluorine above 190°C to form tin tetrafluoride [7783-62-2] SnF. Titanium reacts appreciably above 150°C at a rate dependent on the size of the particles the conversion to titanium tetrafluoride [7783-63-3] TiF, is complete above 200°C. Fluorine reacts with zirconium metal above 190°C. However, the formation of a coating of zirconium tetrafluoride [7783-64 ] ZrF, prevents complete conversion, the reaction reaching... 

The abihty of magnesium metal to reduce oxides of other metals can be exploited to produce metals such as zirconium, titanium [7440-32-6] and uranium [7440-61-1] (see ZiRCONiUMAND ZIRCONIUM COMPOUNDS Titaniumand titanium alloys Uraniumand uranium compounds). These reactions are... 

Chlorination. In some instances, the extraction of a pure metal is more easily achieved from the chloride than from the oxide. Oxide ores and concentrates react at high temperature with chlorine gas to produce volatile chlorides of the metal. This reaction can be used for common nonferrous metals, but it is particularly useful for refractory metals like titanium (see Titanium and titanium alloys) and 2irconium (see Zirconium and zirconium compounds), and for reactive metals like aluminum. 

Condensation of metal vapors followed by deposition on cooler surfaces yields metal powders as does decomposition of metal hydrides. Vacuum treatment of metal hydrides gives powders of fine particle size. Reaction of a metal haHde and molten magnesium, known as the KroU process, is used for titanium and zirconium. This results in a sponge-like product. 

Transesterification of methyl methacrylate with the appropriate alcohol is often the preferred method of preparing higher alkyl and functional methacrylates. The reaction is driven to completion by the use of excess methyl methacrylate and by removal of the methyl methacrylate—methanol a2eotrope. A variety of catalysts have been used, including acids and bases and transition-metal compounds such as dialkjitin oxides (57), titanium(IV) alkoxides (58), and zirconium acetoacetate (59). The use of the transition-metal catalysts allows reaction under nearly neutral conditions and is therefore more tolerant of sensitive functionality in the ester alcohol moiety. In addition, transition-metal catalysts often exhibit higher selectivities than acidic catalysts, particularly with respect to by-product ether formation. 

Metallocene Catalysts. Polymerization of cycloolefins with Kaminsky catalysts (combinations of metallocenes and methylaluminoxane) produces polymers with a completely different stmcture. The reactions proceeds via the double-bond opening in cycloolefins and the formation of C—C bonds between adjacent rings (31,32). If the metallocene complexes contain bridged and substituted cyclopentadienyl rings, such as ethylene(hisindenyl)zirconium dichloride, the polymers are stereoregular and have the i j -diisotactic stmcture. 

In this process, catalysts, such as boric acid, molybdenum oxide, zirconium, and titanium tetrachloride or ammonium molybdate, are used to accelerate the reaction. The synthesis is either carried out in a solvent (aUphatic hydrocarbon, trichlorobenzene, quinoline, pyridine, glycols, or alcohols) at approximately 200°C or without a solvent at 300°C (51,52). 

In pigments, 2irconium sHicate serves as the host lattice for various chromophores, such as vanadium, praseodymium, iron, etc. Zirconium sHicate crystals are usuaHy formed in situ during pigment preparation by a high temperature reaction of Zr02 and Si02 ... 

Enols and alkoxides give chelates with elimination of alcohol. For example, in the reaction of the enol form of acetylacetone [123-54-6] all four alkoxide groups attached to zirconium can be replaced, but only two of the four attached to titanium (Fig. 3). Acetoacetic esters react similarly. 

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