Chromium oxide fluorides

Treatment of pentachloropyridine with hydrogen fluoride at 180 °C under pressure gives a mixture of trichloro-2,6-difluoro- and dichIoro-2,4,6-tri-fluoro-pyridine. Gas-phase fluorination of pentachloropyridine and of 2,4, 2,4,6-trichlorop3rrimidine with hydrogen fluoride over chromium oxide-fluoride and aluminium oxide-fluoride (or A1203) has also been achieved tetrachloropyrimidine gives 5-chloro-2,4,6-trifluoro- (61%), 5,6-dichloro-2,4-difluoro- (33 %), and 4,5,6-trichloro-2-fluoro-pyrimidine (6 %). ... 

The fluorine titration of chromium oxide was carried out at the Elf-Atochem Research Center, Pieire-Benite.The catalyst mineralization was carried out in a Parr bomb by reaction with sodium peroxyde. Fluorine ions were then titrated by a potentiometric method with a specific fluoride electrode. 

In this state, chromium compounds are usually coloured yellow or red (but due to charge transfer (p. 60) and not to the presence of d electrons on the chromium ion). The only halide known is the unstable chromium(VI) fluoride CrF6, a yellow solid. However, oxide halides are known, for example Cr02Cl2 ( chromyl chloride ), formed as a red vapour when concentrated sulphuric acid is added to a chromate(VI) (or dichromate) mixed with a chloride ... 

Disproportionation of halogenatcd aliphatic hydrocarbons can also be accomplished using catalysts based on chromium oxides with the formula Cr01 2F2 1.18 This catalytic material is obtained by fluorination w ith hydrogen fluoride of an oxide hydrate of trivalent chromium with the formula Cr203 xH,0 (with x = 1-9). The chromium oxide hydrate is reacted with a mixture of hydrogen fluoride/nitrogen gas in a molar ratio of 1 10 in an externally heated nickel tubular reactor at 350 450 C. 

Chromium Compounds and the Lower Oxide Fluorides of Molybdenum and Tungsten... 

The reaction of Cr203 with HF gives a dark olive-green compound at high temperatures, and this has been identified as CrOF (88). Further evidence for the existence of this chromium(III) oxide fluoride was provided during the study of the Cr203-CrF3 system (89). The related... 

It is frequently said that the behavior of trivalent chromium resembles that of trivalent aluminum, presumably because of the similarity in radii of the tw o ions (A1+8, 0.45 A Cr+3, 0.55 A). The structures of the solid oxides, fluorides, and chlorides are analogous for the two metals, and both form salts of the alum type (for example, KA1(S04)2 12H20, KCr(SeC>4)2 12H20). Chromium(III) sulfide, (Cr2S3) like aluminum sulfide, cannot be made in aqueous systems. 

Fig. 2. Chromium chemical state plot for chromium oxide and fluoride samples before and after activation with halocarbons (static charge reference C Is 284.8 eV) (reproduced with permission from J. Catal., 159 (1996) 270 (39]).

Trifluoroacetyl fluoride and hexafluoroacctone have been made commercially by reaction of their pcrchloro analogs with hydrogen fluoride and chromium oxide catalysLs. 

In addition to the antimony fluorides, silver, mercury, thallium, aluminium, zinc, zirconium, chromium and other fluorides [7] such as mercury(II) fluoride, vanadium pentafluoride [24] and various transition metal oxide fluorides [25] have been used in exchange processes, although much less widely. 

Oxides, oxyfluorides and fluorides of trivalent metals among which aluminium, chromium and iron, are generally used as catalysts, also supported or in mixture with other metals, usually bivalent metals [8]. In particular chromium oxide and fluorinated chromium oxide have found industrial applications [9] but the study relative to the catalytic mechanism and to the catal34ic nature of the species involved is still under way. 

An inorganic solid lubricant has been described as a self-lubricating composite comprising a chromium oxide (e.g. chromic oxide, Cr203), a metal binder comprising Cr/Ni or Cr/Co alloy, a metal fluoride, and, optionally, a metal lubricant [60]. A milled powder mix of the composite is deposited onto a substrate, e.g. by plasma spraying. 

Vapor-phase fluorination using hydrogen fluoride and, for example, a chromium or aluminum oxide/fluoride catalyst (i.e., the procedures used for manufacture of fluorochloroalkanes16) has also been applied (Eq. 7).17... 

These observations are in some respects contrary to the behavior of chromium oxide catalysts, which are quite active when supported on silica alone. But even the activities of chromium oxide/silica catalysts tend to improve when they are acidified by the addition of compounds of Ti, Al, F, or other ions. As noted in Sections 14.7-14.9, the addition of fluoride or sulfate or silica to Cr/alumina catalysts also increases the activity several fold. Being attached by two, rather than just one, oxide link could also decrease the electron density on chromium oxide catalysts, particularly after calcination at high temperatures when strain can be introduced [52,65,387],... 

For example, the trimethylsilylmethyl derivative of chromium(II) is well suited to this purpose. Although it produces a highly active catalyst on aluminophosphate or fluoride-treated alumina supports, it is barely active on silica by itself. Nevertheless, when added to silica-supported Cr(II) oxide, the result is a highly active catalyst that produces branched polymer. In addition to reacting with silanol groups, the chromium alkyl may also react with chromium oxide to again produce mono-attached species, such as is shown in Scheme 44. Coordination between one Cr atom and its chromium or oxide neighbor also seems likely. 

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