Aluminophosphates , chromium oxide

Although there are many differences between chromium oxide catalysts and the organochromium catalysts, when they are bonded to the support, organochromium catalysts usually display a similar, but exaggerated, MW response in the polymer produced relative to what is observed with chromium oxide catalysts. For example, the MW of polymer produced with each type of catalyst usually decreased as the support calcination temperature was raised. Similarly, when both chromium oxide and the organochromium compounds were deposited onto aluminophosphate supports, they always yielded lower-MW polymer as the amount of phosphate in the support was raised. 

Chromium oxide on aluminophosphate produces polymers having a broader MW distribution than its Cr/silica counterparts, which is evidence of greater heterogeneity of Cr species on the catalyst surface. Organochromium compounds on aluminophosphate also produce polymers having broad MW distributions, and with these catalysts these same trends become unusually clear. Perhaps because the chromium tends to bind through only one link to the surface instead of two, it is often possible to obtain more detailed information about the catalyst from the resultant polymer. 

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. 

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... 

Soluble chromium compounds are known to catalyze the allylic oxidation of olefins [22,23] and benzylic oxidations of alkyl aromatics [22,24] using tert-butyl-hydroperoxide as the primary oxidant. Chromium-substituted aluminophosphates, e. g. CrAPO-5, were shown to catalyze the allylic oxidation of a variety of terpene substrates with TBHP to give the corresponding enones [25,26]. For example, a-pinene afforded verbenone with 77% selectivity (Eq. 6) and 13% of the corresponding alcohol. 

In the as-synthesized CrAPO-5, the chromium(III) is octahedrally coordinated within the framework (four framework oxygen atoms and two water molecules are in the Ciflll) coordination sphere). During calcination, oxidation of chromium(III) to chromiumCVI) occurs leading to the formation of diox-ochromium(VI), which is still bonded to the internal aluminophosphate lattice. The acidic P-OH groups derived from the decomposition of template balance the charges (216). 

Chromium(VI) catalyzes the oxidation of alcohols with alkyl hydroperoxides . Chromium-incorporated molecular sieves, in particular chromium-substituted aluminophosphate-5 (Cr-APO-5) were shown to be effective for the aerobic oxidation of secondary alcohols to the corresponding ketones (Reaction 19). This, and related catalysts, were first believed to be heterogeneous but more detailed investigations revealed that the observed catalysis is due to small amounts of soluble chromium that are leached from the framework by reaction with hydroperoxides. Reaction 19 may involve initial chromium-catalyzed free radical autoxidation of the alcohol to the a-hydroxyalkyl hydroperoxide followed by chromium-catalyzed oxygen transfer with the latter and/or H202 (formed by its dissociation) via an oxochromium(VI)-chromium(IV) cycle. 

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