Chromium substituted

Dissolution behaviour of chromium-substituted hematite in an oxidative/reduc-tive complexing agent environment. J. Nuclear Sci. Techn. 36 798-804 Joseph,Y. Kuhrs, C. Ranke,W. Weiss,W. 

Sheldon and coworkers have developed chromium-substituted molecular sieves (CrAPO-5) as recyclable solid catalysts for several selective oxidations, among them also the allylic" and benzylic ° " ° " ° oxidations using TBHP or O2 as the terminal oxidants (equation 63), which yielded the corresponding benzylic ketones in moderate yield (conv. 13-70%) and moderate to good selectivity (41%, 65-97%). The benzylic alcohols were formed as side products. Allylic oxidation also proceeded with good conversions, while selectivities were lower and both possible products, the allylic ketone (31-77% selectivity) and the allylic alcohol (0-47% selectivity), were formed. Chromium sUicalite showed activity for selective benzylic oxidation in the presence of TBHP as well as giving mainly the allylic ketone (2-cyclohexen-l-one with 74% selectivity) and the allylic alcohol as minor product (2-cyclohexen-l-ol with 26% selectivity) -. ... 

Cr(VI) compounds, like Cr03,414 PDC,415 PCC,416 (OCMe2CH2 CMe20)Cr02417 and a chromium substituted aluminophosphate (CrAPO-5) 418... 

Selective decomposition of cyclohexenyl hydroperoxide to 2-cyclohexen-l-one catalyzed by chromium substituted molecular sieves... 

Chromium substituted aluminophosphate-5 is an active and recyclable catalyst for the selective decomposition of cyclohexenyl hydroperoxide to 2-cyclohexen-l-one. The product is of potential industrial interest for the synthesis of caprolactam. 

The synthesis of cyclohexanone, which is an intermediate in the manufacture of nylon 6 and nylon 6,6 is an important industrial process [1], One of the major current routes for the synthesis of cyclohexanone is the liquid-phase autoxidation of cyclohexane at 125-160 °C and 10 bar followed by the selective decomposition of the intermediate cyclohexyl hydroperoxide, using a soluble cobalt catalyst, to a mixture of cyclohexanol and cyclohexanone [2]. These severe conditions are necessary due to the low reactivity of cyclohexane towards autoxidation. Due to the high reactivity of the products in the autoxidation step conversions must be kept low (<10%) [3,4]. Heterogeneous catalysts potentially offer several advantages over their homogeneous counterparts, for example, ease of recovery and recycling and enhanced stability. Recently we found that chromium substituted aluminophosphate-5 and chromium substituted silicalite-1 (CrS-1) are active, selective and recyclable catalysts for the decomposition of cyclohexyl hydroperoxide to cyclohexanone [5j. 

Chromium-substituted molecular sieves contain Cr after calcination, which might be expected to give reactions typical of oxometal pathways with ROjH, as is observed in solution... 

We recendy performed a didailed investigation of the stability of chromium-substituted molecular sieves [82]. As a model reaction we chose the allylic oxidation of a-pinene with TBHP (Reaction 19) which gives verbenone in high selectivity. 

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. 

Similarly, the CrAPO-5- and chromium silicalite-1 (CrS-l)-catalyzed oxidation of aromatic side-chains with TBHP or O2 as the primary oxidant [27-31] almost certainly arises as a result of soluble chromium(VI) leached from the catalyst. The same probably applies to benzylic oxidations with TBHP catalyzed by chromium-pillared montmorillonite [32]. More recently, a chromium Schiff s base complex tethered to the mesoporous silica, MCM-41, was claimed [33] to be an active and stable catalyst for the autoxidation of alkylaromatic side-chains. It would seem unlikely, however, that Schiff s base ligands can survive autoxidation conditions. Indeed, on the basis of our experience with chromium-substituted molecular sieves we consider it unlikely that a heterogeneous chromium catalyst can be developed that is both active and stable to leaching under normal oxidizing conditions with O2 or RO2H in the liquid phase. Similarly, vanadium-substituted molecular sieves are also unstable towards leaching under oxidizing conditions in the liquid phase [6,34]. 

P-12 - Synthesis and characterization of highly ordered chromium-substituted MCM-48 materials with tailored pore sizes... 

J. Chappert, R.B. Frankel, Mossbauer study of ferrimagnetic ordering in nickel ferrite and chromium-substituted nickel ferrite, Phys. Rev. Lett. 1967, /9(I0), 570. 

Nakato T, Toyoshi Y, Kimura M, Okuhara T (1999) Unique catalysis of an acidic salt of heteropolyacid and consisting of microciystallites. Catal Today 52 23-28 Rong C, Anson FC (1994) Simplified preparations and electrochemical behavior of two chromium-substituted heteropolytungstate anions. Inorg Chem 33 1064-1070 Harrup MK, Hill CL (1994) polyoxometalate catalysis of the aerobic oxidation of hydrogen-sulfide to sulfur. Inorg Chem 33 5448-5455... 

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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