Chromium-containing molecular sieves

Table 3. ESR parameters of Cr + in chromium containing molecular sieve and their assignments...

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... 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

The use of heterogeneous catalysts in the liquid phase offers several advantages compared with homogeneous counterparts, in that it facilitates ease of recovery and recycling. A chromium-containing medium-pore molecular sieve (Si Cr > 140 1), CrS-2, efficiently catalyzes the direct oxidation of various primary amines to the corresponding nitro compounds using 70% t-butylhydroperoxide (TBHP).110... 

Chromium-containing mesoporous silica molecular sieves (Cr-HMS) with tetrahedrally coordinated isolated chromium oxide (chromate) moieties can operate as efficient photocatalysts for the decomposition of NO and the partial oxidation of propane with molecular oxygen under visible light irradiation (Yamashita et al., 2001). 

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... 

The as-synthesized and calcined CrAPO-5 and CrS-1 were characterized by XRD which showed that the samples were pure and had an API and MFI structure respectively. ICP analysis showed that both catalysts contained about 1 % chromium. The results observed in the decomposition of cyclohexenyl hydroperoxide over several redox active moleular sieves are presented in Table 1. CrAPO-5 and CrS-1 displayed rougly equal activity and selectivity in the decomposition of cyclohexenyl hydroperoxide. Blank reactions carried out with Silicalite-1 (S-1) and silicon incorporated Aluminophosphate-5 (SAPO-5) show low conversions confirming that the chromium was responsible for the catalysis. Other transition- metal subsituted molecular sieves showed low conversions. 

P-07 - Photocatalytic reactions on chromium containing mesoporous molecular sieves under visible light irradiation Decomposition of NO and partial oxidation of propane... 

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