Chromium-containing silicalite

Chromium-containing silicalite-2, CrS-2, has been synthesized and shown to catalyze similar reactions using TBHP as an oxidant [109]. Notably, TBHP has been reported to be an ineffective oxidant in TS-1 catalyzed oxidations [26]. However, it has been claimed that aTS-1-TBHP combination exhibits activity in the oxidative cleavage of the C=C double bond of silyl enol ethers to produce dicarboxylic acids [110]. 

The discovery in the early 80 s of titanium silicalites [62-64] opened the new application perspective of zeolitic materials as oxidation catalysts. Several reactions of partial oxidation of organic reactants using dilute solutions of hydrogen peroxide could for the first time be performed selectively in very mild conditions. Other elements inserted in the lattice of silicalites have since been shown to have similarly interesting catalytic properties including, vanadium, zirconium, chromium and more recently tin and arsenic [65]. Titanium silicalites with both MFI (TS-1) and MEL (TS-2) structures have however been the object of more attention and they still seem to display unmatched properties. Indeed some of these reactions like the oxyfunctionalization of alkanes [66-69] by H2O2 are not activated by other Ti containing catalysts (with the exception of Ti-Al-Beta [70]). The same situation... 

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. 

Recently, Cr incorporation was claimed in silicalite-2 (CrS-2) prepared by heating at 170 °C an alkali-free gel containing TEOS, chromium nitrate and TBA-... 

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