Titanium "zeolite" catalyst
Selective mono N-oxidation of substituted pyrazines in good yields using 30% dilute H2O2 as an oxidant with a specially prepared titanium silicate
catalyst is possible.
Preparation of the Catalyst:
Add a solution of titanium peroxide to ethyl silicate (with or without an organic solvent) to obtain a gel. Hydrolyze the homogeneous gel previously
obtained, by adding an organic base to the gel. The ammount of organic base should be only 6-15% of the ammount of silica gel. Next, add deionised
water after the yellowish-white color of the gel begins to turn into a greenish-white color. Stir the greenish-white gel for about 1 hour, then heat
the gel in a pressure cooker at 100 -110 C. The gel must be constantly heated in this way for at least 20 hours. Further heating, up to 2 days, is
preferable. This will result in a solid composite product. Separate out the resultant solid composite material, dry, and bake at a 350-500C
temperature to obtain the final product. This is a catalyst and so only a small quantity need be prepared. The organic base should be an organic amine
with lots of bulky organic groups on it, either a tri- or tetra-alkyl amine, such as tetrapropyl ammonium hydroxide. Alternatively positively charged
coated silica particles can be used instead of the ethyl silicate. These can be prepared by mixing an aqueous colloidal silica with stabilized basic
aluminum acetate. The aluminum composition is stabilized with a small quantity of boric acid, which controls the hydrolysis of the aqueous solution of
basic aluminum acetate.
The catalyst produced above is known as TS-1 and is basically a porous titanium silicate crystal with a structure analogous to zeolite. TS-1 is not
yet commercially available. It can also catalyze the oxidation by H2O2 of imines R2C=NH into oximes R2C=NOH.
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