Some observations on the preparation and properties of colloidal silicalites Part II: Preparation, characterisation and properties of colloidal silicalite-1, TS-1, silicalite-2 and TS-2

Colloidal silicalite-1 and TS-1 have been prepared by hydrothermal synthesis in the 100-175 degreesC temperature range using both conventional and microwave heating. Optimum conditions for heteroatom incorporation were found at high temperatures (175 degreesC) under microwave irradiation and this procedure was subsequently applied to the synthesis of colloidal silicalite-2 and TS-2. A combination of XRD, IR and Raman methods were used to define product purity and the extent of titanium incorporation into the tetrahedral zeolite frameworks. The substitution limits for titanium were found to be in the region of 2.5 at.% for the MFI structure (TS-1) and 3.5 at.% for the MEL framework (TS-2). These studies have confirmed the dependence of the degree of framework substitution upon synthesis temperature. Higher temperatures favour the formation of products with greater levels of framework titanium but there is also an increase in the formation of extra-framework anatase impurity. Increasing levels of titanium in the synthesis composition reduce the overall growth rate of the colloidal TS-1 product and also tend to increase the crystal size. Comparison of the catalytic activity in 2-butanol oxidation for TS-1 samples of crystal size 80, 300 and 560 nm did not show the expected incremental increase for the 80 nm catalyst. This is probably related to the difficulty in achieving an efficient re-dispersion of the colloidal-sized catalyst after isolation from its original sol and subsequent calcination. The catalytic performance of nanosized TS-1 and TS-2 crystals was found to be very similar. (C) 2004 Elsevier Inc. All rights reserved.