Framework composition of titanium silicalite-1

Abstract

The preparation of a titanium silicate with MFI structure (TS-1) was reported by Taramasso et al. and the effective insertion of Ti in the zeolite framework was proved by structural investigations. A titanium silicate with MEL structure (titanium silicalite-2, TS-2) was obtained for the first time in these laboratories and subsequently by others. The framework structure of TS-2 is consistent with the disordered model proposed by Perego et al. The catalytic activity of TS-1 is affected by the possible presence of extraframework Ti, which can promote extensive homolytic side reactions including decomposition of hydrogen peroxide. In the hydroxylation of phenol, this causes a variation in the distribution of isomeric products and an increase in tar production. Extraframework Ti could also be responsible for discrepancies concerning some results coming from oxidation of alkanes. The framework composition of TS-1 can be defined as xTiO{sub 2} {center dot} (1 {minus} x) SiO{sub 2}, x varying between 0 and 0.025. Attempts to produce TS-1 with x > 0.025 fail, since the excess titanium segregates as TiO{sub 2}. Recently, Thangaraj et al. reported the synthesis of pure TS-1 samples with x up to 0.091, but the supporting data disagree with those reported elsewhere. This inducedmore » the authors to reconsider the problem of the characterization of TS-1, with particular attention to the determination of the real framework composition and to the evaluation of the maximum titanium content of TS-1. Among the techniques available for this purpose XRD was chosen for its effectiveness.« less