Titanium silicalite defects

Infrared spectroscopy has proven to be a valuable tool in characterization of titanosiUcates. In the case of TS-1 and other Ti-substituted molecular sieves the infamous band at -960 cm was held to be evidence of titanium incorporation into the framework [20,21]. However, the same band was found with titanium free silicalite [22] and not observed when sihcalite was modified from aqueous fhiorotitanate solutions [23]. Most would agree now that this band is an Si-O stretch derived from defect sites in the sihcate yet there may be some overlap and contribution from Si-O-Ti stretches. It seems in some cases... 

More recently, Kooyman et al. tried to react pure silica MFI (silicalite-1) with TiCl4 both in the gas and in liquid phase [92]. Incorporation of Ti was expected to occur in the structural defects formed during the fast crystallization method used for synthesizing the precursor. Apparently, the attempt was unsuccessful, the titanium having been found to segregate as Ti02 on the crystal surface. 

Another deeply investigated system is that of Ti-MFI (or Ti silicalite) [255]. Only a very small amount of Ti actually enters the silicalite structure in normal tetrahedral sites, although some of them can be bonded to hydroxy groups in open defects [256]. Anatase-like extraframework Ti oxide particles are formed when excess titanium is present upon the preparation. The presence of Ti makes Ti-silicalite less defective than pure silicalite and also changes its room temperature structure from monoclinic to orthorombic. A recent neutron diffraction study showed that Ti sitting is preferential on the sites where defects are frequently present in pure silicalite [257]. 

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