Catalytic properties of crystalline titanium silicalites I. Synthesis and characterization of titanium-rich zeolites with MFI structure

Crystalline titanium silicalites with the MFI structure and high titanium content (Si/Ti=10) have been prepared using improved synthesis procedures. The monoclinic lattice symmetry of silicalite-1 changes to orthorhombic on introduction of titanium. The titanium silicalites retain their orthorhombic symmetry even after calcination in air. On progressive incorporation of titanium, (i) the unit cell dimensions and volume (from XRD) increase linearly, (ii) the crystal size decreases progressively, (iii) the intensity of the framework IR band at 960 cm−1 increases, (iv) a band around 47000 cm−1 appears in the electronic spectra due to charge transfer transitions involving Ti(IV) sites, (v) the hydrophilicity of the zeolite (from water adsorption measurements) increases, (vi) the adsorption capacity for cyclohexane increases, (vii) the 29Si MAS NMR lines exhibit progressive broadening, and finally (viii) the catalytic activity in the hydroxylation of phenol to catechol and hydroquinone also increases markedly. Based on these observations, it is concluded that a significant fraction of titanium in these samples is situated in framework positions. The catalytic properties of these well-defined titanium-modified zeolites will be described in more detail in the subsequent parts of this series.