Catalytic Activity of Titanium Silicates Synthesized in the Presence of Alkali-Metal and Alkaline-Earth Ions

Abstract

Titanium containing aluminum-free ZSM-5 (TS-1) synthesized in the presence and absence of alkali metal and alkaline earth ions are investigated as catalysts for the selective oxidation of alkanes and alkenes using aqueous H_2O_2 as oxidant at temperatures below 100°C. Sodium-exchanged TS-1 is not active for alkane oxidations. Similarly, no catalytic activity is observed for TS-1 synthesized in the presence of high concentrations of alkali metal ions (Si/Na <20). For both cases, the catalytic activity can be restored by washing the solid with acid solution prior to catalytic evaluation. Similar results are obtained for TS-1 synthesized in the presence of other alkali metal ions, e.g., Li, K. However, the presence of alkaline earth ions, e.g., Mg, in the synthesis gel does not have a significant effect on the catalytic activity of TS-1. Based on the catalytic data and the physicochemical properties of the samples, a plausible explanation for the above results is described. The acid treatment may be useful in overcoming the problems of synthesizing TS-1 from reagents that contain alkali metal ions, e.g., TPAOH solutions. More importantly, this treatment opens the possibility of synthesizing other titanium-containing silicate structures that require the presence of alkali metal ions in the synthesis gel for their formation.