CRYSTALLINE, MICROPOROUS ZIRCONIUM SILICATES WITH MFI STRUCTURE

Zirconium-containing medium-pore, Al-free molecular sieves (Si/Zr mola r ratios > 50) with MFI structure have been synthesized both in alkali ne and HF/CH3NH2 medium. The samples synthesized in HF/CH3NH2 medium e xhibit a high crystallinity, bigger particle size, lower surface area and lower catalytic activity in the hydroxylation of phenol as compare d to those synthesized in alkaline medium. An expansion in unit cell v olume (XRD), an absorption at 965 cm(-1) (FTIR) and at 212 nm (UV-Visi ble) as well as the catalytic activity data indicate that upto 0.6 zir conium atom per unit cell. (Si/Zr = 158) could be substituted isomorph ously within the silicalite-1 framework. The IR study of adsorbed pyri dine has shown that strong Lewis acid and weak Bronsted acid sites exi st on all zirconium silicate samples, synthesized in alkaline medium. The relative stability of cluster models representing various possible lattice structures and the ordering of molecular orbitals derived fro m extended Huckel molecular orbital calculations are discussed to unde rstand the nature of active sites in the zirconium silicates.