TI SUBSTITUTION IN MFI TYPE ZEOLITES - A QUANTUM-MECHANICAL STUDY

Ab initio quantum mechanical calculations were performed to study isomorphous substitution of Ti for Si in orthorhombic MFI type zeolite. A minimum energy was found for pentameric Ti(OSiO3H3)(4) clusters, having the same geometry determined in MFI framework by crystal structure analysis, corresponding to a Ti-O bond length of 1.80 Angstrom. The latter value is in good agreement with the experimental value derived by both XRD and EXAFS analysis. No clear evidence in favour of preferential crystal sites for Ti substitution was achieved.