ON THE STRUCTURE AND STABILITY OF TI-ZEOLITES - A COMPARISON OF CLUSTER AND PERIODIC AB-INITIO CALCULATIONS

A periodic Hartree-Fock study of Ti-containing zeolites was performed. The zeolitic framework was modeled by means of the chabazite structur e. Two framework compositions were considered (Ti/(Si + Ti) = 0 and 0. 5) and the corresponding periodic structures were optimized in order t o estimate the geometric and energetic features of the Ti sites. Clust er models were also considered for comparison. It arises that the main structural distortion of the zeolite lattice upon Si/Ti substitution comes from the increment of the T-site volume. From the obtained resul ts, the limitations of the different models are discussed.