Modification of Offretite and Omega-zeolites .1. Structural Characterization

Offretites and omegas were dealuminated by steaming and acid-leaching treatments. XRD, Si-29 MAS n.m.r. and i.r. spectroscopy were used in order to follow the structural evolution of these solids. The presence of extraframework Al species causes a random distribution of the structural data (u.c. parameters, structural band position) as a function of the mole Al fraction determined by chemical analysis (m(total)). However, totally linear plots are hardly ever obtained, even if we plot the structural data against the skeletal Al fraction (m(IV)). This nonlinearity is supposedly due to the existence of two non-equivalent crystallographic sites and to a preferential dealumination of Al atoms located in these two tetrahedral sites. As a consequence, the u.c. contraction appears to be anisotropic. Acid leaching has been shown to give rise to very contrasted structural modifications according to the nature of the zeolite and the dealumination degree. Some dilation of the u.c. during acid-leaching treatments seems to demonstrate that an Al reinsertion into the lattice takes place in acidic media. Extraframework residues are believed to have a very minor influence on structural data, if any.