Lamellar-zeolitic transformations mediated by diffusionless and recrystallization mechanisms

Abstract

The interzeolitic conversions can be divided according to the mechanisms that govern them: competing phases, intergrowth, recrystallization and diffusionless (reconstructive and topotactic). Lamellar silicates have been used as precursors in these transformations, since they share many similarities with zeolites and can also perform a bulky seed effect. Thus, the main objective of this work was the investigation of the mechanisms involved in the hydrothermal conversion of Na-RUB-18 lamellar silicate into MOR zeolite and other zeolitic structures. For this, the effect of the concentration and the origin of the aluminum atoms were evaluated, as well as their role in the lamella activation. The samples were characterized by XRD, XRF, SEM, FTIR, 29Si and 27Al MAS-NMR. An unprecedent panorama showed that the MOR phase formation was related to a classical reconstructive conversion of the precursor lamella, resulting in a topotactic growth, supported by the common to both structures, mor (t-tes) unit. On the other hand, Na-P1 and ZSM-34 structures came from a nonclassical recrystallization of Na-RUB-18 in the reaction medium. The deep understanding of these synthetic mechanisms emerges as the key to the intelligent design of new and useful materials. Pure MOR samples were obtained as nanometric prismatic crystals, a feature that combined with the higher yield when preparing MOR from Na-RUB-18 makes this synthetic route a potential path for the production of chemical catalysts.