Organic Structure Directing Agents (OSDAs) play a central role in synthesis of zeolites. Typically, OSDAs consist in tetraalkylammonium cations that are trapped inside of the voids of zeolites. The positive charge of the occluded cations is compensated i) by the isomorphic substitution of Si atoms by trivalent atoms resulting in negatively charged zeolite framework; ii) by the presence of framework defects consisting in the Si-O– species and Si-OH (silanols) and iii) by the incorporation of fluoride anions in small zeolite cages. The charge compensation method depends on the synthesis gel composition and crystallization conditions.
The exact location of the occluded OSDA has been matter of interest because it is accepted that there is a pairing between the negative zeolite and OSDA positive charges in order to maximize coulombic interactions. Thus, the distance between the highest positive charge density of the OSDA (typically on the N atom) and the negative charge associated to presence of T(III) atoms, Fluoride anions or defects must be as short as possible. This results in the frequently reported charge matching compensation effect.
Here, we will present ITQ-66, which is a new zeolite that we have succeeded in preparing it as Borosilicate and Gallosilicate, but not as Aluminosilicate. Importantly, B and Ga atoms are not located at the same T sites of the zeolite framework, but they are isomorphically incorporated at the T4 and T3 of the ITQ-66 structure, respectively. One could argue that the difference stability of Ga and B in these particular sites of the framework is the driving force for this difference, but also that the charge matching between OSDA and zeolite could take place from two different positions of positive and negative charges.
Simultaneously, we have found that compensating fluoride anions can ‘jump’ from one particular crystallographic location to another during the course of crystallization or just by gentle heating of pure silica RTH samples. Again, charge compensation of the occluded OSDA cations could take place by negative charges placed at different crystallographic positions.
These findings make us to conclude that charge matching mechanism is not a biunivocal relation between positive and negative charge locations. Here, we have found that inorganic compensating defects (i.e. T(III) or interstitial F anions) may be placed at different locations in the zeolite structure in presence of the same OSDA.
Prof. Fernando Rey is the Director of the Instituto de Tecnología Química in Valencia (Spain) and Research Professor at the Spanish National Research Council (CSIC). His main research area includes the synthesis and characterization of zeolites with well-tailored properties, and particularly their application for gas separation processes of industrial interest, such as CO2/methane or olefin/paraffin separations among others.