Effect of HF concentration on the composition and distribution of Ge species in the framework of ITQ-13 and ITQ-17 zeolites

Abstract

Two germanosilicates with zeolitic structures, namely ITQ-13 and ITQ-17, have been synthesized from gels containing various amounts of hydrofluoric acid. Although both zeolites possess similar compositions, they differ not only by their pore size and framework topology but also by the nature of the cavities surrounding fluoride species in the structure. For ITQ-17, in which fluoride is almost exclusively located in D4R units, a decrease in HF concentration in the gel has no influence on the fluoride content in the zeolite. However, it favors the incorporation of germanium species in the framework, particularly in D4R units. Zeolites obtained at low HF concentrations are characterized by high Ge contents and Si/Ge atomic ratios close to 1 in D4R units. In the case of ITQ-13, the possibility for fluoride to reside not only in D4R units but also in the larger [415262] cages minimizes the influence of the HF concentration on the zeolite framework composition. Reducing the HF concentration in the gel has no effect on the Si/Ge ratio in the final zeolite but it decreases the fluoride content in the structure. At low HF concentration, fluoride is absent from [415262] cages and is almost exclusively present in all-silica D4R units. As the concentration increases, fluoride starts to occupy Ge-rich D4R and [415 262] cages, as clearly evidenced by 19F NMR. By contrast to ITQ-17, the amount of HF in the gel does not influence the distribution of Ge species in the framework. © 2012 Published by Elsevier Inc.