Synthesis, Characterization, and Framework Heteroatom Localization in ITQ-21

ITQ-21 has been synthesized in a wide range of compositions. By rationally modifying the
synthesis variables and zeolite composition, it is possible to fine-tune the crystallite size from nanocrystals
(<80 nm) up to microns and to avoid the competition of other phases such as CIT-5, SSZ-24, or a laminar
phase that can also be synthesized with the same organic structure directing agent. By means of XRD and
19F MAS NMR, Ge and Si have been localized among the different crystallographic positions, and it is
shown that Ge preferentially occupies T1 positions at the D4R cages, avoiding formation of Ge−O−Ge
pairs. However, at high Ge loadings (Si/Ge = 1.7), a new 19F MAS NMR signal at −14 ppm has been
observed and assigned to the presence of Ge−O−Ge in Ge-rich D4R cages. Energetic configurations
obtained by theoretical calculations fully agree with experimental observations, with the following increasing
order in energy for Ge substitution:  T1 < T2 < Ge−O−Ge in T1 < T3.