| Abstract
| - ITQ-21 has been synthesized in a wide range of compositions. By rationally modifying thesynthesis 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 laminarphase that can also be synthesized with the same organic structure directing agent. By means of XRD and19F MAS NMR, Ge and Si have been localized among the different crystallographic positions, and it isshown that Ge preferentially occupies T1 positions at the D4R cages, avoiding formation of Ge−O−Gepairs. However, at high Ge loadings (Si/Ge = 1.7), a new 19F MAS NMR signal at −14 ppm has beenobserved and assigned to the presence of Ge−O−Ge in Ge-rich D4R cages. Energetic configurationsobtained by theoretical calculations fully agree with experimental observations, with the following increasingorder in energy for Ge substitution: T1 < T2 < Ge−O−Ge in T1 < T3.
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