Preferential Location of Ge in the Double Four-Membered Ring Units of ITQ-7 Zeolite

ITQ-7 is a tri-dimensional twelve-membered ring zeolite which presents double four-membered ring units (D4MR) in its structure. On the basis of theoretical ab initio calculations, which indicate that isomorphic substitution of Ge for Si atoms in the double four-membered ring units stabilizes such small cages, we have carried out the synthesis of ITQ-7 in the presence of Ge. It is found that the incorporation of Ge reduces the crystallization time from 7 days to less than 12 h, while a detailed analysis of the 19F and 29Si MAS NMR leads to the conclusion that Ge selectively occupies positions at the D4MR. An hypothesis has been introduced which assumes that the increase in the crystallization rate is due to the preferential occupancy of D4MR sites by Ge, and this allows relaxation of the constrained T−O−T bonds of these small D4MR cages.