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

Abstract

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 (19)F 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 (19)F 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.

Location of Ge and extra-framework species in the zeolite ITQ-24

“…This is consistent with the location of Ge found in other zeolites (e.g. ITQ-7 5 , ITQ-22 9 and ITQ-21 10 ),…”

“…3,4 It has been demonstrated in numerous syntheses in the last decade, that the inclusion of Ge in the synthesis mixture can have a profound effect on the crystalline product. [5][6][7][8][9][10] A Ge atom, being larger and somewhat less rigid than Si, allows some frameworks to form that would not crystallize in a pure silicate system. In particular, Ge has been found to favor the formation of double 4-ring (d4r) units.…”

Computational Study of 19F NMR Spectra of Double Four Ring‐Containing Si/Ge‐Zeolites

“…The Ge substitution in Si/Ge-AST zeolite was studied by Wang et al [38] using 19 F and 29 Si NMR spectroscopy, where the À8 and À19 ppm signals were assigned to 4Si4Ge-D4R and 6Si2Ge-D4R respectively. However, a recent publication on the synthesis, characterization and Ge location in ITQ-21 [40] shows that the À8 and À20 ppm 19 F NMR signals arise from the presence of 5Si3Ge-D4R and 7Si1Ge-D4R units, respectively. Therefore, controversial assignments have been reported, as is summarized in Table 1, where the main discrepancy refers to the signal due to 6Si2Ge-D4R units.…”

Catálisis con zeolitas: Desde el laboratorio a su aplicación industrial

“…Un procedimiento de cristalización típico utiliza H 2 O, una fuente de átomos Si (normalmente sílice coloidal, sílice del tipo aerosil o alcóxidos de silicio), un agente mineralizante de la sílice (OH -, F -), y un agente narios por fosfonios y más recientemente por fosfacenos, ha abierto una nueva dirección para la síntesis de zeolitas (18). Finalmente la combinación de efectos directores de estructura de determinados átomos de la red, en conjunción con ADE orgánicos ha proporcionado los resultados más espectaculares en la síntesis de nuevas zeolitas (9)(10)(11)(12)(13)(14).…”

“…Con estas técnicas y partiendo de hipótesis científicas en lugar de procedimientos puramente combinatorios, ha sido posible sintetizar un número elevado de nuevas estructuras (9)(10)(11)(12)(13)(14).…”