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

Abstract

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.

Extra-large pore zeolite (ITQ-40) with the lowest framework density containing double four- and double three-rings

“…However, when Ge is also present in D4Rs, the band shifts to higher field depending on the Si/Ge ratio (11). The presence of a signal at −6.5 ppm in the 19 F MAS-NMR spectra of the ITQ-40 sample (Fig.…”

“…Previously, Davis et al considered using zinc as a framework cation to promote the formation of three-rings, and while three zincosilicates containing three-rings were subsequently synthesized, none of them contain extra-large pores (7)(8)(9). Later, we have shown by theoretical and experimental work, that Ge has a directing effect toward the formation of double four-rings (D4Rs) (10,11), and the presence of these secondary building units (SBUs) has allowed the synthesis of several large and extra-large pore zeolites (12)(13)(14)(15). Among those, Instituto de Tecnologia Química number 33 (ITQ-33) contains D4Rs and three-ring units in the structure and has a low framework density (12.3 T∕1;000 Å 3 ).…”

An Overview of Zeolite, Zeotype and Mesoporous Solids Chemistry: Design, Synthesis and Catalytic Properties

“…Blasco et al used Hartree-Fock ab initio methods to discover that the presence of small amounts of Ge 4þ in the silica framework stabilizes double fourmembered rings (D4MR), cubic subunits formed by two rings each containing four silicon atoms. [27] D4MR are absent in most known silicate frameworks, [28] as the strain present in this arrangement makes them highly unstable. By replacing some of the SiÀ ÀOÀ ÀSi linkages by SiÀ ÀOÀ ÀGe, some of the strain can be released.…”

“…Secondly, the Al present in the MCM-41 parent material allows the introduction of TPA þ cations, which is the template for ZSM-5, via an ion exchange step. On the basis of IR (the appearance of a band at 550 cm À1 , characteristic of the five-membered rings present in the MFI structure) and 27 Al NMR (an increase in the signal related to tetrathedrally coordinated Al), it was concluded that part of the MCM-41 silicate material was, indeed, converted into 'embryonic' ZSM-5 domains. The catalytic activity was compared with that of the parent MCM-41 and it appeared that the modified material had a significantly higher activity in the cracking of cumene.…”

The Assembly‐Disassembly‐Organization‐Reassembly Mechanism for 3D‐2D‐3D Transformation of Germanosilicate IWW Zeolite

“…We describe this process using the ADOR (assembly-disassembly-organization-reassembly) acronym. [3] In this way, two zeolites were prepared: IPC-2 [3] (which is isostructural to COK-14 [4] with the IZA code [5] OKO) and IPC-4 [3] (code PCR). We proposed that for successful application of the ADOR mechanism to other parent zeolite structures, the following structural features are required: 1) the presence of doublefour-ring units (D4R) and 2) that these D4R units are preferentially occupied by Ge.…”