María-José Díaz-Cabañas

Synthesis of all-silica and high-silica molecular sieves in fluoride media

Recent advances in the synthesis of all-silica and high-silica crystalline molecular sieves in fluoride media, with special regard to low framework density phases, are presented. The fundamental differences between the synthesis in hydroxide and fluoride media with respect to the properties of the materials obtained and the phase selectivity of the crystallization are discussed.

High silica zeolites with three-dimensional systems of large pore channels

Abstract Several high silica zeolites with three-dimensional systems of large pore channels have been prepared in aqueous fluoride media: Beta, ITQ-7, ITQ-10 and ITQ-14. Pure silica beta can crystallize using up to 17 organic cations considerably different in size, shape, rigidity and C/N ratios. There are clear differences in the powder X-ray diffraction patterns of calcined beta materials prepared with different cations, suggesting structural differences between them. However, this technique cannot be readily used to get further insights into these differences. 19 F MAS NMR spectroscopy shows the existence of fluoride occluded in double four ring cages in most of the different betas, thus demonstrating for the first time the real occurrence in these intergrown materials of polymorphs different from A and B. Double four ring cages are also present in ITQ-7 and in the new ITQ-10 and ITQ-14 zeolites.

SSZ‐23: An Odd Zeolite with Pore Openings of Seven and Nine Tetrahedral Atoms

Single-crystal X-ray diffraction with synchrotron radiation enabled the structure of microcrystalline SSZ-23 (see drawing on the right), the first zeolite with channels bounded by seven- and nine-membered rings, to be solved.

Synthesis and structure of pure SiO2 chabazite: the SiO2 polymorph with the lowest framework density

A SiO2 material with the lowest framework density (15.4 SiO2 nm–3) and the largest void volume fraction (nearly 50%) ever reported for crystalline silica polymorphs has been synthesised, and its structure solved by direct methods and fully refined using low-resolution powder X-ray diffraction data.

Synthesis in fluoride and hydroxide media and structure of the extra-large pore pure silica zeolite CIT-5

The new extra-large pore molecular sieve CIT-5 has been synthesised in both hydroxide and fluoride aqueous media. It has been found, contrary to previous reports, that Li+ is not an essential component of the synthesis gel, although it accelerates the crystallisation. The pure silica polymorph synthesised in the F− medium presents almost no connectivity defects, in contrast to that synthesised in the OH− medium. IR and 29Si MAS NMR spectra of the new samples synthesised in the F− medium show very good resolution of bands owing to the high local order which arises from the very low concentration of connectivity defects. The structure of this phase has been reinvestigated and solved successfully by direct methods and refined using synchrotron X-ray powder diffraction data. The framework topology previously proposed for this phase is found to be correct, although the structure is more accurately described in Im2a rather than Imma symmetry. Despite the increase in the number of tetrahedra forming the minimum window of the pore, from 12 to 14, the effective pore diameter of CIT-5 appears to be almost identical to that of the unidimensional 12-ring zeolite SSZ-24.

Incommensurate modulation in the microporous silica SSZ-24

A detailed investigation of the structure of microporous silica, SSZ-24, is presented. It is shown by X-ray powder diffraction and (29)Si MAS NMR experiments that the structure deviates from the previously proposed AlPO(4)-5-type structure. At room temperature, electron diffraction (ED) patterns exhibit extra diffraction spots, which can be attributed to an incommensurate structural modulation along the c axis. This in turn results in a pleat pattern in real space with two different intervals arranged aperiodically along the c axis, as observed with high-resolution electron microscopy (HREM). The modulated structure may easily turn into a disordered one through excessive electron irradiation or heat-treatment. In order to understand the origin of the modulation, soft phonon-modes of the ideal premodulated structure were analyzed by the use of the rigid-unit-mode model. The distribution of soft modes in reciprocal space might correspond roughly to diffuse streaks that could be observed in the diffraction patterns at higher temperatures. It was found that several phonon branches soften at specific wave vectors, which are incommensurate with respect to the original period and might be responsible for the modulation. We present a simple analytic treatment to deduce the wave vectors and associated displacement eigenvectors for the incommensurate soft-modes.

Synthesis and catalytic activity of Ti-ITQ-7: a new oxidation catalyst with a three-dimensional system of large pore channels

Isomorphous substitution of Si by Ti in the framework of nITQ-7, the crystalline silica polymorph of lowest density, has been nachieved by direct synthesis, affording its three dimensional system of nlarge pore channels to be used in selective oxidation reactions.

Synthesis of ITQ-21 in OH− media

ITQ-21 has been synthesized from F- free gels; aluminum can be incorporated and this does not affect the rate of nucleation. The introduction of stoichiometric amounts of NH4F with respect to the number of double four rings (D4R) present in the structure leads to an increase in the nucleation and crystallization rate with a quantitative incorporation of fluoride ions into the solid.