Philip A. Barrett

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.

ITQ-12: a new microporous silica polymorph potentially useful for light hydrocarbon separationsElectronic supplementary information (ESI) available: details of the structure solution, Rietveld refinements in space groups C2/m and Cm and energy minimisation calculations in C2/m, Cm and C2. See http://www.rsc.org/suppdata/cc/b3/b306440a/

The new synthetic form of microporous crystalline silica, denoted as ITQ-12, shows a high potential for the separation of propane and propene from its mixtures.

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.

Synthesis, characterization and control of faulting in STF/SFF topologies, a new family of intergrowth zeolites

A new family of zeolites, which can be described as intergrowths between the STF and SFF structure types has been prepared. The degree of faulting between the two end-member structures can be varied across the full range of possibilities, from almost pure STF through to almost pure SFF. This control over the faulting in the materials is accomplished by carefully changing the synthesis conditions and by addition of fluoride ions as a mineralising agent, but for the first time using the same structure directing agent and the same framework composition in all preparations. The materials have been characterized by a number of techniques, including X-ray diffraction, transmission electron microscopy and MAS that supports the intergrowth nature of the samples.

ITQ‐7: ein neues, reines Silicatpolymorph mit dreidimensionalem, großporigem Kanalsystem

Eine neuartige Topologie, ein dreidimensionales System aus grosporigen Kanalen, weist der Zeolith ITQ-7 auf (Struktur siehe Bild). Das Material wurde durch Kombination der strukturdirigierenden Einflusse von Fluoridionen und einem organischen Kation synthetisiert, das dafur konzipiert wurde, die Kristallisation einer Silicatphase mit geringer Dichte zu fordern. Bei ITQ-7 handelt es sich um die bisher am wenigsten dichte kristalline Silicatphase.

Synthesis of microporous gallosilicates with the CGS topology

Zeolite TNU-1 (Taejon National University Number 1), a gallosilicate possesing the CGS topology with a three dimensional system of medium and small pores, crystallises in the presence of Ga in conditions in which the presence of Al yields merlinoite-type zeolites.

Synthesis of a merlinoite-type zeolite with an enhanced Si/Al ratioviapore filling with tetraethylammonium cations

A merlinoite type zeolite with an enhanced Si/Al ratio (around 3.8) compared to known isostructural materials has been synthesised hydrothermally from a gel with Si/Al=5 in the presence of tetraethylammonium and K+ cations. The zeolite has been characterised by a number of techniques and in its calcined form the structure was refined from synchrotron powder X-ray diffraction data [a=14.1291(10), b=14.1308(10), c=9.9274(5)A, Immm]. The different techniques reveal the enhanced Si/Al ratio of the zeolite. Tetraethylammonium is occluded within the tridimensional small pore channels, forcing the K+ and hence the Al content to diminish by 30–50% compared to conventional merlinoite-type zeolites. The thermal stability of the material is greatly improved, although if K+ cations in the zeolite are exchanged by NH4+ the stability decreases as a function of the degree of exchange, suggesting a low stability for the acidic form of merlinoite.

Rational design of a solid acid catalyst for the conversion of methanol to light alkenes: synthesis, structure and performance of DAF-4

By careful choice of the appropriate organic template, new shape-selective solid acid catalysts—framework-substituted microporous aluminium phosphates (AIPO4) that are structurally analogous to the mineral levyne—are designed, synthesised and characterized; they convert methanol preferentially to ethane and propene.

SYNTHESIS AND STRUCTURE OF ITQ-9 : A NEW MICROPOROUS SIO2 POLYMORPH

The new pure silica polymorph ITQ-9 has been synthesised and its structure, comprising medium pore channels and cages, has been solved by direct methods using low resolution powder X ray diffraction data.