David E. W. Vaughan

Studies of Fe(III) incorporated into AlPO4‐20 by X‐ and W‐band EPR spectroscopies

The incorporation of Fe(III), during the synthesis, into aluminosilicate sodalite (FeSOD) and aluminophosphate sodalite, AlPO4‐20 (FAPO), was investigated by continuous wave (CW) and pulsed electron paramagnetic resonance (EPR) techniques at X‐ and W‐band. Specifically, the effect of the framework composition and the presence of occluded template molecules (tetramethyl ammonium hydroxide, TMAOH) in the β cages on the distribution of the Fe(III) species was explored. The X‐band CW EPR spectrum of FAPO shows the existence of two types of species, one with a large (g ≈ 6.3,4) and the other with a small (g ≈ 2) zero field splitting (ZFS) interaction. These species were also found in FeSOD synthesized with TMAOH. The X‐band field‐sweep echo‐detected (FS‐ED) EPR spectrum shows contributions only from the Fe(III) species in the more symmetric environment (g ≈ 2). The other was not detected due to fast relaxation. This spectrum is very broad and suffers from distortions due to the nuclear modulation effect. In contrast, the W‐band FS‐ED EPR spectrum of the same species was significantly narrower and free from distortions. Analysis of the temperature dependence of the width and relative intensity of the peak corresponding to the |−½ 〉 → | +½ 〉 EPR transition shows that the g ≈ 2 signal arises from a number of Fe(III) species with a distribution of ZFS parameters. Calcination significantly reduces the ZFS parameter, D, suggesting that the distortions of the T sites are due to specific interactions with the template. Electron spin echo envelope modulation (ESEEM) experiments shows the presence of weak dipolar interaction between Fe(III) and template 14N and 1H template nuclei, as well as framework 27Al and 31P nuclei. This indicates that the species characterized by small ZFS are well dispersed and are located within the inner structure of the zeolite. These g≈ 2 species are most probably Fe(III) in framework sites. A small fraction that occupies highly asymmetric sites (g ≈ 6.3,4), situated at ‘defect’ framework or extraframework sites, and some Fe(II) produced due to the reduction of Fe(III) by the organic template (detected by Mössbauer spectroscopy), were found as well. The possible presence of some extraframework Fe(III) with a g ≈ 2 signal cannot be excluded. Copyright

Hydrothermal synthesis of mixed octahedral-tetrahedral oxides: Synthesis and characterization of sodium stannosilicates

Abstract New microporous framework structures comprised of octahedra and tetrahedra have been investigated in low temperature, hydrothermal syntheses in the Na 2 O-SnO 2 -SiO 2 system. Initial studies have resulted in several new and highly crystalline stannosilicate ion-exchange capacity, and have remarkable variability depending on the choice of cation or the source of SnO 2 or SiO 2 employed in a reaction. Solid state NMR studies have been initiated, along with other spectroscopic work, to structurally characterize these microcrystalline phases.

The structure of zeolite ZSM-20: mixed cubic and hexagonal stackings of faujasite sheets

Zeolite ZSM-20 comprises faulted block intergrowths of cubic and hexagonal stackings of faujasite sheets, providing a first authentic example of regions of this hexagonal faujasite variant.

Recent Advances in Techniques for Characterizing Zeolite Structures

In developing new zeolites for catalysis and separations, the characterization of structure is an important step in the identification of possible applications. By analogy with the properties of zeolites for which structures are known (of which there is a large and growing data base), possible applications for a new zeolite can be identified once its essential structural features are known. Structural details at several degrees of precision and over a range of length scales are usually sought. These range from local active site geometries or framework cation orderings, through descriptions of the framework structure and pore architecture, to questions of crystal perfection, homogeneity and mesoporosity. Frequently the most time-consuming step is the initial definition of the framework topology. The unavailability of large single crystals of practically all synthetic zeolites, the relative complexities of zeolite structures, and a series of materials-related difficulties combine to limit the usefulness of conventional structure determination methods.

The structure of zeolite CSZ-1 interpreted as a rhombohedrally distorted variant of the faujasite framework

In contrast to earlier suggestions that the structure of CSZ-1 corresponds to the theoretical hexagonal structure 6 of Breck, evidence from a combination of techniques demonstrates that it is instead topologically similar to the faujasite framework, differing in having rhombohedral, rather than cubic, symmetry.

Structure Direction in Zeolite Synthesis

Developments in instrumental and computational techniques, combined with a major expansion in zeolite synthesis experiments around the world, are promoting a wider understanding of zeolite synthesis mechanisms. This paper explores an approach to structure synthesis by design using several examples of zeolites known to co-crystallize in the same, or similar, synthetic gel compositions, including LTA:SOD:FAU; FAU:BSS; ZSM-5/11; ABC 6-ring series; LTL:OFF:ERI. Extended structures interacting with templates as strong structure directing entities play a major role in specific crystal formation. Particular attention is given to phases which develop at overlapping crystallization field boundaries, where two or more extended building structures may interact to form new zeolites, as in the cases of MAZ: MOR: ECR-1 and MER: RHO: PAU.

Orientation of Chemical Properties by Direct Synthesis of Molecular Sieves

This subject was discussed on the basis of a broad collective experimental experience and not only that of the presented papers (1). The classical concept of a nuclei as a critically sized “species” allowing spontaneous growth in a system by virtue of lowering the free energy of the “solution” — was accepted. This may occur spontaneously in a supersaturated system or by the addition of such material promoters from an external source. The issue of whether the latter are nuclei or seeds was discussed at length, and depending on the specific zeolite or synthesis slurry in question, both mechanisms may be invoked. Nuclei in this case are structureless in the sense that such a source may induce crystal growth of many different structures in several different gel systems, whereas seeds are viewed as being structure specific. They direct the crystallization to only one structure — that of the seed.