Biomedical subjects

Richard J Darton

Publications and source records attributed to Richard J Darton.

3 recordsLinked to original sources

PubMed2006

Structure-directing agent location and non-centrosymmetric structure of fluoride-containing zeolite SSZ-55.

Single crystals of pure silica zeolite SSZ-55 were prepared using the fluoride route. Single-crystal X-ray diffraction at a synchrotron source revealed the framework structure of the material, but the unit cell (orthorhombic a = 12.905(2) A, b = 21.344(4) A, c = 5.1279(10)) is too small to accommodate ordered arrays of the organic structure-directing agent. Molecular modeling was used to simulate the docking of the structure-directing agent in the channels of the material, and this revealed a strong space-filling interaction with a number of possible orientations of the organic cation. The overall non-centrosymmetric structure of the solid (spacegroup C222(1)) was confirmed using second harmonic generation experiments.

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PubMed2005

A solid-state NMR method for solution of zeolite crystal structures.

Since zeolites are notoriously difficult to prepare as large single crystals, structure determination usually relies on powder X-ray diffraction (XRD). However, structure solution (i.e., deriving an initial structural model) directly from powder XRD data is often very difficult due to the diffraction phase problem and the high degree of overlap between the individual reflections, particularly for materials with the structural complexity of most zeolites. Here, we report a method for structure determination of zeolite crystal structures that combines powder XRD and nuclear magnetic resonance (NMR) spectroscopy in which the crucial step of structure solution is achieved using solid-state (29)Si double-quantum dipolar recoupling NMR, which probes the distance-dependent dipolar interactions between naturally abundant (29)Si nuclei in the zeolite framework. For two purely siliceous zeolite blind test samples, we demonstrate that the NMR data can be combined with the unit cell parameters and space group to solve structural models that refine successfully against the powder XRD data.

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PubMed2005

Studies on the role of fluoride ion vs reaction concentration in zeolite synthesis.

This work is an experimental response to an intriguing paper recently published by Catlow and co-workers, which looked at the computational feasibility of fluorine location in three different all-silica zeotypes (Attfield, M. P.; Catlow, C. R. A.; Sokol, A. A. Chem. Mater. 2001, 13, 4708). The materials were chosen as representative of three unique host locations. Our present work examined the synthesis of zeotypes AST, IFR, and MTT using organo-cations with a strong preference for crystallizing these structures. We studied the effect of reaction time and the H(2)O/SiO(2) reactant ratio. The latter is probably the most important function in these zeolite crystallizations that use HF. As reaction conditions became more dilute, AST gave way to SGT and IFR to MTW as host structures, while the MTT synthesis was invariant. Our reactions were studied in terms of product yield vs time, product organo-cation content, fluorine content, and the representative (29)Si and (19)F NMR spectra for certain samples. A single crystal study was carried out for a sample of MTT. Our results showed that, consistent with other recent studies, low H(2)O/SiO(2) reactant ratios lead to more open framework host structures (i.e., IFR vs MTW), and there is typically a higher uptake of organo-cation and fluorine. The structure may well contain a higher population of 4-rings within the silicate substructure. While MTT that contains no 4-rings was chosen as the best possible candidate to achieve an ion-pair for the organo-cation and fluoride anion within the silicate host, both NMR and single crystal work confirm that fluoride is bonded to a 5-coordinate silica center within the lattice.

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