Synthesising novel microporous materials
Two scientific groups, from the Universities of Liverpool and St Andrews respectively, have been using Diamond’s X-ray Powder Diffraction beamline (I11) to help solve the structure of novel ordered microporous materials, prepared using two different routes.
Porous materials are used in a wide range of applications, such as catalysis, ion exchange, molecular separation or gas storage, and their structural properties, combined with the nature of the atoms that make them up, directly relate to their ability to perform the desired function. However, the fabrication conditions usually determine the crystallographic structure of the end products and, in particular the sizes and shapes of their pores and lattice voids.
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| Crystal structure for porous molecular cage illustrating interconnected diamondoid pore network in yellow |
The chemistry group from St Andrews synthesised a novel aluminophosphate, a zeolite-like material frequently used as a molecular sieve. The large pore inorganic solid, named STA-15 (for St Andrews microporous solid-15), was prepared by hydrothermal synthesis using the tetrapropylammonium cation (TPA+) as a structure directing agent (SDA). The cation had already been used as SDA for the preparation of several other aluminophosphates (such as the widely used AlPO4-5) or silicoaluminophosphates and the great success of the Scottish group resides in the fact that a new material was obtained through the modification of reactant ratios under conditions that had previously been investigated. X-ray powder diffraction performed on I11 on as-prepared and calcined STA-15 samples helped the group obtain the unit cell, the space group and the structural model for the novel crystals. In particular, they established that STA-15 is the fourth AlPO4 known with a one dimensional channel system bounded by 12-membered rings (12 tetrahedral cations and 12 O atoms – 12MRs) and with unconnected channels. In this case, the channels display a markedly elliptical geometry.
Dr Paul Wright is the Principal Investigator for the study.
catalytic conversion of larger molecules. We are now working on synthesising solids doped with metals, such as magnesium, manganese or iron, in order to measure their catalytic properties.”
Dr Paul Wright
The chemists from the University of Liverpool propose a new design principle for preparing porous organic material: they assembled prefabricated tetrahedral organic cages into microporous crystals, capable of adsorbing small gas molecules, such as nitrogen, hydrogen, methane and carbon dioxide. The crystal porosity is then a consequence of both the molecular voids in the cages and the inefficient packing of tetrahedra. Using 3 cages with different vertex functionalities, the group managed to direct the porosity of the organic materials: powder diffraction performed on I11 helped determine that it ranged from closed voids, to a combination of voids and 1D pore channels through to 3D pore networks. Moreover all three cage structures were found to adsorb significant quantities of gas albeit under different physical conditions.
Professor Andrew Cooper is from the University of Liverpool.
produced by combustion before it reaches the atmosphere.”
Professor Andrew Cooper, University of Liverpool
Novel Large-Pore Aluminophosphate Molecular Sieve STA-15 Prepared Using the Tetrapropylammonium Cation As a Structure Directing Agent, Zhongxia Han et al., Chem. Mater., 2010, 22 (2), pp 338–346
DOI: 10.1021/cm902528y
Porous Organic cages. T. Tozawa et al., Nature Materials, 2009, 8, pp 973–978
DOI:10.1038/nmat2545
For more information on the Powder Diffraction beamline, please contact chiu.tang@diamond.ac.uk