The design and understanding of the mechanism of formation of nanoporous catalytic materials

• Main Author: Simmance, K.
• Published: University College London (University of London) 2011
• Subjects: 540
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.565442

The work described in this thesis is concerned with characterisation of metal-ion substituted aluminophosphates using synchrotron radiation techniques. The aim of this work is to expand the knowledge on the mechanistic aspects of formation and the framework substitution of nanoporous materials thereby enabling the design of the synthesis strategy. The simultaneous measurement of X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) was used to follow the formation of cobalt substituted AlPO-5 (AFI). The results obtained revealed that a cobalt-phosphorus network is formed prior to crystallization and with the aid of an in situ XRD study it was possible to propose a solid hydrogel transformation mechanism. A study on the effect of the type of metal ion, organic template and metal concentration on the crystallisation of AFI materials revealed a distinct trend in the kinetics of formation with increasing metal concentration. This provided conclusive evidence that the trend seen in the kinetics is not related to the appearance of the chabazite phase, as previously suggested; the kinetic study also indicated that Zn(II) ions promote the formation of smaller rings in aluminophosphate materials. While XAS provides information on the local structure, it is an average of all the environments including extra-framework cations, therefore XRD methods, which are sensitive to the long-range order, were employed to study the extent of metal substitution in large-pore aluminophosphates. An expansion of the lattice was observed upon framework incorporation of metal ions; however results highlighted the need for the use of combined XRD and XAS techniques for the determination of the structure of multi-ion substituted materials. Furthermore, XAS and FTIR studies of cobalt substituted SAPO-34 materials highlighted the importance of the synthesis method on the incorporation of metal ions into the framework and their effect on the conversion of methanol to olefin.