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

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

Metal-Substituted Microporous Aluminophosphates

This chapter aims to present the zeotypes aluminophosphates (AlPOs) as a complementary alternative to zeolites in the isomorphic incorporation of metal ions within all-inorganic microporous frameworks as well as to discuss didactically the catalytic consequences derived from the distinctive features of both frameworks. It does not intend to be a compilation of either all or the most significant publications involving metal-substituted microporous aluminophosphates. Families of AlPOs and zeolites, which include metal ion-substituted variants, are the dominant microporous materials. Both these systems are widely used as catalysts, in particular through aliovalent metal ions substitution. Here, some general description of the synthesis procedures and characterization techniques of the MeAPOs (metal-contained aluminophosphates) is given along with catalytic properties. Next, some illustrative examples of the catalytic possibilities of MeAPOs as catalysts in the transformation of the organic molecules are given. The oxidation of the hardly activated hydrocarbons has probably been the most successful use of AlPOs doped with the divalent transition metal ions Co2+, Mn2+, and Fe2+, whose incorporation in zeolites is disfavoured. The catalytic role of these MeAPOs is rationalized based on the knowledge acquired from a combination of the most advanced characterization techniques. Finally, the importance of the high specificity of the structure-directing agents employed in the preparation of MeAPOs is discussed taking N,N-methyldicyclohexylamine in the synthesis of AFI-structured materials as a driving force. It is shown how such a high specificity could be predicted and how it can open great possibilities in the control of parameters as critical in catalysis as crystal size, inter-and intracrystalline mesoporosity, acidity, redox properties, incorporation of a great variety of heteroatom ions or final environment of the metal site (surrounding it by either P or Al)

Small pelagic fish supply abundant and affordable micronutrients to low- and middle-income countries

Wild-caught fish provide an irreplaceable source of essential nutrients in food-insecure places. Fishers catch thousands of species, yet the diversity of aquatic foods is often categorized homogeneously as ‘fish’, obscuring an understanding of which species supply affordable, nutritious and abundant food. Here, we use catch, economic and nutrient data on 2,348 species to identify the most affordable and nutritious fish in 39 low- and middle-income countries. We find that a 100 g portion of fish cost between 10 and 30% of the cheapest daily diet, with small pelagic fish (herring, sardine, anchovy) being the cheapest nutritious fish in 72% of countries. In sub-Saharan Africa, where nutrient deficiencies are rising