Solid-state NMR and powder XRD studies of the structure of SAPO-40 upon hydration-dehydration cycles

It is well known that after the removal of the template many porous aluminophosphates and related materials are very sensitive to water.' Depending on the type of structure, reversible or irreversible phase transitions, loss of crystallinity and changes in the coordination of some framework A1 upon rehydration are observed. For example, solid-state NMR shows that the rehydration of SAPO-5 leads to the formation of octahedral Al. Subsequent dehydration restores the initial tetrahedral coordination of Al. Template-free SAPO-37 becomes totally amorphous to X-rays after exposure to water and stays so after subsequent thermal treatment^.,,^ In contrast, Barthomeuf and co-workers have shown recently, that, on hydration, template-free SAPO-34, an analogue of chabasite, shows the opening of some Si-0-A1 bonds, the effect being reversible upon dehydrati~n.T~h e hydrated distorted structure was found to be stable for months with no further modifications and the ordered material could be regenerated by removal of water. Here we wish to report that the structure of template-free SAPO-40 undergoes a similar reversible modification.

An overview of nonlinear identification and control with neural networks.

The aim of this chapter is to introduce background concepts in nonlinear systems identification and control with artificial neural networks. As this chapter is just an overview, with a limited page space, only the basic ideas will be explained here. The reader is encouraged, for a more detailed explanation of a specific topic of interest, to consult the references given throughout the text. Additionally, as general books in the field of neural networks, the books by Haykin [1] and Principe et al. [2] are suggested. Regarding nonlinear systems identification, covering both classical and neural and neuro-fuzzy methodologies, Reference 3 is recommended. References 4 and 5 should be used in the context of B-spline networks.