Structural characterization of zeolitic catalysts in non-ambient conditions by X-ray and neutron diffraction

This PhD thesis strives to give an exhaustive characterization of the behaviour of zeolites in nonambient conditions. Zeolites presents a lot of peculiar characteristics, such as shape selectivity, high thermal stability, but the most important is their catalytic activity, which have been evaluated for many petrochemical reactions, such as gas-oil cracking and hydrocracking, aromatic alkylation and isomerisation ecc. The catalytic activity of zeolites is due to their particular structure. These minerals, in fact, are characterized by large pores and channels. Only molecules with determined dimensions, consistent with the pores dimensions, can have access to the acid sites located inside these cavities and can be processed. The importance of a detailed structural characterization is due to the fact that it is necessary to explain the efficiency of these materials, their stability as well as the different types of shape selectivity seen in a wide range of catalytic systems. The understanding of the zeolite behaviour upon heating is of particular importance since the sorptive and catalytic properties, molecular sieve effects are enhanced in the dehydrated/calcined phases. Besides, it is important to study the response to heating because reactions involving hydrocarbons need high temperature (about 300-500°C), so it is important that zeolites used for these reactions have a good thermal stability, and it is important to localize acid sites. The study of the characterization of these materials in non-ambient conditions was carried out starting from structure analysis of diffraction data collected on powder or single crystals, using Xray (conventional source or synchrotron radiation) or neutron diffraction. The main results obtained in this PhD thesis can be summarize as follows: 1) zeolite tschernichite. It has been evaluated how defects due to the simultaneous presence of two different polytypes affect the thermal stability of this material; 2) zeolite gmelinite. The most important results is that it was possible to observe that gmelinite, when heated at T>330°C transform into a new phase (AFI-type) which is very important for catalysis and has a good thermal stability; 3) zeolite omega. It was possible to follow in real time the activation of this important catalyst and its acid sites were localized; 4) zeolite ferrierite and heulandite: protons bonded to framework oxygens were located, and it is put in evidence the importance of water interaction with these catalysts.