Electron Microscopy characterisation of fluorinated zeolites

Abstract

Zeolites constitute one of today 's most important materials in Catalysis and their characterisation, in every respect, remains a challenge for scientists in the field. Generally speaking, zeolites can be classified as follows: a) High Si/Al ratio zeolites and b) Zeolites with a small Si/Al relationship. Obviously, many of the properties are quite different for the two types of zeolites since low Si/Al zeolites have less acidic sites of high catalytic strength. Thus, it is important to devise a simple and efficient way to modify low Si/Al zeolites to have higher Si to A1 ratios. One way for achieveing this is via the modification with F of 4A-type zeolites. Figures 1 and 2 show a comparative EDX analysis of an unchanged zeolite and of a fluorinated one, respectively. As can be observed there, the Si/Al ratio remains basically the same, result that is contrary to some previous reports which claimed a loss of A1 during the fluorination. High Resolution Electron Microscopy, on the other hand, shows interesting changes at the microstructural level since some amorphization of the fluorinated samples was detected, as can be observed in the digitally processed high resolution image of figure 3. From our electron microscopy studies we conclude that, for low-pressure fluorination conditions, the zeolites behaves as an inert support for the F whereas for higher pressures the F is incorporated to the zeolite structure