Determination of the 5A zeolite acidity: an infrared study

Abstract The acidic character of 5A zeolite as a function of the calcium content has been explored by different techniques: propylene adsorption experiments, ammonia thermodesorption followed by microgravimetry and FTIR spectroscopy. Propylene is chemisorbed and slowly transformed in carbonaceous compounds (coke) which remain trapped inside the zeolite pores. The coke quantities increase with the Ca 2+ content. Olefin transformation results from an oligomerization catalytic process involving acidic adsorption sites. Ammonia thermodesorption studies as well as FTIR experiments have revealed the presence of acidic sites able to protonate NH3 molecules. This site number is also correlated to the Ca 2+ ion content. As it has been observed for FAU zeolite exchanged with di- or trivalent metal cations, these sites are probably CaOH + species whose v as (OH) mode have a spectral signature around 3567 cm −1 .