Hydrothermal aging of cracking catalysts. II. Effect of steam and sodium on the structure of LaY zeolites

The crystal structure analysis of a highly-exchanged LaY zeolite at 660°C, in the presence of steam, shows that there are two La3+ cations per sodalite cage bridged by two hydroxyl groups which form TOLa(OH)2LaOT (T = Si, Al) linkages stretching across the cages. A radial electron distribution study of the structure of partially or completely destroyed LaY zeolites shows that the TOLa bonds are broken, however, the La3+ ions remain associated in the form of an oxide dimer. Therefore, the mechanism of zeolite aging involves hydrolysis of the TOLa bonds and the transient, local formation of HY zeolite domains which are not stable at high temperature. The occlusion of NaCl salt in the zeolite framework leads to a lower thermal stability. Crystal structure analysis indicates that the linkages involving La3+ ions in sodalite cages are not present after salt occlusion which may account for the lower thermal stability.