Structural and morphological evolutions of spent FCC catalyst pellets toward NaA zeolite

摘要

Spherically shaped pellets, usually discarded after use in fluidized-bed catalytic cracking units (FCCs), were converted to zeolite A-surface-enriched pellets. The final pellets were obtained through a two-step treatment consisting of: (1) alkaline thermal activation followed by (2) a hydrothermal crystallization in selected reaction conditions. The alkaline thermal activation provided pellets mainly constituted by silico-aluminate compounds. When the pellets are heated at 800 A degrees C in contact with sodium carbonate, a structural rearrangement occurs which includes nepheline, crystalline aluminosilicate, and an amorphous fraction expected to be silicon-enriched, preserving the pellet original geometry. Afterward, reaction mixtures were prepared by adding sodium hydroxide solution to the heat-treated product. Then, commercial sodium aluminate was added. During the hydrothermal synthesis at 85 +/- A 3 A degrees C, zeolite A was formed from the calcined product and NaAlSiO4 turned out to be an intermediary crystalline compound. The LTA crystals were already observed for a reaction time of 0.5 h, but the highest conversion to pure zeolite A was reached after 6 h. Those final round solid pellets showed an external surface fully covered by well anchored zeolite A cubic crystals about 1-2 mu m in size. Such materials may be very useful in ion exchange, molecular sieving, and adsorption processes. The crystalline ordering was followed by XRD, SEM, NMR, and water adsorption.