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

Abstract

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 °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 ± 3 °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 μm 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.