Recycling of Spent FCC Catalysts: Conversion of Leached Residues to Zeolite ZSM-5

The commercial recycling of spent FCC catalyst typically focuses on recovering only 1-3 % of rare-earth elements, with the remaining residues often disposed of in landfills. Here, we present a novel method to close a recycling loop for spent FCC catalyst. The method involves a series of leaching steps: Firstly, the spent catalyst material is leached with HNO₃ to remove rare-earth elements such as La; second, solvothermal leaching with HCl removes most of Al and impurities like Fe, Ni and V; finally, a third leaching with H₂SO₄ removes Ti. The solid residues are then used to synthesize ZSM-5 without the addition of any extra silicon or aluminum sources after mild activation. The impurities in the synthesis gel strongly modify the properties of the zeolite, with ZSM-5 crystals containing higher levels of impurities exhibiting lower crystallinities, surface areas, acidities, cracking activities, as well as larger particle sizes.