Influence of Superacid Sites in Ultrastable Y Zeolites on Gas Oil Cracking

Gasoil Cracking has been carried out on two USY samples with unit cell sizes of 24.47 and 24.31 Å, which were steamed at 650° and 650° plus 725°C, respectively. The results were compared with those obtained with the same series of samples after removing most of the extraframework aluminum (EFAL) by (NH4)2SiF6 treatment. In the zeolite steamed at lower temperature, the chemical treatment produces a strong decrease on the cracking activity. This decrease is not due to surface framework dealumination, as shown by XPS analysis and n-heptane cracking, but to the absence of EFAL. The removal of EFAL also produces an increase in gas and coke formation. Much less marked effects are observed on the high temperature steamed zeolites after being chemically treated. Gasoil cracking, activity and selectivity of the 24.47 Å USY sample after removing EFAL and being subject to an ulterior high temperature steaming gives slightly less gases and coke and more gasoline than a USY sample with the same unit cell size but containing all the EFAL.