Effect of Ionic Radius of Rare Earth on USY Zeolite in Fluid Catalytic Cracking: Fundamentals and Commercial Application

The ultrastable Y zeolite (USY) in fluid cracking catalyst is commonly stabilized by ion-exchange with rare earth (RE) cations. The RE-exchange provides hydrothermal stability to the zeolite by improving surface area retention, as well as inhibiting dealumination, resulting in greater preservation of acid sites. Though La and Ce are commonly used in fluid catalytic cracking (FCC) catalysts, we have observed that the stability of REUSY catalysts improves as the ionic radius of the RE cation decreases. In this paper, we compare the activity and selectivity of REUSY catalysts, stabilized with La and heavy (Ho, Er, and Yb) rare earth cations, the latter having a smaller ionic radius, due to the well-known phenomenon of lanthanide contraction. The experimental data show that a significant improvement in catalytic activity is achieved when RE elements having a smaller ionic radius are used to make the REUSY catalyst. Yttrium is even more effective than the heavier lanthanides in stabilizing Y-zeolite, leading to higher cracking activity and gasoline selectivity under a variety of deactivation conditions. These benefits of yttrium exchange does not only result from a larger resistance to dealumination, but also to an increase of the catalyst intrinsic cracking activity, which may be explained by changes in the adsorption of hydrocarbons at the active sites. Examples of commercial applications of yttrium-based FCC catalysts are given.