Role of Rare Earth Ions in the Prevention of Dealumination of Zeolite Y for Fluid Cracking Catalysts

In this work, we use quantum-mechanical calculations to examine a number of possible dealumination routes in a model reminiscent of a commercial Y zeolite (Si/Al ratio of 3) as used in fluid catalytic cracking. First, we determine the distribution of Al over the zeolite lattice. The thermodynamically most stable distribution found in our calculations does not match the aluminum distribution found experimentally with NMR. We then describe the design of a periodic structure model to better fit the experimental distribution for zeolite Y with a Si/Al ratio of 3. This new model is used to determine the mechanism of dealumination in the absence and presence of stabilizing La3+ ions. It was found that the dealumination pathways with La3+ present in this model lead to higher activation energies, supporting a stabilizing effect of rare earth ions on the dealumination. The local environment (with both Bronsted and Lewis acid sites) has a large effect on the transition states and intermediates. While the local environment destabilizes pathways that involve protonation of oxygen atoms near another Bronsted acid site, it reduces the barriers of other pathways by coordinating newly formed OH groups in the transition states. These findings imply that the realistic aluminum distributions provided by our model are a prerequisite for this type of study.