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

Abstract

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 Brønsted 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 Brønsted 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.

Emphasis on the Properties of Metal‐Containing Zeolites Operating Outside the Comfort Zone of Current Heterogeneous Catalytic Reactions

“…[51] Using quantum mechanical calculations, Louwen et al reported that the improved stability of zeolite Y(Si/Al = 3) upon the introduction of La 3+ can be attributed to the increase in the energy barrier of dealumination. [14] Overall, the literature concerning the introduction of RE metal ions into faujasite suggests an increase in the stability of Al under harsh conditions.I mproved understanding of the nature of RE species and their electronic interaction with the zeolite framework over the last decade will serve to guide future advancements in catalyst stability under harsh conditions ( Figure 2). In particular,the improved stability of the zeolite framework conferred by particular metals provides new research opportunities for FCC catalysts and the next generation of naphtha conversion catalysts as refiners will increasingly need to consider the direct conversion of crude to chemicals.…”

“…Significant progress has been made in the preparation of new zeolite structures and morphologies,such as nanozeolites and hierarchical zeolites,y et their stability and regeneration require further investigation and improvement. Several strategies to address the hydrothermal stability of zeolites have been developed: [10] synthesis in fluoride media affording hydrophobic zeolites with al ow number of defects, [11] postsynthetic silane treatment to protect the zeolite surface from attack by condensed water, [12] post-synthetic modification with phosphorous, [13] or ion-exchange with rare-earth metals, [14] resulting in reduced dealumination. However, some of these methods require the use of extremely hazardous reagents,s uch as HF,a nd can result in changes to the concentration, strength and access to acid sites.…”

Emphasis on the Properties of Metal‐Containing Zeolites Operating Outside the Comfort Zone of Current Heterogeneous Catalytic Reactions

“…Using quantum mechanical calculations, Louwen et al. reported that the improved stability of zeolite Y (Si/Al=3) upon the introduction of La 3+ can be attributed to the increase in the energy barrier of dealumination . Overall, the literature concerning the introduction of RE metal ions into faujasite suggests an increase in the stability of Al under harsh conditions.…”

Recent progress in the improvement of hydrothermal stability of zeolites

“…Small cations La, Pr and Nd were mainly found in the site I′ in the SOD cage, whereas Ce was also located in the supercages forming CeO 2 that inhibits the migration of the cation. Louwen et al 166 found that the activation energy for zeolite Y dealumination was higher in the presence of La 3+ cations. The study was done with a realistic model of the Y zeolite, where the Al distribution matches with previously reported studies by NMR spectroscopy and differs from the thermodynamically most stable distribution, which highlights the importance of knowing the local environment of the active sites.…”