Fundamental Insights into the Impact of Extra-Framework Cations on the Stability of Zeolite Structures and Propane Cracking Kinetics

Abstract

Fluid catalytic cracking is one of the most important processes in petroleum refining, breaking long chain hydrocarbons into smaller and higher value products. The efficiency of this process is governed by the stability and activity of zeolites which can be enhanced through the introduction of rare earth (e.g. lanthanum, cerium, and yttrium in FAU zeolites) and phosphoric species (in MFI zeolites) into their pore structure. These additives, in addition to extra-framework aluminum, have been observed to modify product selectivity. This study focuses on using density functional theory to understand the role of these zeolite additives and precisely determine their nature and location within the zeolite framework. Here it is found that in zeolite Y (FAU) lanthanum species preferentially occupy highly confined sites with low hydroxide coordination at low lanthanum loading. However, at higher loadings, lanthanum clusters will form. Phosphorus in MFI is found to preferentially adsorb on framework acid sites through hydrogen bonding interactions. If it becomes dehydrated, the phosphorus species can bind directly to the framework through Al-O-P bonds. These added species have less acid strength than a framework Bronsted acid site and are not likely to have comparable reactivity but may act as Lewis acid sites. Extra-framework aluminum in MFI zeolites have been found to stabilize the propane cracking transition state through dipole-dipole interactions. By elucidating the location of zeolite additives and their influence on reactivity, we can better characterize zeolite behavior and tune selectivity to industrial applications.