Abstract
Zeolite-supported metal catalysts are an integral part of heterogeneous catalysis, with applications in petrochemical processing, biomass conversion, conversion of aromatics, and automobile exhaust aftertreatment. In addition to displaying excellent catalytic activity, selectivity, and hydrothermal stability, zeolites enable tuning of the metal distribution and speciation, ranging from isolated cations to nanoparticles, by means of variation in synthesis, compositional parameters such as Si/Al ratio, metal loading, and gas treatments. Metals in zeolites can exist as (a) cations exchanged in the zeolitic framework, (b) clusters (< 2 nm) encapsulated in the zeolite cages, and (c) extra-crystalline nanoparticles located on the outer surface of zeolite crystallites. The dynamic chemical nature of the metal active sites in ion-exchanged zeolites warrants a molecular level investigation of their speciation and interaction with gas molecules under reaction conditions of interest, to understand the deactivation and regeneration mechanisms of zeolite-supported metal catalysts. Here, we studied zeolites ion-exchanged with Pd and Cu by employing computational modeling and tools such as density functional theory (DFT), wave function theory (WFT), ab initio molecular dynamics (AIMD) simulations, first principles based thermodynamic calculations, and kinetic Monte Carlo (kMC) simulations. We first investigated hydrothermal deactivation and regeneration thermodynamics and mechanisms for Pd-zeolites. We determined Pd cation speciation under different conditions and used this information to study the redispersion of extra-crystalline Pd nanoparticles to ion-exchanged Pd2+ cations in zeolites. We found that H2O pressure plays a critical role in determining the phase boundary between Pd cations and Pd-oxide, and redispersion from Pd-oxide to Pd cations follows an Ostwald ripening-like mechanism. Next, we studied chemical deactivation via sulfur for commercial Cu-zeolites to determine Cu species that are particularly susceptible to irreversible poisoning. Ab initio thermodynamic models for sulfur poisoning of Cu-CHA zeolites showed that Cu dimers that form at high temperatures strongly bind SO2 and SO3, forming (bi)sulfated Cu dimers with remarkable thermodynamic stabilities, that require high temperature (> 870 K) desulfation treatments for catalyst regeneration. Taken together, these studies point towards molecularly detailed design rules for improving the thermal and chemical stability of metal-containing zeolite catalysts.
