In exploring the location of acid sites in zeolites, as the nature of the local environment proximal to the active site is particularly significant due to strong interactions of reactants, intermediates, and transition state species with micropores of molecular dimension. Surface Brønsted acid sites (SBAS) on external crystallite surfaces sometimes contribute negligibly to overall rates and selectivities due to the absence of these confinement effects but can dominate rates in diffusion-limited reactions such as polyolefin hydrocracking. Published correlations for alkylation reactions between trimethylbenzene and dibenzyl ether were utilized to determine SBAS densities in (modified) microporous MFI zeolites and extended to large-pore zeolites using 1‑pyrenemethanol (PM) etherification, which is diffusion-prohibited from accessing microporous BAS. These techniques were also extended to mesopore-containing hierarchical zeolites and silica-alumina catalysts to determine the impact of mesopores on site accessibility and catalyst activity.
Reaction-based techniques utilizing micropore-inaccessible molecules to determine BAS distributions can also be applied to bifunctional zeolite-metal nanoparticle catalysts. Ligand-assisted hydrothermal syntheses were developed for encapsulation of Au, Pd, and AuPd nanoparticles within MFI and BEA zeolites. Although S/TEM and high temperature exposure studies indicated the prominence of nanoparticles within micropores, the use of reaction probes was necessary for quantification of external and internal metal. Benzyl alcohol (BA) oxidation reactions with dibenzothiophene (DBT), where catalytic turnovers only proceed on internal metal, were not optimal for determining encapsulation efficiency, since DBT does not completely poison nanoparticles on external surfaces. This was probed using Pd on silica for BA oxidation in varying solvents (i.e., aliphatic, aromatic) with titrants of different adsorption propensities (i.e., thiol, aromatic) to Pd. Notably, DBT exhibited lower site coverages in neat BA solvent relative to fluorene, and these differences which were rooted in selective adsorption to edges/corners and facets, respectively. However, DBT and fluorene site coverages converged in n-decane solvent due to weaker titrant-solvent interactions that dominated site-selective adsorption behavior and rendered similar adsorption equilibria. Given these insights, the assessment of nanoparticle encapsulation in zeolites was ultimately enabled by PM reactions; this reaction occurred on silica-supported Pd, but insignificant oxidation activity was observed on zeolite-encapsulated Pd nanoparticles.
Research Interests
My graduate research investigated how active site environment and accessibility affect catalyst activity through metal and acid site characterization, (bifunctional) zeolite synthesis, and development of structure-function relationships for various chemistries catalyzed by metal-free and metal-loaded zeolites. Development of these thrusts throughout my PhD journey has impassioned me particularly in research that combines aspects of catalyst synthesis with kinetics to inform reaction behavior. Although I have studied how deliberate selection of reaction probes is a powerful method to characterize a single (suite of a) catalyst, I am also interested in exploratory catalyst synthesis research to further a specific chemistry. Ultimately, I would like to utilize my catalysis research experience for development of sustainable chemistries for greener fuels, chemicals, and pharmaceuticals production.