Research Interests
I use quantum chemistry techniques including density functional theory (DFT) in conjunction with classical molecular simulations to elucidate the kinetics of chemical reactions and the diffusion of reactants and products in nanoporous materials. In close collaboration with experimentalists, I have developed fundamental insight into how the structure and function of nanoporous catalysts can be controlled to enable new chemical transformations.
Microkinetic Modeling of Zeolite Active Sites
Zeolites with incorporated metal atoms can behave as Lewis acids and catalyze a variety of chemical reactions. The acid site identity, size and shape of the pore environment, and framework defects can be modified to affect the stability of adsorbates and transition states. Certain adsorbates can even reversibly dissociate at metal sites producing distinct active site speciations that differ in their catalytic properties. I have shown using DFT simulations and kinetic modeling with rate constants derived from DFT simulations (microkinetic modeling) that these acid site speciations have a prominent role on reaction kinetics. These results were developed in collaboration with experimentalists where we compared computer simulated and experimental reaction rates to validate our conclusions. I’ve also shown that accurately determining the entropy of confined reactive intermediates is central to predicting turnover rates.
See the following publications to learn more!
B.C. Bukowski, J. Bates, R. Gounder, J. Greeley, “First Principles, Microkinetic, and Experimental Analysis of Lewis Acid Site Speciation During Ethanol Dehydration on Sn-Beta Zeolites”, Journal of Catalysis 365, 261-276, 2018.
J.S. Bates, B.C. Bukowski, J.W. Harris, J.P. Greeley, R. Gounder, “Distinct Catalytic Reactivity of Sn Substituted in Framework Locations and at Defect Grain Boundaries in Sn-Zeolites”, ACS Catalysis 9 (7) 6146-6168, 2019.
B.C. Bukowski, J. Greeley, “Scaling relationships for molecular adsorption and dissociation in Lewis acid zeolites”, The Journal of Physical Chemistry C 120 (12), 6714-6722, 2016.
Ab-initio Molecular Dynamics of Solvents Confined in Microporous Voids
Catalyst active site identity and pore/size shape directly influence adsorbate and transition state stability, but they also influence the structure of solvents that in turn affect reaction kinetics. I used ab-initio molecular dynamics simulations to study the structure and stability of water networks around common zeolite acid sites and defects. I found that water networks were significantly perturbed by the defect type, with some strongly localizing small water clusters and others nucleating extended clusters. I further showed that these clusters were locally stable thermodynamic phases, introducing a computational methodology where only the local solvent structure is modeled. By finding the locally stable solvent structure, other researchers could simplify their solvent models in porous catalysts while still representing thermodynamic equilibrium.
We tested these models with ethanol dehydration in H-Beta near pore condensation conditions where I used metadynamics to show how these confined solvents dramatically change the shape and orientation of dehydration transition states from their gas phase analogues. These results agreed with our experimental collaborators both validating our results and providing deeper intuition into rationalizing the role of pore size and shape on kinetics.
See the following publications to learn more!
B.C. Bukowski, J.S. Bates, R. Gounder, J. Greeley, “Defect-Mediated Ordering of Condensed Water Structures in Microporous Zeolites”, Angewandte Chemie International Edition 58 (46), 16422-16426, 2019.
J.S. Bates† and B.C. Bukowski†, J.P Greeley, R. Gounder, “Structure and Solvation of Confined Water and Water-Alkanol Clusters within Microporous Brønsted Acids and their Effects on Alkanol Dehydration Catalysis”, Chemical Science. in press. †denotes equal contribution from both authors.
Dependence of Micropore Void Connectivity and Shape on Molecular Diffusion
While metal sites and framework defects can affect the structure of solvents in porous catalysts, the connectivity and shape of pores will also affect molecular diffusion. Diffusion in nanoporous materials is especially important for reactants and products that may inhibit the observed rate, or favor the formation of unselective side products. Metal-organic frameworks (MOFs) are comprised of metal oxide nodes connected by organic linkers, and the spatial arrangement of nodes, linkers, and their connectivites define a topology. MOFs can have polymorphism where the same set of nodes and linkers produce different topologies that can have dramatically different pore environments. Using MOFs, we can systematically study how different topologies (different pore architectures) impact diffusion. I use classical molecular simulations and data-science techniques including machine learning to develop design criteria for picking optimal MOF topologies based on adsorbate diffusion.
See the following publications to learn more!
B.C. Bukowski and R.Q. Snurr “Topology-dependent Alkane Diffusion in Zirconium Metal-organic Frameworks”, ACS Applied Materials and Interfaces, in press.
R. Wang, B.C. Bukowski, J. Duan, T.R. Sheridan, A. Atilgan, K. Zhang, R.Q. Snurr, J.T. Hupp, “Investigating the Process and Mechanism of Molecular Transport Within a Representative Solvent-filled Metal-Organic Framework” Langmuir 36 (36) 10853-10859.