Designing Reactive Micro-Environments for Selective Chemical Transformations
Zeolites are quintessential catalysts for a diverse range of applications due to their unique size exclusion properties that, when coupled with catalytically active sites, can facilitate highly selective chemical transformations. The tunable physicochemical properties and macroscopic functionality of these microporous solids enables them to be tailored for specific reactions by engineering the molecular binding sites and surrounding void environments.
Our goal is to design structurally specific zeolite catalysts to enable new, selective pathways in unconventional gas- and liquid-phase reactions. We approach this catalysis and materials design challenge by:
- Identifying relevant chemical descriptors of catalytic behavior
- Understanding processes that regulate zeolite self-assembly and crystal growth
- Developing synthetic methods to control zeolite nanoscale fabrication
Current research topics in our laboratory are described below. A summary of our research capabilities can be found here.
Acid-Base Cooperativity for Carbon-Carbon Coupling
Forming carbon-carbon (C-C) bonds is essential in organic synthesis, and aldol condensation reactions provide pathways to convert carbonyl compounds and oxygenates into higher value products for fuels, chemicals, and pharmaceuticals by combing C-C coupling with oxygen removal. These reactions are catalyzed by Lewis acid-base site pairs found in metal oxides and zeolite Si-O-M ensembles. Basic lattice oxygen (O) sites function cooperatively with isolated metal cations (M) to activate 𝛼-C-H bonds and generate bound enolates that react with polarized carbonyl molecules at the Lewis acid site to form 𝛼,𝛽-unsaturated aldehydes and ketones. Our objective is to understand how the structure and binding strength of acid-base site pairs in zeolites, along with cooperative interactions between solvent molecules, reactive intermediates, and the extended catalyst surface, can be modulated to activate the target adsorbates and affect differences in condensation turnovers and product selectivity between competitive C-C coupling reactions.
Catalytic Oxidation to Reduce Anthropogenic Methane Emissions
Methane (CH4) is a vital energy carrier and chemical feedstock, but contributes to greenhouse gas emissions with a global warming potential more than 80 times that of CO2 over a 20-year span. Complete catalytic oxidation of CH4 to CO2 reduces the environmental impact of CH4 by more than 96% while generating fewer NOx emissions compared to thermal combustion processes. Pd-containing silica zeolites are promising catalysts to effectively oxidize dilute CH4 emissions from remote sources at low temperatures (< 400 ºC) in the presence of water. However, understanding the preferred active sites (metal, support), maintaining catalyst durability in the presence of accompanying poisons (H2S, SOx), and protecting active Pd nanoclusters against sintering remain key challenges. Our objective is to encapsulate monometallic Pd and bimetallic domains within zeolites through direct hydrothermal synthesis to introduce physical barriers to limit sintering, improve sulfur and water tolerance by addition of oxophilic promoters, and tune metal-zeolite interactions to influence CH4 oxidation kinetics and rate-determining steps.
Tandem Catalysis for Direct Epoxidation of Alkenes
Developing multi-functional catalysts to selectively transform light hydrocarbons and their derivatives into a diverse range of energy carriers and chemical intermediates through direct, single-step processes that combine multiple reactions in tandem requires fundamentally examining catalytic behavior at the molecular level. The direct, partial oxidation of propylene to propylene oxide (PO) with hydrogen and oxygen over monometallic Au and AuPd bimetallic nanoclusters on titanosilicate zeolites integrates in situ oxidant generation with alkene epoxidation to yield an intensified process that is safer, greener, and more efficient than multi-step synthetic routes. Unfortunately, current Au catalysts suffer from instability and lack the required combination of kinetics in terms of single pass propylene conversion, PO formation rate, PO selectivity, and hydrogen efficiency to compete with existing commercial processes. Our research aims to advance the chemical understanding of this tandem system by: (1) examining how the confining micropores of titanosilicate zeolites can regulate the formation and stability of Au and AuPd nanoclusters through encapsulation, and (2) determining how the surface geometric and electronic structures of the encapsulated nanoclusters along with cooperative metal-support interactions influence rate-controlling reaction steps, alkene epoxidation rates, and product selectivity.
News and Press Releases
- CEP Magazine: AIChE Journal Highlight – A Chemical Engineer’s Guide to Initiating an Academic Career (April 2020)
- CEP Magazine: Young Professionals Point of View – An Alternate Route: Working Before Graduate School (January 2019)
- Advanced Photon Source: Why Zinc Makes Platinum a Better Catalyst (August 2018)