Research

Our Approach to Catalysis and Materials Science Research

Our research group studies the science of heterogeneous catalysis and the molecular details of chemical reactions.  

Our choice of research problems is inspired and guided by applications of catalysis to convert lower carbon-footprint feedstocks into more useful forms of energy and chemicals, and to protect our natural environment from harmful pollutants and emissions from automotive and other sources.

We use an integrated and balanced approach to experimental catalysis research that is complemented by a global network of collaborators in academia, national laboratories, and industry.

Research projects in the Gounder group combine state-of-the-art methods in the synthesis of catalytic materials, the structural and functional characterization of active sites and supports (during reaction when possible), and the quantitative evaluation of kinetic function and mechanistic behavior. 

This integrated and balanced experimental approach is used to study active site requirements, reactive intermediates and transition state structures, and elementary steps in reaction mechanisms.  

We augment the quality and impact of our research through a rich network of collaborations with other scientific experts, including theoretical and experimental research groups in academia, research teams and user facilities in national laboratories and other research centers, and researchers and practitioners in industrial organizations.

Our overarching goal is to use fundamental insights from experiment, together with guidance from theory, to develop structure-function relations that can predict how reactant and catalyst structures influence reactivity and selectivity. 

Ultimately, we aim to use this information to inform catalyst design and selection for new and existing applications and technologies, and develop synthesis-structure-function relations to more precisely guide efforts in catalyst design and discovery.

Synthesis of Zeolite and Molecular Sieve Catalysts

We are particularly interested in the synthesis and catalysis of microporous and mesoporous materials, zeolites, and molecular sieves. These crystalline oxides contain catalytically active sites that are confined within ordered void spaces (e.g., channels, cages, pockets) of molecular dimensions (typically <2 nm for zeolites). 

Zeolites and molecular sieves are ubiquitous as catalysts in the petrochemical refining and chemical industries. Historically, they have been chosen for specific catalytic processes based on the ability of their pore structures to selectively allow or prevent reactants, products, or transition states from accessing active sites (i.e., shape selectivity).

Molecular sieves, however, are remarkably diverse in structure beyond their crystal framework topology. Oftentimes, materials with nominally identical bulk structure and composition, but prepared via different synthetic routes, show dramatic differences in catalytic behavior that reflect the diversity present at atomic and molecular length scales.  

Our research has shown how the structure-directing agents used to guide zeolite crystallization can be chosen to manipulate the atomic arrangement of heteroatoms and catalytic active sites in zeolites, even among materials of fixed elemental composition:

Organic and inorganic structure-directing agents used together or in isolation provides a synthetic strategy to manipulate framework Al distribution in chabazite (CHA) zeolites (top) and ZSM-5 (MFI) zeolites (bottom).

Consequences of Active and Defect Site Arrangement and Positioning for Catalysis

Different arrangements of Brønsted acid sites in zeolites influence alkanol dehydration turnover rates by stabilizing clustered intermediates and transition states via hydrogen bonding.

We develop synthetic and post-synthetic methods that systematically and predictably influence the function and strength of active sites, and their relative arrangement and proximity. We also study methods to modify their surrounding environments, including the size, shape and polarity (hydrophobicity or hydrophilicity) of secondary confining environments that influence catalytic behavior.

We use kinetic and mechanistic studies to connect these site and structural properties to catalytic behavior. As new chemical insights into molecular sieve catalysis develop, we envision that they can guide the preparation of materials that expand the range of catalytic opportunities possible for reactions in gaseous phases and in condensed media.

Our research has shown how the arrangement and positioning of binding sites and defect groups within the confining active site pockets of zeolites can influence their adsorption and catalytic properties for catalytic reactions of hydrocarbons and oxygenates involved in upgrading abundant shale gas and renewable biomass resources to chemicals and fuels:  

Dynamic Evolution of Active Sites During Catalysis

We aim to advance the understanding of reaction mechanisms and active site requirements for catalytic reactions that occur within confined spaces. The properties of both the binding sites and their surrounding and confining environments influence catalysis when reactive intermediates and transition states sense electrostatic and dispersion forces differently. Our previous work has shown that synthetic molecular sieves can show catalytic reactivity and specificity reminiscent of enzymes, and that they can mediate reactions without precedent in biological catalysts. 

One long-term goal of our research is to understand fundamentally why and when synthetic materials containing voids of molecular dimension exhibit such remarkable catalytic control and diversity. Another goal is to determine the mechanistic reasons underlying the diverse catalytic behavior observed among materials of the same bulk structure and composition, but different provenance or history.

Our research has also provided new mechanistic insights into the catalytic function of materials at the interface of heterogeneous and homogeneous catalysis, through studies of industrially-used Cu-zeolites for nitrogen oxide reduction with ammonia. Our work explains mechanistically how metal ions supported on zeolites change structure during reaction to form homogeneous-like complexes, and how electrostatic interactions with the support endow sufficient mobility to enable dynamic and reversible formation of multinuclear complexes during steady-state catalysis:

These findings have broader implications for designing selective oxidation catalysts based on earth-abundant transition metals that can use dioxygen (air) under mild conditions.

Metal ion sites exchanged into zeolites can become solvated and mobilized by reactants during catalysis (top), as occurs in the case of Cu ions by ammonia during the selective catalytic reduction (SCR) of nitrogen oxides (NOx) in diesel exhaust aftertreatment (bottom).

The Applications and Practice of Catalysis Science

Hydrocarbon conversion to liquid fuels and chemicals

Hydrocarbons sourced from conventional petroleum reserves have historically been used as feedstocks for liquid fuel and chemical production in the refining and petrochemical industries. New technological advances in harvesting natural and shale gas resources have granted access to light alkanes as potential new feedstocks in these industries. The projects in this research area study the catalytic conversion of alkanes, alkenes and arenes on acidic and bifunctional catalysts, and both oxidative and non-oxidative routes to convert methane.

Oxygenate conversion to liquid fuels and chemicals

Concerns about the environmental impacts associated with using fossil-based carbon feedstocks have motivated efforts to develop renewable and carbon-neutral forms of energy. The projects in this research area study the development of synthetic catalysts that mediate (stereo)selective reactions of highly-functionalized compounds such as sugars, oxygenates and alkanols derived from lignocellulosic biomass or waste plastic streams. Since such conversion routes often occur in liquid media, another research goal is to understand how liquid solvents interact with surfaces and confined spaces, and in order to design new materials for catalytic processing in liquid media.

Pollution abatement from automotive and other sources

Automobiles generate significant amounts of atmospheric nitrogen oxide (NOx, x = 1, 2) and other pollutants. Promising NOx abatement technologies include selective catalytic reduction (SCR) with ammonia and passive NOx adsorption (PNA) using metal-exchanged zeolites. Cu-CHA zeolites are used commercially for NOx SCR in diesel exhaust aftertreatment, but their structure and reactivity change unpredictably throughout their lifetime. The projects in this research area develop new methods to tailor catalyst structure and characterize catalysts under working conditions, and to understand the mechanistic details of NOx SCR reactions.

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