(3fm) Engineering Dynamic Active Sites and Microenvironments for Sustainable Catalytic Chemistries

Catalytic technologies that accelerate desired chemical transformations will play a central role in addressing urgent sustainability challenges of the 21^st century, such as producing fuels and chemicals from renewable carbon sources with lower CO₂ footprint, and mitigating emissions that harm human health and climate. Biomass is an abundant, renewable carbon source that can be derived from inedible plant matter. If economically viable conversion technologies can be developed, biomass could serve as a feedstock for the production of fuels and chemicals in a circular carbon economy, where the CO₂ released during combustion is recaptured by plants. Nitrogen oxides (NO_x) in automotive engine exhaust are a major cause of air pollution and smog worldwide. Aftertreatment systems in diesel and lean-burn engines are used to catalytically convert NO_x to benign N₂ and water, but struggle to do so at low temperatures; improvements in low-temperature emissions control are hindered by a lack of understanding of the reaction mechanism. The research vision for my future group involves combining the precise synthesis of catalytic materials with well-defined active sites and associated microenvironments, operando characterization of their dynamic structures and interactions under working conditions, and kinetic and mechanistic studies of complex reaction networks, to elucidate the reaction chemistries and active site requirements of catalytic reactions and thereby enable the development of sustainable catalytic technologies.

PhD Research (University of Wisconsin-Madison, advised by James Dumesic & George Huber):

In my graduate work, I showed that the biomass-derived intermediate levoglucosenone can be upgraded to a variety of oxygenated commodity chemicals using bifunctional metal-acid catalysts [1]. A combination of mechanistic tools, including analysis of key intermediates, reaction kinetics, stereochemistry, and isotopic labeling, revealed the catalytic chemistry underlying biomass conversion processes [2-5]. Systematic variation of metal-acid site properties identified the roles of these sites in hydrogenolysis reactions [5], while synthesis of model acid catalysts by atomic layer deposition uncovered differences in the intrinsic reactivity of Brønsted and Lewis acid sites [6]. This fundamental understanding of biomass conversion mechanisms provides a roadmap for the production of new bio-based polymer precursors with controlled structure and stereochemistry [7].

Postdoctoral Research (Purdue University, advised by Rajamani Gounder):

In my postdoctoral work, I am interrogating the mechanism by which zeolite-supported Cu ions reduce NO_x emissions from automotive engine exhaust. By measuring reaction rates across widely varying reaction conditions and monitoring Cu oxidation states with operando synchrotron-based X-ray spectroscopy during reaction [8], I show how Cu²⁺ reduction and Cu⁺ oxidation steps are independently influenced by both the density of Cu cations, and the density of charge-balancing Al anions in the zeolite framework. Both the proximity and mobility of Cu ions affect their reactivity, because Cu ions dynamically pair during one of the reaction steps; such “non-mean field” behavior falls outside the conventional definition of heterogeneous catalysis. Understanding the dynamic interactions between mobile ionic active sites provides directions to design catalysts with improved emissions control performance.

Teaching Interests:

I value the opportunity to share my knowledge and passion for scientific principles and their application to practical problems, both in the classroom and the laboratory. My teaching philosophy is centered on the concept of an adaptable, living classroom that caters to students’ diverse learning needs, by i) incorporating a variety of teaching styles such as inquiry-based and flipped classroom models, ii) emphasizing underlying concepts rather than rote memorization, iii) providing engaging examples from everyday life and emerging fields, and iv) fostering an inclusive learning environment that celebrates diversity in both backgrounds and perspectives. I have numerous teaching experiences including TAing for two undergraduate (Thermodynamics I; Materials Science) and one graduate (Kinetics and Catalysis) course, as well as formal training in teaching and mentorship strategies. I have mentored six undergraduate researchers who made significant contributions to our research efforts and gained valuable skills in the process. I am willing to teach any undergraduate chemical engineering course, but am especially excited to teach undergraduate courses in kinetics/reaction engineering, mass and energy balances, and thermodynamics, as well as a graduate course in kinetics and catalysis. I also look forward to developing elective courses in sustainable chemistry and advanced methods in catalysis. Because chemical engineering should be accessible to all members of our society, throughout the past 10 years I have facilitated hands-on STEM activities with local K-12 students to share my passion for science with the broader community.

References:

1. S. H. Krishna, K. Huang, K. J. Barnett, J. He, C. T. Maravelias, J. A. Dumesic, G. W. Huber, M. De bruyn, B. M. Weckhuysen, "Oxygenated Commodity Chemicals from Chemo‐catalytic Conversion of Biomass derived Heterocycles", AIChE Journal, 2018, 64, 1910-1922. (cover article)
2. S. H. Krishna, M. De bruyn, Z. R. Schmidt, B. M. Weckhuysen, J. A. Dumesic, G. W. Huber, “Catalytic Production of Hexane-1,2,5,6-tetrol from Bio-renewable Levoglucosanol in Water: Effect of Metal and Acid Sites on (Stereo)-Selectivity”, Green Chemistry, 2018, 20, 4557-4565.
3. S. H. Krishna*, T. W. Walker*, J. A. Dumesic and G. W. Huber, "Kinetics of Levoglucosenone Isomerization", ChemSusChem, 2017, 10, 129-138. *equal contributions.
4. S. H. Krishna, D. J. McClelland, Q. A. Rashke, J. A. Dumesic and G. W. Huber, "Hydrogenation of Levoglucosenone to Renewable Chemicals", Green Chemistry, 2017, 19, 1278-1285. (inside front cover)
5. S. H. Krishna, R. S. Assary, Q. A. Rashke, Z. R. Schmidt, L. A. Curtiss, J. A. Dumesic, G. W. Huber, “Mechanistic Insights into the Hydrogenolysis of Levoglucosanol over Bifunctional Platinum Silica-alumina Catalysts”, ACS Catalysis, 2018, 3743-3753.
6. S. H. Krishna*, L. Zhang*, I. Hermans, G. W. Huber, T. F. Kuech, J. A. Dumesic, “Rates of Levoglucosanol Hydrogenolysis over Brønsted and Lewis Acid Sites on Platinum Silica-Alumina Catalysts Synthesized by Atomic Layer Deposition”, Journal of Catalysis 2020, 389, 111-120. *equal contributions
7. S. H. Krishna, J. Cao, M. Tamura, Y. Nakagawa, M. De bruyn, G. S. Jacobson, B. M. Weckhuysen, J. A. Dumesic, K. Tomishige, G. W. Huber, “Synthesis of Hexane-Tetrols and -Triols with Fixed Hydroxyl Group Positions and Stereochemistry from Methyl Glycosides over Supported Metal Catalysts”, ACS Sustainable Chemistry & Engineering, 2020, 8, 800–805.
8. S. H. Krishna, C. B. Jones, J. T. Miller, F. H. Ribeiro, R. Gounder, “Combining Kinetics and Operando Spectroscopy to Interrogate the Mechanism and Active Site Requirements of NO_x Selective Catalytic Reduction with NH₃ on Cu-Zeolites”, Journal of Physical Chemistry Letters, 2020, 11, 5029-5036.