(6bd) Nature of Active O2-Derived Species in Selective Oxidation Catalysis

Catalytic selective oxidation is an essential industrial process to produce key chemical intermediates, such as alkenes, alcohols, epoxides, aldehydes, ketones, and organic acids. Rates and selectivities of the oxidation catalysis depend on the type and nature of O₂-derived species, specifically their nucleophilicity and electrophilicity, which determine their H-abstraction and O-insertion abilities and selectivities. In contrast to nucleophilic surface O-atoms in bulk oxides that tend to abstract H-atoms from organic molecules, electrophilic O₂-derived species, such superoxo, peroxo, or hydroperoxo, catalyze O-insertion reactions as in the case of monooxygenase enzymes that convert methane to methanol² and titanosilicates that form epoxides from alkenes.³ Such electrophilic O₂-derived species formed via O₂ activation steps also influence product selectivities in alkane oxidative dehydrogenations (ODH) by converting the primary ODH product, alkene, into more reactive epoxides that ultimately convert to undesired CO/CO₂. Detailed knowledge of catalytic properties that determine the preference of various O₂ activation routes that form different types of active oxygen species and their role in oxidation catalysis will provide a platform in designing more reactive and selective catalysts.

Recent advances in computational chemistry, atomic-level synthetic methods, and characterization tools have brought the Renaissance in catalysis, allowing us to understand and control catalytic reactions on a molecular level. This synergy, however, can be achieved only when these tools are combined rigorously and effectively to solve problems. One of my recent research projects with Prof. Iglesia revealed the involvements of two types of O₂ activation routes, inner and outer sphere pathways, during re-oxidation of Mo-based polyoxometalate (POM) clusters, by combining kinetic and scavenging experiments with density functional theory (DFT) calculations.⁴ Kinetic responses of the scavenging reactions of bound peroxo (OO*) species during alkanol ODH reactions provided information about mechanistic details of re-oxidation steps, which cannot be probed using steady-state measurements because of the fast nature of re-oxidation compared to the reduction part of the ODH cycles. In junction with these experimental inquires, DFT calculations on the energetics of intermediates and transition states evaluated the plausible reaction pathways that are otherwise indistinguishable from experiments. These calculations, in turn, revealed that OO* species react with gaseous reductants (alkanols or alkenes) to re-form lattice O-atoms (O*) for next ODH turnovers, instead of with another reduced center via conventionally proposed routes, in order to circumvent kinetic hurdles required for OO* hopping steps to bring OO* next to another reduced center.

A rational design of effective catalytic systems requires detailed understanding of catalytic properties that determine the rates and selectivities. Such assessments, however, are not trivial and need to involve a rigorous use of both experiments and theory. My current project with Prof. Iglesia uses DFT calculations to assess inner and outer sphere O₂ activation barriers for POM clusters with different metal atoms and charge-balancing cations. In doing so, we show that the activation barriers mediating the two routs depend differently on the stability of the reduced centers, leading to their respective rates that strongly depend on the strengths of the M-O bonds, which determine the contribution of inner sphere routes, and of O-H bonds, which determine the contribution of outer sphere routes. The experimental confirmation of such findings is currently underway by synthesizing and analyzing the POM clusters with various metal atoms (Mo, Mo-V, and W).

Controlled synthetic methods with atomic precisions are critical to implement design strategies predicted from DFT methods and to provide catalysts with homogeneity. A part of my graduate research projects with Prof. Stair and Prof. Snurr involved atomic layer deposition (ALD) techniques, which can be applied to synthesize highly dispersed metal oxides (mono oxide or mixed oxides with various combinations) and metal nanoparticles on solid supports. The projects for my own research group will combine these research tools to study the type and nature of active oxygen species involved in selective oxidation catalysis and to identify the catalytic properties that determine the activities and selectivities of the involved oxygen species. The research will include, but not limited to, the effects of mixed M-O bonds in ALD synthesized mixed oxides for O₂ activation pathways, the confinement effects in selective oxidation catalysis, and the role of active O₂-derived species in selective oxidation of light alkanes and alkenes.

Postdoctoral Projects: “Dioxygen activation routes in Mars-van Krevelen redox cycles catalyzed by metal oxides” and “Mechanistic details of formic acid decomposition routes on metals and metal oxides” under the supervision of Enrique Iglesia, Chemical and Biomolecular Engineering, University of California, Berkeley

PhD Dissertation: “Gas-phase alkene oxidation by hydrogen peroxide: the nature of active oxygen species in heterogeneous catalysis” under the supervision of Randall Snurr and Peter Stair, Chemical and Biomolecular Engineering, Northwestern University

Teaching Interests:

Throughout my studies in graduate school and postdoctoral research, I have actively mentored several undergraduate and graduate students, which I found myself truly enjoying and led me to pursue an academic career. I have taught in classrooms and guest lectured undergraduate and graduate level main courses (statistical mechanics, Matlab programming, and computational chemistry) at Northwestern university and UC Berkeley. I am confident in my ability to teach any undergraduate and graduate level course in chemical engineering, with preference for kinetics and reactor design, thermodynamics, and transport, based on my experiences and expertise in heterogeneous catalysis. I am also interested in designing and teaching graduate level courses related to applications in catalysis and in computational chemistry.

References

(1) Serrano-Plana, J.; Company, A.; Costas, M. O–O Bond Activation in Cu- and Fe-Based Coordination Complexes: Breaking It Makes the Difference, 1st ed.; Elsevier Inc., 2017; Vol. 70.

(2) Kopp, D. A.; Lippard, S. J. Soluble Methane Monooxygenase: Activation of Dioxygen and Methane. Curr. Opin. Chem. Biol. 2002, 6 (5), 568–576.

(3) Kwon, S.; Schweitzer, N. M.; Park, S.; Stair, P. C.; Snurr, R. Q. A Kinetic Study of Vapor-Phase Cyclohexene Epoxidation by H2O2 over Mesoporous TS-1. J. Catal. 2015, 326, 107–115.

(4) Kwon, S.; Deshlahra, P.; Iglesia, E. Dioxygen Activation Routes in Mars-van Krevelen Redox Cycles Catalyzed by Metal Oxides. J. Catal. 2018, 364, 228–247.