(337cj) Understanding and Designing Sustainable Catalytic Processes: Influence of Solvation on Alkene Epoxidations within Titanium Silicates

Transition metal-incorporated zeolite catalysts are widely introduced in liquid and gas phase catalytic reactions, such as glucose isomerization and alcohol dehydration. The hydrogen peroxide propylene oxide (HPPO) process is a well-established industrial process that utilizes microporous titanosilicates to produce propylene oxide with propylene and hydrogen peroxide (H₂O₂). Across HPPO and various alkene epoxidation processes, catalytic rates of epoxide formation can change up to five orders of magnitude depending upon the changes in reaction microenvironments (i.e., zeolite topology, hydrophilicity, and solvent identity). Most epoxidation processes occur in the liquid-phase and utilize organic solvents (e.g., acetonitrile (CH₃CN)) that fills micropores and solvate species at active sites through interactions between solvent molecules, zeolite pore walls, and the reactive intermediates. These interactions contribute directly to free energy differences that determine reaction rates, selectivities, and intraparticle transports. Here, we investigate these solvent-pore-reactant interactions and design the epoxidation system to interrogate the influence of each change in the reaction microenvironment on catalytic rates and selectivity.

1) Synthesize Ti-Zeolite Catalysts with Varying Physical Properties

We control the physical properties of Ti-incorporated zeolite catalysts. By introducing both hydrothermal synthesis and post-synthetic modification of commercial zeolite framework, we synthesize industrially relevant catalysts (Ti-MFI and Ti-MWW) and zeolites that exhibit drastic changes in response to physical property changes (Ti-BEA and Ti-FAU). More specifically, we vary the pore hydrophilicity by controlling densities of intrapore silanol ((SiOH)_x) defects and topological locations of Ti active sites. X-ray diffraction and diffuse reflectance UV-Vis spectroscopy reveal the crystallographic structures of the zeolite framework and the dispersity of Ti atoms, respectively. In situ titration with bulky molecules that block Ti sites during batch epoxidation (e.g., 1,2-diphenylethylenediamine) shows the preferential location of Ti atoms tetrahedrally coordinated inside pore sites (i.e., 12-membered supercage and 10-membered channels in MWW zeolite). Transmission infrared spectroscopy provides a tool to quantitatively compare the density of intrapore (SiOH)_x densities across catalysts. Collectively, these changes in pore size, active site location, and hydrophilicity lead to significant changes in epoxidation rates (100-fold) and product selectivity (4-fold), reflecting the solvent-pore-reactant interactions.

2) Control the Quantity of Solvent Condensed in Pores under Steady-State

Organic solvents condense spontaneously inside zeolite pores and stabilize the reactive species, and the magnitude responds to the changes in physical properties and reaction conditions. The exact solvent structures and the influence on catalytic performances remain elusive, because many prior studies used liquid-batch systems that do not permit control of the quantity of solvent inside zeolite pores and its coordination to determine the effects of solvent identity and density within the pore on catalysis. In order to directly control these factors without introducing further complications, we conduct steady-state tests in fixed bed reactors that contact only gaseous reactants and “solvent” molecules, and by doing so, we control the quantity of solvent by intentionally changing solvent partial pressures to fix the density of molecules that condense near active sites. These changes in the solvent quantity and interactions between extended pore surfaces can influence reaction chemistry but do not change reaction mechanisms. Yet these interactions lead to significant differences in rates, selectivities, and activation barriers. In a specific alkene substrate (1-hexene), for example, activation enthalpies (ΔH^Ç‚) and entropies (ΔS^Ç‚) increase by 11 kJ∙mol^-1 and 48 J∙mol^-1∙K^-1, respectively, by condensing pore with CH₃CN and these changes in free energies lead to 20-fold and 2-fold increase in turnover rates and epoxide selectivities compared to the absence of intrapore solvents. These changes in performances reflect noncovalent interactions induced by the van der Waals interactions between liquid-like solvents, pore walls, and reactive species, as well as the structural changes in solvents that must reorganize to accommodate reactive species and transition states.

3) Investigate Interactions between Pore, Solvent, and Reactant Molecules via Kinetics and Spectroscopy

Kinetic results of alkene epoxidation reflect convoluted effects of solvent-pore-reactant interactions, but a spectroscopic study (both in and ex situ) can provide insight into deconvoluting the contribution of solvent responding to changes in pore physical properties. We measure the quantity of condensed CH₃CN molecules by comparing the infrared absorbance of CH₃CN physisorbed onto Ti-zeolite pores and mass changes obtained from dynamic vapor sorption. The quantity of CH₃CN depends on the zeolite pore size, intrapore (SiOH)_x density, and the temperature and partial pressures of the system (e.g., ranging between 0 to 10 molecules per unit cell of *BEA pore, 0-50 kPa CH₃CN, 383 K). Among these condensed pores, adsorption of epoxidation product (epoxide) leads to the displacement of CH₃CN molecules from the pores, thereby inducing the reorganization of solvent structures. This reorganization confers entropically-driven free energy benefit to the formation of the epoxidation transition state (that structurally mimics epoxide adsorption), resulting in significant increases in rates and product selectivity.

Collectively, modifying the physical properties of Ti-zeolite catalysts and fine-tuning reaction conditions lead to orders of magnitude differences in alkene epoxidation rates and selectivities. These are supported by the ex situ characterizations of synthesized catalyst materials and in situ spectroscopic evidence of interactions between solvent, zeolite pore wall, and reactive species. These findings provide deeper insight and tangible improvement in catalytic performances that involve organic solvent-mediated reactions in confined structures.

Research Interests

I developed insights for syntheses of metal-incorporating zeolite catalysts (both hydrothermal synthesis and post-synthetic modification), characterization techniques (X-ray diffraction and fluorescence, diffuse-reflectance UV-Vis spectroscopy, transmission infrared spectroscopy, scanning electron microscopy, dynamic vapor sorption). I have also been designing and operating liquid and vapor phase packed-bed reactor systems, as well as in situ spectroscopic analyses to understand interactions among reactive species near active sites.

Beyond these backgrounds as an experimental catalyst researcher during my Ph.D. program, I look forward to applying my knowledge to the broader area of industry, including electrochemical utilization, where I possess expertise with an M.S. degree (e.g., electrochemical metal surface treatment and optimization of pollutant gas reduction system), kinetic modeling, and simulations.

References

1. Ohsung Kwon, E. Zeynep Ayla, David S. Potts, and David W. Flaherty, “Effects of Solvent-Pore Interaction on Rates and Barriers for Vapor-Phase Alkene Epoxidation with Gaseous H₂O₂ in Ti-BEA catalysts” (ACS Catalysis 2023, 13, 6430-6444)
2. David S. Potts, Vijaya Sundar Jeyaraj, Ohsung Kwon, Richa Ghosh, Alexander V. Mironenko, and David W. Flaherty, “Effect of Interactions Between Alkyl Chains and Solvent Structures on Lewis-Acid Catalyzed Epoxidations” (ACS Catalysis 2022, 12, 13372-13393)
3. David S. Potts, Chris Torres, Ohsung Kwon, and David W. Flaherty, “Engineering Intraporous Solvent Environments: Effects of Aqueous-Organic Solvent Mixtures on Competition Between Zeolite-Catalyzed Epoxidation and H₂O₂ Decomposition Pathways” (Chemical Science 2023, 14, 3160-3181)
4. Daniel T. Bregante, Matthew C. Chan, Jun Zhi Tan, E. Zeynep Ayla, Christopher P. Nicholas, Diwakar Shukla, and David W. Flaherty, “The shape of water in zeolites and its impact on epoxidation catalysis” (Nature Catalysis 2021, 4, 797-808)
5. Daniel T. Bregante, Alayna M. Johnson, Ami Y. Patel, E. Zeynep Ayla, Michael J. Cordon, Brandon C. Bukowski, Jeffrey Greeley, Rajamani Gounder, David W. Flaherty, “Cooperative Effects between Hydrophilic Pores and Solvents: Catalytic Consequences of Hydrogen Bonding on Alkene Epoxidation in Zeolites” (Journal of the American Chemical Society 2019, 141, 7302-7319)
6. Ohsung Kwon, KiHo Bae, Jinuk Byun, Taeho Lim, and Jae Jeong Kim, “Study on effect of complexing agents on Co oxidation/dissolution for chemical-mechanical polishing and cleaning process” (Microelectronic Engineering 2020, 227, 111308)
7. Oh Sung Kwon, Seungyeon Baek, Hyeonsu Kim, Insoo Choi, Oh Joong Kwon, and Jae Jeong Kim, “Optimization of solution condition for an effective electrochemical reduction of N₂O” (Electroanalysis 2019, 31, 1-8)