Influence of solvation and microenvironment on alkene epoxidation with hydrogen peroxide within titanium silicates

Kwon, Ohsung

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https://hdl.handle.net/2142/125662 Copy

Description

Title

Influence of solvation and microenvironment on alkene epoxidation with hydrogen peroxide within titanium silicates

Author(s)

Kwon, Ohsung

Issue Date

2024-05-28

Director of Research (if dissertation) or Advisor (if thesis)

Flaherty, David W

Doctoral Committee Chair(s)

Flaherty, David W

Committee Member(s)

• Yang, Hong
• Mirica, Liviu M
• Mironenko, Alexander V

Department of Study

Chemical & Biomolecular Engr

Discipline

Chemical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Zeolite
• Selective oxidation
• Defect engineering
• Pore condensation
• Gas-solid interface
• Pore polarity
• Kinetics

Abstract

Epoxides (e.g., propylene oxide) have a significant role in the petrochemical industry as building blocks for commodity chemicals. The processes of epoxide synthesis utilize heterogeneous catalysts to promote reactions in mild conditions. For example, titanosilicate materials catalyze the industrial epoxidation of propylene (C3H6) with hydrogen peroxide (H2O2) in the presence of organic solvents. In these catalytic epoxidation processes, rates of epoxide formation and product selectivities depend on multiple variables controlled in the system, such as the physical properties of Ti-incorporated zeolite catalysts, the identity of solvents, and the type or phase of the reactor setup. Differences in catalytic rates and product selectivities may therefore reflect variations in the electronic properties of framework Ti atoms, covalent stabilizations of reactive species, and intermolecular interactions (“noncovalent interactions”) between zeolite pores, reactive species, and solvent molecules. Studies have shown that the stabilities of reactive species and transition states respond to specific changes in reaction microenvironments (e.g., zeolite topology, solvent identity), resulting in orders of magnitude differences in catalytic rates, barriers, and product selectivities. However, a fundamental understanding of these convoluted interactions and the role of each modified variable remains elusive. This report explains that differences in catalytic rates, product selectivities, and activation barriers reflect changes in interactions among reactive species, solvent molecules, and extended pore surfaces at gas-solid and solid-liquid interfaces. We deconvolute these effects by controlling variables, which include catalyst hydrophilicity (silanol ((SiOH)x) density), topological location of active sites, pore dimensions, intentional pore condensation with solvents, and the adsorptions of reactive species. Chapter 1 reviews the history and significance of zeolite catalysts, epoxides, and epoxidation chemistry that motivated this study. An introduction to the free energies of activation in epoxidation chemistry follows to aid understanding of key concepts (energetic contributions of local microenvironment changes) covered in subsequent chapters. We demonstrate how changes in the physical properties of zeolite catalysts influence alkene epoxidation kinetics under the liquid phase. Chapter 2 demonstrates that 1-hexene (C6H12) epoxidation kinetics and thermodynamics in the presence of organic solvents reflect changes in (SiOH)x defect densities and active site locations. These properties are fine-tuned by controlling the hydrothermal synthesis conditions of industrially relevant Ti-MWW zeolites. In situ site titration with bulky molecules that poison active sites during batch epoxidation demonstrates the preferential siting of Ti atoms tetrahedrally coordinated inside distinct pore regions (i.e., 12-membered supercage and 10-membered sinusoidal channels), while ex situ characterizations reveal the crystallographic structures of the MWW framework, (SiOH)x defect densities, and the dispersity of Ti atoms. Turnover rates of alkene epoxidation exhibit up to 40-fold differences based on the topological location of active sites, but do not respond to changes in (SiOH)x densities. These kinetic results agree with the free energies of activation that show compensatory effects responding to the reorganization of solvent structures, affected by (SiOH)x defect densities and active site locations. The following chapters focus on the noncovalent interactions between pores, reactant, and solvent molecules at the gas-solid interfaces. Chapter 3 shows that the intentional condensation of gaseous acetonitrile (CH3CN) forms clusters within pores of hydrothermally synthesized and post-synthetically modified Ti-BEA zeolites during C6H12 epoxidation with H2O2 and leads to significant changes in epoxidation turnover rates, epoxide selectivities, and activation barriers. In situ infrared spectroscopy and dynamic vapor sorption reveal that increasing (SiOH)x densities within *BEA zeolite pores lead to a 7-fold rise in the quantity of CH3CN spontaneously condensed inside pore structures. During the epoxidation, condensed CH3CN molecules must reorganize to accommodate the formation of epoxidation transition states to different extents. By increasing the quantity of CH3CN from 0.4 to 10 molecules per *BEA unit cell, the reorganization of intrapore CH3CN entropically stabilizes transition states by 48 J∙mol-1∙K-1, thereby increasing turnover rates and epoxide selectivities by 20-fold and 2-fold, respectively. These kinetic results agree with the infrared spectra showing that the adsorption of epoxide products to the catalysts leads to the displacement of CH3CN molecules and disruption of hydrogen bonds with (SiOH)x defects. Chapter 4 demonstrates that the rates and product selectivities for vapor phase C3H6 epoxidation and sequential reaction networks reflect changes in the physical properties of Ti-incorporated zeolites and pore condensation. Hydrothermally synthesized and post-synthetically modified Ti-zeolites (MFI, *BEA, FAU) exhibit differences in pore sizes and relative (SiOH)x densities. These differences in physical properties lead to changes in C3H6 epoxidation turnover rate and epoxide product (propylene oxide) selectivity by 10- and 3-fold, respectively, under identical kinetic regimes and reaction mechanisms. Kinetic analyses reveal that secondary reactions (epoxide ring-opening) that follow by C3H6 epoxidation respond more sensitively to varying physical properties of zeolite catalysts. Intentional pore condensation with gaseous CH3CN also significantly decreases product selectivity while maintaining identical mechanisms and rates, by facilitating the secondary ring-opening reaction that decomposes epoxide products across all Ti-zeolite reported. These kinetic results agree with previous findings mainly investigated in the bulk liquid phase and expand them to the system without condensed solvents. Chapter 5 aims to numerically deconvolute the influences of interactions of reactive species (covered in early chapters) on the catalytic rates and selectivities, by controlling carbon numbers of alkene substrates (C3-C10) and pore fillings with gaseous CH3CN. Turnover rates of longer-chain alkenes (C6-C10) epoxidation increase systematically by 3-fold with CH3CN density, while rates for short-chain alkenes (C3-C4) remain unchanged. Apparent activation enthalpies and entropies for epoxidations (at a fixed CH3CN density) decrease with carbon numbers from C3 to C6, then increase from C6 to C10, attributable to the facilitated van der Waals (vdW) interactions. Solvation of reactive species by CH3CN leads to an enthalpic stabilization of short-chain alkene substrates, while more significant reorganization of CH3CN to accommodate bulkier transition states results in entropic benefits to the epoxidation kinetics. In situ infrared spectroscopy of alkene substrate adsorptions agrees with findings in epoxidation kinetics, corroborating that the combinations of covalent and noncovalent interactions significantly affect kinetics, which responds to changes in pore condensations and substrate species. Chapter 6 concludes the findings on the effects of local microenvironments on the epoxidation chemistry covered in prior chapters that can be applied to optimize heterogeneous catalysis. Also, we introduce structural transformation methods of hydrothermally synthesized TS-1 (Ti-MFI) catalysts that can provide tangible improvements in both epoxidation rates and selectivities in the absence of condensed solvents. Here, modified Ti-MFI catalysts, where the Ti atoms are specifically located on the surface of the MFI pore (egg-shell) and on the nanosized protrusions outside MFI pores (fin), exhibit comparable turnover rates of alkene epoxidation and epoxide selectivity to commercial TS-1 catalysts even in the absence of bulk solvent condensed in the system, which provides a green alternative to the current processes with minimized consumptions of organic solvents. Collectively, this report aims to understand the complicated interactions between catalyst materials, solvents, and reactive species, thereby providing guidance in optimizing heterogeneous catalysis processes that utilize organic solvents. Combined kinetic and spectroscopic analyses demonstrate the significance of energetic contributions of fine-tuned physical properties of catalyst materials and the organization of reactants and solvents during catalytic reaction cycles.

Graduation Semester

2024-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/125662

Copyright and License Information

Copyright 2024 Ohsung Kwon

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