Strategies to tune active site environments for alkene epoxidations with H2O2 over supported transition metal atoms

Ayla, Ece Zeynep

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https://hdl.handle.net/2142/116195 Copy

Description

Title

Strategies to tune active site environments for alkene epoxidations with H2O2 over supported transition metal atoms

Author(s)

Ayla, Ece Zeynep

Issue Date

2022-07-12

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

Flaherty, David W

Doctoral Committee Chair(s)

Flaherty, David W

Committee Member(s)

• Kenis, Paul J
• Peters, Baron G
• Murphy, Catherine J

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)

• epoxidation
• transition metals
• zeolites
• metal oxides
• H2O2
• in situ Raman spectroscopy
• kinetics
• calorimetry

Abstract

Atomically dispersed transition metal atoms within zeolite frameworks or grafted to surfaces of catalytically inactive mesoporous supports can activate hydrogen peroxide (H2O2) to form intermediates that selectively epoxidize alkenes, including propylene. Despite being reported four decades in the past, titanium-silicalite 1 remains the most widely used and benchmark heterogeneous catalyst for this chemistry. Principles to guide the development of more productive and selective dispersed metal catalysts remain elusive. Reported turnover rates, activation barriers, and the reasons for differences among ostensibly similar sets of materials often disagree across studies. These discrepancies may reflect unrecognized structural distinctions between catalysts or consequential differences among the catalytic conditions used. This report examines the electronic and topological factors that control rates, selectivities, and apparent activation enthalpies for alkene epoxidations with H2O2 by examining series of Groups 4–6 metals incorporated into the BEA zeolite framework and Ti atoms grafted onto non-microporous supports. Measurable electronic properties (inner-sphere interactions) and excess thermodynamic contributions (outer- sphere interactions) at or near the metal active site, assessed quantitatively by spectroscopic and calorimetric methods, correlate strongly with rates, selectivities and activation barriers. Chapter 1 reviews the importance of inner- and outer-sphere interactions that control energy barriers for epoxidation. These interactions can be decoupled and manipulated through choice of the reactive metal, selection of the supporting oxide, and size of the substrate in relation to confined catalytic environments. This is followed by a discussion of extending the understanding of these interactions to more complex active site environments for enantioselective epoxidation and direct epoxidation of propene in Au/Ti-zeolite catalysts with H2 and O2. Rates and selectivities for alkene epoxidations depend sensitively on the identity of the active metal center for both heterogeneous and homogeneous catalysts. While group 6 metals (Mo, W) have greater electronegativities and the corresponding molecular complexes have greater rates for epoxidations than group 4 or 5 metals and molecular complexes, these relationships are not established for zeolite catalysts. Chapter 2 combines complementary experimental methods to determine the effects of metal identity on the catalytic epoxidation of 1-hexene with H2O2 for active sites within the BEA framework. Post-synthetic methods were used to incorporate groups 4−6 transition-metal atoms (Ti, Nb, Mo, W) into the framework of zeolite BEA. In situ Raman and UV−vis spectroscopies show that H2O2 activates to form peroxides (M-( 2-O2)) and hydroperoxides (M-OOH) on all M-BEA but also metal-oxos (M=O) on W- and Mo-BEAs, the latter of which leaches rapidly. Changes in turnover rates for epoxidation as functions of reactant concentrations and the conformation of cis-stilbene epoxidation products indicate that epoxide products form by kinetically relevant O-atom transfer from M-OOH or M-(2-O2) intermediates to the C=C bond and show two distinct kinetic regimes where H2O2-derived intermediates or adsorbed epoxide molecules prevail on active sites. Ti-BEA catalyzes epoxidations with turnover rates 60 and 250 times greater than Nb-BEA and W-BEA, which reflect apparent activation enthalpies (ΔH‡) for both epoxidation and H2O2 decomposition that are lower for Ti-BEA than for Nb- and W-BEAs. Values of ΔH‡ for epoxidation differ much more between metals than barriers for H2O2 decomposition and give rise to large differences in 1-hexene epoxidation selectivities that range from 93% on Ti-BEA to 20% on W-BEA. Values of ΔH‡ for both pathways scale linearly with measured enthalpies for adsorption of 1,2-epoxyhexane from the solvent to active sites measured by isothermal titration calorimetry. These correlations confirm that linear free energy relationships hold for these systems, despite differences in the coordination of active metal atoms to the BEA framework, the identity and number of pendant oxygen species, and the complicating presence of solvent molecules. Chapter 3 investigates the deconvolution of inner- and outer- sphere effects by examining atomically disperse Ti sites on metal oxides (MOx, including SiO2, -Al2O3, ZnO, GeO2) that activate H2O2 to create intermediates active for alkene epoxidations. Turnover rates for 1-hexene epoxidation in acetonitrile vary 1000-fold at identical conditions due to differences in apparent activation enthalpies (ΔH‡epox) and entropies (ΔS‡epox). Ligand-to-metal charge transfer energies and vibrational frequencies of reactive species assessed by in situ UV-Vis and Raman spectroscopy, respectively, indicate supports do not detectably change electronic properties of H2O2-derived intermediates. However, isoelectric points and solution-phase water uptakes for these metal oxides correlate with ΔH‡epox and suggest that non-covalent interactions at the solid-liquid interface influence the stability of epoxidation transition states. Supports with lower pKa values concentrate water near the solid-liquid interface and enthalpically stabilize the transition state. These findings illustrate that outer sphere interactions impact epoxidation reactions upon metal oxide catalysts including titanium silicates. Interactions among fluid-phase molecules, reactive intermediates, and solid surfaces contribute to apparent activation barriers for alkene epoxidations with H2O2 through outer-sphere interactions, which lead to differences in turnover rates even in the absence of a bulk liquid-phase. Chapter 4 demonstrates the significance of these interactions through comparisons of the kinetics for gas-phase epoxidations of alkenes (C3-C10) over Ti atoms substituted within the framework of BEA zeolite. The microporous and hydrophilic environment of zeolite BEA induces capillary condensation and stabilizes liquid-like densities of solvent molecules (e.g., CH3CN) nearby Ti-atom active sites. Although the reaction mechanism, dominant reactive intermediates, and kinetically relevant steps remain identical as the alkyl chain length increases for C3H6 to C10H20, the turnover rates differ by a factor of 30-fold. Formation of epoxidation transition states disturbs these solvating molecules, which leads to changes in apparent activation enthalpies (ΔH‡) and entropies (ΔS‡) that change with a complex dependence on the number of methylene groups (-CH2-) within the alkene. Moreover, we observe less solvent (CH3CN) displacement with shorter alkyl chains, indicating a greater number of molecular interactions between reactants, solvent, and catalyst pore walls for smaller substrate molecules. These changes influence excess enthalpies and entropies which are reflected in the free energy of epoxidation transition states, leading to differences in turnover rates. These results provide guidelines to control the effects of fluid-solid interactions near catalytic active sites, which control the free energies of intermediate states that impact apparent activation barriers or epoxidation reactions in microporous environments. Chapter 5 reviews the findings from Chapters 2-4 on the effects of inner- and outer-sphere interactions on selective liquid- and vapor-phase alkene epoxidation with H2O2 over transition metal atoms dispersed on solid metal oxides. Chapter 5 also includes proposals for potential future investigations to further explore active site environments, specifically for (1) enantioselective epoxidation with H2O2 on supported group 5 transition metals as well as (2) direct propylene epoxidation from H2 and O2 over TS-1 supported metal nanoparticles. Enantioselective chemistry is important to produce biologically active molecules used in the manufacture of drugs and other pharmaceuticals. Enantioselective catalysis combines the effects of electronic and solvent structure at or near the active site with the presence of chiral ligands or an inherently chiral catalyst. Moreover, insight into the direct epoxidation propylene with H2 and O2 over Au-incorporated Ti-zeolites motivates examination of multiple active site environments in proximity and will provide tools to evaluate similar concurrent tandem or “one-pot” catalytic systems. The vapor-phase epoxidation system and methods already developed, introduced in detail in Chapter 4, allow for facile experimental set-up for this study. These future projects examine industrially relevant-chemistries that add another level of complexity to active site surroundings and will provide additional insight into understanding the influence of inner- and outer-sphere effects on oxidation catalysis. Collectively, these studies explore methods to deconvolute inner- and outer-sphere effects that influence electronic and excess thermodynamic properties of active site environments for alkene epoxidations with H2O2 over supported transition metal atoms. The kinetic, spectroscopic, and calorimetric methods introduced can be further applied to provide design principles for the design of solid catalysts for various industrial reactions.

Graduation Semester

2022-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Ece Ayla

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