Solvent and surface effects on alkene oxidation catalysis over transition metal incorporated zeolites

Torres, Chris

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https://hdl.handle.net/2142/121208 Copy

Description

Title

Solvent and surface effects on alkene oxidation catalysis over transition metal incorporated zeolites

Author(s)

Torres, Chris

Issue Date

2023-07-05

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 A
• Murphy, Catherine J
• Guironnet, Damien S

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
• solvation
• confinement

Abstract

Reactions of alkenes and epoxides over zeolite materials benefit from solvent and framework derived stabilization effects, which confer excess free energy (Gε) contributions towards reactive intermediates and transition states. Liquid-phase reactions over Ti incorporated MFI type zeolites (Ti-MFI) feature catalytic rates and selectivities that closely sense the presence of spectating molecules and hydrogen bonding functionalities. Multiple prior reports described the outcome of experiments and the effects of changing individual solvent or framework descriptors and proposed plausible reasons for changes in catalytic rates and selectivities. Many of these studies communicate results impacted by mass transport that obscure intrinsic kinetic effects and rates and activation barriers (ΔG_App҂) obtained under well-defined kinetic regimes. Here, we probe the dependence of alkene epoxidation kinetics on solvent and framework features through turnover rate and activation barrier measurements taken as functions of reactant concentration, zeolite defect silanol ((SiOH)x) densities, water concentrations, solvent identity, and alkene structure combined with spectroscopic and calorimetric measurements that report on related phenomena for Ti-MFI catalysts. The presence of extended hydrogen bond networks in aqueous methanol (CH3OH) over (SiOH)x ¬dense Ti-MFI (Ti-MFI-OH) yields the greatest turnover rates over the span of conditions investigated via Gε contributions that entropically stabilize epoxide transition states as solvent molecules rearrange. Chapter 1 presents a brief history on the usage of zeolites to provide context for the industrial relevance of catalysis over microporous materials and the dearth of knowledge to be obtained through investigation of Gε contributions. Liquid-phase catalysis represents a significant opportunity for rational design of catalyst reaction systems with fine-tuned solvent environments and surface topologies. Alkene epoxidation emerges as a candidate reaction sensitive to the effects of solvent on catalytic rates, which motivates further investigations that use this reaction to develop broad knowledge of these effects while simultaneously advancing technology for environmentally benign and atom-efficient oxidations with H2O2 over Ti-MFI zeolites. Epoxidation turnover rates of 1-hexene (C6H12) with H2O2 depend on solvent identity and Ti-MFI (SiOH)x density, correlating with extent of solvent hydrogen bonding and concomitant disruption of solvent structures with transition state formation. Chapter 2 investigates the impact of protic solvent choice using classical batch kinetics experiments together with in situ infrared spectroscopy and isothermal titration calorimetry (ITC) to explain contributions from solvent molecules on epoxidation catalysis. Turnover rates for C6H12 epoxidation range over three orders of magnitude over Ti-MFI zeolites with distinct (SiOH)x densities across five unique organic solvents (CH¬3OH, ethanol, 1-propanol, 1-butanol, acetonitrile). Values of ΔH_App҂ and entropies ΔS_App ҂ range over 80 kJ mol-1 and 200 J mol-1 K-1, respectively. The most positive ΔH_App҂ and ΔS_App҂ values for a given Ti-MFI catalyst appear for CH3OH solvent, likely due to the large number of hydrogen bonds in CH3OH filled pores. Entropic gains associated with hydrogen-bond disruption corroborates in situ infrared spectra that show the adsorption of 1,2-epoxyhexane (C6H12O) to Ti sites disturbs the equilibrium solvent structure within pores in ways that reflect the quantity of local hydrogen bond moieties. Adsorption enthalpies (ΔH_Ads) of C6H12O over Ti-MFI negatively correlate with ΔH_App҂ across solvent and catalyst combinations. Solvent molecule displacement and the associated breaking and forming of hydrogen bonds among solvents in condensed pores during catalysis and adsorption events incur substantial Gε contributions that control reaction turnover rates and activation barriers through stabilization of transition states. These findings rationalize high conversion and turnover rates of alkene epoxidation in CH3OH solvent over hydrophilic Ti-MFI for industrial processes (e.g., the hydrogen peroxide – propylene oxide process). The role of water (H2O) on catalysis is crucial for understanding liquid phase oxidations which take place with aqueous H2O2 to design reaction systems which take place with high rates and desired product selectivities. Chapter 3 investigates solvent CH3OH and H2O molecule impact on C6H12 epoxidation rates with H2O2 at discrete H2O concentrations ([H2O]) over Ti-MFI catalysts. Turnover rates are 10-fold greater upon Ti-MFI-OH than on Ti-MFI-F at very low [H2O] (39 mM H2O). Liquid-like methanol forms solvent structures over (SiOH)x dense Ti-MFI (Ti-MFI-OH) with more hydrogen bonds than (SiOH)x free Ti-MFI (Ti-MFI-F), which confer Gε contributions that stabilize hydrogen peroxide derived reactive intermediates and epoxide transition states. Hydrogen bonds among intrapore molecules couple to catalytic events nearby solvent molecules, leading to turnover rates for C6H12 epoxidation which increase by ten-fold when [H2O] increases over Ti-MFI catalysts. Values for ΔH_App҂ and ΔS_App҂ are similar at the lowest [H2O] (5 mM H¬2O) between Ti-MFI materials, which implies that similar solvation environments appear near active sites in the absence of water and these weakly sense changes in (SiOH)x density. Increasing [H2O] (0.005 - 5 M H2O) leads to diverging, nonmonotonic changes in barriers that reflect stabilizing contributions near and away from active sites as H2O displaces CH3OH. Values for ΔH_Ads of C6H12¬O become more exothermic with increasing [H2O] over Ti-MFI, and values for ΔH_App҂ generally increase as epoxide adsorption enthalpies become more exothermic. These negative correlations indicate that excess contributions (Hε, Gε) impact hydrogen peroxide derived reactive intermediates (Ti-OOH) within the 10-membered ring pores of MFI to a greater extent than the kinetically relevant transition states that agree with trends observed in Chapter 2, which differs from observations over larger pore materials such as BEA that consist of 12-membered ring pores. Application of solvent design principles, informed by insight to the origin of Gε contributions, provides opportunities to increase reaction rates for other reactions and molecules so long as molecular transport with zeolites does not limit catalytic rates of conversion. Chapter 4 probes the extent to which Ti-MFI zeolites epoxidize linear alkenes with aqueous H2O2 in CH3OH solvent by systematically increasing the alkene carbon length. Zeolites with MFI topologies feature confining spaces that promote linear alkene (CnH2n) epoxidation catalysis as well as product adsorption affinity with increasing reactant size. Combinations of van der Waals interactions between pore features and alkyl carbon chains and solvent-mediated specific interactions (e.g., hydrogen bonds) influence both kinetics and thermodynamics of elementary steps involved in these reactions. Turnover rates for alkene epoxidation with H2O2 in CH3¬OH solvent increase by 20-fold with the chain length of alkenes from 1-pentene to 1-octene at low CnH2n concentrations (1 mM CnH2n), but these changes appear together with decreases in the dependence of epoxidation rates on [CnH2n] (CnH2n power law expression drops from 0.9 to 0.1) during reaction of 1-pentene compared to 1-octene. The reaction order of CnH2n appears to inversely correlate with H2O2 reaction orders at [H2O2] to [CnH2n] ratios greater than 10, demonstrating that Ti atoms saturate with H2O2 derived reactive intermediates on 1-pentene and 1-hexene but alkene or alkene-derived product cover sites during reactions with 1-heptene and 1-octene. The transition towards higher coverages of alkene-derived surface intermediates coincides with increasingly exothermic adsorption enthalpies of the product epoxides over Ti-MFI. These positive correlations indicate that concentration gradients may exist near surfaces in MFI zeolites within the liquid phase that inhibit catalysis for reactants larger than C6H12 and that catalytic measurements should be scrutinized appropriately before inferring structure-catalysis relationships, which may be corrupted by mass transfer artifacts. Chapter 5 summarizes the impact of Chapters 2-4 on design principles for liquid phase alkene epoxidation with aqueous H2O2 over Ti-MFI zeolites. Chapter 5 describes future work and preliminary findings for application of first kinetics principles towards other liquid phase chemistries. Epoxide ring opening (ERO) catalysis with methanol as a nucleophile provides functionalized ethers and alcohols with product distributions that sense local reacting environments. Zeolite frameworks with different pore diameters react 1,2-epoxybutane (C4H8O) with CH3OH to form terminal ethers with larger selectivities over medium pore Sn incorporated MFI (Sn-MFI) compared to Sn incorporated zeolite BEA and FAU. Identical concentration dependencies between these three unique and well characterized Sn-zeolites provides a facile means of measuring ΔG_App҂ to understand how framework confinement effects selectively stabilize terminal ethers via Gε contributions. Altogether, this collection of unique yet intricately related findings contribute to understanding of solvent and framework effects near and away from active sites within microporous materials for liquid-phase catalysis. This ensemble of kinetics, spectroscopy, and calorimetric assessments over synthesized and meticulously characterized catalysts provides molecular, experimental insight into stabilization events during catalytic cycles which inform rational design principles for chemical reaction engineering.

Graduation Semester

2023-08

Type of Resource

Thesis

Copyright and License Information

© 2023 Chris Torres

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