Browsing by Subject "catalysis"

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C-H Activation via Direct Oxidative Routes over Molecular Metal-oxo Species Situated in Metal-Organic Frameworks
(2021-07) Simons, Matthew
Metal Organic Frameworks (MOFs), crystalline materials composed of inorganic nodes connected by organic linker molecules, afford an opportunity to synthesize new biomimetic catalysts engendering the oxidative activation of light alkanes, opening new pathways for the enhancement of underutilized chemical feedstocks. We aim to demonstrate in this dissertation the ability of Fe(II) centers, bearing a similar geometric and electronic structure to sites found in non-heme enzymes, situated in the trimeric iron-oxo nodes of a family of MOFs to activate light alkanes at near ambient temperatures. The identity and quantity of the active site was determined using in situ X-ray Absorption and ex situ Mössbauer Spectroscopy, in concert with in situ chemical titrations. Reaction kinetics, measured by varying reactant concentration and temperature using a recirculating batch reactor, are consistent with the rate limiting reaction of this Fe(II) site with the oxidant, N2O, to form a highly reactive Fe(IV)=O species (k = 1.2-0.8 x10-6 mol molFe(II)-1 kPaN2O-1 s-1 at 378 K) capable of activating C-H bonds homolytically, in agreement with Density Functional Theory calculations that predict subsequent radical-mediated pathways for upgrading propane and methane. A major challenge in this chemistry is resisting the over-oxidation of desired products formed by these pathways, which is often both kinetically and thermodynamically favored. We evince, using in situ Infrared Spectroscopy, that methanol, the desired product of the oxidation of methane, is stabilized as methoxy groups on the MOF through reactions with surface hydroxyl species. Pursuant to this, we added a zeolite (H-ZSM-5, Si/Al = 11.5) in inter- and intra- pellet mixtures with the MOF, observing monotonic increases in methanol selectivity with increasing ratio and proximity of zeolitic H+ to MOF-based Fe(II) sites, signaling increased amounts of methanol being dehydrated and protected within the zeolite. This work demonstrates (i) the radical-rebound mechanism commonly invoked in this chemistry is insufficient to explain the reactivity of these systems, (ii) the selectivity controlling steps involve both chemical and physical rate phenomena, and (iii) offers a strategy to mitigate over-oxidation in these and other similar systems.
Computationally Driven Characterization of Magnetism, Adsorption, and Reactivity in Metal-Organic Frameworks
(2016-06) Borycz, Joshua
Metal-organic frameworks (MOFs) are a class of nanoporous materials that are composed of metal-containing nodes connected by organic linkers. The study of MOFs has grown in importance due to the wide range of possible node and linker combinations, which allow tailoring towards specific applications. This work demonstrates that theory can complement experiment in a way that advances the chemical understanding of MOFs. This thesis contains the results of several investigations on three different areas of MOF research: 1) magnetism, 2) CO2 adsorption, and 3) catalysis. The calculation of magnetic properties within MOFs is quite problematic due to the weak nature of the interactions between the metal centers. The metal atoms in MOFs can be far apart due to the organic linkers and are often in unique chemical environments that are diffcult to characterize. These weak interactions mean that the computational methods must be carefully selected and tested to attain adequate precision. The objective of the work in this thesis was to determine the single-ion anisotropy and magnetic ordering of Fe-MOF-74 before and after oxidation. MOFs have desirable properties for CO2 adsorption such as large pores and high surface areas. Accurate force fields are required in order to make predictions for adsorption interactions with the internal surface of MOFs. Therefore it is important to have computational protocols that enable the derivation of reliable interaction parameters in order to study the trends of adsorption for different metal centers. In the research herein we used ab initio calculations to compute parameters for classical force fields for members of the IRMOF-10 and the MOF-74 series. MOFs have been considered for catalysis due to their thermal stability, reactive metal sites, and large diameter pores. In this thesis we report a series of studies that advance the understanding of the reactivity of MOFs containing Zr6 and Hf6 polyoxometalate nodes. In the first study the proton topology of the nodes within NU-1000 was determined. Several other studies that make use of these MOFs as supports for single-site metal catalysts are also reported. Finally, research where NU-1000 serves as a template for a thermally stable nanocasted material used for high temperature Lewis acid catalysis is also discussed.
Data for: Catalytic Resonance Theory: Negative Dynamic Surfaces for Programmable Catalysts
(2021-08-14) Gathmann, Sallye R; Ardagh, M Alexander; Dauenhauer, Paul J; hauer@umn.edu; Dauenhauer, Paul J; Dauenhauer Group
Catalysts that change with time via programmed variation of their electronic occupation to accelerate surface reactions were evaluated in the case of negative adsorption energy scaling relations. Defined as the relative change in adsorption enthalpy, the gamma linear scaling parameter is negative when two adsorbates alternatively weaken and strengthen as catalysts are electronically perturbed. Simulations were conducted of a single transition state connecting two generic adsorbates representative of multiple reaction classes to understand the resulting negative gamma catalytic ratchet mechanism and its ability to accelerate catalytic reactions above the Sabatier peak and away from equilibrium. Relative to conventional positive gamma catalytic ratchets, the Sabatier volcanoes of negative gamma catalysis are narrower with greater enhancement of dynamic turnover frequency when catalysts are electronically oscillated. Promotion of the catalytic surface reaction forwards or backwards was predictable by a descriptor accounting for the relative rates of forward and reverse kinetics under oscillatory conditions.
Designing Metal-Organic-Frameworks For Selective Biomass Catalysis
(2020-03) Dorneles de Mello, Matheus
Metal-organic frameworks (MOFs) are microporous materials with a wide range of pore sizes and functionalities, making them attractive for a variety of potential applications in catalysis, separations, sensing, and gas storage. Associated with the global demand for clean energy sources to find alternatives to fossil fuels, their use as catalysts for biomass conversion to chemicals finds potential application. The performance of MOFs in these applications is dependent on how stable they are upon modifications to their tunable frameworks. Such modifications include acid treatment, ligand, and cluster functionalization that can be performed by direct synthesis or post-synthesis modification, as desired for optimum performance. This dissertation focuses on using synthetic methods that may enable the tailoring the microstructure of MOFs towards their use for catalysis of biomass. We discuss the use of a method called acid modulation to introduce missing-ligands defects and open Lewis acid sites into the framework of UiO-66 to make it an active and selective catalyst for glucose isomerization to fructose in alcohol media. We demonstrate that upon the alcohol choice, the selectivity of the reaction to fructose can change drastically by favoring other reaction pathways. Furthermore, we investigate the reaction mechanism of glucose isomerization into UiO-66 and identify that glucose reacts via a 1,2-hydride transfer mechanism similar to what was reported for Sn-zeolites. We report the synthesis and installation of phosphonic acid moieties into the ligands of UiO-66 and UiO-67 as a way to introduce Brønsted acidity to these materials by a post-synthesis ligand exchange method. The active sites of P-UiO-66 are elucidated by a combination of solid-state NMR and DFT calculations. P-UiO66 is reported to be active and selective for several acid-catalyzed reactions such as alcohol dehydration and furans dehydra-decyclization with site-time yields approaching that of highly selective phosphoric acid zeolites, holding promise for its use in this and other reactions for the biomass conversion to chemicals. The tunability of MOFs combined with the PSM method and synthesis of phosphonic acids can provide accurate control of the density of active sites with a uniform distribution throughout the framework.
Development and Mechanistic Studies of Ti-Mediated C-N and N-N Coupling Reactions
(2022-11) Cheng, Yukun
This thesis describes our recent efforts on C-N and N-N coupled nitrene transfer reactions via Ti imido complexes. These studies aim to explore the compatibility of Ti(IV/II) redox catalysis with various chemical transformations through mechanistic interrogation and reactivity of model compounds, for the development of new Ti-catalyzed nitrene transfer reactions. Chapter 1 explains the background and challenges in Ti-mediated C-N and N-N coupling reactions. Chapter 2 describes the application of boryl and stannyl alkynes in Ti-catalyzed [2+2+1] pyrrole synthesis and the subsequent Pd-catalyzed cross-coupling reactions for highly functionalized pyrroles. Chapter 3 covers the exploration of alternative nitrene sources in Ti-catalyzed hydroamination via a Ti(IV/II) redox process. Chapter 4 details the mechanistic investigation of the sequential one-electron oxidation processes in the oxidative pyrazole synthesis from diazatitanacyclohexadiene. In chapter 5, a preliminary study on N–N coupled indazole synthesis from diazatitanacyclohexadiene with an extended aromatic system is reported.
Electrostatically Enhanced Thioureas: Synthesis, Reactivity and Selectivity
(2018-07) Fan, Yang
Hydrogen bonding exhibits its importance in enzyme-catalyzed chemical transformations, naturally occurring three-dimensional architectures and molecular recognition. In recent years, synthetic chemists have successfully exploited hydrogen bonds and developed many enantioselective organocatalysts. As a result, small molecule hydrogen bond donors along with organometallic species and enzymes are now recognized as playing a major role in asymmetric synthesis. Thiourea derivatives are among the most common and widely-developed hydrogen bond catalysts. Impressive results in terms of both yields and enantioselectivities in asymmetric syntheses have been obtained. A key feature in their success is the ability of these compounds to simultaneously donate two hydrogen bonds to a substrate, despite their relatively weak acidity. This provides highly stereoconfined environments when chiral moieties are incorporated into the thiourea and has made them the subject of extensive research efforts. The work described in this thesis focuses on the development of a class of positively charged acidity-enhanced thiourea catalysts which make use of an alkylated pyridinium substituent and an appropriate non-coordinating counteranion to enhance their N‒H acidities and improve their catalytic activities by orders of magnitude in a variety of transformations. A series of these catalysts have been synthesized and their reactivities in both asymmetric and non-asymmetric transformations were explored. Simple and highly efficient synthetic schemes and excellent catalytic results have been discovered for these novel species.
Evolution in structure and function of transition metal carbides during the catalytic activation of C-O, C-H, and C-C bonds
(2017-08) Sullivan, Mark
Transition metals such as Mo or W can form interstitial carbides and related interstitial formulations by occlusion of atoms such as C*, H*, O* and N*. Diverse combinations of bulk and surface stoichiometries catalyze a multitude of reactions ranging from entirely metallic hydrogenolysis and (de)hydrogenation to entirely acidic dehydration. Transition metal carbides exhibit extreme oxophilicity, and oxygen-modification results in persistent suppression of metallic characteristics and genesis of acidic properties. The carbidic penchant for cleaving C-O bonds makes these formulations promising catalysts for hydrodeoxygenation or COx activation chemistries. Bulk structure and catalytic function modification resulting from varied synthesis and reaction conditions necessitates in situ studies to quantify the effects of catalyst evolution. Isopropanol (IPA) and acetone reactions with and without O2 co-feed at 354 - 430 K were used to quantify the effects of surface oxidation on acidic and metallic characteristics of MoCx and WCx catalysts including a-Mo2C, b-Mo2C, WC, and W2C. Exposure of a freshly synthesized carbide to O* (O2, IPA or acetone) initiated suppression of metallic and basic functionality (de/hydrogenation and carbonyl condensation, respectively). Dehydration rates per gram could be reversibly tuned by over an order of magnitude using an O2 co-feed over MoCx and WCx catalysts by reversibly creating Brønsted acid sites on the carbide surface without altering bulk carbidic structure as evinced by X-ray diffraction. Oxidation decreased surface areas by an order of magnitude from ~68 to 9 m2 g-1. MoCx and WCx catalysts exhibited IPA-saturated surfaces with zero-order dependence upon IPA pressure from 0.05 – 5 kPa, and intrinsic activation energies over all MoCx and WCx catalysts under all reaction conditions ranged from 89 – 116 kJ mol-1. Brønsted acid site densities were assessed using in situ 2,6-di-tert-butylpyridine (DTBP) titrations; DTBP-normalized turnover frequency (TOF) values for MoCx and WCx catalysts were all within the range of 0.07 – 0.31 propylene s-1 (DTBP site)-1 while dehydration rates per gram varied by nearly two orders of magnitude. IPA dehydration proceeded via an E2 elimination mechanism limited by β-hydrogen scission as evinced by a kinetic isotope effect (KIE) of ~1.8. Carbonyls (acetone and propanal) are deoxygenated over H2-activated Mo2C via sequential C=O hydrogenation to equilibrium and subsequent rate-determining IPA dehydration over Brønsted acid sites at 369 K that is kinetically independent of H2 pressure from 10 – 82 kPa. Mo2C catalyzes propane dehydrogenation and hydrogenolysis with >95% selectivity to dehydrogenation in absence of H2, >95% selectivity to hydrogenolysis to CH4 with H2 co-feed, or >80% selectivity to CO via reforming with H2/CO2 co- feed at 823 K. The fraction of oxidized (O*) and carbidic (*) catalytically active surface sites can be assessed from measured effluent PCO2/PCO ratios due to an established reverse water gas shift equilibrium under steady state reaction conditions. Dehydrogenation rates can be quantitatively described using a two-site reverse Horiuti-Polyani mechanism over three distinct site pairings: (i) O*-O*, (ii) O*-*, and (iii) *-*, as evidenced by distinct dehydrogenation rate constants and activation energies in absence of co-feed, in presence of CO2 co-feed, and in presence of combined H2/CO2 co-feed. O*-* site pairs exhibited the highest dehydrogenation rate constant per gram of catalyst (kO*-* = 0.078 μmol s-1 gcat-1 kPaC3H8-1 > kO*-O* ~ k*-*). Propane dehydrogenation can be used as a probe to rigorously model transient evolution of the extent of surface oxidation and to extract kinetic dehydrogenation rate constants with near quantitative agreement with rate parameters obtained from steady state modeling (kO*-* = 0.065 μmol s-1 gcat-1 kPaC3H8-1).
Greater than the Sum of Its Parts: Tuning Nickel for Uncommon Small Molecule Reactivity and Catalysis via Dative Bonds with Group 13 Lewis Acidic Metalloligands
(2018-06) Cammarota, Ryan
In transitioning to an energy infrastructure which is less reliant on fossil fuels and less deleterious to the environment, it will be critical to couple renewable energy sources to chemical reactions, like the multi-electron conversions of water to dihydrogen (H2) and of carbon dioxide (CO2) to liquid fuels, to allow for efficient energy storage and transport. Many of these essential chemical reactions require expensive metal catalysts to proceed; catalysts featuring multiple Earth-abundant metals are utilized in biological enzymes to facilitate these reactions, and offer underexplored possibilities in synthetic and industrial settings for replacing precious metals. Although inexpensive metals are often poor catalysts for challenging multi-electron processes, there are a multitude of possible metal-metal combinations, which may exhibit more desirable properties when paired together compared to those of their constituent metals. In this vein, an isostructural series of bimetallic complexes which feature a dative bond between Ni and a varied group 13 supporting metal has been systematically studied. The steric and electronic effects of larger group 13 supporting metals were found to poise Ni for the binding and activation of H2, with the Ni center rendered more electron-deficient due to stronger Ni→M dative bonds and more favorably positioned geometrically for small molecule binding. Pairing Ni with Ga was found to be optimal for catalyzing the hydrogenations of olefins to alkanes and of CO2 to formate, both of which often require precious metal catalysts and are challenging two-electron processes that a similarly-ligated mononuclear Ni center without a supporting metal is unable to mediate. By quantitatively comparing structure, redox properties, and the reactivity of key catalytic intermediates, the effects of the supporting metal on the properties of Ni and the catalytic activity of the Ni−M bimetallic complexes have been elucidated. Collectively, experimental and computational results demonstrate that modulating an active transition metal center via a direct interaction with a Lewis acidic supporting metal can be a powerful strategy for favorably altering the properties of inexpensive metals and promoting new reactivity paradigms in base-metal catalysis.
Redox non-innocence and formation of nitrogen-nitrogen bonds in organometallic reactions
(2020-01) Pearce, Adam
The utilization of redox non-innocent ligands and substrates can grant access to new modes of reactivity in organometallic chemistry, allowing for the development of new sustainable methods with earth-abundant catalysts. Through the use of valence tautomerism in nitrogenous ligands and substrates (azides, azobenzene, isodiazenes, etc.), new organometallic reactions have been developed—the atom economical synthesis of important organic molecules such as pyrroles, pyrazoles and imines through titanium redox catalysis and an exploration of the valence tautomerism of an iridium(III)-hydrazido(2-) capable of oxidative addition.
Tunable Synthesis and Characterization of Oleo-Furan Sulfonate Surfactants from Renewable Furan and Fatty Acids
(2018-05) Joseph, Kristeen Esther
An important advance in fluid surface control was the amphiphilic surfactant composed of coupled molecular structures (i.e., hydrophilic and hydrophobic) to reduce surface tension between two distinct fluid phases. Surfactants are widely used in household detergents, cleaners, emulsifiers, foaming agents, and personal care products. Anionic surfactants constitute 50% of the $30 billion global surfactant industry and are widely used in household detergents, and personal care products. Linear alkylbenzene sulfonates (LAS) are widely used due to their low cost and high detergency. Current LAS production methods rely on toxic catalysts and petrochemical-based constituents, such as benzene and long chain hydrocarbons. The reaction has low selectivity to the prescribed linear structure thereby rendering minimal control over the desired composition and properties. Additionally, implementation of simple surfactants such as LAS has been hindered by the broad range of applications in water containing alkaline earth metals (i.e., hard water), which disrupt surfactant function and require extensive use of undesirable and expensive chelating additives. Despite years of technology development, most large-volume surfactants are made from petrochemical sources, while efforts to make renewable surfactants are focused on making existing surfactant structures from renewable sources. In this work, we demonstrate a new surfactant based on the natural structure and chemistry of plant-based oils and sugars with superior function and suitability as a replacement to petrochemicals. Furans obtained from lignocellulosic biomass can be acylated with triglyceride-derived fatty acids and anhydrides in the presence of a heterogenous zeolite catalyst. The results obtained for the reaction of lauric anhydride with furan show that different pore sizes, structures and acidity of zeolites result in varying acylation activity. Preliminary kinetic studies of the indirect acylation using anhydrides provide insight into reaction orders and product inhibition resulting in lowering of catalytic activity. Following acylation, the molecule can be upgraded via several independent and sequential chemistries such as etherification, hydrogenation and aldol condensation and finally subjected to sulfonation to yield surfactant molecules termed as oleo-furan sulfonates (OFS) in high yield. Evaluation of surfactant performance of OFS revealed hundredfold better detergency and stability in hard water conditions in comparison with petroleum-derived counterparts. The synthesis of OFS molecules is, highly tunable and selective where the number of carbon atoms in the linear or branched chain can be easily varied without compromising on reaction yields to achieve desired surfactant properties.
Understanding metal carbide-based catalysts for alternate routes to valuable chemicals
(2019-02) Kumar, Anurag
Depleting fossil fuel reserves and adverse environmental effects of current crude-oil-based processes have governed the development of sustainable energy resources. Biomass and natural gas are promising alternate sources for precursors used in chemical industry. Biomass upgrading is limited by its high oxygen content which reduces its energy density and brings forth significant challenges in its downstream processing. Chemistries that eliminate oxygen selectively while keeping the carbon backbone intact are required for development of technologies for conversion of low-quality, low-price waste product, biomass, to high-value specialty chemicals. Non-oxidative direct conversion of methane, major component of abundant natural gas reserves, to aromatics faces intrinsic thermodynamic constraints. This dissertation reports on (i) the kinetic, mechanistic, and site requirement studies performed on low temperature hydrodeoxygenation of biomass precursors and (ii) a novel polyfunctional catalyst formulation addressing the persistent thermodynamic limitations in high temperature methane dehydroaromatization. Transient kinetic measurements and temperature-programmed-surface reactions were utilized to establish accumulation of oxygen during vapor phase anisole hydrodeoxygenation (HDO) on molybdenum carbide (Mo2C) catalysts at 423 K and atmospheric pressure resulting in suppressed hydrogenation functionality of Mo2C. Kinetic studies on as-synthesized Mo2C (without ambient exposure prior to kinetic measurements) and oxygen treated Mo2C (oxygen incorporation of O:Mobulk ~ 0.075) demonstrated that oxygen only reduces the number of anisole HDO active sites at these low O* concentrations. Anisole HDO reactions on as-synthesized Mo2C and oxygen-treated Mo2C (O:Mobulk ≈ 0.076 – 0.276) resulted in varying benzene and phenol selectivity elucidating that O* content can be used to tune the product selectivity in hydrodeoxygenation reactions on transition metal carbides. These changes in catalytic reactivity were plausibly ascribed to the formation of MoOx/MoOxCy species that disrupt ensembles required for selective aromatic C-O bond cleavage. In-situ CO chemical titration was developed as an operando technique to obtain an accurate count of active sites and thus estimate turnover frequency for anisole HDO reactions on Mo2C catalysts (1.1±0.3 x 10-3 mol molMo-1 s-1). As-synthesized molybdenum carbide showed >98% selectivity towards deoxygenated products and stable chemical conversion for >30 h time-on-stream for vapor phase hydrodeoxygenation of acetic acid at low temperature (403 K) and atmospheric pressure. Space time variation experiments explicated the sequential reaction pathway for acetic acid deoxygenation on Mo2C. Kinetic studies established that the catalytic sites for H2 and acetic acid activation are distinct on Mo2C. Temperature programmed surface reaction (TPSR) with hydrogen post acetic acid HDO reaction evinced the catalyst surface evolution due to oxygen and carbon deposition. A comparison of the results in this thesis with prior reports suggested that the identity of the feed oxygenate determined its proficiency for heteroatom accumulation on/in fresh carbidic materials. 2,2-dimethylpropanoic acid (DMPA) was used as a selective titrant to estimate the catalytic site densities and calculate a turnover frequency (TOF) of (9 ± 1) × 10−4 mol s−1 molDMPA−1 for acetic acid HDO on Mo2C. Catalyst characterization using chemical transient experiments, high-angle annular dark-field imaging (HAADF-STEM), and Raman spectroscopy evidenced the formation of molybdenum carbide nanoclusters inside zeolite pores on high temperature (973K) methane exposure of MoO3/H-ZSM-5 physical mixtures air treated at 973 K. Coupling the catalytic function of MoCx/ZSM-5 with the hydrogen absorptive function of Zr metal in polyfunctional formulations resulted in circumvention of thermodynamic barriers to methane dehydroaromatization without perturbing the reaction pathways and aromatics product selectivity (70 % benzene and 20% naphthalene). Addition of Zr metal to MoCx/ZSM-5 in the form of staged-bed, stratified-bed, and interpellet physical mixtures effectively scavenges H2 formed in the catalyst bed, thereby, enhancing single-pass benzene + naphthalene yield to 14-16% compared to 8% in formulations without zirconium. Isothermal treatment of the MoCx/ZSM-5 + Zr formulation in helium post-reaction resulted in desorption of absorbed hydrogen and regeneration of the Zr absorbent leading to partial regeneration of the polyfunctional catalyst formulation yielding above equilibrium methane conversions in multiple reaction-regeneration cycles. The critical role of dispersive/diffusive H2 transport in lab-scale methane DHA experiments was demonstrated through a detailed reaction-transport model capturing the interplay of kinetic, diffusive, and convective length scales. Current catalytic technologies are faced with new challenges due to shift in available feedstocks towards chemically diverse and renewable sources. This research addresses the challenges in large-scale deployment of biomass and methane upgrading chemistries at fundamental and applied levels by examining the concepts of deoxygenation and C-H activation on catalytic surfaces.
Understanding the role of local condensed phase environments in pyrolytic and catalytic biomass conversion
(2021-05) Maliekkal, Vineet
Biomass conversion generally involves two major sets of chemical transformations – (1) thermal breakdown of macromolecules in the feedstock, such as cellulose, to smaller sugars and oxygenates via fast pyrolysis followed by (2) catalytic upgrading to the desired fuels or precursor chemicals. These reactions of biomass conversion usually occur in the condensed phase – either in the melt phase for pyrolytic reactions or in the solvent phase for catalytic upgrading reactions. The work in this thesis sheds light on the molecular complexity of such condensed phase environments. Explicit molecular modeling of these condensed phase environments coupled with first-principles simulation techniques such as density functional theory (DFT) and ab initio molecular dynamics (AIMD) are used to elucidate the influence of such environments on the kinetics of biomass conversion reactions. Examples from cellulose pyrolysis and hydrogenation chemistry are studied to demonstrate the critical importance of considering the role of condensed phase environments in biomass conversion.Using DFT calculations, constrained AIMD and experimental kinetics from the Pulsed Heated Analysis of Solid Reactions (PHASR) set-up, it is shown that vicinal hydroxyl groups which are present in the cellulose matrix in abundance can directly participate in the activation of cellulose by promoting facile proton transfer as well as stabilizing transition states through hydrogen bonding. The kinetic influence of calcium ions, naturally present in such feedstocks, is also examined in this thesis. It is shown that calcium interacts with cellulosic melt environment such that the native hydrogen bonding is disrupted. Such disruption of the hydrogen bonding network coupled with Lewis acid stabilization of the transition states leads to dual catalytic cycles for cellulose activation and second order rate dependence on calcium. Explicit modeling of the cellulosic environment is critical towards capturing such kinetic behavior. Furthermore, the influence of hydroxyl groups, calcium ions and more generally the cellulosic condensed phase environment, is examined more broadly and extended to other ring opening and fragmentation pathways that lead to glycolaldehyde, a side product of pyrolysis. The work from this part of the thesis helps establish the ubiquitous involvement of the local condensed phase environment in mediating biomass pyrolysis reactions. Finally, aqueous phase hydrogenation of C=C bonds in phenol over Pt particles inside zeolites is studied as a model reaction to demonstrate the importance of solvent environment in catalytic upgrading. Through explicit modeling of local water clusters around the reaction centers, it is shown that increasing the acidity of the zeolite supports can alter the local acidity of the water clusters. This in turn is shown to not just open up proton coupled electron transfer (PCET) pathways but also improve the efficacy of such mechanisms for hydrogenation. Thus, this study helps demonstrate that one can alter the solvent environment to enhance reactions of biomass conversion, especially those that involve proton transfer. More generally, the collective body of work in this thesis could act as a framework for future studies that seek to understand the role of condensed phase environments in biomass conversion as well as to develop strategies that use such environments for improved reactivity and selective chemical transformations.
Unsaturated Hydrocarbons as Building Blocks for Polymers and Pyrroles via Homogeneous Organometallic Catalysis
(2019-04) Chiu, Hsin-Chun
This thesis covers two common applications of organometallic catalysis: polymerization and small molecule synthesis. The first part of my thesis, Chapters 2 and 3, discusses secondary coordination effects on Ni-catalyzed ethylene polymerizations via the development of two new families of heterobimetallic Ni complexes. The second major section, Chapter 4 and 5, is focused on selective pyrrole synthesis through the modification of our recently discovered Ti-catalyzed pyrrole synthesis from alkynes and azobenzenes. Two different strategies, stereoelectronic control and dative directing group effects, have been found to play a significant role in the chemo- and regiocontrol of this catalysis. Lastly, a new project on the combination of Pd-catalyzed polyketone formation and hydroesterification has been carried out as a novel route of making polyketoesters. Some early screenings will be presented in Chapter 6 with various diphosphine ligands.

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