The selective catalytic reduction (SCR) of NO_X (x = 1,2) using Cu-exchanged zeolites is used commercially for the abatement of NO_X from on-road lean-burn diesel engines. At the low exhaust temperatures during cold-start and idle operation (<523 K), the SCR reaction proceeds via a Cu²⁺/Cu⁺ redox cycle of NH₃-solvated and mobilized Cu ions. Reduction of Cu²⁺ species proceeds via NO-assisted reduction of isolated NH₃-solvated Cu²⁺ ions. To complete Cu⁺ to Cu²⁺ oxidation, two [Cu(NH₃)₂]⁺ species react together with O₂ to form a dimeric O₂-bridged Cu²⁺ species that is subsequently reduced by NO and NH₃ to complete the SCR catalytic turnover. NH₃-solvated Cu ion species are nominally isolated under <i>ex-situ</i> conditions, however, motivating the critical research question studied in this work regarding how Cu ion mobility and dynamic interconversion of mononuclear and binuclear active sites facilitate SCR chemistry. In particular, this work focuses on understanding how active site proximity, zeolite pore connectivity and dimensionality, and catalyst poisons impact the number and reactivity of active Cu sites.<br> Steady-state SCR kinetics (473 K) measured at fixed gas conditions (10 kPa O₂) on a series of Cu-chabazite (CHA) zeolites with varied density of isolated Cu ions (0.078-0.35 Cu per 10³ ų) exhibit non-single site behavior because of changes in the kinetic relevance of Cu⁺ oxidation and Cu²⁺ reduction half-cycles, and the non-mean field nature of the Cu⁺ oxidation process. Measurement of SCR rates at dioxygen pressures (1-60 kPa O₂) far removed from typical operating conditions (3-17 kPa O₂) allows isolating the kinetic behavior under primarily Cu⁺ oxidation-limited and Cu²⁺ reduction-limited conditions, and estimating rate parameters for these two regimes by regressing SCR rates as a function of O2 pressure to an empirical Langmuirian rate expression. Apparent rate constants that are first-order in O₂ (k_first) increase systematically with Cu density, consistent with the dual-site Cu⁺ oxidation mechanism. Apparent rate constants that are zero-order in O2 (k_zero) show a weak dependence on Cu density, similar to the fraction of Cu that can be oxidized by O₂ at 473 K in transient experiments, suggesting that changes in k_zero reflect changes in the fraction of active Cu given the single-site nature of Cu²⁺ reduction mechanisms. The measured apparent activation energy in the Cu⁺ oxidation limit (E_app,first) increases systematically with Cu density, highlighting the non-mean field nature of Cu⁺ oxidation over the range of Cu densities studied. The measured apparent activation energies in the Cu²⁺ reduction limit are constant above a threshold Cu density (0.17 Cu per 10³ ų), consistent with mean-field behavior, but begin to deviate at lower densities (0.084-0.10 Cu per 10³ ų).<br> A series of Cu-zeolites with 2D (LEV, FER) and 1D (MOR) pore connectivity were synthesized to quantify how the framework topology and pore structure influences the mobility and reactivity of Cu ions during SCR. When compared to Cu-CHA, a 3D pore structure, values of k_first and k_zero (per total Cu) were several factors lower on the 2D and 1D zeolites, indicating that decreasing the effective volumetric footprint of Cu ions during SCR decreases both the rate of dual-site Cu⁺ oxidation and the fraction of Cu⁺ that oxidizes. When compared to other 3D double-six membered ring (d6r) zeolites with different pore shape (AEI) and size (AFX), rates (per total Cu) were generally a factor of 1.5 to 2 times higher on Cu-CHA, indicating that the open pore structure of cylindrical cages in CHA are favorable for low-temperature SCR reactivity.<br> The arrangement and density of framework Al atoms in CHA influences low-temperature SCR, as the framework Al atoms mediate Cu ion mobility and the arrangement of Al in the framework determines the chemical identity of the Cu active site precursors as either [CuOH]⁺ exchanged at an isolated framework Al center or Cu²⁺ exchanged at paired framework Al in a six-membered ring (6-MR). Synthesis of CHA zeolites with mixtures of Na⁺ and TMAda⁺ provides a strategy to alter the amount of Al centers in 6-MR paired configurations, because Na+ co-occludes in 6-MR voids adjacent to TMAda⁺ occluded within the cha cage. In contrast, synthesis of CHA zeolites with mixtures of K⁺ and TMAda⁺ results in primarily 6-MR isolated Al configurations because K⁺ cations displace TMAda⁺ from residing in cha cages. Thus, the use of different mixtures of organic and inorganic structure directing agents (SDAs) provide routes to synthesize CHA zeolites that favor the formation of either [CuOH]⁺ or Cu²⁺ species. The Cu speciation influences both hydrothermal stability and resistance to sulfur poisoning. SO₂ is a catalyst poison ubiquitous in automotive exhaust and is found to bind to [CuOH]⁺ sites more strongly than Cu²⁺ sites, both before and after high-temperature de-sulfation treatments. <br> Together, these findings reveal several of the important structural and active site requirements for low-temperature NO_X SCR with NH₃ on Cu-zeolites. The non-mean field nature of the SCR redox cycle on Cu²⁺/Cu⁺ ion sites, and the requirement for Cu ions to be located in proximal and accessible locations of zeolite void spaces becomes more favorable in 3D highly connected pore structures, highlighting a primary reason why low-temperature SCR rates (per Cu) are higher on Cu-CHA than on other Cu-zeolites. The synthetic procedures presented here to influence the Al arrangement in CHA zeolites provide new strategies to alter the speciation and density of isolated Cu ion sites, even among Cu-CHA zeolites of nominally identical elemental composition, which have implications for the stability and resistance to poisons of the catalyst under realistic operating conditions. Together, synthetic strategies to manipulate the proximity of active sites, methods to quantify transient and steady-state kinetics, and <i>in situ</i> and <i>operando</i> characterization are invaluable tools to study and understand the non-mean field dynamic interconversion of isolated and multinuclear sites during low-temperature SCR catalysis.<br>

<p>Catalysis has a responsibility to provide solutions to the growing grand challenge of sustainability in the fuels and chemical industry to help combat climate change. These changes; however, cannot be realized without a more fundamental understanding of the active sites that catalyze chemical reactions, and how they can be tuned to control rates and selectivities. Four specific examples of active site modification will be considered in this work: the speciation of isolated metals in zeolite frameworks, solvent thermodynamics and structure at defects in zeolite frameworks, the electronic modification of platinum through alloying in well-defined intermetallic nanoparticles, and the mobility and shape of gold nanoparticles in zeolite channels. Each will highlight how quantum chemistry calculations can provide a fundamental understanding of how these active site modifications influence the kinetics of chemical reactions, and how they can be controlled to pursue solutions to the reduction of carbon through sustainable utilization of shale gas as well as renewable chemicals production through biomass upgrading.</p> <p>Zeolites exchanged with metal heteroatoms can behave as solid Lewis or Br<a>ø</a>nsted acids depending on heteroatom identity. Lewis acid heteroatoms can adsorb water and hydrolyze to speciate into “open sites” which have been shown to differ in their ability to catalyze reactions such as glucose isomerization as compared to “closed sites” which are fully coordinated to the zeolite framework. The structure and catalytic properties of these sites are interrogated by a gas phase reaction, ethanol dehydration, in Sn-Beta by a combined Density Functional Theory (DFT) and experimental study. DFT is used to map the possible reaction mechanisms for ethanol dehydration, including the speciation of Sn sites into hydrolyzed configurations from water or ethanol. A microkinetic model for ethanol dehydration including unselective and inhibitory intermediates is constructed. This microkinetic model predicts the population of reactants and products on the catalyst surface as well as the sensitivity of individual elementary steps to the total rates. Powerful anharmonic entropy methods using <i>ab-initio </i>molecular dynamics (AIMD) is used to capture the entropy of confined reactive intermediates, which is shown to be necessary to compare with experiment. Results on closed and hydrolyzed open zeolite sites can then be compared with ethanol dehydration on “defect open” sites which were shown experimentally to occur at material stacking faults. A grain boundary model is constructed of zeolite Beta, where unique sites have similar ligand identity as hydrolyzed open sites. These defect open sites are found to not contribute to the observed reaction rate as they cannot stabilize the same transition state structures that were observed in internal Beta sites. </p> <p>Intuition about the ethanol dehydration reaction in Sn-Beta was then used to map a more expansive and diverse chemical network, the synthesis of butadiene from acetaldehyde and ethanol. For elementary reactions in this mechanism, which included aldol condensation, MPV reduction, and crotyl alcohol dehydration in addition to ethanol dehydration, the hydrolyzed open sites were found to be crucial reactive intermediates. Hydrolyzed sites were necessary to stabilize favorable transition states, which requires reconstruction of the local framework environment. Methods to preferentially stabilize hydrolyzed sites were then explored, using a screening algorithm developed to consider all possible sites in each zeolite framework. It was found that the stability of these hydrolyzed sites could be correlated to the local strain exerted by the surrounding silica matrix. This provides a new descriptor that stabilizes intermediates relevant to the synthesis of butadiene and ethanol dehydration.</p> <p>Next, the structure and thermodynamic stability of water networks around Sn-Beta defects and heteroatom active sites was considered using AIMD. As many biomass reactions occur in the presence of water, the interactions of water with hydrophobic and hydrophilic functionalized defects dictate how the stability of reactive intermediates and transition states is affected by a solvating environment. Locally stable and strongly nucleated clusters of water were observed to form at Sn defects, with less densely packed water structures stable at hydrophilic defects. This is in comparison to defect-free siliceous Beta, where significantly less water uptake is observed. These local clusters are in equilibrium with the less dense liquid-like phase that extends between defects. These results motivate localized cluster models around active sites in Lewis acids, as well as advance the fundamental understanding of hydrophobic/hydrophilic interactions in microporous materials. The local cluster models are then applied to the ethanol dehydration reaction in protonated aluminum Beta zeolites where experimentally observed non-unity coefficient ratios are rationalized by quantifying a different degree of solvation for the ethanol reactant state as opposed to the transition state, validated by a thermodynamic phase diagram.</p> <p>Changes in the electronic energy levels of <i>d</i> electrons upon alloying was studied in conjunction with a new spectroscopic technique being performed at Argonne National Laboratory to develop new descriptors to predict the degree of coking for different alloys. Resonant Inelastic X-ray Scattering (RIXS) simultaneously probes the occupied and unoccupied valence states of platinum in nanoparticles at ambient conditions. The specific excitation process of this spectroscopy is particularly amendable to DFT modeling, which was used to provide richer chemical insight into how changes in observed RIXS signature related to the electronic structure changes of platinum upon alloying. From a suite of multiple 3d alloy promoter catalysts synthesized, a quantitative comparison with DFT modeled spectroscopy was developed. The stability of DFT calculated coke precursors, relevant to dehydrogenation catalysts to convert light alkanes into olefins, was then correlated to DFT modeled RIXS spectra, which is a better descriptor for adsorption of unsaturated chemical intermediates that used previously, as well as being a descriptor accessible to direct experimental usage.</p> <p>Finally, the diffusion of gold nanoparticles in the TS-1 catalyst was studied using AIMD to help understand what structural motifs of gold are present under reaction conditions and how the shape and binding sites of gold is strongly influenced by deformation by the zeolite framework. This is used to help predict new zeolites for use in direct propylene epoxidation using molecular oxygen and hydrogen. The optimization of this catalyst is environmentally relevant to reduce the usage of inorganics and reduce the cost associated with production of hydrogen peroxide. Following these discussions, the role of computation in the prediction of active site structures and kinetics in conjunction with experiment was included. The broader impact of these findings will also be considered, which span beyond these specific reactions and materials.</p>

<p>Cyclopropanes are found in an array of synthetic and natural products. The Simmons–Smith reaction has been one of the most common methods used to synthesize cyclopropanes since it was first discovered in the 1950s. The Simmons–Smith reaction entails the transfer of a carbene (:CH2) from a zinc carbenoid to an alkene, forming a cyclopropane. However, there are still many limitations to the Simmons–Smith method, including poor functional group tolerance and poor regioselectivity in polyalkene substrates. </p> <p>To address the weaknesses in the Simmons–Smith reactions, we have pursued a transition metal-catalyzed method. Our group has reported a cobalt pyridinediimine (PDI) catalyst system to carry out cyclopropanation reactions using gem-dichloroalkanes and gem-dibromoalkanes in order to access nonstabilized carbenes. This method also offers an advantage over diazo transfer chemistry since diazo chemistry requires a stabilizing group to be present in most cases.  This established work has demonstrated a complimentary reactivity to the Simmons–Smith reaction.</p> <p>In this work, we demonstrate that we could expand upon the existing methods of dimethylcyclopropanation to access spirocyclopropanated products by changing the identity of the dichloroalkane. In addition to this reactivity, an enantiopure catalyst that is able to catalyze an enantioselective cyclopropanation was found. We were able to show a broad scope of this new reaction, and mechanistic experiments are carried out in order to probe the mechanism of this reaction. Overall, this thesis offers a new way to access enantiopure dimethylcyclopropane and spirocyclopropanated products.</p>

<div>Cyclopropanes are commonly found throughout synthetic and natural biologically active compounds. The Simmons–Smith cyclopropanation reaction is one of the most useful methods for converting an alkene into a cyclopropane. Zinc carbenoids are the active intermediate in the reaction, capable of delivering the methylene unit to a broad variety of substrates. Significant advances have been made in the field to increase overall efficiency of the reaction including the use of diethyl zinc as a precursor and allylic alcohols as directing groups.</div><div>Despite the many notable contributions in zinc carbenoid chemistry, persistent limitations of the Simmons–Smith reaction still exist. Zinc carbenoids exhibit poor steric discrimination in the presence of a polyolefin with minimal electronic bias. Additionally, due to the electrophilic nature of zinc carbenoid intermediates, the reaction performs inefficiently with electron-deficient olefins. Finally, alkyl-substituted zinc carbenoids are known to be quite unstable, limiting the potential for substituted cyclopropanation reactions.</div><div>In this work, we demonstrate that cobalt catalysis can be utilized to access novel cyclopropane products through the activation of dihaloalkanes. The content of this thesis will focus on the limitations of Zn carbenoid chemistry and addressing them with cobalt catalyzed, reductive cyclopropanations. In addition to this reactivity, we also demonstrate the dimethylcyclopropanation of activated alkenes to furnish valuable products applicable to natural product synthesis and pharmaceutically relevant compounds. Finally, we will show the unique character of the cobalt catalyzed cyclopropanation reaction through mechanistic experiments and characterization of reaction intermediates. In whole, these studies offer a complementary method to zinc carbenoid chemistry in producing novel and diverse cyclopropane products.</div>

Lewis acidic zeolites are silica-based, crystalline microporous materials containing tetravalent heteroatoms (M4+=Ti, Sn, Zr, Hf) substituted in framework locations, and have been reported to catalyze a wide range of reactions involving oxygenates and hydrocarbons. The synthetic protocols used to prepare Lewis acid zeolites determine the structures of the active sites and the reaction pockets that confine them, which in turn influences reactivity, product selectivity, and catalyst stability. Specifically, aqueous-phase reactions of biomass-derived molecules, such as glucose isomerization, are sensitive to the hydrophobicity of confining environments, leading to changes in turnover rates. As a result, precise evaluation of the structure and behavior of reaction environments and confined active sites among catalysts of varying provenance or treatment history requires quantitative descriptions of active Lewis acid site densities, of densities of surface functional groups that determine the polarity of microporous confining environments, and of the kinetic behavior of these catalytic materials.<div><br></div><div>Methods for quantifying Lewis acid sites and silanol defects are developed here by analyzing infrared (IR) spectra collected after Lewis base (CD3CN, pyridine) titrations of Lewis acidic zeolite surfaces and are compared to vapor-phase methanol and water adsorption isotherms. Additionally, IR spectra collected under ex situ (flowing vapor-phase water) and in situ (aqueous-phase, 373 K, 0-50 wt% glucose) conditions are used to compare co-adsorbed water densities and structures within hydrophobic (low silanol density) and hydrophilic (high silanol density) confining environments within M-Beta zeolites. Under reaction conditions relevant for sugar conversion in aqueous media (353-398 K, 1-50 wt% glucose), hydrophilic reaction pockets stabilize liquid-like extended water structures within microporous environments, while hydrophobic channels stabilize vapor-phase water at lower intraporous water densities. Higher aqueous-phase glucose isomerization rates (368-383 K, 1-50 wt% glucose, per kinetically relevant active site) are observed on hydrophobic Ti-Beta (~6-12x, per Lewis acidic Ti) and Sn-Beta (~50x, per Lewis acidic Sn in open configuration) zeolites over their hydrophilic analogs. Higher turnover rates on hydrophobic M-Beta zeolites reflect the absence of an extended, hydrogen-bonded network of waters, which entropically destabilizes kinetically relevant hydride shift transition states by reducing the flexibility of their primary solvation spheres. These findings suggest catalyst design strategies to minimize the generation of silanol groups within confining reaction environments would lead to increases in turnover rates.<br></div><div><br></div><div>The methods derived herein can be applied to understanding the role of the confining environment and the associated co-adsorbed water on zeolitic materials of different topology and Lewis acid site identity. For example, the transient formation of silanol defects under aqueous-phase operating conditions is primarily responsible for the deactivation of Sn-Beta catalysts observed during aqueous-phase glucose isomerization. Further, quantifying the role of the confining environment geometry and hydrophobicity on aqueous-phase glucose isomerization rates can be used as guidance for catalyst design to increase reaction rates and selectivities toward specific isomerization products. These findings show that both the active site identity and its confining environment, which vary with zeolite topology and micropore polarity, combine to influence reactivity, selectivity and stability for aqueous-phase glucose isomerization catalysis.<br></div>

<p>Acetolactate synthase (ALS) and acetohydroxyacid synthase (AHAS) are two thiamin diphosphate (ThDP)-dependent enzymes that catalyze the formation of acetolactate from two molecules of pyruvate. In addition to acetolactate, AHAS can catalyze the formation of acetohydroxybutyrate from pyruvate and α-ketobutyrate. When formed by AHAS, these compounds are important precursors to the essential amino acids valine and isoleucine. Conversely, ALS forms acetolactate as a precursor to 2,3‑butanediol, a product formed in an alternative pathway to mixed acid fermentation.</p> <p>While these enzymes catalyze the same reaction, they have been found to be quite different. Such differences include: biological function, pH optimum, cofactor requirements, reaction kinetics and quaternary structure. Importantly, AHAS has been identified as the target of the widely-used sulfonylurea and imidazolinone herbicides, which has led to many structural and kinetic studies on AHAS enzymes from plants, bacteria, and fungi. ALS, on the other hand, has only been identified in bacteria, and has largely not seen such extensive characterization. Finally, although some bacteria contain both enzymes, they have never been studied in detail from the same organism. </p> <p>Here, the ALS and AHAS enzymes from <i>Klebsiella pneumoniae</i> were studied using steady-state kinetic analyses, X-ray crystallography, site-directed and site‑saturation mutagenesis, and cell growth complementation assays to i) compare the kinetic parameters of each enzyme, ii) compare the active sites to probe their differences in substrate profile and iii) test the ability of ALS to function in place of AHAS <i>in vivo</i>.</p>

<div>As shale gas exploitation has been developed, production of shale gas in the US has rapidly increased during the last decade. This has motivated the development of techniques to covert shale gas components (mainly C₁ to C₃) to liquid fuels by catalytic conversion. The main goal of the dissertation is to study the geometric and electronic structures of the metal catalysts, which are crucial for understanding the structure-property relationship.</div><div><br></div><div><div>The fi?rst project studies bimetallic Pt-Bi catalyst for non-oxidative coupling of methane. In a recent publication published in ACS catalysis, Pt-Bi/ZSM-5 catalyst</div><div>has been shown to stably convert methane into C2 for 8 hours under non-oxidative conditions. In this thesis, structure of the Pt-Bi/ZSM-5 was shown with HAADF</div><div>imaging, synchrotron XAS and XRD. A new surface cubic Pt₃Bi phase on Pt nanoparticles with Pt-Bi bond distance of 2.80 A was formed. Formation of noble metal intermetallic alloys such as Pt₃M may be the clue for non-oxidative conversion of methane.</div></div><div><br></div><div><div>The second and third project highlight strong metal-support interaction catalysts for propane dehydrogenation. Chemisorption showed partial coverage of the SMSI oxides</div><div>on the surface of the nanoparticles. In situ X-ray absorption near edge (XANES), resonant inelastic X-ray scattering (RIXS), X-ray photoelectron spectroscopy (XPS)</div><div>have shown that little electronic effect on the metal nanoparticles. The catalyst activity per mol of metal decreased due to the partial coverage of the SMSI oxides on the surface of the catalysts. The catalysts, however, had higher selectivity due to smaller ensembles inhibiting hydrogenolysis.</div></div><div><br></div><div><div>In the fourth project Pt-P catalyst was investigated to understand the promoting effect of P.Pt-P catalysts had much higher selectivity for propane dehydrogenation</div><div>(>95%). These give two types of catalysts, a PtP₂-rich surface on Pt core and full PtP₂ ordered structure, which were con?rmed by scanning transmission electron microscopy (STEM), and in situ methods of EXAFS, synchrotron XRD, XPS, and Resonant Inelastic X-ray Spectroscopy (RIXS). The PtP₂ structure has isolated Pt</div><div>atoms separated by P₂atoms. In addition XANES, XPS and RIXS indicate a strong electronic modi?cation in the energy of the valence orbitals.</div></div><div><br></div><div><div>It can be concluded from the Pt-Bi catalyst that intermetallic alloys might be selective for NOCM. Therefore, promoters with higher reduction temperature, such as Mn and Cr, should be used to have stable catalysts at high temperature. Moreover, both Pt-Bi and Pt/CeO₂suggest that selective catalysts for propane dehydrogenation and NOCM may have some correlation. Further studies would be conducted to understand the correlation between the two reactions.</div></div>

Zeolites are crystalline microporous solid acids composed of silica-rich frameworks with aliovalent Al heteroatoms substituted in crystallographically-distinct location sand arrangements, which generate anionic lattice charges that can be compensated by protons and extra framework metal cations or complexes that behave as catalytic active sites. Protons that charge-compensate Al are similar in Brønsted acid strength, yet differ in reactivity because their bound intermediates and transition states are stabilized by van der Waals interactions with confining microporous cavities, and by electrostatic interactions with proximal heteroatoms and adjacent protons. A diverse array of framework Al and extra framework H⁺site ensembles are ubiquitous in low-silica and low-symmetry zeolite frameworks (e.g., MFI, MOR), which cause measured turnover rates to reflect the reactivity-weighted average of contributions from each distinct site ensemble. The reactivity of distinct sites can be further masked by diffusion barriers often imparted by microporous domains and secondary reactions of primary products, which become increasingly prevalent as products encounter higher numbers of active sites during diffusion prior to egress from zeolite crystallites. Consequently,catalytic behavior often depends on zeolite material properties at orders-of-magnitude different length scales, which depend on the specific protocols used in their synthesis and crystallization.<div><br></div><div><div>In this work, CHA zeolites that contain only one symmetrically-distinct lattice site for Al substitution are used as model materials to decouple the effects of proton</div><div>location and proximity in vibrational spectra and turnover rates for acid catalysis. Interactions between proximal protons influence their equilibrium distribution among anionic lattice O atoms in AlO⁻_4/2tetrahedra, and result in temperature-dependent changes to vibrational frequencies and intensities of the asymmetric OH stretching region in infrared spectra measured experimentally and computed by density functional theory (DFT). Protolytic propane cracking and dehydrogenation, a catalytic probe reaction of the intrinsic reactivity of Brønsted acid protons, occur with turnover rates (748 K, per H⁺) that are an order-of-magnitude higher on paired protons than isolated protons, resulting from entropic benefits provided to late carbonium ion-pair transition states by proximal protons. These results indicate that cationic transition states can be stabilized entropically through multi-ion interactions with lattice anion and cation sites. Precise interpretation and quantification of the reactivity of different types and ensembles of Brønsted acid protons in zeolites requires that protolytic chemistry prevails in the absence of secondary active sites or other kinetically-relevant processes, a requirement generally met for alkane cracking but not dehydrogenation on H-form zeolites. Propane dehydrogenation activation energies vary widely (by >100 kJ mol⁻¹) among H-form zeolites of different structure (MFI, MOR, CHA) and composition (Si/Al = 10 – 140) because reactant-derived carbonaceous deposits form in situ and catalyze alkane dehydrogenation under non-oxidative conditions through hydride transfer pathways. Contributions of reactant-derived active sites to propane dehydrogenation rates are quantified through a series of transient and steady-state kinetic experiments with co-fed alkene and dihydrogen products, and are found to depend on gradients in product pressures that are present in integral reactors under non-ideal plug-flow hydrodynamics. Propane dehydrogenation rates collected at initial time-on-stream and in the presence of co-fed H₂ solely reflect protolytic reaction events and can be used to interpret differences in the reactivity of distinct proton sites and ensembles for alkane activation catalysis. The reaction conditions identified here can be used to remove or suppress the reactivity of carbonaceous active sites during catalysis, or to engineer the formation of organocatalysts on zeolite surfaces for selective dehydrogenation or hydride transfer reactions.</div></div><div><br></div><div><div>Synthetic strategies to decouple bulk and active site properties at disparate length scales, which are typically correlated in MFI zeolites crystallized hydrothermally, are developed by adding a second heteroatom and organic structure directing agent (SDA) to synthesis media. Crystallite size and morphology are independently varied from Al content by incorporating B heteroatoms into zeolitic frameworks, which generate protons that are catalytically irrelevant compared to those compensating Al, and NH₃ temperature-programmed desorption methods are developed to differentiate between these two types of proton sites. The siting of Al heteroatoms in distinct locations and ensembles is influenced by the decrease in cationic charge density among occluded SDAs, in cases where ethylenediamine is co-occluded with tetra-<i>n</i>-propylammonium cations. The co-occlusion of organic SDAs enables crystallizing MFI zeolites with different bulk properties but similar Al distributions, or with similar bulk properties and different Al distributions. MFI zeolites crystallized with these methods provide model materials that can be interrogated to decouple the effects of bulk and atomicscale properties on acid catalysis, and open opportunities to exploit these material properties by designing active site ensembles and crystallite diffusion properties for catalytic chemistries that depend on coupled reaction-transport phenomena.</div></div>

<div>Carbenes are reactive organic intermediates comprised of a neutral, divalent carbon atom. The reactivity of carbenes is often orthogonal to polar functional groups (nucleophiles and electrophiles), making them valuable intermediates for organic synthesis. For example, carbenes can engage in cheletropic reactions with olefins to form cyclopropane rings or undergo insertions into weak element-hydrogen bonds. The most established strategy for accessing carbene intermediates is through a redox-neutral decomposition of diazoalkanes to form a transient M=CR₂ species. Over the course of nearly a half-century of development, many instrumental synthetic methods have emerged that operate on this basis. Despite the combined utility of these methods, the scope of catalytic carbene transfer reactions remains largely constrained by the inherent instability of the starting materials. Diazoalkanes often require electron-withdrawing groups to provide stability through resonance effects.</div><div>Contrary to redox-neutral methods, reductive carbene transfer reactions utilize non-stabilized 1,1-dihaloalkanes as carbene precursors. The Simmons-Smith cyclopropanation reaction represents the most documented example of this class, and remains today as the most practical method for parent methylene (:CH₂) transfer. Nevertheless, reductive carbene transfer processes have proven to be remarkably resistant to catalysis. Our group is interested in developing first-row transition metal catalysts which can initiate an oxidative addition into 1,1-dihaloalkanes, followed by a two-electron reduction with an outer-sphere reductant to provide access to a M=CR₂ intermediate for carbene transfer.</div><div>The application of this mechanistic hypothesis toward reductive methylene transfer using CH₂Cl₂ as the carbene source and a Ni catalyst is outlined in chapter one. The discovery of an unexpected cyclooligomerization of methylene carbenes is discussed. Mechanistic studies are presented, which are consistent with a pathway in which carbenes are iteratively inserted into an expanding metallacycle. In chapter two, the corresponding activation of 1,1-dichloroalkenes for vinylidene transfer in [5+1]-cycloadditions with vinylcyclopropanes is outlined. Finally, in the third and final chapter, organic reactions catalyzed by complexes which feature metal-metal bonds are reviewed.</div>

<p>Zeolites are microporous crystalline solids with tetrahedrally bonded Si⁴⁺ atoms linked together with bridging oxygens, interconnected in various geometries and arrangements to generate a diversity of microporous topologies. The substitution of Al³⁺ into framework tetrahedral sites (T-sites) generates anionic lattice charges that can be counterbalanced by protons (Brønsted acid sites) or extraframework metal cations and complexes that can act as catalytic active sites. The local arrangement of Al ensembles can be categorized by the size of the (alumino)silicate rings and the number and order of the Al atoms they contain, which are critical structural features that influence their ability to serve as binding sites for extraframework cations of different size and oxidation state. The ability to exercise control over the isomorphic substitution of Al³⁺ into the zeolite framework during hydrothermal crystallization has long been envisioned, but recognized to depend on complex and kinetically-controlled nucleation and crystal growth events that challenge the development of reproducible synthesis routes and predictive synthesis-structure relations. Here, we present the results of extensive experimental and theoretical investigation of the chabazite (CHA) zeolite topology, which contains a single crystallographically distinct T-site that enables studying effects of Al arrangement independent of T-site location. We then extend these findings and methodologies to investigate more complex zeolite topologies with larger numbers of distinct T-sites, including other small-pore (AEI, LEV) and medium-pore zeolites (MFI, MEL), with a specific focus on MFI zeolites because of their versatility in commercial applications. Cationic species are often present during hydrothermal zeolite crystallization, in the form of inorganic and organic structure directing agents (SDAs), to help guide formation of the intended zeolite topology and to compensate charge when Al is incorporated into the lattice. Variations in the type and amount of cationic SDAs have been shown to influence both the Al siting within different void locations of a given zeolite and the local Al arrangement. In order to make quantitative assessments of the number of Al-Al site pairs formed in a given zeolite, experimental protocols to titrate the specific Al-Al site ensembles are required. We specifically explore the use of Co²⁺titrants at saturation uptakes, verifying the sole presence of Co²⁺ cations via spectroscopic identification and a cation site balance that is closed by quantifying residual Brønsted acid sites by NH₃titration. We then investigate the role of the cationic SDA content in the synthesis mixture on the Al arrangement in MFI zeolites. Depending on the specific mixture of the</p><p>organic cation tetrapropylammonium (TPA⁺) or various neutral organic molecules when used together with smaller Na⁺ cations, MFI zeolites can be crystallized over a range of Al content. Moreover, the fraction of Co²⁺-titratable Al-Al pairs correlates with the amount of occluded Na⁺ cations when the total Al content is held approximately constant (Si/Al ~ 50). These results are consistent with our prior reports of CHA zeolites, wherein the occlusion of smaller Na⁺ cations correlates positively with the formation of Al-Al pairs in six-membered ring (6-MR) locations. Unlike the N,N,N trimethyl-1-adamantylammonium (TMAda⁺) cation used to crystallize CHA, which alone does not form Co²⁺titratable Al-Al site pairs, the organic TPA+ alone can form Al-Al site pairs in MFI. DFT calculations of Al siting energies, using a 96 T-site MFI unit cell containing either one or two Al charge-balanced by one or two occluded TPA⁺respectively, reveal the dominant influence of electrostatic interactions between the cationic N of TPA⁺ and the anionic lattice charge. DFT calculations of probable Co²⁺exchange sites are used to identify a subset of Al-Al site pairs with favorable energies when compensated either by Co²⁺ or by two TPA⁺ molecules in adjacent MFI channel intersections. MFI crystallized with one cationic species (TPA⁺ or Na⁺) with a neutral organic species (ethylenediamine, pentaerythritol, or a mixture of methylamine and 1,4-diazabicyclo[2.2.2]octane) contain significantly lower fractions of Co²⁺-titratable Al-Al pairs at similar bulk Al content (Si/Al = 43–58), demonstrating the role of neutral organic species to occupy void spaces without providing the capacity to compensate charge, thus serving to increase the average spatial separation of framework Al sites. The kinetics of methanol dehydration to dimethyl ether can be quantified by first-order and zero order rate constants (415 K, per H⁺) to probe acid strength and confinement effects in solid Brønsted acids. Here, we use this quantitative probe reaction to investigate how Al arrangements in MFI and CHA affect the mechanism and kinetics of this reaction, in order to connect synthetic protocols to structure and to catalytic function. This effort first involved measurement of methanol dehydration kinetics on a suite of commercially sourced MFI samples to benchmark results obtained on our kinetic instruments to prior literature reports. CHA zeolites with 6-MR isolated protons show zero-order rate constants similar to those for commercial MFI zeolites and other topologies previously studied in the literature, reflecting the invariance in Brønsted acid strength with zeolite topology. First-order rate constants on isolated acid sites in CHA are an order of magnitude higher than acid sites in MFI, reflecting the smaller confining environments present in CHA than in the medium-pore zeolite MFI. In contrast, both first-order and zero-order rate constants among CHA samples increase systematically with the fraction of 6-MR Al pairs, even for samples of nominally similar composition (Si/Al ~ 15). DFT provides evidence for lower activation barriers at protons of 6-MR paired Al sites in CHA, which stabilize transition states via H-bonding interactions through co-adsorbed methanol bound at the proximal acid site, in a manner dependent on the specific Al arrangement and ring size and structure. Such favorable configurations are identified for 6-MR paired Al sites in CHA, but were not identified within MFI zeolite, which shows first-order and zero-order rate constants that are invariant with varying Al-Al site pair content. These findings and conclusions demonstrate how quantitative experimental characterization and kinetic data, augmented by theory insights, can aid in the development of more predictive synthesis-structure-function relations for zeolite materials and help transform empirical efforts in active site design and engineering into a more predictive science.</p>