<p> Ring opening of heterocyclic compounds through C-X bond cleavage is a useful strategy that provides rapid access to highly functionalized acyclic building blocks. In recent years, much work has focused on using transition metal catalysts to activate the C-X bonds of heterocycles and initiate ring opening. Metal-catalyzed ring opening of unstrained heterocycles is less prevalent than catalytic activation of strained heterocycles, which is advantageously driven by relief of ring strain. Methods for catalytic ring activation of unstrained heterocycles exist but are limited. Herein, we report the use of chiral cobalt complexes as catalysts for enantioselective ring opening of dihydrofuran and nitrogen-protected pyrrolines by utilizing dichloroalkenes as vinylidene precursors, Zn as a reductant, and ZnCl2 as an additive. Based on preliminary mechanistic studies, we believe this method proceeds through [2 + 2] cycloaddition between the ligated cobalt vinylidene species and the heterocycle, followed by β-heteroatom elimination, cobalt to zinc transmellation, and protonation to give rise to synthetically useful chiral allylic alcohol and amine products. </p>

<p>The expanded production of shale gas has increased the desire for developing methods for converting light alkanes, especially propane and ethane, into aromatic species (i.e., benzene, toluene, and xylene). A multi-step process for conversion of light alkanes to aromatics may be developed, where the first stage converts light alkanes into olefins and hydrogen, and the second stage converts olefins to aromatics. However, to determine the viability of this process, better understanding of the performance of olefin aromatization in the presence of equimolar hydrogen is necessary. </p> <p><br></p> <p>Previous studies on the conversion of olefins to aromatics with bifunctional ZSM-5 catalysts have concluded that benzene, toluene, and xylenes (BTX) yields are significantly higher than for ZSM-5 alone. These results were attributed to the presence of a dehydrogenation function of Ga or Zn leading to higher rates of aromatics formation. In this study, a highly active, bifunctional PtZn/SiO2 (1.3 wt% Pt, 2.6 wt% Zn) with H-ZSM-5 (Si/Al = 40) catalyst is investigated for propene aromatization at 723 K and 823 K. At low to moderate propene conversions, in addition to BTX, light alkanes and olefins are produced. Many of these may also be converted to aromatics at higher propene conversion while others are not, for example, light alkanes. When compared at equivalent space velocity and propylene conversion, the bifunctional catalyst has a much higher selectivity to aromatics than ZSM-5; however, when compared at equivalent conversion of all reactive intermediates, the bifunctional catalyst exhibits very similar BTX selectivity. At 723 K, for both ZSM-5 and the bifunctional catalyst, the primary non-reactive by-products are propane and butane. At 823 K, both ZSM-5 and the bifunctional catalyst convert propane and butane to aromatics increasing the aromatic yields, and the by-products are methane and ethane.</p> <p><br></p> <p>Additionally, previous studies have investigated the H-ZSM-5 and Ga/H-ZSM-5 in the absence of H2, which is necessary to understand in order to develop a process for the conversion of light alkanes to aromatics. Herein, proton-form ZSM-5 and Ga modified H-ZSM-5 are compared for propylene aromatization in the presence and absence of equimolar hydrogen at 1.9 kPa and 50 kPa partial pressures. At 1.9 kPa, the presence of H2 is shown to have no impact on the product distribution on H-ZSM-5 or Ga/H-ZSM-5. At 50 kPa, H2 is shown to have no significant impact on H-ZSM-5 and has no impact on Ga/H-ZSM-5 at conversions <80%. Additionally, the addition of Ga to H-ZSM-5 is shown to have no impact on the product distribution in the presence or absence of H2, contrary to previous reports. The disagreement with previous literature stems from previous literature comparing H-ZSM-5 and Ga/H-ZSM-5 at equivalent space velocity rather than equivalent propylene conversion despite previous studies showing that the presence of Ga increases the conversion at equivalent space velocity for olefin aromatization. </p>

<p>Dimerization and oligomerization reactions of alkenes are promising catalytic strategies to convert light alkenes, which can be derived from light alkane hydrocarbons (ethane, propane, butane) abundant in shale gas resources, into heavier hydrocarbons used as chemical intermediates and transportation fuels. Nickel cations supported on aluminosilicate zeotypes (zeolites and molecular sieves) selectivity catalyze ethene dimerization over oligomerization given their mechanistic preference for chain termination over chain propagation, relative to other transition metals commonly used for alkene oligomerization and polymerization reactions. Ni-derived sites initiate dimerization catalytic cycles in the absence of external activators or co-catalysts, which are required for most homogeneous Ni complexes and Ni²⁺ cations on metal organic frameworks (MOFs) that operate according to the coordination-insertion mechanism, but are not required for homogeneous Ni complexes that operate according to the metallacycle mechanism. Efforts to probe the mechanistic details of ethene dimerization on Ni-containing zeotypes are further complicated by the presence of residual H⁺ sites that form a mixture of 1-butene and 2-butene isomers in parallel acid-catalyzed pathways, as expected for the coordination-insertion mechanism but not for the metallacycle mechanism. As a result, the mechanistic origins of alkene dimerization on Ni cations have been ascribed to both the coordination-insertion and metallacycle-based cycles. Further, different Ni site structures such as exchanged Ni²⁺, grafted Ni²⁺ and NiOH⁺ cations are proposed as precursors to the dimerization active sites, based on analysis of kinetic data measured in different kinetic regimes and corrupted by site deactivation, leading to unclear and contradictory proposals of the effect of Ni precursor site structures on dimerization catalysis.</p> <p> Dimerization of ethene (453 K) was studied on Ni cations exchanged within Beta zeotypes in the absence of externally supplied activators, by suppressing the catalytic contributions of residual H⁺ sites via selective pre-poisoning with Li⁺ cations and using a zincosilicate support that contains H⁺ sites of weaker acid strength than those on aluminosilicate supports. Isolated Ni²⁺ sites were predominantly present, consistent with a 1:2 Ni²⁺:Li⁺ ion-exchange stoichiometry, CO infrared spectroscopy, diffuse reflectance UV-Visible spectroscopy and <i>ex-situ</i> X-ray absorption spectroscopy. Isobutene serves a kinetic marker for alkene isomerization reactions at H⁺ sites, which allows distinguishing regimes in which 2-butene isomers formed at Ni sites alone, or from Ni sites and H⁺ sites in parallel. 1-butene and 2-butenes formed at Ni sites were not equilibrated and their distribution was invariant with ethene site-time, revealing the primary nature of butene double-bond isomerization at Ni sites as expected from the coordination-insertion mechanism. <i>In-situ</i> X-ray absorption spectroscopy showed that the Ni oxidation state was 2+ during dimerization, also consistent with the coordination-insertion mechanism. Moreover, butene site-time yields measured at dilute ethene pressures (<0.4 kPa) increased with time-on-stream (activation transient) during initial reaction times, and this activation transient was eliminated at higher ethene pressures (≥ 0.4 kPa) and while co-feeding H₂. These observations are consistent with the <i>in-situ</i> formation of [Ni(II)-H]⁺ intermediates involved in the coordination-insertion mechanism, as verified by H/D isotopic scrambling and H₂-D₂ exchange experiments that quantified the number of [Ni(II)-H]⁺ intermediates formed.</p> <p> The prevalence of the coordination-insertion cycles at Ni²⁺ cations provides a framework to interpret the kinetic consequences of the structure of Ni²⁺ sites that are precursors to the dimerization active sites. Beta zeotypes predominantly containing either exchanged Ni²⁺ cations or grafted Ni²⁺ cations show noteworthy differences for ethene dimerization catalysis. The deactivation transients for butene site-time yields on exchanged Ni²⁺ cations indicate two sites are involved in each deactivation event, while those for grafted Ni²⁺ cations indicate involvement of a single site. The site-time yields of butenes extrapolated to initial time, and then further extrapolated to zero ethene site-time, rigorously determined initial ethene dimerization rates (453 K, per Ni) that showed a first-order dependence in ethene pressure (0.05-1 kPa). This kinetic dependence implies the β-agostic [Ni(II)-ethyl]⁺complex to be the most abundant reactive intermediate for the Beta zeolites containing exchanged and grafted Ni²⁺ cations. Further, the apparent first-order dimerization rate constant was two orders of magnitude higher for exchanged Ni²⁺ cations than for grafted Ni²⁺ cations, reflecting differences in ethene adsorption or dimerization transition state free energies at these two types of Ni sites. </p> <p> The presence of residual H⁺ sites on aluminosilicate zeotypes, in addition to the Ni²⁺ sites, causes formation of saturated hydrocarbons and oligomers that are heavier than butenes and those containing odd numbers of carbon atoms. The reaction pathways on Ni²⁺ and H⁺ sites are systematically probed on a model Ni-exchanged Beta catalyst that forms a 1:1 composition of these sites <i>in-situ</i>. The quantitative determination of apparent deactivation orders for the decay of product space-time yields provides insights into the site origins of the products formed. Further, Delplot analysis systematically identifies the primary and secondary products in the reaction network. This strategy shows linear butene isomers to be primary products formed at Ni²⁺-derived sites, while isobutene is formed as a secondary product by skeletal isomerization at H⁺ sites. In addition, propene is formed as a secondary product, purportedly by cross-metathesis between linear butene isomers and the reactant ethene at Ni²⁺-derived sites. Also, ethane is a secondary product that forms by hydrogenation of ethene at H⁺ sites, with the requisite H₂ generated <i>in-situ</i> likely by dehydrogenation and aromatization of ethene at H⁺ sites.</p> <a>The predominance of the coordination-insertion mechanism at Ni²⁺-derived sites implies kinetic factors influence isomer distributions within the dimer products, providing an opportunity to influence the selectivity toward linear and terminal alkene products of dimerization. In the case of bifunctional materials, reaction pathways on the Ni²⁺ and H⁺sites dictate the interplay between kinetically-controlled product selectivity at Ni sites and thermodynamic preference of product isomers formed at the H⁺ sites. </a>In summary, through synthesis of control catalytic materials and rigorous treatment of transient kinetic data, this work presents a detailed mechanistic understanding of the reaction pathways at the Ni²⁺ and H⁺ sites, stipulating design parameters that have predictable consequences on the product composition of alkene dimerization and oligomerization.

<p>The increase in shale gas exploitation has motivated the studies towards new processes for converting light alkanes into higher valuable chemicals, including fuels. The works in this dissertation focuses on two processes: propane dehydrogenation and ethylene oligomerization. The former involves the conversion of propane into propylene and hydrogen, while the latter converts light alkenes into higher molecular weight products, such as butylene and hexene. </p> <p>The thesis project focuses on understanding the effect of geometric effects of Pt alloy catalysts for propane dehydrogenation and the methodologies for their characterization. Pt-Co bimetallic catalysts were synthesized with increasing Co loadings, characterized and evaluated for its propane dehydrogenation performance. In-situ synchrotron X-Ray Powder Diffraction (XRD) and X-Ray Absorption (XAS) were used to identify and differentiate between the intermetallic compound phases in the nanoparticle surface and core. Difference spectra between oxidized and reduced catalysts suggested that, despite the increase in Co loading, the catalytic surface remained the same, Pt₃Co in a Au₃Cu structure, while the core became richer in Co, changing from a monometallic Pt fcc core at the lowest Co loading to a PtCo phase in a AuCu structure at the highest loading. Co^II single sites were also observed on the surface, due to non-reduced Co species. The catalytic performance towards propane dehydrogenation reinforced this structure, as propylene selectivity was around 96% for all catalysts, albeit the difference in composition. The Turnover Rate (TOR) of these catalysts was also similar to that of monometallic Pt catalysts, around 0.9 s^-1, suggesting Pt was the active site, while Co atoms behaved as non-active, despite both atoms being active in their monometallic counterparts.</p> <p>In the second project, a single site Co^II catalyst supported on SiO₂ was evaluated for ethylene oligomerization activity. The catalyst was synthesized, evaluated for propane dehydrogenation, propylene hydrogenation and ethylene oligomerization activities and characterized <i>in-situ</i> by XAS and EXAFS and H₂/D₂ exchange experiments. The catalysts have shown negligible conversion at 250^oC for ethylene oligomerization, while a benchmark Ni/SiO₂ catalyst had about 20% conversion and TOR of 2.3x10^-1 s^-1. However, as the temperature increased to above 300^oC, ethylene conversion increased significantly, reaching about 98% above 425^oC. <i>In-situ</i> XANES and EXAFS characterization suggested that H₂ uptake under pure H₂ increased in about two-fold from 200^oC to 500^oC, due to the loss of coordination of Co-O bonds and formation of Co-H bonds. This was further confirmed by H₂/D₂ experiments with a two-fold increase in HD formation per mole of Co. <i>In-situ</i> XAS characterization was also performed with pure C­₂H₄ at 200^oC showed a similar trend in Co-O bond loss, suggesting the formation of Co-alkyl, similarly to that of Co-H. The <i>in-situ</i> XANES spectra showed that the oxidation state remained stable as a Co²⁺ despite the change in the coordination environment, suggesting that the reactions occurs through a non-redox mechanism. These combined results allowed the proposition of a reaction pathway for dehydrogenation and oligomerization reactions, which undergo a similar reaction intermediate, a Metal-alkyl or Metal-Hydride intermediates, activating C-H bonds at high temperatures.</p>

Brønsted and Lewis acid sites located within microporous solids catalyze a variety of chemical transformations of oxygenates and hydrocarbons. Such reactions occur in condensed phases in envisioned biomass and shale gas upgrading routes, motivating deeper fundamental understanding of the reactivity-determining interactions among active sites, reactants, and solvents. The crystalline structures of zeolites, which consist of SiO₄ tetrahedra with isomorphously-substituted M⁴⁺ (e.g., Sn⁴⁺, Ti⁴⁺) as Lewis acid sites, or Al³⁺ with charge-compensating extraframework H⁺ as Brønsted acid sites, provide a reasonably well-defined platform to study these interactions within confining voids of molecular dimension. In this work, gas-phase probe reactions that afford independent control of solvent coverages are developed and used to interpret measured rate data in terms of rate and equilibrium constants for elementary steps, which reflect the structure and stability of kinetically relevant transition states and reactive intermediates. The foundational role of quantitative kinetic information enables building molecular insights into the mechanistic and active site requirements of catalytic reactions, when combined with complementary tools including synthetic approaches to prepare active sites and surrounding environments of diverse and intended structure, quantitative methods to characterize and titrate active sites and functional groups in confining environments, and theoretical modeling of putative active site structures and plausible reaction coordinates.<br><div><br></div><div>Bimolecular ethanol dehydration to diethyl ether was developed as a gas-phase catalytic probe reaction for Lewis acid zeolites. A detailed mechanistic understanding of the identities of reactive intermediates and transition states on Sn-Beta zeolites was constructed by combining experimental kinetic measurements with density functional theory treatments. Microkinetic modeling demonstrated that Sn active site configurations undergo equilibrated interconversion during catalysis (404 K, 0.5–35 kPa C₂H₅OH, 0.1–50 kPa H₂O) from hydrolyzed-open configurations ((HO)-Sn-(OSi≡)₃---HO-Si) to predominantly closed configurations (Sn-(OSi≡)₄), and identified the most abundant productive (ethanol-ethanol dimer) and inhibitory (ethanol-water dimer) reactive intermediates and kinetically relevant transition state (S_N2 at closed sites). Mechanism-based interpretations of bimolecular ethanol dehydration turnover rates (per Lewis acidic Sn, quantified by CD₃CN IR) enabled measuring chemically significant differences between samples synthesized to contain high or low densities of residual Si-OH defects (quantified by CD₃CN IR) within microporous environments that confine Sn active sites. Hydrogen-bonding interactions with Si-OH groups located in the vicinity of Sn active sites in high-defect Sn-Beta zeolites stabilize both reactive and inhibitory intermediates, leading to differences in reactivity within polar and non-polar micropores that reflect solely the different coverages of intermediates at active sites. The ability of confining microporous voids to discriminate among reactive intermediates and transition states on the basis of polarity thus provides a strategy to mitigate inhibition by water and to influence turnover rates by designing secondary environments of different polarity via synthetic and post-synthetic techniques. </div><div><br></div><div>Despite the expectation from theory that Sn active sites adopt the same closed configurations after high-temperature (823 K) oxidation treatments, distinct Sn sites can be experimentally identified and quantified by the ν(C≡N) infrared peaks of coordinated CD₃CN molecules, and a subset of these sites are correlated with first-order rate constants of aqueous-phase glucose-fructose isomerization (373 K). In contrast, <i>in situ</i> titration of active sites by pyridine during gas-phase ethanol dehydration catalysis (404 K) on a suite of Sn-zeolites of different topology (Beta, MFI, BEC) quantified the dominant active site to correspond to a different subset of Sn sites than those dominant in glucose-fructose isomerization. An extensive series of synthetic and post-synthetic routes to prepare Sn-zeolites containing Sn sites hosted within diverse local coordination environments identified a subset of Sn sites located in defective environments such as grain boundaries, which are more pronounced in Beta crystallites comprised of intergrowths of two polymorphs than in zeolite frameworks with un-faulted crystal structures. Sn sites in such environments adopt defect-open configurations ((HO)-Sn-(OSi≡)₃) with proximal Si-OH groups that do not permit condensation to closed configurations, which resolves debated spectroscopic assignments to hydrolyzed-open site configurations. Defect-open Sn sites are dominant in glucose-fructose isomerization because their proximal Si-OH groups stabilize kinetically relevant hydride shift transition states, while closed framework Sn sites are dominant in alcohol dehydration because they stabilize S_N2 transition states via Sn site opening in the kinetically relevant step and re-closing as part of the catalytic cycle. The structural diversity of real zeolite materials, whose defects distinguish them from idealized crystal structures and allows hosting Lewis acid sites with distinct local configurations, endows them with the ability to effectively catalyze a broad range of oxygenate reactions.</div><div><br></div><div>During aqueous-phase catalysis, high extra-crystalline water chemical potentials lead to intra-pore stabilization of H₂O molecules, clusters, and extended hydrogen-bonded networks that interact with adsorbed intermediates and transition states at Lewis acid sites. Glucose-fructose isomerization turnover rates (373 K, per defect-open Sn, quantified by CD₃CN IR) are higher when Sn sites are confined within low-defect, non-polar zeolite frameworks that effectively prevent extended water networks from forming; however, increasing exposure to hot (373 K) liquid water generates Si-OH groups via hydrolysis of siloxane bridges and leads to lower turnover rates commensurate with those of high-defect, polar frameworks. Detailed kinetic, spectroscopic, and theoretical studies of polar and non-polar titanosilicate zeolite analogs indicate that extended water networks entropically destabilize glucose-fructose isomerization transition states relative to their bound precursors, rather than influence the competitive adsorption of water and glucose at active sites. Infrared spectra support the stabilization of extended hydrogen-bonded water networks by Si-OH defects located within Si- and Ti-Beta zeolites, consistent with ab initio molecular dynamics simulations that predict formation of distinct thermodynamically stable clustered and extended water phases within Beta zeolites depending on the external water chemical potential and the nature of their chemical functionality (closed vs. hydrolyzed-open Lewis acid site, or silanol nest defect). The structure of water confined within microporous solids is determined by the type and density of intracrystalline polar binding sites, leading to higher reactivity in aqueous media when hydrogen-bonded networks are excluded from hydrophobic micropores.</div><div><br></div><div>Aluminosilicate zeolites adsorb water to form (H₃O⁺)(H₂O)_n clusters that mediate liquid-phase Brønsted acid catalysis, but their relative contributions to the solvation of reactive intermediates and transition states remain unclear. Bimolecular ethanol dehydration turnover rates (per H⁺, quantified by NH₃ temperature-programmed desorption and <i>in situ</i> titrations with 2,6-di-<i>tert</i>-butylpyridine) and transmission infrared spectra measured on Brønsted acid zeolites under conditions approaching intrapore H₂O condensation (373 K, 0.02–75 kPa H₂O) reveal the formation of clustered, solvated (C₂H₅OH)(H⁺)(H₂O)_n intermediates, which are stabilized to greater extents than bimolecular dehydration transition states by extended hydrogen-bonded water networks. Turnover rates deviate sharply below those predicted by kinetic regimes in the absence of extended condensed water networks because non-ideal thermodynamic formalisms are required to account for the different solvation of transition states and MARI. The condensation of liquid-like phases within micropores that stabilize reaction intermediates and transition states to different extents is a general phenomenon for Brønsted acid-catalyzed alcohol dehydration within zeolites of different topology (CHA, AEI, TON, FAU), which governs the initial formation and structure of clustered hydronium-reactant and water-protonated transition state complexes. Systematic control of liquid-phase structures within confined spaces by gas-phase measurements around the point of intrapore condensation enables more detailed mechanistic and structural insights than those afforded by either kinetic measurements in the liquid phase, or structural characterizations of aqueous systems in the absence of reactants.</div>

Reported herein is a brief summary of the history, properties, and applications of amine-boranes. The past methods devised for their preparation are described and the routes used to produce the compounds used in the work presented here are detailed. Building on prior synthetic approaches to amine-boranes, a new carbon dioxide mediated synthesis is presented. Proceeding through a monoacyloxyborane intermediate, the borane complexes of ammonia, primary, secondary, tertiary, and heteroaromatic amine are provided in 53-99% yields. Utilizing the amine-boranes obtained from the methods described, two divergent methods for direct amidation are introduced. The first uses amine-boranes as dual-purpose reagents, where the carboxylic acid is first activated by the borane moiety to form a triacyloxyborane-amine complex. This allows the delivery of the coordinated amine to form the amide products. A series of primary, secondary, and tertiary amides were prepared in 55-99% yields using this protocol, which displays a broad functional group tolerance. Extended from this dual-purpose methodology, a catalytic amidation is described. Utilizing ammonia-borane as a substoichiometric (10%) catalyst, a series of secondary and tertiary amide are prepared directly from carboxylic acids and amines in 59-99% yields, including amines containing typically borane reactive functionalities including alcohols, thiols, and alkenes. Amine-boranes are additionally used in two borylation methodologies. By reaction with <i>n</i>-butyl lithium, the amine-boranes are converted to the corresponding lithium aminoborohydrides, which upon reaction with a terminal alkyne provides the alkynyl borane-amine complexes in 65-98% yields. This process is compatible with both alkenes and internal alkynes, as well as a range of aprotic functionalities. A new strategy for aminoborane synthesis is also described and applied to the borylation of haloarenes. Activation of a series of amine-boranes with iodine produces the iodinated amine-borane, which undergoes dehydrohalogenation with an appropriate base to produce either monomeric or dimeric aminoboranes. Several aminoboranes were synthesized exclusively as the monomeric species, which due to their greater reactivity, were used directly in the synthesis of a series of aryl boronates in 65-99% yields.

<div>Thermochemical processing using fast-pyrolysis technology has been used to upgrade feedstocks like biomass and natural gas and more recently studied for plastic recycling. This work aims to improve the selectivity to desired products from a pyrolysis process through better catalysts and reactor design.</div><div>Fast-pyrolysis of biomass to fuels is considered a promising technology due to the higher yields to liquid fuel products. However, the process suffers from low carbon efficiency to hydrocarbon products due to carbon losses to biochar, accounting for 25-40 wt.% of the product stream depending on the biomass type. Using a combination of inorganic free-model compounds, biomass pretreatments and mass spectrometric analyses coupled with lab-scale reactor experiments, the char contribution from the lignocellulosic components (cellulose, hemicellulose, and lignin) and mineral content was investigated. The lignocellulosic components were found to follow the order: Lignin > Hemicellulose > Cellulose. Addition of inorganic salts (K, Na and Ca) to cellobiose, a model compound for cellulose, was found to catalyze additional dehydration reactions on primary pyrolysis products (e.g., levoglucosan) to yield secondary products (e.g., 5-HMF), and produce more char. This knowledge of char formation contributors can enable optimization of the bio-refining process sequencing using process system engineering tools and thus achieve higher carbon efficiency for biomass conversion.</div><div>While biomass has been viewed as a future energy source, there is a need for a transition fuel with the lowest possible greenhouse gas (GHG) footprint. Shale gas, consisting primarily of methane, is a potential candidate due to its large availability and high hydrogen to carbon ratio. Recently, single-atom catalysts have been studied as stable and non-coking catalysts for the non-oxidative coupling of methane (NOCM) to higher hydrocarbons (like ethylene). However, lack of post reaction catalyst characterization and rigorous kinetic testing have raised questions on the stability of these materials. This work combines homogenous (Chemkin simulations, gas phase kinetics) and heterogeneous reaction kinetic studies (reaction orders, steady state kinetics), coupled with microscopy (Scanning and Transmission Electron Microscopy (SEM, TEM)) and surface characterization tools (BET, TGA, Raman spectroscopy, CO-IR spectroscopy) to understand the role of the solid materials during NOCM. Post reaction catalyst characterization using transmission electron microscopy (TEM) analysis on the spent samples (CH4 treated at 975 deg C for 3 hours) reveals that the materials containing Pt single atoms (SA) and Pt nanoparticles (NP) are found to sinter to particles approximately 5-7 nm in size. Ethylene hydrogenation experiments, a kinetic probe for surface Pt, shows initial ethane formation rates that are four orders of magnitude lower on the isolated Pt+2 sites, found on Pt SAs, when compared to the rates obtained if all the surface Pt were assumed to be metallic. These results suggest that single atoms are not the active sites. However, under same reaction conditions (50 mL min-1 CH4 flow and 975 deg C), the ethylene formation rates (in mol h-1) on the solid materials are 2-7 times higher than the empty tube rates, indicating that the surface plays a role during NOCM. Addition of incremental amounts of the solid material increases methane conversion, extrapolating to the bare tube conversion at zero loading. This indicates that the solid materials improve the NOCM performance.</div><div>Experiments with pure methane feeds indicate that the solid materials are found to deactivate due to coking on the surface, evidenced by the coke buildup observed using thermogravimetric analysis (TGA) and the initial time-on-stream kinetic results showing rapid methane deactivation. Raman spectroscopy on the spent catalysts indicate at the development of a similar graphite-like surface intermediate under steady state conditions on all the materials. When compared under the same reaction conditions (975 deg C, 60 mL min-1 Pure CH4 with 10% UHP N2 feed, space velocity = 39.6 L h-1 gcat-1), these coked surfaces show a linear dependence for the ethylene formation rate (in mol h-1 gcat-1) with the spent surface area of the material (in m2 gcat-1). This observation is irrespective of the type of the material studied (alpha Al2O3, Davisil SiO2, 1 wt.% Pt/CeO2, Graphene, Graphite, etc.). In conclusion, these results prove that the spent surface area is critical for NOCM.</div><div>Similar experimental setup was used to study the dehydrogenation of methane, ethane, and propane mixture in the gas phase. Initial experiments at 1 bar pressure and reaction temperatures ranging from 650-850 deg C revealed that ethylene and hydrogen are the main gas phase products, with methane acting as a diluting agent under these reaction conditions. These results could enable direct processing of the shale gas without the use of a conventional ethane/propane separation step. These results were further studied by the system engineers using ANSYS ChemkinPro. For practical applications, these experiments were suggested to be performed at much higher operating pressures (~30 bar) and low residence time (~0.2 s), with a quick quenching step added after the reactor to prevent change in the exit stream compositions. A new reaction system was built to experimentally validate these recommendations.</div>

<p>The strong metal support interaction (SMSI) is among the best-known classes of metal-oxide interfacial interactions in heterogeneous catalysis, which is defined by the coverage of surface oxide on metal nanoparticles, forming a metal-oxide interface. However, there is limited insight in the atomic scale understanding of the structure of the SMSI oxide. In this work, surface science techniques including scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS) and low energy electron diffraction (LEED) were employed to investigate interfacial interactions in multiple catalytic systems, including ZnO-Pd, ZnO-Pt, and MoOx-Pt. To utilize the capabilities of the surface science techniques and to mimic a catalytic metal nanoparticle in SMSI state, ultrathin oxide films were prepared on metal single crystals as inverse model catalysts.</p> <p>The structural and chemical transformations of ultrathin zinc (hydroxy)oxide films on Pd(111) were studied under varying gas phase conditions (UHV, 5×10−7 mbar of O2 and D2/O2 mixture). Under oxidative conditions, zinc oxide forms partially hydroxylated bilayer islands on Pd(111). Sequential treatments of the submonolayer ZnOxHy films in D2/O2 mixture (1:4) at 550 K evoked structural transformations from bilayer to monolayer and to a PdZn near-surface alloy, in accompany with the reduction of Zn, demonstrating that zinc oxide as a non-reducible oxide, can spread on metal surface and show an SMSI-like behavior in the presence of hydrogen. A mixed canonical – grand canonical phase diagram revealed that the monolayer intermediate structure is a metastable structure formed during the kinetic transformation, and the near-surface alloys are stable under the D2/O2 conditions. Grand canonical phase diagram predicted that under real SMSI conditions zinc oxide films on Pd nanoparticles would be stabilized by hydroxylation with stoichiometries such as ZnOH and Zn2O3H3. Based on the experimental and theoretical observations, we propose that the mechanism of metal nanoparticle encapsulation involves both surface (hydroxy)oxide formation as well as alloy formation, depending on the environmental conditions.</p> <p>Hydroxylation plays a more important role in the ZnO/Pt(111) system. Different from Pd(111), zinc oxide tends to form monolayer graphite-like ZnO films on Pt(111) under oxidative conditions at submonolayer coverages. This structure is extremely susceptible to hydroxylation at room temperature, leading to spontaneous formation of honeycomb-like Zn6O5H5 films in hydrogen. The interaction of the two distinct structures with Pt were investigated by XPS, STM, and HREELS with CO, C2H4, and NO as probe molecules. Zn exhibits a partially reduced oxidation state in Zn6O5H5 and donates negative charge to surface Pt in the confined rings, leading to a switch from linear CO adsorption to bridged CO adsorption in accompany with a 50 cm-1 shift of ν(CO) towards lower frequencies. C2H4 readily forms ethylidyne (*CCH3) species at room temperature once adsorbed on Pt(111), while the formation of ethylidyne is weakened on the Zn6O5H5/Pt(111) surface. In summary, this study demonstrated a unique metal-hydroxide interaction, which serves as a novel approach for the modification of metal catalysts.</p> <p>The partial coverage of metal surfaces by oxides could be utilized to passivate specific sites of catalysts, improving the activity and stability. Herein, we studied the structure of surface Mo oxides on Pt(111) and Pt(544) using STM, XPS, and HREELS. At 0.08 ML coverage, Mo oxide tends to form 1~2 nm clusters and the majority of Mo is in +5 oxidation state. The Mo oxide clusters tend to aggregate near the monoatomic Pt steps, showing a higher local density compared to the wide terraces. Therefore, our results provide experimental evidence for the site-selective growth of Mo oxides at step sites, which could prevent the leaching of active component in catalysts under real reaction conditions.</p> <p>Overall, through atomic-level characterization of inverse model catalysts, we provided insights into the nature of metal-oxide interactions in multiple systems. The surface oxide films influence the property of metal surfaces in various ways, including migration, alloy formation, electronic perturbation, geometric confinement, and site-selective blocking. These findings emphasize the necessity of understanding the real structure of catalytic surfaces under different reaction conditions and shed light on rational design of oxide supported metal nanoparticle catalysts.</p>

<div><div><div><p>Pincer ligands are a cornerstone of modern transition metal catalysis. An increasing interest in bimetallic catalysis motivated us to develop binucleating variants of these mononucleating ligands. Expanded variants of the PDI and PyBOX ligands were targeted, leading to the development of the Naphthyridine Diimine (“NDI”) and Naphthyridine Bisoxazoline (“NapBOX”) ligands, respectively. Metalation of NDI with appropriate metal precursors yielded Fe2, Co2 and Ni2 complexes which exhibited unique stoichiometric and catalytic reactivity. Metalation of the NapBOX ligand with nickel carboxylate salts yielded Ni2 complexes which were capable of catalyzing an asymmetric intermolecular [4+1] cycloaddition reaction between 1,1-dichloroalkene-derived vinylidenes and 1,3-dienes. Each of these processes were studied experimentally and computationally in order to understand the fundamental reactivity of organic substrates across metal-metal bonds.</p></div></div></div>

<p>Various organic reactions, including important synthetic reactions involving C–C, C–N, and C–O bond formation as well as reactions of biomolecules, are known to be accelerated when the reagents are present in confined volumes such as sprayed or levitated microdroplets or thin films. This phenomenon of reaction acceleration and the key role of interfaces played in it are of intrinsic interest and potentially of practical value as a simple, rapid method of performing small-scale synthesis. This dissertation has three focusing subtopics in the field of reaction acceleration: (1) application of reaction acceleration in levitated droplets and mass spectrometry to accelerate the reaction-analysis workflow of forced degradation of pharmaceuticals at small scale; (2) fundamental understanding of mechanisms of accelerated reactions at air/solution interfaces; (3) discovery the use of glass particles as a `green' heterogeneous catalysts in solutions and systematical study of solid(glass)/solution interfacial reaction acceleration as a superbase for synthesis and degradation using high-throughput screening.</p><p><br></p><p>Reaction acceleration in confined volumes could enhance analytical methods in industrial chemistry. Forced degradation is critical to probe the stabilities and chemical reactivities of therapeutics. Typically performed in bulk followed by LC-MS analysis, this traditional workflow of reaction/analysis sequence usually requires several days to form and measure desirable amount of degradants. I developed a new method to study chemical degradation in a shorter time frame in order to speed up both drug discovery and the drug development process. Using the Leidenfrost effect, I was able to study, over the course of seconds, degradation in levitated microdroplets over a metal dice. This two-minute reaction/analysis workflow allows major degradation pathways of both small molecules and therapeutic peptides to be studied. The reactions studied include deamidation, disulfide bond cleavage, ether cleavage, dehydration, hydrolysis, and oxidation. The method uses microdroplets as nano-reactors and only require a minimal amount of therapeutics per stress condition and the desirable amount of degradant can be readily generated in seconds by adjusting the droplet levitation time, which is highly advantageous both in the discovery and development phase. Built on my research, microdroplets can potentially be applied in therapeutics discovery and development to rapidly screen stability of therapeutics and to screen the effects of excipients in enhancing formulation stabilities.</p><p><br></p><p>My research also advanced the fundamental understanding of reaction acceleration by disentangles the factors controlling reaction rates in microdroplet reactions using constant-volume levitated droplets and Katritzky transamination as a model. The large surface-to-volume ratios of these systems results in a major contribution from reactions at the air/solution interface where reaction rates are increased. Systems with higher surface-active reactants are subject to greater acceleration, particularly at lower concentrations and higher surface-to-volume ratios. These results highlight the key role that air/solution air/solution interfaces play in Katritzky reaction acceleration. They are also consistent with the view that reaction increased rate constant is at least in part due to limited solvation of reagents at the interface.</p><p><br></p><p><br></p><p>While reaction acceleration at air/solution interfaces has been well known in microdroplets, reaction acceleration at solid/solution interfaces appears to be a new phenomenon. The Katritzky reaction in bulk solution at room temperature is accelerated significantly by the surface of a glass container compared to a plastic container. Remarkably, the reaction rate is increased by more than two orders of magnitude upon the addition of glass particles with the rate increasing linearly with increasing amounts of glass. A similar phenomenon is observed when glass particles are added to levitated droplets, where large acceleration factors are seen. Evidence shows that glass acts as a ‘green’ heterogeneous catalyst: it participates as a base in the deprotonation step and is recovered unchanged from the reaction mixture. </p><p><br></p><p>Subsequent to this study, we have systematically explored the solid/solution interfacial acceleration phenomena using our latest generation of a high-throughput screening system which is capable of screening thousands of organic reactions in a single day. Using desorption electrospray ionization mass spectrometry (DESI-MS) for automated analysis, we have found that glass promotes not only organic reactions without organic catalysts but also reactions of biomolecules without enzymes. Such reactions include Knoevenagel condensation, imine formation, elimination of hydrogen halide, ester hydrolysis and/or transesterification of acetylcholine and phospholipids, as well as oxidation of glutathione. Glass has been used as a general `green' and powerful heterogeneous catalyst.</p>