The Bayer process for the production of alumina (A1203) from bauxite involves a perennial gibbsite (y-Al(OH)3) precipitation step, relating to an inherently slow crystal growth from supersaturated sodium aluminate solutions (pregnant Bayer liquors). The kinetics and mechanisms involved in the transformation of the tetrahydroxo, Al(III)-containing species in solution into octahedrally-coordinated Al(OH)3 crystals in the presence of NA+ and excess of ions, are as yet not fully known. To gain further knowledge and better understanding of the nature of solution species, their specific interaction and participation in the gibbsite crystallization mechanisms, the role alkali ions play in the kinetic behaviour and mechanisms of nucleation, growth and aggregation/agglomeration from caustic aluminate solutions of industrial strength has been investigated.

Dehydroquinate synthase (DHQS) catalyses the five-step transformation of the seven carbon sugar 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAH7P) to the carbacycle dehydroquinate (DHQ). Multiple studies have described in detail the mechanism of most of the steps carried out by DHQS with the exception of the final cyclisation step. In this study, (3S)-3-fluoro-DAH7P and (3R)-3-fluoro-DAH7P (fluorinated analogues of DAH7P) were produced and assayed across three phylogenetically distinct sources of DHQS in order to determine the role of the enzyme during the cyclisation step of the reaction. Incubation of (3S)-3-fluoro-DAH7P with DHQS from Escherichia coli, Pyrococcus furiosus, and Kiwifruit resulted in the production of different ratios of (6S)-6-fluoro-DHQ and 1-epi-(6S)-6-fluoro-DHQ for each enzyme. In addition, enzyme catalysis showed a slowing of reaction rates when (3S)-3-fluoro-DAH7P was used, suggesting that the fluorine at C-3 is stabilising the enol pyranose. An increase in the stabilisation of the fluoro-enol pyranose would allow release of this substrate intermediate from the enzyme to compete with the on-going on-enzyme reaction. The differences in the ratio of products formed suggest that the cyclisation occurs in part on the enzyme and that the epimeric product arises only by an abortive reaction pathway where the (3S)-3-fluoro-enol pyranose is prematurely released and allowed to cyclise free in solution. Once in solution, the (3S)-3-fluoro-enol pyranose could undergo a conformational change in the ring leading to the formation of the epimeric product. Furthermore, it is suspected that the position of fluorine influences the likely transition-state in carbacycle formation leading to the production of the epimeric product. This research has illuminated the role of the enzyme in guiding the correct stereochemistry of the product and illustrates the important molecular interplay between the enzyme and substrate.

This dissertation reports a study of the dissolution mechanism which governs the leaching of Cu from chalcopyrite (CuFeS2) in acidic media at atmospheric pressure and examines the differences between chemical (abiotic), leaching and bioleaching. An array of solution, solid surface and bulk speciation studies were used to make a comprehensive study of the CuFeS2 leaching process(es). / Thesis (PhD)--University of South Australia, 2008.

<div>Metal nanoparticles have long been of paramount importance in many areas such as: emission reduction in cars, hydrogen production via the water-gas shift reaction, and lithium-ion storage in batteries. For these purposes, the size and shape of the nanoparticles have been shown to play a crucial role in improving nanoparticle performance. </div><div><br></div><div>For characterization of nanostructures, the use of transmission electron microscopy (TEM) has been shown to be extremely useful. Via a TEM instrument, one can learn about nanoparticle properties such as: particle size, 3D morphology, chemical composition, fine structure, crystallography, even to atomic resolution. No other technique boasts such ability at such a high xyz resolution. This work includes TEM work for many different applications within catalysis and energy storage fields.</div><div><br></div><div>In catalytic applications, the <1 nm particle sizes often sought after generally lead to higher activity per unit mass of the catalyst, but also have the tendency to sinter due to concomitant increases in the surface free energy, leading to catalyst deactivation especially at elevated temperatures. To investigate the sintering, (Pt,Au)-iron oxide heterodimer nanoparticles were heated in the microscope with simultaneous imaging. For that purpose, the sample was irradiated with a 532 nm pulsed laser, with laser powers of 4-25 mW within a TEM microscope to investigate particle sintering as it happens. The Au and Pt phases were both found to wet over the Fe₃O₄ phase, a behavior opposite to the Strong Metal Support Interactions (SMSI - caused by oxide wetting the metal) which were expected from well-known literature reports. This new behavior demonstrates that not only nanoparticle size, but also the support particle size can affect catalytic properties. This is shown by the fact that the size of the support oxide in these heterodimer nanoparticles is only 3 times the diameter of the active metal nanoparticles, compared to a greater than 20 times size difference for a standard metal oxide supported nanoparticle system. </div><div> </div><div>Nanoparticle metal catalysts can also undergo significant catalytic improvement via the addition of promoting metals. Kinetics were measured on a series of Pt/Co on carbon nanotube support catalysts, and addition of Co was seen to improve the turnover frequency by 10 times. Leaching of the bulk Co phases, while preserving PtCo alloy structures, reduced activity by more than 18 times demonstrating the need for a Pt/CoO_xH_y interface for catalytic promotion, and showing that PtCo alloying did not produce the promotion effect.</div><div><br></div><div>Although, for the PtCo catalysts for WGS, the formation of a Co-oxyhydroxide phase was proved to be vital, nanoparticle alloying is also well-known to improve dehydrogenation kinetics. This was shown for a series of PtM catalysts with core/shell structures, which were found to be highly selective for propane dehydrogenation as a result of the PtM intermetallic phase. XAS studies of these materials led to the discovery that formation of a continuous PtM alloy surface layer that is 2–3 atomic layers thick was sufficient to obtain identical catalytic properties between those of the core–shell and full alloy catalysts. TEM characterization was also performed to determine the core/shell nature of these catalysts.</div><div><br></div><div>Another interesting morphological "tuning knob" of nanoparticle catalysts is related to Reactive metal–support interactions (RMSI). RMSI can have electronic, geometric and compositional effects that can be used to tune catalytic active sites. Generally, non-oxide supports are disregarded as unable to undergo RMSI. However, we report an example of non-oxide-based RMSI between platinum and Nb₂CT_x MXenes--a recently developed, two-dimensional metal carbide, with a dopant labeled as T. The surface functional groups can be reduced, and a Pt–M surface alloy is formed. WGS reaction kinetics reveal that these RMSI supports stabilize the relevant nanoparticles and generate higher H₂O activation ability and thus higher rates compared with a non-reducible support or a bulk niobium carbide. This RMSI between platinum and the niobium MXene support can be extended to other members of the MXene family and opens new avenues for the facile design and manipulation of functional bimetallic nanoparticle catalysts.</div><div><br></div><div>Other important catalytic nanostructures are Au/TS-1 (Titanosilicalite-1, a zeolite with the MFI structure) catalysts which can be used to make propylene oxide (PO), an important industrial intermediate, and are extremely interesting due to the potential for one-pot chemical reactions, which will save on capital costs. The kinetics of propylene epoxidation over these Au/TS-1 catalysts were measured in a continuous stirred tank reactor (CSTR) free from temperature and concentration gradients. Apparent reaction orders were measured at 473 K for H₂ (0.7 order), O₂ (0.2), and C₃H₆ (0.2) for a series of Au/TS-1 catalysts with varied Au (0.02–0.09 wt%) and Ti (Si/Ti: 75–143) contents. These measured orders were consistent with those reported previously. Co-feeding propylene oxide enabled measurement of the apparent reaction order in propylene oxide (−0.4 to −0.8 order), showing, for the first time, and it was found that relevant pressures of propylene oxide reversibly inhibit propylene epoxidation over Au/TS-1, while co-feeding carbon dioxide and water has no effect on the propylene epoxidation rate. Analysis of previously proposed two-site reaction mechanisms in light of these new reaction orders for O₂ (0.4), H₂ (1), and C₃H₆ (0.4), corrected to account for propylene oxide inhibition, provides further evidence that propylene epoxidation over Au/TS-1 occurs via a simultaneous mechanism requiring two distinct, but adjacent, types of sites, and not by a sequential mechanism that invokes migration of H₂O₂ formed on Au sites to PO forming Ti sites. H₂ oxidation rates are not inhibited by propylene oxide, implying that the sites required for hydrogen oxidation are distinct from those required for propylene epoxidation. Those who intend on performing kinetics in the future are encouraged to perform a simple conversion-based tau-test, outlined in the relevant chapter of this thesis, to determine whether products inhibit reaction rates.</div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div><div>Yet another important field in which nanoparticle morphology research is essential is that of development of lithium-ion batteries. The current commercial graphite anode for lithium batteries is unfortunately prone to formation of lithium plating during use, from which well-documented safety issues arise. We demonstrated the use of an alternative anode, antimony, to have a measured specific capacity that is 1.6x higher than the theoretical capacity of graphite. Antimony, however, suffers from low cyclability due to large volumetric changes (~150%) upon the expansion caused by lithiation. To combat this problem, several different synthesis methods to produce nanoparticles of differing structures were tested and it was found that amine boranes produce a unique 3D nanochain structure with stable particle sizes of ~30 nm. These “3D nanochains” were found to have a stable charge capacity retention (98%) after 100 cycles due to their unique morphology which accommodates the lithiation expansion.</div><div><br></div><div>The role of sulfur nanostructures in lithium–sulfur batteries was also examined. Carbon–sulfur composites without crystalline sulfur demonstrate a high specific capacity of ≈1000 Ah kg⁻¹after 100 cycles with a gravimetric current of 557 A kg⁻¹. This high rate capacity is found to depend on sulfur distribution which, in turn, is controlled by the synthesis pathway.</div><div><br></div><div>In conclusion, the morphology of nanostructures affects many different aspects of performance, rate, and stability. Further study into these details are expected to generate additional knowledge of a wide variety of interesting nanomaterials.</div>

<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 dir="ltr">A major goal of applied electrocatalysis research has been the development of electrode materials that are active, selective, stable, and cost effective in producing electricity or desired products. In recent years, developments in <i>ab initio</i> methods for the simulation of catalyst surfaces, and electrochemical reactions occurring on them, have enabled the development of a fundamental understanding of the processes occurring at the solid-liquid interface at an atomistic scale. In combination with experiments, these calculations are helpful in elucidating design principles that can then inform electrocatalyst design. In this work, we describe the application of density functional theory, <i>ab initio</i> molecular dynamics, and high throughput materials informatics approaches to understand oxygen and carbon based electrochemistries, with relevance to electricity conversion and environmental protection. We also introduce an approach, based on a Born-Haber cycle analysis, to quantify adsorbate stabilization from solvent molecules that are ubiquitous for any electrochemical reaction occurring at solid-liquid interfaces.</p><p dir="ltr">The oxygen reduction reaction (ORR) occurs at the cathode in hydrogen fuel cells and, in conjunction with the hydrogen oxidation reaction (HOR) at the anode, produces electricity and water. While platinum group metals are the current state-of-the-art catalysts for the ORR, their high cost has necessitated an extensive search for alternatives. To this end, we investigated iron-nitrogen-carbon (Fe-N-C) catalysts, which are platinum group metal-free and have been shown experimentally to have reasonable activity compared to platinum. Despite their potential as cost effective materials, however, these catalysts are not durable over long-term operation of fuel cells, impeding their commercial adoption. The mechanisms of deactivation of the iron-nitrogen-carbon catalysts under aqueous acidic electrochemical reaction conditions remain debated, and deciphering them is complicated due to the complex structure of the catalyst. We attempt to address these challenges by first examining the structural aspects of the catalyst, sampling numerous potential active site configurations, determining their in-situ structure, and linking them to intrinsic activity and intrinsic stability descriptors. Our findings reveal that an activity-stability tradeoff exists, with the most active sites being most prone to stability issues. Additionally, we explored the role of hydrogen peroxide, a side product of ORR, in degrading Fe-N-C catalysts. This analysis demonstrated that hydrogen peroxide strongly oxidizes the catalyst surface, resulting in an activity loss in the catalyst. Based on these insights, we propose design principles to enhance the activity and stability of Fe-N-C catalysts.</p><p dir="ltr">In additional work, we compared the predictions for the Fe-N-C catalysts with ORR analysis on platinum catalysts, and we further analyzed the oxygen evolution reaction (OER) on iridium oxides and the carbon dioxide reduction reaction (CO₂R) on copper catalysts in water electrolyzers. For ORR on platinum, we identified the formation of hydroxyl and water adsorbate rings on stepped surfaces, akin to hexagonal rings found on terraces but largely absent on Fe-N-C catalysts. The ORR follows an associative mechanism involving proton coupled electron transfer to these ring structures. Furthermore, we provided activity descriptors that aligned with experimental observations, showing a higher activity on stepped surfaces compared to terraces. For OER on iridium oxides, we examined transformations of IrO₂ (110) surfaces, and we pinpointed oxidation of bridge and coordinatively unsaturated top sites as key charge transfer steps that correlate with peaks in cyclic voltammograms. Finally, for CO₂R on copper, we investigated the role of water as a proton source under neutral or alkaline conditions, providing insights into the effect of coverages of surface species on the kinetics of water dissociation that, in turn, can provide protons for CO₂ reduction and the competing hydrogen evolution reaction.</p><p dir="ltr">Through this work, we have gained a deeper understanding of the properties of various catalytic materials under conditions specific to each type of electrochemistry. We elucidated the relationships between the in-situ structure, activity, and stability for the electrocatalysts, and identified key factors influencing catalyst performance. Integrating such insights from a computational perspective with experimental approaches holds great potential in making significant advancements in developing sustainable energy technologies and ultimately contributing to a greener and more energy-efficient future.</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>