Browsing by Subject "Heterogeneous catalysis"

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    First principles simulations of hydrocarbon conversion processes in functionalized zeolitic materials
    (2013-05) Mazar, Mark Nickolaus
    With increasing demand for chemicals and fuels, and finite traditional crude oil resources, there is a growing need to invent, establish, or optimize chemical processes that convert gasifiable carbon-based feedstocks (e.g., coal, natural gas, oil sands, or biomass) into the needed final products. Catalysis is central to almost every industrial chemical process, including alkane metathesis (AM) and the methanol-to-hydrocarbons (MTH) process, which represent final steps in a sequence of hydrocarbon conversion reactions. An in depth understanding of AM and MTH is essential to the selective production of the desired end products. In this dissertation, ab initio density functional theory simulations provide unique mechanistic and thermodynamic insight of specific elementary steps involved in AM and MTH as performed on zeolite supports. Zeolites have been employed throughout the petroleum industry because of their ability to perform acid-catalyzed reactions (e.g., cracking or MTH). The crystalline structure of zeolites imparts regular microporous networks and, in turn, the selective passage of molecules based on shape and functionality. Many different elements can be grafted onto or substituted into zeolites, resulting in a broad range of catalytic behavior. However, due to the variety of competing and secondary reactions that occur at experimental conditions, it is often difficult to extract quantitative information regarding individual elementary steps. ab initio calculations can be particularly useful for this purpose. Alkane metathesis (i.e., the molecular redistribution or chain length averaging of alkanes) is typically performed by transition metal hydrides on amorphous alumina or silica supports. In Chapter 3, the feasibility of AM in zeolites is assessed by using a grafted Ta-hydride complex to explore the full catalytic cycle in the self-metathesis of ethane. The decomposition of a Ta-metallacyclobutane reaction intermediate that forms during olefin metathesis is responsible for the largest activation energy of the catalytic cycle. This assessment is similar to the findings of alkane metathesis studies on alumina/silica supports and indicates that the entire AM cycle can be performed in zeolites by isolated single-atom transition metal hydrides. Performed over acid form zeolites, MTH is used in the conversion of methanol into a broad range of hydrocarbons, including alkenes, alkanes, and aromatics. For reasons that are not yet rigorously quantified, product selectivities vary dramatically based on the choice of catalyst and reaction conditions. The methylation of species containing double bonds (i.e., co-catalysts) is central to the overall process. Distinct structure-function relationships were found with respect to the elementary steps in the methylation and β-scission of olefins. In Chapter 4, the role of zeolite topology in the step-wise methylation of ethene by surface methoxides is investigated. Elementary steps are studied across multiple frameworks (i.e., BEA, CHA, FER, MFI, and MOR) constituting a wide variety of confinement environments. The reaction of surface methoxides with ethene is found to require a transition state containing a primary carbocation. The barrier height is found to decrease nearly monotonically with respect to the degree of dispersion interactions stabilizing the primary carbocationic species in the transition state. In addition, quantification of the ``local'' dispersion energy indicates that confinement effects can not be simply correlated to pore size. The β-scission of olefins plays an important role in the product selectivities of many important chemical processes, including MTH. In Chapter 5, β-scission modes involving C6 and C8 isomers are investigated at a single, isolated Bronsted acid site within H-ZSM-5. We find that the relative enthalpic barriers of β-scission elementary steps can be rationalized by the substitution order of the two different carbocationic carbon atoms that are present in the reactant (C+) and transition states (βC). In fact, the increase in charge required by the βC atom to go from the physi/chemi-sorbed reactant state to the β-scission transition state (+0.23e-0.33e) is found to correlate almost linearly with the intrinsic activation energy (89-233 kJ mol-1). The charge of the βC atom depends, to a large extent, on the substitution order of both the C+ and βC atoms and, therefore, each $beta$-scission mode is a sub-category onto itself. Isomerization reactions, which are fast with respect to β-scission, enable reactant hydrocarbons to explore and find low barrier β-scission pathways. Selectivities predicted on the basis of the relative barrier heights of β-scission modes accessible to C6 and C8 species indicate general agreement with experimental observations.
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    Mechanistic Insights into Controlling the Molecular Transformations of Oxygenates
    (2019-07) Chemburkar, Ashwin
    The detrimental environmental impact of excessive use of petroleum and fossil-fuel has motivated efforts to manufacture chemicals and fuels from biomass, a renewable feedstock. In addition, a large emphasis has been placed on developing chemistries that are not only atom-efficient, but also involve one-pot synthesis, green solvents, and easy separations, thereby, driving next generation technologies towards sustainability and green manufacturing. To that effect, this dissertation elucidates reaction mechanisms of such new chemistries, identifies catalytic features that enable fast, yet selective chemical transformations, as well as explores the complex role of greener solvents in mediating reactions, in a series of collaborative studies involving density functional theory (DFT) simulations and experiments. Biomass-derived precursors are shown to possess chemical and structural flexibility to generate a novel slate of structurally diverse intermediates that can be used as scaffolds in the synthesis of current chemical feedstocks as well as new molecules with advanced functionality for the design of new chemical and materials intermediates. Chemical diversification is demonstrated by using heterogeneous catalysts together with solvents to achieve high reactivity and selectivity. The results presented in this dissertation aim to guide the design of more active, selective, stable and cheaper catalytic materials, as well as synthesis of new molecules and establish new strategies to control the specific products that are formed. A significant part of this dissertation focusses on chemically diversifying coumalic acid (CMA)- a bio-privileged molecule, using a selective, one-pot Diels-Alder-decarboxylation-dehydrogenation domino sequence to generate a library of aromatic compounds. Not only petroleum-derived aromatics, but also a host of novel structures can be synthesized using this approach. These new aromatics have functionalities and structures inaccessible via petroleum and have applications as pharmaceuticals, insecticides, and antimicrobials. Here, we combine in-situ NMR, DFT, and solid-state NMR to guide one-pot synthesis of aromatics from biomass, thus, providing a sustainable means to access these compounds. Our efforts lie at engineering the reactivity by identifying the rate-limiting step in the domino sequence and then subsequently catalyzing it using a heterogeneous catalyst. Selectivity is fine-tuned by esterification of CMA as well as by screening different solvents. Finally, during synthesis of more complex aromatics, structure-reactivity relationships are developed to guide stereo-selective synthesis. This dissertation also discusses aldol condensation and esterification, reactions that are can be utilized to make longer molecules from bio-derived oxygenates to synthesize fuel-grade precursors and additives respectively. Previous studies have shown that these reactions can be catalyzed when pre-equilibrated mixture of alcohol-aldehyde-H2 is fed over metallic Cu catalysts. Here, we reveal the nature of active sites on metallic Cu that enable C-C and C-O bond formation reactions that traditionally been thought to require explicit base or acid assistance. DFT simulations, together with kinetic analyses show the prevalence of Lewis-basic alkoxide sites as well as Lewis-acidic aldehyde sites that co-operatively carry out proton and hydride transfer reactions that enable C-C and C-O bond formation reactions. The studies not only demonstrate the use of greener, earth-abundant transition metals such as Cu in biomass processing, but also opens opportunities to utilize active sites unique to Cu, and possibly other coinage metals, in reduction reactions that selectively require hydrides (for example, C-H formation). Reactions that allow for selective C-O bond hydrogenolysis are central to biomass upgradation. This dissertation also reports hydrogenolysis of fatty acid over bimetallic rhenium oxide catalysts promoted by Pd. Such bimetallic catalysts allow for near-atmospheric pressure operations, contrary to commercial processes. The role of the constituent metals is elucidated here through DFT and multiple experimental probes including steady state and transient rate measurements, extensive characterization, and kinetic isotope effects. The results demonstrate a synergistic role of Pd and Re; while Pd directly provides hydrides (electrons) for C-H formation, Re catalyzes dehydration reactions by providing protons via Brønsted acid sites (ReOH). These results can be extended to over oxophilic metals to screen more selective, cheap and durable materials in the processing of fatty acids and other oxygenates. The last part of this dissertation highlights the role of protic solvents in mediating reduction reactions of oxygenates. Reduction reactions are not only at the heart of biomass upgradation, but also are involved in a host of important applications such as organic and pharmaceutical synthesis. The use of greener solvents such as water, methanol or their mixtures instead of organic solvents is of critical importance from an environmental viewpoint. Here we combine DFT, kinetic isotope effects, flow reactor measurements, and in operando X-ray spectroscopy to show that protic solvents directly participate in hydrogenation reactions over transition metal surfaces. Protic solvents are shown to activate characteristically different routes that can enable transfer of hydrogens as protons and electrons in proton coupled electron transfer (PCET) reactions that favor the formation of O-H bonds on oxygenates. In such reactions, solvent molecules react with co-adsorbed hydrogen atoms on the metal catalyst as well as enable proton-shuttling, while the metal catalyst is found to conduct electrons. In addition, we present evidence for the prevalence of heterogeneous mediator-based chemistry that governs reduction reactions when the protic solvent can decompose on the host catalyst. The structure and reactivity of these heterogeneous surface mediators are found to resemble well-known solution-phase mediators such as TEMPOH (in organic synthesis) and NADPH (in biology). These results demonstrate the ability to impact selectivity during hydrogenation of oxygenates and perhaps other unsaturated compounds by tuning the metal and the protic solvent.
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    A mechanistic understanding of light olefins selectivity in methanol-to-hydrocarbons conversion on MFI
    (2016-11) Khare, Rachit
    Methanol-to-hydrocarbons (MTH) conversion is the final processing step in converting alternative feedstock such as coal, natural gas, and biomass, to hydrocarbon fuels and petrochemicals. Methanol reacts on acidic zeolite catalysts via an indirect “hydrocarbon-pool” mechanism to form a wide variety of hydrocarbons including light olefins, gasoline-range hydrocarbons, and aromatics. The hydrocarbon-pool mechanism involves two reaction cycles simultaneously operating inside the zeolite pores: an olefins-based reaction cycle and an aromatics-based reaction cycle. The observed product distribution in MTH can be rationalized as an effect of the relative rates of propagation of the aromatics-based and the olefins-based reaction cycles. Quantifying the relative propagation of these two catalytic cycles and understanding how these cycles contribute to the overall product distribution under varying reaction conditions, varying feed composition, and on different zeolite topologies or morphologies, is critical for developing structure-function relationships for MTH catalysts. In this work, the effects of independently varying (i) the feed composition (by co-feeding hydrocarbons or oxygenates), (ii) the concentration of active sites (by changing the chemical composition of the zeolite), and (iii) the diffusion characteristics of the zeolite (by changing the crystallite size or silylating the external surface), on the relative rates of propagation of the aromatics- and olefins-based cycles, and consequentially on the observed MTH product selectivity are presented.
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    Nanostructured Metal Oxides for Desulfurization and Heterogeneous Catalysis
    (2021-10) Zhao, Wenyang
    Metal oxides have broad applications in industry and manufacturing. In order to maximize the performance of the metal oxides for targeted applications, it is critical that the preparation process is tailored and optimized. This thesis demonstrates the design, synthesis, characterization, and optimization of two types of porous metal oxides for applications for the removal of H2S in natural gas processing and for high temperature heterogenous catalysis.During natural gas production, the use of metal oxides as solid sorbents in the tail gas treatment unit is economically and operationally beneficial compared to the commercialized liquid sorption process. In the first part of this thesis, a type of porous mixed metal oxide (MMO) sorbent with active CuO is prepared through coprecipitation, and is demonstrated to have superior H2S sorption capacity. This sorbent can be recycled in a continuous adsorption–desorption process with stable performance. In addition, a facile pelletization approach was established, and the regeneration conditions of the pellets were optimized. This study demonstrated good reliability and applicability of this sorbent material in simulated testing conditions. The second part of this thesis describes the preparation of MMOs through nanocasting metal–organic frameworks (MOFs). The metal oxo clusters in MOFs are a source of metal species for MMOs, and the casting organometallic precursors provide another component. Nanocasting largely retains the morphological and structural information of the MOF template in the final MMOs, and provides well-defined MOF-derived catalytically active centers with enhanced thermal stability suitable for high temperature catalysis.