Browsing by Subject "Methanol to hydrocarbons"

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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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    Kinetics and mechanisms of methanol to hydrocarbons conversion over zeolite catalysts
    (2013-05) Hill, Ian Michael
    The methanol-to-hydrocarbons (MTH) process over zeolite catalysts is the final step in the synthesis of commodity chemicals and fuels from alternative carbon sources via synthesis gas intermediates. Emerging research has shown that olefins and aromatics are critical intermediates, acting as scaffolds for the addition of methyl groups from methanol or dimethyl ether (DME) in an indirect "hydrocarbon pool" mechanism. Outstanding questions in this research pertain to (i) the quantitation of reaction rates for C1 homologation and (ii) the mechanism of activating methanol or DME for the formation of carbon-carbon bonds. This research reports rate constants and activation energies of olefin and aromatic methylation steps over zeolites of different pore sizes and geometries from steady-state methylation reactions, as well as isotopic and post-reaction titration studies to determine mechanistic details regarding the structure of the active zeolite surface species. Specifically, isolated steady-state methylation of C2 to C4 olefins over zeolites at differential conditions have shown that reactions producing higher degrees of substitution of intermediate carbocations have rate constants that are an order of magnitude higher than less substituted intermediates. Benzene and toluene methylation reactions show similar kinetic behavior to propylene and linear butene, respectively, over H-ZSM-5. Pressure-dependent studies show a first-order rate dependence on olefin or aromatic pressures which is invariant of DME partial pressures, indicating a surface saturated in DME-derived species reacting with a gas phase co-reactant in the rate-limiting step. These surface species have been identified as methoxides, as observed using post-reaction titration and isotopic studies. The methylation of para- and ortho-xylene with DME at low conversions showed linear dependence of the reaction rate at low pressures of xylene, but the reaction rate became zero-order at higher xylene pressures over H-ZSM-5. The reaction rate remained zero-order in DME pressure, and when taken in conjunction with results from isotopic studies and surface titrations, indicates that the surface methoxides become saturated in adsorbed xylene isomers. A reduction in the critical diffusion length by a factor of >150 did not increase the reaction rate, indicating that the effect is adsorption and not one of transport limitations. Arguments based on derived rate equations modeling observed trends in kinetic, isotopic, and titration studies set a basis for building a microkinetic model for MTH reactions over H-ZSM-5, which can predict expected product distributions for a given set of reaction conditions.