First principles simulations of hydrocarbon conversion processes in functionalized zeolitic materials

Abstract

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, <italic>ab initio</italic> 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. <italic>ab initio</italic> 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 &beta;-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 &beta;-scission of olefins plays an important role in the product selectivities of many important chemical processes, including MTH. In Chapter 5, &beta;-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 &beta;-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 (&beta;C). In fact, the increase in charge required by the &beta;C atom to go from the physi/chemi-sorbed reactant state to the &beta;-scission transition state (+0.23e-0.33e) is found to correlate almost linearly with the intrinsic activation energy (89-233 kJ mol<super>-1</super>). The charge of the &beta;C atom depends, to a large extent, on the substitution order of both the C₊ and &beta;C atoms and, therefore, each $beta$-scission mode is a sub-category onto itself. Isomerization reactions, which are fast with respect to &beta;-scission, enable reactant hydrocarbons to explore and find low barrier &beta;-scission pathways. Selectivities predicted on the basis of the relative barrier heights of &beta;-scission modes accessible to C₆ and C₈ species indicate general agreement with experimental observations.