Mechanistic Insights into Alkane C-H Activation and Functionalization by Metal Oxide Surfaces and Organometallic Complexes
<p>Alkanes are the major components of natural gas and petroleum; however, there are only few practical processes that can functionalize them into more valuable products such as alkene or alcohols. The reason for this difficulty is because alkanes possess strong and inert C-H bonds. The deve...
| Summary: | <p>Alkanes are the major components of natural gas and petroleum; however, there are only few practical processes that can functionalize them into more valuable products such as alkene or alcohols. The reason for this difficulty is because alkanes possess strong and inert C-H bonds. The development of such a process that can convert alkanes to other more valuable functionalized hydrocarbons in a catalytic fashion would produce enormous economic benefits. The key to achieve this goal is to develop a proper catalyst. The catalysts can be organometallic complexes or metal oxide surfaces that catalyze alkane C-H activation and functionalization in homogeneous or heterogeneous conditions.</p>
<p>In this thesis, we apply quantum mechanics to study the known alkane functionalization reactions to provide more insight into those catalytic processes, and we further utilize our computational results to design new reaction pathways for alkane functionalization. Each chapter presented herein constitutes an independent publication focusing on different aspects of the problem.</p>
<p>Chapter 1: Single-Site Vanadyl Activation, Functionalization, and Reoxidation Reaction Mechanism for Propane Oxidative Dehydrogenation on the Cubic V<sub>4</sub>O<sub>10</sub> Cluster: Vanadium oxide is a powerful heterogeneous catalyst that can convert oxidative dehydrogenation (ODH) of propane. Despite numerous studies, either computational or experimental, on this topic, no complete catalytic cycle is provided. In this paper, we examined the detailed mechanism for propane reacting with a V<sub>4</sub>O<sub>10</sub> cluster to model the catalytic oxidative dehydrogenation (ODH) of propane on the V<sub>2</sub>O<sub>5</sub>(001) surface. We reported the mechanism of the complete catalytic cycle, including the regeneration of the reduced catalyst using gaseous O<sub>2</sub>, in which only a single vanadyl site is involved. This mechanism is applicable to propane ODH on the supported vanadium oxide catalysts where only monovanadate (O=V-(O)<sub>4-</sub>) species is present.</p>
<p>Chapter 2: The Magnetic and Electronic Structure of Vanadyl Pyrophosphate from Density Functional Theory: We have studied the magnetic structure of the high-symmetry vanadyl pyrophosphate, focusing on the spin exchange couplings, applying density functional theory with exact exchange and the full three-dimensional periodicity to this system for the first time. Based on the local density of states and the response of spin couplings to varying the cell parameter a, we found that two major types of spin exchange couplings originate from different mechanisms: one from a super-exchange interaction and the other from a direct exchange interaction. Based on the variations in V–O bond length as a function of strain along a, we found that the V–O bonds of V–(OPO)<sub>2</sub>–V are covalent and rigid, whereas the bonds of V–(O)<sub>2</sub>–V are fragile and dative.</p>
<p>Chapter 3: The Para-Substituent Effect and pH-Dependence of the Organometallic Baeyer-Villiger Oxidation of Rhenium-Carbon Bonds: Organometallic Baeyer-Villiger represents another means of oxidizing M-R to M-OR. In this work, we conducted a series of calculations with the goal of providing more insights into the reaction. We find that during this organometallic BV oxidation, the migrating phenyl plays the role of a nucleophile and the leaving group OH is nucleophile. Moreover, we also find that for R = Ph the reaction rate is much faster than that for R = Me, which is later confirmed by experiments.</p>
<p>Chapter 4: Carbon-Oxygen Bond-Forming Mechanisms in Rhenium Oxo-Alkyl Complexes: Intramolecular 1,2-migration of hydrocarbyl across metal-oxo bonds is one of the few means of oxy-functionalizing M-R to M-OR bonds. This strategy works for R = Ph, but fails for R = Me and Et. In this work, we study these systems with the goal of understanding the reason. We find that when R = Me and Et the α-hydrogen is very acidic and easy to abstract even with weak base, such as the counter ion of the complex, leading to unwanted by-products. We find that these side reactions can be avoided by two means: (1) use counter ions with weaker basicity to increase proton abstraction barriers, and (2) use R = iPr, which has a higher migratory aptitude, to accelerate the 1,2-migration rate.</p>
<p>Chapter 5: A Homolytic Oxy-Functionalization Mechanism: Intermolecular Hydrocarbyl Migration from M-R to Vanadyl Oxo: Oxy-functionalization M<sup>δ+</sup>-R<sup>δ-</sup> to M-OR bonds is one of the key challenges in the development of hydrocarbon hydroxylation catalysts. This can be achieved by limited means: (1) organometallic Baeyer-Villiger oxidation, and (2) intramolecular 1,2-migration of hydrocarbyl across metal-oxo bonds. In this work, we have examined C-O bond formation in the reaction of OVCl<sub>3</sub> with Ph<sub>2</sub>Hg to generate phenol using quantum mechanics. Surprisingly, we find this reaction is through an unprecedented bimolecular, one-electron oxidation of the V-Ph bond by a second V=O moiety, not through the experimentally proposed intramolecular phenyl 1,2-migration across V=O bonds. Our calculations on the oxidation of Rh-CH<sub>3</sub> and Ir-CH<sub>3</sub> complexes by OVCl<sub>3</sub> further suggest that the possibility of integrating this new oxidation mechanism into alkane oxidation catalytic cycles. We also give guidelines to choose the systems in which this oxidation mechanism may play an important role.</p> |
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