Theoretical Insights into the Molecular Transformations Governing Redox Chemistry

Abstract

Redox reactions are one of the most prevalent chemistries and a ubiquitous component in naturally occurring processes as well as in the production of fuels and various chemicals involving pharmaceuticals, agrochemicals, and other industrially relevant chemicals. Research efforts in the past have focused on the development of safe and environmentally benign redox processes using alternative resources to deal with issues pertaining to global warming and depletion of the existing crude oil reserves. In addition, in the area of organic synthesis, new synthesis strategies are being developed to access important redox derived chemical intermediates via use of less toxic and easily available redox reagents. However, as is the case with most newly developed catalytic processes, the fundamental challenges associated with poor reaction selectivity and activity hinder the effectiveness of these new reactive transformations and technologies, making them less desirable compared to the currently used approaches. Understanding the molecular transformations occurring during these processes thus remains the key ingredient in the further development of these new processes. Gaining mechanistic and kinetic insights into the reaction would allow us to identify factors that control the selectivity and activity for the given reaction, which would then guide the development of more active and selective reaction systems. Along these lines, electronic structure calculations based mainly on density functional theory (DFT) has emerged as a powerful tool to model the reaction kinetics and also to gain important perspectives about the chemical reactions that are not observable experimentally. My dissertation thus focuses on using density functional theory (DFT) to gain important molecular level insights into the transformations that control the oxidation and reduction pathways occurring via thermal or electrocatalytic routes for five independent redox chemistries. Further, we also look into the salient features of redox chemistry that come into play during fabrication of electronic devices, particularly for MoS2 based transistor devices, which ultimately affect its ensuing performance. The first part of the thesis focuses on ab-initio study of three particular systems of interest in oxidation chemistry. Each of these studies separately report on three distinct catalytic features including the nature of the sites, molecularity of the catalyst, and the presence of surface coverages that affect the selectivity towards the desired product. The initial focus is on processes involving partial oxidation of feedstock chemicals which suffer from issues of low selectivity since the processes of over-oxidation is thermodynamically more favorable. One such example of a system that suffers from over-oxidation is sulfur oxidative coupling of methane (SOCM), which involves selective production of ethylene from methane using sulfur as an oxidant. Herein, we report a detailed kinetic and mechanistic study for SOCM reaction done over a sulfided Fe3O4 catalyst (FeS2). Experimental mechanistic analysis involving Delplot and contact time studies reveal a reaction network for SOCM that is different compared to the traditional OCM. Further computational analysis done for this reaction network suggests a site-specific formation of the selective C2 products and unselective carbon-di-sulfide, allowing for potential tuning of the reaction selectivity. The second system of interest that displays challenges with over-oxidation is the dehydrogenation of cyclohexanone. In this study, we examine the role of the homogeneity of the catalyst (Pd(DMSO)2(TFA)2) in selective control of the dehydrogenation reaction of cyclohexanone. Our DFT calculations indicate that the dehydrogenation proceeds via a rate determining intermolecular deprotonation step, activating the C-H bond at the alpha position. Using distortion-interaction analysis, the further activation of the cyclohexenone intermediate formed is shown to be inhibited due to its higher C-H bond strengths as well as due to its weaker interactions with the hydrogen abstracting entity, TFA- compared to cyclohexanone. Besides issues pertaining to over-oxidation affecting the selectivity, for reacting species with multiple reaction centres or functionalities, multiple parallel reaction pathways can emerge which could also lead to lowering of selectivity towards the desired intermediate. Primary electro-oxidation of glycerol over Pt catalyst is one such example wherein the product distribution is dependent on the carbon center that is being activated (primary versus secondary). Using DFT calculations, we hereby report the influence of coverages on directing the oxidation of the C-H bond at the primary versus the secondary position. We show that under high coverage conditions, oxidation at the primary C-H bond to form glyceraldehyde is preferred despite the stronger C-H bond strengths whereas under low coverage conditions, the weaker secondary C-H bond is preferentially activated to form dihydroxyacetone. We further examine the influence of the reaction conditions such as the pH, metal surface, and the operating potentials on the resultant product distribution for electrocatalytic glycerol oxidation over Pt. The second part of the thesis involves a mechanistic investigation of some important reduction chemistries, in particular - the hydrogen evolution reaction (HER) and the electrochemical Birch reduction. In the HER, we hereby examine MoS2 as a non-precious catalyst for use in fuel cells for production of hydrogen from water, and report enhancements in its electrocatalytic activity via application of an external electric field normal to the MoS2 electrode surface (back-gating). DFT studies aimed to understand the mechanistic nuances of the observed increased activity show that the excess electron densities induced via back-gating increase the binding energies of the hydrogen on the MoS2 surface, which in turn lead to improvements in the electrocatalytic activity of MoS2 for HER. In the next topic, we look at the Birch reduction process wherein we report a practical, safe, and scalable electrosynthetic strategy for the reduction of the arenes to dienes and for other similar reductive transformations and subsequently determine the mechanistic nuances of the reaction in the presence of various reagents and additives. Through electroanalytical and computational investigations, we have shown the reaction pathway to proceed via the reduction of the substrate near the electrode surface with the protonation step as the rate limiting step of the reaction. Further, we have deciphered the unique role of each of the reagents used in the study – electrolyte, lithium bromide (LiBr); solvent, tetrahydrofuran (THF); proton source, dimethyl urea (DMU); and the additive, tris(pyrrolidino)phosphoramide (TPPA), to understand how these components co-operatively aid in promoting the desired transformations towards higher reaction yields. In the final part of the thesis, diverging from the theme of redox reactions in chemical production, we investigate the influence of the redox reactions in device fabrication processes with the aim to mitigate some of the issues concerning the contact problem typically observed at the interface. We study here a system of MoS2-metal contacts, which have applications in development of sub-nanometric transistors. Via our “single-atom addition” approach used here, we show that the nature of the metal deposited strongly influences the resulting structural and electronic properties of the MoS2-metal interface. In conjunction with analytical scanning tunnelling electron microscopy (STEM) studies, we screen and characterize the interfacial properties for different metal contacts including Sc, Ti, Cu, In, and Au, with the aim to design systems that minimize the contact resistance at these interfaces.