Thermal and Electrochemical Light Alkane Upgrading
The dramatic increase in supply of natural gas in the U.S. presents a unique opportunity to develop new catalytic processes that utilize light hydrocarbons for chemical manufacturing and fuel synthesis. Conventional catalytic processes utilize extreme conditions requiring high energy input for the initial C-H bond activation and multiple-step processes for the conversion of activated compounds into higher-value products. Direct conversion routes, on the other hand, have been investigated and have shown promising results toward more-efficient utilization of natural gas resources. In this dissertation, direct conversion routes are classified into three different categories: liquid-phase thermal catalysis, electrocatalysis, and gas-phase thermal catalysis. Investigations involving each of these conversion routes are presented, adding new fundamental information on light alkane upgrading to contribute to the existing literature.First, liquid-phase thermal catalysis was studied by utilizing unsupported AuPd nanoparticles and H2O2 as an oxidant in water. Numerous C2 oxygenates, including ethyl hydroperoxide/ethanol, acetaldehyde, and acetic acid were generated with maximum-observed oxygenate yield 7707 μmol gAuPd-1 h-1 from ethane at 1 bar and 21 °C. In addition, continuous supply of low H2O2 concentrations to a semi-batch reactor over 50 h showed that on-site generated H2O2 can serve as an oxygenate-selective oxidant of ethane. Given that, electrochemical approaches could serve as the basis to continuously and sustainably generate the H2O2 fed into the chemical reactor to partially oxidize light hydrocarbons. In fact, utilizing air and water under electochemical potential can simply produce H2O2. Chapter 3.2 introduces an electrocatalyst that is engineered from electrochemical deposition of Pd ions during O2 reduction. This catalyst showed an extreme activity towards selective production of H2O2 which is determined as the second-highest partial kinetic current density for H2O2 production in acidic media reported in the known literature. The second direct conversion system, electrocatalysis, was also investigated to provide fundamental knowledge about how to effectively activate and functionalize the C-H bond. For this purpose, a prototypical electrocatalyst surface, polycrystalline Pt, was employed to assess the interaction of it with methane and water. Experiments performed by anodic stripping voltammetry techniques coupled with operando surface-enhanced infrared absorption spectroscopy (SEIRAS) indicated that the sites with lower Pt coordination number (defects or step-edges) are potentially responsible for initial C-H bond activation and its decomposition to surface-bond C-H-O or C-O fragments. For the last direct conversion route, oxidative coupling of methane (OCM) was considered as the model system and proposed to be investigated by near-surface optical and mass spectrometry techniques. These diagnostic techniques were initially gauged for the partial oxidation of methanol over a silver catalyst to demonstrate the value of the near-surface gas-phase measurements. The success in mapping out the coupling between the gas-phase and surface in a heterogeneous reaction encouraged us to work on the identical system with different metal surfaces. Chapter 4 mainly exhibits the studies on conversion of methanol near atmospheric pressure using molecular beam mass spectrometry on Pd film surfaces. Methoxymethanol, a rarely observed intermediate, was detected, and provided as supporting evidence to previously proposed reaction networks. Later, in order to shift the direction to OCM reaction, the near surface techniques were modified and optimized with the knowledge gathered during methanol oxidation experiments. Preliminary experiments were conducted on OCM and initial results were included in the Appendix of Chapter 4.3.5. More promising results including the radical detection are expected to be obtained in order to elucidate the complex reaction network taking place on the surface and near-surface. Overall this thesis extensively covers the direct conversion methods for the utilization of light hydrocarbons and provides the detailed experimental procedures in the context of the comprehensive literature.