This Is AuburnElectronic Theses and Dissertations

Conversion of Methane Using Two and Three Dimensional Metal Carbide Catalysts

Date

2020-04-22

Author

Thakur, Raj

Type of Degree

PhD Dissertation

Department

Chemical Engineering

Restriction Status

EMBARGOED

Restriction Type

Auburn University Users

Date Available

04-01-2022

Abstract

After various decades of research aiming to discover desirable catalysts for the direct conversion of methane into valuable products, the modern chemical industry is calling forth finding alternative materials to on-purpose produce commodity chemicals. The emergence of new materials with attractive bulk, surface, and thermal properties, no matter its initial intended application, is becomes appealing to explore their catalytic activity in the upgrading of natural gas. The discovery of two-dimensional metal carbides, so-called MXenes, has represented a breakthrough in various fields, especially in the energy conversion-storage sectors, and we aim to motivate the heterogeneous catalysts community by showing the attractiveness of MXenes towards upgrading natural gas. Multilayered vanadium carbide (m-V2CTx MXene) exhibits a unique morphology as compared to its three-dimensional counterpart, thus giving rise to novel and unique properties that interestingly were not yet explored in heterogeneous catalysis until our studies were reported. We found that at high temperatures ( 600 0C), the multilayered structure persists up-to a certain extent when reacting with nitrogen and hydrogen, while partial (V2CTx decorated with V2O3) and total (V2CTx transform into V2O5) oxidation occurs in the presence of CO2 and air, respectively. The unprecedented thermal and chemical stability of m-V2CTx triggered our interest in investigating the catalytic reactivity for methane conversion. We focused primarily on two of the most relevant reactions that the modern chemical industry is still waiting for the “eureka” moment for large scale implementation, the Dry Reforming of Methane (DRM), and Methane Dehydroaromatization (MDA). We discovered that under DRM, the pristine m-V2CTx serves as a precursor to generating V2O3-V8C7/m-V2CTx, which shows attractive activity, unprecedented selectivity (H2/CO~1), and stability, which are amongst the major challenges faced when using the state-of-the-art nickel-based catalysts. Combining kinetic and spectroscopy studies, complemented with fundamentally designed isotopic labelled experiments, we provide mechanistic insights into the genesis of m-V2CTx as a selective and coke-resistant catalyst for DRM. While performing the studies mentioned above, we also discovered that under certain conditions, the intrinsic confined space within the m-V2CTx leads to the conversion of methane into aromatics. Herein, we show that the m-V2CTx can efficiently convert CH4 into C6H6 with comparable activity to the-state-of-the-art Mo/ZSM-5. Finally, our unique results and the knowledge accumulated using m-V2CTx will be further expanded to develop supported metal oxide-based catalysts for industrially attractive reactions at elevated temperatures (>500°C).