Haoyi Wang, Ph.D. Candidate supervised by Prof. Mani Sarathy
Date: Thursday, November 17, 2022
Time: 3.30 - 5.30 PM
Location: Room 5220, Level 5, Building 5
Converting of methane, the main component of natural gas, into more valuable chemicals has gained interest over its direct combustion for energy generation due to the abundance of natural gas with newly discovered sources and increasing production capability. Oxidative coupling of methane (OCM) represents a potentially viable method to convert methane directly into more desirable products such as ethane, and ethylene. In this dissertation, a comprehensive kinetic study of oxidative coupling of methane (OCM) was performed over La2O3-based catalysts experimentally and numerically.
Since the catalytic activity of La2O3-based catalysts could be improved by doping with other metals. The first section consists of a detailed microkinetic analysis study on La2O3-based catalysts with dopants of Ce and Sr. Different catalysts were synthesized and characterized using different techniques, showing the addition of Ce has the greatest boost over the performance. The kinetics at low methane conversion regimes were analyzed and correlated to the catalysts’ properties. The activation energy for methane hydrogen abstraction from surface oxygen was estimated, as well as the formation rate of primary products including C2, CO, and CO2, which suggested that the initiation reaction steps and activation energy for methane conversion were similar for La2O3-based catalyst.
The next step is to develop homogenous-heterogeneous OCM reaction networks for the La2O3-CeO2 catalyst. Due to the difficulty of accurate prediction of surface mechanism, an accurate and reliable gas-phase model is critical for the entire mechanism. In the second section, the gas-phase kinetics for OCM was studied using a jet-stirred reactor in the absence of a catalyst. Both experiments and simulations were conducted at OCM-relevant operating conditions under various operating conditions, by implementing various gas-phase models related to methane oxidation. Quantities of interest (QoI) and rate of production analyses on hydrocarbon products were also performed to evaluate the models. The gas-phase models taken from catalytic reaction networks showed great deviations from the experimental gas-phase performances. NUIGMech1.1 was selected as the most comprehensive model to describe the OCM gas-phase kinetics and used for the next study.
In the third section, after evaluating the gas-phase kinetics, a homogeneous-heterogeneous kinetic model for La2O3/CeO2 catalyst was constructed. By applying in situ X-ray diffraction, the doping of CeO2 not only enhanced catalytic performance but also improved catalyst stability from CO2 and H2O. A wide range of operating conditions was investigated experimentally and numerically, where a packed bed reactor model was constructed based on the dimensions of experimental setup and catalyst characterization. The rate of production (ROP) was also performed to identify the important reactions and prove the necessity of surface reactions for the OCM process. Laser-induced fluorescence was implemented to directly observe the presence of formaldehyde, for the first time reported in OCM.
The last section includes the implementation of in situ laser diagnosis techniques at the near-surface regions to solve the existing challenges within an optical reactor. Raman scattering was implemented to quantitate the concentration profiles of major stable species near the surface at different heights above the catalyst surface and measure the in situ local temperatures, to study the kinetics transiting from the surface edge to the near-surface gas-phase while laser-induced fluorescence was conducted as complementary to detect the intermediate specie, which provide a new perspective in OCM kinetic studies.
Haoyi Wang is a Ph.D. candidate in the chemical engineering program under the supervision of Prof. Mani Sarathy. He received his Bachelor's degree in Chemical and Biological Engineering from the University of Wisconsin-Madison in 2015. He joined KAUST in 2016, with his research interests in oxidative coupling of methane and hydrocarbon combustion.