Active sites and morphology of MgO and Li-doped MgO in oxidative coupling of methane
Creators: name: Pechsombut, Supanee
Publisher: name: University Of Tasmania
Year: 2023

description: The relationship between morphology and catalytic perfo1¬¨‚àëmance in the oxidative coupling of methane over MgO and Li-doped MgO catalysts has been studied, and the active sites for methane coupling on the catalyst smface have been identified. MgO catalysts of various morphologies were obtained from different Mg precursors using a variety of preparation methods. For Li-doped MgO catalysts, preparation techniques used included coprecipitation, hydrolysis, physical mixing and wet impregnation followed by calcination. All MgO and Li-doped MgO catalysts were characterised prior to the catalytic screening. Atomic absorption spectroscopy was used to determine the Li loading on Li/MgO catalysts. The morphologies of the catalysts, including surface area, particle profile/particle size distribution and pore structures were examined. Temperature programmed desorption (TPD) of phenol on MgO was studied using several detection techniques including the1mal conductivity, infrared spectroscopy and thenno-gravimetric analysis to explore the surface sites of the catalysts. A simple thermo-gravimetric technique was then developed and found to be effective in dete1mination of the smface sites on MgO surfaces. The different surface sites of different basic strength can be distinguished via the desorption profiles obtained as phenol is desorbed with the increasing temperature. Two major surface sites were observed on MgO. They could be attributed to flat surface sites exposure { 100} and surface sites of low coordination number (edges and corners). For Li/MgO catalysts the TPD of phenol was also employed to examine the nature of the smface sites. The amount of phenol adsorbed was small due to the low surface area of the catalysts; however, a significant distinction between sites was still able to be observed. It was noted that apart from the desorption temperature ranges attributed to the flat surfaces and to the surface sites of low coordination of MgO matrix, another desorption range at higher temperatures was also observed. This indicated the presence of a new site of higher basic strength, generated by the addition of Li to MgO. At low Li loadings - 0.1-0.2 % (wt % after calcination), this new site was observed at a very high concentration on the surface. As the Li loading was increased this new site was initially replaced by the smface sites of low coordination number. However, with further increased in the Li loading to greater than - 4.0-5.0 %, the number of low coordination sites also diminished. Catalytic activities of all MgO and Li/MgO catalysts were determined using a conventional flow reactor operating at atmospheric pressure. The products were analysed at various times-on-stream dming the reaction. Any change in the nature of the surface sites was investigated for most catalysts after catalytic testing. The correlation between sites and the catalytic activities of all catalysts was then conducted. It was likely that the methane activation and selectivity to C2-hydrocarbon products arose from the high density of smface sites of low coordination. It was found that catalyst morphology, catalyst surface sites of low coordination and catalytic pe1fonnance for the oxidative coupling of methane are closely inter-related. The active surface sites of low coordination were presumed to originate from defects in the MgO crystal structure that result from the preparation methods and precursors used, along with the influence of the addition of Li. The presence of defects could be related to the physical characteristics of the catalysts such as smface area, particle size dist1ibution profiles and pore structure.