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Active sites and morphology of MgO and Li-doped MgO in oxidative coupling of methane

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Pechsombut, Supanee 1995 , 'Active sites and morphology of MgO and Li-doped MgO in oxidative coupling of methane', PhD thesis, University of Tasmania.

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Abstract

The relationship between morphology and catalytic perfo1·mance 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.

Item Type: Thesis - PhD
Authors/Creators:Pechsombut, Supanee
Keywords: Metal catalysts, Magnesium compounds, Methane, Catalysis
Copyright Holders: The Author
Copyright Information:

Copyright the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1995. Includes bibliographical references.

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