Evaluation of alkali and alkaline earth doped samarium oxide catalysts for the oxidative coupling of methane
Creators: name: Knights, Shanna D.
Publisher: name: University of British Columbia
Year: 2008

description: The catalytic oxidative coupling of methane involves the reaction of methane and oxygen at high temperatures (650°C to 900°C) in the presence of a solid metal oxide catalyst to produce the desirable products, ethane and ethylene, as well as the undesirable products, carbon monoxide and carbon dioxide. Although the homogeneous reactions are well understood, the heterogeneous reactions and their effect on the homogeneous reactions are still the subject of much research and discussion. The effect of the catalyst characteristics on the heterogeneous reactions is also an active area of research. The objective of this thesis was to characterize a series of catalysts, and to determine the effect of various catalyst properties on the oxidative coupling reactions. The experimental part of this thesis consisted of preparing and testing samarium oxide and alkali (Na and K) and alkaline earth (Mg and Ca) doped samarium oxide catalysts for the oxidative coupling of methane. The effects of the specific dopant used, varying dopant concentration (1:100 and 1:10 dopant:Sm mole ratio), and catalyst preparation were evaluated. The catalysts were tested in a bench scale packed bed reactor under conditions of varying temperature (650°C, 750°C, and 850°C) and methane to oxygen mole ratio (2 to 16). The catalysts were characterized by scanning electron microscopy, powder x-ray diffraction, surface area, estimated basicity, ability to form carbonates, and ionic radius of dopant. The addition of dopants to the samarium oxide catalyst resulted in changes in catalyst performance. No new phases were observed in the Sm203 crystal upon addition of the dopant cations, indicating that the cations were dispersed throughout the crystal, although probably not uniformly. The dopant concentration affected the catalyst performance; for example, at 750°C, the C2+ yield increased from 13.1% to 14.3% when the Ca:Sm mole ratio was increased from 1:100 to 10:100. A change in the catalyst preparation procedure resulted in an increase in the crystal dimensions and an improved combustion catalyst (e.g., the methane conversion increased from 9.8% for the standard catalyst to 17.4% for the revised catalyst) with, however, a decrease in the C2yield from 3.2% to 2.7%. The results of this study indicated that, over the range of surface areas tested (2.0 to 3.1 m2/g), surface area did not have a significant effect on the catalyst performance. The basicity of the catalyst appears to have a significant effect on the catalyst performance, with an increase in basicity resulting in an increase in C2+ selectivity (from 54.3% for the least basic catalyst, undoped samarium oxide, to62.8% for the most basic catalyst, 1:10 mole ratio Na:Sm oxide). The catalysts displayed temperature dependent behaviour, and there existed an optimum temperature for maximum C2+ yield, which is dependent on the amount and nature of the dopant, and is likely associated with the formation of carbonates on the catalyst surface. The ionic radius of the cation dopant must be similar to or smaller than the support cation to achieve effective inclusion in the crystal lattice.