The current steam reforming process for the production of CH<sub>3</sub>OH is complicated and difficult, and therefore the direct partial oxidation of CH<sub>4</sub> to CH<sub>3</sub>OH would be economically desirable. In previous work a design approach for a selective partial oxidation catalyst has been investigated, which comprises the combination of components with a desired reactivity, producing a successful selective partial oxidation catalyst. In this approach, it is considered a successful partial oxidation catalyst must activate methane, activate oxygen and not destroy the desired product, methanol. All these properties could not be found in a single catalyst, so it was proposed that two synergistic components could be combined, one responsible for methane activation and the other for oxygen activation/insertion. Previous work has studied the CH<sub>4</sub>/D<sub>2</sub> exchange reaction as an indication of the ability of a metal oxide surface to activate CH<sub>4</sub>. Two metal oxides demonstrated appreciable activity for the activation of CH<sub>4</sub>, these being Ga<sub>2</sub>C<sub>3</sub> and ZnO. These oxides were then doped with different metals in order to try and increase the activity of the catalyst. The doping of Ga<sub>2</sub>O<sub>3</sub> with Zn or Mg did not improve the methane oxidation properties of Ga<sub>2</sub>C<sub>3</sub>, and the doping of ZnO with Ga significantly lowered the light off temperature, the temperature at which CH<sub>4</sub> was first detected, and increased its oxidative capacity. The addition of precious metals significantly affected the catalysts ability to activate CH<sub>4</sub>. The addition of Au to the Ga and Zn catalysts dramatically reduced the light off temperature, and increased its rate of oxidation at lower temperatures, with the optimum loading 2% for both catalysts. For GaO(OH) and ZnO, the addition of 1%Au and l%Pt by coprecipitation produced a synergistic effect, producing lower light offs and higher CH<sub>4</sub> conversion than the singly doped catalysts with Au and Pt separately. When the methane activation catalysts were combined with MoO<sub>3</sub> in a physical mixture, a number of the mixtures produced higher methanol per pass percentage yields than its constituent parts. It is concluded that the increased methane activation properties beneficially interact with the oxygen activation and insertion properties of MoO<sub>3</sub>. However, none of the yields reported were significantly higher. A dual bed system, with the lower layer comprising the methane activation catalysts, and the upper layer consisting of MoO<sub> 3</sub> was tested. The results for this system were promising, with the low temperature activation of CH<sub>4</sub>, combined with the oxygen insertion ability of MoO<sub>3</sub>, producing high selectivities of CH<sub>3</sub>OH at much lower temperatures. The best results were obtained when the ratio of the two layers was 50:50 with respect to 2%Au ZnO and MoO<sub>3</sub>. In previous work a design approach for a selective partial oxidation catalyst has been investigated, by combining components with a desired reactivity to produce a successful selective partial oxidation catalyst, which must activate methane and oxygen, and not destroy methanol. All these properties could not be found in a single catalyst, so it was proposed that two synergistic components could be combined, one responsible for methane activation and the other for oxygen activation/insertion. The doping of ZnO with Ga significantly lowered the light off temperature, and increased its oxidative capacity, an effect which was not seen with the doping of Ga<sub>2</sub>O<sub>3</sub> with Zn or Mg. The addition of Au to the Ga and Zn catalysts dramatically reduced the light off temperature, and increased its rate of oxidation at lower temperatures, both with optimum loading of 2%. The addition of l%Au and l%Pt produced a synergistic effect, producing lower light offs and higher CH<sub>4</sub> conversion than the singly doped catalysts with Au and Pt separately. When the methane activation catalysts were combined with MoO<sub>3</sub> in a physical mixture, a number of the mixtures produced higher methanol per pass percentage yields than its constituent parts. It is concluded that the increased methane activation properties beneficially interact with the oxygen activation and insertion properties of MoO<sub>3</sub>. The dual bed system, with the lower layer comprising the methane activation catalysts, and the upper layer consisting of MoO<sub> 3</sub> produced promising results, with the low temperature activation of CH<sub>4</sub>, combined with the oxygen insertion ability of MoO<sub>3</sub>, producing high selectivities of CH<sub>3</sub>OH at much lower temperatures. The best results were obtained when the ratio of the two layers was 50:50 with respect to 2%Au ZnO and MoO<sub>3</sub>. (Abstract shortened by UMI.).
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:583388 |
| Date | January 2004 |
| Creators | Hammond, Charles Rhodri |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/54537/ |
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